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

Abstract 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 KCl. Strongly adsorbed SO 4 was extracted with 10 mol c  m −3 NaOH after the extraction with 1000 mol c  m −3 KCl. 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 mol c  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.