- Email: [email protected]
Thermodynamic assessment of adsorptive fouling with the membranes modified via layer-by-layer self-assembly technique
Journal of Colloid and Interface Science 494 (2017) 194–203
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Thermodynamic assessment of adsorptive fouling with the membranes modified via layer-by-layer self-assembly technique Liguo Shen a, Xia Cui a, Genying Yu a, Fengquan Li a, Liang Li b, Shushu Feng a, Hongjun Lin a,⇑, Jianrong Chen a,⇑ a b
College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, PR China Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 30 November 2016 Revised 8 January 2017 Accepted 16 January 2017 Available online 18 January 2017 Keywords: Membrane fouling Membrane modification Interfacial interaction Layer-by-layer assembly Membrane bioreactor
a b s t r a c t In this study, polyvinylidene fluoride (PVDF) microfiltration membrane was coated by dipping the membrane alternatingly in solutions of the polyelectrolytes (poly-diallyldimethylammonium chloride (PDADMAC) and polystyrenesulfonate (PSS)) via layer-by-layer (LBL) self-assembly technique to improve the membrane antifouling ability. Filtration experiments showed that, sludge cake layer on the coated membrane could be more easily washed off, and moreover, the remained flux ratio (RFR) of the coated membrane was obviously improved as compared with the control membrane. Characterization of the membranes showed that a polyelectrolyte layer was successfully coated on the membrane surfaces, and the hydrophilicity, surface charge and surface morphology of the coated membrane were changed. Based on the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) approaches, quantification of interfacial interactions between foulants and membranes in three different scenarios was achieved. It was revealed that there existed a repulsive energy barrier when a particle foulant adhered to membrane surface, and the enhanced electrostatic double layer (EL) interaction and energy barrier should be responsible for the improved antifouling ability of the coated membrane. This study provided a combined solution to membrane modification and interaction energy evaluation related with membrane fouling simultaneously. Ó 2017 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (H. Lin), [email protected] (J. Chen). http://dx.doi.org/10.1016/j.jcis.2017.01.051 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
1. Introduction
2. Materials and methods
Membrane fouling contributes to enormous cost, and hence becomes the bottleneck for the large-scale industrial application of membrane separation technology [1–3]. In order to attenuate the membrane fouling problem, significant efforts have been executed to fabricate excellent performance membranes [3–5]. The approaches which have been explored include optimizing the membrane pore or surface structures by controlling the membrane preparation conditions [6,7], chemical grafting of hydrophilic functional monomers [8,9], and organic-inorganic or organic-organic blending [10,11]. Among them, layer-by-layer (LBL) self-assembly technique has been frequently used to coat advantageous polyelectrolytes on membrane surface for membrane fouling mitigation [12–14] due to its high efficiency and the ability to regulate the coated layer structure and composition [15,16]. It was reported that, the electronegative and hydrophilic polyelectrolyte networks could be easily coated on membrane surfaces by this technique [17,18]. For example, Su et al. [19] prepared a polyelectrolyte ultrafiltration (UF) membrane with the materials of poly(acrylonitrile and 2-dimethylaminoethyl methacrylate) (PAN-DMAEMA). The modified membrane showed excellent antifouling property to bovine serum albumin (BSA), and lysozyme in certain range of pH and ionic strength. Zhao et al. [20] prepared polyvinylidene fluoride (PVDF) UF membranes by assembling chitosan (CS) and sodium alginate (SA) polyelectrolytes. Under the optimized experimental conditions, BSA adsorption on the membrane surface is only 4 lg cm2 and the flux recovery ratio after washing reaches to 89%, 99% and 98% for the three typical pollutants of BSA, SA and humic acid (HA), respectively. Ilyas et al. [21] modified the UF membrane by alternately assembling Poly(allylamine hydrochloride) (PAH) and poly (acrylic acid) (PAA). The modified membranes were much easily recovered by washing after fouled. These results strongly suggested that membrane surface properties could be tailored by coating different polyelectrolytes with LBL self-assembly technique, which is flexible for membrane modification to meet various demands. The polyelectrolytes containing ASO 3 functional group have been proved to be an efficient hydrophilic and negatively charged function group in lots of literature studies [22–24]. While the experimental results in literature confirmed the improved antifouling ability of the coated membranes, its underlying causes have not been well investigated. A better understanding of mechanisms regarding antifouling ability is essential to optimize LBL self-assembly technique as well as prepare membranes with high antifouling property. Previous studies generally reported that the reduced fouling of the coated membrane was mainly caused by the reduced adsorption of foulants on membrane surface [19–21,25]. Meanwhile, it is widely accepted that thermodynamic/interfacial interactions between foulants and membrane surface are critical predictors for the susceptibility of a membrane to foulants adsorption [26–28]. Interfacial interactions between two solid substances in water can be described by the extended Derjaguin–Landau–Ver wey–Overbeek (XDLVO) theory [29–31]. In this context, XDLVO theory may provide a probable way to explore the underlying mechanisms of the improved antifouling performance of the coated membranes by LBL self-assembly technique. Nevertheless, no specific study has been conducted to investigate this issue for LBL self-assembly technique. The objective of this study is, therefore, to modify PVDF membrane by coating PDADMAC and PSS polyelectrolytes alternatively through LBL self-assembly technique, and to assess the interfacial interactions between foulants and the modified PVDF membrane. The interfacial mechanisms for the antifouling property of the coated membranes were investigated based on XDLVO theory and its extensions.
2.1. Modification of membranes
195
The PVDF membrane (Pore size 0.1 lm, Jiangsu Dafu Co. Ltd.) was alternately immersed into polyelectrolyte solutions for LBL assembling. It is known that PDADMAC and PSS are the typical positively charged and negatively charged polyelectrolytes, respectively. Since the PVDF membrane was negatively charged (21.19 ± 1.67 mV) in this study, the assembling was initiated by the positively charged PDADMAC solution (1.00 g L1, MW = 100,000–200,000 Da, Aldrich Chemical Co. Inc). The PVDF membranes were immersed into the PDADMAC solution for 1 min to obtain the positively charged PVDF-PDADMAC membranes. After 5 min rinsing by water to remove the superfluous PDADMAC, the positively charged PVDF-PDADMAC membrane was immersed into the negatively charged PSS solution (1.00 g L1, MW = 70,000 Da, Aldrich Chemical Co.) for 1 min to obtain the negatively charged PVDF-PDADMAC-PSS membrane. By the electrostatic interactions and van der Waals forces, PSS was adhered onto the surface to form the negatively charged PVDF-PDADMAC-PSS membrane. Another rinsing by water was lasted for 5 min to remove the superfluous PSS. To minimize the flux decline, the number of bilayers was limited to 1 via the reported strategy [32]. In this study, the negatively charged membrane was alternately assembled with positively charged PDADMAC and then negatively charged PSS. The coated PDADMAC together with PSS means one bilayer. The negatively charged PVDF-PDADMAC-PSS membrane was then used for characterization, filtration, and antifouling experiments. The coating process is shown in Fig. 1. 2.2. Analytical methods Surface of the membranes was measured using a scanning electron microscope (LEO1530vp SEM (Germany)). Previous to observation, samples were attached by carbon tape to the sample stage and sputtered with gold. The voltage was set at 25 kV, and the current was set at 10 mA. Chemical components of the membranes were measured by an Energy dispersive X-ray spectroscopy (7426 EDX OXFORD). The measurement voltage was 20 kV. A commercially available atomic force microscopy (AFM) instrument (Nanoscope 8, Bruker) equipped with a J scanner was used to observe the morphology of the membrane surface. AFM imaging of the membranes was conducted in tapping mode with silicon nitride cantilevers (NP-S, Bruker). The contact angles of the sludge foulant and membrane samples were measured by a contact angle meter (Kino industry Co., Ltb, USA). Ultrapure water, glycerol and diiodomethane were chosen as the probe liquids in the experiments. A drop (5 lL) was lowered onto the membrane’s surface from a needle tip. A magnified image of the droplet was recorded by a digital camera. Static contact angles were determined from these images with a calculation software. The contact-angle measurement was taken as the mean value of 5 different points on each membrane. For measurement of the contact angles on the sludge foulant surface, the sludge liquor obtained from a membrane bioreactor (MBR) was firstly filtered through the membrane to form a cake layer. The formed cake layer was fixed within two glass slides to form a relatively flat surface, and then kept in an oven for 24 h at 40 °C to get rid of water. The foulant samples were used for contact angle measurement. The zeta potential of the membrane was analyzed by a zeta potential analyzer (Zeta 90 Plus, Brookhaven, UK). The zeta potential of the sludge foulants was determined by a microelectrophoresis (Zetasizer Nano ZS, Malvern, UK). The measurement method
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
PVDF membrane
196
PDADMAC
PSS bilayer
PVDF membrane
+ PDADMAC →
positively charged PVDFnegatively charged PVDF+ PSS → PDADMAC membrane PDADMAC-PSS membrane
Fig. 1. The schematic illustration of the coating process.
referred to Idil Mouhoumed et al. [33]. In the experiments, an electrolyte solution (103 mol L1) was prepared by KCl (Aldrich Chemical) and deionized water (18 MX cm). The solution pH was adjusted to 7.0 with 0.1 mol L1 HCl and KOH solutions (Aldrich Chemical) via automatic titrating system. Each sample was measured for three times to obtain the average value. The particle size distribution (PSD) of the sludge liquor sampled from the MBR treating synthetic municipal wastewater was measured by a Mastersizer 2000 Laser particle size analyzer (Malvern, UK). An analysis of variance (ANOVA) was used to test for difference between treatment means. The tests were performed by using the software of Statistical Product and Service Solutions (SPSS) V18.0.
