Comprehensive strategy for the degradation of anti-inflammatory drug diclofenac by different advanced oxidation processes

Accepted Manuscript Comprehensive strategy for the degradation of anti-inflammatory drug diclofenac by different advanced oxidation processes Emilio Rosales, Silvia Diaz, Marta Pazos, M. Angeles Sanromán PII: DOI: Reference:

S1383-5866(18)30159-X https://doi.org/10.1016/j.seppur.2018.04.014 SEPPUR 14510

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

14 January 2018 29 March 2018 3 April 2018

Please cite this article as: E. Rosales, S. Diaz, M. Pazos, M. Angeles Sanromán, Comprehensive strategy for the degradation of anti-inflammatory drug diclofenac by different advanced oxidation processes, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.04.014

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Comprehensive strategy for the degradation of anti-inflammatory drug diclofenac by different advanced oxidation processes Emilio Rosalesa, Silvia Diaza,b, Marta Pazosa, M. Angeles Sanromána,*,[email protected] a

Department of Chemical Engineering, University of Vigo, Campus As Lagoas-Marcosende,

36310 Vigo, Spain b

Instituto Politécnico Nacional, Av. Instituto Politécnico Nacional S/N, Gustavo A. Madero,

Lindavista, 07738 Ciudad de México, D.F., México

Abstract The aim of this study was to develop a comprehensive strategy for the degradation of a pharmaceutical compound, i.e., diclofenac (DCF), in which the solubility is strongly related to the solution pH. Therefore, in this study, alternative treatments according to DCF concentration ranges were developed. Working at the concentration of 7.5 mg/L and pH below 4, the considered treatments by sonolysis, anodic oxidation and electro-Fenton resulted in a good performance with high abatement levels. At high concentrations (140 mg/L), it was necessary to operate at near neutral pH, and the three proposed treatments were sonolysis, anodic oxidation and electro-Fenton performed using iron-alginate spheres as the heterogeneous catalyst. Heterogeneous electro-Fenton (HEF) resulted in the best option for the pollutant removal, and three new chitosan-based heterogeneous catalysts were synthesized and evaluated. The operational conditions (intensity and dosage of catalyst) were optimized resulting in an intensity of 300 mA and catalyst dose of 20 g/L. Operating at these conditions, high levels of DCF degradation and mineralization were achieved. Several treatment cycles, using the same catalyst, were successfully carried out, and the catalysts demonstrated good stability and the potential to operate in a continuous way. Finally, the

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evolution of carboxylic acids and intermediate generation were analysed leading to the proposal of a plausible DCF degradation pathway for HEF. Keywords: Anodic oxidation, chitosan beads, diclofenac, heterogeneous electro-Fenton, sonolysis.

1. Introduction Emerging pollutants are suspected of causing adverse effects in human health and ecosystems. These contaminants include pharmaceutical steroids and hormones, flame retardants, surfactants and personal care products [1]. Their release into the environment even in small quantities provokes an impact on aquatic organisms or human beings if they accumulate in water bodies. In the last years, the European Union established regulations on these substances and a watch list was elaborated according to the Directive on Environmental Quality Standards. In this list, three substances, i.e., the natural hormone oestradiol (E2), the anti-inflammatory drug diclofenac (DCF) and the synthetic hormone ethinyl oestradiol (EE2), were included (Decision 2015/495) in order to facilitate the determination of appropriate measures to address the risk evidenced by those substances [2]. Among them, DCF has attracted attention in the last years due to that it is often detected in wastewater treatment plants effluents and groundwater. The occurrence of DCF in different aquatic compartments and effluents of wastewater, surface water and groundwater is detected in a wide low-concentration range from 0.8 ng/L up to 4.4 mg/L [3]. This pollutant is poorly degraded by traditional wastewater treatments [3, 4]. For this reason, different treatment alternatives have been proposed in the literature for the removal of this substance including advanced oxidation processes (AOPs). AOPs are characterized by the use of non-selective and powerful oxidants such as hydroxyl radicals (●OH), which are able to degrade a wide range of pollutants. In the literature, several

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AOPs have been presented as alternatives including sonolysis [4, 5], electro-Fenton [6, 7],photo-Fenton [8], solar photo-Fenton, and sonochemical [9]. Sonolysis (SO) degradation is an effective treatment for different types of compounds including pharmaceuticals [4, 5]. The generation of ●OH by SO of water has been reported to be due to cavitation (Eq. (1)). During the formation, growth and collapse of small bubbles, high pressures and temperatures are reached, leading to the thermal dissociation of water resulting in the generation of ●OH and ●H [10]. H2O + ))) → ●OH + ●H

Eq. (1)

Anodic oxidation (AO) is another AOP with promising performance in the removal of several organic compounds [11, 12]. This technique is based on the generation of ●OH at the anode as a result of water oxidation onto its surface. The electro-Fenton (EF) process has been considered as a promising alternative due to the in situ generation of H2O2 on the surface of the cathode (Eq. (2)) and the regeneration of the catalyst by the effect of the electric field (Eq. (3)), promoting an eco-friendly treatment system [7, 13]. Thus, the Fenton’s reaction (Eq. 4) is promoted by the electric field action. O2 + 2H+ + e-  H2O2

Eq. (2)

Fe3+ + e-  Fe2+

Eq. (3)

Fe2+ + H2O2 + H+  Fe3+ + H2O + ●OH

Eq. (4)

The main drawbacks associated with EF is a narrow pH range of operation to avoid the formation and subsequent precipitation of iron. To overcome this limitation, heterogeneous electro-Fenton (HEF) has arisen as an alternative to the homogeneous process due to the prevention of sludge formation and extension of the pH range and catalyst life, providing an easy recovery with possibility of reuse in batch or a continuous way without requiring regeneration or replacement.

