Dissipation of Pendimethalin in Soil Under Direct Seeded and Transplanted Rice Field.

The dissipation of pendimethalin applied in direct seeded rice (DSR) and transplanted rice (TPR) field at 1.0 and 2.0 kg a.i. ha-1 followed biphasic first order kinetics (R2 > 0.91) and was comparatively faster under flooded TPR than DSR. The half-life (DT50) of pendimethalin in the soil ranged from 2.22 to 2.80 days in the initial phase and 23.51 to 24.66 days in the final phase in TPR for both application rates. However in DSR, DT50 varied from 3.67 to 4.35 days in the initial phase and 34.19 to 34.99 days in the final phase. Residues of pendimethalin in soil samples analyzed by HPLC and GC-MS/MS were below the detection limit (< 0.003 µg g-1) for both the application rates in DSR and TPR whereas 0.003-0.009 µg g-1 and 0.003-0.008 µg g-1 residues of pendimethalin were found in rice grain and straw samples, respectively.

Temperature Dependent Adsorption-Desorption Behaviour of Pendimethalin in Punjab Soils.

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 (R2 > 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 µg1 - 1/n g-1 mL1/n 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.