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UVC system – Kinetics study, transformation products and toxicity assessment
Accepted Manuscript Title: ADVANCED OXIDATION OF PRESERVATIVE AGENTS IN H2 O2 /UVC SYSTEM – KINETICS STUDY, TRANSFORMATION PRODUCTS AND TOXICITY ASSESSMENT Authors: Magdalena Olak-Kucharczyk, Stanisław Ledakowicz PII: DOI: Reference:
S0304-3894(17)30213-3 http://dx.doi.org/doi:10.1016/j.jhazmat.2017.03.047 HAZMAT 18463
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
5-12-2016 24-2-2017 22-3-2017
Please cite this article as: Magdalena Olak-Kucharczyk, Stanisław Ledakowicz, ADVANCED OXIDATION OF PRESERVATIVE AGENTS IN H2O2/UVC SYSTEM – KINETICS STUDY, TRANSFORMATION PRODUCTS AND TOXICITY ASSESSMENT, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.03.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ADVANCED OXIDATION OF PRESERVATIVE AGENTS IN H2O2/UVC SYSTEM – KINETICS STUDY, TRANSFORMATION PRODUCTS AND TOXICITY ASSESSMENT Magdalena Olak-Kucharczyka,b,*, Stanisław Ledakowiczb a
Textile Research Institute, ul. Brzezinska 5/15, 92-103 Lodz, Poland (*Corresponding author. E-mail address: [email protected]). b Lodz University of Technology, Faculty of Process and Environmental Engineering, Department of Bioprocess Engineering, ul. Wolczanska 213, 90-924 Lodz, Poland. Graphical abstract
Highlights:
The H2O2/UVC process is efficient method for phenylphenols removal from aqueous solutions. The advanced oxidation of phenylphenols led to achieving non-toxic post-reaction solutions. The position of hydroxyl group in the aromatic ring influences on the reaction rate of phenylphenols degradation. Phenol, dihydroxybiphenyls and 2,4-hexadienoic-6-oxo-6-phenyl acids (or trihydroxybiphenyls) are the by-products. The mechanism of phenylphenols reaction with hydroxyl radicals was proposed.
Abstract Phenylphenol isomers (hydroxylated derivatives of biphenyl) are widely used as a preservative agents and disinfectants in many branches of industry. This work focuses on removal of phenylphenol isomers from aqueous solution using H2O2/UVC process. The influence of different operating variables such as pH, initial concentration of hydrogen peroxide, initial concentration of target compounds, fluence rate and presence of radical scavengers on degradation rate was investigated. Moreover,
transformation products and toxicity of the reaction solutions were studied. The reaction rate constants of hydroxyl radicals with phenylphenol isomers were determined by using classical and competition kinetics. The value of reaction rate constant depends on the position of the hydroxyl group in the aromatic ring and follows the order: ortho- phenylphenol < metaphenylphenol < para- phenylphenol. The main by-products of initial degradation steps were dihydroxybiphenyl isomers, phenol and isomers of 2,4-hexadienoic-6-oxo-6-phenyl acid or trihydroxybiphenyl. A properly selected degradation time led to mineralization of reaction solution, and after 2 hours of reaction the total organic carbon reduction was equaled to 89, 87 and 71 % for ortho- phenylphenol, meta- phenylphenol and para- phenylphenol solution, respectively. The application of advanced oxidation in H2O2/UVC system led to achieving non-toxic reaction solutions. Abbreviations: A, absorbance; AT, atrazine; b, optical path length; C, molar concentration; E0, volumic photon fluence rate; EDCs, endocrine disrupting compounds; FH2O2, the fraction of UVC radiation absorbed by hydrogen peroxide; H2O2, hydrogen peroxide; %I, percent of respiration inhibition; k, reaction rate constants; kz, pseudo-first order rate constant; m-PP, meta-phenylphenol; •OH, hydroxyl radical; o-PP, ortho-phenylphenol; PP, phenylphenol; pPP, para-phenylphenol; r, reaction rate; TOC, total organic carbon; tr, retention time during chromatographic analysis; t-BuOH, tert-butanol; ε, molar absorption coefficient; φ, quantum yield; Subscripts: 0, initial conditions; t, time conditions.
Keywords: phenylphenols, H2O2/UVC assessment, transformation by-products.
system,
photodegradation
kinetics,
toxicity
1. Introduction The problem of the aquatic environment pollution is inextricably linked with the industry development. Industrial and municipal wastewater contain many chemicals that adversely affect living organism. Over the past few decades, numerous researches have been focused on substances which interfere with endocrine system of humans and animals [1, 2, 3, 4]. Group of these substances, called as endocrine disrupting compounds (EDCs), may cause changes in the reproductive and immune system, as well as tumors formation [1, 3, 4]. The worrying thing is that EDCs are biologically active at concentration level which is comparable with their presence in wastewater effluents (1.0 ng L−1 - 1.0 µgL−1) [3]. The ability to disrupt the endocrine system show organohalogens, pesticides, phthalates, synthetic and natural steroids, alkylphenols and phytoestrogens [2]. Phenylphenol (PP) isomers (phenylphenols: ortho- phenylphenol (o-PP), metaphenylphenol (m-PP) and para-phenylphenol (p-PP)), hydroxylated derivatives of biphenyl, are widely used as a preservative agents and disinfectants [5, 6]. These compounds were
detected in the water environment [7, 8, 9], as well as in canned beer and canned soft drinks in United States and Germany [6]. Studies showed that phenylphenols have got potential to bioaccumulation in aquatic organisms, e.g. o-PP was detected in bile of deep-sea fish at the concentration ranged from 8.4 to 192.7 µgL−1 [10]. These compounds show estrogenic and antiandrogenic activity, which means that they can disrupt the endocrine system both male and female organisms [5]. Moreover, phenylphenols possess carcinogenic and cytotoxic properties [11, 12]. The list of methods used for phenylphenols degradation includes biodegradation processes [13, 14], ozonation [15, 16], photocatalytic ozonation [17], photocatalysis [18, 19], photosensitized oxidation [20, 21], photolysis [22, 23] and adsorption [24, 25]. Although above processes are efficient, some challenges eg. long process time or formation of toxic byproducts are still unsolved. The advanced oxidation in H2O2/UV system has been successfully applied for the degradation of many water pollutants including endocrine disrupting compounds [26, 27, 28, 29, 30]. This environmental friendly process is based on the in situ generated hydroxyl radicals (•OH) as a main oxidant. These reactive oxygen species are formed as a result of hydrogen peroxide (H2O2) photolysis, and react with organic compounds non-selectively with the rate constants ranging from 108 to 1010 M-1 s-1 [31]. Hydroxyl radicals may react with chemicals in three ways, by hydrogen abstraction (1), electrophilic addition (2) and electron transfer (3). 1 OH SH S H2O
OH FX HOFX
OH SX SX HO
2 3
The predominant type of interaction of •OH with organic compounds is hydrogen abstraction (1). The products of this reaction are organic radicals (S•), which react with dissolved oxygen initiating subsequent thermal oxidation reactions. Electrophilic addition of hydroxyl radicals to organic π-system lead to formation of organic radicals (HOFX•). While, electron transfer to the hydroxyl radicals results in reduction of •OH to hydroxide anions (HO-), and takes place mainly in case of inorganic compounds [32]. It should be emphasized, that the reactions described above are the initial step of pollutants degradation. Along the process course in the reaction system may appear other reactive species (eg. O2•-) and intermediates, then the mechanism of tested compounds degradation becomes more complex. In addition, process conditions (eg. presence of oxygen and organic matter etc.) are also important. This paper presents the results of phenylphenols advanced oxidation in H2O2/UVC system. The influence of different reaction parameters on degradation rate was examined. Moreover, transformation products and toxicity of the reaction solutions were studied. The obtained results allowed us to determine the rate constants of phenylphenols reaction with hydroxyl radicals.
2. Experimental 2.1.
Materials
o-phenylphenol (≥ 98.0%), p-phenylphenol (> 98%), 2,3-dihydroxybiphenyl (≥ 98.0%), atrazine (AT) (97.5%), tert-butanol (t-BuOH) (≥ 99.7%) and sodium formate (≥ 98.0%) were purchased from Fluka. Phenol (> 99%), 2,5-dihydroxybiphenyl (> 97%), 2,2'dihydroxybiphenyl (> 99%) and m-phenylphenol (85%) were from Aldrich. m-phenylphenol was used after recrystallization from methanol (POCh, Poland). The aqueous solutions of PP isomers were prepared in distilled water treated in Millipore Milli-Q Plus System (18.2 MΩ), under sonication. The initial concentration of the o-PP, m-PP and p-PP ranged from 0.64 to 24.7×10-5 M, from 2.54 to 24.4×10-5 M and from 0.64 to 10.0×10-5 M, respectively. The concentrations of model pollutants were selected from the range between the values of solubility in water (0.7 g L-1 (4.1×10-3 M) [33], 7.9 g L-1 (4.64×10-2 M) [34] and 0.056 g L-1 (3.29×10-4 M) [35] for o-PP, m-PP and p-PP, respectively) and value of determined quantification limit (7.46×10-7 M (0.127 mg L-1). The pH of reaction solution was adjusted by phosphate buffer (H3PO4, KH2PO4, Na2HPO4, NaOH, all p.a., POCh, Poland) in the range from 4 to 12. The initial concentrations of hydrogen peroxide ranged from 10-3 to 10-1 M were applied, because in cases of many endocrine disrupting compounds the optimal value of hydrogen peroxide concentration is usually 10-2 M order of magnitude [36, 27, 28]. 2.2.
