Ultraviolet-Photoassisted Advanced Oxidation of Parabens Catalyzed

Article pubs.acs.org/IECR

Ultraviolet-Photoassisted Advanced Oxidation of Parabens Catalyzed by Hydrogen Peroxide and Titanium Dioxide. Improving the System ,‡ ́ Eduardo M. Cuerda-Correa,† Joaquıń R. Domınguez-Vargas,* María J. Muñoz-Peña,‡ and Teresa González‡ †

Department of Organic and Inorganic Chemistry and ‡Department of Chemical Engineering and Physical Chemistry, Faculty of Sciences, University of Extremadura, Avda. de Elvas, s/n, 06006 Badajoz, Spain S Supporting Information *

ABSTRACT: Parabens are widely used as preservatives in personal care products, food, and pharmaceuticals. This paper presents the results of the photochemical degradation of four hazardous water contaminants (methylparaben, ethylparaben, propylparaben, and butylparaben) using an ultraviolet (UV) radiation source in the presence of hydrogen peroxide and/or titanium dioxide. Oxidation kinetics and overall performance of the removal process for the four parabens in Milli-Q water using different advanced oxidation processes (namely, UV/ H2O2, UV/TiO2, and UV/TiO2/H2O2) were studied. The most efficient oxidation system to remove these pollutants was UV/TiO2/H2O2. A factorial central composite orthogonal and rotatable design of experiments was used to improve the parabens removal using the UV/TiO2/H2O2 process. Initial concentrations of H2O2 and TiO2 were the selected operational variables to optimize the process in Milli-Q water. Similarly, experiments were carried out under optimal conditions in different surface water matrices, namely, Guadiana River water (Badajoz, SW Spain), Peña del Á guila reservoir water (Villar del Rey), and wastewater treatment plant effluent (Badajoz).

1. INTRODUCTION Parabens are esters of p-hydroxybenzoic acid that are commonly used preservatives in cosmetics, food packaging, and pharmaceuticals because of their antimicrobial activity.1,2 Parabens may act as weak endocrine disrupter chemicals.3 Parabens were initially used as preservatives in the pharmaceutical industry since the beginning of the 20th century. Because of their good performance, food and cosmetic industries started using parabens shortly thereafter.4−7 Parabens were considered to be very effective preservatives because of their wide spectrum of activity, chemical stability, relative safety of use, and low cost.7,8 However, in the late 1990s and early 2000s, different studies revealed that parabens could be involved in the incidence of serious diseases such as breast cancer9 and metabolic anomalies because of their endocrinedisrupting effects.10−12 The results of some other studies point out in the same direction, although most of them are still inconclusive.4,7,8 Furthermore, parabens can be regarded as virtually ubiquitous pollutants because their presence in air, dust, and soils8 has been confirmed. Also, a wide variety of industrial products (cigarettes, varnishes, glue, animal feed, and health care products)4,7,13 include parabens among their constituents. The main sources of this kind of pollutants are factories, although differences in the concentrations of parabens in urban wastewater samples have been reported depending on the © XXXX American Chemical Society

source (domestic, industrial, or hospital) and the size of the urban area.14−17 The presence of parabens in surface water and sediments has also been reported. Such a presence is attributable to the consumption of a wide variety of paraben-containing products, which eventually results in an unceasing release of these pollutants into the environment.3 Although a number of treatments may remove parabens from wastewater in a relatively efficient manner, the presence of parabens in effluents of wastewater as well as in other aquatic media at low (micrograms to nanograms per liter) concentrations has been detected. This latter indicates the inefficiency of conventional wastewater treatment.8 Hence, the development of new treatment technologies more efficient in the removal of such contaminants is necessary. The simplicity and cost-efficiency make the advanced oxidation processes (AOPs),18,19 such as photochemical technologies,20,21 suitable and attractive options. Moreover, Special Issue: International Conference on Chemical and Biochemical Engineering 2015 Received: November 30, 2015 Revised: March 18, 2016 Accepted: March 21, 2016

A

DOI: 10.1021/acs.iecr.5b04560 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research AOPs have been identified as a successful alternative for the destruction and mineralization of some recalcitrant organic compounds in water.22−25 In these processes, strongly oxidizing species such as hydroxyl radicals (·OH) are generated. These radicals may degrade a wide variety of emerging contaminants. In this context, AOPs based on UV-photodegration are interesting. In the present study, the results obtained from the study of different UV-AOPs such as UV/H2O2, UV/TiO2, and UV/TiO2/H2O2 are reported. Statistical design of experiments was used to optimize the UV/TiO2/H2O2 system and study the interactions between the operational variables. As is well-known, titanium dioxide absorbs radiation in the near-ultraviolet range to form electron−hole pairs, as shown in eq 1. TiO2 + hv → TiO2 + (e− + h+)

Figure 1. Molecular structure of parabens.

