Michael K. Richmann

Complexation of Nd(III) with tetraborate ion and its effect on actinide (III) solubility in WIPP brine

Abstract The potential importance of tetraborate complexation on lanthanide(III) and actinide(III) solubility is recognized in the literature but a systematic study of f-element complexation has not been performed. In neodymium solubility studies in WIPP brines, the carbonate complexation effect is not observed since tetraborate ions form a moderately strong complex with neodymium(III). The existence of these tetraborate complexes was established for low and high ionic strength solutions. Changes in neodymium(III) concentrations in undersaturation experiments were used to determine the neodymium with tetraborate stability constants as a function of NaCl ionic strength. As very low Nd(III) concentrations have to be measured, it was necessary to use an extraction pre-concentration step combined with ICP-MS analysis to extend the detection limit by a factor of 50. The determined Nd(III) with borate stability constants at infinite dilution and 25 °C are equal to logβ1=4.55±0.06 using the SIT approach, equal to logβ1=4.99±0.30 using the Pitzer approach, with an apparent logβ1=4.06±0.15 (in molal units) at I=5.6 m NaCl. Pitzer ion-interaction parameters for neodymium with tetraborate and SIT interaction coefficients were also determined and reported.

Determination of ferrous and ferric iron in aqueous biological solutions

A solvent extraction method was employed to determine ferrous and ferric iron in aqueous samples. Fe(3+) is selectively extracted into the organic phase (n-heptane) using HDEHP (bis(2-ethylhexyl) hydrogen phosphate) and is then stripped using a strong acid. After separation, both oxidation states and the total iron content were determined directly by ICP-MS analysis. This extraction method was refined to allow determination of both iron oxidation states in the presence of strong complexing ligands, such as citrate, NTA and EDTA. The accuracy of the method was verified by crosschecking using a refinement of the ferrozine assay. Presented results demonstrate the ability of the extraction method to work in a microbiological system in the presence of strong chelating agents following the bioreduction of Fe(3+) by the Shewanella alga BrY. Based on the results we report, a robust approach was defined to separately analyze Fe(3+) and Fe(2+) under a wide range of potential scenarios in subsurface environments where radionuclide/metal contamination may coexist with strongly complexing organic contaminants.

Interaction of Nd(III) and Cm(III) with borate in dilute to concentrated alkaline NaCl, MgCl2 and CaCl2 solutions: solubility and TRLFS studies

The interaction of lanthanides and trivalent actinides with borate in dilute to concentrated alkaline NaCl, MgCl2 and CaCl2 solutions was investigated at 22 ± 2 °C by a comprehensive series of solubility experiments with Nd(OH)3(am), and complemented with Cm(III)–TRLFS studies (TRLFS: time resolved laser fluorescence spectroscopy) under analogous pH and ionic strength conditions. Although there was clear evidence of borate complexation in the pH range of 8.5 to 10, overall no significant increase in Nd(III) solubility occurred in any of the investigated salt systems in the presence of [B]tot ≤ 0.4 M, compared with analogous borate-free solutions. On the contrary, a significant decrease in Nd(III) concentration was observed at pHc ≤ 9 in NaCl and MgCl2 systems with [B]tot ≥ 0.16 M (diluted salt systems) or [B]tot ≥ 0.04 M (concentrated salt systems). This observation, together with a clear change in the slope of the solubility curve and the further confirmation by XPS analyses, indicates the transformation of Nd(OH)3(am) into a so far unknown Nd(III)–borate solid phase with significantly lower solubility. Similar Nd(III) concentrations in the aqueous phase are obtained in undersaturation solubility experiments conducted with a synthesized crystalline phase Nd[B9O13(OH)4](cr). TRLFS confirmed the formation of aqueous Cm(III)–borate complexes in dilute to concentrated NaCl and MgCl2 systems at pHc = 8 and [B]tot ≥ 0.04 M. Two different Cm(III)–borate species are proposed based on the peak shift of the spectra, although the resulting fluorescence emission bands do not allow the definition of an unequivocal chemical model for this system. TRLFS also shows that no Cm(III)–borate complexes form under hyperalkaline conditions (pHc = 12), due to the stronger competition posed by hydrolysis and the predominance of weakly coordinating B(OH)4− in the aqueous phase. These results show the impact of An(III)–borate interactions on An(III) speciation and highlight the hitherto unknown role of borate in the immobilization of trivalent actinides under repository-relevant conditions due to the formation of borate-bearing solid phases with significantly lower solubility than the corresponding hydroxides.

Actinide (III) solubility in WIPP Brine: data summary and recommendations

The solubility of actinides in the +3 oxidation state is an important input into the Waste Isolation Pilot Plant (WIPP) performance assessment (PA) models that calculate potential actinide release from the WIPP repository. In this context, the solubility of neodymium(III) was determined as a function of pH, carbonate concentration, and WIPP brine composition. Additionally, we conducted a literature review on the solubility of +3 actinides under WIPP-related conditions. Neodymium(III) was used as a redox-invariant analog for the +3 oxidation state of americium and plutonium, which is the oxidation state that accounts for over 90% of the potential release from the WIPP through the dissolved brine release (DBR) mechanism, based on current WIPP performance assessment assumptions. These solubility data extend past studies to brine compositions that are more WIPP-relevant and cover a broader range of experimental conditions than past studies.

Uranium(VI) solubility in carbonate-free WIPP brine

Abstract The solubility of uranium(VI) was determined in WIPP-relevant brines as a function of pCH+ and ionic strength, in the absence of carbonate. Carbonate concentration was below 2 × 105 M, measured using the gas chromatography method. In the absence of carbonate, the uranium(VI) solubilities were about × 106 M in GWB at pCH+≥7 and about × 108– × 107 M in ERDA-6 brine at pCH+≥8. Solubility of uranium(VI) was also measured in NaCl media at the same levels as in ERDA-6 brine. The data established a uranium solubility that was 10–100 times lower than published results from Diaz-Arocas and Grambow [13], and they are in good agreement with modeling results and other literature data [11,12]. In the absence of carbonate, hydrolysis was the main complexation and precipitation mechanisms for uranium(VI) solubility at high ionic strength and pCH+≥7. However, the effect of borate complexation was noticeable at pCH+∼8–9.

Iron Corrosion Observations: Pu(VI)-Fe Reduction Studies

Iron and Pu Reduction: (1) Very different appearances in iron reaction products were noted depending on pH, brine and initial iron phase; (2) Plutonium was associated with the Fe phases; (3) Green rust was often noted at the higher pH; (4) XANES established the green rust to be an Fe2/3 phase with a bromide center; and (5) This green rust phase was linked to Pu as Pu(IV).