Nanotechnology in Membrane Processes

The use of nanoparticles by humans is not new and it has an old history. However, the use of nanoparticles for the human benefit is new, and its use is friendly with environment. The properties of nanoparticles depend on the sizes of particles. Nanotechnology is the process of manipulating atoms on a nanoscale. The ability to tailor the structure of materials at very small scales can create specific properties, which is otherwise impossible. Thus, the nanotechnology has emerged as a boon in various realms and sectors including industry, food, healthcare, pharmaceutical, electronics and construction etc.

A membrane is a selective barrier; it allows some things to pass through but stops others. Thus, membranes are used in many industrial separation processes, with a number of pros and cons compared to alternative means of performing separations.

The properties/performances of membranes can be changed by incorporating nanoparticles within their matrices. For example, nanoparticles are used to increase the membrane surface hydrophilicity, to control membrane bio-fouling etc. As well, the use of nanofibers opened a new avenue for nanotechnology in membrane processes.

Both nanotechnology and membrane processes are among the most advanced fields of science and technology. But, like every advanced field, both are full of risks and opportunities.

Although the application of nanotechnology in the membrane processes may reduce the cost and maintenance of the membranes used, the use of nanoparticles can result in toxic side effects on humans. There is still very little awareness about the side effects of nanotechnology and there is need for further research in this area.

At present researchers and scientists are spending more and more time applying nanotechnology in various membrane processes such as water treatment, environment, military zone, food industry, space etc.

This chapter describes the materials used for the preparation of synthetic membranes, including also their preparation methods. Synthetic membranes are either ceramic or polymeric. The membranes can be prepared in various shapes such as flat sheet, tubular, hollow fiber, and spiral wound, each with its own special features. Many novel functional nanomaterials are being explored to enhance the performance of membranes.

The nanoparticles can be synthesised by various methods for both research and commercial uses. These are physical, chemical and mechanical. Nanoparticles can be derived from larger molecules or synthesized by ‘bottom-up’ methods that, for example, nucleate and grow particles from fine molecular distributions in liquid or vapour phase. The synthesis method can also include functionalization by conjugation to bioactive molecules.

Electrospun nanofibers have emerged as important fibrous materials for reinforcing or modifying polymer matrices. Nanofibers have gained much interest for use in various biomedical applications over the past few decades due to their unique functional properties such as large surface area and high aspect ratio, which play a vital role in cellular and molecular activities, and their structural similarity to native cellular micro environment. Nowadays nanofibers are being used in industrial scale for the treatment of air and water. Different techniques for the manufacturing nanofibers are presented in this chapter but electrospinning has been shown to be the most effective method.

Carbon nanomaterials (CNMs) have received tremendous attention in the field of novel membrane science and technology. Application of CNMs could improve the membrane separation process. As well, TiO₂ nanoparticles are popular as fillers to the polymeric membranes.

Membrane characterization is an important part of membrane research, development, and engineering. It provides a crucial link between the preparation and performance of the membranes and their structure, chemistry, morphology, transport properties, and other characteristics, with the ultimate goal of understanding how to make the best membrane and use it in the best way. An ideal characterization method should be non-destructive, accurate, repeatable, and fast and should maximize data. Many methods of characterization have been devised. A small change in one of the membrane formation parameters can change the (top layer) structure and consequently have a drastic effect on membrane performance. Characterization techniques can be classified into static and dynamic techniques. The static techniques mainly give information on membrane morphology and structure, and chemical and physical properties. The dynamic techniques are of fundamental importance when investigating membrane performance. For example, the most widely used characterization method for desalination membrane is the measurement of water flux and solute (usually NaCl) rejection; these can be easily measured and so give a quick indication of the suitability of the membrane for the desalination purpose. For surface hydrophilicity, contact angle measurements are conducted via the sessile drop technique, and the angle obtained determines the nature of the membrane surface. New techniques such as AFM, ATR-FTIR, X-ray Photoelectron Spectroscopy (XPS), Positron Annihilation Lifetime Spectroscopy (PALS), Zeta Potential Measurement, Ellipsometry, Fluorescence Microscopy, Nuclear Magnetic Resonance (NMR), Electron Paramagnetic Resonance (EPR), Raman Spectroscopy etc. are available to go deep into understanding of the structure, mechanism, morphology etc. of the membrane, which can help to make desirable membranes.

