Radiation Chemistry of Methyltert-Butyl Ether in Aqueous SolutionS T E P H E N P . M E Z Y K , * , † J A C E J O N E S , †

W I L L I A M J . C O O P E R , ‡ T H O M A S T O B I E N , ‡

M I C H A E L G . N I C K E L S E N , ‡

J . W E S L E Y A D A M S , ‡ K E V I N E . O ’ S H E A , §

D A V I D M . B A R T E L S , |

J A M E S F . W I S H A R T , ⊥

P A U L M . T O R N A T O R E , #

K I M B E R L E Y S . N E W M A N , #

K E L L I E G R E G O I R E , # A N DD A N I E L J . W E I D M A N [ , f

Department of Chemistry and Biochemistry, California StateUniversity at Long Beach, 1250 Bellflower Boulevard,Long Beach, California 90840, Department of Chemistry,University of North Carolina at Wilmington, 601 SouthCollege Road, Wilmington, North Carolina 28403, Departmentof Chemistry, Florida International University, Miami, Florida33199, Chemistry Division, Argonne National Laboratory,Argonne, Illinois 60439, Chemistry Department, BrookhavenNational Laboratory, Upton, New York 11973, Haley andAldrich of New York, 200 Town Centre Drive, Suite 2,Rochester, New York 14623, and Science Research Laboratory,Inc., 15 Ward Street, Somerville, Massachusetts 02143

The chemical kinetics of the free-radical-induceddegradation of the gasoline oxygenate methyl tert-butylether (MTBE) in water have been investigated. Rate constantsfor the reaction of MTBE with the hydroxyl radical,hydrated electron, and hydrogen atom were determinedin aqueous solution at room temperature, using electronpulse radiolysis and absorption spectroscopy (•OH and e-

aq)and EPR free induction decay attenuation (•H) measurements.The rate constant for hydroxyl radical reaction of (1.71( 0.02) × 109 M-1 s-1 showed that the oxidative processwas the dominant pathway, relative to MTBE reactionwith hydrogen atoms, (3.49 ( 0.06) × 106 M-1 s-1, or hydratedelectrons, <8.0 × 106 M-1 s-1. The hydroxyl radicalreaction gives a transient carbon-centered radical whichsubsequently reacts with dissolved oxygen to form peroxylradicals, the rate constant for this reaction was (2.17 (0.06) × 109 M-1 s-1. The second-order decay of the MTBEperoxyl radical was 2k ) (6.0 ( 0.3) × 108 M-1 s-1.These rate constants, along with preliminary MTBEdegradation product distribution measurements, wereincorporated into a kinetic model that compared the predictedMTBE removal from water against experimental measure-ments performed under large-scale electron beam treatmentconditions.

IntroductionThe use of high-octane oxygenated additives to gasoline hasincreased markedly since their introduction in the late 1970s(1). Methyl tert-butyl ether (MTBE) is the most commonlyused oxygenate in the United States (2, 3) comprising of upto 15 vol % of gasoline (4). While initial environmentalconcerns were over MTBE occurrence in the air due to vehicleexhaust (3), there is now an ever-increasing concern over itspresence in groundwaters. MTBE in groundwater has twomajor sources; leaking underground gasoline storage tanks(5) and transport from air (6). MTBE is extremely soluble inwater, with a maximum limit of ∼48 000 mg/L (∼0.54 M) (5),and thus there is practically no retardation of this chemical’smovement due to soil adsorption.

The presence of MTBE in groundwater poses a potentialhealth problem (7, 8). Buckley et al. (9) have shown thatMTBE can accumulate in the blood stream and can bedetected in breath. The documented effects of MTBEexposure are headaches, vomiting, diarrhea, fever, cough,muscle aches, sleepiness, disorientation, dizziness, and skinand eye irritation (8). MTBE is a suspect carcinogen; however,considerable additional work is necessary to better define itshealth effects. Another issue that may be a “driving force”for control of MTBE in drinking water is the organolepticsensitivity of MTBE. According to a recent study (10), humanscan smell MTBE at concentrations between 13.5 and 45.4µg/L; however, the lowest concentration known to have anadverse health effect on any organism is 145 µg/L (11). TheEPA suggested limit is 20-40 µg/L in drinking water (2, 11)and the California Department of Health Services has adopteda secondary maximum concentration level for this chemicalof 5 µg/L (12).

