Chromium Aluminum Technetium (Cr)³ (Al) (Tc) Calcium Iron Thorium (Ca)
Superscript legend: 1. Reference 4. 2. Includes metals tested in the field. 3. Measured at CWL in 1992. 4. Scheduled for measurement at CWL in 1993.
Description
Adsorptive Stripping Voltammetry (ASV) has been demonstrated in the field to measure the concentrations of leachable chromium (Cr), lead, calcium, copper , nickel, zinc, and cobalt in soils and sediments. The demonstration is being done at the Chemical Waste Landfill (CWL) at Sandia National Laboratories (SNL) in Albuquerque, NM. Samples are taken from beneath the CWL during drilling operations and analyzed in a portable laboratory at the site. If successful, this technique will allow decisions to be made in the field during characterization and remedial activities by determining the areal and vertical extent of contamination within hours of the sampling.
Stripping voltammetry has been used for several decades in the laboratory. Recent advances associated with nonelectrolyte (i.e., adsorptive) pre-concentration schemes and the development of miniaturized electrodes have increased the potential for practical use of the technique in the field.
In this program, the samples are dried with a microwave system and digested with a nitric acid leachate. The resulting solution is diluted with distilled or deionized water and analyzed with the ASV technique. For a particular analysis, an electrode(s), with its surface coated with an agent specifically selected to adsorb the specie of interest, is immersed in the solution. The voltammetric response (potential change) of the adsorbed species during the stripping is a function of the concentration of the specie on the electrode. This, in turn, is directly related to the bulk concentration of the specie in the solution through the adsorption isotherm.
The ASV equipment uses 1 A at 120 VAC (120 W). Ten square feet of bench space and a vent hood are also required for operations in the field. Since a different coating must be used on the electrode for adsorption of a particular specie, ASV is not suitable for screening a site to identify the contaminants, but it does measure the concentration of the contaminants after they have been identified by other techniques or from a priori knowledge.
Technical Performance Data
The ASV system has a detection limit of 10^[-10] to 10^[-11] moles/L. Concentrations of chromium and uranium have been measured to 1 ppb in both laboratory and field testing.
The concentrations of two of three trace metals may be measured simultaneously from a single sample. Four hours is quoted as typical for obtaining the results in the field for 12 samples. However, the time required for a single analysis is very test specific. For example, 20 to 30 min. were required to determine the concentrations of Cr(III) and Cr(VI) from a single sample in a field test. However, when the Cr(III) was converted to Cr(VI) by the addition of KMnO4 prior to the analysis, only eight min. were required to measure the total concentration of chromium.
Cost. Start-up cost depends on the analytical requirements for the specific task. The cost for the stripping voltammetry equipment would range from $5K for single element to $37K for multi-element, multi-sampling capability. A microwave digestion unit is approximately $12K. The operations and maintenance costs are dominated by the need for two operators, one of whom may be a technician. The annual cost for chemicals and supplies ranges from $1K to $2K. Life-cycle costs without labor would vary from $20K to $50K for a single ASV system.
Projected Performance
The ASV technique has the potential to measure trace concentrations of approximately 14 metals in addition to those associated with the CWL as shown in the table below. Improvements in the detection limits for some metals may be anticipated.
The following table shows trace metals measured in laboratory and field tests.
Field Tests at CWL Laboratory Testing¹ ²
Chromium Aluminum Technetium (Cr)³ (Al) (Tc) Calcium Iron Thorium (Ca)(Fe) (Th) Cobalt Gold Tin (Co)
Superscript legend: 1. Reference 4. 2. Includes metals tested in the field. 3. Measured at CWL in 1992. 4. Scheduled for measurement at CWL in 1993.
Modifications to the electrodes and processes have also resulted in demonstrated capability of measuring trace levels of electroactive drugs (anti-cancer antibiotics, cardiac glycosides, etc.) and large macromolecules such as insulin and ferritin.
Waste Applicability
The ASV technique is being developed specifically to measure trace concentrations of metals in soils and sediments. However, it will probably be extended to make contaminant measurements of radioactive mixed waste materials in the Mixed Waste Landfill (MWL) at SNL. It could also be used to analyze the contaminants in ground and reactor cooling water and in rain.
Status
The concentration of chromium was measured to the 1 ppb level, and the ability to distinguish between Cr(III) and Cr(VI) was demonstrated during the field tests in June 1992 at the CWL site. During FY 93, the ASV technique was scheduled for extension to the other six trace metals listed in the table. Shortly thereafter the technology should be transferred to a commercial contractor.
Regulatory Considerations
Dilute (1%) nitric acid and very small amounts of elemental mercury (50 mL) are contained in the 100 mL sample used for each analysis. Because of these small quantities, no environmental impacts or risks to the public are anticipated for the ASV process.
Potential Commercial Applications
All the equipment used for the ASV analysis is ``off-the-shelf,'' and at the conclusion of the demonstration it is planned to transfer the technology for an operational system. Thus, the ASV technique should soon be available to the commercial sector for use in measuring the concentration metals in water, soils, and sediments.
The capabilities of the ASV system could also be extended to the metals that have been laboratory tested and are listed in the table. Additional applications in the fields of medicine and pharmacology are possible.
Baseline Technology
The baseline technologies are acid digestion plus inductively coupled argon plasma (ICAP) or atomic adsorption (AA) analyses; both require the use of an off-site laboratory. In addition to being less convenient for field work than ASV, they are slower. Furthermore, they are less sensitive. For example, experiments with chromium indicated that the detection limit for the ICAP technique was an order of magnitude greater than that for ASV.
Intellectual Property Rights
Patent Ownership: None
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References
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