Elimination of phenol and aromatic compounds by zero valent iron and EDTA at low temperature and atmospheric pressure
Introduction
Industrial effluents containing refractory or biotoxic compounds must be treated prior to biological depuration. In this context, the destruction of aromatic and phenolic compounds from wastewater is of interest due to their biotoxic and recalcitrant properties preventing direct biological treatment. Also, processes conducted at mild operating conditions are gaining more attention because of lower equipment and operation costs. Among these treatments, catalytic wet air oxidation (CWAO) (Levec, 1997, Fortuny et al., 1999) is attractive because it has demonstrated to be an effective technique for eliminating organic compounds at relative mild pressures and temperatures, although often above 140 °C and 2000 kPa. Likewise, oxidation techniques carried out at low temperature as Fenton process, i.e. H2O2–Fe(II), also achieve satisfactory results even at room temperatures (Bigda, 1995, De et al., 1999). Nevertheless, both CWAO and Fenton-like processes have still unacceptable cost because of the operating conditions or an excessive peroxide consumption, which limit their interest in real conditions. In addition, the hydrogen peroxide is not very effective at room temperature because it decomposes to form inactive molecular oxygen.
In this sense, the direct use of metals can reduce the process cost. Iron metal, as it is well known, is an effective mediator for the reductive dehalogenation of chlorinated organic compounds in aqueous solution (Farrell et al., 2000). Zero valent iron (ZVI) metal also has proved to degrade nitro aromatic compounds in anaerobic conditions (Agrawal and Tratnyek, 1996). In the same way, powdered ZVI was capable to dechlorinate 1,1,1-trichloro-2,2-bis(pchlorophenyl)ethane (DDT) and related compounds at room temperature in buffered anaerobic conditions (Sayles et al., 1997). Finally, nanoscale metallic iron can oxidize some herbicides as carbothiate or molinate (Joo et al., 2004). During the oxidation of the metallic iron in oxic conditions, both ferrous iron and superoxide radicals appear to be generated, leading to the production of strong oxidant species that are able to degrade the contaminant.
In a totally different approach, ion enzymes are found capable to catalyse biological oxidations (Bianchi et al., 2003). These systems represent a source of inspiration for new catalytics routes. Most of them are based on iron compounds attached to nitrogen-based ligands. On the other hand, the effect of chelated iron on biomolecules is of particular interest in biomedical applications because iron is involved in a wide variety of biochemical roles (Welch et al., 2002). Adding a chelator to the reaction system can result in an increase or decrease of the oxidation potential of iron (Miller et al., 1990). As reported (Bucher et al., 1983), the auto-oxidation of some iron chelates produces a powerful oxidant, which is stronger than the hydroxyl radical. For instance, the oxidation of methanol using iron chelates does not directly depend on the hydrogen peroxide used, since it is believed that ferryl species are formed during oxidation. These species are able to oxidize high reduction potential compounds as methanol. Other authors suggest the occurrence of iron–oxo complexes during the process or a iron(III)–hydroperoxo complex (Bianchi et al., 2003). Therefore, this simple method could be applied to the degradation of refractory organic compounds such as aromatic compounds, like phenol and derivatives.
Simultaneous use of insoluble catalysts and complexes combines the advantages of this approaches. For instance, Noradoun et al. (2003) utilize the combination of metallic iron with ethylenediamine tetraacetic acid (EDTA). This process was able to destruct completely mixtures of 4-chlorophenol and pentachlorophenol under room temperature conditions with ambient air. In this study, the authors propose three possible dioxygen activation schemes, an heterogeneous activation at the metallic iron surface, an homogeneous activation by EDTA and finally, an heterogeneous activation producing ferryl species in the surface of the particles. In all cases, iron(II)–EDTA complex is involved in the scheme. The two first schemes are modified iron mediated Haber–Weiss reaction (Welch et al., 2002) in the presence of EDTA, and the last one, can confirm the presence of an hypervalent iron complex that must be the ferryl ion (Saran et al., 2000). In this chemistry, a variable with high importance is the pH of the solution (Welch et al., 2002). In fact the pH along chelation regulates the reactivity of iron species in solution. The same ZVI–EDTA process was used to the detoxification of malathion, model compound with analogies to some chemical warfare agents (Noradoun et al., 2005). The malathion degradation obtained is greater than 98% whereas the final reaction products are low molecular-weight carboxylic acids.
These promising results, overcoming problems with EDTA degradation that should compete with aromatic compounds degradation (Noradoun and Cheng, 2005), were the starting point of this work, this preliminary study of the oxidation of phenol and other aromatic compounds using metallic iron in the presence of EDTA. The oxidation process presented in this study aims to give some alternatives to high cost oxidizing processes (Fortuny et al., 1999).
Section snippets
Chemicals
Phenol (Ph) was purchased from Panreac (>99% purity), while o-cresol (98% purity), 2-clorophenol (98% purity), aniline (99.5% purity) and p-nitrophenol (98% purity) were all purchased from Aldrich. EDTA disodium salt dihydrate was provided by Panreac (98% purity). Finally, metallic ZVI was purchased from Panreac. Deionised water was used to prepare all the aqueous solutions.
Experimental set-up and procedure
A 600 ml jacketed stirred batch reactor was used for all oxidation runs. Reaction temperature was maintained constant by
Effect of initial phenol concentration
Fig. 1a displays the evolution of phenol conversion for a 150 mg l−1 phenol solution treated at 20 °C of temperature with 10 g of ZVI and an initial concentration of EDTA of 100 mg l−1. A phenol conversion of 53% was attained after 180 min and increased to 85% at the end of the experiment (after 360 min).
In the first 60 min of reaction the solution turned out to be of a brown colour certainly due to the formation of quinones, as confirmed by HPLC analysis. This solution got progressively darker with
Conclusions
An experimental investigation was carried out to evaluate the feasibility of the EDTA enhanced ZVI oxidation process at low temperature and atmospheric pressure. The results demonstrated that this oxidation process was suitable to eliminate phenol and others aromatics contaminants. An increase of the ZVI mass or EDTA concentration to phenol ratio increased in both cases the reaction rates and the phenol conversion attained. A preliminary kinetic analysis of the data showed that the rate of
Acknowledgements
Financial support for this research was provided in part by the Spanish Ministerio de Educación y Ciencia, project “CTM2005-01873”. Irama Sanchez is indebted to the Universitat Rovira i Virgili for providing a pre-doctoral scholarship. Also, Christophe Bengoa thanks Ramón y Cajal program of Spanish Ministerio de Educación y Ciencia for its economic support.
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