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Performance evaluation of electrocoagulation process using iron-rod electrodes forremoving hardness from drinking water

M. Malakootian , H.J. Mansoorian, M. Moosazadeh

Department of Environmental Health, School of Public Health, Kerman University of Medical Sciences, Iran

a b s t r a c ta r t i c l e i n f o

Article history:

Received 11 September 2009

Received in revised form 11 January 2010Accepted 15 January 2010

Available online 19 February 2010

Keywords:

Electrocoagulation

Hardness removal

Iron-rod electrode

Drinking water

Hard water causes many problems in domestic and industrial usage. The growing demands for water of high

quality necessitate the development of modern and cost-effective technologies for softening hard and very

hard waters. One of these techniques is the electrocoagulation process (EC). The purpose of this study was to

investigate the efficiency of EC process in removal of water hardness through iron-rod electrodes in different

circumstances. This study was conducted as a pilot plant. Experimental water sample was taken from water

distribution network of Anar City located in northwestern part of Kerman Province, Iran. The indices for

calcium and total hardness removal in pH (3.0, 7.0, and 10.0), electrical potential of 6, 12, and 24 V and

reaction times of 10, 20, and 30 min were measured. The maximum efficiency of hardness removal which

was obtained in pH 10.0, voltage of 12 and reaction time of 60 min are equal to 98.2% and 97.4% for calcium

and total hardness, respectively. Final pH of remained solution has also increased which rises with acidic pH

and decreases in alcoholic pH, so the results demonstrate the direct effect of pH, potential difference and

reaction time on hardness removal using EC process.

2010 Elsevier B.V. All rights reserved.

1. Introduction

Suitable and available water for human consumption is highly

limited and likewise, available drinking water has been reduced

because of the pollution created naturally and artificially [13].

Among water quality parameters, hardness has always been

investigated as an important factor [4]. Moreover, water hardness

is an essential parameter in industrial water consumption in

manufacturing of high-quality products [5]. Water hardness origi-

nates from existence of cations such as calcium, magnesium; and in

lower traces; aluminum, iron and other bivalent and trivalent

cations. Among hardness causes, ions, calcium and magnesium are

identified as main factors of hardness [69]. Hard water causes many

problems in domestic and industrial consumptions like scale

formation in hot water pipes, kitchen devices, water supply facilities,

boilers, cooling towers, membrane clogging, declining efficiency of

heat exchangers and reaction to the soap and formation of hard

foam [4,1013]. Additionally, soft water is preferred to be hard

enough to prevent Nephritis [7]. Hence, water hardness is one of the

compounds which has to be removed and this process is called

water softening [2,13]. WHO Recommendation for drinking water's

hardness is based on maximum 500 mg/l calcium carbonate [10]. Ofdifferent technologies which need adding chemicals for water

softening, are chemical precipitation and ion exchange and those

which do not need to add chemicals; include reverse osmosis,

electrodialysis, nano-filteration, crystallization, distillation and evap-

oration [1,3,6,9,1417]. These techniques have some problems such

as increased sludge, permanent water hardness, water salts like

sodium, annual high operation costs, sediment formation on

membrane, which require an effluent post treatment and disposal

of residual sludge [13,1820]. Recently, growing demand for high-

quality water has justified the development of modern and low cost

technologies for hard and very hard water softening [1,3,19]. One of

these techniques is electrochemical technology such as electrocoa-

gulation process (EC), which is being used for the removal of ions,

organic matters, colloidal and suspended particles, dyes, surfactants,

oil and heavy metals from aqueous environments [2125]. This

procedure has a broader potential to improve the faults of other

water softening equipments [24]. Electrocoagulation process

involves three stages; coagulant formation through dissolution of

metal ions of anode reactor electrode, destabilization of pollutants,

suspended particles and de-emulsification, and aggregation of

instable phases and floc-forming [2,24,2628]. Destabilization of

pollutants, suspended particles and de-emulsification mechanism

can be established through dispersed double layer compression, ion

neutralization species existing in water and wastewaters, and flocs

and sludge forming [23,28]. In this study, iron electrodes have been

used in electrocoagulant process.

Desalination 255 (2010) 6771

Corresponding author. Tel.: +98 341 320 5074; fax: +98 341 320 5105.

