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Utility of adsorbents in the purification of drinking water: A review of characterization,

efficiency and safety evaluation of various adsorbents

Shashi Prabha Dubey1, Krishna Gopal*1 and J.L. Bersillon2

1Aquatic Toxicology Division, Industrial Toxicology Research Centre, P.B. No. 80,

M.G. Marg, Lucknow - 226 001, India2Laboratoire Environment et Mineralurgie, Groupe de Recheerches sur I’Eau et les Solides Divises,

Centre de Recheche Francois Fiessinger, Nancy - F-54501, Cedex, France

(Received: June 21, 2007; Revised received: December 04, 2007; Accepted: January 02, 2008)

Abstract: Clean drinking water is one of the implicit requisites for a healthy human population. However, the growing industrialization and

extensive use of chemicals for various concerns, has increased the burden of unwanted pollutants in the drinking water of developing

countries like India. The entry of potentially hazardous substances into the biota has been magnifying day by day. In the absence of a

possible stoppage of these, otherwise, useful chemicals, the only way to maintain safer water bodies is to develop efficient purifying

technologies. One such immensely beneficial procedure that has been in use is that of purification of water using ‘adsorbents’. Indigenous

minerals and natural plants products have potential for removing many pollutants viz. fluoride, arsenic, nitrate, heavy metals, pesticides as

well as trihalomethanes. Adsorbents which are derived from carbon, alumina, zeolite, clay minerals, iron ores, industrial by products, and

natural products viz. parts of the plants, herbs and algal biomass offer promising potential of removal. In the recent years attention has been

paid to develop process involving screening / pretreatment / activation / impregnation using alkalies, acids, alum, lime, manganese dioxide,

ferric chloride and other chemicals which are found to enhance their adsorbing efficiency. Chemical characterization of these adsorbents

recapitulates the mechanism of the process. It is imperative to observe that capacities of the adsorbents may vary depending on the

characteristics, chemical modifications and concentration of the individual adsorbent. Removal kinetics is found to be based on the

experimental conditions viz. pH, concentration of the adsorbate, quantity of the adsorbent and temperature. It is suggested that isotherm

model is suitable tool to assess the adsorption capacities in batch and column modes. Safety evaluation and risk assessment of the

process/products may be useful to provide guidelines for its sustainable disposal.

Key words: Efficiency, Characterization, Langmuir, Freundlich, BET

PDF of full length paper is available with author (*krishnagopaldubey@gmail.com)

Introduction

Problems in drinking water quality include presence of

excess fluoride, arsenic, natural organic matters, trihalomethanes,

heavy metals and variety of pathogens that are a major cause of

various water-borne diseases. The tremendous increase in the

use of heavy metals over the past few decades has inevitably

resulted in an increased flux of metallic substances in the aquatic

environment (Gaikwad, 2004; Tseng, 2007; Yoon et al., 2008).

These pollutants enter the water bodies through wastewater from

metal plating industries, batteries, phosphate fertilizer, mining,

pigments and stabilizers alloys (Low and Lee, 1991). Although

preventive measures have been adopted to remove pollutants

however, these methods are not found suitable for removing heavy

metals. Much attention is being paid on the various adsorbents,

which have metal binding capacities and are able to remove

unwanted pollutants from contaminated water at low cost. Abundantly

available materials have been studied for the adsorption of pollutants,

for example, bark and other tannin – rich materials, lignin, chitosan,

dead biomass, xanthate, zeolite, clay and peat moss (Orhan and

Buyukgungor, 1993; Bryant et al., 1992; Rorrer et al., 1993; Hsien

and Rorrer, 1995; Roy et al., 1993; Sharma and Foster, 1993).

Among these adsorbents, chitosan, zeolite, lignin and seaweed

(Kertman et al., 1993) showed high adsorption capacities. Since

coffee residues hold several functional groups, they have high

potential for pollutants adsorption. Tea leaves are also used as

adsorbent (Bailey et al., 1992).

Natural zeolite gained a significant interest, due to their

valuable properties such as ion exchange capability. Clay minerals

are found to be an important inorganic component in the soil and

their sorption capabilities are due to high surface area and exchange

capacities. Industrial byproducts such as waste slurry, lignin, ferric

hydroxide and red mud have been explored for their technical

feasibility to remove pollutants from contaminated water (Kumar et

al., 2000).

In recent years adsorption processes have been

developed using a large diversity of adsorbents. Activated carbons,

activated mineral surfaces (silica, bauxite, alumina), fly ash, industrial

waste, agricultural wastes and coral limestone have also been

used as adsorbents.

