Kinetics and Products of Reactions of MTBE with Ozone and Ozone/Hydrogen Peroxide in Water, Papers of Theatre

Download Kinetics and Products of Reactions of MTBE with Ozone and Ozone/Hydrogen Peroxide in Water and more Papers Theatre in PDF only on Docsity! Journal of Hazardous Materials B89 (2002) 197–212 Kinetics and products of reactions of MTBE with ozone and ozone/hydrogen peroxide in water Marie M. Mitani a, Arturo A. Keller a,∗, Clifford A. Bunton b, Robert G. Rinker c, Orville C. Sandall c a Bren School of Environmental Science and Management, University of California, 4666 Physical Sciences North, Santa Barbara, CA 93106-5131, USA b Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, USA c Department of Chemical Engineering, University of California, Santa Barbara, CA, USA Received 30 August 2000; received in revised form 30 June 2001; accepted 9 July 2001 Abstract Methyl-t-butyl-ether (MTBE) has become a prevalent groundwater pollutant due to its high volume use as a nationwide gasoline additive. Given its physicochemical properties, it requires new treatment approaches. Both aqueous O3 and a combination of O3/H2O2, which gives •OH, can remove MTBE from water, making use of O3 a viable technology for remediation of groundwater from fuel contaminated sites. Rate constants and temperature dependencies for reactions of MTBE with O3 or with •OH at pH 7.2, in a range of 21–45◦C (294–318 K) were measured. The second-order rate constant for reaction of MTBE with O3 is 1.4×1018 exp(−95.4/RT) (M−1 s−1), and for reaction of MTBE with •OH produced by the combination of O3/H2O2 is 8.0×109 exp(−4.6/RT) (M−1 s−1), with the activation energy (kJ mol−1) in both cases. At 25◦C, this corresponds to a rate constant of 27 M−1 s−1 for ozone alone, and 1.2 × 109 M−1 s−1 for O3/H2O2. The concentration of •OH was determined using benzene trapping. Products of reactions of O3 and O3/H2O2 with MTBE, including t-butyl-formate (TBF), t-butyl alcohol (TBA), methyl acetate, and acetone, were determined after oxidant depletion. A reaction pathway for mineralization of MTBE was also explored. Under continuously stirred flow reactor (CSTR) conditions, addition of H2O2 markedly increases the rate and degree of degradation of MTBE by O3. © 2002 Elsevier Science B.V. All rights reserved. Keywords: MTBE; Ozone; Reaction pathway; Hydroxyl radical; Kinetics; Byproducts; TBA; TBF 1. Introduction Methyl-t-butyl-ether (MTBE) has been used as a gasoline oxygenate in the US for over two decades. It eliminates the need for leaded gasoline and is the most common fuel oxy- ∗ Corresponding author. Tel.: +1-805-893-7548; fax: +1-805-893-7612. E-mail address: keller@bren.ucsb.edu (A.A. Keller). 0304-3894/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0304 -3894 (01 )00309 -0 198 M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212 Nomenclature CBf concentration of benzene in outlet (mol l−1) CBo concentration of benzene in inlet (mol l−1) Cmf concentration of MTBE in outlet (mol l−1) Cmo concentration of MTBE in inlet (mol l−1) COH concentration of •OH (mol l−1) CO3 outlet concentration of O3 (mol l −1) k1 rate constant for reaction of MTBE with O3 (M−1 s−1) k2 rate constant for reaction of MTBE with •OH (M−1 s−1) k3 rate constant for reaction of benzene and •OH (M−1 s−1) RB global rate of disappearance of benzene (M s−1) Rm global rate of disappearance of MTBE (M s−1) vf volumetric flow rate of outlet stream (ml s−1) vo volumetric flow rate of inlet stream (ml s−1) Vr volume of reactor (l) genate used to reduce air pollution and increase octane ratings [1]. MTBE may comprise up to 15% by volume of gasoline, and it became the second highest volume chemical pro- duced in the US in 1997 [2]. The high volume use as well as the chemical characteristics of this gasoline additive have resulted in contaminated water supplies around the world where MTBE is used as a gasoline additive. MTBE is very water soluble, making its movement in the environment almost as fast as groundwater, with practically no retardation due to sorption on soil particles. Once re- leased, MTBE is quite persistent due to its molecular structure, i.e. the presence of the t-butyl group, which inhibits