Cosorption of Atrazine and a Lauryl Polyoxyethylene Oxide Nonionic Surfactant on Smectite

Commercial atrazine formulations commonly contain nonionic surfactants that serve as solubilizing and wetting agents for enhancing the stability and efficacy of the herbicide. The fate of atrazine in soils has been extensively investigated; yet, few studies have considered the effects of formulation components on the fate of atrazine in soils. In this study, we investigated the influence of the nonionic surfactant, Brij 35 (Brij), on the sorption of atrazine on Ca- and K-saturated samples of a reference smectite, Panther Creek (PC). In general, Brij concentrations of 50 and 200 mg L-1 had little effect on atrazine sorption, but sorption was substantially inhibited by Brij concentrations of 2100 mg L-1. For Brij concentrations of 6300 mg L-1, atrazine sorption was intermediate between that observed for the 200 and 2100 mg L-1 Brij systems. Brij molecules themselves were very strongly sorbed by PC, with sorption maxima exceeding 200 g kg-1. X-ray diffraction analysis of Brij-treated PC indicated that the sorbed Brij was intercalated into interlayers of the smectite. At Brij concentrations of 2100 mg L-1, Brij competed with atrazine for interlayer sorption sites. In contrast, at the initial Brij concentration of 6300 mg L-1, the clay interlayers were largely filled with Brij, and excess Brij probably accumulated on external surfaces of the clay as surface micelles. We hypothesize that atrazine partitioning into surfactant micelles on external surfaces of the clay led to enhanced retention by the solid phase. Keywords: Pesticide formulations; atrazine; PEO surfactants; Brij 35; smectite; crystalline swelling; sorption