High-flux robust ceramic membranes functionally decorated with nano-catalyst for emerging micro-pollutant removal from water

2D-Like Catalyst with a Micro-nanolinked Functional Surface for Water Purification

In water purification, the performance of heterogeneous advanced oxidation processes significantly relies upon the utilization of the catalyst's specific surface area (SSA). However, the presence of the structural "dead volume" and pore-size-induced diffusion-reaction trade-off limitation restricts the functioning of the SSA. Here, we reported an effective approach to make the best SSA by changing the traditional 3D spherule catalyst into a 2D-like form and creating an in situ micro-nanolinked structure. Thus, a 2D-like catalyst was obtained which was characterized by a mini "paddy field" surface, and it exhibited a sharply decreased dead volume, a highly available SSA and oriented flexibility. Given its paddy-field-like mass-transfer routine, the organic capture capability was 7.5-fold higher than that of the catalyst with mesopores only. Moreover, such a catalyst exhibited a record-high O3-to-·OH transition rate of 2.86 × 10-8 compared with reported millimetric catalysts (metal base), which contributed to a 6.12-fold higher total organic removal per catalyst mass than traditional 3D catalysts. The facile scale preparation, performance stability, and significant material savings with the 2D-like catalyst were also beneficial for practical applications. Our findings provide a unique and general approach for designing potential catalysts with excellent performance in water purification.

Membrane-catalysis integrated system for contaminants degradation and membrane fouling mitigation: A review

The integration of catalytic degradation and membrane separation processes not only enables continuous degradation of contaminants but also effectively alleviates inevitable membrane fouling, demonstrating fascinating practical value for efficient water purification. Such membrane-catalysis integrated system (MCIS) has attracted tremendous research interest from scientists in chemical engineering and environmental science recently. In this review, the advantages of MCIS are discussed, including the membrane structure regulation, stable catalyst loading, nano-confinement effect, and efficient natural organic matter (NOM) exclusion, highlighting the synergistic effect between membrane separation and catalytic process. Subsequently, the design considerations for the fabrication of catalytic membranes, including substrate membrane, catalytic material, and fabrication method, are comprehensively summarized. Afterward, the mechanisms and performance of MCIS based on different catalytic types, including liquid-phase oxidants/reductants involved MCIS, gas involved MCIS, photocatalysis involved MCIS, and electrocatalysis involved MCIS are reviewed in detail. Finally, the research direction and future perspectives of catalytic membranes for water purification are proposed. The current review provides an in-depth understanding of the design of catalytic membranes and facilitates their further development for practical applications in efficient water purification.

Fe(II)-Modulated Microporous Electrocatalytic Membranes for Organic Microcontaminant Oxidation and Fouling Control: Mechanisms of Regulating Electron Transport toward Enhanced Reactive Oxygen Species Activation

Regulation of the fast electron transport process for the generation and utilization of reactive oxygen species (ROS) by achieving fortified electron "nanofluidics" is effective for electrocatalytic oxidation of organic microcontaminants. However, limited available active sites and sluggish mass transfer impede oxidation efficiency. Herein, we fabricated a conductive electrocatalytic membrane decorated with hierarchical porous vertically aligned Fe(II)-modulated FeCo layered double hydroxide nanosheets (Fe(II)-FeCo LDHs) in an electro-Fenton system to maximize exposure of active sites and expedite mass transfer. The nanospaced interlayers of Fe(II)-FeCo LDHs within the microconfined porous structure formed by its vertical nanosheets highly boost the micro/nanofluidic distribution of target pollutants to active centers/species, achieving accelerated mass transferability. Aliovalent substitution by Fe(II) activates in-plane metallics to maximize the available active sites and makes each Fe(II)-FeCo LDH nanosheet a geometrical nanocarrier for constructing a fast electron "nanofluidic" to accelerate Fe(II) regeneration in Fe(III)/Fe(II) cycles. As a result, the Fe(II)-FeCo LDHs exhibited improved reactivity in catalyzing H2O2 to •OH and 1O2. Accordingly, the membrane exhibited a higher atrazine degradation kinetic (0.0441 min-1) and degradation rate (93.2%), which were 4.7 and 2.1 times more than those of the bare carbon nanotube membrane, respectively. Additionally, the enhanced hydrophilic and strongly oxidized reactivity synergistically mitigated the organic fouling occurring in the pores and surface of the membrane. These findings clarify the activation mechanism of ROS over an innovative electrocatalytic membrane reactor design for organic microcontaminant treatment.

