Environ. Sci. Technol. 1992, 26, 1460-1462 (3) Baldi, F.; Filippelli, M.; Olson, G. J. Microb. Ecol. 1989, 17, 263. Filippelli, M. Anal. Chem. 1987, 59, 116. . . Decadt, G.; Baeyens, W.; Bradley, D.; Goeyens, L. Anal. Chem. 1985,57,2788. (6) Jensen, S.; Jernelov, A. Nature 1969, 223, 753. (7) Rowland, J. R.; Davies, M. J.; Grasso, P. Nature 1977,265, 718. (8) Craig, P. J.; Bartlett, P. D. Nature 1978, 275, 635. (9) Imura, N.; Sukegawa, E.; Pan, S. K.; Nagao, K.; Kim, J. K.; Kwan, T.; Ukita, T. Science 1972, 172, 1248. (10) Jackson, J. A.; Blair, W. R.; Brinckman, F. E.; Iverson, W. P. Environ. Sci. Technol. 1982, 16, 110. (11) Bricker, J. L. Anal. Chem. 1980,52, 492. (12) Olson, G. J.; Brinckman, F. E.; Blair, W. R. In Waste Testing and Quality Assurance; Friedman, D., Ed.; ASTM STP 999; American Society for Testing and Materials: Philadelphia, PA, 1988; pp 130-145. (13) Baldi, F.; Cozzani, E.; Filippelli, M. Environ. Sci. Technol. 1988, 22, 836. (14) Heller, S. R.; Milne, G. W. A. EPAINIH Mass Spectral Data Base; U.S. Department of Commerce: Washington,
DC, 1975; Vol. 2, pp 1565, 1768. (15) Rao, C. N. R. Chemical Applications of Infrared Spectroscopy; Academic Press: New York, 1963; pp 8-16. (16) Barbaras, G. D.; Dillard, C.; Finholt, A. E.; Wartick, T.; Wilzbach, K. E.; Schlesinger, H. I. J. Am. Chem. Soc. 1951, 73, 4585. (17) Bordwell, F. G.; Douglass, M. L. J . Am. Chem. SOC.1966, 88, 993. (18) Porter, T. L.; Davis, S. P. J. Opt. SOC.Am. 1963,53,338-343. (19) Donard, F. X.; Weber, J. H. Nature 1988, 332, 339. (20) Brinckman, F. E.; Olson, G. J. Mar. Chem. 1990,30, 147. (21) Speciation of Mercury-Third Part: Intercalibration excercise on mercury determination in tuna fmh muscle (T-22); Bureau Communautaire de References: Brussels, Belgium, October 17,1990; Report 7/91. (22) Bloom, N. Can. J. Fish. Aquat. Sci. 1989, 46, 1131. Received for review October 22, 1991. Revised manuscript received February 24,1992. Accepted March 13,1992. This research was supported by two CCE subcontracts EV4V-0136-I and E V4V-0125-I .
COMMUNICATIONS Decomposition of Parathion in Aqueous Solution by Ultrasonic Irradiation A. Kotronarou, G. M l l l ~and , ~ Michael R. Hoffmann" W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 9 1 125
Introduction
Experimental Section
Parathion (0,O-diethyl o-p-nitrophenyl thiophosphate) is a major pesticide that is used in large quantities worldwide. Organophosphate esters such as parathion have been used as alternatives to DDT and other chlorinated hydrocarbon pesticides. However, the organophosphate esters are not rapidly degraded in natural waters. At 20 "C and pH 7.4, parathion has a hydrolytic half-life of 108 days and its toxic metabolite, paraoxon, has a similar half-life of 144 days (1). The chemical effects of ultrasonic irradiation were first reported more than 50 years ago (2). However, little research has been focused on the detailed kinetics and mechanisms of chemical reactions in the presence of ultrasound. Ultrasound has been used to induce or accelerate a variety of reactions (3-5). Although ultrasound has a broad range of industrial applications ( 3 , 4 ) ,its potential as a water and wastewater treatment alternative has not been explored extensively. We have recently presented the results of investigations of the kinetics and mechanisms of the sonochemical transformations of p-nitrophenol(6) and hydrogen suEde (7) in aqueous solutions at 20 kHz and 75 W cm-2. These results indicate that ultrasonic irradiation may present an alternative treatment method for the elimination of selected organic and inorganic contaminants present in water. In this communication, we report on aspects of the sonolytic degradation of parathion in aqueous solution.
