Comparative study of arsenic removal by iron using electrocoagulation and chemical coagulation

Comparative study of arsenic removal by iron using electrocoagulation and chemical coagulation

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Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/watres

Comparative study of arsenic removal by iron using electrocoagulation and chemical coagulation Divagar Lakshmanan*, Dennis A. Clifford, Gautam Samanta Department of Civil and Environmental Engineering, N 107 Engineering Bldg 1, University of Houston, Houston, TX 77204-4003, USA

article info

abstract

Article history:

This research studied As(III) and As(V) removal during electrocoagulation (EC) in comparison

Received 23 December 2009

with FeCl3 chemical coagulation (CC). The study also attempted to verify chlorine production

Received in revised form

and the reported oxidation of As(III) during EC. Results showed that As(V) removal during

22 April 2010

batch EC was erratic at pH 6.5 and the removal was higher-than-expected based on the

Accepted 8 June 2010

generation of ferrous iron (Fe2þ) during EC. As(V) removal by batch EC was equal to or better

Available online 15 June 2010

than CC at pH 7.5 and 8.5, however soluble Fe2þ was observed in the 0.2-mm membrane filtrate at pH 7.5 (10e45%), and is a cause for concern. Continuous steady-state operation of the EC

Keywords:

unit confirmed the deleterious presence of soluble Fe2þ in the treated water. The higher-

Arsenic

than-expected As(V) removals during batch mode were presumed due to As(V) adsorption

Iron

onto the iron rod oxyhydroxides surfaces prior to the attainment of steady-state operation.

Electrocoagulation

As(V) removal increased with decreasing pH during both CC and EC, however EC at pH 6.5 was

Chemical coagulation

anomalous because of erratic Fe2þ oxidation. The best adsorption capacity was observed with

Competing ions

CC at pH 6.5, while lower but similar adsorption capacities were observed at pH 7.5 and 8.5 with CC and EC. A comparison of As(III) adsorption showed better removals during EC compared with CC possibly due to a temporary pH increase during EC. In contrast to literature reports, As(III) oxidation was not observed during EC, and As(III) adsorption onto iron hydroxides during EC was only 5e30% that of As(V) adsorption. Also in contrast to literature, significant Cl2 was not generated during EC, in fact, the rods actually produced a significant chlorine demand due to reduced iron oxides on the rod. Although Cl2 generation and As(III) oxidation are possible using a graphite anode, a combination of graphite and iron rods in the same EC unit did not produce As(III) oxidation. However, a two-stage process (graphite anode followed by iron anode in separate chambers) was effective in As(III) oxidation and removal. The competing ions, silica and phosphate interfered with As(V) adsorption during both CC and EC. However, the degree of interference depends on the concentration and presence of other competing ions. In particular, the presence of silica lowered the effect of phosphate with increasing pH due to silica’s own significant effect at high pHs. ª 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Previous studies on arsenic removal have reported coagulation to be effective and economical compared with adsorption and ion exchange (Clifford, 1997; Ghurye et al., 2004) and have

shown factors such as pH, As(III/V) speciation, and coagulant dose to affect arsenic removal during coagulation (Cheng et al., 1994; Scott et al., 1995; McNeill and Edwards, 1995, 1997; Hering et al., 1996; Tong, 1997). Based on arsenic uptake by the iron oxide/hydroxide surfaces, preformed Fe

* Corresponding author. Tel.: þ1 713 743 0753; fax: þ1 713 743 4260. E-mail address: [email protected] (D. Lakshmanan). 0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.06.018

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(OH)3 and granular ferric oxide/hydroxide media were found to have less available surface area in comparison to in-situ formed Fe(OH)3(s) which have significantly higher adsorptive capacities (Lakshmanan, 2007). Coagulationefiltration is an effective and widely used technology for treating drinking water to remove arsenic. For small systems, however, adsorption onto media (granular ferric hydroxide (GFH) or granular ferric oxide (GFO)) is currently the process of choice. Even though coagulation processes have greater adsorption capacities and less disposal problems, they are not the process of choice for small systems, because of complexity and the continuous requirement of chemicals for dosing and pH adjustment. Adsorptive processes are preferred because of simple design and operation, and a relatively small space requirement. These factors outweigh disposal issues, lower adsorptive capacity, and high media replacement costs. Electrocoagulation (EC) is a promising technology, which resembles coagulation in that the hydroxides are produced in-situ, but without manual addition of coagulant. EC is expected to bring in the advantages of higher adsorption capacity, no manual chemical addition, less land-area requirement, and no media replacement. In addition, compared with CC, the EC process reportedly requires less coagulant, produces less sludge (Horner and Duffey, 1983; Mills, 2000), and requires less space and capital costs (Mills, 2000). Based on the literature, the electrolytic oxidation of iron rods followed by hydrolysis was expected to result in production of the solid iron hydroxides, necessary for contaminant removal (Chen et al., 2000; Mills, 2000; Balasubramanian and Madhavan, 2001; Mollah et al., 2004; Kumar et al., 2004; Parga et al., 2005; Hansen et al., 2006; Kobya et al., 2006). However during the course of research, it was found that the formation of solid iron hydroxides was erratic at pH 6.5e7.5, and soluble iron was present in the treated water at pH 6.5 due to slow oxidation rate at low pHs. A detailed study on the iron hydroxides formed during iron EC is discussed in detail elsewhere (Lakshmanan et al., 2009). EC has been reported to be efficient in treatment of potable water (Nikolaev et al., 1982; Vik et al., 1984; Holt et al., 2002), urban waste water (Novikova and Shkorbatova, 1982; Tyrina and Morozov, 1982; Pouet and Grasmick, 1995; Kobya et al., 2006), heavy-metal contaminated waters (Osipenko and Pogorelyi, 1977; Mills, 2000), microorganisms (Mills, 2000; Zhu et al., 2005), turbidity (Han et al., 2002) and color (Jiang et al., 2002) have been removed by EC. Some studies have reported EC to be effective in removing arsenic from water and waste water (Balasubramanian and Madhavan, 2001; Kim et al., 2002; Kumar et al., 2004; Parga et al., 2005; Hansen et al., 2006). Arienzo et al. (2002) and Kumar et al. (2004) reported simultaneous oxidation and removal of As(III) during EC. The literature on electrochemical oxidation indicates possible anodic generation of chlorine, which has the capacity to oxidize As(III) (Kim et al., 2002). Because H3AsO3 exists at pH < 9, it is well accepted that for efficient As(III) removal, a pre-oxidation step is essential (Clifford, 1990). So if the literature about possible oxidation is true, EC is expected to be an advantageous process in simultaneously oxidizing and removing As(III). The objectives of this study were to (a) verify the reported ability of EC to oxidize As(III)eAs(V) and remove As(V) by

adsorption, (b) compare EC with CC for As(V) and As(III) removal, and (c) study the effect of competing ions on arsenic removal during EC and CC.

