Ni-Al Layered Double Hydroxide and Oxide Adsorbents for Fluoride Removal and CO 2 Capturing: The Role of Dopants (Mn 2+and Ce4+)

Abstract

Environmental pollution, a major global concern, is caused by various harmful pollutants including fluoride and CO2. They pose specific threats. CO2, a major greenhouse gas, causes climate change and global warming, while excessive fluoride intake can lead to detrimental health problems like fluorosis. This study explored the potential of layered double hydroxides (LDHs) and layered double oxides (LDOs) for removing fluoride from water and capturing CO2 from flue gas. Mn2+ and Ce4+ doped Ni-Al LDHs and LDOs were synthesized through the co-precipitation method. The synthesized materials were characterized using advanced microscopic, spectroscopic, X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA)/Differential Scanning Calorimetry (DSC) and Brunauer-Emmitt-Teller (BET) characterization techniques. The crystallographic structural analysis by XRD showed that the intended LDHs and LDOs were obtained. ATR FT-IR results indicated the presence of essential functional groups in the LDHs and LDOs. XPS confirmed the existence of all the constituent elements in their intended oxidation states. The EDS analysis confirmed a uniform distribution of the intended elements throughout the materials. The composition closely matched the target ratio. The TEM/HRTEM characterization confirmed the successful formation of nano-sized sheet-like LDHs and LDOs. The BET analysis indicated a high surface area of 189 m²/g for the Ce4+-doped Ni-Al LDO. These materials exhibited significant adsorption performance towards fluoride. The LDOs showed good saturation capacities in CO2 capture. Doping with Mn2+ and Ce4+ significantly enhanced fluoride and CO2 adsorption. LDHs outperformed LDOs, demonstrating an adsorption capacity of 238.27 mg/g for fluoride removal, as confirmed by the Langmuir model. The proposed mechanisms for F- adsorption involved a combination of ion exchange, hydrogen bonding and surface complexation. Furthermore, a remarkable CO2 saturation capacity of 14.09 mmol/g was achieved for Ce4+-doped Ni-Al LDO. Regeneration studies indicated good stability for Ce4+-doped LDOs. Fixed-bed models provided further support for the experimental observations. The study revealed that CO2 capture likely involved a combination of mechanisms such as bidentate and monodentate interactions, and bulk carbonate formation, as evidenced by the ATR FT-IR and XRD analysis of the used adsorbents. In conclusion, this study successfully demonstrated the potential of Mn2+ and Ce4+ doped Ni-Al LDHs and LDOs as efficient and sustainable adsorbents for fluoride removal and CO2 capture. Their performance highlighted their potential for mitigating environmental pollution and contributing to a cleaner and healthier future.