Characterization of natural organic foulants removed by microfiltration

Desalination 277 (2011) 370–376

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Characterization of natural organic foulants removed by microfiltration Liu Zheng a, Chu Huaqiang a,⁎, Dong Bingzhi b,⁎, Liu Hao a a b

Faculty of Environmental Science and Engineering, Tongji University, Shanghai 200092, China Key Laboratory of Yangtze River Water Environment of the Ministry of Education, Tongji University, Shanghai 200092, China

a r t i c l e

i n f o

Article history: Received 3 February 2011 Received in revised form 12 April 2011 Accepted 23 April 2011 Available online 17 May 2011 Keywords: Microfiltration Natural water Molecular weight distribution Hydrophobicity Fluorescence EEM spectroscopy

a b s t r a c t In order to characterize microfiltration foulants from natural waters, 12 isolated fractions from three natural waters were analyzed in this study. Hydrophobicity, molecular size distribution and fluorescence excitationemission matrix (EEM) spectral parameters were employed in characterizing membrane foulants. It was found that natural water which contained more large molecules (N 10 kDa) in neutral hydrophilic (N-HPI) fraction could lead to greater flux reduction. However, hydrophobicity and molecular size distribution cannot be analyzed separately in respect of flux decline. Comparatively, fluorescence EEM spectroscopy is better at characterizing foulants because it combined both hydrophobicity and molecular size characters in the spectral parameters. It was found that the most fouling fraction could have a very special fluorescent region between 350 nm b Em b 400 nm and 220 nm b Ex b 250 nm (Em: emission wavelength; Ex: excitation wavelength). The subtraction of fluorescence EEM spectra between feed water and permeate water confirmed that most of the retained foulants came from this region. The proportion of fluorescence intensity in the special region could be a quantitative indicator for the prediction of MF fouling potential. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Natural organic matter (NOM) is a very complex mixture. The extent of MF fouling is highly variable when the characters of feed water are different [1,2]. Therefore, characterizing foulants are critical to the understanding of membrane fouling in natural water filtration. Generally, molecular size distribution and hydrophobicity are considered as the most useful characters in describing membrane foulants. But previous findings were usually very different when these two characters were involved. Some suggested that dissolved organic matter (DOM) ranged from a few hundreds to over 100 kDa caused significant fouling in membrane filtration [2]. In the other researches, only a small proportion of organic matters in the colloidal range could contribute to fouling [3,4]. In the study performed by Carroll et al., DOM with small molecules and neutral hydrophilic character caused the most serious flux decline [5]. On the contrary, Fan et al. identified that large molecules (N30 kDa) in the neutral hydrophilic fraction took the major responsibility for flux decline [6]. Therefore, further researches need to be done on the characters of natural water which could lead to the serious membrane fouling.

⁎ Corresponding authors. Tel.: + 86 21 65982691; fax: + 86 21 65983616. E-mail addresses: [email protected] (L. Zheng), [email protected] (C. Huaqiang), [email protected] (D. Bingzhi), [email protected] (L. Hao). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.04.061

It is considered that the labor-intensive and time-consuming procedures in characterizing NOM hydrophobicity and molecular weight could lead to some disagreement of the results. Recently, fluorescence excitation-emission matrix (EEM) spectroscopy has been widely used in water analysis. It is good at distinguishing water samples from different sources without any complicated pretreatment [7]. N. Lee et al. employed three-dimensional (3D) fluorescence EEM spectroscopy in the research of MF and UF (ultrafiltration) membrane foulants [4]. It was reported that the most problematic foulants were in the lower range of excitation and emission wavelengths. In another study, a detail EEM analyzing procedure was described by R.H. Peiris et al. and three different components of UF foulants were found afterwards [8]. K. Kimura et al. investigated the irreversible fouling of UF membrane and revealed that different NOM fractions could have different EEM locations [9]. Accordingly, components of membrane foulants could be detected by fluorescence EEM scanning. However, there are few universal conclusions drawn by analyzing membrane foulants from fluorescence EEM spectra. Therefore, the usage of fluorescence EEM spectroscopy to identify foulants needs further discussion. The main objective of this study was to discuss effective methods for characterizing natural water foulants in MF filtration process. Hydrophobicity and molecular size distribution were analyzed as the major characters for MF membrane fouling. EEM parameters were analyzed for the better usage of fluorescence spectrum. The relationship between fouling characters and EEM parameters was combined afterwards. According to the analyses, a direct characterizing method for MF foulants from EEM parameters was concluded.

