Chitosan immobilised granular FeOOH-MnxOy bimetal-oxides nanocomposite for the adsorptive removal of lead from water

Mixed metal-oxides are an important class of materials studied for the adsorptive removal of contaminants in water. However, the lack of affordable and versatile immobilisation techniques limits its large-scale deployment in water purification. The present work describes the use of protonated chitosan as a template and reinforcing agent to fabricate FeOOH-MnxOy bimetal-oxide granules formed via complexation of metal-oxides with hydroxyl and amine groups in chitosan. The synthesis procedure also enables the modulation of nanocomposite into granules of different shapes and sizes. XRD and XPS analyses confirm the existence of MnO, Mn2O3, MnO2, and FeOOH phases in the composite. The granules showed excellent affinity to scavenge the model pollutant, Pb2+, through inner-sphere complexation with an estimated Sips isotherm capacity of 108.12 mg/g. The chemical stability analysis of the composite showed that the leaching of Fe and Mn from the composite is pH, contact time, and temperature dependent. The background ions can also affect the chemical stability of the composite. However, Fe and Mn leaching were well within the permissible limit under typical groundwater pH and temperature. There is no significant effect on Pb2+ intake by competing ions such as Cl-, NO3-, HCO3-, Ca2+, Fe2+, and Mn2+. In short, the improved adsorption capacity, the ability to form hydraulically stable granules, and ease of synthesis make the composite a practical adsorbent for removing heavy metal ions in water.