Ethylparaben (EPB) has been classified by different research groups as a potential endocrine-disrupting chemical, implying that it can potentially interfere with the normal balance of the endocrine system of living beings, which with its presence in different effluents, including drinking water, generates the need to seek methods that allow its removal from different water bodies. Advanced oxidation processes have been employed widely to remove organic compounds from different matrices. In this way, Fenton technology (process based on the reaction between ferrous ions and hydrogen peroxide) has been able to degrade different substrates, but due to the Fe2+ requirements to carry out the reaction optimally, combination of the conventional Fenton process with visible light radiation (photo-Fenton) is an alternative used in the treatment of pollution due to the presence of chemicals. In this way, the effectiveness of photo-Fenton on EPB degradation was assessed using a face-centered central composite experimental design that allowed assessment of the effects of Fe2+ and H2O2 initial concentrations on process. In general, results indicated that after 180 min of reaction almost all EPB was eliminated, the dissolved organic carbon in solution was reduced and the sample biodegradability index was increased.

INTRODUCTION

Parabens (PB, alkyl and aryl esters of p-hydroxybenzoic acid) are a group of chemical compounds widely used as antimicrobial preservatives in the manufacture of cosmetics, foodstuffs, and different personal care products due to their broad-spectrum activity against yeasts, molds and bacteria, low cost and stability to acid-base (Lin et al. 2009; Liu et al. 2014; Gmurek et al. 2015; Haman et al. 2015). However, PB, including ethylparaben (EPB, 4-hydroxybenzoic acid ethyl ester), have been classified as potential endocrine-disrupting chemicals (substances that can potentially interfere with the normal balance of the endocrine system) (Liu et al. 2014). In this way, in vivo and in vitro studies have confirmed that this kind of substance may present estrogenic activity and also can cause carcinogenic response at very low concentrations, which seems to increase with the length of the alkyl chain (Boberg et al. 2010; Gmurek et al. 2015). EPB also has been associated with the decrease in the mean lifespan of fruit flies, and has been detected in human urine, serum and milk (Błędzka et al. 2014; Liu et al. 2014).

The mentioned wide use of PB has led to the introduction of these substances to different water bodies such as rivers, and some studies indicate that they may be present even in drinking water, air, dust and soils. In the specific case of the aquatic environments, it has been shown that PB occur in effluents from wastewater treatment plants, which seem to be not able to remove efficiently those compounds (Gmurek et al. 2015; Haman et al. 2015). This raises the need to evaluate different technologies that allow PB elimination from different compartments, including natural waters and wastewater.

Advanced oxidation processes (AOPs) have been employed by different research groups in removing a large number of organic pollutants from different matrices. These technologies are characterized by generating different radical species, especially the hydroxyl radical , which may oxidize, with excellent performance, different molecules (Rodriguez et al. 2011; Nidheesh & Gandhimathi 2012; Demarchis et al. 2015; Gmurek et al. 2015). The Fenton process is one of the most popular AOPs due to the simplicity of the technology, reagents' low cost and lack of toxicity, high extent of substrate elimination and the fact that the reaction can be carried out at near ambient temperature and atmospheric pressure (Nidheesh & Gandhimathi 2012; Babuponnusami & Muthukumar 2014). In general, during Fenton treatment, ferrous ions (Fe2+) catalyze hydrogen peroxide decomposition generating free radicals and ferric ions (Fe3+) as indicated in Equation (1). However, conventional Fenton treatment has some disadvantages; one of these is that during the reaction, Fe3+ ions accumulate in the system and the process does not proceed once all Fe2+ ions are consumed (Babuponnusami & Muthukumar 2014). For that reason, combination of the conventional Fenton process with visible light radiation (photo-Fenton) has been reported as a highly efficient technology for elimination of different types of organic substrates, in which Fe2+ is regenerated due to the reaction of Fe3+ with light (hv) (Equation (2)) and the reaction continues, meaning that extra ferrous ion addition is not necessary (Alalm et al. 2015; Punzi et al. 2015; Schenone et al. 2015). 
formula
1
 
formula
2
The main goal of this work was to analyze the feasibility of using the photo-Fenton oxidation process to achieve the degradation of EPB in aqueous solutions, assessing the effects of H2O2, Fe2+ and pollutant initial concentrations. Mineralization and biodegradability studies were carried out as well, and optimal treatment conditions were determined.