Table 1. Physicochemical Characterization of the Different Aqueous Matrices

(1)

In the presence of redox species adsorbed on the semiconductor particle and under illumination, oxidation and reduction reactions occur simultaneously on its surface. Photoreduction reactions are generated by the so photogenerated holes. After emigrating to the surface, holes react with adsorbed substances, particularly water (eq 2) or OH− ions (eq 3) to generate ·OH radicals. TiO2 (h+) + H 2Oads → TiO2 + ·OHads + H+

(2)

TiO2 (h+) + OH−ads → TiO2 + ·OHads

(3)

pH conductivity (μS/cm) total hardness (mg CaCO3/L) nitrates (mg/L) orthophosphates (mg/L) chlorides (mg/L) magnesium (mg/L) alkalinity (mg/L) ammonium (mg/L) COD (mg/L) TOC (mg/L)

Adding hydrogen peroxide to the UV radiation produces a considerable increase in the rate of photodegradation. This effect may be due to extra ·OH radical generation (eq 4). Furthermore, these radicals can also be formed from photolysis of H2O2 (eq 5). TiO2 (e−) + H 2O2 → TiO2 + OH− + ·OH

(4)

H 2O2 + hv → 2·OH

(5)

river water

reservoir water

WWTP effluent

8.1 415 210 0.10 0.45 40.4 12.8 84.0 0.05 27.5 8.10

7.0 126 38.0 0.23 0.02 25.5 4.05 24.5 0.02 18.1 6.70

8.3 550 287 0.06 1.53 87.4 21. 9 222 5.80 37.2 13.5

2.2. Experimental Procedure. A cylindrical glass reactor was used to perform all the experiments. The reactor is equipped with a radiation lamp (Heraeus TNN 15/32 lowpressure Hg vapor lamp, λ = 254 nm) housed inside a quartz sleeve and located in axial position. Air was continuously bubbled into the reactor to keep the solution stirred. The temperature of the reactor was kept constant (±0.2 °C) with the aid of an external jacket. In all cases, 350 mL of a mixture of the four parabens (5 ppm each) was used to fill the reactor. Quenching was done by addition of sodium bisulphite at preset time intervals. 2.3. Analytical Method. Solutions consisting of a mixture of the four parabens [methylparaben (MP), ethylparaben (EP), propylparaben (PP), and butylparaben (BP)] present in each sample were quantified by high-performance liquid chromatography (HPLC) in a Waters chromatograph. A 996 photodiode array detector and a Waters Nova-Pak C-18 column (5 mm × 150 mm × 3.9 mm) were used. Four peaks were well-defined at retention times of 2.2, 2.8, 4.2, and 6.8 min corresponding to MP, EP, PP and BP, respectively. An isocratic method was used, with a mobile phase consisting of a 60:40 (methanol:water) mixture and 10−2 mol·L−1 orthophosphoric acid. The injection rate was 1 mL·min−1 in all cases. 2.4. Design of Experiments. The statistical design of experiments (DoE) can be used as a powerful tool to analyze the effects that several operational variables may exert on one or more target variables within a certain range. DoE makes it possible to minimize the total number of experiments, too. The use of response surface methodology (RSM) allows the formulation of an equation that correlates the operational variables and the response variable investigated. Each response, Y, can be represented by a mathematical equation as follows:

Both the photocatalytic oxidation process (UV/TiO2) and UV/H2O2 oxidation are widely known and have been applied for the removal of a wide variety of emerging contaminants. This paper aims to demonstrate the synergistic effect of TiO2 and H2O2 combined with UV radiation. Also, the potential occurrence of interactions between the operational variables and the optimization of the advanced oxidation process have been investigated.

