Elucidation of the 1,4-Dioxane Decomposition Pathway at Discrete Ultrasonic Frequencies

Transcription

1 Environ. Sci. Technol. 2000, 34, Elucidation of the 1,4-Dioxane Decomposition Pathway at Discrete Ultrasonic Frequencies MICHAEL A. BECKETT AND INEZ HUA* School of Civil Engineering, Purdue University, West Lafayette, Indiana The sonolytic decomposition chemistry of the refractory compound 1,4-dioxane in aqueous solution has been investigated at four ultrasonic frequencies (205, 358, 618, and 1071 khz). To maintain fully saturated solutions, argon and oxygen were used as sparge gases. Using a frequency of 358 khz, the observed first-order kinetic rate constants for 1,4-dioxane destruction were highest with a sparge gas ratio of 75% Ar/25% O 2 (k ) 4.32 ( s -1 ) and lowest in the presence of pure argon (k ) 8.67 ( s -1 ). Ethylene glycol diformate, methoxyacetic acid, formaldehyde, glycolic acid, and formic acid were found to be the major intermediates of 1,4-dioxane degradation. A reaction mechanism involving these byproducts was proposed concerning primarily reactions with oxidizing species ( OH, OOH, O) in and near the interfacial region of the cavitation bubble. The highest observed first-order 1,4- dioxane decomposition rate occurred at 358 followed by 618, 1071, and 205 khz. At each frequency, approximately 85% of the initial carbon is accounted for as the parent compound, as an intermediate, or as CO 2. The major byproducts formation was investigated at all four frequencies, and the results indicate that free radical mechanisms are significant over the entire range of frequencies studied. Introduction 1,4-Dioxane (C 4H 8O 2) is of environmental concern for assorted reasons. It is currently employed as an organic solvent in numerous chemical processes (1) and is classified as a toxic chemical and hazardous pollutant by the U.S. Environmental Protection Agency (2). Because 1,4-dioxane persists in the natural environment and can migrate swiftly through aquifers, its presence in both surface water and groundwater is problematic (3, 4). The compound has been detected in groundwater in Japan (5), Canada (6), and the United States (7). Structurally related compounds (1,3- dioxanes) have contaminated drinking water resources in Spain (8). Dioxane is an ether and is therefore also of interest because it is structurally related to oxygenated fuel additives, which are groundwater contaminants in the United States (9). 1,4-Dioxane is resistant to both aerobic (10) and anaerobic biological processes (11). A current treatment technology for removing 1,4-dioxane from contaminated waste streams is by distillation, but this process is rather expensive (12). Other treatment methods, including activated carbon (13) and air stripping (12), provide inefficient removals due to the high aqueous solubility ( mg/l) and low vapor * Corresponding author phone: (765) ; fax: (765) ; hua@ecn.purdue.edu. pressure (37 mmhg at 25 C) that are characteristic of 1,4- dioxane (14). Moreover, the aforementioned treatment methods do not destroy the target compound but rather transfer it from one phase to another. Fenton s reagent (15) and γ-irradiation (16) have been explored as possible destructive treatment methods for 1,4- dioxane. Recently, Stefan and Bolton (17) have studied the decomposition of 1,4-dioxane subject to irradiation with ultraviolet light (UV) combined with H 2O 2 and have discovered ethylene glycol diformate (EGD) as the primary intermediate along with glyoxal, formaldehyde, and various other short-chain organic acid byproducts. Similarly, studies of heterogeneous photocatalysis (18, 19) have also revealed EGD as the principal oxidation byproduct along with minor amounts of formic, glycolic, and oxalic acids. Sonolysis is also a potential destructive technique that has been effectively applied to other ethers (20). Most of these processes exploit the reactivity of the hydroxyl radical ( OH), a nonselective oxidant that rapidly attacks organic compounds. Ultrasonic irradiation has been investigated in the transformation of organic pollutants such as phenols (21, 22), chlorinated organics (23), and humic acids (24). Chemical decomposition may be enhanced when an appropriate ultrasonic frequency is employed (25, 26). Sonochemical reactions result from acoustic