Microorganisms are commonly bonded to various soil minerals, which may influence the redox processes and
bacterial metabolism. However, little is known about the impact of particle size of soil minerals on these redox
processes in the subsurface environment. In this study, the Cr(VI) bioreduction by Shewanella oneidensis MR-1
(S. oneidensis) was investigated in the presence of various hematite (α-Fe2O3) particles with average diameters
of 1.0 µm (hem-1 µm) and 80.0 nm (hem-80 nm) under different pH conditions. Fourier transformed infrared
spectroscopy coupled with two-dimensional correlation spectroscopy (FTIR-2D-COS) analysis and isothermal
titration calorimetry (ITC) were used to explore the interaction between S. oneidensis and hematite and monitor
the bacterial metabolic activity, respectively. X-ray photoelectron spectroscopy (XPS) was used to elucidate the
Cr(VI) removal mechanisms. Our results showed that 78% of chromate can be reduced to Cr(III) by S. oneidensis
alone. Whereas, chromate reduction rates were 62% and 85% in the presence of hem-1 µm and hem-80 nm,
respectively. The enhancement of Cr(VI) reduction by S. oneidensis-hem-80 nm complex may be due to the large
surface area as well as the positive charge of hem-80 nm at neutral pH, which influences the physical contact
between S. oneidensis and iron oxides. The microcalorimetric results showed that both hem-1 µm and hem-80 nm
promoted the normal physiological functions of S. oneidensis. XPS confirmed the gradual FeCr2O4 formation and
Fe(II) depletion during the Cr(VI) reduction process. This work expands our understanding of microbial-mineral
interaction and its role in Cr(VI) removal mechanisms in the subsurface environment.