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Table 1 Water quality of source waters. DOM source

DOC (mg.L−1)

UV254 (cm−1)

SUVA (L.mg−1.m−1)

HPO DOC (%)

TPI DOC (%)

C-HPI DOC (%)

N-HPI DOC (%)

SHW HPJ GY

4.8 5.4 4.2

0.075 0.152 0.108

1.562 2.815 2.571

16.8 28.2 19.9

11.4 13.1 17.8

13.1 23.0 25.7

58.7 35.7 36.6

the ratio of the permeate flux (J) to the initial flux (J0) for comparing the effect of membrane fouling potential. For each test, over 800 mL of permeate volumes were measured, samples were collected before and after filtration.

2. Materials and methods 2.1. Preparation of water samples Three natural waters were selected for this study. They are the Sanhaowu Lake (SHW) water in Tongji University on April, which represented the surface water polluted by allochthonous NOM; Huangpujiang River (HPJ) in Shanghai City, on July, which represented the heavy polluted river by waste of industry and human beings; and the Gaoyou (GY) in Jiangsu province, which presented the lower polluted reservoir water. Reverse osmosis (RO) with a fiber pre-filter was used to concentrate source water into 20–25 mg/L (as dissolved organic carbon). Then, the concentrated samples were filtered through a 0.45 μm membrane (regenerated cellulose) and stored at 4 °C in a refrigerator. The water quality of three natural waters is summarized in Table 1. The RO concentrates were adjusted into pH 2 and filtered successively through Supelite DAX-8, Amberlite XAD-4, and (after neutralization to pH 8) Amberlite IRA-958 according to S. Wong's method [10]. HCl and NaOH were used to adjust pH in the experiment. The NOM fractions eluted from the DAX-8 and XAD-4 resins with 0.1 M NaOH were denoted as hydrophobic fraction (HPO) and transphilic fraction (TPI), respectively. Negatively charged hydrophilic fraction (C-HPI) adsorbed on IRA-958 was eluted with 1 M NaOH and 1 M NaCl. The remaining was considered as neutral hydrophilic fraction (N-HPI) which was unadsorbed on all of them. Because the NOM hydrophobicity is pH dependent, prior to filtration, all the concentrated source waters and fractions were neutralized to pH 7.0 and adjusted the concentration into 5 mg/L before experiment.

2.3. Analytical methods The dissolved organic carbon (DOC) concentrations were measured using a Shimadzu TOC-VCPH analyzer with a high-sensitive hot platinum catalyst (standard deviation ±0.10 mg/L). Absorbance at 254 nm (UV254) was determined using a UV–Visible spectrophotometer (Hach D5000). SUVA254 values were calculated by the ratio of UV254 to DOC. The apparent molecular weight distributions of DOM were determined by ultrafiltration fractionation (UF fractionation method) using a series of regenerated cellulose membranes supplied by Amicon® with the MWCOs of 1000 (YM1), 3000 (YM3), 10,000 (YM10) and 30,000 (YM30). To obtain fluorescence EEM spectra, a Hitachi F-4500 luminescence spectrometer was used with a xenon lamp as the excitation source. Both excitation and emission slits were set at 10 nm in this study. Wavelengths were at sequential increments of 5 nm between 200 and 400 nm for excitation and 275–575 nm for emission. The spectra were recorded at a scan speed of 12,000 nm/min, and PMT voltage was set at 400 V. Contour plots of samples were output using Matlab 7.0 and control in the wavelength range in Ex + 10 nm b Em b 2Ex-10 nm (Ex and Em represent the wavelengths of excitation and emission, respectively) for eliminating Rayleigh and Raman scatter peaks. 3. Results and discussion

