Fundamental Principles of Electrochemical Oxidation

With the rise of advanced oxidation processes (AOPs), electrochemical technologies for wastewater treatment have developed rapidly. A number of stable and highly efficient electrode materials are now available, including DSA electrodes (dimensionally stable anodes), BDD electrodes (boron-doped diamond anodes), Ebonex electrodes and NTA electrodes (nanotube arrays).

Electrochemical oxidation refers to the degradation of pollutants either by direct oxidation in an electric field or indirectly through the generation of radicals such as ·OH and O₂^-·. The mechanism is mainly divided into direct anodic oxidation and indirect radical oxidation. In the former, organic molecules adsorbed on the anode are oxidised by losing electrons, but the degradation capacity is limited. In the latter, radicals such as ·OH and O₂^-·, or oxidants such as O₃ and H₂O₂, are generated through electrode reactions and oxidise the organic molecules. In the aqueous phase, H₂O is oxidised at the anode surface into adsorbed ·OH, and may also decompose to produce O₂ and H₂O₂. In addition, the anode material itself may be further converted into a higher-valence oxidation state MO, as shown in Equations (78)-(83).

Factors Influencing Electrochemical Oxidation

The electrochemical oxidation process is mainly affected by the electrode material, the operating conditions and the characteristics of the medium.

Electrode material. The properties of the electrode material directly influence pollutant degradation performance in terms of catalytic activity, reaction rate and competing reactions. Reaction rates can vary by orders of magnitude between different electrode materials. Selecting an electrode material with a high oxygen evolution potential also helps to avoid energy being wasted on the oxygen evolution reaction.

Operating conditions. Key operating parameters include current density and electrode spacing. In a study on the electrochemical degradation of sulfamethazine, Sun Fengkun found that, within the same treatment time, a higher current density produced more ·OH per unit time and therefore a higher removal rate. When the electrode spacing was 1-2 cm the removal rate increased steadily, whereas at 2-5 cm it gradually declined. Too small a spacing readily causes concentration polarisation, while too large a spacing reduces the reaction rate; a spacing of 1-2 cm is therefore generally recommended.

Medium conditions. Electrolyte concentration, pH and the presence of other ions all affect degradation efficiency. The electrolyte provides electrical conductivity; too low a concentration slows degradation, whereas increasing the concentration improves conductivity and voltage efficiency. pH affects the speciation of ions in the system, the surface charge of organic molecules and electrode service life, and its influence differs according to electrode type and pollutant. Other ions in the system, such as Cl^-, SO₄^2- and HCO₃^-, compete with the target pollutants, but the weakly oxidising species they generate at the anode (Cl₂, S₂O₈^2-, C₂O₆^2-) also contribute to pollutant removal.

Advantages and Disadvantages

The advantages of electrochemical oxidation include its ability to remove perfluorinated organic compounds (PFOCs) and to recover high-concentration, valuable metals, thereby avoiding secondary pollution while generating economic benefit. Pollutant degradation pathways are diverse, and the process simultaneously provides disinfection and electro-adsorption effects. The reactor has a small footprint, is simple to operate, works under mild reaction conditions and offers good controllability, allowing current and voltage to be adjusted in real time according to the organic load.

The main challenges lie in electrode fouling and service life, reactor design, and capital and operating costs. In practice, electrodes are prone to fouling, which reduces their activity and requires regular cleaning and maintenance. Soluble electrodes have a short service life, are difficult to recover, may pollute the environment and offer low current efficiency. For engineering applications there is a lack of large-scale electrochemical reactors with uniform mass transfer and stable operation; in addition, electrode materials are expensive and power consumption is high, mostly in the range of 10-60 kW·h/m³.

Applications and Development

Moraes et al. used TiO₂-RuO₂ titanium electrodes to electrolyse landfill leachate; after 3 h at a current density of 116 mA/cm², removal rates for COD, TOC, colour and ammonia nitrogen reached 73%, 57%, 86% and 49% respectively. Liu et al. degraded coking wastewater and its biological effluent using a purpose-built pulsed corona discharge device: at a pulse frequency of 800 Hz the phenol concentration in the raw water fell from 611 mg/L to 227 mg/L and thiocyanate from 348 mg/L to 64 mg/L, the B/C ratio rose from 0.14 to 0.43, and energy efficiency was 3-4 times that of conventional ozonation. Eleotério et al. applied DSA electrodes with different dopant ratios to industrial wastewater containing antibiotics with an initial COD of 670 mg/L; after 4 h at a current density of 40 mA/cm² in 0.5 mol/L Na₂SO₄, COD degradation rates for bromhexine, sulfamethoxazole and trimethoprim were 58%, 48% and 40% respectively.

To address the limitations of the technology, coupling with Fenton reagents, illumination and ultrasound has been explored to enhance oxidation capacity and broaden the treatment range. On the mechanistic side, research into the degradation pathways of different pollutant types supports the design of dedicated electrodes or reactors for specific, highly toxic and refractory wastewaters. The principal technical bottlenecks at present are low current efficiency and short electrode life, and the key to overcoming them lies in the development of electrode materials and reactors.

Common electrodes include DSA and BDD types. DSA electrodes are produced by depositing metal oxide films such as SnO₂, PbO₂ and IrO₂ onto a metal substrate such as Ti or Zr. They offer good catalytic activity but have relatively low oxygen and chlorine evolution potentials; moreover, differences in thermal expansion coefficients during fabrication can leave cracks between coating and substrate, leading to delamination. The former issue can be mitigated by metal doping to raise the oxygen evolution potential, for example Ti/SnO₂-Sb₂O₅ and Ti/RuO₂-Gd; the latter by adding an intermediate layer between substrate and coating to extend electrode life.

BDD electrodes offer high catalytic activity and corrosion resistance, with current efficiencies typically between 51% and 90%. Even at high temperature, a sulfuric acid concentration of 3 mol/L and a current density of 10 000 A/m², electrode life still reaches 264 h. However, they are expensive, costing EUR 12 000-18 000/m², roughly ten times the cost of DSA electrodes. Inexpensive Ebonex electrodes with good conductivity and strong corrosion resistance but short service life have been developed, along with low-cost, highly catalytic blue TiO₂ nanotube array (NTA) electrodes and titanium suboxide electrodes; the operating conditions, industrial-scale fabrication and environmental impact of these new electrodes nevertheless remain unclear and require further study.

In reactor design, factors that must be considered include mass transfer, heat transfer, reaction kinetics, and the distribution of current density and electromotive force across the electrode surface - all of which become significant once the reactor is scaled up. Although a small improvement in treatment efficiency may appear insignificant, it can be highly meaningful in reducing the cost per tonne of water treated. Three-dimensional electrode reactors have addressed the long mass-transfer distances and low current efficiency of conventional two-dimensional reactors, raising COD degradation rates by 10%-50% with clear energy savings; however, uneven voltage and current distribution within the reactor and a tendency towards electrode clogging still require further resolution.

Source: compiled from public literature.