This paper describes the results of experiments on the decomposition of selected nonylphenols (NPs) in aqueous solutions using the UV, UV/H2O2, O3 and UV/O3 processes. The goal of the research was to determine the kinetic parameters of the above-mentioned processes, and to estimate their effectiveness. These substances were selected because of their ubiquitous occurrence in the aquatic environment, resistance to biodegradation and environmental significance. As a result of the experiments, the quantum yields of the 4-n-nonylphenol (4NP) and NP (technical mixture) photodegradation in aqueous solution were calculated to be 0.15 and 0.17, respectively. The values of the second-order rate constants of the investigated compounds with hydroxyl radical and NP with ozone were also determined. The estimated second-order rate constants of 4NP and NP with hydroxyl radicals were equal to 7.6 × 108–1.3 × 109 mol−1 L s−1. For NP, the determined rate constant with ozone was equal to 2.01 × 106 mol−1 L s−1. The performed experiments showed that NP was slightly more susceptible to degradation by the UV radiation and hydroxyl radicals than 4NP. The study demonstrated also that the polychromatic UV-light alone and also in combination with selected oxidizers (i.e. hydrogen peroxide, ozone) may be successfully used for the removal of selected NPs from the aqueous medium.

INTRODUCTION

Nonylphenols (NPs) are substances which are widely detected in the aquatic environment. Their occurrence in the environment is exclusively associated with anthropogenic activities – NPs are not produced by living organisms (Ternes & Joss 2006). NPs are introduced into the environment mainly via industrial effluents and municipal wastewater treatment plant (WWTP) effluents. The concentrations of NPs in WWTP effluents vary widely from undetectable levels to hundreds of micrograms per litre (Teske & Arnold 2008). For instance, as reported by Solé et al. (2000), the concentrations of NP in effluents from WWTPs located in north-eastern Spain varied from about 6 to 343 μg L−1. The occurrence of NPs in surface, ground and drinking water was confirmed by many authors (e.g. Kuch & Ballschmiter 2001; Hohenblum et al. 2004; Vethaak et al. 2005; Liu et al. 2009). The NP concentrations in surface and ground water ranged from a few ng L−1 to a few μg L−1 and the NP concentrations in drinking water were at the level of a few nanograms per litre (Liu et al. 2009). Wagner & Oehlmann (2009) reported the detection of NPs in bottled mineral water at concentrations from 16 to 456 ng L−1. NPs may pose a risk to living organisms (including humans) because they presumably interact with the estrogenic receptors, disturbing the proper functioning of the living organisms. Therefore, they are often classified as endocrine disrupting compounds. Owing to their physicochemical properties, they are included in the list of priority substances (Decision No. 2455/2001/EC) in the field of water policy.

Despite, the fact that the information on biological and chemical transformations of these compounds during wastewater treatment is available in the literature, the data are not easily comparable due to various treatment parameters, sampling procedures and analytical methods (Bertanza et al. 2011). The operating conditions were found to affect the removal of NPs in several activated sludge WWTPs, which varied from 9 to 94% (Ahel et al. 1994). For example, relatively high-removal efficiency of NPs was observed in nitrifying WWTPs. Some authors suggested that NPs removal can be attributed not only to biodegradation, but also to sorption on activated sludge flocs (Ahel et al. 1994; Teske & Arnold 2008).

In the literature, oxidation processes such as ozonation, indirect photolysis, photocatalytic oxidation, O3/H2O2, O3/TiO2 are mentioned as an effective tool to remove NPs from the aquatic environment and a potential complement of the biological wastewater treatment processes (Gültekin & Ince 2007; Mosteo et al. 2010). The efficiency of NPs ozonation is dependent on the pH of the reaction solution and the presence of other substances, which may react with ozone. For example, Deborde et al. (2005) calculated that the reaction rate constant of molecular ozone with a neutral molecule of 4-n-nonylphenol (4NP) (pH < 5.0) was equal to 3.8 × 104 M−1 s−1, whereas the value of the reaction rate constant of ozone with a dissociated molecule of 4NP (pH > 10.0) was about 105 times higher than with a neutral molecule of this substance and it was equal to 6.8 × 109 M1 s−1. Huber et al. (2003) estimated that at neutral pH, the value of reaction rate constant of ozone with 4NP is expected to be at the level of (1–10) × 106 M−1 s−1. The results obtained by the above-mentioned authors show that 4NP is able to react quickly with ozone, however, there is a lack of information about the reaction of ozone with other isomers of NP.