A novel copper oxide/titanium membrane integrated with peroxymonosulfate activation for efficient phenolic pollutants degradation

Herein, a novel CuO catalyst functionalized Ti-based catalytic membrane (FCTM) was prepared via the regulated electro-deposition technique followed with low-temperature calcination. The morphology of CuO catalyst and oxygen vacancy (OV) content can be controlled by adjusting the preparation conditions, under optimal condition (400 °C, electrolyte as sulfuric acid), the fern-shaped CuO catalyst was formed and the OV content was up to its highest level. Under the optimal treatment condition, the 4-chlorophenol (4-CP) removal of the membrane filtration combined with peroxymonosulfate (PMS) activation (MFPA) process was up to 98.2% (TOC removal of 88.2%). Mechanism studying showed that the enhanced performance in this system was mainly due to the increased production of singlet oxygen (1O2) via the co-effect of fern-shaped CuO (increased specific surface area) and its fine-tuned OV (precursor of 1O2), which not only synergistically enhanced adsorption ability but also offered more active sites for PMS activation. Theoretical calculations showed that the OV-rich CuO displayed high adsorption energy for PMS molecule, leading to the change in OO and OH bond (tend to 1O2) of the PMS molecule. Finally, the possible three degradation pathways of 4-CP were formed by the electrophilic attacking of 1O2.

Robust ceramic-based graphene membrane for challenging water treatment with enhanced fouling and scaling resistance

Membrane fouling and scaling are two challenges for efficient treatment of hypersaline wastewater, greatly hindering separation performance and operation stability of desalination membranes. In this work, we report a smooth ceramic-based graphene desalination membrane, exhibiting enhanced anti-fouling and anti-scaling ability and operational performance for efficient treatment of both synthetic and real industrial wastewaters, outperforming polypropylene (PP) membrane. For treatment of hypersaline waters containing organic or inorganic substance, we demonstrate that the graphene membrane exhibits more stable water flux and almost complete salt rejection (>99.9%) during constant operation. Enhanced anti-fouling and desalination performance of graphene membrane could be attributed to the lower attractive interaction force with foulant (-4.65 mJ m-2), lower surface roughness (Ra = 2.2 ± 0.1 nm) and higher affinity with water than PP membrane. Furthermore, an anti-scaling mechanism enabled by graphene membrane is evidenced, with a highlight on the roles of smooth graphene surface with lower roughness, less nucleation sites and lower binding force with scaling crystals. Importantly, even for industrial petrochemical wastewater, such a graphene membrane also exhibits relatively more stable water flux and promising oil and ions rejection during long-term operation, outperforming PP membrane. This study further confirms a promising practical application potential of robust ceramic-based graphene membrane for efficient treatment of more challenging hypersaline wastewater with complicated compositions, which is not feasible by conventional desalination membranes.

Peroxymonosulfate activated by composite ceramic membrane for the removal of pharmaceuticals and personal care products (PPCPs) mixture: Insights of catalytic and noncatalytic oxidation

A composite manganese-based catalytic ceramic membrane (Mn-CCM) was developed by a solid-state sintering method, and its effectiveness toward activation of peroxymonosulfate (PMS) for the degradation of 11 pharmaceutical and personal care products (PPCPs) mixture was tested. The optimized Mn-CCMs/PMS system showed remarkable degradation efficiencies for PPCPs mixture with total removal >90% in ultrapure water, river water and natural organic matter (NOM) solution. The Mn-CCMs/PMS system showed the contribution of different phenomena in PPCPs removal in the order of catalytic oxidation (54.7%, Mn-CCMs/PMS) > noncatalytic oxidation (42.3%, PMS oxidation) > adsorption (3.0%, by Mn-CCMs). The singlet oxygen (¹O₂) was the dominant reactive oxygen specie for the degradation of PPCPs in all water matrices proved by the quenching experiments and electro-paramagnetic resonance (EPR) spectroscopy. The extraordinary stability of Mn-CCMs for the activation of PMS has been noted in terms of repeatability experiments for PPCPs degradation with fewer leaching of Mn (1.9 to 3.6 µg/L). Mineralization was achieved in the range of 28-65% for different water matrices. The toxicity of the PPCPs mixture was reduced by 85.9%. The Mn-CCMs/PMS system showed a reduction (25-100%) in precursors of different carbon- and nitrogen-based disinfection by-products. This study found the Mn-CCMs/PMS system as a feasible purification unit for removing trace concentrations of PPCPs (ng/L) in real drinking water matrices.

