Paraben Compounds—Part II: An Overview of Advanced Oxidation Processes for Their Degradation

: Water scarcity represents a problem for billions of people and is expected to get worse in the future. To guarantee people’s water needs, the use of “ﬁrst-hand water” or the reuse of wastewater must be done. Wastewater treatment and reuse are favorable for this purpose, since ﬁrst-hand water is scarce and the economic needs for the exploration of this type of water are increasing. In wastewater treatment, it is important to remove contaminants of emerging concern, as well as pathogenic agents. Parabens are used in daily products as preservatives and are detected in different water sources. These compounds are related to different human health problems due to their endocrine-disrupting behavior, as well as several problems in animals. Thus, their removal from water streams is essential to achieve safe reusable water. Advanced Oxidation Processes (AOPs) are considered very promising technologies for wastewater treatment and can be used as alternatives or as complements of the conventional wastewater treatments that are inefﬁcient in the removal of such contaminants. Different AOP technologies such as ozonation, catalytic ozonation, photocatalytic ozonation, Fenton’s, and photocatalysis, among others, have already been used for parabens abatement. This manuscript critically overviews several AOP technologies used in parabens abatement. These treatments were evaluated in terms of ecotoxicological assessment since the resulting by-products of parabens abatement can be more toxic than the parent compounds. The economic aspect was also analyzed to evaluate and compare the considered technologies.


Introduction
Water scarcity is experienced by 1.1 billion people and about 2.7 billion people suffer from lack of water at least once a year [1]. It is expected that in 2025, about 1.8 billion people will be living in areas suffering from this problem [2].
To guarantee sustainability and all the needs for all forms of life, particularly in the areas affected by water scarcity, it is mandatory to ensure correct treatment and reuse of wastewater [3]. Effective wastewater treatment is not easy to obtain with the traditional treatments installed in the municipal wastewater treatment plants (WWTP) [4]. The exploitation of "first-hand water" could be a solution to overcome water scarcity, but this is expensive and such water is also scarce. In addition, the recovery of wastewater can be more economically viable than the exploration of "first-hand water"; even considering the economic aspects, the treatment of wastewater can present advantages [3].
Treated wastewater is mostly used in agricultural applications [2], since this activity requires high water consumption [3]. Additionally, the irrigation systems that are used in agriculture could lead to environmental, soil, and water pollution problems and can even present health risks due to the existence of contaminants and pathogenic microorganisms in treated water [2,3].
regulations. The main disadvantage of the use of these technologies is their expensive costs since they demand a significant amount of energy or reagents [41]. Moreover, in general, when real wastewater is considered, the AOP treatments can present efficiency problems, since the presence of carbonate or bicarbonate ions (or other scavenging compounds) can act as scavengers of hydroxyl radicals, which can negatively affect their reaction mechanism and decrease the pollutants removal [42,45].
Hydroxyl radical is the most common radical produced by AOPs and presents a high oxidative power and non-selective behavior. Furthermore, the generation of other oxidant species can occur during the use of AOPs, due to the existence of specific operational conditions. Examples of oxidizing radicals can be seen in Table 1 [42,46]. Other radicals such as ozonide (O 3− ), superoxide (O 2− ), or hydroperoxyl radical (HO 2− ) can also appear [40,47]. The radical's generation occurs in situ and can occur due to the existence of reagents, catalysts, or radiation [41].
The group of AOPs is composed of several technologies, such as ozonation, photocatalysis, Fenton, wet air oxidation, radiation, oxidation by reagents, electrochemical oxidation, anodic oxidation, sonochemical methods, and others [41,46]. The combination and integration of several single techniques is also part of AOP, like photo-Fenton, photocatalytic ozonation, sono-Fenton, electro-Fenton, sono-photocatalysis, photo-ozonation, catalytic ozonation, and peroxone, among others [41][42][43]. Figure 1 presents a scheme summarizing some of the AOPs technologies reported in the literature.
Several studies are being carried out with the objective of lowering the operational costs, using new reactors [41], catalysts, reagents, and even radiation, particularly the use of solar radiation for being free, infinite, and considered as a "green-energy" [44].

Photolysis and Photocatalysis
Photolysis and photocatalysis are two technologies that use radiation for radicals generation, and they differ since photocatalysis also has a catalyst present, to increase the radical's formation rate.
Photolysis can be classified as direct or indirect photolysis, and it usually uses UV radiation from 200-400 nm. Direct photolysis allows the degradation of pollutants due to the photons generated by the irradiation source, and indirect photolysis (known also as photochemical degradation) causes the conversion of pollutants by the presence of radiation and different reagents that enhance the hydroxyl radicals' production [48]. Nowadays, UV radiation is already used for disinfection (inactivation of pathogenic organisms) purposes [49]. Photocatalysis requires radiation to activate a catalyst, generally a heterogeneous semiconductor, and only this activation allows the radical's generation and the pollutants' elimination [50]. To activate the catalyst, the radiation source must emit photons with equal or higher energy than the catalyst's bandgap, which are absorbed into the catalyst and cause the excitation and migration of electrons from the valence band (VB) to the conduction band (CB). This electron migration generates electron-hole pairs (e − /h + ), which will interact with the pollutants present in the reactional medium or in the catalyst surface [44,47,51]. This interaction causes the generation of radicals, which react with the pollutants and promote their oxidation.
In photocatalysis, several catalysts, like TiO 2 , ZnO, ZnS, or CdS, among others, have already been used [44,50]. Titanium dioxide (TiO 2 ) is one of the most common catalysts, due to its low cost, high photo-activity, and chemical stability [52,53]. This catalyst can present three different crystallite phases: Anatase (the most wanted phase, since it is more stable at low temperatures and more photo-active), Rutile (more stable at high temperatures with lower band-gap energy), and Brookite phase (does not have catalytic properties and is rare) [54][55][56].
The costs related to photolysis or photocatalysis can be a drawback for its application, since the artificial UV light is expensive [57], and some semiconductor materials, like the TiO 2 P25, requires this type of light, since this catalyst presents high band-gap energy, which can only be activated by UV [57]. Furthermore, the UV radiation represents only 3-5% of the solar spectrum which can make the use of solar light inefficient, despite the fact that it ensures new advantages as mentioned before [57][58][59]. The solar spectrum is mainly constituted by visible radiation (about 45%) [44], so the use of catalysts that can be activated by this type of radiation can reduce the operational costs of photocatalysis.
One possibility to decrease the catalyst's band-gap energy is doping the material with metals or non-metals elements due to the introduction of oxygen vacancies or impurity energy levels [59], although this makes the catalyst production more expensive. Cationic metals (such as Fe, Ag, Cu, Pd, Pt, and Au) can create an intermediate band between CB and VB, which will decrease the band-gap energy [44,60]. Non-metal doping (as C, N, F, S, . . . ) allows the use of solar radiation and can promote better electron trapping [44,61].
However, the use of H 2 O 2 in peroxone must be careful, since this compound can act as a scavenger of hydroxyl radical and decrease its generation, which could lead to a worse technology performance than single ozonation [41]. Regarding this process, some researchers have considered the optimum H 2 O 2 /O 3 ratio to be 0.5 mol/mol [49]. The use of UV with peroxone can increase the pollutants removal, due to higher generation of OH radicals, as presented in Equation (4) [42].
Catalytic ozonation combines the power of ozone with a catalyst. The presence of catalysts can initiate and enhance the decomposition of ozone molecules into oxidizing radicals, can allow a better absorption of ozone into the liquid phase [41], and, in general, can lead to lower ozone consumption, when compared to single ozonation [49].
Photocatalytic ozonation has a synergetic effect, since it combines the power of photocatalysis with ozone, allowing a higher production of radicals [64]. In this technology, the ozone molecule will react with the catalyst conduction band forming ozonide radicals (·O 3 − ), which will posteriorly generate ·OH radicals [65]. Other radicals such as oxygen atom radicals (·O) or superoxide radicals (·O 2 − ) can also be produced as intermediate radicals, interacting directly with the pollutants or forming hydroxyl radicals [66]. This technology can also prevent the recombination of generated electron-hole pairs [67].