Experimental procedures Experiments were carried out in quartz test tubes of the 10 mL capacity (optical path length (b) equaled 0.85 cm), placed in a merry-go-round device which was located between two exposure panels with UVC lamps (Luzchem) emitting mainly wavelength at 254 nm (88.59%). The volumic photon fluence rate (E0) entering the reaction space was determined in our previous study by using uranyl oxalate actinometer and ranged from 4.3 to 15.0×10-6 einstein L-1 s-1 for 2 and 10 lamps [27], respectively. These values correspond to irradiance from 11.8 to 44.4 Wm-2 [27]. 2.3.
Analytical procedures The total organic carbon (TOC) was measured on a HACH IL 550 TOC-TN apparatus. The progress of PP isomers degradation was monitored by chromatographic analysis performed on HPLC Waters apparatus with UV diode array detector using Nova-Pak 150/C18 column, operated in isocratic mode. The mobile phase was a mixture of methanol (Baker HPLC Analyzed, gradient grade) and acidified water (0.01% H3PO4) at the flow rate 1 mL min-1. The percentage of methanol and acidified water in the solvent was equal to 60% v/v and 40% v/v, respectively. The injection volume of the standards and the samples was 50 µL. Analyses were performed at the constant temperature of 25 oC. The phenylphenol isomers were detected at 243 nm. The limit of detection (LOD) and limit of quantification (LOQ) was equal to 3.2×10-7 M (0.0545 mg L-1) and 7.46×10-7 M (0.127 mg L-1), respectively. The degradation by-products were separated by using UHPLC (Waters) apparatus with photodiode array detector and Acquity UPLC BEH Shield RP18 column (1.7 μm, 2.1 mm ×100 mm). UHPLC apparatus was connected with Q-TOF Mass Spectometer Synapt G2 (Waters), operated in electrospray negative ionization mode (ESI-) with nitrogen as the desolvation gas. The details of above analytical method are presented in our previous work [23].
The toxicity of phenylphenol isomers solutions before and after degradation was assayed towards Escherichia coli sp. strain ATCC 11775 using ToxTrakTM Toxicity tests [37]. This method is based on the spectrophotometric determination of the redox-active dye resazurine at the wavelength of 603 nm. As a result of bacterial respiration resazurin is reduced, and color of the reaction solution changes from blue to pink. Toxic substances inhibit this process [37]. According to this method, toxicity test was repeated four times for each sample. Absorbance measurements were made with a HACH DR/2000 spectrophotometer. The initial reaction rates (r0) were calculated by differentiating exponential curve that fitted experimental points (concentration, time) at the correlation factor higher than 0.97.
3. Results and discussion 3.1.
The determination of optimal degradation conditions
The advanced oxidation in UVC/H2O2 system is multifactor process, therefore at the beginning the influence of different reaction parameters on decay rate was investigated. These studies allowed to establish the optimal conditions of target compounds degradation. The concentration of target compounds did not change in presence of 0.05 M H2O2 (Fig. 1A) even after 24 hours of reaction (data not shown). Therefore, the occurrence of direct reaction of PP isomers with H2O2 called “dark reaction” was excluded. Phenylphenol isomers absorb UVC radiation and they can undergo direct photolysis. The values of apparent quantum yield (φ) of o-PP, m-PP and p-PP photolysis at pH 7 are equal to 0.019±0.00043, 0.023±0.00038 and 0.014±0.00083 [23], respectively. Our previous study presents also the influence of different reaction parameters on phenylphenols photolysis rate, as well as transformation products determination and toxicity assessment before and after photolysis [23]. It is well known that addition of hydrogen peroxide to the UVC irradiated reaction solution accelerates degradation of chemicals. In case of PP isomers about ten-fold acceleration of degradation process was observed (Fig.1A). 50 % decrease in the initial concentration of o-PP, m-PP and p-PP was achieved in less than 30 seconds of advanced oxidation in H2O2/UVC system (Fig. 1A). While the TOC in this period was only reduced by about 0.7, 6 and 1.2 % (Fig. 1B), respectively for o-PP, m-PP and p-PP solution. A very high degree of TOC reduction was achieved after 2 hours of reaction (89, 87 and 71 %, respectively for o-PP, m-PP and p-PP solution) (Fig. 1B). 3.1.1 The influence of hydrogen peroxide concentration An important reaction parameter during advanced oxidation in H2O2/UVC system is concentration of hydrogen peroxide. It is well known that both too low or too high concentration of H2O2 slows down the degradation. It is necessary to select the concentration of hydrogen peroxide when the photolysis of H2O2 produces plenty of OH radicals and their consumption by H2O2 is negligible. The obtained results are shown in Fig. 2A. The highest rate of o-PP decay rate was achieved at hydrogen peroxide concentration equal to 0.03 M, while in case of m-PP and p-PP the optimal concentration of H2O2 was 0.05 M. The occurrence of differences in the optimal H2O2 concentration is associated with the differences
in the molar extinction coefficient of PP isomers at 254 nm (Tab. 1). The smaller absorption coefficient of the target compound, the higher fraction of UVC radiation absorbed by hydrogen peroxide (FH2O2) (eq. (4)) and the smaller H2O2 concentration is needed to achieve optimal degradation rate. H 2O 2 C H 2O 2 (4) FH 2O2 H 2O2 C H 2O2 PP C PP where: C and ε are the molar concentration (M) and molar absorption coefficients at 254 nm (M-1 cm-1) of target compounds (CPP and εPP) and hydrogen peroxide (CH2O2 and εH2O2).
In order to determine another optimal reaction parameters, further studies were carried out using 0.03 M H2O2 for all phenylphenol isomers.
Figure 1.
3.1.2 The influence of pH and irradiation intensity The degradation of PP isomers was studied in pH ranged from 4 to 12. The increase in alkalinity causes a decrease in the PP isomers decay rate (Fig. 2B). This dependence is typical for advanced oxidation in H2O2/UVC system, and is mainly related to the dissociation of hydrogen peroxide at higher pH (pKa=11.3 [38]). Hydroperoxide anions (HO2-) formed in this reaction can inhibit the degradation of PP isomers in few ways. First of all, they reduce the amount of hydrogen peroxide. Secondly, they absorb UVC irradiation more effectively (ε254nm=240 M-1 cm-1 [32]) than hydrogen peroxide (ε254nm=18.6 M-1 cm-1 [39]). Additionally, hydroperoxide anions react with H2O2 and •OH, which results in reduction of •OH concentration in the reaction solution. Experiments carried out by using different irradiation intensity demonstrated that the initial reaction rate of PP isomers degradation increase rectilinearly with the increase of fluence rate (data not shown).
Figure 2.
3.1.3 The influence of initial PP isomers concentration Fig. 3A. presents the changes of initial reaction rate versus initial concentration of PP isomers. The reaction rate increased to the limit PP isomer concentration and then slowed down, what can be explained by a competition between hydrogen peroxide and PP isomers for UVC radiation. With increase of initial PP isomers concentration the amount of absorbed irradiation by hydrogen peroxide decreases and in the reaction solution appear less hydroxyl radicals, what lead to decrease of degradation rate. Figure 3.
3.1.4 The influence of hydroxyl radicals scavengers The next series of experiments were performed in the presence of hydroxyl radicals scavengers. In these studies tert-butyl alcohol and sodium formate were employed. Obtained results were similar for all PP isomers, and an example of such dependencies is shown in Fig. 3B. In case of application optimal H2O2 concentration (0.03M), sodium formate and tert-butyl alcohol slowed down the decomposition of PP isomers, however the reaction was not inhibited to the direct photolysis level. This phenomena, was also observed during parabens [27] and fluorene [40] degradation in H2O2/UV system, as well as in case of phydroxybenzoic acid, 4-hydroxycinnamic acid and p-hydroxyphenethyl alcohol degradation by using photo-Fenton and H2O2/UV processes [41], and may suggest that an additional reaction (“third pathway”) occurs. However, presence of 0.5 M of tert-butanol during degradation of parabens in H2O2/UV system by using hydrogen peroxide excess (1 M) confirmed the very relevant role of hydroxyl radicals [42]. In case of application of optimal H2O2 concentration for degradation phenylphenol isomers (0.03M) (Fig. 3B), the fraction of UVC radiation absorbed by hydrogen peroxide calculated from eq. (4) was equal to 0.63, 0.50 and 0.37 for o-PP, m-PP and p-PP experiments, respectively. Increase of H2O2 concentration (0.5 M) in reaction solution leads to increase the fraction of UVC radiation absorbed by hydrogen peroxide - 0.97, 0.97 and 0.96 for o-PP, m-PP and p-PP experiments, respectively. For these conditions, t-BuOH inhibited degradation process (Fig. 3B), what confirmed that degradation of target compounds proceed by reaction with hydroxyl radicals. 3.2.
