Fe ratio, initial As concentration and co-existing solutes

Separation and Purification Technology 92 (2012) 106–114

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Arsenate and arsenite removal by FeCl3: Effects of pH, As/Fe ratio, initial As concentration and co-existing solutes Junlian Qiao a, Zheng Jiang b, Bo Sun c, Yuankui Sun a, Qi Wang c, Xiaohong Guan a,⇑ a

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, PR China Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, PR China c State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, PR China b

a r t i c l e

i n f o

Article history: Received 3 January 2012 Received in revised form 14 March 2012 Accepted 18 March 2012 Available online 24 March 2012 Keywords: Coagulation Phosphate Silicate Sulfate Humic acid Calcium

a b s t r a c t Coagulation is one of the most commonly used technologies for arsenic removal from water and wastewater. Jar tests were carried out to determine the influence of pH, initial As/Fe molar ratio, equilibrium As concentration and co-occurring solutes on the crossover pH, where As(V) removal was equivalent to As(III) by FeCl3 coagulation. As(V) was more effectively removed than As(III) at lower pH values in FeCl3 coagulation process but the opposite was true at higher pH level. Increasing As/Fe ratio from 0.12 to 0.50 progressively lowered the crossover pH from 8.5 to 7.4. The arsenic isotherms revealed that As(V) removal was favored at low pH and low equilibrium concentration yet the opposite was true for As(III). The co-existing solutes had different influences on the crossover pH, depending on their ability to compete for sorption sites and to hinder or facilitate the aggregation of ferric hydroxide flocs. The presence of sulfate broadened the pH range where As(III) removal was better than As(V) removal. However, As(V) removal was always superior to As(III) removal in the presence of Ca2+ and no crossover pH was observed. The results from this study revealed that arsenic speciation, arsenic loading, and pH should be considered when predicting and managing arsenic removal by coagulation. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Arsenic contamination of groundwater is a widespread occurrence affecting vast regions in India, Bangladesh, China, Mexico, Argentina, and the United States [1]. Sources of arsenic are both natural and anthropogenic. Arsenic is ubiquitous in the earth’s crust with a mean concentration of 1.5–3 mg kg 1 and weathering of arsenic containing rocks releases arsenic into soils, sediments and natural waters [2]. Petroleum refineries, fossil fuel power plants, nonferrous smelting activities and ceramics, semiconductors, pesticides, and fertilizer production are major industrial processes leading to anthropogenic arsenic contamination [3]. Although maximum mobilization of arsenic in soil and groundwater occurs under natural conditions such as weathering reactions and biological activities in soils, contamination of a vast region of developing or underdeveloped countries is attributed to the percolation of industrial wastewater effluents through soils and landfills [4]. Thus, optimizing treatment technologies for As-containing drinking water and industrial wastewater is currently of great urgency and high priority in many countries. Coagulation and ⇑ Corresponding author. Tel.: +86 21 65980596. E-mail addresses: [email protected] (J. Qiao), [email protected] (Z. Jiang), [email protected] (B. Sun), [email protected] (Y. Sun), wangqilefty@ 163.com (Q. Wang), [email protected] (X. Guan). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.03.023

filtration is cost effective for large flow rates or high arsenic (>20 lg L 1) waters and thus has been studied in a large number of investigations [5–12]. The major mechanism of As-elimination is that of adsorption onto the adsorption sites provided by polymerizing iron or aluminum oxy-hydroxide molecules during coagulation [13]. The similar mechanism of arsenic removal in coagulation process as that in adsorption process suggested that the behavior of arsenate (As(V)) and arsenite (As(III)) removal in these two processes should be similar. As(III) removal during coagulation with ferric coagulant has been shown to be less efficient than As(V) under comparable conditions [5,6,11]. Thereby, oxidation was generally performed prior to coagulation to convert As(III) to As(V) species to remove As(III) effectively by coagulation [9,12,14,15]. However, the literatures investigating the adsorption of As(V) and As(III) on various iron (hydr)oxides refutes the conventional wisdom that ‘‘As(III) adsorption to oxides is less effective than As(V) adsorption’’ [16]. Generally there is a trend of increasing adsorption with increasing pH for As(III) while As(V) adsorption decreases with increasing pH. The adsorption envelopes of As(V) and As(III) crossed at certain pH value (crossover pH, above this pH As(III) removal was more favored than As(V) removal), depending on the initial arsenic concentration and the affinity of As(V) and As(III) for the adsorbent surface [16–20]. Dixit and Hering [16] declared that in the pH range of 6–9, typical of natural environments, As(III) was sorbed to a similar

