Influence of phosphorus levels and pH on arsenate adsorption on Gray Lowland soil

<p>There are limited studies that investigate the competitive adsorption between arsenate and phosphate and the impact of pH on arsenate adsorption on Gray Lowland soil, a common paddy soil in Japan. Furthermore, the effect of the native soil phosphorus on arsenate adsorption warrants evaluation. This study aims to 1) assess the characteristics of arsenate adsorption on Gray Lowland soil by comparing it with that on Andosols, which was reported in a previous study and 2) assess the dissolution of native soil phosphorus and its impact on arsenate adsorption. The findings revealed that the amount of arsenate adsorption on Gray Lowland soil was approximately 10% of that on Andosols. The maximum arsenate adsorption on Andosols and Gray Lowland soil was observed at an equilibrium pH of ~3.8 and ~3.0, respectively. Moreover, competitive adsorption between arsenate and phosphate was observed on Gray Lowland soil. The fraction of weakly adsorbed arsenate accounted for 32% of the total adsorbed arsenate, despite adding only arsenate to the soil. It is suggested that arsenate competed with phosphorus, which was released from phosphorous bound to calcium and/or unstable iron contained in soil, in the adsorption on Gray Lowland soil. Native soil phosphorus in Gray Lowland soil continued to be released following repeated input of distilled water. These findings demonstrate that arsenate is not retained for extended periods by the topsoil, which has a low adsorption capacity of arsenate and whose adsorption sites are already saturated with phosphorous. This is because the added arsenate is either weakly or hardly adsorbed to the soil.</p>