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CHAPTER 1

1.0 Introduction

Water comprises over 70% of the Earth’s surface and it is undoubtedly the

most precious natural resource that exists on our planet (Krantz and Kifferstein,

1996). Without the seemingly invaluable compound comprised of hydrogen and

oxygen, life on Earth would be non-existent (Krantz and Kifferstein, 1996). It is

essential for everything on our planet to grow and prosper. Although we as humans

recognize this fact, we disregard it by polluting our rivers, lakes, and oceans.

Subsequently, we are slowly but surely harming our planet to the point where

organisms are dying at a very alarming rate (Krantz and Kifferstein, 1996). In

addition to innocent organisms dying off, our drinking water has become greatly

affected as is our ability to use water for recreational purposes. In order to combat

water pollution, we must understand the problems and become part of the solution

(Krantz and Kifferstein, 1996).

Water pollution is the contamination of water bodies such as lakes, rivers,

oceans, and groundwater by anthropogenic activities, thereby harming the organisms

and plants that live in these water bodies which occur when pollutants are discharged

directly into water bodies without treating them first. Water pollution is a major

problem in the global context. It has been suggested that it is the leading worldwide

cause of deaths and diseases, and that it accounts for the deaths of more than 14,000

people daily. In addition to the acute problems of water pollution in developing

countries, industrialized countries continue to struggle with pollution problems as

well. In the most recent national report on water quality in the United States, 45

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percent of assessed stream miles, 47 percent of assessed lake acres, and 32 percent of

assessed bay and estuarine square miles were classified as polluted (U.S. EPA, 2002).

Water is typically referred to as polluted when it is impaired by anthropogenic

contaminants and either does not support a human use, like serving as drinking water,

and/or undergoes a marked shift in its ability to support its constituent biotic

communities, such as fish. Natural phenomena such as volcanoes, algae blooms,

storms, and earthquakes also cause major changes in water quality and the ecological

status of water. Water can be polluted from many sources. Faecal contamination from

sewage makes water unpleasant and unsafe for recreational activities such as

swimming, boating or fishing. Many organic pollutants, including sewage effluent

and farm and food-processing wastes consume oxygen, suffocating fish and other

aquatic life. Nutrients such as nitrates and phosphates, from everything from farm

fertilizers to household detergents, can 'overfertilize' the water causing the growth of

large mats of algae, some of which are directly toxic. When the algae die, they sink to

the water bottom, decomposing, consuming oxygen and damaging ecosystems.

Chemical contaminants including heavy metals, pesticides and some industrial

chemicals can threaten wildlife and human health. Sediment run-off from the land can

make water muddy, blocking sunlight and, as a result, killing wildlife. Irrigation also,

especially when used improperly, can bring flows of salts, nutrients and other

pollutants from soils into water.

Heavy metals such as Pb and Cd are pollutants of very high priority concern in

the scientific community because apart from being toxic to the entire ecosystem, they

are non-biodegradable and have ability to bioaccumulate in biological species even at

very low concentrations (Yurtsever and Sengil, 2009). Exposures to Lead and

cadmium has been reported to have deleterious health effects on humans including

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damage to liver, kidney and a painful disease called “itai-itai” (Yurtsever and Sengil,

2009; Forstner and Wittman, 1979).

These metals get into natural surface and ground waters from industrial

effluents such as those from the oil and gas, plastic, pharmaceutical, storage-battery

manufacture, paper and pulp, mining, electroplating, lead smelting, other

metallurgical finishing, automobile industry, agricultural run-off, chemical spills and

municipal wastewaters (Yurtsever and Sengil, 2009). Various traditional methods of

removing heavy metals from wastewaters have been reported including the use of

precipitation and coagulation, chemical oxidation, sedimentation, filtration, osmosis,

ion exchange, etc.(Aziz et al., 2009).

Adsorption technology is currently being used extensively for the removal of

heavy metals from aqueous solutions because it is a cleaner, more efficient and

cheaper technology. A review of some recent low cost adsorbents material has been

done by (Yurtsever and Sengil, 2009). With the vast array of biosorbents currently

available for removal of metal ions, there is the dearth of information on the

adsorptive potential of C. papaya seeds

In this project work, Carica Papaya seeds (common name: Pawpaw Seeds)

was used as a adsorbent for the removal of metal ions (Pb2+ and Cd2+) from an

aqueous solution containing metal ions. The seeds were defatted before they were

used in other to remove all the fat and oil present in the seed because the adsorbent

was expected to be a waste material and an undefatted C. papaya seed is still a useful

material because of its oil content which is valuable to man.

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1.1 Aims/Objectives

(i) Defatted C. papaya seeds (DPS), a new biosorbent was used to remove Pb2+ and

Cd2+ from aqueous solution.

(ii) Study of the Sorption parameters such as contact time, equilibrium, pH and

temperature on the adsorbent (Carica papaya) were investigated.

1.2 Importance of this Project

This research work has introduced a cheap, readily available and also an

effective new adsorbent to the adsorption scientific community for the removal of

lead, cadmium and other heavy metals from polluted waste water. This is in line with

the Federal Government of Nigeria’s vision 2020, which says that by year 2020, there

will be portable water supply to every home.

1.3 Brief History of Pollution

Much of what we know of ancient civilizations comes from the wastes they

left behind. Refuse such as animal skeletons and implements from Stone Age cave

dwellings in Europe, China, and the Middle East have helped to reveal hunting

techniques, diet, clothing, tool usage, and the use of fire for cooking that was carried

out by people of old. As humans developed new technologies, the magnitude and

severity of pollution increased. Many historians speculate that the extensive use of

lead plumbing for drinking water in Rome caused chronic lead poisoning in those

who could afford such plumbing. The mining and smelting of ores that accompanied

the transition from the Stone Age to the Metal Age resulted in piles of mining wastes

that spread potentially toxic elements such as mercury, copper, lead, and nickel

throughout the environment. Evidence of pollution during the early Industrial

Revolution is widespread. Samples of hair from historical figures such as Newton and

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Napoleon show the presence of toxic elements such as antimony and mercury. By the

1800s, certain trades were associated with characteristic occupational diseases:

Chimney sweeps contracted cancer of the scrotum (the external sac of skin enclosing

the testes, or reproductive glands) from hydrocarbons in chimney soot; hatters became

disoriented, or “mad,” from nerve-destroying mercury salts used to treat felt fabric;

and bootblacks suffered liver damage from boot polish solvents.

By the 21st century, pollution had evolved from a mainly localized problem to

one of global consequences in which pollutants not only persisted in the environment,

but changed atmospheric and climatic conditions. The Minamata Bay disaster was the

first major indication that humans would need to pay more attention to their waste

products and waste disposal practices, in particular, hazardous waste disposal. In the

years that followed, many more instances of neglect or carelessness resulted in

dangerous levels of contamination. In 1976 an explosion at a chemical factory in

Seveso, Italy, released clouds of toxic dioxin into the area, exposing hundreds of

residents and killing thousands of animals that ate exposed food.

The 1986 Chernobyl’ nuclear reactor accident demonstrated the dangerous

contamination effects of large, uncontained disasters. In an unprecedented action,

pollution was used as a military tactic in 1991 during the conflict in the Persian Gulf.

The Iraqi military intentionally released as much as 1 billion liters (336 million

gallons) of crude oil into the Persian Gulf and set fire to more than 700 oil wells,

sending thick, black smoke into the atmosphere over the Middle East.

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1.4 Types of Pollution

There are three major types of Pollution, which are: air, land and water.

1.4.1 Air: The growth of population centers coupled with the switch from wood-

burning to coal-burning fires created clouds of smoke over cities as early as the

eleventh century (Asimov et al., 1991). This problem is not confined to developed

countries only, but as the Industrial Revolution swept across countries, and as coal

became common in private residences all over the world, smoke and industrial

pollution claimed more and more lives (Leinwand and Gerald, 1990). In the United

States, Donora, Pennsylvania, became famous for a tragedy that symbolized the

dangers of industrial air pollution. In October 26, 1948, a thick, malodorous fog

enveloped the small industrial town. Unlike usual fogs, it did not burn off as the day

progressed but instead, it stayed on the ground for five days which resulted to the

death of twenty people in Donora and about 7,000 were hospitalized with respiratory

problems (Asimov et al., 1991). Smoke from coal-fired power plants creates the

related problem of acid rain and gases (sulfur dioxide and nitrogen oxides) released

by burning fossil fuels, making the rain more acidic and therefore corrosive. Acid rain

kills plants and trees and damages structures. It also accumulates in rivers and

streams, and has resulted in lakes that are already devoid of life in areas where

industrial processes abound (Leinwand and Gerald, 1990).

All around the world, the advent of the internal combustion engine-powered

vehicles compounded air pollution, adding particulate and gaseous contaminant to the

air people breath. The use of leaded gasoline raised lead levels in populations around

the world (Leinwand and Gerald, 1990).

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1.4.2. Land Pollution: This occurs as a result of chemical/ non chemical waste dumps

on land surfaces by anthropogenic activities, leading to environmental disasters. The

practice of chemical dumping persisted for years in the early twentieth century, in

many places, without a thought to the possible risks or consequences of these actions.

Across the globe, developing countries have been buying hazardous waste from

developed nations, where disposal is more expensive. Historically, there has been

little or no regulation of hazardous waste disposal in developing nations; as the world

becomes more of a global community, however, this problem will no doubt haunt

future generations, this is because the buried waste over time will become toxic and

when rain falls, it will leach these toxins to several water bodies and if such waters are

not treated properly before use, there will be crises of loss of life (:ebel et al., 2000).

1.5. Water Pollution

Water is essential to life and that is why most human settlements always began

near a water source. Unfortunately, the importance of clean water was not understood

until the second half of the nineteenth century, a relatively recent development

(Markham and Adam, 1994). The pollution of water with raw sewage was the

catalyst for many typhoid and cholera outbreaks throughout the centuries, in many

parts of the world. Even today, in numerous developing nations, cholera still kills tens

of thousands each year because clean drinking water is not available, or accessible, to

everyone (Ponting and Clive, 1992).

In the developed countries, human wastes was carried in rivers for centuries,

and not only were freshwater sources used as sewage dumps in most of the Western

world, but industrial waste was also discarded in rivers and streams (Leinwand and

Gerald, 1990). Leather tanning waste and butchering waste were frequent early

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polluters of water sources and as the Industrial Revolution progressed, water pollution

became a major crisis (Leinwand and Gerald, 1990). Factories found water sources,

especially rivers, a convenient means of waste disposal, and the trend continued well

into the twentieth century. Recently, many industrial waste byproducts find their way

into the water, either through direct dumping by companies, or through leaching into

groundwater from dumping sites, resulting into massive wildlife die-offs, and are also

blamed for elevated cancer rates, birth defects, and a lower IQ in people who

subsisted on water polluted by heavy industries (Asimov et al., 1991).

1.5.1. Sources of Water Pollution: There are many sources of water pollution but

two general categories exist: direct and indirect contamination sources. Direct sources

include effluent outfalls from factories, refineries, and waste treatment plants and so

on, that emit fluids of varying quality directly into urban water supplies. In the

developed and some developing countries, these practices are regulated, although this

does not mean that pollutants cannot be found in their waters. Indirect sources include

contaminants that enter the water supply from soils/groundwater systems and from the

atmosphere via rain water. Soils and ground waters contain the residue of agricultural

practices (fertilizers, pesticides, etc.) and improperly disposed industrial wastes.

Atmospheric contaminants are also derived from human practices such as gaseous

emissions from automobiles, factories and even bakeries.

Contaminants can be broadly classified into:

• Organic pollutants which are by-products of auto mobile, petroleum and

chemical industries, emissions from waste incinerators, service stations,

domestic solid fuel and gas combustion, spray painting, dry-cleaning and other

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solvent usage, and cigarette smoke, and these pollutants include: benzene,

toluene, xylene and formaldehyde.

• Inorganic pollutants which are of primary concern in urban storm water

containing cadmium, copper, lead, zinc, nitrogen, nitrate, nitrite, ammonia,

phosphorous, and phosphate. These are toxic to humans and the entire

ecosystem.

• Radioactive pollutants from natural sources of radiation are radon gas, medical

X-rays and nuclear medicine; and

• Acid/base pollutants, which occur wherever there are many cars and factories,

producing dangerous chemical by-products that include: sulphur dioxide,

carbon dioxide, and nitrogen oxides which are mixed with moisture in the air.

Towns and municipalities are also major sources of water pollution. In many

public water systems, pollution exceeds safe levels. One reason for this is that much

groundwater has been contaminated by wastes pumped underground for disposal or

by seepage from surface water. When contamination reaches underground water

tables, it is difficult to correct and spreads over wide areas. In addition, many U.S.

communities discharge untreated or only partially treated sewage into the waterways,

threatening the health of their own and neighbouring populations. Along with

domestic wastes, sewage carries industrial contaminants and a growing tonnage of

paper and plastic refuse. Although thorough sewage treatment would destroy most

disease-causing bacteria, the problem of the spread of viruses and viral illness

remains. Additionally, most sewage treatment does not remove phosphorus

compounds, contributed principally by detergents, which cause Eutrophication of

lakes and ponds. Excreted drugs and household chemicals also are not removed by

present municipal treatment facilities, and can be recycled into the drinking water

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supply. Rain drainage is another major polluting agent because it carries such

substances as highway debris (including oil and chemicals from automobile exhausts),

sediments from highway and building construction, and acids and radioactive wastes

from mining operations into freshwater systems as well as into the ocean. Also

transported by rain runoff and by irrigation return-flow are animal wastes from farms

and feedlots, a widespread source of pollutants impairing rivers and streams,

groundwater, and even some coastal waters. Antibiotics, hormones, and other

chemicals used to raise livestock are components of such animal wastes. Pesticide and

fertilizer residues from farms also contribute to water pollution via rain drainage.