Most of the published information on NPs removal by means of electromagnetic radiation concerns an application of monochromatic UV light (alone or combined with H2O2) and photocatalytic methods, in which plain or various modifications of TiO2 are used (Ike et al. 2002; Neamtu & Frimmel 2006; Pereira et al. 2012). For instance, Pereira et al. (2012) estimated that the value of quantum yield of a photochemical decomposition of 4NP initiated by means of monochromatic UV light was equal to 0.27, however, the decay of the investigated substance in the direct low-pressure photolysis was only 23%. The efficiency of the process was improved by the addition of hydrogen peroxide (100 mg L−1). After this operation, the percentage of 4NP degradation exceeded the value of 90%.

Noteworthy, most of the published studies focused only on 4NP degradation by different physical–chemical methods and furthermore the photochemical methods, which were reported in the literature were frequently induced by monochromatic UV radiation. Thus, the aim of this study was to compare the behavior of 4NP and NP technical mixture (NP; mixtures of nonylphenol isomers) in the UV and UV/H2O2 processes, which are induced by means of a polychromatic ultraviolet radiation.

Because of the lack of information on the behaviour of NP in the presence of ozone, this work focused also on determining the effectiveness of NP decomposition in the O3 and UV/O3 processes. The obtained results were used to estimate quantum yields and second-order rate constants of the investigated compounds with hydroxyl radicals. In the case of NP, the obtained results allowed to determine the second-order rate constant of this substance with ozone.

MATERIALS AND METHODS

All the experiments were performed in distilled water containing the standards of 4NP and NP (technical mixture), respectively. The standards of 4NP and NP were purchased from Sigma-Aldrich (Poznań, Poland) and they meet purity requirements for LC analysis (PESTANAL® grade, analytical standard). 4NP was available in the form of a white powder and NP in the form of a yellow liquid (ρ = 0.937 g/cm3). The others chemicals used in the experiments were of the highest purity commercially available.

The experiments were performed in triplicate. The reported results are arithmetic mean values. No statistically significant differences between subsequent replications of each experiment were observed.

The concentrations of 4NP and NP during all the processes were confirmed by a high-performance liquid chromatograph (HPLC) coupled with UV detector. The HPLC system was equipped with a C18 Hypersil Gold column (Thermo Scientific, Polygen, Poland). The mobile phase consisted of mixtures of acetonitrile and water, in volume ratios 90:10 (v/v) and 75:25 (v/v) during the analysis of 4NP and NP, respectively. The flow rate of the mobile phase was set at 1 mL min−1. In the above-described conditions, the retention times of the investigated compounds were equal to 4.3 ± 0.1 and 6.6 ± 0.1 min for 4NP and NP, respectively. The limit of quantification, both in the case of 4NP and NP, was equal to 0.1 mg L−1. It was established as the second lowest calibration point of their calibration curves (linear regression, R2 > 0.99), and the calculated ‘signal to noise’ ratio (S/N) of the compounds was greater than 10. The limits of detection (LOD) of 4NP and NP were defined when S/N was at the level of 3. LODs in both cases were equal to 0.02 mg L−1. The analyses were performed at the wavelengths λ = 225 and λ = 220 nm for 4NP and NP, respectively.

All the experiments (UV, UV/H2O2, O3 and UV/O3 processes) were carried out in a 350 mL glass reactor equipped with a polychromatic medium-pressure mercury lamp, which emitted the photons in the broad emission spectra from 255 to 597 nm (VitaTech, Münsing, Germany). During direct ozonolysis, the UV lamp was turned off. The nominal power of the light source was equal to 400 W, but all the tests were performed using half of its nominal power (200 W). A detailed description of the photo-reactor was presented in the previous publication (Felis et al. 2011). The lamp irradiance (E0) was determined using actinometric investigation and it was equal to 754.87 W m−3 (1.77 × 10−6 Einstein·L−1 s−1). Uranyl oxalate was used as the actinometer. The average path length of light through the solution was equal to 10 mm.

