Determination of diffusion coefficients based on Ficks second law for various boundary conditions

Based on Ficks second law the evaluation of diffusion experiments leads to two distinct apparent diffusion coefficients for each of the ions CS+, Rb+ and Pb2+. On the other hand, only one diffusion coefficient was found for Sr2+, I-, TcO4- and SeO42-. To get an insight into these results diffusion experiments with rock plates were carried out which were equilibrated for several weeks with the corresponding natural groundwater. Either the water-saturated plates were fixed immediately between two volumes of groundwater, one of them spiked with a tracer, or the equilibrated plates were additionally contacted with a tracer solution and then installed into the diffusion cells. The transfer of the diffusing substances through the rock material was evaluated by the non-steady state estimate based on Ficks second law and compared with the steady state evaluation based on Ficks first law. The results are discussed for the two types of experiments. (orig.)