Heavy metal contamination of water remains a global environmental challenge, creating an urgent need for sustainable adsorbents with high capacity and well-defined adsorption mechanisms. Here, chitosan was thermally reconstructed at 200–500 °C to reprogram its metal-binding chemistry. The 400 °C-treated chitosan delivered a high Cu(II) uptake of 120 mg g⁻¹ under optimized conditions (30 mg L⁻¹ Cu(II), 25 °C, pH 5) with robust recyclability, retaining 84% of its removal efficiency over five adsorption–desorption cycles. Thermal treatment reconstructed the framework, enriched surface oxygen functionalities, and improved structural stability. Adsorption followed pseudo‑second‑order kinetics (R²>0.99), consistent with a chemisorption-dominated process. Mechanistic analysis revealed a clear transition from amino- and hydroxyl-group coordination in pristine chitosan to oxygen-mediated chemisorption after thermal treatment. Oxygen-containing surface species emerged as the dominant active sites for Cu(II) capture. These findings demonstrate that controlled thermal reconstruction can effectively tune the metal-binding chemistry, providing a sustainable strategy toward engineering high-performance adsorbents for heavy-metal remediation.
As(III) and As(V) pose distinct challenges for water treatment, and existing layered double hydroxide (LDH)–biochar composites remain limited in removing these species. A bimetallic Fe/Al-LDH–biochar composite (LDH-BC) was fabricated via sequential hydrothermal treatment and mechanochemical ball milling. Under optimized conditions, LDH-BC effectively removed As(III) and As(V), and maintained stable As(III) removal across pH 3–12. The adsorption of both As species followed pseudo-second-order kinetics, and the equilibrium data were well described by the Langmuir–Freundlich model (QmAs(III) = 69.8 mg g–1 and QmAs(V) = 56.9 mg g–1). In addition, coexisting anions inhibited As removal in the order of PO43– > SO42– > SiO32– > Cl–. Both As species were removed mainly via inner-sphere complexation at Fe–OH/Al–OH sites; As(V) additionally involved electrostatic attraction and possibly interlayer exchange, while part of the adsorbed As(III) was oxidized to less toxic As(V) through an interfacial Fe(III)/Fe(II) redox interaction. The LDH-BC retained stable reusability for As(III) over five cycles, whereas As(V) removal declined after the first cycle and stabilized at ~30%, indicating species-dependent regeneration behavior. In summary, Fe/Al-LDH-BC demonstrates effective As removal, and this study provides molecular-level insights into bimetallic LDH-biochar systems for treating As-contaminated water.