Kinetics
In the reaction solution, during advanced oxidation of the target compounds in the H2O2/UVC system the following reaction may occurs:
H 2 O 2 h 2 OH r5 H2O2 E 0 FH2O2 1 exp 2.303b i Ci
OH H 2 O 2 H 2 O HO2
OH HO2 H 2 O O 2
r6 k 6 C H2O2 C OH r7 k 7 C HO2 C OH
(5) (6) (7)
PP OH products r8 k OH C PP C OH
(8)
PP h products r9 PP E 0 FPP 1 exp 2.303b i Ci
(9)
When concentration of hydrogen peroxide is high enough, the occurrence of direct photolysis of PP isomers can be neglected and the concentration of H2O2 in the initial stage of process can be treated as constant. Kinetic calculations were done for cases when the fraction of UVC radiation absorbed by H2O2 calculated from equation (4) equaled to 0.97, 0.97 and 0.96 for oPP, m-PP and p-PP experiments, respectively. In this case, the reaction rate can be described by eq. (8). Assuming steady state for hydroxyl radicals concentration, the reaction rate can be expressed by pseudo-first order equation:
dC PP k z C PP dt
(10)
where: kz is pseudo-first order rate constant defined as follows: k z k OH C OH
(11)
The plots of integrated form of equation (10) are presented in fig.4A. The slope of this lines corresponds to kz.
Figure 4.
The combination of equation (5) - (8) under the assumption that the formation and consumption rates of hydroxyl radicals are equal leads to the expression of stationary concentration of hydroxyl radicals: 2 H 2O2 E 0 FH 2O2 1 exp 2.303b i C i (12) C OH k 6 C H 2O2 k 7 C HO 2 k OH C PP Combining of equation (12) with (11) and after rearranging we get: k OH
k z k 6 C H k 7 C HO2
2 H E 0 FH 1 exp 2.303b i C i k z C PP
(13)
Above relationship was used for calculation of reaction rate constants (k) of hydroxyl radicals with PP isomers. The values of constants necessary for calculations and obtained results are given in Tab. 1 and Tab. 2, respectively. Table 1.
In the second approach to determine the kinetic parameters the competition kinetics procedure was used. This method based on competition between target and reference compounds for hydroxyl radicals. In our experiments atrazine was used as competitor. Photolysis of target and reference compounds was not taken into account because the process was carried out under reaction conditions when hydrogen peroxide absorbed almost all incident radiation. For the case shown in Fig. 4B fraction of UVC radiation absorbed by H2O2 was equal to 0.97, 0.98 and 0.98 for o-PP, m-PP and p-PP experiments, respectively. Competition kinetics method assumes that reaction between the oxidant and each competing compounds is first-order, and the two reactions proceed independently in parallel [45]. Under these conditions, the decay rates of PP isomers and reference compound can be expressed as follows: dC PP (14) k OH C PP C OH dt
dC AT k OH C AT C OH dt
(15)
Integration, transformation and combination equations (14) and (15) leading to the formula which describes the ratio between the reaction rate constants: ln
C PP 0 C ln AT 0 C PPt C ATt
k OH k OHAT
(16)
(17)
The plots of equation (16) are presented in fig.4B. The slope of these lines, which correspond to , and equation (17) were used to calculate rate constants of reaction hydroxyl radicals with PP isomers. The rate constant of reaction hydroxyl radicals with atrazine (kOHAT = 2.4×109 M-1 s-1) was taken from the literature [46]. Obtained results (Tab. 2) are in good agreement with the values determined by using classical kinetics. The determined rate constant values depend on the position of the hydroxyl group in the aromatic ring and increase in the following order: ortho- < meta- < para-phenylphenol. The order of magnitude of these constants is typical for reaction of hydroxyl radicals with organic compounds (108-1010 M-1 s1 ) [31]. For example, the rate constant of reaction hydroxyl radicals with phenol and its ortho(catechol) and meta-oriented hydroxy derivatives (resorcinol) is equal to 1.4 × 1010 M−1 s−1 [47], 1.1 × 1010 M−1 s−1 and 1.2 × 1010 M−1 s−1 [48], respectively. While, the rate constant of reaction hydroxyl radicals with biphenyl is equal to 1.0 × 1010 M−1 s−1 [49]. Table 2.
3.3.
Degradation products and toxicity As already mentioned, the degradation of chemicals in H2O2/UVC system may occur
through direct photolysis and via reaction with hydroxyl radicals, however the second pathway is usually much more faster. The main intermediates of PPs photolysis are dihydroxybiphenyl isomers, and mechanism of their formation is presented in our earlier work [23]. In order to determine the products of phenylphenols reaction with hydroxyl radicals, the samples collected after 1, 4 and 20 min of advanced oxidation performed in presence of hydrogen peroxide excess were analyzed. The transformation products are listed in Tab. 3, while Fig. 5 presents their mass spectra in negative ion mode. All of the obtained transformation products are characterized by shorter retention time (tr) (Tab. 3) than phenylphenol isomers (retention time equal to 8.86 min, 8.83 and 8.82 for o-PP, m-PP and p-
PP, respectively), what may indicate on their higher hydrophilicity in comparison to target compounds.
Table 3.
Figure 5.
Figure 6.
Based on the determined products the mechanism of phenylphenols reaction with hydroxyl radicals was proposed (Fig. 6). Hydroxyl radicals may react with chemicals by their addition and hydrogen or electron transfer. The last type of mechanism is typical for reaction of hydroxyl radicals with inorganic ions, therefore it was omitted in further considerations. Addition of hydroxyl radicals to the double bond leads to generation of organic radicals (reaction 2 Fig. 6), which may react with PP isomers leading to the formation of phenylphenoxyl radicals and dihydroxybiphenyls (reaction 3 Fig. 6). Experiments performed by Lv et al. [50] showed that the oxidation of phenol by hydroxyl radicals led mainly to the ortho(catechol) and para-oriented hydroxy derivatives (hydro-quinone), whereas the meta-oriented hydroxy derivative (resorcinol) became the main product when holes were used as the active species [50]. The main hydroxy derivatives formed during reaction of o-PP with hydroxyl radicals were 2,3-dihydroxybiphenyl (3-phenylpyrocatechol), 2,5-dihydroxybiphenyl (2phenylhydroquinone) and 2,2'-dihydroxybiphenyl (Tab. 3). Phenyl-phenoxyl radicals can be also generated as a result of hydrogen atom abstraction (reaction 4 Fig. 6), and in further reaction with oxygen may be turned into superoxide radicals (reaction 5 Fig. 6). Superoxide radicals may undergo a few different reactions. First of all, the reversible reaction can take place (reaction 6 Fig. 6). Secondly, superoxide radicals can react with PP isomers, what lead to generation of phenyl-phenoxyl radical as well as trihydroxybiphenols or organic acids (reaction 7 Fig. 6), and the latter ones can be transformed into phenol (reaction 8 and 9 Fig. 6). The third pathway of superoxide radicals reaction is hydrogen transer into oxygen and homolysis of oxygen-oxygen bond, what leads to generation of hydroxyl radical and phenylbenzoquinone (reaction 10 Fig. 6), which is further transformed into dihydroxybiphenyl (reaction 11 Fig. 6). It should be noted, that mentioned above products are formed in the initial steps of degradation. Further reactions of these
transformation products with hydroxyl radicals may led to formation of ring opened products and mineralization, which proceeds indeed it was shown in Fig. 1B. In the literature can be found information about toxicity of determined by-products [5154]. At first, attention should be paid on phenol, which is well-known harmful, carcinogenic, cytotoxic and teratogenic pollutant [51]. The EC50 value for phenol determined for fish, mixed bacteria culture, algae and crustacea is equal to 13.1, 510, 403 and 25 mg L-1 [52], respectively. Phenol was successfully degraded by using different advanced oxidation methods [55-58], but in some cases more toxic products were generated [56, 58]. It was reported that 2,5-dihydroxybiphenyl (2-phenylhydroquinone) poses cytotoxic and genotoxic activity, what is caused by reactive oxygen species derived from autooxidation of this compounds to phenyl-benzoquinone [53]. The phenyl-benzoquinone was more toxic to rat liver, kidney and isolated hepatocytes than its precursors: phenylhydroquinone and ophenylphenol [53]. Dihydroxybiphenyls,
metabolites
generated
also
during
biodegradation
of
polychlorobiphenyls, are very toxic for bacteria and affect the cell viability much more than chlorobiphenyls. The exposition of E. coli cells to 2 mM of 2,3-dihydroxybiphenyl caused decrease of bacterial viability after 1 h in more than four orders of magnitude and after 24 h in approximately seven orders of magnitude [54]. Therefore, in order to evaluate the effectiveness of the applied degradation methods in toxicity lowering, the toxicity assessment before and after advanced oxidation was performed (Fig. 7). The results are given in terms of percent of respiration inhibition (%I) (relative measurements) (18). A sample (18) %I 1 100 A control where: A A initial A final ; A represents absorbance measurements made using the spectrophotometers.
