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Failure of membrane distillation (MD) due to pore wetting by amphiphilic molecules has recently received growing interests because it is a critical challenge to overcome for MD to be applicable for treating unconventional feed water. Recent MD studies using feed solutions containing surfactants have elucidated fundamental mechanism of wetting and generated practical solutions for wetting mitigation. However, what remains unclear is the impact of surfactant species on pore wetting kinetics. Based on a recently developed kinetic model for surfactant-induced pore wetting in MD, we hypothesize that the surface excess concentration of a surfactant is the most important surfactant property in affecting the pore wetting kinetics. In this study, we performed controlled MD wetting experiments using seven different types of surfactants and measured their respective breakthrough time as a quantitative metric for wetting kinetics. Our experiments reveal a good linear correlation between the surface excess concentration and breakthrough time for most but one tested surfactant. When surface excess concentration and diffusion coefficient are both considered, the model-simulated breakthrough time matches the experimentally measurement remarkably well for all tested surfactants.

Membrane scaling is detrimental to the efficiency of membrane desalination. While both membrane surface modification and the addition of anti-scalants have been investigated as possible methods to minimize scale formation on membrane surfaces, the efficacies of these two strategies have not been compared in the literature. In this work, we first modified a commercial polyamide reverse osmosis (RO) membrane with four different polymers to create various surface functionalities and properties. Then the anti-scaling efficiencies of the modified membranes were evaluated and compared with the use of poly(acrylic) acid (PAA) as anti-scalant in a bench-scale RO system fed with gypsum supersaturated solutions. The modified membrane with the best performance displayed limited effectiveness in delaying gypsum scaling, whereas gypsum scaling was greatly retarded in the presence of PAA. In contrary to the passive effect of membrane surface modification on gypsum scaling, anti-scalants actively disrupt gypsum crystallization. Further, we applied membrane distillation (MD) to desalinate the concentrated brines generated from RO desalination with and without anti-scalants. We demonstrated that the residual PAA in the RO brines was highly effective to alleviate gypsum scaling in MD, greatly enhancing the total water recovery of the combined RO-MD system. Our results provide comparative insights on the efficiencies of different scaling mitigation strategies in membrane desalination, and demonstrate the promising potential of applying anti-scalants to achieve high water recovery.

Appendix CMembrane pore wetting is a unique and important technical challenge for membrane distillation (MD). While the general principle of pore wetting is well known, the detailed mechanism of pore wetting induced by surfactants that can actively adsorb onto membrane pore surface has not been theoretically elucidated. In this study, we developed a theoretical model, based on surfactant transport in a partially wetted membrane pore under the pseudo-steady state assumption, to quantify the kinetics of pore wetting. The theoretical model predicts several key dependences of wetting kinetics on operating parameters and solution properties, which are highly consistent with results from MD experiments using feed solution containing sodium dodecyl sulfate. It was found that kinetics of pore wetting is strongly dependent on vapor flux, surfactant concentration, but relatively independent of the transmembrane hydraulic pressure. The critical surfactant concentration below which pore wetting does not occur was also predicted by the wetting model. Finally, the impact of surfactant species on wetting kinetics was also discussed.

Pore wetting is a unique and important technical challenge that can lead to process failure in membrane distillation (MD) using hydrophobic membranes. While it is well known that both low-surface-tension and water miscible liquids, such as alcohols, and amphiphilic molecules, such as surfactants, are effective wetting agents, the detailed mechanisms for these agents to induce pore wetting remain unclear. In particular, the role of surface adsorption in surfactant-induced wetting remains to be elucidated. This study provides fundamental insights to understanding the mechanism of pore wetting induced by these two different wetting agents. Using a recently developed wetting monitoring technique based on single-frequency transmembrane impedance, we experimentally probe the kinetics of wetting frontier propagation. We demonstrate that ethanol-induced wetting is instantaneous whereas surfactant-induced wetting is dynamic with its kinetic rate dependent on several critical factors. We also develop a theoretical model, based on the assumption of quasi-equilibrium adsorption, to successfully explain the important features experimentally observed in surfactant-induced wetting. Specifically, it was found that the kinetics of surfactant-induced wetting strongly depends on the vapor flux and the bulk concentration of surfactants in the feed solution, but surprisingly not on the transmembrane hydraulic pressure difference. The results from our study also suggest that while the presence of surfactants promotes wetting, adsorption of surfactants onto pore surface actually deters pore wetting instead of promoting it by rendering the surface hydrophilic.