E-mail address: m.malakootian@yahoo.com (M. Malakootian).

0011-9164/$ see front matter 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.desal.2010.01.015

Contents lists available at ScienceDirect

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mailto:m.malakootian@yahoo.comhttp://dx.doi.org/10.1016/j.desal.2010.01.015http://www.sciencedirect.com/science/journal/00119164http://www.sciencedirect.com/science/journal/00119164http://dx.doi.org/10.1016/j.desal.2010.01.015mailto:m.malakootian@yahoo.com

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Following equations, describe EC process in relation to iron

electrode [24]:

1) Anode

FesFe2aq 2e

Fe2aq 2OH

aqFeOH2s

2) Cathode

2H2Ol 2e

H2g 2OH

aq

3) Overall

Fes 2H2OlFeOH2s H2g

From iron electrodes, Ferro ions are released into the solution

through electrolytic oxidation of anode electrode and produce metal

hydroxides after reacting with hydroxide monomer and polymer ions

which completely depend on pH of the solution [24,26]. Flocs created

through this process are much larger than chemical flocs and contain

more stable acid duric and less water bonds [24]. Gases produced

through anode and cathode electrodes during electrolysis cause

flotation and better removal of pollutants [24,29]. EC technology,

compared with other techniques, enjoys some advantages like plain

equipment, easy functionality, short resistance time, no need of

chemicals, low sludge production, sludge stability, suitable sedimen-

tation of sludge, dewatering and environmental compatibility

[21,23,2831]. The aim of this study was to investigate electrocoagu-

lation process efficiency to remove water hardness using rod-iron

electrodes as a substitute for other water softening techniques and

determining pH and optimal current density.

2. Materials and methods

The present study was performed as a pilot experiment in a mini-

plant inside the chemical laboratory of water and wastewater of

Faculty of Health in Kerman University of Medical Sciences, Kerman,Iran. Water samples were taken from water distribution system of

Anar Town located in northwestern tip of Kerman province.

Properties of consumed water sample have been illustrated in

Table 1. Fig.1 shows an overview of Electrocoagulant Equipments

which include power supply (alternating current transformer to

direct current) and iron-rod electrodes with a diameter of 2 mm

connected at a distance of 2 cm into a glass tank with a dimensions of

110100150 mm and a volume of 1.3 L [21]. Electrodes were

connected to the power supply in a monopolar and parallel

arrangement which consume less energy than the series arrangement.

For the purpose of accuracy, each electrode was connected to positive

and negative poles directly and alternately [28,30]; and Mixing speed

was set to be 400 rpm [28]. Water sample hardness rate was

measured using EDTA Titrimetric method based on techniquesmentioned in the book for standard techniques of water and

wastewater experiment [10]. pH of the sample was adjusted using

sulphuric acid and normal sodium hydroxide; the reactor was tested

with water samples of different pHs (3, 7, and 10) under three

voltages (6, 12, and 24). Under each testing conditions, three reaction

durations were tested: 10, 30, and 60 min. Samples were chosen

(25 mL) from themiddle of thereactor using pipettes. Then, preferred

samples were passed through a membranefilterwitha sizeof 0.45 m

in order to remove the formed flocs. Finally, filtered samples were

analyzed concerning their calcium and total hardness. Reactor's pH

Solution was also analyzed at the end of the experiment.

3. Results

In this study, the efficiency of EC process using rod-iron electrodes

for removing water hardness was studied as a substitute for other

hardness removing techniques while using different voltages, pH and

reaction times. The results of present study have been shown in

Figs. 24. Concentration of hardness of water sample used in all

experiments was constant. Fig. 2 shows the efficiency of removing

water hardness in electrical potentials of 6, 12, and 24 V and pH of 3.0.As it was shown, maximum removal efficiency accomplished in

voltage of 24 and reaction time of 60 min with 94.6% and 97.2% for

calcium and total hardness, respectively. Furthermore, final pH of the

solution was increased from 3.0 to 10.37. Figs. 3 and 4 show the

removal efficiencyin thementioned electrical potentials and in pH 7.0

and 10, respectively. In pH 7.0, the maximum removal efficiency was

achieved in voltage 6 and reaction time of 60 min, which was equal to

95.4% and 95.7% for calcium and total hardness, respectively. In pH 10,

the maximum removal efficiencywas 97.4% and 98.2% forcalcium and

Table 1

Properties of water sample, used in experiment.