Potential remediation technologies for MTBE-contami-nated water have been reviewed in depth (2). Air strippingis not readily applicable due to the high MTBE solubility,and this ether only has a moderate affinity for adsorptiononto granulated activated carbon (13). MTBE is not readilybiodegraded (14) due to the presence of its tert-butyl group;however, some aerobic (15) and anaerobic (16) biodegrada-tion has been previously reported. Therefore, there have beena number of recent studies that investigated the use ofAdvanced Oxidation Technology (AOT) processes for thedestruction of MTBE in groundwater (5, 13, 17-34). Severalstudies have been performed considering UV light/TiO2

slurries (17-19), Fenton’s reagent (20-22), UV/H2O2 (13, 23,24) and O3/H2O2 (25-27), sonolytic destruction (28, 29), andhigh energy radiation (30-34). A recent comprehensivecomparison of all these techniques (35) with a major emphasison cost-effectiveness showed that the performance of all ofthem depended on site-specific conditions such as waterflow, observed influent, and required effluent concentrations.

Common to all of these AOT processes are free radicalreactions that degrade the dissolved MTBE. While the workin this area has generated detailed MTBE degradationmechanisms, there is a lack of absolute rate constants foreven the initial reactions involved in this process. Rateconstants for the reaction of the hydroxyl radical, the majorradical involved in all the above studies, have been recentlyestimated from ozone studies as (3.9 ( 0.7) × 109 (23), 1.9× 109 (25), and 1.2 × 109 M-1 s-1 (26). Only one specificinvestigation of the reaction of the hydroxyl radical withMTBE (36) has been reported, with a measured rate constantreported as 1.6 × 109 M-1 s-1 at room temperature. No rateconstants for the reactions of the other AOT species, thereducing hydrated electron or the hydrogen atom, could befound in the literature. In addition, to our knowledge there

* Corresponding author phone: (562)985-4649; fax: (562)985-8557;e-mail: Smezyk@csulb.edu.

† California State University at Long Beach.‡ University of North Carolina at Wilmington.§ Florida International University.| Argonne National Laboratory.⊥ Brookhaven National Laboratory.# Haley and Aldrich of New York.[ Science Research Laboratory, Inc.f Present address: KLA-Tencor Corp., e-Beam Review Division,

160 Middlesex Tpke, Bedford, MA 01730-1491.

Environ. Sci. Technol. 2004, 38, 3994-4001

3994 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 14, 2004 10.1021/es034558t CCC: $27.50 2004 American Chemical SocietyPublished on Web 06/05/2004

has not been any report of the kinetics of the subsequentprocesses that occur in the overall MTBE degradation.

The focus of this paper was to measure free radicalbimolecular rate constants for MTBE reaction with thehydroxyl radical (•OH), the solvated electron (eaq

-) and thehydrogen atom (H•), and the subsequent reaction of thecarbon-centered radicals with oxygen. We have also incor-porated these rate constants into an electron-beam AOTkinetic model to compare the predicted MTBE removal fromnatural waters under large-scale treatment conditions againstexperimental measurements.

Experimental SectionMethyl tert-butyl ether (MTBE, Aldrich, 99.8%, anhydrous),2-methyl-2-propanol (tert-butyl alcohol, TBA, Fisher Scien-tific, certified), and methanol (Aldrich, HPLC grade) wereobtained at the highest purity available and used as received.

The electron radiolysis of pure water (or dilute solutions)forms hydroxyl radicals, hydrated electrons, and hydrogenatoms according to the stoichiometry

where the numbers in eq 1 are G values (yields) in units ofµM J-1 (37). Absolute radical concentrations (dosimetry) werebased on the transient absorption of (SCN)2

•- at 472 nm,using 10-2 M thiocyanate (KSCN) in N2O-saturated solutionat natural pH (with Gε ) 5.2 × 10-4 m2 J-1 (38)).

MTBE solutions for hydroxyl radical and hydrated electronexperiments used Millipore-quality water (>18.0 MΩ) thatwas saturated by sparging with either high purity N2O or N2

to remove dissolved oxygen. MTBE peroxyl radical formationexperiments, requiring a mixture of N2O and O2 gases, wereprepared by mixing appropriate volumes of separatelysaturated solutions, with the resulting gas concentrationscalculated using the volume ratio and initial gas concentra-tions of [N2O] ) 2.2 × 10-2 M and [O2] ) 1.25 × 10-3 M (39).All of these rate constant measurements were performed atthe Radiation Laboratory, University of Notre Dame, usinga Model TB-8/16-1S linear electron accelerator, with 3-5 nspulses of 8 MeV electrons generating radical concentrationsof 1-3 µM per pulse. A detailed description of the experi-mental setup has been given elsewhere (40). Solution flowrates were adjusted so that each pulse irradiation was per-formed on a fresh sample, and multiple traces (5-15) wereaveraged to produce a single kinetic trace. All of these experi-ments were conducted at ambient temperature (22 ( 2 °C).

