Frank Bok

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.

Uranium contamination is a major global concern, as its chemical and radiological effects threaten ecosystems and human health, particularly in mining-impacted regions. Pentavalent uranium is a key but often overlooked intermediate in U biogeochemistry, challenging the conventional view of a direct U(VI) to U(IV) transition and precipitation in natural waters. Here we demonstrate the formation and stability of U(V) under environmentally relevant mine-water conditions using advanced spectroscopic and microscopic analyses. Our data reveal concurrent U(VI) reduction to U(IV), as biogenic uraninite nanoparticles, and to U(V) as FeU ^(V) O₄ nanoparticles and U(V)-carbonate complexes. U(V) persists for at least 130 days under anoxic conditions and four weeks after exposure to oxygen. Microbial community analysis reveals enrichment of fermentative and sulphate-reducing taxa, consistent with redox conditions favouring U reduction. By demonstrating the stability of immobilised U(V) alongside U(IV), this work advances the understanding of uranium biogeochemistry and offers new insights for sustainable remediation strategies.

Understanding the behavior of contaminants in soils and their transport in water is crucial. This study focuses on the mobility of uranium (U) in a buried contaminated sediment layer, deposited in a wetland 70 years ago by the first U mining activities in France. U mobilization is kinetically limited in the contaminated layer with a small fraction of labile U as deduced from laboratory desorption-based studies and in situ studies performed by DET (Diffusive Equilibration in Thin films)/DGT (Diffusive Gradients in Thin films) passive sampler data monitored over time. The smart-K_d approach, combined with a comprehensive system characterization, identified this labile fraction as U(VI) primarily adsorbed on 2:1 clay minerals, which is in line with X-ray Absorption Spectroscopy (XAS) data. The inert fraction was found to correspond to U minerals and adsorbed U(IV) and U(VI) species exhibiting desorption hysteresis. Combining laboratory and field studies, this multiscale approach allows to connect molecular processes to operational K_d(labile) parameters.

This paper presents a comprehensive data-to-model workflow, including a findable, accessible, interoperable, reusable (FAIR) community sorption database (newly developed LLNL Surface Complexation/Ion Exchange (L-SCIE) database) along with a data fitting workflow to efficiently optimize surface complexation reaction constants with multiple surface complexation model (SCM) constructs. This workflow serves as a universal framework to mine, compile, and analyze large numbers of published sorption data as well as to estimate reaction constants for parameterizing reactive transport models. The framework includes (1) data digitization from published papers, (2) data unification including unit conversions, and (3) data-model integration and reaction constant estimation using geochemical software PHREEQC coupled with the universal parameter estimation code PEST. We demonstrate our approach using an analysis of U(VI) sorption to quartz based on a first L-SCIE implementation, concluding that a multisite SCM construct with carbonate surface species yielded the best fit to community data. Surface complexation reaction constants extracted from this approach captured all available sorption data available in the literature and provided insight into previously published reaction constants and surface complexation model constructs. The L-SCIE sorption database presented herein allows for automating this approach across a wide range of metals and minerals and implementing novel machine learning approaches to reactive transport in the future.

In this study, we investigated the interaction of U(VI) and Eu(III) with Brassica napus suspension plant cells as a model system. Concentration-dependent (0–200 μM) bioassociation experiments showed that more than 75% of U(VI) and Eu(III) were immobilized by the cells. In addition to this phenomenon, time-dependent studies for 1 to 72 h of exposure showed a multistage bioassociation process for cells that were exposed to 200 μM U(VI), where, after initial immobilization of U(VI) within 1 h of exposure, it was released back into the culture medium starting within 24 h. A remobilization to this extent has not been previously observed. The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used to correlate the bioassociation behavior of Eu and U with the cell vitality. Speciation studies by spectroscopy and in silico methods highlighted various U and Eu species over the course of exposure. We were able to observe a new U species, which emerged simultaneously with the remobilization of U back into the solution, which we assume to be a U(VI) phosphate species. Thus, the interaction of U(VI) and Eu(III) with released plant metabolites could be concluded.