Sorption of Cesium on Compacted Bentonite

Abstract

Abstract--Sorption parameters are important components of models used to predict mass transport through dense or compacted earthen materials. These parameters are, however, generally determined in batch tests with loose, unconsolidated materials. Here we directly measure, using a specially designed cell, the extent ofCs + sorption on bentonite compacted to a series of densities ranging from 0.50 to 1,50 Mg/m 3, and compare the results with those obtained from batch tests with loose bentonite. The clay was saturated with a Na-Ca-Cl-dominated solution with an effective ionic strength of 220 mol/m 3. The sorption data were expressed as distribution coefficients, I~. Over the clay density range examined, Ko values for Cs + with compacted clay are about one-half to one-third the value of those with loose clay. The lower sorption on compacted clay is attributed to small and occluded pores that Cs + cannot enter; thus it cannot access the entire volume, or all the sorption sites, of compacted clay. The data suggest that reasonable estimates of K~ with compacted clay can be obtained by scaling down the I~ values measured on loose clay by a factor na/n, where n a is the accessible porosity and n the total porosity of compacted clay.

Surface Diffusion: Is It an Important Transport Mechanism in Compacted Clays?

“…This is because the sorbate cannot enter small and occluded pores in compacted clay, and hence it cannot access the entire clay volume or all sorption sites. Oscarson et al (1994) proposed that a K~ value determined on loose clay be scaled down by a factor na/n (na is the porosity of compacted clay accessible to the sorbate and n the total porosity) to get an estimate of the K~ value for the same clay when compacted. For example, from the accessible porositydensity relationship for Avonlea bentonite given by Oscarson et al (1994), na/n is about 0.35 at p = 1.25 Mg/m 3.…”

Study of the Interaction of Ni2+and Cs+ on MX-80 Bentonite; Effect of Compaction Using the “Capillary Method”

“…The comparison with the results obtained with dispersed bentonite shows that material consolidation has in some cases no effect on retention, in other cases it has a positive (increase of the retention) or negative (decrease of the retention) effect (3,5,6,7,8,9,10). The origin of the effect appears unclear; either the transport or the retention model is critizised (2)(3)(4)(5)(6)(7)9,10). Concerning the retention model, site accessibility is advanced as an explanation (9).…”

“…The origin of the effect appears unclear; either the transport or the retention model is critizised (2)(3)(4)(5)(6)(7)9,10). Concerning the retention model, site accessibility is advanced as an explanation (9).…”

“…To better understand the relation between the retention of contaminants in dispersed and compacted sytems, it appears fundamental to develop new methodologies to measure sorption data directly on compacted materials (9).…”

“…pH edges and isotherms measured in the presence of a synthetic groundwater are compared with those measured in the dispersed state (S/L=0. [5][6][7][8][9][10] kg/m 3 ). The results were modeled based on published models obtained with pure clay materials (11,12,13).…”