MTBE peroxyl radical decay kinetics were investigatedusing the 2.0 MeV Van de Graaff pulse radiolysis system atBrookhaven National Laboratory using a PC-controlled,CAMAC-based data acquisition and control system. Theseexperiments were done using an all-quartz pulse radiolysiscell, consisting of a 50 mL degassing reservoir with an outletthat drains into a 20 × 10 × 5 mm rectangular optical cell.Electron pulse widths used ranged from 0.2 to 2 µs. Theelectron beam passed through the 5 mm dimension and theanalyzing light produced by a Xe arc lamp was passed throughthe long dimension three times to give an effective path lengthof 6 cm. The cell was thermostated at 25.0 °C. MTBE solutionswere prepared using MilliQ water (>18.0 MΩ resistance) thatwas saturated with a premixed gas consisting of 4:1 N2O:O2.

For the kinetic investigation of hydrogen atom reactionswith MTBE, direct electron paramagnetic resonance (EPR)detection of the change in the hydrogen atom concentrationfollowing pulse radiolysis was the monitoring method ofchoice, as conventional pulse radiolysis/optical transientabsorption methodology is difficult to use for the weakabsorption at short wavelengths of both the •H atom and

product radicals. The pulsed EPR-based free induction decay(FID) attenuation method developed at Argonne NationalLaboratory was used because of the pseudo-first-orderscavenging kinetics obtained (41-43).

This system consisted of a 3.0 MeV Van de Graaffaccelerator, which produced 5-55 ns electron pulses thatgenerated hydrogen atoms in aqueous solution within anEPR cavity. A 35 ns microwave pulse was applied to the sampleimmediately after irradiation, and the resulting free inductiondecay of the hydrogen atom low-field (m1 ) 1/2) EPRtransition was recorded on a digital oscilloscope. Multiplepulses (500-2000) were averaged to measure each FID at arepetition rate of 120 Hz.

Stock solutions in ASTM Type 1 purified water were madewith methanol (10-2 M), to scavenge hydroxyl radicals, andacidified to pH 2.0 with HClO4 to increase the initial yield ofhydrogen atoms via quantitative conversion of hydratedelectrons (37). Solutions were recirculated through the EPRcavity. Scavenging experiments were conducted by succes-sively adding known volumes of MTBE to a known volumeof stock solution that was fully saturated with argon.

Large-scale aqueous MTBE removal experiments wereconducted at the Miami Electron Beam Research Facility(EBRF) (44). The irradiation source consists of a 1.5 MeV, 50mA, horizontally scanned beam capable of treating 150gallons per minute (0.57 m3 min-1). The applied dose iscontinuously variable from 0.25 to 8 kGy (25-800 krads) bychanging the beam current. Each experiment consisted oftreating a minimum of 3000 gallons (11.4 m3). A stainlesssteel tanker was used to prepare the solutions, using standardlime-softened Miami tap water (45), and mixing was ac-complished using a 200 gpm (0.76 m3 min-1) pump tocirculate the solution prior to treatment. MTBE concentra-tions were determined using a computer controlled GCequipped with a headspace sampler and flame ionizationdetector (46).

Measurements on the formation and removal of degra-dation products produced in the steady-state irradiation ofMTBE solutions were also conducted in this study, using theScience Research Laboratory (SRL) EB-10 RF electron beamlinear accelerator (47). Fully aerated 520 µg/L MTBE sampleswere prepared using steam-distilled water, with individualsolutions of 280 mL sealed in 9” × 12” Teflon pouches. TheLINAC irradiation consisted of 4.0 MeV electrons, at arepetition rate of 15-60 pulses per second (7 µs pulse width),to give total doses up to 4 kGy. Total irradiation times were5-6 min. Absolute dosimetry, and uniformity of the sweepwidth of the electron beam, was monitored by radiochromicfilm dosimeters, with the total dose found to be accurate to(10%. Quantitative product analysis was by purge and trapGC/MS for MTBE, tert-butyl formate, tert-butyl alcohol, andtert-butyl acetate. Formaldehyde and acetaldehyde wereanalyzed by HPLC, using 2,4-dinitrophenylhydrazine deriva-tives, and formic and oxalic acids were measured directlyusing ion chromatography.

