Electro-oxidation of methyl paraben on DSA®-Cl2: UV irradiation, mechanistic aspects and energy consumption

Electrochimica Acta 338 (2020) 135901

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Electro-oxidation of methyl paraben on DSA®-Cl2: UV irradiation, mechanistic aspects and energy consumption Dawany Dionisio a, b, Lucas H.E. Santos a, Manuel A. Rodrigo b, Artur J. Motheo a, * ~o Carlos Institute of Chemistry, University of Sa ~o Paulo, P.O. Box 780, CEP 13560-970, Sa ~o Carlos, SP, Brazil Sa Department of Chemical Engineering, Faculty of Chemical Sciences & Technologies, Universidad de Castilla - La Mancha, Campus Universitario S/n, 13071, Ciudad Real, Spain a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 December 2019 Accepted 12 February 2020 Available online 14 February 2020

The electro-oxidation of methylparaben (MeP) was studied using a DSA®-Cl2 in simple electrolysis and hybrid process (electrolysis followed by UV light irradiation), aiming at evaluating the oxidation mechanism, the removal of organic matter and the energy consumption. Analysis of the results revealed that MeP removal is rapid in both processes. However, the formation of a solid byproduct of a polymeric nature, which is probably related to MeP oxidation products, has occurred. Irradiation of UV light in the solution improved the mineralization process by facilitating the degradation of byproducts, including the solid one. Using the hybrid process, mineralization increased by 40% with low additional energy costs. In addition, new aspects about the MeP electrooxidation mechanism were found. The use of the DSA®-Cl2 appears to favor oxidative coupling reactions, resulting in higher molecular weight products prior to aromatic ring breakdown and further mineralization. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Hybrid process Chloride medium Buffer solution Oxidation pathway Oligomerization

1. Introduction Parabens are part of an important class of preservatives widely used in food, pharmaceutical and cosmetic industries. Darbre et al. [1] reported in 2004 that these compounds are related to human breast cancer, which was restated by Dagher et al. [2] in 2012. Since that, parabens are classified as endocrine disrupting compounds and various studies have been published considering the adverse effects of these substances on the environment and human health [3e9]. According to that, some organizations such as US Food and Drug Administration (FDA), World Health Organization (WHO), European Union (EU), restricted the permitted concentrations of parabens in several products [9,10]. However, due to the high benefit-cost ratio, these preservatives are still largely used, resulting in the contamination of several environmental matrices [11e14]. Different types of methods are reported for the removal of organic pollutants, such as parabens, from wastewater. Electrochemical advanced oxidation processes (EAOPs) have showed higher efficacy to achieve complete mineralization and good versatility for technologies coupling [15e20]. Combining other

* Corresponding author. E-mail address: [email protected] (A.J. Motheo). https://doi.org/10.1016/j.electacta.2020.135901 0013-4686/© 2020 Elsevier Ltd. All rights reserved.

technologies to electrolysis, for instance irradiation of UV light or ultrasound, it is becoming a common strategy to enhance the production of oxidants in the media and, thus, improving the removal of contaminants [21e23]. Nevertheless, the efficiencies of these processes are strongly dependent on the system coupling, the electrodes material and the matrix composition. A few types of electrodes have been used to study the electrochemical oxidation of parabens [15,17,24e30]. However, borondoped diamond (BDD) anodes are still the most employed, which are very powerful but also expensive. On the other hand, dimensionally stable anodes (DSA®-Cl2) are a commercial type of mixed metal oxides electrode (MMO), which are cheap, robust and very stable. These electrodes are widely used in the chlor-alkali industry and several studies reported their good performance for the degradation of pollutants in chloride media [31e36]. Both, DSA®-Cl2 and BDD, represent the boundaries in the behavior of electrodes for the electrolysis of organics. In fact, a very important model proposed in the nineties about the mechanisms in the oxidation of organics suggests diamond and mixed metal oxides as examples of active and non-active electrodes [37]. For active electrodes, the formation of hydroxyl radicals by anodic oxidation of water was not as effective as in the case non-active electrodes. This is a consequence of these radicals’ characteristics, which are key to understand the performance of the electrolysis of

2

D. Dionisio et al. / Electrochimica Acta 338 (2020) 135901

wastewater. They interact with the metal oxides, leading to the formation of unstable metal oxide species at higher oxidation state, and this unstable oxides becomes the real responsible for the oxidation of organics [38]. This means that the oxidation is really chemical and non radicalary or electrochemical and this explains its softer character. Conversely, in the case of the non-active electrodes, hydroxyl radicals are not combined with the species contained on the surface of the electrode and they attack directly to the organic pollutants in a hasher way [39]. In addition, from the beginning of the research studies focused on the treatment of wastewater [40e43], the significance of mediated oxidation has been pointed out, in particular in the presence of chlorides. These anions allows the formation of chlorine and other oxidants [44e46], which does not always have a positive impact on the treatment because of the reported formation of hazardous byproducts. At this point, key differences between the mechanisms involved in the oxidation of organics using active and non-active electrodes are not related to the oxidative reactions but to the significance of the progress in each of them. Thus, in non-active electrodes, combination of intermediates for form polymers is not promoted and carboxylic acids are easily degraded [47]. Conversely, in active electrodes, with the same oxidation routes, accumulation of carboxylic acid is typically found, because of the difficulties found in the chemical oxidation of these highly oxidized organic molecules. In addition, stability of polymers is much higher than that of the parent molecules and because of that they remain in the treated solution, sometimes fouling the electrode surface and leading to important inefficiencies [48]. It is possible to find several mechanistic proposals of parabens oxidation using either electrochemical or non-electrochemical processes [15,18,20,28,49e53]. However, to the best knowledge of the authors, for the electrooxidation process, all-mechanistic considerations are proposed using BDD anodes, i.e., there is a lack of information on the use of MMO as anodes, specifically for this system. According to this, the aim of the present study was to analyze the electrooxidation mechanism of methyl paraben (MeP) using a DSA®-Cl2, in order to verify the feasibility of this technology for the removal of parabens from wastewaters. A hybrid process with irradiation of UV light during electrolysis was also considered, focusing on the removal of organic matter and energy consumption.

experiments were carried out in a simpler system based on the previous one (see Fig. S1c). The reservoir of the other system was used now as electrochemical cell to treat 0.6 L of solution, without recirculation, using the same electrodes as before. Monochloroacetic acid/chloroacetate (0.1 mol L1, BMCAc) was added to the solution MeP þ NaCl in order to maintain its pH at 3 during the 2 h electrolysis. Determination of MeP, BMCAc and TOC. Concentration of methyl paraben was monitored in a HPLC Shimadzu SPD-10A VP, with a Zorbax SB-C18 (25 cm  4.6 mm) column and UV detector set in 254 nm. Acetonitrile and water (40:60 v/v) were used as mobile phase, at 30  C and flow rate of 1 mL min1. Monochloroacetic acid was also monitored in this system but using a Aminex HPX-87H (Bio-Rad) column (at 25  C) and H2SO4 0.005 mol L1 as mobile phase (1 mL min1), with detection in 214 nm. Total organic carbon (TOC) was determined in a carbon analyzer Sievers InnovOx, General Electric Company (FAPESP 2014/02739e6). Byproducts analysis. For solid product analysis, 1.7 L of treated solution were centrifuged in an Himac CR 2OB2 centrifuge (Hitachi) at 8000 rpm during 15 min. Precipitate was dried for 12 h in a vacuum oven (Precision model 19) at 60  C. The solid was analyzed by infrared spectrophotometry (Shimadzu IRAffinity) in KBr pellets. Cyclic voltammetry was performed with an Autolab PGSTAT 128 N (Metrohm B$V.), using the same anode and cathode as in electrolysis and an Ag/AgCl reference electrode. A cathodic sweep was carried out at 50 mV s1 ranging from 1.3 to 0.3 V. Aromatic byproducts were extracted and concentrated by solid phase extraction (SPE) using C18ec cartridges (1 mL/100 mg, Chromabond®, Macherey-Nagel) and methanol as eluent. These samples were analyzed by HPLC-MS in a Shimadzu Prominence 20A series chromatographer with Zorbax SB-C18 (25 cm  4.6 mm) column, at 40  C, with H2O:ACN (60:40, v/v) acidified by 0.1% formic acid as mobile phase (1.0 mL min1) coupled to a hybrid mass spectrometer quadrupole/TOF with electrospray ionization (Microtof-QII, Bruker Daltonics) (FAPESP 2004/09498e2). 3. Results and discussion 3.1. Effect of current density Fig. 1 shows the removals of MeP (MePR) by electrochemical

