Advancements in the Fenton Process for Wastewater Treatment

Abstract

Fenton is considered to be one of the most effective advanced treatment processes in the removal of many hazardous organic pollutants from refractory/toxic wastewater. It has many advantages, but drawbacks are significant such as a strong acid environment, the cost of reagents consumption, and the large production of ferric sludge, which limits Fenton’s further application. The development of Fenton applications is mainly achieved by improving oxidation efficiency and reducing sludge production. This chapter presents a review on fundamentals and applications of conventional Fenton, leading advanced technologies in the Fenton process, and reuse methods of iron containing sludge to synthetic and real wastewaters are discussed. Finally, future trends and some guidelines for Fenton processes are given.

1. Introduction

The presence of many organic pollutants in wastewater, surface water, and groundwater may result from contaminated soil, agricultural runoff, industrial wastewater, and hazardous compounds storage leakage [1]. These organic pollutants, such as volatile phenols, benzene, and benzene derivatives, are considered highly toxic and low biodegradable. In some instances, conventional treatment methods such as biological processes are not sufficient to remove them. In order to improve water quality, advanced treatment is needed to remove the refractory organics.

Fenton is an effective advanced treatment process. The hydroxyl radical (·OH) can be generated from the reaction between aqueous ferrous ions and hydrogen peroxide (H₂O₂), and it can destroy refractory and toxic organic pollutants in wastewater [2]. Fenton discovered the Fenton reaction in 1894, and he reported that H₂O₂ could be activated by ferrous (Fe²⁺) salts to oxidize tartaric acid. However, its application as an oxidizing process for destroying toxic organics was achieved until the late 1960s [3]. Fenton was mainly used to treat wastewater by radical oxidation and flocculation. H₂O₂ is catalyzed by ferrous ions to decompose into HO· and to trigger the production of other radicals, which can fully oxidize organic matters. The hydroxyl radical (·OH) has a strong oxidation capacity (standard potential = 2.80 V versus standard hydrogen electrode) [4]. Hydroxyl radicals can effectively oxidize refractory organic pollutants in wastewater and even completely mineralized them into CO₂, water, and inorganic salts [5]. Meanwhile, the iron complex produced in the treatment of wastewater by Fenton will play the role of flocculant.

The conventional Fenton continuous flow process configuration, as illustrated in Figure 1, including acid regulation, catalyst mixing, oxidation reaction, neutralization, and solid–liquid separation. Fenton has many advantages, such as its high performance and simplicity (operated at room temperature and atmospheric pressure) for the oxidation of organics [6] and its non-toxicity [7] (H₂O₂ can break down into environmentally safe species like H₂O and O₂). However, Fenton also has some inherent disadvantages, which limit its application and promotion. For example, strict pH range, high H₂O₂ consumption, and the accumulation of ferric sludge that affects the oxidation efficiency [8, 9]. In order to overcome these disadvantages, the enhancement of the Fenton process has attracted much attention from researchers. Both heterogeneous and homogeneous catalysts were used to replace Fe²⁺, including ferric oxide [10], iron minerals [11], and nano zero-valent iron [12].

Figure 1.

Schematic diagram of the Fenton process.

On the one hand, the loss of iron and the consequent sludge generation can be reduced by using reductant or transition metal, giving rise to a heterogeneous Fenton process. On the other hand, some external energy was introduced into the Fenton and Fenton-like processes to form photo-Fenton/Fenton-like processes [13, 14], electro-Fenton-like processes [15, 16], and so on [17, 18]. Hence, this work mainly summarizes the recent advancements in the Fenton process related to improving Fenton oxidation efficiency and minimizing sludge production. It also describes the main drawbacks and potential applications based on recent developments. Some recommendations are also stated in the Conclusions section.

2. Fenton process

Currently, two mechanisms have been proposed to explain the degradation of organic matters by Fenton reaction. One is the Harber-Weiss mechanism [19], which considered that active oxide species ·OH are generated to degrade organics in Fenton reaction. The other is the mechanism of high iron oxide intermediates, which was proposed by Bray and Gorin [20]. They suggested that the strong oxidizing iron substances (FeO²⁺, FeO³⁺) were produced in Fenton reaction, rather than ·OH. With the development of spectroscopy and chemical probe method, it is generally accepted that the formation of ·OH initiates the Fenton oxidation.