2.

Materials and methods

2.1.

Reagents

Analytical reagent chemicals and Milli-Q (18 MU cm) water were used to prepare all stock solutions. Primary standards of 100 mg As/L of each species were prepared from sodium m-arsenite for As(III) and sodium arsenate for As(V) from which the required amount (0.05 mg/L) of As(III) or As(V) was spiked into the challenge water just prior to the coagulation experiments. Salts of sodium nitrate (NaNO3), sodium bicarbonate (NaHCO3), sodium phosphate monohydrate (NaH2PO4$H2O), sodium fluoride (NaF), disodium metasilicate nona-hydrate (Na2SiO3$9H2O), Magnesium sulfate heptahydrate (MgSO4$7H2O), and calcium chloride dihydrate (CaCl2$2H2O) were used to prepare concentrated stock solutions that were diluted for preparing the challenge water. The chemical coagulant stocks, 1 g/L Fe2þ or Fe3þ were prepared in DI water using ferrous sulfate (FeSO4) and ferric chloride (FeCl3), respectively. The coagulant stocks were preserved with acid and the exact metal concentration analyzed using flame atomic absorption spectroscopy (AAS).

2.2.

Preparation of challenge water and coagulant stocks

Analytical reagent chemicals and Milli-Q (18 MU cm) water were used for the studies. All studies were carried out with NSFI-53 challenge water (Table 1), standard challenge water for testing arsenic adsorbents. The NSFI challenge water (NSF/ ANSI, 2007) with composition shown in Table 1 contains representative groundwater concentrations of silica, sulfate, phosphate, fluoride, and hardness, which are known to affect the arsenic adsorption capacity of adsorbents. This is the standard challenge water for testing arsenic adsorbents. Stability of the challenge water was not an issue because it was prepared on the day of experiment, and studies have showed that the challenge water was stable with regard to CaCO3 precipitation and As(III) oxidation for at least a week (Tripp, 2001). The pH of the synthetic groundwater was adjusted using dilute HCl or NaOH solution. The required

Table 1 e Composition of NSFI-53 challenge water. Cation 2þ

Ca Mg2þ Naþ



mg/L

meq/L

40.1 12.6 88.87

2.00 1.04 3.864

141.57

6.904

Anion HCO 3 SO2 4 

Cl NO3-N F PO4-P SiO3-SiO2 As(III)/(V) S¼

mg/L

meq/L

183.0 50 71.0 2.0 1.0 0.040 20.0 0.05 327.09

3.0 1.04 2.0 0.143 0.053 0.0013 0.666 6.904

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amount (0.050 mg/L) of As(III) or As(V) was spiked into the challenge water just prior to the coagulation experiments.

2.3. Electrocoagulation unit setup and arsenic removal procedure A bench-scale EC unit with three anodeecathode pairs was used for this research (Fig. 1). It had a 490-mL active volume including flow-through electrode chamber and the recirculation loop. The plan view of the rod-shaped iron anodes (22 cm long, 5 mm dia.) and porous cylindrical stainless steel cathodes used is shown in Fig. 1. The iron anode (98.5% Fe) and stainless steel cathode were made using commercially available materials. The total active surface area of the iron anode was 110 cm2. The recycle pump discharged into the annular area between the cathode and anode of each of the three cathodeeanode pairs in order to flush the anode where iron coagulant was generated. By adjusting the operating current and generation time, the desired iron concentration was obtained. A 12 V, 1.2 A direct current power supply (HP 6214A) supplied controlled current to the electrodes. The results of deliberate anode rusting/deterioration experiments showed that pre-cleaning the iron rods was required for stable iron generation (Lakshmanan et al., 2009). Thereafter, the iron rods were cleaned on the day of the experiment by scrubbing with sand paper to remove all of the rust and the oxy/hydroxides coating. NSFI challenge water spiked with As(V) or As(III) adjusted to the specific pH (6.5, 7.5, or 8.5) was poured into the top of the EC unit, and was recycled at about 500 mL/min to ensure that the iron generated was quickly released from the surface and hydrolyzed. Most of the arsenic removal experiments were operated in batch mode, i.e., the EC unit was filled with challenge water and a constant current was applied for specific electrolysis time depending on the iron dose to be generated. The challenge water was recirculated for a total of 2 min, including the electrolysis time. After EC, the water was immediately drained into a beaker, and the samples preserved and analyzed for arsenic and iron. Although batch EC experiments were carried out under non-steady-state batch conditions, as has been the practice of other researchers, a limited number of continuous-flow, steady-state EC experiments for As(V) removal were carried

Source Water

Plan View of Anode and Cathode Internal Diameter: 0.85 cm Annular space for water flow

EC Unit

Stainless Steel Cathode Iron Anode Diameter: 0.48 cm

Recycle Pump

Fig. 1 e Schematic diagram of batch bench-scale electrocoagulation system.

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out at pH 6.5, 7.5, and 8.5. The procedure used was as follows: 245  10 mL/min of challenge water spiked with 50 mg/L As(V) was continuously fed into the top of EC unit (residence time ¼ 2 min) and recycled at about 500 mL/min with a current of 28  2 mA applied continuously. The experiments were conducted for 1-h in order to ensure attainment of steady state. At regular intervals, samples were collected at the overflow outlet. Un-filtered samples were preserved for total iron analysis, and the filtered samples were preserved for arsenic and soluble iron analysis as required.

2.4. Arsenic removal procedure during chemical coagulation The chemical coagulation studies were conducted using bench-scale jar tests with ferric (FeCl3) and ferrous (FeSO4) coagulants in the NSFI challenge water. Because of coagulant acidity, the pH of the challenge water dropped after dosing. Hence, pre-titrations were performed to determine the amount of NaOH necessary to achieve the desired equilibrium pHs following dosing. Challenge water spiked with arsenic and adjusted to a specific pH (6.5, 7.5, or 8.5) was added to each jar. With the stirring apparatus preset at the rapid mix speed of 100 rpm (G ¼ 100 s1), the coagulant and a predetermined amount of NaOH solution were added. After mixing for 2 min, the filtered and un-filtered samples were collected and preserved as required.

2.5.

Preservation and analysis

2.5.1.