2. MATERIALS AND METHODS 2.1. Chemicals and Aqueous Matrices. Methylparaben (C 8 H 8 O 3 ), ethylparaben (C 9 H 1 0 O 3 ), propylparaben (C10H12O3), and buthylparaben (C11H14O3) were provided by Sigma−Aldrich (Spain) of the highest purity available. The molecular structure of parabens is illustrated in Figure 1. Initial parabens solution (5 ppm each) was prepared with ultrapure Milli-Q water. H2O2 (33% w/v) was purchased from Merck (White House Station, NJ, United States) and TiO2 (Degussa P-25) was obtained from Degussa Spain. On the other hand, three aqueous matrices were used in this work, namely, water from the Guadiana River (Badajoz, SW Spain), surface water from a public reservoir (Peña del Á guila, Villar del Rey, Badajoz, SW Spain), and a secondary effluent from the municipal wastewater treatment plant (WWTP) of Badajoz. The physicochemical characterization of these aqueous matrices is shown in Table 1. B


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Industrial & Engineering Chemistry Research Y = bo +

∑ bjxj + ∑ bijxixj + ∑ bjjxj2 j=1

i,j=1

Table 3. Values of the Operating Variables in Each System (6)

j=1

where Y is the predicted response, b0 the offset term, bj the linear effect, bij the first-order interaction effect, and bjj the squared effect. In this study, a second-order, factorial, central, composite, orthogonal and rotatable design (FCCORD) was used. Particularly, a two-level (k = 2) design including 2k factorial points, 2k axial points, and n center runs was planned. The total number of experiments, N, is given by eq 7. N = 2k + 2k + n

(7)

where n is considered to be 8 and the axial distance (a = 2 ) is 1.414 to guarantee orthogonality and rotatability. Hence, in this particular case, the total number of experiments was 16. To convert the natural variables into dimensionless codified values, the following equation was used:

Xi − Xi X ΔXi

(8)

where xi is the coded value of the ith independent variable, Xi the natural value of the ith independent variable, XiX the natural value of the ith independent variable at the center point, and ΔXi the value of step change. To perform this study, two independent variables (namely, initial concentrations of H2O2 and TiO2) were used. Statistical validation was achieved by analysis of variance (ANOVA test; α = 0.95). The removal efficiency of parabens (%) after 15 min was considered as the response variable to be statistically analyzed. It was calculated as follows: Y (%) =

C0 − C t × 100 C0

(9)

where C0 is the initial concentration of each of the parabens and Ct is the final concentration of pollutant after 15 min of treatment. Table 2 shows the operation levels of the DOE. In Table S1 (Supporting Information), both coded and real variables of the Table 2. Operating Levels of the DOE UV/TiO2/H2O2 System lower level

upper level

center point

variable (M)

(−1.414)

(+1.414)

(0.000)

[TiO2] × 103 [H2O2] × 103

0.00 0.00

1.34 2.00

0.67 1.00

UV/H2O2

[H2O2] × 104 [Fe2+] × 104 [TiO2] × 104

2.90

UV/TiO2

UV/TiO2/H2O2 2.90

0.38

0.38

Figure 2 shows that the most effective process is the UV/ TiO2/H2O2 system. H2O2 is a powerful oxidant for pollutant removal because of the generated amount of ·OH radicals in the aqueous medium.26 Furthermore, TiO2 catalyzes the photodegradation of organic contaminants because its intrinsic nature and electrical structure27 facilitate the production of hydroxyl radicals in the medium. In heterogeneous photocatalysis, the mechanism of the semiconductor-catalyzed oxidative degradation of organics can be explained by the band gap model.28 It has been demonstrated that the addition of hydrogen peroxide enhances the rate of photodegradation, probably via reactions 4 and 5. Organic species that may be adsorbed on the TiO2 surface or in its close vicinity can react with the ·OH radicals, thus leading to the oxidation of the pollutant(s). In the UV/TiO2 system, ·OH radicals can be generated by reduction of the positive holes of the TiO2 surface as shown by eqs 2 and 3. Hydroxil radicals may also be formed by reaction of electrons on titania surface with water (see eq 4). Addition of H2O2 improved the effectiveness of the process to a concentration limit because H2O2 is a source of ·OH radicals and also an efficient electron scavenger. In this connection, Gupta et al.29 investigated the photocatalytic degradation of an azo dye, Amaranth, in aqueous suspensions using UV alone, UV/H2O2, UV/TiO2, and UV/ TiO2/H2O2. These authors reported that the fastest decolorization is achieved for the UV/TiO2/H2O2 system. The systems UV, UV/H2O2, UV/TiO2, and UV/TiO2/H2O2 reached decolorization efficiencies of 17%, 26%, 38%, and 64%, respectively, in the runs. Harir et al.30 reached the same conclusion when investigating the photocatalytic transformation of the herbicide Imazamox in aqueous solution containing TiO2, H2O2, or the combination of TiO2/H2O2 under simulated sunlight irradiation. The experimental results obtained determine that the degradation rate constant depends on the different conditions, namely, the catalyst load, the concentration of pollutant, and the pH. In all cases, heterogeneous catalysis (i.e., UV/TiO2 and UV/TiO2/H2O2) yielded faster processes compared to the homogeneous systems (UV alone and UV/H2O2). In this investigation, the most efficient method for the abatement of parabens was UV/TiO2/H2O2. Therefore, a factorial central composite orthogonal and rotatable design (FCCORD) was employed to optimize the operating variables. 3.2. Improving the UV/TiO2/H2O2 System. 3.2.1. Experimental Design and Data Analysis. Numerical Analyses. The ANOVA analysis makes it possible to determine if the different parameters exert a statistically significant effect on the response variable. Among the five factors here considered (namely, concentrations of titania and hydrogen peroxide, their respective quadratic terms, and the interaction term) TiO2 dose and H2O2 concentration exhibit a p-value below 0.05 (significance limit); hence, they may be regarded as statistically significant at 95% (see Table S3, Supporting Information). As a consequence, it may be stated that the model used for this study is consistent.

