Challenges and opportunities in functional carbon nanotubes for membrane-based water treatment and desalination

Carbon Nanotubes (CNTs): A Potential Nanomaterial for Water Purification

Nanomaterials such as carbon nanotubes (CNTs) have been used as an excellent material for catalysis, separation, adsorption and disinfection processes. CNTs have grabbed the attention of the scientific community and they have the potential to adsorb most of the organic compounds from water. Unlike, reverse osmosis (RO), nanofiltration (NF) and ultrafiltration (UF) membranes aligned CNT membranes can act as high-flow desalination membranes. CNTs provide a relatively safer electrode solution for biosensors. The article is of the utmost importance for the scientists and technologists working in water purification technologies to eliminate the water crisis in the future. This review summarizes about the application of CNTs in water purification.

Ultrafine Re/Pd nanoparticles on polydopamine modified carbon nanotubes for efficient perchlorate reduction and reusability

This study synthesized nanocomposite catalysts via a modification of Re/Pd codoped carbon nanotubes (CNTs) with different concentrations of polydopamine (PDA), which were used for perchlorate (ClO₄^-) reduction. The loads, dispersion and reducibility of Re/Pd nanoparticles increased yet their particle sizes significantly decreased with the increase of PDA concentrations. The average diameter of Re/Pd codoped D₂CNT (CNT modified by 2 mg/mL PDA) with a narrow size distribution was measured to be 2 nm. The ultrafine Re/Pd codoped D₂CNT catalysts represented outstanding catalytic reduction activity for the conversion of ClO₄^- to Cl^- with TOF of 17.34 h^-1 under the room H₂ atmospheric pressure, which was about 8 times than that of the unmodified catalysts. Furthermore, PDA modification minimized the dissociation of Re by chemical bonding between Re and CNTs carrier and maintained good stability of nanocomposite. This study inspires us to apply green bionic methods to enhance the catalytic reduction of perchlorate by changing the physical properties of Re/Pd nanoparticles.

Recent Advances in Applications of Carbon Nanotubes for Desalination: A Review

As a sustainable, cost-effective and energy-efficient method, membranes are becoming a progressively vital technique to solve the problem of the scarcity of freshwater resources. With these critical advantages, carbon nanotubes (CNTs) have great potential for membrane desalination given their high aspect ratio, large surface area, high mechanical strength and chemical robustness. In recent years, the CNT membrane field has progressed enormously with applications in water desalination. The latest theoretical and experimental developments on the desalination of CNT membranes, including vertically aligned CNT (VACNT) membranes, composited CNT membranes, and their applications are timely and comprehensively reviewed in this manuscript. The mechanisms and effects of CNT membranes used in water desalination where they offer the advantages are also examined. Finally, a summary and outlook are further put forward on the scientific opportunities and major technological challenges in this field.

Towards single-species selectivity of membranes with subnanometre pores

Synthetic membranes with pores at the subnanometre scale are at the core of processes for separating solutes from water, such as water purification and desalination. While these membrane processes have achieved substantial industrial success, the capability of state-of-the-art membranes to selectively separate a single solute from a mixture of solutes is limited. Such high-precision separation would enable fit-for-purpose treatment, improving the sustainability of current water-treatment processes and opening doors for new applications of membrane technologies. Herein, we introduce the challenges of state-of-the-art membranes with subnanometre pores to achieve high selectivity between solutes. We then analyse experimental and theoretical literature to discuss the molecular-level mechanisms that contribute to energy barriers for solute transport through subnanometre pores. We conclude by providing principles and guidelines for designing next-generation single-species selective membranes that are inspired by ion-selective biological channels.

Development of bacterial cellulose-ZnO-MWCNT hybrid membranes: a study of structural and mechanical properties

Self-supported and flexible bacterial cellulose (BC) based hybrid membranes were synthesized and decorated with zinc oxide/multi-walled carbon nanotube (ZnO-MWCNT) composite additives in order to modify and tune their surface and bulk properties. Two types of ZnO-MWCNT additives with different morphologies were used in a wide concentration range from 0 to 90% for BC-based hybrids produced by filtration. The interaction between BC and ZnO-MWCNT and the effect of concentration and morphology of additives on the properties like zeta potential, hydrophilicity, electrical conductivity, etc. would be an important factor in various applications. Furthermore, the as-prepared hybrid membranes were characterized with the use of scanning electron microscopy (SEM), focused ion beam scanning electron microscopy (FIB-SEM), energy dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD) and surface area measurement (BET). Applying the presented synthesis routes, the surface properties of BC-based membranes can be tailored easily. Results reveal that the as-prepared BC-ZnO-MWCNT hybrid membranes can be ideal candidates for different kinds of applications, such as water filtration or catalysts.

