Anhydrous volatile fatty acid extraction through omniphobic membranes by hydrophobic deep eutectic solvents: Mechanistic understanding and future perspective.

Volatile fatty acids (VFAs) derived from arrested anaerobic digestion (AD) can be recovered as a valuable commodity for value-added synthesis. However, separating VFAs from digestate with complex constituents and a high-water content is an energy-prohibitive process. This study developed an innovative technology to overcome this barrier by integrating deep eutectic solvents (DESs) with an omniphobic membrane into a membrane contactor for efficient extraction of anhydrous VFAs with low energy consumption. A kinetic model was developed to elucidate the mechanistic differences between this novel omniphobic membrane-enabled DES extraction and the previous hydrophobic membrane-enabled NaOH extraction. Experimental results and mechanistic modeling suggested that VFA extraction by the DES is a reversible adsorption process facilitating subsequent VFA separation via anhydrous distillation. High vapor pressure of shorter-chain VFAs and low Nernst distribution coefficients of longer-chain VFAs contributed to DES-driven extraction, which could enable continuous and in-situ recovery and conversion of VFAs from AD streams.

Control the hydrophilic layer thickness of Janus membranes by manipulating membrane wetting in membrane distillation.

Janus membranes with asymmetric wettability have attracted wide attentions for their robust anti-oil-wetting/fouling abilities in membrane distillation (MD). Compared to traditional surface modification approaches, in this study, we provided a new approach which manipulated surfactant-induced wetting to fabricate Janus membrane with a controllable thickness of the hydrophilic layer. The membranes with 10, 20, and 40 µm of wetted layers were obtained by stopping the wetting induced by 40 mg L-1 Triton X-100 (J = 25 L m-2 h-1) at about 15, 40, and 120 s, respectively. Then, the wetted layers were coated using polydopamine (PDA) to fabricate the Janus membranes. The resulting Janus membranes showed no significant change in porosities or pore size distributions compared with the virgin PVDF membrane. These Janus membranes exhibited low in-air water contact angles (< 50°), high underwater oil contact angles (> 145°), and low adhesion with oil droplets. Therefore, they all showed excellent oil-water separation performance with ∼100% rejection and stable flux. The Janus membranes showed no significant decline in flux, but a trade-off existed between the hydrophilic layer thicknesses and the vapor flux. Utilizing membranes with tunable hydrophilic layer thickness, we elucidated the underlying mechanism of such trade-off in mass transfer. Furthermore, the successful modification of membranes with different coatings and in-situ immobilization of silver nanoparticles indicated that this facile modification method is universal and can be further expanded for multifunctional membrane fabrication.

Unique Behaviors and Mechanism of Highly Soluble Salt-Induced Wetting in Membrane Distillation.

Scaling-induced wettinggreatly limits the application of membrane distillation (MD) for the desalination of high-salinity feed. Although highly soluble salts (e.g., NaCl) have high concentrations in this water, their scaling-induced wetting remains overlooked. To unravel the elusive wetting behaviors of highly soluble salts, in this study, we systematically investigated the scaling formation and wetting progress by in situ observation with optical coherence tomography (OCT). Through examining the influence of salt type and vapor flux on the wetting behavior, we revealed that highly soluble salt-induced wetting, especially under high vapor flux, shared several unique features: (1) occurring before the bulk feed reached saturation, (2) no scale layer formation observed, and (3) synchronized wetting progress on the millimeter scale. We demonstrated that a moving scale layer caused these interesting phenomena. The initial high vapor flux induced high concentration and temperature polarizations, which led to crystallization at the gas-liquid interface and the formation of an initial scale layer. On the one hand, this scale layer bridged the water into the hydrophobic pores; on the other hand, it blocked the membrane pores and reduced the vapor flux. In this way, the decreased vapor flux mitigated the concentration/temperature polarizations, and consequently led to the dissolution of the feed-facing side of the scale layer. This dissolution prevented the membrane pores from being completely blocked, facilitating the transportation and crystallization of salts at the distillate-facing side of the scale layer (i.e., the gas-liquid interface), thus the proceeding of the wetting layer.

