Scale formation and control in high pressure membrane water treatment systems: A review

Contrasting mixed scaling patterns and mechanisms of nanofiltration and membrane distillation

Oriented towards the pressing needs for hypersaline wastewater desalination and zero liquid discharge (ZLD), the contrasting mixed scaling of thermal-driven vacuum membrane distillation (VMD) and pressure-driven nanofiltration (NF) were investigated in this work. Bulk crystallization was the main mechanism in VMD due to the high salinity and temperature, but the time-independent resistance by the adsorption of silicate and organic matter dominated the initial scaling process. Surface crystallization and the consequent pore-blocking were the main scaling mechanisms in NF, with the high permeate drag force, hydraulic pressure, and cross-flow rate resulting in the dense scaling layer mainly composed of magnesium-silica hydrate (MSH). Silicate enhanced NF scaling with a 75% higher initial flux decline rate attributed to the MSH formation and compression, but delayed bulk crystallization in VMD. Organic matter presented an anti-scaling effect by delaying bulk crystallization in both VMD and NF, but specifically promoted CaCO3 scaling in NF. Furthermore, the incipient scaling was intensified as silicate and organic matter coexisted. The scaling mechanism shifted from surface to bulk crystallization due to the membrane concentration in both VMD and NF. This work fills the research gaps on mixed scaling mechanisms in different membrane processes, which offers insights for scaling mitigation and thereby supports the application of ZLD.

Scaling behavior in membrane distillation: Effect of Biopolymers and Antiscalants

Fouling and scaling are inherent characteristics of membrane-based separation. They lead to a reduced membrane throughput. In the case of membrane distillation (MD), they can possibly result in pore wetting and irreversible failure to sustain the mass transfer interface. Most prior research on understanding fouling and scaling uses indirect measurements (flux) or ex-situ analyses methods (such as SEM and EDX), which limit the outcomes to indirect qualitative conclusions. Particularly, studying scaling tends to be more challenging due to the complexity of the experiments and the method of investigation; it is imperative to distinguish the contributions from the bulk phase and heterogeneous nucleation. In this work, we established a non-invasive, in-situ, real-time imaging experimental apparatus to study the scaling mechanism. Our experimental setup assisted us in distinguishing distinct phases of scaling during the filtration tests. We studied the scaling mechanism of various single-component systems (sodium chloride, strontium sulfate, calcium sulfate, and calcium carbonate) in vacuum MD filtration. The effect of natural organic matter and antiscalants on gypsum scaling were systematically investigated. Overall, organic fouling on the membrane surface expedited heterogeneous crystallization while decelerating crystal growth in the bulk phase. For instance, deposited humic acid (HA) on the membrane surface promoted gypsum heterogeneous nucleation on the membrane surface due to the interactions between HA carboxylic functional groups and calcium ions. The adsorption of HA on the salt crystal also decelerated crystal growth in the bulk phase. Antiscalants delayed and decelerated both crystal nucleation and crystal growth. PAA, a polycarboxylate antiscalant at 5 ppm, was found to effectively delay the onset of nucleation and crystal growth in the bulk phase, while phosphorous antiscalants at 5 ppm only delayed the onset of nucleation in the bulk phase with a negligible influence on crystal growth. Real-time, in-situ, and non-invasive monitoring shed light on the scaling mechanism and can further be used to identify mitigation strategies.

Modeling Framework for Cost Optimization of Process-Scale Desalination Systems with Mineral Scaling and Precipitation

Cost-optimization models are powerful tools for evaluating emerging water treatment processes. However, to date, optimization models do not incorporate detailed chemical reaction phenomena, limiting the assessment of pretreatment and mineral scaling. Moreover, novel approaches for high-salinity and high-recovery desalination are typically proposed without direct quantification of pretreatment needs or mineral scaling. This work addresses a critical gap in the literature by presenting a modeling framework that includes complex water chemistry predictions with process-scale optimization. We use this approach to conduct a technoeconomic assessment on a conceptual high-recovery treatment train that includes chemical pretreatment (i.e., soda ash softening and recarbonation) and membrane-based desalination (i.e., standard and high-pressure reverse osmosis). We demonstrate how to develop and integrate accurate multidimensional surrogate models for predicting precipitation, pH, and mineral scaling tendencies. Our findings show that cost-optimal results balance the costs of pretreatment with reverse osmosis system design. Optimizing across a range of water recoveries (i.e., 50-90%) reveals multiple cost-optimal schemas that vary the chemical dosing in pretreatment and the design and operation of reverse osmosis. Our results reveal that pretreatment costs can be more than double the cost of the primary desalination process at high recoveries due to the extensive pretreatment required to control scaling. This work emphasizes the importance of and provides a framework for including chemistry and mineral scaling predictions in the evaluation of emerging technologies in high-recovery desalination.

