Control of natural organic matter fouling of ultrafiltration membrane by adsorption pretreatment: Comparison of mesoporous adsorbent resin and powdered activated carbon

Control of natural organic matter fouling of ultrafiltration membrane by adsorption pretreatment: Comparison of mesoporous adsorbent resin and powdered activated carbon

Journal of Membrane Science 471 (2014) 94–102 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.c...

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Journal of Membrane Science 471 (2014) 94–102

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Control of natural organic matter fouling of ultrafiltration membrane by adsorption pretreatment: Comparison of mesoporous adsorbent resin and powdered activated carbon Kai Li, Heng Liang n, Fangshu Qu, Senlin Shao, Huarong Yu, Zheng-shuang Han, Xing Du, Guibai Li State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), Harbin Institute of Technology, 73 Huanghe Road, Nangang District, Harbin 150090, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 27 March 2014 Received in revised form 31 July 2014 Accepted 2 August 2014 Available online 11 August 2014

This paper focused on the effects of mesoporous adsorbent resin (MAR) and powdered activated carbon (PAC) pretreatments on ultrafiltration (UF) membrane fouling caused by natural organic matter (NOM). Three model foulants, humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA), were adopted to represent different NOM fractions in natural waters. Moreover, the impact of the presence of adsorbent particles in UF feed water on membrane fouling was also evaluated. The results indicated that MAR adsorption exhibited remarkable performance in alleviating HA and BSA fouling, no matter whether MAR particles were removed before UF or not. In contrast, PAC pretreatment slightly ameliorated HA fouling when PAC particles were removed before UF, whereas HA fouling was exacerbated by PAC pretreatment with PAC particles present in UF feed water. BSA fouling was moderately controlled by PAC adsorption irrespective of the presence or absence of PAC particles in UF feed water. However, neither of these two pretreatments visibly influenced SA fouling. Overall, the results obtained in the current research would provide relevant information on adsorbent selection and process design of the hybrid adsorption/UF process according to the composition and properties of NOM. & 2014 Elsevier B.V. All rights reserved.

Keywords: Membrane fouling Natural organic matter (NOM) Adsorption pretreatment Mesoporous adsorbent resin (MAR) Powdered activated carbon (PAC)

1. Introduction Membrane fouling is one of the major impediments for the widespread application of ultrafiltration (UF) membrane in water and wastewater treatment [1–3]. Extensive studies have been undertaken for more insights into UF membrane fouling, and the natural organic matter (NOM), including allochthonous humic substances and autochthonous biopolymers (mainly consisting of proteins and polysaccharides), has been generally considered as the major culprit responsible for membrane fouling [4–7]. Adsorption is an efficient technology for NOM removal, therefore it has been widely adopted as pretreatment for UF to enhance the performance of UF process [1,8–15]. However, although adsorption

Abbreviations: BSA, bovine serum albumin; CA, cellulose acetate; EOM, extracellular organic matter; HA, humic acid; MAR, mesoporous adsorbent resin; MW, molecular weight; MWCO, molecular weight cut-off; NOM, natural organic matter; PAC, powdered activated carbon; PES, polyethersulfone; SA, sodium alginate; TMP, trans-membrane pressure; UF, ultrafiltration n Corresponding author. Tel./fax: þ 86 451 86283001. E-mail addresses: [email protected] (K. Li), [email protected] (H. Liang), [email protected] (F. Qu), [email protected] (S. Shao), [email protected] (H. Yu), [email protected] (Z.-s. Han), [email protected] (X. Du), [email protected] (G. Li). http://dx.doi.org/10.1016/j.memsci.2014.08.006 0376-7388/& 2014 Elsevier B.V. All rights reserved.

pretreatment always improved the quality of product water, its impact on membrane fouling is still under debate [1,13,14]. Powdered activated carbon (PAC) is the most common type of commercially available adsorbent in water treatment, and thus it has been widely applied in hybrid adsorption/UF process [14]. It was manifested in several studies that PAC adsorbed a significant proportion of NOM and efficiently controlled the membrane fouling [9,10,12,16]. However, in some other studies, PAC adsorption was reported to exert minor influence on membrane fouling although it indeed removed some NOM [8,17]. Moreover, when PAC particles were present in UF feed water, the membrane fouling in the hybrid PAC/UF process was even found to be exacerbated in comparison with that in the individual UF process [8,15,17]. The contradictory influence of PAC on membrane fouling was generally ascribed to the diversity of NOM characteristics in feed water and/ or the membrane properties (e.g., surface hydrophobicity), but systematical studies on this subject were very limited [11,14]. Unlike PAC, mesoporous adsorbent resin (MAR) is an adsorbent specially developed for membrane fouling control [18]. It was reported that MAR significantly reduced both the reversible and irreversible fouling of 20 kDa polyethersulfone (PES) membrane while filtering lake water, although only a small amount of NOM was adsorbed by MAR [19]. Size fractionation results suggested

