Different effects of humic substances on photodegradation of p,p′-DDT on soil surfaces in the presence of TiO2 under UV and visible light

Different effects of humic substances (HS) on TiO2 photocatalytic degradation of p,p′-DDT on soil surfaces were observed under the irradiation of UV and visible light. Under UV light irradiation, HS inhibited the photodegradation of p,p′-DDT on natural soil surfaces as well as photocatalytic degradation induced by TiO2. The decrease of UV-Vis absorbance values and total organic carbon indicated that HS underwent degradation and were mineralized in the presence of TiO2. The adsorption of 2 wt.% HS onto TiO2 extend the response of TiO2 to visible light evidenced by the UV-Vis diffuse reflectance spectra. Under visible light irradiation, p,p′-DDT degradation on HS-sensitized TiO2 was observed. By injecting electrons from the photoexcited HS to the conduction band of TiO2, the sensitized TiO2 degraded p,p′-DDT under the irradiation of visible light. In addition, photodegradation of p,p′-DDT was accelerated in the presence of HS. On the analysis of photoproducts using a gas chromatography coupled with a mass spectrometer analysis, DDE and DDD were detected under irradiation of UV light. It is suggested that DDE could be produced by active radicals and the produced DDD may be due to direct photodegradation of p,p′-DDT. Under irradiation of visible light, produced electrons by photosensitization reaction could attack the aliphatic Cl of p,p′-DDT, leading to the production of DDD. The different photoproducts further confirmed that different mechanism of p,p′-DDT photodegradation was involved under irradiation of UV and visible light.