Effect of Hydration Forces on Protein Fouling of Ultrafiltration

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Effect of Hydration Forces on Protein Fouling of Ultrafiltration Membranes: The Role of Protein Charge, Hydrated Ion Species, and Membrane Hydrophilicity Rui Miao, Lei Wang,* Miao Zhu, Dongxu Deng, Songshan Li, Jiaxuan Wang, Tingting Liu, and Yongtao Lv 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 investigate the influence of hydration forces on the protein fouling of membranes and the major influence factors of hydration forces during the ultrafiltration process, bovine serum albumin (BSA) was chosen as model foulant. For various pH levels and hydrated ion and membrane species, the membrane−BSA and BSA−BSA interaction forces, and fouling experiments with BSA, as a function of ionic strength, were measured. Results showed that hydration forces were a universal phenomenon during the membrane filtration process, when the levels of pH, ion species, and membrane performances were appropriate. First, for the BSA negatively charged or neutral, hydration forces caused a decrease in the membrane fouling. Conversely, for the BSA positively charged, the hydration forces were absent because the counterions were not hydrated, and membrane fouling was enhanced. For different hydrated ions, the smaller the radii of the ions were, the stronger the hydration forces that were produced, and the membrane fouling observed was less, indicating that hydration forces are closely correlated with the size of the hydrated ions. Moreover, in comparison with a hydrophobic membrane, it is more difficult to observe hydrophilic membrane−BSA hydration forces because the hydrophilic membrane surface adsorbs water molecules, which weakens its binding efficiency to hydrated ions.

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INTRODUCTION Ultrafiltration (UF) technology has been widely used in the field of wastewater treatment and water reclamation because of its innate advantages over traditional technologies, such as no phase changes, low operating pressures, and space savings.1 However, membrane fouling is primarily caused by the accumulation of dissolved organic matter (proteins, humic acid, polysaccharides, etc.) in wastewater to be treated, which constitutes a major limitation to the widespread use of UF technology.2 Organic matter fouling of membranes is a complex process, which is closely related to many factors, such as feedwater solution chemistry, membrane properties, and hydrodynamic conditions.3 Cations, such as Ca2+, K+, and Na+, which are widely found in surface water, municipal wastewater, industrial wastewater, and other wastewater or water to be treated, have been identified as a crucial factor that can seriously affect organic matter fouling of membranes.4 This is because cations can easily interact with functional groups in organic matter molecules through complexation, neutralization, or electrical double layers, which can significantly influence the membrane− foulant and foulant−foulant interaction, and can eventually affect the corresponding membrane fouling behavior.5,6 Therefore, an in-depth insight is required into the influence mechanisms of cations on organic matter fouling of membranes, which will be crucially important for controlling and mitigating membrane fouling during the UF process. © XXXX American Chemical Society

Many investigations have been carried out on the role of cations in organic matter fouling of membranes.7−15 For the most part, it has been shown that the rate of organic matter fouling of membranes and the fouling extent could effectively be enhanced by cations, as is explained by the classical Derjaguin−Landau−Verwey−Overbeek (DLVO) theory. That is, with the membrane surface and organic foulants charged negatively, charge screening, complexation, or bridging action would be strengthened with increasing cation content, which weakens the electrostatic repulsion forces of membrane− foulant and foulant−foulant. As a result, there is an increase in the deposition rate of the organic foulants on the membrane surface or in the membrane pores, leading to a significant increase in the fouling rate and the fouling extent of membranes.7−10 In recent years, however, a few researchers have verified that organic matter fouling of membranes was significantly mitigated as the cation content was increased. This phenomenon cannot be explained by the DLVO theory.11−14 Moreover, Zhou et al. observed that the organic matter fouling rate and the fouling extent of membranes first increased and then decreased with increasing cation content.15 It may be clear from the above that Received: Revised: Accepted: Published: A

