Enhancement of Fenton and photo-Fenton processes at initial circumneutral pH for the degradation of the β-blocker metoprolol

Water Research 88 (2016) 449e457

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Enhancement of Fenton and photo-Fenton processes at initial circumneutral pH for the degradation of the b-blocker metoprolol
nez*, S. Esplugas V. Romero, S. Acevedo, P. Marco, J. Gime Departamento de Ingeniería Química, Facultad de Química, Universidad de Barcelona, C/ Martí i Franqu es, 1, 08028 Barcelona, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 July 2015 Received in revised form 9 October 2015 Accepted 18 October 2015 Available online 21 October 2015

The need for acidification in the Fenton and photo-Fenton process is often outlined as one of its major drawbacks, thus in this work the acidification of the Metoprolol (MET) is avoided by the addition of resorcinol (RES), which is used to simulate model organic matter. The experiments were carried out at natural pH (6.2) with different Fe2þ (1, 2.5, 5, and 10 mg/L) and H2O2 (25, 50, 125 and 150 mg/L) concentrations. The performance of MET and RES degradation was assessed along the reaction time. Working with the highest concentrations (5 and 10 mg/L of ferrous iron and 125 and 150 mg/L of H2O2) more than 90% of MET and RES removals were reached within 50 and 20 min of treatment, respectively, by Fenton process. However a low mineralization was achieved in both cases, likely, due to by-products accumulation. Regarding to photo-Fenton process, within 3 min with the highest iron and hydrogen peroxide concentrations, a complete MET degradation was obtained and 95% of RES conversion was achieved. Parameters such Total Organic Carbon, Chemical Oxygen Demand, and AOS were measured. Intermediates were identified and MET degradation path was proposed in the presence of resorcinol. Finally, a comparison between Fenton and photo-Fenton processes at acid pH and at initial circumneutral pH was discussed. The positive effect of RES on Fenton and photo-Fenton systems has been confirmed, allowing the work at circumneutral pH. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Metoprolol Circumneutral pH Fenton Photo-Fenton Resorcinol

1. Introduction In the last years, water scarcity and water quality have become a worldwide concern. Every day large amounts of water are contaminated by different pollutants coming from domestic or industrial uses. Among those pollutants found in water, there is a special group of pharmaceuticals (b-blockers) used for the treatment of the cardiovascular diseases. The major pathway through which b-blockers reach the environment is from pharmaceutical industry. It is true that detected concentrations of pharmaceuticals in surface waters are normally in the order of mg/L or ng/L (Andreozzi et al., 2003; Gros et al., 2006; Vieno et al., 2007). However, the concentrations measured at the discharge point of some industries can reach concentrations around several mg/L (Polar, 2007). In these cases, an effective treatment before discharge can avoid the dispersion of relative high concentrations of pharmaceuticals in superficial waters and mitigate their adverse effects on the environment. The treatments that are currently

carried out in some industries, in some cases, cannot ensure adequate quality effluents output from the environmental point of view. Advanced Oxidation Processes (AOPs) may be a solution in the treatment of these contaminants (Hollender et al., 2009; Miralles-Cuevas et al., 2014). As known, AOPs comprise a range of techniques which can generate highly reactive species able to oxidize recalcitrant pollutants to CO2, H2O and inorganic salts (Neyens and Baeyens, 2003). Among these processes, Fenton and photo-Fenton can be an efficient route for the degradation of a wide variety of organic re
n et al., 2011). fractory compounds in water (Gonz
alez-Bahamo Fenton process (Fe2þ/H2O2/dark) involves the reaction between dissolved Fe2þ and H2O2 in acidic aqueous solution leading to oxidation of Fe2þ to Fe3þ and the production of hydroxyl radicals (HO) (Li et al., 2012; Litter and Quici, 2010; Parsons, 2004). The main reaction involved in the production of free hydroxyl radicals is spontaneous and can occur without the presence of light:

Fe2þ þ H2 O2 /Fe3þ þ HO: þ HO * Corresponding author.
nez). E-mail address: [email protected] (J. Gime http://dx.doi.org/10.1016/j.watres.2015.10.035 0043-1354/© 2015 Elsevier Ltd. All rights reserved.