3

Currently, numerous approaches have been investigated for the preparation of new efficient catalysts involving the use of natural polymeric matrixes such as alginate, chitosan, agar, etc. [6, 13, 14]. Among them, chitosan presents good properties for the preparation of catalysts due to the presence of amine and hydroxyl groups that can act as chelating agents for metal immobilization [15]. The crosslinking reaction of chitosan and a crosslinking agent such as epichlorohydrin leads to the formation of a polymer (Figure 1), which after synthesis enhances the particle resistance, maintaining a good contact area [16] and reinforcing its potential for use as heterogeneous catalysts. In the present study, the degradation of DCF was investigated. However, DCF solubility depends strongly on the pH of the surrounding solution. It is very water soluble in neutralalkaline medium but has low solubility (below 10 mg/L) at pH below pKa value (approximately 4) [17]. Thus, the aim of this work is to evaluate the potential of different treatment alternatives according to the DCF solubility at different pH values. Initially, the removal of the pollutant at concentrations lower than 10 mg/L by SO, AO and EF was evaluated. Then, high concentration (140 mg/L) at near neutral pH was considered, and the three previous treatments were evaluated, with HEF replacing EF. To improve the HEF, new catalysts based on iron chitosan-epichlorohydrin spheres were synthesized. After that, the catalysts were characterized by IR and SEM-EDS and their ability in the HEF was evaluated. Based on the obtained results, the best catalyst was considered for the optimization of the operational conditions (intensity and catalyst dosage), and a semi-continuous batch treatment was ascertained for several cycles. Finally, the mineralization of pollutants was studied following the carboxylic acids trend and a plausible degradation pathway was proposed. 2. Materials and methods 2.1 Reagents

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DCF, sodium sulphate, sulphuric acid, chitosan (CH), epichlorohydrin (ECH) and iron (II) sulphate heptahydrate, and iron (III) oxide were provided by Sigma-Aldrich (Barcelona, Spain). Alginate was purchased from Analema (Spain). All the chemicals used in the experimental procedures were reagent grade. 2.2 Polysaccharide spheres catalysts 2.2.1 Synthesis procedure Alginate spheres (ABE) were prepared according to the procedure described by Iglesias et al. [14]. Chitosan-epichlorohydrin beads (CB) were prepared according to the method reported by Wan Ngah et al. [16]. Initially, 0.8016 g of chitosan flakes were dissolved in 25 mL of 5% (v/v) acetic acid solution until ensuring all of the chitosan was dissolved. The solution was sonicated at 40 kHz 30% power for 5 hours in order to remove air bubbles. Then, the chitosan solution was added dropwise into a precipitation solution containing 100 mL of 0.5 M NaOH generating the spherical chitosan beads. Those CB were filtered, rinsed with distilled water, and air-dried. Secondly, the crosslinking of CH with ECH was carried out by immersing the prepared CB in 50 mL of an ECH solution 0.1M at pH 10 (fitted with NaOH 0.067 M) under stirring (200 rpm and 45ºC) for 2 hours. Latterly, the CB were washed sequentially with hot and cold distilled water, and finally dried. As result of the synthesis procedure four different catalysts were obtained. Three different catalysts based on modified chitosan spheres were obtained using iron impregnation (CBI), coprecipitation (CBP) and entrapment (CBE) processes and another catalyst based on modified alginate spheres iron-entrapment (ABE). Iron sulphate was used for the impregnation and coprecipitation, and iron (III) oxide was used for entrapment process. 2.2.2 Characterization

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Fourier-transform infrared (FT-IR) spectra of the catalysts were recorded on an FT-IR spectrometer (model FT-IR/4100, Jasco). The samples were previously dried in an oven at 60 °C for 40 min. Scanning electron microscopy (SEM) was performed on a JEOL JSM-6700F equipped with an EDS Oxford Inca Energy 300 SEM. The beads in the wet state were frozen in liquid nitrogen and freeze-dried. The freeze-dried beads were coated with C for the SEM observation. Iron content of catalysts was measured by phenanthroline spectrophotometric method, previous digestion of spheres by digestion following the EPA’s acid digestion procedure 3050. 2.3 Experimental setup 2.3.1 SO assays The SO experiments were carried out using a Weber Ultrasonics MG 1000 TDMF. The degradation assays were carried out in a cell of 250 mL containing 150 mL of DCF solution (7.5 mg/L). The power was fixed to 100 W and different frequencies were selected (40, 80 and 120 kHz). The pH of the solution was not corrected during the assays. 2.3.2 AO and EF assays The experiments were performed in 250 mL cylindrical reactor with a working volume of 150 mL. Two different DCF concentrations were used: 7.5 and 140 mg/L. Na2SO4 (0.01M) was added as electrolyte and the pH was adjusted to 3 and kept into its natural value, 6, operating with a phosphate-NaOH buffer, respectively. A current intensity of 300 mA was applied with a power supply using carbon felt (surface 96 cm2) and double faced boron-doped diamond electrode (5.0 cm x 2.5 cm, NeoCoat with a 4-5 μm diamond thickness and a doping level around 2500 ppm, surface 10 cm2) as cathode and anode, respectively. In the EF and HEF experiments, different amounts of the iron modified beads were added as catalyst and in

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order to favour the H2O2 generation continuous saturation of air at atmospheric pressure was ensured by bubbling air (1 L/min) on the solution. 2.4 Analytical procedures 2.4.1 DCF concentration It was determined by means of HPLC (Agilent 1260) equipment with a Kinetex Biphenyl (4.6 x 150 mm, 5 μm). The column was kept at room temperature. The injection volume was set at 10 μL, and the gradient eluent (water/methanol/ammonium formate) was pumped at a rate of 1 mL/min for 15 min. Detection was performed with a diode array detector at 282 nm. Prior to injection, the samples were filtered through a 0.45-μm Teflon filter. 2.4.2 Carboxylic acids determination In order to identify the aliphatic carboxylic acids generated in the DCF degradation process, several samples were analyzed with an ion-exclusion HPLC (Agilent 1100) equipped with a Rezex™ ROA-Organic Acid H+ (8%), column (300 x 7.8 mm i.d., 8 µm). A 0.005 N H2SO4 solution at a flow rate of 0.5 mL/min was used as the mobile phase. Detection was performed with a diode array detector at 206 nm, and the column temperature was maintained at 60 ºC. The identification of intermediates was made by comparison of retention time and UV spectra with those of pure standards. 2.4.3 GC/MS Analysis Extractions of 150 mL of the aqueous sample were performed three times with 30 mL of ethyl acetate each time. After extraction, the samples were dried with a rotary evaporator. Derivatization of the samples was performed by using N,O-Bis-(trimethylsilyl)trifluoroacetamide (BSTFA) to prepare the trimethylsilyl derivatives of the aromatic intermediates. The samples were mixed with up to 200 μL of ethyl acetate for their analysis by GC-MS. Thus, the identification of the degradation products formed during the HEF treatment of the DCF was performed using an Agilent 6850 GC coupled to 5975C VL MSD

