REACTIVE TRANSPORT MODELING OF PLUG-FLOW REACTOR EXPERIMENTS - QUARTZAND TUFF DISSOLUTION AT 240-DEGREES-C

Extension of reactive transport modeling to predict the coupled therma l, hydrological, and chemical evolution of complex geological systems is predicated on successful application of the approach to simulate we ll-constrained physical experiments. In this study, steady-state efflu ent concentrations and dissolution/precipitation features associated w ith crushed quartz and tuff dissolution at 240 degrees C have been det ermined experimentally using a plug-flow reactor (PFR) and scanning el ectron microscopy (SEM) techniques, then modeled with the reactive tra nsport simulator GIMRT (Steefel and Yabusaki, 1996) using a linear rat e law from transition State theory (TST). For quartz dissolution, inte rdependence of the specific surface area (A(m)) and reaction rate cons tant (k(m)) predicted from the modeling agrees closely with that obtai ned from an analytical solution to the reaction-transport equation wit hout diffusion/dispersion, verifying the advection-dominant nature of the PFR experiments. Independently-determined A(qtz) and k(qtz) from t he literature are shown to be internally consistent with respect to th e model and analytical interdependence, implying appropriateness of th e linear TST rate law and adequacy of BET-determined A, for use in mod eling PFR experiments. Applications of this integrated approach for mo nomineralic dissolution include assessment of internal consistency amo ng independent A(m) and k(m) data, estimation of k(m) from BET-determi ned A(m), and rapid evaluation of alternative rate laws. For such diss olution, accurate simulation of the experimental steady-state effluent concentrations (to within 3% for Na, Al and K; to within 15% for Si a nd Ca) and dearth of alteration phases (<1 vol.% in the model) defines minimum supersaturation thresholds for nucleation of primary minerals (to suppress their secondary precipitation) and the (non-unique) rela tive k(m) of hydrous aluminosilicates. For such multicomponent, multip hase experiments, reactive transport simulators provide the only pract ical means of estimating and constraining these kinetic parameters. De spite the approximations and uncertainties inherent in present-day mod els, close agreement between the experimental and simulation results r eported herein provides an important measure of confidence for extendi ng the modeling approach to address increasingly complex systems for w hich development of experimental analogs is impractical or impossible. Published by Elsevier Science B.V.