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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 equilibrium constants were calculated according to the two-layer model by Dzombak & Morel (1990). Near equilibrium between protons/hydroxyls in solution and the ferrihydrite surface was obtained within minutes while equilibration with silicate required days-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 showed 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 the range 940-960 cm-i The shift of the absorption maximum to higher wavenumbers with increasing amount of silicate adsorbed is probably due to an increase in the frequency of Si-O-Si bonds between orthosilicate adsorbed at adjacent sites. Small amounts of goethite were identified in the adsorption products.Silicate, like numerous other oxyanions, binds covalently through ligand exchange at hydrous surfaces of Fe(llI)-(hydr)oxides under the formation of an inner-sphere surface complex (Sigg & Stumm, 1981). Due to its high specific surface area, ferrihydrite is one of the most important adsorbents of silicate evidenced by the often substantial amounts of silicate found in naturally occurring ferrihydrites (Childs, 1992). Because dissolved silicate is abundant in most soils and sediments, it effectively competes with other adsorbates for adsorption sites at the ferrihydrite surface, and hence influences the overall chemical composition of the pore-water (Anderson & Benjamin, 1990; Goldberg, 1985;Sigg & Stumm, 1981). Furthermore, dissolved silicate plays an important role in soil development, and the soil solution concentration of silicate may be used for estimation of the soil mineral weathering rates when extent and rates of silicate adsorption/ desorption and precipitation processes are known (Hansen et al., 1994). Hence, knowledge about the affinity of silicate for the ferrihydrite surface is important.Although adsorption isotherm equations, like the Langmuir expression, may accurately des...