Fenton's Reagent
The traditional Fenton's reaction uses H 2 O 2 and Fe 2+ as a catalyst for ·OH radical generation. During this process, Fe 2+ is oxidized to Fe 3+ and regenerated to Fe 2+ , through the reaction of H 2 O 2 and Fe 3+ [40]. The decomposition of H 2 O 2 is catalyzed by Fe 2+ , forming the oxidative radicals that are going to attack the pollutants. However, the use of Fe 2+ must be done with care, since it can react with the generated radicals, producing Fe 3+ , and work as a scavenger of the reaction itself [46]. Furthermore, in Fenton's technology, the iron can be used in homogeneous or heterogeneous form [48]. Equations (5) and (6) present Fenton's process mechanism [40,41]: The Fenton's oxidation rate is higher at pH = 3 since the hydroxyl radicals are more present at acidic pH, allowing a fast reaction [48]. This technology is effective but can present negative aspects related to the operational costs, due to the reagent's consumption and pH adjustments [46]. As disadvantages, it also presents difficult recovery and reuse of iron and limited operational pH range, which can restrict Fenton's applications and decrease its efficiency [68,69].
Moreover, the use of H 2 O 2 can present negative effects. In low amounts, it can decrease the efficiency of pollutants degradation due to insufficient radical's production, and in excessive quantities, it can form less reactive radicals, such as HO 2 ·, and decompose itself into water and oxygen, reducing the technology performance, as presented in Equations (7)-(11) [62,70].
The advantages of Fenton's reaction are the use of environmentally friendly reagents, and the fact that the technology is easy to operate and implement, is efficient, and does not need energy to initiate the reaction. However, this technology also allows a quicker consumption of Fe 2+ than its regeneration, has a limited operation pH range, and the complexation of iron species and the production of secondary sludge generally occur [71].
The use of photo-Fenton (UV and Fenton) could lead to lower reagent costs since the generation of radicals is enhanced, the reaction time is lower, and it requires less reagent compared to single Fenton's [48]. Photo-Fenton reactions are presented in Equations (12) and (13) [41]: This technology also leads to a lower generation of secondary iron sludge, and to a faster iron regeneration [71]. The use of heterogeneous catalysts in Fenton's technology allows the occurrence of this reaction at higher pH [46].
Other methodologies can be used to overtake the traditional Fenton's drawbacks. The use of other metals (such as Cu, Mn, Cr, Ce, Co, Ru, or Al) instead of Iron (II) can be an option, and this substitution is widely known as Fenton-like processes [68,69,72].
Equations (14)- (20) present the Fenton's mechanism using Cu 2+ and Mn 2+ [69,72]: Mn 3+ + e − -> Mn 2+ (20) Catalyst regeneration also occurs when Cu 2+ and Mn 2+ are used, due to the interaction with H 2 O 2 . The use of copper allows the existence of neutral and alkaline pH operations [69], which can be an advantage over the traditional Fenton's since this pH range would cause the precipitation of Fe 3+ (in Fe(OH) 3 form) in the conventional Fenton [40]. Moreover, an increase in hydroxyl radical's generation can occur when magnetite substituents with transition metals are used [72].
However, Fenton-like processes also present disadvantages such as the existence of cytotoxic characteristics of metals, complex mechanisms, related high-costs, difficult anaerobic conditions, pH limitations, metal leaching, and loss of catalyst efficiency, depending on the type of metal used [72]. In general, metals do not allow good catalyst reuse and their reuse is only possible for a few cycles [72].

Persulfate, Peroxymonosulfate, and Hydrogen Peroxide
Oxidizing agents can be used in AOP, such as the persulfate anion (S 2 O 8 2− ), hydrogen peroxide (H 2 O 2 ), or peroxymonosulfate anion (HSO 5 − ), which are the most used oxidizing agents in wastewater treatment systems [73]. The objective is the same as other technologies, that is, the generation of hydroxyl and/or other radicals for the elimination of contaminants [42].
The ersulfate anion is stable at ambient temperatures, is selective, and is a strong oxidant, which can work in a wide pH range, but it is cost-effective [74]. Its high solubility and longer lifetime allow it to decompose several organic pollutants [42].
Persulfate can react in an acidic medium, enhancing the production of sulfate radicals as presented in Equation (22) Usually, the performance of persulfate alone is low, so to achieve good efficiency, the presence of catalyst, heat, or radiation is desired [42]. The presence of catalyst enhances the sulfate radical's production. The use of Fe 2+ is an option, since this metal can activate the PS for contaminants degradation, but this is not so efficient, probably due to the generation and accumulation of Fe 3+ [42].
Peroxymonosulfate (PMS) is also a strong oxidant. It can be used in a wide pH range, and in the presence of light, heat, or catalysts, it is activated and can generate hydroxyl and sulfate radicals as presented in Equations (23) and (24), and in Equation (25) when exposed in an acidic medium [42], which may represent an advantage compared to the persulfate reaction.
Additionally, the sulfate radicals can be decomposed into more hydroxyl radicals in basic pH conditions, as shown in Equations (26) and (27) [42]. Furthermore, the radical's production is favorable at basic pH regarding PS and PMS reactions, since this pH allows the formation of hydroxyl radicals, which are more reactive than the sulfate radicals formed in acidic media [42].
Hydrogen peroxide is a reagent with relatively high oxidative power that is used for radical's generation but may be inefficient when used alone since it cannot produce hydroxyl radicals by itself. It can also decompose into water and oxygen (as mentioned in Equations (28)-(30)), showing low performance in the removal of recalcitrant and refractory compounds [42].
2 H2O2 -> 2 H2O + O2 (30) The radicals generated by hydrogen peroxide are less reactive than ·OH radicals [45], and its oxidation potential is higher at low pH, so this reagent is favored when used in an acidic medium, and the combination of H 2 O 2 with other reagents or radiation can enhance its efficiency [42]. However, the presence of hydrogen peroxide must be carefully dosed, since high and low amounts can negatively affect its performance, as explained in Section 2.3. The optimum ratio of H 2 O 2 needs to be determined experimentally, and it is dependent on the experimental conditions [42,45,62] and effluent characteristics.
This reagent can only be used as an AOP technology when it is in the presence of radiation (UV, solar, etc.) or catalyst [42], and its good characteristics include good market availability, good thermal stability, infinite solubility in water, its ability to be stored in the operational site, and lack of mass transfer problems associated with gases [42,45].
The use of UV with H 2 O 2 reagent leads to the generation of two hydroxyl radicals (Equation (31)) which should enhance the removal performance [35,75,76] compared to a single H 2 O 2 reagent that cannot produce these types of radicals. Furthermore, organic and inorganic peroxyl radicals can be generated using H 2 O 2 /UV in the presence of oxygen and other organic compounds [45]. However, this technology also shows disadvantages, since low-pressure UV lamps require high concentrations of H 2 O 2 for good OH generation, but the existence of H 2 O 2 at high concentrations can lead to a scavenging effect of hydroxyl radicals [49]. Therefore, the use of H 2 O 2 with UV radiation may not be recommended since it can have a high scavenging effect in secondary and tertiary wastewater treatments [49].
At alkaline pH, several studies report an interesting behavior of H 2 O 2 reagent, even when it is used alone. In fact, it has already obtained degradation efficiencies higher than 90% in 60 min regarding dibutylsulfide (DBS) degradation [77], higher DBS disappearance rate for single H 2 O 2 when compared to single ozone or O 3 /H 2 O 2 system for pH < 12 [77], and about 52% of 4-chloro-2 nitrophenol degradation at pH = 10 [78], which showed better results than at pH < 10, with UV use, or in Fenton's (from pH = 6 until pH = 10) [78].

Electrochemical Oxidation
Electrochemical oxidation (EO) is the most used electrochemical technology for wastewater treatment [82]. In EO, an electric charge is produced between two electrodes (solid conductors) and is discharged in an electrolyte solution used as a medium [46]. One of the electrodes is positively charged (anode), while the other is negatively charged (cathode), and the oxidation of pollutants occurs in the anode, while the reduction occurs in the cathode [46]. This technology has the advantages of requiring less space, needing lower temperatures in the operation, and having a low by-product generation rate [83].
The organic compounds oxidation can occur by direct anodic oxidation (classified as "heterogeneous" when it occurs the direct electron transfer to the anode or classified as "homogeneous" when the oxidants are produced on anode surface from components of the electrolyte solution) or by chemical reaction with electrogenerated species (·OH radicals or other oxidizing species). The first one leads to weak removal, while the second leads to high-performance removal [41,82,84].
The ·OH radicals generation starts with the oxidation of water in the anode, and only occurs at 2 V or above. Below this value, only direct oxidation can happen [46]. The indirect oxidation can also occur by the interaction of the pollutants with strong oxidants (such as chlorine), which must be generated during electrolysis by Cl − -containing species present in the water matrix and/or in the electrolyte solution [41,82,85].
The occurrence of oxidative species is dependent on the anode oxygen-evolutionoverpotential, in which the anodes can be classified as "active", presenting low oxygenoverpotential, or as "non-active", showing high oxygen-overpotential [41]. The "nonactive" anodes produce hydroxyl radicals adsorbed on their surface as presented in Equation (32), allowing the direct oxidation of the pollutants with M(·OH), while in the "active" anodes, the M(·OH) (M is the anode material) is chemisorbed and transformed into superoxide or a higher oxide (MO) as presented in Equation (33) [41,82].
A "non-active" anode can only act as an inert substrate and a sink for electron removal, and, consequently, it cannot participate in direct anodic oxidation or provide catalytic active sites for the adsorption of organic molecules from the reactional medium [82].
The BDD electrode presents the advantage of high stability even in strongly acidic media, low adsorption properties, inert surface, and high O 2 evolution overvoltage, being the most preferable anode of electrochemical oxidation. The most responsible pathway for pollutants degradation using this electrode is by OH generation, and the BDD oxidation power is higher than for other anodes [49,82,87]. However, this material also presents disadvantages such as high cost and electrode instability during operation, and it is not easy to obtain BDD with large dimensions [88]. For wastewater treatment, only high voltages are used, since the use of low voltages can cause anode poisoning, because the direct anodic oxidation promotes by-products formation, which can be adsorbed into the anode [82].
The advantages of the electrochemical AOP are that the removal of the pollutants and the mineralization occurs using the OH radical in situ. However, as disadvantages, in general, these technologies are associated with considerable electric costs, they need specific pH to obtain optimal conditions, the polluted solution must have good conductivity or it will need electrolytes to be added, and the catalyst is not usually recyclable or reused [89]. Other technologies are also part of the group of electrochemical AOPs, such as electrocoagulation, electrochemical reduction, electro-Fenton, electro-PS, electro-PMS, and electro-peroxone, among others, as well as the integration of radiation with these processes [41,89,90].

