Mechanochemical valorization of grape vinegar pulp into functional activated carbon for efficiency removal of malachite green and Hg(II) from water

Industrial effluents containing synthetic dyes and toxic metal ions pose persistent risks to aquatic ecosystems and human health. Herein, grape vinegar pulp, an agro-industrial residue, was valorized into mechanochemically activated carbon (aGVP) through H₃PO₄-assisted activation, N₂-assisted carbonization, and high-energy ball milling for the removal of malachite green (MG) dye and Hg(II) ions from water. FTIR, BET, SEM, and pH pzc analyses revealed a porous carbonaceous structure enriched with oxygen-containing functional groups and a pH pzc of approximately 4.06. Batch experiments demonstrated strong dependence of adsorption on solution pH, adsorbent dosage, initial concentration, contact time, and temperature, with rapid uptake and near-equilibrium attained within 90 min. Adsorption kinetics were best described by the pseudo-second-order model (R² = 0.995–0.999). Equilibrium data were better represented by Freundlich model for both pollutants, supporting adsorption on energetically heterogeneous surface sites. The Langmuir model estimated maximum adsorption capacities of 107.76 mg g⁻¹ for MG and 1244.17 mg g⁻¹ for Hg(II), though depicting low correlation coefficient values. Thermodynamic analyses confirmed that the adsorption processes were spontaneous and endothermic. Process optimization using Central Composite Design-based Response Surface Methodology predicted maximum removal efficiencies of 97.26% for MG and 96.10% for Hg(II), closely agreeing with experimental validation. Despite the relatively low BET surface area, aGVP exhibited high removal performance attributed to oxygen-rich surface functionalities, favorable adsorbate–surface interactions, and enhanced pore accessibility. This work presents a cost-effective waste-to-resource approach for producing multifunctional biomass-derived adsorbents with significant potential for sustainable wastewater treatment and circular bioeconomy applications.