Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX-XXX
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In Situ Healing of Compromised Membranes via PolyethylenimineFunctionalized Silica Microparticles Sang-Ryoung Kim, Bezawit A. Getachew, and Jae-Hong Kim* Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States S Supporting Information *
ABSTRACT: Microscale damages to membranes used in large-scale filtration processes for water treatment can result in severe degradation of product water quality. One promising technology to address this issue is in situ healing of compromised membranes via healing agents that are added to the feed side of a membrane system and seal the defect site because of increased hydraulic drag through damage site during filtration. We herein introduce an improved in situ membrane healing method using amine-functionalized silica microparticles that is effective under varying operating conditions, overcoming limitations faced by previous healing agents such as chitosan agglomerates. The silica microparticles are functionalized with branched polyethylenimine (PEI) molecules for efficient interparticle cross-linking with glutaraldehyde. The PEI-decorated silica microparticles (SiO2@PEI MPs) were characterized using scanning electron microscopy, dynamic light scattering, and zeta potential analysis. This study investigates the selective deposition of the SiO2@PEI MPs on the damage area using confocal laser scanning microscopy under variable cross-flow rate (0.5−2.0 L/min) and flushing time (10 to 30 min) conditions. The in situ healing technique recovered the particle rejection of compromised membranes to 99.1% of the original membrane’s performance without any flux decline. The results of this study show that the use of SiO2@PEI MPs is a promising and practical approach to ensure membrane process integrity.
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INTRODUCTION Low-pressure membranes, including microfiltration (MF) and ultrafiltration (UF) membranes, have increasingly become a standard in water treatment processes over the past decade.1,2 Low-pressure membranes are primarily used for micro-sized pathogens/particles removal as stand-alone units or as a pretreatment for nanofiltration/reverse osmosis. The effective removal by membranes.3 If this selective layer’s integrity is compromised, pathogens/particles may pass through membranes and deteriorate the product water quality.4−6 As a result, the membrane industry has developed various direct/indirect tests for monitoring the integrity of low-pressure membranes (e.g., pressure hold/decay test,7 particle counting,6,8 or turbidity monitoring6), and an increasing number of regulatory agencies (e.g., Environmental Protection Agency in the United States,9 Drinking Water Inspectorate in the United Kingdom,10 etc.) require water treatment utilities to conduct membrane integrity monitoring on a regular basis. While research on the integrity testing of membranes is abundant, there are few technologies that provide the ability to repair membranes identified to be compromised.4,5,11−18 To address this issue, we have developed an in situ healing technique that repairs damaged membranes by the filtration of a suspension of chitosan agglomerates through a damaged membrane.4,5 The chitosan agglomerates block damage sites on a membrane because of increased hydraulic drag and © XXXX American Chemical Society
subsequent cross-linking by glutaraldehyde forms a sealing matrix without requiring module disassembly.5 While this in situ healing technique showed promising results on both flat sheet and hollow fiber membranes, there are a few limitations in using chitosan as a healing agent in a practical setting. First, the in situ healing technique can only be applied to membranes with smaller pores than the chitosan agglomerates. Chitosan agglomerates smaller than membrane pores can cause a decrease in membrane permeability by clogging pores in undamaged areas of the membrane. While the size of agglomerates can be controlled to be between 0.5 and 2.2 μm by adjusting the pH,4,5 in a full-scale practice, it is difficult to precisely control the pH of an entire feed. Without precise control of the pH, the size of chitosan agglomerates can vary widely, making the in situ healing technique less effective.4 There is also a possibility of chitosan agglomerate size changes in the membrane system due to various causes, such as residual chemicals, intensive air scouring, or coagulation between chitosan agglomerate and anionic substances. Second, the chemical stability of cross-linked chitosan in water treatment systems can be low due to the its pH-sensitive characteristics Received: July 6, 2017 Revised: September 1, 2017 Accepted: October 10, 2017
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DOI: 10.1021/acs.est.7b03436 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
Figure 1. Reaction scheme of the formation of (a) PEI-functionalized silica microparticles via coupling with DVS as a cross-linker and (b) FITClabeled silica microparticles via thiourea linkage between isothiocyanate and a primary amine. Abbreviations mean the following: SiO2 MPs (bare silica microparticles); SiO2-APTES MPs (amine-functionalized SiO2 MPs by using APTES); SiO2-APTES-DVS MPs (vinyl sulfone-functionalized SiO2-APTES MPs by using DVS); SiO2-APTES-DVS-PEI MPs (SiO2@PEI MPs, branched PEI decorated SiO2-APTES-DVS MPs); SiO2@FITC MPs (FITC labeled SiO2@PEI MPs).
