Phenol oxidation by Fenton's reagent (H2O2 + Fe2+) in aqueous solution has been studied in depth for the purpose of learning more about the reactions involved and the extent of the oxidation process, under various operating conditions. An initial phenol concentration of 100 mg/L was used as representative of a phenolic industrial wastewater. Working temperatures of 25 and 50 degrees C were tested, and the initial pH was set at 3. The H2O2 and the Fe2+ doses were varied in the range of 500-5000 and 1-100 mg/L, respectively, corresponding to 1-10 times the stoichiometric ratio. A series of intermediates were identified, corresponding mainly to ring compounds and short-chain organic acids. Most significant among the former were catechol, hydroquinone, and p-benzoquinone; the main organic acids were maleic, acetic, oxalic, and formic, with substantially lower amounts of muconic, fumaric, and malonic acids. Under milder operating conditions (H2O2 and Fe2+ at lower concentrations), a great difference was found between the measured total organic carbon (TOC) and the amount of carbon in all analyzed species in the reaction medium. This difference decreased as the doses of H2O2 and Fe2+ increased, indicating that the unidentified compounds must correspond to oxidation intermediates between phenol and the organic acids. To establish a complete oxidation pathway, experiments were carried out using each of the identified intermediates as starting compounds. Dihydroxybenzenes were identified in the earlier oxidation stages. Muconic acid was detected in catechol but not in the hydroquinone and p-benzoquinone oxidation runs; the last two compounds were oxidized to maleic acid. Oxalic and acetic acid appeared to be fairly refractory to this oxidation treatment. A detailed knowledge of the time evolution of the oxidation intermediates is of environmental interest particularly in the case of hydroquinone and p-benzoquinone because their toxicities are several orders of magnitudes higher than that of phenol itself. The time evolution of the intermediates and TOC was fitted to a simple second-order kinetic equation, and the values of the kinetic constants were determined. This provides a simplified approach useful for design purposes.

This review provides updated information on the application of the Fenton process as an advanced oxidation method for the treatment of industrial wastewaters. This technology has been used in recent decades as a chemical oxidation process addressed to meet a variety of objectives including final polishing, reduction of high percentages of organic load in terms of chemical oxygen demand or total organic carbon and removal of recalcitrant and toxic pollutants thus allowing for further conventional biological treatment. The efficiency and flexibility of this technology has been proven with a wide diversity of effluents from chemical and other related industries or activities, including pharmaceutical, pulp and paper, textile, food, cork processing, and landfilling among others.

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The effect of temperature on the Fenton process has been studied within the range of 25−130 °C using phenol (100 mg/L) as target compound, 10 mg/L Fe2+, and a dose of H2O2 corresponding to the theoretical stoichiometric amount (500 mg/L) for mineralization. The TOC reduction was considerably improved as temperature increased. Whereas at 25 °C the TOC decreased less than 28%, a reduction of almost 80% was achieved at 90 °C. Beyond this temperature no significant improvement of mineralization was observed, although the rate of the process was considerably enhanced. Increasing the temperature leads to a more efficient consumption of H2O2 which indicates an enhanced iron-catalyzed H2O2 decomposition into radicals as temperature increases rather than the generally accepted thermal breakdown of H2O2 into O2 and H2O. Therefore, working at a temperature well above the ambient provides a way of intensifying the Fenton process since it allows a significant improvement of the oxidation rate and the mineralization percentage with reduced H2O2 and Fe2+ doses. Furthermore it would not represent a serious drawback in the case of many industrial wastewaters which may be already at that temperature. Besides, partial recovery of heat from the treated off-stream would always allow saving energy. The TOC time-evolution was well described by a kinetic model based on TOC lumps with apparent activation energy values in the range of 30−50 kJ/mol.

This work deals with the evolution of intermediates and ecotoxicity upon Fenton's oxidation of phenol in aqueous solution. The EC50 values of the intermediates identified in the oxidation pathway of phenol have been measured. Some of these compounds, mainly hydroquinone and p-benzoquinone, showed toxicity levels much higher than phenol itself. Depending on the operating conditions, these intermediates could be completely transformed into organic acids, mainly oxalic and formic. Ecotoxicity values substantially lower than those expected from the chemical composition were measured in the reaction samples. This is explained by a reduction of the concentration of aromatic intermediates when the pH was adjusted at 6-8 (according to what is required by the standard bioassay ISO 11348-3). Formation of complexes between hydroquinone and p-benzoquinone at increasing pH can remove from solution those highly toxic intermediates whose very low EC50 values give rise to a high ecotoxicity even at fairly low concentrations. This together with the enhanced decomposition of residual H202 at increasing pH represent important beneficial effects of the neutralization step following Fenton treatment which allow a complementary cleaning of the effluent.