Environ. Sci. Techno/. 1995, 29, 1373-1379
Use of Sonochemistry in Monitoring Chlorinated Hydrocarlions in Water G R A Z Y N A E. O R Z E C H O W S K A , + , + EDWARD J . POZI 0ME K , * ft,* VERNON F. HODGE,§ AND WILLIAM H. ENGELMANN" Harry Reid Center for Environmental Studies, University of Nevada-Las Vegas, Las Vegas, Nevada 89154-4009, Department of Chemistry, University of Nevada-Lus Vegas, Las Vegas, Nevada 89154-101 7, and Environmental Monitoring Systems Luboratoy, US.EPA, Las Vegas, Nevada 89193-3478
The U.S. Environmental Protection Agency (EPA) has been examining the potential of combining sonication with available measurement technologies for monitoring chlorinated hydrocarbons in water. The chloride ion (CI-) concentration, conductivity, and pH were measured before and after sonication. CIcould be detected in aqueous solutions of 3-80 ppm carbon tetrachloride (CC14), chloroform (CHCM, and trichloroethylene (TCE) after 1 min of sonication. The increases of CI- were accompanied by increases in conductivity and decreases in pH. The conductivity changes were higher than expected based on measured CI-. Ion chromatography of solutions before and after sonication showed that formate ion (HCOO-) was also formed. Other ions may have formed as well, but the concentrations were too low to allow their detection relative to HCOO- and CI-. The results achieved serve as proof-of-principle and form a base of information which can be used to develop ultrasound monitoring methods for these compounds. Aromatic and polyaromatic chloro compounds represented by chlorobenzene (Ph-CI) and polychlorobiphenyls (PCBs), respectively, did not release CI- upon sonication as readily as did CClc CHC13, and TCE. The PCB solutions gave no measurable changes in either CI-, conductivity, or pH under the conditions of the experiments described.
Introduction Excellent summaries of the fundamentals of ultrasound are available ( 1 , 2). The classical use of sonication with environmental samples has been to assist in the extraction of semivolatile organic contaminants from soil (3). Interest has also emerged recently in the possibility of using sonication to remediate groundwater (4- 7). Various organochlorine compounds have been sonicated either as aqueous solutions, as dispersions, or in nonaqueous solutions with the formation of a wide range of highly degraded products (1). Early investigations of the effects of ultrasonic irradiation on aqueous solutions of organochlorine compounds and KI were made in 1929by Smith et al. (8). At the end of 1 min of sonication of an aqueous solution of CC4 in the presence of starch and KI, the solution became opaque. A blue color formed which became much more intense than that obtained by 3 min sonication of KI and starch alone. This was cited as evidence of C-Cl bond cleavage causing the acceleration of 12 formation due to presence of Clz. In the early 1950s,Weissler et al. (9)carried out more detailed studies on the ultrasonic oxidation of Iby CCl, as an effect of C-Cl bond cleavage. It was also concluded that the main reaction was between water and dissolved CC4 to give Clz, which then oxidized I-. The oxidation of K I solutionsproceeded only about one-fifteenth as rapidly in the absence of CCk as in its presence. Cheung and co-workers (5)reported preliminary results on the use of ultrasound to destroy organochlorine compounds at the 100-1000 ppmv range in aqueous solution. The GUMS results indicated rapid decreases of methylene chloride (CH2C12)after sonication. Decreases in pH were noted as the CH2C12was destroyed. It was concluded that sonochemical destruction of organochlorine compounds appears to be a potentially powerful method of remediation, which may compete with or serve as an adjunct to other advanced oxidation processes. Toy and co-workers ( 7 ) have considered the use of sonolysis to decompose l,l,l-trichloroethane (C13CCHs) in water at 1-10 ppm levels. The interest was in defining process conditions. It was found that as the volume of the same concentration of C13CCH3 was increased, the sonolysis digestion efficiencydecreased. Petrier and co-workers (10) examined the potential applications of sonochemical treatment as a way to treat toxic wastes containing sodium pentachlorophenate (PCP). The main feature was rapid cleavage of the C-C1 bond giving C1-. Sonication of aqueous solutions of organochlorine compounds leads to different products depending on the compound and conditions. In most of the reported studies the common product is HCl. One report on the sonochemistry of 3-chloropropionitrile in a biphasic system with water, using an ultrasound cleaner, indicated that the major reaction was hydrolysis of the nitrile group to give an amide (11).
