Heavy metal and radionuclide sequestration by minerals: Spectroscopic investigations and environmental implications.

摘要

The main objective of this dissertation is to investigate the structures and mechanisms of heavy metals and radionuclides interacting with different materials, utilizing complementary techniques such as batch uptake, X-ray diffraction (XRD), thermogravimetric - differential thermal analysis (TG-DTA), direct current plasma-atomic emission spectrometer (DCP-AES), pair distribution function (PDF) analysis of X-ray total scattering and X-ray absorption spectroscopy (XAS). Contaminants of interest include UO22+, Cr 3+ and Zn2+. Sorbent materials studied include aluminum oxide, the common industrial remediation byproduct iron-oxyhydroxide (ferrihydrite), and naturally occurring hydroxylapatite.;The effects of co-existing ligands such as phosphate/arsenate on uranyl sorption on alumina surface were examined over a range of pH and concentration conditions. In the presence of arsenate, uranyl sorption was greatly enhanced at acidic pH ranges, and the amount of enhancement is positively correlated to the initial arsenate and uranyl concentrations. At pH 4-6, U LIII - and As K-edge EXAFS results suggest the formation of surface-sorbed uranyl species as well as uranyl arsenate surface precipitates(s) with a structure similar to trogerite. Uranyl polymeric species or oxyhydroxide precipitate(s) become more important with increasing pH values.;The bulk structure of products of Cr coprecipitation with nanocrystalline iron oxyhydroxide (ferrihydrite) was characterized over a range of Cr concentrations (0--100%). Thermal analysis shows the pure Cr end member has higher water content than ferrihydrite, and show three stages of weight loss probably related to the loss of surface/structural water and hydroxyl groups. Pair distribution function (PDF) analysis shows progressive structural changes and a decrease of coherent domain size with increasing Cr content, from ∼2 nm for ferrihydrite to ∼1 nm for the pure Cr end member, which is likely to have only short-range order and is amorphous. X-ray absorption near edge structure (XANES) analysis suggests both Cr(III) and Fe(III) being the dominant oxidation state, and therefore no redox reactions occur during the Cr incorporation. XANES also shows progressive changes in the local structure around Cr and Fe atoms.;Structure of Zn incorporation into hydroxylapatite (HAP) was characterized using complementary techniques. XAS results demonstrate that the incorporated Zn occurs in tetrahedral coordination in HAP. EXAFS fit results were consistent with theoretical models calculated using density function theory, and suggest the incorporated Zn occupies the Ca2 site with large local distortion. Great similarities of local structure were observed between Zn-doped synthetic and biological apatite samples, suggesting similar mechanism(s) of Zn incorporation into hydroxylapatite.;Effects and mechanisms of pre-treating alumina surfaces with arsenate on uranyl sorption were investigated. Surface modification using inorganic ligands such as arsenate can greatly enhance uranyl sorption under acidic conditions. Positive correlations were observed between U(VI) uptake and the ratio between initial As solution concentration for pretreatment and initial U solution concentration, suggesting the formation of ternary surface complex(es) and/or precipitates. Extended X-ray absorption fine structure (EXAFS) results at both the U LIII- and As K-edges suggest the formation of U-As precipitate(s) with a structure similar to UO2HAsO4 ˙4H2O and likely U polymeric species at high U concentrations. The ratios between surface sorbed uranyl, U-As precipitate(s) and uranyl polymeric species are dependent on the [As]ini/[U]ini ratio and absolute U initial concentration.;Overall, the studies addressed in this dissertation provide new insights into characterization of the mechanisms of heavy metal and radionuclide sequestration by natural or engineered materials in complex systems. The results might also have direct applications for contaminant remediation based on surface sorption or structural incorporation processes. Pretreatment processes may have applications for the design and selection of fill materials for permeable reactive barriers (PRB), which can be used for removing dissolved uranium from groundwater.