Comparative study of nanofiltration membrane characterization devices of different dimension and configuration (cross flow and dead end)

Microplastics in aquatic systems: An in-depth review of current and potential water treatment processes

Plastic products, despite their undeniable utility in modern life, pose significant environmental challenges, particularly when it comes to recycling. A crucial concern is the pervasive introduction of microplastics (MPs) into aquatic ecosystems, with deleterious effects on marine organisms. This review presents a detailed examination of the methodologies developed for MPs removal in water treatment systems. Initially, investigating the most common types of MPs in wastewater, subsequently presenting methodologies for their precise identification and quantification in aquatic environments. Instruments such as scanning electron microscopy, dynamic light scattering, Fourier transform infrared spectroscopy, Raman spectroscopy, surface-enhanced Raman spectroscopy, and Raman tweezers stand out as powerful tools for studying MPs. The discussion then transitions to the exploration of both existing and emergent techniques for MPs removal in wastewater treatment plants and drinking water treatment plants. This includes a description of the core mechanisms that drive these techniques, with an emphasis on the latest research developments in MPs degradation. Present MPs removal methodologies, ranging from physical separation to chemical and biological adsorption and degradation, offer varied advantages and constraints. Addressing the MPs contamination problem in its entirety remains a significant challenge. In conclusion, the review offers a succinct overview of each technique and forwards recommendations for future research, highlighting the pressing nature of this environmental dilemma.

Response of Ionic Hydration Structure and Selective Transport Behavior to Aqueous Solution Chemistry during Nanofiltration

The effect of aqueous solution chemistry on the ionic hydration structure and its corresponding nanofiltration (NF) selectivity is a research gap concerning ion-selective transport. In this study, the hydration distribution of two typical monovalent anions (Cl- and NO3-) under different aqueous solution chemical conditions and the corresponding transmembrane selectivity during NF were investigated by using in situ liquid time-of-flight secondary ion mass spectrometry in combination with molecular dynamics simulations. We demonstrate the inextricable link between the ion hydration structure and the pore steric effect and further find that ionic transmembrane transport can be regulated by breaking the balance between the hydrogen bond network (i.e., water-water) and ion hydration (i.e., ion-water) interactions of hydrated ion. For strongly hydrated (H2O)nCl- with more intense ion-water interactions, a higher salt concentration and coexisting ion competition led to a larger hydrated size and, thus, a higher ion rejection by the NF membrane, whereas weakly hydrated (H2O)nNO3- takes the reverse under the same conditions. Stronger OH--anion hydration competition resulted in a smaller hydrated size of (H2O)nCl- and (H2O)nNO3-, showing a lower observed average hydration number at pH 10.5. This study deepens the long-overlooked understanding of NF separation mechanisms, concerning the hydration structure.

Tutorial review of error evaluation in experimental water research at the example of membrane filtration

Experimental water research lacks clear methodology to estimate experimental error. Especially when natural waters are involved, the characterization tools bear method-specific artifacts while the varying environmental conditions prevent regular repeats. This tutorial review identifies common mistakes, and proposes a practical procedure to determine experimental errors at the example of membrane filtration. Statistical analysis is often applied to an insufficient number of repeated measurements, while not all error sources and contributions are considered. This results in an underestimation of the experimental error. Variations in relevant experimental parameters need to be investigated systematically, and the related errors are quantified as a half of the variation between the max and min values when standard deviation is not applicable. Error of calculated parameters (e.g. flux, pollutant removal and mass loss) is estimated by applying error propagation, where weighing contributions of the experimental parameters are considered. Appropriate judgment and five-time repetition of a selected experiment under identical conditions are proposed to validate the propagated experimental error. For validation, the five repeated data points should lie within the estimated error range of the error bar. The proposed error evaluation procedure is adaptable in experimental water research and intended for researchers to identify the contributing factors of an experimental error and carry out appropriate error quantification and validation. The most important aim is to raise awareness of the necessity to question error methodology and reproducibility of experimental data, to produce and publish high quality research.

