Toxicology Letters 133 (2002) 103– 111 www.elsevier.com/locate/toxlet Combined effects of anions on arsenic removal by iron hydroxides Xiaoguang Meng a,*, George P. Korfiatis, Sunbaek Bang, Ki Woong Bang b a Center for En6ironmental Engineering, Ste6ens Institute of Technology, Hoboken, NJ 07030, USA b Department of En6ironmental Engineering, Hanbat National Uni6ersity, Taejon, South Korea Abstract Batch experiments were conducted to investigate the combined effects of phosphate, silicate, and bicarbonate on the removal of arsenic from Bangladesh groundwater (BGW) and simulated groundwater by iron hydroxides. The apparent adsorption constants indicated that the affinity of the anions for iron hydroxide sites decreased in the following order arsenate \phosphate\ arsenite\silicate\bicarbonate. Phosphate, silicate, and bicarbonate decreased the removal of As(III) even at relatively low concentrations and low surface site coverage. Phosphate (0 – 0.08 mM), silicate (0–0.8 mM), and bicarbonate (0–14 mM) in separate solutions had none to moderate effects on As(V) removal in a solution containing 6.7 mg/l Fe and 0.3 ppm As(V). In the presence of bicarbonate and silicate the adverse effect of phosphate on As(V) adsorption was magnified. The residual As(V) concentration after iron hydroxide treatment increased from less than 13 mg/l in separate bicarbonate (2.2 mM) and phosphate (0.062 mM) solutions to 110 mg/l in the solution containing both anions. The results suggested the combined effects of phosphate, silicate, and bicarbonate caused the high mobility of arsenic in Bangladesh water. © 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Arsenic; Iron hydroxides; Adsorption; Bangladesh; Phosphate; Silicate; Bicarbonate; Anion effects; Selectivity 1. Introduction Arsenate [As(V)] and arsenite [As(III)] are common arsenic species in naturally contaminated groundwater and surface water in many countries. Millions of wells are drilled into Ganges alluvial deposits for public water supply in Bangladesh and West Bengal (Nickson et al., 1998; Das et al., 1996). The release of arsenic from the arsenic* Corresponding author. Tel.: +1-201-216-8014; fax: + 1201-216-8303. E-mail address: [email protected] (X. Meng). bearing aquifer sediments may have polluted more than 3 million of the approximately 5 million existing wells in Bangladesh, affecting upto 70 million people (Lepkowski, 1999). The mobility of arsenic in groundwater sediment and water treatment sludge is governed by the redox potential (Meng et al., 2001b; Nickson et al., 1998). Arsenic is typically associated with iron oxides under oxic environment and with pyrite minerals under anoxic conditions. Arsenic can be released as the results of pyrite mineral oxidation or reduction of iron oxides in an intermediate redox range (i.e. − 4B peB 0) (Meng et al., 2001b). 0378-4274/02/$ - see front matter © 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 3 7 8 - 4 2 7 4 ( 0 2 ) 0 0 0 8 0 - 2 104 X. Meng et al. / Toxicology Letters 133 (2002) 103–111 Ferric chloride and sulfate are commonly used for the removal of arsenic from water because iron hydroxides have high removal capacity for arsenic (Gulledge and O’Conner, 1973; Cheng et al., 1994; McNeill and Edwards, 1995). The chemical composition of water such as phosphate and silicate concentrations can significantly affect the removal of arsenic by iron hydroxides. Silicate decreases the removal of As(III) and As(V) in potassium nitrate solutions by coprecipitation with ferric chloride (Meng et al., 2000). Phosphate enhances the mobility of As(V) in soils contaminated with lead arsenate (Peryea and Kammereck, 1997). The removal of SO24 − , SeO23 − , PO34 − , and CrO24 − by iron hydroxides was affected adversely by silicate (Meng and Letterman, 1996; Goldberg, 1985; Zachara et al., 1987). Recent experimental results have shown that arsenic in Bangladesh well groundwater is difficult to remove by iron hydroxides because of elevated phosphate and silicate concentrations (Meng et al., 2001a). An iron to arsenic mass ratio of greater than 40 is required to remove arsenic to less than 50 mg/l which is the current drinking water standard in Bangladesh. On the other hand, a Fe/As ratio of less than 12 is sufficient to remove greater than 99% of arsenic from groundwater collected from New Hampshire (NH), USA. In the present study, batch coagulation tests were conducted with Bangladesh well water and simulated groundwater to evaluate the combined effects of phosphate, silicate, and bicarbonate on the removal of As(V) and As(III) by iron hydroxides. The results reported in the present work will benefit the design of more effective arsenic treatment processes and the development of more accurate models for predicting the transport of arsenic in aquifers. 