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Geochemistry of inorganic arsenic and selenium in a tropical soil effect of reaction time, ph, and comp
Geochemistry of inorganic arsenic and selenium in a tropical soil: effect of reaction time, pH, and competitive anions on arsenic and selenium adsorption Kok-Hui Goh, Teik-Thye Lim * Environmental Engineering Research Centre, School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 8 May 2003; received in revised form 12 November 2003; accepted 18 November 2003 Abstract Factors that can affect As and Se adsorption by soils influence the bioavailability and mobility of these elements in the subsurface. This research attempted to compare the adsorption capacities of As(III), As(V), Se(IV), and Se(VI) on a tropical soil commonly found in Singapore in a single-species system. The effect of reaction time, pH, and competitive anions at different concentrations on the adsorption of both As and Se species were investigated. The As and Se adsorption isotherm were also obtained under different background electrolytes. The batch adsorption experiments showed that the sequence of the As and Se adsorption capacities in the soil was As(V) > Se(IV) > As(III) > Se(VI). The adsorption kinetics could be best described by the Elovich equation. The adsorption of As(V), Se(IV), and Se(VI) appeared to be influenced by the variable pH-dependent charges developed on the soil particle surfaces. Phosphate had more profound effect than SO 2À 4 on As and Se adsorption in the soil. The competition between PO 3À 4 and As or Se oxyanions on adsorption sites was presumably due to the formation of surface complexes and the surface accumulation or precipitation involving PO 3À 4 . The thermodynamic adsorption data for As(V) and Se(IV) adsorption followed the Langmuir equation, while the As(III) and Se(VI) adsorption data appeared to be best-represented by the Freundlich equation. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Arsenic; Selenium; Adsorption; Kinetics; Elovich model; Competitive anions 1. Introduction Arsenic and selenium are among the inorganic con- taminants that have become of evolving environmental concern lately. The accumulation of As and Se in soils, aquifer sediments and drinking water through various pathways has threatened the health of plants, wildlife, and human beings. The presence of these ions in the environment is regulated by many environmental and public health agencies or authorities. For example, under the Safe Drinking Water Act of United States, Maximum Contaminant Levels (MCLs) in drinking water established for As and Se are 10 and 50 lg/l, respectively. The new standard for As (Arsenic Rule) was adopted by USEPA on January 22, 2001 to replace the old standard of 50 lg/l in drinking water. The rule became effective on February 22, 2002. Sources of As and Se contamination are predominately associated with anthropogenic activities, arising from application of agricultural pesticides, disposal of industrial wastes, landfilling of sewage sludges, and combustion of fuels (Grossl et al., 1997; Pezzarossa and Petruzzelli, 2001). Public and political concerns have arisen as a result of * Corresponding author. Tel.: +65-6790-6933; fax: +65-6791- 0676. E-mail address: cttlim@ntu.edu.sg (T T. Lim). 0045-6535/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2003.11.041 Chemosphere 55 (2004) 849–859 www.elsevier.com/locate/chemosphere the potential groundwater and surface water contami- nation by these contaminants. Arsenic can exist in inorganic form, organic form, and gaseous state. The oxidation states of As in the natural systems are )3, 0, +3, and +5. The main inor- ganic forms of As in contaminated soils and sediments are +5 and +3 (Harper and Haswell, 1988) but some- times the oxidation states of )3 and 0 are expected to be found in highly reducing conditions (McBride, 1994). Oxidation states play a significant role in determining the potential mobility and sensitivity of As toward changes of the environmental conditions in soils. Arse- nite is much more toxic (Ferguson and Gavis, 1972), more soluble and therefore more mobile as compared to As(V). Arsenite (as H 3 AsO 3 and H 2 AsO À 3 ) normally predominate in slightly reduced soils whereas As(V) (as H 2 AsO À 4 and HAsO 2À 4 ) occur predominantly in well- oxidized soils. Selenium can also be present in inorganic and or- ganic forms, and sometimes in inorganic–organic form. Selenium can easily form compounds with metals and occurs in about 50 minerals (Kabata-Pendias and Pen- dias, 2001). It is present in four different oxidation states in aqueous and subsurface systems, namely )2, 0, +4, and +6. The fate and transport of Se in contaminated sites are very much influenced by its chemical form and speciation. Selenite and Se(VI) are the predominantly Se species found in contaminated soils. According to Neal and Sposito (1989), Se(IV) is