The measurement of leach rates: A review

NUCLEAR ANDCHEMICAL WASTEhiANAGEMENT, Vol. 3, pp. 117-123.1982 Printed in the USA. All rights reserved.

THE MEASUREMENT

0191-815X/82/020117-07W3.00/0 Copyright o 1982 Pergamon Press Ltd.

OF LEACH RATES: A REVIEW

J. E. Mended Pacific Northwest Laboratory, Richland, Washington 99352, USA

ABSTRACT. A historical perspective of the techniques that can be used to measure the leach rate of radioactive waste forms is presented. The achievement of leach rates that are as low as possible has been an important goal ever since the development of solidification processes for liquid radioactive wastes began in the 1950’s. Leach tests can be divided into two major categories, dynamic and static, based on whether or not the leachant in contact with the test specimen is changed during the course of the test. Both types of tests have been used extensively. The results of leach tests can be used to compare

waste forms, and that has been a major purpose of leach data heretofore; increasingly, however, the data now are needed for predicting long-term leaching behavior during geologic disposal. This requirement is introducing new complexities into leach testing methodology.

1. INTRODUCTION

had an adequate understanding been available.

To understand

how leach testing has evolved, we have to separate the ultimate goal from the immediate purpose, because it is the immediate purpose that has gotten the most attention up until now. Morey, in his classic book on glass, has an excellent chapter on the measurement of the leachability of glass, in which he defines what I am terming the immediate purpose very well,

2. MEASUREMENT TECHNIQUES Leach tests can be divided into two major categories, dynamic and static, based on the criterion of whether or not the leachant in contact with the test specimen changes during the course of the test. If the entire leachant is withdrawn and replaced, either in a batch operation as in the International Atomic Energy Agency (IAEA) procedure (2) or by a slow feed and bleed technique, then the test belongs in the dynamic category. In static tests the leachant is not changed.

“. * . the test (to be practical) shall give such quantitative information that the glass-maker can have a categorical answer to the questions: whether or not a given change in

composition or treatment is beneficial, and which of several possible changes is the one which will give the most useful glass”(1).

2. I. Dynamic Tests Some of the earliest leach testing of radioactive solids utilized dynamic techniques. For instance, the waste glass used for the burial experiment at Chalk River in Canada was preleached in the dynamic apparatus shown in Fig. 1 (3). The apparatus was constructed so that the distilled water leachant could either pass by the specimen just once, or be recycled, using an airlift arrangement. In the actual experiment the leachant passage was once-through. It is interesting to note that the apparatus also had a provision to pass the leachate through a soil column to determine the sorption-desorption behavior of the leached radionuclides. If temperature control were added, this leach apparatus constructed almost 25 years ago would compare favorably with the most sophisticated in use today. It is also instructive to look at the results of those

Substitute “waste form” for “glass” in Morey’s statement, and it is a succinct statement of the immediate purpose of our leach testing. The ultimate goal, of course, is to understand the behavior of the entire leaching system, which includes not only the waste form, but the other elements of the waste package and the surrounding geological formations, well enough so that predictions of radionuclide release can be made for long into the future. As I discuss leach measurement techniques I will try to relate them to both the immediate purpose and ultimate goals. There should be a close correspondence. Otherwise, some near-term decisions concerning waste forms may be made that perhaps would be different RECEIVED12/12/80;

of the whole system

ACCEPTED3/15/82.

117

118

J. E. MENDEL

L t

FIGURE 1.

SAMPLE

Early Canadian leach test apparatus (3).

early leach tests as they were reported by the Canadians (see Fig. 2). Note the familiar decrease in leach rate with time. Note also the units used to express the leach rate. The units are grams of glass (removed)/ cm’-day, calculated from the analysis of gross gamma activity in the leachate, and from a knowledge of how much gross gamma activity was in the original glass. The assumption is then made that the fraction of total glass removed by leaching is the same as the fraction of gross gamma activity removed. This method of expressing leach rates, although in common use for almost 25 years, still causes confusion today. Finally, note that the results in Fig. 2 form a broad band, approximately an order of magnitude wide. This early leach data is worth reviewing since it points up questions that are still with us today: ?? How does leach rate change with time? ?? What is the least confusing method of reporting leach test data? ?? What is the precision and accuracy of leach test data? I’ll come back to all of these questions, but now let me continue this brief historical review of leach testing. In the introduction an immediate purpose and ultimate goal for leach testing were described. It

DAYS

FIGURE 2.

25 “C (3).

