Systematic comparison of the hydration and dehydration of Na+-, K+-, and NH4+-saturated montmorillonite, nontronite, hectorite, saponite, and Fe-saponite by in situ X-ray diffraction measurements

Understanding the swelling properties of smectites is important for geoengineering, biochemical applications, and planetary sciences. The aim of this study was to construct a general model to predict the swelling behavior of smectites saturated with ubiquitous monovalent cations (Na+, K+, and NH4+). The swelling behavior of homo-ionized montmorillonite, nontronite, Al-bearing nontronite, hectorite, saponite, and Fe-saponite during hydration and dehydration were examined by means of relative-humidity-controlled X-ray diffraction. The basal spacings of Na+- and K+-saturated smectites measured during dehydration exhibited very similar curves irrespective of the smectite species, whereas those for hydration were different for different smectite species under lower relative humidity. The similarities of the dehydration curves suggest that dehydration of Na+ and K+ in the smectite interlayers are solely controlled by the intrinsic nature of the interlayer cations, i.e., the hydration energies of the cations. However, hydration of dehydrated Na+ and K+ in the smectite interlayers is controlled by both the intrinsic nature of the interlayer cations and the nature of the host phase. In contrast to Na+- and K+-saturated smectites, the hydration curves of NH4+-saturated smectites were consistent with the dehydration curves. This absence of hysteresis most likely results from the fact that dehydrated NH4+ is bound to the basal oxygen via hydrogen bonding, which was supported by the different lengths of the basal spacings of smectites after complete drying. In turn, this finding suggests that the notable hysteresis observed for Na+- and K+-saturated smectites under low relative humidity was caused by smaller layer-to-layer distances of Na+- and K+- saturated smectites, which may limit the accessibility of water molecule to the interlayer of smectites for hydration. The dehydration behaviors of NH4+-saturated smectites were almost the same as those of K+-saturated smectites except for one saponite specimen, consistent with the similar ionic radii of NH4+ and K+.