strongly adsorbed by soils while Se(VI) is only weakly sorbed and leaches easily. Selenide and elemental Se are usually found in reducing environments and are unavailable to plants and animals. Selenite is present in mildly oxidizing, neutral pH envi- ronments and typical humid regions, while Se(VI) is the predominant form under ordinary alkaline and oxidized conditions. Adsorption is one of the most commonly reported, and possibly the initial reaction to occur when As or Se interacts with soils. Adsorption of inorganic As and Se on soil is of paramount importance because this process regulates the fate and mobility of As and Se in soil. Although the overall rate of adsorption relies on numerous factors, the adsorption study of As and Se on soils has been mainly linked to environmental factors such as pH (Pierce and Moore, 1982; Xu et al., 1988; Ticknor and McMurry, 1996; Kuan et al., 1997; Man- ning and Goldberg, 1997; Garcia-Sanchez et al., 2002; Goldberg, 2002), redox potential, reaction time (Prasad, 1994; Carbonell Barrachina et al., 1996; Lo and Chen, 1997; Su and Suarez, 2000; O’Reilly et al., 2001), and oxidation states of As and Se. Competitive ions and soil properties are also being emphasized due to their key roles in controlling As and Se mobility in soils. Adsorption processes involving As and Se are con- sidered to be rapid. According to Prasad (1994), it was found that the removal of As(V) from aqueous solution was rapid in the initial stages of contact and reached a maximum in the range of 35–60 min for soil minerals such as hematite and feldspar. Carbonell Barrachina et al. (1996) observed that 80% of the total amount of As(III) adsorbed was sorbed by Spanish soil in the first 30 min. O’Reilly et al. (2001) discovered that As(V) sorption on goethite was initially rapid, with over 93% As(V) adsorption within 24 h. There is noticeably little information reported on the kinetics of both As(III) and As(V) adsorption in the same soil system. This infor- mation is essential because both As(III) (predominantly in the reduced condition) and As(V) (predominantly in the oxidized condition) are often discovered in either redox environments because of the relatively slow redox transformation of As (Masscheleyn et al., 1991). The conflicting results were also observed for the kinetic study on Se adsorption. Lo and Chen (1997) found that Se(IV) and Se(VI) achieved adsorption equilibrium at different times. They reported that Se(IV) adsorption on an iron-coated sand was attained rapidly within 10 min while Se(VI) adsorption needed a duration of 1.5 h to reach equilibrium. Conversely, Su and Suarez (2000) reported that sorption of both Se(IV) and Se(VI) on iron oxides reached equilibrium at the same time which was less than 25 min. These contradictory observations indicate the necessity to further elucidate the effect of reaction time on Se(IV) and Se(VI) adsorption. Numerous studies of pH influence on As adsorption on oxides and hydroxides suggested that As(III) had a sorption maximum at around pH 7 (Pierce and Moore, 1982; Xu et al., 1988; Goldberg, 2002), whereas As(V) sorption reached a maximum sorption around pH 4–7 and then decreased with more alkaline pH (Manning and Goldberg, 1997; Garcia-Sanchez et al., 2002; Goldberg, 2002). On the other hand, a few researchers studied the adsorption of Se onto the soil minerals (oxides, hydroxides, or clays) and found that adsorption decreased with increasing pH (Ticknor and McMurry, 1996; Kuan et al., 1997). The mobility, bioavailability, and toxicity of As and Se in soils may also be greatly affected by the presence of competitive anions. Anions such as PO 3À 4 ,SO 2À 4 ,CO 2À 3 , and Cl À can compete with As and Se for sorption sites. Xu et al. (2002) found that the addition of AsO 3À 3 ,Cl À ,NO À 3 ,SO 2À 4 , CrO 2À 4 , and CH 3 COO À hardly affected the As(V) adsorption on zeolite. However, Qafoku et al. (1999) reported that SO 2À 4 was able to displace some of the adsorbed As(V) on an amended soil. Violante and Pigna (2002) studied the competitive sorption of PO 3À 4 and As(V) on selected clay minerals. They found that PO 3À 4 could inhibit As(V) sorption on clay minerals such as gibbsite and kaolinite. Smith et al. (2002) observed that the presence of PO 3À 4 greatly decreased As(V) sorption by soils containing low amounts of Fe oxides but had little effect on the amount of As(V) adsorbed by soils with high Fe content. Goldberg (2002) found that there was no competitive 850 K H. Goh, T T. Lim / Chemosphere 55 (2004) 849–859 effect of the presence of equimolar As(III) on As(V) adsorption. On the other hand, the competitive effect of equimolar As(V) on As(III) adsorption was small and apparent only on kaolinite and illite in the pH range 6.5– 9. Obviously, the study of competitive anion effects on As(III) adsorption has not gained much attention as compared to As(V) adsorption due to