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equipped with a HP-5-MS column. Hydrogen was the carrier gas at a flow rate of 1.2 mL/min. For the GC separation, the GC injection port temperature was set at 280°C. The program temperature started at 50°C (held for 5 min). Subsequently, the temperature ramp was set at 5°C/min to 280°C. The temperature was maintained at 280°C for 5 min. The MS detector was operated in EI mode (70 eV). 2.4.4 Total organic carbon analysis Total organic carbon (TOC) was determined using Lange cuvette test in a Hach Lange DR 2800. From these results, the TOC percentages abatements were calculated from the following equation (Eq. (5)):

TOC reduction (%) 

TOC 100 TOC0

Eq. (5)

where ΔTOC is the variation of TOC (mg/L) during the treatment and TOC 0 (mg/L) is the TOC value at the beginning of the treatment. 2.4.5 Kinetic studies Kinetic studies were performed to evaluate the model of the behaviour of DCF degradation by the ●OH generated in the selected AOPs. Thus, a second-order kinetic equation model can express the abatement rate of the DCF: -dCDCF/dt = k·C•OH·CDCF

Eq. (6)

where C·OH is the ●OH concentration, CDCF is the concentration of DCF (mg/L), t is the reaction time (min) and kw is the kinetic coefficient. After considering the steady state approximation for

●

OH concentration [18], the

concentration reduction of DCF can be accurately described by pseudo-first order reaction kinetic: -dCDCF/dt = ki·CDCF

Eq. (7)

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where ki (1/min) is the apparent rate constant of the reaction and i represents the used AOP or catalyst. The suitability of the model is determined by a regression coefficient (R2) close to the unit. 2.4.6 Measurement of process efficiency To determine the efficiency of the different processes performed, different energy parameters were evaluated using the following equations (Eqs. (8-10)) [18]:



EC kWh / m3



 I ·V ·t / Vs

ECDCF  kWh / kg DCF   I·V ·t / mDCF ECTOC (kWh/kgTOC) = I·V·t/∆mTOC

Eq. (8) Eq. (9) Eq. (10)

where EC (kWh/m3) is the energy consumption per treated volume, ECDCF (kWh/kgDCF) is the energy consumption per degraded DCF mass, ECTOC (kWh/kgTOC) is the energy consumption per degraded TOC, I is the cell applied current (A), V is the average voltage (V), t is the treatment time (h), and ΔmDCF is the DCF mass removed (g), ΔmTOC is the decay in TOC (g) and Vs is the solution volume (L). The economical cost of the treatment was calculated from the electric current consumption and considering the electricity price for industrial consumers of Spain in 2016, i.e., 0.103 € per KWh [19]. 3. Results and discussion The stability of the target pollutant with pH is an important factor to consider in the development of an effective treatment process. DCF solubility depends strongly on the pH of the surrounding solution. It has weak acidic properties (pKa of approximately 4) and therefore, its solubility depends on the pH of the medium. For this reason, the treatment of DCF at different pH values was evaluated at two concentration levels: 7.5 mg/L at acid pH and a higher one 140 mg/L at near neutral pH. 3.1. Treatment of DCF at acid pH 9

In this study, it was determined that the solubility of the DCF decreases, achieving a value of approximately 10 mg/L working at acidic pH, which is in accordance with other authors who also reported the same decrease of solubility operating with concentration values of approximately 5-9 mg/L at pH 3 [20-22]. To perform the treatment of DCF at this pH, an initial concentration of 7.5 mg/L was selected in order to avoid the precipitation of this compound and keep the concentration close to the levels reported for some hospital wastewater [21]. Thus, different treatment alternatives such as SO, AO and EF were considered in order to stablish the best process. 3.1.1 SO degradation of DCF The efficiency of this technique in the generation of ●OH and consequently in the degradation of some organic compounds has been reported in the literature [5]. Therefore, the degradation of DCF by SO was evaluated using 100 W at different frequencies and the results are displayed in Figure 2. It can be observed that there is an increase of the degradation rate with the increase of the frequency from 40 to 120 kHz. Operating at 120 kHz, degradation levels higher than 80% were attained in less than 30 min. Meanwhile, the alternative studied frequencies reached values between 50% and 55% in the same treatment time. Moreover, the obtained profile is very different with a sharper slope at the beginning and reaching total removal after 90 min. Similar degradation levels were reported by Güyer et al. [22] in the removal of DCF operated with high power (120 W) and frequency (861 kHz). The degradation kinetics were calculated and the fitting to the data was performed following a pseudo-first order reaction model, which is in agreement with the results obtained previously and by other authors [23]. Operating at the selected conditions, the kinetic constants obtained were kSO-120kHz = 0.0557 min-1 and R2 = 0.9694, kSO-80kHz = 0.0275 min-1 and R2

=

0.9759, and kSO-40kHz = 0.0191 min-1 and R2 = 0.9372 for the three assayed

frequencies. Those values are in the range of that reported by Güyer et al. (k 0.0493 min-1)

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[22] working at similar DCF concentrations, pH, but with higher power and frequencies. In the developed treatment and operating at the optimal frequency, the degradation rate increased more than 10%. This fact may be attributed to the fact that the process is able to generate a larger number of active bubbles presenting a larger efficiency in the generation of ●

OH [22].

