Hydrochar for Cadmium Removal from Aqueous Systems: Functional Groups, Sorption Pathways, and Environmental Performance

Heavy metal contamination of water poses serious environmental and public health risks due to its persistence, toxicity, and tendency to bioaccumulate. Developing sustainable sorbents capable of removing cadmium under realistic environmental conditions is therefore essential. Hydrochar produced from food waste biomass offers a promising solution, yet the mechanistic relationships among its surface chemistry, mineral composition, and cadmium-binding pathways remain poorly understood. This study investigates hydrochar obtained through controlled hydrothermal carbonization and evaluates its performance in removing Cd2+ from aqueous solutions. Comprehensive characterization using FTIR, SEM–EDS, and XRD was conducted to elucidate functional groups, the porous morphology that facilitates metal binding, and the crystalline phases that influence cadmium sorption. Batch adsorption experiments examined the effects of contact time and initial Cd2+ concentration, while kinetic modeling (pseudo-first-order, pseudo-second-order, and intraparticle diffusion) and isotherm modeling (Langmuir and Freundlich) provided insight into surface complexation, electrostatic interactions, and mineral-assisted binding. Thermodynamic parameters (ΔG°, ΔH°, ΔS°) were evaluated to determine the spontaneity and energetic nature of Cd2+ adsorption. The hydrochar exhibited measurable affinity for cadmium, driven by contributions from hydroxyl, carbonyl, and ether groups, as well as embedded inorganic constituents. Overall, the findings establish structure–function relationships governing cadmium removal and demonstrate hydrochar’s potential as a sustainable material for water treatment and environmental remediation.