Bacteria-assisted preparation of nano α-Fe₂O₃ red pigment powders from waste ferrous sulfate

Highlights: • A route to prepare nano α-Fe₂O₃ red pigment from waste ferrous sulfate is proposed. • Acidithiobacillus ferrooxidans is introduced for accelerating iron oxidation. • The particle size of synthetic α-Fe₂O₃ is ranged from 22 nm to 86 nm. • The prepared nano α-Fe₂O₃ red pigment fulfills ISO 1248-2006. - Abstract: Massive ferrous sulfate with excess sulfuric acid is produced in titanium dioxide industry each year, ending up stockpiled or in landfills as solid waste, which is hazardous to environment and in urgent demand to be recycled. In this study, waste ferrous sulfate was used as a second raw material to synthesize nano α-Fe₂O₃ red pigment powders with a bacteria-assisted oxidation process by Acidithiobacillus ferrooxidans. The synthesis route, mainly consisting of bio-oxidation, precipitation and calcination, was investigated by means of titration, thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray fluorescence (XRF) to obtain optimum conditions. Under the optimum conditions, nano α-Fe₂O₃ red pigment powders contained 98.24 wt.% of Fe₂O₃ were successfully prepared, with a morphology of spheroidal and particle size ranged from 22 nm to 86 nm and averaged at 45 nm. Moreover, the resulting product fulfilled ISO 1248-2006, the standards of iron oxide pigments.