Further treatment of coking wastewater treated in A2O-MBR by the nanofiltration-powder activated carbon hybrid system

In this study, further treatment of coking wastewater treated in anoxic-oxic-membrane bioreactor (A2O-MBR) was investigated to meet the standards of the ministry by means of nanofiltration (NF) (with two different membranes and different pressures), microfiltration -powder activated carbon (MF-PAC) hybrid system and NF-PAC (with two different membranes and five different PAC concentrations) hybrid system. In addition to the parameters determined by the ministry, other parameters such as ammonium, thiocyanate (SCN-), hydrogen cyanide (HCN), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), color were also examined to evaluate the flux performance and treatment efficiency of the hybrid processes. According to the results, chemical oxygen demand (COD) in the NF process, COD and total cyanide (T-CN) in the MF-PAC process could not meet the discharge standards. As for the NF-PAC hybrid system, XN45 membrane met the discharge standards in all parameters (COD = 96±1.88 mg/L, T-CN =<0,02 mg/L, phenol =<0.05 mg/L), with a recovery rate of 78% at 0.5 g/L PAC concentration.

Adsorption Isotherm and Mechanism of Ca2+ Binding to Polyelectrolyte

Polyelectrolytes, such as poly(acrylic acid) (PAA), can effectively mitigate CaCO3 scale formation. Despite their success as antiscalants, the underlying mechanism of binding of Ca2+ to polyelectrolyte chains remains unresolved. Through all-atom molecular dynamics simulations, we constructed an adsorption isotherm of Ca2+ binding to sodium polyacrylate (NaPAA) and investigated the associated binding mechanism. We find that the number of calcium ions adsorbed [Ca2+]ads to the polymer saturates at moderately high concentrations of free calcium ions [Ca2+]aq in the solution. This saturation value is intricately connected with the binding modes accessible to Ca2+ ions when they bind to the polyelectrolyte chain. We identify two dominant binding modes: the first involves binding to at most two carboxylate oxygens on a polyacrylate chain, and the second, termed the high binding mode, involves binding to four or more carboxylate oxygens. As the concentration of free calcium ions [Ca2+]aq increases from low to moderate levels, the polyelectrolyte chain undergoes a conformational transition from an extended coil to a hairpin-like structure, enhancing the accessibility to the high binding mode. At moderate concentrations of [Ca2+]aq, the high binding mode accounts for at least one-third of all binding events. The chain's conformational change and its consequent access to the high binding mode are found to increase the overall Ca2+ ion binding capacity of the polyelectrolyte chain.

Flow-through electrochemically assisted reverse-osmosis: A new process towards low-chemical desalination

Two-pass reverse osmosis (RO) process is prevailing in seawater desalination, but each process must consume considerable amounts of chemicals to secure product water quality. Caustic soda is used to raise the pH of the first-pass RO permeate (also the second-pass RO feed) to ensure adequate removal of boron in the subsequent second-pass RO, while antiscalants and disinfectants such as hypochlorite are added in the feed seawater for scaling and biofouling control of the first-pass RO membranes. Here, we report for the first time a flow-through electrochemically assisted reverse osmosis (FT-EARO) module system used in the first-pass RO, aiming to dramatically reduce or even eliminate chemical usage for the current RO desalination. This novel system integrated an electroconductive permeate carrier as cathode and an electroconductive feed spacer as anode on each side of the first-pass RO membrane. Upon applying an extremely low-energy (< 0.005 kWh/m3) electrical field, the FT-EARO module could (1) produce a permeate with pH >10 with no alkali dosage, ensuring sufficient boron removal in the second-pass RO, and (2) generate protons and low-concentration free chlorine near the membrane surface, potentially discouraging membrane scaling and biofouling while maintaining satisfactory desalination performance. The current study further elucidated the high scalability of this novel electrified high-pressure RO module design. The low-chemical manner of FT-EARO presents an attractive practical option towards green and sustainable seawater desalination.