MATERIALS AND METHODS

Materials

Solutions were prepared using Millipore water (18.2 MΩ cm). EPB containing more than 99% of pure compound was purchased from Alfa-Aesar. Hydrogen peroxide (35% w/w) and isopropanol were supplied by Merck. Iron(II) chloride tetrahydrate (Sigma Aldrich) was used as ferrous ion source. pH was controlled using an HCl concentrated solution. Sodium thiosulfate pentahydrate (Sigma Aldrich) was employed for quenching remaining H2O2 after sampling process. EPB-d4 (CDN Isotopes) was used as internal standard in the analytical methods. Methanol and formic acid of liquid chromatography–mass spectrometry grade and ultra-high purity grade argon (Praxair) and nitrogen were employed for chromatographic analysis.

Experimental design

The effects of the concentrations of Fe2+ and H2O2 on substrate removal were evaluated using a face-centered central composite design which allows identification of the significant factors or interactions to determine the conditions under which EPB suffers the highest rates of degradation. In short, the experimental design is used to find the best set of values, for a set of factors, giving an optimal response and to assess the influence of a parameter in a process (Pabari & Ramtoola 2012; Prakash Maran et al. 2013). In order to define the experimental levels of each variable, some preliminary experiments were carried out and Table 1 shows the parameter settings employed in the experimental process. Statgraphics Centurion XVI software was used to analyze data. The chosen experimental response was the extent of pollutant elimination after 30 minutes of treatment, and the total of experiments was 11 (three center points). Process pH was fixed at 3, which has been widely reported as optimal for maximum efficiency in treatments involving the use of Fenton reagent (Feng et al. 2004; Karale et al. 2014).

Table 1

Levels of experimental design factors

Factor Levels
 
Low Medium High 
Fe2+ (mg L−17.0 14.0 21.0 
H2O2 (mg L−150 125 200 
Factor Levels
 
Low Medium High 
Fe2+ (mg L−17.0 14.0 21.0 
H2O2 (mg L−150 125 200 

Data analysis was performed with a confidence level of 95% and all the runs were conducted in triplicate.

Photodegradation experiments

A Suntest CPS/CPS+ (Atlas) photosimulator, equipped with a xenon lamp capable of emitting radiation of light in a spectrum similar to the sun (wavelengths greater than 310 nm), was used to conduct photo-Fenton experiments. Irradiance was 350 ± 10 W m−2 during all the experiments. Pyrex flasks containing 200 mL of solution were used for light exposition. EPB initial concentration was 1 mg L−1 in almost all the tests.

Analytical methods

Samples of 1 mL were withdrawn at different time intervals during the experimental process, and analyzed after quenching remnant H2O2. EPB concentration was determined using an Acquity UPLC system (Waters Corporation), which was coupled to a triple quadrupole detection mass spectrometer with an orthogonal Z-spray-electrospray (electrospray ionization). Chromatographic separation was performed using an Acquity UPLC BEH C18 column (1.7 μm, 50 mm, 2.1 mm, Waters). Mobile phase consisted of an isocratic mix (50/50) of water +0.01% HCOOH (v/v) and methanol +0.01% HCOOH (v/v). Flow rate was 0.3 mL min−1 and EPB-d4 was used as internal standard. Masslynx 4.1 (Micromass) software was used to process quantitative data.

Mineralization and biodegradability studies

Organic matter conversion into CO2 and H2O was evaluated by quantifying the dissolved organic carbon (DOC) using an Apollo 9000 series TOC analyzer (Teledyne Tekmar). Chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) were determined according to the methodology described in Standard Methods for the Examination of Water and Wastewater (2012), methods 5220D (closed reflux, colorimetric method) and 5210D (respirometric method), respectively.