25 000 Da), 2-propanol (anhydrous, 99.5%), fluorescein isothiocyanate isomer I (FITC, ≥ 97.5%), and dimethyl sulfoxide (DMSO, ≥ 99.5%) were purchased from SigmaAldrich (USA). All chemicals were used as supplied without further purification. Ultrapure water from a Milli-Q Integral water purification system (Millipore Co., USA) was used to prepare all experimental aqueous solutions and water permeability test. Synthesis of Surface-Functionalized Silica Microparticles. Bare SiO2 MPs (2.2 μm diameter, Superior Silica LLC, USA) were first washed by 80% v/v ethanol and filtered with polyvinylidene fluoride (PVDF) disc membrane filter (HVLP Durapore Membrane Filter, EMD Millipore, USA) with a nominal pore size of 0.45 μm followed by drying in a vacuum oven overnight. To covalently attach PEI to bare SiO2 MPs,23−25 pretreated SiO2 MPs were first transferred to a pure ethanol solution by a sonication (30 min)-centrifugationredispersion process and dried in a vacuum oven. The surfacefunctionalization steps are schematically illusted in Figure 1a. The washed SiO2 MPs (0.5 g) were added to 200 mL of hexane and ultrasonicated for 30 min to achieve a monodispersed silica suspension, followed by reaction with 0.8 mL of APTES for 2 h under continuous stirring. The resulting suspension was then washed twice to remove excess APTES by centrifugation (6000 rpm, 30 min) and redispersion (sonification 30 min) in pure ethanol. Wahsed SiO2-APTES MPs suspension was then dried and redispersed in 50 mL of 2-propanol. The suspension was completely dried in a vacuum oven at 100 °C for 20 h. For further functionalization, the aminopropyl-functionalized silica microparticles (SiO2-APTES MPs) were transferred to 200 mL of 2-propanol. DVS (400 μL) was then added into a sonicated SiO2-APTES MPs suspension. The mixture was stirred for 2 h at room temperature, followed by one cycle of washing and redispersing in 2-propanol. To introduce the branched amine
and the biodegradable β-(1,4) glycosidic linkages between Dglucosamine and N-acetyl-D-glucosamine groups.19,20 Under acidic conditions, cross-linked chitosan swells due to the protonation of amino groups but contracts to its original size under neutral conditions. This reversible expansion and contraction can degrade the chitosan over time. The β-(1,4)linkages can be hydrolyzed by lysozyme, chitinase, chitosanase, etc., which are found in water treatment systems because of the presence of bacteria and fungi.20,21 Therefore, the chemical stability of chitosan may not be reliable enough under extended exposure to wastewater in real world applications. We herein report an advanced in situ healing technique that overcomes the aforementioned limitations and can be applied to various operating conditions. The improved in situ healing technique uses silica microparticles (SiO2 MPs), which can maintain their size and chemical stability under a wide range of conditions. SiO2 MPs are readily available in the commercial market at low cost22 and can be surface-functionalized with polyethylenimine (PEI),23−25 a cationic polymer that can react with aldehyde functional groups in glutaraldehyde for crosslinking.26 We synthesized PEI-decorated SiO2 MPs (SiO2@PEI MPs) and tested in situ healing of a damaged hollow fiber membrane system with a suspension of the microparticles. Since the performance recovery depends heavily on the deposition of SiO2 MPs on the damage site, this study also investigated factors that could affect particle deposition dynamics under realistic hollow fiber membrane operating conditions.
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MATERIALS AND METHODS Chemicals. Tetraethyl orthosilicate (TEOS, 99.999%), hexane (for HPLC, > 95%), (3-aminopropyl)-triethoxysilane (APTES, 99%), ethanol (200 proof, ACS reagent), divinyl sulfone (DVS, 97%), polyethylenimine (PEI, branched, MW ≈ B
DOI: 10.1021/acs.est.7b03436 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 2. Schematic diagram of hollow fiber membrane process with pressurized module. Water flux and cross-flow rate were monitored using a digital weighing balance and a flow meter, respectively. The filtration system was operated under four different conditions to confirm the healing performance. All experimental sets were operated at 20 °C.
