Surface-functionalized magnetic nanoparticles are promising adsorbents due to their large surface areas and ease of separation after contaminant removal. In this work, the affinity of Cr(VI) adsorption to 8 nm surface-functionalized superparamagnetic magnetite nanoparticles was determined for surface coatings with amine (trimethyloctadecylammonium bromide, CTAB) and carboxyl (stearic acid, SA) functional groups. Cr(VI) adsorbed more strongly to the CTAB-coated nanoparticles than to the SAcoated materials due to electrostatic interactions between positively charged CTAB and anionic Cr(VI) species. The adsorption of Cr(VI) by CTABand SA-coated nanoparticles increased with decreasing pH (4.5−10), which could be simulated by a surface complexation model. Cr(VI) removal performance by the nanocomposite was evaluated for two realistic drinking water compositions. The cooccurrence of divalent cations (Ca 2+ and Mg 2+ ) and Cr(VI) resulted in decreased Cr(VI) adsorption as particles were destabilized, leading to the aggregation and lower effective surface area, confirming the importance of the overall water composition on the performance of novel engineered nanomaterials for water treatment applications.

Microbial reduction of soluble hexavalent uranium (U(VI)) to sparingly soluble tetravalent uranium (U(IV)) has been explored as an in situ strategy to immobilize U. Organic ligands might pose a potential hindrance to the success of such remediation efforts. In the current study, a set of structurally diverse organic ligands were shown to enhance the dissolution of crystalline uraninite (UO 2 ) for a wide range of ligand concentrations under anoxic conditions at pH 7.0. Comparisons were made to ligand-induced U mobilization from noncrystalline U(IV). For both U phases, aqueous U concentrations remained low in the absence of organic ligands (<25 nM for UO 2 ; 300 nM for noncrystalline U(IV)). The tested organic ligands (2,6-pyridinedicarboxylic acid (DPA), desferrioxamine B (DFOB), N , N ′-di(2-hydroxybenzyl)ethylene-diamine- N , N ′-diacetic acid (HBED), and citrate) enhanced U mobilization to varying extents. Over 45 days, the ligands mobilized only up to 0.3% of the 370 μM UO 2 , while a much larger extent of the 300 μM of biomass-bound noncrystalline U(IV) was mobilized (up to 57%) within only 2 days (>500 times more U mobilization). This work shows the potential of numerous organic ligands present in the environment to mobilize both recalcitrant and labile U forms under anoxic conditions to hazardous levels and, in doing so, undermine the stability of immobilized U(IV) sources.

Adsorption and subsequent reduction of U(VI) on Fe(II)bearing clay minerals can control the mobility of uranium in subsurface environments. Clays such as montmorillonite provide substantial amounts of the reactive surface area in many subsurface environments, and montmorillonite-containing materials are used in the storage of spent nuclear fuel. We investigated the extent of reduction of U(VI) by Fe(II)bearing montmorillonite at different pH values and sodium concentrations using X-ray absorption spectroscopy and chemical extractions. Nearly complete reduction of U(VI) to U(IV) occurred at a low sodium concentration at both pH 3 and 6. At pH 6 and a high sodium concentration, which inhibits U(VI) binding at cation-exchange sites, the extent of U(VI) reduction was only 70%. Surface-bound U(VI) on unreduced montmorillonite was more easily extracted into solution with bicarbonate than surface-bound U(IV) generated by reduction of U(VI) on Fe(II)-bearing montmorillonite. We developed a nonelectrostatic surface complexation model to interpret the equilibrium adsorption of U(IV) on Fe(II)-bearing montmorillonite as a function of pH and sodium concentration. These findings establish the potential importance of structural Fe(II) in low iron content smectites in controlling uranium mobility in subsurface environments.

Adsorption of uranium onto goethite is an important partitioning process that controls uranium mobility in subsurface environments, for which many different surface complexation models (SCMs) have been developed. While individual models can fit the data for which they are parameterized, many perform poorly when compared with experimental data covering a broader range of conditions. There is an imperative need to quantitatively evaluate the variations in the models and to develop a more robust model that can be used with more confidence across the wide range of conditions. We conducted an intercomparison and refinement of the SCMs based on a metadata analysis. By seeking the globally best fit to a composite dataset with wide ranges of pH, solid/sorbate ratios, and carbonate concentrations, we developed a series of models with different levels of complexity following a systematic roadmap. The goethite−uranyl−carbonate ternary surface complexes were required in every model. For the spectroscopically informed models, a triple-plane model was found to provide the best fit, but the performance of the double-layer model with bidentate goethite−uranyl and goethite−uranyl−carbonate complexes was also comparable. Nevertheless, the models that ignore the bidentate feature of uranyl surface complexation consistently performed poorly. The goodness of fitting for the models that ignore adsorption of carbonate and the charge distributions was not significantly compromised compared with that of their counterparts that considered those. This approach of model development for a large and varied dataset improved our understanding of U(VI)−goethite surface reactions and can lead to a path for generating a single set of reactions and equilibrium constants for including U(VI) adsorption onto goethite in reactive transport models.

The mobility of uranium(VI) in subsurface environments can be limited by its adsorption to clay minerals and by chemical reduction to less mobile U(IV) solid phases. Even for adsorbed and precipitated U species that control the net release of U(VI) to the aqueous phase, dynamic exchange between phases at equilibrium conditions can occur with a pool of exchangeable solid-associated uranium considered as the labile fraction. In this study, the lability of uranium adsorbed to montmorillonite and uranium present in UO2(s) was examined by an isotope exchange technique. Crystalline UO2(s) containing depleted uranium (predominantly 238U and with small amounts of 235U) was contacted with a U(VI) aqueous solution enriched in 236U under oxygen-free (95% N2(g) + 5% H2(g)) conditions that altered the aqueous phase isotopic composition while minimally affecting the overall aqueous U(VI) concentration. Complete isotope exchange between U(VI) adsorbed onto montmorillonite and aqueous U(VI) was observed within 3.5 h. In contrast, no isotope exchange of aqueous U(VI) with UO2(s) was observed even after 47 days of contact. X-ray photoelectron spectroscopy of the UO2(s), both before and after its reaction with aqueous U(VI), showed the presence of U(VI), U(V), and U(IV) on the surface. Although U(VI) was present on the UO2(s) surface, it was nonlabile over a timescale of weeks. This study highlights the difference in lability between adsorbed and precipitated U-containing species, and it illustrates that U(VI) at the surface of a UO2 precipitate does not behave the same as U(VI) adsorbed to minerals not composed of U.