Application of Boron-doped Diamond Electrodes for Wastewater Treatment

Boron-doped diamond (BDD) thin film is a new electrode material that has received great attention recently because it possesses several technologically important characteristics such as an inert surface with low adsorption properties, remarkable corrosion stability, even in strong acidic media, and an extremely wide potential window in aqueous and non-aqueous electrolytes. Due to these properties, diamond electrodes are promising anodes for electrochemical treatment of wastewater containing organic pollutants. The objective of this review is to summarise and discuss the recent results available in the literature concerning the application of diamond electrodes to environmentally-oriented electrochemistry. A mechanism of the electrochemical incineration and a kinetic model for organics oxidation on BDD is also presented. Moreover, fundamentals and applications to wastewater treatment of the powerful electro-oxidation method using an undivided cell with BDD anode and the cathodic electrogeneration of hydrogen peroxide by electro-Fenton or photoelectro-Fenton processes are discussed. In these processes, organics are effectively destroyed by the large amounts of hydroxyl radical (•OH) produced on the BDD surface by water oxidation and by Fenton’s reaction between added Fe and H2O2 electrogenerated at the cathode. The presence of Cu as co-catalyst with UVA irradiation enhances the degradation of organics, making the photoelectro-Fenton with BDD anode viable for industrial application. *Corresponding author Email: [email protected] INTRODUCTION As many industries produce wastewater containing toxic organic pollutants, there has been a notable increase in both research and the number of businesses concerned with the treatment of such industrial effluents and, nowadays, many new technologies are available, including biological, physical and chemical processes. In this field, oxidative electrochemical technologies, providing versatility, energy efficiency, amenability to automation and environmental compatibility have reached a promising stage of development and can now be effectively used for the destruction of toxic or biorefractory organics [1]. The overall performance of the electrochemical processes is determined by the complex interplay of parameters that may be optimized to obtain an effective and economical incineration of pollutants. The principal factors determining the electrolysis performance are electrode potential and current density, mass transport regime, cell design, electrolysis medium and, above all, electrode materials. The ideal electrode material for the degradation of organic pollutants should be totally stable in the electrolysis medium, cheap and exhibit high activity towards organic oxidation and low activity towards secondary reactions (e.g. oxygen evolution). Consequently, many anodic materials have been tested to find the optimum one. According to the 140 J. Environ. Eng. Manage., 18(3), 139-153 (2008) model proposed in previous works [2-4] anode materials can be divided into two extreme classes as follows: (i) Active anodes, which have low oxygen evolution overpotential and consequently are good electrocatalysts for the oxygen evolution reaction and only favour partial and selective oxidation (i.e. conversion), include carbon and graphite, platinum-based anodes, iridium-based oxides and ruthenium-based oxides. And (ii) Non-active anodes have high oxygen evolution overpotential and consequently are poor electrocatalysts for the oxygen evolution reaction but enable the complete and non-selective oxidation of organics to CO2 by electrogenerated hydroxyl radicals, such as antimony-doped tin dioxide, lead dioxide and borondoped diamond (BDD). Among the non-active anodes, BDD exhibits several technologically important properties that distinguish it from conventional electrodes, such as: (1) An extremely wide potential window in aqueous and non-aqueous electrolytes: in the case of high-quality diamond, hydrogen evolution commences at about –1.25 V vs. SHE and oxygen evolution at +2.3 V vs. SHE, then the potential window may exceed 3 V [5]; (2) Corrosion stability in very aggressive media: the morphology of diamond electrodes is stable during long-term cycling from hydrogen to oxygen evolution even in acidic fluoride media [6]; (3) An inert surface with low adsorption properties and a strong tendency to resist deactivation: the voltammetric response towards ferri/ferrocyanide is remarkably stable for up to two weeks of continuous potential cycling [7]; and (4) Very low double-layer capacitance and background current: the diamond-electrolyte interface is ideally polarisable and the current between –1000 and +1000 mV vs. SCE is < 50 μA cm. The double-layer capacitance is up to one order of magnitude lower than that of glassy carbon [8]. Due to these properties, conducting diamond seems to be a promising electrode material and so, in the last decades, it has been widely studied with the goal of developing applications in the electrochemical oxidation of organics for wastewater treatment [9,10]. In fact, during electrolysis in the potential region of water discharge, BDD anodes involve the production of weakly adsorbed hydroxyl radicals that unselectively and mineralise organic pollutants with high current efficiency [11]. In the last years, several indirect electrooxidation methods based on the cathodic electrogeneration of hydrogen peroxide like electro-Fenton (EF) and photoelectro-Fenton (PEF) are being developed for the remediation of acidic wastewaters containing toxic and non-biodegradable organic pollutants. They are environmentally friendly techniques since the main oxidant of organics is the in situ electrogenerated hydroxyl radical, which is a very strong oxidizing species due to its high standard potential (Eo = 2.80 V vs. SHE). This radical is able to non-selectively react with most organic pollutants yielding dehydrogenated or hydroxylated products up to total mineralization. When an undivided electrolytic cell is used, the EF method involves the production of hydroxyl radical in the bulk solution from the catalytic Fenton’s reaction between Fe with electrogenerated H2O2, simultaneously to the generation of radical •OH from water oxidation at the surface of the anode. This method thus profits the oxidation ability of both anode and cathode reactions and then, it is expected to be more efficient to destroy organics than direct anodic oxidation (AO). Its oxidation power depends on the electrodes and metallic ions (Fe, Fe, Cu, etc.) used as catalyst. The effectiveness of EF can be enhanced under irradiation of the solution with UVA light in the PEF treatment, which can be even more efficient using sunlight as energy source in the solar PEF (SPEF) process. The aim of this review is to elucidate the basic principles of electro-oxidation of organics with BDD anode and to discuss the recent progress dealing with the application of diamond electrodes in the electrochemical treatment of wastewater containing organic pollutants. A kinetic model for the prediction of the evolution of COD (chemical oxygen demand) during organics incineration on BDD is also presented. Finally, fundamentals and applications of the AO on BDD, as well as of the EF or PEF processes using the same anode with an undivided cell, will also be discussed herein. The influence of applied current density and metallic ions used as catalyst (Fe and/or Cu) in the EF and PEF processes is also explored. MECHANISM OF THE ELECTROCHEMICAL INCINERATION In electrochemical incineration (EI) reactions oxygen is transferred from water to the organic pollutant using electrical energy. This is the so-called electrochemical oxygen transfer reaction (EOTR). A typical example of EOTR is the anodic EI of phenol (Eq. 1). C6H5OH + 11H2O → 6CO2 + 28H + 28e (1) In this reaction water is the source of oxygen atoms for the complete oxidation of phenol to CO2 at the anode of the electrolytic cell. The liberated protons in this reaction are discharged at the cathode to dihydrogen (Eq. 2).