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Degradation of parabens in different aqueous matrices by several O3-derived advanced oxidation processes Eduardo M Cuerda-Correa, Joaquín R Dominguez-Vargas, Maria Jesus Muñoz Peña, and Teresa Gonzalez Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00740 • Publication Date (Web): 18 Apr 2016 Downloaded from http://pubs.acs.org on April 27, 2016

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Industrial & Engineering Chemistry Research

Degradation of parabens in different aqueous matrices by several O3-derived advanced oxidation processes Eduardo M. Cuerda-Correa (1), Joaquín R. Domínguez (2)*, María J. Muñoz-Peña (2), Teresa González (2) (1)

(2)

Department of Organic and Inorganic Chemistry. Avda. de Elvas, s/n. Faculty of Sciences. 06006 Badajoz, 406 (Spain).

Department of Chemical Engineering and Physical Chemistry. Avda. De Elvas, s/n. Faculty of Science. 06071 Badajoz (Spain).

KEYWORDS: ozonation, parabens, advanced oxidation process, ozone combined process. ABSTRACT Ozonation is a promising technology for the treatment of bio-refractory compounds. Parabens belong to a chemical family of apparently innocuous preservatives that have been used in the food and cosmetic industries for several years. Oxidation of different parabens (methylparaben (MP), ethylparaben (EP), propylparaben (PP), and butylparaben (BP)) was carried out using O3 in ultrapure water. A design of experiments procedure has been carried out in order to optimize the ozonation process as well as to study the interaction between the studied variables: pH and T. In addition to the application of the single ozonation, ozone-combined processes were studied under the optimal conditions obtained for O3 (namely, pH=8 and T= 20ºC). The tested processes were

O3/Fenton,

O3/H2O2,

O3/UV,

O3/UV/H2O2,

O3/photo-Fenton,

O3/UV/TiO2

and

O3/UV/TiO2/H2O2. According to the experimental results, single ozonation is a more effective treatment than O3/H2O2 and O3/Fenton processes. Additionally, the introduction of UV irradiation results, in all cases, in a faster and more efficient removal of the parabens due to the

*Corresponding author. Phone: +34-924-289300 Extn 86888; Fax: +34-924-289395. ACS Paragon Plus Environment

Email address: [email protected] (Joaquín R. Dominguez)


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contribution of ozone photolysis to the radical-mediated pathway. The most efficient method for the degradation of these emerging pollutants was O3/UV/TiO2/H2O2. The influence of different aqueous matrices (namely, ultrapure water, river water, WWTP effluent and reservoir water) on the removal of the target pollutants has been analyzed. The direct ozonation pathway is the predominant degradation mechanism and a remarkable synergistic effect of bicarbonate ions has been demonstrated.

1. INTRODUCTION. The presence of pharmaceuticals and personal care products (PPCPs) in the environment has recently drawn the attention of the scientific community due to their ubiquity and effects on health.1 Parabens are esters of 4-hydroxybenzoic acid, also called p-hydroxybenzoic acid (Figure 1). In this research work the most frequently used parabens (i.e., methylparaben, ethylparaben, propylparaben and butylparaben) have been selected as the target pollutants to be removed from water by O3-combined processes. All of them are very effective antimicrobial agents, with a broad spectrum of activity,2 and are used as preservatives mainly in personal care products, food, and pharmaceuticals.2,3 However, the number of reports confirming the estrogenic activity of parabens is rising in the last few years.4-6 A potential relationship between breast cancer and the application of paraben-containing products on skin is speculated since these compounds have been reported to be involved in the development of breast tumors.7 Even though parabens are expected to be removed using conventional water and sewage treatments,8 their presence in river water9 and effluent of waste treatment plants10,11 has expelled this perception. However, the main sources of human exposure to parabens are personal care products and pharmaceuticals.

ACS Paragon Plus Environment

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A literature review indicates the presence of parabens in all parts of the environment and also in human fluids such as urine, milk, serum, and semen.12-19 The presence of parabens has also been reported in influents and effluents of sewage treatment plants and in river water.20,21 Several studies have demonstrated that the extent to which parabens can be degraded under typical biological treatment plants used in wastewater treatment plant (WWTP) is limited. Advanced oxidation processes (AOPs) are a good choice for the treatment of hazardous nonbiodegradable pollutants. These processes are based on the generation of highly reactive species such as hydroxyl radical (•OH), which degrades a broad range of organic contaminants quickly and in an unselective manner. Among these oxidation processes, ozone (O3) with higher standard oxidation potential is expected to oxidize organic pollutants more efficiently than chlorine and chlorine dioxide.22 The use of advanced oxidation technology based upon ozone is a good option because ozone itself is a good oxidation agent that can degrade a wide variety of organic contaminants and is capable of producing hydroxyl radical when it is coupled with hydrogen peroxide or UV irradiation.23 Furthermore, ozone is also widely used in drinking water treatment.24,25 It has been shown to provide microbial disinfection and oxidation of trace contaminants during wastewater and water reuse applications.26-28 Ozone has been traditionally used in the treatment of drinking water.29 In other works30,26 it was observed that ozone dose of 2 mg · L-1 was capable of removing over 80% of some drugs present in an effluent from the secondary treatment in a sewage treatment plant. During ozonation of wastewater, some drugs are predominantly removed via direct reaction with ozone whereas other are oxidized and transformed largely by hydroxyl radicals.31,32


