Degradation of Nitroaromatic Compounds by Homogeneous AOPs

Abstract

Nitroaromatic compounds are environmental contaminants associated with anthropogenic activities such as production and use of dyes, explosives, pesticides and pharmaceuticals. Many of these substances, such as nitrobenzene and nitrophenols, usually found in wastewaters of these industries are considered potentially toxic. Because nitro-substituted aromatic compounds have strong electron withdrawing groups, they are poorly biodegradable by aerobic treatments. The detoxification of wastewaters containing these hazardous substances is very difficult since, due to their high stability, they are usually refractory to conventional biological treatments. Research on alternative or additional methods of wastewater treatment is of current interest. Wastewater treatment by means of advanced oxidation processes (AOPs) has become one of the issues of major interest in modern environmental chemistry. Various AOPs are nowadays available and applicable at laboratory, pilot or even technical levels for achieving oxidative degradation of organic pollutants in aqueous media. These processes are based on the production of highly reactive species. Among them, the hydroxyl radicals (HO•) are the main oxidizing species. Hydroxyl radicals are able to oxidize most organic compounds due to their high reactivity and low selectivity. The reaction of HO• with organic compounds (by addition to double bonds and/or by hydrogen abstraction) generates C-centered radicals that are subsequently trapped by dissolved oxygen to yield peroxides and peroxyl radicals. These intermediates initiate thermal chain autooxidation reactions and the overall processes may, if necessary, lead to complete mineralization. A clear understanding of the effect of reagent concentrations on the evolution of reaction byproducts is critical for producing proper engineering designs. Therefore, the optimization of AOP-methods for waste water treatment requires a comprehensive understanding of the chemical events that govern the transformation rates of the pollutants. The main objective of this chapter is to provide a comprehensive description of physicochemical phenomena that govern both the transformation rates of nitroaromatic pollutants and the overall degradation efficiencies during waste water treatments by different advanced oxidation processes in homogeneous phase. The chapter summarizes the results obtained in studies related with the degradation of nitroaromatic compounds of environmental relevance by different homogeneous AOPs. Simple tools for describing the