Regulation of the Sulfur Environment in Clusters to Construct a Mn-Sn₂S₆ Framework for Mercury Bonding

The remarkable chemical activity of metal-sulfur clusters lies in their unique spatial configuration associated with the abundant unsaturated-coordination nature of sulfur sites. Yet, the manipulation of sulfur sites normally requires direct contact with other metal atoms, which inevitably changes the state of the coordinated sulfur. Herein, we facilely construct a Mn-Sn₂S₆ framework by regulating the sulfur environment of the [Sn₂S₆]^4- cluster with metal ions. Mn-Sn₂S₆ showed superior removal performance to gaseous elemental mercury (Hg⁰) at low temperatures (20-60 °C) and exhibited high resistance against SO₂. Moreover, Mn-Sn₂S₆ can completely remove liquid Hg²⁺ ions with low or high concentrations from acid wastewater. In addition, the adsorption capacities of Mn-Sn₂S₆ toward Hg⁰ and Hg²⁺ reached 21.05 and 413.3 mg/g, respectively. The results of physico-chemical characterizations revealed that compared with Cu²⁺, Co²⁺, and Fe²⁺, the moderate regulation of Mn²⁺ led to the special porous spherical structure of Mn-Sn₂S₆ with uniform element distribution, due to the difference of electrode potentials [E^θ(Mn²⁺/Mn) < E^θ(S/S^2-) < E^θ(Sn⁴⁺/Sn²⁺)]. The porous structure was beneficial to Hg⁰ and Hg²⁺ adsorption, and the presence of Mn⁴⁺/Mn³⁺ and S^1- promoted the oxidation of Hg⁰, resulting in stable HgS species. The constructed Mn-Sn₂S₆, thus, is a promising sorbent for both Hg⁰ ang Hg²⁺ removal and provides guidelines for cluster-based materials design and tuning.