Unraveling the Kinetics and Mechanism of Surfactant-Induced Wetting in Membrane Distillation: An In Situ Observation with Optical Coherence Tomography.

In this study, we performed a direct contact membrane distillation and successfully demonstrated the non-invasive imaging of surfactant-induced wetting using optical coherence tomography. This method enabled us to investigate the wetting kinetics, which was found to follow a "three-region" relationship between the wetting rate and surfactant concentration: the (i) nonwetted region, (ii) concentration-dependent region, and (iii) concentration-independent region at low, intermediate, and high surfactant concentrations, respectively. This wetting behavior was explained by the "autophilic effect", i.e., the wetting was caused by the transfer of surfactants from the water-vapor interface to the unwetted membrane and rendered this membrane hydrophilic, and then the wetting frontier moved forward under capillary forces. At region-(i), the surfactant concentration in the water-vapor interface (Clv) was too low to make the unwetted membrane sufficiently hydrophilic; thereby, the membrane could not be wetted. At region-(ii), due to the fast adsorption of the surfactant on the newly wetted membrane, the wetting rate was determined by the advection/diffusion of surfactants from the feed stream. Consequently, the wetting rate increased with the increases in the water flux and surfactant concentration. At region-(iii), the advection/diffusion provided excess surfactants for adsorption, and thus Clv reached its upper limit (maximum surface excess) and the wetting rate leveled off.

Functional catalytic membrane development: A review of catalyst coating techniques.

Catalytic membranes combine catalytic activity with conventional filtration membranes, thus enabling diverse attractive benefits into the conventional membrane filtration processes, such as easy catalyst reuse, antifouling, anti-microbial, and enhancing process efficiency. Up to date, tremendous progresses have been made on functional catalytic membrane preparation and applications, which significantly advances the competitiveness of membrane technologies in process industries. The present article provides a critical and holistic overview of the current state of knowledge on existing catalyst coating techniques for functional catalytic membrane development. Based on coating mechanisms, the techniques are generally categorized into physical and chemical surface coating routes. For each technique, we first introduce fundamental principle, followed by a critical discussion of their applications with representative case studies. Advantages and drawbacks are also emphasized for different surface coating technologies. Finally, future perspectives are highlighted to provide deep insights into their future developments.

Engineering Interface with a One-Dimensional RuO2/TiO2 Heteronanostructure in an Electrocatalytic Membrane Electrode: Toward Highly Efficient Micropollutant Decomposition.

Decomposition of micropollutants using an electrocatalytic membrane reactor is a promising alternative to traditional advanced oxidation processes due to its high efficiency and environmental compatibility. Rational interface design of electrocatalysts in the membrane electrode is critical to the performance of the reactor. We herein developed a three-dimensional porous membrane electrode via in situ growth of one-dimensional RuO2/TiO2 heterojunction nanorods on a carbon nanofiber membrane by a facile hydrothermal and subsequent thermal treatment approach. The membrane electrode was used as the anode in a gravity-driven electrocatalytic membrane reactor, exhibiting a high degradation efficiency of over 98% toward bisphenol-A and sulfadiazine. The superior electrocatalytic performance was attributed to the 1D RuO2/TiO2 heterointerfacial structure, which provided the fast electron transfer, high generation rate of the hydroxyl radical, and large effective surface area. Our work paves a novel way for the fundamental understanding and designing of novel highly effective and low-consumptive electrocatalytic membranes for wastewater treatment.

Control of organic and surfactant fouling using dynamic membranes in the separation of oil-in-water emulsions.

