SIT coefficients

As the ion interaction coefficients for the Th ion are important parameters for the evaluation of standard state equihbrium constants from experimental data in perchlorate, nitrate and chloride media, an additional experimental study was undertaken [2006NEC/ALT] where trace activity coefficients of the Th ion in dilute to concentrated NaC104, NaNOs and NaCl solutions (PJ = 0.01-0.02 M) were determined at 22°C from hquid-liquid distribution equilibria between aqueous 

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SIT coefficients referring to this approach, mostly derived from osmotic coefficients in binary metal chloride or nitrate solutions [1980CIA], are reported in the previous NEA-TDB reviews. However, only the more recent reviews [2001LEM/FUG], [2003GUI/FAN] used the corresponding values of 8(An02,Cl), e(An, Cr), and 8(An", Cr) to calculate equilibrium constants for the solubility, hydroxide and carbonate complexes of actinides from experimental data in chloride media. 

The SIT coefficients used by this review for the determination of standard state solubility constants and equilibrium constants for hydroxide, fluoride, and carbonate complexes from experimental data in chloride and nitrate media refer to the strict ion interaction approach. They imphcitly include the formation of weak Th(IV) complexes or ion pairs with nitrate and chloride ions. However, they must not be combined with... 

Figure VI-3 SIT coefficients 8(An", CIO4) and 8(An", Cl) of tetravalent actinide aquo...

Figure VI-4 Correlations between SIT coefficients of non-complexed cations (a) NO3) and CIO4), (b) Cl ) and CIO4). Open points are...

Figure VI-5 Trace activity coefficients of in NaCl solution at 25°C, calculated with the SIT coefficient 8(Th", Cr) = (0.25 + 0.03) kg-mol [1980CIA], [1992GRE/FUG] (A) and with the different sets of the Pitzer parameters reported by Pitzer and Mayorga [1973PIT/MAY] (B) and by Roy et al. [1992ROYA OG] and Rai et al. [1997RAI/FEL] (C).

Hietanen and Sillen [1968HIE/SIL] tested different speciation models for their data in 3.0 M NaCl, including Th4(OH)g as minor complex. The calculated value of logic A,4 is consistent for the different models. The value of log,o p, = - (21.1 + 0.2) in 3 M NaCl can be used to evaluate the SIT coefficients... 

These calculations show that the present selection of standard state equihbrium constants for both complexes, logn, V°5,6 (Thg(OH) ) and log,o Vrs.a (TheiOH) ), combined with reasonable estimates for the unknown SIT coefficients, does not create an inconsistency when both complexes are included in the calculations. The calculations are consistent with the speciation models accepted for the evaluation of the original data in the corresponding media. Moreover the present selections allow predictions for low ionic strength where, as expected, the calculated speciation for perchlorate, nitrate and chloride media is very similar. 

The SIT coefficients selected in this table refer to a strict ion interaction approach where the effect of... 

The equilibrium constants and SIT coefficients selected in this table refer to a strict ion interaction approach where the effect of chloride or nitrate complexation is included in the interaction coefficients. Therefore these values must not be combined with the formation constants of chloride and nitrate complexes discussed in Sections VIII.2.2.1 and X. 1.3.3. 

Figure VII-15 Solubility data for Th02(am, hyd) in 0.5 M NaC104 a) from [1989MOO] and b) from [19940ST/BRU] (filled squares eaibonate-free solution open squares under C02(g) atmosphere which has, however, no effect on the speciation at pH < 4.5). The solubility constant and aqueous speciation are calculated using the hydrolysis constants and SIT coefficients selected in the present review...

Table VII-19 Solubility constants logj at zero ionic strength re-evaluated from solubihty studies with Th(OH)4(am) or Th02(am,hyd) at 17-25°C (based on logio (Th COH) ") values and SIT coefficients selected in the present review.)...

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,...

Table VIII-19 SIT coefficients e(Th", Cl) based on a reanalysis of hteratuie data from [1950DAY/STO] and [1952WAG/STO] for the extraction of thorium from (H, Na ) (CT, CIO4) solutions using the strict ion interaction approach, i.e. without assuming the formation of chloride complexes. The calculations were performed with the NONLINT-SIT code and alternatively by hnear regression in plots of logiof) vs. m -see the Appendix A entry for [1950DAY/STO].

The following SIT coefficients should be used in calculations involving this... 

Table XI-4 SIT coefficients for Th(IV) carbonate and hydroxide-carbonate complexes estimated from charge type analogues [2005ALT/NEC] except otherwise stated.

Figure XI-2 Solubility of Th02(am, hyd) and speciation under 0.1 and 1.0 bar C02(g) in 0.5 M NaCl [2005ALT/NEC]. The calculations are based on the equilibrinm constant and SIT coefficients selected in the present review and the solubility constants calcnlated from the data in carbonate-free solutions, logj X° = -(47.6 + 0.5). The dotted curves are calculated with the equilibrium constants proposed by Osthols et. al. [19940ST/BRU] for Th(OH)4(aq) and Th(0H)3(C03) (see text for notation of Th(IV) hydroxide-carbonate complexes).

Hence, the following formation constants and SIT coefficients are selected ... 

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.

Method 2 (strict ion interaction model). In distinction to the complexation model, the ion interaction model takes into account the fact that the SIT coefficients 8(M, CIO4) differ from s(M, XA but ignores the formation of complexes ([Th]aq = [Th" ]). The equations for logioT (at (/Wao )o)arethns ... 

Table A-8 SIT coefficients obtained from linear regression (Figure A-1 to Figure A-4).

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