Macroporous membranes doped with micro-mesoporous β-cyclodextrin polymers for ultrafast removal of organic micropollutants from water

Removal of contaminants present in water and wastewater by cyclodextrin-based adsorbents: A bibliometric review from 1993 to 2022

Cyclodextrin (CD), a cyclic oligosaccharide from enzymatic starch breakdown, plays a crucial role in pharmaceuticals, food, agriculture, textiles, biotechnology, chemicals, and environmental applications, including water and wastewater treatment. In this study, a statistical analysis was performed using VOSviewer and Citespace to scrutinize 2038 articles published from 1993 to 2022. The investigation unveiled a notable upsurge in pertinent articles and citation counts, with China and USA contributing the highest publication volumes. The prevailing research focus predominantly revolves around the application of CD-based materials used as adsorbents to remove conventional contaminants such as dyes and metals. The CD chemistry allows the construction of materials with various architectures, including cross-linked, grafted, hybrid or supported systems. The main adsorbents are cross-linked CD polymers, including nanosponges, fibres and hybrid composites. Additionally, research efforts are actually concentrated on the synthesis of CD-based membranes, CD@graphene oxide, and CD@TiO2. These materials are proposed as adsorbents to remove emerging pollutants. By employing bibliometric analysis, this study delivers a comprehensive retrospective review and synthesis of research concerning CD-based adsorbents for the removal of contaminants from wastewater, thereby offering valuable insights for future large-scale application of CD-based adsorption materials.

β-cyclodextrin polymer composites for the removal of pharmaceutical substances, endocrine disruptor chemicals, and dyes from aqueous solution- A review of recent trends

Cyclodextrin (CD) and its derivatives are receiving attention as a new-generation adsorbent for water pollution treatment due to their external hydrophilic and internal hydrophobic properties. Among types of CD, β-Cyclodextrin (βCD) has been a material of choice with a proven track record for a range of utilities in distinct domains, owing to its unique cage-like structural conformations and inclusion complex-forming ability, especially to mitigate emerging contaminants (ECs). This article outlines βCD composites in developing approaches of their melds and composites for purposes such as membranes for removal of the ECs in aqueous setups have been explored with emphasis on recent trends. Electrospinning has bestowed an entirely different viewpoint on polymeric materials, comprising βCD, in the framework of diverse functions across a multitude of niches. Besides, this article especially discusses βCD polymer composite membrane-based removal of contaminants such as pharmaceutical substances, endocrine disruptors chemicals, and dyes. Finally, in this article, the challenges and future directions of βCD-based adsorbents are discussed, which may shed light on pragmatic commercial applications of βCD polymer composite membranes.

Polycage membranes for precise molecular separation and catalysis

The evolution of the chemical and pharmaceutical industry requires effective and less energy-intensive separation technologies. Engineering smart materials at a large scale with tunable properties for molecular separation is a challenging step to materialize this goal. Herein, we report thin film composite membranes prepared by the interfacial polymerization of porous organic cages (POCs) (RCC3 and tren cages). Ultrathin crosslinked polycage selective layers (thickness as low as 9.5 nm) are obtained with high permeance and strict molecular sieving for nanofiltration. A dual function is achieved by combining molecular separation and catalysis. This is demonstrated by impregnating the cages with highly catalytically active Pd nanoclusters ( ~ 0.7 nm). While the membrane promotes a precise molecular separation, its catalytic activity enables surface self-cleaning, by reacting with any potentially adsorbed dye and recovering the original performance. This strategy opens opportunities for the development of other smart membranes combining different functions and well-tailored abilities.

β-Cyclodextrin functionalized adsorbents for removal of organic micropollutants from water

The presence of organic micropollutants in water is an ongoing concern due to the potential risks to living organisms. β-Cyclodextrin-based adsorbents have been developed to remove organic micropollutants from water as they are deemed to be efficient, selective and reusable. This literature review establishes the current state of the knowledge on the application of β-Cyclodextrin adsorbents for the removal of organic micropollutants from water and determines knowledge gaps and recommendations for future studies. An inventory of organic micropollutants that have been studied was developed and it revealed that bisphenol-A has been the most commonly studied. Adsorbent configurations were reviewed and modifications to the adsorbent structures that have provided enhanced adsorption properties were identified. The size and shape of the organic micropollutants was found to affect the adsorption behavior. The surface charge of β-Cyclodextrin adsorbents influence adsorption when repulsive forces are present and the extent of repulsion can depend on the pH of the solution. Common competitors such as natural organic matter and inorganic ions do not significantly impact the adsorption of organic micropollutants however relatively small fulvic acids may compete for the β-Cyclodextrin cavity depending on the adsorbent type. Desorption of organic micropollutants from these adsorbents has been accomplished with alcohols and most adsorbents have been recovered and reused in adsorption/desorption cycles. The need for enhanced recovery processes that maintain water quality and adsorbent integrity was identified. The use of quantitative structure-activity relationships and molecular computational tools could potentially guide future environmental applications of β-Cyclodextrin adsorbents.

