7654
Ind. Eng. Chem. Res. 2008, 47, 7654–7662
Comprehensive Kinetic Model for the Degradation of Methyl tert-Butyl Ether by an Ozone/UV Process Temesgen Garoma* and Mirat D. Gurol CiVil, Construction, and EnVironmental Engineering Department, College of Engineering, San Diego State UniVersity, 5500 Campanile DriVe, San Diego, California 92182
The paper presents a comprehensive kinetic model that describes the degradation of methyl tert-butyl ether (MTBE) in an ozone/UV process. First, the degradation pathways for MTBE were proposed on the basis of major reaction intermediates identified during the oxidation of MTBE. The yield for each major reaction intermediate was determined by fitting experimental data to the model prediction. Accordingly, 43, 20, 15, 11, and 6% of MTBE oxidized resulted in the generation of TBF, MMP, TBA, acetone, and methyl acetate, respectively. TBF oxidation resulted in the generation of HiBA, acetone, and TBA as primary intermediates at 47, 24, and 20%, respectively. During the oxidation of TBA, 66% resulted in the generation of HiBA and 34% in acetone. The kinetic model was verified using different sets of experimental data by varying the initial concentration of MTBE, influent ozone gas concentration, and incident UV light intensity. The model predicted well the degradation of MTBE by an ozone/UV process. In addition, the model predicted the accumulation and decay of primary intermediates (TBF, TBA, MMP, methyl acetate, and acetone) with slight variations. Introduction Methyl tert-butyl ether (MTBE) was introduced in the late 1970s and early 1980s as an antiknocking agent when lead was phased out of gasoline. Since 1990, it has been used to fulfill the fuel oxygenate requirements set by the U.S. Congress in the Clean Air Act Amendment (CAAA). The CAAA mandated compounds that increase oxygen be added either in the winter or all year to gasoline in specific parts of the United States where concentrations of carbon monoxide in the winter or ozone in the summer exceed established air quality standards. As a result, MTBE has escaped into the environment from leaking underground storage tanks or through accidental discharge.1,2 Once in the environment, MTBE dissolves readily in water and moves easily in urban stormwater, surface water, and groundwater systems. The widespread detection of MTBE in drinking water sources (surface water and groundwater) and treated water has caused great concern in the scientific communities and public health agencies because MTBE is suspected as a possible human carcinogen.3 Consequently, 25 states including California, which accounts for approximately 32% of U.S.’s MTBE consumption, have passed legislation that would completely ban or partially restrict the use of MTBE in gasoline.4 However, MTBE that has already been released into the environment will likely continue to degrade the groundwater and soils for several years to come because MTBE is known to degrade slowly by microorganisms.5-7 Several studies investigated the oxidation of MTBE using biodegradation,5-9 chemical oxidation,10 and advanced oxidation processes (AOPs).11-21 The objective of these studies was to establish the degree of degradation of MTBE, to identify reaction intermediates, or in some cases to understand the reaction mechanisms. The main objective of the current study is to develop a comprehensive kinetic model that describes the degradation of MTBE during an ozone/UV treatment process. * To whom correspondence should be addressed. E-mail: tgaroma@ mail.sdsu.edu.
The degradation of MTBE by an ozone/UV process has been investigated by the authors.21 Approach To establish the degradation pathways for MTBE, first MTBE was oxidized in an ozone/UV process and major reaction intermediates were identified. The major reaction intermediates identified were also oxidized by an ozone/UV process under the same experimental condition as for MTBE. Then, the degradation pathways for MTBE were proposed on the basis of the major reaction intermediates identified and data reported in the literature for the oxidation of some of the reaction intermediates using other AOPs which rely on hydroxyl radical (OH · ) as a main reaction mechanism. The kinetic model for the degradation of MTBE was then developed on the basis of the proposed degradation pathways, and the yield for each major intermediate was determined by fitting the experimental data to the model prediction. Finally, the kinetic model was verified using different sets of experimental data by varying the initial concentration of MTBE, influent ozone gas concentration, and incident UV light intensity. As pointed out in the introduction, the oxidation of MTBE by an ozone/UV process have been investigated by the authors.21 In the study, the reaction intermediates during the oxidation of MTBE were identified, and a good organic carbon balance was obtained (Figure 8 of ref 21), indicating that most reaction intermediates have been identified and quantified. Readers are referred to prior publication21 for the description of the experimental setup and analytical methods. Proposed Degradation Pathways In the literature, degradation pathways for MTBE have been reported using other AOPs, such as H2O2/UV,16 O3/H2O2,18 and TiO2 slurries;12 however, no information on the degradation pathways for MTBE is available for an ozone/UV process. The degradation of MTBE and its intermediates can be initiated by the attack of an OH · at methyl (-CH3), methoxy (-OCH3),
10.1021/ie800721t CCC: $40.75 2008 American Chemical Society Published on Web 09/13/2008
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7655
Figure 1. Structural formula of MTBE and its reaction intermediates.
