MINERALOGICAL AND SPECTROSCOPIC INVESTIGATIONS OF SOILS AND SEDIMENTS: FROM TOXIC TRACE METALS TO FREE RADICALS (Invited Presentation)

It is well recognized that fine-grained soils, sediments, and natural organic matter play major roles in trace-metal cycling through various molecular-level processes. We will discuss these fundamental processes observed in situ based on our field studies of heavy metal(oid)s in a variety of contaminated environments over the past two decades. We will show that careful sampling and detailed mineralogical analysis, including Rietveld refinement of powder XRD patterns, Electron Microscopy and EXAFS spectroscopy provide direct clues about the mineral and organic species controlling trace metal mobility in natural or heavily contaminated environments. Comparison of various field sites revealed major minerals, surface and organic-bound species involved in the immobilization of toxic elements such as lead, arsenic and uranium. For instance, sorbed species can form in recently contaminated soils/sediments, whereas poorly ordered phases may precipitate in highly contaminated environments, which can facilitate metal(oid)s sequestration. In addition to non-crystalline forms, low-solubility lead-phosphate minerals and arsenic-iron-sulfide minerals observed in hundreds to thousands-year-old soils and sediments are also important to consider for improving the long-term management of contaminated areas. Uranium in contaminated soils and sediments at environmentally relevant concentrations tends to occur in a continuum of sorbed species ranging from mononuclear sorbates to polynuclear clusters that include phosphate, silicate, or NOM ligands. These ill-defined species are the major forms of U in thousand-year-old soils and sediments under oxidizing and reducing conditions. Accordingly, we infer that they may control long-term U binding to solid phases in a variety of surface environments. Finally, EPR spectroscopy coupled with spin-trapping revealed radical species and putative iron reactive species formed at the mineral/water interface from air oxidation of iron oxide and sulfide minerals. We will show that such reactions can also occur in soils and sediments, with a focus on the role of organic and inorganic ligands. Based on these findings, we will discuss the putative role of such reactive processes in the natural, long-term degradation of organic pollutants in redox-active environments.