Sorption and degradation of tebutam in soil under controlled laboratory conditions

This study investigates tebutam (N-benzyl-N-isopropylpivalamide) sorption and degradation in the surface level (0-20 cm) of a brown silty clay soil. All the experiments were conducted under controlled laboratory conditions using C-14 uniformly ring labelled tebutam. The sorption process was evaluated by adsorption kinetics, adsorption and desorption isotherms using a batch equilibration method. In our conditions, equilibration time for 95% of herbicide adsorbed was 0.21 h. This initial rapid sorption pattern, was followed by slower sorption continuing over 48 h. The adsorption isotherm was well described by the Freundlich equation with K-f = 1.48 and n(f) 0.95. This corresponds to a K-d of 1.48 l.k(g-)1 and a K-oc of 148 l.kg(-1), comparable to the coefficients measured for atrazine. Desorption of tebutam was easy and weakly hysteretic, particularly for large adsorbed amounts. For degradation, data showed that 50% of tebutam was degraded in 14 days (as a percentage of extractable residues), and nearly 41% was mineralized in the same interval. Non-extractable residues reached 46% of initial applied amounts in 12 days. Their slow and gradual release determined after 12 days, appears to supply the mineralization process. Degradation products were formed from the fourth incubation day and three of them appeared to persist in soil media. They reached nearly 80% of extractable residual radioactivity at the end of incubation.