the assumption that As(III) is less strongly adsorbed on the soil or mineral surface. However, the anion competition effect on As(III) adsorption should not be neglected because of its high potential toxicity and environmental rele- vance. Some studies revealed that SO 2À 4 could compete with Se(IV) and Se(VI) for adsorption sites on oxides or hydroxides (Saeki et al., 1995; You et al., 2001). The presence of PO 3À 4 was also found to reduce Se sorption by direct competition (Dhillon and Dhillon, 2000; Monteil-Rivera et al., 2000). Although much research has been devoted to study the effect of reaction time, pH and competitive anions on As and Se adsorption on individual soil minerals, comparatively few adsorption studies have been focused on the soil itself which may comprise various types of minerals. In addition, the abundance of soils in a variety of geochemical environ- ments and their influence on adsorption of contaminants suggested the need for more characterization of As and Se adsorption on different soils. This study compared the adsorption capacities of As(III), As(V), Se(IV), and Se(VI) on a tropical soil in a single-species system. The influence of reaction time, pH, and competitive anions on the adsorption of As(III), As(V), Se(IV), and Se(VI) were examined. The adsorption isotherms with different background elec- trolytes were also obtained. 2. Materials and methods 2.1. Soil sample and characterization An uncontaminated reddish brown tropical soil sample was obtained from a site in the western part of Singapore where heavy industries are located. The mineral composition of the soil includes quartz, clay minerals of predominantly kaolinite, montmorillonite and chlorite (Parashar, 1998), and other fine materials such as Fe and Al oxides (Lim et al., 2004). The geo- logical formation of the soil is known as the sedimentary Jurong Formation (Pitts, 1984). The soil sample was prepared by pulverizing the soil so that all passed through a 150-lm stainless steel sieve. There were no soil particles retained on the sieve, and therefore it preserved the mineral composition of the original bulk soil. The soil powder was then thoroughly mixed to ensure homogeneity in mineral composition throughout, and kept in a dry condition for subsequent characterization and As and Se adsorption studies. Pulverization of the soil was essential as it ensured that each small sample (as small as 0.1 g) taken from the soil for the characteriza- tion or adsorption experiments would adequately rep- resent the original bulk soil in term of its composition. Particle size distribution of the original bulk soil was carried out by sieving the soil through the sieves ranging from the sizes of 14 mm to 425 lm. The soil fraction that passed through the 425-lm sieve was further collected for particle size analysis using Malvern Microplus Mastersizer. For the pulverized soil, the pH was mea- sured in soil slurries with soil:water ratios of 1:1 and 1:20 using Corning pH meter model 145. HORIBA conduc- tivity meter was used to determine the electrical con- ductivity (EC) of the soil. The organic content in the soil was determined as chemical oxygen demand (COD) using the method modified from the APHA Standard Method Part 5220c (APHA, 1998). The content of Si, Al, Fe, Mn, As, and Se in the soil was determined by microwave-assisted acid digestion (EPA Method 3052) and subsequent analysis using Inductively Coupled Plasma-Optical Emission Spectrometer (ICP–OES) of Perkin-Elmer Optima 2000. The cation exchange capacity (CEC) and anion exchange capacity (AEC) of the soil at various pH values were evaluated using a buffer salt extraction method adopted by Lim et al. (1997). The soil surface charge density was determined in a batch system with a constant soil:water ratio of 1:20, using a method modified from alkalimetric and acidimetric titration methods described by Stumm (1992). The BET surface area of the soil was determined using Micropore System QUANTA CHROME (AUTOSORB-1). 2.2. Adsorption experiments All the glassware used was soaked in 5% HNO 3 for overnight and then rinsed with Milli-Q (18.2 MX cm resistivity) water before being used in this study. The plastic labware was washed several times with RO/DI water before use. Analytical reagent grade chemicals and RO/DI water were used for preparation of all solutions throughout the study. All experiments were performed using the batch adsorption technique at room tempera- ture in the 90-ml polypropylene (PP) centrifuge bottles. The batch adsorption experiments were performed in triplicate and carried out by using the pulverized soil as the adsorbent and As(III), As(V), Se(IV), and Se(VI) as the adsorbates. NaAsO 2 (Merck, FW 129.92, Assay min 98.5%), Na 2 HAsO 4 Æ 7H 2 O (Sigma, FW 312, Assay 99.4%), Na 2 SeO 3 (Fluka Chemika, FW 172.94, Assay >95%), and Na 2 SeO 4 (Sigma, FW 188.9) were used as the As(III), As(V), Se(IV), and