J. Qiao et al. / Separation and Purification Technology 92 (2012) 106–114

or greater extent than As(V) on HFO and goethite. Grafe et al. [17,18] revealed that more As(III) than As(V) could be removed at pH >4 and pH >3 by adsorption on goethite and ferrihydrite, respectively, under comparable conditions. The discrepancy in the removal behavior of As(V) and As(III) observed in ferric coagulation and that in adsorption processes may be associated with the very low concentration of arsenic and narrow pH range employed in the coagulation studies. Hering et al. [5,6] and Lakshmanan et al. [7] concluded that As(V) was more effectively removed than As(III) during ferric coagulation but their studies were carried out at arsenic concentration 6100 lg L 1 and at near neutral pH. The range of As concentration in natural waters is large and can be as high as 5000 lg L 1 and the pH of natural waters can be far beyond the neutral pH range [1,21]. Moreover, the As concentration in copper smelting wastewater can be as high as 100–1000 mg L 1 even after Ca(OH)2 addition [22]. However, a systematic comparative study on removal of As(V) and As(III) in ferric coagulation process as functions of As/Fe ratios, pH and initial arsenic concentration is currently lacking. The removal of arsenic by FeCl3 may be influenced by the presence of other solutes, including cations and competing anions, in the system. One of the most common cations in natural water is calcium and it is well known that calcium has great impacts on anions removal by adsorption or coagulation [6,9]. The most abundant competing anions naturally present are humic acid (HA), bicarbonate, silicate, sulfate and phosphate [23]. Although many studies had investigated the influence of these anions and cations on arsenic removal in coagulation process [5,6,10,12], very few of them determined the influence of co-existing solutes on the crossover pH. Among these anions, carbonate anions do compete with As for adsorption sites but the competitive effect is relatively small with regard to the potential competitive effects of other anions [10,24]. Therefore, batch coagulation experiments were carried out to compare the adsorption behavior of As(V) and As(III) by FeCl3 as functions of As/Fe ratios, pH, initial arsenic concentration and co-occurring solutes (calcium, phosphate, silicate, sulfate and HA). 2. Materials and methods 2.1. Materials All chemicals were reagent-grade and used without any purification. All chemicals were purchased from Sigma if it was not otherwise specified. All solutions were prepared with double distilled water. The stock solutions of As(III) and As(V) were prepared from Na2HAsO3 and Na3AsO4
7H2O, respectively. FeCl3, Na2SO4, Na2SiO3
9H2O, NaH2PO4, CaCl2 stock solutions containing 5.6 g L 1 Fe3+, 50 g L 1 SO24 , 5 g L 1 Si, 0.4 g L 1 P, and 10 g L 1 Ca2+, respectively, were prepared weekly. Background electrolyte solutions were prepared from the reagent-grade salts NaCl and NaHCO3. A commercial HA purchased from Jiangxi Yuanzhi HA Co., Ltd., China, was purified by repeated pH adjustment, precipitation, and centrifugation to remove ash, humin, and fulvic acid. The SUVA of this HA at 254 nm is 10.73 (mg C L 1) 1 cm 1. 2.2. Jar test procedure and chemical analysis Jar tests were performed open to the air with a jar testing device (TA2-2, Wuhan Hengling Co., Ltd.) to simulate a conventional coagulation/flocculation process. The jar testing procedure was initiated with a rapid mixing at 120 rpm for 1 min, followed by 40 rpm for 20 min and finally there was a 30 min settling. Rapid mixing was started immediately after dosing the pre-determined aliquots of FeCl3 to 1 L synthetic water containing As(V) or As(III). If it was

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not otherwise specified, the initial concentration of arsenic was 2.36 mg L 1 and the dosage of FeCl3 was 4.73 mg Fe3+ L 1. Blank tests without coagulant application were also conducted in parallel and it was confirmed that arsenic lost through adsorption onto the glassware was negligible. Each experiment was carried out in (at least) duplicate. All experiments were performed with a constant ionic strength of 0.01 mol/L NaCl and 0.001 mol/L NaHCO3 to provide necessary alkalinity. Sodium hydroxide and hydrochloric acid were employed to adjust solution pH before the jar test to the predetermined level and kept pH constant during the coagulation process. The influence of equilibrium arsenic concentration on the amount of arsenic removed at different pH levels was determined by varying initial arsenic concentration from 0 to 7.5 mg L 1 while keeping the dosage of FeCl3 constant at 4.73 mg Fe3+ L 1. To determine the influence of co-occurring solutes on arsenic removal by FeCl3, phosphate of 0–1.0 mg P L 1, silicate of 0–21.6 mg Si L 1, sulfate of 0–40 mg L 1, humic acid of 0–10.4 mg C L 1, or Ca2+ of 0– 104.8 mg L 1 was dosed to the arsenic solution before coagulant application. After each test, the supernatant was sampled, filtrated immediately through a 0.45 lm membrane filter, and acidified with concentrated HNO3 for determination of total As and Fe with inductively coupled plasma atomic emission spectroscopy (ICP-AES, Perkin Elmer Inc.). The solution pH was measured with pHS-3C meter (Shanghai Hongyi Inc.). The electrophoretic mobility of the flocs formed in the coagulation process was investigated at room temperature with a Zetasizer2000 (Malvern Instruments Ltd., UK). The dynamic floc sizes were monitored following the methods presented by Zhao et al. [25] but we employed a laser diffraction instrument Bettersize2000 (Dandong Baite, China). 3. Results and discussion 3.1. pH-dependent arsenic removal Fig. 1 shows that As(V) removal in FeCl3 coagulation process was strongly influenced by pH and initial As(V)/Fe ratio. More than 99% As(V) removal was observed at pH 5.0–7.0, pH 5.0–6.0, pH 5.0, and pH 5.0 corresponding to the As(V)/Fe ratio of 0.12, 0.25, 0.37, and 0.50, respectively. At all As(V)/Fe molar ratios, As(V) removal experienced a considerable decrease as pH decreased from 5.0 to 4.0, which should be ascribed to the enhanced dissolution of ferric hydroxide at pH 4.0 since only the precipitated iron could mediate arsenic removal, as shown in Fig. S1(a). When pH increased from 5.0 to 10.0, the removal of As(V) decreased at higher pH, more sharply at higher As(V)/Fe ratio, which should be associated with the aqueous As(V) speciation and the abundance of positively charged sorption sites [9,20,26]. Considering the pKa values of arsenic acid, H2 AsO4 and HAsO24 are the dominant species of As(V) in the pH range of 4.0–6.8 and 6.8–10.0, respectively [26]. Thus, As(V) species become more negatively charged with increasing pH. The surface charge of iron precipitates decreased as pH increased from 4.0 to 10.0, as shown in Fig. S2. In addition, Fig. S1(a) revealed that the amount of precipitated iron decreased as pH increased from 6.0 to 10.0, especially at higher As(V) concentration. The presence of high concentration of HAsO24 at pH P6.8 could sequester the agglomeration and precipitation of iron hydroxide [27,28], leading to the reduction in sorption sites. Therefore, As(V) removal by FeCl3 was determined by the following factors: the amount of precipitated Fe, As(V) speciation and surface charge of the precipitates. The pH-dependent As(III) removal by FeCl3 was very different from that of As(V), as illustrated in Fig. 1. As(III) removal was minor at low pH and increased to maximum at pH 9.0 and then decreased sharply with further increase in pH. As(III) was mainly present as neutral molecule at pH <9.2 and as H2 AsO3 at pH 9.2–12.7. In contrast to As(V), the high pKa values for As(III) reflect a strong tendency for protons to outcompete positively charged specific