Sonochemical Methods
Sonication or sonochemical methods, also known as sonolysis when it is used as a single technology, is an AOP technology where the ·OH radicals are generated from water pyrolysis due to the high intensity of acoustic cavity bubbles [40]. This technology, a type of hydrodynamic cavitation, is a cavitation method (i.e., a process involving the formation of micro-bubbles or cavities and quick collapse of bubbles to release energy), but until now, only ultrasound is used at the industrial level [46]. This process uses ultrasound frequencies (16 kHz-100 MHz) to produce a great amount of energy and promote substrate oxidation due to the formation and collapse of microbubbles from acoustical-wave-induced compression and rarefaction [46,49]. A violent implosion occurs after these microbubbles reach a critical resonance size, releasing high pressure and temperatures (>5000 K and >1000 atm), leading to highly reactive radical generation, due to the dissociation of water molecules into hydroxyl ions and hydroxyl radicals [46,49]. Furthermore, these radicals can react with the substrate, forming substrate radicals [46]. The oxidation of pollutants mainly occurs due to the action of the generated radicals but can also happen by thermal decomposition [49]. However, in this case, the chemical structure of the pollutant must also be considered.
Equations ( This technology presents as advantages a low interference from water matrix, low heat transfer required, and not needing chemicals. However, it has the disadvantage of using high amounts of energy ad having low efficiency and mineralization, so this technology must be combined with other AOPs to overcome these problems, benefiting also from synergistic effects [40,46,49]. Therefore, other ultrasounds-based technologies such as sonophotolysis (UV + ultrasounds), sonocatalysis (catalyst + ultrasounds), sonophotocatalysis (UV + catalyst + ultrasounds), or even the presence of oxidation reagents with ultrasounds can present advantages compared to the single sonolysis [49].

AOP for Parabens Removal
After considering the characteristics of several AOPs technologies, this section overviews their application for paraben's abatement.

Photolysis and Photocatalysis
The use of radiation is known to increase the oxidizing radical's generation, but it is also needed when a semi-conductor catalyst is present, since it requires energy to be activated (i.e., to overpass the catalyst's bandgap). Although these technologies have disadvantages related to the application of UV radiation, the results of the reviewed studies show in general a good performance regarding the parabens abatement, with photocatalysis showing better efficiency. Furthermore, overall, the parabens adsorption or their removal by photolysis presents poor results, although some experiments using photolysis have led to interesting removal outcomes. Some authors already report studies using solar radiation, although most studies use solar simulators.
In the literature, there are some studies using photolysis and particularly photocatalysis with different catalysts for parabens removal.

Parabens Abatement by Photolysis
Photolysis only uses radiation for contaminants degradation. UVA, UVC, and solar light (natural and simulated) have already been used in parabens abatement.
Regarding the parabens abatement, the photolysis performed by Gomes et al. [91] showed negligible results (lower than 4%), while the use of catalyst and radiation led to better results. Lin et al. [7] removed MP using photolysis with UV radiation, achieving less than 5% after 120 min in a pilot plant, while Lin et al. [32] removed 10% of BeP in a laboratory installation.
Gmurek et al. [35] used photolysis with UVC radiation, achieving 10% removal for MP, EP, PP, and BuP and about 60% degradation for BeP in 120 min for individual parabens and about 20% for MP and BuP, 30% for EP and PP in 120 min, and 80% for BuP in 20 min, when the parabens mixture was considered. Complete mixture removal was achieved at 480 min of UVC photolysis.
In general, several studies such as those of Gomes et al. [58], Lin et al. [7], Lin et al. [32], and Gomes et al. [91] present low performance when UVA photolysis was used in parabens abatement. Solar light studies by Gomes et al. [58] led to 16% of removal of MP, EP, and PP in 60 min, and the study by Vela et al. [3] led to 30% and 35% of MP and EP removal in 240 min using real wastewater effluent as water matrix, suggesting an interesting behavior of solar light by itself, although the study by Zúñiga-Benitez et al. [92] led to poor EP abatement.
The study by Gomes et al. [93] led to almost complete removal of PP and BuP using UV-Vis radiation, and the study by Álvarez et al. [94] achieved 80-99.8% of single degradation for MP, EP, or BuP in 120 min using UV (254 nm) radiation. Both studies performed good results regarding the parabens degradation, but the study by Gmurek et al. [35] using UVC radiation only led to about 20-30% mixture degradation for MP, EP, PP, and BuP removal in 120 min (BeP was 80% in 20 min), and the total removal was only achieved after 480 min.
In fact, Gmurek et al. [35] used a higher concentration of parabens (10 mg/L of MP, EP, PP, BuP, and BeP each and 10 mg/L of p-HBA), while Gomes et al. [93] only applied 10 mg/L of PP and BuP each (20 mg/L total). Moreover, Álvarez et al. [94] only used about 45.65 mg/L of MP, 49.85 mg/L of EP, and 58.29 mg/L of BuP. This should be responsible for the differences in the results, and, the presence of more compounds may make the degradation of each compound difficult.
In general, it seems that UVC must be preferred to UVA light when used alone to achieve better results. However, this implies higher energy costs, besides the danger associated with such strongly energetic radiation. Table 2 presents several studies' results using photolysis in parabens abatement.

