Enhancing the adsorptive capacity of carbon nanofibers by impregnation with ferric oxide for the removal of cadmium from aqueous solution

Carbon nanofibers (CNF) possess remarkable surface properties and have been used in many applications, including water treatment. They are used for the removal of many contaminants from polluted water, including heavy metals. Further research is ongoing to enhance their removal efficiency for heavy metals. Previous studies involving the impregnation of metal oxides onto carbon adsorbents have reported an increase in CNF adsorption capacity. The incorporated ferric oxide, Fe₂O₃, can exist in different polymorphs. In this study, CNF impregnated with 10 wt% Fe₂O₃ nanoparticles was studied for its capacity to remove cadmium ions (Cd²⁺) from water. Both raw and Fe₂O₃-modified CNF (Fe-CNF) were characterized using several techniques. The SEM-EDX and XRF confirmed a successful impregnation of Fe₂O₃, while XRD, FTIR, and TGA showed that Fe₂O₃ was incorporated in the form of γ-Fe₂O₃ (a non-toxic and biodegradable polymorph). The effect of pH, contact time, temperature, and agitation speed on Cd²⁺ ions removal was tested in batch-mode adsorption experiments. The results revealed that Cd²⁺ ions removal efficiency achieved by Fe-CNF was around 80 % within 30 min of treating of 2.0 mg/L Cd solution at pH 8, adsorbent dosage of 5 g/L, and agitation speed of 150 rpm. The sorption of Cd²⁺ ions followed a pseudo-second-order model, and the thermodynamic studies indicated that iron impregnation changed the adsorption mechanism from a primarily endothermic process to a more efficient exothermic physical adsorption mechanism. Furthermore, the potential to regenerate and re-use the prepared Fe-CNF was established in 3 cycles of adsorption-desorption experiments. The 79 % cadmium recovery rate from Fe-CNF indicated its potential suitability for analytical purposes. The study demonstrated the advantage of impregnating CNF with ferric oxide to enhance the removal efficiency of Cd²⁺ ions from an aqueous solution.