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

Abstract

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–S_x (KCMS) chalcogel as a highly efficient sorbent for heavy metal ions and details its sorption mechanisms. Its incoherent structure comprises Mo₂^V(S₂)₆ and Mo₃^IVS(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 K_d ≥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.