Engraving Polyamide Layers by In Situ Self-Etchable CaCO3 Nanoparticles Enhances Separation Properties and Antifouling Performance of Reverse Osmosis Membranes.

Nanovoids within a polyamide layer play an important role in the separation performance of thin-film composite (TFC) reverse osmosis (RO) membranes. To form more extensive nanovoids for enhanced performance, one commonly used method is to incorporate sacrificial nanofillers in the polyamide layer during the exothermic interfacial polymerization (IP) reaction, followed by some post-etching processes. However, these post-treatments could harm the membrane integrity, thereby leading to reduced selectivity. In this study, we applied in situ self-etchable sacrificial nanofillers by taking advantage of the strong acid and heat generated in IP. CaCO3 nanoparticles (nCaCO3) were used as the model nanofillers, which can be in situ etched by reacting with H+ to leave void nanostructures behind. This reaction can further degas CO2 nanobubbles assisted by heat in IP to form more nanovoids in the polyamide layer. These nanovoids can facilitate water transport by enlarging the effective surface filtration area of the polyamide and reducing hydraulic resistance to significantly enhance water permeance. The correlations between the nanovoid properties and membrane performance were systematically analyzed. We further demonstrate that the nCaCO3-tailored membrane can improve membrane antifouling propensity and rejections to boron and As(III) compared with the control. This study investigated a novel strategy of applying self-etchable gas precursors to engrave the polyamide layer for enhanced membrane performance, which provides new insights into the design and synthesis of TFC membranes.

NaHCO3 addition enhances water permeance and Ca/haloacetic acids selectivity of nanofiltration membranes for drinking water treatment.

The existence of disinfection by-products such as haloacetic acids (HAAs) in drinking water severely threatens water safety and public health. Nanofiltration (NF) is a promising strategy to remove HAAs for clean water production. However, NF often possesses overhigh rejection of essential minerals such as calcium. Herein, we developed highly selective NF membranes with tailored surface charge and pore size for efficient rejection of HAAs and high passage of minerals. The NF membranes were fabricated through interfacial polymerization (IP) with NaHCO3 as an additive. The NaHCO3-tailored NF membranes exhibited high water permeance up to ∼24.0 L m - 2 h - 1 bar-1 (more than doubled compared with the control membrane) thanks to the formation of stripe-like features and enlarged pore size. Meanwhile, the tailored membranes showed enhanced negative charge, which benefitted their rejection of HAAs and passage of Ca and Mg. The higher rejection of HAAs (e.g., > 90%) with the lower rejection of minerals (e.g., < 30% for Ca) allowed the NF membranes to achieve higher minerals/HAAs selectivity, which was significantly higher than those of commercially available NF membranes. The simultaneously enhanced membrane performance and higher minerals/HAAs selectivity would greatly boost water production efficiency and water quality. Our findings provide a novel insight to tailor the minerals/micropollutants selectivity of NF membranes for highly selective separation in membrane-based water treatment.

Demystifying the Role of Surfactant in Tailoring Polyamide Morphology for Enhanced Reverse Osmosis Performance: Mechanistic Insights and Environmental Implications.

Surfactant-assisted interfacial polymerization (IP) has shown strong potential to improve the separation performance of thin film composite polyamide membranes. A common belief is that the enhanced performance is attributed to accelerated amine diffusion induced by the surfactant, which can promote the IP reaction. However, we show enhanced membrane performance for Tween 80 (a common surfactant), even though it decreased the amine diffusion. Indeed, the membrane performance is closely related to its polyamide roughness features with numerous nanovoids. Inspired by the nanofoaming theory that relates the roughness features to nanobubbles degassed during the IP reaction, we hypothesize that the surfactant can stabilize the generated nanobubbles to tailor the formation of nanovoids. Accordingly, we obtained enlarged nanovoids when the surfactant was added below its critical micelle concentration (CMC). In addition, both the membrane permeance and selectivity were enhanced, thanks to the enlarged nanovoids and reduced defects in the polyamide layer. Increasing the concentration above CMC resulted in shrunken nanovoids and deteriorated performance, which can be ascribed to the decreased stabilization effect caused by micelle formation. Interestingly, better antifouling performance was also observed for the surfactant-assisted membranes. Our current study provides mechanistic insights into the critical role of surfactant during the IP reaction, which may have important implications for more efficient membrane-based desalination and water reuse.