Seawater-Based Fracturing Fluid: A Review

Hydraulic fracturing uses a large amount of fresh water for its operation; conventional wells can consume up to 200 000 gallons of water, while unconventional wells could consume up to 16 million gallons. However, the world's fresh water supply is rapidly depleting, making this a critical and growing problem. Freshwater shortages during large-scale hydraulic fracturing in regions that lack water, such as the Arabian Peninsula and offshore operations, need to be addressed. One of the ways to address this problem is to substitute fresh water with seawater, which is a sustainable, cheap, and technically sufficient fluid that can be utilized as a fracturing fluid. However, its high salinity caused by the multitude of ions in it could induce several problems, such as scaling and precipitation. This, in turn, could potentially affect the viscosity and rheology of the fluid. There are a variety of additives that can be used to lessen the effects of the various ions found in seawater. This review explains the mechanisms of different additives (e.g., polymers, surfactants, chelating agents, cross-linkers, scale inhibitors, gel stabilizers, and foams), how they interact with seawater, and the related implications in order to address the above challenges and develop a sustainable and compatible seawater-based fracturing fluid. This review also describes several previous technologies and works that have treated seawater in order to produce a fluid that is stable at higher temperatures, that has a considerably reduced scaling propensity, and that has utilized a stable polymer network to efficiently carry proppant downhole. In addition, some of these previous works included field testing to evaluate the performance of the seawater-based fracturing fluid.

Dissolution mechanism of Fe3O4 scale by 1-hydroxyethane-1,1-diphosphonic acid: an ab initio molecular metadynamics study

Using the ab initio molecular metadynamics method, the adsorption of the structure of 1-hydroxyethane-1,1-diphosphonic acid (HEDP) on the Fe3O4 surface and subsequent detachment of Fe atoms from the surface were simulated, and the dissolution mechanism by which HEDP dissolves Fe3O4 scale at room temperature while other organic acids cannot was elucidated. The adsorbed hydroxyl groups, water and HEDP on the Fe3O4 surface play a synergistic role in detaching the Fe ions, which increases the coordination number of the Fe atoms and weakens the original Fe-O bond strength. In addition, the strong coordination ability and flexible molecular structure of HEDP also facilitate dissolution of Fe3O4 scale by breaking down the chemical bonds and forming Fe-HEDP complexes. The free energy surface for the dissolution reaction shows a low barrier, and the descaling reaction is easily accomplished.

Silica mitigated calcium mineral scaling in brackish water reverse osmosis

Although the autopsies of reverse osmosis (RO) membranes from full-scale, brackish water desalination plants identify the co-presence of silica and Ca-based minerals in scaling layers, minimal research exists on their formation process and mechanisms. Therefore, combined scaling by silica and either gypsum (non-alkaline) or amorphous calcium phosphate (ACP, alkaline) was investigated in this study for their distinctive impacts on membrane performance. The obtained results demonstrate that the coexistence of silica and Ca-based mineral salts in feedwaters significantly reduced water flux decline as compared to single type of Ca-based mineral salts. This antagonistic effect was primarily attributed to the silica-mediated alleviation of Ca-based mineral scaling. In the presence of silica, silica skins were immediately established around Ca-based mineral precipitates once they emerged. Sheathing by the siliceous skins hindered the aggregation and thus the morphological evolution of Ca-based mineral species. Unlike sulfate precipitates, ACP precipitates can induce the formation of dense and thick silica skins via an additional condensation reaction. Such a phenomenon rationalized the notion concerning a stronger mitigating effect of silica on ACP scaling than gypsum scaling. Meanwhile, coating by silica skins altered the surface chemistries of Ca-based mineral precipitates, which should be fully considered in regulating membrane surface properties for combined scaling control. Our findings advance the mechanistic understanding on combined mineral scaling of RO membranes, and may guide the appropriate design of membrane surface properties for scaling-resistant membrane tailored to brackish water desalination.

Facilitated OH¯ diffusion via bubble motion and water flow in a novel electrochemical reactor for enhancing homogeneous nucleation of CaCO3

Electrochemistry is a potential method for water softening. An essential disadvantage is OH¯ ions from water electrolysis accumulate on cathode surface, inducing the generation of the insulating CaCO3 layer and then interrupting the electrochemical reaction. In order to propel OH¯ diffusion into the bulk solution instead of aggregation at cathode, we designed an electrochemical reactor, whose electrodes were placed horizontally in the middle of the reactor, and the bubbles created by water electrolysis move upward, while the water flows downward. The visual evidence displayed that the unique reactor structure allowed OH¯ to spread to almost all the solution rapidly. Average pH value of bulk solution reached 10.6 in only 3 min. Therefore, homogeneous nucleation of CaCO3 in bulk solution would take primary responsibility for water softening, and the softening efficiency is up to 212.9 g CaCO3/h/m2, higher than reported results. The reactor is easy to scale up, providing a new idea for the softening of circulating cooling water.