Tailored CNTs Buckypaper Membranes for the Removal of Humic Acid and Separation of Oil-in-Water Emulsions

Carbon nanotubes (CNTs) are a robust material and proven as a promising candidate for a wide range of electronic, optoelectronic and environmental applications. In this work, two different methods were utilized for the preparation of CNTs exhibiting different aspect ratios via chemical vapor deposition (CVD). The as-prepared CNTs were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂adsorption isotherms, thermogravimetric analysis and Raman spectroscopy in order to investigate their morphological and structural properties. Free-standing CNTs "buckypaper" membranes were fabricated, characterized and tailored to meet the requirements of two applications, i.e., (1) the removal of humic acid (HA) from water and (2) separation of oil-in-water emulsions. It was revealed that the hydrophobic buckypapers showed high separation performance for Shell oil-in-water emulsions filtration, with up to 98% through the accumulation of oil droplets onto the membrane surface. The absorption capacity of buckypaper membranes for various organic liquids (oil, chloroform and toluene) was evaluated over 10 absorption cycles to investigate their recyclability and robustness. Moreover, surface modification was introduced to the pristine CNTs to increase their surface hydrophilicity and improve the pure water permeability of buckypapers. These modified buckypapers showed high flux for HA solutions and excellent HA rejection efficiency up to 95%via size exclusion and electrostatic repulsion mechanisms.

The Recent Progress in Modification of Polymeric Membranes Using Organic Macromolecules for Water Treatment

For decades, the water deficit has been a severe global issue. A reliable supply of water is needed to ensure sustainable economic development in population growth, industrialization and urbanization. To solve this major challenge, membrane-based water treatment technology has attracted a great deal of attention to produce clean drinking water from groundwater, seawater and brackish water. The emergence of nanotechnology in membrane science has opened new frontiers in the development of advanced polymeric membranes to enhance filtration performance. Nevertheless, some obstacles such as fouling and trade-off of membrane selectivity and permeability of water have hindered the development of traditional polymeric membranes for real applications. To overcome these issues, the modification of membranes has been pursued. The use of macromolecules for membrane modification has attracted wide interests in recent years owing to their interesting chemical and structural properties. Membranes modified with macromolecules have exhibited improved anti-fouling properties due to the alteration of their physiochemical properties in terms of the membrane morphology, porosity, surface charge, wettability, and durability. This review provides a comprehensive review of the progress made in the development of macromolecule modified polymeric membranes. The role of macromolecules in polymeric membranes and the advancement of these membrane materials for water solution are presented. The challenges and future directions for this subject are highlighted.

Modeling and simulation of fertilizer drawn forward osmosis process using Aspen Plus-MATLAB model

Although experimental studies on the impact of feed (FS) and draw solutions (DS) on the forward osmosis (FO) applications are reported in literature, systematic mathematical modeling considering the dynamic change in solution properties is lacking. In this study, asymmetric FO membrane simulation model was established using Aspen Plus-MATLAB subroutines algorithm to account for the effect of concentration polarization (CP), types of FS and DS and in their properties on FO performance. The developed model was validated by comparing the simulation with experimental results. The model successfully predict the performance of FO process under wide varieties of operational conditions, FS and DS flow rates and concentrations. The model showed that the variation of MCFDS concentration had a marked effect on water flux (WF) in contrast to flow rate. The WFs obtained from seawater (SW) increased from 5.28 L/m².h to 42.08 L/m².h as MCFDS changes from 150 g/L to 300 g/L which corresponding to 11.66% to 45.33% of water recovery. As for synthetic aquaculture wastewater (SAWW), 9.70 L/m².h to 37.32 L/m².h of WFs were exhibited with the increase of MCFDS concentration from 50 g/L to 200 g/L, respectively. The effect of concentrated external CP (CECP) was found to be significant in case of SW and negligible with SAWW. Whereas, increasing MCFDS concentration increases the severity effect of dilutive internal CP (DICP). The degree of DICP depends on the solute resistivity (K_D) of porous layer, which were elevated (4.22-5.88 s/m) as MCFDS concentration increases (150-300 g/L). The study demonstrated the effectiveness and suitability of the developed Aspen Plus-MATLAB model simulating the FO process.

