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Experiments and modeling of boric acid permeation through double-skinned forward osmosis membranes Lin Luo, Zhengzhong Zhou, Tai-Shung Chung, Martin Weber, Claudia Staudt, and Christian Maletzko Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b06166 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 12, 2016
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Environmental Science & Technology
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Experiments and modeling of boric acid permeation through double-skinned forward
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osmosis membranes
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Lin Luo1,2 , Zhengzhong Zhou3, Tai-Shung Chung1,2,4*,
7
Martin Weber5, Claudia Staudt5, Christian Maletzko6
8 9 10 11 12 13 14 15 16 17 18 19
1
2
NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Singapore 117456
Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 3
4
Water Desalination & Reuse (WDR) Center, King Abdullah University of Science and Technology, Thuwal 23955–6900, Saudi Arabia
20 21 22 23 24 25 26 27
School of Chemistry & Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang, Jiangsu Province, P. R. China 212013
5
Advanced Materials & Systems Research, BASF SE, GM-B001, 67056 Ludwigshafen, Germany 6
Performance Materials, BASF SE, G-PM/PU-F206, 67056 Ludwigshafen, Germany
*Corresponding author
28 29
Tel: +65-65166645; fax: +65-67791936; Email:
[email protected] 30
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Abstract
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Boron removal is one of the great challenges in modern wastewater treatment, owing to the
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unique small size and fast diffusion rate of neutral boric acid molecules. As forward osmosis
34
(FO) membranes with a single selective layer are insufficient to reject boron, double-skinned
35
FO membranes with boron rejection up to 83.9% were specially designed for boron
36
permeation studies. The superior boron rejection properties of double-skinned FO membranes
37
were demonstrated by theoretical calculations, and verified by experiments. The double-
38
skinned FO membrane was fabricated using a sulfonated polyphenylenesulfone (sPPSU)
39
polymer as the hydrophilic substrate and polyamide as the selective layer material via
40
interfacial polymerization on top and bottom surfaces. A strong agreement between
41
experimental data and modeling results validates the membrane design and confirms the
42
success of model prediction. The effects of key parameters on boron rejection, such as boron
43
permeability of both selective layers and structure parameter, were also investigated in-depth
44
with the mathematical modelling. This study may provide insights not only for boron removal
45
from wastewater, but also open up the design of next generation FO membranes to eliminate
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low-rejection molecules in wider applications.
47 48
Keywords
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Forward osmosis; double-skinned FO membranes; boron transport model; boron removal
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efficiency
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1. Introduction
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Recently, boron has come to the forefront as a possible drinking water contaminant. The
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global average concentration of boron in seawater is approximately 4.6 mg/L, while peaks of
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about 100 mg/L boron have been reported on produced water streams.1 To meet the final user
58
specifications, boron-containing wastewater needs to be regulated, and the boron
59
concentration in drinking water is suggested to be 2.4 mg/L by the World Health
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Organization. However, boron in the form of boric acid (B(OH)3 or H3BO3, a weak Lewis
61
acid of pKa=9.24,2 cannot be easily removed by conventional treatments.3 Alternatively,
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membrane technology is one potential solution; however, its removal efficiency still remains
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a challenge.4 At neutral pH, boric acid exists as an undissociated molecule with zero charge.
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It has a low molecular weight of only 61.8 g/mol, and a tendency to hydrogen bond with the
65
membrane matrix.1 Consequently, commercial brackish water reverse osmosis (BWRO)
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membranes were reported to be ineffective to remove neutral boron with rejection ratios less
67
than 65%, while seawater reverse osmosis (SWRO) operations can achieve a fair rejection
68
rate of 90%.1, 3, 5 In the case of treating high boron-containing wastewater or obtaining a high
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water recovery, even the permeate of SWRO may not be able to meet the strict requirements
70
of boron content. Thus, some RO processes are modified by multi-step approaches at
71
evaluated pH or with the aid of boron selective resins. 4, 6
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As an emerging membrane technology, forward osmosis (FO) has been investigated for boric
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acid permeation at neutral conditions.7-10 FO is driven by the osmotic pressure difference
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between the draw and feed solutions across the semi-permeable membrane.11-14 Under no or
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low pressure operation, FO membranes have advantages of low fouling tendency and high
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rejections to a wide range of contaminants.15-20 In terms of boron removal, both cellulose-
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based and thin film composite (TFC) FO membranes have been studied.7-10 The boron
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passage through FO membranes is significantly affected by membrane orientation, water flux,
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boron permeability of the selective layer, structure parameter, as well as the pH value and ion
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strength of feed solutions. However, most laboratory FO membranes can only achieve a
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boron rejection similar to BWRO (≤ 65%) at neutral pH. Considering the insufficient boron
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rejection of a single selective layer, the focus in this study lies on specially designed novel
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double-skinned FO membranes to hinder the boron passage and improve the separation
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performance.
86 87
The original concept of double selective FO membranes was invented by means of phase
88
inversion of cellulose acetate (CA) solutions cast on glass plates.21 It has been extended to
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form double selective FO hollow fibers22 and double selective FO flat-sheet membranes
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consisting of two TFC layers23, two different selective layer materials24 or layer-by-layer
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polyelectrolytes25, 26. These double-skinned FO membranes have shown mitigated internal
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concentration polarization (ICP)23, 25 and better fouling resistance24, 27 than single-selective
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FO membranes. Since the former also possesses superior boron transport resistance to the
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latter, this study aims to take the advantages of the dual-skin configuration to mitigate boron
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transport for a higher boron rejection. Therefore, double-skinned TFC FO membranes were
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fabricated and their boron flux and rejection performance were investigated. A transport
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model describing boron passage was also derived in order to simulate the experimental
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results and to understand the science among separation performance, transport phenomena
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and crucial membrane parameters. This work may provide useful insights for better FO
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membrane design to remove trace contaminants in water treatment.
