Environ. Sci. Technol. 2004, 38, 4856-4859
Mechanisms and Inorganic Byproducts of Trihalomethane Compounds Sonodegradation
OH• + OH• f H2O2
(4)
HO2 + •HO2 f H2O2 + O2
(5)
H2O2 + OH• f •HO2 + H2O
(6)
•
HILLA SHEMER* AND NAVA NARKIS Environmental and Water Resources Engineering, Technions Israel Institute of Technology, Haifa 32000, Israel
Organic pollutants can be sonodegraded by two pathways: pyrolysis, oxidation by free radicals, or the combination of both. The sonolytic degradation mechanisms and byproducts formation of aqueous trihalomethanes (THMs) were investigated at acoustic frequency of 20 kHz. The main sonodegradation mechanism of the chloroform, dichlorobromomethane, dibromochloromethane, and bromoform was found to be pyrolysis. The sonolysis degradation pathway of iodoform is free radicals oxidation. Hydrogen peroxide, nitrate, chloride, bromide, iodide, and iodate ions were detected and quantified as the inorganic products of the THMs sonication. A total of 48% TOC removal was achieved after a 180-min sonication of the THMs mixture.
Introduction Ultrasound provides an unusual mechanism for generating high-energy chemistry (1). Ultrasonic irradiation in liquids induces acoustic cavitation, a process at which bubbles nucleate, grow, and implode in a liquid medium. As a result of the cavitation bubbles collapse, extreme temperatures and pressure are generated. The temperature and pressure inside the gas phase of the cavitation bubble can reach 5200 K and 500 atm. The temperature in the interfacial region, between the solution and the collapsing bubble was estimated at 1900 K. Furthermore, very fast heating and cooling rates, calculated at 1010 K/s, were estimated for the cavitation bubbles (1). In the aqueous solution, organic pollutants can undergo sonodegradation by two possible mechanisms: (i) direct pyrolysis in the gas phase of the cavitation bubbles due to high-localized temperature and pressure or (ii) reactions with free radicals in the bubble interfacial region or in the bulk solution. Free •OH, •H, and •HO2 radicals are formed as a result of water molecules thermolysis during the collapse of the cavitaion bubble, as shown in eqs 1-3. In the presence of oxygen, •O radicals are generated, as shown in eqs 3 and 7. In addition, the strong oxidizing agents hydrogen peroxide and ozone are formed as a result of the reactions either between free radicals or oxygen molecule as shown in eqs 4, 5, and 8 (2): ))))
H2O 98 OH• + H• •
•
(1)
H + O2 f HO2
(2)
H• + O2 f OH• + O•
(3)
* Corresponding author telephone: +972 4 8292145; fax: +972 4 8293925; e-mail:
[email protected]. 4856
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))))
O2 98 O• + O•
(7)
O• + O2 f O3
(8)
Organic compounds sonolysis depends on the characteristics of the organic compounds and at the same time on the sonication conditions. Hydrophobic and volatile organic compounds degrade mainly by thermal decomposition, while hydrophilic and less volatile compounds degrade mainly by OH• radicals oxidation (3). Often there is no simple relation between the various sonication parameters, which include acoustic intensity and frequency, bubbled gas, solvent, solution temperature, pH, contact time, and solute concentration. The trihalomethanes (THMs), which are primarily formed by the reaction of free chlorine with natural organic materials, are considered as priority pollutants by the U.S. EPA (4). Being nonbiodegradable, the THMs are often found in contaminated groundwaters. Therefore, a reliable and destructive method for THMs removal from drinking water supplies is required, since in most physicochemical treatments the THMs are removed from the aqueous medium but are not destroyed. For example, aeration, which was found to be the most efficient treatment for THMs removal, involves the transfer of the contaminants from the aqueous phase to air (5). In this study the removal of the THMs chloroform (CHCl3), dichlorobromomethane (CHBrCl2), dibromochloromethane (CHBr2Cl), bromoform (CHBr3), and iodoform (CHI3) by ultrasonic irradiation was investigated. The sonodegradation mechanisms and byproducts formation were studied.