Parabens Abatement by Photocatalysis
The presence of catalysts should improve the degradation of parabens, particularly when UVA radiation is used since it seems that this kind of light poorly removes parabens when only light is applied. With this, it is expected to obtain better results when UVA photocatalysis is used.
Gomes et al. [91] used doped TiO 2 catalysts and pure TiO 2 to remove methylparaben (MP), ethylparaben (EP), propylparaben (PP), butylparaben (BuP), and benzylparaben (BeP) by UVA photocatalysis, reaching parabens degradation values higher than 15%. In fact, this was higher than the results obtained by the photolysis of the same mixture, which proves their good catalyst effect.
Vela et al. [3] used TiO 2 catalysts (P25 and Vlp-7000) for the degradation of six CECs, including MP and EP. Results showed that P25 is the most efficient catalyst, with degradations higher than 65%, and for which has the best dose of 200 mg/L and whose pH efficiency is in the order 7 > 5.5 > 8.5. Moreover, the experiments using a pilot installation led to good results regarding the pollutants removal as well. Better degradation was achieved in the presence of catalyst as well.
Comparing these results to the photolysis results presented in Section 3.1.1, the presence of catalyst seems to show an obvious improvement in parabens abatement. The studies of Gomes et al. [91], Lin et al. [7], and Lin et al. [32] used UVA radiation, while Vela et al. [3] applied sunlight instead. The presence of UVA light (also present in the solar spectrum) is important, since this radiation is the only one able to activate the TiO 2 catalyst due to the high bandgap that is associated with this material. In fact, if its bandgap is reduced, then the more efficient use of visible radiation or solar light should be possible, which could decrease the operational costs of this technology. With this, several studies using doped TiO 2 catalyst or also other catalysts are also discussed.
Zúñiga-Benítez and Peñuela [92] used TiO 2 catalyst to remove EP through photocatalysis, using simulated solar radiation in laboratory experiments and sunlight for experiments at pilot scale. The use of catalyst under lab conditions led to 63.1% removal in 120 min reaction, with the application of 0.95 g/L of catalyst and initial EP concentration of 1 mg/L. Regarding the pilot-scale experiment, 45.88% degradation was achieved after 6 h, using the same experimental conditions, but a higher volume (100 L). The authors also used hydrogen peroxide with TiO 2 , resulting in a better EP removal performance using the best operational conditions, achieving 96.9% removal in 2 h and 81.38% in 6 h, for laboratory and pilot-scale experiments, respectively. Frontistis et al. [96] achieved better results, reaching almost total removal in 20 min using ZnO, while TiO 2 P25 led to about 90% abatement in 30 min, using 1 mg/L of EP and 0.25 g/L of catalyst with a solar simulator as a radiation source. Although the experiments present several differences, these studies led to interesting results using solar radiation or solar simulated radiation, which should encourage more studies using these catalysts with the sun as a source of light. Furthermore, the study of Frontistis et al. [96] suggests a better performance of ZnO over the traditional titanium dioxide.
Several authors such as Gomes et al. [91], Kotzamanidi et al. [34], and Petala et al. [97] experimented with doped TiO 2 catalysts in parabens abatement. Kotzamanidi et al. [34] tested Al-TiO 2 , which had the best PP removal using a catalyst dose of 1 g/L with 95% abatement in 90 min, while Petala et al. [97] used N-TiO 2 getting EP abatement of 32% and 13% in 240 min for N-TiO 2 and TiO 2 with Vis radiation, while solar radiation was used for 10 min and achieved about 34% and 43% for TiO 2 and N-TiO 2 , respectively, for the best calcination temperature. In fact, the use of N-TiO 2 seems to enhance the parabens degradation under visible or solar radiation compared to pure TiO 2 . Although the experiment by Kotzamanidi et al. [34] achieved almost complete removal, the authors did not perform experiments with pure TiO 2 , and say that the presence of Al did not considerably change the band-gap energy. Moreover, for the best Al-TiO 2 (0.04 Al w%), the band-gap energy was about 3 eV, while the common TiO 2 catalyst usually has about 3.2 eV. This may suggest that Al it is not advantageous to make titanium dioxide more efficient under Vis or solar radiation.
Kotzamanidi et al. [34] used 1 g/L of catalyst and initial PP dose of 0.42 mg/L, and Petala et al. [97] used 0.5 g/L of catalyst and 0.3 mg/L of EP, while Gomes et al. [91] used 0.07 g/L of catalyst, 10 mg/L of each paraben (MP, EP, PP, BuP, and BeP), and UVA radiation. In Gomes et al.'s [91] study, they used a mixture with higher parabens concentrations and had lower catalyst concentration, but obtained removals of about 35-70%, 40-48%, 25-40%, 5-15%, and 1-10% for Pd, Ag, Pt, Au, and pure TiO 2 , respectively, after 180 min. In this work, the presence of metals enhanced the degradation when compared to bare TiO 2 , and Pd and Ag catalysts had better performance than the one obtained by Petala et al. [97] using Vis radiation. In fact, the use of these noble metals seems to enhance the photocatalytic activity of titanium dioxide, which the authors suggest may be related to the electron charge transfer from the CB to the metallic nanoparticles band, less electron-hole recombination, or higher hydroxyl radical's production. The authors did not determine the catalysts' band-gap energy, but studies have shown where the deposition of metals could lower the catalyst band-gap energy. However, the presence of noble metals should increase the catalysts' price, since these metals are usually expensive, so the N-TiO 2 may probably be more economically attractive, and it also showed good results under Vis and sunlight. Nevertheless, more tests using different concentrations and/or mixtures should be performed to confirm this theory.
Other catalysts have also been investigated for the same purpose. Fernandes et al. [6] tested single and mixture degradation of three parabens (MP, EP, and PP) using TiO 2 P25 and metal-free, graphite-like carbon nitride as catalysts with 417 nm radiation provided by LEDs. The GNC-500 catalyst had better performance, with about total removal in 20 min for single parabens and 99% removal for parabens mixture in 25 min. TiO 2 P25 only had total removal after 120 min for both experiments. Xiao et al. [33] removed single MP, EP, PP, and BuP by photocatalysis using I x -Bu 4 O 5 Br 2 as a catalyst, leading to a higher PP removal of 94.5% for I 0.7 -Bu 4 O 5 Br 2 in 60 min. The authors used >420 nm radiation, 1 g/L of catalyst, and 10 mg/L initial concentration of each paraben. In fact, this catalyst led to better abatement and had 2.51 eV, although not being the catalyst with lower band-gap energy (I 1 -Bu 4 O 5 Br 2 with 2.41 eV that led to 91.2%). Furthermore, Petala et al. [98] achieved a better EP removal value of 97.6%, using CuO 0.75 /BiVO 4 , by photocatalysis in 60 min, when the catalyst dose was 0.5 g/L and the paraben concentration was 0.5 mg/L, using a solar simulator as a radiation source (>280 nm). Kumar et al. [99] used Fe 3 O 4 /BiVO 4 heterostructure (bandgap of Fe 3 O 4 of 2.57 eV and of BiVO 4 of 2.36 eV) for MP abatement, reaching 75% of degradation in 60 min, with 0.1 g/L of catalyst and 5 mg/L of MP using solar light. These studies suggest interesting results for different catalysts compared to the traditional semiconductors, which may be also an option in the application of photocatalysis. Moreover, some of these catalysts present lower bandgaps than the traditional TiO 2 , which may be mainly responsible for the good results using radiation such as solar, simulated solar, or Vis light. Thus, this can be an advantage for the use of natural light and simplify the industrial implementation powered by the sun.
The use of catalysts in powder form makes photocatalysis implementation and catalyst use difficult at an industrial level. This drawback can be overcome using powder catalyst immobilized in a support material, which can allow a good recovery and easy separation of the solid material from the reactional medium. With this, several supported and innovative catalysts have already been used, even for parabens degradation.
Gomes et al. [58] tested TiO 2 nanotubes (NT) arrays for photocatalytic degradation of paraben's mixture. In general, the catalyst presence improved the pollutants' degradation, but only 20% of removal was achieved in 60 min reaction, with 7 TiO 2 /NT plates and UVA. The maximum degradation was 25% when 14 plates were used. The usage of solar radiation enhanced the parabens mixture degradation compared to UVA, allowing a maximum parabens degradation of 80% using seven plates, although the operational installation was different. Furthermore, Sousa et al. [100] used photocatalysis catalyzed by nanostructures (titanate nanowires (TNW) and modified titanate nanowires with Mn) for MP removal in 90 min. The initial pollutant concentration was 10 mg/L, and the technology removed 62%, 50%, and 40% for Mn-doped TNW, Mn-adsorbed TNW, and single TNW, respectively, using UV-Vis (40-48% UV and 40-43% Vis). The use of these types of structures seems to allow good removal rates, combining also important advantages for industrial application. Gomes et al. [58] had 80% parabens mixture degradation using TiO 2 /NT, Zúñiga-Benítez and Peñuela [92] had 45.88% of EP degradation in 6 h using TiO 2 , Petala et al. [97] determined about 34% and 43% EP degradation for TiO 2 and N-TiO 2 in 10 min and achieved total removal in 60 min with N-TiO 2 catalyst, and Kumar et al. [99] used Fe 3 O 4 /BiVO 4 reaching 75% of MP degradation in 60 min. In all these studies, solar light was used as an energy source. Comparing these studies, the study of Gomes et al. [58] had better results than Zúñiga-Benítez and Peñuela [92] and Kumar et al. [99], and although total degradation was never achieved, contrary to what happened in Petala et al.'s [97] work, it should be noticed that in this study, it was used a parabens mixture, while in the others, only single parabens degradation was evaluated. Thus, the complexity of the reaction mixture highly influences the process efficiency. Table 3 summarizes photocatalysis studies in parabens abatement.

Ozone-Based Technologies
The treatment of parabens has been also investigated with the oxidant power of ozone to ensure the degradation of pollutants. Ozone-based techniques can bring different treatment perspectives and features, since the use of this molecule can allow good performance in the removal of different and persistent pollutants but can also bring problems related to ozone properties (explosive, toxic, and corrosive). Moreover, generally high production costs are necessary, the O 3 generation must be in situ, and there is usually a low efficiency of generation.

Single Ozonation
The ozone molecule is known for being a powerful oxidant with very effective degradation results for organic molecules in wastewater treatment, despite having some drawbacks as poor mineralization. Besides wastewater treatment, this technology is also used for water disinfection.
Tay et al. [25] used ozonation to remove a mixture of five parabens. The authors achieved 99% parabens degradation in 12 min and 61% and 32% for COD and TOC removal in 3 h. As expected, while ozone was able to efficiently remove unsaturated molecules (such as the parabens), total mineralization was not possible to achieve. In fact, from the abatement of the parent molecules, saturated compounds were produced, which are refractory to further ozone oxidation. Furthermore, Gomes et al. [26] performed ozonation using the same mixture but with a concentration of 10 mg/L of each paraben, achieving total removal in 120 min. In fact, the results of ozonation should be analyzed as a function of transferred ozone dose (TOD), since this parameter allows one to understand the amount of ozone transferred to the reactional medium and it represents an important parameter for the economic evaluation using ozone [102].
Cuerda-Correa et al. [23] treated a mixture of four parabens (MP, EP, PP, and BuP), achieving 82% removal in 20 min reaction. Asgari et al. [28] only achieved 47-52% removal for the best results. In fact, these results were obtained at pH = 9 and were higher than those reached for pH = 3 or pH = 5. This should be expected, since, in a basic medium, the generation of radicals is enhanced. This radical pathway is desired due to its higher performance when compared to direct ozone reaction at acidic pH [41]. Fernandes et al. [103] also removed 10 mg/L of parabens mixture, reaching a degradation higher than 97% in 120 min (a natural parabens solution has pH of 5 [104]), and Asgari et al. [28] achieved 36-42% for similar pH in 60 min, which is a concordant result. Furthermore, Gomes et al. [102] had total removal of 1 mg/L of each paraben (in mixture) in 15 min, which represents 10 times less than the concentration used by Asgari et al. [28] and Fernandes et al. [103].
In general, it seems that ozonation is able to eliminate parabens for whichever concentration is used. However, this does not imply that total mineralization was reached. In fact, ozone can react with unsaturated molecules, leading to low-molecular-weight by-products refractory to further ozonation. Thus, the results of the parent compounds degradation should be complemented with some information about COD or TOC removal as well as toxicological studies. This w]could be very important regarding the potential environmental impact of the treated water.
An overview of works dealing with single ozonation for parabens abatement is presented in Table 4.