Hydrophilization by treating the membrane surface with water soluble solvent (acids, alcohols, and mixtures of acids, alcohols, and water) is one of the surface modification techniques. Heating and coating are also used to modify the membrane surface.

In chemical treatment, the polymeric membrane surface is modified through covalent bonding. Among many such approaches, grafting to and grafting from are two distinct surface modification approaches used to modify membranes with responsive polymers. The grafting-to technique is based on the attachment of end-functionalized responsive materials to the membrane surface, whereas grafting-from involves a polymer chain grown from initiator sites on the membrane surface through a surface-initiated polymerization process. The membrane surface is activated by high energy radiation, followed by the grafting of hydrophilic modifiers. In this way, the membrane bulk is not significantly affected, but the membrane surface properties can be improved. LBL is also the most common method to modify the membrane surface. It can be conducted by intercalation of positive- and negative-charged polyelectrolytes on the external surface of a porous support.

Membrane surface modification using responsive nanomaterials is a promising technique to overcome the limitations of existing membranes. Depending upon the mode of application, a wide variety of foreign materials has been incorporated into the dope solution for improving the functional properties of membranes. These materials include metal or metal oxide nanoparticles, zeolites, clay nanoparticles, carbon nanotubes and metal organic frameworks (MOFs). The surface of nanofibers fabricated via electrospinning can also be modified by the methods described above.

Gas separation via membrane has been recognized as the main technology that is used for hydrogen recovery, air separation, natural gas sweetening, helium recovery, natural gas dehydration, and so on. The successful application of a membrane-based separation process depends significantly on the appropriate chemical, physical, mechanical, and permeation properties of the membranes. Mathematical modeling of the membrane-based gas separation process can be useful to predict the performance under different conditions. Gas separation through membranes can take place by solution-diffusion mechanisms, depending mainly on the chemical and physical structures of the membrane.

Gas transport through a mixed-matrix membrane (MMM) is a complicated problem. Different modeling attempts have been developed for the prediction of the performance of MMMs by various theoretical expressions depending on the MMM’s morphology and chemistry, including ideal and nonideal MMMs. Attempts have been made to predict the effective permeability of a gaseous penetrant through the MMMs as a function of continuous phase (polymer matrix) and dispersed phase (porous or nonporous particles) permeabilities, as well as volume fraction of the dispersed phase. Also various models and mechanisms for the solvent and solute transport through reverse osmosis membrane have been proposed by a number of investigators.

Nanotechnology in membrane process has emerged as a boon in various realms and sectors including industry, food, healthcare, electronics, water purification, military, clothing, space and construction etc.

There is a significant need for novel advanced water technologies, in particular to ensure a high quality of drinking water, eliminate micropollutants, and intensify industrial production processes by the use of flexibly adjustable water treatment systems. Nanoengineered materials, such as nanoadsorbents, nanometals, nanomembranes, and photocatalysts, offer the potential for novel water technologies that can be easily adapted to required applications. TiO₂ and graphene-based membranes have the potentials to become the preferred candidates to next-generation membranes coupling high permeability to high selectivity. In order to enter the water and wastewater market, aquaporin-based membranes have to be competitive with conventional membranes in terms of stability and useful life. However, technical limitation of nanoengineered water technologies is that they are rarely adaptable to mass processes, and at present, in many cases are not competitive with conventional treatment technologies.

In the medical world, nanotechnology is also seen as a boon since this can help with creating what is called smart drugs. In medicine, nanotechnology is entering in areas like tissue regeneration, bone repair, immunity and even cures for such ailments like cancer, diabetes, and other life threatening diseases. Nanotubes are playing the role to cure paralysis and neurological diseases. Iron oxide nanoparticles, with their superparamagnetic properties, are used in a rapidly expanding number of applications, such as for cell labeling, separation, and tracking; for therapeutic agents in cancer therapy, and for diagnostic agents. There is promising research that indicates that the cure for cancer could lie in the hands of nanoscience.