Production of hydroxyl radicals by electrochemically assisted Fentonʹs reagent: Application to the mineralization of an organic micropollutant, pentachlorophenol

Hydroxyl radicals are very powerful oxidizing agents. They are involved in hydroxylation reactions, in biological and atmospheric phenomena. A recent application of these radicals is their use in decontamination of water polluted by toxic organic substances like pesticides. Chemically, these radicals are produced by the use of a mixture of (H2O2+Fe2+), the so called Fentonʹs reagent. In this work Fentonʹs reagent is generated by electrochemistry in a catalytic way. The reaction of the hydroxyl radicals with pentachlorophenol (PCP) was studied. These radicals generated in situ in aqueous solution react with PCP and thus lead to its degradation. The evolution of the composition of the solution was followed by chromatographic analysis, COD analysis and the measurement of the total organic carbon (TOC) of the studied aqueous solution. Tetrachloro-o-benzoquinone and tetrachloro-p-benzoquinone (TCBQ) are the only aromatic intermediates identified. They result from the oxidation of the corresponding tetrachlorohydroquinones (TCHQ). Just like PCP, the TCBQs are degraded and disappear in their turn. The mineralization of the initial toxic substrate is confirmed on the one hand, by the regular decrease in quantity of the total organic carbon of the solution (TOC analysis) and on the other hand, by the quantitative release of chloride ions according to the electrical charge passed during electrolysis. The degradation of PCP appears relatively slow compared to that of the other organic pollutants studied by the electro-Fenton process and also to the degradation of other less substituted chlorophenols. The kinetic rate of the appearance of chloride ions is slower than that in the degradation of PCP. This phenomenon highlights the formation of chlorated aliphatic intermediates.