Environ. Sci. Technol. 2005, 39, 6101-6108
Kinetics of Arsenate Adsorption-Desorption in Soils HUA ZHANG AND H. M. SELIM* Department of Agronomy and Environmental Management, Sturgis Hall, Louisiana State University, Baton Rouge, Louisiana 70803
Adsorption-desorption of arsenic is the primary factor that impacts the bioavailability and mobility of arsenic in soils. To examine the characteristics of arsenate [As(V)] adsorption-desorption, kinetic batch experiments were carried out on three soils having different properties, followed by arsenic release using successive dilutions. Adsorption of As(V) was highly nonlinear, with a Freundlich reaction order N much less than 1 for Olivier loam, Sharkey clay, and Windsor sand. Adsorption of arsenate by all soils was strongly kinetic, where the rate of As(V) retention was rapid initially and was followed by gradual or somewhat slow retention behavior with increasing reaction time. Freundlich distribution coefficients and Langmuir adsorption maxima exhibited continued increase with reaction time for all soils. Desorption of As(V) was hysteretic in nature and is an indication of lack of equilibrium retention and/or irreversible or slowly reversible processes. A sequential extraction procedure provided evidence that a significant amount of As(V) was irreversibly adsorbed on all soils. A multireaction model (MRM) with nonlinear equilibrium and kinetic sorption successfully described the adsorption kinetics of As(V) for Olivier loam and Windsor sand. The model was also capable of predicting As(V) desorption kinetics for both soils. However, for Sharkey clay, which exhibited strongest affinity for arsenic, an additional irreversible reaction phase was required to predict As(V) desorption or release with time.
Introduction Increasing amounts of arsenic (As) are being introduced into soil and water environments as a result of natural and anthropogenic processes (1). In addition, contamination of ground and surface water by arsenic from soils and aquifers pose significant threat to human health (2). Therefore, it is essential to fully understand the fate and transport characteristics of arsenic in soils and aquifers. Briefly, the major inorganic forms of arsenic in the natural soil environment are arsenate [As(V)] under aerobic conditions and arsenite [As(III)] under anaerobic conditions. However, because the redox reactions between As(V) and As(III) are relatively slow, both oxidation forms are often found in soils regardless of pH and Eh (3). Knowledge of adsorption and desorption of arsenic is necessary for predicting the fate and behavior of arsenic in the soil environments. It is well established that Fe and Al oxides and hydroxides have a high affinity to arsenic. Microscopic studies such as extended X-ray adsorption fine * Corresponding author phone: (225) 578-2110; fax: (225) 5781403; e-mail:
[email protected]. 10.1021/es050334u CCC: $30.25 Published on Web 07/15/2005
2005 American Chemical Society
structure spectroscopy (EXAFS) (4-8) and Fourier transform infrared (FTIR) (9-10) have shown that both As(V) and As(III) form mono- or bidentate inner-sphere surface complexes with iron oxides via a ligand exchange mechanism. Because of their negatively charged surfaces, clay minerals generally have low arsenic adsorption capacity (11-13). Surface complexation models have been employed to describe the adsorption of arsenic on minerals (6, 10, 11). In addition to studies on minerals, several studies have demonstrated that arsenic adsorption on soils is correlated with Al and Fe oxides contents (14-20). Equilibrium experiments carried out in relatively short reaction times (usually 24 h) have been used widely for quantifying the extent of arsenic adsorption in soils (14-19). However, the utility of results from short duration studies for predictions of the fate and transport of arsenic is often limited because equilibrium conditions are rarely achieved in 24 h under laboratory or field conditions due to a wide variety of biological, chemical, and hydrological factors. A literature search revealed that the influence of residence time on retention and release of arsenic in soils has not been adequately investigated. It has been observed that the sorption rate of As(V) and As(III) on minerals was initially rapid and was followed by a slow phase (7, 8, 21-23). Fuller et al. (21) observed that As(V) adsorption on synthesized ferrihydrite had a rapid initial phase (72 h) on As(V) adsorption were not investigated. VOL. 39, NO. 16, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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In contrast to adsorption