Catalytic and Photocatalytic Ozonation
The performance of single ozonation can be enhanced using catalysts, radiation, and reagents, leading to higher degradation rates and reducing some disadvantages of this technology. There are not many studies regarding the abatement of parabens by catalytic and photocatalytic ozonation. However, in general, the presence of catalysts and radiation plus catalyst seems to lead to better results.
Cuerda-Correa et al. [23] used photolytic and photocatalytic ozonation with TiO 2 to remove four parabens. The results showed 95% and 100% removal for photocatalytic ozonation and photolytic ozonation in 20 min, respectively. In fact, these results were higher than for single ozonation (82% degradation) in the same study. Furthermore, the same occurred in the study of Asgari et al. [28], who used ZnO as photocatalyst and had 47-52% degradation (pH = 9) in 60 min for single ozonation, while photolytic and photocatalytic ozonation led to 53-65% and 96-98% degradation, respectively. This behavior was also verified by Gomes et al. [26], who studied the degradation of a mixture of five parabens by ozonation and catalytic ozonation. The presence of catalyst required a lower ozone dose compared to single ozonation. Moreover, a treated solution with lower toxicity was achieved by the catalytic process. These results prove a clear improvement of ozonation when coupled with radiation and/or catalyst. However, in the study of Cuerda-Correa et al. [23], the catalyst did not improve the parabens degradation, which can be related to the generation of radicals (OH or ozonide) or even to a negative interaction between the ozone and the catalyst. The same was reported in the study by Gomes et al. [102], in which they used photocatalytic and catalytic ozonation for the degradation of a parabens mixture in different water matrices using TiO 2 /NT and carried out toxicity assessment analysis using Allivibrio fischeri bacteria and Lepidium sativum seeds. The presence of catalyst only decreased the solutions' toxicity since the luminescence inhibition for A. fischeri was lower and the germination index for L. sativum was higher when the ozone was used with the catalyst.
In addition, Gomes et al. [102] used UVA and sun as light sources. When sunlight was used, the objective was quickly achieved, but more ozone was consumed, while the use of UVA seemed to slightly enhance parabens degradation, although the difference was not as significant. Total parabens removal was achieved for TOD = 4.5 mg/L and TOD = 8.5 mg/L for UVA and sunlight, respectively, using an initial concentration of 1 mg/L.
Gomes et al. [26] required TOD = 70 mg/L (Pt-TiO 2 ) for the removal of 10 mg/L of each paraben (MP, EP, PP, BuP, and BeP) using catalytic ozonation, while the study of Gomes et al. [27] needed about 40 mg/L of TOD (Ag-TiO 2 ) using photocatalytic ozonation for the same paraben's mixture and concentration. Both studies reached total removal of parabens. Furthermore, in the study of Gomes et al. [26], about 94 mg/L of TOD were necessary for catalytic ozonation using Ag-TiO 2 , while in Gomes et al.'s [27] study, the presence of UVA led to a lower TOD of 40 mg/L using the same catalyst [27]. Moreover, in Gomes et al.'s [26] experiment, the best catalysts were Pd and Pt-TiO 2 in the catalytic experiments, while in the photocatalytic experiments of Gomes et al. [27], this was not verified, since the best catalysts were Ag and Pt-TiO 2 [27]. The catalyst's performance and behavior can be influenced by the photogenerated electrons and the generation of radicals. In fact, the authors suggest that the Au-TiO 2 had low performance in photocatalytic ozonation due to a higher retention of photogenerated electrons related to its higher electronegativity, which could lower the generation of ozonide radicals, and, therefore, the generation of hydroxyl radicals [27]. Furthermore, Ag-TiO 2 has lower electronegativity, so its good performance could also be related to this fact, and this can explain why the presence of UVA did not have any effect on Au-TiO 2 [27]. In addition, in the study of Gomes et al. [27], TiO 2 also did not have any improvement when UVA was added, which could be related to poor photoactivity of the catalyst due to its high band-gap, while in the other doped catalysts, the results were better than the pure TiO 2 and without UVA, which means that UVA had a positive influence on them, maybe to their lower band-gap and high photoactivity [26,27].
Fernandes et al. [103] had almost total removal using 10 mg/L for different synthesis methods of N-TiO 2 reaching TOD of 30-44 mg/L, and about 26-45 mg/L, for the different methods, using photocatalytic ozonation. These results were better than the ones provided by Gomes et al. [27], who used a catalyst doped with noble metals (best results for Ag-TiO 2 with TOD = 40 mg/L). This may suggest a good behavior of non-metal doping for photocatalytic ozonation, which may be interesting due to the costs associated with noble metals. Table 5 overviews works dealing with ozone-based technologies for parabens removal.

Fenton's Process
There are not many studies regarding the paraben's abatement using Fenton's process. The most conventional Fenton's oxidation uses Fe 2+ and H 2 O 2 for the elimination of organic molecules through the generation of oxidative radicals. The presence of iron allows the continuous generation of hydroxyl radicals that are the most wanted radicals for pollutants removal.
Gmurek et al. [30] used Fenton for the degradation of a mixture of parabens (10 mg/L). In this experiment, the initial pH was 6, and after the addition of Fe 2+ and H 2 O 2 , the pH decreased to 3, which is generally the optimal condition for this reaction, due to a better generation of oxidizing radicals. This reaction led to the complete removal of parabens. Zúñiga-Benítez et al. [105] used Fenton's reagent to remove 3-benzophenone and methylparaben. Several conditions were tested, and the MP removal was between 45.39% and 89.96% after 30 min. Using Fenton's reagent and light, the removal was 98% for the best operational conditions, while the use of Fe 2+ and H 2 O 2 only removed 35%, which shows a clear improvement when UV radiation was applied to the Fenton's process.
Dominguez et al. [106] treated 150 mL mixture with 5 mg/L of each paraben (MP, EP, PP, and BuP). An experimental design was applied in which the initial concentration of the reagents (Fe 2+  In general, the studies suggest a good performance of Fenton's over parabens abatement, since high removal rates were generally reported. Table 6 presents an overview of the use of Fenton's technology regarding parabens removal.

