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Destruction of propyl paraben by persulfate activated with UV-A light emitting diodes
Journal of Environmental Chemical Engineering 6 (2018) 2992–2997
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Destruction of propyl paraben by persulfate activated with UV-A light emitting diodes
T
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Alexandra Ioannidia,b, Zacharias Frontistisa,b, , Dionissios Mantzavinosa,b a b
Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, GR-26504 Patras, Greece INVALOR: Research Infrastructure for Waste Valorization and Sustainable Management, Caratheodory 1, University Campus, GR-26504 Patras, Greece
A R T I C LE I N FO
A B S T R A C T
Keywords: LED Parabens Persulfate Radicals Water matrix
UV-A light emitting diodes (LEDs: 10 W of nominal power, 7.9 × 10−7 einstein/(L.s) photon flux) were employed as the radiation source to activate sodium persulfate (SPS) for the degradation of endocrine disruptor propyl paraben (PP). Experiments in ultrapure water (UPW) were conducted varying SPS concentration between 0 and 500 mg/L, PP concentration between 100 and 800 μg/L, initial solution pH between 3.2 and 9.2, as well as adding various organic (alcohols and humic acid) and inorganic (bicarbonate, chloride), non-target species. Besides UPW, experiments were performed in two actual matrices, i.e. secondary treated wastewater and bottled water. Degradation was found to increase with increasing SPS concentration and decreasing PP concentration, while it was favored as near-neutral, inherent solution pH. The addition of methanol and t-butanol, known radical scavengers, partly restricted the reaction, thus implying that both sulfate and hydroxyl radicals are formed and participate in PP degradation. The addition of humic acid in UPW completely quenched the reaction and so did the use of wastewater as the water matrix. Less significant were the effects of bicarbonate and chloride. SPS can also be activated by the UV-A part of simulated solar radiation (7.3 10−7 einstein/(L s) photon flux), potentially improving process sustainability.
1. Introduction In recent decades due to the rapid development of analytical chemical techniques, traces of many resistant substances have been identified in environmental samples. The concentration of these pollutants is of the order of ng/L–μg/L and, therefore, they are called micropollutants [1–4]. Among various micropollutants, parabens have been accused of having a potential impact on human and animal health. According to some studies, parabens have a weak estrogenic effect mimicking the hormones secreted by the human body and they are classified as endocrine disruptors [5–7]. Parabens have various uses, mainly in the cosmetics industry, as well as food preservatives [8]. Although parabens can partly be biologically removed, they have been detected in environmental samples including the secondary effluents of conventional wastewater treatment plants due to their wide use and consequent accumulation. Therefore, there is a need for advanced treatment in order to minimize the accumulation of micropollutants in the environment [9–11]. Advanced oxidation processes (AOPs) are a family of physicochemical processes based on the in situ production of very reactive
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oxygen species (radicals) with high oxidizing power that have the ability to convert most harmful organic compounds into carbon dioxide and water [12,13] AOPs include many different technologies such as heterogeneous photocatalysis, Fenton reaction, sonochemistry, electrochemical oxidation, UV-C/H2O2 etc. Some of the processes mentioned above are based on the consumption of oxidants like hydrogen peroxide or sodium persulfate in order to produce the reactive oxygen species [14]. In recent years, the use of sodium persulfate (SPS) as an oxidizing agent for the production of reactive oxygen species is constantly gaining ground. This is mainly due to its considerable advantages such as its solid form and high solubility. In addition, it can also be activated in many different ways, while its price is competitive with other oxidants like hydrogen peroxide [15,16]. Sodium persulfate is a mild oxidant with a redox potential of 2.1 V, therefore it is unable to oxidize directly refractory compounds. However, SPS activation (with the help of transition metals, microwave (MW), ultrasound (US) or ultraviolet (UV) radiation, heat, or even pH) leads to sulfate radical production with a redox potential of (2.5–3.1) V [16,17].
Corresponding author at: Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, GR-26504 Patras, Greece. E-mail address: [email protected] (Z. Frontistis).
https://doi.org/10.1016/j.jece.2018.04.049 Received 22 November 2017; Received in revised form 16 February 2018; Accepted 15 April 2018 Available online 22 April 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 6 (2018) 2992–2997
A. Ioannidi et al. Heat/UV/MW/US
S2 O82 − ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2SO∘− 4
Table 1 Characteristics of the water matrices used in this work.
