Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships.

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Exploitation of Amine Groups Cooped up in Polyamide Nanofiltration Membranes to Achieve High Rejection of Micropollutants and High Permeance of Divalent Cations.

To enhance the use of nanofiltration in the production of quality drinking water, particularly through the efficient removal of micropollutants yet still preserving essential minerals, the targeted nanofiltration membranes (NFMs) are required to have small pore dimensions coupled with a high, net-negative charge density. Herein, after the formation of a separation layer using piperazine interfacially polymerized with trimesoyl chloride, the exploitation of residual amine groups was systematically investigated by different diacyl chlorides in an organic milieu, which caused the upper part of the final separation layer to be denser and highly negatively charged. Hence, this protocol offers a novel means to fabricate NFMs simultaneously endowed with a low molecular cutoff (MWCO) of 145-238 Da and a reduced rejection of MgCl2 (48%-80%) as well as a competitive water permeance. Those features are ideally applicable to the goal of removing small micropollutants while preserving mineral ions, as needed for the energy-efficient production of safe, quality drinking water. Furthermore, an attempt was made to correlate MWCO with MgCl2 rejection, which provides some insights on the nexus of the electrostatic effects constrained by size exclusion. The significance of residual amine groups and the modification environment was unveiled, and this method paves a new avenue for designing functional NFMs.

Modulating the Asymmetry of the Active Layer in Pursuit of Nanofiltration Selectivity via Differentiating Interfacial Reactions of Piperazine.

Nanofiltration (NF), highly prospective for drinking water treatment, faces a challenge in simultaneously removing emerging contaminants while maintaining mineral salts, particularly divalent cations. To overcome this challenge, NF membranes possessing small pores concomitant with highly negatively charged surfaces were synthesized via a two-step fabrication strategy. The key is to generate a polyamide active layer having a loose and carboxyl group-abundant segment on top and a dense barrier segment underneath. This was achieved by restrained interfacial polymerization between trimesoyl chloride and partly protonated piperazine to form a highly depth-heterogeneous polyamide network, followed by second amidation in an organic environment to remove untethered polyamide fragments and associate malonyl chlorides with reserved amine groups to introduce more negative charges. Most importantly, on first-principle engineering the spatial architecture of the polyamide layer, amplifying asymmetric charge distribution was paired with the thinning of the vertical structure. The optimized membrane exhibits high salt/organic rejection selectivity and water permeance superior to most NF membranes reported previously. The rejections of eight emerging contaminants were in the range of 66.0-94.4%, much higher than the MgCl2 rejection of 41.1%. This new fabrication strategy, suitable for various diacyl chlorides, along with the new membranes so produced, offers a novel option for NF in potable water systems.

Boosting the Performance of Nanofiltration Membranes in Removing Organic Micropollutants: Trade-Off Effect, Strategy Evaluation, and Prospective Development.

In view of the high risks brought about by organic micropollutants (OMPs), nanofiltration (NF) processes have been playing a vital role in advanced water and wastewater treatment, owing to the high membrane performance in rejection of OMPs, permeation of water, and passage of mineral salts. Though numerous studies have been devoted to evaluating and technically enhancing membrane performance in removing various OMPs, the trade-off effect between water permeance and water/OMP selectivity for state-of-the-art membranes remains far from being understood. Knowledge of this effect is significant for comparing and guiding membrane development works toward cost-efficient OMP removal. In this work, we comprehensively assessed the performance of 88 NF membranes, commercialized or newly developed, based on their water permeance and OMP rejection data published in the literature. The effectiveness and underlying mechanisms of various modification methods in tailoring properties and in turn performance of the mainstream polyamide (PA) thin-film composite (TFC) membranes were quantitatively analyzed. The trade-off effect was demonstrated by the abundant data from both experimental measurements and machine learning-based prediction. On this basis, the advancement of novel membranes was benchmarked by the performance upper-bound revealed by commercial membranes and lab-made PA membranes. We also assessed the potentials of current NF membranes in selectively separating OMPs from inorganic salts and identified the future research perspectives to achieve further enhancement in OMP removal and salt/OMP selectivity of NF membranes.

Enzymatic Cleaning Mitigates Polysaccharide-Induced Refouling of RO Membrane: Evidence from Foulant Layer Structure and Microbial Dynamics.

Traditional harsh chemical cleaning-in-place (CIP) is corrosive to membranes but has limited inhibition on refouling, a tough problem for long-term operation of reverse osmosis (RO). Mild enzymatic cleaning (at pH 9) is a promising alternative but lacks long-term verification and insightful elucidation. In this study, we investigated the instantaneous efficiency, postcleaning refouling, and biological effect of enzymatic CIP (compounded with lipase, protease, and sodium dodecyl sulfate) on practical RO membranes during a 500 h multicycle operation. The enzymatic CIP had an average cleaning efficiency of 77%, which is comparable to a commercial harsh CIP benchmark (pH > 12). It mitigated refouling by shaping the biofilm into a loose and porous architecture where newly arrived organics conformed standard blocking, whereas harsh chemicals rendered a smooth and dense gel layer with quick refouling in intermediate blocking or cake filtration mode. Such structural disparities were dominated by polysaccharides according to quantitative chemical analyses. Gene sequencing and ecological network analysis further proved that the behavior of polysaccharide-related keystone species (such as Sphingomonas and Xanthomonas) significantly changed after long-term enzymatic treatment. In this regard, the mild selective pressure of enzymatic reagents can directionally regulate microbial dynamics, alter foulant layer structure via bio-organic synchronicity, mitigate refouling, and eventually improve the sustainability of RO operation.

A Y(III)-based Metal-Organic Framework as a Carrier in Chemodynamic Therapy.

A biocompatible Y(III)-based metal-organic framework [Y4(TATB)2]·(DMF)3.5·(H2O) (ZJU-16, H3TATB= 4,4',4''-(1,3,5-triazine-2,4,6-triyl) tribenzoic acid) was synthesized, and it was adopted to load Mn2+ for chemodynamic therapy. Meanwhile, ibuprofen sodium (IBUNa), an anti-inflammatory drug, was introduced to increase the amount of Mn2+ (about 5.66 wt %) due to the low loading capacity of Mn2+. Mn&IBUNa@ZJU-16 which was loaded by Mn2+ and IBUNa exhibited significant effects of chemodynamic therapy and excellent inhibition of the 4T1 tumor cell growth, implying its long-term prospects in chemodynamic therapy and its possibility in bimodal cancer therapy.

Electrically Tuning Ultrafiltration Behavior for Efficient Water Purification.

Conventional ultrafiltration (UF) technology suffers from membrane fouling and limited separation performance. This work demonstrates a novel electrical tuning strategy to improve the separation efficiency of the UF process. An electrically enhanced UF (EUF) system with two sets of oppositely placed membrane-electrode modules was set up. A series of multicycle treatment experiments were conducted to reveal the performance and tuning mechanism of the EUF system. The applied electrical tuning operation brought about an up to 68% reduction of average transmembrane pressure increasing rate (Rp), indicating a strong capability in inhibiting membrane fouling. This fouling reduction can be mainly ascribed to the applied electrophoretic force, changes in solution chemistry, and generation of peroxide, which repulses foulants away from the membrane, hampers foulant adsorption owing to enhanced electrostatic repulsion, and degrades foulants, respectively. The 1.2 V voltage was identified as an effective voltage for stably inhibiting membrane fouling. Besides, the electrical tuning operation led to an up to ∼32% increase in foulant retention rate (φ) owing to both non-Faradaic effects (including electrosorption and electrophoretic repulsion) and Faradaic oxidative degradation. Moreover, the electrical tuning operation allowed a remarkable desalination capability with a significantly higher desalination rate and an up to ∼43% greater salt adsorption capacity as compared with a conventional capacitive deionization process. Additionally, the EUF system achieved a good performance in removing heavy metals (Ag, Cu, Pb, Se, and Sb). The overall enhanced EUF performance suggests promising prospects for practical applications.

A Facile and Scalable Fabrication Procedure for Thin-Film Composite Membranes: Integration of Phase Inversion and Interfacial Polymerization.

Conventional dense thin-film composite (TFC) membranes evince a universally low water permeability, the increase of which typically relies on introducing additional transport channels based on intricate steps within a membrane preparation process. In this study, we reported a novel and simplified procedure for the fabrication of high-performance TFC membranes. Specifically, the dissolution of aqueous monomers in the casting solution was utilized for the following interfacial polymerization (IP). Since the monomers diffused to the water bath during phase inversion, the control of precipitation time enabled an effective regulation of the monomer concentration in the formed polymeric substrates, where the IP reaction was initiated by the addition of the organic phase. The entire and uniform embedment of aqueous monomers inside the substrates contributed to the formation of ultrathin and smooth selective layers. An excellent separation performance (i.e., water permeability: 34.7 L m-2 h-1 bar-1; Na2SO4 rejection: ∼96%) could be attained using two types of aqueous monomers (i.e., piperazine and ß-cyclodextrin), demonstrating the effectiveness and universality of this method. Compared to the conventional immersion-based process, this novel procedure shows distinct advantages in reducing monomer usage, shortening the production cycle, and achieving a more superior membrane performance, which is highly promising for large-scale membrane manufacture.

