Asymmetric Faradaic system based on organometallic electrodes shows suppression of parasitic water reactions and remarkable selectivity in redox-mediated electrosorption of micropollutants.

Global water security is jeopardized by the presence of anthropogenic contaminants, which can persist resiliently in the environment and adversely affect human health. Surface adsorption of polluting species is an effective technique for water purification. In this work, redox-active magnetic compounds were designed for the targeted removal of inorganic and organic anions in water via polymeric redox-active vinylferrocene (VFc) and pyrrole (Py) moieties. An Fe3O4@SiO2@PPy@P­(VFc-co-HEMA) composite was prepared in a four-step process, with the outermost layer possessing heightened hydrophilicity as a result of the optimized incorporation of 2-hydroxyethylmethacrylate (HEMA) monomers into the backbone of the ferrocene macromolecule. The synthesized materials are able to separate carcinogenic hexavalent chromium oxyanions and other charged micropollutants, and exhibit a 2-fold or greater enhancement in adsorption uptake once the redox-active ferrocene groups are oxidized to ferrocenium cations, with capacities of 23, 49, 66, and 95 mg/g VFc for maleic acid, 2-(6-methoxy-2-naphthyl)­propionic acid (Naproxen), (2,4-dichlorophenoxy)­acetic acid (2,4-D), and (2-dodecylbenzene)­sulfonic acid (DBS), respectively, and a > 99% extractability of chromium in the 1 ppm range. The application of redox-active components to a magnetic particulate scaffold improves maneuverability and phase contact, giving rise to new potential aqueous separation process frameworks for water or product purification.

Nitrosamines make up a major class of contaminants of emerging concern that are toxic, present at trace levels in aqueous environments, and challenging to destroy because of their chemical stability. We report novel redox electrodes based on hemin-functionalized carbon nanotubes showing high electrocatalytic activity for nitrosamine reduction at low potentials (−0.5 V vs Ag/AgCl or −0.27 vs the standard hydrogen electrode) and with turnover numbers of >700. The redox electrodes were tested under a range of electrolyte and pH conditions and demonstrated high conversion of nitrosamines at high reaction rates, even at parts per billion levels in secondary effluent from a wastewater treatment plant. We propose that the pathway for nitrosamine reduction involves a proton-mediated conversion of the nitroso group to hydrazines and secondary amines. These high-performance biomimetic electrocatalysts for nitrosamine reduction are based on complexes containing earth-abundant metals and, potentially, have broad applications in environmental remediation, water treatment, and industrial organo-electrochemical processes.

A framework of ferrocene-containing polymers bearing adjustable pH-and redox-active properties in aqueous electrolyte environments was developed. The electroactive metallopolymers were designed to possess enhanced hydrophilicity compared to the vinylferrocene (VFc) homopolymer, poly-(vinylferrocene) (PVFc), by virtue of the comonomer incorporated into the macromolecule, and could also be prepared as conductive nanoporous carbon nanotube (CNT) composites that offered a variety of different redox potentials spanning a ca. 300 mV range. The presence of charged non-redox-active moieties such as methacrylate (MA) in the polymeric structure endowed it with acid dissociation properties that interacted synergistically with the redox activity of the ferrocene moieties to impart pH-dependent electrochemical behavior to the overall polymer, which was subsequently studied and compared to several Nernstian relationships in both homogeneous and heterogeneous configurations. This zwitterionic characteristic was leveraged for the enhanced electrochemical separation of several transition metal oxyanions using a P(VFc 0.63 -co-MA 0.37 )-CNT polyelectrolyte electrode, which yielded an almost twofold preference for chromium as hydrogen chromate versus its chromate form, and also exemplified the electrochemically mediated and innately reversible nature of the separation process through the capture and release of vanadium oxyanions. These investigations into pH-sensitive redox-active materials provide insight for future developments in stimuliresponsive molecular recognition, with extendibility to areas such as electrochemical sensing and selective separation for water purification.

The unprecedented increase in atmospheric CO 2 concentration calls for effective carbon capture technologies. With distributed sources contributing to about half of the overall emission, CO 2 capture from the atmosphere [direct air capture, (DAC)] is more relevant than ever. Herein, an electrochemically mediated DAC system is reported which utilizes affinity of redox-active quinone moieties towards CO 2 molecules, and unlike incumbent chemisorption technologies which require temperature or pH swing, relies solely on the electrochemical voltage for CO 2 capture and release. The design and operation of a DAC system is demonstrated with stackable bipolar cells using quinone chemistry. Specifically, poly(vinylanthraquinone) (PVAQ) nega-tive electrode undergoes a two-electron reduction reaction and reversibly complexes with CO 2 , leading to CO 2 sequestration from the feed stream. The subsequent PVAQ oxidation, conversely, results in release of CO 2 . The performance of both small-and meso-scale cells for DAC are evaluated with feed CO 2 concentrations as low as 400 ppm (0.04 %), and energy consumption is demonstrated as low as 113 kJ per mole of CO 2 captured. Notably, the bipolar cell construct is modular and expandable, equally suitable for small and large plants. Moving forward, this work presents a viable and highly customizable electrochemical method for DAC.

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