K. Li et al. / Journal of Membrane Science 471 (2014) 94–102

that MAR preferentially adsorbed a fraction of NOM that had an apparent MW of 20–200 kDa. Li et al. [20] found that MAR mitigated the fouling of 100 kDa PES membrane caused by algal extracellular organic matter (EOM) much more efficiently than PAC did because MAR selectively removed high-MW fraction of EOM. Besides, both of the studies demonstrated that the presence of MAR particles in UF feed water at concentrations as high as 100 mg/L would not bring negative effects on membrane fouling [19,20]. The aforementioned studies indicated that MAR might be a more promising adsorbent in membrane fouling control in comparison with PAC. But the studies with respect to MAR were restricted to the fouling of PES membrane caused by NOM in lake water and algal EOM, and the comparative studies of two types of adsorbents were limited. The performances of MAR and PAC in mitigating membrane fouling caused by diverse NOM components and the underlying mechanisms are still unclear. The main objective of this study was to obtain a comprehensive understanding of the effects of MAR and PAC pretreatments on NOM fouling of UF membrane. Because the complexity of natural water matrices made it difficult to elaborate the influence of adsorption pretreatment on membrane fouling by different NOM fractions, model foulants were employed in this study and caution should be taken when extrapolating the results obtained here to natural waters. Two types of commonly used UF membranes with different surface hydrophobicities were used in the tests. The adsorption capacities of MAR and PAC towards three foulants were investigated and UF experiments were carried out with NOM solutions before and after adsorption pretreatment. Moreover, the contribution of adsorbent particles in UF feed water to membrane fouling was also examined.

95

Table 1 Main properties of MAR and PAC. Adsorbent

Average particle size (d50, μm)

Zeta potential (mV)

BET surface area (m2/g)

Average pore size (nm)

MAR PAC

25.2 7 0.8 32.17 0.7

 22.4 71.0  23.9 70.9

1087 9 12197 13

16.4 70.5 2.2 70.1

Note: values represent average7 standard deviation, n¼ 3.

preparation could be found in reference [20]. Wood-based PAC was purchased from Bench Chemicals (Tianjin, China) and was used without further purification. Stock solutions of MAR and PAC with mass concentrations of 10 g/L were prepared and stored in refrigerator for use. The main properties of MAR and PAC are listed in Table 1. It can be seen that they had similar particle size and surface charge, but they displayed substantial difference in the pore structure characteristics. The BET surface area of MAR was less than 10% of that of PAC, while the pore size of MAR was much larger than that of PAC. Adsorption tests were conducted to characterize the adsorption of the NOM fractions onto two types of adsorbents. The concentrations of HA, BSA and SA employed in adsorption tests were 1–30 mg/L and the concentration of the adsorbent was 50 mg/L. The adsorbent was added to NOM solutions and the flasks were shaken in a rotary shaker at 120 rpm and 20 1C for 12 h. Finally, the samples were filtered with 0.45 μm mixed cellulose filters (Taoyuan, China) to remove adsorbent particles and the concentrations of HA, BSA and SA in the filtrate were measured. The retention of these NOM fractions by this filter has been proved to be negligible in preliminary tests. The adsorbed amounts of HA, BSA and SA onto the adsorbents were calculated by mass balance.

2. Materials and methods 2.3. UF membranes and experimental setup 2.1. NOM solutions Humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA) purchased from Sigma-Aldrich (USA) were used as representatives of humic substances, proteins and polysaccharides, respectively. To prepare HA stock solution, 2 g of HA was added to 800 mL of 0.01 M NaOH solution, followed by stirring for 24 h and adjusting pH of the solution to 7.0 using 1 mol/L HCl. The solution was then diluted to 1000 mL to get the HA stock solution with a concentration of 2 g/L. The stock solutions of BSA and SA were prepared by dissolving 1 g of BSA and SA in 1000 mL Milli-Q water, respectively, followed by stirring for 24 h. The stock solutions were all stored in dark at 4 1C. The stock solutions were diluted with Milli-Q water to obtain the NOM solutions used for adsorption tests and UF experiments. In order to simulate the solution chemistry of natural waters, 1 mmol/L NaHCO3, 6 mmol/L NaCl and 1 mmol/L CaCl2 were added and the pH was adjusted to 7.570.1 with 0.1 mol/L HCl and NaOH. The concentrations of HA, BSA and SA employed in UF experiments were 10, 2 and 2 mg/L, respectively. The corresponding dissolved organic carbon concentrations were 4.3870.11, 0.8170.07 and 0.7670.12 mg C/L, respectively. Unless otherwise specified, the concentrations of model foulants reported in this paper were on the basis of the mass of model foulants rather than the content of carbon. The concentrations of BSA and SA used in UF experiments were much lower than that of HA because the concentration levels of proteins and polysaccharides were usually very low in natural surface water [21,22]. 2.2. Adsorbents and adsorption tests MAR is a type of mesoporous adsorbent synthesized following the method proposed by Clark et al. [18] and detailed steps of