HYPOTHESIS: A superhydrophilic membrane with rough and hierarchical structures is possibly fouled by surfactant-stabilized oil and organic foulants, because these foulants could not be hindered by the water layer formed on superhydrophilic membrane surface. A dynamic membrane was possibly an effective method to address this fouling problem. EXPERIMENTS: A microfiltration membrane, a nanofiber membrane, and a dynamic membrane were used for the separation of surfactant-free emulsions, surfactant-stabilized emulsions, and the surfactant-stabilized emulsions containing typical organic foulants. The oil rejection and membrane fouling were compared. FINDINGS: The microfiltration membrane, nanofiber membrane, and dynamic membrane had high resistances to the fouling by surfactant-free emulsions because these membranes were underwater superoleophobic. However, these membranes showed low resistances to the fouling by surfactant-stabilized oil droplets and organic foulants. For the dynamic membrane, the oil droplets and organic foulants trapped in the separation layer could be readily removed in the detachment-washing-recoating steps; therefore, almost no physically irreversible fouling was observed in the multi-cycle filtration. With the size distributions of oil droplets in the emulsions and the particle of the dynamic membrane, the rejection of oil by the dynamic membrane could be calculated by simply assuming that the particle was spherical, uniform, and tightly packed.

Omniphobic Nanofibrous Membrane with Pine-Needle-Like Hierarchical Nanostructures: Toward Enhanced Performance for Membrane Distillation.

Wetting and fouling phenomena are the main concerns for membrane distillation (MD) in treating high-salinity industrial wastewater. This work developed an omniphobic membrane by growing titanium dioxide (TiO2) nanorods on polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers using a hydrothermal technique. The TiO2 nanorods form a uniform pine-needle-like hierarchical nanostructure on PVDF-HFP fibers. A further fluorination treatment provides the membrane with a low-surface-energy omniphobic surface, displaying contact angles of 168° and 153° for water and mineral oil, respectively. Direct contact MD experiments demonstrated that the resulting membrane shows a high and stable salt rejection of >99.9%, while the pristine PVDF-HFP nanofibrous membrane suffers a rejection decline caused by intense pore wetting and oil fouling in the desalination process in the presence of surfactant and mineral oil. The superior antiwetting and antifouling behaviors were ascribed to a nonwetting Cassie-Baxter state established by the accumulation of a great deal of air in the hydrophobized hierarchical re-entrant structures. The development of omniphobic membranes with pine-needle-like hierarchical nanostructures provides an approach to mitigate membrane wetting and fouling in the MD process for the water reclamation from industrial wastewater.

One-step tailoring surface roughness and surface chemistry to prepare superhydrophobic polyvinylidene fluoride (PVDF) membranes for enhanced membrane distillation performances.

Superhydrophobic polyvinylidene fluoride (PVDF) membrane is a promising material for membrane distillation. Existing approaches for preparing superhydrophobic PVDF membrane often involve separate manipulation of surface roughness and surface chemistry. Here we report a one-step approach to simultaneously manipulate both the surface roughness and surface chemistry of PVDF nanofibrous membranes for enhanced direct-contact membrane distillation (DCMD) performances. The manipulation was realized in a unique solvent-thermal treatment process, during which a treatment solution containing alcohols was involved. We demonstrate that by using different chain-length alcohols in the treatment solvent, surface roughness can be promoted by creating nanofin structures on the PVDF nanofibers using an alcohol which has moderate affinity with PVDF. Meanwhile, surface chemistry can be tuned by adjusting the fraction distribution of crystal phases (nonpolar α phase and polar ß phase) in the membrane using different alcohols. PVDF membranes with different surface wettabilities were used to evaluate the effects of surface roughness and surface energy on the DCMD performances. Combining both low surface energy and multi-scale surface roughness, pentanol-treated PVDF membrane achieved best anti-water property (water contact angle of 164.1° and sliding angle of 8.1°), and exhibited superior water flux and enhanced anti-wetting ability to low-surface-tension feed in the DCMD application.

Fast polydopamine coating on reverse osmosis membrane: Process investigation and membrane performance study.