2. Experimental Chemicals. Methyl paraben was obtained from Sigma-Aldrich. NaCl (Synth) and H2SO4 (PanReac) were used as supporting electrolyte and for pH adjustment. Monochloroacetic acid (AnalytiCals) was used to prepare a buffer solution with pH ¼ 3 (named monochloroacetate buffer, BMCAc). Acetonitrile and methanol, HPLC grade were obtained from PanReac. All reactants were used as received. High-purity water obtained from a Millipore Milli-Q system (resistivity >18 M cm at 25  C) was used for the preparation of all solutions. Electrochemical treatments. Experiments were carried out in a recirculation batch system (see Fig. S1a) constituted of an electrochemical cell with 1.0 L of working solution, a jacketed reservoir with 0.7 L of solution and a circulator pump working at 5.0 mL s1 flow rate. A commercial DSA® of Ti/Ru0,3Ti0,7O2 (purchased from DeNora do Brasil) was used as anode and a Ti plate as cathode, both with 54.4 cm2 and separated by 5 cm in the cell. Solutions of MeP (100 mg L1, pH ¼ 3.0) þ NaCl (0.15 mol L1) were treated by 2 h electrolysis using an Autolab PGSTAT 128 N (Metrohm B$V.). For UV light irradiation, a germicidal UVC lamp (4 W, l ¼ 254 nm, Philips) was introduced into a quartz tube and centralized inside the reservoir (see Fig. S1b). For the study of pH using buffer solution,

Fig. 1. Electrochemical removal of MeP (100 mg L1) at different current densities as a function of the instant applied charge. Japp ¼ (B) 1.0, ( ) 2.5, (D) 5.0, (A) 10 mA cm2. DSA®-Cl2 in NaCl 0.15 mol L1 at 25  C.

▪

D. Dionisio et al. / Electrochimica Acta 338 (2020) 135901

treatment at current densities (japp) ranging from 1.0 to 10.0 mA cm2, which are equivalent to maximum electrical charges (Q) of 0.06e0.64 A h L1, respectively. As expected, the higher the japp, the higher the removal of the pollutant: MePR of 29 and 76% were obtained for japp ¼ 1.0 and 2.5 mA cm2, respectively, whereas for higher japp, complete removal was achieved with 80 and 40 min of treatment for 5.0 and 10 mA cm2 (Table 1). In the studied system, chloride is oxidized to chlorine gas (Eq. (1)) on the DSA®-Cl2 surface; dissolved Cl2 is hydrolyzed forming hypochlorous acid (Eq. (2)), which further dissociates to hypochlorite (Eq. (3)) depending on the solution pH (pKa (HClO) ¼ 7.5) [54,55]. 2 Cl / Cl2 þ 2 e

(1)

Cl2 þ H2O # HClO þ Cl þ Hþ

(2)

HClO # ClO þ Hþ

(3)

Hydroxyl radicals are also produced in the system from water electrolysis (Eq. (4)), however, they are strongly adsorbed on DSA®Cl2 surface and will only oxidize species near to this interface [56]. The main species on this region are chloride anions and, thus, the main OH reaction might be the formation of Cl radicals (Eq. (5)) [57]. On the other hand, due to its higher stability, active chlorine species (Cl2, HClO, ClO) will be present in the whole volume of working solution and, therefore, will be the mainly responsible for the indirect oxidation of methyl paraben. Accordingly, as the japp increases, MeP is converted to other compounds at higher rates, due to the higher production of active chlorine species. In the present work, the quantitative determination of free chlorine was not made; however, numerous works report that the degradation of organics increases with the increase of Cl concentration in the medium and the increase of the applied current density (when using DSA®-Cl2) [58e60]. H2O / Hþ þ OH þ e

(4)

Cl þ OH þ Hþ / Cl þ H2O

(5)

At higher japp values, it is noticeable that for each value of applied electrical charge the same MePR is attained, which indicates that the removal process is limited by mass transfer [61,62]. This limitation can be a result of the transport of Cl to the anode surface, controlling the oxidants electrogeneration, and the transport of oxidants from the anode to the bulk solution and vice versa, limiting the oxidation of MeP. On the other hand, for japp < 5.0 mA cm2, charge transfer is facilitated on the electrode surface, nonetheless a small concentration of chlorine is electrogenerated, resulting in low MeP conversion. The elimination of organic matter was evaluated by TOC removal (TOCR), which is presented in Table 1. Those removals seem to be enhanced by higher japp values, however for 5.0 and

Table 1 Kinetic constant, MeP and TOC removals, and mineralization current efficiency for the single electrochemical process (E) at different current densities. DSA®-Cl2 in NaCl 0.15 mol L1 at 25  C. japp (mA cm2)

kE (103 min1)

MePR (%)

TOCR (%)

MCE (%)

1.0 2.5 5.0 10.0

2.5 12 35a

29 (120 76 (120 100 (80 100 (40

3.0 17 29 27

28 59 50 24

a

b

min) min) min) min)

Value calculated for the first kinetic region (k1); k2 was not determined due to the lack of values measured. b Kinetic constant could not be determined due to the fast MeP abatement.

3

10.0 mA cm2 similar TOCR is observed, because some products of MeP degradation may be more recalcitrant than MeP itself. Usually, short-chain carboxylic acids and organochlorine compounds are found as byproducts of oxidation processes, since they are highly oxidized and recalcitrant molecules [63,64]. From TOC results, it is also possible to determine the mineralization current efficiency of the process (MCE), which is indicative of how efficiently the applied electrical charge promotes TOC removal. MCE can be calculated by Eq. (6) [65], where TOCi and TOCf refer to the initial and final concentrations of organic carbon (mg C L1), respectively, F is the Faraday constant (96,485C mol1), V is the volume of working solution (L), I is the applied current (A), t is time (h), n is the number of electrons exchanged in the mineralization process of the organic compound, m is the number of carbon atoms of the molecule under study, and 4.32  107 is the conversion factor for units homogenization (3600 s h1  12,000 mg C mol1). Table 1 shows that MCE sharply decreased for japp > 2.5 mA cm2, since increasing current densities did not result in increasing TOCR. Consequently, optimal MCE was observed at japp ¼ 2.5 mA cm2 although this current density did not provide the highest TOCR. This behavior is related to the occurrence of competitive reactions (i.e. oxygen evolution reaction) and other oxidation processes that do not lead to mineralization (i.e. production of highly oxidized, recalcitrant byproducts).