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High removal efficiencies are achieved with ozone and/or hydroxyl radicals for compounds with electron-rich aromatic rings (e.g. hydroxyl, amino, acylamino, alkoxy, etc.) and alkyl aromatics, as well as for compounds with amino groups and non-aromatic alkenes.33 Among the POAs, those involving the use of ozone form a well-defined group and are characterized by the high reactivity of ozone and its ability to combine with different agents to increase the yield of the oxidation process. The combination of O3 with other agents allows raising the concentration of radicals. For instance, perozone processes, photolysis of ozone or combinations of ozone with hydrogen peroxide and ozone photolysis, mainly by near UV radiation, such as the O3/UV are among the most studied processes to the date process.34 One of the advanced oxidation processes that are currently being investigated is the catalytic ozonation and even more recently, the photocatalytic ozonation. In this work, the results obtained from an advanced oxidation process of parabens based on ozone treatment are presented. The design of experiments was used to study the effect of pH (in the range 2.5-9.54) ant the temperature of the process (in the range 11-39 ºC). Response Surface Methodology (RSM) technique was used to optimize parabens degradation (Y, %). The aim of the present study is to determine the optimum conditions of the O3 process. Such optimum is applied to several O3-combined processes, namely O3/Fenton, O3/H2O2, O3/UV, O3/UV/H2O2, O3/photo-Fenton, O3/UV/TiO2 and O3/UV/TiO2/H2O2 in ultrapure water.

2. MATERIALS AND METHODS. 2.1. Chemicals and aqueous matrices. Methylparaben

(C8H8O3),

ethylparaben

(C9H10O3),

propylparaben

(C10H12O3),

and

butylparaben (C11H14O3) were purchased from Sigma–Aldrich, Spain, in analytical purity grade.


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Pollutants stock solution (5 ppm each) was prepared with ultrapure water. In order to guarantee pH stability, all solutions were adjusted with HPO42-/H2PO4- buffer solution. For the O3/Fenton and O3/photo-Fenton systems the pH was adjusted using the referred perchloric acid/perchlorate buffer solution. Apart from ultrapure water, another three aqueous matrices were used to investigate the removal of the four parabens. The first one was water from the Guadiana River (Badajoz, SW Spain). The second one was a surface water from the public reservoir Peña del Águila (Villar del Rey, Badajoz, SW Spain), and the third one was a secondary effluent generated in the municipal WWTP of Badajoz. The results of the physico-chemical characterization of the aqueous matrices used in this work are shown in Table 1. 2.2. Experimental procedure. All experiments were performed in a semi-continuous agitated glass reactor submerged in a thermostatic bath to keep the temperature at the desired value within ±0.2ºC range. For each of the experiments, the reactor was filled with a mixture of four parabens in aqueous solution (5 ppm each). Previous experiments (not shown for the sake of brevity) confirmed that the experimental time of equilibration was reached within the first 15 min. For all experiments the reactor was filled with 350 mL (except to O3/Fenton process, 250 mL). The reaction was quenched by adding sodium bisulphite at preset time intervals. The reactor had four inlets at the top for sampling, bubbling the gas feed, venting and measuring the temperature. Ozone was produced from an oxygen stream in an ozone generator (Sander Ozonisator mod. 501, Germany). In every experience in which UV radiation was involved, basically, the reactor was equipped with a radiation lamp located inside the reactor in axial position and protected by a quartz sleeve which housed the lamp. A low pressure vapor mercury lamp (TNN 15/32, nominal electrical


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power 15W; Heraeus, Madrid, Spain) which emitted monochromatic radiation at 254 nm was used. The optimal conditions obtained through the experimental design (namely, pH = 8 and T = 20° C) were used to carry out the elimination of compound by combining ozone and other oxidation agents. For the O3/Fenton and O3/photo-Fenton systems the pH was adjusted using the referred perchloric acid/perchlorate buffer solution (pH=3.5) to avoid Fe2+ and Fe3+ precipitation. 2.3. Analytical method. Solutions consisting of an admixture of the four parabens were used. The concentrations of methylparaben, ethylparaben, propylparaben and butylparaben were analytically determined by HPLC in a Waters chromatograph, equipped with a 996 photodiode Array detector and a Waters Nova-Pak C-18 column (5 µm, 150 mm 9 3.9 mm). Parabens peaks were obtained at retention times of 2.2, 2.8, 4.2 and 6.8 min for MP, EP, PP and BP, respectively. Samples of 50 µL solution were introduced (1 mL/min) into the chromatograph and 60:40 (methanol:water 10−2 mol·L-1 orthophosphoric acid) was used as the mobile phase. Isocratic operation mode was chosen, that is, the proportion of the mobile phase was kept constant throughout the experiment. 2.4. Mathematical and statistical procedures. The DOE is a common methodology used to examine the influence of operating condition of O3 oxidation of pollutants and to determine the optimum condition using RSM (response surface methodology). Using RSM, the aggregate mix proportions can be arrived with minimum number of experiments without the need for studying all possible combination experiments. Therefore, the application of RSM allowed the establishment of a mathematical relationship between variables and the response variable investigated. To convert the natural variables into dimensionless codified values, the following equation was used:


6

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xi =

Xi − Xi ∆X i

X

[1]

where xi is the coded value of the i-th independent variable, Xi the natural value of the i-th independent variable, XiX the natural value of the i-th independent variable at the center point, and ∆Xi is the value of step change. Each response Y can be represented by a mathematical equation that correlates the response surface:

Y = bo + ∑ b j x j + ∑ bij xi x j + ∑ b jj x 2j j =1

i , j =1

[2]

j =1

where Y is the predicted response, b0 is the offset term, bj is the linear effect, bij is the firstorder interaction effect, bjj is the squared effect, and k is the number of independent variables. For this work, Central Composite Design (CCD) was selected. It involves the use of a twolevel factorial design with 2k points combined with 2k axial points and n center runs, k being the number of factors, N, is obtained according to Eq. [3].

N = 2k + 2 ⋅ k + n

[3]

where n is considered to be twelve and the axial distance is 2 in order to guarantee an orthogonal and rotatable design. To perform this study, two independent variables (pH and Temperature) were used, with 8 replicates of central point, so that the total number of experiments (N) was 16. The experimental results were statistically analyzed and the statistical validation was achieved by ANOVA test at 95% confidence. Table 2 shows the operation levels of the DOE. In Table S1 (Supplementary Material) the design of experiments with coded and real variables are shown. On the other hand, Table S2 (Supplementary Material) shows the experimental obtained response in each experiment (Yparaben (%)).


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The obtained parabens removal at 15 minutes was considered as the response to perform the statistical analysis in the O3 process. It was calculated as follows: Y (%) =

C0 − Ct x100 C0

[4]

where C0 represents the concentration of each of the parabens initially present in solution and Ct is the final concentration of pollutant determined once the contact time (15 minutes) has elapsed. To determine the concentration, a calibration curve is used In order to determine the concentration range in which the system complies with the Lambert–Beer’s law as well as the optimal range of applicability, Ringbom’s plot was used.

3. RESULTS AND DISCUSSION. 3.1. Experimental design and data analysis of the ozonation process. 3.1.1. Numerical analysis The results obtained in each of the experiments described in the previous section are summarized in Table S2. The ANOVA analysis provides the significance of the different parameters. According to the RSM, five factors are considered (pH, temperature and combinations of all them) and five or four of them have a p-value below 0.05 (significance limit), which indicate that such factors are statistically significant at 95% (see Table S3 in Supplementary Material). This means that the model used to represent the behaviour of the interactive factors is consistent. A nonlinear polynomial regression is carried out taking into account Eq.2, and the following expression was obtained accordingly as performed and the following expression for each


8

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paraben. The Table 3 shows the values of the fitting coefficients obtained for the removal of the four parabens.

YParaben(%) = b0 + A ⋅ pH − B·T − C·( pH) 2 − D·pH·T − E·(T ) 2

[5]

These values reveal that the pH is preceded by a positive (+) sign in all cases and, hence it exert a positive effect on the removal of parabens. On the contrary, the remaining factors exhibit a negative (-) sign in Eq. 5, which is indicate a negative influence of these factors on the removal process. The factors negatively influencing parabens removal are the temperature, the combination of pH and temperature, the squared pH, and the temperature. The influence of temperature on paraben degradation by ozonation has been studied within a temperature range of 11 to 39°C (Table 2). In general, the removal of the compounds increases with temperature until approximately 18°C, however, at a temperature higher than this (optimum value), the removal efficiency achieved of the compound decreases. Generally the elimination of paraben increases with pH. Under acidic conditions, ozone itself is the reactive species in the degradation process. Due to the selectivity of ozone in the degradation process, parabens degradation has low efficiency under acidic conditions. Furthermore, the fraction of ozone decomposes before reacting with the compounds, is catalysed by OH- ions, and thus occurs more rapidly and to a greater extent with increasing pH. Therefore, the degradation rate of the parabens increases with increasing pH. The obtained correlation factors r2 were among 0.910 and 0.959. This regression leads to an optimum YParaben (100%) In Table 4, it can be seen that for the four parabens, the pH have an optimum value which is within the working region. Also for the other variable (temperature), the optimal value is in the working region, except to propylparaben, the value is one corner of the study area (-1,414), so


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there is not an optimal value for that variable in that range. It can only talk about the trend or influence positive of the variable of study. Figure S1 (Supplementary Material) presents for each experiment, the difference between experimental and predicted Y (%). This is the graphical expression of adjusted correlation factor (according to Eq. 2). It can be appreciates a good correlation between both parameters. Therefore, the model is able to predict the experimental results reasonably well. Finally, it is also possible to determine the experimental conditions that provide a simultaneous optimization of all of the response variables. With such an aim, the so-called desirability function approach is commonly used. In this case, this method has been used to determine the operational

conditions that would maximize the simultaneous removal of the four parabens. Briefly, according to the literature35 for each response yi(x), a desirability function di(yi) assigns numbers between 0 and 1 to the possible values of yi, with di(yi)= 0 representing a completely undesirable value of yi and di(yi) = 1 representing a completely desirable or ideal response value. The individual desirabilities are then combined using the geometric mean, which gives the overall desirability D: D = (d1(y1) · d2(y2) · d3(y3)... · dz(yz))1/z

[6]

with z denoting the number of responses. Notice that if any response yi is completely undesirable di(yi) = 0, and then the overall desirability, D, is zero. The desirability function can adopt different expressions if a given response yi has to reach a maximum, a minimum or a specific value. Table 4 also lists the coded and natural values of pH and temperature that maximize the removal of each of the four parabens simultaneously. Such coded values are 0.522374 and -0.647001, respectively. Operating under these conditions, at least theoretically, an optimum removal of the four parabens can be achieved simultaneously.