Se(VI) sources, respec- tively. A concentration of 200 lM of the adsorbate in single-species system was used in all the adsorption experiments, except in the experiment investigating adsorption isotherms. The soil-to-solution ratio used in K H. Goh, T T. Lim / Chemosphere 55 (2004) 849–859 851 this study was 1:20. The natural pH (pH around 4.7) was selected during the adsorption experiments with the aim of minimizing the number of dissolved As or Se species (to only one or two predominant species), reduce the number of possible adsorbing species and eventually simplify the overall system. pK a values of arsenious acid (H 3 AsO 3 ) and arsenic acid (H 3 AsO 4 ) are as follows: pK 1 ¼ 9:22, pK 2 ¼ 12:13, and pK 3 ¼ 13:4; pK 1 ¼ 2:20, pK 2 ¼ 6:97, and pK 3 ¼ 11:53, respectively (O’Neill, 1995). While pK a values of selenious acid (H 2 SeO 3 ) and selenic acid (H 2 SeO 4 ) are as follows: pK 1 ¼ 2:55 and pK 2 ¼ 8:15; pK 1 ¼À3:0 and pK 2 ¼ 1:66, respectively (Faust and Aly, 1999). Hence, at pH 4.7, H 3 AsO 0 3 and H 2 AsO À 4 are the respective predominant dissolved As(III) and As(V) species. While HSeO À 3 and SeO 2À 4 are the predominant dissolved Se(IV) and Se(VI) species at the same pH. 2.2.1. Effect of reaction time, pH and competitive anions The purpose of this kinetic adsorption experiment was to identify the reaction time required for adsorption to reach equilibrium. This experiment was carried out by determining the amount of the adsorbates adsorbed at various reaction times, ranging from 10 min to 24 h. Initially, solutions containing the adsorbates in single- species system were prepared with 0.01 M of NaCl as background electrolyte. The soils and the solutions were then added into each PP bottle to make up a soil:solu- tion ratio of 1:20. Soil-less blanks (with addition of the adsorbate only) were also prepared for determination of initial concentration of the adsorbates (C 0 ). The sus- pensions in the bottles were continuously agitated at 35 oscillations per minute in a reciprocating shaker and their pH values were periodically measured and adjusted back by adding 0.01 M HCl or 0.01 M NaOH to the natural pH of the soil. This was to eliminate the effect of changes in pH taking place throughout the adsorption process. At the end of the desired reaction times, the bottles were centrifuged and the supernatants were filtered through the 0.45-lm Whatman membrane filters. The filtrates were subsequently acidified with HNO 3 and kept in the refrigerator before ICP–OES analysis. The effect of pH on adsorption was studied by determining the amount of the adsorbate adsorbed within the pH range of 3–7. The adsorption at more basic pH values was not studied because soil properties would be damaged at this pH values and influence the adsorption behavior of the soil. The soil was first added into each PP bottle and followed by the addition of 1 M sodium acetate for pH stabilization purpose. The adsorbate with 0.01 M NaCl was then added into each PP bottle. Soil-less blanks were also kept for C 0 deter- mination. The suspensions in the bottles were then ad- justed to the desired pH values by adding 0.01 M HCl or 0.01 M NaOH. The suspensions were continuously agitated for 24 h. The pH values of the suspensions were periodically measured and adjusted if necessary to en- sure that adsorption had taken place consistently at the desired pH values. The 24-h adsorption equilibration period was chosen based on the results of the earlier kinetic adsorption experiment. At the end of the 24-h pH equilibration, the pH values of the suspensions were recorded again. The bottles were subsequently centri- fuged and the supernatants filtered. The filtrates were then acidified and kept for ICP–OES analysis. The competitive anions that were employed in this study were SO 2À 4 and PO 3À 4 . The effect of these compet- itive anions on adsorption was investigated by deter- mining the amount of the As or Se adsorption at different concentrations of SO 2À 4 and PO 3À 4 ranging from 0.002 to 0.05 M. The soils and the solutions were added into each PP bottle. Soil-less blanks were also prepared to measure C 0 . The suspensions were contin- uously agitated for 24 h and their pH values were peri- odically measured and adjusted back to the natural pH of the soil. After 24 h, the samples were treated as those carried out in the aforementioned adsorption experi- ments. The amount of the adsorbate adsorption was calcu- lated as follows: Sð%Þ¼ ðC 0 À C f Þ C 0  100% ð1Þ where S is the adsorption of the adsorbate (%); C 0 is the initial concentration of the adsorbate in the soil-less blank (mg/l); C f is the final concentration of the adsor- bate in the filtrate (mg/l). Sðmg=kg soilÞ¼ ðC 0 À C f ÞV M ð2Þ where S is the adsorption of the adsorbate (mg/kg soil), V is the volume of the solution added (l), and M is the soil added into each bottle (kg). 