| Sorption of toxic trace metals on soil materials is an important process influencing the environmental quality of soils by decreasing the mobility and bioavailability of the metals. The recent development of atomic-resolution techniques has stimulated efforts to determine sorption mechanisms. In this thesis, the sorption mechanisms of potentially toxic metals by minerals and humic substances are examined using x-ray absorption spectroscopy.; This thesis includes investigations of the: (1) reaction of trivalent lanthanide ions, serving as analogs of radioactive trivalent actinide ions, in an effort to understand the sorption behavior of hazardous nuclear waste elements, and (2) complexation of methylmercury by humic substances, in an effort to determine the dominant methylmercury-binding mechanism in soil organic matter. The hypotheses tested in this study are: (1) Ln 3+ reacts with phosphate on mineral surfaces to yield phosphate surface precipitates, and (2) CH3Hg+ reacts with humic substances, binding to reduced-sulfur ligands.; The results described in this thesis reveal that the amount of lanthanide ions (Eu3+, Gd3+, and Dy3+) sorbed on boehmite (γ-AlOOH) surfaces dramatically increases in the presence of phosphate. Trivalent gadolinium and dysprosium ions are precipitated on boehmite surfaces, in the presence of phosphate, and, most likely, in the absence of phosphate. In the presence of phosphate, these rare-earth cations react to form uniform, ultra-fine particles of LnPO4 surface precipitates. To the extent that Ln3+ ions serve as suitable chemical analogs for Ac3+ ions, the results of this thesis predict the ubiquitous phosphate, adsorbed to all mineral surfaces in the environment, would enhance the sorption of Ac3+ ions leading to the precipitation of ultra-fine AcPO4 particles on mineral surfaces.; The results of the humic complexation experiments reveal that when CH 3Hg+ concentrations exceed a certain fraction of the reduced-sulfur ligands in organic matter, nitrogen or oxygen ligands replace reduced-sulfur ligands as the predominant ligands complexing this toxic organometallic cation. An important result relates to the number of reactive reduced-sulfur binding sites; namely assuming every reduced sulfur is equally reactive toward CH3Hg+ leads to an overestimation of the reactive reduced-sulfur ligands. Finally, CH3Hg+ molecules form 1:1 complexes with humic ligands, retaining intact C–Hg bonds. |
