H_2xMn_xSn_3-xS₆ (x = 0.11-0.25): A novel reusable sorbent for highly specific mercury capture under extreme pH conditions

The H_2xMn_xSn_3-xS₆ (x 0.11-0.25) is a new solid acid with a layered hydrogen metal sulfide (LHMS). It derives from K_2xMn_xSn_3-xS₆ (x 0.5.....0.95) (KMS-1) upon treating it with highly acidic: solutions. We demonstrate that LHMS-1 has enormous affinity for the very soft metal ions such as Hg ²⁺ and Ag⁺ which occurs via a rapid ion exchange process. The tremendous affinity of LHMS-1 for Hg²⁺ is reflected in very high distribution coefficient K_d^Hg values(>10⁶ mL g^-1). The large affinity and selectivity of LHMS-1 for Hg ²⁺ persists in a very wide pH range (from less than zero to nine} and even in the presence of highly concentrated HCI and HNO₃ acids. LHMS-1 is significantly more selective for Hg²⁺ and Ag⁺ than for the less soft cations Pb²⁺ and Cd²⁺. The Hg ²⁺ ions are immobilized in octahedral sites between the sulfide layers of the materials via Hg-S bonds as suggested by pair distribution function (PDF) analysis. LHMS-1 could decrease trace concentrations of Hg ²⁺ (e.g.<100 ppb) to well below the acceptable limits for the drinking water in less than two min. Hg-laden LHMS-1 shows a remarkable hydrothermal stability and resistance in 6 M HCI solutions. LHMS-1 could be regenerated by treating Hg-loaded samples with 12 M HCI and reused without loss of its initial exchange capacity.