Arsenic sample preservation and analysis

After EC and CC experiments, the treated water was collected using a syringe and filtered through a 0.2-mm membrane filter. For determining total soluble arsenic (As(Tot)), samples were preserved in dilute HNO3 (final conc. 0.1%, pH < 2) and for As (III) determination, the samples were preserved in EDTAAcetic acid as required by inorganic arsenic speciation (Samanta and Clifford, 2005). Arsenic analyses were performed using flow injection-hydride generation-atomic absorption spectrometry (FI-HG-AAS). A flow injection analysis system coupled with an atomic absorption spectrophotometer equipped with an electrodeless discharge lamp at a wavelength of 193.7 nm was used. To determine total arsenic [As(Tot) ¼ As(III) þ As(V)], samples were treated with L-cysteine in 2 M HCl and kept for 15 min at room temperature to reduce As(V)eAs(III). To determine As(III) in the presence of As(V), the carrier HCl solution was replaced by 2 M citric acid/ citrate buffer at pH 5.0. The As(V) was then calculated from the difference of As(Tot) and As(III) (Samanta and Clifford, 2005).

2.5.2.

Iron preservation and analysis

For the determination of total iron, un-filtered samples from EC and FeCl3 coagulant stocks were dissolved in dilute HNO3 (final conc. 0.1%). The exact concentration of total iron was analyzed using flame atomic absorption spectroscopy (AAS) according to Standard Methods (Clesceri et al., 1998). The Fe2þ concentration was also analyzed by UV Spectrophotometer using the 1,10 phenanthroline method (Vogel, 1978), by taking un-filtered samples dissolved in 0.1% HCl. The insoluble iron

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concentration (Fe3þ) was then calculated from the difference between total iron and Fe2þ.

3.2. Arsenic removal during chemical coagulation in NSFI challenge water 3.2.1.

2.6.

Effect of competing ions on arsenic removal

CC and EC experiments discussed above were repeated for the following compositions of challenge water spiked with As(V) at pH 6.5, 7.5, and 8.5 to determine the effect of silica and phosphate on As(V) adsorption.  NSFI-53 without silica (i.e., Si ¼ 0 mg/L, PO4-P ¼ 40 mg/L)  NSFI-53 without phosphate (i.e., P ¼ 0 mg/L, Si ¼ 20 mg/L)  NSFI-53 without silica and phosphate (i.e., Si ¼ 0 mg/L and PO4-P ¼ 0 mg/L)

3.

Results and discussion

3.1.

Operation of electrocoagulation unit

The EC unit was first tested for its ability to generate iron in proportion to the current by operating at different current values, both in batch and continuous mode. Samples for analysis of total iron, generated by EC unit, were taken from the effluent of the EC unit at each different operating-current condition. To determine the current efficiency, the amount of iron generated was calculated using Faraday’s Law. The experimental details and results are given comprehensively elsewhere (Lakshmanan, 2007). Both in batch and continuous mode, the iron generated experimentally was consistent with Fe2þ production at the anodes. By adjusting the operating current and flow rate of source water to be treated, the desired iron concentration was obtained. The iron generation experimental data did not fit the theoretical curve perfectly and was scattered during batch mode as compared with continuousmode operation probably due to unsteady-state operation. The calculated current efficiency with respect to total iron generation was >95% during continuous mode and 80e95% during batch mode based on the slope of the linear best-fit curve with zero intercept.

Arsenic removal with ferric chloride as coagulant

The As(V) and As(III) removal efficiencies in NSFI challenge water during CC with FeCl3 as a function of coagulant dose and pH are shown in Fig. 2. The As(V) removal efficiencies were pH dependent and the removals increased with decreasing pH (Fig. 2a). As(III) removal was independent of pH in the 6.5e8.5 pH range (Fig. 2b). The As(III) removal efficiency was found to be the same as the removal efficiency of As (Total) (Fig. 2b), confirming no As(III) oxidation during FeCl3 coagulation.

3.2.2.

As(V) removal with ferrous sulfate as coagulant

To understand the importance of dosing Fe3þ in comparison with Fe2þ for arsenic removal, which proved to be critical during Fe2þ generation in EC, CC experiments were conducted with FeSO4 as coagulant in NSFI challenge water at pH 6.5, i.e., experimental conditions similar to CC with FeCl3 (rapid mix at 100 rpm for 2 min). The comparison of As(V) removal efficiencies with FeSO4 (Fe2þ) and FeCl3 (Fe3þ) showed insignificant removals with Fe2þ (<10% removal for 2.5 mg/L Fe2þ) compared to removals obtained with Fe3þ (z100% removal for 2.5 mg/L Fe3þ) at pH 6.5 (Figure S1 in Supporting material). The analysis of filtrate from CC with Fe2þ showed >95% soluble iron (Fe2þ) remaining in the treated challenge water for all doses. The results were consistent with Fe2þ oxidation studies using FAS as coagulant in challenge water which showed insignificant oxidation (<5%) at the end of 2 min of mixing at pH 6.5 (Lakshmanan et al., 2009).

3.3. Arsenic(V) removal during electrocoagulation in NSFI challenge water 3.3.1.

As(V) removal during batch mode

The As(V) removals in NSFI challenge water due to iron oxides/hydroxides generated in EC showed erratic removals at pH 6.5, while at pH 7.5 and pH 8.5 the removals were more consistent (Fig. 3a). The As(V) removal efficiencies at pH 6.5 and 7.5 were similar, and comparison of removal

Fig. 2 e As(V) (a) and As(III) (b) removal efficiency during CC in NSFI challenge water.

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Fig. 3 e (a) Removal efficiency of As(V) during EC in NSFI challenge water as a function of iron dose and pH (b) Percentage iron species after 2 min of mixing in NSFI challenge water during EC as a function of pH.