July 21, 2016 December 4, 2016 December 7, 2016 December 7, 2016 DOI: 10.1021/acs.est.6b03660 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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MATERIALS AND METHODS Chemicals. Commercial bovine serum albumin (BSA, 98% purity, Sigma-Aldrich, St. Louis, MO) was used in this study. The molecular weight and isoelectric point (IEP) of BSA were specified to be about 67 kDa and 4.7, respectively, by the manufacturer. The stock solution (1 g/L) was prepared by dissolving BSA in ultrapure water, which was stored in sterilized glass bottles at 4 °C until required for use. For each fouling experiment, a fresh protein working solution at a dissolved organic carbon (DOC) concentration of 10 mg/L was prepared. For controlling the charge of the BSA, the pH level of the protein working solution was adjusted with either 0.01 mol/L NaOH or 0.01 mol/L HCl solutions. Unless specified otherwise, the ionic strength of the BSA working solution was adjusted with 1 M NaCl, and all working solutions were prepared with ultrapure water (resistivity of 18.2 Mohm.cm, TOC < 0.001 mg/L), supplied by an Elga Purelab Ultra water purification system (Elga, U.K.). All reagents and chemicals were analytical grade, with purity >99%. UF Membranes. Relatively hydrophobic PVDF and more hydrophilic EVOH UF membranes were used in the current study, which were prepared via a nonsolvent-induced phase separation method as follows. PVDF (Solef 1015; Solvay Advanced Polymers Co., U.S.A.) or EVOH (ethylene content of 32%, Kuraray, Japan) was mixed with a certain proportion of N,N-dimethylacetamide (DMAc; Tianjin Fuchen Chemical Reagent Co., China) and stirred vigorously for 16 h at 60 °C to obtain a homogeneous solution. The polymer solution was then allowed to stand for 12 h in vacuum oven at 60 °C to allow air bubbles to escape. The resulting homogeneous polymer solution was uniformly spread onto a glass plate using a casting knife, and the glass plate was then immersed immediately in a deionized (DI) water bath set at a temperature of 40 °C. The membrane precipitated on the glass plate, which was thoroughly rinsed to remove residual solvent and then immersed in DI water for 5−7 days before use. The characteristics of PVDF and EVOH flat sheet UF membranes are summarized in Table S1. Fouling Experiments. The BSA fouling experiments were performed using a dead-end filtration setup as previously described.22 The protocol for the BSA fouling experiments was as follows. First, a new UF membrane was compacted at 0.15 MPa with DI water until the membrane flux reached a stable value. Then, the transmembrane pressure was lowered to 0.10 MPa, and the membrane was stabilized with DI water to establish a stable permeate flux that is referred to as the purewater flux (J0). Finally, the BSA solution with desired chemical condition was filtered through the membrane under 0.1 MPa for 2 h, and at this stage, changes in the membrane flux (J) were monitored online, which was normalized to the pure water flux (J/J0). The fouled membrane was removed from the filtration setup and used for AFM force measurements. All BSA fouling experiments were conducted at 20 °C. At least six replicates of fouling experiments were carried out for each chemical condition. Interfacial Force Measurement. A MultiMode 8.0 atomic force microscope (Bruker, Germany), in conjunction with BSA colloidal probes, was used to quantify the membrane−BSA and BSA−BSA interaction forces as a function of ionic strength for specific pH, hydrated ion species, and membrane conditions.

