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

Mixing 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 PO 4 ‐AsO 4 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 AsO 4 . 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 PO 4 concentration but was not significantly influenced by PO 4 source. Ammonium sulfate did not enhance As release. Arsenic solubility was consistent with a mechanism of specific PO 4 ‐AsO 4 exchange, while PO 4 solubility was consistent with control by metastable PO 4 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.