Structure and reactivity assessment of dioctahedral montmorillonite during provoked variable sequential cation exchange process via XRD modelling approach

Abstract

The intrinsic cation exchange capacity (CEC) of clays is a crucial characteristic that offers a tool to assess their overall chemical properties. For the confinement of industrial and radioactive waste, clays are deployed as geological barriers, hence it is important to consider how they will react under external stresses. With respect to the electronegativity rate and cation ionic potential, the impact of a series of sequential cation exchange processes with heavy metals exchangeable cations was the main emphasis of this work. Using X-ray diffraction (XRD) profile modeling to assess interlayer space (IS) deformation and highlight geochemical alteration, the structural reactivity of Na-rich montmorillonite was investigated. To determine the ideal structural characteristics defining IS configuration, the study contrasted estimated reflections produced from theoretical models with those produced experimentally from 00l reflections. Per each exchanged cation, the results showed a partial CEC saturation and a heterogeneous mixed layer structure (MLS), with heterogeneous hydration behavior continuing to exist regardless the type of exchangeable cation. The established chemical constraint had no effect on the cation hydration sphere for Co (II), Ni (II), Mg (II), Cu (II), and Zn (II) exchangeable cations. However, significant interlamellar water molecule development was observed for the Cd (II), Pb (II), and Ba (II) cations, indicating a major alteration in the discretization of the hydration states and particular hydration heterogeneities. The Cd (II), Pb (II), and Ba cations were most affected by these alterations, which resulted in a constrained and inhibited CEC performance. The theoretical decomposition of the observed XRD profiles was used to show the existence of multiple layer type populations and their stacking mode, with most of the studied samples showing a tendency toward segregated stacking configuration. Moreover, as a function of the applied stress, the CEC of the stress samples gradually dropped.