Kinetics and adsorption of metolachlor and atrazine and the conversion products (deethylatrazine, deisopropylatrazine, hydroxyatrazine) in the soil profile of a river basin

The adsorption kinetics and adsorption parameters of metolachlor, atrazine, deethylatrazine (DEA), deisopropylatrazine (DIA) and hydroxyatrazine (HA) were investigated in a soil profile in a maize field formed from recent alluvial deposits in a river basin in Greece. We used the batch equilibrium method modified to simulate field conditions as closely as possible for the use and practices related to soil applied pre-emergence herbicides. Pseudo-equilibrium times, determined by kinetic studies, were achieved after 16, 16, 24, 24 and 48 hours for metolachlor, DIA, DEA, HA and atrazine, respectively. At pseudo-equilibrium the percentage of the adsorbed amount increased in the order of DEA (10%) < DIA (14%) < atrazine (27%) < metolachlor (43%) < HA (94%) which indicates that more than 57% of all compounds except for HA are in solution and available for transport to deeper soil layers when conditions similar to those simulated in the laboratory exist in the field. Adsorption isotherms of all compounds and in most of the cases correlated well with the Freundlich model and adsorption coefficients (K-f) decreased with increased soil depth. Principal component and multiple regression analyses confirmed the importance of the soil organic carbon content on the adsorption capacity of soils for all compounds except HA in the plough layers (0-40 cm). In the subsurface soils (40-110 cm) variables such as clay content and pH were more important. For HA, the K-f values determined for the plough and subsurface soil layers were better correlated with clay content and pH. Also in the subsurface soils, the variation in organic carbon content was not correlated with the variation of K-f values. Thus calculated Koc-f-values misrepresent the adsorptive capacity of these soils towards the compounds studied.