Sorption of Metals onto Natural Sediments and Engineered Iron Oxide Nanoparticles

Metal contamination of subsurface environments and engineered water systems can be derived from natural processes and anthropogenic activities associated with industrial processes, past weapons production, and mining works. The toxic and carcinogenic effects of uranium and chromium pose a significant risk to the environment and human health. For uranium contamination in subsurface environments, phosphate addition has been performed for in-situ immobilization, which can avoid the costs associated with pump-and-treat or excavation-based remediation strategies. The interactions of uranium and phosphate in Hanford sediments had been insufficiently explored in terms of its site-specific groundwater chemistry and aquifer sediment properties. For water treatment system, novel materials such as engineered magnetite nanoparticles have gained attention due to their promising performance in separating heavy metals from the aqueous phase. As a result, the study of the interaction between metals with either sediments or nanocomposites is imperative in designing and implementing subsurface in-situ remediation and improving water treatment processes. To investigate the impact of phosphate on the immobilization of U(VI) in Hanford sediments, batch and column experiments were performed with artificial groundwater prepared to emulate the conditions at the site. Batch experiments revealed enhanced U(VI) sorption with increasing phosphate addition. X-ray absorption spectroscopy (XAS) measurements of samples from the batch experiments found that U(VI) was predominantly adsorbed at conditions relevant to most field sites (low U(VI) loadings, \u3c 25 µM), and U(VI) phosphate precipitation occurred only at high initial U(VI) (\u3e25 µM) and phosphate loadings. While batch experiments showed the transition of U(VI) uptake from adsorption to precipitation, the column study was more directly relevant to the subsurface environment because of the high solid:water ratio in the column and the advective flow of water. In column experiments, more U(VI) was retained in sediments when phosphate-containing groundwater was introduced to U(VI)-loaded sediments than when the groundwater did not contain phosphate. This enhanced retention persisted for at least one month after cessation of phosphate addition to the influent fluid. Sequential extractions and laser-induced fluorescence spectroscopy (LIFS) of column sediments suggested that the retained U(VI) was primarily in adsorbed forms. These results indicate that in-situ remediation of groundwater by phosphate addition provides lasting benefit beyond the treatment period via enhanced U(VI) adsorption to sediments. U(VI) transport through sediment-packed columns have been demonstrated to be kinetically controlled and the heterogeneous system contributed to the transport behavior under different flow rates. In water treatment processes, surface-functionalized magnetite nanoparticles have high capacity for U(VI) and Cr(VI) adsorption and can be easily separated from the aqueous phase by applying a magnetic field. A surface-engineered bilayer structure enables the stabilization of nanoparticles in aqueous solution. Functional groups such as carboxylic or amine groups in stearic acid (SA), oleic acid (OA), octadecylphosphonic acid (ODP), and trimethyloctadecylammonium bromide (CTAB) coatings led to different adsorption extents towards U(VI) and Cr(VI). The adsorption of U(VI) to OA-coated nanoparticles was examined as a function of initial loading of U(VI) (5-15 μM), pH (4.5 to 10), and the presence or absence of carbonate. CTAB-coated nanoparticles possess higher Cr(VI) adsorption affinity than nanoparticles with carboxyl groups (SA), due to the strong electrostatic interactions between opposite charges. For both U(VI) and Cr(VI), the entire adsorption dataset were successfully simulated with surface complexation models with a small set of adsorption reactions. The results show that the adsorption behavior was related to the changing aqueous species and properties of surface coatings on nanoparticles. The models could also capture the trend of pH-dependent surface potential that are consistent with measured zeta potentials. While developing novel materials for metal removal, the stability and treatment efficiency of the material need to be tested in real water systems. The application of CTAB-coated nanoparticles was tested with the presence of two drinking water supplies, and decreases in Cr(VI) adsorption were associated with the presence of Ca2+. When the Ca2+ concentration increased from 0 to 3.3 mM, adsorption decreased. Because only slight aggregation was associated with Ca2+ and an observed increase in zeta potential with Ca2+ addition should actually enhance Cr(VI) adsorption, the causes of inhibition of Cr(VI) by Ca2+ are not associated with particle size or surface charge. Instead it is likely that Ca2+ influences the structure of the organic bilayer on the nanoparticle surfaces in a way that decreased the availability of surface sites. The information gained from these research projects improved our understanding of metal interactions with both sediments from subsurface environments and engineered nanoparticles. It broadened knowledge of the controlling processes during the in-situ remediation of field sites and the separation of heavy metals from in water treatment. For remediation, the results illustrate the consideration of optimizing the timing and doses of phosphate addition in remediation strategies could lead to slower U(VI) release with effectively controlled levels. For water treatment the application of the material-based treatment processes needs more consideration of its stability and treatment performance with real water resources