Catalytic ozonation of real textile wastewater by magnetic oxidized g-C3N4 modified with Al2O3 nanoparticles as a novel catalyst

•The graphitic carbon nitride was oxidized by a novel method.•The BET surface area increased from 57.31 to 115.41 m2/g after Al2O3 immobilization.•Catalytic ozonation using O@g-C3N4/Al2O3 nanocatalyst resulted in 75% COD removal.•Textile wastewater effluent resulted in meeting EPA discharge criteria.•BI enhanced from 0.27 to 0.74 after 60 min of catalytic ozonation.