J. Qiao et al. / Separation and Purification Technology 92 (2012) 106–114

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108

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100

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80

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60

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40 As(V)/Fe=0.12 As(III)/Fe=0.12

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As(V)/Fe=0.25 As(III)/Fe=0.25

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As(V)/Fe=0.50 As(III)/Fe=0.50

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pH

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7

pH

Fig. 1. Effect of pH on As(V) and As(III) removal by FeCl3 at different As/Fe molar ratios (Fe3+ = 4.73 mg L

sorption sites for the AsO33 moiety at low pH [20]. This results in relatively minor removal of As(III) at low pH and greater removal at higher pH in coagulation process. Pallier et al. [29] noted that As(III) removal depended on coagulant dose and on the number of sites available on hydroxide surfaces rather than on coagulation pH, which was different from our observation, since they employed a much higher coagulant dosage and a much narrower pH range. The sudden decrease in As(III) removal as pH increased from 9.0 to 10.0 should be associated primarily with the incomplete precipitation of Fe(III) and secondly with the repulsion between H2AsO3and negatively charged iron hydroxide flocs. Only 1.0–12.9% of total iron was precipitated at pH 10.0, as shown in Fig. S1(b). Generally, As(V) removal was more favorable than As(III) at lower pH values in FeCl3 coagulation process but the opposite was true at higher pH values. However, the actual pH where As(V) removal was equivalent to As(III) was dependent on the level of arsenic loading. At As/Fe ratio as low as 0.12, the crossover pH was 8.5. Increasing As/Fe ratio from 0.12 to 0.50 progressively lowered the crossover pH from 8.5 to 7.4. Burton et al. [20] examined the adsorption of As(V) and As(III) on schwertmannite and observed that the crossover pH shifted from 8.0 to 4.5 as As/Fe molar ratio increased from 0.03 to 0.38. Similar observations were reported for As(V) and As(III) adsorption on amorphous iron oxide, goethite and ferrihydrite [16–18]. However, the crossover pH shifted to a lesser extent in the coagulation process by varying As/Fe molar ratio compared to that in the adsorption process, which may be ascribed to different mechanisms of coagulation and adsorption. Many researchers have noticed the differences between arsenic adsorption to metal oxyhydroxide solids during coagulation as compared to addition of preformed solids. The mechanisms of arsenic removal by coagulation include direct precipitation, surface precipitation, solid-solution formation, or an increase in the number of adsorption sites provided by

1

).

polymerizing iron oxy-hydroxide molecules during in situ precipitation [6,13]. 3.2. Arsenic sorption isotherms The amounts of As(V) and As(III) removed by FeCl3 as functions of equilibrium As concentration and pH are shown in Fig. 2 and the amounts of precipitated iron collected in this process are illustrated in Fig. S3. At pH 5 or 6, the amount of As(V) removed increased significantly at equilibrium As(V) concentration below 0.50 mg L 1 and it slowly increased with increasing equilibrium As(V) concentration. However, much different phenomena were observed at pH 7 or 8. The amount of As(V) removed increased rapidly to maxima of 0.41 and 0.27 mg As/mg TFe (TFe stands for the total concentration of iron added to the solution), respectively, at pH 7 and 8 and then experienced a gradual drop with increasing equilibrium As(V) concentration, which should be ascribed to the drop in the amount of precipitated iron at high equilibrium As(V) concentration (as illustrated in Fig. S3). At low equilibrium As(V) concentration, the amount of As(V) removed was mainly determined by the equilibrium As(V) concentration. As the equilibrium As(V) concentration increased progressively, it inhibited iron precipitation significantly by forming highly dispersed colloids difficult to be removed by filtration, as demonstrated in Fig. S3, and the influence of precipitated iron outcompeted that of equilibrium As(V) concentration. Thus, the significant drop in the amount of precipitated iron resulted in a gradual drop in the amount of As(V) removed. For the case of As(III) removal, the adsorption isotherm can be divided into two stages at all pH levels examined in this study. In the first stage, As(III) removal increased gradually with increasing equilibrium As(III) concentration and reached an intermediate flat region before a linear increase in As(III) removed was observed

109

Arsenic removed (mmol As/mmol T Fe)

J. Qiao et al. / Separation and Purification Technology 92 (2012) 106–114

.8

.8

pH=5

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As(V) As(III)

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Equilibrium arsenic concentration (mg/L)

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.6 Fe(III) alone Fe(III) + 0.8 mg L-1 As(III) Fe(III) + 7.5 mg L-1 As(III)

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Equilibrium arsenic concentration (mg/L)

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Equilibrium arsenic concentration (mg/L)

