Journal of Environmental Sciences21(2009)285–290Abiotic degradation of four phthalic acid esters in aqueous phase undernatural sunlight irradiationRuttapol Lertsirisopon,Satoshi Soda,Kazunari Sei,Michihiko Ike∗Division of Sustainable Energy and Environmental Engineering,Graduate School of Engineering,Osaka University,2-1Yamada-oka,Suita565-0871,Osaka,Japan.E-mail:ike@see.eng.osaka-u.ac.jpReceived24April2008;revised02June2008;accepted17June2008AbstractAbiotic degradability of four phthalic acid esters(PAEs)in the aquatic phase was evaluated over a wide pH range5–9.The PAE solutions in glass test tubes were placed either in the dark and under the natural sunlight irradiation for evaluating the degradation rate via hydrolysis or photolysis plus hydrolysis,respectively,at ambient temperature for140d from autumn to winter in Osaka,Japan. The efficiency of abiotic degradation of the PAEs with relatively short alkyl chains,such as butylbenzyl phthalate(BBP)and di-n-butyl phthalate(DBP),at neutral pH was significantly lower than that in the acidic or alkaline condition.Photolysis was considered to contribute mainly to the total abiotic degradation at all pH.Neither hydrolysis nor photolysis of di-ethylhexyl phthalate(DEHP) proceeded significantly at any pH,especially hydrolysis at neutral pH was negligible.On the other hand,the degradation rate of di-isononyl phthalate(DINP)catalyzed mainly by photolysis was much higher than those of the other PAEs,and was almost completely removed during the experimental period at pH5and9.As a whole,according to the half-life(t1/2)obtained in the experiments,the abiotic degradability of the PAEs was in the sequence:DINP(32–140d)>DBP(50–360d),BBP(58–480d)>DEHP(390–1600d) under sunlight irradiation(via photolysis plus hydrolysis).Although the abiotic degradation rates for BBP,DBP,and DEHP are much lower than the biodegradation rates reported,the photolysis rate for DINP is comparable to its biodegradation rate in the acidic or alkaline condition.Key words:phthalic acid esters;abiotic degradation;photolysis;hydrolysis;first-order kineticsDOI:10.1016/S1001-0742(08)62265-2IntroductionIn recent years,considerable attention has been paid totoxicity and degradability of phthalic acid esters(PAEs)(Staples et al.,1997),which have been frequently de-tected throughout aquatic environment(Fromme et al.,2002;Yuan et al.,2002).PAEs undergoing hydrolysis,photolysis,and aerobic/anaerobic biodegradation could beremoved from aquatic environment.Among those differentdegradation processes,relatively abundant data are avail-able on biodegradation of PAEs(Chang et al.,2007;Liet al.,2005;Yuan et al.,2002;Wang et al.,1996,2000).On the other hand,limited information has been knownon the abiotic processes(hydrolysis and photolysis).Thereare several researchers who have focused on photolysis ofPAEs using artificial irradiation sources,such as xenon arclamps(Bajt et al.,2001),mercury lamps(Mailhot et al.,2002),and ultraviolet light(Lau et al.,2005),to cleanup the PAEs-contaminated water.However,those irradi-ation sources had very different radiation intensities andwavelength distributions from natural sunlight irradiation.As a rare study for describing the abiotic degradation of*Corresponding author.E-mail:ike@see.eng.osaka-u.ac.jpPAEs under natural aquatic environment,Gledhill et al.(1980)have reported that butylbenzyl phthalate(BBP)exposing to sunlight irradiation for28d resulted in lessthan5%degradation,but limited information is availableto date.Wolfe et al.