Red Mud Modified Peanut Shell Biochar for Pb(II) and Cu(II) Removal: Adsorption Mechanisms and Sustainability Assessment

Red mud-modified peanut shell biochar (RPC) was prepared by oxygen-limited pyrolysis using peanut shells and red mud as feedstocks. Its physicochemical properties and Pb2+/Cu2+ adsorption performance were investigated using SEM-EDS, XRD, FTIR, XPS, and batch experiments. Red mud modification altered the mineral composition and surface chemistry of the biochar by introducing Fe- and Al-containing mineral phases and oxygen-containing surface functionalities. Under the tested batch conditions, RPC showed experimentally observed equilibrium adsorption capacities of 75.26 and 34.16 mg·g−1 for Pb2+ and Cu2+, respectively, approximately 1.85- and 1.81-fold those of pristine biochar. Adsorption was well described by the Freundlich isotherm and pseudo-second-order kinetic models, with surface complexation, precipitation, and ion exchange as the main mechanisms. Life cycle assessment (LCA) gave a global warming potential of 1.53 kg CO2-eq kg−1 RPC, with drying and pyrolysis as the main hotspots. Techno-economic assessment (TEA) identified pyrolysis heat demand and avoided red mud handling credit as key cost drivers. Overall, RPC offers a promising route for combined agricultural and industrial waste valorisation in heavy-metal wastewater treatment.