Adsorption of Atrazine on Smectites

Smectites may strongly influence the fate of pesticides in soils due to their large surface area and abundance in agricultural soils. This research was undertaken to determine the effects of smectite properties on the affinity of smectitic clays for atrazine (2-chloro-4-ethylamino-6-isopropylamino-1, 3, 5 triazine). Samples of the <2-µm fraction were separated by sedimentation from 13 reference smectites and treated with H₂O₂ for removal of organic matter. A soil smectite sample was separated (<0.02-µm fraction) from a H₂O₂ and dithionite-citrate-bicarbonate-treated Webster (fine-loamy, mixed, mesic Typic Haplaquoll) soil by a dispersion-centrifugation-decantation technique. All 14 samples were Ca saturated, dialyzed free of excess electrolyte, and freeze dried. The mineralogy of each sample was evaluated by a combination of x-ray diffraction and chemical analysis. Values for cation-exchange capacity (CEC), surface charge density (SCD), surface area (SA), and percent tetrahedral charge (TC) were based on structural interpretations of the chemical analyses. Freundlich adsorption isotherms for atrazine adsorption on each sample were determined using the batch-equilibration method. Smectite was the dominant mineral in 13 of the clay samples; the 14th sample was a mixture of interstratified smectite-illite, quartz, and kaolinite. For the smectitic clays, atrazine adsorption ranged from 0 to 100%, and the logarithm of the Freundlich adsorption constant [log(Kf)] decreased linearly with the CEC (r² = 0.82) of the clays. Stepwise multiple-regression analysis indicated that log (Kf) values were correlated with SCD and SA values (r² = 0.83). Inclusion of TC and suspension pH as independent variables in the regression analysis did not significantly improve the correlations. The results indicate that the affinity of smectites for atrazine decreases with increasing SCD, which suggests that atrazine is primarily adsorbed by smectites as a neutral species.