Ferric pyrophosphate-activated N and S self-doped keratin biochar derived from rabbit hair enables efficient Cr(VI) removal: Synergistic effect of Fe(Ⅱ) with N and S

The removal of Cr(VI) from wastewater by biochar is limited by the surface chemical structure. The doping structure is the key for biochar to improve the surface chemical properties. In this study, the N,S,Fe self-doped biochar for Cr(VI) removal was prepared by pyrolysis and activation by Fe₄(P₂O₇)₃ of waste rabbit hair. The results show that Fe-RBC successfully retains the oxygen-containing and N functional groups in keratin, simultaneously generating ${\mathrm{SO}}_4^{2-}$and$S^{2-}$, and introducing Fe^2 + . The findings of Cr(VI) removal study indicated that the 1.2 g/L biochar exhibited a removal efficiency of approximately 98.33% for Cr(VI) in wastewater within 6 h. The XPS and CV results show that the carbon skeleton constitutes a stable conductive and adsorption substrate. S participates in and promotes the cycling of Fe (II) and Fe (III), and then serves as a direct electron transfer medium to reduce Cr(VI). The significantly enhanced aromatic π electron cloud obtained by graphite-n contributing electrons to the carbon skeleton π system provides adsorption and fixation sites for the reduction product Cr(III) through cation -π interactions. This research fully exploited the natural structural advantages of protein fibers and achieved in-situ controllable preparation of heteroatom self-doped biochar.