Arsenic removal from drinking water by electrocoagulation using iron electrodes- an understanding of the process parameters

Arsenic removal from drinking water by electrocoagulation using iron electrodes- an understanding of the process parameters

Journal of Environmental Chemical Engineering 4 (2016) 3990–4000 Contents lists available at ScienceDirect Journal of Environmental Chemical Enginee...

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Journal of Environmental Chemical Engineering 4 (2016) 3990–4000

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Arsenic removal from drinking water by electrocoagulation using iron electrodes- an understanding of the process parameters Tuhin Banerjia,b,* , Sanjeev Chaudharia a b

Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India National Environmental Engineering Research Institute, Mumbai Zonal Centre, 89/B, Dr. A.B. Road, Worli, Mumbai 400018, India

A R T I C L E I N F O

Article history: Received 17 June 2016 Received in revised form 2 August 2016 Accepted 2 September 2016 Available online 3 September 2016 Keywords: Arsenic removal Electrocoagulation Iron electrodes Oxidation Interfacial reactions

A B S T R A C T

Various methods exist for arsenic removal from water, but most are not viable as they require addition of oxidants and use adsorbents which have limited adsorption capacities. Electrocoagulation using iron electrodes (ECFe) is a promising technology for arsenic removal. Efficiency of arsenic removal by ECFe may be affected by parameters such as pH, current intensity, initial arsenic concentration and cooccurring ions like phosphate, silicate, natural organic matter (NOM), bicarbonate, sulphate, nitrate and chloride but the causal action is not well understood. Thus experiments were designed and carried out to observe these effects and get a better understanding of ECFe. The results indicate that the oxidation of Fe (II) generated during ECFe is essential for high efficiency (with respect to iron dose) of arsenic removal. Lower current intensities and pH7 were found to be most efficient for arsenic removal per unit weight of iron dissolved. Observations indicate that arsenic reacts with ferric (hydr)oxides and phosphate complexes with Fe(II). With time these complexes and precipitates are reordered to give a much higher affinity of iron (hydr)oxides for phosphate. Silicate, upto 20 mg/L, had negligible effect but at 30 mg/L was seen to reduce arsenic removal by ECFe. Increasing NOM concentrations also reduced arsenic removal but bicarbonate, sulphate, nitrate and chloride had no effect on arsenic removal by ECFe. Presence of old ferric (hydr)oxides in the reactor led to lesser requirement of Fe(II) to achieve less than 10 ppb concentration as it promoted interfacial reactions. Formation of FeAsO4 was observed by FTIR analysis of the precipitates. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Arsenic is a known carcinogen and arsenic contamination of drinking water is affecting people in many parts of the world. The regulatory drinking water Indian standard and World Health Organisation (WHO) guideline value is 10 ppb. Many states in India and almost whole of Bangladesh are affected by arsenic in groundwater. It has also been predicted that the spread of arsenic might be much higher but due to lack of analysis as all the areas haven’t been tested [1]. The installation of tube wells in an effort to provide drinking water free of microbial pathogens has resulted in widespread arsenic poisoning of people living in India (West Bengal) and Bangladesh [2,3]. Provision of Arsenic safe water source is one of the best solutions to mitigate arsenic related health problems. However, such water sources in affected

* Corresponding author at: National Environmental Engineering Research Institute, Mumbai Zonal Centre, 89/B, Dr. A.B. Road, Worli, Mumbai 400018, India. E-mail address: [email protected] (T. Banerji). http://dx.doi.org/10.1016/j.jece.2016.09.007 2213-3437/ã 2016 Elsevier Ltd. All rights reserved.

areas are scarce. Various treatment technologies have been developed for arsenic removal from drinking water. Arsenite can make upto more than 90% of the total Arsenic content in groundwater in certain areas [4]. Thus as ground water usually contains an appreciable proportion of As(III), an efficient arsenic removal by most of the technologies necessitates conversion of As (III) to As(V) by chemical oxidation [5,6]. Electrocoagulation using iron electrodes (ECFe) is a relatively new technology which offers a lot of promise for arsenic removal. Electrocoagulation is an alternative to using chemical coagulants or conventional adsorbents for arsenic removal, as it requires no additional chemicals and hence the continuous dependence of users on shops/sellers/ suppliers is removed. Arsenic removal by ECFe under various conditions of current intensity, pH, electrode connectivity, aeration [dissolved oxygen (DO)], temperature, sedimentation [or reaction time before separation of hydrous ferric oxide (HFO) flocs from aqueous phase] and initial arsenic concentration [7,8,9] has been reported. But there exists little coherence amongst the various designs developed [10–13]. Thus a better understanding of ECFe process for

