GES 166, Soil Chemistry

Winter, 2002

 

MINTEQA2 Tutorial #1

 

In this tutorial you will become familiar with the equilibrium speciation program MINTEQA2. Subsequent tutorials will expand on what we learn today, so time invested now will make future tutorials easier. This program (or others like it) is an indispensable tool for the aqueous geochemist because we can use the program to make calculations that would otherwise not be possible using a calculator.

 

Description of MINTEQ

 

MINTEQA2 is a geochemical equilibrium speciation model for dilute aqueous systems. The model can be used to calculate the mass distribution between the dissolved, adsorbed, and multiple solid phases under a variety of conditions including a gas phase with constant partial pressure. Seven adsorption models are available in MINTEQA2. Input data consist of total dissolved concentrations for the components of interest, and optionally, parameters such as pH, pE, or the partial pressure 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 at equilibrium with the solution and either 1) subject to dissolution if conditions warrant, or 2) not subject to complete dissolution. 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 user can also easily modify the standard database.

 

Computer codes of this type are generally composed of three parts:

  1. Input File: The input file that contains the data input by the user. Typically, this file contains dissolved (i.e. Ca concentrations, pH, temperature) and solid phase (i.e. minerals, sorption sites) information for a water sample.
  2. Database file(s): This file contains the thermodynamic constants that govern the processes of interest (i.e. complexation constants, mineral solubilities, activity constants) which will be used to conduct calculations.
  3. Algorithm or Executable Files: These files contain the workings of the code, which solve the specified problem (usually using an iterative numerical approach) within the constraints imposed by the Database files and the information in the Input file.
  4. Output File: This file contains the results of the calculations performed by the Algorithm Files.

 

For these tutorials we primarily limit our manipulations to the Input File, however there are times when we will want to change or add to the Database files.

 

Where to Get a Copy of MINTEQ

MINTEQ was developed by the U.S. E.P.A. and is available free of charge at:

http://www.epa.gov/ceampubl/softwdos.htm

A copy of the users manual can also be downloaded at this site.

The Swedish research councils VR and MISTRA, have developed a windows based version of the program (called Visual MINTEQ) which is distributed via Internet free of charge:

http://www.lwr.kth.se/english/OurSoftware/Vminteq/

This windows interface assists the user in creating an Input File, changing the Database Files and examining the Output File data. For this tutorial we will use this windows based version of MINTEQ.

 

Installation

For this tutorial, this step will not be necessary as the program is already installed on the lab computers.

 

First we need to download the program from the web site.

Go to the web site above and download the zip file: vminsetup.zip, save to your folder.

Now extract the zip file.

Double click on setup.exe and follow the directions to install the program.

Note: You may find it necessary to use the default file locations specified by the setup program to get Visual Minteq to work.

If all goes well, you should now be able to double click on the vminteq.exe file and the visual minteq screen will open.

 

Getting Started

 

Log on to the computer using:

 

User Name: ges166

Password: 166ges02

 

Start the program:

Click: Start, Programs, vminteq

 

The Visual Minteq window should now be open.

 

This is the interface you will use to create you input file. Like most windows programs, there are a series of drop-down menus across the top of the screen. In addition, there are a number of input fields across the middle of the screen.

 

Our task in this tutorial is to determine the activity, the speciation distribution for the aqueous species, and the saturation indices for relevant mineral phases for our water sample. This tutorial will give you the basic skills necessary to complete the homework assignment.

 

Here is the composition of a water sample that we need to enter into our Input File:

 

Ca2+                             49        mg/L

Mg2+                            23        mg/L

Na+                              15        mg/L

K+                                13        mg/L

Cl-                                                    37        mg/L

SO42-                            76        mg/L

pH                               7.3

Alkalinity                    123      mg/L CaCO3

Temperature                15        oC

 

