Technical assistance: What is the role of hydrogen peroxide in the wastewater treatment process?

1 Fenton reagent: The Fenton reagent is composed of ferrous salts and hydrogen peroxide; when the pH is low enough, hydrogen peroxide is decomposed into •OH under the catalysis of Fe2+, thereby triggering a series of chain reactions. 　　 The roles of Fenton reagent in water treatment mainly include the oxidation of organic substances and coagulation [1, 2]. 　　 Fenton’s reagent can remove organic pollutants from water to varying degrees; as found in a kinetic study on the treatment of 4 types of trihalomethanes in drinking water [3], for different concentrations of bromoform, at pH 3.5 the optimal molar ratio of H2O2 to Fe2+ is 3.7–1.9. Bromoform at different initial concentrations achieved degradation rates of 65%–85% after 3 minutes, and the degradation mechanism followed pseudo-first-order kinetics; chloroform did not undergo degradation. Tests conducted by Sheng et al. [4] on the removal of two anionic surfactants, ABS and LAS, showed that the optimal operating conditions for this system are 90 mg/L of FeSO4, 60 mg/L of H2O2, and a pH value of around 3; after 50 minutes of operation, the removal rates for both ABS and LAS exceeded 95%. 　　 Recent studies have shown that the decomposition of H2O2 to produce •OH can also be achieved using homogeneous catalysts such as Fe(III) and Mn(II), as well as heterogeneous catalysts like iron powder, graphite, and oxidized iron-manganese minerals, thereby enabling the oxidation and removal of organic pollutants [5, 6]. When exposed to light (such as ultraviolet light), the oxidation capacity of Fenton-like reagents improves (especially in aqueous solutions with high concentrations of pollutants) [7, 8]. When Fe(III) is used in place of Fe(II), since Fe(II) is generated immediately, the chance of •OH being reduced by Fe(II) is reduced, thereby **improving the efficiency of •OH utilization. If certain chelating agents (such as C2O2-4, EDTA, etc.) are added to this system, the removal efficiency of organic substances can be improved. Marianne studied the effect of oxalate on the oxidation of atrazine by the Fenton system under simulated sunlight, and the results showed that at higher oxalate concentrations, the degradation rate of atrazine was faster ； In the absence of oxalate, atrazine degrades only at pH < 4.1, and the degradation rate is slow. This is because the complex formed between oxalic acid and iron controls the distribution of iron in the solution, also has an impact on the pH value of the solution, and since oxalic acid acts as a •OH scavenger, it affects the oxidation efficiency of the system as well. 　　 Numerous experimental studies have shown that Fenton reagent or Fenton-like systems can be used to degrade many organic compounds, such as pentachlorophenol, phenols, trichloroethylene, azo dyes, nitrophenols, chlorobenzene, aromatic amines, trihalomethanes, mitoxantrone, methyl parathion, surfactants, and others. The main parameters affecting the Fenton reagent reaction include the pH value of the solution, residence time, temperature, and the concentrations of hydrogen peroxide and Fe2+. During operation, the pH value should not be too high (between 2 and 4). 2 UV/H2O2: The system exhibits a stronger ability to remove organic substances compared to the use of hydrogen peroxide or ultraviolet light alone. Treatment experiments on chlorinated phenol compounds showed [9] that when the wavelength of the light used was >290 nm and the H2O2 concentration was 55 mg/L, the removal efficiency of 2-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol could be improved; the removal rate of trichlorophenol after 3 hours of treatment could reach 100%. This system is highly influenced by the properties of the treated water. For example, oxidation tests on atrazine showed [10] that 99% of atrazine can be degraded within 15 minutes; however, when HCO3- or humic acid is present in the solution, their ability to capture •OH radicals, along with the fact that humic acid can absorb ultraviolet light, results in a significant reduction in the degradation rate of atrazine. Liao et al.’s degradation experiments on n-chlorobutane showed that as both the amount of hydrogen peroxide and UV intensity increased, the degradation efficiency of n-chlorobutane improved ； However, as the pH value of the solution increases, total inorganic carbon rises, and humic acid appears, the degradation efficiency decreases. 　　 