Considering ligand pKa in uranium solubility calculations

[Inho_Lee]

Dear all,

I am currently working on a modeling project to evaluate the binding strength between uranium and a specific ligand. In particular, I would like to estimate uranium solubility in a solution containing both uranium and the ligand under the following conditions: pH 8.2 and ionic strength 0.5 M (NaCl).

In this context, I am wondering how to properly incorporate the ligand?s pKa values into the speciation/solubility calculation. Is there a recommended way or typical approach to include the ligand acid?base equilibria when modeling uranium solubility under these conditions?

The PHREEQC script below is intended to estimate uranium solubility in a system where 1 mM oxalate is present together with a uranium solid phase. For reference, the dissociation constants of oxalic acid are pKa1 = 1.27 and pKa2 = 4.28.

Sincerely,

SOLUTION_MASTER_SPECIES
    Oxalate   Oxalate-2   -2   C2O4   88.02

SOLUTION_SPECIES
   Oxalate-2 = Oxalate-2
        log_k  0
    UO2+2 + Oxalate-2 = UO2Oxalate
        log_k   7.38
    UO2+2 + 2 Oxalate-2 = UO2Oxalate2-2
        log_k  11.72
    UO2+2 + 3 Oxalate-2 = UO2Oxalate3-4
        log_k  13.6
END

PHASES
    Na2U2O7
        Na2U2O7 + 6 H+ = 2 Na+ + 2 UO2+2 + 3 H2O
        log_k 22.59        # 25 ?C, NEA-TDB errata
    Fix_pH
        H+ = H+
        log_k 0
END

SOLUTION 1
    temp   25
    pH     8.2
    pe     4
    redox  pe
    units  mol/L
    Na     0.5
    Cl     0.5
    water  1       # kg

EQUILIBRIUM_PHASES
    Na2U2O7   0     1    # SI target, initial moles
    Fix_pH   -8.2   NaOH  10  # keep pH with NaOH reservoir

REACTION 1
Oxalate-2  0.001
END

SELECTED_OUTPUT
  -file U_oxalate_model.sel
  -totals U Oxalate
  -molalities UO2+2 UO2Oxalate UO2Oxalate2-2 UO2Oxalate3-4
  -si Na2U2O7
END

[dlparkhurst]

I can't say whether the log Ks are right, but the definitions look reasonable.

For the REACTION, you should add a charge-balanced reaction: something like Na2Oxalate. PHREEQC does not recognize charge in the REACTION definition.