T. Banerji, S. Chaudhari / Journal of Environmental Chemical Engineering 4 (2016) 3990–4000

efficient arsenic removal is required. Arsenic contaminated groundwater typically contains either, As(III) or As(V). In the pH range of groundwater (pH 6–8), the predominant As(V) species present are H2AsO4 and HAsO42 and for As(III) is H3AsO30. When electric current is passed through Fe electrodes (ECFe), Fe(II) is dissolved, which in the presence of DO, gets oxidized, and if As(V) is present in the system, Fe(III)-As precipitates are formed. Thus arsenic gets removed by separation of the precipitates from the aqueous phase. Fe(III)-As precipitates formed are expected to be only or combination of bonds similar to FeAsO4, monodentate and bidentate complexes with Fe(III) and uncomplexed hydrous Fe(III) oxides. Much efficient use of iron was observed due to its coprecipitation in presence of arsenic [14–16]. It is also known that Fe(II), during oxidation, forms highly reactive oxidizing species [Fe(IV)] which can oxidize As(III) to As (IV) and if sufficient Fe(IV) is available then it may further oxidize As(IV) to As(V) [17,18]. This As(V) complexes with Fe(III) and precipitates from the system. Thus factors affecting oxidation of Fe(II) and effective Fe(IV) utilization, such as pH, and DO would affect arsenic removal. Similarly, co-occurring ions (eg. PO43, SiO32) which affect oxidation of Fe(II) and scavenge Fe(IV) are likely to affect arsenic removal. Tokoro et al. [19] performed coprecipitation and adsorption (preformed HFO as adsorbent) experiment under identical aqueous phase conditions and when 15 mg/L Fe(III) was co-precipitated in the presence of As(V), arsenic concentration in treated water reduced from 0.15 mM (11.25 mg/L) to 0.003 mM (0.225 mg/L), whereas with the same Fe dose when As(V) was adsorbed on pre-formed HFO, arsenic concentration reduced only to 0.126 mM (9.45 mg/L), indicating much higher arsenic removal per unit weight of iron in coprecipitation. In ECFe, as Fe(II) oxidizes and forms Fe(III) in the presence of arsenic, the phenomenon can be considered similar to co-precipitation. Fe(II) oxidation is characterised by homogeneous (bulk aqueous) and heterogeneous (near or on the Fe(III)As precipitate surface) phase by DO [20]. Additionally, in ECFe, the Fe(III)-As precipitates formed, can adsorb As(III), As(V), Fe(II) and DO, which would lead to interfacial reactions. Thus the presence of Fe(III)-As precipitate is also likely to affect oxidation of Fe(II) and As(III). As oxidation of Fe(II) and As(III) is dependent on pH, thus system pH would also be an important variable which has not yet been studied in detail. It is also known that phosphate and silicate reduce arsenic removal but their exact mode of action is not known. Various authors have reported varying capacities for arsenic removal in ECFe [7–9,21,22] as there is a general lack of understanding of the ECFe system. Thereby experiments were performed to understand the various factors affecting arsenic removal by ECFe and the results are presented in this paper. 2. Materials and methods The chemicals were analytical reagent grade and were used without any further purification. All glassware were cleaned with distilled water and 1 N H2SO4 and then rinsed with distilled water. Stock solutions of arsenite were prepared by dissolving appropriate quantity of arsenic trioxide, As2O3, (Merck, India) in distilled water containing 1% (w/w) NaOH and the solution was then diluting up to 1L with distilled water before use. The arsenate stock solution was prepared from the sodium arsenate, Na2HAsO47H2O (Merck, India). The intermediate and secondary standards of arsenic solutions were prepared freshly for each experiment. The working solutions containing arsenic were prepared by dissolving appropriate amount of arsenic from stock solutions in tap water. Tap water was tested for the pH, alkalinity, and the presence of arsenic, iron and phosphate. It was found that the pH of the water varied from 7.2 to 7.5, bicarbonate alkalinity was approximately

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45–50 mg/L as CaCO3, the dissolved iron, phosphate and arsenic concentration were not detectable in tap water. 2.1. Electrocoagulation experiment setup The ECFe reactor consisted of a 2 L glass beaker with two iron plates immersed in spiked tap water. The plates were 15 cm by 10 cm in size with a thickness of 3 mm. They were placed 1 cm apart in the arsenic containing tap water. The total submerged surface area of each electrode was 110 cm2. Before each experiment, the electrodes were abraded with sand paper to remove scales and then cleaned with successive rinses of water and 0.1 N H2SO4. Constant direct electric current was applied to iron plates by a DC source. To provide enough oxygen for formation of Fe(III) precipitates, the solution was sparged with air throughout experiments. DO in the ECFe reactor had been maintained between 4 and 6 mg/L for all the experiments using an air pump with 8 L/ min air flow rate in addition to magnetic stirring. The pH was maintained at 7(0.1), unless otherwise mentioned, with the addition of dilute HCl or NaOH during ECFe. Faraday’s law, given in Eq. (1), is used to calculate the amount of iron which goes into the solution (g Fe). W¼

itM ZF

ð1Þ

where w = metal dissolving (g Fe), i = current (A), t = time (sec), M = molecular weight of Fe, Z = number of electrons involved in the oxidation/reduction reaction (Z = 2), F = Faraday’s constant, 96485 (C mol1) 2.2. Arsenic, phosphate and iron analysis Molybdenum blue method was adopted here for routine analysis of As(III) and As(V) and phosphate, which is spectroscopic measurement of arsenic-phosphate-molybdenum complexes, developed by [23]. Random checks were also carried out by FIAS-AAS (Flow Injection Analysis System for Atomic Absorption Spectrophotometer) (Perkin Elmer model 400 AA with FIAS 100) and Wagtech Arsenator field test kits, to verify the results obtained by spectroscopic method. The minimum detection limit of FIASAAS is of 1 ppb. Dissolved iron [Fe(II)] and total iron concentrations were determined using the phenanthroline method [24]. 3. Results and discussion 3.1. Comparison of As(III) and As(V) removal by ECFe Arsenic removal by ECFe is achieved by the formation of As(V)Fe(III) complexes when current is passed in the ECFe setup in presence of DO. Affinity of Fe(III) precipitates for As(V) is much higher than As(III) [16], but natural ground water, in GMB plain, contains significant fraction of As(III) [25], which makes it difficult to reduce arsenic to meet drinking water standards. Also as As(III) is more toxic than As(V), As(III) removal is very important [26]. Therefore, as a first step oxidation of As(III) to As(V) by chemical oxidants has been suggested [27]. Several researchers have also mentioned the possibility of As(III) oxidation during the ECFe process [7,28,29]. Experiments were carried out to observe the oxidation of As(III) by ECFe. As(V) and As(tot) measurements were carried out and the difference in As(tot) and As(V) gives As(III) concentration. Batch ECFe was performed with initial arsenic concentration of 500 ppb at pH 7 with applied current of 30 mA and results are presented in Fig. 1 where ‘Co’ in the figure denotes initial concentration of arsenic. When Feadded: 22 mg/L and 32 mg/ L, treated water As(tot)residual reduced to less than 10 ppb for As(V) and As(III) respectively, and this can be clearly observed in the inset

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T. Banerji, S. Chaudhari / Journal of Environmental Chemical Engineering 4 (2016) 3990–4000

40

Charge loading (C/L) 80

in the following sections. Aqueous phase parameters such as pH of the solution [6,334], initial arsenic concentration [21,8,22] and presence of other co-occurring ions [35,16,9] may affect arsenic removal by Fe(III) precipitates. The effect of these aqueous phase parameters on arsenic removal by ECFe was observed and has been presented in the following sections.