environmental degradation under normal conditions and strongly in- hibits natural biodegradation [3,4]. This results in widespread contamination when MTBE escapes into the environment. A major concern arises from leaking underground fuel tanks that contaminate groundwater at much higher concentrations than surface sources. Con- tamination of lakes and rivers by two-stroke gasoline engines is also a problem [5]. MTBE uncontained in the environment inevitably results in groundwater pollution, and it was the second most frequently detected chemical in samples of shallow ambient groundwater from the US Geological Survey’s National Water Quality Assessment Program [6]. Although, recent progress in in situ treatment has been reported, there are many cir- cumstances where aboveground treatment is required [7]. Some methods simply separate MTBE from water, such as air stripping or GAC adsorption, while others involve oxida- tion to harmless products [8]. Although, separation techniques may be less expensive than oxidation, they require an additional step for the treatment or disposal of the pollutant. Ozonation has been shown to be a viable option in the treatment of waste and drinking water. With the development of large scale ozone (O3) generators and lower operating costs, there has been increasing interest in using O3 to remove compounds that are difficult or too expensive to remove by other methods. In some cases, O3 treatment alone adequately degrades contaminants to meet water quality standards [9]. M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212 201 injection block of the GC, kept at 250◦C [18]. A 65
m polydimethylsiloxane/divinylbenzene SPME fiber was used for most of the analyses and reproducibly and quantifiably extracted MTBE, benzene, TBF, TBA, acetone (Me2CO), and methyl acetate (MeOAc). Concentra- tions for all organic compounds were based on calibration standards which were carried out at the same pH and temperature as the reaction conditions. Formic acid and acetic acid were detectable by using this fiber, but the extraction was not reproducible. The 100
m polydimethylsiloxane SPME fiber gave a higher extraction of MTBE, but did not extract the more soluble organic compounds and therefore was not generally used. For both fibers, an exposure time of 2 min with stirring and a desorption time of 1 min were used. The fiber was injected into a VOCOL (Supelco) capillary column (30 m × 0.25 mm × 1.5
m) with a temperature ramp programmed to 100◦C for 3 min and in- creased 20◦C min−1 to 150◦C. All analyses were made in duplicate with a reproducibility of ±10%. 3. Results 3.1. Kinetic studies The initial rates of reaction of aqueous O3 and MTBE show that reactions are first order with respect to O3 and MTBE individually; i.e. second-order overall. Mechanis- tically, this is most likely due to the activating effect of O3 attack on methoxy hydro- gen [19]. However, decomposition products of aqueous O3, namely •OH, may react with MTBE or intermediates, and in some cases may be the predominant oxidant during ox- idation by O3. The formation of •OH involves reaction of O3 and the hydroxide ion (initiation). The calculated rate of O3 decomposition by reaction with hydroxide ion (OH−), produc- ing •OH using Eq. (1), accounts for approximately 10% of the rate of disappearance of O3 in our system. This indicates that reaction of MTBE with •OH is not negligible and must be included in the rate expression. Also, the disappearance of O3 is approximately three to four times faster than the disappearance of MTBE, indicating that O3 is reacting with other species such as OH−, various oxygen radicals produced by O3 decomposition, and other products of MTBE oxidation. MTBE may also react with species other than O3 or •OH, but we assume that these reactions are negligible. Based on these considerations, the following kinetic rate expression in the stirred flow reactor is applicable: Rm = k1CO3Cmf + k2COHCmf (2) where Rm is global rate of disappearance of MTBE (M s−1), k1 the rate constant for reac- tion of MTBE with O3 (M−1 s−1), k2 the rate constant for reaction of MTBE with •OH (M−1 s−1), Cm the concentration of MTBE (mol l−1), Cmf the outlet