Cadmium adsorption performance and mechanism from aqueous solution using red mud modified with amorphous MnO(2)

In this study, red mud modified by manganese dioxide(MRM) was utilized as an adsorbent to effectively remove Cd(2+) from aqueous solution. The characteristics were analysed by SEM–EDS, XRD, BET, FTIR and XPS. Different factors that affected the Cd(2+) removal on MRM, such as dosage, initial pH, initial Cd(2+) concentration, were investigated using batch adsorption experiments. Simultaneously, the adsorption kinetics, adsorption isotherms and adsorption thermodynamics of Cd(2+) 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 m(2) g(−1), 0.02 cm(3) g(−1)) than RM (10.22 m(2) g(−1), 0.73 cm(3) g(−1)). The adsorption experiments found that the equilibrium adsorption capacity of MRM for Cd(2+) 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 Cd(2+) in the wastewater treatment process.