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3.1.2. Graphical analysis. Modelization is made on the basis of five factors which correspond to Eq.2. A graphical expression of the ANOVA test results may be the Pareto graphic (Fig. S2). Bars represent the standardized effects of each involved factor, considering them as the pH and the temperature, and combinations of them. Filled bars are a graphical representation of negatively affecting factors, such as the squared of pH, the temperature, the squared of temperature and the combinations of pH and temperature. On the other hand, unfilled bar represents the factor which affect positively, such as the pH. The Pareto graphic also gives us an idea on how is the influence the factor on the final response (% Y removal parabens). The vertical line determines the significance level of the ANOVA test at 95 % confidence. In this case, all factors (except BP) are above this line, so affect the removal of the different parabens. The evaluation of the CCORD model leads also to the study of the main effects of the involved variables. This can be observed in Figure S3 (Supplementary Material). Two curves are drawn representing the effect of varying each variable while the other ones keeps constant. In all cases, the effect of pH is positive and the temperature presents a negative tendency. A maximum is shown in the final part of the curve in the case of pH and in the beginning part of the curve in the case of temperature. With regard to the interaction between variables, Figure S4 (Supplementary Material) shows the interaction between the two variables studied. Each curve is a representation of the evolution of percentage removal by varying one variable in the CCORD model, that is, with its pair variable equal to +1 and equal to -1. Parallel lines mean there is no interaction between them. Crossing lines indicate the contrary, it means, the modification of one of the variable affects the


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other. At high pH values, the other variable (temperature of process) affects the process, but at low pH values, the temperature has little effect on the target variable. The response surface plot is the most important graphical representation in the RSM (see Figure 2). It plots Eq.5 and allows evaluating the behavior of the whole system under study from a qualitative point of view. As it can be appreciate in the Fig.6, within this region the studied response is represented by a convex surface. The maximum appears in the positive side of coded pH and the negative side of coded T. The removing of paraben increased when the pH is increased and when the T is decreased. 3.1.3. Rationalization of the experimental results. The statistical robustness of the experimental data discussed until now has been demonstrated and the aim of this section is not to call into question the results of the graphical and numerical statistical analyses of the experimental data. On the contrary, this section aims at explaining and justifying the experimental results. When possible, the experimental data obtained in this work have been compared with other previously reported in the literature. With respect to the influence of pH on the ozonation process, this particular topic has been widely discussed in the literature. Nevertheless, Response Surface Methodology has been scarcely used as a tool to analyze the influence of this operational variable on the removal efficiency of organic pollutants by ozonation processes. In this connection, Wilde el al.36 reported that at pH values below 4, the ·OH generation is limited, which results in the prevalence of the direct reaction of molecular ozone with organics. On the contrary, within the pH range comprised between 4 and 9 both, direct ozonolysis and ·OH mechanism take place. Finally, at pH above 9, ·OH generation is predominant. This is probably so because, as the pH value increases, interactions between O3 and hydroxide ions lead to the formation of hydroxyl radicals.37 Hence, low values of pH results in


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predominant direct ozonation pathway whereas high values of pH results in an extensive decomposition of the O3 molecules giving rise to large amounts of ·OH radicals thus hindering direct ozonation and enhancing indirect (radical-mediated) degradation of the pollutants. Similar results have been reported by El-Din et al.38. The experimental results obtained in this work are in good accordance with these assertions. In fact, optimal removal efficiencies of the four parabens separately are attained at pHs comprised between 7.5 and 8.4 and the simultaneous optimized removal is achieved at pH = 7.3. The study of the influence of temperature on ozonation processes has attracted the attention of the scientific community for several years. Nevertheless, controversies have arisen when dealing on the influence of temperature on the removal efficiency of organic pollutants by ozonation processes. According to Perkins et al.39, the difference in the removal efficiency of organic acid dyes by ozone within the temperature range comprised between 24 and 50ºC would be of little practical significance. Koyuncu and Afşar40 reached the same conclusion when studying the ozonation of azo dyes in textile wastewaters. Similar results were obtained by Kornmüller et al.41 for the ozonation of polycyclic aromatic hydrocarbons. No influence of temperature on the ozonation of the target pollutant was found by these authors within the temperature range comprised between 20 and 40ºC. More recently, Hsing et al.42 reported that the effect of temperature on the removal rate of acid orange 6 by ozonation was limited. On the contrary, Quan et al.43 found that that temperature exerts a noticeable effect on the removal of 4-chloro-2-methyl phenoxyacetic acid (MCPA) by ozonation processes both, in the presence and in the absence of a well-known ·OH scavenger as it is tert-butyl alcohol. These authors conclude that the pseudo first-order rate constant in both systems increases as temperature rises, thus leading to an enhanced and faster removal of the target pollutant.


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Finally, several researchers have reported a negative influence of temperature on the degradation of different organic molecules by ozonation treatments. Elovitz et al.44 attribute the negative effect of temperature on the ozonation process to the fact that ozone depletion rates increase with increasing temperature. Perkins et al.39 conclude that such a negative effect is due to the fact that the solubility of ozone in water decreases as temperature rises. The experimental data reported in the present work indicate that although, as a rule (excepting, perhaps for PP), temperature may exert a positive effect on the ozonation process within the range comprised between 10 and approximately 16-18 ºC, a further increase of temperature above 20ºC and up to 40ºC results in a decrease of the removal efficiency of the parabens (see Figure S3). A plausible explanation for this experimental evidence could be that ozone solubility in water, stability in aqueous solution and reactivity towards the organic pollutants are strongly dependent on temperature. These three factors affect the efficiency of the ozonation process in a remarkable manner. As indicated above, solubility of ozone in water decreases as temperature rises. The same applies to the stability of ozone in aqueous solution. On the contrary, the reactivity of ozone towards the organic molecules in aqueous solution increases as temperature does. Hence, according to the experimental data, the positive effect of temperature on the reactivity of ozone predominates at the lower part of the operational range (i.e., 10 to 18-20ºC) and the removal efficiency rise as temperature does. From 20ºC and above, as temperature rises the ozone dissolved in water becomes progressively more unstable and, in addition, the solubility of this gas in water decreases, too. As a consequence, once the optimal operational temperature (i.e., 16-18ºC) is reached, the removal efficiency of the parabens begins to decrease if temperature rises. Similar results have been reported in the literature.45,46 It must also be kept in one’s mind that the indirect (i.e., radical chain reaction-mediated) ozonation is remarkably