Two types of flat-sheet UF membranes with the same molecular weight cut-off (MWCO) of 100 kDa, i.e., a PES membrane (OM100076, Pall, USA) and a cellulose acetate (CA) membrane (PLHK07610, Millipore, USA), were used in this study. They have similar surface charge and roughness, but are apparently different in surface hydrophobicity [23]. The contact angles of the PES and the CA membrane were 58.21 and 19.31, respectively, indicating that the PES membrane was much more hydrophobic than the CA membrane. UF experiments were performed in a filtration cell (Amicon 8400, Millipore, USA) in dead-end mode at room temperature (20 71 1C). UF membrane was placed on the bottom of the cell with its glossy side towards the bulk solution during filtration. A peristaltic pump was used as the suction pump to maintain a constant permeate flux of 150 L/(m2 h). The trans-membrane pressure (TMP) was monitored by a pressure transducer (PTP708, Tuopo Electric, Foshan, China) mounted between the filtration cell and the suction pump. The pressure transducer was connected to a computer and the data was automatically logged every five seconds. Average fouling rate was calculated by dividing the increment of TMP by filtration time. 2.4. UF experiments and fouling resistance analysis Fouling reversibility (i.e. the response of membrane fouling to physical cleaning strategies) is of great significance from the perspective of application, while the analysis of fouling resistance distribution (external vs. internal fouling) would favor the identification of fouling mechanisms [3,24,25]. In this study, external fouling was operationally classified into loosely- and stronglyattached external fouling by its response to shear stress, and

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internal fouling was divided into reversible internal fouling and irreversible fouling through backwashing. Therefore, membrane filtration resistance was described using the resistance-in-series model as shown below [26,27]: Rt ¼ TMP=ðμJÞ ¼ Rm þ Rf ¼ Rm þ Ref þ Rif

ð1Þ

Ref ¼ Ref l þ Ref s

ð2Þ

Rif ¼ Rif r þ Rirr

ð3Þ 1

where, Rt is the total hydraulic resistance (m ); TMP is the transmembrane pressure (Pa); μ is the dynamic viscosity of the feed water (Pa s); J is the permeate flux of the membrane (m/s); Rm is the clean membrane resistance (m  1); Rf is the total fouling resistance (m  1); Ref is the external fouling resistance (m  1); Rif is the internal fouling resistance (m  1); Ref-l is the looselyattached external fouling resistance (m  1); Ref-s is the stronglyattached external fouling resistance (m  1); Rif-r is the reversible internal fouling resistance (m  1); Rirr is the physically irreversible fouling resistance (m  1). During this study, UF experiments were conducted as follows to obtain the filtration resistance mentioned above. 1) The filtration of 100 mL Mill-Q water was carried out and the average of TMP values was recorded as TMP0. Rm could be calculated as follows : Rm ¼ TMP 0 =ðμJÞ

adsorbent particles in UF feed water on membrane fouling, UF experiments were conducted in two modes for each NOM solution: 1) the adsorbent particles were removed before UF by filtering through 0.45 μm mixed cellulose filters (Taoyuan, China) and the filtrate, which was denoted as MAR-treated HA/BSA/SA and PAC-treated HA/BSA/SA, was used as UF feed water; 2) the mixed solution of NOM and adsorbent particles was fed into UF cell and was denoted as MAR þ HA/BSA/SA and PAC þ HA/BSA/SA.

ð4Þ

2.5. Analytical methods The concentration of HA was determined in terms of UV absorbance at 254 nm using a UV/vis spectrophotometer (T6, China). The concentrations of BSA and SA were measured by dissolved organic carbon analysis using a total organic carbon analyzer (multi N/C 2100, Jena, Germany). The particle size distributions of MAR and PAC were characterized using a MasterSizer 2000 (Malvern, UK). Zeta potentials of MAR and PAC were determined using Nano S90 (Malvern, UK) at the pH of 7.570.1. The surface areas and pore size distributions of MAR and PAC were measured using a surface area and porosity analyzer (ASAP 2020, Micromeritics, USA) with a nitrogen adsorption method. All the analyses were done in triplicate.