In the present research, we examined the potential of combining sonication with existing measurement technologies for monitoring hazardous chemicals in aqueous t Present address: Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA 23529-0126. Harry Reid Center for Environmental Studies. 5 Department of Chemistry, University of Nevada-Las Vegas. U.S. EPA.
*
0013-936~95/0929-1373$09.00/00 1995 American Chemical Society
VOL. 29, NO. 5, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1
1373
solutions. Target organochlorine compounds such as TCE, CHC13, and CC4 were selected since these are the most common pollutants found at hazardous waste sites. Reviewing the literature on the sonochemistry of organochlorine compounds did not lead to any reports on the use of ultrasound for chemical monitoring. However, the reported sonochemistry of organochlorine compounds in water gave much support for the possibility of using sonication in combination with C1-, conductivity, and/or pH changes as a way of monitoring for the pollutants.
TABLE 1
Methods
meters were calibrated dailyfor each series of experiments. The limit of detection for the Cl- ISE electrode was 1.8 ppm. The conductivity and the C1- ISE electrode meters were calibrated in concentration units, i t . , milligrams per liter (pprn),using C1- standard solutions. Calibrationcurve equations obtained with regression analysis showed correlation coefficients of 0.998-0.999. Standard solutions used for calibrating the C1- ISE were in a range 1-15 ppm C1- for deionized water and 50-250 ppm C1- for tap water. Though one of the points (1 ppm) was below the sensitivity limit of the C1- ISE, excellent correlation coefficients were obtained. This allowed estimates through extrapolation down to 0.5ppm C1-. Calibrations of the conductivitymeter were made with standard solutions of 1-25 ppm C1- for deionized water and 100-1000 ppm C1- for tap water. Measurements of conductivity, C1-, and pH were made in samples before and after sonication. The effects of sonication were noted as changes in these parameters. Relative errors of conductivity and C1- measurements ranged from 1%to 10%and were concentration related, i.e., higher errors were found at lower concentrations. The errors were 1% or less in the pH measurements. Examples of levels of ion concentrations and pH for the deionized water and tap water used for sample preparations are presented in Table 1; these represent blanks. The results are averages of six samples each. There were no changes in ion concentrations and pH after sonication of the deionized and tap water blanks. However, there were some variations of those parameters among different samples of tap water. These variations were of no consequence since measurements were always performed before and after sonication. The sample volume used with the cup horn system was 8 mL. This corresponded to the minimum volume needed for measurements with the conductivity cell. The optimum sample volume for use with the 112-in.horn probe was 15 mL. This allowed proper immersion of the probe. In the pulse mode, ultrasonic vibrations are transmitted to the test solution at a rate of one pulse per second. The pulse duration can be adjusted from 10% to 90%, enabling a solution to be processed at full ultrasonic intensity while limiting temperature buildup. The average output power in the 1/2-in. horn probe was 120 W, and in the cup horn it was 100 W. The temperature of a sample after sonication was 30-40 "C and was 27-30 "C for the 112-in. horn probe and the cup horn, respectively.
