Fenton Technology for Wastewater Treatment: Dares and Trends


Fenton reaction remains an efficient technique for decomposing recalcitrant organic contaminants. Nevertheless, traditional Fenton response has many lim-itations like the necessity of acidic pH circumstance, the formation of iron sludge and the need for elevated chemical introductions. Procedures like het-erogeneous Fenton, fluidized-bed Fenton, employment of chelating products and in situ formations of Fenton’s reagent have been examined as likely solu-tions to such drawbacks. Bello et al. [1] presented an excellent discussion of the restrictions of Fenton reaction and the fresh manners for dealing with them and this work focuses on its main findings. The heterogeneous Fenton method stays the most largely examined thanks to the expansion achieved in catalysis. The fluidized-bed Fenton method has the capacity to diminish sludge formation and ameliorate technology efficiency. Chelating chemicals are employed to performing homogeneous Fenton at circumneutral pH, even if the potentially decisive impact of many chelating products remains a source of worry. In situ formation of Fenton’s reagent via bio-electrochemical technique (bio-electro-Fenton) seems to be a likely manner to diminish the price related to Fenton’s reagent. Despite the progress registered in the Fenton technologies, the classical process, and its ameliorated versions, membranes processes remain fundamental for secure wastewater treatment. As sure barriers towards pollution dispersal, processes such as nanofiltration should be coupled to Fen-ton techniques.

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Ghernaout, D. , Elboughdiri, N. and Ghareba, S. (2020) Fenton Technology for Wastewater Treatment: Dares and Trends. Open Access Library Journal, 7, 1-26. doi: 10.4236/oalib.1106045.

1. Introduction

There is an increasing endeavor to expand efficient techniques able of eliminating recalcitrant pollutants from wastewaters [1] [2]. Several organic contaminants, like pharmaceuticals and personal care products (PPCPs), are recalcitrant to the traditional wastewater treatment techniques [3] [4] [5]. Numerous investigations have proved the existence of diverse recalcitrant and emerging contaminants in the effluents of traditional wastewater treatment plants (WWTPs) [6] [7] [8]. Such contaminants might restrict the possible reuse of the treated wastewater and constitute dangers to public health and nature [9] [10] [11] [12] [13]. For instance, chronic toxicity and endocrine disruption have been related to the existence of even a low level of PPCP [14].

Because of the imperfections of traditional wastewater treatment techniques in dealing with recalcitrant contaminants [15] [16] [17] [18], attempts have been escalated to discover substitutional techniques. Even if physicochemical processes like adsorption might reduce recalcitrant contaminants from wastewater, they are mainly separation techniques and additional remediation may be needed [19] [20] [21]. Biodegrading recalcitrant contaminants are frequently slow and mainly transform the organic contaminants into several intermediates, which may therewith cumulate in nature [22] [23]. Consequently, efficient remediation necessitates the total mineralization of such recalcitrant contaminants [1] [24] [25]. This may be attained by advanced oxidation processes (AOPs) [26] [27] [28], which are capable of efficaciously oxidizing organic chemicals.

During AOPs, hydroxyl radicals (ŸOH) are produced through a series of methods [29]. The ŸOH (oxidation potential = 2.8 eV) could enter in mutual actions with the contaminants (rate constant: 109 M−1∙s−1), oxidizing them to simpler intermediates and probably to CO2 and H2O [30]. During the last decade, AOPs have attracted huge attention. They are viewed as the preferable techniques for decomposing recalcitrant contaminants [31]. Usually, AOPs are categorized following the procedure of producing the ŸOH. Such procedures comprise Fenton oxidation [30], photocatalysis [32], UV-founded techniques, ozonation [29], sonolysis [33] and electrochemical oxidation [1] [34].

Fenton oxidation is an efficacious AOP and implies a catalytic degradation of hydrogen peroxide (H2O2) by ferrous iron (Fe2+) to form ŸOH following Equation (1) [18]. The produced ŸOH after that oxidizes the organic contaminant (Equation (2)). The initial step of the technique is a rapid degradation of H2O2 by Fe2+ [35], which forms a significant quantity of ŸOH and transform Fe2+ to Fe3+ [36] [37] (Equation (3)) [1]. Even if Fenton oxidation is mostly defined in such straightforward stages, the technology is considerably more combined since it implicates numerous additional reactions. Usually, such techniques may be largely classified into initiation, propagation and termination reactions [38].