Adsorption: The search for new technologies involving the removal

of toxic pollutants from wastewater has directed attention to adsorption,

Review Paper

Journal of Environmental Biology May 2009, 30(3) 327-332 (2009)

©Triveni Enterprises, Lucknow (India) For personal use only

Free paper downloaded from: www.jeb.co.in Commercial distribution of this copy is illegal

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Dubey et al.

based on binding capacities of various biological materials (Babel

and Kurniawan, 2003). The major advantages of sorption over

conventional treatment methods include low cost, high efficiency,

minimization of chemical and biological sludge, regeneration of

sorbents, and possibility of metal recovery (Ahalya et al., 2003).

Adsorption equilibrium: Adsorption capacity of the adsorbent

can be determined by making a contact between the adsorbate and

adsorbent. If adsorption is the removal mechanism, then the residual

concentration will be reached that will remain unchanged with time,

which is also known equilibrium, and the process is adsorption

equilibrium. Two types of adsorption processes are mentioned: a)

Physical adsorption, which is a reversible phenomenon. It results

from the action of van der Waals forces. It is usually dominant at low

temperatures and is multilayered. b) Chemisorption is generally

irreversible, because chemical interactions are involved between

the adsorbate and adsorbent moiety. Factors affecting the adsorption

process are pH, temperature, adsorbent quantity, and particle size

including other chemical properties of the adsorbate and adsorbent.

Adsorption isotherms: It is the relationship between the amount

of adsorbate adsorbed on the surface of adsorbent and equilibrium

concentration of the adsorbate at a certain temperature and other

condition. The equilibrium data is formulated into an isotherm model.

Brunauer (Brunauer et al., 1972) classified adsorption isotherm in

six types. These types may be monolayer, multilayer or

condensation in pores/ capillaries. An isotherm model is suitable tool

to assess the adsorption capacities in batch and column study.

Batch study consists of contacting an adsorbate with a definite quantity

of adsorbent in batch stirred system. The mixture is agitated to

facilitate the adsorption process. In column study, adsorbent is packed

in a column reactor and almost no flow or movement of adsorbent

takes place inside the column.

Different theoretical and empirical models have been

proposed to describe the different types of isotherms in batch study.

Most commonly used models are discussed here which are generally

used for the interpretation of adsorption isotherms.

Langmuir isotherm: According to this isotherm surface is

homogeneous. It assumes that all the adsorption sites have equal

affinity for the adsorbate molecules and adsorption at one site does

not affect adsorption at an adjacent site (Langmuir, 1918; Weber

1972). The Langmuir equation may be written as:

Qo bCe

qe = (non – linear form) .....(1)

1+ bCe

Ce

1 1

= + Ce

(linear form) .....(2)

qe Qo b Qo

where qe

is the amount of solute adsorbed per unit weight of

adsorbent (mg g-1), Ce = Equilibrium concentration of solute in bulk

solution (mg l-1), Qo = Monolayer adsorption capacity (mg g-1) and

b = constant related to the free energy of adsorption/ desorption

(bαe- ∆G / RT ). Sorbents with highest possible Qo and high b value

are desirable for a process.

Freundlich isotherm: Herbert Max Finley Freundlich, a German

physical chemist, presented an empirical adsorption isotherm. It

describes the equilibrium on heterogeneous surfaces. The

Freundlich isotherm is the earliest known relationship describing

the adsorption equation and is often expressed as:

qe = K

FC

e1/n (non –linear form) .....(3)

where qe is the adsorption density (mg of adsorbate per g of

adsorbent), Ce is the concentration of adsorbate in solution (mg l-1),

KF

and n are the empirical constants dependent on several

environmental factors and n is greater than one.

This equation is conveniently used in the linear form by

taking the logarithmic of both sides as:

1

lnqe = ln K

F + lnC

e......(4)

n

A plot of lnCe against lnq

e yielding a straight line indicates

the confirmation of the Freundlich isotherm for adsorption. The

constants can be determined from the slope and the intercept. 1/n is

the intensity of the adsorption process, KF is the relative adsorption

capacity of the adsorbent (mg g-1) (Freundlich, 1906).

BET isotherm: This isotherm assumes the multilayer adsorption of

adsorbate on adsorbent surfaces (Brunauer et al., 1972; Weber

1972).

BCQo

qe = (non – linear form) (5)

(Cs – C ) [1+ (B –1)(C/Cs)]

C 1 B –1

qe = = + C/Cs (linear form) (6)

(Cs – C)qe BQo BQo

where qe is the amount of solute adsorbed per unit weight of

adsorbent (mg g-1), B = A constant related to the energy of interaction

with the surface, C = The equilibrium concentration of adsorbate in

solution (mg l-1), Qo = The number of moles of adsorbate per unit

weight of adsorbent to form a complete monolayer, and Cs = The

saturation concentration of the adsorbate.