We report a novel membrane surface modification method using a fast polydopamine coating (fPDAc) strategy. Specifically, NaIO4 was introduced in the coating process to accelerate the polydopamine deposition rate. Surface properties and separation performances of fPDAc-coated reverse osmosis membranes were characterized and compared to those obtained using the conventional slow polydopamine coating (sPDAc) strategy. Quartz crystal microbalance measurements showed greatly increased polydopamine deposition rate using the fPDAc method, resulting in a reduction of 97% coating time to reach an areal mass of 2000 ng/cm2. Both fPDAc and sPDAc enhanced the surface hydrophilicity and reduced the membrane surface charge. At relatively low areal mass deposition (<1000 ng/cm2), fPDAc-coated membranes showed improved NaCl rejection together with only mild loss of pure water flux. Nevertheless, this rejection enhancement effect was not noticeable when extensive polydopamine coating was applied due to the undesirable cake-enhanced concentration polarization effect. The extensive polydopamine coating was further accompanied with severe loss of membrane permeability, suggesting that shorter coating time (e.g., 4 min) is preferred using the fPDAc method. Our study provides a more rapid and effective membrane surface coating method compared to the conventional sPDAc method.

Factors and mechanisms that influence the reactivity of trivalent copper: A novel oxidant for selective degradation of antibiotics.

Trivalent copper complexes are active intermediates in aquatic redox reactions that involve copper ions or structural copper, but their reactivity and selectivity toward pollutants remain unknown. We characterized copper(III) periodate, a representative trivalent copper compound, with phenol and several antibiotics as model contaminants. The results show that Cu(III) is highly reactive to phenol degradation; near-complete degradation was achieved after 10 min at a molar ratio of 3:1 (Cu[III]: phenol). Common alcohols, including methanol and 2-propanol, showed pH-dependent reactivity for Cu(III). In contrast to aquo trivalent copper ions that react rapidly with tert-butanol, Cu(III) showed limited reactivity toward tert-butanol. A mechanistic investigation showed that the degradation was caused by direct oxidation by Cu(III) and that no hydroxyl radicals were involved. Common water components such as chloride ions did not influence the reaction, which suggests that the use of Cu(III) may help mitigate the generation of chlorinated products. As a one-electron oxidant, Cu(III) showed high reactivity to degrade electron-rich compounds; the concentrations of sulfamethazine, sulfamethoxazole, and sulfadiazine (100 µg/L) were reduced to 1.8, 7.5, and 42.5 ng/L, respectively, after 2 min of reaction with 10 µM Cu(III). These results demonstrate a novel and highly efficient oxidant for selective removal of ubiquitous micropollutants from water bodies.

Influences of Air, Oxygen, Nitrogen, and Carbon Dioxide Nanobubbles on Seed Germination and Plant Growth.

Nanobubbles (NBs) hold promise in green and sustainable engineering applications in diverse fields (e.g., water/wastewater treatment, food processing, medical applications, and agriculture). This study investigated the effects of four types of NBs on seed germination and plant growth. Air, oxygen, nitrogen, and carbon dioxide NBs were generated and dispersed in tap water. Different plants, including lettuce, carrot, fava bean, and tomato, were used in germination and growth tests. The seeds in water-containing NBs exhibited 6-25% higher germination rates. Especially, nitrogen NBs exhibited considerable effects in the seed germination, whereas air and carbon dioxide NBs did not significantly promote germination. The growth of stem length and diameter, leave number, and leave width were promoted by NBs (except air). Furthermore, the promotion effect was primarily ascribed to the generation of exogenous reactive oxygen species by NBs and higher efficiency of nutrient fixation or utilization.

Lipase immobilized catalytically active membrane for synthesis of lauryl stearate in a pervaporation membrane reactor.

A composite catalytically active membrane immobilized with Candida rugosa lipase has been prepared by immersion phase inversion technique for enzymatic synthesis of lauryl stearate in a pervaporation membrane reactor. SEM images showed that a "sandwich-like" membrane structure with a porous lipase-PVA catalytic layer uniformly coated on a polyvinyl alcohol (PVA)/polyethersulfone (PES) bilayer was obtained. Optimum conditions for lipase immobilization in the catalytic layer were determined. The membrane was proved to exhibit superior thermal stability, pH stability and reusability than free lipase under similar conditions. In the case of pervaporation coupled synthesis of lauryl stearate, benefited from in-situ water removal by the membrane, a conversion enhancement of approximately 40% was achieved in comparison to the equilibrium conversion obtained in batch reactors. In addition to conversion enhancement, it was also found that excess water removal by the catalytically active membrane appears to improve activity of the lipase immobilized.