Surface-Modified Nanofibrous PVDF Membranes for Liquid Separation Technology

Recent developments in the application of membrane separation technology and its challenges in oil-water separation: A review

In the development and production process of domestic and foreign oil fields, large amounts of oil-bearing wastewater with complex compositions containing toxic and harmful pollutants are generated. These oil-bearing wastewaters will cause serious environmental pollution if they are not effectively treated before discharge. Among these wastewaters, the oily sewage produced in the process of oilfield exploitation has the largest content of oil-water emulsion. In order to solve the problem of oil-water separation of oily sewage, the paper summarizes the research of many scholars in many aspects, such as the use of physical and chemical methods such as air flotation and flocculation, or the use of mechanical methods such as centrifuges and oil booms for sewage treatment. Comprehensive analysis shows that among these oil-water separation methods, membrane separation technology has higher separation efficiency in the separation of general oil-water emulsions than other methods and also exhibits a better separation effect for stable emulsions, which has a broader application prospect for future developments. To present the characteristics of different types of membranes more intuitively, this paper describes the applicable conditions and characteristics of various types of membranes in detail, summarizes the shortcomings of existing membrane separation technologies, and offers prospects for future research directions.

Chemical Cleaning Process of Polymeric Nanofibrous Membranes

Membrane fouling is one of the most significant issues to overcome in membrane-based technologies as it causes a decrease in the membrane flux and increases operational costs. This study investigates the effect of common chemical cleaning agents on polymeric nanofibrous membranes (PNM) prepared by polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), and polyamide 6 (PA6) nanofibers. Common alkaline and acid membrane cleaners were selected as the chemical cleaning agents. Membrane surface morphology was investigated. The PAN PNM were selected and fouled by engine oil and then cleaned by the different chemical cleaning agents at various ratios. The SEM results indicated that the use of chemical agents had some effects on the surface of the nanofibrous membranes. Moreover, alkaline cleaning of the fouled membrane using the Triton X 100 surfactant showed a two to five times higher flux recovery than without using a surfactant. Among the tested chemical agents, the highest flux recovery rate was obtained by a binary solution of 5% sodium hydroxide + Triton for alkaline cleaning, and an individual solution of 1% citric acid for acidic cleaning. The results presented here provide one of the first investigations into the chemical cleaning of nanofiber membranes.

Polymeric Membranes for Oil-Water Separation: A Review

This review is devoted to the application of bulk synthetic polymers such as polysulfone (PSf), polyethersulfone (PES), polyacrylonitrile (PAN), and polyvinylidene fluoride (PVDF) for the separation of oil-water emulsions. Due to the high hydrophobicity of the presented polymers and their tendency to be contaminated with water-oil emulsions, methods for the hydrophilization of membranes based on them were analyzed: the mixing of polymers, the introduction of inorganic additives, and surface modification. In addition, membranes based on natural hydrophilic materials (cellulose and its derivatives) are given as a comparison.

Investigation of the Morphological Structure of Needle-Free Electrospun Magnetic Nanofiber Mats

Electrospun magnetic nanofibers are promising for a variety of applications in biomedicine, energy storage, filtration or spintronics. The surface morphology of nanofiber mats plays an important role for defined application areas. In addition, the distribution of magnetic particles in nanofibers exerts an influence on the final properties of nanofiber mats. A simple method for the production of magnetic nanofiber mats by the addition of magnetic nanoparticles in an electrospinning polymer solution was used in this study. In this work, magnetic nanofibers (MNFs) were prepared by needle-free electrospinning technique from poly(acrylonitrile) (PAN) in the low-toxic solvent dimethylsulfoxide (DMSO) and 20 wt% Fe3O4 at different parameter conditions such as PAN concentration, voltage and ultrasonic bath. The distribution of nanoparticles in the fiber matrix was investigated as well as the chemical and morphological properties of the resulting magnetic nanofibers. In addition, the surface morphology of magnetic nanofiber mats was studied by confocal laser scanning microscope (CLSM), scanning electron microscope (SEM), Fourier transform infrared microscope (FTIR) and ImageJ software, and distribution of Fe3O4 particles in the matrix was investigated by energy dispersive X-ray spectroscopy (EDX).