formyl or carbonyl (-HCO), carboxyl (-COOH), and/or hydroxyl (-OH) groups.16,22-24 The structural formula for MTBE and its intermediates are presented in Figure 1. The nomenclature for each target chemical is provided in parenthesis. Degradation Pathways for MTBE. Abstraction of H-atoms at one of the equivalent methyl groups or methoxy group of MTBE could lead to the formation of 2-methoxy-2-methyl propionaldehyde (MMP) and tert-butyl formate (TBF), respectively. Additionally, abstraction of a methoxy group, two methyl groups, or methyl and methoxy group in MTBE could lead to the generation of tert-butyl alcohol (TBA), methyl acetate, and acetone, respectively. The methyl (CH•3) and methoxy (OCH3 · ) radicals, formed as a result of these abstractions, lead to the formation of formaldehyde and formic acid through series of reaction intermediates,25,26 and upon oxidation, formaldehyde leads to formic acid.27 Time profiles of products identified and presented in Figure 7 of ref 21 during an ozone/UV treatment of MTBE also reveal that TBF, TBA, MMP, methyl acetate, and acetone are primary reaction intermediates of MTBE oxidation. The proposed degradation pathway for MTBE is summarized in eq 1. Degradation Pathways for TBA. The degradation of TBA can be initiated by an OH · attack at any of the equivalent methyl groups or hydroxyl group and lead to the formation of hydroxyiso-butyraldehyde (HiBA), acetone, formaldehyde, and formic acid. This agrees with time profiles of reaction intermediates identified and presented in Figure 9 of ref 28 during TBA oxidation using an ozone/UV process. Degradation Pathways for TBF. The reaction intermediates formed during an ozone/UV treatment tert-butyl formate (TBF) are presented in the Supporting Information. The degradation of TBF can be initiated by the attack of an OH · at any of the equivalent methyl groups or formyl group. Time profiles of reaction intermediates formed during TBF oxidation indicate that TBA, HiBA, acetone, formaldehyde, and formic acid are the primary reaction intermediates. Degradation Pathways for Methyl Acetate. An OH · attack at methyl or methoxy group of methyl acetate can generate acetic acid, oxalic acid, formaldehyde, and formic acid as primary reaction intermediates. This is supported by time profiles of reaction intermediates identified during an ozone/UV treatment of methyl acetate. The time profiles are presented in the Supporting Information. Degradation Pathways for Acetone. The reaction intermediates formed during an ozone/UV treatment of acetone are presented in the Supporting Information. An OH · attack at either
of the equivalent methyl groups in acetone can lead to the formation of pyruvaldehyde, acetic acid, formaldehyde, and formic acid. Degradation of MMP and HiBA. For 2-methoxy-2-methyl propionaldehyde (MMP) and hydroxy-iso-butyraldehyde (HiBA), no product study was conducted since they are not commercially available. The degradation of MMP could be initiated by an OH · attack at methyl groups, methoxy group, or carbonyl group and lead to the generation of hydroxyiso-butyraldehyde (HiBA), methyl acetate (MAC), acetone (AC), formaldehyde, and formic acid.16 The degradation of HiBA can be initiated by an OH · attack at the methyl, carbonyl, or hydroxyl group and may lead to the formation of acetone, pyruvaldehyde, and formic acid. Degradation Pathways for Other Intermediates. The degradation of other reaction intermediates, such as pyruvaldehyde, formaldehyde, and organic acids can be initiated by OH · attack at methyl, carbonyl, carboxyl, or hydroxyl group. The carbonyl and carboxyl radicals formed by the abstraction process can lead to the generation of carbon dioxide.29 In eq 1 through eq 13, the proposed degradation pathways for MTBE and its reaction intermediates are summarized. In these equations mass balance is provided only for the carbon atom.
