Electrochemical treatment of aqueous solutions of catechol by various electrochemical advanced oxidation processes: Effect of the process and of operating parameters
Introduction
Conservation of the environment, which requires a sustainable development to avoid compromising existing natural resources, is gradually becoming a major objective. As a result, a special research area of environmental electrochemistry has been developed, which is based on the utilization of electrochemical techniques to prevent or minimize environmental pollution. Traditional physical, chemical and biological techniques are widely used for the treatment of wastewater containing biodegradable organic pollutants. They are often inadequate for treating various kinds of industrial and agricultural effluents, as they are relatively expensive, inefficient, time consuming or a secondary cause of pollution. Consequently, over the past three decades, researchers have tried to develop new and more environmentally friendly technologies for the total elimination of persistent organic pollutants from wastewater and for the quantitative and rapid mineralization of non-biodegradable organics. In this context, the use of more innovative processes such as the advanced oxidation processes (AOPs) has acquired great relevance [1]. These processes have proved to have high efficiency with advantages such as versatility, high energy efficiency, amenability of automation, and safety because they operate at mild conditions with limited use of chemicals [2], [3], [4]. AOPs produce in situ hydroxyl radical (OH), the strongest oxidizing agent (E° = 2.80 V / SHE) after fluorine. These processes are especially efficient for aromatic molecules thanks to the non-selective electrophilic aromatic substitution of hydroxyl radical to aromatic moieties, leading finally to the ring opening reactions. Among them, electrochemical AOPs (EAOPs) such as anodic oxidation (AO) and electro-Fenton (EF) processes have gained much interest for the removal of organic compounds. In AO, the destruction of pollutants is mediated by hydroxyl radicals generated on the surface of the anode by the oxidation of water and its efficiency depends strongly on the anode material. In EF process, H2O2, produced at the cathode by oxygen reduction (Eq. (1)), reacts with catalytic amounts of Fe2 + to generate the OH radicals in the solution (Eqs. (2a), (2b)) [5].
These formed OH react rapidly on organics, leading to their oxidation/mineralization according to the following equations:
The advantages of this process with respect to conventional Fenton process is the continuous electro-generation of H2O2 inside the reactor and the cathodic regeneration of Fe2 + from cathodic reduction of Fe3 +.
Catechol is one of the most plentiful compounds in olive mills wastewaters (OMW) [6]. Furthermore, it is generated in the Fenton degradation of aromatic compounds [7] and it is the first intermediate produced by the degradation of tyrosol by electro-Fenton [8] (one of the most relevant compounds in OMW). It is a toxic, persistent pollutant and it is not readily biodegradable under environmental conditions because of its aromatic structure [9]. The International Agency for Research on Cancer (IARC) has classified catechol as possibly carcnogenic to humans (Group 2B) [10]. For these reasons, several technologies have been investigated for the removal of catechol from wastewater and aqueous solutions, including biological methods [11], [12], adsorption [13], [14], ozonation process [15], [16], advanced photo-oxidation process [17] and Fenton and photo-Fenton [9], [18].
In this experimental work, an in-depth study on the electrochemical treatment of catechol is reported. The main aim of the work is to evaluate the potential utilization of various electrochemical processes (namely, AO, EF, oxidation by electro-generated active chlorine and coupled processes) for the degradation of such phenolic compound in aqueous solutions. Furthermore, the effect of various operating conditions (including the nature of anode for AO, the initial pH and the current density) was widely investigated in order to optimize the selected electrochemical processes. For EF process, the effect of the nature of the catalyst (FeSO4, pyrite and chalcopyrite) was also analysed. At the end of this work, the two more promising processes (AO and EF) were coupled to achieve a more effective degradation of catechol.
Section snippets
Electrolysis system
Electrolyses were performed in a cylindrical, undivided tank glass cell under vigorous stirring performed by a magnetic stirrer with 50 mL of solution. Saturated calomel electrode (SCE) was used as reference electrode and all potentials reported in this study are referred to it. Different kinds of anode and cathode were used depending on the selected process:
- (i)
Direct anodic oxidation (AO): Boron-doped diamond (BDD, wet surface area 1.53 cm2, from Condias, supported on Nb, 5000–6000 ppm boron) or
Results and discussion
As previous mentioned, various electrochemical processes can be potentially used for the treatment of wastewater contaminated by organic pollutants resistant to conventional biological processes. Hence, in this work we have evaluated the utilization of various approaches including the direct anodic oxidation, the indirect oxidation by electro-generated active chorine, the electro-Fenton and a coupled process (direct anodic oxidation and EF), with the main aim of selecting the most suitable
Conclusions
The treatment of aqueous solutions of catechol was performed by various electrochemical routes. It was found that the performances of the process, in terms of removal of catechol and TOC and of CE, dramatically depend on the adopted electrochemical route:
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quite high abatements of both catechol and TOC were achieved by electro-Fenton (EF) and further higher with direct anodic oxidation (AO) at BDD;
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lower abatements of catechol and TOC were obtained by AO at DSA anode;
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high abatements of catechol
Acknowledgments
The Tunisian authors acknowledge partial financial support from the University of Gabes (Tunisia). Università di Palermo is acknowledged for financial support.
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