MCE ð%Þ ¼

 TOCi  TOCf :n:F:V 4:32  107 :m:I:t

:100

(6)

It is worth noting that the persistence of byproducts observed for high current densities could also be related to medium composition in terms of oxidizing species. For japp > 5.0 mA cm2, the final pH of the wastewater was shifted from 3.0 to 7.0e8.0. At those final pH values, the ratio [ClO]/[HClO] is higher than at the initial pH. Since ClO is a weaker oxidant than HClO (E0 ¼ 0.89 and 1.49 V, respectively [58]), mineralization is possibly impaired in spite of the high concentration of oxidizing species. Nevertheless, the low TOCR and MCE observed at 1.0 mA cm2 is most likely related to the low production of oxidizing species than to their relative concentration, since the pH increase was too small in this case (0.3 pH units). 3.2. Effect of UV light irradiation on electrolysis A germicide lamp (l ¼ 254 nm) was coupled to the solution tank in order to improve organic matter removal by means of the UV/Cl2 process. Irradiation of UV light into the solution may contribute to the removal of organic matter by producing highly oxidizing species (such as OH and Cl) and degrading photosensitive molecules. Fig. 2 shows the kinetics for the removal of MeP by electrochemical process single (E) and coupled to UV/Cl2 (E-P). Photolysis of MeP in Cl medium (inset of Fig. 2) was not efficient since MePR was 17% and almost no TOC removal was observed after 2 h. On the other hand, when photolysis is coupled to electrolysis, MeP is completely removed from the solution at japp ¼ 5.0e10.0 mA cm2. However, no improvement was observed for the E-P process in comparison to single electrolysis. It is possible to observe a pseudofirst order kinetic for MePR by the electrolytic processes, which the kinetic constants (k) are presented in Table 1 (E process) and Table 2 (E-P process). For japp ¼ 5.0 mA cm2, two different kinetic regions can be observed because indirect oxidation is facilitated by low pollutant concentration. As the process advances, the abatement of MeP changes its behavior because of the lower MeP concentration in the medium. Even though, the reaction still follows pseudo-first order model but with a different kinetic coefficient, thus, it is observed k1 and k2 for each regime, respectively [66]. When lower

4

D. Dionisio et al. / Electrochimica Acta 338 (2020) 135901

major for 10.0 mA cm2. Because of better TOC removals, the MCE is also improved, in which 71% of efficiency was achieved for 5.0 mA cm2. In order to evaluate the coupling of processes from an energetic point of view, the synergistic index (SI) was correlated with the energy consumption (EC) for TOC removal (Fig. 3). The SI is used to evaluate the effect of the combination of different processes and, in this case, was calculated specifically for TOC removal, according to Eq. (10), where TOCR,E-P, TOCR,E and TOCR,P are the removals of TOC (%) for the coupled process (E-P) and the individual processes E and P, respectively. The energy consumption of the processes per unit of TOC mass was estimated by Eq. (11) (adapted from Martínez-Huitle et al. [69]), where U is the cell voltage (V), I is the applied current (A), t is time (h), Wlamp is the UV lamp consumption (V), V is volume of working solution (L) and DTOC is the experimental TOC decay (in mg L1).

SI ¼ Fig. 2. Kinetic of MeP (100 mg L1) removal by electrochemical processes (black symbols) single and (white symbols) with UV irradiation. Japp ¼ () 1.0, ( ) 2.5, (:) 5.0 mA cm2. Inset: Removal of MeP (100 mg L1) by photolysis in chloride medium. DSA®-Cl2 in NaCl 0.15 mol L1 at 25  C; UVC lamp (l ¼ 254 nm, 4 W).

▪

Table 2 Kinetic constant, MeP and TOC removals, and mineralization current efficiency for electrochemical process with UV irradiation (E-P). DSA®-Cl2 in NaCl 0.15 mol L1 at 25  C; UVC lamp (l ¼ 254 nm, 4 W). japp (mA cm2)

kE-P (103 min1)

MePR (%)

TOCR (%)

MCE (%)

1.0 2.5 5.0 10.0

2.9 12 33a

31 78 100 100

4.4 16 38 42

42 61 71 38

b

a

Value calculated for the first kinetic region (k1); k2 was not determined due to the lack of values measured. b Kinetic constant could not be determined due to the fast MeP abatement.

japp values were applied, this behavior was not observed because the lower removal of MeP. This effect was also observed on previous studies of MeP degradation using boron doped diamond (BDD) anodes [26,67]. Differently from MeP removal, Table 2 shows that the removal of organic matter was enhanced by the irradiation of UV light. It is possible to observe an increase on TOCR up to 55% with relation to single electrolysis (Table 1). Active chlorine species are produced by the electrochemical process, and under UV irradiation they can be activated to even more power oxidizing species, such as OH and Cl (Eqs. (7)e(9)). As it was mentioned before, these species will also be formed near to the anode surface. However, the irradiation of UV light promotes the activation effect at the whole volume of solution, which facilitates the mass transport of the system and, thus, the removal of contaminants. HOCl þ hn / OH þ Cl

(7)

ClO þ hn / O þCl

(8)

O þ H2O / OH þ OH

(9)

Considering that some byproducts may be organochlorinated compounds, TOCR may also be improved under UV irradiation due to the rupture of CeCl bonds [68]. Once higher japp represent higher concentrations of byproducts, the UV effect on electrolysis was

TOCR;EP TOCR;E þ TOCR;P

(10)

 U:I:t þ W
lamp :t EC kW h gTOC 1 ¼ V:DTOC

(11)

According to the correlation presented in Fig. 3, the higher the SI, the lower is the energy required for removing 1 g of TOC from the working solution and, hence, the higher is the efficiency of the process. An antagonistic effect can be observed for low values of japp due to the low TOCR obtained under those conditions. This result can be explained by the greater effect of UV light on the degradation byproducts than on the MeP, as the results of Table 2 suggest. On the other hand, for higher current values the efficiency of E-P is improved: at 10 mA cm2 the coupling enhances the individual process by almost 40% (SI ¼ 1.38), and the EC is only 1.4 times higher than the single electrolysis. Under this condition, the extra energy required for the use of UV lamp is counterweighed by the better mineralization removal. Depending on the aim of the treatment, the hybrid process might be more interesting for application, even if the EC is slightly higher. The different MCE results obtained for each treatment (Tables 1 and 2) indicate that the composition of the treated solutions is different. Hence, chemical oxygen demand (COD) and toxicity might also be different, which are important parameters to consider for an effluent disposal.

Fig. 3. Energy consumption of the electrochemical processes (black bars) single and (gray bars) with UV irradiation, and ( ) synergistic index of the coupled process. DSA®-Cl2, [MeP] ¼ 100 mg L1 in NaCl 0.15 mol L1 at 25  C; UVC lamp (l ¼ 254 nm, 4 W).

