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May 4, 2015 - ABSTRACT: To determine further the enhancement and mitigation mechanisms of protein fouling, filtration experiments were carried out wit...
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Enhancement and Mitigation Mechanisms of Protein Fouling of Ultrafiltration Membranes under Different Ionic Strengths Rui Miao, Lei Wang,* Na Mi, Zhe Gao, Tingting Liu, Yongtao Lv, Xudong Wang, Xiaorong Meng, and Yongzhe Yang School of Environmental & Municipal Engineering, Xi’an University of Architecture and Technology, Yan Ta Road. No. 13, Xi’an 710055, China S Supporting Information *

ABSTRACT: To determine further the enhancement and mitigation mechanisms of protein fouling, filtration experiments were carried out with polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes and bovine serum albumin (BSA) over a range of ionic strengths. The interaction forces, the adsorption behavior of BSA on the membrane surface, and the structure of the BSA adsorbed layers at corresponding ionic strengths were investigated. Results indicate that when the ionic strength increased from 0 to 1 mM, there was a decrease in the PVDF−BSA and BSA−BSA electrostatic repulsion forces, resulting in a higher deposition rate of BSA onto the membrane surface, and the formation of a denser BSA layer; consequently, membrane fouling was enhanced. However, at ionic strengths of 10 and 100 mM, membrane fouling and the BSA removal rate decreased significantly. This was mainly due to the increased hydration repulsion forces, which caused a decrease in the PVDF−BSA and BSA−BSA interaction forces accompanied by a decreased hydrodynamic radius and increased diffusion coefficient of BSA. Consequently, BSA passed more easily through the membrane and into permeate. There was less accumulation of BSA on the membrane surface. A more nonrigid and open structure BSA layer was formed on the membrane surface.

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INTRODUCTION

Although previous investigations have reported the effects of ionic strength on protein fouling,10−17 the results are somewhat confusing. For the most part, it has been shown that protein fouling is enhanced with increasing ionic strength; this phenomenon can be well explained by the classical Derjaguin−Landau−Verwey−Overbeek (DLVO) theory. As the ionic strength increases, higher counterion concentration in the solution leads to compression of the electric double layer (EDL) due to charge screening, and the net charge of protein molecules is reduced, which weakens the electrostatic repulsion between bovine serum albumin (BSA) molecules and BSA molecules and the membrane. As a result, there is an increase in the adsorption and deposition of the BSA molecules on the membrane surface or in the membrane pores, leading to significantly enhanced membrane fouling.10−12 However, a few reports13−17 contradict the findings of those above-mentioned studies10−12 in that membrane fouling is shown to be significantly mitigated with an increase in ionic strength. The DLVO theory fails to describe the experimental observations. Chan et al. carried out microfiltration of BSA solutions at various ionic strengths and suggested that membrane fouling was less serious at high ionic strength,

Ultrafiltration (UF) technology is increasingly used in wastewater treatment and water reclamation and as pretreatment in seawater desalination because it offers significant advantages compared with conventional separation processes. However, membrane fouling is still a major problem that restricts its popularity and application.1,2 Protein-like substances, which abound in wastewater, have been identified as one of the major types of membrane foulants.3,4 Understanding protein fouling behavior and its key influencing factors is crucial for the control and mitigation of membrane fouling.5 Ions are present in high and low concentrations in surface water, municipal wastewater, industrial wastewater, and other wastewater or water to be treated.6 In particular, the ionic strength of seawater can be as high as 400 mM or even higher.7 Ionic strength has been identified as a crucial factor that can seriously affect the fouling rate in membrane operation.8 Since the ionic strength is also closely related to the properties of the membrane surface and the proteins used, as well as the protein−membrane and protein−protein interactions, it can affect protein deposition and fouling potential.9 Therefore, obtaining in-depth insight into the effect of ionic strength on protein fouling behavior could lead to the development of strategies for fouling control and mitigation, which is crucial for the successful and widespread application of UF technology in the field of wastewater treatment. © 2015 American Chemical Society

