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-(ch lorophenyl)-3-ethyl-2,5-dihydro-5-oxopyridazine-4-carboxylic acid) was inve stigated in batch equilibrium experiments and compared to that of two other organic acids (2,4-D and salicylic acid). Sorption coefficients K-d for th e three compounds were determined in 18 Cambisols and Ferralsols. K-d value s for clofencet were 0.3-9.4 l/kg for Cambisols and 2.1-68 l/kg for Ferrals ols. Sorption of clofencet was strongly related statistically to that of sa licylic acid. Sorption of clofencet and salicylic acid decreased exponentia lly with increasing solution pH in Cambisols whereas a bell-shaped curve wa s obtained for the sorption of salicylic acid in Ferralsols. Sorption of 2, 4-D (2,4-dichlorophenoxyacetic acid) was not statistically related to the p H of the different soils. Positively charged oxide surfaces were shown to p lay a significant role in the sorption of clofencet and salicylic acid. The use of simple correlation and multiple linear regressions suggested that t he main sorption mechanisms of clofencet in soils were likely to be ligand exchange on oxide surfaces and, to a lesser extent, cation bridging. Differ ences in the sorption behaviour of clofencet/salicylic acid and 2,4-D might be attributed to the possibility of the two former compounds forming biden tate complexes with metals. (C) 2001 Elsevier Science Ltd. All rights reser ved.