efficiencies at pH 7.5 and 8.5 showed that, with increasing pH, the As(V) removal decreased as was the case with CC. It was expected, based on literature reports (Chen et al., 2000; Mills, 2000; Balasubramanian and Madhavan, 2001; Mollah et al., 2004; Kumar et al., 2004; Parga et al., 2005; Hansen et al., 2006; Kobya et al., 2006) that insoluble iron oxides/ hydroxides (Fe(OH)2(s) and/or Fe(OH)3(s)) would be the end products after electrolysis and hydrolysis during EC. Hence, As(V) removals were expected to be similar if not better than CC. However, the results observed were in contrast to expectations particularly at pH 6.5, and to an extent, at pH 7.5. Thus, to determine if insoluble iron oxide/hydroxides were formed during EC, detailed studies on the fundamental mechanisms involved and the possible products formed during EC were carried out in a smaller, easier to manage single-cell EC unit (Lakshmanan et al., 2009). The results showed increasing soluble Fe2þ concentration with decreasing pH. The concentration of soluble ferrous iron (Fe2þ) and insoluble iron oxide/hydroxide (Fe(OH)3(s)/FeOOH(s) as Fe3þ) formed after 2 min of mixing are shown in Fig. 3b. Even though there were significant As(V) removal efficiencies, significant soluble iron concentrations were observed in the filtrate at pH 6.5 (70e85% Fe2þ) and pH 7.5 (20e30% Fe2þ). The solid ferric oxides/hydroxides formed would help in arsenic removal, while the soluble Fe2þ would pass through the filter and not help in removal. At pH 6.5, most of the iron present after 2 min was soluble Fe2þ, which would not adsorb As(V). A comparison of As(V) removals at pH 6.5 during EC and Fe2þ CC showed >50% removal during EC, while <10% was possible with Fe2þ coagulant. This higher adsorption during EC was possibly due to the iron oxide/hydroxide surfaces of the iron rods in the EC unit under unsteady-state batch operation, which was confirmed by continuous EC experiments discussed below. Another reason for the greater Fe2þ oxidation during EC (15e30%) compared to CC (<5%) was the temporary pH increase during electrolysis which hastened Fe2þ oxidation (Lakshmanan et al., 2009). At pH 7.5 about 70e80% iron was insoluble Fe(OH)3(s) capable of adsorbing As (V) after 2 min and so the overall As(V) removal was good, however the possible soluble iron in the effluent is still a cause for concern.

3.3.2. As(V) removal during continuous-flow steady-state operation conditions In actual practice, EC systems will be operated continuously, ultimately reaching steady state. However, most results presented in this paper as well as those of other researchers (Tsouris et al., 2000; Han et al., 2002; Holt et al., 2002) have been obtained by unsteady-state operation of the EC unit in batch mode. In order to verify that the production of soluble iron in the effluent during batch mode also occurs under steady-state continuous-mode conditions, and that higher As(V) removals at pH 6.5 during batch mode were due to unsteady-state operation, limited continuous-flow experiments were performed for As(V) removal at pH 6.5, 7.5, and 8.5. Theoretically, current of 28 mA, flow rate of 245 mL/min and residence time of 2 min should attain an iron concentration of 2.0 mg/L at steady state based on Fe2þ generation. Samples were taken from the overflow of the EC unit for analysis of total iron and soluble iron (Fe2þ). At the beginning, the total iron concentration built up to a steady state value after about 7e8 EC-unit volumes, and reached the theoretical capacity of 2.0 mg/L (Figure S2a in Supporting material). Based on experimental total iron generation, it appears that the current efficiency approached 100% at steady state at all pHs compared with lower efficiencies (80e95%) during batch mode. Although the total iron concentration reached the theoretical value at all pHs, the soluble iron (Fe2þ) concentration at the outlet decreased with increasing pH as expected based on faster Fe2þ oxidation with increasing pH (Figure S2a in Supporting material). This is consistent with observations made during batch-mode operation (Fig. 3b). A comparison of soluble iron (Fe2þ) and insoluble iron oxide/hydroxide at the outlet under steady state condition at pH 6.5e8.5 shows that ferric oxide/ hydroxide increases with increasing pH (Figure S2b in Supporting material), as was the case with batch-mode operation. Although 2.0 mg/L total iron was generated in the EC unit continuously at all pHs, the As(V) removals were only due to Fe(OH)3(s) produced at that pH, because the remaining soluble iron (Fe2þ) in the filtrate would not remove As(V). Subtracting the soluble iron (Fe2þ) from total iron generated, the As(V) removal percentages of 56, 72, and 56 attained at pH 6.5, 7.5, and 8.5 were due to 0.45, 1.2, and 1.96 mg/L Fe3þ (precipitated

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as Fe(OH)3(s)/FeOOH(s)), and not due to 2 mg/L total iron (Fig. 4a). Even though steady state with respect to As(V) removal was attained in <5 min at pH 7.5 and pH 8.5, it took approximately 20e30 min at pH 6.5 for attainment of steady state. In the case of batch-mode operation, As(V) removals of approximately 80e95%, 75e85%, and 60% were obtained at pH 6.5, 7.5, and 8.5 for 2.0 mg/L total iron and it was clear that the As(V) removals were greater during batch compared with continuous operation on a total iron basis, particularly at pH 6.5. So it appeared that the higher removals during batch mode at pH 6.5 were due to the non-attainment of steady state and the As(V) adsorption by the oxyhydroxides on the iron surfaces prior to attainment of steady state. The comparison of As(V) removal during EC-continuous mode operation (considering only the Fe3þ generated) and batch CC with FeCl3 based on Fe3þ dose showed similar removals (Fig. 4b), which confirms that the As(V) removals at steady state during EC were nearly the same as FeCl3 CC. However, for attainment of similar removal efficiencies to chemical coagulation, higher amount of total iron has to be generated to produce the necessary solid ferric oxide/ hydroxides needed for As(V) removal.

3.4. Arsenic(III) removal during electrocoagulation in NSFI challenge water The As(III) removal efficiencies in NSFI challenge water during EC showed a slight dependency on pH with removal efficiency increasing slightly with increasing pH (Fig. 5a). EC was expected to simultaneously oxidize and remove As(III) (Arienzo et al., 2002; Kumar et al., 2004). However based on experimental results, it was found that there was no significant oxidation during EC (Fig. 5b). The percent of speciated As (III) and As(Total) removals were similar except at high pH where there was a small difference, probably due to As(III) oxidation and temporary pH increase during electrolysis. However the difference was not significant enough to draw a conclusion. The equal As(III) and As(V) removals during EC reported were purported to be due to As(III) oxidation by Cl2 produced or by redox reaction at the anode (Kim et al., 2002).

However, it was found Cl2 was not produced at the anode, in fact there was a significant chlorine demand during EC with iron anodes. Studies also showed insignificant As(III) oxidation due to redox reaction even at inert anodes. The attempted As(III) oxidation studies are discussed in detail later.