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dependent on temperature and that the rate of the chain reactions involved increases as temperature does.44,47 The fact that in this work the removal efficiency decreases markedly at temperatures of 20ºC and above suggests that for the degradation of the four parabens by ozonation process the direct ozonation is the prevalent mechanism. 3.2. Degradation of parabens dissolved in real water matrices using ozonation. In order to analyze the influence of the aqueous matrix on the degradation of the parabens by the ozonation process, three different aqueous matrices (namely, surface water from the Guadiana River and Villar del Rey reservoir, and an effluent from the WWTP of Badajoz) were used. All the experiments were performed under the optimal conditions determined for the simultaneous removal of the parabens in ultrapure water by the ozonation process. Figure 3 shows the evolution of the removal efficiency vs. time of the four parabens for the ozonation process in the four aqueous matrices. From Figure 3 it can be easily seen that, as a rule, the removal efficiency (in %) of the four parabens in the aqueous matrices follows the trend ultrapure water < river water ≃ WWTP effluent ≃ reservoir water. Moreover, in general, it can be stated that the removal of the four pollutants is slower when ultrapure water is used as the aqueous matrix. Hence, a synergistic effect of the substances present in real water matrices is pointed out by the experimental results. In other words, although a nearly complete removal of the pollutants is achieved in the four aqueous matrices under study, the treatment times required are remarkably shorter in real water matrices (15 minutes) than in ultrapure water (30 minutes). In order to investigate the effect of the different ions present in the real water matrices on the removal rate of the pollutants, different solutions were prepared by dissolving chlorides, nitrates, carbonates and/or phosphates in ultrapure water. Since, as a rule, WWTP and reservoir water were the matrices leading to the fastest removal of parabens, the concentrations of the referred


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anions dissolved in ultrapure water were those summarized in Table 1 for WWTP. Figure 4 depicts the removal of the four parabens vs time in the different solutions. From the data plotted in this figure, several experimental features may be highlighted. Firstly, the effect of chlorides and nitrates on these parameters is negligible. Secondly, the presence of phosphates results in slightly lower removal efficiencies as well as in slower removal rates of the four pollutants with respect to ultrapure water. And, finally, the presence of carbonates leads to enhanced removal efficiencies and speeds up the removal process. The limited effect of chlorides and nitrates on the removal efficiency of organic pollutants has been previously reported in the literature. For instance, Grebel et al.48 suggest that the scavenging effect of halides may dramatically reduce the treatment efficiency of electron-poor contaminants, but the extent of the reduction with electron-rich contaminants may be less dramatic. This latter appears to be the case in the removal of the four parabens under study in this work. Regarding the role of nitrate ions in ozonation processes, the importance of a strong oxidizing agent such as peroxynitrite (ONOO−) has been reported in the literature.49 Peroxynitrite is a nitrogen oxyanion that contains a peroxo (O-O) bond. Peroxynitrite is a structural isomer of the nitrate ion and its ability to react with a wide variety of inorganic and organic reductants has been reported previously.50 However, to take place in a kinetically significant manner, isomerization of nitrate into peroxynitrite requires the presence of UV radiation. Hence, the synergetic effects of nitrate through the formation of peroxynitrite can be neglected in this case as UV radiation was not used. With respect to the effect of the phosphate ions in solution, it is well known that this chemical species constitutes a scavenger for ·OH radicals according to the following reactions:51 H2PO4- + ·OH → H2PO4· + OH-

k ≈105 M-1s-1

[7]


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HPO42- + ·OH → HPO4-· + OH-

k ≈107 M-1s-1

[8]

Under the operational conditions (i.e., pH=8.3) approximately 95% of phosphate present in solution is in its HPO42- form whereas the remaining 5% is as H2PO4-. According to equations [7] and [8], HPO42- acts as a more efficient scavenger of ·OH radicals. Nevertheless, the decrease in the removal efficiency of parabens that can be attributed to the scavenger effect of phosphate is very low, which suggests that the ·OH-mediated degradation of these pollutants is not the predominant mechanism in this particular case. On the contrary, as indicated above, the presence of carbonates in solution results in an enhancement of both, the removal efficiency and the removal rate of the parabens. Carbonate and bicarbonate are also known as scavengers of ·OH radicals. The radical scavenging effect of carbonate is more remarkable than that of bicarbonate due to its higher reaction rate constant with ·OH radicals:52 • OH + HCO3− → OH− + • HCO3

k ≈ 8.5·106 M-1 s-1

[9]

• OH + CO32− → OH− + • CO3−

k ≈ 3.9·108 M-1 s-1

[10]

Under the operational conditions (pH = 8.3), bicarbonate is by far the predominant species. Hence, the scavenging effect is not as efficient in this case but it should result in a decreased removal efficiency of parabens. The fact that, on the contrary, the four pollutants are removed to a larger extent and more rapidly in the presence of carbonates also points out that the ·OHmediated degradation is not the predominant pathway. In other words, the degradation of the pollutants by ozonation is not attributable to hydroxyl radical oxidation. Although oxidation of bicarbonate into bicarbonate radical by ·OH radicals is feasible (see equation [9]), ·HCO3 is not likely to exert a remarkable oxidation of parabens. It must be kept in one’s mind that the reducing potential of ·HCO3 is lower (1.78 V at pH 7.0) than that of ozone (2.08 V).53 Additionally, it has been reported48,54 that carbonate and, particularly, bicarbonate


