Magnesium Silicate Co-precipitation

[PAILLEF]

Hi everyone,

Currently in pHd, I am starting with phreeqc.
My problem is the Following :
I have two solution : One of Potassium Silicate  and One of Magnesium Nitrate + HNO3
And I mix the same volume of each

SOLUTION 1 Si
    temp      20
    pH        7 charge
    pe        4
    redox     pe
    units     mol/l
    density   1
    K         0.125
    Si        0.0625
    -water    1 # kg
SOLUTION 2
    temp      25
    pH        7 charge
    pe        4
    redox     pe
    units     mol/l
    density   1
    Mg        0.03125
    N(5)      0.125
    -water    1 # kg

MIX 1
    1    0.5
    2    0.5
 
Experimentally when I mixed this two solutions I obtained a pH =8.8 and concentrations in the resulting solution of [Si] = 0,002 mol.L-1 and [Mg] = 0,013 mol.L-1 + a solid phase.

So I create a Solid phase in the data base matching the stochiometry of my solid and adjust the log K for a saturation indexe of 0 in this experimental conditions

Mg0.34SiO2.6H0.520 + 0.68 H+ + 1.4 H2O = 0.34 Mg++ + 1 H4(SiO4)
   log_k 2.84

When I try to precipitate this phase with PHREEQC it does not match neither the pH or the concentrations.
The simulation even give a pH higher than the mix (10,9 for the mix and 11,6 for the post precipitation solution).

Thank you in advance if you have a solution for my problem.

[dlparkhurst]

I'm not sure how you got your log K. If you can enter your final solution composition, then you can calculate the SI for your phase, and then adjust your log K.

Did you exclude air from your experiment? Addition of CO2 from the atmosphere would have a large affect on the solution composition and pH. Adding atmospheric CO2(g) to your system drops the pH to about 9.

[PAILLEF]

Sorry for my bad explanation.
Yes I took my final composition and adjust the log K to obtain SI =0.

Sorry to ask but how can I add atmospheric CO2 in the solution ?

Tks

[dlparkhurst]

Code: [Select]


SOLUTION 1 Si
    temp      20
    pH        7 charge
    pe        4
    redox     pe
    units     mol/l
    density   1
    K         0.125
    Si        0.0625
    -water    1 # kg
SOLUTION 2
    temp      25
    pH        7 charge
    pe        4
    redox     pe
    units     mol/l
    density   1
    Mg        0.03125
    N(5)      0.125
    -water    1 # kg
END

MIX 1
    1    0.5
    2    0.5
EQUILIBRIUM_PHASES
Mg-Si 0 0
CO2(g) -3.5 10
END


[PAILLEF]

Thank you for your quick answer !