Decontamination of hexavalent chromium in aqueous systems through reduction with silver doped zinc oxide nanoparticles

Hexavalent chromium (Cr(VI)) is the most toxic state of chromium that can lead to serious ecological and human health consequences. This research investigated the efficacy of visible-light-active silver-doped zinc oxide (Ag-Doped ZnO) nanoparticles to remove Cr(VI) via reduction to a non-toxic state. The study was focused on determining the optimum Ag-Doped ZnO dose for Cr(VI) removal under the effect of pH, and investigating the kinetics of Cr(VI) reduction. The ZnO NPs were prepared using the co-precipitation method and doping with Ag was achieved with KOH and thermal decomposition. Cr(VI) removal was efficient at NP:Cr(VI) ratios of 50:1 and 25:1 achieving 100 % removal in 24 h and 48 h respectively. The removal process was hindered at pH below 6 and above 9 at a lower NP dose. The test for contact time, showed that 42 ± 9 % of the removal was attained in the first 6 h while most of the removal (73 ± 8 %) occurred in the first 12 h. The rate constant revealed that the reduction with Ag-Doped ZnO exhibited a pseudo-first-order kinetic rate with k values of −0.065 ± 0.01 mg/L.h⁻¹.