Radiolytic degradation of 4-nitrophenol in aqueous solutions_ Pulse and steady state radiolysis
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Radiolytic degradation of4-nitrophenol in aqueous solutions:Pulse andsteady state radiolysis studyJayashree Biswal a,n,Jhimli Paul b,D.B.Naik c,S.K.Sarkar c,S.Sabharwal da Isotope Applications Division,Bhabha Atomic Research Centre,Trombay,Mumbai400085,Indiab Radiation Technology Development Division,Bhabha Atomic Research Centre,Trombay,Mumbai400085,Indiac Radiation and Photochemistry Division,Bhabha Atomic Research Centre,Trombay,Mumbai400085,Indiad Industrial Applications and Chemistry Section,Division of Physical and Chemical Sciences,Department of Nuclear Sciences and Applications,International Atomic Energy Agency, Wagramer Str.5,A-1400Vienna,AustriaH I G H L I G H T Sc98%degradation of100micro molar4-NP has been achieved by4.4k Gy radiation dose.c Radical anion of4-nitrophenol was found to be reducing in nature.c The degradation of4-NP was found to be more effective in presence of oxygen.a r t i c l e i n f oArticle history:Received4September2012Accepted2January2013Available online11January2013Keywords:4-nitrophenolPulse radiolysisGamma radiolysisDegradationa b s t r a c tThe radiation induced degradation of4-nitrophenol(4-NP)has been studied by gamma irradiation,while the reactivity and spectral features of the short lived transients formed by reaction with primarytransient radicals at different pHs has been investigated by pulse radiolysis technique.In steady stateradiolysis a dose of4.4k Gy is able to degrade98%of1Â10À4mol dmÀ34-NP.4-NP has pK a at7.1,above which it is present in the anionic form.At pH5.2, OH and N3 radicals were found to react with4-NP with rate constants of 4.1Â109dm3molÀ1sÀ1and 2.8Â108dm3molÀ1sÀ1,respectively.Differences in the absorption spectra of species formed in the reactions of4-NP with OH and N3radicals suggested that OH radicals add to the aromatic ring of4-NP along with electron transferreaction,whereas N3 radicals undergo only electron transfer reaction.At pH9.2,rate constants for thereaction of OH radicals with4-NP was found to be higher by a factor of2compared to that at pH5.2.This has been assigned to the deprotonation of4-NP at pH9.2.&2013Elsevier Ltd.All rights reserved.1.IntroductionNitrophenols are listed as major water pollutants.These areinvolved in the synthesis of many industrial products and appearin the degradation of pesticides like parathion(Gal et al.,1992)and nitrofen(Nakagawa and Crosby,1774).The purification ofwastewaters contaminated with these pollutants is difficult sincethey are resistant to the traditional water treatment techniques.Long periods of incubation are required for the microbial degra-dation of4-nitrophenol(4-NP)(Zaidi et al.,1996).Advancedoxidation processes(AOP)have recently appeared as a remark-able technique for accelerating the oxidation and degradation of abroad range of organic matter in wastewater(Wojnarovits andTakacs,2009;Alkdasi et al.,2004;Pera-Titus et al.,2004;Andreozzi et al.,1999).This process involves the in situ genera-tion of extremely strong oxidants such as hydroxyl radicals inaqueous solution by different techniques.Different methods forAOPs include ozonation(Yu and Yu,2001;Kuosa et al.,2007),Fenton reaction(Oturan et al.,2000;Du et al.,2007)electro-chemistry(Canizares et al.,2004),electrocatalysis(Shen et al.,2008),photocatalysis(Vargas and Nunez,2009),ultraviolet irra-diation(Zhang et al.,2003;Einschlag et al.,2009),sonolysis(Kotronarou et al.,1991),microwave(Bo et al.,2008),electronbeam irradiation(Follut and Leitner,2007)and gamma irradiation(Chmielewski and Haji-Saeid,2004).Among these methods,ionizing radiation appears to be one promising alternative forhazardous organic wastewater treatment.High energy radiationinduced degradation of organic compounds is an environmentallyfriendly process for decontamination of water and soil(Wojnarovits and Takacs,2008;Tolgyessy,1990;Mezyk et al.,Contents lists available at SciVerse