FROM

START

might be said that the Canadian’s started right off by addressing the ultimate goal with their burial experiments started in 1959 and 1960. But in the interim there have also been a great many tests performed that had only the immediate short-term purpose of deciding if waste form A is “better” than waste form B under certain limited conditions. One dynamic test, in particular, that has been widely adopted for this purpose is the Soxhlet test. Thomas (4) suggested the Soxhlet test for leaching radioactive solids as early as 1955. In recent years there has been a tendency to modify the Soxhlet test to obtain more positive temperature control over a broader range of temperatures (Figs. 3a and 3b). Temperature control can be achieved by varying the pressure in the apparatus so that the water boils at different temperatures (5) or, by physically controlling the temperature of the chamber in which the specimen is located (6). The major feature of Soxhlet leaching is that the leachant is always freshly distilled water. This represents the dilute boundary condition for leaching; the specimen surface contacts essentially pure water throughout the test. This, of course, is an artificial situation, of important theoretical significance, but not representative of a condition that a waste form could encounter in nature.

CLOSED CHAMBERPRESSURE ADJUSTED TO OBTAIN DIFFERENT BOILING POINTS

SPECIMEN 4s

HEAT

OF LEACHING

Leach rate of early Canadian glass, distilled water at

I Ill

FIGURE 3%

Soxhlet apparatus temperature control.

using pressure adjustment

for

119

>

\r

been ignored in recognition that, [I] for tests of practical duration, materials that are durable enough to be considered as waste forms will leach as if they were semi-infinite slabs even when all sides are exposed, and [2] mathematical expressions can be derived to treat the leaching of shapes more complex than semi-infinite slabs. The IAEA proposal addressed the two different goals of leach testing by distinguishing between an A and a B procedure. Procedure A was for the immediate purpose of comparing materials and used distilled water as the leachant. Procedure B was to determine expected long-term behavior during geologic disposal and called for the use of site-specific leachants representative of groundwater from the disposal sites. The IAEA proposal was an excellent beginning toward achieving some standardization in leach testing. Subsequent efforts to standardize leaching have built on the foundation provided by the IAEA proposal. Examples are as follows: A draft International Standards Organization (ISO) standard leach test for radioactive solids (7) includes provisions for testing at four temperatures between 23 and lOO”C, and at three buffered pH’s, 2.3, 5.7, and 9.5.

HEAT FIGURE 3b. chamber (6).

Soxhlet-like apparatus using constant temperature

By the late 1960’s the IAEA recognized the need for standardization in leach testing and in 1971 a proposal for a standard test was published (2). The proposed IAEA test was a dynamic test performed at ambient laboratory temperaure. Thus, it was similar to the early Canadian test in some respects, but the apparatus requirements were much simplified, as shown in Fig. 4. Flow of the leachant past the specimen was approximated by replacing the leachant with fresh solution on the schedule shown. The ratio of solution volume to sample surface area was restricted to 10 cm or less. This ratio, although strictly arbitrary, is now quite commonly used in many leach tests. The proposed IAEA test required that only one side of the specimen be exposed to the leachant, so that the data could be treated as if a semi-infinite slab were being leached. This requirement adds experimental complexity and has generally

A draft American Nuclear Society (ANS) standard leach test for low-level waste solids (8) adapts the first week of an IAEA-type leach test for quality control purposes. The draft also proposes the use of a single value leaching index for each waste constituent that is based on its apparent diffusion coefficient in the waste solid (9). The constant-flow single-pass leach test in use at Lawrence Livermore Laboratory (10) and being developed as the MCC-4 leach test by the Materials Characterization Center (11). The constant flow tests go back to the early Canadian concept and they address the major problem with the proposed IAEA procedure. Its gradation of the leachant change schedule with time in effect creates a variable flow rate, and it has been found that the leach rate changes as flow rate changes. The question then arises, if leach rate changes with flow rate, what flow rates should be studied? The answer must be that the whole range of conceivable flow rates should be covered, but attention should be concentrated on the most likely flow rates. For a geologic repository the most likely flow rates are expected to be very low, that is, water in contact with the waste form will be virtually static. This is the reason for the importance of the other major class of leach tests, the static tests, since they define what happens at the limiting lowflow condition.

120

J. E. MENDEL TIGHTLY-COVERED LEACHANT CONTAINER LEACHANT EXPOSED SAMPLE SURFACE

??

TEMPERATURE

= 25 + 5OC

??

