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

Architecting highly hydratable and permeable dense Janus membrane for rapid and robust membrane distillation desalination

Dense Janus membranes (JMs) are potential candidates in hypersaline wastewater treatments for membrane distillation (MD). However, dense surface layers generally add obvious membrane mass transfer resistance, limiting its practical application. In this study, a novel dense JM was facilely developed by controlled interfacial polymerization utilizing a phosphonium functional monomer (THPC) on hydrophilic polyvinylidene fluoride (PVDF) substrate. MD desalination performance results showed that the optimized THPC/PVDF JM surprisingly achieved a flux even 21.3 % higher than that of its substrate and exhibited robust stability to surfactants, oil, and gypsum. Importantly, potential mechanism of remarkable flux enhancement of dense JMs was revealed. Raman and DSC analyses showed that THPC/PVDF JM with elevated intermediate water content had minimal enthalpy for water evaporation, thus facilitating water vaporization. Dynamic water contact angle measurements, advanced microstructural characterization by positron annihilation spectroscopy and water permeability experiments etc. showed that THPC/PVDF JM with high hydrophilicity and appropriately enlarged effective pore sizes provided strong water-absorption capacity and water permeability, contributing to rapid water replenishment. Both synergistically contributed to an incredible increase in membrane flux. Meanwhile, the dense surface layer, featured by its strong hydrophilicity and abundant hydroxyl groups, effectively prevented complex contaminants from intruding hydrophobic substrate and discouraged oil/gypsum adhesion, ensuring stable MD operation. This study shall provide useful insights and strategies to design high-flux dense JMs with minimal membrane failure propensity, highlighting its great potential for efficient hypersaline wastewater treatments.

Improved in-situ characterization for the scaling-induced wetting in membrane distillation: Unraveling the role of crystalline morphology

Despite being recognized as a promising technique for treating high salinity water, membrane distillation (MD) has been plagued by the scaling of sparingly soluble salts. The growth of crystals can not only create additional resistance to evaporating water at the feed-membrane interface, but also alter the hydrophobic network to bridge the feed and distillate (i.e., result in the phenomenon of wetting). When recognizing the uncertain behaviors of calcium sulfate (CaSO4) scaling in MD, this study was motivated to ascertain whether the crystal-membrane interactions could be dependent on the variation in crystalline morphology. In particular, optical coherence tomography (OCT) was employed to characterize the scaling-induced wetting via a direct-observation-through-the-membrane (DOTM) mode, which mitigated the effects of developing an external scaling layer on resolving the crystal-membrane interactions. The improved in-situ characterization suggests that the crystalline morphology of CaSO4 could be effectively regulated by varying the stoichiometry of crystallizing ions; the richness of calcium in the aqueous environment for crystallization would be in favor of weakening the crystal-membrane interactions. The stoichiometry-dependent growth of CaSO4 crystals can be exploited to develop an effective strategy for preventing the hydrophobic network from being wetted or irreversibly damaged.

Unveiling the spatiotemporal dynamics of membrane fouling: A focused review on dynamic fouling characterization techniques and future perspectives

Membrane technology has emerged as a crucial method for obtaining clean water from unconventional sources in the face of water scarcity. It finds wide applications in wastewater treatment, advanced treatment, and desalination of seawater and brackish water. However, membrane fouling poses a huge challenge that limits the development of membrane-based water treatment technologies. Characterizing the dynamics of membrane fouling is crucial for understanding its development, mechanisms, and effective mitigation. Instrumental techniques that enable in situ or real-time characterization of the dynamics of membrane fouling provide insights into the temporal and spatial evolution of fouling, which play a crucial role in understanding the fouling mechanism and the formulation of membrane control strategies. This review consolidates existing knowledge about the principal advanced instrumental analysis technologies employed to characterize the dynamics of membrane fouling, in terms of membrane structure, morphology, and intermolecular forces. Working principles, applications, and limitations of each technique are discussed, enabling researchers to select appropriate methods for their specific studies. Furthermore, prospects for the future development of dynamic characterization techniques for membrane fouling are discussed, underscoring the need for continued research and innovation in this field to overcome the challenges posed by membrane fouling.