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ions enhance the stability of aqueous molecular ozone, which in results in the presence of more ozone molecules available to degrade the pollutants. Although the mechanism of this promotion effect has not been completely elucidated to the date, it can be at least partially ascribed to the fact that bicarbonate can terminate the self-decomposition cycle of ozone by consuming hydroxyl radicals generated from ozone–OH− reaction.53 From all the above exposed it may be concluded that the contribution of indirect ozonation (i.e., the ·OH radical chain reaction mediated mechanism) is very scarce in the case of the removal of parabens from aqueous solution whereas direct ozonation is the main responsible of the oxidizing of the pollutants. In order to corroborate this assertion, ozonation of the four parabens has been performed in the presence of t-butyl-alcohol (tBuOH), a powerful scavenger of ·OH radical that is commonly used to study this kind of processes from a mechanistic point of view. Figure 5 plots, as an example, the removal efficiencies of MP and BP in the presence and absence of tBuOH. It can be clearly seen that the possibility of ·OH-mediated oxidation (i.e., indirect ozonation) results in a slightly larger removal efficiency only at the earliest stages of the ozonation process. For treatment times larger than 5 minutes, the presence or absence of hydroxyl radicals does not exert any important effect and after 12-15 minutes of ozonation the removal efficiencies are virtually the same regardless tBuOH is added or not to the system. These experimental evidences are in good agreement with the fact that the direct ozonation of organic molecules is a selective reaction with slow reaction rate constants, typically being in the range of 1.0−106 M-1 s-1.55 Nevertheless, the reaction rate of ozone with organic pollutants is enhanced if the target molecule includes aromatic rings with electron-supplying substituents as, for instance, hydroxyl groups. This is precisely the case of parabens (see Figure 1).


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The mechanism of parabens degradation by AOPs has already been investigated56. Results showed that the four parabens can be initially attacked by •OH through •OH-addition and Habstraction routes. The •OH-addition route was more important for the degradation of shorter alkyl-chain parabens like MP and EP, while the H-abstraction route was predominant for the degradation of parabens with longer alkyl-chain for example PP and BB. A degradation mechanism of parabens by ozonation was proposed57.Hydroxylation was found to be a significant reaction in the ozonation of parabens, mostly at the aromatic ring and the ester chain of parabens. Hydroxylation at the aromatic ring produced a series of aromatic ring hydroxylated products which can occur via a direct reaction with ozone or ·OH which was generated from the decomposition of ozone in water.

3.3. Comparison of the degradation of parabens by different O3-combined advanced oxidation processes. In order to analyze the concomitant effect of other reactant(s) as well as of the UV irradiation on the removal efficiency of the organic micropollutants under study, a set of 8 experiments were performed in ultrapure water. Apart from single ozonation, two binary processes (i.e., O3/H2O2 and O3/UV), three ternary processes namely, O3/H2O2/Fe2+ (or O3/Fenton), O3/UV/TiO2 and O3/UV/H2O2, and two quaternary processes, O3/UV/H2O2/Fe2+ (or O3/photo Fenton) and O3/UV/H2O2/TiO2 were studied. All experiments were performed under the optimal conditions obtained from the analysis of the design of experiments, that is, pH = 8 and T = 20ºC excepting those involving Fe2+. These latter were carried out at pH = 3.5 to avoid Fe2+ precipitation. The specific experimental conditions for each of the systems under analysis are summarized in Table S4 (Supplementary Material).


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Figure 6 shows the average elimination of MP, EP, PP and BP in ultrapure water for each of the different AOPs in which ozone is involved. Two experimental facts are to be mentioned in this connection. Firstly, single ozonation is a more effective treatment than O3/H2O2 and O3/Fenton processes. And, secondly, the introduction of UV irradiation results, in all cases, in a faster and more efficient removal of the parabens. The fact that the O3/H2O2 process is the slowest among those studied here and the second less efficient one is attributable to an inhibitory effect of H2O2. In this connection, Beltrán et al.58 reported on the negligible and even negative effect of hydrogen peroxide in terms of oxidation rate with respect to single ozonation. These authors attribute this neutral or even negative influence to a detrimental effect of H2O2 on direct ozonation reactions so that the contribution of ·OH radical mediated oxidation (due to the presence of hydrogen peroxide) cannot compensate the former. Lee at al.59 reached the same conclusion when dealing with the degradation of chlorotetracycline by ozone-based advanced oxidation processes. With respect to the O3/Fenton system a more efficient removal of the parabens would be foreseeable due to the contribution of the Fenton reagent to the formation of ·OH radicals and the scarce interference of this reagent with the direct ozonation reactions. Nevertheless, this is not the case in the present study. In fact, the O3/Fenton system is slightly slower and less efficient than single ozonation, as indicated above. It must be taken into consideration, however, that this experiment was performed under acidic pH (3.5) to avoid Fe2+ precipitation. Zeng et al.60 reported that the performance of O3 alone at pH = 8 is somewhat better than that observed for O3/Fenton at pH = 3.5, which is in good agreement with the results of this work. Closely related with this latter experiment, the O3/UV/H2O2/Fe2+ (or O3/photo Fenton) system exhibits a slightly higher efficiency than ozonation alone. This change is attributable to the role