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2. Experimental section
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2.1 Materials and membrane formation
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Sulfonated polyphenylenesulfone polymer (sPPSU) with a sulfonation degree of 1.5 mol %
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(IEC value 5.0 meq/100g polymer) was kindly provided by BASF and used as the support
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material for the fabrication of FO membranes. It was synthesized using the directly
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copolymerized sulfonation method developed by McGrath28. The sPPSU polymer has been
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utilized for ultrafiltration29, nanofiltration30 and forward osmosis31.
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The flat sheet membrane support was cast on a glass plate using the phase inversion method.
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Details of polymer dope formula and casting conditions are described in supporting
112
information (SI). To fabricate the double-skinned TFC membrane, a polyamide TFC layer
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was first synthesized on the top surface of the support by interfacial polymerization between
114
MPD and TMC monomers, then on the bottom surface following the same protocol as
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described in the SI. Between the syntheses of two polyamide layers, the membranes were
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kept hydrated by soaking in DI water. For comparison, the single-skinned FO membrane was
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fabricated with a TFC layer only formed on the top surface of the support.
118 119
2.2 FO membrane performance
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FO performance of the newly fabricated TFC FO membranes was evaluated through a lab-
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scale cross-flow setup. Details can be found elsewhere31, 32. The flow rates at both the draw
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and feed sides were kept at 0.2 L min-1, and all the experiments were conducted at room
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temperature (23±0.5 ◦C). NaCl solutions of different concentrations and a boron solution of
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100 ppm were prepared as draw and feed solutions, respectively. The boron feed solution was
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prepared by dissolving boric acid into DI water, without adjusting the pH. At this pH (~6.6),
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boric acid is poorly hydrated and exists in the form of undissociated molecules. The average
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testing duration for each FO operation is around 1 hour. The single-skinned FO membrane
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was tested under two modes: (1) the active layer facing draw solution (AL-DS) mode where
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the selective layer faced the draw solution and (2) the active layer facing feed solution (AL-
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FS) mode where the TFC selective layer faced the boron feed. While the double-skinned FO
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membrane was tested with the top polyamide layer facing the boron feed solution, referred to
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as the double-skinned (DS) mode.
133 134
The water permeation flux (Jw, L m−2 h−1 or LMH) was calculated as follows:
ܬ௪ =
∆
∆௧
135
(1)
136
where ∆V is the volume of permeation water collected in the draw side over a predetermined
137
time ∆t, and Am is the effective membrane area. The back diffusion of the draw solute to the
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feed side; namely, the salt reverse flux (JS, gm-2h-1 or gMH), can be calculated by measuring
139
the conductivity in the feed solution at the beginning and the end of each experiment:
ܬ௦ =
140 141
∆( ) ∆௧
(2)
where Ct and Vt are the salt concentration and feed volume at the end of tests, respectively.
142 143
Boron solute flux (JB, mg s-1 m-2) and boron rejection (R) are calculated by the following
144
equations:
145
146
ಳ ൗ ೢ
ܴ = ቆ1 −
ಷ
ܬ =
ವೝೌೢ ವೝೌೢ ∆௧
ቇ × 100% = ቀ1 −
(3) ವೝೌೢ ವೝೌೢ ಷ ∆
ቁ × 100%
(4)
147
where CDraw and CFeed are the boron concentrations of the draw and feed solutions at the end
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of tests, respectively, VDraw is the volume of the draw solution. In order to estimate the boron
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rejection, the effective boron concentration diffusing across the membrane could be
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expressed as the ratio of JB/Jw7, which could be experimentally calculated as the change of the
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boron amount in the draw solution side (CDrawVDraw) divided by the permeate volume ∆V.
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The azomethine-H colorimetric method was utilized to detect the boron concentration in the
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feed or draw solution. This detection method has been reported to be the most sensitive
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spectrometric technique with a detection limit of 0.02 ppm and least suffering from
155
interferences.33, 34 More details could be found in the SI.
156 157
2.3 Mass transport characteristics of selective layers
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The pure water permeability coefficient, A, NaCl permeability coefficient, Bs, and boron
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permeability coefficient, BB, of the FO membranes were evaluated in a dead-end RO cell
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under a transmembrane pressure of 3 bar. A was determined from the pure water permeation
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flux, while the salt rejection (Rs) and boron rejection (RB) were evaluated respectively using
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1000 ppm NaCl and 100 ppm boron solutions as feeds. Then A, Bs and BB were determined
163
with the aid of the following equations:
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165
166
=ܣ
ܴ = ൬1 − ଵିோ ோ
=
ொ
∆
൰ × 100%
(∆ି∆గ)
(5) (6) (7)
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where Q is the water permeation volumetric flow rate, Cp and Cf are the solute concentrations
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in the permeate and feed solutions, individually. ∆P and ∆π are the hydraulic pressure and
169
osmotic pressure differences across the membrane, respectively. NaCl and boron
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concentrations were detected respectively via conductivity and azomethine-H colorimetric
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methods. For clarity, the TFC layer fabricated on the top surface of the substrate was
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considered as the active layer1 with a boron permeability coefficient of B1, while the bottom
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TFC layer was referred to as the active layer2, which has a boron permeability coefficient of
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B2.
