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• The solid material properties not only control the degree to which arsenic is adsorbed at <br />a given pH, but also the amount of arsenic that can be adsorbed before the surface of the <br />solid becomes saturated. The process is described mathematically by the Langmuir <br />Isotherm, which is as follows: <br />C (solid) = KI*Am*C(soln)/(1+K1*C(soln)) (2) <br />Where, <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 />KI = Langmuir adsorption constant <br />Examples of 1--angmuir Adsorption Isotherms for three different solid materials are <br />illustrated in Figure 3. <br />80000 <br />Fe(OH)3 Am = 82,412 mg/kg KI = 0.03 mg/L <br />m 70000 <br />Y <br />E 60000 <br />in 50000 <br />c 40000 <br />0 <br />I "- <br />M <br />30000 <br />c <br />a) <br />U 20000 - - - <br />v° Montmorillonite Am = 742 mg/kg KI = 20.2 mg/L <br />10000 - -- <br />0 Am = 539 mg/kg KI = 21.6 mg/L <br />0 <br />0 10 20 30 40 50 <br />Concentration in Solution (mg/L) <br />Figure 3 - Langmuir Isotherms illustrating arsenate adsorption capacities of <br />Fe(OH)3(s), kaolinite, and montmorillonite at a pH of 5 su. Langmuir adsorption <br />constants (KI and Am) are from Pierce and Moore (1982) for Fe(OH).;(s) and Frost and <br />Griffin (1977) for kaolinite and montmorillonite. <br />The adsorption of arsenate as illustrated in Figure 3 can be understood by imagining a <br />"clean" soil or sediment which is subjected to waters with increasing arsenate <br />. COTICCntratiolls (such as a with the C\hansioll cat all ai-s,enatc-bcaring; groundwater