Electrocatalytic reduction of nitrate (NO3−) to ammonia (NH3) is garnering increasing interest due to its potential to reduce CO2 emissions as a substitute for the Haber-Bosch process, while also mitigating NO3− pollution. However, it remains a challenge to achieve a current density exceeding 300 mA cm−2 while maintaining the stability of catalysts. Additionally, the anodic oxygen evolution reaction, characterized by slow kinetics and high energy barriers, severely impedes the widespread adoption of NH3 formation from NO3− reduction. Therefore, in this study, we introduce amorphous phosphorus-doped cobalt catalysts (Co–P@NF) prepared via a facile electrodeposition process for efficient NO3− reduction and hydrazine oxidation. The incorporation of phosphorus in Co–P@NF facilitates electron migration from phosphorus to cobalt, enhancing *H provision for efficient hydrogenation of the intermediate *NO2−. This results in a current density of 2 A cm−2 at −0.3 V, with a faradaic efficiency for NH3 of 91% in an electrolyte containing 1 M NO3−. Moreover, the Co–P@NF catalyst exhibits remarkable long-term stability, maintaining an NH3 faradaic efficiency exceeding 90% and a current density of 799 mA cm−2 after 82 hours of electrolysis. Furthermore, Co–P@NF displays high catalytic activity in promoting the rate-determining step of hydrazine oxidation, from *N2H2 to *N2H. The incorporation of the HzOR (hydrazine oxidation reaction)-assisted NO3−RR (nitrate reduction reaction) unit significantly reduces the cell voltage to 0.34 V at 300 mA cm−2.@en

Membrane technology presents an effective solution for treating oily wastewater, a significant environmental hazard stemming from industries such as food processing, metalworking, and oil extraction. Compared to polymeric membranes, ceramic ones exhibit superior mechanical, chemical, and thermal stability, enabling more effective oil removal and easier cleaning. Despite their advantages, membrane fouling remains a challenge, impacting the efficiency of oily wastewater treatment. This review explores oily wastewater characteristics and ceramic membrane applications in treatment processes. It examines the factors influencing ceramic membrane fouling, including wastewater properties (e.g., oil concentration, pH), membrane characteristics (e.g., surface hydrophilicity, charge), and operational parameters (e.g., cross-flow velocity, permeate flux). Strategies to mitigate fouling, such as pretreatment, backpulsing/backwashing for sustained operation, and chemical cleaning for fouling removal, are discussed. By using pretreatment, membrane fouling can be reduced. Backpulsing/backwashing is effective to maintain a long-term operation. Chemical cleaning is effective in removing irreversible fouling and restoring the performance of the ceramic membranes. Moreover, membrane modification techniques that enhance performance are highlighted. Ultimately, the review identifies that effective fouling control is crucial for optimizing ceramic membrane use in oily wastewater treatment, underscoring the need for ongoing research in this area.

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Atomic layer deposition (ALD) is known for its unparalleled control over layer thickness and 3D conformality and could be the future technique of choice to tailor the pore size of ceramic nanofiltration membranes. However, a major challenge in tuning and functionalizing a multichannel ceramic membrane is posed by its large internal pore volume, which needs to be evacuated during ALD cycling. This may require significant energy and processing time. This study presents a new reactor design, operating at atmospheric pressure, that is able to deposit thin layers in the pores of ceramic membranes. In this design, the reactor wall is formed by the industrial tubular ceramic membrane itself, and carrier gas flows are employed to transport the precursor and co-reactant vapors to the reactive surface groups present on the membrane surface. The layer growth for atmospheric-pressure ALD in this case proceeds similarly to that for state-of-the-art vacuum-based ALD. Moreover, for membrane preparation, this new reactor design has three advantages: (i) monolayers are deposited only at the outer pore mouths rather than in the entire bulk of the porous membrane substrate, resulting in reduced flow resistances for liquid permeation; (ii) an in-line gas permeation method was developed to follow the layer growth in the pores during the deposition process, allowing more precise control over the finished membrane; and (iii) expensive vacuum components and cleanroom environment are eliminated. This opens up a new avenue for ceramic membrane development with nano-scale precision using ALD at atmospheric pressure.

