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TECHNICAL REPORTS

Heavy Metals in the Environment

Desorption Kinetics of Arsenate from Kaolinite as Influenced by pH

M. Quaghebeur^a,b,*, A. Rate^a, Z. Rengel^a, and C. Hinz^a

^a Soil Science and Plant Nutrition, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia
^b Current address: Flemish Institute for Technological Research (Vito), Boeretang 200, BE-2400 Mol, Belgium

^* Corresponding author (mieke.quaghebeur{at}vito.be)

Received for publication January 29, 2004.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Arsenic is highly toxic and therefore represents a potential threat to the environment and human health. The mobility and bioavailability of arsenic in soil is mostly controlled by adsorption and desorption reactions. Even though adsorption and traditional batch desorption experiments provide information about the environmental fate of As, the equilibrium conditions imposed in these studies would usually not be reached in the natural environment. Flow-through desorption techniques, where the desorbed species are removed from the substrate, can therefore be used to provide information about the rate and mechanisms of As desorption. The effect of pH on As adsorption reactions is relatively well understood; however, desorption of As and the effect of pH on As desorption remain unexplored. Desorption of As(V) (the most dominant arsenic species in aerated soils) was therefore investigated using batch and flow-through desorption experiments. Traditional batch desorption experiments underestimated the desorption rate of As(V) from kaolinite. The pH had a large effect on the amount of As(V) desorbed from kaolinite, with both an increase and a decrease in pH (from the initial pH 6.4) enhancing As(V) desorption. Modeling desorption over time revealed that the pH can influence As(V) desorption over extended periods of time.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
ELEVATED arsenic concentrations in soil can originate from anthropogenic sources (mining, agriculture, coal combustion) and from natural occurrence of arsenic in the soil parent material. The weathering of arsenic-containing soil minerals and desorption of arsenic from soil particles will increase arsenic concentration in soil solution and can contaminate drinking and irrigation water. Therefore, understanding arsenic desorption from soil minerals will provide important information about the fate of arsenic in the environment.

The major oxidation states of As in the soil are As(V) (arsenate) and As(III) (arsenite), with As(V) the most dominant species in aerobic soils. Arsenate is a chemical analogue of phosphate and will therefore adsorb strongly to Fe and Al oxides and hydroxides (Hingston et al., 1971). Recent studies have shown that arsenate forms inner-sphere complexes via ligand exchange reactions (bidentate and monodentate) with the goethite surface (Waychunas et al., 1993; Manceau, 1995; Grossl and Sparks, 1995; Fendorf et al., 1997). Xu et al. (1988) observed similar adsorption envelopes for As(V) on alumina (-Al₂O₃) and kaolinite, hence As(V) anions are likely to interact mostly with the aluminol functional group in kaolinite. A combination of macroscopic and spectroscopic studies found that As(V) predominantly forms inner-sphere bidendate binuclear complexes at the aluminum oxide–water interface, regardless of pH, ionic strength, and aging time (Arai et al., 2001; Arai and Sparks, 2002).

In contrast to the relatively good understanding of the adsorption mechanisms of P and As(V) on Fe and Al oxides, the adsorption of As(V) on kaolinite is less well understood. Early work by Muljadi et al. (1966) proposed that P adsorption on kaolinite occurred mainly at the edge of the crystal face at low surface coverage, whereas at high surface coverage P could penetrate into some amorphous regions on the clay surface. In more recent work, the adsorption of As(V) on kaolinite was successfully modeled using a constant capacitance model, suggesting that As(V) formed inner-sphere complexes with the kaolinite surface via ligand exchange mechanisms (Manning and Goldberg, 1996; Goldberg, 2002).

