自由基诱导的水溶液中氟西汀的降解：脉冲辐解及稳态辐照研究

吉天翼 刘艳成 赵剑锋,3 徐 刚 王文锋,* 吴明红,*

自由基诱导的水溶液中氟西汀的降解：脉冲辐解及稳态辐照研究

吉天翼1,2刘艳成2赵剑锋2,3徐 刚1王文锋2,*吴明红1,*

(1上海大学环境与化学工程学院，上海200444；2中国科学院上海应用物理研究所，上海201800；3中国科学院大学，北京100049)

本文运用脉冲辐解探究了不同自由基与药物氟西汀(FLX)之间的反应。羟基自由基(·OH)与FLX反应生成苯环上的羟基加成物，而硫酸根阴离子自由基则通过单电子氧化FLX生成苯阳离子自由基，该中间产物再进一步与水反应生成苯环上的羟基加成物。本研究测定了三种自由基·OH，水合电子以及与 FLX反应的反应速率常数分别为：7.8×109,2.3×109和1.1×109mol·L－1·s－1。本文还运用电子束辐照技术探究了不同辐照条件下的FLX降解效果，结合HPLC和紫外可见光谱仪进行分析。在N2O和空气饱和的两种条件下，FLX溶液经1.5 kGy辐照后降解效率均达到90%以上，而N2饱和条件下，加入0.1 mol·L－1的叔丁醇的FLX溶液经1.5 kGy辐照后仅有43%分解。此外，酸性和中性条件下FLX的降解效率均大于碱性条件下的。结果阐明了饱和空气的FLX溶液在中性条件下的降解效果最佳，且·OH诱导的反应比更有利于FLX的分解。本研究期望对于进一步探究FLX的降解反应提供有益的帮助。

氟西汀；脉冲辐解；羟基自由基；硫酸根阴离子自由基；降解

Key Words: Fluoxetine;Pulse radiolysis;Hydroxyl radical;Sulfate radical anion;Degradation

1 Introduction

Recently,social and scientific concerns about the occurrence of pharmaceutical and personal care products(PPCPs)in the environmental water have increased1,2.Many drugs have been detected in environmental water due to the widespread use of pharmaceuticals and the insufficient removal processes in ordinary water and wastewater treatment3,4.Furthermore,concerns have also been raised about the potential impacts of their parent compounds and biologically active metabolites on environmental and human health5.Therefore,PPCPs have been recognized as environmental pollutants6.

Fluoxetine(FLX)(N-methyl-3-(p-trifluoromethylphenoxy)-3-phenylpropylamine,shown in Fig.1),also named Prozac,is widely used for treating depression and other neurological or mental diseases.As a selective serotonin reuptake inhibitor(SSRI), fluoxetine(FLX)and its demethylated active metabolite norfluoxetine(NFLX)were proposed as being potentially dangerous to the environment in a list of 10 pharmaceuticals7.Since they undergo incomplete decomposition in the wastewater treatment process,FLX and NFLX have been detected with the concentration level of ng·L－1in surface waters of most of countries8－11. Hence,it implied that wastewater effluents are an important source of FLX and NFLX residue in the surface water12,13.Furthermore, it was reported that some freshwater fishes were toxic and the copulation and maturity of microorganisms were distributed after exposure to FLX14,15.Therefore,although FLX and its metabolites are present in the environment in very low concentrations,they may present a potential hazard to the environmental water as well as to human health.

