Surface restructuring of red mud to produce FeO x (OH) y sites and mesopores for the efficient complexation/adsorption of β-lactam antibiotics

Abstract

In this work, iron oxide in the red mud (RM) waste was restructured to produce mesopores with surface [FeO_x (OH)_y ] sites for the efficient complexation/adsorption of β-lactam antibiotics. Red mud composed mainly by hematite was restructured by an acid/base process followed by a thermal treatment at 150–450 °C (MRM150, MRM200, MRM300, and MRM450) and fully characterized by Mössbauer, XRD, FTIR, BET, SEM, CHN, and thermogravimetric analyses. The characterization data showed a highly dispersed Fe³⁺ oxyhydroxy phase, which was thermally dehydrated to a mesoporous α-Fe₂O₃ with surface areas in the range of 141–206 m² g⁻¹. These materials showed high efficiencies (21–29 mg g⁻¹) for the adsorption of β-lactam antibiotics, amoxicillin, cephalexin, and ceftriaxone, and the data was better fitted by the Langmuir model isotherm (R ² = 0.9993) with monolayer adsorption capacity of ca. 39 mg g⁻¹ for amoxicillin. Experiments such as competitive adsorption in the presence of phosphate and H₂O₂ decomposition suggested that the β-lactamic antibiotics might be interacting with surface [FeO_x (OH)_y ] species by a complexation process. Moreover, the OH/Fe ratio, BET surface area and porosity indicated that this complexation is occurring especially on [FeO_x (OH)_y ]_surf sites contained in the mesopore space.