UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

Thorium undergoes hydrolysis in aqueous solutions at pH values above 3. The distribution of
thorium hydrolytic species, shown in Figure 5.4, was calculated as a function of pH using the
MINTEQA2 code and the thermodynamic data tabulated in Langmuir and Herman (1980). The
aqueous species included in the speciation calculations are listed in Table 5.14. The species
distribution in Figure 5.4 was determined for a concentration of 1 µg/l total dissolved thorium for
a water free of any complexing ligands other than hydroxide ions. The chosen thorium
concentration is based on Hem (1985, p. 150) who gives 0.01 to 1 µg/l as the range expected for
thorium concentrations in fresh waters. The calculated species distribution shows that the
uncomplexed ion Th4+ is the dominant ion at pH values less than ~3.5. At pH values greater than
3.5, the hydrolysis of thorium is dominated, in order of increasing pH, by the aqueous species
2+ + "
, Th(OH)3,
and Th(OH)4(aq).
The latter 2 hydrolytic complexes have the widest range
Th(OH) 2
of stability with pH.
The large effective charge of the Th4+ ion can induce hydrolysis to the point that polynuclear
complexes may form (Baes and Mesmer, 1976). Present knowledge of the formation of
polynuclear hydrolyzed species is poor because there is no unambiguous analytical technique to
determine these species. However, polynuclear species are believed to play a role in mobility of
thorium in soil/water systems. Langmuir and Herman (1980) list estimated thermodynamic values
6+ 8+ 9+
for the thorium polynuclear hydrolyzed species Th2(OH) 2 , Th4(OH) 8 , and Th6(OH) 15
based on
the review of Bases and Mesmer (1976).
Table 5.14. Thorium aqueous species included in the
speciation calculations.
Aqueous Species
Th 4+ , ThOH 3+ 2+ +
, Th(OH) 2 , Th(OH)3,
Th(OH)4°(aq),
6+ 8+ 9+
Th2(OH) 2 , Th4(OH) 8 , Th6(OH) 15
- 6-
Th(OH) 3CO3 and Th(CO3) 5
ThF 3+ 2+ +
, ThF2 , ThF3,
ThF4°(aq)
ThCl 3+ 2+ +
, ThCl2 , ThCl3,
ThCl4°(aq)
2+ 2- 4-
ThSO4 , Th(SO4) 2°(aq), Th(SO4) 3 , Th(SO4) 4
4+ 3+ 2+
ThH3PO4 , ThH2PO4 , Th(H2PO4) 2 ,
2-
Th(HPO4) 2°(aq), Th(HPO4) 3
5.56