Binding interactions of herbicide metabolites with clay minerals and humic acids in an aqueous system

Interactions with dissolved organic matter or suspended clay minerals can both affect the transport of organic xenobiotics in soil. Therefore, dialysis experiments were carried out to investigate such binding interactions of two triazine herbicide metabolites (desethyl terbuthylazine, hydroxydesethyl terbuthylazine) with dissolved humic acids and montmorillonite. Using 14C-labeled compounds in a small-scale dialysis device allowed sensitive determination of both free and bound analytes. Adsorption of desethyl terbuthylazine to montmorillonite steadily increased with falling pH from 10 to 2, whereas adsorption of hydroxydesethyl terbuthylazine showed a maximum at pH 4-4.5. This was ascribed to different protonation of the compounds, owing to pKA values of approximately 2 and 5 for desethyl terbuthylazine and hydroxydesethyl terbuthylazine, respectively. Addition of dissolved humic acids to the montmorillonite suspension did not change the pH-dependency of binding for both compounds. Distribution of herbicide metabolites between humic acids and montmorillonite was investigated by dialysis experiments in a system of humic acid solution versus montmorillonite suspension. Dissolved humic acids did not diminish the adsorption of either compound to montmorillonite.