▪

D. Dionisio et al. / Electrochimica Acta 338 (2020) 135901

3.3. Identification of byproducts 3.3.1. Solid product By the end of the degradations, with both E and E-P processes, the initially clear solutions became turbid because of the presence of a yellow, suspended solid; some foam was also formed on the surface of the solution. Higher accumulation of the solid is obtained for lower electrical charges after 2 h of E process (see Fig. S2). The increase of Q resulted in less turbid solutions, which means that those byproducts can be degraded by the electrochemical process. The suspended solid produced during E process (Q ¼ 0.45 A h L1) was separated from the solution by centrifugation and analyzed by infrared (IR) spectroscopy. IR spectrum of the solid is presented in Fig. 4a and interpreted as follows [70]. The peaks at 1462 cm1 and 849 cm1, and the shoulder at 1120 cm1 (all identified by squares) are attributed to 1,4- and 1,2,4-substituted aromatic rings. The peaks between 1740 cm1 and 1540 cm1 (circles) account for the presence of carboxylic acids (aliphatic and aromatic), and carboxylate salts. The peaks at 3462 cm1, with shoulders at 3262 cm1 and 3050 cm1, and around 1362 cm1 (triangles) are related to carboxylic and phenolic OH groups. The shoulders at 1060 cm1 and 1038 cm1 (diamonds) are assigned to aryl chlorides. According to that, the solid byproduct seems to consist of chlorinated degradation products of MeP. It was further noticed that the solid is formed on the cathode surface since a strong, yellow color is observed around this electrode during the first minutes of the degradations. Hence, the cathode was studied by cyclic voltammetry (Fig. 4b) in the degradation medium, after 0, 10 and 20 min of applied current (5.0 mA cm2). A redox process occurs on the cathode and its intensity increases as the degradation proceeds. It is possible that MeP oxidation products, formed at the solution bulk, are reduced on the cathode. When phenolic compounds and their oxidized counterparts are present in the same solution, they can undergo polymerization reactions [71,72]. Therefore, it is possible that the yellow solid observed is a polymerization product, most likely oligomers, of some intermediates of MeP degradation. Also, in this case, the relatively high molecular weight of those oligomers would cause their insolubility in an aqueous medium. In order to diminish this effect studies can be carried out in a divided cell, preventing that the oxidation product of MeP reaches the cathodic region. However, it is important to consider that for some wastewater

5

types divided cells present lower efficiency due to mass transfer problems. As mentioned before, pH has an important effect on the active chlorine species and, thus, on the oxidant power of the medium. In this work, when a non-buffered solution was used, it was observed an increase of the solution pH (from 3 to 8 after 4 h), which indicates alkalinizing and loss of oxidant power (because the ratio [ClO]/[HClO] increases with increasing pH, and ClO is a weaker oxidizing agent than HClO). Considering that, several experiments were carried out using a buffer (BMCAc) to maintain the pH and the ratio [ClO]/[HClO] constant during the process. Solution of MeP (100 mg L1) þ NaCl (0.15 mol L1) was treated by electrochemical process for 2 h and compared to the degradation of the buffered solution (monochloroacetic acid/chloroacetate (0.1 mol L1) þ MeP þ NaCl). Initially, the monochloroacetic acid (MCAA) was treated in NaCl medium in order to observe the efficacy of electrochemical process, with DSA®-Cl2, to remove it from water. It can be observed by the inset of Fig. 5a that this process is not efficient to remove MCAA under the studied conditions, resulting in a small variation of 5% on its concentration. After 80 min there is a slight increase on the concentration, which can be attributed to intrinsic errors in the method. Therefore, this acid is considered stable and feasible to be used as buffer solution (BMCAc) on MeP degradation, since it will not compete with MeP oxidation under the conditions used. Fig. 5a shows the removal of MeP in non-buffered (NaCl) and buffered (NaCl þ BMCAc) media. No difference was observed for the removal of MeP, achieving complete its removal after 80 min of treatment. This means that pH does not affect MeP oxidation. However, in Fig. 5b it is possible to observe that TOC removal is favored by the buffered medium, as well as the values of MCE and the EC. Hence, the removal of byproducts is affected by pH, presenting better efficiency at acidic conditions. Several electrooxidation studies [18e23] have shown that degradation of organic compounds are facilitated by acidic and oxidant conditions, because reaction steps, as decarboxylation and aromatic ring opening, require such conditions. As a result of the constant ratio [ClO]/[HClO] during the electrolysis, it was observed a improve of 40% in TOCR and of 67% in MCE, with almost half of the energy consumed, because alkalinizing and loss of oxidizing power did not take place. Furthermore, regarding the solid product formation it was observed that in the presence of BMCAc the final solution was more

Fig. 4. (a) IR spectrum of the yellow, solid byproduct, obtained in KBr pellet. (b) Cyclic voltammograms of the Ti cathode in 0.15 mol L1 NaCl þ 100 mg L1 MeP (100 mg L1) solution, scan rate: 50 mV s1, after 0, 10 and 20 min of degradation (japp ¼ 5.0 mA cm2).

6

D. Dionisio et al. / Electrochimica Acta 338 (2020) 135901

Fig. 5. Comparison of MeP degradation by electrochemical process in non-buffered (NaCl 0.15 mol L1) and in buffered (NaCl 0.15 mol L1 þ BMCAc 0.1 mol L1) media with japp ¼ 2.5 mA cm2 at 25  C. (a) Removal of MeP (black symbols) without and (white symbols) with BMCAc; (b) removal of TOC, MCE and EC obtained (black bars) without and (gray bars) with BMCAc. Inset: Removal of monochloroacetic acid (0.1 mol L1) by electrochemical process in NaCl medium (0.15 mol L1).

clear than for the non-buffered solution (see Fig. S3) and did not present formation of foam. Also, in comparison with Fig. S2, it can be seen that the aspect of the final solution using BMCA and 0.45 A h L1 is similar to that obtained with non-buffered medium and 3.63 A h L1, which is 8 times more applied charge. It is interesting to note that during the experiments in buffered medium, the solution turbidity increased in a first moment followed by its decrease. This effect indicates that the solid product was formed and degraded by the electrochemical process and it reinforces that the buffered medium improves the byproducts oxidation.

3.3.2. Mechanistical proposal A 4 h electrolysis using japp ¼ 15 mA cm2 was performed in order to achieve higher organic matter removal. MeP was completely removed after 40 min of treatment; however, the mineralization was not improved. A plateau at 36% of TOCR is reached after the first hour of electrolysis. As a result, low MCE (11%) and high EC for TOC removal (0.64 kW h gTOC1) were observed. During this electrolysis, samples for liquid chromatography coupled to mass spectrometry (LC-MS) were collected every hour for the investigation of the mechanism of MeP electro-oxidation on DSA®-Cl2. Only MeP was detected in the initial solution (t ¼ 0 h) and no compound was detected for t ¼ 4 h, which indicates that aromatic substances were completely removed after that time. Six main intermediates were detected (those with more intense peaks in the mass spectra) and identified (Fig. 6). The compounds named as 1 to 4 were identified from the mass spectrum for t ¼ 1 h. After 2 h of electrolysis, the compound 4 remains in solution and a new compound, 5, was detected. For t ¼ 3 h, only 4 was detected and completely removed within the next hour. Compounds 1 to 4 (m/z ¼ 267, 221, 301 and 255, respectively) have higher molecular weight than MeP (152.15 u), which is only possible if MeP molecules react toward other species by addition or similar reactions. By a retrosynthesis approach, it is possible to suggest that compounds 1 and 2 are formed from compounds A and B, after a sequence of hydrolysis, hydroxylation and decarboxylation steps (see Figs. S3aeS3b). Compounds A and B were not detected, however, retrosynthetic analysis shows that A may be formed by the oxidative coupling of two molecules of C, whereas B

