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Effect of BSA and sodium alginate adsorption on decline of filtrate flux through polyethylene microfiltration membranes
Journal of Membrane Science 594 (2020) 117469
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Effect of BSA and sodium alginate adsorption on decline of filtrate flux through polyethylene microfiltration membranes
T
Kazuki Akamatsua,∗, You Kagamia, Shin-ichi Nakaoa,b a
Department of Environmental Chemistry and Chemical Engineering, School of Advanced Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo, 192-0015, Japan b Research Institute for Science and Technology, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo, 192-0015, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Microfiltration membrane Adsorption Filtrate flux BSA Sodium alginate
To demonstrate the impact of the interaction between fouling substances and membrane materials on the decline of filtrate flux quantitatively, we developed a novel experimental system consisting of (1) closed-loop cross-flow filtration tests to demonstrate the relationship between the concentration of fouling substances in the feed solution and the steady-state flux, which is a traditional evaluation approach, and (2) pure water flux tests with membranes that are immersed in solutions of the same fouling substances prior to the tests to demonstrate the effect of their adsorption on the membrane performance. Bovine serum albumin and sodium alginate were used as model substances for proteins and polysaccharides, respectively, and polyethylene microfiltration membranes were used. Interestingly, the fluxes in both experiments were comparable to each other when the concentration of the fouling substances and the initial flux were equal. Because the only factor to increase the filtration resistance in the second experiments was adsorption, which was not affected by the filtration conditions, the results directly proved that the adsorption of these substances on the membrane surfaces and pore walls was dominant in the reduction of filtrate fluxes, even though the extent of the reduction was not severer in the case of sodium alginate.
1. Introduction One of the biggest challenges for wastewater treatment, water purification, and sea water desalination using membranes, is fouling, which drastically reduces membrane performance. Fouling phenomena are very complex because they are affected by numerous factors, including membrane material, pore structure, and surface roughness [1–3], and also because there are a wide variety of fouling potential materials [4,5]. It is therefore quite challenging to quantitatively and systematically understand fouling phenomena. Proteins and polysaccharides are often highlighted as major fouling substances. In membrane bioreactors (MBRs) for wastewater treatment, for instance, extracellular polymeric substances (EPS) are regarded as one of the dominant substances contributing to fouling [6,7], and of course EPS contain a number of proteins and polysaccharides. However, owing to the complexity of the reactions in MBRs, controlled by various kinds of microorganisms, it is difficult or almost impossible to identify each protein and polysaccharide that potentially affects fouling. Furthermore, fouling depends on the operation conditions and temperature [8,9] because these parameters affect the microbial reactions. It is
∗
important to evaluate fouling behavior in practical operation even under conditions where the fouling materials are not identified, however it is also important to investigate fouling behavior using model substances as fouling materials. Many researchers use bovine serum albumin (BSA) as a model for proteins [10–12], and sodium alginate as a model for polysaccharides [13–15]. And it has been generally postulated that nonspecific adsorption of proteins onto the membrane surface or inside of membrane pores (i.e. the interaction between the fouling materials and the membrane materials, which is not affected by filtration conditions) is an origin of fouling that triggers the subsequent evolution of severe fouling [16,17], even though other types of fouling, such as pore blocking, pore clogging, gel layer formation, would be occurred. Under this assumption, a lot of researchers have carried out the adsorption tests using proteins to evaluate the adsorption amounts and used these values as an index for fouling propensity [18–23]. Various analytical techniques such as the quartz crystal microbalance with dissipation (QCM-D) [24,25], the atomic force microscope (AFM) analysis [25,26], the electrical impedance spectroscopy (EIS) [27,28], the ultrasonic frequency-domain reflectometry [29], as well as the conventional adsorption test, have also been employed to discuss
Corresponding author. E-mail address: [email protected] (K. Akamatsu).
https://doi.org/10.1016/j.memsci.2019.117469 Received 27 May 2019; Received in revised form 4 September 2019; Accepted 12 September 2019 Available online 12 September 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 594 (2020) 117469
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fouling propensity. This assumption has also been made in the development of biomaterials [30,31], which often require anti-thrombogenic properties. Some researchers have reported that the adsorption of polysaccharides can be the origin of subsequent severe fouling [32,33]. However, previous studies generally showed just two things: (i) if a target membrane has an adsorption propensity against a target fouling substance or not, and (ii) if the target membrane are fouled in the filtration test using the target fouling substance or not. And they just imagined that the adsorption of proteins contributes dominantly to the reduction of membrane performance, in particular filtrate fluxes. There are few reports that tried providing a direct evidence that the adsorption of proteins is the dominant factor to the reduction of filtrate fluxes in microfiltration. And for polysaccharides it is currently not clear whether or not adsorption is a main contributing factor in membrane performance. In this study, we demonstrate a new experimental system that enables us to clearly discuss the extent of the contribution of adsorption of organic substances to decreases of filtrate flux in cross-flow microfiltrations. The experimental system is quite simple and consists of (1) closed-loop cross-flow filtration tests to clarify the relationship between the concentration of fouling substances in the feed solution and the steady-state flux and (2) pure water flux tests with membranes that are immersed in solutions of the same fouling substances prior to the tests to clarify the effect of adsorption on the membrane performance. The latter experiments enable us to correlate quantitatively the adsorption with the filtration resistances measured by the pure water flux in response to the applied pressure, which only focuses on the contribution of the adsorption to the decline of filtrate flux. Furthermore, comparison of the flux profiles in the closed-loop cross-flow filtration tests to demonstrate the relationship between the concentration of fouling substances in the feed solution and the steady-state flux with those obtained in the pure water flux tests enables us to prove directly how much the contribution of the adsorption to the reduction of filtrate flux is during practical cross-flow microfiltrations. We use BSA as a model for protein and sodium alginate as a model for polysaccharide. We provide clear evidence of the extent of the contributions of BSA and sodium alginate adsorption to filtrate flux decreases in cross-flow microfiltration using polyethylene membranes.