FLX shows the most absorbance in the range of UV spectrum, but its photodegradation is limited in environmental water,even under appropriate conditions of pH and temperature.Kwon and Armbrust16illustrated the low biological degradability of FLX in wastewater treatment plants,as it was not only stable during hydrolysis and photolysis but also resistant to micro-biodegradation.Nowadays,advanced oxidation processes(AOPs)are a rapid and high-efficiency technology that have been used successfully to remove multiple pollutants by forming strong oxidants such as hydroxyl radicals(·OH)to eliminate contaminants and mineralization.To improve this degradation efficiency,a study reported that using sonochemical treatment as a mean of pretreatment combined with biological treatment to remove FLX17. FLX was eliminated in an Ar-saturated solution after 60 min of sonication,and 15%was mineralized after 360 min of ultrasonic irradiation.Radiation technology is considered to be an advanced oxidation processes(AOP)technique,and the radicals formed by radiolysis of water can degrade pollutants18,19.Silva et al.20reported FLX eliminated completely by electron beam irradiation at a dose of more than 2.5 kGy,while TOC was removed only 22%even at a dose of 20 kGy.Garrido et al.21discovered that FLX was oxidized mainly through the oxidations of the secondary amine group and aromatic ring,which yielded a transient cation-radical and then conducted further reactions.

Fig.1 Molecular structure of FLX

In this paper,we studied that different intermediates of water radiolysis reacted with FLX by monitoring the growth/decay of transient intermediates by using pulse radiolysis.The rate constants of radical reactions with FLX were determined,and the yield of FLX decomposition was investigated in different conditions by electron beam irradiation.Finally,we compared the rate constants of different radical reactions with FLX and the degradation rates of FLX under different conditions to discern the optimal conditions for eliminating FLX.

2 Materials and methods

2.1 Materials

Fluoxetine hydrochloride(FLX·HCl)was purchased from Tokyo Chemical Industry(＞98%purity).Tert-butanol and K2S2O8were obtained from Sigma-Aldrich.NaOH and phosphate(used for preparation of buffers,pH=7.1)were purchased form J&K Chemical Ltd.All chemicals were analytical reagents and employed without further purification.Sample solutions were prepared using ultra-pure water,and experiments were carried out at ambient temperature.Solutions were bubbled with N2O or N2(high purity,99.999%)for at least 20 min.

2.2 Pulse radiolysis and steady state radiolysis

The nanosecond pulse radiolysis experiments were conducted using a 10 MeV linear electron accelerator with high-energy electron pulse duration of 8 ns,and the details were described elsewhere22,23.As a thiocyanate dosimeter,0.1 mol·L－1KSCN solution bubbled with N2O was used to measure the pulse dosimetry using G[(CNS)2·－]=5.8 and by taking ε480nm=7600 dm3· m－1·cm－122.The dose of each electron pulse was 10 Gy.A500 W xenon lamp was used as the source of analyzing light,and the electron pulse and the detecting beam passed vertically through a quartz cell with an optical path length of 10 mm.

Main radicals generated by water radiolysis were shown in Eq. (1),in which the G-values(μmol·J－1)shown in brackets are the radiation chemical yields of radicals24－26.To study the hydroxyl radical(·OH)reaction,sample solutions were pre-saturated with N2O to convert the hydrated electron(e－aq)and hydrogen atom(·H) to·OH under pulse radiolysis,as shown in Eqs.(2)and(3)24,27,28. To research the reducing reactions oftert-butanol was used to scavenge·OH in the N2saturated solutions as shown in Eq.(4)27,29.

H2O⇝·OH(0.28),·H(0.06),H3O+(0.27),H2(0.05),

Electron beam irradiation was accomplished utilizing a GJ-2-II electron accelerator with a 1.8 MeV beam energy during the steady state radiolysis study.The experiments were irradiated with a dose range of 0.5－20 kGy and a dose rate of 0.045 kGy·s－1.

2.3 Analytical procedures

The UV-visible experiments were performed using a Hitachi U-3900 spectrophotometer with the detection wavelength in the range of 190－500 nm.The concentrations of FLX before and after irradiation were measured using an HPLC system(Agilent 1200 series)equipped with a reversed C18column(250 mm×4.6 mm); the detection wavelength of the VW monitor was set as 226 nm. The mobile phase was a mixture of acetonitrile(ACN)and 10 mmol·L－1potassium monophosphate(50:50)at an isocratic mode(1 mL·min－1)30.The injection volume of the auto-sampler was set to 10 μL.