1.5.2. Methods of Controlling / Treatment of Water Pollution

Domestic Sewage Treatment: In urban areas, domestic sewage is typically treated by

centralized sewage treatment plants. In the U.S., most of these plants are operated by

local government agencies. Municipal treatment plants are designed to control

conventional pollutants like BOD (Biochemical Oxygen Demand) and suspended

solids. Well-designed and operated systems (i.e., secondary treatment or better) can

remove 90 percent or more of these pollutants. Some plants have additional sub-

systems to treat nutrients and pathogens. Most municipal plants are not designed to

treat toxic pollutants found in industrial wastewater (U.S. EPA, 2004).

Cities with sanitary sewer overflows or combined sewer overflows employ one or

more engineering approaches to reduce discharges of untreated sewage, including:

Utilizing a green infrastructure approach to improve storm water management

capacity throughout the system (U.S. EPA, 2008); repair and replacement of leaking

and malfunctioning equipment(U.S. EPA, 2004); increasing overall hydraulic

capacity of the sewage collection system (often a very expensive option).

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Industrial Wastewater Treatment: Industries that generate wastewater with high

concentrations of conventional pollutants (e.g. oil and grease), toxic pollutants (e.g.

heavy metals, volatile organic compounds) or other non - conventional pollutants such

as ammonia, need specialized treatment systems. Some of these facilities can install a

pre-treatment system to remove the toxic components, and then send the partially-

treated wastewater to the municipal system. Industries generating large volumes of

wastewater typically operate their own complete on-site treatment systems.

Some industries have been successful at redesigning their manufacturing processes to

reduce or eliminate pollutants, through a process called pollution prevention.

Heated water generated by power plants or manufacturing plants may be controlled

with: Cooling ponds, man-made bodies of water designed for cooling by evaporation,

convection, and radiation; cooling towers, which transfer waste heat to the

atmosphere through evaporation and/or heat transfer; cogeneration, a process where

waste heat is recycled for domestic and/or industrial heating purposes.

Agricultural Wastewater Treatment

:on point Source Control: Farmers utilize erosion controls to reduce runoff flows

and retain soil on their fields. Common techniques include contour plowing, crop

mulching, crop rotation, planting perennial crops and installing riparian buffers.( U.S.

EPA, 2003). Nutrients (nitrogen and phosphorus) are typically applied to farmland as

commercial fertilizer; animal manure; or spraying of municipal or industrial

wastewater (effluent) or sludge. Nutrients may also enter runoff from crop residues,

irrigation water, wildlife, and atmospheric deposition(U.S. EPA, 2003). Farmers can

develop and implement nutrient management plans to reduce excess application of

nutrients (U.S. EPA, 2003).

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Point Source Wastewater Control: Farms with large livestock and poultry

operations, such as factory farms, are called concentrated animal feeding operations

or confined animal feeding operations in the U.S. and are being subject to increasing

government regulation( U.S. EPA, 2008). Animal slurries are usually treated by

containment in lagoons before disposal by spray or trickle application to grassland.

Constructed wetlands are sometimes used to facilitate treatment of animal wastes, as

are anaerobic lagoons. Some animal slurries are treated by mixing with straw and

composted at high temperature to produce a bacteriologically sterile and friable

manure for soil improvement.

Construction Site Storm Water

Sediment from construction sites is managed by installation of: erosion

controls, such as mulching and hydro seeding, and sediment controls, such as

sediment basins and silt fences. Discharge of toxic chemicals such as motor fuels and

concrete washout is prevented by use of: spill prevention and control plans, and

specially-designed containers (e.g. for concrete washout) and structures such as

overflow controls and diversion beams (U.S. EPA, 2006).

Urban runoff (storm water)

Effective control of urban runoff involves reducing the velocity and flow of

stormwater, as well as reducing pollutant discharges. Local governments use a variety

of stormwater management techniques to reduce the effects of urban runoff. Pollution

prevention practices include low impact development techniques, installation of green

roofs and improved chemical handling (e.g. management of motor fuels & oil,

fertilizers and pesticides)( U.S. EPA, 2008). Runoff mitigation systems include

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infiltration basins, bioretention systems, constructed wetlands, retention basins and

similar devices.

1.6. Heavy Metals

There are 35 metals that concern us because of occupational or residential

exposure; 23 of these are the heavy elements or "heavy metals": antimony, arsenic,

bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, gold, iron, lead,

manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium,

vanadium, and zinc (Glanze, 1996). Interestingly, small amounts of these elements

are common in our environment and diet and are actually necessary for good health,

but large amounts of any of them may cause acute or chronic toxicity (Glanze, 1996).

Heavy metal toxicity can result in damaged or reduced mental and central nervous

function, lower energy levels, and damage to blood composition, lungs, kidneys, liver,

and other vital organs (International Occupational Safety and Health Information

Centre 1999). Long-term exposure may result in slowly progressing physical,

muscular, and neurological degenerative processes that mimic Alzheimer's disease,

Parkinson's disease, muscular dystrophy, and multiple sclerosis. Allergies are not

uncommon and repeated long-term contact with some metals or their compounds may

even cause cancer (IOSHIC, 1999).

For some heavy metals, toxic levels can be just above the background

concentrations naturally found in nature. Therefore, it is important for us to inform

ourselves about the heavy metals and to take protective measures against excessive

exposure. In most parts of the world, heavy metal toxicity is an uncommon medical

condition; however, it is a clinically significant condition when it does occur. If

unrecognized or inappropriately treated, toxicity can result in significant illness and

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reduced quality of life (Ferner, 2001). For persons who suspect that they or someone

in their household might have heavy metal toxicity, testing is essential. Appropriate

conventional and natural medical procedures may need to be pursued (Dupler, 2001).

The association of symptoms indicative of acute toxicity is not difficult to

recognize because the symptoms are usually severe, rapid in onset, and associated

with a known exposure or ingestion (Ferner, 2001): cramping, nausea, and vomiting;

pain; sweating; headaches; difficulty breathing; impaired cognitive, motor, and

language skills; mania; and convulsions. The symptoms of toxicity resulting from

chronic exposure (impaired cognitive, motor, and language skills; learning

difficulties; nervousness and emotional instability; and insomnia, nausea, lethargy,

and feeling ill) are also easily recognized; however, they are much more difficult to

associate with their cause. Symptoms of chronic exposure are very similar to

symptoms of other health conditions and often develop slowly over months or even

years. Sometimes the symptoms of chronic exposure actually abate from time to time,

leading the person to postpone seeking treatment, thinking the symptoms are related

to something else.

1.6.1. Definition of Heavy Metals

Heavy metals are chemical elements with a specific gravity that is at least 5

times the specific gravity of water (Lide, 1992). The specific gravity of water is 1 at

4°C (39°F). Simply stated, specific gravity is a measure of density of a given amount

of a solid substance when it is compared to an equal amount of water. Some well-

known toxic metallic elements with a specific gravity that is 5 or more times that of

water are arsenic, 5.7; cadmium, 8.65; iron, 7.9; lead, 11.34; and mercury, 13.546

(Lide, 1992). Heavy metals could also be defined generally as collective term which

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applies to the group of metals and metalloids with an atomic density greater than 4

g/cm³. Although it is a loosely defined term (Duffus, 2002), it is widely recognized and

usually applies to the widespread contaminants of terrestrial and freshwater

ecosystems. The heavy metals which are included in Apis are cadmium, chromium,

copper, mercury, lead, zinc, arsenic, boron and the platinum group metals, which

comprises Platinum, Palladium, Rhodium, Ruthenium, Osmium, and Iridium. Unlike

almost all organic pollutants, such as organochlorines, heavy metals are elements

which occur naturally in the Earth’s crust. They are therefore found naturally in soils

and rocks with a subsequent range of natural background concentrations in soils,

sediments, waters and organisms. Anthropogenic releases can give rise to higher

concentrations of the metals relative to the normal background values.

1.6.2. Beneficial Heavy Metals

In small quantities, certain heavy metals are nutritionally essential for a

healthy life. Some of these are referred to as the trace elements (e.g., iron, copper,

manganese, and zinc). These elements, or some form of them, are commonly found

naturally in foodstuffs, in fruits and vegetables, and in commercially available

multivitamin products (IOSHIC, 1999). Diagnostic medical applications include

direct injection of gallium during radiological procedures, dosing with chromium in

parental nutrition mixtures, and the use of lead as a radiation shield around x-ray

equipment (Roberts, 1999). Heavy metals are also common in industrial applications

such as in the manufacture of pesticides, batteries, alloys, electroplated metal parts,

textile dyes, steel, and so forth (IOSHIC, 1999). Many of these products are in our

homes and actually add to our quality of life when properly used.

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1.6.3. Toxic Heavy Metals

Heavy metals become toxic when they are not metabolized by the body and

accumulate in the soft tissues. Heavy metals may enter the human body through food,

water, air, or absorption through the skin when they come in contact with humans in

agriculture and in manufacturing, pharmaceutical, industrial, or residential settings.

Industrial exposure accounts for a common route of exposure for adults. Ingestion is

the most common route of exposure in children (Roberts, 1999). Children may

develop toxic levels from the normal hand-to-mouth activity of small children who

come in contact with contaminated soil or by actually eating objects that are not food

(dirt or paint chips) (Dupler, 2001). Less common routes of exposure are during a

radiological procedure, from inappropriate dosing or monitoring during intravenous

(parental) nutrition, from a broken thermometer (Smith et al., 1997), or from a

suicide or homicide attempt (Lupton et al., 1985).

As a rule, acute poisoning is more likely to result from inhalation or skin

contact of dust, fumes or vapors, or materials in the workplace. However, lesser levels

of contamination may occur in residential settings, particularly in older homes with

lead paint or old plumbing (International Occupational Safety and Health Information

Centre 1999). The Agency for Toxic Substances and Disease Registry (ATSDR) in

Atlanta, Georgia, (a part of the U.S. Department of Health and Human Services) was

established by congressional mandate to perform specific functions concerning

adverse human health effects and diminished quality of life associated with exposure

to hazardous substances. The ATSDR is responsible for assessment of waste sites and

providing health information concerning hazardous substances, response to

emergency release situations, and education and training concerning hazardous

substances (ATSDR, 2001).

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1.6.4. Sources of Heavy Metal

Motivations for controlling heavy metal concentrations in gas streams are

diverse. Some of them are dangerous to health or to the environment (e.g. Hg, Cd, As,

Pb, Cr). Some may cause corrosion (e.g. Zn, Pb), some are harmful in other ways (e.g.

Arsenic may pollute catalysts). Within European community the 13 elements of

highest concern are As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sn and Tl, the emissions of

which are regulated in waste incinerators. Some of these elements are actually

necessary for humans in minute amounts (Co, Cu, Cr, Ni) whilst others are

carcinogenic or toxic, affecting, among others, the central nervous system (Hg, Pb,

As), the kidneys or liver (Hg, Pb, Cd, Cu) or skin, bones or teeth (Ni, Cd, Cu, Cr).

Heavy metal pollution can arise from many sources but most commonly arises

from the purification of metals, e.g., the smelting of copper and the preparation of

nuclear fuels. Electroplating is the primary source of chromium and cadmium.

Through precipitation of their compounds or by ion exchange into soils and muds,

heavy metal pollutants can localize and lay dormant. Unlike organic pollutants, heavy

metals do not decay and thus pose a different kind of challenge for remediation.

Currently, plants or microorganisms are tentatively used to remove some

heavy metals such as mercury. Plants which exhibit hyper accumulation can be used

to remove heavy metals from soils by concentrating them in their bio mater. Some

treatment of mining tailings has occurred were the vegetation is then incinerated to

recover the heavy metals.

Broadly, heavy metals can be considered to come from two sources. These

are:

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:atural Sources: These are as a result of natural processes such as weathering of

heavy metal- containing rocks and those that are found as constituents of soil parent

materials and aquatic environment.

Anthropogenic Sources: These are sources of heavy metals linked with man’s

activities such as mining, farming, use of chemicals such as pesticides, fertilizers and

other chemicals; municipal wastes and industrial big-products, contaminated

sediments and mines wastes, accidental spills, fuels, paints, fuels, smelting processes,

battery manufacture etc, and metal processing which is the main source of heavy

metal contamination.

1.6.5. Effects of Heavy Metals

It is difficult to summarize all the adverse effects due to heavy metals because of

their large number and the fact that each element can cause a number of different

problems. The effects of heavy metals can be grouped into four broad categories;

these are:

Human Health Hazards: The primary results of exposure of humans to heavy metals

is through inhalation of dust, consumption of food grown on contaminated soils or

through direct ingestion of soil. Chronic low-level intakes of heavy metals have

damaging effect on humans as there is no good mechanism for their elimination.

Metals such as lead, copper, mercury cadmium are cumulative poisons. The

introduction of lead and copper into food chain affect human health and studies

concerning their accumulation vegetables is of increasing importance. Lead is known

to cause impaired motor skill development, delay growth and affect

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neurotransmission in children. In humans, lead toxicity sometimes causes the

formation of a bluish line along the gums, which is known as the “Burton line” or

"lead line" (James et al., 2005), although this is very uncommon in young children.