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3. Modeling
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3.1 Internal concentration polarization (ICP)
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In FO processes using a single-skinned FO membrane (Figure S1), salt concentration
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polarization can happen internally inside the support layer and externally on the selective
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layer.35 Usually, the ICP is a severer problem than the external concentration polarization as
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the latter may be mitigated by increasing the flow rate. Equations had been developed to
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model the water flux under ICP in single-skinned FO processes36-38:
183
In AL-DS mode:
184
(8)
185
In AL-FS mode:
186
(9)
187
where πD,b and πF,b are the osmotic pressures of the bulk draw and feed solutions, while πD,m
188
and πF,m represent the osmotic pressures on the surfaces of the selective layer facing the draw
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and feed solutions, respectively. A and Bs refer to water permeability and salt permeability
190
coefficients, which can be determined by methods described in section 2.3. Since the osmotic
191
pressure of the bulk feed is very low, πF,m is assumed to be 0 in the AL-FS mode. Then the
192
solute diffusion coefficient within the porous layer Km,s can be calculated from the
193
experimental FO performance according to eq. 9. Thus, the membrane structural parameter S
194
is obtained by dividing the salt diffusivity Ds by Km,s:
195
ܬ௪ = ܭ,௦ ݊ܫ
గವ, ିೢ ାೞ గಷ,್ ାೞ
ܬ௪ = ܭ,௦ ݊ܫగ
గವ,್ ାೞ
ܵ=ೄ
,ೞ
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(10)
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Since the same membrane substrates and interfacial polymerization methods were utilized to
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fabricate single- and double-skinned membranes in this study, it was reasonable to assume
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that they have the same structure parameter.
199 200
3.2 Boron transport through double-skinned FO membranes
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An analytical model for boron transport through double-skinned FO membranes is derived by
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combining the solution-diffusion model for the selective layers39 and the diffusion-convection
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transport in the support layer40, 41. A schematic of boron transport in a double-skinned FO
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membrane is shown in Figure 1. During FO operations, active layer1 faces the boron feed
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solution, while active layer2 faces the draw solution. Once boron permeates through the
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active layer1, it would be carried by the water flux from the interface to the support layer.
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Due to the retention of boron by the active layer2, boron concentration builds up at the
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interface between the support and active layer2. This phenomenon can be considered as the
209
internal concentration polarization (ICP) of boron. Within the support, boron transport is
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dominated by the convection induced by the water flow but also affected by the diffusion due
211
to the concentration gradient.40 At any distance x away from the interface between the
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support and active layer1, the boron flux JB,dx can be expressed as:
213
ܬ, ௗ௫ = − ܦௗ௫ + ܬௐ ܥ
214
where C represents the boron concentration at the distance x, JW is the water flux and D is the
215
effective boron diffusion coefficient within the porous layer. It is defined as D=DB·ε, where
216
DB is the boron diffusion coefficient and ε is the porosity. At steady state, the boron solute
217
flux JB can be expressed as:
218
ௗ
ܬ = ܤଵ (ܥி − ܥᇱ ) = ܤଶ ൫ " ܥ− ܥ ൯
(11)
(12)
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where B1 and B2 are the boron permeability of active layer1 and 2, while C’ and C” represent
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the boron concentrations at the interfaces facing active layer1 and layer2, respectively. CF is
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the boron concentration in the feed, while CD is the boron concentration at active layer2
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facing the draw solution7:
ܥ =
223
ಳ
ೈ
(13)
224
In eq. 11, the boundary conditions of ICP are taken into consideration:, C(0)=C’ at x=0; and
225
C(τt)=C” at x=τt (τ is the tortuosity and t is the thickness of support layer)42. By solving these
226
equations, boron flux JB and boron rejection R could be expressed as:
227
228
ܬ =
ಳభ ೈ ಳమ౮౦൫ೈ /಼ ൯
ܴ = 1 − ವ = 1 −
ಷ
భ
ಳ ଵା భ ା
ಳ
ೈ ಷ
= 1−
ܥி
(14) భ
ೈ ಳభ ಳమ ౮౦൫ೈ /಼ ൯
ೈ ାభ ା
(15)
229
Here, Km is the mass transfer coefficient of boron within the support, which is equal to the
230
ratio of DB to S (the structure parameter):
231
ܭ = =ݏ
232
ಳ ௌ
(16)
௧∙ఛ ఌ
(17)
233
The calculation of Km is based on eq. 10 and 16. As the structural characteristics of the
234
membrane support layer t, τ and ε are invariant within experimental conditions and S is the
235
intrinsic property of the membrane support, the same S value is employed in the ICP
236
calculation of both boron and NaCl transports:
237
=ݏ
௧∙ఛ ఌ
= ೄ = ಳ
,ೞ
(18)
238
For comparison, the passage of boron through the single-skinned TFC membrane is also
239
simulated and summarized in the SI, the same equations have also been reported7. Figure S2
240
illustrates the boron transports across the single and double TFC membranes under different
241
modes. Table 1 lists the modeling equations to calculate the normalized boron flux and boron
242
rejection under these three modes. Experimentally, B1 and B2 were characterized according to
243
section 2.3 and Km was calculated from eq. 18.
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4. Results and discussion
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4.1 As-fabricated FO membranes
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The morphologies of the sPPSU membrane support and the double-skinned FO membrane
248
are shown in Figure 2. The membrane substrate exhibits a denser top surface with small pores
249
but a relatively porous bottom surface with a thickness of around 50µm. Due to the delayed
250
demixing induced by the sulphonated material during phase inversion, the as-fabricated
251
membrane substrate displays a fully sponge-like structure29, 30. The membrane substrate has a
252
pure water permeability of 450.7 ± 21.9 LMH bar−1 with a mean pore size of 15.6 nm. Its
253
molecular weight cut-off is 94.5 kDa measured by solute rejection experiments29. In the case
254
of the double-skinned FO membrane, its top and bottom TFC layers exhibit similar ridge-
255
and-valley morphologies. However, they are different in thickness. The two inserted FESEM
256
images in Figure 2 enlarge their cross section morphologies. The polyamide thin film on the
257
bottom has a thickness of ~750 nm, which is significantly thicker than the top one (~350 nm).
