Mepiquat–acetochlor formulations: sorption and leaching

Our aim was to study the adsorption of the plant regulator mepiquat to montmorillonite and test the suitability of montmorilonite–mepiquat complexes in formulations to reduce leaching of the non-polar herbicide, acetochlor. Adsorption of the monovalent organic cation mepiquat to montmorillonite through a range of ionic strengths was determined and modeled. Mepiquat adsorption reached the cation exchange capacity (CEC) of the montmorillonite. The binding coefficients K=300 M−1 for neutral mepiquat complex formation and K̄=10 M−1 for positively charged complex formation were employed. Increased NaCl and CsCl electrolyte solution concentrations reduced mepiquat adsorption which was also affected by the inorganic cation binding coefficients. An adsorption model, which includes the electrostatic Gouy–Chapman equations and specific binding in a closed system, effectively simulated of the adsorption isotherms and yielded quantitative predictions for the effect of electrolyte concentration on mepiquat adsorption. Acetochlor adsorption to mepiquat–montmorillonite complexes was about 20% of the amount added, and increasing the mepiquat concentration from 0.5 to 0.8 mmol mepiquat g−1 clay made no difference. Acetochlor desorption from formulations was rather rapid with 38–67% released during 1 day. The rate of acetochlor release was reduced to 8.5% during 1 day after a brief pre-washing of the formulations. Despite the rate reduction in acetochlor release, current formulations are still not satisfactory for reduced acetochlor leaching.