PS, PMS, and H 2 O 2 Oxidation
PS and PMS anions are widely used oxidizing reagents. In fact, these agents are applied as other compounds in their molecular form (such as sodium persulfate (SPS) or potassium peroxymonosulfate), in which their dissociation into the anions and their interaction with light, heat, or catalysts generates sulfate radicals that will promote the pollutant's oxidation. However, the efficiency of persulfate anions alone is not as good, and the presence of other materials is required to enhance the generation of sulfate radicals, so, in general, and as presented in the referred studies, the use of these agents is accompanied by the presence of radiation and/or catalyst. Furthermore, the basic pH is favorable when these anions are used, since at this pH it is possible to form OH radicals that are more reactive than the ·SO 4 2− radicals. Hydrogen peroxide is another widely used reagent for the generation of oxidizing radicals, and, at low or high amounts, its efficiency can be negatively affected due to low pollutants oxidation or scavenging behavior of hydroxyl radicals. This reagent is mainly used in the presence of other materials considered as an AOP technology, and its presence can enhance the technology's efficiency, particularly with the use of UV radiation, which allows the generation of hydroxyl radicals.
Gmurek et al. [35] used hydrogen peroxide for the degradation of a mixture of five parabens and p-HBA. The individual parabens using UVC/H 2 O 2 technology occurred in about 6 min, and the mixture of parabens was totally removed in 30 min.
Dhaka et al. [73] tested ethylparaben conversion using radiation and three different reagents (persulfate, peroxymonosulfate, and hydrogen peroxide). In 90 min, EP removal was about 97%, 98.1%, 81.3% and 36% for UV/H 2 O 2 , UV/PS, UV/PMS, and UVC photolysis, respectively. Furthermore, Dhaka et al. [74] removed about 98.9% and 34% of methylparaben using UV/PS and UVC at the same time. These results are very similar and prove the good performance of persulfate anion when coupled with UVC light. Compared to photolysis results, the existence of oxidizing agent seems to significantly increase the oxidation of the parabens compounds, which may be related to a higher generation of oxidizing radicals. Moreover, the use of UV/PMS was less effective than UV/PS in the study of Dhaka et al. [73], which is not expected, since PMS can generate hydroxyl and sulfate radicals and PS can only produce sulfate radicals [42]. Total removal of PP using UV/PS oxidation was obtained by Ioannidi et al. [107], which used UVA radiation and simulated solar light, but only 12% was achieved using Vis radiation. However, these results suggest a good performance of these technologies regarding the parabens abatement for different radiation sources, which may be an advantage for this purpose.
The presence of a catalyst may also enhance this degradation. Yang et al. [108] removed BuP by peroxymonosulfate oxidation with Mn-Fe oxycarbide catalyst, achieving the highest removal of 99% on 180 min, while Bekris et al. [109] used graphene-based catalysts with PS to remove 1 mg/L of PP, achieving 95% degradation for the best catalyst in 20 min. Moreover, the use of SPS alone was tested, reaching 10% degradation in 120 min, which shows the poor performance of this agent when used alone. Matthaiou et al. [110] used 1 g/L of SPS and different iron materials as a catalyst for the removal of 0.4 mg/L of PP, achieving the best degradation, 90%, in 90 min. Comparing these studies shows that the presence of catalyst led to almost total removal in general, which shows a good degradation performance when a catalyst material is present, and the study by Bekris et al. [109] also presented a poor performance for SPS when used alone, which is the expected behavior. Moreover, these performances were also obtained for different types of catalysts, which may suggest that it is possible to use a wide range of different catalytic materials. Table 7 presents the use of reagents for parabens abatement. • PP removal was about 90% for S1, S2, and S3 catalyst in 90 min and was 50-55% for S4 and S5 catalyst. [110]

Electrochemical Technologies
Electrochemical methods are also part of advanced oxidation processes and are widely used for degradation of pollutants. The use of these technologies on an industrial scale can be difficult, due to high costs, since one of the most used anodes is the boron-doped diamond (BDD), which is a very expensive material.
Martins et al. [36] used photo-electrocatalysis for PP removal using TiO 2 nanotubes as a catalyst. At pH = 3, total removal was achieved in only 30 min. The authors also showed that the presence of WO 3 in TiO 2 nanotubes enhances the photo-activity by 20%. Frontistis et al. [85] used electrochemical oxidation with BDD anode and achieved total EP removal in about 15 min for 200 µg/L of EP and 30 min when 600 µg/L was used as the initial concentration.
Dionisio et al. [111], Steter et al. [112], and Dionisio et al. [113] removed 100 mg/L of methylparaben using electrochemical oxidation with BDD [111,112] and DSA-Cl 2 [113]. Dionisio et al. [111] had 100% removal in 20 min for electrochemical oxidation and photoelectrocatalysis with UVC radiation, while photochemical degradation led to 20% removal in almost 500 min reaction. When 3 g/L of Na 2 SO 4 was used as an electrolyte solution, the efficiency decreased, since total removal was achieved for photo-electrocatalysis in about 240 min, for electrocatalysis in 300 min, and the photochemical oxidation led to less than 10% in almost 500 min. This clearly shows the influence of electrolyte solution on the technology's performance. Moreover, it seems that NaCl presents a good behavior for these experiments. Dionisio et al. [113] used 0.15 mol/L of NaCl as electrolyte solution, which also had 100% MP removal, but it was obtained in 40 min for electrocatalysis and photoelectrocatalysis, which did not show improvement when UVC radiation was added to the degradation process. UVC photolysis was also tested, but only 17% of MP was removed in 120 min. In the studies by Dionisio et al. [111,113], the presence of radiation was not able to increase the electrochemical experiments' efficiency, but when compared to single radiation, the high performance of the electrochemical route was noticed [113]. Steter et al. [112] used BDD as an anode and 0.05 mol/L of K 2 SO 4 as an electrolyte solution but only achieved 34.65% of MP removal in 20 min for the best conditions. In fact, compared to the study of Dionisio et al. [111], the choice of electrolyte solution may explain the difference in the obtained results. With this, it is possible to notice that NaCl and Na 2 SO 4 are good choices at a first sight, and the use of K 2 SO 4 should not be considered due to low performance.
The removal of BuP by electrochemical technologies was also investigated. Pueyo et al. [114] used electrochemical oxidation, using different materials as anode and stainless-steel as cathode, to remove 0.5 mg/L of BuP in an electrolyte solution of 0.1 M of Na 2 SO 4 . BDD anode allowed a BuP removal of 100% in 15 min, while stainless steel and platinum only achieved 40% and 18% degradation, respectively. Gomes et al. [115] also removed BuP in river water with this technology, but using platinum or glassy carbon anodes, for different electrolyte solution (KCl, H 2 SO 4 and K 4 P 2 O 7 ). In this condition, 95% of BuP was degraded in 180 min in the presence of 0.1 M of K 4 P 2 O 7 as electrolyte and 40 µM of CTAC on glassy carbon. The study by Pueyo et al. [114] proved the good performance of BDD over platinum and stainless-steel anodes. This should be expected, since one of the good characteristics of BDD is its good working performance, but the disadvantages such as its high cost can make it difficult to implement electrochemical experiments with this anode. Furthermore, the study of Gomes et al. [115] suggested an easy degradation of the target compound when used in a real water matrix, which is a positive sign for real application purposes.
As well as electrochemical oxidation or photoelectrocatalysis methodologies, electro-Fenton and photo-electro Fenton were also used in parabens abatement. Steter et al. [88] removed MP, EP, and PP by photo-electro Fenton with BDD or RuO 2 anode. Regarding the BDD anode, a removal of 65% (360 min), 100% (360 min), and 100% (180 min) was achieved for H 2 O 2 , electro-Fenton, and photoelectron-Fenton with solar radiation. Furthermore, the authors studied the influence of the water matrix in the paraben's abatement. When real wastewater was tested, BDD led to total removal in 150 min, while the RuO 2 anode led to a removal higher than 95% in 180 min, using solar light photoelectron-Fenton. This study proved the good efficiency of electro-Fenton and Photo-electro Fenton, since the removal rate was higher when Fenton's reagent was used compared to single H 2 O 2 , and the same conclusion was reached when radiation was added. These results can be explained by an improvement in the generation of hydroxyl radicals. Furthermore, this study also showed high degradation results when real wastewater was use,d which may suggest a good efficiency for these experimental conditions when applied to real treatment approaches. Rosales et al. [116] tested the MP abatement for electro-Fenton and anodic oxidation, using BDD/Nb anode, and graphite felt (GF) or Nb as cathode. Results showed that anodic oxidation led to about 60% removal for Ru and GF cathodes, while the electro-Fenton using the same cathodes led to about 78% and 98% removal, respectively, in which the electro-Fenton process led to higher performance when compared to anodic oxidation. The results are related to the different technologies' mechanisms. Table 8 presents the use of electrochemical technologies in parabens removal.

Sono-Based Technologies
Ultrasound can generate oxidizing radicals from water pyrolysis due to the creation and collapse of microbubbles. This technology can present good advantages but has the disadvantages of high amount of energy needed, low mineralization, and low efficiency, which can make this technology infeasible with difficult implementation at industrial scale.
Daghrir et al. [37] used photosonochemical degradation for BuP abatement (20 mg/L). The experiments led to a removal of 61.3%, 60.02%, and 87.96% for single UV light, ultrasound, and the combination of both in 120 min. Furthermore, for different experimental conditions (BuP concentration = 10 mg/L, US/UV experiment, 40 W, pH = 7) a removal of 99.2% removal was achieved. Nikolau et al. [38] removed about 92% of MP using the best operational parameters of T = 20 • C, pH = 6, power density of 60 W/L and the presence of biochar. This result was similar to the conclusions obtained by Sasi et al. [118], which showed 99% MP removal using an ultrasound frequency of 350 and 620 kHz, or a power density of 40.25 and 22.75 W/mL.
Ultrasound was also tested in the presence of a catalyst. Zanias et al. [119] studied sonocatalysis for MP removal using bimetallic Co-Fe carbon xerogel as a catalyst. Adsorption into catalyst was 29.2% of MP in 60 min, and sonocatalysis led to 54.6% removal in 60 min for a power density of 25 W/L, but almost full removal was obtained in 45-60 min, when the power density was increased to 52 W/L.
In general, the removal of parabens was achieved, and high parabens degradation was reported. Nevertheless, the drawbacks associated with sonochemical processes as the high energy requirement or poor mineralization may affect their choice and their use in pollutants degradation at full-scale. In fact, other AOPs with better performances and lower energy requirements can be used in wastewater treatments.
An overview of parabens degradation by sono-based technologies is presented in Table 9.

