Chitosan/carbon/ZnMn₂O₄ nanocomposite for efficient removal of chromium(VI) and anionic/cationic dyes

This work advances waste-to-resource circularity by transforming battery waste into an efficient sorbent, addressing mixed-pollutant challenges in industrial wastewater. An innovative mesoporous ternary chitosan/carbon/ZnMn₂O₄ nanocomposite (Cs/C/ZMO) was engineered through a two-step synthesis process for the removal of both anionic and cationic pollutants. First, zinc‑carbon battery waste was hydrothermally converted in ZnSO₄ solution to carbon/ZnMn₂O₄ (C/ZMO), followed by chitosan integration (1:2 wt/wt, respectively) via alkaline co-precipitation and epichlorohydrin crosslinking. Structural characterization (SEM, BET, XRD, EDX, CHN/O, XPS, and pHzpc) confirmed morphology, mesoporosity, ZnMn₂O₄ spinel phase, elemental composition, surface functionality, and charge. Sorption capacities were exceptional: Cr(VI) reached ∼6.565 mmol/g (pH 2.5, 328 K), while methyl orange (MO) and methylene blue (MB) achieved ∼0.621 mmol/g (pH 2.5) and ∼ 0.790 mmol/g (pH 6.0), respectively, at 298 K. Equilibrium was attained within 50–75 min, following pseudo-first-order kinetics. Cr(VI) removal proceeded via synergistic sorption, coordination, and electro-redox mechanisms. Thermodynamically, Cr(VI) sorption was endothermic (reciprocal to MO/MB), entropy-driven, and a spontaneous process. Reusability tests demonstrated good stability and durability, retaining ∼88 % capacity over eight cycles with >90 % desorption efficiency. In acidic tannery effluent (pH 2.5), Cs/C/ZMO sorbent exhibited exceptional selectivity for Cr(VI) removal over competing cations, underscoring its practical utility.