Development of a natural iron-based mineral/biochar composite for efficient simultaneous removal of Cr(VI) and methylene blue in the presence of oxalic acid

The complexity of industrial wastewater, heavy metals and organic pollutants often co-exist, making it difficult for conventional water processing to effectively purify water. Herein, natural siderite and bagasse were facilely developed as an iron-based mineral/biochar composite (Fe@BC=1:2) via one-step co-pyrolysis. This process achieved the simultaneous removal of hexavalent chromium (Cr(VI)) and methylene blue (MB) in the presence of oxalic acid (OA). Biochar perfectly implemented the reduction and effective dispersion of iron ore. The formation of iron species optimized the Zeta potential and electrochemical performance of biochar, which facilitated the adsorption and electron transfer process for oxalate and Cr(VI). The Fe@BC=1:2/OA system demonstrated excellent performance under various conditions, and the pseudo-second-order kinetic model indicated that chemisorption was the dominant mechanism for the Cr(VI) and MB removal. The Cr(VI) reduction was synergistically driven by the electron transfer of OA over the conjugated structure of biochar and the iron species redox processes. This was accompanied by the generation and intensification of biochar-induced persistent free radicals (PFRs), which consequently triggered the oxidative degradation of the adsorbed MB. This work provides a broader perspective on preparing cost-effective iron-based biochar from natural minerals and biomass for the simultaneous removal of heavy metals and organic pollutants.