It is a widely accepted principle that a thermodynamically reversible desalination process should consume the Gibbs free energy of separation. This principle has been shown in reverse osmosis and has important practical implications in reducing its energy consumption. Capacitive deionization (CDI) with carbon electrodes, a desalination process based on electrical double layer (EDL) formation, should also follow such a principle when it operates in a thermodynamically reversible way. Inspired by a previous thermodynamic analysis on a three-stage reversible CDI process using the Gouy-Chapman-Stern model, we conducted a thermodynamic analysis of a four-stage reversible CDI cycle using the modified Donnan model. This analysis better reflects the cyclic nature of practical CDI operations and account for the significant EDL overlap in nanosized micropores of realistic CDI electrodes. Our analysis of CDI cycles with different separations and final discharge voltages shows that the electrical work to complete a four-stage cycles is numerically exactly identical to the Gibbs free energy of separation, as long as the cycle is operated in a thermodynamically reversible manner.

Significant progress has been made over recent years in capacitive deionization (CDI) to develop novel system configurations, predictive theoretical models, and high-performance electrode materials. To bring CDI to large scale practical applications, it is important to quantitatively understand the intrinsic tradeoff between kinetic and energetic efficiencies, or the relationship between energy consumption and the mass transfer rate. In this study, we employed both experimental and modeling approaches to systematically investigate the tradeoff between kinetic and energetic efficiencies in membrane CDI (MCDI). Specifically, we assessed the relationship between the average salt adsorption rate and specific energy consumptions from MCDI experiments with different applied current densities but a constant effluent salinity. We investigated the impacts of feed salinity, diluted water salinity, diluted water volume per charging cycle, and electrode materials on the kinetics-energetics tradeoff. We also demonstrate how this tradeoff can be employed to optimize the design and operation of CDI systems and compare the performance of different electrode materials and CDI systems.

Fouling on submerged surfaces is a major limiting factor for membranes, heat exchangers, and marine vessels as it induces mass and heat transfer resistances that increase operating costs and lead to system failures. While the role of surface roughness on fouling has been extensively studied, the specific effect of surface roughness on fouling is debated in literature. In this study, we employed force spectroscopy based on atomic force microscopy with two model colloidal probes to elucidate the role of surface roughness on foulant-surface interactions. Specifically, we quantified the strength and characteristic lengths of the interactions between the colloidal probes and hydrophilic and hydrophobic surfaces with and without surface texture. We found that hydrophilic surfaces are generally less prone to foulant adhesion than hydrophobic surfaces and that increasing roughness of a hydrophilic surface mitigates foulant adhesion. In comparison, we found that increased roughness of a hydrophobic surface increases the attractive foulant-surface interaction, and thus, its fouling propensity. Based on the results from this study, the implications for developing surfaces with fouling resistance are also examined.

In this study, we developed composite membranes with a hydrophobic substrate and a hydrophilic top surface using electrospinning to mitigate oil fouling in membrane distillation (MD). The electrospinning approach can be universally applied to any hydrophobic membrane substrate and to ensure the non-wetting condition of the substrate due to the electrospun fibrous structure. Using this approach, polytetrafluoroethylene (PTFE) hydrophobic substrate was coated with two different hydrophilic fibrous networks, including a cellulose acetate (CA) fibrous network and a nanocomposite fibrous network comprising CA and silica nanoparticles (SiNPs). We characterized the pristine and the modified membranes using contact angle measurements and tensiometer-based oil probe force spectroscopy, and tested the anti-fouling performance of the different membranes in MD experiments using a saline crude-oil emulsion as the feed solution. While both coatings offered significant improvement in oil fouling resistance compared to the substrate PTFE membrane, the nanocomposite CA-SiNPs fibrous coating outperformed the CA coating in terms of hydrophilicity, oil adhesion resistance, and anti-oil-fouling performance in MD experiments.