(1980)inferred that according to amathematical model data the photolysis is the primarydegradation process of PAEs in oligotrophic lakes.Forbetter understanding of the fate of PAEs in aquatic envi-ronment,there is a need to conduct more realistic studieson abiotic degradation with natural solar intensity.The aim of this study was to assess the contributionof abiotic degradation of PAEs under the sunlight irra-diation at ambient temperature over a wide pH rangethat normally found in natural aquatic environments.Fourcommercial PAEs(BBP,di-n-butyl phthalate(DBP),di-ethylhexyl phthalate(DEHP),and di-isononyl phthalate(DINP)),were subjected to the abiotic degradation tests,and the half-lives of the PAEs by hydrolysis and photolysiswere estimated by thefirst-order degradation kinetics.Therole of abiotic degradation in the fate of the PAEs wasdiscussed by comparing the estimated half-lives with thoseby aerobic/anaerobic biodegradation reported previously.286Ruttapol Lertsirisopon et al.V ol.211Materials and methods1.1Phthalic acid eatersAnalytical grade BBP(Tokyo Chemical,Japan),DBPand DEHP(Kishida Chemical,Osaka,Japan),and DINP(Wako,Osaka,Japan)were utilized in the abiotic degra-dation tests.BBP and DBP are suspected to be endocrinedisruptors(Jobling et al.,1995;Harris et al.,1997;Beres-ford et al.,2000).DEHP is a probable human carcinogen(class B)and BBP is a possible human carcinogen(class C)(USEPA,2000).DINP is one of the PAEs with the largestproduction rate as well as DEHP.1.2Abiotic degradation testsOutdoor experiments were carried out on the roof ofa building of Department of Environmental Engineering,Graduate School of Engineering,Osaka University,Japan(34◦N,135◦E)for140d from September2004to March2005.All the selected PAEs were prepared at total organiccarbon(TOC)concentration of100g/L in artificial riverwater(g/L deionized water)(K2HPO421.8,KH2PO48.5,Na2HPO444.6,NH4Cl1.7,MgSO4·7H2O22.5,CaCl2 27.5,and FeSO4·6H2O0.25).The concentration of BBP, DBP,DEHP,and DINP at100g TOC/L corresponds to0.44,0.52,0.35,and0.32mmol/L,respectively.Sincesolubility of the PAEs is extremely low,the solutionswere treated by an ultrasonicator(TOMY UD201,Seiko,Japan)with output20kHz and stirred for several hoursfor homogenization.The pH value was adjusted by HClor NaOH to designed pH value(5.0,6.0,7.0,8.0,and9.0).A30-mL of the solution was placed in a50-mLpyrex glass test tube(IWAKI,Chiba,Japan)in duplicate.All tubes were sealed tightly with a rubber stopper.Thetest tube was in2-mm thick and90%–100%transparentfor solar radiation.All test tubes were divided into twogroups,one was exposed to the sunlight irradiation and theother was covered with aluminum foil to keep in the darkat the same place.The radiation energy and the ambienttemperature were measured by a pyranometer EKO MS-802(EKO Instruments,Japan).1.3Analytical proceduresA0.75-mL was periodically sampled from the testtube and mixed with0.75mL acetonitrile to extract thePAE,followed by centrifugation(15000×g)to obtainsupernatant.The0.75mL of supernatant was used for thePAE analysis by high performance liquid chromatography(HPLC).The HPLC apparatus consisted of two LC-10ADvp solvent delivery pump with a DGU-14A solventcontroller and a SPd−10Avp UV-Vis spectrophotometricdetector connected to an advanced computer interface forthe analyses by a chromatography workstation(LCsolu-tion,ver.1.02J,Shimadzu,Japan).Samples were injectedvia a SIL-10AF autosampler into Shim-pack VP-ODScolumn(150mm×4.6mm,i.d.5μm;Shimadzu,Kyoto)with the mobile phase of acetonitrile and water mixture(90:10,V/V)at aflow rate of0.5mL/min.The PAEs weredetected by the UV detector at a wavelength of254nm.The errors in the PAEs measurements were less than5%. 