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Title: GEOCHEMICAL MODULATION OF PESTICIDE SORPTION ON SMECTITE CLAY

Author
item LI, HUI - MICHIGAN STATE UNIVERSITY
item TEPPEN, BRIAN - MICHIGAN STATE UNIVERSITY
item Laird, David
item JOHNSTON, CLIFF - PURDUE UNIVERSITY
item BOYD, STEPHEN - MICHIGAN STATE UNIVERSITY

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2004
Publication Date: 10/15/2004
Citation: Li, H., Teppen, B.J., Laird, D.A., Johnston, C.T., Boyd, S.A. 2004. Geochemical modulation of pesticide sorption on smectite clay. Journal of Environmental Science and Technology. 38(20):5393-5399.

Interpretive Summary: The risk that soil applied pesticides will have an adverse effect on groundwater or the environment depends to a large extent on how strongly the pesticide interacts with the soil. If the pesticide is strongly adsorbed by the soil it will be less likely to leach to the groundwater. For many years scientists thought that most pesticides were primarily adsorbed by soil organic matter and that soil minerals contributed little to sorption. In recent years, research has shown that certain soil clay minerals can adsorb large amounts of certain pesticides. The research, however, suggests that the process is very complicated. Some clays adsorb large amounts of pesticides while other clays do not, and some pesticides are strongly adsorbed on clays while others are not. This study is part of a concerted effort to figure out the mechanisms controlling adsorption of pesticides by soil clays. We discovered that the type of exchangeable cations (calcium versus potassium) has a big influence on sorption of weakly polar pesticides but has little effect on sorption of non-polar pesticides (a polar molecule will dissolve in water while a non-polar molecule is "oil-like" and will not mix with water). Also our results demonstrate that the concentration of potassium in the internal galleries of clays greatly increases the amount of weakly polar pesticides that the clay will adsorb. This research will help scientist to better understand how organic chemicals interact with soil clays. The research may also help industry design better formulations for pesticides and environmental engineers design containment systems, which trap organic contaminants in soils and aquifers.

Technical Abstract: Pesticide adsorption by soil clays is influenced by the exchangeable cations. Potassium-saturated clays frequently demonstrate the strongest affinity for pesticides. We hypothesize that the magnitude of pesticide sorption to soil minerals is proportional to the fraction of clay interlayers saturated with potassium (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/CaC12 aqueous solutions. The presence of different amounts of KCl and CaCl2 resulted in varying populations of K and Ca 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), 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 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.