(1)

In the presence of H2O2 and at pH  3, the reaction system is autocatalytic, because Fe (III) reacts with H2O2 giving Fe2þ, which is

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V. Romero et al. / Water Research 88 (2016) 449e457

generated at a slow rate (Litter and Quici, 2010): Fe 3þ þ H2 O2 /Fe2þ þ HO:2 þ Hþ (Bautista et al., 2008; Burbano et al., 2008). Photo-Fenton or photo-assisted Fenton (Fe2þ/H2O2/light) process involves irradiation with sunlight or an artificial light source, which increases the rate of contaminant degradation by stimulating the reduction of Fe3þ to Fe2þ. Photo-Fenton process starts with the classical Fenton's reagent Eq. (1). When the system is irradiated with UV or visible light, the photo-reduction of ferric to ferrous ions is promoted concomitantly with the generation of
n et al., additional HO$, according to Eq. (2) (Parsons, 2004; De Leo 2014; Guz et al., 2014; Carra et al., 2014; Pupo Nogueira et al., 2007).

Fe

3þ

þ H2 O þ hv/Fe2þ þ $OH þ Hþ

(2)

Photo-Fenton is a complex process where several reactions can take place between the species in solution (Parsons, 2004). The optimal pH for this process is 2.8, because the solubility of Fe3þ hydroxy complexes decreases for pH values higher than 3. In addition, [Fe(OH)]2þ, the most photoactive species (with radiation absorption between wavelengths 290 and 400 nm), reaches its maximum concentration around the aforementioned pH (Pignatello et al., 2006). Moreover at this pH the reaction between Fe2þ and H2O2 produces OH (~60%) and ferryl anion, both species, the reactive OH and [Fe(OH)]2þ, account for the degradation of the pollutant in the ferrous step (Minero et al., 2013). The need for acidification in the Fenton and photo-Fenton process is often outlined as one of its major drawbacks because this means additional cost through the consumption of reagents for acidification and possibly for the subsequent neutralization. Therefore, there is strong interest in working at neutral pH to avoid pH adjustment. The positive effect of natural organic matter (NOM) constituents on photo-Fenton systems, which allows working at near neutral pH, has been previously studied (Murray and Parsons, 2004; Georgi et al., 2007; Lipczenska-Kochany and Kochany, 2008; MoncayoLasso et al., 2009; Vermilyea and Voelker, 2009; Huang et al., 2013). The problems related to the lack of efficiency of Fenton and photo-Fenton at neutral pH could be solved by adding compounds able to form stable complexes with iron. It is known that iron forms complexes with dissolved organic matter (DOM), like polycarboxylates and aminopolycarboxylates, and these complexes typically have higher molar adsorption coefficients in the near-UV and visible regions than the aquocomplexes (Miralles-Cuevas et al., 2014; Pignatello et al., 2006). Consequently, both effects could substantially improve photo-Fenton. In addition, the complexes undergo photoreduction via ligand metal charge transfer leading to the production of Fe2þ necessary for the Fenton's cycle.

h i h i Fe3þ L þ hv/ Fe3þ L /Fe2þ þ L:

(3)

Thus, photo-Fenton process at neutral pH in waters containing DOM can increase the feasibility of using this AOP at large commercial scale, since costs and drawbacks of acidification and the subsequent neutralization are eliminated (De la Cruz et al., 2012). As commented before, some industries, in particular some pharmaceutical industries, have problems from the point of view of debugging, especially due to the complex and toxic waste generated and its difficult treatment. In this context, the aim of this work is the removal of the b-blocker Metoprolol (MET), as model pollutant, by Fenton and photo-Fenton at initial circumneutral pH. Relatively high MET concentrations were used to simulate an industrial aqueous solution. In this work the described photo-Fenton is actually a variant of the common oxidation advanced process that starting with Fe2þ the most used presently for practical applications. The acidification of the solution is avoided by the addition of


