Kinetics of degradation and adsorption–desorption isotherms of thiobencarb and oxadiargyl in calcareous paddy fields

Herbicides are an important source of contamination in paddy fields. Monitoring their fate and chemical interactions is therefore imperative for sustaining the environment and human health. To meet this purpose, field experiments were conducted to investigate kinetics of thiobencarb and oxadiargyl dissipation in soil and water of two paddy fields. Their adsorption and desorption isotherms were also determined in the soil samples. Variation in concentration was monitored for 60 d in soil solution phase and for 315 d in soil solid phase. In soil solution, concentrations of both herbicides were rapidly reduced within 5 d and reached steady state within 20–30 d. Analysis of experimental data resolved a half-life ≈2–4 d for both herbicides. In soil solid phase, adsorption reaction played a dominant role in the first 10 d. Afterwards, degradation reactions regulated the process. Variation in concentration was minimized after about 150 d for thiobencarb and 80 d for oxadiargyl. The half-lives were calculated ≈50 d for thiobencarb and ≈20 d for oxadiargyl, indicating that association with soil particles protect them effectively against degradation reactions. Adsorption isotherms confirmed that both herbicides were strongly adsorbed on soil particles. Furthermore, desorption data indicated that after four successive desorption steps, less than 9% thiobencarb and 1% oxadiargyl were released. This denotes that electrolyte ions in solution cannot adequately compete with and replace adsorbed thiobencarb and oxadiargyl molecules. This would lead to a considerable hysteresis between adsorption and desorption isotherms as was observed experimentally. Overall, it was concluded that both herbicides are among non-persistent and immobile herbicides in the paddy soils.

► Thiobencarb and oxadiargyl are strongly adsorbed on soil particles.
► Association of herbicides with soil particles protect them against degradation.
► Desorption isotherms deviate strongly from adsorption isotherms.
► Total desorption is less than 9% for thiobencarb and 1% for oxadiargyl.
► Thiobencarb and oxadiargyl are among non-persistent and immobile herbicides in soil.