Experimental Section Materials. Chloroform, tert-butyl methyl ether (MTBE), tertbutyl alcohol (t-BuOH), and sodium sulfate were purchased from Merck, Germany; reagent grade dibromochloromethane and bromodichloromethane were from Fluka, Germany; bromoform reagent grade was from BDH Chemicals, England; iodoform (99%) was from Aldrich, Milwaukee, WI; pentane spectranal was from Riedel-deHae¨n, Germany. All the reagents were used without further purification. Analytical Methods. THMs concentrations in the aqueous solutions were determined by using Varian CP 8410 gas chromatograph with a capillary column 30 m in length, 0.25 µm film thickness, and 0.32 mm i.d. Helium was used as a carrier gas, with a flow rate of 2 mL min-1. Initial oven temperature was 40 °C. The temperature was increased at the rate of 12 °C min-1 up to 90 °C and at 20 °C min-1 to 160 °C. Aqueous samples were extracted using pentane (6) or MTBE combined with sodium sulfate salt (7). Hydrogen peroxide concentration was determined by the iodide method (8). Chloride, bromide, iodide, iodate, nitrite, and nitrate anions in the aqueous solution were determined by Dionex Al 405 ion chromatograph with IonPac AS11 column. TOC was measured with a Shimadzu 5000A, total organic carbon analyzer. Experimental Setup. The ultrasonic irradiation, at 20 kHz, was carried out by Sonics and Materials VCX-400 vibracell. 10.1021/es049852f CCC: $27.50
2004 American Chemical Society Published on Web 07/30/2004
The sonication was performed with a titanium probe (diameter of 2.5 cm) at a power of 18.4 W, as determined calorimetrically (9). The acoustic intensity was 3.75 W/cm2, and the power density was 0.184 W/mL. A 100-mL aqueous solution was sonicated in a 200-mL conical closed glass reactor kept in a temperature-controlled bath. The aqueous solution temperature during sonication was 25 ( 0.5 °C. A 2-mL sample was withdrawn periodically from the aqueous solution using a sampling Teflon tube and a glass syringe. A glycerol trap was used in order to avoid pressure buildup in the reactor. A digital pH meter and a thermometer were used to monitor these parameters in the aqueous solution. All the aqueous solutions were prepared in deionized water. The initial pH was 5.4-5.8 without buffer addition. The sonication was started 10 min after the reactor was sealed, for up to 180 min continually. The initial THMs weight concentration was 10 mg/L. Hence, the molar concentrations differ among the organic compounds studied. Sonolysis experiments were performed using tert-butyl alcohol, which is a known OH• radical scavenger, to suppress radicals reactions. The t-BuOH concentration was in the range of 1.2-15 mM, based on the initial molar concentration of the organic compound investigated, which resulted in molar ratios of 1:30 and 1:100 THM to t-BuOH. All experiments were carried out at least in triplicate.
Results and Discussion Reaction Mechanisms Determination. Sonolysis experiments were performed using t-BuOH, which is a known OH• radical scavenger, to determine whether the sonodegradation of the CHCl3, CHBrCl2, CHBr2Cl, CHBr3, and CHI3 occur via pyrolysis or by free radicals oxidation. The t-BuOH scavenger OH• radicals inside the cavitation bubble and prevent their accumulation at the interfacial region of the bubble (10). Hence, t-BuOH should suppress radicals degradation of the THMs. Since all the trihalomethanes have one carbon atom, the sonodegradation products of both mechanisms are expected to be similar. Choi and Hoffmann (11) identified C2Cl4 and C2Cl6 as the organic radicals reaction products formed during the photocatalytic reaction of chloroform in aqueous TiO2 suspension. C2Br4 was formed as the photodegradation product of the bromoform (11). Respectively, CCl4, C2Cl4, C2Cl6, and HCl were identified as the main products during pyrolysis of chloroform at temperatures of 600-1300 K. The other pyrolysis byproducts detected at trace levels were C2HCl3, c-C6Cl6, C3Cl4, C4Cl6, and C2HCl5 (12). Therefore, the addition of a radical scavenger enabled us to establish the THMs sonolysis mechanisms rather than byproducts identification. Varying concentrations of t-buOH were added to the THMs aqueous solutions, which provided molar ratios of 30:1 and 100:1 t-buOH to the initial THM molar concentration. The residual to initial THM compound concentration ratios (C/C0) during sonication in the absence and presence of various t-BuOH concentrations is plotted in Figures 1-3. From Figure 1, it may be observed that the sonolysis of CHCl3, CHBrCl2, and CHBr2Cl was not inhibited by the addition of t-BuOH. Nevertheless, a 20% reduction in the CHBr3 sonodegradation occurred at molar ratios of 100:1 t-buOH to CHBr3, as shown in Figure 2. These results suggest that the predominated degradation mechanism of CHCl3, CHBrCl2, and CHBr2Cl compounds was ultrasonic irradiation pyrolysis. The CHBr3 was sonodegraded by the combination of pyrolysis and free radicals oxidation. It can be assumed that 80% of the bromoform was thermally degraded by the 20 kHz ultrasonic irradiation and that the remaining 20% was radically decomposed. The CHI3 was found to behave differently. At the absence of t-BuOH, 60% of the initial 0.03 mM CHI3 was sonodegraded
FIGURE 1. Residual to initial THM compound concentration ratios during sonication in the presence of various t-BuOH concentrations, which provided molar ratios of 1:30 and 1:100 THM to t-buOH.