RATIO OF LEACHANT VOLUME TO EXPOSED SAMPLE SURFACE NOT TO EXCEED 10 cm

SAMPLE CONTAINER

LEACHANT A. 6.

DEMINERALIZED WATER DISPOSAL ENVIRONMENT

LEACHANT ?? ?? ?? ??

SAMPLING

AND

(INTERCOMPARISON METHOD) WATER (ENVIRONMENTAL METHOD) CHANGE-OUT

DAILY DURING FIRST WEEK ONCE/WEEK FOR FOLLOWING 8 WEEKS ONCE/MONTH FOR FOLLOWING 6 MONTHS TWICE/YEAR THEREAFTER FIGURE 4.

2.2.

SCHEDULE

Standard leach method proposed by IAEA (2).

Static Tests

The leach tests developed for commercial materials by the American Society for Testing and Materials (ASTM) are static tests (12). Many static tests have also been utilized for the leaching of radioactive waste forms (13). Until recently these were usually short-term scouting tests and often utilized buffered leachant solutions, buffered at pH 4 and 9, for instance, since these are considered the pH bounds for normal groundwater ( 14). Static tests are closed systems that approach a steady-state condition fairly rapidly, which may be considered the “saturated” boundary condition. At room temperature several years may be required to reach the steady state, thus autoclaves are often used to accelerate the rate of approach. One of the MCC tests, MCC-3, is designed to define the “saturated” boundary condition for waste forms in a variety of leachants.

3. METHODS RESULTS

OF EXPRESSING

LEACH TEST

From the preceding brief review, it can be seen that the measurement of leach rates is a relatively straight forward operation. Presenting the data from the leach tests is another matter entirely. Most of us have struggled with explaining what g cm-’ d-l means, and have wished there was some other way of expressing our results besides using a rate that changes with time, and is often illusionary because it assumes all of the constituents in the waste form leach at the same rate as the constituent used to calculate the rate.

Rate =

fraction of

x

constituent A leached X

g of waste form cm2 of surface area

1 days of leaching’

(1)

It is usually found that the fraction of A that is leached is different from the fraction of B that is leached, which in turn is different from the fraction of C that is leached, etc., so of course it is very important to be specific whether the leach rate being reported is based on the behavior of constituent A, B, C, or another. Of course the information used to calculate the leach rate by Eq. 1 can be used to calculate other values. Dividing by the density will give a corrosion or penetration rate, with the units of cm/day. This can be summed over time to yield cumulative penetration in cm. For many waste forms it is found that the cumulative penetration varies as the aand it is possible to calculate an effective diffusion coefficient for constituents A, B, C, etc. (15). Other mechanisms besides diffusion are usually affecting the leach rate, so the diffusion coefficient does not tell the whole story. I want to emphasize that although there are many ways of reporting leach rates, they all begin with the same experimental data. If that raw data is presented clearly and in detail, then it is relatively unimportant what other computational techniques are applied in the initial reporting of the data; the users of the data can easily go back and do their own preferred computations. 4. ANALYTICAL As

SENSITIVITY

more durable waste forms are developed, increased

TWE MEASUREMENT OF LEACH RATES

121

sensitivity is required of leach tests. Techniques that have been used commonly to increase leach testing sensitivity include the following: ?? elevate test temperatures; ?? increase surface area of test specimens; ?? add aggressive chemicals to leachant; ?? use more sensitive leachate analytical techniques. All of these techniques will enhance the sensitivity of leach testing and enable the testing of more durable waste forms. Important considerations often rule out the first three of these techniques, however. For instance, care must be used in the extrapolation of leach rates obtained at elevated temperatures to lower temperatures because different leaching mechanisms may predominate at the two temperatures. The ratio of surface of test speciment to leachant volume is usually specified and held constant in standard leach tests because it has been found to be a parameter that affects the value obtained for the leach rate. When aggressive leachants are resorted to, they are usually acids, sometimes organic acids, such as citric acid. Such aggressive leachants may attack materials by mechanisms that are not typical of the milder leachant solutions waste forms could normally be anticipated to encounter. Thus, although all of these techniques may be used at times, emphasis should properly be placed on augmenting the sensitivity of the leachate analytical techniques. The question then is how sensitive should the analytical techniques be? Currently the answer would seem to be sensitive enough to demonstrate the Nuclear Regulatory Commission’s proposed criterion of 10T5fractional release per year, assuming that the limiting case is when this maximum permissable release rate is achieved by the waste form alone. This fractional release translates roughly into a leach rate of 1 x IO+ g of waste form/cm’ day. Table 1 shows the type of analytical sensitivity that is required for

