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 (K_d) (∼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 Mo₃S₁₃ ^2- 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.