Chronic lead poisoning can produce a lead hue of the skin with a lividity and pallor

(James et al., 2005). Mercury is known to cause irritability, lack of calmness,

memory loss, fatigue, learning disorders, arthritis, and inflammation while, cadmium

poisoning causes softening of the bones and kidney failure, causing severe pains in the joints

and spine, the disease is named “itai-itai disease” (:ordberg, 2004).

Animal Health: Animal health effects include domestic animals and wildlife.

Toxicities of Molybdenum in livestock (molybdenosis) and Selenium in livestock and

wildlife (selenosis) are common having metal animal health problems. Ruminant

animals are the most susceptible to Molybdenosis, a Mo – Induced Cu deficiency

Selemium and other metal toxicity are harmful to animals if consumed regularly.

Phytotoxicities: Phytotoxicity refers to reduce yields or death of plants in the soil.

Symptoms of heavy metal induced phytoxicities include stunting, chlorisis, necrosis,

and death of the plant. Heavy metals often associated with phytoxiaties include

stunting, chlorosis, necrosis and death of the plant. Heavy metals often associated

with physotoxicities include Cu, Ni and Zn Phytixicity problems are of concern for

two primary reasons. First, the reduction in soil quality included by elevated heavy

metal concentrations can reduce both the quantity and quality of the food produced

from that soil; secondly, in areas where irrigation is spare due to heavy metal

toxicitives, wind and water erosion occurs uninhabited.

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Aquatic Environment: Effect of elevated concentrations of heavy metals in aquatic

environment are difficult to assess due in part to the mobility of some of the species

and the difficulty in separating out the effect of contaminated sediments. Soil erosion

and industrial effluent discharge are the main source of heavy metals in aquatic

environments. The effects of heavy metals in aquatic environment are generally to

reduce diversity, productivity and density of aquatic organisms.

1.6.6. Methods of Heavy Metal Removal from Industrial Effluents.

Discharge of metals to aquatic environment is of major concern and treatment of

industrial effluents has consequently attracted considerable attention. Common

methods of metal removal from industrial effluents include chemicals precipitation,

dialysis, solvent extraction, electrolytic extraction, cementation, reverse osmosis,

membrane filtration, ion exchange, adsorption and co-precipitation. Conventional

physical and chemical treatment of low-concentration, large volume wastes tend to be

very costly. Consumptive process such as chemical precipitation entails large capital

and operating cost. Attention has thus been focused on non-consumptive methods that

include ion-exchange and other sorption process. Even then, commercially available

resins and activated carbons can be expensive, hence: the need to development other

methods that are relatively cheap.

Ion Exchange: An ion-exchange system consists of a pair of column or pressure

vessel, one containing an anion exchange resin, the other, a cation exchange resin.

The effluent is continuously pumped through the two columns in a series to yield the

treated effluent. Where an exceptionally high quality effluent is required, a third

21

mixer-bed exchange column can be placed after the cation exchange resin. It is

particularly suitable where low metal concentration is present.

Solvent Extraction: This involves separation of components of a liquid mixture by

treatment with a solvent in which one or more of the desired component is

preferentially soluble. It is a process widely used in the petroleum and chemical

industry but which has only recently been adopted for recovery of metals from

aqueous solutions. At present, it finds only little application in this sphere largely

because of cost of solvent loss.

Reverse Osmosis: On a practical scale, it is still at infancy. The process requires high

pressure (up to 100 atmospheres) and is thus fairly costly in terms of energy. The

delicacy of the membrane is restrictive with regard to solid content and pH of the

material being treated. However, the process has been used on effluents from

electroplating in the electronic component industry.

Electrodialysis: In this process, the ionic components (heavy metals) are separated

through the use of semi-permeable ion-selective membranes. Application of an

electrical potential between the two electrodes causes a migration of cations and

anions towards respective electrodes. Because of the alternate spacing of cation and

anion permeable membranes, cells of concentrated and dilute salts are formed. The

disadvantage is the formation of metal hydroxides, which clog the membrane. The

cost of this process is very dependent on dissolved solids concentration. It does not

find common use in effluent treatment but may be appropriate in certain

22

circumstances where the concentrate is of high value. A recent development is the

rotatory electrode.

Evaporation: This is one of the most common methods used in industry for

concentrating aqueous solution. Nevertheless use of this method as a means of

effluents treatment is rare and occurs only under special circumstances where

effluents contains a high concentrations of valuable material e.g. the concentration of

static rinses from electroplating processes especially chromium.

Ultrafiltration: They are pressure driven membrane operations that use porous

membranes for the removal of heavy metals. The main disadvantage of this process is

the generation of sludge.

Chemical Precipitation: Precipitation of metals is achieved by the addition of

coagulants such as alum, lime, iron salts and other organic polymers. The large

amount of sludge containing toxic compounds produced during the process is the

main disadvantage.

Phytoremediation: Phytoremediation is the use of certain plants to clean up soil,

sediment, and water contaminated with metals. The disadvantages include that it takes

a long time for removal of metals and the regeneration of the plant for further

biosorption is difficult.

23

1.7. Adsorption

This process can be cheap, relatively simple and effective if a suitable

adsorbent is found. The use of adsorption as a method of removal of heavy metals

from industrial effluent is the focus of this project work. Adsorption is the

accumulation of atoms or molecules on the surface of a material. This process creates

a film of the adsorbate (the molecules or atoms being accumulated) on the adsorbent's

surface. It is different from absorption, in which a substance diffuses into a liquid or

solid to form a solution. The term sorption encompasses both processes, while

desorption is the reverse process. Adsorption is present in many natural physical,

biological, and chemical systems, and is widely used in industrial applications such as

activated charcoal, synthetic resins, and water purification. Adsorption, ion exchange,

and chromatography are sorption processes in which certain adsorbates are selectively

transferred from the fluid phase to the surface of insoluble, rigid particles suspended

in a vessel or packed in a column.

Similar to surface tension, adsorption is a consequence of surface energy. In a

bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the

constituent atoms of the material are filled by other atoms in the material. However,

atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent

atoms and therefore can attract adsorbates. The exact nature of the bonding depends

on the details of the species involved, but the adsorption process is generally

classified as physisorption (characteristic of weak van der Waals forces) or

chemisorption (characteristic of covalent bonding).

24

1.8. Isotherms

Adsorption is usually described through isotherms, that is, the amount of

adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if

liquid) at constant temperature. The quantity adsorbed is nearly always normalized by

the mass of the adsorbent to allow comparison of different materials.

The first mathematical fit to an isotherm was published by Freundlich and Küster

(1894) and is a purely empirical formula for gaseous adsorbates:

nkPm

x /1=

where x is the quantity adsorbed, m is the mass of the adsorbent, P is the pressure of

adsorbate and k and n are empirical constants for each adsorbent-adsorbate pair at a

given temperature.

The function has an asymptotic maximum as pressure increases without

bound. As the temperature increases, the constants k and n change to reflect the

empirical observation that the quantity adsorbed rises more slowly and higher

pressures are required to saturate the surface.

In 1916, Irving Langmuir published a new model isotherm for gases adsorbed on

solids, which retained his name. It is a semi-empirical isotherm derived from a

proposed kinetic mechanism. It is based on four assumptions:

(i) The surface of the adsorbent is uniform, that is, all the adsorption sites are

equivalent.

(ii) Adsorbed molecules do not interact.

(iii) All adsorption occurs through the same mechanism.

(iv)At the maximum adsorption, only a monolayer is formed: molecules of

adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only

on the free surface of the adsorbent.

25

These four assumptions are seldom all true: there are always imperfections on the

surface, adsorbed molecules are not necessarily inert, and the mechanism is clearly

not the same for the very first molecules to adsorb as for the last. The fourth condition

is the most troublesome, as frequently more molecules will adsorb on the monolayer;

this problem is addressed by the BET isotherm for relatively flat (non-microporous)

surfaces. The Langmuir isotherm is nonetheless the first choice for most models of

adsorption, and has many applications in surface kinetics (usually called Langmuir-

Hinshelwood kinetics) and thermodynamics.

Langmuir suggested that adsorption takes place through this mechanism:

ASSAg ⇔+

Where Ag is a gas molecule and S is an adsorption site. The direct and inverse rate

constants are k and k-1. If we define surface coverage, θ, as the fraction of the

adsorption sites occupied, in the equilibrium we have

KP

KPor

PK

KK

+=

−=

−=

1)1(1θ

θθ

Where P is the partial pressure (gas) or the molar concentration of the solution (gas).

For very low pressures KP≈θ and for high pressures, 1≈θ , θ is difficult to measure

experimentally; usually, the adsorbate is a gas and the quantity adsorbed is given in

moles, grams, or gas volumes at standard temperature and pressure (STP) per gram of

adsorbent. If we call vmon the STP volume of adsorbate required to form a monolayer

on the adsorbent (per gram of adsorbent), monv

v=θ and we obtain an expression for a

straight line: monmon vPKvv +

+=1

1111

Through its slope and y-intercept we can obtain vmon and K, which are constants for

each adsorbent/adsorbate pair at a given temperature. vmon is related to the number of

26

adsorption sites through the ideal gas law. If we assume that the number of sites is just

the whole area of the solid divided into the cross section of the adsorbate molecules,

we can easily calculate the surface area of the adsorbent. The surface area of an

adsorbent depends on its structure; the more pores it has, the greater the area, which

has a big influence on reactions on surfaces.

If more than one gas adsorbs on the surface, we define θE as the fraction of empty

sites and we have

∑=

+=

n

i

ii

jj

j

PK

PK

1

1

θ and

∑=

+=

n

i

ii

E

PK1

1

1θ

where i is each one of the gases that adsorb.

1.8.1. Types of Isotherms

Bet Isotherm Equation: Brunauer et al., (1938) developed an equation for

multimolecular adsorption, so-called the BET equation:

seeS

ee

CCBCC

QBCQ

/)1(1)[(

0

−+−=

Where, Q0 is the amount of solute adsorbed per unit weight of adsorbent in forming a

complete monolayer on the surface, B is a constant relating to the energy of

interaction with the surface and Cs is the saturation concentration of the solute.

Preetha and Viruthagiri (2005) applied the BET model to describe biosorption of

Zn2+ by Rhizopus arrhizus.

Pseudo-First-Order Equation: The pseudo-first-order kinetic equation or the so-

called Lagergren equation has the following formulation:

27

)( te

e QQKdt

dQ−=

in which Qt is the amount of adsorbate adsorbed at time t, Qe is its value at equilibrium and k1 is a constant. The pseudo-firs torder Lagergren equation is indeed in line with the concept of linear driving force.

Pseudo-Second-Order Equation: The pseudo-second-order kinetic equation was

first proposed by Blanchard et al. and since then it has been frequently employed to

analyze biosorption data obtained from various experiments using different adsorbates

and biosorbents as reviewed by Ho et al., (2000):

22

' )( te

t QQKdt

dQ−=

in which k’2 is a constant.

Toth Isotherm Equation: This isotherm equation was derived from the potential

theory, and has been widely employed to describe biosorption on heterogeneous

biosorbent surface:

nTn

bTC

bTCQQ

e

e

e

]1

)(1[max

+=

in which bT and nT are two constants. Obviously, if nT =1, the equation reduces to

the Langmuir-type isotherm equation, but the T´oth isotherm equation could not

reflect the feature of the Freundlich-type biosorption. It has been reported that the

above equation could reasonably describe the Ni2+ biosorption by Sargassum wihtii

(Vijayaraghavan et al., 2006).

Sips Isotherm Equation: In study of the distribution of adsorption energies of the

sites of a catalyst surface, Sips proposed an empirical isotherm equation which is

often expressed as:

28

CnseK

eCnKQQ

eq

eq

ethe +=

1

5

in which ns is the Sips constant. At the time of proposing the above empirical

isotherm, Sips noted that “we do not know whether or not this form of isotherm

actually represents any experimental results”. Based on a comparison study of a

number of isotherm equations applied to the sorption of methylene blue on lemon

peel, Kumar and Porkodi concluded that the Sips isotherm provided the best fitting of

experimental data, followed by the Langmuir and the Redlich– Peterson isotherm

equations. In the following sections, two approaches were developed for derivation of

the empirical Sips equation.

Freundlich Isotherm Equation: Freundlich proposed an empirical isotherm

equation:

nF

ee kFEQ/1=

in which kF and nF are Freundlich constants. As the Freundlich isotherm equation is

exponential, it can only be reasonably applied in the low to intermediate concentration

ranges. Similar to the Langmuir isotherm the equation, has also been widely

employed in biosorption research.

Langmuir Isotherm Equation: Langmuir theoretically examined the adsorption of

gases on solid surfaces, and considered sorption as a chemical phenomenon.

Basically, the Langmuir isotherm equation has a hyperbolic form:

eeq

eeq

etheCK

CKQQ

+=

1

29

in which Qe is the adsorption capacity by weight at equilibrium, Qeth is the theoretical

maximum adsorption capacity by weight, and Keq represents the equilibrium constant

of adsorption reaction, while Ce is concentration of adsorbate at equilibrium. This

isotherm equation has been most frequently applied in equilibrium study of

biosorption, however, it should be realized that the Langmuir isotherm offers no

insights into the mechanism aspects of biosorption.

