Defective MOFs-based electrocatalytic self-cleaning membrane for wastewater reclamation: Enhanced antibiotics removal, membrane fouling control and mechanisms

Sustainable and ultrafast antibiotics removal, self-cleaning and disinfection with electroactive metal-organic frameworks/carbon nanotubes membrane

Although conventional nanofiltration (NF) membrane is widely applied in water treatment, it faces the challenges of insufficient selectivity toward emerging contaminants, low permeability and non-sustainable fouling control. Herein, a novel electroactive metal-organic frameworks/carbon nanotubes membrane was constructed by facile and green nanobubbles-mediated non-solvent-induced phase separation (NIPS) strategy for ultrafast antibiotics removal. It presented 3-fold to 100-fold higher permeability (101.3-105.7 L·h-1·m-2·bar-1) without compromising rejection (71.8 %-99.3 %) of common antibiotics (tetracycline, norfloxacin, sulfamethoxazole, sulfamethazine) than most commercial and state-of-the-art NF membranes. The separation mechanism was due to the synergy of loose selective layer with three-dimensional interconnected networks and UiO-66/CNTs with unique pore sieving and charge property. It also presented excellent antibiotics selectivity with high NaCl/tetracycline separation factor of 194 and CuCl2/tetracycline separation factor of 316 for remediation of antibiotics and heavy metal combined pollution. Meanwhile, it possessed efficient anti-fouling, antibacterial and electro-driven self-cleaning ability, which enabled sustainable fouling control and disinfection with short process, low energy and chemical consumption. Furthermore, potential application of UiO-66/CNTs membrane in wastewater reclamation was demonstrated by stable antibiotics rejection, efficient flux recovery and long-term stability over 260 h. This study would provide useful insights into removal of emerging contaminants from water by advanced NF membrane.

Assembling 99% MOFs into Bioinspired Rigid-Flexible Coupled Membrane with Significant Permeability: The Impacts of Defects

Assembling metal-organic frameworks (MOFs) into high-performance macroscopic membranes is crucial but still challenging. MOF-containing hybrid membranes can effectively integrate the advantages of flexible guest materials and MOFs. Nevertheless, the inherent limitations in fully harnessing the distinct characteristics of MOFs persist due to the substantial guest material content necessitated in membrane fabrication. Herein, inspired by the rigid and flexible structures in biological systems, rigid MIP-202(Zr) and defective MIP-202(Zr) (D-MIP-202(Zr)) modified flexible graphene oxide (GO) sheets are synthesized in situ and then assembled into a rigid-flexible coupled MOF-based membrane. The defects in D-MIP-202(Zr) are introduced by using acetic acid as the modulation agent. The obtained GO@MIP-202(Zr) membrane possesses a hierarchical porous structure with a 99 wt% MOF proportion, which is higher than the GO@D-MIP-202(Zr) (75 wt%) membrane with a compact bulge-structured surface. The water permeability of the GO@MIP-202(Zr) membrane attains remarkedly 5762.92 L h-1 m-2 bar-1 , which is 960 and 2.6 times higher than that of the GO membrane and GO@D-MIP-202(Zr) membrane. Additionally, benefiting from the superhydrophilicity and underwater superoleophobicity, the resultant membrane not only demonstrates high rejection for oil-water emulsions but also exhibits exceptional recyclability and anti-fouling ability. These findings provide valuable insights into the assembly of MOFs into high-performance membranes.

Contact-Electro-Catalysis-Assisted Separation via a Dancing PTFE Membrane for Fouling Control

Advanced oxidization processes (AOPs) offer promising solutions for addressing the fouling issues in membrane separation systems. However, the high energy requirements for electrical or light power in the AOPs can be a drawback. In this study, we present a contact-electro-catalysis (CEC)-based approach for controlling membrane fouling, which is stimulated by mild ultrasonic irradiation. During this process, electrons are transferred between a dancing polytetrafluoroethylene membrane and water or oxygen molecules, resulting in the formation of free radicals •OH and •O2-. These free radicals are capable of degrading or inactivating foulants, eliminating the need for additional chemical cleaners, secondary waste disposal, or external stimuli. Furthermore, the time-dependent voltage spikes/oscillations (peak, +7.8/-8.2 V) generate a nonuniform electric field that drives dielectrophoresis, effectively keeping contaminants away from the membrane surface and further enhancing the antifouling performance of the dancing membrane. Therefore, the CEC-assisted membrane separation system offers a green and effective strategy for controlling membrane fouling through mild mechanical stimulation.

Multifunctional Roles of Dihydrotetrazine-Decorated Zr-MOFs in Photoluminescence and Colorimetrism for Discrimination of Arsenate and Phosphate Ions in Water

