Degradation of nitroaromatic compounds by homogeneous AOPs

FERNANDO S. GARCÍA EINSCHLAG; LUCIANO CARLOS; DANIELA NICHELA

Waste Water-TREATMENT AND REUTILIZATION

InTech. Open Access Publisher.

Lugar: Rijeka; Año: 2011; p. 1 - 434

Intech Book: Waste WaterCHAPTER ABSTRACTTitle:Degradation of nitroaromatic compounds by homogeneous AOPsAuthors:Fernando S. García Einschlag, Luciano Carlos and Daniela NichelaMission of the chapter:The main objective of the chapter is to provide a comprehensive description ofphysicochemical phenomena that govern both the transformation rates of nitroaromatic pollutants and the overall degradation efficiencies during waste water treatments by different advanced oxidation procedures (AOPs) in homogeneous phase.Concerns of the chapter:The chapter summarizes the results obtained in studies related with the degradation ofnitroaromatic compounds of environmental relevance by different homogeneous AOPs. Simple tools for describing the main kinetic features in different systems are presented. In addition, the influence of reaction conditions in the transformation pathways of nitrobenzene is discussed.Short description:1.- Introduction1.1.- Environmental relevance of nitroaromatic compounds. Nitroaromatic compounds are environmental contaminants associated with anthropogenic activities such as production and use of dyes, explosives, pesticides, and pharmaceuticals, among others. Many of these substances, such as nitrobenzene and nitrophenols, are usually found in wastewaters of these industries and are considered potentially toxic. Because nitro-substituted aromatic compounds have strong electron withdrawing groups, they are poorly biodegradable in aerobic treatment systems. 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.1.2.- Advanced Oxidation Processes (AOPs). 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 organic radicals that are subsequently trapped by dissolved molecular oxygen to yield peroxides and peroxyl radicals. These intermediates initiate thermal chain autooxidation reactions and the overall process may eventually lead to complete mineralization (to CO2, H2O and mineral acids).1.3.- Physicochemical characterization. It has been recently shown that physicochemical properties of the organic pollutants (i.e., absorption coefficients, rate constants, and acid-base behavior, among others) must be known to develop reaction models capable of predicting the efficiency of oxidation. In addition, since the treatment effectiveness is not only related to the substrate consumption but also to the distribution of intermediate reaction products, 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 governthe transformation rates of the pollutants.It is well known that rather complex reaction manifolds with many reaction steps are involved in the degradation of aromatic pollutants. However, results obtained in degradation experiments of nitroaromatic compounds using different homogeneous AOPs can be analyzed by using simplified models that take into account only a reduced number of kinetically key steps. These models are capable of correctly describing the main kinetic features of the studied systems by using only a few parameters as  redictive tools. This kind of approach has important implications from a practicaltechnological viewpoint since it may be used for the rational design of efficient processes.2.- Methods2.1.- Substrate characterization. Speciation of the model pollutants may influence the kinetic trends observed since both the spectral behavior and the reactivity towards HO• radicals depend on speciation. Speciation studies presented include the analysis of acid base and complexation equilibriums performed by UV/vis spectroscopy. It is worth mentioning that spectral behavior of reaction mixtures allows evaluating inner filter effects for photoenhanced technologies. The reactivity towards hydroxyl radicals governs the fraction of HO• that effectively attack the model pollutant in a given reaction mixture. Therefore competition experiments are routinely performed inorder to asses the rate constants that are required to evaluate HO• scavenging effects. Finally, the characterization of the initial toxicity and its evolution along the treatment by means of toxicity tests is recommended.2.2.- Monitoring the substrate transformation. Different analytical techniques used to follow substrate consumption and product formation are presented. Among them UV/vis, HPLC-UV/vis, HPLC-MS, GC-MS, selective electrodes (i.e., Cl- and pH), IC and TOC are commonly used. The reaction rates calculated from the concentration profiles are used to obtain kinetic information, whereas the analysis of the distribution of reaction intermediates is used for drawing mechanistic conclusions.2.3.- Analysis of product distributions. For a detailed study of the contribution of different reaction channels of substrate degradation it should be taken into account that the initial attack of HO• to nitroaromatic substrates produces hydroxynitrocyclohexadienyl-like radicals (HNCHD•). These radicals subsequently form different primary products through parallel reaction pathways.The yield of the i-th primary product (ηi) is defined as the degraded  substrate fraction that converts into the corresponding product (Xi) as a result of the aforementioned reaction steps. Mathematical expressions for the evaluation of absolute and relative yields are presented as valuable tools for studying the effect of reaction conditions on the relative contribution of different reaction pathways.2.4.- Kinetic modeling. In order to obtain quantitative expressions describing simplifiedreaction models, the application of the steady state approximation for HO• radicals is a very useful strategy. Thus, equations governing the production and fate of HO• radicals (i.e., rProd HO• & kapp HO•) should be taken into account. The evaluation of rProd HO•is presented for both dark and irradiated systems; whereas the HO• scavenging factor, that governs the HO• lifetimes, is calculated by taking into account the main decay pathways.3.- Reaction rates and simplified reaction schemes for homogeneous AOPsSimplified schemes and model equations describing the experimental trends in differenthomogeneous AOP systems are presented. The studied substrates include: 4-Chloro 3,5-dinitrobenzoic acid (CDNBA), nitrobenzene (NB), 1-chloro-2,4-dinitrobenzene (CDNB), 2,4- dinitrophenol (DNP), 4-nitrophenol (PNP), 2-hydroxybenzoic acid (2HBA), 2,4- dihydroxybenzoic acid (24DHBA), 2-hydroxy-5-nitrobenzoic acid (2H5NBA), 4-hydroxy-3-nitrobenzoic acid (4H3NBA) and 2-hydroxy-4-nitrobenzoic (2H4NBA) acid. Among the studied technologies are the following ones:3.1.- UV Photolysis. CDNBA, NB, PNP, DNP and CDNB were used as model compound. The results show that UV-Photolysis is rather inefficient.3.2.- VUV Photolysis. The irradiation of water in the vacuum ultraviolet region (VUVphotolysis) using irradiation wavelengths of 185 nm (low pressure Hg arc with Suprasil envelope) and/or 172 nm (Xe-excimer lamp) produces HO• and H• radicals. CDNBA was decomposed by this technique and the results have shown that VUV photolysis is an efficient method for CDNBA mineralization.3.3.- UV/H2O2 systems. It is well known that UV-C photolysis of H2O2 efficiently produces hydroxyl radicals. However, the rather small absorption coefficient of hydrogen peroxide in the UVC region results in a serious drawback that limits the efficiency of this technique. In order to diminish inner-filter effects of organic matter, high H2O2 concentrations are usually required. Since at high H2O2 concentrations the scavenging of HO• by the added oxidant cannot be neglected, the rates of substrate degradation as a function of initial H2O2 concentration usually display a maximum. The optimal concentration of hydrogen peroxide ([H2O2]OPT) leading to the fastest oxidation rate could be evaluated using a simplified expression based on the main reactions involved in the first stages of the degradation process. As model substrates for UV/H2O2 systems we used CDNBA, NB, CDNB, DNP and PNP3.4.- UV/NO3 - & UV/NO2 - systems. Hydroxyl radicals may also be generated by UV photolysis of NO3 - and NO2 - ions in aqueous solutions. The results of a set of experiments dealing with the degradation of NB and PNP in these systems showed that the simplified model derived for UV/H2O2 systems can be easily extended to obtain a proper description of UV/NO3 - and UV/NO2 - systems.3.5.- Fenton systems. Kinetic and mechanistic studies of NB degradation by the Fenton’s reagent were conducted. The concentration profiles usually showed biphasic kinetics where an initial fast phase is followed by a much slower one. The relive importance of each phase strongly depended on the initial concentrations of Fe+2, H2O2 and NB.3.6.- Fenton-like systems. The decomposition of H2O2 in the presence of catalytic amounts of Fe(III) or Cu(II) salts can also be used to produce HO• radicals. The degradation of organic pollutants in the so called Fenton-like systems usually displays an autocatalytic behavior. This trend was confirmed for the substrates NB, PNP, 2HBA, 24DHBA, 2H5NBA, 4H3NBA and 2H4NBA. A simple kinetic model capable of quantitatively predicting the experimental results associated with the initial phase is proposed. Since the overall efficiency of the autocatalytic system is limited by the slowness of the initial phase, the equations derived provide valuable insights for reactor designpurposes. In addition, an empirical equation, which may be used as a valuable tool for asemiquantitative analysis of the main kinetic features of the inverted ‘‘S’’ profiles, is also presented.3.7.- Photo-Fenton systems. UV or visible irradiation can markedly enhance the overalldegradation rates of organic pollutants in the well known Photo-Fenton systems. Photo-Fenton technique is probably the AOP most frequently used in technical applications since irradiation accelerates the rates of the slow phases in Fenton-like systems and also allows a complete decrease of the total organic content present in waste waters. The advantages of photo-Fenton systems with respect to Fenton and Fenton-like systems are discussed and a method for evaluating the relative importance of photo-induced processes (i.e., Photo-enhancement factors) is given.4.- Product yields and mechanism of substrate transformation4.1.- General transformation scheme for NB. From the analysis of the distribution of NB reaction products obtained at different conversion degrees and from the comparison of the molecular structures, a general scheme of NB degradation in homogeneous AOP systems is proposed. The mechanism considers two main transformation pathways: the hydroxylation pathways which yields phenolic derivatives and the nitration pathway which mainly yields 1,3- dinitrobenzene.4.2.- Hydroxylation Pathways. The elementary steps associated to the mechanisms of NB hydroxylation are presented. In addition, in order to explain the differences in the relative proportions of the primary reaction products, the role O2, H2O2 and Fe+2/Fe+3 concentrations on the relative yields of phenolic derivatives of NB was evaluated by different experimental approaches including: Fenton (Fe+2/H2O2), Fenton-like (Fe+3/H2O2), photo-Fenton (Fe+3/UV/H2O2) and UV/H2O2 systems.4.3.- Nitration Pathway. As NB degradation proceeds in AOP systems, the organic nitrogen is mainly released as nitrite ions. Therefore, •NO2 radicals can be in situ formed by NO2 – oxidation trough either thermal or photochemical reactions. •NO2 is a well known nitrating agent which is capable of reacting with phenol to yield nitrophenols (mainly orto and para isomers). However, owing to the electron withdrawing nature of the nitro group, the aromatic ring of nitrobenzene  molecule cannot be nitrated by an elementary bimolecular step involving NB and •NO2 under mild AOP conditions. With the aim of elucidating the mechanism of NB nitration, the role O2, H2O2, Fe+2/Fe+3, NO2 - and NO3-   concentrations on the relative 1,3-DNB yields was evaluated. Differentexperimental approaches including, Fenton (Fe+2/H2O2), Fenton-like (Fe+3/H2O2), UV/H2O2, UV/NO2 - and UV/NO2 - systems were used. The results indicate that, in the experimental domain tested, the prevailing NB nitration pathway involves the reaction between the •OH-NB adduct and •NO2 radicals.