Electro-Fenton process for treating electroplating wastewater

1 Introduction to Electro-Fenton Method

The concept of Advanced Oxidation Processes (AOPs) was first proposed by Glaze et al. in 1987, characterized by the generation of hydroxyl radicals (·OH). Advanced oxidation technologies based on the Fenton reaction, which have developed in recent years, mainly include the Fenton method and Fenton-like methods. Traditional Fenton reactions cannot achieve iron cycling, have a narrow pH range (optimal 2.0-3.5), iron ions themselves capture free radicals, and the utilization rate of hydrogen peroxide is not high, limiting their application in water treatment. Due to the short lifespan of hydroxyl radicals, their treatment effects are often enhanced by external conditions such as ultraviolet light and electric fields, broadening their application in water treatment. These include photocatalytic Fenton, electro-Fenton, photo-electro-Fenton, and UV-Fenton, among others. Among these, the electro-Fenton method not only possesses all the properties of electrochemical methods but also utilizes the strong oxidizing power of hydroxyl radicals, gradually becoming the main development direction for Fenton reagents and also the mainstream direction for electrochemical technology.

2 Principle of the Fenton Method

In 1894, French scientist Fenton H. J. H. discovered that ferrous ions and hydrogen peroxide could effectively degrade tartaric acid under acidic conditions. With the increasing severity of organic pollution, this method provided a new avenue for the treatment of organic wastewater, marking a significant milestone. To commemorate Fenton H. J. H.'s outstanding contributions, Fe2+/H2O2 was named Fenton's reagent, and this reaction was called the Fenton reaction. Fenton's reagent can effectively and non-selectively oxidize organic matter and possesses extremely strong oxidizing properties. However, the mechanism of the Fenton reaction was not well understood, and scientists proposed various possible hypotheses. Americans used 2,4-dimethyl-2-nitrosopentane (DMPO) as a free radical scavenger and employed nuclear magnetic resonance to capture free radical signals, proposing a free radical and oxidant fragment mechanism. Subsequently, research by Walling, Norman, and Jefcoate, among others, also confirmed this conclusion. David R. et al. summarized and compiled previous research on the mechanism of the Fenton reaction (see Table 1-1). The currently widely accepted reaction mechanism is as follows: hydrogen peroxide reacts with ferrous ions to produce free radicals (.OH) and hydroxide ions (OH-). The free radicals have a very high oxidation electrode potential (see Table 1.2), thus, Fenton's reagent primarily utilizes the strong oxidizing power of free radicals in water treatment. In addition to ferrous ions, other transition metal ions (such as Cu2+, etc.) can also catalyze the decomposition of hydrogen peroxide into free radicals and hydroxide ions.

Electro-Fenton technology is one of the electrochemical treatment technologies developed based on Fenton reagents. Electro-Fenton (EF) can be divided into two forms. One is where soluble ferrous salts react with H2O2 generated on the cathode in a slightly acidic solution to undergo the Fenton reaction, effectively combining electrochemistry and Fenton. Electrodes used in this method are often graphite, carbon felt, etc. The other method is the sacrificial anode method, where a metal electrode acts as the anode, dissolving out Fe2+, which then reacts with externally added H2O2 in a Fenton reaction. The reaction mechanism is as follows:

The mechanism for pollutant removal by the electro-Fenton reaction is also very complex. The generally accepted mechanism is based on the strong oxidizing action of hydroxyl radicals. Due to the different forms of electro-Fenton, the ways in which hydroxyl radicals are generated also differ. However, in the degradation of pollutants, researchers generally believe that it is similar to the action of Fenton reagents, mainly involving the strong oxidizing action of free radicals generated by the cathode and anode to oxidize and decompose pollutants.

3 Characteristics of the Electro-Fenton Method

Compared to the traditional Fenton technology, electro-Fenton technology has the following advantages:

(1) Using graphite, carbon felt, etc. as the cathode for electrochemical reactions, under acidic conditions, the oxygen introduced into the cathode is converted into hydrogen peroxide. Therefore, hydrogen peroxide does not need to be added externally, reducing treatment costs and also minimizing the danger of transporting hydrogen peroxide.