3. Results and discussion 3.1. Adsorption of NOM fractions onto MAR and PAC

Ref l ¼ ðTMP 1  TMP 2 Þ=ðμJÞ

ð5Þ

3) To remove the strongly-attached cake/gel layer remaining after stirring, the membrane was taken out and wiped with a wet sponge carefully. Afterwards, the membrane was reinstalled and the filtration of Milli-Q water was conducted again with the average of TMP values recorded as TMP3. Then, Ref-s derived from strongly-attached cake/gel layer could be calculated as follows: Ref s ¼ ðTMP 2  TMP 3 Þ=ðμJÞ

ð6Þ

4) The membrane was backwashed with 20 mL Mill-Q water by placing the reverse side of the membrane upwards and then the TMP value of filtering Mill-Q water was determined and recorded as TMP4. Therefore, Rif-r and Rirr could be calculated by Eqs. (7) and (8), respectively, as follows: Rif r ¼ ðTMP 3  TMP 4 Þ=ðμJÞ

ð7Þ

Rirr ¼ ðTMP 4 TMP 0 Þ=ðμJÞ

ð8Þ

Fig. 1 shows the adsorption of different NOM fractions onto MAR and PAC. It can be observed that the HA adsorption capacity of MAR was much larger than that of PAC. The adsorbed amount of HA onto MAR reached 279.6 mg/g at an equilibrium concentration of 15.95 mg/L while that onto PAC was just 103.4 mg/g at a much higher equilibrium concentration of 24.8 mg/L. For BSA, the adsorption capacity of MAR was also larger than that of PAC, but the gap was not as significant as that for HA. With regard to SA, the adsorption capacities of MAR and PAC were both quite small. The different adsorption capacities of MAR and PAC for each NOM fraction could be attributed to diverse pore structures of the two adsorbents and various chemical compositions and MW distributions of NOM fractions. PAC is a typical microporous adsorbent, whereas the pore size of MAR is much larger (Table 1). HA is a mixture of polyelectrolytes with a wide MW distribution. As reported previously, the hydrodynamic size of a large proportion

300

Adsorbed amount (mg/L)

2) NOM solution with a volume of 350 mL was fed into the cell and then the filtration begun. When the permeate volume increased to 300 mL or the TMP reached 80 kPa, the filtration experiment was terminated and the TMP at the end was recorded and designated as TMP1. After that, the stirrer in the filtration cell was stirred at a rate of 200 rpm for 2 min to remove loosely-attached cake/gel layer on the membrane surface, followed by filtration of 100 mL Mill-Q water with the average of TMP values recorded as TMP2. Ref-l (including the resistance caused by concentration polarization) could be calculated as follows:

MAR-HA PAC-HA

MAR-BSA PAC-BSA

5

15

MAR-SA PAC-SA

250 200 150 100 50 0 0

10

20

25

30

Equilibrium concentration (mg/L) Prior to UF experiments, the adsorbent was added to NOM solutions at a dose of 50 mg/L and the contact time was set at 30 min. In order to elucidate the effects of the presence of

Fig. 1. Adsorption of NOM fractions onto MAR and PAC (the solid lines represent the fitting of the data according to Freundlich isotherm model, the equilibrium concentration and adsorbed amount are on the basis of the mass of model foulants).

80

60

60

45

TMP (kPa)

40

20

PAC-treated HA PAC+HA

5

20

0

0

10

30

15

Raw HA MAR-treated HA MAR+HA 15

25

97

Raw HA MAR-treated HA MAR+HA

PAC-treated HA PAC+HA

5

20

0 0

30

10

Ref-l

Rif-r

11

12

Fouling resistance (10

9 6 3

Ref-l

25

30

Ref-s

Rif-r

Rirr

8 6 4 2

C+ PA

R+

H

H

A

A

A A M

tr C-

R-

tr

ea te d

ea te d

H

H

A w A M

C PA

R+ A M

H

A +H

A H

H ed at tre C-

PA

R A

A

A H

A H

-tr ea te d

w Ra M

A

0

0

Ra

11

Fouling resistance (10

10

Rirr

m -1)

Ref-s

-1

m )

15

15

Filtration time (min)

Filtration time (min)

PA

TMP (kPa)

K. Li et al. / Journal of Membrane Science 471 (2014) 94–102

Fig. 2. Effects of MAR and PAC pretreatments on HA fouling of PES membrane: (a) TMP buildup (only mean values were reported) and (b) fouling resistance analysis (error bars represent standard deviations from the means) (n ¼3).

Fig. 3. Effects of MAR and PAC pretreatments on HA fouling of CA membrane: (a) TMP buildup (only mean values were reported) and (b) fouling resistance analysis (error bars represent standard deviations from the means) (n¼ 3).

of Sigma-Aldrich HA molecules is larger than the size of micropores (wo2 nm) [28]. As a result, although the BET surface area of PAC was much larger than that of MAR, MAR was superior to PAC in HA adsorption because micropores of PAC might be unaccessible to high-MW HA molecules [20,29]. BSA is a standard model protein with MW of  67 kDa [30]. Only parts of the surface pores of PAC were accessible to BSA molecules, and thus the BSA adsorption capacity of PAC was lower than that of MAR. Unlike HA and BSA, SA molecules exhibited high hydrophilicity, and the hydrophobic interaction between the adsorbent and SA was weak [31,32]. Therefore, both MAR and PAC displayed very small SA adsorption capacity.