studies, only a limited number of studies have investigated release or desorption of arsenic from minerals and soils (7, 13, 21, 23). Fuller et al. (21) suggested that As(V) desorption from ferrihydrite was limited by diffusional processes within soil aggregates. Their data indicated that only a small portion of As(V) desorbed from ferrihydrite after 144 h of reaction. Lin and Puls (13) found that the desorption rate of As(III) and As(V) from clay minerals was significantly decreased with increasing aging time. They explained this phenomenon with the diffusion of arsenic into internal sorption sites, which may not be readily accessible by the bulk solution. O’Reilly (7) found that a significant amount of As(V) bound to goethite (>60%) was not readily dissociated through exposure to 6 mM phosphate solution after 5 months of exposure. Arai and Sparks (23) found in their research that the rate of As(V) desorption from aluminum oxide surfaces decreased with increasing initial adsorption time (3 days-1 year). Furthermore, their EXAFS studies provided microscopic evidence of rearrangement of surface complexes and surface precipitation. Relatively few studies have been conducted to investigate arsenic desorption from soils. For example, Jacobs et al. (14) found that the extractability of sorbed As(V) by 1 M NH4OAc and Bray P solution (0.03 M NH4F and 0.025 M HCl) decreased with increasing sorption time, indicating increased binding strength with increasing reaction time. Elkhatib et al. (24) reported that following 24-h sorption, As(III) desorption from soils was hysteric and only small amounts of the sorbed As(III) were slowly released from five soils. In contrast, CarbonellBarrachina et al. (25) reported that As(III) desorption was reversible from three soils having low sorption affinity to As(III). They suggested that different results could be explained through differences in sorption capacities of the soils used. The objectives of our study were (i) to quantify the kinetics of adsorption and desorption of arsenate in three soils having different physiochemical properties, and (ii) to assess multireaction (equilibrium-kinetic) modeling for its capability of describing the retention as well as the desorption or release behavior of As(V) in different soils.
Materials and Methods Surface soils from the Ap horizon (0-10 cm) of Olivier loam (fine-silty, mixed, thermic Aquic Fragiudalf), Sharkey clay (very fine, montmorillonitic, nonacid, thermic, Vertic Haplaquept), and Windsor sand (mixed, mesic Typic Dipsamment) were used in this study (see Table 1). Both citratebicarbonate-dithionite (CBD) and ammonium oxalate extractions were carried out on these soils by shaking duplicate 50-mL centrifuge tubes containing 0.50 g of soil in 25 mL of extractant for 24 h (26). Soil properties for these benchmark soils, for examplee, pH, CEC, and particle size distribution, were determined earlier in our laboratory by Bucher et al. (17) and are given in Table 1. Kinetic batch experiments were conducted to determine adsorption and desorption isotherms for As(V) to the three soils at constant room temperature of 25 °C under aerobic conditions. Six initial As(V) concentrations Co (5, 10, 20, 40, 80, and 100 mg L-1) of KH2AsO4 were prepared in 0.01 M KNO3 background solution to maintain constant ionic strength. Batch experiments were initiated by mixing 3 g of air-dry soil with 30 mL of As(V) solution in a 40-mL Teflon tube. For each input concentration Co, the tests were performed in triplicate, and the average and coefficient of variation in the amount of As(V) adsorbed is reported. The mixtures were shaken at 150 rpm on a reciprocal shaker and subsequently centrifuged for 10 min at 4000 rpm after each specified reaction time. A 1-mL aliquot was sampled from the supernatant at reaction times of 2, 6, 12, 24, 72, 168, 336, and 504 h. After sampling, the slurry was agitated using a 6102
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TABLE 1. Selected Physical and Chemical Properties of the Studied Soils soila
pH TOCb (%) CECc (cmol kg-1) sandd (%) silt (%) clay (%) selective extraction by ammonium oxalate (pH 3.0) Fe (g kg-1) Al (g kg-1) selective extraction by citratebicarbonate-dithionite (CBD) Fe (g kg-1) Al (g kg-1)
Olivier
Sharkey
Windsor
5.80 0.83 8.6 5 89 6
5.77 1.41 29.6 3 36 61
6.11 2.03 2.0 77 20 3
0.32 0.08
0.83 0.23
0.36 0.69
4.09 1.29
7.77 2.42
3.68 3.65
a Soil samples were collected from Louisiana (Sharkey and Olivier) and New Hampshire (Windsor). b TOC ) total organic carbon. c CEC ) cation exchange capacity. d Grain size distribution: sand (2.00-0.05 mm), silt (0.05-0.002 mm), and clay (
