Comparison of the Solution Behaviour of a Pyrite/Calcite Mixture in Batch and Unsaturated Sand Column

The successful application of reaction transport algorithms to calculating the chemical evolution of natural systems requires accurate methods to compute the rates of mineral/fluid surface reactions. Regarding the transport of radio-nuclides in mining dumps the dissolution of minerals is of special importance. The use of a kinetic rate law of the mineral dissolution verified for unsaturated conditions will allow a realistic modeling of the mineral weathering in the environment. Dissolution rates of minerals in an aqueous solution are determined by surface reaction rates, the morphology of the mineral’s surface and, in case of the unsaturated zone, by the degree of the water saturation. For this process, it is decisive which portion of the particle surfaces is in con-tact with percolating water. To study the differences of mineral dissolution under saturated and unsaturated conditions batch and column experiments were carried out with a pyrite/calcite mixture. The experimental results were verified by calculations. Comparing the dissolution in batch with that in the column experiment, which was performed with a water flow velocity of 0.64 cm/d and analyzed in the region with a water saturation of 0.11, one can conclude that only the small portion of about five per cent of the grain surface is chemically reactive in this unsaturated flow.