Abstract
Electrochemical oxidation using Boron-doped diamond (BDD) electrodes is a promising purification technique to remove organic pollutants, such as benzoic acid, from wastewaters. In this study the oxidation of benzoic acid has been investigated on rotating disk electrodes and in flow cells with well-defined mass transfer. The oxidation of benzoic acid on BDD proceeds via the formation of 4-hydroxybenzoic acid, which reacts in consecutive steps towards hydroquinone and benzoquinone. The ring opening in the oxidation of benzoquinone results in the formation of aliphatic carboxylic acids. It was shown that the intermediates 4-hydroxybenzoic acid and hydroquinone can be oxidized prior to water oxidation, which suggests the reaction occurs via a DET mechanism. Benzoquinone is not oxidized before water oxidation and can presumably solely be oxidized by OH radicals. This means that in order to oxidize benzoic acid completely to CO2 the formation of OH radicals is needed. Finally, the effects of mass transfer were analyzed. It was shown that the conversion rates of benzoic acid, benzoquinone and 4-hydroxybenzoic acid were below the limiting mass transfer rates, implying that these reactions are primarily limited by kinetics.
Highlights
The purification of industrial wastewater is an important topic in the chemical industry and is expected to become even more important in the decades to come due to the stricter environmental regulations
The reaction mechanism we suggest for the oxidation of benzoic acid on Boron-doped diamond (BDD) is comparable to the electrochemical oxidation on PbO2 electrodes, for which He et al reported the formation of 4-hydroxybenzoic acid, hydroquinone and benzoquinone [17]
Benzoic acid oxidation on BDD can be oxidized prior to water oxidation, low oxidation rates are observed in this region
Summary
The purification of industrial wastewater is an important topic in the chemical industry and is expected to become even more important in the decades to come due to the stricter environmental regulations. Various purification methods have been reported for the removal of benzoic acid from (industrial) wastewaters such as adsorption, reactive extraction, photo-catalytic degradation and electrochemical oxidation [1,2,3,4]. The latter method is attracting increasing interest, since it can be carried out under mild conditions and does not generate any secondary waste. Electrochemical oxidation is a complex process that can occur in different ways, either directly or indirectly via intermediates [5,6,7,8,9,10,11]. In studies where intermediates were reported, this was done in a qualitative rather than quantitative way [12,13]
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