258 259
The difference in layer thickness is probably caused by different interfacial polymerization
260
kinetics in both layers. Since the MPD aqueous solution is first introduced and absorbed by
261
the membrane support, the MPD monomer would diffuse out of the porous support into the
262
organic phase and react with TMC when contacting with the TMC solution.43-45 Thus, the
263
polyamide layer grows in protuberances and ridges instead of flat films. Once the thickness
264
of the polyamide film reaches a certain limit (ca. 20 nm), the MPD diffusion rate drops
265
quickly and the polymerization reaction is terminated46. Since the pore size and surface
266
porosity of the membrane substrate also affect MPD diffusion rate, the termination process
267
would be delayed on the bottom surface, where the incomplete coverage of the polyamide
268
films on large pores allows MPD to continuously diffuse out for reaction. As a result, layers
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of protuberances and ridges form on the bottom surface, leading to a thick TFC layer. For
270
comparison, membrane morphologies of the single-skinned FO membrane are presented in
271
Figure S3.
272 273
The water permeability and salt rejection of TFC layers in the double-skinned membrane are
274
characterized separately through RO tests. As illustrated in Table 2, a pure water permeability
275
(PWP) of 1.64 LMH/bar is characterized for the TFC layer formed on the top, while the
276
bottom one exhibits a lower PWP of 1.19 LMH/bar. The different PWPs of two selective
277
layers could be explained by their different polyamide thicknesses (Figure 2). Both TFC
278
layers have good NaCl rejection rates of ~ 95% but relatively poor boron rejections of lower
279
than 45%. Also, the boron permeability coefficients of the top and bottom selective layers are
280
calculated based on eq. 7, and represented by B1 and B2, individually. Both layers exhibit
281
much higher boron permeability coefficients than NaCl, confirming that boron can pass
282
through the membrane more easily and freely. Unlike the hydrated sodium and chlorine ions
283
of NaCl in water, boric acid exists as an undissociated molecule with no charge at the pH of
284
6.6. Therefore, the primary separation mechanism for boron removal by RO tests is size
285
exclusion. Due to the small size of poorly hydrated boron molecules, size exclusion becomes
286
less efficient, resulting in the poor boron rejection and the high boron permeability coefficient.
287 288
4.2 Experimental FO performance
289
Figure 3 demonstrates the FO performance of as-fabricated FO membranes by adjusting the
290
draw solution concentrations under different operation modes. Importantly, AL-DS and AL-
291
FS modes are two operation modes of the single-skinned FO membrane that comprises a TFC
292
layer on its top surface, while the DS mode refers to the double-skinned FO membrane with
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its top surface facing the feed solution. In our study, a 100 ppm boron solution is employed as
294
the feed solution, while the concentration of NaCl draw solutions varies from 0.5 M to 4 M.
295
All water fluxes and reverse salt fluxes rise up as a function of draw solution concentration in
296
three modes. Obviously, the AL-DS mode exhibits the highest water flux and reverse salt flux
297
under the same NaCl concentration, while the DS mode the lowest. Since the double-skinned
298
FO membrane has an additional selective layer, the water and salt fluxes are reduced by the
299
greater transport resistance and the ICP within the membrane support.
300
301
By using eq. 9 with aid of FO data, the solute diffusion coefficient within the porous layer
302
Km,s is calculated to be 5.51 µm/s, from which the structure parameter S is estimated to be
303
268.4 µm according to eq. 10 and a known DS from literatures7. Then the mass transfer
304
coefficient of boron within the support Km is determined to be 5.44 µm/s based on eq. 16 and
305
18. Both single- and double-skinned FO membranes are considered to share the same S and
306
Km parameters. The experimentally measured boron flux and boron rejection will be
307
illustrated in the following section and compared with the modeling results.
308
309
4.3 Theoretical and experimental boron flux
310
As shown in Table 1, the feed boron concentration CF directly affects the boron flux but has
311
little or negligible influence to boron rejection since the boron concentration here is highly
312
diluted.7,9,10 Figures 4 and 5 display the normalized boron flux (JB/CF) and boron rejection as
313
a function of water flux (JW) in FO operations using various draw solution concentrations,
314
respectively. The normalized boron fluxes in all operating modes increase with water flux to
315
different extent. The increment of JB/CF with JW in the AL-DS mode is the most significant,
316
followed by AL-FS and DS modes. This order could be explained by the different boron
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transport behaviors in different operation modes. Under the AL-DS mode (Figure S2 A),
318
boric acid enters the porous support layer from the feed, driven by water convective flow and
319
diffusion. However, due to the retention by the active layer, boron concentration at the
320
interface between the support layer and active layer becomes higher than that in the feed,
321
which is reported as the concentrative ICP of boron 7. This elevates the boron concentration
322
gradient across the dense layer and thus enhances boron passage. As JW increases, the
323
magnitude of ICP grows exponentially, leading to a greater increment in boron flux. However,
324
in the AL-FS mode, the boron transport is mainly influenced by JW and the boron
325
permeability of the active layer. Thus, its
326
the DS mode, when boron passes through the active layer1 (Figure S2 C) and enters into the
327
porous support, concentrative ICP occurs due to the boron retention by the second active
328
layer. Thus an increasing boron concentration profile from active layer1 to active layer2
329
within the porous layer is established. Consequently, the boron concentrations at both
330
interfaces (i.e., C’ and C”) are raised by the ICP effect so that the boron concentration
331
gradient across the active layer1 is reduced, resulting in a lower boron flux, in comparison to
332
the AL-FS mode. It is noteworthy that although the boron concentration gradient across the
333
active layer2 is increased, the resulting boron flux across active layer2 is limited by the boron
334
flux across active layer1, since the two values must be equal at steady state. Hence, the boron
335
flux in the DS mode is always lower than that in the AL-FS mode.