concentration of MTBE (mol l−1), CO3 the outlet concentration of O3 (mol l−1), COH is the concentration of •OH (mol l−1). Rm can also be related to the operating conditions, using a mass balance: Rm = voCmo − vfCmf Vr (3) 202 M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212 where Cmo is the concentration of MTBE in inlet (mol l−1), vo the volumetric flow rate of inlet stream (ml s−1), vf the volumetric flow rate of outlet stream (ml s−1), Vr is the volume of reactor (l). The concentration of •OH, COH, is determined using a relationship developed by Elovitz et al. [21] which defines the ratio of exposures of •OH and O3, ∫ [•OH] dt/ ∫ [O3] dt . They measured the concentration of •OH as a function of O3 concentration over time by using a probe (decarboxylation of p-chlorobenzoic acid), which rapidly and quantitatively traps •OH and does not react with O3. A calibration of this probe system has been made [10]. The study indicates that •OH concentration does not change significantly with temperature or pH, although O3 concentration is a stronger function of these parameters. The value of k1 can be determined from Eqs. (2) and (3) using measurable parameters: k1 = voCmo − vfCmf − k2COHCmf VrCO3Cmf (4) The value of k2 was obtained from experiments with O3, H2O2, benzene and MTBE, described below. In these experiments, •OH was generated by reaction of O3 and H2O2 and was trapped competitively by benzene and MTBE, k2 was found to be ∼1.2×109 M−1 s−1. Fig. 1 presents the Arrhenius plot for ozonation of MTBE in the absence of H2O2. The apparent activation energy (Ea) for ozonation of MTBE is 95.4 kJ mol−1. The experimental data is presented in Table 1. The measured temperature dependence of k1 (M−1 s−1) is k1 = 1.4 × 1018 exp−95.4 RT (5) Fig. 1. Arrhenius plot for the reaction between MTBE and ozone. M .M .M itanietal./JournalofH azardous M aterials B 89 (2002) 197–212 203 206 M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212 Fig. 2. Arrhenius plot for the reaction between MTBE and the hydroxyl radical, generated by ozone and hydrogen peroxide. batch reactor are presented in Table 3. The system of MTBE and O3/H2O2 resulted in similar products, but in different proportions. As can be seen in Table 3, the amount of organic products decreased upon addition of H2O2 to the system, indicating more complete oxidation. Formic acid was initially found in the batch reactor when O3 was used alone, but it was undetectable after oxidant depletion. For the O3/H2O2 system, formic acid was never detected at any stage of the reaction, probably because of its rapid reaction with •OH which gives •COO−. This radical then reacts with oxygen or other oxidants to ultimately form CO2 [28,29]. Based on identification of intermediates, products and studies of rates, we postulate a main pathway for mineralization of MTBE, TBF, and TBA. Fig. 3 shows the proposed reaction Table 3 Mole ratios of products to reactants from experiments in a batch reactor at pH 7.2 and ∼23◦C Initial reactant Products Product/reactant with O3 (mole ratio) Product/reactant with O3/H2O2 (mole ratio) Decrease with O3/H2O2 (%) MTBE TBF 0.50 0.34 32 TBA 0.14 0.10 29 Acetone 0.20 0.12 40 Methyl acetate 0.13 0.10 23 TBF TBA 0.24 0.15 38 Acetone 0.34 0.24 29 TBA Acetone 0.28 0.20 29 M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212 207 Fig. 3. Suggested pathway for reaction of MTBE with ozone reaction: Pathway A shows attack on the t-butyl group to form methyl acetate. The major reaction is at the -hydrogen of MTBE to form TBF, pathway B [18]. pathway of MTBE with O3 based on our experimental results and literature. The main initial product of the reaction of O3 with MTBE is TBF. TBF can be generated by insertion of O3 at the -hydrogen to form a hydrotrioxide intermediate, as shown for ozonation of other ethers [19]. Subsequently, reactions may follow either of two pathways after O3 insertion. TBF and H2O2 may be formed from the trioxide intermediate. The other possibility is formation of a TBF radical, which ultimately forms TBF, again producing H2O2 by (1) simple electron transfer or (2) reaction with MTBE itself and propagating a chain reaction. O3 could attack a -hydrogen of