1.9. Biosorption

The search for new technologies involving the removal of toxic metals from

wastewaters has directed attention to biosorption, based on metal binding capacities of

various biological materials. Biosorption can be defined as the ability of biological

materials to accumulate heavy metals from wastewater through metabolically

mediated or physico-chemical pathways of uptake (Fourest and Roux, 1992). Algae,

bacteria and fungi and yeasts have proved to be potential metal biosorbents (Volesky,

1986). The major advantages of biosorption over conventional treatment methods

include (Kratochvil and Volesky, 1998): Low cost, High efficiency, Minimisation of

chemical and/or biological sludge, No additional nutrient requirement, Regeneration

of biosorbent, and Possibility of metal recovery.

The biosorption process involves a solid phase (sorbent or biosorbent;

biological material) and a liquid phase (solvent, normally water) containing a

dissolved species to be sorbed (sorbate, metal ions). Due to higher affinity of the

sorbent for the sorbate species, the latter is attracted and bound there by different

mechanisms. The process continues till equilibrium is established between the amount

of solid-bound sorbate species and its portion remaining in the solution. The degree of

30

sorbent affinity for the sorbate determines its distribution between the solid and liquid

phases.

1.9.1. Biosorbent material: Strong biosorbent behaviour of certain micro-organisms

towards metallic ions is a function of the chemical make-up of the microbial cells.

This type of biosorbent consists of dead and metabolically inactive cells. Some types

of biosorbents would be broad range, binding and collecting the majority of heavy

metals with no specific activity, while others are specific for certain metals. Some

laboratories have used easily available biomass whereas others have isolated specific

strains of microorganisms and some have also processed the existing raw biomass to a

certain degree to improve their biosorption properties; Recent biosorption

experiments have focused attention on waste materials, which are by-products or the

waste materials from large-scale industrial operations. For e.g. the waste mycelia

available from fermentation processes, olive mill solid residues (Pagnanelli et al.,

2002), activated sludge from sewage treatment plants (Hammaini et al., 2003),

biosolids (:orton et al., 2003), aquatic macrophytes (Keskinkan et al., 2003), etc.

:orton et al., (2003) used dewatered waste activated sludge from a sewage treatment

plant for the biosorption of zinc from aqueous solutions. The adsorption capacity was

determined to be 0.564 mM/g of biosolids. The use of biosolids for zinc adsorption

was favourable compared to the biosorption rate of 0.299 mm/g by the seaweed

Durvillea potatorum (Aderhold et al., 1996). Keskinkan et al., (2003) studied the

adsorption characteristics of copper, zinc and lead on submerged aquatic plant

Myriophyllum spicatum. The adsorption capacities were 46.69 mg/g for lead, 15.59

mg/g for zinc and 10.37 mg/g for copper. Pagnanelli et al., (2002) have carried out a

preliminary study on the 'Use of olive mill residues as heavy metal sorbent material

31

and the results revealed that copper was maximally adsorbed in the range of 5.0 to

13.5 mg/g under different operating conditions.

The simultaneous biosorption capacity of copper, cadmium and zinc on dried

activated sludge (Hammaini et al., 2003) were 0.32 mmol/g for metal system such as

Cu-Cd; 0.29 mmol/g for Cu-Zn and 0.32 mmol/g for Cd-Zn. The results showed that

the biomass had a net preference for copper followed by cadmium and zinc.

Another inexpensive source of biomass where it is available in copious

quantities is in oceans as seaweeds, representing many different types of marine

macro-algae. However most of the contributions studying the uptake of toxic metals

by live marine and to a lesser extent freshwater algae focused on the toxicological

aspects, metal accumulation, and pollution indicators by live, metabolically active

biomass. Focus on the technological aspects of metal removal by algal biomass has

been rare. Although abundant natural materials of cellulosic nature have been

suggested as biosorbents, very less work has been actually done in that respect. The

mechanism of biosorption is complex, mainly ion exchange, chelation, adsorption by

physical forces, entrapment in inter and intrafibrilliar capillaries and spaces of the

structural polysaccharide network as a result of the concentration gradient and

diffusion through cell walls and membranes.

There are several chemical groups that would attract and sequester the metals

in biomass: acetamido groups of chitin, structural polysaccharides of fungi, amino and

phosphate groups in nucleic acids, amido, amino, sulphhydryl and carboxyl groups in

proteins, hydroxyls in polysaccharide and mainly carboxyls and sulphates in

polysaccharides of marine algae that belong to the divisions Phaeophyta, Rhodophyta

and Chlorophyta. However, it does not necessarily mean that the presence of some

32

functional group guarantees biosorption, perhaps due to steric, conformational or

other barriers.

1.9.2. Choice of metal for biosorption process

The appropriate selection of metals for biosorption studies is dependent on the

angle of interest and the impact of different metals, on the basis of which they would

be divided into four major categories: (i) toxic heavy metals (ii) strategic metals (iii)

precious metals and (iv) radio nuclides. In terms of environmental threats, it is mainly

categories (i) and (iv) that are of interest for removal from the environment and/or

from point source effluent discharges.

Apart from toxicological criteria, the interest in specific metals may also be

based on how representative their behaviour may be in terms of eventual

generalization of results of studying their biosorbent uptake. The toxicity and

interesting solution chemistry of elements such as chromium, arsenic and selenium

make them interesting to study. Strategic and precious metals though not

environmentally threatening are important from their recovery point of view.

1.10. Review of Past Works on Adsorption

Heavy metals are some of the major factors of environmental contaminations.

Physical and chemical methods have been proposed and applied to remove metal ions

from effluents, but in general, these methods are commercially impractical, either

because of high operating cost or the difficulty in treating. For example, the use of

conventional technologies, such as ion exchange, chemical precipitation, reverse

osmosis, and evaporative recovery, for this purpose is often inefficient and/or very

expensive (Cheung et. al., 1997). Some of the works carried-out by a number of

33

researchers will be reported as a form of review on past works on the adsorption of

metals from aqueous solutions containing metal ions and how effective the various

adsorbent used have been able to remove or adsorb the metal ions. Ions of toxic

metals can bind to the mineral surface in a number of ways, including absorption,

adsorption, precipitation and cation exchange (Steele et al., 2000).

Trivedi et al., (2001) investigated adsorption of metals onto goethite surface

and found a connection between metal affinities for goethite and order Cu > Pb > Zn

> Cd > Co > Ni > Mn > Ca > Mg, which tends to follow the decrease in

electronegativity and radii of the hydrated cations, with a slight variation.

Morera et al., (2001) evaluated the mobility and distribution of Cd, Cu, Ni,

Pb and Zn in four different types of soils. They concluded that soils have a higher

affinity with Pb and Cu than with Cd, Ni or Zn. Sequential extraction analyses

showed that in the case of soils with a higher content of carbonates, metals were

extracted predominantly from the carbonate fraction. The authors also confirmed the

high affinity of Cu with organic matter.

Chemical contaminants at low concentrations are difficult to remove from

water. Chemical precipitation, reverse osmosis and other methods become inefficient

when contaminants are present in trace concentrations. The process of adsorption is

one of the few alternatives available for such situations (Huang and Morehart,

1991). A number of adsorbent materials have been studied for their capacity to

remove heavy metals, including activated carbon, activated alumina, ion exchange

resins, crushed coals etc. Some of these materials, such as ion exchange resins are

totally effective but expensive and others, such as coal and straw, are inexpensive but

ineffective. Activated carbon is very effective in removing heavy metals, but is

readily soluble under extreme pH conditions (Huang and Blankenship, 1989).

34

Peat moss has been found as very effective in adsorbing heavy metals (Ho,

1995). In this study Slow Sand Filters (SSFs) were tried to remove heavy metals and

found very effective (Muhammad et al., 1997). Experimental results on the

influences of process variables in removing heavy metals by SSFs demonstrated that

adsorption was one of the mechanisms of the removal of heavy metals (Muhammad

et al., 1997). To confirm this prediction/hypothesis, batch adsorption tests were

carried out. This research deals with the results of the batch adsorption tests to

establish adsorption isotherms and adsorption capacity of the sand for the selected

heavy metals.

The removal of some heavy metals ions (Mn2+, Fe2+, Ni2+ and Cu2+) from

aqueous solution by adsorption was investigated by Uzun and Guzel (2000). The

commercial activated carbon (Merck 2514), chitosan and agar were used as

adsorbents. All experiments were carried out in a pH range from 5.3 to 5.5 and the

equilibrium adsorption contact times were determined for M - 2514. The adsorption

rate constants were determined from obtained kinetics curves suitable for first degree

of rate kinetics. Adsorption isotherms of heavy metals on M - 2514 from aqueous

solution were determined. These adsorption isotherms were seen to be consistent with

Freundlich's adsorption isotherm. They compared M - 2514, chitosan and agar

according to their percent uptake yields of the heavy metals. The adsorption kinetics

of Mn2+,Fe2+,Ni2+ and Cu2+ on M - 2514 commercial activated carbon were

investigated. Sufficient amounts of sample were taken from solutions containing

adsorbent. The solution was diluted and analyzed with a UNICAM 929 Atomic

Absorption Spectrometer (AAS). Equilibrium adsorption times without any change of

concentrations (Cr) remaining without adsorption of heavy metals were obtained from

kinetic curves related to Mn2+, Fe2+, Ni2+ and Cu2+. The adsorption isotherms of heavy

35

metals were studied. The equilibrium concentrations of heavy metals were determined

after taking sufficient amount of samples from clear parts of solutions containing

adsorbent and doing proper dilutions. The amount adsorbed (Ca) was calculated from

the difference between initial and equilibrium concentrations.

Heavy-metal adsorption reactions, in a competitive system, are important to

determine heavy-metal availability to plants and their mobility throughout the soil. It

is now well established that the free metal ion concentration, which is of relevance in

metal bioavailability and toxicity studies, is often controlled by metal ion binding to

natural organic matter (Kinniburgh et al., 1990). The objective of this work was to

evaluate heavy metal sorption capacity of two subsurface horizons from an Eutric

Regosol (ER) and a Distric Regosol (DR). Ten solutions of mixtures of Cd, Cr, Cu,

Ni, Pb, and Zn nitrates (between 10 and 400mg L−1) were prepared to obtain

adsorption isotherms. Twelve grams of soil samples were treated with 200mL of

solution and shaken for 24 hours at 25ºC (Gomes et al., 2001). After centrifugation,

metal in solution was determined by ICP-OES (Perkin Elmer Optima 4300 DV).

Desorption experiments were performed using the pellets resulting from adsorption

experiments (Madrid and Diez, 1992). Each sample was treated with 200mL of an

acetic acid (0.02M) and sodium acetate (0.02M) solution, buffered at pH 4.5 and

shaken for 24 hours at 25ºC. Kd100 was used to establish the adsorption and retention

selectivity sequences of the heavy metals (Covelo et al., 2004) by these soils. Both

studied Regosols have low organic matter contents (5.3 and 19.29 g kg−1 in RD and

RE respectively) and low cation exchange capacity (1.08 and 0.38 cmol(+) kg−1 in RD

and RE respectively). The clay proportion in soils mineral fraction is 21.94% in

Distric Regosol and 26.29% in Eutric Regosol. Kaolinite is the most frequent mineral

in RD clay fraction (>50%) whereas in RE is between 10 and 30% in their clay

36

fraction. RD has low mineralogical variety and oxides content than RE. Pb > Zn > Cd

> Cr > Ni > Cu is the adsorption selectivity sequence obtained in RE. Their retention

selectivity sequence is Cr > Zn > Pb > Ni > Cd > Cu. Selectivity sequences obtained

in DR are Pb > Zn > Cr > Cd > Ni > Cu (after adsorption experiments) and Cr > Zn >

Ni > Pb > Cd > Cu (after desorption ones). Cu is the metal adsorbed and retained in

lesser amounts in both studied soils. This fact is attributable to their low organic

matter content because this soil property presents high affinity to Cu sorption and

retention. Both retention selectivity sequences are very similar. Only third and fourth

positions change, Pb is retained in higher amounts than Ni by RE whereas RD retains

higher amounts of Ni than Pb. RE mineralogical variety influences the high Pb

retention of this soil.

Coconut coir pith, an agricultural solid waste was used by :amasivayam and

Sureshkumar (2007) as biosorbent for the removal of chromium(VI) from water and

waste water after modification with a Cationic surfactant,

hexadecyltrimethylammonium bromide. Optimum pH for Cr(VI) adsorption was to be

2.0. Reduction of Cr(VI) to Cr(III) occurred to a slight extent during the removal.

Langmuir, Freundlich and dubinin Radushkevic(D-R) isotherms were used to model

the adsorption equilibrium data and the system follow three isotherms. The adsorption

of the biosorbent was found to be 76.3mg/g, which is higher or comparable to the

adsorption capacity of various adsorbents reported in literature. Kinetic studies

showed that the adsorption obeyed a second order and Elovich model.