Fig. 1. Conceptual illustration of the experimental setup.
Fig. 2. Conceptual illustration of the time course of flux for (a) the closed-loop cross-flow filtration test and (b) the pure water flux test.
2. Experimental
that correspond to the increase of the filtration resistance caused by the filtration test. The retentate and permeate were fully recycled to keep the concentration of the feed constant during the tests. The flow rate was 2.0 L min−1, and the temperature was 25 °C.
2.1. Materials BSA and sodium alginate were purchased from Wako Pure Chemical Industries Ltd., Japan. Porous polyethylene microfiltration membranes (Asahi Kasei Corp., Japan, pore size 0.06 μm, thickness 30 μm, porosity 41%) were used.
2.3. Pure water flux tests with membranes that were immersed in solutions containing BSA and sodium alginate in advance
2.2. Closed-loop cross-flow filtration tests using BSA and sodium alginate
The membranes were immersed in solutions containing a known concentration of BSA or sodium alginate in a temperature-controlled shaking bath at 25 °C for 5 h (BSA) or 20 h (sodium alginate) to achieve sufficient adsorption. Pure water flux tests were then carried out with the immersed membranes in cross-flow mode using the set-up shown in Fig. 1. To enable comparison of the results of the cross-flow microfiltration tests, the applied pressure was first set at ΔP1, which would give a pure water flux of 4.0 × 10−6 m3 m−2 s−1 with a pristine membrane. After observing the steady state under these conditions, the applied pressure was increased to ΔP2, which would give a pure water flux of 6.0 × 10−6 m3 m−2 s−1 with a pristine membrane. Next, the pressure was set at ΔP3, which would result in a flux of 1.0 × 10−5 m3 m−2 s−1 with a pristine membrane. In this experiment, it is expected that we can obtain the time course of flux as shown in Fig. 2(b) for each concentration and that we can discuss the flux decline from the initial value that correspond to the increase of the filtration resistance only caused by the adsorption. The flow rate was 2.0 L min−1, and the temperature was 25 °C, which are the same as for
A conceptual illustration of the experimental setup is shown in Fig. 1. The concentrations of BSA and sodium alginate in the feed solution ranged from 10 to 5000 ppm and 50 to 1000 ppm, respectively. First, the applied pressure (ΔP1) was tuned to achieve a pure water flux of 4.0 × 10−6 m3 m−2 s−1 for 30 min, then the feed was changed from pure water to the solution containing BSA or sodium alginate with a known concentration. After observing the steady-state flux under these conditions, the applied pressure was increased to 1.5 times the initial pressure (ΔP2 = 1.5 ΔP1), which would achieve a flux of 6.0 × 10−6 m3 m−2 s−1 if the membrane was not fouled. When the steady-state flux had been observed under these conditions, the applied pressure was increased to 2.5 times the initial pressure (ΔP3 = 2.5 ΔP1), which would achieve a flux of 1.0 × 10−5 m3 m−2 s−1 if the membrane was not fouled [34–36]. In this experiment, it is expected that we can obtain the time course of flux as shown in Fig. 2(a) for each concentration and that we can discuss the flux decline from the initial value 2
Journal of Membrane Science 594 (2020) 117469
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Fig. 4. Pure water flux over time for membranes that had been immersed in solutions with different BSA concentrations. The flow rate was 2.0 L min−1, and the temperature was 25 °C. (△10 ppm, ▲50 ppm, □300 ppm, ■500 ppm, ○1000 ppm, ●5000 ppm).
Fig. 3. Flux of different BSA feed solution concentrations over time. The flow rate was 2.0 L min−1, and the temperature was 25 °C. (△10 ppm, ▲50 ppm, □300 ppm, ■500 ppm, ○1000 ppm, ●5000 ppm).
the closed-loop cross-flow filtration tests in Section 2.2.
