Sorption of weak organic acids in soils: clofencet, 2,4-D and salicylic acid

The sorption behaviour of a new wheat hybridising agent (clofencet, 2-4-(chlorophenyl)-3-ethyl-2,5-dihydro-5-oxopyridazine-4-carboxylic acid) was investigated in batch equilibrium experiments and compared to that of two other organic acids (2,4-D and salicylic acid). Sorption coefficients Kd for the three compounds were determined in 18 Cambisols and Ferralsols. Kd values for clofencet were 0.3-9.4 l/kg for Cambisols and 2.1-68 l/kg for Ferralsols. Sorption of clofencet was strongly related statistically to that of salicylic acid. Sorption of clofencet and salicylic acid decreased exponentially with increasing solution pH in Cambisols whereas a bell-shaped curve was obtained for the sorption of salicylic acid in Ferralsols. Sorption of 2,4-D (2,4-dichlorophenoxyacetic acid) was not statistically related to the pH of the different soils. Positively charged oxide surfaces were shown to play a significant role in the sorption of clofencet and salicylic acid. The use of simple correlation and multiple linear regressions suggested that the main sorption mechanisms of clofencet in soils were likely to be ligand exchange on oxide surfaces and, to a lesser extent, cation bridging. Differences in the sorption behaviour of clofencet/salicylic acid and 2,4-D might be attributed to the possibility of the two former compounds forming bidentate complexes with metals.