EXAMPLES
TITLE Example 2.--Temperature dependence of solubility of gypsum and anhydrite SOLUTION 1 Pure water pH 7.0 temp 25.0 EQUILIBRIUM_PHASES 1 Gypsum 0.0 1.0 Anhydrite 0.0 1.0 REACTION_TEMPERATURE 1 25.0 75.0 in 51 steps SELECTED_OUTPUT -file ex2.pun -si anhydrite gypsum ENDA set of 51 temperatures is specified in the REACTION_TEMPERATURE data block. The input data specify that for every degree of temperature, beginning at 25oC and ending at 75oC, the phases defined by EQUILIBRIUM_PHASES (gypsum and anhydrite) will react to attain equilibrium, if possible, or until both phases are completely dissolved. Finally, SELECTED_OUTPUT is used to write the saturation indices for gypsum and anhydrite to the file ex2.pun after each calculation. This file was then used to generate figure 1.
The results of the initial solution calculation and the first reaction step are shown in table 6. The distribution of species for pure water is shown under the heading "Beginning of initial solution calculations". The equilibration of the system with the given amounts of gypsum and anhydrite at 25oC is the first reaction step, which is displayed after the heading "Beginning of reaction calculations". Immediately following this heading, the reaction step is identified, followed by a list of the identity of the keyword data used in the calculation. In this example, the solution composition stored as number 1, the pure-phase assemblage stored as number 1, and the reaction temperatures stored as number 1 are used in the calculation. Conceptually, the solution and the pure phases are put together in a beaker, which is regulated to 25oC, and allowed to react to system equilibrium.
Under the subheading "Phase assemblage", the saturation indices and amounts of each of the phases defined by EQUILIBRIUM_PHASES are listed. In the first reaction step, the final phase assemblage contains no anhydrite, which is undersaturated with respect to the solution (saturation index equals -0.22), and 1.985 mol of gypsum, which is in equilibrium with the solution (saturation index equals 0.0). All of the anhydrite has dissolved and most of the calcium and sulfate have reprecipitated as gypsum. The "Solution composition" indicates that 15.67 mmol/kg water of calcium and sulfate remain in solution, which defines the solubility of gypsum in pure water. However, the total number of moles of each constituent in the aqueous phase is only 15.11 because the mass of water is only 0.9645 kg ("Description of solution"). In precipitating gypsum (CaSO4.2H2O), water has been removed from solution. Thus, the mass of solvent water is not constant in reaction calculations as it was in PHREEQE; reactions and waters of hydration in dissolving and precipitating phases may increase or decrease the mass of solvent water.
The saturation indices for all of the reaction steps are plotted in figure 1. In each step, pure water was reacted with the phases at a different temperature (the reactions are not cumulative). The default database for PHREEQC indicates that gypsum is the stable phase (saturation index equals 0.0) at temperatures below about 57oC; above this temperature, anhydrite is the stable phase.