ScienceDirectjournal homepage:/locate/radphyschemRadiation Physics and Chemistry0969-806X/$-see front matter&2013Elsevier Ltd.All rights reserved./10.1016/j.radphyschem.2013.01.003n Corresponding author.Tel.:þ912225592548;fax:þ912225505151.E-mail address:jbiswal@.in(J.Biswal).Radiation Physics and Chemistry85(2013)161–1662004;Popov and Getoff,2005;Hu et al.,2006).Aqueous solution when irradiated with high energy radiation,major part of the energy is absorbed by water.This leads to the generation of reactive transient radicals by water radiolysis.In the pH range of 6–8.5,radiolysis of air-free water leads to the formation of a different radical species (Eq.(1))H 2O *g 0:27Þe aq Àþð0:06ÀÁH þ0:27ðÞ OH þð0:04ÞH 2þð0:07ÞH 2O 2þ0:27ðÞH 3O þð1ÞThe numbers in brackets (G values)indicate the number ofmicromoles of each species formed per one Joule of absorbedenergy.The reactive species are H , OH and e aq Àin nitrogenpurged aqueous solution and OH,HO 2 ,O 2 Àin the presence of oxygen.These radicals interact with organic molecules present in the aqueous solution leading to degradation.Hence the effect of high energy radiation on dissolved organic molecule is indirect.The present work deals with the steady state and pulse radiolysis study of degradation of 4-nitrophenol at different pHs and under different ambient conditions.The reactions of different transient radicals produced by water radiolysis have been studied by pulse radiolysis technique.2.Materials and methods 2.1.ReagentsAqueous solutions were prepared using nanopure water (Resistivity ¼18M O cm).4-nitrophenol (499%)was obtained from Aldrich and all other chemicals used in the present study were of highest purity and were used as received.pH of the solutions was adjusted using borate and phosphate buffers or by adding plain NaOH and perchloric acid in appropriate concentra-tions.Prior to use,glassware was cleaned with aquaregia (volume ratio HNO 3/HCl ¼1:3)and thoroughly rinsed with nanopure water.High pure N 2,O 2and N 2O gases were used for saturating the reaction mixture.2.2.Irradiation and instrumentationGamma irradiations were carried out in the 60Co gamma chamber having dose rate of 2.2k Gy h À1determined using Fricke dosimetry (McLaughlin et al.,1980).Absorption spectra were recorded on a Thermoelectron-Evolution-300recording spectro-photometer.Pulse radiolysis experiments were carried out by using linear electron accelerator (LINAC)having 7MeV energy at Radiation &Photochemistry Division of Bhabha Atomic Research Centre with pulse duration of 500ns and 2m s (Guha et al.,1987).The dose delivered by the pulse was determined by using 0.01mol dm À3potassium thiocyanate dosimeter solution for G.e value 2.6Â10À4m 2J À1at 475nm (Buxton and Stuart,1995).3.Results and discussionOrganic free radicals often exhibit acid–base properties so that their reactivity and absorption properties change with pH.The quantitative behavior of acidic and basic form of an organic compound in solution can be understood only if their pK a values are known.4-nitrophenol undergoes deprotonation at higher pH to give corresponding phenoxide ion.3.1.pK a measurementAbsorption spectra of aqueous solutions of 1Â10À4mol dm À34-NP were recorded at different pHs.The pK a was determined fromthe inflexion point of the plot of absorbance at 400nm vs.pH and was found to be 7.1.This pK a value is for acid dissociation at the phenolic OH group of 4-NP and it is a reference point to obtain completely protonated or deprotonated 4-NP at different pH.3.2.Pulse radiolysis studiesPulse radiolysis experiments were carried out by exposing aqueous 4-NP solution to electron beam (EB)from LINAC.The aim of doing pulse radiolysis is to follow the kinetics of reaction of different radicals with 4-NP by observing the transient absorptionspectra.The rate constants for the reactions of e aq Àwith 4-NP weredetermined by following the decay of e aq Àabsorption and the rate constants for reactions of OH ,N 3 ,2-hydroxy propyl radicals with 4-NP were determined by following the built up of transient absorption,respectively,(Table 1).Absorption spectra of the transient formed in these reactions were recorded and also corrected for the depletion of the parent molecule (4-NP).3.2.1.Reaction of 4-nitrophenol with OH radicalsThe reaction between the hydroxyl radicals ( OH)with 4-NP was investigated by irradiating 1Â10À4mol dm À3aqueous 4-NPsolution saturated with N 2O,so that N 2O converts e aq Àto OH radicals (Eq.