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

Impact of residual aluminum on nanofiltration gypsum scaling: Mitigation roles played by different species

Residual aluminum (Al) is a growing pollutant in nanofiltration (NF) membrane-based drinking water treatment. To investigate the impact of distinct Al species fouling layers on gypsum scaling during NF, gypsum scaling tests were conducted on bare and three Al-conditioned (AlCl3-, Al13, and Al30-) membranes. The morphology of gypsum, the role of Al species on Ca2+ adsorption during gypsum scaling, and the interactions between gypsum crystals and Al-conditioned membranes were investigated. Results indicated that Al-conditioned membranes had lower flux decline than the bare membrane, with the order of AlCl3- Collapse

Key Words

• Brackish water
• Gypsum scaling
• Interface interaction
• Nanofiltration
• Residual aluminum

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MESH Headings

• Calcium Sulfate/chemistry
• Aluminum/chemistry
• Water Purification/methods
• Filtration/methods
• Adsorption
• Membranes, Artificial
• Water Pollutants, Chemical
• Calcium/chemistry
• Crystallization

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Affiliation(s)

Jinjin Jia State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, PR China
Daliang Xu State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, PR China
Jiaxuan Yang State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, PR China
Dachao Lin School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, 510006, PR China
Longfeng Hu State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, PR China
Wenxing Jin State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, PR China
Jinlong Wang State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, PR China
Weijia Gong School of Engineering, Northeast Agricultural University, 600 Changjiang Street, Xiangfang District, Harbin, 150030, PR China
Guibai Li State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, PR China
Heng Liang State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, PR China.

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Regulating gypsum scaling-induced wetting in membrane distillation by heterogeneous crystallization: Role of filter media

Mineral scaling and scaling-induced wetting are critical issues in membrane distillation (MD) during treatment of saline wastewaters. Gypsum scaling and scaling-induced wetting in MD were successfully regulated by heterogeneous crystallization with in-line granular filtration in this study. Stable water recovery increased from 32.5 % to more than 52.5 % in one-cycle operation, depending on filter media properties. Because a large mass of crystals were retained or/and adsorbed in the granular filter, the scaling mass on membrane surface was reduced by 41.2 %, 23.1 %, 54.7 % and 78.1 % by filter charged with activated carbon, sand, fiber and activated alumina, respectively. When activated carbon, sand, fiber and activated alumina were used, the final MD fluxes were 1.58, 1.04, 1.96 and 3.43 times that without filter, and permeate conductivity decreased by 43.0 %, 46.8 %, 83.2 % and 81.3 %, respectively. The multi-cycle tests showed that heterogeneous crystallization gradually occurred in the granular filter, thereby promoting seeding-induced crystallization that reduced gypsum scaling and scaling-induced wetting in MD. Excellent anti-scaling and anti-wetting performance of in-line granular filtration was also confirmed for synthetic and real industrial wastewater. The results of this study provide guidance for mineral scaling control in MD to allow resource utilization for saline wastewater.

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.

Gypsum heterogenous nucleation pathways regulated by surface functional groups and hydrophobicity

Gypsum (CaSO4·2H2O) plays a critical role in numerous natural and industrial processes. Nevertheless, the underlying mechanisms governing the formation of gypsum crystals on surfaces with diverse chemical properties remain poorly understood due to a lack of sufficient temporal-spatial resolution. Herein, we use in situ microscopy to investigate the real-time gypsum nucleation on self-assembled monolayers (SAMs) terminated with -CH3, -hybrid (a combination of NH2 and COOH), -COOH, -SO3, -NH3, and -OH functional groups. We report that the rate of gypsum formation is regulated by the surface functional groups and hydrophobicity, in the order of -CH3 > -hybrid > -COOH > -SO3 ≈ - NH3 > - OH. Results based on classical nucleation theory and molecular dynamics simulations reveal that nucleation pathways for hydrophilic surfaces involve surface-induced nucleation, with ion adsorption sites (i.e., functional groups) serving as anchors to facilitate the growth of vertically oriented clusters. Conversely, hydrophobic surfaces involve bulk nucleation with ions near the surface that coalesce into larger horizontal clusters. These findings provide new insights into the spatial and temporal characteristics of gypsum formation on various surfaces and highlight the significance of surface functional groups and hydrophobicity in governing gypsum formation mechanisms, while also acknowledging the possibility of alternative nucleation pathways due to the limitations of experimental techniques.