functional group, 5 mL of PEI solution (500 mg of PEI in 5 mL of 2-propanol) was added and sonicated for 5 min. After overnight reaction under stirring, excess PEI was washed by three cycles of centrifugation-redispersion with 2-propanol. Finally, the SiO2-APTES-DVS-PEI MPs (SiO2@PEI MPs) were redispersed in 100 mL of ethanol (final concentration of 5 g/L). FITC-labeled silica microparticles (SiO2@FITC MPs) were fabricated to provide visual evidence of selective deposition of SiO2 MPs at the site of damage. To synthesize SiO2@FITC MPs,23,27 5.25 mg of FITC was dissolved in 1 mL of DMSO and mixed with 20 mL of SiO2@PEI MPs suspension (Figure 1b). The mixture was stirred at room temperature for overnight and then filtered with excess ethanol to remove impurities. Finally, the filtered mixture was dispersed in 20 mL of ethnaol to form a suspension with a concentration of 5 g/L. Characterization of Functionalized Silica Microparticles. The hydrodynamic diameter and zeta potential of bare and surface-functionalized SiO2 MPs were measured (NanoBrook Omni, Brookhaven Instruments, USA) in dilute suspensions in water after sonication for 30 min. Experiments to measure all zeta potentials of SiO2 MPs suspensions were performed using ultrapure water at pH 6.2. The SEM samples were prepared by dropping a dilute MPs suspension onto a silicon wafer plate and drying the plate in a vacuum oven at room temperature overnight. Surface morphologies of MPs were imaged using a scanning electron microscope (SEM, Hitachi SU-70, Japan) after coating with 4 nm thick layer of
iridium (208HR, Cressington, USA). The resulting SEM images were analyzed by ImageJ software (NIH, USA) to determine the actual size of MPs. Hollow Fiber Membrane Filtration. A cross-flow labscale filtration system was configured to operate at constant pressure using a pressurized PVDF hollow fiber module and a peristaltic pump (Figure 2). A single, 28 cm-long strand of hollow fiber (Cleanfil, Kolon Industry, Inc., 0.1 μm nominal pore size) with an effective filtration area of 17.6 cm2 was mounted in the custom-built module and continuously supplied with a feed solution from a dispensing vessel in an outside-in configuration as shown in Figure S1a and S1b. The operating pressure (28 to 72 kPa) and cross-flow velocity (3.79 × 10−2 to 1.52 × 10−1 m/s) were controlled by a peristaltic pump and retentate valve (Figure 2). Operating pressure and flow rate of the retentate were measured by a digital pressure gauge (ISE40A, SMC, Japan) and a flow meter (Flow S-110, McMillan, USA), respectively. Installed hollow fiber membranes were precompacted at 50 kPa for 2 h before measuring the water permeability on a digital weighing balance every 1 min. Rejection measurements were conducted with a fluorescent microsphere feed solution (Fluoresbrite YG Microspheres, 1.0 μm, Polyscience Inc., USA) at a concentration of 0.025 g/L.11,12 The concentration of fluorescent microspheres in the permeate was measured with a spectrofluorophotometer (Shimadzu RF-5031PC, Japan) at an excitation wavelength of 441 nm and an emission wavelength of 486 nm.11−13 C
DOI: 10.1021/acs.est.7b03436 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 3. Characterization of synthesized SiO2@PEI MPs by dynamic light scattering for (a) zeta potential and (b) size. Error bars represent standard deviation (n = 10 for zeta potential, n ≥ 9 for size). Representative SEM images of (c) bare SiO2 MPs and (d) SiO2@PEI MPs. Scale bars correspond to 10 μm. Inset images show a magnified MP (1 μm scale bar).
