11:15 AM - 11:30 AM
[HCG32-02] Expansion and alteration behaviors of NH4-montmorillonite
Keywords:Expansion, Alteration, Ammonium, Potassium, Montmorillonite, Molecular Dynamics
To improve understanding of the swelling behavior of NH4-MMT, the expandability of a suite of homoionic MMT, including Na-MMT, K-MMT, Cs-MMT, and NH4-MMT were investigated by XRD under controlled relative humidity (RH) conditions and supported by molecular dynamics (MD) calculations.
Differences in the swelling behavior of the suite of homoionic MMT were observed by XRD and could be largely explained in terms of the valence, the radius and the hydration energy of the interlayer cations. All these properties, however, are very similar for K+ and NH4+ and so could not be used to explain the reduced swelling of K-MMT compared to NH4-MMT under low RH conditions. Although it has been well known that K-MMT lost expandability by dehydration, it has not been well known whether this process would occur on NH4-MMT or not.
First-principles MD calculations were conducted in a previous study (Shi et al., 2013) for NH4-MMT containing only a small number of water molecules (less than 1.25 molecules per NH4-MMT half unit cell). The hydrogen bonding and the network between the surface oxygen, interlayer NH4+ ion and water were reported to be increased by adding more water molecules to the interlayer. The basal spacing measured by XRD under controlled RH in the current study are consistent with the interlayer containing less than one layer of water, although the interlayer charge may be different between samples. It is also difficult to make direct comparisons with Shi et al.'s calculations and experimental results because the number of water molecules in the interlayer cannot be precisely controlled experimentally.
The classical MD simulations in the current study described the mixing enthalpy of MMT / water molecules system by including the number of interlayer water molecules as a variable. The potential functions and parameters of MMT and water molecules proposed by Nakano and Kawamura (2006) and Kawamura (2008) were employed in this study. Parameters on NH4+ are determined to reproduce NH4Cl structure. The mixing enthalpy was used as an index to evaluate the swelling state stability. To consider the RH effect, the mixing enthalpy (Hmix) was calculated by: Hmix=H(MMT+nH2O)-(HMMT+nµH2O), where the chemical potential of water, µH2O includes the effect of RH by: µH2O= µ0+RTln(P/P0) (µ0:standard chemical potential, R:gas constant, T:temperature(K), P0/P:partial pressure of water) The minimum of Hmix indicates the stable hydration state of the MMT. In this study, n = 0 to 20 water molecules were added to homoionic MMT to understand the different swelling behavior of K-MMT and NH4-MMT under each RH condition. It was found that the series of calculated mixing enthalpy was consistent with the XRD results under controlled RH. In the NH4-MMT system, the hydrogen bond between NH4+ and surface oxygen on clay was confirmed from the radial distribution function. These hydrogen bonds make large basal spacing of NH4-MMT rather than that of K-MMT at 0%RH, which lead to hydration at low RH condition compare to K-MMT because the expansion of smectite occurs when hydration of interlayer cation exceeds the electrostatic attraction between silicate layer and cation. This result indicates that NH4-MMT is more difficult to become non-expandable or less expandable mineral than K-MMT.
This study was partly funded by the Ministry of Economy, Trade and Industry of Japan through “The project for validating assessment methodology in geological disposal system” in JFY 2016.