> I'm a PhD student in the Research Center Karlsruhe at the Institute of Nuclear Waste Management. I'm working on a DFG project regarding the bassanite-gypsum-transition together with Dr. Dirk Bosbach. > We try to model the reaction kinetics of bassanite dissolution, gypsum nucleation and gypsum growth to develop a predictive reaction model for plaster setting with PhreeqC. We determined the kinetic constants for the reaction rates by batch and flow-through experiments. Then we used PhreeqC succesfully to model bassanite dissolution and gypsum growth, but the nucleation is still missing. We have determined the nucleation rate. > Now my question: Can you give me hint, how to integrate gypsum nucleation/nucleation rate in PhreeqC? I've attached my input file and the database we use. In your input file you have M0 = 1e-20, which is then included several places in the rate. This makes the rate very sensitive to M0, which I don't think is a good idea. So you should reconsider how to take account of M0. I don't know much about nucleation kinetics, so I can't give you a definitive answer. It seems like you need a more complex rate expression. Perhaps you want to keep track of the mass and surface area of a number of particles. If you know the nucleation rate and the initial diameter of the particles, you could add particles of a given diameter at a rate (probably dependent on SI) and store them with PUT statements for each time step. I guess you could just store particle diameter as PUT(n,diameter). You would have to revisit previously nucleated particles to calculate rates and update their diameters. It might be tricky to apportion the overall rate to the collection of diameters. A simpler approach would be to make the rate a function of total moles precipitated (but not M0) and SI (or SR). I know Dennis Eberl has a set of programs called Galloper (I think) that implements a model of crystal growth, though not connected to a aqueous model. It uses a law of proportionate growth for the particles. I have not seen a mechanistic explanation for the applicability of the law, but it seems to account for crystal sizes. You may want to try to implement his approach with a number of particles. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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