Synergistic valorization of red mud and spent coffee grounds via one-step hydrothermal carbonization for high-efficiency Pb(II) and Cd(II) removal from wastewater

The escalating contamination of water resources by toxic heavy metals necessitates sustainable remediation strategies. This study presents a novel adsorbent, RHC-180/0.5, synthesized via a one-step hydrothermal carbonization under mild conditions, utilizing red mud (RM) and spent coffee grounds (SCG) as raw materials. The biopolymers in SCG, including cellulose, hemicellulose, and lignin, transformed into hydroxyl, carboxyl, and phenolic groups, enhancing the material's adsorption capacity. Batch adsorption experiments showed optimal performance under alkaline conditions. To minimize precipitation interference, the adsorption mechanism was further investigated at pH = 6. Pseudo-second-order kinetics and Freundlich isotherm models indicated chemical adsorption and heterogeneous multilayer adsorption, respectively. Simultaneously, the Langmuir model yielded maximum theoretical adsorption capacities of 302.33 mg/g and 154.97 mg/g for Pb(II) and Cd(II), respectively, confirming the excellent adsorption performance of RHC-180/0.5. Intraparticle diffusion model analysis showed that surface diffusion dominated in the three-phase adsorption process. Thermodynamic analysis confirmed that the adsorption process was endothermic. Spectroscopic analysis validated the roles of chemical precipitation, ion exchange, and cation-π interactions, surface complexation, and redox reactions in the adsorption mechanism. At pH = 6, RHC-180/0.5 retained approximately 80 % of its initial removal efficiency after ten regeneration cycles, demonstrating excellent stability and reusability. This work highlights a sustainable, cost-effective approach to converting agricultural and industrial waste into high-performance adsorbents for heavy metal-contaminated wastewater treatment.