Efficient capture of endocrine-disrupting compounds by a high-performance nanofiltration membrane for wastewater treatment

Conventional polyamide (PA) nanofiltration (NF) membranes can readily adsorb aromatic compounds, such as endocrine disrupting compounds (EDCs). Therefore, these substances can easily be transported across the membrane by solution-diffusion, resulting in a poor EDC-rejection. In this work, a novel thin film nanocomposite (TFN) membrane was fabricated by incorporating covalent organic frameworks (COFs) into the PA layer via an interfacial polymerization reaction. COFs with functional groups can provide abundant active binding sites for highly efficient EDC-capture. The rejection of the optimal TFN-COF membrane for bisphenol A, bisphenol AF, and sodium 2-biphenylate was 98.3%, 99.1%, and 99.3%, respectively, which was much higher than of the rejection of the pristine NF-membrane (82.4%, 95.5%, and 96.4%, respectively). Additionally, the TFN-COF membrane could be regenerated fast and efficiently by washing with ethanol for some minutes. COF nanofillers with porous structures provide additional water channels, making it possible to overcome the permeability-selectivity trade-off of NF membranes. The water permeance (17.1 L m^-2 h^-1 bar^-1) of the optimal membrane was about two times higher than for the pristine NF-membrane (8.7 L m^-2 h^-1 bar^-1). In addition, the TFN-COF membrane with a COF-loading of 0.05% w/v had an excellent Na₂SO₄ rejection (95.2%) due to size exclusion and strong Donnan effect. This work combines traditional NF membranes and adsorption materials to achieve efficient capture and rapid release of EDCs without sacrificing salt rejections, which opens the door to develop fit-for-purpose adsorptive NF membranes.

Microfiltration Membranes for the Removal of Bisphenol A from Aqueous Solution: Adsorption Behavior and Mechanism

This study mainly investigated the adsorption behavior and mechanism of microfiltration membranes (MFMs) with different physiochemical properties (polyamide (PA), polyvinylidene fluoride (PVDF), nitrocellulose (NC), and polytetrafluoroethylene (PTFE)) for bisphenol A (BPA). According to the adsorption isotherm and kinetic, the maximum adsorption capacity of these MFMs was PA (161.29 mg/g) > PVDF (80.00 mg/g) > NC (18.02 mg/g) > PTFE (1.56 mg/g), and the adsorption rate was PVDF (K1 = 2.373 h−1) > PA (K1 = 1.739 h−1) > NC (K1 = 1.086 h−1). The site energy distribution analysis showed that PA MFMs had the greatest adsorption sites, followed by PVDF and NC MFMs. The study of the adsorption mechanism suggested that the hydrophilic microdomain and hydrophobic microdomain had a micro-separation for PA and PVDF, which resulted in a higher adsorption capacity of PA and PVDF MFMs. The hydrophilic microdomain providing hydrogen bonding sites and the hydrophobic microdomain providing hydrophobic interaction, play a synergetic role in improving the BPA adsorption. Due to the hydrogen bonding force being greater than the hydrophobic force, more hydrogen bonding sites on the hydrophobic surface resulted in a higher adsorption capacity, but the hydrophobic interaction contributed to improving the adsorption rate. Therefore, the distribution of the hydrophilic microdomain and hydrophobic microdomain on MFMs can influence the adsorption capacity and the adsorption rate for BPA or its analogues. These consequences provide a novel insight for better understanding the adsorption behavior and mechanism on MFMs.