detachment of BSA from the membrane occurred. When the applied pressure was increased to 1.5 times the initial pressure (at 80 min in this case), the pure water flux became 1.7 × 10−6 m3 m−2 s−1, which is ~1.5 times that at the initial pressure, and remained constant at this pressure over time. This is because the amount of adsorbed BSA did not change even when the applied pressure was changed. Thus, the filtration resistance remained unchanged, and accordingly the pure water flux was proportional to the applied pressure. Next, when the applied pressure was increased to 2.5 times the initial pressure (at 150 min in this case), the pure water flux increased to 3.0 × 10−6 m3 m−2 s−1, which is ~2.5 times as large as that measured at the initial pressure, and the flux remained constant over time for the same reason as described for the 1.5 times increase case. In addition, the pure water flux under the conditions that would give 4.0 × 10−6 m3 m−2 s−1 for a pristine membrane, became lower as the BSA concentration used for immersion increased. This trend was also true in the cases where the pure water flux would be 6.0 × 10−6 m3 m−2 s−1 and 1.0 × 10−5 m3 m−2 s−1 if a pristine membrane was used. These results indicate that the decrease of the flux due to adsorption became more severe with increasing BSA concentration used for immersion. To evaluate the extent of the contribution of BSA adsorption to the reduction of membrane performance, we compared the results of the two tests. Fig. 5 shows the relationships between the concentration of BSA used for both tests and the steady-state flux under the conditions where the flux would be (a) 4.0 × 10−6 m3 m−2 s−1, (b) 6.0 × 10−6 m3 m−2 s−1, and (c) 1.0 × 10−5 m3 m−2 s−1 if the membrane was not fouled. As discussed above, using the case of 1000 ppm as an example, the steady-state flux in the closed-loop cross-flow filtration test and the pure water flux using the immersed membrane were 1.1 × 10−6 and 1.2 × 10−6 m3 m−2 s−1 for (a), 1.6 × 10−6 and 1.7 × 10−6 m3 m−2 s−1 for (b), and 2.5 × 10−6 and 3.0 × 10−6 m3 m−2 s−1 for (c), respectively. In each case the two values are almost the same. The comparisons at the other BSA concentrations show similar results, and the steady-state flux in the closedloop cross-flow filtration test and the pure water flux using the immersed membrane overlapped, in particular for concentrations higher than 500 ppm. Because the pure water flux tests focus on the contribution of adsorption only, this result indicates that the dominant, and almost only, factor contributing to the reduction of filtrate flux in crossflow microfiltration, was the physical adsorption of BSA, particularly in the higher concentration range. In addition, we note that the steadystate flux decreased significantly with increasing BSA concentration in
3. Results and discussion 3.1. BSA fouling Fig. 3 shows the flux over time for feed solutions with different BSA concentrations. When the concentration was 1000 ppm, a significant decrease of flux was observed just after the addition of BSA, and the steady-state flux was 1.1 × 10−6 m3 m−2 s−1. Under these conditions, the flux would be 4.0 × 10−6 m3 m−2 s−1 if the membrane was not fouled. This result shows that the membrane was severely fouled by BSA under these conditions. The applied pressure was then increased to 1.5 times the initial pressure (at 150 min in this case), and accordingly the flux increased to 1.6 × 10−6 m3 m−2 s−1, which is approximately 1.5 times the steady-state flux under the previous pressure conditions. The flux then remained stable, indicating that the steady-state flux was 1.6 × 10−6 m3 m−2 s−1. When the applied pressure was increased to 2.5 times the initial pressure (at 240 min in this case), which would achieve a flux of 1.0 × 10−5 m3 m−2 s−1 for a membrane that showed no fouling, the flux increased to 2.5 × 10−6 m3 m−2 s−1. This is ~2.5 times greater than the steady-state flux under the initial pressure conditions. The flux then remained stable, and the steady-state flux was measured to be 2.5 × 10−6 m3 m−2 s−1 under these conditions. Owing to the severe fouling, the fluxes were much lower than those expected in the absence of fouling. We should also note that the steady-state flux at each pressure decreased with increasing BSA concentration. This indicates that BSA fouling became more severe as concentration increased. However, the steady-state flux for 1000 ppm and 5000 ppm did not differ greatly even though the concentration differed five-fold, indicating that the effect of BSA concentration on filtrate flux was not as pronounced in this concentration range. Fig. 4 shows the flux of pure water through membranes that had been immersed in solutions with different BSA concentrations over time. The membranes exhibited fouling from the beginning of these experiments owing to the adsorption of BSA, thus the pure water flux was expected to be lower than that for a pristine membrane. When the membrane was immersed in 1000 ppm BSA solution, the pure water flux was 1.2 × 10−6 m3 m−2 s−1 under the initial conditions, where the flux would be 4.0 × 10−6 m3 m−2 s−1 if a pristine membrane was used. The flux remained constant throughout the period where pressure was applied, indicating that the permeation resistance caused by BSA adsorption did not change during this period. In other words, no 3
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Fig. 5. Relationships between the concentration of BSA in the closed-loop crossflow microfiltration test (closed symbols) and the pure water test (open symbols) and the steady-state flux under conditions where the flux would be (a) 4.0 × 10−6 m3 m−2 s−1, (b) 6.0 × 10−6 m3 m−2 s−1, and (c) −5 3 −2 −1 1.0 × 10 m m s , shown as the dashed lines, if the membrane was not fouled.
Fig. 7. Flux over time for feed solutions with different sodium alginate concentrations. The flow rate was 2.0 L min−1, and the temperature was 25 °C. (△50 ppm, ▲100 ppm, □500 ppm, ■1000 ppm).
Based on the results discussed, we can conclude that the adsorption of BSA on the membrane surfaces and pore walls is the dominant factor in filtrate flux decrease. There are many reports describing the development of membranes that can suppress fouling as a result of materials properties designed to hinder protein adsorption [37–42], and our findings demonstrate the validity of this approach. 3.2. Sodium alginate fouling Fig. 7 shows the flux over time in response to the concentration of sodium alginate in the feed solution. Fouling did occur, however it is clear that decreases in flux as a result of sodium alginate fouling were not as severe as those caused by BSA fouling shown in Fig. 3. For example, when the concentration of sodium alginate was 1000 ppm, a decrease in flux was observed just after the addition of sodium alginate, and the steady-state flux was 2.4 × 10−6 m3 m−2 s−1. This flux was more than twice as large as that in the case of BSA with the same concentration. The applied pressure was then increased to 1.5 times the initial pressure (at 170 min in this case), and the flux accordingly increased and stabilized at 3.6 × 10−6 m3 m−2 s−1. This was 1.5 times higher than under the initial conditions. When the applied pressure was increased to 2.5 times the initial pressure (at 290 min in this case), the flux increased and became stable at 6.0 × 10−6 m3 m−2 s−1, which was 2.5 times higher than at the initial pressure. In addition, similar to the case for BSA, the steady-state flux at each stage became lower with increasing sodium alginate concentration. This indicates that fouling by sodium alginate also became more severe with increasing concentration in this range. Fig. 8 shows the pure water flux over time for membranes that had been immersed in solutions with different concentrations of sodium alginate. When the immersion concentration was 1000 ppm, the pure water flux was 2.0 × 10−6 m3 m−2 s−1 under the initial pressure conditions, where the flux would be 4.0 × 10−6 m3 m−2 s−1 for a pristine membrane. The flux did not change under these conditions, which means that the permeation resistance caused by the adsorption of sodium alginate did not change during this period. When the applied pressure was increased to 1.5 times the initial pressure (at 110 min in this case), the constant flux was 3.1 × 10−6 m3 m−2 s−1, which is ~1.5 times as large as the initial flux. This is the same as was observed for BSA. Next, the pure water flux became 5.4 × 10−6 m3 m−2 s−1 when the applied pressure was increased to 2.5 times the initial pressure (at 220 min in this case). The flux also remained constant over time, and the value was ~2.5 times the initial value. Furthermore, the pure water
Fig. 6. Interpretation of the results shown in Fig. 5.
