Modeling cadmium and sulfate adsorption by Fe(OH)₃/SiO₂ mixed oxides

Adsorption of cadmium and sulfate in a mixed Fe(OH)₃/SiO₂ system was studied using the triple-layer model. The uptake of H₄SiO₄ dissolved from the SiO₂ particles was considered in modeling the cation and anion adsorption. The mixed oxides were formed with a range of Fe(OH)₃:SiO₂ 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 SiO₂ 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 SiO₂ 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 SiO₂ surface at pH > 4. The uptake of H₄SiO₄ reduced sulfate adsorption, but it had a negligible effect on cadmium absorption due to a combination of sorbate competition and electrostatic interactions.