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With a view to developing a UV-LED photocatalytic reactor for small and remote water systems, the degradation of three representative agriculturally produced contaminants, two antibiotics and an endocrine disruptor hormone, was evaluated in a bench 365 nm LED photoreactor using a slurry of the well-known Degussa P25® (TiO2) as photocatalyst. Use of an additional electron capture additives O2 and H2O2 was also assessed. Loss of the parent organic compounds was tracked by HPLC or UV absorbance and mineralization, where feasible, was studied with TOC analysis with conventional instrumentation. In all cases, degradation is significant with moderate light dose. Lab data suggest log reduction with light delivery less than 2.2 kWhr per cubic meter light delivery.

Sensitized photodechlorination of polychlorinated biphenyl, PCB 138, in three different surfactant solutions was studied. The sensitizer of choice was leuco-methylene blue, which was produced in situ from methylene blue using either triethylamine or sodium borohydride. Three types of surfactants, anionic (SDS), neutral (TWEEN 80), and cationic (CTAB) at different concentrations were investigated. The neutral and cationic surfactants were found to be more effective than anionic. In each case the surfactant concentration was found to play a significant role in the rate of dechlorination. For different sensitized systems (triethylamine or sodium borohydride), a different product distribution and a different pathway of dechlorination was observed., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

A study of dechlorination of PCB 138, under visible light employing methylene blue (MB) and triethylamine (TEA) in acetonitrile/water has been conducted to investigate the details of the mechanism of dechlorination and to determine the efficiency of the process for this representative congener. Two other amines, N-methyldiethanolamine (MEDA) and (triethanolamine) TEOA also replaced TEA and two other solvents, methanol and ethanol replacing acetonitrile were examined for effects on reaction rates. The results show that PCB 138 can be dechlorinated efficiently in this photocatalytic reaction. Clarifying ambiguities in several previous reports, the reduced form of MB, leuco-methylene blue (LMB) was identified as responsible for the photoreaction with its excited state transferring an electron to PCBs; oxidized LMB (i.e. MB) is reduced back to LMB by the excess amine present. The reaction depends on a cycle driven by the amine as a sacrificial electron donor. MEDA proved to be the most efficient electron donor; apparently in consequence of the most favourable steady state concentration of LMB. Methanol and ethanol may be used to replace acetonitrile with little change in the efficiency of the reaction., (Copyright 2010 Elsevier B.V. All rights reserved.)

As part of a program aimed at developing a field process for cleanup of PCB contaminated soils using photochemistry in basic 2-propanol, additional details of the dechlorination pathway are presented. The mechanism involves a chain reaction with both homolytic photochemical C-Cl bond fission and electron transfer steps producing PCB anion radicals. Kinetics of dechlorination of various congeners show patterns of relative rates associated with the basic 2-propanol medium that are not found in other media because both electron transfer and photochemical homolysis steps determine overall rates of dechlorination and govern the pathways and relative concentrations of intermediates. The electron transfer steps display opposite structure-reactivity correlations to the photo-homolysis, C-Cl bond fission steps. Oxygen quenching is shown to differentially affect both types of steps. In contrast to the suggestion that inter system crossing can be highly efficient with reaction originating from a PCB triplet, oxygen quenching data suggest that a significant minimum of the quantum yield is non-quenchable, presumably because of a reaction path from the PCB singlet. This may help to explain why exclusion of air is not entirely necessary in practice.

A study on dechlorination of PCB138 in transformer oil (TO) and 2-propanol (IPA) using 254 nm ultraviolet (UV) light as well as dye sensitized visible light has been conducted. Studies on dechlorination of polychlorinated biphenyls (PCBs) in TO using visible light in the presence of methylene blue (MB) and triethylamine (TEA) (providing a ‘photocatalytic’ cycle) in both deaerated and aerated conditions have been conducted to determine effects of TO, MB and TEA on reaction rates. The results show that photolytic methods are effective in treating PCBs in TO, and that the oil plays a limited adverse role. Under UV irradiation, PCB 138 can be >99% dechlorinated in the presence 0.06% (w/w) TO in IPA within 1 h with a rate constant of 0.0853 min−1, while 47% of PCB138 can be dechlorinated in 92.1% (w/w) TO in IPA within 2 h with a rate constant of 0.0051 min−1. In the ‘photocatalytic’ system, 94% reduction of PCB 138 was achieved within 30 min with a rate constant of 0.0968 min−1 when the solvent was 60.70% (w/w) TO in IPA, while 71% dechlorination of PCB138 was achieved within 30 min with a rate constant of 0.0382 min−1 when 81.62% (w/w) TO was present. In treatment of 30–73 ppm PCBs in TO, the optimal concentration of MB and TEA were found to be 0.5 g/L and 58.08 g/L respectively. Because of quenching by oxygen, deaeration of the solution is necessary for an efficient reaction. The photocatalytic system is especially adapted for treating lower concentration of PCBs in TO. [ABSTRACT FROM AUTHOR]

Several oxidative methods were studied to degrade sulfolane in an aqueous medium. These include UVA and UVC irradiation with suitable photoactive oxidants, including ozone, H 2 O 2 , and TiO 2 based photocatalysis and their combinations. Since sulfolane lacks absorption bands in the UV range beyond 200 nm, initiation of reactions depends on the spectra and photochemistry of the oxidants. Among all the advanced oxidation processes investigated, combinations of (a) UVC with H 2 O 2 and O 3 (b) UVC with H 2 O 2 and (c) UVC with O 3 led to the highest rate of sulfolane loss in synthetic water samples. Experiments on sulfolane contaminated groundwater samples also indicated that these three combinations can efficiently degrade sulfolane. Furthermore, a synergistic effect was observed in the combination of H 2 O 2 and O 3 photolysis. [ABSTRACT FROM AUTHOR]