H 2 O 2 + Fe 2 + Fe 3 + + O H + OH (1)

O H + Organic Products (2)

O H + Fe 2 + Fe 3 + + OH (3)

The ŸOH may enter in mutual reactions with organic matter and/or Fenton catalyst through either of three routes: 1) hydroxyl addition, 2) hydrogen abstraction or 3) electron transfer [39]. Hydroxyl addition happens with organic chemicals possessing aromatic systems or carbon-carbon multiple bonds (Equation (4)). Hydrogen abstraction takes place with unsaturated organic chemicals (Equation (5)) while electron transfer happens if ŸOH interacts with inorganic ions (Equation (6)) [1].

O H + R R ( OH ) (4)

O H + RH R + H 2 O (5)

Fe 2 + + O H Fe 3 + + OH (6)

The most important benefits of Fenton process are the simplicity of the method and the efficient decomposition of contaminants. Further, both Fe2+ [40] and H2O2 are easily obtainable, simple to manipulate and environmentally harmless. Fenton oxidation has been largely tried for dealing with diverse wastewater streams like textiles [41] [42], pharmaceuticals [43] [44], olive mill [45] [46], leachate [47], agrochemicals [48] [49] and other recalcitrant pollutants. Moreover, Fenton oxidation was presented as an environmentally secure choice for sludge conditioning and treatment [50]. For instance, researchers [51] tried Fenton process for the conditioning of sewage sludge and examined the contribution of iron [52] species and pH optimization. The Fenton oxidation transformed most of the sludge-bound water to free water, improving its dewater-capacity. The process was more assisted by the coagulating impact of Fe3+ [1] [53] [54] [55] [56].

Fenton process is influenced by diverse running factors, comprising the level and ratio of Fenton’s reagent (H2O2 and Fe2+), solution pH, time, temperature, contaminant concentration and the type of the reaction matrix [1]. Nevertheless, Fenton’s reagent and pH are frequently viewed as the most crucial factors. The level of H2O2 dictates the possible ŸOH that will be formed in the method. This implies that the more H2O2, the more ŸOH will be possibly produced, following the level of Fe2+ and different working circumstances. Nevertheless, a surplus level of H2O2 may conduct to a scavenging impact where the H2O2 reacts with the ŸOH, transforming it to the less reactive HO2Ÿ [21]. Further, Fe2+ works as a catalyst for the degradation of H2O2. Consequently, augmenting the level of Fe2+ may conduct a more important catalytic degradation of H2O2. Nevertheless, overabundant quantities of Fe2+ may as well conduct to scavenging of ŸOH, which will curb the technique efficiency [57]. Moreover, an elevated level of Fe2+ may as well conduct to an elevated sludge formation at the end of the method. As a result, the ratio of H2O2 to Fe2+ is a critical precaution to guarantee the optimum formation of ŸOH and avert the scavenging impact.

The wastewater pH is an additional crucial factor touching Fenton process. Generating ŸOH is governed by the pH of the reaction medium; indeed, more elevated pH conducts to the precipitation of Fe (III) in the form of iron hydroxideflocs [1] [58]. Therefore, the formation of ŸOH and the dependent decomposition of contaminants via the Fenton process is efficient below acidic pH, i.e., in the interval of 2.8 - 3.5 [59] [60] [61]. In other words, the pH of the wastewater should be regulated inside this domain to guarantee ferrous and ferric irons with a view to preserve their catalytic capacity [62]. As mentioned above, the temperature was counted to possess an unfavorable influence on Fenton response. This was attributed to the worry for probable degradation of H2O2 above 90˚C and the energy necessity [63]. Therefore, researches on Fenton methods have widely been performed below ambient temperature. Nevertheless, modern investigations have proved that augmented temperatures may elevate the oxidation rate and improve Fenton’s response. For instance, scientists [64] noted an amelioration of the transformation of linear alkylbenzenesulfonate through Fenton reaction if the temperature was elevated from 20˚C to 94˚C. At the lower temperature, 61% transformation was reached, which augmented to 99.9% at the higher temperature. Many fresh investigations have as well shown the improvement of Fenton response at more increase temperature [65] [66]. The favorable impact of higher temperatures may be addressed to a more performant utilization of H2O2, which conducts to the more important formation of ŸOH [67]. The type of reaction matrix stays a crucial factor as the existence of organic/inorganic chemicals in wastewater may lead to a scavenging impact on the ŸOH. Mineral chemicals like NaCl, Na2CO3, and Na2SO3 are detected in different levels in genuine wastewaters and may react with ŸOH, diminishing their accessibility [68].