The traditionally accepted Fenton mechanism is represented by Eqs. (1)–(7) [21].

$${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{3+}+·\mathrm{OH}+{\mathrm{OH}}^{-}{\mathrm{k}}_1=63.5{\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$E1

$${\mathrm{Fe}}^{2+}+·\mathrm{OH}\to {\mathrm{Fe}}^{3+}+{\mathrm{OH}}^{-}{\mathrm{k}}_2=3.2\times 108{\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$E2

$$\mathrm{RH}+·\mathrm{OH}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{R}\to \text{further oxidation}$$E3

$$\mathrm{R}·+{\mathrm{Fe}}^{3+}\to {\mathrm{Fe}}^{2+}+{\mathrm{R}}^{+}$$E4

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{2+}+·\mathrm{OOH}+{\mathrm{H}}^{+}{\mathrm{k}}_2=0.001–0.01{\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$E5

$$·\mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to ·\mathrm{OOH}+{\mathrm{H}}_2\mathrm{O}{\mathrm{k}}_2=3.3\times 107{\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$E6

$$·\mathrm{OH}+·\mathrm{OH}\to ·\mathrm{OOH}+{\mathrm{OH}}^{-}{\mathrm{k}}_2=6.0\times 109{\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$$E7

According to the above equation, ferrous iron (Fe²⁺) was rapidly oxidized to ferric ions (Fe³⁺), while Fe²⁺ is regenerated from the so-called Fenton-like reaction between Fe³⁺ and H₂O₂ at a very slow rate. Equation (1) is usually considered as the core of the Fenton reaction.

The Fenton process is usually operated under the solution pH value of 3. The oxidation activity of ·OH is related to the solution pH. The oxidation potential of ·OH increases and the oxidation capacity is enhanced with decreasing pH [22, 23]. In addition, the activity of Fenton reagent is reduced with increasing pH due to the lack of active Fe²⁺, in which the formation of inactive iron oxohydroxides and ferric hydroxide precipitate. Meanwhile, auto-decomposition of H₂O₂ appears at high pH [24].

At very low pH values, iron complex species [Fe(H₂O)₆]²⁺ and stable oxonium ion [H₃O₂]⁺ exist, which reduces the reactivity between Fe²⁺ and H₂O₂ [25, 26]. Therefore, the efficiency of the Fenton process to degrade organic compounds is reduced both at high and low pH. In addition, there are many competitive reactions in the Fenton reaction system. In Fenton oxidation, the reaction rate is dependent on the iron dosage, while the extent of mineralization is directly related to the concentration of oxidant.

It is important to understand the mutual relationships between Fenton reagent in terms of ·OH production and consumption. These relationships were investigated and classified them into three categories according to the quantity of the [Fe²⁺]₀/[H₂O₂]₀ ratio (initial concentration of Fe²⁺ versus initial concentration of H₂O₂) [27]. Their results showed that [Fe²⁺]₀/[H₂O₂]₀ ratio and organics can affect the competition in Fenton reaction paths.

The main disadvantages for the application of the Fenton process are the relatively high cost of H₂O₂ and the high amount of ferric sludge produced in the neutralization step of the treated solution before disposal. These drawbacks and the more increasingly stringent water regulations are a challenge to develop solutions addressed to improve the Fenton technology. On one hand, the energy was introduced into Fenton to enhance the ·OH generation, such as photo-Fenton, electro-Fenton, and so on. On the other hand, iron-based catalysts and reuse of Fenton sludge were developed as a Fenton-like reaction.

