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Methyl paraben is commonly employed as a preservative in pharmaceutical preparations, personal care products and some processed foods. However, the ester constitutes a potential pollutant in aquatic environments and has been classified as an endocrine disruptor. This study describes the degradation of methyl paraben (100 mg L -1 in 0.05 mol L -1 aqueous potassium sulfate at pH 5.7) by means of an electrochemical process (employing a boron-doped diamond anode) either alone or coupled with sonolysis. Electrolyses were performed at 25, 30 and 35 ± 1°C during 120 min using applied constant current densities of 10.8 and 21.6 mA cm -2 . The hybrid sonoelectrochemical processes were conducted under similar conditions with the application of ultrasound at a frequency of 20 kHz and a power intensity of 523 W cm -2 . Although mineralization of methyl paraben could be achieved using either process, in comparison with the electrochemical method, the hybrid technique showed a higher mineralization efficiency (around 60 %) with approximately 50 % removal of total organic carbon, thereby confirming the synergistic effect of sonolysis.

This work focuses on the competitive oxidation of two very different molecules, when they underwent electrochemical oxidation with diamond electrodes. To shed light on the mechanisms of this competitive oxidation, solutions containing methyl paraben and propylene glycol at different ratios are electrolyzed (using sulfate or chloride supporting electrolytes). Results obtained pointed out that removal of both species can be easily attained by the electrochemical process, being promoted the mineralization by the action of the sulfate derivative products and the formation of chlorinated hydro-carbons by the action of chlorine oxidants, although the mechanisms of the oxidation do not depend on the primary anion contained in the waste. The higher the concentration of species to be oxidized, the higher is the amount of intermediates and the slower is the mineralization the ratio influences. An important outcome is that there is a limit concentration in each one organic compound interferes on the degradation of a pollutant. Thus, the interference effect of PG on MeP oxidation was only observed for low MeP/PG ratios.

In this paper, electrooxidation of methyl paraben (MeP) is studied by electrolysis only and electrolysis coupled with sonolysis, using a diamond electrode. Complete mineralization of MeP was achieved for both processes, in chloride and sulfate media. Results showed that, although the oxidation of pollutant is faster in the presence of chloride, the mineralization is favored in sulfate medium. Ultrasound irradiation enhanced the removal of organic matter due to the activation of oxidant species in both supporting electrolytes. Moreover, the formation of chlorine gas in the chloride containing medium improves the ultrasound cavitation effect, promoting faster depletion of the total organic carbon in the first hour of treatment. Regarding the formation of more toxic products, all possible organochlorinated intermediates were removed, since complete mineralization was attained in less than 5 hours. Ultrasonic coupling to the electrolysis process accelerates the destruction of the intermediates and delays the formation of perchlorate, which only begins after the complete removal of the total organic carbon. Low and high ultrasound frequencies were evaluated and were found to produce different effects of cavitation, which affect the electrolysis in different ways. The final result will be a balance between these effects and thus, an optimum frequency can be established for different systems.

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The electrochemical oxidation of aqueous solution containing methyl paraben (100 mg L-1) in Na2SO4 (0.05 mol L-1) was performed in a one compartment cell using a BDD (boron-doped diamond) anode. Electrolyses were carried out at different current densities (25 at 100 mA cm-2) and the results indicated that the process is supported by mass transport. The kinetic analysis based on methyl paraben concentration monitored during electrolysis shows that the concentration decay has a pseudo first-order behavior.