Roles of membrane and organic fouling layers on the removal of endocrine disrupting chemicals in microfiltration.

To understand the adsorption behavior of endocrine disrupting chemicals (EDCs) is important for enhancing the treatment performance and preventing potential secondary pollution caused by EDCs desorption in a microfiltration system. The dynamic adsorption of four representative EDCs, namely estriol (E3), 17ß-estradiol (E2), 17α-ethynylestradiol (EE2), and 4-nonylphenol (4-NP) in a microfiltration system was investigated using the Thomas' model. The product of the equilibrium constant and the total adsorption capacity of the membrane, Ka, for E3, E2, EE2, and 4-NP were 4.91, 9.78, 15.6, and 826, respectively, strongly correlating with the compound octanol-water partition coefficient (KOW). Adsorption appeared to be enhanced when organic fouling formed on the surface of membrane, indicating the role of an additional adsorption column for EDCs acted by a fouling layer in microfiltration. Results of a comparison between the Ka values for clean membrane and fouled membrane illustrated that the significant contribution made by fouling layers may be attributed to the foulant layer's hydrophobicity (in the case of calcium humate layer) and thickness (in the case of calcium alginate layer). This study provided a novel perspective to quantitatively analyze the dynamic adsorption behavior of trace pollutants in membrane process.

Pro-Inflammatory Cytokine TNF-α Attenuates BMP9-Induced Osteo/ Odontoblastic Differentiation of the Stem Cells of Dental Apical Papilla (SCAPs).

BACKGROUND/AIMS: Periapical periodontitis is a common oral disease caused by bacterial invasion of the tooth pulp, which usually leads to local release of pro-inflammatory cytokines and osteolytic lesion. This study is intended to examine the effect of TNF-α on BMP9-induced osteogenic differentiation of the stem cells of dental apical papilla (SCAPs). METHODS: Rat model of periapical periodontitis was established. TNF-α expression was assessed. Osteogenic markers and ectopic bone formation in iSCAPs were analyzed upon BMP9 and TNF-α treatment. RESULTS: Periapical periodontitis was successfully established in rat immature permanent teeth with periapical lesions, in which TNF-α was shown to release during the inflammatory phase. BMP9-induced alkaline phosphatase activity, the expression of osteocalcin and osteopontin, and matrix mineralization in iSCAPs were inhibited by TNF-α in a dose-dependent fashion, although increased AdBMP9 partially overcame TNF-α inhibition. Furthermore, high concentration of TNF-α effectively inhibited BMP9-induced ectopic bone formation in vivo. CONCLUSION: TNF-α plays an important role in periapical bone defect during the inflammatory phase and inhibits BMP9-induced osteoblastic differentiation of iSCAPs, which can be partially reversed by high levels of BMP9. Therefore, BMP9 may be further explored as a potent osteogenic factor to improve osteo/odontogenic differentiation in tooth regeneration in chronic inflammation conditions.

[Oral absorption of asiatic acid nanoparticles modified with PEG].

A solvent diffusion method was used to prepare pegylated asiatic acid (AA) loaded nanostructured lipid carriers (p-AA-NLC), and the ligated intestinal circulation model was established to observe the absorption and distribution in small intestine. The concentration of AA in bile after oral administration of p-AA-NLC was detected by HPLC in healthy SD rats to indirectly evaluate the oral absorption promoting effect of PEG-modified namoparticles. The results showed that the penetration of p-AA-NLC was enhanced significantly and the transport capacity was increased greatly in small intestinal after PEG modification. As compared with the normal nanoparticles (AA-NLC), the Cmax of the drug excretion was increased by 76%, the time to reach the peak (tmax ) was decreased and the elimination half-life t1/2 was doubled in the rats after oral administration of p-AA-NLC, and the AUC0→t was 1.5 times of the AA-NLC group, indicating that the oral bioavailability of AA-NLC was significantly improved by hydrophilic modification of PEG.

