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.