Arsenic (As) that has been accumulated from irrigation groundwater to paddy field soil by sorption has the potential for food contamination by plant uptake and recontamination of the groundwater. This study evaluated the effect of calcium (Ca) and magnesium (Mg) addition on As leaching from paddy field soil collected from the southwest region of Bangladesh. Batch experiments were employed to systematically investigate the role of Ca and Mg addition on the leaching behavior of As under different concentrations of Ca and Mg, pH conditions and anaerobic incubation. Results indicated that As leaching was highly decreased with the increase of Ca and Mg addition, at pH greater than 9.0 and during anaerobic incubation. In contrast, Iron (Fe) leaching was decreased by Ca and Mg addition. Adsorption of Ca and Mg was observed and significant correlation with adsorbed As was obtained in all batches. The probable mechanism was precipitation of As due to the increase in the positive surface charge of the Fe hydroxide solids by Ca and Mg adsorption. This study also indicated that Ca and Mg addition could decrease As leaching even under the presence of phosphorus
Journal of Water and Environment Technology, Vol. 8, No.4, 2010 Address correspondence to Jun Nakajima, Department of Environmental Systems Engineering, Faculty of Science and Engineering, Ritsumeikan University, E-mail: jnt07778@se.ritsumei.ac.jp Received May 7, 2010, Accepted July 21, 2010. - 329 - Effect of Calcium and Magnesium Addition on Arsenic Leaching from Paddy Field Soil of Bangladesh Mohammad Shafiul AZAM*, Md. SHAFIQUZZAMAN*, Jun NAKAJIMA* *Department of Environmental Systems Engineering, Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu 525-8577, Japan ABSTRACT Arsenic (As) that has been accumulated from irrigation groundwater to paddy field soil by sorption has the potential for food contamination by plant uptake and recontamination of the groundwater. This study evaluated the effect of calcium (Ca) and magnesium (Mg) addition on As leaching from paddy field soil collected from the southwest region of Bangladesh. Batch experiments were employed to systematically investigate the role of Ca and Mg addition on the leaching behavior of As under different concentrations of Ca and Mg, pH conditions and anaerobic incubation. Results indicated that As leaching was highly decreased with the increase of Ca and Mg addition, at pH greater than 9.0 and during anaerobic incubation. In contrast, Iron (Fe) leaching was decreased by Ca and Mg addition. Adsorption of Ca and Mg was observed and significant correlation with adsorbed As was obtained in all batches. The probable mechanism was precipitation of As due to the increase in the positive surface charge of the Fe hydroxide solids by Ca and Mg adsorption. This study also indicated that Ca and Mg addition could decrease As leaching even under the presence of phosphorus. Keywords: arsenic, calcium, groundwater contamination, leaching, magnesium, paddy field soil. INTRODUCTION Arsenic (As) has long been recognized as a threat to human health. It is known to cause skin cancer and has also been linked to liver, lung, bladder and kidney cancer (Smith, 1992). Arsenic contamination in groundwater as well as paddy field soil through irrigation is a major concern in Bangladesh and India (Chakraborti et al., 2002). Understanding the leaching behavior of As in paddy field soil is important in evaluating its potential impact on food contamination (Abedin et al., 2002). It is well-known that cations can affect the behavior of anions in environmental systems and vice versa (Stumn, 1992). The most important cations in environmental systems from a quantitative point of view are often calcium (Ca) and magnesium (Mg). These two cations can influence the behavior of important anions, such as As, in a complex manner since both precipitation and adsorption equilibriums are potentially important. Although there have been extensive