The Fenton process undergoes many restrictions that prevent its large-scale usages. One of such barrier remains the immoderate formation of iron sludge that causes troubles in the recycling and can lead to resultant contamination. Additional restrictions involve the need for a limited domain of working pH and elevated chemical inputs [69]. As a consequence, handling the disadvantages of the Fenton process constitutes an endless study attempt in the area of the Fenton method for recalcitrant wastewater treatment. Many of the procedures to deal with such restrictions comprise the employment of heterogeneous Fenton process [70] [71], fluidized-bed Fenton technique [72], and electro-Fenton [73].

2. Fenton Technology for Recalcitrant Wastewater Treatment: Review Papers’ State of the Art

The domain of Fenton technology for recalcitrant wastewater treatment was the subject of numerous review papers. Such literature discussions focused on diverse features of the Fenton technique comprising the basis and usages of conventional Fenton oxidation, heterogeneous Fenton process, electro-Fenton with its related processes, fluidized-bed Fenton process, Fenton process at circumneutral pH employing chelating agents and hybrid techniques, where Fenton reaction is integrated with additional techniques. Several review papers embraced the large domain of AOPs, like those mentioned by Oturan and Aaron [74], in which the Fenton process is evaluated with different AOPs. Nevertheless, Bello et al. [1] in their work largely encompassed those reviews where Fenton oxidation is the main interest (see in [1] (their Table 1) for a summary of many of these reviews).

Neyens and Baeyens [30] presented an evaluative survey of the conventional Fenton process, concentrating on the kinetics, the impact of running factors, the importance of the ratio of Fenton’s reagent and the usage of Fenton method in sludge dewatering. In the following publication, Pignatello et al. [75] offered a global discussion of the pathways of Fenton and Fenton-similar processes, reaction routes and intermediates, changed configurations of Fenton process (heterogeneous Fenton, photo-helped Fenton [76], etc.) and their usages to water and soil remediation. More lately, scientists [77] [78] displayed analysis of Fenton-similar technologies for organic wastewater remediation. They concentrated on running factors, changed Fenton-similar methods, catalysts, and economic considerations [1].

Brillas et al. [79] published a thorough discussion of the electro-Fenton and similar electrochemical techniques that are founded on Fenton’s response. They focused on the basics of Fenton’s chemistry, the progression of the electro-Fenton method, cell arrangement, working factors, pathways, integrated electro-Fenton techniques, and their ecological utilizations. Newly, Poza-Nogueiras et al. [80] suggested an analysis of the present tendencies and developments in electro-Fenton technology focusing on the implementation of heterogeneous catalysts. They concentrated on the diverse sorts of catalysts and elaboration procedures, the influence of running indicators, pathways and usages in treating water, soil and adsorbent renewal. Researchers [81] evaluated the basic concepts and usages of electro-Fenton technology. The review concentrated on the kinds of catalyst and functionalized cathodic materials, route of catalytic activation of H2O2 and setup arrangement. A discussion of the bio-electro-Fenton was lately published by Li et al. [82] [83] who focused on its basics, working factors, reactor conception, and economic features [1].

Table 1. Probable impacts of pH on Fenton process [1].

Even if such analyses have embraced the basics of Fenton reaction and identical methods, a discussion concentrating on the main restrictions of Fenton technology and the new development to treating them has not been given until the excellent work done by Bello et al. [1]. Indeed, they discussed the fresh advance to dealing with the main restrictions of Fenton’s response for recalcitrant wastewater treatment. Primarily, they analyzed the restrictions of Fenton technology, pursued by a pertinent discussion of the modern references on probable procedures for dealing with such restrictions. For each action plan, they assessed the basics and exemplary implementations in decomposing recalcitrant organic contaminants. They also suggested viewpoints on potential orientations for the next research. In fact, Bello et al. [1] ’s analysis remains appropriate because of the augmenting investigation works to resolving the restrictions of the Fenton technology.