Thermodynamic parameters such as ∆G0, ∆H0, and ∆S0 were evaluated, indicating

that the overall adsorption process was endothermic and spontaneous. Effects of

foreign anions were also examined. The adsorbent was also tested for the removal of

Cr(VI) from electroplating effluent. :amasivayam and Sureshkumar (2007) used

37

heaxdecyltrimethylammonium to modify the surface of coir pit. The removal of

chromium ion from aqueous solution by surfactant modified coir pit was found to be

effective. They went further that chromium removal was substantially greater for

modified coir pit than for a raw coir pit. optimum ph for chromium removal was

found to be 2.0 and the reduction of Cr(VI) to Cr(III) also took place during

adsorbtion process. Equilibrium adsorbtion data showed good fit to all the three

isotherms, Langmuir, freundlich and dubinin raduskevich. Adsorbtion process flowed

second order kinetics and elovich model, and it was spontaneous. Desorption studies

showed that apart from ion exchange, chemisorptions might also govern the

chromium uptake by the surfactant modified adsorbent

Biosorption of Cd(II) and Cr(VI) ions in single solution using staphylococcus

xylosus and pseudomonas sp., and their selectivity in binary mixtures was

investigated by Ziagova et al., (2006). In this work, Langmuir and freundlich models

were applied to describe metal biosorption and the influence of pH, biomass

concentration and contact time was determined. maximum uptake capacity of

cadmium was estimated to 250 and 278 mg/g, whereas that of chromium to 143 and

95mg/g for S.xylolus and pseudomonas sp., respectively. In binary mixtures with Cd

(II) ions as the dominant species, there is a profound selectivity for cadmium

biosorption, reaching 96% and 89% for pseudomonas sp. and S.xylosus, respectively,

at 10 mg/L Cd(II) and 5mg/L Cr(VI). It was shown that when Cr(VI) ions are the

dominant species, there is selectivity towards chromium around 92% with S.xylosus

only. In Cd(II) biosorption experiments, it was observed that pH optima were very

close, 6.0 for S.xylosus and 7.0 for Pseudomonas sp. comparing to Cr(VI),where the

estimated values are 1.0 and 4.0 for S.xylosus and Pseudomonas respectively. From

Cadmium uptake values 278 and 250mg/g for Pseudomonas sp. and S.xylosus,

38

respectively, it was concluded that both bacteria can be used successfully for

cadmium removal. Also, in the binary systems examined when Cd(II) ions are the

dominant species in the solution, they observed a profound selectivity for cadmium

biosorption against chromium, using both bacteria. Also, when Cr(VI) ions are the

dominant species in the solution, there is selectivity towards chromium with S.xylosus

only. It was concluded that this is a first step towards creation of highly selective

biosorbents for Cd(II), Cr(VI), and other toxic metals.

Abdel-Ghani et al., (2008) carried out adsorption of Cr(III) and Pb(II) from a

solution onto Casuarina Glauca tree leaves as the adsorbent. Effects of contact time

and pH on the adsorption process were investigated and the isothermal studies were

carried out using 1g of leaves in 50ml synthetic waste water at different metal ions

concentrations while the initial pH of the synthetic waste water was about 5. They

used Langmuir, Freundlich, and Temkin isotherms to examine their experimental

results in other to obtain the appropriate model and they found out that Temkin

isotherms represent the measured sorption data well. They concluded that Casuarina

Glauca tree leaves were able to simultaneously remove chromium and lead ions from

the aqueous solutions. They observed after 120minutes of contact between the

adsorbent and the adsorbate that an increase in the pH resulted in a higher metal

removal which showed that tree leaves can be used for the sorption of Cr(III) and

Pb(II) from mixed solutions.

Panda et al., (2007) fractionalized husk of lathyrus sativus(HLS) by

introducing thio groups with the help of carbon disulphide treatment in alkaline

environment. Their elemental analysis indicated that there is an increase in the

sulphur content of the fractionalized biomass from initial 0.36% to 3.7% of the

priastine biomass, suggesting the incorporation of thio group on HLS. The

39

involvement of the hydroxyl groups mainly in the fractionalization process is

demonstrated by Fourier transform infrared (FTIR) spectroscopic study. They

observed that the adsorption capacity of the fractionalized biomass with respect to

Cadmium and Nickel increased by about 50% compared to that of pristine one. They

also deduced that in the case of unmodified HLS, adsorption process involving the

fractionalized biomass obeys Langmuir isotherm model and attains equilibrium in

10minutes compared with 60minutes attainment of the unmodified biomass. They

found out that at pH 5.0, 1g of fractionalized HLS adsorbs 52.8 and 23.6mg of

Cadmium and Nickel, respectively, thereby registering about 50% adsorption

increment compared with that of control biomass. They concluded that fractionalized

biomass enhances the biosorption capacity of HLS in the treatment of industrial

effluent.

Jacques et al., (2006) used a yellow passion-fruit shell (YPFS- Passiflora

edulis Sims. F. flavicarpa Dedener) as an alternative biosorbent for the removal of

Cr(III) and Pb(II) ions from aqueous solutions. The effects of pH and contact time on

the biosorption capacities were studied and they observed that the biosorption kinetics

follows a pseudo second order kinetic model, obtaining the biosorption constant rates,

1.48×10-2 and 4.70×10-3g/mg/min using metallic ion solution, for Cr(III) and Pb(II),

respectively. It was observed that the maximum biosorption capacities of YPFS were

85.1 and 151.6mg/g for Cr(III) and Pb(II), respectively. They reported that the high

biosorption capacities of YPFS places this biosorbent as one of the best biosorbents

for the removal of Cr(III) and Pb(II) from aqueous effluents. They concluded that

YPFS is an excellent alternative biosorbent for the removal of Cr(III) and Pb(II) from

aqueous solutions because of its direct use without any chemical treatment, presenting

a biosorption capacities for Cr(III) and Pb(II) which in-turn makes its use in

40

commercial view point interesting because of its low preparation cost and large

availability in the tropical countries.

Removal of Nickel(II) from aqueous solutions through adsorption onto

biopolymer sorbents, such as calcium alginate(CA), Chitosan coated calcium

alginate(CCCA) and chitosan coated silica(CCS), was studied by Vijaya et al.,

(2007), using equilibrium batch and column flow techniques. The biosorbent were

characterized by FTIR, SEM, TGA and surface area analysis. The extent of

adsorption was found to be a function of the pH of the solution, contact time, sorbate

concentration and adsorbent dose. They noted that the optimum pH was found to be

5.0. the adsorption of Ni(II) ions on CA was comparatively higher than CCCA and

CCS. They observed that the adsorption of Ni(II)ions onto the biopolymers follows a

pseudo second order kinetics and that the equilibrium adsorption data for Ni(II) on

CA,CCCA and CCS were fitted to Freundlich, and Langmuir isotherms. They

concluded that biomaterials are more effective for the removal of Ni(II) from aqueous

medium.

Mesoporous materials were prepared by Maeda and Ishida (2007) using

hydrothermal treatment of powder-compacts consisting of metakaolinite, quartz and

slaked-lime with different forming pressures. They observed (Hirotaka et al., 2008)

that the hydrothermally solidified materials developed strength despite the formation

of hydrogarnet, exhibited a broad pore size distribution of more than 3.4nm, and the

volume and specific surface area increased with decreasing the forming pressure.

They remarked that the hydrothermally solidified materials showed an excellent

humidity-controlling ability due to adsorption and desorption of water vapor by

utilizing their mesopores. They concluded that the mesopore volume increased, thus

enhancing the water vapor adsorption–desorption ability of the materials. This result

41

agreed well with the prediction made using the Kelvin equation and Arai’s correction

equation. The hydrothermally solidified materials prepared at 15MPa showed

excellent humidity-controlling properties in the airtight chamber.

In this work, Meral and Ayhan (2008) studied the effect of temperature, pH

and initial metal concentration on Pb(II) biosorption on modified quebracho tannin

resin (QTR). Scanning electron microscopy (SEM) and Fourier transform infrared

spectroscopy (FTIR) were used to investigate QTR structure and morphology.

Besides, the specific BET surface area and zeta-potential of the QTR were analysed.

Thermodynamic functions, the change of free energy (∆G◦), enthalpy (∆H◦) and

entropy (∆S◦) of Pb adsorption on modified tannin resin were calculated as −5.43 kJ

mol−1 (at 296± 2 K), 31.84 kJ mol−1 and 0.127 J mmol−1 K−1, respectively, indicating

the spontaneous, endothermic and the increased randomness nature of Pb2+

adsorption. The kinetic data was tested using pseudo-first-order, pseudo-second-order,

Elovich and intra-particle diffusion model. The results suggested that the pseudo-

second-order model (R2 > 0.999) was the best choice among all the kinetic models to

describe the adsorption behavior of Pb(II) onto QTR. Langmuir, Freundlich and

Tempkin adsorption models were used to represent the equilibrium data. The best

interpretation for the experimental data was given by the Langmuir isotherm and the

maximum adsorption capacity (86.207 mg g−1) of Pb(II) was obtained at pH 5 and

296 K. They concluded that, chemically modified tannin resin as a sorbent is an

efficient and economical alternative in Pb2+ ion removal from water. Also, it is

apparent that QTR can serve as an appropriate adsorbent in removal process of Pb(II)

ions from aqueous solution in terms of its high sorption capacity, naturality and

abundance.

42

Tae et al., (2008) used hydrocarbon (H/C) trap developed using a multi-

layered zeolite, in which zeolite A and zeolite beta was selected for the water trap and

the hydrocarbon trap, respectively, with the goal of reducing hydrocarbon emissions

during a cold-start. The adsorption and desorption of hydrocarbons was examined in

the monolithic adsorber during a vehicle test as well as in a lab-scale micro-reactor as

a powder form. The competitive adsorption of water vapor and hydrocarbons in the

exhaust gas from a gasoline vehicle on the hydrocarbon adsorption sites at low

temperatures caused the decrease in the H/C removal efficiency during a cold-start. It

was reported that, in a multi-layered adsorption system composed of the water trap

and the hydrocarbon trap, the hydrocarbon removal efficiency was greatly improved

owing to the elimination of water with the hydrophilic zeolite A in the upper layer,

also the hydrocarbon desorption can be delayed because of the latent heat of

desorption of adsorbed water in the upper layer. Therefore, this double-layered

zeolite-based hydrocarbon trap showed the better performance compared with the

conventional hydrocarbon trap during a cold-start. They concluded that, competitive

adsorption of water vapor and hydrocarbons in the exhaust gas from a gasoline

vehicle on the hydrocarbon adsorption sites at low temperatures causes decrease in the

H/C removal efficiency during a cold-start and in the multi-layered adsorption system

composed of the water trap and the hydrocarbon trap, the hydrocarbon removal

efficiency was greatly improved owing to the elimination of water with the

hydrophilic zeolite A in the upper layer. Furthermore, the hydrocarbon desorption can

be delayed because of the latent heat of desorption of adsorbed water in the upper

layer during evaporation. Therefore, this double-layered zeolite-based hydrocarbon

trap showed the better performance compared with the conventional hydrocarbon trap

during a cold-start.

43

In this work, Su-Hsia and Ruey-Shin (2008) reviewed the technical

feasibility of the use of activated carbon, synthetic resins, and various low cost natural

adsorbents for the removal of phenol and its derivatives from contaminated water.

They noted that, instead of using commercial activated carbon and synthetic resins,

researchers have worked on inexpensive materials such as coal fly ash, sludge,

biomass, zeolites, and other adsorbents, which have high adsorption capacity and are

locally available. They observed from their survey of about 100papers that low- cost

adsorbents have demonstrated outstanding removal capabilities for phenol and its

derivatives compared to activated carbons (Su-Hsia and Ruey-Shin, 2008).

Adsorbents that stand out for high adsorption capacities are coal-reject, residual coal

treated with H3PO4, dried activated sludge, red mud, and cetyltrimethylammonium

bromide-modified montmorillonite. Of these synthetic resins, they remarked that,

HiSiv 1000 and IRA-420 display high adsorption capacity of phenol and XAD-4 has

good adsorption capability for 2-nitrophenol, which make them suitable for industrial

effluents containing phenol and its derivatives. They noted that the adsorption

capacities of the adsorbents presented here vary significantly depending on the

characteristics of the individual adsorbent, the extent of chemical modifications, and

the concentrations of solutes. Chemical contamination of water from a wide range of

toxic compounds, in particular aromatic molecules, is a serious environmental

problem owing to their potential human toxicity (Su-Hsia and Ruey-Shin, 2008).

Therefore, low-cost materials are sorely needed that are comparable to activated

carbon or synthetic resins in-terms of adsorption capacity and should be locally

available. The present review shows that some such materials have equivalent or even

more adsorption capacity to activated carbon and synthetic resins. On the other hand,

some solid waste such as sludge has become one of the society’s most vexing

44

problems (Su-Hsia and Ruey-Shin, 2008). If the solid waste can be converted into

low-cost adsorbents for the treatment of discharged wastewater, the cost of removal

might decrease. Although the organoclays revealed good adsorption capability, they

were still non-economic. Preliminary studies have also revealed the applicability of

the substances such as dried activated sludge, red mud, fertilizer wastes, different

types of coal, and soon, as scavengers of phenol and its derivatives. Last but not the

least, if the alternative adsorbents mentioned previously are found to be highly

efficient for the removal of phenol and its derivatives, not only the industries but the

living organisms and the surrounding environment will also benefit from the potential

toxicity due to phenol and its derivatives. Su-Hsia Lin and Ruey-Shin Juang

concluded that, the use of low-cost adsorbents may contribute to the sustainability of

the surrounding environment and undoubtedly, low-cost adsorbents will offer alot of

promising benefits for commercial purpose in the future.

Ozlem et al., (2008) studied the potential use of cotton plant wastes-stalk (CS)

and hull (CH) as sorbents for the removal of Remazol Black B (RB5), a vinyl sulfone

type reactive dye. The results indicated that adsorption was strongly pH-dependent

but slightly temperature-dependent for each sorbent-dye system. The Freundlich,

Langmuir, Redlich-Peterson and Langmuir-Freundlich adsorption models were used

for the mathematical description of adsorption equilibrium and isotherm constants

were evaluated at 25 °C. All models except the Freundlich model were applicable for

the description of dye adsorption by both sorbents in the concentration range studied.