3.5. Comparison of arsenic adsorption during chemicaland electro-coagulation Graphs showing percent arsenic removal as a function of coagulant dose and pH are of practical importance, but the equilibrium adsorption isotherm provides a more fundamental comparison. Although the Freundlich isotherm equation (qe ¼ KC1/n e ) has a theoretical basis, it is used here only as an empirical power-curve fitting equation. The adsorption isotherms of As(V) and As(III) on the in-situ e formed hydroxides of Fe during CC and EC at pH 6.5, 7.5, and 8.5 are compared in Fig. 6aeb. The most favorable As(V) adsorption isotherm in the case of CC was observed at pH 6.5 compared with pH 7.5, which was favorable compared with pH 8.5 (Fig. 6a). In the case of EC, the most favorable As(V) adsorption isotherm was observed at pH 7.5 (Fig. 6a), while at pH 6.5, the adsorption isotherm was erratic and less reproducible. The As (III) adsorption isotherms on the in-situ formed iron hydroxides were pH independent during CC, while the adsorption isotherms were more favorable but less reproducible during EC (Fig. 6b). As discussed previously, the scatter in the EC data was due to production of Fe2þ in the EC unit and its variable oxidation depending on pH, mixing time and DO in the challenge water. The best-fit isotherm equations were used to calculate the arsenic adsorption capacities of the iron hydroxides for an equilibrium concentration of 10 mg/L As(V) or As(III). The comparison of EC and CC showed similar As(V) adsorption capacities at pH 7.5 and 8.5. However at pH 6.5, CC exhibited more than twice the As(V) adsorption compared with EC which showed lower As(V) adsorption at pH 6.5 than at pH 7.5 (Fig. 7a). The comparison of As(III) adsorption capacities during CC and EC showed a higher adsorption capacity for As (III) during EC (Fig. 7b). The increase was not significant enough

Fig. 4 e (a) As(V) removal as a function of Fe(OH)3(s) produced during continuous-flow operation in EC unit with NSFI water at pH 6.5, 7.5, and 8.5 (Current [ 28 mA; Flow rate [ 245 ± 10 mL/min; Residence time [ 2 min) (b) Comparison of As(V) removal by EC-continuous mode due to Fe3D produced with CC for similar dose.

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Fig. 5 e As(III) removal (a) removal/conversion (b) efficiency during EC in NSFI challenge water as a function of iron dose and pH.

to avoid pre-oxidation of As(III), however the higher As(III) oxidation at high pH during EC is due to the temporary pH increase occurring during EC. A comparison of As(III) and As(V) adsorption capacities (Fig. 7a and b) shows 3e20 times higher As(V) adsorption for both EC and CC, which (a) confirms no significant oxidation during both EC and CC, and (b) reinforces the importance of pre-oxidation of As(III) for efficient removal.

3.6. Effect of silica and phosphate on As(V) adsorption in NSFI challenge water CC and EC experiments were performed using NSFI challenge water with and without competing ions (silica and phosphate) to study their effect on As(V) adsorption. The arsenic removal percentages as a function of coagulant dose and pH were transformed to adsorption isotherms in order to provide meaningful data on the effect of competing ions. As was observed with NSFI challenge water during EC, arsenic removals were erratic at pH 6.5 (Figure S3b, 4b, 5b, 6b in Supporting material) and significant soluble iron was observed in the filtrate at pH 6.5 and 7.5 with all variations of NSFI challenge water tested. When coagulating with FeCl3 in the presence and absence of silica in challenge water, it was found that silica does have

a significant effect on the As(V) adsorption during CC, and the effect increased with increasing pH (Figure S3a in Supporting material). Similar observation was made during EC where silica had a significant effect on the adsorption of As(V) and the effect increased with increasing pH as was the case with CC (Figure S3b in Supporting material). Phosphate in NSFI water containing silica had a significant effect at pH 6.5, however the effect was less observable with increase in pH during CC with FeCl3. This was observable through the more favorable adsorption isotherms in the absence of phosphate at pH 6.5 as compared with the influence of phosphate on the adsorption isotherms at pH 7.5 and 8.5 (Figure S4a in Supporting material). Similar to CC, EC experiments also showed a slight effect on the adsorption of As(V) at pH 7.5 and no significant effect at pH 8.5, which was observed through the more favorable adsorption isotherms in the absence of phosphate at pH 7.5 as compared to pH 8.5 (Figure S4b in Supporting material). The adsorption isotherms were used to calculate the effect of silica and phosphate in NSFI challenge water on As(V) adsorption capacities of iron hydroxides formed for an equilibrium concentration of 10 mg/L As(V) in the pH range 6.5e8.5 (Fig. 8a and b for CC and EC). As can be observed from the figures, silica showed a significant effect on As(V) adsorption

Fig. 6 e Comparison of As(V) (a) and As(III) (b) adsorption isotherms onto in-situ formed iron oxide/hydroxide during CC and EC at pH 6.5, 7.5 and 8.5.

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Fig. 7 e Comparison of As(V) (a) and As(III) (b) adsorption capacities for an equilibrium concentration of 10 mg/L as a function of pH during CC and EC.

in NSFI challenge water, and the effect increased with increasing pH during both CC and EC. Phosphate showed a significant effect on As(V) adsorption in NSFI challenge water at pH 6.5 during CC, but had a lesser effect at pH 7.5 and 8.5 during CC and EC.

3.7. Individual effect of silica and phosphate on As(V) adsorption The above reported effects of silica and phosphate were determined in the presence of other competing ions, e.g., the effect of silica was studied using challenge water containing phosphate. The individual effects of competing ions were evaluated by studying the effect of each competing ion under study (silicate or phosphate) in the absence of the other competing ion, e.g., effect of silica studied using challenge water without phosphate. Based on CC experiments in the presence and absence of silica in NSFI challenge water without phosphate, it was found that silica does have a significant impact on As(V) adsorption and the effect increased with increasing pH. The effect of silica was significant at pHs 7.5 and 8.5, and it had a minor

effect at pH 6.5 (Figure S5a in Supporting material). Similar to CC, EC experiments also showed a significant effect of silica on As(V) adsorption and the effect increased with increasing pH. The decrease in removal efficiency was slightly more significant in the presence of silica at pH 8.5 compared with pH 7.5, which was observable through the more favorable adsorption isotherms in the absence of silica at pHs 7.5 and 8.5 (Figure S5b in Supporting material). In the absence of silica in NSFI challenge water, phosphate affected As(V) adsorption during CC with FeCl3 and the effect decreased with increasing pH (Figure S6a in Supporting material). Similar to CC, EC experiments showed a significant effect of phosphate on As(V) adsorption at pH 7.5 compared with the effect at pH 8.5 (Figure S6b in Supporting material). To summarize, the adsorption isotherms were used to calculate the individual effect of silica and phosphate in NSFI challenge water on As(V) adsorption capacities of iron hydroxides formed for an equilibrium concentration of 10 mg/L As(V) in the pH range 6.5e8.5 (Fig. 9a and b for CC and EC). In the absence of phosphate, silica significantly reduced the adsorption capacity of As(V) and the effect increased significantly with increasing pH. Phosphate also significantly

