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mass imbalance problem

Dear Dr. Parkhurst -
I have been using PHREEQC for Windows (v1.5.10) and the WATEQ4F database to explore the expected speciation of uranium in low alkalinity waters having various hardness, to assist some colleagues in the design of uranium toxicity tests.  The test solutions are based on a recipe to achieve a given alkalinity and hardness.  Uranyl dinitrate hexahydrate [UO2(NO3)2:6H2O] are added to reach the desired uranium concentration, and the test solution is neutralized with NaOH as needed.
I have come across a situation that I can neither explain nor correct.  For a given solution recipe, as the uranium concentration is increased the sum of the uranium species no longer matches the total aqueous uranium in the system.  This appears to happen to only the uranium species ? all the other species have good mass balance when comparing the sum of the aqueous species with the mass added in making the test solution.  I have been speciating the stock solution and then adding the uranium as a dissolution reaction.  The mass balance for U varies as:
     [U]                 solution pH              [U] balance
0.01 mg/L,             6.998                         99.8%
1.00 mg/L              6.817                         53%
100 mg/L               4.242                         87%
100 mg/L               7.783                         32%      (neutralized with NaOH  2:1 molar ratio with added uranyl dinitrate)
This is for a hardness of 15 mg/L CaCO3 and a target alkalinity of 5 mg/L (these specifications are from the experimental design).  The uranium has a decreasing mass balance for increasing [U] at neutral conditions, but has a better balance for acidic conditions (which are not realistic for toxicity testing).
This pattern in uranium mass balance persists even when:
- the order of neutralization reaction is changed (add NaOH to the stock solution before adding the uranium) 
- O2 and CO2 saturation are forced,
- pe is not entered explicitly
- the uranium, nitrate and neutralizing sodium are declared as aqueous concentrations instead of using dissoultion reactions
- pH is set to 'charge balance' (this changes slightly the mass balance as the pH of the stock solution jumps to 8.1 and I have not tried to force it back to neutral, but the pattern remains)
- uranium minerals are allowed to precipitate (schoepite is the only mineral that precipitates, but the mass balance of the uranium remaining in solution follows the same pattern except the final mass balance is about 59% instead of about 32%)
For further information I have attached 2 input '.txt ' files and their corresponding '.out' output files: A1-15-03 has explicit uranyl nitrate dissolution reactions and neutralization, and file A1-15-02 has the uranium loadings and neutralization included in the solution concentrations.  I hope you will be able to find the time to look at this and tell me where I have gone astray.
Peter Flavelle
Senior Specialist
Wastes and Geosciences Division
Canadian Nuclear Safety Commission
P.O. Box 1046 Station B
280 Slater Street
Ottawa, Canada
K1Z 6R2
(613) 995-3816 / fax (613) 995-5086
TITLE U Speciation Calculations, input file A1-15d.txt
SOLUTION 1   U Tox test stock solution: Table A1 - Hardness 15
# Stock Solution 8.4 mg NaHCO3, 10.2 mg CaSO4, 9.0 mg MgSO4, 4.0 mg KCl
# in 1 liter deionized water
     units     mg/l
     pH   7.0
     Ca   3.003
     Cl   1.902
     C    6
     K    2.098
     Mg   1.818
     Na   2.299
     S    14.38
     NaOH + H+ = Na+ + H2O
     log_k     14.18 
     UO2(NO3)2:6H2O = UO2+2 + 2NO3- + 6H2O
     log_k     2.3
     delta_h   4.77 kcal
USE Solution  1
REACTION 1 0.01 mg U
    UO2(NO3)2:6H2O     1.0
    0.000000042     moles
USE Solution  1
REACTION 2 1 mg U 
    UO2(NO3)2:6H2O     1.0
    0.0000042     moles
USE Solution  1
REACTION 3 100 mg U 
    UO2(NO3)2:6H2O     1.0
    0.00042      moles
SAVE Solution 2
USE Solution 2
REACTION 4 Neutralize with NaOH
    NaOH               1.0
    0.00084   moles

Attachment: A1-15-03.out