120

600 As(tot) concentraon (ppb)

As(tot)residual : Co=563 ppb As(V) 500

(A)

As(tot)residual : Co=557 ppb As(III) As(V)residual : Co=557 ppb As(III)

400 300 200 100 0 10

0

20 Iron added (mg/L)

30

40

As(tot) concentraon (ppb)

50 40

(B) 30 20 10 0 15

20

25 30 Iron added (mg/L)

35

Fig. 1. (A) Removal of 500 ppb As(V) and As(III) by batch ECFe at pH 7 with DO 4– 5 mg/L and 30 mA current; (B) Inset showing the difference of iron requirement for As(V) and As(III).

of Fig. 1. It can be observed from the figure, for the run with As(III), that when Feadded: 2.64 mg/L, As(tot)residual: 323 ppb but As (V)residual increased to 52 ppb from 20 ppb indicating oxidation of As(III). Similar increase in As(V) concentration while performing ECFe for As(III) removal has been reported by others [7–9]. For the run with initial As(V), As(III) was below detection limit, indicating no formation of As(III) by ECFe in the conditions provided. At the end of experimental run, arsenic from arsenic laden Fe(III) precipitates was solubilised under acidic conditions, and it showed presence of only As(V), indicating oxidation of As(III). The possibility of oxidation of As(III) by DO is insignificant as the reaction kinetics are too slow to contribute to As(III) oxidation by ECFe [30]. Oxidation of As(III) to As(V) during oxidation of Fe(II) to Fe(III) has been reported earlier [16,31]. Formation of Fe(IV), which is an oxidizing intermediate, during oxidation of Fe(II) to Fe(III) and the use of this Fe(IV) for oxidation of As(III) to As(V) via As(IV) has been proposed by others [32,18,17]. It may be observed from Fig. 1 that at Fe dose of 32 mg/L, As(III) oxidation was greater than 99%, which is without the use of any external chemicals. Whereas, As (III) concentration of 50 ppb required 0.1 mg/L Cl2 and 0.25 mg/L ozone in distilled water for greater than 99% As(III) oxidation [33]. Thus, the conversion of As(III) to As(V) without the use of chemicals in ECFe is quite efficient and can be considered as a major advantage of this process. 3.2. Operational parameters affecting As(III) removal by ECFe As previously mentioned, As(III) oxidation and removal by ECFe is dependent on Fe(II) oxidation and Fe(III) precipitation, thus As (III) removal by ECFe would be affected by operational parameters affecting Fe(II) oxidation in ECFe. The effect time provided for Fe(II) oxidation (standing time) and current intensity has been presented

3.2.1. Effect of time for Fe(II) oxidation (Standing time) on arsenic removal As mentioned previously that in ECFe reactor, with the passage of current Fe(II) is dissolved which gets oxidized by dissolved oxygen forming oxidizing reactive intermediate [Fe(IV)] and Fe(IV) oxidizes As(III) to As(V). The removal of As(V) occurs by forming an insoluble complex with Fe(III). Thus for arsenic removal, Fe(II) should form Fe(III) and should be separated from the aqueous phase. Efficient arsenic removal by ECFe can occur only with more Fe(III) formation as complexes of Fe(II)-As(V) are not separated from aqueous phase. Experiments were carried out where different standing times post ECFe were provided. The term standing time means the time provided post ECFe after which the Fe-As precipitates have been separated from the aqueous phase, i.e. time provided for Fe(II) oxidation reactions to proceed. In these experiments samples from the ECFe reactor were collected at regular time intervals (to vary the amount of Fe added) and then allowed to stand for increasing time before filtering. The results of the experiment are shown in Fig. 2. It can be observed from the results that the arsenic removal efficiency was higher on allowing the reaction mixture longer standing time before filtration. As(tot) in treated water reduced to less than 50 ppb after addition of 62 mg/L and a reaction time of 3 h. But addition of 39 mg/L and 25 mg/L Fe by ECFe showed less than 50 ppb As(tot) in treated water when standing time post ECFe was 6 h and 12 h respectively. Several researchers have not considered the complete oxidation of Fe(II) and reported poor arsenic removal by EC [34,36]. While others have monitored Fe(II) coming out of EC reactor and low concentration of Fe(II) coming out has been assumed to indicate that Fe(II) oxidation was complete [21]. However, it may be mentioned that Fe(II) adsorbs on Fe(III) precipitates [37,38], thereby even though very less Fe(II) might be present in aqueous phase a significant fraction of adsorbed Fe(II) may remain unoxidized on Fe(III) precipitates, and not contribute to arsenic removal. It is not known how the presence of arsenic [As(III) or As(V)] affects oxidation of Fe(II), but the interaction of Fe(II) with As(III) and As(V) are reported to be quite weak [40,41]. However, for simplification the Fe(II) oxidation behaviour, it is assumed that Fe

0

Charge loading (C/L) 100 150

50

200

1000

Arsenic concentraon (ppb)

0

100

10

3 h standing me aer EC 6 h standing me aer EC 12 h standing me aer EC 18 h standing me aer EC

1 0

10

20

30 40 Iron added (mg/L)

50

60

Fig. 2. Effect of Fe(II) oxidation time on As(III) removal by ECFe.

