Onn Rabinovitz

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 K=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.

Chapter 5.2 - Clays, Clay Minerals, and Pesticides

Abstract Design and test of clay-based formulations of pesticides for solving environmental and economical problems are described. Organoclays were mainly designed to promote the adsorption of neutral and hydrophobic pesticides and slow their release. Adsorption of organic cations modifies the nature of the clay mineral surface, transforming it from hydrophilic to hydrophobic. The modified clay mineral surface can have enhanced affinity for adsorbing neutral organic molecules of hydrophobic characteristics. The adsorption of the hydrophobic herbicides alachlor, metolachlor norflurazon, and acetochlor, which include a phenyl ring, was maximal for montmorillonite preadsorbed by a small cation, for example, phenyl trimethylammonium at a loading corresponding to 5/8 of the cation-exchange capacity (CEC). Loading of the organic cations above the CEC of the clay can promote the adsorption of certain anionic herbicides, such as imazaquin. Reduction of volatilization and photodegradation of herbicides was also achieved by certain organoclay formulations. In certain cases, an organic cation adsorbed on the clay mineral can act as an energy acceptor of the photoexcited molecule of the pesticide, which returns to its ground state before its photodecomposition occurs, thus becoming photostabilized. Clay mineral–micelle and –liposome formulations were introduced for obtaining slow release formulations of certain anionic herbicides, which could not be achieved by organoclay ones. The procedure involves incubation of the clay mineral with organic cations, which are mostly in micelles or liposomes. The complex formed between ODTMA (octadecyl trimethylammonium) and montmorillonite in the presence of excess of micelles is very different from the complex formed in the exclusive presence of ODTMA monomers, as shown by electron microscopy, XRD, and adsorption measurements. Unlike the monomer–clay mineral complex, which was not efficient for the adsorption of anionic organic molecules, such as sulfometuron, the micelle–clay mineral complex was highly efficient. Liposome–clay mineral formulations were prepared by employing the positively charged didodecylammonium and the neutral and EPA approved phosphatidylcholine. Efforts to develop slow release formulations also focused on encapsulation of herbicides in clay mineral polymer nanocomposites. The efficacy of the bypiridil herbicides paraquat (PQ) and diquat (DQ), which are divalent organic cations, used for post-emergence weed control was enhanced by addition to the herbicide formulation of monovalent organic cations which could compete for adsorption to the dust with DQ and PQ, thus making them more available for herbicidal activity.