It can be observed in Fig. 2(b) that raw HA mainly caused external fouling, with the Ref (i.e., the sum of Ref-l and Ref-s) accounting for 77.5% of the total fouling, and the percentage of Rirr in Rf was just 16.8%. With adsorbent particles removed before UF, MAR pretreatment dramatically reduced both the external and internal fouling, but PAC pretreatment exerted little influence on the external fouling and just lowered the internal fouling by 35.1%. MAR particles present in UF feed water slightly increased the Ref-l and lowered the Ref-s, whereas the Ref was obviously elevated by PAC particles and the increase overwhelmed the decrease in internal fouling due to HA removal. In addition, MAR pretreatment reduced the Rirr more efficiently than PAC pretreatment did, and the adsorbent particles present in UF feed water showed negligible influence on the Rirr. Fig. 3 shows the effects of MAR and PAC pretreatments on HA fouling of CA membrane. As compared with the fouling of PES membrane, the fouling of CA membrane developed more slowly and the percentages of internal fouling and irreversible fouling in total fouling were lower, which might be ascribed to the high hydrophilicity of the CA membrane. Nevertheless, the influence of adsorption pretreatment on fouling of CA membrane was analogous to that on PES membrane. For better understanding of the difference between the efficiency of MAR and PAC pretreatments in HA fouling control, HA concentrations in the feed and permeate of PES and CA membranes were measured (Fig. 4(a)). For raw HA, the rejection coefficients of PES and CA membranes were 81.6% and 67.4%,

3.2. Effects of MAR and PAC pretreatments on HA fouling The effects of MAR and PAC pretreatments on HA fouling of PES membrane are shown in Fig. 2. As shown in Fig. 2(a), raw HA resulted in significant membrane fouling and the average fouling rate was 1.744 kPa/min. When adsorbent particles were removed before UF, MAR and PAC pretreatments reduced the fouling rate by 61.5% and 9.3%, respectively. The TMP evolution of MARþHA was almost the same as that of MAR-treated HA, indicating that the presence of MAR particles in UF feed water exerted minor influence on membrane fouling. In contrast, PAC particles present in UF feed water obviously contributed to membrane fouling, and the final TMP of PAC þHA was higher than those of PAC-treated HA and raw HA.

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K. Li et al. / Journal of Membrane Science 471 (2014) 94–102

Feed water Permeate of PES membrane Permeate of CA membrane

HA concentration (mg/L)

10 8 6 4 2

M

PA

A

C+

R+

H

H

A

A

A H tre at ed

M

PA

A

C-

R-

Ra

tre at

w

ed

H

H

A

A

0

BSA concentration (mg/L)

2.5 Feed water Permeate of PES membrane Permeate of CA membrane

2.0 1.5 1.0 0.5

A

A PA

A

C+

R+

BS

BS

A M

Feed water Permeate of PES membrane Permeate of CA membrane

2.5

SA concentration (mg/L)

BS ed at

M

PA

A

C-

R-

tre

tre

Ra

at

w

ed

BS

BS

A

A

0.0

3.3. Effects of MAR and PAC pretreatments on BSA fouling

2.0 1.5 1.0 0.5

PA

C+

SA

SA R+ A M

Ctre at ed

at tre RM A

PA

w

ed

SA

SA

SA

0.0 Ra

than the percentage of high-MW HA, indicating that part of HA molecules smaller than membrane pore size were also rejected by the membrane. Several mechanisms, including adsorption, coagulation, gel layer formation and electrostatic repulsion, might be responsible for the rejection of low-MW HA [5,36]. The higher rejection coefficient of PES membrane in comparison with that of CA membrane indicated that adsorption might be more pronounced for the more hydrophobic membrane, which was in line with the higher percentage of internal fouling in total fouling for PES membrane (Figs. 2 and 3). When adsorbent particles were removed before UF, membrane fouling was determined by the residual HA in the solution after adsorption pretreatment. MAR pretreatment reduced HA concentration in feed water from 9.91 mg/L to 5.05 mg/L, but the concentrations in PES and CA membrane permeate just declined by 0.20 and 0.23 mg/L, respectively, demonstrating that most of the HA adsorbed by MAR would have otherwise been rejected by the membrane. Thus, MAR pretreatment significantly reduced the deposition of HA on/within the membrane and mitigated HA fouling substantially. In contrast, PAC pretreatment just reduced HA concentration in feed water by 1.76 mg/L, and the concentrations in PES and CA membrane permeate were lowered by 0.91 and 1.15 mg/L, respectively. The effects of PAC pretreatment on HA transmission suggested that a large proportion of HA removed by PAC adsorption could permeate through the membrane and HA deposition on the membrane was not reduced significantly, which could explain the poor performance of PAC pretreatment in HA fouling control. In consideration of the pore structure characteristics of the adsorbents (Table 1), it was reasonable to speculate that MAR pretreatment preferentially removed high-MW HA, and that PAC adsorbed both high- and low-MW of HA though it was less efficient in HA removal. The HPSEC analysis results of HA before and after adsorption pretreatment (Fig. S1 in the Supplementary Information) proved the above speculation. When adsorbent particles were not removed before UF, membrane fouling involved the interactions among the adsorbent particles and unadsorbed HA, which will be discussed in Section 3.5.

