Surface chemical model of the bentonite-water interface and its implications for modelling the near field chemistry in a repository for spent fuel

摘要

Understanding the surface chemical properties of montmorillonite in near-neutral and alkaline media is essential for establishing a chemical model of the bentonite/water interaction applicable for repository conditions. A pretreated and well-characterised Wyoming MX-80 bentonite has been used for investigating the acid/base characteristics of Na-montmorillonite. The CEC of Na-montmorillonite was determined to 108 meq/100 g for pretreated bentonite and to 85 meq/100 g for the bulk material. The BET surface area was (31.53 + or - 0.16) sq m/g. Potentiometric titrations of montmorillonite suspensions at ionic strengths I=0.005 M, 0.05 M and 0.5 M were conducted as batch-type experiments. Deprotonation of surface OH groups possibly exposed at the edge surface causes an overall negative charge on the surface of montmorillonite in the alkaline pH range. In this pH range, the protolysis degree of OH groups increases with increasing pH and ionic strength. The proton density on the surface of montmorillonite increases with decreasing pH in the acidic pH range (pH<7.5). In this pH range, two simultaneously occurring surface reactions account for the observed proton density on montmorillonite: Protonation of edge OH groups and ion exchange of the major cations for H(+) at the structural-charge sites. The experimental results are interpreted in terms of a two-site model with structural-charge surface sites (X layer sites) and variable-charge surface sites (edge OH groups) as the reactive surface functionalities. The total population of the surface sites are estimated to TOT-OH=2.84 x 10 to the -5th power mol/g, TOT-X=2.22 x 10 to the -5th power mol/g. The intrinsic acidity constants for the OH groups are determined to pK(sup int)(sub al)= (5.4 + or - 0.1) and pK(sup int)(sub a2)=(6-7 + or - 0.1), respectively, using th configuration of the diffuse double layer model (DDLM). 43 refs, 18 figs, 11 tabs. (Atomindex citation 26:042121)