Precipitation, adsorption and rhizosphere effect: The mechanisms for Phosphate-induced Pb immobilization in soils-A review.

Lead (Pb) is one of the most toxic heavy metals that pose a direct threat to organisms and it can not been degraded through microbial activities or chemical reaction. Bioavaibility and eco-toxicity of Pb which mostly depend on Pb chemical speciation play an important role in the remediation of Pb-contaminated soils. Phosphate (P) amendments which could transfer Pb from unstable fraction to stable fraction are commonly used to immobilize Pb in soils and have been extensively studied by researchers during decades. Based on the previous study, it can be concluded that three principal mechanisms may be responsible for P-induced Pb immobilization: 1) the precipitation of Pb-phosphates, including direct precipitation, ion-exchange (or substitution) effect and liming effect; 2) the adsorption of Pb, including the direct adsorption and the adsorption of Pb to iron (hydr)oxides; 3) the rhizosphere effect, including acidification effect and mycorrhizae effect. In this review, these mechanisms have been completely discussed and the internal relationships among them were summarized to give a better understanding of P-induced Pb immobilization in soils and promote the development of P-based remediation technology.