K–Co–Mo–Sx chalcogel: high-capacity removal of Pb2+ and Ag+ and the underlying mechanisms

Chalcogenide-based aerogels, known as chalcogels, represent a novel class of nanoparticle-based porous amorphous materials characterized by high surface polarizability and Lewis base properties, exhibiting promising applications in clean energy and separation science. This work presents a K–Co–Mo–Sx (KCMS) chalcogel as a highly efficient sorbent for heavy metal ions and details its sorption mechanisms. Its incoherent structure comprises Mo2V(S2)6 and Mo3IVS(S6)2 anion-like clusters with four- and six-coordinated Co–S polyhedra, forming a Co–Mo–S covalent network that hosts K+ ions through electrostatic attraction. The interactions of KCMS with heavy metal ions, particularly Pb2+ and Ag+, reveal that KCMS is exceptionally effective in removing these ions from ppm concentrations down to trace levels (≤5 ppb). KCMS rapidly removes Ag+ (≈81.7%) and Pb2+ (≈99.5%) within five minutes, achieving >99.9% removal within an hour, with a distribution constant Kd ≥108 mL g−1. KCMS exhibits an impressive removal capacity of 1378 mg g−1 for Ag+ and 1146 mg g−1 for Pb2+, establishing it as one of the most effective materials known to date for heavy metal removal. This material is also effective for the removal of Ag+ and Pb2+ along with Hg2+, Ni2+, Cu2+, and Cd2+ from various water sources even in the presence of highly concentrated and chemically diverse cations, anions, and organic species. Analysis of the post-interacted KCMS by synchrotron X-ray pair distribution function (PDF), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) revealed that the sorption of Pb2+, Ag+, and Hg2+ mainly occurs by the exchange of K+ and Co2+. Despite being amorphous, this material exhibits unprecedented ion-exchange mechanisms both for the ionically and covalently bound K+ and Co2+, respectively. This discovery advances our knowledge of amorphous gels and guides material synthesis principles for the highly selective and efficient removal of heavy metal ions from water.