T. Banerji, S. Chaudhari / Journal of Environmental Chemical Engineering 4 (2016) 3990–4000

(II) oxidation is not affected by the presence of arsenic. The equation given by [42] is mentioned below: dFeðIIÞ ¼ k½FeðIIÞ½DO½OH 2 dt

0

20

3993

Charge loading (C/L) 40 60

80

100

500 Arsenic concentraon (ppb)

ð2Þ

It is clear from Eq. (2) that Fe(II) concentration, DO and pH govern the overall Fe(II) oxidation rate. The above mentioned equation is mostly used to model homogeneous Fe(II) oxidation, i.e. oxidation occurring in aqueous phase. Further, it may be mentioned Fe(II) oxidation can also occur in heterogeneous phase [20]. It may be mentioned that most of the literature for estimating oxidation rate have used Fe(II) concentrations upto 10 mg/L [43,42,44]. Most of the literature deals with the initial oxidation phase of Fe(II) and for short duration as most Fe(II) oxidation data at around pH 7 have been reported only upto 10–60 min [20,45,42]. Though homogenous reaction rate information is available but scanty information is available for heterogeneous Fe(II) oxidation rate. Calculated aq. Fe(II) remaining has been obtained using kinetic rate constants of Sung and Morgan [43] and it has been compared to aq. Fe(II) (experimental) in the 3 h reaction time samples (shown in Table 1). It can be observed from Table 1 that there was a considerable difference in the aq Fe(II) values obtained from the experiment and calculated from the rate constants of Sung and Morgan [43] at 3 h, 6 h and 12 h standing time. From Table 1 it is clear that for 3 h standing time the difference between calculated and experimental value is quite less upto Feadded: 11.57 mg/L but beyond this concentration the experimental and calculated values don’t match. Whereas after 6 h standing time, the experimental and calculated values don’t match for low Fe concentration but match at higher Fe concentration. Thus it is apparent that the rate equation does not predict Fe(II) oxidation reasonably well, which might be due to limitation of considering only homogeneous oxidation. As it was observed that As(III) removal did not significantly improve beyond 12 h standing time (data points of 12 h standing time and 18 h standing time almost overlapped in Fig. 2), therefore 12 h standing time before separation of the precipitates was provided in all the subsequent experiments.

50

5 As(tot) at 15mA As(tot) at 30mA As(tot) at 60mA

1 0.5 0

5

10

As(III) at 15mA As(III) at 30mA As(III) at 60mA 15 20 Iron added (mg/L)

25

30

Fig. 3. Effect of current intensity on As(III) removal [the hollow symbols with connecting lines represent As(tot) concentration and solid symbols represent As(III) remaining in solution].

same order—15 mA > 30 mA > 60 mA which can possibly be due to ineffective utilization of Fe(IV). When Fe(II) is added in multiple small doses with sufficient time for the oxidation of Fe(II) after each addition, the As(III)/Fe(II) ratio in solution is always higher than if the same amount of Fe(II) is added in one dose and with a larger As(III)/Fe(II) ratio, a smaller fraction of the reactive oxidants produced during the oxidation of Fe(II) is consumed by Fe(II), and a larger fraction of As(III) is oxidized [16,21]. Similar inference was drawn from results another experiment with an As(III)initial: 1000 ppb and current dose of 30 mA, 60 mA and 120 mA (data not shown). 3.2.3. Effect of pH Apart from speciation of As(III), As(V) and Fe(III), oxidation rate of Fe(II) is also affected by pH [47,42]. At high pH, high Fe(II) oxidation rate would lead to higher Fe(IV) generation rate. Experiments were performed to observe the effect of pH on As(III) removal by maintaining the pH in the ECFe reactor setup at 6, 7 and 8 (pH range of groundwater). The results of the experiment are presented in Fig. 4. When Feadded: 16.9 mg/L, at pH 7—As (tot)residual: 29 ppb out of which As(III)residual: 7 ppb, at pH 8—As (tot)residual: 49 ppb out of which As(III)residual: 48 ppb and at pH 6—As(tot)residual: 73 ppb out of which As(III)residual: 22 ppb. From the results it is clear that the highest As(III) oxidation took place at pH 7 leading to highest arsenic removal for same Feadded. The concentration of As(III)residual and As(tot)residual is almost same at pH 8, hence the legends in the figure are overlapping. As most efficient oxidation of As(III) was achieved at pH 7 and it is known

3.2.2. Effect of current intensity Various researchers have evaluated the effect of current intensity on arsenic removal by ECFe [21,7,46]. Experiments were carried out to evaluate the effect of current intensity on As(III) removal by ECFe. The results are presented in Fig. 3 where As(tot) means concentration of As(III) + As(V) and the results are compared based on the total iron dissolved. It can be observed from the figure that lower current intensity gave better arsenic removal and the As(III) oxidation efficiency also increased in the

Table 1 Calculated and observed Fe(II) concentrations in ECFe after given standing time. Fe added by ECFe (mg/L)

5.48 8.44 11.57 17.85 24.50 31.56 39.05 50.39 62.48

Fe(II) concentration after 3 h

Fe(II) concentration after 6 h

Fe(II) concentration after 12 h

Experimental

Calculated

Experimental

Calculated

Experimental

Calculated

0.77 1.03 1.22 1.35 1.52 1.49 2.10 2.25 2.66

0.70 1.08 1.49 2.29 3.15 4.05 5.02 6.47 8.03

0.28 0.36 0.55 0.69 0.76 0.82 0.92 1.00 1.00

0.09 0.14 0.19 0.29 0.40 0.52 0.64 0.83 1.03

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

0.02 0.04 0.05 0.08 0.10 0.13 0.16 0.21 0.26

Calculated using the equation [Fe(II)] = [Fe(II)0]exp(-k1t) where: t = time in minutes and k1 = 0.0114 min1 calculated for our experimental conditions from constant k = 9.4  1015 M3 min1 given in Table 3 of [43]; n.d.= not detectable (minimum detection limit = 0.05 ppm).