{
(CH3)3COCHO (TBF)
CCHO(CH3)2OCH3 (MMP)
OH· 3)3COH + HCHO ⁄ HCOOH (1) (CH3)3COCH3 98 (CH(TBA) (FAD⁄FOA) (MTBE) (CH3)3COH + HCHO ⁄ HCOOH (TBA)
(FAD⁄FOA)
(AC)
(FAD⁄FOA)
CH3COCH3 + 2HCHO ⁄ HCOOH
{
(CH3)3COH + HCOOH (FOA)
(TBA)
3)2CHO + HCOOH (CH3)3COCHO 98 HOC(CH (2) (FOA) (HiBA) (TBF) CH3COCH3 + HCHO + HCOOH OH·
OH·
(FAD)
(AC)
(FOA)
CCHO(CH3)2OCH3 98 (MMP)
{
HOC(CH3)2CHO + HCHO ⁄ HCOOH
(FAD⁄FOA)
(AC) OH·
(CH3)3COH 98 (TBA)
{
HOC(CH3)2CHO
HOC(CH3)2CHO 98 (HiBA)
OH·
CH3COOCH3 98 (MAC)
OH·
CH3COCH3 98
{
(HiBA)
CH3COCH3 + HCHO ⁄ HCOOH
OH·
(AC)
(FAD⁄FOA)
(HiBA)
CH3COOCH3 + HCHO ⁄ HCOOH + CO2 (3) (FAD⁄FOA) (MAC) CH3COCH3 + HCHO ⁄ HCOOH + CO2
{
(AC)
(4)
(FAD⁄FOA)
CH3COCH3 + CO2 (AC)
CH3COCOH + HCHO ⁄ HCOOH (PYD)
(FAD⁄FOA)
(5)
CH3COOH + HCHO ⁄ HCOOH (ACA)
(FAD⁄FOA)
HOCOCOOH + HCHO ⁄ HCOOH
{
(OXA)
(6)
(FAD⁄FOA)
CH3COCOH (PYD)
CH3COOH + HCHO ⁄ HCOOH (ACA)
(FAD⁄FOA)
(7)
7656 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008
OH·
{
CH3COCOOH (PYA)
CH3COCOH 98 CH3COOH + CO2 (PYD)
OH·
CH3COCOOH 98 (PYA) OH·
CH3COOH 98 (ACA)
(ACA)
HCHO ⁄ HCOOH + 2CO2
{
{
(8)
(FAD⁄FOA)
CH3COOH + CO2 (ACA)
HCHO ⁄ HCOOH + 2CO2
(9)
(FAD⁄FOA)
HOCOCOOH (OXA)
HCHO ⁄ HCOOH + CO2
(10)
(FAD⁄FOA)
OH·
HCHO 98 HCOOH (FAD)
(FOA)
(11)
OH·
HCOOH 98 CO2 (FOA)
(12)
OH·
HOCOCOOH 98 2CO2 (OXA)
(13)
The Comprehensive Kinetic Model On the basis of the proposed degradation pathways discussed in the previous section and summarized in eqs 1 through 13, the overall rate of change in the concentration of MTBE and its reaction intermediates in a completely mixed batch reactor can be represented by the following expressions. It is wellknown that the reaction of an OH · with organic compounds is second-order overall.30 In the current study, the ozone/UV system was designed to result in an aqueous ozone concentration below the minimum detectable limit of 5 µg/L by the Indigo method.31 In addition, the reactivity of molecular ozone toward MTBE and its reaction intermediates is very low compared to that of OH · .18 Therefore, the reaction of OH · with MTBE and its reaction intermediates is the main mechanism through which they are oxidized in the ozone/UV process. d[MTBE] ) -kMTBE[OH · ][MTBE] dt
(14)
d[TBF] ) l1kMTBE[OH · ][MTBE] - kTBF[OH · ][TBF] dt
(15)
d[MMP] ) l2kMTBE[OH · ][MTBE] - kMMP[OH · ][MMP] (16) dt d[TBA] ) l3kMTBE[OH · ][MTBE] + m1kTBF[OH · ][TBF] dt kTBA[OH · ][TBA] (17) d[HiBA] ) m2kTBF[OH · ][TBF] + n1kMMP[OH · ][MMP] + dt p1kTBA[OH · ][TBA] - kHiBA[OH · ][HiBA] (18) d[MAC] ) l4kMTBE[OH · ][MTBE] + n2kMMP[OH · ][MMP] dt kMAC[OH · ][MAC] (19) d[AC] ) l5kMTBE[OH · ][MTBE] + m3kTBF[OH · ][TBF] + dt n3kMMP[OH · ][MMP] + p2kTBA[OH · ][TBA] + q1kHiBA[OH · ][HiBA] - kAC[OH · ][AC] (20) d[PYD] ) q2kHiBA[OH · ][HiBA] + s1kAC[OH · ][AC] dt kPYD[OH · ][PYD] (21)
d[PYA] ) t1kPYD[OH · ][PYD] - kPYA[OH · ][PYA] (22) dt d[ACA] ) r1kMAC[OH · ][MAC] + s2kAC[OH · ][AC] + dt t2kPYD[OH · ][PYD] + u1kPYA[OH · ][PYA] - kACA[OH · ][ACA] (23) d[FAD] ) a1kMTBE[OH · ][MTBE] + b1kTBF[OH · ][TBF] + dt c1kMMP[OH · ][MMP] + d1kTBA[OH · ][TBA] + e1kHiBA[OH · ][HiBA] + f1kMAC[OH · ][MAC] + g1k[OH · ]AC[AC] + t3kPYD[OH · ][PYD] + u2kPYA[OH · ][PYA] + V2kACA[OH · ][ACA] kFAD[OH · ][FAD] (24) d[FOA] ) a2kMTBE[OH · ][MTBE] + b2kTBF[OH · ][TBF] + dt c2kMMP[OH · ][MMP] + d2kTBA[OH · ][TBA] + e2kHiBA[OH · ][HiBA] + f2kMAC[OH · ][MAC] + g2kAC[OH · ][AC] + t4k[OH · ]PYD[PYD] + u3kPYA[OH · ][PYA] + V3kACA[OH][ACA] + kFAD[OH · ][FAD] - kFOA[OH · ][FOA] (25) d[OXA] ) r2kMAC[OH · ][MAC] + V1kACA[OH · ][ACA] dt kOXA[OH · ][OXA] (26) where kMTBE, kTBF, kMMP, etc., are the second-order rate constants for the reaction of OH · with MTBE, TBF, MMP, etc., and the values reported in the literature and summarized in Table 1 are used. li, mi, ni, pi, qi, ri, si, ti, ui, and Vi represent the yield of reaction intermediates generated from carbon-centered parent chemicals, such as MTBE, TBF, MMP, TBA, HiBA, MAC, AC, PYD, PYA, and ACA, respectively, while ai, bi, ci, di, ei, fi, and gi represent the yield of reaction intermediates generated from methyl or methoxy groups of MTBE, TBF, MMP, TBA, HiBA, MAC, and AC, respectively. All yield factors are determined by fitting experimental data to model predictions in the next section. Calibration of the Kinetic Model To predict the degradation of MTBE and its intermediates by an ozone/UV process for given influent ozone gas concentration, incident UV light intensity, and water quality parameters, such as pH and bicarbonate concentration, the kinetic model developed for aqueous ozone photolysis by the authors40 and presented in eqs 27-30 below were coupled with eqs 14-26 and solved numerically. Detailed derivation of eqs 27-30, description of parameters, and sensitivity of the model to operational and process parameters are provided in the referenced publication.40 The systems of ODEs were solved numerically using ode15s, an ODE solver available in MATLAB. Ode15s is a stiff and a variable-step solver based on the numerical differentiation formula. The variable-step feature allows ode15s to control error by varying the step size during the simulation, reducing the step size to increase accuracy when a model’s values are changing rapidly and increasing the step size to avoid taking unnecessary steps when the model’s values are changing slowly. d[O3] ) nO3 - k[O3]1[OH-] - ΦPIa - 2k∗2 [O3][H2O2] dt 2k6[O3][OH · ] - k∗7 [H2O2][OH · ] - k∗9 [OH · ][HCO3]-
∑k
di[O3][Mi]
(27)
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7657
d[H2O2] ) γ1{k1[O3][OH-] + ΦPIa - ΦP Ia - k∗2 [O3][H2O2] dt k∗7 [H2O2][OH · ] - k∗9 [OH · ][HCO3 ]} (28)
Table 3. Estimated Yield Factors
d[OH · ] ) 2ΦP Ia + 2k∗2 [O3][H2O2] dt
(CH3)3COCH3 (MTBE)
∑k
bi[Mi][OH · ] -
∑
kj[Sj][OH · ] (29)
The values of the yield factors were estimated by fitting experimental data to the model predictions using lsqcurvefit, an optimization function available in MATLAB. To estimate the values of each parameter set, two sets of experimental data conducted under different experimental conditions were used and the values were averaged. For each set, the initial concentration of the target chemical, influent ozone gas concentration, and incident UV light intensity were varied while the alkalinity, initial pH, and ozone gas flow rate were kept constant at 2 mM, 7.0, and 1.5 L/min, respectively. The experimental conditions for the data sets used in the calibration of the model are presented in Table 2. The parameters (ri and fi) and Vi, which represent the yield of intermediates from methyl acetate and acetic acid, respectively, were first estimated by using