Characterization of model soil colloids by cryo-scanning electron microscopy

The cryo-scanning electron microscopy (cryo-SEM) consists of the SEM examination of samples whose microstructure has been fixed by plunge-freezing into a liquid with a high thermal conductivity, such as nitrogen slush. The advantage of this cryo-preparation method is that it can transform the interstitial water of the samples into ice whose crystalline domain dimensions (a few nm to a few tens of nanometers) do not exceed those of the finest details that can be observed, and thus do not modify sample morphology at the observation scale. Cryo-SEM was applied to the study of wet-state model soil colloids: a soil humic acid, a commercial smectite, a synthetic ferrihydrite and their binary and ternary associations. The cryo-SEM images have given the following information: The smectite suspension behaved as a non-oriented network, the ferrihydrite suspension was formed from particles of various shapes and dimensions, and the humic acid exhibited a sponge-like structure. The micromorphology of the ferrihydrite–humic acid association was close to that of pure ferrihydrite, while the smectite-containing associations recalled the basic network of the smectite. The comparison of the images obtained by cryo-SEM, and by conventional SEM performed on the same samples after they had been freeze-dried, points out the morphologic modifications deriving from the conventional drying process. Besides SEM investigation, the soil colloids have been characterized by X-ray diffractometry, FTIR spectroscopy, elementary analysis and surface area determination.