Removal of strontium by nanofiltration: Role of complexation and speciation of strontium with organic matter

Strontium (Sr) removal from water is required because excessive naturally occurring Sr exposure is hazardous to human health. Climate and seasonal changes cause water quality variations, in particular quality and quantity of organic matter (OM) and pH, and such variations affect Sr removal by nanofiltration (NF). The mechanisms for such variations are not clear and thus OM complexation and speciation require attention. Sr removal by NF was investigated with emphasis on the role of OM (type and concentration) and pH (2-12) on possible removal mechanisms, specifically size and/or charge exclusion as well as solute-solute interactions. The filtration results show that the addition of various OM (10 types) and an increase of OM concentration (2-100 mgC.L-1) increased Sr removal by 10-15%. The Sr-OM interaction was enhanced with increasing OM concentration, implying enhanced size exclusion via Sr-OM interaction as the main mechanism. Such interactions were quantified by asymmetric flow field-flow fractionation (FFFF) coupled with an inductively coupled plasma mass spectrometer (ICP-MS). Both extremely low and high pH increased Sr removal due to the enhanced charge exclusion and Sr-OM interactions. This work elucidated and verified the mechanism of OM and pH on Sr removal by NF membranes.

Interplay of the forces governing steroid hormone micropollutant adsorption in vertically-aligned carbon nanotube membrane nanopores

Vertically-aligned carbon nanotube (VaCNT) membranes allow water to conduct rapidly at low pressures and open up the possibility for water purification and desalination, although the ultralow viscous stress in hydrophobic and low-tortuosity nanopores prevents surface interactions with contaminants. In this experimental investigation, steroid hormone micropollutant adsorption by VaCNT membranes is quantified and explained via the interplay of the hydrodynamic drag and friction forces acting on the hormone, and the adhesive and repulsive forces between the hormone and the inner carbon nanotube wall. It is concluded that a drag force above 2.2 × 10-3 pN overcomes the friction force resulting in insignificant adsorption, whereas lowering the drag force from 2.2 × 10-3 to 4.3 × 10-4 pN increases the adsorbed mass of hormones from zero to 0.4 ng cm-2. At a low drag force of 1.6 × 10-3 pN, the adsorbed mass of four hormones is correlated with the hormone-wall adhesive (van der Waals) force. These findings explain micropollutant adsorption in nanopores via the forces acting on the micropollutant along and perpendicular to the flow, which can be exploited for selectivity.

Towards effective recovery of humate as green fertilizer from landfill leachate concentrate by electro-neutral nanofiltration membrane

Under the environmental sustainability concept, landfill leachate concentrate can be up-cycled as a useful resource. Practical strategy for effective management of landfill leachate concentrate is to recover the existing humate as fertilizer purpose for plant growth. Herein, we designed an electro-neutral nanofiltration membrane to separate the humate and inorganic salts for achieving a sufficient humate recovery from leachate concentrate. The electro-neutral nanofiltration membrane yielded a high retention of humate (96.54 %) with an extremely low salt rejection (3.47 %), tremendously outperforming the state-of-the-art nanofiltration membranes and exhibiting superior promise in fractionation of humate and inorganic salts. With implementation of the pressure-driven concentration process, the electro-neutral nanofiltration membrane enriched the humate from 1756 to 51,466 mg∙L-1 at a fold of 32.6, enabling 90.0 % humate recovery and 96.4 % desalination efficiency from landfill leachate concentrate. Furthermore, the recovered humate not only exerted no phytotoxicity, but also significantly promoted the metabolism of red bean plants, serving as an effective green fertilizer. The study provides a conceptual and technical platform using high-performance electro-neutral nanofiltration membranes to extract the humate as a promising nutrient for fertilizer application, in view of sustainable landfill leachate concentrate treatment.