mez et al., resorcinol (RES, di-hydroxy benzene isomer) (Ortega-Go
mez et al., 2014), which is used 2013; Spuhler et al., 2010; Ortega-Go to simulate organic matrix in water and to generate ferricarboxylates complexes. The efficiency of this process was also compared with the classical photo-Fenton process at acid pH. 2. Materials and experimental SET-UPS 2.1. Chemicals and reagents The initial solutions of 50 mg/L of Metoprolol Tartrate Salt (MET, CAS number: 56392-17-7, 99% purity from SigmaeAldrich) were prepared using deionized water. This high concentration (50 ppm) was selected because it is the real concentration that can come from some industries. Acetonitrile (analytical reagent grade from Fischer Chemical) and orthophosporic acid (85% from Panreac Quimica) were used for HPLC analysis. H2O2 (30% w/w, from Merck), FeSO4.7H2O (PA from Panreac), NaHSO3 and MeOH (PAI from Panreac) reagents were used without further purification. Resorcinol (RES, CAS number 108-46-3) was purchased from SigmaeAldrich. 2.2. Techniques and analytical determination Metoprolol (MET) concentration was monitored by HPLC 1260 from Waters using a SEA18 (250  4.6 mm i.d.; 5 mm particle size) Teknokroma column, and 80 Hz UV detector. The mobile phase was composed of water (pH 3) and acetonitrile (80:20), injected with a flow-rate of 0.85 mL/min. MET concentration was followed at UV maximum absorbance (221.9 nm). TOC was analyzed with a Shimadzu TOC-V CNS analyzer. To analyze COD, the Standard Methods 5220D procedure was followed, using a spectrophotomer (Hach Lange DR 2500) at 420 nm. H2O2 consumption was followed using the metavanadate spectrophotometric method at 450 nm (Pupo Nogueira et al., 2005). For the intermediates identification, samples were analyzed by electrospray ionization/mass spectrometry using an electrospray (ion spray) ESI-MS and a LC/MSD-TOF (Agilent Technologies) mass spectrometer. All samples were filtered with a polyethersulfone membrane filter (0.45 mm, Chemlab) to remove the catalyst before analytical procedures except for iron measures. The iron (II) content was determined by o-phenanthroline standardized procedure (ISO 6332). 2.3. Experimental setup All experiments for Fenton and photo-Fenton were carried out in a blacklight blue lamps reactor (BLB), with or without irradiation, depending on the treatment required. The photochemical reactor was a 2 L Pyrex-jacketed thermostatic vessel (inner diameter 11 cm, height 23 cm), equipped with three 8 W BLB lamps (Philips TL 8W08 FAM). The photon flux was measured with o-Nitrobenzaldehyde actinometry (De la Cruz et al., 2013) and it was 6.0 mE/s at 365 nm. The reactor was covered with aluminum foil to avoid loss of radiation. During the experiments the temperature was kept constant at 25  C with a thermostatic bath (Haake C-40). To assure a good mixing of the solution the vessel was provided with a magnetic stirrer. In photo-Fenton experiments, when hydrogen peroxide was added, light was switched on immediately. 2.4. Sample preparation The aqueous solution of 50 mg/L of MET with 50 mg/L of RES was prepared in Milli-Q water. Then, the iron was added, and after 15 min of stirring, the hydrogen peroxide was added into the solution. In the case of photo-Fenton once the hydrogen peroxide was added the lamps were switched on. During all Fenton and photo-