Fig. 2. Influence of equilibrium arsenic concentration on the amount of arsenic removed at different pH levels (the inset shows the growth rate of aggregated flocs at different initial As(III) concentration) (Fe3+ = 4.73 mg L 1).

with further increase in equilibrium As(III) concentration. The amount of removed As(III) did not attain a maximum in the pH range of 5.0–8.0 and within the initial As concentration range of 0–7.5 mg L 1, consistent with the results of adsorption study [19,20] and may be due to the polymerization of removed As(III) at high concentration [23]. As illustrated in Fig. S3, the amount of precipitated iron kept almost constant at various equilibrium As(III) concentrations. However, the growth rate of aggregated flocs was obviously slowed down at high initial As(III) concentration (as shown by the inset in Fig. 2), thereby decreasing the particle size of the flocs and increasing the number of apparent adsorption sites at the beginning of flocculation stage, which facilitated As(III) removal. The largest amounts of As(III) removed by Fe(III) are up to 0.33 mol As/mol TFe at pH 7.0 and 0.23 mol As/ mol TFe at pH 5.0, respectively, which were much smaller than that reported by Raven et al. [19]. The discrepancy should be ascribed to the different initial As/Fe ratios employed in these two studies. Comparing the adsorption isotherms of As(V) and As(III) during coagulation, it was revealed that much more As(V) was removed than As(III) at pH 5.0–6.0 and the difference was greater at lower pH. As(V) could be more effectively removed than As(III) by ferric coagulation at pH 7 and pH 8 when the equilibrium As concentrations were below 2.13 and 0.60 mg L 1, respectively. However, As(III) was removed in larger quantities than As(V) during ferric coagulation at pH 7 and pH 8 when the equilibrium As concentrations were higher than 2.13 and 0.60 mg L 1, respectively. At pH 9, the amount of As(III) removed was always higher than that of As(V) removed throughout the range of arsenic concentration examined in this study (the results were not shown). Raven et al. [19] reported that As(V) adsorbed in equal or larger amounts than As(III) at pH 4.6 when As solution concentration was less than

approximately 1 mol As kg 1 ferrihydrite but As(III) was adsorbed in larger amounts than As(V), even at the lowest As concentrations at pH 9.2, somewhat similar to the behaviors of As(V) and As(III) in coagulation process. Thus, the amount of As(V) or As(III) removed by ferric coagulation was strongly dependent on pH and equilibrium arsenic concentration, i.e., As(V) was more preferentially removed than As(III) at lower pH and lower equilibrium arsenic concentration and the converse was true for As(III). 3.3. Influence of the co-occurring solutes on As(V) and As(III) removal by FeCl3 The presence of co-existing anions and cations may influence the removal of As(V) and As(III) by FeCl3 coagulation and thus affect the crossover pH, where As(V) removal was equivalent to As(III). This study examined the influence of co-occurring anions and cations of environmental relevant concentrations on As(V) or As(III) removal by FeCl3 in the pH range of 6.0–8.0 when the initial As/Fe molar ratio was 0.37 and the results are shown in Figs. 3–5 and Figs. S4–S6. Different from their influence on adsorption of As(V) or As(III) onto various adsorbents, the co-occurring solutes not only compete with As(V) or As(III) for sorption sites but also alter the amount of precipitated iron, which is the effective fraction for arsenic removal, by sequestering or facilitating the precipitation of Fe(III) [9,10,12,26,30]. 3.3.1. Influence of co-occurring anions on As(V) and As(III) removal by FeCl3 Fig. 3 revealed that phosphate had slight effect on As(V) removal but lowered As(III) removal significantly at pH 6, indicating that phosphate competed more strongly with As(III) than with

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PO4 (mg P

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.6 3-

PO4 (mg P

.8

1.0

L-1)

Fig. 3. Influence of phosphate concentration on As(III) or As(V) removal by FeCl3 coagulation: (a) As(III) removal; (b) As(V) removal; (c) amount of precipitated iron in the process of As(III) removal by coagulation; (d) amount of precipitated iron in the process of As(V) removal by coagulation (As/Fe = 0.37).

As(V) since the amount of precipitated iron kept constant at this pH. Martinson and Reddy [31] also reported that phosphate applied at 0–1 mg L 1 decreased As(III) adsorption on cupric oxide nanoparticles to a greater magnitude than As(V) adsorption. As shown in Fig. 3(c), in the FeCl3 coagulation process for As(III) removal, the precipitation of Fe(III) was slightly hindered at pH 7 and considerably sequestered at pH 8 by phosphate when phosphate concentrations were P0.63 and P0.42 mg L 1, respectively, which led to a greater drop in As(III) removal at pH 8 than that at pH 7. However, when As(V) was removed by FeCl3, the amount of precipitated iron was significantly drop due to the presence of phosphate of 0.21–1.04 mg L 1 at pH 7.0–8.0, which resulted in a sharp decrease in As(V) removal at pH 7.0–8.0. The decrease in the amount of precipitated iron should be ascribed to the formation of soluble complexes or highly dispersed colloids, which were not retainable by the 0.45 lm filter paper, between phosphate and Fe(III) [10,12,26,32]. This speculation was confirmed by the determination of As(V) removal and precipitated iron at pH 8 with 0.22 lm filter paper instead of 0.45 lm filter paper, as demonstrated Fig. 3(b and d). The difference in the amount of precipitated iron obtained with 0.45 and 0.22 micron filter papers should be ascribed to the formation of finely dispersed colloids with size of 0.22–0.45 lm. The influence of phosphate on As(V) removal at pH 8 shrunk greatly by decreasing the pore size of filter paper from 0.45 to 0.22 lm since a portion phosphate was removed with the flocs with size ranging from 0.22 to 0.45 lm. This indicates that improving the separation of iron precipitates from water can enhance arsenic removal by FeCl3 coagulation.