ಳ,ಲಽషಷೄ ಷ
value is less sensitive to JW. In contrast, in
336 ಳ
ಷ
337
Interestingly, Figure 4 shows that the
values of both DS and AL-FS modes are
338
approaching the same plateau as water flux increases further. This phenomenon can be
339
explained from their modeling equations of
340
ܭ, ൯ increases exponentially with JW, so that a large JW could bring the two terms
ಳ,ಲಽషಷೄ ಷ
and
ಳ,ವೄ ಷ
in Table 1. The term exp൫ܬ௪ /
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భ
మ ୣ୶୮൫ೢ /,ಳ ൯
and
భ
ೈ
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to approach zero. Consequently, the denominators in these two
342
equations approach 1. Thus, at high JW values, the normalized boron fluxes of AL-FS and DS
343
modes approach the same limiting value of B1, which is 6.844 LMH or 1.901 µm/s. On the
344
other hand, the modeling equation in the AL-DS mode could be re-written as
345
భ ಳ భ ା భ ౮౦(ೢ /಼,ಳ ) ೈ
346
exponentially so that the denominator is dominated by the term
347
As shown in the figure, the
348
fluxes. Physically, as Jw increases, the water convective flow increases, directly bringing
349
more boron into the FO membrane. This phenomenon not only enhances the concentrative
350
ICP of boron but also increases the boron concentration gradient across the active layer. As a
351
result, it increases boron passage. On the other hand, the AL-FS mode does not express such
352
concentrative ICP phenomenon. While in the DS mode, concentrative ICP of boron also
353
happens within the porous layer as boron flux increases by the convective flow. However, the
354
active layer 1 prevents boron from entering and results in lower ICP.
ಳ,ಲಽషವೄ ಷ
=
. If JW increases to a certain high value, the term exp൫ܬ௪ /ܭ, ൯ increases ಳ,ಲಽషವೄ ಷ
భ
ೈ
, and then
ಳ,ಲಽషವೄ ಷ
≈ ܬௐ .
value increases linearly with water flux at high water
355 356
Figure 5 presents experimental and modeled boron rejections as a function of water flux
357
under various modes. The boron rejections in AL-FS and DS modes increase with an increase
358
in JW and finally reach almost the same plateau. When Jw is small, both active layers play
359
their roles in rejecting boron, with little influence from concentrative ICP of boron within the
360
support. However, once Jw becomes larger, the boron concentration gradient within the
361
support increases due to the larger convective flow and ICP of boron, so the rejection
362
contributed by the active layer 2 is weakened. Thus, the active layer 1 determines the trend of
363
overall boron rejection. On the contrary, the boron rejection in the AL-DS mode initially
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increases with JW, but decreases with a further increment in JW, due to the significant boron
365
ICP at high water fluxes. Experimental data match well with the model results. The highest
366
boron rejection is 83.9% which is achieved under the DS mode at a water flux of 21.9 LMH.
367
To our best knowledge, this is the highest rejection ever achieved by experimental FO
368
membranes in neutral boron separation as summarized in Table S1 with literature data7-10.
369
The as-prepared double-skinned TFC FO membrane demonstrate a higher boron rejection
370
than commercial BWRO membranes (rejection ratios ≤ 65%), but slightly lower than SWRO
371
membranes (rejection ratios ~ 90%) at neutral pH values1.
372 373
Among the modeling curves in Figure 5, the DS mode seems to always exhibit the highest
374
boron rejection under the same JW, while the AL-DS mode has the lowest rejection. This
375
trend could be further corroborated by comparing their modeling equations. By comparing
376
RAL-FS and RAL-DS, RDS and RAL-FS according to Table 1, we can express their differences as
377
follows:
ܴିிௌ − ܴܮܣ−= ܵܦ
378
ܴௌ − ܴܮܣ−= ܵܨ
379
380
ଵ ೈ /,ಳ ൯
For JW>0, the term ୣ୶୮൫
ܤ1 ೈ ଵି
ܤ1 +
1
expቀ ܹܬ/ܭ
ܹܬ expቀ ܹܬ/ܭ
݉,ܤ
ቁ
݉,ܤ
ቁ
൩
(19)
൩(భ ାೈ )
భమ ೈ
మ ୣ୶୮൬ ೢ ൰(భ ାೈ )൦ೈ ାభ ା ಼,ಳ
ೈ ಳభ
(20)
൪
ಳమ౮౦ቆ ೢ ቇ ಼,ಳ
is smaller than 1, thus both eq. 19 and 20 are positive, i.e.,
381
RAL-FS-RAL-DS>0 and RDS-RAL-FS>0. The magnitudes of RAL-FS, RAL-DS and RDS is therefore
382
following the order:
383
ܴௌ = 1 −
భ
ೈ ಳభ ಳమ ౮౦൫ೢ /಼,ಳ ൯
ೈ ାభ ା
> ܴିிௌ = 1 −
భ
భ ାೈ
> ܴିௌ = 1 −
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భ
భ ାೣ( ೈ /಼ ೢ
,ಳ )
(21)
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384
Consistent with our experimental data, the boron rejection rate in the DS mode is always the
385
highest, while the one in the AL-DS mode is the lowest under the same JW. Overall, the
386
double-skinned FO membrane exhibits better boron separation performance; namely, a lower
387
boron flux and a higher boron rejection, compared with the single-skinned one. However,
388
deviations still exist between the theoretical modelling and experimental data in Figures 4 &5.
389
This is perhaps due to the uncertainty during the membrane casting, the fabrication of TFC
390
layers which were handled manually. Overall, the close agreement between experimental and
391
modeling results implies that the newly derived model equations could help predict the boron
392
transport behaviors through double-skinned FO membranes. In the following section, the
393
effects of key parameters on the separation performance of double-skinned FO membranes
394
are modelled in order to theoretically simulate and design a better double-skinned FO
395
membrane.
