In order to make sound decisions regarding arsenate contamination rish, 1979, 1983; Jacobs et al., 1970). Arsenate sorption in soil and water environments, it is necessary to have a thorough understanding of the mechanisms of arsenate sorption and desorption on soils and soil components vs. pH increases until maxiover extended periods. The major objectives of this study were to mum sorption is reached and then sorption decreases determine the effects of aging or residence time on the kinetics of with further pH increase (Goldberg and Glaubig, 1988; arsenate sorption and desorption on goethite, and to combine spectro-Xu et al., 1988). For example, arsenate sorption on scopic x-ray absorption fine structure (XAFS) and macroscopic studies montmorillonite and kaolinite increased at low pH, disin order to determine sorption and desorption mechanisms over time played a peak near pH 5, and decreased at higher pH at pH 4 and 6. Sorption studies, conducted from 4 min to ≈12 mo, values (Goldberg and Glaubig, 1988). showed that arsenate sorption on goethite increased with time. Sorp-Goethite (␣-FeOOH), the most common iron oxide tion was initially rapid, with over 93% arsenate being sorbed in a 24-h in soils, has double bands of FeO 3 (OH) 3 octahedra period at pH 6. Similar arsenate adsorption behavior was observed which share edges and corners to form 2 by 1 octahedra at pH 4. Analysis of the samples with extended x-ray absorption fine structure (EXAFS) revealed that there exist two distinct atomic shells tunnels (only large enough to accommodate the passage surrounding the adsorbed As. The closest atomic shell was identified of protons) partially bonded by H bonds (Cornell and as an O atom, the next shell out was identified as an Fe atom. The
make better predictions about the mobility of contaminants, it is critical that time-dependent metal sorption To improve predictions of the toxicity and threat from Pb contamiand desorption behavior on soil be understood, as well nated soil, it is critical that time-dependent sorption and desorption behavior be understood. In this paper, the sorption and desorption as the mechanism of the sorption-desorption reactions. behavior (pH ϭ 5.50, I ϭ 0.05 M) of Pb in a Matapeake silt loam Lead sorption behavior is often initially fast, followed soil (Typic Hapludult) were studied by stirred-flow and batch experiby a slow reaction (Benjamin et al., 1981; Hayes et al., ments. In addition, we studied the effects of soil organic matter (SOM) 1986; Strawn et al., 1998). The fast reaction is most likely on sorption and desorption behavior by treating the soil with sodium adsorption via electrostatic attraction, and/or innerhypochlorite to remove the SOM fraction, and using a soil with six sphere complexation with functional groups present on times as much SOM (St. Johns loamy sand [Typic Haplaquods]) as the soil components. There are several possible reasons the Matapeake soil. Lead sorption consisted of a fast initial reaction for the slow sorption steps, such as: slow interparticle in which all of the Pb added to the stirred-flow chamber was sorbed. diffusion in porous minerals and organic matter, forma-Following this initial fast reaction, sorption continued and appears to tion of precipitates on surfaces which can sometimes be be rate limited (indicated by a decrease in the outflow concentration when the flow rate was decreased, or when the flow was stopped). slower than typical sorption, and adsorption onto sites The total amount of Pb sorbed was 102, 44, and 27 mmol kg Ϫ1 for that have relatively large activation energies (Fuller et the St. Johns soil and the untreated and treated Matapeake soils,
The fate of Pb in the environment is highly dependent on sorption and desorption reactions on solid surfaces. In this study Pb sorption and desorption kinetics on γ-Al 2 O 3 at pH 6.50, I ) 0.1 M, and [Pb] initial ) 2 mM were investigated using both macroscopic and spectroscopic measurements. X-ray absorption fine structure (XAFS) spectroscopy revealed a Pb-Al bond distance of 3.40 Å, consistent with an inner-sphere bidentate bonding mechanism. XAFS results show no change with time in the average local atomic structure surrounding the Pb and no indication of the formation of Pb surface precipitates. Adsorption kinetics were initially fast, resulting in 76% of the total sorption occurring within 15 min, followed by a slow continuous sorption reaction likely resulting from diffusion through micropores. Desorption at I ) 0.1 M and pH 6.50 was studied using a cation-exchange resin as a sink for Pb(aq). Under these conditions, Pb desorption was 98% reversible within 3 days of incubation time. Furthermore, desorption and adsorption kinetics demonstrated similar trends: a fast reaction followed by a slow reaction. The use of spectroscopy combined with adsorption and desorption kinetic studies has revealed important information on the interaction between lead and aluminum (hydr)oxides. This information is valuable for predicting the fate of Pb in the environment. FIGURE 3. Desorption of Pb from γ-Al2O3 by replenishing background electrolyte every 24 h. Data points are from three separate experiments.FIGURE 4. Lead desorption kinetics from γ-Al2O3 using Na-saturated resins, pH 6.50, I ) 0.1 M. Error bars are standard deviations of three samples.dSPb + 2H + a dSH 2 + Pb 2+
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