may be formed by the oxidative coupling of C and D. Those compounds derive from E and F, respectively, which correspond to hydrolysis and oxidation products of MeP and are very likely to be formed in this medium. It is known that oxidizing conditions, in a wide range of pH values, may lead to oligo- and even polymerization of phenolic compounds [73,74]. Dimers, trimers and tetramers may be the main products in mildly acid and mildly oxidizing conditions [75]. As mentioned before, in this work, when a non-buffered solution was used, it was observed an increase of the solution pH (from 3 to 8 after 4 h), which indicates both alkalinizing and loss of oxidant power. Therefore, oxidative coupling steps involving compounds C and D are likely to happen in the studied conditions. A similar retrosynthesis logic can be used for compounds 3 and 4 (see Figs. S3ceS3d). It can be suggested that compound G is as a common precursor, which is possibly formed by the oxidative coupling of H and C. Compound H can be formed by chlorination of D or derive from I, which is a chlorination product of F. Finally, it is suggested that compound 5 derives from compounds 1 and/or 2, after hydrolysis and decarboxylation steps (see Fig. S3e). Compounds C, D, E, F, and other similar products, were detected by Rosales et al. [28] for MeP degradation using BDD anode and an iron-carbonaceous cathode for electro-Fenton process. Steter et al. [18,20] have also detected compounds D, F, and other similar products, during the first stages of MeP oxidation, on BDD anode, in sulfate and chloride media. Moreover, compounds similar to H and I were also found by Steter et al. [20] in their study on the electrooxidation of MeP on different anodes and different media. However, oxidative coupling products were not observed by those authors [18,20,28]. These studies report that MeP degradation mechanism follows few decarboxylation steps until the formation of hydroquinone and the further rupture of the aromatic ring. In the presence of chloride, a similar mechanism was observed, though with formation of several polychlorinated phenols and chlorinated aliphatic carboxylic acids [20]. On the other hand, Frontistis et al. [15,52] observed the formation of oxidative coupling products on the treatments of ethyl paraben by electrochemical process with a BDD anode and by heat-activated persulfate oxidation. In both studies, it is reported the formation of several oligomers either using persulfate or chloride media. The oligomers were identified by LC-TOF-MS as dimers and trimers of ethyl paraben and its

D. Dionisio et al. / Electrochimica Acta 338 (2020) 135901

7

Fig. 6. Mechanistic proposal for the electrochemical degradation of MeP (100 mg L1, 4 h treatment) on DSA®-Cl2. Japp ¼ 15 mA cm2, NaCl 0.15 mol L1, 25  C.

chlorination products. According to the intermediates identified by MS, the retrosynthetic analysis and the literature, a possible mechanistic route for MeP oxidation is proposed as shown in Fig. 6. First, MeP originates its precursor, 4-hydroxybenzoic acid (F) by nucleophilic addition and releasing of a eOCH3 group. F can undergo addition of a eOH or a -Cl group, forming compounds E and I. Those three compounds combine and form more complex molecules, which happen by oxidative coupling at the position 2 of the aromatic ring, and three main pathways are suggested:  Two molecules of C react and form intermediate A, which further reacts to form product 1 (m/z ¼ 267);  C and D react and form intermediate B, which further reacts to form product 2 (m/z ¼ 221);  H and C react and form intermediate E, which further reacts to form products 3 (m/z ¼ 301) and 4 (m/z ¼ 255). Product 1 is likely formed by hydrolysis of intermediate A, whereas 3 are likely formed by hydroxylation and hydrolysis of G. Products 2 and 4 are possibly formed from intermediates B and G, respectively, by hydrolysis, aromatic ring opening, decarboxylation and cyclizing steps. Product 5 is formed from products 1 and 2 by similar reaction pathways. Therefore, products 3, 4 and 5 are the main last aromatic compounds that remain in the medium before all aromatic rings are open and produce aliphatic structures.

It is worth noting that in the present study only aromatic compounds were determined by LC-MS. Compounds 1 to 5 are expected to be relatively stable, even in the presence of oxidants, because of the conjugation between double bonds and nonbonding electron pairs. This is one reason why those compounds would last long enough to be isolated and detected. Even though, all aromatic compounds were completed removed after 4 h of electrochemical treatment. After the opening of aromatic rings, aliphatic compounds are expected to be formed (including chlorinated carboxylic acids) [18,20,64,76,77]. In fact, aromatic pollutants always follow the formation of aromatic intermediates (which can couple and form polymer) and the ring opening to form carboxylic acids [41,44,47,64,78,79], which in case of using active electrodes are more difficult to be removed than aromatic and they accumulate in the electrolyte [21,54]. The nature of the carboxylic acids formed is very well known since the turn of the century, consisting mainly in oxalic acid, which is very slowly mineralized. Despite the mechanistic complications, it was observed that part of the aliphatic byproducts was oxidized and mineralized to CO2, H2O e Cl, since 36% of the TOC was removed from the medium.

4. Conclusions From the results presented, the following conclusions can be drawn:

8

D. Dionisio et al. / Electrochimica Acta 338 (2020) 135901

 Methyl paraben can be removed from the aqueous medium containing chloride anion by the electrochemical process using the DSA®-Cl2, mainly by mediated oxidation.  Irradiation of UV light facilitates the removal of byproducts, but not of MeP itself. Thus, the synergism was better for higher japp values. Mineralization can be increased by 40% with low additional energy costs.  The degradation process resulted in the formation of a solid byproduct, which probably derives from the oxidation of MeP products and is of polymeric nature. However, the electrolysis itself seems to be effective to remove this product.  MeP oxidation initially happens through the chlorination and hydroxylation of 4-hydroxybenzoic acid. Those compounds undergo oxidative coupling reactions to form higher molecular weight products before the aromatic ring breakdown. Thereafter, several aliphatic acids (including their chlorination products) are expected to be formed as the last byproducts prior to mineralization.

[12]

[13]

[14]

[15]

[16]

[17]

Acknowledgements [18]

Financial support from the Brazilian Federal Agency for the Support and Improvement of Higher Education (CAPES), Saeo Paulo Research Foundation (FAPESP, Brazil) [2016/19662e1, 2016/ 04825e2, 2017/10118e0 and 2017/20444e1], National Council for Scientific and Technological Development (CNPq, Brazil) [140669/ 2014e0] and from the Spanish Ministry of Economy, Industry and Competitiveness, European Union through project CTM201676197-R (AEI/FEDER, UE) are gratefully acknowledged. Authors are also thankful to Prof. Antonio C. B. Burtoloso for the discussion about the final mechanistic proposal.

[19]

[20]

[21]

[22]

