CHARACTERIZATION OF FRESH AND AGED BIOGENIC IRON OXIDES (BIOS)

Biogenic iron oxides, known as BIOS, have recently been the subject of several studies because of their potential role in contaminant sorption in natural environments. Other studies have also looked at the microbial population diversity of BIOS and at the key chemical factors controlling their formation and occurrence. Fresh natural BIOS were collected in a groundwater seepage area from pH-neutral Cu-Zn-Pb mine tailings in Quebec, Canada. The amount of soluble Fe (II) and Fe total in the stream where BIOS formed decreased with distance from the source of the seepage area and the surface waters in contact with BIOS contained lower levels of heavy metals than waters collected at a BIOS-free site. Chemical digestion of the BIOS samples indicated that all heavy metals were sequestered by the matrix of cells and iron oxides, while X-ray diffraction analyses showed that ferrihydrite was a common component of all BIOS samples, but trace amounts of goethite and lepidocrocite were also detected. SEM imaging revealed that they were essentially composed of small particles of iron oxides in close association with tube-like structures reminiscent of Leptothrix cells, a well known neutrophilic iron oxidizing bacterium.

BIOS from an another site, i.e., a wetland in Chalk River, Ontario, were aged at 4 C for a period of 5 years in the dark in order to simulate diagenesis. X-ray diffraction of the fresh and aged BIOS displayed similar patterns, indicating no change in mineralogy upon ageing, which was corroborated by Fe-EXAFS. Microbial reduction of the aged BIOS however showed that the rates of reaction were slower when compared to those of the original fresh BIOS. TEM observations of the aged BIOS revealed the presence of coarse Fe-oxides particles onto the cells, which were much larger that the original iron oxides. Our results therefore indicate the coarsening of the iron oxides slows down the reduction rates, which implies that aged BIOS are efficient sorbents of metal contaminants and than upon ageing, they are capable of retaining more metals into their structures than previously thought.