A simplified method for elucidating the effect of size exclusion on nanofiltration membranes

Separation and Purification Technology 85 (2012) 1–7

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A simplified method for elucidating the effect of size exclusion on nanofiltration membranes E.-E. Chang a, Chung-Huei Liang b, Chin-Pao Huang c, Pen-Chi Chiang b,⇑ a

Department of Biochemistry, Taipei Medical University, Taipei, Taiwan Graduate Institute of Environmental Engineering, National Taiwan University, Taipei, Taiwan c Department of Civil and Environmental Engineering, University of Delaware, USA b

a r t i c l e

i n f o

Article history: Received 16 August 2010 Received in revised form 30 April 2011 Accepted 3 May 2011 Available online 19 May 2011 Keywords: Nanofiltration Mass transfer coefficient Natural organic matter

a b s t r a c t Molecular weight, cross-flow velocity, and transmembrane pressure were considered among the main factors affecting the rejection of target compounds and natural organic matter (NOM) by the nanofiltration (NF) process. The mass transfer coefficient was derived from MW and used to interpret the rejection of the target compounds so as to elucidate the rejection contributed by size exclusion of NOM during the NF process. Results showed that increasing the cross-flow velocity or the transmembrane pressure results in an increase in the Sherwood and Peclect numbers, thereby enhancing solute and water transport through the membrane pores. The experimental results showed that increasing the cross-flow velocity or transmembrane pressure could improve rejection, but the error between experimental result and the estimated value increased for larger target compounds due to the neglect of adsorption effect. For the removal of NOM under high cross-flow velocity, rejection had already reached its maximum; further increase in transmembrane pressure did not increase the rejection. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Nanofiltration (NF) is a cost-effective technique for the removal of low-molecular-weight micropollutants such as endocrine -disrupting chemicals and pharmaceutical and personal care products (PPCP) from water at operating pressure lower than that of reverse osmosis. In water treatment engineering, it is important to control the formation of chlorinated disinfection by-products as well as color and odor, which are caused mostly by natural organic matter (NOM). Many investigators have shown that NF performs very well in the removal of NOM [1–3]. For example, Owen et al. [1] reported that NF with a relatively lower molecular weight cut-off (MWCO) of 400–800 Da showed effective removal of NOM. Many factors such as the characteristics of solute (e.g., molecular weight, size, and geometry) and membrane (e.g., MWCO distribution and zeta potential), water chemistry (e.g., pH, ionic strength, calcium hardness) and hydrodynamic condition (e.g., transmembrane pressure, cross-flow velocity, and channel configurations) can affect the removal mechanisms, i.e., size exclusion, electrostatic repulsion, and adsorption, of NF filtration [4–8]. For the size exclusion mechanism, researchers attempted to develop an easy but effective approach to elucidate the molecular characteristics, e.g., shape, dimension and MW, of a molecule to evaluate ⇑ Corresponding author. Address: No. 71, Chou-shan Rd., Taipei 106, Taiwan. Tel.: +886 2 2362 2510; fax: +886 2 2366 1642. E-mail address: [email protected] (P.-C. Chiang). 1383-5866/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.05.002

the effect of steric hindrance on the rejection of certain solutes during the NF process [9–12]. The results showed that the molecular weight (MW) of a non-charged compound can be a useful parameter to predict rejection. In 2002, Ozaki and Li [13] reported that the rejection of non-charged and non-polar compounds could be predicted using the MW of the compound. However, although it is easy to determine the MW of a compound, MW alone cannot provide enough information on the geometry of a molecule to enable the influence by steric hindrance to be evaluated. For a better predictive result of the steric hindrance effect on rejection, several studies have found that molecular size parameters such as molecular length, Stokes radii, and mean molecular size, are better indicators than MW [13–17]. Among these size -related parameters, the Stokes radius has been used most commonly for evaluating the steric hindrance [18]. However, the drawback of the Stokes radius is that it is estimated by the diffusivity of the solute, which can be a difficult task. Quantitatively, the transport of organic solutes across the NF membrane is controlled by both diffusion and convection and can be correlated with the mass transfer coefficient. The contribution by diffusion is dominant for compounds having a high solute-radius to membrane-radius ratio, whereas the contribution of convection is dominant for compounds having a small solute-radius to membrane-radius ratio [12]. For NOM removal, the NF process combines the advantages of a low demand of chemical addition and a continuous production of quality water; specifically, it is possible to achieve almost total