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135901. References [1] P.D. Darbre, A. Aljarrah, W.R. Miller, N.G. Coldham, M.J. Sauer, G.S. Pope, Concentrations of parabens in human breast tumours, J. Appl. Toxicol. 24 (2004) 5e13, https://doi.org/10.1002/jat.958. [2] Z. Dagher, M. Borgie, J. Magdalou, R. Chahine, H. Greige-Gerges, p-Hydroxybenzoate esters metabolism in MCF7 breast cancer cells, Food Chem. Toxicol. 50 (2012) 4109e4114, https://doi.org/10.1016/j.fct.2012.08.013. [3] M.G. Soni, I.G. Carabin, G.A. Burdock, Safety assessment of esters of phydroxybenzoic acid (parabens), Food Chem. Toxicol. 43 (2005) 985e1015, https://doi.org/10.1016/j.fct.2005.01.020. [4] D. Bledzka, J. Gromadzinska, W. Wasowicz, Parabens. From environmental studies to human health, Environ. Int. 67 (2014) 27e42, https://doi.org/ 10.1016/j.envint.2014.02.007. [5] J. Boberg, C. Taxvig, S. Christiansen, U. Hass, Possible endocrine disrupting effects of parabens and their metabolites, Reprod. Toxicol. 30 (2010) 301e312, https://doi.org/10.1016/j.reprotox.2010.03.011.  andez-Diaz, R. Hauser, Medications as [6] L.E. Dodge, J.W. Choi, K.E. Kelley, S. Hern a potential source of exposure to parabens in the U.S. population, Environ. Res. 164 (2018) 580e584, https://doi.org/10.1016/j.envres.2018.03.035. [7] N. Aubert, T. Ameller, J.-J. Legrand, Systemic exposure to parabens: pharmacokinetics, tissue distribution, excretion balance and plasma metabolites of [14C]-methyl-, propyl- and butylparaben in rats after oral, topical or subcutaneous administration, Food Chem. Toxicol. 50 (2012) 445e454, https:// doi.org/10.1016/j.fct.2011.12.045. [8] J.C. Xue, K. Kannan, Accumulation profiles of parabens and their metabolites in fish, black bear, and birds, including bald eagles and albatrosses, Environ. Int. 94 (2016) 546e553, https://doi.org/10.1016/j.envint.2016.06.015. rska, E. Jabłon  ska, Parabens and their [9] K. Nowak, W. RatajczakeWrona, M. Go effects on the endocrine system, Mol. Cell. Endocrinol. 474 (2018) 238e251, https://doi.org/10.1016/j.mce.2018.03.014. [10] T. Velegraki, E. Hapeshi, D. Fatta-Kassinos, I. Poulios, Solar-induced heterogeneous photocatalytic degradation of methyl-paraben, Appl. Catal. B Environ. 178 (2015) 2e11, https://doi.org/10.1016/j.apcatb.2014.11.022. [11] A.V. Marta-Sanchez, S.S. Caldas, A. Schneider, S.M.V.S. Cardoso, E.G. Primel, Trace analysis of parabens preservatives in drinking water treatment sludge,

[23] [24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

treated, and mineral water samples, Environ. Sci. Pollut. Res. (2018), https:// doi.org/10.1007/s11356-018-1583-4. J. Chen, B.F.G. Pycke, B.J. Brownawell, C.A. Kinney, E.T. Furlong, D.W. Kolpin, R.U. Halden, Occurrence, temporal variation, and estrogenic burden of five parabens in sewage sludge collected across the United States, Sci. Total Environ. 593 (2017) 368e374, https://doi.org/10.1016/j.scitotenv.2017.03.162. M. Honda, M. Robinson, K. Kannan, Parabens in human urine from several Asian countries, Greece, and the United States, Chemosphere 201 (2018) 13e19, https://doi.org/10.1016/j.chemosphere.2018.02.165. W.H. Li, Y.L. Shi, L.H. Gao, J.M. Liu, Y.Q. Cai, Occurrence, fate and risk assessment of parabens and their chlorinated derivatives in an advanced wastewater treatment plant, J. Hazard Mater. 300 (2015) 29e38, https://doi.org/ 10.1016/j.jhazmat.2015.06.060. Z. Frontistis, M. Antonopoulou, M. Yazirdagi, Z. Kilinc, I. Konstantinou, A. Katsaounis, D. Mantzavinos, Boron-doped diamond electrooxidation of ethyl paraben: the effect of electrolyte on by-products distribution and mechanisms, J. Environ. Manag. 195 (2017) 148e156, https://doi.org/10.1016/ j.jenvman.2016.06.044. J.R. Dominguez, M.J. Munoz-Pena, T. Gonzalez, P. Palo, E.M. Cuerda-Correa, Parabens abatement from surface waters by electrochemical advanced oxidation with boron doped diamond anodes, Environ. Sci. Pollut. Res. 23 (2016) 20315e20330, https://doi.org/10.1007/s11356-016-7175-2. F.E.R.R. Gomes, N.E. de Souza, C.A. Galinaro, L.O.R.R. Arriveti, J.B. de Assis, G. Tremiliosi-Filho, Electrochemical degradation of butyl paraben on platinum and glassy carbon electrodes, J. Electroanal. Chem. 769 (2016) 124e130, https://doi.org/10.1016/j.jelechem.2016.03.016. J.R. Steter, R.S. Rocha, D. Dionísio, M.R.V. V Lanza, A.J. Motheo, Electrochemical oxidation route of methyl paraben on a boron-doped diamond anode, Electrochim. Acta 117 (2014) 127e133, https://doi.org/10.1016/ j.electacta.2013.11.118. J.R. Steter, D. Dionisio, M.R.V. Lanza, A.J. Motheo, Electrochemical and sonoelectrochemical processes applied to the degradation of the endocrine disruptor methyl paraben, J. Appl. Electrochem. 44 (2014) 1317e1325, https://doi.org/10.1007/s10800-014-0742-7. J.R. Steter, E. Brillas, I. Sires, On the selection of the anode material for the electrochemical removal of methylparaben from different aqueous media, Electrochim. Acta 222 (2016) 1464e1474, https://doi.org/10.1016/ j.electacta.2016.11.125. M. Panizza, G. Cerisola, Direct and mediated anodic oxidation of organic pollutants, Chem. Rev. 109 (2009) 6541e6569, https://doi.org/10.1021/ cr9001319. C.A. Martinez-Huitle, M.A. Rodrigo, I. Sires, O. Scialdone, C.A. Martínez-Huitle, s, O. Scialdone, Single and coupled electrochemical proM.A. Rodrigo, I. Sire cesses and reactors for the abatement of organic water pollutants: a critical review, Chem. Rev. 115 (2015) 13362e13407, https://doi.org/10.1021/ acs.chemrev.5b00361. R. Dewil, New perspectives for advanced oxidation processes, J. Environ. Manag. 195 (2017) 93e99, https://doi.org/10.1016/J.JENVMAN.2017.04.010. A.S. Martins, P.J.M. Cordeiro-Junior, G.G. Bessegato, J.F. Carneiro, M.V.B. Zanoni, M.R. de V. Lanza, Electrodeposition of WO3 on Ti substrate and the influence of interfacial oxide layer generated in situ: a photoelectrocatalytic degradation of propyl paraben, Appl. Surf. Sci. 464 (2019) 664e672, https://doi.org/10.1016/j.apsusc.2018.09.054. s, Solar photoelectro-Fenton treatment of a mixture J.R. Steter, E. Brillas, I. Sire of parabens spiked into secondary treated wastewater effluent at low input current, Appl. Catal. B Environ. 224 (2018) 410e418, https://doi.org/10.1016/ j.apcatb.2017.10.060. D. Dionisio, A.J. Motheo, C. S aez, M.A. Rodrigo, Effect of the electrolyte on the electrolysis and photoelectrolysis of synthetic methyl paraben polluted wastewater, Separ. Purif. Technol. 208 (2018) 201e207, https://doi.org/ 10.1016/j.seppur.2018.03.009. ez, P. Can ~ izares, M.A. Rodrigo, Effects of ultraD. Dionisio, A.J. Motheo, C. Sa sound irradiation on the electrochemical treatment of wastes containing micelles, Appl. Catal. B Environ. 248 (2019) 108e114, https://doi.org/10.1016/ J.APCATB.2019.02.004. E. Rosales, G. Buftia, M. Pazos, G. Lazar, M.A. Sanrom an, Highly active based iron-carbonaceous cathodes for heterogeneous electro-Fenton process: application to degradation of parabens, Process Saf. Environ. Protect. 117 (2018) 363e371, https://doi.org/10.1016/j.psep.2018.05.014. M. Neumann-Spallart, S.S. Shinde, M. Mahadik, C.H. Bhosale, Photoelectrochemical degradation of selected aromatic molecules, Electrochim. Acta 111 (2013) 830e836, https://doi.org/10.1016/j.electacta.2013.08.080. J.R. Steter, D. Dionísio, R.S. Rocha, D.W. Miwa, M.R. Lanza, A. Jesus Motheo, Electrochemical degradation of methyl paraben using a boron-doped diamond anode, ECS Trans. (2012) 111e117, https://doi.org/10.1149/1.4704947. G.R.P. Malpass, R.S. Neves, A.J. Motheo, A comparative study of commercial and laboratory-made Ti/Ru0.3Ti0.7O2 DSA® electrodes: in situ and ex situ surface characterisation and organic oxidation activity, Electrochim. Acta 52 (2006) 936e944, https://doi.org/10.1016/j.electacta.2006.06.032. G.R.P. Malpass, D.W. Miwa, A.C.P. Miwa, S.A.S. Machado, A.J. Motheo, PhotoAssisted electrochemical oxidation of atrazine on a commercial Ti/ Ru0.3Ti0.7O2 DSA electrode, Environ. Sci. Technol. 41 (2007) 7120e7125, https://doi.org/10.1021/es070798n. G.R.P. Malpass, D.W. Miwa, A.C.P. Miwa, S.A.S. Machado, A.J. Motheo, Study of photo-assisted electrochemical degradation of carbaryl at dimensionally

