Environ. Sci. Techno/. 1995, 29, 1576- 1580
On-line Monitoring of Aromatic Near-Uhwiilet Fiber-optic Absorptisn Sensor TYE E. BARBER, WALTER G. FISHER, AND E R I C A . WACHTER* Health Sciences Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831-61 13
A prototype ultraviolet absorption sensor was used to continuously monitor the composition of vapor extractant from soil remediation wells at the Lawrence Livermore Dynamic Underground Stripping Site over a period of five weeks. The sensor measured absorption in the spectral region of 230.0-300.0 nm, allowing direct detection of benzene and its derivatives (including toluene, xylenes, and ethylbenzene). The spectra of these compounds contain considerable fine structure, and deconvolution of the mixture spectra allowed a quantitative determination of the concentration of benzene to be made. Relative levels of total aromatic hydrocarbons were also determined. The trends measured by the on-line sensor were in agreement with standard off-line laboratory analyses and were obtained continuously in real time. Continuous monitoring allowed transient events as well as midto long-term trends in the extraction process to be measured.
Introduction Aromatic hydrocarbons are among the most commonly encountered environmentally hazardous chemicals at Department of Energy (DOE), Department of Defense (DOD),and private sites throughout the United States due to their widespread use in solvents, paints and fuels ( 1 ) . The potential carcinogenicity and other health problems associated with these compounds has resulted in great concern over possible exposure and has led to public demand for cleanup of contaminated sites. Remediation is frequently attempted either by excavation followed by ex situ treatment of contaminated soil or by in situ treatment based on continuous pumping and treatment of contaminated groundwater (2). Unfortunately, excavation can be extremely expensive or impractical at many sites, while passive extraction methods tend to be very time consuming ( 3 ) . In the arid western United States, where the vadose zone often extends to a depth of 100 ft or more and experiences huge seasonal variations in depth, such methods are almost completely ineffective. A promising new approach for site cleanup is dynamic underground stripping ( 4 ) . In this process, ground temperature is raised by resistive heating or steam injection (or a combination of both), causing mobilization and volatilization of organic contaminants trapped in the saturated and vadose zones. The volatilized contaminants and steam are then removed from the soil using a vapor and liquid extraction system. Because of the greatly increased extraction efficiency of this approach, it is estimated that for many sites, cleanup time could be reduced from decades to months (5). At a site on the grounds of the Lawrence Livermore National Laboratory in Livermore, CA, it is estimated that between 10 000 and 17 000 gallons of various hydrocarbon fuels leaked from a ruptured underground petroleum storage tank over an extended period. When the leak was discovered in the late 1970s, the fuels had migrated to a depth of about 140 ft below the surface to a position at the interface between the saturated and vadose zones. Furthermore, a subsequent rise in groundwater level spread the contamination and trapped it throughout -40 ft of soil below the present groundwater level (at 100ft). If standard pump and treat methods were used to remove this contamination, it has been estimated that it would require at least 200 years to completely remediate the site. Hence, to facilitate remediation, an experimental dynamic stripping system was installed at the site. The system began operation in November 1992, and during 21 weeks of demonstration tests, over 7000 gallons of fuel were extracted (6). To gain a better understanding of the dynamic stripping process and to monitor its progress, several analytical methods were used. The quantity and composition of the contaminants extracted were measured by both off-line laboratory and experimental on-line techniques (7). Offline methods required sample collection and analysis and included total hydrocarbon measurement by flame ionization and photoionization and individual hydrocarbon speciation by gas chromatography/flame ionization and * Author to whom correspondence should be addressed; e-mail address:
[email protected].
1576 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6,1995
0013-936x/95/0929-1576$09.00/0
C 1995 American Chemical Society
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FIGURE 1. Flow-through sensor head. Monochromatic UV light emerging fmm the optical fiber is collimated by a lens and then passes axially through the optical absorption cell to the photodiode detector.
