Diepoxyoctane-cross-linked and spermidine-functionalized chitosan beads for the removal of hexavalent chromium and...

Complex industrial effluents often contain coexisting contaminants that compete for adsorption sites and complicate wastewater treatment. Here, diepoxyoctane-cross-linked and spermidine-functionalized chitosan beads (CHI-DEO-SPD) were developed as acid-stable, amine-rich adsorbents for removing Reactive Blue 4 (RB4) and Cr(VI) from single and binary aqueous systems. The novelty of this strategy lies in combining DEO cross-linking, which improves structural stability under acidic conditions, with spermidine functionalization, which increases the density of protonatable amine sites for anionic pollutant binding. The beads were optimized at 1.5 g/L SPD, 40 °C, and 3 h. FTIR, XRD, TGA, SEM, BET, and pHpzc analyses confirmed successful modification, reduced crystallinity, improved thermal stability, enhanced surface roughness, increased surface area (4.59 to 34.60 m²/g), and a positively charged surface below pH 6.3. Langmuir capacities reached 872.35 mg/g for RB4 and 785.19 mg/g for Cr(VI) in single-solute systems and remained high in binary systems at 715.26 and 699.92 mg/g, respectively. Kinetic and equilibrium data were best described by pseudo-second-order and Freundlich models, indicating heterogeneous chemisorption driven mainly by electrostatic attraction, hydrogen bonding, and dye-specific π-π interactions. Thermodynamic analysis confirmed spontaneous adsorption in all systems. RB4 adsorption was weakly exothermic in single systems but became endothermic under competition, while Cr(VI) adsorption was endothermic in both systems and more energy-intensive in binary systems. After five regeneration cycles, the beads retained 89.8% and 82.9% of their initial RB4 and Cr(VI) capacities, respectively. These results demonstrate the strong potential of CHI-DEO-SPD beads for treating acidic wastewater containing coexisting dyes and heavy metals.