There is little research on the oxidation intermediates and mechanisms of UV/H2O2; Stefan and others have studied the oxidation pathways of propanone, MTBE, etc. [11–14]. The experiment was carried out in a Rayox reactor (with a H2O2 concentration of 16 mmol/L). The intermediate products of propionate degradation were propionic acid, propionaldehyde, hydroxypropionic acid, formic acid, acetic acid, and oxalic acid; after some time, they mineralized into water and carbon dioxide. Studies have found that the competition between the formed intermediate products and propylidene•OH results in a relatively slow degradation rate of propylidene. Oxidation tests on MTBE showed that it can be completely mineralized; its main intermediate products are tert-butyl formic acid, 2-methoxy-2-methylpropanal, methyl acetate, propanone, tert-butanol, and formaldehyde. Some carbonyl compounds and organic carboxylic acids are also produced. In the study of the degradation mechanism of 1,4-hexanedioxol, Stefan found that its main intermediate products were various aldehydes such as formaldehyde, acetaldehyde, and glyoxal, as well as organic acids like formic acid, methoxyacetic acid, acetic acid, glyoxylic acid, and oxalic acid, and the formate ester of 1,2-ethanediol; this was confirmed by comparing the TOC of the solution with the carbon balance in the system. 　　 Experimental studies on the removal of trichloromethane from drinking water using a combination of hydrogen peroxide and ultraviolet light have shown that, while removing trichloromethane, it is also possible to reduce the total organic carbon content in the drinking water, thereby further improving the water quality. Tests using UV/H2O2 to treat groundwater contaminated with tetrachloroethylene showed that when the concentration of tetrachloroethylene in the groundwater was 76–227 μg/L, the removal rate could reach 97.3%–99%, and the cost was comparable to that of activated carbon treatment. In addition, there have been reports on the removal of trichloroethylene from groundwater, which can reduce its concentration from 3000–4000 μg/L to 0.68–0.83 μg/L, achieving satisfactory results. 　　In the UV/H2O2 system, each molecule of H2O2 can generate two molecules of •OH, offering a better cost-benefit ratio compared to Fenton reagent. Compared with other methods such as Fenton reagent and adsorption, it can effectively remove organic pollutants from water without causing secondary pollution, nor is any subsequent treatment required. The 3 H2O2/O3 system is the most widely used advanced oxidation technology in drinking water, as only hydrogen peroxide needs to be added to the ozone reactor. Japan began research in the late 1970s, while the United States adopted it for urban wastewater treatment in the 1980s. 　　 Ozone itself possesses strong oxidizing properties and can remove a large amount of organic substances, but its oxidizing effect is weaker on certain halogenated hydrocarbons and pesticides. Using ozone in combination with hydrogen peroxide can **improve the oxidation efficiency**. For example, O3 oxidizes chlorobenzene very slowly at pH=2 (with a kinetic rate constant of 0.06–3 mol/(L•s) [15]), but the degradation efficiency is **improved when using O3/H2O2 [with a kinetic rate constant of (4–5)×10^9 mol/(L•s)]. H2O2/O3 also exhibits excellent removal efficiency for the pesticide chlorsulfuron, with a removal rate of over 95% within 20 minutes. Nelieu et al. studied the oxidation mechanism of atrazine at a concentration of 0.46×10-5 mol/L in a 10 L batch reactor [16], and analyzed the effects of ozone addition conditions, the O3/H2O2 ratio, solution pH, and the presence of HCO3- ions on the formation of intermediate products. 　　 O3/H2O2 also has a certain effect on the treatment of industrial wastewater. Beltran et al. found that O3/H2O2 significantly increases the rate of COD degradation when used to treat tomato processing wastewater [17]; at a pH of 6, the COD removal rate was 86%, but it had no effect on wine factory wastewater. 　　 In terms of drinking water, research focuses mainly on the treatment of halogenated hydrocarbons in groundwater. In a pilot study conducted in 1986 by Aeita et al. [18] on groundwater contaminated with trichloroethylene (TCE) and tetrachloroethylene (PCE), it was shown that the combined use of hydrogen peroxide and ozone could increase the mass transfer of ozone into the water (by a factor of 1.7); moreover, the amount of ozone required to achieve a 95% removal rate of TCE and PCE was only 56%–64% of that needed when using ozone alone. Duguet conducted experiments on the removal of benzene compounds, o-dichloroaniline, 2-methylisobutanol, trichloroethylene, and tetrachloroethylene from groundwater, achieving good results in all cases. 　　Compared with UV advanced oxidation, the O3/H2O2 method does not require UV to activate molecules; therefore, its main advantage is that it can still function effectively in water with high turbidity. 