gas chromarographyhass spectrometry. Because of the high cost and hazards associated with sampling and analysis, samples were collected only once or twice daily. Such a low sampling frequency provides limited insight into the dynamic stripping process and cannot provide sufficient data for real-time optimization and control of the system. To provide effective feedback to the extraction system, continuous, real-time compositional monitoring is required. In this work, a prototype near-ultraviolet absorption sensor was evaluated for use in on-line, real-time measurement of aromatic hydrocarbons extracted during dynamicundergroundstipping. The sensorwasamodified version of other near-ultraviolet absorption devices previously developed at the Oak Ridge National Laboratory (8, 9). This sensor was designed to monitor monoaromatic hydrocarbons (benzene, toluene, ethylhenzene, and xylenes) because of their relatively high water solubility and high concentrations in fuels (10). Compared to other potential on-line methods (such as gas chromatography or flame ionization detection), optical absorptionspectroscopy represents an excellent compromisebetween overall system cost, sensitivity, selectivity, and ruggedness. Spectral information useful for monitoring aromatic hydrocarbons may be obtained in both the near-ultraviolet (uv) and mid-infrared (IR) spectral regions. Unlike IR, however, near-W absorption can be measured in the presence of large concentrations of water with minimal spectral interference. This is an especially important point when working with moisture-saturated samples from the subsurface. In addition, near-W spectroscopy utilizes relatively simple and inexpensive instrumentation that is compatible with remote measurement using fiber-optic sensors. While the absorption spectra of most organic compounds are too similar to allow compound-specific identification based solely on W absorption, aromatic hydrocarhons are a notable exception (8, 9, 11). In the spectral region from 230 to 300 nm, monoaromatic hydrocarbonspossess unique absorption features. allowing identification of individual aromatic species. In addition, most branched and cyclic alkanes are transparent in this band (12),virtually eliminating background interferences from nonaromatic hydrocarbons, such as those from other fuel components present at the site. Both liquid and vapor phases were extracted from wells at the Livermore site. The liquid phase, which contained only a small fraction of the extracted fuel, was sent to a waste water treatment facility. The vapor phase, which contained a majority of the extracted fuel, was directed into an internal combustion engine (ICE) designed to provide a negative pressure (vacuum) on the extraction
wells; the engine was partially powered by the extracted hydrocarbons,cleanlybumingthefuelvapors. A Wsensor head was installed in the vapor phase extraction line at a point immediately ahead of the ICE. The head consisted of an optical absorption cell remotely connected to the rest of the instrumentation by fiber-optic and electrical connections. This configuration was necessary because the sensor head was located in a harsh, outdoor construction site environment. Thesensorwasinstalled,andcontinuous operation beganduringthefirst weekof extraction, allowing collection of W absorption spectra every 30 min over a period of 5 weeks. From these absorption spectra, the absolute concentration of benzene was determined, and relative measurements were made for total aromatics. Since laboratory samples were collected daily, a comparison of sensor performance with standard off-line analytical methods was possible.
Experimental Section The determination of aromatic hydrocarbons was based on measurement of the absorption of vapor components in a flow-throughcell, shown in Figure 1. A small fraction of the extractant was passively diverted from the main extraction line through the cell using 9.5-mm4.d. vacuum tubing. Inlet and outlet sampling ports were separated on the main line by approximately 1.5 m and had a pressure drop of-70 Pabetweenports. Themainlinewasoperated at a vacuum of -10 kPa below ambient. The absorption cell was constructed from commercially available pipe fittings. Quartz windows were held in place at each end of the cell using face seal fittings (Park Fluid Connectors, Huntsville, AL),giving an optical path length of 30 cm. A Wdetectorwas mounted at one end ofthe cell, consisting of a 1-cm-diameter photodiode packaged with a current to voltage operational amplifier (HW-4000B, EG&G Judson, Montgomeryville, PA). A second operational amplifier in avoltage-follower configurationwas mounted at the output of the detector to isolate it from the other electronics and to amplifythesignalbyafactorof 10. The photodiodeand electronics were enclosed in a sealed aluminum housing attached directly to the weld gland of the face seal fitting. Light was transmitted to the other end ofthe cell by ahighOH 600-pm-diameter fused silica optical fiber (Polymicro Technologies, Phoenix, AZ). A single 12.7-mm-diameter f l 2 lens was used to collimate light emitted from the fiber. The collimated beam was directed through the cell to the active surface of the photodiode. The cell was connected to the remainder ofthe instrumentationby25-m-long fiberoptic and photodiode electrical connections, which were sheathedforprotectionina 1.25-cm-diameter rubberhose. VOL. 29. NO. 6.1995 iENVIRONMENTAL SCIENCE &TECHNOLOGY m 1577