at certain levels of cation contents membrane fouling will decrease with increasing cation content. However, most explanations of this phenomenon appear to be simple descriptions or speculation based on experimental observations and visual comparison of the normalized permeate flux decline curves during the filtration process. There appears to be no further information or explanation why organic matter fouling of membranes decreases with increasing cation content. On the basis of the combination of atomic force microscopy (AFM) and quartz crystal microbalance with dissipation monitoring technology, our research group investigated the protein fouling behaviors of polyvinylidene fluoride (PVDF) ultrafiltration membranes over a range of Na+ concentrations.16 Results verified that when the Na+ concentration increased above a critical value, the significant increase in hydration repulsion forces (non-DLVO forces) between the membrane and BSA molecules, and between BSA molecules, was the major reason for the mitigation of membrane protein fouling. Hydration forces are relatively short-ranged repulsive forces, which are difficult to distinguish from the electrostatic force and van der Waals forces at lower hydrated cation concentrations, and they become stronger with increasing hydrated ion content.17 When the membrane/BSA-solvent surface is surrounded by hydrated cations, which retain some water of hydration, this contributes to the formation of a structured water layer around the membrane and foulant surface. Then, the hydration repulsion forces will be provoked when the hydration layers of two interacting surfaces overlap each other.18−20 The hydration repulsion forces could weaken the membrane−foulant and foulant−foulant interaction forces effectively, which decreases the adsorption and deposition rate of foulants onto the membrane surface.16 It may be an effective strategy for mitigating membrane fouling. However, although there is currently broad interest in the topic of hydration repulsion forces in many fields, such as in cell biology, clay swelling and geochemical processes in geology, biolubrication, and nanotribology, only a few research reports exist on hydration repulsion forces in membrane fouling.21 The influence mechanisms of hydration forces on membrane fouling and the major influence factors of hydration forces during the membrane filtration process is still unclear. Therefore, it is essential to unravel the above issues as they may be critically important for using hydration repulsion forces to prevent/ control membrane fouling. In this study, bovine serum albumin (BSA) was chosen as the model organic matter foulant, and a BSA colloidal probe was prepared by adhering a BSA microsphere onto the end of a cantilever. An AFM, in conjunction with the BSA colloidal probe, was used to measure the membrane−BSA and BSA− BSA interaction forces as a function of ionic strength for various pH levels, hydrated ion species, and UF membranes with different hydrophilicity levels. Fouling experiments using BSA with specific chemical compositions were conducted, and the results were combined with corresponding AFM force measurement results to investigate the role of hydration repulsion force on membrane fouling. This also provided a visual insight into the influence mechanisms that different BSA compositions, hydrated ions species, and membrane hydrophilicity have on hydration repulsion forces during the ultrafiltration process, which would provide a theoretical basis for improved control of membrane fouling. B

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Figure 1. Representative PVDF−BSA and BSA−BSA interaction force curves at pH 3 as a function of ionic strengths.

Adhesion Forces as a Function of Ionic Strengths with BSA Positively Charged. First, with the BSA positively charged at pH 3.0, the adhesion forces of PVDF−BSA and BSA−BSA at different ionic strengths were measured, corresponding representative adhesion force curves are presented in Figure 1. At ionic strengths 0, 1, 10, and 100 mM, the average adhesion forces between PVDF and BSA were 19.1 ± 0.91, 18.3 ± 0.89, 18.0 ± 0.90, and 15.6 ± 0.83 nN, and those for BSA− BSA were 0.53 ± 0.023, 0.78 ± 0.036, 0.98 ± 0.043, and 1.37 ± 0.069 nN, respectively. From the above, the following information is obtained. The PVDF−BSA interaction forces decreased with increasing ionic strengths. This observation could be reasonably explained by the extent of electrostatic attraction forces between PVDF and BSA. At pH 3, the BSA was positively charged while the PVDF membrane was negatively charged (as displayed in Table S2−S3), which would lead to the PVDF−BSA electrostatic attraction forces. The magnitude of the electrostatic attraction force is proportional to the charges of PVDF and BSA.25 The increase in ionic strengths resulted in a weakening of the electrostatic attraction force between PVDF and BSA, as a result of the electric double compression effect, which would then cause a decrease in the PVDF−BSA adhesion forces.26,27 In contrast to PVDF−BSA, the BSA−BSA interaction forces increased with increasing ionic strength. This is also because the BSA molecules became less negative with increasing ionic strength, accompanied by a decrease in the electrostatic repulsion force between BSA and BSA, and produced an increase in the BSA−BSA adhesion forces. From what has been discussed above, with the BSA in a positively charged condition and with increasing ionic strength, the changes in the adhesion forces of PVDF−BSA and BSA− BSA could be satisfactorily explained by the classical DLVO theory. It is important to note that hydration repulsion forces were not observed at higher ionic strengths. This is because when the BSA is positively charged, the counterions near the BSA molecular surface are the Cl− anions, which are practically not hydrated.26 Therefore, a structured water layer cannot be formed around the BSA surface, and the hydration forces do not appear. The results of the flux decline curves of BSA-fouled membranes at pH 3 are displayed in Figure S2. At ionic strengths 0, 1, 10, and 100 mM, the flux of BSA-fouled membranes declined by 61%, 66%, 70%, and 80% within 120 min of filtration time, respectively. It is obvious that the membrane fouling rate and extent were enhanced with increasing ionic strength. This variation characteristic is in agreement with the order of the BSA−BSA adhesion forces, which also indicates that BSA fouling of the PVDF UF membrane was mainly dependent on BSA−BSA interaction

