THE IMPORTANCE OF NANOPARTICLES AND THEIR UNUSUAL PROPERTIES IN SEDIMENTS AND SOILS FROM HEAVY-METAL CONTAMINATED SITES

Nanoparticles of various oxides, sulfates, sulfides, and clay minerals, in samples collected from the riverbed and floodplain of the river draining the largest mining contaminated site in the United States (the Clark Fork River Superfund Complex), have been discovered, identified, and studied with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. Ferrihydrite and a vernadite-like mineral (Fe and Mn oxide hydrates, respectively) are poorly crystalline nanophases and environmentally critical in this system because they readily and consistently take up contaminant heavy metals (As, Cu, Pb, and/or Zn). In several field specimens, the ferrihydrite and vernadite-like minerals are intimately mixed on the nanoscale, but they can also occur separately. It is suggested that the vernadite-like mineral, found separately, is produced biogenically by Mn oxidizing bacteria, while the same phase associated with ferrihydrite is produced abiotically via the heterogeneous oxidation of Mn2+(aq) starting on ferrihydrite surfaces. Laboratory experiments show that the rate of this important abiotic heterogeneous Mn oxidation reaction is greatly increased in the presence of smaller and smaller nanoparticles of iron oxides. This is probably due to a change in the geometric and electronic structure of surface atoms as the particle size changes through the nanoregime. The sulfate mineral jarosite has also been observed via TEM, forming as a result of primary sulfide breakdown, and environmentally important because it incorporates Pb into its structure. Nanoparticulate secondary sphalerite and chalcopyrite, both containing toxic heavy metals, were observed forming in the anoxic streambeds. The solubilities of jarosite and these secondary sulfides are expected to be significantly higher for nanograins vs. crystals that are coarser, important due to the release of Pb and As during dissolution. Finally, nontronite and smectite, primary minerals from hydrothermal veins in the main ore body and found in nearly every sample studied, occasionally contain Cu and Zn. Further testing is needed to determine how exchangeable, and therefore bioavailable, Cu and Zn are from these clays.