MINTEQA2 is a geochemical equilibrium speciation model that can solve a broad range of chemical equilibrium problems in an environmental setting. MINTEQA2 is capable of calculating equilibrium aqueous speciation, adsorption, gas phase partitioning, solid phase saturation states, and precipitation-dissolution of metals. The model contains an extensive thermodynamic database and seven different algorithms for performing its calculations. Whereas the MINTEQA2 program does the calculations, PRODEFA2, the Problem Definition Program for MINTEQA2, is an interactive program that assists the user in developing and building an input file for the MINTEQA2 model.

These two programs are completely separate, however, both programs use the same thermodynamic database files. These files contain the pre-defined set of components and the reactions in which those components serve as reactants. Because the model has its own database, the primary information that must be conveyed through the input file to model a particular system is the total dissolved concentration or fixed activity of each component of the system. There are four choices for units of concentrations for the input data; molal, mg/l, ppm, and meq/l. The user may enter portions of the data in different units, however, regardless of the units chosen, the output data are always expressed in molal. It is also possible for the user to insist that certain conditions prevail at equilibrium. For example, the equilibrium partial pressure of a gas, the solution pH, pE, or precipitation of solids may all be specified.

The sequence of operation for the two programs is generally PRODEFA2 followed by MINTEQA2. At start-up, PRODEFA2 prompts for the name of the MINTEQA2 input file to be created. It also prompts for the name of an existing MINTEQA2 input file to be used as an optional seed file (or template). After inquiring for file names, PRODEFA2 goes automatically to Edit Level I to display the settings and parameters of the default problem or of the seed problem if one has been specified. PRODEFA2 is divided into four distinct sections called edit levels. Upon the users acceptance of the Edit Level I settings, a main menu screen is displayed from which the user may choose to enter any of the four edit levels.

At start-up, MINTEQA2 prompts for the name of the MINTEQA2 input file on which to perform the calculations and of a name for the output file that will contain the derived results. MINTEQA2 solves the equilibrium problem iteratively by computing mole balances from estimates of component activities, that is, activities of the free species represented by the components. Hence, it is necessary to provide an initial estimate or guess for the activity of each component. However, PRODEFA2 makes this guess automatically for every component as equal to the component total dissolved concentration but also provides the means for the user to change the guess.

PROGRAM EXECUTION

(for MINTEQA2 Version 3.11)

Suggestions for running Version 4.0 can be found on the MINTEQA Help Page.

PRODEFA2

To begin execution of PRODEFA2, the interactive program that will develop and build an input file for the MINTEQA2 model, perform the following steps: (Note: indicates a press of the return key)

• Exit windows to get to the DOS prompt or select the MSDOS prompt from the Programs file in WINDOWS.
• Type cd\ at the C:\WINDOWS> prompt and hit Enter.
• From the C:\> prompt, type cd MINTEQA2 and hit Enter.
• From the C:>MINTEQA2> prompt, type PRODEFA2 and hit Enter.
• PRODEFA2 starts and prompts for the name of the MINTEQA2 input file to be created, any acceptable DOS file name can be used (a name up to eight characters in length followed by an optional extension up to three characters in length, example SkipLab1.inp)
• PRODEFA2 then prompts for a seed file. Type the appropriate file name or hit Enter to create a new file. (Hit Enter for Lab Prob 1 and others which don't require a seed file.)
• After inquiring for file names, PRODEFA2 goes automatically to Edit Level I to display the settings and parameters of the default problem or the seed problem represented by a seed file if one has been specified. Upon the users acceptance of the Edit Level I settings, a main menu screen is displayed from which the user may choose to enter any of the four edit levels or to Exit the program.

MINTEQA2

Once you have created an input file in Prodefa, you may run MINTEQA. To execute the MINTEQA2 program follow these steps:

• Type minrun at the C:\MINTEQA2> prompt.
• Answer the filename prompts by entering the input file name which was previously created in PRODEFA2. Then enter another filename for the output file, example; SkipLab1.out
• Edit your output file to view your problems. At the C:\MINTEQA2> prompt, type edit then the output file name.

LABORATORY EXERCISES

Each of the following problems is followed by a "how to" step by step description. The latter problems do not contain the basic information learned in the beginning problems, and also some self-explanatory steps are not included.

Laboratory Problem 1

Pb(OH)₂ is a species in suspension in fresh natural water at pH 8.0.

