INFLUENCE OF SMECTITE HYDRATION AND SWELLING ON ATRAZINE SOPTION BEHAVIOR

Authors: CHAPPELL, MARK - IOWA STATE UNIVERSITY; Laird, David; THOMPSON, MICHAEL - IOWA STATE UNIVERSITY; LI, HUI - MICHIGAN STATE UNIVERSITY; TEPPEN, BRIAN - MICHIGAN STATE UNIVERSITY; JOHNSTON, CLIFF - PURDUE UNIVERSITY; BOYD, STEPHEN - MICHIGAN STATE UNIVERSITY

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2005
Publication Date: 5/1/2005
Citation: Chappell, M.A., Laird, D.A., Thompson, M.L., Li, H., Teppen, B.J., Johnston, C.T., Boyd, S.A. 2005. Influence of smectite hydration and swelling on atrazine soption behavior. Environmental Science and Technology. 39(9) 3150-3156.

Interpretive Summary: The fate of pesticides and chemical contaminants in soils is strongly influenced by the way the chemicals interact with constituents of the soil. For many years it was thought that the pesticides were primarily adsorbed by soil organic matter, however, over the last decade there has been growing evidence that soil clays contribute substantially and sometimes dominate pesticide adsorption. The mechanisms controlling sorption of pesticides by soil clays is not clearly understood. We discovered that the extent of clay swelling has a big impact on the adsorption of atrazine by clay. As clay swells the internal surfaces hold large amounts of water and we determined that this internal water inhibits the adsorption of atrazine. Conversely, as clay dries and looses its internal water, the clay was shown to have a much larger ability to adsorb atrazine. We also found that air-drying a soil sample can substantially influence the ability of the soil to adsorb atrazine, however, the effect was variable. Scientists will use this new information to better understand the interactions of pesticides and other contaminants with soils. The results also demonstrate a fundamental weakness in existing environmental fate models used by scientists, the EPA, and other action and regulatory agencies.

Technical Abstract: Sorption studies are important both for investigations of interaction mechanisms between organic molecules and soil surfaces and to parameterize models that predict the fate of organic contaminants in soils. Experiments showed that air-drying caused an irreversible decrease in the basal spacing of potassium (K)-saturated Panther Creek smectite (PC), while basal spacings of never-dried and air-dried-rehydrated calcium (Ca)-PC were similar. The affinity of atrazine sorption was twice as high for the never-dried K-PC as the never-dried Ca-PC. Air-drying K-PC increased its sorption affinity by one order of magnitude, while no significant effect of the air-drying treatment was observed for Ca-PC. Air-drying resulted in no significant difference in the affinity of Ca-humic acid (Aldrich humic acid) samples for atrazine. Choice of background solution (20 mM KCl versus 10 mM CaCl2) and air-drying treatments significantly affected atrazine sorption affinities for the three-smectitic soils, however the trends were not consistent with those observed for the reference smectite. Further, extending the initial rehydration time from 24 to 240 h (prior to adding atrazine) significantly decreased the soil's sorption affinity for atrazine. We conclude that the hydration status of smectites has a large influence on the affinity of atrazine sorption, and air-drying treatments have the potential to modify the sorption affinity of smectitic soils for organic molecules such as atrazine.