Surface exchange phase composition and nonionic surfactant effects on the nonequilibrium transport of atrazine

Numerous studies have been reported regarding atrazine (2-chloro-4-isopropyl-6 ethyl-diamino-triazine) sorption/desorption and mobility in soil using the pure atrazine compound. Relatively little research has been carried out on atrazine behavior in soil minerals in the presence of surfactants. We hypothesized that clay mineralogy and surfactants may influence sorption/mobility of atrazine. In this study, we conducted miscible displacement experiments to elucidate the influence of the surfactant Brij 35 (C12H25[C2H4O](23)OH), clay mineral type, and exchange phase composition on atrazine mobility in mixtures of specially prepared 2:1 clays. Chromatography columns were packed in duplicate with sand and Na- or Ca-saturated illite or mixtures of Na- or Ca-bentonite (0:1, 1:1, 9:1). The columns were presaturated with defined CaCl2 (154 mmol(C) L-1) or CaCl2 plus NaCl (SAR = 75 (mmol L-1)(-1/2)) solutions and adjusted to pH 6.3. Columns were later eluted with a pulse of a 6 mg L-1 atrazine solution with or without 0.1% Brij 35, incubated for 48 h after atrazine breakthrough, and then washed with d-H2O. Results showed that atrazine breakthrough was incomplete (max C/Co < 1.0) in all systems. In the bentonite systems, the atrazine breakthrough curve (BTC) in the presence of Brij 35 was shifted to the left of the atrazine BTC in the absence of Brij 35 (at one pore volume) when the Na- to Ca-bentonite ratio = 9:1, suggesting that the surfactant enhanced atrazine mobility. In contrast, surfactant was shown to reduce atrazine mobility with a diminishing Na- to Ca-bentonite ratio. Similar results were found in the illite systems, but in reverse order with respect to Na- and Ca-clay exchange phase composition. Based on our work, we concluded that the exchange phase composition impacted the surfactant's ability to mobilize atrazine in soils.