Runnan Zhang

Antifouling, High-Flux Nanofiltration Membranes Enabled by Dual Functional Polydopamine

A facile method for fabricating antifouling and high-flux nanofiltration (NF) membranes was developed based on bioinspired polydopamine (PDA). Polyethersulfone (PES) ultrafiltration membrane as the support was first deposited a thin PDA layer and then chemically modified by a new kind of fluorinated polyamine via Michael addition reaction between fluorinated polyamine and quinone groups of PDA. PDA coating significantly reduced the pore sizes of the PES support membrane and endowed the NF membrane with high separation performance (flux about 46.1 L/(m(2) h) under 0.1 MPa, molecular weight cutoff of about 780 Da). The grafted fluorinated polyamine on the PDA layer could form low free energy microdomains to impede the accumulation/coalescence of foulants and lower the adhesion force between foulants and the membrane, rendering the membrane surface with prominent fouling-release property. When foulant solutions (including bovine serum albumin, oil and humic acid) were filtered, the resultant NF membrane exhibited excellent antifouling properties (the minimal value of total flux decline ratio was ∼8.9%, and the flux recovery ratio reached 98.6%). It is also found that the structural stability of the NF membrane could be significantly enhanced due to the covalent bond and other intermolecular interactions between the PDA layer and the PES support.

Coordination-enabled synergistic surface segregation for fabrication of multi-defense mechanism membranes

The antifouling mechanism lies at the heart of a number of surface-governed applications ranging from biomedical implants and devices, marine coatings, to membrane separations. However, the multi-defense mechanism has not been ingeniously employed to design and fabricate high-performance antifouling membranes. In this study, a coordination chemistry-enabled approach is explored to manipulate the synergistic surface segregation of amphiphilic copolymers and hydrophilic inorganic nanoparticles during the membrane formation process, thus constructing membrane surfaces with an optimally integrated fouling-resistant mechanism and fouling-release mechanism. Moreover, the metal–organic coordination interaction ensures the stable coexistence of copolymers and inorganic nanoparticles on the membrane surface, as well as the high mechanical strength of membranes. Consequently, the membranes display superior antifouling properties and long-term stability in oil/water emulsion separation.

Fabrication of antifouling polymer–inorganic hybrid membranes through the synergy of biomimetic mineralization and nonsolvent induced phase separation

Membrane-based technology is regarded as the most promising approach for oil/water separation, but suffers from severe membrane fouling. Hybrid membranes may have great opportunities in dealing with fouling problems due to their hierarchical structures and multiple functionalities. In this study, novel kinds of hybrid membranes with both inorganic hydrophilic microdomains and organic low surface free energy (LSFE) microdomains are fabricated through the synergy of in situ biomimetic mineralization and nonsolvent induced phase separation. The as-prepared hybrid membrane exhibits a homogeneous dispersion of nanoparticles, higher mechanical strength, underwater superoleophobicity and surface heterogeneity. Owing to its concomitant collaborative fouling-resistant mechanism and fouling-release mechanism, it is difficult for oil foulants to approach or attach to the membrane surface, and consequently the membranes display significantly enhanced antifouling properties and separation performance. Particularly, the permeation flux decline approaches zero during oil-in-water emulsion filtration. This study may endeavor to provide a facile and generic strategy to manipulate the structure–property relationship of membranes for efficient water treatment processes.

Green coating by coordination of tannic acid and iron ions for antioxidant nanofiltration membranes

A novel green coating method was proposed to prepare composite nanofiltration (NF) membranes without using organic solutions or toxic reagents in the formation of the active layer compared with traditional interfacial polymerization. Tannic acid (TA) and iron(III) chloride (FeCl3) were chosen as the two reactive monomers dissolved in the aqueous phase. The stable metal–polyphenol complex coating was formed via the coordination reaction between TA and iron ions (FeIII) upon porous support. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and water contact angle were used to characterize the chemical features of the prepared TA–FeIII/polyethersulfone (PES) composite NF membranes. Scanning electron microscope (SEM) and atomic force microscopy (AFM) were utilized to observe the surface morphologies. The effects of reactive monomer concentration and reaction time on the permeability of water and rejection of dyes and inorganic salts were investigated, respectively. The TA–FeIII/PES composite NF membranes possessed good structural stability and oxidation resistance ability.

