Imparting Selective Fluorophilic Interactions in Redox Copolymers for the Electrochemically Mediated Capture of Short-Chain Perfluoroalkyl Substances

Abstract

With increasing regulations on per- and polyfluoroalkyl substances (PFAS) across the world, understanding the molecular level interactions that drive their binding by functional adsorbent materials is key to effective PFAS removal from water streams. With the phaseout of legacy long-chain PFAS, the emergence of short-chain PFAS has posed a significant challenge for material design due to their higher mobility and hydrophilicity and inefficient removal by conventional treatment methods. Here, we demonstrate how cooperative molecular interactions are essential to target short-chain PFAS (from C4 to C7) by tailoring structural units to enhance affinity while modulating the electrochemical control of capture and release of PFAS. We report a new class of fluorinated redox-active amine-functionalized copolymers to leverage both fluorophilic and electrostatic interactions for short-chain PFAS binding. We combine molecular dynamics (MD) simulations and electrosorption to elucidate the role of the designer functional groups in enabling affinity toward short-chain PFAS. Preferential interaction coefficients from MD simulations correlated closely with experimental trends: fluorination enhanced the overall PFAS uptake and promoted the capture of less hydrophobic short-chain PFAS (C ≤ 5), while electrostatic interactions provided by secondary amine groups were sufficient to capture PFAS with higher hydrophobicity (C ≥ 6). The addition of an induced electric field showed favorable kinetic enhancement for the shortest PFAS and increased the reversibility of release from the electrode. Integration of these copolymers with electrochemical separations showed potential for removing these contaminants at environmentally relevant conditions while eliminating the need for chemical regeneration.

Investigating the Electrochemically Driven Capture and Release of Long-Chain PFAS by Redox Metallopolymer Sorbents

“…Also, the hydrophobicity or polarity of the metallopolymer can influence the partitioning of PFOA between the bulk solution and the metallopolymer surface, with an increase of hydrophobicity enhancing adsorption and polarity decreasing it. A more detailed mechanistic study on the balance between electrostatic and hydrophobic studies can be further investigated in the future through computational investigations – an approach that was recently applied to correlate the binding between fluorophilic copolymers and a range of short-chain PFAS . Going forward, we believe molecular dynamic simulations can assist in elucidating the differential contributions of charge transfer, hydrophobicity, and sterics.…”

“…A more detailed mechanistic study on the balance between electrostatic and hydrophobic studies can be further investigated in the future through computational investigations − an approach that was recently applied to correlate the binding between fluorophilic copolymers and a range of short-chain PFAS. 61 Going forward, we believe molecular dynamic simulations can assist in elucidating the differential contributions of charge transfer, hydrophobicity, and sterics.…”

Molecularly Selective Polymer Interfaces for Electrochemical Separations

“…Given the currently limited selection of stable and efficient redox-active groups, the incorporation of non-redox-active groups within copolymers facilitates the development of unique physicochemical properties, without direct participation in electron-transfer reactions. Consequently, the design of the redox-active copolymer can exhibit exceptional selectivity based on the combination of electrostatic interactions of redox-active groups with the hydrophilicity, ,, solubilities, electrostatic repulsions, , and specific molecular interactions , of the non-redox-active pendant groups (Figure a). Thus, the transition from a homopolymer to a copolymer design enables a broader range of material choices, ultimately expanding the scope of potential applications of electrochemical separation technologies.…”

“…Redox-copolymer design has also evolved as a promising approach to the effective removal of emerging contaminants, perfluoroalkyl substances, and polyfluoroalkyl substances (PFAS), known as forever chemicals. PFAS removal is challenging due to their chemical persistence and dilute concentrations found in water, typically within the parts per billion range. − ,, A redox copolymer composed of 4-meth­acryloyloxy-2,2,6,6-tetra­methyl­piperidin-1-oxyl (TMA) and 4-meth­acryloyl­oxy-2,2,6,6-tetra­methyl­piperidine (TMPMA) has exhibited exceptional performance in the removal of long-chain PFAS by tailoring the hydrophobicity, N–H interactions, and electrostatic interactions. For instance, P­(TMA 51 - co -TMPMA 49 ) has demonstrated a remarkable uptake capacity (>1,000 mg/g) of perfluorooctanoic acid (PFOA) and electrochemical reversibility over 90% .…”

“…For instance, P­(TMA 51 - co -TMPMA 49 ) has demonstrated a remarkable uptake capacity (>1,000 mg/g) of perfluorooctanoic acid (PFOA) and electrochemical reversibility over 90% . On top of the electrostatic interactions of TMA and amine interactions of TMPMA, the fluorophilic interaction of 2,2,3,3,4,4,4-hepta-fluoro­butyl methacrylate (HFBMA) was utilized as a specific molecular interaction to target short-chain PFAS, with carbon chain lengths ranging from C 4 to C 7 (Figure d) . Fluorocarbon groups on the PFAS facilitate dipole–dipole or point–dipole interactions with a fluorinated nonredox moiety, HFBMA, through intermolecular dispersion interactions, commonly referred to as fluorophilic interactions.…”