Silicate dosed at 4.3–21.6 mg L 1 had negligible influence on As(V) removal but it decreased As(III) removal by 26.6% at pH 6.0 although it did not affect iron precipitation, as demonstrated in Fig. S4. Silicate had minor influence on As(V) removal and lowered As(V) removal considerably at pH 7.0 when its concentrations were 68.6 and P13.0 mg L 1, respectively, ascribed to the drop in the amount of precipitated iron at high silicate concentration. Increasing pH from 7.0 to 8.0 exemplified the reduction in the amount of precipitated iron by the presence of silicate and silicate had greater affinity for iron hydroxide surface at higher pH, thus only 8.0– 18.0% As(V) was removed by FeCl3 in the presence of silicate at pH 8.0. The inhibitory effect of silicate on ferric oxyhydroxide precipitation has been also reported in the literature [10,12,30,33]. As(III) removal was decreased by 34% and 55%, respectively, at pH 7.0 and 8.0 by the presence of 21.6 mg L 1 silicate although the amount of precipitated iron was not affected at pH 7.0 and was decreased by only 15.0% at pH 8.0. Thus, the reduction in As(III) removal caused by silicate was primarily due to the competition of silicate with As(III) for iron hydroxide surface sites. Humic acid decreased As(III) removal by FeCl3 to a greater extent at higher pH and the increase in humic acid concentration also resulted in a more significant reduction in As(III) removal, as illustrated in Fig. S5, which should be mainly associated with the reduction in the amount of precipitated iron as well as the competition of HA with As(III) for the adsorption sites [17]. As(V) removal and the amount of precipitated Fe(III) at pH 6.0 experienced a sharp decrease as HA concentration increased from 4.16 to 6.24 mg L 1. However, the presence of HA as low as 0.2 mg L 1

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Fig. 4. Influence of Ca2+ on As(V) or As(III) removal as functions of Ca2+ concentration and pH levels: (a) As(III) removal; (b) As(V) removal; (c) amount of precipitated iron in the process of As(III) removal by coagulation; (d) amount of precipitated iron in the process of As(V) removal by coagulation (As/Fe = 0.37).

resulted in a sharp drop in As(V) removal and the amount of precipitated Fe(III) at pH 8.0, mainly ascribed to the decrease in the amount of precipitated Fe(III). Less than 16.5% of the applied Fe(III) was precipitated and As(V) removal efficiency was below 9.0% at pH 7.0–8.0 when HA was over 2.0 mg L 1. Fig. S6 shows that the influence of sulfate on As(III) removal by FeCl3 at pH 6.0–8.0 was negligible but As(V) removal was decreased from 98.4% to 93.6%, from 82.3% to 38.8%, and from 45.4% to 12.3% at pH 6.0, 7.0, and 8.0, respectively, as sulfate concentration increased from 0 to 40 mg L 1. The decrease in As(V) removal caused by the presence of sulfate was mainly ascribed to the reduction in the amount of precipitated iron, as illustrated in Fig. S6(d). Moreover, the competition between sulfate and As(V) for iron hydroxide surface sites also contributed to the reduction in As(V) removal since the decrease in As(V) removal caused by the presence of sulfate was greater than the reduction in the amount of precipitated iron. Meng et al. [10] reported that the removal of As(V) and As(III) was almost not affected in a pH range of 4–10 and Jain et al. [34] also indicated that the presence of 1– 50 mg L 1 sulfate had little impact on As(III) removal by Fe(VI)/ Al(III). Our previous study [26] found that the presence of sulfate had negligible effect on As(III) removal by KMnO4/Fe(II) at pH of 4–5 while As(V) removal was decreased by 6.5–36.0% over pH 6– 9 by the presence of 50 mg L 1 SO24 . The different influences of sulfate on As(V) removal in different studies may be ascribed to the different As(V)/coagulant molar ratios. A much higher As(V)/ coagulant molar ratio was employed in this study compared to those in other studies.

The influence of various ligands on As(III) and As(V) removal is mainly determined by the effect of ligands on iron precipitation and the affinity of ligands for iron hydroxide surface compared to As(III) or As(V). Generally, As(V) removal is less affected by the presence of ligands than As(III) under acidic conditions and low ligands concentration when the reduction in As(III) and As(V) removal is mainly determined by competition between ligands and As(III)/ As(V) for surface sites. At pH 7.0–8.0, the drop in As(V) removal caused by the presence of anions was predominantly associated with the reduction in the amount of precipitated iron and secondly associated with the competition of the anions for surface sites. 3.3.2. Influence of co-occurring Ca2+ on As(V) and As(III) removal by FeCl3 The influence of Ca2+ on As(III) and As(V) removal by FeCl3 was very different, as illustrated in Fig. 4. The presence of Ca2+ of 5.2 mg L 1 lowered As(III) removal by 13% over the pH range of 6.0–8.0 and further increase in Ca2+ concentration had no influence on As(III) removal. However, the reason is unknown at this moment. The application of Ca2+ did not affect As(V) removal at pH 6.0 but it increased As(V) removal by 8% and 39%, respectively, at pH 7.0 and 8.0. Although Ca2+ had similar influence on As(V) removal in adsorption process as that in coagulation process, the mechanisms are different. Enhanced As(V) adsorption by applying Ca2+ under neutral and alkaline conditions was mainly ascribed to the reduced repulsive potential between the negatively charged adsorbent surface and the negatively charged As(V) species [35]. However, the improvement in As(V) removal at pH P7.0 due to

