In-situ hydrothermal synthesis of Fe-doped hydroxyapatite photocatalyst derived from converter slag toward xanthate photodegradation and Cr(VI) reduction under visible-light irradiation

In this work, the salicylic acid-pretreated artificial converter slag-an end product of the steel-making process containing rich Fe and Ca-was successfully utilized for in-situ fabrication of Fe(III)-doped HAp (Fe(III)-HAp) using a modified hydrothermal reaction. Structure, morphology, chemical composition, and photo-electrochemical properties were investigated experimentally and theoretically. The experimental results showed that the successful in-situ Fe(III) doping, along with the formation of oxygen vacancies (OVs), narrowed the bandgap of HAp and broadened its light response range, thus improving its photocatalytic activity. In simulated mineral flotation effluents, 18 % Fe(III)-HAp showed superior performance in xanthate photooxidation under visible light irradiation, giving a 99.4 % removal rate and 87 % mineralization rate within 180 min. While Fe(III)-HAp_CS showed optimum activity in Cr(VI) photoreduction (similar to 100 %) in the presence of a hole scavenger within 30 min. To understand the mechanism, we thus concentrated on the synergistic effects of the Fe(III) dopants and OVs on Fe(III)-HAp. DFT calculation revealed that Fe(III) occupying the Ca atom locations adjacent to the OV could form bandgaps close to experimental results. Intermediary energy levels generated by OVs and Fe(III) doping were responsible for greater visible-light absorption and excellent separation of photogenerated carriers. Scavenger tests and EPR results demonstrated that superoxide radicals (center dot O-2(-)) and holes (h(+)) were dominant photoactive species in xanthate oxidation. Photo-produced electrons effectively reduced Cr(VI) in the presence of a hole scavenger. This work enabled Fe(III)-HAp with OVs to be synthesized from Fe and Ca-enriched converter slag via a viable approach and applied to the treatment of emulated floatation wastewater, which had significant potential in both solid waste recycling and actual industrial wastewater treatment.