Compared with the numerous studies on sorption of arsenate by soil minerals (e.g., Frost and Griffin, 1977; Fordham and Norrish, 1983; Xu et al., 1988; Goldberg, 1986; Manning and Goldberg, 1996; Arai et al., 2001; Violante and Pigna, 2002), there are relatively few that examine desorption. Lin and Puls (2000) reported desorption of 86 to 97% of As from arsenated kaolinite, using 1 mM phosphate as extracting solution. However, the desorption of As(V) from kaolinite and Al oxides was found to be affected by long-term storage or the aging process (Lin and Puls, 2000; Arai and Sparks, 2002). Determination of As species, which were extracted from As(V)-treated kaolinite, showed that no reduction of As(V) to As(III) took place at the clay surface. However, it was observed that oxidation of As(III)-treated kaolinite occurred even when clay minerals were in anaerobic conditions (Lin and Puls, 2000). Given that changes in speciation of As will result in different mobility of As in the environment, the fact that clay minerals might influence As speciation is important for understanding the fate of As in the environment.

Traditionally, desorption studies are performed using batch desorption experiments (Sparks, 1988; O'Reilly et al., 2001). However, the equilibrium conditions reached in batch desorption experiments (Hinz and Selim, 1999) generally do not reflect naturally occurring processes. In the natural environment, it is most likely that the desorbed species will be removed from the substrate by leaching or plant uptake over time. Hence, the desorption processes will also be driven by a concentration gradient over time (i.e., controlled mainly by time-dependent rather than equilibrium reactions).

Flow-through desorption techniques, where the desorbed species are removed from the substrate, have recently been used often to study the kinetics of desorption processes in soils and can provide information about the rate and mechanisms of the desorption reactions. However, the rate constants obtained in flow-through setups often depend on the flow rate and mixing conditions. Therefore, only apparent rates are studied, and care must be taken when extrapolating results to conditions other than those used experimentally (Sparks, 1988). The use of flow-through systems is, however, suitable for studying the effect of adsorption period, temperature, and pH on desorption, as the aim of these studies is to understand the factors controlling desorption kinetics rather than just describing the kinetics per se. In the study presented here, a flow-through ("thin disc") system was therefore used to investigate the effect of pH on desorption of arsenate from kaolinite.

Recent studies observed significant changes in rhizosphere pH when canola (Brassica napus L.) and velvet grass (Holcus lanatus L.) took up As(V) and P adsorbed on kaolinite (Quaghebeur and Rengel, 2004). Therefore, it was of particular interest to study the effect of pH on As(V) desorption from kaolinite. Changes in pH greatly influence desorption of metals from soils; however, it has been suggested that pH effects on desorption of anionic As species are much less pronounced (Livesey and Huang, 1981; Wenzel et al., 2001). A change in pH can influence the desorption of As(V) from kaolinite (i) directly by changing the amounts of ligands (OH^–) competing with As(V) for desorption sites, or (ii) indirectly by altering the properties (charge) of the kaolinite surface and changing the charge (speciation) of the adsorbed arsenate species (Ioannou and Dimirkou, 1997; Jain et al., 1999).

The aim of this study was to investigate the effect of pH on the desorption kinetics of As(V) from kaolinite using a flow-through system. An initial characterization of the flow-through system was done employing nonreactive tracers to evaluate the effect of mixing in the flow-through cell on the desorption results. Flow interruption was used to test whether arsenate adsorbed to kaolinite was close to equilibrium with arsenate in solution when the flow stopped.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Preparation and Selected Properties of Arsenated Kaolinite
Kaolinite was sampled at a clay pit in Goomalling (Western Australia) (31°24' 56'' S, 116°43' 5'' E). Material was crushed in a stainless steel grinder and washed several times with deionized water until the electric conductivity was below 20 mS m^–1. X-ray diffraction revealed that the clay was kaolinite with minor impurities of quartz and muscovite. It had a specific surface area of 26 m² g^–1 (as determined by the adsorption of N₂ [BET method with a Gemini instrument; Micromeritics, Norcross, GA]); the crystals were mainly euhedral hexagonal platy with <0.2% tubular crystals (probably halloysite) (Model 430 transmission electron microscope, LaB₆ filament, 300 kV; Philips, Eindhoven, the Netherlands). It was the intention of this study to use kaolinite with a minimum of pretreatment to preserve its natural mineralogy. However, impurities such as Fe and Al oxide coatings could enhance oxyanion adsorption on kaolinite (Manning and Goldberg, 1996). In our study, the Fe oxide content (9.2 g Fe₂O₃ kg^–1) in the kaolinite was rather low. In addition, the As(V) adsorption on kaolinite was much lower than that on pure Fe and Al oxides (Manning and Goldberg, 1996). Hence, it can be assumed that Fe and Al coatings in our experimental system were, in the worst-case scenario, only minor constituents.