Surface Functional Groups Affect Iron (Hydr)oxide Heterogeneous Nucleation: Implications for Membrane Scaling

Because of its favorable thermodynamics and fast kinetics, heterogeneous solid nucleation on membranes triggers early-stage mineral scaling. Iron (hydr)oxide, a typical membrane scale, initially forms as nanoparticles that interact with surface functional groups on membranes, but these nanoscale phenomena are difficult to observe in real time. In this study, we utilized in situ grazing incidence small angle X-ray scattering and ex situ atomic force microscopy to examine the heterogeneous nucleation of iron (hydr)oxide on surface functional groups commonly used in membranes, including hydroxyl (OH), carboxyl (COOH), and fluoro (F) groups. We found that, compared to nucleation on hydrophilic OH- and COOH-surfaces, the high hydrophobicity of an F-modified surface significantly reduced the extents of both heterogeneously and homogeneously formed iron (hydr)oxide nucleation. Moreover, on the OH-surface, the high functional group density of 0.76 nmol/cm² caused faster heterogeneous nucleation than that on a COOH-surface, with a density of 0.28 ± 0.04 nmol/cm². The F-surface also had the highest heterogeneous nucleation energy barrier (26 ± 0.6 kJ/mol), followed by COOH- (23 ± 0.8 kJ/mol) and OH- (20 ± 0.9 kJ/mol) surfaces. The kinetic and thermodynamic information provided here will help us better predict the rates and extents of early-stage scaling of iron (hydr)oxide nanoparticles in membrane processes.

Removal of Trace Uranium from Groundwaters Using Membrane Capacitive Deionization Desalination for Potable Supply in Remote Communities: Bench, Pilot, and Field Scale Investigations

The performance of membrane capacitive deionization (MCDI) desalination was investigated at bench, pilot, and field scales for the removal of uranium from groundwater. It was found that up to 98.9% of the uranium can be removed using MCDI from a groundwater source containing 50 μg/L uranium, with the majority (94.5%) being retained on the anode. Uranium was found to physiochemically adsorb to the electrode without the application of a potential by displacing chloride ions, with 16.6% uranium removal at the bench scale via this non-electrochemical process. This displacement of chloride did not occur during the MCDI adsorption phase with the adsorption of all ions remaining constant during a time series analysis on the pilot unit. For the scenarios tested on the pilot unit, the flowrate of the product water ranged from 0.15 to 0.23 m³/h, electrode energy consumption from 0.28 to 0.51 kW h/m³, and water recovery from 69 to 86%. A portion (13-53% on the pilot unit) of the uranium was found to remain on the electrodes after the brine discharge phase with conventional cleaning techniques unable to release this retained uranium. MCDI was found to be a suitable means to remove uranium from groundwater systems though with the need to manage the accumulation of uranium on the electrodes over time.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

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.

Colloidal aspects of calcium carbonate scaling in water-in-oil emulsions: A fundamental study using droplet-based microfluidics

HYPOTHESIS

As a mainstream process in the extraction and recovery of crude oil, water is injected into reservoirs in the so-called waterflooding process to facilitate the oil displacement through the wellbore, typically generating water-in-oil (W/O) emulsions. Based on economic considerations, sea water is used in the flooding process; however, the ionic incompatibility between the injected water and the formation water inside the reservoir may precipitate sparingly-soluble inorganic salts (scale). We hypothesize that calcium carbonate (CaCO₃) scale dynamically interacts with cationic surfactants in W/O emulsions, resulting in (i) scale growth retardation and (ii) emulsion destabilization.

EXPERIMENTS

We developed stable W/O emulsions via combining droplet-based microfluidics with multifactorial optimizations to investigate the influence of emulsion properties, such as surfactant type and concentrations, temperature, and pH, as well as calcium ions on the CaCO₃ scaling kinetics and emulsion stability. The CaCO₃ scale was characterized based on particle size and charge, lattice structure, interactions with the surfactant, and time-dependent effects on emulsion stability.

FINDINGS

The interfacial interactions between the cationic surfactant (cetyltrimethylammonium bromide, CTAB) and CaCO₃ retarded scale growth rate, decreased crystal size, and destabilized emulsion within hours as a result of surfactant depletion at the water-oil interface. The surfactant did not affect the crystal structure of scale, which was formed as the most thermodynamically stable crystalline polymorph, calcite, at the ambient condition. This fundamental study may open new opportunities for engineering stable W/O emulsions, e.g., for enhanced oil recovery (EOR), and developing scale-resistant multiphase flows.

