Enhanced photocatalytic reduction of toxic Cr(VI) with Cu modified ZnO nanoparticles in presence of EDTA under UV illumination

Abstract

In this study, the photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution using 1 wt% Cu/ZnO photocatalyst in presence of ethylenediaminetetraacetic acid (EDTA) under UV irradiation was investigated. The photocatalytic Cr(VI) reduction with ZnO under UV irradiation was significantly low. A fast Cr(VI) reduction with ZnO was recorded in presence of EDTA. Results demonstrated that the rate constant raised from 1.6 × 10−2 to 7.5 × 10−2 min−1 with 20 ppm EDTA addition. The optimum reduction (90%) was observed within 30 min at pH 3. The reduction efficiency of Cr(VI) was evidently increased from 6% to 99%, with increasing ZnO dosages up to 40 mg. The role of co-catalysts such as Au, Ag, Cu and NiO was also studied. The addition of Cu ion remarkably accelerated the reduction of Cr(VI), compared with ZnO/EDTA system alone. The Cu motivated ZnO/EDTA system completed the 99% reduction within only 30 min. The Cu ions were reduced to metallic copper and Cu2O by conduction band electrons of ZnO. The electron–hole recombination was gently declined by the shallow electron trapping of Cu. On the other hand, EDTA could be performed in two ways by forming Cr(III)-complex and separating Cr(III) from ZnO surface and by consuming valence band holes. The Cu modified ZnO photocatalyst was characterized by N2 adsorption- desorption isotherm, TEM, EDX, XPS, DRS and PL spectra. We evaluated the influence of pH, photocatalyst dosage, initial concentration of EDTA and initial concentration of Cu2+ ion (Cu doping amount) on photocatalytic reduction of Cr(VI). The pseudo first order kinetic reaction was confirmed for this reduction process. Herein, a probable mechanism of this photocatalytic reduction process was also presented.