the elements typically analyzed in leachates from tests with simulated nonradioactive waste forms. Analytical techniques such as inductively-coupled, plasma-emission spectroscopy, neutron activation analysis, and atomic absorption are seen to be only marginally capable of meeting the analytical requirements for a typical commercial waste product. The fission product concentrations are so low in a typical defense high-level waste product that their nonradioactive isotopes cannot be used to demonstrate a leach rate of 1 x lo-* g/cm’-day. Other means would be required, such as arbitrarily increasing the fission product concentration in the test specimens. Ultimately, leach rates must be determined on waste form samples loaded with actual radioactive waste. Radioactive counting techniques applied to the leachates from these tests will in most cases permit the measurement of considerably lower leach rates. In these tests, where the leachate may contain only parts-per-billion of the constituent of interest, extreme experimental care is required. Blanks must be run to detect unexpected contaminants. But most importantly, adsorption or plating out on the surfaces of the leach vessel must be minimized. Vessels made from hydrophobic plastics are often used, such as the Teflon vessels required for the MCC leach tests. Leachates should be acidified to maintain constituents in solution; the leach vessel should be treated with acid to remove adsorbed or plated-out material. 5. SURFACE ANALYSES Up to now this review has emphasized solution analysis, which is the standard technique for measuring leach rates, although weight loss has also been used, particularly in screening tests and as a check on the results obtained by solution analysis. But modern

TABLE 1 Analytical Sensitivity Requirements Typical Concentration in Solid, wt.% Element cs Sr Nd U Zr Na Si Ti

Defense Waste Prod.

Commercial Waste Prod.

0.01 0.005 0.1 2 0.1 10 20 4

2 1 3 1 3 10 20 2

Leachate Concentration After 1 DaP, mg/l Defense Waste Prod. 0.0001 O.OOOO5 0.001 0.02 0.001 0.1 0.2 0.04

aAssuming leach rate of 1 x lo-’ g waste form/cm’ day. b Inductively-coupled, plasma-emission spectroscopy, unless indicated. CAtomic absorption. d Pulsed laser fluorescence.

Commercial Waste Prod. 0.02 0.01 0.03 0.01 0.003 0.1 0.2 0.02

Analytical Detection Limitb, mg/l 0.005c 0.001 0.02 0.OOOOO5d 0.01 O.OlC 0.02 0.002

J. E. MENDEL

122

surface analysis techniques allow much more than weight loss to be determined from the specimens after leaching is finished. The new techniques enable analyses to be performed that yield a profile of the specimens from their leached surfaces into the undisturbed interiors. A partial listing of the surface analytical techniques that are being employed is shown in Table 2. Some spectroscopy techniques permit analyses of just the first five to ten angstroms of the surface. With ion milling the analysis can be repeated as layers are removed to yield a profile that penetrates thousands of angstroms into the specimen. In conjunction with this, the specimen can be sectioned and the cross-section can be examined and analyzed by electron microscope techniques. The spectroscopic and microscopic techniques complement one another, the former being particularly useful for studying the early stages of leaching, while the latter finds more application in the study of welldeveloped alterations of the surface due to more prolonged rigorous leaching. As described earlier, leaching is almost always found to be at least somewhat incongruent. The result is that an altered layer at the surface has a different compositon than the bulk of the waste form. The form that this difference in composition takes, as determined by surface analysis, can offer clues concerning the leaching mechanism. Concentration of constituents in the surface layer can be detected. These concentrated constituents can originate in the waste and reprecipitate in the surface layer, or they can come from the leachant, possibly displacing waste constituents as they deposit. A diffusion front can usually be detected for the alkali metals. Ideally the location and shape of the diffusion front can be correlated with the buildup of leached alkalis in the leachate. Thus, surface analysis can both give clues as to why certain constituents are almost nonexistant in the leachate, and corroborate the diffusion behavior of other ions. In many cases the surface layers act as protective

layers that hinder leaching in a very desirable way. Continued utilization of the latest surface analysis techniques should help us understand better why some surface layers are more protective than others. 6. PRECISION

