PHREEQC unable to predict the precipitation of NaCl

[Tameka D]

Hi,

I am very new to PHREEQC and I am in need of some guidance. I am attempting to predict the precipitation of minerals from synthetic urine. To validate my model, I added MgCl2.6H2O to my synthetic urine (see below code). The model predicts the precipitation of Brucite, HAP and Mg3(PO4)2 but in my experiment, NaCl or halite also precipitated. I am unable to determine where the error is in my model. Does anyone have any suggestions?
Aside: I am using a modified wate4qf database.

SOLUTION 1 Synthetic urine
    temp      25
    pH        5.6
    pe        4
    redox     pe
    units     mol/l
    density   1
    Ca        0.004
    Cit       0.002
    Cl        0.1342
    K         0.052
    Mg        0.003
    N(-3)     0.019 as NH4
    Na        0.118
    Ox        0.0001
    P         0.031 as PO4
    S(6)     0.016 as SO4
    Urea      0.42
    -water    0.25 # kg

USE SOLUTION 1

REACTION 1 Addition of Mg source
   MgCl2:6H2O   1
   NaOH      1.64     
   0.27 moles

EQUILIBRIUM_PHASE
   Halite   0   0
   Struvite   0   0
   ACP      0     0
   HAP      0     0
   TCP      0     0
   DCPA      0     0
   DCPD      0     0
   OCP      0     0
   Brucite   0   0
   Mg3(PO4)2   0   0
   MgHPO4:3H2O   0   0

SAVE SOLUTION 2

END

[dlparkhurst]

If Cl and Na are approximately equal, it takes a molality of about 6 to be in equilibrium with halite. I think your concentrations are 1 to 2 molal, the SI(Halite) is less than zero, and halite would not precipitate.

So, check your concentrations and reaction to see if Na and Cl concentrations should be higher.

Altenatively, you need to remove water from the system. Evaporation alone would require about 2/3 of the water to be removed before halite precipitation. Perhaps you are precipitating hydrated minerals that would effectively increase the Na and Cl concentrations. Finally, water could be tied up in aqueous complexes, which effectively sequesters some of the water. Given about 15 mol of water (0.25 kg), it is difficult to see how 2/3 of the water (7-10 mol) could be lost or sequestered in these processes, unless there was substantial evaporation.

Assuming your observation of halite precipitation is correct, I don't have a good theory for its formation; concentrations of Na and Cl just don't seem quite high enough.

[Tameka D]

Thank you. That was very helpful!