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 environments. Atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) adsorption-desorption was determined on 11 reference smectites (<2-mu m size fraction), and a soil smectite (<0.02-mu m size fraction from 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 desorption 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 positive hysteresis was observed with some clays, and the magnitude of the hysteresis, evaluated using the ratio of the Freundlich isotherm coefficients for adsorption and desorption, increased with SCD. This suggests that atrazine is retained by stronger binding mechanisms on smectites with high SCD, in spite of its lower adsorption capacity. On clays with high atrazine adsorption coefficients, the amount of atrazine desorbed was larger than would be predicted from the adsorption isotherms, resulting in a ''negative hysteresis'' whereby the desorption isotherm slope was greater than the adsorption isotherm slope. ''Negative hysteresis'' can be explained if atrazine is assumed to be excluded from interlayer or intraquasi-crystal water, where only external or interquasi-crystal water would be available during the desorption process.