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 effect s of ionic strength on ion adsorption, it is possible to distinguish b etween inner and outer sphere ion surface complexes. This study was co nducted to evaluate ionic strength effects on SO4 and PO4 adsorption o n gamma-Al2O3 and kaolinite at varying solution pH (3-11), and to infe r SO4 and PO4 adsorption mechanisms at the mineral-water interface. Th e adsorption of SO4 on gamma-Al2O3 and kaolinite decreased monotonical ly with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO4 ad sorption on gamma-Al2O3 and kaolinite increased from pH 3 to 4 and dec reased 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 increas ing ionic strength, whereas at high pH, PO4 adsorption increased sligh tly with increasing ionic strength, resulting in a crossover point whe re there was no ionic strength effect. The triple-layer model (TLM) wa s applied to model the adsorption of SO4 and PO4 with both inner and o uter sphere complexes using the FITEQL 3.1 computer program. Sulfate a dsorption was better modeled by assuming outer sphere complex formatio n, while PO4 adsorption was better modeled by assuming inner sphere co mplex formation.