Fenton process

It was developed in the 1890s by Henry John Horstman Fenton. He reported that H₂O₂ could be activated by Fe²⁺ salts to oxidize tartaric acid. In 1934, Haber and Weiss

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proposed that the active oxidant produced by Fenton reaction is ‧OH , one of the most powerful oxidants known. Fenton treatment has received even more extensive attention because of its broad-spectrum of target compounds, strong oxidation ability, fast reaction rate, and the simplicity of the treatment equipment. Besides, the process is simple and non-expensive, operating at low temperature and at atmospheric pressure [27].

The Fenton method requires H2O2, Fe²⁺ salts and acidic pH. Comparing with other bulk oxidants, H₂O₂ is inexpensive, safe and easy to handle and no lasting environmental threat since it readily decomposes to water and oxygen. The mechanism of the Fenton reaction with H₂O₂ and Fe²⁺ is shown in Eq. 2-1 to 2-3. Furthermore, Fe³⁺

produced can react with H2O2 to form Fenton-like reaction (Eq. 2-2) and generate less powerful hydroperoxyl radical (HO2‧, E⁰ 1.42 V/SHE) [33,67-69]. Therefore, the ferric ion can be used instead of ferrous ion in Fenton process.

Fe²⁺ + H2O2 → Fe³⁺ + ‧OH + OH^-, k1 = 53-76 M^-1 S^-1 (2-1)

Fe³⁺ + H2O2 → Fe²⁺ + HO2‧ + H⁺, k2 = 0.01-0.02 M^-1 S^-1 (2-2)

Fe³⁺ + HO2‧ → Fe²⁺ + O2 + H⁺, k3 = (0.33-2.1) × 10⁶ M^-1 S^-1 (2-3)

The reaction rate of the Fenton reaction (k1) is much faster than the Fe²⁺

regeneration rate (k₂ and k₃), therefore the addition of Fe²⁺ and H₂O₂ is required to keep the reaction proceed. The major advantages of the Fenton’s reagent are: (1) both iron and H2O2 are cheap and non-toxic; (2) there is no mass transfer limitation due to its homogeneous catalytic nature; (3) there is no additional energy involved in using catalyst; (4) the process is technologically simple. The main disadvantages of Fenton

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reaction are (1) the continuous addition of Fe²⁺ and H₂O₂ for further oxidation of organic compound owing to the very slow regeneration rate of Fe²⁺; (2) the production of large volume of ferric hydroxide sludge [29-31]; (3) requirement of an acidic reaction condition (usually pH at 2.5-3.5) which could consume a lot of acid and require neutralization of low pH effluent prior to drainage [70]. In order to overcome these drawbacks and to improve treatment efficiency, several modified Fenton process like electro-Fenton, anodic Fenton, heterogeneous Fenton and photo-Fenton treatment are developed. The advantages and disadvantages of each Fenton process are listed in Table 2-5. Using Fenton process for the treatment of organic compounds in water and wastewaters reveals high removal efficiency under optimal experimental conditions.

Even with optimal removal efficiency, operation costs can be an important limitation in the selection of treatment. For this reason, Fenton reaction may be recommended as a pre-treatment process to enhance biodegradability for further biological treatment and reduce the operational costs to an acceptable extent [71].

2.4.1 Electro-Fenton treatment

Recently, indirect electro-chemical oxidation methods such as electro-Fenton are being developed for wastewater remediation. In these environmentally clean electrochemical techniques, H₂O₂ is continuously generated in an acidic contaminated solution (Eq. 2-4) from the two-electron reduction of O2 at reticulated vitreous carbon, mercury pool, carbon-felt and O₂-diffusion cathodes [72].

O₂ + 2H⁺ + 2e^- → H₂O₂ (2-4)

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Table 2-5. The advantage and disadvantage of each Fenton process.

Process Advantage Disadvantage Reference

Fenton

1. Cost-effective 2. Simple handling

3. No additional energy involved

1. Continuous addition of

1. Allowing better control of the process

3. Low current density and low reaction rate

[17,22,72]

Anodic Fenton

1. Avoiding the addition of iron ion

2. Effluent pH is neutral

1. Additional energy

1. Recycle of iron source 2. Lower H₂O₂ consumption

1. Requirement of acidification and neutralization

2. Low reaction rate

3. Iron source leaching at acid pH

[42,76,77]

Photo-Fenton

1. Reduction of ferric hydroxide sludge formation

2. Without continuous addition of iron ion

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The two main sources of ‧OH are anodic water oxidation (Eq. 2-5) and classical Fenton’s reaction between Fe²⁺ and H2O2 by adding small amounts of Fe²⁺ as catalyst into the acidic solution. Fe³⁺ species generated from Fenton’s reaction revert to Fe²⁺ by different reduction processes, involving H2O2 and/or organic intermediate radicals, as well as the direct reduction of Fe³⁺ on the cathode, allowing the propagation of the Fenton’s reaction by a catalytic cycle [80]. The reduction of Fe³⁺ by H2O2 takes place in two steps (Eq. 2-6 and Eq. 2-7), and also produces HO2‧ as the anodic oxidation of H2O2 (Eq. 2-8).