3.1.2 AO degradation of DCF AO is an AOP that permits the electrochemical degradation of pollutants with free hydroxyl radicals generated on the surface of a proper anode material (Eq. (11) and (12)). The best anodes for this procedure are non-active boron-doped diamond (BDD) thin-film electrodes due to that they possess greater O2-overpotential than other anodic materials, thus generating a greater amount of physiosorbed •OH (Eq. (11)), and because they interact very weakly with the produced radicals. BDD + H2O → BDD(•OH) + H+ + e−

Eq. (11)

BDD(•OH) + organic compounds → BDD + oxidized products

Eq. (12)

Initially, an intensity current of 300 mA was selected. As it is observed in Figure 2, the degradation rate is faster than the previous evaluated treatments reaching removal values of 97% in only 10 min, which is substantially higher if compared with approximately 80% achieved after 30 min for SO. It has to be noted that the solution pH remained practically constant up to the final value of 3.21. This fact can be explained due to that the same amounts of H+ in the anode and OH− in the cathode from water oxidation and reduction, respectively, are produced [11]. An increase in the current applied leads to a higher degradation rate that can be associated with the concomitant acceleration of Eq. (11), thereby generating larger quantities of BDD(•OH) that destroy faster the organic compounds [12]. However, in these

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experiments, due to the high degradation levels achieved, it was not necessary to evaluate its behaviour at high intensity, and this value was selected to compare with the EF process. 3.1.3 EF degradation of DCF In EF, a catalytic amount of Fe2+ is added to the polluted solution to react with H2O2, thus producing hydroxyl radicals in the bulk from Fenton's reaction (Eq. (4)). Although it is possible to reduce iron sludge due to the cathodic regeneration of Fe2+ (Eq. (3)) to avoid its generation, it is necessary to keep the pH of the solution around the optimum pH 3 [18]. As it is shown in Figure 2, the degradation profile is analogous to AO with a rapid degradation in a short treatment time. Similar to AO, during the experiment, the starting colourless solution always become clear yellow from the initial time of treatment due to the generation of more or less soluble aromatic compounds, although it becomes colourless again after 30 min because of the overall destruction of such species by •OH in the bulk and adsorption on the BDD surface. This behaviour is in agreement with that reported by GarcíaMontoya et al. [24]. In addition, the pH was stable along with the process increasing up to 3.18. Based on these results, AO presented a good behaviour in the removal of the DCF without addition of external reagents in comparison with EF, and it seems to be a suitable technology for the degradation of DCF at low concentrations. Similar results were reported by Dobrin et al. [25] achieving a total removal after 10 min., and using an alternative treatment, i.e., pulsed corona discharge. 3.1.4 Energetic and economical cost analysis As a consequence of the previous presented results, the cost of AO and EF treatments was estimated. The major operational cost related to the AO and EF is associated with the energy consumption. For this reason, different energy parameters were evaluated. In addition, other

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indirect costs should also be considered in EF treatment related to the energy consumption of aeration and gas bubbling (2.19 kWh and 0.22 €/h). The total EC, ECDCF and economical cost of the AO treatment were calculated resulting in values of 6.298 kWh/m3, 372.91 kWh/kgDCF, 38.41 €/kgDCF, respectively. When the parameters were calculated for the EF treatment, EC, ECDCF and economical cost values were 6.331 kWh/m3, 428.89 kWh/kgDCF, and 44.14 €/kgDCF. The obtained EC values are more than five-fold lower than that reported by Abdul et al. [26] for the treatment of DCF and with a low treatment cost. Both treatments resulted in fast, sustainable and clean alternatives for the DCF treatment with low economical cost. 3.2 Treatment of DCF at natural pH As it has been mentioned before, the limited solubility of the DCF at low pH made the treatment of higher pollutant concentrations at pH of approximately 3 impossible. Thus, to evaluate the efficiency of the studied technologies at high DCF concentration, it is required to work at the natural pH and to keep this pH value by buffering the solution, avoiding the interference on the measurement and not obtaining false values [27]. The selected concentration was 140 mg/L in order to evaluate the degradation of DCF in more concentrated levels that can possibly be obtained, for example, after one treatment of concentration of pharmaceuticals present in wastewater [28]. In the literature, several examples are based on the adsorbent regeneration-desorption process operating at neutral pH [29, 30]. In addition, at this initial DCF concentration, the TOC level is 80 mg/L, which allows for a reliable study of the rapid degradation kinetics of DCF and the detection of transformation products. 3.2.1 Preliminary assays

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Initially, the selected pH was the natural pH of the solution containing the DCF (pH 6.19) and a phosphate buffer was added to the system in order to keep the pH stable in the treatment. The SO and AO were performed directly to the DCF solution of initial concentration of 140 mg/L. However, in the case of EF treatment, the operation at this pH demands the use of a heterogeneous catalyst (HEF) in order to avoid iron precipitation due to the pH of the solution. For this reason, a classical HEF using iron fixed in alginate beads (ABE) was considered. As it is shown in Figure 3, SO treatment provoked almost no effect in the removal of the pollutant with removal values lower than 3%. Meanwhile, AO presented significant degradation values reaching degradation levels of approximately 60% after 120 min. Using HEF, significant improvement of the DCF abatement can be observed, attaining values higher than 80% after 120 min with a heterogeneous catalyst. Comparing the feasibilities of the studied techniques in the removal, HEF was selected as the better option for the treatment of high concentration DCF solutions. However, the evaluation of ABE catalysts in the final treatment revealed the loss of physical integrity due to the presence in the bulk of buffer phosphate. For this reason, it is necessary to explore other catalysts with high activity and stability along with the treatment time. 3.2.2 Preparation and characterization of new catalysts One of the main disadvantages of the ABE is the fact that they are not thermodynamically stable under certain conditions [31] and their solubilization occurs [32]. Operating at a pH higher than 6 and in the presence of phosphate buffer solution, the complexation of the polyvalent cations in the alginate beads with the buffer and their replacement by the monovalent ones in the alginate beads cause a decrease in the mechanical resistance of the alginate beads and subsequently increase their porosity until they are finally dissolved [33]. Thus, as it was detected in our study, the use of buffer such as phosphate decreases the

