Cation diffusion in the interlayer space of swelling clay minerals – A combined macroscopic and microscopic study

Ionic Transport in Nano-Porous Clays with Consideration of Electrostatic Effects

“…It was possible to reproduce the data with the same level of quality as the Brownian dynamics calculations by considering an interlayer Ca 2+ diffusion coeffi cient value of 0.8 × 10 -11 m 2 ·s -1 , together with an interlayer Na + diffusion coeffi cient of 4 × 10 -11 m 2 ·s -1 , 1 × 10 -11 m 2 ·s -1 , 0.1 × 10 -11 m 2 ·s -1 , and 0.1 × 10 -11 m 2 ·s -1 , for the experiments at NaCl concentration of 1, 0.1, 0.05 and 0.003 mol·L -1 respectively. The decrease of the Na + interlayer diffusion coeffi cient with NaCl concentration is in agreement with MD results from Tertre et al (2015), which show a decrease of its value with an increase of the Ca 2+ /Na + occupancy ratio in the interlayer (Fig. 21c).…”

“…Recently, Tertre et al (2015) published data from their diffusion experiments carried out with centimeter-size mono-crystal of vermiculite. Vermiculite is a swelling clay mineral that has a high CEC (~1.8 mol·kg -1 ) originating primarily from isomorphic substitutions in the tetrahedral sheet.…”

“…The data from Tertre et al (2015) were reinterpreted using the interlayer diffusion option of PHREEQC (Fig. 21b).…”

“…Experiments fromTertre et al (2015): a) setup; b) modeling of the Ca 2+ out-diffusion results using the interlayer diffusion option of PHREEQC (this study); c) self-diffusion coeffi cients of Na + and Ca 2+ in the vermiculite interlayer as a function of the equivalent fraction of Ca on the surface (X Ca ), and obtained from molecular dynamics calculations (fromTertre et al 2015).…”

Specific ion effects of incomplete ion-exchange by electric field-induced ion polarization

“…However, many studies have demonstrated that achieving complete ionexchange is challenging; inversely, the pronounced incomplete ion-exchange is generally observed. [6][7][8][9][10][11][12][13] Currently, the reasons for incomplete ion-exchange are as follows: the slow migration of ions arising from the immobilization of edgeinterlayers or the collapse of frayed edges in clay mineral, 8,9,13,14 the incompatibility of the cation size with the space available in specic sites of the materials, 15 and insufficient external disturbances lead to the blocking of the diffusion of exchange ions. 12,16 Although these attempts to make sense of incomplete ion exchange, systematic and theoretical explanations regarding specic ion effects are still relatively scarce.…”