In this study, red mud modified by manganese dioxide(MRM) was utilized as an adsorbent to effectively remove Cd2+ from aqueous solution. The characteristics were analysed by SEM–EDS, XRD, BET, FTIR and XPS. Different factors that affected the Cd2+ removal on MRM, such as dosage, initial pH, initial Cd2+ concentration, were investigated using batch adsorption experiments. Simultaneously, the adsorption kinetics, adsorption isotherms and adsorption thermodynamics of Cd2+ were also investigated using adsorption experiments data. The characterization results showed that MRM had a rougher, larger specific surface area and pore volume (38.91 m2 g−1, 0.02 cm3 g−1) than RM (10.22 m2 g−1, 0.73 cm3 g−1). The adsorption experiments found that the equilibrium adsorption capacity of MRM for Cd2+ was significantly increased to 46.36 mg g−1, which was almost three times that of RM. According to the fitting results, the pseudo-second-order kinetic model described the adsorption process better than the pseudo-first-order kinetic model. The Langmuir model fitted the adsorption isotherms well, indicating that the adsorption process was unimolecular layer adsorption and the maximum capacity was 103.59 mg g−1. The thermodynamic parameters indicated that the adsorption process was heat-trapping and spontaneous. Finally, combined XPS and FTIR studies, it was speculated that the adsorption mechanisms should be electrostatic attachment, specific adsorption (i.e., Cd–O or hydroxyl binding) and ion exchange. Therefore, manganese dioxide modified red mud can be an effective and economical alternative to the removal of Cd2+ in the wastewater treatment process.
In this study, red mud (RM) was used as a support for LaFeO 3 to prepare LaFeO 3 -RM via the ultrasonic-assisted sol–gel method for the removal of methylene blue (MB) assisted with bisulfite (BS) in the aqueous solution. Characterization by scanning electron microscopy and the Brunauer–Emmett–Teller method indicated that LaFeO 3 -RM exhibited a large surface area and porous structure with a higher pore volume (i.e. 10 times) compared with the bulk LaFeO 3 . The XRD, XPS and FTIR results revealed that the support of porous RM not only dispersed LaFeO 3 particles but also increased Fe oxidation capability, oxygen-containing functional groups and chemically adsorbed oxygen (from 44.3% to 90.3%) of LaFeO 3 -RM, which improved the catalytic performance in structure and chemical composition. MB was removed through the synergistic effect of adsorption and catalysis, with MB molecules first absorbed on the surface and then degraded. The removal efficiency was 88.19% in the LaFeO 3 -RM/BS system under neutral conditions but only 27.09% in the LaFeO 3 /BS system. The pseudo-first-order kinetic constant of LaFeO 3 -RM was six times higher than that of LaFeO 3 . Fe(III) in LaFeO 3 -RM played a key role in the activation of BS to produce SO 4 ⋅ − by the redox cycle of Fe(III)/Fe(II). Dissolved oxygen was an essential factor for the generation of SO 4 ⋅ − . This work provides both a new approach for using porous industrial waste to improve the catalytic performance of LaFeO 3 and guidance for resource utilization of RM in wastewater treatment.
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