(2) Only a portion of the oxygen introduced into the electrolyte is converted into hydrogen peroxide; the rest is released in gaseous form, which stirs the electrolyte, ensuring uniform mixing and preventing polarization.

(3) Electrochemical reactions can continuously provide Fe2+. The plates dissolve directly to produce Fe2+, or Fe3+ receives electrons and is reduced to Fe2+, allowing Fe2+ to be recycled. The Fenton method requires the addition of iron salts, resulting in high costs and a large amount of sludge. The treated water has high color and high anion content, which is not an issue with electro-Fenton.

(4) Electro-Fenton degradation of pollutants involves not only the oxidation by free radicals but also processes such as electrocoagulation, electrooxidation, electroreduction, and electroflotation, or a combined synergistic effect of these processes. See specificallySewage Treasure Mallinformation orhttp://www.dowater.commore related technical documents.

4 Application of Electro-Fenton Method in Water Treatment

Huang Y H et al. used the electro-Fenton method to treat petrochemical wastewater. The experimental results showed that the electro-Fenton method, which uses a sacrificial iron anode to provide Fe2+ and externally added H2O2, has a high removal effect on COD in wastewater. Compared with O3, O3/H2O2, sodium hypochlorite, and traditional Fenton methods, the electro-Fenton method showed significantly better removal effects than the other methods. Brillas E et al. used the electro-Fenton method to degrade aniline. Cristina Flox et al. used electro-Fenton and photo-electro-Fenton with Pt as the anode to treat indigo dye wastewater. Both electro-Fenton with BDD as the anode and photo-electro-Fenton with a combined catalyst of Fe2+ and Cu2+ and Pt as the anode could completely mineralize indigo dye. Marco Panizza et al. used electro-Fenton with hydrogen peroxide generated at the cathode to degrade synthetic dyes and investigated the influence of various factors on the degradation effect. Zhou M H et al. used the electro-Fenton method to treat methyl red, using PTFE as the cathode, and investigated the influence of electrolyte Na2SO4 concentration, pH, ferrous ion concentration, and pollutant concentration on methyl red removal. The results showed that the degradation of methyl red occurred in two stages, with the second stage having a slower degradation rate than the first stage. Zhang H et al. used electro-Fenton to remove COD from landfill leachate and investigated the influence of reaction time, plate spacing, current density, and H2O2/Fe2+ molar ratio on degradation. The results showed that electro-Fenton can effectively degrade organic matter in landfill leachate and is not only highly efficient but also more economical than conventional Fenton. Brillas E et al. studied the herbicide 3,6-dichloro-2-methoxybenzoic acid using anodic oxidation, electro-Fenton, and photo-electro-Fenton. Zhao X et al. used electrocoagulation and electro-Fenton technology to degrade wastewater from printed circuit boards. Zhao X et al. used electrocoagulation and electro-Fenton technology to remove arsenic, using DSA and iron plates as electrodes. Arsenic was oxidized on the DSA electrode and then removed by coagulation and precipitation. The influence of coexisting ions on arsenic removal was studied, and it was found that Ca and Mg ions promoted arsenic removal, while ions such as Cl-, CO32-, and PO43- inhibited arsenic removal. Boye B et al. used anodic oxidation, electro-Fenton, and photo-electro-Fenton methods to degrade the herbicide 2,4,5-T. Panizza M et al. used the electro-Fenton method to degrade organic industrial wastewater containing naphthalene sulfonic acid and anthraquinone sulfonic acid. Lian Yu et al. used the electro-Fenton oxidation method to treat simulated acidic orange II wastewater. The electro-Fenton oxidation method can efficiently decompose the azo bond and naphthalene ring in the molecular structure of acidic orange II, improving the biodegradability of the wastewater.