D. Dionisio et al. / Electrochimica Acta 338 (2020) 135901

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

stable anodes (DSA®), J. Hazard Mater. 167 (2009) 224e229, https://doi.org/ 10.1016/J.JHAZMAT.2008.12.109. A.J. Motheo, E.R. Gonzalez, G. Tremiliosi-Filho, P. Olivi, A.R. de Andrade, ger, E.M. Belgsir, C. Lamy, The oxidation of formaldehyde on B. Kokoh, J.-M. Le high overvoltage DSA type electrodes, J. Braz. Chem. Soc. 11 (2000) 16e21, https://doi.org/10.1590/S0103-50532000000100004. P.A. Alves, G.R.P. Malpass, H.D. Johansen, E.B. Azevedo, L.M. Gomes, W.F.D. Vilela, A.J. Motheo, Photo-assisted electrochemical degradation of real textile wastewater, Water Sci. Technol. 61 (2010) 491e498, https://doi.org/ 10.2166/wst.2010.870. J.M. Aquino, D.W. Miwa, M.A. Rodrigo, A.J. Motheo, Treatment of actual effluents produced in the manufacturing of atrazine by a photo-electrolytic process, Chemosphere 172 (2017) 185e192, https://doi.org/10.1016/ j.chemosphere.2016.12.154.  ti, D. Gandini, C. Comninellis, A. Perret, W. Haenni, Oxidation of organics G. Fo by intermediates of water discharge on IrO2 and synthetic diamond anodes, Electrochem. Solid State Lett. 2 (1999) 228e230, https://doi.org/10.1149/ 1.1390792. O. Simond, V. Schaller, C. Comninellis, Theoretical model for the anodic oxidation of organics on metal oxide electrodes, Electrochim. Acta 42 (1997) 2009e2012, https://doi.org/10.1016/S0013-4686(97)85475-8. I. Duo, P.A. Michaud, W. Haenni, A. Perret, C. Comninellis, Activation of borondoped diamond with IrO2 clusters, Electrochem. Solid State Lett. 3 (1999) 325e326, https://doi.org/10.1149/1.1391137. R.R. Keenan, J.D. Stuart, Anodic oxidation of DDT, methoxychlor, and related compounds, J. Electrochem. Soc. 123 (1976) 1637e1642, https://doi.org/ 10.1149/1.2132659. C.H. Comninellis, C. Pulgarin, Anodic Oxidation of Phenol for Waste Water Treatment, 1991. https://link.springer.com/content/pdf/10.1007% 2FBF01034049.pdf. (Accessed 6 November 2018). C. Comninellis, C. Pulgarin, Electrochemical oxidation of phenol for wastewater treatment using SnO2, anodes, J. Appl. Electrochem. 23 (1993) 108e112, https://doi.org/10.1007/BF00246946. ringer, C. Comninellis, Electrochemical detoxificaC. Pulgarin, N. Adler, P. Pe tion of a 1,4-benzoquinone solution in wastewater treatment, Water Res. 28 (1994) 887e893, https://doi.org/10.1016/0043-1354(94)90095-7. C. Comninellis, A. Nerini, ANODIC-OXIDATION OF phenol IN the presence OF NaCl for waste-water treatment, J. Appl. Electrochem. 25 (1995) 23e28. % 3CGo. M. Panizza, G. Cerisola, Application of diamond electrodes to electrochemical processes, Electrochim. Acta 51 (2005) 191e199, https://doi.org/10.1016/ j.electacta.2005.04.023. M. Panizza, C.A. Martinez-Huitle, Role of electrode materials for the anodic oxidation of a real landfill leachate e comparison between TieRueSn ternary oxide, PbO2 and boron-doped diamond anode, Chemosphere 90 (2013) 1455e1460, https://doi.org/10.1016/j.chemosphere.2012.09.006. D. Gandini, E. Mahe, P.A. Michaud, W. Haenni, A. Perret, C. Comninellis, Oxidation of carboxylic acids at boron-doped diamond electrodes for wastewater treatment, J. Appl. Electrochem. 30 (2000) 1345e1350. C. Comninellis, A. Kapalka, S. Malato, S.A. Parsons, I. Poulios, D. Mantzavinos, Advanced oxidation processes for water treatment: advances and trends for R&D, J. Chem. Technol. Biotechnol. 83 (2008) 769e776, https://doi.org/ 10.1002/jctb.1873. K.S. Tay, N.A. Rahman, M.R. Bin Abas, Ozonation of parabens in aqueous solution: kinetics and mechanism of degradation, Chemosphere 81 (2010) 1446e1453, https://doi.org/10.1016/j.chemosphere.2010.09.004. X.N. Feng, Y. Chen, Y. Fang, X.Y. Wang, Z.P. Wang, T. Tao, Y.G. Zuo, Photodegradation of parabens by Fe(III)-citrate complexes at circumneutral pH: matrix effect and reaction mechanism, Sci. Total Environ. 472 (2014) 130e136, https://doi.org/10.1016/j.scitotenv.2013.11.005. C. Papadopoulos, Z. Frontistis, M. Antonopoulou, D. Venieri, I. Konstantinou, D. Mantzavinos, Sonochemical degradation of ethyl paraben in environmental samples: statistically important parameters determining kinetics, by-products and pathways, Ultrason. Sonochem. 31 (2016) 62e70, https://doi.org/ 10.1016/j.ultsonch.2015.12.002. Z. Frontistis, M. Antonopoulou, I. Konstantinou, D. Mantzavinos, Degradation of ethyl paraben by heat-activated persulfate oxidation: statistical evaluation of operating factors and transformation pathways, Environ. Sci. Pollut. Res. 24 (2017) 1073e1084, https://doi.org/10.1007/s11356-016-6974-9. J. An, C. Xia, J. He, H. Feng, Journal of Environmental Science and Health, Part A Toxic/Hazardous Substances and Environmental Engineering Oxidation of Propyl Paraben by ferrate(VI): Kinetics, Products, and Toxicity Assessment Oxidation of Propyl Paraben by ferrate(VI): Kinetics, Products, and Toxicity Assessment, 2018, https://doi.org/10.1080/10934529.2018.1459074. s, E. Brillas, M.A. Oturan, M.A. Rodrigo, M. Panizza, Electrochemical I. Sire advanced oxidation processes: today and tomorrow. A review, Environ. Sci. Pollut. Res. 21 (2014) 8336e8367, https://doi.org/10.1007/s11356-014-27831. J.M. Aquino, R.C. Rocha-Filho, L.A.M. Ruotolo, N. Bocchi, S.R. Biaggio, Electrochemical degradation of a real textile wastewater using b-PbO2 and DSA® anodes, Chem. Eng. J. 251 (2014) 138e145, https://doi.org/10.1016/ J.CEJ.2014.04.032. C. Comninellis, Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment, Electrochim. Acta 39 (1994)

