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

The mechanisms of SO(4) adsorption on clays have been investigated by many researchers. However, few researches have focused on the fraction of SO(4) that is adsorbed in the diffuse layer to the total adsorbed SO(4). We investigated SO(4) adsorption in detail on an allophanic Andisol (volcanic ash soil.), especially the fraction of SO(4) adsorbed in the diffuse layer to the total adsorbed SO(4), conducting experiments under conditions of low pH (pH 3.3 and 4.3) and low ion concentrations (1.0 and 0.1 mol(c) m(-3)) to avoid a strong negative surface charge of the soil particles. SO(4) and NO(3) adsorption under their competitive conditions were measured by a batch method using mixtures of HNO(3) and H(2)SO(4). Exchangeable SO(4) and NO(3) were extracted with 1000 mol(c) m-3 KCI. Strongly adsorbed SO(4) was extracted with 10 mol(c) m(-3) NaOH after the extraction with 1000 mol(c) m-3 KCI. The exchangeable SO(4) made up to 72-77% of the total adsorbed SO(4). These results suggested that both inner-sphere and outer-sphere complexes co-exist in the allophanic Andisol at low pH. SO(4) was strongly selective over NO(3) 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 1.0 mole m-3. The model overestimated NO(3) adsorption and underestimated SO(4) adsorption. The difference is due to the fact that SO(4) adsorption in the Stern layer is neglected. Next, we calculated SO(4) adsorbed in the diffuse layer using the Stern-Gouy-Chapman model under the assumption that all the measured NO(3) adsorbed was in the diffuse layer. Our results indicated that the SO(4) in the diffuse layer made up only less than 6% of the total adsorbed SO(4). Most of the adsorbed SO(4) is likely to be found in direct contact with the soil surface. (C) 2011 Elsevier B.V. All rights reserved.