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played by UV irradiation. In fact, as mentioned above, the use of UV light results in an enhanced removal efficiency of all the systems with respect to single ozonation. This fact is probably due to the UV-mediated photolysis of ozone. Particularly, Monteagudo et al.61 reported that the combination of ozonation with photo-Fenton at low pH results in a remarkable increase of the contribution of the radical-mediated pathway (up to ~77%) to the detriment of the direct ozonation (~22%). For O3 alone the direct-to-radical reaction ratio was ~96:3. In this connection, Benitez et al.62 concluded that the combinations of ozone with UV radiation or even UV and H2O2 result in a synergistic effect of the individual reactions, namely, direct ozonation, direct photolysis, and hydroxyl radical oxidation. The ·OH radicals can be generated through photolysis of ozone and/or H2O2. The referred authors also analyzed the effect of TiO2 and UV radiation coupled to ozonation and concluded that the increase in the removal efficiency could be due to the formation of the ozonide radical, which in turn generates ·OH radicals. The experimental results obtained in this work suggest that the addition of H2O2 or TiO2 to the O3/UV system, however, exerts a very limited effect (if any) in terms of kinetics and removal efficiency when compared with the O3/UV system. Only in the case of the quaternary O3/UV/H2O2/TiO2 system the removal rate is remarkably higher, which suggests that the simultaneous presence of a well-known ·OH radical generator as H2O2 and an effective photocatalyst as TiO2 accelerates the degradation process but without influencing in a noticeable manner the overall efficiency of the process as depicted in Figure 6. These results are also in accordance with other previously reported in the literature.63,64


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4. CONCLUSIONS. From the results obtained in the present study the following conclusions may be drawn: •

A statistical design of experiments procedure has been carried out in order to optimize the removal of four parabens (MP, EP, PP, and B) in ultrapure water by ozonation process. The influence of pH and T (ultrapure) was analyzed. Under the optimal operational conditions (pH=7.3 and T= 18.5ºC) 94.85-99.22% removal of the four parabens simultaneously can be achieved.

•

The influence of different aqueous matrices (namely, ultrapure water, river water, WWTP effluent and reservoir water) on the removal of the target pollutants has been analyzed. From the results obtained it can be concluded that for the particular case of the removal of parabens, the direct ozonation pathway is the predominant degradation mechanism and a remarkable synergistic effect of bicarbonate ions has been demonstrated.

•

Up to seven O3-combined processes such as O3/H2O2, O3/UV, O3/Fenton, O3/UV/TiO2, O3/UV/H2O2, O3/photo-Fenton, and O3/UV/TiO2/H2O2 have been studied. The method more efficient to combating this emerging pollution was O3/UV/TiO2/H2O2. According to the experimental results, single ozonation is a more effective treatment than O3/H2O2 and O3/Fenton processes. Additionally, the introduction of UV irradiation results, in all cases, in a faster and more efficient removal of the parabens due to the contribution of ozone photolysis to the radical-mediated pathway.


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ASSOCIATED CONTENT Supporting Information. Design of experiments (Table S1), Response in each experiment (Table S2), Results of the ANOVA test (Table S3), Values of operating variables of POAs (Table S4), Figure S1 according to Eq. [2], Pareto graphic (Figure S2), Main effects (Figure S3) and Interaction graphic (Figure S4).

ACKNOWLEDGMENT The authors gratefully acknowledge financial support of this research work through the Comisión Interministerial de Ciencia y Tecnología (CICYT) -CTM2013-41354-R project.

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(58) Beltrán, F.J.; Ovejero, G.; Rivas, J. Oxidation of polynuclear aromatic hydrocarbons in water. 4. Ozone combined with hydrogen peroxide. Ind. Eng. Chem. Res. 1996, 35, 891. (59) Lee, H.; Lee, E.; Lee, C.H.; Lee, K. Degradation of chlorotetracycline and bacterial disinfection in livestock wastewater by ozone-based advanced oxidation. J. Ind. Eng. Chem. 2011, 17, 468. (60) Zeng, Z.; Zou, H.; Li, X.; Arowo , M.; Sun, B.; Chen, J.; Chu, G.; Shao, L. Degradation of phenol by ozone in the presence of Fenton reagent in a rotating packed bed. Chem. Eng. J. 2013, 229, 404. (61) Monteagudo, J.M.; Carmona, M.; Durán, A. Photo-Fenton-assisted ozonation of pCoumaric acid in aqueous solution. Chemosphere. 2005, 60, 1103. (62) Benitez, F.J.; Acero, J.L; Real, F.J.; Roldan, G.; Casas, F. Comparison of different chemical oxidation treatments for the removal of selected pharmaceuticals in water matrices. Chem. Eng. J. 2011, 168, 1149.

(63)

Benitez,

F.J.;

Beltran-Heredia,

J.;

Acero,

J.L.;

Pinilla,

M.L.

Simultaneous

photodegradation and ozonation plus UV radiation of phenolic acids-major pollutants in agroindustrial wastewaters. J. Chem. Technol. Biotechnol. 1997, 70, 253. (64) Méndez-Arriaga, F.; Otsu , T.; Oyama, T.; Gimenez, J.; Esplugas, S.; Hidaka, H.; Serpone, N. Photooxidation of the antidepressant drug Fluoxetine (Prozac®) in aqueous media by hybrid catalytic/ozonation processes. Water Res. 2011, 45, 2782.