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stability of ABE [32], and for this reason, it is necessary to search other catalysts that are able to keep the efficiency and physical integrity. For this reason, new chitosan-based catalysts were developed. Three different fixation processes (Fe impregnation, co-precipitation and entrapment) were selected for the preparation of the new chitosan-based catalysts resulting in three different chitosan beads (CBI, CBP, CBE, respectively). All of the prepared chitosan beads exhibited almost spherical shapes with a size between 3-5 mm. They were characterized by the determination of their iron content, FT-IR spectra, SEM microscopy and EDS. Initially, the amount of iron fixed on the chitosan-ECH beads was determined, and the results indicated concentrations between 3.08 and 3.88 mg of Fe/g of bead, which are comparable with the results obtained for alginate beads (2.69 mg/g) reported by Iglesias et al. [14]. These chitosan beads were characterized by scanning electron microscopy, and the results achieved for CBE are shown in Figure 4a-e as an example. As it is depicted in Figure 4b, increasing the zoom of the picture (x25000), a porous structure with an irregular tube shape can be observed. EDS was utilized to study the element composition of the particle and its distribution over the surface of the particle (Figure 4c-f). The catalyst showed a homogeneous distribution of iron and a distribution of element weights of C 43.03%, O 35.28%, Si 0.34%, and Fe 21.34% (Figure 4g). Then, FT-IR spectra of the prepared chitosan-ECH beads were obtained (Figure 5). The spectra indicate the presence of different types of functional groups in the cross-linked beads of chitosan-ECH in comparison with the raw chitosan. A strong band located between 3200 to 3600 cm-1 indicates the presence of OH and NH2 groups. Two different peaks are observed at 1636 cm-1 and 1646 cm-1 which are characteristics of amine deformation and vibration of

15

amide groups, respectively. The peak at 1155 cm-1 is assigned to CN stretching vibration which is in accordance with the broad band detected from 3200-3600 cm-1.

3.2.3 DCF degradation by HEF with different catalysts The performance of the different prepared catalysts in the degradation of DCF at natural pH by the HEF process was evaluated and the results are shown in Figure 6. It can be observed that a rapid degradation was achieved in the first 60 min with removal values of approximately 60-75%, and then the reaction rate decreases until the end of the process at 120 min attaining degradation values between 80 and 95%. In all cases, the possible leaching of iron from the beads was also studied without noticeable iron release in the prepared catalyst. The next step of the work was the analysis of these experimental data after fitting to a pseudo-first order kinetic model as shown in equation (9). In this particular case, the values obtained and the apparent rate constants are presented in Table 1. In all cases, the values are higher than 0.95, which indicates the validity of the proposed kinetic models, in agreement with that reported by several authors on the ●OH degradation of different organic compounds [34, 35]. CBE presented the highest apparent rate constant reaching a value 3-times that obtained for AO. A similar behaviour was obtained for the other catalysts with kinetic constants between 2 and 2.5 times the kAO. The order of the kinetic constant was kCBE > kCBI > kABE > kCBP > kAO. When the ABE and chitosan beads were compared, the degradation rate was improved with two of the last ones showing a better performance in the assayed conditions. The degradation iron entrapment (CBE) presented better results than that achieved by iron impregnation (CBI) or coprecipitation (CBP). As a consequence, the method selected for the iron fixation in the synthesized CH-ES catalysts proved to be an important factor that affects the extension of the treatment and the strength of the iron fixation on the catalyst surface by chelating the metal to the amine and hydroxyl groups [15]. Finally, to 16

confirm the mineralization achieved during the treatment, the TOC was measured at the end of the experiments and resulted in a TOC decay of 21.21%, 62.03% and 74.39% for CBP, CBI and CBE, respectively. Another parameter representative of the mineralization process is the presence of carboxylic acids as by-products of the DCF breakage and as a previous step for the total mineralization. In the different samples, several carboxylic acids such as glycolic, oxalic, acetic, and oxalic formic acids were identified proving the mineralization of the pollutant. In the next section, the profile of the carboxylic acids determined during the HEF degradation of DCF will be shown and discussed. Based on these results, CBE seems to be the catalyst that presents the best catalytic properties giving good DCF removal, TOC removal and high value of the kinetic constant. According to these data, entrapment provided better results than impregnation or precipitation in the preparation of the catalyst. Thus, further studies were developed using this catalyst. 3.2.4 HEF optimization using CBE as the catalyst The HEF process was optimized, and operational conditions such as catalyst dosage and applied current and operation in successive cycles were considered. 3.2.4.1 Effect of amount of catalyst dosage The amount of catalyst affects the degradation process due to its importance in the ●OH production providing the iron required for the Fenton reaction. As a consequence, its influence was analysed. Figure 7a shows the DCF removal after 120 min for four different amounts of catalyst. As it is seen in the figure, a significant improvement was obtained when the amount was increased from 1 to 3 g (6.67 to 20 g/L) and then the maximum removal value was obtained reaching removal values close to 96%. Further increases in the catalyst dosage (5 and 7 g) provoked a reduction in the efficiency of the treatment in comparison with 3 g assays with elimination

17

values of approximately 90%. However, the removal achieved was higher than that attained for 1 g (85%). The kinetics of the degradation were also studied, and the results are shown in Table 1. The TOC analysis was carried out at the end of the experiments, and resulted in a reduction of 74.39%, 60.37%, and 60.69% for 3, 5 and 7 g of catalyst dosage, respectively. These results showed that operating with 3 g of CBE (approximately 0.20 mM Fe), the maximum removal was obtained, and a significant TOC reduction was achieved. Moreover, the energy consumption parameters (EC, ECDCF and ECTOC) and treatment cost were calculated for the different catalyst dosages. As it is observed in Table 1, all of the parameters increased with the catalyst amount between 3 and 7 g, and when the amount was reduced from 3 to 1 g, the same occurred. According to the previously obtained result, 3 g (20 g/L) of catalyst implied a lower energy consumption and treatment cost and high DCF removal and TOC reduction. These obtained results are in accordance with the heterogeneous catalyst concentration range reported in the literature for EF processes by Bae et al. [20] and Iglesias et al. [36]. 3.2.4.2 Effect of applied current Current intensity is an operational factor of great importance due to the in situ electrogeneration of H2O2 required for the DCF removal which is dependent on it. Three different current intensities (100, 300 and 500 mA) were selected to evaluate their effect on the efficiency of the treatment (Figure 7b). A high removal percentage was attained for the current intensities tested with values from 81.97% to 95.11%. As could be expected, a higher current intensity increased the removal efficiency due to the higher electrogeneration of H2O2 and thus ●OH production. However, the parasitic reactions at 500 mA caused the reduction in the removal level obtained with the treatment. Other authors, such as Meijide et al. [34], reported a similar effect of the current intensity. When the TOC reduction was evaluated, the results showed the same trend with a