polymeric substances-rough membrane surface, particle-smooth membrane surface and particle-rough membrane surface, were considered (Fig. 2). For the scenario of two infinite planar surfaces, XDLVO approach enables to calculate LW, AB and EL interaction energy per unit area (specific energy, mJ m2) at separation distance (h) (DGLW(h), DGAB(h) and DGEL(h)) by using the following equations [29]:
2.3. Filtration tests
DGEL ðhÞ ¼ jfm ff er e0
For lab-scale studies, it is a general operation to conduct shortterm filtration tests for assessment of membrane performance. The performance of the prepared membranes was analyzed through a dead-end filtration system. The valid membrane area in this system is 0.002 m2, and the efficient volume of the filtration cell (MSC300, Mosu Corp., Shanghai, China) is 325 ml. The pressure of the measurement is 20 kPa. The antifouling property was measured by recording the flux decline process during continuous filtration of the sludge liquor with a concentration of 2.68 g L1 mixed liquid suspended solids (MLSS). The sludge liquor was obtained from the MBR treating synthetic wastewater. Detailed information regarding the operation of MBR, and the sludge properties can refer to our previous study [34]. The susceptibility of a membrane to foulant adhesion can be directly reflected by the remained sludge amount on membrane surface after a washing process. A cake layer was formed on the membrane surface by filtering 200 ml 2.68 gMLSS L1 sludge liquor in the filtration cell. Thereafter, washing process was performed by filling filtration cell (where the cake layer formed) with 100 ml tap water, followed with shaking the filtration cell, and discarding the filling water. The washing process was repeated for 3 times for each membrane. While the shaking operation is difficult to quantify, the washing process was identical for both of the control and coated membranes. 2.4. Methods to calculate interfacial interactions According to XDLVO theory, the noncovalent interfacial interactions between two surfaces in a medium include three kind of interactions: Lifshitz-Van der waals (LW), acid-based (AB), and electrostatic double layer (EL) interactions [29]. In order to provide a comprehensive comparison between the control and coated membranes, herein, three interaction scenarios, including dispersed
2
DGLW ðhÞ ¼ DGLW h0
h0 h
ð1Þ
2
DGAB ðhÞ ¼ DGAB h0 exp
h0 h k
ð2Þ
! f2m þ f2f 1 ð1 coth jhÞ þ sinh jh 2fm ff
ð3Þ
Two planar surfaces are assumed to contact each other at a minAB imum separation distance (h0 = 0.158 nm) [35]. DGLW h0 , DGh0 and
DGEL h0 mean the specific LW, AB and EL energy between two planar surfaces at separation distance of h0, respectively, which can be obtained through Eqs. (4)–(6), respectively [29]. qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiqffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi ð4Þ DGLW cLW cLW cLW cLW m w w h0 ¼ 2 f
hpffiffiffiffiffiffipffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffiqffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi DGAB cþw cf þ cm cw þ cw cþf þ cþm cþw h0 ¼ 2 qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi i cf cþm cþf cm
DGEL h0
¼
e0 er j 2
n2m
þ
n2f
"
1 cothðjh0 Þ þ
2nm nf n2m þ n2f
# cschðjh0 Þ
ð6Þ
where cLW, c+and c are the LW, electron donor and electron acceptor surface tension components of a substance (the subscripts m, f and w denote membrane, foulant and water, respectively), respectively. These surface tension components of a solid substance can be calculated by solving a set of three Young’s equations (Eq. (7)), which requires to measure contact angle (/) of three probe liquids (ultrapure water, glycerol and diiodomethane used in this study) on the substance surface.
ð1 þ cos /Þ tol cl ¼ 2
qffiffiffiffiffiffiffiffiqffiffiffiffiffiffiffiffi
cLW cLW þ s l
ffi ffipffiffiffiffiffi pffiffiffiffiffi þ
cl
cs þ
qffiffiffiffiffiffipffiffiffiffiffiffi
cþl cs
ð7Þ
For the scenario of particle-smooth membrane surface system, AB EL the interfacial interactions of U LW fwm ðDÞ; U fwm ðDÞ; U fwm ðDÞ (unit: kT) can be described by the Derjaguin approximation (DA) method [36,37]:
197
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
(a)
(b)
dispersed polymeric substances
rough membrane
(c)
sludge floc
sludge floc
interaction
interaction
smooth membrane
rough membrane
Fig. 2. Schematics of the interaction scenarios of (a) dispersed polymeric substances-rough membrane surface, (b) particle foulants-smooth membrane surface, and (c) particle foulants-rough membrane surface.
LW U LW fwm ðDÞ ¼ 2pDGh0
2
h0 R D
AB U AB fwm ðDÞ ¼ 2pRDDGh0 exp
ð8Þ
h0 D k
ð9Þ
1 þ ejD 2 2 2jD U EL þ n Þ þ n lnð1 e m f fwm ðDÞ ¼ per e0 R 2nm nf ln 1 ejD
the interval [a, b] of variable x and the interval [c, d] of variable y, respectively. Herein, h = (b a)/2m, and k = (d c)/2n. Defining fi,j to be the function value of f(xi, yj), the double integral could be obtained as follows:
Z
b
Z
d
f ðx; yÞdxdy ¼ a
c
i¼1 j¼1
ð10Þ where D is the closet distance between a foulant particle and membrane surface; R means the particle radius; the subscript ‘‘fwm” represents the interaction between foulant particles and membrane surfaces in water. For the scenario of particle-rough membrane surface system, a combined method developed by Lin et al. [38] was adopted to calculate the interfacial interactions:
Z 2p Z 0
R
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DGLW D þ R þ z R2 r2 f ðr; hÞ rdrdh
0
ð11Þ U AB fwm ðDÞ ¼
Z 2p Z 0
R
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DGAB D þ R þ z R2 r 2 f ðr; hÞ rdrdh
0
ð12Þ U EL fwm ðDÞ ¼
Z 2p Z 0
R
x2i2
Z
y2j
f ðx; yÞdxdy
y2j2
m X n hk X ðf þ f 2i;2j2 þ f 2i;2j 9 i¼1 j¼1 2i2;2j2
þ f 2i2;2j1 Þ þ 16f 2i1;2j1
ð15Þ
Generally, the greater the value of m and n, the higher the accuracy of the computation. Although the composite Simpson’s rule is an approximate method, it is possible to get results with high accuracy through setting high value of m and n. Detailed calculations showed that, setting m = n = 1600 or above could make the calculation error negligible. However, such an operation would result in an enormous computation amount such that the help of computers has to be sought. MATLAB software was adopted to conduct the calculations in this study. 3. Results and discussion 3.1. Filtration performance
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DGEL D þ R þ z R2 r 2 f ðr; hÞ rdrdh
0
ð13Þ where, z is the roughness of membrane surface; r is the radius of differential circular ring on particle surface; dh is the differential angle of the differential circular arc in the circular ring; f (r, h) is local amplitude directly below the circular arc as a function of the position of the differential circular arc defined by r and h.f (r, h) can be also viewed as the function of membrane surface morphology. Basically, the randomness of surface roughness precludes a rigorous mathematical model describing membrane surface morphology [39]. However, it is possible to propose a hypothetical surface topology that represents some pertinent statistical properties of the rough membrane surface. According to literature [38,40,41], the following function was adopted for the hypothetical membrane surface morphology:
f ðr; hÞ ¼ z sinðpr cos h=2z þ uÞ
x2i
þ f 2i2;2j Þ þ 4ðf 2i1;2j2 þ f 2i;2j1 þ f 2i1;2j
ð14Þ
where u is a phase shift of the sine function (assumed to be zero for simplicity in this study). The two integrals in Eqs. (11)–(13) were estimated through composite Simpson’s rule. The details can refer to Lin et al. [38]. The composite Simpson’s rule was applied to numerically estimate the double integrals in Eqs. (11)–(13). As for double integral, certain point x1 = a, xi = x1 + ih (i = 1, 2, . . ., 2m + 1) and y1 = b, yj = y1 + jk (j = 1, 2, . . ., 2n + 1) were used to subdivide
Both the control and coated membranes are subjected to filtration tests under the identical experimental conditions. Fig. 3 shows the changes of remained membrane flux ratio (RFR) with operational time for the control membrane and coated membrane when they were used to filter the sludge liquor. Herein, RFR, which
100 Control Coated
90
Remained Flux Ratio (%)
U LW fwm ðDÞ ¼
m X n Z X
80 70 60 50 40 30
0
20
40
60
80
100
Time (min) Fig. 3. Variation of remained membrane flux ratio (RFR) with operational time when the sludge liquor was filtrated (MLSS = 2.68 g L1, pressure = 20 kPa).
198
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
Fig. 4. The illustration of the fouled membrane surface (the left is the surface of the control membrane after washing; the right is the surface of the coated membrane after washing).
Fig. 5. The SEM images of membrane surface: (a), (b) and (c) the images of the control membrane under different amplifications; (d), (e) and (f) the images of the coated membrane under different amplifications.
means the ratio of the instantaneous flux to the initial water flux, was used to assess the antifouling performance. This parameter can exclude other effects, and better reflect the differences in the
whole filtration process between the two membranes [42,43]. Under the same conditions in this study, the initial water flux for the control and coated membranes are 0.133 kg m2 s1and
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
0.087 kg m2 s1, respectively. As shown in Fig. 3, the two membranes have the similar profile which can be characterized by a rapid flux decline followed by a transition to a plain state. This suggests that both control and coated membranes undergo fouling during the filtration process. However, the RFR of the coated membrane is obviously higher than that of the control membrane, indicating the enhanced antifouling performance of the coated membrane. It should be noted that, comparison of the long-term filtration performance would provide more convincing results, and this will be conducted in near future. To evaluate the adhesion of sludge foulants on the membrane surface, filtration tests were carried out by formation of a cake layer on the membrane surface through filtration of 200 ml sludge solution, followed by the identical washing process (referring to the methodology section). The sludge cake layer on the control membrane surface stayed firmly, while the cake layer on the coated membrane fell off completely (Fig. 4). This result clearly shows the much higher interaction strength between the sludge foulants and the control membrane than that between the sludge foulants and the coated membrane. 3.2. Characterization of the membranes Fig. 5 shows the SEM images of the surface morphology of the control and coated membranes. By comparing the images, it can be found that a smooth layer was successfully coated on the membrane surfaces, and moreover, certain amounts of pores disappeared on the coated membrane surface. This implicated that coating of polyelectrolytes could block the pores to some extent. This phenomenon has been also observed in previous studies [44,45]. Therefore, in order to modify membrane surface properties
199
and simultaneously avoid the severe clogging of pores, only one PDADMAC/PSS bilayer rather than multilayers was coated on the membrane surface in this study. Fig. 5 also indicates that the polyelectrolytes are rather evenly coated on the membrane surface although the coated layer is thin. The EDX spectra of the control membrane in Fig. 6(a) indicate that there is no signal of S element. In contrast, while signal of S element is weak due to thin coated layer, EDX spectra of the coated membrane in Fig. 6(b) clearly show existence of S element. Since EDX spectra have provided convincing information, X-rays photoelectron spectroscopy (XPS) analysis has not been conducted although it may provide more significant signal for existence of S element. S element should come from the ASO 3 function group in PSS, indicating the success of coating PSS on membrane surface. Assembling with ASO 3 function groups on membrane surface would improve surface hydrophilicity, but may also cause Carelated fouling. Fig. 7 shows the AFM images of the control and coated membranes. The enormous fluctuations could be found for both control and coated membranes. The roughness values obtained by the AFM analysis (dimension: 10 lm 10 lm) are 38.5 nm and 37.2 nm for control and coated membranes, respectively (Table 1). Although the difference in average roughness is small, ANOVA analysis shows that the difference is statistically significant (p < 0.05). It is, therefore, evident that, the surfaces of the both membranes are significantly rough, and the coated membrane surface is slightly smoother than the control membrane surface. The surface properties in terms of zeta potential and contact angle of the control, coated membrane and sludge layer are characterized, and the results are listed in Table 1. As compared with the control membrane, the coated membrane possesses
Fig. 6. The EDX spectra of the membrane surface composite: (a) spectra for the control membrane; (b) spectra for the coated membrane.
