1993 Volume 30 Issue 4 Pages 314-332
The sensitivity of uranium solubility to variation of ligand concentrations in the groundwater was systematically evaluated. Predominant dissolution reactions between uranium solubility limiting solids and predominant aqueous species were obtained by means of calculating uranium solubility and speciation using the NEA Thermochemical Data Base of uranium and the geochemical code SOLGASWATER. Logarithm of the mass law equations for the predominant dissolution reactions allowed that logarithmic concentration of total dissolved uranium species, log[UO22+]Τ, was a linear function of pH and logarithm of ligand concentration. The equation for the difference of log[UO22+]Τ between the U-H2O system and U-ligand-H2O system was derived from the linear functions in the common pe-pH area of the two systems. Differential coefficients obtained from the difference of log[UO22+]Τ allowed to judge which predominant dissolution reactions are sensitive to the variation of ligand concentrations as follows:
(1) The uranium solubility increases most sensitively with ligand concentrations, according to the following equations;
at pe 10,
UO3•2H2O(cr)+3HCO-3, =UO2(CO3)34-+3H2O(1)+H+ (the CO2-closed system),
UO3•2H2O(cr)+3H2CO3(aq)=UO2(CO3)34-+3H2O(1)+4H+ (the CO2-open system).
and at pe -4,
UO2(cr)+4H++4F-=UF4(aq)+2H2O(1).
(2) The uranium solubility decreases with ligand concentrations, according to the following equations;
at pe 10,
UO2CO3(cr)+2H+=UO22++H2CO3(aq)
(UO2)3(PO4)2•6H2O(cr)+4H+=-3UO22++2H2PO-4+6H2O(1),
(UO2)3(PO4)2•6H2O(cr)=3UO2(OH)2(aq)+2H2PO-4+2H+,
and at pe -4,
USiO4(cr) +4H2O(1) =U (OH)4(aq)+H4SiO4(aq).
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