Received: Revised: Accepted: Published: 6574

December 1, 2014 April 26, 2015 May 4, 2015 May 4, 2015 DOI: 10.1021/es505830h Environ. Sci. Technol. 2015, 49, 6574−6580

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Environmental Science & Technology

in the protein working solution was 10 mg/L. The solution was stored in sterilized glass bottles at 4 °C until required for use. A fresh solution was prepared for each fouling experiment. The ionic strength of the protein working solution was adjusted with 1 M NaCl. Unless specified otherwise, all working solutions were prepared with ultrapure water supplied by an Elga Purelab Ultra water purification system (Elga, UK). All reagents and chemicals were analytical grade, purity >99%. PVDF UF Membranes. The flat sheet PVDF UF membranes used in this study were prepared via phase inversion. In brief, specific quantities of PVDF (Solef 1015; Solvay Advanced Polymers Co., USA) and lithium chloride (LiCl, Tianjin Chemical Reagent Co., China) were dissolved in N,N\-dimethylacetamide (DMAc; Tianjin Fuchen Chemical Reagent Co.) at room temperature, then heated to 60 °C under vigorous stirring for 12 h to obtain a homogeneous solution. The polymer solution was left to stand for 1 day to allow air bubbles to escape. The resulting homogeneous polymer solution was uniformly spread onto a glass plate using a casting knife. The glass plate with the cast solution was immediately immersed into a deionized (DI) water bath set at 20 °C; the membrane soon precipitated on the glass plate. The membrane was thoroughly rinsed and then immersed in DI water for 4−5 days until used. The characteristics of PVDF UF membranes are summarized in Table S1 of the Supporting Information. Fouling Experiments. A dead-end filtration setup previously described was used for the fouling experiments.2 The core component of the dead-end filtration setup is a stirred cell used to hold the feed solution. The test membrane was placed on the bottom of the stirred cell. Constant pressure was applied throughout the filtration period using a compressed nitrogen gas cylinder. The permeate flux data were continuously recorded using an electronic balance connected to a computer. A new membrane was used in each experiment. The protocol for the fouling experiments was as follows. First, the membrane was precompacted with DI water under 0.15 MPa until the permeate flux reached a stable value. Then, the transmembrane pressure was reduced to 0.10 MPa, and the membrane was stabilized with DI water to establish a stable pure water flux (J0). Finally, the BSA solution with desired ionic strength (pH = 7.0 ± 0.2, DOC = 10 mg/L) was filtered through the membrane under 0.1 MPa for 2 h. The membrane flux (J) during filtration of the BSA solutions was monitored online, which was normalized to the pure water flux (J/J0). At least six replicates of fouling experiments were carried out for each fouling condition investigated. QCM-D Experiments. The amount of BSA adsorbed on the PVDF surface and the structure of the BSA adsorption layer at different ionic strengths were investigated with a QCM-D (E1, Q-Sense, Sweden) system. A PVDF membrane was first formed on the surface of a goldcoated sensor crystal (QSX 301 Au, Q-Sense). The following method was used: (i) A homogeneous PVDF solution was prepared by dissolving a certain amount of PVDF in DMAc. (ii) A gold-coated sensor crystal was treated for 10 min in a UV chamber, then soaked in a mixture of ultrapure water, ammonia (25% v/v), and hydrogen peroxide (30% v/v) at 75 °C for 10 min, rinsed thoroughly with ultrapure water, and dried under pure nitrogen gas. (iii) The cleaned gold-coated sensor crystal was fixed on the rotary platform of a spin coater (KW-4A, Institute of Microelectronics of the Chinese Academy of Sciences, China), 5 μL of PVDF solution was dropped onto the

