Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet Biosynthesis of iron oxide nanoparticles from mineral coal tailings in a stirred tank reactor Danielle Maass a, ⁎ ,1 , Alexsandra Valério b,1 , Luís Antonio Lourenço b,1 , Débora de Oliveira b , Dachamir Hotza b a Institute of Science and Technology (ICT), Federal University of São Paulo (UNIFESP), 12231-280 São José dos Campos, SP, Brazil b Department of Chemical and Food Engineering (EQA), Federal University of Santa Catarina (UFSC), 88040-900 Florianópolis, SC, Brazil ARTICLE INFO Keywords: Biomining Iron oxide nanoparticles Mineral coal tailings Rhomboclase Rhodococcus erythropolis ABSTRACT Chemical oxidation of mineral coal tailings is one of the most important environmental issues during the lifetime of a mine. The presence of sulfur compounds favors the occurrence of metal acid leaching, which contaminates water with bioaccumulative metals, rendering it unsuitable for domestic and agricultural use. The biomining of residual iron present in these tailings and its transformation into high added-value by-products is economically and environmentally attractive. The extraction of residual iron from rhomboclase and its transformation into nanoparticles by Rhodococcus erythropolis ATCC 4277 free-cells in a stirred tank reactor was studied. R. ery- thropolis ATCC 4277 biomining capacity was improved by diminishing stirring rate and oxygen flow rate of stirred tank reactor. According to the results of the 2 2 full factorial design, smaller sizes of iron-based nano- particles (< 50 nm) were achieved when a stirring rate of 100 rpm and an oxygen flow rate of 0.1 L.min -1 were used. Composition analyses (XRD, FTIR, TEM, EDS and Mössbauer spectroscopy) showed that the synthesized nanoparticles are formed by iron oxide (β-Fe 2 O 3 and α-Fe 2 O 3 ). The proposed biomining process represents an environmental-friendly and sustainable process for the transformation of mineral coal tailings into products with greater added value. 1. Introduction Mining activity has been considered one of the major indicator of industrial development. However, this activity produces large amounts of waste and the disposal of mine tailings is one of the most important environmental issues during a mine lifetime. Some of these wastes are inert and hence not likely to represent a significant impact to the en- vironment (Alp et al., 2009). The mine tailings are usually stored as a slurry of high water content into large tailing ponds (Evangelou, 1995; Ferrow et al., 2005; Oliveira et al., 2016). The harmfulness of tailing slurries can be decreased by adding neutralizing substances, so that they can be chemically stabilized into a form in which harmful sub- stances cannot release to the environment (David Bonen, 1995; Oliveira et al., 2016; Poon and Lio, 1997). Other fractions of mining waste, especially tailings from processing of sulfide minerals like pyrite (FeS 2 ) and pyrrhotite (Fe 1-x S 2 ), can cause environmental risks due to their tendency to oxidize in the presence of water or air (Hansen et al., 2013; Sánchez-Andrea et al., 2014). The stabilization of these iron sulfide minerals causes considerable costs for mining companies since their discharge should be strictly controlled. Therefore, the reprocessing of iron sulfate minerals tailings as a source of metals and chemicals or as a raw material of construction boards has been studied (Klein et al., 1993; Nehdi and Tariq, 2007; Sánchez- Andrea et al., 2014). Biomining has emerged as an innovative approach for sulfide mi- neral processing. This process utilizes biological systems (chiefly pro- karyotic microorganisms) to facilitate the extraction and recovery of metals from ores (Johnson, 2014). Biomining is considered an en- vironmentally-friendly process when compared to traditional physical- chemical methods since less energy and chemical reagents are neces- sary (Brierley and Brierley, 2013). The application of these biological process for metal recovery has been widely applied commercially throughout the world to enhance the extraction of base metals from sulfide ores (Brierley and Brierley, 2013; Lang and Schüler, 2006). Currently, it is responsible for approximately 15–25% of the world's copper production, 5% of gold and smaller percentages of cobalt, nickel, uranium, and zinc (Colica et al., 2012; Nancharaiah et al., 2016). https://doi.org/10.1016/j.hydromet.2019.01.010 Received 7 June 2018; Received in revised form 19 December 2018; Accepted 21 January 2019 ⁎ Corresponding author. E-mail address: danielle.maass@unifesp.br (D. Maass). 1 These authors contributed equally to this work. Hydrometallurgy 184 (2019) 199–205 Available online 23 January 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved. T