The critical role of the operating conditions on the Fenton oxidation of 2-chlorophenol: Assessment of PCDD/Fs formation

Abstract

This work assesses the influence of the operating conditions H2O2 dose (20 or 100% of the stoichiometric amount), temperature (20 or 70°C), and the presence of chloride in the oxidation medium in the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during Fenton treatment of aqueous samples of 2-chlorophenol, 2-CP, one of the strongest precursor of PCDD/Fs. After 4h of oxidation in the experiments carried out with 20% H2O2 chlorinated phenoxyphenols and biphenyls, which are intermediates in PCDD/Fs formation, as well as PCDD/Fs were observed, resulting in concentrations 11 times higher than in the untreated sample. Additionally, when NaCl was also present in the reaction medium, PCDD/Fs were formed at higher extent, with a total concentration 74.4 times higher than in the untreated 2-CP solution. Results depicted a preferential formation of PCDFs over PCDDs, with dominance of lower chlorinated PCDD/Fs (tetra and penta-PCDD/Fs). Besides, the formation of the most toxic PCDD/Fs congeners (2,3,7,8-PCDD/Fs) was not favored under the operating conditions used in this work.

Assessment of PCDD/Fs formation in the Fenton oxidation of 2-chlorophenol: Influence of the iron dose applied

“…Therefore, this study extends the work of Vallejo et al (2014) by determining the effect of iron dose on the degradation of 2-CP and the potential formation of PCDD/Fs during the Fenton oxidation process.…”

“…In the literature, the observed differences between the measured and the calculated TOC have been frequently explained by the formation of coupling-reaction aromatic species (Zazo et al, 2005), which confer a dark brownish color to the reaction medium (Sedlak and Andren, 1991). Some of these condensation substances, such as chlorinated biphenyl, chlorinated phenoxyphenols, chlorinated diphenyl ethers and chlorinated dioxins and furans have been identified by GC-MS during Fenton and Fenton-like treatments of solutions containing CPs at substoichiometric conditions of solutions containing CPs (Detomaso et al, 2003;Poerschmann et al, 2009;Munoz et al, 2011Munoz et al, , 2012Vallejo et al, 2014). Nevertheless, there is still lack of deep knowledge in the understanding of the formation and identification of reaction products that occur at substoichiometric H 2 O 2 to CP molar ratios (Poerschmann et al, 2009).…”

“…Other intermediates, such as phenyl radicals and phenoxyl diradicals, can also be formed from 2-CP and contribute to the formation of PCDD/Fs (Pan et al, 2013;Zhang et al, 2014). In addition, as a result of the chlorine content of 2-CP as well as the ubiquitous chloride in the reaction medium, the effectiveness of the Fenton process may be diminished owing to the formation of iron(III)-chlorocomplexes and the scavenging capacity of the chloride ion, although the inhibition caused by ferric complexation may be avoided at chloride concentrations lower than 200 mM as it was observed by Lu et al (2005) and Vallejo et al (2014). Chloride ions compete with organic compounds for the reaction with Å OH leading to free chlorine, which exists mainly as chlorine and dichloride anion radical at pH 3 (Lu et al, 2005;De Laat and Le, 2006;Anglada et al, 2009;Urtiaga et al, 2014).…”

“…Although the formation of coupling-reaction aromatic byproducts, such as the chlorinated biphenyls, dibenzofurans and dibenzodioxins have been addressed during the treatment of CPs under nonstoichiometric Fenton system conditions (Fukushima and Tatsumi, 2001;Poerschmann et al, 2009;Munoz et al, 2011Munoz et al, , 2012, these studies are covered basically from a qualitative and/or semi-quantitative approach. Recently, Vallejo et al (2014) assessed the influence of several operating conditions on the formation of PCDD/Fs during the treatment of 2-CP using the Fenton process, including H 2 O 2 concentration, temperature and the presence of chloride, and found that working at 20% of the stoichiometric H 2 O 2 concentration led to the formation of PCDD/Fs, with increasing concentrations when chloride ions were present in the reaction medium.…”

“…These species may participate in the chlorination of chlorophenol and its oxidized intermediates. Therefore, highly chlorinated PCDD/Fs must have been generated not only by the oxidative coupling of CPs and their oxidation intermediates, but also by prevalent chlorination reactions (Briois et al, 2007;Altarawneh et al, 2009;Vallejo et al, 2014). Nevertheless, while PCDD/Fs emissions as a result of combustion processes as well as their formation mechanisms from precursors in gas phase have been studied at length, a detailed understanding of PCDD/F formation via non-combustion technologies requires further investigation (Weber, 2007).…”