200
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
Fig. 7. The AFM images of membrane surface: (a) and (b) images of the control membrane from different views; (c) and (d) images of the coated membrane from different views.
Table 1 The surface properties of the control, coated membranes and sludge foulants. Substances
Roughness (nm)
Zeta potential (mV)
Control membrane Coated membrane Sludge foulants
38.5 ± 0.3 37.2 ± 0.6 –
21.19 ± 1.67 27.61 ± 1.45 23.37 ± 3.40
higher absolute value of zeta potential and lower contact angle of three probe liquors. These results are reasonable as considering that the plentiful SO 3 groups coated on the membrane surface facilitate to the improvement of the negative charge and hydrophilicity [46]. However, the differences in zeta potential and contact angles between the coated and control membrane are not great enough. This phenomenon may be attributed to the fact that some PDADMAC groups are not covered by PSS, and the thin coated bilayer. It should be noted that, although the differences in zeta potential and contact angles are not great enough, they are statistically significant (based on one way analysis of
Contact angle (°) Water
Glycerol
Diiodomethane
75.45 ± 5.86 71.71 ± 2.68 67.56 ± 1.47
69.54 ± 3.47 63.34 ± 3.21 76.11 ± 0.43
46.06 ± 6.07 28.01 ± 2.01 42.86 ± 8.86
variance (ANOVA) method). As the sludge foulants were also negatively charged (23.37 ± 3.40 mV), the more negative charge and more hydrophilic surface of the coated membrane would certainly improve antifouling property of membrane. The control membrane was characterized to possess an average roughness of 38.5 nm. The surface properties of the sludge foulants were also characterized. It can be seen that the results are comparable with those of sludge foulants in literature [47,48], indicating the representativeness of the sludge foulants for membrane fouling study. All of these data were then used for calculation of interfacial interactions between the membranes and sludge foulants.
201
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
tively, showing 36.2% improvement in DGEL h0 under conditions in this study. The improved EL interaction is expected to provide enhanced repulsive force against foulants adhesion, improving the antifouling ability. It should be noted that real membrane surface is rough, and the scenario of two infinite planar surfaces is atypical in reality. However, the soluble microbial products (SMPs), usually as the abundant foulants in wastewater treatment systems, are chainlike substances, which can flex to fit membrane surface [49]. Therefore, the interactions between SMPs and rough membrane surface can be regarded as the interactions between two planar surfaces. Other researchers also observed the reduced adsorption of chain molecular foulants like BSA and humic acids to the membranes coated by LBL assembly technique [19,20,25]. They simply attributed the reduced fouling to the modified surface charge but didn’t provide a quantitative analysis. This study numerically calculated the interaction energy between foulants and membrane surface, thus not only provided approaches to quantitatively assess interaction energy, but also gave a more plausible explanation to the reduced fouling of the membranes coated by LBL assembly technique. The interaction profiles in the scenario of particle-smooth membrane combination for the control and coated membranes are shown in Fig. 9. For both membranes, different property and distribution of the individual interaction component result in a distinct distribution of total interaction versus separation distance. There apparently exists an energy barrier when a foulant particle closes to membrane surface. This means that the eventual adhesion of a foulant particle on membrane surface should overcome a repulsive energy barrier. Obviously, the higher the energy barrier is, the more difficult the adhesion of a particle becomes. It can be seen from Fig. 9, the energy barrier for the coated membrane is 2426 kT, significantly higher than the value of 1843 kT for the control membrane in this study. Therefore, the interaction energy analysis can explain the reduced fouling of the coated membrane. Fig. 10 shows the interaction profiles in the scenario of particlerough membrane combination for control and coated membranes. The profiles are quite similar to the profiles shown in Fig. 9. However, the interaction strength for rough membrane surface is only about tenth of the strength for smooth membrane surface. This indicates membrane roughness would significantly reduce the interaction strength. For both membranes, an energy barrier also exists although its strength significantly decreases as compared with that in Fig. 9. The coated membrane possesses higher energy barrier (280.5 kT) than the control membrane (217.5 kT). It should be noted that the comparison is conducted based on the assumption of same membrane surface topography. Actually, as shown in Section 3.2, the LBL assembly technique would yield relatively
6
Energy per unit area (mJ m-2) Energy per unit area (mJ m-2)
brane counterpart. This is particularly apparent that DGEL h0 for the coated and control membranes is 2.86 and 2.10 mJ m2, respec-
(b)
control membrane 4 EL interaction
2 0
total interaction -2
AB interaction
LW interaction
-4 -6 6 Separation distance (nm)
4
coated membrane
EL interaction
2 0
total interaction -2
AB interaction
LW interaction
-4 -6 Separation distance (nm)
Fig. 8. The profiles of specific interactions between membrane and sludge foulants as a function of the separation distance in the scenario of two infinite planar surfaces for (a) the control membrane, and (b) the coated membrane.
(a)
4000
control membrane
3000
Interaction energy (kT)
It is widely accepted that interfacial interactions are the critical predictors for the susceptibility of a membrane to foulant adhesion [26–28]. Fig. 8 shows the profiles of interactions between the membrane and sludge foulants in the scenario of two infinite planar surfaces for control and coated membranes. The two membranes show the similar profile. LW and AB interactions are attractive while EL interaction is repulsive. Moreover, the total interaction energy is positive at large separation distances. This phenonmenon is reasonable for the interaction scenario of two infinite planar surfaces because the EL interaction is positive and plays a predominant role over the AB interaction and LW interaction at large separation distances. The main difference between the two profiles is the significance of EL interaction. The coated membrane-sludge foulants combination shows higher EL interactions than the control mem-
(a)
total interaction
2000 1000
EL interaction
energy barrier
0
LW interaction
-1000 -2000
AB interaction
-3000 4000 (b) -4000 3000
Interaction energy (kT)
3.3. Underlying mechanisms of the reduced fouling
total interaction
2000
coated membrane EL interaction
energy barrier
1000 0
LW interaction
-1000 -2000
AB interaction
-3000 -4000 0
2
4
6 8 10 Separation distance (nm)
12
14
16
Fig. 9. The profiles of specific interactions between membrane and sludge foulants as a function of the separation distance in the scenario of particle foulants-smooth membrane surface for (a) the control membrane, and (b) the coated membrane.
smooth membrane surface. Considering this effect, the difference in the energy barrier for the two membranes would be more signif-
202
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
(a)
EL interaction
Interaction energy (kT)
300
References
200
energy barrier
100 0 -100
LW interaction
-200 AB interaction
-400 400
EL interaction
300
Interaction energy (kT)
control membrane
total interaction
-300
(b)
(2016C31G2030032) and Natural Science Foundation of Zhejiang Province (No. LQ16B060001) is highly appreciated.
400
coated membrane
total interaction
200
energy barrier
100 0
-100 LW interaction
-200 AB interaction
-300 -400 0
2
4
6 8 10 12 Separation distance (nm)
14
16
Fig. 10. The profiles of specific interactions between membrane and sludge foulants as a function of the separation distance in the scenario of particle foulants-rough membrane surface for (a) the control membrane, and (b) the coated membrane.