studies on the general leaching characteristics of As from soil (Masscheleyn et al., 1991; Shaw, 2006), quantification of Ca and Mg effects on As leaching has been less well studied. Smith et al. (2002) investigated the effect of Ca on As sorption in soils and explained that sorption of Ca 2+ lead to increased positive charge of the adsorption surface thereby increasing the anion sorption. Bothe et al. (1999) showed that lime addition to As containing wastes is beneficial in reducing the mobility of dissolved As, through the formation of low solubility calcium arsenate (Ca 3 (AsO 4 ) 2 ). Wang et al. (2008) conducted batch tests to understand the role of Ca on the leaching characteristics of As from coal fly ash and concluded that Ca precipitation Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 330 - played the most important role in reducing As leaching in the experimental pH (2 - 12) range. In our previous study, As leaching experiments were conducted with paddy field soil (Azam et al., 2009). Both batch and column leaching tests were carried out with de- ionized water (simulate the natural conditions of rainfall) and synthetics groundwater (simulate the natural groundwater of Bangladesh). Results showed that As leaching was significantly lower when using synthetics groundwater. It was concluded that the groundwater in Bangladesh containing high amounts of Ca and Mg played an important role in reducing As leaching. A detailed study is needed to clarify underlying mechanisms that control As leaching under different conditions of Ca and Mg addition. The objective of this study was to investigate the effect of Ca and Mg addition on the leaching behavior of As from highly contaminated paddy field soil under different concentrations of Ca and Mg addition, pH conditions and anaerobic incubation. The influence of phosphorus (P) on As release with and without the addition of Ca and Mg was also studied. Accordingly, several batch leaching experiments had been conducted with original As contaminated soil collected from the paddy field of Bangladesh. MATERIALS AND METHODS Soil sample collection and characterizations Surface soil samples (0 - 10 cm) collected from Bagerhat district, southwestern region of Bangladesh, were used in this study. Soil samples collected from a paddy field were air-dried and crushed to pass through a 0.5 mm sieve and stored in airtight polythene bags. The samples were oven dried before every experiment. Important physical and chemical properties, including particle size, pH, organic matter (OM) content and total concentration of major elements, such as As, Fe, Ca, Mg, and P were determined following aqua regia digestion of soils. Synthetic groundwater Synthetic groundwater (GW) was prepared by the dissolution of specific chemicals in de-ionized water (DW). The chemical composition of GW was similar to the main characteristics of Bangladesh groundwater (BGS, 2000) which consisted, commonly, of NH 4 Cl, MgSO 4 ·7H 2 O, NaCl, KH 2 PO 4 , CaCl 2 ·2H 2 O, MnSO 4 ·5H 2 O and NaHCO 3 . In order to clarify the relationship between As leaching and Fe content of the soil sample, As and Fe salts were not included in GW. Batch leaching experiments Several batch leaching experiments were conducted to clarify the effect of Ca and Mg addition on As leaching. In all batches, 1.00 g of soil sample was mixed with 100 mL of GW solution in 100 mL Teflon bottle and was shaken at 140 rpm for 24 h. After shaking, the pH of the mixed liquor was measured and filtered through 1.0 µm filter paper (No. 5C, Advantec, Japan) for the analysis of As, Fe, Ca, Mg and P contents of the filtrate. The amount of Ca and Mg adsorbed per gram of soil was calculated from the difference of Ca and Mg concentration in the initially added Ca and Mg solution and the supernatant equilibrium solution taking into account the amount of Ca and Mg present in solution of the control (no Ca and Mg) experiment. All the experiments were conducted in duplicate. The conditions of the batch experiments performed are listed in Table 1. Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 331 - Table1 - Details of batch experiments Batch Type Ca or Mg Concentration (mg/L) P Concentration (mg/L) Other Conditions Effect of Ca and Mg concentrations 0, 5, 10, 20, 50, 100, 150 and 200 0.9 pH 7.0 Effect of pH 0 and 100 0.9 pH adjusted to 3.0, 5.0, 7.0, 9.0 and 11.0 Effect of P 0 and 100 0 and 2.0 pH 7.0 Leaching in anaerobic incubation 0 and 100 0.9 Glucose addition at 100 mg/L ; pH 7.0 Analytical methods Arsenic standard stock solution (1,000 ppm), HCl (35%), HNO 3 (60%), NH 4 Cl (99.0%), KH 2 PO 4 (99.0%) and NaOH (96%) were purchased from Nacalai Tesque, Inc., Kyoto, Japan. The stock solution of Fe, P, Ca and Mg (1,000 ppm), NaHCO 3 (99.5%), MgSO 4 ·7H 2 O (99.5%), NaCl (99.5%), CaCl 2 ·2H 2 O (99.0 - 103.0%), MnSO 4 .5H 2 O (99.0%) were obtained from Wako Pure Chemical Industries Ltd., Japan. Fresh calibration standards were prepared by diluting the analytical standards in 5% nitric acid. Particle size distribution was measured by the laser diffraction method (Shimadzu, Japan, SALD 3000). Soil pH was determined with 1 : 2 soil/water suspension using pH meter (Horiba, Japan). Oxidation reduction potential (ORP) was measured by UC-23 digital pH/ORP meter (CKC) and converted to Eh. Organic matter was determined by the percentage of weight loss after ignition (600ºC for 1 hr). Arsenic was analyzed by ICP-MS (Hewlett Packard 4500, USA). Cross checking was conducted with high range of Cl - to investigate whether this ion interfered in the As measurement by ICP-MS. Phosphorus was determined by Molybdenum Blue colorimetric method (JIS K 0102, 1993). Determination of Fe, Ca and Mg was done by ICP-AES (SPS 4000, Seiko, Japan). RESULTS AND DISCUSSION Soil Sample Characterization The chemical and physical properties of soil sample are shown in Table 2 indicating that the soil was slightly acidic in nature (pH 6.4) and the OM content was high (7.6%). In addition, the particle size distribution indicates the soil texture as silty sand. The background concentration of total As in the studied soil was 109 µg/g which was higher than the As concentration of irrigation contaminated soil (46 µg/g) in the most affected zone of Bangladesh (Mehrag and Rahman, 2003). High As (250 - 300 µg/L) contaminated groundwater was used for irrigation in which As seemed to be accumulated on the topsoil of the paddy fields. The Fe, Ca, Mg and P content of the soil sample were 43.2 mg/g, 5.98 mg/g, 8.77 mg/g and 0.97 mg/g, respectively (Table 2). Effect of the Addition of Different Concentrations of Ca and Mg Fig. 1(a) clearly shows a decrease in the leaching of As with an increase in the added amount of Ca and Mg. In the absence of Ca and Mg, leached As was 51.1 µg/L. Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 332 - Table 2 - Properties of soil in the studied site Characteristics Content Sand (%) 44.3 Silt (%) 47.8 Clay (%) 7.9 Soil pH 6.4 Organic matter (%) 7.6 As a (µg/g) 109 ± 4 Fe a (mg/g) 43.2 ± 0.9 Ca a (mg/g) 5.98 ± 0.17 Mg a (mg/g) 8.77 ± 0.18 P a (mg/g) 0.97 ±0.01 a average ± standard deviation of 4 samples However, upon addition of 100 mg/L each of Ca and Mg leaching of As decreased to 24.6 and 23.3 µg/L, respectively. Leaching of As was decreased to more than 50%. This effect became less significant upon addition of more than 100 mg/L of Ca and Mg. Fig. 1(b) shows the Fe concentration profiles of leachate with different Ca and Mg additions. Data indicated that leached Fe concentrations decreased with the increase in Ca and Mg addition and it became zero when more than 100 mg/L of Ca and Mg was added. It seemed that the decrease in the soluble Fe concentration was associated with a corresponding decrease in the soluble As concentration with the increase of Ca and Mg addition. Strong correlation was obtained between leachate Fe and As for Ca addition (R 2 = 0.96; p < 0.01) and for Mg addition (R 2 = 0.97; p < 0.01). Leaching of Fe could be inhibited by Ca and Mg addition and Ca 2+ could facilitate the formation of larger Fe (III) hydroxide flocs (Lui et al., 2007) which seemed to result in the decrease of Fe as well as As concentration in the leachate. Fig. 2 shows the adsorbed amount of Ca and Mg with different additions of Ca and Mg. Results indicated that Ca and Mg adsorption increased with the increase in Ca and Mg addition and the amount adsorbed increased up to 50 mg/L. Beyond Ca and Mg addition up to 50 mg/L, no more adsorption was observed (data not shown). Significant correlation was obtained between the adsorbed Ca and As (calculated from the decrease of As in the leachate) (R 2 = 0.87; p = 0.02) and between Mg and As (R 2 = 0.82; p = 0.03). Two hypotheses could be formulated as to why soluble As decreased with the addition of Ca and Mg. The first hypothesis was that As, not bound to solids, reacted with Ca and Mg to form solid Ca and Mg arsenate (Voigt and Brantley, 1996; Bothe et al., 1999; Raposo et al., 2004). The second hypothesis was that the specific sorption of Ca 2+ and Mg 2+ leads to increased positive surface charge. Increasing the valency of the cation (Ca 2+ and Mg 2+ ) makes the potential in the plane of sorption less negative, thereby increasing anion [arsenate (AsO 4 3- )] sorption in soil (Smith et al., 2002). In another study, Parks et al. (2003) showed that Ca arsenate could not be formed at a pH < 11.5 and might form at pH 12 and 12.5. Leachate pH obtained ranged between 7.0 and 8.0 which might not support the first hypothesis. On the other hand, adsorption of Ca and Mg from GW and subsequent decrease in leachate As concentrations supported the second hypothesis. In the case of high concentrations of Ca and Mg added (more than 50 mg/L), As leaching continued to decrease. With no Ca and Mg adsorption it could be Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 333 - 0 0.5 1 1.5 2 0 20 40 60 80 100 120 140 160 180 200 Ca, Mg addition (mg/L) Leachate Fe (mg/L) Ca addition Fe Mg addition Fe 0 10 20 30 40 50 60 0 20 40 60 80 100 120 140 160 180 200 Ca, Mg addition (mg/L) Leachate As (µg/L) Ca addition As Mg addition As (a) (b) 0 0.5 1 1.5 2 0 20 40 60 80 100 120 140 160 180 200 Ca, Mg addition (mg/L) Leachate Fe (mg/L) Ca addition Fe Mg addition Fe 0 10 20 30 40 50 60 0 20 40 60 80 100 120 140 160 180 200 Ca, Mg addition (mg/L) Leachate As (µg/L) Ca addition As Mg addition As (a) (b) Amount of Ca and Mg added (mg/L) Amount of Ca and Mg added (mg/L) Leached As (µg/L) Leached Fe (mg/L) Ca addition Mg addition Ca addition Mg addition Fig. 1 - Concentration profiles of (a) As and (b) Fe in leachate with the addition of Ca and Mg in different concentrations 0 0.5 1 1.5 2 2.5 5102050 Ca, Mg addition (mg/L) Adsorbed Ca, Mg (mg/g) Ca Mg Fig. 2 - Adsorbed amount of Ca and Mg with different concentrations of Ca and Mg added hypothesized that the larger Fe hydroxide flocs formed by Ca 2+ and Mg 2+ could enhance co-precipitation of As with Fe hydroxides. As the batch results indicated that the effect of Ca and Mg addition more than 100 mg/L was less significant we therefore considered that the maximum absorbable concentration of Ca and Mg was 100 mg/L for other batch experiments. Effect of pH Fig. 3(a) shows the leachate As concentrations under different pH conditions with Ca and Mg additions of 0 and 100 mg/L. Adjusted pH of the soil solution was slightly altered after 24 hrs of shaking which was indicated in the figures. Results indicated that in all cases As leaching was relatively low in pH range of 3.5 - 7.1 and then increased when the pH increased. Addition of Ca and Mg significantly lowered As leaching at pH higher than 9.0, and larger difference was observed at pH 10.5. At pH 7.1, the difference of As leaching with and without the addition of 100 mg/L of Ca was 23.0 µg/L while at pH 10.5 it was 103 µg/L. Similar results were observed in another