Unconventional Mechanism in the Selective Removal of Lead and Cadmium from Acidic Media Using Na _{<i>x</i> +2 <i>y</i>} Sn _{4– <i>y</i>} S ₈ ·3H ₂ O (NMS-7)

Metal sulfide ion exchangers (MSIEs) are promising sorbents for the selective removal of toxic heavy metals from water, yet their ion exchange mechanisms in acidic media remain poorly understood. Here we revisit the layered MSIE Na₂Sn_2.5S₆·3H₂O (NMS-7), establish its true composition and structure by MicroED and complementary spectroscopy, and contrast it with the related but nonexchanging phase Na₄Sn₂S₆. NMS-7 combines accessible Na⁺ sites in a SnS₂-derived framework with interlayer water, enabling rapid chemisorptive uptake of Pb²⁺ and Cd²⁺, with equilibrium reached within 20 min and capacities of 382 and 217 mg/g at pH ≈ 5, consistent with an ideal 2Na⁺/M²⁺ exchange stoichiometry. The material maintains high removal efficiencies across a broad pH window and in complex matrices, while the protonated solid acid H₂Sn_2.5S₆ (HMS-7) formed in strong acid, exhibits reduced, incomplete exchange. PXRD, S/TEM, solid state NMR, and Pb L₃-edge EXAFS reveals that Pb and Cd uptake in acidic media does not proceed by classical intercalation but through an inhomogeneous mechanism where protonated and metal-rich domains coexist within the same crystallites; in particular, atomically resolved HRTEM directly images nanoscale banded contrast associated with alternating metal rich and metal poor regions, while EXAFS analysis reveal two distinct first shell Pb-S coordination environments distinct from bulk PbS. Our findings define NMS-7 as a structurally well-defined, high-performance sorbent and show that local probes are essential to resolve coupled protonation and metal exchange governing soft divalent cation behavior in layered MSIEs.