Lead, zinc, nickel and chromium ions removal from polluted waters using zeolite formed from bauxite, obsidian and their combination with red mud: Behaviour and mechanisms

Synthetic zeolites obtained combining natural sources (bauxite and obsidian), pure alumina/silica reagents, and waste material (red mud) were tested for heavy metals (i.e., Pb2+, Zn2+, Ni2+, Cr3+) removal. The adsorption capabilities of the formed sodalite, zeolite A and zeolite X (LTA and FAU topology, respectively), were compared through thermodynamic and kinetic experiments. Although all the newly-formed zeolites were able to remove the pollutant elements within 24 h, Zeolite X and sodalite synthesized combining obsidian (natural material as silica source) and red mud (waste material as alumina source) proved to be a better sorbent phase (qmax 20–25 mg g−1) compared to Zeolite A formed from treated bauxite (qmax 4–18 mg g−1). Their removal efficiency was also evaluated in polluted waters (wastewater treatment plant [WWTP] effluent, heavy metals mixture, native pH). Depending on synthesized zeolite type, the adsorption mechanism was accredited to ion exchange and precipitation mechanisms. The location of metal-ions inside the zeolite channels was defined by X-ray Powder Diffraction (XRPD) Rietveld analysis. Host–guest interactions among the framework oxygen atoms, co-adsorbed water molecules, and metal-ions were highlighted by the refined bond distances. Finally, magnetic characterization allowed the recognize of different magnetic properties as a function of raw materials used for zeolite synthesis.