study (Wang et al., 2008). In case of Mg addition at pH 7.1, the difference of Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 334 - 0.0 1.0 2.0 3.0 4.0 3.0 5.0 7.0 9.0 11.0 pH Leachate Fe (mg/L) No addition Ca=100mg/L addition Mg=100mg/L addition 0 50 100 150 200 250 3.0 5.0 7.0 9.0 11.0 pH Leacha te As (µg/L) No addition Ca=100mg/L addition Mg=100mg/L addition (a) (b) Fig. 3- Concentration profiles of (a) As and (b) Fe in leachate as a function of pH with Ca and Mg addition As leaching with and without the addition of 100 mg/L of Mg was 23.0 µg/L while at pH 10.5 it was 140 µg/L. Fig. 3(b) shows that Fe leaching was not similar to As leaching in the alkaline pH range. In all cases, leaching of high concentrations of Fe occurred in acidic condition and it decreased to almost zero with the increase in pH. With an increase in pH to neutral condition, precipitation of Fe as hydroxides occurred. This resulted in the decrease of Fe leaching. Poor correlation was obtained between the concentrations of Fe and As leached upon addition of Ca (R 2 = 0.05) and Mg (R 2 = 0.15). Fig. 4 shows the adsorbed amount of Ca and Mg as a function of pH. Calcium and Mg adsorption was low in the pH range of 3.5 - 8.5 and higher adsorption was observed at pH 10.5. At higher pH, the negative charge of soil increases providing an increased number of exchangeable sites with a higher affinity for divalent cations (Chan et al., 1979). Calcium was adsorbed on the ferric hydroxide surfaces in appreciable amounts at high pH (Smith and Edwards, 2002). Significant correlation was obtained between adsorbed Ca and As (R 2 = 0.98) and Mg and As (R 2 = 0.99). The general leaching behavior exhibited shown in the results as a function of pH, was typical for the adsorption of anionic elements, such as As (Goldberg and Glaubig, 1988; Wang et al., 2008). The increase in As release when pH was greater than 7.1 without the addition of Ca and Mg was mostly caused by a decrease in the protonated surface sites that serve as binding sites for anionic As species. With the addition of Ca and Mg, formation/precipitation of several less soluble Ca-As and Mg-As compounds occurred, especially under high pH conditions when arsenate (AsO 4 3- ) was the dominant aqueous species. Bothe et al. (1999) reported the formation/precipitation of arsenate apatite at the pH range of 9.0 - 12.0. Parks et al. (2003) stated that as the pH increased, the surface properties of ferric hydroxide solids became more negative and repulsion resulted between As and ferric hydroxide. At high pH, divalent cations (Ca 2+ ) reduced the electrostatic repulsion for negatively charged As and were retained on the surface. Results of this study indicated that higher difference in the As leaching with and without the addition of Ca and Mg supported the mechanism of arsenate apatite formation at high pH (greater than 9.0). Concurrently, the increase in the adsorption of Ca and Mg with the increase in pH enhanced the higher adsorption of As with Ca and Mg. Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 335 - 0.0 3.0 6.0 9.0 12.0 3.5 5.7 7.1 8.5 10.5 pH Adsorbed Ca, Mg (mg/g) Ca = 100 mg/L addition Mg = 100 mg/L addition Fig. 4 - Adsorbed amount of Ca and Mg as a function of pH Effect of Phosphorus In batch tests, the decrease in the concentrations of leached P (data not shown) was indicated in the GW adsorbed onto soil surface. Fig. 5 shows the profiles of leached As in different P concentrations in GW with and without the addition of Ca and Mg. In the absence of P without the addition of Ca and Mg, leached As was 32.9 μg/L. While at P concentrations of 1.0 and 2.0 mg/L, leached As increased to 51.1 and 60.8 μg/L, respectively. With the addition of 100 mg/L of Ca at P concentrations of 0, 1.0 and 2.0 mg/L, leached As decreased to 22.6, 24.6 and 47.1 μg/L, respectively. On the other hand, upon the addition of 100 mg/L Mg, the concentrations of leached As were 20.7, 23.3 and 45.0 