Modeling of sulfate adsorption on andisols for implementation in the SAFE model

Sulfate adsorption models were investigated in this study to simulate the pH dependence of SO₄^2- and H⁺ adsorption on Andisols and thereby show the effects on acidification and recovery processes of Andisols. An extended constant capacitance model (ECCM), which assumed protonation of surface -OH groups and outer-sphere complexation of SO₄^2-, proved valid to simulate SO₄^2- and H⁺ adsorption on Andisols when evaluated using batch experiment data of SO₄^2- and base cation adsorption at pH 4 to 5 and low ionic strengths. According to the ECCM calculation, SO₄^2- adsorption was reproduced by an empirical equation using SO₄^2- and H⁺ concentrations and H⁺ adsorption was approximated by another empirical equation using pH and SO₄^2- adsorption. Since these two empirical equations are quite simple, they will be an efficient SO₄^2- adsorption submodel in SAFE, the dynamic soil chemistry model to simulate the soil acidification and recovery processes.