Highly-effective removal of Pb by co-pyrolysis biochar derived from rape straw and orthophosphate

When used separately, biochar and orthophosphate are good materials to remove Pb from water, but few studies have been done on Pb removal by biochar-orthophosphate composite. Here biochar-orthophosphate composites were prepared by co-pyrolyzing rape straw with orthophosphate (Ca(H₂PO₄)₂·H₂O / KH₂PO₄) at ratio of 5:1 (W:W), noted as WBC-Ca and WBC-K, respectively, so as to explore the Pb removal capacities and mechanisms of co-pyrolysis biochars. The sorption isotherms of Pb were well fitted with Langmuir model and the maximum sorption capacities of Pb by original biochar, WBC-Ca, and WBC-K were 184.1, 566.3 and 1559 mmol kg^-1, respectively. The results of FTIR, XRD, and XPS analyses showed that phosphorus in biochar played an important role to remove Pb by forming lead-precipitates. However, the species of lead-precipitates in three types of Pb-loaded biochars were Pb₅(PO₄)₃Cl, Pb₂P₂O₇, and Pb_n/2(PO₃)_n, individually, and that was because speciation of phosphorus had undergone significant thermochemical transformation during pyrolysis process. Orthophosphate in WBC-Ca was mainly transformed to pyrophosphate, while orthophosphate in WBC-K was transformed to both metaphosphate and pyrophosphate. The present results warrant the promising application of co-pyrolysis biochar derived from rape straw and orthophosphate in removal of Pb from wastewater.