Mineral reactivity contributes to the global biogeochemical cycling of elements. CO2is consumed during the chemical weathering of many silicates and during formation of carbonates in the oceans. The balance of nutrients in soils and waters is coupled to the release of elements during weathering and the retention of elements during mineral precipitation. Dissolution of many minerals involves acid neutralization and the rate of most mineral reactions is dependent on the pH of the solution. The acid input to the atmosphere has increased drastically during the last century affecting our environment severely, with problems such as deterioration of the bronze-age rock carvings in the world heritage area Tanum, SW Sweden.
This thesis compiles studies of quantum chemical calculations of the siloxane bond strength, laboratory dissolution experiments of quartz and brucite as well as field studies of the water chemistry of a rock surface in Tanum, SW Sweden.
Quartz consists of only SiO2and provides the possibility to investigate specifically the siloxane bond, which is present in most silicates. Quartz dissolution is performed using plug-flow reactors during long-term experiments. This produced among the slowest rates reported in the literature, which suggests that long runtime is needed to approach natural dissolution rates. A rate minimum was obtained at pH 3-4, which was observed also after a short reaction time. Alkali metal cations increased the rate at pH>3, and KCl decreased the rate at pH<3, similar to results obtained for feldspar dissolution. The alkali metal effects and the pH dependence of the quartz dissolution emphasize the importance of surface species for the dissolution kinetics of silicate minerals, which may indicate the rate determining mechanisms in natural weathering environments.
Quantum chemical methods are used to investigate the effects of different surface species on the siloxane bond strength. Direct adsorption of alkali metal cations reduces the bond strength for alkaline surface models and increases the bond strength for acid surface models, indicating that the surface species contribute to the rate of the quartz dissolution. Interaction with water molecules proved to be very important for the ion specific effects. For effects from protonation and deprotonation of the quartz surface, the hydration was essential. Further quantum chemical calculations showed that the pH dependence of the dissolution rate at high surface charge coverage could be due to acid-base repulsion within the surface layer.
Brucite is a fast dissolving mineral that gives the possibility to investigate the rate dependence on the transport of ions. Rotating disc experiments are used to separate between surface reaction controls and molecular diffusion controls. Numerical modeling of the fluid equations is used to model a reaction zone near the brucite surface. The experiments and the model revealed that the solution pH determines the reaction mechanism in terms of surface reaction or diffusion reaction control. This is important information for the conclusion of the rate controlling mechanism of fast dissolving minerals on a rock surface.
The water chemistry on the rock surface in Tanum gives information of rate controlling factors that can be used to reduce the chemical weathering rate. Extensive sampling of deposition, influenced by sea salt and acid rain, and runoff is performed on a rock surface in Tanum, SW Sweden. Two rock surfaces are investigated and compared; a roof protected, washed surface and a reference surface. The chemical weathering rate calculated as anorthite dissolution was 10 times slower than in literature data, due to a difference between the specific surface area and the reactive surface area in field. The chemical weathering rate of both surfaces was correlated with temperature and large differences in dissolution rate between the two ponds are due to differences in temperature and pH. Control of these parameters should give a surface that is preserved longer than an unprotected rock surface.
Stockholm: Department of Geological Sciences, Stockholm University , 1999. , 26 p.