Processes > Dissolution and precipitation

Temperature effects on Uraninite precipitation

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Jeonghwan Hwang:
The purpose of my model is to confirm the uraninite precipitation tendency of dissolved uranium with increasing temperature.
A simple model was created by changing the temperature to 15, 35, 55, 75, and 95 degC.
The initial pe is -2.26, which indicates a reducing environment.

As a result of the model, the concentration of dissolved U(4) increased from 4.29E-10 mol/kgw to 1.06E-9 mol/kgw as the temperature increased from 15 degC to 95 degC.
Also, the amount of uraninite decreased from 9.9957E-7 mol to 9.9891E-7 mol as increasing temperature.
These results seem to be because the solubility of uraninite increases as the temperature increases.

However, when the 'Analytical equation' of Uraninite provided by LLNL DB is solved according to temperature (A1=-7.58E+01, A2=-1.06E-02, A3=5.97E+03, A4=2.19E+01, A5=9.31E+01), the log_K of the 'UO2 + 4H+ = U++++ + 2H2O' reaction is -4.36 at 15 degC and -7.38 at 95 degC. It means that the solubility of uraninite decreases with increasing temperature.

In addition, the pe values ​​of the equilibrium solutions were significantly different from the initial values (pe=-2.26), and showed values ​​of -0.5, -0.48, -0.49, -0.53, and -0.62 at 15, 35, 55, 75 and 95 degC, respectively. It seems to be indicative of an oxidized environment compared to the initial condition, but I could not find any reason for pe change in my model.

Is there any chemical equation I'm missing in this part?

Sincerely,
Jeongwhan Hwang

=================== Model Code =====================
Database C:\phreeqc\database\LLNL.dat

Selected_output
reset false
file hwang.txt
pe true
pH true
user_Punch
10 Punch TOT("U(4)") TOT("U(6)") EQUI("Uraninite")
end
end

Solution 1
unit mol/L
temp 15
pe -2.26
U 1E-6
equilibrium_phases
Uraninite 0 0
end
Solution 2
unit mol/L
temp 35
pe -2.26
U 1E-6
equilibrium_phases
Uraninite 0 0
end
Solution 3
unit mol/L
temp 55
pe -2.26
U 1E-6
equilibrium_phases
Uraninite 0 0
end
Solution 4
unit mol/L
temp 75
pe -2.26
U 1E-6
equilibrium_phases
Uraninite 0 0
end
Solution 5
unit mol/L
temp 95
pe -2.26
U 1E-6
equilibrium_phases
Uraninite 0 0
end

=================== Model Results=====================
pH             pe           U(4)           U(6)      Uraninite
7          -2.26     9.9999e-07     3.0976e-12     0.0000e+00
6.99999      -0.503644     4.2979e-10     4.3349e-12     9.9957e-07
7          -2.26     9.9999e-07     6.4682e-12     0.0000e+00
6.99999      -0.480287     4.3420e-10     1.0183e-11     9.9956e-07
7          -2.26     9.9997e-07     1.0585e-11     0.0000e+00
6.99999      -0.487828     5.0808e-10     1.8834e-11     9.9947e-07
7          -2.26     9.9995e-07     1.4260e-11     0.0000e+00
6.99999      -0.530985     6.8482e-10     2.8038e-11     9.9929e-07
7          -2.26     9.9994e-07     1.6568e-11     0.0000e+00
7      -0.617153     1.0563e-09     3.3790e-11     9.9891e-07

dlparkhurst:
If you look at pure water, without any U, pH decreases and pe increases as you increase temperature. Even though the logK for uraninite decreases with temperature, I think the pH decrease (H+ increase) due to the change in log K of H2O = H+ + OH- (with temperature) causes the solubility of uraninite to increase (with temperature) (UO2 + 4H+ = U+4 + 2H2O).

--- Code: ---Solution 1
unit mol/L
temp 15
pe -2.26
#U 1E-6
END
USE solution 1
EQUILIBRIUM_PHASES 1
Uraninite 0 0
REACTION_TEMPERATURE
15 95 in 9 steps

USER_GRAPH 2
-axis_titles            "Temperature, Celsius" "pH, pe"
-initial_solutions      false
-connect_simulations    true
-plot_concentration_vs  x
-start
10 GRAPH_X TC
30 GRAPH_Y -LA("H+"), -LA("e-")
-end
-active                 true
END

--- End code ---

Jeonghwan Hwang: