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Efficient usage of hydrogen peroxide in tetracycline degradation via photo-fenton process

Enhanced photo-Fenton Process for Tetracycline Degradation using
Efficient Hydrogen Peroxide Dosage
Evelyn Yamal-Turbay*, Esther Jaén, Moisès Graells, Montserrat Pérez-Moya Departament d'Enginyeria Química, EUETIB, Universitat Politècnica de Catalunya, c/Comte d'Urgell, 187, 08036 Barcelona, Spain Abstract
The remediation of water solutions containing Tetracycline antibiotic (TC) using photo-Fenton treatments has received scarce attention in the literature. However TC deserves attention due to its condition of emerging contaminant. In this work, TC oxidation in water solutions (12 L, 40 mg L-1) by means of photo-Fenton reaction under variable hydrogen peroxide dosage is investigated. This shows a more efficient use of the hydroxyl radicals produced in the reaction medium and enhances treatment performance. First, a suitable range of Fenton reagent loads is determined in a preliminary study. The hydrogen peroxide dosage is parameterized through two variables: the initial release of the total load, y0 (kick-off), and the time tini at which the continuous dosage of the rest of the load starts. Hence, a design of experiments (22) can be used to characterize the performance of the process under different hydrogen peroxide dosage protocols. The results obtained this way show that total TC remediation is attained in all the cases studied, but alike amounts of hydrogen peroxide lead to total mineralization only when the dosage scheme is conveniently tuned. Therefore, the photo-Fenton treatment has proved to be effective in removing TC from water solutions, and the opportunity for an efficient dosage to reduce the requirements of hydrogen peroxide has also been confirmed. Keywords: Tetracycline antibiotic, emerging contaminants, photo-Fenton treatment, hydrogen
peroxide dosage, design of experiments. * Corresponding author: Escola Universitària d'Enginyeria Tècnica Industrial de Barcelona. Comte d'Urgell 187. 08036 Barcelona, Spain. +34 934137275. [email protected] Highlights
 Tetracycline in water solutions is shown to be totally degraded via photo-Fenton treatment  Subject to dosage, a H2O2 load can mineralize a tetracycline solution to different extents  H2O2 dosage is modeled via two parameters, upon which a design of experiments is proposed  Design of experiments dosage conditions yield mineralization rates ranging from 77% to almost  If conveniently dosed, stoichiometric H2O2 loads can achieve total mineralization 1. Introduction
In recent years, a new group of contaminants called emerging contaminants has been detected in water sources. Although they are found in very low concentrations (in the order of ng-μg L-1) they can become a real environmental problem because of their high persistence and ecotoxicity [1, 2]. Among these emerging contaminants are pharmaceuticals and personal care products (PPCPs) that can act as endocrine disruptors and affect bacteria used in conventional water treatments [3]. In particular, tetracyclines (TCs) are broad-spectrum antibiotics with activity against gram-positive and gram-negative bacteria. The presence of such antibiotics in the environment is mainly due to their discharge through excretion whether or not metabolized. As conventional treatments cannot be expected to process this kind of contaminants, they come to appear in surface water, groundwater and even in drinking water, developing antibiotic-resistant pathogens, among several other environmental issues [3-5]. According to Wang et al. [6] they have been detected at concentrations at 0.07-1.34 µg L-1 in surface water samples. In the last thirty years, advanced oxidation processes (AOPs) have acquired increasing interest as an option either to remediate recalcitrant contaminants before or after conventional treatments [7-9]. AOPs consist in the in situ generation of highly oxidant species to degrade organic matter, and have shown to provide excellent results, especially those processes based on the use of the Fenton reagent. Fenton and related processes (photo-enhanced Fenton process, electro-Fenton, sono-Fenton and so on) have proved to be efficient in mineralizing antibiotics [3, 5, 10, 11]. Tetracycline degradation by means of AOPs has been investigated in the past, especially by using UV and UV/H2O2 treatments, and heterogeneous photocatalysis, (contaminant concentrations ranging from 10-100 mg L-1). UV has shown to be effective in degrading this contaminant and the presence of H2O2 has revealed to increase TC degradation and mineralization rates [9, 12-45]. Heterogeneous photocatalysis with TiO2 has also been investigated, and a lower performance with respect to UV and UV/H2O2 processes has been reported [8, 16]. Electrochemical processes have also shown to be effective for TC degradation [17, 18]. Conversely, TC degradation via photo-Fenton process has received scarce attention, although TC is among the components of some wastewaters whose photo- Fenton treatment has been investigated [19] and the research by Rossi and Nogueira [5] reported total remediation of 24 mg L-1 TC solutions. In Fenton