k/4

xi =

operating variables (M)

statistical design are shown. On the other hand, Table S2 (Supporting Information) shows the experimental response obtained in each experiment (Yparaben (%)).

3. RESULTS AND DISCUSSION 3.1. Degradation of Parabens by Different UV-Based AOPs. The degradation of the four parabens, simultaneously dissolved in ultrapure water, was performed by different UVbased AOPs: UV/H2O2, UV/TiO2, and UV/TiO2/H2O2. To perform a comparison of efficiency, a preliminary set of experiments was carried out according to the operational conditions summarized in Table 3. Figure 2 shows the removal efficiency of the four parabens versus time for the different AOPs. C


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Industrial & Engineering Chemistry Research

Figure 2. Removal efficiency (%) of parabens in each UV-based AOP.

Nonlinear polynomial regression is carried out taking into account eq 6. In this sense, a particularized form of the referred regression equation was obtained for each paraben (eq 10).

Table 5. Coded and Natural Optimal Values for the Removal of MP, EP, PP, and BP Milli-Q Water for 15 Min of Treatment, UV/TiO2/H2O2 System

YParaben (%) = b0 + A ·[TiO2 ] + B ·[H 2O2 ] + C·[TiO2 ]2

Separate Optimization

2

− D·[H 2O2 ]·[TiO2 ] + E ·[H 2O2 ]

coded values

(10)

optimun Y (%)

Table 4 shows the values of the fitting coefficients obtained for the removal of the four parabens. In all cases, the

MP EP PP BP

Table 4. Fitting Coefficients Obtained for the Removal of the Four Parabens

75.45 84.59 89.45 90.02

UV/TiO2/H2O2 System

[TiO2]

natural values

[H2O2]

coded values

paraben

b0

A

B

C

D

E

MP EP PP BP

47.3 56.0 65.6 67.2

4.35 4.40 4.80 3.90

13.8 15.5 16.0 15.3

0.60 0.10 2.30 1.10

4.60 4.33 4.25 2.90

3.40 2.25 0.53 0.62

optimun D=1 MP EP PP BP

coefficients A and B that precede [TiO2] and [H2O2] are behind a positive (+) sign. This means that both factors exert a positive effect on the removal of parabens by the UV/TiO2/ H2O2 system. The obtained correlation factors, r2, were excellent, reaching 0.94, which indicates that the mathematical model used is able to predict the experimental results in an accurate manner. On the other hand, Table 5 shows the values of initial [H2O2] and [TiO2] that maximize the removal of each of the four parabens separately. It can be appreciated that for propylparaben and butylparaben, the initial concentration of TiO2 reaches an optimum value which is within the working region (−1, +1). However, for the other variable (initial concentration of H2O2), an optimal value is predicted at the highest value of this variable (i.e., + 1.414), which suggests that it is not possible to attain an

[TiO2] M

−1.414 1.414 0.000 −1.414 1.414 0.000 −0.249 1.414 5.53 × 10−4 −0.127 1.414 6.11 × 10−4 Joint Optimization

[H2O2] M 2.00 2.00 2.00 2.00

× × × ×

10−3 10−3 10−3 10−3

natural values

[TiO2]

[H2O2]

[TiO2] M

[H2O2] M

−0.6179

1.4142

3.78 × 10−4

2.00 × 10−3

optimal operational value for that variable in this particular range. 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 literature,31 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


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Figure 3. Main effect plots of initial [H2O2] and [TiO2].

