Effects of in situ biostimulation on iron mineral speciation in a sub-surface soil

The in situ alteration of Fe redox states in subsurface soils by bacteria, otherwise known as bioreduction, may play a key role in the immobilization of hazardous redox active metals such as U, Tc, and Cr. The objective of this study was to characterize changes in Fe mineralogy occurring in a subsurface soil as a result of biostimulation in order to evaluate the bioremediation potential of this approach. Biostimulation was achieved by injecting glucose into the soil through a small well next to a sampling well. Cores taken from the sampling well were analyzed by variable-temperature 57Fe Mössbauer spectroscopy. Results revealed that biostimulation resulted in an overall loss of Fe from the system and major changes in the distribution of its oxide and oxyhydroxide mineral forms. Compared to the non-biostimulated soil, the spectral components assigned to goethite were greatly diminished in intensity in the samples that had been biostimulated, whereas the hematite component was appreciably increased. The Fe(II):Fe(III) ratio in the non-oxide phase (aluminosilicate clay minerals) also increased, indicating that the bioreduction processes in the soil also affected the redox state of Fe in the constituent clay minerals.