(2)).e aq ÀþN 2O þH 2O - OH þOH ÀþN 2(2)The rate constants were determined at two different pH values,viz.5.2and 9.2.Since the H concentration is only $10%of total radical concentration in N 2O saturated solution, OH is the main reacting species in this system.4-NP has absorption up to 480nm in the pH range 4–11.For this reason,observed spectrum of the transient was corrected for parent absorption using the Eq.(3),e T ¼e P þOD T G T ÂG e SCN 2OD SCN 2 !ð3Þwhere,e T and e P are the molar extinction coefficients of thetransient and of parent,respectively,OD T and G T are the absor-bance and the yield of transient species,respectively.The corrected absorption spectra of the transient species formed after irradiation are shown in Figs.1and 2for pH 5.2and 9.2,respectively,and the corresponding observed absorp-tion spectra are given in Fig.1a and Fig.2(inset),respectively.At pH 5.2the spectrum shows strong absorption at 290nm and at pH 9.2the spectrum shows two prominent bands at 300nm and 400nm.In principle OH radicals can undergo two types of reactions with 4-NP.1)It can add to the phenyl ring of 4-NP leading to the formation of p complex,which can subsequently transform to hydroxyl cyclohexadienyl radical or it can dissociate.2)The other way is to cause one electron oxidation of 4-NP.Table 1Characteristics of the transient species formed on reaction of 4-nitrophenol with different transient species.RadicalspH k max (nm)k (formation)(dm 3mol À1s À1)OH5.22904.1Â1099.2300,4008.7Â109e aqÀ 5.2310 2.8Â10109.24002Â1010N 35.23202.7Â1089.2300,400 6.5Â109(CH 3)2 CO 5.2320 1.3Â109MV 2þ5.26055.3Â108J.Biswal et al./Radiation Physics and Chemistry 85(2013)161–166162It is reported that OH addition to phenyl ring results in formation of hydroxy cyclohexadienyl type of radical (Wojnarovits and Takacs,2008;Alfassi and Schuler,1985;Terzian et al.,1995),which has an absorption maximum at $420nm (Cercek and Ebert,1968;Linde,1977).But in the case of 4-NP at pH 5.2no such absorption band was observed at this wavelength.Further,Cyclohex-adienyl radicals are known to react by a second order process (Terzian et al.,1995;Cercek and Ebert,1968),whereas transient produced in the reaction of 4-NP with OH decayed by a first order process having rate constant 4Â103s À1.Therefore based on the above two reasons formation of cyclohexadienyl type of radical is excluded in case of 4-NP.At pH 5.2one electron oxidation of 4-NP byOH may be hindered due to protonation of 4-NP.Linde proposed that in case of 4-nitroaniline the reaction between aromatic ring of 4-nitroaniline with OH leads to the formation of a p complex,which subsequently decays to produce aminophenoxyl type of radical (Linde,1977).Hence it is suggested that the reaction of OH with the aromatic ring of 4-NP results in formation of a p complex shown in the Scheme 1,which may absorb at 290nm.Afterwards the transient may decay by a first order process to give semiquinone radical (Eiben and Fessenden,1971)absorbing at 400–500nm which might have appeared at much higher time scale.One electronoxidation by OH is more favorable in case of phenolate anion giving an oxidized species having absorption peak at 400nm (Roder et al.,1999)at pH 9.2.Along with this,the p complex is also formed by the reaction of 4-NP with OH (Scheme 2),which has absorption at 300nm.The formation of one electron oxidized species,which absorbs at 400nm is confirmed later by the reaction of N 3 with 4-NP discussed in the Section 3.2.3.The pseudo first order rate constants (k f )for the reaction of hydroxyl radical with 4-NP was determined