Novel red mud waste-derived magnetic geopolymer spheres, made without the addition of any extra iron compounds, for the removal of wastewater contaminants

In this work, and for the first time, the production of waste-based magnetic geopolymer spheres by a simple heat treatment under a reductive atmosphere is reported. Upon heat treatment, the iron oxide present in bauxite wastes (red mud), used as a solid precursor, is reduced to magnetite, thus producing magnetic spheres without the need for the addition of any secondary magnetic materials. The magnetisation of these mm-size materials reached 5.34 A m² kg^-1, suggesting they contain up to 6 wt% magnetite. This was demonstrated to be sufficient for their magnetic separation/removal, without the need for the addition of any extra magnetic iron oxides to the waste material. The magnetic spheres were then evaluated as sorbent materials for the removal of lead from water, selected as a model pollutant compound. These porous bulk-type sorbents showed high metal removal efficiency reaching an uptake value of 19 mg/g at pH 5 after 24 h of contact time. These promising results, and the easy post-treatment recovery of the waste-based magnetic spheres by the use of inexpensive permanent magnets, demonstrate the potential of the proposed strategy to address environmental concerns.