In preliminary experiments the adsorption isotherm of arsenate on kaolinite was determined to identify the maximum adsorption capacity. Arsenate was supplied as Na₂HAsO₄ in a background of 10 mM KCl. The initial As concentrations in the preliminary experiment ranged from 1 to 15 mg L^–1, and the kaolinite concentration in suspension was 10 g L^–1. The suspensions were shaken for 24 h in the dark at 20°C, and the initial pH of the suspension was adjusted to 6.00 with HCl or KOH (0.01 M) after 1 h. The tubes were centrifuged at 20000 x g for 5 min at 20°C in a temperature-controlled centrifuge. The supernatant was filtered (Whatman [Maidstone, UK] no. 1) and analyzed for As by hydride generation inductively coupled plasma–mass spectrometry (ICP–MS) (see below).

Arsenate was adsorbed onto kaolinite (according to the procedure described above) to prepare a large quantity of arsenated kaolinite with approximately 30% of the maximum adsorption capacity occupied by arsenate. However, in contrast to the preliminary experiment described above, the contact time was increased to 48 h, and the solid-to-liquid ratio was 30 g L^–1. The arsenated kaolinite was then washed with deionized water to remove excess salts, centrifuged (4000 x g for 1 h), dried at 50°C for 8 d, and ground in a mortar and pestle. To determine the exact amount of As adsorbed on kaolinite, the minerals were digested (aqua regia [3:1 HCl to HNO₃]), and the digests were analyzed for As (as described below). The arsenated kaolinite contained 1.51 µmol As g^–1 and the final pH was 6.4 ± 0.1.

To determine the pH buffering capacity of the arsenated kaolinite, 0.25 g of the substrate was suspended in 50 mL of nutrient solution (see below) (5 g L^–1). The pH of the substrate was then automatically recorded after repeated additions of KOH (0.05 M) by an automated titrator (DL55 titrator; Mettler Toledo, Columbus, OH). The small increments (0.02 mL) of KOH (0.05 M) were added when dE was <0.0004 mV s^–1.

Batch Desorption Experiment to Equilibrate the pH of Kaolinite
The desorption experiments were initially designed to investigate whether changes in rhizosphere pH affected arsenate availability and hence influenced uptake by canola of arsenate adsorbed on kaolinite; therefore, the composition of the desorption solution was similar to the nutrient solution used in the canola study (Quaghebeur and Rengel, 2004). The nutrient solution contained 10 µM H₃BO₃, 2 µM MnCl₂, 1 µM ZnSO₄, 0.2 µM CuCl₂, 0.05 µM Na₂MoO₄, 1 mM MgSO₄, 2 mM KNO₃, 2 mM NH₄NO₃, and 2 mM CaCl₂ (ionic strength 0.018 M). Although the nutrient solution was quite complex, its composition was unlikely to have modified the effect of pH on the desorption of As(V) from kaolinite. The anions MoO^2–₄ and SO^2–₄ were of most concern; however, the concentration of MoO^2–₄ was very low, whereas recent studies showed that SO^2–₄ did not affect desorption of As(V) from goethite (O'Reilly et al., 2001).

The arsenated kaolinite was mixed with nutrient solution (2 g L^–1) in plastic batch reactors, and the suspensions were gently mixed with a small stir bar. The pH was adjusted to 5.5, 6.5, or 8.5 by addition of an appropriate amount of 0.1 M HCl or KOH as determined in separate titration experiments. Desorption was conducted in a temperature-controlled room (20°C), and CO₂ was controlled via N₂ purging. The pH was adjusted manually every hour initially (for 6 h) and every 12 h later by small additions of 0.01 M HCl or 0.01 M KOH. Desorption solution (1 mL) was sampled in periods ranging from 15 to 60 min initially for the first 8 h and then every 24 h thereafter and filtered immediately (0.22 µm). The arsenate desorbed from the kaolinite was quantified by determining total As in the desorption solution.

