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Partitioning of Alkali Metal Salts and Boric Acid from Aqueous Phase into the Polyamide Active Layers of Reverse Osmosis Membranes Jingbo Wang, Ryan Scott Kingsbury, Lamar A. Perry, and Orlando Coronell Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04323 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 15, 2017
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Partitioning of Alkali Metal Salts and Boric Acid from Aqueous Phase into the Polyamide Active Layers of Reverse Osmosis Membranes
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Jingbo Wanga, Ryan S. Kingsburya, Lamar A. Perrya, b, Orlando Coronella,* Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7431, USA b Department of Applied Physical Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3216, USA *Corresponding author [tel:+1-919-966-9010; fax:+1- 919-966-7911; e-mail:
[email protected]]
Active layer mass
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a
Active layer mass
4 5 6 7
Before Time
Pure water Membrane active layer QCM sensor
After Time
Solution
NaCl
NaCl
NaCl
NaCl
QCM sensor
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Abstract: The partition coefficient of solutes into the polyamide active layer of reverse osmosis
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(RO) membranes is one of the three membrane properties (together with solute diffusion
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coefficient and active layer thickness) that determine solute permeation. However, no well-
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established method exists to measure solute partition coefficients into polyamide active layers.
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Further, the few studies that measured partition coefficients for inorganic salts report values
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significantly higher than one (~3-8), which is contrary to expectations from Donnan theory and
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the observed high rejection of salts. As such, we developed a bench-top method to determine
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solute partition coefficients into the polyamide active layers of RO membranes. The method uses
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a quartz crystal microbalance (QCM) to measure the change in the mass of the active layer
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caused by the uptake of the partitioned solutes. The method was evaluated using several
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inorganic salts (alkali metal salts of chloride) and a weak acid of common concern in water
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desalination (boric acid). All partition coefficients were found to be lower than 1, in general
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agreement with expectations from Donnan theory. Results reported in this study advance the
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fundamental understanding of contaminant transport through RO membranes, and can be used in
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future studies to decouple the contributions of contaminant partitioning and diffusion to
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contaminant permeation.
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Key Words: thin-film composite, polyamide, partitioning, Manning, quartz crystal microbalance
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Introduction
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Reverse osmosis (RO) membranes have the capability of removing a broad range of
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contaminants (e.g., inorganic salts, small organics) from water, and are among the most
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promising processes for clean water production from impaired sources such as seawater and
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treated wastewater.1 Most commercial RO membranes have a thin-film composite (TFC)
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structure with a polyamide active layer (~20-200 nm) which constitutes the main barrier to water
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and solute transport, an intermediate polysulfone support layer (~20-50 µm), and a polyester
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backing layer (~50-150 µm).2
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Within the framework of solution-diffusion theory, solute permeation through the active layer is
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the result of solute partitioning into the active layer, diffusion through the active layer, and
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partitioning out of the active layer.3,4 The permeability of the active layer material to solutes (PS,
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m2·s-1) is a function of the solute partition coefficient between water and active layer (KS,
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unitless) and the solute diffusion coefficient within the active layer (DS, m2·s-1) as given by
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PS = KS DS .
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Therefore, the ability to quantify KS and DS is essential to understand from a fundamental
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perspective the mechanisms of contaminant permeation through RO membranes, and ultimately
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enable construction of predictive transport models. Being able to quantify KS and DS would also
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facilitate the development of improved active layer materials for specific applications, as it
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would enable the identification of the transport property that must be targeted for modification in
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the active layer to improve performance (i.e., KS, DS, or both).
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While PS can be easily obtained from permeation tests and measurements of active layer
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thickness,5 independent measurements of KS and DS remain challenging. Given that the time
(1)
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scale of diffusion of solutes through active layers is very short (18 MΩ.cm), and stored in ultrapure water in amber glass bottles until used.
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Membrane sample preparation. The polyamide active layer was isolated from membrane
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coupons onto three 5 MHz quartz crystal microbalance (QCM) sensors. The isolation of the
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polyamide active layer enabled the performance of partitioning measurements in the absence of
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potential artifacts that could be caused by the presence of the much thicker polysulfone support
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and polyester backing layers. The polyamide area isolated on each sensor was 1.54 cm2. The
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active layer isolation procedure was described in detail in our previous work,19 is based on a
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protocol proposed by Freger,20 and has been successfully applied in different studies.21–23 It has
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been shown that characterization results for bulk active layer properties using isolated active
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layers are equivalent to corresponding results for active layers in intact membranes.5,6,19,24,25 In
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brief, the polyester backing layer was first peeled off manually, leaving behind the composite of
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polysulfone support layer and polyamide active layer. Then, the polyamide-polysulfone
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composite was placed against the surface of the QCM sensor with the polyamide side facing the
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sensor, and the polysulfone layer was dissolved using dimethylformamide (DMF). Next, the
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QCM sensor coated with the polyamide active layer was air dried, rinsed with ultrapure water,
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dried with ultrapure nitrogen gas, and stored in a sealed plastic box until use.
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Measurement of areal mass of active layer and change in areal mass of the active layer when
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exposed to test solutions. QCM analyses were used to measure the areal mass of active layers
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isolated on QCM sensors (mAL, ng.cm-2) and the change in areal mass (∆m, ng.cm-2) of the active
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layers when exposed to test solutions during partitioning tests. For each sensor, the mAL value
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was obtained as the difference between QCM measurements for the sensor exposed to air before
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and after active layer isolation. Also for each sensor, ∆m values were obtained as the difference
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between QCM measurements for the coated sensor exposed to ultrapure water and to aqueous
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solutions containing the solute of interest (see next section). The mAL values were used to obtain
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the thicknesses of the active layers isolated on QCM sensors, and the mAL and ∆m values were
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used to calculate partition coefficients as described in the Results and Discussion section. Only
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mass changes greater than the limit of quantification (LOQ=33 ng.cm-2, see Supporting
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Information (SI)) were used for calculations.26 All values and corresponding errors reported in
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this manuscript correspond to the average and standard deviation, respectively, of measurements
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from the triplicate coated sensors.
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Partitioning tests. Partitioning tests at pH=5.3 were performed for all solutes. We chose pH=5.3
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because it is relevant for scaling prevention applications,27,28 and because at pH values below
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≈5.5 aromatic polyamide active layers have been shown29,30 to have relatively low (
