Experimentelle Untersuchung zur Auflösungskinetik von Kaolinit und Montmorillonit in Anwesenheit von Sulfat, Phosphat, Amino- und Carbonsäuren sowie Harnstoff im offenen und geschlossenen System

Abstract

The aim of the present study is to decipher mechanisms and kinetics for the reactions at the solid/liquid interface by experiments at well defined physical/chemical conditions. For this purpose, dissolution experiments with the widespread clay minerals kaolinite and montmorillonite were carried out. These minerals are ubiquitous components of sediments and considerably involved in geochemical cycles of soils. The study is focused on the boundary conditions of open (flow experiments) and closed systems (batch experiments) with respect to the fluid. Results permit to simulate reaction processes and mechanisms in highly permeable horizons up to completely closed interstitial pore spaces. Inorganic (sulfuric acid, phosphate) and organic components (amino and carboxylic acids as well as urea) were selected as model substances. These components comprise different and well known structures and functional groups. Accordingly, a systematic investigation of the interaction between aqueous components and respective solid surfaces can be carried out.The results clearly show both, mainly proton controlled dissolution reactions and ligand controlled dissolution mechanisms. The dissolution starts with sorption of the aqueous component at the mineral surface (outer-sphere), followed by a formation of surface complexes (inner-sphere) and partly liberation of hydrated or complexed metal cations into the solution. Montmorillonite dissolves at the edges of the crystals, whereas for kaolinite the aluminol groups at the gibbsite layer may be also involved in dissolution mechanisms.Interactions between the clay minerals and aqueous solutions are verified by fixation of dissolved components at the mineral surface, intercalation of dissolved components, and change of solution chemistry during an experimental run.The decomposition of the silicate lattice can be followed by the obtained dissolution rates for silicic acid, whereas liberation of aluminum and iron is controlled by additional reactions, especially in the pH-range from 2 to 3. Such secondary reactions are referred to sorption at the solid surface and ion exchange, which are strongly controlled by the specific affinities of the aqueous components. Formation of aqueous aluminium complexes by ligands such as oxalic acid leads to a significant sorption reduction.In the case of montmorillonite, intercalated sodium ions are rapidly transferred into the solution by ion exchange. For the release of magnesium ions no simultaneously reactions occur.In both sets of experiments, the presence of oxalic acid reveals a strong increase of dissolution rates for silica compared to the other model substances, whereas acetic acid causes a significant decrease of the respective rates.In general, dissolution rates at the initial stage of batch experiments (closed system) show the same order of magnitude compared to those obtained at the steady state in open systems. However, dissolution rates for batch experiments continuously decrease with ongoing reaction time due to accumulation of dissolved ions.