The preparation method of the BSA colloidal probe was as follows. First, the free end of a tipless cantilever probe (NP-10, Bruker, Germany) was coated with a small amount of Epikote. Then, a BSA-modified 3 μm microsphere made of silica (micromod Partikeltechnologie GmbH Co., Germany) was glued to the end of the cantilever using the Epikote. According to the manufacturer, the IEP of the BSA microsphere was about pH 4.7. Finally, to strengthen the connection between the BSA microsphere and the cantilever, the BSA colloidal probes were stored at 4 °C for at least 1 week prior to use. A scanning electron microscope (SEM) image of a BSA colloidal probe made in this way is shown in Figure S1. As in previously published procedures, the AFM interaction force measurements were carried out in a fluid cell using a closed inlet/outlet loop under contact mode.23,27 First, a membrane sample (PVDF, EVOH, or BSA-fouled membrane) was mounted in the bottom of the fluid cell and then rinsed with the test solution at least three times. Then, the force between the BSA colloidal probe and the sample surface was measured after the fluid cell was fully filled with the test solution. The test solution was the aqueous solution, the chemical conditions of which were similar to that used in the corresponding BSA fouling experiments. For the membrane− BSA interaction force, the clean PVDF or EVOH membrane was used as sample, while for the BSA−BSA interaction force, the BSA-fouled membrane was used as sample. For each type of membrane sample, force measurements were carried out at six locations, and more than 10 force curves were obtained at each location. The average force values were used in the Results and Discussion section. Analytical Methods. The zeta potentials of BSA with different chemical conditions were determined with a Zetasizer Nano instrument (ZS90, Malvern, U.K.). The zeta potentials of the membranes were determined by an electrokinetic analyzer for solid surface analysis (SurPASS, Anton Paar GmbH, Austria) for corresponding chemical conditions. An adjustable gap cell was also used, and the zeta potential was then calculated according to the Helmholtz−Smoluchowski relationship.24 All measurement experiments were performed at least three times.

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RESULTS AND DISCUSSION Effect of BSA Charged Performance on Hydration Forces During UF Process. In this section, with the BSA positively charged (pH 3), neutral (pH 4.7), and negatively charged (pH 9), the PVDF−BSA and BSA−BSA interaction forces as a function of ionic strengths were measured. These results were combined with results for corresponding BSA fouling of PVDF UF membranes to investigate the effect of BSA charge on the hydration forces during the UF process. C

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Figure 2. Representative PVDF−BSA and BSA−BSA interaction force curves at pH 4.7 as a function of ionic strengths.

Figure 3. Representative PVDF−BSA and BSA−BSA interaction force curves at pH 9 as a function of ionic strengths.