mainly due to the higher solubility of the protein at higher salt concentration.13 Salgin et al. suggested that the electrostatic interactions (repulsive or attractive forces) of protein− membrane and protein−protein decrease with an increase in ionic strength, which then increases the mass transfer of protein from the membrane surface back to the bulk solution. Thus, the concentration of BSA at the membrane surface is reduced, and the steady-state permeate flux of a fouled membrane is improved.14,15 In contrast, after investigating the effect of ionic strength on the BSA fouling behavior of a UF membrane, She et al. suggested that the conformational changes and greater solubility of protein molecules may be the reason why the membrane flux decline rate decreased with an increase in ionic strength.16 Similar results were obtained by Wang et al. after studying the effect of the ionic strength of a BSA/ lysozyme (LYS) mixture on the fouling of a nanofiltration membrane.17 They attributed the greater flux at higher ionic strength to the major difference between a single protein system and a mixed protein system. In a mixed protein system, BSA and LYS are oppositely charged, which increases the ionic strength and hence suppresses the electrostatic attraction between adjacent BSA and LYS molecules due to compression of the EDL, thus retarding membrane fouling. To date, there appears to be no consistent explanation as to why protein fouling of membranes exhibits different characteristics at different ionic strengths. All existing explanations appear to be simple descriptions or speculation based on experimental observations and visual comparison of the normalized permeate flux decline curves during filtration of protein solutions at different ionic strengths.9 No clear theoretical knowledge exists to support the experimental observations. Therefore, further investigations are required to explain the mechanism of membrane fouling at different ionic strengths. In the present study, one of the most extensively applied membrane materials in UF systems, polyvinylidene fluoride (PVDF), was chosen as the membrane material to prepare flat sheet UF membranes. A PVDF colloidal probe and a PVDFcoated sensor crystal were also used. One of the most widely used model proteins during membrane fouling, BSA, was selected as a model protein. The interaction forces of PVDF− BSA and BSA−BSA at different ionic strengths were directly measured by atomic force microscopy (AFM) in conjunction with colloidal probes. A quartz crystal microbalance with dissipation monitoring (QCM-D) combined with a PVDFcoated sensor crystal was used to investigate the effect of ionic strength on the deposition and adsorption behavior of BSA on the PVDF surface and the structure of the BSA adsorption layers. These results were combined with those of fouling experiments and the BSA removal rate (at corresponding ionic strengths) to obtain visual insight into the BSA fouling behavior at different ionic strengths and then to explain the effect of ionic strength on membrane fouling by proteins. The ultimate goal is to provide technical strategies for the prevention/control and mitigation of membrane fouling by proteins during wastewater treatment.

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MATERIALS AND METHODS Protein and Chemicals. Commercial BSA (98% purity, Sigma-Aldrich, St. Louis, MO) was used as a model protein foulant to represent the protein-like substances in wastewater. A stock solution thereof (1 g/L) was prepared by dissolving BSA in ultrapure water. The dissolved organic carbon (DOC) 6575

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was mounted on the bottom of the fluid cell and then rinsed with the test solution at least three times. After the fluid cell was fully filled with the test solution, the force between the colloidal probe and the sample surface was measured. In this study, the PVDF colloidal probe was used as a PVDF membrane to determine the interaction forces between PVDF and BSA at the desired ionic strengths. The BSA-fouled membrane was used as the sample surface. The test solution was the aqueous solution, the ionic strength of which was similar to that used in the corresponding fouling experiments. The interaction force between PVDF membrane and carboxyl was measured using the carboxyl colloidal probe; a clean PVDF membrane was used as sample, and an aqueous solution of desired ionic strength was used as test solution. A BSA-coated colloidal probe was used as BSA foulant to measure the interaction force between BSA and BSA; the BSA-fouled membrane was used as sample, and the test solution was the BSA solution that had been used in the fouling experiments. For each type of membrane sample, force measurements were performed at six different locations, and more than 15 force curves were obtained at each location. Analytical Methods. The DOC concentrations of samples of each BSA solution were measured with a TOC analyzer (TOC-L, CPN, Shimadzu, Japan). The zeta potentials and hydrodynamic diameters of BSA at different ionic strengths were determined with a Zetasizer Nano instrument (ZS90, Malvern, UK). An electrokinetic analyzer for solid surface analysis (SurPASS, Anton Paar GmbH, Austria) was used to characterize the zeta potentials of the PVDF membrane surface at different ionic strengths. An adjustable gap cell was also used, and the zeta potential was then calculated according to the Helmholtz−Smoluchowski relationship.19 All measurement experiments were performed at least three times.