[Response surface method for optimization of asiatic acid nanoparticles modified with PEG and its enhancing effects on intestinal absorption].

A solvent diffusion method was used to prepare pegylated asiatic acid (AA) loaded nanostructured lipid carriers (p-AA-NLC). Then central composite design-response surface method was used to obtain optimum condition for preparation technology of p-AA-NLC, where PEG/lipid ratio was 8.0% and AA/lipid ratio was 22.0%. Under the optimum condition, the system had particle size of (111.2±2.9) nm, Zeta potential of (-37.1±0.9) mV, drug loading of (15.4±0.2)% and entrapment efficiency greater than 90%. The deviations between observed values and predicated values were all below 5%, indicating that the established model had a good predictability. Meanwhile, a low-speed single pass perfusion model of rat in situ was set up to estimate the absorption kinetics of p-AA-NLC in small intestine, where the effective permeability (Peff), absorption rate constant (Ka) and other parameters were used to evaluate the drug absorption. It turned out that Peff and Ka in p-AA-NLC group were significantly higher than those in unmodified group (P<0.05), indicating that asiatic acid loaded nanostructured lipid carriers (AA-NLC) could enhance the effects on intestinal absorption after being modified with hydrophilic PEG.

Methane production in microbial reverse-electrodialysis methanogenesis cells (MRMCs) using thermolytic solutions.

The utilization of bioelectrochemical systems for methane production has attracted increasing attention, but producing methane in these systems requires additional voltage to overcome large cathode overpotentials. To eliminate the need for electrical grid energy, we constructed a microbial reverse-electrodialysis methanogenesis cell (MRMC) by placing a reverse electrodialysis (RED) stack between an anode with exoelectrogenic microorganisms and a methanogenic biocathode. In the MRMC, renewable salinity gradient energy was converted to electrical energy, thus providing the added potential needed for methane evolution from the cathode. The feasibility of the MRMC was examined using three different cathode materials (stainless steel mesh coated with platinum, SS/Pt; carbon cloth coated with carbon black, CC/CB; or a plain graphite fiber brush, GFB) and a thermolytic solution (ammonium bicarbonate) in the RED stack. A maximum methane yield of 0.60 ± 0.01 mol-CH4/mol-acetate was obtained using the SS/Pt biocathode, with a Coulombic recovery of 75 ± 2% and energy efficiency of 7.0 ± 0.3%. The CC/CB biocathode MRMC had a lower methane yield of 0.55 ± 0.02 mol-CH4/mol-acetate, which was twice that of the GFB biocathode MRMC. COD removals (89-91%) and Coulombic efficiencies (74-81%) were similar for all cathode materials. Linear sweep voltammetry and electrochemical impedance spectroscopy tests demonstrated that cathodic microorganisms enhanced electron transfer from the cathode compared to abiotic controls. These results show that the MRMC has significant potential for production of nearly pure methane using low-grade waste heat and a source of waste organic matter at the anode.

Tunable Light-Responsive Polyurethane-urea Elastomer Driven by Photochemical and Photothermal Coupling Mechanism.

Light-driven soft actuators based on photoresponsive materials can be used to mimic biological motion, such as hand movements, without involving rigid or bulky electromechanical actuations. However, to our knowledge, no robust photoresponsive material with desireable mechanical and biological properties and relatively simple manufacture exists for robotics and biomedical applications. Herein, we report a new visible-light-responsive thermoplastic elastomer synthesized by introducing photoswitchable moieties (i.e., azobenzene derivatives) into the main chain of poly(ε-caprolactone) based polyurethane urea (PAzo). A PAzo elastomer exhibits controllable light-driven stiffness softening due to its unique nanophase structure in response to light, while possessing excellent hyperelasticity (stretchability of 575.2%, elastic modulus of 17.6 MPa, and strength of 44.0 MPa). A bilayer actuator consisting of PAzo and polyimide films is developed, demonstrating tunable bending modes by varying incident light intensities. Actuation mechanism via photothermal and photochemical coupling effects of a soft-hard nanophase is demonstrated through both experimental and theoretical analyses. We demonstrate an exemplar application of visible-light-controlled soft "fingers" playing a piano on a smartphone. The robustness of the PAzo elastomer and its scalability, in addition to its excellent biocompatibility, opens the door to the development of reproducible light-driven wearable/implantable actuators and lightweight soft robots for clinical applications.