Flow-Through Desorption
Preliminary Experiment: Evaluation of the Flow-Through Reactor
To investigate the effect of mixing conditions in the Swinnex filter holder (Millipore, Billerica, MA) on desorption, a nonreactive pulse experiment was performed with an empty vessel and a vessel containing 0.075 g of kaolinite. A pulse of 0.1 M CaCl₂ was applied to a background of 0.01 M CaCl₂ and the electrical conductivity (EC) was measured continuously with a flow-through EC meter (158 BL; Lazer Research Laboratories, Los Angeles, CA). Data were collected every 10 s by a data logger connected to a computer. The effect of flow rate on mixing conditions was investigated using 3 different flow rates (1, 2.7, and 4 mL min^–1). The flow rate was kept constant during each pulse.

Experiment 1: Desorption of Arsenic(V) from Kaolinite Without Pre-Equilibration of Initial pH of Kaolinite
The arsenated kaolinite (0.075 g) was suspended in 25 mL of the nutrient solution described above. This mixture was quantitatively filtered through a weighed Swinnex filter holder containing a 0.22-µm filter (Millipore). The total concentration of As in the filtrate was measured by hydride generation ICP–MS (see below) so that the amounts of As both sorbed and entrained could be determined. The amounts of entrapped As were then subtracted from the amount present in the first desorption fraction. The Swinnex filter holder containing the substrate (and some entrained liquid) was then connected to a peristaltic pump and positioned on top of a fraction collector. The nutrient solution was pumped through the filter at a rate of 3 mL min^–1 and sampled at 3-min intervals. The total amount of arsenate desorbed from kaolinite was calculated by determining total As in the desorption solution collected (measurements by hydride generation ICP–MS, see below). The total desorption time was 15 h, and the system was maintained at 20°C during the whole desorption period. During the desorption run, the nutrient solution was stirred constantly, and CO₂ was controlled via N₂ purging. The initial amount of As adsorbed on kaolinite was 1.51 µmol g^–1. The pH of desorption solution was monitored continuously and adjusted manually if it deviated more than 0.05 pH units from the desired pH (5.5, 6.5, or 8.5). The initial pH of the arsenated kaolinite was 6.4 ± 0.1.

Experiment 2: Desorption of Arsenic(V) from Kaolinite with pH Pre-Equilibration of Kaolinite
In Experiment 1 there was no pH pre-equilibration of the kaolinite. Therefore, the pH of the arsenated-kaolinite could have changed slightly and gradually (due to the buffer capacity of kaolinite) toward the pH of the desorption solution during the flow-through desorption run. Hence, for a certain pH level, the kinetics of As desorption were influenced by a gradual change in pH together with a maximum concentration gradient. In contrast, in this experiment, the pH of the substrate was pre-equilibrated during 7 d (see batch desorption experiment) to stabilize pH before the flow-through desorption run.

At the end of the batch desorption experiment (7 d), the suspension was mixed, and half (approximately 25 mL) of the amount was filtered through a weighed Swinnex filter holder containing a 0.22-µm Millipore filter. The amount of substrate on the filter ranged from 60 to 80 mg and was weighed accurately at the end of the adsorption run. The initial amounts of As(V) on kaolinite varied for the different pre-equilibration pH values (1.16 µmol g^–1 at pH 5.5; 0.86 µmol g^–1 at pH 6.5; 0.71 µmol g^–1 at pH 8.5). The desorption setup and experimental conditions were similar to those described for the flow-through desorption without pre-equilibrating the initial pH.