Progress in alumina ceramic membranes for water purification: Status and prospects

Ceramic membranes have gained increasing attention in recent years for the removal of various contaminants from water. Alumina membrane is considered as one of the most important ceramic membranes, which plays important roles not only in separation processes such as microfiltration, ultrafiltration, and nanofiltration, but also in catalysis- and adsorption- enhanced separation applications in water purification and wastewater treatment. However, there is currently still lack of a comprehensive critical review about alumina membranes for water purification. In this review, we first discuss recent developments of alumina membranes, and then critically introduce the state-of-the-art strategies for lowering fabrication cost, improving membrane performances and mitigating membrane fouling. Especially, aiming to improve membrane performance, some emerging methods are summarized such as tailoring membrane structure, developing flexible membranes, designing nano-pores for precise separation, and enhancing multi-functionalities. In addition, engineering applications of alumina membranes for water purification are also briefly introduced. Finally, the prospects for future research on alumina membranes are proposed, such as economic preparation/application, challenging precise separation, enriching multi-functionalities, and clarifying separation mechanisms.

Ceramic nanofiber membrane anchoring nanosized Mn2O3 catalytic ozonation of sulfamethoxazole in water

Catalytic ceramic nanofiber membranes (Mn@CNMs) were prepared by anchoring Mn₂O₃ nanoparticles on the pits of attapulgite (APT) nanofibers via an impregnation and in-situ precipitation method. An integrated catalytic ozonation/membrane filtration process applying Mn@CNM was employed to degrade sulfamethoxazole (SMX) and the removal achieved up to 81.3% during a 7-h continuous filtration. The reactive oxygen species (ROS) quenching and radical detection experiments were conducted to determine the contribution of ¹O₂, ·OH and O₂^·- towards the catalytic degradation of SMX. Moreover, Mn@CNM exhibited wide applicability for real water matrix and the total removal of various kinds of emerging contaminants in real hospital wastewater reached up to 98.5%. The excellent performances of Mn@CNM were attributed to the nano-confinement effect in the membrane layer. First, anchoring Mn₂O₃ nanoparticles on the pits of the APT surface suppressed the growth and aggregation of nanosized Mn₂O₃, providing abundant reactive sites for catalytic ozonation. Second, the interlaced APT nanofibers formed nano-sized network structures, where ROS and SMX were confined in close vicinity and ROS have more chances to attack SMX. This work provides a promising strategy for the preparation of catalytic ceramic membrane with high catalytic efficiency for degradation of emerging contaminants in water.

Cost and efficiency perspectives of ceramic membranes for water treatment

More robust ceramic membranes with tailorable structures and functions are increasingly employed for water treatment, particularly in some harsh applications for their ultra-long service lifespan due to their high mechanical, structural, chemical and thermal stability and anti-fouling properties. Decreasing cost and enhancing efficiency are two key but quite challenging application-oriented issues for broader and larger-scale engineering application of current ceramic membranes, and are required to make ceramic membranes a highly efficient and economic water treatment technique. In this review, we critically discuss these two significant concerns of both cost and efficiency for water treatment ceramic membranes, focusing on an overview of various advanced strategies and mechanism insights. A brief up-to-date discussion is first introduced about recent developments of ceramic membranes covering the major advances of novel membranes and applications. Then some promising strategies for decreasing the cost of ceramic membranes are discussed, including membrane material cost and processing cost. To fully address the issue of moderate efficiency with single separation function, valuable and considerable insights are provided into recent major progress and mechanism understandings in application with other unit processes, such as advanced oxidation and electrochemistry techniques, to significantly enhance treatment efficiency. Subsequently, a review of recent ceramic membrane applications emphasizing harsh operating environments is presented, such as oil-water separation, saline water, refractory organic and emerging contaminant wastewater treatment. Finally, engineering application, conclusions, and future perspectives of ceramic membrane for water treatment applications are critically discussed offering new insight based on understanding the issues of cost and efficiency.