(1)
Many researchers have studied the activation of SPS for the advanced oxidation of parabens in water. Chen et al. [18] studied the heat activated oxidation of ethyl and methyl parabens with particular emphasis on the mechanism of parabens decomposition. Recently, our group has demonstrated that persulfate activated by heterogeneous catalysts such as iron-containing carbon xerogels [19] or pristine graphene [20] can efficiently decompose propyl paraben. Moreover, we have studied [21] the elimination of ethyl paraben by the hot persulfate process developing a semi-empirical model based on the operating parameters of the process. In other studies, Dhaka et al. [22] studied the destruction of methyl paraben by the UV-C-activated persulfate process. Although the use of UV-C radiation seems attractive since the persulfate absorbs mainly at 254 nm it suffers from several drawbacks. Among others, the UV-C light has low penetration, which is affected by interferences with other constituents of the water matrix [23]. In addition, most UV-C lamps contain mercury, thus from an environmental point of view the process cannot be classified as environmentally friendly. In contrast, UV-A light can be employed for persulfate activation; despite that UV-A has theoretically lower efficiency than UV-C, it allows the use of solar radiation. In addition, the use of UV-A light emitting diodes (LEDs), a new generation of lighting source with increased lifetime (10.000–100.000 h) that (i) does not contain toxic metals and (ii) is characterized by very high efficiency and low power consumption, can lead to the construction of a new generation of small portable reactors [24]. Recently, Ahmadi et al. [25] demonstrated that the peroxymonosulfate/Fe2+ system can decompose carmoisine. a common food dye. However, their work focused on the enhancement of the metal/persulfate system under UVA-LED irradiation and it was found that UVA-LED was not the crucial factor for the activation of persulfate. In other studies, Rasoulifard et al. [26] investigated the UV/ thermal/S2O82− process for the degradation of pesticide diazinon in the soil. They found that, amongst several operational parameters tested, temperature was the single most significant factor for the degradation of diazinon. In this perspective, the purpose of this work was to study the use of UV-A radiation emitted by LED or simulated solar light for the degradation of propyl paraben from aqueous solutions in the presence of sodium persulfate. Particular emphasis was given to the effect of operating parameters like propyl paraben and persulfate concentration, irradiation type and intensity, pH and the water matrix. As far as we know, this is the first report concerning the oxidation of parabens based on UV-A LED-activated persulfate in order to examine the possible use of solar light for the elimination of endocrine disruptors.
Parameter
Secondary Effluent
Bottled Water
pH TOC (mg/L) COD (mg/L) Sulfates (mg/L) Chlorides (mg/L) Bicarbonates (mg/L)
8.8 7 21 30 74 189
7.5 – – 15 9.8 211
performed with a solar simulator (Oriel, model LCS-100) equipped with a 100 W xenon, ozone-free lamp. The photon flux in the UV-A part of the electromagnetic spectrum, determined by chemical actinometry, was 7.9 10−7 and 7.3 10−7 einstein/(L.s) for the UV-A LED lamp and the solar simulator, respectively. Experiments were conducted in a cylindrical pyrex reaction vessel of 120 mL capacity, which was open to the atmosphere (open air equilibrium). During the experiment, the aqueous solution was under magnetic stirring. Samples of 1.2 mL were periodically withdrawn from the vessel, quenched with methanol, filtered with a 0.22 μm PVDF syringe filter and analyzed by HPLC. The temperature was left uncontrolled during the experiments but it never exceeded 27 °C. 2.3. Analytical methods High-performance liquid chromatography (HPLC: Alliance 2695, Waters) was employed to monitor the concentration of PP. Separation was achieved on a Kinetex XB-C18 100A column (2.6 μm, 2.1 mm × 150 mm) and a 0.5 μm inline filter (Phenomenex). The mobile phase consisting of 75:25 water:acetonitrile eluted isocratically at 0.25 mL/min and 45 °C, while the injection volume was 100 μL. Detection was achieved through a photodiode array detector (Waters 2996 PDA detector, detection λ = 254 nm) [20]. 3. Results and discussion 3.1. Effect of oxidant concentration
Propyl paraben (PP, C10H12O3, CAS number: 80-05-7) and sodium persulfate (SPS, Na2S2O8, 99+%, CAS number: 7775-27-1) were purchased from Sigma-Aldrich. Humic acid (technical grade), sodium chloride (99.8%), sodium hydroxide (98%), sodium bicarbonate and sulphuric acid (95%) were also obtained from Sigma-Aldrich. All chemicals were used as received, without further purification. Most experiments were conducted in ultrapure water (UPW), while secondary effluent from the University of Patras campus wastewater treatment plant and bottled water were also used; some characteristics of the two matrices are included in Table 1.
In a preliminary series of experiments, the effect of the concentration of the oxidant on the destruction of propyl paraben in ultrapure water and inherent pH of 6 was investigated. Increasing the persulfate concentration expectedly accelerated the degradation rate of paraben, while the destruction due to direct photolysis (i.e. in the absence of oxidant) was insignificant. For example, the removal of 200 μg/L PP after 30 min of irradiation for SPS concentration equal to 0, 100, 250 and 500 mg/L was 1%, 27.6%, 88.7% and 100%, respectively (Fig. 1a). This behavior has been reported repeatedly in the literature [20,27,28] and it is due to the increased production of reactive oxygen species at increased concentrations of oxidant. However, the use of excessive concentrations of persulfates must be avoided since, apart from the cost associated with the additional oxidant, sulfate radicals are converted to sulfate anions. According to the guidelines for drinking water quality by the World Health Organization (WHO) [29], the limit above which sulfate anions affect the taste of drinking water is 250 mg/ L. Moreover, increased concentrations of oxidants may delay the degradation due to self-scavenging reactions involving the reactive oxidizing species [17] at the range of SPS concentrations employed in this work, this phenomenon was not observed. The decomposition of organic pollutants by various AOPs can be modeled by a pseudo-first order kinetic expression [30]:
2.2. Experimental procedure
rate = −
The radiation source was a LED with a nominal power of 10 W that emits predominantly at 365 ± 5 nm. Additional experiments were
Fig. 1b shows the computed apparent rate constants from the data of Fig. 1a and according to Eq. (2). A 5-fold SPS concentration increase led
2. Materials and methods 2.1. Chemicals
2993
d[C] = k obs [C] dt
(2)
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A. Ioannidi et al. 1 0.9 0.8 0.7
C/Co
0.6 0.5 0.4 0.3 0.2 61 mM MeOH 61 mM t-BuOH UP water
0.1 0
0
10
20
30
40
50
60
Time, min
Fig. 3. Effect of radical scavengers on 200 μg/L PP degradation with 250 mg/L SPS under UV-A LED irradiation in UPW and inherent pH.