Design and fabrication of three-dimensional chiral nanostructures based on stepwise glancing angle deposition technology.

The chiral structures have displayed some inevitable and fascinating properties in many research fields, such as chemistry, biology, mathematics, and physics. In this Article, we report the use of stepwise glancing angle deposition technology to produce the 3D chiral nanostructures. Through the optimization of deposition parameters (such as the orientation angle of poly styrene spheres (PSs) array, the deposition angle, thickness, and number), a great number of chiral structures have been achieved, and their size depends on the diameter of PS spheres. These chiral structures all can be simulated and predesigned through the use of a 3D geometrical model, which greatly improves the efficiency of this method. In addition, the circular dichroism spectrum shows that these chiral structures own an obvious Cotton effect, indicating their potential application as 3D chiral metamaterials.

Patterned membranes for improving hydrodynamic properties and mitigating membrane fouling in water treatment: A review.

Membrane technologies have been widely applied in water treatment over the past few decades. However, membrane fouling remains a hinderance for the widespread use of membrane processes because it decreases effluent quality and increases operating costs. To mitigate membrane fouling, researchers have been exploring effective anti-fouling strategies. Recently, patterned membranes are gaining attention as a novel non-chemical membrane modification for membrane fouling control. In this paper, we review the research on patterned membranes used in water treatment over the past 20 years. In general, patterned membranes show superior anti-fouling performances, which mainly results from two aspects: hydrodynamic effects and interaction effects. Due to the introduction of diversified topographies onto the membrane surface, patterned membranes yield dramatic improvements on hydrodynamic properties, e.g., shear stress, velocity field and local turbulence, restraining concentration polarization and foulants' deposition on the membrane surface. Besides, the membrane-foulant and foulant-foulant interactions play an important role in the mitigation of membrane fouling. Due to the existence of surface patterns, the hydrodynamic boundary layer is destroyed and the interaction force as well as the contact area between foulants and surface are decreased, which contributes to the fouling suppression. However, there are still some limitations in the research and application of patterned membranes. Future research is suggested to focus on the development of patterned membranes appropriate for different water treatment scenarios, the insights into the interaction forces affected by surface patterns, and the pilot-scale and long-term studies to verify the anti-fouling performances of patterned membranes in practical applications.

Interface interaction between silica and organic macromolecule conditioned forward osmosis membranes: Insights into quantitative thermodynamics and dynamics.

Silica scaling is a rising concern in forward osmosis membrane-based water treatment process. The coexistence of ubiquitous organic macromolecules causes complex silica scaling. The silica scaling mechanism on the surface of the organic conditioned membrane remains unclear. An integrated multi scale thermodynamic and dynamic approach was used in this study to provide in-depth insights into the binding effect at the interface between the silica and the organic conditioned membrane at the molecular level. Sodium alginate (SA) was used as the model polysaccharide, bovine serum albumin (BSA) and lysozyme (LYZ) were chosen as two oppositely charged proteins. The results show that the silica scaling degree of different organic conditioned membranes follows the order LYZ > BSA > SA. The binding strength between silica and organic macromolecules and the membrane surface charge are the major factors governing the degree of silica scaling. Quartz crystal microbalance with dissipation (QCM-D), isothermal titration calorimetry (ITC), and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) model analyses were conducted to quantify the binding capacity of silica to the organic conditioned membrane. The LYZ conditioned membrane exhibits the highest affinity for silica adsorption, and electrostatic interaction was the main molecular interaction force. This study provides fresh insights into how silica and an organic conditioned membrane interact and induce silica scaling, providing new information on potential mechanisms and control strategies to prevent membrane scaling.

MiR-199a-5P promotes osteogenic differentiation of human stem cells from apical papilla via targeting IFIT2 in apical periodontitis.