The water-energy nexus has motivated the quest for new membrane-based technologies that target potable water and energy production. To this end, membrane distillation (MD), forward osmosis (FO) and pressure retarded osmosis (PRO) provide alternative means for the sustainable production of freshwater and electricity from feed water with high fouling potential such as wastewater. MD is a thermally driven process that can utilize low grade (latent) heat sources, while FO and PRO harness osmotic gradients as the driving force. High rejection of contaminants, compact modular design and low fouling propensity make these membrane technologies suitable for treating different types of wastewater. However, the application of feed solutions with high loads of organic matter and bacteria prompts the development of microbial fouling (biofouling), which significantly reduces system performance. Therefore, mitigating biofouling by minimizing bacterial attachment and enhancing the biofilm cleaning efficiency is imperative. We stress that in-depth exploration of the impacts imposed by biofilm in MD, FO and PRO systems is essential before developing new approaches for biofouling mitigation. This comprehensive review compiles the driving forces of these non-pressurized membrane systems, while focusing on the current knowledge regarding the various impacts of biofouling. Moreover, we highlight current and future research directions that focus on the development of new approaches to minimize MD, FO and PRO biofouling.

Membrane distillation (MD) is a promising desalination technology that is capable of utilizing low-grade thermal energy to treat highly saline feed water. Conventional hydrophobic MD membranes limit the application of MD to desalination of relatively clean water without amphiphilic contaminants, as those amphiphilic constituents promote wetting of MD membrane and failure of the MD process. Here, we report a facile approach to fabricate superamphiphobic MD membranes with anti-surfactant-wetting property based on coaxial electrospinning. Silica nanoparticles were used as the sheath solution to create electrospun fibers with nanoscale roughness on individual fibers. After surface fluorination, the fabricated membrane exhibited superamphiphobicity as reflected by very high sessile drop contact angles with both water and oil. Robust MD performance in the presence of surfactants was observed with the superamphiphobic membrane, but not with commercial hydrophobic membranes or amphiphobic membrane without local nanoscale roughness.

Membrane distillation (MD) has been identified as a promising technology to desalinate the hypersaline wastewaters from fracking and other industries. However, conventional hydrophobic MD membranes are highly susceptible to fouling and/or wetting by the hydrophobic and/or amphiphilic constituents in these wastewaters of complex compositions. This study systematically investigates the impact of the surface wetting properties on the membrane wetting and/or fouling behaviors in MD. Specifically, we compare the wetting and fouling resistance of three types of membranes of different wetting properties, including hydrophobic and omniphobic membranes as well as composite membranes with a hydrophobic substrate and a superhydrophilic top surface. We challenged the MD membranes with hypersaline feed solutions that contained a relatively high concentration of crude oil with and without added synthetic surfactants, Triton X-100. We found that the composite membranes with superhydrophilic top surface were robustly resistant to oil fouling in the absence of Triton X-100, but were subject to pore wetting in the presence of Triton X-100. On the other hand, the omniphobic membranes were easily fouled by oil-in-water emulsion without Triton X-100, but successfully sustained stable MD performance with Triton X-100 stabilized oil-in-water emulsion as the feed solution. In contrast, the conventional hydrophobic membranes failed readily regardless whether Triton X-100 was present, although via different mechanisms. These findings are corroborated by contact angle measures as well as oil-probe force spectroscopy. This study provides a holistic picture regarding how a hydrophobic membrane fails in MD and how we can leverage membranes with special wettability to prevent membrane failure in MD operations.

Recent progress in developing anti-fouling membranes suggests that integrating low-surface-energy functional groups into a hydrophilic matrix can generate a robust fouling resistant surface that not only mitigates the attachment, but also facilitates the detachment, of hydrophobic foulants. Excellent anti-fouling performance with such surfaces with heterogeneous surface energy has been reported in pressurized membrane processes. However, the impact of low-surface-energy functional groups on the membrane fouling resistance in membrane distillation (MD) has not been investigated and is the goal of the current study. In this study, we compared the fouling behaviors between a reference hydrophobic polyvinylidene fluoride (PVDF) membrane and three composite membranes with chitosan based hydrogel surface but different amount of perfluoroalkyl functional groups. We first fabricated and characterized the three composite membranes, then challenged them in MD experiments with a crude-oil-emulsion as the feed solutions, and finally conducted force spectroscopy to probe the interaction between an oil droplet and the three composite membranes as well as the reference PVDF membrane. The results from MD fouling tests and force spectroscopy suggest that incorporating perfluoroalkyl functional groups into the hydrogel matrix did significantly improve the anti-fouling performance and reduce oil adhesion onto the membrane surface as long as they were not excessive on the surface.