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Sodium hypochlorite (NaClO) is widely used for the chemical cleaning of fouled ultrafiltration (UF) membranes. Various studies performed on polymeric membranes demonstrate that long-term (>100 h) exposure to NaClO deteriorates the physicochemical properties of the membranes, leading to reduced performance and service life. However, the effect of NaClO cleaning on ceramic membranes, particularly the number of cleaning cycles they can undergo to alleviate irreversible fouling, remains poorly understood. Silicon carbide (SiC) membranes have garnered widespread attention for water and wastewater treatment, but their chemical stability in NaClO has not been studied. Low-pressure chemical vapor deposition (LP-CVD) provides a simple and economical route to prepare/modify ceramic membranes. As such, LP-CVD facilitates the preparation of SiC membranes: (a) in a single step; and (b) at much lower temperatures (700–900 °C) in comparison with sol-gel methods (ca. 2000 °C). In this work, SiC ultrafiltration (UF) membranes were prepared via LP-CVD at two different deposition temperatures and pressures. Subsequently, their chemical stability in NaClO was investigated over 200 h of aging. Afterward, the properties and performance of as-prepared SiC UF membranes were evaluated before and after aging to determine the optimal deposition conditions. Our results indicate that the SiC UF membrane prepared via LP-CVD at 860 °C and 100 mTorr exhibited excellent resistance to NaClO aging, while the membrane prepared at 750 °C and 600 mTorr significantly deteriorated. These findings not only highlight a novel preparation route for SiC membranes in a single step via LP-CVD, but also provide new insights about the careful selection of LP-CVD conditions for SiC membranes to ensure their long-term performance and robustness under harsh chemical cleaning conditions.@en

Ceramic membranes have been increasingly employed in water treatment owing to their merits such as high-stability, anti-oxidation, long lifespan and environmental friendliness. The application of ceramic membranes mainly focuses on microfiltration and ultrafiltration processes, and some precise separation can be achieved by introducing novel porous materials with superior selectivity. Recently, metal–organic frameworks (MOFs) have developed a wide spectrum of applications in the fields of the environment, energy, water treatment and gas separation due to the diversity and tunable advantages of metal clusters and organic ligands. Although the issue of water stability in MOF materials inhibits the development of MOF membranes in water treatment, researchers still overcome many obstacles to advance the application of MOF membranes in water treatment processes. To the best of our knowledge, there is still a lack of a reviews on the development process and prospects of ceramic-based MOF membranes for water treatment. Therefore, in this review, we mainly summarize the fabrication method for ceramic-based MOF membranes and their application in water treatment, such as water/salt separation, pollutant separation, heavy metal separation, etc. Following this, based on the high structural, thermal and chemical stability of ceramic substrates, and the high controllability of MOF materials, the superiority and insufficient use of ceramic-based MOF membranes in the field of water treatment are critically discussed.@en

