DESORPTION OF DIURON AND ISOPROTURON FROM UNDISPERSED CLAY LOAM SOIL

Studies were conducted to investigate the desorption of diuron and iso proturon adsorbed on undispersed clay loam soil, and the influence of residence time in soil on desorption. The soil was treated at 0.6 or 3 mg kg(-1), at 70% moisture content and in the presence of sodium azid e to prevent degradation. Measurement of herbicide concentrations in s oil solution sampled by means of glass microfibre filters showed that adsorption mainly occurred for one day but long-term sorption proceede d for >two weeks. After a one-day or three-week residence time, soil s olution was partly replaced (28%). Measurement of concentrations in so lution showed rapid desorption, with equilibria being achieved within 1 h (diuron) or a few hours (isoproturon). After 16 successive desorpt ions done at 30-min or 12-h intervals, equilibration times tended to b e longer. For the short residence time, desorption and long-term sorpt ion could occur simultaneously and equilibration might be faster. Resi dence time had no significant effect on desorption kinetics nor on the small hysteresis observed for diuron. The aging effect, involving lon g-term sorption only, decreased the proportion of diuron removed from the soil by successive desorptions but, for isoproturon, desorption fr equency and desorption kinetics were more important.