EFFECTS OF CLAY MICROSTRUCTURE ON URANIUM(VI) SORPTION AND DIFFUSION

A prediction of metal adsorption and diffusion processes in clay-rich media is complicated by (1) the complexity of the mineralogical structure of montmorillonite clay, in terms of its pore-size distributions and available surface site types, and (2) the often complex metal solution speciation, which can include cationic, uncharged, and anionic complexes, depending on solution conditions. Clay particles consist of stacks of negatively-charged smectite layers, which leads to two types of porosities: (1) large pores between clay particles, with little influence of electric-double-layer forces, and (2) very thin interlayer spaces within individual clay particles, where diffusion is impacted by surface charge and ionic strength. Furthermore, these two porous regimes provide different surface environments for contaminant sorption reactions. Electrostatic and hydration forces only are thought to govern cation exchange reactions in interlayer spaces, whereas chemical bonding with surface ligands is dominant for surface complexation reactions at edge sites of clay particles. Finally, a ‘spillover’ effect may occur, where the electrostatic surface potential of basal cation exchange sites influences the surface potential of neighboring edge sites. As sorption and diffusion processes are expected to take place differently in these two volumes, this essentially creates two ‘small-scale diffusion pathways’, where each one becomes dominant under different system conditions. For instance, at high pH a partial or full exclusion of anions from negatively charged clay interlayer spaces could decrease the effective ‘anion-accessible’ porosity and diffusive flux under steady state conditions, while at low pH the ‘surface diffusion’ of weakly-adsorbed cations could increase the overall flux.

We will discuss the effects of these clay microstructure characteristics on metal sorption and diffusion processes, using uranium(VI) as example. Our results from lab-scale U(VI)-montmorillonite diffusion experiments at alkaline pH demonstrate the importance of anion exclusion effects. A new U(VI)-montmorillonite surface complexation model, that specifically accounts for the ‘spillover’ effect, allows us to predict U(VI) sorption under varying conditions with a minimum number of fitting parameters.