3 Results and discussion

3.1 Pulse radiolysis

3.1.1 Hydroxyl radical reactions

The concentration of 0.5 mmol·L－1FLX in the N2O-saturated solution at pH=7.1 was studied by pulse radiolysis.As shown in Fig.2,the transient absorption spectrum for the reaction of·OH with FLX depicts a characteristic absorption at 340 nm.After 1 μs, it was quenched rapidly with time increased.Merga et al.31reported that the absorption peak in the range of 300－350 nm corresponded to the·OH adduct,which was generated by the·OH attack on the aromatic ring.According to a previous report,FLX degraded to produce the hydroxylated and O-dealkylated intermediates under indirect photodegradation32.It is possible that·OH reacted with FLX as shown in the following equation:

Fig.2 Transient absorption spectra obtained from hydroxyl radical oxidation with 5×10－4mol·L－1FLX in N2O-saturated aqueous solutions(pH=7.1)

The inset of Fig.2 shows the buildup rate constant(kobs)monitored at 340 nm,with various concentrations of FLX ranging from 0.02 to 1 mmol·L－1.Therefore,the rate constant was determined to be 7.8×109mol·L－1·s－1based on the linear trend of the pseudo-first-order transient rate constant.The value of the rate constant of·OH reaction with FLX is similar to those reported about·OH reaction with benzene32,demonstrating that the formation of the hydroxylcyclohexadienyl radical is the first step in the reaction of·OH with FLX24,33.This result also suggests that the majority of·OH added to the benzene ring,rather than reacting with alkylbenzene in the abstraction of the hydrogen atom.

3.1.2 Hydrated electron reactions

To investigate the reaction of FLX with hydrated electrons,the experiment was performed in an N2-saturated sample solution with the addition of 0.1 mol·L－1tert-butanol to scavenge·OH,where e－aqis main reactor partner.Astrong broad band at the peak of 690 nm was observed after electron pulse irradiation(as shown in Fig.3a).And the spectrum exhibits the decay ofat 690 nm with different time in the presence and absence of FLX solution.Thedecay ofwas faster with 0.5 mmol·L－1FLX solution than without the addition of FLX solution.After 1 μs,the characteristic absorption ofdecayed completely in the 0.5 mmol·L－1FLX solution.Hence,the hydrated electron decay appears to be accelerated in the presence of FLX.

Fig.3 (a)Time-resolved absorption spectra obtained from thereaction with 5×10－4mol·L－1FLX in N-saturated solutions2containing 0.1 mol·L－1tert-butanol(pH=7.1);(b)plot of the observed decay rate constant(kobs)as monitored by the reaction ofwith different concentrations of FLX at 690 nm

Fig.3b shows that the plot of decay rate constant for the reaction ofwith different concentrations of FLX was observed in the decay signal ofat 690 nm.The curve was fitted to a linear trend of the pseudo-first-order rate constant,the value of the reaction ofwith FLX was determined to be 2.3×109mol·L－1·s－1.The

3.1.3 Sulfate radical anion

aq,with a yield of G(SO4·－)=2.7 μmol·J－1(Eq.(6))35.Fig.4 depicts the time-resolved absorption spectra of the SO4·－reaction with FLX recorded at different time,which shows strong absorption peaks at 350 and 460 nm.The characteristic absorption ofwas reported to be at 460 nm in previous studies36.Compared to the absorption spectrum of transient intermediate in the absence of FLX at 1 μs, it has a new absorption peak at 350 nm in the 0.5 mmol·L－1FLX solution.The characteristic absorption ofdecayed rapidly with increasing time,while the absorbance of transient intermediate increased at 350 nm(shown in Eq.(7)).Theradicalinduced degradation of some benzene compounds formed the intermediates of hydroxylated adducts of the benzene ring18.In this study,we conjectured that the SO4·－attacked to the aromatic ring by single electron oxidation,forming benzene radical cation and then further reacted with H2O,forming·OH adduct37.The bimolecular rate constant of the SO4·－radical reaction with FLX was estimated with the range concentration from 0.06－0.22 mmol·L－1, based on the pseudo-first-order decay rate constant(inset of Fig.4).And the value is 1.1×109mol·L－1·s－1,as determined from the decay of SO4·－at 460 nm.