Fig. 4. Effects of MAR and PAC pretreatments on NOM concentrations in the feed and permeate of PES and CA membranes (a) HA, (b) BSA and (c) SA (error bars represent standard deviations from the means, n¼ 3).

respectively. According to previous studies regarding SigmaAldrich HA and conducted under similar solution environment, the proportion of HA fraction with MW larger than 100 kDa (the MWCO of membrane used in this study) was in the range 48–62% [33–35]. This high-MW HA fraction could be rejected by the membrane and mainly cause external fouling. But the observed rejection coefficients of PES and CA membranes were both higher

Fig. 5 shows the effects of MAR and PAC adsorptions on BSA fouling of PES membrane. As shown in Fig. 5(a), PES membrane was severely fouled by raw BSA. MAR pretreatment almost eliminated BSA fouling, and PAC pretreatment reduced BSA fouling significantly. The presence of MAR particles in UF feed water showed little influence on TMP buildup while PAC particles slightly contributed to membrane fouling. Unlike the dominance of external fouling in HA fouling, the internal fouling accounted for 52.3% of BSA fouling of PES membrane. Regardless of the presence or absence of MAR particles in UF feed water, MAR pretreatment substantially decreased both the external and internal fouling. PAC pretreatment reduced both the external and internal fouling moderately, and PAC particles present in UF feed water increased external fouling to some extent. In the case of CA membrane which was more hydrophilic, the fouling caused by raw BSA was so slight that the efficiency of the adsorption pretreatments could not be significantly distinguished (Fig. 6). BSA concentrations in the feed and permeate of PES and CA membranes are shown in Fig. 4(b). For raw BSA, the rejection coefficients of PES and CA membranes were 53.1% and 16.8%, respectively. BSA is a homogeneous model protein with MW of  67 kDa and there are usually some BSA aggregates in the solution [37]. The much higher rejection coefficient of PES membrane suggested that hydrophobic adsorption played an important role in BSA rejection and membrane fouling. According to the distribution of fouling resistance shown in Fig. 5, it can be inferred

K. Li et al. / Journal of Membrane Science 471 (2014) 94–102

99

30

80

25

TMP (kPa)

40

20

Raw BSA MAR-treated BSA MAR+BSA

0 0

5

10

15

25

15 Raw BSA MAR-treated BSA MAR+BSA

10 5

PAC-treated BSA PAC+BSA 20

20

0

30

0

5

10

Filtration time (min) Ref-l

Ref-s

Rif-r

Rirr

4

11

m -1) 12

Fouling resistance (10

Ref-l

20

25

30

8

4

Ref-s

Rif-r

Rirr

3

2

1

A

PA

C+

B

SA

SA +B R A

tr C-

M

ea te d

d te ea -t r

R A

BS

BS

A BS Ra w

A BS C+ PA

B SA Rtre at ed BS PA A C -tr ea te d BS A M A R+ BS A

w

A

Ra

M

A

0

0

M

11

Fouling resistance (10

15

Filtration time (min)

m -1)

16

PAC-treated BSA PAC+BSA

PA

TMP (kPa)

60

Fig. 5. Effects of MAR and PAC pretreatments on BSA fouling of PES membrane: (a) TMP buildup (only mean values were reported) and (b) fouling resistance analysis (error bars represent standard deviations from the means) (n ¼3).

Fig. 6. Effects of MAR and PAC pretreatments on BSA fouling of CA membrane: (a) TMP buildup (only mean values were reported) and (b) fouling resistance analysis (error bars represent standard deviations from the means) (n¼ 3).

that BSA fouling of PES membrane was initially caused by pore constriction due to adsorptive fouling, followed by pore blockage and cake formation [38,39]. For the hydrophilic CA membrane, the adsorptive fouling was very limited at the BSA concentration employed in this study (2 mg/L) and the subsequent pore blockage and cake formation were also minimized [23,40], thus the BSA fouling of CA membrane was much lower than that of PES membrane. As shown in Figs. 1 and 4(b), MAR was more efficient than PAC in removing BSA. Therefore, it was rational that MAR pretreatment mitigated BSA fouling more effectively than PAC pretreatment did. When adsorbent particles were present in UF feed water, the contribution of MAR and PAC particles to membrane fouling were different and the mechanisms will be discussed later.