Results and DiscussionReaction of MTBE with Hydroxyl Radicals. The radicalsproduced in the electron pulse radiolysis of water can beselectively isolated by adding chemicals to allow measure-ment of the rate constant of a single radical with a substrate.In an aqueous nitrous oxide (N2O) saturated solution, theinitially produced solvated electrons and hydrogen atoms(eq 1) are quantitatively converted into hydroxyl (•OH) radicalsvia the reactions (48):

eaq- + N2O + (H2O) f •OH + OH- + N2

k2 ) 9.1 × 109 M-1 s-1 (2)

•H + N2O f •OH + N2 k3 ) 2.1 × 106 M-1 s-1 (3)

H2O ∧∧∧ f 0.28•OH + 0.27eaq- + 0.06•H + 0.05H2 +

0.07H2O2 + 0.27H+ (1)

VOL. 38, NO. 14, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3995

The pulse radiolysis of aqueous, nitrous oxide-saturatedMTBE solutions yielded a radical spectrum with a maximumat 260 nm (filled squares in Figure 1). This absorption isattributed to the formation of the carbon-centered radicalsby hydrogen atom abstraction from MTBE (49)

and showed a dose dependent decay. However, this transientabsorption was too weak to conveniently be used todetermine radical formation kinetics, and so the overallbimolecular rate constant for the reaction of hydroxyl radicalswith MTBE was determined using competition kinetics, usingthiocyanate (SCN-) in N2O-saturated solution as a standard.The reaction of thiocyanate with the hydroxyl radical in N2O-saturated solution occurs by (48)

and the transient (SCN)2-• species has a strong absorption

whose maximum is at 472 nm. Upon addition of MTBE to

this solution, reactions 4 and 5 occur which lowers the totaltransient (SCN)2

-• absorption intensity. The competition forthe hydroxyl radicals follows the equation

where Abs°(SCN)2-• is the peak transient absorption mea-

sured in only the SCN- solution, and Abs(SCN)2-• is the

reduced absorbance of (SCN)2-• when MTBE is present. As

this technique is dependent upon the initial hydroxyl radicalconcentration being constant for all the measurements, atthe higher added MTBE concentrations, slight correctionsfor intraspur scavenging of radicals were made (50). Thetransformed kinetic data are shown in Figure 2, and from theslope of this linear plot, the bimolecular rate constant for thehydroxyl radical with MTBE was determined as k4+5 ) (1.71( 0.02) × 109 M-1 s-1, based upon k6 ) 1.05 × 1010 M-1 s-1

at this temperature (48). Our measured rate constant is invery good agreement with the value of 1.6 × 109 M-1 s-1 forthis reaction reported by Eibenberger (36), who utilized thesame methodology. Our rate constant also agrees well withthe middle value determined from the ozone-based estimatedmethodology (1.9 × 109 M-1 s-1) by Acero et al. (25). OurMTBE measured rate constant is also consistent withliterature rate constants (36, 48, 51, 52, 56) for hydroxyl radicalreactions with analogous compounds. A comparison sum-mary of all these values is given in Table 1.

TABLE 1. Rate Constants (M-1 s-1) for Hydroxyl Radical, Hydrated Electron and Hydrogen Atom Reaction with MTBE, and MTBERadical Reaction with Dissolved Oxygen Determined in This Study in Comparison to Analogous Literature Valuesa

compound •OH e-aq H• R• + O2

MTBE (methyl tert-butyl ether) (1.71 ( 0.02) × 109 <8.0 × 106 (3.49 ( 0.06) × 106 (2.17 ( 0.06) × 109

(3.9 ( 0.7) × 109 (23)1.9 × 109 (25)1.2 × 109 (26)1.6 × 109 (36)

ETBE (ethyl tert-butyl ether) (1.81 ( 0.03) × 109 (51) e 107 (51) (7.0 ( 0.1) × 106 (51) -TAME (tert-amyl methyl ether) (2.37 ( 0.04) × 109 (51) e 4 × 106 (51) (3.1 ( 0.1) × 106 (51) -DIME (dimethyl ether) 1.0 × 109 (52) 4.9 × 109 (53)DIEE (diethyl ether) 2.9 × 109 (36) < 1.0 × 107 (54) 4.3 × 107 (55)DIPE (diisopropyl ether) (2.49 ( 0.04) × 109 (51) e 6.7 × 106 (51) (6.7 ( 0.1) × 106 (51)

(3.7 ( 0.4) × 109 (56)TBA (tert-butyl alcohol) 6.0 × 108 (48) <4.0 × 105 (57) 1.7 × 105 (58) 1.4 × 109 (60)

1.8 × 109 (61)a Bold values are rate constants determined in this study.

FIGURE 1. Absorption spectrum of 10-3 M MTBE solutions at pH7 saturated with N2O (9) and 1:1 N2O/O2 (O) 15 µs after the electronpulse.