the lower concentration range and was almost independent of the concentration in the higher concentration range in both tests. This trend can be easily interpreted by Langmuir-adsorption as shown in Fig. 6; the adsorbed amount increases almost linearly with concentration and then deviates from the linear relationship and is finally constant. The adsorbed amount is closely related to the permeation resistance. In this assumption, the flux should decrease almost linearly with the concentration of BSA and then become constant at the higher BSA concentration. Before drawing a conclusion, we have to consider a case where the steady-state flux in the closed-loop cross-flow filtration test would be smaller than the pure water flux using the immersed membrane, even though such a phenomenon was not observed in this study. In such a case, two possibilities should be considered. One is the existence of other fouling mechanism. As described in the introduction part, other types of fouling such as pore blocking, pore clogging, gel layer formation would be occurred independently or simultaneously. The other is the effect of the flow rate. When the flow rate had been much smaller than 2.0 L min−1 in this study, the steady-state flux in the closed-loop cross-flow filtration test would become smaller, which would result in smaller values than that in the pure water flux test. This is because the filtration resistance caused by the increase of the deposited fouling substances would be generated in the closed-loop cross-flow filtration test, while the pure water flux with immersed membrane would not be affected by the flow rate, in principle. Of course, it is suitable to eliminate such effects because we would like to focus on the effect of adsorption. In the series of the experiments, we did not obtain such data, so we don't care these possibilities. 4
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BSA. The findings discussed have clearly proved that the adsorption of BSA and sodium alginate on the membrane surfaces and pore walls is the dominant factor in the reduction of the filtrate flux in the microfiltration. This adsorption may be the origin of the subsequent evolution of severe fouling when processing actual wastewater, although this was not tested in this study. However, the new experimental systems we have tested in this study have allowed us to confidently conclude that the strategy of developing membranes that suppress the adsorption of proteins and polysaccharides is important. 4. Conclusion This study aimed to clearly demonstrate the extent of the contribution of fouling substance adsorption to the reduction of membrane performance—in particular filtrate fluxes—using polyethylene microfiltration membranes with BSA as a model substance for proteins and sodium alginate as a model substance for polysaccharides. We carried out (1) closed-loop cross-flow filtration tests by changing the concentration of fouling substances in the feed solution and (2) pure water flux tests with membranes that were immersed in solutions of the same fouling substances prior to the tests. The aim of the former experiments was to demonstrate the relationships between the concentration of the fouling substances and the steady-state flux, and that of the latter was to demonstrate the effect of the adsorption of the fouling substances on the membrane performance. Thus, the comparison of these results clearly proves the contribution of adsorption to the reduction of filtrate flux. Throughout the discussion of the experimental results, we were able to conclude that adsorption on the surface of the membranes and pore walls was a dominant factor in membrane fouling in both cases, even though the impact of the adsorption on the reduction of the filtrate flux was dependent on the fouling substances. The experiments shown in this study are relatively simple, however the experimental system demonstrated in this study is a powerful tool for both understanding the fouling mechanism and developing low-fouling membranes.
Fig. 8. Pure water flux over time for membranes that had been immersed in solutions with different sodium alginate concentrations. The flow rate was 2.0 L min−1, and the temperature was 25 °C. (△50 ppm, ▲100 ppm, □500 ppm, ■1000 ppm).
Acknowledgments Microfiltration membranes were kindly supplied by Asahi Kasei Corp., Japan. A part of this research was supported by a Grant-in-Aid for Scientific Research (B) (No. 18H01770) from Japan Society for the Promotion of Science (JSPS).
Fig. 9. Relationships between the concentration of sodium alginate in the closed-loop cross-flow microfiltration test (closed symbols) and in the pure water test (open symbols) and the steady-state flux under the conditions where flux would be (a) 4.0 × 10−6 m3 m−2 s−1, (b) 6.0 × 10−6 m3 m−2 s−1, and (c) 1.0 × 10−5 m3 m−2 s−1, shown as the dashed lines, if the membrane was not fouled.