Interpenetrating polymer networks for desalination and water remediation: a comprehensive review of research trends and prospects

Interpenetrating polymer network (IPN) architectures have gained a lot of interest in recent decades, mainly due to their wide range of applications including water treatment and environmental remediation. IPNs are composed of two or more crosslinked polymeric matrices that are physically entangled but not chemically connected. In polymer science, the interpenetrating network structure with its high polymer chain entanglement is commonly used to generate materials with many functional properties, such as mechanical robustness and adaptable structure. In order to remove a targeted pollutant from contaminated water, it is feasible to modify the network architectures to increase the selectivity by choosing the monomer appropriately. This review aims to give a critical overview of the recent design concepts of IPNs and their applications in desalination and water treatment and their future prospects. This article also discusses the inclusion of inorganic nanoparticles into traditional polymeric membrane networks and its advantages. In the first part, the current scenario for desalination, water pollution and conventional desalination technologies along with their challenges is discussed. Subsequently, the main strategies for the synthesis of semi-IPNs and full-IPNs, and their relevant properties in water remediation are presented based on the nature of the networks and mechanism, with an emphasis on the IPN membrane. This review article has thoroughly investigated and critically assessed published works that describe the latest study on developing IPN membranes, hydrogels and composite materials in water purification and desalination. The goal of this critical analysis is to elicit fresh perspectives regarding the application and advantages of IPNs in desalination and water treatment. This article will also provide a glimpse into future areas of research to address the challenges relating to advanced water treatment as well as its emerging sustainable approaches. The study has put forward a convincing justification and establishes the relevance of IPNs being one of the most intriguing and important areas for achieving a sustainable generation of advanced materials that could benefit mankind.

Recent approaches and advanced wastewater treatment technologies for mitigating emerging microplastics contamination - A critical review

Microplastics have been identified as an emerging pollutant due to their irrefutable prevalence in air, soil, and particularly, the aquatic ecosystem. Wastewater treatment plants (WWTPs) are seen as the last line of defense which creates a barrier between microplastics and the environment. These microplastics are discharged in large quantities into aquatic bodies due to their insufficient containment during water treatment. As a result, WWTPs are regarded as point sources of microplastics release into the environment. Assessing the prevalence and behavior of microplastics in WWTPs is therefore critical for their control. The removal efficiency of microplastics was 65 %, 0.2-14 %, and 0.2-2 % after the successful primary, secondary and tertiary treatment phases in WWTPs. In this review, other than conventional treatment methods, advanced treatment methods have also been discussed. For the removal of microplastics in the size range 20-190 μm, advanced treatment methods like membrane bioreactors, rapid sand filtration, electrocoagulation and photocatalytic degradation was found to be effective and these methods helps in increasing the removal efficiency to >99 %. Bioremediation based approaches has found that sea grasses, lugworm and blue mussels has the ability to mitigate microplastics by acting as a natural trap to the microplastics pollutants and could act as candidate species for possible incorporation in WWTPs. Also, there is a need for controlling the use and unchecked release of microplastics into the environment through laws and regulations.

New insight into scale inhibition during tea brewing: Ca2+/Mg2+ complexing and alkalinity consumption

Scale not only affects the taste and color of water, but also increases the risks of osteoporosis and cardiovascular diseases associated with drinking it. As a popular beverage, tea is rich many substances that have considerable potential for scale inhibition, including protein, tea polyphenols and organic acids. In this study, the effect of tea brewing on scale formation was explored. It was found that the proteins, catechins and organic acids in tea leaves could be released when the green tea was brewed in water with sufficient hardness and alkalinity. The tea-released protein was able to provide carboxyl groups to chelate with calcium ions (Ca²⁺), preventing the Ca²⁺ from reacting with the carbonate ions (CO₃^2-). The B rings of catechins were another important structure in the complexation of Ca²⁺ and magnesium ions (Mg²⁺). The carboxyl and hydroxyl groups on the organic acids was able to form five-membered chelating rings with Ca²⁺ and Mg²⁺, resulting in a significant decrease in Ca²⁺ from 100.0 to 60.0 mg/L. Additionally, the hydrogen ions (H⁺) provided by the organic acids consumed and decreased the alkalinity of the water from 250.0 to 131.4 mg/L, leading to a remarkable reduction in pH from 8.93 to 7.73. It further prevented the bicarbonate (HCO₃^-) from producing CO₃^2- when the water was heated. The reaction of the tea constituents with the hardness and alkalinity inhibited the formation of scale, leading to a significant decrease in turbidity from 10.6 to 1.4 NTU. Overall, this study provides information to help build towards an understanding of the scale inhibition properties of tea and the prospects of tea for anti-scaling in industrial applications.