396 397
4.4 Modeling predictions
398
According to eq. 15, the boron rejection of the double-skinned FO membrane is affected by
399
JW and its intrinsic properties such as boron permeability of each active layer and structure
400
parameter. Theoretically, more ideal double-skinned FO membranes should have active
401
layers with lower boron permeability (i.e., smaller B1 and B2 values) and a thinner substrate
402
with a higher porosity and lower tortuosity (i.e., smaller S parameter). Figure 6A~C presents
403
the simulated boron rejection as a function of JW by changing B1, B2 and S parameters
404
separately. In each figure, only one parameter is reduced to half or by 10 times while the
405
other two parameters are invariant. As the boron permeability coefficient decreases from B1
406
to 0.1B1 in Figure 6A, or from B2 to 0.1B2 in Figure 6B, both boron rejection curves shift up
407
and the rejections increase with increasing water flux. An ideal rejection of almost 100%
408
could be achieved in the case of 0.1B1 in the high water flux region. The influence of S
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409
parameter on boron rejection is presented in Figure 6C. The ICP effects are reduced by
410
lowering the S parameter, leading to a higher boron rejection. The improvement becomes
411
more pronounced at higher water fluxes. Among the three key parameters, the influence of
412
changing boron permeability of active layers appears to be more significant than changing the
413
S parameter within the experimental range of this study.
414 415
Figure 6D compares the boron rejection as a function of water flux between double-skinned
416
FO membranes and single-skinned ones under the AL-FS mode. The black solid line labeled
417
with “B1, B2 and S” represents the curve simulated with parameters obtained from
418
experiments, while the other colored dotted lines are generated with one or more parameters
419
decreased by half. As observed, the rejection difference between DS and AL-FS modes (i.e.,
420
RDS-RAL-FS) is strongly dependent on water flux. The highest rejection difference always
421
appears within the low Jw range, while the value of RDS-RAL-FS decreases with a further
422
increase in Jw. As water flux increases, the rejection rates of both membranes also increase
423
and both approach to 1, inevitably leading to a smaller difference. Moreover, the ICP of
424
boron within the support under the DS mode becomes severer with high water fluxes, which
425
also contributes to the diminished difference in rejection rate. On the other hand, the highest
426
rejection difference are affected by the modeling variables B1, B2 and S. As B1 is replaced by
427
0.5B1, the value of RDS-RAL-FS is decreased to only 12%, indicating the increased similarity of
428
DS and AL-FS operations. Due to the improved boron retention by active layer1, most of the
429
boron will be rejected there, making the second active layer in the double-skinned FO
430
membrane less important. As a result, the double-skinned FO membrane behaves more like a
431
single-skinned membrane in the AL-FS mode. When further decreasing B2 to 0.5B2, an
432
enlarged boron rejection difference of 20.6% can be achieved.
433
parameter of the additional active layer in the DS mode, a reduced B2 particularly increases
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Since B2 is a special
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the rejection ability of the double-skinned FO membrane. Moreover, the RDS-RAL-FS value is
435
further improved to 21.3% when the S parameter is also reduced, corresponding to a
436
reduction in ICP effects. Future works should aim to enhance double-skinned FO membranes
437
with a higher boron rejection by improving the boron retention ability of active layers and
438
reducing the S parameter of the membrane support.
439 440
In summary, double-skinned FO membranes are proved to be superior in boron rejection.
441
However, if operated under the same clean draw /feed solutions (such as NaCl solution/boron
442
feed in this study), double-skinned FO membranes naturally exhibit a lower water flux than
443
single-skinned ones, due to the additional skin layer. But it is worth noting that double-
444
skinned membranes are reported to have a similar or higher water flux when dealing with
445
viscous draw solutions23 or foulant-containing feed solutions25. The skin layers on both sides
446
of the membrane could help to prevent the clogging or penetration of foulants into the
447
support layer. Also, organic fouling (e.g. alginate fouling) during FO is reported to have no
448
discernible influence on boron rejection8. Thus, double-skinned FO membranes are not
449
necessarily more energy-intensive or having low separation efficiency, but could be
450
promising in providing both high boron rejection and good water flux when treating high
451
fouling tendency solutions, which requires further efforts to explore.
452 453
Nomenclature
454
A
water permeability, L m-2 h-1 bar-1 or LMH/bar
455
Am
effective membrane area, m2
456
AL-DS active layer facing draw solution
457
AL-FS active layer facing feed solution
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Bs
NaCl permeability coefficient, L m-2 h-1 or LMH
459
BB
boron permeability coefficient, LMH
460
B1
boron permeability of active layer1, LMH
461
B2
boron permeability of active layer2, LMH
462
C
boron concentration at the distance x
463
C’
boron concentration at the interfaces facing active layer1
464
C”
boron concentration at the interfaces facing active layer2
465
CF
boron concentration in the feed
466
CD
boron concentration at active layer2 facing the draw solution
467
Cp
solute concentration in the permeate solution, mg/L
468
Cf
solute concentration in the feed solution, mg/L
469
Ct
NaCl concentration at the end of tests, mg/L
470
CDraw boron concentration of the draw solution at the end of tests, mg/L
471
CFeed
boron concentration of the feed solution at the end of tests, mg/L
472
D
effective boron diffusion coefficient within the porous layer, m2/s
473
DB
boron diffusion coefficient, m2/s
474
Ds
NaCl diffusion coefficient, m2/s
475
DS
double-skinned
476
ICP
internal concentration polarization
477
FO
forward osmosis
478
JB
boron flux, mg s-1m-2
479
JB,dx
boron flux at the distance x
480
JS
salt reverse flux, gm-2h-1, abbreviated as gMH
481
JW
water flux, LMH
482
Km
mass transfer coefficient of boron within the support, µm/s
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483
Km,s
NaCl diffusion coefficient within the support, µm/s
484
∆P
hydraulic pressure across the membrane, bar
485
Q
water permeation volumetric flow rate, L/h
486
R
boron rejection, %
487
S
membrane structural parameter, µm
488
∆t
the time duration of one FO experiment operation, h
489
TFC
thin-film-composite
490
∆V
volume of permeation water collected in the draw side over ∆t, L
491
VDraw volume of the draw solution, L
492
Vt
feed volume at the end of tests, L
493
ε
porosity
494
π
osmotic pressure, bar
495
∆π
osmotic pressure difference across the membrane, bar
496
τ
tortuosity
497
t
thickness of support layer, m
498 499
Acknowledgments
500
This research was funded by BASF SE, Germany with a grant number of R-279-000-363-597.