2.2.2. Adsorption isotherm Adsorption isotherms were obtained by carrying out adsorption experiments with fixed amount of soil but varying initial adsorbate concentrations prepared by adding various volumes of stock solutions of As(III), As(V), Se(IV), and Se(VI) and making up with solution containing Cl À ,SO 2À 4 or PO 3À 4 and RO/DI water to the desired volume. The concentration of Cl À ,SO 2À 4 or PO 3À 4 in the adsorbate was 0.01 M, which served as the background electrolyte. Soil-less blanks were also pre- pared to determine C 0 for each adsorption determina- tion. The suspensions in the bottles were continuously agitated for 24 h and their pH values were periodically measured and adjusted back to the natural pH of the soil. At the end of the agitation, the samples were cen- trifuged, filtered, and acidified before ICP–OES analysis. 852 K H. Goh, T T. Lim / Chemosphere 55 (2004) 849–859 3. Results and discussion 3.1. Geochemical properties of tropical soil The tropical soil used in this study consisted of 38% gravel, 22% sand, 21% silt, and the remaining 19% clay and other colloidal particles. This soil is grouped as gravelly clay loam in accordance with the USDA soil classification system. The characteristics of the tropical soil used in this study are shown in Table 1. It can be seen from Table 1 that the soil was very acidic with pH values of about 4.5 for a soil:water ratio of 1:1 and about 4.7 for a soil:water ratio of 1:20. The parent material, weathering processes, and absence of carbon- ate content were factors that contribute to the acidity of the soil. The EC value of the soil was low. Organic matter was insignificant in the soil. Significant amount of Fe in the soil was possibly due to the forms of Fe oxides, Fe oxyhydroxides, or hydrous Fe oxides. The amount of Mn in the soil was insignificant. The native As and Se contents in the soil were less than 30 mg/kg. The CEC value of the soil was 34 cmol/kg soil at natural pH (pH 4.7) whereas the AEC value of the soil was 21 cmol/kg soil at the same pH value. The CEC value of the soil was higher than the CEC of a typical clay loam texture, which ranged from 15 to 30 cmol/kg soil (Do- nahue et al., 1977). According to Evangelou (1998), this might be due to the presence of montmorillonite and oxides of Fe and Al that have high CEC values. The soil has a point of zero charge (pzc) at pH of about 4.6 (Fig. 1). The low pzc could be due to kaolinite clay mineral in the soil which typically has a pzc ranging from 2 to 4.6 (Evangelou, 1998). The BET surface area of the soil was 9.17 m 2 /g. 3.2. Time-dependent As and Se adsorption The kinetics of As and Se adsorption on the tropical soil in 0.01 M NaCl medium are depicted in Fig. 2. In general, the adsorption capacities of the adsorbates in decreasing order were: As(V) < Se(IV) < As(III) < Se(VI). It was observed that the adsorption rates of As(III), As(V), Se(IV), and Se(VI) were rapid in the first hour and then decelerated noticeably as the reaction plateaued after 8 h. The increases in adsorption beyond 8 h were marginal and seemed to approach equilibrium at about 24 h, in which the percentages of As(III), As(V), Se(IV), and Se(VI) adsorbed at this time were 58%, 92%, 72%, and 25%, respectively. It could be de- duced from the results that As(III), As(V), Se(IV), and Se(VI) virtually attained adsorption equilibrium at Table 1 Geochemical properties of the tropical soil used in this study Soil property Particle size distribution (%) Sand 22 Silt 21 Clay 19 Soil color Reddish brown pH at room temperature (1:1 soil:water) 4.5 (1:20 soil:water) 4.7 Electrical conductivity at room temperature (1:1 soil:water) (mS/cm) 0.08 Organic content (COD, %) BDL Chemical content (mg/kg soil) Si 309 000 Al 15 400 Fe 26 500 Mn 30 As <30 Se <30 CEC (cmol/kg soil) at natural pH 34 AEC (cmol/kg soil) at natural pH 21 BET surface area (m 2 /g) 9.17 BDL ¼ below detection limit. -3.00E-01 -2.00E-01 -1.00E-01 0.00E+00 1.00E-01 2.00E-01 3.00E-01 02468101214 pH Surface Charge Density (mmol/g soil) Fig. 1. Surface charge density of the tropical soil as a function of pH. Fig. 2. Changes of As(III), As(V), Se(IV), and Se(VI) adsorp- tion with time. K H. Goh, T T. Lim / Chemosphere 55 (2004) 849–859 853 about 24 h in a single-species system under comparable operating condition. The 24-h adsorption period was therefore chosen as the reaction time for the subsequent adsorption experiments. The time-dependent adsorption results of As and Se were also analyzed using several kinetic models including zero-order model, Lagergren (first-order) model, squared-driving force mass transfer model, Elovich model, Ritchie (second-order) model etc. The Elovich model was found to best describe the adsorption kinetics of both As and Se species on the tropical soil within a 24 h time-frame. The linear form of the Elovich equation can be expressed as S ¼ 1 a lnðaaÞþ 1 a ln t þ 1 aa ð3Þ or it can be simplified as (if