2. Experimental methods All chemicals used in the experiments were reagent grade. A Fe(III) stock solution containing 2000 mg/l Fe(III) and 0.1% HCl was prepared from FeCl3 · 6H2O (Fisher, Pittsburgh, PA) and trace metal grade HCl (Fisher). As2O5 · 3H2O (Aldrich, Milwaukee, WI) was dissolved in dis- tilled-deionized (DI) water to prepare a primary stock solution containing 1000 mg/l As(V). A secondary stock solution of 10 mg/l As(V) was prepared every week by dilution of the primary stock solution with DI water. NaAsO2 (Fisher) stock solution containing 1000 mg/l As(III) was prepared every 2 weeks. Simulated Bangladesh groundwater (BGW) containing 0.82 mM MgCl2, 2.5 mM CaCl2, and different amounts of Na2SiO3 · 5H2O (0– 0.8 mM), NaH2PO4 (0–0.08 mM), NaHCO3 (0–14 mM), and NaCl (0 or 2.2 mM) were used to test the effects of phosphate, silicate, and bicarbonate on arsenic removal. The As(III) or As(V) stock solution was added to the water in 1 l beakers to reach an arsenic concentration of 300 mg/l in the batch experiments. Then ferric chloride solution was added to the beakers to reach an iron concentration of 6.7 mg/l. The solution pH was controlled at 6.8 by addition of NaOH or HCl. After 30 min of mixing the suspension was filtered through a 0.45 mm membrane filter for the analysis of soluble arsenic, iron, phosphate, and silicate. Removal of arsenic from Bangladesh well waters was tested by oxidizing ferrous ion and As(III) through aeration or addition of sodium hypochlorite solution and subsequent filtration. Water samples were collected from four tube wells in Kishoreganj and Munshiganj districts of Bangladesh. The samples were aerated and stored in 125 ml polypropylene bottles without acidification. After the samples were oxidized by exposure to air for 10 days, they were passed through a 0.45 mm membrane filter to remove the ferric hydroxide precipitate. In another set of field experiments, sodium hypochlorite solution was added to the water samples to oxidize As(III) and Fe(II). A residual chlorine content of approximately 1 mg/l was maintained by the addition of the hypochlorite solution during approximately 5 min of mixing. After the samples were mixed for 30 min, they were filtered through 0.4 mm membrane filter. Similar arsenic removal was obtained when As(III) and Fe(II) were completely oxidized by air and hypochlorite. Groundwater collected from a well in NH was used to represent groundwater with low phosphate concentration. The water sample contained X. Meng et al. / Toxicology Letters 133 (2002) 103–111 105 Table 1 Chemical composition of the waters used in coprecipitation tests Watersa As (mg/l) Fe (mg/l) P (mg/l) Si (mg/l) Na (mg/l) Ca (mg/l) Mg (mg/l) BGW NH SB 280–587 70 300 5.5–7.8 0.7 6.7 1.6–2.7 0.02 0–2.5 14–20 6.6 0–22 15–78 13 50 65–151 16 100 14–42 2.9 20 a BGW, Bangladesh well water; NH, New Hampshire well water; SB, Simulated Bangladesh water. approximately 70 mg/l As(V) and 0.7 mg/l iron (Table 1), respectively. As(III) and Fe(II) were added into the NH samples to reach similar total As and Fe concentrations as in the Bangladesh water samples (BGW). The water samples was treated with sodium hypochlorite and then filtered to removal particulate arsenic with the same procedures as those used for the treatment of BGW. Arsenic and iron concentrations were determined using a Furnace Atomic Absorption Spectrometer (FAAS) (Varian SpectrAA-400) and inductively coupled plasma (ICP) emission spectrometer (Varian Liberty-200). As(V) and As(III) in water samples were separated using arsenic