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Design strategies of smart membranes for switchable gas separation performance.

Membrane separation has been an efficient and energy saving technique in dealing with greenhouse gases, but the traditional membrane designs might not be able to handle the concomitant environmental pollution due to their fixed properties. Correspondingly, the use of an active-responsive smart membrane appears to be a new trend for membrane development in the coming future, which shows great potential to deal with the obstacles. In this research, we demonstrate a smart graphene-organic framework membrane to enable reversed electrical-switchable permselectivity in CO2 separation. The addition of polydopamine (PDA) to the polyvinylidene fluoride/Graphene (PVDF/G) membranes was done to (i) induce the β-phase of PVDF, since –NH2-functionalized graphene has specific interactions (dipole induced dipole interaction) between graphene-PDA and PVDF; (ii) modify the organic PVDF-inorganic graphene interface; and (iii) facilitate CO2 selective separation. Permeability and permselectivity was increased after applying voltage that resulted in the increase of gas permselectivity in response to the lowest applied voltage range (0–3 V) to the membrane. Digital image correlation method depicted the response of the membrane to voltage and proved that the membrane has high piezoelectric properties that is switchable. Furthermore, PALS studies confirmed the free volume and interlayers in the membrane. This membrane has unique properties because the pore changes from bimodal to single pore distribution.

Graphene possesses a set of unique physicochemical properties including exceptional mechanical, thermal, and electrical properties, making it an excellent candidate for constructing materials for a wide range of applications including the vast field of separation technology using membranes. Separation membranes based on graphene and its derivatives have shown satisfactory results over the years when applied to environmental systems such as wastewater treatment and gas purification, which is a great contribution in sustainable development. However, membrane separation research is generally performed using conventional membranes with fixed driving force and separation properties that suffer from fouling and decline in performance upon long-term use or when a change in the environmental conditions occur. These issues can be solved by employing an emerging technology that makes use of graphene and its derivatives combined with polymeric materials to construct stimuli-responsive or “smart” membranes that respond to the changes in their environment such as chemical cues, temperature, pressure, and external fields, and have a self-regulated separation performance due to reversible physicochemical properties. In this review, we present a report on the recent advancements on graphene-based separation technology including a concise discussion on the basic structure and properties of graphene and its derivatives, various membrane fabrication methods, and their employment on the different areas of membrane separation. More importantly, the main focus of this paper is to evaluate the design and utilization of graphene-based smart separation membranes.

The development of science and technology is accompanied by a complex composition of multiple pollutants. Conventional passive separation processes are not sufficient for current industrial applications. The advent of active or responsive separation methods has become highly essential for future applications. In this work, we demonstrate the preparation of a smart electrically responsive membrane, a poly(vinylidene difluoride) (PVDF)-graphene composite membrane. The high graphene content induces the self-assembly of PVDF with a high β-phase content, which displays a unique self-piezoelectric property. Additionally, the membrane exhibits excellent electrical conductivity and unique capacitive properties, and the resultant nanochannels in the membrane can be reversibly adjusted by external voltage applications, resulting in the tailored gas selectivity of a single membrane. After the application of voltage to the membrane, the permeability and selectivity toward carbon dioxide increase simultaneously. Moreover, atomic-level positron annihilation spectroscopic studies reveal the piezoelectric effect on the free volume of the membrane, which helps us to formulate a gas permeation mechanism for the electrically responsive membrane. Overall, the novel active membrane separation process proposed in this work opens new avenues for the development of a new generation of responsive membranes.

