Characterization of the atrazine's bound (nonextractable) residues using fractionation techniques for soil organic matter

Atrazine's bound residues (BR), in the range of 10-40% of the applied atrazine, were obtained by laboratory incubation (56 d) of four soils having different capacities to degrade atrazine in relation to the presence or absence of a microflora able to mineralize the triazinic ring, Soil size fractionation followed by alkaline extraction, before and after HF treatment, and then acid hydrolysis with 2 M HCl in reflux conditions was applied to the soils containing BR. Most of the BR were in the finest fraction (<20 mum) that contained the humified organic matter (from 61 to 77% of the total BR), and between 78 and 89% was made soluble during the different steps of the chemical fractionation procedure. From 20 to 50% of the BR of the fraction <20 mum was identified as the intact atrazine and its main derivatives, indicating that this proportion of the BR was probably formed by entrapment in voids of the soil organic matter. Between 13 and 30% of the BR was associated to humic acids (HA); they were not dialyzable and were released by acid hydrolysis with HCl, indicating that these BR were chemically bound to HA by an heteroatomic bond after the substitution of the chlorine atom of atrazine. Comparison of the results obtained for the four soils indicated that (i) an important activity of microorganisms able to mineralize the triazinic ring favors the formation of highly degraded products that can form BR; (ii) a soil pH <6 favors the formation and stabilization of hydroxylated derivatives of atrazine, and (iii) a high content of humic acids favors the formation of chemically bound residues.