V. Romero et al. / Water Research 88 (2016) 449e457

Fenton experiments, the MET, RES, TOC, hydrogen peroxide, Fe2þ, total Fe concentrations, pH and temperature were measured. 3. Results and discussion Preliminary assays were done in BLB reactor to assess possible MET degradation, not referable to Fenton and photo-Fenton process or to dismiss possible interferences or complexations between MET-RES-Fe2þ. All preliminary experiments lasted 60 min at natural pH (6.2) and the different reagent concentrations used were 50 mg/L of MET, 50 mg/L of RES, 12.5 mg/L of Fe2þ and 125 mg/L of H2O2. Thus, MET/Fe2þ/hv, MET/H2O2 in dark and MET/Fe2þ in dark showed negligible MET degradation and mineralization. RES/H2O2 in dark presented negligible RES degradation and mineralization. MET/RES and MET/RES/Fe2þ in dark exhibited negligible MET, RES and TOC degradation. In all experiments where Fe2þ was used, it was added at initial working pH 6.2 (circumneutral pH), without adjustment to pH 3. The Ligand-Fe (RES-Fe) molar ratio is an important parameter to establish the best operational conditions, and it should be stoichiometrically correct and at least 1:1 or higher (De Luca et al., 2014). To verify these condition three different concentrations of the Ligand (Resorcinol) were tested (12.5, 25 and 50 mg/L) to evaluate the best percentage of iron chelation with the minimum quantity of ligand, leaving the necessary free Fe2þ to catalyse the reaction. Thus, RES was mixed with 50 mg/L of MET, 12.5 mg/L of Fe2þ and 125 mg/L of H2O2 during 60 min. It was observed that a complete MET degradation was reached within 10 min for the three RES concentrations tested. However, for 12.5 and 25 mg/L of RES, the Fe2þ concentration dropped after 5 and 35 min, respectively. Thus, this behavior shows that RES-Fe molar ratio was inadequate because the uncomplexed iron precipitated or the Fe2þ concentration is not enough to complex with RES. This could be because of the oxidation rate of Fe2þ to Fe3þ by the Fenton reaction is higher than the reduction rate of Fe3þ to Fe2þ by photo-Fenton process (Baba et al., 2015). However, Fe2þ concentration was kept constant during all the experiments when 50 mg/L of RES were used, as this ratio allows the highest percentage of iron chelate among the RES-Fe ratios tested. Thus, 50 mg/L of RES was selected as the best resorcinol concentration to carry out the following experiments. 3.1. Influence of Fe2þ and hydrogen peroxide concentrations in Fenton process in the presence of resorcinol All experiments have been carried out with 50 mg/L of MET and 50 mg/L of RES at circumneutral pH. Increasing the initial Fe2þ concentration (1, 2.5 or 5 mg/L), a positive effect in MET and RES degradations can be observed. The series of experiments with 25 and 50 mg/L of hydrogen peroxide were carried out up to total hydrogen peroxide consumption and the experiments with 125 mg/L of H2O2 up to 180 min, as maximum reaction time. In all the experiments the initial Fe2þ concentration remained almost constant during the reaction time. The different iron and hydrogen peroxide concentrations tested are summarized in Table 1. It can be observed the positive influence of iron and H2O2 in MET and RES degradations, which increase when H2O2 concentration increases. On the other hand, the pH trends showed that, at the beginning of the treatment, the pH drops from 6.2 to 3.6 and it keeps constant during the reaction time, as a result of the acidic nature of the RES by-products in the media (the decreased pH slows the rate of Fe2þ oxidation by O2 (Barona et al., 2015)). Moreover, we can assume that the by-products form complexes with ferric ions (Moncayo
mez et al., Lasso et al., 2009; Spuhler et al., 2010; Ortega-Go

451

Table 1 MET and RES conversions achieved for the different Fe2þ and H2O2 concentrations tested in Fenton process. Fe2þ (mg/L)

H2O2 (mg/L)

MET removal (%)

RES removal (%)

Time (min)