Peracetic acid integrated catalytic ceramic membrane filtration for enhanced membrane fouling control: Performance evaluation and mechanism analysis

Endowing ceramic membrane (CM) catalytic reactivity can enhance membrane fouling control in the aid of in situ oxidation process. Peracetic acid (PAA) oxidant holds great prospect to integrate with CM for membrane fouling control, owing to the prominent advantages of high oxidation efficacy and easy activation. Herein, this study, for the first time, presented a PAA/CM catalytic filtration system achieving highly-efficient protein fouling alleviation. A FeOCl functionalized CM (FeOCl-CM) was synthesized, possessing high hydrophilicity, low surface roughness, and highly-efficient activation towards PAA oxidation. Using bovine serum albumin (BSA) as the model protein foulant, the PAA/FeOCl-CM catalytic filtration notably alleviated fouling occurring in both membrane pores and surface, and halved the flux reduction degree as compared with the conventional CM filtration. The PAA/FeOCl-CM catalytic oxidation allows quick and complete disintegration of BSA particles, via the breakage of the amide I and II bands and the ring opening of the aromatic amino acids (e.g., Tryptophan, Tyrosine). In-depth investigation revealed that the in situ generated ^•OH and ¹O₂ were the key reactive species towards BSA degradation during catalytic filtration, while the organic radical oxidation and the direct electron transfer pathway from BSA to PAA via FeOCl-CM played minor roles. Overall, our findings highlight a new PAA/CM catalytic filtration strategy for achieving highly-efficient membrane fouling control and provide an understanding of the integrated PAA catalytic oxidation - membrane filtration behaviors.

The nitrogen-doped multi-walled carbon nanotubes modified membrane activated peroxymonosulfate for enhanced degradation of organics and membrane fouling mitigation in natural waters treatment

The synthesized catalyst nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) were introduced into membrane technology for peroxymonosulfate (PMS) activation. The enhanced permeability of the N-MWCNTs-modified membrane might be attributed to the increase in hydrophilicity and membrane porosity. The catalytic degradation and membrane filtration performance for the N-MWCNTs-modified membrane/PMS system in treating different types of natural waters were evaluated. The removal of phenol by the N-MWCNTs-modified membrane was 83.67% in 2 min, which was greater than the phenol removal by the virgin membrane (3.39%) and N-MWCNT powder (41.42%), respectively. Moreover, the resultant membrane coupled with PMS activation exhibited outstanding removal effects on the fluorescent organics in the secondary effluent and Songhua River water. The combination effectively reduced the total membrane fouling caused by the secondary effluent, Songhua River water, and three typical model organics by 28.19-61.98%. Electron paramagnetic resonance and classical quenching tests presented that the active species (SO4·-, ·OH, and 1O2) and other non-radical processes generated by N-MWCNTs activated PMS decreased the foulants deposition on the membrane surface. Meanwhile, the membrane interception accelerated the aggregation of pollutants and PMS towards the membrane surface through applied pressure, facilitating their mass transfer to the N-MWCNTs surface for the catalysis exerted more effectively. This study demonstrated the potential application of the coupling of N-MWCNTs catalytic oxidation and the UF, which offers a promising prospect to improve the permeate quality and simultaneously overcome the membrane fouling barriers.

Accelerated organics degradation by peroxymonosulfate activated with biochar co-doped with nitrogen and sulfur

Engineered biochar is increasingly regarded as a cost-effective and eco-friendly peroxymonosulfate (PMS) activator. Herein, biochar doped with nitrogen and sulfur moieties was prepared by pyrolysis of wood shavings and doping precursor. The doping precursor consists of either urea, thiourea or 1:1 w/w mixture of urea and thiourea (denoted as NSB-U, NSB-T and NSB-UT, respectively). The physicochemical properties of the NSBs were extensively characterized, revealing that they are of noncrystalline carbon with porous structure. The NSBs were employed as PMS activator to degrade organic pollutants particularly methylene blue (MB). It was found that NSB-UT exhibited higher MB removal rate with k_app = 0.202 min^-1 due to its relatively high surface area and favorable intrinsic surface moieties (combination of graphitic N and thiophenic S). The effects of catalyst loading, PMS dosage and initial pH were evaluated. Positive enhancement of the MB removal rate can be obtained by carefully increasing the catalyst loading or PMS dosage. Meanwhile, the MB removal rate is greatly influenced by pH due to electrostatic interactions and pH dependent reactions. The NSB-UT can be reused for several cycles to some extent and its catalytic activity can be restored by thermal treatment. Based on the radical scavenger study and XPS analysis, the nonradical pathway facilitated by the graphitic N and thiophenic S active sites are revealed to be the dominant reaction pathway. Overall, the results of this study show that engineered biochar derived from locally available biowaste can be transformed into PMS activator for environmental applications.

