Thorium speciation

Thorium. Experimental and theoretical studies of thorium speciation, solubility, and sorption in low-ionic-strength waters are described by Langmuir and Herman (1980), Laflamme and Murray (1987), Osthols et ai (1994), Osthols (1995), and Quigley et al. (1996). Langmuir and Herman (1980) provide a critically evaluated thermodynamic database for natural waters at low temperature that is widely used. However, it does not contain information about important thorium carbonate complexes, and the stability of phosphate complexes may be overestimated (US EPA, 1999b). 

In the environment, thorium and its compounds do not degrade or mineralize like many organic compounds, but instead speciate into different chemical compounds and form radioactive decay products. Analytical methods for the quantification of radioactive decay products, such as radium, radon, polonium and lead are available. However, the decay products of thorium are rarely analyzed in environmental samples. Since radon-220 (thoron, a decay product of thorium-232) is a gas, determination of thoron decay products in some environmental samples may be simpler, and their concentrations may be used as an indirect measure of the parent compound in the environment if a secular equilibrium is reached between thorium-232 and all its decay products. There are few analytical methods that will allow quantification of the speciation products formed as a result of environmental interactions of thorium (e.g., formation of complex). A knowledge of the environmental transformation processes of thorium and the compounds formed as a result is important in the understanding of their transport in environmental media. For example, in aquatic media, formation of soluble complexes will increase thorium mobility, whereas formation of insoluble species will enhance its incorporation into the sediment and limit its mobility. 

Tetravalent. The hydrolysis of tetravalent actinide ions can begin to occur in solutions with pH levels < 2. Under dilute conditions, species of the form An(OH) " (n = 1 4) are predicted however, most hydrolysis studies have only been able to identily the first hydrolysis product, An(OH) +. It should be noted that in all of these compounds the remainder of the coordination sphere is made up of bound H2O molecules. The end member of the speciation is the neutral An(OH)4 or An02-2H20. This complex has low solubihty but has been postulated to exist in solutions from solubihty experiments when using the isolated solid as the starting material. Under more concentrated conditions, polymeric materials have been postulated. In modeling the hydrolysis of thorium at concentrations greater than mM, polynuclear species of the form Th2(OH)2 +, Th2(OH)4 +, Th4(OH)g +, Th6(OH)i4 +, and so on, have been included. 

Tetravalent. The best-studied tetravalent actinide carbon-ato complex is An(C03)5 (An = Th, U, Pu). This anion has been isolated using a variety of cations, including Na+, K+, T1+, [Co(NH3)6] + and C(NH2)3+/NH4+. In solution, the pentacarbonato complex is the end member of the series An(C03) " " (n = 1-5) however, in the mineral tuhokite, Na6BaTh(C03)6-6H20, thorium exists as a hexacarbonato complex. The analysis of the thermodynamic data for these actinide carbonate systems has led to differences of opinion on the actual speciation. The data appear to support both the stepwise addition of C03 and subsequent loss of H2O molecules within the An + cation coordination sphere as well as the formation of mixed hydroxo carbonato complexes, for example Pu(C03)3(0H) . 

Figure 10 An example of speciation calculation. Distribution of thorium complexes versus pH at 25 °C with 2Th = 0.01 p,g L in pure water (after Langmuir and Herman, 1980).

Suzuki and Banfield (1999) discuss the similarities between the uranium-microbe interactions and transuranic-microbe interactions. Macaskie (1991) notes that it is possible to extrapolate the data for microbial uranium accumulation to other actinides. Hodge et al. (1973) observe that the biological behavior of uranium, thorium, and plutonium resemble that of ferric iron. Microbes can also affect the speciation and transport of multivalent fission products. For example, Fe " -reducing bacteria and sulfate-reducing bacteria can reduce soluble pertechnetate to insoluble Tc(IV), as discussed by Lloyd et al. (1997). For additional information about these topics, the reader is referred to the references cites above. Applications of these principles are described in the section on bioremediation later in this chapter. 