Flow Interruption for Experiments 1 and 2
To determine whether the desorption process studied in the flow-through setup was controlled by pH gradients and/or concentration gradients, flow interruption was performed. The flow was stopped for 12 h after 6 h 40 min (Experiment 1 [without pH equilibration]) or 4 h 10 min (Experiment 2 [with pH equilibration]). After the interruption period, desorption was continued at the same flow rate. The pH of desorption solution was pH 5.5. The initial As concentration on kaolinite was 1.51 µmol As g^–1 in Experiment 1 (without pH equilibration), whereas kaolinite pre-equilibrated at pH 5.5 had an initial As concentration of 1.16 µmol g^–1 (Experiment 2).

Chemical Analysis
Total Amount of Arsenic Adsorbed on Kaolinite
The substrate was dried at 105°C for 3 d and ground by hand in a mortar and pestle. The dry substrate was then weighed accurately (1 g) in a glass tube (200 mL) to which 10 mL of aqua regia was added. The mixtures were heated in a digestion block (AIM500; A.I. Scientific, Queensland, Australia) (slowly [20 min] heated to 100°C and kept at that temperature for 30 min followed by further 2 h at 125°C or until the volume of acid was reduced to 4 to 5 mL). The tubes were then allowed to cool to room temperature, and the volume was made up to 20 mL with MilliQ water (Millipore). The mixtures were mixed (vortex) and filtered (Whatman no. 1). Stream sediment (STSD-3) (Mining and Mineral Sciences Laboratories, Ottawa, ON, Canada) was included as a certified reference material. It contained 22 mg As kg^–1 (recovery 109 ± 1%).

Total Arsenic Concentrations in Desorption Solutions and Kaolinite Digests
Total arsenic concentration in desorption solutions was measured by mixing 9 mL of desorption solution with 1 mL of a reducing solution containing 15% (w/v) potassium iodide, 15% (w/v) ascorbic acid, and 40% (v/v) hydrochloric acid. This mixture was heated in a water bath at 50°C for 1 h. Total arsenic was determined by a hydride generation technique using a PerkinElmer B050-5540 continuous flow vapor system interfaced with a SCIEX ELAN 6000 series ICP–MS (PerkinElmer, Wellesley, MA) (Quaghebeur et al., 2003). A few samples were analyzed for As speciation (Quaghebeur et al., 2003) to confirm that no reduction of As(V) to As(III) occurred during the desorption experiments. Total As in the kaolinite digest (see above) was determined by hydride generation ICP–MS as described above, except that 1 mL of digest was mixed with 9 mL of reducing solution containing 1.5% (w/v) potassium iodide, 1.5% (w/v) ascorbic acid, and 10% (v/v) hydrochloric acid.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Characterization of the Flow-Through Setup
Evaluating the Mixing Conditions in the Flow-Through Cell
The aim of investigating As(V) desorption kinetics was to quantify the amount of As(V) desorbed over time. However, when using a flow-through setup, the desorbed As(V) species were not removed directly from the desorption cell. Therefore, it was necessary to characterize the duration of the mixing conditions in the desorption cell. The presence of kaolinite (0.075 g) in the flow-through cell increased the time during which mixing conditions in the flow-through cell influenced the desorption results, although the effect was much more pronounced for the high and medium flow rates (4 and 2.7 mL min^–1) compared with the low flow rate (1 mL min^–1) (results not presented). As expected, increasing the flow rate reduced the time during which mixing conditions in the flow-through cell could affect desorption. In the flow-through setup, the mixing durations were quite short; hence, recommended minimum eluent collection times ranged from 350 s (for low flow rate [1 mL min^–1]) to 125 s (for high flow rate [4 mL min^–1]) (Fig. 1) . Based on this information, eluent was sampled every 180 s (with a flow rate of 3 mL min^–1) to investigate As(V) desorption from kaolinite. In studies using similar flow-through setups, collection times ranged from 120 to 360 s with flow rates from 1.5 to 3 mL min^–1 (Backes et al., 1995; McLaren et al., 1998).