According to the Langmuir model, (CS) and (CH) sorbents exhibited the highest RB5

dye uptake capacities of 35.7 and 50.9 mg g−1, respectively, at an initial pH value of

1.0. Simple mass transfer and kinetic models were applied to the experimental data to

examine the mechanisms of adsorption and potential rate-controlling steps. It was

45

found that both external mass transfer and intra-particle diffusion played an important

role in the adsorption mechanisms of dye, and adsorption kinetics followed the

pseudo second-order type kinetic model for each sorbent. Using the Langmuir model

parameters, thermodynamic constant ∆G° was also evaluated for each sorption

system. They concluded that, Results obtained from this study showed that cotton

stalk or cotton hull can be used as an adsorbent for the removal of RB5 dye from the

aqueous solution in a static batch system. Since cotton stalk and hull, the wastes of

cotton plant, are freely, abundantly and locally available, they can be put in use as

economical sorbents for the wastewater treatment.

Zhang et al., (2008) used extracellular polymeric substance (EPS) of Proteus

mirabilis TJ-1 as a novel biosorbent to remove dye from aqueous solution in batch

systems. As a widely used and hazardous dye, basic blue 54 (BB54) was chosen as

the model dye to examine the adsorption performance of the EPS (Zhang et al.,

2008). The effects of pH, initial dye concentration, contact time and temperature on

the sorption of BB54 to the EPS were examined. At various initial dye concentrations

(50–400 mg/L), the batch sorption equilibrium was obtained within 5 minutes and

kinetic studies suggested that the sorption followed the internal transport mechanism.

According to the Langmuir model, the maximum BB54 uptake of 2.005 g/g was

obtained and chemical analysis of the EPS indicated the presence of protein (30.9%,

w/w) and acid polysaccharide (63.1%, w/w). Scanning electron microscopy (SEM)

images showed that the EPS with a crystal-linear structure was whole enwrapped by

adsorbed dye molecules. FTIR spectrum result revealed the presence of adsorbing

groups such as carboxyl, hydroxyl and amino groups in the EPS. High-molecular

weight of the EPS with more binding-sites and stronger van der Waals forces together

with its specific construct led to the excellent performance of dye adsorption (Zhang

46

et al., 2008). They concluded that, EPS of P. mirabilis TJ-1 is an effective biosorbent

to remove BB54 from aqueous solution which also shows potential board application

as a biosorbent for both environmental protection and dye recovery, owing to the

presence of carboxyl, hydroxyl and amino groups in the EPS, which have been

approved to be the preferred groups for most sorption processes.

The sorption of Cr(VI) from aqueous solutions with macroporous resins which

contain quarternary amine groups (Lewatit MP 64 and Lewatit MP 500) was studied

by Pehlivan and Cetin (2008) using varying Cr(VI) concentration, adsorbent dose,

pH, contact time and temperature. Batch shaking sorption experiments were carried

out to evaluate the performance of Lewatit MP 64 and Lewatit MP 500 anion

exchange resins in the removal of Cr(VI) from aqueous solutions. The concentration

of Cr(VI) in aqueous solution was determined by UV–visible spectrophotometer. The

ion exchange process, which is dependent on pH, showed maximum removal of

Cr(VI) in the pH range 3–7 for an initial Cr(VI) concentration of 1 × 10−3 M. The

optimum pH for Cr(VI) adsorption was found as 5.0 for Lewatit MP 64 and 6.0 for

Lewatit MP 500. The maximum Cr(VI) adsorption at pH 5.0 is 0.40 and 0.41 mmol/g

resin for Lewatit MP 64 and Lewatit MP 500 anion exchangers, respectively. The

maximum chromium sorption occurred at approximately 60 min for Lewatit MP 64

and 75 min for Lewatit MP 500. The suitability of the Freundlich and Langmuir

adsorption models was also investigated for each chromium–sorbent system. The

uptake of Cr(VI) by the anion exchange resins was reversible and so it has good

potential for the removal of Cr(VI) from aqueous solutions (Pehlivan and Cetin,

2008). They concluded that, both resins have a large collective sorption with Cr (VI)

ion, but Lewatit MP 500 shows stronger binding. The isotherm plots showed a high

sorption for pH 3–5 for both of the resins. Optimum pH for ion exchange was 5.0 for

47

Lewatit MP 64 and 6.0 for Lewatit MP 500. The adsorption of Cr(VI) increased with

agitation period and attained an optimum at about 60 min for Lewatit MP 64, 75 min

for Lewatit MP 500. These ion exchangers can be used as an efficient sorbent for the

removal of Cr(VI) from aqueous solution and are thus attractive sorbents for the

treatment of wastewater containing Cr(VI) ion at trace levels and can also be used for

reversible uptake of Cr(VI) and successfully applied to water and industrial

wastewater samples(Pehlivan and Cetin, 2008).

In this work, sorption efficiency of treated olive stones (TOS) towards

cadmium and safranine removal from their respective aqueous solutions was

investigated. TOS material was prepared by treatment of olive stones with

concentrated sulphuric acid at room temperature followed up by a subsequent

neutralization with 0.1 M NaOH aqueous solution. The resulting material has been

thoroughly characterized by SEM, energy-dispersive X-ray (EDX), MAS 13C NMR,

FTIR and physicochemical parameters were calculated. The sorption study of TOS at

the solid–liquid interface was investigated using kinetics, sorption isotherms, pH

effect and thermodynamic parameters. The preliminary results indicate that TOS

exhibit a better efficiency in terms of sorption capacities toward the two pollutants

(128.2 and 526.3 mg/g for cadmium and safranine, respectively) than those reported

so far in the literature. Moreover, the sorption process is ascertained to occur fast

enough so that the equilibrium is reached in less than 15 min of contact time. The

results found in the course of this study suggest that ion exchange mechanism is the

most appropriate mechanism involved in cadmium and safranine removal. Finally, the

sorption efficiency of TOS is compared to those of other low-cost sorbents materials

yet described in the literature. They concluded that, raw olive stones were treated with

concentrated sulphuric acid without heating or using any special atmosphere

48

environment, leading to a low-cost sorbent with a good affinity for positively charged

species. The obtained material was characterized and used for basic dye and heavy

metal removal from aqueous solutions. The kinetic and sorption data fitted well the

second-order kinetic model and the Langmuir model, respectively, with good values

of the determination coefficient. The sorption process was found to be pH

independent. The proposed ion exchange sorption mechanism is discussed on the

basis of SEM, MAS 13C NMR, EDX, FTIR and mass balance results. The obtained

sorption capacities toward safranine and cadmium were better than many low-cost

materials yet reported in the literature.

The removal of heavy metals from plating factory wastewater with economical

materials such are montmorillonite, kaolin, tobermorite, magnetite, silica gel and

alumina as adsorbents for the removal of Cd(II), Cr(VI), Cu(II) and Pb(II) from waste

water was investigated by Katsumata et al., (2003) using column method. This

removal method of heavy metals proved highly effective as removal efficiency tended

to increase with increasing pH and decrease with increasing metal concentration

(Katsumata et al., 2003). They observed that the removal percentages by adsorption

onto montmorillonite, tobermorite, magnetite, and silica gel showed high values for

all metals. They remarked that, based on heat of adsorption, the adsorption process

shows a chemisorption reaction process. They concluded that, the column method was

effective in the removal of Cd (II), Cr (VI) and Cu (II) in rinsing wastewater from

plating factory and since the economical adsorbents used can be obtained

commercially because they are easily synthesized, the wastewater treatment system

developed is rapid, simple and cheap for the removal of heavy metals.

49

Grape waste generated in wine production is a cellulosic material rich in

polyphenolic compounds which exhibits a high affinity for heavy metal ions (Rumi et

al., 2008). Rumi et al., (2008), cross-linked a grape waste using a concentrated

sulphuric acid form an adsorption gel which was then characterized and utilized for

the removal of Cr(VI) from synthetic aqueous solution. Adsorption tests were

conducted in batch mode to study the effects of pH, contact time and adsorption

isotherm of Cr(VI), which followed the Langmuir type adsorption and exhibited a

maximum loading capacity of 1.91 mol/kg at pH 4. The adsorption of different metal

ions like Cr(VI), Cr(III), Fe(III), Zn(II), Cd(II) and Pb(II) from aqueous solution at

different pH values 1–5 was also investigated. It was observed that the cross-linked

grape waste gel was found to selectively adsorb Cr (VI) over other metal ions tested.

They remarked that the cross-linked grape waste gel has high possibility to be used as

effective adsorbent for Cr (VI) removal. They concluded that, Cross-linked grape

waste gel has been found to be selective to Cr (VI) ion and highly effective for its

removal from synthetic aqueous solution and that, since most of the industrial

wastewaters contaminated with Cr (VI) are highly acidic, cross-linked grape waste gel

can be good candidate for waste water treatment and due to large generation of grape

waste each year, simple production process of cross-linked grape waste gel, seems to

be a promising adsorbent.

In this study the ability of Cercis siliquastrum L. leaves for the adsorption of

Pb (II), Cu (II) and Ni (II) ions were studied by Salehi et al., (2000). The effects of

different parameters such as contact time of biosorbent and sorbents, pH of metal

solution, and initial metal ion concentration on the biosorption were investigated. The

maximum sorption of all metals was carried out in pH 4. Increasing the initial metal

concentration in lower values caused a steep growth in biosorption, which was not

50

observed in higher values. In the optimum sorption condition, the affinity of the

leaves to metal ions was in the order of Pb (II)> Cu (II)> Ni (II). The biosorption of

the metal ions were studied by Langmuir and Freundlich adsorption isotherm models.

It was observed that the data were fitted very well to Langmuir adsorption isotherm

model. According to the obtained correlation coefficient values, Freundlich model

could predict Pb (II) and Cu (II) adsorption adequately but it was not suitable for Ni

(II) sorption. Experimental data were exploited for kinetic evaluations related to the

sorption process. According to our results, second-order kinetic provided a good

description of biosorption for the tested metals with regression correlation coefficients

more than 0.9998 for all the sorbate-sorbent systems. They concluded that, the

potential of Cercis siliquastrum L. leaves has been investigated for removing heavy

metals, such as Pb (II), Cu (II) and Ni (II) from aqueous solution. The maximum

uptake for all metal ions was obtained in pH 4. The results obtained in this study

indicated the highest adsorption ability of C. siliquastrum for Pb (II), among the

tested metal ions. The study of metals biosorption by C. siliquastrum leaves proved

that the process conform Langmuir better than Freundlich isotherm model. The

overall adsorption rate of the Pb (II), Cu (II) and Ni (II) can be best described by the

second-order kinetic.

Preliminary studies on the antifertility effect of pawpaw seeds (Carica

papaya) on the gonads of male albino (Wistar) rats was investigated by Udoh and

Kehinde (1999). An oral dose of crude ripe pawpaw seeds at 100 mg/kg body weight

and 50 mg/kg body weight were administered orally for 8 weeks and histological

observations at a high dose of 100 mg/kg body weight showed degeneration of the

germinal epithelium and germ cells, a reduction in the number of Leydig cells and the

presence of vacuoles in the tubules and at a low dose of 50 mg/kg body weight little

51

effect was observed, however, there was disorganization in some of the seminiferous

tubules while others appeared normal.

Carica papaya seed extract is currently being marketed as a nutritional

supplement with purported ability “to rejuvenate the body condition and to increase

energy”. The product claims to improve immunity against common infection and

body functioning. Mariluz et al., (2003) analyzes the chemical constituents of the

Carica® Seed Extract and determine the potential immunomodulatory properties of

the different bioactive fractions. These immunomodulatory activities of crude

Carica® Seed Extract and its bioactive fractions were examined in vitro using

lymphocyte proliferation assays and complement-mediated hemolytic assay. Three

major observations were made in this study: (1) the crude Carica® Seed Extract and

two other bioactive fractions significantly enhanced the phytohemagglutinin

responsiveness of lymphocytes; (2) none of the Carica® Seed Extract (at the

concentrations used in this study) was able to protect the lymphocytes from the toxic

effects of chromium; and (3) some of the bioactive fractions of Caricas seed extract

were able to significantly inhibit the classical complement-mediated hemolytic

pathway. These findings provide evidence for Immunostimulatory and anti-

inflammatory actions of Carica® Seed Extract (Mariluz et al., 2003).

Robert and Dane (2008) tested the effectiveness of the Methanolic extracts

from under-ripped, ripped, and ripped then dried pawpaw seeds (PPSE) as an

antioxidative agent. Under-ripped seeds contained more total phenolic compounds

than ripe seeds, but ripe seeds showed the highest reducing potential. The

development of value added products from pawpaw seeds may lead to successful

commercialization of this underutilized fruit (Robert and Dane, 2008).