501
Special thanks are given to Dr. Gang Han of NUS and Dr. Natalia Widjojo of BASF
502
(Singapore) for their help and suggestions. This research was also partially funded by the
503
Singapore National Research Foundation under its Competitive Research Program for the
504
project entitled, “Advanced FO Membranes and Membrane Systems for Wastewater
505
Treatment, Water Reuse and Seawater Desalination” (grant numbers: R-279-000-336-281
506
and R-279-000-339-281).
507
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Supporting Information Available
509
Information addressing membrane formation, boron detection, modeling of single-skinned
510
FO membranes. Water and boron transport mechanisms of different operational modes in FO
511
processes (Figure S1 and Figure S2); morphologies of single-skinned FO membranes (Figure
512
S3); experimental membrane performance of boron flux and boron rejection (Figure S4);
513
predictions of boron rejection of three modes under the influence of different parameters
514
(Figure S5); comparison of FO membranes in boron rejection studies (Table S1).
515 516
References
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30. Zhong, P. S.; Widjojo, N.; Chung, T. S.; Weber, M.; Maletzko, C., Positively charged nanofiltration (NF) membranes via UV grafting on sulfonated polyphenylenesulfone (sPPSU) for effective removal of textile dyes from wastewater. J. Membr. Sci. 2012, 417, 52-60. 31. Widjojo, N.; Chung, T. S.; Weber, M.; Maletzko, C.; Warzelhan, V., A sulfonated polyphenylenesulfone (sPPSU) as the supporting substrate in thin film composite (TFC) membranes with enhanced performance for forward osmosis (FO). Chem. Eng. J. 2013, 220, (0), 15-23. 32. Anastasio, D.; R. McCutcheon, J., Using forward osmosis to teach mass transfer fundamentals to undergraduate chemical engineering students. Desalination 2013, 312, (0), 10-18. 33. López, F. J.; Giménez, E.; Hernández, F., Analytical study on the determination of boron in environmental water samples. Fresen. J. Anal. Chem. 1993, 346, (10-11), 984-987. 34. Sah, R. N.; Brown, P. H., Boron Determination—A Review of Analytical Methods. Microchem. J. 1997, 56, (3), 285-304. 35. Gray, G. T.; McCutcheon, J. R.; Elimelech, M., Internal concentration polarization in forward osmosis: role of membrane orientation. Desalination 2006, 197, (1-3), 1-8. 36. McCutcheon, J. R.; Elimelech, M., Modeling water flux in forward osmosis: Implications for improved membrane design. Aiche J. 2007, 53, (7), 1736-1744. 37. Zhao, S. A. F.; Zou, L. D., Relating solution physicochemical properties to internal concentration polarization in forward osmosis. J. Membr. Sci. 2011, 379, (1-2), 459-467. 38. Luo, L.; Wang, P.; Zhang, S.; Han, G.; Chung, T. S., Novel thin-film composite tribore hollow fiber membrane fabrication for forward osmosis. J. Membr. Sci. 2014, 461, 2838. 39. Geise, G. M.; Paul, D. R.; Freeman, B. D., Fundamental water and salt transport properties of polymeric materials. Progress in Polymer Science 2014, 39, (1), 1-42. 40. Mehta, G. D.; Loeb, S., Internal polarization in the porous substructure of a semipermeable membrane under pressure-retarded osmosis. J. Membr. Sci. 1978, 4, (2), 261265. 41. Tang, C. Y. Y.; She, Q. H.; Lay, W. C. L.; Wang, R.; Field, R.; Fane, A. G., Modeling double-skinned FO membranes. Desalination 2011, 283, 178-186. 42. Lee, K. L.; Baker, R. W.; Lonsdale, H. K., Membranes for power-generation by pressure-retarded osmosis J. Membr. Sci. 1981, 8, (2), 141-171. 43. Cadotte, J. E. Interfacially synthesized reverse osmosis membrane. US patent 4277344, 1981. 44. Ghosh, A. K.; Jeong, B.-H.; Huang, X.; Hoek, E. M. V., Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties. J. Membr. Sci. 2008, 311, (1-2), 34-45. 45. Li, X.; Wang, K. Y.; Helmer, B.; Chung, T. S., Thin-film composite membranes and formation mechanism of thin-film layers on hydrophilic cellulose acetate propionate substrates for forward osmosis processes. Ind. Eng. Chem. Res. 2012, 51, (30), 10039-10050. 46. Yan, H.; Miao, X.; Xu, J.; Pan, G.; Zhang, Y.; Shi, Y.; Guo, M.; Liu, Y., The porous structure of the fully-aromatic polyamide film in reverse osmosis membranes. J. Membr. Sci. 2015, 475, 504-510.