Introduction: Periapical alveolar bone loss is the common consequence of apical periodontitis (AP) caused by persistent local inflammation around the apical area. Human stem cells from apical papilla (hSCAPs) play a crucial role in the restoration of bone lesions during AP. Studies have recently identified the critical role of microRNAs (miRNAs) involved in AP pathogenesis, but little is known about their function and potential molecular mechanism, especially in the osteogenesis of hSCAPs during AP. Here, we investigated the role of clinical sample-based specific miRNAs in the osteogenesis of hSCAPs. Methods: Differential expression of miRNAs were detected in the periapical tissues of normal and patients with AP via transcriptomic analysis, and the expression of miR-199a-5p was confirmed by qRT-PCR. Treatment of hSCAPs with miR-199a-5p mimics while loaded onto beta-tricalcium phosphate (ß-TCP) ceramic particle scaffold to explore its effect on osteogenesis in vivo. RNA binding protein immunoprecipitation (RIP) and Luciferase reporter assay were conducted to identify the target gene of miR-199a-5p. Results: The expression of miR-199a-5p was decreased in the periapical tissues of AP patients, and miR-199a-5p mimics markedly enhanced cell proliferation and osteogenic differentiation of hSCAPs, while miR-199a-5p antagomir dramatically attenuated hSCAPs osteogenesis. Moreover, we identified and confirmed Interferon Induced Protein with Tetratricopeptide Repeats 2 (IFIT2) as a specific target of miR-199a-5p, and silencing endogenous IFIT2 expression alleviated the inhibitory effect of miR-199a-5p antagomir on the osteogenic differentiation of hSCAPs. Furthermore, miR-199a-5p mimics transfected hSCAPs loaded onto beta-tricalcium phosphate (ß-TCP) scaffolds induced robust subcutaneous ectopic bone formation in vivo. Discussion: These results strengthen our understanding of predictors and facilitators of the key AP miRNAs (miR-199a-5p) in bone lesion repair under periapical inflammatory conditions. And the regulatory networks will be instrumental in exploring the underlying mechanisms of AP and lay the foundation for future regenerative medicine based on dental mesenchymal stem cells.

Improved antifouling properties of polyamide nanofiltration membranes by reducing the density of surface carboxyl groups.

Carboxyls are inherent functional groups of thin-film composite polyamide nanofiltration (NF) membranes, which may play a role in membrane performance and fouling. Their surface presence is attributed to incomplete reaction of acyl chloride monomers during the membrane active layer synthesis by interfacial polymerization. In order to unravel the effect of carboxyl group density on organic fouling, NF membranes were fabricated by reacting piperazine (PIP) with either isophthaloyl chloride (IPC) or the more commonly used trimesoyl chloride (TMC). Fouling experiments were conducted with alginate as a model hydrophilic organic foulant in a solution, simulating the composition of municipal secondary effluent. Improved antifouling properties were observed for the IPC membrane, which exhibited lower flux decline (40%) and significantly greater fouling reversibility or cleaning efficiency (74%) than the TMC membrane (51% flux decline and 40% cleaning efficiency). Surface characterization revealed that there was a substantial difference in the density of surface carboxyl groups between the IPC and TMC membranes, while other surface properties were comparable. The role of carboxyl groups was elucidated by measurements of foulant-surface intermolecular forces by atomic force microscopy, which showed lower adhesion forces and rupture distances for the IPC membrane compared to TMC membranes in the presence of calcium ions in solution. Our results demonstrated that a decrease in surface carboxyl group density of polyamide membranes fabricated with IPC monomers can prevent calcium bridging with alginate and, thus, improve membrane antifouling properties.

Two strategies of stubborn biofouling strains surviving from NaClO membrane cleaning: EPS shielding and/or quorum sensing.

The declined performance of repeated chemically-enhanced-backwashing (CEB) seriously hampered the sustainable operation of membrane bioreactor (MBR) in long-term, and could be partially attributed to the strengthened anti-cleaning properties of residual stubborn microbes. Although plenty of research has been done towards either the model strains or the whole post-CEB microbial community, little was known about the resisting behavior of practical stubborn strains when confronting oxidative stresses induced by NaClO. Hence, this study isolated 21 strains from samples in a large-scale MBR plant with routine CEB treatment. To unravel how they survive and affect membrane fouling, their anti-oxidation ability, fouling potential and quorum sensing (QS) effect before and after NaClO stimuli were evaluated. The composition and molecular weight distribution of extracellular polymeric substance (EPS) were also investigated to understand their roles during the anti-CEB process. It was found that typical stubborn strains tended to secrete more EPS as protective shields, where polysaccharides (especially the ones >1 kDa) made major contribution. However, sometimes EPS could not well resist the stimuli, with consequent low survival rate and high intracellular ROS level. Under such circumstances, stubborn strains would rather choose to be sensitive with surged QS level and quick population regrowth to maintain vitality under the oxidative stresses. Both strategies aggravated biofouling and eventually enhanced the anti-cleaning properties of biofilm.