cavitation, the rapid expansion and collapse of gas- and vapor-filled bubbles. Temperatures as high as 4300 K (27) and pressures approaching 1000 atm (28) can exist within each bubble upon collapse. This transient phenomenon in turn promotes compound degradation through various possible mechanisms (29, 30): high temperature reactions, including combustion; supercritical water oxidation; and free radical oxidation. The last is important when the pollutant does not partition significantly into the vapor phase. A more complete discussion of aqueous sonochemistry and environmental applications is given by Hoffmann and co-workers (31), and a comprehensive discussion of ultrasound and sonochemistry can be found by Crum (32) and Suslick (33). There still remain many unanswered questions regarding reaction mechanisms under a variety of ultrasonic conditions. The research reported in this paper was directed toward elucidating decomposition products and pathways for 1,4- dioxane during sonolysis at discrete ultrasonic frequencies. The viability of ultrasound and the chemical pathways comprising sonochemical destruction of dioxane are explored. Ultrasonic frequency and sparge gas variation (argon and oxygen) are also examined as a component of optimal ultrasonic reactor design and operation. Experimental Materials and Methods Ultrapure water (R ) 18 MΩ cm -1 ) was obtained through a Barnstead Nanopure Ultrapure water system. Reagent sodium chloride (Fisher), grade potassium biphthalate (J. T. Baker, Inc.), potassium iodide (VWR Scientific), ammonium molybdate (Fisher Scientific), 1,4-dioxane (Sigma), ethylbenzene (Aldrich), and GC resolve grade hexane, and methylene chloride (Fisher Scientific) were used as received. Fresh solutions of 1.0 mm 1,4-dioxane were used for each individual study. Acetic, methoxyacetic, glycolic, formic, and oxalic acids and formaldehyde (Sigma) and 2,4-dinitrophenylhydrazine (Aldrich) were analytical reagent grade and also used as received. Ethylene glycol diformate (EGD) was purchased from Fluka and used as received. All of the sonolytic experiments were performed with an AlliedSignal URS ultrasonic transducer powered by an Allied Signal R/F generator (256 W maximum output). Four ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 18, /es000928r CCC: $ American Chemical Society Published on Web 08/15/2000

2 TABLE 1. Formation of Principal Free Radical Species during Ultrasonic Irradiation (33, 36-38) Argon Sparge (M is an Inert Third Molecule) H 2 O f OH + H (1) OH + M H 98 H2 O (2) 2 M OH + 98 H2 O 2 (3) 2 M H 98 H2 (4) 2 OH + T O + H 2 O (5) 2 M O 98 O2 (6) Additional Reactions in the Presence of Oxygen O 2 f 2 O (7) O 2 + O f O 3 (8) O 2 + H f OOH (or OH + O) (9) FIGURE 1. 1,4-Dioxane degradation with various gas sparges ratios. Reactor conditions: power intensity ) 5.1 W/cm 2, frequency ) 358 khz, C o ) 1.0 mm, C t ) concentration at time t. Results are averaged from experiments performed in duplicate. Error bars represent the 95% confidence interval. O + OOH T OH + O 2 (10) O + H 2 O f 2 OH (11) 2 OOH f H 2 O 2 + O 2 (12) ultrasonic frequencies were employed during the experiments (205, 358, 618, and 1071 khz). The active acoustical vibration area of the transducer was 25 cm 2, and the output power of the generator (indicated on the instrument itself) was 128 W. Therefore, the effective transducer intensity during each experiment was 5.1 W/cm 2 (power/vibrational area). The irradiated aqueous solutions were contained in a waterjacketed glass reactor (AlliedSignal, Germany) with an inner diameter of 6 cm (25), which was connected to a circulating cooling bath (Fisher Scientific). While the maximum volume for the glass reactor was 700 ml, the sonicated volume for each experiment was 500 ml. Kinetics experiments were performed in the batch mode by sonicating an aqueous 500-mL solution of dioxane. The temperature was monitored continuously and was maintained constant at 25 ( 1 C. Four ports were located on the reactor cover and were used to fully saturate the solution with argon and/or oxygen, to monitor liquid temperature, and to withdraw samples periodically for analysis. Solutions were sparged 20 min prior to the beginning of each sonication at a gas flow rate of 100 ml/min and were continuously sparged throughout the entire