Effect of soil and sediment composition on acetochlor sorption and desorption

Herbicide fate and its transport in soils and sediments greatly depend upon sorption-desorption processes. Quantitative determination of herbicide sorption-desorption is therefore essential for both the understanding of transport and the sorption equilibrium in the soil/sediment-water system; and it is also an important parameter for predicting herbicide fate using mathematical simulation models. The total soil/sediment organic carbon content and its qualitative characteristics are the most important factors affecting sorption-desorption of herbicides in soil or sediment. Since the acetochlor is one of the most frequently used herbicides in Slovakia to control annual grasses and certain annual broad-leaved weeds in maize and potatoes, and posses various negative health effects on human beings, our aim in this study was to investigate acetochlor sorption and desorption in various soil/sediment samples from Slovakia. The main soil/sediment characteristics governing acetochlor sorption-desorption were also identified.

The sorption-desorption of acetochlor, using the batch equilibration method, was studied on eight surface soils, one subsurface soil and five sediments collected from the Laborec River and three water reservoirs. Soils and sediments were characterized by commonly used methods for their total organic carbon content, distribution of humus components, pH, grain-size distribution, and smectite content, and for calcium carbonate content. The effect of soil/sediment characteristics on acetochlor sorption-desorption was examined by simple correlation analysis.

Sorption of acetochlor was expressed as the distribution coefficient (K (d)). K (d) values slightly decreased as the initial acetochlor concentration increased. These values indicated that acetochlor was moderately sorbed by soils and sediments. Highly significant correlations between the K (d) values and the organic carbon content were observed at both initial concentrations. However, sorption of acetochlor was most closely correlated to the humic acid carbon, and less to the fulvic acid carbon. The total organic carbon content was found to also significantly influence acetochlor desorption.

Since the strong linear relationship between the K (d) values of acetochlor and the organic carbon content was already released, the corresponding K (oc) values were calculated. Considerable variation in the K (oc) values suggested that other soil/sediment parameters besides the total soil organic carbon content could be involved in acetochlor sorption. This was revealed by a significant correlation between the K (oc) values and the ratio of humic acid carbon to fulvic acid carbon (C(HA)/C(FA)).

When comparing acetochlor sorption in a range of soils and sediments, different K (d) values which are strongly correlated to the total organic carbon content were found. Concerning the humus fractions, the humic acid carbon content was strongly correlated to the K (d) values, and it is therefore a better predictor of the acetochlor sorption than the total organic carbon content. Variation in the K (oc) values was attributed to the differences in distribution of humus components between soils and sediments. Desorption of acetochlor was significantly influenced by total organic carbon content, with a greater organic carbon content reducing desorption.

This study examined the sorption-desorption processes of acetochlor in soils and sediments. The obtained sorption data are important for qualitative assessment of acetochlor mobility in natural solids, but further studies must be carried out to understand its environmental fate and transport more thoroughly. Although, the total organic carbon content, the humus fractions of the organic matter and the C(HA)/C(FA) ratio were sufficient predictors of the acetochlor sorption-desorption. Further investigations of the structural and chemical characteristics of humic substances derived from different origins are necessary to more preciously explain differences in acetochlor sorption in the soils and sediments observed in this study.