Post-irradiation effect and reductive dechlorination of chlorophenols at oxygen-free TiO2/water interfaces in the presence of prominent hole scavengers

Heterogeneous photocatalysis with TiO2 nanoparticles for the detoxification of waste-waters and other media is typically achieved by oxidative pathways involving ultimately photogenerated surface-trapped holes (i.e, surface-bound (OH)-O-. radicals). However, reductive pathways may coexist. The degradation of 2,4-dichlorophenol (2,4-DCP) and pentachlorophenol (PCP) was examined in irradiated O-2-free aqueous media in the presence of prominent hole scavengers such as polyethyleneimine (PEI), triethylamine (TEA), 2-propanol (IPA) and ethylenediaminetetraacetic acid (in the EDTA(4-) state) to assess the role of photogenerated electrons. The presence of these scavengers imparts a blue-violet color to the nanoparticles corresponding to a broad absorption band in the 400-850 nm region. A post-irradiation effect was observed when irradiation was terminated, which we ascribe to an electron transfer from radical species formed by hole scavenging to the TiO2 particles (reminiscent of the doubling current phenomena in semiconductor electrodes). The two chlorophenols degraded in oxygen-free media, a process inhibited by PEI, TEA and IPA. In the presence of EDTA, degradation of 2,4-dichorophenol occurred by dechlorination only after a 120-min induction period, a process attributed to the accumulated electrons on the particle surface. HPLC analyses revealed that the intermediate products were not those expected from typical oxidative pathways in aerated aqueous media. The degradative process is inferred to take place first by electron attachment to the halophenol to yield phenoxy related radicals and then by secondary reactions with water and/or with each other. (C) 2000 Elsevier Science S.A. All rights reserved.