Hydrophobicity-Tuned Polymeric Redox-Active Materials for Selective Electrochemical Metal Ion Recovery in Aqueous Environments

The control of bulk surface and colloidal particle interfacial properties was a constant thread permeating the careers of Professors Eli Ruckenstein and Dennis Prieve, who made many widely acknowledged seminal contributions to their respective fields. In this work, and in their honor, we describe the use of adaptable surface-immobilized redox-active materials for control of electrochemically mediated separation processes via targeted molecular recognition and reduced energy requirements. We introduce molecularly tunable vinylferrocene metallopolymers (P(VFc-co-X)) with modifiable operating potentials, charge storage capacities, capacity retentions, and analyte affinities in various electrolyte environments based on the hydrophobicity of X. The styrene (St) co-monomer impedes hydrophobic anions from ferrocene access, providing P(VFc-co-St) with specific response capabilities for and greatly improved cyclabilities in hydrophilic anions. This adjustable electrochemical stability enables preferential chromium and rhenium oxyanion separation from both hydrophobic and hydrophilic electrolytes that significantly surpasses capacitive removal by an order of magnitude, with a robust perrhenate uptake capacity of 329 mg/g VFc competitive with established metal-organic framework physisorbents and 17-fold selectivity over 20-fold excess nitrate. Pairing P(VFc-co-X) with other solution-specific electroactive macromolecules such as the pH-dependent poly(hydroquinone) (PHQ) and the cesium-selective nickel hexacyanoferrate (NiHCF) allows the preparation of dual-functionalized electrosorption cells. P(VFc-co-X)//PHQ offers optimizable energetics based on X and pH for a substantial 4.6-fold reduction from 0.21 to 0.04 kWh/mol rhenium in acidic versus near-neutral media, and P(VFc-co-St)//NiHCF facilitates simultaneous extraction of chromium, rhenium, and cesium ions. Proof-of-concept reversible perrhenate separation in flow further highlights such frameworks as promising approaches for next-generation water purification technologies.