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T. Banerji, S. Chaudhari / Journal of Environmental Chemical Engineering 4 (2016) 3990–4000

0

20

40

Charge loading (C/L) 60 80

1000 100

120

As(V):1060 ppb

Arsenic concentraon (ppb)

Arsenic concentraon (ppb)

500

50

5

pH 6: As(tot) pH 7: As(tot) pH 8: As(tot)

pH 6: As(III) pH 7: As(III) pH 8: As(III)

As(V):560 ppb 100

As(V):260 ppb

10

1

0.1 0

1 0.5

0 0

5

10

15 20 Iron added (mg/L)

25

30

10

35

20

30 40 Iron added (mg/L)

50

60

Fig. 5. Effect of initial concentration of As(V) on its removal by ECFe. Fig. 4. Effect of pH As(III) removal ECFe [the hollow symbols with connecting lines represent As(tot) concentration and solid symbols represent As(III) remaining in solution].

that Fe(IV) is the oxidizing radical generated during ECFe, thus it indicates that Fe(IV) is most effectively utilized at pH 7 in comparison to pH 6 and 8. This is a combined effect of Fe(IV) production, Fe(IV) stability and heterogeneous phase reactions and thus oxidation of As(III) to As(V) and its complexation to Fe (III) precipitates. As observed, at pH 8, As(III) oxidation was poorer in comparison to 7 and 6 which probably is due to relatively quick oxidation of Fe(II) [42] leading to insufficient utilization of Fe(IV). Although Fe(II) oxidation would be slower at pH 6 in comparison to pH 7 and 8 [42], Fe(IV) is unstable at pH 6 [38,39] which might be the reason for lesser As(III) oxidation at pH 6 as compared to pH 7. At pH 6 it seems that As(V) complexes less with the Fe(III) precipitates, as compared to at pH 7, which is clear from the less arsenic removal obtained and also reported by Jain and Loeppert [48]. Similar results are reported for higher oxidation of methanol and ethanol with Fe(II) at pH 7 compared to pH 6 and 8 [49]. 3.2.4. Effect of initial arsenic concentration As mentioned above co-precipitation and adsorption by preformed HFO have different mechanisms of arsenic [As(V)] removal [19]. In co-precipitation, as As(V) interacts during precipitation (hydrolysis) of Fe(III), it offers As(V) complexation possibilities with Fe(III), Fe(III)-hydroxo species before the complete hydrolysis of Fe(III) [for eg. Fe(OH)3]. Analysis of As (V)-Fe(III) co-precipitation literature [19], indicates that amount of As(V) complexed/removed per unit weight of iron, was influenced by initial aqueous phase arsenic concentration. Experiments were performed with varying initial As(V) and As (III) concentration in ECFe systems. The results are presented in Fig. 5. For all initial arsenic concentrations, the removal efficiency (mg As removed/g Fe added by ECFe), was initially high followed by a gradual decrease, consistent with other published literature [21,8,22]. It can be observed from the figure, that to reach less than 10 ppb As(tot) Feadded was 21 mg/L Fe, for As(V)initial: 260 ppb; was 25 mg/L Fe, for As(V)initial: 560 ppb; and was 29 mg/L Fe, for As(V)initial: 1060 ppb. This shows that there was no linear relationship between initial arsenic concentration and iron required to reach less than 10 ppb. Thus the practice of normalizing results from experiments conducted at different initial concentrations as a method of comparing removal efficiencies of various iron based (ECFe, ZVI) arsenic removal technologies independent of initial arsenic concentration seems improper. Ignoring the dependence of arsenic removal efficiency on initial arsenic concentration can lead to error in estimating the

Fe dose required to reach less than 10 ppb level. Similar observations have also been made by Amrose et al. [21]. From Fig. 5, it is clear that arsenic removal per unit weight of iron to achieve aqueous phase As concentration of 50 ppb or 10 ppb is higher for higher initial As(V) concentration. The observed behaviour can be explained by considering that most of the arsenic removed by Fe(III) at higher As concentration does not re-dissolve/ desorb into aqueous phase even at quite low As concentration. In other words, arsenic removed from aqueous phase by Fe(III) is held strongly and irreversibly sorbed/co-precipitated. The aforementioned statement is supported by observations made by Langmuir et al. [50]. Langmuir et al. [50], during neutralization of raffinate, As (V):54 mg/L, Fe(III): 1440 mg/L, initial pH: 2.18, obtained residual arsenic concentration less than 1 mg/L at pH 7.37. The results could not be explained by the diffuse layer model which assumes desorption of arsenic from HFO. However, the same model could explain the results when one step adsorption (no desorption of arsenic from HFO) of As(V). The same could also be explained when Fe(III) precipitates during co-precipitation was considered to proceed with no back reaction (re-dissolution) of precipitated ferric arsenate. Further, whether precipitated as ferric arsenate [50], co-precipitated with an Fe(III) phase [51] or adsorbed, most of the As(V) held by solids following neutralization to pH values near 7 is held irreversibly. Fuller et al. [14] have employed Freundlich model for comparison of arsenic removal by co-precipitation and preformed HFO. Similarly, here also Freundlich model was used to compare arsenic removal capacity of Fe for different initial arsenic concentration by ECFe. Table 2 shows the data of Freundlich fit and it can be observed that kf is varying but 1/n is same. The kf obtained for initial As(V) concentration of 1060 ppb is higher than for initial As(V) concentration of 263 ppb, indicating that arsenic removal capacity per unit of Fe decreases with decrease in initial arsenic concentration. If it is assumed that as the ECFe

Table 2 Data of Freundlich fit for removal of different initial concentrations of As(V) by ECFe. Initial As(V) concentration

1060 ppb

560 ppb

260 ppb

kf 1/n

13.21 0.424

9.20 0.411

4.86 0.421

Initial As(III) concentration

1063 ppb

557 ppb

111 ppb

kf 1/n

9.57 0.310

7.18 0.358

2.61 0.373

T. Banerji, S. Chaudhari / Journal of Environmental Chemical Engineering 4 (2016) 3990–4000

3.2.5. Effect of co-occurring ions and humic acid on As(III) removal by ECFe Several researchers have reported detrimental effects of cooccurring ions on arsenic removal by iron based adsorbents, coagulants or ECFe process [48,52–54,35,16,9]. Phosphate, silicate and humic acid are most reported to be causing a reduction in the arsenic removal efficiency and emphasis has been given in this study to observe the effect of co-occurring ions and humic acid on As(III) oxidation and its subsequent removal by ECFe.

