| The in situ immobilization of Pb and Zn via the formation of sparingly soluble Pb- and Zn-P minerals has been regarded as one of the most environmentally friendly and economical option to remediate Pb and Zn contaminated soils. However, some uncertainties remain unclear in the use of P amendments to immobilize excess Pb and Zn in soils. This work was initiated to investigate some of these uncertainties including the long-term stability of the immobilized Pb-and Zn-P in the rhizosphere soil, factors contributing to the poor efficiency Pb-and Zn-P formation in soils, and the interaction between plants, immobilized Pb- and Zn-P and soil biota, specifically arbuscular mychorrizal fungi. A one-year long batch dissolution study revealed that organic acids (oxalic, citric, and malic acid) abundant in rhizosphere soil were able to dissolve pyromorphite (a synthetic Pb-P mineral) and liberate Pb and P into solution. Similarly, tall fescue plants grown with pyromorphite as a sole source of P were able to solubilize pyromorphite and consequently accumulate Pb in their biomasses. In an incubation experiment, the formation of target Pb- and Zn-P minerals after P amendment was significantly reduced in the presences of oxalic and citric acids. The results suggest that organic acids in rhizosphere soils are capable of releasing Pb from Pb- and Zn-P minerals, and are significant factors contributing to the poor efficiency of P amendments to effectively immobilize high levels of Pb and Zn in contaminated soils. In a complementary study, an alternative metal immobilization technique by co-pyrolysis of a contaminated soil with woody biomass revealed great potential to further exploit pyrolytic processes to immobilize metals such as Zn and Cd in contaminated soils with highly resilient charred biomass. |