Fig.4 Time-resolved absorption spectra obtained in the reaction of SO·4－with 5×10-4mol·L－1FLX in N2-saturated solutions containing 0.1 mol·L－1K2S2O8and 0.1 mol·L－1tert-butanol(pH=7.1)

3.2 Steady state radiolysis

The initial concentration of 0.29 mmol·L－1FLX in air,N2O or N2bubbled solutions were irradiated with different doses by the electron beam irradiation.In the N2O-saturated solution,·OH is the dominant oxidant to oxidizes pollutants.While e－aqis an important reducing agent in the N2-saturated solution containing 0.1 mol·L－1tert-butanol as the selected radical scavenger.In the presence of dissolved O2,and H·were both converted into O2·－/ HO2·(Eqs.(8,9)),therefore,·OH+O2·－/HO2·reactions occur in the aerated solution19.

Fig.5 displays the·OH-induced degradation efficiency of FLX in the N2O-saturated solution at pH=7.At a dose of 1.5 kGy,the decomposition yield of FLX was approximately 90%;at an absorbed dose of 5 kGy,more than 99%FLX was consumed.With the increasing dose,the characteristic absorption of FLX decreased at 226 nm,indicating the decomposition of FLX in the aqueous solution(inset of Fig.5).Meanwhile,when the absorbed dose was increased,the absorption peak at 265 nm also increased. It was also observed that the peak at approximately 265 nm was slightly redshifted after irradiation,and this same phenomenon also was observed in the spectrum of the air-saturated solution (data not shown).The peak at 265 nm was denoted the formation of changed aromatic rings38.It was also illustrated the hydroxylated product formed by·OH attacked to the aromatic ring.

To study reactions of individual radical with FLX,the atmo-

Fig.5 Effect of various doses on the yield of decomposition of the initial concentration of 0.29 mmol·L－1FLX in the

N2O-saturated solution as determined by the HPLC system and integrating the area under the chromatographic peaksphere condition was changed to produce reactive radical intermediates.And the above experiments suggested that SO4·－can oxide with FLX,so we also further explored the efficiency ofoxidation with FLX.From the Fig.6,the efficiency of the·OH-induced reaction was slightly higher than the·OH+O2·－/HO2· reaction in the N2O and air atmospheres,but both reactions were much higher than theandreactions in the N2atmosphere. After being irradiated with a dose of 1.5 kGy,the initial FLX molecules deceased by 95%and 93%in N2O and air bubbled

solutions,respectively,in contrast with 43%reaction)and 73%reaction)reductions in the N2-saturated solution.FLX were decomposed completely with·OH and·OH+reactions at a dose of 5 kGy,and more than 90%FLX were decomposed withandreactions.It was reported that the mineralization of ibuprofen by

radical is better than·OH at pH=7 since the yield of oxidizing radicals increased about 2.2 times in the presence of K2S2O818.However,as proved by our transient study,·OH reaction with FLX was observed to be faster than SO4·－.Meanwhile,as shown by the steady state results,·OH-induced degradation of FLX is more efficient thanradicalinduced degradation.This is probably due to two reasons listed

adical could not fully or mostly react with FLX because of the competitive reaction between the selfdecay of radical andradical reaction with FLX.The other reason is that the addition reaction of·OH radical is more efficient than the single electron oxidation ofradical in the ring opening reaction of FLX.

The effect of degradation efficiency of FLX at different pH values was also examined.Fig.7 displays the decomposition yield of FLX in air-saturated solutions at pH 4,7 and 11.At a dose of 2 kGy,FLX had decomposed by more than 95%at pH 4 and 7. The decompositions of FLX both under acidic condition and the neutral condition were better than alkaline condition at a low absorbed dose.Additionally,it has been reported that the degradation of FLX increased at a condition of acidic pH by sonochemical treatment,which has been interpreted to reflect the

Fig.6 Dependence of the yield of FLX radiolytic decomposition on the·OH reaction(■)in the N2O-saturated solution,the

