Lab- and pilot-scale wet scrubber study on the redox-mediated simultaneous removal of NOx and SO2 using a CaCO3-based slurry with KI as a redox catalyst.

This study presents a novel approach that integrates ozone-driven chemical oxidation to convert NO into soluble NO2, followed by the simultaneous absorption of NO2 and SO2 into a CaCO3-based slurry using the redox catalyst potassium iodide (KI). Using cyclic voltammetry, we demonstrate the redox properties of the I2/2I- couple, which facilitates NO2 reduction into soluble NO2- and catalyst regeneration through sulfite (SO32-)-driven reduction, thus establishing a closed catalytic cycle within the components of flue gas. In lab-scale wet-scrubbing tests, we explore the effect of various operational parameters (i.e., KI concentration, pH, and SO2 concentration), with a 15 h stability test demonstrating >60% NOx and >99% SO2 removal efficiency when the pH is controlled between 7.5 and 8.5. A successful pilot-scale implementation conducted at an inlet flow rate of 1000 m3 h-1 further confirmed the reproducibility of the proposed redox-catalytic cycle. Our study offers a cost-effective, sustainable, and scalable solution for effectively mitigating NOx and SO2 emissions at low temperatures.

Electrochemical recovery of nutrients from human urine coupled with biogas upgrading.

This study explored the coupling of electrochemical nutrient recovery from human urine with biogas upgrading. Ammonia nitrogen-rich (≥300 mM) and alkaline (≥pH 9) hydrolyzed urine (HU) is a promising alternative CO2 solvent. Spent urine after biogas upgrading (SU), with neutralized pH and increased conductivity resulting from CO2 absorption, is advantageous over HU for recovering total ammonia nitrogen (TAN) through electro-concentration. Experiments using synthetic urine at varying applied current densities (13-77 A/m2) demonstrated effective TAN recovery from both HU and SU, with greater enrichment factors at higher currents (2.1-3.3-fold, 1.2-1.8 M TAN concentrate). Validation experiments using real urine at the optimized current density of 52 A/m2, considering energy consumption, exhibited superior TAN recovery and energy efficiency when using SU (3.7-fold enrichment, 1.6 M TAN concentrate; suitable for liquid fertilizer) compared to HU. These findings offer an advanced strategy for maximizing urine valorization, contributing to a circular economy.

Electrochemically-mediated reactive separation of nitric oxide into nitrate using iron chelate.

Valorization of nitric oxide is a promising solution for addressing the environmental and resource issues related to the nitrogen cycle. However, low concentrations of nitric oxide combined with impurities in exhaust streams limit its potential, and it requires extensive energy to produce high-purity nitric oxide. Here, we propose a synergistic reactive separation system that combines iron-chelate selective absorption with an electrochemical reaction to convert nitric oxide to nitrate. Among the iron-based chelates tested, EDTA was found to be the most effective in capturing gas-phase nitric oxide. Direct electrochemical oxidation of Fe-EDTA-NO solution exhibited Faradaic efficiency and a partial current density toward nitrate of 70% and 30.1 mA cm-2 at 2.2 V vs RHE and pH 7, resulting in a 43-fold enhancement of nitrate partial current density and a 2-fold improvement in Faradaic efficiency compared to simple purging without selective absorbent. Nitrate was then selectively recovered from the Fe-EDTA system using simple polarity reversal following electrooxidation with a separation factor of 13 over background sulfate. This study offers a new approach to gas-phase NO remediation and valorization using an electrified means.

Coupling nitrate capture with ammonia production through bifunctional redox-electrodes.

Nitrate is a ubiquitous aqueous pollutant from agricultural and industrial activities. At the same time, conversion of nitrate to ammonia provides an attractive solution for the coupled environmental and energy challenge underlying the nitrogen cycle, by valorizing a pollutant to a carbon-free energy carrier and essential chemical feedstock. Mass transport limitations are a key obstacle to the efficient conversion of nitrate to ammonia from water streams, due to the dilute concentration of nitrate. Here, we develop bifunctional electrodes that couple a nitrate-selective redox-electrosorbent (polyaniline) with an electrocatalyst (cobalt oxide) for nitrate to ammonium conversion. We demonstrate the synergistic reactive separation of nitrate through solely electrochemical control. Electrochemically-reversible nitrate uptake greater than 70 mg/g can be achieved, with electronic structure calculations and spectroscopic measurements providing insight into the underlying role of hydrogen bonding for nitrate selectivity. Using agricultural tile drainage water containing dilute nitrate (0.27 mM), we demonstrate that the bifunctional electrode can achieve a 8-fold up-concentration of nitrate, a 24-fold enhancement of ammonium production rate (108.1 ug h-1 cm-2), and a >10-fold enhancement in energy efficiency when compared to direct electrocatalysis in the dilute stream. Our study provides a generalized strategy for a fully electrified reaction-separation pathway for modular nitrate remediation and ammonia production.

