Chelation-based metal cation stabilization of graphene oxide membranes towards efficient sieving of mono/divalent ions

Multicomponent Nacre-Like Heterogeneous Nanochannels with Ion Sieving and Light-Sensitivity Properties for Improved Energy Conversion

Nanochannel membranes are currently being employed for selective ion transport and salinity gradient energy capture. However, the poor functionality of nanochannel membranes and the unfavorable influence of multivalent cations result in low energy conversion efficiency, limiting the energy conversion performance of nanochannel membranes. Herein, multicomponent nacre-like heterogeneous nanochannels composed of carboxymethyl chitosan (CMC)-intercalated composite two-dimensional (2D) GO, C3N4 nanosheets, and polyethylene terephthalate (PET) channels are developed using an interfacial assembly strategy. Benefiting from the asymmetric structure, the heterogeneous membrane achieves enhanced ion diffusion and unidirectional ion transport behaviors. Subsequently, a high power density of 8.46 W/m2 is achieved in artificial seawater and river water by the heterogeneous membrane. Notably, the introduction of CMC into 2D lamellar channels endows the heterogeneous membrane with outstanding ion sieving performance, thereby mitigating the unfavorable influence of divalent cations on energy conversion, reducing it from 32.08% to 19.85%. Furthermore, the heterogeneous membrane exhibits sensitive and stable light-responsive ion transport behavior, profiting from the light sensitivity of GO and C3N4 nanosheets, and an improvement of 11.29% in energy conversion performance can be accomplished with light irradiation. This work proposes an idea to design multifunctional nanochannel membranes to achieve highly efficient salinity gradient energy conversion.

PEI-Reinforced GO/g-C3N4 Composite Membrane for Salt Separation

Membrane separation is an important separation and purification technology, membrane materials are the key to membrane separation technology, and the rapid development of two-dimensional materials in recent years has brought new opportunities for membrane separation technology. Graphene oxide (GO) is a hydrophilic two-dimensional material, and its unique physical and chemical structure and properties give it the potential to become a high-performance separation membrane; however, the instability limits its application. In this study, a two-dimensional material, graphite-phase carbon nitride (g-C3N4), was integrated into graphene oxide (GO) to address the challenges associated with its practical applications. This modification not only endows the material with more advantageous properties as a membrane but also introduces a heterogeneous structure that bolsters the composite membrane's stability. This structure also enhances the permeability and selectivity of the membrane, leading to impressive results in self-driven permeation tests and staggered-flow filtration tests under applied pressure. Following additional surface charge modifications, the composite membrane demonstrated a separation efficiency of approximately 90.4% in permeation tests and an impressive separation efficiency of 63.8% in filtration tests for binary mixed salt solutions. These results indicate a significant capability for mono- and divalent ion separation, suggesting promising potential for salt separation applications.

2D Membranes Interlayered with Bimetallic Metal-Organic Frameworks for Lithium Separation from Brines

Efficient lithium extraction from salt lakes is essential for a sustainable resource supply. This study tackles the challenge of separating Li+ from Mg2+ in complex brines by innovatively integrating two-dimensional (2D) graphene oxide (GO) with bimetallic metal-organic frameworks (MOFs). Zn2+ and Co2+ ions are confined within GO interlayers through an in situ synthesis, forming a 2D Zn-Co MOFs/GO membrane (Zn-Co-GOM). This design exploits the unique advantages of bimetallic MOFs, including enhanced structural stability and superior ion separation capabilities due to the synergistic effects of Zn and Co. The Zn-Co-GOM demonstrates an impressive separation factor of 191 for Li+ over Mg2+, significantly surpassing traditional membranes. This exceptional selectivity is achieved through a combination of size exclusion effects and ion transport energy barriers. Our approach not only enhances the practical application of membrane technology for lithium extraction from salt lakes but also provides valuable insights into the underlying separation mechanisms.

Effectively Sieving Alkali Metal Ions Using Functionalized Graphene Oxide Membranes by Exploiting Water-Repellent Interactions

Sieving membranes capable of discerning different alkali metal ions are important for many technologies, such as energy, environment, and life science. Recently, two-dimensional (2D) materials have been extensively explored for the creation of sieving membranes with angstrom-scale channels. However, because of the same charge and similar hydrated sizes, mostly laminated membranes typically show low selectivity (<10). Herein, we report a facile and scalable method for functionalizing graphene oxide (GO) laminates by dually grafting cations and water-repellent dimethylsiloxane (DMDMS) molecules to achieve high selectivities of ∼50 and ∼20 toward the transport of Cs+/Li+ and K+/Li+ ion pairs, surpassing many of the state-of-the-art laminated membranes. The enhanced selectivity for alkali metal ions can be credited to a dual impact: (i) strong hydrophobic interactions between the incident cations' hydration shells and the water-repellent DMDMS; (ii) the efficient screening of electrostatic interactions that hamper selectivity.

Selective separation of monovalent anions by PPy/pTS membrane electrodes in redox transistor electrodialysis

Selective separation of nitrate over chloride is crucial for eutrophication mitigation and nitrogen resource recovery but remains a challenge due to their similar ionic radius and the same valence. Herein, a polypyrrole membrane electrode (PME) was fabricated by polymerization of pyrrole (Py) and p-toluenesulfonate (pTS), which was used as a working electrode in redox transistor electrodialysis. The anions in the source solution were first incorporated into the PME at reduction potentials and then released to receiving solution at oxidation potentials. Pulse widths and potentials were optimized to maximize the ion separation performance of PME, resulting in the improvement of NO₃^-/Cl^- separation factor up to 6.93. The ion distributions in various depths of PME indicated that both NO₃^- and Cl^- were incorporated into PME at negative potentials. Then, NO₃^- was preferentially released from PME at positive potentials, but most Cl^- was retained. This was ascribed to the high binding energy between Cl^- and PPy/pTS structure, which was 51.4% higher than that between NO₃^- and PPy/pTS structure. Therefore, the higher transport rate of NO₃^- in comparison with Cl^- was achieved, leading to a high NO₃^- selectivity over Cl^-. This work provides a promising avenue for the selective separation of nitrate over chloride, which may contribute to nitrogen resource recycling and reuse.

A review on recent advancements on removal of harmful metal/metal ions using graphene oxide: Experimental and theoretical approaches

Graphene oxide is a two-dimensional carbon nanomaterial and has gained huge popularity over the last decade. Because, the graphene oxide can be dispersed in water easily and it is one of the most researched two-dimensional materials in the current time. The extraordinary properties shown by graphene oxide (GO) are due to its unique chemical structure; includes various hydrophilic functional groups containing oxygen such as carboxyl, hydroxyl, carbonyl and tiny sp² carbon domains surrounded by sp³ domains. These groups are very peculiar for various applications as they allow covalent functionalisation with a plethora of compounds. Large surface area, intrinsic fluorescence, excellent surface functionality, amphiphilicity, improved conductivity, high adsorption capacity and superior biocompatibility are some of the chemical properties have drawn research from various fields. Graphene oxide has various interactions such as coordination, chelation, hydrogen bonding, electrostatic interaction, hydrophobic effects, π-π interaction, acid base interaction etc., with various metal ions. This review is focused on the removal of metals and metal ions due to their interactions mentioned above. Further, potential of composites of graphene oxide in the removal of metal and metal ions is also discussed. Further, the current challenges in this field at industrial-scale are also discussed.