Activated carbon (prepared from secondary sludge biomass) supported semiconductor zinc oxide nanocomposite photocatalyst for reduction of Cr (VI) under visible light irradiation

Activated carbon prepared from waste tannery sludge biomass was employed in the synthesis of carbon supported ZnO photocatalyst (ZnO-SBAC). Different ratios of ZnO-SBAC (ZC) were synthesized by hydrothermal method. The prepared materials were characterized thoroughly. From the UV-vis-DRS data, zinc: carbon 2:3 ratio (ZC-2:3) was found to have lower band gap energy of 2.96 eV compared to other ratios. FE-SEM analysis revealed the ZnO flakes to lie on the surface of the carbon. The prepared ZC-2:3 nanocomposites has higher surface area of 216.35 m(2)/g compared to pristine ZnO 11.60 m(2)/g. The in-situ generation of OH radicals was detected by EPR analysis. The prepared ZC nanocomposite material was employed as a visible active photo-catalyst for the reduction of Cr(VI) ions. Among the prepared ratios, ZC-2:3 exhibited the highest activity. The preliminary photocatalytic studies showed acidic medium (pH 2.25) and a catalyst dosage of 10 mg/10 mL to be optimum. The kinetic studies showed the photo reduction of Cr(VI) using pristine (P-ZnO) and ZnO-SBAC to follow pseudo-first order kinetics. The effect of adsorption (dark reaction) was investigated using Langmuir and Freundlich isotherms. ZC has better adsorption capacity (3.54 mg/g) compared to P-ZnO (0.76 mg/g) which may increase the photoreduction of Cr(VI) ions. The ZC-2:3 nanocomposite showed better reusability photocatalyst without affecting its performance up to three cycles when compare to P-ZnO. The prepared ZnO-SBAC exhibits good synergetic effect of adsorption followed by photoreduction for the conversion of Cr(VI) to Cr(III).