Surface structural ion adsorption modeling of competitive binding of oxyanions by metal (hydr)oxides

Spectroscopy has provided a progressive flow of information concerning the binding mechanism(s) of ions and their surface-complex structure. An import ant challenge in surface complexation models (SCM) is to connect the molecu lar microscopic reality to macroscopic adsorption phenomena. This is import ant because SCM alone provide insufficient insight in the binding mechanism s, and moreover, it is a priori not obvious that SCM, which describe the pH dependent adsorption correctly in simple systems, will predict the ion int eraction under multicomponent conditions. This study elucidates the primary factor controlling the adsorption process by analysing the adsorption and competition of PO4, AsO4, and SeO3. We show that the structure of the surfa ce-complex acting in the dominant electrostatic field can be ascertained as the primary controlling adsorption factor. The surface species of arsenate are identical with those of phosphate and the adsorption behavior is very similar. On the basis of the selenite adsorption, we show that the commonly used 2pK models are incapable to incorporate in the adsorption modeling th e correct bidentate binding mechanism found by spectroscopy. The use of the bidentate mechanism leads to a proton-oxyanion ratio and corresponding pH dependency that are too large. The inappropriate intrinsic charge attributi on to the primary surface groups and the condensation of the inner sphere s urface complex to a point charge are responsible for this behavior of commo nly used 2pK models. Both key factors are differently defined in the charge distributed multi site complexation (CD-MUSIC) model and are based in this model on a surface structural approach. The CD-MUSIC model can successfull y describe the macroscopic adsorption phenomena using the surface speciatio n and binding mechanisms as found by spectroscopy. The model is also able t o predict the anion competition well. The charge distribution in the interf ace is in agreement with the observed structure of surface complexes. (C) 1 999 Academic Press.