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 H(2)O(2) 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 H(2)O(2) 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 H(2)O(2) reoxidation are appropriate targets for detailed studies of natural soil/sediment Fe(III) phyllosilicate reduction.