Water scarcity, population growth, and climate change are causing a shortage of water resources globally. Industries are turning to the reclamation and reuse of wastewater, including oily wastewater, which is a major byproduct of oil and gas extraction. The small droplet size of oil-in-water emulsions, however, makes them difficult to remove using traditional methods like coagulation and flocculation, gravitational settling, dissolved air flotation, hydrocyclone, and adsorption. Membrane separation has emerged as one of the most promising techniques to deal with oil-in-water emulsions due to its high removal efficiency and small footprint. The main challenge for the wider adoption of membrane technology for oily wastewater treatment is membrane fouling. Membrane fouling is a pervasive problem in water purification membranes. It could cause serious negative effects, such as a decline in water production, higher operational pressure and associated higher energy consumption. Ceramic membranes, particularly SiC membranes, are a promising method for removing small oil droplets from water. They are physically and chemically stable and have high fouling resistance to oil droplets. SiC membranes have better permeability and lower fouling tendency compared to other ceramic membranes. However, their high cost limits their widespread application in the market. In this research, extensive literature reviews were first performed (Chapters 2 and 3) and then we proposed a new method, low-pressure chemical vapor deposition (LPCVD), to prepare SiC-deposited ceramic membranes for oily wastewater treatment. With LPCVD, a layer of SiC was deposited on alumina supports at a lower temperature (750 ˚C), compared to 2000 ˚C for commercial SiC preparations. Due to the low water contact angle (< 5˚) and negatively charged surface, these SiC-deposited alumina ceramic membranes are expected to be more fouling resistant to oil emulsions than the pristine alumina membranes. The performance of deposited membranes is influenced not only by the coated SiC layer but also by the filtration modes used for evaluation. As a result, we respectively used constant pressure and constant flux filtration to assess the fouling of ceramic membranes with and without SiC deposition. Additionally, the emulsion chemistry, such as surfactant concentration, pH, salinity, and Ca2+, plays a crucial role in the interactions between oil droplets and the membrane surface, which can cause membrane fouling. Understanding these mechanisms can be a crucial step towards the feasibility of using LPCVD to prepare SiC membranes for treating oily wastewater with lower fouling. First, novel SiC-deposited ceramic membranes were developed by LPCVD at a relatively low temperature (750 ˚C) (Chapter 4). Different deposition times varying from 0 to 150 min were used to tune membrane pore size. The pure water permeance of the membranes only decreased from 350 L m-2 h-1 bar-1 to 157 L m-2 h-1 bar-1 when the deposition time was increased from 0 to 120 min. Correspondingly, the membrane pore size was narrowed down from 71 to 47 nm. Increasing the deposition time from 120 to 150 min mainly resulted in the formation of a thin, dense layer on top of the support instead of in the pores. Notably, the SiC layer rendered the pristine membrane surface more hydrophilic and negatively charged, effectively reducing membrane fouling during oil emulsion filtration. Next, the fouling of SiC-deposited ceramic membranes and the pristine alumina membrane was respectively compared at constant pressure and constant flux filtration conditions (Chapter 5). The threshold flux of the membranes was first determined by flux-stepping experiments. Afterwards, membrane filtration was respectively conducted at below and above the threshold flux. In single cycle constant flux filtration experiment, the fouling tendency of the membranes was consistent with the results of threshold flux experiments. However, the inclusion of backwash in constant flux experiments led to a change in the fouling tendency, which was also dependent on the permeate flux. The improved surface hydrophilicity and charge made backwash more efficient for the modified membranes while extensive modification has a negative effect on membrane fouling resistance due to the huge loss in membrane permeance. In contrast, constant transmembrane pressure experiments showed that the order of membrane fouling was only related to membrane permeance, and no effect of surface properties was observed. Therefore, constant flux filtration experiments with backwash are recommended to be applied to evaluate the performance of the membranes with and without modification. Finally, the impact of emulsion chemistry and operational parameters on the fouling of alumina membranes with and without a SiC deposition was systematically studied under constant flux filtration mode with backwash (Chapter 6). The results showed that the SiC-deposited membrane had a lower reversible and irreversible fouling when permeate flux was below 110 Lm-2h-1. In addition, a higher permeance recovery after physical and chemical cleaning was observed, as compared to the alumina membranes. The fouling of both membranes was decreased with the increase of sodium dodecyl sulphate (SDS) concentration in the feed, but to a higher extent in the alumina membranes. Increasing the pH of the emulsion could reduce the fouling of both membranes due to the enhanced electrostatic repulsion between oil droplets and membrane surface. Under high salinity conditions (100 mM NaCl), the screening of surface charge resulted in only a small difference in irreversible fouling between the alumina and SiC-deposited membranes. The presence of Ca2+ in the emulsion led to high irreversible fouling of both membranes, because of the compression of diffusion double layer and the interactions between Ca2+ and SDS. The low fouling tendency and/or high cleaning efficiency of the SiC-deposited membranes indicated their potential for oily wastewater treatment. Overall, this dissertation shows that the fouling of SiC-deposited ceramic membranes is lower than that of the pristine alumina membranes towards oil-in-water emulsion treatment. Although there are still limitations, these SiC-deposited membranes show the potential for further development. @en

Ceramic membranes have drawn increasing attention in oily wastewater treatment as an alternative to their traditional polymeric counterparts, yet persistent membrane fouling is still one of the largest challenges. Particularly, little is known about ceramic membrane fouling by oil-in-water (O/W) emulsions in constant flux filtration modes. In this study, the effects of emulsion chemistry (surfactant concentration, pH, salinity and Ca²⁺) and operation parameters (permeate flux and filtration time) were comparatively evaluated for alumina and silicon carbide (SiC) deposited ceramic membranes, with different physicochemical surface properties. The original membranes were made of 100% alumina, while the same membranes were also deposited with a SiC layer to change the surface charge and hydrophilicity. The SiC-deposited membrane showed a lower reversible and irreversible fouling when permeate flux was below 110 L m⁻² h⁻¹. In addition, it exhibited a higher permeance recovery after physical and chemical cleaning, as compared to the alumina membranes. Increasing sodium dodecyl sulfate (SDS) concentration in the feed decreased the fouling of both membranes, but to a higher extent in the alumina membranes. The fouling of both membranes could be reduced with increasing the pH of the emulsion due to the enhanced electrostatic repulsion between oil droplets and membrane surface. Because of the screening of surface charge in a high salinity solution (100 mM NaCl), only a small difference in irreversible fouling was observed for alumina and SiC-deposited membranes under these conditions. The presence of Ca²⁺ in the emulsion led to high irreversible fouling of both membranes, because of the compression of diffusion double layer and the interactions between Ca²⁺ and SDS. The low fouling tendency and/or high cleaning efficiency of the SiC-deposited membranes indicated their potential for oily wastewater treatment.

