Sulfur minerals originating from coal mining represent an important environmental problem. Turning these wastes into value-added by-products can be an interesting alternative. Biotransformation of coal tailings into iron-containing nanoparticles using Rhodococcus erythropolis ATCC 4277 free-cells was studied. The influence of culture conditions (stirring rate, biomass concentration and coal tailings ratio) in the particle size was investigated using a 2 3 full factorial design. Statistical analysis revealed that higher concentrations of biomass produced larger sized particles. Conversely, a more intense stirring rate of the culture medium and a higher coal tailings ratio (% w/w) led to the synthesis of smaller particles. Thus, the culture conditions that produced smaller particles (<50 nm) were 0.5 abs of normalized biomass concentration, 150 rpm of stirring rate and 2.5% w/w of coal tailings ratio. Composition analyzes showed that the biosynthesized nanoparticles are formed by iron sulfate. Conversion ratio of the coal tailings into ironcontaining nanoparticles reached 19%. The proposed biosynthesis process, using R. erythropolis ATCC 4277 free-cells, seems to be a new and environmental-friendly alternative for sulfur minerals reuse.

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