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Arsenic removal (%)

100

80

60

40 HA=0 mg L-1 HA=0.2 mg L-1 HA=2.0 mg L-1

20

0 0

20

40

60

80

b

a 0

100

20

40

60

80

100

Precipitated iron (%)

100 80 60 40 20

c

0 0

20

40

60

80

Ca2+ (mg L-1)

100

d 0

20

40

60

80

100

Ca2+ (mg L-1)

Fig. 5. Influence of Ca2+ concentration on As(V) or As(III) removal at pH 8.0 in the presence of humic acid: (a) As(III) removal; (b) As(V) removal; (c) amount of precipitated iron in the process of As(III) removal by coagulation; (d) amount of precipitated iron in the process of As(V) removal by coagulation (As/Fe = 0.37).

the application of Ca2+ was primarily associated with the enhancement in the amount of precipitated iron at low Ca2+ concentration and secondly with the decreased repulsive potential between the negatively charged flocs and the negatively charged As(V) species at high Ca2+ concentration. The improvement of As(V) removal in coagulation process by the presence of Ca2+ under neutral and alkaline conditions had been reported by many researchers [6,9]. 3.3.3. Influence of Ca2+ on As(V) and As(III) removal by FeCl3 in the presence of humic acid Ca2+ could also partially counteracted the adverse impact of anions on arsenic removal in FeCl3 coagulation under neutral and alkaline conditions. The influences of Ca2+ on arsenic removal by FeCl3 coagulation in the presence of HA were examined and demonstrated in Fig. 5. The application of Ca2+ had almost no effect on As(III) removal in the presence of 0.2 mg L 1 HA. However, As(III) removal in the presence of 2.1 mg L 1 HA was enhanced from 20.6% to 50.1% by the application Ca2+, which was mainly ascribed to the increase in the amount of precipitated iron. Fig. 5(b) showed that the application of Ca2+ improved As(V) removal by FeCl3 coagulation to a greater extent at pH 8.0 in the presence of HA of higher concentration, which corresponded well with the variation of the amount of precipitated iron. Previous studies have reported that Ca2+ could reduce the negative effect of anions on arsenic removal in coagulation process under neutral and alkaline conditions by facilitating the agglomeration of ferric hydroxide flocs and its influence was greater at higher pH [36,37]. As(V) removal and the amount of precipitated iron experienced sharp improvements as Ca2+ concentration was increased from 0 to 5.2 and from 5.2 to 10.4 mg L 1 in the presence of 0.2 and 2.0 mg L 1 HA, respectively, indicating that higher Ca2+ concentration was necessary to counteract the adverse influence of ligands of higher concentration on As(V) removal by coagulation. The combined influence of Ca2+

and HA indicates that simultaneous application of FeCl3 and Ca2+ under alkaline conditions is beneficial to As(V) removal, especially in the presence of competing anions. 3.3.4. Influence of co-occurring solutes on crossover pH Because of different influences of co-occurring solutes on As(V) and As(III) removal by FeCl3, the crossover pH was affected by the presence of co-occurring solutes, as illustrated in Fig. 6 and Figs. S7–S10. The presence of phosphate progressively decreased the crossover pH from 7.7 in the absence of phosphate to 7.2 with phosphate dosed at 0.4 mg L 1, as shown in Fig. S7. Dixit and Hering [16] also reported that the presence of 3.1 mg L 1 phosphate decreased the crossover pH from 8.5 (with HFO and 10 lM total As) or 8.0 (with goethite and 25 lM total As) to about 7.5 (for HFO) or 7.0 (for goethite) due to its competition with As(V) for sorption sites. The crossover pH shifted to 7.9 as phosphate concentration was increased to 0.6 mg L 1 and further increase in phosphate concentration resulted in no crossover pH since phosphate at P0.6 mg L 1 induced a greater reduction in As(III) removal than in As(V) removal in the pH range of 6.0–8.0. The presence of silicate dosed at 4.3–21.6 mg L 1 had no much influence the crossover pH. The crossover pH shifted significantly from 7.7 to 6.8 due to the presence of 2.1 mg L 1 HA but no crossover pH was observed at higher concentration of HA. As demonstrated in Fig. 6, increasing sulfate concentration from 0 to 20 mg L 1 lowered crossover pH from 7.7 to 6.8 and further increase in sulfate concentration had little influence on crossover pH, indicating that the presence of sulfate broadened the pH range where As(III) removal was more efficient than As(V) removal. As(V) removal was always superior to As(III) removal in the presence of Ca2+ and no crossover pH was observed, as demonstrated in Fig. S10. The combined influence of HA and Ca2+ revealed that the As(V) and As(III) removal edges crossed at certain pH below 8.0 in the presence of

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100

As(V) As(III)

As(V) As(III)

As(V) As(III)

80 60 40 20 2-

SO4 =0 mg L 0 5.5 6.0 6.5

2-

-1

SO4 =5 mg L 7.0

7.5

100

8.0

8.5 5.5

6.0

2-

-1

6.5

SO4 =10 mg L 7.0

7.5

8.5 5.5

6.0

6.5

7.0

7.5

8.0

8.5

As(V) As(III)

As(V) As(III)

As(V) As(III)

80

8.0

-1

60 40 20 2-

2-

-1

SO4 =20 mg L 0 5.5 6.0 6.5 7.0

SO4 =30 mg L 7.5

8.0

8.5 5.5

6.0

6.5

2-

-1

SO4 =40 mg L

7.0

7.5

8.0

8.5 5.5

pH

pH

6.0

6.5

-1

7.0

7.5

8.0

8.5

pH

Fig. 6. Influence of sulfate on As(V) or As(III) removal as functions of sulfate concentration and pH levels (As/Fe = 0.37).

