Effects of Increasing Potassium Chloride and Calcium Chloride Ionic Strength on Pesticide Sorption by Potassium- and Calcium-Smectite

In aqueous suspension, the affinity of many pesticides for smectites is influenced by the clay properties such as surface area, surface charge density and location, exchangeable cations, and hydration status of exchangeable cations in clay interlayers. The amount and the type of salts present in the aqueous phase influence clay quasicrystal structures and hydration status, which we hypothesize as major determinants of pesticide sorption. To test this hypothesis, we measured sorption isotherms of alachlor, atrazine, dichlobenil, and diuron by K–saturated smectite (K-SWy-2) in KCl solution and Ca–saturated smectite (Ca-SWy-2) in CaCl2 solution at several ionic strengths. The results indicated that pesticide sorption by K-SWy-2 significantly increased with increasing aqueous KCl concentration. In contrast, sorption by Ca-SWy-2 at different CaCl2 ionic strengths remained nearly constant. The “salting-out” effect on the reduction of dissolution of pesticides failed to account for the significantly increased sorption by K-SWy-2 in aqueous KCl solutions. We conclude that formation of better-ordered clay quasicrystals and shrinkage of clay interlayer distance owing to increasedKCl ionic strength facilitate the intercalation of pesticides leading to greater sorption by the clay, while the salting-out effect plays a minor role (if any) in the observed sorption enhancement. SORPTION TO SOILS and sediments is a major factor influencing transport, bioavailability, and hence fate of pesticides in the environment. Awidely accepted view is that soil organic matter (SOM) functions as the predominant sorptive phase whereas soil mineral fractions play a relatively minor role in the sorption of neutral organic contaminants and pesticides from water (Chiou et al., 1979; Chiou et al., 1983; Karickhoff et al., 1979; Kile et al., 1995; Weber and Huang, 1996; Xia and Ball, 1999; Xing and Pignatello, 1997). This view is exemplified by the reliance on soil organic carbon-normalized sorption coefficients (KOC) to predict the fate and transport of organic compounds in soils. However, several recent studies and a few earlier ones provide clear evidence that soil minerals can function as effective adsorbents for pesticides and organic contaminants under environmentally relevant conditions (Bailey et al., 1968; Bowman, 1973; Boyd et al., 2001; Celis et al., 1998; Haderlein et al., 1996; Hundal et al., 2001; Johnston et al., 2002; Laird et al., 1992; Mortland, 1970; Sheng et al., 2002; Zhu et al., 2004). For important categories of pesticides (e.g., triazines, carbamates, ureas, nitrophenols, benzonitrile) and organic contaminants (e.g., nitroaromatics), sorption by clays can be equal or greater than that by SOM based on comparisons of sorption by a unit mass of isolated clay minerals vs. SOM and by synthetic clay–organic matter complexes (Li et al., 2003; Sheng et al., 2001). Mineral fractions in soils can also function as effective adsorptive domains for pesticides and organic contaminants, particularly for the soils containing smectite clays (Charles et al., 2006). Among the naturally occurring soil minerals examined, smectite clays manifested the greatest sorption (on unit mass of sorbent basis) for nitroaromatic compounds due to their large surface areas and expandability (Weissmahr et al., 1998). Among smectites, less surfacecharge-density clays typically provide larger hydrophobic nanosites on siloxane surface between exchangeable cations for adsorption leading to greater uptake (Jaynes and Boyd, 1991; Laird et al., 1992; Lee et al., 1990; Sheng et al., 2002). The origin of negative charges in smectite clays also affects pesticide sorption. Negative charges arising from tetrahedral sheet isomorphic substitution are more localized, which appears to have a favorable effect on the interlayer environment for pesticide sorption compared to smectites with isomorphic substitution predominantly in the octahedral sheet (Aggarwal et al., 2006). The neutral siloxane surfaces of smectite clays are important adsorption domains for organic compounds; however, water molecules associated with exchangeable cations and clay surfaces may obscure these sites. Organic compounds must be able to effectively compete with these water molecules to access the mineral sorptive sites. The type of exchangeable cation strongly influences the hydration status of clay minerals. Among the inorganic cations commonly occurring in natural environments, less strongly hydrated cations (e.g., K) manifest smaller hydration spheres than more strongly hydrated cations (e.g., Ca) hence leaving more neutral clay siloxane surfaces available for adsorption (Li et al., 2003, 2004a). Weakly hydrated cations also facilitate adsorption by easing the displacement of hydration waters thereby promoting solute interactions with exchange cations (e.g., formations of innerand outer-sphere complexes) (Boyd et al., 2001). In addition, the basal spacings of more weakly hydrated cation-exchanged smectites, for example, K-smectite, often approximate the thickness of aromatic molecules such as nitroaromatic compounds. For example, K-smectite with sorbed H. Li, B.J. Teppen, and S.A. Boyd, Dep. of Crop and Soil Sciences, and Environmental Science and Policy Program, Michigan State Univ., East Lansing, MI 48824; D.A. Laird, USDA-ARS, National Soil Tilth Lab., Ames, IA 50011; C.T. Johnston, Crop, Soil and Environmental Sciences, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907. Received 5 Dec. 2005. *Corresponding author (boyds@