Rows Parameter Quantity

I Total hardness (mg/L CaCO3) 300

II Calcium hardness (mg/L CaCO3) 138

I II Phenolp ht halein alka linity (m g/L C aCO3) 22

IV Methyl orange alkali nity (mg/L CaCO3) 300

V Turbidity (NTU) 3

VI EC (s/cm) 1612

VII pH 8.35

Fig. 1. Bench-scale EC reactor with monopolar electrodes in parallel connection.

Fig. 2. Efficiency of hardness removal during EC process using iron-rod electrodes;

(initial concentration for total and calcium hardness, respectively: 300, 138 mg/L and

pH=3).

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total hardness, respectively and reached the potential difference of

12 V and reaction time of 60 min, which considered as the maximum

removal efficiency in this study. Final pH for remained solution has

increased from 7 and 10 to 10.54 and 10.63 respectively, as pH does

during the electrocoagulation process.

4. Discussion

4.1. Effect of current density

Density of electricity current is one of the most important

parameters to manage reaction speed in EC processes as the

determining coagulant dosage injected into the solution. In high and

low densities, the alum released into the aqueous environment

decreased while the produced flocs increased, respectively; likewise,

the rate of removal has also been decreased and increased,

respectively [2224]. By increasing the density, speed and efficiency

of removal process, energy and electrode consumption, the amount ofproduced sludge and operating costs was increased, while, reaction

time was decreased [3235]. Thus, the effect of electrical current

intensity on hardness removal from water was obtained in this

research. As it was observed, the removal efficiency was increased;

and the current density and the time considered to be optimal for

similar efficiencies werealso improved, leading to decreased electrical

potential differences.These results are in accordance with thefindings

of arsenic removal study conducted in India, and of chromium carried

out in Iran, mercury removal from water using EC with Al and Fe

electrodes in France and removing humic acid from groundwater

waters using EC in China [22,3538].

In present study, voltage 12 in reaction time of 60 min indicated

maximum removal efficiency, i.e. 98.2% for total hardness and 97.4%

for calcium hardness. Minimum removal efficiency was obtained in

potential difference of 6 V; in general, voltage 12 is suggested to

achieve the desired efficiency.

4.2. pH effect

Inprevious studies, it isproved that pH is an important factor in EC

process and this process is highly dependent on the pH of solution and

has a significant effect on forming metal hydroxide species and

removal mechanism of ions and pollutants [22,26,32,33,39]. Gener-

ally, pH changes during EC process depend on the type of used

electrode and the primary pH [22,26]. pH increase in this process is

attributed to the formationof H2 in cathode electrode and aggregation

of hydroxide ions in the solution [27,33,35]. Therefore, EC process

could act as a pH regulator [33,35]. In this study, three pH-ranges of 3,

7, and 10 were examined in order to investigate the effect of pH on

hardness removal.

Reactions for these three ranges are as follows [40]:

1) Reaction 1 (acid pH)

2Fes 6H2OlO2g 4H2g 2FeOH2s

2) Reaction 2 (neutral pH)

3Fes 8H2OlFeOH2s 2FeOH3s 4H2g

3) Reaction 3 (alkaline pH)

2Fes 6H2Ol2FeOH3s 3H2g

Other reactions may be observed at high pH near the cathode and

provoked the precipitation of the carbonate salt on this electrode,

reaction equation are as these [34] :

HCO3 OH

CO23 H2O

CO23 Ca

2CaCO3

CO2

3 Mg2MgCO3

In these reactions, produced H2 goes upward and causes flotation

and Al (OH)3 and Al (OH)2 precipitate. The results indicated that

maximum and minimum removal efficiency was obtained in pH of

10.0 and 3.0, respectively. pHs (3.0, 7.0, and 10.0) of the remaining

solution were increased to 7.37, 10.54, and 10.63 respectively, which

were high in acidic pH and low in alkaline pH. Concerning iron-rod

electrodes, final pH was always higher than the primary pH. The

results of this study are consistent with the results of studies of

decolorization conducted in Korea, arsenic removal in India, anddiazinon and chromium removal in Iran using EC process [28,33,36].