Does Surface Roughness Necessarily Increase the Fouling Propensity of Polyamide Reverse Osmosis Membranes by Humic Acid?

Surface roughness has crucial influence on the fouling propensity of thin film composite (TFC) polyamide reverse osmosis (RO) membranes. A common wisdom is that rougher membranes tend to experience more severe fouling. In this study, we compared the fouling behaviors of a smooth polyamide membrane (RO-s) and a nanovoid-containing rough polyamide membrane (RO-r). Contrary to the traditional belief, we observed more severe fouling for RO-s, which can be ascribed to its uneven flux distribution caused by the "funnel effect". Additional tracer filtration tests using gold nanoparticles revealed a more patchlike particle deposition pattern, confirming the adverse impact of "funnel effect" on membrane water transport. In contrast, the experimentally observed lower fouling propensity of the nanovoid-containing rough membrane can be explained by: (1) the weakened "funnel effect" thanks to the presence of nanovoids, which can regulate the water transport pathway through the membrane and (2) the decreased average localized flux over the membrane surface due to the increased effective filtration area for the nanovoid-induced roughness features. The current study provides fundamental insights into the critical role of surface roughness in membrane fouling, which may have important implications for the future development of high-performance antifouling membranes.

Nanofiltration Membranes with Crumpled Polyamide Films: A Critical Review on Mechanisms, Performances, and Environmental Applications.

Nanofiltration (NF) membranes have been widely applied in many important environmental applications, including water softening, surface/groundwater purification, wastewater treatment, and water reuse. In recent years, a new class of piperazine (PIP)-based NF membranes featuring a crumpled polyamide layer has received considerable attention because of their great potential for achieving dramatic improvements in membrane separation performance. Since the report of novel crumpled Turing structures that exhibited an order of magnitude enhancement in water permeance ( Science 2018, 360 (6388), 518-521), the number of published research papers on this emerging topic has grown exponentially to approximately 200. In this critical review, we provide a systematic framework to classify the crumpled NF morphologies. The fundamental mechanisms and fabrication methods involved in the formation of these crumpled morphologies are summarized. We then discuss the transport of water and solutes in crumpled NF membranes and how these transport phenomena could simultaneously improve membrane water permeance, selectivity, and antifouling performance. The environmental applications of these emerging NF membranes are highlighted, and future research opportunities/needs are identified. The fundamental insights in this review provide critical guidance on the further development of high-performance NF membranes tailored for a wide range of environmental applications.

Cosolvent-Assisted Interfacial Polymerization toward Regulating the Morphology and Performance of Polyamide Reverse Osmosis Membranes: Increased m-Phenylenediamine Solubility or Enhanced Interfacial Vaporization?

Cosolvent-assisted interfacial polymerization (IP) can effectively enhance the separation performance of thin film composite (TFC) reverse osmosis (RO) membranes. However, the underlying mechanisms regulating the formation of their polyamide (PA) rejection films remain controversial. The current study reveals two essential roles of cosolvents in the IP reaction: (1) directly promoting interfacial vaporization with their lower boiling points and (2) increasing the solubility of m-phenylenediamine (MPD) in the organic phase, thereby indirectly promoting the IP reaction. Using a series of systematically chosen cosolvents (i.e., diethyl ether, acetone, methanol, and toluene) with different boiling points and MPD solubilities, we show that the surface morphologies of TFC RO membranes were regulated by the combined direct and indirect effects. A cosolvent favoring interfacial vaporization (e.g., lower boiling point, greater MPD solubility, and/or higher concentration) tends to create greater apparent thickness of the rejection layer, larger nanovoids within the layer, and more extensive exterior PA layers, leading to significantly improved membrane water permeance. We further demonstrate the potential to achieve better antifouling performance for the cosolvent-assisted TFC membranes. The current study provides mechanistic insights into the critical roles of cosolvents in IP reactions, providing new tools for tailoring membrane morphology and separation properties toward more efficient desalination and water reuse.

Nanofiltration for drinking water treatment: a review.