The complexation of thorium(IV) and plutoniumflV) with a tetrahydroxamate ligand based on the cyclohexane-1,2-diyldinitrilotetraacetate complexon, with hydroxamate instead of carboxylate groups has been reported. The speciation appears to be pH dependent. Up to pH 9 the complexes... 

Figure VII-16 Solubility of thorium hydroxide determined by Higashi [1959HIG] at room temperatnre from oversaturation after 1, 3, 7, and 100 days. The solnbihty and speciation lines are calcnlated with the hydrolysis constants and SIT coefficients selected in this review, logj,...

The available thermodynamic data are of two types stabihty constants, enthalpy and entropy of reaction for the formation of soluble complexes Th(S04) " " and solubihty data for various solid phases. The two sources are linked because the solubility of the solid phases depends on the chemical speciation, i.e., the sulphate complexes present in the aqueous phase. The analysis of the experimental stability constants has been made using the SIT model however, this method cannot be used to describe the often very high solubility of the solid sulphate phases. In order to describe these data the present review has selected a set of equilibrium constants for the formation of Th(S04) and Th(S04)2(aq) at zero ionic strength based on the SIT model and then used these as constants in a Gibbs energy minimisation code (NONLINT-SIT) for modelling experimental data to determine equilibrium constants for the formation of Th(S04)3 and the solubility products of different thorium sulphate solids phases. 

Figure XI-7 Solubility and speciation of thorium at a total carbonate concentration of Ctot = 10 M. Experimental data for Na6[Th(C03)5] 12H20(s) in 1.0 M...

Figure XI-8 Solubility and speciation of thorium at a total carbonate concentration of Ctot = 0.2 M. Experimental data for Na6[Th(C03)5]-12H20(s) in 0.2 M NaHCOs /1.8 M NaNOs and in 0.2 M NazCOs /1.6 M NaNOs [1973DER/FAU3] and calculated solubility for Th02(am, hyd) in 0.2 MNa2C03 /1.6 MNaNOs containing 0.01-1.0 M NaOH. The calculations are based on the equilibrium constants and SIT coefficients selected in the present review taking into accounting the variation of the solution composition and ionic strength.

BIS/KRA] Bischoff, H., Kramer-Schnabel, U., Esser, V., Xi, R. H., Heppner, P. M., Marx, G., Determination of formation constants, solubilities, and ionic speciation of uranium, plutonium and thorium in aqueous and saturated salt solutions for modelling safety aspects of ILW repositories, Sci. Basis Nucl. Waste Management XV, vol. 257, pp.331-336, (1992). Cited on pages 172, 618. 

FigmeXl-7 Solubility and speciation of thorium at a total carbonate... 

Clay Mineralogy. Hydrothermal Vent Deposits. Platinum Group Elements and their Isotopes in the Ocean. Pore Water Chemistry. Rare Earth Elements and their Isotopes in the Ocean. River Inputs. Tracers of Ocean Productivity. Transition Metals and Heavy Metal Speciation. Uranium-Thorium Decay Series in the Oceans Overview. 

Studies of the speciation of actinides in environmental waters are made difficult by the very low concentrations involved and the possibility that minor, undetected contaminants may dominate the binding of a particular metal ion. The environmental behaviour of the actinides has been reviewed. Americium and thorium exhibit simpler behaviour than other actinides since their oxidation states under such conditions are limited to Am and Th. Both are readily adsorbed by granitic rocks and tend to exhibit low solubilities, The thermodynamic solubility product of amorphous Am(OH)3 has been measured as log = 17.5 0.3 and no evidence for amphoteric behaviour or the formation of Am(OH)4 was found below pH 13. Stability constants for the binding of Am to humic acid have been found to vary with the degree of ionization, a, and were given by log = 10.58a -1-3.84 and log 2 = 5.32a -b 10.42. These were larger than the corresponding values for Eu. Humic acids also bind Th as described in Section 65.2.1. 

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