Before this assay, hydrogen peroxide was removed from the samples by adding catalase, which catalyzes hydrogen peroxide decomposition to water and oxygen [59]. Performed studies demonstrated that this enzyme is not toxic against E. coli bacteria (data not shown). The solutions of all tested compounds before the degradation process inhibited the respiration process of E. coli bacteria (inhibition values >10% or <-10%) (Fig. 7). The toxicity test performed after 4 min of degradation, when the complete removal of 10-4 M of each phenylphenol isomer was observed, confirmed the adverse effect of transformation by-
products on indicator microorganisms (Fig. 7). Whereas, the application of advanced oxidation process within 60 min led to achieving non-toxic solutions (inhibition values within -10% to +10%) (Fig. 7), what indicate on correctness of degradation time selection. Moreover, it should be noted, that the exposition to UVC radiation (without H2O2 addition) by 60 min practically did not change the toxicity of o-PP and m-PP solutions, while in case of pPP the toxicity decreased [23].
Figure 7.
4. Conclusions Advanced oxidation in H2O2/UVC system is effective and very quick method for phenylphenol isomers removal from aquatic environment. 50% decrease in the initial concentration of target compounds was achieved in less than 30 seconds of degradation. While the extent of mineralization in this period was very weak. A high degree of total organic carbon reduction was achieved after 2 hours of reaction (89, 87 and 71 %, respectively for o-PP, m-PP and p-PP solution). The highest rate of o-PP decay rate was achieved at hydrogen peroxide concentration equal to 0.03 M, while in case of m-PP and p-PP the optimal concentration of H2O2 was equal to 0.05 M. The increase in alkalinity causes a decrease in the PP isomers decay rate. The initial reaction rate of PP isomers degradation increase rectilinearly with the increase of fluence rate. The reaction rate increase to the limit concentration of PP isomers and then slow down, what can be explained by a competition between hydrogen peroxide and PP isomers for UVC irradiation. The presence of hydroxyl radical scavengers slows down the reaction course, and in case of application tert-butyl alcohol and hydrogen peroxide excess the degradation was inhibited to the direct photolysis level. The reaction rate constants of hydroxyl radicals with phenylphenol isomers determined by two different method: classical and competition are in good agreement with each other. The value of reaction rate constant depends on the hydroxyl group position in the aromatic ring and follows the order: ortho- phenylphenol < meta- phenylphenol < para- phenylphenol. The main products of initial degradation steps were dihydroxybiphenyl isomers, phenol and isomers of 2,4-hexadienoic-6-oxo-6-phenyl acid or trihydroxybiphenyl. The application of advanced oxidation in H2O2/UVC system led to achieving non-toxic reaction solutions. Acknowledgements This work was supported by the Polish National Science Centre (Poland) within a project number N523 748540.
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Figure captions:
Figure 1. Changes of relative PP isomers concentration during different processes (A) and TOC during degradation in H2O2/UVC system (B) (CoPP0=5.0×10-5 M, CmPP0=5.0×10-5 M, CpPP0=4.9×10-5 M, CH2O20=5.0×10-2 M, E0=10.6×10-6 einstein L-1 s-1, pH=7).
Figure 2. The effect of hydrogen peroxide concentration (CoPP0=5.2×10-5 M, CmPP0=5.0×10-5 M, CpPP0=5.1×10-5 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (A) and pH of reaction solution (CoPP0=5.0×10-5 M, CmPP0=5.0×10-5 M, CpPP0=4.9×10-5 M, CH2O20=3.0×10-2 M, E0=10.6×10-6 einstein L-1 s-1) (B) on initial reaction rates of PP isomers degradation.
Figure 3. The influence of initial concentration of PP isomers on the initial reaction rate during degradation in H2O2/UVC system (CH2O20=3.0×10-2 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (A). The influence of radical scavengers presence on degradation of o-PP (CoPP0=4.9×10-5M, Ct-BuOH=1.0×10-1 M, CHCOONa=1.0×10-1 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (B).
Figure 4. Determination of pseudo-first order rate constants for PP isomers decay in the H2O2/UVC system (CoPP0=4.6×10-5 M, CmP0=1.41×10-5 M, CpPP0=2.13×10-5 M, CH2O2=5.0×10-1 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (A). Results of competition kinetics (CoPP0=2.33×10-5 M, CAT 2PP=2.4×10-5 M, Cm3PP0=1.03×10-5 M, CAT 3PP=1.01×10-5 M, CpPP0=1.0×10-5 M, CAT 4PP=1.03×10-5 M, CH2O2=5.0×10-1 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (B)
Figure 5. Mass spectra in negative ion mode of reaction products of phenylphenols with hydroxyl radicals.
Figure 6. Potential reaction pathways of phenylphenols with hydroxyl radicals on the example of o-PP.
Figure 7. Toxicity of o-PP (A), m-PP (B) and p-PP (C) solutions before and after advanced oxidation in H2O2/UVC system (CoPP0=1×10-4 M, CmPP0=1×10-4 M, CpPP0=1×10-4 M, CH2O2=5.0×10-2 M, E0=10.6×10-6 einstein L-1 s-1), pH 7). *Rn- is the number of next repetition.
TABLES Table 1. The literature data used for kinetic calculations.
Constant
φH2O2
εH2O2 at 254 nm (M-1cm-1)
εPP at 254 nm and pKa for pH 7 H2O2 (M-1cm-1)
Value
0.5
18.6
o-PP: 6740 m-PP: 16900 p-PP: 16820
References
[43]
[39]
[23]
11.3
[38]
pKa
k6
k7
for PP
(M-1s-1)
(M-1s-1)
o-PP: 9.67 m-PP: 9.17 2.7×107 7.5×109 p-PP: 9.27 [23]
[44]
[45]
Table 2. The determined values of reaction rate constants of PP isomers with hydroxyl radicals. Rate constants k•OH (M-1s-1)
Method
o-PP
m-PP
p-PP
classical kinetics
(9.83±0.318) × 109
(1.17±0.056) × 1010
(1.18±0.036) × 1010
competition kinetics
(9.27±0.34) × 109
(1.2±0.039) × 1010
(1.29±0.054) × 1010
Table 3. Products of reaction phenylphenols with hydroxyl radicals identified by UHPLC-MS.
Product name (molecular formula)
Retention time (tr), min
Detected mass of M-H ion (m/z)
Theoretic Error of mass mass of M-H M-H ion ion (m/z) determination
o-PP phenol* (C6H6O)
5.48
93.0367
93.03405
0.00265
2,4-hexadienoic-6-oxo-6-phenyl acid isomers (C12H10O3) or trihydroxybiphenyl isomers (C12H10O3)
6.21
201.054
201.05517
-0.00117
6.33
201.054
201.05517
-0.00117
2,5-dihydroxybiphenyl* (C12H10O2)
7.55
185.0574
185.06027
-0.00287
2,3-dihydroxybiphenyl* (C12H10O2)
7.83
185.0574
185.06027
-0.00287
2,2'-dihydroxybiphenyl* (C12H10O2)
8.14
185.0574
185.06027
-0.00287
dihydroxybiphenyl (C12H10O2)
8.51
185.0574
185.06027
-0.00287
m-PP phenol* (C6H6O)
5.48
93.0367
93.03405
0.00265
2,4-hexadienoic-6-oxo-6-phenyl acid (C12H10O3) or trihydroxybiphenyl (C12H10O3)
6.21
201.054
201.05517
-0.00117
7.86
185.0574
185.06027
-0.00287
8.14
185.0634
185.06027
0.00313
dihydroxybiphenyl isomers (C12H10O2)
p-PP phenol* (C6H6O)
5.48
93.0367
93.03405
0.00265
2,4-hexadienoic-6-oxo-6-phenyl acid isomers (C12H10O3) or trihydroxybiphenyl isomers (C12H10O3)
6.21
201.054
201.05517
-0.00117
dihydroxybiphenyl isomers (C12H10O2)
7.83 7.93 8.14
185.0574 185.0634 185.0574
185.06027 185.06027 185.06027
-0.00287 0.00313 -0.00287
*- confirmed by the standard.
