The effects of phosphate and pH on arsenate adsorption on allophanic Andosols in Miyazaki

ABSTRACT Arsenic (As) contamination in agricultural soils is a serious problem. The mobility and bioavailability of As in agricultural soil are mainly controlled by phosphate and pH. Although there is a potential risk of soil contamination in Japan by geogenic As distributed along the volcanic front, few studies have investigated arsenate adsorption on Andosols, a typical agricultural soil in Japan. In this study, we evaluated the effects of phosphate and pH on arsenate adsorption on allophanic Andosols. The adsorption isotherms of Andosols for arsenate and phosphate were determined in batch experiments at different phosphate concentrations and pH. In addition, following the competitive adsorption experiment of arsenate and phosphate, we determined three fractions of the adsorbed arsenate by sequential extraction as easily soluble As fraction (F1; 0.01 mol L−1 NaNO3 extraction), specifically adsorbed As and As mainly bound to poorly crystalline materials (F2; 0.2 mol L−1 NH4-oxalate buffer extraction), and As mainly bound to crystalline materials (F3; 0.2 mol L−1 NH4-oxalate buffer+0.1 mol L−1 ascorbic acid extraction). Andosols adsorbed~96.0% arsenate and~98.3% phosphate at the initial arsenate concentration of 214 µmol L−1 and phosphate concentration of 323 µmol L−1, respectively in each single-anion system. The maximum arsenate adsorption on the Andosols occurred at an equilibrium pH of~3.8 and it decreased with further increasing or decreasing equilibrium pH. Upon 300 µmol L−1 of arsenate addition, the Andosols adsorbed 98.6, 99.4, 91.0, and 65.8% of arsenate at an equilibrium pH of 3.0, 3.8, 6.2, and 7.6, respectively. The effect of pH was incorporated into the parameter of the modified Freundlich model for arsenate adsorption on Andosols. The competitive adsorption experiment results indicated a greater affinity of phosphate than arsenate for Andosols. Sequential extraction revealed that competition with phosphate not only decreased the amount of adsorbed arsenate, but also increased the amount of arsenate in F1, which is mobile and readily available to plants. Thus, phosphate fertilization can increase the mobility and bioavailability of arsenate in agricultural soils.