9

1857e1862, https://doi.org/10.1016/0013-4686(94)85175-1. [57] F.L. Souza, J.M. Aquino, D.W. Miwa, M.A. Rodrigo, A.J. Motheo, Photo-assisted electrochemical degradation of the dimethyl phthalate ester on DSA® electrode, J. Environ. Chem. Eng. 2 (2014) 811e818, https://doi.org/10.1016/ j.jece.2014.02.003. [58] S. Garcia-Segura, J.D. Ocon, M.N. Chong, Electrochemical oxidation remediation of real wastewater effluents d a review, Process Saf. Environ. Protect. 113 (2018) 48e67, https://doi.org/10.1016/j.psep.2017.09.014. [59] G.R.P. Malpass, D.W. Miwa, L. Gomes, E.B. Azevedo, W.F.D. Vilela, ~es, R. Bertazzoli, S.A.S. Machado, A.J. Motheo, M.T. Fukunaga, J.R. Guimara Photo-assisted electrochemical degradation of the commercial herbicide atrazine, Water Sci. Technol. 62 (2010) 2729e2736, https://doi.org/10.2166/ wst.2010.207. [60] O. Scialdone, S. Randazzo, A. Galia, G. Silvestri, Electrochemical oxidation of organics in water: role of operative parameters in the absence and in the presence of NaCl, Water Res. 43 (2009) 2260e2272, https://doi.org/10.1016/ j.watres.2009.02.014. [61] P. Canizares, J. Garcia-Gomez, J. Lobato, M.A. Rodrigo, Modeling of wastewater electro-oxidation processes part I. General description and application to inactive electrodes, Ind. Eng. Chem. Res. 43 (2004) 1915e1922, https:// doi.org/10.1021/ie0341294. [62] P. Canizares, J. Garcia-Gomez, J. Lobato, M.A. Rodrigo, Modeling of wastewater electro-oxidation processes part II. Application to active electrodes, Ind. Eng. Chem. Res. 43 (2004) 1923e1931, https://doi.org/10.1021/ie0341303. s, E. Brillas, Treatment of a mixture of food color additives [63] A. Thiam, I. Sire (E122, E124 and E129) in different water matrices by UVA and solar photoelectro-Fenton, Water Res. 81 (2015) 178e187, https://doi.org/10.1016/ j.watres.2015.05.057. [64] P. Canizares, R. Paz, C. Saez, M.A. Rodrigo, Electrochemical oxidation of alcohols and carboxylic acids with diamond anodes - a comparison with other advanced oxidation processes, Electrochim. Acta 53 (2008) 2144e2153, https://doi.org/10.1016/j.electacta.2007.09.022. s, M.A. Oturan, Electro-fenton process and related electro[65] E. Brillas, I. Sire chemical technologies based on fenton’s reaction chemistry, Chem. Rev. 109 (2009) 6570e6631, https://doi.org/10.1021/cr900136g. [66] A.M. Polcaro, S. Palmas, F. Renoldi, M. Mascia, On the performance of Ti/ SnO2and Ti/PbO2anodes in electrochemical degradation of 2-chlorophenol for wastewater treatment, J. Appl. Electrochem. 29 (1999) 147e151, https:// doi.org/10.1023/A:1003411906212. ez, P. Canizares, M.A. Rodrigo, Coupling ultra[67] D. Dionisio, A.J. Motheo, C. Sa sound to the electro-oxidation of methyl paraben synthetic wastewater: effect of frequency and supporting electrolyte, ChemElectroChem 6 (2019) 1199e1205, https://doi.org/10.1002/celc.201801199. [68] O. Legrini, E. Oliveros, A.M. Braun, Photochemical processes for water treatment, Chem. Rev. 93 (1993) 671e698, https://doi.org/10.1021/cr00018a003. [69] C.A. Martínez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review, Appl. Catal. B Environ. 87 (2009) 105e145, https://doi.org/10.1016/ J.APCATB.2008.09.017. [70] R.M. Silverstein, F.X. Webster, D.J. Kiemle, Infrared spectrometry, in: Spectrom. Identif. Org. Compd., seventh ed., John Wiley & Sons, Inc., Hoboken, NJ, USA, 2005, pp. 72e126. s, R. Lapuente, L.G. Andio  n, F. Cases, E. Morallo n, J.L. V [71] P. Garce azquez, Electropolymerization of phenol on carbon steel and stainless steel electrodes in carbonate aqueous medium, Polym. J. 32 (2000) 623, https://doi.org/10.1295/ polymj.32.623. di, Electropolymerization of phenol, o-nitro[72] Y. Samet, D. Kraiem, R. Abdelhe phenol and o-methoxyphenol on gold and carbon steel materials and their corrosion protection effects, Prog. Org. Coating 69 (2010) 335e343, https:// doi.org/10.1016/J.PORGCOAT.2010.07.006. [73] G. Mengoli, M.M. Musiani, Protective coatings on iron by anodic oxidation of phenols in oxalic acid medium, Electrochim. Acta 31 (1986) 201e210, https:// doi.org/10.1016/0013-4686(86)87109-2. [74] G. Gao, C.D. Vecitis, Doped carbon nanotube networks for electrochemical filtration of aqueous phenol: electrolyte precipitation and phenol polymerization, ACS Appl. Mater. Interfaces 4 (2012) 1478e1489, https://doi.org/ 10.1021/am2017267. [75] J. Heinze, B.A. Frontana-Uribe, S. Ludwigs, Electrochemistry of conducting polymersdpersistent models and new concepts y, Chem. Rev. 110 (2010) 4724e4771, https://doi.org/10.1021/cr900226k. [76] S.Z. Wang, J.L. Wang, Comparative study on sulfamethoxazole degradation by Fenton and Fe(II)-activated persulfate process, RSC Adv. 7 (2017) 48670e48677, https://doi.org/10.1039/c7ra09325j. s, Reaction sequence for the [77] M.A. Oturan, M. Pimentel, N. Oturan, I. Sire mineralization of the short-chain carboxylic acids usually formed upon cleavage of aromatics during electrochemical Fenton treatment, Electrochim. Acta 54 (2008) 173e182, https://doi.org/10.1016/J.ELECTACTA.2008.08.012. [78] P. Canizares, R. Paz, C. Saez, M.A. Rodrigoz, Electrochemical oxidation of wastewaters polluted with aromatics and heterocyclic compounds, J. Electrochem. Soc. 154 (2007) E165eE171, https://doi.org/10.1149/ 1.2772415. [79] P. Canizares, C. Saez, J. Lobato, R. Paz, M.A. Rodrigo, Effect of the operating conditions on the oxidation mechanisms in conductive-diamond electrolyses, J. Electrochem. Soc. 154 (2007) E37eE44, https://doi.org/10.1149/1.2424409.