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DIVISION S-2—SOIL CHEMISTRY

Influence of Exchangeable Cations on Water Adsorption by Soil Clays

^a Dep. of Agronomy, Purdue Univ., West Lafayette, IN and USDA-ARS, National Soil Erosion Research Laboratory, West Lafayette, IN
^b School of Natural Resources, The Ohio State Univ., 2021 Coffey Rd., Columbus, OH 43210
^c Dep. of Agronomy, Purdue Univ., West Lafayette, IN
^d School of Natural Resources, The Ohio State Univ., Columbus, OH

^* Corresponding author (dontsova.1{at}osu.edu).

The interaction of water with the clay fractions (<2 µm) from two midwestern soils was studied using Fourier transform infrared (FTIR) spectroscopy and gravimetric methods. The soil clay fractions were obtained from a Blount loam (fine, illitic, mesic Aeric Epiaqualfs) and a Fayette silty clay loam (fine-silty, mixed, mesic, superactive Typic Hapludalfs). These clay fractions were exchanged with Mg²⁺, 50:50 Ca²⁺/Mg²⁺, Ca²⁺, Na⁺, and K⁺ to determine the influence of the exchangeable cation on their water sorption behavior. Water sorption isotherms and FTIR spectra of the clays were collected simultaneously using a gravimetric-spectroscopic cell. Overall, the amount of water sorbed by the samples increased as the ionic potential of the exchangeable cation increased and was strongly correlated to the hydration energy of the cations (P > F < 0.0001). The position of the H–O–H bending band (₂ mode) also increased with increasing ionic potential of the exchangeable cation indicating strengthening of water H bonds. In addition, it was observed that the position of this band decreased with increasing water content for the Mg-exchanged clays compared with an overall increase for the Ca-exchanged samples. X-ray diffraction patterns indicated an expansion of the phyllosilicate clay minerals as the water activity increased; however, no differences were observed between the Ca- and Mg-exchanged samples. This study shows that the molecular properties of water on Ca- and Mg-exchanged soil clays are similar to that on specimen clays and provides new insight about the role of exchangeable cations in soils.

Abbreviations: BET, Brunauer-Emmett-Teller • CEC, cation exchange capacity • EGME, ethylene glycol monoethyl ether • EIRM, Environmental Infrared Microbalance • FTIR, Fourier transform infrared • IR, infrared • I/S, randomly interstratified illite-smectite • OM, organic matter • P/P₀, partial pressure of water vapor • RH, relative humidity

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