Understanding molecular-scale interactions between natural organic matter (NOM) and nuclide species is essential for predicting contaminant fate and designing effective remediation strategies in aquatic environments. Molecular simulations were used to investigate the complexation and aggregation behaviors of four representative nuclide species (UO22+, Sr2+, Cs+, and TcO4−) with NOM under environmentally relevant aqueous conditions. A clear hierarchy in binding affinity was observed (UO22+ > Sr2+ > Cs+ ≈ TcO4−), with distinctly ion-specific complexation mechanisms. UO22+ predominantly forms stable, multidentate inner-sphere complexes with oxygen-containing functional groups of NOM, whereas Sr2+ mainly exhibits monodentate inner-sphere coordination. In contrast, Cs+ shows weak, predominantly outer-sphere association, while TcO4− is electrostatically excluded, leading to unfavorable association with NOM. The resulting free-energy profiles elucidate the thermodynamic controls governing these ion-specific interactions. Furthermore, background Ca2+ promotes NOM aggregation, which may alter effective nuclide retention in a nuclide-dependent manner, enhancing immobilization of strongly complexing cations while providing limited attenuation for weakly interacting species. These findings highlight the role of NOM as a selective geosorbent and scavenger in aquatic systems and emphasize the need for nuclide-specific risk assessments that explicitly consider ambient water chemistry when predicting nuclide mobility.
Comprehensive solubility experiments with niobium(V) were conducted from oversaturation conditions in weakly alkaline, carbonated solutions equilibrated with calcite and carbonated cement. These conditions are expected in advanced degradation stages of cement, and are representative of cementitious environments as those foreseen in underground and near-surface repositories for short-lived low- and intermediate-level radioactive wastes (L/ILW-SL). Moderate niobium concentrations ([Nb]aq ≈ 10−6 M) were quantified at short contact times (t < 3 days), matching thermodynamic calculations with Nb2O5(am) as solubility-controlling solid phase. At longer equilibration times (t > 10 days), a further decrease in solubility ([Nb]aq ≤ 10−8 M) is observed, which evidences the formation of a sparingly soluble secondary phase transformation. Comparison of solubility data determined in this work with previous studies in carbonate-free systems support that Nb(V)-carbonate complexes can be neglected in the investigated range of carbonate concentrations (≈10−3–10−4 M). Systematic characterization of the resulting solid phases by means of Raman spectroscopy and powder X-ray diffraction unequivocally point towards the predominance of hydrated Ca-Nb(V) solid phase/s of rather amorphous character and small (sub-nm) particle size. A conclusive assignment of the stoichiometry of this/these solid phase/s is not feasible at this stage, although available evidence points towards a Ca: Nb ratio clearly below 1.
The potential for waste rock from mining activities to leach poor quality drainage has been well documented. However, despite the potential for leaching to occur over many decades or centuries, few studies have investigated the relevant geochemical processes over long timescales. This study looks at the porewater geochemistry from a ∼10 Ktonne waste-rock pile at an abandoned prospecting property, the Ore Chimney mine, that has been in place for approximately 100 years. Waste-rock samples were collected at 0.5 m intervals from three trenches that transected the ∼5 m depth of the pile; samples were centrifuged to extract porewaters and analyzed for standard geochemical parameters, anions, cations, and metals. The geochemical modelling software PHREEQC was used to explore possible secondary controls on porewater chemistry. The results indicate near-neutral pH throughout the depth of the waste rock and abundant alkalinity (average: 58.6 mg/L CaCO3), indicating that carbonate buffering is still active within the waste rock. Zn concentrations, as high as 87.5 ppm (16.5 ppm on average), accounted for 92% [M] of heavy metals and is attributed to the oxidative dissolution of sphalerite, the primary sulfide mineral in the waste rock. The preferential weathering of sphalerite over pyrite produces less acidity and plays a large role in maintaining carbonates and neutral pH in the system. Despite the long period of weathering within the Ore Chimney waste rock, weathering processes are similar to those observed in other waste-rock studies and differences in geochemical parameters are largely ascribed to differences in lithology, principally sulfide mineralogy and carbonate content, rather than the age of the waste-rock. High alkalinity levels suggest that the dissolution of calcite and dolomite controlpH, and Pb, Zn, Cd, and Cu concentrations in the porewaters. Results from this study indicate that the source lithology of sulfides plays a strong role in weathering. This study shows that a detailed understanding of in situ solid- and aqueous-phase geochemistry is required to delineate waste-rock weathering as a function of long-term exposure.
