STABIL ITY CONSTANTS FOR S IL ICATE ADSORBED TO FERRIHYDRITE

A B ST R ACT: Intrinsic surface acidity constants (Kal intr, Ka2 imr) and surface complexation constant for adsorption of orthosilicate onto synthetic ferfihydrite (Ksi for the complex-=FeOSi(OH)3) have been determined from acid/base titrations in 0.001-0.1 M NaCIO4 electrolytes and silicate adsorption experiments in 0.01 M NaNO3 electrolyte (pH 3-6). The surface quilibrium constants were calculated according to the two-layer model by Dzombak & Morel (1990). Near equilibrium between protons/hydroxyls in olution and the ferrihydrite surface was obtained within minutes while equilibration with silicate required ays-weeks, both reactions probably being diffusion controlled. Applying the values for specific surface area and site densities for ferrihydrite used by Dzombak & Morel (1990) (600 m 2 g- l, 3.4/zmole m-e) the constants pKal intr = 6.93 + 0.12, pKa2 intr = 8.72 + 0.17 and log Ksi = 3.62 were calculated by using the FITEQL optimization routine. Use of the specific surface area actually measured (269 m 2 g- 1) gave a poorer fit of the experimental data. Due to the slow adsorption of silicate and hence long shaking times, changes in the surface characteristics of the ferrihydrite seem to take place, probably a decrease in the concentration of surface sites. Adsorption isotherms calculated using the derived equilibrium constants howed that approximately twice the amount of silicate was adsorbed at pH 5 compared with pH 3. Infrared spectroscopy of silica adsorbed to ferrihydrite showed Si-O stretching absorption maxima in th