Iron modified chitosan/coconut shell activated carbon composite beads for Cr(VI) removal from aqueous solution

Iron modified chitosan/coconut shell activated carbon (Fe/CSCC) composite bead is synthesized to remove Cr(VI) and is characterized to reveal the influencing factors and reaction mechanism. Results show that the adsorption capacity (Q_e) of Cr(VI) increases with the increase of iron loading, contact time (t), Cr(VI) initial concentration (C₀), and temperature (T), but decreases with the increase of pH, and mass and volume ratio (m/v). After 0.1 mol FeCl₃ modification, the removal efficiency of Cr(VI) by Fe/CSCC reaches as high as 97.25 % at pH = 3, m/v = 1.0 g/L, t = 2880 min, C₀ = 25 mg/L, and T = 25 °C. The coexisting ions of SO₄^2-, HPO₄^-, and Ca²⁺ lead to the decrease of Q_e by 7.82, 5.05, and 5.50 mg/g, respectively, and the inhibition effect increases with their increasing concentrations. Fe/CSCC adsorption for Cr(VI) is an endothermic spontaneous process, and a chemical and monolayer adsorption, which is better fitted to a pseudo-second-order kinetic. The fitted maximum Q_e is 64.49 mg/g by using the Langmuir model. Moreover, after five cycles of regeneration, the Q_e value only drops about 3.46 mg/g. Characterization analysis of BET, XRD, FTIR, XPS, and SEM-EDS indicates that Cr(VI) is mainly adsorbed by Fe/CSCC through electrostatic attraction and complexation, which is related to the -COOH and - NH₂ groups, and Fe - O groups, respectively.