Processes > Dissolution and precipitation

Seawater evporation

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Peng:
I am working on a seawater evaporation model using the Pitzer database. I found that the water pH continuously decreases with the evaporation, reaching 4.6 at the end of the simulation. This may result in dissolution of early precipitated carbonate minerals. Is this a modeling artifact? How to ensure the modeled pH is realistic? The input file is attached below.

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Title Modern Seawater
Solution 1
        units   mmol/kgw
        pH      8.22
        temp    25.0
        Ca              10.66
        Mg              55.08
        Na              485.4
        K               10.58
        Cl              565.7
        S(6)            29.26
        C(4)            2.406
        -water          1
REACTION 1
        H2O -1.0
        54.0 moles in 10000 steps
INCREMENTAL_REACTIONS true
EQUILIBRIUM_PHASES  1
CO2(g) -3.386    10.0 #411 ppm
SELECTED_OUTPUT 1
-file Modern.out
-totals C(4) Ca Cl K Mg Na S 
-equilibrium_phases CO2(g)
-saturation_indices CO2(g)
End

dlparkhurst:
If you want to find out the sequence of mineral precipitation and dissolution, you should include the minerals in the simulation. Hopefully, you can find experimental evidence for the minerals that actually form, but PHREEQC will find the stable phase assemblage from the set of minerals that you include in the calculation.

See example 17 (second half) for an example simulation of the evaporation of Black Sea water.

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