Epichlorohydrin cross-linked chitosan for adsorption of reactive red 2 dye: Optimization and adsorption mechanism

The direct discharge of wastewater containing chemical dyes seriously affects human health. Here, we prepared novel adsorbent cross-linked chitosan beads for adsorption of chemically synthesized dyes. The prepared chitosan beads were tested for a series of characterization and adsorption properties. The adsorption conditions were also optimized. In addition, the adsorption mechanism, kinetics, thermodynamics and adsorption modelling of the adsorbent were investigated. The cross-linked chitosan beads have a regular shape and uniform size. The surface has a plurality of amino and hydroxyl groups. The results showed that epichlorohydrin increased the adsorption rate and reusability of chitosan. Adsorption of RR2 dye by cross-linked chitosan beads was spontaneous. The adsorption process was both physisorption and chemisorption. Under the optimal adsorption conditions (T = 303 K, pH = 3, dye concentration 55 mg/L, 50 mg of adsorbent dose), the maximum adsorption of RR2 dye by crosslinked chitosan beads ECS1 was 485.1 mg/g. The adsorption thermodynamics of the adsorbent was calculated and ΔH⁰ and ΔS⁰ were −41.22 kJ/mol and −0.103 kJ/mol·K, respectively. The surface of ECS 1 is rich in amino groups, hydroxyl groups and heteroatoms (N and O). The adsorption of RR2 molecules is facilitated by electrostatic forces, hydrogen bonding and n-π interactions.