3. Restrictions of Fenton Process and Procedure to Deal with Them

3.1. Fluidized-Bed Fenton Process

The Fenton process has many obstacles that prevent its industrial implementation. Fenton technique is greatly touched by the solution pH, which requires to be fixed in the acidic domain to avert the precipitation of Fe3+ into iron hydroxide (Fe(OH)3(s)). Table 1 recaps the impacts of pH outside the optimum domain. The necessity of delicate dominance of pH and the hardness of running in acidic circumstances stop the workable implementations of Fenton technology [84]. Bigger pH conducts to complexation reactions and precipitation of iron oxides, causing the formation of excessive sludge. As a result, sludge production constitutes one more inconvenient of the traditional Fenton process [85] [86]. Sludge formation opens the hazard of additional contamination and the necessity for sludge remediation and recycling. The necessity of supplementary remediation of sludge is an origin of worry since the cost of sludge remediation may be above to 35% - 50% of the total operating cost of the wastewater treatment [30]. More difficulties of the Fenton process involve elevated chemical utilization, instability of the Fenton’s reagent, undesirable reactions and loss of oxidant, hardness in regulating the reagent concentrations and the exigency to neutralize the treated wastewater prior recycling [75] [87]. Table 2 underlines the main restrictions of the Fenton technique and several of the likely procedures to deal with them [1].

There is increasing attention in investigations to resolve the restrictions of the traditional Fenton process. Not many steps have appeared as likely solutions as listed in Table 2. One such procedure remains the expansion of heterogeneous Fenton oxidation, in which iron oxides or other metal oxides are employed as heterogeneous catalysts [38]. Fluidized-bed Fenton technique is second planning, in which a fluidized-bed reactor is integrated with the homogeneous Fenton reaction to enhance process efficiency and decrease sludge formation. An additional manner implicates performing Fenton oxidation at near-neutral pH

Table 2. Mainrestrictions of classical Fenton process and their likely solutions [1].

via the introduction of a chelating chemical to produce iron-complexes [60] [61]. Manners able of in situ generation of oxidants (H2O2) like electro-Fenton and bio-electro-Fenton (microbial fuel cell) stay interesting techniques for treating the elevated chemical necessity. Bello et al. [1] analyzed such procedures below four classes: 1) fluidized-bed Fenton process, 2) heterogeneous Fenton processes, 3) Homogeneous Fenton at neutral pH using chelating agents and, 4) in situ production of Fenton’s reagent.

Immoderate sludge formation is between the main imperfections of the conventional Fenton reaction [1]. Several scientists have tried the application of a fluidized-bed reactor to realize Fenton’s response, the so-named fluidized-bed Fenton process. In the fluidized-bed Fenton process, solids carriers, such as SiO2 (Figure 1), are employed to trigger the crystallization of iron oxide and diminish sludge production [95] [98]. The fluidization improves mixing and raises ŸOH-contaminant interaction. Further, recirculation gives more oxygen into the setup, which may improve the technique efficiency by keeping ŸOH [99].

The first investigation on the fluidized-bed Fenton process was published by Chou and Huang [100]. They examined the oxidation of benzoic acid. Huang and Huang [101] [102] applied Fenton oxidation for decomposing phenol in a fluidized-bed setup. In the same way, Anotai et al. [103] noted the decomposition of nitrobenzene and iron crystallization employing a fluidized-bed Fenton process [1]. Following researches were mainly oriented to sludge decrease and process efficiency in decomposing diverse recalcitrant contaminants like dimethyl sulfoxide [98], dyes [104], bisphenol A [105] and phthalocyanine (Figure 2) [106].

3.2. Electro-Fenton Process

Using electrochemical technology in the Fenton oxidation is rising as an encouraging option. In electrochemical technology, the electron is employed as the

Figure 1. Various processes that occur in fluidized-bed Fenton process using SiO2 as carriers, 1) homogeneous reaction between Fe2+ and H2O2, 2) crystallization of Fe3+ on the surface of SiO2, 3) heterogeneous reaction between crystallized iron oxide and H2O2 and, 4) reductive dissolution of iron oxide [1].

Figure 2. Mainrunning factors of fluidized-bed Fenton process [1].

reagent to push the treatment method. Various choices may be acquired if electrochemical technology is implemented to Fenton oxidation. Between such, two manners could give an in situ production of Fenton’s reagent. During the first manner, the electro-Fenton process, H2O2 is constantly formed at the cathode while the iron catalyst is externally introduced. The second procedure, the anodic Fenton process, implicates the utilization of sacrificial iron anode for the electrogeneration of Fe2+ while H2O2 is formed at the cathode or externally injected [1]. As numerous researches have usually grouped the latter as an electro-Fenton technology, it is really a peroxi-coagulation method, in which the contaminant is eliminated by the integrated oxidation and coagulation caused by the existence of Fe(OH)3 [79]. A thorough analysis of the electro-Fenton process and similar electrochemical techniques was published by Brillas et al. [79] ; however, more fresh discussions of the basic concepts and environmental utilization of the electro-Fenton process were suggested by Poza-Nogueiras et al. [80] and Ganiyu et al. [81].

