Industrial waste against pollution: mill scale-based magnetic hydrogels for rapid abatement of Cr(vi)

Abstract

This work studies the adsorption of hexavalent chromium from water using magnetic composite hydrogels obtained by combining magnetite micropowders, synthesized from industrial by-products, with a natural biopolymer (alginate) and other active species that could be obtained from food waste, like ascorbic acid. In particular, mill scale was collected from a steel hot strip mill, converted into magnetite microparticles and encapsulated in alginate-based hydrogel bubbles, in combination with activated carbon (AC) and L-ascorbic acid (VitC). The synergistic combination of each active component creates an efficient, multifunctional hybrid adsorbing unit, which enables the total abatement of Cr(VI) from contaminated water, even at a high concentration range (up to 50 mg L⁻¹). Alginate plays the double role of water-absorbing and Cr(VI)-adsorbing matrix as well as that of carrier and container for the other active components, among which AC is utilized as an extra-adsorbing unit for Cr(VI), while VitC enables reduction of Cr(VI). Furthermore, the presence of Fe₃O₄ microparticles endows the hydrogel bubbles with magnetic responsiveness, enabling their easy removal at the end of their decontaminant activity by means of small magnets. The adsorption capability of the Fe₃O₄/AC/VitC-alginate hydrogels was tested under different conditions: Cr⁶⁺ was completely abated within 60 minutes when the Cr(VI) concentration in ultrapure water was equal to 20 mg L⁻¹, and within 360 minutes when the concentration in ultrapure water was 50 mg L⁻¹, without need of any pH-regulating treatment. The role of each different active unit was investigated and compared to the adsorption performances of whole composite systems, elucidating cooperative effects and deriving optimized formulations. The composite hydrogels were also tested in real contaminated water in the presence of other possible common competitive inorganic species and in non-acidic pH. Under these conditions it was also possible to obtain complete Cr(VI) removal, even if with slower kinetics, and 93% Cr(VI) (50 mg L⁻¹) was abated within 60 minutes.