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E.-E. Chang et al. / Separation and Purification Technology 85 (2012) 1–7

NOM rejection and high recovery (85–90%) with NF membranes having a 300 Da MWCO [19]. NF membranes are able to effectively remove NOM through a combination of size exclusion and physicochemical interactions such as electrostatic repulsion and adsorption [20]. The analysis of important physico-chemical properties of NOM such as the distribution of MW and hydrophobic/hydrophilic ratio enables determination, design and operation of the treatment processes [21]. However, establishment of a simplified and direct approach is still needed for evaluating NOM rejection by NF process. The objective of this study was to investigate the effect of crossflow velocity and transmembrane pressure on size exclusion using MW as a surrogate parameter to determine the mass transfer coefficient. The rejection of neutrally charged target compounds including resorcinol, phloroglucinol, glucose, sucrose, and two PPCPs, i.e., acetaminophen and triclosan, were analyzed using the Spieler–Kedem equation without the electrostatic influence to determine the extent of the size exclusion mechanism. The performance of NOM removal during the NF process in terms of rejection ratio was evaluated and interpreted based on the size exclusion mechanism with respect to the target compounds studied.

area of 46.2 cm2 each were used. The MWCO of the two membranes were estimated by the polyethylene glycol (200–1500 Da, Aldrich) with concentration of 20 mg/L. The results show that the estimated MWCO of NF70 and NF270 were 250 and 300 Da, respectively [23]. The performance of the membrane was determined by permeate flux and target compound rejection. The concentration of target compounds in the permeate and feed were measure at fixed time intervals (0, 1, 2, 3, 4, 5, 6, 12, 16, 20, and 24 h). A detailed schematic diagram of the filtration module has been reported previously [23]. All experiments were carried out at a constant temperature of 25 ± 1 °C with both permeate and concentrate being recycled. Background permeate flux was determined using de-ionized water. Control experiments were carried out to determine the degree of adsorption of target compounds onto the internal walls of the apparatus, and the results showed that the effect of materials loss by adsorption was negligible. A new membrane was used to minimize membrane fouling under otherwise the same experimental conditions. All membranes used in this study were pre-pressurized by pure water for 24 h. 2.3. Determination of the rejection by size exclusion

2. Materials and method 2.1. Target compounds and NOM Resorcinol (1,3-dihydroxybenzene), phloroglucinol (1,3,5-trihroxybenzene), glucose, sucrose, and two PPCPs (acetaminophen and triclosan) were selected as the target compounds for the evaluation of rejection. The water samples were prepared using organic-free and de-ionized water (Milli-Q SP) for the dissolved organic carbon (DOC) concentrations of 5.0–6.0 mg/L. The pH of all solutions was adjusted to 7–8 using 0.1M hydrochloric acid or sodium hydroxide. The concentrations of acetaminophen and triclosan were analyzed by an HPLC system equipped with a C-18 column (with diameter, length and pore size of 4.6, 150 mm, and 5 lm, respectively). A UV detector was used with the wavelength set at 254 nm for acetaminophen and 225 for triclosan, respectively. The mobile phases consist of methanol and KH2PO4 buffer solution. The injection volume of a sample was 25 lL. Resorcinol, phloroglucinol, glucose, and sucrose samples were filtered through a prewashed 0.45 lm filter and then determined by a TOC analyzer (O.I. Corp., model 700). The NOM sample was taken from source water in the Tai Lake water treatment plant King-men, Taiwan. All water samples used in the membrane process were pre-filtered with 0.45-lm filters to remove colloidal particles. The MW distribution of NOM was quantified by gel filtration chromatograph using a 30-cm TSKG2000SWXL column with an inner diameter of 7.8 mm. The HPLC (LCP 4100, ECOM) was coupled with a refractive index detector (LR 125, VISCOTEK). The hydrophobicity of NOM was fractionated based on the method delineated by Marhaba et al. [22]. Water samples were pre-filtered with 0.45 lm filters to remove particulates, and then fractionated by adsorption using three resins, i.e., DAX-8 (non-ionic species), Dowex 500WX8-200 (cationic species), and Diaion WA 10 (weak anionic species). Table 1 shows the characteristics of the target compounds studied. 2.2. Membrane filtration system A cross-flow mode filtration apparatus with a flat-sheet membrane cell was used for the filtration tests. The apparatus was made of stainless steel to minimize undesirable adsorption of target compounds. Two plate form membranes (NF70 and NF270 manufactured by DOW FILMTEC™ (Hollywood, CA) with an effective

