Description
A fiber-optic sensor is being developed to monitor carbon tetrachloride (CCl4) at the Hanford Site. Fiber-optic based sensors are a relatively new type of detection and monitoring technology that facilitates in situ analyses of inaccessible and inhospitable environments. This new technology has already been applied to monitoring and a variety of environmental parameters and toxic contaminants found in the vadose zone and groundwater. The technology is based on the ability of fused quartz optical fibers to transmit probe signals of visible or near infrared light long distances to remotely located sensors. The signals passing through the optical fibers are immune to electromagnetic interferences and can be readily multiplexed to a single optical detector to provide a real-time and multipoint monitoring capability.
The most common fiber-optic sensors incorporate transduction mechanisms that either monitor a wavelength-dependent optical attenuation of the probe beam or the production of fluorescence emissions in regions of the optical spectrum that are distinct from the probe's frequency. Generally, the chemical species that are targets of environmental analyses do not absorb visible light or produce fluorescence emissions even when they are highly concentrated, so detecting these analytes at the relatively low, parts-per-million levels normally found in contaminated environments requires an indirect method of detection.
The most successful approach to detecting these low analyte concentrations use organic dye molecules that interact reversibly and specifically with the chemical species of interest. The physical-chemical reaction of the analyte with the organic transducer produces a change in the photophysical characteristics of the dye that modulates the probe beam. The magnitude of the optical response is correlated to the concentration of the analyte. When the nature of the analyte-dye interactions is known, specific dye molecules can be selected to detect a narrow range of related chemical species.
The configuration of a fiber-optic sensor system is not complicated and requires a simple light source (a light bulb or a light emitting diode), a detector (silicon photodiode), and simple optics (lens, filter, and mirrors) to focus and guide light into and out of the fiber-optic conduit. The same fiber can be used to transmit the probe beam to the sensor, as well as to carry the modulated signal back to the detector. Consequently, at the proximal end of the fiber is a small calculator-size box of optics and electronics that contains both the light source and signal detection equipment. The electronics box is configured to a small central processing unit (CPU) or a lap-top computer that collects and analyzes the sensor signals and provides useful information on the analyte concentration. At the distal and working end of the fiber is the sensor, normally encased in a protective metal shield to prevent damage.
The efficient transmission qualities of optical fibers, the small cross-sectional area of ruggedized fiber-optic cables and sensors, and the simple and small-scale sensor optics and electronics enlist fiber-optic based sensors as ideal candidates for monitoring vadose zone contaminants in wells, boreholes, and other remote and hard-to-reach locations.
Technical Performance Data
This technology provides a real-time sensor for monitoring volatile organic compounds (VOCs) in the gas phase. The sensors can be in situ and operate continuously to provide trend data, as well as immediate fluctuations in CCl4 levels. Fiber-optic sensors can operate automatically and unattended, have sensors placed in remote or hazardous areas, are low in cost and low in maintenance, and can continuously log data for subsequent analysis. The sensors are very small and can be solar powered.
Cost. These units are not yet available; however, a central electronic package that can monitor sensors from several locations may cost approximately $10K. The sensors will be comparatively inexpensive and should cost less than $100. The costs associated with fiber optic based sensors will depend upon the sampling protocols and application for which the technology will be used. The estimated cost for daily operation is negligible. The routine maintenance will involve weekly or monthly checks on the system and should be negligible other than manpower expenditures. Life-cycle costs are estimated at $25K/yr (5 yrs.) and $1.25K/yr (10 yrs.).
Projected Performance
The sensors being developed for measuring CCl4 at the Hanford Site are intended to detect low part-per-million to part-per-billion gas-phase levels of this contaminant. As yet, direct measurement of VOCs dissolved in water has not been accomplished, and all measurements have been made in the vapor phase above contaminated aqueous solutions. However, the issue of direct measurement of VOC's in water will be addressed.
These sensors are not anticipated to satisfy the Environmental Protection Agency's (EPAs) analytical procedures for low level analysis.
Waste Applicability
Fiber-optic sensors are applicable to the monitoring of CCl4 and other classes of VOCs in groundwater, vadose zone, and vapor extraction off-gases. Monitoring is applicable in situ or ex situ.
Status
The system is currently available.
Regulatory Considerations
This technology does not involve any chemical or physical hazards to workers, is inherently safe, and poses no risk to the environment. This technology reduces sample handling, thus further reducing worker exposure to contaminants.
Potential Commercial Applications
This technology is applicable to the monitoring of CCl4 and VOCs in wells and at remediation sites. In addition, remote monitoring of waste sites or on-line process streams may also be possible. The sensors can provide trend data so that changes in contamination levels can be tracked, or the sensors can function as alarms to provide an indication of CCl4 breakthrough, as in granular activated carbon (GAC) beds or aquifers.
Baseline Technology
The baseline technology is gas chromatography.
Intellectual Property Rights
Patent Ownership: DOE
Other Owners: Patent licensed to Purus, San Jose, CA
Patent Number: 4,834,497 (May 30, 1989)
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Principal Investigator
None at present.
References
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