Effect of Adsorbed Phosphate on Transport of Arsenate through an Oxisol

Competitive oxyanion adsorption can increase the mobility of toxic trace elements in soils leading to groundwater contamination and a potential threat to the environment. We conducted a study to determine how phosphate fertilization might affect arsenate mobility in soil. The competitive nature of sorbed phosphate on arsenate mobility was investigated using laboratory soil columns constructed from an aggregated Oxisol. The soil was incubated with four levels of phosphate (0–48.6 mmol P kg −1 ) before being leached with 0.66 mmol KH 2 AsO 4 L −1 in 0.05 M KCl. Arsenate mobility was greatly enhanced by treatment with increasing amounts of phosphate. The As breakthrough curves (BTCs) were divided into two phases. The initial phases of arsenate interaction was consistent with ligand exchange of Me‐OH + 2 groups, where Me is the coordinating metal atom at the oxide surface. This phase was characterized by the constant pH of the effluent. The second phase was consistent with ligand exchange of As with Me‐OH groups and surface‐completed P. The asymmetry of the As BTCs suggested that the physical structure of the matrix influenced the convective flow path of the As and subsequently its reaction and movement through the soil.