Metal oxide and carbon nanomaterial based membranes for reverse osmosis and membrane distillation: A comparative review

Sequential Infiltration Synthesis of Silicon Dioxide in Polymers with Ester Groups-Insight from In Situ Infrared Spectroscopy

New strategies to synthesize nanometer-scale silicon dioxide (SiO2) patterns have drawn much attention in applications such as microelectronic and optoelectronic devices, membranes, and sensors, as we are approaching device dimensions shrinking below 10 nm. In this regard, sequential infiltration synthesis (SIS), a two-step gas-phase molecular assembly process that enables localized inorganic material growth in the targeted reactive domains of polymers, is an attractive process. In this work, we performed in situ Fourier transform infrared spectroscopy (FTIR) measurements during SiO2 SIS to investigate the reaction mechanism of trimethylaluminum (TMA) and tri(tert-pentoxy) silanol (TPS) precursors with polymers having ester functional groups (poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), polycaprolactone (PCL), and poly(t-butyl methacrylate) (PBMA)), for the purpose of growing patterned nanomaterials. The FTIR results show that for PMMA and PEMA, a lower percentage of functional groups participated in the reactions and formed weak and unstable complexes. In contrast, almost all functional groups in PCL and PBMA participated in the reactions and showed stable and irreversible interactions with TMA. We discovered that the amount of SiO2 formed is not directly correlated with the number of interacting functional groups. These insights into the SiO2 SIS mechanism will enable nanopatterning of SiO2 for low-dimensional applications.

Synergetic effect on fouling alleviating of membrane distillation in urine resource recovery by thermally activated peroxydisulfate pretreatment

Given that the spontaneous precipitation of minerals caused by urea hydrolysis and abundant organic compounds, membrane fouling became a major obstacle for urine recovery by membrane distillation (MD). Herein, this study developed a combined system (TAP-MD) by integrating thermally activated peroxydisulfate (TAP) and MD process to inhibit membrane fouling and improve separation efficiency. Based on the TAP-MD system, the separation performance was improved significantly, improving nutrient recovery efficiency and quality of reclaimed water. More than 80% of water could be recovered from urine, and about 94.13% of total ammonia nitrogen (TAN), 99.02% of total nitrogen (TN), 100% of total phosphate (TP), and 100% of K+ were rejected. The mechanism for alleviating urine-induced fouling was systematically and intensively studied. With TAP pretreatment, the TAN concentration of pretreated urine was kept at a low level steadily and the pH was at neutral or weakly acidic. Hence, inorganic scaling represented by carbonate and phosphate precipitates were significantly inhibited by creating unfavorable solvent environment for crystallization with TAP pretreatment. Additionally, aromatic proteins were found as the main organic foulants. According to the secondary structure of protein, the proteins were degraded by the cleavage of peptide bonds by TAP pretreatment. Meanwhile, the hydrophilicity of protein increased, which reduced the hydrophobic interaction of protein and membrane surface and thus alleviated protein-induced membrane fouling. This study revealed the inorganic and organic foulants in urine that caused membrane fouling and demonstrated the mechanism of membrane fouling alleviation by TAP-MD system. The experimental results will be instrumental in better understanding the mechanisms of membrane fouling induced by urine and optimize MD process for resource recovery from urine.

Emerging advances of composite membranes for seawater pre-treatment: a review

As the population continues to grow, the preservation of the world's water resources is becoming a serious challenge. The seawater desalination process is considered a sustainable option for the future. The two most common technologies used in desalination are reverse osmosis (RO) and membrane distillation (MD). However, membrane fouling caused by the accumulation of contaminants on the membrane surface is an emerging and growing problem. A pre-treatment stage is required to reach optimal efficiency during the desalination process since this stage is crucial for a successful desalination process. In this regard, the development of new material-based composite membranes has the potential to upgrade the anti-fouling features of RO membranes thereby enhancing desalination efficiency due to their high permeability, hydrophilicity, selectivity mechanical strength, thermal stability, and anti-bacterial properties. The objective of this review is to present various techniques for seawater pre-treatment. The results of the use of several membrane types and methods of modification have also been discussed. The performance of composite membranes for seawater pre-treatment is defined and the future perspectives have been highlighted.