NTA is present and at pH 8.0 is predominantly HT^2- ion. Consider the reaction:

Pb(OH)₂(s) + HT^2- <---> PbT^- + OH^- + H₂O

Define Pb(OH)₂ as an infinite solid and NTA at a concentration of 25 mg/L. Determine the ratio of [PbT^-]/[HT^2-]. Notice the difference in concentrations of the NTA ions.

• Run PRODEFA2
• In Edit Level I
  - Choose option #4 to change molal to mg/L
  - Choose option #8 and allow over saturated solids to precipitate. Then select #1 to allow this precipitation only after the final answer is reached
  - Choose option #10 to set the method used to compute activity coefficients to the Debye-Huckel (this will be used in all of the following problems); for a description of the method see the appendix under Activity Coefficient Corrections
  - Choose option #12 to specify the equilibrium pH. Set the pH to 8.00
• In Edit Level II
  - Select option #6, then choose #1 to search the thermodynamic database for the appropriate Pb species; see the appendix for MINTEQA2's definitions of solids
  (1) The ID number for the species is not known
  (2) Pb(OH)₂ is in group #20 (Oxide or Hydroxide)
  (3) Choose the letter 'P' for the first letter of the cation
  (4) The desired equation is #5
  (5) Do not change the log K value or the enthalpy value. Select R to return to the main menu.
  - Choose option #1, to specify the total dissolved concentration of NTA: Choose #1 to select the Total Dissolved Concentration. Enter N as the first letter of the cation. Select #8 NTA+3. Enter 25 mg/l. Then hit Enter to terminate component entry. Return to the previous Options Menu. Then Return to the Main Menu.
• Exit PRODEFA2
• Run MINTEQA2 as in the execution steps listed above for the program.
• Edit the output file; Part 3 lists the concentrations of PbNTA and NTAH.
• The MINTEQA2 value for [PbT^-]/[HT^2-] is 27/1 (value in notes is 20/1) (NOTE: You will not see any data that specifically states the 27/1 ratio. You must compute that yourself.)

Laboratory Problem 2

Consider the same reaction as in Problem 1. However now consider the effect of changing the pH from 8.0 to 7.0 to 6.0. Determine the ratio of [PbT^-]/[HT^2-] at each pH level. Why does changing the pH have this effect?

• Open Prodefa2. Enter a new input file name (eg. LabProb2.inp).
• Use Problem 1 input file as a seed file (eg. SkipLab1.inp)
• Accept all settings in Edit Level I.
• In Edit Level IV, to sweep the pH range
  - Specify the total H⁺concentration (First letter of the sweep component is 'H') by choosing the fixed equilibrium activity. Choose Option #1 to" Specify that the total concentration or fixed log activity..." Enter H as the first letter of the component. Then select #5 H+.
  - Choose Option #2 so the values will represent "Fixed Equilbrium Activity eg. pH, pE) Choose three values (8.0, 7.0, and 6.0) and specify them explicitly.
• Hit R to accept all settings. Return to the Main Menu.
  
• Exit PRODEFA2 and run MINTEQA2
• Edit the output file. Again look at part 3 of the output file, but for each individual value of pH
• [PbT^-]/[HT_2-] at pH=8.0 is 27/1; at pH =7.0 is 267/1; at pH=6.0 is 2570/1

Laboratory Problem 3

Use the same pH condition (pH=8.0) and NTA concentration (NTA = 25mg/L) as in Problem 1, but now define Pb(OH)₂ as a finite solid at a concentration of 1.0x10^-4 mol/L.

Note that the Pb(OH)² dissolves and that MINTEQA2 then recalculates the equilibrium.

• Do not use Problem 1 as a seed file
• Do the same steps as in Problem 1 for Edit Level I and for setting NTA concentration
  - With the exception of option #8 in Edit Level I; allow over saturated solid to precipitate each time a mineral precipitates or dissolves (to understand this effect see the appendix under General Description of MINTEQA2)
  
• After the previous step, go back to Edit Level I, and change option #4 back to molal
• In Edit Level II
  - Choose option #7 and define Pb(OH)₂ as a finite solid
  - Go through the same steps as an infinite solid, but enter the concentration when prompted
  
• In the output file after part 6, the saturation indices, MINTEQA2 should say that Pb(OH)₂ devolves then a recalculation of the equilibrium should follow

Laboratory Problem 4

Consider a second example with the chelator NTA by doing the same examination using lead carbonate.