PP degradation is favored at the solution’s inherent pH of 6, while it is partly impeded at acidic and alkaline environments. PP has a pKa value of 8.4; when the solution pH is above this value, PP is negatively charged. Sodium persulfate is a weak acid with a pKa value of 6.3. Since the reaction is homogeneous, the pH effect is mainly associated with the relative distribution of reactive radicals and, consequently, the different mechanisms of paraben decomposition involved. It is well known that the oxidation with hydroxyl radicals is the dominant mechanism at high pH values, while sulfate radicals usually dominate the process at acidic pH values [17] These results are consistent with the work of Gao et al. [33] who studied the degradation of sulfamethazine with the UV-C/SPS process and reported that the decomposition was favored at pH 6.5. Chen et al. [18], who studied the oxidation of ethyl and methyl parabens with thermally activated persulfate, reported an optimum pH value of 5.
Fig. 1. Effect of sodium persulfate concentration on 200 μg/L PP degradation under UV-A LED irradiation in UPW and inherent pH. (a) PP removal (b), apparent kinetic constants.
to a 17-fold increase in the degradation rate. Bu et al. [31] who studied the decomposition of oxcarbazepine using the UV-C/SPS process, reported that when persulfate concentration increased from 0.2 to 5 mM the apparent kinetic constant increased from 0.007 to 0.039 min−1.
3.3. Radical scavengers In order to shed light on the mechanism of propyl paraben oxidation, further experiments were conducted with an excess of t-butanol and methanol (61 mM) and the results are shown in Fig. 3. t-Butanol and methanol are often used as scavengers since they react with reactive oxygen species at different rates [34,35] the former has a greater affinity to scavenge hydroxyl radicals, while the latter sulfate radicals kMeOH , SO4∘− = 107M−1s−1 kBuOH , ∘OH = (kMeOH , ∘OH = 8108M−1s−1, 5.2108M−1s−1, kBuOH , SO4∘− = 106M−1s−1). As seen in Fig. 3, the presence of either alcohol significantly delayed propyl paraben oxidation. These results indirectly prove that the oxidation of propyl paraben occurs through reactions induced by both sulfate and hydroxyl radicals. Chen et al. [18] also reported, in their study, the detrimental effect of t-butanol and ethanol on ethyl paraben decomposition by the heatactivated persulfate process; in particular, the effect of ethanol on the kinetics was an order of magnitude greater than that of t-butanol.
3.2. Effect of pH Another set of experiments was carried out to study the effect of initial pH on the decomposition of PP and the results are depicted in Fig. 2. The experiments were conducted without buffering the solution to alkaline or acidic conditions, since there are reports in the literature discussing unwanted interactions between the phosphates typically found in buffers and the reactive species [32].
3.4. Effect of propyl paraben initial concentration In additional experiments, the effect of initial PP concentration on process efficiency was studied in the range 200–800 μg/L with 250 mg/ L SPS and inherent pH (Fig. 4a). The 45-min degradation of PP was 100%, 91.7%, 86.7% and 53.9% at 100, 200, 400, and 800 μg/L, respectively. Fig. 4b shows the respectively computed rate constants, whose values decrease as PP concentration increases. This is typical in AOPs and is associated with the concentration of the oxidant relative to that of the pollutant. At the conditions where the oxidant is not the limiting reactant (e.g. at relatively low pollutant concentrations), the reaction appears to be first (or near first) order with respect to the
Fig. 2. Effect of initial pH on 200 μg/L PP degradation with 250 mg/L SPS under UV-A LED irradiation in UPW. 2994
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Fig. 5. Effect of (a) bicarbonate and (b) chloride on 200 μg/L PP degradation with 250 mg/L SPS under UV-A LED irradiation in UPW and inherent pH.
Fig. 4. Effect of initial PP concentration on its degradation with 250 mg/L SPS under UV-A LED irradiation in UPW. (a) PP removal, (b) apparent kinetic constants.