3. Enhanced Fenton process

3.1 Photo-Fenton process

The classic Fenton reaction efficiency was affected by the conversion rate from Fe³⁺ to Fe²⁺. Recent methods have promoted the in situ circulation from Fe³⁺ to Fe². A combination of hydrogen peroxide and UV radiation with Fe²⁺ or Fe³⁺ oxalate ion [photo-Fenton (PF) process] produces more ·OH compared to the conventional Fenton method [28]. The hydroxy-Fe³⁺ complexes after Fenton reaction mainly exists in the form of Fe(OH)²⁺ at pH 2.8–3.5. The photochemical regeneration of Fe²⁺ by photo-reduction (Eq. (8)) of ferric Fe³⁺ occurs in the photo-Fenton reaction. The newly generated Fe²⁺ reacts with H₂O₂ and generates ·OH and Fe³⁺, and the cycle continues:

$$\mathrm{Fe}{\left(\mathrm{OH}\right)}^{2+}+\mathrm{hv}\to {\mathrm{Fe}}^{2+}+·\mathrm{OH}$$E8

Direct photolysis of H₂O₂ (Eq. (9)) produces ·OH, which can be used for the degradation of organic compounds, and in turn increases the rate of degradation of organic pollutants [29].

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{hv}\to 2·\mathrm{OH}$$E9

However, photo-Fenton gives a better degradation of low concentration organic pollutants. Because the high concentration organic pollutants could reduce the absorb radiation of iron complex, which needs a longer radiation time and more H₂O₂ dosage.

Excess H₂O₂ can easily capture ·OH. In order to improve the efficiency of photo-Fenton, several organic ligands such as EDTA, EDDS, oxalate, and other organic carboxylic acid were added and complexed with Fe³⁺ under photocatalysis [29, 30]. The positive effects achieved by these ions can be attributed to the following aspects: (i) iron-ligands having higher ability compete for UV light in a wide wavelength range compared to other Fe³⁺-complexes, and promoting the reduction of ferric ion to ferrous ion and accordingly, regeneration of higher amounts of ·OH, (ii) Promoting H₂O₂ activation and ·OH radical generation, (iii) improving iron dissolution at pH 7.0, and (iv) operating over the broad range of the solar radiation spectrum [14].

Compared with the classic Fenton, photo-Fenton has many advantages. A photo-induced Fe³⁺/Fe²⁺ redox cycle could decrease the dosage of catalyst in Fenton, which effectively reduce the formation of iron sludge [31, 32, 33]. Meanwhile, solar or UV light can increase the utilization of H₂O₂, and possess photolysis on several small molecule organics. However, photo-Fenton has many disadvantages, such as low utilization of visible light, the required UV energy for a long time, high energy consumption, and cost.

3.2 Electro-Fenton process

Electrochemical processes can be combined with Fenton processes (EF processes) during WW treatment to improve the Fenton processes. Fe²⁺ and H₂O₂ were produced by the electrochemical method as Fenton reagent. The electro-Fenton process follows the reaction shown below [34, 35], where H₂O₂ can be generated in situ via a two-electron reduction of dissolved oxygen on the surface of the cathode in an acidic solution when the electrochemical process is applied (reaction Eq. (10)).

$${\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$E10

Also, the produced ferric ion from Eq. (1) can be reduced to Fe²⁺ by electrochemical regeneration of Fe³⁺ ions on the cathode surface:

$${\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}\to {\mathrm{Fe}}^{2+}$$E11

Water was oxidized to oxygen ta the anode (Eq. (12)):

$$2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}$$E12

·OH was also generated at the surface of a high-oxygen overvoltage anode from water oxidation:

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}}^{-}+·\mathrm{OH}+{\mathrm{e}}^{-}$$E13

Compared with tradition Fenton reaction, electro-Fenton has certain advantages, including (i) the production of H₂O₂ in situ via an electrochemical processes is beneficial for an increase in the organics degradation efficiency, a decrease in the cost, and a reduction in the risks associated with transportation; (ii) ferrous ion is regenerated through the reduction of ferric ions on the cathode, which reduces the production of iron sludge; and (iii) realizing the diversification of organics degradation pathway, such as Fenton oxidation, anodic oxidation, flocculation, and electric adsorption [36].

Electro-Fenton gives a better degradation of alachlor than the tradition Fenton. However, electro-Fenton processes have some problems with respect to H₂O₂ production. The production of H₂O₂ is slow because oxygen has low solubility in water and the current efficiency under reduced pH (pH < 3) is low. In addition, the efficiency of the electro-Fenton process depends on electrode nature, pH, catalyst concentration, electrolytes, dissolved oxygen level, current density, and temperature [37].