Micro/nano plastics inhibit the formation of barium sulfate scale on metal surface

Mineral scale (scale) is the crystalline inorganic precipitate from aqueous solution. Scale formation in pipelines has long been a challenge in various industrial systems. Micro/nano plastics (MNPs) have the potential to strongly influence scale formation process. However, comprehensive studies and mechanistic understanding of the interactions between MNPs and scales remain significantly underexplored. To fill this gap, we firstly adopted quartz crystal microbalance with dissipation (QCM-D) technology to monitor the in situ formation of barium sulfate (BaSO4) (0.001 M, saturation index 2.5) scale influenced by MNPs on metal surfaces. Microplastic (MP) (5 µm)-loaded surface exhibits hydrophobicity (contact angle > 123.1º), which reduces the rate of scale formation (90.86 ± 11.01 (ng cm-2 min-1)). Electrostatic repulsion impeded crystal growth while ion adsorption has a limited effect. Experiments on BaSO4 formation on metal pipes loaded with foam packaging debris were conducted over 30 days, and similar inhibition results were obtained. This study highlights the important role of MNPs in controlling heterogeneous nucleation and crystal growth of scale on metal surfaces, providing valuable insights for both MNPs and scale research.

Influence of zwitterionic amphiphilic copolymers on heterogeneous gypsum formation: A promising approach for scaling resistance

This study aims to investigate the influence of zwitterionic amphiphilic copolymers (ZACs) in the nucleation and growth of heterogeneous CaSO4 at the zwitterion-water interface, which is crucial for the prevention of mineral scaling and consequent downtime or suboptimal performance in industries like membrane desalination, heat exchangers, and pipeline transportation. In situ grazing incidence small angle X-ray Scattering (GISAXS), and quartz crystal microbalance with dissipation (QCM-D) techniques were used to analyze the evolution of CaSO4 particles on two new ZAC coatings: poly-(trifluoroethyl methacrylate-random-sulfobetaine methacrylate) (PTFEMA-r-SBMA, or PT:SBMA) and poly(trifluoroethyl methacrylate-random-2-methacryloyloxyethyl phosphorylcholine) (PTFEMA-r-MPC, or PT:MPC). The results showed that PT:MPC coatings promoted nucleation but inhibited crystal growth, resulting in slower overall reaction kinetics on PT:MPC coatings compared to PT:SBMA coatings. Interfacial interactions involving the substrates, sulfate minerals, and ions were examined, revealing that calcium ion adsorption, primarily governed by electrostatic attraction, played a crucial role in the nucleation and growth processes on both ZAC coatings. The crystal characterization revealed a phase transition from bassanite to gypsum on both ZAC coatings, suggesting that these zwitterionic materials can influence the mineral phase of heterogeneously formed CaSO4 crystals. These findings enhance our understanding of the fundamental mechanisms underlying heterogeneous CaSO4 scaling in the presence of zwitterionic materials.

Ultra-antiwetting Membrane for Hypersaline Water Crystallization in Membrane Distillation

Membrane distillation (MD) has great potential in the management of hypersaline water for zero liquid discharge (ZLD) due to its high salinity tolerance. However, the membrane wetting issue significantly restricts its practical application. In this study, a composite membrane tailored for extreme concentrations and even crystallization of hypersaline water is synthesized by coating a commercial hydrophobic porous membrane with a composite film containing a dense polyamide layer, a cation exchange layer (CEL), and an anion exchange layer (AEL). When used in direct contact MD for treating a 100 g L-1 NaCl hypersaline solution, the membrane achieves supersaturation of feed solution and a salt crystal yield of 38.0%, with the permeate concentration at <5 mg L-1. The composite membrane also demonstrates ultrahigh antiwetting stability in 360 h of long-term operation. Moreover, ion diffusion analysis reveals that the ultrahigh wetting resistance of the composite membrane is attributed to the bipolar AEL and CEL that eliminate ion crossover. The literature review elucidates that the composite membrane is superior to state-of-the-art membranes. This study demonstrates the great potential of the composite membrane for direct crystallization of hypersaline water, offering a promising approach to filling the gap between reverse osmosis and conventional thermal desalination processes for ZLD application.

Scaling-Free Cathodes: Enabling Electrochemical Extraction of High-Purity Nano-CaCO3 and -Mg(OH)2 in Seawater

For electrochemical application in seawater or brine, continuous scaling on cathodes will form insulation layers, making it nearly impossible to run an electrochemical reaction continuously. Herein, we report our discovery that a cathode consisting of conical nanobundle arrays with hydrophobic surfaces exhibits a unique scaling-free function. The hydrophobic surfaces will be covered with microbubbles created by electrolytic water splitting, which limits scale crystals from standing only on nanotips of conical nanobundles, and the bursting of large bubbles formed by the accumulation of microbubbles will cause a violent disturbance, removing scale crystals automatically from nanotips. Benefiting from the scaling-free properties of the cathode, high-purity nano-CaCO3 (98.9%) and nano-Mg(OH)2 (99.5%) were extracted from seawater. This novel scaling-free cathode is expected to eliminate the inherent limitations of electrochemical technology and open up a new route to seawater mining.