2.2. Membranes and fouling procedure 3.1. Membrane fouling by source water and fractions

VCWP

Mixed cellulose ester

1.0

SHW HPJ

.8

GY

.6 .4 .2

Table 2 Characteristics of flat sheet membranes used in unstirred cell unit. Membrane Material

The flux decline with three source waters is shown in Fig. 1. As it is shown, significant difference in flux behavior can be observed. The flux of SHW water declined most rapidly, followed by HPJ water, and GY water showed the least. As can be seen in Table 1, SHW water

Normalized Flux (J/Jo)

A commercially available flat sheet microfiltration membrane (MF-Millipore™) was employed in this study, which consisted of 80– 100% nitrocellulose (Cas No. 9004-70-0) and 0–20% cellulose acetate (Cas No. 9004-35-7). The membrane specifications from the supplier are shown in Table 2. Prior to performance, all virgin membranes were pre-soaked and stored in ultrapure (Milli-Q) water for 24 h at 4 °C to remove impurities. A dead-end unstirred cell was used in all membrane fouling experiments. High purity N2 gas provided transmembrane pressure (TMP) at 0.1 MPa for filtration. Prefiltration was conducted before membrane fouling using ultrapure water until a constant permeate flux (J0) was achieved. All experiments were conduced at room temperature (20–25 °C). A fresh membrane disk was used for each performance. The permeate flux was recorded using an electronic balance (Shimadzu, VW2200H, accuracy ±0.1 g) connected with a computer at a fixed interval. The relative flux (J/J0) was employed as

Pore size

Hydrophobicity Porosity Manufacturer

0.1 μm

Hydrophilic

0

200

400

600

800

Filtration Volume (mL) 74%

Millipore Fig. 1. Flux decline of three source waters (DOC 5.0 mg/L, TMP 0.1 MPa, pH 7.0, 25 °C).

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showed low SUVA value and its N-HPI fraction proportion was higher than that of other water sources, which appears to imply that N-HPI fraction is responsible for flux decline. However, N-HPI fraction in HPJ was slightly less than that in GY water, while HPO fraction in HPJ was more than GY, which suggested that HPO also contributes to fouling. The flux decline for different DOM fractions of source waters is shown in Fig. 2. For all water sources, the flux with N-HPI declined most rapidly, followed by HPO and TPI fractions, and C-HPI showed the least. This result is consistent with previous study on MF fouling potential in which same fractionation method was employed [6]. It is interesting to note that, for the same fraction, different water sources showed different flux declines. For all fractions, SHW exhibited the greatest flux decline, followed by HPJ, and GY the least. This result is in agreement with the finding inferred from source waters and implies that water source is more important to the membrane fouling than the DOM hydrophobicity. That is because, as a complex mixture of various organic matters, the concentration, chemistry and composition of DOM are highly dependent on the source of water. Although DOM with similar functional groups or structures can be characterized as the same hydrophobicity fraction, they are still different in many other characters. For this reason, the better understanding of the source waters by the use of more characteristics would be necessary. 3.2. Relationship between MW and flux decline Molecular weight distribution of each fraction was determined by UF fractionation method. As is summarized in Fig. 3, SHW water contained the highest percentage of large MW (N10 kDa), followed by HPJ water, and GY water showed the least. This order of large MW is in agreement with that of flux decline, suggesting that large molecules have major contribution to flux decline. As was confirmed in the other

SHW

N-HPI

HPJ C-HPI

GY

TPI HPO 0%

5%

10%

.4

TPI Fraction

.8 .6 .4 .2

.2 0

200

400

600

800

0

Filtration Volume (mL)

400

600

800

N-HPI Fraction

1.0

Normalized Flux (J/Jo)

Normalized Flux (J/Jo)

200

Filtration Volume (mL)

C-HPI Fraction

1.0 .8 .6 .4 .2

.8 .6 .4 .2

0.0 0

200

400

600

Filtration Volume (mL)

