Effects of different secondary nano-scaled roughness on the properties of omniphobic membranes for brine treatment using membrane distillation

Insights into the silica scaling behaviors in membrane distillation and anti-scaling mechanism of functional polymers

Silica scaling imposes a significant limitation on the efficacy of membrane distillation (MD) in the treatment of hypersaline wastewater. The complex dynamic behaviors of silica at the membrane-water-air interface and the poor understanding of molecular-level anti-scaling mechanism hampers the development of effective antiscalants for mitigating silica scaling in MD. Despite using functional polymers to prevent silica polymerization, the inhibition mechanisms are unclear. Here, the kinetic process of silica scaling during MD and the potential anti-scaling mechanism of poly-ethylenimine (PEI) were investigated at the molecular level via molecular dynamics simulations. The investigation reveals that silica scales were more likely to adhere to the water-PTFE interface with a free energy potential well of -40.0 kJ mol-1 than that of the water-air interface with a -11.4 kJ mol-1 potential well. Silica scales falling at the water-air interface also migrated on the water-air interface until captured by the PTFE membrane. In this work, a representative functional amino-rich polymer PEI was constructed as silica inhibitors and its scale inhibition mechanism was elucidated. Notably, the inclusion of PEI increased the free-energy barriers for the silica polymerization reaction from 72.0 kJ mol-1 to 86.1 kJ mol-1, compared to scenarios without the antiscalants. Moreover, quantitative structure-activity relationships (QSAR) model of ΔGwater-silica was developed to predict the anti-scaling efficiencies of typical antiscalants based on machine learning method. These findings provide valuable insights into enhancing the efficiency of silica scaling mitigation strategies.

Constructing spherical-beads-on-string structure of electrospun membrane to achieve high vapor flux in membrane distillation

Hydrophobic membranes with a reentrant-like structure have shown high hydrophobicity and high anti-wetting properties in membrane distillation (MD). Here, PVDF spherical-beads-on-string (SBS) fibers were electrospun on nonwoven fabric and used in the MD process. Such a reentrant-like structure was featured with fine fibers, a low ratio of bead length to bead diameter, and high bead frequency. It was revealed that the SBS-structured membranes exhibited an exceptional capability for vapor flux, due to the formation of a network of more interconnected macropores than that of fibers and fusiform-beads-on-string structures, ensuring unimpeded vapor diffusion. In the desalination of formulated seawater (3.5 wt.% NaCl solution), a vapor flux of 61 ± 3 kg m-2 h-1 with a salt rejection of >99.98 % was achieved at a feed temperature of 60 °C. Furthermore, this SBS structured membrane showed satisfactory seawater desalination performance with a stable flux of 40 kg m-2 h-1 over a 27 h MD process. These findings suggest a viable approach for fabricating SBS-structured membranes that significantly enhance vapor flux in MD for desalination applications. Besides, the hydrophobic membranes with SBS structure can be prepared by single-step electrospinning, and it is facile to scale-up manufacture. This strategy holds promise for advancing the development of high-performance MD membranes tailored for efficient seawater desalination processes.

Applications of electrically conductive membranes in water treatment via membrane distillation: Joule heating, membrane fouling/scaling/wetting mitigation and monitoring

Membrane distillation (MD) is a thermally driven separation process that is driven by phase change. The core of this technology is the hydrophobic microporous membrane that prevents mass transfer of the liquid while allowing the vapor phase to pass through the membrane's pores. Currently, MD is challenged by its high energy consumption and membrane degradation due to fouling, scaling and wetting. The use of electrically conductive membranes (ECMs) is a promising alternative method to overcome these challenges by inducing localized Joule heating, as well as mitigating and monitoring membrane fouling/scaling/wetting. The objective of this review is to consolidate recent advances in ECMs from the standpoint of conductive materials, membrane fabrication methodologies, and applications in MD processes. First, the mechanisms of ECMs-based MD processes are reviewed. Then the current trends in conductive materials and membrane fabrication methods are discussed. Thereafter, a comprehensive review of ECMs in MD applications is presented in terms of the different processes using Joule heating and various works related to membrane fouling, scaling, and wetting control and monitoring. Key insights in terms of energy consumption, economic viability and scalability are furnished to provide readers with a holistic perspective of the ECMs potential to achieve better performances and higher efficiencies in MD. Finally, we illustrate our perspectives on the innovative methods to address current challenges and provide insights for advancing new ECMs designs. Overall, this review sums up the current status of ECMs, looking at the wide range of conductive materials and array of fabrication methods used thus far, and putting into perspective strategies to deliver a more competitive ECMs-based MD process in water treatment.