Two-Dimensional Interlayer Space Induced Horizontal Transformation of Metal-Organic Framework Nanosheets for Highly Permeable Nanofiltration Membranes

Laminar membranes comprising graphene oxide (GO) and metal-organic framework (MOF) nanosheets benefit from the regular in-plane pores of MOF nanosheets and thus can support rapid water transport. However, the restacking and agglomeration of MOF nanosheets during typical vacuum filtration disturb the stacking of GO sheets, thus deteriorating the membrane selectivity. Therefore, to fabricate highly permeable MOF nanosheets/reduced GO (rGO) membranes, a two-step method is applied. First, using a facile solvothermal method, ZnO nanoparticles are introduced into the rGO laminate to stabilize and enlarge the interlayer spacing. Subsequently, the ZnO/rGO membrane is immersed in a solution of tetrakis(4-carboxyphenyl)porphyrin (H2 TCPP) to realize in situ transformation of ZnO into Zn-TCPP in the confined interlayer space of rGO. By optimizing the transformation time and mass loading of ZnO, the obtained Zn-TCPP/rGO laminar membrane exhibits preferential orientation of Zn-TCPP, which reduces the pathway tortuosity for small molecules. As a result, the composite membrane achieves a high water permeance of 19.0 L m-2  h-1  bar-1 and high anionic dye rejection (>99% for methyl blue).

Preparation of Nanoparticle-Loaded Extracellular Vesicles Using Direct Flow Filtration

Extracellular vesicles (EVs) have shown great potential as cell-free therapeutics and biomimetic nanocarriers for drug delivery. However, the potential of EVs is limited by scalable, reproducible production and in vivo tracking after delivery. Here, we report the preparation of quercetin-iron complex nanoparticle-loaded EVs derived from a breast cancer cell line, MDA-MB-231br, using direct flow filtration. The morphology and size of the nanoparticle-loaded EVs were characterized using transmission electron microscopy and dynamic light scattering. The SDS-PAGE gel electrophoresis of those EVs showed several protein bands in the range of 20-100 kDa. The analysis of EV protein markers by a semi-quantitative antibody array confirmed the presence of several typical EV markers, such as ALIX, TSG101, CD63, and CD81. Our EV yield quantification suggested a significant yield increase in direct flow filtration compared with ultracentrifugation. Subsequently, we compared the cellular uptake behaviors of nanoparticle-loaded EVs with free nanoparticles using MDA-MB-231br cell line. Iron staining studies indicated that free nanoparticles were taken up by cells via endocytosis and localized at a certain area within the cells while uniform iron staining across cells was observed for cells treated with nanoparticle-loaded EVs. Our studies demonstrate the feasibility of using direct flow filtration for the production of nanoparticle-loaded EVs from cancer cells. The cellular uptake studies suggested the possibility of deeper penetration of the nanocarriers because the cancer cells readily took up the quercetin-iron complex nanoparticles, and then released nanoparticle-loaded EVs, which can be further delivered to regional cells.

Chromium (III) and chromium (VI) removal and organic matter interaction with nanofiltration

Chromium (Cr) is a toxic inorganic contaminant for drinking water, in which the concentration has to be controlled for human health and safety. Cr retention was investigated with stirred cell experiments using sulphonated polyethersulfone nanofiltration (NF) membranes of different molecular weight cut-off (MWCO). Cr(III) and Cr(VI) retention follow the order of the MWCO of the studied NF membranes; HY70-720 Da > HY50-1000 Da > HY10-3000 Da with a pH dependency, especially for Cr(III). The importance of the charge exclusion was highlighted when Cr(OH)₄^- (for Cr(III)) and CrO₄^2- (for Cr(VI)) was the predominant species in the feed solution. In presence of organic matter, namely humic acid (HA), Cr(III) retention increased by 60 %, while no influence of HA was observed for Cr(VI). HA did not induce major modifications on the membrane surface charge for these membranes. Solute-solute interaction, in particular Cr(III)-HA complexation, was the responsible mechanism for the increase in Cr(III) retention. This was confirmed by asymmetric flow field-flow fractionation, coupled with inductively coupled plasma mass spectrometry (FFFF-ICP-MS) analysis. Cr(III)-HA complexation was significant at HA concentrations as low as 1 mgC/L. The chosen NF membranes were able to achieve the EU guideline (25 μg/L) for Cr in drinking water for a feed concentration of 250 μg/L.

