Dissolution reaction of uranium rocks depending on the redox state

[inhee]

Hello. I don't know how to apply dissolution reaction using EQUILIBRIUM_PHASES in PHREEQC

I want to model the situation that oxidizing water flow through uranium-bearing rocks. Therefore, U(VI) is dissolved in water and uranium concentration will increase.

The uranium concentration of rock sample is 24 ppm. If this uranium is come from only uraninite(UO2), how I can code to calculate this reaction in PHREEQC? I assume that the dissolution reaction is equilibrium.

The SI of existing uraninite(originally, reducing state) is -3.39. I tried coding like below, but I think it is not right. (Also, I changed pe value to that of oxidizing water(1.4 -> 5)).

Code: [Select]

EQUILIBRIUM_PHASES
Uraninite -3.39 1e-7 # 1e-7 is 24 ppm / 238 g of 1 mole U


[dlparkhurst]

To model a roll-front deposit, you need an oxidizing water and a reducing zone. Below is an example. The infilling solution contains dissolved oxygen, which dissolves uraninite. The kinetic reaction of organic matter--CH2O--reduces U(6) to U(4) resulting in the precipitation of uraninite in the reducing zone. The graph shows a zone of U(6) that decreases with distance and the accumulation of uraninite downstream from the front.

All of the concentrations, distances, velocity, and reaction rates are arbitrarily chosen to demonstrate the process. I used the wateq4f.dat database.

Code: [Select]

RATES
CH2O
10 k = 1 / 3.15e7 # 1/y
20 rate = k*TOT("U(6)")
30 moles = rate * TIME
40 SAVE moles
END

SOLUTION 0
EQUILIBRIUM_PHASES 0
CO2(g) -3.4 1
Calcite 0 1
O2(g) -0.7 1
SAVE solution 0
END
SOLUTION 1
EQUILIBRIUM_PHASES 1-10
Calcite 0 1
Uraninite(c) 0 1e-3
SAVE solution 1-10
END
KINETICS 1-10
CH2O
-formula CH2O 1
-M 1
END

TRANSPORT
-cells 10
-shifts 30
-time 3.15e7
-length 1
-punch_cells 1-10
-punch_frequency 30
-print_frequency 30
USER_GRAPH 1
    -headings               dist U(6) U(4) Uraninite
    -axis_titles            "Distance, m" "Molality" "Uraninite"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X DIST
20 GRAPH_Y TOT("U(6)"), TOT("U(4)")
30 GRAPH_SY EQUI("Uraninite(c)")
  -end
    -active                 true
END



[inhee]

Thank you for answering my question, D. L. parkhurst!

I'm curious about setting rock sample U concentration.

Is rock sample moles 1e-7, right? I said that "EQUILIBRIUM_PHASES Uraninite 0 1e-7" when uranium concentration of rock is 24 ppm.

Also, SI of uraninite is 0 or previous value in EQUILBIRIUM_PHASES? I think 0 means equilibrium about uraninite, but SI of uraninite is not 0 in reducing zone. What is right set up? 

[dlparkhurst]

I did not try to replicate your system, I only meant to show you how a roll front deposit could be modeled. Equilibrium is a reasonable start for your modeling, but you will have to modify the example as you see fit.

The infilling solution will have a specified amount of dissolved oxygen. That provides a maximum for the amount of uranium that can be oxidized. The amount may be less if oxygen reacts with other electron acceptors or the uranium reaction is kinetically slow and does not proceed to completion.

Once U(6) is dissolved, it is another question as to how fast uraninite precipitates once it is beyond the oxidizing zone.