No abstract

The removal of highly toxic, ultra-dilute contaminants of concern has been a primary challenge for clean water technologies. Chromium and arsenic are among the most prevalent heavy metal pollutants in urban and agricultural waters, with current separation processes having severe limitations due to lack of molecular selectivity. Here, we report redox-active metallopolymer electrodes for the selective electrochemical removal of chromium and arsenic. An uptake greater than 100 mg Cr/g adsorbent can be achieved electrochemically, with a 99% reversible working capacity, with the bound chromium ions released in the less harmful trivalent form. Furthermore, we study the metallopolymer response during electrochemical modulation by in situ transmission electron microscopy. The underlying mechanisms for molecular selectivity are investigated through electronic structure calculations, indicating a strong charge transfer to the heavy metal oxyanions. Finally, chromium and arsenic are remediated efficiently at concentrations as low as 100 ppb, in the presence of over 200-fold excess competing salts.

Redox species have been explored extensively for catalysis, energy storage, and molecular recognition. It is shown that nanostructured pseudocapacitive electrodes functionalized with ferrocene‐based redox polymers are an attractive platform for the selective sorptive separation of dilute organic anions from strong aqueous and organic electrolyte solutions, and subsequent release of the sorbed ions to a stripping phase through electrochemical control of the specific binding processes. A remarkable degree of selectivity is shown for carboxylates (–COO–), sulfonates (–SO3−), and phosphonates (–PO3−2) over inorganic anions such as PF6− and ClO4− (separation factor >140 in aqueous and >3000 in organic systems), and between carboxylates with various substituents, based on differences in electronic structure and density of the adsorbates, beyond size, and charge. Our organometallic redox electrodes are a promising platform for targeting aqueous and organic systems requiring high separation factors and fast throughput, such as in the recovery of value‐added products from organic synthesis and isolation of dilute yet highly toxic organic contaminants. The combination of spectroscopic experiments and quantum chemistry sheds light on a selective binding mechanism based on redox‐enhanced hydrogen bonding between the cyclopentadienyl ligand and the carboxylate functional group, with broader implications for molecular design, supramolecular recognition, and metallocene catalysis.

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

in water as anionic arsenate (As(V)) or arsenite (As(III)), with the latter being acutely toxic and difficult to remove. [3] Commonly employed techniques to remove arsenic are coagulation-flocculation or chemical adsorption, both which require significant chemical input, and extensive pretreatment steps for As(III) to As(V) conversion. [3c] Thus, novel removal technologies that integrate removal and conversion of arsenic are critical for sustainable environmental management.The development of advanced materials for water purification, selective contaminant removal, and improved energy efficiency is critical to tackling water-energy nexus challenges, including through the design of more effective membranes and field-assisted adsorbents. [4] Electrochemical methods for water treatment such as capacitive deionization (CDI) have garnered increased attention as a desalination technology, and also as a heavy metal removal platform, due to their efficiency and low environmental footprint compared to typical methods. [5] Electrosorption systems benefit from inherent modularity and scalability, which opens the door to point of source remediation systems. Electrochemical conversion of As(III) to As(V) on carbon electrode has been investigated previously for CDI-based arsenic remediation. [5l,6] However, low arsenic selectivity in the presence of competing ions has limited the total uptake capacity of carbon-based CDI, [5c,h-l] as most arsenic contaminated water sources are composed of 10 to 1000-fold excess salts. [7] Thus, the design of molecularly selective functional adsorbents is necessary to address these materials chemistry limitations.Recent work has shown redox-active/Faradaic materials as an attractive platform for selective water contaminant removal. [8] Redox-active metallopolymers have demonstrated remarkable uptake of anions with significant selectivity, both of organic anions and heavy metal oxyanions. [8b,9] At the same time, asymmetric electrochemical systems have traditionally been proposed in energy-storage applications to enhance capacitance and electrochemical properties. [10] Here, we leverage this electrochemical design for the first time to integrate both the separation and the reactions step electrochemically at functionalized electrodes. We seek to combine two redox-active polymer Advanced redox-polymer materials offer a powerful platform for integrating electroseparations and electrocatalysis, especially for water purification and environmental remediation applications. The selective capture and remediation of trivalent arsenic (As(III)) is a central challenge for water purification due to its high toxicity and difficulty to remove at ultra-dilute concentrations. Current methods present low ion selectivity, and require multistep processes to transform arsenic to the less harmful As(V) state. 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). D...