The rejection model incorporated the steric effect, convection and diffusion in cylindrical pores perpendicular to the membrane surface, which was simplified from the Spieler–Kedem equation without the electrostatic influence. Coupled with the two-film theory, the relation between the real rejection, Rr, and the observed rejection, R0, can be expressed as follows [24]:

  1  Rr 1  R0 J v ¼ exp Rr R0 k

ð1Þ

or

  Rr exp Jkv    R0 ¼ 1  Rr 1  exp Jkv

ð2Þ

where Jv is the permeate flux of the solution, and k is the mass transport coefficient. Real rejection is defined as where concentration in the upstream solution at the membrane surface. It should be noted that the real rejection is not the same as the observed rejection because of concentration polarization. The real rejection, Rr, is difficult to measure and is usually expressed in terms of the Peclet number, Pe, and the equilibrium partition coefficient for hard-sphere particles considering only steric interaction, U, that is,

Rr ¼ 1 

Pe ¼

KC U 1  expðPeÞð1  K C UÞ

K c J v Dx K d D ek

U ¼ ð1  kÞ2

ð3Þ

ð4Þ ð5Þ

where Kc and Kd are the hindrance nature of ions inside the membrane due to convection and diffusion, respectively; D is the diffusivity of the solute in bulk solution; Dx is the thickness of the membrane; ek is the membrane porosity; and k is the ratio of the size of solute to pore radius. The hindrance factors Kd and Kc, which are functions of the ratio of solute size to pore radius, k, can be related to the hydrodynamic coefficients of enhanced drag and the lag coefficient of a spherical solute moving inside a cylindrical pore of infinite length [25], that is,

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E.-E. Chang et al. / Separation and Purification Technology 85 (2012) 1–7 Table 1 Characteristics of the organic compounds used for the establishment of the relationship between MW and mass transfer coefficient.

a b c

Target compound

CAS number

Molecular formula

Molecular weight

Acidity (pKa)

Stokes radius (nm)

Diffusivity(109 m2/ s)

Initial concentration, (mg/L) (as DOC)

Resorcinol Phloroglucinol Glucose Sucrose Acetaminophen Triclosan Creatine 2-(2-Butoxyethoxy)ethanol Caprolactam 2-Propanol Formaldehyde Methanol Urea

108-46-3 108-73-6 921-60-8 57-50-1 103-90-2 3380-34-5 57-00-1 112-34-5

C6H6O2 C6H6O3 C6H12O6 C12H22O11 C8H9NO2 C12H7Cl3O2 C4H9N3O2 C8H18O3

110.1 126.1 180.2 342.3 151 289.6 131.2 162.2

9.15 8.45 n/a n/a 9.51 7.9 3.43 n/a

0.34a 0.35 0.37 0.47 0.35 0.42 0.37c 0.32

0.99b 0.96 0.76 0.51 0.94 0.58 0.66c 0.77

7.6 (5.0) 8.8 (5.0) 12.5 (5.0) 11.9 (5.0) 9.4 (6.0) 12.1 (6.0) Not used

105-60-2 67-63-0 50-00-0 67-56-1 57-13-6

C6H11NO C3H8O CH2O CH4O CH4N2O

113.2 60.1 30 32 60.1

n/a 16.5 13.3 15.5 n/a

0.28 0.26 0.22 0.19 0.18

0.87 0.93 1.11 1.28 1.38

Derived from molecular modeling software ChemDraw (CambridgeSoftÒ). Calculated using Wike and Chang equation [36]. See Ref. [12].