icant. Nevertheless, the higher repulsive energy barrier for the coated membrane would make the particle foulants more difficult to adhere to membrane surface, mitigating membrane fouling. From viewpoint of interaction energy and energy barrier, the reduced fouling of the coated membrane in the experiments could be well explained. 4. Conclusion In this study, PVDF microfiltration membrane was coated by a polyelectrolyte layer- PDADMAC and PSS via the LBL selfassembly technique to mitigate membrane fouling. It was found that, sludge cake layer on the coated membrane could be more easily washed off, and moreover, the RFR of the coated membrane was obviously improved as compared with the control membrane. SEM and EDX analysis suggested that polyelectrolyte layer was successfully coated on the membrane surfaces. The coated membrane possessed an advantageous surface morphology. Due to the abundant SO 3 group in polyelectrolyte layer, the hydrophilicity and negative charge of the coated membrane was enhanced. XDLVO analysis showed existence of a repulsive energy barrier when a foulant particle closed to membrane surface. The enhanced EL interaction and energy barrier should be responsible for the improved antifouling ability of the coated membrane. These results suggested the potential to tailor surface properties and to optimize antifouling ability of membranes by LBL self-assembly technique combined with series of analytical methods provided in this study. Acknowledgements Financial support of National Natural Science Foundation of China (Nos. 21506195, 51578509), Public Welfare Project of the Science and Technology Department of Zhejiang Province
[1] X. Wang, V.W.C. Chang, C.Y. Tang, Osmotic membrane bioreactor (OMBR) technology for wastewater treatment and reclamation: advances, challenges, and prospects for the future, J. Membr. Sci. 504 (2016) 113–132. [2] Z. Wang, J. Ma, C.Y. Tang, K. Kimura, Q. Wang, X. Han, Membrane cleaning in membrane bioreactors: a review, J. Membr. Sci. 468 (2014) 276–307. [3] F. Meng, S.-R. Chae, A. Drews, M. Kraume, H.-S. Shin, F. Yang, Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material, Water Res. 43 (2009) 1489–1512. [4] B. Van der Bruggen, Chemical modification of polyethersulfone nanofiltration membranes: a review, J. Appl. Polym. Sci. 114 (2009) 630–642. [5] M. Amirilargani, M. Sadrzadeh, E.J.R. Sudhölter, L.C.P.M. de Smet, Surface modification methods of organic solvent nanofiltration membranes, Chem. Eng. J. 289 (2016) 562–582. [6] K.-J. Hwang, C.-Y. Liao, K.-L. Tung, Effect of membrane pore size on the particle fouling in membrane filtration, Desalination 234 (2008) 16–23. [7] K. Riedl, B. Girard, R.W. Lencki, Influence of membrane structure on fouling layer morphology during apple juice clarification, J. Membr. Sci. 139 (1998) 155–166. [8] A. Mehrparvar, A. Rahimpour, Surface modification of novel polyether sulfone amide (PESA) ultrafiltration membranes by grafting hydrophilic monomers, J. Ind. Eng. Chem. 28 (2015) 359–368. [9] Y. Hu, K. Lu, F. Yan, Y. Shi, P. Yu, S. Yu, S. Li, C. Gao, Enhancing the performance of aromatic polyamide reverse osmosis membrane by surface modification via covalent attachment of polyvinyl alcohol (PVA), J. Membr. Sci. 501 (2016) 209– 219. [10] M. Safarpour, A. Khataee, V. Vatanpour, Preparation of a novel polyvinylidene fluoride (PVDF) ultrafiltration membrane modified with reduced graphene oxide/titanium dioxide (TiO2) nanocomposite with enhanced hydrophilicity and antifouling properties, Ind. Eng. Chem. Res. 53 (2014) 13370–13382. [11] L. Shen, X. Bian, X. Lu, L. Shi, Z. Liu, L. Chen, Z. Hou, K. Fan, Preparation and characterization of ZnO/polyethersulfone (PES) hybrid membranes, Desalination 293 (2012) 21–29. [12] G.-R. Xu, S.-H. Wang, H.-L. Zhao, S.-B. Wu, J.-M. Xu, L. Li, X.-Y. Liu, Layer-bylayer (LBL) assembly technology as promising strategy for tailoring pressuredriven desalination membranes, J. Membr. Sci. 493 (2015) 428–443. [13] T. Ishigami, K. Amano, A. Fujii, Y. Ohmukai, E. Kamio, T. Maruyama, H. Matsuyama, Fouling reduction of reverse osmosis membrane by surface modification via layer-by-layer assembly, Sep. Purif. Technol. 99 (2012) 1–7. [14] P. Kaner, D.J. Johnson, E. Seker, N. Hilal, S.A. Altinkaya, Layer-by-layer surface modification of polyethersulfone membranes using polyelectrolytes and AgCl/ TiO2 xerogels, J. Membr. Sci. 493 (2015) 807–819. [15] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (1997) 1232–1237. [16] F. Caruso, H. Lichtenfeld, M. Giersig, H. Möhwald, Electrostatic self-assembly of silica nanoparticle-polyelectrolyte multilayers on polystyrene latex particles, J. Am. Chem. Soc. 120 (1998) 8523–8524. [17] J. Bhattacharjee, S. Banik, S.A. Hussain, D. Bhattacharjee, A study on the interactions of cationic porphyrin with nano clay platelets in Layer-by-Layer (LbL) self assembled films, Chem. Phys. Lett. 633 (2015) 82–88. [18] N. Ghavidel Mehr, C.D. Hoemann, B.D. Favis, Chitosan surface modification of fully interconnected 3D porous poly(e-caprolactone) by the LbL approach, Polymer 64 (2015) 112–121. [19] Y.L. Su, C.X. Mu, C. Li, Z.Y. Jiang, Antifouling property of a weak polyelectrolyte membrane based on poly(acrylonitrile) during protein ultrafiltration, Ind. Eng. Chem. Res. 48 (2009) 3136–3141. [20] X.Z. Zhao, A.W. Qin, D.P. Liu, C.J. He, Tuning the antifouling property of PVDF ultrafiltration membrane with surface anchored polyelectrolyte complexes for sewage treatment, RSC Adv. 5 (2015) 63580–63587. [21] S. Ilyas, J. de Grooth, K. Nijmeijer, W.M. de Vos, Multifunctional polyelectrolyte multilayers as nanofiltration membranes and as sacrificial layers for easy membrane cleaning, J. Colloid Interf. Sci. 446 (2015) 386–393. [22] M. Kumar, M. Ulbricht, Low fouling negatively charged hybrid ultrafiltration membranes for protein separation from sulfonated poly(arylene ether sulfone) block copolymer and functionalized multiwalled carbon nanotubes, Sep. Purif. Technol. 127 (2014) 181–191. [23] B.P. Tripathi, N.C. Dubey, M. Stamm, Polyethylene glycol cross-linked sulfonated polyethersulfone based filtration membranes with improved antifouling tendency, J. Membr. Sci. 453 (2014) 263–274. [24] D. Song, J. Xu, Y. Fu, L. Xu, B. Shan, Polysulfone/sulfonated polysulfone alloy membranes with an improved performance in processing mariculture wastewater, Chem. Eng. J. 304 (2016) 882–889. [25] C.Y. Ba, D.A. Ladner, J. Economy, Using polyelectrolyte coatings to improve fouling resistance of a positively charged nanofiltration membrane, J. Membr. Sci. 347 (2010) 250–259. [26] Q. Wang, Z. Wang, C. Zhu, X. Mei, Z. Wu, Assessment of SMP fouling by foulant–membrane interaction energy analysis, J. Membr. Sci. 446 (2013) 154– 163. [27] L.-M. Whang, Y.-J. Wu, Y.-C. Lee, H.-W. Chen, T. Fukushima, M.-Y. Chang, S.-S. Cheng, S.-F. Hsu, C.-H. Chang, W. Shen, C.K. Huang, R. Fu, B. Chang, Nitrification performance and microbial ecology of nitrifying bacteria in a full-scale
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
[28]
[29] [30]
[31]
[32]
[33]
[34]
[35]
[36]
[37] [38]
membrane bioreactor treating TFT-LCD wastewater, Bioresour. Technol. 122 (2012) 70–77. H. Hong, W. Peng, M. Zhang, J. Chen, Y. He, F. Wang, X. Weng, H. Yu, H. Lin, Thermodynamic analysis of membrane fouling in a submerged membrane bioreactor and its implications, Bioresour. Technol. 146 (2013) 7–14. C.J. van Oss, Hydrophobicity of biosurfaces – origin, quantitative determination and interaction energies, Colloids Surf. B 5 (1995) 91–110. C.J. van Oss, Long-range and short-range mechanisms of hydrophobic attraction and hydrophilic repulsion in specific and aspecific interactions, J. Mol. Recognit. 16 (2003) 177–190. V.T. Nguyen, T.W.R. Chia, M.S. Turner, N. Fegan, G.A. Dykes, Quantification of acid–base interactions based on contact angle measurement allows XDLVO predictions to attachment of Campylobacter jejuni but not Salmonella, J. Microbiol. Meth. 86 (2011) 89–96. F. Diagne, R. Malaisamy, V. Boddie, R.D. Holbrook, B. Eribo, K.L. Jones, Polyelectrolyte and silver nanoparticle modification of microfiltration membranes to mitigate organic and bacterial fouling, Environ. Sci. Technol. 46 (2012) 4025–4033. E. Idil Mouhoumed, A. Szymczyk, A. Schäfer, L. Paugam, Y.H. La, Physicochemical characterization of polyamide NF/RO membranes: insight from streaming current measurements, J. Membr. Sci. 461 (2014) 130–138. L.-G. Shen, Q. Lei, J.-R. Chen, H.-C. Hong, Y.-M. He, H.-J. Lin, Membrane fouling in a submerged membrane bioreactor: impacts of floc size, Chem. Eng. J. 269 (2015) 328–334. J.M. Meinders, H.C. van der Mei, H.J. Busscher, Deposition efficiency and reversibility of bacterial adhesion under flow, J. Colloid Interf. Sci. 176 (1995) 329–341. S. Bhattacharjee, M. Elimelech, Surface element integration: a novel technique for evaluation of DLVO interaction between a particle and a flat plate, J. Colloid Interf. Sci. 193 (1997) 273–285. E.M.V. Hoek, G.K. Agarwal, Extended DLVO interactions between spherical particles and rough surfaces, J. Colloid Interf. Sci. 298 (2006) 50–58. H. Lin, M. Zhang, R. Mei, J. Chen, H. Hong, A novel approach for quantitative evaluation of the physicochemical interactions between rough membrane surface and sludge foulants in a submerged membrane bioreactor, Bioresour. Technol. 171 (2014) 247–252.