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All other instrumentation for the sensor was located in a shed situated at the edge of the dynamic stripping site. A30-W deuterium lamp (L2196,Hamamatsu, Bridgewater, NJ) served as the optical source for the sensor. Light from the lamp was collimated and focused onto the entrance slit of a 0.3-m-focallength monochromator (HR320with 2400 groove/mm, 250-nm blazed grating, Instruments SA, Inc., Edison, NJ) using a dual lens aperture matching system. Slit widths of 50 pm were used throughout the field tests. Light transmitted through the monochromator was launched into the optical fiber (at the end distal from the sensor head) using a second dual lens configuration. This light was modulated at 200 Hz using a mechanical chopper (Model 230, Ithaco, Ithaca, NY) located immediately prior to the coupling optics. A lock-in amplifier (Model 3921, Ithaco) was used to demodulate the photodiode signal. To facilitate data acquisition, the output of the lock-in amplifier was converted to frequency by a function generator (FG500, Tektronix, Beaverton, OR) and subsequently digitized using a frequency counter (SpectraLink IFCNT signal acquisition module with Prism Software, Instruments SA). To obtain a spectrum, the monochromator was scanned from 230 to 300 nm in 0.1-nm steps, acquiring data for 0.2 s at each wavelength. A complete scan required approximately 5 min, and spectra were collected every 30 min. Since only a single detector channel was used, it was necessary to periodically purge the cell with air so that a reference spectrum could be obtained. Timer-controlled solenoid valves installed on the sampling lines allowed the cell to be purged for a period of 1 h every 4 h. Absorption spectra were obtained by taking the negative log of the ratio of the sample and reference spectra. Figures 2 and 3 show representative reference and sample spectra and the resulting absorption spectrum, respectively. To calibrate system response, a syringe pump was used to inject benzene, toluene, and xylene into a calibrated air stream (13). Linearity of this approach was independentlyverified using flame ionization detection (OVA 128, Foxboro, Norwalk, CT). In addition, photometric linearity of the optical system was verified up to 1.25 AU using neutral density fliers. Analytical figures of merit includeinstrument response of 0.000 18 AUlppm (at atmospheric pressure) with a correlation of 0.994, a standard deviation of blank (20observations)ofO.OOO 76AU, and anestimated detection limit for benzene of 13 ppm (30at atmospheric pressure). 1578 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 6,1995
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FIGURE 4. Daily average total aromatic concentration based on integrated area of the absorption spectrum from 234 to 284 nm. "S" represents time when steam injection stopped "R" is when injection resumed. The time axis refers to cumulative site operating days for the dynamic stripping process. The sensor was installed on day 14.
Results and Discussion The near-UV fiber-optic absorption sensor was operated continuously at the Livermore site for 5 weeks. The sensor head was exposed to the outdoor environment and to substantial electrical noise and vibration generated by equipment at the site. The cell was also exposed to rain as well as low humidity conditions and ambient temperatures that varied between 12 and 43 "C. Even under these adverse conditions, the system operated over the course of the 5 week demonstration without hardware failure. To determine relative changes in total aromatic hydrocarbon concentration, the absorption curve was integrated from 234 to 284 nm, a region of common absorption for aromatic species. Figure 4 shows the trends observed for this integrated area averaged over 24-h periods plotted against cumulative time of site operation. The dynamic stripping process utilizes periodic steam injection in conjunctionwith vacuum extraction,and several interesting trends in the efficiency of extraction were discovered with the help of continuous UV monitoring. For example, the rate of aromatic hydrocarbon removal usually decreased upon initiation of steam injection and continued to decrease for the duration of steam injection. After steam injection
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was stopped, the concentration of extracted hydrocarbons present in the vapor stream rapidly increased. The trend observed during injection may be the result of an increase in well pressure that suppresses the rate of contaminant volatilization and migration. Conversely, when vacuum is applied, the drop in vapor pressure at the elevated soil temperatures may cause the residual pore fluids and volatile hydrocarbons to boil, thus increasing the contaminant extraction rates. Test data also showed significant diurnal fluctuations in the absorption of total aromatics as determined by total peak area (Figure 5). These fluctuations corresponded exactlywith recorded fluctuations in ambient temperature along with changes in pressure and flow rate within the vapor extraction system. The correlation between ambient temperature and sensor response led to an analysis of the efficiency of the vapor extraction system. The diurnal fluctuations appear to be caused by changes in condensation rate on the extraction system walls resulting from variations in ambient temperature (higher condensation rates correspond closely with cooler nighttime