However, when the ionic strengths increased to 10 and 100 mM, the BSA molecules were approximately neutrally charged, so the electrostatic repulsive forces of PVDF−BSA and BSA− BSA were nearly negligible. By this time, as the accumulated amount of hydrated Na+ cations on the surfaces of the BSA and membrane increased, the hydration repulsion forces of PVDF− BSA and BSA−BSA further increased and became critical, which contributed to the decline of PVDF−BSA and BSA− BSA adhesion forces.30 The corresponding deposition rate and extent of the BSA on the membrane surface were decreased, and membrane fouling was mitigated. Given the above, at the IEP of the BSA and with the increasing ionic strengths, the PVDF−BSA and BSA−BSA hydration repulsion forces increased significantly, which would quickly shield the changes in the electrostatic forces, and the interaction forces between membrane and foulant and between foulant and foulant could be weakened drastically, resulting in the decrease in the corresponding membrane fouling. Adhesion Forces as a Function of Ionic Strengths with BSA Negatively Charged. At pH 9, the BSA was negatively charged, and the adhesion forces between PVDF and BSA and between BSA and BSA at different ionic strengths are displayed in Figure 3. At ionic strengths 0, 1, 10, and 100 mM, the average adhesion forces between PVDF and BSA were 1.7 ± 0.11, 2.5 ± 0.13, 3.4 ± 0.17, and 1.6 ± 0.13 nN, and those for BSA−BSA were 0.16 ± 0.012, 0.17 ± 0.011, 0.50 ± 0.023, and 0.31 ± 0.019 nN, respectively. It is interesting to observe that similar to the phenomenon that was observed for the IEP of BSA, at pH 9.0, the PVDF−BSA and BSA−BSA interaction forces decreased significantly when the ionic strength was above a critical value. This suggests that the hydration repulsion forces were also effectively raised with the BSA negatively charged. Considering Figure 2, it was found that at pH 4.7, the corresponding interaction forces and membrane fouling decreased when the ionic strength reached 10 mM. However, it is important to note that at pH 9, the decrease in corresponding interaction forces and membrane fouling were

forces. This is mainly because, at pH 3, the PVDF−BSA interaction force was much stronger (more than 10 times) than the BSA−BSA interaction force for each ionic strength condition. The stronger PVDF−BSA interaction force caused the PVDF membrane surfaces to be completely covered by BSA in a very short period of filtration, and soon membrane fouling was dominated by the BSA−BSA interaction forces.27 Adhesion Forces as a Function of Ionic Strengths at the IEP of BSA. At pH 4.7 (the IEP of the BSA), the PVDF−BSA and BSA−BSA adhesion forces at different ionic strengths were measured. The corresponding representative adhesion force curves are presented in Figure 2. At ionic strengths 0, 1, 10, and 100 mM, the average adhesion forces between PVDF and BSA were 4.7 ± 0.25, 5.2 ± 0.24, 3.5 ± 0.17, and 2.0 ± 0.13 nN, while for BSA−BSA, they were 6.4 ± 0.33, 10.6 ± 0.49, 7.4 ± 0.39, and 1.4 ± 0.11 nN, respectively. Obviously, for both PVDF−BSA and BSA−BSA, when the ionic strengths ranged from 0 to 1 mM, the adhesion forces increased significantly, whereas they decreased gradually when ionic strengths increased from 1 to 100 mM, which is in contrast to the DLVO theory. These results could be explained by the presence of hydration forces at high ionic strengths.28 At the IEP of the BSA, as presented in Table S2−S3, the PVDF membrane was negatively charged while the BSA was slightly negatively charged. Therefore, the PVDF membrane and the BSA were surrounded by Na+ cations, which are very hydrated ions.29 When the ionic strengths increased from 0 to 1 mM, the electrostatic repulsive forces of PVDF−BSA and BSA−BSA decreased, while the hydration repulsion forces between PVDF and BSA, and between BSA and BSA were weak; they were masked by changes in the electrostatic force. These resulted in the PVDF−BSA and BSA−BSA adhesion forces increasing with increasing ionic strengths, and the deposition rate and extent of BSA onto the PVDF membrane surface were increased, and the corresponding membrane fouling was enhanced (Figure S2). D

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Figure 4. Representative PVDF−BSA and BSA−BSA interaction force curves at LiCl, NaCl, KCl (ionic strength 100 mM) and ionic absence conditions at pH 7.0.