203
[39] E.M.V. Hoek, S. Bhattacharjee, M. Elimelech, Effect of membrane surface roughness on colloidmembrane DLVO interactions, Langmuir 19 (2003) 4836–4847. [40] J. Verran, A. Packer, P. Kelly, K.A. Whitehead, The retention of bacteria on hygienic surfaces presenting scratches of microbial dimensions, Lett. Appl. Microbiol. 50 (2010) 258–263. [41] K.A. Whitehead, J. Verran, The effect of surface topography on the retention of microorganisms, Food Bioprod. Process. 84 (2006) 253–259. [42] A. Córdova, C. Astudillo, C. Guerrero, C. Vera, A. Illanes, Assessment of the fouling mechanisms of an ultrafiltration membrane bioreactor during synthesis of galacto-oligosaccharides: effect of the operational variables, Desalination 393 (2016) 79–89. [43] O. Sanyal, Z. Liu, J. Yu, B.M. Meharg, J.S. Hong, W. Liao, I. Lee, Designing foulingresistant clay-embedded polyelectrolyte multilayer membranes for wastewater effluent treatment, J. Membr. Sci. 512 (2016) 21–28. [44] F. Diagne, R. Malaisamy, V. Boddie, R.D. Holbrook, B. Eribo, K.L. Jones, Polyelectrolyte and silver nanoparticle modification of microfiltration membranes to mitigate organic and bacterial fouling, Environ. Sci. Technol. 46 (2012) 4025–4033. [45] T. Laakso, M. Kallioinen, A. Pihlajamäki, M. Mänttäri, J.-E. Wong, Polyelectrolyte multilayer coated ultrafiltration membranes for wood extract fractionation, Sep. Purif. Technol. 156 (Part 2) (2015) 772–779. [46] Y. Hanafi, A. Szymczyk, M. Rabiller-Baudry, K. Baddari, Degradation of poly (ether sulfone)/polyvinylpyrrolidone membranes by sodium hypochlorite: insight from advanced electrokinetic characterizations, Environ. Sci. Technol. 48 (2014) 13419–13426. [47] X. Su, Y. Tian, H. Li, C. Wang, New insights into membrane fouling based on characterization of cake sludge and bulk sludge: an especial attention to sludge aggregation, Bioresour. Technol. 128 (2013) 586–592. [48] L. Feng, X. Li, G. Du, J. Chen, Adsorption and fouling characterization of Klebsiella oxytoca to microfiltration membranes, Process Biochem. 44 (2009) 1289–1292. [49] H. Cai, H. Fan, L. Zhao, H. Hong, L. Shen, Y. He, H. Lin, J. Chen, Effects of surface charge on interfacial interactions related to membrane fouling in a submerged membrane bioreactor based on thermodynamic analysis, J. Colloid Interf. Sci. 465 (2016) 33–41.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Thermodynamic assessment of adsorptive fouling with the membranes modified via layer-by-layer self-assembly technique Liguo Shen a, Xia Cui a, Genying Yu a, Fengquan Li a, Liang Li b, Shushu Feng a, Hongjun Lin a,⇑, Jianrong Chen a,⇑ a b
College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, PR China Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 30 November 2016 Revised 8 January 2017 Accepted 16 January 2017 Available online 18 January 2017 Keywords: Membrane fouling Membrane modification Interfacial interaction Layer-by-layer assembly Membrane bioreactor
a b s t r a c t In this study, polyvinylidene fluoride (PVDF) microfiltration membrane was coated by dipping the membrane alternatingly in solutions of the polyelectrolytes (poly-diallyldimethylammonium chloride (PDADMAC) and polystyrenesulfonate (PSS)) via layer-by-layer (LBL) self-assembly technique to improve the membrane antifouling ability. Filtration experiments showed that, sludge cake layer on the coated membrane could be more easily washed off, and moreover, the remained flux ratio (RFR) of the coated membrane was obviously improved as compared with the control membrane. Characterization of the membranes showed that a polyelectrolyte layer was successfully coated on the membrane surfaces, and the hydrophilicity, surface charge and surface morphology of the coated membrane were changed. Based on the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) approaches, quantification of interfacial interactions between foulants and membranes in three different scenarios was achieved. It was revealed that there existed a repulsive energy barrier when a particle foulant adhered to membrane surface, and the enhanced electrostatic double layer (EL) interaction and energy barrier should be responsible for the improved antifouling ability of the coated membrane. This study provided a combined solution to membrane modification and interaction energy evaluation related with membrane fouling simultaneously. Ó 2017 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (H. Lin), [email protected] (J. Chen). http://dx.doi.org/10.1016/j.jcis.2017.01.051 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
1. Introduction
2. Materials and methods
Membrane fouling contributes to enormous cost, and hence becomes the bottleneck for the large-scale industrial application of membrane separation technology [1–3]. In order to attenuate the membrane fouling problem, significant efforts have been executed to fabricate excellent performance membranes [3–5]. The approaches which have been explored include optimizing the membrane pore or surface structures by controlling the membrane preparation conditions [6,7], chemical grafting of hydrophilic functional monomers [8,9], and organic-inorganic or organic-organic blending [10,11]. Among them, layer-by-layer (LBL) self-assembly technique has been frequently used to coat advantageous polyelectrolytes on membrane surface for membrane fouling mitigation [12–14] due to its high efficiency and the ability to regulate the coated layer structure and composition [15,16]. It was reported that, the electronegative and hydrophilic polyelectrolyte networks could be easily coated on membrane surfaces by this technique [17,18]. For example, Su et al. [19] prepared a polyelectrolyte ultrafiltration (UF) membrane with the materials of poly(acrylonitrile and 2-dimethylaminoethyl methacrylate) (PAN-DMAEMA). The modified membrane showed excellent antifouling property to bovine serum albumin (BSA), and lysozyme in certain range of pH and ionic strength. Zhao et al. [20] prepared polyvinylidene fluoride (PVDF) UF membranes by assembling chitosan (CS) and sodium alginate (SA) polyelectrolytes. Under the optimized experimental conditions, BSA adsorption on the membrane surface is only 4 lg cm2 and the flux recovery ratio after washing reaches to 89%, 99% and 98% for the three typical pollutants of BSA, SA and humic acid (HA), respectively. Ilyas et al. [21] modified the UF membrane by alternately assembling Poly(allylamine hydrochloride) (PAH) and poly (acrylic acid) (PAA). The modified membranes were much easily recovered by washing after fouled. These results strongly suggested that membrane surface properties could be tailored by coating different polyelectrolytes with LBL self-assembly technique, which is flexible for membrane modification to meet various demands. The polyelectrolytes containing ASO 3 functional group have been proved to be an efficient hydrophilic and negatively charged function group in lots of literature studies [22–24]. While the experimental results in literature confirmed the improved antifouling ability of the coated membranes, its underlying causes have not been well investigated. A better understanding of mechanisms regarding antifouling ability is essential to optimize LBL self-assembly technique as well as prepare membranes with high antifouling property. Previous studies generally reported that the reduced fouling of the coated membrane was mainly caused by the reduced adsorption of foulants on membrane surface [19–21,25]. Meanwhile, it is widely accepted that thermodynamic/interfacial interactions between foulants and membrane surface are critical predictors for the susceptibility of a membrane to foulants adsorption [26–28]. Interfacial interactions between two solid substances in water can be described by the extended Derjaguin–Landau–Ver wey–Overbeek (XDLVO) theory [29–31]. In this context, XDLVO theory may provide a probable way to explore the underlying mechanisms of the improved antifouling performance of the coated membranes by LBL self-assembly technique. Nevertheless, no specific study has been conducted to investigate this issue for LBL self-assembly technique. The objective of this study is, therefore, to modify PVDF membrane by coating PDADMAC and PSS polyelectrolytes alternatively through LBL self-assembly technique, and to assess the interfacial interactions between foulants and the modified PVDF membrane. The interfacial mechanisms for the antifouling property of the coated membranes were investigated based on XDLVO theory and its extensions.
2.1. Modification of membranes
195
The PVDF membrane (Pore size 0.1 lm, Jiangsu Dafu Co. Ltd.) was alternately immersed into polyelectrolyte solutions for LBL assembling. It is known that PDADMAC and PSS are the typical positively charged and negatively charged polyelectrolytes, respectively. Since the PVDF membrane was negatively charged (21.19 ± 1.67 mV) in this study, the assembling was initiated by the positively charged PDADMAC solution (1.00 g L1, MW = 100,000–200,000 Da, Aldrich Chemical Co. Inc). The PVDF membranes were immersed into the PDADMAC solution for 1 min to obtain the positively charged PVDF-PDADMAC membranes. After 5 min rinsing by water to remove the superfluous PDADMAC, the positively charged PVDF-PDADMAC membrane was immersed into the negatively charged PSS solution (1.00 g L1, MW = 70,000 Da, Aldrich Chemical Co.) for 1 min to obtain the negatively charged PVDF-PDADMAC-PSS membrane. By the electrostatic interactions and van der Waals forces, PSS was adhered onto the surface to form the negatively charged PVDF-PDADMAC-PSS membrane. Another rinsing by water was lasted for 5 min to remove the superfluous PSS. To minimize the flux decline, the number of bilayers was limited to 1 via the reported strategy [32]. In this study, the negatively charged membrane was alternately assembled with positively charged PDADMAC and then negatively charged PSS. The coated PDADMAC together with PSS means one bilayer. The negatively charged PVDF-PDADMAC-PSS membrane was then used for characterization, filtration, and antifouling experiments. The coating process is shown in Fig. 1. 2.2. Analytical methods Surface of the membranes was measured using a scanning electron microscope (LEO1530vp SEM (Germany)). Previous to observation, samples were attached by carbon tape to the sample stage and sputtered with gold. The voltage was set at 25 kV, and the current was set at 10 mA. Chemical components of the membranes were measured by an Energy dispersive X-ray spectroscopy (7426 EDX OXFORD). The measurement voltage was 20 kV. A commercially available atomic force microscopy (AFM) instrument (Nanoscope 8, Bruker) equipped with a J scanner was used to observe the morphology of the membrane surface. AFM imaging of the membranes was conducted in tapping mode with silicon nitride cantilevers (NP-S, Bruker). The contact angles of the sludge foulant and membrane samples were measured by a contact angle meter (Kino industry Co., Ltb, USA). Ultrapure water, glycerol and diiodomethane were chosen as the probe liquids in the experiments. A drop (5 lL) was lowered onto the membrane’s surface from a needle tip. A magnified image of the droplet was recorded by a digital camera. Static contact angles were determined from these images with a calculation software. The contact-angle measurement was taken as the mean value of 5 different points on each membrane. For measurement of the contact angles on the sludge foulant surface, the sludge liquor obtained from a membrane bioreactor (MBR) was firstly filtered through the membrane to form a cake layer. The formed cake layer was fixed within two glass slides to form a relatively flat surface, and then kept in an oven for 24 h at 40 °C to get rid of water. The foulant samples were used for contact angle measurement. The zeta potential of the membrane was analyzed by a zeta potential analyzer (Zeta 90 Plus, Brookhaven, UK). The zeta potential of the sludge foulants was determined by a microelectrophoresis (Zetasizer Nano ZS, Malvern, UK). The measurement method
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
PVDF membrane
196
PDADMAC
PSS bilayer
PVDF membrane
+ PDADMAC →
positively charged PVDFnegatively charged PVDF+ PSS → PDADMAC membrane PDADMAC-PSS membrane
Fig. 1. The schematic illustration of the coating process.
referred to Idil Mouhoumed et al. [33]. In the experiments, an electrolyte solution (103 mol L1) was prepared by KCl (Aldrich Chemical) and deionized water (18 MX cm). The solution pH was adjusted to 7.0 with 0.1 mol L1 HCl and KOH solutions (Aldrich Chemical) via automatic titrating system. Each sample was measured for three times to obtain the average value. The particle size distribution (PSD) of the sludge liquor sampled from the MBR treating synthetic municipal wastewater was measured by a Mastersizer 2000 Laser particle size analyzer (Malvern, UK). An analysis of variance (ANOVA) was used to test for difference between treatment means. The tests were performed by using the software of Statistical Product and Service Solutions (SPSS) V18.0.