Sample Preparation. High purity (99%) chemicals were used for sample solution preparation. The chlorinated hydrocarbons including Ph-C1, CHC13, TCE, CCh, and two PCBs (4,4'-dichlorobiphenyland 3,3'-dichlorobiphenyl)and Triton X-100 (a surfactant) were purchased from Aldrich ChemicalCompany,Inc. An aqueous solution of potassium chloride (1000 ppm C1-) was obtained from Orion. Stock solutions of all analyteswere prepared in methanol (MeOH) and used for preparation of the aqueous test samples (1:lOOdilution).ForthePCBsamples, a l : l mixture of the 3,3'- and 4,4'-isomers in methanol was diluted in 1% aqueous solution of Triton X-100. The concentrations of stock solutions were calculated to obtain roughly the same concentrations (ppm) of organic chlorine among samples, except for those of PCBs, which were somewhat lower due to limited availability of reagents. Deionized water was used for preparation of the sample solution unless otherwise stated. Equipment. A Branson Ultrasonic Corporation Sonifer Model 450 (20 kHz) was used for sonication of samples. The unit was equipped with a power supply, a soundproof box, a converter, a cup horn, and a 112-in. horn probe. Though data are presented with both the cup horn and the horn probe, there was no intention of making a real comparison. Emphasis was placed on elucidation of the individual performance of these commercially available items. Sonication was performed in glass screw-cap reaction tubes. The tubes were modified from commercially available screw-cap vials by rounding their bottoms and adding glass supporting rings. The design of the tubes allowed them to be placed into the sonicator cup horn at the same depth. Also, the 1/2-in. horn probe was always immersed to the same depth in the reaction tubes. Sonication Procedures and Measurements. Sample solutionswere pipetted into reaction tubes. Each individual sample tube was closed and then placed in the cup horn system for sonication. In the case of the 1/2-in. horn probe system, the horn was directly immersed in the sample tube. The tube was placed into the cup horn, which also served as a cooling bath. The coolant was pumped through the cup horn using a constant temperature circulator and a peristaltic pump. The temperature of the circulator was set at -10 "C. The cup horn and the 1/2-in. horn probe were operated at the maximum output control setting, Le., 10, during experiments. The experimental parameters investigated included continuous vs pulse sonics (pulse mode 10-go%), time of sonication (1-90 min), different concentrations of analytes, temperature of samples after sonication, and different water sources (deionized vs tap). At least two series of three replicates of each sample concentration and blank were sonicated and measured. The conductivity and pHlion selective electrode USE) 1374 rn ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 5 , 1995
Average levels of Ion Concentrations and pH in Samples of Deioaizerl Water a d Las Vegas Tap Water before Sonication water
conductivity (ppm)
CI- (ppm)
PH
deionized
0.00f 0.00 330 i 4
0.40 f 0.05a 98.3i 1.1
6.30f 0.01 8.37f 0.02
ta P a
Results below the limit of quantitation (ref 15).
Results and Discussion Optimization of Conditionswith the Cup Horn System. Changes in conductivity, C1- content, and pH of TCE solutions with the cup horn at various pulse modes are shown in Figure 1. The sonication time was 10 min. The greatest increase of ion concentrations and the greatest decrease of pH were observed at the 60%pulse mode. Much
'-1
A
CUP-HORN
--y Concentration of CI- Obtained from Ion TABLE 2
..U.M!.N.........................................................................................................................
,o
Chromatography and CI- ISE Measurementsa method 0
sample
E
--. *-*-
. _
0
* - - - * - - -Y
--
---.-Ib.03
4----*
pH
-A
TCE (37 pprn) CC14 (40 pprn) CHC13 (37 pprn) Ph-CI (94 pprn) PCB (55 pprnP 1% aq solution Triton X-100
ion chromatographyb CI- (ppm)
lSECCI(PPm)
2.80
2.67 i 0.18 3.81 i 0.50 4.37 0.25 0.40 i 0.05 0.20 i 0.08 0.20 i 0.02
3.74 4.55 0.50 0.30 0.30
*
a Samples sonicated in the cup horn at 60% pulse mode at 10 min. One sample per compound. CThreesamples per compound, PCB solution in 1% aqueous solution Triton X-100.
CUP-HORN PULSE MODE,60%
g 25 6 20
.....................................................................................................................
0 ,5
......................................................................................................................... ...............................................................
=I
...........................................................................................................................