In recent decades, nanofiltration (NF) is considered as a promising separation technique to produce drinking water from different types of water source. In this paper, we comprehensively reviewed the progress of NF-based drinking water treatment, through summarizing the development of materials/fabrication and applications of NF membranes in various scenarios including surface water treatment, groundwater treatment, water reuse, brackish water treatment, and point of use applications. We not only summarized the removal of target major pollutants (e.g., hardness, pathogen, and natural organic matter), but also paid attention to the removal of micropollutants of major concern (e.g., disinfection byproducts, per- and polyfluoroalkyl substances, and arsenic). We highlighted that, for different applications, fit-for-purpose design is needed to improve the separation capability for target compounds of NF membranes in addition to their removal of salts. Outlook and perspectives on membrane fouling control, chlorine resistance, integrity, and selectivity are also discussed to provide potential insights for future development of high-efficiency NF membranes for stable and reliable drinking water treatment.

Mechanistic Insights into the Role of Polydopamine Interlayer toward Improved Separation Performance of Polyamide Nanofiltration Membranes.

Interlayered thin-film nanocomposite membranes (TFNi) are an emerging type of membranes with great potential to overcome the permeability-selectivity upper bound of conventional thin-film composite (TFC) nanofiltration and reverse osmosis membranes. However, the exact roles of the interlayer and the corresponding mechanisms leading to enhanced separation performance of TFNi membranes remain poorly understood. This study reports a polydopamine (PDA)-intercalated TFNi nanofiltration membrane (PA-PSF2, PDA coating time of 2 h) that possessed nearly an order of magnitude higher water permeance (14.8 ± 0.4 Lm-2 h-1 bar-1) than the control TFC membrane (PA-PFS0, 2.4 ± 0.5 Lm-2 h-1 bar-1). The TFNi membrane further showed enhanced rejection toward a wide range of inorganic salts and small organic molecules (including antibiotics and endocrine disruptors). Detailed mechanistic investigation reveals that the membrane separation performance was enhanced due to both the direct "gutter" effect of the PDA interlayer and its indirect effects resulting from enhanced polyamide formation on the PDA-coated substrate, with the "gutter" effect playing a more dominant role. This study provides a mechanistic and comprehensive framework for the future development of TFNi membranes.

Dissecting the Role of Substrate on the Morphology and Separation Properties of Thin Film Composite Polyamide Membranes: Seeing Is Believing.

Recent studies show that the surface morphology of a thin film composite (TFC) polyamide membrane depends strongly on its porous substrate. Nevertheless, the underlining mechanisms and the effects on membrane separation performance remain controversial. To dissect the exact role of pore properties, we synthesized TFC polyamide membranes on polycarbonate substrates with cylindrical track-etched pores (PCTE) of well-defined pore size ranging from 10 to 800 nm. Leaf-like roughness features were most prominent for polyamide films formed on substrates of intermediate pore sizes (80 and 100 nm). Smaller pores inhibited leaf-like features as a result of insufficient storage of m-phenylenediamine (MPD) monomers for the interfacial reaction, whereas larger pores resulted in diminished surface roughness due to the lack of confinement to the interfacially degassed nanobubbles. Substrate porosity plays a critical role on membrane water permeability, while smaller pores with greater pore density are favored to improve membrane rejection. TFC polyamide membranes prepared on sponge-like poly(ether sulfone) and polysulfone substrates exhibit better water permeability and salt rejection compared to the PCTE-TFC membranes thanks to the simultaneously enhanced confinement and MPD storage effects. The mechanistic insights gained in this study reveal the huge potential of substrate design toward high-performance TFC RO membranes.

Engineering Interface with a One-Dimensional RuO2/TiO2 Heteronanostructure in an Electrocatalytic Membrane Electrode: Toward Highly Efficient Micropollutant Decomposition.

Decomposition of micropollutants using an electrocatalytic membrane reactor is a promising alternative to traditional advanced oxidation processes due to its high efficiency and environmental compatibility. Rational interface design of electrocatalysts in the membrane electrode is critical to the performance of the reactor. We herein developed a three-dimensional porous membrane electrode via in situ growth of one-dimensional RuO2/TiO2 heterojunction nanorods on a carbon nanofiber membrane by a facile hydrothermal and subsequent thermal treatment approach. The membrane electrode was used as the anode in a gravity-driven electrocatalytic membrane reactor, exhibiting a high degradation efficiency of over 98% toward bisphenol-A and sulfadiazine. The superior electrocatalytic performance was attributed to the 1D RuO2/TiO2 heterointerfacial structure, which provided the fast electron transfer, high generation rate of the hydroxyl radical, and large effective surface area. Our work paves a novel way for the fundamental understanding and designing of novel highly effective and low-consumptive electrocatalytic membranes for wastewater treatment.

