Tunable cation coordination by polyoxometalates enables selective heavy metal separation and critical mineral recovery at redox electrochemical interfaces

Separating isovalent heavy metals such as lead (Pb²⁺) and cadmium (Cd²⁺) from strongly acidic, multicomponent aqueous feeds presents a critical challenge for both toxic metal remediation and value recovery, as extensive hydration shells and competitive coordination among coexisting cations suppress conventional selectivity mechanisms. Here, we demonstrate that Keggin-type polyoxometalates (PMo₁₂O₄₀³⁻) enable redox-driven heavy metal discrimination in 0.1 M HCl through solvation-dependent coordination that modulates electron transfer kinetics and cation transport. Collision-induced dissociation mass spectrometry (CID-MS) reveals that [Pb–PMo₁₂O₄₀]⁻ exhibits the highest gas-phase stability, while Fourier-transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations show that solution-phase coordination shifts toward terminal oxo sites with magnesium (Mg²⁺) and Pb²⁺ inducing the largest spectral perturbations. Cyclic voltammetry (CV) establishes that [Cd–PMo₁₂O₄₀]⁻ exhibits the slowest electron transfer rate and lowest diffusion coefficient, whereas [Pb–PMo₁₂O₄₀]⁻ demonstrates stronger electronic coupling and enhanced redox activity, with [Mg–PMo₁₂O₄₀]⁻ showing intermediate behavior. These differences define a kinetic selectivity factor (S_A/B^K), derived from CV-measured transport and electron transfer metrics, which stabilizes after 300 cycles and persists through 1000 cycles with S_Pb/Mg^K ∼ 2.1 and S_Cd/Mg^K ∼ 1.8, confirming that the coordination environment remains durable under prolonged electrochemical cycling. Chronoamperometric electroextraction performed on a [PMo₁₂O₄₀]/C redox electrode at −0.2 V vs. Ag/AgCl in acidic binary (Pb²⁺ + Cd²⁺) and ternary (Pb²⁺ + Cd²⁺+ Mg²⁺) feeds, followed by ICP-MS analysis, quantifies a process selectivity factor (S_A/B^P) that reflects real separation performance. In the binary mixture, enrichment factors favor Pb²⁺ over Cd²⁺, but adding Mg²⁺ in the ternary feed reduces Pb/Cd discrimination and increases Mg enrichment, demonstrating that multicomponent competition under fixed-time, fixed-potential operation suppresses S_A/B^P despite higher S_A/B^K. Together, these findings provide fundamental insight into how solvation, coordination geometry, and redox activity collectively govern ion selectivity, and they establish design rules for tailoring kinetics and electrode architectures to enable advanced electrode materials for heavy metal remediation, wastewater valorization, and critical mineral recovery from unconventional feedstocks.