/
ion was not detected in samples before sonication except for PCB and Triton X-100. This is due to an impurity. Formate ion was detected in all samples after sonication. No other major sonication products were noted in the ion chromatograms. Formate concentrations after sonication were estimated to be the following: 0.29 ppm (TCE),0.88 ppm (CCW, 0.69 ppm (CHC13),and 0.20 ppm (Ph-Cl). These results are semiquantitative at best but give an appreciation of the HCOO- levels. For the PCB and TritonX-100samples, the concentration of HCOO- was the same before and after sonication andwas estimated to be 0.39 ppm. On the basis of the ion chromatogram of Triton X- 100,HCOO-, C1-, and an unidentified substance were concluded to be impurities. The concentration ratios of HCOO-ICl- using the ion chromatography results were calculated as follows: TCE = 0.10 (0.29/2.80),CCb=O.23 (0.88/3.74),CHC13=0.15 (0.69/ 4.55), and Ph-Cl= 0.40 (0.20/0.50). The highest ratio was for Ph-C1, a compound that did not give a high C1- yield. Differences in sonication mechanisms might be reflected by the ratio variations; however,time did not allowa detailed study of this phenomenon. A source of HCOO- ion could also be through the sonochemistry of MeOH, which was used to prbpare solutions of analytes. However, this possibility was eliminated since sonication of water containing MeOH without the chlorinated hydrocarbons did not lead to the appearance of HCOO-. Changes in C1- Concentrations. Changes in C1concentrationversus sonication time in the cup horn system at 60% pulse mode are illustrated in Figure 4. The greatest relative increases of C1- were observedwith CHC13 solutions at each sonication time. The longer the sonication times, the greater were the increases for all analyte solutions.There was measurable but low C1- (less than 0.40 ppm) in Ph-Cl solution after 10 and 20 min of sonication. Changes in C1concentration in PCB solutions were not noted after sonication for 10 min. Changes in C1- concentration for sample solutions after sonication in the 1/2-in. horn probe system are illustrated in Figure 5. The greatest increase of C1- was noted for CC&; smaller changes of C1- were observed for CHC13and TCE. This parallels conductivity changes. There were measurable but very small increases of C1- concentration for Ph-C1 solutions (estimated lower than 0.3 ppm) after 2-min sonication at the constant mode. Changes in C1- vs concentration of CC4, CHC13, and TCE in the range of 3-80 ppm resulted in linear relationships with excellent correlation coefficients (Figure 6). Changes in Conductivity. Changes in conductivitywith sonication time using the cup horn system at 60% pulse
' 1 ilo /
I
........................................................................................................................
-
.......................................................................... q 5 ............................................................................................................................. 0
.5
.03
~---------A---------*---------.
J
I
I
10
, 20
!
60
'
PH
3.9.
Sonication Time (mln) FIGURE 2. Changes upon sonication of 37 ppm TCE vs time.
lower changes were noticed at the constant mode (loo%), which may be explained by losses of power output during sonication. The same pattern of changes was found as the result of 5- and 20-min sonication. The longer the sonication time, the greater the observed changes. Sonication time-dependent changes with the cup horn system at the 60% pulse mode in TCE solution are shown in Figure 2. The greatest changes in ion concentrations were observed at the 60-min sonication time; however, the 5-min sonication time was sufficient to allow increases of conductivity and C1- as well as decreases of pH to be noted. Sonication times greater than 60 min were not examined. Ion ChromatographyAnalysis. Increases in C1- in test samples after sonication were used as evidence that C-Cl bond cleavage occurred. Organic chlorine became measurable as chloride ion. The increases in conductivitywere due to the formation of C1- and other ionic products. Ion chromatography was used to identify products resulting from cup horn sonication experiments at the 60% pulse mode and 10-min sonication time. Four sample solutions of each analyte were examined including a 1% aqueous solution of Triton X-100 that served as the blank for PCB solutions. (Triton X-100 was used to ensure solubility of the PCBs.) One sample solution from each set was analyzed by ion chromatography. Ion chromatograms before and after sonication of a 40 ppm CC14 are presented in Figure 3.
Data of C1- concentrations in samples after sonication obtained from ion chromatography and C1- ISE measurements are presented in Table 2. The data show very good agreement for C1- between the two methods. This confirmed credibility of C1- ISE measurements in the present work. Chloride ion was not detected by either method in the deionized water used for these experiments. Formate
VOL. 29, NO. 5,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1375
pS Before sonication
After sonication Cup-horn Pulse mode, 60% Time, 10 min
cr
;ii
ii ii ii
HCOQ
w
FIGURE 3. Ion chromatograms of
ii
! .--i,w.d-~J I I
I
I
I
40 ppm CCI. Correlation Coefficient 0.995
I.
, ,,' ,'
............................ ...................