Innovative Membrane Technologies for the Treatment of Wastewater Polluted with Heavy Metals: Perspective of the Potential of Electrodialysis, Membrane Distillation, and Forward Osmosis from a Bibliometric Analysis

A bibliometric analysis, using the Scopus database as a source, was carried out in order to study the scientific documents published up to 2021 regarding the use of electrodialysis, membrane distillation, and forward osmosis for the removal of heavy metals from wastewater. A total of 362 documents that fulfilled the search criteria were found, and the results from the corresponding analysis revealed that the number of documents greatly increased after the year 2010, although the first document was published in 1956. The exponential evolution of the scientific production related to these innovative membrane technologies confirmed an increasing interest from the scientific community. The most prolific country was Denmark, which contributed 19.3% of the published documents, followed by the two main current scientific superpowers: China and the USA (with 17.4% and 7.5% contributions, respectively). Environmental Science was the most common subject (55.0% of contributions), followed by Chemical Engineering (37.3% of contributions) and Chemistry (36.5% of contribution). The prevalence of electrodialysis over the other two technologies was clear in terms of relative frequency of the keywords. An analysis of the main hot topics identified the main advantages and drawbacks of each technology, and revealed that examples of their successful implementation beyond the lab scale are still scarce. Therefore, complete techno-economic evaluation of the treatment of wastewater polluted with heavy metals via these innovative membrane technologies must be encouraged.

Fouling in reverse osmosis membranes: monitoring, characterization, mitigation strategies and future directions

Water scarcity has been a global challenge for many countries over the past decades, and as a result, reverse osmosis (RO) has emerged as a promising and cost-effective tool for water desalination and wastewater remediation. Currently, RO accounts for >65% of the worldwide desalination capacity; however, membrane fouling is a major issue in RO processes. Fouling reduces the membrane's lifespan and permeability, while also increases the operating pressure and chemical cleaning frequency. Overall, fouling reduces the quality and quantity of desalinated water, and thus hinders the sustainable application of RO membranes by disturbing its efficacy and economic aspects. Fouling arises from various physicochemical interactions between water pollutants and membrane materials leading to foulants' accumulation onto the membrane surfaces and/or inside the membrane pores. The current review illustrates the main types of particulates, organic, inorganic and biological foulants, along with the major factors affecting its formation and development. Moreover, the currently used monitoring methods, characterization techniques and the potential mitigation strategies of membrane fouling are reviewed. Further, the still-faced challenges and the future research on RO membrane fouling are addressed.

Polymeric membranes functionalized with nanomaterials (MP@NMs): A review of advances in pesticide removal

The excessive contamination of drinking water sources by pesticides has a pernicious impact on human health and the environment since only 0.1% of pesticides is utilized effectively to control the and the rest is deposited in the environment. Filtration by polymeric membranes has become a promising technique to deal with this problem; however, the scientific community, in the need to find better pesticide retention results, has begun to meddle in the functionalization of polymeric membranes. Given the great variety of membrane, polymer, and nanomaterial synthesis methods present in the market, the possibilities of obtaining membranes that adjust to different variables and characteristics related to a certain pesticide are relatively extensive, so it is expected that this technology will represent one of the main pesticide removal strategies in the future. In this direction, this review focused on, - the main characteristics of the nanomaterials and their impact on pristine polymeric membranes; - the removal performance of functionalized membranes; and - the main mechanisms by which membranes can retain pesticides. Based on these insights, the functionalized polymeric membranes can be considered as a promising technology in the removal of pesticides since the removal performance of this technology against pesticide showed a significant increase. Obtaining membranes that adjust to different variables and characteristics related to a certain pesticide are relatively extensive, so it is expected that functionalized membrane technology will represent one of the main pesticide removal strategies in the future.

