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Soil Chemistry

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

^a Dep. of Crop and Soil Sciences, and Environmental Science and Policy Program, Michigan State Univ., East Lansing, MI 48824
^b USDA-ARS, National Soil Tilth Lab., Ames, IA 50011
^c Crop, Soil and Environmental Sciences, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907

^* Corresponding author (boyds{at}msu.edu)

In aqueous suspension, the affinity of many pesticides for smectites is influenced by the clay properties such as surface area, surface charge density and location, exchangeable cations, and hydration status of exchangeable cations in clay interlayers. The amount and the type of salts present in the aqueous phase influence clay quasicrystal structures and hydration status, which we hypothesize as major determinants of pesticide sorption. To test this hypothesis, we measured sorption isotherms of alachlor, atrazine, dichlobenil, and diuron by K⁺–saturated smectite (K-SWy-2) in KCl solution and Ca²⁺–saturated smectite (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 CaCl₂ ionic strengths remained nearly constant. The "salting-out" effect on the reduction of dissolution of pesticides failed to account for the significantly increased sorption by K-SWy-2 in aqueous KCl solutions. We conclude that formation of better-ordered clay quasicrystals and shrinkage of clay interlayer distance owing to increased KCl ionic strength facilitate the intercalation of pesticides leading to greater sorption by the clay, while the salting-out effect plays a minor role (if any) in the observed sorption enhancement.

Abbreviations: Ca-SWy-2, Ca²⁺–saturated Wyoming montmorillonite reference smectite • CEC, cation exchange capacity • HPLC, high-performance liquid chromatography • K-SWy-2, K⁺-saturated Wyoming montmorillonite reference smectite • SOM, soil organic matter • SWy-2, Wyoming montmorillonite reference smectite • XRD, X-ray diffraction