Author
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Laird, David |
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Submitted to: Clays and Clay Minerals
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/19/1999 Publication Date: N/A Citation: N/A Interpretive Summary: Soil clay minerals can adsorb over 100% of their weight in water. This swelling phenomenon influences the ability of soils to supply water and nutrients to growing plants. The swelling of clays also influences the stability of soil structure and hence the quality of soils. In the past much research has been published on how relative humidity and the type of cations adsorbed on clays influences the ability of clays to adsorb water. This research is the first to demonstrate that the amount of charge on a clay surface (layer charge) also influences their ability to adsorb water. We now know that the amount of water that can be adsorbed inside of a clay mineral is constant while the amount of water that can be adsorbed on the outside surfaces of the clay varies with the relative humidity, the type of cation adsorbed on the clay, and the amount of charge on the outside surfaces of the clay. A new mathematical model describes the way in which clays adsorb water. This information is important to scientists who will use the new understanding of how clays adsorb water to better understand how soils retain water and are able to supply water to growing plants. The ability of soils to retain water is one of their most important properties and directly impacts soil productivity, the economic viability of farmers, and the supply of food to consumers. Technical Abstract: The capacity to adsorb water vapor is a critical property of 2:1 phyllosilicates. Previous studies have investigated the effects of water vapor pressure, hydratable surface area, and nature of the saturating cations on the hydration of 2:1 phyllosilicates. This study was designed to quantify the influence of layer charge on the hydration of 2:1 phyllosilicates. The amount of water retained by 12 Mg-saturated clays at 54% RH was quantified gravimetrically. X-ray diffraction and total chemical analysis were used to determine the hydratable surface area (447 to 759 m**2 g**-1) and layer charge (0.327 to 0.754 e f.u.**-1) of each sample. The amount of water retained by the clays at 54% RH increased with both hydratable surface area and layer charge of the clays. However, the increase in water content with layer charge occurred entirely on the external surfaces of the clay. This finding suggests that all of the water ron external surfaces is localized around the cation/charge sites rather than forming multi-layers as has been previously suggested. A simple model is proposed for the hydration of 2:1 phyllosilicates. The model assumes that interlayer volume controls interlayer hydration and that the number of cation/charge sites on external surfaces controls hydration of external surfaces. |