Silicic Acid Removal by Metal-Organic Frameworks for Silica-Scale Mitigation in Reverse Osmosis

Reverse osmosis (RO) membranes are susceptible to silica scaling, resulting in irreversible degradation of membrane performance. This work covered the fabrication of MIL-101(Fe) for silicic acid adsorption to alleviate the silica scaling of RO membranes. The effect of pH, mixing time and initial concentration on silicic acid adsorption of MIL-101(Fe) was appraised in detail. The adsorption experiments demonstrated that MIL-101(Fe) possessed an excellent adsorption ability for silicic acid with the maximum adsorption capacity reaching 220.1 mgSiO₂·g^-1. Data fitting confirmed the pseudo-second-order equation and Freundlich equation were consistent with silicic acid adsorption on MIL-101(Fe). Finally, a simulated anti-scaling experiment was carried out using a feed solution pretreated by MIL-101(Fe) adsorption, and the permeance exhibited a much lower decline after 24 h filtration, confirming that MIL-101(Fe) exhibits an excellent application potential for silica-scale mitigation in RO systems.

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.

Boosting the Performance of Nanofiltration Membranes in Removing Organic Micropollutants: Trade-Off Effect, Strategy Evaluation, and Prospective Development

In view of the high risks brought about by organic micropollutants (OMPs), nanofiltration (NF) processes have been playing a vital role in advanced water and wastewater treatment, owing to the high membrane performance in rejection of OMPs, permeation of water, and passage of mineral salts. Though numerous studies have been devoted to evaluating and technically enhancing membrane performance in removing various OMPs, the trade-off effect between water permeance and water/OMP selectivity for state-of-the-art membranes remains far from being understood. Knowledge of this effect is significant for comparing and guiding membrane development works toward cost-efficient OMP removal. In this work, we comprehensively assessed the performance of 88 NF membranes, commercialized or newly developed, based on their water permeance and OMP rejection data published in the literature. The effectiveness and underlying mechanisms of various modification methods in tailoring properties and in turn performance of the mainstream polyamide (PA) thin-film composite (TFC) membranes were quantitatively analyzed. The trade-off effect was demonstrated by the abundant data from both experimental measurements and machine learning-based prediction. On this basis, the advancement of novel membranes was benchmarked by the performance upper-bound revealed by commercial membranes and lab-made PA membranes. We also assessed the potentials of current NF membranes in selectively separating OMPs from inorganic salts and identified the future research perspectives to achieve further enhancement in OMP removal and salt/OMP selectivity of NF membranes.

Application and Visualization of Fluorescent-Tagged Antiscalants in Electrodialysis Processing of Aqueous Solutions Prone to Gypsum Scale Deposition

Membrane scaling is a serious problem in electrodialysis. A widely used technique for controlling scale deposition in water treatment technologies is the application of antiscalants (AS). The present study reports on gypsum scale inhibition in electrodialysis cell by the two novel ASs: fluorescent-tagged bisphosphonate 1-hydroxy-7-(6-methoxy-1,3-dioxo-1Hbenzo[de]isoquinolin-2(3H)-yl)heptane-1,1-diyl-bis(phosphonic acid), HEDP-F and fluorescein-tagged polyacrylate, PAA-F2 (molecular mass 4000 Da) monitored by chronopotentiometry and fluorescent microscopy. It was found that cation-exchange membrane MK-40 scaling is sufficiently reduced by both ASs, used in 10^-6 mol·dm^-3 concentrations. PAA-F2 at these concentrations was found to be more efficient than HEDP-F. At the same time, PAA-F2 reveals gypsum crystals' habit modification, while HEDP-F does not noticeably affect the crystal form of the deposit. The strong auto-luminescence of MK-40 hampers visualization of both PAA-F2 and HEDP-F on the membrane surface. Nevertheless, PAA-F2 is proved to localize partly on the surface of gypsum crystals as a molecular adsorption layer, and to change their crystal habit. Crystal surface coverage by PAA-F2 appears to be nonuniform. Alternatively, HEDP-F localizes on the surface of a deposit tentatively in the form of [Ca-HEDP-F]. The proposed mechanisms of action are formulated and discussed. The application of antiscalants in electrodialysis for membrane scaling mitigation is demonstrated to be very promising.

Gypsum scale formation and inhibition kinetics with implications in membrane system

