Remediation of Pb, Cd, and Cu contaminated soil by co-pyrolysis biochar derived from rape straw and orthophosphate: Speciation transformation, risk evaluation and mechanism inquiry.

Biochars are widely used in the remediation of soil heavy metals, but there has been no clear understanding to the effects of novel co-pyrolysis biochars derived from biomass and orthophosphate on soil heavy metals. In this study, co-pyrolysis biochars derived from rape straw and orthophosphate (Ca (HPO)·HO/KHPO) were prepared and used to explore their effects on the speciations and ecological risks of Pb, Cd, and Cu in contaminated agricultural soil. The results showed that the addition of these co-pyrolysis biochars significantly decreased TCLP extracted concentrations (decreased by 5.9-81.7%) and ecological risks of heavy metals (Pb, Cd, and Cu) by transforming the metals from available speciation to stable speciation in soils. Co-pyrolysis biochar derived from rape straw and KHPO showed the highest immobilization capacities, and the immobilization capacities of biochars for three metals were in the order of Pb > Cu > Cd. Co-pyrolysis biochars could precipitate and complex with heavy metals directly by the phosphate and -OH on their surface, and also could promote immobilization of heavy metals indirectly by increasing soil pH value and available P content. During incubation, the content of carboxyl groups on biochars increased significantly, which was beneficial to the further complexation of heavy metals. In summary, the application of co-pyrolysis biochar derived from rape straw and orthophosphate (especially for KHPO) could effectively reduce ecological risks of Pb, Cd, and Cu in contaminated soil.