Synthesis of in-situ Al3+-defected iron oxide nanoflakes from coal ash: A detailed study on the structure, evolution mechanism and application to water remediation

Abstract

The recovery of value-added materials from coal ash waste is of highly economic value and sustainable significance. However, researches on the synthesis of defect-engineering nanomaterials from coal ash are still blank. Herein, iron oxide (Fe1.72Al0.28O3, simplified as FAO) nanoflakes were successfully synthesized from a brown coal fly ash (BCFA) waste. The obtained FAO nanoflakes possess a round-shape morphology with a diameter of around 300 nm and 50 nm in thickness. With the progress of hydrothermal treatment, the impure Al3+ gradually replaced part of the Fe3+ in the α-Fe2O3 crystal. Specifically, Al3+ was preferentially adsorbed on the (001) facet, hindering the growth of Fe3+ on the [001] direction and thus causing the flattening of the resultant FAO. The introduced Al3+ also serves as the disordered defects on the hematite surface, leading to decreased crystal parameters for hematite, the formation of a compact first shell and a reduced periodical symmetry for the central cation Fe3+. The defects were also found to significantly improve the adsorption capacity of the resultant FAO for Cr(VI), As(V), As(III) and Congo red in waste water, with the maximum adsorption capacity of 68.3, 80.6, 61.1 and 213.8 mg g−1, respectively. Cyclic tests also confirmed a relatively strong stability for the as-synthesised adsorbents.