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Page 32 of 42

Table 1. Physico-chemical characterization of the aqueous matrices. River water Reservoir water WWTP effluent pH

8.11

7

8.3

Conductivity (µS/cm)

415

126.2

550

Total Hardness (mg CaCO3/L)

210

38

287

Nitrates (mg/L)

0.1

0.235

0.06

Orthophosphates (mg/L)

0.45

0.029

1.53

Chlorides (mg/L)

40.4

25.5

87.4

Magnesium (mg/L)

12.88

4

21. 87

84

24

222

Ammonium (mg/L)

0.052

0.02

5.8

COD (mg/L)

27.5

18

37

TOC (mg/L)

8

6.7

13

Alkalinity (mg/L)


32

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Table 2. Operating levels of the DOE. Axial

Factorial Central Factorial Axial

Variable (-1.4142) (-1)

(0)

(+1)

(+1.4142)

pH

2.5

3.5

6

8.5

9.5

T (ºC)

11

15

25

35

39


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 42

Table 3. Fitting coefficients obtained for the removal of the four parabens. Paraben

b0

A

B

C

D

E

Methylparaben 58.0

8.9

10.9

10.5

8.6

11.0

Ethylparaben

69.8

10.4

12.3

14.5

11.0

10.9

Propylparaben 83.5

13.4

14.9

19.1

16.8

10.3

Butylparaben

15.4

12.1

15.8

4.35

8.9

90.0


34

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Table 4. Coded and natural optimal values for the removal of MP, EP, PP and BP ultrapure water for 15 minutes of treatment. Separate optimization Optimum Y Coded values (%) pH T

Natural values pH

T, ºC

Methylparaben 65.64

0.745

-0.785

7.9

17.1

Ethylparaben

0.708

-0.922

7.8

15.8

Propylparaben 100.00

0.971

-1.414

8.4

11.0

Butylparaben

0.601

-0.825

7.5

16.8

79.09

99.62

Joint optimization Optimum Y Coded values (%)

Natural values

pH

T

pH

T, ºC

0.522

-0.647

7.3

18.5

Methylparaben 94.81 Ethylparaben

97.97

Propylparaben 99.20 Butylparaben

99.22


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Methyl 4-hydroxybenzoate (MP)

Propyl 4-hydroxybenzoate

Page 36 of 42

Ethyl 4-hydroxybenzoate (EP)

Buthyl 4-hydroxybenzoate (BP)

Figure 1. Molecular structure of target parabens studied.


36

100 80 60 40 20 0,6

0,2

-0,2

-0,6

0,6

-1 1

0,2

-0,2

-0,6

-1

80 60 40

0.2 -0.2

Coded pH

-0.6

-1 1

60 40 20 1

0,6

0.6

0.2

-0.2

Coded T

0,2

-0,2

-0,6

Coded pH Y (%) PP 40 50 60 70 80 90 100

0.6

80

Coded T

100

20 1

Y(%) EP 30 40 50 60 70 80

100

-0.6

-1

% Rem oval efficiency BP

1

% Rem oval efficiency EP

Y (%) MP 20 30 40 50 60

Coded pH

% Removal efficiency PP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


% R em oval efficiency M P

Page 37 of 42

-1 1

0,6

-0,2

0,2

-0,6

-1

Coded T

Y (%) BP 40 50 60 70 80 90 100

100 80 60 40 1

0,6

0,2

-0,2

Coded pH

-0,6

-1 1

0,6

0,2

Figure 2. Response surface and contour plots for the removal of MP, EP, PP and BP.


-0,2

Coded T

-0,6

-1

100

80

80

Removal efficiency, %

100

60

a)

40

Methylparaben Ultrapure water River water WWTP effluent Reservoir water

20

Page 38 of 42

60

b)

40

Ethylparaben Ultrapure water River water WWTP effluent Reservoir water

20

0

0 0

5

10

15

20

25

30

35

0

5

10

15

time, min

20

25

30

35

time, min

100 100

80

80



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



60

c)

40

Propylparaben Ultrapure water River water WWTP effluent Reservoir water

20

60

d)

40

Butylparaben Ultrapure water River water WWTP effluent Reservoir water

20

0

0

0

5

10

15

20

25

30

35

0

5

10

time, min

15

20

25

time, min

Figure 3. Removal efficiency of the four parabens in different aqueous matrices.


30

35

100

80

80


100

60

a)

Methylparaben Ultrapure water Ultrapure + nitrate Ultrapure + phosphate Ultrapure + chloride Ultrapure + carbonate WWTP water

40

20

60

b)

Ethylparaben Ultrapure water Ultrapure + nitrate Ultrapure + phosphate Ultrapure + chloride Ultrapure + carbonate WWTP water

40

20

0

0 0

5

10

15

20

25

30

35

0

5

10

time, min

15

20

25

30

35

time, min

100

100

80

80



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 39 of 42

60

c) 40

Propylparaben Ultrapure water Ultrapure + nitrate Ultrapure + phosphate Ultrapure + chloride Ultrapure + carbonate WWTP water

20

60

d)

Ultrapure water Ultrapure + nitrate Ultrapure + phosphate Ultrapure + chloride Ultrapure + carbonate WWTP water

40

20

0

Butylparaben

0 0

5

10

15

20

25

30

35

0

5

10

time, min

15

20

25

30

35

time, min

Figure 4. Removal efficiency of the four parabens in the presence of different anions.


39


100

100 Without t BuOH With tBuOH

Without tBuOH With t BuOH 80


80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 42

60

40

20

60

40

20

0

0 1

2

3

4

5

7

10

12

15

20

25

30

1

2

3

time, min

4

5

7

10

12

15

20

25

30

time, min

Figure 5. Removal efficiency of MP (left) and BP (right) in the presence and absence of tBuOH.


40

Page 41 of 42

100

Average removal efficiency, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60 O3 O3/H2O2 O3/UV

40

O3/H2O2/Fe2+ O3/UV/TiO2 O3/UV/H2O2

20

2+

O3/UV/H2O2/Fe O3/UV/H2O2/TiO2 0 0

5

10

15

20

time, min

Figure 6. Average removal efficiency of MP, EP, PP and BP in ultrapure water by O3-combined processes.


41


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Degradation of Parabens in Different Aqueous ... - ACS Publications

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