Photodegradation of xylene isomers: Kinetics, mechanism, secondary pollutant formation potential and health risk evaluation.

Photodegradation technology has been widely applied in the purification of industrial aromatic hydrocarbons. However, whether this technology efficiently removes the pollutants to prevent secondary pollution and health risk is still unclear. Here, the photodegradation processes of three xylenes were compared under designed reaction atmospheres and light sources. Xe lamp showed poor photodegradation ability toward xylenes, no matter in N2 or N2+O2 system, while much higher photodegradation performance of xylenes were obtained under ultraviolet (UV) and vacuum ultraviolet (VUV) irradiation, especially in N2+O2+VUV system, where 97.9% of m-xylene, 99.0% of o-xylene or 87.5% of p-xylene with the initial concentration of 860 mg/m3 was removed within 240 min. The xylenes underwent three processes of photo-isomerization, photodecomposition and photo-oxidation to produce intermediates of aromatics, alkanes and carbonyls. Among them, the photo-isomerization products showed the highest concentration percentage (e.g., ≥50% in o-xylene system), confirming that photo-isomerization reaction was the dominated photodegradation process of xylenes. Moreover, these isomerized products not only contributed about 97% and 91% to the formation potential of O3 (OFP) and secondary organic aerosols (SOAFP), but also displayed obvious non-carcinogenic risk, although one of photodecomposition product-benzene showed the highest occupational exposure risk. Therefore, the secondary pollution and health risks of photodegradation products of xylenes were non-ignorable, although the OFP, SOAFP and health risks of the generated products reduced at least 4.5 times in comparison with that of the degraded xylenes. The findings are helpful for the appropriate application of this technology in the purification of industrial organic waste gas.