ADSORPTION OF SMALL WEAK ORGANIC-ACIDS ON GOETHITE - MODELING OF MECHANISMS

The adsorption of lactate, oxalate, malonate, phthalate, and citrate h as been determined experimentally as a function of concentration, pH, and ionic strength. The data have been described with the CD-MUSIC mod el of Hiemstra and Van Riemsdijk [J. Colloid Interface Sci. 179, 488-5 08 (1996)] which allows a distribution of charge of the organic molecu le over the surface and the Stern layer. Simultaneously, the concentra tion, pH, and salt dependency as well as the basic charging behavior o f goethite could be described well. On the basis of model calculations , a distinction is made between inner and outer sphere complexation of weak organic acids by goethite. The results indicate that the affinit y of the organic acids is dominated by the electrostatic attraction. T he intrinsic affinity constants for the exchange reaction of surface w ater groups and organic acids, expressed per bond, increases with incr easing number of reactive groups on the organic molecule. Ion pair for mation between noncoordinated carboxylic groups of adsorbed organic ac ids and cations of the background electrolyte proved to be important f or the salt dependency. The knowledge obtained may contribute to the i nterpretation of the binding of larger organic acids like fulvic and h umic acids. (C) 1997 Academic Press.