An Overview of the Modification Strategies in Developing Antifouling Nanofiltration Membranes

Freshwater deficiency has become a significant issue affecting many nations' social and economic development because of the fast-growing demand for water resources. Nanofiltration (NF) is one of the promising technologies for water reclamation application, particularly in desalination, water, and wastewater treatment fields. Nevertheless, membrane fouling remains a significant concern since it can reduce the NF membrane performance and increase operating expenses. Consequently, numerous studies have focused on improving the NF membrane's resistance to fouling. This review highlights the recent progress in NF modification strategies using three types of antifouling modifiers, i.e., nanoparticles, polymers, and composite polymer/nanoparticles. The correlation between antifouling performance and membrane properties such as hydrophilicity, surface chemistry, surface charge, and morphology are discussed. The challenges and perspectives regarding antifouling modifiers and modification strategies conclude this review.

Partially oxidised boron nitride as a 2D nanomaterial for nanofiltration applications

Boron nitride (BN) based 2D nanomaterials are an emerging class of materials for the development of new membranes for nanofiltration applications. Here, we report the preparation, characterisation and testing of highly promising nanofiltration membranes produced from partially oxidised BN (BNOx) 2D nanosheets. In our work, the partial oxidation of BN was successfully achieved by heating the bulk h-BN powder in air at 1000 °C, resulting in BNOx product. The characterisation of the sample showed the presence of B-OH groups corresponding to the partial oxidisation of the BN. The BNOx material was then exfoliated in water and used to produce membranes, using vacuum filtration. These membranes were characterised using electron microscopy, BET and mercury porosimetry techniques. The membranes have also been tested in water purification and removal of several typical water-soluble dyes, demonstrating outstanding retention values close to 100%. We believe that this research opens up new opportunities for further production, as well as chemical functionalisation and modification of membranes for nanofiltration and separation technologies.

Gravimetric analysis of stability of polymeric materials during exposure to chemical disinfectants at different temperatures

The goal of this study was to evaluate the impact of thermal and chemical aging processes on high-density polyethylene (HDPE), low-density polyethylene (LDPE), unplasticized polyvinyl chloride (U-PVC), and high-impact polyvinyl chloride (Hi-PVC) pipes. The materials were exposed to 1-10 ppm chemical disinfectants [chlorine dioxide (ClO₂) and hypochlorite (HOCl)] at 40-80 °C for 1200 h. The diffusion properties of the materials were systematically analyzed based on the change in their sorption characteristics and activation energies according to the Arrhenius model. Moreover, the structural changes were analyzed with scanning electron microscopy (SEM), Fourier transform infrared (FTIR) radiation, and thermogravimetric analysis (TGA). The results show that the materials have Fickian characteristics in the aging environment. Specifically, the water sorption rates of HDPE and LDPE increase first and then decrease after reaching saturation (Ms); those of U-PVC and Hi-PVC its increasing continuously with different rate. This behavior of materials was prominent for ClO₂ at high temperature and disinfectant dose because of polymeric chains crosslinking and rearrangement, extraction of monomers, and stable compounds removal during aging under exposed conditions. The deleterious effects decreased the activation energies of the materials and increased the concentrations of carbonyl groups [CO] via the formation of ketones, aldehydes, and carboxylic acids. The decomposition temperature increased with the changes in the material morphology and elemental contents under the investigated conditions. Moreover, LDPE and Hi-PVC were more severely affected in the thermal aging process with 10 mg.L^-1 ClO₂ at 80 °C.

Adsorptive Membrane for Boron Removal: Challenges and Future Prospects

The complexity of removing boron compounds from aqueous systems has received serious attention among researchers and inventors in the water treating industry. This is due to the higher level of boron in the aquatic ecosystem, which is caused by the geochemical background and anthropogenic factors. The gradual increase in the distribution of boron for years can become extremely toxic to humans, terrestrial organisms and aquatic organisms. Numerous methods of removing boron that have been executed so far can be classified under batch adsorption, membrane-based processes and hybrid techniques. Conventional water treatments such as coagulation, sedimentation and filtration do not significantly remove boron, and special methods would have to be installed in order to remove boron from water resources. The blockage of membrane pores by pollutants in the available membrane technologies not only decreases their performance but can make the membranes prone to fouling. Therefore, the surface-modifying flexibility in adsorptive membranes can serve as an advantage to remove boron from water resources efficiently. These membranes are attractive because of the dual advantage of adsorption/filtration mechanisms. Hence, this review is devoted to discussing the capabilities of an adsorptive membrane in removing boron. This study will mainly highlight the issues of commercially available adsorptive membranes and the drawbacks of adsorbents incorporated in single-layered adsorptive membranes. The idea of layering adsorbents to form a highly adsorptive dual-layered membrane for boron removal will be proposed. The future prospects of boron removal in terms of the progress and utilization of adsorptive membranes along with recommendations for improving the techniques will also be discussed further.

