Calcium impact on uranium

[Chemistry]

Hi, I am an undergraduate student majoring in nuclear engineering.

I have a question. I am simulating the effect of calcium on uranium speciation in aquatic open system (P_O2 = 10^(-3.4) atm).

My intention is to explore the effect of calcium on uranium concentration by comparing the results with and without calcium ions (i.e., (a) of Fig. 11. in reference "https://www.sciencedirect.com/science/article/pii/S0045653523033192#fig11")

So I wrote the following code:

Code: [Select]

PHASES
NaNO3
NaNO3 = Na+ + NO3-
log_k -20
Fix_H+
H+ = H+
log_k 0
NaCl
NaCl = Na+ + Cl-
log_k 0

END

USER_GRAPH 1
 -headings U U(OH)4 UO2+ UO2(CO3)2-2 UO2(CO3)3-4 CaUO2(CO3)3-2 Ca2UO2(CO3)3
 -chart_title "Uraninite Solubility"
 -axis_scale x_axis auto auto
 -axis_scale y_axis -30 auto
 -axis_titles "pH" "Concentration"
 -connect_simulations true
 -initial_solutions false
 -start
10 x = -LA("H+")
20 PLOT_XY x, LOG(TOT("U")), symbol = None
30 PLOT_XY x, LOG(MOL("U(OH)4")), symbol = None
40 PLOT_XY x, LOG(MOL("UO2+")), symbol = None
50 PLOT_XY x, LOG(MOL("UO2(CO3)2-2")), symbol = None
60 PLOT_XY x, LOG(MOL("UO2(CO3)3-4")), symbol = None
70 PLOT_XY x, LOG(MOL("CaUO2(CO3)3")), symbol = None
80 PLOT_XY x, LOG(MOL("Ca2UO2(CO3)3-2")), symbol = None

-end

SOLUTION 0
     temp    25
     pH  7.000
     units   mmol/kgw
     density 1
     Ca 1
     -water  1 # kg

EQUILIBRIUM_PHASES 0
     Fix_H+  -7.000  HCl    10
     NaCl  -20    10
     CO2(g)  -3.4   10
     UO2(am,hyd) 0 1000
     Calcite 0 1000

END
...

But it does not match my intention. It just looks like the solubility of uraninite.

Almost all calcium exists in the form of calcium-free ions, not as calcium uranyl carbonate species.

Please explain what I need to do to match my code with Fig. 11. in the reference.

Thank you.

[dlparkhurst]

I don't know what you expected. The script specifies equilibrium with UO2, so you should not be surprised that you calculated a solution in equilibrium with UO2.

If you wanted to see the effect of Ca, here is a script that adds CaCl2 while maintaining a fixed PCO2, fixed pH, and equilibrium with calcite if it becomes saturated. Maybe you can adapt the script to your needs.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0
NaCl
NaCl = Na+ + Cl-
log_k 0

END

USER_GRAPH 1
 -headings Ca U U(6) U(5) U(4)
 -chart_title "Uraninite Solubility"
 -axis_scale x_axis auto auto auto auto log
 #-axis_scale y_axis -30 auto
 -axis_titles "pH" "Concentration"
 -connect_simulations true
 -initial_solutions false
 -start
10 GRAPH_X TOT("Ca")
20 GRAPH_Y TOT("U"), TOT("U(6)"), TOT("U(5)"), TOT("U(4)")
-end

SOLUTION 1
     temp    25
     pH  7.000
     units   mmol/kgw
     density 1
C(4) 1 CO2(g) -3.4
     -water  1 # kg

EQUILIBRIUM_PHASES 1
     Fix_H+  -7.000  HCl    10
     NaCl  -20    10
     CO2(g)  -3.4   10
     UO2(am,hyd) 0 10
     #Uraninite 0 10
     Calcite 0 0
REACTION 1
CaCl2 1
1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 2e-1 1e0
END

[dlparkhurst]

I should mention that the database you use may make a big difference in the modeling results. Different databases have different combinations of aqueous species and log Ks.

If you need to compare different databases, you may want to look at the program lsp at phreeplot.org. It allows you to extract species and log Ks from a database, or databases. For instance, you could pull all of the Ca--UO2--CO3 species to compare among databases.

[Chemistry]

Thank you dlparkhurst!

I think I made a mistake. And as you answered me, one of the reasons why the result is different from what I intended is because the database is different.

Anyway, thanks for the answer. I will think more about the problem with your PHREEQC code.