Jessica Lessing

Gallium(III) sorption onto hematite was investigated as a function of pH using batch sorption experiments, precipitation controls, zeta potential measurements, extended X-ray absorption fine structure spectroscopy, and thermodynamic surface complexation modeling. Sorption experiments at initial Ga(III) concentrations of 40 and 100 µM showed a steep sorption edge at low pH and nearly complete gallium retention over a broad pH range, from pH 3.5–8.0. Parallel precipitation experiments and thermodynamic calculations demonstrated that bulk precipitation alone cannot account for the observed retention behavior, particularly at low pH and under alkaline conditions where hematite strongly immobilizes gallium. Zeta potential measurements revealed a systematic increase in surface charge in the presence of Ga(III), indicating strong specific interaction with the hematite surface. EXAFS spectroscopy showed a highly consistent local coordination environment over the entire investigated pH range, characterized by octahedral Ga–O coordination and distinct Ga–Fe backscattering paths. The spectroscopic results indicate a structurally robust retention mechanism and are consistent with either strongly bound inner-sphere surface complexes or partial incorporation of Ga(III) into near-surface Fe(III) sites of the hematite structure. Surface complexation modeling reproduced the sorption data for both initial concentrations using a single hydrolyzed Ga surface species and lead to a logK° value of −1.23 ± 0.04 for the surface complexation formation. The combined results demonstrate that Ga(III) is strongly retained by hematite over a broad pH range, leading to substantial stabilization of gallium surface species relative to dissolved species and bulk hydroxide precipitation.

The adsorption of radionuclides (RNs) onto mineral surfaces is a key retardation mechanism and plays a critical role in the long-term safety assessment of radioactive waste repositories. While numerous studies[1,2,3] have investigated the sorption behavior of trivalent actinides (e.g. Am, Cu) and lanthanides (e.g. Eu, Y) on various minerals, most experiments rely on geochemically simplified systems, typically in binary configurations involving single minerals and single sorbing species. Natural systems, however, are considerably more complex, with competitive sorption, bulk and surface precipitation, incorporation, and co precipitation processes all influencing RN retardation.In this study, we examine the competitive sorption of Eu (a chemical analogue for trivalent actinides) and Al onto quartz using a combination of batch and column experiments supported by mechanistic surface complexation modeling (SCM). Aluminum, an abundant component in natural groundwater and porewater, may compete with RNs for sorption sites. In addition, due to its low solubility at near neutral pH, Al may undergo surface precipitation, altering mineral surface charge and modifying sorption behavior.Batch experiments were performed under varying geochemical conditions, including pH, ionic strength, and initial Al and Eu concentrations. The results show that Al sorption onto quartz arises at lower pH compared to Eu, with sorption edges at approximately pH 4.5 for Al and pH 5.5 for Eu (50% sorbed). In the presence of Al, Eu sorption onto quartz is significantly reduced. Batch sorption data is used for SCM calibration via inverse modeling. These models are then validated using experimental data from column experiments to assess the applicability, advantages, and limitations of derived parameters. In addition, column experiments with K‑feldspar are planned and will be evaluated using SCM to further examine competing sorption effects. Ongoing work focuses on further model development. First modelling outcomes, together with initial assessments of the applicability, strengths and limitations of the emerging parameter set will be presented.These investigations contribute to improving previous modelling approaches in which competitive Al sorption may have influenced radionuclide behavior. More broadly, the results highlight the importance of understanding geochemical surface reactions to enhance the reliability of long-term safety assessments for radioactive waste repositories.[1] J. Neumann, J. Colloid Interface Sci. (2020) (https://doi.org/10.1016/j.jcis.2020.11.041)[2] J. Lessing, Colloids and Surfaces A (2024) (https://doi.org/10.1016/j.colsurfa.2024.133529) [3] S. Britz, PhD Thesis, 2018 (https://doi.org/10.24355/dbbs.084-201806051207-0)

