PhD Graduation

Ion adsorption is highly relevant in science, technology, and the environment. For many elements, the ion concentration is regulated by adsorption and desorption. In nature, three classes of reactive materials are present each with a specific characteristic in relation to ion binding. For organic matter, the main keyword is chemical heterogeneity, for clay minerals, it is permanent negative charge with corresponding cation exchange, and for metal (hydr)oxides, it is the dominance of electrostatics in ion binding.

In this thesis, a new framework for ion binding to metal (hydr)oxides is described that allows linking of the microscopic processes of ion binding to macroscopic adsorption phenomenon. The new framework is based on a structural approach of mineral surfaces as well as of surface complexes, being described with respectively with the MUlti-SIte Complexation (MUSIC) model and the Charge Distribution (CD) approach.

In the MUSIC model, surface groups are distinguished based on metal coordination that creates differences in charge. This leads to variation in affinity of the surface oxygens for protons. With the MUSIC model, the intrinsic proton affinity of the various types of groups can be derived, which is essential to calculate the overall surface charge of metal (hydr)oxides due to proton adsorption. For the calculation, electrostatic theory is applied since accumulation of protons at the surface will create an electrostatic field that is experienced by the adsorbing protons themselves as well as by other ions. The field will very strongly affect the ion binding and therefore it is essential to account for this.

Ions form complexes at the surface and the structure of the complexes can be elucidated with in-situ spectroscopy and molecular modeling. At the scale of the interface, surface complexes will experience a gradient of repulsion and/or attraction by the electrostatic field. To account for this important phenomenon, a new approach, known as the CD model, has been developed, which is a logical extension of the MUSIC model. The charge distribution is tightly linked to the microscope structure of the surface complexes. The CD model is able to calculate accurately the electrostatic energy involved in ion binding. This is essential since ion binding at mineral - solution interfaces is dominated by changes in the electrostatic field at variation of solution conditions such as pH, ionic strength, and the concentration of the ion as well as of its competitors.

The new ion adsorption framework is able to describe the main macroscopic adsorption phenomena. The CD-MUSIC model has been tested successfully for a series of ions bound in competition to different mineral surfaces, in particular to the various Fe and Al hydroxides that play an important role in the environment. To apply the model in field samples, a methodology has been developed to measure the equivalent reactive surface area of the natural oxide fraction. It revealed that metal oxide fraction can be considered as a collection of nanoparticles that are covered and/or embedded in a matrix of natural organic matter. To apply the new framework in soil, a practical solution has been suggested to account for the interaction of oxide particles with natural organic matter.

Promotor: Prof.dr. W.H. van Riemsdijk