S0304-3894(17)30213-3 http://dx.doi.org/doi:10.1016/j.jhazmat.2017.03.047 HAZMAT 18463
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
5-12-2016 24-2-2017 22-3-2017
Please cite this article as: Magdalena Olak-Kucharczyk, Stanisław Ledakowicz, ADVANCED OXIDATION OF PRESERVATIVE AGENTS IN H2O2/UVC SYSTEM – KINETICS STUDY, TRANSFORMATION PRODUCTS AND TOXICITY ASSESSMENT, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.03.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ADVANCED OXIDATION OF PRESERVATIVE AGENTS IN H2O2/UVC SYSTEM – KINETICS STUDY, TRANSFORMATION PRODUCTS AND TOXICITY ASSESSMENT Magdalena Olak-Kucharczyka,b,*, Stanisław Ledakowiczb a
Textile Research Institute, ul. Brzezinska 5/15, 92-103 Lodz, Poland (*Corresponding author. E-mail address: [email protected]). b Lodz University of Technology, Faculty of Process and Environmental Engineering, Department of Bioprocess Engineering, ul. Wolczanska 213, 90-924 Lodz, Poland. Graphical abstract
Highlights:
The H2O2/UVC process is efficient method for phenylphenols removal from aqueous solutions. The advanced oxidation of phenylphenols led to achieving non-toxic post-reaction solutions. The position of hydroxyl group in the aromatic ring influences on the reaction rate of phenylphenols degradation. Phenol, dihydroxybiphenyls and 2,4-hexadienoic-6-oxo-6-phenyl acids (or trihydroxybiphenyls) are the by-products. The mechanism of phenylphenols reaction with hydroxyl radicals was proposed.
Abstract Phenylphenol isomers (hydroxylated derivatives of biphenyl) are widely used as a preservative agents and disinfectants in many branches of industry. This work focuses on removal of phenylphenol isomers from aqueous solution using H2O2/UVC process. The influence of different operating variables such as pH, initial concentration of hydrogen peroxide, initial concentration of target compounds, fluence rate and presence of radical scavengers on degradation rate was investigated. Moreover,
transformation products and toxicity of the reaction solutions were studied. The reaction rate constants of hydroxyl radicals with phenylphenol isomers were determined by using classical and competition kinetics. The value of reaction rate constant depends on the position of the hydroxyl group in the aromatic ring and follows the order: ortho- phenylphenol < metaphenylphenol < para- phenylphenol. The main by-products of initial degradation steps were dihydroxybiphenyl isomers, phenol and isomers of 2,4-hexadienoic-6-oxo-6-phenyl acid or trihydroxybiphenyl. A properly selected degradation time led to mineralization of reaction solution, and after 2 hours of reaction the total organic carbon reduction was equaled to 89, 87 and 71 % for ortho- phenylphenol, meta- phenylphenol and para- phenylphenol solution, respectively. The application of advanced oxidation in H2O2/UVC system led to achieving non-toxic reaction solutions. Abbreviations: A, absorbance; AT, atrazine; b, optical path length; C, molar concentration; E0, volumic photon fluence rate; EDCs, endocrine disrupting compounds; FH2O2, the fraction of UVC radiation absorbed by hydrogen peroxide; H2O2, hydrogen peroxide; %I, percent of respiration inhibition; k, reaction rate constants; kz, pseudo-first order rate constant; m-PP, meta-phenylphenol; •OH, hydroxyl radical; o-PP, ortho-phenylphenol; PP, phenylphenol; pPP, para-phenylphenol; r, reaction rate; TOC, total organic carbon; tr, retention time during chromatographic analysis; t-BuOH, tert-butanol; ε, molar absorption coefficient; φ, quantum yield; Subscripts: 0, initial conditions; t, time conditions.
Keywords: phenylphenols, H2O2/UVC assessment, transformation by-products.
system,
photodegradation
kinetics,
toxicity
1. Introduction The problem of the aquatic environment pollution is inextricably linked with the industry development. Industrial and municipal wastewater contain many chemicals that adversely affect living organism. Over the past few decades, numerous researches have been focused on substances which interfere with endocrine system of humans and animals [1, 2, 3, 4]. Group of these substances, called as endocrine disrupting compounds (EDCs), may cause changes in the reproductive and immune system, as well as tumors formation [1, 3, 4]. The worrying thing is that EDCs are biologically active at concentration level which is comparable with their presence in wastewater effluents (1.0 ng L−1 - 1.0 µgL−1) [3]. The ability to disrupt the endocrine system show organohalogens, pesticides, phthalates, synthetic and natural steroids, alkylphenols and phytoestrogens [2]. Phenylphenol (PP) isomers (phenylphenols: ortho- phenylphenol (o-PP), metaphenylphenol (m-PP) and para-phenylphenol (p-PP)), hydroxylated derivatives of biphenyl, are widely used as a preservative agents and disinfectants [5, 6]. These compounds were
detected in the water environment [7, 8, 9], as well as in canned beer and canned soft drinks in United States and Germany [6]. Studies showed that phenylphenols have got potential to bioaccumulation in aquatic organisms, e.g. o-PP was detected in bile of deep-sea fish at the concentration ranged from 8.4 to 192.7 µgL−1 [10]. These compounds show estrogenic and antiandrogenic activity, which means that they can disrupt the endocrine system both male and female organisms [5]. Moreover, phenylphenols possess carcinogenic and cytotoxic properties [11, 12]. The list of methods used for phenylphenols degradation includes biodegradation processes [13, 14], ozonation [15, 16], photocatalytic ozonation [17], photocatalysis [18, 19], photosensitized oxidation [20, 21], photolysis [22, 23] and adsorption [24, 25]. Although above processes are efficient, some challenges eg. long process time or formation of toxic byproducts are still unsolved. The advanced oxidation in H2O2/UV system has been successfully applied for the degradation of many water pollutants including endocrine disrupting compounds [26, 27, 28, 29, 30]. This environmental friendly process is based on the in situ generated hydroxyl radicals (•OH) as a main oxidant. These reactive oxygen species are formed as a result of hydrogen peroxide (H2O2) photolysis, and react with organic compounds non-selectively with the rate constants ranging from 108 to 1010 M-1 s-1 [31]. Hydroxyl radicals may react with chemicals in three ways, by hydrogen abstraction (1), electrophilic addition (2) and electron transfer (3). 1 OH SH S H2O
OH FX HOFX
OH SX SX HO
2 3
The predominant type of interaction of •OH with organic compounds is hydrogen abstraction (1). The products of this reaction are organic radicals (S•), which react with dissolved oxygen initiating subsequent thermal oxidation reactions. Electrophilic addition of hydroxyl radicals to organic π-system lead to formation of organic radicals (HOFX•). While, electron transfer to the hydroxyl radicals results in reduction of •OH to hydroxide anions (HO-), and takes place mainly in case of inorganic compounds [32]. It should be emphasized, that the reactions described above are the initial step of pollutants degradation. Along the process course in the reaction system may appear other reactive species (eg. O2•-) and intermediates, then the mechanism of tested compounds degradation becomes more complex. In addition, process conditions (eg. presence of oxygen and organic matter etc.) are also important. This paper presents the results of phenylphenols advanced oxidation in H2O2/UVC system. The influence of different reaction parameters on degradation rate was examined. Moreover, transformation products and toxicity of the reaction solutions were studied. The obtained results allowed us to determine the rate constants of phenylphenols reaction with hydroxyl radicals.
2. Experimental 2.1.
Materials
o-phenylphenol (≥ 98.0%), p-phenylphenol (> 98%), 2,3-dihydroxybiphenyl (≥ 98.0%), atrazine (AT) (97.5%), tert-butanol (t-BuOH) (≥ 99.7%) and sodium formate (≥ 98.0%) were purchased from Fluka. Phenol (> 99%), 2,5-dihydroxybiphenyl (> 97%), 2,2'dihydroxybiphenyl (> 99%) and m-phenylphenol (85%) were from Aldrich. m-phenylphenol was used after recrystallization from methanol (POCh, Poland). The aqueous solutions of PP isomers were prepared in distilled water treated in Millipore Milli-Q Plus System (18.2 MΩ), under sonication. The initial concentration of the o-PP, m-PP and p-PP ranged from 0.64 to 24.7×10-5 M, from 2.54 to 24.4×10-5 M and from 0.64 to 10.0×10-5 M, respectively. The concentrations of model pollutants were selected from the range between the values of solubility in water (0.7 g L-1 (4.1×10-3 M) [33], 7.9 g L-1 (4.64×10-2 M) [34] and 0.056 g L-1 (3.29×10-4 M) [35] for o-PP, m-PP and p-PP, respectively) and value of determined quantification limit (7.46×10-7 M (0.127 mg L-1). The pH of reaction solution was adjusted by phosphate buffer (H3PO4, KH2PO4, Na2HPO4, NaOH, all p.a., POCh, Poland) in the range from 4 to 12. The initial concentrations of hydrogen peroxide ranged from 10-3 to 10-1 M were applied, because in cases of many endocrine disrupting compounds the optimal value of hydrogen peroxide concentration is usually 10-2 M order of magnitude [36, 27, 28]. 2.2.