from the formation signal of the transient species in the presence of different concentrations of 4-NP and these were plotted against concentrations of 4-NP as shown in Fig.1b (inset)for pH 5.2.The bimolecular rate constant was obtained from the slope of the plot of k F vs.concentrations and the value were found to be 4.1Â109and 8.7Â109dm 3mol À1s À1for pH 5.2and 9.2,respectively,(Table 1).3.2.2.Reaction of 4-nitrophenol with hydrated electronThe primary radicals produced by radiolysis of water can be selectively scavenged by certain solutes to allow measurement of the rate constant of a single radical species with the substrate.The reaction between 4-NP and e aq Àwas investigated in N 2saturated solution of 1Â10À4mol dm À34-NP containing 0.2mol dm À3tert -butanol.Tert -butanol converts OH to less reactive CH 2(CH 3)2COH by Eq.(4).(CH3)3COH þ OH - CH 2(CH 3)2COH þH 2O(4)The corrected absorption spectrum of the semi-reduced species formed after irradiation is shown in Fig.3and 4at pH 5.2and 9.2,respectively.The corresponding transient absorption peaks were observed at 310nm and 400nm.At pH 5.2hydratedelectrons react with 4-NP producing radical anion OHC 6H 4NO 2 À,which has absorption at 310nm.At pH 9.2the absorption maximum of the radical anion formed in the reaction between hydrated electron and 4-NP absorbs at 400nm.The bimolecular rate constants of reaction (5)were found to be 2.8Â1010and 2Â1010dm 3mol À1s À1at pH 5.2,9.2,respectively.A small decrease in the value of the rate constant of the reaction of 4-NP with e aq Àmay be attributed due to the columbic repulsion between negative charges on the 4-NP and that on the hydrated electron ****at pH 9.2.4-NP þe aq À-4-NP À-k 2-Productð5Þ3.2.3.Reaction of 4-NP with N 3 radicalTo investigate the reaction of N 3 radicals with 4-NP pulse radiolysis experiments were carried out with N 2O purged aqueous solution of 2Â10À4mol dm À34-NP containing 2Â10À2mol dm À3sodium azide (NaN 3).N 2O converts hydrated electrons to hydroxyl300400500600700800200040006000800010000t r a n s i e n t e x t i n c t i o n c o e f f i c i e n t (d m 3 m o l -1 c m -1)wavelength(nm)Fig.1.Corrected absorption spectrum of transient formed by reaction of OH radicals with 4-nitrophenol at pH 5.2,Inset (a):Observed absorption spectrum,(b):Plot of k exp ,monitored at 420nm vs.4-nitrophenol concentration.300400500600700800200040006000800010000T r a n s i e n t e x t i n c t i o n c o e f f i c i e n t (d m 3 m o l -1 c m -1)Wavelength(nm)Fig.2.Corrected absorption spectrum of transient formed by reaction of OH radicals with 4-nitrophenol at pH 9.2,Inset:Observed absorption spectrum.Scheme 1.OH reaction with 4-NP at pH 5.2(pie complex formation).Scheme 2.OH reaction with 4-NP at pH 9.2(one electron oxidation).J.Biswal et al./Radiation Physics and Chemistry 85(2013)161–166163radicals by reaction (2)and reactions of azide ions (N 3À)withhydroxyl radicals give N 3radicals by reaction (6)OH þN 3À-OH ÀþN 3(6)The corrected absorption spectra of the transient species formed after irradiation are shown in Fig.5and 6at pH 5.2and 9.2,respectively.The spectra show strong absorption peak at 320nm at pH 5.2and at 300nm and 400nm at pH 9.2.N 3 radicals are known to react with phenolic compounds by one electron oxidation reaction.At pH 5.2,the observed absorption peak at 300nm might be due to the oxidation product,4-nitro phenoxyl radical formed in the reaction of 4-NP with N 3 radicals (Scheme 3).The bimolecular rate constant was found to be 2.7Â108dm 3mol À1s À1(Table 1).Differences in the absorption spectra of species formed in the reactions of 4-NP with OH andN 3 radicals at pH 5.2is because ofOH radicals add to the aromatic ring of 4-NP,whereas N 3radicals reacts by one electron oxidation.At pH 9.2deprotonated 4-NP is easily get oxidized by one electron oxidation,which is also reflected in the rate constant obtained between the reaction of N 3 radicals with 4-NP.The bimolecular rate constant for this reaction was found to be 6.5Â109dm 3mol À1s À1(Table 1).At pH 