RESULTS AND DISCUSSION

Photodegradation of EPB

As was mentioned, EPB photodegradation was evaluated considering the effects of Fe2+ and H2O2 initial concentrations. Table 2 shows all the tests included in the experimental design and the pollutant removal after 30 min of treatment. Additionally, Figure 1 corresponds to the contours of estimated response surface for substrate elimination. Results indicate that both parameters affect analyte elimination, reaching degradation percentages between 45 and 68%. Likewise, Figure 1 suggests that Fe2+ and H2O2 initial concentrations that lead to higher contaminant photodegradation are, with respect to the evaluated experimental range, intermediate values.
Table 2

Experimental design for pollutant photodegradation (EPB initial concentration: 1.0 mg L−1, temperature: 35 °C, irradiance: 350 W m−2, reaction time: 30 min)

Test Fe2+ concentration (mg L−1H2O2 concentration (mg L−1EPB removal (%) Experimental EPB removal (%) Calculated by model 
21 200 58.240 60.721 
14 125 65.832 65.840 
50 34.956 34.060 
14 200 68.388 62.906 
21 50 61.258 59.842 
21 125 68.156 67.090 
14 125 64.890 65.840 
200 45.678 48.679 
14 50 52.845 55.157 
10 125 50.283 48.178 
11 14 125 63.626 65.840 
Test Fe2+ concentration (mg L−1H2O2 concentration (mg L−1EPB removal (%) Experimental EPB removal (%) Calculated by model 
21 200 58.240 60.721 
14 125 65.832 65.840 
50 34.956 34.060 
14 200 68.388 62.906 
21 50 61.258 59.842 
21 125 68.156 67.090 
14 125 64.890 65.840 
200 45.678 48.679 
14 50 52.845 55.157 
10 125 50.283 48.178 
11 14 125 63.626 65.840 
Figure 1

Contours of estimated response surface for EPB removal (pollutant initial concentration: 1.0 mg L−1, temperature: 35 °C, irradiance: 350 W m−2, reaction time: 30 min).

Figure 1

Contours of estimated response surface for EPB removal (pollutant initial concentration: 1.0 mg L−1, temperature: 35 °C, irradiance: 350 W m−2, reaction time: 30 min).

Effect of Fe2+ initial concentration

Figure 2 shows the main effects plot for EPB removal, which indicates how the variation in one variable influences the process. In the case of Fe2+ initial concentration, it can be seen that, increasing this factor from 7 to ∼17 mg L−1, the increase in pollutant removal is appreciable, but at higher catalyst concentrations an inhibitory effect takes place. This behavior is likely due to the fact that higher Fe2+ concentrations promote a higher generation and subsequently a probable EPB oxidation, but when there is an Fe2+ excess, this may react with (scavenging effect), reducing the number of available radicals (Equation (3)) and the pollutant removal (Bautista et al. 2007; Karale et al. 2014). 
formula
3
Figure 2

Main effects plot for EPB removal (pollutant initial concentration: 1.0 mg L−1, temperature: 35 °C, irradiance: 350 W m−2, reaction time: 30 min).

Figure 2

Main effects plot for EPB removal (pollutant initial concentration: 1.0 mg L−1, temperature: 35 °C, irradiance: 350 W m−2, reaction time: 30 min).

Effect of H2O2 initial concentration

Hydrogen peroxide presence in the reaction medium influences notably the Fenton and photo-Fenton treatments because, as indicated by Equation (1), this is the source of hydroxyl free radicals (Alalm et al. 2015). From Figure 2, it can be noted that an increment in the H2O2 concentration from 50 to around 135 mg L−1 led to an enhancement in the substrate elimination due to a higher production of hydroxyl free radicals from H2O2 decomposition after reaction with Fe2+. But, if H2O2 initial concentration is higher, EPB degradation decreases, which can be attributed to scavenging by the excess of H2O2 and recombination (Equations (4) and (5)). H2O2 may react with free radicals to generate hydroperoxyl radical , which is a weaker oxidant that also can react with the remaining to form oxygen and water (Equation (6)), reducing in this way the number of available hydroxyl free radicals (Chu et al. 2007; Batista & Nogueira 2012; Affam & Chaudhuri 2013). 
formula
4
 
formula
5
 
formula
6

Optimization of ethylparaben photo-degradation

In order to optimize the pollutant photo-degradation process, factors and interactions between them that significantly affect EPB elimination were determined. A Pareto chart (Figure 3) shows the magnitude and the significance of the effects of factors and interactions. Any effect or interaction that surpasses the chart reference line has an important influence on the studied process. A positive effect (+) indicates that substrate removal increases in the presence of high levels of the respective factor or interaction, while a negative effect (−) indicates that degradation decreases in the presence of high values of the factor or interaction. As can be seen from Figure 3, the most important parameter of the overall treatment procedure was the Fe2+ concentration (A), followed by the squares of reagent initial concentrations (AA and BB) and the H2O2 concentration (B). Also, the Pareto chart allows us to deduce that iron-(II) and hydrogen peroxide initial concentrations have a positive effect, whereas their squares have a negative effect on the EPB removal.
Figure 3