run. The sparge gases included argon and oxygen and varied according to the type of experiment being performed. To study the degradation kinetics of 1,4-dioxane, 2.5-mL aliquots were taken initially and every 10 min for each experiment. The samples were then extracted with a 3-mL mixture of hexane and methylene chloride (80:20, v:v). Ethylbenzene was used as an internal standard. The organic extracts were analyzed by injection (1 µl) into a Hewlett- Packard (HP) 5890 gas chromatograph equipped with a flame ionization detector (FID) and an HP 7673 autosampler. The GC contained a Vocol column from Supelco (30 m, 0.53 mm i.d., 3.0 µm film thickness) and was programmed with an initial oven temperature of 60 C for 2 min followed by a temperature ramp of 15 C/min for 4 min and holding at 120 C for 1 min. The injection port and detector temperatures were maintained at 150 and 250 C, respectively. FIGURE 2. Formation of sonolytic byproducts of 1,4-dioxane over time. Reactor conditions: frequency ) 358 khz, sparge gas ratio ) 75%/25% (Ar/O 2), power intensity ) 5.1 W/cm 2, C o ) 1.0 mm. Results are averaged from experiments performed in triplicate. Error bars represent the 95% confidence interval. EGD was identified using the solid-phase extraction (SPME) method from Supelco combined with GC/MS (17). A 10-mL aliquot of sonicated sample was vigorously mixed with a salted (NaCl) solution (10 ml/3.0 g, solution/salt). The SPME fiber (DVB/CAR/PDMS) was inserted into the solution, and the compound was allowed to adsorb onto the fiber for 20 min. The fiber was then inserted into an HP 5890 series II GC in tandem with an HP 5972 series mass spectrometer. SPME was necessary in order to introduce a detectable quantity of EGD into the mass spectrometer. EGD was then quantified using a direct aqueous injection into the GC/FID with a J&W Scientific DB-624 column (30 m, 0.53 mm i.d., 3.0 µm film thickness). Using this method, EGD eluted at 2.9 min. All organic acids were detected and quantified using a Dionex ion chromatograph (IC) equipped with a CD 20 conductivity detector (17). Methoxyacetic (retention time (rt) ) 3.6 min), acetic (rt ) 3.9 min), glycolic (rt ) 4.2 min), and formic acids (rt ) 5.0 min) were all measured using an IonPac VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

3 FIGURE 3. Reaction scheme of the initial degradation mechanism for 1,4-dioxane. Reactions took place under the following conditions: sparge gas ratio ) 75%/25% (Ar/O 2), power intensity ) 5.1 W/cm 2, frequency ) 358 khz, C o ) 1.0 mm. The major intermediates observed in this study are highlighted in boxes. AS4A ion exchange column with an AG4A guard column in a 2.5 mm sodium borate eluent (flow rate ) 1.5 ml/min). Oxalic acid (rt ) 3.7 min) was determined using an IonPac AS11 ion-exchange column and AG11 guard with a 12 mm NaOH eluent (flow rate ) 1.5 ml/min). Formaldehyde and glyoxal were derivatized with 2,4- dinitrophenylhydrazine (DNPH) and then measured by highperformance liquid chromatography (HPLC) (34). The glyoxal and formaldehyde hydrazones eluted at 5.2 and 7.5 min, respectively, when injected into a Varian 9012 HPLC with a Varian 9050 variable wavelength UV-Vis detector. The hydrazones were separated on a mm Alltech Econosil column (C 18 reversed phase) with isocratic elution (30/70 vol % water/methanol solvent mixture) and a mobilephase flow rate of 1.0 ml/min. The total organic carbon (TOC) was determined with a Shimadzu model TOC-5000A analyzer according to Standard Methods (35). All experiments (analysis by GC/FID, HPLC, TOC) except the organic acid studies (IC detection) were performed in duplicate, and two injections were made for each sample. The organic acid investigations were performed in triplicate with two injections made for each sample ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 18, 2000

