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plume). As the solution arsenate concentrations increase, increasingly greater amounts <br />of arsenate can be "forced" onto the solid surface. The steep part of the curve where soils <br />arsenate concentration increase rapidly describes the process. As the arsenate <br />concentrations on the soil continue to increase, a point is eventually reached where the <br />solid surfaces are completely saturated with arsenate and there is no more capacity for <br />additional; arsenate adsorption. No matter how high the dissolved arsenate <br />concentrations become, the solid arsenate concentration remains constant. The flat part <br />of the curve describes the saturation point of the solid. The Langmuir Am constant is the <br />adsorption capacity and determines the level of the flat portion of the curve, while the Kl <br />constant determines the rate at which Am is reached (the steepness of the initial segment <br />of the curve). <br />Figure 3 shows that at pH 5, iron hydroxide has a much higher arsenate adsorption <br />capacity than montmorillonite or kaolinite clays. Theoretically, a sample of ferric <br />hydroxide could be analyzed, and the concentration of arsenic could be compared to <br />Am. If the analytical result of the solid is significantly higher than Am, then arsenate is <br />likely controlled by coprecipitation rather than adsorption. However, in practice, soils <br />and sediments are rarely composed of a single phase, but are instead heterogeneous <br />mixtures of different minerals with varying amounts of iron hydroxide present. <br />However, the affinity of arsenate for iron minerals such as iron hydroxide can be used to <br />evaluate the fate and transport of arsenate when exposed to soils of varying iron <br />contents. <br />pH also has a significant effect on the adsorption capacity of arsenic, as shown in Table • <br />2. <br />Table 2 - Adsorption Capacity of Arsenate and Arsenite vs. H <br /> <br />Arsenate Adso tion Capacity (mg/kg) Arsenite Adsorption <br />Capacity m /k <br />H Fe(OH)3 s' AI(OH)3 s 2 Fe(OH)3 s' <br />5 82,412 119,872 34,688 <br />6 63,682 110,732 37,685 <br />7 34,014 88,331 38,434 <br />8 16,932 62,783 36,561 <br />9 10,189 37,535 31,242 <br />1. Pierce and Moore (1982) <br />2. Anderson et al. (1976) <br />The pH dependence is due to the speciation of arsenic and the surface charge of the solid <br />at different pH values. Arsenate is a negatively charged ion (anion) at pH values greater <br />than about 2 (Figure 1), while the aluminum and iron hydroxides tend to be positively <br />charged. However, as the pH increases, the surfaces of the solids become less positive <br />and the arsenate species become increasingly negative resulting in fewer adsorption <br />sites. Arsenite, being a neutral species below pH 9 (Figure 2), is relatively insensitive to <br />changes in pH. <br />0