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. D

Surface analysis of cryofixation-vacuum-freeze-dried polyaluminum chloride–humic acid (PACl–HA) flocs

“…In addition, the ten nanometers of pores in dried PACl-HA flocs are close to the size of ice crystals formed in flocs during ultrarapid freezing, which suggests that cryofixation could be an effective technique, at least preserving the morphology of flocs at the most part between few nanometers to hundreds of nanometers [22], which is the maximum scales for pore surface fractal dimension. Table 4.…”

“…But according to Nègre et al study [22], the microstructure in less than microns scales of flocs may be partly preserved after above dried floc preparation process. So the following surface morphology study could be based on these scales, and only could give a primary attempt on the floc surface properties such as surface fractal, specific surface area, pore diameter and volume.…”

“…Recently, cryofixation is becoming a promising technique in many research and industrial fields. Plunging sample into a liquid with high thermal conductivity can result in ultrarapid freezing of sample and transform the water into micro-ice crystals with dimensions of a few nanometers to a few tens of nanometers, and thus distortion to the morphology of the sample could be minimized [21][22][23]. After floc cryofixation, vacuum-drying technique has been used to sublimate the micro-ice crystals from sample.…”

“…Cryofixation has also been applied to characterize humic substances and their binary and ternary associations with clay minerals, ferrihydrite and smectite. With the aid of this technique, better understanding of the role of organic soil polymers in the aggregation processes of soil components is obtained [22].…”

Effect of polyferric chloride (PFC) doses and pH on the fractal characteristics of PFC–HA flocs

“…Cryofixation of the PFC-HA floc samples was conducted by plunge-freezing in liquid nitrogen at 77 K. A 15-mL plastic centrifugal tube containing 10 mL of flocculated water with flocs was plunged into a liquid-nitrogen tank for 30 min, after which the cryofixation samples were placed in a vacuum-freeze dryer (FD-1A, Beijing Boyikang Laboratory Instruments Ltd., China) for 24-48 h. Sublimation of the ice formed from void or interstitial water of the sample was then induced by the vacuum-freeze drying process [28]. Finally, dry floc powder samples were obtained and stored in a desiccator.…”