Enhancement of photocatalytic reduction of Cr(VI) using hetero-system NiAl2O4/ZnO under visible light

Abstract

Hexavalent chromium Cr(VI) is well known to be a toxic and non-biodegradable contaminant and can cause significant environmental damage if it is not eliminate from wastewater. However, it can be reduced to Cr(III), which is less toxic by photocatalysis process using the heterosystem NiAl2O4/ZnO. NiAl2O4 prepared by nitrate method crystalize in a spinel structure and was characterized, by XRD, FTIR, and SEM techniques. NiAl2O4 acts as electrons pump and the electron transfer to chromate is mediated via ZnO. Under the optimized conditions, the percentage of Cr(VI) reduction was 62 % for 20 mg/L, NiAl2O4/ZnO ratio (1/1) at pH~3.7 and under visible light. An improvement up to 72% was obtained when the reaction occurs in a Rishton reactor with six bleds after 6 h illumination. It is therefore concluded that the Cr(VI) photocatalytic reduction followed a pseudo first order kinetic model, in agreement with the Langmuir–Hinshelwood mechanism. This work revealed that the NiAl2O4/ZnO heterosystem exhibits a better photocatalytic efficiency for the photoreduction of Cr(VI) mainly due to the good separation of electron-hole pairs (e-/h+) in this combination.