Molecular design of redox-materials provides a promising technique for tuning physicochemical properties which are critical for selective separations and environmental remediation. Here, the structural tuning of redox-copolymers, 4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TMA) and 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine (TMPMA), denoted as P(TMA x-co-TMPMA 1−x), is investigated for the selective separation of anion contaminants ranging from perfluorinated substances to halogenated aromatic compounds. The amine functional groups provide high affinity toward anionic functionalities, while the redox-active nitroxyl radical groups promote electrochemically-controlled capture and release. Controlling the ratio of amines to nitroxyl radicals provides a pathway for tuning the redox-activity, hydrophobicity, and binding affinity of the copolymer, to synergistically enhance adsorption and regeneration. P(TMA x-co-TMPMA 1−x) removes a model perfluorinated compound (perfluorooctanoic acid (PFOA)) with a high uptake capacity (>1000 mg g −1) and separation factors (500 vs chloride), and demonstrates exceptional removal efficiencies in diverse perand polyfluoroalkyl substances (PFAS) and halogenated aromatic compounds, in various water matrices. Integration with a boron-doped diamond electrode allows for tandem separation and destruction of pollutants within the same electrochemical cell, enabling the energy integration of the separation step with the catalytic degradation step. The study demonstrates for the first time the tuning of redox-copolymers for selective remediation of organic anions, and integration with an advanced electrochemical oxidation process for energy-efficient water purification.

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The remediation of perfluoroalkyl substances (PFAS) is an urgent challenge due to their prevalence and persistence in the environment. Electrosorption is a promising approach for wastewater treatment and water purification, especially through the use of redox polymers to control the binding and release of target contaminants without additional external chemical inputs. However, the design of efficient redox electrosorbents for PFAS faces the significant challenge of balancing a high adsorption capacity while maintaining significant electrochemical regeneration. To overcome this challenge, we investigate redox-active metallopolymers as a versatile synthetic platform to enhance both electrochemical reversibility and electrosorption uptake capacity for PFAS removal. We selected and synthesized a series of metallopolymers bearing ferrocene and cobaltocenium units spanning a range of redox potentials to evaluate their performance for the capture and release of perfluorooctanoic acid (PFOA). Our results demonstrate that PFOA uptake and regeneration efficiency increased with more negative formal potential of the redox polymers, indicating possible structural correlations with the electron density of the metallocenes. Poly(2-(methacryloyloxy)ethyl cobaltoceniumcarboxylate hexafluorophosphate) (PMAECoPF 6 ) showed the highest affinity toward PFOA, with an uptake capacity of more than 90 mg PFOA/g adsorbent at 0.0 V vs Ag/AgCl and a regeneration efficiency of more than 85% at −0.4 V vs Ag/AgCl. Kinetics of PFOA release showed that electrochemical bias greatly enhanced the regeneration efficiency when compared to open-circuit desorption. In addition, electrosorption of PFAS from different wastewater matrices and a range of salt concentrations demonstrated the capability of PFAS remediation in complex water sources, even at ppb levels of contaminants. Our work showcases the synthetic tunability of redox metallopolymers for enhanced electrosorption capacity and regeneration of PFAS.

Per- and polyfluorinated alkyl substances (PFAS) are persistent contaminants that have been continuously detected in groundwater and drinking water around the globe. Hexafluoropropylene oxide dimer acid (tradename GenX) has been...

The removal of per- or polyfluorinated alkyl substances (PFAS) has received increasing attention because of their extreme stability, our increasing awareness of their toxicity at even low levels, and scientific challenges for traditional treatment methods such as separation by activated carbon or destruction by advanced oxidation processes. Here, we performed a direct and systematic comparison of two electrified approaches that have recently shown promise for effective degradation of PFAS: plasma and conventional electrochemical degradation. We tailored a reactor configuration where one of the electrodes could be a plasma or a boron-doped diamond (BDD) electrode and operated both electrodes galvanostatically by continuous direct current. We show that while both methods achieved near-complete degradation of PFAS, the plasma was only effective as the cathode, whereas the BDD was only effective as the anode. Compared to the BDD, plasma required more than an order of magnitude higher voltage but lower current to achieve similar degradation efficiency with more rapid degradation kinetics. All these factors considered, it was noted that plasma or BDD degradation resulted in similar energy efficiencies. The BDD electrode exhibited zero-order kinetics, and thus, PFAS degradation using the conventional electrochemical method was kinetically controlled. On the contrary, analysis using a film model indicated that the plasma degradation kinetics of PFAS using plasma were mass-transfer-controlled because of the fast reaction kinetics. With the help of a simple quantitative model that incorporates mass transport, interfacial reaction, and surface accumulation, we propose that the degradation reaction kinetically follows an Eley–Rideal-type mechanism for the plasma electrode, and an intrinsic rate constant of 2.89 × 108 m4 mol–1 s–1 was obtained accordingly. The investigation shows that to realize the true kinetic potential of plasma degradation for water treatment, mass transfer to the interface must be enhanced.