xerogels; the 30-min PP removal decreased from 88% to 62.3% when UPW was spiked with 500 mg/L bicarbonate. Chen et al. [18] found that the presence of bicarbonate up to 10 μM did not alter the degradation rate of 20 μM ethyl or methyl paraben with 1 mM persulfate at 50 °C. On the contrary, Lu et al. [27] showed that the addition of bicarbonate could increase significantly the oxidation rate of diclofenac using UV-C activated persulfate. This was attributed to the fact that carbonate radicals are more selective than hydroxyl and sulfate radicals, although their oxidation potential is lower. The effect of chloride on PP degradation is depicted in Fig. 5b. A slightly detrimental effect appears to occur during the early stages of the reaction and this is more pronounced at higher concentrations of chloride; nonetheless, quantitative PP removal occurs within 45–60 min of reaction irrespective of the presence of chloride. The effect of chloride is controversial and is associated with the possible formation of chloride radicals: [36,37]
pollutant. However and as the concentration of pollutant increases (e.g. the oxidant becomes the limiting reactant), the rate constant changes with the pollutant concentration, implying a shift towards zeroth order kinetics [20]. 3.5. Effect of the water matrix One of the major factors behind the implementation of AOPs on a large scale is the reduction of process efficiency in the case of real wastewaters due to the unselective nature of hydroxyl radicals and other reactive oxygen species. In this perspective, further experiments were conducted to study the influence of typical anions and organic matter contained in environmental samples, like bicarbonate, chloride and humic acid spiked in UPW; moreover, experiments were performed with two real matrices, i.e. bottled water and secondary treated municipal effluent Τhe presence of bicarbonate reduces the rate of propyl paraben removal (Fig. 5a). For example, the 30-min extent of propyl paraben degradation was reduced from 79% to 72.3% and 53% for bicarbonate concentrations equal to 0, 250 and 500 mg/L, respectively. The respective apparent kinetic constants were computed equal to 0.0637, 0.056 and 0.036 min−1. Reactions between radicals and bicarbonate can occur as follows, leading to the formation of less reactive carbonate radicals: HCO3− + SO4%− → CO3%− + SO42− + H+ HCO3− +
−
HO% → CO3% + H2O
2− − ∘ SO∘− k = 2.7 × 108 M−1s−1 4 +Cl → Cl +SO4
(5)
HO∘+Cl− → Cl∘+HO− k = 4.3 × 109 M−1s−1
(6)
Outsiou et al. [28] who studied the degradation of bisphenol A by persulfate oxidation activated by Fe/Co-containing carbon xerogels, reported that the reaction rate increased six times when the solution was added 200 mg/L chloride. Conversely, Chen et al. [18] reported that the addition of 0.5 mM of sodium chloride reduced the removal of 20 μΜ methyl paraben with 1 mM persulfate at 50 °C from 54.3% to 33.3%, while a further increase of chloride to 10 mM did not cause any further reduction. Fig. 6 shows the effect of adding 10 mg/L humic acid on PP degradation; humic acid, a compound that is often used to examine the effect of organic matter other than the probe compound, quenched PP
(3) (4)
Metheniti et al. [18] reported a similar effect of bicarbonate on PP degradation by means of persulfate oxidation activated by carbon 2995
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photon flux in the UV-A part of the spectrum. The experiment with solar radiation was repeated using an appropriate filter to cut wavelengths below 420 nm; as can clearly be seen, persulfate cannot be activated in the visible part of the spectrum, while the recorded 10–15% degradation is due to the oxidative action of molecular persulfate. Nonetheless, these results suggest that sunlight can replace artificial radiation since it consists of 5–7% UV-A radiation [39]. A final experiment was performed with a UV-A LED of 9.6 × 10−7 einstein/(L s), clearly showing that process efficiency can be increased with increasing intensity 4. Conclusions The conclusions drawn from this study are summarized as follows: (i) UV-A radiation from LEDs or solar light can activate persulfate to produce sulfate and hydroxyl radicals that can readily degrade parabens at environmentally relevant concentrations (i.e. at the low μg/L level) in UPW. (ii) Unlike the high process efficiency attained in UPW, extrapolation of the results in environmental matrices is difficult due to the interactions between the micropollutant, the reactive species and the non-target constituents of the matrix. A characteristic effect in this work was the wastewater matrix, where paraben degradation was completely impeded; strong adverse effects were also recorded upon the addition of humic acid and alcohols in UPW. Anions such as bicarbonate and chloride also had an effect, albeit less pronounced, on degradation. (iii) To enhance process efficiency, which seems to be important for actual matrices, a possible combination with other light-based AOPs, like heterogeneous photocatalysis, can become an option. Besides the matrix effect, operating parameters such as the micropollutant to oxidant concentration ratio and the solution pH can alter reaction rates.
Fig. 6. Effect of humic acid and actual water matrix on 200 μg/L PP degradation with 250 mg/L SPS under UV-A LED irradiation and inherent pH.
Fig. 7. Effect of type and intensity of radiation on 200 μg/L PP degradation with 250 mg/L SPS (except the photolysis run) in UPW and inherent pH.
Acknowledgments Dr Zacharias Frontistis would like to thank the Greek State Scholarship Foundation (IKY) for providing him a fellowship for conducting post-doctoral research in Greece through the “IKY Fellowships of Excellence for Postgraduate Studies in Greece – Siemens Programme” in the framework of the Hellenic Republic – Siemens Settlement Agreement. We acknowledge support of this work by the project “INVALOR: Research Infrastructure for Waste Valorization and Sustainable Management” (MIS 5002495) which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme “Competitiveness, Entrepreneurship and Innovation” (NSRF 20142020) and co-financed by Greece and the European Union (European Regional Development Fund).
degradation. This is due to (i) the competition between propyl paraben and humic acid molecules for the reactive oxygen species. The organic carbon content of 10 mg/L humic acid is 4.7 mg/L, while that of 0.2 mg/L PP is only 0.133 mg/L, i.e. the ratio of humic acid to paraben concentration ratio, in terms of organic carbon content, is 35.3; this implies that unselective radicals are more likely to react with humic acid than propyl paraben; (ii) it is well known that humic acid absorbs UV light [38] and due to this filtering effect fewer photons are available to activate persulfate. In their study, Chen et al. [18] demonstrated that the presence of 10 mg/L humic acid decreased the degradation of methyl paraben from 50.6% to 10.5% after 240 min of persulfate oxidation. Fig. 6 also shows the effect of actual water matrices on PP degradation. Similarly to the effect of humic acid in UPW, PP degradation in wastewater was completed impeded; this is consistent with the fact that wastewater contains 7 mg/L of organic carbon (Table 1), which is about 25% higher than the organic carbon contained in humic acid. Conversely, the effect of bottled water, containing mainly but not exclusively bicarbonate (Table 1), is not clear and this may be associated with the interplay amongst the various anions, propyl paraben and the reactive oxygen species.