This review introduces the development history of the widely used NF model, i.e., the Donnan-Steric Pore Model with Dielectric Exclusion (DSPM-DE), from the emergence of its predecessors to its current form. We present the details of DSPM-DE with assumptions and equations that account for each mechanism. Model inputs and outputs, as well as the key parameters and the experimental procedures for determining these parameters are also explained. Furthermore, the DSPM-DE is applied to investigate NF performance under various conditions. Specifically, the DSPM-DE is employed to provide a mechanistic interpretation of the well-known selectivity-permeability tradeoff that is more typically applied to describe the behavior of non-porous membrane. Based on the simulations using the DSPM-DE, we also discuss strategies on enhancing NF performance via tuning membrane properties. NF membranes with thin active layer and small nanopores that are strongly hydrophilic and charged are beneficial to achieving a high perm-selectivity. Analysis is also performed on the separation of divalent and monovalent cations using DSPM-DE. Finally, we discuss the uncertainties of model parameters and their impacts on the model prediction. We also discuss the validity of fundamental model assumptions and the prospects on potential improvements of NF modeling to enhance the utility in predicting NF performance. Machine learning-based models, once trained with a large set of data, are likely more powerful than DSPE-DE or any physical NF model in future application of performance prediction.

Wetting induced by salt-scaling and surfactants is the Achilles heel of membrane distillation, especially for concentration of high salinity wastewater. Herein, we rationally developed a membrane with robust wetting resistance by integrating superhydrophobic-omniphobic surface and anti-deformable pores into one system. The membrane was first developed by electrospinning, which was then modified with surface roughness, and followed by coating of polydimethylsiloxane to weld the intersecting fibers and fluoroalkylsilane to lower the membrane surface energy. The product exhibits excellent wetting resistance when concentrating the high salinity NaCl solution from 20 to 38 wt% (saturation condition), the simulated reverse osmosis concentrated water, the gypsum and the low-surface-tension high salinity wastewater. Moreover, the mechanism of membrane wetting resistance was also systematically discussed based on the experiment and computer simulation. It reveals that the superhydrophobic-omniphobic surface could stabilize the surface-bound air layer chemically, thus reducing the contact of crystals and surfactant with the membrane surface. Simultaneously, the anti-deformable pore also helps it overcome the asymmetrical hydraulic disturbance on enlarging membrane pore size, thus stabilizes the surface-bound air layer structurally. The presented development will provide a platform to understand and achieve wetting and scaling inhibition MD membrane for high salinity wastewater treatment.

We herein report a novel approach based on electrochemical impedance spectroscopy (EIS) for in-situ monitoring of the adsorption kinetics in preparing polyelectrolyte multilayer nanofiltration membranes using layer-by-layer (LbL) deposition. Unlike existing methods for monitoring adsorption kinetics, this new approach is non-destructive and applicable to various substrates (as it does not use the substrate as a sensor). The model nanofiltration membrane used in this study is prepared by alternate depositions of Poly(diallyldimethylammonium chloride) and Poly(sodium 4-styrenesulfonate) on a polyacrylonitrile ultrafiltration membrane as the support. In each deposition step, the EIS measurements yield two important parameters, including the interfacial layer solution resistance and the film resistance, for probing the extent of polyelectrolyte deposition and membrane performance. The extent of polyelectrolyte deposition as probed by the EIS measurements is well corroborated by independent measurements of the membrane surface potential and the nanofiltration performance including water permeability and Na2SO4 rejection. This EIS-based approach enables the optimization of membrane fabrication using LbL deposition by conveniently identifying the minimum deposition time required to attain surface saturation.

Optimizing system design and operation of reverse osmosis (RO) systems requires an in-depth comprehension of the intrinsic tradeoff between RO mass transfer kinetics and energetics. In this study, we demonstrate that this critical trade-off can be quantified using the relationship between the average water flux and the specific energy consumption (SEC). We derive analytical expressions to quantify the average water flux and SEC for single stage, two stage, and closed circuit RO processes. These analytical expressions are useful for system design and operation optimization as they facilitate direct comparison of the kinetic and energetic efficiencies between RO processes with different operation conditions and system configurations. Finally, we compare the kinetics and energetics of the three system configurations using these analytical expressions and discuss their relative advantages and disadvantages in RO desalination.