K c ¼ b2  ð1  kÞ2 cð1 þ 0:054k  0:988k2 þ 0:441k3 Þ

ð6Þ

3. Results and discussion

K d ¼ 1  2:3k þ 1:154k2 þ 0:224k3

ð7Þ

3.1. Determination of the mass transfer coefficient, k

Since it is also difficult to directly measure the thickness and porosity of the membrane, one can apply the Hagen–Poisseuille equation to calculate Dx/ek, that is,

Jv ¼

r 2p P 8lðDx=ek Þ

ð8Þ

where rp is the pore radius of the membrane, and P is the applied pressure across the membrane or the transmembrane pressure, and l is the viscosity of the solution. From the experimental data of P and the corresponding Jv, the value of Dx/ek can be determined. The Dx/ek of NF70 and NF270 membranes from the experimental results are shown in Table 2. The mass transfer coefficient, k, a function of feed flow rate, cell geometry and solute system can be calculated for a flat channel and smooth walls with turbulent flow [26]:

Sh ¼ 0:023Re0:875 Sc0:25

ð9Þ kdh ), D

where Sh, Re, and Sc are the Sherwood number (Sh ¼ Reynolds number (Re ¼ qdlh Uc ), and Schmidt number (Sc ¼ qlD), respectively, and dh is the hydraulic diameter of the membrane cell, q is the density of water, and Uc is the cross-flow velocity. In addition, the mass transfer coefficient can be rearranged as follows:

 0:625

q K ¼ 0:023 l

0:125 0:875 0:75 dh Uc D

ð10Þ

Eq. (10) shows that k can be expressed as the function of solution characteristics (q/l), apparatus setup (dh), flow condition (Uc), and solute diffusivity. Table 2 Determination of Dx/ek for NF70 and NF270 membranes. Membrane

Pressure (kPa)

Pure water permeate flux (106 m/s)

Dx/ek (106 m)

NF70

413.6 482.5 698.3 861.6 1033.9 413.6 482.5 698.3 861.6 1033.9

7.8 9.0 14 18 23 7.5 9.5 10 11 12

1.71 1.73 1.65 1.55 1.48 1.13 1.00 1.24 1.38 1.35

NF270

Fig. 1a shows the relationship between the MW and the Stokes radius for the six target compounds. Besides the six target compounds, this study also added seven other organic compounds, which are common and electro-neutral, to show the relationship among MW, Stokes radius, and diffusivity. The result in Fig. 1a shows that the Stokes radius of a compound is in a positive relationship to its MW. In general, the geometric property of a compound is usually derived by the application of computer software, e.g., ChemDraw (CambridgeSoftÒ) used in this study. Although most software can provide an accurate data to calculate the diffusivity and then determine the mass transfer coefficient, inconvenience is the limitation in usual. Fig. b and c show the diffusivity and mass transfer coefficient as a function of MW, respectively. It is obvious that both the diffusivity and the mass transfer coefficient are linearly proportional to the MW for the six target compounds; compounds of higher MW yield lower k or diffusivity. Based on Eq. (10), k is a function of the solution characteristics, apparatus setup, flow condition, and solute diffusivity, in which the solute diffusivity could be replaced by MW according to the result of Fig. 1. Therefore, k can be estimated as follows:

k ¼ 0:023

 0:625

q l

0:125

dh

 0:75 U 0:875 2  1012 MW þ 1  109 c ð11Þ

Kimura et al. [27] reported that the rejection of non-charged compounds was influenced mainly by the molecular size of the compound. Recently, Verliefde et al. [25] have applied a convection-diffusion model to calculate the rejection of neutral organic solutes and concluded that the higher the molar weight, the higher the rejection. Since the removal mechanism was sieving only in the absence of electrolytes at pH 7, the lower k value represented larger molecular size and higher rejection. Van der Bruggen et al. [28] and Kosutic and Kunst [29] reported that the rejection of uncharged (organic) molecules was determined by the relative size of the molecules and the membrane pores. Models describing the rejection of organic molecules based on the sieving-only mechanism, i.e., molecule size, have been reported elsewhere [30–32]; however, the rejection could be underestimated when a target compound interacts strongly with the membrane and form fouling.

