Modeling the adsorption of weak organic acids on goethite : the ligand and charge distribution model

A detailed study is presented in which the CD-MUSIC modeling approach is used in a new modeling approach that can describe the binding of large organic molecules by metal (hydr)oxides taking the full speciation of the adsorbed molecule into account. Batch equilibration experiments were performed using the iron (hydr)oxide goethite to determine the adsorption of a series of weak organic acids (e.g. lactic acid, oxalic acid, malonic acid, phthalic acid, citric acid, and fulvic acid). In order to develop the new modeling approach, the binding of weak organic acids with a well-defined structure and charging behavior is first described using the classical CD-MUSIC model approach. The adsorption can be described accurately with a limited number of surface species, which differ in the degree of protonation and in the number of inner sphere and outer sphere complexes formed between the reactive groups of the organic molecule and the oxide surface. For the formation of inner and outer sphere complexes, a constant distribution of charge over the solid/water interface is assumed. With the same charge distribution for inner and outer sphere complexes, the adsorption of FA is described with the CD-MUSIC model using a small set of discrete surface species. This approach is not fully satisfactory since it does not take the full speciation of the adsorbed weak organic acids into account as can be inferred from spectroscopic data. In order to take the full possible speciation of the adsorbed organic molecule into account, a new model concept was developed. In the ligand and charge distribution (LCD) model concept, the number of inner sphere, outer sphere and proton complexes of the reactive groups of one adsorbed organic molecule is calculated using the NICCA equation. From the resulting speciation of the adsorbed molecule, the main input parameters of the CD-MUSIC model are obtained. The new model concept is successfully tested on the adsorption and infrared data of benzenecarboxylic acid adsorption on goethite by Boily et al. (2000a,b). The LCD model was extended for the surface complexation of the phenolic groups in order to describe the previously determined data of FA adsorption by goethite. Simultaneously, the concentration, pH, and salt dependency of the FA adsorption are described well. Furthermore, the co-adsorption of protons upon the adsorption of FA by goethite is predicted accurately. The developed model approach offers new insights in the fundamental understanding of ion adsorption under natural conditions.