D = (d1(y1) ·d 2(y2 ) ·d3(y3 )...·dz(yz ))1/ z

The application of the FCCORD model leads also to the study of the main effects of the involved variables. The main effect plots corresponding to the removal of the four pollutants are shown in Figure 3. Two curves are drawn representing the effect of varying each variable while the other one is kept constant in its central value. In all cases, the effect of initial concentration of H2O2 and TiO2 is positive. Figure 3 also illustrates that initial H2O2 concentration is the main factor governing the parabens removal. In all cases, the effect of initial [H2O2] is positive along the whole operational range. With respect to [TiO2], for PP and BP the curve clearly shows a maximum. This suggests that, as a rule, the presence of TiO2 in solution favors the oxidation reactions because of the catalytic role of TiO2, but only up to a coded value of [TiO2] around −0.25 or −0.13. Larger coded values of this variable exert a negative effect on the removal efficiency of all the parabens. Figure S3 (Supporting Information) shows the interaction plots between the two variables here studied. Each pair of curves represents the values of Yparaben when one of the variables is kept in the extremes of the FCCORD model, namely, with a coded value equal to +1 (upper plot) and equal to −1 (lower plot), and the other one is varied between −1 and +1. If the curves intersect, interaction occurs. This means that changes in one of the variables affects the other one in a significant manner. Taking this latter into consideration, from the interaction plot it may be concluded that interactions clearly appear between variables A and B. At high H2O2 values, the other variable (concentration of TiO2) has little effect on the process, but at low H2O2 values, TiO2 concentration affects the target variable. The synergistic effect of the simultaneous presence of titania and hydrogen peroxide is indicated by the increase in the removal efficiencies of the four pollutants when relatively high concentrations of both reagents are used. For instance, removal

(11)

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 5 also lists the coded and natural values of initial [TiO2] and [H2O2] that maximize the removal of each of the four parabens simultaneously. Such coded values are −0.6179 and 1.4142, respectively. Operating under these conditions, at least theoretically, a total (100%) removal of the four parabens can be achieved. 3.2.2. Experimental Design and Data Analysis. Graphical Analyses. Modelization was made on the basis of five factors which correspond to eq 6. The so-called Pareto plot (see Figure S2, Supporting Information) graphically represents the results of the ANOVA test. The standardized effects of the different factors (i.e., initial concentration of H2O2 and TiO2 and combinations of them) are represented by bars. Gray bars correspond to positively affecting factors (namely, H2O2 and TiO2 concentration and the square of H2O2 concentration), whereas white bars represent negatively affecting factors, such as the square of TiO2 and the combinations of H2O2−TiO2. These latter are included in eq 6 behind a negative sign. The signification of the ANOVA test (at a confidence level of 95%) is graphically represented by the vertical rule that stands close to 2 in the Pareto plot. Hence, if the bar corresponding to a given factor reaches this rule (in this case, A and B) it may be stated that this particular factor exerts a statistically significant influence on the response variable, whereas the remaining ones exert no significant effect on the final response. The Pareto plot also provides information regarding the relative influence of the factors on the final response, YParaben. E


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Figure 4. Response surface and contour plots for the removal of MP, EP, PP, and BP.

rate of removal of this contaminant increased proportionally with the concentration of TiO2 (up to 550 mg/L). However, with larger values of catalyst charge, a decrease was observed in the degradation of the pollutant, independently of the concentration of H2O2. Furthermore, when the concentration of H2O2 increased, the rate of degradation also decreased. At high concentrations of H2O2, the removal of 4-methylbenzylidenecamphor decreased because hydroxyl radicals could act as a radical scavenger.36 For each experiment, Figure S1 (Supporting Information) presents the difference between experimental YParaben and calculated YParaben according to eq 6. A good correlation between both parameters can be seen. Therefore, the model is able to predict the experimental results reasonably well. Additional experimental work has been performed, and the rate of consumption of H2O2 has been included in the Supporting Information (see Figure S4). The experimental results indicate that after 15 min (considered as the response to perform the statistical analysis in the process), approximately 30% of H2O2 had already been consumed. 3.2.3. Experimental Confirmation of the Theoretical Maximum. To corroborate that the experimental conditions predicted by the model are adequate for the removal of the four parabens, complementary experiments were performed under operational conditions corresponding of the simultaneous removal of all pollutants (see Table 5). Table 6 includes the experimental removal efficiencies reached for MP, EP, PP, and

