Solvent-free preparation of α-Fe2O3 nanoparticles from metallurgical iron sludge for the treatment of Cr(VI)-contaminated wastewater

Metallurgical iron sludge (MIS) is an industrial by-product that needs high value-added utilization without harmless to the environment. Here, α-Fe2O3 nanoparticles were successfully prepared from MIS via a novel solvent-free method (SN-α-Fe2O3) and applied to Cr(VI) ions removal. Compared to conventional methods using pure reagents, the as-prepared SN-α-Fe2O3 aggregated into uniform cake-like mesoporous structures with a higher specific surface area (115.03 m2/g) and smaller crystal size (32.87 nm), thus providing a large number of active adsorption sites. Therefore, SN-α-Fe2O3 exhibited superior Cr(VI) removal performance (removal efficiency 98.21%) in Cr(VI)-contaminated wastewater compared to previous studies. Furthermore, Cr(VI)-saturated SN-α-Fe2O3 exhibited great stability under acidic conditions and maintained excellent adsorption performance after five cycles. The proposed SN-α-Fe2O3 preparation process forwent the use of water solvent or organic solvents, thus improving reactor efficiency and avoiding wastewater generation. Particularly, the growth mechanism of SN-α-Fe2O3 crystal followed the solid-phase transition mechanism, whereby small amounts of crystal water contained in pretreated MIS was sufficient for SN-α-Fe2O3 crystallization. Furthermore, SN-α-Fe2O3-mediated Cr(VI) adsorption was attributed to the electrostatic interaction and reduction of hydroxyl group on the surface of SN-α-Fe2O3. This study thus provides a win-win strategy for the conversion of industrial by-products into valuable nanoparticles for the cost-efficient treatment of heavy metal-polluted wastewater. [Display omitted] •Low-cost MIS was sustainably converted into high value-added α-Fe2O3 nanoparticles.•The SN-α-Fe2O3 preparation method improved efficiency without wastewater generation.•SN-α-Fe2O3 provided many active adsorption sites due to its structural properties.•SN-α-Fe2O3 exhibited great Cr(VI) removal efficiency (98.21%) at optimal pH (2.00).•SN-α-Fe2O3 removed Cr(VI) via electrostatic interaction and OH group reduction.