Fabricating hydroxyapatite functionalized biochar composite using steel slag and Hami melon peel for Pb(II) and Cd(II) removal

With rising global concerns about the discharge of large volumes of waste materials derived from industrial manufacture and human life, methods for utilizing waste materials have been greatly developed. In this study, an ingenious hydroxyapatite (Hap) functionalized hierarchical porous biochar (HB) composite is fabricated using a filtrate rich in Ca 2+ ions after steel slag reuse and spent Hami melon peel as raw materials. Microstructural analyses indicate that Hap nanoparticles are loaded on HB with hierarchical pores produced from a pyrolysis process of Hami melon peel. A series of HB-Hap materials by tuning HB addition amount (0.5, 1, and 2 g) can be obtained through a coprecipitation process. As a case study, HB-0.5Hap exhibits high specific surface area of 66.92 m 2 /g and high-efficiency removal performance for Pb(II) and Cd(II) ions in wastewater. The maximum adsorption capacities of HB-0.5Hap are 134.4 (Pb(II)) and 60.4 (Cd(II)) mg/g, which are 1.77 and 1.23 times higher than that of pure HB, respectively. It is attributed to a synergistic mechanism involving ion exchange, chemical precipitation , surface complexation, and cation-π interaction. According to a semi-quantitative calculation, ion exchange plays a major role in the removal of Pb(II) (78.61%, the relative contribution of ion exchange mechanism) and Cd(II) (77.77%). This study demonstrates a facile, economical, and sustainable strategy for the value-added utilization of steel slag and spent Hami melon peel, which can be critical in solving environmental problems.