Competitive sorption of arsenate and phosphate on allophanic Andosols

Arsenic (As) contamination in agricultural soils is a pressing environmental concern, particularly in volcanic regions where Andosols dominate and phosphorus (P) fertilizers are intensively applied. Because arsenate and phosphate are chemically similar oxyanions, they compete for sorption sites on soil minerals, thereby controlling their mobility and bioavailability. In this study, we investigated the competitive sorption of arsenate and phosphate on three allophanic Andosols and synthesized allophane under varying pH conditions. Batch sorption experiments were conducted using both single- and binary-component systems. Results showed that arsenate and phosphate sorption were strongly pH-dependent. Maximum sorption on allophane occurred at around pH 4.5, coinciding with the onset of aluminum dissolution. In the binary system, phosphate effectively reduced arsenate sorption. For synthesized allophane, the total amount of arsenate and phosphate sorbed in the binary system (1 mM arsenate +1 mM phosphate) was comparable to the amount of arsenate sorbed in the single-component system (2 mM arsenate), indicating that arsenate and phosphate compete for the same sorption sites. While the order of anion addition had only a minor influence on arsenate sorption compared with the competitive effect of phosphate, phosphate sorption was clearly higher when phosphate was added first. Furthermore, differences among the soils in their sorption behavior toward arsenate and phosphate reflected their contents of active aluminum and iron (operationally defined as oxalate-extractable Al and Fe). These results provide mechanistic insight into the competitive and exchangeable nature of arsenate and phosphate sorption on allophanic Andosols.