STABILITY-CONSTANTS FOR SILICATE ADSORBED TO FERRIHYDRITE

Intrinsic surface acidity constants (K(a1)intr, K(a2)intr) and surface complexation constant for adsorption of orthosilicate onto synthetic ferrihydrite (K(Si) for the complex = FeOSi(OH)3) have been determined from acid/base titrations in 0.001-0.1 m NaClO4 electrolytes and sili cate adsorption experiments in 0.01 m NaNO3 electrolyte (pH 3-6). The surface equilibrium constants were calculated according to the two-lay er model by Dzombak & Morel (1990). Near equilibrium between protons/h ydroxyls in solution and the ferrihydrite surface was obtained within minutes while equilibration with silicate required days-weeks, both re actions probably being diffusion controlled. Applying the values for s pecific surface area and site densities for ferrihydrite used by Dzomb ak & Morel (1990) (600 m2 g-1, 3.4 mumole m-2) the constants pK(al)int r 6.93 +/- 0.12, pK(a2)intr = 8.72 +/- 0.17 and log K(Si) = 3.62 were calculated by using the FITEQL optimization routine. Use of the specif ic surface area actually measured (269 m2 g-1) gave a poorer fit of th e experimental data. Due to the slow adsorption of silicate and hence long shaking times, changes in the surface characteristics of the ferr ihydrite seem to take place, probably a decrease in the concentration of surface sites. Adsorption isotherms calculated using the derived eq uilibrium constants showed that approximately twice the amount of sili cate was adsorbed at pH 5 compared with pH 3. Infrared spectroscopy of silica adsorbed to ferrihydrite showed Si-O stretching absorption max ima in the range 940-960 cm- 1. The shift of the absorption maximum to higher wavenumbers with increasing amount of silicate adsorbed is pro bably due to an increase in the frequency of Si-O-Si bonds between ort hosilicate adsorbed at adjacent sites. Small amounts of goethite were identified in the adsorption products.