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Membrane filtration is considered to be one of the most promising methods for oily wastewater treatment. Because of their hydrophilic surface, ceramic membranes show less fouling compared with their polymeric counterparts. Membrane fouling, however, is an inevitable phenomenon in the filtration process, leading to higher energy consumption and a shorter lifetime of the membrane. It is therefore important to improve the fouling resistance of the ceramic membranes in oily wastewater treatment. In this review, we first focus on the various methods used for ceramic membrane modification, aiming for application in oily wastewater. Then, the performance of the modified ceramic membranes is discussed and compared. We found that, besides the traditional sol-gel and dip-coating methods, atomic layer deposition is promising for ceramic membrane modification in terms of the control of layer thickness, and pore size tuning. Enhanced surface hydrophilicity and surface charge are two of the most used strategies to improve the performance of ceramic membranes for oily wastewater treatment. Nano-sized metal oxides such as TiO2, ZrO2 and Fe2O3 and graphene oxide are considered to be the potential candidates for ceramic membrane modification for flux enhancement and fouling alleviation. The passive antifouling ceramic membranes, e.g., photocatalytic and electrified ceramic membranes, have shown some potential in fouling control, oil rejection and flux enhancement, but have their limitations.@en

Silicon carbide (SiC) ceramic membranes are of particular significance for wastewater treatment due to their mechanical strength, chemical stability, and antifouling ability. Currently, the membranes are prepared by SiC-particle sintering at a high temperature. The production suffers from long production time and high costs. In this paper, we demonstrated a more economical way to produce SiC ultrafiltration membranes based on low-pressure chemical vapor deposition (LPCVD). SiC was deposited in the pores of alumina microfiltration supports using two precursors (SiH₂Cl₂ and C₂H₂/H₂) at a relatively low temperature of 750 °C. Different deposition times varying from 0 to 150 min were used to tune membrane pore size. The pure water permeance of the membranes only decreased from 350 Lm⁻²h⁻¹bar⁻¹ to 157 Lm⁻²h⁻¹bar⁻¹ when the deposition time was increased from 0 to 120 min due to the narrowing of membrane pore size from 71 to 47 nm. Increasing the deposition time from 120 to 150 min mainly resulted in the formation of a thin, dense layer on top of the support instead of in the pores. Oil-in-water emulsion filtration experiments illustrated that both the reversible and irreversible fouling of the SiC-deposited UF membrane was considerably lower as compared to the pristine alumina support. The unique feature that pore sizes decrease linearly as a function of SiC deposition time creates opportunities to produce low-fouling SiC membranes with tuned pore sizes on relatively cheap support.

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Highly efficient and economic treatment of wastewater sludges and wastewaters in one way is a challenging issue in the water treatment field. Herein we present a waste-to-resource strategy for rational fabrication of low–cost ceramic membranes, which simultaneously addresses the treatment of heavy metal-laden sludges and the separation of oil-in-water (O/W) emulsions. A thermal conversion mechanism is proposed for complicated reactions between simulated nickel-laden wastewater sludge and bauxite mineral. In addition to full stabilization and recycling of heavy metal wastewater sludges, rational tailoring of ceramic membrane structures can also be realized to achieve high water flux and favorable mechanical and surface properties. With rational structure design, the tailored spinel-based ceramic membranes exhibited high rejection and high flux (7473 LMH·bar⁻¹) simultaneously for separation of oily wastewater, outperforming other reported state-of-the-art ceramic membranes. The membrane fouling mechanism revealed the dominance of cake layer formation at low cross flow velocities, while a combined model of cake layer formation and pore blocking dominated membrane fouling at high cross-flow velocities. The proposed strategy can be potentially extended toward design of functional ceramic membranes derived from other heavy metal wastewater sludges and for other water treatment applications.

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