MONOSILICATE ADSORPTION BY FERRIHYDRITE AND GOETHITE AT PH 3-6

The constant capacitance model and the Elovich equation were combined in the following mathematical expression enabling calculation of the a mount of silicic acid adsorbed by iron oxides as a function of the Si concentration, pH, soil:solution ratio, and reaction time: [GRAPHICS] K(a1) is the protolytic surface constant and K(Si) is the stability co nstant for the Fe oxide-silicate surface complex; -logK(a1) = 6.40-0.5 4(8-pH) and logK(Si) = 3.85 for ferrihydrite and goethite. Good agreem ent was found between calculated Si adsorption and the amount actually found to be adsorbed by synthetic ferrihydrite and goethite at differ ent pH (3-6), Si concentration (1-600 muM), solid:solution ratio (1:30 0, 1:1200) and reaction time (1-480h).