View larger version (24K): [in this window] [in a new window]   Fig. 1. Nonreactive pulse (0.1 M CaCl2 applied to a background of 0.01 M CaCl2) to investigate the mixing conditions in a flow-through desorption cell with the presence of 0.075 g kaolinite. The flow rates used were 1 mL min–1 (low), 2.7 mL min–1 (medium), and 4 mL min–1 (high). Changes in electric conductivity over time were presented relative to the electric conductivity of the applied pulse (EC/EC0). Data represent the means of two replicates.

View larger version (31K): [in this window] [in a new window]   Fig. 2. Relative As concentration in the eluent during flow-through desorption from kaolinite before and after a 12-h flow-interruption period. The flow interruption (arrow) was applied to flow-through desorption experiments without or with pre-equilibration of pH of the substrate. The pH of the desorption solution was 5.5 (in the flow-through desorption experiment and in the 7-d pre-equilibrating period).

View larger version (17K): [in this window] [in a new window]   Fig. 3. The (a) pH and (b) buffer capacity of arsenated kaolinite determined by addition of KOH (0.05 M) to 0.25 g substrate suspended in 50 mL nutrient solution (same as that used in desorption experiment). KOH (0.05 M) was added in small increments (0.02 mL when dE was <0.0004 mV s–1) to the suspensions by a pH-stat apparatus.

Without pH pre-equilibration, the solution of pH 8.5 desorbed slightly more As(V) from kaolinite (initial pH 6.4) compared with desorption solutions with pH 5.5 or 6.5. There was no significant difference in the total amount of As(V) desorbed using desorption solutions with pH 5.5 and 6.5 (Table 2, Fig. 4) . Therefore, it seemed that the difference in As(V) desorption from kaolinite caused by differences in pH of the desorption solution was only minor. As mentioned earlier, in the flow-through desorption without pH pre-equilibration, the pH of the substrate most likely changed slowly during the desorption run, with desorption hence controlled by both a gradient in pH and a concentration gradient. Therefore, conclusions about the relative importance of the mechanisms with regards to the effect of pH on As(V) desorption from kaolinite that was not pH pre-equilibrated cannot be made.

View larger version (29K): [in this window] [in a new window]   Fig. 4. The effect of pH on desorption of As(V) from kaolinite using a flow-through desorption setup. Desorption solutions of different pH levels (5.5, 6.5, or 8.5) were used to desorb As(V) from kaolinite. Each pH level was replicated two times (R1, R2). The initial pH of the arsenated kaolinite was 6.4 ± 0.1 and the pH was not pre-equilibrated.

View larger version (24K): [in this window] [in a new window]   Fig. 5. The effect of pH on desorption of As(V) from kaolinite using a flow-through desorption setup. Desorption solutions of different pH levels (5.5, 6.5, or 8.5) were used to desorb As(V) from kaolinite. The initial pH of the arsenated kaolinite was 6.4 ± 0.1; however, the pH was pre-equilibrated during 7 d in a batch desorption experiment to obtain arsenated substrate with initial pH of 5.5, 6.5, or 8.5 before the start of the flow-through desorption period. Each desorption run was replicated two times, but only one representative replicate is shown (replicates produced overlapping data). The results modeled according to Backes et al. (1995) (Table 3) are presented as a solid line at each pH.

Kinetics of Arsenic(V) Desorption from Kaolinite
A wide range of models have been used to describe ion adsorption to, and desorption from, soil minerals (Sparks, 1988; Kithome et al., 1998). The Elovich equation and a two rate-constant equation both appear successful in describing phosphate release from soils (Chien and Clayton, 1980; Onken and Matheson, 1982). However, conformity of kinetic data to a particular equation does not necessarily result in the best model, nor can one propose mechanisms based on this fit alone (Sparks, 1988). In the study presented here, the kinetic data will be discussed according to a two rate-constant model that provided excellent fit to the experimental data.

