dissipation kinetics and ad-desorption isotherms of thiobencarb in paddy fields

abstract herbicides are the main sources of soil and water pollution in paddy fields. herbicides mobility to the surface and ground water is a major concern for human health and the environment. it is, therefore, necessary to investigate their fate and their interactions in natural ecosystems. to meet this purpose, field experiments were conducted to measure kinetic of thiobencarb dissipation in soil and water at dashtnaz and gharakhail agricultural research stations, mazandaran. also, its adsorption and desorption isotherms were determined in the soil samples. thiobencarb concentration was monitored in soil and water (soil solution) via sampling during 315 and 60 days periods, respectively. adsorption isotherms were carried out in equilibrium concentration ranged from 0 to 26 mg l-1. desorption isotherms were measured in three equilibrium concentrations. the results of field experiments revealed that thiobencarb concentration is declined with time rapidly and reaches to a steady state after approximately 30 days. experimental data were successfully described using the first order kinetics equation. thiobencarb half-lives were determined 7 and 10 days in dashtnaz and gharakhail water, respectively. in both soils, thiobencarb concentration was strongly varied in early stages of soil sampling. the variations were reduced with discontinuation of irrigation. similar to water, thiobencarb concentration in soils was gradually decreased with time, and arrived to a relatively constant concentration after about 110 days. first order kinetic equation described correctly the change in thiobencarb dissipation in soils. thiobencarb half-lives were calculated 93 and 114 days for dashtnaz and gharakhail soils, respectively. adsorption isotherms indicated that thiobencarb is strongly adsorbed on soil particles. thiobencarb was desorbed very slowly and a strong hysteresis was observed between adsorption and desorption isotherms. calculations revealed that thiobencarb is a persistence and non-leacher herbicide in the soil, due to its strong adsorption and high hydrophobic coefficient.