Electrospinning Nanofiber Mats with Magnetite Nanoparticles Using Various Needle-Based Techniques

Electrospinning can be used to produce nanofiber mats containing diverse nanoparticles for various purposes. Magnetic nanoparticles, such as magnetite (Fe₃O₄), can be introduced to produce magnetic nanofiber mats, e.g., for hyperthermia applications, but also for basic research of diluted magnetic systems. As the number of nanoparticles increases, however, the morphology and the mechanical properties of the nanofiber mats decrease, so that freestanding composite nanofiber mats with a high content of nanoparticles are hard to produce. Here we report on poly (acrylonitrile) (PAN) composite nanofiber mats, electrospun by a needle-based system, containing 50 wt% magnetite nanoparticles overall or in the shell of core–shell fibers, collected on a flat or a rotating collector. While the first nanofiber mats show an irregular morphology, the latter are quite regular and contain straight fibers without many beads or agglomerations. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) reveal agglomerations around the pure composite nanofibers and even, round core–shell fibers, the latter showing slightly increased fiber diameters. Energy dispersive X-ray spectroscopy (EDS) shows a regular distribution of the embedded magnetic nanoparticles. Dynamic mechanical analysis (DMA) reveals that mechanical properties are reduced as compared to nanofiber mats with smaller amounts of magnetic nanoparticles, but mats with 50 wt% magnetite are still freestanding.

Statistical Analysis of Nanofiber Mat AFM Images by Gray-Scale-Resolved Hurst Exponent Distributions

Two-dimensional structures, either periodic or random, can be classified by diverse mathematical methods. Quantitative descriptions of such surfaces, however, are scarce since bijective definitions must be found to measure unique dependency between described structures and the chosen quantitative parameters. To solve this problem, we use statistical analysis of periodic fibrous structures by Hurst exponent distributions. Although such a Hurst exponent approach was suggested some years ago, the quantitative analysis of atomic force microscopy (AFM) images of nanofiber mats in such a way was described only recently. In this paper, we discuss the influence of typical AFM image post-processing steps on the gray-scale-resolved Hurst exponent distribution. Examples of these steps are polynomial background subtraction, aligning rows, deleting horizontal errors and sharpening. Our results show that while characteristic features of these false-color images may be shifted in terms of gray-channel and Hurst exponent, they can still be used to identify AFM images and, in the next step, to quantitatively describe AFM images of nanofibrous surfaces. Such a gray-channel approach can be regarded as a simple way to include some information about the 3D structure of the image.

Hydrophilic Surface-Modified PAN Nanofibrous Membranes for Efficient Oil-Water Emulsion Separation

In order to protect the environment, it is important that oily industrial wastewater is degreased before discharging. Membrane filtration is generally preferred for separation of oily wastewater as it does not require any specialised chemical knowledge, and also for its ease of processing, energy efficiency and low maintenance costs. In the present work, hybrid polyacrylonitrile (PAN) nanofibrous membranes were developed for oily wastewater filtration. Membrane surface modification changed nitrile groups on the surface into carboxylic groups, which improve membrane wettability. Subsequently, TiO₂ nanoparticles were grafted onto the modified membranes to increase flux and permeability. Following alkaline treatment (NaOH, KOH) of the hydrolysed PAN nanofibres, membrane water permeability increased two- to eight-fold, while TiO₂ grafted membrane permeability increase two- to thirteen-fold, compared to unmodified membranes. TiO₂ grafted membranes also displayed amphiphilic properties and a decrease in water contact angle from 78.86° to 0°. Our results indicate that modified PAN nanofibrous membranes represent a promising alternative for oily wastewater filtration.

Potential Applications of Magnesium-Based Polymeric Nanocomposites Obtained by Electrospinning Technique

In the last few decades, the development of new electrospun materials with different morphologies and advanced multifunctional properties are strongly consolidated. There are several reviews that describe the processing, use and characterization of electrospun nanocomposites, however, based on our knowledge, no review on electrospun nanocomposites reinforced with nanoparticles (NPs) based on magnesium, Mg-based NPs, are reported. Therefore, in the present review, we focus attention on the fabrication of these promising electrospun materials and their potential applications. Firstly, the electrospinning technique and its main processing window-parameters are described, as well as some post-processing methods used to obtain Mg-based materials. Then, the applications of Mg-based electrospun nanocomposites in different fields are pointed out, thus taking into account the current trend in developing inorganic-organic nanocomposites to gradually satisfy the challenges that the industry generates. Mg-based electrospun nanocomposites are becoming an attractive field of research for environmental remediation (waste-water cleaning and air filtration) as well as for novel technical textiles. However, the mayor application of Mg-based electrospun materials is in the biomedical field, as pointed out. Therefore, this review aims to clarify the tendency in using electrospinning technique and Mg-based nanoparticles to huge development at industrial level in the near future.