polymeric substances-rough membrane surface, particle-smooth membrane surface and particle-rough membrane surface, were considered (Fig. 2). For the scenario of two infinite planar surfaces, XDLVO approach enables to calculate LW, AB and EL interaction energy per unit area (specific energy, mJ m2) at separation distance (h) (DGLW(h), DGAB(h) and DGEL(h)) by using the following equations [29]:
2.3. Filtration tests
DGEL ðhÞ ¼ jfm ff er e0
For lab-scale studies, it is a general operation to conduct shortterm filtration tests for assessment of membrane performance. The performance of the prepared membranes was analyzed through a dead-end filtration system. The valid membrane area in this system is 0.002 m2, and the efficient volume of the filtration cell (MSC300, Mosu Corp., Shanghai, China) is 325 ml. The pressure of the measurement is 20 kPa. The antifouling property was measured by recording the flux decline process during continuous filtration of the sludge liquor with a concentration of 2.68 g L1 mixed liquid suspended solids (MLSS). The sludge liquor was obtained from the MBR treating synthetic wastewater. Detailed information regarding the operation of MBR, and the sludge properties can refer to our previous study [34]. The susceptibility of a membrane to foulant adhesion can be directly reflected by the remained sludge amount on membrane surface after a washing process. A cake layer was formed on the membrane surface by filtering 200 ml 2.68 gMLSS L1 sludge liquor in the filtration cell. Thereafter, washing process was performed by filling filtration cell (where the cake layer formed) with 100 ml tap water, followed with shaking the filtration cell, and discarding the filling water. The washing process was repeated for 3 times for each membrane. While the shaking operation is difficult to quantify, the washing process was identical for both of the control and coated membranes. 2.4. Methods to calculate interfacial interactions According to XDLVO theory, the noncovalent interfacial interactions between two surfaces in a medium include three kind of interactions: Lifshitz-Van der waals (LW), acid-based (AB), and electrostatic double layer (EL) interactions [29]. In order to provide a comprehensive comparison between the control and coated membranes, herein, three interaction scenarios, including dispersed
2
DGLW ðhÞ ¼ DGLW h0
h0 h
ð1Þ
2
DGAB ðhÞ ¼ DGAB h0 exp
h0 h k
ð2Þ
! f2m þ f2f 1 ð1 coth jhÞ þ sinh jh 2fm ff
ð3Þ
Two planar surfaces are assumed to contact each other at a minAB imum separation distance (h0 = 0.158 nm) [35]. DGLW h0 , DGh0 and
DGEL h0 mean the specific LW, AB and EL energy between two planar surfaces at separation distance of h0, respectively, which can be obtained through Eqs. (4)–(6), respectively [29]. qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiqffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi ð4Þ DGLW cLW cLW cLW cLW m w w h0 ¼ 2 f
hpffiffiffiffiffiffipffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffiqffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi DGAB cþw cf þ cm cw þ cw cþf þ cþm cþw h0 ¼ 2 qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi i cf cþm cþf cm
DGEL h0
¼
e0 er j 2
n2m
þ
n2f
"
1 cothðjh0 Þ þ
2nm nf n2m þ n2f
# cschðjh0 Þ
ð6Þ
where cLW, c+and c are the LW, electron donor and electron acceptor surface tension components of a substance (the subscripts m, f and w denote membrane, foulant and water, respectively), respectively. These surface tension components of a solid substance can be calculated by solving a set of three Young’s equations (Eq. (7)), which requires to measure contact angle (/) of three probe liquids (ultrapure water, glycerol and diiodomethane used in this study) on the substance surface.
ð1 þ cos /Þ tol cl ¼ 2
qffiffiffiffiffiffiffiffiqffiffiffiffiffiffiffiffi
cLW cLW þ s l
ffi ffipffiffiffiffiffi pffiffiffiffiffi þ
cl
cs þ
qffiffiffiffiffiffipffiffiffiffiffiffi
cþl cs
ð7Þ
For the scenario of particle-smooth membrane surface system, AB EL the interfacial interactions of U LW fwm ðDÞ; U fwm ðDÞ; U fwm ðDÞ (unit: kT) can be described by the Derjaguin approximation (DA) method [36,37]:
197
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
(a)
(b)
dispersed polymeric substances
rough membrane
(c)
sludge floc
sludge floc
interaction
interaction
smooth membrane
rough membrane
Fig. 2. Schematics of the interaction scenarios of (a) dispersed polymeric substances-rough membrane surface, (b) particle foulants-smooth membrane surface, and (c) particle foulants-rough membrane surface.
LW U LW fwm ðDÞ ¼ 2pDGh0
2
h0 R D
AB U AB fwm ðDÞ ¼ 2pRDDGh0 exp
ð8Þ
h0 D k
ð9Þ
1 þ ejD 2 2 2jD U EL þ n Þ þ n lnð1 e m f fwm ðDÞ ¼ per e0 R 2nm nf ln 1 ejD
the interval [a, b] of variable x and the interval [c, d] of variable y, respectively. Herein, h = (b a)/2m, and k = (d c)/2n. Defining fi,j to be the function value of f(xi, yj), the double integral could be obtained as follows:
Z
b
Z
d
f ðx; yÞdxdy ¼ a
c
i¼1 j¼1
ð10Þ where D is the closet distance between a foulant particle and membrane surface; R means the particle radius; the subscript ‘‘fwm” represents the interaction between foulant particles and membrane surfaces in water. For the scenario of particle-rough membrane surface system, a combined method developed by Lin et al. [38] was adopted to calculate the interfacial interactions:
Z 2p Z 0
R
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DGLW D þ R þ z R2 r2 f ðr; hÞ rdrdh
0
ð11Þ U AB fwm ðDÞ ¼
Z 2p Z 0
R
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DGAB D þ R þ z R2 r 2 f ðr; hÞ rdrdh
0
ð12Þ U EL fwm ðDÞ ¼
Z 2p Z 0
R
x2i2
Z
y2j
f ðx; yÞdxdy
y2j2
m X n hk X ðf þ f 2i;2j2 þ f 2i;2j 9 i¼1 j¼1 2i2;2j2
þ f 2i2;2j1 Þ þ 16f 2i1;2j1
ð15Þ
Generally, the greater the value of m and n, the higher the accuracy of the computation. Although the composite Simpson’s rule is an approximate method, it is possible to get results with high accuracy through setting high value of m and n. Detailed calculations showed that, setting m = n = 1600 or above could make the calculation error negligible. However, such an operation would result in an enormous computation amount such that the help of computers has to be sought. MATLAB software was adopted to conduct the calculations in this study. 3. Results and discussion 3.1. Filtration performance
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DGEL D þ R þ z R2 r 2 f ðr; hÞ rdrdh
0
ð13Þ where, z is the roughness of membrane surface; r is the radius of differential circular ring on particle surface; dh is the differential angle of the differential circular arc in the circular ring; f (r, h) is local amplitude directly below the circular arc as a function of the position of the differential circular arc defined by r and h.f (r, h) can be also viewed as the function of membrane surface morphology. Basically, the randomness of surface roughness precludes a rigorous mathematical model describing membrane surface morphology [39]. However, it is possible to propose a hypothetical surface topology that represents some pertinent statistical properties of the rough membrane surface. According to literature [38,40,41], the following function was adopted for the hypothetical membrane surface morphology:
f ðr; hÞ ¼ z sinðpr cos h=2z þ uÞ
x2i
þ f 2i2;2j Þ þ 4ðf 2i1;2j2 þ f 2i;2j1 þ f 2i1;2j
ð14Þ
where u is a phase shift of the sine function (assumed to be zero for simplicity in this study). The two integrals in Eqs. (11)–(13) were estimated through composite Simpson’s rule. The details can refer to Lin et al. [38]. The composite Simpson’s rule was applied to numerically estimate the double integrals in Eqs. (11)–(13). As for double integral, certain point x1 = a, xi = x1 + ih (i = 1, 2, . . ., 2m + 1) and y1 = b, yj = y1 + jk (j = 1, 2, . . ., 2n + 1) were used to subdivide
Both the control and coated membranes are subjected to filtration tests under the identical experimental conditions. Fig. 3 shows the changes of remained membrane flux ratio (RFR) with operational time for the control membrane and coated membrane when they were used to filter the sludge liquor. Herein, RFR, which
100 Control Coated
90
Remained Flux Ratio (%)
U LW fwm ðDÞ ¼
m X n Z X
80 70 60 50 40 30
0
20
40
60
80
100
Time (min) Fig. 3. Variation of remained membrane flux ratio (RFR) with operational time when the sludge liquor was filtrated (MLSS = 2.68 g L1, pressure = 20 kPa).
198
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
Fig. 4. The illustration of the fouled membrane surface (the left is the surface of the control membrane after washing; the right is the surface of the coated membrane after washing).
Fig. 5. The SEM images of membrane surface: (a), (b) and (c) the images of the control membrane under different amplifications; (d), (e) and (f) the images of the coated membrane under different amplifications.
means the ratio of the instantaneous flux to the initial water flux, was used to assess the antifouling performance. This parameter can exclude other effects, and better reflect the differences in the
whole filtration process between the two membranes [42,43]. Under the same conditions in this study, the initial water flux for the control and coated membranes are 0.133 kg m2 s1and
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
0.087 kg m2 s1, respectively. As shown in Fig. 3, the two membranes have the similar profile which can be characterized by a rapid flux decline followed by a transition to a plain state. This suggests that both control and coated membranes undergo fouling during the filtration process. However, the RFR of the coated membrane is obviously higher than that of the control membrane, indicating the enhanced antifouling performance of the coated membrane. It should be noted that, comparison of the long-term filtration performance would provide more convincing results, and this will be conducted in near future. To evaluate the adhesion of sludge foulants on the membrane surface, filtration tests were carried out by formation of a cake layer on the membrane surface through filtration of 200 ml sludge solution, followed by the identical washing process (referring to the methodology section). The sludge cake layer on the control membrane surface stayed firmly, while the cake layer on the coated membrane fell off completely (Fig. 4). This result clearly shows the much higher interaction strength between the sludge foulants and the control membrane than that between the sludge foulants and the coated membrane. 3.2. Characterization of the membranes Fig. 5 shows the SEM images of the surface morphology of the control and coated membranes. By comparing the images, it can be found that a smooth layer was successfully coated on the membrane surfaces, and moreover, certain amounts of pores disappeared on the coated membrane surface. This implicated that coating of polyelectrolytes could block the pores to some extent. This phenomenon has been also observed in previous studies [44,45]. Therefore, in order to modify membrane surface properties
199
and simultaneously avoid the severe clogging of pores, only one PDADMAC/PSS bilayer rather than multilayers was coated on the membrane surface in this study. Fig. 5 also indicates that the polyelectrolytes are rather evenly coated on the membrane surface although the coated layer is thin. The EDX spectra of the control membrane in Fig. 6(a) indicate that there is no signal of S element. In contrast, while signal of S element is weak due to thin coated layer, EDX spectra of the coated membrane in Fig. 6(b) clearly show existence of S element. Since EDX spectra have provided convincing information, X-rays photoelectron spectroscopy (XPS) analysis has not been conducted although it may provide more significant signal for existence of S element. S element should come from the ASO 3 function group in PSS, indicating the success of coating PSS on membrane surface. Assembling with ASO 3 function groups on membrane surface would improve surface hydrophilicity, but may also cause Carelated fouling. Fig. 7 shows the AFM images of the control and coated membranes. The enormous fluctuations could be found for both control and coated membranes. The roughness values obtained by the AFM analysis (dimension: 10 lm 10 lm) are 38.5 nm and 37.2 nm for control and coated membranes, respectively (Table 1). Although the difference in average roughness is small, ANOVA analysis shows that the difference is statistically significant (p < 0.05). It is, therefore, evident that, the surfaces of the both membranes are significantly rough, and the coated membrane surface is slightly smoother than the control membrane surface. The surface properties in terms of zeta potential and contact angle of the control, coated membrane and sludge layer are characterized, and the results are listed in Table 1. As compared with the control membrane, the coated membrane possesses
Fig. 6. The EDX spectra of the membrane surface composite: (a) spectra for the control membrane; (b) spectra for the coated membrane.