FIGURE 2. Residual to initial bromoform concentration ratios during sonication in the presence of various t-BuOH concentrations, which provided molar ratios of 1:30 and 1:100 CHBr3 to t-buOH.
FIGURE 3. Residual to initial iodoform concentration ratios during sonication in the presence of various t-BuOH concentrations, which provided molar ratios of 1:30, 1:100, and 1:500 CHI3 to t-buOH. within a 180-min reaction. In the presence of 3.0 mM t-BuOH, little degradation of CHI3 took place; approximately 20% sonolysis was achieved. A further increase of the t-buOH concentration, up to 15 mM, did not show any significant change in the CHI3 sonolysis, as shown in Figure 3. From these results it can be concluded that the main degradation of CHI3 proceeds via reaction with free radicals. The reaction at which hydroxyl radicals oxidize the THMs compounds to form halomethyl radical is shown in eq 9 (5):
CHX3 + OH• f •CX3 + H2O
(9)
Once the halomethyl radicals are formed, they may react further according to the following reactions. VOL. 38, NO. 18, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Dihalocarbene has been postulated as an intermediate of THMs sonolysis (eq 10) (13): •
CX3 f :CX2 + Cl•
(10)
Phosogene generation is possible at an oxygen-saturated aqueous solution, as shown in eqs 11 and 12 (13): •
•
CX3 + O2 f •O2CX3
(11)
O2CX3 + H2O f COX2 + HOX
(12)
The COX2 is expected to hydrolyze rapidly into carbon dioxide and HX (eq 13) (13):
COX2 + H2O f CO2 + 2HX
(13)
Dihalocarbene hydrolysis leads to carbon monoxide formation (eq 14) (13):
:CX2 + H2O f CO + 2HX
(14)
The pyrolysis of the THMs is proposed (12) to include the formation of the halomethyl radicals •CX3 and dihalocarbene : CX2, as shown in eqs 15-17. The •CX3 radicals are generated mainly by hydrogen atom abstraction from the THM molecule by the halide anions (eq 16) (13):
CHX3 a : CX2 + HX
(15)
CHX3 + X- a •CX3 + HX
(16)
: CX2 + X- a •CX3
(17)
Dimerization by recombination of •CX3 or : CX2 radicals was postulated by Taylor et al. (12) as the main reactions generating tetrahalomethylene and hexahaloethane, such as C2Cl4 and C2Cl6, respectively (eqs 18 and 19):
2•CX3 f C2X6
(18)
2: CX2 f C2X4
(19)
Some of the expected organic byproducts, based on the reactions described above, are toxic and health hazardous and, therefore, are unwanted in drinking water. In this study, carbon tetrachloride, hexachloroethane, and tetrachloroethylene were not detected experimentally during the THMs sonication. These results indicate that these organic intermediates undergo sonodegradation (13-15) and, hence, did not accumulate in the aqueous solution. THMs Mineralization. The total organic carbon (TOC) and THMs ultrasonic removal is compared in Figure 4. The results refer to a 180-min sonication of the THMs mixture of CHCl3 + CHBrCl2 + CHBr2Cl + CHBr3 + CHI3, at initial concentration of 10 mg/L of each compound. The THMs concentration, measured by the gas chromatograph, indicated that 92% of the total initial concentration of the parent THMs was sonodegraded, whereas only 48% of the TOC was removed. The TOC measurements account for 48% of the mass balance on carbon. The total concentration of all organic intermediates in the aqueous solution was also expressed as the TOC. Thus, 44% of the initial TOC represents halogenated intermediates and products of the THMs ultrasonic decomposing. The remaining 8% are attributed to the parent THMs that were not degraded. The main mineralization was observed in the first 60 min of sonication, which may be due to the production of stable compounds such as organic acids. 4858
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FIGURE 4. TOC and THMs ultrasonic removal during a 180-min sonication of the mixture of THMs in deionized water solution.