Water desalination using membrane technology is one of the main technologies to resolve water pollution and scarcity issues. In the membrane treatment process, mineral scale deposition and fouling is a severe challenge that can lead to filtration efficiency decrease, permeate quality compromise, and even membrane damage. Multiple methods have been developed to resolve this problem, such as scale inhibitor addition, product recovery ratio adjustment, periodic membrane surface flushing. The performance of these methods largely depends on the ability to accurately predict the kinetics of mineral scale deposition and fouling with or without inhibitors. Gypsum is one of the most common and troublesome inorganic mineral scales in membrane systems, however, no mechanistic model is available to accurately predict the induction time of gypsum crystallization and inhibition. In this study, a new gypsum crystallization and inhibition model based on the classical nucleation theory and a Langmuir type adsorption isotherm has been developed. Through this model, it is believed that gypsum nucleation may gradually transit from homogeneous to heterogeneous nucleation when the gypsum saturation index (SI) decreases. Such transition is represented by a gradual decrease of surface tension at smaller SI values. This model assumes that the adsorption of inhibitors onto the gypsum nucleus can increase the nucleus superficial surface tension and prolong the induction time. Using the new model, this study accurately predicted the gypsum crystallization induction times with or without nine commonly used scale inhibitors over wide ranges of temperature (25-90 °C), SI (0.04-0.96), and background NaCl concentration (0-6 mol/L). The fitted affinity constants between scale inhibitors and gypsum show a good correlation with those between the same inhibitors and barite, indicating a similar inhibition mechanism via adsorption. Furthermore, by incorporating this model with the two-phase mineral deposition model our group developed previously, this study accurately predicts the gypsum deposition time on the membrane material surfaces reported in the literature. We believe that the model developed in this study can not only accurately predict the gypsum crystallization induction time with or without scale inhibitors, elucidate the gypsum crystallization and inhibition mechanisms, but also optimize the mineral scale control in the membrane filtration system.

Analysis of Calcium Sulfate Scaling Phenomena on Reverse Osmosis Membranes by Scaling-Based Flux Model

In this study, the behavior of permeate flux decline due to scale precipitation of calcium sulfate on reverse osmosis membranes was investigated. The proposed scaling-based flux model is able to explain that permeate fluxes attributed to three mechanisms of scale precipitation-cake formation, surface blockage, and mixed crystallization-converge to the same newly defined scaling-based critical flux. In addition, a scaling index is defined, which determines whether scale precipitates on the membrane. The experimental results were analyzed based on this index. The mass-transfer coefficients of flat membrane cells used in the experiments were measured and, although the coefficients differed, they could be summarized in the same form as the Leveque equation. Considering the results of the scale precipitation experiments, where the operating conditions of pressure, solute concentration, temperature, and Reynolds number were varied, the convergent values of the permeate fluxes are explained by the scaling-based critical fluxes and the scale precipitation zones by the scaling indexes.

Distinct impacts of natural organic matter and colloidal particles on gypsum crystallization

Gypsum scaling via crystallization is a major obstacle limiting the applications of membrane-based technologies and heat exchangers in engineered systems. Herein, we perform the first comparative investigation on the impacts of natural organic matter (Suwannee River humic acid, SRHA) and colloidal particles on the gypsum crystallization process in terms of induction time and crystal morphology. Results show that the presence of SRHA significantly increases the induction time of gypsum crystallization. Specifically, at a solution saturation index of 4.92, the induction time increases 6.5-fold in the presence of 6 mg/L SRHA, compared to the case without SRHA. SRHA also alters the morphology of the formed calcium sulfate crystals, resulting in a polygon-like shape, differing from the characteristic needle-like shape of gypsum in the absence of additives. These changes in crystal morphology are attributed to the adsorption of SRHA on the gypsum crystal surface, blocking the active sites for gypsum growth. In contrast, in the presence of colloidal particles, the observed induction time of gypsum crystallization either decreases or increases, depending on the competitive interplay between the enhancement effect in the nucleation step and the inhibition effect in the subsequent crystal growth step. Furthermore, the formed gypsum crystals in the presence of colloidal particles exhibit a needle-like morphology similar to the crystals formed in the absence of any additives. Our study provides fundamental understanding of gypsum crystallization in feedwaters containing natural organic matter and colloidal particles, highlighting the importance of feedwater composition in gypsum scaling.

Exploring the Limitations of Osmotically Assisted Reverse Osmosis: Membrane Fouling and the Limiting Flux

Osmotically assisted reverse osmosis (OARO) has shown great potential for low-cost and energy-efficient brine management. However, its performance can be significantly limited by membrane fouling. Here, we performed for the first time a comprehensive study on OARO membrane fouling, explored the associated fouling mechanisms, and evaluated fouling reversibility via simple physical cleaning strategies. First, internal membrane fouling at the draw (permeate) side was shown to be insignificant. Flux behavior in short-term operation was correlated to both the evolution of fouling and the change of internal concentration polarization. In long-term operation, membrane fouling constrained the OARO water flux to a singular, common upper limit, in terms of limiting flux, which was demonstrated to be independent of operating pressures and membrane properties. Generally, once the limiting flux was exceeded, the OARO process performance could not be improved by higher-pressure operation or by utilizing more permeable and selective membranes. Instead, different cyclic cleaning strategies were shown to be more promising alternatives for improving performance. While both surface flushing and osmotic backwashing (OB) were found to be highly effective when using pure water, a full flux recovery could not be achieved when a nonpure solution was used during OB due to severe internal clogging during OB. All in all, the presented findings provided significant implications for OARO operation and fouling control.