MTBE, but the tertiary methyl group sterically disfavors the reaction. Another indication of O3 insertion at the -hydrogen is the dominant formation of TBF as the initial product. If O3 did preferentially attack a -hydrogen, TBF would not be the main product. However, formation of methyl acetate in MTBE oxidation indicates that there may be attack on the t-butyl group of MTBE by O3 or an oxidant from O3 decomposition. Esters are formed from ketones in the Baeyer–Villiger reaction with hydroperoxides or peroxy acids, but we excluded this reaction by performing control experiments with acetone and O3 and O3/H2O2, since only unreacted acetone was identified after oxidant depletion. In separate experiments, reactions of TBF and TBA with O3 and O3/H2O2 did not generate methyl acetate. Therefore, methyl acetate is not derived directly from acetone, TBF or TBA, 208 M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212 but indirectly from MTBE, by attack on the t-butyl group. Other studies have also detected methyl acetate as a by-product in the reaction of O3 and MTBE [11]. As mentioned previously, TBF is hydrolyzed in aqueous media to TBA and formic acid in the absence of oxidants. Acids and bases catalyze this reaction, where a larger deviation from neutral pH causes an increase in the rate of hydrolysis, especially at higher pH. Recent work by Church et al. have estimated the half-life of TBF at pH 7 to be 5 days [30]. Since TBA is a radical scavenger, it is not surprising that it is not the major product of reaction of MTBE with O3. Once TBA is formed, its oxidation can generate acetone [31]. The reaction of TBA with •OH and/or other radicals can form •OHCH2C(CH3)2 and oxygen may react with the TBA radical to form •OOCH2C(CH3)2OH [31]. Both these radicals should decompose giving acetone and •CH2OH (hydroxymethyl radical), which is ultimately oxidized to CO2. Acetic acid is a product of TBA oxidation by O3 and O3/H2O2, but it is undetectable after oxidant depletion. Control studies indicate that the acetic acid detected in our reactions is derived by oxidation of TBA at the t-butyl group. In a control experiment of acetone oxidation by O3 or O3/H2O2, acetic acid was not seen as a product with the concentrations of reactants used in this work. In addition, the formation of methyl acetate from MTBE could involve oxidation of two methyl groups from the t-butyl group and replacement by oxygen, in a multistep process [19], and the corresponding oxidation of the t-butyl group of TBA should generate acetic acid. 3.3. Product formation during oxidation of TBF or TBA In these experiments, TBA or TBF were reacted with O3 or O3/H2O2. Degradation of TBF or TBA followed similar pathways to complete oxidation as MTBE, through intermediates identified in initial and final stages of ozonation. In the reaction of TBF with O3 or O3/H2O2, TBA and acetone were the identified organic products. In the reaction of TBA with O3 or O3/H2O2, acetone was the only detectable organic product after oxidant depletion. Table 3 presents the mole ratios of reactants and products after oxidant depletion and also confirms that the amount of organic products always decreased for the O3/H2O2 system. Based on identifiable products at different stages of oxidation of TBF and TBA, we conclude that TBF and TBA follow the reactions shown in Fig. 3. 4. Conclusion The major products of environmental concern in the oxidation of MTBE by O3 and O3/H2O2 are TBF and TBA. Because oxidants were limiting reactants under our batch con- ditions, there were significant residual organic products, especially when O3 is used alone. Addition of H2O2 reduced the organic products, and increased mineralization of MTBE. The yield of TBF in the stirred flow reactor, with a mean residence time of ∼850 s, was consistently ∼25% of the initial MTBE, in moles. This indicates that the TBF produced reacts with O3, which explains why the transformation of O3 is greater than that of MTBE. As temperature increased from 22 to 41◦C in the CFSR, the rate of disappearance of MTBE increased by 11% with O3 alone. It would be necessary to increase the residence time, M.M. 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