center of the sensor crystal, followed by rotation for 15 s at a speed of 1200 r/min. The sensor crystal was then heated to 60 °C, the rotation speed was increased to 6000 r/min, and it was maintained for 40 s. Following this procedure, a PVDF membrane was successfully formed on the surface of the sensor crystal. The PVDF-coated sensor crystal was rinsed thoroughly with ultrapure water and then dried under pure nitrogen gas for about 60 s. The surface topography of the sensor crystal before and after coating with PVDF is shown in Figure S1 of the Supporting Information. For all QCM-D experiments, the flow rate was maintained at 0.1 mL/min using a peristaltic pump (Ismatec, Switzerland), which maintained laminar flow in the module. The temperature was set at 23 °C. In each experiment, a clean PVDF-coated sensor crystal was first mounted in the flow modules of the QCM-D system. Ultrapure water was pumped through the measurement chamber until a stable baseline was achieved. Then a BSA solution at the desired ionic strength (identical to that used in the fouling experiments) was introduced into the system for 25 min. Both the frequency and dissipation at the third overtone were recorded online and then used to determine the amount of BSA adsorbed on the PVDF surface and the structure of the BSA adsorption layer. The deposition and adsorption of BSA on the PVDF membrane surface result in an increase in mass, which is recorded as a decrease in the crystal sensor frequency (Δf). The relationship between the crystal sensor frequency and the mass adsorbed onto the crystal sensor surface followed the Sauerbrey relationship:18 C Δm = − Δf n

(1)

where Δm is the adsorbed mass, n is the overtone number, and C is the crystal constant (17.7 ng Hz−1 cm−2). Hence, the measurement of variations in the crystal sensor frequency could provide a parameter to immediately monitor changes in the mass of deposited BSA with time. Dissipation could be used to reflect the energy dissipation of adsorbed material during deposition, which could provide insight into the structure of the deposited BSA. Adhesion Force Measurements. The interaction forces of PVDF−BSA, BSA−BSA, and PVDF−carboxyl (carboxyl groups of BSA) at different ionic strengths were determined with a MultiMode 8.0 atomic force microscope (Bruker, Germany) in conjunction with PVDF, BSA-coated, and carboxyl colloidal probes, respectively. The methods used to prepare the PVDF and BSA-coated colloidal probes were based on previously published procedures.2 The PVDF colloidal probe was prepared by sintering a 5 μm diameter PVDF microsphere onto the cantilever free end of a tipless AFM probe (NP-10, Bruker, Germany). The BSA-coated colloidal probes were prepared by adsorbing the corresponding foulants onto the surface of the PVDF microspheres that had been sintered onto the cantilever. The carboxyl colloidal probes were prepared by gluing a 5 μm diameter carboxyl microsphere to the end of a cantilever. After each fouling experiment, the fouled membrane with a foulant layer formed on the surface was removed from the filtration setup and immediately soaked in a solution identical to that used in the corresponding fouling experiment. Measurements of the interaction force (between colloidal probe and sample surface) were performed in a fluid cell using a closed inlet/outlet loop under contact mode. First, a sample

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RESULTS AND DISCUSSION Effect of Ionic Strength on BSA Fouling. The effect of ionic strength on the BSA fouling of PVDF UF membranes was systematically investigated. The permeate flux decline curves of BSA-fouled membranes over the entire filtration period at the different ionic strengths are presented in Figure 1. The greatest flux reduction occurred at 1 mM; the flux declined by almost 90% within 10 min of filtration time. The membrane fouling was significantly enhanced at ionic strengths ranging from 0−1 mM. This is consistent with the findings of previous reports, namely, that the BSA fouling of reverse

Figure 1. Normalized flux versus filtration time for BSA solutions at different ionic strengths. Experimental conditions: the DOC and pH of the BSA solutions were 10 mg/L and 7.0 ± 0.2, respectively; the applied pressure was 0.1 MPa; and the test temperature was 23 °C. 6576