SA is a typical type of hydrophilic polysaccharide [31,39]; according to Kim and Dempsey [34], 86.0% of SA molecules are larger than 100 kDa and 11.2% of them are in the range of 30–100 kDa. Therefore, SA caused significant external fouling as a result of cake/gel formation and the percentage of irreversible fouling was very small. As shown in Fig. 4(c), the rejection coefficients of PES and CA membranes for raw SA were 90.8% and 87.5%, respectively, which were in accordance with the properties of SA. As shown in Fig. 4(c), the SA concentration hardly declined after MAR and PAC pretreatments, and thus neither of them were able to reduce SA fouling. Moreover, neither the presence of MAR nor PAC particles exerted detectable influence on SA fouling, which was distinctive from the situation of HA and BSA and will be discussed later.

3.4. Effects of MAR and PAC pretreatments on SA fouling

3.5. Summary of the effects of MAR and PAC pretreatments on membrane fouling caused by different NOM fractions

The effects of MAR and PAC pretreatments on SA fouling of PES membrane are shown in Fig. 7. The average fouling rate during raw SA filtration was 2.260 kPa/min, and most of the fouling was loosely-attached external fouling. The effects of both MAR and PAC pretreatments on SA fouling were insignificant, irrespective of the presence or absence of adsorbent particles in UF feed water. As shown in Fig. 8, SA fouling of CA membrane was less severe than that of PES membrane, and the influence of adsorption pretreatment was also inconsiderable.

For each NOM fraction investigated in this study, fouling of PES membrane was more severe than that of CA membrane more or less. The results were consistent with previous studies and the differences could be associated with the lower adsorptive fouling of the CA membrane due to its high hydrophilicity [23,40]. Nevertheless, the fouling of the two types of membranes was similarly affected by each adsorbent. The varied influence of MAR and PAC pretreatments on PES membrane fouling caused by different NOM fractions is summarized in Fig. 9.

K. Li et al. / Journal of Membrane Science 471 (2014) 94–102

80

60

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45

TMP (kPa)

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20

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Fig. 7. Effects of MAR and PAC pretreatments on SA fouling of PES membrane: (a) TMP buildup (only mean values were reported) and (b) fouling resistance analysis (error bars represent standard deviations from the means) (n¼ 3).

Fig. 8. Effects of MAR and PAC pretreatments on SA fouling of CA membrane: (a) TMP buildup (only mean values were reported) and (b) fouling resistance analysis (error bars represent standard deviations from the means) (n¼ 3).

As the adsorbent particles were removed before UF, the efficiency of adsorption pretreatment in fouling control depended on the adsorption capability and preference of the adsorbent. As for HA which was heterogeneous in MW distribution, MAR pretreatment decreased HA concentration in feed water much more efficiently than PAC pretreatment did and MAR preferentially adsorbed high-MW HA. As a result, MAR pretreatment significantly reduced the deposition of HA on the membrane and mitigated HA fouling substantially, whereas PAC pretreatment just slightly mitigated HA fouling, which was in accordance with previous studies [8,17]. BSA molecules were relatively uniform in MW, therefore MAR pretreatment removed more BSA and controlled BSA fouling more efficiently than PAC pretreatment did. For SA which was hydrophilic and was hardly adsorbed by both MAR and PAC, neither of the pretreatments apparently mitigated its fouling. When there was no separation step before UF, adsorbent particles present in UF feed water were retained by the membrane along with part of the unadsorbed NOM. As presented in Sections 3.2–3.4, the influence of the presence of PAC particles on NOM fouling was specific to the types of NOM fraction while the presence of MAR particles exerted minor influence on all the three fractions. On the basis of the SEM images of the particle layer formed on the surface of PES membrane with and without NOM foulant (Fig. S2 in the Supplementary Information), one possible explanation can be given as the following. The size of adsorbent particles were more than three orders of magnitude larger than

that of NOM foulants (μm vs. nm), thus it can be assumed that NOM molecules would pass through the adsorbent particle layer if there were no interactions between adsorbent particles and NOM molecules. As reported previously, both the resistance of MAR and PAC layers in the absence of NOM were negligible in comparison with the clean membrane resistance [14,20]. Therefore, the influence of the presence of adsorbent particles on membrane fouling mainly depended on the interactions between adsorbent particles and NOM molecules and the subsequent change in the structure and resistance of the particle layer. Several mechanisms might be responsible for the change in the structure and resistance of the particle layer: alteration of particle surface properties due to organic adsorption, hindered back diffusion of NOM molecules and rejection of NOM molecules by the particle layer [41–44]. These mechanisms were not exclusive and more than one of them might act at the same time [42,45]. As shown in Fig. 4(a), both the presence of MAR and PAC particles slightly decreased HA concentration in the permeate, suggesting that both MAR and PAC layers rejected some HA molecules. But the hindered back diffusion mechanism could not be excluded because HA molecules entrapped in the particle layer might have been adsorbed by the adsorbent. Nonetheless, the internal pores of MAR were wide enough to accommodate HA with different MWs, therefore, most HA molecules adsorbed by MAR entered the internal pores of MAR and the resistance of MAR layer might be unaffected by the presence of HA, which means that the resistance of the MAR layer formed in the presence of HA was also negligible [14,20]. However,