Reversible filtration redox of methylene blue in dimethylsulfoxide by manganese oxide loaded carbonaceous nanofibrous membrane through Fenton-like oxidation

The reversible redox of methylene blue in organic solvents was highly attractive, yet was rarely reported. In this study, we realized the continuous filtration redox of methylene blue (MB) in dimethylsulfoxide (DMSO) through Fenton-like oxidization by using MnO₂ loaded carbonaceous nanofibrous membrane (cPAN-MnO₂). The carbonaceous nanofibrous membrane (cPAN) was fabricated through electrospun of polyacrylonitrile and subsequent carbonization. The obtained cPAN nanofibrous membrane showed excellent stability in polar DMSO. MnO₂ can be readily coated on cPAN nanofibers through an in situ redox reaction between cPAN and potassium permanganate. The fabricated cPAN-MnO₂ membrane exhibited instantaneous reduction property towards MB in DMSO during a gravity-driven continuous filtration process. Interestingly, MB reduction was initiated by a typical Fenton-like oxidization, where hydroxyl radicals were firstly generated from hydrogen peroxide catalyzed by MnO₂ in DMSO. Then hydroxyl radicals attacked DMSO to further produce methyl radicals, which resulted in the reduction of MB. In addition, MB reduction process in DMSO was reversible. Our study provides a novel strategy for continuous redox of MB in polar organic solvent and might give new ideas for MB applications.

Recent development of pressure retarded osmosis membranes for water and energy sustainability: A critical review

With the goal of zero-liquid discharge and green energy harvest, extraction of abundant green energy from saline water via pressure retarded osmosis (PRO) technology is a promising but challenging issue for water treatment technologies to achieve water and energy sustainability. Development of high performance PRO membranes has received increased concerns yet still under controversy in practical applications. In this review, a comprehensive and up-to-date discussion of some key historical developments is first introduced covering the major advances of PRO engineering applications and novel membranes especially made in recent years. Then the critical performance indicators of PRO membranes including water flux and power density are briefly discussed. Subsequently, sufficient discussion on four performance limiting factors in PRO membrane and process is presented including concentration polarization, reverse solute diffusion, membrane fouling and mechanical stability. To fully address these issues, an updated insight is provided into recent major progresses on advanced fabrication and modification techniques of novel PRO membranes featuring enhanced performance with different configurations and materials, which are also reviewed in detail based on the viewpoint of design rationales. Afterwards, antifouling strategies and engineering applications are critically introduced. Finally, conclusions and future perspective of PRO membrane for practical operation are briefly discussed.

Investigation of the interfacial behavior of organics on sulfide semiconductor surfaces by quantum chemical calculations and molecular dynamics simulations

The presence of organic pollutants in the world is harmful to our existence and the environment.

Flexible Superhydrophobic Metal-Based Carbon Nanotube Membrane for Electrochemically Enhanced Water Treatment

Treatment of highly saline wastewaters via conventional technology is a key challenging issue, which calls for efficient desalination membranes featuring high flux and rejection, low fouling, and excellent stability. Herein, we report a high-strength and flexible electro-conductive stainless steel-carbon nanotube (SS-CNT) membrane, exhibiting significantly enhanced anticorrosion and antifouling ability via a microelectrical field-coupling strategy during membrane distillation. The membrane substrates exhibited excellent mechanical strength (244.2 ± 9.8 MPa) and ductility, thereby overcoming the critical bottleneck of brittleness of traditional inorganic membranes. By employing a simple surface activation followed by self-catalyzed chemical vapor deposition, CNT was grown in situ on SS substrates via a tip-growth mechanism to finally form robust superhydrophobic SS-CNT membrane. To address the challenging issues of significant corrosion and fouling, using a negative polarization microelectrical field-coupling strategy, simultaneously enhanced antifouling and anticorrosion performance was realized for treatment of organic high salinity waters while exhibiting stable high flux and rejection via an electrostatic repulsion and electron supply mechanism. This application-oriented rational design protocol can be potentially used to extend toward high performance composite membranes derived from other electro-conductive metal substrates functionally decorated with CNT network and to other applications in water treatment.