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E.-E. Chang et al. / Separation and Purification Technology 85 (2012) 1–7

(a) 0.7 Stokes radius (× 10-9 m)

0.6 0.5

13 12

0.4

7 8 9

5 3

6

1

0.2

11 10

0.3

4

2

0.1

creased, indicating a high mass transfer through the membrane pores. During the NF process, convection is the dominant mechanism for solute transport, but the contribution of diffusion increases when the convective transport is hindered [33]. Meanwhile, Van der Bruggen and Vandecasteele [9] indicated that for a membrane with smaller pores, the contribution of diffusion to the transport could be greater. On the other hand, an increase in Pe yielded an increase in the real rejection, Rr, so that the observed rejection decreased simultaneously. On the other hand, both Re and Sh increased as the cross-flow velocity increased. A high Sherwood number indicates a high mass transfer rate and implies that more solute can transfer across the membrane. Lee et al. in 2004 [34] concluded that the Sherwood correlation can be used to determine the mass transfer coefficient of NOM, with relatively reliable accuracy, when supplemented with the corresponding diffusion coefficient. Fig. 2 shows the relationship between the mass transfer coefficient and rejection of the six target compounds for the two membranes. The k values used in the estimation were determined by Eq. (10). The results suggest that target compounds with higher k should exhibit lower rejection for the two membranes. Since the observed rejection is determined by both real rejection and the mass transfer coefficient, it would increase as k increases at constant real rejection for an individual target compound. In Table 3, it was observed that an increase of transmembrane pressure increased the extent of solute convection as expressed by Pe, and the experimental results also revealed rejection increasing with the mass transfer coefficient for an individual target compound. It should be noted that lower k represents higher rejection and is attainable only when comparing various target compounds or membranes. Because real rejection increases with increasing MW, larger target compounds with smaller k but larger Rr will lead to higher rejection.

1. Formaldehyde 2. Methanol 3. 2-Propanol 4. Urea 5. Resorcinol 6. Caprolactam 7. Ploroglucinol 8. Creatine 9. Acetaminophen 10. 2-(2-Butoxyethoxy)ethanol

0

Diffusivity (×10-9 m2/s)

(b) 1.6 11. Glucose 12. Triclosan 13. Sucrose

4

1.4

2

1.2 1

1

5 3

0.8

7

9

Fitness (R2)

6 10 11

0.6

12

8

13

0.4

MW - Stokes radius: 0.82 MW - Diffusivity: 0.70 MW - k: 0.73

0.2 0

(c)

60 2

k (× 10-6 m/s)

50 40

4 5

1 3

7

9

6

30

10

11

12

8

13

20 10

3.3. Effect of cross-flow velocity and transmembrane pressure on rejection

0 0

100

200

300

400

500

600

MW (Da)

Fig. 3 shows the observed and estimated rejections of resorcinol, acetaminophen, glucose, triclosan and sucrose under various cross-flow velocities by NF270. The k values used in the estimation were determined by Eq. (11). Both experimental and estimated results show that the rejection increased as the cross-flow velocity increased. It should be noted that the experiments and estimation were under the same permeate flux, Jv. Meanwhile, the real rejection did not change with the cross-flow velocity. Consequently, from Eq. (2) it can be understood that the observed rejection increased as the cross-flow velocity increased. From Fig. 3 it is obvious that most experimental results are higher than the estimated results. One possible reason can be attributed to the effect of adsorption which can provide extra rejection efficiency for virgin membranes.

Fig. 1. Relationship between MW and (a) Stokes radius; (b) diffusivity; and (c) mass transfer coefficient.

3.2. The relationship between mass transfer coefficient and rejection Table 3 shows the dimensionless parameters, i.e., Re, Sc, Sh and Pe, estimated for the four neutral target compounds. Various pressures were simulated to determine the effect of transmembrane pressure on the dimensionless parameters as shown in Table 3. Apparently, the Reynolds, Schmidt and Sherwood numbers remained constant as the transmembrane pressure changed, but the Peclet number increased as the transmembrane pressure in-

Table 3 List of dimensionless parameters, namely, Re, Sc, Sh, and Pe under the experimental conditions of this study. Transmembrane pressure (kPa)

414

Cross-flow velocity (m/s)

0.3 0.9

689

0.3 0.9

Membrane type

NF270 NF70 NF270 NF70 NF270 NF70 NF270 NF70

Re

Sc

Sh

6.1  103

9.0  102

2.6  102

1.8  104

9.0  102

6.8  102

6.1  103

9.0  102

2.6  102

1.8  104

9.0  102

6.8  102

Pe Resorcinol

Phloroglucinol

Glucose

Sucrose

0.30 0.45 0.30 0.45 0.50 0.78 0.50 0.78

0.33 0.52 0.33 0.52 0.56 0.90 0.56 0.90

0.53 0.92 0.53 0.92 0.90 1.60 0.90 1.60

8.7 847 8.7 847 14.6 1467 14.6 1467

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E.-E. Chang et al. / Separation and Purification Technology 85 (2012) 1–7 1.0

