Pesticides Sorption Kinetics, Equilibria, and Column Transport Using Fertilizer Mixtures Applied to Soils from Florida and Nigeria.

Sorption kinetics (SK) and sorption equilibria (SE) of organic pesticides in soils are normally conducted in solutions containing one pesticide and supporting electrolytes such as CaCl₂ and KCl. Since soils normally have complex mixtures in the soil solution, our objective was to compare SK-SE data obtained from single-pesticide mixtures (the standard protocol) and complex mixtures of pesticides and nutrients. For this purpose, Atrazine, Imidacloprid, Imidacloprid urea, and Pentafluorobenzoic acid (non-sorbed tracer) were dissolved in a fertilizer mixture containing 8 mM NH₄NO₃, KH₂PO₄, and KCl (Mix#1). Also, single-pesticide mixtures were prepared in 8 mM KCl (Mix#2). The SK-SE experiments were conducted in three surface soils, two from Florida (Candler sand and Immokalee fine sand) and one from northern semi-arid Nigeria (Tulluwa upland). The pesticides' SK and SE data in Mix#1 and Mix#2 were similar across soils, showing kinetic partitioning and reaching equilibrium before 24 hours. The SK data for the organics were mostly described by the one-site nonequilibrium model (OSNE) or by the two-site nonequilibrium (TSNE) model. The SE data mostly followed the Freundlich isotherm model, where parameters N and K_f were essentially the same between Mix#1 and Mix#2. The results suggest that these pesticides did not interact in solution, or they did not compete for the same sorption sites. Pesticide breakthrough curves (BTCs) were also analyzed in soil columns packed with the mentioned soils at steady-saturated water flow. Pulse-inputs using Mix#1 showed that the PFBA BTCs were described by the convective-dispersive model. The pesticides’ BTCs showed nonequilibrium transport described by either OSNE or TSNE transport models. In general, Tulluwa upland soils from Nigeria showed the lowest sorption capacity, followed by Candler and Immokalee, a trend that was mostly explained by the soil organic matter content. Imidacloprid-urea was less sorbed than Imidacloprid across soils, and Atrazine showed the highest sorption overall. In essence, the data obtained from Mix#1 and Mix#2 were the same, implying that SK and SE parameters obtained from either mixture could be used for modeling sorption and transport processes in soil when these pesticides and nutrients exist in solution simultaneously.