CCI,
PULSE MOOE, 80% 1 MINUTE
. . . . . . . . .
I
.......................
lZ1 10
............................................... ..........
~
0.987
.....................
0.957 20
Sonication Time (min) FIGURE 4. Changes in CIV concentration upon sonication vs time.
Chlorinated Hydrocarbon (ppm) FIGURE 6. Changes in chloride ion upon sonication vs initial concentration of chlorinated hydrocarbon in water.
.......................................................................
PULSE MODE, 60% ................................................................................................
................................................................
................................ ...............................
Sonication Time (mln) 8 Pulse Mode (%) FIGURE 5. Changes in CI- concentration upon sonication vs time and pulse mode.
mode are illustrated in Figure 7. The greatest changes of ion concentration were observed at 20 min, and the lowest changes were ohsenred at 5 min. Changes in conductivity for Ph-CI solutions were noted only after 10 and 20 min of sonication. The changes however were very small (1-2.5 ppm). PCB solutionswere sonicatedfor 10min; no changes in conductivity were noted. The greatest changes of ion concentration were ohsenred with solutions of CHCIJ and CCb. Conductivity changes using the 1/2-in.horn probe are illustrated in Figure 8. Sonicationwas performed at 2 min in constant and 60%pulse modes and at 1 min in constant and 80% pulse modes. In contrast to the cup horn system, 1376. ENVIRONMENTAL SCIENCE 5 TECHNOLOGY i VOL. 29. NO. 5, 1995
Sonication Time (mln) FIGURE 7. Changes in conductivity upon sonication vs time.
it was noted that the constant mode is more effectivethan the 60%pulse mode. The changes of ion concentration at 2-min sonication at the 60% pulse mode were at least half those ohsenred at the constant mode. There were no significant differences noted in conductivity changes between 1-min sonication at the constant and 80% pulse modes; they were higher however than those found after 2-minsonicationat the60%pulsemode. Forbothsystems, effectivenessincreased with sonication time. The greatest changes of ion concentration with the horn probe were noted with solutions of CCb, i.e., 21 ppm at 2 min and 8.5 ppm at 1 min at constant mode. In contrast, the greatest
HORN 112"
2511tn" HORN PROBE
........................................................................................................
...................
2min~1009~ 2min~6GA
I min~l0G%
1 min-80%
2.w 2 min-lQQ%
Sonication Time (min) & Pulse Mcde (%) FIGURE 8. Changes in conductivity upon sonication vs time an pulse mode. CUP-HORN PULSE MODE, 60% 6.30
'
'
2 min-6rX
1 min-lOOK
'
1 min-8rX
I
Sonicatlon Time (min) & Pulse Mode (%) FIGURE 10. pH vs time and pulse mode in the sonication of various chlorinated hydrocarbons. 6.23qm 5.774\
,.'
~ m HORN-PROBE ' PULSE HODE.BO% lMiNUTE
5.85
5.40
Ip 4.95
4.w
Sonlcation Time (mln) FIGURE 9. pH vs time in the sonication of various chlorinate hydrocarbons,
changes with the cup horn were observed for CHC solutions. The results were found to be repeatablt Increases in ion concentrations were observed for TC solutionswiththehornprobe, buttherewerenomeasurabl changes for Ph-CI solutions. Changes in pH. Changes of pH in sample solution upon sonication in the cup horn at 60% pulse mode versu sonication time are illustrated in Figure 9. The change were greater with longer sonication times. At 20 min th pH decreased by 2.3 units for CCb solutions. The smalle! relative changes of pH were with solutions of Ph-Cl i sonication time of 10and 20 min. These changes, howeve were much larger than would have been predicted base on C1- or conductivity results (mequiv of C1- = mequiv c H+). The implication is that Ph-CI is reacting but CI- is nc a major product. Hydroxylation may be occurring at a open position of the aromatic ring. Changes in pH were measurable but smallwith solution of PCB. This parallels the lack of changes in CI- an conductivity for PCB solutions mentioned earlier. Changes in pH upon sonication versus time and puls mode in the 1/2-in. horn probe are illustrated in Figure 11 As noted with the cup horn, pH changes were larger wit longer sonication times. Smaller changes were noted 2 the 60% pulse mode in comparison to the constant mod for the same sonication time (2 rnin). There were n differences in pH changes between the constant and 80' pulse modes at 1-min sonication. The changes in pH i 1-min sonication in the horn probe were comparable wit
0
10
20
30
40
50
60
70
80
Chlorinated Hydrocarbon (ppm) FIGURE 11. pH vs initial concentration of chlorinated hydrocarbon in aqueous sonication studies.