processes, hydroxyl radicals are produced by the reaction between hydrogen peroxide and a ferrous salt [20]. In the specific case of photo-Fenton reaction, an external source of energy (UV irradiation) can enhance the process by accelerating iron ions to be regenerated (from Fe3+ to Fe2+) and recycled [21]. However, since the hydroxyl radical is extremely unstable and non-selective, it is frequently scavenged by undesired secondary reactions. This indicates the convenience to continuously produce hydroxyl radicals in the reaction media instead of using high hydrogen peroxide concentrations at the beginning of the process [22]. Recent efforts have been devoted to the use of continuous or semi-continuous hydrogen peroxide dosage for reducing the scavenging of hydroxyl radicals, thus reducing costs and environmental impact as well [21, 23]. Examples of this are the work by Chu et al. [24], who proposed a stepwise dosage of hydrogen peroxide that improved mineralization; the work by Zazo et al. [25], who applied a continuous dosage protocol to the Fenton treatment to increase abatement of TOC with the same overall H2O2 consumption; and the work by Prato-Garcia and Buitrón [26], who addressed the decolorization of azo dye mixtures by means of two different continuous dosage strategies that improved effluent quality in terms of toxicity and biodegradability. Hydrogen peroxide dosage has been investigated for Fenton treatments aimed at increasing the efficiency of the remediation of solutions of other six selected antibiotics [27]. However, regarding the oxidation of TC antibiotic in water solutions via photo-Fenton treatment, to the best of the authors' knowledge, no study has been reported on the influence of hydrogen peroxide dosage. More recently, Yamal-Turbay et al. [28] proposed a practical parameterization of hydrogen peroxide dosage protocol that allowed setting an easy design of experiments to characterize the influence of hydrogen peroxide dosage on the performance of a photo-Fenton treatment. This technique was tested in the remediation of coffee solutions and led to the determination of more efficient dosage conditions for hydrogen peroxide and enhanced process performance. This work aims to apply this systematic dosage protocol for improving the performance of the photo-Fenton treatment of TC solutions in water. 2. Methods
95% purity TC hycrochloride from Sigma Aldrich was purchased to prepare 40 mg L-1 solutions in tap water. Fenton reagents: hydrogen peroxide 33% w/v (Panreac) and 7-hydrated iron (II) sulfate (Merck) were used as received. HCl (J.T. Baker) and NaOH (Panreac) 1 M were used for automated Oxalic acid 0.01 M (Panreac) prepared in filtered milli Q grade water and J.T. Baker ultragradient HPLC acetonitrile and methanol were used as HPLC mobil phases, while a stock of ammonium metavanadate (Riedel-de-Haën) 0.062 M in H2SO4 (Panreac) 0.58 M according to Nogueira et al. [29] was kept refrigerated for hydrogen peroxide measurement. 2.2 Pilot plant description The photochemical pilot plant consists in a 12 L system: glass reservoir and piping system. The light source is a 230 W medium pressure mercury lamp UV TQ718 (irradiating from 300 to 550 nm) placed in a glass cooling jacket in the center of the reservoir. The incident photon flux, determined by potassium ferrioxalate actinometry [30], was 3.4033× 10−5 Einstein s−1. The system is provided with a centrifugal pump which keeps a recirculation flow of 12 L min-1. The reaction pH was maintained at 2.7±0.1 by using a proportional-integral-derivative (PID) controller. Figure 1a shows a diagram of the pilot plant. 2.3 Analytical methods Process performance was evaluated by withdrawing aliquots from the pilot plant and measuring the following variables at regular time intervals:  Total organic carbon (TOC) concentration was measured by means of a Shimadzu VCHS/CSN  TC concentration was determined via HPLC, using an Agilent 1200 series with UV-DAD array detector. The chromatographic conditions and data analysis were performed using the Agilent Chem-Station (Rev.B.04.06 SP1[647]) software. The chromatographic column was a 5 µm 4.6x150 mm Zorbax Eclipse XDB-C18 (Agilent Technologies), maintained at 30ºC, and the diode array detector was set at 271 nm. 20 μL samples, injected by a manual injector, were eluted by a mixture (70% 0.01 M oxalic acid, 10% methanol and 20% acetonitrile) flowing at 1.0 mL min-1 [31]. Tetracycline retention time under these conditions was 2.4 minutes. A five- level calibration curve (range 2-30 mg L-1) was used for TC quantification.  H2O2 concentration was measured by a UV-vis spectrophotometer Hitachi U-2001 after reaction with ammonium metavanadate, following the technique proposed by Nogueira et al. 2.4 Hydrogen peroxide dosage definition The continuous dosage of H2O2 was executed by means of a peristaltic pump (Watson Marlow 300 series OEM) programmed from the pilot plant SCADA. The dosage protocol was previously proposed and stated [28] and is characterized (Eq. 1) by the fraction of the total dose of H2O2 added initially, y0, the dosage interval, tadd, and the time at which the dosage starts, tini. Figure 1b shows a graphical representation of the addition protocol. t  0 0  t   1  y  t  tini  t  t   add   t  TS After a preliminary study which will be explained in further sections, the total amount of H2O2 was fixed to 858 mg, which corresponds to an equivalent concentration of 71.5 mg L-1. The treatment span and the dosage interval were also fixed (TS=90 min; tadd =30 min). Hence, the remaining two degrees of freedom (y0, tini) were used in the design of experiments. The quantitative performance index selected to rank the output of the assays and to discuss the results is the conversion attained at TS, defined as follows: 3. Results and discussion