Efficiency of the adsorbents: Natural products as adsorbents

for the removal of heavy metals, nitrates, arsenic, fluorides, natural

organic materials, trihalomethanes have gained important credibility

during recent years because of their good performance and low

cost. The adsorbent developed offers a viable alternative to

traditional metal removal technologies. There are several adsorbents

commonly used for the removal of arsenic (Katsoyiannis and

Zouboulis, 2002; Elson et al., 1980), fluorides (Srimurali et al.,

328

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Utility of adsorbents in the purification of drinking water

Scanning electron microscopy (SEM): Fig. 1: Uncoated sand, Fig. 2: FeCl3 coated sand, Fig. 3: Activated alumina, Fig. 4: Iron oxide coated activated

alumina, Fig. 5: Activated carbon powder, Fig. 6: Carbonized ground nut husk

329

Fig. 1 Fig. 2

Fig. 3 Fig. 4

Fig. 5 Fig. 6

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1998; Ramos et al., 1999; Fan et al., 2003; Chaturvedi et al.,

1990), heavy metals, natural organic matter (Korshin et al., 1997;

Mall and Upadhyay, 1998; Chiang et al., 2002; Daifullah and Girgis

1998; Bernard et al., 1997) and trihalomethanes from drinking

water, which give better results after impregnation with certain

chemicals. The most common adsorbent widely used is activated

carbon. Most of the activated carbon that is used for pollution control

is manufactured from bituminous coal. Activated carbon can be also

be made from agricultural wastes, wood and petroleum. Activated

alumina is commonly used to remove oxygenates and mercaptans

from hydrocarbons and fluorides from water. The adsorption

properties of materials of biological origin have been investigated:

chitosan (Elson et al., 1980) amine modified coconut coir (Baes et

al., 1997). Natural organic matter can be adsorbed on iron oxide.

Factors that effect the capacity of an adsorbent include its

surface area, pore size, and polarity. Activation is the process that

enhances the porous structure essential for effective adsorption.

The two types of activation process widely used are chemical and

physical activation. Carbonization of the material of the vegetable

origin wit the addition of the activating agents is generally known as

chemical activation. Physical activation or gas activation is a process

in which inactive carbonized product is allowed to react with suitable

substance, usually gaseous (carbon dioxide or steam). The sorption

capacities for the sorbents are usually very low before activation and

the maximum sorption capacity rarely exceeds 0.1-0.2 mmol g-1. In

the case of chitosan, arsenic sorption capacity is significantly lower

than the levels reached with other metal ions (Guibal et al., 1995;

Guibal et al., 2000; Jansson Charrier et al., 1996).

Several processes have been developed to increase the

efficiency of sorbents for metal ion uptake especially by ligand grafting.

The chemical modification of silica surface by using chemical ligating

groups provides scopes for trapping metal ions specially and

selectively even at ultra trace level (Jal et al., 2004).

NOx adsorption on KOH impregnated activated carbon

also give positive support to this concept that chemical impregnation

increase the removal efficiency of the adsorbent (Lee et al., 2002).

Activated carbon was produced from almond shells through

chemical activation, by several activating agents (H3PO

4, ZnCl

2,

K2CO

3 and Na

2CO

3). ZnCl

2 activated material was noticed to be the

best product with high adsorption capacity (Bevla et al., 1984a;

Bevla et al., 1984b).