·OH+O2·－/HO2·reaction(▲)in the air-saturated solution,and the(●)and? Fig.7 Dose dependence of the decomposition yield of the initial concentration of 0.29 mmol·L－1FLX in the

air-saturated solution(·OH+O2·－/HO2·reaction) dominance of the hydrophilic form of FLX17.The pKavalue of FLX is 10.0530.Therefore,the substance exists mainly in its neutral form at pH=11,which is more stable at the time of radical attacking. 4 Conclusions

This study has shown the transient reactions of FLX with different radicals in pulse radiolysis,and the degradation efficiencies of FLX by electron beam irradiation under different conditions. The·OH radical,solvated electrons,and sulfate radical anions quickly reacted with FLX with the rate constants of 7.8×109, 2.3×109,and 1.1×109mol·L－1·s－1,respectively.The experiments illustrated that the degradation of FLX was occurred both by oxidative and reducing radicals,and the oxidative radicals tend to be more efficient for the decomposition of FLX.Based on the results obtained in this study,we thought that hydroxylated adduct was formed by hydroxyl radical attacking the aromatic ring directly.While it was found that SO4·－reaction preferentially formed a benzene radial cation by single electron oxidation,the intermediates were further transformed into the·OH adduct by reacting with H2O.

For the steady study,over 90%FLX degraded with an absorbed dose of 1.5 kGy both in the presence of oxygen(·OH+O2·－/HO2· reaction)and in its absence(·OH reaction).In comparing different oxidants,it was observed that the degradation rates of FLX with·OH were higher than that with SO4·－radical.It is possible that the yield of SO4·－radical reacted with FLX was not as much as the yield of·OH,and·OH adduct was more efficient for the ring opening reaction of FLX.Therefore,radiolytic degradation is likely an effective way of eliminating FLX in aqueous solution. And it is also recommended that the radiolytic degradation of FLX molecule was performed by·OH-induced reaction at a neutral condition.

Acknowledgment: The authors gratefully thank the Shanghai Institute of Applied Physics,Chinese Academy of Sciences and the University of Shanghai.References

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JI Tian-Yi1,2LIU Yan-Cheng2ZHAO Jian-Feng2,3XU Gang1WANG Wen-Feng2,*WU Ming-Hong1,*
(1School of Environment and Chemical Engineering,Shanghai University,Shanghai 200444,P.R.China;2Shanghai Institute of Applied Physics,Chinese Academy of Sciences,Shanghai 201800,P.R.China;3University of Chinese Academy of Sciences,Beijing 100049,P.R.China)

The reactions of the pharmaceutical fluoxetine(FLX)with different radicals were investigated by pulse radiolysis.The reaction of hydroxyl radical(·OH)with FLX formed hydroxylated adduct of the aromatic ring,while oxidation of FLX by sulfate radical anion(SO4·－)formed benzene radical cation that further reacted with H2O to yield the·OH adduct.The determined rate constants of·OH,hydrated electrons(e－aq),and SO4·－with FLX were 7.8×109,2.3×109,and 1.1×109mol·L－1·s－1,respectively.In the steady-state radiolysis study, the degradation of FLX in different radiolytic conditions by electron beam irradiation was detected by HPLC and UV-Vis spectra techniques.It was found that FLX concentration decreased by more than 90%in both N2O and air-saturated solutions after 1.5 kGy irradiation.In contrast,only 43%of FLX was decomposed in N2-saturated solution containing 0.1 mol·L－1tert-butanol.The degradation rates of FLX in acidic and neutral solutions were higher than those in alkaline solutions.Our results showed that the degradation of FLX is optimal in air-saturated neutral solution,and·OH-induced degradation is more efficient than SO4·－oxidation of FLX.The obtained kinetic data and optimal conditions give some hints to understand the degradation of FLX.

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*Corresponding authors.WANG Wen-Feng,Email:wangwenfeng@sinap.ac.cn.WU Ming-Hong,Email:mhwu@shu.edu.cn.国家自然科学基金(21173252,41430644,11675098)资助项目

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