Co-pyrolysis of biomass and phosphate tailing to produce potential phosphorus-rich biochar: efficient removal of heavy metals and the underlying mechanisms

Application of biochar to treat heavy metal polluted wastewater has received increasing attention; however, the immobilization ability of pristine biochar for metal ions is still limited. In this study, phosphate tailing was co-pyrolyzed with sawdust and peanut shell to acquire phosphorus-rich biochars with high removal rates for Cd, Zn, Pb, and Cu. Meanwhile, the improvement mechanisms by phosphate tailing were clarified by XRD, FTIR, SEM-EDS, BET-N₂, and model fitting. Results showed that after phosphate tailing impregnation, surface area of sawdust, and peanut shell biochars increased from to 11.6 m² g^-1, and from 43.5 to 53.4 m² g^-1, respectively. Functional groups of -COOH and CO₃^2- on biochar increased and the P₂O₇^4- newly generated. Besides, large amounts of Ca(PO₃)₂ and Ca₂P₂O₇ crystals were detected in biochar ash. As for sawdust biochar, loading of phosphate tailing raised the sorption rates of Cd, Zn, Pb, and Cu by 0.35, 0.61, 1.10, and 2.64 times, respectively, as for peanut shell biochar, it was raised by 0.12, 0.47, 0.11, and 1.98 times, respectively. The sorption isotherms by phosphate tailing-loaded biochars were better fitted to Langmuir (R² = 0.85-1.00) than Freundlich model (R² = 0.58-0.91). Heavy metals could bind with -OH and -COOH on phosphate tailing-loaded biochars, meanwhile generated phosphorus-rich precipitation with PO₃^- and P₂O₇⁴⁺, including Cd₂P₂O₇, Cd(PO₃)₂, Zn (PO₃)₂, Pb (PO₃)₂, Pb₂P₂O₇, Cu(PO₃)₂, and Cu₂P₂O₇. This study proposed an innovative method to produce phosphorus-rich biochars by loading phosphate tailing for highly efficient removal of heavy metals from water bodies, and also realized the resource utilization of phosphate tailing, which was of great significance to reduce environmental pollution.