Electro-oxidation
When an energy input and sufficient supporting electrolyte are provided to the system, strong oxidizing species are formed, which interact with the contaminants and degrade them.
[2] Electro-oxidation has recently grown in popularity thanks to its ease of set-up and effectiveness in treating harmful and recalcitrant organic pollutants, which are typically difficult to degrade with conventional wastewater remediation processes.
[2] Electro-oxidation has been applied to treat a wide variety of harmful and non-biodegradable contaminants, including aromatics, pesticides, drugs and dyes.
[9] Electro-oxidation can additionally be paired with other electrochemical technologies such as electrocoagulation, consecutively or simultaneously,[10] to further reduce operational costs while achieving high degradation standards.
Typical values of salts concentration are in the range of few grams per liter, but the addition has a significant impact on power consumption and can reduce it by up to 30%.
Although some expressions have been proposed to evaluate the instantaneous current efficiency, they have several limitations due to the presence of volatile intermediates or the need for specialized equipment.
Where EC is the cell voltage (V), I is the current (A), t is the treatment time (h), (ΔCOD)t is the COD decay at the end of the process (g/L) and Vs is the solute volume (L).
[17] When voltage is applied to the electrodes, intermediates of oxygen evolution are formed near the anode, notably hydroxyl radicals.
[19] The surface of "active" anodes strongly interacts with hydroxyl radicals, leading to the production of higher state oxides or superoxides.
Hydroxyl radicals are physisorbed on the electrode surface by means of weak interaction forces and thus available for reaction with contaminants.
[9] The organic pollutants are converted to fully oxidized products, such as CO2, and reactions occur in a much less selective way with respect to active anodes:[19]
Such species are strongly reactive with many organic compounds, promoting their mineralization, but they can also produce several unwanted intermediates and final products.
Although sulfates can be involved in mediated oxidation as well, electrodes with high oxygen evolution overpotential are required to make it happen.
[22] Electrodes based on carbon or graphite are common due to their low cost and high surface area.
However, they are not suited for working at high potentials, as at such conditions they experience surface corrosion, resulting in reduced efficiency and progressive degradation of the exposed area.
[11] As a result, electro-oxidation with Platinum electrodes usually provides low yield due to partial oxidation of the compounds.
The contaminants are converted into stable intermediates, difficult to be broken down, thus reducing current efficiency for complete mineralization.
[12] Mixed metal oxides, also known as dimensionally stable anodes, are very popular in electrochemical process industry, because they are very effective in promoting both chlorine and oxygen evolution.
In the case of wastewater treatment, they provide low current efficiency, because they favor the competitive reaction of oxygen evolution.
[24] Similarly to Platinum electrodes, formation of stable intermediates is favored over complete mineralization of the contaminants, resulting in reduced removal efficiency.
[25] Lead dioxide electrodes have long been exploited in industrial applications, as they show high stability, large surface area, good conductivity and they are quite cheap.
Also, lead dioxide electrodes were found to be able to generate ozone, another strong oxidizer, at high potentials, according to the following mechanism:[11]
Also, the electrochemical properties and the stability of these electrodes can be improved by selecting the proper crystal structure: the highly crystalline beta-phase of lead dioxide showed improved performance in the removal of phenols, due to the increased active surface provided by its porous structure.
[26] Moreover, incorporation of metallic species, such as Fe, Bi or As, within the film was found to increase the current efficiency for mineralization.
Once doped, BDD electrodes show high chemical and electrochemical stability, good conductivity, great resistance to corrosion even in harsh environment and a remarkable wide potential window (2.3 V vs SHE).
[11] Once the hydroxyl radicals are formed on the electrode surface, they rapidly react with organic pollutants, resulting in a lifetime of few nanoseconds.
If the mass transfer coefficient for the system is known, the limiting current density can be defined for a generic organic pollutant according to the relation:[29]
Hence, systems with low COD are likely to be operating in diffusion control, exhibiting pseudo-first order kinetics with exponential decrease.
[29] If the limiting current density is obtained with other analytical procedures, such as cyclic voltammetry, the proposed equation can be used to retrieve the corresponding mass transfer coefficient for the investigated system.
[29] Given the thorough investigations on the process design and electrodes formulation, electro-oxidation has already been applied to both pilot-scale and full-stage commercially available plants.
