Preparation of Layer-by-Layer Nanofiltration Membranes by Dynamic Deposition and Crosslinking

Modification Approaches of Polyphenylene Oxide Membranes to Enhance Nanofiltration Performance

Presently, water pollution poses a serious threat to the environment; the removal of organic pollutants from resources, especially dyes, is very important. Nanofiltration (NF) is a promising membrane method to carry out this task. In the present work, advanced supported poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) membranes were developed for NF of anionic dyes using bulk (the introduction of graphene oxide (GO) into the polymer matrix) and surface (the deposition of polyelectrolyte (PEL) layers by layer-by-layer (LbL) technique) modifications. The effect of PEL combinations (polydiallyldimethylammonium chloride/polyacrylic acid (PAA), polyethyleneimine (PEI)/PAA, and polyallylamine hydrochloride/PAA) and the number of PEL bilayers deposited by LbL method on properties of PPO-based membranes were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact angle measurements. Membranes were evaluated in NF of food dye solutions in ethanol (Sunset yellow (SY), Congo red (CR), and Alphazurine (AZ)). The supported PPO membrane, modified with 0.7 wt.% GO and three PEI/PAA bilayers, exhibited optimal transport characteristics: ethanol, SY, CR, and AZ solutions permeability of 0.58, 0.57, 0.50, and 0.44 kg/(m²h atm), respectively, with a high level of rejection coefficients-58% for SY, 63% for CR, and 58% for AZ. It was shown that the combined use of bulk and surface modifications significantly improved the characteristics of the PPO membrane in NF of dyes.

Polystyrene Sulfonate Particles as Building Blocks for Nanofiltration Membranes

Today the standard treatment for wastewater is secondary treatment. This procedure cannot remove salinity or some organic micropollutants from water. In the future, a tertiary cleaning step may be required. An attractive solution is membrane processes, especially nanofiltration (NF). However, currently available NF membranes strongly reject multivalent ions, mainly due to the dielectric effect. In this work, we present a new method for preparing NF membranes, which contain negatively and positively charged domains, obtained by the combination of two polyelectrolytes with opposite charge. The negatively charged polyelectrolyte is provided in the form of particles (polystyrene sulfonate (PSSA), d ~300 nm). As a positively charged polyelectrolyte, polyethyleneimine (PEI) is used. Both buildings blocks and glycerol diglycidyl ether as crosslinker for PEI are applied to an UF membrane support in a simple one-step coating process. The membrane charge (zeta potential) and salt rejection can be adjusted using the particle concentration in the coating solution/dispersion that determine the selective layer composition. The approach reported here leads to NF membranes with a selectivity that may be controlled by a different mechanism compared to state-of-the-art membranes.

Optimization of Nanofiltration Hollow Fiber Membrane Fabrication Process Based on Response Surface Method

Layer-by-layer (LBL) self-assembly technology has become a new research hotspot in the fabrication of nanofiltration membranes in recent years. However, there is a lack of a systematic approach for the assessment of influencing factors during the membrane fabrication process. In this study, the process optimization of LBL deposition was performed by a two-step statistical method. The multiple linear regression was performed on the results of single-factor experiments to determine the major influencing factors on membrane performance, including the concentration of Poly (allylamine hydrochloride) (PAH), glutaraldehyde, and the NaCl concentration in PAH solution. The Box–Behnken response surface method was then used to analyze the interactions between the selected factors, while their correlation with the membrane performance was obtained by polynomial fitting. The R² value of the regression models (0.97 and 0.94) was in good agreement with the adjusted R² value (0.93 and 0.86), indicating that the quadratic response models were adequate enough to predict the membrane performance. The optimal process parameters were finally determined through dual-response surface analysis to achieve both high membrane permeability of 14.3 LMH·MPa⁻¹ and MgSO₄ rejection rate of 90.22%.