Engineering amphiphilic nanofiltration membrane surfaces with a multi-defense mechanism for improved antifouling performances

Antifouling nanofiltration (NF) membrane surfaces capable of combating membrane fouling caused by different foulants are highly desirable for their broad applications. In this study, amphiphilic NF membranes with both hydrophilic domains and low surface energy domains were prepared to optimally integrate the fouling-resistant defense mechanism and fouling-release defense mechanism for enhanced antifouling performance. The construction of amphiphilic surfaces involved two-step surface modification of the hydrophilic polyamide NF membrane: (1) the introduction of primary amine groups as active sites by grafting triethylenetetramine (TETA) onto the carboxyl groups of the polyamide membrane; (2) the subsequent grafting of 2,2,3,4,4,4-hexafluorobutyl methacrylate (HFBM) through the Michael addition reaction. The amphiphilic membranes had better antifouling properties compared with the hydrophilic polyamide membrane during the filtration of bovine serum albumin (BSA) protein solution, humic acid solution and oil/water emulsion, which exhibited lower flux decline during utilization and higher flux recovery after water cleaning. Hopefully, this study is applicable to prepare a broad spectrum of antifouling NF membranes.

Improving Permeation and Antifouling Performance of Polyamide Nanofiltration Membranes through the Incorporation of Arginine

Inspired by the hydrophilicity effect of arginine (Arg) in water channel aquaporins (AQPs), Arg was incorporated into the polyamide layer during interfacial polymerization to enhance the permeation and antifouling performance of the nanofiltration (NF) membranes. Due to the presence of active amine groups, Arg became another aqueous phase monomer along with piperazine (PIP) to react with trimesoyl chloride (TMC) during interfacial polymerization, which was incorporated into the polyamide network. The resulting polyamide NF membranes were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), static water contact angle, zeta potential, and positron annihilation spectroscopy (PAS) measurement. The effects of incorporating Arg in aqueous phase on water permeability and the rejection of dyes and inorganic salts of the NF membranes were studied, respectively. Similar to its function in AQPs, Arg apparently increased the hydrophilicity and the negative charges of the membrane surface and, consequently, the permeation performance. When the addition of Arg reached 40% to PIP, the water flux was doubled and the rejection ratios of Congo red and Orange GII were still >90%. Meanwhile, the antifouling experiments verified that the modified polyamide NF membranes possessed excellent fouling-resistant performance for negatively charged foulants of BSA, emulsified oil droplet, and humic acid. The flux was decreased below 15%, and recovery even rose to 89%.

Manipulating the multifunctionalities of polydopamine to prepare high-flux anti-biofouling composite nanofiltration membranes

A high-flux anti-biofouling composite nanofiltration (NF) membrane was facilely prepared by simultaneously manipulating the three functionalities (adhesion, reaction and separation) of polydopamine (PDA) via a two-step dip-coating method. Stemming from the adhesion and separation functionalities, PDA was in situ deposited on a polyethersulfone (PES) substrate and formed an ultrathin dense layer, which endowed the membrane with a satisfying rejection for a broad range of small organic molecules (methyl orange of 65.65%, orange GII of 79.05%, congo red of about 98.78%, methyl blue and alcian blue of nearly 100%) and a relatively high water flux of 249.45 L m−2 h−1 MPa−1 under the operation pressure of 0.2 MPa. Stemming from the reaction functionality, PDA could reduce silver ions when exposed to AgNO3 solution and trigger the in situ generation of silver nanoparticles (AgNPs) on the membrane surface, which conferred excellent anti-biofouling properties to the membrane with low bioadhesion and high antibacterial efficiency (almost 100%) against Escherichia coli. Moreover, the adhesion and reaction functionalities have a close relation to the separation functionality of PDA. The effects of PDA deposition time, AgNO3 treatment time and AgNO3 concentration on the separation performance of the resulting composite NF membranes were systematically investigated.

