Phosphate-enhanced movement of arsenic out of lead arsenate-contaminated topsoil and through uncontaminated subsoil

Abstract

Past use of lead arsenate insecticides has resulted in elevated concentrations of lead (Pb) andarsenic (As) in topsoils of many existing and former deciduous tree fruit orchard sites throughoutthe world. Application of phosphate (PO4)-containing fertilizers to these soils can increase soil Assolubility, phytoavailability and downward mobility. A laboratory soil column experiment wasconducted to determine if As released by phosphate additions to a topsoil artificially contaminatedwith lead arsenate (1.65 mmol total Pb/kg; 1.10 mmol total As/kg) would be appreciably resorbedby the underlying uncontaminated subsoil. Treatments were a factorial combination of topsoilamendment with monoammonium phosphate (MAP, 0 or 16.67 mmol PO4/kg), and amount ofleaching (1, 5 and 10 pore volume displacements (PVD) with distilled water under saturated flowconditions). Soil As decreased in the topsoil with increasing amount of leaching and increased inthe subsoil. Addition of MAP substantially increased loss of topsoil. As, promoted As transportinto and through the subsoil, and increased dissolved As concentrations in the column leachates.After 10 PVDs, 95% of the initial soil As remained in the -MAP columns, while 56% of the initialsoil As remained the +MAP columns. Dissolved Pb concentrations were <0.05 µmol/Lin all column leachates. The data are consistent with a mechanism of PO4-enhanced release of Asin the topsoil and subsequent promotion of As movement through the subsoil by continuingcompetition of dissolved As and PO4 for ion adsorption sites. The experimental results indicatethat use of PO4-containing fertilizers on lead arsenate-contaminated soils has the potential togreatly enhance downward movement of soil As.