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flux under the conditions that would give 4.0 × 10−6 m3 m−2 s−1 for a pristine membrane, became lower with increasing concentration of sodium alginate used for the immersion. This trend was also observed for the cases where the pure water flux would be 6.0 × 10−6 m3 m−2 s−1 and 1.0 × 10−5 m3 m−2 s−1 if a pristine membrane was used. These observations are similar to those for BSA. Fig. 9 shows the relationships between the concentration of sodium alginate used for both tests and the steady-state flux under the conditions where flux would be (a) 4.0 × 10−6 m3 m−2 s−1, (b) 6.0 × 10−6 m3 m−2 s−1, and (c) 1.0 × 10−5 m3 m−2 s−1 if the membrane was not fouled; in order to clearly demonstrate the extent of the contribution of sodium alginate adsorption to the decrease of filtrate flux. As was the case for BSA, both of the fluxes were comparable at every concentration condition tested in this study. This experimental finding shows that the dominant factor in the reduction of filtrate flux was the adsorption of sodium alginate on the membrane surface and pore walls, even though the impact was smaller than that in the case of 5
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Effect of BSA and sodium alginate adsorption on decline of filtrate flux through polyethylene microfiltration membranes
T
Kazuki Akamatsua,∗, You Kagamia, Shin-ichi Nakaoa,b a
Department of Environmental Chemistry and Chemical Engineering, School of Advanced Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo, 192-0015, Japan b Research Institute for Science and Technology, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo, 192-0015, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Microfiltration membrane Adsorption Filtrate flux BSA Sodium alginate
To demonstrate the impact of the interaction between fouling substances and membrane materials on the decline of filtrate flux quantitatively, we developed a novel experimental system consisting of (1) closed-loop cross-flow filtration tests to demonstrate the relationship between the concentration of fouling substances in the feed solution and the steady-state flux, which is a traditional evaluation approach, and (2) pure water flux tests with membranes that are immersed in solutions of the same fouling substances prior to the tests to demonstrate the effect of their adsorption on the membrane performance. Bovine serum albumin and sodium alginate were used as model substances for proteins and polysaccharides, respectively, and polyethylene microfiltration membranes were used. Interestingly, the fluxes in both experiments were comparable to each other when the concentration of the fouling substances and the initial flux were equal. Because the only factor to increase the filtration resistance in the second experiments was adsorption, which was not affected by the filtration conditions, the results directly proved that the adsorption of these substances on the membrane surfaces and pore walls was dominant in the reduction of filtrate fluxes, even though the extent of the reduction was not severer in the case of sodium alginate.
1. Introduction One of the biggest challenges for wastewater treatment, water purification, and sea water desalination using membranes, is fouling, which drastically reduces membrane performance. Fouling phenomena are very complex because they are affected by numerous factors, including membrane material, pore structure, and surface roughness [1–3], and also because there are a wide variety of fouling potential materials [4,5]. It is therefore quite challenging to quantitatively and systematically understand fouling phenomena. Proteins and polysaccharides are often highlighted as major fouling substances. In membrane bioreactors (MBRs) for wastewater treatment, for instance, extracellular polymeric substances (EPS) are regarded as one of the dominant substances contributing to fouling [6,7], and of course EPS contain a number of proteins and polysaccharides. However, owing to the complexity of the reactions in MBRs, controlled by various kinds of microorganisms, it is difficult or almost impossible to identify each protein and polysaccharide that potentially affects fouling. Furthermore, fouling depends on the operation conditions and temperature [8,9] because these parameters affect the microbial reactions. It is
∗
important to evaluate fouling behavior in practical operation even under conditions where the fouling materials are not identified, however it is also important to investigate fouling behavior using model substances as fouling materials. Many researchers use bovine serum albumin (BSA) as a model for proteins [10–12], and sodium alginate as a model for polysaccharides [13–15]. And it has been generally postulated that nonspecific adsorption of proteins onto the membrane surface or inside of membrane pores (i.e. the interaction between the fouling materials and the membrane materials, which is not affected by filtration conditions) is an origin of fouling that triggers the subsequent evolution of severe fouling [16,17], even though other types of fouling, such as pore blocking, pore clogging, gel layer formation, would be occurred. Under this assumption, a lot of researchers have carried out the adsorption tests using proteins to evaluate the adsorption amounts and used these values as an index for fouling propensity [18–23]. Various analytical techniques such as the quartz crystal microbalance with dissipation (QCM-D) [24,25], the atomic force microscope (AFM) analysis [25,26], the electrical impedance spectroscopy (EIS) [27,28], the ultrasonic frequency-domain reflectometry [29], as well as the conventional adsorption test, have also been employed to discuss
Corresponding author. E-mail address: [email protected] (K. Akamatsu).
https://doi.org/10.1016/j.memsci.2019.117469 Received 27 May 2019; Received in revised form 4 September 2019; Accepted 12 September 2019 Available online 12 September 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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fouling propensity. This assumption has also been made in the development of biomaterials [30,31], which often require anti-thrombogenic properties. Some researchers have reported that the adsorption of polysaccharides can be the origin of subsequent severe fouling [32,33]. However, previous studies generally showed just two things: (i) if a target membrane has an adsorption propensity against a target fouling substance or not, and (ii) if the target membrane are fouled in the filtration test using the target fouling substance or not. And they just imagined that the adsorption of proteins contributes dominantly to the reduction of membrane performance, in particular filtrate fluxes. There are few reports that tried providing a direct evidence that the adsorption of proteins is the dominant factor to the reduction of filtrate fluxes in microfiltration. And for polysaccharides it is currently not clear whether or not adsorption is a main contributing factor in membrane performance. In this study, we demonstrate a new experimental system that enables us to clearly discuss the extent of the contribution of adsorption of organic substances to decreases of filtrate flux in cross-flow microfiltrations. The experimental system is quite simple and consists of (1) closed-loop cross-flow filtration tests to clarify the relationship between the concentration of fouling substances in the feed solution and the steady-state flux and (2) pure water flux tests with membranes that are immersed in solutions of the same fouling substances prior to the tests to clarify the effect of adsorption on the membrane performance. The latter experiments enable us to correlate quantitatively the adsorption with the filtration resistances measured by the pure water flux in response to the applied pressure, which only focuses on the contribution of the adsorption to the decline of filtrate flux. Furthermore, comparison of the flux profiles in the closed-loop cross-flow filtration tests to demonstrate the relationship between the concentration of fouling substances in the feed solution and the steady-state flux with those obtained in the pure water flux tests enables us to prove directly how much the contribution of the adsorption to the reduction of filtrate flux is during practical cross-flow microfiltrations. We use BSA as a model for protein and sodium alginate as a model for polysaccharide. We provide clear evidence of the extent of the contributions of BSA and sodium alginate adsorption to filtrate flux decreases in cross-flow microfiltration using polyethylene membranes.