52

Wahba and Zaghloul (2007) compared three different soil minerals

(montmorillonite, kaolonite and calcite) for their ability to remove heavy metals

(HM). These minerals were applied in chloride form dissolved in aqueous solutions of

these metals at two concentrations (2000 and 6000 ppm). The effect of contact time,

initial metal concentration and type of natural soil minerals on the adsorption process

at 20±2C° was studied using kinetic approach. Data gathered from Electrical Stirred

Flow Unit (ESFU) used for kinetic study indicated that almost steady state adsorption

conditions were reached after 2-4 h of unit working for the adsorption of Pb (II), Cd

(II) and Zn (II). At 2000 ppm metal concentration and 2h reaction time, they observed

that the maximum Pb metal removed from the solution was found in calcite by about

74.2%, followed by montmorillonite and kaolonite by about 66% and 58%

respectively. With an increase in the concentrations of these metals, the same trend of

HM retention was detected. The constants rate represented capacity and intensity

factors of the best fitted equations showed that the increasing trend of adsorption

capacity have the order: calcite> montmorillonite> kaolonite in their tendency to

retain HM. They concluded that, Calcite tend to absorb Pb more than other heavy

metals at both concentrations 2000ppm and 600ppm and the type of mineral has a

great role on the adsorption of heavy metals: i.e. calcite < montmorillonite< kaolinite.

These explain the low effect of pollution of heavy metals on the calcareous soils,

compared to the alluvial clay soils.

Prasad and Freitas (2000) investigated the potential of Quercus ilex

phytomass from stem, leaf and root as an adsorbent of chromium (Cr), nickel (Ni),

copper (Cu), cadmium (Cd) and lead (Pb) at ambient temperature. The metal uptake

capacity of the root for different metals was found to be in the order:

53

Ni>Cd>Pb>Cu>Cr; stem Ni>Pb>Cu>Cd>Cr; and leaf Ni>Cd>Cu>Pb>Cr. The

highest amount adsorbed was Ni (root>leaf>stem). It was shown that Ni is

sequestered mostly in the roots, where concentrations can be as high as 428.4 ng/g dry

wt., when 1-year-old seedlings were treated with Ni (2000 mg/l) in pot culture

experiments, compared to 7.63 ng/g dry wt., control (garden and greenhouse soil)

topsoil where Ni was present in trace amounts. This proves that the root biomass of Q.

ilex has the capacity for complexing Ni. Cr exhibited the least adsorption values for

all the three types of phytomass compared to other metals. The trend of adsorption of

the phytomass was similar for Ni and Cd, i.e. root>leaf>stem. Desorption with 10 mM

Na4 EDTA was effective (55–90%) and, hence, there exists the possibility of

recycling the phytomass. The biosorption results of recycled phytomass suggested

that the selected adsorbents are re-usable. The advantages and potential of the Q. ilex

phytomass as a biofilter of toxic trace metals and the need for enhancing the

efficiency of the Q. ilex phytomass as an adsorbent of metals were evident.

54

CHAPTER 2

2.0. Materials and Methods

2.1. Preparation of DPS adsorbent

The seeds of Carica papaya were collected from the open markets in Benin

City, Nigeria and left under the sun to dry. After seven days, the seeds were collected

and crushed. The crushed seeds were then defatted using the soxhlet extraction

method with hexane as the solvent. The defatted sample of Carica papaya was

collected and dried in a fume hood to allow residual hexane to go off the sample. The

sample collected thereafter is therefore referred to as Defatted Carica Papaya Seeds

(DPS).

2.2 Surface Chemistry of DPS adsorbents

Surface chemistry of the DPS adsorbent was characterized by adopting the

Boehm titration and pH drift (or pHpzc – pH of point of zero charge) methods. Boehm

titration method is described as follows: 0.5 g of a well grinded DPS were dispersed

in duplicate in 50 mL each of 0.05 M NaHCO3, 0.025 M Na2CO3, 0.05 M NaOH and

0.05 M HCl contained in 250 mL capacity glass bottles with tight glass corks. The

bottles were shaken using an end-to–end shaker (Grant instrument ltd, England) at

180 rpm for 24 hr. After 24 hr, the samples were filtered using whatman No.1 filter

paper and titrated with 0.05 M NaOH or 0.05 M HCl depending on the starting

solution used. The amount of acidic groups on the surface of the DPS adsorbent were

approximately probed as follows: NaHCO3, (carboxylic group), Na2CO3 (carboxylic

and lactonic groups) and NaOH (carboxylic, lactonic and phenolic groups).The

number of surface basic sites was calculated from the amount of HCl that reacted with

the filtrate.

55

The method used for pHpzc of DPS adsorbent was a modification of (Devarly

et al., 2008). The method is described as follows: 50 mL of 0.01 M KCl were

prepared and added into a series of glass bottles with corks. Their pH values were

adjusted in range between 2 and 12 at interval of 0.5 using either 0.01 M HCl or 0.01

M NaOH. The pH of initial solutions were measured with a pH meter and noted as

pHinitial. After constant value of pHinitial had been reached; 0.15 g of the ground DPS

sample was added to each bottle and then corked. This set up was shaken for 48 hr.

After 48 hr, the second pH of the extracts noted as pHfinal was measured with a pH

meter. A plot of pHinitial on the x-axis against the difference between pHinitial and

pHfinal on the y-axis was made as shown in Fig.1 pHPZC was a point where the curve

cut the x -axis implying the point when pHinitial = pHfinal.

The specific surface area of DPS adsorbent was calculated from the following

equation (Gregg and Singh, 1982).

M

-a-S

MBg

MB

2010−×××= (1)

where SMB is the specific surface area in 10-3 km2 kg-1; -g is the number of molecules

of metal ion adsorbed at the monolayer of fibers in kg kg-1 (or -g = -m*M); aMB is the

occupied surface area of one molecule of metal ion in Ų; - is Avogadro’s number,

6.02 x 1023 mol-1; and M is the molecular weight of metal ion in g mol-1. This method

has been proven to be as efficient as the BET and Ethylene Glycol Monoethyl Ether

(EGME) methods of determining SSA (Yukselen and Kaya, 2006).

2.3 Infrared Spectroscopy

The technique of Devarly et al. (2008) was used with slight modification. The

vibrational frequency of Carica papaya seeds was obtained from FTIR transmission

56

spectra of defatted and undefatted seeds by KBr method. The grinded KBr powder

was pressed with SHIMADZU MHP-1 mini hand press to form homogenized pellet

for background measurement. A 10% dilution of Carica papaya seed with KBr was

grinded with agate mortar to have a homogenized mixture. A pellet suitable for IR

measurement was produced using the SHIMADZU MHP-1 mini hand press. The

sample measurement was taken in the percent transmittance mode. SHIMADZU

8400S FTIR instrument in the vibrational absorption range of 4000 – 500 cm-1 was

employed.

2.4 Adsorption Studies

Stock solutions of 1000 mg/L of Pb2+ and Cd2+ were prepared from analar grades of

Pb (NO3)2 and Cd (NO3)2.4H2O respectively. Thereafter, other lower concentrations

were prepared from the stock solutions.

2.4.1. Initial Concentration

In studying the effect of initial metal ion concentration, 50 mL of both metal

ion in the range of 10 – 500 mg/L and 0.5 g of DPS adsorbent were agitated for 2

hours at 180 rpm at 298 K. The effect of temperature on the adsorption of Pb2+ and

Cd2+ onto DPS adsorbent was studied by repeating the procedure for initial

concentration at 313 K and 323 K. The thermodynamic parameters, ∆H, ∆S and ∆G

for the adsorption process were obtained from the relation lnb = ∆S/R − ∆H/RT with

all the letters having their usual meanings.

57

2.4.2. Adsorbent Dose

The effect of adsorbent dose was carried out by taking known weights of the

DPS biosorbent ranging from 0.1 g to 3.0 g into glass bottles containing 50 mL of 250

mg/L each of Pb2+ and Cd2+. These mixtures were agitated for 2 hours at 180 rpm at

room temperature (298 K). The effect of pH on adsorption capacity of DPS biosorbent

was studied by dispersing 0.5 g each of DPS biosorbent in 50 mL of 250 mg/L of both

metal ions contained in a series of glass bottles and adjusting the pH of the adsorbate

solution with either 0.01 M NaOH or 0.01 M HNO3 in the range of 3 to 8.These were

agitated for 2 hours at 180 rpm at room temperature (298K).

2.4.3. Particle size

The effect of particle size on adsorption was studied with four different

particle sizes (75, 300, 500 and 750 µm) of DPS biosorbent by dispersing 0.5 g of

each particle size in 50 mL of 250 mg/ L of both Pb2+ and Cd2+. These were agitated

for 2 hr at 180 rpm at 298 K. Fifty milliliters of 250 mg/L each of Pb2+ and Cd2+

containing 0.5 g of DPS adsorbent were agitated at 180 rpm at 298 K and samples

were taken out of the shaker at different time intervals between 30 seconds and 120

minutes to study the effect of contact time. Kinetic data were further used to

investigate the slow step occurring in the present adsorption system using Bangham’s

equation (Faust and Aly, 1987). The Langmuir and Freundlich isotherm models were

used to study the adsorption isotherms of DPS adsorbent.

Samples were collected in duplicate and the averages of the results were used for

analysis. All filtrates were analyzed using Atomic Absorption Spectrophotometry

(AAS).

58

2.4.4. Effect of pH

The pH effect was studied on suspensions of 0.5g of the DPS in 50mL of Lead

and cadmium solutions (250ppm each). The pH was adjusted to 3, 4, 5, 6, 7, and 8

Lead and cadmium solutions, respectively. The resulting mixtures were stirred for 1hr

in the Grant Shaker instrument. After 1hr, the solutions were then filtered and the

filtrates were then analysed, using AAS.

2.4.5. Effect of Temperature

The influence of temperature on the removal process was studied at three

different temperature values (40 and 50 °C). This is done by weighing 0.5g of the

sorbent into series of shaking bottles containing different concentrations of Pb2+ (10,

50, 100, 200, and 250ppm) which are then placed inside the Grant Shaker Instrument

and shaken for 1hr. After an hour, the solutions were then filtered and the filtrates

were taken for analysis. This process was also carried out on Cd2+ at the same reaction

conditions.

2.5 Kinetic study

Fifty millimeters (50 mL) of 250 mg/L each of Pb and Cd containing 0.5 g of

DPS adsorbent were agitated at 180 rpm at 298 K and samples were taken out of the

shaker at different time intervals between 30 seconds and 120 minutes to study the

effect of contact time. The filtrates were analyzed using AAS.

Kinetic data were further used to investigate the slow step occurring in the

present adsorption system using Bangham’s equation. The Langmuir and Freundlich

isotherm models were used to study the adsorption isotherms of DPS adsorbent.

59

Kinetic data were further used to investigate the slow step occurring in the present

adsorption system using Bangham’s equation.

)log(303.2

logloglog tV

mk

mqC

C o

to

o α+

=

− (3)

where V is the volume of solution (L) and α (<1) and k0 (g) are constants, m is the

weight (g) of DPS adsorbent used for the adsorption reaction, and t is time (min).

Other kinetic models used for the analysis of kinetic data include:

The Pseudo First Order Model (PFOM) which is obtained as follows

( )tet qqk

dt

dq−= 2 (4)

On rearrangement

kdtqq

dq

te

t =− (5)

which on integration under the boundary conditions of t = 0 to t, and qt = 0 to qt,

gives a linear logarithmic expression,

ktqqq ete =+−− ln)ln( (6)

The Pseudo Second Order Kinetic Model (PSOM) which is obtained as follows:

( )22 te

t qqkdt

dq−=

(7)

On rearrangement and integration

tkqqq ete

2

11=−

− (8)

60

When this is linearized, it gives

eet q

t

qkq

t+=

22

1 (9)

where qe is the amount of Pb2+ and Cd2+ adsorbed at equilibrium (mg/g), qt is the

amount of Pb2+ and Cd2+ adsorbed at time t (min) in mg/g and k2 is the rate constant

of the PSOM for sorption of both metal ions.

The initial sorption rate h, can be obtained by the following equation:

22 eqkh = (10)

Where h is the initial sorption rate (mg/g min).

tqk

tqkq

e

et

2

22

1 += (11)

2.6. Adsorption Isotherms

The Langmuir isotherm is a valid monolayer sorption on a surface containing

a finite number of binding sites. It assumes uniform energies of sorption on the

surface and no transmigration of adsorbate in the plane of the surface. The Langmuir

equation may be written as

qe = bCe

bCeQo

+1 (Non-linear form) (10)

oo

e QbCeQq

111+=

(Linear form) (11)

Where qe is the amount of solute adsorbed per unit weight of adsorbent (mg/g), Ce is

the equilibrium concentration of solute in the bulk solution (mg/L), Qo is the

61

monolayer adsorption capacity (mg/g) and b is the constant related to the energy of

adsorption (L g-1). It is the value reciprocal of the concentration at which half the

saturation of the adsorbent is attained.

The other equilibrium isotherm models were used for adsorption data fittings

Freundlich isotherm model

n

Fe CeKq /1= (12)

efe CnKq log/1loglog += (13)

The non-linear chi-square error method of analysis was used to fit experimental qt

data against theoritcal qt data. The equivalent mathematical statement for the non-

linear chi square as given by (Kang et al., 2008) is:

me

mee

q

qq

,

2,2 )( −

Σ=χ (14)

Where qe,m is equilibrium capacity obtained by calculating from the model (mg/g) and

qe is experimental data of the equilibrium capacity (mg/g). If data from model are

similar to the experimental data, χ2 will be small in number, while if they differ, χ2

will be a bigger number.

62

CHAPTER 3

3.0 Result and Discussions

3.1 Surface Chemistry

The surface of DPS adsorbent contains heteroatom like hydrogen, oxygen,

nitrogen, sulphur, halogen and phosphorus (Devarly et al., 2008).These atoms to a

very great extent determine the surface chemistry of the seeds. Surface chemistry

study is basically done to probe the acidity and basicity of the material under study.