Poly(vinylidene fluoride) membrane with immobilized TiO2 for degradation of steroid hormone micropollutants in a photocatalytic membrane reactor

The lack of effective technologies to remove steroid hormones (SHs) from aquatic systems is a critical issue for both environment and public health. The performance of a flow-through photocatalytic membrane reactor (PMR) with TiO₂ immobilized on a photostable poly(vinylidene fluoride) membrane (PVDF-TiO₂) was evaluated in the context of SHs degradation at concentrations from 0.05 to 1000 µg/L under UV exposure (365 nm). A comprehensive investigation into the membrane preparation approach, including varying the surface Ti content and distribution, and membrane pore size, was conducted to gain insights on the rate-limiting steps for the SHs degradation. Increasing surface Ti content from 4 % to 6.5 % enhanced the 17β-estradiol (E2) degradation from 46 ± 12-81 ± 6 %. Apparent degradation kinetics were independent of both TiO₂ homogeneity and membrane pore size (0.1-0.45 µm). With optimized conditions, E2 removal was higher than 96 % at environmentally relevant feed concentration (100 ng/L), a flux of 60 L/m²h, 25 mW/cm², and 6.5 % Ti. These results indicated that the E2 degradation on the PVDF-TiO₂ membrane was limited by the catalyst content and light penetration depth. Further exploration of novel TiO₂ immobilization approach that can offer a larger catalyst content and light penetration is required to improve the micropollutant removal efficiency in PMR.

Membrane-organic solute interactions in asymmetric flow field flow fractionation: Interplay of hydrodynamic and electrostatic forces

The structure and size characterization of organic matter (OM) using flow field-flow fractionation (FFFF) is interesting due to the numerous interactions of OM in aquatic systems and water treatment processes. The estimation of hydrodynamic and electrostatic forces involved in the fractionation of OM over different molecular weight cut-off (MWCO) membranes is vital for a better understanding of the FFFF process. This work aims to understand the membrane-OM interactive forces with respect to membrane MWCO, solute molecular weight, flow rates, solution pH and ionic strength. Polystyrene sulfonate sodium salt (PSS) of molecular weights 10, 30 and 65 kDa were used as model organic solutes for fractionation over ultrafiltration (UF) membranes of MWCO 1-30 kDa. Maximum fractionation of PSS was achieved by using a tight membrane of 1 kDa MWCO at the conditions of high permeate flow rate (1.5-2.0 mL·min^-1), low concentrate flow rate (0.2-0.3 mL·min^-1) and low ionic strength (10 mM). The better fractionation corresponds to high permeate drag force and low concentrate drag force. A low membrane-solute DLVO interaction is favourable for the retention of a small solute. This study illustrated that FFFF characteristics can be analyzed based on membrane-solute interactive forces controlled by selected flow, size and charge parameters.

Development of Functional Hybrid Polymers and Gel Materials for Sustainable Membrane-Based Water Treatment Technology: How to Combine Greener and Cleaner Approaches

Water quality and disposability are among the main challenges that governments and societies will outside during the next years due to their close relationship to population growth and urbanization and their direct influence on the environment and socio-economic development. Potable water suitable for human consumption is a key resource that, unfortunately, is strongly limited by anthropogenic pollution and climate change. In this regard, new groups of compounds, referred to as emerging contaminants, represent a risk to human health and living species; they have already been identified in water bodies as a result of increased industrialization. Pesticides, cosmetics, personal care products, pharmaceuticals, organic dyes, and other man-made chemicals indispensable for modern society are among the emerging pollutants of difficult remediation by traditional methods of wastewater treatment. However, the majority of the currently used waste management and remediation techniques require significant amounts of energy and chemicals, which can themselves be sources of secondary pollution. Therefore, this review reported newly advanced, efficient, and sustainable techniques and approaches for water purification. In particular, new advancements in sustainable membrane-based filtration technologies are discussed, together with their modification through a rational safe-by-design to modulate their hydrophilicity, porosity, surface characteristics, and adsorption performances. Thus, their preparation by the use of biopolymer-based gels is described, as well as their blending with functional cross-linkers or nanofillers or by advanced and innovative approaches, such as electrospinning.