3.3 Sono-Fenton process

The combined treatment using ultrasound with Fenton reagent is known as sono-Fenton, which provides a synergistic effect on organic degradation [38, 39]. Ultrasound can enhance the Fenton’s oxidation rate due to the generation of more ·OH caused by the cavitation within ultrasonic irradiation.

The physical effect of cavitation is the generation of intense convection in the medium through the phenomena of microturbulence and shock waves, whereas the chemical effect of cavitation is the generation of radical species, such as oxygen (·O), hydroperoxyl (·OOH), and ·OH through the dissociation of solvent vapor during transient collapse of cavitation bubbles [40]. On the other hand, Fe³⁺ continuously reacts with H₂O₂, according to Eq. (5). A part of Fe³⁺ after a Fenton reaction exists in the form of Fe▬OOH². Fe▬OOH² can be quickly decomposed into Fe²⁺ and ·OOH; thereby, the phenomenon promotes the Fe³⁺/Fe²⁺ redox cycle [41]. In addition, ultrasound provides stirring and mass transfer effects to promote the diffusion of reactants in solution and improve the efficiency of Fenton reaction. However, sono-Fenton has some disadvantages, such as high cost and energy- intensive, so it is limited in practical application.

3.4 Fenton combined with other wastewater treatment technologies

The physical, chemical, and biological technologies have been widely used to treat wastewater. However, most of the real wastewater contains many organic pollutants with high toxicity and low biodegradability [42]. So that biological technologies are not enough. Moreover, the physical and chemical techniques are often effective for color, macromolecular organics, and suspended matter removal.

In order to improve the wastewater quality, advanced treatment needs the removal of refractory organics from the wastewater. In recent years, Fenton has been used to treat refractory wastewater by combining it with other wastewater-treatment technologies. The application of the Fenton oxidation process, as a pre-treatment, oxidizes refractory organics and improves the biodegradability, solubility, and coagulation, which are beneficial to the subsequent treatment.

Fenton was proved to be a feasible technique as the pre-oxidation for polluted pharmaceutical wastewater. Fenton as a pre-treatment process increased the BOD₅/COD value from 0.26 to 0.5, and also removed chloramphenicol, diclofenac, p-aminophenol, benzoic acid, and other toxic organics in the final effluent results from the Fenton-biological treatment processes [43].

Compared with biological treatment alone, Fenton-biological treatment processes improved COD removal [44]. Fenton combined with membrane filtration was also used to treat pharmaceutical wastewater. Although single nanofiltration (NF) and Fenton could effectively remove pharmaceutical active compounds, high organic load favored membrane fouling, and resulting in flux decline. The calcium salts were found to be the main fouling on the NF membrane surface. Fenton as the pre-treatment could ensure higher flux for the NF process. Also, the Fenton-NF system was found to be a promising method for wastewater from the pharmaceutical industry containing etodolac [45].

On the other hand, lime/unhair effluent, which contain highly loaded organic hazardous wastes, have been effectively treated by the Fenton-membrane filtration system at the pilot-scale [46]. Fenton was used also as a post-treatment of other treatment technologies for degrading residual organics, including physico-chemical and biological treatment. Compared with the aerobic sequencing batch reactor (SBR), SBR-Fenton could improve the TOC removal rate of textile wastewater up to 12% [47]. The solar photo-Fenton process was found to be an efficient process in removing phytotoxicity from Olive mill wastewater samples [48]. A COD and phenol removal as high as 94 and 99.8% could be achieved in coagulation/flocculation combined with the solar photo-Fenton system [48]. This combined technology, taking into account its reasonable overall cost, can be applied in somewhere with plenty of sunshine. Based on the technological and economic analysis performed, all combined treatment technologies will provide better performance than single treatment.

The Fenton process, combined with biological technologies, has shown quite low operational cost. The integrated Fenton-membrane processes were found efficient in removing organics from the industry wastewater. The phenolic compounds concentrated in the concentrate flow by membrane filtration could be recovered and further valorized in various industries. Although the combined Fenton and membrane technologies show the relatively higher overall cost of the combined membrane technologies, it is necessary to estimate the accuracy of the potential profit from the sale and valorization of these by-products recovered by this process.

