Photocatalytic reduction of Cr (VI) in aqueous solution over ZnO/HZSM-5 nanocomposite: optimization of ZnO loading and process conditions

The photo-reduction of Cr (VI) was investigated over a series of ZnO-impregnated H-ZSM-5 catalysts varying in ZnO loadings. All the samples were characterized by XRD, FESEM, TEM, EDX, BET, ICP and FT-IR techniques. The XRD analysis confirmed the existence of ZnO and HZSM-5 as crystalline phases in the nanocomposite structure. TEM images illustrated that fine nanoparticles of active metal oxide that were uniformly dispersed over HZSM-5 support. In addition, activity tests were carried out and effects of operational parameters such as initial concentration, reaction time, UV intensity and pH were evaluated. Among the prepared samples, the ZnO/H-ZSM-5 nanocomposite containing 40 wt% ZnO demonstrated the best photocatalytic activity. The full factorial design was applied to optimize the effect of variable factors in the presence of the best photocatalyst. The ANOVA results indicated that at the 95% confidence level, all four variables and their interactions significantly affected the Cr (VI) reduction. In addition, the most effective variable was the initial concentration of Cr (VI). The optimum initial concentration of Cr (VI), pH, UV intensity and reaction time were found to be 10 mg/L, 3, 125 W and 60 min, respectively. Under optimal conditions, complete reduction (>98%) was approximately obtained for Cr (VI).