Application of sequential injection-square wave voltammetry (SI-SWV) to study the adsorption of atrazine onto a tropical soil sample

Square wave voltammetry automated by sequential injection analysis was applied to determine the Freundlich adsorption coefficients for the adsorption of atrazine onto a clay rich soil. The detection limit in soil extracts was between 0.18 and 0.48 μmol L−1, depending on the medium used to prepare the extracts (0.010 mol L−1 KCl, CaCl2 or HNO3 and 0.0050 mol L−1 H2SO4), all of them conditioned in 40 mmol L−1 Britton-Robinson buffer at pH 2.0 in presence of 0.25 mol L−1 NaNO3. Also in soil extracts the linear dynamic range was between 1.16 and 18.5 μmol L−1 (0.25-4.0 μg mL−1), with a sampling frequency of 190 h−1. The Kf Freundlich adsorption coefficient was 3.8 ± 0.2 μmol1−1/n Ln kg−1 in medium of 0.010 mol L−1 KCl or CaCl2, but increased to 7.7 ± 0.1 and 9.0 ± 0.3 μmol1−1/n Ln kg−1 in 0.010 mol L−1 HNO3 and 0.0050 mol L−1 H2SO4, respectively. The increase of Kf was related to the decrease of pH from 6.4-6.7 in KCl and CaCl2 to 3.7-4.0 in presence of HNO3 or H2SO4, which favors protonation of atrazine, facilitating electrostatic attractions with negative charges of the clay components of the soil. The 1/n parameters were between 0.76 and 0.86, indicating that the isotherms are not linear, suggesting the occurrence of chemisorption at specific adsorption sites. No statistically significant differences were observed in comparison to the adsorption coefficients obtained by HPLC. The advantage of the proposed SI-SWV method is the great saving of reagent because it does not use organic solvent as in the case of HPLC (50% (v/v) acetonitrile in the mobile phase). Additionally the start up of SI-SWV is immediate (no column conditioning necessary) and the analysis time is only 19 s.