|The potential negative effects of adding phosphate amendments for lead (Pb) remediation in Pb-contaminated soils were studied using column and batch experiments. Soils contaminated with high levels of lead as well as moderate levels of arsenic (As) and antimony (Sb) were amended with triple superphosphate (TSP) and tested to determine the effects of P on Sb and As. It is generally known that P amendments can increase As mobility availability, and that As and Sb tend to behave similarly, chemically. In addition, tests were employed to determine if a correlation between Sb and Pb existed at small-arms firing range sites due to the corrosion of Sb-Pb ammunition.
Multiple column studies were performed to assess mobility changes in soil bound Sb and As, as well as Pb, with the addition of TSP. These tests showed elevated concentrations of Sb and As in column effluents for the P-amended contaminated soil, indicating an increase in Sb and As mobility as a result of the TSP addition. Minimal As and Sb leaching was observed during column tests performed on non-amended soil samples. Pb leaching was minimal in TSP amended and non-amended column tests, indicating no significant difference in soil-Pb mobility with and without the presence of TSP.
Batch experiments were performed to assess the changes in bioaccessibility for Pb, Sb and As with varying amounts of TSP addition, by using a streamlined version of the physiologically based extraction test (PBET). Minimal changes in both As and Sb bioaccessibility were observed with TSP additions. As was expected, Pb bioaccessibility was lowered with increasing amounts of TSP addition.
Strong correlations between Sb and Pb were detected for multiple soil samples collected from three small-arms firing ranges (R2 > 0.90; p-value < 0.05 for all three tested sites). Laser ablation was also performed on several bullets collected from one of the sites in question, detecting a higher concentration of Sb in the bullet core compared to all other elements, excluding lead. Arsenic was also measured in one of the sites in question; however, no correlation between As and Pb was detected suggesting the presence of As to be naturally occurring or background As.
These experiments concluded that while phosphate amendments decrease bioavailability of Pb and slightly decrease the mobility of Pb, competition between P, As and Sb drives the release of Sb and As from soils, increasing their mobility and potentially their bioavailability as well.