2Results2.1Daily changes of radiation energy and ambienttemperatureThe daily average radiation and ambient temperature widely varied from17.1to242.8W/m2(107.9W/m2by mean)and0.4to27.4°C(10.8°C by mean)(Fig.1).These conditions can reflect a moderate autumn and winter in Japan in the temperate zone.2.2Abiotic degradationThe results of the abiotic degradation tests of the PAEs at different initial pH values are shown in Fig.2.The pH value of the mixture did not change significantly during the experimental period.The PAE concentrations in the dark were affected only by hydrolysis,while those under the sunlight irradiation were affected by both hydrolysis and photolysis.Degradation of all the PAEs proceeded slowly in the dark,and more than80%of the initial concentration remained after140d at all pH,especially degradation of DEHP and DINP in the dark was negligible at neutral pH. On the other hand,BBP,DBP,and DINP demonstrated the relatively high abiotic degradation rate under the sunlight irradiation at acidic and alkaline pH values compared with those under the dark condition,suggesting the higher photolysis rate than the hydrolysis rate of the three PAEs. DINP was effectively degraded under sunlight irradiation even at neutral pH,where more than50%was removed. BBP and DBP were degraded by about20%under same condition.On the other hand,DEHP degradation proceed-ed slowly even under sunlight irradiation at any pH,and more than80%of its initial concentration remained even after140d.Any distinctive intermediate could not be detected under the experimental condition.2.3Kinetic analysisAlthough there was a large variation in solar radia-tion and temperature during the experimental period,the abiotic degradation curves could be well described by thefirst-order kinetic model compared to the zero-order kinetic that could also describe the abiotic degradation under dark condition.Thefirst-order kinetic parameters(k) and half-lives(t1/2)are estimated and summarized in Table 1,and the simulated degradation curves are described in Fig.2.The k values and half-lives determined for degradation under sunlight irradiation represent typical values for the abiotic PAEs degradation under moderate or calm climates such as autumn or winter in Osaka,Japan. According to the kinetic analysis,abiotic degradation of the four PAEs would be ranked by half-lives as:BBP(390–1500d) DBP(430–1300d)>DINP(>460d) DEHP (>830d)in the dark and DINP(32–140d)>DBP(50–360 d),BBP(58–480d)>DEHP(390–1600d)under sunlight irradiation.No.3Abiotic degradation of four phthalic acid esters in aqueous phase under natural sunlight irradiation287Fig.1Daily average radiation and ambient temperature at Osaka from September 2004to March 2005.Experiments were beaked by typhoons (A)and equipment maintenance(B).Fig.2Degradation of the PAEs in sunlight irradiation (upper panels)and in the dark (lower panels)at pH 5,6,7,8,and 9.The regression lines by first-order kinetics are shown.DBP:di-n -butyl phthalate;BBP:butylbenzyl phthalate;DEHP:di-ethylhexyl phthalate;DINP:di-isononyl phthalate.3DiscussionThis research aimed at assessing the contribution of abiotic degradation of the PAEs to total degradation in aquatic environment.The abiotic degradation tests were carried out for selected PAEs at ambient temperature in the dark and under natural sunlight irradiation.Degradation observed in the former test refers to hydrolysis solely,while that in the latter test to hydrolysis plus photolysis,that is total abitotic degradation.The half-lives of the PAEs by abiotic degradation under ambient conditions which have been reported previously (Table 2).Our resultsconsist with the previous data that PAEs are hydrolyzed at negligible rates and photolysis is minor degradation in aquatic environment at neutral pH.Fig.3illustrates the contribution of hydrolysis and photolysis and pH to total abiotic degradation of each PAE during 140d experiment.It could be said,as a whole,that photolysis is the main process for abiotic PAE degradation and the acid-and alkali-catalyzed photo-hydrolysis processes enhance abiotic degradation considerably.The e ffect of pH on the abiotic degradation rate was rarely reported in aquatic u et al .