New Smart Magnetic Ionic Liquid Nanocomposites Based on Chemically Bonded Imidazole Silica for Water Treatment

New magnetic silica imidazolium ionic liquid nanocomposites were synthesized by a sol-gel technique. The (3-aminopropyl)triethoxysilane (APTS) was condensed with glyoxal and p-hydroxybenzaldehyde in acetic acid to produce an amino-modified silica ionic liquid (Si-IIL). The APTS was condensed with TEOS in ethanol and water to prepare amino-modified SiO₂ nanoparticles. The produced amino-modified SiO₂ silica was condensed with glyoxal and p-hydroxybenzaldehyde in acetic acid to produce chemically bonded silica SiO₂-IIL. The SiO₂-IIL and Si-IIL were used as capping agents during and after the formation of magnetite nanoparticles in ammonia to produce magnetic SiO₂-IIL-Fe₃O₄ and Fe₃O₄-Si-IIL adsorbents, respectively. Their chemical structure, morphology, crystalline lattice structure, surface charges, particle sizes, and magnetic characteristics elucidated the formation of core-shell and highly dispersed magnetic nanocomposites. The saturation magnetization values of Fe₃O₄-Si-IIL and SiO₂-IIL-Fe₃O₄ were 35.3 and 30.8, respectively. The uniform dispersed disconnected spherical morphologies appeared for Fe₃O₄-Si-IIL hybrid and the core-shell spherical morphology obtained with SiO₂-IIL-Fe₃O₄ hybrid NPs. The Fe₃O₄-Si-IIL and SiO₂-IIL-Fe₃O₄ show an excellent high chemical adsorption capacities as 460.3 and 300.9 mg·g^-1, respectively (not reported in the literature) when used as an adsorbent to remove CB-R250 water pollutant under optimum conditions. Their applicability and reusability as fast and highly effective adsorbents for Coomassie blue (CB-R250) organic water pollutants were investigated.

Carbon Nanomaterials for the Treatment of Heavy Metal-Contaminated Water and Environmental Remediation

Nanotechnology is an advanced field of science having the ability to solve the variety of environmental challenges by controlling the size and shape of the materials at a nanoscale. Carbon nanomaterials are unique because of their nontoxic nature, high surface area, easier biodegradation, and particularly useful environmental remediation. Heavy metal contamination in water is a major problem and poses a great risk to human health. Carbon nanomaterials are getting more and more attention due to their superior physicochemical properties that can be exploited for advanced treatment of heavy metal-contaminated water. Carbon nanomaterials namely carbon nanotubes, fullerenes, graphene, graphene oxide, and activated carbon have great potential for removal of heavy metals from water because of their large surface area, nanoscale size, and availability of different functionalities and they are easier to be chemically modified and recycled. In this article, we have reviewed the recent advancements in the applications of these carbon nanomaterials in the treatment of heavy metal-contaminated water and have also highlighted their application in environmental remediation. Toxicological aspects of carbon-based nanomaterials have also been discussed.

Forward osmosis membrane processes for wastewater bioremediation: Research needs

Increasing research and development works have been made to develop forward osmosis (FO) processes as a cost-effective substitute for energy intensive water vacuum suction facility in submerged membrane bioreactor (MBR) applications. Perceived to be a spontaneous water driven process without external applied pressures, the FO has been applied in lab and pilot scales for wastewater bioremediation. This paper reviewed the state-of-the-art developments on the FO unit, the process, and ways of enhancing process performance, particularly on the aspects of flux enhancement, flow resistance reduction, and draw solute with low reverse salt diffusion, which are relevant to enhanced osmotic MBR performance. The perspective to realize the use of FO processes in revision of currently existing energy intensive osmotic MBR processes is discussed with research needs being highlighted.

Next-Generation Multifunctional Carbon-Metal Nanohybrids for Energy and Environmental Applications

Nanotechnology has unprecedentedly revolutionized human societies over the past decades and will continue to advance our broad societal goals in the coming decades. The research, development, and particularly the application of engineered nanomaterials have shifted the focus from "less efficient" single-component nanomaterials toward "superior-performance", next-generation multifunctional nanohybrids. Carbon nanomaterials (e.g., carbon nanotubes, graphene family nanomaterials, carbon dots, and graphitic carbon nitride) and metal/metal oxide nanoparticles (e.g., Ag, Au, CdS, Cu₂O, MoS₂, TiO₂, and ZnO) combinations are the most commonly pursued nanohybrids (carbon-metal nanohybrids; CMNHs), which exhibit appealing properties and promising multifunctionalities for addressing multiple complex challenges faced by humanity at the critical energy-water-environment (EWE) nexus. In this frontier review, we first highlight the altered and newly emerging properties (e.g., electronic and optical attributes, particle size, shape, morphology, crystallinity, dimensionality, carbon/metal ratio, and hybridization mode) of CMNHs that are distinct from those of their parent component materials. We then illustrate how these important newly emerging properties and functions of CMNHs direct their performances at the EWE nexus including energy harvesting (e.g., H₂O splitting and CO₂ conversion), water treatment (e.g., contaminant removal and membrane technology), and environmental sensing and in situ nanoremediation. This review concludes with identifications of critical knowledge gaps and future research directions for maximizing the benefits of next-generation multifunctional CMNHs at the EWE nexus and beyond.