Fig. 1. Conceptual illustration of the experimental setup.
Fig. 2. Conceptual illustration of the time course of flux for (a) the closed-loop cross-flow filtration test and (b) the pure water flux test.
2. Experimental
that correspond to the increase of the filtration resistance caused by the filtration test. The retentate and permeate were fully recycled to keep the concentration of the feed constant during the tests. The flow rate was 2.0 L min−1, and the temperature was 25 °C.
2.1. Materials BSA and sodium alginate were purchased from Wako Pure Chemical Industries Ltd., Japan. Porous polyethylene microfiltration membranes (Asahi Kasei Corp., Japan, pore size 0.06 μm, thickness 30 μm, porosity 41%) were used.
2.3. Pure water flux tests with membranes that were immersed in solutions containing BSA and sodium alginate in advance
2.2. Closed-loop cross-flow filtration tests using BSA and sodium alginate
The membranes were immersed in solutions containing a known concentration of BSA or sodium alginate in a temperature-controlled shaking bath at 25 °C for 5 h (BSA) or 20 h (sodium alginate) to achieve sufficient adsorption. Pure water flux tests were then carried out with the immersed membranes in cross-flow mode using the set-up shown in Fig. 1. To enable comparison of the results of the cross-flow microfiltration tests, the applied pressure was first set at ΔP1, which would give a pure water flux of 4.0 × 10−6 m3 m−2 s−1 with a pristine membrane. After observing the steady state under these conditions, the applied pressure was increased to ΔP2, which would give a pure water flux of 6.0 × 10−6 m3 m−2 s−1 with a pristine membrane. Next, the pressure was set at ΔP3, which would result in a flux of 1.0 × 10−5 m3 m−2 s−1 with a pristine membrane. In this experiment, it is expected that we can obtain the time course of flux as shown in Fig. 2(b) for each concentration and that we can discuss the flux decline from the initial value that correspond to the increase of the filtration resistance only caused by the adsorption. The flow rate was 2.0 L min−1, and the temperature was 25 °C, which are the same as for
A conceptual illustration of the experimental setup is shown in Fig. 1. The concentrations of BSA and sodium alginate in the feed solution ranged from 10 to 5000 ppm and 50 to 1000 ppm, respectively. First, the applied pressure (ΔP1) was tuned to achieve a pure water flux of 4.0 × 10−6 m3 m−2 s−1 for 30 min, then the feed was changed from pure water to the solution containing BSA or sodium alginate with a known concentration. After observing the steady-state flux under these conditions, the applied pressure was increased to 1.5 times the initial pressure (ΔP2 = 1.5 ΔP1), which would achieve a flux of 6.0 × 10−6 m3 m−2 s−1 if the membrane was not fouled. When the steady-state flux had been observed under these conditions, the applied pressure was increased to 2.5 times the initial pressure (ΔP3 = 2.5 ΔP1), which would achieve a flux of 1.0 × 10−5 m3 m−2 s−1 if the membrane was not fouled [34–36]. In this experiment, it is expected that we can obtain the time course of flux as shown in Fig. 2(a) for each concentration and that we can discuss the flux decline from the initial value 2
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Fig. 4. Pure water flux over time for membranes that had been immersed in solutions with different BSA concentrations. The flow rate was 2.0 L min−1, and the temperature was 25 °C. (△10 ppm, ▲50 ppm, □300 ppm, ■500 ppm, ○1000 ppm, ●5000 ppm).
Fig. 3. Flux of different BSA feed solution concentrations over time. The flow rate was 2.0 L min−1, and the temperature was 25 °C. (△10 ppm, ▲50 ppm, □300 ppm, ■500 ppm, ○1000 ppm, ●5000 ppm).
the closed-loop cross-flow filtration tests in Section 2.2.