Experimental procedures Experiments were carried out in quartz test tubes of the 10 mL capacity (optical path length (b) equaled 0.85 cm), placed in a merry-go-round device which was located between two exposure panels with UVC lamps (Luzchem) emitting mainly wavelength at 254 nm (88.59%). The volumic photon fluence rate (E0) entering the reaction space was determined in our previous study by using uranyl oxalate actinometer and ranged from 4.3 to 15.0×10-6 einstein L-1 s-1 for 2 and 10 lamps [27], respectively. These values correspond to irradiance from 11.8 to 44.4 Wm-2 [27]. 2.3.
Analytical procedures The total organic carbon (TOC) was measured on a HACH IL 550 TOC-TN apparatus. The progress of PP isomers degradation was monitored by chromatographic analysis performed on HPLC Waters apparatus with UV diode array detector using Nova-Pak 150/C18 column, operated in isocratic mode. The mobile phase was a mixture of methanol (Baker HPLC Analyzed, gradient grade) and acidified water (0.01% H3PO4) at the flow rate 1 mL min-1. The percentage of methanol and acidified water in the solvent was equal to 60% v/v and 40% v/v, respectively. The injection volume of the standards and the samples was 50 µL. Analyses were performed at the constant temperature of 25 oC. The phenylphenol isomers were detected at 243 nm. The limit of detection (LOD) and limit of quantification (LOQ) was equal to 3.2×10-7 M (0.0545 mg L-1) and 7.46×10-7 M (0.127 mg L-1), respectively. The degradation by-products were separated by using UHPLC (Waters) apparatus with photodiode array detector and Acquity UPLC BEH Shield RP18 column (1.7 μm, 2.1 mm ×100 mm). UHPLC apparatus was connected with Q-TOF Mass Spectometer Synapt G2 (Waters), operated in electrospray negative ionization mode (ESI-) with nitrogen as the desolvation gas. The details of above analytical method are presented in our previous work [23].
The toxicity of phenylphenol isomers solutions before and after degradation was assayed towards Escherichia coli sp. strain ATCC 11775 using ToxTrakTM Toxicity tests [37]. This method is based on the spectrophotometric determination of the redox-active dye resazurine at the wavelength of 603 nm. As a result of bacterial respiration resazurin is reduced, and color of the reaction solution changes from blue to pink. Toxic substances inhibit this process [37]. According to this method, toxicity test was repeated four times for each sample. Absorbance measurements were made with a HACH DR/2000 spectrophotometer. The initial reaction rates (r0) were calculated by differentiating exponential curve that fitted experimental points (concentration, time) at the correlation factor higher than 0.97.
3. Results and discussion 3.1.
The determination of optimal degradation conditions
The advanced oxidation in UVC/H2O2 system is multifactor process, therefore at the beginning the influence of different reaction parameters on decay rate was investigated. These studies allowed to establish the optimal conditions of target compounds degradation. The concentration of target compounds did not change in presence of 0.05 M H2O2 (Fig. 1A) even after 24 hours of reaction (data not shown). Therefore, the occurrence of direct reaction of PP isomers with H2O2 called “dark reaction” was excluded. Phenylphenol isomers absorb UVC radiation and they can undergo direct photolysis. The values of apparent quantum yield (φ) of o-PP, m-PP and p-PP photolysis at pH 7 are equal to 0.019±0.00043, 0.023±0.00038 and 0.014±0.00083 [23], respectively. Our previous study presents also the influence of different reaction parameters on phenylphenols photolysis rate, as well as transformation products determination and toxicity assessment before and after photolysis [23]. It is well known that addition of hydrogen peroxide to the UVC irradiated reaction solution accelerates degradation of chemicals. In case of PP isomers about ten-fold acceleration of degradation process was observed (Fig.1A). 50 % decrease in the initial concentration of o-PP, m-PP and p-PP was achieved in less than 30 seconds of advanced oxidation in H2O2/UVC system (Fig. 1A). While the TOC in this period was only reduced by about 0.7, 6 and 1.2 % (Fig. 1B), respectively for o-PP, m-PP and p-PP solution. A very high degree of TOC reduction was achieved after 2 hours of reaction (89, 87 and 71 %, respectively for o-PP, m-PP and p-PP solution) (Fig. 1B). 3.1.1 The influence of hydrogen peroxide concentration An important reaction parameter during advanced oxidation in H2O2/UVC system is concentration of hydrogen peroxide. It is well known that both too low or too high concentration of H2O2 slows down the degradation. It is necessary to select the concentration of hydrogen peroxide when the photolysis of H2O2 produces plenty of OH radicals and their consumption by H2O2 is negligible. The obtained results are shown in Fig. 2A. The highest rate of o-PP decay rate was achieved at hydrogen peroxide concentration equal to 0.03 M, while in case of m-PP and p-PP the optimal concentration of H2O2 was 0.05 M. The occurrence of differences in the optimal H2O2 concentration is associated with the differences
in the molar extinction coefficient of PP isomers at 254 nm (Tab. 1). The smaller absorption coefficient of the target compound, the higher fraction of UVC radiation absorbed by hydrogen peroxide (FH2O2) (eq. (4)) and the smaller H2O2 concentration is needed to achieve optimal degradation rate. H 2O 2 C H 2O 2 (4) FH 2O2 H 2O2 C H 2O2 PP C PP where: C and ε are the molar concentration (M) and molar absorption coefficients at 254 nm (M-1 cm-1) of target compounds (CPP and εPP) and hydrogen peroxide (CH2O2 and εH2O2).
In order to determine another optimal reaction parameters, further studies were carried out using 0.03 M H2O2 for all phenylphenol isomers.
Figure 1.
3.1.2 The influence of pH and irradiation intensity The degradation of PP isomers was studied in pH ranged from 4 to 12. The increase in alkalinity causes a decrease in the PP isomers decay rate (Fig. 2B). This dependence is typical for advanced oxidation in H2O2/UVC system, and is mainly related to the dissociation of hydrogen peroxide at higher pH (pKa=11.3 [38]). Hydroperoxide anions (HO2-) formed in this reaction can inhibit the degradation of PP isomers in few ways. First of all, they reduce the amount of hydrogen peroxide. Secondly, they absorb UVC irradiation more effectively (ε254nm=240 M-1 cm-1 [32]) than hydrogen peroxide (ε254nm=18.6 M-1 cm-1 [39]). Additionally, hydroperoxide anions react with H2O2 and •OH, which results in reduction of •OH concentration in the reaction solution. Experiments carried out by using different irradiation intensity demonstrated that the initial reaction rate of PP isomers degradation increase rectilinearly with the increase of fluence rate (data not shown).
Figure 2.
3.1.3 The influence of initial PP isomers concentration Fig. 3A. presents the changes of initial reaction rate versus initial concentration of PP isomers. The reaction rate increased to the limit PP isomer concentration and then slowed down, what can be explained by a competition between hydrogen peroxide and PP isomers for UVC radiation. With increase of initial PP isomers concentration the amount of absorbed irradiation by hydrogen peroxide decreases and in the reaction solution appear less hydroxyl radicals, what lead to decrease of degradation rate. Figure 3.
3.1.4 The influence of hydroxyl radicals scavengers The next series of experiments were performed in the presence of hydroxyl radicals scavengers. In these studies tert-butyl alcohol and sodium formate were employed. Obtained results were similar for all PP isomers, and an example of such dependencies is shown in Fig. 3B. In case of application optimal H2O2 concentration (0.03M), sodium formate and tert-butyl alcohol slowed down the decomposition of PP isomers, however the reaction was not inhibited to the direct photolysis level. This phenomena, was also observed during parabens [27] and fluorene [40] degradation in H2O2/UV system, as well as in case of phydroxybenzoic acid, 4-hydroxycinnamic acid and p-hydroxyphenethyl alcohol degradation by using photo-Fenton and H2O2/UV processes [41], and may suggest that an additional reaction (“third pathway”) occurs. However, presence of 0.5 M of tert-butanol during degradation of parabens in H2O2/UV system by using hydrogen peroxide excess (1 M) confirmed the very relevant role of hydroxyl radicals [42]. In case of application of optimal H2O2 concentration for degradation phenylphenol isomers (0.03M) (Fig. 3B), the fraction of UVC radiation absorbed by hydrogen peroxide calculated from eq. (4) was equal to 0.63, 0.50 and 0.37 for o-PP, m-PP and p-PP experiments, respectively. Increase of H2O2 concentration (0.5 M) in reaction solution leads to increase the fraction of UVC radiation absorbed by hydrogen peroxide - 0.97, 0.97 and 0.96 for o-PP, m-PP and p-PP experiments, respectively. For these conditions, t-BuOH inhibited degradation process (Fig. 3B), what confirmed that degradation of target compounds proceed by reaction with hydroxyl radicals. 3.2.
Kinetics
In the reaction solution, during advanced oxidation of the target compounds in the H2O2/UVC system the following reaction may occurs:
H 2 O 2 h 2 OH r5 H2O2 E 0 FH2O2 1 exp 2.303b i Ci
OH H 2 O 2 H 2 O HO2
OH HO2 H 2 O O 2
r6 k 6 C H2O2 C OH r7 k 7 C HO2 C OH
(5) (6) (7)
PP OH products r8 k OH C PP C OH
(8)
PP h products r9 PP E 0 FPP 1 exp 2.303b i Ci
(9)
When concentration of hydrogen peroxide is high enough, the occurrence of direct photolysis of PP isomers can be neglected and the concentration of H2O2 in the initial stage of process can be treated as constant. Kinetic calculations were done for cases when the fraction of UVC radiation absorbed by H2O2 calculated from equation (4) equaled to 0.97, 0.97 and 0.96 for oPP, m-PP and p-PP experiments, respectively. In this case, the reaction rate can be described by eq. (8). Assuming steady state for hydroxyl radicals concentration, the reaction rate can be expressed by pseudo-first order equation:
dC PP k z C PP dt
(10)
where: kz is pseudo-first order rate constant defined as follows: k z k OH C OH
(11)
The plots of integrated form of equation (10) are presented in fig.4A. The slope of this lines corresponds to kz.
Figure 4.
The combination of equation (5) - (8) under the assumption that the formation and consumption rates of hydroxyl radicals are equal leads to the expression of stationary concentration of hydroxyl radicals: 2 H 2O2 E 0 FH 2O2 1 exp 2.303b i C i (12) C OH k 6 C H 2O2 k 7 C HO 2 k OH C PP Combining of equation (12) with (11) and after rearranging we get: k OH
k z k 6 C H k 7 C HO2
2 H E 0 FH 1 exp 2.303b i C i k z C PP
(13)
Above relationship was used for calculation of reaction rate constants (k) of hydroxyl radicals with PP isomers. The values of constants necessary for calculations and obtained results are given in Tab. 1 and Tab. 2, respectively. Table 1.
In the second approach to determine the kinetic parameters the competition kinetics procedure was used. This method based on competition between target and reference compounds for hydroxyl radicals. In our experiments atrazine was used as competitor. Photolysis of target and reference compounds was not taken into account because the process was carried out under reaction conditions when hydrogen peroxide absorbed almost all incident radiation. For the case shown in Fig. 4B fraction of UVC radiation absorbed by H2O2 was equal to 0.97, 0.98 and 0.98 for o-PP, m-PP and p-PP experiments, respectively. Competition kinetics method assumes that reaction between the oxidant and each competing compounds is first-order, and the two reactions proceed independently in parallel [45]. Under these conditions, the decay rates of PP isomers and reference compound can be expressed as follows: dC PP (14) k OH C PP C OH dt
dC AT k OH C AT C OH dt
(15)
Integration, transformation and combination equations (14) and (15) leading to the formula which describes the ratio between the reaction rate constants: ln
C PP 0 C ln AT 0 C PPt C ATt
k OH k OHAT
(16)
(17)
The plots of equation (16) are presented in fig.4B. The slope of these lines, which correspond to , and equation (17) were used to calculate rate constants of reaction hydroxyl radicals with PP isomers. The rate constant of reaction hydroxyl radicals with atrazine (kOHAT = 2.4×109 M-1 s-1) was taken from the literature [46]. Obtained results (Tab. 2) are in good agreement with the values determined by using classical kinetics. The determined rate constant values depend on the position of the hydroxyl group in the aromatic ring and increase in the following order: ortho- < meta- < para-phenylphenol. The order of magnitude of these constants is typical for reaction of hydroxyl radicals with organic compounds (108-1010 M-1 s1 ) [31]. For example, the rate constant of reaction hydroxyl radicals with phenol and its ortho(catechol) and meta-oriented hydroxy derivatives (resorcinol) is equal to 1.4 × 1010 M−1 s−1 [47], 1.1 × 1010 M−1 s−1 and 1.2 × 1010 M−1 s−1 [48], respectively. While, the rate constant of reaction hydroxyl radicals with biphenyl is equal to 1.0 × 1010 M−1 s−1 [49]. Table 2.
3.3.
Degradation products and toxicity As already mentioned, the degradation of chemicals in H2O2/UVC system may occur
through direct photolysis and via reaction with hydroxyl radicals, however the second pathway is usually much more faster. The main intermediates of PPs photolysis are dihydroxybiphenyl isomers, and mechanism of their formation is presented in our earlier work [23]. In order to determine the products of phenylphenols reaction with hydroxyl radicals, the samples collected after 1, 4 and 20 min of advanced oxidation performed in presence of hydrogen peroxide excess were analyzed. The transformation products are listed in Tab. 3, while Fig. 5 presents their mass spectra in negative ion mode. All of the obtained transformation products are characterized by shorter retention time (tr) (Tab. 3) than phenylphenol isomers (retention time equal to 8.86 min, 8.83 and 8.82 for o-PP, m-PP and p-
PP, respectively), what may indicate on their higher hydrophilicity in comparison to target compounds.
Table 3.
Figure 5.
Figure 6.
Based on the determined products the mechanism of phenylphenols reaction with hydroxyl radicals was proposed (Fig. 6). Hydroxyl radicals may react with chemicals by their addition and hydrogen or electron transfer. The last type of mechanism is typical for reaction of hydroxyl radicals with inorganic ions, therefore it was omitted in further considerations. Addition of hydroxyl radicals to the double bond leads to generation of organic radicals (reaction 2 Fig. 6), which may react with PP isomers leading to the formation of phenylphenoxyl radicals and dihydroxybiphenyls (reaction 3 Fig. 6). Experiments performed by Lv et al. [50] showed that the oxidation of phenol by hydroxyl radicals led mainly to the ortho(catechol) and para-oriented hydroxy derivatives (hydro-quinone), whereas the meta-oriented hydroxy derivative (resorcinol) became the main product when holes were used as the active species [50]. The main hydroxy derivatives formed during reaction of o-PP with hydroxyl radicals were 2,3-dihydroxybiphenyl (3-phenylpyrocatechol), 2,5-dihydroxybiphenyl (2phenylhydroquinone) and 2,2'-dihydroxybiphenyl (Tab. 3). Phenyl-phenoxyl radicals can be also generated as a result of hydrogen atom abstraction (reaction 4 Fig. 6), and in further reaction with oxygen may be turned into superoxide radicals (reaction 5 Fig. 6). Superoxide radicals may undergo a few different reactions. First of all, the reversible reaction can take place (reaction 6 Fig. 6). Secondly, superoxide radicals can react with PP isomers, what lead to generation of phenyl-phenoxyl radical as well as trihydroxybiphenols or organic acids (reaction 7 Fig. 6), and the latter ones can be transformed into phenol (reaction 8 and 9 Fig. 6). The third pathway of superoxide radicals reaction is hydrogen transer into oxygen and homolysis of oxygen-oxygen bond, what leads to generation of hydroxyl radical and phenylbenzoquinone (reaction 10 Fig. 6), which is further transformed into dihydroxybiphenyl (reaction 11 Fig. 6). It should be noted, that mentioned above products are formed in the initial steps of degradation. Further reactions of these
transformation products with hydroxyl radicals may led to formation of ring opened products and mineralization, which proceeds indeed it was shown in Fig. 1B. In the literature can be found information about toxicity of determined by-products [5154]. At first, attention should be paid on phenol, which is well-known harmful, carcinogenic, cytotoxic and teratogenic pollutant [51]. The EC50 value for phenol determined for fish, mixed bacteria culture, algae and crustacea is equal to 13.1, 510, 403 and 25 mg L-1 [52], respectively. Phenol was successfully degraded by using different advanced oxidation methods [55-58], but in some cases more toxic products were generated [56, 58]. It was reported that 2,5-dihydroxybiphenyl (2-phenylhydroquinone) poses cytotoxic and genotoxic activity, what is caused by reactive oxygen species derived from autooxidation of this compounds to phenyl-benzoquinone [53]. The phenyl-benzoquinone was more toxic to rat liver, kidney and isolated hepatocytes than its precursors: phenylhydroquinone and ophenylphenol [53]. Dihydroxybiphenyls,
metabolites
generated
also
during
biodegradation
of
polychlorobiphenyls, are very toxic for bacteria and affect the cell viability much more than chlorobiphenyls. The exposition of E. coli cells to 2 mM of 2,3-dihydroxybiphenyl caused decrease of bacterial viability after 1 h in more than four orders of magnitude and after 24 h in approximately seven orders of magnitude [54]. Therefore, in order to evaluate the effectiveness of the applied degradation methods in toxicity lowering, the toxicity assessment before and after advanced oxidation was performed (Fig. 7). The results are given in terms of percent of respiration inhibition (%I) (relative measurements) (18). A sample (18) %I 1 100 A control where: A A initial A final ; A represents absorbance measurements made using the spectrophotometers.