Activated carbons were prepared from untreated and

phosphoric acid treated coconut shells, and increased adsorption

capacity was found (Laine et al., 1989). Ammonium chloride

impregnated and untreated almond shell and hazelnut shell samples

were tested for their surface area at different temperatures. Chemical

activation carried out at 350oC gave products with surface area

values above 500 m2 g-1. However, the surface area values

observed for the products obtained from untreated raw materials

were reported about half of this value. It was also observed that, the

surface area of products obtained from NH4Cl impregnated samples

reached values of over 700 m2 g-1 when the carbonization

temperature was increased 700oC (Balci et al., 1994). The granular

activated carbon samples activated, either chemically, with H3PO

4,

or physically, with CO2, under a variety of conditions. The product

obtained by chemical activation had higher BET surface areas and

greater product yields than the CO2-activated carbons. The products

were also compared with the commercial activated carbon, which

have higher adsorption ability (Toles et al., 1997). Phosphoric acid

impregnated apricot stones were carbonized at 300, 400 and 500oC

respectively, were found to increase the BET surface area increased

from 700 m2 g-1 up to 1400 m2 g-1 with rise in temperature. The

highest BET surface area was obtained from the sample that was

impregnated with 30% (vol) phosphoric acid and carbonized at

500oC (Girgis and Daifullah, 1998). Activated carbons were

produced from canes from Arundo donax, a rapid-growing plant,

by phosphoric acid activation under four different activation

atmosphere, to develop carbons with substantial capability to adsorb

Cd(II) and Ni(II) ions from dilute aqueous solutions. Surface areas

and total pore volumes of the activated carbons used were of around

1100 m2 g-1. It is established that, the content of carbons’ polar or

acidic surface oxygen functional groups, with their development

depending on the atmosphere used, influenced predominantly metal

adsorption. Carbons derived under flowing air, possessing the

largest total content of these groups, showed the best adsorption

effectiveness (>90%) for both ions, even superior to that determined

for a commercial sample used as a reference (Basso et al., 2002).

Denizli et al. (1997) incorporated dye ligand onto synthetic polymers

to enhance their metal ion sorption properties. Similar modifications

have been performed on activated carbon; the impregnation with

metal ions significantly enhances arsenic sorption on activated carbon

(Huang and Vane, 1989; Rajakovic, 1998).

Characterization of the adsorbents: Chemical characterization

of the products and spent adsorbent may be explored to understand

the mechanism of the process involved. Amorphous and crystalline

nature of the adsorbent can be determined by the use of X-ray

Diffraction XRD and identification of various constituents in adsorbents

can be determined with the help of IR spectra. The specific surface

area of the adsorbent can be calculated from the N2 - BET equation

(Mohan et al., 2005). Porosity can be investigated using surface

area analyzer and mercury porositimeter. Scanning electron

Microscopy (SEM), Extended X-ray adsorption fine structure,

nuclear magnetic resonance (NMR) and high performance liquid

chromatography (HPLC) may give the details regarding the texture

(Huggins et al., 2002; Singh et al., 2003) and delineate the

mechanism of process of the water purification. The trace element

contents of the coals have been determined by a combination of

instrumental neutron activation analysis (INAA), inductively coupled

plasma mass spectrometry (ICP – MS) and ICP atomic adsorption

spectroscopy (ICP-AES) (Shtepenko et al., 2005). X-ray

photoelectron spectroscopy (XPS) analysis was used for the study

of sorbent and the metal speciation on the sorbent. Desorption

studies were also carried out to elucidate the interactions between

the metal and the sorbents. A sorbent for metal removal has been

Dubey et al.330

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produced from anhydrous calcium silicate/ aluminate by means of

an accelerated carbonation process. Iron treatment increases the

adsorption capacity by 10 – fold. XPS studies supported that iron is

into the protein matrix. Shifts in XPS spectra suggest that dichromate

binding occur with the iron at active adsorption sites (Fathima et al.,

2005).

Surface–reaction phenomenon of adsorbent caused by the

KOH impregnation and NOx adsorption were investigated by using

SEM, time of flight secondary ion mass spectrometry (ToF – SIMS)

and SIMS depth profile. In the results of ToF – SIMS and SIMS

depth profile, NO2- and NO3- increased at the surface distribution by

forming oxide crystals of KNO2 and KNO

3 due to bond formation

with potassium ions (Lee et al., 2002).

Safety evaluation of the adsorbents: It is prerequisite to assure

the water quality as recommended by regulatory and health

agencies, which are desired in the daily intake of the drinking water

(Gopal et al., 2004). The physico-chemical and bacteriological

characteristics of the produced water may be compared with the test

water as per the standard methods (APHA, 2005). In the process of

adsorption of the minerals/ plant products it is apparent that synthesis

of large amount of the product is required, in which large amount of

sludge is also generated. Once this sludge is added to the soil, the

leachates ultimately find its way to the surface water. The soil and

water contains living organism, which are responsible for the

biotransformation and biodegradation of the material and also the

translocation of the energy into food chain. Therefore the standard

protocols are to be developed for the safety evaluation of the products

and spent wash/ solid waste.

Multispecies testing e.g. fish bioassay chironomus, daphnia,

earthworm and algal bioassays are recommended protocols that

may be carried out for ecotoxicological assessment (OECD, 2002).

Identification of suitable disposal sites are prerequisite in case of

community water supply.

Acknowledgments

The authors are thankful to the Director, Industrial Toxicology

Research Centre, Lucknow, for providing all necessary facilities for

this work. One of the author is thankful to CSIR for providing fellowship

for the research work.

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