In this study, we exploit the nitrogen-sulfur elemental contrast of thin-film composite (TFC) polyamide membranes and present, for the first time, the application of two elemental analysis techniques, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDX) and X-ray photoelectron spectroscopy (XPS) C60⁺ ion-beam sputtering, to elucidate the nanoscale structure and chemical composition of the polyamide-polysulfone interface. Although STEM-EDX elemental mapping depicts the presence of a dense polyamide layer at the interface, it is incapable of resolving the elemental contrast at nanoscale resolution at the interfacial zone. Depth-resolved XPS C60⁺ ion-beam sputtering enabled nanoscale characterization of the polyamide-polysulfone interface and revealed the presence of a heterogeneous layer that contains both polyamide and polysulfone signatures. Our results have important implications for future studies to elucidate the structure-property-performance relationship of TFC membranes.

Xinglin Lu    Siamak Nejati    Youngwoo Choo    Chinedum Osuji    马军    Menachem Elimelech    

ACS Applied Materials and Interfaces

2015

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.

Shihong Lin    Menachem Elimelech    

Desalination

2015

Understanding the interactions of graphene oxide (GO) with biological membranes is crucial for the evaluation of GO's health and environmental impacts, its bactericidal activity, and to advance graphene-based biological and environmental applications. In an effort to understand graphene-induced bacterial inactivation, we studied the interaction of GO with bacterial (Escherichia coli) cell membranes using atomic force microscopy (AFM). Toward this goal, we devised a polydopamine-assisted experimental protocol to functionalize an AFM probe with GO nanosheets, and used AFM-based force spectroscopy to measure cell membrane-GO interaction forces. Our results show that GO-cell interactions are predominantly repulsive, with only sporadic adhesion forces being measured upon probe pull-off, which we attribute to lipopolysaccharide bridging. We provide evidence of the acellular oxidation of glutathione by GO, underscoring the role of oxidative pathways in GO-mediated bacterial cell inactivation. Our force spectroscopy results suggest that physicochemical interactions do not underlie the primary mode of action of GO in bacteria.

Santiago Romero-Vargas Castrillón    François Perreault    Andréia fonseca De    Menachem Elimelech    

Environmental Science and Technology Letters

2015

This study elucidates the relationship between membrane properties and the rejection of trace organic contaminants (TrOCs) in forward osmosis (FO). An asymmetric cellulose triacetate (CTA) and a thin-film composite (TFC) polyamide FO membrane were used for this investigation. The effective average pore radius (r), selective barrier thickness over porosity parameter (l/ε), surface charge, support layer structural parameter (S), pure water permeability coefficient (A) and salt (NaCl) permeability coefficient (B) of the two membranes were systematically characterised. Results show that measured rejection of TrOCs as a function of permeate water flux can be well described by the pore hindrance transport model. This observation represents the first successful application of this model, which was developed for pressure-driven nanofiltration, to an osmotically-driven membrane process. The rejection of charged TrOCs by the CTA and TFC membranes was high and was governed by both electrostatic repulsion and steric hindrance. The TFC membrane exhibited higher rejection of neutral TrOCs with low molecular weight than the CTA membrane, although the estimated pore size of the TFC membrane (0.42nm) was slightly larger than that of the CTA membrane (0.37nm). This higher rejection of neutral TrOCs by the TFC membrane is likely attributed to its active layer properties, namely a more effective active layer structure, as indicated by a larger l/ε parameter, and pore hydration induced by the negative surface charge. © 2013.

Ming Xie    Nghiem L.    William Price    Menachem Elimelech    

Water Research

2014

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) and a feed 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 27.6kJkg, 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. © 2013 Elsevier B.V.

Shihong Lin    Ngai yin Yip    Menachem Elimelech    

Journal of Membrane Science

2014

Fouling behaviour and its impact on the rejection of trace organic contaminants (TrOCs) by forward osmosis (FO) were investigated. Membrane fouling was simulated using humic acid and colloidal particles as model foulants at different initial permeate water fluxes. Water flux decline was insignificant at an initial permeate flux of 9L/mh and the fouling layer was loose and fluid-like. By contrast, the water flux decline was substantial at an initial permeate flux of 20L/mh, resulting in the formation of a compact fouling layer. Water flux recovery after physical cleaning for both humic acid and colloidal particle fouled membranes was consistently higher at an initial permeate flux of 9L/mh compared to 20L/mh. The results suggest that the fouling layer structure varied from a fluid-like loose layer at low initial permeate flux to a more cohesive and compact layer at high initial permeate flux. We surmise that the fluid-like loose layer formed at low initial permeate flux contributed to pore blockage and thus enhanced steric hindrance, thereby leading to an increase in TrOC rejection. By contrast, the cohesive and compact fouling layer formed at high initial permeate flux exacerbated cake-enhanced concentration polarisation, resulting in a decrease in TrOC rejection. © 2014.