Constructing a zwitterionic ultrafiltration membrane surface via multisite anchorage for superior long-term antifouling properties

Zwitterions bearing balanced charge groups have attracted increasing attention to fabricate antifouling membranes due to their highly hydrated structure. The simple and efficient method of covalent connection to enhance the stability of the zwitterions on membrane surfaces is still challenging. Herein, s branched polyethyleneimine (PEI) was employed to synthesize the zwitterion by a quaternary amination reaction with sodium chloroacetate. A novel zwitterionic surface with neutral surface charge was constructed by grafting the PEI-based zwitterion (Z-PEI) onto the hydrolyzed PAN (H-PAN) membrane surface. Covalent bonds were formed between the Z-PEI and H-PAN membrane while the electrostatic attraction would promote this reaction. The zwitterionic membranes exhibited superior antifouling ability (flux recovery ratio of about 99.8% and total flux decline of about 31.4%) due to the formation of a tight hydration layer by zwitterions on the membrane surface. Furthermore, the flux recovery ratio was not changed obviously during the long term experiment and could be maintained at as high as 96.3 and 98.4% after immersion in acid and alkali solution, respectively. These results demonstrated that the long term and chemical stability of zwitterionic functional surfaces were significantly enhanced via multisite covalent anchorage.

Fabrication and characterization of antifouling carbon nanotube/polyethersulfone ultrafiltration membranes

In this study, carbon nanotubes coated with poly(sulfobetaine methacrylate) (SBMA@CNT) particles were synthesized via a precipitation polymerization method. The SBMA@CNT particles were used as a novel kind of modifier to fabricate polyethersulfone (PES) ultrafiltration membranes by a non-solvent induced phase separation method (NIPS). During the membrane formation process, the surface segregation phenomenon of the SBMA@CNT particles was found, which was demonstrated by an energy dispersive spectrometer (EDS) mapping and X-ray photoelectron spectroscopy (XPS). This phenomenon was ascribed to the self-organization of hydrophilic SBMA@CNT particles spontaneously on the interface of membrane/water in the formation of membranes. In the ultrafiltration of a bovine serum albumin (BSA) feed solution, the best-performing membrane was found to effectively reduce protein adhesion. Its irreversible and reversible flux declines were remarkably decreased and the flux recovery was as high as 98.9%.

Precise nanopore tuning for a high-throughput desalination membrane via co-deposition of dopamine and multifunctional POSS

The demand for fractionating organic molecules and salt ions via a membrane is increasing in various industries, but the sub-nanoscale difference in solute size makes it a critical challenge. Herein, amino-functionalized polyhedral oligomeric silsesquioxane (POSS) nanoparticles were employed as a molecular-level regulator to manipulate the nanopores of a polydopamine (PDA) membrane via a facile co-deposition process. By physical intercalation and chemical bonding of multifunctional POSS, increased nanoporosity was achieved within a narrow mean nanopore size range of 1.04–1.07 nm. The optimized POSS-PDA/PAN membrane exhibited desirable dye rejection (>90%) and salt permeation (>90%) with a high permeance of 1099 L m−2 h−1 MPa−1, which was 2–4 times higher than that of previously reported membranes with a similar dye rejection. Additionally, the rigid cage-like POSS nanoparticles cross-linked with the PDA membrane contributed to the reinforced alkali and compaction resistance. The synergy of mussel-inspired chemistry and multifunctional nanomaterials provided a novel strategy to engineer nanoporous membranes with a favorable physiochemical nanostructure for efficient and selective transport of both molecules and ions.