(CH3)3COCH3 + •OH f •CH2(CH3)2COCH3 + H2O 29%(4)

(CH3)3COCH3 + •OH f (CH3)3COCH2• + H2O 71% (5)

•OH + SCN- f SCNOH•- (6)

SCNOH•- f SCN• + OH- (7)

SCN• + SCN- T (SCN)2-• (8)

2(SCN)2-• f (SCN)2 + 2SCN- (9)

FIGURE 2. Competition kinetics plot for rate constant determinationof hydroxyl radical reaction with 1.0 × 10-2 M MTBE in N2O-saturatedsolution at pH 7.0. Solid line is weighted linear fit, with a calculatedslope of 0.163 ( 0.001, corresponding to a second-order rate constantfor hydroxyl radical reaction with MTBE of (1.71 ( 0.02) × 109 M-1

s-1.

Abso(SCN)2-•

Abs(SCN)2-•

) 1 +k4+5[MTBE]

k6[SCN]-(10)

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In the presence of oxygen the MTBE radical absorptionat 260 nm increased by a factor of 2-3 (see open circles inFigure 1). The absorption growth is attributed to thesubsequent formation of alkylperoxyl radicals by addition ofoxygen to the initially formed carbon-centered radicals:

and

The consistent wavelength of the absorption maximum forboth the alkyl and alkylperoxyl (which we generically writeas ROCH2O2

•) radical has also been observed in MTBE gas-phase measurements (59). The observed growth rate of this260 nm absorption in solution was found to be dependentupon the dissolved oxygen concentration, and a plot of thepseudo-first-order growth rate constant against oxygenconcentrations gave a straight line (Figure 3) of slope k11+12

) (2.17 ( 0.06) × 109 M-1 s-1. A search of the literature forcomparative reactions of analogous species (see Table 1)found similar rate constants for the peroxyl radical formationfrom tert-butyl alcohol radicals, k ) 1.4-1.8 × 109 M-1 s-1

(60, 61), but that our value was slower than the measuredrate constant for the methoxymethyl radical, k ) 4.9 × 109

M-1 s-1 (53).At longer times, the 260 nm absorption was seen to decay

(see Figure 4). Data measured out to 0.5 s showed acomplicated mixture of decay kinetics, including at least oneother dose-dependent process. The resolution of all thesekinetics was beyond the scope of this project. The kineticsof the initial decay (<1 ms, see Figure 4a) were second-order,and by fitting a range of these short-time data to a combinedsecond-order decay to a sloping baseline (to account for thefurther decays) an averaged rate constant for the overallreaction

was obtained. Unfortunately comparison of this rate constantwith measured rate constants for peroxyl radical decay forthe analogous species in Table 1 is difficult, due to the rangeof values reported for diethyl ether (DIEE, 2k ) 1.7-4.4 ×109 M-1 s-1 (62, 63)) and tert-butyl alcohol (TBA, 2k ) 1.8-8.0 × 108 M-1 s-1 (60, 64, 65)). These peroxyl radicals areknown (66-68) to undergo self-recombination to produce

a tetroxide intermediate that subsequently decomposes togive a variety of products. These reactions include

where TBF is tert-butyl formate. The fractional yield offormation of TBF from MTBE formed by UV/hydrogenperoxide water treatment has been measured as 0.28 ( 0.03(23), and TBF degradation kinetics in water by AOT meth-odologies have been reported elsewhere (24, 25, 69).

Reaction of MTBE with Hydrated Electrons. For selectivemonitoring of hydrated electrons, experiments were per-formed in a N2-saturated, 0.5 M TBA solution, whichscavenges •OH radicals and •H atoms to form the relativelyinert 2-methyl-2-propanol radical (48):

The pseudo-first-order decay rate of the isolated hydratedelectron absorption at 650 nm was dependent upon MTBEconcentration (see Figure 5), corresponding to a second-order rate constant for the reaction

FIGURE 3. Second-order rate constant determination for the reactionof the MTBE radical with dissolved oxygen. Pseudo-first-ordergrowth rate constants obtained at 260 nm using 1.0 × 10-2 M MTBEat pH 7.0 with various N2O/O2 gas mixtures. Solid line correspondsto weighted linear fit, giving a rate constant of (2.17 ( 0.06) × 109

M-1 s-1.FIGURE 4. Decay kinetics of 3.0 × 10-3 M MTBE solution saturatedwith a 4:1 N2O:O2 gas mixture at neutral pH and 25 °C. (a) Short-timesecond-order decay of 260 nm absorbance of MTBE-peroxyl radical.Initial MTBE peroxyl radical concentration was 7.78 µM, basedupon a calculated G-value of 5.63 (50). Solid line is second-orderfit to a sloping baseline, corresponding to a decay rate constantof 2k ) (6.0 ( 0.3) × 108 M-1 s-1. (b) Longer time decay(s) of MTBE-peroxyl radical absorbance. Dose-dependent measurements showedthat there was at least one more second-order process in thismixed-order decay.