The membrane distillation (MD)-based desalination process is thought to be a promising strategy to address global challenges, such as safe water-energy crisis and environmental pollution. Here, we demonstrate a novel self-heated vacuum (MD) system employing a graphene–PVDF flat sheet membrane Joule heater that enables the combination of the interfacial heating of a feed stream and conventional VMD for energy efficient sea water desalination. In this system, the direct delivery of low voltage DC power to a superhydrophobic graphene–PVDF flat sheet membrane Joule heater was placed in a filtration module, and effectively heats the feed stream directly at the feed/membrane interface. Thus, it eliminates the preheating of feed solution and temperature polarization (TP), along with enhanced permeate flux, which in turn significantly reduces the energy requirement for the desalination process. The graphene–PVDF flat sheet membrane Joule heater was obtained via simple phase inversion technique, and exhibited excellent Joule heating performance, superhydrophobicity, and graphene laminar stacking-induced high porosity. While treating a saline feed without preheating, and with optimum membrane and experimental variables, our self-heated MD system achieved a permeate flux of 23.44 L m−2 h−1 and salt rejection of 99.41%, with the lowest specific heating energy (Qsh) consumption of 0.109 kW h L−1 and the highest gain output ratio (GOR) of 5.72, respectively. This is the greatest performance achieved so far, compared to the performance of solar heating (photothermal) and induction heating-based self-heated MD systems reported previously. The optimum membrane surface temperature (T* = 38 °C) and effective membrane area (71.1%) were assessed using FVM simulation, and were validated by the corresponding experimental results. This energy efficient self-heated vacuum MD system is suitable for high purity water production and effective sea water desalination. Furthermore, by harvesting the electrical energy required for membrane Joule heating from renewable energy (solar energy) and using feed streams with low grade waste heat, we could further increase the economic viability and application potential of the system. The self-heated vacuum MD system demonstrated here constitutes an effective approach for a sustainable, decentralized water desalination technology.

With rapid advancement in water filtration materials, several efforts have been made to fabricate electrospun nanofiber membranes (ENMs). ENMs play a crucial role in different areas of water treatment due to their several advantageous properties such as high specific surface area, high interconnected porosity, controllable thickness, mechanical robustness, and wettability. In the broad field of water purification, ENMs have shown tremendous potential in terms of permeability, rejection, energy efficiency, resistance to fouling, reusability and mechanical robustness as compared to the traditional phase inversion membranes. Upon various chemical and physical modifications of ENMs, they have exhibited great potential for emerging applications in environment, energy and health sectors. This review firstly presents an overview of the limiting factors influencing the morphology of electrospun nanofibers. Secondly, it presents recent advancements in electrospinning processes, which helps to not only overcome drawbacks associated with the conventional electrospinning but also to produce nanofibers of different morphology and orientation with an increased rate of production. Thirdly, it presents a brief discussion about the recent progress of the ENMs for removal of various pollutants from aqueous system through major areas of membrane separation. Finally, this review concludes with the challenges and future directions in this vast and fast growing area.

Nanocrystalline mixed spinel ferrite of Zn 0.5 Ni 0.5 Fe 2 O 4 nanoparticles has been successfully synthesized by coprecipitation method and encapsulated by silica and PEG-4000 with various concentrations. X-Ray Diffraction (XRD) patterns showed that nanoparticles contained Zn 0.5 Ni 0.5 Fe 2 O 4 spinel ferrite with crystallite size of 15.2 nm. After silica encapsulation the new phase of SiO 2 appearance and the crystallite size became 14.1 nm. After PEG-4000 encapsulation also appeared a new phase of a-Fe 2 O 3 and the crystallite size became 13.3 nm. Transmission Electron Microscopy (TEM) image showed that the nanoparticles more dispersed and decrease agglomeration after silica and PEG-4000 encapsulation. The coercivity (H c ) of Zn 0.5 Ni 0.5 Fe 2 O 4 was 50.7 Oe. The H c decrease after silica and PEG-4000 encapsulation became 45,2 Oe and 47.9 Oe, respectively. It is due to the decrease of particle size. The saturation magnetization (M s ) of Zn 05 Ni 0 . 5 Fe 2 O 4 was 18.4 emu/g, and decrease to 16.8 emu/g after silica encapsulation because SiO 2 is non-magnetic properties. M s decrease to 17.1 emu/g after PEG-4000 encapsulation. This is because PEG-4000 has paramagnetic properties. Fourier Transform Infra Red (FTIR) spectra for Zn 05 Ni 05 Fe 2 O 4 show that the absorbtion peaks is around 300–600 cm−1 which is an M-O bond vibration. After a silica encapsulation, there are new bond vibration typical of silica like Si-O-Si (1087.85 cm−1), Si-OH (794.67 cm−1), and Si-O-Fe (574.64 cm−1). The PEG-4000 encapsulation has new vibration for typical of PEG like C-O (1064.71 cm−1). Both of encapsulation has M-O bond vibration indicates the presence of Zn 0.5 Ni 0.5 Fe 2 O 4 nanoparticles.