4 FIGURE 4. Reaction scheme for the sonolytic degradation of ethylene glycol diformate (EGD). Scheme continued from Figure 3. The tetroxide intermediate mentioned here is structurally different than the one shown in Figure 3. Results and Discussion Ultrasonic reactor performance can be enhanced by the appropriate choice of sparge gas and ultrasonic frequency, among other parameters. A number of reports indicate that sparging with appropriate gases during sonolysis will significantly impact the efficiency of bubble collapse during acoustic cavitation (30, 36). Gases with a higher polytropic index, K (K ) C p/c v), and lower thermal conductivity will lead to more intense conditions within a collapsing bubble because less heat is dissipated to the surrounding aqueous environment during the rapid implosion. During this implosion, a number of reactions may occur within the cavitation bubble (see Table 1). The thermolysis of H 2O (reaction 1) into the hydroxyl radical and hydrogen atom is one of the most prevalent reactions during sonication of aqueous solutions (33). The resulting OH radical may oxidize organic substrates, react with hydrogen to form water (reaction 2), or react with another OH radical to form hydrogen peroxide (reaction 3). The hydrogen atom is also known to recombine and form hydrogen gas (reaction 4) (36). Reactions 5 and 6, however, are very unlikely to occur in the absence of oxygen (36-38). Reactive gases such as O 2 will also undergo numerous reactions within the gaseous bubble phase (Table 1) (38). Sonolytic reactions of O 2 lead to OH, OOH, and O radical species (reactions 7-11). These reactive intermediates formed in the presence of O 2, along with the thermolysis of water provide additional radical species that attack the target compound. In addition, the OOH radical can recombine to produce hydrogen peroxide (reaction 12). To determine the optimum sparging conditions for the reactor system, sonication was performed employing different sparge ratios of Ar and O 2. Ar is an inert gas, and the high-temperature decomposition of O 2 is well characterized VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

5 FIGURE 5. Reaction scheme for the pathway to methoxyacetic acid and decomposition. Scheme continued from Figure 3. (39). A primary objective of the experiments was to identify byproducts and to propose a decomposition pathway, which would have been more difficult in the presence of a complex mixture such as air. Thus, these gases were also chosen in order to simplify data interpretation during the byproduct studies. Pseudo-first-order plots of 1,4-dioxane degradation with varying ratios of sparge gas at a frequency of 358 khz are shown in Figure 1. 1,4-Dioxane degrades more quickly in the presence of pure oxygen than with pure Ar, an inert as. To account for this difference, the polytropic index, K, and thermal conductivity of both Ar and O 2 must be considered. The polytropic index, K, correlates to the heat released upon gas compression and is higher for Ar than for O 2 (1.66 vs 1.41) (40). The thermal conductivity of O 2 is higher than that of Ar (48.1 vs 30.6 mw m -1 K -1 )(25). This leads to the conclusion that bubble implosion in the presence of Ar leads not only to a higher overall internal temperature but, during the acoustic cavitation process, less heat is dissipated to the immediate surroundings by each bubble as well. While higher cavitation temperatures should lead to more effective compound decomposition, the results however appear to show something different (Figure 1). This occurrence can be explained by the production of additional radical species during decomposition of O 2 that can compensate for the lower internal cavitation temperatures which arise from the use of O 2 over Ar. Nevertheless, it is clear that a combination of the sparge gases O 2 and Ar lead to higher degradation rates than either gas individually; the highest degradation rate occurs at a 75% Ar/25% O 2 ratio. With the 75% Ar/25% O 2 sparge ratio, there is an optimum balance created between the higher temperatures generated during acoustic cavitation from Ar and the generation of active radical species from O 2 that promotes the most effective conditions for compound destruction. This approximate ratio has also been demonstrated to be optimal by other investigators (36). To elucidate the reaction mechanisms of 1,4-dioxane degradation during sonolysis, the byproducts of 1,4-dioxane ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 18, 2000