The Roles of Nanomaterials in Conventional and Emerging Technologies for Heavy Metal Removal: A State-of-the-Art Review

Heavy metal (HM) pollution in waterways is a serious threat towards global water security, as high dosages of HM poisoning can significantly harm all living organisms. Researchers have developed promising methods to isolate, separate, or reduce these HMs from water bodies to overcome this. This includes techniques, such as adsorption, photocatalysis, and membrane removal. Nanomaterials play an integral role in all of these remediation techniques. Nanomaterials of different shapes have been atomically designed via various synthesis techniques, such as hydrothermal, wet chemical synthesis, and so on to develop unique nanomaterials with exceptional properties, including high surface area and porosity, modified surface charge, increment in active sites, enhanced photocatalytic efficiency, and improved HM removal selectivity. In this work, a comprehensive review on the role that nanomaterials play in removing HM from waterways. The unique characteristics of the nanomaterials, synthesis technique, and removal principles are presented. A detailed visualisation of HM removal performances and the mechanisms behind this improvement is also detailed. Finally, the future directions for the development of nanomaterials are highlighted.

Ar/O2 plasma treatment of carbon nanotube membranes for enhanced removal of zinc from water and wastewater: A dynamic sorption-filtration process

In this study, pristine multi-walled carbon nanotubes (MWCNTs) were functionalized by using Ar/O₂ plasma treatment technique, which enhanced adsorptive membrane filtration of zinc ions from water and wastewater. The XPS analysis showed that plasma treatment largely increased the surface oxygen groups content of MWCNTs from 2.78% to 6.79%. This change increased the surface negative charged, dispersion and adsorption properties of MWCNTs without causing any damages to the integrity of the nanotube pattern. Pressure-driven filtration of plasma-treated MWCNT (P-CNT) dispersion formed a stable layer inside the lumen of a hollow fiber membrane. The contact angle analysis demonstrated that after incorporation of P-CNT into the HF membrane increased membrane hydrophilicity. The P-CNT membrane effectively removed almost 100% of zinc from synthetic waters and approximately 80% of zinc from a wastewater effluent by surface complexation reaction. A follow-up regeneration study demonstrated that the adsorptive removal of zinc by the CNT membrane was reversible under selected conditions, thus making it possible to repeatedly use the membrane for long-term zinc filtration. This study suggests that HF membranes modified with P-CNTs possess superb adsorption properties for metal ions, allowing the operation of CNT membranes for water treatment.

Efficient removal of zinc from water and wastewater effluents by hydroxylated and carboxylated carbon nanotube membranes: Behaviors and mechanisms of dynamic filtration

In this work, a bench scale study was designed to investigate the removal of zinc (Zn²⁺) and regeneration efficiencies of functionalized-MWCNT (f-MWCNT) membranes. The f-MWCNTs were incorporated into polyvinylchloride (PVC) hollow fiber membranes (HFMs), which acted as a substrate and a barrier for MWCNTs leaching to water. The results revealed that the removal capacity of Zn²⁺ through f-CNT membranes were above 98% for the synthetic water and over 70% for real wastewater effluents; predominantly involved surface complexation reaction. The acquired removal efficiency of CNT membrane is attributed to high absolute zeta potential followed by the hydrophilicity of the nanotubes coated the inside surface of HFMs and high concentration of oxygen functional groups on CNT surfaces. Later on, different regenerating solutions were used to desorb Zn²⁺ ions repeatedly from the inner surface of membranes and to recycle the CNT membranes for continuous removal of Zn²⁺ from water. The XPS analysis revealed that, Zn²⁺ ions were completely recovered owing to the ion exchange interactions. The results further confirmed that f-CNT membranes retained their original removal capacity after several successive cycles. Therefore, we recommend that, f-CNTs-based membranes have the potential to be used for large-scale removal and recovery of heavy metal ions from water or wastewater.