Environ. Sci. Technol. 1994, 28, 1054-1061
Sorption of Atrazine on Soil Clay Components David A. Laird,’ Pau Y. Yen, William C. Koskinen, Tom R. Steinhelmer, and Robert H. Dowdy US. Department of Agriculture, Agricultural Research Service, National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, Iowa 500 11
Distribution coefficients for sorption and desorption of atrazine were determined for all combinations of three chemical treatments and four particle size fractions prepared from a typical soil clay. Organic and inorganic components were 11and 89 5% of the mass and contributed 68 and 32% of the affinity of the soil clay for atrazine, respectively. Organic matter associated with the coarse clay had substantially greater affinity for atrazine and exhibited greater sorption-desorption hysteresis than organic matter associated with the fine clay. Silicate minerals had a moderate affinity for atrazine and exhibited little sorption-desorption hysteresis. Free Fe compounds interacted with silicate minerals in the coarse and medium clay to reduce affinity of the silicate minerals for atrazine. The results suggest that atrazine is primarily retained on silicate clays by physical sorption but that both physical sorption and chemisorption contribute to retention of atrazine by soil organic matter.
Introduction
Development of a mechanistic understanding of pesticide sorption in soils is complicated by the heterogeneous nature of soil components. Soil organic components include both living organisms and a continuum form dead organisms to amorphous humic substances. Soil inorganic components include nearly amorphous oxyhydroxide materials and both crystalline and quasicrystalline minerals. The particle size of soil inorganic components typically varies over 4-5 orders of magnitude (e.g., 10-2-10-7 m). Numerous studies have demonstrated correlations between organic matter content and sorption of nonionic organic compounds by soils (1-3). Chiou (4, 5 ) has hypothesized that partitioning into the volume of soil organic matter is the predominant mechanism for sorption of nonpolar organic compounds by soil materials in aqueous systems. However, most pesticides have some polarity, and there is substantial evidence that polar organic compounds are sorbed by both organic and inorganic soil components (3,6-9). Furthermore, surface properties of soil components are more important than the amount of those components in determining pesticide affinities (9). Recently, we found sorption ranging from 0 to -100% of added atrazine ( K fvalues ranging from (0.01 to 1334) on 14 different smectites (10). Two parameters, surface charge density and surface area, accounted for 82 % of the variability in atrazine sorption by the smectites. Such data indicate that knowledge of the clay content or even the percentage smectite in a soil may not be enough to predict the contribution of inorganic soil components to sorption of pesticides by the whole soil. The various soil components may contribute independently to the sorption of pesticides; however, interactions between components are likely. For instance, humic substances may compete with pesticides for sorption sites, and both organic matter and metal oxyhydroxides may 1054
Environ. Sci. Technol., Vol. 28, No. 6, 1994
coat surfaces of silicate minerals altering the affinity of those surfaces for pesticides. To date, little is known about the relative contributions of the various soil components to pesticide sorption and even less is known about the interactions between soil components that influence pesticide sorption. Mechanisms by which nonionic pesticides bind to soil components depend on pesticide chemistry and the nature and properties of the soil components (11,12). In general, binding by physical sorption is reversible while binding by chemisorption is irreversible. However, an apparent irreversibility, or “hysteresis” in desorption of a solute from a sorbent, may be caused by factors other than chemisorption, such as failure to reach equilibrium during desorption and losses of the pesticide due to chemical or biological degradation, volatilization, or precipitation (13, 14). Furthermore, chemisorbed pesticides can be partially desorbed depending on the nature of the chemical bonds and the desorption methodology (11,15). With caution, however, hysteresis measurements can be used for distinguishing sorption mechanisms (13). Analyses of sorption-desorption hysteresis indicate that nonionic pesticides are retained on surfaces of soil minerals through relatively weak electrostatic (physical sorption) interactions (6,12,16),whereas both physical and chemical interactions may bind nonionic pesticides to soil organic matter (6,17,18). For instance, high clay samples exhibit less sorption-desorption hysteresis with metribuzin [4-amino-6-(l,l-dimethylethyl)-3-(methylthio)-l,2,4-triazin-5(4H)one] than high organic matter samples (6,17). In another study (18),increasing sorption-desorption hysteresis was reported for diuron [3’-(3,4-dichlorophenyl)-1,l-dimethylureal with increasing organic matter content in freshwater sediments. Such evidence suggests that polar organic compounds form stronger bonds with soil organic matter than with soil minerals. The objectives of this study were to (i) quantify the relative contributions of organic matter, silicate clays, and free Fe compounds to the sorption of atrazine by clay separated from a prominent agricultural soil; (ii) quantify interactions between soil clay components that influence atrazine sorption; and (iii) elucidate binding mechanisms between atrazine and soil clay components. Only the clay fraction is considered in this study as sorption is a surface reaction, and the vast majority of both organic and inorganic surfaces are associated with the clay fraction (19, 20). Experimental Methods Sample Preparation. The soil sample used in this study was obtained from the Aphorizon of a Webster (fineloamy, mixed, mesic Typic Haplaquoll) pedon. The pedon is located on the University of Minnesota Southern Agricultural Experiment Station, near Waseca, MN. The soil was mechanically dispersed in distilled water without chemical pretreatments, and the soil clay (