Table 1 Influences of coexisting ions on crossover pH. 1

Co-existing ion

Concentration (mg L

Phosphate

0.2 0.4 0.6 0.8 1.0 4.3 8.6 13.0 17.3 21.6 2.1 4.1 6.0 8.1 10.1 5

7.6 7.2 7.9 N.A. N.A. 7.7 7.8 7.7 7.7 8.0 6.8 N.A. N.A. N.A. N.A. 7.5

10 20 30 40 5.2 21.0 42.9 62.9 83.8

7.2 6.8 6.9 6.9 N.A. N.A. N.A. N.A. N.A.

Silicate

Humic acid

SO24

Ca2+

)

Crossover pH

2.1 mg L 1 HA and 5.2 mg L 1 Ca2+ but As(V) removal was always superior to As(III) removal as long as Ca2+ concentration was P10.4 mg L 1. In sum, the crossover pH shifts to lower values in the presence of phosphate of 0.2–0.4 mg L 1, sulfate of 5–40 mg L 1 or HA of 2.1 mg L 1. The crossover pH for different cases was summarized in Table 1. 4. Conclusions and environmental implications The data presented in this study clearly show that the oftenstated generalization that As(III) is much more difficult to be removed than As(V) and should be oxidized to As(V) before coagulation is too simplistic, when FeCl3 is employed as coagulant. As(III) is removed in larger quantities than As(V) during ferric coagulation at pH 7 and pH 8 when the equilibrium As concentra-

tions are higher than 2.13 and 0.60 mg L 1, respectively. The pH range where As(III) is removed preferentially by FeCl3 to As(V) is broader at larger initial As/Fe molar ratios. In the presence of phosphate of 0.2–0.4 mg L 1, sulfate of 5–40 mg L 1 or HA of 2.1 mg L 1, the crossover pH shifts to lower values, indicating that As(III) was removed to a greater extent by FeCl3 than As(V) over a broader pH range. Therefore, whether As(III) should be oxidized to As(V) before removal by FeCl3 coagulation depends on pH, As/Fe molar ratio, equilibrium arsenic concentration and co-occurring solutes. Since much more As(V) is removed than As(III) at pH 5.0 and As(V) removal is more resistant to the influence of co-existing solutes under acidic conditions, it is recommended that As(III) be converted to As(V) and then removed by coagulation at pH 5.0 to achieve maximum removal of arsenic by FeCl3. Different from their influence on adsorption of As(V) or As(III) onto various adsorbents, the co-occurring ligands not only compete with As(V) or As(III) for sorption sites but also sequester iron precipitation by forming finely dispersed colloids. Therefore, some measures, including application of high valence cations or sludge recycling or microfiltration to replace the traditional sand filtration, should be taken to enhance iron precipitation or the separation of finely dispersed colloids from water to improve arsenic removal by FeCl3 coagulation in the presence of competing anions. Acknowledgements This work was supported by the National Natural Science Foundation of China (50908060), the State Key Laboratory of Pollution Control and Resources Reuse (PCRRY11001) and the Fundamental Research Funds for the Central Universities (0400219206). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2012.03. 023. References [1] P.L. Smedley, D.G. Kinniburgh, A review of the source, behaviour and distribution of arsenic in natural waters, Appl. Geochem. 17 (2002) 517–568. [2] B.K. Mandal, K.T. Suzuki, Arsenic round the world: a review, Talanta 58 (2002) 201–235.

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[3] M.L. Pierce, C.B. Moore, Adsorption of arsenite on amorphous iron hydroxide from dilute aqueous solution, Environ. Sci. Technol. 14 (1980) 214–216. [4] A.K. Gupta, D. Deva, A. Sharma, N. Verma, Fe-grown carbon nanofibers for removal of arsenic(V) in wastewater, Ind. Eng. Chem. Res. 49 (2010) 7074– 7084. [5] J.G. Hering, P.Y. Chen, J.A. Wilkie, M. Elimelech, Arsenic removal from drinking water during coagulation, J. Environ. Eng. – ASCE 123 (1997) 800–807. [6] J.G. Hering, P.Y. Chen, J.A. Wilkie, M. Elimelech, S. Liang, Arsenic removal by ferric chloride, J. Am. Water Works Assn. 88 (1996) 155–167. [7] D. Lakshmanan, D. Clifford, G. Samanta, Arsenic removal by coagulation – with aluminum, iron, titanium, and zirconium, J. Am. Water Works Assn. 100 (2008) 76–88. [8] J. Gregor, Arsenic removal during conventional aluminium-based drinkingwater treatment, Water Res. 35 (2001) 1659–1664. [9] X.H. Guan, J. Ma, H.R. Dong, L. Jiang, Removal of arsenic from water: effect of calcium ions on As(III) removal in the KMnO4-Fe(II) process, Water Res. 43 (2009) 5119–5128. [10] X.G. Meng, S. Bang, G.P. Korfiatis, Effects of silicate, sulfate, and carbonate on arsenic removal by ferric chloride, Water Res. 34 (2000) 1255–1261. [11] M. Edwards, Chemistry of arsenic removal during coagulation and Fe–Mn oxidation, J. Am. Water Works Assn. 86 (1994) 64–78. [12] D. Laky, I. Licsko, Arsenic removal by ferric-chloride coagulation – effect of phosphate, bicarbonate and silicate, Water Sci. Technol. 64 (2011) 1046– 1055. [13] K.L. Mercer, J.E. Tobiason, Removal of arsenic from high ionic strength solutions: effects of ionic strength, pH, and preformed versus in situ formed HFO, Environ. Sci. Technol. 42 (2008) 3797–3802. [14] Y. Lee, I.H. Um, J. Yoon, Arsenic(III) oxidation by iron(VI) (ferrate) and subsequent removal of arsenic(V) by iron(III) coagulation, Environ. Sci. Technol. 37 (2003) 5750–5756. [15] X. Zhao, B. Zhang, H. Liu, J. Qu, Removal of arsenite by simultaneous electrooxidation and electro-coagulation process, J. Hazard. Mater. 184 (2010) 472– 476. [16] S. Dixit, J.G. Hering, Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenic mobility, Environ. Sci. Technol. 37 (2003) 4182–4189. [17] M. Grafe, M.J. Eick, P.R. Grossl, Adsorption of arsenate (V) and arsenite (III) on goethite in the presence and absence of dissolved organic carbon, Soil Sci. Soc. Am. J. 65 (2001) 1680–1687. [18] M. Grafe, M.J. Eick, P.R. Grossl, A.M. Saunders, Adsorption of arsenate and arsenite on ferrihydrite in the presence and absence of dissolved organic carbon, J. Environ. Qual. 31 (2002) 1115–1123. [19] K.P. Raven, A. Jain, R.H. Loeppert, Arsenite and arsenate adsorption on ferrihydrite: kinetics, equilibrium, and adsorption envelopes, Environ. Sci. Technol. 32 (1998) 344–349. [20] E.D. Burton, R.T. Bush, S.G. Johnston, K.M. Watling, R.K. Hocking, L.A. Sullivan, G.K. Parker, Sorption of arsenic(V) and arsenic(III) to schwertmannite, Environ. Sci. Technol. 43 (2009) 9202–9207.