Toxicological Studies of Parabens Abatement
As proven by the previous sections, the advanced oxidation processes can promote the total abatement of parabens (single or mixture), but some studies reveal that in some conditions, the total parabens removal cannot be achieved. Moreover, also the formation of by-products during these processes can interfere with the initial toxicity or even increase the toxic character of the treated solution [24,30,75]. In this way, it is important to consider the impact of treatments in target species, such as aquatic species, since the resulting by-products can be more toxic than the parent compounds [120]. With this, the toxicological study must be a parameter of consideration in the design and application of AOP technologies for wastewater treatment, since although the target substances may be eliminated, the solution toxicity can increase, causing negative impacts if discharged into the environment. Table 9 presents toxicity studies of initial and treated solutions of single or mixtures of parabens, using different AOP technologies. In general, all the treated samples had less toxicity compared to the initial ones, concluding that photolysis, photocatalysis, and ozone-based technologies can help in reducing toxicity.
Gomes et al. [102] tested the abatement of MP, EP, and PP mixture at 1 mg/L each, by photocatalytic and catalytic ozonation, using TiO 2 /NT. For catalytic ozonation, the light inhibition (LI) of A. fischeri decreased about 57%, while L. sativum had a germination index (GI) increase of 90%, while for single ozonation, the toxicity of A. fischeri decreased by 46% and occurred a GI increase of 79% for L. sativum seeds. For both species, the toxicity decreased when the treatments were applied.
Gomes et al. [121] treated 10 mg/L of each paraben in a five parabens mixture using photolysis and photocatalytic ozonation, and the toxicity was evaluated using V. fischeri, C. fluminea, and L. sativum. Regarding the V. fischeri, the treatment reduced the initial LI of 96% to 43% for the best catalyst, while C. fluminea's total mortality was reduced to zero mortality in 72 h, except for 1%Pd-TiO 2 catalyst, which led to a mortality of 29% . L. sativum had an initial GI of 42% and the best results showed a GI of 112%. Furthermore, Gomes et al. [26] removed a mixture of five parabens at 10 mg/L each by ozonation and catalytic ozonation using several doped TiO 2 catalysts. Toxicity decreased for all the treated samples, achieving a maximum of 53.7% of inhibition decrease for V. fischeri, 79% mortality decrease for C. flumínea, and an increase of 50% for L. sativum, while Gomes et al. [27] tested the same but using ozonation, photolysis, photolytic ozonation, and photocatalytic ozonation, achieving a maximum decrease for photocatalytic ozonation of 60% for V. fischeri, 100% for C. flumínea (no mortality observed), and an increase of 70%, achieving a maximum of 112% for L. sativum. Moreover, Gomes et al. [91] used photocatalysis with different TiO 2 catalysts for the degradation of a mixture of five parabens, reaching the best results of decrease from 100% to 21% (79% decrease) in mortality of C. flumínea and an increase from 40% to 70% (30% increase) for L. sativum, while V. fischeri showed 80% inhibition after treatment.
Comparing these studies, Gomes et al. [121] had a higher V. fischeri LI decrease of 53%, which was similar to the results obtained by Gomes et al. [26] (53.7%) and Gomes et al. [27] (60%) for the same species. This represents an important behavior for the treatment technologies regarding toxicity removal, and the difference obtained by photocatalytic ozonation in Gomes et al. [121] and Gomes et al. [26] was similar to the ones obtained by catalytic ozonation of Gomes et al. [121] but better than the results obtained for photocatalysis performed by Gomes et al. [91], which showed about 80% light inhibition after treatment, which represent the worst result of these studies.
Moreover, photolytic ozonation seems to be worse than photocatalytic ozonation, which may suggest a good behavior of catalysts presence regarding the toxicity elimination. This may also suggest that the presence of catalyst can be an interesting advantage in wastewater treatment, since it can lower the treatment costs (when compared to single ozone) [27] and can also enhance the toxicity decrease. For C. fluminea total mortality, this was reduced to zero in almost all experiments in 72 h in the study of Gomes et al. [121], which was concordant with the results of Gomes et al. [27], but Gomes et al. [26] obtained mortality of 29% and 21% for single ozonation and catalytic ozonation with TiO 2 . Photocatalysis showed the worst results once again, with the worst clam mortality of 55% [91]. These studies show a better behavior of photocatalytic ozonation and photolytic ozonation than catalytic ozonation and photocatalysis when doped TiO 2 catalysts were used, which showed even worse results. In fact, this is interesting, since the use of ozone should easily generate intermediaries from parabens' abatement that can be more toxic than the parent compounds. However, these contaminants may have been completely degraded without affecting the toxicity. Furthermore, the behavior of photocatalytic ozonation regarding this species is similar to the one observed for A. fischeri. For L. sativum, Gomes et al. [121] achieved the best GI of 112%, Gomes et al. [26] of 90%, Gomes et al. [91] of 70%, and best GI of 112% for Gomes et al. [27] study, which presents similar results and behaviors than the observed for the other species.
Fernandes et al. [103] treated the same solution than Gomes et al. [26,27,91,121] but used catalytic ozonation with N-TiO 2 and single ozone. The authors achieved better results of 30%, 30%, and 20% for C. fluminea mortality using ozonation, catalytic ozonation with TiO 2, and catalytic ozonation with N-TiO 2 , respectively, while the initial mortality was 84% in 48 h. Regarding L. sativum, it showed an initial GI of 41.8% and had GI of 74.6%, 108.6%, and 123.4% for O 3 , O 3 + TiO 2, and O 3 + N-TiO 2 . The toxicity evaluation using A. fischeri showed an initial LI of 95.6% and LI of 70%, 73.9%, and 63% for O 3 , O 3 + TiO 2, and O 3 + N-TiO 2 . In fact, compared to Gomes et al. [26], the results obtained by Fernandes et al. [103] were the worst, which may indicate a weak performance of N-TiO 2 for toxicity decreased when compared to TiO 2 doped with noble metals.
Velegraki et al. [75] used photocatalysis for MP removal, and the toxicity was evaluated using Artemta franciscana nauplii. In this study, the formation of more toxic intermediaries occurred when the MP removal was about 39%, in which an increase of 10% in toxicity was observed. However, after the complete MP removal, the intermediaries were also removed, and the solution's toxicity was similar to the initial one. Furthermore, Orak et al. [31] used cress seeds for toxicity evaluation of an MP solution that was treated with photo-Fentonlike reaction (using best operational conditions). In fact, the observed inhibition was about 1% for the treated water and about 30% for the initial MP solution.
Gmurek et al. [30] used several AOPs for the degradation of a mixture of MP, EP, PP, BuP, and BeP at 10 mg/L each. Considering the treatments, regarding A. fischeri, the worst result was achieved for photocatalysis using several catalysts that had LI higher than 80%, and the best results occurred for ozone with Au-TiO 2 with a LI of 31.4%. For L. sativum, the worst GI was obtained by 43% for photocatalysis with TiO 2 and the best was 112% for O 3 with UVA radiation and Ag-TiO 2 . Furthermore, the toxicity of C. fluminea was worst for UVA with Pt-TiO 2 , with 55%, and the best occurred for photocatalytic ozonation, in which all the catalysts led to 0% mortality.

