Miscible Displacement, Sorption and Desorption of Atrazine in a Brazilian Oxisol

Abstract

We investigated atrazine [2-chloro-4-(ethylamino)-6-(isopropyl-amino)- s -triazine] leaching, sorption, and desorption in a Brazilian Oxisol under no-till (NT) and conventional (CON) agricultural management. The specific objectives were (i) to infer characteristics of the sorption process (equilibrium vs. nonequilibrium sorption, reversible vs. nonreversible sorption) from breakthrough experiments, (ii) to compare sorption parameters derived from breakthrough and batch experiments, (iii) to predict the distribution of atrazine on reversible and irreversible sorption sites within the soil columns using transport parameters derived from breakthrough data, and (iv) to evaluate the atrazine metabolites during breakthrough experiments. Breakthrough curves of radiolabeled atrazine and of a nonreactive tracer, Br − , were measured during miscible displacement experiments in unsaturated soil columns. The concentration of metabolites in the leachate was determined at three different times. At the end of the displacement experiment, which lasted 494 h, the distributions of atrazine and its metabolites within the soil columns and their partitioning in desorbable, extractable and nonextractable fractions were determined. Atrazine breakthrough curves were fitted with a three-site chemical nonequilibrium convective dispersive transport model considering irreversible sorption. The three-site chemical nonequilibrium model predicted that around 40% of the applied atrazine was irreversibly sorbed at the end of the leaching experiment. This corresponded with sum of the measured extractable and nonextractable fractions. The K d values for reversible sorption sites derived from the BTCs were similar to those derived from 24-h batch adsorption experiments. However, irreversible sorption was not observed in a four-step consecutive batch desorption experiment that lasted 120 h in total. Differences in desorption between the column and batch experiments were probably due to differences in time-scale and sorption-desorption conditions. More than 90% of the radioactivity in the leachate and 80% remaining in the soil column, respectively, were characterized as atrazine, indicating a slow decay. Hydroxyatrazine was the most important metabolite of atrazine and showed higher retention in the column than the other metabolites.