Fig. 8 e Effect of silica and phosphate in NSFI challenge water on As(V) adsorption capacities of in-situ formed iron oxides/ hydroxides formed during CC (a) and EC (b) for an equilibrium concentration of 10 mg/L As(V) in the 6.5e8.5 pH range. *Adsorption capacities during EC were not calculated at pH 6.5 because of likely As(V) adsorption by iron oxide/hydroxide on iron rod surfaces.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 6 4 1 e5 6 5 2

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Fig. 9 e Individual effects of silica and phosphate on As(V) adsorption capacities of in-situ formed iron oxides/hydroxides during CC (a) and EC (b) for an equilibrium concentration of 10 mg/L As(V) in the 6.5e8.5 pH range. *Adsorption capacities during EC were not calculated at pH 6.5 because of likely As(V) adsorption by iron oxide/hydroxide on iron rod surfaces.

lowered As(V) adsorption at all pHs in the absence of silica and the effect increased with decreasing pH. In summary, a comparison of individual and combined effect studies shows that silica significantly affected As(V) adsorption regardless of the presence or absence of phosphate during both EC and CC and the effect increased with increasing pH. Silica, particularly in its monovalent state (H3SiO 4 ), competes with arsenate for adsorption sites. The effect of silica is greater at pH 8.5 than at pH 6.5 due to the increase in the concentration of negatively charged H3SiO 4 with increasing pH (Table S1 in Supporting material). In spite of the fact that the neutral H4SiO4 species dominates at pH 6.5e8.5, the concentration of silica is so high (333 mM) that the negatively charged silicate ion concentration (H3SiO 4 ) is very high in comparison with the arsenic species at pHs of 7.5 and 8.5 and ionized silicate is nearly equal to arsenate at pH 6.5 (Table S1 in Supporting material). Phosphoric acid has similar pKa values to those of arsenic acid, and phosphate exists as 2 monovalent (H2PO 4 ) and divalent (HPO4 ) at pH 6.5e8.5, which 2 are similar in chemical behavior to H2AsO 4 and HAsO4 . Both  2 H2PO4 and HPO4 can have ligand-exchange reactions with the ferric oxide hydroxides formed and compete for adsorption sites with the arsenates due to concentration of phosphorous (1.29 mM) being higher than that of arsenic (0.67 mM) (Table S1 in Supporting material). However, the presence of silica lowered the effect of phosphate at pH 7.5 and 8.5 due to its own significant effect in that pH range.

3.8. Reason for erratic behavior during electrocoagulation As was observed with NSFI challenge water, As(V) removals were erratic at pH 6.5 for all variations of NSFI challenge water. To better understand this behavior, the total iron concentrations in the filtrate during EC experiments were analyzed. CC experiments were also conducted with FeSO4 as coagulant for all variations of challenge water mentioned above. The iron in the filtrate after 2 min of mixing at pH 6.5e8.5 during EC showed that with decrease in pH, the soluble iron

(Fe2þ) concentration increased (Figure S7a in Supporting material). At pH 6.5, most of the iron generated passed though the filter as soluble Fe2þ (z80%), which would not adsorb As(V). CC experiments with Fe2þ showed insignificant removals compared with Fe3þ as coagulant for all variations of challenge water tested (Figure S7b in Supporting material). These results showing little if any As(V) adsorption during CC with Fe2þ confirmed that the higher-than-expected scattered As(V) adsorption during EC at pH 6.5 probably resulted from As (V) adsorption onto iron oxide/hydroxide coated anodes in the EC unit, as also observed with NSFI challenge water (Fig. 3b). At pH 7.5 (Figure S7a in Supporting material) iron present in the EC unit after 2 min was mostly Fe(OH)3(s) capable of adsorbing As(V). Thus, the overall As(V) removal was good due to formation of insoluble iron oxides/hydroxides. Nevertheless, the soluble Fe2þ (10e40%) in the effluent is a cause for concern, and optimizing the EC system would be necessary.

3.9.

Studies on As(III) oxidation

3.9.1.

Chlorine generation and utilization in Fe-EC unit

Contrary to literature on As(III) oxidation (Arienzo et al., 2002; Kumar et al., 2004) no As(III) oxidation was observed during EC. The literature on electrochemical oxidation also indicates possible Cl2 generation (Kim et al., 2002; Chen et al., 2000; Han et al., 2002), which has the potential to oxidize As(III). Hence, experiments were conducted to verify Cl2 production at the anode. The experimental procedure was the same as that of As(III) removal except that As(III) was not spiked in the challenge water and the EC unit operated in the range of 50e800 mA. The samples collected after 2 min of mixing were analyzed for free Cl2 (DPD Photometric method). The experimental results showed no measurable Cl2 generation in the EC outlet which was not surprising in light of Fe2þ generation in the EC unit and presence in the EC outlet. Experiments were also conducted to study likely Cl2 consumption due to Fe2þ production at the iron anode in the EC unit. Chlorinated water (1.6 mg/L Cl2) prepared by adding sodium hypochlorite solution (10e13% available Cl2 solution) to deionized water was passed through the EC unit for a total

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time of 2 min, and total iron in the range of 0e3 mg/L was generated during several batch experiments. The samples collected and analyzed for Cl2 and iron showed a decrease in Cl2 with increasing iron concentration (Figure S8a in Supporting material). There was a significant decrease (w65%) in Cl2 concentration even when iron was not generated in the EC unit, due to Cl2 consumption by the partially oxidized iron rods. With increasing iron generation, the Cl2 content in the water decreased due to Cl2 oxidation of Fe2þ produced at the anode to Fe3þ. Thus, it appears that significant Cl2 was not being produced, and even if it was, it was used up due to Cl2 demand of the Fe0 and Fe2þ in the EC unit.

3.9.2.

As(III) oxidation using graphite rod as anode

Due to insignificant As(III) oxidation and no noticeable Cl2 generation using iron anodes, the possibility of Cl2 generation and As(III) oxidation using an inert graphite anode was studied. The experimental procedure was the same using NSFI challenge water containing 71 mg/L Cl without As(III) except that a single graphite-rod anode was used in place of the three ironrod anodes (25e150 mA with electrolysis time of 60 s). The experimental results showed Cl2 production with a graphite anode, however the Cl2 produced was only 2e5% of theoretical amount based on Faraday’s law. Possible reasons for the low Cl2 residual were the low Cl ions and possible side reactions that produce oxygen and other water electrolysis products. Since Cl2 production was possible with graphite rods, experiments were conducted with challenge water spiked with 50 mg/L As(III) (25e150 mA with electrolysis time of 60 s). Although the current efficiency was quite low, based on the determination of As(III) by speciation, it was found that As(III) oxidation increased with increasing current both at pH 7.5 and pH 8.5 (Figure S8b in Supporting material) and complete oxidation was possible. To determine the mechanism of As(III) oxidation (As(III) redox reaction at the anode or possible oxidation by Cl2 generated at anode), experiments were carried out in the absence of chloride ions in NSFI challenge water. The results showed significantly less As(III) oxidation (<20%) at even higher currents in the absence of chloride ions as compared with oxidation in the presence of chloride ions in NSFI challenge water at both pH 7.5 and 8.5 (Figure S8b in Supporting material), which indicated that As(III) oxidation was likely due to Cl2 produced from the chloride ions at the anode. Experiments conducted with NSFI-53 challenge water spiked with As(III) at two different initial arsenic concentrations (50 and 100 mg/L) showed faster As(III) oxidation at low As (III) concentrations due to the higher ratio of Cl2 to As(III) (Lakshmanan, 2007). These differences in rates for low As(III) concentrations are understandable because of the extremely poor efficiency of Cl2 generation (2e5% based on the residuals observed), which makes the Cl2 demand of As(III) significant in comparison with the actual amount of Cl2 generated.

