Comparisons of structural iron reduction in smectites by bacteria and dithionite: II. A variable-temperature Mossbauer spectroscopic study of Garfield nontronite

The reduction of structural Fe in smectite may be mediated either abiotically by reaction with chemical reducing agents or biotically by reaction with various bacterial species. The effects of abiotic reduction on clay surface chemistry are much better known than the effects of biotic reduction, and differences between them are still in need of investigation. The purpose of the present study was to compare the effects of dithionite (abiotic) and bacteria (biotic) reduction of structural Fe in nontronite on the clay structure as observed by variable-temperature Mossbauer spectroscopy. Biotic reduction was accomplished by incubating Na-saturated Garfield nontronite (sample API 33a) with Shewanella oneidensis strain MR-1 (Fe-II/total Fe achieved was similar to 17 %). Partial abiotic reduction (Fe-II/total Fe similar to 23 %) was achieved using pH-buffered sodium dithionite. The nontronite was also reduced abiotically to Fe-II/total Fe similar to 96 %. Parallel samples were reoxidized by bubbling 02 gas through the reduced suspensions at room temperature prior to Mossbauer analysis at 77 and 4 K. At 77 K, the reduction treatments all gave spectra composed of doublets for structural Fe-II and Fe-III in the nontronite. The spectra for reoxidized samples were largely restored to that of the unaltered sample, except for the sample reduced to 96 %. At 4 K, the spectrum for the 96 % reduced sample was highly complex and clearly reflected magnetic order in the sample. When partially reduced, the spectrum also exhibited magnetic order, but the features were completely different depending on whether reduced biotically or abiotically. The biotically reduced sample appeared to contain distinctly separate domains of Fe-II and Fe-III within the structure, whereas partial abiotic reduction produced a spectrum representative of Fe-II-Fe-III pairs as the dominant domain type. The 4 K spectra of the partially reduced, fully reoxidized samples were virtually the same as at 77 K, whereas reoxidation of the 96 % reduced sample produced a spectrum consisting of a magnetically ordered sextet with a minor contribution from a Fe-II doublet, indicating significant structural alterations compared to the unaltered sample.