Biomass-derived carbon/iron composite (FexOy-BC (RM)) with excellent Cd(II) adsorption from wastewater – Red mud resource utilization

Red mud is an industrial by-product produced in the process of alumina refining. It contains many iron elements, and the traditional resource utilization method is to extract it. This study took an alternative approach by using red mud's iron content as the substrate to synthesize a novel iron-biochar adsorbent for removing heavy metals from wastewater. Walnut shell biochar was reacted with the iron in red mud via an in-situ reduction–oxidation process to produce an iron-carbon composite adsorbent (FexOy-BC). A series of characterization analyses (e.g., SEM, FTIR, XPS, etc.) and batch adsorption experiments were conducted to investigate the properties and Cd(II) removal performance of the adsorbent, respectively. Response surface methodology was further employed to optimize the adsorption conditions, identifying the ideal 6 g/L adsorbent dose, 10 mg/L initial Cd(II) concentration (C0), and pH 6. The optimized quadratic model demonstrated excellent fit, with an average Cd(II) removal efficiency of 92.59 % under optimal conditions. The adsorption mechanism, renewability, and preliminary cost-benefit analysis indicate that FexOy-BC has the potential to be a highly efficient and sustainable adsorbent. In summary, the new method can recover resources from red mud waste and synthesize heavy metal adsorbents, which comprehensively solves two long-standing problems of industrial waste.