Usually, electro-Fenton is planned to possess a constant electrogeneration of H2O2 at the cathode. In a regular process, air/O2 is constantly furnished to the cathode; at the same time, the production of H2O2 is caused by a two-electron reduction of O2 in acidic medium as illustrated in Equation(7) [63]. The introduction of Fe2+ leads to the catalytic degradation of H2O2 to form ŸOH founded on Fenton’s reaction. Moreover, there is a constant electroregeneration of Fe2+ at the cathode as defined by Equation (8) [1].

O 2 + 2 H + + 2 e H 2 O 2 (7)

Fe 3 + + e Fe 2 + 8)

As Fenton’s reagent is produced in situ employing electron, which is a clean species, many of the restrictions of the classical Fenton oxidation may be avoided. With the constant in situelectrogeneration of H2O2, the prices related to direct external injection of H2O2 will be avoided. Contrasted to the price of the external addition of H2O2, Huang and Chu [107] affirmed that up to 80% cost decrease may be attained with the in situelectrogeneration of H2O2. This price diminution will even augment when the costs related to transportation and storage of H2O2 are factored in [1]. This constitutes one of the benefits of electro-Fenton process contrasted to the conventional Fenton oxidation. An additional benefit of electro-Fenton process is the constant formation of Fe2+ (Equation (8)), which could reduce the potential sludge production from the precipitation of Fe3+. Further, the constant regeneration of Fe2+ will diminish the catalyst input and improve the formation of ŸOH [63].

The performance of electro-Fenton is a function of numerous factors like applied current, solution pH, sort of electrolyte, the quantity of catalyst and the initial level of target contaminant. The impacts of solution pH, catalyst level and initial level of contaminant on electro-Fenton are identical to the classical Fenton oxidation. As an illustration, the optimum pH for efficient electro-Fenton was mentioned to be in the interval of 2.5 - 3.5 [74]. Applied current is a basic factor that touches the efficiency of electro-Fenton. The electrogeneration of H2O2 and the constant regeneration of Fe2+ are a function of the applied current. As a result, the performance of electro-Fenton augments with the augmentation in the applied current. Nevertheless, elevating the applied current over specific end may add parasitic reactions into the technique, decreasing the contaminant decomposition performance. The optimum quantity of applied current is application-specific and could be evaluated by early trials [1].

Several investigations have proved the performance of electro-Fenton to produce in situ Fenton’s reagent for following decomposition of organic contaminants. Aside from the benefit of price diminution, efficacious decompositions of pollutants have been noted. On the other hand, the energy demand stays a worry in the electro-Fenton process [1].

3.3. Bio-Electro-Fenton (Microbial Fuel Cell)

The employment of microbial fuel cells (MFCs) to produce H2O2 for Fenton oxidation, the so-named bio-electro-Fenton, is rising as an encouraging technique to diminish the cost linked to H2O2 purchase and transportation [77] [78]. In MFCs including an anode and cathode compartments, microorganisms are utilized to transform the chemical energy stored in organic pollutants to produce electric current and chemicals [108] [109]. The electrons are formed at the anaerobic anode compartment comprising the organic substrate and transported to the cathode compartment via an external circuit [110]. At the cathode, the two-electron oxygen reduction conducts to the generation of H2O2. With the formation of H2O2, Fenton catalyst (Fe2+, iron oxide) can be introduced into the setup for the catalytic degradation of the H2O2 to form ŸOH [1]. Figure 3 illustrates a schematic of a typical MFC employed in Fenton process (bio-electro-Fenton process). The simultaneous formation of H2O2 and electricity production make bio-electro-Fenton an interesting technique for wastewater treatment. Information about the basics of bio-electro-Fenton method may be read in the reviews published by Li et al. [82] [83].