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Fig. 9. Schematic of the influence of MAR and PAC pretreatments on membrane fouling caused by different NOM fractions.

although high-MW HA was excluded from the internal pores of PAC, a considerable part of the HA adsorbed by PAC was high-MW HA (Fig. 4(a) and Fig. S1), implying that this part of HA was adsorbed in the external pores and/or on the particle surface. Moreover, there was a large amount of high-MW HA in the solution pretreated by PAC (Fig. S1). More high-MW HA would adhere on the surface of PAC particles if hindered back diffusion and rejection of HA molecules by particle layer occurred. HA molecules adhered on the surface of PAC particles might interact with each other, resulting in a PAC layer with lower porosity and higher resistance in comparison with PAC layer formed in the absence of NOM. BSA is also a hydrophobic NOM fraction, but its MW is smaller than high-MW HA and the concentration used in the experiment was much lower than that of HA. Therefore, the decrease of PAC layer porosity in BSA fouling was not as significant as that in HA fouling, and the presence of PAC particles just resulted in a moderate increase in membrane fouling. For the reason similar to that stated in HA fouling, the presence of MAR particles exerted minor influence on BSA fouling. In the case of SA which could be regarded as a non-interacting foulant, due to the relatively low concentration of SA used in this study (2 mg/L), the structures of MAR and PAC layers might remained almost the same as that formed in the absence of NOM and membrane fouling was not influenced visibly by the presence of MAR and PAC particles. Overall, the influence of the presence of adsorbent particles on membrane fouling was closely related with the textural properties of the particle and the interactions between the particle and NOM molecules; further studies are needed to explore the underlying mechanisms. It should be noted that the results obtained in this study were based on the model foulants used in this experiment. There might be some discrepancies in properties (e.g., MW, hydrophobicity) between the model foulants used in this study and NOM in natural waters or other model foulants, therefore, caution should be taken in extrapolating the results to other systems. For instance, the MW of HA from IHSS is smaller than that of Sigma-Aldrich HA used in this study [36], and thus the efficiency of PAC pretreatment in fouling control might be improved based on the above analysis; meanwhile, the efficiency of MAR and PAC pretreatments might vary with the source of feed water to some extent (Fig. S3 in the Supplementary Information). Nevertheless, this study demonstrated

directly that the efficiency of adsorption pretreatment in NOM fouling control was related to the characteristics of NOM and adsorbent, and the underlying mechanisms were discussed preliminary. The findings reported here would contribute to the understanding of membrane fouling in the hybrid adsorption/UF process.

4. Conclusions Performance of MAR and PAC pretreatments in mitigating UF membrane fouling caused by different NOM fractions was systematically investigated in this study. The following conclusions can be drawn. (1) The adsorption capacity of MAR for different NOM fractions was in the order of HA 4BSA4SA, while that of PAC followed the order of BSA4 HA4SA. In addition, the adsorption capacity of MAR was higher than that of PAC for HA and BSA, and both MAR and PAC exhibited very small SA adsorption capacities. (2) Although the fouling of CA membrane was lower than that of PES membrane due to the high hydrophilicity of CA membrane, fouling of the two membranes was similarly influenced by each pretreatment. (3) MAR pretreatment efficiently removed high-MW HA and significantly reduced HA fouling, no matter whether MAR particles were removed before UF or not. In contrast, PAC pretreatment with PAC particles removed before UF slightly alleviated HA fouling, whereas HA fouling was exacerbated when PAC particles were present in UF feed water. (4) MAR pretreatment mitigated BSA fouling more effectively than PAC pretreatment did because MAR adsorption removed more BSA. Moreover, the presence of MAR particles in UF feed water caused little additional resistance, while PAC particles contributed to membrane fouling moderately. By contrast, regardless of the presence or absence of adsorbent particles in UF feed water, neither MAR nor PAC pretreatments exerted significant influence on SA fouling. (5) The efficiency of adsorption pretreatment in NOM fouling control was associated with the characteristics of NOM and adsorbent. As a result, from the view of membrane fouling control, the design

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of a hybrid adsorption/UF system should fully consider the composition and properties of NOM in the feed water.

Acknowledgment This research was jointly supported by the National Natural Science Foundation of China (51138008, 51308146), Program for New Century Excellent Talents in University (NCET-13-0169), State Key Laboratory of Urban Water Resource and Environment (2014DX04), the Funds for Creative Research Groups of China (51121062) and the Fundamental Research Funds for the Central Universities (AUGA5710050713). Anonymous referees are greatly acknowledged for their constructive and helpful suggestions.

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