Enhanced Anti-Wetting Methods of Hydrophobic Membrane for Membrane Distillation

Increasing issues of hydrophobic membrane wetting occur in the membrane distillation (MD) process, stimulating the research on enhanced anti-wetting methods for membrane materials. In recent years, surface structural construction (i.e., constructing reentrant-like structures), surface chemical modification (i.e., coating organofluorides), and their combination have significantly improved the anti-wetting properties of the hydrophobic membranes. Besides, these methods change the MD performance (i.e., increased/decreased vapor flux and increased salt rejection). This review first introduces the characterization parameters of wettability and the fundamental principles of membrane surface wetting. Then it summarizes the enhanced anti-wetting methods, the related principles, and most importantly, the anti-wetting properties of the resultant membranes. Next, the MD performance of hydrophobic membranes prepared by different enhanced anti-wetting methods is discussed in desalinating different feeds. Finally, facile and reproducible strategies are aspired for the robust MD membrane in the future.

The enhancement of dye filtration performance and antifouling properties in amino-functionalized bentonite/polyvinylidene fluoride mixed matrix membranes

Trade-off issue and membrane fouling remain two major issues in the utilization of membrane technology for the water treatment due to reduced membrane permeability and lifetime. In our study, we employed 3-aminopropyltriethoxysilane modified bentonite (BNTAPS) as an anti-fouling modifier to prepare polyvinylidene fluoride (PVDF)-based membranes via the phase inversion method. The effects of BNTAPS concentration on the physical, mechanical, morphological, and filtration performance of the hybrid membranes have been investigated. It was found that the addition of BNTAPS improved the hydrophilicity of the membrane revealed by the decreased water contact angle. Consequently, the pure water flux of PVDF membrane containing 0.5% BNTAPS (PVDF/BNTAPS0.5%) increased to 35.5 L m^-2 h^-1. Moreover, the PVDF/BNTAPS membrane showed a smaller pore diameter and porosity compared to pristine PVDF. The membrane performance evaluation was carried out using cationic and anionic dyes, i.e., methylene blue (MB) and acid yellow (AY17), respectively. Our study revealed that the rejection of each dye was slightly increased for the PVDF/BNTAPS0.5%. However, the flux recovery rate of the PVDF/BNTAPS membrane significantly improved, which directly prolonged the membrane lifetime.

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.

Which Surface Is More Scaling Resistant? A Closer Look at Nucleation Theories for Heterogeneous Gypsum Nucleation in Aqueous Solutions

Developing engineered surfaces with scaling resistance is an effective means to inhibit surface-mediated mineral scaling in various industries including desalination. However, contrasting results have been reported on the relationship between scaling potential and surface hydrophilicity. In this study, we combine a theoretical analysis with experimental investigation to clarify the effect of surface wetting property on heterogeneous gypsum (CaSO₄·2H₂O) formation on surfaces immersed in aqueous solutions. Theoretical prediction derived from classical nucleation theory (CNT) indicates that an increase of surface hydrophobicity reduces scaling potential, which contrasts our experimental results that more hydrophilic surfaces are less prone to gypsum scaling. We further consider the possibility of nonclassical pathway of gypsum nucleation, which proceeds by the aggregation of precursor clusters of CaSO₄. Accordingly, we investigate the affinity of CaSO₄ to substrate surfaces of varied wetting properties via calculating the total free energy of interaction, with the results perfectly predicting experimental observations of surface scaling propensity. This indicates that the interactions between precursor clusters of CaSO₄ and substrate surfaces might play an important role in regulating heterogeneous gypsum formation. Our findings provide evidence that CNT might not be applicable to describing gypsum scaling in aqueous solutions. The fundamental insights we reveal on gypsum scaling mechanisms have the potential to guide rational design of scaling-resistant engineered surfaces.

Review: Brine Solution: Current Status, Future Management and Technology Development

Desalination brine is extremely concentrated saline water; it contains various salts, nutrients, heavy metals, organic contaminants, and microbial contaminants. Conventional disposal of desalination brine has negative impacts on natural and marine ecosystems that increase the levels of toxicity and salinity. These issues demand the development of brine management technologies that can lead to zero liquid discharge. Brine management can be productive by adopting economically feasible methodologies, which enables the recovery of valuable resources like freshwater, minerals, and energy. This review focuses on the recent advances in brine management using various membrane/thermal-based technologies and their applicability in water, mineral, and energy recoveries, considering their pros and cons. This review also exemplifies the hybrid processes for metal recovery and zero liquid discharge that may be adopted, so far, as an appropriate futuristic strategy. The data analyzed and outlook presented in this review could definitely contribute to the development of economically achievable future strategies for sustainable brine management.

