The constant capacitance model and the Elovich equation were combined in the following mathematical expression enabling calculation of the amount of silicic acid adsorbed by iron oxides as a function of the Si concentration, pH, soil:solution ratio, and reaction time:
Ka1 is the protolytic surface constant and Ksi is the stability constant for the Fe oxide-silicate surface complex; -logKa1 = 6.40–0.54(8-pH) and logKsi = 3.85 for ferrihydrite and goethite. Good agreement was found between calculated Si adsorption and the amount actually found to be adsorbed by synthetic ferrihydrite and goethite at different pH (3–6), Si concentration (1–600 μM), solid:solution ratio (1:300, 1:1200) and reaction time (1–480h).
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