Energy Barriers for Steroid Hormone Transport in Nanofiltration

Nanofiltration (NF) membranes can retain micropollutants (MPs) to a large extent, even though adsorption into the membrane and gradual permeation result in breakthrough and incomplete removal. The permeation of MPs is investigated by examining the energy barriers (determined using the Arrhenius concept) for adsorption, intrapore diffusion, and permeation encountered by four different steroid hormones in tight and loose NF membranes. Results show that the energy barriers for steroid hormone transport in tight membrane are entropically dominated and underestimated because of the high steric exclusion at the pore entrance. In contrast, the loose NF membrane enables steroid hormones partitioning at the pore entrance, with a permeation energy barrier (from feed toward the permeate side) ranging between 96 and 116 kJ/mol. The contribution of adsorption and intrapore diffusion to the energy barrier for steroid hormone permeation reveals a significant role of intrapore diffusive transport on the obtained permeation energy barrier. Overall, the breakthrough phenomenon observed during the NF of MPs is facilitated by the low energy barrier for adsorption. Experimental evidence of such principles is relevant for understanding mechanisms and ultimately improving the selectivity of NF.

Fouling Analysis in One-Stage Ultrafiltration of Precipitation-Treated Bacillus subtilis Fermentation Liquors for Biosurfactant Recovery

Primary recovery of surfactin from precipitation-pretreated fermentation broths of Bacillus subtilis ATCC 21332 culture by one-stage dead-end and cross-flow ultrafiltration (UF) was studied. Dead-end experiments were first performed to select suitable conditions, including the amount of added ethanol-a micelle-destabilizing solvent (0-70 vol%), type (polyethersulfone, polyacrylonitrile, poly(vinylidene fluoride)) and molecular-weight cut-off (MWCO, 30-100 kDa) of the membrane in the surfactin concentration range of 0.25-1.23 g/L. Then, the cross-flow UF experiments were conducted to check the recovery performance in the ranges of feed surfactin concentration of 1.13-2.67 g/L, flow velocity of 0.025-0.05 m/s, and transmembrane pressure of 40-100 kPa. The Hermia model was also used to clarify membrane fouling mechanisms. Finally, three cleaning agents and two in situ cleaning ways (flush and back-flush) were selected to regain the permeate flux. As for the primary recovery of surfactin from the permeate in cross-flow UF, a polyethersulfone membrane with 100-kDa MWCO was suggested, and the NaOH solution at pH 11 was used for membrane flushing.

Selenium species removal by nanofiltration: Determination of retention mechanisms

Selenium (Se) is a dissolved oxyanion drinking water contaminant requiring appropriate removal technologies. The removal of selenite (Se^IV) and selenite (Se^VI) with nanofiltration (NF) was investigated with an emphasis on the role of Se speciation and membrane charge screening on the retention mechanisms. The pH (2 to 12) showed strong pH dependence of Se retention, which was due to the speciation. No significant impact of salinity was observed by increasing NaCl concentration from 0.58 to 20 g/L. Application of the Donnan steric pore partitioning model with dielectric exclusion (DSPM-DE) showed that Donnan exclusion was the dominant retention mechanism for the oxyanions Se species. Nine different organic matter (OM) types were investigated at 10 mgC/L to determine if OM affects Se retention. Only OM characterised by negatively charged fractions, such as humic acid (HA), enhanced Se retention with NF270 of up to 20% for Se^IV and 10% for Se^VI. This was explained by enhanced Donnan exclusion. NF270 was effective in removing Se from real water (Gahard groundwater, Ille et Vilaine, France). The EU guideline (20 μg/L) of Se in drinking water was achieved with comparable performance to OM-free experiments using synthetic waters.

Photocatalytic degradation of steroid hormone micropollutants by TiO2-coated polyethersulfone membranes in a continuous flow-through process

