Characterization of model soil colloids by cryo-scanning electron microscopy

-Abstract 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 cryopreparation 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.