A model assuming the occurrence of two simultaneous first-order reactions described by Backes et al. (1995) was used to describe the desorption of As(V) from kaolinite. The mathematical expression of this model is C_M = C₁ exp(–k₁t) + C₂ exp(–k₂t), where C_M is the concentration of As(V) adsorbed to kaolinite at time t; and C₁ and C₂ are the initial concentrations (t = 0) of As(V) bound to sites with first-order desorption rate constants k₁ and k₂, respectively (Table 3). It was assumed that all the As(V) ions would eventually (t = ) desorb, meaning that C₁ + C₂ = total As(V) adsorbed. The fit to experimental data was achieved by optimizing values for C₁, k₁, and k₂. Since the total As(V) adsorbed at t = 0 was fixed by the experimental data, C₂ was in effect fixed by the fitted value of C₁.

The flow interruption experiments revealed that without pre-equilibration of the substrate pH, the surface was far from equilibrium. It was therefore likely that desorption observed was driven by a combination of processes (see earlier) and did not necessarily follow a simple first-order reaction. When the substrate pH was pre-equilibrated for 7 d before the flow-through desorption, flow interruption experiments revealed that the substrate was closer to equilibrium. Therefore, desorption at a certain pH was mainly driven by a concentration gradient, making it more likely that the desorption reaction obeyed the first-order kinetics. Consequently, only flow-through desorption after pH pre-equilibration can be interpreted when fitted to two simultaneous first-order reactions.

The rate constant for desorption from the slow sites (k₂) decreased in the order pH 8.5 > 5.5 > 6.5 (Table 3). Similarly, the total amount of As(V) desorbed from kaolinite was higher at pH 8.5 (60% of the initial amount adsorbed) compared with the amount desorbed at pH 5.5 (47%) and pH 6.5 (38%). Therefore, the desorption rate constant at the slow sites (k₂) for each pH corresponded well with the total amount of As(V) desorbed after 15 h. However, the desorption rate (k₁) at the fast reaction sites for each pH did not correspond to the total amount of As(V) desorbed at each pH. The rate constant for the fast reaction (k₁) followed the order pH 6.5 = 8.5 > 5.5.

For all pH levels, the k₁ desorption rate constant was approximately 100 times higher than the desorption rate constant at the slow sites (k₂). Therefore, one would expect that the total amount of As desorbed would be mainly controlled by the fast reaction (k₁). The kaolinite, however, contained a greater number of slow desorption sites (C₂) (65–78% of the total sites) controlled by k₂ compared with fast desorption sites (C₁) controlled by k₁ (Table 3); moreover the t_1/2 for the fast reaction was 20 min, therefore dominating the initial desorption (0–60 min). In addition, the amount of As(V) desorbed from kaolinite at different pH levels corresponded well with the slow reaction rate constant (k₂). Therefore, we can assume that changes in pH mainly influence the amount of As(V) desorbed from the slow desorption sites (C₂), which indicates that the effect of pH can have an effect on As(V) desorption from kaolinite over a long period of time.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The amount of As(V) desorbed over time using a flow-through setup (desorbed species are removed over time) was higher in comparison with the amount desorbed over time during a traditional batch desorption setup. Therefore, batch desorption underestimated the amount of As(V) desorbed over time. Flow-through desorption experiments can hence be useful to predict As desorption behavior for soils with significant aluminosilicate content. This type of desorption could also be used for studying As(V) desorption behavior of other common soil minerals.

Changes in pH (both an increase and a decrease from the initial pH 6.4) can increase the amount of As(V) desorbed from kaolinite. Parameters obtained from modeling the kinetic data showed that the pH will mainly effect the slow desorption sites, and therefore a change in pH can have a long-term effect on As(V) desorption. Recent studies observed significant changes in the rhizosphere pH when canola took up As(V) and P adsorbed on kaolinite (Quaghebeur and Rengel, 2004). The results presented here therefore suggest that changes in the rhizosphere pH are likely to affect As(V) uptake by plants. Hence, rhizosphere chemistry should be taken into account when studying the fate of As in the plant–soil system (plant toxicity, phytoremediation, etc.).

	ACKNOWLEDGMENTS

 
The authors would like to thank Mr. Edgardo Alarcon Leon for technical help during the evaluation of the flow-through setup.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Corresponding author (zrengel{at}fnas.uwa.edu.au).

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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