The first published investigation on the bio-electro-Fenton was the research of Zhu and Ni [111]. They studied the decomposition of p-nitrophenol with simultaneous electricity production. Since then, numerous researches have noted the efficient employment of MFC to produce H2O2 for Fenton oxidation with concomitant electricity production. Mai and Li [110] concentrated on the usage of bio-electro-Fenton for decomposing Orange II at neutral pH. The maximum H2O2 formed was 3.24 mg/L, leading to total decomposition of the dye following 14 h and simultaneous power production of 230 mW/m2. In identical research, Feng et al. [112] attained an H2O2 formation of 2.86 mg/L, conducting to total mineralization of Orange II. As an alternative of the dual-chamber usually utilized in the bio-electro-Fenton, Zhu and Logan [113] proposed a single-chamber MFC setup for decomposing phenol. The technology reduced more than 75% of total organic carbon (TOC) from the phenol-containing solution. Even if the

Figure 3. Schematic of microbial fuel cell applied to Fenton oxidation (PEM = Proton Exchange Membrane) [1].

single-chamber device is depicted to possess higher performance and lower operational cost, the quantity of H2O2 formed was not evaluated [1].

More fresh investigations of bio-electro-Fenton comprise the research of Li et al. [114]. Their main focus was the technique of employing a microbial reverse-electrodialysis electrolysis cell for decomposing Orange G. The fundamental characteristic of the device is that the electrons are formed from the exoelectrogens and salinity-gradient among seawater and freshwater in the cell. The technology conducted to the TOC elimination performance of 99.6%. Zhang et al. [115] tried the bio-electro-Fenton oxidation of paracetamol employing a dual-chamber MFC. The method eliminated until 70% of the initial paracetamol under optimum condition [1].

One main worry with the bio-electro-Fenton process remains the formation of O2 as an alternative to the wanted H2O2. To make sure the selectivity of cathode material for H2O2 generation, few scientists have proposed the employment of carbonaceous cathode as an alternative to the highly expensive Pt cathode. Carbon-founded materials are encouraging thanks to low price, plenty and electro-reduction activity. Wang et al. [77] [78] tried the reduction of bisphenol A, estrone, sulfamethazine and triclocarban employing bio-electro-Fenton with a graphite electrode. The maximum concentration of H2O2 formed was 2.06 mg/L. On the other hand, Asghar et al. [116] reached a maximum H2O2 production of 140 mg/L with a simultaneous electricity formation of 33.52 W/m2 employing a heat-treated graphite cathode. Because of poor catalytic activity of carbonaceous material, higher yields of H2O2 has not been attained [1].

To obtain enhancement in the production of H2O2, heteroatom doping of carbonaceous cathode is one of the proposed solutions [117]. In this attention, nitrogen doping has proved to be an efficacious procedure as it changes the electronic features of the carbon material and elevates the electrochemical surface area. Numerous researchers have established the contribution of nitrogen doping on H2O2 formation and power yields. For example, Asghar et al. [118] have proved a 25% augmentation in the yield of H2O2 formation through treating graphite cathode with NH3 at elevated temperature. In addition, in many publications, H2O2 formation rates of 1.7 - 121 mmol/h/g are reached employing N-doped porous carbon [119]. Lately, Wang et al. [120] altered activated carbon air cathode by co-pyrolyzing with glucose and doping with nano-zero-valent iron to enhance two-electron oxygen reduction in the bio-electro-Fenton process. The changed electrode showed higher decomposition performance and more power production contrasted to pure activated carbon. Further, the anti-biofouling impact was noted on the altered cathode following an extended period of running [1].

Even if researches have proved the feasibility of the in-situ formation of H2O2 in the bio-electro-Fenton setup, there are numerous dares like the low generation of H2O2, membrane fouling [121] [122], high internal resistance and reactor arrangements that require more revision prior its commercial implementations [1].

4. Electro-Fenton Process for Disinfecting Water

Ren et al. [123] tested a breakthrough by one method of new flow-through electro-Fenton with a graphene-modified cathode, which is frequently considered to be inefficient. They established that this technique was cost-effective for concomitant sulfadiazines (SDZs) decomposition and disinfection [124] [125] from urban secondary effluent with an extremely low electrical energy consumption (EEC) of 0.21 kWh/m3, explained to the elevated H2O2 formation of 4.41 mg/h/cm2 on the fresh graphite felt cathode altered by electrochemically exfoliated grapheme (EEGr) with a low EEC of 3.08 kWh/(kg H2O2). Contrasted with the inefficient SDZs decomposition by the traditional flow electro-Fenton, this technology was more cost-efficient and overpass the harsh needs of the electrolyte level. Further, it depicted excellent performance in decomposing diverse antibiotics, and the graphene-modified cathode still kept stable performance following eight subsequent trials. Taking into consideration the integrated work of ŸOH and active chlorine [126] [127] [128] [129] [130], the generation of hydroxylated and chlorine-containing by-products [131] was proved, and a likely decomposition route for SDZs was suggested. This flow-through electro-Fenton technique presented a substitutional process for killing pathogens and decomposing antibiotics through single technology for treating and reusing domestic secondary effluent (Figure 4).

