Ionic strength effects on sulfate and phosphate adsorption on gamma-alumina and kaolinite: triple-layer model

It is known that PO4 is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO4 adsorption at the mineral-water interface has been in debate. By studying the effects of ionic strength on ion adsorption, it is possible to distinguish between inner and outer sphere ion surface complexes. This study was conducted to evaluate ionic strength effects on SO4 and PO4 adsorption on gamma-Al2O3 and kaolinite at varying solution pH (3-11), and to infer SO4 and PO4 adsorption mechanisms at the mineral-water interface. The adsorption of SO4 on gamma-Al2O3 and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO4 adsorption on gamma-Al2O3 and kaolinite increased from pH 3 to 4 and decreased from pH 6 to 11, with an adsorption plateau between pH 4 and 6. Effects of change in ionic strength on PO4 adsorption varied with pH. At low pH, PO4 adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO4 adsorption increased slightly with increasing ionic strength, resulting in a crossover point where there was no ionic strength effect. The triple-layer model (TLM) was applied to model the adsorption of SO4 and PO4 with both inner and outer sphere complexes using the FITEQL 3.1 computer program. Sulfate adsorption was better modeled by assuming outer sphere complex formation, while PO4 adsorption was better modeled by assuming inner sphere complex formation