ORGANIC-CHEMICALS IN THE ENVIRONMENT - DISSOLVED ORGANIC-MATTER AND ADSORPTION-DESORPTION OF DIMEFURON, ATRAZINE, AND CARBETAMIDE BY SOILS

Dissolved organic matter (DOM) from several sources influenced the adsorption and the desorption by soil of three herbicides: dime-furon {4-[2-chloro-4-(3,3-dimethylureido) phenyl]-2-t-butyl-1,3,4-oxasiazolin-5-one}, atrazine (2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine), and carbetamide [D-N-ethyl-2-(phenyl-carbamoyloxy)propioamide)]. The DOM was endogenous (water soluble and humic compounds directly extracted from soil) or exogenous (water soluble organic matter from compounds applied to soils through agricultural practices: sewage sludge, fermented straw, and farmyard slurry; and two model compounds-tannic acid and an anionic surface active agent). Experiments were conducted to quantify the importance of herbicide sorption in three separate systems: DOM-soil, DOM-herbicide in solution, and DOM-adsorbed herbicide. Herbicide adsorption strongly increased in the presence of water soluble materials from fermented straw, which explained the lowest desorption obtained with this DOM. On the other hand, the water soluble extracts of liquid or solid sludge favored herbicide desorption. Generally, adsorption of the less soluble herbicides (dimefuron and atrazine) was favored when soil was pretreated with DOM, and their adsorption generally decreased when DOM was preincubated with herbicides before soil contact. Opposite effects were generally noted for the most soluble herbicide (carbetamide). A new empirical, two-compartment model provided a good description of desorption isotherms. The first compartment (a linear desorption) was similar for most DOM types. The differences in desorption isotherms were mainly related to the different behavior of the second compartment (an exponential desorption), in which the molecules were more strongly adsorbed.