Journal of Environmental Chemical Engineering 9 (2021) 105096 Available online 18 January 2021 2213-3437/© 2021 Elsevier Ltd. All rights reserved. Size effect of hematite particles on the Cr(VI) reduction by Shewanella oneidensis MR-1 Abdelkader Mohamed a, b , Boya Sun a , Cheng Yu a , Xuemeng Gu a , Noha Ashry a , Yassine Riahi a , Ke Dai a, *, 1 , Qiaoyun Huang a a Key Laboratory of Arable Land Conservation (Middle and Lower Reach of Yangtze River), Ministry of Agriculture and Rural Affairs, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China b Soil and Water Research Department, Nuclear Research Center, Atomic Energy Authority, Abou Zaabl 13759, Egypt A R T I C L E INFO Editor: Dr. Zhang Xiwang Keywords: Hematite particle size Shewanella oneidensis MR-1 Cr(VI) reduction Microcalorimetry 2D-COS studies ABSTRACT Microorganisms are commonly bonded to various soil minerals, which may infuence 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-Fe 2 O 3 ) 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 infuences 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 confrmed the gradual FeCr 2 O 4 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. 1. Introduction Chromium (Cr), has become a hot area of interest due to its mounting release into the environment from various industrial activities such as wood preservation, leather tanning, electroplating, alloy manufacturing, pigment, and dye synthesis [1]. Chromium mainly exists in two valence states: hexavalent Cr(VI) and trivalent Cr(III) forms in the contaminated environment [2]. Since Cr(VI) is more toxic, soluble, mobile, and carcinogenic than Cr(III), therefore, it is highly recom- mended to convert Cr(VI) to Cr(III) due to the low toxicity, solubility, and mobility of Cr(III) as well as its precipitation potential as Cr(III) (oxy)hydroxides [3–5]. Microbial Cr(VI) reduction has been regarded as the most suitable chromium remediation approach because of the natural widespread bacteria in the soil and water environment under anaerobic conditions. Moreover, it has been reported that the Cr(VI) bioreduction is a more eco-friendly strategy than the conventional physico-chemical strategies which often result in secondary sludge [6,7]. Recently, many bacteria have been reported to reduce the more toxic Cr(VI) to less toxic Cr(III) including dissimilatory metal reducing bacteria (DMRB) [8,9]. It is well known that the functional groups such as hydroxyl, carboxyl, amino, etc., which are important participants in the biological adsorption pro- cess on the surface of microbial cell walls are infuenced by the system pH [10]. Iron minerals are ubiquitous in the soil environment and have a high surface area and surface activity, which can be used for heavy metal remediation due to their high redox activity [11,12]. A previous study demonstrated that the effect of minerals on the microbial Cr(VI) * Corresponding author. E-mail address: dk@mail.hzau.edu.cn (K. Dai). 1 ORCID: 0000–0003-0727–0399 Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece https://doi.org/10.1016/j.jece.2021.105096 Received 4 November 2020; Received in revised form 30 December 2020; Accepted 17 January 2021