PbCO₃(s) + HT^2- <---> PbT^- + HCO₃^-

PbCO₃ is in equilibrium with the CO₂, HCO₃, CO₃^2- system and the NTA species. At pH 7.0 the HT^- species is again the dominant species of NTA. Given a concentration of NTA of 25mg/L and that in natural waters HCO₃- frequently has a concentration of 1.0 x 10^-3 M/L, determine the ratio of lead complexed as PbT^- to uncomplexed free NTA of the form HT^-.

• In Edit Level I, do the same as Problem 1 (set pH to 7.0) also choose option #9 and set the maximum number of iterations to 100
• In Edit Level II
  - Enter the NTA concentration
  - Choose to define an infinite solid
  - Group #50 is the carbonates
  - The desired reaction between Pb²⁺and CO₃^2- is reaction #1
  - In Edit Level I, change mg/L back to molal
  
• In Edit Level III, choose CO₃^2- and set the total concentration to 0.001M

• The MINTEQA2 value for [PbT^-]/[HT^2-] is 32/1 (In the notes it is 41/1)

Laboratory Problem 5

Calcium, one of the most common ions, also forms the NTA complex from the dominant form of NTA (HT^2-), given a pH of 7.0. The thermodynamic databases in MINTEQA2, however, do not contain the following reaction:

Ca²⁺ + T3- <---> CaT^-       logK= 8.17

and therefore it needs to be added in PRODEFA2 under Edit Level II (Molecular weight of complex = 228.14g/mol) so that MINTEQA2 can determine the desired reaction;

Ca²⁺ + HT^2- <---> CaT^- + H⁺

Given a concentration of Ca²⁺ of 1.0x10^-3 mol/L and NTA of 25mg/L, determine the ratio of [CaT^-]/[HT^2-].

• In Edit Level I, set options #8 and #10 as in Problem 1, and also set the pH
• In Edit Level II, choose to specify an aqueous species not in the database
  - Choose a conventional reaction
  - Search the thermodynamic database for the species you want
  - Ca²⁺ for the cation, NTA3^- for the other major ion; note that the reaction is not in MINTEQA2 (choose 0), but define a new species comprised of the specified constituents and accept the ID #
  - Enter the name for the aqueous species as CaT^-
  - Enter the following parameters:
  (1) Charge on the species = -1.0
  (2) Debye-Huckel values = 0.0
  (3) Alkalinity factor = 0.0
  (4) Molecular weight = 228.14
  (5) Stoichiometry of NTA^-3 and Ca⁺² are both 1.0
  (6) No other components in the reaction
  (7) Log K value for formation of species is = 8.17
  (8) Enthalpy for formation = 0.0

• In Edit Level III, change the Ca²⁺ concentration to 0.001 molal
• In Edit Level I, change molal to mg/L then in Edit Level III enter the NTA concentration
• The MINTEQA2 value for [CaT^-]/[HT^2-] is 43/1 (In the notes it is 76/1)

Laboratory Problem 6

Consider the same situation in Problem 5, but now also consider the influence that lead carbonate would have on the reaction.

Ca²⁺ + HT^2- <---> CaT^- + H⁺

and PbCO₃(s) + HT^2- <---> PbT^- + HCO₃^-

Specifying the PbCO₃(s) as a finite solid with a given concentration of 1.0x10^-4 mol/L and the carbonate component as 0.001 mol/L. Describe the competition between the lead and the calcium to complex with NTA by determining the ratio of [PbT^-]/[CaT^-].

• Use the Problem 5 input file as a seed file
• In Edit Level II, define PbCO₃ as a finite solid and set the concentration
  - Remember to first check the units in Edit Level I
• In Edit Level III, set the CO₃^-2 concentration
• The MINTEQA2 value for [PbT^-]/[CaT^-] is 0.74 (The value in the notes is 0.5)

  
  

  Laboratory Problem 7

  To begin to appreciate just how complex a system can be and the need for mathematical models to assist in combining all of the competing equilibrium simultaneously, use Problem 6 as a seed file, but now add Pb(OH)₂ as an infinite solid to the system at a concentration of:
  1.0x10^-4 mol/L and also sweep the pH from 8.0 to 7.0 to 6.0.

