Biosynthesis of iron oxide nanoparticles from mineral coal tailings in a stirred tank reactor

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. erythropolis ATCC 4277 biomining capacity was improved by diminishing stirring rate and oxygen flow rate of stirred tank reactor. According to the results of the 22 full factorial design, smaller sizes of iron-based nanoparticles (<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-Fe2O3 and α-Fe2O3). The proposed biomining process represents an environmental-friendly and sustainable process for the transformation of mineral coal tailings into products with greater added value.