temperatures). Benzene concentrations were determined from peak height, using the sharp absorption feature at 243 nm. In this wavelength region, the other aromatic components in the mixture show only broad, featureless background absorptions. Since the total absorption at any particular wavelength is a linear summation of the absorption of the individual components present, the peak observed at 243 nm can be attributed to benzene absorption alone. By approximating the background contributions using a second-order polynomial, it was possible to calculate the absorption due to benzene, as shown in Figure 6. The concentration of benzene as measured by both the U V sensor and off-line gas chromatography is shown versus time in Figure 7. Qualitative trends are in excellent agreementwith results obtained bythe analyticallaboratory. Note, however,that the absolute concentrations measured by the sensor were consistently less than those measured by the analytical laboratory. This may be due to uncertainty associated with the pressure and temperature of the cell or errors in calibration of the sensor. Alternatively,systematic errors in sampling or in the laboratory analyses might account for the differences. Unfortunately, time limits at the site necessitated calibration of the sensor after all field measurements were completed. This required complete disassembly, shipment, and reassembly between on-site measurements and calibration, resulting in the possibility
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FIGURE 6. Graphical representation of the procedure used to calculate benzene concentration from mixture absorption spectra obtained at the dynamic striping site. 600 Off-line
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FIGURE 7. Comparison of the benzene concentrations measured by the on-line sensor and off-line laboratory analyses.
of a systematic error in calibration. Significantly, though, this would not affect relative performance of the instrument throughout the course of the demonstration. In addition to benzene, the concentration of each of the other aromatic components could be determined from the existing absorption data by use of multivariate analysis methods. These methods allow the deconvolution of overlapping absorption bands and subsequent determination of concentrations of individual components (14).In order to utilize multivariate methods, the instrument must be calibrated using a set of standards that resemble the matrix and contain independently varying concentrations of the components of interest. Initially, we planned to use off-line gas chromatographic analyses of the vapor stream to develop a calibration set using the on-line sensor data. Unfortunately, the relative concentrations of the target components remained constant throughout the period on site, invalidating this approach, while project constraints precluded subsequent preparation of an accurate calibration set. One of the main advantages of any on-line sensor is the relatively low cost per analysis, which can translate into a much greater sampling frequency than is practical with off-line sampling methods. Standard laboratory analyses for volatile aromatic hydrocarbons frequentlycost between $100 and $500 per sample. In contrast, the total cost per VOL. 29, NO. 6,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of expensive laboratory samples. Also, on-line results should be useful for revealing transient and short-term trends in system performance. And while it was not the subject of this trial, sensors such as the near-W device could be used to obtain real-time feedback for control and optimization of process operation.
ICE Fallure
Acknowledgments
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FIGURE8. Spikes observedin the absorption due to benzene resulting from short-term fluctuations in the extraction system performance. sample using the UV absorption sensor during the course of this field test was approximately $30, including initial instrument assembly, installation, site staffing, and data analysis. We estimate that this cost could easily be reduced by a factor of 10-100 if the instrumentation was fully automated and manufactured as a modular package. It is important to note that the laboratory analyses in this field test represent samples taken only once or twice a day and thus represent only a small fraction of time compared to the overall demonstration. Analyses obtained at this frequency are generally unsuitable for detecting short-term fluctuations in system performance. For example, detection of fluctuations in aromatic concentration with ambient temperature would have been difficult if not impossible. More significantly, off-line sampling cannot detect transient events that may indicate system malfunctions or other rapid process variations. The spikes shown in Figure 8 represent large, very short-term fluctuations in benzene concentration. One of the spikes can be attributed to extraction system failure which occurred on day 33. The other spikes are due to rapid changes in the temperature and pressure of the main vapor line. Such data are extremely useful for optimization of system performance, failure diagnosis and prediction, and compliancewith safety and environmental regulations. On-line data from sensors such as the near-UV absorption device can be used for cost-effective system documentation or, under more sophisticated circumstances, to provide direct feedback for real-time system control.
