SORPTION-DESORPTION OF HERBICIDES IN SOIL AS INFLUENCED BY ELECTROLYTE CATIONS AND IONIC-STRENGTH

Accurate prediction of pesticide leaching potential and contamination of groundwater required improved understanding of the influence of various soil factors on pesticide sorption/desorption. Effects of varying ionic strength and cation composition of electrolyte on sorption/desorption of bromacil [5-bromo-6-methyl-3-(1-methylpropyl)-2,4 (1H, 3H) pyrimidinedione], simazine [6-chloro-N-N'-diethyl-1,3,5-triazine-2,4-diamine], norflurazon [4-chloro-5-(methyl-amino)-2-(3-(trifluoromethyl) phenyl)-3(2H)-pyridazinone], and diuron [N'-(3,4-dichlorophenyl)-N,N-dimethylurea] by a Candler fine sand (0-15 cm; hyperthermic, uncoated Typic Quartsipsamment) were investigated. Among the herbicides used, sorption decreased in the order: diuron > norflurazon > simazine > bromacil. Sorption of all four herbicides increased with increasing ionic strength of the electrolyte. Using 1.0 M CaCl2 as an electrolyte for equilibration of soil with herbicides increased the sorption of bromacil, simazine, norflurazon, and diuron by 33, 32, 14, and 17%, respectively, as compared to the sorption using 0.01 M CaCl2. Sorption of all four herbicides showed very little difference when using either CaCl2 or KCl as an electrolyte for equilibration at 1.0 M concentration. Sorption of bromacil, simazine, norflurazon and diuron were increased by 25, 27, 21, and 20%, respectively when electrolyte KCl concentration was increased from 1.0 to 2.0 M. Desorption of all four herbicides in 1.0 M NH4OAc increased significantly as the electrolyte concentration during sorption was increased. The effects of electrolyte cation species on the sorption of herbicides ranked in the following order: (1) Fe >> Al = Cu > K = Ca = Mg = Na for bromacil, simazine, and norflurazon and (2) Fe >> Al = Cu > K > Ca = Mg = Na for diuron. Desorption of all four herbicides in 1.0 M NH4OAc was greater when either Fe, Al, or Cu electrolyte was used during sorption as compared to using basic cations including K, Ca, Mg, or Na electrolytes.