Xueting Zhao

Antifouling membranes for sustainable water purification: strategies and mechanisms

One of the greatest challenges to the sustainability of modern society is an inadequate supply of clean water. Due to its energy-saving and cost-effective features, membrane technology has become an indispensable platform technology for water purification, including seawater and brackish water desalination as well as municipal or industrial wastewater treatment. However, membrane fouling, which arises from the nonspecific interaction between membrane surface and foulants, significantly impedes the efficient application of membrane technology. Preparing antifouling membranes is a fundamental strategy to deal with pervasive fouling problems from a variety of foulants. In recent years, major advancements have been made in membrane preparation techniques and in elucidating the antifouling mechanisms of membrane processes, including ultrafiltration, nanofiltration, reverse osmosis and forward osmosis. This review will first introduce the major foulants and the principal mechanisms of membrane fouling, and then highlight the development, current status and future prospects of antifouling membranes, including antifouling strategies, preparation techniques and practical applications. In particular, the strategies and mechanisms for antifouling membranes, including passive fouling resistance and fouling release, active off-surface and on-surface strategies, will be proposed and discussed extensively.

Efficient Wastewater Treatment by Membranes through Constructing Tunable Antifouling Membrane Surfaces

In the present study, a facile in situ approach for constructing tunable amphiphilic or hydrophilic antifouling membrane surfaces was demonstrated by exquisitely manipulating the microphase separation and surface segregation behavior of the tailor-made ternary amphiphilic block copolymers during the commonly utilized wet phase inversion membrane-formation process. Under dead-end filtration for oily wastewater treatment, the membrane with amphiphilic surface exhibited over 99.5% retention ratio of chemical oxygen demand (COD) without appreciable membrane fouling: the water permeation flux was slightly decreased during operation (total flux decline was 6.8%) and almost completely recovered to the initial value (flux recovery ratio was more than 99.0%) after simple hydraulic washing. While for the proteins-containing wastewater treatment, the membrane with hydrophilic surface exhibited about 52.6% COD retention ratio and superior antifouling performance: only 17.0% total flux decline and also more than 99.0% flux recovery ratio. Hopefully, the present approach can be developed as a competitive platform technology for the preparation of robust and versatile antifouling membrane, leading to the high process efficiency of wastewater treatments.

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.

Free-Standing Graphene Oxide-Palygorskite Nanohybrid Membrane for Oil/Water Separation.

Graphene oxide (GO) is an emerging kind of building block for advanced membranes with tunable passageway for water molecules. To synergistically manipulate the channel and surface structures/properties of GO-based membranes, the different building blocks are combined and the specific interfacial interactions are designed in this study. With vacuum-assisted filtration self-assembly, palygorskite nanorods are intercalated into adjacent GO nanosheets, and GO nanosheets are assembled into laminate structures through π-π stacking and cation cross-linking. The palygorskite nanorods in the free-standing GOP nanohybrid membranes take a 3-fold role, rendering enlarged mass transfer channels, elevating hydration capacity, and creating hierarchical nanostructures of membrane surfaces. Accordingly, the permeate fluxes from 267 L/(m(2) h) for GO membrane to 1867 L/(m(2) h) for GOP membrane. The hydration capacity and hierarchical nanostructures synergistically endow GOP membranes with underwater superoleophobic and low oil-adhesive water/membrane interfaces. Moreover, by rationally imparting chemical and physical joint defense mechanisms, the GOP membranes exhibit outstanding separation performance and antifouling properties for various oil-in-water emulsion systems (with different concentration, pH, or oil species). The high water permeability, high separation efficiency, as well as superior anti-oil-fouling properties of GOP membranes enlighten the great prospects of graphene-based nanostructured materials in water purification and wastewater treatment.

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.

Improved antifouling property of PVDF membranes by incorporating an amphiphilic block-like copolymer for oil/water emulsion separation

An amphiphilic block-like copolymer bearing hydrophobic poly(butyl methacrylate) (PBMA), hydrophilic poly(poly(ethylene glycol) methyl ether methacrylate) (PEGMA) and low surface energy poly(hexafluorobutyl methacrylate) (PHFBM) segments was synthesized by free radical polymerization. The copolymer was then used as an additive to fabricate antifouling polyvinylidene difluoride (PVDF) membranes by the non-solvent induced phase separation (NIPS) method. During the membrane preparation process, the low surface energy PHFBM segments were dragged to the membrane surfaces by the surface segregated hydrophilic PEGMA segments. The presence of the PHFBM segments on the membrane surfaces significantly enhanced the antifouling property of the PVDF membranes during oil/water emulsion filtration. The total flux decline (Rt) was drastically decreased to 10.6% and the flux recovery ratio (FRR) was 99.4%. In addition, the influences of different operating conditions (including agitating speed, operating pressure and oil concentration) on the antifouling property were extensively investigated. The fluxes were nearly completely recovered after simple hydraulic cleaning even under a low agitating speed, high operating pressure and high oil concentration.

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.

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.