Pareto chart for EPB degradation (pollutant initial concentration: 1.0 mg L−1, temperature: 35 °C, irradiance: 350 W m−2, irradiation time: 30 min).

Figure 3

Pareto chart for EPB degradation (pollutant initial concentration: 1.0 mg L−1, temperature: 35 °C, irradiance: 350 W m−2, irradiation time: 30 min).

After a multiple regression analysis using the statistical software, a polynomial model that relates pollutant removal to the significant parameters and interactions was obtained (Equation (7)). Coefficients of each variable and interaction in the equation indicate the significance of their respective effect on the process. 
formula
7
where [Fe2+] and [H2O2] are ferrous ion and hydrogen peroxide initial concentrations (mg L−1), respectively.

In the model, the negative coefficients of [H2O2]2 and [Fe2+]2 imply that the optimal concentration of reagents are intermediate values, suggesting that very high or very low initial doses of H2O2 and Fe2+ have inhibitory effects on EPB elimination. Additionally, Table 2 shows the results calculated from the model, which with the coefficient of determination (R2 = 94.27%) indicate that indeed Equation (7) adequately predicts EPB elimination under studied conditions.

Table 3 corresponds to optimal conditions for compound degradation under the studied range, which were determined taking into account the polynomial regression and the response surface methodology.

Table 3

Optimum levels for EPB removal

Factor Optimum 
Fe2+ (mg L−117.818 
H2O2 (mg L−1136.011 
Factor Optimum 
Fe2+ (mg L−117.818 
H2O2 (mg L−1136.011 

Pollutant removal under optimal conditions

Experiments under conditions that favor higher substrate elimination were carried out. Figure 4 shows that, after 180 min of photo-Fenton treatment, practically all the pollutant is removed. From the figure it also can be appreciated that using the conventional Fenton technology just ∼53% of EPB degradation is reached after 300 min of reaction, indicating that indeed light irradiation contributes positively to the process owing to the cyclic regeneration of ferrous ions, as indicated in Equation (2). In addition, the combination of Fe2+ and light radiation lead to a 19.3% substrate degradation, which is related to the fact that Fe2+ can react, under light irradiation, with dissolved oxygen and generate Fe3+ and superoxide anion radical (O2), which promotes the formation of H2O2, as Equations (8) and (9) indicate. Generated H2O2 can react with ferrous ions and hydroxyl radicals are formed (Equation (1)) (Liu et al. 2015). 
formula
8
 
formula
9
Figure 4

EPB removal under optimal conditions (pollutant initial concentration: 1.0 mg L−1, Fe2+ initial concentration: 17.81 mg L−1, H2O2 initial concentration: 136.0 mg L−1, pH: 3.0, temperature: 35 °C, irradiance: 350 W m−2).

Figure 4

EPB removal under optimal conditions (pollutant initial concentration: 1.0 mg L−1, Fe2+ initial concentration: 17.81 mg L−1, H2O2 initial concentration: 136.0 mg L−1, pH: 3.0, temperature: 35 °C, irradiance: 350 W m−2).

Additional experiments showed that hydrolysis, photolysis, and oxidation with hydrogen peroxide processes are scarcely responsible for the observed pollutant elimination under photo-Fenton treatment.

On the other hand, in order to clarify the role of radicals on EPB degradation, some tests were carried out under the presence of isopropanol (i-PrOH), which is known as a good free radical scavenger (high rate constant of reaction between the radical and i-PrOH: 1.9 × 109 mol L−1 s−1) (Chen et al. 2005; Santiago et al. 2013). Results, in effect, illustrated that i-PrOH presence reduces notably the substrate degradation, suggesting that hydroxyl radicals are the main oxidizing agent of EPB.

