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LANL Develops TRU Waste Mobile Analysis Methods for RCRA-Listed Metals

DOE plans to dispose of approximately 6.2 million cubic feet of transuranic (TRU) waste at the Waste Isolation Pilot Plant (WIPP) site over a 25-year disposal period. Analytical characterization is a critical compliance activity required for most TRU waste destined for treatment and/or disposal. This waste is located at various DOE sites throughout the nation and, generally speaking, the required analytical capabilities for characterization do not exist at the majority of storage sites. Thus, the waste will require costly, difficult, and highly regulated transport to a facility with the requisite capabilities. To help DOE in its characterization activities, researchers in CST and NMT Divisions are working to develop a mobile analytical laboratory capable of measuring metals that fall under the Resource Conservation and Recovery Act (RCRA)(i.e., As, Ba, Cd, Cr, Hg, Pb, Se, Ag,). In addition, Sb, Be, Ni, Tl, V, and Zn are also regulated and must be characterized. The estimated savings of using a mobile laboratory as an alternative to transporting the waste is nearly one hundred million dollars.

In this study, several direct chemical analysis techniques were chosen as potentially suitable for mobile deployment. Direct chemical analysis techniques measure analytes in a solid sample without the need for acid digestion, thus minimizing sample handling and improving analytical throughput (i.e., turn-around time). This research targeted the following techniques: glow discharge mass spectrometry (GDMS), laser induced breakdown spectroscopy (LIBS), dc arc atomic emission spectroscopy (DC-ARC-AES) using a charge injection device (CID) as a detector, laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), and energy dispersive x-ray fluorescence (EDXRF). This project was supported by Laboratory-Directed Research and Development funds.

The method-development phase of this study involved devising sample preparation procedures, instrument parameter optimization (e.g., analyte wavelength selection, laser power settings, etc.), and determination of analytical figures of merit. Each technology developer (see sidebar) was provided with surrogate cemented waste samples. These surrogates, obtained from the Idaho National Engineering and Environmental Laboratory, are actual samples used in the WIPP Analytical Laboratory Performance Demonstration Program (PDP). All laboratories that intend to characterize WIPP waste must qualify their analytical methods through this program.

Following successful development of the analytical method, another set of PDP samples was distributed to each of the researchers in a blind, round-robin format. The Quality Assurance Program Plan for the Transuranic Waste Characterization Program (TWCP) describes the performance criteria for TRU waste characterization, including PDP analysis, that must be met to demonstrate effective analytical performance. The concentrations of analytes in the PDP samples were not known at the time of analysis. After the EDXRF and DC-ARC analytical results were compiled, values for the expected concentration of analytes in each of the evaluation samples were obtained from the Idaho National Energy Laboratory, and results were compared to the known values. Accuracy and precison of the analysis were evaluated using TWCP data-quality objectives.

Evaluation of EDXRF

All analyses were performed using a commercial EDXRF spectrometer. A nearly identical field-transportable instrument is also available, so performance of this method should be similar under field conditions. The instrument has an x-ray tube source with variable source current and voltage up to 1 mA and 50 kV, which permits optimization of excitation conditions for the element of interest. It also uses a high-resolution, electrically cooled Si(Li) detector, which permits simultaneous collection of x-rays of variable energy with minimal spectral interference. The instrument is capable of rapid, multi-element standardization and quantitation of the acquired spectra. Samples were analyzed using an auto-sampling turret in the automated mode overnight; throughput is presently about 10 to 15 samples per day (see figure).

Figure 1. Steve Goldstein (CST-9) loads the autosampler on an EDXRF instrument.

The EDXRF technique meets detection limit requirements for 11 of the 14 metals. Exceptions are V, Hg, and Be; the latter is not detected by EDXRF. The detection limit for vanadium is only a factor of 2 above the required limit, so it is likely that the required limit could be met by increasing data acquisition time by a factor of 4 or by increasing sample size. However, the required detection limit for mercury is a factor of 40 lower and apparently unattainable by direct EDXRF techniques. Additional field-based techniques, which can more sensitively measure both mercury and beryllium, are required for waste characterization of these RCRA metals.

Results of the round-robin analysis show the majority of results meet the TWCP data quality objectives for percent recovery (+20%) and relative percent difference (+30%). Only chromium and mercury deviated slightly from the required recovery for two of the blind samples. Vanadium recoveries for two samples were low, but not enough to reduce the analytical score. Further studies are needed to determine the effects on method performance of inherent radiation of the samples and matrix variability, although these effects are expected to be relatively minor.

Evaluation of DC-ARC-AES

DC-ARC-AES is a bulk-solids analytical technique that uses a solid-state integrating detector to measure the spectral emission intensities produced when a sample is vaporized and excited by a dc arc. The CID detector represents relatively new technology that has the potential to improve analytical performance over the traditional dc arc and conventional spectroscopic techniques. Samples are pulverized, mixed with graphite powder, and burned in the lower of two vertically mounted graphite electrodes. The detector chip is similar to a photographic plate in that it provides for continuous wavelength coverage; hence, most elements in the periodic table can be determined if present in sufficient quantity. Potential analytical benefits over conventional spectroscopic methods include full elemental fingerprinting of the sample, the ability to detect weak spectral lines in the midst of strong matrix signals, improved sample throughput, simultaneous background correction, minimal sample preparation, and instrumental ruggedness.

The DC-ARC technique did not perform as well as expected on the round-robin test. Only 3 analytes (Ba, Be, and Ag) met the detection limit criterion. Detection limits obtained for Sb, Cd, Cr, Pb, and Tl were high by approximately a factor of 2. They may be improved by further optimization. The reason for the high detection limits is that interfering elements, especially Fe, produce background spectral interference on most of the target analytes. Fe is present in these samples at relatively high concentrations (>6000 ppm). The low detection criterion for Hg precludes analysis by dc arc as well as any other emission technique. Results of the blind round- robin test show element recoveries ranging from 72%­133%, except for the 47% recovery for V on one of the blind samples. The majority of elements that failed the TWCP objective for percent recovery failed by less than 10%.

The correction of two major problems may improve the results. First, background correction must be more precise. The five-fold dilution of the matrix, as tested, results in concentrations of interfering elements at very high levels. Over- or undercorrection of these interferences results in biases. Secondly, the technique has poor precision. It is believed this is due to a nonhomogenous sample with varying particle sizes. Better sampling, grinding, and sieving processes could result in improved precision and accuracy. An improved method with a greater dilution of the sample with graphite powder has been tested, and the preliminary results show improvements in the detection limits and recoveries for several of the elements.

Conclusion

EDXRF appears to have great potential for TRU waste characterization for the majority of regulated metals, with the exception of Hg and Be. A solid-sampling, automated, cold vapor, atomic absorption spectrometer has been used to qualify Hg analysis for the PDP program. This instrument is small, rugged, and can be readily installed in a mobile laboratory. While the GDMS, LA-ICPMS, and LIBS work is not complete, emission techniques can measure Be accurately at very low concentrations. LIBS, which is compact, portable, and cost-efficient, will probably prove to be the technique to use for Be determinations.

This article was contributed by Cynthia Mahan (NMT-1). Other technology developers on this project are Randy Drake, Debbie Figg, and Dave Wayne (NMT-1), and Steve Goldstein (CST-9). This project was funded by Environmental Management-LDRD.


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