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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

View larger version (24K): [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 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 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.

View larger version (29K): [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 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.