Effect of EPB initial concentration

Substrate initial concentration was varied in the range 0.5–2.0 mg L−1. Experimental results indicated that, regardless of this parameter, after 120 min of treatment extent of pollutant removal was higher than 80% (Figure 5). Degradation of different organic compounds using Fenton and photo-Fenton technologies has been described as a pseudo first-order reaction kinetics with regard to pollutant concentration, as indicated in Equation (10) (Tamimi et al. 2008; Alalm et al. 2015). 
formula
10
where r is pollutant degradation/reaction rate, C0 is EPB initial concentration and k is the degradation rate constant.
Figure 5

Effect of pollutant initial concentration (Fe2+ initial concentration: 17.81 mg L−1, H2O2 initial concentration: 136.0 mg L−1, pH: 3.0, temperature: 35 °C, irradiance: 350 W m−2).

Figure 5

Effect of pollutant initial concentration (Fe2+ initial concentration: 17.81 mg L−1, H2O2 initial concentration: 136.0 mg L−1, pH: 3.0, temperature: 35 °C, irradiance: 350 W m−2).

The relationship between Ln(C/C0) and t for pollutant degradation is depicted in Figure 6. In this way, it can be seen that the kinetic model fits the experimental results very well, getting coefficients of determination (R2) in the interval 97.05–98.41%. However, the inset plot in Figure 6 indicates that, at low pollutant concentration, the degradation rate constant is higher compared with that at higher concentrations, whereas EPB removal rate increases with an increase in the substrate initial concentration. This situation is probably due to the fact that when the initial pollutant concentration increases, the hydroxyl radical concentrations remain constant for all pollutant molecules and hence the removal rate constant decreases (Tamimi et al. 2008).
Figure 6

Relationship between Ln(C/C0) and time for EPB removal. Inset plot: degradation rate constant as function of pollutant initial concentration (Fe2+ initial concentration: 17.81 mg L−1, H2O2 initial concentration: 136.0 mg L−1, pH: 3.0, temperature: 35 °C, irradiance: 350 W m−2).

Figure 6

Relationship between Ln(C/C0) and time for EPB removal. Inset plot: degradation rate constant as function of pollutant initial concentration (Fe2+ initial concentration: 17.81 mg L−1, H2O2 initial concentration: 136.0 mg L−1, pH: 3.0, temperature: 35 °C, irradiance: 350 W m−2).

Analysis of mineralization and biodegradability

Mineralization and biodegradability index (ratio BOD5/COD) evolution during EPB photo-treatment is depicted in Figure 7. In general, it can be appreciated that during 300 min of reaction ∼66% of DOC is reduced, implying that organic matter is transformed into CO2 and water. The BOD5/COD ratio increased markedly during treatment, suggesting that photo-Fenton treatment enhances sample biodegradability.
Figure 7

Mineralization and biodegradability evolution during EPB photo-treatment (pollutant initial concentration: 1.0 mg L−1, Fe2+ initial concentration: 17.81 mg L−1, H2O2 initial concentration: 136.0 mg L−1, pH: 3.0, temperature: 35 °C, irradiance: 350 W m−2).

Figure 7

Mineralization and biodegradability evolution during EPB photo-treatment (pollutant initial concentration: 1.0 mg L−1, Fe2+ initial concentration: 17.81 mg L−1, H2O2 initial concentration: 136.0 mg L−1, pH: 3.0, temperature: 35 °C, irradiance: 350 W m−2).

CONCLUSIONS

Photo-Fenton technology seems to be an effective method for removing EPB from aqueous solutions. Results indicated that Fe2+ and H2O2 initial concentrations are fundamental parameters to reach pollutant total degradation. Analysis of reaction kinetics indicated that regardless of EPB initial concentration more than 80% of substrate is removed after 120 min of treatment. Mineralization variation during the process allowed us to infer that more than 65% of the organic matter is transformed into CO2 and H2O, and BOD5/COD index indicated that biodegradability of samples was enhanced.

ACKNOWLEDGEMENTS

The authors would like to thank the Colombian Administrative Department of Science, Technology and Innovation (COLCIENCIAS) and the ‘Fondo de Sostenibilidad 2014–2015’ of the Research Vice-rectory of the Universidad de Antioquia for support of this work.

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