Sorption and Desorption of Pesticides by Clay Minerals and Humic Acid-Clay Complexes

small amounts of water to soil reduced the sorption of organic compounds relative to the sorption by the In soils, organic matter and minerals are often associated such that dehydrated soil in hexane, and that organic vapor adit is unclear how the presence of the former component influences sorption by soil was significantly depressed by increasing the sorptive properties of the latter one. In this study, sorption and desorption of the herbicides 4,6-dinitro-o-cresol (C7H6N2O5) and dichrelative humidity. Based on these observations, they lobenil (C7H3Cl2N) by Ca2 –, K –smectites, and humic acid–smectite concluded that soil behaves as a dual sorbent in which complexes, was investigated using batch-equilibrations and x-ray difsoil organic matter functions as a partitioning medium fraction (XRD). Greater sorption of 4,6-dinitro-o-cresol compared and mineral fractions as conventional adsorbents. Furwith dichlobenil was observed for both smectites and humic acid– thermore, the adsorption of organic compounds by the smectite complexes. For both pesticides, K smectites were more mineral fractions was thought to be restrained by ambieffective sorbents than Ca2 smectites, with the lower charge-density ent moisture because water molecules preferentially occlay (SWy-2) displaying a greater sorption capacity than the higher cupied the adsorptive sites on mineral surfaces. This charge-density clay (SAz-1). The presence of humic acid did not imhypothesis may be valid for nonpolar organic compact pesticide sorption by K clays, but could enhance or suppress pounds (e.g., aromatic hydrocarbons) and those conpesticide sorption by Ca2 –clays. A composite model for estimating pesticide sorption, which assumes mineral and organic matter function taining slightly polar functional groups (e.g., –Cl), howindividually as sorbent phases, predicted sorption within a factor of ever, for moderately and strongly polar functional 0.8 to 1.5 times the measured values. Humic acid did not contribute group-containing compounds including many pesticides, to pesticide desorption hysteresis in K –humic acid–clay complexes, multiple sorption mechanisms may be operative. These but was a source of hysteresis in the corresponding Ca2 complexes. include solute partitioning into soil organic matter as The basal spacings of K-SWy-2 and humic acid-modified K-SWy-2 well as specific interactions with mineral components increased gradually from approximately 10.4 to 12.2 Å with increasing such as clays (Karickhoff, 1984). In some cases, mineral 4,6-dinitro-o-cresol loadings. Also, XRD patterns of humic acid-modifractions may contribute more than soil organic matter fied and unmodified K-SWy-2 smectite clays were found identical. to the retention of certain neutral organic contaminants These results demonstrate the intercalation of 4,6-dinitro-o-cresol and and pesticides (Sheng et al., 2001). suggest that humic acids are restricted to the external surfaces of clay tactoids. Together, these results indicate that clay mineral fractions Polar functional group-containing compounds interin soils, including those with organic coatings, may play an important act with clays through a variety of mechanisms (Mortrole in the retention of certain pesticides. land, 1970). These include interactions with exchangeable cations on clay surfaces via ion-dipole interactions, and with siloxane surfaces via surface adsorption. The O matter and clay minerals are generally conion-dipole mechanism may involve direct interactions sidered as the two most important soil components of organic compounds with exchangeable cations or inin the retention of soil-applied pesticides. It is well estabdirect interactions through the intermediation of water lished that the uptake of nonpolar organic compounds molecules surrounding the cations. Ion-dipole interacfrom aqueous solution is strongly correlated with soil tions may be enhanced with increasing charge valence organic matter (or organic C) content (Chiou et al., of exchangeable cations. For example, pesticide (e.g., 1979, 1983; Karickhoff et al., 1979; Kile et al., 1995; clomazone, atrazine) adsorption on montmorillonite Kleineidam et al., 1999; Weber and Huang, 1996; Xia saturated with a variety of cations decreased in the orand Ball, 1999; Xing and Pignatello, 1997). Reliance on der: Al3 Mg2 Ca2 Li Na (Bowman, 1973; soil-organic-matter (or organic-C) normalized sorption Loux et al., 1989; Sawhney and Singh, 1997). For the coefficients (KOM, KOC) to predict the transport of neuAl3 –saturated clays, strong polarization of water moletral organic compounds in soil and subsurface environcules associated with Al3 manifests strong H-bonding ments implies that soil organic matter is the singular with pesticides leading to greater adsorption (Sawhney sorptive domain, and ignores the potential contributions and Singh, 1997). Alternatively, water molecules surof soil mineral fractions. Previous research by Chiou rounding strongly hydrated cations such as Ca2 and and coworkers (Chiou and Shoup, 1985; Chiou et al., Mg2 may weaken ion-dipole interactions by inhibiting 1985) noted that in hexane-soil systems, the addition of the direct interaction between polar functional groups and exchangeable cations (Johnston et al., 2001, Sheng et al., 2002). Nonpolar surface interactions between orH. Li, B.J. Teppen, and S.A. Boyd, Department of Crop and Soil ganic compounds and the siloxane surfaces of clays may Sciences, Michigan State University, East Lansing, MI 48824; G. also occur (Jaynes and Boyd, 1991; Laird and Fleming, Sheng, Department of Crop, Soil and Environmental Sciences, University of Arkansas, Fayetteville, AR 72701; C.T. Johnston, Depart1999). In clays exchanged with less strongly hydrated ment of Agronomy, Purdue University, West Lafayette, IN 47907. Received 1 Apr. 2002. *Corresponding author ([email protected]). Abbreviations: EDA, electron donor acceptor; XRD, x-ray diffraction. Published in Soil Sci. Soc. Am. J. 67:122–131 (2003).