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Treatment of wastewater containing chlorinated solvents by solar photocatalysis and photo-Fenton

Article 16 of the European Union Water Framework Directive (2000/60/EC) set out a "Strategy against pollution of water". The first stage in the strategy was a list of priority substances which would become Annex X of the Directive. Once the list of priority substances has been adopted, the Commission will propose community-wide water quality standards and emission controls for those priority substances (European Commission, 2001). The 32 substances or groups of substances on the proposed list of priority substances include selected chemicals, plant protection products, biocides, metals and other groups, such as polyaromatic hydrocarbons. Finally, Decision No 2455/2001/EC of the European Parliament and of the Council of 20 November 2001 was made by establishing the list of priority substances in the field of water policy and amending Directive 2000/60/EC. Several non-biodegradable chlorinated solvents (NBCS) are included in these substances: 1,2-dichloroethane, dichloromethane and trichloromethane (chloroform). In the same context, the EU IPPC Directive (96/61/EC) required the development of technologies and management practices for specific industrial sectors (see Annex I of the Directive) for the minimisation of pollution and for the development of water recycling. Due to the lack of available on-site treatment technologies, a large number of industrial activities do not treat such wastewater adequately. As consequence, simple, available low-cost technologies are in great demand.

Advanced Oxidation Processes (AOPs) have been proposed as an alternative for the treatment of biorecalcitrant wastewater. Many studies have concentrated on this goal, pointing out that these processes, while making use of different reacting systems are all characterised by the same chemical feature: production of OH radicals (·OH). Hydroxyl radicals can be generated with a semiconductor which absorbs UV radiation when it is in contact with water. Whenever different semiconductor materials have been tested under comparable conditions for the degradation of the same compounds, TiO2 has generally been demonstrated to be the most active. OH radicals can also be produced by the Fenton reagent (addition of H2O2 to Fe2+ salts). Photo-Fenton combines Fenton and UV-VIS light. The photolysis of Fe3+ complexes enables Fe2+ regeneration. Under these conditions iron can be considered a real catalyst. Both catalytic systems are of special interest because solar light can be used.

TiO2 has often been used for the treatment of NBCS, but photo-Fenton has not been applied as often. In any case, most of this research has been done in small, air-tight and very often cooled (to avoid NBCS volatilisation) laboratory photoreactors illuminated artificially. Results have been successful, but there is one aspect not very often addressed in these articles. The future application of photoreactors for the treatment of wastewater containing NBCS must be done in large systems, where cooling would not be economically feasible and air-tightness very often impossible, more so in a solar photoreactor. To work under air-tight conditions it would be necessary to: (i) inject oxygen (or air) to maintain its concentration in the reactor (for organic carbon mineralisation), (ii) continuously purge the CO2 produced and (iii) increase the plant construction cost (tanks, tube connections, photoreactor, etc) to prevent gas leaks. It must therefore be demonstrated that an AOP is suitable for application to volatile or semi-volatile compounds (dissolved in water) without exorbitant increase in plant and operating costs. For this reason, the main objective of this work was to demonstrate that NBCS treatment is feasible in an open-air pilot-plant photoreactor illuminated by sunlight.

In CADOX Project, it has been demonstrated that both photo-Fenton and TiO2 photocatalytic treatment of wastewater containing NBCS is possible, but if an unsealed reactor (or photoreactor) is to be used, operating parameters must be very carefully selected. Anaerobic conditions are the only way to guarantee that NBCS will not be released into the atmosphere during TiO2 treatment. But, under these conditions the NBCS are not mineralised and other toxic compounds could be formed. In this context, a photo-Fenton treatment with an iron concentration of around 56 mg L-1 could be a suitable treatment. Special care must be taken in selecting reactor operating temperature (which promotes both NBCS volatilisation and reaction rate) and initial hydrogen peroxide concentration. Besides, chloride determination is proposed as key parameter for controlling NBCS photocatalytic treatment.

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