t ) 1=aa): S ¼ 1 a lnðaaÞþ 1 a ln t ð4Þ where S is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/kg) at time t while a and a are the constants. The time-dependent adsorption results that fitted to the Elovich kinetic model are shown in Fig. 3. The testing of the Elovich equation is carried out by plotting S versus ln t, in which a straight line can be obtained. The constant a can be calculated from the slope of the straight line. The constant a can be obtained from the a value and the intercept on the y-axis (i.e., ln t ¼ 0) plot. The respective values of a, a, and the coefficient of correlation (r) of the adsorbates are given in Table 2. According to Elovich rate law, the rate of As and Se adsorption decreased exponentially with the increasing As and Se coverage on the soil surface: dS dt ¼ a Á e ÀaS ð5Þ where a is the reaction rate at zero coverage (initial condition of S ¼ 0att ¼ 0 which gives Elovich equation in the form of Eq. (3) when solving Eq. (5)). The cov- erage scale factor a is the reciprocal of the coverage S 1=e at which the adsorption rate has fallen to 1=e of its initial value (Wang et al., 2000): dS dt 1=e ¼ a Á e ÀaS 1=e ¼ a=e ð6Þ It is also worthwhile noted from the literature that the Elovich model was used by Carbonell Barrachina et al. (1996) to adequately describe the adsorption kinetics of As(III) on several Spanish soils. 3.3. Effect of pH on As and Se adsorption The adsorption of As(III), As(V), Se(IV), and Se(VI) on the tropical soil as a function of pH is shown in Fig. 4. The adsorption of As(V) increased with pH up to a maximum of 92% or equivalent to 337 mg/kg soil at pH 4.5 and then gradually decreased with further increase in pH. However, the adsorption rate of As(III) contin- ued to increase when the pH values increased from 3 to 7. In the case of Se, both Se(IV) and Se(VI) decreased in adsorption when pH increased. The adsorption of Se(IV) fell from 83% at pH 3 to 59% at pH 7, while for Se(VI) the percentages of adsorption dropped from 46% at pH 3 to 15% at pH 7. It was found that the tropical Fig. 3. Kinetic study of As(III), As(V), Se(IV), and Se(VI) adsorption using Elovich kinetic model. Table 2 Regression analysis results of the Elovich kinetic model for the adsorption of As(III), As(V), Se(IV), and Se(VI) Adsorbate a a r As(III) 4.78 · 10 2 0.0516 0.985 As(V) 4.30 · 10 3 0.0366 0.998 Se(IV) 2.72 · 10 4 0.0566 0.988 Se(VI) 2.01 · 10 7 0.1906 0.938 Fig. 4. Effect of pH on the adsorption of As(III), As(V), Se(IV), and Se(VI). 854 K H. Goh, T T. Lim / Chemosphere 55 (2004) 849–859 soil had greater adsorption of As(V) than the adsorption of As(III), Se(IV), and Se(VI) from pH 3–7. According to thermodynamic calculations reported by O’Neill (1995), when pH < pzc (pH 4.6), the positively charged surfaces on the soil would likely prefer the adsorption of the monovalent As(V) oxyanion, H 2 AsO À 4 , to the un- charged As(III) molecule H 3 AsO 0 3 . The cause of greater adsorption of As(V) than As(III) when pH > pzc was believed to be partly due to the high affinity of Fe oxides to As(V), in which pzc for Fe oxide is in the pH range of 6–8 according to Evangelou (1998). On the other hand, Se(IV) was more favorably adsorbed than Se(VI) by the soil from pH 3 to 7, and this observation suggested that Se(IV) was adsorbed more than Se(VI) over a rather wide pH range. The declining trend in the adsorption of As(V), Se(IV), and Se(VI) with increasing pH showed evidence of similar behavior to that of surface charge density of the soil (Fig. 1). The soil surface charge density de- creased as the pH of the system rose, with which the adsorption results showed agreement. This implied that the decreasing adsorption as pH increased could be due to the increasing amount of the OH À . This was because OH À could result in electrostatic repulsion between As(V), Se(IV), or Se(VI) oxyanions and hydroxylic functional groups (generated by accumulation of OH À ) on the soil surface. Since As(III) has less negative charge character as compared to As(V), Se(IV), and Se(VI) in the investigated pH range, it did not exhibit repulsion and conversely its adsorption increased with increasing pH which matched the findings reported by earlier researchers (Frost and Griffin, 1977; Goldberg and Glaubig, 1988; Goldberg, 2002). On the other hand, the adsorption of As(III), As(V), Se(IV), and Se(VI) oc- curred at pH below pzc could be attributed to attraction between opposite charges by electrostatic force. In short, it could be deduced that adsorption of As and Se by the soil was dependent on the variable charge developed on the soil particle surface. The decline of As(V) adsorption with decreasing pH at pH less than 4.5 could be attributed to (i) dissolution of the soil minerals (mainly amorphous Fe and clay minerals) which would release the adsorbed As(V) or reduce its adsorption and (ii) high solubility of Fe ars- enates at acidic condition (Matera and H echo, 2001). Iron arsenate was a potential sink for As(V) in the aqueous solution. At low pH, the high solubility of this compound inhibited its precipitation and therefore re- duced As(V) retention by the soil. The shift from anionic As species from H 2 AsO À 4 to neutral As species H 3 AsO 4 when the pH decreased could also contribute to the decrease in As(V) adsorption at extremely acidic con- dition. It was also observed that the maximum As(V) adsorption occurred at pH around 5 according to pre- vious works (Manning and Goldberg, 1997; Garcia- Sanchez et al., 2002; Goldberg, 2002). 