Experimental and modeling studies on sorption and diffusion of radium in bentonite

The sorption and desorption behavior of radium on bentonite and purified sm ectite was investigated as a function of pH. ionic strength and liquid to s olid ratio by batch experiments. The distribution coefficients (K-d) were i n the range of 10(2) to > 10(4) mi g(-1) and depended on ionic strength and pH. Most of sorbed Ra was desorbed by 1 M KCl. The results for purified sm ectite indicated that Ra sorption is dominated by ion exchange at layer sit es of smectite, and surface complexation at edge sites may increase Ra sorp tion at higher pH region. Reaction parameters between Ra and smectite were determined based on an interaction model between smectite and groundwater. The reaction parameters were then used to explain the results of bentonite by considering dissolution and precipitation of minerals and soluble impuri ties. The dependencies of experimental Kd values on pH, ionic strength and liquid to solid ratio were qualitatively explained by the model. The modeli ng result for bentonite indicated that sorption of Ra on bentonite is domin ated by ion exchange with smectite. The observed pH dependency was caused b y changes of Ca concentration arising from dissolution and precipitation of calcite. Diffusion behavior of Ra in bentonite was also investigated as a function of dry density and ionic strength. The apparent diffusion coeffici ents (D-a) obtained in compacted bentonite were in the range of 1.1 X 10(-1 1) to 2.2 X 10(-12) m(2) s(-1) and decreased with increasing in dry density and ionic strength. The K-d values obtained by measured effective diffusio n coefficient(D-e) and modeled D-e were consistent with those by the sorpti on model in a deviation within one order of magnitude. (C) 2001 Elsevier Sc ience B.V. All rights reserved.