Ultrasonic irradiation of 25-mL parathion-saturated deionized water solutions was conducted in a water-jacketed stainless steel cell (Sonics & Materials, 50-mL capacity) with a Branson 200 sonifier, operating at 20 kHz and -75 W cm-2. The temperature of the sonicated solution was kept constant at 30 "C (the temperature of the flow-through cooling water was 15 "C). All sonolytic reactions were carried out in air-saturated solutions in a stainless steel reactor with the sealing sampling ports. The concentration of the parathion hydrolysis product, p-nitrophenol (PNP), was determined spectrophotometrically in alkaline solution with a Shimadzu MPS-2000 UV-vis spectrophotometer. The measured absorbance at 401 nm [ E = 19 200 M-l cm-' (S)] was corrected for small contributions due to 4-nitrocatechol absorption (4-NC) at this wavelength [ E = 6500 M-l cm-' (S)]. Nitrite, nitrate, sulfate, phosphate, and oxalate ions were determined with a Dionex 2020i ion chromatograph and a Dionex AS4-A column. The eluent was a mixture of 14.7 mM ethylenediamine, 10 mM NaOH, 10 mM H3B03,and 1 mM Na2COB. Water was purified with a Milli-Q/RO system to 18 MQ resistivity.
'Present address: Department of Chemistry, Auburn University, Auburn, AL 36849. 1460
Environ. Sci. Technol., Vol. 26, No. 7, 1992
Results and Discussion Sulfate, nitrite, nitrate, p-nitrophenol (PNP), phosphate, and oxalate were identified as products of parathion sonolysis. Figure l shows the time-dependent variations in the concentrationsof the major products and intermediates in the sonolysis of a saturated parathion solution. The
0013-938X/92/0926-1460$03.00/0
Q 1992 American Chemical Society
1.5
1 :0
0.5
2.0
Sonication time (hours) Flgure 1. Products of parathion sonication at 75 W cm-', T , = 30 82 pM. O C , pH, = 6.0, and [parathion],
maximum solubility of parathion in water is 82 pM (24 pg mL-') at 25 O C (9). After 2 h of sonolysis, [Sot-] was found to be 82 p M . From this result, we concluded that all of the initial parathion was degraded in 1 2 h at 30 "C under sonolysis at 20 kHz with an intensity of 75 W ern-,. In addition, we found that the [S042-]increased linearly with sonolysis time with a zero-order rate constant of k,, = 0.68 pM m i d . Figure 1 also shows that the sum of [NO,] + [p-NP],where [NO ] = [NO,-] + [NO,], was formed at the same rate as Sot. PO:- and C2042-were formed at slower rates. We showed previously (6)that PNP degrades sonochemically by denitration to yield NO2- and NO3-. Therefore, [NO,] + [p-NP] can be accepted to represent the total amount of PNP that was formed from the decomposition of parathion. Since the reactants were unbuffered, the pH of the solution changed during sonication. After 30 min of sonication, the pH dropped from 6.1 to 4.1 and remained close to that value thereafter (pH = 3.9 after 1 h and pH = 3.7 after 2 h). The observed decrease in pH is consistent with the formation of H+ during ultrasonic irradiation of PNP under similar experimental conditions (6). Over a broad pH range, p-nitrophenol is found to be the major hydrolysis product of parathion (1). When hydrolysis is catalyzed by metal ions, deethylation of parathion is also observed with the formation of O-ethyl-O(pnitropheny1)monothiophosphoric acid as a secondary product (10). The stoichiometric equations for these two reaction pathways are
i
-
( C , H s O ) , - P 4 1 ~ 0 ,
-
1
( C 2 H s O ) , P 4 H t BO
'' 0
NO,
(l)
In alkaline solution, eq 1 proceeds via nucleophilic substitution of the nitrophenolate group by OH-, while in acidic solution, rupture of the 0-C bond occurs as shown in eq 2 (11). Our experimental results suggest that when parathion is exposed to electrohydraulic cavitation during sonolysis, both the P=S and the P-nitrophenolate bonds are broken and Sod2and PNP are formed. Deethylation of the resulting diethylphosphate moiety, which leads to the formation of Po43and C204,-, appears to proceed at a