μg/L, respectively. Arsenic and P are usually associated with amorphous Fe oxyhydroxides in soils and compete for adsorption sites (Woolson et al., 1973). At increasing P concentrations, the strongly binding P competed effectively for the limited sites available for sorption and enhanced the As leaching (Livesey and Huang, 1981; Roy et al., 1986; Manning and Goldberg, 1996). With the addition of Ca and Mg, leaching of As was decreased at all P concentrations but at high P concentration, it had the possibility to form hydroxyapatite Ca 5 (PO 4 ) 3 OH and other calcium phosphate compounds which might decrease As adsorption. More phosphates could be sorbed in the presence of Ca than in its absence, and more Ca could be sorbed in the presence of phosphate. This effect might be explained with the reduction of positive surface charge by the adsorption of phosphate and less repulsion for the positive Ca ions. Although the presence of P enhanced the leaching of As from soil, results indicated that Ca and Mg addition could effectively decrease As leaching even in the presence of P. Effect of anaerobic incubation Fig. 6 shows the concentrations of leached As at different incubation time (1, 5, 10 and 15 days) along with Eh at 0 and 100 mg/L of Ca and Mg. The Eh value of the mixed liquor decreased from highly oxidized condition of 380 ± 20 mV (mean ± SD, n = 6) after 1day to 110 ± 35 mV after 15 days of incubation. Leachate pH values ranged between 7.7 ± 0.1 and 6.8 ± 0.1. Results showed an increase in As leaching with Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 336 - 0 50 100 150 200 250 0 5 10 15 Time (day) Leached As (µg/L) 0 100 200 300 400 500 Eh (mV) No addition Ca =100 mg/L addition Mg = 100 mg/L addition Eh Fig. 6 - Concentrations of leached As at different incubation time and Eh with Ca and Mg addition 0 20 40 60 80 0.0 1.0 2.0 P (mg/L) Leached As (µg/L) No addtion Ca=100 mg/L addition Mg=100 mg/L addition Fig. 5 - Profiles of leached As as a function of P with Ca and Mg addition increasing time and decreasing Eh. Without the addition of Ca and Mg, leaching of As in 1 day was 48.8 µg/L and it increased to 199 µg/L after 15 days of incubation time. With the addition of Ca and Mg in the soil sample, As leaching was decreased significantly with incubation time and Eh. In 1 day, the leaching difference with and without the addition of Ca and Mg was small (12.8 μg/L for Ca and 12.3 μg/L for Mg) but it increased to 75 μg/L and 88 μg/L, respectively, after 15 days of incubation. According to the Eh-pH diagram (Masscheleyn et al., 1991) it was indicated that without Ca and Mg addition, the influence of redox on As leaching in soils was governed by the conversion of As (V) to As (III) followed by desorption. With Ca and Mg addition, Ca 2+ and Mg 2+ were sorbed on the soil surface which increased the positive charge and resulted in the adsorption/precipitation of As with Ca and Mg. Significant correlation was obtained between adsorbed Ca and As (R 2 = 0.98) and that between Mg and As (R 2 = 0.59). Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 337 - CONCLUSIONS Investigations revealed that Ca and Mg addition was effective in reducing As leaching from soil. Leaching of As was decreased with the increase of Ca and Mg concentration in the solution. In high pH (11.0) the effect of Ca and Mg addition was the maximum. The reduction of As leachability by Ca and Mg addition was most likely due to the divalent cation effect of Ca and Mg. In anaerobic incubation, As leaching probably decreased due to the adsorption of Ca and Mg. Adsorbed As and Ca and Mg were correlated well in the batch experiments. Ca and Mg addition could decrease As leaching even under the presence of P in the synthetic ground water. High amounts of Ca and Mg naturally present in ‘hard’ water could be a practical and viable method for immobilizing As by its adsorption to ferric hydroxide in soil. 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