The O3 and UV/O3 processes were conducted as homogeneous ozonolysis, which means that ozone was introduced into the reactor as its stock solution. Ozone was generated from the oxygen in the BMT 802 ozonator. Ozone stock solutions (≈0.2 mmol−1 L) were prepared by sparging of O3-containing oxygen through 0.5 L distilled water (buffered to pH = 7.0 by means of 10 mmol−1 L phosphate buffer). The buffered water was cooled in an ice bath. The flow rate of the O3-containing oxygen through the water was equal to 37 L h−1. The concentration of ozone in the O3-containing oxygen ranged from 42.0 to 47.0 gO3m−3 and it was controlled by ozone analyzer (BMT Vent, Berlin, Germany). After 2 h of the treatment, the concentration of ozone in the ozone stock solutions was confirmed by means of the indigo and/or spectrophotometric method (Huber 2004). The appropriate volume of the ozone stock solution was added to the reactor, to which an aqueous solution of the investigated substances had been introduced beforehand, to get the final concentration of ozone and test substances. The ozone stock solutions were prepared prior to the experiments (they were not stored).

The determination of reaction rate constant between NP and ozone was carried out in 10 mL glass vials based on the assumptions of the competition kinetic model. Benzene was used as a reference substance and tert-butyl alcohol was used as the hydroxyl radical scavenger. The preparation of the ozone stock solution and determination of the reaction rate constant between NP and ozone were performed according to the procedure described by Huber (2004).

All the experiments were performed at pH = 7.0 and ambient temperature. The investigations were carried out in the presence of 10 mmol−1 L phosphate buffer, which provides a constant pH during all the experiments.

The lamp emission spectrum was measured by means of the radiophotometer USB 4000 (Ocean Optics, Dunedin, FL, USA). The absorption spectra of 4NP and NP in distilled water (10 mg L−1, pH = 7.0) were measured by means of the spectrophotometer V-650 (ABL&E-JASCO, Kraków, Poland).

RESULTS

Direct photolysis (UV process)

The average initial concentrations of 4NP and NP, during their direct photolysis were equal to 10.01 ± 0.11 and 9.83 ± 0.17 mg L−1. The photolysis of 4NP and NP resulted in a decomposition of the investigated compounds moieties. Within 5 min of the process, 30% decay of 4NP was observed. After 60 min of irradiation, the 4NP concentration was equal to 1.11 ± 0.32 mg L−1. It corresponded to 89% removal of this compound from the solution. In the case of direct photolysis of NP, after 5 min of the process, the NP decay was at the level of 45%. Within 60 min of the UV process, NP concentration decreased to 0.72 ± 0.35 mg L−1. These results showed that NP removal after 60-min exposition to UV radiation was equal to 96% (Figure 1(a)). Comparing the above-mentioned results with the results presented by Pereira et al. (2012), who obtained only 37% removal of 4NP in the low-pressure direct photolysis, it can be stated that the photolysis induced by means of polychromatic light is more effective in terms of NP decomposition.

Figure 1

Photodegradation of 4NP and NP (a), and absorption spectra of 4NP and NP against the background of the lamp emission (b).

Figure 1

Photodegradation of 4NP and NP (a), and absorption spectra of 4NP and NP against the background of the lamp emission (b).

The absorption spectra of 4NP and NP appeared in the range from 200 to 297 nm. The medium-pressure Hg lamp emitted photons from 254 to 579 nm. This means that only the radiation from 254 to 297 nm was actively used in these experiments (Figure 1(b)).

Owing to the application of a polychromatic radiation source, the molar extinction coefficients of 4NP and NP (ɛ4NP and ɛNP) were calculated as a weighted average of single molar extinction coefficients determined at selected wavelengths in the range of the active spectrum. The values of the weighted averaged molar extinction coefficients were calculated to be 1.04 × 103 L mol−1 cm−1 and 9.80 × 102 L mol−1 cm−1 for 4NP and NP, respectively.

The direct photolysis rate is usually described by a relationship which is a combination of the Stark–Einstein law and the Lambert–Beer law (1) 
formula
1
where Φ – quantum yields; E0 – lamp irradiance, Einstein L−1 s−1; C – concentration, mol−1 L; b – light path, cm; ɛ – weighted average molar extinction coefficient, L mol−1 cm−1.
When the concentration of the absorbing substance in the solution is relatively low, i.e. its absorbance does not exceed the value of 0.1, Equation (1) may be simplified to the first-order Equation (2) (Miller & Olejnik 2001) 
formula
2
where k – pseudo-first-order rate constant, s−1.
The values of the absorbance of 4NP and NP were calculated according to the Lambert–Beer law (3) 
formula
3
where A – absorbance.

