Geochemical Modulation of Pesticide Sorption on Smectite Clay HUI LI, † BRIAN J. TEPPEN, † DAVID A. LAIRD, ‡ CLIFF T. JOHNSTON, § AND STEPHEN A. BOYD †, * Environmental Science and Policy Program, and Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824, National Soil Tilth Laboratory, USDA-ARS, Am es, Iowa 50011, Crop, Soil and Environm ental Sciences, Departm ent of Agronom y, Purdue University, West Lafayette, Indiana 47907 Pesticide adsorption by soil clays can be dramatically influenced by the exchangeable cations present. Among the common exchangeable base cations in soils (Ca 2+ ,Mg 2+ , K + , and Na + ), K + -saturated clays frequently demonstrate the strongest affinity for pesticides.In the presence of multiple exchangeable cations in the system, we hypothesize that the magnitude of pesticide sorption to soil minerals is proportional to the fraction of clay interlayers saturated with K + ions. To test this hypothesis, we measured sorption of three pesticides with different polarities (dichlobenil,monuron,and biphenyl) by homoionic K- and Ca- smectite (SWy-2) in KCl/CaCl 2 aqueous solutions. The presence of different amounts of KCl and CaCl 2 resulted in varying populations of K + and Ca 2+ on the clay exchange sites. The sorption of dichlobenil and, to a lesser extent monuron, increased with the fraction of K + on clay mineral exchange sites. Ca- and K-SWy-2 displayed the same sorption capacities for nonpolar biphenyl. X-ray diffraction patterns indicated that at lower fractions of K + -saturation, exchangeable K + ions were randomly distributed in clay interlayers and did not enhance pesticide sorption. At higher populations of K + (vs Ca 2+ ), demixing occurred causing some clay interlayers, regions, or tactoids to become fully saturated by K + , manifesting greatly enhanced pesticide sorption. The forward and reverse cation exchange reactions influenced not only K + and Ca 2+ populations on clays but also the nanostructures of clay quasicrystals in aqueous solution which plays an important, if not dominant, role in controlling the extent of pesticide sorption. Modulating the cation type and composition on clay mineral surfaces through cation exchange processes provides an environmental-safe protocol to manipulate the mobility and availability of polar pesticides, which could have applications for pesticide formulation and in environmental remediation. Introduction It has been well established that the retention of nonpolar organic contaminants in soil-water systems is strongly correlated with soil organic matter (SOM) content and that soilmineralfractionsplaya comparativelyminorrole except in the absence ofwater.SOM is viewed as providinga partition phase for the uptake oforganic contaminants and pesticides (1-9). The presence of natural and anthropogenic high- surface-area carbonaceous materials (HSACM, e.g., soots, humin, kerogen) in soils/sediments also contributes to a strong sorption of organic contaminants at low relative concentrations (10-17).However,the HSACM materials are typically present in soils/sediments at low concentrations compared to the totalSOM,hence,their overallcontribution to sorption is low, especially at higher relative contaminant concentrations or in the presence of multiple solutes (13, 18). The SOM-partition model appears valid for organic contaminantscontainingnonpolarorslightlypolarfunctional groups (e.g., -Cl). It is, however, frequently extended to organiccontaminantsin general,includingthose containing polar functional groups such as many pesticides. This is illustrated by the common use of soil-organic-matter (or carbon)-normalized sorption coefficients(KOM, KOC)to predict pesticide mobility in soils. It is now clear that for important categories of pesticides (e.g., triazines, carbamates, ureas, nitrophenols)and organiccontaminants(e.g.,nitroaromatic compounds), sorption by clays may equal or exceed that by SOM, based on the estimates of sorption by the isolated sorbents (19-26). For example, Sheng et al. noted that K + - saturated smectite (i.e., SWy-2) was a more effective sorbent for pesticides such as 4,6-dinitro- o-cresoland 2,6-dichlobenil compared to sorption by an organic soil (20). Furthermore, humic coatings on reference smectites (i.e., SWy-2 and SAz-1) did not significantly impact sorption and desorption of 4,6-dinitro- o-cresol and 2,6-dichlobenil by clay fractions for synthetic K + -saturated humic-smectite complexes with low organic carbon contents (<1.7%) (25). Pesticide adsorption bysoilmineralsisoften dramatically influenced by the types of exchangeable cations commonly found in nature (Ca 2+ , Mg 2+ , Al 3+ ,K + ,NH4 + ,and Na + ). These cations on clays control clay interlayer environments and may interact with sorbed organic contaminants/pesticides, leading to greater adsorption (21, 23-27). Am o n g the common exchangeable cations in soils, K + -saturated clay minerals frequently demonstrate the strongest sorption of pesticides. This appears to be a manifestation of the comparatively weak hydration of K + . The enthalpy of hydration for K + is -314 kJ/mol, smaller than that of Na + (-397 kJ/mol) and much smaller than that of Ca 2+ (-1580 kJ/mol) and Mg 2+ (-1910 kJ/mol). Uncharged siloxane surfaces between charged sites on smectite surface are relatively hydrophobic and can interact with nonpolar moietiesoforganic contaminantswhen theyare able to access these mineral surfaces (22, 28-30). In contrast, when smectites are saturated with strongly hydrated divalent cations (e.g., Ca 2+ , Mg 2+ ), the hydration sphere surrounding exchangeable cations diminishes the size of adsorptive domains between cations (21, 23, 26, 31) and reduces the strength ofinteractionsbetween exchangeable ionsand polar functional groups in organic contaminants (24, 25, 27, 32). Last, for K + -saturated smectites,the basalspacings are often observed at ∼12.3 Å, which appears optimal for adsorption of organic contaminants (21, 24). This spacing is just large enough to allow intercalation of the sorbate (e.g., nitroaro- matics), while minimizing its interaction with water mol- ecules.This energeticallyfavorable process occurs when the aromatic solute directly contacts the opposing clay siloxane surfaces. Larger interlayer spacings associated with Na + or *Correspondingauthor phone: (517)355-0271,ext 252;fax: (517)- 355-0270; e-mail: boyds@msu.edu. † Michigan State University. ‡ USDA-ARS. § Purdue University. Environ. Sci. Technol. 2004, 38, 5393-5399 10.1021/es0494555 CCC: $27.50  2004 American Chemical Society VOL. 38, NO. 20, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5393 Published on Web 09/16/2004