Ultrasonically modified alumina industry waste-derived red mud beads coated with a Z-scheme Bi12O15Cl6/Fe2O3@C photocatalyst for enhanced degradation of antibiotics in wastewater

Abstract

Bauxite residue, or red mud (RM), is a highly alkaline by-product produced during alumina extraction from bauxite. The global accumulation and unscientific disposal of RM raise concerns, considering their negative impact on the environment. In addition, due to their recalcitrance to biological treatment, the unprecedented rise in the concentration of pharmaceuticals in the aquatic biota poses a threat to non-targeted species. To address these issues, RM was sustainably utilized to prepare beads and subsequently surface-modified (MRM beads) through alkali-assisted ultrasonication to immobilize a Z-scheme Bi₁₂O₁₅Cl₆/Fe₂O₃@C heterostructure for the photocatalytic degradation of a mixture of norfloxacin (NLX) and doxycycline (DCL) in a continuous-flow photocatalytic reactor. Under optimal conditions, the photocatalyst-coated MRM beads achieved degradation efficiencies of 91.6% for NLX and 86.2% for DCL after a residence time of 240 min, with corresponding degradation rate constants of 0.0103 min⁻¹ and 0.0083 min⁻¹, respectively. The main active species responsible for NLX and DCL degradation were found to be O₂^˙−, with subsequent roles played by HO˙ and h⁺, as confirmed through EPR. Moreover, the BFC-II coated MRM beads exhibited remarkable reusability for up to six cycles and could partially restore the photocatalytic activity when heated. Moreover, XRD analysis indicated the retention of the crystallographic properties, while micro-Raman spectra revealed carbon loss due to repeated calcination during regeneration. Real water matrices negatively affected the degradation of NLX and DCL due to their intrinsic constituents. This study advocates the sustainable utilization of RM as a catalyst support in a continuous-flow photocatalytic reactor, promoting waste management and scalability.