Microporous polymer adsorptive membranes with high processing capacity for molecular separation

Trade-off between permeability and nanometer-level selectivity is an inherent shortcoming of membrane-based separation of molecules, while most highly porous materials with high adsorption capacity lack solution processability and stability for achieving adsorption-based molecule separation. We hereby report a hydrophilic amidoxime modified polymer of intrinsic microporosity (AOPIM-1) as a membrane adsorption material to selectively adsorb and separate small organic molecules from water with ultrahigh processing capacity. The membrane adsorption capacity for Rhodamine B reaches 26.114 g m⁻², 10–1000 times higher than previously reported adsorptive membranes. Meanwhile, the membrane achieves >99.9% removal of various nano-sized organic molecules with water flux 2 orders of magnitude higher than typical pressure-driven membranes of similar rejections. This work confirms the feasibility of microporous polymers for membrane adsorption with high capacity, and provides the possibility of adsorptive membranes for molecular separation.

Trade-off between permeability and nanometer-level selectivity is an inherent shortcoming of membrane-based separation of molecules. Here, the authors report a membrane adsorption material based on hydrophilic amidoxime modified polymer of intrinsic microporosity to selectively adsorb and separate small organic molecules from water with ultrahigh processing capacity

Recent Advancements in Cyclodextrin-Based Adsorbents for the Removal of Hazardous Pollutants from Waters

Water is an essential substance for the survival on Earth of all living organisms. However, population growth has disturbed the natural phenomenon of living, due to industrial growth to meet ever expanding demands, and, hence, an exponential increase in environmental pollution has been reported in the last few decades. Moreover, water pollution has drawn major attention for its adverse effects on human health and the ecosystem. Various techniques have been used to treat wastewater, including biofiltration, activated sludge, membrane filtration, active oxidation process and adsorption. Among the mentioned, the last method is becoming very popular. Moreover, among the sorbents, those based on cyclodextrin have gained worldwide attention due to their excellent properties. This review article overviewed recent contributions related to the synthesis of Cyclodextrin (CD)-based adsorbents to treat wastewater, and their applications, especially for the removal of heavy metals, dyes, and organic pollutants (pharmaceuticals and endocrine disruptor chemicals). Furthermore, new adsorption trends and trials related to CD-based materials are also discussed regarding their regenerative potential. Finally, this review could be an inspiration for new research and could also anticipate future directions and challenges associated with CD-based adsorbents.

Removal of Contaminants from Water by Membrane Filtration: A Review

Drinking water sources are increasingly subject to various types of contamination due to anthropogenic factors and require proper treatment to remove disease-causing agents. Public drinking water systems use different treatment methods to provide safe and quality drinking water to populations. However, they are ineffective in removing contaminants that are considered a danger to the environment and therefore to humans. Several alternative treatment processes have been proposed, such as membrane filtration, as final purification methods. This paper aims to summarize the type of pollutant compounds, filtration processes, and membranes that have been most studied in this area with particular emphasis on how the modification of membranes, either the manufacturing process or the incorporation of nanomaterials, influences their performance.

Room Temperature Fabrication of Macroporous Lignin Membranes for the Scalable Production of Black Silicon

Rising global demand for biodegradable materials and green sources of energy has brought attention to lignin. Herein, we report a method for manufacturing standalone lignin membranes without additives for the first time to date. We demonstrate a scalable method for macroporous (∼100 to 200 nm pores) lignin membrane production using four different organosolv lignin materials under a humid environment (>50% relative humidity) at ambient temperatures (∼20 °C). A range of different thicknesses is reported with densely porous films observed to form if the membrane thickness is below 100 nm. The fabricated membranes were readily used as a template for Ni²⁺ incorporation to produce a nickel oxide membrane after UV/ozone treatment. The resultant mask was etched via an inductively coupled plasma reactive ion etch process, forming a silicon membrane and as a result yielding black silicon (BSi) with a pore depth of >1 μm after 3 min with reflectance <3% in the visible light region. We anticipate that our lignin membrane methodology can be readily applied to various processes ranging from catalysis to sensing and adapted to large-scale manufacturing.

Ultrahigh Flux and Strong Affinity Poly(N-vinylformamide)-Grafted Polypropylene Membranes for Continuous Removal of Organic Micropollutants from Water

The rapid and effective removal of organic micropollutants (OMPs) from water remains a huge challenge for traditional water treatment techniques. Compared with powder adsorbents such as polymers and nanomaterials, the free-standing adsorptive membrane is possible for large-scale applications and shows promise in removing OMPs. Herein, inspired by aquatic plants, a novel free-standing adsorptive membrane (NPPM) with high water flux, strong adsorption affinity, and excellent reproducibility was prepared by one-step UV surface grafting. N-Vinylformamide (NVF) was employed to introduce multiple hydrophilic and hydrogen bonding sites on the surface of commercial polypropylene fiber membranes (PPM). The NPPM exhibits excellent water permeability and ultrahigh water flux (up to 40 000 L/(m² h)) and could continuously remove a broad spectrum of OMPs from water. Its adsorption performance is 5-100 times higher than that of PPM and commercial membranes. Even in natural water sources such as tap water and river water, the NPPM shows unchanged adsorption performance and high OMPs removal efficiency (>95%). Notably, the NPPM has excellent regeneration performance and can be regenerated by hot water elution, which provides an environmentally friendly regeneration method without involving any organic solvent. Moreover, the synergy between hydrogen bonding and hydrophobic interaction is revealed, and the hydrophobic interaction provided by the hydrophobic substrate is proved to play a fundamental role in OMPs adsorption. The strong hydrogen bonds between the grafts and the OMPs are demonstrated by variable-temperature FTIR spectroscopy (vt-FTIR), ¹³C nuclear magnetic resonance spectroscopy (¹³C NMR), and simulation calculations. The strong hydrogen bonds could increase the enthalpy change and enhance the adsorption affinity, so the NPPM has a strong adsorption affinity, which is 100 times that of similar adsorption membranes. This study not only presents an adsorptive membrane with great commercial potential in the rapid remediation of a water source but also opens a pathway to develop an adsorptive membrane with high water flux and strong adsorption affinity.

Preparation, characterization and dyeing wastewater treatment of a new PVDF/PMMA five-bore UF membrane with β-cyclodextrin and additive combinations

A new type of polyvinylidene fluoride (PVDF)/polymethyl methacrylate (PMMA) hollow fiber membrane (HFM) with five bores was prepared. The effects of polyvinylpyrrolidone (PVP), β-cyclodextrine (β-CD), polyethylene glycol (PEG) and polysorbate 80 (Tween 80) and their combinations on the PVDF/PMMA five-bore HFMs were investigated. The performance and fouling characteristics of five-bore HFMs for dyeing wastewater treatment were evaluated. Results indicated that adding 5 wt.% PVP increased the porosity and water flux of the membrane but decreased the bovine serum albumin (BSA) rejection rate. Adding 5 wt.% β-CD significantly improved the tensile strength and rejection of the HFMs with no effect on the increase of water flux. The characteristic of the HFMs with different additive combinations proved that the mixture of 5 wt.% PVP and 1 wt.% β-CD gave the best membrane performance, with a pure water flux of 427.9 L/ m²·h, a contact angle of 25°, and a rejection of BSA of 89.7%. The COD_cr and UV₂₅₄ removal rates of dyeing wastewater treatment were 61.10% and 50.41%, respectively. No breakage or leakage points were found after 120 days of operation, showing their reliable mechanical properties. We set the operating flux to 55 L/m²·h and cross-flow rate to 10%, which can effectively control membrane fouling.

Efficient removal of various coexisting organic pollutants in water based on β-cyclodextrin polymer modified flower-like Fe3O4 particles

HYPOTHESIS

The construction of porous β-cyclodextrin polymer (β-CDP) modified flower-like Fe₃O₄ particles (CDP@Fe₃O₄) is expected to remove various organic pollutants from water, based on the larger specific surface area of flower-like Fe₃O₄ particles and the active sites provided by β-CDP. With the help of various noncovalent interactions, the removal ability of CDP@Fe₃O₄ for various water-soluble and water-insoluble organic pollutants were systematically studied.

EXPERIMENTS

CDP@Fe₃O₄ were successfully synthesized and applied for the simultaneous removal of various organic pollutants with different electrical properties, structure and hydrophobicity. Adsorption efficiency, adsorption process, adsorption mechanism and the reusability of CDP@Fe₃O₄ for single pollutant and mixed pollutants were comprehensively investigated.

FINDINGS

CDP@Fe₃O₄ exhibited excellent adsorption capabilities for various pollutants. Importantly, when these pollutants were coexisting, CDP@Fe₃O₄ still maintained a comparable removal ability for various pollutants. Efficient removal of organic pollutants was attributed to varieties of noncovalent interactions between organic pollutants and CDP@Fe₃O₄, including hydrophobic interactions, hydrogen bonds, π-π and electrostatic interactions. These results revealed that the excellent adsorption ability and convenient regeneration make CDP@Fe₃O₄ being a potential candidate in various complex organic wastewater purification.