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Environmental Science & Technology osmosis (RO) membranes becomes more significant with an increase in ionic strength.10 As both BSA and the PVDF membrane become less negative with an increase in ionic strength (see Table S2, Supporting Information), the decrease of PVDF−BSA and BSA−BSA electrostatic repulsion forces causes more serious membrane fouling. In contrast, at ionic strengths >1 mM, the zeta potentials of both BSA and the PVDF membrane continued to decrease, which indicated that electrostatic repulsion between BSA molecules and between BSA molecules and the membrane was still decreasing. However, the flux decline rate and extent of the BSA-fouled membrane clearly decreased with an increase in ionic strength. In particular, the flux of BSA-fouled membranes at 100 mM ionic strength had declined by 23% at the end of filtration; this flux was four times lower than in the case of 1 mM ionic strength. Similar results were obtained by Chan et al.13 They explained that the decrease in membrane fouling at higher ionic strength was mainly due to the greater solubility of the protein at higher salt concentration.13 She and co-workers suggested that the rate of membrane flux decline with an increase in ionic strength was mainly due to the conformational changes and greater solubility of protein molecules at higher ionic strength.16 Wang et al. suggested that the greater flux at higher ionic strength was mainly due to decreased electrostatic attraction between BSA and LYS at higher ionic concentration.17 Overall, the above offer some simple and provisional explanations for the experimental phenomenon that membrane flux decline is slower at higher ionic strength. However, further information and an explanation of membrane fouling mechanisms at high ionic strength are limited, and thus further studies are required. Adsorption of BSA to PVDF and Adsorbed BSA Layer Structure. The adsorption behavior of BSA on the PVDF membrane surface and structural changes of the corresponding BSA adsorbed layers were investigated over a range of ionic strengths. The representative normalized frequency (Δf) and dissipation (ΔD) at the third overtone under each of the ionic strength conditions are presented in Figure 2, panels a and b, respectively. In stage A, ultrapure water is injected to achieve a stable baseline. In stage B, a BSA solution with desired ionic strength is introduced. A decrease in Δf and increase in ΔD, from the baseline, were monitored in stage B. Figure 2, panel a shows Δf as a function of the ionic strength on a PVDF-coated surface. The extent of Δf decrease of the PVDF-coated sensor crystal increased from 18.6 to 24.3 Hz when the ionic strength increased from 0 to 1 mM. However, at ionic strengths of 10 and 100 mM, the extent of Δf decrease was 17.2 and 14.3 Hz. Clearly, when the ionic strength exceeded 1 mM, the extent of Δf decrease of the PVDF-coated sensor crystal decreased with an increase in ionic strength. The measured Δf shift was proportional to the amount of mass on the sensor crystal surface. It was clear that the rate of adsorption and the amount of BSA on the PVDF surface increased initially and then decreased with increasing ionic strength. This characteristic variation is in agreement with the changing characteristic of a decrease in flux rate and extent of corresponding fouled membranes. A similar trend was observed by several other groups who investigated the deposition behavior of colloidal particles on a sensor crystal surface; however, the mechanisms by which this occurs were neither fully elucidated nor researched further.20−23 Figure 2, panel b depicts ΔD for a PVDF-coated sensor crystal as a function of ionic strength. Note that there is a

Figure 2. Normalized change in (a) Δf (decreased) and (b) ΔD (increased) as a function of time for BSA adsorption on a PVDF surface, at different ionic strengths. Experimental conditions: the DOC and pH of the BSA solutions were 10 mg/L and 7.0 ± 0.2, respectively; the test temperature was 23 °C; and the flow rate was 0.1 mL/min.