1 1 1 2.5 2.5 2.5 5 5 5 10

25 50 125 25 50 125 25 50 125 150

26.6 72.8 87.1 54.5 85.0 96.5 64.6 90.5 98.7 100

29.6 66.1 80.3 51.0 73.7 87.0 56.0 72.6 90.0 92.0

60 60 60 60 60 60 60 60 50 20

2014), thus, permitting their solubilisation. The initial pH (6.2) of the solution subserve RES degradation, suggesting that it could be
mez mainly catalysed by iron-carboxilic acid complexes (Ortega-Go et al., 2013; Barona et al., 2015). In this process, the ferrous iron in solution is present during all the reaction time (see Fig. 1) likely, because of the resorcinol and its by-products present in solution (Barona et al., 2015). The Fenton process has the drawback that the regeneration of ferrous iron from ferric iron is the rate limiting step in the catalytic iron cycle. However, using resorcinol there was always ferrous iron during the treatment, thereby suggesting that Fe3þ reduction to Fe2þ was fast enough. 3.2. Mineralization and AOS in the presence of resorcinol Regarding mineralization, Table 2 summarizes TOC and COD conversion at the end of the processes and it also shows the corresponding Average Oxidation State (AOS) values. AOS was calculated using Eq. (4):

AOS ¼ 4½ðTOC  CODÞ=TOC

(4)

Where, TOC and COD are reported in mol/L of carbon and oxygen, respectively. AOS takes values between þ4 for CO2, the most oxidized state of C, and 4 for CH4, the most reduced state of C (Sarria et al., 2002). As it could be appreciated in Table 2, not very promising TOC and COD removals were achieved possibly due to RES and/or MET recalcitrant nature (Barona et al., 2015). When Fenton was carried out with 10 mg/L of iron and 150 mg/L of hydrogen peroxide, the highest mineralization was reached: 17.2% within 60 min. The initial value of AOS of the solution (MET þ RES) was 0.26 and it can be seen in Table 2 that this value increases at the end of the treatments, reaching the highest value with 10 mg/L of iron and

Fig. 1. Trend of Fe2þ present in the experiments carried out with 50 mg/L of MET, 50 mg/L of RES during the Fenton process and without RES (pH 2.8).

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Table 2 Mineralization at 60 min, COD removal and AOS at the end of Fenton process under different work conditions. Fenton Fe2þ (mg/L)

H2O2 (mg/L)

TOC removal (%)

Time (min)

COD removal (%)

AOS

Time (min)

1 1 1 2.5 2.5 2.5 5 5 5 10

25 50 125 25 50 125 25 50 125 150

2.6 7.6 9.6 4.7 10.2 10.7 9.1 10.2 11.6 17.2

60 60 60 60 60 60 60 60 60 60

9 e e 9.5 21 e 17 19 e 33

0.4 e e 0.4 0.7 e 0.5 0.6 e 1.0

110 150 180 60 120 120 60 60 180 120

consequent formation of MET-219. The hydrogen abstraction and elimination of water from MET-267 possibly generates MET-249, which drives to MET-207 by loss of the isopropyl moiety. Afterwards a possible oxidation of MET-207 produces MET-225. Moreover MET-207 can generate MET-192 by a loss of ammonia after the hydrogen abstraction. MET-133 was identified as amino-diol which can be produced by breaking of the CeC bond in the aliphatic part of the MET-267. MET-149 could be produced by the oxidation of MET-133. MET115 was identified as fragment of the ethanolamine side by abstraction of aromatic chain. Several ion masses were also identified during RES degradation (Fig. 3) by Fenton and photo-Fenton processes (Table 5) at the end of the treatment. Some of them, such as oxalic acid (E), maleic acid (H) and succinic acid (I) are in agreement with Barona and coworkers (Barona et al., 2015). Resorcinol was also identified (G).

150 mg/L of H2O2. The increase in AOS means that more oxidized species are formed, thus, more potentially biodegradable compounds are produced during the MET and RES degradation. 3.3. Influence of resorcinol in photo-Fenton process In order to see the influence of the resorcinol in photo-Fenton process, 2.5 mg/L of Fe2þ/25 mg/L of H2O2 and 10 mg/L of Fe2þ/ 150 mg/L of H2O2 concentrations were used, since they have been used without resorcinol in previous studies (Romero Olarte, 2015) (see Table 3). Photo-Fenton exhibits higher efficiency in MET and RES degradation than Fenton, possibly due to the continuous OH generation. Concerning to TOC and COD conversions, low removals were achieved possible due to RES and/or MET recalcitrant nature as it was mentioned before. Table 3 shows an increase in AOS, particularly with 10 mg/L Fe2þ and 150 mg/L H2O2. It means that more oxidized species are formed.