The efforts of developing anti-oil-fouling membranes have so far been focusing on tailoring membrane surface wetting properties, whereas the impacts of surface charge are often eclipsed. In this study, we investigated the impacts of surface charge and wetting property on oil fouling kinetics in membrane distillation (MD). Two composite membranes with in-air hydrophilic and underwater oleophobic surfaces, one of positive charge and the other of negative charge, were fabricated by modifying a hydrophobic polyvinylidene fluoride (PVDF) membrane with hydrophilic polyelectrolytes with different charges. The modified composite membranes were compared with the reference PVDF membrane for their contact angles, adhesion force curves, and fouling kinetics in MD processes. It was found that the negatively charged composite membrane performed the best in mitigating fouling by the negatively charged oil emulsion, followed by the positively charged composite membrane, with the pristine PVDF membrane being the most susceptible to oil fouling in MD experiments. The results from underwater oil CA measurements and oil probe force spectroscopy corroborated the results in the fouling experiments. The impact of surface charges on oil-membrane interaction and associated mechanism were also discussed.

The recent development of high performance thin film composite membrane has been a major technological thrust in the research of forward osmosis (FO). While most of the recently developed FO membranes are of flat sheet (FS) geometry, hollow fiber (HF) FO membranes have attracted significant attention in recent years due to their promising prospect in full-scale applications. Existing studies on HF FO membrane fabrication and characterization exclusively apply the mass transfer equations developed for FS membranes. Whether or not these mass transfer equations for FS membranes are applicable for membranes with HF geometry remains theoretically unclear. In this paper, accurate analytical equations are derived to describe mass transfer of water and solute across an HF membrane. These equations take into account the curvature effect of the HF membranes and thus have very different mathematical forms from those for FS membranes. A systematic comparison of the mass transfer equations between HF and FS membranes was also conducted using both simulated and experimentally measured flux data. The results from such a comparison suggest that the mass transfer equations for FS membranes are in general applicable for an HF geometry, which provides the theoretical basis for the application of the well-established FS mass transfer equations in characterizing HF membranes.

Although significant technological advances have been made in recent years on pressure retarded osmosis (PRO), its practical viability remains unclear as few studies have been conducted at an integrated system level to quantify the potential of net energy output. In this study, we develop a framework to assess the net energy output of a PRO system by first quantifying the gross energy output via solving the mass transfer equations for a full-scale PRO module, and then incorporating the major energy losses from pretreatment, flow circulation, and inefficient energy recovery. We also propose a novel concept called net membrane power density that is strongly relevant to the capital cost of a PRO system. Finally, we describe an approach, based on the quantifiable specific net energy and net membrane power density, for assessing the economic viability of a PRO system. Albeit using seawater/river water PRO as the context for illustrating our approach, the assessment framework developed is universally applicable to PRO systems with any solution pairing. The results from this study clearly show the impacts of various parameters on the practical performance of a PRO system, thereby providing important guidance to the improvement of its design and operation.

Bipolar membrane electrodialysis (BMED) is a promising, cost-effective technology for treating highly saline wastewater toward zero-liquid-discharge. Instead of producing low-value minerals of sodium chloride and sulfate, BMED can produce a valuable concentrated acid and base for industrial reuse. In this study, we first develop a numerical model to describe the mass transfer and energy consumption of a BMED process. We then present a systematic framework based on performance tradeoff curves for evaluating the performance of a BMED process that captures the inherent tradeoff between energy efficiency and the kinetic rate of the process. Such a performance tradeoff provides the technical basis for comparison between different BMED operations and systems and for technoeconomic analysis. Using such a framework and Na2SO4 as a model feed solution, we evaluate the impacts of initial feed concentration, target concentration of NaOH, and the ratio between the initial volumes of feed and seed base solutions on the BMED performance. Finally, we also demonstrate that a novel 3D junction bipolar membrane can enhance the performance of the BMED process.