OF LEACH TEST DATA

How well our understanding of leach behavior progresses is somewhat dependent on advancements in the precision and reproducibility of leach test data. This is a responsibility of the experimentalist that has not yet been addressed very well. Mathematical modelers have difficulty deducing defensible long-range extrapolations using experimental data with error bars that often span a factor of two or three or even more. The rigor being introduced by the MCC leach test procedures will help improve data precision and the reproducibility between laboratories. The Round Robin testing of the MCC-1 static leach procedure that is currently underway will document the degree of reproducibility obtained in some 15 different laboratories. 7. SUMMARY In summarizing this review of leach test methodology, I believe it can be fairly said that leach testing in the United States is nearing an important milestone. The comparison of waste forms on a common basis, via the Department of Energy’s HLW and TRU lead laboratories and the MCC, is well underway. Now we’re advancing toward the next big step- developing an understanding of mechanisms so that long-term behavior, particularly in geologic repositories, can be predicted. Here, there are very real experimental problems that will require some ingenuity to solve. Techniques utilized by geochemists for controlling pH during mechanism-elucidating experiments can be adopted, for instance. But we also need to control oxygen fugacity at very low levels to duplicate the conditions expected in a filled and sealed repository.

TABLE 2 Techniques for Analysis of Leached Surfaces Depth of Analysis

output Data

ESCA (Electron Spectroscopy for Chemical Analysis) Auger Electron Spectroscopy SIMS (Secondary Ion Mass Spectroscopy)

5-10 K

Qualitative to Quantitative

AR Ion Beam Milling (with the above techniques)

To several thousand .k

Qualitative to Quantitative

Scanning Electron Microscope with EDAX (Energy Dispersion Analysis by X-Ray)

(can be applied to class sections)

Method

1w

Quantitative (to 0.1%)

123

In addition, there are the problems of pressure, radiation, exposed surface configuration on a realistic scale and interactions with other components of the waste packages. Of course, all of these tests must be performed with representative geologic fluids. And finally there is a pressing need for very long-term (several years) confirmatory tests. These can be most reliably and economically conducted with equipment that is automated to the highest degree possible. I believe it can be predicted safely that before the next big step in leach testing is completed, the equipment being used will be much more complex than the relatively simple equipment that has been described today. I am also confident the experimentalists will be able to meet the challenge.

8. REFERENCES Morey, G. W., The Properties of Glass, 2nd Ed. Reinhold Publishing, New York (1954). Hespe, E. D. Leach testing of immobilized radioactive waste solids-A proposal for a standard method. Atomic Energy Rev. 9: 195-207 (1971).

Bancroft, A. R. and Gamble, J. D. Initiation of a field burial test of disposal of fission products incorporated into glass. CRE-808, Atomic Energy of Canada Limited, Chalk River, Ontario (1958). Thomas, H. C. Proposed standard leaching test, pp. 87-89. TID-7550, U.S. Atomic Energy Commission, Washington, DC (1958).

5. Boult, K. A., Dalton, J. T., Hall, A. R., Hough, A., and Marples, J. A. C. The Leaching of Radioactive Waste Storage Glasses. AERE-R9188, AERE, Harwell, England (1978). 6. Lanza, F., Jacquet-Francillon, N., and Marples, J. A. C. Methodology of leach testing to borosilicate glasses in water, Presented at the First European Community Conference on Radioactive Waste Management and Disposal held in Luxembourg (1980). 7. Internation Standards Organization. Draft International Standard on Long-term leach testing of radioactive waste solidification products. ISO/DIS 6%1 (1979). 8. Neilson, R. M., Jr. Experimental procedure for proposed ANS standard on low-level waste leachability. Trans. Am. Nucl. Sot. 33: 203-205 (1979).

9. Godbee, H. W. and Compere, E. L. A figure-of-merit determined from the ANS leachability standard. Trans. Am. Nucl. Sot. 33: 205-207 (1979).

10. Weed, H. C. and Jackson, D. D. Design of a variable-flowrate, single-pass leaching system. UCRL-52785, Lawrence Livermore Laboratory, Livermore, California (1979). 11. Mendel, J. E., ed. Ross, W. A., Strachan, D. M., Turcotte, R. P. and Westsik, J. H. Jr. Materials characterization center workshop on leaching of radioactive waste forms summary report. PNL-3318, Pacific Northwest Laboratory, Richland, Washington (1980). 12. American Society for Testing and Materials. Standard test methods for resistance of glass containers to chemical attack. ANSI/ASTM C-225-73, ASTM, Philadelphia, Pennsylvania (1978). 13. Mendel, J. E. A review of leaching test methods and the leachability of various solid media containing radioactive wastes. BNWL-1765, Pacific Northwest Laboratory, Richland, Washington (1973). 14. Ross, W. A. et al. Annual report on the characterization of high-level waste glasses. PNL-2625, Pacific Northwest Laboratory, Richland, Washington (1978).