Iron oxides decorated graphene oxide/chitosan composite beads for enhanced Cr(VI) removal from aqueous solution

This study is the first to evaluate the effects of Iron oxides (FeO_x) species and their decoration on graphene oxide/chitosan (GO/CS) composites for Cr(VI) removal and the possibility of Fe secondary pollution. Results show that Fe(III) is a better decoration material than Fe(II) and decoration through immersion-evaporation shows a higher adsorption capacity of Cr(VI) (Q_e) than co-precipitation. Fe₂O₃-GO/CS as the only eco-friendly composite for enhanced Cr(VI) removal is further used for batch adsorption experiments, characterization, kinetics, isotherms, and thermodynamic studies. It is found that Cr(VI) removal mainly includes electrostatic attraction between Cr(VI) oxyanions and surface -NH₃⁺ and -OH₂⁺, and the adsorbed Cr(VI) partially reduces to Cr(III). Q_e increases with the increasing initial Cr(VI) concentration, contact time, and temperature, while decreases with the increasing pH and mass and volume ratio (m/v). The coexisting ions (Cl^-, NO₃^-, SO₄^2-, PO₄^3-, As, Fe, and Pb) can cause an obvious decrease of Q_e. The removal efficiency (R_e) and Q_e are 94.3% and 83.8 mg/g, respectively under the optimal conditions. After five times of regeneration, R_e is still as high as 84% and Q_e drops about 2.6%. Cr(VI) adsorption is spontaneous and endothermic, which is best fitted with the Sips model, and the fitted maximum Q_e is 131.33 mg/g.