As the values of the absorbance both for 4NP and NP were equal to 0.0472 and 0.0445, the rates of the direct photolysis reaction (ruv) were calculated using Equation (2), after the mathematical transformation. The calculated values of the quantum yields of the photochemical decay of 4NP (Φ4NP) and NP (ΦNP) were equal to 0.15 and 0.17, respectively. The results showed that both investigated substances, despite some differences in their chemical structure and the resulting differences between their absorption spectra, are susceptible to degradation by means of the radiation emitted by the polychromatic medium-pressure mercury lamp. The obtained values of quantum yields for both 4NP and NP are lower than the value of 4NP quantum yield, which was obtained by Pereira et al. (2012) during the investigation of 4NP decay by means of radiation emitted by monochromatic low-pressure mercury lamp (Φ4NP = 0.27). However, in the case of the medium-pressure photolysis, in spite of obtaining lower values of quantum yields, a higher NP decomposition efficiency was observed than in the low-pressure photolysis. A hypothetical elucidation of this phenomenon is that in the case of monochromatic radiation the photochemical decay of 4NP was mainly affected by the processes resulting from direct absorption of photons by this compound. Conversely, in the case of polychromatic radiation, the emitted photons might be absorbed by the intermediate products of NP photo-transformation to a greater extent than during the photolysis induced by monochromatic radiation. This stems from the fact that a greater range of emitted UV radiation is available for the by-products occurring in the reaction solution. In addition, NP intermediates formed during the primary (or secondary) photochemical processes may affect the efficiency of the photochemical decomposition of NPs (the so-called photo-sensibilization effect).

UV/H2O2 process

Decomposition of the substances in the UV/H2O2 system requires establishing an optimal hydrogen peroxide concentration, i.e. when its photolysis produces a sufficiently large amount of OH· radicals and their consumption by H2O2 molecules is negligible. The UV/H2O2 experiments were performed at various initial concentrations of hydrogen peroxide, this is, 100, 1,000, or 10,000 mg L−1 during experiments with 4NP and 10, 100, or 1,000 mg L−1 during NP photooxidation. In all these experiments, the H2O2 addition caused the increase of 4NP and NP photodegradation efficiencies in comparison to the results obtained during direct photolysis of these compounds. However, it was observed that the higher doses of H2O2 caused an improvement of the reaction efficiency only during the NP photooxidation, especially in the first 10 min of the process. Irrespective of the initial dose of H2O2, after 30 min of the process, the removal of NP was higher than 90%.

In the case of 4NP, during the first 5 min of the UV/H2O2 process, the decay of the investigated substance proceeded almost identically, irrespective of the initial H2O2 concentrations. After this period, in the UV/H2O2 process assisted by 10,000 mg L−1 of H2O2, the decay of 4NP was retarded in comparison to the results obtained with the other applied doses of H2O2. After 30 min of the process, in the case of H2O2 doses equal to 100–1,000 mg L−1, the observed 4NP removal was higher than 90%, while in the case of H2O2 dose equal to 10,000 mg L−1, the 4NP removal was below 80%. The observed phenomenon can be relatively simply explained – H2O2 present in the solution at a high-concentration absorbed most of the electromagnetic radiation emitted from the UV lamp causing inhibition of direct photolysis of 4NP. In addition, the consumption of hydroxyl radicals by H2O2 might be significant. The removal efficiencies of 4NP and NP in the UV/H2O2 process at various initial H2O2 concentrations are presented in Figure 2.

Figure 2

Removal of 4NP (a) and NP (b) in the UV/H2O2 process.

Figure 2

Removal of 4NP (a) and NP (b) in the UV/H2O2 process.

The second-order rate constants of the investigated compounds with hydroxyl radicals (kOH) were determined based on the data obtained during photochemical decay of 4NP and NP in the UV/H2O2 process. In general, the decay of the substance by hydrogen peroxide combined with UV radiation consists of several individual processes, namely direct oxidation by H2O2, direct photolysis and reaction with hydroxyl radicals generated during H2O2 photolysis. The reaction rate (r) of the substance decay in the UV/H2O2 system may be expressed by Equation (4) 
formula
4
where rUV1 – direct photolysis rate (in the UV/H2O2 process), rOH – reaction rate with hydroxyl radicals, rd – initial reaction rate with H2O2, ‘dark reaction’ rate (including rate of sorption process).
The initial reaction rates (r, rd) of the investigated substances were calculated by differentiating exponential curves that fitted experimental points (C, t). The rates of direct photolysis (rUV1) were calculated from modified Equation (1), taking into account the distribution of UV radiation between the investigated substances (f) and hydrogen peroxide (fH) determined according to Equation (5) 
formula
5
The values of rOH were calculated according to Equation (4).
Assuming the quasi-stationary concentration of OH (steady state), the reaction of decay of the investigated compounds proceeded as a pseudo-first-order reaction, where the apparent rate constant (kapp) of the reaction with OH can be presented by means of Equation (6) (Bledzka et al. 2010) 
formula
6
where [OH] – quasi-stationary concentration of OH, kOH – rate constant of the substance with OH.
The reaction rate of the substances with OH may be expressed as follows (7) (Bledzka et al. 2010): 
formula
7
After mathematical transformation of Equation (7), the values of the apparent reaction rate (kapp) reaction of the investigated substances with OH were calculated as (8) 
formula
8