1.0

Rejection (Ro)

0.8

NF70

0.6

resorcinol

phloroglucinol

glucose

sucrose

acetaminophen

triclosan

resorcinol

phloroglucinol

glucose

sucrose

acetaminophen

triclosan

0.4

Sucrose 0.8

Triclosan Glucose

Rejection (Ro)

NF270

NF70

0.6

Acetaminophen 0.4

Experimental data resorcinol acetaminophen

0.2

NF270

0.2

Resorcinol

glucose triclosan sucrose

0.0 400

0.0 60

8

10

12

14

16

18

500

k (× 10-6 m/s) Fig. 2. Relationship between mass transfer coefficient k and diffusivity for the six target compounds. (Lines: model calculation; symbols: experimental data.)

600

700

800

900

Transmembrane pressure (kPa)

20

Fig. 4. Influence of transmembrane pressure on NF270 rejection (symbol: experimental data, line: estimated value).

(a) 15 1.0

Experimenatl data Uc-0.15 (m/s)

Solution flux (×10-6 m/s)

Sucrose

0.8

Rejection ( R o )

Triclosan Glucose

0.6

Acetaminophen

Experimental data

0.4

resorcinol acetaminophen

Uc-0.60 (m/s)

10

Resorcinol

glucose

0.2

Uc-0.30 (m/s)

triclosan sucrose

5

0.0 0

0.1

0.2

0.3

0.4

0.5

Cross-flow velocity, U c (m/s)

(b) 1.00

Fig. 3. Influence of cross-flow velocity on NF270 rejection (symbol: experimental data, line: estimated value).

Rejection (Ro)

Fig. 4 shows the effect of transmembrane pressure on the observed rejections of resorcinol, acetaminophen, glucose, triclosan and sucrose under various transmembrane pressures by NF270. The k values used in the estimation were also determined by Eq. (11). For the experimental results, the observed rejections increased as the transmembrane pressure increased for the five target compounds. On the other hand, the observed rejections of only smaller target compounds, i.e., resorcinol, acetaminophen and glucose, increased slightly with the transmembrane pressure for the estimation results. In the estimation procedure, both permeate flux and real rejection increased as the transmembrane pressure increased, and k did not change with transmembrane pressure. When increasing transmembrane pressure, the contribution from the increase of real rejection was offset by the increase of permeate flux. Consequently, the effect of increasing transmembrane pressure on the observed rejection was insignificant for larger target compounds, e.g., triclosan and sucrose. Another possible explanation is that increasing transmembrane pressure can render the membrane structure compact, which results in larger organic solute hindered to pass through the membrane. Meanwhile, the estimated results for the influence of transmembrane pressure on rejection were similar to that shown in

0.96

0.92

Prediction 0.88

Uc-0.15

(m/s)

Uc-0.30

(m/s)

(m/s) Uc-0.60 (m/s) 0.84 200 0

400

600

800

1000

1200

Transmembrane pressure (kPa) Fig. 5. Effect of transmembrane pressure and cross-flow velocity on (a) solution flux; and (b) rejection of NOM (symbol: experimental data, line: estimated value).

Fig. 3, i.e., the fitness of acetaminophen was good while that of triclosan was underestimated. The estimation error of triclosan can be attributed to the smaller acidity constant of triclosan (8.0) than that of acetaminophen (9.7). That is, at pH near 8, triclosan was more depronated than acetaminophen, which resulted in a slight increase in its rejection due to electrostatic repulsion.