4. The application of modified iron source as heterogeneous catalysts in Fenton reactions

The Fenton reaction in which iron salts are used as a catalyst is defined as a homogeneous Fenton process. Nevertheless, there are some disadvantages, including (i) the formation of ferric hydroxide sludge at pH values above 4.0 and its removal, (ii) difficulty in catalyst recycle and reuse, (iii) high energy consumption, and (iv) limitation of operating pH range. Therefore, the application of modified iron source as heterogeneous catalysts in Fenton reaction to overcome the shortcomings of homogeneous catalysis has been widely studied. Different heterogeneous catalysts have been used in Fenton reactions, including zero valent iron [49], iron pillared clays [50], and iron minerals [51]. Figure 2 shows various types of heterogeneous Fenton-like catalysts.

Figure 2.

Iron-containing catalysts.

4.1 Fenton-like reactions using zero-valent iron

Recently, zero-valent iron (Fe⁰) has been increasingly used in the heterogeneous Fenton system, due to its large specific surface area and high reactivity. It is reported that the removal of contaminants in the Fe⁰ induced heterogeneous Fenton system involves two steps [52, 53, 54]: (i) H₂O₂ decomposes on or near the Fe⁰ surface to form Fe²⁺ (Eq. (14)); (ii) then, Fenton reaction occurs, Fe²⁺ reacts with H₂O₂ to produce ·OH (Eq. (1)), and contaminants are degraded. Meanwhile, the produced Fe³⁺ is further reduced to Fe²⁺ (Eq. (15)).

$${\mathrm{Fe}}^0+{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{H}}^{+}\to {\mathrm{Fe}}^{2+}+2{\mathrm{H}}_2\mathrm{O}$$E14

$${\mathrm{Fe}}^{3+}+{\mathrm{Fe}}^0\to 3{\mathrm{Fe}}^{2+}$$E15

The degradation of trichloroethylene (TCE) in nano-scale zero-valent iron (nZVI) Fenton systems with Cu(II) was investigated [55]. TCE was significantly degraded (95%) in 10 min in the nZVI Fenton system with 20 mM Cu(II) at initial pH 3, while slight degradation (25%) was observed in nZVI Fenton system without Cu(II) at the same experimental condition. Because of the high activity on the Fe⁰ surface, Fe⁰ could easily coalesce into aggregates, which reduced the reactivity. The particle size of Fe⁰ is too small to recycle and separate at the end of the treatment.

4.2 Fenton-like reactions using iron oxides

Different physicochemical characteristics of these oxides make them favorable for oxidative reactions, where the surface area, pore size/volume, and the crystalline structure have significant effects on their activities. The amorphous Fe₂O with the largest surface area has the lowest catalytic efficiency, while the higher efficiency was achieved with crystalline α-Fe₂O₃, which has a significantly lower surface area [56]. In the Fenton process, magnetite has gained considerable attention than other iron oxides due to its unique characteristic: the magnetically easy separation of magnetite catalysts from the reaction system as a result of its magnetic property.

In many cases, magnetite offered better performance due to the presence of Fe²⁺ cations in its structure [57]. A comparison of the catalytic activity of amorphous iron (III) oxide, maghemite, magnetite, and goethite mixed with quartz was carried out by Hanna et al. [58] for methyl red degradation in presence of H₂O₂. The authors indicated that the oxidation state of iron in the oxides was the critical parameter, considering that Fe²⁺ is superior to Fe³⁺ in Fenton processes. In this study, magnetite exhibited the highest rate constant normalized to surface area per unit mass of oxide (SSA) at neutral pH value.