Zn x Ni 1−x Fe 2 O 4 magnetic nanoparticles with various x from 0.2 to 0.8 have been synthesized by a chemical co-precipitation method. The X-ray diffraction (XRD) patterns confirmed that nanoparticles have spinel cubic structure with some impurity phases. The lattice parameter of the sample for x = 0.2 was 8.599 A and then increases with the increase of Zn concentration. This is due to the replacement of smaller Ni2+ by larger Zn2+ cation. The crystallite size of the sample for x = 0.2 about 21.5 nm and then decrease by the increase in Zn content. It is due to the lower bond energy of Zn2+-O2- (159 kJ/mol) as compared with Ni2+-O2- (451 kJ/mol). The Fourier transform-infrared (FT-IR) spectra between 400 and 4000 cm−1 confirmed the intrinsic cation vibrations of the spinel structure. Transmission electron microscopy (TEM) pattern showed that the sample seem to be agglomeration with spherical shape. Vibrating sample magnetometer (VSM) measurement showed that the values of the maximum magnetization at 15 kOe increased from 5.9 to 18.5 emu/g by decreasing of Zn content. The coercivity (Hc) values of of the sample for x = 0.2 was 48.4 Oe and then decreases with the increasing of Zn content. It is due to the decrease in the magnetocrystalline anisotropy attributed to the absence of Ni2+ cation.

Nanocrystalline mixed spinel ferrite of Zn0.5Ni0.5Fe2O4 magnetic nanoparticles (MNPs) has been successfully synthesized by coprecipitation method and encapsulated and PEG-4000 with various concentrations. X-Ray Diffraction (XRD) patterns showed that nanoparticles contained Zn0.5Ni0.5Fe2O4 spinel ferrite with the particles size of 15.2 nm. After PEG-4000 encapsulation, particles size decrease became 13.3 nm. Transmission Electron Microscopy (TEM) image showed that the nanoparticles still aglomerate after PEG-4000 encapsulation. The coercivity (Hc ) of Zn0.5Ni0.5Fe2O4 was 50.7 Oe. The Hc decrease after PEG-4000 encapsulation became 47.9 Oe, respectively. It is due to the decrease of particle size. The saturation magnetization (Ms ) of Zn0.5Ni0.5Fe2O4 was 18.4 emu/g, and decrease to 17.1 emu/g after PEG-4000 encapsulation. This is because PEG-4000 has paramagnetic properties.

Nickel Zinc Ferrite (Ni0.5Zn0.5Fe2O4) magnetic nanoparticles have been successfully synthesized using the co-precipitation method with variation of synthesis temperature and concentration of NaOH. X-ray diffraction (XRD) analysis confirmed that Ni0.5Zn0.5Fe2O4 nanoparticles have spinel cubic crystal structure. The XRD profile of sample showed that the crystallinity increases with the increase of synthesis temperature. Meanwhile the XRD profile of sample showed the crystallinity decreases with the increase of NaOH concentration. Crystallite size of samples were in the range of 11.1-16.0 nm. There is no significant change in the lattice parameter size for the samples synthesized with increase of temperatures and NaOH concentrations. Transmission electron microscope (TEM) image showed that the sample was agglomerated. The selected area electron diffraction (SAED) image showed the diffraction ring as representation of Miller plane and confirmed that sample was polycrystalline. The maximum magnetization (σs ) at 15 kOe of the samples increases with the increase of crystallite size and crystallinity. The coercivity (Hc ) of the samples increases with the increase of crystallite size.