High-performance nanofiltration membrane with excellent perm-selectivity and fouling resistance was fabricated by layer-by-layer deposition of polyelectrolytes, polyethylenimine (PEI) and polystyrene sulfonate (PSS), with the intercalation of self-assemblies of zwitterionic surfactants, 3-(N, N-Dimethylmyristylammonio) propane sulfonate (SB3-14). The integration of SB3-14 to the polyelectrolyte active layer dramatically enhanced the water permeability of the low-pressure nanofiltration (LNF) membrane without compromising the rejection of humic acid (HA). Specifically, the LNF membrane with intercalation of zwitterionic surfactant self-assemblies achieved a water permeability of up to 131 L m−2 h−1 bar−1 and an HA rejection over 99%. In addition, the intercalation of SB3-14 surfactants also made the LNF membrane significantly smoother and less prone to fouling in long-term LNF operation, leading to higher water flux and HA rejection when the LNF process reached a steady state.

In this study, we analyze the effects of membrane properties, namely water permeability, solute permeability, and structural parameter, on the overall performance of an FO membrane module to extract water from simulated seawater (0.6 M NaCl). By considering the thermodynamic limit of operation, we demonstrate that the maximum achievable water recovery is practically independent of membrane properties, and higher maximum water recovery is achievable with counter-current compared to co-current mode. Analysis of the module-scale model indicates that reducing the support layer structural parameter offers substantial reductions in the membrane area required to achieve a specified water recovery. For example, a 25% reduction of the structural parameter of a state-of-the-art thin-film composite (TFC) membrane (from 400 to 300 μm) yields a sizable 20% reduction in membrane area. In contrast, quintupling the water permeability coefficient (from 2.0 to 10.0 L m−2 h−1 bar−1) of a modern TFC membrane generates only a modest 10% saving in membrane area. In addition, because of the permeability-selectivity trade-off that governs current polymeric membranes, doubling the water permeability coefficient would cause crippling ~7-fold increases in forward and reverse solute permeation. This quantitative study models the potential performance of a module-scale FO desalination process and firmly highlights the need to prioritize the reduction of support layer mass transport resistances over water permeability increases in membrane development.

Reverse osmosis (RO), currently the most energy efficient desalination process, is inherently more energy intensive compared to conventional fresh water treatment technologies, as it is constrained by the thermodynamics of separation of saline solutions. Therefore, pushing the energy consumption towards the thermodynamic limit of separation would lead to significant long-term savings in energy cost. In this work, we quantitatively demonstrate the potential of energy reduction for RO desalination using staged operations with both multi-stage direct pass and closed-circuit configurations. We relate the minimum specific energy of desalination (i.e., the minimum energy required to generate a unit volume of permeate water) to the number of stages in each configuration, and elucidate the fundamental difference between the two configurations. Our analysis suggests that although it is theoretically impossible to reach the thermodynamic minimum energy of separation with closed-circuit RO, this configuration is robust and much more practical to implement than the multi-stage direct pass RO.

Forward osmosis (FO) has been extensively investigated in the past decade. Despite significant advancements in our understanding of the FO process, questions and challenges remain regarding the energy efficiency and current state of the technology. Here, we critically review several key aspects of the FO process, focusing on energy efficiency, membrane properties, draw solutes, fouling reversibility, and effective applications of this emerging technology. We analyze the energy efficiency of the process, disprove the common misguided notion that FO is a low energy process, and highlight the potential use of low-cost energy sources. We address the key necessary membrane properties for FO, stressing the importance of the structural parameter, reverse solute flux selectivity, and the constraints imposed by the permeability–selectivity tradeoff. We then dispel the notion that draw solution regeneration can use negligible energy, highlighting the beneficial qualities of small inorganic and thermolytic salts as draw solutes. We further discuss the fouling propensity of FO, emphasizing the fouling reversibility of FO compared to reverse osmosis (RO) and the prospects of FO in treating high fouling potential feed waters. Lastly, we discuss applications where FO outperforms other desalination technologies and emphasize that the FO process is not intended to replace RO, but rather is to be used to process feed waters that cannot be treated by RO.