The removal of toxic halogenated pollutants in water is a topic of increasing urgency nowadays. Anthropogenic organic pollutants, especially per- and polyfluoroalkyl substance (PFAS), have seen increased concentrations in groundwater1 and drinking water.2 Due to their chemical persistence, bioaccumulation, and potential adverse health effects,3 these molecules are at the center of attention for environmental management. New selective separation technologies for targeting these contaminants from very diluted streams must be developed since traditional treatment methods lack molecular selectivity, may require extensive chemicals for regeneration, and often involve energy-intensive processes. Therefore, we developed a system that utilizes a redox copolymer capable of selectively adsorbing long-chain4 and short-chain PFAS, and mediating adsorption and release solely by electrochemical control. These redox electrosorbent is composed of poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl) (PTMA) and poly(4-methacryloyloxy2,2,6,6-tetramethylpiperidine) (PTMPMA), shortly denoted as PTMA-co-TMPMA. We were able to adsorb and desorb perfluorooctanoic acid (PFOA) up to 1,000 mg PFOA/g adsorbent in a batch system. We also found electrocatalytic breakdown for the shorter chain PFAS molecule hexafluoropropylene oxide dimer acid (GenX) during electrochemical reduction. Tandem electrosorption and electrochemical oxidation effect using boron-doped diamond (BDD) was also studied, by combining our redox-polymer electrodes with advanced oxidation counter electrodes. Finally, we introduced our redox copolymer electrode into a flow cell system capable of successfully adsorbing and desorbing fluorinated substances. We envision these integrated electrochemical reactive separation technologies to play an important role going forward for emerging contaminant remediation. References: Bruton, T. A.; Sedlak, D. L., Treatment of Aqueous Film-Forming Foam by Heat-Activated Persulfate Under Conditions Representative of In Situ Chemical Oxidation. Environ Sci Technol 2017, 51 (23), 13878-13885.Gebbink, W. A.; van Asseldonk, L.; van Leeuwen, S. P. J., Presence of Emerging Per- and Polyfluoroalkyl Substances (PFASs) in River and Drinking Water near a Fluorochemical Production Plant in the Netherlands. Environ Sci Technol 2017, 51 (19), 11057-11065.Chou, W.-C.; Lin, Z., Probabilistic human health risk assessment of perfluorooctane sulfonate (PFOS) by integrating in vitro, in vivo toxicity, and human epidemiological studies using a Bayesian-based dose-response assessment coupled with physiologically based pharmacokinetic (PBPK) modeling approach. Environment International 2020, 137, 105581.Kim, K.; Baldaguez Medina, P.; Elbert, J.; Kayiwa, E.; Cusick, R. D.; Men, Y.; Su, X., Molecular Tuning of Redox-Copolymers for Selective Electrochemical Remediation. Advanced Functional Materials 2020, 30 (52), 2004635.

In article number 2004635, Xiao Su and co‐workers report the molecular design of redox copolymers with tunable affinity, redox activity, and hydrophobicity for the electrochemical separation of organic pollutants, including highly persistent contaminants such as perfluoroalkyl substances. The proposed strategy highlights new directions in the rational design of electroactive polymers for next‐generation water purification and environmental remediation.

The modular nature of an electrochemical system makes it a promising platform for process intensification, enabling to combine reactions and separations. Redox-functionalized materials can play a major role in integrating electrochemical reactions and separations, especially for water purification and environmental applications.1, 2 In this approach, molecular engineering of redox-active materials provides a way of tuning physicochemical properties which are critical for selective separations, regeneration, and conversion. In our work, we discuss the development of redox-active electrodes and systems for electrochemical reactive separations of heavy metal and organic micropollutants. First, the tandem selective capture and conversion of As(III) to As(V) is achieved using an asymmetric design of two redox‐active polymers, poly(vinyl)ferrocene (PVF) and poly‐TEMPO‐methacrylate (PTMA).3 During capture, PVF selectively removes As(III) with exceptional uptake (>100 mg As/g adsorbent), and during release, synergistic electrocatalytic oxidation of As(III) to As(V) with >90% efficiency can be achieved by PTMA, a radical‐based redox polymer. The system demonstrates >90% removal efficiencies with real wastewater and concentrations of arsenic as low as 10 ppb. By integrating electron‐transfer through the judicious design of asymmetric redox‐materials, an order‐of‐magnitude energy efficiency increase can be achieved compared to non‐faradaic, carbon‐based materials. Second, the molecular tuning of redox-copolymers is leveraged for controlling synergistic electrostatic and hydrophobic interactions, enabling selective capture and reversible release of organic micropollutants of emerging concern including per- and polyfluoroalkyl substances (PFAS).4 Various PFASs are separated in different water matrices, with the high separation factor of > 500. The integrated capture and degradation of PFAS is enabled based on judicious design of electrode materials and electrochemical parameters. Our findings are expected to provide an energy-efficient and sustainable platform for integrating reactions and separations electrochemically, for the process intensification in water purification and environmental remediation. References: Su, X.; Bromberg, L.; Tan, K.-J.; Jamison, T. F.; Padhye, L. P.; Hatton, T. A., Electrochemically Mediated Reduction of Nitrosamines by Hemin-Functionalized Redox Electrodes. Environmental Science & Technology Letters 2017, 4 (4), 161-167.Su, X.; Kulik, H. J.; Jamison, T. F.; Hatton, T. A., Anion-Selective Redox Electrodes: Electrochemically Mediated Separation with Heterogeneous Organometallic Interfaces. Advanced Functional Materials 2016, 26 (20), 3394-3404.Kim, K.; Cotty, S.; Elbert, J.; Chen, R.; Hou, C.-H.; Su, X., Asymmetric Redox-Polymer Interfaces for Electrochemical Reactive Separations: Synergistic Capture and Conversion of Arsenic. Advanced Materials 2020, 32 (6), 1906877.Kim, K.; Baldaguez Medina, P.; Elbert, J.; Kayiwa, E.; Cusick, R. D.; Men, Y.; Su, X., Molecular Tuning of Redox-Copolymers for ...