Micro-electrolysis is widely studied and applied as a low-pollution, low-cost advanced oxidation technology. Its principle is that wastewater is used as electrolyte, iron and carbon are used as electrodes to undergo redox reactions, thereby degrading pollutants in wastewater. Its earliest prototype came from the zero-valent iron theory proposed by Robert W. Gillham in groundwater treatment, and it has been widely studied and applied in the remediation of groundwater micro-pollution in the United States and Northern Europe. my country introduced this technology in the 1980s and expanded the research field from groundwater remediation to industrial wastewater treatment research, especially organic wastewater that is difficult to directly biodegrade.
1. Mechanism of micro-electrolysis technology
Currently, the mainstream theoretical views on the removal of pollutants in wastewater by micro-electrolysis believe that there are galvanic cell theory, redox theory, adsorption flocculation theory and micro-electric field theory.
1.1 Galvanic cell theory
The cast iron (iron-carbon alloy) mainly used in the micro-electrolysis process can form a micro-battery (microscopic battery) in the wastewater. When additional macroscopic cathode materials such as carbon are added to the system, a macroscopic battery is formed. Various corrosion phenomena occur during the reaction, and corrosion batteries are also formed. The cathode reaction of micro-electrolysis electrode reaction is mainly divided into three conditions: acidic (anaerobic), acidic (aerobic) and neutral alkaline. For details, see the reaction equation.
The electrode potential difference generated under acidic (aerobic) conditions is 1.22V higher than that generated under acidic (anaerobic) conditions. Aeration can increase the oxidation capacity of the primary cell; the Fe2+ ions continuously generated by the anode Fe avoid anode passivation, and the Fe2+ ions have certain oxidizing properties, which promote electrochemical corrosion and improve the treatment effect.
1.2 Redox theory
From equations 1-1 and 1-3, it can be seen that the Fe2+ ions and atomic H and anode Fe0 generated under acidic conditions can change the properties of certain pollutants in the wastewater to improve the biodegradability of the wastewater, such as nitrobenzene and azo organic matter are reduced to produce amine groups. Fe0 is an active metal that can effectively reduce wastewater containing Cu2+, Pb2+ and other elements. Fe2+ ions can reduce the toxicity of wastewater containing Cr2O72-, and Fe0 can also reduce nitrates.
1.3 Redox theory
The adsorption flocculation theory can be divided into two situations: the material used in the electrode has a certain ability to adsorb substances and the reaction process produces some chemical substances with adsorption capacity. The material used in the anode is generally cast iron chips, which has a porous structure and a relatively large comparative area. Its surface activity is strong and can adsorb some pollutants. When activated carbon is added as a cathode material, the activated carbon will also adsorb pollutants in the wastewater.
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