1000

mg As removed/g Fe added

a) 100

R2= 0.968

10

1060ppb As(V) 560 ppb As(V)

1

260 ppb As(V) 0.1 100

mg As removed/g Fe added

b) 10

1060 ppb As(III) 1 R2= 0.945

550 ppb As(III) 110 ppb As(III)

0.1

0.01 1

10 100 Arsenic concentraon (ppb)

3995

1000

3.2.5.1. Effect of phosphate. Phosphate is ubiquitous in nature. The application of phosphatic fertilizers [55] and sewage disposal on ground/surface water [56] leads to an increase in phosphate concentration in groundwater. Phosphate adversely affects arsenic removal during co-precipitation with Fe(II) and Fe(III) and among the co-occurring ions phosphate has been reported to be most important factor for reduction in arsenic removal in Fe(II) co-precipitation systems [16] which are similar to ECFe systems. Thus to evaluate the effect of phosphate on the ECFe system, arsenic removal with ECFe in the presence of phosphate was performed. More than 2 ppm PO4-P was observed in only 8.5% tubewells in Bangladesh [57]. Fig. 7 shows the effect on performance of ECFe system for removal of 500 ppb As(III) in the presence of PO4-P concentrations varying from 0 to 2 ppm. From the figure it can be seen that in the presence of 2 ppm PO4-P, after Feadded: 26 mg/L, As(tot)residual: 172 ppb out of which As (III)residual was 146 ppb. Whereas at same Fe dose without PO4-P As(tot)residual: 6 ppb out of which As(III)residual was 4 ppb. This indicates that As(III) oxidation and removal by ECFe is reduced in presence of phosphate. As phosphate can scavenge oxidizing intermediates formed during the Fenton reaction [58] so possibly

Fig. 6. Freundlich fit of a) As(V) and b) As(III) removal by ECFe considering irreversible sorption.

As(III) concentration (ppb)

500

50 As(III) - 0 ppm P As(III) - 1 ppm P As(III) - 2 ppm P 5

0.5 1

500

As(tot) concentration (ppb)

experiment proceeds, the arsenic present in aqueous phase complexes with Fe(III) and is precipitated, which is the primary assumption of co-precipitation, and if it is considered to form a solid inert phase that does not participate/affect in further arsenic removal, then also it can be visualised that it is similar to irreversible sorption/co-precipitation. Considering irreversibility of arsenic removal, i.e. arsenic complexed with Fe and removed from ECFe reactor, does not influence the subsequent arsenic removal by ECFe. In Fig. 6, arsenic removal by Fe is computed by considering only aqueous phase arsenic concentration, i.e. for initial As(V) 1060 ppb and addition of Fe of (2.64 + 2.73 = ) 5.37 mg/L, Asresidual: 278 ppb then Asremoved/Feadded: 146 mg/g. Whereas, while considering irreversible behaviour, with initial As (V): 1060 ppb, Fe added: 2.64 mg/L, Asresidual: 550 ppb, and Asremoved/Feadded: 193 mg/g; as it is considered that arsenic removed by Fe is irreversible, then initial As for additional Fe addition: 2.73 mg/L can be considered as 550 ppb. Considering initial As: 550 ppb, Asresidual: 278 ppb then Asremoved/Feadded: 100 mg/g, this is comparable Asremoved/Feadded value of 115 mg/g with initial As of 560 ppb. Accordingly the computations were done and results are plotted in Fig. 6a. From the figure it is clear that data of all initial As concentration are no more distinct/ parallel, but overlap and fall in a straight line. The plot makes evident that arsenic removal by Fe(III), produced by ECFe, is significantly irreversibly held by Fe(III) precipitates. Similar calculations were performed for As(III) removal also considering irreversible sorption as explained above and the results are presented in Fig. 6b. From the figure it is clear that data of all initial As concentration are no more distinct/parallel, but overlap and fall in a straight line. Thus, whether As(III) or As(V) removal by ECFe, both forms are irreversibly held in the Fe(III) precipitates.

50

5 As(tot) - 0 ppm P As(tot) - 1 ppm P As(tot) - 2 ppm P 0.5 1

0

20

40 Iron added (mg/L)

60

Fig. 7. Effect of phosphate on As(III) removal by ECFe.

80

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3.2.5.2. Effect of silicate. It has been reported that Bangladesh region has silicate of greater than 30 ppm in only 8.14% tubewells [57]. Thus the experiments were performed to determine the effect of silicate on As(III) removal efficiency of ECFe with silicate concentrations of upto 30 ppm. The results of this experiment are shown in Fig. 9. From the figure it can be seen that when Feadded: 55 mg/L, As(tot)residual: 2 ppb in presence of 0 ppm and 10 ppm silicate out of which As(III)residual: 2 ppb and in the presence of 20 ppm As(tot)residual: 5 ppb out of which As(III)residual: 4 ppb. This indicates that no significant adverse effect on As(III) oxidation

0

20

Iron added (mg/L) 40 60

80

As(tot) concentraon (ppb)

100

3h reacon me aer EC 12h reacon me aer EC 18h reacon me aer EC 24h reacon me aer EC

1

P concentraon (ppb)

1000

100

10

1 0

50

100

150

200

250

Charge loading (C/L) 100

50

150

200

As(III) concentration (ppb)

1000

100

As(III): 0 ppm Si

10

As(III): 10 ppm Si As(III): 20 ppm Si

As(III): 30 ppm Si 1

1000

100

10

As(tot): 0 ppm Si As(tot): 10 ppm Si As(tot): 20 ppm Si As(tot): 30 ppm Si

1 0

18

36 Iron added (mg/L)

54

Fig. 9. Effect of silicate on As(III) removal by ECFe.

and removal was observed at silicate concentration of 10 and 20 ppm. But in presence of 30 ppm silicate, when Feadded: 55 mg/L, As(tot)residual: 64 ppb and As(III)residual: 63 ppb. This indicates that As(III) oxidation by ECFe was significantly reduced in presence of 30 ppm silicate, thereby reducing its removal by ECFe. Other researchers also have reported that silicate adversely affects arsenic removal by Fe(III) precipitates [35,54]. As(III) removal may not be affected by silicate during co-precipitation with Fe(II) but can be negatively affected during co-precipitation with Fe(III), as during co-precipitation with Fe(II), As(III) was oxidized by the Fe(IV) formed and this did not happen during co-precipitation with Fe(III) [16]. Co-precipitation with Fe(II) is similar to ECFe. This indicates that silicate can compete with As(III) but not As(V) during co-precipitation. As(III) gets oxidized in ECFe, there was less effect of presence of silicate on As(III) removal by ECFe [9]. At higher concentrations (i.e. 30 mg/L Si) and comparatively much lower arsenic concentration [1000 ppb As(III) i.e. 30 times more Si in solution] negative effect of presence of silica was observed possibly due to silica polymerization and bonding with Fe(II) to reduce As(III) oxidation thereby reduction in efficiency of ECFe. Other researchers have reported high concentrations of silica polymerize to physically block access to sorption sites and not through competitive sorption [60].

