SLOW-RELEASE OF S-ETHYL DIPROPYLCARBAMOTHIOATE FROM CLAY SURFACES

The adsorption/desorption behavior of the volatile herbicide S-ethyl d ipropylcarbamothioate (EPTC) to/from montmorillonite and sepiolite was studied. The clays were used as such or with their surface properties modified by adsorption of the cationic dye methyl green (MG). At 30 d egrees C, the half-life time (T-1/2) of the herbicide in its free form was 10 h, whereas when adsorbed to montmorillonite the T-1/2 was more than 5 days. When EPTC was incorporated into the soil, the T-1/2 valu es were 4 and 9 days for the free and adsorbed forms, respectively. Th e interactions between EPTC and clay surface were studied by Fourier-t ransform infrared spectroscopy. Vibrational spectra revealed two diffe rent populations of EPTC in the clay-organic complexes: one in which t he molecules are adsorbed, interacting with the clay surface, and anot her in which they do not interact with the clay, their spectrum being almost identical to that of free EPTC. Coadsorbed MG impairs the EPTC- clay interactions, causing an increase in free EPTC, indicating that f ree EPTC is released from the organoclay at a rate higher than that of adsorbed EPTC. Oat bioassays showed that the herbicidal activity of c lay-complexed EPTC was extended by more than 1 week. These data indica te that clay-EPTC interactions can be employed to improve herbicidal p erformance of EPTC and to control its release to the environment.