ATRAZINE DESORPTION FROM SMECTITES

In soils low in organic matter, pesticide adsorption and desorption by clay minerals may strongly influence the fate of pesticides in soil e nvironments. Atrazine loro-4-ethylamino-6-isopropylamino-1,3,5-triazin e) adsorption-desorption was determined on 11 reference smectites (<2- mu m size fraction), and a soil smectite (<0.02-mu m size fraction fro m the Ap horizon of a fine-loamy, mixed, mesic Typic Haplaquoll). For each clay sample, adsorption and desorption isotherms were determined using batch equilibration. Atrazine adsorption on the clays decreased with increasing surface change density (SCD) of the smectites. The des orption isotherms indicated that adsorption was generally reversible. This suggests that atrazine is primarily adsorbed on smectite surfaces through relatively weak van der Waals or H bonds. However, a small po sitive hysteresis was observed with some clays, and the magnitude of t he hysteresis, evaluated using the ratio of the Freundlich isotherm co efficients for adsorption and desorption, increased with SCD. This sug gests that atrazine is retained by stronger binding mechanisms on smec tites with high SCD, in spite of its lower adsorption capacity. On cla ys with high atrazine adsorption coefficients, the amount of atrazine desorbed was larger than would be predicted from the adsorption isothe rms, resulting in a ''negative hysteresis'' whereby the desorption iso therm slope was greater than the adsorption isotherm slope. ''Negative hysteresis'' can be explained if atrazine is assumed to be excluded f rom interlayer or intraquasi-crystal water, where only external or int erquasi-crystal water would be available during the desorption process .