Before this assay, hydrogen peroxide was removed from the samples by adding catalase, which catalyzes hydrogen peroxide decomposition to water and oxygen [59]. Performed studies demonstrated that this enzyme is not toxic against E. coli bacteria (data not shown). The solutions of all tested compounds before the degradation process inhibited the respiration process of E. coli bacteria (inhibition values >10% or <-10%) (Fig. 7). The toxicity test performed after 4 min of degradation, when the complete removal of 10-4 M of each phenylphenol isomer was observed, confirmed the adverse effect of transformation by-
products on indicator microorganisms (Fig. 7). Whereas, the application of advanced oxidation process within 60 min led to achieving non-toxic solutions (inhibition values within -10% to +10%) (Fig. 7), what indicate on correctness of degradation time selection. Moreover, it should be noted, that the exposition to UVC radiation (without H2O2 addition) by 60 min practically did not change the toxicity of o-PP and m-PP solutions, while in case of pPP the toxicity decreased [23].
Figure 7.
4. Conclusions Advanced oxidation in H2O2/UVC system is effective and very quick method for phenylphenol isomers removal from aquatic environment. 50% decrease in the initial concentration of target compounds was achieved in less than 30 seconds of degradation. While the extent of mineralization in this period was very weak. A high degree of total organic carbon reduction was achieved after 2 hours of reaction (89, 87 and 71 %, respectively for o-PP, m-PP and p-PP solution). The highest rate of o-PP decay rate was achieved at hydrogen peroxide concentration equal to 0.03 M, while in case of m-PP and p-PP the optimal concentration of H2O2 was equal to 0.05 M. The increase in alkalinity causes a decrease in the PP isomers decay rate. The initial reaction rate of PP isomers degradation increase rectilinearly with the increase of fluence rate. The reaction rate increase to the limit concentration of PP isomers and then slow down, what can be explained by a competition between hydrogen peroxide and PP isomers for UVC irradiation. The presence of hydroxyl radical scavengers slows down the reaction course, and in case of application tert-butyl alcohol and hydrogen peroxide excess the degradation was inhibited to the direct photolysis level. The reaction rate constants of hydroxyl radicals with phenylphenol isomers determined by two different method: classical and competition are in good agreement with each other. The value of reaction rate constant depends on the hydroxyl group position in the aromatic ring and follows the order: ortho- phenylphenol < meta- phenylphenol < para- phenylphenol. The main products of initial degradation steps were dihydroxybiphenyl isomers, phenol and isomers of 2,4-hexadienoic-6-oxo-6-phenyl acid or trihydroxybiphenyl. The application of advanced oxidation in H2O2/UVC system led to achieving non-toxic reaction solutions. Acknowledgements This work was supported by the Polish National Science Centre (Poland) within a project number N523 748540.
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Figure captions:
Figure 1. Changes of relative PP isomers concentration during different processes (A) and TOC during degradation in H2O2/UVC system (B) (CoPP0=5.0×10-5 M, CmPP0=5.0×10-5 M, CpPP0=4.9×10-5 M, CH2O20=5.0×10-2 M, E0=10.6×10-6 einstein L-1 s-1, pH=7).
Figure 2. The effect of hydrogen peroxide concentration (CoPP0=5.2×10-5 M, CmPP0=5.0×10-5 M, CpPP0=5.1×10-5 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (A) and pH of reaction solution (CoPP0=5.0×10-5 M, CmPP0=5.0×10-5 M, CpPP0=4.9×10-5 M, CH2O20=3.0×10-2 M, E0=10.6×10-6 einstein L-1 s-1) (B) on initial reaction rates of PP isomers degradation.
Figure 3. The influence of initial concentration of PP isomers on the initial reaction rate during degradation in H2O2/UVC system (CH2O20=3.0×10-2 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (A). The influence of radical scavengers presence on degradation of o-PP (CoPP0=4.9×10-5M, Ct-BuOH=1.0×10-1 M, CHCOONa=1.0×10-1 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (B).
Figure 4. Determination of pseudo-first order rate constants for PP isomers decay in the H2O2/UVC system (CoPP0=4.6×10-5 M, CmP0=1.41×10-5 M, CpPP0=2.13×10-5 M, CH2O2=5.0×10-1 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (A). Results of competition kinetics (CoPP0=2.33×10-5 M, CAT 2PP=2.4×10-5 M, Cm3PP0=1.03×10-5 M, CAT 3PP=1.01×10-5 M, CpPP0=1.0×10-5 M, CAT 4PP=1.03×10-5 M, CH2O2=5.0×10-1 M, E0=10.6×10-6 einstein L-1 s-1, pH 7) (B)
Figure 5. Mass spectra in negative ion mode of reaction products of phenylphenols with hydroxyl radicals.
Figure 6. Potential reaction pathways of phenylphenols with hydroxyl radicals on the example of o-PP.
Figure 7. Toxicity of o-PP (A), m-PP (B) and p-PP (C) solutions before and after advanced oxidation in H2O2/UVC system (CoPP0=1×10-4 M, CmPP0=1×10-4 M, CpPP0=1×10-4 M, CH2O2=5.0×10-2 M, E0=10.6×10-6 einstein L-1 s-1), pH 7). *Rn- is the number of next repetition.
TABLES Table 1. The literature data used for kinetic calculations.
Constant
φH2O2
εH2O2 at 254 nm (M-1cm-1)
εPP at 254 nm and pKa for pH 7 H2O2 (M-1cm-1)
Value
0.5
18.6
o-PP: 6740 m-PP: 16900 p-PP: 16820
References
[43]
[39]
[23]
11.3
[38]
pKa
k6
k7
for PP
(M-1s-1)
(M-1s-1)
o-PP: 9.67 m-PP: 9.17 2.7×107 7.5×109 p-PP: 9.27 [23]
[44]
[45]
Table 2. The determined values of reaction rate constants of PP isomers with hydroxyl radicals. Rate constants k•OH (M-1s-1)
Method
o-PP
m-PP
p-PP
classical kinetics
(9.83±0.318) × 109
(1.17±0.056) × 1010
(1.18±0.036) × 1010
competition kinetics
(9.27±0.34) × 109
(1.2±0.039) × 1010
(1.29±0.054) × 1010
Table 3. Products of reaction phenylphenols with hydroxyl radicals identified by UHPLC-MS.
Product name (molecular formula)
Retention time (tr), min
Detected mass of M-H ion (m/z)
Theoretic Error of mass mass of M-H M-H ion ion (m/z) determination
o-PP phenol* (C6H6O)
5.48
93.0367
93.03405
0.00265
2,4-hexadienoic-6-oxo-6-phenyl acid isomers (C12H10O3) or trihydroxybiphenyl isomers (C12H10O3)
6.21
201.054
201.05517
-0.00117
6.33
201.054
201.05517
-0.00117
2,5-dihydroxybiphenyl* (C12H10O2)
7.55
185.0574
185.06027
-0.00287
2,3-dihydroxybiphenyl* (C12H10O2)
7.83
185.0574
185.06027
-0.00287
2,2'-dihydroxybiphenyl* (C12H10O2)
8.14
185.0574
185.06027
-0.00287
dihydroxybiphenyl (C12H10O2)
8.51
185.0574
185.06027
-0.00287
m-PP phenol* (C6H6O)
5.48
93.0367
93.03405
0.00265
2,4-hexadienoic-6-oxo-6-phenyl acid (C12H10O3) or trihydroxybiphenyl (C12H10O3)
6.21
201.054
201.05517
-0.00117
7.86
185.0574
185.06027
-0.00287
8.14
185.0634
185.06027
0.00313
dihydroxybiphenyl isomers (C12H10O2)
p-PP phenol* (C6H6O)
5.48
93.0367
93.03405
0.00265
2,4-hexadienoic-6-oxo-6-phenyl acid isomers (C12H10O3) or trihydroxybiphenyl isomers (C12H10O3)
6.21
201.054
201.05517
-0.00117
dihydroxybiphenyl isomers (C12H10O2)
7.83 7.93 8.14
185.0574 185.0634 185.0574
185.06027 185.06027 185.06027
-0.00287 0.00313 -0.00287
*- confirmed by the standard.