9.2the peak positionof transient absorption peaks for reaction of 4-NP with OH radicals and N 3 radicals are similar,but the relative intensities are quite different.N 3 radicals bring about one electron oxidation of 4-NP at pH 9.2,whereas OH radicals react by electron transfer as well as addition reaction.The adduct has higher extinction coefficient at 300nm compared to 400nm,whereas the nitro phenoxyl radicals have higher extinction coefficient at 400nm compared to 300nm.3.2.4.Reaction of 4-NP 2-hydroxy propyl radicalTo study the reaction of 2-hydroxy propyl radicals with 4-NP the pulse radiolysis experiment was carried out in N 2O saturated200040006000800010000T r a n s i e n t e x t i n c t i o n c o e f f i c i e n t (d m 3 m o l -1 c m -1)wavelength(nm)Fig.3.Corrected absorption spectrum of transient formed by reaction of e aq Àwith 4-nitrophenol at pH 5.2.30040050060070080020004000600080001000012000140001600018000T r a n s i e n t e x t i n c t i o n c o e f f i c i e n t (d m 3 m o l -1 c m -1)Wavelength(nm)Fig.4.Corrected absorption spectrum of transient formed by reaction of e aq Àwith 4-nitrophenol at pH 9.2.200040006000800010000T r a n s i e n t e x t i n c t i o n c o e f f i c i e n t (d m 3 m o l -1 c m -1)Wavelength(nm)Fig. 5.Corrected absorption spectrum of transient formed by reaction of N 3 radicals with 4-nitrophenol at pH 5.2.T r a n s i e n t e x t i n c t i o n c o e f f i c i e n t (d m 3 m o l -1 c m -1)Wavelength(nm)Fig. 6.Corrected absorption spectrum of transient formed by reaction of N 3 radicals with 4-nitrophenol at pH 9.2.Scheme 3.N 3 reaction with 4-NP at pH 5.2(one electron oxidation).J.Biswal et al./Radiation Physics and Chemistry 85(2013)161–166164solution of5Â10À4mol dmÀ34-NP containing0.5mol dmÀ32-propanol at pH5.2.The H and OH radicals are scavenged by2-propanol(Eq.(7))giving2-hydroxy propyl radicals.(CH3)2CHOHþH / OH-(CH3)2 COHþH2/H2O(7) The corrected absorption spectrum of the transient species matches very well with that given in Fig.3for the anion radical formed in the reaction of4-NP with e aqÀ.Thus it can be concluded that2-hydroxy propyl radicals quantitatively transfer electron to 4-NP.The bimolecular rate constant was found to be 1.3Â109dm3molÀ1sÀ1(Table1).3.2.5.Reaction of transient radical anion of4-NP with methyl viologenExperiments were carried out to check the reducing nature of the electron adduct of4-NP(4-NPÀ)by studying the electron transfer reaction with an organic molecule having low reduction potential value.Methyl viologen(MV2þ)is a well-known oxidant having l max at605nm(e¼12,800dm3molÀ1cmÀ1)with reduc-tion potential value(E0)À0.45V vs.NHE.The transient species formed on pulse radiolysis of N2saturated aqueous4-NP solution containing0.5mol dmÀ3tert-butanol and2.8Â10À5mol dmÀ3 methyl viologen at pH5.2was observed to react with methyl viologen(Eq.(8)).The bimolecular rate constant was found to be5.3Â108dm3molÀ1sÀ1(Table1).Hence the reduction potential of4-NPÀis more negative thanÀ0.45V.4-NP ÀþMV2þ-MV þþ4-NP(8) 3.3.Steady state gamma radiolysisThe steady state degradation of aqueous solution of4-NP has been studied to know the radiation induced degradation behavior of4-NP in aerated solution,solution saturated with N2,O2,N2O,N2O:O2(1:1) and N2saturated with tert-butanol.1Â10À4mol dmÀ3aqueous 4-NP were irradiated with gamma rays at a dose rate2.2k Gy hÀ1 and uv-visible spectra of these irradiated solutions were recorded at different time intervals.The degradation of4-NP can be seen from the decrease in absorbance value of4-NP at317nm with increase in dose.This decrease in absorbance is in the entire wavelength range. Since there is no indication of development of any other spectral peak with increase in dose,hence there is no interference from formation of radiolytic products.The degradation of4-NP in aqueous solution is governed by the nature of reactive species present in the system. Initially the reaction of these radicals with4-NP results in formation of stable or unstable intermediates and ultimately causes complete degradation and mineralization of the compound.The degradation behavior of4-NP in different environment has been shown in Fig.7 and is discussed below.3.3.1.Degradation of4-nitrophenol in aerated/O2saturated solutionIn the presence of air/O2,H and e aqÀreact with dissolved oxygen so that the reactive radicals present in the system are HO2 ,O2 À(Eqs.