Anodic oxidation of 2,4-dihydroxybenzoic acid for wastewater treatment: study of ultrasound activation

Abstract

The electrooxidation at a platinum electrode of 2,4-dihydroxybenzoic acid (2,4-DHBA) assisted by ultrasound was investigated at two frequencies (20 and 500 kHz ). At high frequency, hydroxyl radicals are generated, which directly oxidise organic pollutants whereas, at low frequency, mass transfer rates of the electroactive species from the bulk solution to the electrode surface as well as adsorption/desorption mechanisms are considerably accelerated. For a 300 mg l −1 initial concentration and a 300 A m −2 current density, the TOC decrease was 47% after passing an electricity amount of 1.5 A h at low frequency and only 32% after passing 3.5 A h at high-frequency sonoelectrooxidation or electrooxidation. At low frequency, 2,4-DHBA degradation is accelerated and final TOC is lower: cavitation phenomena ensure the cleaning of the electrode surface thus increasing the active electrode surface. Observed by-products of sonoelectrodegradation are the same as for electrooxidation alone, including the following: 2,3,4- and 2,4,5-trihydroxybenzoic acids (THBA), maleic acid, glyoxylic acid and oxalic acid. Fewer intermediate aromatic compounds are formed at low-frequency irradiation. Moreover, the faradaic yield increases under low-frequency sonication, showing a more efficient use of electrochemical energy. Nevertheless, the overall energy consumption remains high (>200 kW kg −1) .