minerals Article Bioleaching Coal Gangue with a Mixed Culture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans Zihao Chen 1 , Xinying Huang 1 , Huan He 1, *, Jielin Tang 1 , Xiuxiang Tao 1 , Huazhou Huang 2 , Rizwan Haider 3 , Muhammad Ishtiaq Ali 4 , Asif Jamal 4 and Zaixing Huang 1,5, *   Citation: Chen, Z.; Huang, X.; He, H.; Tang, J.; Tao, X.; Huang, H.; Haider, R.; Ali, M.I.; Jamal, A.; Huang, Z. Bioleaching Coal Gangue with a Mixed Culture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. Minerals 2021, 11, 1043. https://doi.org/10.3390/min11101043 Academic Editor: Jean-François Blais Received: 27 August 2021 Accepted: 23 September 2021 Published: 26 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Key Laboratory of Coal Processing and Efficient Utilization of Ministry of Education, School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China; ts19040055a31@cumt.edu.cn (Z.C.); HXY19852125760@163.com (X.H.); c15396828196@163.com (J.T.); taoxx163@163.com (X.T.) 2 Key Laboratory of Coal Bed Methane Resource & Reserving Process of the Ministry of Education, School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221008, China; huazhouh@163.com 3 Institute of Energy & Environmental Engineering, University of the Punjab, Lahore 54590, Pakistan; rizwan.ieee@pu.edu.pk 4 Department of Microbiology, Quaid-i-Azam University, Islamabad 45320, Pakistan; ishimrl@qau.edu.pk (M.I.A.); asifjamal@qau.edu.pk (A.J.) 5 Department of Civil and Architectural Engineering, University of Wyoming, Laramie, WY 82072, USA * Correspondence: hehuan6819@cumt.edu.cn (H.H.); zhuang@uwyo.edu (Z.H.); Tel.: +86-516-83883194 (H.H.) Abstract: A mixed culture of A. ferrooxidans and A. thiooxidans isolated from a coal gangue dump was used to bioleach coal gangue in a column reactor to investigate the leaching of elements. The changes of metal ions (Fe, Mn and Cr) and sulfate in the leaching solution, elemental composition, mineral components and sulfur speciation of the coal gangue before and after bioleaching were analyzed by atomic absorption, anion chromatography, XRF, XRD and XPS. The results show that the mixed culture could promote the release of metal ions in coal gangue, with a leaching concentration of Fe > Mn > Cr. EC and Eh have significantly increased with the increase of metal ion concentrations in the leaching solution. XRF analyses show that the contents of Fe, Mn and S decreased in coal gangue after bioleaching. XRD results suggest that the bioleaching has impacts on minerals in coal gangue, particularly the Fe-containing components. XPS analyses show that sulfur speciation in the raw gangue samples was associated with sulfate, dibenzothiophene and pyrite sulfur. After continuous leaching by the mixed culture, the total sulfur, pyrite sulfur and sulfate sulfur in coal gangue decreased from 2.06% to 1.18%, 0.66% to 0.14% and 1.02% to 0.52%. The desulfurization rates of the pyrite and sulfate were 78.79% and 49.02 %. It is concluded that the mixed culture of these two microorganisms could effectively leach metals from coal gangue coupling with the oxidation of sulfide to sulfate. This study has provided fundamental information as a potential application in the recovery of valuable metals from coal gangue or environmental remediation related to gangue in the future. Keywords: bioleaching; coal gangue; mixed culture; metals; sulfur speciation 1. Introduction Coal mining and processing have generated massive coal gangue and accumulated as gangue dumps. The leaching, weathering and biological catalysis of coal gangue can release metals and sulfur into the environment. Furthermore, the formation of acid mine drainage (AMD) from oxidation of pyrites and other sulfur-bearing minerals in coal gangue causes serious environmental pollution to the air, surface water and surrounding soil [1,2]. There has been concerns over the pollution caused by coal gangue dumps. As such, the number of reports related to environmental pollution and risk assessment of sulfur and metals has increased in the past thirty years [3,4]. However, coal gangue also contains Minerals 2021, 11, 1043. https://doi.org/10.3390/min11101043 https://www.mdpi.com/journal/minerals