Emerging advances and current applications of nanoMOF-based membranes for water treatment

Metal-organic frameworks (MOFs) are significantly tunable materials that can be exploited in a wide range of applications. In recent years, a large number of studies have been focused on synthesizing nano-scale MOFs (nanoMOFs), thus taking advantage of these unique materials in various applications, especially those that are only possible at nano-scale. One of the technologies where nanoMOF materials occupy a central role is the membrane technology as one of the most efficient separation techniques. Therefore, numerous reports can be found on the enhancement of the physicochemical properties of polymeric membranes by using nanoMOFs, leading to remarkably improved performance. One of the most considerable applications of these nanoMOF-based membranes is in water treatment systems, because freshwater scarcity is now an undeniable crisis facing humanity. In this in-depth review, the most prominent synthesis and post-synthesis methods for the fabrication of nanoMOFs are initially discussed. Afterwards, different nanoMOF-based composite membranes such as thin-film nanocomposites (TFN) and mixed-matrix membranes (MMM) and their various fabrication methods are reviewed and compared. Then, the impacts of using MOFs-based membranes for water purification through growing metal-organic frameworks crystals on the support materials and utilization of metal-organic frameworks as fillers in mixed matrix membrane (MMM) are highlighted. Finally, a summary of pros and cons of using nanoMOFs in membrane technology for water treatment purposes and clear future prospects and research potentials are presented.

Preparation of ECTFE Porous Membrane for Dehumidification of Gaseous Streams through Membrane Condenser

Due to the good hydrophobicity and chemical resistance of poly(ethylene trifluoroethylene) (ECTFE), it has been an attractive potential material for microfiltration, membrane distillation and more. However, few porous hydrophobic ECTFE membranes were prepared by thermally induced phase separation (TIPS) for membrane condenser applications. In this work, the diluent, di-n-octyl phthalate (DnOP), was selected to prepare the dope solutions. The calculated Hassen solubility parameter indicated that ECTFE has good compatibility with DnOP. The corresponding thermodynamic phase diagram was established, and it has been mutually verified with the bi-continuous structure observed in the SEM images. At 30 wt% ECTFE, the surface contact angle and liquid entry pressure reach their maximum values of 139.5° and 0.71 MPa, respectively. In addition, some other basic membrane properties, such as pore size, porosity, and mechanical properties, were determined. Finally, the prepared ECTFE membranes were tested using a homemade membrane condenser setup. When the polymer content is 30 wt%, the corresponding results are better; the water recovery and condensed water yield is 17.6% and 1.86 kg m⁻² h⁻¹, respectively.

Bio-inspired super liquid-repellent membranes for membrane distillation: Mechanisms, fabrications and applications

With the aggravation of the global water crisis, membrane distillation (MD) for seawater desalination and hypersaline wastewater treatment is highlighted due to its low operating temperature, low hydrostatic pressure, and theoretically 100% rejection. However, some issues still impede the large-scale applications of MD technology, such as membrane fouling, scaling and unsatisfactory wetting resistance. Bio-inspired super liquid-repellent membranes have progressed rapidly in the past decades and been considered as one of the most promising approaches to overcome the above problems. This review for the first time systematically summarizes and analyzes the mechanisms of different super liquid-repellent surfaces, their preparation and modification methods, and anti-wetting/fouling/scaling performances in the MD process. Firstly, the topology theories of in-air superhydrophobic, in-air omniphobic and underwater superoleophobic surfaces are illustrated using different models. Secondly, the fabrication methods of various super liquid-repellent membranes are classified. The merits and demerits of each method are illustrated. Thirdly, the anti-wetting/fouling/scaling mechanisms of super liquid-repellent membranes are summarized. Finally, the conclusions and perspectives of the bio-inspired super liquid-repellent membranes are elaborated. It is anticipated that the systematic review herein can provide readers with foundational knowledge and current progress of super liquid-repellent membranes, and inspire researchers to overcome the challenges up ahead.

Recent Improvement Strategies on Metal-Organic Frameworks as Adsorbent, Catalyst, and Membrane for Wastewater Treatment

The accumulation of pollutants in water is dangerous for the environment and human lives. Some of them are considered as persistent organic pollutants (POPs) that cannot be eliminated from wastewater effluent. Thus, many researchers have devoted their efforts to improving the existing technology or providing an alternative strategy to solve this environmental problem. One of the attractive materials for this purpose are metal-organic frameworks (MOFs) due to their superior high surface area, high porosity, and the tunable features of their structures and function. This review provides an up-to-date and comprehensive description of MOFs and their crucial role as adsorbent, catalyst, and membrane in wastewater treatment. This study also highlighted several strategies to improve their capability to remove pollutants from water effluent.