[21] V.K. Sharma, M. Sohn, Aquatic arsenic: toxicity, speciation, transformations, and remediation, Environ. Int. 35 (2009) 743–759. [22] H.K. Hansen, L.M. Ottosen, Removal of arsenic from wastewaters by airlift electrocoagulation: part 3: copper smelter wastewater treatment, Sep. Sci. Technol. 45 (2010) 1326–1330. [23] Jain, R.H. Loeppert, Effect of competing anions on the adsorption of arsenate and arsenite by ferrihydrite, J. Environ. Qual. 29 (2000) 1422–1430. [24] T. Radu, J.L. Subacz, J.M. Phillippi, M.O. Barnett, Effects of dissolved carbonate on arsenic adsorption and mobility, Environ. Sci. Technol. 39 (2005) 7875– 7882. [25] Y.X. Zhao, B.Y. Gao, H.K. Shon, B.C. Cao, J.H. Kim, Coagulation characteristics of titanium (Ti) salt coagulant compared with aluminum (Al) and iron (Fe) salts, J. Hazard. Mater. 185 (2011) 1536–1542. [26] X.H. Guan, J.M. Wang, C.C. Chusuei, Removal of arsenic from water using granular ferric hydroxide: macroscopic and microscopic studies, J. Hazard. Mater. 156 (2008) 178–185. [27] X.H. Guan, H.R. Dong, J. Ma, L. Jiang, Removal of arsenic from water: effects of competing anions on As(III) removal in KMnO4-Fe(II) process, Water Res. 43 (2009) 3891–3899. [28] L.C. Roberts, S.J. Hug, T. Ruettimann, M. Billah, A.W. Khan, M.T. Rahman, Arsenic removal with iron(II) and iron(III) waters with high silicate and phosphate concentrations, Environ. Sci. Technol. 38 (2004) 307–315. [29] V. Pallier, G. Feuillade-Cathalifaud, B. Serpaud, Influence of organic matter on arsenic removal by continuous flow electrocoagulation treatment of weakly mineralized waters, Chemosphere 83 (2011) 21–28. [30] R. Liu, J. Qu, S. Xia, G. Zhang, G. Li, Silicate hindering in situ formed ferric hydroxide precipitation: inhibiting arsenic removal from water, Environ. Eng. Sci. 24 (2007) 707–715. [31] C.A. Martinson, K.J. Reddy, Adsorption of arsenic(III) and arsenic(V) by cupric oxide nanoparticles, J. Colloid Interface Sci. 336 (2009) 406–411. [32] X.H. Guan, H.R. Dong, J. Ma, Influence of phosphate, humic acid and silicate on the transformation of chromate by Fe(II) under suboxic conditions, Sep. Purif. Technol. 78 (2011) 253–260. [33] M.C. Ciardelli, H. Xu, N. Sahai, Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by coprecipitation in synthetic and natural groundwater, Water Res. 42 (2008) 615–624. [34] Jain, V.K. Sharma, O.S. Mbuya, Removal of arsenite by Fe(VI), Fe(VI)/Fe(III), and Fe(VI)/Al(III) salts: effect of pH and anions, J. Hazard. Mater. 169 (2009) 339– 344. [35] Y. Masue, R.H. Loeppert, T.A. Kramer, Arsenate and arsenite adsorption and desorption behavior on coprecipitated aluminum:iron hydroxides, Environ. Sci. Technol. 41 (2007) 837–842. [36] R. Liu, X. Li, S. Xia, Y. Yang, R. Wu, G. Li, Calcium-enhanced ferric hydroxide coprecipitation of arsenic in the presence of silicate, Water Environ. Res. 79 (2007) 2260–2264. [37] H.R. Dong, X.H. Guan, D.S. Wang, J. Ma, Individual and combined influence of calcium and anions on simultaneous removal of chromate and arsenate by Fe(II) under suboxic conditions, Sep. Purif. Technol. 80 (2011) 284–292.