significant difference between the ΔD values at the different ionic strengths. The ΔD values were 0.49 × 10−6, 0.42 × 10−6, 1.01 × 10−6, and 1.95 × 10−6 at ionic strengths 0, 1, 10, and 100 mM, respectively. It was interesting to see that the changing trend of ΔD with an increase in ionic strength was reversed at ionic strength 1 mM, which is similar to what occurred for Δf and the flux decline. ΔD could therefore provide insight into the structure of the deposited BSA and be closely related to the adsorption amount. The energy dissipation per unit change of adsorbed mass, |ΔD/Δf |, was used to compare the structural characteristics of the BSA layer at different ionic strengths.24 A lower value of |ΔD/Δf | suggests formation of a dense and compact layer, while a higher value indicates a “soft” open structure and dissipative layer.25 The |ΔD/Δf| ratio as a function of ionic strength, which is calculated using ΔD and Δf data from stage B, is presented in Table S3 of the Supporting Information. The |ΔD/Δf | ratio decreased from 0.026 × 10−7 to 0.017 × 10−7 Hz−1 when the ionic strength increased from 0 to 1 mM, indicating that the BSA layer deposited on the PVDF surface became more compact. However, in contrast, the |ΔD/Δf | values at ionic strengths 10 and 100 mM were about 3.5- and eight-times greater, respectively, than at ionic strength 1 mM; the |ΔD/Δf | ratio increased sharply with an increase in ionic strength from 1 to 100 mM. This observation suggests that a BSA layer with a nonrigid and open structure is formed on the PVDF surface at higher ionic strength. By combining these results with the results from Figure 1, it is interesting to note that the more “soft” and open the BSA layer is, the higher the pseudostable flux of the corresponding fouled membrane. This is consistent with previous reports; namely, a membrane’s pseudostable flux is mainly dependent on the structure of the cake layer.2 Interfacial Adhesion Force Changes at Different Ionic Strengths. Several research groups have clearly demonstrated that the measurement of membrane−foulant and foulant− 6577

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Figure 3. Representative normalized adhesion forces versus distance curves of (a) PVDF−BSA and (b) BSA−BSA at different ionic strengths.

adhesion force between latex microspheres and proteins decreases with increasing ionic strength.30 The reasons why the adhesion forces of PVDF−BSA and BSA−BSA were reduced at higher ionic strength were investigated further. Since the carboxyl group is one of the predominant functional groups of BSA, and its size and conformation are constant, a carboxylate-modified latex probe was prepared and used to determine the adhesion force of carboxyl−PVDF. Representative normalized adhesion forces versus distance curves of PVDF−carboxyl at different ionic strengths are presented in Figure 4.

foulant interaction forces is a powerful method to explain membrane fouling behavior.26−28 To better understand the BSA fouling mechanism of PVDF UF membranes at different ionic strengths, especially under higher ionic strength conditions, the adhesion forces of PVDF−BSA and BSA− BSA at ionic strengths 0, 1, 10, and 100 mM were determined. Figure 3 shows the representative normalized adhesion forces (F/R) versus distance curves of PVDF−BSA and BSA−BSA at different ionic strengths. At ionic strengths 0, 1, 10, and 100 mM, the average adhesion forces between PVDF and BSA were 0.71, 0.88, 0.32, and 0.29 mN/m, while for BSA−BSA, they were 0.36, 0.42, 0.28, and 0.25 mN/m, respectively. Obviously, the adhesion force of PVDF−BSA was stronger than that of BSA−BSA at each of the ionic strengths considered. Combined with the fact that the flux decline of BSA-fouled membranes and BSA deposition mainly occur at an early stage, this indicates that membrane fouling is dominated by the interaction forces between membrane and foulant rather than between foulant and foulant. This is in general agreement with the findings of Hashino et al., who measured the adhesion forces of BSA−PVDF and BSA−BSA using a BSA colloidal probe.29 The above conclusion suggests that the selection of an appropriate membrane surface modification method that could decrease the PVDF−BSA interaction force would be an effective strategy to mitigate the BSA fouling of PVDF membranes. It is interesting to note that the change in trend of the adhesion force of PVDF−BSA with ionic strength followed the same trend as that for the adhesion force of BSA−BSA. When the ionic strength was in the range 0−1 mM, the PVDF−BSA and BSA−BSA adhesion forces increased with increasing ionic strength. This phenomenon can be well explained by the classical DLVO theory. In other words, the compression of the EDL was enhanced with increasing ionic strength, resulting in a weakening of the electrostatic repulsion forces between BSA molecules and between BSA and the PVDF membrane, which would then cause an increase of the PVDF−BSA and BSA− BSA adhesion forces. When the ionic strength was increased from 1 to 100 mM, the electrostatic repulsions of BSA−BSA and BSA−PVDF decreased further as the net charge of BSA and the PVDF membrane surface was significantly reduced. However, the PVDF−BSA and BSA−BSA adhesion forces decreased significantly with increasing ionic strength, which is contrary to the DLVO theory. This observation is consistent with the findings of Xu’s group, who surmised that changes in the size and conformation of the proteins may be the reason why the

