Reversible and Irreversible Dehydration of Hydroxy-Interlayered Vermiculite from Coastal Plain Soils
Dehydration behavior of hydroxy-interlayered vermiculite (HIV) from three coastal plain soils was studied using x-ray diffraction (XRD) and thermal analysis. Relative humidity (RH) at 25 °C had little effect on HIV 001 XRD peaks, except for a 0.01 to 0.05° 2θ shift at RH ≈ 0%. Heating to 50 °C shifted peak positions 0.10 to 0.20° 2θ for fine silts and 0.05 to 0.10° 2θ for coarse clays. Effects of 50 °C treatment were largely reversed by re-wetting, indicating that H2O re-entered interlayers. Heating to 115 °C broadened peaks and produced shifts of up 0.40° 2θ. Dehydration at ≥ 115 °C was essentially irreversible, as indicated by lack of 001 expansion and failure to regain full sample weight at 100% RH. Peaks were markedly broadened and attenuated by heating to 165 and 225 °C, with full width at half maxima approaching 1.0° 2θ. No significant peak in the 1.0-nm region developed from interlayer collapse, even at 550 °C. Reversible dehydration may be attributable to free-H2O loss from interpolymer zones at temperatures below the onset of significant polymer dehydration. The data suggest that attraction between dehydrated polymers and 2:1-layer surfaces exceeds hydration forces, resulting in irreversible collapse and variable interlayer thickness. Re-entry of H2O into interlayers could be sterically impeded by the dehydrated polymers themselves or by their binding effect on layers. The 1.4-nm mineral in the soils studied exhibits thermal properties more consistent with HIV than an alternative vermiculite-kaolin intergradient structure.
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