Efficient adsorption and photocatalytic removal of hexavalent chromium using chitosan-functionalized graphene oxide-MnO2 nanocomposite.

Graphene oxide/MnO2/chitosan (GO/MnO2/CS) nanocomposite showed distinct morphological changes and improved adsorption properties in wastewater treatment as compared to individual GO. GO was synthesized using modified Hummer's method and then functionalized with MnO2 nanoparticles and chitosan to prepare two nanocomposites: GO/MnO2 and GO/Mn2/CS. Nanocomposites characterized by UV/Visible spectroscopy, XRD, FTIR, SEM, and TGA. The average crystallite sizes calculated from XRD analysis were 30 nm for GO/MnO2 and 31 nm for GO/MnO2/CS. The nanocomposite showed a red shift in band gap from 3.85 eV (GO) to 2 eV (GO/MnO2/CS), which led to enhanced adsorption capacity and photocatalytic activity. Adsorption experiments were conducted to assess the elimination of Cr (VI) ions by synthesized nanomaterials at various operational conditions. The maximum adsorption capacities (qmax) were 131.58, 150.64 and 179.28 mg/g for GO, GO/MnO2, and GO/MnO2/CS respectively, at pH 2. The adsorption behavior of Cr (VI) onto the synthesized nanocomposites followed the Freundlich isotherm model, indicating multilayer adsorption on a heterogeneous surface. Kinetic studies revealed that the process followed a pseudo-second-order model, suggesting chemisorption as the dominant mechanism.