4.3. Effect of resistance time

In accordance with Faraday Act, the time of electrolysis in EC

process affects the rate of metal ion released into the system [22].

Iron-rod electrodes need shorter resistance time to achieve a desirable

removal efficiency and are cost-effective concerning energy and

electrode consumption, compared with aluminum ones [39]. In this

study, considering the effect of resistance time on hardness removal

through EC process using rod-iron electrodes, it has been shown that

by increasing the reaction time, an enhanced rate of removal resulted;

so that, maximum removal efficiency was achieved in voltage of 12,

pH 10 and the resistance time of 60 min which is in agreement with

Fig. 3. Efficiency of hardness removal during EC process using iron-rod electrodes

(Initial concentration for total and calcium hardness, respectively: 300, 138 mg/L;

pH=7).

Fig. 4. Efficiency of hardness removal during EC process using iron-rod electrodes

(initial concentration for total and calcium hardness, respectively: 300, 138 mg/L;

pH=10).

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the results obtained from arsenic removal in India, electrolytical

removal of Cr+6 in UK, the removal of diazinon from aqueous

environments and chromium (VI) of synthetic solutions using EC

process in Iran, removal of indium ions using EC with iron electrodes in

Taiwan, optimization of oil removal from oily wastewaters by EC using

response surface methodand thestudyon thetreatmentof photovoltaic

wastewater using electrocoagulation in Algeria [3436,4143].

4.4. Effect of electrode spacing

In case of solution's boosted resistance, increased distance

between each couple of anode and cathode electrodes leads to

increased voltage. Because of diminished ions aggregation, hydroxide

polymers, decreased rate of suspended solids and absorption of ions

causing water hardness as well and electrostatic force, enhancing

electrolyte constancy and spacing will eventually lead to decreased

removal efficiency [44]. In the current study, distance between anode

and cathode electrodes was chosen to be 2 cm, and by decreasing the

distance, reactions were improved due to the topical increase of

concentration, and as a result, removal efficiency was increased [44].

Our result is in accordance with data from other works like mercury

removal from water using EC with Al and Fe electrodes in France,

removing humic acid from groundwater waters using EC in china andalso Removal of Fe (II) from tap water by electrocoagulation

technique in India [37,43,45].

4.5. The effect of selected electrode's category

In electrochemicalprocesses, the type of the selected electrode has

a significant effect on removal efficiency. Therefore, it is important to

select a suitable type of electrode. The electrode used for drinking

water treatment must be non-toxic, so, iron, aluminum and titanium

electrodes were selected because of their non-toxic nature, cheap and

easy accessibility [33,36]. In this study, regarding the removal of

arsenic, iron-rod electrodes were 62% more efficient than its

aluminum equivalents [35] and due to their high efficiencies, iron

electrodes were used in this study. Initially, the color of the effl

uenttreated by iron-rod electrodes was greenish and then changed into

dark-yellowish. In electrode electrolysis, green and yellow colors

resulted due to ions' ferric and ferro natures. These findings were

similar to those obtained from treatment study of wastewater in

potato-chip manufacturing factory using EC process [33].

4.6. Effect of mixing

Mixing is regarded as an important unit of water treatment which

significantly affects reactions and controlling processes like sedimen-

tation [46]. Using rod electrodes instead of flat equivalent, it was

possible to put more electrodes inside reactor and had a better mixing

process; moreover, time needed to accomplish the operation

problems would be reduced due to the production of more metalhydroxide flocs.

5. Conclusion

Results of this study showed that EC could be used for removing

ions responsible for water hardness. The greater removal effective-

ness has occurred in pH 10, voltage 12 and in time course of 60 min;

which was 98.2% and 97.4% for total calcium hardness and finally

demonstrated the importance of direct pH, potential difference and

the reaction time course on removal of hardness using EC.

These results indicate that using rod-iron electrodes in electro-

coagulation process can be effective in removing water hardness in

different situations like potential difference, acidity, reaction time and

electrode type and spacing.

Acknowledgements

The authors are grateful to Environmental Health research

Commission of Kerman University of Medical Sciences for the project

approval. Moreover, we appreciate the assistance of Ms. Mahshid

Loloei and Ms. Marzieh Gharib.

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