ROCH2-O4-CH2OR f O2 + ROCH2OH + TBF (14)

f H2O2 + 2TBF (15)

f O2 + 2ROCH2O• (16)

ROCH2OH f TBA + HCHO (17)

•OH + (CH3)3COH f •CH2(CH3)2COH + H2O

k18 ) 6.6 × 108 M-1 s-1 (18)

•H + (CH3)3COH f •CH2(CH3)2COH + H2

k19 ) 1.7 × 105 M-1 s-1 (19)

eaq- + (CH3)3COCH3 f products (20)

(CH3)3COCH2• + O2 f (CH3)3COCH2O2

• (11)

•CH2(CH3)2COCH3 + O2 f •O2CH2(CH3)2COCH3 (12)

2ROCH2O2• f ROCH2-O4-CH2OR

2k13 ) (6.0 ( 0.3) × 108 M-1 s-1 (13)

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of k20 ) (8.0 ( 0.3) × 106 M-1 s-1. For this slow reaction itis possible that that hydrated electron reactions withimpurities might also be significant, and therefore thismeasured rate constant should only be considered an upper-limit value. Our rate constant is consistent with previousupper-limit values determined for analogous ether speciesin aqueous solution (see Table 1).

Reaction of MTBE with Hydrogen Atoms. Initial absorp-tion experiments performed using N2-saturated 0.5 M TBAsolutions at pH 1.0 again only gave a very weak signal at 260nm. This absorption was presumably due to MTBE radicalsformed by both hydroxyl radical and hydrogen atom reac-tions; however, no kinetic resolution of these two processeswas obtained in this study. Competition kinetics experimentsperformed using the •H-adduct of p-benzoquinone (54) werealso unsuccessful. Therefore the hydrogen atom reaction rateconstant with MTBE was measured using the FID-attenuationEPR method (41, 42). These MTBE measurements wereperformed in argon saturated, pH 2.0 solution, where theradiolytically formed hydrated electrons are quantitativelyconverted to hydrogen atoms, and in the presence of 10-2

M methanol to ensure complete scavenging of hydroxylradicals.

The general expression for the effective damping rate ofthe FID in these experiments is given by (41-43)

where k° is the first-order natural spin relaxation rate constantin the absence of added solute, k22 is the hydrogen atomscavenging rate constant for the MTBE reaction

and kexi [Ri] represents the spin-dephasing contribution of

second-order spin exchange and recombination reactionsbetween hydrogen atoms and other free radicals. Rateconstants previously measured using this method have beenfound to be slightly dependent on radiation dose when thelast term in equation 21 is not sufficiently constant over theexperimental time scale of ca. 5 µs. This effect was also notedin this study (Figure 6a), where specific second-order rateconstants of (4.14 ( 0.03) × 106, (3.77 ( 0.04) × 106, and (3.54( 0.07) × 106 M-1 s-1 were obtained for the Van de Graaffpulse widths of 55, 25, and 12 ns, respectively. These pulsewidths correspond to relative doses of 16.5, 6.40, and 2.95;the approximate average cell dose was 30 Gy/pulse for the

55 ns pulse used. These relative values were simply themeasurements of the average beam current (µA) on a shutterpositioned before the irradiation cell and by assuming thatthese currents were proportional to the actual dose given tothe sample we could extrapolate the measured rate constantsto “zero-dose” (see Figure 6b) to give a hydrogen atomreaction rate constant with MTBE of k22 ) (3.49 ( 0.06) ×106 M-1 s-1 (see Table 1). This rate constant is in goodagreement with analogous values determined in the literature.Thermodynamic considerations suggest that the hydrogenatom reaction with MTBE would be similar to that for thehydroxyl radical, consisting of abstraction of a hydrogen atomto create a primary carbon-centered radical.

Steady-State Irradiation MeasurementsThe development of kinetic computer models for chemicalremoval using the electron beam AOT process is one of thelong-term goals of our research group. Therefore, large-scaleMTBE removal experiments were conducted at the MiamiElectron Beam Research Facility (44). MTBE loss measure-ments were performed using natural Miami tap water at twodifferent pHs, 5.3 and 8.4, and these results are shown inFigures 7 and 8.

The computer code MAKSIMA-CHEMIST (70) was usedfor the kinetic modeling. Further details of the integrationalgorithm and validation tests can be found elsewhere (70,71). The input to the kinetic model includes a list of all reactingspecies, their initial concentrations that were obtained fromthe experimental measurements, and the rate constants. Thewater residence time in the irradiated region was estimatedfor this facility as 0.091 s. MTBE removal simulations alsoshown in Figures 7 and 8 are based upon the electron beamkinetic model detailed by Mak et al. (72), with the extra MTBEreaction rate constants with the hydroxyl radical, hydratedelectron, hydrogen atom, and the formation and decay of

FIGURE 5. Second-order rate constant determination for the reactionof the hydrated electron with MTBE in neutral pH, N2-saturated,0.50 M tert-butanol solution. Solid line corresponds to weightedlinear fit of 8.0 × 106 M-1 s-1.

