Fertilizer Amendment of Mining-Impacted Soils from Broken Hill, Australia: Fixation or Release of Contaminants?

The aim of the study was to appraise various types of phosphate fertilizers (bone meal, superphosphate, triple superphosphate, and potassium orthophosphate) for immobilizing metals and metalloids in mining-impacted soils from Broken Hill, Australia. Soils were rich in metals (Pb, Zn, Cu, and Cd) and metalloids (As and Sb) which were mainly contained in minor to trace amounts of coronadite [PbMn8O16], kintoreite [PbFe3(PO4)2(OH,H2O)6], Pb, and Zn sulfides and sulfates (possibly sphalerite, galena, and anglesite) as well as in unidentified soluble metal-bearing phases. Phosphate stabilization experiments were conducted as kinetic column leaching experiments, and chemical and mineralogical changes were assessed using elemental, sulfur isotope, and XRD analyses as well as electron microprobe phase mapping. The application of phosphate fertilizer to the metal-contaminated topsoils led to mineralogical changes, including the formation of secondary metal-bearing phosphates. The elemental concentrations of leachates were used as a criterion to assess the performance of phosphate treatments. Potassium orthophosphate fertilizer was the most effective amendment for Cd stabilization; superphosphate and triple superphosphate fertilizers were the most effective amendments for Pb stabilization. By contrast, the release of As, Cu, Mn, Sb, and Zn were not significantly suppressed, and in several cases, increased, using bone meal, superphosphate, triple superphosphate, and potassium orthophosphate amendments. This study indicates that in situ phosphate stabilization of mining-impacted soils at Broken Hill would most likely be a complex and impractical undertaking in residential areas due to the risk of substantial metal, metalloid, phosphate, and sulfate release.