The values of the parameters used in the calculations of rate constants of NP and 4NP with hydroxyl radicals are listed in Table 1.

Table 1

The values of the parameters used in calculations of rate constants of NP and 4NP with OH

Parameter NP 4NP 
Initial substance concentration, C, mol−15.98 × 10−5 4.33 × 10−5 
Initial H2O2 concentration, CH, mol−12.94 × 10−2 2.94 × 10−2 
r, mol−1 L s−1 1.30 × 10−7 6.46 × 10−8 
rd, mol−1 L s−1 1.91 × 10−8 8.19 × 10−9 
ruv1, mol−1 L s−1 2.56 × 10−8 1.75 × 10−8 
rOH, mol−1 L s−1 8.56 × 10−8 3.98 × 10−8 
fH 0.866 0.894 
fC 0.134 0.106 
kapp, s−1 1.43 × 10−3 8.97 × 10−4 
Parameter NP 4NP 
Initial substance concentration, C, mol−15.98 × 10−5 4.33 × 10−5 
Initial H2O2 concentration, CH, mol−12.94 × 10−2 2.94 × 10−2 
r, mol−1 L s−1 1.30 × 10−7 6.46 × 10−8 
rd, mol−1 L s−1 1.91 × 10−8 8.19 × 10−9 
ruv1, mol−1 L s−1 2.56 × 10−8 1.75 × 10−8 
rOH, mol−1 L s−1 8.56 × 10−8 3.98 × 10−8 
fH 0.866 0.894 
fC 0.134 0.106 
kapp, s−1 1.43 × 10−3 8.97 × 10−4 
In the UV/H2O2 system, the following reactions can occur (Christensen et al. 1982; Legrini et al. 1993; Bledzka et al. 2010) 
formula
9
 
formula
10
 
formula
11
 
formula
12
 
formula
13
 
formula
14
where φH – H2O2 quantum yield (0.5; according to Legrini et al. 1993), k2 = 2.7 × 107 and k4 = 7.5 × 109s−1 (according to Christensen et al. 1982), fH, fC – fractions of the UV radiation absorbed by H2O2 and substance, respectively; K – equilibrium constant.
Using the steady-state assumption, in which the generation and disappearance rates of hydroxyl radicals are equal to each other, the quasi-stationary concentration of OH may be calculated by combining the reaction rates from Equations (9), (10), (12) and (13) (steady state → r1 = r2 + r4 + rOH) according to Equation (15) 
formula
15
The combination of Equation (6) with (15), after mathematical conversion, allowed calculating the rate constant of the investigated substances in reaction with hydroxyl radicals (kOH) (16) 
formula
16
The estimated second-order rate constants of 4NP and NP with hydroxyl radicals were equal to 7.6 × 108 mol−1 L s−1 and 1.3 × 109 mol−1 L s−1 (kOHNP), respectively. This means that, both investigated compounds are very prone to decay by means of hydroxyl radicals; however, NP was slightly more susceptible to degradation both by the UV radiation and hydroxyl radicals than 4NP.