  1. First, select the mg/L concentration unit.
  2. Set the temperature
  3. Now set the pH as ‘Fixed at…’ and specify the value.
  4. Add the components Ca, Mg, Na, K, Cl and SO4 using the ‘Add components’ section and specifying a total concentration (NOT a fixed activity) and clicking the ‘Add to List’ button for each component after entering a concentration.
  5. Add the alkalinity by opening the drop-down menu ‘Parameters’, clicking ‘specify alkalinity’, selecting ‘yes’ to the question ‘do you want to specify inorganic carbon in this problem’, clicking ‘OK’ to the statement ‘When alkalinity is specified, you are not allowed to specify any solids.’ Then select the appropriate units and specify the concentration and click ‘OK.”
  6. Save your file using the ‘File’ bar at the top of the screen. You should create a folder with your name on it so that you do not clutter up the MINTEQ folder with your input and output data.
  7. At this point, you should be ready to go, but to double-check your input, click the ‘View/Edit List’ button. This opens a new view that contains your input data in a table. Compare these values to those you meant to enter; you can edit your values in this view if necessary. Note that H+ has been added to the list for you since it is needed for pH, a value of ‘0’ has been assigned; you do not need to change this. Also, CO3-2 has been added based on your entered alkalinity value. Recall that alkalinity is the sum of the non-protonated carbonate species (as equivalents). MINTEQ has already made the carbonate equilibrium calculation and is expressing the alkalinity as CO3-2 and this value is obviously different from the value you input (it should read 73.74). Once you are convinced that all is well, return to the main menu by clicking the ‘Back to Main Menu’ button.
  8. Resave your input file if you have made any changes.
  9. Now you are ready to launch the Algorithm Files, so click the ‘Run MINTEQ’ button. You should get a message indicating the program ran correctly. There is some chance that if you made a bunch of changes that the program will crash- it is a little buggy- if so you will need to start over (bummer dude).
  10. Click the ‘OK’ button and the program will automatically dump you into the output interface where you can look at the results of your calculations.
  11. First thing, compare your charge difference value (upper right of screen) with your neighbor to be sure that your input was correct. If your values are different, it indicates that someone has an incorrect value or setting in their input file that requires fixing before proceeding.
  12. Once you have a consistent charge balance value, save your output file. You may want to make a note of how you are naming your files to keep track.
  13. OK, so what have you done? MINTEQ has essentially taken your raw concentration data and converted it into activities by using the Davies Equation to calculate activities and iterating over the aqueous complexation constants and mass and charge balance expressions until the all the equations are satisfied. So, check out the activity values, note that they are different from the concentration (recall why?).
  14. Click on the ‘View species distribution’ tab. This table shows the distribution of each component within the various complexes contained in the MINTEQ database. What are the major complexes for Ca? SO4? Some species are almost entirely present as the free species; other species form more complexes. Is there a pattern to this? Why?
  15. Click on the ‘Display saturation indices’ tab. This shows us more calculations that MINTEQ has preformed for us. Using the calculated activities, MINTEQ has searched its mineral database for all the possible mineral phases that can form with the specified species and calculated the ion activity product (IAP) for each mineral. MINTEQ has then compared this value to the Ksp for each mineral (using the saturation indices –SI‑ formulation). So now we can tell whether these minerals are thermodynamically favored to precipitate or dissolve in our water (SI < 0 is undersaturated and is favored to dissolve, SI > 0 is over saturated and favored to precipitate). Note that all the SI values are <0 indicating that our water is undersaturated with respect to these mineral phases. Assuming thermodynamic equilibrium, all of these minerals would dissolve in our water sample. However, recall that no system is at perfect equilibrium and that the database Ksp values are never perfect. Given those uncertainties, there is one mineral in the list that would be considered to be at or near equilibrium within our water sample. What is it? If this mineral is actually in contact with our water is it dissolving or precipitating?
  16. You should output each of these output tables (concentration, species distribution and saturation indices) to Excel using the ‘Options’ bar (next to the ‘File’ bar at the top of the screen).
  17. OK, lets return to the main menu and make a few changes. First, lets increase the Na and Cl concentrations to 1500 and 3700 mg/L, respectively. Remember that editing is easily done using the ‘View/edit list’ button. Run MINTEQ and go to the output file. Comparing the newly calculated values to your old values now in the excel file, how has the activities of Na and Cl changed? How about Ca? Does that make sense? Are there any changes to the SI values?
  18. Now increase the Ca and SO4 concentrations to 4900 and 7600 mg/L, respectively. How does this impact the activities, complexes, and saturation indices?
  19. What would happen if we increased the pH? Decreased the pH?
  20. How about changing the temperature? Compare the impact on the SI for calcite and halite; why does temperature impact these to phases differently?