FIGURE 6. (a) Dose dependence of the hydrogen atom scavengingrate constants for Ar-saturated MTBE solutions containing 10-2 Mmethanol at pH 2.0 and 21.6 °C. Solid lines correspond to fittedvalues of (3.54 ( 0.07) × 106 (4), (3.77 ( 0.04) × 106 (O), and (4.14( 0.03) × 106 (0) M-1 s-1 for the 12, 25, and 55 ns pulse widths,respectively. (b) Rate constant extrapolation to “zero dose” forMTBE at pH 2.0 and 21.6 °C, to give a limiting value of k22 ) (3.49( 0.06) × 106 M-1 s-1. The approximate average cell dose was 30Gy/pulse for the 55 ns pulse used. Error bars correspond to onestandard deviation obtained from weighted linear fits to thescavenging plots.

keff ) ko + k22[MTBE] + ∑kexi [Ri] (21)

•H + (CH3)3COCH3 f products (22)

3998 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 14, 2004

the MTBE peroxyl radical taken as the values of this study.Initial solute conditions are as given in Table 2.

For the purposes of this kinetic modeling, irreversibleloss of an organic chemical was assumed to occur upon itsreaction with the hydrated electron or upon formation ofthe corresponding peroxyl radical.

Rate constants for the natural water scavengers listed inTable 2 were obtained from standard literature data (48). Fordissolved organic carbon (DOC), rate constants for thereaction of the hydrated electron and the hydrogen atomwere assumed to be 1.00 × 107 M-1 s-1. A rate constant forhydroxyl radical reaction with DOC has been reported (73),as (6.6 ( 5.4) × 108 L mg-1 s-1. The large error in this rateconstant poses a problem in simulating destruction resultsbecause the DOC concentration is relatively high compared

to other scavengers in the system. We have addressed thisproblem by minimizing the error in the predicted removalof several contaminants, including benzene, phenol, andtoluene, using the data of Zele et al. in the same water matrix(74), from which a “best” rate constant of 2.0 × 108 M-1 s-1

was derived. Our hydroxyl radical reaction rate constant isin good agreement with two other recent values reported inthe literature: 3.6 × 108 L (mol C)-1 s-1, averaged over 18different sources of DOC (75), and 2.4 × 108 L (mol C)-1 s-1

(76).The MTBE removal curves generated by this kinetic model

are in excellent agreement with experiment at pH 5.3 (seeFigure 7). However, the same model predicts a much higher

TABLE 2. Initial Solute Species and Concentrations Assumed for Computer Kinetic Modeling of MTBE Removal by the ElectronBeam Process

species concentration

pH 5.3 pH 8.4

solute mg L-1 M mg L-1 M k•OH, M-1 s-1 ke-(aq), M-1 s-1 k•H, M-1 s-1

alkalinity (as CaCO3) 0 45.00bicarbonate (HCO3

-) 0 0 52.8 8.65 × 10-4 8.50 × 106 1.00 × 106 4.40 × 104

carbonate (CO32-) 0 0 0.73 1.22 × 10-5 3.90 × 108 3.90 × 105 0

chloride (Cl-) 41.6 5.64 × 10-4 41.6 5.64 × 10-4 3.0 × 109 1.00 × 106 0bromide (Br-) 0.29 2.50 × 10-6 0.29 2.50 × 10-6 1.10 × 1010 0 2.80 × 107

nitrate (NO3- as N) 0.36 2.10 × 10-6 0.36 2.10 × 10-6 0 9.70 × 109 1.40 × 106

DOC (as C) 6.00 5.00 × 10-4 6.00 5.00 × 10-4 2.00 × 108 1.00 × 107 1.00 × 107

DO (as O2) 5.60 9.38 × 10-5 5.60 9.38 × 10-5 0 1.90 × 1010 2.10 × 1010

chloramine (NH2Cl) 0 0 2.75 5.44 × 10-5 2.8 × 109 2.1 × 1010 1.2 × 109

TABLE 3. Summary of MTBE Loss and Product Formation/Destruction in SRL Experimentsa,b

dose(kGy)

[MTBE]µM

[tert-butylformate]

µM

[tert-butylalcohol]

µM[formaldehyde]

µM[formate]

µM[oxalate]

µM

0 5883 0 0 0 0 00.25 997 951.6 468.6 6.2 13.2 0.70.50 55.3 478.0 415.9 5.8 11.1 2.01.00 BMDL 27.4 BMDL 2.7 5.5 5.71.50 BMDL BMDL BMDL 2.2 4.4 9.12.00 BMDL BMDL BMDL 1.2 2.9 14.02.50 BMDL BMDL BMDL 0.7 2.2 15.83.00 BMDL BMDL BMDL BMDL 3.0 15.64.00 BMDL BMDL BMDL BMDL 2.0 13.9

a See text for details. b BMDL - below method detection limit.

