Abstract
Adsorption of cadmium and sulfate in a mixed Fe(OH) 3 SiO 2 system was studied using the triple-layer model. The uptake of H 4SiO 4 dissolved from the SiO 2 particles was considered in modeling the cation and anion adsorption. The mixed oxides were formed with a range of Fe(OH) 3:SiO 2 ratios by controling the precipitation of ferric hydroxide in a suspension of fumed silica (Cab-O-Sil). Observation of the micromorphology of the mixed oxides by transmission electron microscopy showed that Fe(OH) 3 formed separately from the SiO 2 particles, therefore, an independent oxide model was used to simulate cation and anion adsorption. In this model Cd adsorption occurred on separate Fe(OH) 3 and SiO 2 surfaces. When the pH was lower than 8, Fe(OH) 3 dominated Cd adsorption in the mixed system. Sulfate adsorption on the independent Fe(OH) 3 surface was modeled for the mixed oxide system because no sulfate was adsorbed by the SiO 2 surface at pH > 4. The uptake of H 4SiO 4 reduced sulfate adsorption, but it had a negligible effect on cadmium absorption due to a combination of sorbate competition and electrostatic interactions.
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