A Theoretical Study of the Effect of the Leach Interval on a Semidynamic Leach Test (Ansi/Ans-16.1-1986)
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A THEORETICAL STUDY OF THE EFFECT OF THE LEACH INTERVAL ON A SEMIDYNAMIC LEACH TEST (ANSI/ANS-16.1-1986)
ROGER D. SPENCE* AND RICHARD L. COX** *Oak Ridge National Laboratory, Post Office Box 2003, Oak Ridge, TN 37831-7273 **Oak Ridge Gaseous Diffusion Plant, Post Office Box 2003, Oak Ridge, 37831-7053
TN
ABSTRACT Numerical solution of Fick's second law of diffusion demonstrated that the error introduced in the ANSI/ANS-16.1-1986 procedure, by assuming a zero surface concentration, varied from about 30% for a leachability index of 5 to about 0.1% for an index of 10. Waste forms typically have indexes of 7 or greater, implying errors of less than 5% with this assumption. The estimated leachability index differs from the actual value by less than 0.2 when the numerical solution is analyzed as input experimental data, except for a leachability index as low as 5 (essentially the value for water).
INTRODUCTION The semidynamic leach test under consideration is the ANSI/ANS-16.1-1986 leach procedure.' The data analysis for this procedure assumes that zero concentration exists at the surface of the leaching specimen, commonly referred to as dynamic leaching. If leaching is diffusion controlled, as assumed, then several analytical solutions are available in the literature to 23 model the leaching. . The ANS-16.1 procedure uses these solutions for dynamic leaching to estimate diffusion coefficients from experimental data. To simulate dynamic leaching but still generate measurable concentrations, the procedure utilizes a semidynamic technique (i.e., the specimen is leached in a specified volume of leachant for a finite interval and is then removed and placed in an equal volume of fresh leachant). Thus, for the ANS-16.1 procedure, a specimen is sequentially leached statically in 10 separate volumes of leachant, but the results are analyzed and interpreted as if the leaching were done dynamically. This paper explores the effect of allowing concentration buildup in the leachate during each interval as opposed to true dynamic leaching (i.e., maintaining zero concentration at the surface at all times). This comparison was done by assuming diffusion control and calculating the solution to the diffusion model for the different boundary conditions [i.e., dynamic (zero surface concentration) compared to static (buildup of surface concentration)].
MODEL The slab geometry (region between two parallel planes) in Fig. 1 with one-dimensional diffusion was selected for study. With the assumption that diffusion is the only mechanism of leaching, then leaching in this case can be modeled by Fick's second law as given by:
ac_ D 82C 2
where C t D x
-
at 8x concentration, g/cm ; time, s; 2 diffusion coefficient, cm /s; distance from slab centerline,
(1)
3
cm.
Mat. Res. Soc. Symp. Proc. Vol. 176. e1990 Materials Research Society
102
The two major parallel surfaces of the slab each had cross-sectional 2 areas of 50 cm . Only these major surfaces were assumed in2 contact with the The ANS-16.1 leachant and leaching (i.e., total leachi
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