speciation cartridges (Meng et al., 2001b) immediately after the samples were collected. Soluble phosphate and silicate concentrations were determined by the ascorbic acid and heteropoly blue methods, respectively (Clesceri et al., 1989). much higher than the current maximum contaminant level of 50 mg/l arsenic for drinking water in Bangladesh. Speciation analysis of the soluble arsenic showed that As(III) was completely oxidized to As(V). The removal of iron was greater than 99%, indicating a complete conversion of ferrous ions to ferric hydroxide. Arsenic removal by coprecipitation with iron hydroxide from spiked NH water samples was more efficient than from Bangladesh water (Fig. 1). When As(III) and Fe(II) was added into the NH water to reach the same total arsenic and iron concentrations as in Bangladesh well 1 water, arsenic concentration was reduced from 587 to 15 mg/l by oxidation and filtration. In contrast, the arsenic in the treated well 1 water sample was only reduced to 187 mg/l. The final pH of the NH water samples was controlled at similar values to that of the well water samples. The results suggested that some coexisting solutes in Bangladesh waters adversely affected the removal of arsenic. 3. Results and discussion 3.1. Effects of anions on As remo6al from Bangladesh water The removal of arsenic from BGW and spiked NH groundwater is shown diagrammatically in Fig. 1. Total arsenic and iron concentrations in the well waters ranged from 280 to 587 mg/l and 5.5 to 7.8 mg/l, respectively. Approximately, 86% of the arsenic in the well waters was As(III). Greater than 95% of the Fe was in soluble form. The pH of the well water was approximately 6.9. After the water samples were oxidized and passed through the membrane filter, 51– 75% of the arsenic was removed. The residual arsenic concentrations ranged from 100 to 187 mg/l, which was Fig. 1. Removal of arsenic from BGW and spiked NH water samples by ferric hydroxides at equilibrium pH 7.0 90.2. Total Fe: 7.7 mg/l in well 1 water; 6.7 mg/l in well 2; 5.5 mg/l for well 3; and 7.8 mg/l for well 4. 106 X. Meng et al. / Toxicology Letters 133 (2002) 103–111 Fig. 2. Effects of anions on As(V) and As(III) removal from SBW (2.5 mM CaCl2, 0.82 mM MgCl2, and 2.2 mM NaCl) containing single anions. Initial As = 300 mg/l; total Fe(III) = 6.7 mg/l; equilibrium pH 6.9 9 0.1. The concentrations of selected anions and cations are listed in Table 1 for Bangladesh and NH water samples. Both cation and anion concentrations in BGW were obviously higher than in NH water. Phosphate concentration in Bangladesh water was approximately two orders of magnitude higher than in NH water. Silicate concentration in Bangladesh water was two to three times higher than that in NH water. The effects of the anions on the removal of arsenic were evaluated using simulated Bangladesh water (SB) containing the same arsenic, iron, and cation concentrations as Bangladesh well 2 water (Table 1). Bicarbonate, silicate, and phosphate were added into the SB water samples separately. The effect of sulfate was not investigated since its concentration was less than 3 mg/l in most of Bangladesh well waters (2). As(V) and As(III) (in separate solutions) were effectively removed to 2 and 16 mg/l, respectively, by iron hydroxides from the NaCl solution (Fig. 2). In the bicarbonate, silicate, and phosphate solutions the residual As(III) concentration was 79, 156, and 186 mg/l, respectively. The adverse effects of the anions on As(III) removal decreased in the following order: phosphate\ silicate\ bicarbonate\ chloride. The chloride and bicarbonate concentrations were much higher than silicate concentration in the water. The phosphate concentration was lower than silicate concentration by an order of magnitude. It should be noted that phosphate had only a slight effect on the removal of As(V). Bicarbonate and silicate had none and moderate effects on As(V) removal, respectively. None of the anions inhibited As(V) removal to the extent observed in Bangladesh well waters (Fig. 1). The results in Fig. 3 illustrate the combined effects of the anions on arsenic removal. The concentrations of arsenic, iron, and the