TABLE 1. THMs Removal Efficiency and Halide Ions Yield after a 180-min Sonication compound
THMs removal (%)
CHCl3 CHBrCl2 CHBr2Cl CHBr3 CHI3 CHI3 in a mixture
100.0 97.3 91.6 80.2 60.1 88.7
Cl93.0 88.0 82.4
halide yield of (%) BrIIO388.0 84.0 76.0 38.5 77.0
Inorganic Byproducts. Bromide, chloride, iodide, and iodate ions were found to be dominant products of the sonodegradation of the THMs. The halide ions yield is considered as an indication to the ultrasonic decomposition of the THMs. Table 1 shows the halide yield calculated by the following equation:
% halide ion released )
(
100 × [halide ion]IC
)
AW(halide ion) × [organic compound]i MW(organic compound)
Where AW is the halogen atomic weight; MW is the THM compound molecular weight; [halide ion]IC is the halide anion concentration determined by the ion chromatograph (mg/L); [organic compound]i is the initial concentration of the THM compound (mg/L). Based on the THMs input, the halide ions in the aqueous solution were less than those calculated. When CHBr2Cl > CHBr3 > CHI3, as shown in Figure 4. A correlation between the sonodegradation efficiencies of the THMs and their physicochemical properties, such as vapor pressure and boiling point, was observed. Selected physical properties of the THMs are listed in Table 2. The THMs degradation and the sonolytic yield of chloride ions from CHCl3, CHBrCl2, and CHBr2Cl and of bromide ions from CHBrCl2, CHBr2Cl, and CHBr3 increases as their vapor pressures increase and boiling points decrease. These results are in correlation with the determined THMs degradation
mechanisms. The main degradation mechanisms of CHCl3, CHBrCl2, CHBr2Cl, and CHBr3, which more rapidly enter the gas phase because of their relatively high vapor pressures, are pyrolysis inside or in the interfacial region of the collapsing cavitation bubbles. In contrast, the main decomposition pathway of CHI3, which exhibit relatively low vapor pressure, is radicals reactions in the interfacial region and the bulk solution.
Literature Cited (1) Suslick, K. S. Science 1990, 247, 1439-1445. (2) Hoffmann, M. R.; Hua, I.; Hochemer, R. Ultrason. Sonochem. 1996, 3, S163-S172. (3) Drijvers, D.; Van Langenhove, H.; Nguyen Thi Kim, L.; Bray, L. Ultrason. Sonochem. 1999, 6, 115-121. (4) Symons, J. M.; Stevens, A. A.; Clark, R. M.; Geldreich, E. E.; Love, T. O.; Demarco, J. Water Eng. Manage. 1981, July, 50-64. (5) Tang, W. Z.; Tassos, S. Water Res. 1997, 31 (5), 1117-1125. (6) Standard Method for the Examination of Water and Wastewater, 20th ed.; APHA, WPCF, and AWWA: Washington, DC, 1998. (7) Munch, D. J.; Hautman, D. P. U.S. EPA Method 551.1; U.S. Government Publications Office: Washington, DC, 1995. (8) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Environ. Sci. Technol. 1988, 22, 798-806. (9) Kimura, T.; Sakamoto, T.; Leveque, J. M.; Sohmiya, H.; Fujita, M.; Ikeda, S.; Ando, T. Ultrason. Sonochem. 1996, 3, S157S161. (10) Peller, J.; Wiest, O.; Kamat, P. V. J. Phys. Chem. A. 2001, 105, 3176-3181. (11) Choi, W.; Hoffmann, M. R. Environ. Sci. Technol. 1997, 31 (1), 89-95. (12) Taylor, P. H.; Dellinger, B.; Tirey, D. A. Int. J. Chem. Kinet. 1991, 23, 1051-1074. (13) Francony, A.; Pe´trier, C. Ultrason. Sonochem. 1996, 3, S77-S82. (14) Yim, B.; Okuno, H.; Nagata, Y.; Maeda, Y. J. Hazard. Mater. 2001, 81 (3), 253-263. (15) Wu, C. D.; Liu, X. H.; Fan, J. C.; Wang, L. S. J. Environ. Sci. Health, Part A 2001, 36 (6), 947-955. (16) Hua, I.; Hoffmann, M. R. Environ. Sci. Technol. 1996, 30, 864871. (17) Shemer, H.; Narkis, N. Abstracts of the 226th ACS National Meeting; American Chemical Society: Washington, DC, 2003; Vol. 43, pp 1022-1024. (18) Hua, I.; Hoffmann, M. R. Environ. Sci. Technol. 1997, 31, 22372243. (19) Wakeford, C. A.; Blackburn, R.; Lickiss, P, D. Ultrason. Sonochem. 1999, 6, 141-148. (20) Lide, D. R. CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2003.
Received for review January 29, 2004. Revised manuscript received May 30, 2004. Accepted June 19, 2004. ES049852F
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