In Situ Healing Process. To create consistently sized damages on the hollow fiber membranes, a custom damaging device was used as shown in Figure S2. The hollow fiber membrane was laterally positioned between support cover slides, and a vertically positioned microtome blade (MB35 Premier, Thermo Scientific, USA) was used to damage the membranes.4 The in situ healing process was carried out similar to the previously established healing sequences with chitosan, with minor changes.4,5 Briefly, 5 mL of SiO2@PEI MPs suspension was injected into the dispensing vessel that contained 2 L of DI water. Filtration through the damaged membrane was carried out for 5 min, followed by flushing with DI water for 10 min and filtration of 3 wt % glutaraldehyde solution for 10 min, under various pressure and cross-flow rates. The membrane was then kept at room temperature for 1 h without any filtration for the cross-linking reaction to occur. Finally, the membrane system was washed with DI water for 30 min under the same operating condition as SiO2@PEI MPs filtration. The healed membrane area morphology was observed using an SEM. The sampled hollow fiber specimens were dried in a vacuum oven at room temperature for 24 h and sputter coated with 4 nm of iridium coating before SEM analysis. Imaging of FITC-Labeled Silica Microparticle Deposition on Damaged Hollow Fibers. The synthesized SiO2@ FITC MPs suspension (0.0025 wt %) was injected into the dispensing vessel and filtered through a damaged membrane at various pressure and cross-flow velocity conditions. To avoid photobleaching of FITC by external light sources, the filtration was performed in the dark by covering the module with aluminum foil. After filtration, the hollow fiber membranes were imaged using a confocal laser scanning microscope (CLSM, Nikon C2, Japan).11,12 The obtained images were analyzed using 3D image process/analysis software (IMARIS
6.1.5, Bitplane, Switzerland). The details of the CLSM experiment are described in the Supporting Information.
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RESULTS AND DISCUSSION Synthesis and Characterization of Surface-Functionalized Silica Microparticles. Successful functionalization of SiO2 MPs with amine groups (Figure 1a) was confirmed by zeta potential measurements (Figure 3a).23−25 Prior to modification, SiO2 MPs have a negative zeta potential (−51.6 ± 4.7 mV) because of surface silanol groups.28 The addition of APTES, which binds to the surface of SiO2 MPs through the hydrolysis of the ethoxysilane group and condensation with the hydroxyl group, increased the zeta potential value to −6.0 ± 2.7 mV. This increase is expected due to the positively charged primary amine groups in APTES.24,25 After the second surface modification step in which the bifunctional cross-linker, DVS, reacts rapidly with APTES through a Michäel addition,29 the MPs became more negatively charged due to the introduction of vinyl sulfone groups. The cross-linker is necessary to further functionalize the MPs with the branched polyamine, PEI, which results in MPs with a much larger ratio of amine terminal groups compared to the functionalization with APTES.24,25,30 After this final step, the branched PEI introduces sufficient primary amines, which provide sites for cross-linking with glutaraldehyde to form damage-sealing matrix, to result in a net positive charge of 59.7 ± 3.9 mV. The results of the dynamic light scattering analysis showed that the mean particle size of SiO2 MPs after APTES surface modification was similar to that of the original MPs (2.6 ± 0.3 μm), and the mean particle diameters of SiO2 MPs after DVS and PEI modifications were 2.9 ± 0.6 and 3.4 ± 8.5 μm, respectively (Figure 3b). The hydrodynamic diameter of SiO2 MPs (2.6 ± 0.3 μm) is expected to be larger than the average microparticle diameter measured by the SEM (2.2 ± 0.5 μm, D
DOI: 10.1021/acs.est.7b03436 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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which PEIs form bridges between the SiO2@PEI MPs (Figure 4e), which are formed when the primary amine functional group of the branched PEIs reacts with the aldehyde functional group in glutaraldehyde to form a macromolecular structure (Figure S4).31−33 Localized Deposition of Silica Microparticles on Damage Site. Preferential deposition of SiO2@PEI MPs on the damage site, rather than the rest of the membrane, during in situ healing process is necessary to successfully restore damages without particles depositing on the defects-free membrane area. Particle deposition on the defect-free membrane area would result in a decline in flux through the membrane after the in situ healing process to levels much lower than the pristine membrane’s permeability. During in situ healing using SiO2@ PEI MPs, the water flux through the damaged membrane increased by 46.6% (relative to the pristine membrane) and rapidly recovered to the original value within 5 min after SiO2@ PEI MPs filtration (Figure 5a). Since the flux did not decrease further in the subsequent 60 min of operation, it can be assumed that particles sitting on undamaged sites were washed out during the flushing process. This is corroborated by the SEM image of the membrane after in situ healing, where undamaged membrane areas look identical to the pristine membrane (Figure 5b and 5c). In contrast, during in situ healing of compromised membranes with chitosan agglomerates