Electrosorption of cadmium ions in aqueous solutions using a copper-gallate metal-organic framework.

Recently, there is a recognized need for green technologies for the effective decontamination of toxic heavy metal ions in wastewater. This study demonstrates the electrochemically assisted uptake and release of cadmium ions (Cd2+) using a redox-active Cu-based metal-organic framework (MOF) electrode. Copper gallate (CuGA), which was synthesized in an aqueous solution, is a water-stable and cost-effective MOF adsorbent in which naturally abundant gallic acid is used as a linker. This work utilized copper within the CuGA structure as a redox center to attract Cd2+ by means of Cu2+/Cu+ reduction, exhibiting rapid uptake kinetics and a much higher capacity (>60 mg g-1) compared to the case without electrochemical assistance (~15 mg g-1). In addition, by applying an opposite overpotential to induce the re-oxidation of copper, the facile recovery of Cd2+ and the regeneration of the electrode were possible without regenerants. Physicochemical characterizations including XPS were conducted to investigate the chemical oxidation states and stability of the electrode after the effective electrosorption-regeneration process. This work presents the feasibility of a Cu-based MOF electrode as a reusable platform for the efficient removal of Cd2+, supporting the continued discovery and development of new Faradaic electrodes for electrochemical wastewater treatments.

Selective cobalt and nickel electrodeposition for lithium-ion battery recycling through integrated electrolyte and interface control.

Molecularly-selective metal separations are key to sustainable recycling of Li-ion battery electrodes. However, metals with close reduction potentials present a fundamental challenge for selective electrodeposition, especially for critical elements such as cobalt and nickel. Here, we demonstrate the synergistic combination of electrolyte control and interfacial design to achieve molecular selectivity for cobalt and nickel during potential-dependent electrodeposition. Concentrated chloride allows for the speciation control via distinct formation of anionic cobalt chloride complex (CoCl42-), while maintaining nickel in the cationic form ([Ni(H2O)5Cl]+). Furthermore, functionalizing electrodes with a positively charged polyelectrolyte (i.e., poly(diallyldimethylammonium) chloride) changes the mobility of CoCl42- by electrostatic stabilization, which tunes cobalt selectivity depending on the polyelectrolyte loading. This strategy is applied for the multicomponent metal recovery from commercially-sourced lithium nickel manganese cobalt oxide electrodes. We report a final purity of 96.4 ± 3.1% and 94.1 ± 2.3% for cobalt and nickel, respectively. Based on a technoeconomic analysis, we identify the limiting costs arising from the background electrolyte, and provide a promising outlook of selective electrodeposition as an efficient separation approach for battery recycling.

Electrochemical approaches for selective recovery of critical elements in hydrometallurgical processes of complex feedstocks.

Critical minerals are essential for the ever-increasing urban and industrial activities in modern society. The shift to cost-efficient and ecofriendly urban mining can be an avenue to replace the traditional linear flow of virgin-mined materials. Electrochemical separation technologies provide a sustainable approach to metal recovery, through possible integration with renewable energy, the minimization of external chemical input, as well as reducing secondary pollution. In this review, recent advances in electrochemically mediated technologies for metal recovery are discussed, with a focus on rare earth elements and other key critical materials for the modern circular economy. Given the extreme heterogeneity of hydrometallurgically-derived media of complex feedstocks, we focus on the nature of molecular selectivity in various electrochemically assisted recovery techniques. Finally, we provide a perspective on the challenges and opportunities for process intensification in critical materials recycling, especially through combining electrochemical and hydrometallurgical separation steps.

Electrochemical ammonia accumulation and recovery from ammonia-rich livestock wastewater.

