Synthesis of functionalised biochar using red mud, lignin, and carbon dioxide as raw materials

Abstract The massive generation of red mud has been considered as a serious environmental burden because of its toxicity, alkaline nature, and complex compositional matrix. Accordingly, diverse technical approaches for red mud utilization have been extensively developed, but their practical implementation has not been fully established because of technical incompleteness. In these respects, establishing reliable strategies for disposing red mud is of great importance. To enhance the technical viability of red mud valorisation, utilizing an existing disposal platform for solid wastes can be an alternative option. Therefore, co-pyrolysis of red mud and lignin was conducted in this study. Furthermore, the possible utilization of CO2 during the co-pyrolysis was explored to valorise the end-product (biochar), which enhanced its porosity. In addition to the enhanced porosity, CO2 utilization during the co-pyrolysis of red mud and lignin led to surplus generation of CO by shifting the carbon distribution from pyrolytic oil to CO. In detail, CO generation in the CO2 environment was enhanced 24 times more than that in the N2 environment. Thus, the surplus CO in the CO2 environment was used to transform iron oxides in the red mud into zero-valent iron. In sum, two functionalities (enhanced porosity and zero-valent iron content) were leveraged by the CO2, which synergistically enhanced the reduction capability of the biochar. Reduction of p-nitrophenol and Cr(VI) was successfully completed using biochar, of which removal efficiency by reduction reached up to 99 and 69.7%, respectively. Therefore, the experimental findings provide a breakthrough for valorising two widespread waste materials, red mud and lignin.