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E.-E. Chang et al. / Separation and Purification Technology 85 (2012) 1–7

3.4. Elucidation of the rejection of NOM The flux experiments in the presence of NOM were conducted at three cross-flow velocities (i.e., 0.15, 0.3 and 0.6 m/s) and five transmembrane pressures (between 517 and 1,034 kPa). The results shown in Fig. 5a for NF270 indicated that the flux increased as both the cross-flow velocity and the transmembrane pressure increased. That is, at the same transmembrane pressure, increasing the cross-flow velocity resulted in a high solution flux; however, the difference was insignificant. For NOM removal, Table 4 lists the characteristics of NF270 and NOM controlling the rejection. Among various physical factors, the pore size, expressed as MWCO and pore radius, was directly related to size exclusion. The surface roughness was related to colloid adhesion (or fouling), which was indirectly related to size exclusion and adsorption. The contact angle of NF270 was measured as 55.9 ± 5.8o, which showed that NF270 was classified as ‘‘hydrophobic’’ and implied that NF270 had better adsorption capacity for NOM. For the chemical characteristics, the surface functional groups possessed the most carboxylic groups. Considering the effect of electrostatic repulsion, zeta potential is a good indicator. NF270 membrane had a negative zeta potential (22 mV) at neutral pH with a pHzpc (point-of-zero charge) of near pH 4. Since information on the charge characteristics of NOM was unavailable, only size exclusion at a fixed pH value was considered in estimating the rejection of NOM. Fig. 5b shows the rejection of NOM, including the experimental data (symbols) and the estimated values (lines), and the calculation procedures are shown in Table 5. In Fig. 5b, the experimental results show that solute rejection increased with transmembrane pressure when the cross-flow velocity was low and that increasing

cross-flow velocity did not increase rejection significantly. It was noted that at lower cross-flow velocity (e.g., 0.15 m/s), the measured rejection of NOM rose from 0.89 to 0.95 as the transmembrane pressure increased from 517 to 1,034 kPa. In contrast, the rejection remained relatively constant at 0.95 when the cross-flow velocity was increased to 0.6 m/s under the same range of transmembrane pressure of 517 and 1,034 kPa. Table 5 shows the estimated rejection by size exclusion under various cross-flow velocities and a fixed transmembrane pressure of 800 kPa based on the MW distribution. Table 4 shows that 70% of the NOM molecules were larger than 1 kDa. It was assumed that half of the remaining 30% was smaller than the nominal MWCO of NF270, so it was expected that at least 85% of the total could be removed by size exclusion or by other mechanisms. In the fraction with MW less than 300 Da, the k and the corresponding size exclusion of each molecular fraction was estimated based on the MW, and total rejection contributed by only size exclusion could then be determined. The results show that the average rejection by size exclusion for MW less than 300 Da ranged from 0.42 to 0.45, which was about 7% of the total rejection. Furthermore, approximately 7– 9% of NOM could not be removed by size exclusion, which was in agreement with experimental results. Our results also agreed with the findings reported by Radjenovic et al. [35], who suggested that the mechanism of size exclusion should yield greater rejection (i.e. >85%) for uncharged solutes having MW greater than the MWCO of the NF membranes. From Table 5 it is clearly observed that our approach can simulate the rejection of NOM since most of NOM are larger than the nominal MWCO of the membrane. In other words, the fraction less than the nominal MWCO contributed no more than 10% of the total rejection.

Table 4 Summary of results on analyzing the major factors affecting NOM removal. Parameter

Physical

Analytical result

Size Surface roughness Hydrophobicity

Chemical

a b c d e f g

Surface functional group Surface charge

Removal mechanism

Membrane characteristicsa

NOM characteristics

MWCOb: 300 Da; Pore radiusc: 0.54 nm Scan area 1  1 lm2: 2.89 nm; scan area 10  10 lm2: 10.13 nmd Contact angle: 55.9 ± 5.8o; classified as ‘‘hydrophobic’’e

<1 kDa: 30%; 1–5 kDa: 32%; >5 kDa: 38%

Size exclusion

Not available

Size exclusion and adsorption Adsorption

Highest peak around 1246 cm1 (carboxylic groups)f The isoelectric point is near pH 4 and negative values at neutral pHg

Hydrophilic: 21%; transphilic: 21%; hydrophobic: 58% Not available Not available

Adsorption Electrostatic repulsion

NF270 used in this study. Standard polyethylene glycol at constant transmembrane pressure of 483 kPa. Saccharides at various levels of transmembrane pressure: 345–1034 kPa. Root-mean-square roughness by atomic force microscopy. (DI-3100, Veeco Instruments Inc., USA) Automatic Contact Angle Meter. (CA-VP, Kyowa Interface Science Co., Japan) Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy. (Varian 800 FTIR spectrometer) Zeta potential by measuring the streaming potential with the Fairbrother and Mastin approach [37].

