In a broad sense, electrochemical oxidation refers to the entire process of electrochemistry, which involves direct or indirect electrochemical reactions occurring at the electrode based on the principles of oxidation-reduction reactions. These reactions aim to reduce or remove pollutants from wastewater.
Narrowly defined, electrochemical oxidation specifically refers to the anodic process. In this process, an organic solution or suspension is introduced into an electrolytic cell, and through the application of direct current, electrons are extracted at the anode, leading to the oxidation of organic compounds. Alternatively, low-valence metals can be oxidized to high-valence metal ions at the anode, which then participate in the oxidation of organic compounds. Typically, certain functional groups within organic compounds exhibit electrochemical activity. Under the influence of an electric field, the structure of these functional groups undergoes changes, altering the chemical properties of the organic compounds, reducing their toxicity, and enhancing their biodegradability.
Electrochemical oxidation can be categorized into two types: direct oxidation and indirect oxidation. Direct oxidation (direct electrolysis) involves the direct removal of pollutants from wastewater by oxidizing them at the electrode. This process includes both anodic and cathodic processes. The anodic process involves the oxidation of pollutants at the anode surface, converting them into less toxic substances or substances that are more biodegradable, thereby reducing or eliminating pollutants. The cathodic process involves the reduction of pollutants at the cathode surface and is primarily used for the reduction and removal of halogenated hydrocarbons and the recovery of heavy metals.
The cathodic process can also be referred to as electrochemical reduction. It involves the transfer of electrons to reduce heavy metal ions such as Cr6+ and Hg2+ into their lower oxidation states. Additionally, it can reduce chlorinated organic compounds, transforming them into less toxic or non-toxic substances, ultimately enhancing their biodegradability:
R-Cl + H+ + e → R-H + Cl-
Indirect oxidation (indirect electrolysis) involves the use of electrochemically generated oxidizing or reducing agents as reactants or catalysts to convert pollutants into less toxic substances. Indirect electrolysis can be further classified into reversible and irreversible processes. Reversible processes (mediated electrochemical oxidation) involve the regeneration and recycling of redox species during the electrochemical process. Irreversible processes, on the other hand, utilize substances generated from irreversible electrochemical reactions, such as strong oxidizing agents like Cl2, chlorates, hypochlorites, H2O2, and O3, to oxidize organic compounds. Irreversible processes can also generate highly oxidative intermediates, including solvated electrons, ·HO radicals, ·HO2 radicals (hydroperoxyl radicals), and ·O2- radicals (superoxide anions), which can be used to degrade and eliminate pollutants such as cyanide, phenols, COD (Chemical Oxygen Demand), and S2- ions, ultimately transforming them into harmless substances.
In the case of direct anodic oxidation, low reactant concentrations can limit the electrochemical surface reaction due to mass transfer limitations, while this limitation does not exist for indirect oxidation processes. During both direct and indirect oxidation processes, side reactions involving the generation of H2 or O2 gas may occur, but these side reactions can be controlled through the selection of electrode materials and potential control.
Electrochemical oxidation has been found to be effective for treating wastewater with high organic concentrations, complex compositions, a multitude of refractory substances, and high coloration. By utilizing anodes with electrochemical activity, this technology can efficiently generate highly oxidative hydroxyl radicals. This process leads to the decomposition of persistent organic pollutants into non-toxic, biodegradable substances and their complete mineralization into compounds like carbon dioxide or carbonates.
Post time: Sep-07-2023
