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The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes Jay Ryan Werber, Akshay Deshmukh, and Menachem Elimelech Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.6b00050 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 10, 2016
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Environmental Science & Technology Letters
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The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes
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Environmental Science & Technology Letters
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Jay R. Werber,† Akshay Deshmukh,† and Menachem Elimelech*,†,‡
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†
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286
‡
Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University
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* Corresponding author: Menachem Elimelech, Email:
[email protected], Phone: (203) 432-2789
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ABSTRACT
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Desalination membranes are essential for the treatment of unconventional water sources, such as
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seawater and wastewater, to alleviate water scarcity. Promising research efforts on novel
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membrane materials may yield significant performance gains over state-of-the-art thin-film
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composite (TFC) membranes, which are constrained by the permeability–selectivity tradeoff.
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However, little guidance currently exists on the practical impact of such performance gains,
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namely enhanced water permeability or enhanced water–solute selectivity. In this critical review,
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we first discuss the performance of current TFC membranes. We then highlight and provide
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context for recent module-scale modeling studies that have found limited impact of increased
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water permeability on the efficiency of desalination processes. Next we cover several important
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examples of water-treatment processes where inadequate membrane selectivity hinders process
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efficacy. We conclude with a brief discussion of how the need for enhanced selectivity may
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influence the design strategies of future membranes.
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INTRODUCTION
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As the impact of water scarcity grows in regions around the globe, there is an ever-increasing
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need to augment municipal, industrial, and agricultural water supplies through the purification of
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unconventional water sources, such as seawater and municipal wastewater.1,
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inextricable linkage between water and energy consumption — often called the water–energy
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nexus — the augmentation of water supplies must not come at the cost of large amounts of
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consumed energy. As such, with their high energy efficiency and often superior efficacy,
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membrane-based technologies have gained widespread implementation in various water-
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treatment processes.2, 3
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Due to the
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Desalination membranes — membranes that allow passage of water but largely reject salt and
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most other solutes — play a critical role in many of these processes.3-5 These membranes lie at
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the heart of traditional reverse osmosis (RO) processes, including (i) seawater reverse osmosis
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(SWRO), which is the dominant seawater desalination technology globally, and (ii) brackish
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water reverse osmosis (BWRO), which allows for desalination of low salinity water, such as
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brackish groundwater that makes up 55% of global groundwater supplies.4, 6 In addition, RO is a
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key step in advanced municipal wastewater treatment schemes that allow for industrial and
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potable reuse.2, 7 Lastly, the emerging technology of forward osmosis (FO), which also relies on
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desalination membranes, has enabled treatment of highly saline wastewaters, such as shale-gas
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produced waters, that are untreatable by RO due to high required hydraulic pressures.8
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The increased use of desalination membranes has come with a renewed focus on membrane
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materials research.9 Due to recent advances in nanomaterial synthesis and assembly, potential
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step-change improvements in performance may be possible. However, the existing body of
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literature lacks guidance on the practical impact of improvements in the critical active layer
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properties, namely membrane water permeability and water–solute selectivity. In other words, in
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the design of novel desalination membranes, what active layer properties are most desired?
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In this critical review, we first cover the performance of state-of-the-art desalination
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membranes. We then review recent analyses and modeling studies that have found limited
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impact of improvements in membrane water permeability on the performance of RO and FO
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processes. Next, we highlight several important examples of processes that are adversely affected
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by inadequate solute retention, demonstrating the need for enhanced water–solute selectivity. 2 ACS Paragon Plus Environment
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Lastly, we discuss how current and potential future membranes fit into this landscape. Lessons
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gained from this critical review should influence the design strategies of novel desalination
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membranes.
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SELECTIVE LAYER PERFORMANCE OF CURRENT MEMBRANES
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Aromatic thin-film composite (TFC) polyamide membranes, the current state-of-the-art, serve as
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the benchmark for any novel desalination membrane.5 The selective layer — also called the
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active layer — in TFC membranes is a dense, highly-crosslinked polyamide film, formed via the
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interfacial polymerization of two aromatic monomers: m-phenylenediamine and trimesoyl
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chloride. Water and solute transport through the active layer is governed by the solution-
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diffusion model.10, 11 In this model, transport through the active layer, which is considered non-
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porous, is diffusive in nature. Water and solutes partition into the polymeric active layer, diffuse
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down their chemical potential gradient, and desorb into the permeate solution.