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Destruction of propyl paraben by persulfate activated with UV-A light emitting diodes
T
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Alexandra Ioannidia,b, Zacharias Frontistisa,b, , Dionissios Mantzavinosa,b a b
Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, GR-26504 Patras, Greece INVALOR: Research Infrastructure for Waste Valorization and Sustainable Management, Caratheodory 1, University Campus, GR-26504 Patras, Greece
A R T I C LE I N FO
A B S T R A C T
Keywords: LED Parabens Persulfate Radicals Water matrix
UV-A light emitting diodes (LEDs: 10 W of nominal power, 7.9 × 10−7 einstein/(L.s) photon flux) were employed as the radiation source to activate sodium persulfate (SPS) for the degradation of endocrine disruptor propyl paraben (PP). Experiments in ultrapure water (UPW) were conducted varying SPS concentration between 0 and 500 mg/L, PP concentration between 100 and 800 μg/L, initial solution pH between 3.2 and 9.2, as well as adding various organic (alcohols and humic acid) and inorganic (bicarbonate, chloride), non-target species. Besides UPW, experiments were performed in two actual matrices, i.e. secondary treated wastewater and bottled water. Degradation was found to increase with increasing SPS concentration and decreasing PP concentration, while it was favored as near-neutral, inherent solution pH. The addition of methanol and t-butanol, known radical scavengers, partly restricted the reaction, thus implying that both sulfate and hydroxyl radicals are formed and participate in PP degradation. The addition of humic acid in UPW completely quenched the reaction and so did the use of wastewater as the water matrix. Less significant were the effects of bicarbonate and chloride. SPS can also be activated by the UV-A part of simulated solar radiation (7.3 10−7 einstein/(L s) photon flux), potentially improving process sustainability.
1. Introduction In recent decades due to the rapid development of analytical chemical techniques, traces of many resistant substances have been identified in environmental samples. The concentration of these pollutants is of the order of ng/L–μg/L and, therefore, they are called micropollutants [1–4]. Among various micropollutants, parabens have been accused of having a potential impact on human and animal health. According to some studies, parabens have a weak estrogenic effect mimicking the hormones secreted by the human body and they are classified as endocrine disruptors [5–7]. Parabens have various uses, mainly in the cosmetics industry, as well as food preservatives [8]. Although parabens can partly be biologically removed, they have been detected in environmental samples including the secondary effluents of conventional wastewater treatment plants due to their wide use and consequent accumulation. Therefore, there is a need for advanced treatment in order to minimize the accumulation of micropollutants in the environment [9–11]. Advanced oxidation processes (AOPs) are a family of physicochemical processes based on the in situ production of very reactive
⁎
oxygen species (radicals) with high oxidizing power that have the ability to convert most harmful organic compounds into carbon dioxide and water [12,13] AOPs include many different technologies such as heterogeneous photocatalysis, Fenton reaction, sonochemistry, electrochemical oxidation, UV-C/H2O2 etc. Some of the processes mentioned above are based on the consumption of oxidants like hydrogen peroxide or sodium persulfate in order to produce the reactive oxygen species [14]. In recent years, the use of sodium persulfate (SPS) as an oxidizing agent for the production of reactive oxygen species is constantly gaining ground. This is mainly due to its considerable advantages such as its solid form and high solubility. In addition, it can also be activated in many different ways, while its price is competitive with other oxidants like hydrogen peroxide [15,16]. Sodium persulfate is a mild oxidant with a redox potential of 2.1 V, therefore it is unable to oxidize directly refractory compounds. However, SPS activation (with the help of transition metals, microwave (MW), ultrasound (US) or ultraviolet (UV) radiation, heat, or even pH) leads to sulfate radical production with a redox potential of (2.5–3.1) V [16,17].
Corresponding author at: Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, GR-26504 Patras, Greece. E-mail address: [email protected] (Z. Frontistis).
https://doi.org/10.1016/j.jece.2018.04.049 Received 22 November 2017; Received in revised form 16 February 2018; Accepted 15 April 2018 Available online 22 April 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.
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S2 O82 − ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2SO∘− 4
Table 1 Characteristics of the water matrices used in this work.
(1)
Many researchers have studied the activation of SPS for the advanced oxidation of parabens in water. Chen et al. [18] studied the heat activated oxidation of ethyl and methyl parabens with particular emphasis on the mechanism of parabens decomposition. Recently, our group has demonstrated that persulfate activated by heterogeneous catalysts such as iron-containing carbon xerogels [19] or pristine graphene [20] can efficiently decompose propyl paraben. Moreover, we have studied [21] the elimination of ethyl paraben by the hot persulfate process developing a semi-empirical model based on the operating parameters of the process. In other studies, Dhaka et al. [22] studied the destruction of methyl paraben by the UV-C-activated persulfate process. Although the use of UV-C radiation seems attractive since the persulfate absorbs mainly at 254 nm it suffers from several drawbacks. Among others, the UV-C light has low penetration, which is affected by interferences with other constituents of the water matrix [23]. In addition, most UV-C lamps contain mercury, thus from an environmental point of view the process cannot be classified as environmentally friendly. In contrast, UV-A light can be employed for persulfate activation; despite that UV-A has theoretically lower efficiency than UV-C, it allows the use of solar radiation. In addition, the use of UV-A light emitting diodes (LEDs), a new generation of lighting source with increased lifetime (10.000–100.000 h) that (i) does not contain toxic metals and (ii) is characterized by very high efficiency and low power consumption, can lead to the construction of a new generation of small portable reactors [24]. Recently, Ahmadi et al. [25] demonstrated that the peroxymonosulfate/Fe2+ system can decompose carmoisine. a common food dye. However, their work focused on the enhancement of the metal/persulfate system under UVA-LED irradiation and it was found that UVA-LED was not the crucial factor for the activation of persulfate. In other studies, Rasoulifard et al. [26] investigated the UV/ thermal/S2O82− process for the degradation of pesticide diazinon in the soil. They found that, amongst several operational parameters tested, temperature was the single most significant factor for the degradation of diazinon. In this perspective, the purpose of this work was to study the use of UV-A radiation emitted by LED or simulated solar light for the degradation of propyl paraben from aqueous solutions in the presence of sodium persulfate. Particular emphasis was given to the effect of operating parameters like propyl paraben and persulfate concentration, irradiation type and intensity, pH and the water matrix. As far as we know, this is the first report concerning the oxidation of parabens based on UV-A LED-activated persulfate in order to examine the possible use of solar light for the elimination of endocrine disruptors.