18

maximum TOC reduction at 300 mA which decreased at higher and lower current values in the selected conditions. For this reason, further studies were performed at this current intensity. 3.2.4.3 Catalyst reuse One of the main advantages of the heterogeneous catalysts is the possibility of being recovered and reused in several degradation cycles. The performance of the CBE catalyst in wastewater treatment by successive cycles was studied. Three degradation cycles were carried out with the recovered catalyst from easy filtration because of its particle size. As is depicted in Figure 8, during all of the batches, the total elimination of the pollutant was reached after 150 minutes. It was also detected that the degradation profiles were quite similar for the first and second batches with a decrease in the degradation rate in the third batch. A similar behaviour was also detected in the EF degradation of Azure B and Lissamine Green by ABE [13]. The TOC reduction was also analysed, and the results showed a reduction of 74.39% of TOC in the first batch which was maintained in the following batches resulting in an abatement of 73.24% after the third batch, proving the efficiency of the system after several reuses. Apart from the degradation ability of the treatment in the different cycles, the effect of the reuses on the catalyst stability and iron distribution was analysed by SEM-EDS after each cycle and the results are shown in Figure 9. It is observed that the integrity, stability and iron distribution exhibited small alterations and the system can work without operational problems for three cycles before the catalyst showed some structural changes. The presence of P and S in the EDS can be related to the use of the buffer and electrolyte during the treatment process. According to the EDS, there is a small decrease in the iron amount in comparison with the initial degradation treatment which then stayed stable in all of the performed cycles.

19

Therefore, the system was able to work without operational problems for several cycles, attaining high DCF removal degree. 3.2.5 Short chain acids mineralization As it was reported in a previous section, several short chain carboxylic acids can be formed in the treatment. For this reason, the presence of different carboxylic acids was studied operating at the optimal conditions and their concentrations along with time were studied. Different carboxylic acids such as glycolic, malonic, oxalic, acetic, and oxalic formic acids were identified, and their profiles are shown in Figure 10. Most of the detected carboxylic acids (glycolic, formic, malonic, oxalic, acetic acids) appeared during the first 60 min of treatment, reducing their amount after that and almost disappearing in 120 min. This can suggest that initially the degradation process occurs in the DCF and aromatic molecules without much ●OH available for the degradation of these acids. Progressively, the intermediates broke into small size molecules implying more ●OH for the mineralization of these acids. As an example, the formation of a significant amount of oxalic acid, a recalcitrant and not easily degradable intermediate, was observed during the treatment of DCF, reaching values of approximately 16 mg/L in the first hour of treatment. Then, a significant reduction was detected during the second hour followed by a slight reduction until the end of the experiment. In opposition to this and confirming the previously exposed results, succinic acid was not produced in the first hour of treatment time, but its concentration increases with time reaching values of approximately 5-7 mg/L after 120 minutes. This fact could suggest that the degradation of DCF produces a significant amount of aromatic intermediates that are slowly degraded during 60 min, producing succinic acid as the breakage product. Following the TOC reduction, it was detected that the mineralization attained levels of approximately 90% after 270 min where 83.1% of the remaining TOC was due to the carboxylic acids.

20

3.2.6 Degradation pathway To confirm the mineralization of DCF, the intermediate compounds generated during the process were identified by GC-MS. Based on the obtained results, a degradation pathway for the DCF degradation by HEF treatment was proposed (Figure 11). Initially, the rearrangement of DCF, most likely being formed during acidification, before the attack by ●OH, causes an intramolecular cyclation leading to water loss and generating 1(2,6-dichlorophenyl)-indolin-2-one (2) [5, 37]. The addition of ●OH to DCF generated a radical intermediate (3). The preferential attack of the ●OH to the carbon molecules bonded to nitrogen leads to the cleavage of C-N bonds and the breakdown of 1-(2,6-dichlorophenyl)indolin-2-one (2) into different aromatic compounds such as 2,6-dichlorophenol (4), oxindole (5); and DCF radical (3) into 2,6-dichloroaniline (6) as the major product, and 2hydroxyphenyl acetic acid (7). The hydroxylation of 2,6-dichloroaniline (6) causes the apparition of 3,5-dichloro-4-aminophenol (8) in which the amine group could be replaced by ●

OH to generate 2,6-dichlorohydroquinone (9). This product (9) can also be generated by the

hydroxylation of (4). In the literature, some of the intermediate products are described as a result of DCF degradation by other oxidation processes. Pérez-Estrada et al. [8] identified intermediates (6) and (8) in the photo-Fenton degradation of DCF; Ziylan et al. [9] and Cinar et al. [5] found (2), (6), (8), and (9) as intermediate products in the sonochemical and sonolysis degradation of DCF, respectively. Chloride ion is released through numerous hydroxylation and hydrogen abstraction steps [38]. The hydroxyl-radical driven dechlorination of 2,6-dichlorohydroquinone (9) which reflects the ortho specific action of hydroxyl radicals as reported by Schlosser et al. [39] leads to the apparition of 1,2,3,5tetrahydroxybenzene (17). The formation of the different compounds such as benzoic acid (10) and 4-hydroxybenzoic acid (11) not directly derived from 2-hydroxyphenyl acetic acid (7) is suggested. Then, the 21