those using the cup horn for 10 min of sonication. The greatest changes of pH were observed in CCb solutions at 2 min of sonication. The change was a decrease in 3.8 units of pH using the constant mode. There were measurable changes in pH of the Ph-CI solution after sonication. It was interesting to find that the pH changes with Ph-CI, though small (0.3unit), were significant but much smaller than the pH changes for Ph-CI solutions in the cup horn. As mentioned above, HCOOHwas also formed. Though the amounts were relatively small, they would contribute to the pH decrease. Changes in pH upon sonicationvs concentration ofTCE, CHC13.and CCb in the approximate range of 3-80 ppm are shown in Figure 11. Tap Water Samples. Solutions of TCE (37ppm). CCb (40 ppm), CHC13 (37 ppm), and Ph-Cl (94 ppm) were prepared with our laboratory tap water. Sonication was performed in the cup horn system at 60% pulse mode at 10 min and in the 1/2-in. horn probe at 80% pulse mode at 1min. Using tapwatersolutions, changes in conductivity and pH were observed to be far lower than those in deionized water. This may be explained by the initial pH. AS mentioned earlier, the tap water pH was on the average 8.4, whereas the deionized water pH was 6.5. It is known that the rate of organochloro compound sonolysis increases at lower pH, i.e., pH -4 (3.Also, any bicarbonates and carbonates that are present may act as buffering agents. Furthermore, bicarbonate and carbonate may serve as hydroxyl radical scavengers (IZ), thus possibly inhibiting VOL. 29, NO. 5.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
rn
1377
CUPHORN PULSE MODE, W%
I
TlUF 7"YIU . ........
I
% -COH
-CCI
16
HZ0
I
+HCl
I
11
r W I T H H 0 .
12
......................................................... ~~~~~.~~ ................................... ............................................
...................................
~
H,O
a H. +
-C-CI
+ HO.
..................... ........................
-
HO.
+ HOC1
-C.
,
SECONDARY REACTIONS SUCH AS WITH HmOm L30
..s PH
2
- C-CI +
-C.CI
-E
F.
I '~ 12-
i,
=
c
10-
p%L..J
............
1
...............................................
~.
.................
I
....................................................................... 6-
............. ..................... ............................. 625 4.40
PH
I Tap Water
a HzO
+
I
-c.
2
- C.
+ 2 HOC1
I
+ CI.
I
EXAMPLE
...............................
..........................................................
H,O,
Oda"8z.d water
FIGURE 12. Changes upon sonication of 31 ppm CHCI, in tap and deionized water.
sonochemistry processes involving such radicals. Figure 12 shows the differences in sonication of 37 ppm CHCb in tap and deionized water. The changes obsenred with the horn probe were lower than those from the cup horn comparing either tap or deionized water. Thiswas expected from previous experiments under similar conditions. Reaction Mechanism. As mentioned earlier, the common product in the aqueous sonochemistry of organochlorine compounds is usually HCI. Elucidation of reaction mechanism was not part of the objectives of the present work. However, there are several possibilities (Figure 13). Major mechanisms undoubtedly involve hydrogen and hydroxyl radical reactions and pyTolysis. The relative importance of the other reactions shown in Figure 13is unknown; however, they are logical possibilities. Their importance could certainly vary depending on the nature ofthe pollutant and the conditions ofthe sonication. Under the conditions of the present experiments,CI- was the major ionic product, and small amounts ofHCOO- were detected as well. As was mentioned earlier, HCOO- did not originate from the sonochemistry of MeOH solvent. However, the oxidation of MeOH by sonochemistry reaction products such as Clz or HOC1 is a possibility. Use of ultrasound in combination with CI- ISE appears more applicable to monitoringnonaromatic organochlorine compounds such as TCE, CHC13,and CCL than for aromatic compounds such as Ph-C1 and PCBs under the conditions of the present work. Relatively low yields of CI- were obtained from Ph-CI and PCBs. Low yield of CI- does not necessarily mean that the aromatic compounds did not react. Alogicalexplanationisthat hydroxylradicalsoxidized Ph-Clad the PCB mixturewithout dehalogenation. Sedlak andAndren (13)examinedthe oxidationofPCBsbyhydroxyl radicals generated with Fenton's reagent. (Thisis a buffered 1378 m ENVIRONMENTAL SCIENCE 8 TECHNOLOGY i VOL. 29, NO. 5.1995
1-
CCI, + H,O 2CCI.