slower rate. p-Nitrophenol has been shown to degrade during sonication (6) to NO,, benzoquinone, 4-nitrocatechol, and organic acids (formate, acetate, oxalate) and eventually to NO, and C02with a half-life of 30 min under the above experimental conditions. The overall effect of sonication on parathion is shown schematically in Figure 2. The chemical effects of ultrasonic irradiation are a direct result of acoustic cavitation. Sound waves traveling through water with frequencies greater than 15 kHz force the growth and subsequent collapse of small bubbles of gas in response to the passage of expansion and compression waves. The greatest coupling occurs when the natural resonance frequency of the bubble equals the ultrasonic frequency (e.g., 20 kHz equals a bubble diameter of 130 pm). The chemical effects are realized during and immediately after collapse of a vapor-filled cavitation bubble (3,12,13).In the interior of collapsing cavitation bubbles, transient temperatures approaching 5000 K have been measured (3, 4 , 14-16) and pressures of several hundred atmospheres have been calculated (17). Temperatures near 2000 K have been determined for the interfacial region surrounding a collapsing bubble (4,14,15). During bubble collapse, which occurs within 100 ns, H20 undergoes thermal dissociation to give H' atoms and 'OH radicals: H20
A H' + 'OH
(3)
The 'OH radicals, which are produced in the hot vapor phase, may react there or they may diffuse into and react within the surrounding liquid phase. Parathion, which has a low vapor pressure (8),is not expected to enter the gas phase of the cavities during sonication. The reaction pathways for parathion sonolysis appear to be the same as those proposed for the sonolytic degradation of PNP (6). In this case, parathion appears to undergo thermal decomposition in the hot interfacial region of the collapsing cavitation bubbles and degrades secondarily via reaction with 'OH radicals. Sonochemical reactions are normally characterized by the simultaneous occurrence of pyrolysis and radical reactions, especially at high solute concentrations (18). Volatile solutes will undergo direct pyrolysis reactions within the gas phase of the collapsing bubbles or within the hot interfacial region. Pyrolysis (Le., combustion) in the interfacial region is predominant at high solute concentrations, while at low solute concentrations, free-radical reactions are likely to predominate (6). In the bulk solution, the chemical reaction pathways are similar to those observed in aqueous radiation chemistry [as induced by aquated electrons (Eaq), y-rays, or X-rays]. However, evidence for combustion-like reactions at low solute concentrations with nonvolatile surfactants and polymers has been presented (19). Sonolysis is relatively inefficient with respect to the total input energy. Under our experimental conditions, only a small portion of the total energy supplied to the direct immersion horn system results in useful free-radical reactions (7). We have shown (7,20)that sonolysis reactors with larger radiating surfaces result in greater energy efficiencies than the direct immersion probe reactor, which has a small (1cm2)radiating surface. We have found that the same energy input dispersed over a broader area results in a significant enhancement in reaction rate and energy utilization efficiency. The low energy utilization efficiency may limit the use of direct probe sonolysis to special applications such as groundwater remediation or for low-flow pretreatment of hazardous industrial wastes. Environ. Sci. Technoi., Vol. 26, No. 7, 1992
1461
S
II
Sonolysfs
OZN ~ O - ! - ( O C H z C H 3 ) ,
OH
OzN
HO-P-(OCHzCH,)z
t
N r s r Phase
Hfl
H
o
o
o
H
Sonofysis
O
z
N
O
O
-
H
-
t
t
~ 2 0 4 ' - , HCO;,
Second Phase
O S
NOz', NO3-, HC
=
O
CH,CO,'
=
Sonolysfs
II
HO-P-(OCHzCH,),
P043.,
--
sod2., C2H50 H
Second Phase Sonolysfs
-------t Pod3., SO4'., NO3-, C 0 2 , H+ (Final P r o d u c t s )
(Intermediates)
Final Phase
Figure 2. Schematic presentation of the effect of ultrasonic Irradiation on parathion.