anions were the same as in the solutions shown in Fig. 2 except that combinations of the three types of anions were used in the solutions. The residual As(III) concentrations in the multi-anion solutions (Fig. 3) were slightly higher than in the single anion solutions (Fig. 2). For instance, the residual As(III) concentration was increased from 156 mg/l in silicate solution (Fig. 2) to 165 mg/l in a solution containing both silicate and bicarbonate (Si+HCO3, Fig. 3). The residual As(III) concentration was increased from 186 mg/l in the phosphate solution to 221 mg/l in the phosphate– silicate–bicarbonate solution. As seen from Fig. 3, the residual As(V) was dramatically increased in the multi-anion systems except for the bicarbonate–silicate solution. The residual As(V) in the bicarbonate–silicate solution was 13 mg/l, which was lower than that in the silicate solution (Fig. 2). This behavior was not expected since the addition of bicarbonate to the Fig. 3. Combined effects of anions on As(V) and As(III) removal from SBW (2.5 mM CaCl2, 0.82 mM MgCl2, and 2.2 mM NaCl). Initial As =300 mg/l; total Fe(III) =6.7 mg/l; equilibrium pH 6.9 90.1. X. Meng et al. / Toxicology Letters 133 (2002) 103–111 Fig. 4. Removal of As(V) as a function of bicarbonate and silicate concentrations. Initial As(V) = 300 mg/l, Fe(III) = 6.7 mg/l, MgCl2 = 0.82 mM, CaCl2 = 2.5 mM, 2.2 mM NaCl, equilibrium pH 6.8. silicate system should further reduce the amount of surface sites available for As(V) adsorption. All the experimental results were verified through at least three repeated tests. The residual As(V) concentration was increased from less than 13 mg/l in the single phosphate and bicarbonate solutions (Fig. 2) to 111 mg/l in the phosphate– bicarbonate solution (Fig. 3). When silicate and phosphate coexisted, As(V) concentration was also dramatically increased. The residual As(V) concentration in the phosphate– silicate – bicarbonate solution was 124 mg/l. The efficiency of As(V) removal from the multianion solutions (Fig. 3) and from well 2 water (Fig. 1) was similar. The results in Fig. 3 demonstrated that the coexistence of phosphate with silicate and bicarbonate hindered the removal of As(V) from the well waters. Phosphate was a key competing anion affecting the removal of As(V). The effect of phosphate was magnified in the presence of bicarbonate and silicate. 107 affected when silicate concentration was in a range 0–0.6 mM. At high silicate concentration rang, the removal of As(V) was obviously reduced. When silicate concentration increased to approximately 0.7 mM, arsenic removal was reduced from greater than 99% (i.e. residual As: 2 mg/l) to approximately 85% (i.e. residual As: 44 mg/l). The results were consistent with the previously reported data which showed that silicate decreased As(V) removal in KNO3 solution especially at pH greater than 7 (Meng et al., 2000). When silicate and bicarbonate coexisted in the solution, As(V) removal was high than in single silicate solution. The observation agreed with the results shown in Figs. 2 and 3. Additional experiments are required to understand the effects of bicarbonate on As(V) removal in silicate solution. Phosphate had very little effect on the removal of As(V) when its concentration increased from 0 to 70 mM (i.e. 0.07 mM) in the single phosphate solution (Fig. 5). At a phosphate concentration of 80 mM, the removal of As(V) was decreased from 99.4% (residual arsenic As= 1.9 mg/l) to 90.7% (residual As=28 mg/l). In the presence of 2.2 mM HCO− 3 , As(V) removal decreased linearly with the increase of phosphate concentration from 20 to 80 mM. Only 56% of As(V) was removed at a phosphate concentration of 80 mM in the combined solution. When phosphate concentration increased from 0 to 80 mM in a 0.64 mM silicate 3.2. Remo6al of As as a function of anion concentrations The effects of the anions on the arsenic removal were further investigated over a wide concentration range of the anions. When bicarbonate concentration increased from 0 to 13 mM, the removal of As(V) by iron hydroxides was not affected (Fig. 4). The removal of As(V) was not Fig. 5. Removal of As(V) as a function of phosphate concentration in different anion solutions. Initial As(V) = 300 mg/l, Fe(III)= 6.7 mg/l, MgCl2 =0.82 mM, CaCl2 =2.5 mM, 2.2 mM NaCl, equilibrium pH 6.8. 