We are convinced that a sound understanding of retardation and migration processes is essential for the development of any sorption databases. In this context discussions often raise the question, "Haven't we already collected enough data and knowledge on sorption processes over the past decades?" Indeed, as an example, many studies[1,2] have focused on the sorption of trivalent actinides (e.g., Am, Cm) and lanthanides (e.g., Eu, Y) onto various mineral surfaces. However, these studies typically investigate geochemically simple systems, often in binary configurations with single surfaces and single sorbates, to gain a fundamental understanding of surface processes.Natural systems are far more complex, with the possibility of e.g. competitive sorption, bulk and surface precipitation, incorporation, and co-precipitation processes that all potentially affect radionuclide (RN) retardation, not to mention possible organic interactions. Therefore, in an ongoing research project we focus on retardation and mobilization processes under more complex, closer-to-nature geochemical conditions. Here, the challenge lies in the unique combination of spectroscopic, microscopic, chemical, and electrochemical techniques, as along with the corresponding thermodynamic modeling. Consequently, we couple batch, column, and surface charge (streaming potential) experiments with atomic force microscopy (AFM), crystal truncation rod scattering/resonant anomalous X-ray reflectivity (CTR/RAXR), powder X-ray diffraction (PXRD), and quartz-crystal-microbalance (QCM) measurements. Our focus is on competing surface reactions, dissolution kinetics, and surface precipitation. The systems investigated involve trivalent lanthanides competing with Al(III) and Ga(III) on K- and Ca-rich feldspars, as well as on hematite and quartz surfaces.So far, AFM measurements have confirmed the presence of an Al layer on the surface of K-rich feldspar (orthoclase), revealing the formation of a low-density amorphous Al layer, which varies with pH. It is assumed that this layer might influence the mineral’s surface charge behavior as previously reported in [2]. In terms of hematite, competitive sorption studies revealed that Al(III) does not significantly affect Eu(III) retention, except for a slight increase at low pH, suggesting limited competition under environmentally relevant conditions. For the investigated quartz surfaces, Al(III) retardation is relevant at lower pH-values compared to Eu(III) sorption processes resulting in potential competing surface reactions under relevant pH conditions which will be studies in detail. QCM measurements with feldspar-coated sensors have shown that it is possible to observe feldspar dissolution and Al as well as Eu uptake, albeit for relatively high solute concentrations. A new cell design was developed offering the opportunity to directly couple QCM and streaming potential measurements.The experimental data will be used to develop sound and robust thermodynamic surface complexation models (SCMs) consistent with dissolution/precipitation patterns. These models will then be validated using data from column experiments to assess the applicability, advantages, and limitations of derived parameters.[1] J. Neumann et al., J. Colloid Interface Sci. (2020) (https://doi.org/10.1016/j.jcis.2020.11.041) [2] J. Lessing et al., Colloids and Surfaces A (2024) (https://doi.org/10.1016/j.colsurfa.2024.133529)

The transport of radionuclides in the environment is a major problem for the safety assessment of radioactive waste repositories. Storage in deep geological repositories is considered a safe disposal strategy because of their ability to isolate hazardous components from the biosphere for hundreds of thousands of years. Minor actinides (i.e. Am, Cm and Np) dominate the radiotoxicity of spent nuclear fuel over geological timescales. In underground repositories, reducing conditions are expected, and therefore the trivalent oxidation state is dominant for Am and Cm as well as possibly for Pu. For investigations of the mobility of the trivalent actinides Am³⁺ and Cm³⁺, the less toxic trivalent rare earth elements, in particular Eu³⁺, are commonly used. Besides clay and salt, crystalline rock is considered a possible host rock for deep geological repositories. Crystalline rock (e.g. granite) consists mainly of quartz, mica and feldspar. The latter forms common aluminosilicates making up ∼60 vol. % of the earth's crust, but their sorption behaviour is not well understood, especially for the Ca-bearing members of the group. Here, we study the sorption of trivalent actinides and their rare earth element homologues on plagioclase which are Ca-bearing feldspars, quantitatively and mechanistically. Zeta potentials of various Ca feldspars show an unexpected increase at pH 4–7, which becomes more pronounced as the amount of Ca in the crystal lattice increases. This can be interpreted by assuming uptake of Al³⁺ and/or the precipitation of an Al phase, where Al originates from feldspar dissolution at different pH values. Nevertheless, only minor differences were found in the retention and surface speciation of Cm³⁺ on Ca and K feldspars (Neumann et al., 2021). Ca feldspar has a slightly higher potential to retain trivalent metal ions compared to K feldspar. An inner-sphere (IS) complex and its two hydrolysis forms have been identified on both minerals, but the hydrolysis of the IS complex is stronger in the Ca-rich mineral. A surface complexation model for Ca feldspar was developed by combining the batch sorption data and the spectroscopically identified surface complexes to describe the experimental data. These data will be the basis for the improvement of transport simulations for a reliable safety assessment of potential radioactive waste repositories in crystalline rock.