4.3 Fenton-like reactions using other iron-containing catalysts

The characteristic of iron materials mainly determines the oxidation efficiency in the heterogeneous Fenton reaction. The design of iron-containing catalysts can improve the activity and stability of the heterogeneous Fenton reaction. In recent years, heterogeneous Fenton catalysts have also been developed in the following aspects: (1) iron-loaded material, that is, iron is loaded on the porous materials, such as carbon nanotubes, clay and molecular sieves by a simple method. These materials are considered as potential heterogeneous Fenton catalysts because of low cost, high specific surface area, rich active sites, and easy separation, (2) new iron-containing materials. In the homogeneous Fenton-like processes, the catalysts used in the Fenton-like processes include Fe³⁺, Cu²⁺, Mn²⁺, Co²⁺, and Ag⁺. Sometimes organic or inorganic ligands are also used for complexing and stabilizing the metal ion over a wide pH range. The ligands studied include, but are not limited to, citrate, oxalate, edetic acid (EDTA), humic acids, and ethylenediamine succinic acid (EDDS). The catalysts based on different metal elements and ligands were developed to improve the degradation of organics, promote the Fe³⁺/Fe²⁺ redox cycle and decrease the sludge production [59, 60].

5. Reuse of the iron-containing sludge after Fenton reaction

A large amount of ferric sludge generated from the Fenton treatment. The practical applications of the Fenton process are limited, mainly because of neutralization after oxidation. The discharge of ferric sludge easily causes secondary pollution because of residual organics adsorbed and accumulated in ferric sludge from treated wastewater. The disadvantage is, therefore, the main obstacle limiting the development and application of the Fenton process [61].

Two approaches have been studied to minimize the production of sludge as a by-product of the Fenton process, including heterogeneous catalysts and the reuse of the iron-containing sludge [61]. However, the catalytic activity is usually weakened after repetitive use due to active iron leaching [62] or the decay of active catalytic sites [63]. Recently, the reuse of iron-containing Fenton sludge has been drawing increasing interest from researchers world-wide.

5.1 Fenton-like reactions using iron-containing sludge

The iron-containing Fenton sludge was used as an iron source for the synthesis of ferrite catalysts that have drawn much more attention due to their potential application in the fields of catalysis in the Fenton process.

Zhang et al. [64] proposed a novel method for the reuse of Fenton sludge in the synthesis of nickel ferrite particles (NiFe₂O₄). In phenol degradation with H₂O₂, NiFe₂O₄ alone, and NiFe₂O₄▬H₂O₂, the phenol removal was as high as 95 ± 3.4%. However, the phenol removal efficiencies were as low as 5.9 ± 0.1% and 13.5 ± 0.4% in H₂O₂ and NiFe₂O₄ alone, respectively. The leaching of iron ion from heterogeneous ferrite catalysts under the acid conditions is a common phenomenon. The leached iron amounted to 6.3 ± 0.2% of total iron, and the recovery ratio of NiFe₂O₄ catalyst in this study was found to be 97.1 ± 1.7% [64]. Notably, a rapid electron exchange between Ni²⁺ and Fe³⁺ ions in the NiFe₂O₄ structure could accelerate the conversion of Fe³⁺ to Fe²⁺, which was beneficial for the Fenton reaction. In addition, the Fe³⁺ on the surface of NiFe₂O₄ particles and the leaching of iron ion from NiFe₂O₄ could also react with the H₂O₂ to induce Fenton reaction.

Therefore, phenol could be effectively removed [64]. Roonasi and Nezhad [65] compared the catalytic activity of nano ferrite M-Fenton sludge (M = Cu, Zn, Fe, and Mn), and CuFe₂O₄ achieved the best performance. Based on the previous studies, a new catalyst Cu₂O▬CuFeC₂O₄ was synthesized by co-sedimentation. Compared with CuFeC₂O₄, Cu₂O▬CuFeC₂O₄ improved the phenol removal. The phenol removal as high as 97.3 ± 0.4%, and the leached iron amounted to 4.77% of total iron in Cu₂O▬CuFeC₂O₄▬H₂O₂ reaction. The superior catalytic performance was mainly due to the synergistic effect of both Cu⁺ and Cu²⁺ as well as Fe²⁺/Fe³⁺ redox pairs [66]. An electron bridge was formed between Cu⁺ and Fe Fe³⁺, which accelerates the formation of Fe²⁺ species in order to boost the reaction rate [66].