5. Bio-Electro-Fenton Processes for Disinfecting Water

As mentioned previously, the bio-electrochemical technology-assisted advanced oxidation reactions (that is the bio-electro-Fenton system) have found an ideal position where they may be controlling in the next years, particularly for reducing recalcitrant organic contaminants. Contrasted to the classical electro-Fenton techniques, the bio-electro-Fenton setup hugely diminished the expenditures on wastewater treatment in terms of electric energy consumption and running prices. The bio-electro-Fenton setup is beginning to be a flexible technique providing a novel solution for emerging environmental problems linked with wastewater treatment. Li et al. [83] deeply examined the present publications concerning the decomposition of the recalcitrant organic contaminants in the bio-electro-Fenton device, particularly focusing on the treatment efficiency related to reactor design and major working factors.

As bio-electro-Fenton has been widely examined for eliminating recalcitrant organics, its implementation capacity vis-a-vis disinfecting water (that is killing pathogens) staysnot familiar. Zhou et al. [132] tried the demobilization of Escherichia coli [133] - [139] in a microbial electrolysis cell-based bio-electro-Fenton setup (recalled as a microbial electrolytic-Fenton cell) with the objective to widen the utilization of microbial electrochemistry. They observed that a 4-log lowering of E. coli (107 to hundreds CFU/mL) was attained with an external applied voltage of 0.2 V, 0.3 mM Fe2+ and cathodic pH of 3.0 (Figure 5). Nevertheless, unimportant demobilization was detected in the control tests

Figure 4. The Scanning electron microscopy (SEM) of unmodified (a), without electrochemically exfoliated grapheme (EEGr); (b) and EEGr; (c) modified graphite felts; N2 adsorption/desorption isotherms and the pore size distribution of three different graphite felts (unmodified, without EEGr and EEGr) (d) [123].

Figure 5. Schematic illustration of disinfection in Bio-electro-Fenton System [132].

without external voltage or Fe2+ injection. The killing microorganisms’ impact was improved when the cathode airflow rate augmented from 7 to 41 mL/min and was also proportional to the elevation of Fe2+ level from 0.15 to 0.45 mmol/mL. Lethal cell film demolition via ŸOH was described as one probable route for disinfection (Figure 6). Zhou et al. [132] successfully established the likelihood of the bio-electro-Fenton technique for killing pathogens inactivation that provides

Figure 6. SEM images of E. coli (a) (b) untreated, (c) (d) after bio-electro-Fenton treatment for 60 min (arrows indicated that the collapses in cellular structure) [132].

comprehension for the coming expansion of possible, performant, and cost-effective biological [140] [141] water treatment technology.

6. Fenton Technologies: Hybridization as Main Future Trend

Despite the progress registered in the Fenton technologies, the classical process, and its ameliorated versions, adsorption and/or membranes processes [142] [143] [144] [145] remain fundamental for secure wastewater treatment.

In this context, Rogers et al. [146] established the chance of a toilet setup that recycles blackwater for onsite reuse as flush water, in which the blackwater is electrochemically treated to eliminate pathogens due to fecal pollution. Nevertheless, they found that this electrochemical technique needs great energy (48 - 93 kJ/L) to obtain total disinfection of the process liquid, and the disinfected liquid retains color and chemical oxygen demand (COD). Granular activated carbon (GAC) efficaciously diminishes COD in concentrated wastewaters. Rogers et al. [146] supposed that decreasing COD with GAC before the electrochemical application would enhance disinfection energy. They expanded and tried a hybrid system that integrates these techniques and proved its capacity to attain complete disinfection with elevated energy efficiency and liquid quality more appropriate for onsite reuse and/or discharge. Their hybrid setup combining both the pre- and post-treatment GAC filters with electrochemical treatment conducted to a considerable decrease in the steady-state levels of numerouspollutants contrasted to the identical device without GAC (Figure 7). Mostimportantly, 1) the COD of the process liquid was diminished by 69%, and 2) the appearance of the water was hugely enhanced proved by the decreases in turbidity and color. The energy needed for total disinfection was diminished by 71% to 20 kJ/L with the addition of both pre- and post-treatment GAC filters. This energy decrease is probably attributed to the diminution in specifically soluble COD [146].