Magnetic Properties of Electrospun Magnetic Nanofiber Mats after Stabilization and Carbonization

Magnetic nanofibers are of great interest in basic research, as well as for possible applications in spintronics and neuromorphic computing. Here we report on the preparation of magnetic nanofiber mats by electrospinning polyacrylonitrile (PAN)/nanoparticle solutions, creating a network of arbitrarily oriented nanofibers with a high aspect ratio. Since PAN is a typical precursor for carbon, the magnetic nanofiber mats were stabilized and carbonized after electrospinning. The magnetic properties of nanofiber mats containing magnetite or nickel ferrite nanoparticles were found to depend on the nanoparticle diameters and the potential after-treatment, as compared with raw nanofiber mats. Micromagnetic simulations underlined the different properties of both magnetic materials. Atomic force microscopy and scanning electron microscopy images revealed nearly unchanged morphologies after stabilization without mechanical fixation, which is in strong contrast to pure PAN nanofiber mats. While carbonization at 500 °C left the morphology unaltered, as compared with the stabilized samples, stronger connections between adjacent fibers were formed during carbonization at 800 °C, which may be supportive of magnetic data transmission.

A Review on Membrane Technology and Chemical Surface Modification for the Oily Wastewater Treatment

Cleaning of wastewater for the environment is an emerging issue for the living organism. The separation of oily wastewater, especially emulsified mixtures, is quite challenged due to a large amount of wastewater produced in daily life. In this review, the membrane technology for oily wastewater treatment is presented. In the first part, the global membrane market, the oil spill accidents and their results are discussed. In the second and third parts, the source of oily wastewater and conventional treatment methods are represented. Among all methods, membrane technology is considered the most efficient method in terms of high separation performance and easy to operation process. In the fourth part, we provide an overview of membrane technology, fouling problem, and how to improve the self-cleaning surface using functional groups for effectively treating oily wastewater. The recent development of surface-modified membranes for oily wastewater separation is investigated. It is believed that this review will promote understanding of membrane technology and the development of surface modification strategies for anti-fouling membranes.

Electrospinning on 3D Printed Polymers for Mechanically Stabilized Filter Composites

Electrospinning is a frequently used method to prepare air and water filters. Electrospun nanofiber mats can have very small pores, allowing for filtering of even the smallest particles or molecules. In addition, their high surface-to-volume ratio allows for the integration of materials which may additionally treat the filtered material through photo-degradation, possess antimicrobial properties, etc., thus enhancing their applicability. However, the fine nanofiber mats are prone to mechanical damage. Possible solutions include reinforcement by embedding them in composites or gluing them onto layers that are more mechanically stable. In a previous study, we showed that it is generally possible to stabilize electrospun nanofiber mats by 3D printing rigid polymer layers onto them. Since this procedure is not technically easy and needs some experience to avoid delamination as well as damaging the nanofiber mat by the hot nozzle, here we report on the reversed technique (i.e., first 3D printing a rigid scaffold and subsequently electrospinning the nanofiber mat on top of it). We show that, although the adhesion between both materials is insufficient in the case of a common rigid printing polymer, nanofiber mats show strong adhesion to 3D printed scaffolds from thermoplastic polyurethane (TPU). This paves the way to a second approach of combining 3D printing and electrospinning in order to prepare mechanically stable filters with a nanofibrous surface.

Stabilization of Electrospun Nanofiber Mats Used for Filters by 3D Printing

Electrospinning is a well-known technology used to create nanofiber mats from diverse polymers and other materials. Due to their large surface-to-volume ratio, such nanofiber mats are often applied as air or water filters. Especially the latter, however, have to be mechanically highly stable, which is challenging for common nanofiber mats. One of the approaches to overcome this problem is gluing them on top of more rigid objects, integrating them in composites, or reinforcing them using other technologies to avoid damage due to the water pressure. Here, we suggest another solution. While direct 3D printing with the fused deposition modeling (FDM) technique on macroscopic textile fabrics has been under examination by several research groups for years, here we report on direct FDM printing on nanofiber mats for the first time. We show that by choosing the proper height of the printing nozzle above the nanofiber mat, printing is possible for raw polyacrylonitrile (PAN) nanofiber mats, as well as for stabilized and even more brittle carbonized material. Under these conditions, the adhesion between both parts of the composite is high enough to prevent the nanofiber mat from being peeled off the 3D printed polymer. Abrasion tests emphasize the significantly increased mechanical properties, while contact angle examinations reveal a hydrophilicity between the original values of the electrospun and the 3D printed materials.