Soluble phosphate amendments for metal and radionuclide immobilization

Abstract

Intensive mining and processing of metals and radionuclides have resulted in significant soil and sediment contamination. Phosphate-based in-situ immobilization of the metals and radionuclides has been proposed as an alternative to disruptive and expensive cleanup strategies. This dissertation summarizes results of two experiments, which tested potential of four phosphate amendments [trisodium trimetaphosphate, dodecasodium phytate (Na-IP6), calcium phytate (Ca-IP6) and hydroxyapatite] to immobilize Cu, Zn, and Pb in soil and Ni and U in sediment. Stability of immobilized contaminants was tested by selective leaching and solid phase speciation techniques. Hydroxyapatite lowered trace metal and U solubility with increasing treatment level and Ca-IP6 behaved in general similarly, except for a slight increase in Ni and Cu solubilities at higher amendment levels, presumably due to the chelation potential of phytate. Application of metaphosphate and Na-IP6 were proven unsuitable due to dispersion of soil organic matter and colloidal particles, which had a negative impact on contaminant solubility. Leaching techniques revealed amendment-derived changes in element fractionation and indicated that HA was suitable for Cu, Zn, and Pb immobilization, whereas Ca-IP6 only for Pb and U immobilization, and brought into question the efficacy of applying phosphates for immobilization of Ni. Qualitative spectroscopy disclosed Pb speciation in solid phase in the untreated soil and mechanisms of its transformation upon phosphate addition, but failed to identify any crystalline form of U in the sediment. Unamended Pb-contaminated soil showed strong diffraction patterns for anglesite, cerussite, and plumbojarosite. Hydroxyapatite and Ca-IP6 amendments facilitated precipitation of corkite and to a lesser extent drugmanite and pyromorphite. This is significant because pyromorphite is usually proposed as the main sink of Pb in the amended soils.