A library of BASIC scripts of reaction rates for geochemical modeling using phreeqc

Abstract

Rate equations and kinetic parameters for about 100 minerals were programmed into a library of callable Basic language scripts for the geochemical modeling program Phreeqc (version 3.5.0) to facilitate the application of kinetics in geochemical modeling. For most minerals, the following general equation is used: rnet=SA∑jAje−Ea,j/RT∏iai,jni(1−Ωpj)qjwhere rnet stands for the net dissolution rate of a mineral phase (mol kgw−1 s−1); j the jth reaction mechanism; SA the surface area per unit water mass (m2 kgw−1); Aj the Arrhenius pre-exponential factor (mol m−2 s−1); Ea,j the apparent reaction activation energy (J mol−1); R the universal gas constant (8.31446 J mol−1 K−1); T the temperature (K); ai the activity of aqueous species i; Ω the mineral saturation quotient. pj and qj are empirical fitting parameters. j stands for the specific mechanisms of reaction. Other forms of rate equations and associated parameters programmed in the library include parallel mechanisms, Langmuir adsorption isotherm, and empirical rate equations that apply to a specific reaction mechanism or geochemical system. A separate file of PHASEs, which define the chemical stoichiometry of the phases, dissolution reactions, and equilibrium constants of the dissolution reactions, is also provided. PHREEQC requires that the names in PHASES and RATES blocks match with each other.The Basic language scripts can also be used as templates for writing other rate equations which users might wish to use. To illustrate the application of the script library, we simulated the reaction path of albite dissolution at 25 °C and 1 bar, using three rate equations and compared the results. The script and phase library and supporting materials can be downloaded from https://github.com/HydrogeoIU/PHREEQC-Kinetic-Library and doi.org/10.5967/41gq-yr13.