6 TABLE 2. Total Carbon Balance at 20-Min Intervals during Sonolytic Degradation of 1,4-Dioxane as a Function of Time a time (min) dioxane EGD FA MAA F GA OA TOC measd (mg/l as C) b %C accounted for a All concentrations are in mm unless noted otherwise. The TOC remaining in solution is determined by the TOC analyzer. Reactor conditions: sparge gas ratio ) 75%/25% (Ar/O 2), power intensity ) 5.1 W/cm 2, frequency ) 358 khz, C o ) 1.0 mm. EGD, ethylene glycol diformate; FA, formic acid; MAA, methoxyacetic acid; F, formaldehyde; GA, glycolic acid; OA, oxalic acid. b Other carbon losses can be attributed by mineralization to CO 2 (major) and volatilization from sparging effects (minor). Mineralization to CO 2 is accounted for by the TOC measurements. were determined and quantified using a variety of analytical methods as discussed previously. Compounds formed at 358 khz are shown in Figure 2. After 120 min, 96% of the initial 1,4-dioxane concentration was depleted, and five major reaction intermediates were detected: ethylene glycol diformate (EGD) (C 4H 6O 4), methoxyacetic acid (C 3H 6O 3), formic acid (CH 2O 2), glycolic acid (C 2H 4O 3), and formaldehyde (CH 2O). Formate is produced at the fastest rate followed by EGD. Methoxyacetic acid and formaldehyde are formed at similar rates followed by glycolic acid, which appears after an induction period of 30 min. The ph of the solutions also decreased from 7.50 ( 0.25 to 3.75 ( 0.25 during the course of these experiments. This ph decrease is consistent with the formation of organic acids as byproducts of 1,4-dioxane degradation. Investigations into 1,4-dioxane gas phase thermolysis indicate that ethylene (C 2H 4), carbon monoxide (CO), hydrogen gas (H 2), and formaldehyde (CH 2O) are byproducts resulting from direct C-C bond cleavage (41). During sonolysis of 1,4-dioxane, formaldehyde was detected and attributed to an oxidation pathway in these experiments. Ethylene, H 2, and CO were not detected. The possibility of byproduct accumulation however as a consequence of direct 1,4-dioxane ring cleavage has not been completely ruled out (Figure 3). If this cleavage were to occur though, it would predominantly take place between the C-C bond due its lower bond dissociation energy (BDE ) 79 kcal/mol) as compared to that of C-O (85 kcal/mol) and C-H (BDE ) 93 kcal/mol) (42). In the following discussion, we postulate the major pathways leading from 1,4-dioxane to its subsequent mineralization based on the byproducts observed during the sonication of 1,4-dioxane. In view of the complexity of the system, we propose a mechanism of sonolytic 1,4-dioxane destruction that occurs predominantly with reactions involving strongly oxidizing radical species ( OH, OOH, O) at and near the interfacial region of the cavitation bubble. Hydroxyl radicals have been specifically identified in sonicated aqueous solutions via electron spin resonance (43). Oxygen atoms ( O) (36, 39) and hydroperoxyl radicals ( OOH) (36, 44), although not directly quantified or detected, have been inferred from specific sonochemical reactions. The reaction of 1,4-dioxane by other free radicals should not be ruled out, although hydrogen abstraction by OOH and O atom with organic substrates may lead to byproducts that are similar to those resulting from OH radical attack. In addition, the O atom, due to its highly reactive nature in the gaseous phase (38), is more likely to play an indirect role by providing radical species (by reaction with oxygen and water vapor) with longer lifetimes that can subsequently move to the interfacial region and react with the target compound (45, 46). All free radicals formed however may react with the target compound or other molecules that are present within or in close proximity to the cavitation bubble. FIGURE 6. 1,4-Dioxane degradation at four ultrasonic frequencies. Reactor conditions: power intensity ) 5.1 W/cm 2, sparge gas ratio ) 75%/25% (Ar/O 2), C o ) 1.0 mm, T ) 25 C. Error bars represent the 95% confidence interval. With the free radical mechanism, the reaction between 1,4-dioxane (I) and OH initially begins with an H-abstraction at any of the carbons (due to symmetry) leading to the 1,4- dioxanyl radical (II) and water as shown in Figure 3 (47-49). Reactions between the OOH radical and 1,4-dioxane will lead to the same intermediate (II) along with the production of H 2O 2. Subsequent reactions of the 1,4-dioxanyl radicals with O 2 at diffusion-controlled rates lead to the peroxyl radicals (III), which normally undergo head-to-head termination reactions (bimolecular