Effects of Increasing Potassium Chloride and Calcium Chloride Ionic Strength on Pesticide Sorption by Potassium- and Calcium-smectite

Authors: LI, HUI - MICHIGAN STATE UNIVERSITY; TEPPEN, BRIAN - MICHIGAN STATE UNIVERSITY; Laird, David; JOHNSTON, CLIFF - PURDUE UNIVERSITY; BOYD, STEPHEN - MICHIGAN STATE UNIVERSITY

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2006
Publication Date: 9/20/2006
Citation: Li, H., Teppen, B., Laird, D.A., Johnston, C., Boyd, S. 2006. Effects of Increasing Potassium Chloride and Calcium Chloride Ionic Strength on Pesticide Sorption by Potassium- and Calcium-smectite. Soil Science Society of America Journal. 70(6):1889-1895.

Interpretive Summary: The fate of organic chemicals in soils depends on many factors. One of the most important is the ability of an organic chemical to adsorb to soil particles. For many years, organic chemicals were believed to primarily adsorb to soil organic matter, but recent research has demonstrated that soil clay minerals are also very important for adsorption of some classes of organic chemicals. But the amount of an organic chemical adsorbed by soil clays ranges from 0 to 100% depending on the properties of the organic chemical, properties of the clay, and the chemistry of the soil solution. We discovered that sorption of four herbicides on clays increases substantially with the concentration of potassium chloride in the soil solution, but the concentration of calcium chloride in the soil solution has little effect on sorption of the pesticides. We believe that swelling of clays decreases with increasing concentration of potassium chloride and this makes the clays better able to adsorb the pesticides, whereas clay swelling in not effected by calcium chloride concentration. This research will help scientists better understand how organic molecules interact with soil clays and hence better predict the fate of pesticides and other organic chemical contaminants in soil environments. The research may also interest the chemical industry as they seek to produce better pesticide formulations.

Technical Abstract: Smectite clays have demonstrated high affinity for many pesticides in aqueous solution, which is influenced by the intrinsic natures of smectite clays (e.g., surface charge density and location), exchangeable cations, and clay interlayer hydration status. The amount and the type of salts present in the aqueous phase determine clay quasicrystal structures, interlayer swelling property and hydration status, which we hypothesize as a major factor influencing pesticide sorption. To test this hypothesis, we measured sorption isotherms of alachlor, atrazine, dichlobenil and diuron by K-smectite (SWy-2) in KCl solution and Ca-SWy-2 in CaCl2 solution at several ionic strengths. The results indicated that pesticide sorption by K-SWy-2 significantly increased with increasing aqueous KCl concentration. In contrast, sorption by Ca-SWy-2 at different ionic strengths of CaCl2 remained nearly constant. The salting-out effect on the aqueous dissolution of organic compounds fails to account for the significantly increased sorption by K-SWy-2 in aqueous solutions containing relatively high KCl concentrations. We conclude that formation of more ordered clay quasicrystals and/or shrinkage of K-smectite interlayer distance owing to increased KCl ionic strength facilitates the intercalation of pesticides leading to greater sorption by the clay, while the salting-out effect plays a minor role (if any) in this enhanced sorption.