Catalytic wastewater treatment. (conférence invitée)
Résumé
Although the water resource is not exhaustible, the water quality must be preserved, which does represent a major challenge for the coming decades. Water is extremely useful on a daily basis for many different industrial, agricultural and domestic applications. As a result of all these different uses, a wide variety of waste effluents is being produced everyday all over the world. The lecture will concentrate on such wastewater but not on the treatments involved upon the production of drinking water.
While conventional wastewater treatment plants, mainly based on biological processes, are quite efficient for the purification of low concentrated, non-toxic and biodegradable wastewater, new solutions had to be imagined for the treatment of toxic and/or non-biodegradable and/or highly concentrated effluents.
Many different approaches have been investigated, numerous combinations have been tested but none of the alternatives developed up to now might be considered as universal. None of them is really better than the other: the more tools in the toolbox, the better.
Before selecting one technology over another, many different parameters have to be considered including the composition of the effluent, the concentration of the pollutants, the physical and chemical characteristics of the wastewater (pH, ionic force, turbidity, etc.), the volumes to be treated, as well as some engineering and economical aspects, etc. The final objective of the treatment is also to be taken into account, i.e. whether the pollutants should be totally mineralized or the treated effluents should only be made biodegradable or the organic content should be recovered to produce new chemicals and/or materials.
Among the possibilities, catalytic processes may play a crucial role, especially when the reaction conditions need to be made milder and/or when the process has to be ran faster and/or whenever selectivity issues are of major concern. Again, different options have been evaluated, including both reduction and oxidation processes.
Key examples taken from the recent literature will be presented all along the lecture, trying to combine different aspects and approaches, going from the development of innovative catalytic materials till the mechanistic information. Looking at the reduction processes, both the hydrodehalogenation and the catalytic reduction reactions will be briefly reviewed and the major challenges to be overcome will be highlighted both in terms of material science or catalytic efficiency (activity and selectivity). Finally, most catalytic oxidation processes will be listed, especially the advanced oxidation processes (AOPs), involving the hydroxyl radicals as the active oxygenated species; and the catalytic wet air oxidation for the most complex type of effluents. The main achievements and the key issues in the different fields will be identified.