3.9.3. As(III) oxidation and removal using combination of graphite and iron rods Based on the results obtained with graphite rods on As(III) oxidation, experiments were conducted to study simultaneous oxidation and removal of As(III) using various combinations of graphite and iron rods in the same EC-unit housing with NSFI challenge water at pH 7.5. Even though the removal efficiencies

seemed to increase with the use of graphite rods due to the possible As(III) oxidation with generated Cl2, the increase in As (III) removal was not significant (Figure S9a in Supporting material). The As(III) removal efficiency was found to be approximately the same as that of the removal efficiency of As (Tot), thus there was no significant As(III) oxidation in an EC housing containing both graphite and iron rods (Figure S9b in Supporting material). No Cl2 was detected in the outlet, which suggested that the Cl2 produced was immediately consumed by the iron rods and Fe2þ generated. Thus, in order for As(III) to be oxidized by electrically generated Cl2, it has to be done in a separate housing ahead of the iron rods. Based on the failure of Cl2 generation in a single housing, a two-stage process of As(III) oxidation using graphite rod followed by removal of As(V) (oxidized As(III)) using iron rods was designed and tested using NSFI challenge water spiked with 50 mg/L As(III) at pH 7.5 and 8.5. As expected, the test results showed complete oxidation of As(III) in the first stage where Cl2 was generated at a graphite anode and in the second stage the As(V) was removed by the iron generated. The As(III) adsorption capacities using the two-stage electro-oxidationecoagulation process were similar to the As(V) adsorption capacities during EC at pH 7.5 and 8.5. This improved performance for As(III) removal was a result of oxidation in the first stage and subsequent removal in the second stage.

4.

Conclusions

The studies on As(III) oxidation during electrocoagulation and the comparison of arsenic removal during electrocoagulation and chemical coagulation led to the following conclusions:  As(V) removal during EC was erratic at pH 6.5 with all variations of challenge water tested.  The erratic As(V) removal behavior at pH 6.5 was due to (a) > 70% un-oxidized soluble iron (Fe2þ) in the treated water (b) low percent (15e30%) of insoluble iron oxide/hydroxide, and (c) variable amount of As(V) adsorption onto the iron oxide/ hydroxide surfaces of iron rods.  As(V) removal was efficient at pH 7.5 and 8.5 during EC with all variations of challenge water tested, however, a significant and highly variable amount of soluble Fe2þ (10e45%) in the treated water at pH 7.5 is a cause for concern.  The EC-continuous-mode experiments also produced soluble Fe2þ in the treated water as observed in batch mode and indicated that somewhat greater As(V) removal during batch mode was due to arsenic adsorption onto the iron rods before steady state was attained.  As(V) removal was highly pH dependent during both CC and EC, and the efficiency increased with decreasing pH. As(III) removal was slightly pH dependent during EC, compared with CC, which was pH independent.  The best As(V) adsorption was observed with CC at pH 6.5, while EC at pH 6.5 was inconsistent and less efficient. The As (V) adsorption capacities were similar at pH 7.5 and 8.5 during CC and EC.  As(III) adsorption was slightly greater during EC compared with CC. However, the As(III) adsorption capacities were only 5e30% of those attained for As(V) during both CC and EC.

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 6 4 1 e5 6 5 2

 Contrary to literature, there was no significant oxidation of As(III) and subsequent removal of As(V) during EC.  As(V) removal using EC is not advisable at pH 6.5 unless there is a very long oxidation time (5e24 h) for complete oxidation (80e100%) of Fe2þ produced. As(V) removal by EC at pH 7.5 should be exercised with caution due to potential presence of high concentrations of soluble Fe(II) in the effluent.  As expected, the competing ions silica and phosphate did affect the adsorption of arsenic. Silica interference increased with increasing pH whereas phosphate interference decreased with increasing pH. However the significance of the effect depended on the presence of the other competing ion. In particular, the presence of silica lowered the effect of phosphate with increasing pH due to its own significant effect at high pHs.  With iron anodes, Cl2 was not generated in the EC unit, and, in fact, the reduced oxides/hydroxides on the iron anodes and Fe2þ produced during electrolysis produced a very significant source of Cl2 demand.  With a graphite-rod anode, Cl2 generation was possible when typical concentration of chloride ions (71 mg/L) was present in the solution, and complete oxidation of As(III) was possible. Unfortunately, the efficiency of Cl2 production was only 2e5%.  A combination of graphite and iron anodes in the same ECunit housing had no significant advantage for As(III) removal compared with the iron rods alone. However, a two-stage EC process with graphite anode in stage 1 and iron anodes in stage 2 was effective for As(III) oxidation and removal without any need for chemicals.

Acknowledgements The authors thank the AWWA Research Foundation for financial support and are grateful to Project Officer Hsiao Wen Chen for technical and administrative assistance.

Appendix. Supplementary material Supplementary material associated with this article can be found in the on-line version, at 10.1016/j.watres.2010.06.018.

references

Arienzo, M., Adamo, P., Chiaenzelli, J., Bianco, M.R., DeMartino, A., 2002. Retention of arsenic on hydrous ferric oxides generated by electrochemical peroxidation. Chemosphere 48 (10), 1009e1018. Balasubramanian, N., Madhavan, K., 2001. Arsenic removal from industrial effluent through electrocoagulation. Chem. Eng. Technol. 24 (5), 519e521. Chen, X.M., Chen, G., Yue, P.L., 2000. Separation of pollutants from restaurant wastewater by electrocoagulation. Separ. Purif. Technol. 19 (1), 65e76.