200
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
Fig. 7. The AFM images of membrane surface: (a) and (b) images of the control membrane from different views; (c) and (d) images of the coated membrane from different views.
Table 1 The surface properties of the control, coated membranes and sludge foulants. Substances
Roughness (nm)
Zeta potential (mV)
Control membrane Coated membrane Sludge foulants
38.5 ± 0.3 37.2 ± 0.6 –
21.19 ± 1.67 27.61 ± 1.45 23.37 ± 3.40
higher absolute value of zeta potential and lower contact angle of three probe liquors. These results are reasonable as considering that the plentiful SO 3 groups coated on the membrane surface facilitate to the improvement of the negative charge and hydrophilicity [46]. However, the differences in zeta potential and contact angles between the coated and control membrane are not great enough. This phenomenon may be attributed to the fact that some PDADMAC groups are not covered by PSS, and the thin coated bilayer. It should be noted that, although the differences in zeta potential and contact angles are not great enough, they are statistically significant (based on one way analysis of
Contact angle (°) Water
Glycerol
Diiodomethane
75.45 ± 5.86 71.71 ± 2.68 67.56 ± 1.47
69.54 ± 3.47 63.34 ± 3.21 76.11 ± 0.43
46.06 ± 6.07 28.01 ± 2.01 42.86 ± 8.86
variance (ANOVA) method). As the sludge foulants were also negatively charged (23.37 ± 3.40 mV), the more negative charge and more hydrophilic surface of the coated membrane would certainly improve antifouling property of membrane. The control membrane was characterized to possess an average roughness of 38.5 nm. The surface properties of the sludge foulants were also characterized. It can be seen that the results are comparable with those of sludge foulants in literature [47,48], indicating the representativeness of the sludge foulants for membrane fouling study. All of these data were then used for calculation of interfacial interactions between the membranes and sludge foulants.
201
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
tively, showing 36.2% improvement in DGEL h0 under conditions in this study. The improved EL interaction is expected to provide enhanced repulsive force against foulants adhesion, improving the antifouling ability. It should be noted that real membrane surface is rough, and the scenario of two infinite planar surfaces is atypical in reality. However, the soluble microbial products (SMPs), usually as the abundant foulants in wastewater treatment systems, are chainlike substances, which can flex to fit membrane surface [49]. Therefore, the interactions between SMPs and rough membrane surface can be regarded as the interactions between two planar surfaces. Other researchers also observed the reduced adsorption of chain molecular foulants like BSA and humic acids to the membranes coated by LBL assembly technique [19,20,25]. They simply attributed the reduced fouling to the modified surface charge but didn’t provide a quantitative analysis. This study numerically calculated the interaction energy between foulants and membrane surface, thus not only provided approaches to quantitatively assess interaction energy, but also gave a more plausible explanation to the reduced fouling of the membranes coated by LBL assembly technique. The interaction profiles in the scenario of particle-smooth membrane combination for the control and coated membranes are shown in Fig. 9. For both membranes, different property and distribution of the individual interaction component result in a distinct distribution of total interaction versus separation distance. There apparently exists an energy barrier when a foulant particle closes to membrane surface. This means that the eventual adhesion of a foulant particle on membrane surface should overcome a repulsive energy barrier. Obviously, the higher the energy barrier is, the more difficult the adhesion of a particle becomes. It can be seen from Fig. 9, the energy barrier for the coated membrane is 2426 kT, significantly higher than the value of 1843 kT for the control membrane in this study. Therefore, the interaction energy analysis can explain the reduced fouling of the coated membrane. Fig. 10 shows the interaction profiles in the scenario of particlerough membrane combination for control and coated membranes. The profiles are quite similar to the profiles shown in Fig. 9. However, the interaction strength for rough membrane surface is only about tenth of the strength for smooth membrane surface. This indicates membrane roughness would significantly reduce the interaction strength. For both membranes, an energy barrier also exists although its strength significantly decreases as compared with that in Fig. 9. The coated membrane possesses higher energy barrier (280.5 kT) than the control membrane (217.5 kT). It should be noted that the comparison is conducted based on the assumption of same membrane surface topography. Actually, as shown in Section 3.2, the LBL assembly technique would yield relatively
6
Energy per unit area (mJ m-2) Energy per unit area (mJ m-2)
brane counterpart. This is particularly apparent that DGEL h0 for the coated and control membranes is 2.86 and 2.10 mJ m2, respec-
(b)
control membrane 4 EL interaction
2 0
total interaction -2
AB interaction
LW interaction
-4 -6 6 Separation distance (nm)
4
coated membrane
EL interaction
2 0
total interaction -2
AB interaction
LW interaction
-4 -6 Separation distance (nm)
Fig. 8. The profiles of specific interactions between membrane and sludge foulants as a function of the separation distance in the scenario of two infinite planar surfaces for (a) the control membrane, and (b) the coated membrane.
(a)
4000
control membrane
3000
Interaction energy (kT)
It is widely accepted that interfacial interactions are the critical predictors for the susceptibility of a membrane to foulant adhesion [26–28]. Fig. 8 shows the profiles of interactions between the membrane and sludge foulants in the scenario of two infinite planar surfaces for control and coated membranes. The two membranes show the similar profile. LW and AB interactions are attractive while EL interaction is repulsive. Moreover, the total interaction energy is positive at large separation distances. This phenonmenon is reasonable for the interaction scenario of two infinite planar surfaces because the EL interaction is positive and plays a predominant role over the AB interaction and LW interaction at large separation distances. The main difference between the two profiles is the significance of EL interaction. The coated membrane-sludge foulants combination shows higher EL interactions than the control mem-
(a)
total interaction
2000 1000
EL interaction
energy barrier
0
LW interaction
-1000 -2000
AB interaction
-3000 4000 (b) -4000 3000
Interaction energy (kT)
3.3. Underlying mechanisms of the reduced fouling
total interaction
2000
coated membrane EL interaction
energy barrier
1000 0
LW interaction
-1000 -2000
AB interaction
-3000 -4000 0
2
4
6 8 10 Separation distance (nm)
12
14
16
Fig. 9. The profiles of specific interactions between membrane and sludge foulants as a function of the separation distance in the scenario of particle foulants-smooth membrane surface for (a) the control membrane, and (b) the coated membrane.
smooth membrane surface. Considering this effect, the difference in the energy barrier for the two membranes would be more signif-
202
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
(a)
EL interaction
Interaction energy (kT)
300
References
200
energy barrier
100 0 -100
LW interaction
-200 AB interaction
-400 400
EL interaction
300
Interaction energy (kT)
control membrane
total interaction
-300
(b)
(2016C31G2030032) and Natural Science Foundation of Zhejiang Province (No. LQ16B060001) is highly appreciated.
400
coated membrane
total interaction
200
energy barrier
100 0
-100 LW interaction
-200 AB interaction
-300 -400 0
2
4
6 8 10 12 Separation distance (nm)
14
16
Fig. 10. The profiles of specific interactions between membrane and sludge foulants as a function of the separation distance in the scenario of particle foulants-rough membrane surface for (a) the control membrane, and (b) the coated membrane.
