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<br />Amorphous Silica Si 02 2.0 <br />Manganese Dioxide 6-Mn02 2.8 <br />Montmorillonite Clay Nao.2Cao.,A12Si401o OH 2.10 H2O 2.5 <br />Kaolinite Clay A12Si205 OH 4 4.6 <br />1. Data from Stumm and Morgan (1981) <br />The materials with a higher pHzPc are able to maintain a positive charge at a higher pH <br />than for materials with a lower pHzPc. Of the materials listed in Table 1, amorphous <br />ferric hydroxide is the best anion adsorbent at higher pH values (below 8.5). <br />Experimental adsorption/ desorption data for antimony are very sparse in the literature. <br />Nakamaru et al. (2006) obtained partition coefficient values (Kds) for 110 Japanese <br />agricultural soils ranging from 1 to 2065 L/kg, with a geometric mean of 62 L/kg. The <br />Kd value decreased (antimony became more mobile) with increasing pH and phosphate <br />concentrations. The pH effect is likely due to the more negative surface charge of the <br />soils with increasing pH. The phosphate effect was due partially to competitive <br />adsorption between antimony and phosphate. <br />Under typical Eh/pH conditions, Sb(+3) is a neutral ion (Sb(OH)30) and is not attracted <br />to negatively or positively charged surfaces. However, specific adsorption onto specific <br />sites can still occur. Ettler et al. (2007) found that the most effective solution for leaching <br />antimony from smelter impacted soils and stream sediments was a 0.1 M Na2HP04 <br />solution. The effectiveness of the phosphate solution may be attributed to competitive <br />. adsorption between phosphate and antimony. Watkins et al. (2006) found that Sb(+3) <br />adsorbs specifically to goethite (a-Fe00H), which can be described using the Langmuir <br />Isotherm model. <br />The Langmuir Isotherm, is described as follows: <br />C (solid) = Kl*Am*C(soln)/(1+K1*C(soln)) <br />Where, <br />(2) <br />C(solid) = concentration of arsenic adsorbed to the solid phase (mg/kg) <br />C(soln) = concentration of arsenic dissolved in the solution phase (mg/L) <br />Am = maximum adsorption capacity of the solid (mg/kg) <br />Kl = Langmuir adsorption constant <br />Examples of Langmuir Adsorption Isotherms for three different solid materials are <br />illustrated in Figure 4. <br />0