Dependence of Radionuclide Sorption on Sample Grinding Surface Area, and Water Compositionby P. S. Z. Rogers, Los Alamos Natl Lab, Los Alamos, United States,
A. Meijer, Los Alamos Natl Lab, Los Alamos, United States,
Abstract: Experiments are described that were designed to quantify the dependence of sorption properties and surface area on the crushed-particle size of the rock samples used in batch sorption experiments. To this end, nine different size fractions of tuff samples, ranging from 2 to 4 mm down to less than 38 μm, were prepared by grinding, sieving, and washing bulk samples. These were reacted with solutions containing three different radioactive sorbers, 137Cs, 85Sr, and 237Np, the solid and solution fractions were separated, and both fractions were gamma counted to determine partitioning of the radioactive sorber. Surface areas of each sample were determined by BET analysis. From this information, the sorption coefficient (Rd) of each tracer on each particle-size fraction, and its correlation with surface area, was calculated. The two rock types chosen for study differ markedly in their mineralogy, surface area, and sorption characteristics. The first sample, a devitrified tuff from the Topopah Springs, consists of small microcrystallites, generally less than a micron in diameter of feldspar, quartz, crystobalite, and trydimite. Average surface area of these samples is approx.1 m2/g. The second sample is a zeolitized tuff from the Calico Hills. It is made up of large, friable, particles of clinoptilolite and mordenite with quartz, opal-CT, and feldspar. Surface areas for these samples are higher, approx.20 m2/g, because of the high (>50%) zeolite content. In addition, duplicate suites of samples from both rock types were pre-treated by reaction with a groundwater containing added calcium and magnesium. Sorption experiments were then performed using the normal groundwater. The purpose of this pre-treatment was to study the sorption geochemistry of these tuffs after equilibration with a Paleozoic-type water composition. This situation would correspond to a scenario where groundwater leaking from the repository contacted deeper layers of tuffs that had previously been saturated with Paleozoic-type water. Clear dependence of the sorption Rd's on the composition of the pre-treatment water was observed for some samples.
Subject Headings: Sorption | Groundwater | Rocks | Water storage | Waste storage | Rock properties | Surface properties
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