experimental data obtained from the oxidation of methyl acetate. No experimental data was available for acetic acid; however, the experimental data from methyl acetate was used since acetic acid is the primary degradation product of methyl acetate. By using this experimental data, the computer program iteratively found the best estimates of ri, fi, and Vi by simultaneously optimizing the difference between experimental data and model predictions for methyl acetate, acetic acid, formaldehyde, formic acid, and oxalic acid. The range and average values of ri, fi, and Vi obtained from two sets of experimental data are presented in Table 3. Next the values of si, gi, ti, and ui were estimated by using experimental data obtained from oxidation of acetone. Here the Table 1. Reaction Rate Constants with OH · for MTBE and Its Reaction Intermediates chemicals
molecular structure
k (M-1 s-1)
reference
MTBE TBF TBA MMP HiBA MAC AC FAD PYD ACA FOA OXA PYA
(CH3)3COCH3 (CH3)3COCHO (CH3)3COH CCHO(CH3)2OCH3 HOC(CH3)2CHO CH3COOCH3 CH3COCH3 HCHO CH3COCOH CH3COOH HCOOH HOCOCOOH CH3COCOOH
3.9 ×109 1.2 ×109 7.6 ×108 3.0 ×109 3.0 ×109 1.2 ×108 1.4 ×108 1.0 ×109 7.0 ×108 1.0 ×108 2.2 ×109 5.3 ×106 3.1 ×107
14 14 32 18 18 33 34 35 35 36 37 38 39
Table 2. Experimental Conditions for Data Used in Model Calibrations target chemical MTBE TBF TBA MAC AC
parent compound
initial concentration (mM)
influent ozone gas concentration (mg/L)
incident UV light intensity (E/L-s)
1.02 0.96 1.03 0.95 1.07 1.42 1.01 1.42 0.85 1.43
37 50 40 51 40 67 40 57 40 54
5.8 ×10-6 9.0 ×10-6 5.8 ×10-6 9.0 ×10-6 5.8 ×10-6 9.0 ×10-6 5.8 ×10-6 9.0 ×10-6 5.8 ×10-6 9.0 ×10-6
(CH3)3COCHO (TBF)
intermediates
average (%)
(CH3)3COCHO (TBF)a
l1 ) 40-45 l1 ) 43
CCHO(CH3)2OCH3 (MMP)a (CH3)3COH (TBA)a CH3COOCH3 (MAC)a CH3COCH3 (AC)a HCHO (FAD) HCOOH (FOA) (CH3)3COH (TBA)a
l2 ) 18-22 l3 ) 12-18 l4 ) 5-6 l5 ) 10-12 a1 ) 5-8 a2 ) 21-25 m1 ) 19-20
l2 ) 20 l3 ) 15 l4 ) 6 l5 ) 11 a1 ) 7 a2 ) 23 m1 ) 20
m2 ) 45-49 m3 ) 22-26 b1 ) 10 b2 ) 95 n1 ) 72-75
m2 ) 47 m3 ) 24 b1 ) 10 b2 ) 95 n1 ) 75
n2 ) 10-12 n3 ) 10-15 c1 ) 10-13 c2 ) 48-54 p1 ) 62-70 p2 ) 30-38 d1 ) 17-21 d2 ) 10-12 q1 ) 24-30
n2 ) 11 n3 ) 13 c1 ) 12 c2 ) 51 p1 ) 66 p2 ) 34 d1 ) 19 d2 ) 11 q1 ) 27
q2 ) 60 e1 ) 27-35 e2 ) 24-28 r1 ) 55-68
q2 ) 60 e1 ) 31 e2 ) 26 r1 ) 62
r2 ) 13-18 f1 ) 40-41 f2 ) 60-61 s1 ) 16-46 s2 ) 45-45 g1 ) 22-56 g2 ) 18-41 t1 ) 8-15
r2 ) 16 f1 ) 41 f2 ) 61 s1 ) 31 s2 ) 45 g1 ) 39 g2 ) 30 t1 ) 12
t2 ) 71-81 t3 ) 3-5 t4 ) 8-9 u1 ) 100
t2 ) 76 t3 ) 4 t4 ) 9 u1 ) 100
u2 ) u3 ) V1 ) V2 ) V3 ) 100
u2 ) u3 ) V1 ) V2 ) V3 ) 100
HOC(CH3)2CHO (HiBA)a CH3COCH3 (AC)a HCHO (FAD) HCOOH (FOA) CCHO(CH3)2OCH3 HOC(CH3)2CHO (HiBA)a (MMP) CH3COOCH3 (MAC)a CH3COCH3 (AC)a HCHO (FAD) HCOOH (FOA) (CH3)3COH (TBA) HOC(CH3)2CHO (HiBA)a CH3COCH3 (AC)a HCHO (FAD) HCOOH (FOA) HOC(CH3)2CHO CH3COCH3 (AC)a (HiBA) CH3COCOH (PYD)a HCHO (FAD) HCOOH (FOA) CH3COOH (ACA)a CH3COOCH3 (MAC) HOCOCOOH (OXA)a HCHO (FAD) HCOOH (FOA) CH3COCH3 (AC) CH3COCOH (PYD)a CH3COOH (ACA)a HCHO (FAD) HCOOH (FOA) CH3COCOOH (PYA)a CH3COCOH (PYD) CH3COOH (ACA)a HCHO (FAD)a HCOOH (FOA)a CH3COOH (ACA)a CH3COCOOH (PYA) HCHO (FAD) HCOOH (FOA) CH3COOH (ACA) HOCOCOOH (OXA)a HCHO (FAD)a HCOOH (FOA)a HCHO (FAD) HCOOH (FOA)a a
range (%)
0 0 25-29 25-27 46-48
0 0 27 26 47
Primary intermediates.
