Geochemical equilibrium speciation model


MINTEQA2 Introduction
MINTEQA2 Database
MINTEQA2 Requirements

MINTEQA2 Introduction

MINTEQA2 is a geochemical equilibrium speciation model capable of computing equilibria among the dissolved, adsorbed, solid, and gas phases in the laboratory or in natural aqueous systems. MINTEQA2 includes an extensive database of reliable thermodynamic data that is also accessible to PRODEFA2, an interactive program designed to be executed prior to MINTEQA2 for the purpose of creating the required MINTEQA2 input file. Both MINTEQA2 and PRODEFA2 utilize extended memory.

The data required to predict the equilibrium composition in MINTEQA2 consists of a chemical analysis of the sample to be modeled giving total dissolved concentrations for the components of interest and any other relevant invariant measurements for the system of interest, possibly (but not necessarily) including pH, pe, or the partial pressures of one or more gases. A measured value of pH and/or pe may be specified as equilibrium values or MINTEQA2 can calculate equilibrium values. Also, a mineral may be specified as presumed present at equilibrium, but subject to dissolution if equilibrium conditions warrant, or definitely present at equilibrium and not subject to complete dissolution.

MINTEQA2 Database

MINTEQA2 has an extensive thermodynamic database that is adequate for solving a broad range of problems without need for additional user-supplied equilibrium constants. The standard database can be easily modified if it is found to be incomplete or inadequate for a particular problem. The empirical nature of the available metal adsorption data reflects the fact that natural adsorbent phases often occur as mixtures of impure amorphous substances that vary widely in chemical behavior from site to site. For this reason, adsorption data are left to the discretion and problem-specific knowledge of the user. Seven adsorption models are available in MINTEQA2 to match the type of data available for specific problems:

(1) the activity Kd model
(2) the activity Langmuir model
(3) the activity Freundlich model
(4) the ion exchange model
(5) constant capacitance model
(6) the triple-layer model
(7) the diffuse-layer model

Thermodynamic database files of surface reactions are generally not provided for these models as for aqueous and solid species; the user must provide the set of surface reactions and their equilibrium constants. An exception is a database of several surface reactions relevant to the diffuse-layer model for trace metal adsorption onto an iron oxide surface.

The application of a geochemical equilibrium model to an environmental problem involves four steps:

(1) Formulate one or more precise and relevant chemical questions that can be answered if one knows the equilibrium composition of the system. The formulation of the chemical questions must respect the inherent limitations in the site-specific data such as incomplete sampling or incomplete chemical analyses of samples.

(2) Pose the chemical questions to the model in terms of those symbols and formats that it is programmed to understand and from which it may interpret a mathematical problem.

(3) Cause the computer program that is the geochemical equilibrium model (in this case, MINTEOA2) to solve the mathematical problem.

(4) Interpret the output from the model in terms of the original environmental problem.

The first step is almost always the most difficult; the ability to do this well is not obtained from reading a user manual.

PRODEFA2 is designed to perform step 2 by asking questions about the chemical system to be modeled and building the appropriate MINTEQA2 input file from the answers.

Step 3, the actual execution of MINTEQA2, is usually rather automatic, not requiring user intervention once initiated. Occasionally, computational problems will occur during execution that will require the user to make adjustments to the input file and re-execute MINTEQA2.

Step 4 is performed as the user examines the MINTEQA2 output file and relates the result to the initial problem.

The MINTEQA2 package includes both source and executable codes for MINTEQA2 and PRODEFA2 as well as technical support.

MINTEQA2 Requirements: PC 486/Pentium with 2 MB RAM and math coprocessor.

Incredible NetworksΤελευταία Ενημέρωση 27 Ιουλίου 2004 - Last Revised on July 27th 2004
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