(2005)reported that the rates of DBP degradation were high at288Ruttapol Lertsirisopon et al.V ol.21Table 1Kinetic parameters for abiotic degradation of the PAEs under sunlight irradiation and dark conditionsCondition pHDBPBBPDEHPDINPk(d −1)t 1/2(d)k (d −1)t 1/2(d)k (d −1)t 1/2(d)k (d −1)t 1/2(d)Sunlight5 1.4×10−250 1.2×10−258 1.8×10−3390 2.1×10−2326 1.0×10−2669.3×10−375 1.3×10−3550 1.3×10−2527 1.9×10−3360 1.5×10−3480 4.4×10−41600 4.9×10−314087.3×10−394 6.9×10−31009.9×10−4700 1.1×10−2619 1.2×10−257 1.0×10−268 1.5×10−3460 1.9×10−236Dark5 1.4×10−3510 1.6×10−34408.2×10−48409.6×10−47206 1.1×10−3620 1.2×10−3600 5.6×10−41300 5.9×10−412007 5.3×10−41300 4.6×10−415000–0–8 1.3×10−3530 1.5×10−3480 6.8×10−41000 6.9×10−4100091.6×10−34301.8×10−33908.4×10−48301.5×10−4460All the regression resulted in r 2>0.99.Table 2Abiotic degradation half-life of the PAEs (d)DBPBBP DEHP DINP ReferenceHydrolysis 3650–730000–Wolfe et al .,1980a –>100––Gledhill et al .,1980bPhotolysis880–4450–73–550–Wolfe et al .,1980;Howard 1991c –>100––Gledhill et al .,1980d–:No data;a based upon overall rate constant at pH 7and 30°C;b aqueous solutions under dark conditions for 28d;c scientific judgment based upon estimated rate data for alkylperoxy radicals in aqueous solution;d Sunlight exposure in aqueous solutions for 28d.Fig.3Contribution of hydrolysis and photolysis on abiotic degradation of the PAEs under di fferent initial pH after 140d tested period.both the highest pH value (pH =10)and lowest pH (pH =3)under UV irradiation at the initial concentration of 600g TOC /L for 60min.According to the degradation model of DBP (Fig.4),the major degradation pathway of DBP involves the hydrolytic photolysis of the carbon in the α-and /or β-position of the ester chain with the production of aromatic carboxylic derivatives (Lau et al .,2005).As the pH is elevated to the alkali range,a simpleNo.3Abiotic degradation of four phthalic acid esters in aqueous phase under natural sunlight irradiation289Fig.4Model of degradation mechanism of DBP photolysis at different pH(Lau et al.,2005).Six different pathways are marked by numbers.The mechanisms are divided into3major pathways depending on the pH level.Aαand Aβ:acidic catalysis at pH3;Hβ:hydrolysis;H butyl:oxidation/reduction of the butylchain at pH5;Bα:basic catalysis when pH>>7.cleavage dominates the degradation process through a photochemical homolyric decarboxylation.Additionally, multi-degradation pathways explain the fast acid-catalyzed hydrolytic photolysi.This model possibly explains the results of abiotic degradation of the PAEs.However,both hydrolysis and photolysis were not effective for DEHP at acidic/alkaline pH,i.e.,abiotic degradability of DEHP is low at ordinal conditions like in the ranges of solar radiation,temperature and pH tested here.The other three PAEs could be readily degraded under acidic and alkaline conditions mainly via photolysis.DINP could be efficiently degraded even at neutral pH.In contrast, the abiotic degradation of BBP and DBP was significant-ly ineffective compared with that under acidic/alkaline conditions.Therefore,abiotic degradation of DINP might proceed readily regardless of pH when sunlight irradiation is available.It could partly explain that there is no apparent contamination of aquatic environment by DINP in contrast to frequent detection of DEHP,even though DINP is one of the PAEs with the largest production as well as DEHP. The detection,identification,and toxicity evaluation of the intermediates are needed for further studies,although no distinctive intermediate in abiotic degradation of the PAEs could be detected in current experimental conditions.In order to discuss the abiotic degradation contributions to the fate or disappearance of the PAEs in aquatic envi-ronment,the half-lives of PAEs by biodegradation which have been reported