800

30%

research, although most of the DOM came from small and medium size of molecules, membrane fouling was highly related to the large molecules [3]. That is because large molecules of DOM may consist of polysaccharide, proteins, colloidal or even some humic substances in aggregate [11]. They can narrow or block the membrane pores and form the gel layer on membrane surface, resulting in serious flux decline [11,12]. It is also found in Fig. 3 that most of the large molecules in SHW water came from N-HPI fraction, while the major proportion of large molecules in HPJ and GY water derived from both HPO and N-HPI fractions. This implies that large molecules contained in N-HPI fraction have the most effect on the flux decline. The large molecules contained in HPO fraction may lead to fouling to some extent and

Normalized Flux (J/Jo)

.6

25%

Fig. 3. Distribution of large molecules (N 10 kDa) in isolated fractions (DOC 10.0 mg/L, pH 7.0, 25 °C).

1.0

.8

20%

15%

Distribution percentage (%)

HPO Fraction

1.0

Normalized Flux (J/Jo)

Total

Isolated fractions

372

0

200

400

600

Filtration Volume (mL)

Fig. 2. Flux decline of isolated fractions (○: SHW water; ▽: HPJ water; □: GY water) (DOC 5.0 mg/L, TMP 0.1 MPa, pH 7.0, 25 °C).

800

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explain why fouling potential of HPJ was higher than that of GY water. N. Lee et al. reported in their research that HPO fraction with molecular size lower than 10 kDa was not responsible for fouling [4]. However, there was no further analysis on HPO fraction with higher molecular size. It was also found that large hydrophilic molecules, such as polysaccharides and proteins, were considered as important foulants [11,13]. Generally speaking, large molecules of DOM can be easily retained in the microfiltration process because of their physical character. But the influence and mechanism to the membrane flux would be different according to their chemical characters. In previous studies, hydrophobic interaction was considered as the major fouling mechanism between the hydrophobic fractions and membranes [14,15]. However, for a membrane of less hydrophobic tendency, the influence of pore blockage by the large molecular and hydrophilic fractions could be more obvious [13,15]. Therefore, in this research, the N-HPI fractions could give higher flux decline than HPO fractions. Although molecular distribution can be employed as an important character in analyzing membrane fouling potential, it is not the solution once for all. The extent of fouling is still hard to be figured out from it. For example, the proportion of large molecules in HPO fractions of SHW and GY waters is similar, but the flux declines of them are very different. Moreover, it is hard to interpret the flux discrepancy between HPJ and GY source waters if the large molecules in both of the two important fractions have their own advantages. Therefore, it is implied that not only molecular weight but also components of these problematic fractions should be considered for fouling. Previously, fluorescence EEM spectroscopy was used to distinguish component types of membrane foulants [8,9]. Some has reported that fluorescence parameters have positive relations with either hydrophobicity or molecular size of organic matters [16,17]. For this reason, EEM is employed in the study to make further understanding of the source waters. 3.3. Relationship between fluorophores and flux decline Fig. 4 illustrates the 3D fluorescence EEM spectra of three water sources. According to P.G. Coble's naming system, distinct fluorescence peaks can be identified from four regions [18]. SHW water shows two obvious peaks at Ex285/Em320 and Ex235/Em350, which can be labeled as peak T and peak B, respectively, and HPJ water exhibits a similar pattern with a weak response peak at Ex245/Em445. In contrast, fluorescence spectrum of GY water is different with the other waters and shows two major peaks at Ex325/Em420 and Ex255/Em430, which can be labeled as peak C and peak A, respectively. It was reported that peak C and peak A were attributed to humiclike substances such as humic acid and fulvic acid, whereas peak T and peak B were associated with protein-like substances such as tryptophan and tyrosine [7]. For this reason, SHW and HPJ waters are likely to contain similar components and may have similar membrane fouling potential. However, it was obvious that these two waters were different in characters and led to the different flux declines. On the contrary, GY water which shows much less proteinlike fluorophores than the other waters had similar flux decline with HPJ water and had similar hydrophilic proportions (Table 1) with it as well. So, it seems that water characters and fouling potentials are hard to be identified by the distribution of fluorophores in fluorescence EEM spectra. But in previous studies, it has been proved that water characters such as hydrophobicity and molecular size had very close relations with the distribution of fluorophores [19]. The contradiction comes from water source. Generally, natural organic matter is complex mixture with many molecular aggregations and attached functional groups. Fluorophores which are called humic-like are not only humic substances derived from terrestrial materials but also microbial residues which could contain protein-like organic matters [7,20]. In the same way, protein-like fluorophores such as tryptophan could not only be from free amino acid but also from related functional groups

