Ba(x)Ra(1-x)SO4 Solid Solution Precipitation

[lswerhone]

Hello,

I am trying to model coprecipitation of radium with barite in uranium mine tailings using a Barite-RaSO4 solid solution. Essentially I'd like to predict the decrease in porewater radium from fresh tailings as the solid solution precipitates to a target SI for barite measured in aged in-situ tailings porewater. An example of the code is below:

Code: [Select]

#DATABASE C:\Program Files (x86)\USGS\Phreeqc Interactive 3.7.3-15968\database\llnl.dat
#Precipitation of a Ba-Ra-SO4 solid solution in uranium mine tailings

SOLUTION 1
    temp      7
    pH        10.6
    pe        1
    redox     pe
    units     mg/l
    density   1
    Alkalinity 67
    Ca        655
    Cl        29
    K         28.1
    Mg        2.99
    Na        76
    S(6)      1560
    Ba        0.198
    Ra        1.781e-5
    -water    1 # kg

SOLID_SOLUTIONS 1
    Ba(x)Ra(1-x)SO4
        -comp Barite 0
        -comp RaSO4 0

END

When I run this code it precipitates the solid solution until it reaches a barite SI of 0 (down from 1.88 in the initial solution), however I would like to precipitate the solid solution out to a target barite SI of 0.5?is this possible with the SOLID_SOLUTIONS function like it is with the EQUILIBRIUM_PHASES function?

Also, am I correct that setting the moles of each solid solution component to zero forces precipitation of the solid solution based on the porewater ratios of Ba and Ra, or am I misunderstanding this function?

Any insight here would be greatly appreciated.

Thank you,
Lawrence

[dlparkhurst]

You can define a new Barite phase that is 0.5 units more soluble. It is a little awkward, but here is a script that shows the comparison between a Barite solid solution and the new Barite phase solid solution.

At equilibrium, the following relations hold

Code: [Select]

K(Barite) = IAP(Barite)/X(Barite)
K(RaSO4) = IAP(RaSO4)/X(RaSO4)

where X is mole fraction. (Nonideal binary solid solutions include an activity coefficient times the mole fraction.)

Code: [Select]

#DATABASE C:\Program Files (x86)\USGS\Phreeqc Interactive 3.7.3-15968\database\llnl.dat
#Precipitation of a Ba-Ra-SO4 solid solution in uranium mine tailings
NAMED_EXPRESSIONS
MyConstant
-log_k 0.5
PHASES
Barite_obs
BaSO4 = Ba+2 + SO4-2
log_k -9.9711
-delta_H 25.9408 kJ/mol # Calculated enthalpy of reaction Barite
# Enthalpy of formation: -352.1 kcal/mol
-analytic -1.8747e+2 -7.5521e-2 2.079e+3 7.7998e+1 3.2497e+1
# -Range:  0-300
-add_logk MyConstant
END
SOLUTION 1
    temp      7
    pH        10.6
    pe        1
    redox     pe
    units     mg/l
    density   1
    Alkalinity 67
    Ca        655
    Cl        29
    K         28.1
    Mg        2.99
    Na        76
    S(6)      1560
    Ba        0.198
    Ra        1.781e-5
    -water    1 # kg
END
USE solution 1
SOLID_SOLUTIONS 1
    Ba(x)Ra(1-x)SO4
        -comp Barite 0
        -comp RaSO4 0
USER_GRAPH 1
    -headings               Calculation SI(Barite)
    -axis_titles            "Calculation number" "SI(Barite)" ""
    -axis_scale x_axis      0 3 1 1
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 calc = GET(1) + 1
20 PUT(1, calc)
30 GRAPH_X calc
40 GRAPH_Y SI("Barite")
  -end
    -active                 true
END
USE solution 1
SOLID_SOLUTIONS 1
    Ba(x)Ra(1-x)SO4_obs
        -comp Barite_obs 0
        -comp RaSO4 0
END