1000

10

0

As(tot) concentration (ppb)

phosphate can scavenge Fe(IV) and affect As(III) oxidation. Thus it can be said that phosphate was having a detrimental effect on As (III) removal by ECFe in two ways. Namely, due to similar affinity for Fe(III) of both As(V) and phosphate [48,52,53] and also reduction in As(III) oxidation which also leads to reduction in arsenic removal efficiency. As phosphate complexes with Fe(II), therefore available free Fe (II) in aqueous phase would reduce and thus oxidation of Fe(II) would be affected by phosphate. Accordingly, effect of standing time was also observed and results are presented in Fig. 8. From the figure it is clear that arsenic and phosphate removal increase upto 24 h standing time indicating that oxidation rate of Fe(II) reduced in the presence of phosphate, which in the absence of phosphate only required 12 h standing time. From Fig. 8 it can be observed that after 12 h standing arsenic removal was poor (from initial 1092 ppb to 859 ppb) upto Feadded: 5.5 mg/L compared to phosphate (from initial 948 ppb to 593 ppb). The less removal of As(tot) is expected if phosphate and arsenic are competing for sites on Fe(III) precipitates [59]. The time required for oxidation of As(III) allowed phosphate to initially outcompete arsenic for complexation sites on Fe(III) precipitates.

300

Charge loading (C/L) Fig. 8. Effect of standing time for Fe(II) oxidation on As(III) and PO4-P removal by ECFe.

3.2.5.3. Effect of humic acid. Natural Organic Matter (NOM) commonly occurs in groundwater, and in West Bengal region concentrations of around 16 ppm have been reported [25]. Humic acid (HA) is used as a representative compound for NOM for laboratory experiments [61]. Experiments were carried out to study the effect of HA on As(III) removal by ECFe. The results are presented in Fig. 10. When Feadded: 70 mg/L the As(tot)residual was 3 ppb, 80 ppb and 136 ppb for setup without HA, with 10 ppm HA and

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0

Charge loading (C/L) 100 150

50

8

200

250 7

1000

6 DO (mg/L)

100 As(tot) - 0 ppm HA As(tot) - 10 ppm HA As(tot) - 20 ppm HA As(III) remaining: 10 ppm HA As(III) remaining: 20 ppm HA As(III) remaining: 0 ppm HA

10

5 4

Fe added as HFO 1000mg/L

3

no HFO added

2 1 0

1

0

0

20

40 Iron added (mg/L)

60

50

100

80

Fig. 10. Effect of humic acid on As(III) removal by ECFe [the hollow symbols with connecting lines represent As(tot) concentration and solid symbols represent As(III) remaining in solution].

with 20 ppm HA respectively. From the figure it is clear that in the presence of HA As(III)residual and As(tot)residual are overlapping indicating that all the As(V) formed was removed. The figure clearly shows that As(III) oxidation was reduced in presence of HA. Thus it seems that Fe(IV) was possibly consumed for oxidation of HA in ECFe system. 3.2.5.4. Effect of SO42, NO3, HCO3 and Cl. Batch ECFe experiments were performed to observe the effect of presence of SO42 (200 mg/L), NO3 (40 mg/L), HCO3 (250 mg/L) and Cl (250 mg/L) on As(III) removal (data not shown). It was observed that SO42, NO3, and HCO3 did not significantly affect the As(III) removal by ECFe and Cl improved the As(III) removal slightly. Because these ions have not been reported to affect Fe(III) precipitation, do not oxidize As(III) and do not adsorb/react as strongly on Fe(III) precipitates as As(V) and phosphate at pH 7, it was expected that these ions would not greatly affect the performance of the ECFe process. The effect of sulphate and nitrate observed here is consistent with reported literature [35,9,8]. Bicarbonate adsorbs on neutral sites on Fe(III) precipitates whereas As(V) adsorbs on negatively charged sites [62]. However it can reduce As(III) adsorption, but as As(III) is oxidized in ECFe the effect of bicarbonate is not observed and this is consistent with literature repots [63]. Slightly enhanced As(III) oxidation and hence removal by ECFe in the presence of 250 mg/L Cl was observed probably due to formation of oxidants such as hypochlorite (OCl) and hypochlorous acid (HOCl) [8].

200

250

300

Fig. 11. Variation of DO with time in the absence and presence of Fe(III) precipitates (HFO).

in these conditions this difference was possibly due to the adsorption of DO on HFO. To delineate the effect of presence Fe(III) precipitates (HFO), oxygen mass transfer coefficient (KLa) was estimated according to the method described in [66]. The average KLa in absence of HFO was found to be 0.1252 min1 and in presence of HFO was 0.0212 min1. The experiments show that DO is adsorbed onto HFO. Fe(II) adsorbs on HFO [37,38] as does As(III) [67,59]. Thus it is clear that in the ECFe system As(III), Fe(II) and DO would be available at the interface of HFO and are likely to be in the close proximity which would promote the reactions occurring near or at the interface [68]. 3.4. Effect of presence of interface [Fe(III) precipitates] on As(III) removal As mentioned above, Fe(III) precipitates can adsorb DO, Fe(II) and As(III) and thus presence of interface in ECFe systems is likely to promote interfacial reactions. Experiments were carried out to evaluate the effect of presence of Fe(III) precipitates in the ECFe setup and the results are presented in Fig. 12. Fe(III) precipitates are referred to as Fe(III) ppt in the figure. In the absence of Fe(III) precipitates, less than 10 ppb As(tot)residual was achieved after addition of 45 mg/L Fe. The Fe(III) precipitates formed in the experimental run were allowed to remain in the reactor, spiked with As(III) to get arsenic concentration of 1050 ppb As(III) and