efficiencies ranging from 57.5 to 77.2% are reached in experiment 7, with initial concentrations of TiO2 and H2O2 equal to 1.15 × 10−3 and 1.71 × 10−3 M, respectively. Nevertheless, H2O2 exerts a more remarkable effect on the removal efficiency of the pollutants that TiO2 does, as can be seen from the results of experiment 13 (76.9−93.0%), where 2.0 × 10−3 M H2O2 were used. However, again, in this case 6.7 × 10−4 M TiO2 was present in the reaction, and its effect cannot be considered as negligible. The response surface plot is perhaps the most important graphical representation in the RSM (see Figure 4). It represents eq 6 and makes it possible to evaluate the evolution of the whole system under study from a qualitative point of view. As a rule, the removal percent of each paraben increased when the concentration of H2O2 increased. The experimental results reveal that, because of the interaction occurring between TiO2 dose and H2O2 concentration, TiO2 is not effective for the removal of the pollutants when operating at high concentrations of H2O2. Figure 4 shows a similar behavior for the four parabens. Hence, because the removal of parabens increased at large concentration of H2O2, it can be easily concluded that the main oxidation pathway that largely governs the overall process is the photolysis of H2O2. This photolysis is hampered by TiO2 because it will prevent the passage of light to H2O2. Therefore, the optimum value for this variable (TiO2) is at the lowest part of the operational range (i.e., −1.414). In general, the effect of TiO2 is negligible at such high H2O2 concentrations. Nevertheless, the interaction plot suggests that at low values of H2O2 concentration, the effect of TiO2 is clearly positive. Similar results have been reported by other studies of various organic substances.32,33 Moreover, the decrease in the degradation rate when more catalyst is added could be due to the deactivation of activated molecules by collision with molecules in the elemental state.34 In a study of Sakkas et al.,35 based on RSM, the photochemical degradation of 4-methylbenzylidenecamphor was studied. The optimization process determined that the

Table 6. Experimental and Predicted Obtained Response for Each Paraben (Yparaben%) in Conditions of the Desirability = 1, UV/TiO2/H2O2 System

F

paraben

predicted

experimental

MP EP PP BP

74.7 83.3 89.1 89.7

73.1 81.4 85.9 90.4 DOI: 10.1021/acs.iecr.5b04560 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Removal of the four parabens in different aqueous matrices by UV/TiO2/H2O2 process.

Similarly, Austin et al.38 reported the photodegradation of a pesticide by UV/TiO2 system in a wastewater. These authors indicated that both the organic matter and alkalinity could act as scavengers of hydroxyl radicals. Furthermore, the alkalinity exerts a significant inhibitory effect on the process due to blockage of active sites on the surface of TiO2 by high concentrations of organic matter. In this connection, Kanakaraju et al.39 reported that the presence of inorganic anions in river water decreases the rate of removal of naproxen. Phosphate and chloride anions play an important role in the degradation of diverse pollutants. Phosphate ions have a high capacity to be strongly adsorbed on the surface of TiO2, thus reducing its photoactivity.40 Other researchers indicate that the presence of anions in water can influence the efficiency of the photocatalytic degradation of organics.41,42 More recently, Van Doorslaer et al.43 reported the effect of suspended particles and inorganic/organic constituents on the degradation of a pharmaceutical compound by heterogeneous photocatalysis. The presence of organic and inorganic anions reduces the adsorption of the pharmaceutical on the catalyst’s surface. The most pronounced effect was observed when humic acids were present. The main conclusions drawn by these authors were the following: first, an inhibitory effect on the rate of catalytic degradation was caused by the presence of humic acids; and second, the coefficient of the contaminant adsorption−desorption increased with concentration of humic acids. This effect can be somewhat unexpected because the adsorption of pharmaceuticals normally decreases because of the competition between the humic acids and the pharmaceutical compound for the active sites of TiO2. This latter effect cannot explain the decrease in the rate of degradation of the pollutant when the concentration of humic acids increased. Therefore, other effects must be involved: (i) inhibition through decreased accessibility of pharmaceuticals for reactive species or their scavenging by humic acids44 and/or (ii) adsorption of a fraction of the UV light by the chromophore

