Efficient visible light photocatalysis of benzene, toluene, ethylbenzene and xylene (BTEX) in aqueous solutions using supported zinc oxide nanorods

Abstract

Benzene, toluene, ethylbenzene and xylenes (BTEX) are some of the common environmental pollutants originating mainly from oil and gas industries, which are toxic to human as well as other living organisms in the ecosystem. Here we investigate photocatalytic degradation of BTEX under visible light irradiation using supported zinc oxide (ZnO) nanorods grown on glass substrates using a microwave assisted hydrothermal method. ZnO nanorods were characterized by electron microscopy, X-ray diffraction (XRD), specific surface area, UV/visible absorption and photoluminescence spectroscopy. Visible light photocatalytic degradation products of BTEX are studied for individual components using gas chromatograph/mass spectrometer (GC/MS). ZnO nanorods with significant amount of electronic defect states, due to the fast crystallization of the nanorods under microwave irradiation, exhibited efficient degradation of BTEX under visible light, degrading more than 80% of the individual BTEX components in 180 minutes. Effect of initial concentration of BTEX as individual components is also probed and the photocatalytic activity of the ZnO nanorods in different conditions is explored. Formation of intermediate byproducts such as phenol, benzyl alcohol, benzaldehyde and benzoic acid were confirmed by our HPLC analysis which could be due to the photocatalytic degradation of BTEX. Carbon dioxide was evaluated and showed an increasing pattern over time indicating the mineralization process confirming the conversion of toxic organic compounds into benign products.

Fig 1. Schematic representation of a photocatalytic process using zinc oxide nanorods for degrading the monoaromatic hydrocarbons.

Fig 2. (a) Top and cross-sectional (inset) SEM micrographs and (b) XRD pattern of ZnO nanorods.

(a) SEM micrographs of ZnO nanorods and (b) XRD pattern of the microwave assisted hydrothermally grown ZnO nanorods on glass substrate. Samples were annealed at 350°C for 1 h after the hydrothermal growth.

Fig 3. TEM micrograph of a synthesized single nanorod.

The top inset representing the lattice fringes in the ZnO nanorods and the bottom inset is for the high resolution TEM showing the SAED pattern.

Fig 4. (a) Typical optical absorption spectrum and (b) room temperature photoluminescence (PL) spectrum of ZnO nanorods.

Inset in Fig 4a representing the tauc plot and inset in Fig 4b is showing the ZnO surface defect mediated PL bands (excitation: 325 nm) deconvoluted into two Gaussian components centered at 527 nm and 570 nm, respectively.

Fig 5. Reduction in the concentrations of (a) benzene, (b) toluene, (c) ethylbenzene and (d) xylene.

Reduction in the concentration of (a) benzene, (b) toluene, (c) ethylbenzene and (d) xylene under simulated solar light irradiation (incident power: 1 kW/m²) in the presence and absence of ZnO nanorods. The starting concentration of all the monoaromatic hydrocarbons was 25 ppm.

Fig 6. Plots representing the rate constants vs. initial concentrations of (a) benzene, (b) toluene, (c) ethylbenzene and (d) xylene.

Fig 7. Evolution of carbon dioxide over time during BTEX photocatalytic degradation.

Evolution of carbon dioxide over time degradation of 25 ppm BTEX mixture in DI water in the presence of ZnO nanorods under the simulated solar light irradiation as determined by using GC fitted with thermal conductivity detector (TCD). Inset shows the CO₂ chromatogram detected at retention time 3.34 minutes.

Fig 8. Schematic representation of visible light photocatalytic degradation of BTEX.

Visible light photocatalytic degradation of BTEX in aqueous medium in the presence of ZnO nanorods as supported photocatalyst. Upon photo excitation, BTEX molecules are photocatalytically degraded by the ZnO nanorods resulting in various types of intermediate byproduct, which upon successive photo-oxidation produces CO₂, H₂O and mineral acids leading to the complete mineralization of BTEX.