  Examine the percent change of NTA complexing with lead and calcium through the pH sweep and explain if this is what you would have predicted considering only the independent general reactions:

  Pb(OH)₂(s) + HT^2- <---> PbT^- + OH^- + H₂O

  and PbCO₃(s) + HT^2- <---> PbT^- + HCO₃^-

  and Ca²⁺ + HT^2- <---> CaT^- + H⁺

  
  

  MINTEQA2 determines % NTA binding to lead and calcium at the following pH values:

  pH	Pb	Ca
  8.0	5.4	94.4
  7.0	44.0	54.7
  6.0	93.6	5.3

  
  

  
  

  Appendix

  (Excerpted from the MINTEQA2 User's Guide)

  Infinite Solid Definition

  An infinite solid is one that is not subject to complete dissolution. As such, the solution is required to be at equilibrium with the infinite solid.

  Finite Solid Definition

  A finite solid is presumed present at equilibrium, however, unlike the infinite solid above, a solid designated as finite may dissolve if equilibrium conditions warrant. It is entered in the same manner as the infinite solid, with the exception that you may specify an amount present (in moles present in one liter of solution). The amount can be entered as zero because you really do not know how much is present at equilibrium, if any; it is MINTEQA2's job to figure that out! In theory, it doesn't matter to MINTEQA2 whether the system totals for various components are specified at the outset as all dissolved or all bound in precipitated solid(s) of given amount(s). MINTEQA2 will shift mass from the dissolved to precipitated phases or vice versa as required by equilibrium.
  

  
  

  Possible Solid Definition

  Possible solids are solids that are permitted to precipitate if equilibrium conditions warrant. All database solids become possible solids when the precipitation flag in Edit Level I option 8 is so set. In that case there is no need for this option. However, the other setting of the Edit Level I flag dictates that all solids be EXCLUDED SPECIES except those explicitly designated as POSSIBLE SOLIDS through this option. Note that within MINTEQA2, when a possible solid precipitates it is re-defined as a finite solid. Conversely, when a finite solid dissolves, it is re-defined as a possible solid.
  

  
  

  General Description of MINTEQA2 Problem Development using Iterations

  To solve the chemical equilibrium problem, MINTEQA2 uses an initial guess for the activity of each component to calculate the concentration of each species according to mass action expressions written in terms of component activities. The total mass of each component is then calculated from the concentrations of every species containing that component. The calculated total mass for each component is then compared with the known input total mass for each component. If the calculated total mass and the known input total mass for any component differ by more than a pre-set tolerance level, a new estimate of the component activity is made and the entire procedure is repeated. The aqueous phase equilibrium composition is that set of species concentrations which gives a mass imbalance less than the tolerance level for every component.

  After equilibrating the aqueous phase, MINTEQA2 computes the saturation index (SI) for each possible solid with respect to the solution. The solid with the most positive SI is allowed to precipitate by depleting the dissolved concentrations of those components comprising the solid in accordance with the known stoichiometry of each component. The reverse process occurs if an existing solid is found to be undersaturated with respect to the solution. In either case, it is necessary to re-equilibrate the solution after mass has been added to or depleted from the aqueous phase. Thus the aqueous solution is re-equilibrated just as before except with one less degree of freedom if precipitation has occurred or one more if dissolution has occurred. The entire computational loop of iterating to equilibrium, checking for precipitation or dissolution, and shifting mass from the aqueous to the solid phase or vice versa is repeated until equilibrium is achieved and there are no oversaturated possible solids and no undersaturated existing solids.
  

  
  

  Activity Coefficient Corrections of Equilibrium Constants

  Activity coefficients for all species are functions of solution ionic strength (I) and vary as species distributions alter the ionic strength. Unless a fixed ionic strength is specified, successive sets of activity coefficients are calculated for all solution species with each iteration. These are used to generate corrected values of the equilibrium constants that appear in the mole balance expressions. Initial activity guesses for the input components are provided in the input file for a given problem. These initial component activity guesses are used to "crudely" estimate the concentrations of each dissolved species so that the solution ionic strength can be calculated. Each succeeding iteration provides improved estimates of species concentrations and activity corrections.

  The solution ionic strength is used in either the modified Debye-HÄckel equation or the Davies equation to calculate activity coefficients (g) for all charged species. If the user selects the modified Debye-Huckel equation, it will be used for those species that have the necessary parameters in the database. For any species lacking the necessary parameters, the Davies equation will be used to estimate the activity coefficient for that species. If the user selects the Davies equation at the outset, it will be used throughout the problem because it requires no species-specific data other than charge. The activity coefficients are used to compute adjusted equilibrium constants.