CI,+ CO
Ns@ C,CI.
HrO
-GCI
+
H.
-
-C.
+ 2HCl
+ Ch
+ HCI
FIGURE 13. Possible mechanisms in the sonochemistry of organochlorine compounds in water.
solution of H2O2and Fez+.) It was found through GUMS that the halogenated sites were unreactive hut that hydroxyl radical attacked one ofthe PCB nonhalogenated sites. Such a reaction scheme, though applicable to decomposition of Ph-CI and PCBs, does not lead to the immediate formation of c1-. Implications for Chemical Monitoring Methods Development. The use of sonication in combination with measuring changes in C1-, conductivity, and pH in real time is a very simple approach in monitoring organochlorine compounds in water. The major application would he toward fieldscreening oftarget chlorinatedhydrocarbons inwater,thoughthemethodcouldbeusedin thelaboratory as well. Field screening methods are used to determine whether a pollutant of interest is present or absent above or below a predetermined threshold. Screening methods are generally used in situations where the target analytes are already known. Examples would he in determining the progress of remediation at a hazardous waste site and for postclosure monitoring. Screening methods provide relative concentrations for chemical classes, though semiquantitative and quantitative data may he possible. An overview of current field screening technologies and new directions are available (14). The use of sonochemistry in field monitoring of chlorinated hydrocarbons in water may yield semiquantitative data at best, though excellent correlation coefficients were obtained under controlled lahoratoly conditions (seeFigure 6). There are many parameters that can affect the rate of C1- production. These include geometry of the ultrasound cell, temperature, and pH. Also, the presence of humic substances, inorganic salts, and/or suspended particles of various sizes might impede or facilitate the sonochemistry. One may not necessarily he able to provide controls in a field situation to optimize the course of sonochemical
TABLE 3
Summary of CI- Yields (%) under Various Sonication Conditions cup horn compound (ppm)
TCE (37) CHC13 (37) CCli (40) Ph-CI (94)
sonication time
min
5 (60) 3.1 5.6 4.6
10 (60) 10.3 13.3 9.7 0.2
ln-in. horn probe pulse mode ( O h )
sonication time
Deionized Water Solutions 20 (60) 2 (const) 16.8 5.8 25.3 15.2 13.1 22.4 1.8 1.3
2 (60) 3.4 8.2 8.9 0.6
min
pulse mode (%I
1 (const) 2.7 7.5 8.0 1.3
1 (80) 2.6 8.2 6.6 1.8
Tap Water Solutions
TCE (37) CHClj (37) CCl4 (40) Ph-CI (94)
6.7 8.1 10.1 0.2
reactions. In field screening, for situations in which the potential contaminants are known and in which the water system characteristics are understood, optimization may not be needed. Preliminary results in our laboratory indicate that the presence of humic substances does not affect C1- yields. Kotronarou (12) studied the effect of large sand particles (500pm average) and fine particles (7 nm average) on the sonication rate of sulfide oxidation. Large particles might be expected to decrease the rate because of sound attenuation. The fine particles might enhance the rate by providing additional nuclei for bubble formation. The effects of sand particles at the sizes and concentrations studied were insignificant. Destruction of hazardous substances, including organochlorine compounds, using ultrasound has been proposed. References were cited (5- 7 ) . In such cases, high reaction yields are very important. Optimization of C1yield and determination of reactions rates were beyond the scope of the present work. In contrast to remediation scenarios, monitoring scenarios do not require high yields as reflected by the present work. Sufficient C1- was formed under the sonication conditions examined to allow its measurement. Table 3 contains a summary of C1- yields under various sonication conditions for the experiments discussed previously. It is apparent that 5-min sonication with the cup horn at 60% pulse mode or 1-min sonication with the 1/2-in. horn probe resulted in close to 3% or higher yields of C1-. This was sufficient to achieve detection with the commercial C1- ISE for 37-40 ppm of TCE,CHC13,and CCL. Lower concentrations of these compounds could be detectable by increasing the C1- yield or by using more sensitive monitors. Another way would be to increase sonication time. Twenty-minute sonication of the CHC13 solution raised the yield to 25.3%. Bhatnagar and Cheung (6)claim 72-99 % sonochemical destruction of organochlorine compounds under the conditions of their experiments, which involved higher power (approximately 200 W) and longer reaction times (40 min or longer). However, long reaction times and high power may not be practical under field monitoring scenarios.
