Sulfate adsorption on a volcanic ash soil (allophanic Andisol) under low pH conditions

The mechanisms of SO₄ adsorption on clays have been investigated by many researchers. However, few researches have focused on the fraction of SO₄ that is adsorbed in the diffuse layer to the total adsorbed SO₄. We investigated SO₄ adsorption in detail on an allophanic Andisol (volcanic ash soil), especially the fraction of SO₄ adsorbed in the diffuse layer to the total adsorbed SO₄, conducting experiments under conditions of low pH (pH 3.3 and 4.3) and low ion concentrations (1.0 and 0.1mol_cm^-3) to avoid a strong negative surface charge of the soil particles. SO₄ and NO₃ adsorption under their competitive conditions were measured by a batch method using mixtures of HNO₃ and H₂SO₄. Exchangeable SO₄ and NO₃ were extracted with 1000mol_cm^-3 KCl. Strongly adsorbed SO₄ was extracted with 10mol_cm^-3 NaOH after the extraction with 1000mol_cm^-3 KCl. The exchangeable SO₄ made up to 72-77% of the total adsorbed SO₄. These results suggested that both inner-sphere and outer-sphere complexes co-exist in the allophanic Andisol at low pH. SO₄ was strongly selective over NO₃ under these conditions. We compared adsorbed amounts calculated by the Gouy-Chapman model with the measured values at solution conditions of pH 3.3 and 1.0mol_cm^-3. The model overestimated NO₃ adsorption and underestimated SO₄ adsorption. The difference is due to the fact that SO₄ adsorption in the Stern layer is neglected. Next, we calculated SO₄ adsorbed in the diffuse layer using the Stern-Gouy-Chapman model under the assumption that all the measured NO₃ adsorbed was in the diffuse layer. Our results indicated that the SO₄ in the diffuse layer made up only less than 6% of the total adsorbed SO₄. Most of the adsorbed SO₄ is likely to be found in direct contact with the soil surface.