Facet-preferential reduction of hematite nanocrystals by Shewanella oneidensis MR-1: An iron isotope tracer study

Coexistence of crystals with multiple facets on an individual particle or different crystals is prevalent in real environments. However, the reduction efficiency of individual crystal facets in facet-combined system is still enigmatic. This study aimed to determine the reduction efficiencies of different facets of hematite by Shewanella oneidensis MR-1. An iron isotope tracer was used to distinguish Fe(II) produced from reduction of the combined and separate hematite {001} and {100} facets, i.e., Hem {001} nanoplates and Hem {100} nanorods. Hem {001} nanoplates was reduced 1.8 times more significantly than Hem {100} nanorods when the two facets combined, whereas Hem {001} nanoplates was reduced only 1.2 times more significantly than Hem {100} nanorods in separate reactors. Initial ~ 72% and final ~ 63% of reduced atoms from Hem {001} nanoplate demonstrated the facet-preferential reduction of hematite crystal when the facets combined. The pseudo-first-order rate constants (k) of reduced Fe(II) for Hem {001} nanoplates and {100} nanorods were 3.2 and 2.0 × 10−2 d−1, respectively. We attributed the more efficient reduction of Hem {001} nanoplates than Hem {100} nanorods to their differences in surface hydroxyl groups, surface charges, ligand-bound conformation and steric effects. These findings provide new insights into microbe-mineral interaction based on the crystal facet and the overall role of Fe oxides nanocrystals in the environments.