Influence of Exchangeable Cations on Water Adsorption by Soil Clays

little is known about the molecular mechanisms of soil– water interactions underlying this behavior. A considerThe interaction of water with the clay fractions ( 2 m) from two able amount of work has been conducted on specimen midwestern soils was studied using Fourier transform infrared (FTIR) spectroscopy and gravimetric methods. The soil clay fractions were clay minerals and their interaction with water, but it is obtained from a Blount loam (fine, illitic, mesic Aeric Epiaqualfs) not clear if similar mechanisms are operative in soils. and a Fayette silty clay loam (fine-silty, mixed, mesic, superactive There is some recent work showing that soils clays beTypic Hapludalfs). These clay fractions were exchanged with Mg2 , have differently than specimen clays because the solid 50:50 Ca2 /Mg2 , Ca2 , Na , and K to determine the influence of the phases are more complex and include organic matter exchangeable cation on their water sorption behavior. Water sorption (OM) (Zachara et al., 1993). isotherms and FTIR spectra of the clays were collected simultaneously The interaction of water with expandable clay minerusing a gravimetric-spectroscopic cell. Overall, the amount of water als has been an active area of research for more than sorbed by the samples increased as the ionic potential of the exchange60 yr (Buswell et al., 1937; Mooney et al., 1952b; Keren able cation increased and was strongly correlated to the hydration energy of the cations (P F 0.0001). The position of the H–O–H and Shainberg, 1975, 1979; Poinsignon et al., 1978; Hall bending band ( 2 mode) also increased with increasing ionic potential and Astill, 1989; Johnston et al., 1992; Berend et al., 1995; of the exchangeable cation indicating strengthening of water H bonds. Cases et al., 1997; Chiou and Rutherford, 1997; Dios In addition, it was observed that the position of this band decreased Cancela et al., 1997; Xu et al., 2000; Madejova et al., with increasing water content for the Mg-exchanged clays compared 2002). Water condensation and osmotic swelling are the with an overall increase for the Ca-exchanged samples. X-ray diffracdominant mechanisms for retaining water molecules at tion patterns indicated an expansion of the phyllosilicate clay minerals high relative humidity (RH). At low water contents, the as the water activity increased; however, no differences were observed hydration characteristics of smectite depend strongly on between the Caand Mg-exchanged samples. This study shows that the exchangeable cation (Berend et al., 1995; Cases et the molecular properties of water on Caand Mg-exchanged soil clays are similar to that on specimen clays and provides new insight about al., 1997; Dios Cancela et al., 1997; Xu et al., 2000). the role of exchangeable cations in soils. Layer charge also affects the amount of water adsorbed on clay surfaces, with more water being adsorbed on high charged smectites than on low charged ones (Chiou and Rutherford, 1997; Laird, 1999; Xu et al., 2000). T interaction of water with soil colloids plays a critical role in all areas of soil science, and numerous However, Laird (1999) showed that effective hydration studies have shown that exchangeable cations signifinumbers of exchangeable cations decrease with increase cantly influence soil–water relations. Dispersion and clay in layer charge. swelling, which are enhanced by Na and Mg on the soil Water sorption by expandable clay minerals is generexchange complex (Rengasamy, 1983; Rengasamy et al., ally accompanied by an increase in basal spacing or 1986; Shainberg et al., 1988; Heil and Sposito, 1993; swelling caused by hydration of the interlayer cations Curtin et al., 1994; Dontsova and Norton, 2002), can and the exposed clay surfaces at low water contents, block airand water-conducting soil pores, thereby affollowed by osmotic effects when free water is present fecting infiltration (Keren, 1989, 1990, 1991; Dontsova (Keren and Shainberg, 1975; Sposito and Prost, 1982). and Norton, 2002) and hydraulic conductivity (Bakker Berend et al. (1995) and Cases et al. (1997) observed and Emerson, 1973; Alperovitch et al., 1981; Levy et an increase in basal spacing with the hydration energy al., 1988; Curtin et al., 1994; Zhang and Norton, 2002). of the saturating cation: Cs Rb K Na By contrast, Ca is known to promote the flocculation of Li Ba2 Sr2 Ca2 Mg2 . For Mgand Casoil colloids (Rengasamy, 1983; Rengasamy et al., 1986; smectite, a homogeneous two-layer hydrate predomiHeil and Sposito, 1993; Curtin et al., 1994; Dontsova and nated over a wide range of RH, and a two-layer hydrate Norton, 2002) and is often used in various soil remediawas formed for all divalent cations at high RH. Farmer tion strategies. Although exchangeable cations are known (1978) reviewed several studies of the swelling of clays to affect the overall behavior of water in soils, relatively exchanged with different cations, and observed a trend for increased swelling at high RH with an increase in cation hydration energy. Differences in clay basal spacK.M. Dontsova and L.D. Norton, Dep. of Agronomy, Purdue Univ., West Lafayette, IN and USDA-ARS, National Soil Erosion Research ings among different divalent cations, though, were Laboratory, West Lafayette, IN; K.M. Dontsova currently at: School small and inconsistent. Greater differences between the of Natural Resources, The Ohio State Univ., 2021 Coffey Rd., Columd-spacings of Ca and Mg exchanged clays were observed bus, OH 43210; C.T. Johnston, Dep. of Agronomy, Purdue Univ., West Lafayette, IN; J.M. Bigham, School of Natural Resources, The Ohio State Univ., Columbus, OH. Received 27 Aug. 2003. *CorreAbbreviations: BET, Brunauer-Emmett-Teller; CEC, cation exchange sponding author ([email protected]). capacity; EGME, ethylene glycol monoethyl ether; EIRM, Environmental Infrared Microbalance; FTIR, Fourier transform infrared; IR, Published in Soil Sci. Soc. Am. J. 68:1218–1227 (2004).  Soil Science Society of America infrared; I/S, randomly interstratified illite-smectite; OM, organic matter; P/P0, partial pressure of water vapor; RH, relative humidity. 677 S. Segoe Rd., Madison, WI 53711 USA