BP after 15 min operating under the theoretical optimal conditions. The predicted removal efficiencies are also included in this table. The concordance between experimental and theoretical removal efficiencies indicates that the mathematical model used is able to predict the experimental results in an accurate manner. 3.2.4. Oxidation of Parabens in Real Surface Water Matrices. To analyze the influence of the aqueous matrix on the degradation of parabens by the UV/TiO2/H2O2 process, three different aqueous matrices (namely, water from the Guadiana River, Villar del Rey reservoir, and effluent from the WWTP of Badajoz) were used (see Table 1). All the experiments were performed under the optimal conditions determined for the simultaneous removal of the parabens in ultrapure water by the UV/TiO2/H2O2 process. Figure 5 shows the evolution of the removal efficiency versus time of the four parabens in the four mentioned aqueous matrices. From Figure 5 it can be easily seen that, as a rule, the removal efficiency (%) of the four parabens in the different aqueous matrices follows the order ultrapure water > reservoir water > river water > WWTP effluent. When trends of reactivity and pollution load are compared, it follows that the greater the pollution load, the lower the removal efficiency of parabens. This fact can be justified because the oxidizing species (UV light, hydroxyl radicals, and other radical species) are consumed in reacting with other organic compounds different from parabens. Also, it can be explained by the scavenger action of PO43− and HCO3− species. Similar results have been reported by the scientific community in recent years. For instance, Rubio et al.37 studied the use of TiO2 nanoparticles to evaluate the effect of the aqueous matrix on the photodegradation of clofibric acid. The removal efficiency of the compound decreased in real waters (tap, mineral, river, and wastewater) in the presence of inorganic species (NaCl, NaHCO3, etc.) or in the presence of organic compounds (humic acids and surfactants). G



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groups of humics acids making it less available for the catalyst.44,45

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04560. Design of experiments (Table S1), response in each experiment (Table S2), results of the ANOVA test (Table S3), observed Y versus Y calculated according to eq 6 (Figure S1), Pareto graph (Figure S2), interaction graph (Figure S3), and consumption of [H2O2] (Figure S4) (PDF)

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REFERENCES

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4. CONCLUSIONS The following conclusions may be drawn from the results obtained in this work. (1) Degradation rates and removal efficiencies of different parabens (methylparaben, ethylparaben, propylparaben, and buthylparaben) in ultrapure water using different advanced oxidation processes such as UV/H2O2, UV/TiO2, and UV/ TiO2/H2O2 have been studied. The UV/TiO2/H2O2 process was the most efficient method to remove these emerging pollutants. (2) Degradation of parabens by the UV/TiO2/H2O2 process in ultrapure water was optimized with the aid of experimental design and surface response methodology. An orthogonal, rotatable factorial central composite design of experiments was carried out to optimize the system. The influence of hydrogen peroxide concentration was the greatest among the studied variables. The influence of titanium dioxide concentration was variable. At high H2O2 values, TiO2 concentration has little effect on the process, but at low H2O2 values, the TiO2 concentration largely affects the target variable. An optimum oxidation point was found for the mixture of parabens at [TiO2] = 3.78 × 10−4 M and [H2O2] = 2.00 × 10−3 M. (3) To corroborate that the experimental conditions predicted by the model are adequate for the removal of the four parabens in ultrapure water, complementary experiments were performed under the optimal operational conditions. The concordance between experimental and theoretical removal efficiencies indicates that the mathematical model used is able to predict the experimental results in an accurate manner. (4) The removal efficiency (%) of the four parabens in the different aqueous matrices follows the order ultrapure water > reservoir water > river water > WWTP effluent. When the trends of reactivity and pollution load are compared, it follows that the greater the pollution load, the lower the removal efficiency of parabens.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of this research work through the Ministerio de Economiá y Competitividad, Plan Nacional I+D+I, CTM 2013-41354-R project, as well as by Junta de Extremadura through the GR15067 project. H


Article

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Ultraviolet-Photoassisted Advanced Oxidation of Parabens Catalyzed

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