The high chemical and structural stabilities of zirconium (Zr)-based metal-organic frameworks (MOFs) in aquatic media make them ideal candidates for wastewater treatment. Rational decoration or Zr-MOFs with functional groups can significantly extend their application in this area. In this work, two well-known Zr-MOFs, UiO-66 and MIL-140-A, were functionalized with dihydrotetrazine function to increase their capability in water treatment. Investigations reveal that these two dihydrotetrazine (DHTZ)-functionalized MOFs, namely UiO-66(Zr)-DHTZ and MIL-140(Zr)-DHTZ, can be applied as a two-component array for highly selective and sensitive discrimination of arsenate (AsO43-) and phosphate (PO43-) ions in water in the presence of other anions. Photoluminescence (PL) tests using UiO-66(Zr)-DHTZ show that this MOF can detect these two anions via a ratiometric response, 1.74 for arsenate and 1.84 for phosphate at 2 μM, with superior detection limits (7.2 × 10-8 M for AsO43- and 4.3 × 10-8 M for PO43-). The ratiometric PL response of UiO-66(Zr)-DHTZ toward arsenate and phosphate anions arises possibly from the arsenate-dihydrotetrazine hydrogen bonding. In the next step, colorimetric tests using MIL-140(Zr)-DHTZ were conducted to discriminate the arsenate from phosphate with a very low detection limit at nanomolar level. This MOF undergoes a yellow-to-pink color change in the presence of arsenate ions, while no color change is observed in the presence of phosphate. This color change is observed through conversion of dihydrotetrazine sites inside the pores of MIL-140(Zr)-DHTZ into tetrazine. Altogether, the PL response of UiO-66(Zr)-DHTZ is originated from the hydrogen bond-donating/accepting character of DHTZ function, while the colorimetric response of MIL-140(Zr)-DHTZ is based on the chemical conversion of DHTZ function. This work clearly shows that the decoration of Zr-based MOFs with multicharacter functional groups can develop their application in wastewater treatment as multipurpose platforms.

Fundamental Perspectives on the Electrochemical Water Applications of Metal-Organic Frameworks

HIGHLIGHTS

The recent development and implementation of metal-organic frameworks (MOFs) and MOF-based materials in electrochemical water applications are reviewed. The critical factors that affect the performances of MOFs in the electrochemical reactions, sensing, and separations are highlighted. Advanced tools, such as pair distribution function analysis, are playing critical roles in unraveling the functioning mechanisms, including local structures and nanoconfined interactions. Metal-organic frameworks (MOFs), a family of highly porous materials possessing huge surface areas and feasible chemical tunability, are emerging as critical functional materials to solve the growing challenges associated with energy-water systems, such as water scarcity issues. In this contribution, the roles of MOFs are highlighted in electrochemical-based water applications (i.e., reactions, sensing, and separations), where MOF-based functional materials exhibit outstanding performances in detecting/removing pollutants, recovering resources, and harvesting energies from different water sources. Compared with the pristine MOFs, the efficiency and/or selectivity can be further enhanced via rational structural modulation of MOFs (e.g., partial metal substitution) or integration of MOFs with other functional materials (e.g., metal clusters and reduced graphene oxide). Several key factors/properties that affect the performances of MOF-based materials are also reviewed, including electronic structures, nanoconfined effects, stability, conductivity, and atomic structures. The advancement in the fundamental understanding of these key factors is expected to shed light on the functioning mechanisms of MOFs (e.g., charge transfer pathways and guest-host interactions), which will subsequently accelerate the integration of precisely designed MOFs into electrochemical architectures to achieve highly effective water remediation with optimized selectivity and long-term stability.

Polymer-flooding produced water treatment using an electro-hybrid ozonation-coagulation system with novel cathode membranes targeting alternating filtration and in situ self-cleaning

Polymer-flooding produced water is more difficult to treat for reinjection compared with normal produced water because of the presence of residual hydrolyzed polyacrylamide (HPAM). A novel cathode membrane integrated electro-hybrid ozonation-coagulation (CM-E-HOC) process was proposed for the treatment of polymer-flooding produced water. This process achieved in situ self-cleaning by generated microbubbles in the cathode membrane. The CM-E-HOC process achieved a higher suspended solid (SS), turbidity and PAM removal efficiency than the CM-EC process. The SS in the CM-E-HOC effluent was ≤ 20 mg/L SS, which met the reinjection requirements of Longdong, Changqing Oilfield, China (Q/SYCQ 08,011-2019) at different current densities (3, 5 and 10 mA/cm²). The CM-E-HOC process greatly mitigated both reversible and irreversible membrane fouling. Therefore, excellent flux recovery was obtained at different in situ self-cleaning intervals during the CM-E-HOC process. Furthermore, alternating filtration achieved continuous water production during the CM-E-HOC process. On one hand, the effective removal of aromatic protein-like substances and an increase in oxygen-containing functional groups were achieved due to the enhanced oxidation ability of the CM-E-HOC process, which decreased membrane fouling. On the other hand, the CM-E-HOC process showed improved coagulation performance because of the increased oxygen-containing functional groups and polymeric Fe species. Therefore, larger flocs with higher fractal dimensions were generated, and a looser and more porous cake layer was formed on the membrane surface during the CM-E-HOC process. Consequently, the CM-E-HOC process exhibited better in situ self-cleaning performance and lower filtration resistance than the CM-EC process.

Reduction of Ultrafiltration Membrane Fouling by the Pretreatment Removal of Emerging Pollutants: A Review

Ultrafiltration (UF) processes exhibit high removal efficiencies for suspended solids and organic macromolecules, while UF membrane fouling is the biggest obstacle affecting the wide application of UF technology. To solve this problem, various pretreatment measures, including coagulation, adsorption, and advanced oxidation, for application prior to UF processes have been proposed and applied in actual water treatment processes. Previously, researchers mainly focused on the contribution of natural macromolecular pollutants to UF membrane fouling, while the mechanisms of the influence of emerging pollutants (EPs) in UF processes (such as antibiotics, microplastics, antibiotic resistance genes, etc.) on membrane fouling still need to be determined. This review introduces the removal efficiency and separation mechanism for EPs for pretreatments combined with UF membrane separation technology and evaluates the degree of membrane fouling based on the UF membrane's materials/pores and the structural characteristics of the cake layer. This paper shows that the current membrane separation process should be actively developed with the aim of overcoming specific problems in order to meet the technical requirements for the efficient separation of EPs.