Conclusions The potential of combining sonication with commercially availablemeasurement technologies for monitoring specific pollutants in water is judged to be high. The results achieved with the organochlorine compounds CCL, CHClB, and TCE serve as proof of principle and form a base of information that can be used to develop ultrasound
3.0 6.1 6.8
monitoring methods for these compounds. Similar approaches in developing ultrasound monitoring methods for other classes of pollutants is judged to be high as well.
Acknowledgments The authors gratefully acknowledge Drs. Brian Johnson, Spencer Steinberg, and Klaus Stetzenbach, University of Nevada-Las Vegas, for allowing the use of ion chromatographs and for helpful discussions. The US. Environmental Protection Agency (EPA), through its Office of Research and Development (ORD), partially funded and collaborated in the research described here. It has been subjected to the Agency’s peer review and has been approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
literature Cited (1) Suslick, K. S., Ed. Ultrasound: Its Chemical, Physical, and Biological Effect;VCH Publishers, Inc.: New York, 1988;336 pp. (2) Mason, T. J. Chemistry with Ultrasound Elsevier Applied Science: New York, 1990; 195 pp. (3) Test Methods for Evaluating Solid Waste, 3rd ed.; EPA SW-846, Method 3550; U.S. Environmental Protection Agency,. U.S. Government Printing Ofice: Washington, DC, 1990. (4) Kotronarou, A.; Mills, G.; Hoffmann, M. R. Environ. Sci. Technol. 1992, 26 (7),1460-1462. (5) Cheung, M.;Bhatnagar, A.; Jansen, G. Environ. Sci. Technol. 1991, 25, 1510-1512. (6) Bhatnagar, A.; Cheung, M. H. Environ. Sci. Technol. 1994,28 (8), 1481- 1486. (7) Toy, M. S.; Stringham, R. S.; Passel,T. 0.SonolysisTransformation of l,l,l-Trichloroethane in Water and Its Process Analyses. Preprint Extended Abstract; Presented before the Division of Environmental Chemistry, American Chemical Society,Atlanta, GA, Aprl4- 19, 1993;American Chemical Society: Washington, DC, 1993; pp 420-421. (8) Smith, F. 0.;Johnson, C. H.; Olson, A. R.I. Am. Chem. SOC.1929, 51, 370-377. (9) Weissler, A.; Cooper, H. W.; Snyder, S. 1.Am. Chem. SOC.1950, 72, 1769-1775. (10) Petrier, C.; Micolle, M.; Merlin, G.; Luche, J. L.; Reverdy, G. Environ. Sci. Technol. 1992, 26, 1639-1642. (11) Farhart, F.; Berchiesi, G. Synth. Commun. 1992,22, 3137-3140. (12) Kotronarou, A. Ph.D. Dissertation, California Institute of Technology, Pasadena, California, 1992. (13) Sedlak, D. L.; Andren, A. W. Environ. Sci. Technol. 1991, 25, 1419- 1427. (14) Poziomek, E. J.; Koglin, E. N. Environ. Lab 1994/1995, DeclJan, 18-24.
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Received for review October 17, 1994. Revised manuscript received January 31, 1995. Accepted February 3, 1995.@ ES940637G @
Abstract published in Advance ACS Abstracts, March 15, 1995.
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