Ionic Strength Effects on Sulfate and Phosphate Adsorption on γ‐Alumina and Kaolinite: Triple‐Layer Model

It is known that PO 4 is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO 4 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 SO 4 and PO 4 adsorption on γ‐Al 2 O 3 and kaolinite at varying solution pH (3–11), and to infer SO 4 and PO 4 adsorption mechanisms at the mineral‐water interface. The adsorption of SO 4 on γ‐Al 2 O 3 and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO 4 adsorption on γ‐Al 2 O 3 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 PO 4 adsorption varied with pH. At low pH, PO 4 adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO 4 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 SO 4 and PO 4 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 PO 4 adsorption was better modeled by assuming inner sphere complex formation.