Isolation and Microbial Reduction of Fe(III) Phyllosilicates from Subsurface Sediments

Fe­(III)-bearing phyllosilicates can be important sources of Fe­(III) for dissimilatory microbial iron reduction in clay-rich anoxic soils and sediments. The goal of this research was to isolate Fe­(III) phyllosilicate phases, and if possible, Fe­(III) oxide phases, from a weathered shale saprolite sediment in order to permit experimentation with each phase in isolation. Physical partitioning by density gradient centrifugation did not separate phyllosilicate and Fe­(III) oxide phases (primarily nanoparticulate goethite). Hence we examined the ability of chemical extraction methods to remove Fe­(III) oxides without significantly altering the properties of the phyllosilicates. XRD analysis showed that extraction with acid ammonium oxalate (AAO) or AAO in the presence of added Fe­(II) altered the structure of Fe-bearing phyllosilicates in the saprolite. In contrast, citrate-dithionite-bicarbonate (CDB) extraction at room temperature or 80 °C led to minimal alteration of phyllosilicate structures. Reoxidation of CDB-extracted sediment with H₂O₂ restored phyllosilicate mineral d-spacing and Fe redox speciation to conditions similar to that in the pristine sediment. The extent of microbial (Geobacter sulfurreducens) reduction of Fe­(III) phyllosilicates isolated by CDB extraction and H₂O₂ reoxidation (16 ± 3% reduction) was comparable to what took place in pristine sediments as determined by Mossbauer spectroscopy (20 ± 11% reduction). These results suggest that materials isolated by CDB extraction and H₂O₂ reoxidation are appropriate targets for detailed studies of natural soil/sediment Fe­(III) phyllosilicate reduction.