Conclusions The utility of near-UV absorption monitoring was clearly demonstrated during rigorous field trials at the Livermore site, where the UV sensor was successfully deployed as a monitor of aromatic hydrocarbons in a moisture-saturated vapor stream. Although not utilized in this capacity, it could have served equallywell on the aqueous extractant stream because of,the minimal interferences posed bywater in the near-W. The agreement between trends measured by the sensor and the off-line laboratory analyses suggests that on-line measurements can be used to reduce the number
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The authors would like to thank Robin Newmark and Roger Aines of Lawrence Livermore National Laboratory for their help in deployment of the UV absorption sensor. Without their efforts and advocacy on our behalf, this work would not have been possible. Extensive support was provided at the site by Everett Sorenson, Dennis White, Bill Siegal, and Paul Gronner. The support of Dr. S. P. Mathur, EM551, and of Dr. Caroline Purdy, EM-542, DOE Office of Technology Development,is gratefully acknowledged. This research was sponsored by the U.S. Department of Energy under contract DE-AC05-840R21400with Martin Marietta Energy Systems, Inc.
Literature Cited (1) Sittig, M. Handbook of Toxic and Hazardous Chemicals and Carcinogens,3rd ed.; Noyels Publications: ParkRidge, NJ, 1991; Vols. 1 and 2. (2) Ellison, R. In Hydrocarbon Contaminated Soils; Calabrese, E. J., Kostecki, P. T., Eds.; Lewis Publishers, Inc.: Chelsea, MI, 1991; Vol. 1, p 301. (3) Ross, L. D. Ground Water Monit. Rem. 1993, 8 (41, 92. (4) Aines, R. D.; Newmark, R. L. Energy Technol. Rev. LLNL UCRL52000-92-7; Lawrence Livermore National Laboratory: Livermore, CA, 1992; Vol. 7, p 1. (5) Yow, J. L.; Aines, R. D.; Newmark, R. L.; Udell, K. S.; Ziagos, J. P. Proceedings of Geoenvironment 2000, Baton Rouge, LA, February 1995. LLNL UCRL-JC-115345);Lawrence Livermore National Laboratory: Livermore, CA, 1995. (6) Aines, R. D.; Newmark, R. L.; Ziagos, J.; Copeland, A.; Udell, K. Energy and Technology Review. LLNL UCRL-52000-94-5; Lawrence Livermore National Laboratory: Livermore, CA, 1994; VOl. 5, p 11. (7) Newmark, R. L., Ed. Dynamic Underground Stripping Project: LLNL Gasoline Spill Demonstration Report. LLNL UCRL-ID116964; Lawrence Livermore National Laboratory: Livermore CA, 1994. (8) Hawthorne, A. R.; Morris, S. A.; Moody, R. L.; Gammage, R. B. 1.Eniuron. Sci. Health 1984, A19 (31, 253. (9) Haas, J. W.; Gammage, R. B. Proceedings, DOE Real-time Subsurface Monitoring of Groundwater Workshop; Report No. CONF-9004173-1 (NTIS No. DE90009822/HDM);U.S. Department of Energy, U.S. Government Printing Office: Washington, DC, 1990. (10) Poulsen, M.; Lemon, L.; Barker, J. F. Enuiron. Sci. Technol. 1992, 26, 2483. (11) Hage, R. N.; Jones, V. T. In Hydrocarbon Contaminated Soils; Calabrese, E. J., Kostecki, P. T., Eds.; Lewis Publishers, Inc.: Chelsea, MI, 1991; Vol. 1, p 193. (12) Simons, W. W. The Sadlter Handbook of Ultraviolet Spectra; Sadtler Research Laboratories, Inc.: Philadelphia, PA, 1979. (13) Woodfin, W. J. Gas and Vapor Generating Systems for Laboratories;DHHS (NIOSH)PublicationNo. 84-113;National Institute for Occupational Safety and Health, U.S. Government Printing Office: Washington, DC, 1984. (14) Thomas, E. V. Anal. Chem. 1994, 66, 795A.
Received for review September 26, 1994. Revised manuscript received March 6, 1995. Accepted March 9, 1995.@
ES9405991 @
Abstract published in Advance ACS Abstracts, April 15, 1995.
