Boron and salt ion transport in electrically assisted reverse osmosis

Upcycling End-of-Life Polyvinylidene Fluoride Membranes into Reverse Osmosis Membranes for Sustainable Water Purification

Membrane technology has been increasingly applied in water purification to address global water scarcity. However, commercial membranes inevitably reach the end-of-life (EoL) after long-term operation, which constrains the sustainability of membrane technology. Herein, we demonstrated the feasibility of upcycling real EoL poly(vinylidene fluoride) (PVDF) microfiltration (MF) membranes into reverse osmosis (RO) membranes with higher separation precision via the interfacial polymerization (IP) reaction. We highlighted that the EoL MF membrane, with a fouling-induced narrowed pore size and relatively hydrophobic properties, is preferred for upcycling. The resultant upcycled RO membrane exhibited a satisfactory NaCl rejection (98.6 ± 0.4%) with favorable water permeance (2.3 ± 0.7 L m-2 h-1 bar-1), comparable to the performance of commercial RO membranes. Real wastewater treatment evaluations confirmed the membrane stability and permeate safety. Life-cycle assessment and techno-economic analysis showed that this upcycling process promises environmental and economic benefits, potentially reducing CO2-eq emissions by 18.6% and costs by 76.5%-92.2% compared with the conventional membrane approach. This proof-of-concept study paves the way for creating a closed eco-loop of membrane recycling for sustainable water purification.

Harnessing Electrochemistry Synergy in Reverse Osmosis: Modulating Ammonium Localized Oxidation and Restricted Transport

The unsatisfactory selectivity of reverse osmosis (RO) membranes toward ammonium poses a critical challenge in water safety when reclaiming water from domestic wastewater. Herein, we developed a novel integrated electrochemical-assisted RO (ECRO) system using the electrically treated feed spacer and permeate carrier as electrodes. This system enhanced ammonium removal efficiency significantly while maintaining low energy consumption, increasing from 94.36% at 0 V to 99.91% at 4 V. The improvement was primarily attributed to localized oxidation and restricted transport of ammonium ions. Specifically, the permeate carrier anode facilitated the indirect oxidation of ammonium through active chlorine via the breakpoint chlorination pathway, notably localized on the permeate side to prevent damage to the separation layer of the RO membrane and simultaneously avoid additional chemical additives. Furthermore, the restricted ammonium ion transport was responsible for its improved enthalpic barrier, as evidenced by both experimental investigation and Monte Carlo simulation. This rise in enthalpic barrier was primarily driven by the reverse electric field force across the RO membrane, coupled with the constrained ion migration near the membrane surface and ion diffusion within the membrane. This study offers new insights and a theoretical foundation for the optimization of electrochemistry synergy membrane systems, highlighting the potential for enhancing ammonium removal in wastewater reclamation in a green and low-energy manner.

Adsorptive Membrane for Boron Removal: Challenges and Future Prospects

The complexity of removing boron compounds from aqueous systems has received serious attention among researchers and inventors in the water treating industry. This is due to the higher level of boron in the aquatic ecosystem, which is caused by the geochemical background and anthropogenic factors. The gradual increase in the distribution of boron for years can become extremely toxic to humans, terrestrial organisms and aquatic organisms. Numerous methods of removing boron that have been executed so far can be classified under batch adsorption, membrane-based processes and hybrid techniques. Conventional water treatments such as coagulation, sedimentation and filtration do not significantly remove boron, and special methods would have to be installed in order to remove boron from water resources. The blockage of membrane pores by pollutants in the available membrane technologies not only decreases their performance but can make the membranes prone to fouling. Therefore, the surface-modifying flexibility in adsorptive membranes can serve as an advantage to remove boron from water resources efficiently. These membranes are attractive because of the dual advantage of adsorption/filtration mechanisms. Hence, this review is devoted to discussing the capabilities of an adsorptive membrane in removing boron. This study will mainly highlight the issues of commercially available adsorptive membranes and the drawbacks of adsorbents incorporated in single-layered adsorptive membranes. The idea of layering adsorbents to form a highly adsorptive dual-layered membrane for boron removal will be proposed. The future prospects of boron removal in terms of the progress and utilization of adsorptive membranes along with recommendations for improving the techniques will also be discussed further.

Boron Removal from Reverse Osmosis Permeate Using an Electrosorption Process: Feasibility, Kinetics, and Mechanism

Boron is present in the form of boric acid (B(OH)₃ or H₃BO₃) in seawater, geothermal waters, and some industrial wastewaters but is toxic at elevated concentrations to both plants and humans. Effective removal of boron from solutions at circumneutral pH by existing technologies such as reverse osmosis is constrained by high energy consumption and low removal efficiency. In this work, we present an asymmetric, membrane-containing flow-by electrosorption system for boron removal. Upon charging, the catholyte pH rapidly increases to above ∼10.7 as a result of water electrolysis and other Faradaic reactions with resultant deprotonation of boric acid to form B(OH)₄^- and subsequent removal from solution by electrosorption to the anode. Results also show that the asymmetric flow-by electrosorption system is capable of treating feed streams with high concentrations of boron and RO permeate containing multiple competing ionic species. On the basis of the experimental results obtained, a mathematical model has been developed that adequately describes the kinetics and mechanism of boron removal by the asymmetric electrosorption system. Overall, this study not only provides new insights into boron removal mechanisms by electrosorption but also opens up a new pathway to eliminate amphoteric pollutants from contaminated source waters.

A multifunctional and low-energy electrochemical membrane system for chemical-free regulation of solution pH

A proper pH environment is essential for a wide variety of industries and applications especially related to water treatment. Current methods for pH adjustment including addition of acid/base and electrochemical processes demonstrate disadvantages associated with environment and energy. Here, we designed a multifunctional electrochemical membrane system (EMS) with one piece of filtration membrane inserted into an electrochemical cell. When electrical field was applied, OH^- and H⁺ ions were produced from reduction and oxidation reactions at cathode and anode, respectively. The membrane posed a resistance for the transport of OH^- and H⁺ ions and prevented their mixing in the cell. The EMS can be also operated in a filtration mode, which could simultaneously regulate permeate and feed pH and accomplish water filtration. In both non-filtration and filtration modes, EMS could achieve effective control of solution pH over a wide range by exerting different voltages without dosing any chemicals. Under the voltage of 1.2 V, the solution pH could reach and be maintained at 10.7 and 3.3 in cathodic and anodic channels, respectively. Furthermore, it was experimentally demonstrated that the EMS only consumed extremely low energy. This, together with membrane filtration in an integrated manner, highlights the huge potential of the EMS for applications in various water industries.