An attractive alternative to the probe reactor system is the high-intensity near-field acoustical processor (NAP) ( 4 ) . The NAP reactor system consists of two sonicated metal plates that form the sides of a rectangular flowthrough pipe; one plate has a set of transducers operated at 16 kHz while the opposing plate has a similar set of transducers operated at 20 kHz. In this configuration, a liquid flowing between the plates, which may be as large as 0.5 m X 3 m with a plate separation of 0.08 m, is exposed to an ultrasonic intensity that is greater than that expected from a simple doubling of a single plate due to reverberation of the ultrasound. This technology has already been used on a large scale for the extraction of oil from oil shale ( 4 ) with volumetric flow rates approaching 300 L min-'. Sonolysis does not require the addition of chemical additives to achieve viable degradation rates. However, we (7) have shown that iodide can be utilized as an effective sonolytic catalyst for reactions involving 'OH. In the case of H2S oxidation (7), the catalytic effect (e.g., a 3-10-fold enhancement in overall reaction rate) occurs as follows: I-
- + + -+ 'OH
HS- + IOHHS'
IOH-
HS'
0 2
O2
I-
(4)
+ OH-
S042-
(5) (6)
In a similar way, we (20) have used I-to accelerate the = 3 min) sonolytic degradation of carbon tetrachloride as follows:
C C ~ ,,4 cc13* + c1*
+ CC13'
(7)
+ CC130H CC130H C12C=O + H" + C1'C12C=O + H20 C02 + 2H+ + 2C1IOH-
-+
I-
-+
-+
2C1'
- Clz
Hz0
HOC1 + H+
+ C1-
(8) (9)
(10) (11)
These results suggest that sonolysis may be extremely useful for the remediation of groundwaters contaminated with CC,, which is difficult to treat economically by 1462
Environ. Sci. Technol., Voi. 26, No. 7, 1992
conventional methods. Registry No. Parathion, 56-38-2; p-nitrophenol, 100-02-7; benzoquinone, 106-51-4;hydroquinone, 123-31-9 4-nitrocatechol, 3316-09-4; formic acid, 64-18-6; oxalic acid, 144-62-7; diethylmonothiophosphoric acid, 2465-65-8.
Literature Cited (1) Faust, S. D.; Gomaa, H. M. Environ. Lett. 1972, 3, 171. (2) Richards, W. T.; Loomis, A. L. J . Am. Chem. Soc. 1927, 49, 3086. (3) Suslick, K. S.,Ed. Ultrasound Its Chemical,Physical, and Biological Effects; VCH: New York, 1988. (4) Mason, T. J.; Lorimer, J. P. Sonochemistry. Theory, Applications and Uses of Ultrasound in Chemistry; Ellis Horwood, Ltd.: Chichester, W. Sussex, UK, 1988. ( 5 ) Henglein, A. Ultrasonics 1987, 25, 6. (6) Kotronarou, A,; Mills, G.; Hoffmann, M. R. J . Phys. Chem. 1991, 95, 3631. (7) Kotronarou, A,; Mills, G.; Hoffmann, M. R. Environ. Sci. Technol., in press. ( 8 ) Kortum, G. 2.Phys. Chem. 1939, 42, 39. (9) Williams, E. F. Ind. Eng. Chem. 1951, 43, 950. (10) Hilgetag, G.; Teichmann, H. Angew. Chem. 1965, 77,1001. (11) Blumental, E.; Helbert, J. B. Trans. Faraday SOC.1945, 41, 611. (12) Sehgal, C. M.; Wang, S. Y. J. Am. Chem. Soc. 1989,103, 6606. (13) Henglein, A. Ultrasonics 1987, 25, 6. (14) Suslick, K. S.; Hammerton, D. A.; Cline, D. E., Jr. J. Am. Chem. SOC.1986,100, 5641. (15) Flint, E. B.; Suslick, K. S. Science 1991, 253, 1397. (16) Suslick, K. S.; Choe, S. B.; Chichowlas, A. A.; Grinstaff, M. W. Nature 1991, 253, 414. (17) Shutilov, V. A. In Fundamental Physics of Ultrasound; Gordon & Breach Science Publishers: New York, 1988. (18) Henglein, A.; Kormann, C. Int. J . Radiat. Biol. 1985, 40, 251. (19) Alegria, A. E.; Lion, Y.; Kondo, T.; Riesz, P. J. Phys. Chem. 1989, 93, 490. (20) Kotronarou, A. Ultrasonic Irradiation of Chemical Compounds in Aqueous Solutions. Ph.D. Thesis, California Institute of Technology, 1992.
Received f o r review September 6, 1991. Revised manuscript received March 26, 1992. Accepted March 31, 1992.