  The modified Debye-Huckel expression used to calculate the activity coefficients is:

  

  Where:

  A_d and B_d = constants that depend on the dielectric constant and temperature

  Z_i = the charge on each species i

  a_i = ion size parameter

  b_i = ion specific parameter that accounts for the decrease in solvent concentration in concentrated solutions

  I = solution ionic strength

  
  

  The ionic strength (I) is calculated from

  

  Where:

  C_i = concentration of ion species i

  m = number of charged species present in the solution

  Z_i = charge on species i

  The modified Debye-Huckel relation above is used only when the parameters a_i and b_i are available in the database. The current database contains a_i and b_i parameters for many major inorganic ion species and a few important trace metals. Where data are not available or if the user selects it, the Davies equation will be used.

  The Davies equation as implemented in MINTEQA2 is:

  

  in which the variables are defined as in the Debye-Huckel expression.

  Users are cautioned that the activity correction models presented here are generally not intended for use at ionic strengths greater than 0.5. At higher ionic strengths, as in marine conditions (ionic strength = 0.7 M), these correction equations may still provide usable results; this should be verified for the specific system to be modeled. Alternatively, one should consider adding expanded versions of the Debye-Huckel equation, which include terms to account for ion interactions occurring in more concentrated solutions.

  Successive sets of log K values that reflect the temperature corrections (van't Hoff) and activity coefficient corrections (Debye-Huckel or Davies) above are computed and substituted into the mole balance expressions. If no solids are specified, the Jacobian matrix relating changes in mass balance to changes in component activities is used to calculate that set of component activities that will simultaneously minimize the mass imbalance for all species. The procedure used is an iterative Gaussian elimination and back substitution with a convergence test following each iteration.
  

  
  Homework to be handed in for credit in class:
  

  MINTEQA2 Laboratory Exercise (Homework)

  FRAQUIL is a medium that has been developed for conducting experiments on the effect of trace metals on the growth of algae. Algae cells (and other microorganisms) are capable of adsorbing different metals from a variety of solution environments. This biosorption phenomena has been the subject of increased research activity for a variety of reasons, including the use of biomass for metal reclamation and remediation of industrial streams, toxic trace metal accumulation in the food chain, and analytical applications for trace metal analysis.

  Chelating agents are common potential water pollutants. They are of concern primary because of their ability to solubilize heavy metals. EDTA (ethylenediaminetetraacetate) or other chelators can be added to the FRAQUIL medium to determine the effect of the presence of a chelator on the growth of algae and their potential for biosorption. MINTEQA2 can be used to calculate the speciation of the metals in the medium.

  The composition of FRAQUIL is approximately:

  Ca2+	2.50	E-4
  CO32-	1.50	E-4
  Cl1-	5.01	E-4
  Cu2+	9.97	E-10
  Fe3+	4.51	E-7
  Mg2+	1.50	E-4
  Mn2+	2.30	E-8
  NO31-	1.00	E-4
  PO43-	1.00	E-5
  K1+	2.00	E-5
  H4SiO4	1.25	E-4
  Na1+	2.85	E-4
  SO42-	1.50	E-4
  Zn2+	4.00	E-9
  H20
  The pH is fixed at 7.3
  EDTA4-	5.00	E-6

  
  

  
  MINTEQA2 can be used to aid in the determination of what is taking place in the system. Use MINTEQA2 to answer the following questions about the FRAQUIL and EDTA environment. Which metals are bound with EDTA? What are the free concentrations of these metals relative to their total concentrations? What controls the free concentration of EDTA? In doing this problem neglect redox reactions and atmospheric CO₂. Also allow any solids to precipitate.
  

  
  

  Homework Instructions:

  1. Work through the practice problems using Minentqa2 and them do the above homework assignment to be handed in.

  2. After you have run the exercises please hand in the final results pages (only 1 to 5 pages are necessary to provided the final answers to this exercise) of the print out that shows the ratio of the elements in solution or complexed.

  3. Provide a one-paragraph analysis and description or narrative of what this means and what you learned about this environmental water system.

  4. Hand in both of these assignments before the mid term exam to receive full credit for the exercise.

  5. Please do not forget to put your name on the printout and on your narrative description.
  

  
  
  

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