Evidence for Metal Phosphate Formation upon the Addition of Different P Amendments to a Pb-Zn Smelter Contaminated Soil.

The use of P amendments to stabilize heavy metals in soil environments has been well documented, especially for Pb.  We hypothesized that the differences in efficacy of Pb/Zn stabilization in contaminated soils upon fluid or granular P amendment addition is due to different P reaction processes in and around fertilizer granules and fluid droplets.  We used a combination of several synchrotron-based techniques, namely, spatially resolved micro-x-ray fluorescence, micro-x-ray absorption near edge structure spectroscopy, micro-extended-x-ray absorption fine structure spectroscopy, and micro-x-ray diffraction to speciate Pb and Zn at two incubation times in a smelter contaminated soil after the addition of several fluid and granular P amendments.  All fluid and granular P sources were able to induce Pb phosphate formation, but fluid phosphoric acid (PA) was the most effective with time and distance from the treatment.  The acidity from PA increased the presence of aqueous Zn and soluble Zn solid phases, which could be a significant disadvantage because of increased bioavailability of Zn and other metals often found with Pb.  The Pb phosphate mineral plumbogummite is an intermediate phase of pyromorphite formation.  Triple super phosphate (TSP) was more effective than granular phosphate rock (PR) in generating Pb and Zn phosphates at greater distances from the treatment. However, the eutrophication potential from TSP is greater than PR.  Therefore, PR can be used in acidic to slightly acidic soils to stabilize both Pb and Zn, while minimizing the environmental impacts of P.