Adsorption of Uranium(VI) to Manganese Oxides: X-ray Absorption Spectroscopy and Surface Complexation Modeling

Abstract

The mobility of hexavalent uranium in soil and groundwater is strongly governed by adsorption to mineral surfaces. As strong naturally occurring adsorbents, manganese oxides may significantly influence the fate and transport of uranium. Models for U(VI) adsorption over a broad range of chemical conditions can improve predictive capabilities for uranium transport in the subsurface. This study integrated batch experiments of U(VI) adsorption to synthetic and biogenic MnO(2), surface complexation modeling, ζ-potential analysis, and molecular-scale characterization of adsorbed U(VI) with extended X-ray absorption fine structure (EXAFS) spectroscopy. The surface complexation model included inner-sphere monodentate and bidentate surface complexes and a ternary uranyl-carbonato surface complex, which was consistent with the EXAFS analysis. The model could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. U(VI) adsorption to synthetic δ-MnO(2) appears to be stronger than to biogenic MnO(2), and the differences in adsorption affinity and capacity are not associated with any substantial difference in U(VI) coordination.

Adsorption of zinc by biogenic elemental selenium nanoparticles

“…This can be attributed to the relatively small amount of zinc adsorption at an equilibrium pH value of 4. A similar observation was made during adsorption of U(VI) on MnO 2 [48].…”

“…To this point, it was observed that the increase in NaCl concentration from 0.001 M to 0.1 M, there was no significant effect on the zinc ion adsorption by BioSeNPs (Figure 2d). This suggests that the Zn 2+ ions are adsorbed on the surface of BioSeNPs via inner sphere complexation [48].…”

Adsorptive removal of arsenic from aqueous solution by zeolitic imidazolate framework-8 (ZIF-8) nanoparticles

“…S1 (Supporting Information), after As(III) and As(V) loading, the pH IEP of ZIF-8 nanoparticles decreased from 9.6 to about 7.0 and 5.9, respectively. These results indicate that inner-sphere surface complexes may be occurred during the arsenic adsorption process on ZIF-8 nanoparticles, because when outer-sphere surface complexes are formed, the arsenic adsorption is strongly dependent on ionic strength and cannot shift the pH IEP of ZIF-8 nanoparticles [23,24]. To better understand the arsenic adsorption capacity of ZIF-8 nanoparticles, Langmuir and Freundlich isotherm models were applied here to explain the adsorption equilibriums.…”

Uranium(IV) adsorption by natural organic matter in anoxic sediments

“…Using spectroscopy and nanoscale microscopy, we have provided direct evidence that U(IV) adsorbs to organic matter and organic matter-coated clay particles in a complex natural matrix comprising a mixture of organic and mineral phases. Few researchers have investigated the adsorption behavior of U(IV) as a function of U concentration because UO 2+x is highly insoluble, making it difficult to apply traditional measurements (e.g., adsorption isotherms) (36). However, Latta et al (18) found that U(IV) adsorbed to magnetite and rutile at surface loadings less than ∼4,000 ppm; whereas UO 2+x precipitated at surface loadings of 24,000 ppm (magnetite) and 12,000 ppm (rutile).…”