Effect of Hydrated Ion Species on Hydration Forces during UF Process. For a better understanding of the effect of hydrated ion species on hydration forces during the UF process, the PVDF−BSA and BSA−BSA interaction forces and membrane fouling with the PVDF UF membrane at different hydrated ions (ionic strength 100 mM) were investigated. In this section, typical monovalent hydrated ions, i.e., LiCl, NaCl, and KCl, were used. To avoid more complicated interactions, e.g., bridging and complexation, the multivalent ions (like Ca2+ and Mg2+) were not taken into account. PVDF−BSA and BSA−BSA Interaction Forces at Different Hydrated Ion Species. Figure 4 shows the representative adhesion force curves for PVDF−BSA and BSA−BSA at LiCl, NaCl, KCl (ionic strength 100 mM), and ionic absence, respectively. Corresponding to the Li+, Na+, K+, and ionic absence conditions, the average adhesion forces between PVDF and BSA were 1.4 ± 0.09, 1.6 ± 0.10, 1.9 ± 0.12, and 3.1 ± 0.18 nN, while the interaction forces for BSA−BSA were 0.36 ± 0.02, 0.43 ± 0.04, 0.64 ± 0.04, and 0.76 ± 0.06 nN, respectively. It is clear that for Li+, Na+, and K+, the PVDF− BSA or BSA−BSA interaction forces were weaker at ionic strength 100 mM than at ionic strength 0 mM, which indicated that the hydration repulsion forces between the PVDF membrane and the BSA or between BSA and BSA have become dominant at an ionic strength for LiCl or NaCl or KCl of 100 mM. Comparing the interaction forces at three different ion species, it is interesting to find that the presence of Li+, Na+, and K+ leads to obvious differences in interaction forces between PVDF and BSA or between BSA and BSA. These differences may be caused by the differences in the balance of electrostatic repulsion forces and hydration repulsion forces.17 When the ionic strength of LiCl, NaCl, and KCl is at 100 mM, as shown in Table S4, the charges of the PVDF membrane and the BSA were comparable, which indicated that similar levels of electrostatic repulsion forces would be present on PVDF−BSA or BSA−BSA.26 Thus, it is reasonable to conclude that the difference in the PVDF−BSA or BSA−BSA adhesion forces shown in Figure 4 is attributable to the hydration repulsion forces. As shown in Figure 4, under the three ionic species, both PVDF−BSA interaction forces and BSA−BSA interaction forces increased in the following order: Li+ < Na+ < K+. Considering the fact that this difference was attributed to the hydration repulsion forces at corresponding ion conditions, it is interesting to find that the smaller the radius of the hydrated ion is, the stronger the hydration repulsion force is; that is, the hydration repulsion forces decrease with increasing radii of the hydrated ions. This is because Li+ is a small ion and possesses a

only observed when the ionic strength reached 100 mM. This observation is similar to the findings of Jones’s group, who investigated the BSA fouling behaviors of UF membranes as a function of ionic strengths at differing pH conditions. They found that around the IEP of BSA, the membrane fouling rate and fouling extent decreased significantly with increasing ionic strength. In contrast, with increasing pH (BSA more negatively charged), the mitigation phenomenon of membrane fouling seems to disappear; membrane fouling is even enhanced with increasing ionic strength.31,32 This implies that it was more difficult to observe hydration repulsion forces when the BSA was more negatively charged. This phenomenon may be explained by the differences in the extent to which electrostatic repulsion forces decrease at pH 9.0 and 4.7. As shown in Table S2, for the same range of variation of ionic strength, the decrease in the net charge of the BSA at pH 9 was significantly greater than that at pH 4.7. Because the electrostatic force is proportional to the net charges of PVDF and BSA, the decrease in the PVDF−BSA and BSA−BSA electrostatic repulsion force at pH 9.0 is clearly greater than that at pH 4.7. Therefore, compared with pH 4.7, much stronger hydration repulsion forces are required to shield the changes in the electrostatic repulsion force at pH 9.0. Moreover, comparing corresponding adhesion forces as a function of ionic strengths at pH 3, 4.7, and 9.0, it is easy to observe that the BSA charge is one of the key factors affecting hydration repulsion forces. Briefly, for the pH above or around the IEP, the BSA was negatively charged or neutral, while the membrane was negatively charged. The hydrated Na+ ions would accumulate in the vicinity of the BSA and membrane surface via chargeshielding, contributing to the formation of a structured water layer around the protein and membrane surface. The overlap of such water layers leads to the hydration repulsion forces between the membrane and the BSA or between BSA and BSA. These hydration repulsion forces increased with increasing ionic strength, shielding the changes in the electrostatic repulsion force, which caused a decrease in membrane−BSA and BSA−BSA interaction forces, accompanied by the mitigation of membrane fouling. In contrast, for the pH below the IEP, the BSA was positively charged and the counterions near the BSA surface were the Cl− anions, which were practically not hydrated. Therefore, it is difficult to form a structured water layer around the BSA surface, and the hydration forces between the membrane and the BSA and between BSA and BSA do not appear, resulting in membrane fouling behavior in agreement with the classical DLVO theory; that is, membrane fouling was enhanced with increasing ionic strength. E