O3 and UV/O3 processes

Because the information on the 4NP decay by means of ozone in various technological configurations is available in the literature (Huber 2004; Liu et al. 2009), the studies on decomposition by the O3 and UV/O3 processes were conducted only for NP. During the homogeneous ozonolysis of NP, its removal was dependent on the ozone doses. The reaction of NP decay proceeded very rapidly. The mean initial concentration of NP during this experiment was equal to 9.75 ± 0.45 mg L−1. The ozone doses used in the experiment, namely 0.8 mg L−1, 1.0 mg L−1 and 2.0 mg L−1 resulted in degradation of NP (after 3 min of the process) equal to 45%, 52% and 60%, respectively. In the UV/O3 system, the NP oxidation was followed by the reaction with hydroxyl radicals, molecular ozone and direct photolysis. The NP decomposition efficiency was also dependent on the initial ozone doses. For example, after 2 min of the UV/O3 process and ozone dose equal to 1.5 mg L−1, the concentration of NP decreased from 9.06 ± 0.73 to 4.92 ± 0.22 mg L−1 and it corresponded to 59% decay of this compound. After the same time of the UV/O3 process and the ozone dose of 0.8 mg L−1, the NP decomposition was at the level of 44% (Figure 3). The second-order rate constant for the reaction of NP with ozone () was determined using the competition kinetic model described by Huber (2004) (Figure 4). Phenol was used as a reference substance. The reaction rate constant of phenol with ozone at pH = 7 is equal to 1.8 × 106 mol−1 L s−1 (Huber 2004). The calculated value of the reaction rate constant of ozone with NP () was equal to 2.01 × 106 mol−1 L s−1. The obtained value of is similar to the corresponding rate constant of phenol reported by Huber (2004) and other phenol-based endocrine disrupters, e.g. 4NP estimated by Huber et al. (2003). This may suggest that the phenolic ring (in the NP structure) could be the main area of ozone action with the investigated compounds.

Figure 3

NP removal in the O3 (a), and O3/UV processes (b) by various O3 concentrations.

Figure 3

NP removal in the O3 (a), and O3/UV processes (b) by various O3 concentrations.

Figure 4

Determination of the rate constant of NP with ozone using competition kinetic model.

Figure 4

Determination of the rate constant of NP with ozone using competition kinetic model.

The values of reaction rate constants of ozone with organic substances are in the range of 0.1–7 × 109 mol−1 L s−1 (Von Gunten 2003). This means that NP has a relatively high affinity for ozone and it can be effectively decomposed by direct ozonolysis.

CONCLUSIONS

Based on the obtained results, it can be concluded that NPs are susceptible to degradation in the UV, UV/H2O2, O3 and UV/O3 processes. The quantum yields of 4NP and NP photodegradation were equal to 0.15 and 0.17, respectively. This means that the efficiency of the direct photolysis performed using the polychromatic radiation was relatively high and this process may play an important role during the removal of both NP and 4NP from the aquatic environment.

The NP degradation reactions in the UV/H2O2 system required establishing an optimal hydrogen peroxide concentration, i.e. when its photolysis generated a sufficient amount of hydroxyl radicals and their consumption by hydrogen peroxide was insignificant. In all the UV/H2O2 experiments, the hydrogen peroxide addition caused the increase of 4NP and NP photodegradation efficiencies in comparison to the results obtained during direct photolysis of these compounds. However, the concentration of H2O2 equal to 10,000 mg L−1 provoked retardation of the 4NP decay in comparison to the results obtained at lower H2O2 concentrations. According to the results obtained in the UV/H2O2 process, the second-order rate constants of 4NP and NP with hydroxyl radicals were calculated to be 7.6 × 108M−1 s−1 () and 1.3 × 109 mol−1 L s−1 (kOHNP), respectively. Reaction rate constants of various substances with hydroxyl radical in aqueous solution are commonly in the order of 106–109 mol−1 L s−1 (Esplugas et al. 2007). This means that both NP and 4NP react with hydroxyl radicals very rapidly. However, NP reacts with hydroxyl radical slightly faster than 4NP.

In the O3 and UV/O3 processes, the NP removal efficiency was dependent on the ozone doses. The second-order rate constant for the reaction of NP with ozone () was determined using the competition kinetic model with phenol as a reference substance. The value of a second-order rate constant of NP with O3 was estimated to be 2.01 × 106 mol−1 L s−1. The determined value of is in accordance with the corresponding rate constants for phenol and other phenolic endocrine disrupters (e.g. 4NP).

The results obtained in this experiment indicated that the photochemical oxidation induced by means of polychromatic UV radiation both alone and assisted by the addition of the various oxidizers (i.e. hydrogen peroxide or ozone) may help to resolve the problem of the emergent pollutants in the aquatic environment. It should be noted that the results were obtained in laboratory conditions and it is very important to perform such experiments in real environmental samples, in order to verify the obtained results.

ACKNOWLEDGEMENT

The authors thank the Polish Ministry of Science and Higher Education (Grant No. PZB/MNiSW/07/2006/31) for its financial support of this study.

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