Micropollutants in the aquatic environment pose a high risk to both environmental and human health. The photocatalytic degradation of steroid hormones in a flow-through photocatalytic membrane reactor under UV light (365 nm) at environmentally relevant concentrations (50 ng l^-1 to 1 mg l^-1) was examined using a polyethersulfone-titanium dioxide (PES-TiO₂) membrane. The TiO₂ nanoparticles (10-30 nm) were immobilized both on the surface and in the nanopores (220 nm) of the membrane. Water quality and operational parameters were evaluated to elucidate the limiting factors in the degradation of steroid hormones. Flow through the photocatalytic membrane increased contact between the micropollutants and ·OH in the pores. Notably, 80% of both oestradiol and oestrone was removed from a 200 ng l^-1 feed (at 25 mW cm^-2 and 300 l m^-2 h^-1). Progesterone and testosterone removal was lower at 44% and 33%, respectively. Increasing the oestradiol concentration to 1 mg l^-1 resulted in 20% removal, whereas with a 100 ng l^-1 solution, a maximum removal of 94% was achieved at 44 mW cm^-2 and 60 l m^-2 h^-1. The effectiveness of the relatively well-known PES-TiO₂ membrane for micropollutant removal has been demonstrated; this effectiveness is due to the nanoscale size of the membrane, which provides a high surface area and facilitates close contact of the radicals with the very small (0.8 nm) micropollutant at an extremely low, environmentally relevant concentration (100 ng l^-1).

Enhanced modelling and experimental validation of ultra-low pressure reverse osmosis membrane system for treatment of synthetic brackish water

Water purification from brackish water sources has been acknowledged as one of the most promising ways to produce drinkable water in water-scarce areas. In this study, an ultra-low pressure reverse osmosis (ULPRO) membrane was numerically and experimentally investigated to produce drinking water by the removal of sodium chloride salt which provides further validation of the model from a practical perspective. An enhanced predictive model based on the Donnan-Steric Pore Model with dielectric exclusion (DSPM-DE) incorporating the osmotic effects was formulated in process simulation. The feed pressure and concentration were optimized as input variables and interaction between them was observed, while salt rejection and water recovery rate were taken as response attributes. The results obtained on the ULPRO membrane showed that the performance depends on the charge, steric, and dielectric effects. Furthermore, the enhanced model was validated with the experimental data attained from a laboratory-scale filtration system with good accuracy in the salt rejection and water recovery results. Comparing the enhanced DSPM-DE with the existing solution diffusion model reveals that the enhanced model predicts the membrane performance better and thereby qualifies itself as a reliable model for desalination of brackish water using ULPRO membrane.

Removal of arsenic(III) via nanofiltration: contribution of organic matter interactions

The removal of arsenic(III) (As(III)) with nanofiltration (NF) was investigated with emphasis on the role of salinity, pH and organic matter on retention mechanisms. While no measurable impact of salinity on As(III) retention with NF membranes (NF270 and NF90) was observed, a significant increase in As(III) retention occurred from pH 9 to pH 12. This was explained by As(III) deprotonation at pH > 9 that enhanced Donnan (charge) exclusion. Of the five different organic matter types investigated at 10 mgC/L, only humic acid (HA) increased As(III) retention by up to 10%. Increasing HA concentration to 100 mgC/L enhanced As(III) retention by 40%, which was attributed to As(III)-HA complexation. Complexation was confirmed by field-flow fractionation inductively coupled plasma mass spectrometry (FFF-ICP-MS) measurements, which showed that the bound As(III) increased with HA concentration. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that NF90, which exhibited lower permeability reduction than NF270, has accumulated a lower amount of As(III) in the presence of HA, where As(III)-HA complex was formed in the feed solution. This finding implies that As(III) retention with NF technology can be enhanced by complexation, instead of using other methods such as oxidation or pH adjustement.

Organic matter interference with steroid hormone removal by single-walled carbon nanotubes - ultrafiltration composite membrane

Composite adsorbent-ultrafiltration membranes for micropollutant removal are an interesting concept, although water matrix compounds, such as organic matter (OM), may interfere with micropollutant adsorption. To prevent this interference, the adsorbents such as single-walled carbon nanotubes (SWCNTs) can be deposited on the permeate side of an ultrafiltration membrane (UF) that can retain OM. In this study, the 'shielding' effect by different UF MWCOs of such an SWCNT-UF composite was evaluated for nine different types of OM. All nine OM types competed with 17β-estradiol (E2) for adsorption to varying extents, and tannic acid (TA, 1.7 kDa) reduced E2 adsorption by SWCNT-UF most drastically. TA interference mechanisms included direct competition with hormones for adsorption sites, and indirect competition via TA-hormone interactions. TA was not fully retained by the composite membrane with an MWCO range of 3-100 kDa, resulting in ineffective shielding. In contrast, humic acid was mostly retained by the composite with low MWCO of 3-10 kDa and allowed high E2 adsorption. This study demonstrates that tailoring composite membranes by carefully choosing UF MWCO can prevent interference of OM types in micropollutant adsorption by permeate-side adsorbents.