decarboxylation of those compounds leads to the formation of phenol (12) and hydroquinone (13) [40]. The hydroxylation of (12) and (13) generates catechol (15), hydroxyquinol (16) and 1,2,3,5-tetrahydroxybenzene (17). Then, the opening of the ring occurs and several long-chain carboxylic acids were detected depending on the origin compound considered. On the one hand, the cleavage of catechol (15) and hydroxyquinol (16) generates muconic acid (18), maleic acid (19) or fumaric acid (20). As it was reported in the literature [34, 41], the reaction between maleic acid (19) or fumaric acid (20) with ●OH yields the production of malic (21) and succinic (22) acids. The reaction of succinic acid (22) with ●OH causes the breakage of the acid and causes the formation of 3-oxopropanoic (not detected) and glyoxylic (24) acids [42]. The oxidation of this 3-oxopropanoic acid by ●OH generates oxalic (23) and malonic acids (25), which is in agreement with that reported by Charbouillot et al [42]. Oxalic (23) acid is also generated due to the degradation of malonic acid (25) or can be yielded from the oxidation of glyoxylic acid (24). The reaction of malonic acid with ●OH could generate tartronic acid (34) [43]. On the other hand, the opening of 1,2,3,5-tetrahydroxybenzene (17) and other more hydroxylated compounds leads to the apparition of glucaric acid (31). The hydroxylation of glucaric acid (31) generates glyceric acid (32) which is oxidated to tartronic acid (34). The H abstraction of tartronic acid produces the apparition of ketomalonic acid (35) which is rapidly oxidized to oxalic acid in the presence of Fe2+ [44]. Finally, the direct mineralization of oxalic (23) and formic (27) acids to CO2 and H2O could be directly achieved. Due to the difficult oxidation of oxalic acid by ●OH, the mineralization time of this by-product is largely prolonged with the consequent increase of treatment time to achieve total TOC removal [45, 46]. 4. Conclusions

22

As results of this work, viable solutions are proposed for solving the handicap of DCF operating at different conditions. These solutions improve the decision-making process required to tackle the problems regarding the treatment of this emerging pollutant. Operating in the conditions of low solubility, AO and EF proved to be resolute solutions. When the concentration of the pollutant is higher, and the solubility is not a subject of concern, HEF showed to be an efficient option attaining high levels of abatement and mineralization as demonstrated. The use of the prepared modified chitosan beads catalyst such as CBE had a significant effect in the DCF degradation by HEF, improving the treatment of the contaminant and opening promising perspectives for fast and economical treatment of polluted wastewater.

Acknowledgements: This research is financially supported by the Spanish Ministry of Economy and Competitiveness (MINECO), Xunta de Galicia and European Regional Development Fund (Projects CTM2017-87326-R and ED431C 2017/47). The authors are grateful to Water JPI ERA-NET Cofund WaterWorks2015 Call (Project REWATER). Emilio Rosales and Silvia Diaz are grateful to MINECO for the Juan de la Cierva postdoctoral grant and the National Council of Science and Technology, CONACYT for a Beca Mixta grant, respectively, for their financial support.

23

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[31] K.L. Deng, H.B. Zhong, T. Tian, Y.B. Gou, Q. Li, L.R. Dong, Drug release behavior of a pH/temperature sensitive calcium alginate/poly(N-acryloylglycine) bead with core-shelled structure, Express Polym. Lett. 4 (2010) 773-780. [32] L. Segale, L. Giovannelli, P. Mannina, F. Pattarino, Calcium alginate and calcium alginate-chitosan beads containing celecoxib solubilized in a self-emulsifying phase, Scientifica. 2016 (2016). [33] K. Sobecka, A. Bartkowiak, Calcium carbonate as modifier of mechanical properties of alginate/Ca microbeads (2009), XVIIth International Conference on Bioencapsulation, Groninger, Netherlands. [34] J. Meijide, E. Rosales, M. Pazos, M.A. Sanromán, p-Nitrophenol degradation by electro-Fenton process: Pathway, kinetic model and optimization using central composite design, Chemosphere. 185 (2017) 726-736. [35] I. Ouiriemmi, A. Karrab, N. Oturan, M. Pazos, E. Rozales, A. Gadri, M.Á. Sanromán, S. Ammar, M.A. Oturan, Heterogeneous electro-Fenton using natural pyrite as solid catalyst for oxidative degradation of vanillic acid, J Electroanal Chem. 797 (2017) 69-77. [36] O. Iglesias, J. Gómez, M. Pazos, M.A. Sanromán, Electro-Fenton oxidation of imidacloprid by Fe alginate gel beads, Appl. Catal. B Environ. 144 (2014) 416-424. [37] K. Reddersen, T. Heberer, Formation of an artifact of diclofenac during acidic extraction of environmental water samples, J. Chromatogr. A. 1011 (2003) 221-226. [38] W.Z. Tang, Physicochemical Treatment of Hazardous Wastes, Lewis Publishers, Boca Raton, US, 2003. [39] D. Schlosser, K. Fahr, W. Karl, H.-. Wetzstein, Hydroxylated metabolites of 2,4-dichlorophenol imply a Fenton-type reaction in Gloeophyllum striatum, Appl. Environ. Microbiol. 66 (2000) 2479-2483. [40] M.A. Oturan, J. Pinson, Hydroxylation by electrochemically generated OH• radicals. Mono- and polyhydroxylation of benzoic acid: Products and isomers' distribution, J. Phys. Chem. 99 (1995) 13948-13954. [41] E. Mousset, L. Frunzo, G. Esposito, E.D.V. Hullebusch, N. Oturan, M.A. Oturan, A complete phenol oxidation pathway obtained during electro-Fenton treatment and validated by a kinetic model study, Appl. Catal. B Environ. 180 (2016) 189-198. [42] T. Charbouillot, S. Gorini, G. Voyard, M. Parazols, M. Brigante, L. Deguillaume, A.-. Delort, G. Mailhot, Mechanism of carboxylic acid photooxidation in atmospheric aqueous phase: Formation, fate and reactivity, Atmos. Environ. 56 (2012) 1-8. [43] M. Peleg, The chemistry of ozone in the treatment of water, Water Res. 10 (1976) 361-365. [44] J.A. Garrido, E. Brillas, P.L. Cabot, F. Centellas, C. Atias, R.M. Rodríguez, Mineralization of drugs in aqueous medium by advanced oxidation processes, Portugaliae Electrochim Acta. 25 (2007) 19-41. [45] M. Diagne, N. Oturan, M.A. Oturan, Removal of methyl parathion from water by electrochemically generated Fenton's reagent, Chemosphere. 66 (2007) 841-848. [46] S. Garcia-Segura, E. Brillas, Mineralization of the recalcitrant oxalic and oxamic acids by electrochemical advanced oxidation processes using a boron-doped diamond anode, Water Res. 45 (2011) 2975-2984.