Significance of atomic-scale defects in flexible surfaces on local solvent and ion behaviour

Many factors can affect the course of heterogeneous nucleation, such as surface chemistry, flexibility and topology, substrate concentration and solubility. Atomic-scale defects are rarely investigated in detail and are often considered to be unimportant surface features. In this work, we set out to investigate the significance of atomic-scale defects in a flexible self-assembled monolayer surface for the behaviour of clusters of Ca²⁺ and CO₃^2- ions in water. To this end, we use molecular dynamics simulations to estimate the diffusion coefficients of ion clusters at different topological surface features and obtain ionic radial distribution functions around features of interest. Well-tempered metadynamics is used to gain insight into the free energy of ions around selected surface defects. We find that certain defects, which we refer to as active defects, can impair ionic surface diffusion, as well as affect the diffusion of ions in close proximity to the surface feature in question. Our findings suggest that this effect can result in an ability of such topological features to promote ion clustering and increase local ionic concentration at specific surface sites. The work reported here shows how the presence of small atomic-scale defects can affect the role of a surface in the process of heterogeneous nucleation and contributes towards a rational definition of surfaces as effective nucleating agents.

Foulant Identification and Performance Evaluation of Antiscalants in Increasing the Recovery of a Reverse Osmosis System Treating Anaerobic Groundwater

The objectives of this study are to assess the performance of antiscalants in increasing the recovery (≥85%) of a reverse osmosis (RO) plant treating anaerobic groundwater (GW) in Kamerik (the Netherlands), and to identify scalants/foulant that may limit RO recovery. Five different commercially available antiscalants were compared on the basis of their manufacturer-recommended dose. Their ability to increase the recovery from 80% to a target of 85% was evaluated in pilot-scale measurements with anaerobic GW and in once-through lab-scale RO tests with synthetic (artificial) feedwater. A membrane autopsy was performed on the tail element(s) with decreased permeability. X-ray photoelectron spectroscopy (XPS) analysis indicated that calcium phosphate was the primary scalant causing permeability decline at 85% recovery and limiting RO recovery. The addition of antiscalant had no positive effect on RO operation and scaling prevention, since at 85% recovery, permeability of the last stage decreased with all five antiscalants, while no decrease in permeability was observed without the addition of antiscalant at 80% recovery. In addition, in lab-scale RO tests executed with synthetic feed water containing identical calcium and phosphate concentrations as the anaerobic GW, calcium phosphate scaling occurred both with and without antiscalant at 85% recovery, while at 80% recovery without antiscalant, calcium phosphate did not precipitate in the RO element. In brief, calcium phosphate appeared to be the main scalant limiting RO recovery, and antiscalants were unable to prevent calcium phosphate scaling or to achieve a recovery of 85% or higher.

A Case Study of Calcium Carbonate Crystallization during Reverse Osmosis Water Desalination in Presence of Novel Fluorescent-Tagged Antiscalants

Calcium carbonate scaling in reverse osmosis (RO) desalination process is studied in the presence of two novel fluorescent-tagged scale inhibitors 1,8-naphthalimide-tagged polyacrylate (PAA-F1) and 1-hydroxy-7-(6-methoxy-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)heptane-1,1-diyl-bis(phosphonic acid) (HEDP-F) by fluorescent microscopy (FM) and scanning electron microscopy (SEM). Both antiscalants diminished the mean size of calcite crystals relative to the blank experiment. The behavior and localization of HEDP-F and PAA-F1 during calcite scale formation on membrane surface was found to be significantly different from the distribution in similar RO experiments with gypsum, reported earlier. In the former case, both antiscalants are concentrated exactly on the surface of calcium carbonate crystals, while in the latter one they form their own phases (Ca-HEDP-F and Ca-PAA-F1) and are not detected on gypsum scale. The difference is interpreted in terms of interplay between background calcium concentration and sparingly soluble calcium salts’ solubility. HEDP-F reveals slightly higher efficiency than PAA-F1 against calcite scale formation, while PAA-F exhibits a higher ability to change calcite morphology. It is demonstrated that there is a lack of correlation between antiscaling efficacy and ability of antiscalant to change calcium carbonate morphology in a particular case study. An application of fluorescent-tagged antiscalants in RO experiments provides a unique possibility to track the scale inhibitor molecules’ localization during calcite scale formation. Fluorescent-tagged antiscalants are presumed to become a very powerful tool in membrane scaling inhibition studies.