Lei et al. [147] suggested an electrochemical dynamic membrane filtration (EDMF) device for concomitant solid-liquid separation (also preserving electrodes against fouling) and sewage disinfection (Figure 8). At a low voltage of 2.5 V, efficacious disinfection was obtained in the EDMF, with ~100% log elimination performance (no detectable bacteria in the effluent). They proved that the EDMF setup, run at membrane flux of 100 L/(m2∙h), may keep long-lasting bacterial disinfection performance of real wastewater (~100% log removal) in continuous flow trials (Figure 9). Transmembrane pressure (TMP) augmented from 0.8 kPa to 22 kPa during 80 d (one operation cycle), and cleaning of EDMF could efficiently reestablish TMP and biocidal actions for next filtration cycles. On the other hand, without dynamic membrane, the disinfection performance was reduced from the initial ~100% log elimination (with no detectable live bacteria) to ~44.4% log decrease during 7 d. Reactive oxygen species (ROS)-mediated oxidation was responsible for killing microorganisms in the EDMF, and ŸOH and H2O2 formed in this device had a key contribution, occasioning deterioration to cell envelopes and K+ escape from the cytosol. Further, catalase and superoxide dismutase for intracellular ROS attenuation was inhibited, leading to the augmentation of intracellular oxidative stress and therefore high-efficient disinfection. Such findings underline the capacity of EDMF configuration to be

Figure 7. Experimental systems. (A) GAC packed bed column filter lab setup. (B) Schematic of GAC packed bed column filters integrated into the liquid treatment system [146].

Figure 8. Schematic of the prepared EDMF module [147].

Figure 9. SEM images of E. coli after electrochemical treatment [147].

applied for treating and disinfecting wastewater.

Anfruns-Estrada et al. [148] suggested a sequential electrocoagulation (EC)/EF treatment (30 min each) as a more performant for a merged depollution and disinfection of urban wastewater.

7. Conclusions

From this work, the following conclusions can be drawn:

1) Restrictions like the need for acidic circumstance, sludge formation, and elevated chemical inputs have continued to retard the implementations of homogeneous Fenton in dealing with recalcitrant wastewater. Resolving such drawbacks stays, consequently, one of the significant research subjects in the domain of advanced oxidation processes for treating wastewater. This is obvious in the augmenting number of reports on procedures like heterogeneous Fenton oxidation, bio-electro-Fenton, fluidized-bed Fenton process, and homogeneous Fenton at circumneutral pH employing chelating chemicals. Between such procedures, heterogeneous Fenton remains the most largely tested thanks to the development in catalysis and material science. Fluidized-bed Fenton possesses the capacity to diminish sludge production and improve technique efficiency. A homogeneous Fenton process may be realized at circumneutral pH employing chelating chemicals. Nevertheless, because of numerous problems related to the injection of chelating products, homogeneous Fenton at circumneutral pH has not attracted as much interest. Electro-Fenton and bio-electro-Fenton techniques have the potential to diminish the price linked with the external introduction of H2O2 and decrease sludge formation. Heterogeneous Fenton and fluidized-bed Fenton methods seem to be the most realizable strategies to deal with the drawbacks of the Fenton reaction. On the other hand, features like price, ecological consequences, and upscaling possibility will decide their large-scale usability. This work has briefly discussed the fresh progress in dealing with the restrictions of the Fenton process. It will, consequently, besides the excellent review of Bello et al. [1], serve as a reference document in the domain of Fenton reaction for recalcitrant wastewater treatment, especially for specialists looking for improving the effectiveness of Fenton response.

2) The microbial electrolytic-Fenton cell was established as an encouraging substitutional option for disinfecting and treating water [132]. H2O2 was in situ formed in the cathode and reacted with Fe2+ chemicals to excite the Fenton response. Device efficiency was touched by the applied voltage, cathodic aeration rate and the levels of Fe2+ injection. Batch trials divulged that the applied voltage of 0.2 V was seen as more appropriate for total demobilization. Further, disinfection impacts were proportional to Fe2+ potion and aeration rate; however, following economic considerations, 0.3 mmol (Fe2+)/mL and 29.8 mL/min aeration were adopted as optimal. As a prototype pathogenic microorganisms, E. coli hugely demobilized via the Fenton technology during which the bacteria cells were badly damaged through hydroxyl radical’s aggression.

3) Despite the progress registered in the Fenton technologies, the classical process, and its ameliorated versions, membranes processes remain fundamental for secure wastewater treatment. As sure barriers towards pollution dispersal, processes such as nanofiltration should be coupled to Fenton techniques.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.


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