detachment of BSA from the membrane occurred. When the applied pressure was increased to 1.5 times the initial pressure (at 80 min in this case), the pure water flux became 1.7 × 10−6 m3 m−2 s−1, which is ~1.5 times that at the initial pressure, and remained constant at this pressure over time. This is because the amount of adsorbed BSA did not change even when the applied pressure was changed. Thus, the filtration resistance remained unchanged, and accordingly the pure water flux was proportional to the applied pressure. Next, when the applied pressure was increased to 2.5 times the initial pressure (at 150 min in this case), the pure water flux increased to 3.0 × 10−6 m3 m−2 s−1, which is ~2.5 times as large as that measured at the initial pressure, and the flux remained constant over time for the same reason as described for the 1.5 times increase case. In addition, the pure water flux under the conditions that would give 4.0 × 10−6 m3 m−2 s−1 for a pristine membrane, became lower as the BSA concentration used for immersion increased. This trend was also true in the cases where the pure water flux would be 6.0 × 10−6 m3 m−2 s−1 and 1.0 × 10−5 m3 m−2 s−1 if a pristine membrane was used. These results indicate that the decrease of the flux due to adsorption became more severe with increasing BSA concentration used for immersion. To evaluate the extent of the contribution of BSA adsorption to the reduction of membrane performance, we compared the results of the two tests. Fig. 5 shows the relationships between the concentration of BSA used for both tests and the steady-state flux under the conditions where the flux would be (a) 4.0 × 10−6 m3 m−2 s−1, (b) 6.0 × 10−6 m3 m−2 s−1, and (c) 1.0 × 10−5 m3 m−2 s−1 if the membrane was not fouled. As discussed above, using the case of 1000 ppm as an example, the steady-state flux in the closed-loop cross-flow filtration test and the pure water flux using the immersed membrane were 1.1 × 10−6 and 1.2 × 10−6 m3 m−2 s−1 for (a), 1.6 × 10−6 and 1.7 × 10−6 m3 m−2 s−1 for (b), and 2.5 × 10−6 and 3.0 × 10−6 m3 m−2 s−1 for (c), respectively. In each case the two values are almost the same. The comparisons at the other BSA concentrations show similar results, and the steady-state flux in the closedloop cross-flow filtration test and the pure water flux using the immersed membrane overlapped, in particular for concentrations higher than 500 ppm. Because the pure water flux tests focus on the contribution of adsorption only, this result indicates that the dominant, and almost only, factor contributing to the reduction of filtrate flux in crossflow microfiltration, was the physical adsorption of BSA, particularly in the higher concentration range. In addition, we note that the steadystate flux decreased significantly with increasing BSA concentration in
3. Results and discussion 3.1. BSA fouling Fig. 3 shows the flux over time for feed solutions with different BSA concentrations. When the concentration was 1000 ppm, a significant decrease of flux was observed just after the addition of BSA, and the steady-state flux was 1.1 × 10−6 m3 m−2 s−1. Under these conditions, the flux would be 4.0 × 10−6 m3 m−2 s−1 if the membrane was not fouled. This result shows that the membrane was severely fouled by BSA under these conditions. The applied pressure was then increased to 1.5 times the initial pressure (at 150 min in this case), and accordingly the flux increased to 1.6 × 10−6 m3 m−2 s−1, which is approximately 1.5 times the steady-state flux under the previous pressure conditions. The flux then remained stable, indicating that the steady-state flux was 1.6 × 10−6 m3 m−2 s−1. When the applied pressure was increased to 2.5 times the initial pressure (at 240 min in this case), which would achieve a flux of 1.0 × 10−5 m3 m−2 s−1 for a membrane that showed no fouling, the flux increased to 2.5 × 10−6 m3 m−2 s−1. This is ~2.5 times greater than the steady-state flux under the initial pressure conditions. The flux then remained stable, and the steady-state flux was measured to be 2.5 × 10−6 m3 m−2 s−1 under these conditions. Owing to the severe fouling, the fluxes were much lower than those expected in the absence of fouling. We should also note that the steady-state flux at each pressure decreased with increasing BSA concentration. This indicates that BSA fouling became more severe as concentration increased. However, the steady-state flux for 1000 ppm and 5000 ppm did not differ greatly even though the concentration differed five-fold, indicating that the effect of BSA concentration on filtrate flux was not as pronounced in this concentration range. Fig. 4 shows the flux of pure water through membranes that had been immersed in solutions with different BSA concentrations over time. The membranes exhibited fouling from the beginning of these experiments owing to the adsorption of BSA, thus the pure water flux was expected to be lower than that for a pristine membrane. When the membrane was immersed in 1000 ppm BSA solution, the pure water flux was 1.2 × 10−6 m3 m−2 s−1 under the initial conditions, where the flux would be 4.0 × 10−6 m3 m−2 s−1 if a pristine membrane was used. The flux remained constant throughout the period where pressure was applied, indicating that the permeation resistance caused by BSA adsorption did not change during this period. In other words, no 3
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Fig. 5. Relationships between the concentration of BSA in the closed-loop crossflow microfiltration test (closed symbols) and the pure water test (open symbols) and the steady-state flux under conditions where the flux would be (a) 4.0 × 10−6 m3 m−2 s−1, (b) 6.0 × 10−6 m3 m−2 s−1, and (c) −5 3 −2 −1 1.0 × 10 m m s , shown as the dashed lines, if the membrane was not fouled.
Fig. 7. Flux over time for feed solutions with different sodium alginate concentrations. The flow rate was 2.0 L min−1, and the temperature was 25 °C. (△50 ppm, ▲100 ppm, □500 ppm, ■1000 ppm).
