Waste salt-assisted pyrolysis of antibiotic fermentation residue for biochar production and hexavalent chromium reduction mechanism.

The accumulation of antibiotic fermentation residue (AFR) and pharmaceutical waste salt (WS) poses a severe disposal challenge. Herein, a WS-assisted co-pyrolysis strategy is proposed to convert these hazardous waste streams into an active material for hexavalent chromium (Cr(VI)) reduction. During pretreatment, the intrinsic osmotic stress of WS disrupts microbial cells in AFR, thereby promoting deep dehydration. The subsequent thermal process effectively degrades residual antibiotics and antibiotic resistance genes. During co-pyrolysis, the inherent NaCl in WS acts as a mineral-phase regulator. The salt matrix promotes pore development in the carbon matrix, forming a hierarchical pore architecture. Concurrently, the salt matrix converts calcium-rich components into Ca5(PO4)3OH. These Ca5(PO4)3OH domains enrich CrO42- near Fe-containing redox sites, where Fe-mediated electron transfer drives the aqueous reduction of Cr(VI) to Cr(III). The synthesized composite achieves more than 90% reduction of low-concentration Cr(VI). Economic evaluation confirms the financial feasibility of this integrated protocol when avoided hazardous-waste disposal costs are considered. This integrated upcycling strategy mitigates pharmaceutical solid waste while producing functional biochar for environmental remediation.