Cadmium adsorption performance and mechanism from aqueous solution using red mud modified with amorphous MnO2

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 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 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.

Automated summary

In this study, red mud modified by manganese dioxide (MRM) was used as an effective adsorbent for removing Cd2+ from aqueous solutions. The MRM had a larger specific surface area and pore volume compared to ordinary red mud (RM). The adsorption experiments showed that the equilibrium adsorption capacity of MRM for Cd2+ was significantly higher than that of RM. The kinetic and thermodynamic studies suggested that the adsorption process was controlled by a pseudo-second-order kinetic model and was heat-trapping and spontaneous. XPS and FTIR analyses indicated that the adsorption mechanisms involved electrostatic attachment, specific adsorption, and ion exchange. Therefore, MRM could be a promising and cost-effective alternative for Cd2+ removal in wastewater treatment.