Fig. 4. EEM fluorescence spectra of raw waters (DOC 5.0 mg/L, pH 7.0, 25 °C).

bonded within colloids or humic/fulvic-like molecules [21,22] which are in a large range of molecular size. Furthermore, different molecular structures have different excitation responses to the lamp source. Humic-like fluorophores could be very vague if the protein-like fluorophores had very high fluorescence intensities simultaneously. As is shown in Fig. 4, humic-like peaks in SHW water are not as obvious as in GY water, but their intensities (Table 3) in both waters are similar. In addition, it is obvious that waters which contained high intensity of protein-like fluorophores can lead to serious flux decline. Especially the fluorophore of peak B excluded in GY water, there must be some reasons with which the membrane flux decline was associated. For this reason, the distribution of fluorophores cannot be the only indicator for characterizing water sources. It would be a promising way to combine fluorescence intensity and location for characterizing MF foulants from various water sources. As is shown in Fig. 5, isolated fractions from SHW water could be distinguished according to their fluorescence EEM spectra. Although peak A and peak C are very vague in raw water, they are quite obvious in the HPO and TPI fractions. Comparatively, peak B and peak T in Em b350 nm (the emission wavelength less than 350 nm) are dominant

374

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Table 3 Fluorescence spectral parameters of DOM fractions. Water source

Fractions

SHW

Raw water HPO TPI N-HPI Raw water HPO TPI N-HPI Raw water HPO TPI N-HPI

HPJ

GY

a

Peak A

Peak C

Peak T

f450/500a

Peak B

Ex/Em

Intensity

Ex/Em

Intensity

Ex/Em

Intensity

Ex/Em

Intensity

245/415 245/420 245/430 245/400 245/445 245/425 245/445 240/370 255/430 260/440 255/455 245/415

4.45 3.43 4.86 4.26 18.06 13.24 12.73 10.36 3.90 4.94 4.40 2.73

315/415 315/430 315/420 315/400 305/435 315/435 315/430 315/410 325/420 315/430 315/425 315/400

3.85 3.73 4.84 3.50 7.87 7.92 8.29 5.08 3.09 3.62 4.19 2.50

285/320 – 285/325 280/320 280/320 290/350 280/320 280/315 285/320 – 285/320 285/320

8.79 – 5.85 2.83 25.67 9.59 16.50 4.82 2.15 – 1.86 1.51

235/350 – 230/335 – 235/350 240/355 235/335 – – – – –

6.63 – 3.95 – 34.26 24.60 14.75 – – – – –

1.44 1.23 1.42 1.64 1.43 1.23 1.37 1.75 1.30 1.15 1.34 1.58

f450/500 is named as fluorescence index which means the ratio of the emission intensity at a wave length of 450 nm to that at 500 nm, obtained with an excitation of 370 nm [24].

in C-HPI and TPI fractions. Similar shifts could also be observed in HPJ and GY waters (Table 3). Therefore, it is indicated that, to the same source water, hydrophobic fractions have more humic/fulvic-like

fluorophores in the longer emission wavelength (Em N 400 nm) than hydrophilic fractions. Meanwhile, protein-like fluorophores in the range of Em b350 nm are contained mainly in the less fouling

Fig. 5. EEM fluorescence spectra of isolated fractions (DOC 5.0 mg/L, pH 7.0, 25 °C).