Parameter
Secondary Effluent
Bottled Water
pH TOC (mg/L) COD (mg/L) Sulfates (mg/L) Chlorides (mg/L) Bicarbonates (mg/L)
8.8 7 21 30 74 189
7.5 – – 15 9.8 211
performed with a solar simulator (Oriel, model LCS-100) equipped with a 100 W xenon, ozone-free lamp. The photon flux in the UV-A part of the electromagnetic spectrum, determined by chemical actinometry, was 7.9 10−7 and 7.3 10−7 einstein/(L.s) for the UV-A LED lamp and the solar simulator, respectively. Experiments were conducted in a cylindrical pyrex reaction vessel of 120 mL capacity, which was open to the atmosphere (open air equilibrium). During the experiment, the aqueous solution was under magnetic stirring. Samples of 1.2 mL were periodically withdrawn from the vessel, quenched with methanol, filtered with a 0.22 μm PVDF syringe filter and analyzed by HPLC. The temperature was left uncontrolled during the experiments but it never exceeded 27 °C. 2.3. Analytical methods High-performance liquid chromatography (HPLC: Alliance 2695, Waters) was employed to monitor the concentration of PP. Separation was achieved on a Kinetex XB-C18 100A column (2.6 μm, 2.1 mm × 150 mm) and a 0.5 μm inline filter (Phenomenex). The mobile phase consisting of 75:25 water:acetonitrile eluted isocratically at 0.25 mL/min and 45 °C, while the injection volume was 100 μL. Detection was achieved through a photodiode array detector (Waters 2996 PDA detector, detection λ = 254 nm) [20]. 3. Results and discussion 3.1. Effect of oxidant concentration
Propyl paraben (PP, C10H12O3, CAS number: 80-05-7) and sodium persulfate (SPS, Na2S2O8, 99+%, CAS number: 7775-27-1) were purchased from Sigma-Aldrich. Humic acid (technical grade), sodium chloride (99.8%), sodium hydroxide (98%), sodium bicarbonate and sulphuric acid (95%) were also obtained from Sigma-Aldrich. All chemicals were used as received, without further purification. Most experiments were conducted in ultrapure water (UPW), while secondary effluent from the University of Patras campus wastewater treatment plant and bottled water were also used; some characteristics of the two matrices are included in Table 1.
In a preliminary series of experiments, the effect of the concentration of the oxidant on the destruction of propyl paraben in ultrapure water and inherent pH of 6 was investigated. Increasing the persulfate concentration expectedly accelerated the degradation rate of paraben, while the destruction due to direct photolysis (i.e. in the absence of oxidant) was insignificant. For example, the removal of 200 μg/L PP after 30 min of irradiation for SPS concentration equal to 0, 100, 250 and 500 mg/L was 1%, 27.6%, 88.7% and 100%, respectively (Fig. 1a). This behavior has been reported repeatedly in the literature [20,27,28] and it is due to the increased production of reactive oxygen species at increased concentrations of oxidant. However, the use of excessive concentrations of persulfates must be avoided since, apart from the cost associated with the additional oxidant, sulfate radicals are converted to sulfate anions. According to the guidelines for drinking water quality by the World Health Organization (WHO) [29], the limit above which sulfate anions affect the taste of drinking water is 250 mg/ L. Moreover, increased concentrations of oxidants may delay the degradation due to self-scavenging reactions involving the reactive oxidizing species [17] at the range of SPS concentrations employed in this work, this phenomenon was not observed. The decomposition of organic pollutants by various AOPs can be modeled by a pseudo-first order kinetic expression [30]:
2.2. Experimental procedure
rate = −
The radiation source was a LED with a nominal power of 10 W that emits predominantly at 365 ± 5 nm. Additional experiments were
Fig. 1b shows the computed apparent rate constants from the data of Fig. 1a and according to Eq. (2). A 5-fold SPS concentration increase led
2. Materials and methods 2.1. Chemicals
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d[C] = k obs [C] dt
(2)
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C/Co
0.6 0.5 0.4 0.3 0.2 61 mM MeOH 61 mM t-BuOH UP water
0.1 0
0
10
20
30
40
50
60
Time, min
Fig. 3. Effect of radical scavengers on 200 μg/L PP degradation with 250 mg/L SPS under UV-A LED irradiation in UPW and inherent pH.