Omniphobic Nanofibrous Membrane with Pine-Needle-Like Hierarchical Nanostructures: Toward Enhanced Performance for Membrane Distillation.

Wetting and fouling phenomena are the main concerns for membrane distillation (MD) in treating high-salinity industrial wastewater. This work developed an omniphobic membrane by growing titanium dioxide (TiO2) nanorods on polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers using a hydrothermal technique. The TiO2 nanorods form a uniform pine-needle-like hierarchical nanostructure on PVDF-HFP fibers. A further fluorination treatment provides the membrane with a low-surface-energy omniphobic surface, displaying contact angles of 168° and 153° for water and mineral oil, respectively. Direct contact MD experiments demonstrated that the resulting membrane shows a high and stable salt rejection of >99.9%, while the pristine PVDF-HFP nanofibrous membrane suffers a rejection decline caused by intense pore wetting and oil fouling in the desalination process in the presence of surfactant and mineral oil. The superior antiwetting and antifouling behaviors were ascribed to a nonwetting Cassie-Baxter state established by the accumulation of a great deal of air in the hydrophobized hierarchical re-entrant structures. The development of omniphobic membranes with pine-needle-like hierarchical nanostructures provides an approach to mitigate membrane wetting and fouling in the MD process for the water reclamation from industrial wastewater.

Tailoring Polyamide Rejection Layer with Aqueous Carbonate Chemistry for Enhanced Membrane Separation: Mechanistic Insights, Chemistry-Structure-Property Relationship, and Environmental Implications.

Surface roughness and the associated nanosized voids inside the roughness structures have great influence on the separation performance of thin film composite polyamide reverse osmosis (RO) membranes. Inspired by the recent findings that these voids are formed due to the degassing of CO2 nanobubbles during interfacial polymerization, we systematically investigated the role of carbonate chemistry, particularly the solubility of CO2, in the aqueous m-phenylenediamine (MPD) solution for the first time. "Ridge-and-valley" roughness features were obtained when the pH of the MPD solution was between the two acidity constants of the carbonate system (i.e., 6.3 ≤ pH ≤ 10.3), under which condition HCO3- dominates over the other carbonate species. Increasing pH over this range led to both increased water permeability and better rejection of various solutes, thanks to the simultaneously enhanced effective filtration area and cross-linking degree of the polyamide layer. Further increase of pH to 12.5 resulted in more disparate rejection results due to membrane hydrolysis: rejection of neural solutes (B and As(III)) was compromised whereas that of charged solutes (NaCl and As(V)) was maintained. The mechanistic insights gained in the current study reveal the critical need to design RO membranes directly for end applications based on first principles.

Non-Polyamide Based Nanofiltration Membranes Using Green Metal-Organic Coordination Complexes: Implications for the Removal of Trace Organic Contaminants.

Polyamide-based thin film composite (TFC) membranes are generally optimized for salt rejection but not for the removal of trace organic contaminants (TrOCs). The insufficient rejection of TrOCs such as endocrine disrupting compounds (EDCs) by polyamide membranes can jeopardize product water safety in wastewater reclamation. In this study, we report a novel nonpolyamide membrane chemistry using green tannic acid-iron (TA-Fe) complexes to remove TrOCs. The nanofiltration membrane formed at a TA-Fe molar ratio of 1:3 (TA-Fe3) had a continuous thin rejection layer of 10-30 nm in thickness, together with a water permeability of 5.1 Lm2-h-1bar-1 and a Na2SO4 rejection of 89.7%. Meanwhile, this membrane presented significantly higher rejection of EDCs (up to 99.7%) than that of polyamide membranes (up to 81.8%). Quartz crystal microbalance results revealed that the sorption amount of a model EDC, benzylparbaen, by TA-Fe3 layer was nearly 2 orders of magnitude less than that by polyamide, leading to reduced transmission and higher rejection. Further analysis of membrane revealed a much greater water/EDC selectivity of the TA-Fe3 membrane compared to the polyamide membranes.