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Boron flux (convection, coupled with Jw)
CF
Boron flux (diffusion) Net boron flux JB
Feed solution
C”
C’ Water flux Jw
B1
x=0 Active layer1
Draw solution B2 CD
x Support layer Active layer2
Figure 1. Boron transport in a double-skinned FO membrane
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Top surface
Cross section
Bottom surface
Substrate
100nm
1 μm
10 μm Top
Double-skinned
Bottom
1 μm
1 μm
10 μm
Figure 2. Morphologies of the membrane support and double-skinned FO membranes ACS Paragon Plus Environment
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80
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Water flux Reverse salt flux AL-DS AL-FSO DS O
70
12
Water flux (LMH)
60 6 50 3
40
0
30 20
-3
10
-6
0
0.5M 1M 1.5M 2M
AL-DS mode
0.5M 1M 1.5M 2M
AL-FS mode
1M 2M
3M
4M
Reverse salt flux (gMH)
9
-9
DS mode
Figure 3. Experimental membrane performance of water flux and reverse salt flux in different operation modes (Draw solutions: 0.5 M ~ 4 M NaCl, feed solution: 100 ppm boron solution (pH 6.6)) ACS Paragon Plus Environment
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AL-FS AL-DS
14
DS
Modeling Experimental
AL-DS
12
JB/CF (m/s)
10 8 6 4
AL-FS 2
DS
0 0
5
10
15
20
25
30
35
40
45
50
55
60
65
Water flux (LMH)
Figure 4. Experimental and modeled normalized boron fluxes as a function of water flux under various operation modes (Draw solutions: NaCl solutions, feed solution: 100 ppm boron solution (pH 6.6)) ACS Paragon Plus Environment
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100
DS
90
AL-FS
Boron rejection (%)
80 70 60 50
AL-DS
40 30 20
AL-FS AL-DS
10
DS
Modeling Experimental
0 0
5
10
15
20
25
30
35
40
45
50
55
60
65
Water flux (LMH)
Figure 5. Experimental and modeled boron rejections as a function of water flux under various operation modes (Draw solutions: NaCl solutions, feed solution: 100 ppm boron solution (pH 6.6)) ACS Paragon Plus Environment
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B
100
100
90
90
80
80
Boron rejection (%)
Boron rejection (%)
A
70 60 50 40
B1, B2, S
30
0.5B1, B2, S
20
0.1B1, B2, S
10
70 60 50 40
B1 , B2 , S
30
B1, 0.5B2, S
20
B1, 0.1B2, S
10
0
0 0
5
10
15
20
25
30
35
40
45
50
55
60
65
0
5
10
15
20
Water flux (LMH)
C
25
30
35
40
50
55
60
65
Water flux (LMH)
D
100
0.30 0.27
B1, B2, S
0.24
0.5B1, B2, S
90
0.5B1, 0.5B2, S
80 0.21
0.5B1, 0.5B2, 0.5S
70
RDS-RAL-FS
Boron rejection (%)
45
60 50 40
B1, B2, S
30
B1, B2, 0.5s
20
B1, B2, 0.1s
0.18 0.15 0.12 0.09 0.06 0.03
10
0.00
0 0
5
10
15
20
25
30
35
40
Water flux (LMH)
45
50
55
60
65
0
5
10
15
20
25
30
35
40
45
50
Water flux (LMH)
Figure 6. Predictions of boron rejection of double-skinned FO membranes under the influence of (A) boron permeability of the active layer1 B1, (B) boron permeability of active layer2 B2, (C) ACS Paragon Plus Environment structure parameter S, while (D) is the boron rejection difference between DS and AL-FS modes
Environmental Science & Technology
Page 32 of 33
Table 1. Modeling equations for normalized boron flux and boron rejection
Mode
Normalized boron flux
Boron rejection
AL-FS mode
𝐽𝐵,𝐴𝐿−𝐹𝑆 𝐵1 = 𝐵 𝐶𝐹 1+ 1 𝐽𝑊
𝑅𝐴𝐿−𝐹𝑆 = 1 −
AL-DS mode
𝐽𝐵,𝐴𝐿−𝐷𝑆 = 𝐶𝐹
𝑅𝐴𝐿−𝐷𝑆 = 1 −
DS mode
𝐵1 exp(𝐽𝑤 /𝐾𝑚,𝐵 ) 𝐵 exp(𝐽𝑤 /𝐾𝑚,𝐵 ) 1+ 1 𝐽𝑊
𝐽𝐵,𝐷𝑆 𝐵1 = 𝐵 𝐵1 𝐶𝐹 1+ 1 + 𝐽𝑊 𝐵2 exp 𝐽𝑤 /𝐾𝑚,𝐵
𝑅𝐷𝑆 = 1 −
ACS Paragon Plus Environment
𝐵1 𝐵1 + 𝐽𝑊 𝐵1
𝐵1 +
𝐽𝑊 exp(𝐽𝑤 /𝐾𝑚,𝐵 ) 𝐵1
𝐽𝑊 + 𝐵1 +
𝐽𝑊 𝐵1 𝐵2 exp 𝐽𝑤 /𝐾𝑚,𝐵
Page 33 of 33
Environmental Science & Technology
Table 2. Intrinsic separation properties of membrane selective layers
•
Selective layer
Pure Water permeability, A (LMH/bar)
NaCl rejection
NaCl permeability Bs, (LMH)
Boron rejection
Boron permeability BB, (LMH)
TFC layer on the top surface
1.64±0.14
95.5%
0.195
41.7%
6.844 (B1)
TFC layer on the bottom surface
1.19±0.13
94.7%
0.137
43.1%
4.694 (B2)
RO tests conditions: pressure 3bar, 1000 ppm NaCl feed, or 100 ppm Boron feed
ACS Paragon Plus Environment