rate constants >10 9 M -1 s -1 ) to form the tetroxide intermediate (IV) (50). Once formed, this tetroxide intermediate rapidly decomposes to form R-oxyl radicals (V) and molecular oxygen (17, 50). In the case of a tetroxide derived from 1,4-dioxanyl peroxyl radicals, an electrocyclic process leading to H 2O 2 and two carbonyl compounds or a disproportionation reaction leading to a 2-hydroxy alcohol, a corresponding carbonyl compound, and oxygen via the Russell mechanism (51) appear unlikely due to stereochemical hindrance (17). Upon formation, these R-oxyl radicals may undergo two primary types of reactions: (i) β-c-c fragmentation leading to radical VI or (ii) 1,2-H-shift leading to R-hydroxyl radicals (X) (52). Further reactions involving radicals VI consume oxygen and form peroxyl radicals (VII), which can then dimerize to yield a tetroxide intermediate (VIII). The tetroxide rapidly decomposes to ethylene glycol diformate (EGD) (IX) and hydrogen peroxide (Figure 3). Maurino et al. (18) also have evidence that the formation of EGD is indicative of a sequential oxidative ring-opening mechanism. The further decomposition of EGD is initiated with H-abstraction by OH or OOH attack (Figure 4). Transformation may proceed in the presence of O 2 by forming another VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

7 FIGURE 7. Formation of (A) ethylene glycol diformate (EGD) and (B) formaldehyde (F) over time. Conditions used: sparge gas ratio ) 75%/25% (Ar/O 2), power intensity ) 5.1 W/cm 2, C o ) 1.0 mm. Results are averaged from experiments performed in triplicate. Error bars represent the 95% confidence interval. tetroxide intermediate that can decompose via either a free radical mechanism or a concerted mechanism (50). If the free radical pathway takes place, two alkoxyl radicals (XII) and O 2 are formed. The alkoxyl radicals (XII) then undergo β-scission and react to form aldehyde esters (XIV) and formic acid (XV). The aldehyde esters are unstable and quickly hydrolyze to formic acid (XV) and glycolaldehyde (XVI)(17). Additional reactions involving glycolaldehyde (XVI) and free radical species will lead to the rapid formation of glyoxal (XVII) and glycolic acid (XVIII). Because glyoxal was measured in trace concentrations during our investigations, we consider this pathway to be minor. The concerted mechanism involves the formation of H 2O 2 and an ester intermediate (XIII), which can then hydrolyze to glycolic (XVIII) and formic acids (XV) (17). It is not possible to further distinguish the relative importance of each pathway based on H 2O 2 measurements over time because H 2O 2 results from other reactions independent of 1,4-dioxane decomposition (36, 53). Although there are previous investigations of 1,4-dioxane decomposition due to oxidation, photolysis, or other destruction methods, only one study reports methoxyacetic acid (MAA) as an intermediate (17). The researchers examined the degradation of 1,4-dioxane in dilute aqueous solution by irradiation with UV/H 2O 2 and proposed that MAA was eventually formed from an R-oxyl radical in a process involving OOH as a reducing agent under reducing conditions. Furthermore, in their experiments acetic acid was formed as a byproduct of MAA in reducing conditions involving OOH as well. It is unlikely that reducing conditions exist in the sonochemical reactor under the conditions reported in this paper. Moreover, acetic acid was not detected during sonication of aqueous MAA under the same conditions of 1,4-dioxane sonication. It is therefore hypothesized that the formation of MAA occurs via the 1,2-H-shift pathway mentioned previously (Figure 3). The bond cleavage, promoted by the extreme conditions inside the cavitation bubble, occurs between the C 4-O bond (Figure 5). The oxygen can then donate its unpaired electron to the carbon (denoted C 1 in Figure 5) leading to the formation of a double bond between the oxygen and C 1. This ring opening leads to the creation of the carboxyl radical XIX, which will, in a series of ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 18, 2000

