Influence of humic acid on adsorption and desorption of atrazine, hydroxyatrazine, deethylatrazine, and deisopropylatrazine onto a clay-rich soil sample

Adsorption and desorption properties of atrazine and some of its metabolites, hydroxyatrazine (ATOH), deethylatrazine (DEA), and deisopropylatrazine (DIA), were studied with a clay-rich soil sample (clay content of 53%). A part of this soil was treated with humic acid (Soil-HA) to assess the influence of this important component of natural organic matter on adsorption and desorption processes. This study was performed using the batch approach with 1.0 g of soil, or Soil-HA, in 5.0 mL of 0.010 mol L-1 CaCl2 solution containing the herbicide and the metabolites in a concentration range between 0.010 and 5.0 mg L-1. After 24 h of contact time, the suspensions were centrifuged and the four compounds were quantified in the supernatant phases by high-performance liquid chromatography. The adsorption and desorption data of both Soil and Soil-HA were properly fitted by the linearized Freundlich equation. For the untreated soil, the adsorption affinity order evaluated as a function of the K-f values was AT-OH > AT > DIA > DEA, while desorption followed the order DEA > DIA similar to AT > AT-OH. The presence of humic acid increased significantly the adsorption of all compounds, following the same affinity order observed for the untreated soil. Increase in adsorption was especially high for AT-OH and AT. On the other hand, the dealkylated metabolites, DEA and DIA, were more easily desorbed from the Soil-HA sample, suggesting that natural organic matter facilitates the leaching of these compounds. Desorption order in the presence of humic acid was DEA > DIA > AT > ATOH.