(9)and(10))and OH radicals.H þO2-HO2 (9) e aqÀþO2-O2 À(10)The degradation of4-NP by these radicals as well as OH radicals at317nm is shown in Fig.7.97.6%and98.5%degradation has been observed by dose of4.4k Gy in air and O2saturated solution,respectively.Oxygen concentration in aerated and oxy-gen saturated aqueous solution are 2.4Â10À4mol dmÀ3and 1.2Â10À3mol dmÀ3,respectively.Slightly higher extent of degradation of4-NP in oxygen saturated system can be due to more4-NP–OH adduct react with O2giving peroxy species ultimately leading to degradation.3.3.2.Degradation of4-nitrophenol in deoxygenated solutionIn N2saturated solution,where dissolved oxygen is removed from the system the reactive species are e aqÀ,H and OH.The G value for H is lower as compared to e aqÀand OH radicals. Hence the reaction of4-NP with e aqÀand OH will be predominant. In N2saturated system84%degradation has been observed by dose of4.4k Gy.The percent degradation in this case is lower than the aerated condition.The reason for this may be the degradation of4-NP by e aqÀis less effective than the degradation by HO2 ,O2 À.3.3.3.Degradation of4-nitrophenol in deoxygenated solution containing t-butanolIn N2purged aqueous solution of4-NP containing tert-butanol the degradation of4-NP is caused by hydrated electron(Eq.(4)) (Buxton et al.,1988).Initially the degradation of4-NP is slower, but99.5%degradation is achievable at4.4k Gy dose.This value is higher than the degradation in any other case.The reason may be, though the reaction of e aqÀwith4-NP is comparatively slower,but the intermediate formed by this reaction under goes degradation more effectively.3.3.4.Degradation of4-nitrophenol in N2O saturated solutionDegradation of4-NP in N2O saturated solution is caused by OH radicals Eq.(2).More than95%degradation of4-NP has been achieved at dose of4.4k Gy.When aqueous4-NP solution was saturated with N2O:O2(1:1) and irradiated,the extent of degradation was found to be more than the above case,which suggests that initially formed4-NP–OH adduct reacts with O2leading to enhanced degradation.The overall extent of degradation of4-NP in different environ-ment is given by,N2saturated solution o aerated solution o N2O saturated solution o N2O:O2(1:1)saturated solution o O2satu-rated solution o N2saturated solution containing t-butanol.0.00.20.40.60.81.0AbsorbanceDose(kGy)Fig.7.Absorbance values of1Â10À4mol dmÀ3aqueous4-nitrophenol solution at317nm irradiated with different dose in different irradiation condition, (a)aerated,(b)N2,(c)O2,(d)N2O,(e)N2O:O2(1:1)purged and(f)N2purged and tert-butanol.J.Biswal et al./Radiation Physics and Chemistry85(2013)161–1661654.ConclusionsRadiolytic species formed in water were found to react quite fast with4-nitrophenol.Neutral4-nitrophenol on reaction with OH radicals gave adduct as well as one electron oxidized species.Radical anion of4-nitrophenol was found to be reducing in nature.The g-irradiation is an efficient and promising method for the degradation of4-NP in aqueous solutions.The degradation of4-NP was found to be more effective in presence of oxygen.AcknowledgmentsThe authors thank Dr.Gursharan Singh,Associate Director, Radiochemistry and Isotope Group,Bhabha Atomic Research Centre for his support.The authors also thank Dr.L.Varshney, Head,Radiation Technology Development Division,Bhabha Atomic Research Centre and Dr.H.J.Pant,Isotope Applications Division,Bhabha Atomic Research Centre for their helpful suggestions.ReferencesAlfassi,Z.B.,Schuler,R.H.,1985.Reaction of azide radicals with aromatic com-pounds.Azide as a selective oxidant.J.Phys.Chem.89,3359–3363. 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