H2O → ‧OH + H⁺ + e^- (2-5)

Fe³⁺ + H2O2 ↔Fe-OOH²⁺ + H⁺ (2-6)

Fe-OOH²⁺ → Fe²⁺ + HO2‧ (2-7)

H2O2 → HO2‧+ H⁺ + e^- (2-8)

2.4.2 Anodic Fenton treatment

AFT was proposed as an improvement of the classic Fenton treatment and the electrochemical Fenton treatment. The reaction treatment is separated into two half-cells.

Ferrous ion is generated from iron in anodic half-cell by electrolysis (Eq. 2-9), whereas water is reduced in the cathodic half-cell. H2O2 is pumped into the anodic half-cell. In the cathodic half-cell, water is reduced on a graphite cathode (Eq. 2-10).

anode: Fe → Fe²⁺ + 2e^- (2-9)

cathode: 2H₂O + 2e^- → H₂ + 2OH^- (2-10)

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The AFT has several significant advantages over Fenton treatment and electrochemical Fenton treatment. First, the ferrous ion is delivered into the treatment system by electrolysis, overcoming the difficulty of handling hygroscopic ferrous salt.

Secondly, the pH of the treatment effluent can be partially neutralized by combining effluents from the anodic and cathodic half-cells of the AFT. Thirdly, the Fenton reaction can occur in an optimal pH environment in the anodic half-cell, keeping the treatment efficiency high [63].

2.4.3 Heterogeneous Fenton treatment

Homogeneous Fenton processes (such as AFT, electro-Fenton) have a significant disadvantage: homogeneously catalyzed reactions need 50-80 mg L^-1 of iron ions in solution, which is quit above 2 mg L^-1 in treated water regulated by the European Union directives [67]. In addition, the removal/treatment of the sludge-containing iron ions (the precipitate of ferric hydroxide) at the end of the treatment require large amount of chemicals and increase treatment cost. To overcome the disadvantages, the use of heterogeneous catalysts usually called heterogeneous Fenton is developed. Iron oxides such as goethite [81], zeolite [77] and magnetites are effective catalysts for catalytic H2O2 oxidation. However, it has been demonstrated that iron oxide catalysts lose their activity because of leaching effects of metallic catalysts under acidic condition. The leaching and deactivation of the catalyst are still challenges for developing advantageous catalyst for oxidation of wastewaters. Even catalysts used in heterogeneous Fenton treatment can be easily removed from the effluent, Mantzavinos (2003) reported that homogeneous Fenton process is more effective than heterogeneous ones [73].

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2.4.4 Photo-Fenton treatment

The degradation rate of target compound is enhanced with an irradiation applied and the process is called photo-Fenton reaction. The photo-Fenton reaction, a combination of H₂O₂ and UV irradiation less than 450 nm with Fe³⁺ or Fe²⁺, is a promising treatment process which can produce more ‧OH in comparison to Fenton reaction. When Fe³⁺ ions are added to the system, the Fe(OH)²⁺ complex is formed at the acidic environment as shown in Eq. 2-11. The positive effect of irradiation on the degradation rate is due to the photoreduction of Fe(OH)²⁺ to Fe²⁺ as shown in Eq. 2-12 [68]. Subsequently, the regenerated Fe²⁺ can further react with more H2O2 molecules, produce new ‧OH as Eq. 2-1 and form a reaction cycle [32,33]. It has three advantages, i.e. (1) facilitate the Fenton treatment without continuous addition of external Fe²⁺; (2) reduce the ferric hydroxide sludge formation [28,33]; (3) one mole of H2O2 can produce two moles of ‧OH according to Eq. 2-13 [28,33]. The more ‧OH generated than Fenton reaction is considered to be responsible for high efficiency of the photo-Fenton process. With irradiation of a light source, the performance of photo-Fenton was reported to be positively enhanced compared to the Fenton reaction [27,28,66].

Fe³⁺ + H2O → FeOH²⁺ + H⁺ (2-11)

FeOH²⁺ + hν → Fe²⁺ + ‧OH (λ < 450 nm) (2-12)

H₂O₂ + hν → 2‧OH (λ < 400 nm) (2-13)

In the past, the photo-Fenton treatment has shown very high efficiency in the mineralization of biorefractory pesticides and other organic pollutants [13,24-28].

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Recently, it has been proven that the irradiation of Fe³⁺ with H₂O₂ enhances the reaction rate of oxidant production through the involvement of ‧OH and high valence iron intermediates responsible for the direct attack to organic matter [26,68]. The advantages of the photodegradation process as an oxidative treatment are economical, rapid degradation and simple handling. Therefore, the photocatalytic reaction would be applied to wastewater treatment as a new developing methodology for reducing levels of pesticides and endocrine disrupting chemicals. However, high electrical energy demand and chemical reagents consumption are major drawbacks of photo-Fenton process [31]. In order to reduce the operational cost and achieve high performance, the experimental conditions, i.e. pH, temperature and dosages of reactants, must be optimized.