3.4. Effect of competitive anions on As and Se adsorption Arsenite and Se(VI) adsorption decreased signifi- cantly (P < 0:01) as the concentration of SO 2À 4 increased as illustrated by Fig. 5. However, as the SO 2À 4 concen- tration increased beyond 0.01 M, the amount of As(III) and Se(VI) adsorbed was not affected. For the case of As(V), the adsorption fluctuated marginally between 257 and 286 mg/kg soil for SO 2À 4 concentration up to 0.05 M, while Se(IV) adsorption only experienced a minor reduction when SO 2À 4 concentration increased. Arsenate and Se(IV) consistently showed higher adsorp- tion values than As(III) and Se(VI) and they were less affected by SO 2À 4 competition for adsorption sites. Without SO 2À 4 , the maximum adsorption for As(III), As(V), Se(IV), and Se(VI) was 45%, 70%, 76%, and 44%, respectively. These observations suggested that SO 2À 4 could com- pete with As(III) and Se(VI) oxyanions for adsorption sites. Nonetheless, the presence of SO 2À 4 could hardly af- fect the As(V) and Se(IV) adsorption. On the other hand, the influence of SO 2À 4 on As(V) and Se(IV) adsorption was found insignificant when its ionic strength was fur- ther increased. The adsorption behavior of As(V) and Se(IV) which was unaffected by the changes of SO 2À 4 ionic strength is macroscopic evidence for strong specific binding mechanism (inner-sphere complex) between As(V) or Se(IV) and soil minerals (Hayes et al., 1988). Conversely, As(III) and Se(VI) adsorbed were more easily displaced by SO 2À 4 because both of them might only form weak bond (outer-sphere complex) with soil minerals according to Zhang and Sparks (1990). In the presence of PO 3À 4 , there were significant de- creases in As(III), As(V), Se(IV), and Se(VI) adsorption as the concentration of PO 3À 4 increased, as illustrated in Fig. 6. The adsorption of all As and Se oxyanions showed a sharp decrease over a low PO 3À 4 concentration range, and then gradually reached a plateau phase when PO 3À 4 concentration was further increased. In general, Fig. 5. Effect of SO 2À 4 at various concentrations on the adsorption of As(III), As(V), Se(IV), and Se(VI). K H. Goh, T T. Lim / Chemosphere 55 (2004) 849–859 855 the effect of competitive anion on As and Se adsorption on the tropical soil was more evident in the case of PO 3À 4 as compared to SO 2À 4 . Arsenate and Se(IV) adsorption were less influenced by the presence of PO 3À 4 than As(III) and Se(VI) mainly due to the greater adsorption of As(V) and Se(IV) by the soil. The results suggested that the competition between PO 3À 4 and As or Se oxya- nions on adsorption sites possibly involved two different reactions on soil surface. The reactions were the for- mation of surface complex and the surface accumulation or precipitation. The increased competition due to the presence of PO 3À 4 for soil adsorption sites might occur through high affinity of PO 3À 4 or the effect of mass action of the increasing concentration of PO 3À 4 in solution as suggested by Smith et al. (2002). Phosphate could associate with surface functional groups on the surfaces of soil minerals such as Fe oxides, forming strong inner- sphere surface complexes and reducing surface potential. As a result, reduction in available adsorption sites oc- curred and this inhibited the adsorption of As or Se. Accumulation or precipitation of PO 3À 4 on the soil sur- face might also promote the formation of negatively charged surface sites and reduce surface potential. Therefore, the electrostatic repulsion between As or Se and the negatively charged surface sites increased and thus reduced the adsorption of As and Se. 3.5. Adsorption isotherm Various adsorption isotherms were used to describe the thermodynamics of As(III), As(V), Se(IV), and Se(VI) adsorption. In this study, As(V) and Se(IV) adsorption isotherms could be described by Langmuir equation while As(III) and Se(VI) adsorption isotherms could be best represented by Freundlich equation. The Langmuir equation can be expressed as follows: C e S ¼ 1 bS m þ C e S m ð7Þ where C e is the equilibrium concentration of As or Se oxyanions in the solution (mg/l), S is the amount of As or Se adsorbed at equilibrium (mg/kg soil), b is constant, and S m is the maximum possible adsorption at equilib- rium in the Langmuir equation (mg/kg