The presence of surface functional groups like carboxylic, lactonic and phenolic

groups are known to constitute the acidity of the material while oxygen containing

groups and oxygen free Lewis basic sites constitute the basicity.

Table 1. Values of surface acidity and pHPZC of DPS biosorbent using Boehm’s

titration

Sample Carboxylic Lactonic Phenolic Acidic value Basic value pHPZC

Meq/g of DPS adsorbent

DPS adsorbent 0.098 ± 0.013 0.117 ± 0.009 1.864 ± 0.022 2.078 0.587 6.25

Table 1 highlights the quantitative surface chemistry analyses of DPS biosorbent

consisting of the amount of acidic and basic functional groups. The pH at which the

H+ and OH- were equal, known as the pH at point of zero charge (pHPZC), was also

determined (Fig. 1). The acidic pHPZC of 6.25 shown by the DPS adsorbent is

consistent with the Boehm titration result that presented dominance of acidic group of

the surface of the seeds. This is in accordance with Mahir et al., (2004) and Kang et

al., (2008) who observed the same trend on activated carbon fiber samples and

activated carbon from jackfruit peel waste respectively.

63

The specific surface area of DPS adsorbent was found to be 143.27 m2 g-1.

This specific surface area is large when compared with lignocellulosic substrate (185

m2 g-1) (Boudesocque et al., 2008), prawn pond algae treated with acid (2.1 m2 g-1)

(Srinivasa et al., 2007), pure wild cocoyam biomass (32.91 m2 g-1) (Horsfall Jnr et

al., 2007), palm shell powder (2.52 m2 g-1) (Sreelatha et al., 2008), powder orange

peel (128.7 m2 g-1) (Lu et al., 2009) and other biosorbents used for adsorption

recently.

3.1 Infrared Spectroscopy

In Fig. 2 it was observed that there were several small hypsochromic shifts in

peaks when Pb metal ion was adsorbed onto DPS biosorbent. Observed hypsochromic

shifts include a shift in -OH band from 3371 to 3294 cm-1; S=O band from 1057 to

1049 cm-1 with a hypsochromic shift. Also a shoulder band from 1103 to 1091 cm-1

-4

-3

-2

-1

0

1

2

3

4

5

0 2 4 6 8 10 12

Initial pH

Change in pH

Fig. 1: pHpzc plot for biosorption of Pb 2+ and Cd 2+ onto DPS adsorbent

64

was observed. In the defatted spectrum, a sharp and distinct absorption was observed

at 1651 cm-1 (C=O stretching).

Fig. 2. FTIR plots of DPS biosorbent and metal loaded DPS biosorbent.

This is a clear indication of the presence of α,β-unsaturated ketone, β-keto

(enolic) esters, Lactones, quinones and carboxylic acids. There was also an observed

small shift in this region to 1647 cm-1. Ansari et al., (2009) have also observed

similar hypsochromic shifts when they adsorbed Cd2+ on Polypogon monspeliensis

waste biomass. From Boehm titration the amount of Carboxylic, lactonic and phenolic

groups in DPS biosorbent are 0.098, 0.117, and 1.864 meq/g. The basic groups had

0.587 meq/g and the pHPZC of DPS biosorbent was found to be 6.25.

65

3.2 Effect of pH

In Fig. 3 it was observed that there was no significant change in percentage of

metal ion adsorbed especially for Pb2+ with change in pH (Pb2+ from 96 - 99% and

Cd2+ from 85 - 98%). This might suggests that adsorption of the metal ions onto DPS

biosorbent is not largely influenced by pH. This could mean that ion-exchange

mechanism is not strictly the mechanism by which the metal ions are being adsorbed

onto the surface of DPS biosorbent. It is possible that the lone pair of electrons (Lewis

base) on some of the functional groups in DPS biosorbent may have played a major

role in the removal of the metal ions by (Lewis acid) DPS biosorbent. However,

percentage of Pb2+ adsorbed onto DPS biosorbent was more than for Cd2+.

Fig. 3: Adsorption plot of Pb2+ and Cd2+ onto DPS biosorbent due to pH effect.

75

80

85

90

95

100

105

3 4 5 6 7 8

pH

% metal ion adsorbed

% Pb

% Cd

66

3.3 Effect of biosorbent dose

Fig. 4 shows the amount of Pb2+ and Cd2+ adsorbed with varying DPS

biosorbent weights in 250 mg/L of the single metal ion in aqueous solution.

Increasing biosorbent dosage decreased the amount of both metal ion adsorbed with

Pb2+ being more adsorbed than Cd2+ (Fig 4). However, the percentage of metal ion

adsorbed was observed to follow the reverse trend. Similar trend have been observed

by Ho et al., (1995) and Ho and Ofomaja (2005). It is possible that the change in the

solid/liquid ratio from 0.003 to 0.1 may have directly resulted in this trend since

amount adsorbed, qe, has an inverse proportionality function to weight of biosorbent

but a direct proportionality function to percentage adsorbed. 0.5 g of DPS biosorbent

was chosen for use in this work not because it gave the highest adsorption capacity

but because it showed sufficiently high percentage of metal ions adsorbed which were

not significantly different from those obtained at higher dosage of DPS biosorbent.

Fig. 4: Effect of DPS biosorbent dose on adsorption of Pb2+ and Cd2+.

0

2

4

6

8

10

12

14

16

18

20

0.1 0.5 1 1.5 2 3

adsorbent weight

amount adsorbed (mg/g)

80

82

84

86

88

90

92

94

96

98% metal ion adsorbed

qe Pb

qe Cd

% ad Pb

% ad Cd

67

3.4 Effect of Contact time

Fig. 5 suggests that increasing contact time increased the amount of metal ions

adsorbed on DPS biosorbent. Both metal ions showed very high initial sorption rates

(13.8 mg g-1 min-1 for Pb2+ and 44.4 mg g-1 min-1 for Cd2+). The higher initial sorption

rate of Cd2+ is likely because of the smaller ionic radii of Cd2+ which enables it to

compete favourably with Pb2+ for same adsorption site on DPS biosorbent. The ionic

radii (Pauling) of the metal ions are Cd2+ (0.97Å) and Pb2+ (1.20 Å). It has been noted

that the smaller the ionic diameters, the higher the adsorption rate (Uzun and Guzel,

2000). The overall Pseudo second order kinetic rate was low but was faster for Cd2+

(0.028 g mg-1 min-1) than for Pb2+ (0.096 g mg-1 min-1) for same reason adduced

before even though the adsorption capacity of DPS is more for Pb2+ than for Cd2+

(22.17 mg/g for Pb2+ and 21.55 mg/g for Cd2+). The adsorption failed the pseudo-first

order kinetic model implying that adsorption of Pb2+ and Cd2+ onto DPS biosorbent is

not only time dependent but also concentration dependent. From Bangham’s equation

we observed that the adsorption is not strictly by pore diffusion but also film diffusion

because of the reduced linearity of the plots (Pb2+r2=0.6434 and Cd2+ r2=0.8997)

(Faust, 1987).

68

Fig. 5. Pseudo-second order kinetic model plots for the adsorption of Pb2+ and

Cd2+ onto DPS biosorbent.

Recently, Wu et al., (2009), introduced the “approaching equilibrium factor”,

Rw, into the Pseudo-second order kinetic model (PSO). Calculations using the

modified PSO kinetic model for data obtained in this study suggest that Rw for the

adsorption of Pb2+ and Cd2+ is 0.013 and 0.004 respectively. This implies that the

adsorption of Cd2+ approaches equilibrium faster than Pb2+. This further supports the

rate constants and initial sorption rates obtained for the adsorption of both metal ions

onto DPS biosorbent in this study. The half-life (t0.5) for the adsorption process was

found to be 1.61 min and 0.48 min for Pb2+ and Cd2+ respectively (Wu et al., 2009).

3.5 Effect of initial metal ion concentration

Increasing initial metal ion from 50 – 500 mg L-1 increased amount of both

metal ion adsorbed onto DPS biosorbent from 0.998 to 48.46 mg g-1 for Pb2+ and

from 0.992 to 46.44 mg g-1 for Cd2+. This observed trend is because there is

0

5

10

15

20

25

0 20 40 60 80 100 120 140

time (mins)

qt (mg/g)

exp Pb2+

PSOM Pb2+

exp Cd2+

PSOM Cd2+

69

increasing concentration gradient between the biosorbent and the adsorbate. The

adsorption capacity of DPS biosorbent for Pb2+ was observed to be 1666.67 mg/g and

for Cd2+ it was 1000.00 mg/g (Table 1). Adsorption capacities obtained in this study

are higher than those obtained from biosorbents in recent times; Ponkan peel (112.1

mg/g for Pb2+, Pavan et al., 2008 ), macrofungus biomass (38.4 mg/g for Pb2+ and

27.3 mg/g for Cd2+, Ahmet and Mustafa, 2009), Almond Shell (9.00 for Pb2+ and

7.00 mg/g for Cd2+, Mohammad et al., 2009), and Zeolite (175 mg/g for Pb2+ and

137 mg/g for Cd2+, Babel and Kurniawan, 2003). This shows the strong future

potential of this biosorbent in removing metal ions from waste water. With increasing

particle size (75 < 300 < 500 < 750 µm) of DPS biosorbent, adsorption capacity

decreased. This is consistent with literature that the smaller the particles size of an

adsorbents, the greater the rate of diffusion and adsorption. However, intra-particle

diffusion is reduced as the particle size reduces, because of the shorter mass transfer

zone, causing a faster rate of adsorption (Singh et al., 2008; Tarley and Arruda,

2004). With the portion of DPS biosorbent not sieved, adsorption capacity for both

metal ions was observed to be slightly higher (≈14.3%). Fitting experimental data to

Langmuir and Freudlich equilibrium models nonlinear regression, it was observed

that the Freudlich gave better fit than Langmuir (Figs. 6 and 7). This implies that the

adsorption of Pb2+ and Cd2+ onto DPS biosorbent is on heterogeneous adsorption sites

on the surface of DPS biosorbent which further strengthens the fact that DPS

biosorbents has multi-adsorption sites as shown from the FTIR analysis.

70

Fig. 6: Langmuir (LM) and Freudlich (FM) isotherm model plots for the adsorption of

Pb2+ onto DPS biosorbent

Fig. 7: Langmuir (LM) and Freudlich (FM) isotherm model plots for the adsorption of

Cd2+ onto DPS biosorbent

0

50

100

150

200

250

300

350

0 5 10 15 20 25 30 35 40

Ce (mg/L)

qe (mg/g)

exp Cd2+

FM Cd2+

LM Cd2+

0

100

200

300

400

500

600

0 5 10 15 20

Ce (mg/L)

qe (mg/g)

exp Pb2+

FM Pb2+

LM Pb2+

71

3.6 Thermodynamics of Adsorption

Table 2 shows data for the various thermodynamic parameters in the adsorption of

Pb2+ and Cd2+ onto DPS biosorbent. The table suggests that the adsorption of Pb2+ and

Cd2+ onto DPS biosorbent is highly feasible, spontaneous and exothermic in nature. ∆

G was found to increase slightly with increasing temperature.

Table 2: Thermodynamic parameters for the adsorption of Pb2+ and Cd2+ onto DPS biosorbent

-∆ H (kJ mol-1) -∆ S (J mol-1) r2 -∆ G (kJ K-1 mol-1)

298 K 313 K 323 K

Pb2+ 12.32 73.56 0.9286 34.61 35.35 36.09

Cd2+ 19.30 61.63 0.9956 4.55 4.57 4.58

72

CHAPTER 4

Conclusion

The use of a new adsorbent, defatted Carica papaya seeds, in the adsorption of

Pb2+ and Cd2+ gave an adsorption capacity of 1666.67 mg/g and 1000.00 mg/g

respectively. The adsorption of both metal ions was found to be highly feasible,

spontaneous and exothermic in nature. The adsorption of both metal ions was found to

approach equilibrium in ≈20 min. of contact time. Its very high adsorption capacity

suggests the strong potential of this adsorbent in the removal of metal ions from

aqueous solution.

73

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Appendix

Preparation of Stock solution of metal ions

The chemicals used for the preparation of Pb2+ and Cd2+ were Pb(NO3)2 and

Cd(NO3)2.4H2O, respectively. The stock solutions of each metal ions were prepared

as shown below:

For 1000ppm(1000mg/L) Stock Solution of Pb2+

MM of Pb(NO3)2 = 331g

Atomic M. of Pb = 207g

Therefore, 207g Pb → 331g Pb(NO3)2

1g Pb → Xg Pb(NO3)2

Then, Xg Pb(NO3)2 = 331 × 1 = 1.5990g Pb(NO3)2

207

Therefore, 1.5990g Pb(NO3)2 in 1L standard flask is equivalent to 1000ppm Pb2+

For 1000ppm (1000mg/L) Stock Solution of Cd2+

MM of Cd(NO3)2.4H2O = 308.48g

Atomic M. of Cd = 112.41g

Therefore, 112.41g Cd → 308.48g Cd(NO3)2.4H2O

1g Cd → Xg Cd(NO3)2.4H2O

Then, Xg Cd(NO3)2.4H2O = 308.48 × 1 = 2.7442g Cd(NO3)2.4H2O

112.41

Therefore, 2.7442g Cd(NO3)2.4H2O in 1L standard flask is equivalent to 1000ppm

Cd2+