computer program used previously fixed value of Vi and experimental data obtained from oxidation of acetone to optimize the difference between experimental data and model predictions for acetone, pyruvaldehyde, pyruvic acid, acetic acid, formaldehyde, formic acid, and oxalic acid. The estimated values of si, gi, ti, and ui are presented in Table 3. Similarly, to estimate the parameters pi, di, qi, and ei, experimental data from the oxidation of TBA and previously fixed values of Vi, si, gi, ti, and ui were used. Then the experimental data on TBF was used to estimate the yield factors mi and bi. Finally, the yield factors li, ai, ni, and ci were estimated by fitting experimental data obtained from the oxidation of MTBE to model predictions. In Table 3, the range and average of all estimated yield factors are summarized. It is expected the yield factors for intermediates formed from the parent chemical add up to 1. For MTBE, the yield factors
7658 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008
Figure 3. MTBE oxidation by an ozone/UV process.
inferred that about 64% MTBE degradation occurs as a result of OH · attack at the methoxy side of MTBE and about 36% at the methyl sides. This result agrees well with the findings of Stefan et al.,16 who reported that about 40% of MTBE oxidation occurs as a result of OH · attack on the tert-butyl sides and about 60% at the methoxy side. Figure 2. Overall degradation pathways for MTBE (numbers indicate yields in %).
(averaged values) were as follows: l1 ) 0.43, l2 ) 0.20, l3 ) 0.15, l4 ) 0.06, and l5 ) 0.11, which added up to 0.95. The yield factors for TBA (p1 ) 0.66 and p2 ) 0.34) added up to 1. For TBF, MMP, and HiBA the yield factors added up to 0.91, 0.99, and 0.87, respectively. For simple structure chemicals, such as methyl acetate and acetone, the yield factors added up only to 0.78 and 0.76, respectively. This is likely due to simultaneous abstraction of the methyl or methoxy groups in these chemicals by OH · and the formation carbon dioxide directly from the |
carbon atom at the center of the structure ( - c - ). On the other hand, the yield factors for intermediates formed from methyl or methoxy radicals may not necessarily add up to 1. For example, during TBA oxidation, HiBA and acetone are expected to be generated (eq 4). It is only during the formation of acetone that a methyl radical is generated. Thus, the yield factors for formaldehyde (d1 ) 0.19) and formic acid (d2 ) 0.11) from the oxidation of TBA are expected to add up to the yield factor of acetone (p2 ) 0.34). Table 3 shows that 43, 20, 15, 11, and 6% of MTBE degradation resulted in the formation of TBF, 2-methoxy-2methyl propionaldehyde (MMP), TBA, acetone, and methyl acetate, respectively. This result agrees well with data from Acero et al.,18 who found 42, 19, 13, 8, and 18% of MTBE degradation generated TBF, MMP, TBA, methyl acetate, and acetone, respectively, for an O3/H2O2 process. During TBA oxidation, about 66% of it resulted in the formation of hydroxyiso-butyraldehyde (HiBA) while 34% of TBA oxidation generated acetone. Acero et al.18 reported that about 60% and 40% of TBA decay forms HiBA and acetone, respectively. In Figure 2, the overall degradation pathways for MTBE are presented. Included in the figure are the average yields in percent for the major reaction intermediates formed. As previously stated the degradation of MTBE is initiated by an OH · attack at either the three equivalent methyl groups or methoxy group. An OH · attack at the methoxy side of MTBE generates TBF, TBA, and acetone, while an attack at the methyl side generates MMP, methyl acetate, and acetone. Assuming that methyl acetate and acetone are formed in equimolar amounts from an OH · attack on the methoxy side of MTBE, it can be
Verification of the Kinetic Model In this section, an attempt was made to verify the kinetic model using experimental data obtained from the oxidation of MTBE conducted under different experimental conditions from experimental data used in the calibration of the kinetic model. A 0.51 mM MTBE solution was oxidized by using an influent ozone gas concentration of 25 mg/L and incident UV light intensity of 2.9 × 10-6 Einstein/L-s. The concentration of bicarbonate was 2 mM. At the start of the experiment the pH of the solution was adjusted to 7.0, at the end of the oxidation period it was recorded as 8.07, and the kinetic models is capable of taking into account change in pH during the oxidation process. In Figure 3 through 5, the experimental data and model predications for MTBE degradation and accumulation and decay of reaction intermediates are presented. To determine the goodness of fit between the model predictions and experimental data, the chi-square (χ2) and F statistics tests were conducted, and the results are presented in Table 4. The χcr2 and Fcr values were obtained from χ2 and F statistics tables for an R value of 0.05, respectively. The calculated values for χ2 are less than χcr2 values indicating that that the differences between the model predictions and the experimental data are not significant. The experimental results are therefore consistent with the model results with a 95% chance of probability. Similarly, the calculated F values are less than Fcr values indicating that differences between the standard deviations of the model predictions and the experimental data are not significant. The model predicted well the degradation of MTBE in the ozone/UV process, Figure 3. In addition, the model predicted the accumulation and decay of primary intermediates such as TBF, MMP, TBA, methyl acetate, and acetone with slight variations, Figure 4. However, there are major discrepancies between model predications and experimental data for some intermediates generated as a result of further oxidation of primary intermediates, for instance, HiBA, pyruvaldehyde, and oxalic acid. Furthermore, the model underpredicted the accumulation and overpredicted the decay of pyruvaldehyde, Figure 5. This is most likely due to the fact that the model underestimated the decay of acetone and HiBA from which pyruvaldehyde is primarily generated. The model also under-
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7659
Figure 4. Accumulation and decay of primary intermediates during oxidation of MTBE.