icant. Nevertheless, the higher repulsive energy barrier for the coated membrane would make the particle foulants more difficult to adhere to membrane surface, mitigating membrane fouling. From viewpoint of interaction energy and energy barrier, the reduced fouling of the coated membrane in the experiments could be well explained. 4. Conclusion In this study, PVDF microfiltration membrane was coated by a polyelectrolyte layer- PDADMAC and PSS via the LBL selfassembly technique to mitigate membrane fouling. It was found that, sludge cake layer on the coated membrane could be more easily washed off, and moreover, the RFR of the coated membrane was obviously improved as compared with the control membrane. SEM and EDX analysis suggested that polyelectrolyte layer was successfully coated on the membrane surfaces. The coated membrane possessed an advantageous surface morphology. Due to the abundant SO 3 group in polyelectrolyte layer, the hydrophilicity and negative charge of the coated membrane was enhanced. XDLVO analysis showed existence of a repulsive energy barrier when a foulant particle closed to membrane surface. The enhanced EL interaction and energy barrier should be responsible for the improved antifouling ability of the coated membrane. These results suggested the potential to tailor surface properties and to optimize antifouling ability of membranes by LBL self-assembly technique combined with series of analytical methods provided in this study. Acknowledgements Financial support of National Natural Science Foundation of China (Nos. 21506195, 51578509), Public Welfare Project of the Science and Technology Department of Zhejiang Province
[1] X. Wang, V.W.C. Chang, C.Y. Tang, Osmotic membrane bioreactor (OMBR) technology for wastewater treatment and reclamation: advances, challenges, and prospects for the future, J. Membr. Sci. 504 (2016) 113–132. [2] Z. Wang, J. Ma, C.Y. Tang, K. Kimura, Q. Wang, X. Han, Membrane cleaning in membrane bioreactors: a review, J. Membr. Sci. 468 (2014) 276–307. [3] F. Meng, S.-R. Chae, A. Drews, M. Kraume, H.-S. Shin, F. Yang, Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material, Water Res. 43 (2009) 1489–1512. [4] B. Van der Bruggen, Chemical modification of polyethersulfone nanofiltration membranes: a review, J. Appl. Polym. Sci. 114 (2009) 630–642. [5] M. Amirilargani, M. Sadrzadeh, E.J.R. Sudhölter, L.C.P.M. de Smet, Surface modification methods of organic solvent nanofiltration membranes, Chem. Eng. J. 289 (2016) 562–582. [6] K.-J. Hwang, C.-Y. Liao, K.-L. Tung, Effect of membrane pore size on the particle fouling in membrane filtration, Desalination 234 (2008) 16–23. [7] K. Riedl, B. Girard, R.W. Lencki, Influence of membrane structure on fouling layer morphology during apple juice clarification, J. Membr. Sci. 139 (1998) 155–166. [8] A. Mehrparvar, A. Rahimpour, Surface modification of novel polyether sulfone amide (PESA) ultrafiltration membranes by grafting hydrophilic monomers, J. Ind. Eng. Chem. 28 (2015) 359–368. [9] Y. Hu, K. Lu, F. Yan, Y. Shi, P. Yu, S. Yu, S. Li, C. Gao, Enhancing the performance of aromatic polyamide reverse osmosis membrane by surface modification via covalent attachment of polyvinyl alcohol (PVA), J. Membr. Sci. 501 (2016) 209– 219. [10] M. Safarpour, A. Khataee, V. Vatanpour, Preparation of a novel polyvinylidene fluoride (PVDF) ultrafiltration membrane modified with reduced graphene oxide/titanium dioxide (TiO2) nanocomposite with enhanced hydrophilicity and antifouling properties, Ind. Eng. Chem. Res. 53 (2014) 13370–13382. [11] L. Shen, X. Bian, X. Lu, L. Shi, Z. Liu, L. Chen, Z. Hou, K. Fan, Preparation and characterization of ZnO/polyethersulfone (PES) hybrid membranes, Desalination 293 (2012) 21–29. [12] G.-R. Xu, S.-H. Wang, H.-L. Zhao, S.-B. Wu, J.-M. Xu, L. Li, X.-Y. Liu, Layer-bylayer (LBL) assembly technology as promising strategy for tailoring pressuredriven desalination membranes, J. Membr. Sci. 493 (2015) 428–443. [13] T. Ishigami, K. Amano, A. Fujii, Y. Ohmukai, E. Kamio, T. Maruyama, H. Matsuyama, Fouling reduction of reverse osmosis membrane by surface modification via layer-by-layer assembly, Sep. Purif. Technol. 99 (2012) 1–7. [14] P. Kaner, D.J. Johnson, E. Seker, N. Hilal, S.A. Altinkaya, Layer-by-layer surface modification of polyethersulfone membranes using polyelectrolytes and AgCl/ TiO2 xerogels, J. Membr. Sci. 493 (2015) 807–819. [15] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (1997) 1232–1237. [16] F. Caruso, H. Lichtenfeld, M. Giersig, H. Möhwald, Electrostatic self-assembly of silica nanoparticle-polyelectrolyte multilayers on polystyrene latex particles, J. Am. Chem. Soc. 120 (1998) 8523–8524. [17] J. Bhattacharjee, S. Banik, S.A. Hussain, D. Bhattacharjee, A study on the interactions of cationic porphyrin with nano clay platelets in Layer-by-Layer (LbL) self assembled films, Chem. Phys. Lett. 633 (2015) 82–88. [18] N. Ghavidel Mehr, C.D. Hoemann, B.D. Favis, Chitosan surface modification of fully interconnected 3D porous poly(e-caprolactone) by the LbL approach, Polymer 64 (2015) 112–121. [19] Y.L. Su, C.X. Mu, C. Li, Z.Y. Jiang, Antifouling property of a weak polyelectrolyte membrane based on poly(acrylonitrile) during protein ultrafiltration, Ind. Eng. Chem. Res. 48 (2009) 3136–3141. [20] X.Z. Zhao, A.W. Qin, D.P. Liu, C.J. He, Tuning the antifouling property of PVDF ultrafiltration membrane with surface anchored polyelectrolyte complexes for sewage treatment, RSC Adv. 5 (2015) 63580–63587. [21] S. Ilyas, J. de Grooth, K. Nijmeijer, W.M. de Vos, Multifunctional polyelectrolyte multilayers as nanofiltration membranes and as sacrificial layers for easy membrane cleaning, J. Colloid Interf. Sci. 446 (2015) 386–393. [22] M. Kumar, M. Ulbricht, Low fouling negatively charged hybrid ultrafiltration membranes for protein separation from sulfonated poly(arylene ether sulfone) block copolymer and functionalized multiwalled carbon nanotubes, Sep. Purif. Technol. 127 (2014) 181–191. [23] B.P. Tripathi, N.C. Dubey, M. Stamm, Polyethylene glycol cross-linked sulfonated polyethersulfone based filtration membranes with improved antifouling tendency, J. Membr. Sci. 453 (2014) 263–274. [24] D. Song, J. Xu, Y. Fu, L. Xu, B. Shan, Polysulfone/sulfonated polysulfone alloy membranes with an improved performance in processing mariculture wastewater, Chem. Eng. J. 304 (2016) 882–889. [25] C.Y. Ba, D.A. Ladner, J. Economy, Using polyelectrolyte coatings to improve fouling resistance of a positively charged nanofiltration membrane, J. Membr. Sci. 347 (2010) 250–259. [26] Q. Wang, Z. Wang, C. Zhu, X. Mei, Z. Wu, Assessment of SMP fouling by foulant–membrane interaction energy analysis, J. Membr. Sci. 446 (2013) 154– 163. [27] L.-M. Whang, Y.-J. Wu, Y.-C. Lee, H.-W. Chen, T. Fukushima, M.-Y. Chang, S.-S. Cheng, S.-F. Hsu, C.-H. Chang, W. Shen, C.K. Huang, R. Fu, B. Chang, Nitrification performance and microbial ecology of nitrifying bacteria in a full-scale
L. Shen et al. / Journal of Colloid and Interface Science 494 (2017) 194–203
[28]
[29] [30]
[31]
[32]
[33]
[34]
[35]
[36]
[37] [38]
membrane bioreactor treating TFT-LCD wastewater, Bioresour. Technol. 122 (2012) 70–77. H. Hong, W. Peng, M. Zhang, J. Chen, Y. He, F. Wang, X. Weng, H. Yu, H. Lin, Thermodynamic analysis of membrane fouling in a submerged membrane bioreactor and its implications, Bioresour. Technol. 146 (2013) 7–14. C.J. van Oss, Hydrophobicity of biosurfaces – origin, quantitative determination and interaction energies, Colloids Surf. B 5 (1995) 91–110. C.J. van Oss, Long-range and short-range mechanisms of hydrophobic attraction and hydrophilic repulsion in specific and aspecific interactions, J. Mol. Recognit. 16 (2003) 177–190. V.T. Nguyen, T.W.R. Chia, M.S. Turner, N. Fegan, G.A. Dykes, Quantification of acid–base interactions based on contact angle measurement allows XDLVO predictions to attachment of Campylobacter jejuni but not Salmonella, J. Microbiol. Meth. 86 (2011) 89–96. F. Diagne, R. Malaisamy, V. Boddie, R.D. Holbrook, B. Eribo, K.L. Jones, Polyelectrolyte and silver nanoparticle modification of microfiltration membranes to mitigate organic and bacterial fouling, Environ. Sci. Technol. 46 (2012) 4025–4033. E. Idil Mouhoumed, A. Szymczyk, A. Schäfer, L. Paugam, Y.H. La, Physicochemical characterization of polyamide NF/RO membranes: insight from streaming current measurements, J. Membr. Sci. 461 (2014) 130–138. L.-G. Shen, Q. Lei, J.-R. Chen, H.-C. Hong, Y.-M. He, H.-J. Lin, Membrane fouling in a submerged membrane bioreactor: impacts of floc size, Chem. Eng. J. 269 (2015) 328–334. J.M. Meinders, H.C. van der Mei, H.J. Busscher, Deposition efficiency and reversibility of bacterial adhesion under flow, J. Colloid Interf. Sci. 176 (1995) 329–341. S. Bhattacharjee, M. Elimelech, Surface element integration: a novel technique for evaluation of DLVO interaction between a particle and a flat plate, J. Colloid Interf. Sci. 193 (1997) 273–285. E.M.V. Hoek, G.K. Agarwal, Extended DLVO interactions between spherical particles and rough surfaces, J. Colloid Interf. Sci. 298 (2006) 50–58. H. Lin, M. Zhang, R. Mei, J. Chen, H. Hong, A novel approach for quantitative evaluation of the physicochemical interactions between rough membrane surface and sludge foulants in a submerged membrane bioreactor, Bioresour. Technol. 171 (2014) 247–252.
203
[39] E.M.V. Hoek, S. Bhattacharjee, M. Elimelech, Effect of membrane surface roughness on colloidmembrane DLVO interactions, Langmuir 19 (2003) 4836–4847. [40] J. Verran, A. Packer, P. Kelly, K.A. Whitehead, The retention of bacteria on hygienic surfaces presenting scratches of microbial dimensions, Lett. Appl. Microbiol. 50 (2010) 258–263. [41] K.A. Whitehead, J. Verran, The effect of surface topography on the retention of microorganisms, Food Bioprod. Process. 84 (2006) 253–259. [42] A. Córdova, C. Astudillo, C. Guerrero, C. Vera, A. Illanes, Assessment of the fouling mechanisms of an ultrafiltration membrane bioreactor during synthesis of galacto-oligosaccharides: effect of the operational variables, Desalination 393 (2016) 79–89. [43] O. Sanyal, Z. Liu, J. Yu, B.M. Meharg, J.S. Hong, W. Liao, I. Lee, Designing foulingresistant clay-embedded polyelectrolyte multilayer membranes for wastewater effluent treatment, J. Membr. Sci. 512 (2016) 21–28. [44] F. Diagne, R. Malaisamy, V. Boddie, R.D. Holbrook, B. Eribo, K.L. Jones, Polyelectrolyte and silver nanoparticle modification of microfiltration membranes to mitigate organic and bacterial fouling, Environ. Sci. Technol. 46 (2012) 4025–4033. [45] T. Laakso, M. Kallioinen, A. Pihlajamäki, M. Mänttäri, J.-E. Wong, Polyelectrolyte multilayer coated ultrafiltration membranes for wood extract fractionation, Sep. Purif. Technol. 156 (Part 2) (2015) 772–779. [46] Y. Hanafi, A. Szymczyk, M. Rabiller-Baudry, K. Baddari, Degradation of poly (ether sulfone)/polyvinylpyrrolidone membranes by sodium hypochlorite: insight from advanced electrokinetic characterizations, Environ. Sci. Technol. 48 (2014) 13419–13426. [47] X. Su, Y. Tian, H. Li, C. Wang, New insights into membrane fouling based on characterization of cake sludge and bulk sludge: an especial attention to sludge aggregation, Bioresour. Technol. 128 (2013) 586–592. [48] L. Feng, X. Li, G. Du, J. Chen, Adsorption and fouling characterization of Klebsiella oxytoca to microfiltration membranes, Process Biochem. 44 (2009) 1289–1292. [49] H. Cai, H. Fan, L. Zhao, H. Hong, L. Shen, Y. He, H. Lin, J. Chen, Effects of surface charge on interfacial interactions related to membrane fouling in a submerged membrane bioreactor based on thermodynamic analysis, J. Colloid Interf. Sci. 465 (2016) 33–41.