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Sorption and mobility of 14C-labeled imazaquin and metolachlor in four soils as influenced by soil properties

Weber, J.B., McKinnon, E.J., Swain, L.R.
Journal of agricultural and food chemistry 2003 v.51 no.19 pp. 5752-5759
metolachlor, imazaquin, herbicide residues, isotope labeling, carbon, radionuclides, sorption, desorption, clay loam soils, sandy soils, sandy loam soils, soil pH, A horizons, leaching, bulk density, cation exchange capacity, soil pore system, clay fraction, soil organic matter, humic substances, soil pollution, sandy clay loam soils, loamy sand soils
Aqueous batch-type sorption-desorption studies and soil column leaching studies were conducted to determine the influence of soil properties, soil and suspension pH, and ionic concentration on the retention, release, and mobility of [14C]imazaquin in Cape Fear sandy clay loam, Norfolk loamy sand, Rion sandy loam, and Webster clay loam. Sorption of [14C]metolachlor was also included as a reference standard. L-type sorption isotherms, which were well described by the Freundlich equation, were observed for both compounds on all soils. Metolachlor was sorbed to soils in amounts 2-8 times that of imazaquin, and retention of both herbicides was related to soil organic matter (OM) and humic matter (HM) contents and to herbicide concentration. Metolachlor retention was also related to soil clay content. Imazaquin sorption to one soil (Cape Fear) increased as concentration increased and as suspension pH decreased, with maximum sorption occurring in the vicinity of pKa1 = (1.8). At pH levels below pKa1 imazaquin sorption decreased as hydronium ions (H3O+) increased and competed for sites. NaCl was more effective than water in desorption of imazaquin at pH levels near the pKa1. Mechanisms of bonding are postulated and discussed. The mobility of imazaquin through soil columns was in the order Rion Norfolk > or = to Cape Fear Webster, whereas for metolachlor it was Rion > or = to Norfolk >> Webster > or = to Cape Fear. Imazaquin was from 2 to 10 times as mobile as metolachlor.