Identifying the Iron Phases in Biosolids-Amended Soils Via Mössbauer Spectroscopy that Control Cadmium Sorption.
Mark A. Chappell, Aaron G.B. Williams, Kirk G. Scheckel, and James A. Ryan. U.S. Environmental Protection Agency, 5995 Center Hill Ave, Cincinnati, OH 45224-4504
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 is to demonstrate the utility of Mössbauer spectroscopy in identifying the dominant inorganic/organic Fe phases in biosolids-amended soils. In this work, we present investigations into the role of inorganic (Fe) and organic phases in biosolids-amended soils on cadmium sorption. We employed a purely physical density separation to remove organic carbon, and thus diminish its role in cadmium sorption. Preliminary results from batch sorption work showed that cadmium sorption was reduced with decreasing organic carbon content of biosolids-amended soils, yet cadmium sorption was also controlled by total Fe content. Preliminary Mössbauer results show that Fe in biosolids-amended soils is predominantly found as yet-to-be identified ferric (Fe3+) oxides, with only a minor fraction of biosolids Fe associated with humic/organic domains. The results suggest that cadmium sorption and stability in soil is predominantly based on the interaction of the heavy metal with the inorganic ferric-oxide phases, which probably accumulate as precipitates when the soil dries. The lack of organic-Fe complexes suggests a reduced potential for mobilizing biosolids cadmium through humic/organic chelation.