4 UV/H2O2/O3 　　 UV/H2O2/O3 utilizes oxidation and photolysis for the degradation of organic substances, including direct oxidation by O3, oxidation by hydroxyl radicals generated from the decomposition of O3 and H2O2, direct photolysis, as well as the photolysis and dissociation of H2O2. The relative importance of these mechanisms in oxidizing organic substances depends on various operating parameters such as pH, UV intensity and wavelength range, and the ratios between the oxidants and the organic substances. 　　 Zeff applied for a patent in 1988 for the UV/H2O2/O3 method for removing various organic substances [19]. A 200 mg/L methanol solution can remove 97% of DOC within 30 minutes ； When treating groundwater contaminated by chloromethane, dichloromethane, 1,1-dichloroethane, 1,2-dichloroethane, trichloroethylene, tetrachloroethylene, benzene, chlorobenzene, toluene, etc., the removal rate of TOC reaches 98% within 60 minutes. Comparative tests also showed that the UV/H2O2/O3 method is more effective than oxidation systems using UV, H2O2, O3 alone or in combination with each other. When treating groundwater contaminated by VOCs, benzene, toluene, ethylbenzene, xylene, etc., Lewis et al. studied the effects of O3, H2O2, flow rate, and UV irradiation energy on the degradation efficiency, and found that the degradation rate of VOCs could reach 98%, with aromatic compounds being almost completely degraded. 　　 Ozonation or advanced oxidation technologies can increase the biodegradability of organic substances in textile wastewater. Ledakowicz studied the treatment of textile wastewater by combining oxidation methods with biological treatment [20]; he tested the effect of different oxidant concentrations on microbial growth during subsequent biological treatment, and found that O3/UV and UV/H2O2/O3 were the best bio-pretreatment techniques, causing only 10% damage to the microorganisms. 　 Landfill leachate contains a large amount of toxic and harmful substances, among which biologically refractory organic compounds account for a significant proportion. Studies show that after biological treatment, landfill leachate still contains large amounts of organic matter, with a COD level as high as 1290 mg/L, a TOC level of 378 mg/L, and a TOX level of 1.3 mg/L [21]. The UV/H2O2/O3 method is the most effective way to remove TOX, with removal rates of 83% and 69% for COD and TOC respectively; the authors also recommend using a high-pressure mercury lamp as the light source. Wenzel used advanced oxidation technologies to treat landfill leachate containing high concentrations of toxic organic compounds such as chlorophenols, polycyclic aromatic hydrocarbons, and PAHs [22], and found that UV/H2O2/O3 achieved a 10%–20% higher removal rate of TOC compared to UV/H2O2. 5 Applications in drinking water disinfection. Currently, there is extensive research on the use of ClO2 and O3 for drinking water disinfection; in particular, O3 disinfection is already in use in many water treatment plants. However, when Br- is present in the water, O3 disinfection generates suspect carcinogens such as bromates, as well as bromine-containing by-products. 　　 Advanced oxidation technologies offer a new approach for the disinfection of drinking water. The •OH radicals generated by AOPs can consume excess O3 in water and also limit the formation of bromates [23]; moreover, the presence of H2O2 in water enables the reduction of hypobromous acid (or salts), thereby reducing bromate formation. Therefore, the formation of by-products caused by bromination can be controlled by adjusting the amounts of H2O2/O3. Studies have shown that O3/H2O2 is also an effective disinfectant [24, 25]. Carmimeo et al. used H2O2/UV to replace ClO2 for pre-disinfection of untreated water, which helped to suppress the formation of disinfection by-products as well as the regrowth of microorganisms in the water after disinfection.

This post was last edited by hesonchang214 on 2012-6-25 21:30. Many. Control bacteria and algae in water ； Wandering, etc ； What kind of results do you need to achieve? There are now compound hydrogen peroxides available on the market, which are excellent for water treatment!

The oxidant in Fenton oxidation oxidizes organic substances under the catalysis of Fe2+.

This post was last edited by hesonchang214 on 2012-6-25 21:30. Simply put, it involves making use of the strong oxidizing properties of hydrogen peroxide. The advantages of hydrogen peroxide and other oxidants are their lower cost and the absence of residues. It is most commonly used to form Fenton’s reagent with ferrous ions, as well as for breaking down cyanide!

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