Slow Release of S-Ethyl Dipropylcarbamothioate from Clay Surfaces

The adsorption/desorption behavior of the volatile herbicide S-ethyl dipropylcarbamothioate (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 °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 values were 4 and 9 days for the free and adsorbed forms, respectively. The interactions between EPTC and clay surface were studied by Fourier-transform infrared spectroscopy. Vibrational spectra revealed two different populations of EPTC in the clay-organic complexes: one in which the molecules are adsorbed, interacting with the clay surface, and another 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 free EPTC is released from the organoclay at a rate higher than that of adsorbed EPTC. Oat bioassays showed that the herbicidal activity of clay-complexed EPTC was extended by more than 1 week. These data indicate that clay-EPTC interactions can be employed to improve herbicidal performance of EPTC and to control its release to the environment.