EPSc 500 Special Topics Geochemical and Biogeochemical Reaction Modeling TTh 10to 11:30, Rudolph Hall 308 Instructor: Prof. Jeff Catalano Office Hours: T 4 to 5, Th 2:30 to 3:30 Textbook (required): Geochemical and Biogeochemical Reaction Modeling, 2nd edition Website: http://epsc.wustl.edu/~catalano/epsc500/ Course Objectives: • To become familiar with the principles and applications of geochemical reaction modeling in low-temperature and hydrothermal environments. • To learn methods to model geochemical and biogeochemical processes using both equilibrium and kinetic approaches. • To appreciate the dangers of treating geochemical modeling software as a black box.

Week Dates Topic 1 1/14, 16 Introduction; Overview of Geochemical Modeling; Nature of

Equilibrium; Solving for Equilibrium; Equilibrium Models of Aqueous Systems;

2 1/21,23 Working within GWB; Uniqueness 3 1/28, 30 Thermodynamic Databases: Compilation and Editing; Automatic

Reaction Balancing; Activity Corrections 4 2/4,6 Redox Chemistry; Reaction Path Modeling 5 2/11,13 Mass Transfer Reactions; Polythermal, Fixed, and Sliding Reaction

Paths; Geochemical Buffers 6 2/18,20 Hydrothermal Fluids; Microbial Energy Yields; Geothermometry; 7 2/25, 27 Evaporation 8 3/4,6 Stable Isotopes; Sediment Diagenesis 9 3/10-14 SPRING BREAK – NO CLASS 10 3/18, 20 NO CLASS: Prof. Catalano at ACS and LPSC; WORK ON PROJECTS 11 3/25, 27 Kinetics of Dissolution and Precipitation; Kinetics of Water-Rock

Interactions 12 4/1, 3 Sorption and Ion Exchange; Surface Complexation 13 4/8,10 Redox Kinetics 14 4/15,18 Microbial Kinetics; Microbial Communities 15 4/22,24 Student Presentations

Tentative Course Schedule

Recent Examples of Geochemical Modelling

Taken from Geochimica et Cosmochimica Acta

Ca

ppb

Introduction to Geochemical Modeling

What is a model?

“A model is a simplified version of reality that is useful as a tool. A successful model strikes a balance between realism and practicality. Properly constructed, a model is neither so simplified that it is unrealistic nor so detailed that it cannot be readily evaluated and applied to the problem of interest.”

Terminology

• System: The subset of the universe that we are concerned with

• Closed System: Fixed composition, mass cannot enter or leave

• Open System: Variable composition, mass can enter and leave

• Extent: Amounts of fluid and minerals considered in the calculation

Types of Equilibrium in Models • Complete Equilibrium: All reactions are at equilibrium • Metastable Equilibrium: Some reactions happen too slow and

do not equilibrate – In a model we can suppress reactions that do not equilibrate

• Partial Equilibrium: Special case of metastable equilibrium; one component (e.g., the fluid) is equilibrated but not all of the components are equilibrated with each other – Common with redox reactions, which can be decoupled

• Local Equilibrium: Portions of system each in equilibrium, but whole system is not – Often applied to describe chemical changes during transport – System is divided up into a number of small units; each unit is

equilibrated, but units do no equilibrate with each other

Components of Initial System

• Temperature • Mass of solvent (typically 1 kg of H2O) • Amount of minerals present • Gas fugacities (partial pressures) • Amount of dissolved components • Activities of species (e.g., pH) • Oxidation state

The Reaction Path, ξ • Once the initial system is determined a model then follows a

reaction path – This is the course followed by the equilibrium system as it responds to

changes in composition and temperature – This is effectively a very simple reaction rate

• In geochemical modeling the reaction path is typically broken up into 100 steps – If reacting 100 mg of feldspar with a fluid, this will occur in 100, 1 mg steps,

equilibrating at each step

• The length of each step (or really the amount reacted at each step) can be set by a kinetic rate law, or components can be held constant (buffered), or temperature can be varied

Closed-System Models

Titration Models

Fixed Fugacity Models

• Used to describe a system that is buffered by a gas phase

• Most common examples involve water buffered by the atmosphere – Partial pressure of O2 in air sets dissolved oxygen

during redox reaction – Partial pressure of CO2 set dissolved carbonate

concentration during mineral precipitation or dissolution

Sliding-Fugacity Models

Local Equilibrium Models

Local Equilibrium Models

Local Equilibrium Models

Local Equilibrium Models

Local Equilibrium Models

Local Equilibrium Models

Reactive Transport Models

Uncertainties

• Is our chemical analysis sufficiently accurate? • Does the thermodynamic dataset contain the species

and minerals important to the study? • Is the thermodynamic data sufficiently accurate? • Can activity coefficients by calculated accurately? • Are the assumptions regarding equilibrium

appropriate? • Is our conceptual model for the reaction process

correct?

Components of GWB

GWB Modules

• Main Components: – React: Main program for calculations – Act2: Used to make activity-activity diagrams – Tact: Used to make temperature-activity diagrams – Rxn: Automatic reaction balancing

• Other components: – SpecE8: React with some options disabled – Gtplot: Main plotting routine for React – GSS: Spreadsheet for geochemical data