Polypyrrole–Mo3S13: An Efficient Sorbent for the Capture of Hg2+ and Highly Selective Extraction of Ag+ over Cu2+

The new material Polypyrrole–Mo3S13 (abbr. Mo3S13–Ppy) is a new material prepared by ion-exchange between Ppy-NO3 and (NH4)2Mo3S13. The Mo3S13–Ppy was designed to exhibit strong selectivity for Ag+ and highly toxic Hg2+ in mixtures with other ions. It displays an apparent selectivity ranking of Hg2+ > Ag+ ≥ Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+. The strong affinity of Mo3S13–Ppy for Ag+ and Hg2+ was confirmed with extremely high distribution coefficients (Kd) (∼107 mL/g) and remarkable removal efficiencies (>99.99%), resulting in <1 ppb concentrations of these ions. Furthermore, Mo3S13–Ppy achieved excellent separation selectivity for Ag+ from Cu2+ (even at a high Cu2+/Ag+ ratio, the molar ratio of 867 and mass ratio of 500) because of the special structure of Mo3S132– and its component Mo4+ and (S2)2–. This is promising for the direct extraction of low-grade silver from copper-rich minerals. The maximum Ag uptake capacity of 408 mg/g is redox-based and surprisingly involves the deposition of large, millimeter sized, metallic silver (Ag0) crystals on the surface of Mo3S13–Ppy.