Phosphate‐Induced Release of Arsenic from Soils Contaminated with Lead Arsenate

Abstract

AbstractMixing high rates of monoammonium phosphate (MAP) or monocalcium phosphate fertilizers with orchard soil often stimulates growth of newly planted pome fruit trees. This practice may increase As solubility in soils contaminated with lead arsenate pesticide residues because of competitive PO4‐AsO4 exchange. Laboratory experiments were conducted examining As release in batch suspensions of five lead arsenate‐contaminated soils. Dissolved Pb concentrations were <5 µmol/L in all soil suspensions. Redox potential exceeded 500 mV in all suspensions, suggesting that the primary form of dissolved As was probably AsO4. Phosphate‐induced As release was dependent on soil type, occurring in four nonvolcanic soils but not in a pumiceous soil. Arsenic release was positively related to added PO4 concentration but was not significantly influenced by PO4 source. Ammonium sulfate did not enhance As release. Arsenic solubility was consistent with a mechanism of specific PO4‐AsO4 exchange, while PO4 solubility was consistent with control by metastable PO4 minerals. Phosphate‐released As was not appreciably resorbed after multiple wetting and drying cycles. These results explain enhanced As mobility, phytoavailability, and phytotoxicity observed in lead arsenate‐contaminated soils amended with MAP.