Ternary semiconductor photocatalysts for the degradation of benzene-toluene-xylene in aqueous phase

Abstract

Wastewater produced from the petroleum industry contains hazardous organic compounds such as benzene, toluene and xylene (BTX). A method based on advanced oxidation process has been applied to treat the organic compounds using semiconductor photocatalysts. Three types of ternary photocatalysts,namely ZnO/WO3/SnO2(ZWS), ZnO/WO3/Fe2O3(ZWF) andZnO/WO3/TiO2(ZWT) were prepared viamodified sol-gel method with different percentage ofatomic ratios of each metal oxide. Characterization of the catalyst was conducted using X-ray diffraction (XRD), diffuse reflectance ultra-violetspectroscopy (DR-UV), field emission scanning electron microscopy-energy dispersive X-ray (FESEM-EDX) and nitrogen adsorptionanalyses. The XRD analysis revealed that all the prepared catalysts are mostlyin polycrystalline phases. The existence of binary species such as ZnWO4, SnW3O9 and Fe2WO6 were detected in the bulk matrices of the ternary oxide catalysts. The types of physisorption isotherm represented by ZWS (90:9:1), ZWS (90:3:7), ZWS (33:33:34) and ZWT (33:33:34) photocatalysts were of the Type IV isotherm and hysteresis loop assigned for the mesoporousity of the respective catalysts. The band gap energy of each photocatalyst was determined by DR-UV and it showedthat the band gap was affected by the change in ratio of the precursors. FESEM-EDX analysis for all the prepared ternary catalysts exhibited small and dispersed particles. The photocatalytic activity of the prepared ternary catalysts was tested on thesimulated BTX aqueous phase (1000 ppm)irradiated under UV-light. ZWSwith the ratio of 90:3:7, 7:3:90 and 1:90:9gave 66.97%, 59.02% and 61.01% of the BTX degradation, respectively. Meanwhile, ZWF (33:33:34), ZWS (33:33:34), and ZWT (33:33:34)photocatalysts gave43.27%,68.58%, and 70.41% of the BTX degradation, respectively. All the degradation processes were conducted using365 nm UV-light at ambient temperature and pressure. The total organic carbon (TOC) analysis revealed that the percentageof mineralization of the treated compounds were 40.58% (ZWF33:33:34), 64.91% (ZWS33:33:34), and 65.36% (ZWS33:33:34) with slightly increased in the percentage of degradation using 312 nm light source. The optimum concentration of H2O2used was 10 ppmwith72.32% of BTX degradation using ZWTphotocatalystand365 nm UV-light source. The reproducibility study usingZWTcatalyst immobilized on quartz glass showed that the photocatalyticactivity decreased gradually after the first usage