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Figure 5. Normalized flux versus filtration time for EVOH membranes (a) and representative EVOH−BSA interaction force curves (b) at different ionic strengths, pH 7.0.

result in a weaker hydration repulsion force and more serious membrane fouling. These results indicate that the hydration repulsion forces of PVDF−BSA and BSA−BSA were in close correlation with the size of the corresponding hydrated counterions surrounding the membrane and the BSA. Effect of Membrane Hydrophilicity on Hydration Forces during UF Process. To understand better the influence of UF membrane hydrophilicity on hydration forces, another relatively hydrophilic membrane, poly(ethylene-covinyl alcohol) (EVOH) UF membrane, was used. The EVOH membrane was also negatively charged, and its contact angle was about 40°, which is lower than that for the PVDF membrane (75°), as shown in Table S1. The BSA fouling experiments with EVOH UF membranes at ionic strengths 0, 1, 10, and 100 mM were systematically investigated (pH 7.0), and the interaction forces between the EVOH membrane and the BSA at corresponding ionic strengths were determined The results are presented in Figure 5. All the above experiments with hydrophobic PVDF membranes had also been carried out in our previous study. Figure 5(a) shows the normalized flux decline curves of EVOH membranes as a function of filtration time for BSA solutions at different ionic strengths. Over 120 min filtration stages, at ionic strengths 0, 1, 10, and 100 mM, the flux of BSAfouled membranes declined by 62%, 66%, 73%, and 86%, respectively. It was obvious that for the EVOH membrane, the flux decline rate and extent were increased with increasing ionic strengths. However, for the relatively hydrophobic PVDF membrane, at the same pH value and ionic strength range, the flux decline rate and extent were decreased significantly when the ionic strength exceeded 1 mM.16 To improve our understanding of the above differences, the interaction forces between the EVOH membrane and the BSA over a range of ionic strengths were measured, as presented in Figure 5(b). At ionic strengths 0, 1, 10, and 100 mM, the average adhesion forces between the EVOH membrane and the BSA were 3.1 ± 0.16, 4.9 ± 0.29, 6.4 ± 0.35, and 7.6 ± 0.31 nN, respectively. This was completely consistent with the variation characteristics of the membrane fouling rates described earlier. The EVOH−BSA interaction forces increased with increasing ionic strengths, and it seems that the hydration repulsion phenomenon was not observed. In contrast, for the PVDF membrane, when ionic strength reached 10 mM, the hydration forces between the PVDF membrane and the BSA increased sharply and shielded the electrostatic force, which resulted in a decrease in the PVDF−BSA adhesion forces, accompanied by a decrease in membrane fouling. From the above comparison, it is clear that at the same ionic strengths, and comparing this with the hydrophobic PVDF