5.2 Regenerated the iron-containing sludge by the electrochemical process

Fenton process was employed to treat synthetic dye wastewater with a supply of Fe²⁺ electrolytically generated from iron-containing sludge [67]. The concentration of Fe²⁺ increases linearly (r² of 0.94) with increasing electrolysis time, but the amount of total iron provided is enough and not the limiting factor for the electrogeneration of Fe²⁺. In electro-Fenton reaction, Fe³⁺ and O₂ were reduced to Fe²⁺ and H₂O₂ at the same time on the cathode. So, there exists competition between Fe²⁺ and H₂O₂ production. In order to eliminate the competition and decrease the chemical cost of H₂O₂, hypochlorous acid (HOCl) was instead of H₂O₂ [68]. Two iron sludge reuse modes were examined to treat 1,4-dioxane in this study: sequencing batch mode and separation batch mode. The current efficiency (CE) in the electrolytic cell is related to the initial iron concentration, the initial iron species, and operation pH. Fe³⁺ ions were perceived to be more suitable for use as the initial iron species in the electrochemical Fenton-type process, where the CE was found independent of the Fe³⁺ concentration. Compared with the sequencing batch mode, the iron recovery ratio was higher in the separation batch mode. Therefore, the separation batch mode is relatively suitable for iron sludge reuse for both the CE and the iron recovery rate [68].

5.3 Regenerated the iron-containing sludge by the thermal method

In order to remove the residual organics adsorbed in the waste sludge and minimize the sludge production. Baking the sludge was a common method [69, 70]. A higher the baking temperature led to less accumulation of organics in sludge. The iron-containing sludge catalyst showed more activity (superior TPh, COD, and TOC removals) but higher leaching iron and less adsorption at high baking temperature. Also, the BOD₅/COD ratio was dropped by less than 50% [70]. However, the thermal methods will increase the overall cost and operational difficulty. Fe³⁺/Fe²⁺ redox cycle cannot be effectively realized in thermal system, thus, decreasing the catalytic ability of the iron-containing sludge catalyst.

5.4 Regenerated the iron-containing sludge by reduction

Some organic ligands (such as EDTA, EDDS) are used for complexing and regenerating iron-containing sludge to promote the Fe³⁺/Fe²⁺ redox cycle. These organic ligands effectively inhibited the precipitation of Fe³⁺. However, organic ligands such as EDTA were difficult to b biodegraded, which remained in the water and caused the second pollution. These organic ligands cannot efficiently promote the Fe³⁺/Fe²⁺ redox cycle, so reductants were required to help the Fe³⁺ reduction.

Organic reductants such as ascorbic acid, glutamic acid, and catechol were usually used in the Fenton-based process to accelerate the Fe³⁺/Fe²⁺ cycle, enhance the performance of Fenton reaction and expand the range of operation pH. On the other hand, organic reductants can react with ·OH. Because the selectivity of the reaction between reductants and ·OH is different, the degradation efficiency of pollutants is different.

The investigation evaluated the efficacy of the ferric oxy-hydroxide sludge continuous reuse in the Fenton-based treatment of landfill leachate in a sequencing batch reactor with and without the addition of supplementary ferrous iron [71]. The mechanism of the ferric oxy-hydroxide sludge-activated hydrogen peroxide oxidation in the presence of strong complexing and reducing agents was proposed.

Three iron-dissolution mechanisms could be distinguished: protonation, complexation, and reduction. First, in the case of the H₂O₂/sludge system, the probability of iron dissolution by protonation is objectively high at favorable acidic conditions applied. Second, the dissolution of iron by complexation involves the attachment of a complexing ligand onto the ferric oxy-hydroxide surface; humic and fulvic acids, main subclasses in landfill leachate, contain a high density of functional groups, which can conjugate iron ions to form ion-ligand complexes. Third, these complexing ligands were partially dissolved at pH 3, and bound to the protonated OH-group, thus ending in ultimate decomposed of the Fe³⁺-ligand complex into the bulk solution. Last, both humic and fulvic acids, as effective reductants, reduced Fe³⁺ via electron transfer mechanism [72].

Besides, the addition of a Fe²⁺ activator to the ferric oxy-hydroxide sludge-activated Fenton-based systems increases the total phenols removal rate and reuse cycles. The supplementation of the Fe²⁺ activator could ensure the full utilization of the oxidant.