Mineral scaling induced membrane wetting in membrane distillation for water treatment: Fundamental mechanism and mitigation strategies

The scaling-induced wetting phenomenon seriously affects the application of membrane distillation (MD) technology in hypersaline wastewater treatment. Unlike the large amount of researches on membrane scaling and membrane wetting, scaling-induced wetting is not sufficiently studied. In this work, the current research evolvement of scaling-induced wetting in MD was systematically summarized. Firstly, the theories involving scaling-induced wetting were discussed, including evaluation of scaling potential of specific solutions, classical and non-classical crystal nucleation and growth theories, observation and evolution of scaling-induced processes. Secondly, the primary pretreatment methods for alleviating scaling-induced wetting were discussed in detail, focusing on adding agents composed of coagulation, precipitation, oxidation, adsorption and scale inhibitors, filtration including granular filtration, membrane filtration and mesh filtration and application of external fields including sound, light, heat, electromagnetism, magnetism and aeration. Then, the roles of operation conditions and cleaning conditions in alleviating scaling-induced wetting were evaluated. The main operation parameters included temperature, flow rate, pressure, ultrasound, vibration and aeration, while different types of cleaning reagents, cleaning frequency and a series of assisted cleaning measures were summarized. Finally, the challenges and future needs in the application of nucleation theory to scaling-induced wetting, the speculation, monitoring and mitigation of scaling-induced wetting were proposed.

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.

Treatment of Aniline Wastewater by Membrane Distillation and Crystallization

Aniline is a highly toxic organic pollutant with "carcinogenic, teratogenic and mutagenesis" characteristics. In the present paper, a membrane distillation and crystallization (MDCr) process was proposed to achieve zero liquid discharge (ZLD) of aniline wastewater. Hydrophobic polyvinylidene fluoride (PVDF) membranes were used in the membrane distillation (MD) process. The effects of the feed solution temperature and flow rate on the MD performance were investigated. The results showed that the flux of the MD process was up to 20 L·m^-2·h^-1 and the salt rejection was above 99% under the feeding condition of 60 °C and 500 mL/min. The effect of Fenton oxidation pretreatment on the removal rate of aniline in aniline wastewater was also investigated, and the possibility of realizing the ZLD of aniline wastewater in the MDCr process was verified.

Early monitoring of pore wetting in membrane distillation using ultrasonic time-domain reflectometry (UTDR)

Pore wetting induced by surfactants and salt scaling is a major obstacle to the industrial application of membrane distillation (MD). Identifying the transition of wetting stages and achieving early monitoring of pore wetting are crucial for wetting control. Herein, we made a pioneering attempt to use ultrasonic time-domain reflectometry (UTDR) technique to non-invasively detect the pore wetting in a direct contact MD, and explain the UTDR waveform with the help of optical coherence tomography (OCT) imaging. The results showed that the water-vapor interface had a strong reflection to ultrasound (reflection coefficient = 0.9995), while the water-membrane and water-scaling layer interfaces showed relatively weak reflection. Therefore, UTDR could effectively detect the movement of water-vapor interface with the low interference from the signals generated by the membrane and scaling layer. For the surfactant-induced wetting, the occurrence of wetting could be successfully detected by the right-shift in phase and the reduction in amplitude of the UTDR waveform. Moreover, the wetting depth could be accurately calculated by the time of flight (ToF) and ultrasonic velocity. For scaling-induced wetting, the waveform slightly shifted to the left at the beginning due to the growth of scaling layer, then to the right because the left-shift was surpassed by the right-shift of the waveform caused by pore wetting. Both for the surfactant- and scaling-induced wetting, the variation of the UTDR waveform was sensitive to wetting dynamics, and the right-shift of phase and the reduction in amplitude of the waveform could act as early monitoring signals to the occurrence of wetting.

Coaxial Electrospun Nanofibrous Membranes for Enhanced Water Recovery by Direct Contact Membrane Distillation

Membrane distillation (MD) is an emerging technology for water recovery from hypersaline wastewater. Membrane scaling and wetting are the drawbacks that prevent the widespread implementation of the MD process. In this study, coaxially electrospun polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) nanofibrous membranes were fabricated with re-entrant architecture and enhanced hydrophobicity/omniphobicity. The multiscale roughness was constructed by incorporating Al2O3 nanoparticles and 1H, 1H, 2H, 2H Perfluorodecyltriethoxysilane in the sheath solution. High resolution transmission electron microscopy (HR-TEM) could confirm the formation of the core-sheath nanofibrous membranes, which exhibited a water contact angle of ~142.5° and enhanced surface roughness. The membrane displayed a stable vapor flux of 12 L.m−2.h−1 (LMH) for a 7.0 wt.% NaCl feed solution and no loss in permeate quality or quantity. Long-term water recovery from 10.5 wt.% NaCl feed solution was determined to be 8−10 LMH with >99.9% NaCl rejection for up to 5 cycles of operation (60 h). The membranes exhibited excellent resistance to wetting even above the critical micelle concentration (CMC) for surfactants in the order sodium dodecyl sulphate (SDS) (16 mM) > cetyltrimethylammonium bromide (CTAB) (1.5 mM) > Tween 80 (0.10 mM). The presence of salts further deteriorated membrane performance for SDS (12 mM) and Tween-80 (0.05 mM). These coaxial electrospun nanofibrous membranes are robust and can be explored for long-term applications.

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.