Table 5 Estimated mass transfer coefficient (k), rejection by size exclusion and predicted rejections of NOM. Cross-flow velocity (m/s)

0.15 0.3 0.6

Estimated k (105 m/s) in each MW fraction

Estimated rejection by size exclusion in each MW fraction

Sum of estimated rejection by size exclusion

0– 100 Da

100– 200 Da

200– 300 Da

0– 100 Da

100– 200 Da

200– 300 Da

Average

Less than 300 Da

Total

Experimental total rejection

2.3 4.3 7.8

1.9 3.6 6.5

1.5 2.8 5.2

0.05 0.06 0.07

0.35 0.40 0.43

0.87 0.90 0.92

0.43 0.46 0.47

0.09 0.10 0.11

0.88 0.89 0.90

9.4 (%) 10.1 (%) 10.5 (%)

Estimated rejection less than 300

E.-E. Chang et al. / Separation and Purification Technology 85 (2012) 1–7

Since the NOM was composed of a great number of molecules larger than the membrane pore size, the rejection was not affected by the transmembrane pressure, which was different from the case for larger target compounds such as sucrose or triclosan. However, there still was a certain amount of organic matter that could not be removed by the membrane, and it was presumed that this organic matter was smaller than the membrane pores. In this study, approximately 5–10% of NOM passed through the membrane pores. The improvement of rejection under low cross-flow velocity condition was attributed to the increase of flux induced by increasing the transmembrane pressure. Under the high cross-flow velocity condition, increasing the transmembrane pressure would no longer change the degree of rejection. Results suggested that with size exclusion alone, molecules smaller than the membrane pore cannot be removed by adjusting the cross-flow velocity or the transmembrane pressure. In other words, at the maximum rejection, the operators can choose a proper operation mode, saving energy without degrading the permeate quality. 4. Conclusions The mass transfer coefficient k used in the Spieler–Kedem equation can be determined by simplifying the relationship among MW, Stokes radius and diffusivity, to estimate the size exclusion for the neutrally charged target compounds. Thus, mass transfer coefficient derived from MW is an acceptable approach to estimate the observed rejection. The observed rejection can be represented as the function of real rejection, permeate flux and mass transfer coefficient. When increasing cross-flow velocity, only mass transfer coefficient increases but permeate flux and real rejection are invariable, which results in the increase of the observed rejection. On the other hand, when increasing transmembrane pressure, mass transfer coefficient is invariable, but the contribution of the increased real rejection is offset by the increase of permeate flux. Since the effect of adsorption was not considered, most experimental results were higher than the estimated value, especially for larger target compounds such as triclosan and sucrose. In general, neither cross-flow velocity nor transmembrane pressure can affect the rejection of organic matter larger than the membrane pores. For the removal of NOM, the improvement of rejection was attributed to the increase of flux induced by increasing the transmembrane pressure under the low cross-flow velocity condition. Under the high cross-flow velocity condition, rejection reached its maximum, so increasing the transmembrane pressure would no longer change the degree of rejection. References [1] D.M. Owen, G.L. Amy, Z.K. Chowdhury, R. Paode, G. McCoy, K. Viscosil, NOM characterization and treatability, J. Am. Water Works Assoc. 87 (1995) 46–63. [2] J.A. Nilson, F.A. Digiano, Influence of NOM composition on nanofiltration, J. Am. Water Works Assoc. 88 (1996) 53–66. [3] G.L. Amy, J. Cho, Interaction between natural organic matter (NOM) and membranes: rejection and fouling, Water Sci. Technol. 40 (1999) 131–139. [4] J. Cho, G.L. Amy, J. Pellegrino, Membrane filtration of natural organic matter: initial comparison of rejection and flux decline characteristics with ultrafiltration and nonofiltration membranes, Water Res. 33 (1999) 2517– 2526. [5] A. Braghetta, F.A. DiGiano, W.P. Bell, Nanofiltration of natural organic matter: pH and ionic strength effects, J. Environ. Eng. ASCE 123 (1997) 628–641. [6] S.K. Hong, M. Elimelech, Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes, J. Membr. Sci. 132 (1997) 159–181. [7] B. Van der Bruggen, C. Vandecasteele, Removal of pollutants from surface water and groundwater by nanofiltration: overview of possible applications in the drinking water industry, Environ. Pollut. 122 (2003) 435–445.

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