soil). The linear form of Freundlich equation is expressed as log S ¼ log K p þ n log C e ð8Þ where K p and n are constants. The sets of best-fitted adsorption isotherms are plotted in Fig. 7, while Table 3 shows the types of best-fitted adsorption isotherm and the values of their parameters as well as the correlation of determinations (r 2 ). Arsenate and Se(IV) adsorption increased signifi- cantly with increased adsorbate loading initially and then increased gradually when higher adsorbate loadings were added due to surface sites saturation. As the sites on the soil were occupied, it was increasingly more dif- ficult for As(V) and Se(IV) to find available adsorption sites on the soil. In contrast, the adsorption of As(III) and Se(VI) tended to increase more steadily as the equilibrium concentration (C e ) increased, as compared to As(V) and Se(IV) adsorption. Therefore, it could be deduced that both As(III) and Se(IV) adsorption iso- therms did not reach the maximum adsorption under the experimental condition in this study and thus they indicated the higher binding sites and low surface cov- erage. Either at the lower or higher equilibrium con- centrations, the adsorption of As(V) was the greatest and followed by the adsorption of Se(IV), As(III), and Se(VI). 4. Conclusions The adsorption capacities of As and Se on the trop- ical soil investigated generally followed the order: As(V) > Se(IV) > As(III) > Se(VI). In addition, the studies of pH effect, competitive anion effect as well as the adsorption isotherm further verified this sequence of As and Se adsorption capacity on the tropical soil. The adsorption kinetics could be best described with Elovich kinetic model. The adsorption of As(V), Se(IV), and Se(VI) was dependent on the variable charge developed on the soil surface, except for the case of As(III). The reduction in As(V), Se(IV), and Se(VI) adsorption with increasing pH demonstrated evidence of identical behavior to that of the soil surface charge density. While As(III) adsorption increased with increasing pH because it has lesser negative charge character that prohibited electrostatic repulsion between As(III) oxyanion and hydroxylic functional group. From the investigation on the effect of competitive anions, it was found SO 2À 4 could hardly affect As(V) and 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0.05 0.06 Adsorption (%) 0 50 100 150 200 250 300 350 Adsorption (mg/kg soil) As(III) As(V) Se(IV) Se(VI) 0 20 40 0 0.005 0.01 Fig. 6. Effect of PO 3À 4 at various concentrations on the adsorption of As(III), As(V), Se(IV), and Se(VI). 856 K H. Goh, T T. Lim / Chemosphere 55 (2004) 849–859 Se(IV) adsorption but could compete with As(III) and Se(VI) for adsorption sites. Phosphate had more pro- found effect on As(III), As(V), Se(IV), and Se(VI) adsorption than SO 2À 4 , which could be partly attributed to its higher negative charge compared to that of SO 2À 4 . The competition between PO 3À 4 and As or Se oxyanions on adsorption sites was probably caused by two different reactions on surfaces of soil minerals. The reactions were believed to be the formation of surface complex and the surface accumulation or precipitation. In term of adsorption isotherm, Langmuir isotherm could better represent equilibrium adsorption of As(V) and Se(IV) 0.0 0.5 1.0 1.5 2.0 2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 log C e (mg/l) log S (mg/kg soil) 0.01 M Chloride Ion 0.01 M Sulfate Ion 0.01 M Phosphate Ion ` As(III) 0.0 0.1 0.2 0.3 0 1020304050 C e (mg/l) C e /S (kg/l) 0.01 M Chloride Ion 0.01 M Sulfate Ion 0.01 M Phosphate Ion As(V) (a) (b) 0.0 0.1 0.2 0.3 0.4 0.5 0 102030405060 C e (mg/l) C e /S (kg/l) 0.01 M Chloride Ion 0.01 M Sulfate Ion 0.01 M Phosphate Ion Se(IV) (c) 0.0 0.5 1.0 1.5 2.0 2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 log C e (mg/l) log S (mg/kg soil) 0.01 M Chloride Ion 0.01 M Sulfate Ion 0.01 M Phosphate Ion Se(VI) (d) Fig. 7. Best-fitted adsorption isotherms of different adsorbates on the tropical soil: (a) Freundlich isotherms of As(III) adsorption; (b) Langmuir isotherms of As(V) adsorption; (c) Langmuir isotherms of Se(IV) adsorption; (d) Freundlich isotherms of Se(VI) adsorption. Table 3 The parameters of best-fitted adsorption isotherms calculated for As(III), As(V), Se(IV), and Se(VI) adsorption Adsorbate Background electrolyte Langmuir Freundlich S m br 2 K p nr 2 As(III) Cl À 32.5 0.47 0.976 SO 2À 4 16.6 0.70 0.963 PO 3À 4 6.35 0.87 0.980 As(V) Cl À 217 0.45 0.987 SO 2À 4 208 0.40 0.996 PO 3À 4 154 0.16 0.881 Se(IV) Cl À 145 17.3 0.999 SO 2À 4 139 2.77 0.996 PO 3À 4 81.3 0.48 0.977 Se(VI) Cl À 35.9 0.34 0.929 SO 2À 4 18.7 0.34 0.798 PO 3À 4 24.0 0.36 0.933 K H. Goh, T T. Lim / Chemosphere 55 (2004) 849–859 857 while Freundlich isotherm was more suitable in describing As(III) and Se(VI) equilibrium adsorption. References APHA, 1998. 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