Figure 4. Representative normalized adhesion forces versus distance curves of PVDF−carboxyl at different ionic strengths.

The average adhesion force between PVDF and carboxyl in a test solution of zero ionic strength was 6.7 mN/m. It increased to 9.4 mN/m when the ionic strength was increased to 1 mM. This can be well explained by the DLVO theory. However, the average PVDF−carboxyl adhesion force decreased dramatically to 2.2 and 0.77 mN/m when the ionic strength was increased to 10 and 100 mM, respectively. Lee et al. also found that the adhesion force between the carboxyl group and RO membrane decreased dramatically as the concentration of the NaCl solution increased.31 Moreover, the variation in characteristics for the PVDF−carboxyl adhesion force at ionic strengths 1− 100 mM was similar to that for the PVDF−BSA and BSA−BSA adhesion forces; namely, the PVDF−carboxyl adhesion force decreased with an increase in ionic strength. This is mainly because the hydration repulsion force between membrane and BSA, or carboxyl, and between BSA molecules increased significantly at higher ionic strength. Negatively charged PVDF, BSA, and carboxyl surface were surrounded by very hydrated Na+ cations, which retain some 6578

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permeate, which mitigates the adsorption and accumulation of BSA on the membrane surface.38 It is interesting to see that, at higher ionic strength, membrane fouling is significantly reduced due to an increased hydration repulsion force, but it is at the cost of a decline in the rate of foulant removal. Therefore, the mitigation of membrane fouling and the corresponding rate of foulant removal must be suitably adjusted and balanced for optimum design performance during the actual UF process operation.

water of hydration, contributing to the formation of a special hydration layer in the vicinity of PVDF, BSA, and its carboxyl surface.32 The hydration repulsion forces were provoked when the hydration layers of two interaction surfaces overlapped.33 A stronger hydration repulsion force is generated when the accumulated amount of hydrated Na+ cations increases.34 When the ionic strength increased from 0 to 1 mM, the hydration repulsion forces between PVDF and BSA (or carboxyl) and between BSA and BSA were weak; they were masked by changes in the electrostatic force.35 However, when the ionic strength increased to 10 and 100 mM, the hydration repulsion forces of PVDF−BSA, BSA−BSA, and PVDF− carboxyl further increased and masked those changes in the electrostatic force, contributing to the decline of PVDF−BSA, BSA−BSA, and PVDF−carboxyl adhesion forces. The corresponding BSA deposition rate and extent of BSA deposition onto the membrane surface were reduced, and a more nonrigid and open structure BSA layer was formed. This phenomenon suggests that the interaction forces between a membrane and foulant and between a foulant and foulant, which primarily control the fouling behavior in different filtration stages,36 could be effectively weakened by the hydration repulsion forces when the ionic strength reaches a given value. Hydration repulsion may be a universal phenomenon among proteins.33 Ang et al. reported that protein fouling of membranes is greater at higher ionic strength due to screening effects.10,11 This may be because the appropriate conditions of pH or ionic strength, or membrane material performance, are not met, resulting in hydration repulsion not being observed. BSA Removal Rate at Different Ionic Strengths. Figure 5 displays the BSA removal rate of a PVDF membrane at

