Soil variability and temperature-dependent adsorption–desorption behaviour of herbicide bispyribac-sodium

Bispyribac-sodium is a herbicide used to control weeds in paddy crops and could contaminate water bodies. Soil properties play a crucial role in adsorption–desorption of bispyribac-sodium and impact its off-site movements. This study examined the adsorption–desorption of bispyribac-sodium in four differentially textured soils, i.e. clay loam, loam, sandy loam and sand. Additionally, three treatments involving removal of organic matter, clay content and addition of organic matter to clay-removed soil were studied to critically evaluate their role in the process. The Lagergren pseudo-second-order model effectively described the kinetics of the process. Adsorption and desorption isotherms followed the Freundlich equation. Maximum adsorption was observed at 25°C, followed by a significant reduction at higher temperatures. All the untreated soils showed low to mild adsorption, with adsorption coefficients (K_D) ranging from 1.58 to 7.02 mL/g. Even in Alfisol and the treated soils containing high amount of organic matter, the K_D did not varied significantly (ranged from 0.37 to 6.94 mL/g) from those in untreated soils containing low organic matter. The results clearly demonstrated that organic matter had a negative impact on adsorption, whereas clay content and pH contributed to its enhancement. Desorption was very high (up to 87.31%) in all soils. Thermodynamic parameters (ΔH = 1.27 to 6.32 kJ/mol, ΔS = 0.01 to 1.11 J/mol/K, and ΔG = −0.88 to −4.94 kJ/mol) indicated adsorption as endothermic, spontaneous and primarily physical in nature. It is concluded that bispyribac-sodium is weakly adsorbed and readily desorbed across different soils and their treatments. Additionally, even an increase in soil organic matter was found to be ineffective in significantly influencing the adsorption–desorption process and makes bispyribac-sodium highly prone to contaminate environment due to its off-site movements.