Molecularly-porous ultrathin membranes for highly selective organic solvent nanofiltration

Engineering membranes for molecular separation in organic solvents is still a big challenge. When the selectivity increases, the permeability tends to drastically decrease, increasing the energy demands for the separation process. Ideally, organic solvent nanofiltration membranes should be thin to enhance the permeant transport, have a well-tailored nanoporosity and high stability in harsh solvents. Here, we introduce a trianglamine macrocycle as a molecular building block for cross-linked membranes, prepared by facile interfacial polymerization, for high-performance selective separations. The membranes were prepared via a two-in-one strategy, enabled by the amine macrocycle, by simultaneously reducing the thickness of the thin-film layers (<10 nm) and introducing permanent intrinsic porosity within the membrane (6.3 Å). This translates into a superior separation performance for nanofiltration operation, both in polar and apolar solvents. The hyper-cross-linked network significantly improved the stability in various organic solvents, while the amine host macrocycle provided specific size and charge molecular recognition for selective guest molecules separation. By employing easily customized molecular hosts in ultrathin membranes, we can significantly tailor the selectivity on-demand without compromising the overall permeability of the system.

Engineering thin membranes for molecular separation with well tailored nanoporosity and which can withstand harsh conditions is still a big challenge. Here, the authors introduce a trianglamine macrocycle as a molecular building block for cross-linked membranes, prepared by facile interfacial polymerization, for high performance selective separations.

Bibliometric approach to the perspectives and challenges of membrane separation processes to remove emerging contaminants from water

The presence of contaminants in water is concerning due to the potential impacts on human health and the environment, and ingested contaminants cause harm in various ways. The conventional water treatment systems are not efficient to remove these contaminants. Therefore, novel techniques and materials for the removal of contaminants are increasingly being developed. The separation process using modified membranes can remove these micropollutants; therefore, they have attracted significant research attention. Among the materials used for manufacturing of these membranes, composites based on graphene oxide and reduced graphene oxide are preferred owing to their promising properties, such as mechanical resistance, thermal and chemical stability, antifouling capacity, water permeability, high thermal and electrical conductivity, high optical transmittance and high surface area. Membrane separation processes (MSP) can be used as secondary or tertiary treatment during the supply of wastewater. However, the efficient and accessible applications of these technologies are challenging. This study aims to demonstrate the main concepts of membrane separation processes and their application in the removal of emerging contaminants. This study reports bibliometric mapping, relevant data on studies using membranes as water treatment processes, and their viability in industrial applications. The main challenges and perspectives of these technologies are discussed in detail as well.

Emerging pollutants-Part II: Treatment

Emerging pollutants (EPs) refer to a class of pollutants, which are emerging in the environment or recently attracted attention. EPs mainly include pharmaceutical and personal care products (PPCPs), endocrine-disrupting chemicals (EDCs), and antibiotic resistance genes (ARGs). EPs have potential threats to human health and ecological environment. In recent years, the continuous detections of EPs in surface and ground water have brought huge challenges to water treatment and also made the treatment of EPs become an international research hotspot. This paper summarizes some research results on EPs treatment published in 2019. This paper may be helpful to understand the current situations and development trends of EP treatment technologies.

Interfacially crosslinked β-cyclodextrin polymer composite porous membranes for fast removal of organic micropollutants from water by flow-through adsorption

Persistent organic micropollutants have seriously damaged aquatic ecological equilibrium and affected human health. Conventional adsorbents are limited due to slow adsorption rate. Therefore, it's significant to integrate adsorbent into porous membrane to develop a highly efficient continuous filtration method for water purification. Herein, β-cyclodextrin polymer (β-CDP) composite porous membranes were prepared via convenient interfacial cross-linking. The membranes combined the adsorption ability of β-CDP and the convective mass transport process of filtration membrane to quickly remove contaminants from water by flow-through adsorption. In optimized preparation conditions, the composite membrane exhibited a 100% of removal efficiency towards bisphenol A and a high treating capacity up to 440 mg m^-2. The treating capacity kept nearly unchanged in acidic and neutral pH condition, while increased greatly with the addition of salts due to the salting-out effect. Also, the membrane could completely remove pollutants with ultrahigh flux up to 2500 L·m^-2 h^-1. In addition, the used membranes were fully regenerated by mild ethanol cleaning.