Electrooxidation of 2,4-dichlorophenol and other polychlorinated phenols at a glassy carbon electrode
Introduction
The removal of chlorophenols (CPs) from waste water is of environmental interest because CPs have a negative effect on a number of aquatic life forms. CPs, together with chlorobenzenes and polychlorinated biphenyls, are key intermediates in dioxin formation by the reaction of such organic matter as lignin, petroleum, coke or coal with various chlorocarbons in the presence of O2 and Cl2 [1]. Of the existing 19 CPs, 2,4-dichlorophenol (2,4-DCP) and 2,4,5-trichlorophenol (2,4,5-TCP) are used for production of herbicides and pentachlorophenols (PCP) act as fungicides, herbicides and insecticides [2]. In general, the resistance of CPs to microbial degradation, their environmental persistence, and toxicity increase with increasing Cl substitution [3]. It has also been reported that the position of the Cl atom in the aromatic ring determines the rate of microbial degradation, CPs with two chlorine atoms in equivalent positions in the aromatic ring (2,3,6- and 3,4,5-TCP) presenting a lower degradation rate than those with a Cl atom in each of the o-, m- and p- positions (2,3,4- and 2,4,5-TCP) [3]. Therefore, the degradation and analytical detection of CPs is of great interest in the environmental protection field as well as in industrial process control [4].
Phenols are alcohols slightly soluble in water, showing acidity in aqueous solutions:Their pKa can be determined by electrochemical measurements [5].
Gattrell and Kirk [6] reported that phenol electrooxidation produces electrode fouling and that tars formed on the electrode by electropolymerization show a low rate of oxidation, low permeability and strong adhesion to the electrode [7]. Wang et al. [8] found by scanning tunneling microscopy that the morphology of films produced by phenol oxidation varied significantly, in the case of CV, with scan rate, phenol concentration and potential limits, and in the case of constant potential, with holding time.
Recently indirect electrochemical techniques involving electrogeneration of strong oxidants, e.g. Fenton's reagent, have been used for remediation of water containing toxic-persistent-bioaccumulative organic pollutants (e.g. herbicide 2,4-D, the main component of which is 2,4-DCP), obtaining a higher than 95% mineralization [9].
In this work, the electrooxidation of 2,4-DCP at a glassy carbon (GC) rotating disk electrode at different pHs has been studied by cyclic voltammetry (CV) and chronoamperometry. In order to determine the influence of the position of the chlorine atom in the aromatic ring on the reactivity of the CPs, a cyclic voltammetric study of mono (2-CP and 4-CP), di (2,4-DCP and 2,6-DCP), tri (2,4,6-TCP) and PCP was undertaken. The influence of CP concentration, pH and potential was studied.
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Reagents and apparatus
CPs were Aldrich p.a., and Na2SO4, H2SO4, KH2PO4, Na2HPO4·2H2O, Na2CO3, NaHCO3 and NaOH used for buffer solutions were Merck p.a. All the solutions were freshly prepared with twice distilled water and the experiments were carried out at room temperature under nitrogen atmosphere. A pH of 2.2 was preferentially used in order to avoid the protonation of the CPs that occurs at very low pH values [5].
A conventional three-compartment Pyrex cell provided with a Luggin capillary was employed. All
Cyclic voltammetry of 2,4-DCP
Fig. 1A shows five consecutive cyclic voltammograms (CVs) at 0.1 V s−1 (with nitrogen stirring after each scan) of a GC electrode in 0.5 M Na2SO4+10 mM H2SO4+1 mM 2,4-DCP (pH 2.2). The first scan (solid line) shows an anodic peak at 0.55 V (peak a) and a cathodic one at −0.30 V (peak b). In successive scans the height of both peaks a and b decreases, but in the positive scan a small anodic peak at 0.19 V (peak c) and an anodic prepeak (d) near 0.4 V appear. The electrode fouling increased with
Discussion
Electrooxidation of all the CPs is, with the exception of PCP, under diffusion control, with similar values of dIp/dv0.5, indicating that n(D/b)0.5 is about the same for all the CPs studied.
A summary of the CV data for 2,4-DCP at different pHs is given in Table 3. The value of dEp/dlog(v) is in the range 26–50 mV dec−1, the differences being certainly due to electrode fouling by polymerization products, as shown by both chronoamperometric and RDE measurements. However, nitrogen bubbling
Conclusions
The electrooxidation of CPs begins with the formation of the phenoxi radical, and continues by two possible paths: one pathway yields species with quinonic structure, and the other one is the formation of insoluble polymers that passivate the electrode surface. The relative rates of the two pathways depend on the CP concentration, higher CP concentrations favouring polymerization on the electrode surface, and lower CP concentrations the oxidation to quinonic species.
The reactivity of CPs
Acknowledgements
The authors thank DICYT-USACH and FONDECYT (Grant 1980378) for financial support.
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