108 X. Meng et al. / Toxicology Letters 133 (2002) 103–111 3.3. Binding affinity of anions and adsorption density The binding affinity of the anions for the surface sites and the adsorption density were determined in order to understand the effects of the competing anions on As(V) and As(III) removal. The binding affinity was determined using adsorption equilibrium constants. The adsorption of anions on iron hydroxide surface can be described by the equation: SOH + Ln − + mH+USHq L(n − 1 − q) − + H2O Fig. 6. Removal of As(III) as a function of bicarbonate, silicate, and phosphate concentrations. Initial As(III) =300 mg/l, Fe(III)=6.7 mg/l, MgCl2 = 0.82 mM, CaCl2 = 2.5 mM, 2.2 mM NaCl, equilibrium pH 6.8. solution, As(V) removal decreased from 85 to 65%. It is obvious that bicarbonate and silicate magnified the effects of phosphate on As(V) removal by ferric hydroxides. Iron hydroxides were less effective in removing As(III) than As(V) at a neutral pH. In the chloride solution, approximately 95% of the As(III) was removed (Fig. 6). The removal of As(III) decreased significantly as bicarbonate and silicate concentrations increased in the separate solutions. When bicarbonate and silicate concentrations were 13 and 0.8 mM, the removal of As(III) was reduced to 64 and 42%, respectively. The effect of phosphate on As(III) removal was more dramatic than bicarbonate and silicate. When phosphate concentration was 0.08 mM, the removal of As(III) was only 24%. (m= 1 to 4; q= m−1) (1) where SOH denotes the hydroxyl sites on iron hydroxide surface, Ln − indicates the anions, and SHq L(n − 1 − q) − represents the adsorbed anion as surface complexes. The equilibrium constant for reaction Eq. (1) can be expressed as K= [SHq L(n − 1 − q) − ] [SOH][Ln − ][H+]m (2) where [ ] indicates the concentration of the aqueous and surface species. The equilibrium constant is affected by electrostatic or Coulombic interaction between the surface potential and the adsorbed ions (Stumm, 1992). At constant pH, an apparent equilibrium constant can be used to describe the adsorption of the anions. K app = [SHq L(n − 1 − q) − ] = [H+]mK [SOH][Ln − ] (3) K app values for the adsorption of the anions by iron hydroxides were determined from the initial and equilibrium anion concentrations and the surface site concentration (Table 2). The initial con- Table 2 Binding affinity of anions for Fe(OH)3 in SB As(V) Total SOH sites (mM) Initial anion concentration (mM) Equilibrium anion concentration (mM) K app Affinity P As(III) Si 0.108 0.108 0.108 0.108 0.010 0.010 0.010 0.010 4.0×10−5 3.0×10−4 2.1×10−3 7.5×10−3 2.5×106 3.4×105 3.8×104 3.1×103 High Equilibrium pH 6.8, 2.5 mM CaCl2, 0.82 mM MgCl2, 2.2 mM NaCl. HCO3 0.108 NA NA NA Low X. Meng et al. / Toxicology Letters 133 (2002) 103–111 109 Table 3 Surface site coverage by adsorbed anions Anions Total concentration (mM) Anion/SOH molar ratio As(V)+P system As(V)+Si As(V)+P+Si As(V) 4.0×10−3 0.037 Site coverage by adsorbed anions (%) P 5.0×10−2 0.46 3.619 0.03 3.2 90.1 2.429 0.13 45.2 90.03 0 28.4 91.3 Si 6.4×10−1 5.9 Total coverage (%) 0 60 922 42 929 49 63 73 Total Fe(III) concentration = 6.7 mg/l (0.108 mM SOH sites); equilibrium pH 6.8; solution composition: 2.5 mM CaCl2, 0.82 mM MgCl2, 2.2 mM NaCl. centration of the anions, except bicarbonate, was 0.010 mM in separate anion solutions. Total Fe(III) concentration and equilibrium pH was 6.7 mg/l (i.e. 0.12 mM) and 6.8, respectively. Based on a surface site density of 0.9 mmol SOH per mmol Fe (Meng and Letterman, 1993), the total surface site concentration in the suspension was 0.108 mM. As(V) adsorption isotherms also showed that the adsorption capacity of ferric hydroxides was greater than 0.6 mmol As(V) per mmol Fe (Meng et al., 2000). Among the anions tested, As(V) had the strongest binding affinity for iron hydroxides (Table 2). The K app value of As(V) was seven times greater than that of phosphate. Therefore, phosphate had only slight effect on the removal of As(V) when its concentration was approximately 