predicted the accumulation of formaldehyde. This could be due to a possible underestimation of the yield factors of formaldehyde from MTBE and/or its primary intermediates. Overall, the model prediction results are reasonable given the large number of parameters fitted in the model. The sensitivity of the kinetic model to operational conditions, such as incident UV light intensity, influent ozone gas concentration, and water quality in terms of bicarbonate and pH, as well as process parameters, such as effective path length for UV light, has been presented elsewhere.40 The model could provide valuable insight into the design of experiments and AOPs for the oxidation of MTBE and its intermediates. Figure 6a and 6b present the simulation for the oxidation of 0.5 mM of MTBE solution in an ozone/UV process using an influent ozone gas concentration of 48 mg/L and incident
Table 4. χ-Square and F Statistics Tests chi-square test χ2 MTBE TBF MMP TBA HiBA MAC AC PYD PYA ACA FAD FOA OXA
F-test χcr2
-2
3.61 × 10 5.00 × 10-5 6.37 × 10-6 6.68 × 10-5 3.95 × 10-5 1.52 × 10-5 1.16 × 10-04 8.05 × 10-5 1.01 × 10-5 6.83 × 10-4 1.75 × 10-4 1.07 × 10-4 8.75 × 10-2
16.92 16.92 16.92 16.92 16.92 16.92 16.92 16.92 16.92 16.92 16.92 16.92 16.92
F
Fcr -2
3.41 × 10 3.93 × 10-2 1.25 × 10-2 1.84 × 10-4 2.60 × 10-3 4.00 × 10-3 1.90 × 10-3 9.18 × 10-2 3.90 × 10-3 9.26 × 10-4 7.05 × 10-1 1.60 × 10-1 3.70 × 10-1
4.35 4.35 4.35 4.35 4.35 4.35 4.35 4.35 4.35 4.35 4.35 4.35 4.35
7660 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008
Figure 5. Accumulation and decay of secondary/tertiary intermediates during oxidation of MTBE.
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7661
Figure 6. Simulation of the oxidation of MTBE and accumulation and decay of intermediates: (a) major primary intermediates and (b) secondary/tertiary intermediates.
UV light intensity of 5.8 × 10-6 Einstein/L-s. The pH and bicarbonate carbonate concentration are 7.0 and 2 mM, respectively. The simulation shows that of the primary intermediates, MMP peaks early at about 10 min followed by TBF at 15 min. TBA which is generated from both MTBE and TBF peaked approximately at 18 min. The accumulation of acetone is the highest among the major primary intermediates indicating that it is also formed from the degradation of other intermediates, such as, TBF, MMP, TBA, and HiBA. Acetone peaked at 35 min while methyl acetate which is generated from MTBE and MMP peaked at 24 min. The time profiles of formaldehyde and formic acid presented in Figure 6b indicate that they are generated early in the oxidation process. This is expected since they are generated from methyl (-CH3), methoxy (-OCH3), formyl or carbonyl (-HCO), and/or carboxyl (-COOH) groups abstracted from MTBE and its intermediates by OH · . The simulation also shows that the reaction intermediates formed during the oxidation of MTBE react well in the ozone/UV process and complete mineralization could be archived by the process, if desired. However, the simulation result indicates that oxalic acid may require longer reaction time compared with other intermediates. This is expected because of the lower reactivity of oxalic with OH · compared to other intermediates, Table 1.