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Water flux according to the solution-diffusion model is given by10, 11
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ܬ௪ = ܣሺ∆ܲ − ∆ߨ ሻ
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where Jw is the volumetric water flux, A is the water permeability coefficient (also called
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permeance), ∆P is the applied hydraulic pressure, and ∆πm is the osmotic-pressure difference
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across the membrane active layer between the feed and permeate sides. In RO, flow is driven by
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hydraulic over-pressure, i.e. the difference between hydraulic and osmotic pressures. In FO, flow
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is driven by an osmotic-pressure difference created using a highly concentrated draw solution.
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For RO, eq 1 can be modified using film theory to account for concentration polarization in the
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diffusive boundary layer at the feed channel–membrane interface:10
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ܬ௪ = ܣ∆ܲ − ∆ߨܾ exp ൬݇ ݓ൰൨
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Here, ∆πb is the osmotic-pressure difference between the bulk feed and permeate solutions, and kf
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is the overall feed-side mass transfer coefficient averaged for all feed solutes. In both RO and FO,
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solute flux is modeled as Fickian diffusion:
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ܬ௦ = ܿ∆ܤ
(1)
ܬ
(2)
݂
(3)
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where Js is the solute flux, B is the solute permeability coefficient, and ∆cm is the concentration
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difference across the membrane active layer. As can be seen from eqs 1–3, the contribution of
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the membrane active layer to water and solute fluxes is entirely contained in the lumped
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coefficients A and B.
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The impact of membrane properties on water flux differs between RO and FO. In RO, only
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active-layer properties (i.e. A coefficient) affect water flux, while in FO support-layer properties
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are also important. During FO operation, permeating water molecules dilute the draw solution at
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the interface of the active layer and support layer. This dilution, combined with hindered
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diffusion within the support layer, results in a draw-solute concentration gradient — termed
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internal concentration polarization — that sharply decreases the osmotic pressure driving force
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and the achievable water flux.8 Support-layer properties that impact resistance to diffusion,
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including thickness, δs, tortuosity, τ, and effective porosity, εeff, are contained in the structural
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parameter, S:8, 12
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ఋ ఛ
ܵ=ఌೞ
(4)
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Minimizing the structural parameter (i.e., achieving high porosity, low tortuosity, and low
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thickness) maximizes draw-solute diffusion and the resulting water flux in FO.
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Solute retention in RO and FO is predominantly influenced from a materials perspective by
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active-layer properties, namely A and B.8, 11 For example, solute rejection (R = 1 – cpermeate/cfeed)
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in RO can be modeled as a function of A and B (see Supporting Information for derivation):
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ோ ଵିோ
=
ቈ∆ି∆గ್ ୣ୶୮ቆ ೢ ቇ ೖ ୣ୶୮൬ ೢ ൰ ೖೞ
=
ೢ
ୣ୶୮൬ ೢ ൰ ೖೞ
(5)
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where ksol is the feed-side mass transfer coefficient for the solute of interest. It is useful to
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consider eq 5 at fixed hydraulic pressure while neglecting changes in the bulk osmotic-pressure
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difference (i.e. ∆ܲ − ∆ߨ is constant), which is permissible in the case of high salt rejection. At
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very low A, water flux is low, concentration polarization is minimal (Jw1 pH unit from the solute pKa, and only from studies that verified that membrane salt rejection was near expected levels (>98%). NDMA refers to N-nitrosodimethylamine, a disinfection byproduct. Square symbols are from Ozaki and Li.69 Triangle symbols are from Miyashita et al.52
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(For final publication, use the high resolution figures provided as separate files)
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More efficient processes
Selectivity
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Water Permeability
In v e r s e S a lt P e r m e a b ility C o e ffic ie n t 1 / B ( L -1 m 2 h )
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