Facile Surface Modification of Polyamide Membranes Using UV-Photooxidation Improves Permeability and Reduces Natural Organic Matter Fouling

A new optimized ultraviolet (UV) technique induced a photooxidation surface modification on thin-film composite (TFC) polyamide (PA) brackish water reverse osmosis (BWRO) membranes that improved membrane performance (i.e., permeability and organic fouling propensity). Commercial PA membranes were irradiated with UV-B light (285 nm), and the changes in the membrane performance were assessed through dead-end and cross-flow tests. UV-B irradiation at 12 J·cm^-2 enhanced the pure water permeability by 34% in the dead-end tests without decreasing the mono- or divalent ion rejections, as compared with the pristine PA membrane, and led to less fouling by natural organic matter in the cross-flow tests. Scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) confirmed that UV-B irradiation opened the pore structure and created carboxylic and amine groups on the PA surface, leading to increased membrane surface charge and hydrophilicity. Thus, an optimal UV-B dose appears to modify only a thin layer of the PA membrane surface, which favorably enhances the membrane performance. UV-B did not alter the structure, flux, or salt rejection for cellulose triacetate (CTA)-based membranes. While other membrane surface modifications include oxidants, strong acids, and bases, the UV-B facile treatment is chemical-free, thus reducing chemical wastes, and easy to apply in roll-to-roll fabrication processes of PA membranes. The results also showed that a low UV irradiation dose could be applied to PA or CTA membranes for disinfection or photocatalytic oxidation.

Nanofiltration membranes for salt and dye filtration: effect of membrane properties on performances

In this study, commercial nanofiltration membranes (Toray, NF 270, Desal 5 L) were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, optical profilometry, contact angle, mechanical strength and zeta potential measurements. Filtration performance tests were conducted with distilled water, MgSO₄ solution and synthetic dye solutions, respectively. Among three commercial membranes, the Toray membrane was thought to be better choice. Additional experiments were carried out for a more detailed characterization of the selected membrane. Therefore, firstly, flux and removal efficiency was monitored by using dye solutions at different pH values, and then experiments were carried out to observe the effect of different temperatures. Also, another filtration test with NaCl solution was performed for the Toray membrane. As the main purpose of this study, we aimed to establish a significant correlation between the structural properties of membranes and their performances. In light of the results obtained, it was observed that the contact angle, mechanical strength and surface roughness values of the membrane significantly affected the membrane performance. It was concluded that the most important parameter in dye removal was the zeta potential. As a result of this work, a data set of commercial membranes was created and is available to all membrane users.

Scaling Risk Assessment in Nanofiltration of Mine Waters

Nanofiltration can be applied for the treatment of mine waters. One of the main problems is the risk of crystallization of sparingly soluble salts on the membrane surface (scaling). In this work, a series of batch-mode nanofiltration experiments of the mine waters was performed in a dead-end Sterlitech^® HP 4750X Stirred Cell. Based on the laboratory results, the concentration profiles of individual ions along the membrane length in a single-pass industrial-scale nanofiltration (NF) unit was calculated, assuming the tanks-in-series flow model inside the membrane module. These calculations also propose a method for estimating the maximum achievable recovery before the occurrence of the calcium sulfate dihydrate scaling in a single-pass NF 40″ length spiral wound module, simultaneously allowing metastable supersaturation of calcium sulfate dihydrate. The performance of three membrane types (NF270, NFX, NFDL) has been evaluated for the nanofiltration of mine water.

