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• Phosphate competes with arsenate for adsorption sites resulting in less arsenate <br />adsorption and greater mobility. Other ions such as chloride, sulfate, and nitrate have <br />little or no effect on arsenic adsorption. <br />Effect of Silica <br />Silica competes with arsenic for adsorption sites, and can affect both the effectiveness <br />and the adsorption capacity of adsorption media such as Sorb33. As the pH of the <br />solution increases (above about 8.5 su), not only does the surface charge of the media <br />become negative, which tends to repel negatively charged arsenic oxyanions, but the <br />dissolved silica species go from neutral species to predominantly charged anions, which <br />compete with arsenic for specific adsorption sites (see Figure 4). <br />Silica <br />LI <br />H,SiO, <br />H,SiO,° <br />0 2 4 6 8 10 12 14 <br />1 <br />0.9 <br />0.8 <br />0.7 <br />0.6 <br />?a <br />a 0.5 <br />Q <br />0.4 <br />03 <br />0.2 <br />0.1 <br />0 <br />pH <br />Figure 4 - Silica speciation as a function of pH (alpha is the fraction of the total <br />dissolved silica consisting of the given species). <br />References <br />Anderson, M.A., J.F. Ferguson and J. Gavis. 1976. Arsenate adsorption on amorphous <br />aluminum hydroxides. /. Colloid & Inferfi:ce Sci, v. 54 p. 391-399. <br />Duel, L.E. and A. R. Swoboda. 1972. Arsenic solubility in a reducted environment. Soil <br />Sci. Soc. Am. Proc. 36:276-278. <br />0