Direct contact membrane distillation (DCMD) can desalinate saline waters using low-grade heat and is thus economically attractive when low-temperature thermal energy is readily available. Coupling DCMD with a heat exchanger (HX) can significantly enhance the energy efficiency of the process by recovering the latent heat accumulated in the permeate (distillate) stream. This study evaluates the mass recovery rate (i.e., fraction of feed water recovered), γ, and the specific heat duty (i.e., energy input per unit mass of product water), β, of DCMD desalination using low-grade heat coupled with HX. Mass and heat transfer in DCMD and HX were modeled at the module scale and thermodynamic analysis of the system was performed. The relative flow rate (between the permeate and feed streams), α, was found to be a critical operation parameter to optimize process performance, regardless of the mass and heat transfer kinetics. Both numerical evaluation and analytical analysis reveal a critical relative flow rate, α⁎, that demarcates DCMD operation between a permeate limiting regime (when α α⁎), when mass transfer kinetics are not limiting. Similarly, we identified mass-limited and temperature-limited heat recovery regimes in the HX that are dependent on α. Our analysis shows that the highest γ and lowest β achievable are solely determined by the thermodynamic properties of the system and always occur at the critical relative flow rate, α⁎. For example, the thermodynamic limits for γ and β are 6.4% and 27.6 kJ kg−1, respectively, for seawater desalination by single-pass DCMD at 60 °C with HX. However, in practical operation, as the DCMD membrane area and permeability cannot be infinitely large, the process is in a mass-transfer-limiting-regime and performance departs from the thermodynamic limits. Lastly, we demonstrate that heat transfer across a thermally-conductive DCMD membrane further reduces the recovery rate and energy efficiency of the process. The findings from this study have important implications for optimization of the DCMD process and for serving as criteria to assess process performance.

Interactions between organic matter (OM) and engineered polymer coatings as they affect the retention of polyvinylpyrrolidone (PVP) polymer-coated silver nanoparticles (AgNPs) were studied. Two distinct types of OM-cysteine representing low molecular weight multivalent functional groups, and Suwannee River Humic Acid (HA) representing high molecular weight polymers, were investigated with respect to their effects on particle stability in aggregation and deposition. Aggregation of the PVP coated AgNPs (PVP-AgNPs) was enhanced by cysteine addition at high ionic strengths, which was attributed to cysteine binding to the AgNPs and replacing the otherwise steric stabilizing agent PVP. In contrast the addition of HA did not increase aggregation rates and decreased PVP-AgNP deposition to the silica porous medium, consistent with enhanced electrosteric stabilization by the HA. Although cysteine also reduced deposition in the porous medium, the mechanisms of reduced deposition appear to be enhanced electric double layer (EDL) interaction at low ionic strengths. At higher ionic strengths, aggregation was favored leading to lower deposition due to smaller diffusion coefficients and single collector efficiencies despite the reduced EDL interactions.

An extension of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was developed to model the potential energy of interaction between a soft particle (particle coated with either uncharged polymer or polyelectrolyte) and a rigid flat surface prior to the particle-surface contact. An uncharged polymer is predicted to reduce the barrier of interaction energy curve by shifting the contact frontier away from the particle core and as a result increases the affinity between the coated particle and the uncoated surface. For particles coated with polyelectrolyte, the particle–surface interaction is primarily regulated by the thickness and charge density of the polyelectrolyte layer. For either type of coating, increasing ionic strength is predicted to reduce the barrier of interaction potential and thus increase the affinity between the particle and the surface. A more comprehensive model was also established for a polyelectrolyte-coated particle of charged core surface, in which the importance of segment density of the coating layer was demonstrated. Limitations of these models were discussed especially for particles of very low segment density to which the post-contact interaction would become important.

Multi-wall carbon nanotube (MWCNT)/aromatic polyamide (PA) nanocomposite membranes were synthesized by a polymer grafting process. Surface morphology, roughness, and mechanical strength of the resultant nanocomposite membranes were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and micro-strain analysis, respectively. SEM and AFM images showed that MWCNTs were well dispersed in the PA matrix. Measurements of mechanical properties of this composite showed increasing membrane strength with increasing MWCNT content with monotonic increases in Young's modulus, toughness, and tensile strength. The addition of MWCNTs also improved the rejection of both salt and organic matter relative to the 10% PA membrane base case. The nanocomposite membrane synthesized with 15 mg/g MWCNT in a 10% PA casting solution rejected NaCl and humic acid by factors of 3.17 and 1.67 respectively relative to the PA membrane without MWCNTs, while membrane permeability decreased by 6.5%.