FIGURE 7. Large-scale irradiation removal of MTBE (9) at pH 5.3.Solid line (-O-) is predicted removal using kinetic model as describedin text.

FIGURE 8. Large-scale irradiation removal of MTBE (9) at pH 8.4.Solid lines are kinetic model simulations: (-O-) same model asused for pH 5.3 (Figure 7); (-4-) inclusion of 2.75 mg/L chloramines;(-3-) inclusion of 2.75 mg/L chloramines at pH 9.2 and 60.0 mg/Ltotal alkalinity.

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destruction rate than observed at pH 8.4 (see Figure 8).Consideration of the water chemistry at these two differentpHs gave the following differences: at pH 8.4 there was aresidual presence of the water disinfectant monochloramine(NH2Cl, ∼2.5-3.0 mg L-1 as Cl2) and a higher ratio ofcarbonate (CO3

2-) relative to bicarbonate (HCO3-). Therefore,

these variations were also tested in the kinetic model.The rate constants for the reaction of the hydroxyl radical

(77), the hydrated electron, and the hydrogen atom (78) withmonochloramine at room temperature have been recentlydetermined as 5.2 × 108, 2.17 × 1010, and 1.23 × 109 M-1 s-1,respectively. However, including these radical scavengingcompetitive reactions at the highest monochloramine con-centration of 2.75 mg/L (54.4 µM) showed little improvementin the agreement between theory and experiment at pH 8.4(see Figure 8). Varying the solution conditions to themaximum pH (9.2) and alkalinity (60 mg L-1) observed forMiami groundwater to maximize the carbonate to bicarbon-ate concentration ratio also had little impact on the predictedremoval rate. One other possibility for the reduced rate ofMTBE removal could be competition for the primary radicalsby the products formed in the electron beam destruction ofMTBE. To test this hypothesis, preliminary steady-stateexperiments were also performed in this study to establishthe time (dose) dependence of intermediate chemical speciesunder electron beam irradiation conditions. Table 3 showsthe dose-profiles obtained in the continuous irradiationexperiments performed using the SRL accelerator (47). Theseirradiations were performed to a very high dose, well beyondthe complete destruction of the added MTBE. Concentrationsof TBF, TBA, formaldehyde, formate, and oxalate weredetermined.

The two major products, TBF and TBA, were observed togrow in with dose and then be totally removed by furtherirradiation. These two species concentrations were muchhigher than found for formaldehyde, formate, and oxalate,and so were the only two species included in our extendedkinetic modeling. From the observed dose profile both speciesappear to be formed at the same time, with the concentrationof TBF about twice that of TBA. Specific rate constants forhydroxyl radical, hydrated electron, and hydrogen atomreaction with TBF have been evaluated (69), and those forTBA were taken from the established literature (48). Theproduct ratio was incorporated in our kinetic model byallowing two overall separate decomposition pathways forthe MTBE peroxyl radical:

These rate constants gave the observed dose profile forthese two species. Varying the absolute magnitude of theserate constants by an order of magnitude while keeping thesame ratio gave the same results in the model. However, theinclusion of the hydroxyl radical competition from these twoproducts into the kinetic model at the measured alkalinity,pH, and in the presence of chloramines gave no discernibledifference in the predicted removal of MTBE under thesebasic conditions. At this time we cannot quantitatively explainthe slower rate of MTBE removal at pH 8.4.

AcknowledgmentsPartial support for this research was provided by the NationalScience Foundation, under Grant BES 97-29965. Workdescribed herein at the Radiation Laboratory, University ofNotre Dame was supported by the Office of Basic EnergySciences of the U.S. Department of Energy. Work performedat Argonne National Laboratory was supported by the U.S.Office of Basic Energy Sciences, Division of Chemical

Sciences, U.S.-DOE under contract number W-31-109-ENG-38. Work performed at Brookhaven National Laboratory wasfunded under contract DE-AC02-98CH10886 with the U.S.Department of Energy and supported by its Division ofChemical Sciences, Office of Science. We would also like tothank Rance Hardison for performing some of the ionchromatographic measurements.

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Received for review June 4, 2003. Revised manuscript re-ceived March 25, 2004. Accepted March 26, 2004.

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