Field Application of Phosphate Rock for Remediation of Metal-Contaminated Soils
This research project focused on the long-term feasibility of phosphate-induced Pb immobilization in the field. The selected site is located at urban area of Northwest Jacksonville and was contaminated with Pb primarily due to past battery recycling activities. Based on our laboratory results, a pilot-scale field demonstration was initiated in 2000. Phosphate amendment was employed at a P/Pb molar ratio of 4. Half of the P amendment was first applied as H3PO4 to all plots, and six weeks later the other half was applied as H3PO4 in Treatment 1 (T1), Ca(H2PO4)2 in Treatment 2 (T2) and 5% phosphate rock in Treatment 3 (T3).
For this study, soil samples were collected on 8/27/2005, 4.5 years after the initial P application, and were analyzed for pH and total Pb, Ca and P concentrations. In addition, Pb concentrations extracted using the toxicity characteristic leaching procedure (TCLP), synthetic precipitation leaching procedure (SPLP), and physiologically based extraction test (PBET) in the soil samples were determined. No water was available for sampling during the trip, therefore no data will be presented.
After 4.5 years of phosphate application, the acidification effects of phosphoric acid were only observed in Treatment T1 at the top 30 cm, with a pH reduction of 1 unit compared to the control. In all treatments, the highest Pb concentrations were observed at 20-40 cm, ranging from 296 to 36,300 mg kg-1. The total Pb concentration in the control was much lower than those in P-treated soils, which makes it difficult to evaluate the effects of P application in the soil. As expected, P concentrations in all three treatments were elevated, especially at the surface soil and with Treatment T3. As far as P leaching is concerned, Treatment T2 was the most efficient, with the least amount of P being migrated down the soil profile. Though it may have added more risk for P leaching down to the groundwater, the fact that phosphate rock migrated down the soil profile implied that Pb immobilization at subsurface soil is possible by adding phosphate rock to surface soil. This may be significant in terms of soil remediation since phosphate induced Pb immobilization has been limited to surface soil only.
Due to the heterogeneity of soil Pb distribution and lower Pb concentrations in the control sample, evaluation of phosphate-induced Pb immobilization was based on normalized data, i.e. ratios of TCLP-Pb, SPLP-Pb, and PBET-Pb to total Pb were used. Among the three treatments, all three treatments were effective in reducing TCLP-Pb (43-50%) and PBET-Pb (19-75%), with Treatment T3 being most effective partially because phosphate rock remained in the soil even after 4.5 years. On the other hand, Treatment T1 was effective in reducing SPLP-Pb (42-62%) in the soil. Among the three treatments, the PBET (2.85-100% of total Pb) was the strongest in extracting soil Pb, followed by TCLP (0.34-7.43% of total Pb) and SPLP (0.21 to 4.88% of total Pb).
Our research further suggests that P amendments can be a cost-effective and environmental-friendly alternative to treat Pb-contaminated soils. However, caution should excised to maximize lead immobilization and minimize potential adverse impacts caused by application of phosphate amendments to soils. It is recommended to reverse the phases of P application, i.e., to add calcium phosphate and phosphate rock first and apply phosphoric acid second, or add them all simultaneously. This would lead to the dissolution of cerussite and more insoluble P amendments at the same time, favoring lead immobilization and minimizing potential P and Pb leaching.