under similar operating conditions, intact membrane surfaces are partly covered with the healing agent leading to a decline in membrane permeability (Figure 5d and S5). These results are likely due to the relatively large pore size of the membrane used for this study (0.1 μm), compared to the membrane in previous studies (10 kDa of nominal molecular weight cutoff5 or 0.02 μm4). Additionally, the insufficient cleaning power of flushing with a feed pump could have contributed to the increased chitosan deposition. In situ healing with SiO2@PEI MPs did not cause particle accumulation on the intact membrane surface for two reasons. First, the particle diameter (≥2.0 μm) is much larger than the nominal pore diameter of the membrane (0.1 μm). Second, the selectivity of SiO2@PEI MPs arises from the ratio of drag forces that determine the particle’s movement, namely, the ratio between the cross-flow and permeate flow drag forces (Figure 6a). When the membrane is damaged, the permeate flux through the damage site increases significantly (Table S2), increasing the ratio of permeate-flow to cross-flow drag forces in that area (Figure 6b). More specifically, in the intact or undamaged membrane, the cross-flow velocity is 3 orders of magnitude higher than the permeate velocity, while in the damaged membrane the permeate velocity in the damage area is 10 times higher than the crossflow velocity (Figure 6b). Therefore, the SiO2@PEI MPs preferentially migrate toward the damage site driven by the increased hydraulic drag. The local drag force ratio is maintained regardless of the cross-flow velocity, that is, even when the cross-flow velocity is increased, a proportional increase in the permeate flow drag occurs due to the change in operation pressure. Visual Confirmation of the Silica Microparticles Deposition. Further visual evidence for the preferential deposition of SiO2 MPs in the damage area is found in the reconstructed CLSM images of an in situ healed hollow fiber membrane. Figure S6 clearly shows that the FITC labeled microparticles are primarily deposited on the damage site. Additionally, this technique allows for quantification of the amount of SiO2@FITC MPs deposited on the damage area.
Figure 3c) because of the hydration of the particles as well as agglomeration of some particles in the aqueous phase.24 The SEM results show that while the surface morphology of the bare SiO2 MPs was uniform and smooth (Figure 3c), the SiO2@PEI MPs have a rough and irregular morphology due to the attachment of PEI on the surface (Figure 3d and Figure S3). In Situ Healing Performance. The filtration of SiO2@PEI through compromised hollow fiber membranes followed by cross-linking with glutaraldehyde, that is, in situ healing process, successfully recovered the membrane’s original properties on the basis of the water flux and particles rejection. To examine the in situ healing performance, pristine membranes were damaged using a microtome device, resulting in a damage with an approximate area of 0.109 mm2 (Figure 4b). The damage
Figure 4. (a) Water flux and fluorescent microspheres rejection of membranes at pristine, damaged, and healed states under a cross-flow rate of 1.0 ± 0.07 L/min, operating pressure of 34 ± 1.7 kPa, at 20 °C. The average results from triplicate experiments are shown with error bars representing standard deviation. Representative SEM images of (b) damaged membrane and (c) in situ healed membrane. Scale bars correspond to 500 μm. (d) Magnified image of the damaged site with 50 μm scale bar. (e) Higher-magnification image of cross-linked SiO2@PEI MPs in the damage site with 4 μm of scale bar.
increased the water flux from 301 ± 15 L·m−2·h−1 (LMH) to 442 ± 22 LMH and decreased the particle removal from 99% to 70.4 ± 7.8% (Figure 4a). After the in situ healing process, the water flux through the membrane was restored to 293 ± 18 LMH and rejection to 99.1 ± 1.0%, which corresponds to 97% and 99% performance recovery, respectively (Figure 4a). SEM images show that after the in situ healing process, SiO2@PEI MPs are deposited in the damage sites forming a particle network that completely plugs the damage sites (Figure 4c and 4d). Higher magnification SEM image show a structure in E
DOI: 10.1021/acs.est.7b03436 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 5. (a) Relative water flux change during in situ healing process. The filtration system was operated with a cross-flow rate of 1.0 ± 0.1 L/min, operating pressure of 34 ± 1.7 kPa, and temperature at 20 °C. Specific in situ healing process: SiO2@PEI MPs, 5 min; 1st flushing, 10 min; glutaraldehyde, 10 min; cross-linking reaction, 60 min; 2nd flushing, 30 min. SEM images of (b) pristine membrane, (c) defect-free area of SiO2@ PEI MPs in situ healed membrane, and (d) defect-free area of chitosan in situ healed membrane. Scale bars correspond to 4 μm. Detailed chitosan in situ healing condition is described in Figure S5.
Figure 6. (a) Transport of particles in cross-flow filtration model on the vertically installed membrane. (b) Local drag forces ratio estimated by using cross-flow and permeate-flow under different operating conditions. The local permeate drag force near the defect-free area and the damaged area were calculated using measured fluxes through pristine and damaged membranes, respectively. Local drag force = Jdf,defect‑freearea/Jcf,cross‑flow or Jdefectarea/ Jcf. See the Tables S1 and S2 for detail values.