26

Figure and table captions Figure 1. Chitosan–ECH beads structure. Figure 2. DCF degradation profile by SO at different frequencies (40 kHz, ●; 80 kHz, ○; 120 kHz, ▼), AO (Δ), and EF (■). Operational conditions: [DCF] 7.5 mg/L and pH 3. SO: 100 W; AO and EF: I 300 mA, Na2SO4 0.01 M, Fe2+ 0.2 mM (only EF) and air 1 L/min (only EF). Standard deviation of each data is less than 5%. Figure 3. DCF degradation by SO (▼), AO (○) and HEF with ABE (●). Operational conditions: [DCF] 140 mg/L and pH 6. SO: 100 W and 120 kHz; AO and HEF: I 300 mA, Na2SO4 0.01 M, ABE 3g (only HEF) and air 1 L/min (only HEF). Standard deviation of each data is less than 5%. Figure 4. SEM image of CBE (a-c). Distribution of different elements: Fe (d), O (e), C (f) in the surface of the prepared CBE heterogeneous catalyst. EDS identification of elements (g) Figure 5. FTIR spectra of the new prepared chitosan–ECH beads (from bottom to up): CBE (orange), CBP (green), CBI (dark red); and chitosan (brown). Figure 6. HEF treatment of DCF with different heterogeneous catalyst: CBI (●), CBP (○), CBE (▼), and ABE (Δ). Operational conditions: [DCF] 140 mg/L, I 300 mA, pH 6, Na2SO4 0.01 M, catalyst dosage 3 g, air 1 L/min. Inset panel presents the corresponding pseudo-firstorder kinetic analysis. Standard deviation of each data is less than 3%. Figure 7. Effect CBE catalyst dosage (a), and the current intensity (b) in the HEF degradation of DCF. Operational conditions: [DCF] 140 mg/L, pH 6, Na2SO4 0.01 M, air 1 L/min. Standard deviation of each data is less than 3%. Figure 8. Reuse of the catalyst in successive batches for the HEF degradation of DCF. Operational conditions: [DCF] 140 mg/L, I 300 mA, pH 6, Na2SO4 0.01 M, CBE dosage 3 g, air 1 L/min. Standard deviation of each data is less than 3%. Figure 9. SEM-EDS images of CBE obtained after each one of the three treatment cycles. Figure 10. Profile of carboxylic acids determined during the HEF degradation of DCF with CBE at the optimal conditions. Operational conditions: [DCF] 140 mg/L, I 300 mA, pH 6, Na2SO4 0.01 M, catalyst CBE 3 g, air 1 L/min. Standard deviation of each data is less than 3%. Figure 11. Proposed DCF degradation pathway by HEF. The compounds in blue were not detected by GC-MS.

27

Table 1. Experimental conditions, pseudo-first order kinetic constants and energy consumption parameters obtained for the HEF. Operational conditions: pH 6, current intensity 300 mA, air 1 L/min (only in HEF).

28

Figure 1.

NH2 *

O

O

O

*

O CH2 HO CH CH2 O CH2 O *

O

O

OH

n*

NH2

29

Figure 2.

1.0

6

5

4

-ln (C/C0)

0.8

3

2

1

0.6 0

C/C0

0

10

20

30

40

50

time (min)

0.4

0.2

0.0 0

20

40

60

80

100

time (min)

30

Figure 3.

1.0

0.8

C/C0

0.6

0.4

0.2

0.0 0

20

40

60

80

100

120

time (min)

31

Figure 4.

(a)

(b)

(c)

(d)

(e)

(f) C O

Relative counts

Fe

Fe

Si

Fe

5

10

15

20

keV

(g)

32

Figure 5.

Transmitance

C-N strecht

O-H strecht NH2 strecht

4000

Alkyl C-H strecht Amide

3000

2000

C-O strecht

1000

Wavenumber (cm-1)

33

Figure 6.

1.0 4

3

-ln(C/C0)

0.8

0.6

2

C/C0

1

0 0

20

40

60

80

100

120

140

t (min)

0.4

0.2

0.0 0

20

40

60

80

100

120

time (min)

34

100

100

80

80

% Degradation

% Degradation

Figure 7.

60

40

20

40

20

0

0 1g

(a)

60

3g

5g

7g

100 mA

300 mA

500 mA

(b)

35

Figure 8.

1.0

1st batch

3rd batch

2nd batch

0.8

C/C0

0.6

0.4

0.2

0.0 0

50

100

150

200

250

300

350

400

450

time (min)

36

Figure 9. Sample

SEM image

EDS graph

After cycle 1

C O

Relative counts

Fe

P Fe

S

Fe 5

10

15

20

keV

After cycle 2

C O

Relative counts

Fe

P Fe

S

Fe 5

10

15

20

15

20

keV

After cycle 3

C O

Relative counts

Fe

P Fe

S

Fe 5

10

keV

37

Figure 10.

Carboxylic acid concentration (mg/L)

35 Oxalic Oxamic Malonic Succinic Glycolic Formic

30

25

20

15

10

5

0 30

60

150

240

270

300

time (min)

38

Figure 11.

39

Table 1.

Catalyst dosage (g) -

k (1/min)

R2

EC (kWh/m3)

ECDCF (kWh/kgDCF)

ECTOC (kWh/kgTOC)

Treatment cost (€/kgDCF)

Treatment cost (€/kgTOC)

0.0080

0.9970

49.13

328.24

786.36

33.80

80.99

CBI

3

0.0203

0.9940

42.06

291.52

695.73

30.02

71.66

CBP

3

0.0163

0.9830

68.43

519.98

3379.03

53.55

348.13

CBE

1

0.0165

0.9703

30.20

188.90

1296.05

19.45

133.49

CBE

3

0.0242

0.9905

26.91

175.46

195.65

17.97

37.04

CBE

5

0.0189

0.9902

31.48

197.49

595.02

20.34

61.29

CBE

7

0.0189

0.9991

36.50

230.90

622.00

23.68

64.05

ABE

3

0.0182

0.9996

38.31

242.99

572.71

25.02

58.99

Catalyst

None(AO)

Highlights 

Alternative treatments according to diclofenac concentration ranges were developed



Treatments at natural pH are necessary working at high concentration of diclofenac



Fe chitosan-epichlorohydrin beads proved to be viable heterogeneous Fenton catalysts



Reutilisation of catalyst in several treatment cycles were successfully performed

40