0.08 mM (Fig. 5). Hingston (1981) also reported that goethite had higher selectivity for As(V) over phosphate in a neutral pH range. The binding constant of As(V) was 800 times greater than silicate. Silicate had moderate affect on As(V) removal only at high concentrations (i.e. Si\ 0.6 mM, Fig. 4). The affinity of As(III) for iron hydroxides was much weaker than As(V) and phosphate. Therefore, the removal of As(III) was reduced significantly by phosphate, silicate, and bicarbonate (Figs. 2 and 6). The amounts of surface sites occupied by adsorbed anions were determined in suspensions containing the same concentrations of the anions and iron as in well 2 water. The molar ratios of the anions to the surface sites were calculated based on the total anion concentrations and total surface sites in the suspensions (Table 3). The total surface site concentration in the suspension containing 6.7 mg Fe/l was 0.108 mM. According to the anion/SOH molar ratios, if all phosphate (0.05 mM) and As(V) (0.004 mM) were adsorbed on the iron hydroxide surface, they would have occupied 46 and 3.7% of the surface sites, respectively. Analysis of the equilibrium phosphate concentration showed that approximately 98% of the phosphate was removed by iron hydroxides, resulting in a surface site coverage of 45.2%. Therefore, the insignificant effect of phosphate on As(V) removal in the single anion solution (Figs. 1 and 5) could be attributed to low surface site coverage by phosphate and high affinity of As(V) for the surface sites. Silicate concentration in the water was 5.9 times higher than the surface site concentration (Table 3). However, only approximately 10% of the total silicate was removed from the single silicate solution because of weak affinity of silicate for iron hydroxides. The surface sites covered by the adsorbed silicate were approximately 60%. The standard deviation values for the site coverage were calculated from the equilibrium silicate concentrations obtained in repeated experiments. Since the silicate concentrations were much greater than the surface site concentration, small analytical errors for the silicate concentration resulted in large standard deviation in the surface site coverage. When phosphate and silicate coexisted in the solution (As(V)+ P+ Si system, Table 3), the percentage of surface sites occupied by each type of anions decreased due to competitive effects. The total site coverage increased from 49% in single phosphate solution to 73% in the solution con- 110 X. Meng et al. / Toxicology Letters 133 (2002) 103–111 taining both phosphate and silicate. The increased surface site coverage decreased removal of As(V). Phosphate significantly decreased the removal of As(III) even at very low surface site coverage (Fig. 6). As(III) removal decreased from 95 to 73% when phosphate concentration increased from 0 to 0.02 mM (Fig. 6). The phosphate to surface site molar ratio was only 0.18 when phosphate concentration was 0.02 mM. The results suggested that the surface sites on iron hydroxides were not uniform. Phosphate occupied the highly active surface sites because of its higher adsorptive affinity than As(III). Infrared spectroscopy studies revealed that three types of surface OH groups existed on goethite (aFeOOH) (Sun and Doner, 1996). The surface hydroxyl groups of hydrous metal oxides are heterogeneous due to the structural diversity of the crystal faces, and the presence of exposed edges, corners and defects (Kinniburgh and Jackson, 1981). 4. Conclusions Since As(V) had the highest affinity for iron hydroxide surface sites among the anions tested, phosphate and silicate could significantly reduce the removal of As(V) only at high surface site coverage. At normal levels of phosphate in BGW (PB 70 mM), phosphate alone did not have a significant effect on As(V) removal by iron hydroxide. However, the presence of silicate and bicarbonate magnified the effect of phosphate, thus, inhibiting arsenic removal. Phosphate and silicate could substantially reduce the removal of As(III) even at low surface coverage due to low affinity of As(III) for the surface sites. 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