------------------ Reading data base. ------------------ SOLUTION_SPECIES SOLUTION_MASTER_SPECIES PHASES EXCHANGE_MASTER_SPECIES EXCHANGE_SPECIES SURFACE_MASTER_SPECIES SURFACE_SPECIES END ------------------------------------ Reading input data for simulation 1. ------------------------------------ TITLE Example 2.--Temperature dependence of solubility of gypsum and anhydrite SOLUTION 1 Pure water pH 7.0 temp 25.0 EQUILIBRIUM_PHASES 1 Gypsum 0.0 1.0 Anhydrite 0.0 1.0 REACTION_TEMPERATURE 1 25.0 75.0 in 51 steps SELECTED_OUTPUT file ex2.pun si anhydrite gypsum END ----- TITLE ----- Example 2.--Temperature dependence of solubility of gypsum and anhydrite ------------------------------------------- Beginning of initial solution calculations. ------------------------------------------- Initial solution 1. Pure water -----------------------------Solution composition------------------------------ Elements Molality Moles Pure water ----------------------------Description of solution---------------------------- pH = 7.000 pe = 4.000 Activity of water = 1.000 Ionic strength = 1.001e-07 Mass of water (kg) = 1.000e+00 Total alkalinity (eq/kg) = 1.082e-10 Total carbon (mol/kg) = 0.000e+00 Total CO2 (mol/kg) = 0.000e+00 Temperature (deg C) = 25.000 Electrical balance (eq) = -1.082e-10 Iterations = 2 Total H = 1.110124e+02 Total O = 5.550622e+01 ----------------------------Distribution of species---------------------------- Log Log Log Species Molality Activity Molality Activity Gamma OH- 1.001e-07 1.001e-07 -6.999 -7.000 0.000 H+ 1.000e-07 1.000e-07 -7.000 -7.000 0.000 H2O 5.551e+01 1.000e+00 0.000 0.000 0.000 H(0) 1.416e-25 H2 7.079e-26 7.079e-26 -25.150 -25.150 0.000 O(0) 0.000e+00 O2 0.000e+00 0.000e+00 -42.080 -42.080 0.000 ------------------------------Saturation indices------------------------------- Phase SI log IAP log KT H2(g) -22.00 -22.00 0.00 H2 O2(g) -39.12 44.00 83.12 O2 ----------------------------------- Beginning of reaction calculations. ----------------------------------- Reaction step 1. Using solution 1. Pure water Using pure phase assemblage 1. Using temperature 1. 25.00 is current temperature. -------------------------------Phase assemblage-------------------------------- Moles in assemblage Phase SI log IAP log KT Initial Final Delta Anhydrite -0.22 -4.58 -4.36 1.000e+00 -1.000e+00 Gypsum 0.00 -4.58 -4.58 1.000e+00 1.985e+00 9.849e-01 -----------------------------Solution composition------------------------------ Elements Molality Moles Ca 1.567e-02 1.511e-02 S 1.567e-02 1.511e-02 ----------------------------Description of solution---------------------------- pH = 7.062 Charge balance pe = 4.111 Adjusted to redox equilibrium Activity of water = 1.000 Ionic strength = 4.190e-02 Mass of water (kg) = 9.645e-01 Total alkalinity (eq/kg) = 1.122e-10 Total carbon (mol/kg) = 0.000e+00 Total CO2 (mol/kg) = 0.000e+00 Temperature (deg C) = 25.000 Electrical balance (eq) = -1.082e-10 Iterations = 13 Total H = 1.070729e+02 Total O = 5.359687e+01 ----------------------------Distribution of species---------------------------- Log Log Log Species Molality Activity Molality Activity Gamma OH- 1.403e-07 1.155e-07 -6.853 -6.938 -0.085 H+ 1.007e-07 8.666e-08 -6.997 -7.062 -0.065 H2O 5.551e+01 9.996e-01 0.000 0.000 0.000 Ca 1.567e-02 Ca+2 1.047e-02 5.176e-03 -1.980 -2.286 -0.306 CaSO4 5.191e-03 5.242e-03 -2.285 -2.281 0.004 CaOH+ 1.192e-08 9.909e-09 -7.924 -8.004 -0.080 H(0) 6.322e-26 H2 3.161e-26 3.192e-26 -25.500 -25.496 0.004 O(0) 0.000e+00 O2 0.000e+00 0.000e+00 -41.393 -41.388 0.004 S(-2) 0.000e+00 HS- 0.000e+00 0.000e+00 -65.005 -65.090 -0.085 H2S 0.000e+00 0.000e+00 -65.162 -65.158 0.004 S-2 0.000e+00 0.000e+00 -70.633 -70.946 -0.313 S(6) 1.567e-02 SO4-2 1.047e-02 5.075e-03 -1.980 -2.295 -0.315 CaSO4 5.191e-03 5.242e-03 -2.285 -2.281 0.004 HSO4- 5.145e-08 4.276e-08 -7.289 -7.369 -0.080 ------------------------------Saturation indices------------------------------- Phase SI log IAP log KT Anhydrite -0.22 -4.58 -4.36 CaSO4 Gypsum 0.00 -4.58 -4.58 CaSO4:2H2O H2(g) -22.35 -22.35 0.00 H2 H2S(g) -64.16 -105.80 -41.64 H2S O2(g) -38.43 44.69 83.12 O2 Sulfur -47.69 -83.46 -35.76 S