Adjusting the Activity of water

[sharmilan]

Dear Phreeqc experts,

For my crystallisation model, I need to relate the 'Activity of water' in terms of relative humidity. Therefore, could you guide me on whether there is any possibility to assign a value for the 'Activity of water' in PHREEQC calculations?

Thank you
sharmilan.

[dlparkhurst]

To a first approximation, activity of water is proportional to the mole fraction of water in a solution--that is, moles of water divided by the moles of water plus moles of solute species. The pitzer model has a more complicated calculation involving the osmotic coefficient.

You can add or remove water to achieve a specified activity of water. You must have some solutes, and they will be concentrated or diluted to a specified activity of water. The following is an example calculation. It produces an activity of water of approximately 0.7. Using pitzer.dat (osmotic coefficient approach) or phreeqc.dat (mole fraction approach),  the concentrations approach 8 mol/kgw, which is approaching Phreeqc's limit of numerical stability limits. Higher concentrations make it more likely that Phreeqc will fail to converge to a numerical solution.

Code: [Select]

PHASES
H2O
H2O = H2O
END
SOLUTION
-units mmol/kgw
Na 1
Cl 1
END
USE solution 1
EQUILIBRIUM_PHASES
#H2O    -0.022 100 # ~0.95 activity of water
#H2O    -0.046 100 # ~0.9 activity of water
#H2O    -0.1 100 # ~0.8 activity of water
H2O    -0.155 100 # ~0.7 activity of water
END


[sharmilan]

Dear dlparkhurst

Thank you for your reply. I have attached the results of the PHREEQC calculation for your example. For my RTM model, I need to retrieve the mass of water in the solution in the time step of calculation. However, your mentioned method indicates that during chemical equilibrium, the mass of water (5.550e+01 mol) in solution is transferred into H2O in the phase. Consequently, only 1.133e-04 kg of water is available in the solution. Therefore, could you advise me....shall I adjust the value of the activity of water (-ah2o) using the SOLUTION_MODIFY key command?

Thank you,
S.Sharmilan

Code: [Select]

------------------------------------
Reading input data for simulation 2.
------------------------------------

USE solution 1
EQUILIBRIUM_PHASES
H2O    -0.155 100 # ~0.7 activity of water
END
-----------------------------------------
Beginning of batch-reaction calculations.
-----------------------------------------

Reaction step 1.

Using solution 1.
Using pure phase assemblage 1.

-------------------------------Phase assemblage--------------------------------

                                                      Moles in assemblage
Phase               SI  log IAP  log K(T, P)   Initial       Final       Delta

H2O              -0.16    -0.16      0.00    1.000e+02   1.555e+02   5.550e+01

-----------------------------Solution composition------------------------------

Elements           Molality       Moles

Cl                8.828e+00   1.000e-03
Na                8.828e+00   1.000e-03

----------------------------Description of solution----------------------------

                                       pH  =   8.914      Charge balance
                                       pe  =  10.209      Adjusted to redox equilibrium
      Specific Conductance (µS/cm,  25°C)  = 315744
                          Density (g/cm³)  =   1.26361
                               Volume (L)  =   0.00014
                        Activity of water  =   0.700
                 Ionic strength (mol/kgw)  =   8.828e+00
                       Mass of water (kg)  =   1.133e-04
                 Total alkalinity (eq/kg)  =   1.281e-05
                         Temperature (°C)  =  25.00
                  Electrical balance (eq)  =  -1.449e-09
 Percent error, 100*(Cat-|An|)/(Cat+|An|)  =  -0.00
                               Iterations  =  58
                                  Total H  = 1.257480e-02
                                  Total O  = 6.287399e-03

----------------------------Distribution of species----------------------------

                                               Log       Log       Log    mole V
   Species          Molality    Activity  Molality  Activity     Gamma    cm³/mol

   OH-             1.281e-05   5.812e-06    -4.893    -5.236    -0.343      8.29
   H+              1.741e-09   1.219e-09    -8.759    -8.914    -0.155      0.00
   H2O             5.551e+01   6.998e-01     1.744    -0.155     0.000     18.07
Cl            8.828e+00
   Cl-             8.828e+00   5.786e+00     0.946     0.762    -0.183     20.46
H(0)          0.000e+00
   H2              0.000e+00   0.000e+00   -42.278   -41.396     0.883     28.61
Na            8.828e+00
   Na+             8.828e+00   2.321e+01     0.946     1.366     0.420      1.82
   NaOH            1.766e-15   1.349e-14   -14.753   -13.870     0.883     (0) 
O(0)          3.306e-11
   O2              1.653e-11   1.262e-10   -10.782    -9.899     0.883     30.40

------------------------------Saturation indices-------------------------------

  Phase               SI** log IAP   log K(298 K,   1 atm)

  H2(g)           -38.29    -41.40   -3.10  H2
  H2O              -0.16     -0.16    0.00  H2O
  H2O(g)           -1.66     -0.16    1.50  H2O
  Halite            0.56      2.13    1.57  NaCl
  O2(g)            -7.01     -9.90   -2.89  O2

**For a gas, SI = log10(fugacity). Fugacity = pressure * phi / 1 atm.
  For ideal gases, phi = 1.

------------------
End of simulation.
------------------

------------------------------------
Reading input data for simulation 3.
------------------------------------

-------------------------------
End of Run after 0.172 Seconds.
-------------------------------

[dlparkhurst]

You can't adjust the activity of water independently. Adjusting -ah2o will only change the initial estimate of the activity of water in the equilibrium calculation. The final activity of water will be the same regardless of the initial estimate.

The activity of water is a consequence of solute concentrations. If you want a lower specified activity of water in the solution, the only way to do that is to remove water to increase concentrations, which lowers the activity of water.

I think for your modeling, you must consider a GAS_PHASE. If you have a gas phase that has a given partial pressure of H2O(g), the solution will gain or lose H2O, and the gas will lose or gain H2O(g) to come to equilibrium. If the gas volume is small, the P(H2O(g)) will approach equilibrium with the a(H2O) of the solution, which usually will be somewhere near 100% relative humidity. If the gas volume is large and relative humidity is low, the solution will evaporate to high concentration. In a very dynamic system, you may have to treat the transfer of water between gas and solution as a KINETICS process, where the rate depends on the difference from equilibrium between the gas and solution.