Study of the dehydration process of vermiculites by applying a vacuum pressure: formation of interstratified phases

Abstract

Structural transformations between the different hydration states of three vermiculite samples from Sta. Olalla (Huelva, Spain), Paulistana (Piaui, Brasil) and West China, have been observed by X-ray diffraction at atmospheric pressure, P = 1.4610 -2 mbar and P = 2.4610 -4 mbar. The samples were studied in flake and powder forms. The effect of vacuum has been proven to be the same as that of temperature, i.e. it causes dehydration of vermiculite, but with a different evolution through the different hydration states. In fact, under vacuum, the process seems to be inhibited at a one-water layer hydration state (1-WLHS), without a further dehydration of samples to a zero-water layer hydration state (0-WLHS).Furthermore, the dehydration process has been shown to occur through different interstratified states in each vermiculite. This result has been related to the interlayer Mg-cation content, due to its affinity to water molecules. The interstratified states have been analysed by the direct Fourier-transform method. The vermiculite from Sta. Olalla exhibits the most complex process, with formation of three different interstratified phases: two phases characterized by an interstratification of interplanar distances, d = 11.5 -13.8 A Ê and d = 9.6 -11.5 A Ê , respectively, and a practically segregated phase characterized by d = 13.8 A Ê . For the vermiculite from China, an interstratified phase not previously reported has been found, with an interplanar distance of 12.10 A Ê .The inhibition of dehydration at 1-WLHS, as observed, could be used in applications such as adsorption and separation technology of gases and liquids, or in heterogeneous catalysis processes.

Vermiculite: Structural Properties and Examples of the Use

“…Vermiculite structures contain water interlamellar layers which are subjected to the hydration and dehydration processes [19][20][21][22][23][24][25][26]. The hydration properties are controlled by the interlayer cations Mg 2+ and minor amounts of Ca 2+ , Na + , and K + .…”

Ion Exchanged Vermiculites with Lower Expansion Onset Temperatures

“…Exposing vermiculite to a vacuum causes rapid dehydration. Below 120 °C the vacuum dehydration is arrested at a 1-WLHS and under vacuum conditions the exfoliation is not dependent on the presence of water above 120 °C [26].…”

“…It progresses stepwise during elimination and/or accumulation of one or several water layers structures, with some overlap, accompanied by a change in the d-spacing [3,35,37]. Previous studies have shown that during thermal dehydration of vermiculite there is no destruction of the crystals: water molecules are released by interlayer diffusion and escape through the cracks [16,26,28,38]. Exposing vermiculite to a vacuum causes rapid dehydration.…”

“…These WLHS define the basal reflection and are intrinsically related to the interlayer cation(s), relative humidity and temperature [26,31,34,35]. In natural biotite (vermiculite parent mica) the interlayer ions are potassium and the interlayer space contains no water (0-WLHS).…”

“…Compared to pure vermiculite, vermiculite-mica interstratified materials feature higher expansion ratios with the thermal exfoliation also commencing at lower temperatures [21,[24][25][26][27]. Additionally, this modified and interstratified vermiculite is amenable to produce a durable exfoliated material [10].…”