higher surface charge density and can retain its hydration shells very effectively, to produce relatively higher hydration repulsion forces. Conversely, K+, the largest ion in comparison with the Li+ and Na+ ions, possesses the lowest surface charge density and holds the water molecules in its hydration shells less effectively, thus producing relatively weaker hydration effects.33−35 This observation is consistent with the findings of Donose and Bogdan’s group, who investigated the forces between silica surfaces in monovalent ion aqueous solutions and found that corresponding interaction forces increased with increasing radius of the hydrated ions at high ionic strengths.36,37 Membrane Fouling Behavior at Different Hydrated Ion Species. The BSA fouling of PVDF UF membranes at ionic strength 100 mM (LiCl, NaCl, and KCl) and ionic absence were systematically investigated; the results are shown in Figure S3. At ionic strengths of 0 and 100 mM for LiCl, NaCl, and KCl, the flux decline rate of BSA-fouled membranes was 76%, 30%, 37%, and 48% within 120 min of filtration time. As shown in Figure S3, at ionic strength 0 mM, the membrane fouling rate and extent were obviously more serious than for Li+ or Na+ or K+ ionic conditions. This phenomenon was similar to that for the PVDF−BSA or BSA−BSA adhesion forces at corresponding ionic conditions. This is mainly because the PVDF−BSA or BSA−BSA adhesion forces were weakened by hydration repulsion forces for the Li+ or Na+ or K+ ionic conditions (as mentioned earlier), which decreased the adsorption and accumulation of BSA on the surface of the PVDF membrane. A more nonrigid and open structure BSA layer was formed on the membrane surface,16 and the corresponding membrane fouling was thus decreased. Moreover, comparing the flux decline behavior at Li+, Na+, and K+ conditions, it is easy to find that in the whole filtration process, the BSA with K+ ion was found to have the most significant fouling potential and membrane flux decline, while the flux decline rate and extent were the least for the Li+ ionic condition. This phenomenon could be explained by the interaction forces of PVDF−BSA and BSA−BSA at corresponding ionic conditions, as shown in Figure 4. Compared with K+ and Na+ ions, for the Li+ ionic condition, the PVDF− BSA and BSA−BSA interaction forces were the weakest, resulting in decreased membrane fouling rate and extent. By contrast, the strongest PVDF−BSA or BSA−BSA interaction forces occurred for the K+ condition, accompanied by the most serious membrane fouling. From what has been discussed above, it seems that the smaller the radii of hydrated ions were, the stronger the hydration repulsion forces produced are, and the membrane fouling that occurs is less. Conversely, a larger hydrated ion will F

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(Grant No. QN1604), 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).

membrane, it is more difficult to observe hydration repulsion forces between the hydrophilic EVOH membrane and the BSA. This may be because the hydrophilic EVOH membrane possesses a large number of hydrophilic hydroxyl groups; thus, the water molecules would rather be adsorbed onto the membrane surface.38,39 This eventually weakens the binding efficiency between hydrated Na+ ions and the EVOH membrane surface, making it more difficult to form a structured water layer around the membrane surface. Therefore, even at an ionic strength of 100 mM, the hydration repulsion forces between EVOH and BSA were not observed. On the basis of the above discussion, it is clear that the greater the binding efficiency of hydrated ions to the membrane surface is, the stronger the hydration repulsion force will be between the membrane and the BSA. These results imply that the hydration repulsion force is also strongly dependent on the properties of membrane. For treating wastewater with higher salinity, membranes should be selected that possess a higher combining capacity with hydrated Na+ ion. This may be beneficial for enhancing the hydration repulsion force between the membrane and the foulant, thus reducing membrane fouling.

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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.6b03660. Figure S1: Scanning electron microscope image of a BSA colloidal probe (3 μm). Figure S2: Normalized flux versus filtration time for BSA solutions as a function of ionic strengths at pH 3.0 (a), pH 4.7 (b), and pH 9.0 (c). Figure S3: Normalized flux versus filtration time for BSA solutions at different hydrated cations. Table S1: The characteristics of PVDF and EVOH flat sheet UF membranes. Table S2: Zeta potentials of BSA as a function of ionic strength at different pH levels. Table S3: Zeta potentials of PVDF membrane as a function of ionic strength at different pH levels. Table S4: Zeta potentials of BSA and PVDF membranes at different hydrated cations. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 029 8220 2729. Fax: +86 029 8220 2729. E-mail: [email protected]. ORCID

Lei Wang: 0000-0002-7584-8802 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 (Grants No. 51278408, No. 51608429), the China Postdoctoral Science Foundation (Grants No. 2015M580820, No. 2016T90895), the Natural Science Foundation of Shaanxi Province (Grant No. 2016JQ5067), the Educational Commission of Shaanxi Province of China (Grant No. 16JS062), the Young Talent Fund of the University Association for Science and Technology in Shaanxi’ China (Grant No. 20160220), the Youth Foundation of Xi’an University of Architecture and Technology G

DOI: 10.1021/acs.est.6b03660 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.est.6b03660 Environ. Sci. Technol. XXXX, XXX, XXX−XXX