Further reuse of phosphorus-laden biochar for lead sorption from aqueous solution: Isotherm, kinetics, and mechanism

Biochar and engineered biochar have been used for phosphorous recovery from wastewater, but the resulted phosphorous-laden (P-laden) biochar needs further disposal. In this study, the feasibility of reusing P-laden biochar for Pb immobilization as well as the underlying mechanism was explored. Three types of engineered biochar, i.e., Ca modified biochar, Mg modified biochar, and Fe modified biochar, were selected to sorb P and then the exhausted biochar was further used for Pb sorption. Results showed that Mg and Ca modified biochar exhibited considerable Pb sorption capacity after P sorption with the maximum value of 3.36–4.03 mmol/g and 5.49–6.58 mmol/g, respectively, while P-laden Fe modified biochar failed to sorb Pb due to its acidic pH. The removal of Pb by P-laden Mg modified biochar involved more precipitation including PbHPO4, Pb5(PO4)3(OH), and Pb3(CO3)2(OH)2 because of its higher P sorption capacity and more -OH group on the surface. Cation exchange with CaCO3 to form PbCO3 was the main mechanism for Pb removal by P-laden Ca modified biochar despite the formation of Pb5(PO4)3(OH) precipitate. Our results demonstrate that waste P-laden biochar can be further used for the effective removal of Pb, which provides a potential approach for waste adsorbent disposal. [Display omitted] •Reuse of waste P-laden biochar as Pb sorbent was proposed.•P-laden engineered biochar exhibited distinct Pb sorption capacity.•Contribution of P in P-laden biochar to Pb removal was estimated.•Cation exchange and precipitation determined Pb removal by P-laden biochar.