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fractions and have little relations with membrane flux decline. In previous studies, T.F. Marhaba and K. Kimura also compared fluorescence peak locations with different DOM fractions [9,16]. It was found that the excitation and emission wavelengths were longer in hydrophobic fractions than in hydrophilic ones which were similar with this study. However, due to the different fractionation methods of DOM, peak locations of fractions were different. Furthermore, it is worth noting that the EEM spectra of N-HPI fractions from all source waters were similar which contain both humic/fulvic-like and protein-like fluorophores (Fig. 5). But, the locations of their humic/fulvic-like fluorophores are different from those in hydrophobic fractions. In Table 3, it is obvious that emission wavelength of peak C in N-HPI fractions is shorter than in other fractions. Besides, the fulvic-like fluorophore has shifted from Em N400 nm to Em N350 nm. According to the former researches [19,23], the blue shift of hydrophobic fluorophores was due to the break-up of large aromatic molecules or the decrease of aromatic rings in a chain structure. That is to say, the fluorophore in the range of 350 nm b Em b 400 nm is different with fulvic-like fluorophore. In addition, f450/500 could be used to distinguish fulvic acid derived from microbial sources [24]. It was reported that a fluorescence index of around 1.4 was terrestrially derived, while the index of ~ 1.9 was microbially derived [17,24]. Accordingly, to the fluorescence index (f450/500) of N-HPI (Table 3), it is suggested that fulviclike fluorophores in N-HPI fractions are prone to be derived from microbial organic matters such as protein or polysaccharides. So the fluorophore of this range could also be a result of red shift of protein-like fluorophore if it existed. Therefore, the overlapping region between fulvic-like fluorophore and tyrosine-like fluorophore (350 nm b Em b 400 nm) stands for the existence of large molecules with less aromatic structures and more microbial origins which could lead to the serious flux decline. In other words, it could be explained that the source waters which contained fluorophores in the special region between 350 nm b Em b 400 nm and 220 nm b Ex b 250 nm could lead to more serious membrane fouling. In order to confirm the above results, the fluorescence EEM spectra of both feed and permeate waters were obtained successively. The discrepancy of them was represented in Fig. 6 by subtraction of both fluorescence matrixes. As is shown, the retentions of all the three raw waters have fluorophores in the special region. But there are still some differences between them. SHW subtraction is closer to the tyrosinelike fluorophore which implies that the foulants of SHW water were prone to come from microbial origins. GY water is closer to the fulviclike fluorophore which implies that the source of GY water foulants tended to be terrestrial substances. Comparatively, the HPJ foulants may come from both origins because of the intermediate fluorophore range of its retentions. Besides, it is found that by calculating the proportion of fluorescence intensity in this special region, the MF fouling potential could be quantitatively analyzed. The intensity of total fluorophores in the spectrum between 300 nmb λemb 550 nm and 220 nmb λexb 400 nm was divided by the intensity of this special region. Based on the calculation, the proportions are 7.35% (SHW water), 5.71% (HPJ water) and 2.46% (GY water), respectively. That is to say, the more the fluorophores exist in this region, the more serious membrane fouling could be achieved to the source water. 4. Conclusions In this study, natural water foulants which caused MF flux decline were characterized through several methods. Based on the analyses, it is worth noting that hydrophobicity and molecular size have both effect on the membrane flux, and should not be considered separately. Natural water which led to rapid flux decline contained high proportion of large molecules (MW N 10 kDa) with neutral hydrophilic

375

Fig. 6. Subtraction EEM fluorescence spectra of raw waters (DOC 5.0 mg/L, pH 7.0, 25 °C).

character. Large molecules with hydrophobic character could lead to gradual decline which was less serious than the former. Fluorescence EEM spectral parameters contained useful information about hydrophobicity and molecular size. There is a special region in the fluorescence EEM spectrum between 350 nm b Em b 400 nm and 220 nm b Ex b 250 nm. This region was related to the large molecules with less aromatic structures and more microbial origins which were characterized as important foulants by former characters. Therefore, this special region in fluorescence EEM spectrum is very useful to predict foulants from natural waters. Comparatively, the fluorophores in other regions of EEM spectrum had less relation with membrane fouling. Two parameters were proposed in this study for the confirmation of special fluorescence region. The subtraction of fluorescence matrixes was a promising method to analyze source of foulants. And the proportion of fluorescence intensity in special region could be used as a parameter to estimate the extent of membrane fouling. However, the feasibility of these measurements should be discussed under more experiments of natural source waters.

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