5651

Cheng, R.C., Liang, S., Wang, H.C., Beuhler, M.D., 1994. Enhanced coagulation for arsenic removal. J. AWWA 86 (9), 79e90. Clesceri, L.S., Greenberg, A.E., Eaton, A.D., 1998. Standard Methods for the Examination of Water and Wastewater, twentieth ed. American Public Health Association/American Water Works Association/Water Environment Federation, Washington, DC. Clifford, D.A., 1990. In: Pontius, F.W. (Ed.), Water Quality and Treatment: a Handbook of Community Water Supplies, fourth ed. McGraw-Hill, Inc., New York. Clifford, D.A., 1997. Final Report: Phases 1, 2, and 3 City of Albuquerque Arsenic Study. Dept. Of Civil and Environmental Engg., University of Houston, Texas. Ghurye, G., Clifford, D.A., Tripp, A.R., 2004. Pilot study of coagulation microfiltration for arsenic removal from groundwater. J. AWWA 96 (4), 143e152. Han, M., Song, J., Kwon, A., 2002. Preliminary investigation of electrocoagulation as a substitute for chemical coagulation. Water Sci. Technol. 2 (5), 73e76. Hansen, H.K., Nu´n˜ez, P., Grandon, R., 2006. Electrocoagulation as a remediation tool for wastewaters containing arsenic. Miner. Eng. 19 (5), 521e524. Hering, J.G., Chen, P.-Y., Wilkie, J.A., Elimelech, M., Liang, S., 1996. Arsenic removal by ferric chloride. J. AWWA 88 (4), 155e167. Holt, P.K., Barton, G.W., Wark, M., Mitchell, C.A., 2002. A quantitative comparison between chemical dosing and electrocoagulation. Colloid. Surface Physicochem. Eng. Aspect. 211 (2), 233e248. Horner, G., Duffey, J.G., 1983. Electrochemical Removal of Heavy Metals from Wastewater. The American Electroplaters Society Annual Meeting, Denver, CO. Jiang, J.Q., Graham, N., Andre, C., Geoff, H., Brandon, N., 2002. Laboratory study of electrocoagulationeflotation for water treatment. Water Res. 36 (16), 4064e4078. Kim, J., Velichenko, A.B., Korshin, G.V., 2002. Effect of selected water quality parameters on electrochemical oxidation of arsenite in drinking water. In: Proc. Annu. AWWA Conf. Kobya, M., Senturk, E., Bayramoglu, M., 2006. Treatment of poultry slaughterhouse wastewaters by electrocoagulation. J. Hazard. Mater. 133 (1), 172e176. Kumar, P.R., Chaudhari, S., Khilar, K., Mahajan, S.P., 2004. Removal of arsenic from water by electrocoagulation. Chemosphere 55 (9), 1245e1252. Lakshmanan, D., 2007. A Systematic Study of Arsenic Removal from Drinking Water Using CoagulationeFiltration and ElectrocoagulationeFiltration. Ph.D. Dissertation, University of Houston, Houston, TX. Lakshmanan, D., Clifford, D.A., Samanta, G., 2009. Ferrous and ferric ion generation during iron electrocoagulation. Environ. Sci. Technol. 43 (10), 3853e3859. McNeill, L.S., Edwards, M., 1995. Soluble arsenic removal at water treatment plants. J. AWWA 87 (4), 105e113. McNeill, L.S., Edwards, M., 1997. Predicting arsenic removal during metal hydroxide precipitation. J. AWWA 89 (1), 75e86. Mills, D., 2000. A new process for electrocoagulation. J. AWWA 92 (6), 34e43. Mollah, M.Y., Morkovsky, P.G., Gomes, A.G., Kesmez, M., Parga, J., 2004. Fundamentals, present and future perspectives of electrocoagulation. J. Hazard. Mater. 114 (1e3), 199e210. Nikolaev, N.V., Kozlovski, A.S., Utkin, I.I., 1982. Treating natural waters in small water systems by filtration with electrocoagulation. Soviet J. Water Chem. Technol. 4 (3), 244e247. Novikova, S.P., Shkorbatova, T.L., 1982. Purification of effluents from the production of synthetic detergents by electrocoagulation. Soviet J. Water Chem. Technol. 4 (4), 353e357. NSF/ANSI, 2007. Standard No. 53, Drinking Water Treatment Units-Health Effects. NSF International, Ann Arbor, Michigan.

5652

w a t e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 6 4 1 e5 6 5 2

Osipenko, V.D., Pogorelyi, P.I., 1977. Electrocoagulation neutralization of chromium containing effluent. Metallurgist 21 (9e10), 44e45. Parga, R., Cocke, D.L., Valverde, V., 2005. Characterization of electrocoagulation for removal of chromium and arsenic. Chem. Eng. Technol. 28 (5), 605e612. Pouet, M.T., Grasmick, A., 1995. Urban wastewater treatment by electrocoagulation and flotation. Water Sci. Technol. 31 (3), 275e283. Samanta, G., Clifford, D.A., 2005. Preservation of inorganic arsenic species in groundwater. Environ. Sci. Technol. 39 (22), 8877e8882. Scott, K.N., Green, J.F., Do, H.D., Mclean, S.J., 1995. Arsenic removal by coagulation. J. AWWA 87 (4), 114e126. Tong, J., 1997. Development of an Iron(III)CoagulationeMicrofiltration Process for Arsenic Removal from Groundwater. Masters thesis, Dept. Of Civil and Environmental Engineering, University of Houston, Houston. Tsouris, C., DePaoli, D.W., Shor, J.T., Hu, M., Ying, T., 2000. Electrocoagulation for magnetic seeding of colloidal

particles. Colloid. Surface Physicochem. Eng. Aspect. 177 (2), 223e233. Tripp, A.R., 2001. Selectivity Considerations in Modeling the Treatment of Perchlorate Using Ion Exchange Processes. Ph.D. diss., Dept. of Civil and Environmental Engineering, University of Houston, Houston. Tyrina, L.M., Morozov, A.F., 1982. Electrochemical treatment of industrial wastewaters containing copper cyanide and thiocyanate complexes. Soviet J. Water Chem. Technol. Vol.4 (5), 462e465. Vik, E.A., Carlson, D.A., Eikum, A.S., Gjessing, E.T., 1984. Electrocoagulation of potable water. Water Res. 18 (11), 1355e1360. Vogel, A.I., 1978. Vogel’s Textbook of Quantitative Inorganic Analysis, Including Elementary Instrumental Analysis, fourth ed. Longman, New York. Zhu, B., Clifford, D.A., Chellam, S., 2005. Comparison of electrocoagulation and chemical coagulation pretreatment for enhanced virus removal using microfiltration membranes. Water Res. 39 (13), 3098e3108.