Comparison of Photocatalytic Membrane Reactor Types for the Degradation of an Organic Molecule by TiO2-Coated PES Membrane

Photocatalytic membrane reactors with different configurations (design, flow modes and light sources) have been widely applied for pollutant removal. A thorough understanding of the contribution of reactor design to performance is required to be able to compare photocatalytic materials. Reactors with different flow designs are implemented for process efficiency comparisons. Several figures-of-merit, namely adapted space-time yield (STY) and photocatalytic space-time yield (PSTY), specific energy consumption (SEC) and degradation rate constants, were used to assess the performance of batch, flow-along and flow-through reactors. A fair comparison of reactor performance, considering throughput together with energy efficiency and photocatalytic activity, was only possible with the modified PSTY. When comparing the three reactors at the example of methylene blue (MB) degradation under LED irradiation, flow-through proved to be the most efficient design. PSTY1/PSTY2 values were approximately 10 times higher than both the batch and flow-along processes. The highest activity of such a reactor is attributed to its unique flow design which allowed the reaction to take place not only on the outer surface of the membrane but also within its pores. The enhancement of the mass transfer when flowing in a narrow space (220 nm in flow-through) contributes to an additional MB removal.

Separation and degradation detection of nanogram-per-litre concentrations of radiolabelled steroid hormones using combined liquid chromatography and flow scintillation analysis

Detection of micropollutants such as steroid hormones occurring in the aquatic environment at concentrations between ng/L and µg/L remains a major challenge, in particular when treatment efficiency is to be evaluated. Steroid hormones are typically analysed using mass-spectrometry methods, requiring pre-concentration and/or derivatisation procedures to achieve required detection limits. Free of sample preparation steps, the use of radiolabelled contaminants with liquid scintillation counting is limited to single-compound systems and require a separation of hormone mixtures before detection. In this work, a method was developed coupling ultra-high-pressure liquid chromatography (UHPLC) with flow scintillation analysis (FSA) for separation and detection of radiolabelled estrone, 17ß-estradiol, testosterone and progesterone. Adjustment of the flow rate of scintillation liquid and UHPLC mobile phase, gradient time, column temperature, and injection volume allowed the separation of steroid hormones and degradation products. The limit-of-detection (LOD = 1.5–2.4 ng/L) and limit-of-quantification (LOQ = 3.4–4.3 ng/L) for steroid hormones were comparable with the current state-of-the-art technique (LC-MS/MS) for non-derivatised compounds. Although the method cannot be applied to real water samples (unless spiked with radiotracers), it serves as a useful tool for the development of water treatment technologies at laboratory scale as demonstrated via: i) adsorption on polymer-based spherical activated carbon, ii) retention in nanofiltration, iii) photodegradation using a photocatalytic membrane.

Investigation of the reaction kinetics of photocatalytic pollutant degradation under defined conditions with inkjet-printed TiO2 films – from batch to a novel continuous-flow microreactor

Reaction kinetics was studied for photocatalytic pollutant degradation with inkjet-printed TiO₂ in a batch and a continuous-flow microreactor.

Efficient Photocatalytic Removal of Methylene Blue Using a Metalloporphyrin-Poly(vinylidene fluoride) Hybrid Membrane in a Flow-Through Reactor

A novel combination of a poly(vinylidene fluoride) (PVDF) membrane with pore size 0.2 μm and a photosensitizer 5,10,15,20-tetrakis (pentafluorophenyl)-21H,23H-porphine palladium(II) (PdTFPP) makes a promising hybrid material for the generation of singlet oxygen (¹O₂) and, thus, water treatment applications. The fabricated photocatalytic membrane exhibits permeability of 4280 ± 250 L·m^-2·h^-1·bar^-1 and stable photocatalytic degradation performance over a 90 h period, when illuminated with green light (528 ± 20 nm) and operated in a dead-end, single-pass configuration. Methylene blue (MB) degradation of 83% was achieved for MB concentration of 1 mg·L^-1 under the flow rate of 0.1 × 10^-3 L·min^-1 (flux of 30 L·m^-2·h^-1), light intensity of 21 mW·cm^-2, and PdTFPP loading of 25 μmol·g^-1. Due to an enhanced mass transfer, the reaction rate of MB removal (with apparent rate constant of k_app = 6.52 min^-1) results in an efficient photodegradation of MB inside the PdTFPP-PVDF membrane. The influence of experimental parameters such as catalyst loading, flow rate, light intensity, and solute concentration on MB removal was investigated. This research enables the application of photocatalytic PdTFPP-PVDF membranes as a potential technology for water decontamination under visible-light illumination.