Zirconia-Doped Methylated Silica Membranes via Sol-Gel Process: Microstructure and Hydrogen Permselectivity

In order to obtain a steam-stable hydrogen permselectivity membrane, with tetraethylorthosilicate (TEOS) as the silicon source, zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O) as the zirconium source, and methyltriethoxysilane (MTES) as the hydrophobic modifier, the methyl-modified ZrO₂-SiO₂ (ZrO₂-MSiO₂) membranes were prepared via the sol-gel method. The microstructure and gas permeance of the ZrO₂-MSiO₂ membranes were studied. The physical-chemical properties of the membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), and N₂ adsorption–desorption analysis. The hydrogen permselectivity of ZrO₂-MSiO₂ membranes was evaluated with Zr content, temperature, pressure difference, drying control chemical additive (glycerol) content, and hydrothermal stability as the inferred factors. XRD and pore structure analysis revealed that, as n_Zr increased, the MSiO₂ peak gradually shifted to a higher 2θ value, and the intensity gradually decreased. The study found that the permeation mechanism of H₂ and other gases is mainly based on the activation–diffusion mechanism. The separation of H₂ is facilitated by an increase in temperature. The ZrO₂-MSiO₂ membrane with n_Zr = 0.15 has a better pore structure and a suitable ratio of micropores to mesopores, which improved the gas permselectivities. At 200 °C, the H₂ permeance of MSiO₂ and ZrO₂-MSiO₂ membranes was 3.66 × 10⁻⁶ and 6.46 × 10⁻⁶ mol·m⁻²·s⁻¹·Pa⁻¹, respectively. Compared with the MSiO₂ membrane, the H₂/CO₂ and H₂/N₂ permselectivities of the ZrO₂-MSiO₂ membrane were improved by 79.18% and 26.75%, respectively. The added amount of glycerol as the drying control chemical additive increased from 20% to 30%, the permeance of H₂ decreased by 11.55%, and the permselectivities of H₂/CO₂ and H₂/N₂ rose by 2.14% and 0.28%, respectively. The final results demonstrate that the ZrO₂-MSiO₂ membrane possesses excellent hydrothermal stability and regeneration capability.

A figure of merits-based performance comparison of various advanced functional nanomaterials for adsorptive removal of gaseous ammonia

The implementation of sustainable industrial development based on energy/cost-effective techniques with zero/low rate of pollutant emission is an ideal strategy for the proper management of a natural environment. Gaseous ammonia released from a variety of anthropogenic sources (e.g., agriculture, pharmaceuticals, commercial cleaning products, and refrigerant) is estimated to be as high as 150 million tons∙year^-1. To reduce the negative effects of atmospheric ammonia, the great utility of advanced functional nanomaterials (e.g., metal organic frameworks, covalent organic polymers, metal/metal oxide nanoparticles, and carbon nanostructures) has been recognized. To gain a better understanding of the sorptive removal potential of diverse materials, their performance has been evaluated based on the key performance merits (e.g., initial concentration, sorption capacity, and partition coefficient). Generally, the PC values can be applied to significantly estimate the contaminant adsorption potential of NMs via balancing the biased influences of operating parameters (e.g., initial concentration of pollutants) as perceived for the partitioning of compounds between aqueous phases at equilibrium (e.g., Henry's Law). Therefore, in this work, we have proposed the PC as a prosperous performance merit (in terms of heterogeneity of surface and strength of adsorption process) for the selection of high performance nano-adsorbents for gaseous ammonia. Moreover, the water stability, recyclability, economic aspects, and future perspectives have also been discussed for real-world applications of advanced nanomaterial against gaseous ammonia adsorption. The outcome of this evaluation will be expedient for the classification/selection of the most effectual and cost-effective options for mitigation of environmental pollutants like gaseous ammonia.