8 FIGURE 8. Formation of (A) methoxyacetic acid (MAA) and (B) formic acid (FA) over time. Conditions used: sparge gas ratio ) 75%/25% (Ar/O 2), power intensity ) 5.1 W/cm 2, C o ) 1.0 mm. Results are averaged from experiments performed in triplicate. Error bars represent the 95% confidence interval. undetermined oxidation steps involving free radicals possibly involving the formation of a tetroxide intermediate, subsequently form MAA (XXI). MAA will then continue to react with free radicals through H-abstraction at the methyl carbon followed by β-scission. Formaldehyde (XX) and the formyl methyl radical (XXII) are thus created. Formaldehyde is rapidly oxidized to formic acid, which eventually oxidizes to CO 2 (54). The formyl methyl radical can then combine with OH to form glycolic acid (XVIII). Both formaldehyde and glycolic acid were detected when MAA was sonicated alone, thereby giving support for the proposed pathway in Figure 5. Trace amounts of oxalate were measured during the sonication of 1,4-dioxane but only appeared after the formation of glycolic acid. The pathway for glycolic acid oxidation to CO 2 has previously been shown to produce formate and oxalate as intermediates prior to mineralization (17). In this study, when aqueous glycolic acid was sonicated, formate and oxalate were detected in significant amounts, thereby accounting for the production of oxalic acid. As is evident in the proposed reaction schemes, the generation of formic acid results from a number of pathways, which is consistent with its presence at the highest concentration among the intermediates (Figure 2). A total carbon balance on 1,4-dioxane and its byproducts was determined as follows: [total carbon (mm as C)] t ) [1,4-dioxane] t + [intermediates] t + [carbon mineralized] t (13) where the subscript t denotes concentrations measured after t minutes of sonication. The total carbon mineralized (converted to CO 2) was determined using a TOC analyzer. The results of the total carbon analysis at 358 khz are shown in Table 2. For the duration of the experiments, approximately 80-85% of the initial carbon is accounted for as the parent compound, as an intermediate, or as CO 2. Possible physical loss mechanisms include vaporization; continuous sparging is necessary to maintain gas saturation during the experiments. Sparging studies in the absence of ultrasound were performed on 1.0 mm solutions of each identified byproduct for 2 h in order to ascertain volatilization losses. During that VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9 time period, there was a maximum loss of 3-5% for each compound. By assuming a 5% loss for each compound over time, nearly 90% of the organic carbon can be accounted for. The impact of frequency on sonolytic environmental processes has been shown to be significant (55, 56). The results have varied widely due to the use of different types of ultrasonic emitters such as probes or plates and the diversity of reactors incorporated (30, 55). The effect of sonolysis on the degradation rate of 1,4-dioxane was studied over a range of frequencies utilizing the same transducer and reactor system for each experiment to keep the power/ area and power/volume ratios consistent. The ultrasonic reaction conditions included the predetermined optimal sparge gas ratio of 75% Ar/25% O 2 and a power intensity of 5.1 W/cm 2 at four frequencies. The fastest degradation occurs at a frequency of 358 khz followed by 618, 1071, and 205 khz (Figure 6). To explain this phenomenon, the following equation, which reveals that the resonant size of an acoustic bubble is inversely correlated to the emitted frequency, must be considered (57): R r 2 ) 3κP 0 /Fω r 2 (14) where R r is the resonant bubble radius, K (K ) C p/c v)isthe polytropic index, P 0 is the hydrostatic pressure, F is the density of the solution, and ω r is the resonant frequency. Lower frequencies lead to a more violent collapse than higher frequencies due to greater resonant bubble sizes. As the frequency increases, bubble lifetimes are shorter, but there are more cavitation events per unit of time, and the bubble surface area to volume ratio is increased. The effect of this is to increase the mass transfer of OH radicals into the surrounding medium and concurrently increase diffusion of volatile compounds into the bubble (56). At high frequencies, the resonant bubble size is not large enough to produce enough energy upon collapse to form sufficient numbers of OH radicals from water, and a point of diminishing returns is