previously(Table3)were used for comparison with those determined in this study(Table1). Since there have been rarely reported on the biodegra-dation half-lives under acidic/alkaline condition,the data only determined at neutral pH are listed in Table3.Under the aerobic condition,BBP and DBP can be biodegraded by half-lives of a few days,whereas,more than weeks or months are required for DEHP and DINP.The half-lives in the anaerobic condition are2–10times longer than those in aerobic conditions.Photolysis and aerobic degradation occurs simultaneously in the surface layer of aquatic environment.The biodegradation half-lives of the PAEs are much shorter than abiotic degradation at neutral pH,suggesting that biodegradation would be the main process to remove the PAEs from aquatic environment when the aqueous phase is neither acidic nor alkaline. However,low or high pH could lower the microbial activities.Lertsirisopon et al.(2003)reported that the half-lives of BBP,DBP,DEHP and DINP increased about20% under aerobic condition(pH5–9).Chang et al.(2005) have also reported that about15%increase in the half-290Ruttapol Lertsirisopon et al.V ol.21Table3Biodegradation half-life of the PAEs in aquatic systems(d)Condition DBP BBP DEHP DINP ReferenceAerobic0.5–10.10.5–10.57.3–27.5Yuan et al.,2002a 1–32–42–5Fujita et al.,2005b2–54–116–217–40Lertsirisopon et al.,2003c Anaerobic8–1414–33>1300Painter and Jones,1990d11.7–18.99.9–25.529.9–39.1Yuan et al.,2002e2–32–3>208>347Lertsirisopon et al.,2006f a Unacclimated river sediment samples at pH7.0;b unacclimated river,pond,or activated sludge samples at pH7.2,and28°C;c unacclimated pond water samples at28°C;d unacclimated sludge at30°C;e unacclimated river sediment microcosms at pH7and30°C;f natural sediment microcosms at pH7and28°C.lives of BBP and DEHP by anaerobic degradation wasobserved at low or high pH(pH5–9).In contrast,ourstudy showed that abiotic degradability of the PAEs wasconsiderably enhanced under the acidic and alkaline condi-tions.Therefore,the contribution of abiotic degradation todisappearance of the PAEs will increase when pH becomesacidic or alkaline.Even so,the disappearance of BBP,DBP and DEHP might highly depend on biodegradationunless pH becomes extremely acidic/alkaline judging fromcomparison of the half-lives.However,abiotic degradation(photolysis)would play a significant role in removal ofDINP in acidic(pH<6)or alkaline(pH>8)conditionsinstead of biodegradation.4ConclusionsThe abiotic degradation tests under the sunlight ir-radiation over a wide pH range were carried out toestimate the contribution of hydrolysis and photolysis todegradation of the PAEs in aquatic environment.Theexperiments gavefindings that abiotic degradation ofthe PAEs occurs mainly by photolysis under acidic andalkaline conditions rather than neutral pH,as a wholetrend.According to thefirst-order kinetics analysis,theabiotic degradability(hydrolysis plus photolysis)of thePAEs in aquatic environment would be ranked as DINP >DBP,BBP>DEHP.Although the abiotic degradation rate for BBP,DBP,and DEHP is considered much smallerthan their biodegradation rate when pH is not extremelyacidic/alkaline,the photolysis rate for DINP is comparablewith its biodegradation rate in aquatic environment.DINPin aquatic environment would be degraded mainly byphotolysis in both acidic and alkaline conditions but bymicroorganisms at neutral pH.ReferencesBajt O,Mailhot G,Bolte M,2001.Degradation of dibutyl phthalate by homogeneous photocatalysis with Fe(III)in aqueous solution.Applied Catalysis B:Environmental,33:239–248.Beresford N,Routledge E J,Harris C A,Sumper J P,2000.Issues arising when interpreting results from an in vitro assay for estrogenic activity.Toxicology and Applied Pharmacology,162:22–33. 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