Environ. Sci. Technol. 2004, 38, 3994-4001
Radiation Chemistry of Methyl tert-Butyl Ether in Aqueous Solution S T E P H E N P . M E Z Y K , * ,† J A C E J O N E S , † WILLIAM J. COOPER,‡ THOMAS TOBIEN,‡ MICHAEL G. NICKELSEN,‡ J. WESLEY ADAMS,‡ KEVIN E. O’SHEA,§ DAVID M. BARTELS,| JAMES F. WISHART,⊥ PAUL M. TORNATORE,# KIMBERLEY S. NEWMAN,# KELLIE GREGOIRE,# AND D A N I E L J . W E I D M A N [,f Department of Chemistry and Biochemistry, California State University at Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, Department of Chemistry, University of North Carolina at Wilmington, 601 South College Road, Wilmington, North Carolina 28403, Department of Chemistry, Florida International University, Miami, Florida 33199, Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, Haley and Aldrich 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-induced degradation of the gasoline oxygenate methyl tert-butyl ether (MTBE) in water have been investigated. Rate constants for the reaction of MTBE with the hydroxyl radical, hydrated electron, and hydrogen atom were determined in aqueous solution at room temperature, using electron pulse 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 process was the dominant pathway, relative to MTBE reaction with hydrogen atoms, (3.49 ( 0.06) × 106 M-1 s-1, or hydrated electrons, 18.0 MΩ) that was saturated by sparging with either high purity N2O or N2 to remove dissolved oxygen. MTBE peroxyl radical formation experiments, requiring a mixture of N2O and O2 gases, were prepared by mixing appropriate volumes of separately saturated solutions, with the resulting gas concentrations calculated using the volume ratio and initial gas concentrations of [N2O] ) 2.2 × 10-2 M and [O2] ) 1.25 × 10-3 M (39). All of these rate constant measurements were performed at the Radiation Laboratory, University of Notre Dame, using a Model TB-8/16-1S linear electron accelerator, with 3-5 ns pulses of 8 MeV electrons generating radical concentrations of 1-3 µM per pulse. A detailed description of the experimental setup has been given elsewhere (40). Solution flow rates were adjusted so that each pulse irradiation was performed on a fresh sample, and multiple traces (5-15) were averaged to produce a single kinetic trace. All of these experiments were conducted at ambient temperature (22 ( 2 °C). MTBE peroxyl radical decay kinetics were investigated using the 2.0 MeV Van de Graaff pulse radiolysis system at Brookhaven National Laboratory using a PC-controlled, CAMAC-based data acquisition and control system. These experiments were done using an all-quartz pulse radiolysis cell, consisting of a 50 mL degassing reservoir with an outlet that drains into a 20 × 10 × 5 mm rectangular optical cell. Electron pulse widths used ranged from 0.2 to 2 µs. The electron beam passed through the 5 mm dimension and the analyzing light produced by a Xe arc lamp was passed through the long dimension three times to give an effective path length of 6 cm. The cell was thermostated at 25.0 °C. MTBE solutions were prepared using MilliQ water (>18.0 MΩ resistance) that was saturated with a premixed gas consisting of 4:1 N2O:O2. For the kinetic investigation of hydrogen atom reactions with MTBE, direct electron paramagnetic resonance (EPR) detection of the change in the hydrogen atom concentration following pulse radiolysis was the monitoring method of choice, as conventional pulse radiolysis/optical transient absorption methodology is difficult to use for the weak absorption at short wavelengths of both the •H atom and
product radicals. The pulsed EPR-based free induction decay (FID) attenuation method developed at Argonne National Laboratory was used because of the pseudo-first-order scavenging kinetics obtained (41-43). This system consisted of a 3.0 MeV Van de Graaff accelerator, which produced 5-55 ns electron pulses that generated hydrogen atoms in aqueous solution within an EPR cavity. A 35 ns microwave pulse was applied to the sample immediately after irradiation, and the resulting free induction decay of the hydrogen atom low-field (m1 ) 1/2) EPR transition was recorded on a digital oscilloscope. Multiple pulses (500-2000) were averaged to measure each FID at a repetition rate of 120 Hz. Stock solutions in ASTM Type 1 purified water were made with methanol (10-2 M), to scavenge hydroxyl radicals, and acidified to pH 2.0 with HClO4 to increase the initial yield of hydrogen atoms via quantitative conversion of hydrated electrons (37). Solutions were recirculated through the EPR cavity. Scavenging experiments were conducted by successively adding known volumes of MTBE to a known volume of stock solution that was fully saturated with argon. Large-scale aqueous MTBE removal experiments were conducted at the Miami Electron Beam Research Facility (EBRF) (44). The irradiation source consists of a 1.5 MeV, 50 mA, horizontally scanned beam capable of treating 150 gallons per minute (0.57 m3 min-1). The applied dose is continuously variable from 0.25 to 8 kGy (25-800 krads) by changing the beam current. Each experiment consisted of treating a minimum of 3000 gallons (11.4 m3). A stainless steel tanker was used to prepare the solutions, using standard lime-softened Miami tap water (45), and mixing was accomplished using a 200 gpm (0.76 m3 min-1) pump to circulate the solution prior to treatment. MTBE concentrations were determined using a computer controlled GC equipped with a headspace sampler and flame ionization detector (46). Measurements on the formation and removal of degradation products produced in the steady-state irradiation of MTBE solutions were also conducted in this study, using the Science Research Laboratory (SRL) EB-10 RF electron beam linear accelerator (47). Fully aerated 520 µg/L MTBE samples were prepared using steam-distilled water, with individual solutions of 280 mL sealed in 9” × 12” Teflon pouches. The LINAC irradiation consisted of 4.0 MeV electrons, at a repetition rate of 15-60 pulses per second (7 µs pulse width), to give total doses up to 4 kGy. Total irradiation times were 5-6 min. Absolute dosimetry, and uniformity of the sweep width of the electron beam, was monitored by radiochromic film dosimeters, with the total dose found to be accurate to (10%. Quantitative product analysis was by purge and trap GC/MS for MTBE, tert-butyl formate, tert-butyl alcohol, and tert-butyl acetate. Formaldehyde and acetaldehyde were analyzed by HPLC, using 2,4-dinitrophenylhydrazine derivatives, and formic and oxalic acids were measured directly using ion chromatography.
Results and Discussion Reaction of MTBE with Hydroxyl Radicals. The radicals produced in the electron pulse radiolysis of water can be selectively isolated by adding chemicals to allow measurement of the rate constant of a single radical with a substrate. In an aqueous nitrous oxide (N2O) saturated solution, the initially produced solvated electrons and hydrogen atoms (eq 1) are quantitatively converted into hydroxyl (•OH) radicals via 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)
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TABLE 1. Rate Constants (M-1 s-1) for Hydroxyl Radical, Hydrated Electron and Hydrogen Atom Reaction with MTBE, and MTBE Radical 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 (3.9 ( 0.7) × 109 (23) 1.9 × 109 (25) 1.2 × 109 (26) 1.6 × 109 (36) (1.81 ( 0.03) × 109 (51) (2.37 ( 0.04) × 109 (51) 1.0 × 109 (52) 2.9 × 109 (36) (2.49 ( 0.04) × 109 (51) (3.7 ( 0.4) × 109 (56) 6.0 × 108 (48)