reached. The frequency range proximal to 358 khz appears to optimize both energy from bubble implosion and concomitant mass transfer of reactive species in to and out of the bubble. Since there have been few in depth investigations comparing byproduct formation at several frequencies, the byproducts of 1,4-dioxane decomposition were examined at all frequencies (205, 358, 618, and 1071 khz) in this study. The formation of ethylene glycol diformate, methoxyacetic acid, formic acid, and formaldehyde are shown in Figures 7 and 8. TOC measurements at 208, 618, and 1071 khz were also made. Carbon balance calculations accounted for approximately 80-85% of all carbon, commensurate with what was observed at 358 khz. As seen in Figure 6, the degradation rate of 1,4-dioxane is most rapid at 358 followed by 618, 1071, and 205 khz. The accumulation of each intermediate is clearly the greatest at 358 khz, consistent with the parent compound decomposing most rapidly at this frequency. The accumulation of most intermediates follows the ordering of the 1,4-dioxane degradation rates. However, production of ethylene glycol diformate, formaldehyde, and formic acid is slowest at 205 khz. The order of methoxyacetic acid accumulation (Figure 8A) is different however (358, 618, 205, followed by 1071 khz). Methoxyacetic acid accumulates more rapidly at 205 than at 1071 khz during most of the experiment. The most violent conditions and the hottest temperatures and pressures within the cavitation bubble tend to decrease as the frequency is increased. Evidence for this has been corroborated by both studies on sonoluminescence (58), the emission of light during the ultrasonic irradiation of a liquid, and acoustic cavitation field predictions combining mathematical modeling with ultrasonic experimental observations (59). By examining the reaction scheme leading to the formation of methoxyacetic acid (Figure 3), it is possible that these extreme conditions facilitate the scission of radical X more extensively after the 1,2-H-shift takes place at 205 khz than during cavity collapse at 1071 khz. Although extreme conditions in cavitation bubbles decrease with increasing frequency, cavitation intensity at 358 and 618 khz is still substantial along with an increased number of these radicals capable of reaching the gas phase via higher induced mass transfer rates. At 1071 khz, the mass transfer of solutes in to and out of the cavitation bubble is greater than at lower frequencies, but the energy available for bond cleavage upon bubble implosion is diminished. This distinction accounts for overall higher concentrations of methoxyacetic acid produced at 358 and 618 than at 205 and 1071 khz. Nevertheless, the overall conclusion from these byproduct results indicates that the decomposition pathways at each frequency are analogous. By using ultrasound, a refractory contaminant such as 1,4-dioxane can be decomposed into smaller short-chain organics that can then be degraded more effectively by other means such as biological processes. The results of this investigation have shown that 1,4-dioxane is sonolytically decomposed, leading to the formation of major intermediates ethylene glycol diformate and methoxyacetic acid followed by formaldehyde, formic acid, and glycolic acid. Free radical pathways are significant over the entire range of frequencies investigated (205, 358, 618, and 1071 khz). Sparging gases and frequency also play an important role in optimizing sonochemical effects. Acknowledgments We thank the United States Department of Energy (DOE Grant DE-FG07-96ER14710) and the Purdue Research Foundation (Award ) for funding these studies. Furthermore, we thank Dr. Mihaela Stefan of the Siebens Drake Research Institute for helpful advice and Dr. Changhe Xiao of the School of Civil Engineering at Purdue University for analytical laboratory assistance. Finally, we thank Wilmarie Flores for her efforts in performing some of the experiments. Literature Cited (1) The Merck Index. An Encyclopedia of Chemicals, Drugs, and Biologicals, 11th ed.; Budavari, S., Ed.; Merck and Co. Inc.: Rahway, NJ, (2) Handbook of Toxic and Hazardous Chemicals and Carcinogens, 3rd ed.; Sittig, M., Ed.; Noyes Publishers: Park Ridge, NJ, 1991; p1. (3) Abe, A. Sci. 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