3.1. Preliminary assays 3.1.1. Reagent doses As mentioned in the introduction, the concentrations investigated in previous works concerning TC degradation by means of photolysis and photocatalysis processes ranged between 10 and 300 mg L-1 [4, 8, 13, 16], while Rossi and Nogueira [5] proved that photo-Fenton process can be successfully applied for the degradation of 24 mg L-1 TC solutions with hydrogen peroxide and ferrous iron initial concentrations ranging from 35 to 680 mg L-1 and 5.6 and 11 mg L-1, respectively. According to this information and taking into account TOC analyzer sensibility, TC concentration was fixed at 40 mg L-1, which corresponds to a TOC concentration of 23 mg L-1 and requires a stoichiometric H2O2 concentration of 150 mg L-1. A 22 factorial design of experiments (DOE) with star points was performed to decide Fenton reagent doses. It was important to identify a dose which permits a compromise between degradation and dosage interest. For that aim, Fe2+ dose was centered at 5 mg L-1 (10 mg L-1 is the maximum legal value in effluents in Spanish legislation [32]), having minimum and maximum values 2 and 8 mg L-1, Regarding H2O2 load, this ranged between 9% and 90% of the stoichiometric amount given by the TC oxidation just by means of hydrogen peroxide, which is taken as a reference. Sub-stoichiometric hydrogen peroxide loads can be used to ease the identification intermediate species at the early stages of the reaction [33]; furthermore, sub-stoichiometric loads can be enough for total TC oxidation since hydrogen peroxide is not the only source of oxygen [21, 34]. Minimum and maximum values of the H2O2 load were set at 14.3 and 128.7 mg L-1, having a center value of 71.5 mg L-1, which corresponds to a 48% of the stoichiometric amount. Figure 2a presents DOE results considering TS as the system Low iron loads (2 mg L-1) provide degradation rates around 55% despite H2O2 dose (red line). On the contrary, higher iron values (5 or 8 mg L-1) noticeably increase response values as H2O2 doses increase. However, 5 mg L-1 iron loads lead to higher TOC degradation than 8 mg L-1 for equal H2O2 amounts (lines blue and green). Thus, 5 mg L-1 iron loads offer more efficient use of the hydroxyl radicals generated in the system. According to this, iron dose was set at 5 mg L-1. Finally, a hydrogen peroxide dose was set at 71.5 mg L-1 because higher values, when combined with 5 mg L-1 iron doses provide too fast TOC reduction, which complicates the evaluation of the effect of reagent dosage on degradation. In brief, from here on, the following concentrations are used:  Tetracycline initial concentration = 40 mg L-1  Hydrogen peroxide equivalent concentration = 71.5 mg L-1  Iron (II) initial concentration = 5 mg L-1 3.1.2. Blank assays A set of blank assays was initially performed in order to evaluate the influence of each separate reagent on TOC and TC degradation, which are next presented. Figure 2b shows the evolution of TOC, H2O2 and TC concentrations for UV treatment, UV/H2O2, Fenton and photo-Fenton, involving the previously stated reagent doses when applied. Mineralization by means of UV treatment only reaches 35%, UV/H2O2 and Fenton allow almost 60%, and the photo-Fenton process may reach 77%. TC is completely degraded in all the cases where H2O2 is present, being the degradation faster when the ferrous salt is added. On the contrary, only a 40% of TC is degraded by solely using UV irradiation (90 min treatment). Both TC and TOC reduction rates obtained without using H2O2 are comparatively poor. However, it is important to bear in mind that these rates indicate that, to a non-negligible extent, H2O2 may not be the sole responsible of the complete oxidation of organic matter. Once established that the photo-Fenton process is capable of degrading TC and mineralize organic matter, H2O2 dosage was investigated in order to determine to which extent process efficiency can be improved given the same reagent loads. 