Ming Xie    Nghiem L.    William Price    Menachem Elimelech    

Desalination

2014

We present a novel hybrid membrane system that operates as a heat engine capable of utilizing low-grade thermal energy, which is not readily recoverable with existing technologies. The closed-loop system combines membrane distillation (MD), which generates concentrated and pure water streams by thermal separation, and pressure retarded osmosis (PRO), which converts the energy of mixing to electricity by a hydro-turbine. The PRO-MD system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages for heat source temperatures ranging from 40 to 80 °C and working concentrations of 1.0, 2.0, and 4.0 mol/kg NaCl. The factors controlling the energy efficiency of the heat engine were evaluated for both limited and unlimited mass and heat transfer kinetics in the thermal separation stage. In both cases, the relative flow rate between the MD permeate (distillate) and feed streams is identified as an important operation parameter. There is an optimal relative flow rate that maximizes the overall energy efficiency of the PRO-MD system for given working temperatures and concentration. In the case of unlimited mass and heat transfer kinetics, the energy efficiency of the system can be analytically determined based on thermodynamics. Our assessment indicates that the hybrid PRO-MD system can theoretically achieve an energy efficiency of 9.8% (81.6% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and a working solution of 1.0 M NaCl. When mass and heat transfer kinetics are limited, conditions that more closely represent actual operations, the practical energy efficiency will be lower than the theoretically achievable efficiency. In such practical operations, utilizing a higher working concentration will yield greater energy efficiency. Overall, our study demonstrates the theoretical viability of the PRO-MD system and identifies the key factors for performance optimization. © 2014 American Chemical Society.

Shihong Lin    Ngai yin Yip    Tzahi Cath    Chinedum Osuji    Menachem Elimelech    

Environmental Science and Technology

2014

We demonstrate the simultaneous extraction of phosphorus and clean water from digested sludge centrate using a forward osmosis (FO)-membrane distillation (MD) hybrid process. In this FO-MD hybrid process, FO concentrates orthophosphate and ammonium for subsequent phosphorus recovery in the form of struvite (MgNHPO·6HO), while MD is used to recover the draw solution and extract clean water from the digested sludge centrate. A decline in water flux was observed during the FO process, but fouling was largely reversible after a brief, simple membrane flushing using deionized water. The FO process also provides an effective pretreatment capacity to the subsequent MD process, which exhibited stable water flux. The use of MgCl as the draw solute for the FO process is another novel aspect of the system. The reverse salt flux of magnesium to the concentrated digested sludge across the FO membrane and the diffusion of protons away from the digested sludge create favorable conditions for the formation of struvite crystals. The precipitates obtained in the hybrid process were verified to be struvite crystals by examining the crystal morphology, element composition, and crystal structure. Results reported here highlight the potential and robustness of the FO-MD hybrid process for extracting phosphorus from wastewater.

Ming Xie    Nghiem L.    William Price    Menachem Elimelech    

Environmental Science and Technology Letters

2014

Pressure-retarded osmosis (PRO) has the potential to generate sustainable energy from salinity gradients. PRO is typically considered for operation with river water and seawater, but a far greater energy of mixing can be harnessed from hypersaline solutions. This study investigates the power density that can be obtained in PRO from such concentrated solutions. Thin-film composite membranes with an embedded woven mesh were supported by tricot fabric feed spacers in a specially designed crossflow cell to maximize the operating pressure of the system, reaching a stable applied hydraulic pressure of 48 bar (700 psi) for more than 10 h. Operation at this increased hydraulic pressure allowed unprecedented power densities, up to 60 W/m with a 3 M (180 g/L) NaCl draw solution. Experimental power densities demonstrate reasonable agreement with power densities modeled using measured membrane properties, indicating high-pressure operation does not drastically alter membrane performance. Our findings exhibit the promise of the generation of power from high-pressure PRO with concentrated solutions.

Anthony Straub    Ngai yin Yip    Menachem Elimelech    

Environmental Science and Technology Letters

2014

This study systematically investigates the organic fouling behavior of a superhydrophilic polyvinylidene fluoride (PVDF) ultrafiltration membrane functionalized via post-fabrication tethering of surface-tailored silica nanoparticles to poly(methacrylic acid)-grafted PVDF membrane surface. Sodium alginate (SA), Suwannee River natural organic matter (SRNOM), and bovine serum albumin (BSA) were used as model organic foulants to investigate the antifouling behavior of the superhydrophilic membrane with combined-fouling (mixture of foulants) and individual-fouling (single foulant) tests. A membrane bioreactor (MBR) plant supernatant was also used to verify the organic antifouling property of the superhydrophilic membrane under realistic conditions. Foulant size distributions and foulant-membrane interfacial forces were measured to interpret the observed membrane fouling behavior. Molecular weight cutoff measurements confirmed that membrane functionalization did not adversely affect the intrinsic membrane selectivity. Both filtration tests with the synthetic foulant-mixture solution (containing SA, SRNOM, and BSA) and MBR plant supernatant demonstrated the reliability and durability of the antifouling property of the superhydrophilic membrane. The conspicuous reduction in foulant-membrane interfacial forces for the functionalized membrane further verified the antifouling properties of the superhydrophilic membrane, suggesting great potential for applications in wastewater treatment. © 2014 Elsevier B.V.

Shuai Liang    Genggeng Qi    Kang Xiao    Jianyu Sun    Emmanuel Giannelis    黄霞    Menachem Elimelech    

Journal of Membrane Science

2014