Electrocatalytic reduction enables waste stream-free removal of nitrate contamination in water based on reactive transformation of nitrate into harmless dinitrogen (N₂) or valuable products such as green ammonia (NH₃). However, the reaction process is usually limited by slow mass transport, catalyst cost, and product selectivity. To address these challenges, we develop a mechanically robust, metal-free multiwalled carbon nanotube (CNT)-based electrified membrane (EM), and we elucidate how electrofiltration enables tunable matching of mass transport and reaction timescales to enhance conversion efficiency. Specifically, the CNT-EM was employed as a porous flow-through cathode to decrease the diffusion boundary layer to 29.5 nm and thus enhance the mass transport of nitrate and overall reaction rate. The nitrate reduction rate was tuned by controlling the permeate flow rate (0 to 60 L h^-1 m^-2), and the maximum removal efficiency of 86.9% was reached when the reaction rate matched the mass transport rate across a range of applied potentials. This relationship was confirmed by a fitted dynamic model, which showed an excellent correlation between the current density and permeate flux (R²>0.99). Furthermore, defects in CNTs were identified as the catalytic active sites by comparing the activity of EMs containing acid-treated metal-free CNTs and reduced oxidized CNTs. Importantly, the long-term stability, tolerance of environmental interferences, and removal of nitrate to below the EPA drinking water limit with high N₂ selectivity were demonstrated in synthetic surface water. These results suggest that the CNT-EM and flow-through reactor may provide a promising solution for decentralized nitrate destruction in drinking water.

Chronic Kidney Disease (CKD) is a longstanding condition in which declining kidney function prevents the kidneys from filtering waste and excess fluid from the blood, ultimately leading to renal failure. Dialysis, a treatment to replicate kidney function, works by filtering a water-based solution called dialysate through a patient’s bloodstream to remove excess waste; this can be done in a hospital setting or at home. Home dialysis is one of Baxter’s largest product segments despite requiring monthly shipments of 360 liters of dialysate to the patient's home. These deliveries burden the patient as they must allocate storage space for the bags in their homes and transport them to their treatment location, causing further issues if they suffer from mobility or physical constraints. Additionally, the monthly deliveries are extremely costly to Baxter with associated delivery costs accounting for over 40% of the total costs to support a patient yearly. Electrothermal Membrane Distillation (ETMD), a derivative of self-heating MD systems, is a point-of-use, modular technology that has the potential to recover at least 85% of water from the dialysis effluent. Bench Scale tests have demonstrated 99.9% rejection of TOC and conductivity from synthetic dialysis effluent and promising flux performance. The recovery of water from dialysis effluent will greatly reduce the delivery volume of dialysate, ultimately lowering the cost of treatment and will negate the need for patients to store and transport large volumes of dialysate throughout their homes.

Advancing high-performance aqueous separation technologies is a critical step to increase the sustainability in water treatment, energy storage, and environmental remediation. Among the mainstream separation technologies are membrane separation, adsorption/desorption, and solvent extraction. Because different separation processes are evaluated with distinct performance metrics, currently there is no comprehensive framework to compare and evaluate separation performance among technologies for the same use. These discrepancies significantly frustrate the informed development for next-generation separation materials for fit-for-purpose environment-health applications. In all membrane-based, adsorption-based, and solvent-driven separations, tradeoff relationships between productivity and selectivity are increasingly documented: achieving higher throughput of the feed stream is always accompanied by a sacrifice in capacity of discriminating different species. However, there is no universal framework to incorporate and compare the productivity–selectivity upper bounds of different technologies. This study proposes and formulates a set of universal performance metrics to evaluate separation behavior of membrane, adsorption, and solvent extraction. Commonly employed evaluated metrics are discussed, and universal productivity and separation factors are proposed and formulated for membrane, adsorption, and solvent extraction. With collated data from the literature, the state-of-the-art permeability–selectivity performances of membrane, adsorption, and solvent extraction are generated for the first time, and compared and discussed for water purification and resource recovery. Findings of this study lay foundations for systematically advancing separation technologies for water-energy-environment applications, provide rationale for future development of different separation technologies, and guide improved comparison of separation performance.

Membrane technologies that enable the efficient purification of impaired water sources are needed to address growing water scarcity. However, state-of-the-art engineered membranes are constrained by a universal, deleterious trade-off where membranes with high water permeability lack selectivity. Current membranes also poorly remove low molecular weight neutral solutes and are vulnerable to degradation from oxidants used in water treatment. Pressure-driven distillation is a promising technology for water treatment that can directly replace conventional reverse osmosis technology with improved selectivity and oxidation resistance. We investigate the practical implementation of pressure-driven distillation technology, reporting near complete removal of boron, urea, and nitrogenous disinfection byproducts. We then probe fouling behavior with feedwaters containing organic and biological matter and show that cleaning with ozone and chlorine can effectively prevent fouling. Finally, we investigate the water permeability possible with pressure-driven distillation membranes, showing that ultrathin (less than 150 nm thick) membranes can exceed commercial reverse osmosis membrane in permeability and selectivity. We also show the use of hydrophilic polydopamine-based coatings can further enhance flux rates by 140% and reduce fouling on the membrane surface. Overall, this work offers insights that will enable the practical implementation of pressure-driven distillation for advanced water treatment.

State-of-the-art reverse osmosis membranes poorly remove low-molecular-weight, neutral solutes and are prone to degradation by chemical oxidants. Pressure-driven distillation, a novel membrane process using applied hydraulic pressure to drive water vapor transport, enables water separation with ultrahigh selectivity and oxidation tolerance and potentially reduces cost and energy consumption for desalination and water reuse. The process has been demonstrated in preliminary studies using costly and delicate ceramic membranes. However, these materials pose challenges for scaling up due to limited surface area, high cost, and deformation in pressurized modules. Here, we present a scalable approach to fabricating high-performance robust membranes by controlled metal sputtering and hydrophobic modification on different polymeric substrates. The thickness of the selective metal layer can be tuned by adjusting the sputtering angle and time, which allows for increased water throughput without sacrificing solute selectivity. The membranes were fabricated on a polyvinylidene fluoride substrate and can operate under a high hydraulic pressure of 34.5 bar, offering a water permeability of 3.3 L m^-2h^-1bar^-1 and a sodium chloride rejection of more than 98.5%. In addition, the membranes exhibited near-complete removal of hazardous small neutral contaminants including boron, N-nitrosodimethylamine, and haloacetonitrile, and unchanged desalination performance after severe exposure to high doses of chlorine. Such advantages of the membranes will facilitate the implementation of pressure-driven distillation and pave the way for low-cost and energy-efficient water purification.