3.2. DOE for dosage characterization A design of experiments considering the dosage parameters y0 and tini (eq.1) and the response TS was applied; the earliest sample time at which TC is not detected (tTC) was also included as a response. Table 1 lists the levels and results of the planned assays according to a 22 DOE with start points and three center points for statistical validity; minimum and maximum factors were set at 10 and 30% for y0 and 0 and 30 minutes for tini. Reference experiments were included in the study. Figure 3a shows the results of the central experiment of the design, which was repeated six times. Average values are presented along with the corresponding standard deviations. Total remediation of the sample is shown to be achieved with these conditions. Regarding TC degradation, it was achieved within 10 minutes treatment. From this point on, experiments are named according to the following nomenclature: "Experiment- code_y0_tini". Assay R corresponds to the reference experiment (no dosage protocol). Figure 3b compares the evolution of TOC and TC concentrations for the central assays (y0=20%; tini =15 min), an assay without kick-off (experiment code L, y0=0%; tini=0 min), and the reference assay. The performance obtained is analogous to that obtained by previous studies using caffeine [28]. Only 77% mineralization is achieved without dosage. However, the proposed scheme, when conveniently tuned, shows that it is possible to reach up to almost total mineralization (98%), which corresponds to 27 percent points of improvement. According to Prato-Garcia and Buitrón [26], this may be explained because when the entire reagent dose is added at once, the excess of H2O2 promotes secondary reactions that scavenge hydroxyl radicals and "waste" them. Dosage allows reducing this excess and having a more efficient dedication of hydroxyl radicals to degrading organic matter [22, Regarding TC remediation, table 1 shows the earliest sample time (tTC) at which TC is not detected and total degradation is assumed. The slowest degradation is observed in experiments B and J, for which tTC increases up to 45 and 30 minutes respectively, while for the rest of the assays tTC is below 15 minutes. This behavior can be explained because of the combination of low initial hydrogen peroxide doses (y0), which is rapidly exhausted and causes the process to stop, and the late dosage (tini), which slows the process down; actually, better performance of experiment J compared to assay B obeys to sooner tini despite lower y0. Figure 4 shows the relation between conversion at TS, TS, and the time at which total degradation is assumed, tTC. This confirms the opportunity to improve the treatment outcome by adjusting the dosage parameters. When the reagent is dosed proportionally without kick-off addition (experiment L), an improvement of 18 percent points is possible; furthermore, both objectives, maximizing mineralization and minimizing time for degrading TC, are achieved simultaneously. That is the case of experiment C, which is the closest to the low right corner of the plot. Regarding the evident interaction between both factors y0 and tini these results also indicate the higher significance of the latter. When continuous dosage starts soon, better performance is achieved despite y0; conversely, low y0 values may require starting dosage at earlier tini values. Further data on the influence of low y0 values on TC degradation are given in Figure 5. Compared to assay J (tini=15 min), the higher y0 value of new experiment P_8_15 (8%) produces no performance changes; however, the lower y0 value of new experiment Q_4_15 (4%) results in hydrogen peroxide exhaustion and the process interruption until more reagent is dosed after 15 minutes. 4. Conclusions
The oxidation of TC antibiotic in water solutions by means of the photo-Fenton treatment was investigated. The treatment was applied to 12 L of 40 mg L-1 tetracycline samples with total reagent doses of 71.5 mg L-1 hydrogen peroxide (48% of stoichiometric dose) and 5 mg L-1 Fe2+ (half the legal limit in wastewaters in Spain). These conditions achieved total TC remediation and to produce up to 77% solution mineralization within the reaction span studied. Hydrogen peroxide dosage was also investigated in order to improve process performance. Since excess hydrogen peroxide fosters scavenging reactions, dosage is considered to provide the conditions for reducing this unsolicited effect. A dosage protocol introduced in a previous work was applied to the study of the degradation of TC. The systematic parameterization of the dosage led to a DOE for determining the treatment outcome for a given set of assays. Hence, given this fixed H2O2 load, treatment performance was shown to be able to improve total mineralization (up to 22.6 percent points) when dosage parameters were conveniently adjusted. The efficient use of hydrogen peroxide, via dosage, has revealed to significantly improve the performance of the treatment of water solutions containing TC by cutting the H2O2 demand and reducing the cost of photo-Fenton treatments. This work has also shown that the H2O2 demand can be reduced quite below the stoichiometric value, which suggests the importance of other oxygen sources in the photo-Fenton treatment. This additional aspect and its relation with dosage deserve further The authors would like to thank the Spanish Ministerio de Economía y Competitividad and the European Union (through the European Regional Development Fund) for supporting this research, which is part of the activities developed under the Project EHMAN (DPI2009-09386) and Project SIGERA (DPI2012-37154-C02-01). Evelyn Yamal-Turbay also thanks Universidad de Carabobo for financial support through professorial grant CD-4352. References
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