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The paper delineates the field trials conducted to investigate the persistence behaviour and dissipation kinetics of pretilachlor in puddled paddy fields under subtropical humid climatic region. Matrix solid phase dispersion (MSPD) method was used for extraction of the pretilachlor from paddy grain and paddy soil samples collected from the experimental field. Pretilachlor residues were quantified using high-performance liquid chromatography (HPLC) with UV/Vis detector at 210 nm. The average recoveries of pretilachlor extracted from matrix ranged from 80.3 to 103.3% with standard deviation less than 10% and sensitivity of 0.01 μg g(-1). The dissipation rate of pretilachlor in paddy field soil and paddy field water followed first-order kinetics with decrease in pretilachlor residues as a function of time. Faster dissipation of pretilachlor was observed in paddy field water than in paddy field soil with half life of 1.89-2.97 days and 7.52-9.58 days, respectively. At harvest, the residues of pretilachlor in the paddy soil and paddy crop samples were below the detection limit.

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This study shows the effect of soil type and temperature on the adsorption and desorption behaviour of pendimethalin using a batch equilibration technique. Adsorption kinetics followed pseudo-second-order-model (R > 0.99). The shape of adsorption curve for studied soils was S-type at 30 and 40°C and L-type at 50°C. The isotherms were nonlinear and were well described by Freundlich equation. Adsorption capacity ranged from 1.4 to 2.2 μg g mL and the order of adsorption was: clay loam > sandy loam > loamy sand indicating strong affinity of pendimethalin towards organic matter and clay content. Irrespective of soil type, the adsorption of pendimethalin increased with increase in temperature suggesting endothermic process. Freundlich desorption coefficient was greater than adsorption in all soils at studied temperatures indicating hysteresis. Thermodynamic parameters revealed spontaneous adsorption process which becomes more favourable at high temperature. The adsorption of pendimethalin was dominated by surface adsorption at lower equilibrium concentration and partition at high concentrations.