3.3. Adsorption of dissolved oxygen (DO) by Fe(III) precipitates

0

50

Charge loading (C/L) 100

1000

150

200

As(tot) without Fe(III)

80

As(tot) with Fe(III) ppt As(tot) concentraon(ppb)

Earlier sections have discussed the oxidation of Fe(II) in ECFe, in the presence of DO, forming Fe(III) precipitates and an oxidizing radical [Fe(IV)]. The Fe(II) oxidation can be through homogenous phase reactions [42] or through heterogeneous phase reactions [20]. It was felt that heterogeneous reactions could be promoted by the proximity of Fe(II) and DO on Fe(III) precipitates. Fe(III) precipitates can adsorb DO [64,65]. Experiments were carried out to quantify O2 adsorption on Fe(III) precipitates (HFO) maintaining identical aeration, pH and temperature conditions in the experimental setups with and without HFO. The results have been presented in Fig. 11. From the figure it can be observed that in the absence of HFO, initially the DO increased rapidly upto 4.4 mg/L. Whereas in the presence of HFO the DO increased upto 2.5 mg/L and then it became gradual. As the aeration was identical, therefore it was expected that the oxygen transfer was uniform and therefore

150 Time (sec)

As(V) without Fe(III) As(V) with Fe(III) ppt

60

100 40 10

As(V) conc (ppb)

Arsenic concentration (ppb)

3997

20

1

0 0

10

20

30 Iron added (mg/L)

40

50

Fig. 12. Effect of presence of interface [recycled Fe(III) precipitates] on As(III) removal by batch ECFe.

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equilibrated for 12 h, hence As(III) adsorption by recycled Fe(III) precipitates would not give erroneous conclusions. Reactor volume lost to sampling in previous ECFe run was made up with D/W and

As(III) added to get final concentration of 1000 ppb As(III) in aqueous phase. In the experimental run with Fe(III) precipitates, 50% less Feadded can achieve less than 10 ppb As(tot)residual as

Fig. 13. FTIR analysis a) ECFe precipitates with As(V) b) ECFe precipitates without arsenic c) standard FeAsO4 prepared with methodology of [71].

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shown in Fig. 12. From the figure it can be observed that approximately 22 mg/L Fe addition would be sufficient to reduce arsenic concentration in treated water to less than 10 ppb. Thereby recycled Fe(III) precipitates seem to promote the interfacial reactions for As(III) oxidation and hence improve ECFe efficiency. Fe(III)-Fe(II) system often acts as an electron carrier [69]. Shuttling of electrons through the bulk of Fe(III) precipitates is also known to occur [70], hence even if the three reactants are not in close proximity but on the surface of Fe(III) precipitates, the oxidation reactions of As(III) can still be catalyzed. Thus the surface reactions on Fe(III) precipitates will most likely be more important for As(III) oxidation and removal than reactions aqueous phase. These results also show that sludge produced to treat water to less than 10 ppb arsenic concentration would reduce in the presence of Fe(III) precipitates. 3.5. FTIR studies of ECFe precipitates Arsenic-iron co-precipitation has been mostly suggested for remediation of mine drainage. Thus studies on arsenic concentration [As(V) greater than 15 ppm] relevant to mine drainage have been conducted and have shown FeAsO4 formation at near neutral pH conditions [19]. Whereas, the concentration of arsenic in groundwater rarely exceeds 1 ppm [57]. Thus attempts have been made to analyse the Fe-As precipitate formed in ECFe. ECFe experiments were conducted maintaining Fe/As ratio of 5 at pH 7 where the aqueous phase arsenic concentration was not allowed to exceed 1 ppm and sufficient time (3 days) was provided for added Fe(II) oxidation. The precipitates were collected and air dried. The FTIR spectra of the solid precipitates was obtained and is presented in Fig. 13a and of Fe(III) precipitates prepared by ECFe in the absence of arsenic in Fig. 13b and of standard FeAsO4 (prepared as per [70] in Fig. 13c. The spectra obtained were compared with available literature and the peaks are attributed to the OH stretch, Fe OH bond, to OH bend (i.e. H2O), Fe O bond, uncomplexed/unprotonated As (V)O bond, As(V)Fe bond, As(V) OFe bond and As O bend [72–74,71]. Of these, the peak at 828 cm1 indicates that in the given conditions Fe(III) bonds with As(V) before hydrolysis which matches with standard FeAsO4. Further proof of formation of some FeAsO4 in these systems has been obtained by matching the XRD patterns of the precipitates from these experimental runs to those reported by Tokoro et al. [19] and Jia et al. [75] (results not shown). 4. Conclusions The results presented show that ECFe is able to oxidize As(III) to As(V) but this is dependent on the oxidation of Fe(II) dissolving from the anode of the electrochemical cell. If the oxidation is very quick, as in the case of higher pH, then the oxidation of As(III) is reduced. The optimal pH for As(III) removal by ECFe was found to be pH 7. Arsenic removal is also affected by higher current intensity, and although the final concentration of arsenic in treated water is low but due to wastage of oxidizing intermediates the iron requirement is higher. Increasing concentrations of phosphate, silicate and NOM were seen to reduce the arsenic removal efficiency of ECFe either due to competition of sorption sites or by impeding As(III) oxidation by using up the oxidizing intermediates formed during oxidation of Fe(II) to Fe(III). Bicarbonate, nitrate, sulphate and chloride did not affect efficiency of ECFe for arsenic removal. Also DO was observed to be adsorbing onto HFO. Presence of Fe(III) precipitates during ECFe showed enhanced arsenic oxidation and removal possibly due to improved interfacial reactions.

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