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BENEFICIAL EFFECTS OF PHOSPHATE AMENDMENTS IN METALS-CONTAMINATED SOILS

Current remediation activities in residential areas with Pb-contaminated soils have utilized soil removal as the primary strategy. Soil removal is a disruptive activity that has significant disadvantages in terms of cost, the need for a contaminated-soil repository, and the need for a clean source of replacement soil.

Phosphorus (P) amendment has recently been proposed as an in situ alternative to soil removal for Pb- and Cd- contaminated soils. The primary benefit is a reduction in Pb bioavailability such that the primary receptor of concern, the human child, will show less of an increase in blood Pb concentration after incidental consumption of soil that has received P compared to unamended soil.

The proposed mechanism for Pb is the conversion of Pb to pyromorphite, an insoluble Pb phosphate mineral. The potential mechanisms for other metals are less clear, although limited data suggests some reductions in bioavailability occur. Numerous issues need to be resolved before this new technology is adopted.

These include the method of application, the P source (soluble versus insoluble), long term efficacy, the affect of P on other metals of concern that frequently occur simultaneously with Pb, the appropriate P:Pb molar ratio, control of soil pH, methods for estimating reductions in metal bioavailability, and the impacts on ecological risk.

A review of the literature will be presented on research and field studies in which phosphate amendments have been employed to reduce chemical and biological availability of metals in soils contaminated with Pb, Zn, and Cd. Integrated Exposure Uptake Biokinetic model results will be used to demonstrate reductions in Pb bioavailability to children.

Sequential extraction results from P- amended soils will be presented illustrating the influence of P on the bioavailability of Pb, Cd, and Zn. Companion mineralogical analyses and solubility equilibria results illustrating the formation of pyromorphite upon P addition will also be presented. Research needs and major roadblocks for large-scale adaptation of the technology will be discussed.

Key words: phosphate, lead, bioavailability, solubility, soil, cadmium, zinc