Organics as quinone- and hydroquinone-structure compounds (such as tannic acid, lignin, phenol, and among others) may assist the Fenton oxidation by reducing Fe³⁺ to Fe²⁺ The loss iron can be used by these organics to form complexes and reduced in the H₂O₂/sludge cycle. Comparing the reducing efficiency of iron-containing sludge by the quinone-structure organics, it is found that H₂O₂/sludge/TN system gave the best performance after adding the quinone-structure organics. Tannins are considered to be strong metal-chelating and reducing agents [73]. They exhibit anti-oxidation (act as ·OH scavengers) and pro-oxidation (promote ·OH generation in the presence of transition metals) properties in biological systems (living organisms).

A comprehensive study of the catalytic performance of Fe³⁺ in the presence of tannic acid during the Fenton-based treatment of 2,4,6-trichlorophenol (TCP) was performed [74]. The addition of TN significantly promoted the degradation of TCP. H₂O₂ dose and ferric sludge load for water treatment in the presence of TN should be optimized to balance Fe³⁺ reduction to boost the Fenton reaction and ·OH scavenging to remove TN from water. The Fe³⁺ reductive mechanism by tannic acid incorporated tannic acid-Fe³⁺ complex formation and decay through an electron transfer reaction to form Fe²⁺.

TN availability in wastewater allows for the reuse of non-regenerated ferric sludge for Fenton-based oxidation without any supplementary Fe²⁺. The positive effect of TN in the Fe³⁺/H₂O₂ system can be explained by its monomer of gallic acids. Gallic acid complexed with Fe³⁺ to form two complexes that protonated [Fe(LH)]²⁺ and deprotonated [Fe(L)]⁺ from pH 1.0 to 3.0. These complexes were decomposed into Fe²⁺ and the quinone group by electron transfer, while Fe²⁺ reacted with H₂O₂ to accelerate the TCP removal [74].

The reuse methods of iron-containing sludge have been successfully proved with synthetic aqueous solutions of highly toxic compounds, such as phenols and chlorophenols. However, the decontamination of real wastewater by these enhanced technologies has been so far scarcely performed. The main reason is that real wastewater contains many organic pollutants with high toxicity and low biodegradability. These pollutants cannot completely mineralize. Organic pollutants will be continuously accumulated in reused iron-containing sludge that produced from treating real wastewater by the Fenton process. The phenomenon causes the low efficiency of the Fenton reaction.

6. Conclusions

During the last few years, many research efforts have been made toward the improvement of the Fenton process. Hybrid methods such as photo-Fenton, electro-Fenton, and sono-Fenton are not economically viable techniques to degrade large volumes of effluent disposed of by the industries. Most experimental studies have been conducted at the laboratory scale; thus, a more detailed investigation is required for the Fenton process to be considered feasible for industrial treatment plants. Further research on the advancement of the Fenton process is needed to demonstrate the economic and commercial feasibility of this process.

Although heterogeneous catalysts demonstrate considerable advances for the elimination of contaminants, there are still drawbacks related to the low oxidation rates, which appeared when pH values above four along with iron leaching, leading to an increase in the H₂O₂ consumption. Future studies should address the stability of the process for broader operational conditions to avoid metal leaching of into the reaction solution and their negative effects on the environment. These combined methods and heterogeneous systems expected to reduce the production of Fenton sludge. However, the high cost of combined methods, the leaching of active iron, and the decay of active catalytic sites should limit the reduction of Fenton sludge.

At present, regenerating the Fenton sludge is crucial for future studies. The use of Fenton sludge as a catalyst for the Fenton process has been tested mainly after thermal regeneration and subsequent re-dissolution of iron-containing solids by acid, chemical regeneration with reducing agent, and electrochemical reduction. The decontamination of real effluents by these enhanced technologies has been so far scarcely performed. The main reason is that real wastewater contains many complex organic pollutants with high toxicity and low biodegradability. These pollutants cannot completely mineralize. Significant attention should be devoted in the future on the development of rate expressions (based on reaction mechanisms), identification of reaction intermediates, identification of scale-up parameters, and cost-effectiveness analysis.

Acknowledgments

The work is financially supported by the China special S&T project on treatment and control of water pollution (2017ZX07402002).