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ASSOCIATED CONTENT

S Supporting Information *

Surface morphology of a gold-coated sensor crystal and a PVDF-coated sensor crystal. Characteristics of the PVDF UF membrane used in this study. Zeta potentials of BSA and a PVDF membrane surface as a function of ionic strength. |ΔD/ Δf | values as a function of ionic strength. Hydrodynamic diameter of BSA as a function of the ionic strength. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/es505830h.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 029 8220 2729; fax: +86 029 8220 2729; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this study was provided by the National Natural Science Foundation of China (Grant Nos. 51178378, 51278408), the Shanxi Province Science and Technology Innovation Projects (Grant Nos. 2012KTCL03-06, 2013KTCL03-16), the Innovative Research Team of Xi’an University of Architecture and Technology and the Doctorate Foundation of Xi’an University of Architecture and Technology (Grant No. DB03154).

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REFERENCES

(1) Barry, M. C.; Hristovski, K.; Westerhoff, P. Membrane fouling by vesicles and prevention through ozonation. Environ. Sci. Technol. 2014, 48, 7349−7356. (2) Wang, L.; Miao, R.; Wang, X.; Lv, Y.; Meng, X.; Yang, Y.; Huang, D.; Feng, L.; Liu, Z.; Ju, K. Fouling behavior of typical organic foulants in polyvinylidene fluoride ultra filtration membranes: Characterization from microforces. Environ. Sci. Technol. 2013, 47, 3708−3714. (3) Liu, J.; Shen, X.; Zhao, Y.; Chen, L. Acryloylmorpholine-grafted PVDF membrane with improved protein fouling resistance. Ind. Eng. Chem. Res. 2013, 52, 18392−18400. (4) Celik, E.; Liu, L.; Choi, H. Protein fouling behavior of carbon nanotube/polyethersulfone composite membranes during water filtration. Water Res. 2011, 45, 5287−5294. (5) Ding, Y.; Tian, Y.; Li, Z.; Wang, H.; Chen, L. Microfiltration (MF) membrane fouling potential evaluation of protein with different ion strengths and divalent cations based on extended DLVO theory. Desalination 2013, 331, 62−68. (6) Wang, F.; Zhang, M.; Peng, W.; He, Y.; Lin, H.; Chen, J.; Hong, H.; Wang, A.; Yu, H. Effects of ionic strength on membrane fouling in a membrane bioreactor. Bioresour. Technol. 2014, 156, 35−41. (7) Silva, V.; Geraldes, V.; Brites Alves, A. M.; Palacio, L.; Prádanos, P.; Hernández, A. Multi-ionic nanofiltration of highly concentrated salt mixtures in the seawater range. Desalination 2011, 277, 29−39. (8) Yu, Y.; Lee, S.; Hong, S. Effect of solution chemistry on organic fouling of reverse osmosis membranes in seawater desalination. J. Membr. Sci. 2010, 351, 205−213.

Figure 5. Effect of ionic strength on rate of BSA removal.

different ionic strengths: at zero ionic strength, it was 85%, and at 1 mM ionic strength, it increased to 95%. This is because the interaction forces of PVDF−BSA and BSA−BSA increase when the ionic strength increases from 0 to 1 mM, enhancing the aggregation and deposition rate of BSA onto the PVDF membrane surface.37 However, it is surprising to see that the BSA removal rate decreases sharply with a further increase in the ionic strength; it only reached 13% at 100 mM ionic strength. This is consistent with the findings of a previous study, namely that the BSA removal rate was greatly reduced at high salt concentrations.16 The main reason for this is that with an increase in ionic strength, the PVDF−BSA and BSA−BSA interaction forces are weakened due to an increase in the hydration repulsion force, which is accompanied by a decreased hydrodynamic radius and increased diffusion coefficient of BSA (Table S4, Supporting Information). The result is that BSA passes more easily through the membrane and into the 6579

DOI: 10.1021/es505830h Environ. Sci. Technol. 2015, 49, 6574−6580

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DOI: 10.1021/es505830h Environ. Sci. Technol. 2015, 49, 6574−6580