PP degradation is favored at the solution’s inherent pH of 6, while it is partly impeded at acidic and alkaline environments. PP has a pKa value of 8.4; when the solution pH is above this value, PP is negatively charged. Sodium persulfate is a weak acid with a pKa value of 6.3. Since the reaction is homogeneous, the pH effect is mainly associated with the relative distribution of reactive radicals and, consequently, the different mechanisms of paraben decomposition involved. It is well known that the oxidation with hydroxyl radicals is the dominant mechanism at high pH values, while sulfate radicals usually dominate the process at acidic pH values [17] These results are consistent with the work of Gao et al. [33] who studied the degradation of sulfamethazine with the UV-C/SPS process and reported that the decomposition was favored at pH 6.5. Chen et al. [18], who studied the oxidation of ethyl and methyl parabens with thermally activated persulfate, reported an optimum pH value of 5.
Fig. 1. Effect of sodium persulfate concentration on 200 μg/L PP degradation under UV-A LED irradiation in UPW and inherent pH. (a) PP removal (b), apparent kinetic constants.
to a 17-fold increase in the degradation rate. Bu et al. [31] who studied the decomposition of oxcarbazepine using the UV-C/SPS process, reported that when persulfate concentration increased from 0.2 to 5 mM the apparent kinetic constant increased from 0.007 to 0.039 min−1.
3.3. Radical scavengers In order to shed light on the mechanism of propyl paraben oxidation, further experiments were conducted with an excess of t-butanol and methanol (61 mM) and the results are shown in Fig. 3. t-Butanol and methanol are often used as scavengers since they react with reactive oxygen species at different rates [34,35] the former has a greater affinity to scavenge hydroxyl radicals, while the latter sulfate radicals kMeOH , SO4∘− = 107M−1s−1 kBuOH , ∘OH = (kMeOH , ∘OH = 8108M−1s−1, 5.2108M−1s−1, kBuOH , SO4∘− = 106M−1s−1). As seen in Fig. 3, the presence of either alcohol significantly delayed propyl paraben oxidation. These results indirectly prove that the oxidation of propyl paraben occurs through reactions induced by both sulfate and hydroxyl radicals. Chen et al. [18] also reported, in their study, the detrimental effect of t-butanol and ethanol on ethyl paraben decomposition by the heatactivated persulfate process; in particular, the effect of ethanol on the kinetics was an order of magnitude greater than that of t-butanol.
3.2. Effect of pH Another set of experiments was carried out to study the effect of initial pH on the decomposition of PP and the results are depicted in Fig. 2. The experiments were conducted without buffering the solution to alkaline or acidic conditions, since there are reports in the literature discussing unwanted interactions between the phosphates typically found in buffers and the reactive species [32].
3.4. Effect of propyl paraben initial concentration In additional experiments, the effect of initial PP concentration on process efficiency was studied in the range 200–800 μg/L with 250 mg/ L SPS and inherent pH (Fig. 4a). The 45-min degradation of PP was 100%, 91.7%, 86.7% and 53.9% at 100, 200, 400, and 800 μg/L, respectively. Fig. 4b shows the respectively computed rate constants, whose values decrease as PP concentration increases. This is typical in AOPs and is associated with the concentration of the oxidant relative to that of the pollutant. At the conditions where the oxidant is not the limiting reactant (e.g. at relatively low pollutant concentrations), the reaction appears to be first (or near first) order with respect to the
Fig. 2. Effect of initial pH on 200 μg/L PP degradation with 250 mg/L SPS under UV-A LED irradiation in UPW. 2994
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Fig. 5. Effect of (a) bicarbonate and (b) chloride on 200 μg/L PP degradation with 250 mg/L SPS under UV-A LED irradiation in UPW and inherent pH.
Fig. 4. Effect of initial PP concentration on its degradation with 250 mg/L SPS under UV-A LED irradiation in UPW. (a) PP removal, (b) apparent kinetic constants.
xerogels; the 30-min PP removal decreased from 88% to 62.3% when UPW was spiked with 500 mg/L bicarbonate. Chen et al. [18] found that the presence of bicarbonate up to 10 μM did not alter the degradation rate of 20 μM ethyl or methyl paraben with 1 mM persulfate at 50 °C. On the contrary, Lu et al. [27] showed that the addition of bicarbonate could increase significantly the oxidation rate of diclofenac using UV-C activated persulfate. This was attributed to the fact that carbonate radicals are more selective than hydroxyl and sulfate radicals, although their oxidation potential is lower. The effect of chloride on PP degradation is depicted in Fig. 5b. A slightly detrimental effect appears to occur during the early stages of the reaction and this is more pronounced at higher concentrations of chloride; nonetheless, quantitative PP removal occurs within 45–60 min of reaction irrespective of the presence of chloride. The effect of chloride is controversial and is associated with the possible formation of chloride radicals: [36,37]
pollutant. However and as the concentration of pollutant increases (e.g. the oxidant becomes the limiting reactant), the rate constant changes with the pollutant concentration, implying a shift towards zeroth order kinetics [20]. 3.5. Effect of the water matrix One of the major factors behind the implementation of AOPs on a large scale is the reduction of process efficiency in the case of real wastewaters due to the unselective nature of hydroxyl radicals and other reactive oxygen species. In this perspective, further experiments were conducted to study the influence of typical anions and organic matter contained in environmental samples, like bicarbonate, chloride and humic acid spiked in UPW; moreover, experiments were performed with two real matrices, i.e. bottled water and secondary treated municipal effluent Τhe presence of bicarbonate reduces the rate of propyl paraben removal (Fig. 5a). For example, the 30-min extent of propyl paraben degradation was reduced from 79% to 72.3% and 53% for bicarbonate concentrations equal to 0, 250 and 500 mg/L, respectively. The respective apparent kinetic constants were computed equal to 0.0637, 0.056 and 0.036 min−1. Reactions between radicals and bicarbonate can occur as follows, leading to the formation of less reactive carbonate radicals: HCO3− + SO4%− → CO3%− + SO42− + H+ HCO3− +
