Impact of Uniaxial Mechanical Perturbation on Structural Properties and Smectite Porosity Features: Ion Exchanger Efficiency and Adsorption Performance Fate

Abstract

The use of montmorillonite in the context of engineered barriers makes it possible to minimize the spread of heavy metals from industrial and even radioactive waste. An evaluation of the performance of the mechanisms controlling the clay-environment interaction and predicting the dynamics/configuration of the interlayer space (IS) is required. This work focuses on a quantitative identification of the structural changes and porosity alteration in the case of heavy metal-exchanged montmorillonite samples (Co2+ and Cd2+ cations) undergoing mechanical stresses (uniaxial oedometric test (loading/unloading)). Relationships between mechanical stress strength, intrinsic structural response, ion exchanger efficiency, and adsorption performance fate are investigated. This goal is achieved through the correlation of in situ quantitative X-ray diffraction (XRD) analysis (under an extremely controlled atmosphere reached by varying relative humidity rate %rh) and porosity investigation (assured by combining outcomes from BET (Brunauer–Emmett–Teller) and BJH- (Barrett, Joyner, and Halenda-) PSD (pore size distribution) analysis). Obtained results show an upsurge in the structural heterogeneities accompanying the theoretical increase in the mixed layer structure (MLS) number and developing an unconventional hydration behaviour after stress relaxation regardless of exchangeable cation nature. Experimental XRD patterns are reproduced using MLS, which suggests the coexistence of more than one “crystallite” specie and more than one exchangeable cation indicating a complex cation exchange capacity (CEC) saturation. For extremely low %rh value, a new homogeneous dehydrated state trend is observed in the case of the Co2+ cation. Porosity analysis shows mesopore volume growth for the stressed sample and confirms crystallite exfoliation layer trends, results of the layer cohesion damage, and subsequent constraint strength fluctuations.