Applicability of the Kinetic Model. The model was developed and calibrated using MTBE solution prepared in DI water. The model takes into account the scavenging effects of MTBE and its reaction intermediates, and carbonate ions present in the sample. The model does not take into account the scavenging effects of other impurities that may be present in water. Furthermore, the kinetic model is limited to water systems that do not contain UV absorbing species or target chemicals (and intermediates) that do not absorb UV radiation. The kinetic model developed in this study could be used to optimize operational parameters, such as influent ozone gas concentration, incident UV light intensity, and initial concentration of target chemicals, which are critical in the design and operation of an actual ozone/UV system. The kinetic model can also be extended to other AOPs, such as H2O2/UV and H2O2/O3, which rely on OH · as the primary reaction mechanism for degradation of target chemicals. In addition, the kinetic model can be modified to describe the degradation of other pollutants and the accumulation and decay of their reaction intermediates in an ozone/ UV system. In summary, the degradation pathways for MTBE was proposed using major intermediates identified during oxidation of MTBE. On the basis of the proposed degradation pathways, a comprehensive kinetic model that describes the
7662 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008
degradation of MTBE and its intermediates by an ozone/UV process was described. By using experimental data on MTBE and its intermediates, the kinetic model was calibrated by fitting experimental data with model predictions. Accordingly, 43, 20, 15, 11, and 6% of MTBE oxidized resulted in the generation of TBF, MMP, TBA, acetone, and methyl acetate, respectively. TBF oxidation resulted in the generation of HiBA, acetone, and TBA as primary intermediates at 47, 24, and 20%, respectively. During the oxidation of TBA, 66% resulted in the generation of HiBA and 34% in acetone. An attempt was made to verify the kinetic model using different sets of experimental data on MTBE. The model predicted well the removal of MTBE. In addition, the model predicted the accumulation and decay of primary intermediates with slight variations. Supporting Information Available: Reaction intermediates identified during an ozone/UV treatment of tert-butyl formate (TBF), methyl acetate, and acetone. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Mehlman, M. A. Dangerous and cancer-causing properties of products and chemicals in the oil refining and petrochemical industry 0.15. Health-hazards and health risks from oxygenated automobile fuels (MTBE)Lessons not heeded. Int. J. Occup. Med. Toxicol. 1995, 4, 219–236. (2) Gullick, R. W.; LeChevallier, M. Occurrence of MTBE in drinking water sources. J. Am. Water Works Assoc. 2000, 92, 100–113. (3) Mehlman, M. A. Carcinogenicity of methyl-tertiary butyl ether in gasoline. Carcinog. Bioassays Prot. Public Health 2002, 982, 149–159. (4) USEPA, State Actions Banning MTBE (Statewide): EPA-420-B04-009, Washington, DC, 2004, http://www.epa.gov/mtbe/420b04009.pdf. (5) Suflita, J. M.; Mormile, M. R. Anaerobic biodegradation of known and potential gasoline oxygenates in the terrestrial subsurface. EnViron. Sci. Technol. 1993, 27, 976–978. (6) Salanitro, J. P.; Diaz, L. A.; Williams, M. P.; Wisniewski, H. L. Isolation of a bacterial culture that degrades methyl t-butyl ether. Appl. EnViron. Microbiol. 1994, 60, 2593–2596. (7) Steffan, R. J.; McClay, K.; Vainberg, S.; Condee, C. W.; Zhang, D. L. Biodegradation of the gasoline oxygenates methyl tert-butyl ether, ethyl tert-butyl ether, and tert-amyl methyl ether by propane-oxidizing bacteria. Appl. EnViron. Microbiol. 1997, 63, 4216–4222. (8) Hickman, G. T.; Novak, J. T. Relationship between subsurface biodegradation rates and microbial density. EnViron. Sci. Technol. 1989, 23, 525–532. (9) Bradley, P. M.; Landmeyer, J. E.; Chapelle, F. H. Aerobic mineralization of MTBE and tert-butyl alcohol by stream-bed sediment microorganisms. EnViron. Sci. Technol. 1999, 33, 1877–1879. (10) Damm, J. H.; Hardacre, C.; Kalin, R. M.; Walsh, K. P. Kinetics of the oxidation of methyl tert-butyl ether (MTBE) by potassium permanganate. Water Res. 2002, 36, 3638–3646. (11) Wagler, J. L.; Malley, J. P. The Removal of methyl tertiary-butyl ether from a model groundwater using UV/peroxide oxidation. J. New England Water Works Assoc. 1994, 108, 236. (12) Barreto, R. D.; Gray, K. A.; Anders, K. Photocatalytic degradation of methyl-tert-butyl ether in TiO2 slurriessa proposed reaction scheme. Water Res. 1995, 29, 1243–1248. (13) Liang, S.; Palencia, L. S.; Yates, R. S.; Davis, M. K.; Bruno, J. M.; Wolfe, R. L. Oxidation of MTBE by ozone and peroxone processes. J. Am. Water Works Assoc. 1999, 91, 104–114. (14) Chang, P. B. L.; Young, T. M. Kinetics of methyl tert-butyl ether degradation and by-product formation during UV/hydrogen peroxide water treatment. Water Res. 2000, 34, 2233–2240. (15) Cater, S. R.; Stefan, M. I.; Bolton, J. R.; Safarzadeh-Amiri, A. UV/ H2O2 treatment of methyl tert-butyl ether in contaminated waters. EnViron. Sci. Technol. 2000, 34, 659–662. (16) Stefan, M. I.; Mack, J.; Bolton, J. R. Degradation pathways during the treatment of methyl tert-butyl ether by the UV/H2O2 process. EnV. Sci. Technol. 2000, 34, 650–658.
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ReceiVed for reView May 3, 2008 ReVised manuscript receiVed July 31, 2008 Accepted July 31, 2008 IE800721T
