Sorption and desorption kinetics of diuron, fluometuron, prometryn and pyrithiobac sodium in soils

The sorption and desorption characteristics of four herbicides (diuron, flu ometuron, prometryn and pyrithiobac-sodium) in three different cotton growi ng soils of Australia was investigated. Kinetics and equilibrium sorption a nd desorption isotherms were determined using the batch equilibrium techniq ue. Sorption was rapid (> 80% in 2 h) and sorption equilibrium was achieved within a short period of time (ca 4 h) for all herbicides. Sorption isothe rms of the four herbicides were described by Freundlich equation with an r( 2) value > 0.98. The herbicide sorption as measured by the distribution coe fficient (K-d) values ranged from 3.24 to 5.71 L/kg for diuron, 0.44 to 1.1 3 L/kg for fluometuron, 1.78 to 6.04 L/kg for prometryn and 0.22 to 0.59 L/ kg for pyrithiobac-sodium. Sorption of herbicides was higher in the Mores s oil than in Narrabri and Wee Waa soils. When the K-d values were normalised to organic carbon content of the soils (K-oc), it suggested that the affin ity of the herbicides to the organic carbon increased in the order: pyrithi obac-sodium < fluometuron < prometryn < diuron. The desorption isotherms we re also adequately described by the Freundlich equation. For desorption, al l herbicides exhibited hysteresis and the hysteresis was stronger for highl y sorbed herbicides (diuron and prometryn) than the weakly sorbed herbicide s (fluometuron and pyrithiobac-sodium). Hysteresis was also quantified as t he percentage of sorbed herbicides which is not released during the desorpt ion step (omega = [n(ad)/n(de)-1] x 100). Soil type and initial concentrati on had significant effect on omega. The effect of sorption and desorption p roperties of these four herbicides on the off-site transport to contaminate surface and groundwater are also discussed in this paper.