Construction of Loose Positively Charged NF Membrane by Layer-by-Layer Grafting of Polyphenol and Polyethyleneimine on the PES/Fe Substrate for Dye/Salt Separation

The effective separation of dyes and inorganic salts is highly desirable for recycling inorganic salts and water resource in printing and dyeing wastewater treatment. In this work, tannic acid (TA) and polyethyleneimine (PEI) were grafted on the PES/Fe ultrafiltration membrane via the coordination assembly and Michael addition strategy to fabricated a loose nanofiltration membrane (LNM). The effect of PEI concentration on membrane morphologies and properties was systematically investigated. The membrane surface becomes more hydrophilic and transforms into positive charge after the PEI grafting. The optimized PES/Fe-TA-PEI membrane possesses high pure water flux (124.6 L·m⁻²·h⁻¹) and excellent dye rejections (98.5%, 99.8%, 98.4%, and 86.4% for Congo red, Eriochrome black T, Alcian blue 8GX, and Bromophenol blue, respectively) under 2 bar operation pressure. Meanwhile, the LNM showed a high Alcian blue 8GX rejection (>98.4%) and low NaCl rejection (<5.3%) for the dye/salt mixed solutions separation. Moreover, the PES/Fe-TA-PEI LNM exhibited good antifouling performance and long-term performance stability. These results reveal that such LNM shows great potential for effective fractionation of dyes and salts and recycling of textile wastewater.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.

Recent developments in layer-by-layer assembled systems application in water purification

Electrostatically-based layer-by-layer (LbL) assembly is a versatile surface functionalization technique allowing the construction of complex three-dimensional architectures on virtually any type of material using various combinations of nano-bricks. One of the most promising applications of LbL assembled systems is in water purification. The main two strategies developed in this purpose consist in either enhancing the barrier properties of separation membranes and in the construction of core-shell organic/inorganic sorbents. In this review, the recent achievements in this topic are discussed with respect to the use of LbL-based composites in desalination and removal of heavy metal ions or organic pollutants. Finally, some works dealing with economic aspects of using LbL assemblies for water purification are presented, thus highlighting forthcoming strategies to develop economically-viable materials for such applications.

Assessing the nanotechnology on the grounds of costs, benefits, and risks

Background

The technical innovations are based on the principles of science with the assurance of outweighing their cost and risk factors with the benefits to society. But sometimes, the innovation either itself becomes a risk or brings in some risk factors along with it. For most of the alleyway of an innovation from its emergence to its road to societal acceptance and adoption, the focus remains on the benefits majorly. Only when we are at the neck of the hour we think about some of the apparent cost and risk issues. The understanding, proper communication, and address of the basics of risk factors are necessarily required much in advance to deal with this issue.

Main body

Nanoparticles with very small size and huge surface area are being derived from various plants, microbes, chemical compounds, metals, and metal alloys. Without our realizations, nanotechnology has become a vital part of our day-to-day life, and nanoparticles are proving their worth in almost every field ranging from food, water, medicine, agriculture, construction, fashion, electronics, and computers to eco-remediation, but what about the costs involved and the risks associated? We strongly need to recognize these concerns and challenges, and it requires collaborative efforts from academicians, researchers, industries, government, and non-government organizations to involve people in dialogs to deal with them.

Conclusion

Through reviewing various studies and articles on nanotechnology, this review has shown that nanotechnology can productively be used to produce consumer goods for pharma, electronics, food, agriculture, aviation, construction, security, and remediation sectors which are advantages in their characteristics. Regarding the future of nanotechnology, we need to focus on assessment and management of risks associated for its promising market growth.

Graphical abstract

Adsorption and Release of Rose Bengal on Layer-by-Layer Films of Poly(Vinyl Alcohol) and Poly(Amidoamine) Dendrimers Bearing 4-Carboxyphenylboronic Acid

Phenylboronic acid-bearing polyamidoamine dendrimer (PBA-PAMAM)/poly(vinyl alcohol) (PVA) multilayer films were prepared through the layer-by-layer (LbL) deposition of PBA-PAMAM solution and PVA solution. PBA-PAMAM/PVA films were constructed successfully through the formation of boronate ester bonds between the boronic acid moiety in PBA and 1,3-diol units in PVA. When the (PBA-PAMAM/PVA)₅ films were immersed in rose bengal (RB) solution, RB was adsorbed onto the LbL films. The amount of RB adsorbed was higher in the LbL films immersed in acidic solution than in basic solution. The release of RB from the LbL films was also promoted in the basic solution, while it was suppressed in the acidic solution. The boronic acid ester is oxidized to phenol by hydrogen peroxide (H₂O₂) and the carbon-boron bond is cleaved, so that the (PBA-PAMAM/PVA)₅ films can be decomposed by immersion in H₂O₂ solution. Therefore, when RB-adsorbed (PBA-PAMAM/PVA)₅ films were immersed in H₂O₂ solution, the release of RB was moderately promoted when the solution was weakly acidic.