seawater evaporation

[Mr.white]

I currently want to use a set of water samples to conduct a seawater evaporation experiment to obtain the salt precipitation sequence, but I don't know how to reduce the water volume. In the REACTION module, I can only set the volume change of water vapor, not the volume change of water in the solution.
SOLUTION 1
    temp      15
    pH        5.3
    pe        4
    redox     pe
    units     g/kgw
    density   1
    Ca        0.18
    Cl        276.52
    K         2.34
    Li        1.7
    Mg        98.27
    Na        5.03
    S(6)      20.31
    -water    20 # kg
REACTION 1
    H2O(g)     10
    1 moles in 1 steps
EQUILIBRIUM_PHASES 1
    Halite    0 0

[dlparkhurst]

You should look at the latter part of example 17 in the users' manual. It demonstrates and evaporation sequence for seawater. It considers more phases than just halite.

Here are three ways that you can remove water in a simulation. The first simply removes water to reach halite saturation. The second sequentially removes water in increasing amounts and precipitates halite when it is supersaturated. The third specifies activities of water and precipitates halite when it is supersaturated.

If you are considering only major elements, you should use the Pitzer database, which is more accurate at high ionic strengths than the ion-association databases.

Code: [Select]

SOLUTION 1  SEAWATER FROM NORDSTROM AND OTHERS (1979)
        units   ppm
        pH      8.22
        pe      8.451
        density 1.023
        temp    25.0
        Ca              412.3
        Mg              1291.8
        Na              10768.0
        K               399.1
        Cl              19353.0
        Alkalinity      141.682 as HCO3
        S(6)            2712.0
END
### Method 1
USE solution 1
EQUILIBRIUM_PHASES 1
    Halite    0 H2O(g) 10 precipitate_only
END
### Method 2
INCREMENTAL_REACTIONS
USE solution 1
REACTION 1
    H2O     1
    -45 10*-1
EQUILIBRIUM_PHASES
Halite 0 0
USER_GRAPH 1
    -headings               frac SI("Halite")
    -axis_titles            "Fraction of water removed" "SI" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X [1 - TOT("water")]
20 GRAPH_Y SI("Halite")
  -end
    -active                 true
END
USER_GRAPH 1
-detach
END
### Method 3
PHASES
Water_activity
H2O = H2O
log_k 0
END
USER_GRAPH 2
    -headings               frac SI("Halite")
    -axis_titles            "Fraction of water removed" "SI" "ACT(H2O)"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
10 GRAPH_X [1 - TOT("water")]
20 GRAPH_Y SI("Halite")
30 GRAPH_SY ACT("H2O")
USE solution 1
EQUILIBRIUM_PHASES
Water_activity -0.046 10 # 0.9
Halite 0 0
END
USE solution 1
EQUILIBRIUM_PHASES
Water_activity -0.097 10 # 0.8
Halite 0 0
END
USE solution 1
EQUILIBRIUM_PHASES
Water_activity -0.155 10 # 0.7
Halite 0 0
END
USE solution 1
EQUILIBRIUM_PHASES
Water_activity -0.222 10 # 0.6
Halite 0 0
END
USE solution 1
EQUILIBRIUM_PHASES
Water_activity -0.301 10 # 0.5
Halite 0 0
END