High levels of ammonia inhibit microbial activities and lead to process instability of traditional wastewater treatment. Nitrogen recovery via ammonia stripping is the best developed method, but this approach requires large amounts of alkaline chemicals and substantial energy for stripping. In this study, we designed a simple electrochemical system that allows the facile accumulation of a neutral species of ammonia (NH3), resulting in much lower overall stripping costs. In batch operation treatment of synthetic livestock wastewater (LW), the energy efficiency for total ammonia nitrogen (TAN) migration was found to be the best at a current density of 93.8 mA cm-2. Fed-batch operation, using synthetic or real LW, resulted in very high degrees of TAN accumulation (10,158 mg-N L-1 for synthetic and 17,704 mg-N L-1 for real LW) in catholyte after 400 min. It was found that TAN migration was responsible for 0.221 and 0.492 of total charge migration for synthetic and real LW, respectively. The nitrogen flux across a cation exchange membrane was 5975 g-N m-2 d-1 with an energy input of 28.2 kWh (kg-N)-1 when using real LW. All this supported the conclusion that an electrochemical approach such as this makes it possible to achieve highly desirable ammonia recovery from wastewater in a sustainable manner.

Semiconducting Polymer Interfaces for Electrochemically Assisted Mercury Remediation.

Nanostructured polymer interfaces can play a key role in addressing urgent challenges in water purification and advanced separations. Conventional technologies for mercury remediation often necessitate large energetic inputs, produce significant secondary waste, or when electrochemical, lead to strong irreversibility. Here, we propose the reversible, electrochemical capture and release of mercury, by modulating interfacial mercury deposition through a sulfur-containing, semiconducting redox polymer. Electrodeposition/stripping of mercury was carried out with a nanostructured poly(3-hexylthiophene-2,5-diyl)-carbon nanotube composite electrode, coated on titanium (P3HT-CNT/Ti). During electrochemical release, mercury was reversibly stripped in a non-acid electrolyte with 12-fold higher release kinetics compared to nonfunctionalized electrodes. In situ optical microscopy confirmed the rapid, reversible nature of the electrodeposition/stripping process with P3HT-CNT/Ti, indicating the key role of redox processes in mediating the mercury phase transition. The polymer-functionalized system exhibited high mercury removal efficiencies (>97%) in real wastewater matrices while bringing the final mercury concentrations down to <2 µg L-1. Moreover, an energy consumption analysis highlighted a 3-fold increase in efficiency with P3HT-CNT/Ti compared to titanium. Our study demonstrates the effectiveness of semiconducting redox polymers for reversible mercury deposition and points to future applications in mediating electrochemical stripping for various environmental applications.

Asymmetric Redox-Polymer Interfaces for Electrochemical Reactive Separations: Synergistic Capture and Conversion of Arsenic.

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). 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. The study demonstrates for the first time the effectiveness of asymmetric redox-active polymers for integrated reactive separations and electrochemically mediated process intensification for environmental remediation.

Sulfate reducing bacteria-based wastewater treatment system integrated with sulfide fuel cell for simultaneous wastewater treatment and electricity generation.

This study aimed to design a sulfate-reducing bacteria (SRB)-based wastewater treatment system (SWTS) integrated with a sulfide fuel cell (SFC) as an alternative to the energy-intensive aerobic wastewater treatment process. The result showed that the COD/sulfate ratio and hydraulic retention time (HRT) were two important parameters in a SWTS. The highest COD and sulfate removal efficiency rates were at a HRT of 4 h at a COD/sulfate ratio of 0.67, reaching 83 ±â€¯0.2% and 84 ±â€¯0.4% with sulfate removal rates of 4.087 ±â€¯32 mg SO42-/d, respectively. A microbial analysis revealed that the dominance of nine OTUs belonging to SRB closely affected the high sulfate removal efficiency in the SWTS. At the HRT of 8 h, voltage of 0.02 V and a power density level of 130 mW/m2 were obtained with sulfide removal efficiency of 99 ±â€¯0.5%. These results overall demonstrate that SRB can serve as a green and effective route for wastewater treatment.

Electrochemical Synthesis of Ammonia from Water and Nitrogen: A Lithium-Mediated Approach Using Lithium-Ion Conducting Glass Ceramics.

Lithium-mediated reduction of dinitrogen is a promising method to evade electron-stealing hydrogen evolution, a critical challenge which limits faradaic efficiency (FE) and thus hinders the success of traditional protic-solvent-based ammonia electro-synthesis. A viable implementation of the lithium-mediated pathway using lithium-ion conducting glass ceramics involves i) lithium deposition, ii) nitridation, and iii) ammonia formation. Ammonia was successfully synthesized from molecular nitrogen and water, yielding a maximum FE of 52.3 %. With an ammonia synthesis rate comparable to previously reported approaches, the fairly high FE demonstrates the possibility of using this nitrogen fixation strategy as a substitute for firmly established, yet exceedingly complicated and expensive technology, and in so doing represents a next-generation energy storage system.