Application of two surface complexation models to the adsorption of weak organic acids by soil: an additive approach

Summary The description of adsorption of organic acids by soil with models requires many parameters. It is often difficult to obtain their values, and so we might try to test the application of models with published data on soil characteristics. We used two models in our trials, namely the constant capacitance model and the variable surface charge–variable surface potential model. We modelled the sorption data for three weak organic acids [2,4‐D, 2–4‐dichloro‐phenoxy acetic acid; salicylic acid, 2‐hydroxybenzoic acid; and clofencet, 2–4‐(chlorophenyl)‐3‐ethyl‐2,5‐dihydro‐5‐oxopyridazine‐4‐carboxylic acid] in three Cambisols and two Ferralsols. We adopt an additive approach where the five soils were considered to behave as mixtures of goethite and gibbsite. This was done with values taken from the literature on the assumption that the compounds formed monodentate complexes. Variations given by the two models in adsorbed amounts as a result of pH changes were well described for clofencet and 2,4‐D but were less successful for salicylic acid. Simulations of sorption isotherms at various pHs ranging from 5 to 8 matched the experimental data closely for clofencet. Prediction of the adsorption of clofencet and 2,4‐D in a range of Cambisols and Ferralsols at their native pH was generally good. We attribute the somewhat poorer fit observed for salicylic acid to the formation of bidendate complexes. Further improvement to the modelling should involve the formation of bidentate complexes between organic acids and oxide material and the effects of organic matter, particularly in relation to adsorption of non‐ionized compounds.