Environ. Sci. Technol. 2000, 34, 3574-3578
Sorption, Diffusion, and Desorption of Alachlor in Oxidized and Reduced Smectite Membranes NIKOLAI M. KOCHERGINSKY† AND J O S E P H W . S T U C K I * ,‡ Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, and Department of Natural Resources and Environmental Sciences, University of Illinois, W-321 Turner Hall, 1102 South Goodwin Avenue, Urbana, Illinois 61801
The fate of pesticides in the environment depends to a large extent on their interactions with clay mineral surfaces in soils and sediments. Most studies of these interactions typically employ batch suspension methods. Another method, which may be much faster and simpler, is to cast the clay into the form of a self-supporting film immobilized between two porous nitrocellulose filters. The objective of this study was to investigate the kinetics of alachlor transport through such a membrane. We found that at least two kinetic phases (rapid and slow) exist. Herbicide disappearance from donor solutions, its appearance in acceptor solutions (including time lag and the rate at steady state), and its retention by the membrane revealed that adsorption on the clay surface plays a prominent role in transmembrane transport processes. The oxidation state of structural Fe in the clay film changed the surface chemical properties of the clay and modified both the fast adsorption and slow transport of alachlor through the membrane. Transport through the reduced clay was accompanied by chemical alterations of alachlor. No chemical alterations were observed if the clay film was in its oxidized state.
Introduction When fertilizers and other agricultural chemicals are applied to soils, they come into intimate contact with clay mineral surfaces. Sorption, diffusion, and degradation are the main processes determining the long-term persistence and biological activity of herbicides and other contaminants and their rate of movement through the unsaturated zone and potential for groundwater contamination (1). The oxidation state of Fe in clay minerals fundamentally alters the mineral surface chemistry (2, 3). The effects of reducing conditions on clay-organic interactions could, therefore, be an important factor determining the fate and chemical transformations of herbicides and other agricultural chemicals in soils (4). The interactions of herbicides and chemical fertilizers with soils classically are characterized in the laboratory using batch suspension and shaking or stirring techniques and also solid-liquid-phase separations by centrifugation (5, 6). Another popular method involves the use of soil columns in which the solution passes through the soil under an hydraulic gradient (7), which presumes fast sorption equilibrium, but * Corresponding author phone: (217)333-9636; fax: (217)244-7805; e-mail:
[email protected]. † National University of Singapore. ‡ University of Illinois. 3574
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000
is often an oversimplification because both sorption and transport processes have both fast and slow kinetic phases (5). The phenomenon of multiple kinetic phases is well documented in a plethora of literature both for inorganic and nonpolar organic substances, but the underlying mechanisms for these phases are still unclear and may be attributable to physical heterogeneity of the porous system and/or to the presence of different chemical adsorption centers (3). Recently we demonstrated that a supported clay membrane is an effective system to characterize sorption and transport processes in soils that contain smectite clays and organic components (8). A three-layer membrane was constructed by placing a self-supporting clay film between two microporous nitrocellulose filters. A similar filter|clay|filter system was recently shown to be useful in reverse osmosis separation of electrolytes and nonelectrolytes, including amino acids (9). The purpose of the present study was to apply this novel trilayer membrane system for studying the sorption and transport of herbicides in unaltered and redox-treated clays. The ultimate objective is to understand better the processes governing the fate and mobility of herbicides and contaminants in the environment.
Materials and Methods A Fe-rich clay sample (ferruginous smectite SWa-1, obtained from the Source Clays Repository of The Clay Minerals Society, Columbia, MO) was used in this study. This smectite clay, except for its total Fe content, is typical of the swelling clays that exist in soils. A stock supply of the clay was prepared by first separating its
