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Experimentelle Bestimmung der Lo?sungskinetik synthetischen Calciumcarbonats und natu?rlicher Kalkgesteine

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Abstract (Summary)
Within the scope of this work dissolution rates of synthetic calcite and natural limestone (freshly and etched surface) were measured under different boundary conditions. Experiments were conducted with the objective to determine the influence of initial pCO2, temperature, dissolved foreign ions and material on the dissolution kinetics of calcium carbonate. A dataset emerged, which can be implemented into models of karst evolution accounting for different chemical boundary conditions. The batch experiment using the free drift method was chosen for measuring dissolution rates in closed system conditions with respect to CO2. By adjusting the boundary conditions (volumesurface-ratio, diffusion boundary layer thickness) measurement of surface controlled dissolution rates was warranted. The following empirical rate law proves valid for all measured dissolution rates. The reaction rate is a function of the relative saturation, expressed as the relation of the Caconcentration, c, to its equilibrium value, ceq. n1 R = k1 ? (1?cceq ) c ceq < xs n R = k ? 1? c c c ceq ? xs 2 ( ) 2 eq Above a saturation level xs the rate law changes to a higher kinetic order n2. The kinetic parameters n1, n2, k1 and k2 vary with changing boundary conditions (pCO2i, T, material, foreign ion concentration). Values of the reaction order n1 range from 0.5 to 2.5, n2 from 3.4 to 7.8, the kinetic order k1 varies between 7·10-8 and 5·10-7 mmol/cm2s, k2 between 7.5·10-7 and 13 mmol/cm 2s. The dissolution rates of synthetic calcite (Baker Calcite) and pure natural calcite closely follow the rates proposed by the PWP-model of PLUMMER et al. (1978). The dissolution kinetics of fresh natural limestone does not differ signifcantly from etched material. The dissolution kinetics of the studied materials react differently to the presence of foreign ions in solution. Sulfate, phosphate and magnesium inhibit the dissolution rates of synthetic Baker Calcite. The resulting dissolution kinetics resembles that of natural limestone in pure H2O-CO2-solutions. In contrast to this sulfate has no influence on the dissolution rates of Jura Limestone, whereas magnesium and phosphate do inhibit the dissolution. The different dissolution kinetics can be attributed to the various lithologies. Natural limestone often contains impurities such as sulfate, magnesium, phosphate, strontium or alumosilicates. The adsorption energy of physisorbed calcium ions rises through the interaction with impurities on the surface of the mineral resulting in the inhibition of the dissolution process. From this follows that foreign ions can only influence the dissolution kinetics of natural limestone if they are not already intrinsic impurities and have not reached a critical surface concentration. If pure calcite dissolves in solutions containing foreign ions, these adsorb to the mineral surface and cause the inhibition described for natural limestone. This is reflected by similar kinetic parameters of natural limestone and pure calcite in solutions with foreign ions. For the first time mixing corrosion in freshwater and freshwater-saltwater mixtures was reproduced in the laboratory. Under these boundary conditions the linear kinetics of Baker Calcite and nonlinear of natural limestone could be confirmed. Model calculations on the evolution of karst systems were conducted for two different chemical boundary conditions (pCO2i = 0.001 atm, 0.05 atm; 10°C) using the kinetic parameters of Jura Limestone in one- and two-dimensional models. Different evolutions of the aquifer were found for the two cases in all models. These differences are caused by the complex interplay between dissolution rates and hydrology. Inhaltsverzeichnis III
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School:Hochschule Bremen

School Location:Germany

Source Type:Master's Thesis

Keywords:kalkstein lo?slichkeit

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