The Roles of Oil-Water Interfaces in Forming Ultrasmall CaSO4 Nanoparticles

In natural and engineered environmental systems, calcium sulfate (CaSO4) nucleation commonly occurs at dynamic liquid-liquid interfaces. Although CaSO4 is one of the most common minerals in oil spills and oil-water separation, the mechanisms driving its nucleation at these liquid-liquid interfaces remain poorly understood. In this study, using in situ small-angle X-ray scattering (SAXS), we examined CaSO4 nucleation at oil-water interfaces and found that within 60 minutes of reaction, short rod-shaped nanoparticles (with a radius of gyration (Rg) of 17.2 ± 2.7 nm and a length of 38.2 ± 5.8 nm) had formed preferentially at the interfaces. Wide-angle X-ray scattering (WAXS) analysis identified these nanoparticles as gypsum (CaSO4·2H2O). In addition, spherial nanoparticles measuring 4.1 nm in diameter were observed at oil-water interfaces, where surface-enhanced Raman spectroscopy (SERS) revealed an elevated pH compared to the bulk solution. The negatively charged oil-water interfaces preferentially adsorb calcium ions, collectively promoting CaSO4 formation there. CaSO4 particle formation at the oil-water interface follows a nonclassical nucleation (N-CNT) pathway by forming ultrasmall amorphous spherical particles which then aggregate to form intermediate nanoparticles, subsequently growing into nanorod-shaped gypsum. These findings of this study provide insights into mineral scaling during membrane separation and can inform more efficient oil transport in energy recovery systems.

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.

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.

Patterned dense Janus membranes with simultaneously robust fouling, wetting and scaling resistance for membrane distillation

Membrane fouling, wetting and scaling are three prominent challenges that severely hinder the practical applications of membrane distillation (MD). Herein, polyamide/polyvinylidene fluoride (PA/PVDF) Janus membrane comprising a hydrophobic PVDF substrate and a patterned dense PA layer by reverse interfacial polymerization (R-IP) was developed. Direct contact MD experiments demonstrated that PA/PVDF Janus membrane could exhibit simultaneously superior resistance towards surfactant-induced wetting, oil-induced fouling and gypsum-induced scaling without compromising flux. Importantly, the size-sieving effect, rather than the breakthrough pressure of the membrane, was revealed as the critical factor that probably endowed its resistance to wetting. Furthermore, a unique possible anti-scaling mechanism was unveiled. The superhydrophilic patterned dense PA layer with strong salt rejection capability not only prevented scale-precursor ions from intruding the substrate but also resulted in the high surface interfacial energy that inhibited the adhesion and growth of gypsum on the membrane surface, while its relatively low surface -COOH density benefited from R-IP process further ensured the membrane with a low scaling propensity. This study shall provide new insights and novel strategies in designing high-performance MD membranes and enable robust applications of MD facing the challenges of membrane fouling, wetting and scaling.

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.

Revisiting scaling of calcium sulfate in membrane distillation: Uncertainty of crystal-membrane interactions

Scaling of calcium sulfate (CaSO₄) is a stumbling block to the development of membrane distillation (MD), which holds promise for the treatment of saline water/wastewater. Despite increasing efforts made to understand the scaling behavior of CaSO₄ in a process of MD and thereby develop strategies for mitigating the negative effects, considerable uncertainty remains about occurrence of the wetting and structural damage that could result from the strong crystal-membrane interactions. This study combined experimental and theoretical approaches to corroborate that a higher degree of supersaturation could be achieved by concentrating the CaSO₄ in the feed at a faster rate; the elevated supersaturation would be in favor of exerting substantially high crystallization pressure on the membrane structures. In particular, the theoretical analysis established two dimensionless groups for measuring the relative importance of the concentration effect and quantifying the essential role played by the crystalline growth, respectively. In addition to alleviating the uncertainty, this study would be beneficial to the design of MD processes with improved scaling resistance.

A Tale of Two Foulants: The Coupling of Organic Fouling and Mineral Scaling in Membrane Desalination

Membrane desalination that enables the harvesting of purified water from unconventional sources such as seawater, brackish groundwater, and wastewater has become indispensable to ensure sustainable freshwater supply in the context of a changing climate. However, the efficiency of membrane desalination is greatly constrained by organic fouling and mineral scaling. Although extensive studies have focused on understanding membrane fouling or scaling separately, organic foulants commonly coexist with inorganic scalants in the feedwaters of membrane desalination. Compared to individual fouling or scaling, combined fouling and scaling often exhibits different behaviors and is governed by foulant-scalant interactions, resembling more complex but practical scenarios than using feedwaters containing only organic foulants or inorganic scalants. In this critical review, we first summarize the performance of membrane desalination under combined fouling and scaling, involving mineral scales formed via both crystallization and polymerization. We then provide the state-of-the-art knowledge and characterization techniques pertaining to the molecular interactions between organic foulants and inorganic scalants, which alter the kinetics and thermodynamics of mineral nucleation as well as the deposition of mineral scales onto membrane surfaces. We further review the current efforts of mitigating combined fouling and scaling via membrane materials development and pretreatment. Finally, we provide prospects for future research needs that guide the design of more effective control strategies for combined fouling and scaling to improve the efficiency and resilience of membrane desalination for the treatment of feedwaters with complex compositions.