Construction and mechanistic insights of a novel ZnO functionalized rGO composite for efficient adsorption and reduction of Cr(VI)

A series of ZnO decorated reduced graphene oxide (rGO) (ZnrGO_x) with different doping ratios were synthesized by the alkaline hydrothermal method using graphene oxide (GO) and Zn(NO₃)₂·6H₂O as precursors, and subsequently used for the adsorption study of Cr(VI) in water. The morphology, crystalline phase structure, and surface elemental properties of ZnrGO_x composites were revealed by XRD, SEM, BET, FT-IR, and XPS characterizations. The results showed that ZnO nanoparticles can be clearly seen on the surface of layered rGO. Meanwhile, as the doping rate increased, the C = C double bonds were broken and more carboxylic acid groups formed in ZnrGO_x. In addition, the ZnrGO_0.1 composite had the most excellent adsorption performance and good stability, and reusability. The adsorption removal rate of Cr(VI) can reach 99%, and the maximum adsorption amount of Cr(VI) was 68.9655 mg/g in 3 h. The isothermal and kinetic model simulations showed that Cr(VI) adsorption on ZnrGO_0.1 composite is a chemical adsorption process, spontaneous and endothermic. Based on the concentrations of different valence states of Cr in the solid and liquid phases, 40% of Cr(VI) was reduced to Cr(III) on the surface of ZnrGO_0.1 composite. Moreover, the adsorption-reduction mechanisms of Cr(VI) on ZnrGO_0.1 composite were further elucidated. The ZnrGO_0.1 composite manifested great potential as an efficient adsorbent for Cr(VI) removal.