Facile activation of natural calcium-rich sepiolite with oxalic acid for selective Pb(II) removal: Highly-efficient performance, mechanisms and site energy distribution

The design and development of adsorbents with high efficiency, selectivity, and economy for Pb(II) are essential to environmental governance and ecological safety. Herein, an oxalic acid (OA) activated natural sepiolite (nSEP) composite for highly efficient Pb(II) removal was prepared by a facile impregnation strategy. The OA activated nSEP nanocomposite (OA-nSEP) was characterized by various instrumental techniques and its adsorption performance towards Pb(II) was further evaluated through a series of static and dynamic experiments under various environmental conditions. Results revealed that OA reacted with the calcium impurities in nSEP to form calcium oxalate, causing mesoporous structure and larger specific surface area of OA-nSEP. The obtained OA-nSEP possessed super high Pb(II) adsorption capacities (858.4-1252 mg/g), which were much higher than that of most modified clays or conventional materials. The average adsorption site energy and the standard deviation of the site energy distribution were analyzed to investigate the strength of Pb(II) binding onto OA-nSEP and the adsorption site heterogeneity. Mechanism studies confirmed that oxalate groups exerted a primary role in the adsorption process. X-ray diffraction and X-ray photoelectron spectrometry (XPS) unveiled that the coordination of oxalate with Pb(II) and precipitation of lead oxalate was responsible for the high efficiency and selectivity. Distinguishing feature of high adsorption capacity, specific selective adsorption, abundant availability, and splendid reusability make the OA-nSEP a promising candidate for eliminating Pb(II) in practical scenarios.