Based on the results discussed, we can conclude that the adsorption of BSA on the membrane surfaces and pore walls is the dominant factor in filtrate flux decrease. There are many reports describing the development of membranes that can suppress fouling as a result of materials properties designed to hinder protein adsorption [37–42], and our findings demonstrate the validity of this approach. 3.2. Sodium alginate fouling Fig. 7 shows the flux over time in response to the concentration of sodium alginate in the feed solution. Fouling did occur, however it is clear that decreases in flux as a result of sodium alginate fouling were not as severe as those caused by BSA fouling shown in Fig. 3. For example, when the concentration of sodium alginate was 1000 ppm, a decrease in flux was observed just after the addition of sodium alginate, and the steady-state flux was 2.4 × 10−6 m3 m−2 s−1. This flux was more than twice as large as that in the case of BSA with the same concentration. The applied pressure was then increased to 1.5 times the initial pressure (at 170 min in this case), and the flux accordingly increased and stabilized at 3.6 × 10−6 m3 m−2 s−1. This was 1.5 times higher than under the initial conditions. When the applied pressure was increased to 2.5 times the initial pressure (at 290 min in this case), the flux increased and became stable at 6.0 × 10−6 m3 m−2 s−1, which was 2.5 times higher than at the initial pressure. In addition, similar to the case for BSA, the steady-state flux at each stage became lower with increasing sodium alginate concentration. This indicates that fouling by sodium alginate also became more severe with increasing concentration in this range. Fig. 8 shows the pure water flux over time for membranes that had been immersed in solutions with different concentrations of sodium alginate. When the immersion concentration was 1000 ppm, the pure water flux was 2.0 × 10−6 m3 m−2 s−1 under the initial pressure conditions, where the flux would be 4.0 × 10−6 m3 m−2 s−1 for a pristine membrane. The flux did not change under these conditions, which means that the permeation resistance caused by the adsorption of sodium alginate did not change during this period. When the applied pressure was increased to 1.5 times the initial pressure (at 110 min in this case), the constant flux was 3.1 × 10−6 m3 m−2 s−1, which is ~1.5 times as large as the initial flux. This is the same as was observed for BSA. Next, the pure water flux became 5.4 × 10−6 m3 m−2 s−1 when the applied pressure was increased to 2.5 times the initial pressure (at 220 min in this case). The flux also remained constant over time, and the value was ~2.5 times the initial value. Furthermore, the pure water
Fig. 6. Interpretation of the results shown in Fig. 5.
the lower concentration range and was almost independent of the concentration in the higher concentration range in both tests. This trend can be easily interpreted by Langmuir-adsorption as shown in Fig. 6; the adsorbed amount increases almost linearly with concentration and then deviates from the linear relationship and is finally constant. The adsorbed amount is closely related to the permeation resistance. In this assumption, the flux should decrease almost linearly with the concentration of BSA and then become constant at the higher BSA concentration. Before drawing a conclusion, we have to consider a case where the steady-state flux in the closed-loop cross-flow filtration test would be smaller than the pure water flux using the immersed membrane, even though such a phenomenon was not observed in this study. In such a case, two possibilities should be considered. One is the existence of other fouling mechanism. As described in the introduction part, other types of fouling such as pore blocking, pore clogging, gel layer formation would be occurred independently or simultaneously. The other is the effect of the flow rate. When the flow rate had been much smaller than 2.0 L min−1 in this study, the steady-state flux in the closed-loop cross-flow filtration test would become smaller, which would result in smaller values than that in the pure water flux test. This is because the filtration resistance caused by the increase of the deposited fouling substances would be generated in the closed-loop cross-flow filtration test, while the pure water flux with immersed membrane would not be affected by the flow rate, in principle. Of course, it is suitable to eliminate such effects because we would like to focus on the effect of adsorption. In the series of the experiments, we did not obtain such data, so we don't care these possibilities. 4
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BSA. The findings discussed have clearly proved that the adsorption of BSA and sodium alginate on the membrane surfaces and pore walls is the dominant factor in the reduction of the filtrate flux in the microfiltration. This adsorption may be the origin of the subsequent evolution of severe fouling when processing actual wastewater, although this was not tested in this study. However, the new experimental systems we have tested in this study have allowed us to confidently conclude that the strategy of developing membranes that suppress the adsorption of proteins and polysaccharides is important. 4. Conclusion This study aimed to clearly demonstrate the extent of the contribution of fouling substance adsorption to the reduction of membrane performance—in particular filtrate fluxes—using polyethylene microfiltration membranes with BSA as a model substance for proteins and sodium alginate as a model substance for polysaccharides. We carried out (1) closed-loop cross-flow filtration tests by changing the concentration of fouling substances in the feed solution and (2) pure water flux tests with membranes that were immersed in solutions of the same fouling substances prior to the tests. The aim of the former experiments was to demonstrate the relationships between the concentration of the fouling substances and the steady-state flux, and that of the latter was to demonstrate the effect of the adsorption of the fouling substances on the membrane performance. Thus, the comparison of these results clearly proves the contribution of adsorption to the reduction of filtrate flux. Throughout the discussion of the experimental results, we were able to conclude that adsorption on the surface of the membranes and pore walls was a dominant factor in membrane fouling in both cases, even though the impact of the adsorption on the reduction of the filtrate flux was dependent on the fouling substances. The experiments shown in this study are relatively simple, however the experimental system demonstrated in this study is a powerful tool for both understanding the fouling mechanism and developing low-fouling membranes.
Fig. 8. Pure water flux over time for membranes that had been immersed in solutions with different sodium alginate concentrations. The flow rate was 2.0 L min−1, and the temperature was 25 °C. (△50 ppm, ▲100 ppm, □500 ppm, ■1000 ppm).
Acknowledgments Microfiltration membranes were kindly supplied by Asahi Kasei Corp., Japan. A part of this research was supported by a Grant-in-Aid for Scientific Research (B) (No. 18H01770) from Japan Society for the Promotion of Science (JSPS).
Fig. 9. Relationships between the concentration of sodium alginate in the closed-loop cross-flow microfiltration test (closed symbols) and in the pure water test (open symbols) and the steady-state flux under the conditions where flux would be (a) 4.0 × 10−6 m3 m−2 s−1, (b) 6.0 × 10−6 m3 m−2 s−1, and (c) 1.0 × 10−5 m3 m−2 s−1, shown as the dashed lines, if the membrane was not fouled.
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