We further confirmed that SiO2@FITC MPs deposited in the damage area would not be desorbed during the flushing process; the amount of residual SiO2@FITC MPs on the damage site was very similar at a flushing time of 10, 20, and 30 min (Figure 7e−g). In all of these results, there was also no significant attachment of microparticles to the intact surface of the membrane and as a result, a flushing time of 10 min would be optimal for cleaning the membrane without desorbing any of the particles on the damage site. Overall, CLSM images provide
Figure 7a−c shows that the amount of SiO2@FITC MPs deposited on the damage area was very similar under varying cross-flow rates ranging from of 1.0−2.0 L/min. When the permeate velocity is reduced down to zero by closing the filtrate valve (Figure 2), the amount of SiO2 MPs that are deposited on the damage site decreases by 83% (Figure 7d). These results further confirm that it is the local drag force ratio that primarily determines the microparticle deposition on the damage site. F
DOI: 10.1021/acs.est.7b03436 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 7. Observation of SiO2@FITC MPs deposition on damage area by using CLSM. The image was obtained using a 10× objective lens with depth scanning at 10 μm resolution. The three-dimensional images were reconstructed using IMARIS software. Va−Vg correspond to the volume of SiO2@FITC MPs deposited on the membrane surface as calculated by IMARIS software. Filtration was conducted with membranes a−c at 34, 48, and 72 kPa, respectively. Filtration through membranes d−f was conducted at 34 kPa and 1.0 L/min.
various operating conditions in a practical membrane system, marking a notable improvement over the previously developed chitosan-based in situ healing technique. While chitosan is relatively inexpensive and abundant, mass production of chitosan is not suited for controlling the molecular weight and the degree of deacetylation of chitosan, which plays a critical role in the size of agglomerates that are formed. In contrast, SiO2 MP healing agent can be fabricated without concern of fluctuating particle size.34 Therefore, SiO2 MPs offer a distinct advantage over chitosan agglomerates which can cause pore-clogging and subsequent flux decline. In addition, because SiO2 MPs and nanoparticles are commercialized in various sizes,35 this process can be tuned based on the pore size of the membrane, estimated size of damages, and the cross-flow rate. It is also expected that healing agents will be able to treat various sizes of damage at once, using both small and large SiO2 MPs simultaneously or sequentially.
reliable confirmation of the localized deposition of the SiO2@ FITC MPs on the damage site under common operating conditions. Stability of In Situ Healed Membrane. The physical and chemical stability of the in situ healed membranes is a critical requirement for practical application of this healing method. The SiO2@PEI MPs in situ healed membranes maintained a constant flux without any change in rejection in long-term experiments performed over 31 days with a regular chemical (sodium hypochlorite) cleaning (Figures 8 and S7). Under four different operating conditions, the water permeability and rejection rate of the in situ healed membranes remained at ≥95.6% and ≥98.9% of the original membrane, respectively. These results suggest that the SiO2 MPs have sufficient mechanical/chemical stability for in situ healing. The collective results of this study show that SiO2 MPs can be used under
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b03436. Experimental procedure including membrane operation, damaging method, SEM images of SiO2 MPs and SiO2@ PEI MPs, detail CLSM observation method, reaction scheme and SEM images of cross-linked SiO2@PEI MPs via coupling with glutaraldehyde, in situ healing results with chitosan agglomerates, calculation of flow rate through the damage site, particle deposition and estimated drag force, CLSM images of damage area, and chemical stability of in situ healed membrane (PDF)
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Figure 8. Stability of in situ healed membrane with periodic chemical washing. Operating condition of experimental sets was operating pressure of 34 kPa and cross-flow rate of 1.0 L/min. All membranes were chemically cleaned by soaking 100 mg/L of sodium hypochlorite for 1 h every day. Particle rejection and water flux were measured immediately after chemical cleaning.
AUTHOR INFORMATION
Corresponding Author
*Phone: +1-203-432-4386. Fax: +1-203-432-4387. E-mail:
[email protected]. G
DOI: 10.1021/acs.est.7b03436 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Jae-Hong Kim: 0000-0003-2224-3516 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the International Cooperation Program for Industrial Technologies (N0001232) funded by the Korea government Ministry of Trade, Industry and Energy.
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REFERENCES
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DOI: 10.1021/acs.est.7b03436 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