3.5. Comparison Fenton and photo-Fenton with and without resorcinol

3.4. Intermediates oxidation and identification in the presence of resorcinol

Fenton and Photo-Fenton were compared at pH 2.8 and at circumneutral pH (10 mg/L of Fe2þ and 150 mg/L H2O2) (Fig. 4, a and b). At acid pH the process was carried out without resorcinol. In order to show the improvement of MET degradation using resorcinol in Fenton and photo-Fenton processes, Table 6 has presented as the summary of the experimental results obtained at 2.5 mg/L of Fe2þ/25 mg/L of H2O2 and 10 mg/L of Fe2þ/150 mg/L of H2O2 concentrations. A complete MET depletion was obtained by Fenton at circumneutral pH, within 20 min and 67% of MET conversion was achieved working with Fenton at pH 2.8. In photo-Fenton, with resorcinol and circumneutral pH, a complete MET elimination was obtained in 3 min of irradiation and 7 min of irradiation were necessary without resorcinol and pH 2.8. Fig. 4 shows the enhancement of the process when the experiments were carried out with resorcinol in the media, likely because there is the formation of complexes between resorcinol and its degradation intermediates with the ferric iron (Moncayo-Lasso et al., 2009;
mez et al., 2013, 2014) that increases MET degradaOrtega-Go tion. The presence of resorcinol has a chelating effect, thus, the positive effect of natural organic matter (NOM) on Fenton and

Intermediates identification was carried out on samples collected at the end of the treatment. Several ion masses were identified during MET degradation by Fenton and photo-Fenton 
processes (Abramovi
c et al., 2011; Borkar et al., 2012; Soji c et al., 2012; Tay et al., 2013). The m/z (mass-to-charge ratio) is shown in Table 4. According to the proposed structures, a possible pathway for MET degradation can be suggested (Fig. 2). The oxidation of MET m/ z ¼ 267 probably can form MET-281, MET-297 and MET-283 by binding the OH radical in the aromatic ring. The subsequent oxidation drives to di, tri and tetra hydroxylates (MET-299, MET315 and MET-331, respectively). MET-239 can be formed probably by reactions which involve OH attack on the ether side chain followed by elimination. In the next step, MET-239 undergoes the oxidation of alcohols to aldehydes explained by MET-237. The binding of OH radicals in the aromatic ring of MET-237 generates the formation of MET-253. From MET-237 one can also have water elimination generating a carbonyl, followed by an intermolecular electron transfer, which generates a double bond and the

Table 3 MET and RES conversions, mineralization at 60 min, COD removal and AOS at the end of the photo-Fenton process. Photo-Fenton Fe2þ (mg/L)

H2O2 (mg/L)

MET removal (%)

RES removal (%)

Time (min)

TOC removal (%)

COD removal (%)

AOS

Time (min)

2.5 10

25 150

54.0 100

48.9 94.4

30 3

7.3 25

11 45

0.4 1.2

60 60

V. Romero et al. / Water Research 88 (2016) 449e457

453

Table 4 MET by-products identified by HPLC-MS at the end of the treatment by Fenton and photo-Fenton process with 50 mg/L of RES and 50 mg/L of MET.1 2.5 mg/L Fe (II), 25 mg/L H2O2 Fenton;2 2.5 mg/L Fe (II), 25 mg/L H2O2 photo-Fenton;3 10 mg/L Fe (II), 150 mg/L H2O2 Fenton and4 10 mg/L Fe (II), 150 mg/L H2O2 photo-Fenton. m/z

Elemental composition

Proposed structure (Label)

Fenton

Photo-Fenton

Fenton

Photo-Fenton

1

2

3

4 X

267

C15H25NO3

X

X

X

115

C6H13NO

X

X

X

133

C6H15NO2

X

X

X

X

149

C6H15NO3

X

X

192

C12H16O2

X

207

C12H17NO2

X

219

C13H17NO2

X

X

X

225

C12H19NO3

X

231

C10H17NO5

X

X

X

X

237

C13H19NO3

X

X

X

X

239

C13H21NO3

X

X

X

X

(continued on next page)