−
HO% → CO3% + H2O
2− − ∘ SO∘− k = 2.7 × 108 M−1s−1 4 +Cl → Cl +SO4
(5)
HO∘+Cl− → Cl∘+HO− k = 4.3 × 109 M−1s−1
(6)
Outsiou et al. [28] who studied the degradation of bisphenol A by persulfate oxidation activated by Fe/Co-containing carbon xerogels, reported that the reaction rate increased six times when the solution was added 200 mg/L chloride. Conversely, Chen et al. [18] reported that the addition of 0.5 mM of sodium chloride reduced the removal of 20 μΜ methyl paraben with 1 mM persulfate at 50 °C from 54.3% to 33.3%, while a further increase of chloride to 10 mM did not cause any further reduction. Fig. 6 shows the effect of adding 10 mg/L humic acid on PP degradation; humic acid, a compound that is often used to examine the effect of organic matter other than the probe compound, quenched PP
(3) (4)
Metheniti et al. [18] reported a similar effect of bicarbonate on PP degradation by means of persulfate oxidation activated by carbon 2995
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photon flux in the UV-A part of the spectrum. The experiment with solar radiation was repeated using an appropriate filter to cut wavelengths below 420 nm; as can clearly be seen, persulfate cannot be activated in the visible part of the spectrum, while the recorded 10–15% degradation is due to the oxidative action of molecular persulfate. Nonetheless, these results suggest that sunlight can replace artificial radiation since it consists of 5–7% UV-A radiation [39]. A final experiment was performed with a UV-A LED of 9.6 × 10−7 einstein/(L s), clearly showing that process efficiency can be increased with increasing intensity 4. Conclusions The conclusions drawn from this study are summarized as follows: (i) UV-A radiation from LEDs or solar light can activate persulfate to produce sulfate and hydroxyl radicals that can readily degrade parabens at environmentally relevant concentrations (i.e. at the low μg/L level) in UPW. (ii) Unlike the high process efficiency attained in UPW, extrapolation of the results in environmental matrices is difficult due to the interactions between the micropollutant, the reactive species and the non-target constituents of the matrix. A characteristic effect in this work was the wastewater matrix, where paraben degradation was completely impeded; strong adverse effects were also recorded upon the addition of humic acid and alcohols in UPW. Anions such as bicarbonate and chloride also had an effect, albeit less pronounced, on degradation. (iii) To enhance process efficiency, which seems to be important for actual matrices, a possible combination with other light-based AOPs, like heterogeneous photocatalysis, can become an option. Besides the matrix effect, operating parameters such as the micropollutant to oxidant concentration ratio and the solution pH can alter reaction rates.
Fig. 6. Effect of humic acid and actual water matrix on 200 μg/L PP degradation with 250 mg/L SPS under UV-A LED irradiation and inherent pH.
Fig. 7. Effect of type and intensity of radiation on 200 μg/L PP degradation with 250 mg/L SPS (except the photolysis run) in UPW and inherent pH.
Acknowledgments Dr Zacharias Frontistis would like to thank the Greek State Scholarship Foundation (IKY) for providing him a fellowship for conducting post-doctoral research in Greece through the “IKY Fellowships of Excellence for Postgraduate Studies in Greece – Siemens Programme” in the framework of the Hellenic Republic – Siemens Settlement Agreement. We acknowledge support of this work by the project “INVALOR: Research Infrastructure for Waste Valorization and Sustainable Management” (MIS 5002495) which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme “Competitiveness, Entrepreneurship and Innovation” (NSRF 20142020) and co-financed by Greece and the European Union (European Regional Development Fund).
degradation. This is due to (i) the competition between propyl paraben and humic acid molecules for the reactive oxygen species. The organic carbon content of 10 mg/L humic acid is 4.7 mg/L, while that of 0.2 mg/L PP is only 0.133 mg/L, i.e. the ratio of humic acid to paraben concentration ratio, in terms of organic carbon content, is 35.3; this implies that unselective radicals are more likely to react with humic acid than propyl paraben; (ii) it is well known that humic acid absorbs UV light [38] and due to this filtering effect fewer photons are available to activate persulfate. In their study, Chen et al. [18] demonstrated that the presence of 10 mg/L humic acid decreased the degradation of methyl paraben from 50.6% to 10.5% after 240 min of persulfate oxidation. Fig. 6 also shows the effect of actual water matrices on PP degradation. Similarly to the effect of humic acid in UPW, PP degradation in wastewater was completed impeded; this is consistent with the fact that wastewater contains 7 mg/L of organic carbon (Table 1), which is about 25% higher than the organic carbon contained in humic acid. Conversely, the effect of bottled water, containing mainly but not exclusively bicarbonate (Table 1), is not clear and this may be associated with the interplay amongst the various anions, propyl paraben and the reactive oxygen species.
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3.6. Effect of type and intensity of irradiation Fig. 7 shows the effect of the type and intensity of radiation on PP degradation. Radiation under simulated solar radiation is nearly equally effective as that with UV-A LED at 7.9 × 10−7 einstein/(L s) and this is consistent with the fact that both sources have a similar 2996
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