Continuous debate regarding inorganic and organic phases in biosolids as prominent sorbents of metals has yielded limited definitive data. We have demonstrated with X-ray absorption and X-ray fluorescence spectroscopies that metals in biosolids have a significant association with Fe phases within biosolids and biosolids amended soils; however, identification of these Fe phases has not been fully characterized and has been limited to a few samples. Additionally, the detection of Fe crystals in biosolids by X-ray diffraction is nearly impossible. The purpose of this presentation will be to discuss the application of Mössbauer spectroscopy to address several key issues surrounding the chemistry of Fe phases in biosolids.

Mössbauer spectroscopy measures the Mössbauer effect which involves the recoil-free emission and absorption of gamma radiation from the excited states of a nucleus. Since the gamma emission is recoil-free, it can be resonantly absorbed by stationary atoms present in a solid. The nuclear transitions are very sensitive to the local environment of the Fe atom and Mössbauer spectroscopy is a sensitive probe of the different environments an Fe atom occupies in a solid material. The atomic interactions, which include isomer shift, quadrupole splitting and magnetic splitting, are the primary characteristics of Mössbauer spectra to identify Fe speciation.

In our discussion we will present a background review of Mössbauer spectroscopy and provide examples of Mössbauer investigations of various biosolids with different Fe concentrations and components. As in most heterogeneous systems, the chemistry of Fe in biosolids is dependent upon many factors that are influenced by the origination, processing, and the handling of biosolids prior to land application. Speciation of Fe phases by Mössbauer spectroscopy will improve our understanding of metal retention in biosolids amended soils and of biological availability issues.