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V. Romero et al. / Water Research 88 (2016) 449e457

Table 4 (continued ) m/z

Elemental composition

Proposed structure (Label)

Fenton

Photo-Fenton

Fenton

Photo-Fenton

1

2

3

4

249

C15H23NO2

X

251

C14H21NO3

X

X

X

253

C13H19NO4

X

X

X

X

269

C14H23NO4

X

X

281

C15H23NO4

X

X

X

X

283

C15H25NO4

X

X

X

X

297

C15H23NO5

299

C15H25NO5

X

X

315

C15H25NO6

X

X

331

C15H25NO7

X

X

photo-Fenton systems, which allows working at near neutral pH
mez (Georgi et al., 2007; Moncayo-Lasso et al., 2009; Ortega-Go et al., 2013; Spuhler et al., 2010; Rodríguez-Chueca et al., 2014), has been confirmed. Another aspect is that, during the reaction, OH attacked both MET and RES, but there is not complexes destruction because no loss of iron was observed (De Luca et al.,

x

2014). This could be explained likely by the high stability of the complex with resorcinol and its intermediates. Another reason of these increase of MET degradation would be by the acid nature of the resorcinol photo-degradation by-products, the decrease of pH slows the rate of Fe2þ oxidation by O2 to Fe3þ, allowing their solubilisation (Minella et al., 2014; Barona et al., 2015).

V. Romero et al. / Water Research 88 (2016) 449e457

455

Fig. 2. Proposed MET degradation pathways by Fenton and photo-Fenton processes with resorcinol present in the media.

Fig. 3. MS-spectrum of major oxidation products of resorcinol (a-) and liquid chromatogram of MET and RES for photo-Fenton process at neutral pH with 2.5 mg/L of Fe2þ e 25 mg/L H2O2 at 30 min of reaction time (b-).

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V. Romero et al. / Water Research 88 (2016) 449e457

Table 5 RES by-products identified by HPLC-MS at the end of the treatment by Fenton and photo-Fenton process. Compound

m/z

Elemental composition

A B C D E F G H I J K L M N

72 73 74 88 90 101 110 116 118 124 126 128 130 142

C3H4O2 C2HO3 C2H2O3 C3H4O3 C2H2O4 C3HO4 C6H6O2 C4H4O4 C4H6O4 C6H4O3 C6H6O3 C5H4O4 C5H6O4 C6H6O4

Table 6 MET and RES conversions, mineralization at 60 min of the Fenton and photo-Fenton processes at circumneutal pH and acid pH, respectively. Fe2þ (mg/L)

H2O2 (mg/L)

Fenton 2.5 25 10 150 Photo-Fenton 2.5 25 10 150

Neutral pH

pH 2.8 MET removal (%)

TOC removal (%)

Time (min)

MET removal (%)

TOC removal (%)

54.5 100

4.7 17.2

23 67

4 8

60 60

81 100

7.3 76

54 100

7.3 25

60 60

Fenton process. In both cases, Fe2þ concentration remained almost constant during all the reaction time, probably because of the high stability of the resorcinol and its by-products and of the ferrous iron regeneration that can follow, under these conditions, different paths. Biodegradability increased at the end of the processes. Twenty-one intermediates of MET degradation were identified and a possible degradation pathway by Fenton and photoFenton was proposed. References

Fig. 4. MET degradation by a-Fenton and b-photo Fenton process at acid pH and circumneutral pH.

4. Conclusions The addition of resorcinol allowed to work at circumneutral pH and produced an enhancement of Fenton and photo-Fenton processes compared with the process at initial pH 2.8, probably because of the complexes formed between RES by-products and ferric ions. In Fenton process (10 mg/L of Fe2þ and 150 mg/L of H2O2), in presence of resorcinol, a complete MET removal and 92% of RES degradation were achieved in 20 min of treatment. Under the same conditions tested in Fenton, a complete MET depletion and 94.4% of RES conversion were reached in 3 min by photo-


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