Treatment Costs
Besides the efficiency of the paraben's abatement, it is important to evaluate the impact of AOPs in terms of economic aspect since these kinds of processes are mainly associated with high operational costs.
All the presented costs pertain to different paradigms, so the values themselves are affected by several issues such as the location of the study, quantity treated, effluent characteristics, energy price, types of equipment, treatment time, etc.
In this work, the kWh values were converted, considering the price of industrial electricity referred by Gomes et al. [123] of 0.1276 €/kWh, and dollars ($) were converted to Euros (€), considering $1 = €0.84. It should be noted that a complete economic analysis must consider maintenance and costs related to operators, among other factors, such as contingencies, cost of raw materials, utilities, etc. [124]. Furthermore, if it involves the treatment of effluents, it should be noted that the choice of technologies must include reactor design, maintenance costs, and other operating costs [27].
Vela et al. [3] removed 90% of a mixture of methylparaben, ethylparaben, bisphenol A, bisphenol B, diamylphthalate, and butyl-benzylphthalate by photocatalysis with ZnO, TiO 2 P25, and TiO 2 Vlp 7000. The treatment costs were 103 €/m 3 , 149 €/m 3 , and 285 €/m 3 , respectively, considering the facility and project contingency, engineering project, consumables, service, and overhead cost. For the use of ZnO and P25, the treatment related only to parabens costs about 30 €/m 3 and 84 €/m 3 , respectively. This value is slightly more expensive than other values reported in Table 11, but this estimation may be more accurate since it already considers some important project (estimation) costs, and it is regarding a pilot-scale installation.    [125] Gomes et al. [27]  . These technologies are more expensive than the ones studied by Gomes et al. [27], and this can be related to the costs related to the reagents and with radiation. Furthermore, in Gomes et al.'s [27] study, the presence of catalyst and radiation seems to decrease the technologies' costs, and the use of single UVA with ozone is cheaper than catalyst with ozone and then UVA with catalyst and ozone. Catalytic ozonation was the most expensive treatment process, but this was not verified when photocatalytic ozonation was used. Moreover, this study suggests that the use of UVA makes the treatment more expensive but reduces the time of treatment, which can lead to smaller equipment and cheaper investment costs. When volcanic rocks are considered as the catalyst, the technology price decreases to 0.08 €/m 3 , which may indicate a significant decrease in operational costs when low-price catalysts are used and when UV radiation is not considered.
Furthermore, Nian et al. [117] calculated the costs of Lin et al. [7] photocatalysis with TiO 2 for MP removal, achieving treatment costs of 100 $/g (84 €/g), and Sasi et al. [118] used sonochemical oxidation for methylparaben abatement. The cost is dependent on the chosen power density, but the cost varied from 197.92 $/g to 527.78 $/g (570-1050 €/g). These studies report high expensive technologies, which present a huge disadvantage in using these treatments applied at industrial scale.
Steter et al. [112] achieved an MP removal comprised between 19.85% and 34.65%, using electrocatalysis with a BDD anode. The cost is related to the energy consumption, which is dependent on current density and was estimated to be between about 1.49 kWh/kg and 36 kWh/kg (0.19-4.59 €/kg) and 0.26 kWh/m 3 and 7.24 kWh/m 3 (0.03-0.92 €/m 3 ). In fact, the real price for this technology is more expensive, since the costs with the BDD, the other electrodes, the electrolyte solution, which needs to be replaced several times (the costs related to reagents), etc. are not considered. Furthermore, these energy costs are only related to single MP removal, and if more parabens (in mixture) are considered, the price will normally increase. Table 11 shows some associated costs of different advanced oxidation processes in the abatement of parabens mixtures as a pollutant.
Considering the presented studies regarding the treatment costs, a solid comparison and analysis can only be completed if several parameters are considered. In this case, this is difficult to do, since these parameters are different from study and study and due to the fact that not the same parameters are considered by all the studies. However, the state of the art was analyzed based on several studies from literature, and this should motivate more investigation in this subject.
Moreover, from doing an ad hoc analysis, it seems that the technologies using different oxidizing agents are more expensive than others using catalysts or radiation, which can be expected since the use of reagents should enhance the operational costs because these agents cannot be recovered. The use of sonolysis and some photocatalytic degradation treatments seems to be also expensive, but other reported works using ozonation, catalytic ozonation, and photocatalytic ozonation with TiO 2 show low values associated. In general, it seems that the use of catalysts reduces the operational costs, but the use of UV radiation and the catalyst increases the related costs as well. Therefore, a balance between the operational costs, toxicity abatement, and pollutants removal performance should be considered to choose the technology for wastewater treatment. Furthermore, the values reported by Vela et al. [3] exhibit values from a study at a pilot scale regarding a real treatment plant, and it already considers some important project variables, parameters, and costs that must be taken in economic analysis. Compared to the studies at the laboratory scale, higher values were reached, and these values should be closer to real values at the industrial scale.
To understand the real values related to these technologies, extensive in-depth studies at a real scale for the same effluent and the same operational conditions should be done using the different AOP technologies.

Future Perspectives
The elimination of parabens and other CECs has been studied using different AOP technologies over the years by different authors. However, much research must be done before these can be easily implemented with low costs and in an environmentally friendly manner.
In general, and regarding the treatment of parabens, it seems that the use of catalysts should be preferred to the use of radiation or reagents alone. Furthermore, to lower the catalyst cost, it is important to guarantee that these can be used several times and can be recovered, regenerated, and reused. The use of radiation and catalyst should be considered depending on the technique to be applied, since some technologies, such as photocatalysis, need radiation to work. If the catalyst can be activated by solar radiation, the costs related to these technologies should be lower and attractive for industrial implementation. The common catalyst that is reported (TiO 2 ) cannot be activated by this type of radiation or by visible radiation, but it is already known that this can be possible with catalyst doping. Furthermore, another thing to turn the industrial application closer is the use of immobilized catalysts, since the most used ones are present in powder form, and this can make industrial application difficult, since the powder will need more separation and recovery units, which can be traduced into more equipment, energy, labor, or resuming more operational and investment costs.
During the use of different AOPs, it is noticed that the poor mineralization and the increase in toxicity can occur due to the generation of more (toxic) by-products (or, in other words, the lack of complete oxidation into CO 2 and H 2 O). This parameter should always be considered since it is possible to remove the wanted target compounds while, on the other hand, generating more toxic compounds that, when released into the environment, could have a worse impact than the initial pollutant's solution.
Moreover, in the AOP treatment designs, the associated costs (investment and operational costs) are another important evaluation parameter for the choice of the technology to be used. In fact, one major disadvantage of this group of advanced treatments is the associated high costs, especially related to energy and reagents consumption; this problem needs to be overcome in order to make these technologies feasible. Furthermore, in some of the countries already affected by water scarcity, the costs may be the biggest obstacle for advanced wastewater treatment technologies.
With this, there are many parameters that need to be considered and optimized for the implementation of these technologies on an industrial scale, but it is important that in the near future, the implementation of easily operational, efficient, low-cost, and environmentally friendly AOPs technologies be applied for real wastewater treatment.
So, for future perspectives, it is important to Based on the reviewed works, regarding parabens' treatment (which may not be applied to other CECs or real wastewater solutions due to the diversity of compounds presented in wastewaters from different places), authors appear to be focused on photocatalysis and ozone-based solutions. In fact, some disadvantages of other technologies such as the generation of iron sludge in Fenton's process, the high energy needed and low efficiency in sonochemical methods, the high costs and difficult reuse of specific materials (electrodes) that can be related to the use of electrochemical oxidation, the high amount of reagent needed in the use of oxidizing agents, or the needed severe conditions in wet air oxidation may be responsible for the lack of improvement in these technologies. Although photocatalysis and ozone-based technologies present disadvantages, the solution to overcome these problems can be easily explored, which, together with its operational simplicity and high efficiency, can be the reason why these technologies are the subject of several studies by different authors. For example, the poor mineralization of ozonolysis can be diminished by using catalysts. Furthermore, it is important to consider that the use of combined AOPs or the use of AOP technology followed by secondary treatment or other combination can have a synergetic effect. For example, if the wastewater solution is very blurred, the use of photocatalysis will require a pretreatment of coagulation and flocculation (for example) to make the wastewater less blurred and so the radiation can penetrate it. Furthermore, for the poor mineralization of ozonolysis, or even to complete the mineralization of catalytic ozonation or photocatalytic ozonation, a secondary treatment technology can probably be used after this treatment. These are only a few hypotheses of problems that can be related to the photocatalysis or ozone technologies use and their possible solutions.
However, none of these technologies should be removed from consideration until some research for the treatment of a specific wastewater is carried out. In fact, the choice of technology depends on several parameters already mentioned, and others, such as the volume/flow of wastewater to treat, wastewater composition, and wastewater properties (pH, COD, BOD, turbidity, etc.). It is also important to note that AOPs can/should be implemented considering other treatment technologies that are already common in WWTPs (as preliminary, primary, and secondary treatments) to take advantage of their synergetic effect, increasing the treatment station efficiency (as an entire treatment) and possibly reducing costs in project and operational treatments.

Conclusions
Different AOP technologies are discussed in this work for parabens degradation from water. These compounds' detection in the environment is related to effluents discharge and weak parabens elimination in WWTPs. The correct parabens' removal is possible using advanced oxidation processes, so the parabens abatement by different AOP technologies (such as ozone-based technologies, photolysis and photocatalysis, oxidizing agents, Fenton' process, sonochemical technologies, and electrochemical technologies) is reviewed in this work. Literature does not have so many references for the use of some AOP, regarding parabens abatement, which can be related to the technology's characteristics. Furthermore, some of these technologies may be infeasible for industrial application due to some related disadvantages, for example, high operational costs or low efficiencies.
The impact of treated solutions was revealed with an overview of different ecotoxicity tests. This type of experiment is important to choose the technology to use in the treatment of wastewater, since the AOPs interfere with and change the solutions' toxicity, which can make it more dangerous to the environment than the initial solution. Among the technology performance and removal efficiency, the operational costs are another characteristic to consider in the choice of treatment technologies, so with this in mind, several costs presented in the literature were reviewed.
Finally, regarding the parabens removal from wastewater with different technologies, the results are encouraging for efficient use, but there are still opportunities improvement that must be investigated. However, it is important to remember that these results are only for a specific pollutants mixture solution (mainly parabens in most cases), so their behavior for treatment of other CEC compounds or other toxic pollutants must also be analyzed. On the other hand, since these compounds enter wastewater with a diverse composition including several compounds, it will interesting to study further steps before the real application can be achieved, such as the use and analysis of these technologies' efficiency and characteristics for the treatment of real wastewater stream.