Optimization of adsorption of hydrophobic herbicides on montmorillonite preadsorbed by monovalent organic cations: Interaction between phenyl rings

This study aimed to optimize organo-clay formulations for reduction of leac hing of the herbicides alachlor, metolachlor, and norflurazon, which includ e a phenyl ring in the structure. The adsorbed amounts of herbicides increa sed severalfold when montmorillonite was preadsorbed by an organic cation; benzyltrimethylammonium (BTMA) was more effective than benzyltriethylammoni um (BTEA). Fourier transform infrared studies indicated interactions betwee n alachlor molecules and adsorbed BTMA. The adsorption affinity of the herb icides increased with BTEA loading up to the cation exchange capacity (CEC) of montmorillonite but reached a maximum at a BTMA loading of 5/8 of the C EC. The enhanced adsorbed amounts of herbicides are mainly due to interacti ons between the phenyl rings of herbicide molecules and organic cations, wh ich are favored with the smaller cation, BTMA. BTMA preadsorbed on the clay up to the CEC forms a fraction (14-18%) of charged dimers so that less phe nyl rings are available for interacting with herbicide molecules. This effe ct is small for preloading by BTEA, so that the amounts adsorbed increase w ith the degree of preloading. Thus, optimization of clay-based herbicide fo rmulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage.