Two-dimensional FTIR spectroscopic characterization of functional

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Two-dimensional FTIR spectroscopic characterization of functional groups of NaOCl-exposed alginate: Insights into membrane refouling after on-line chemical cleaning Zhong Yu, Zhongbo Zhou, Guocheng Huang, Xing Zheng, Linjie Wu, Shanshan Zhao, and Fangang Meng ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00082 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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ACS Applied Bio Materials

Two-dimensional FTIR spectroscopic characterization of functional groups of NaOCl-exposed alginate: Insights into membrane refouling after on-line chemical cleaning

Zhong Yu a,b, Zhongbo Zhou a,b, Guocheng Huang a,b, Xing Zheng c, Linjie Wu c, Shanshan Zhao a,b, Fangang Meng a,b*

a

School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510006,

PR China b

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation

Technology, Sun Yat-sen University, Guangzhou 510275, China c

Department of Civil and Environmental Engineering, Xi’an University of Technology, Xi’an

710048, China

*

Corresponding author.

Fangang MENG, Ph.D., Email: [email protected] ORCID ID: https://orcid.org/0000-0002-7976-9722

1

ACS Paragon Plus Environment

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Abstract: In-depth understandings of the interactions between NaOCl and foulants (e.g., alginate) at a molecular level can provide a deeper insight into the optimization of chemical cleaning. In this study, two-dimensional Fourier transform infrared correlation spectroscopic (2D-FTIR-COS) analysis was used to probe the hypochlorite oxidation of alginate at normal wastewater treatment operating pH (i.e., 6, 7, and 8). The 2D-FTIR-COS characterization indicated that the groups in alginate were degraded by NaOCl in a similar sequence at all investigated pHs: carboxylate groups (-COOH at pH 6 and 7) > C-O-C bonds, C-O bonds of uronic acids > C-O-C bonds. The results also showed that the presence of hemiacetals, which are formed by uronic acids, can protect alginate chains from oxidation during their initial exposure to NaOCl (30 min) and thus maintain alginate at a constant molecular size. Additionally, the rupture of C-O-C bonds resulting from initial oxidation (30 min) increased the viscosity of alginate solution because of the decreased stiffness of alginate chains and the rise of stronger intermolecular junctions. Long-term exposure to NaOCl (480 min) largely degraded uronic acids and the C-O-C bonds of alginate molecules, leading to decreases in molecular weight and viscosity at pH 6 and 7. However, due to the low degree of oxidation at pH 8, molecular weight and viscosity of alginates were slightly changed after long-term exposure (480 min) to NaOCl. Our results imply that the partial oxidation of alginate during hypochlorite scavenging enhanced its fouling propensity, which could have a potential implication for the implementation of chemical cleaning. Keywords: Chemical cleaning; Membrane fouling; Sodium hypochlorite; Alginate; 2


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2D-FTIR-COS

1. Introduction Over the past few decades, membrane bioreactors (MBRs) have gained increasing popularity in wastewater treatment and reuse

1-2

. However, membrane

fouling, which can result in significant flux decline, is an inherent problem for the practical application of MBRs 3. On-line chemical cleaning by NaOCl mixed with caustic (i.e., cleaning cocktail) is often conducted during MBR operation in order to maintain a constant permeability

4-6

. Nonetheless, NaOCl exposure during on-line

cleaning has been recognized to potentially impose adverse impacts on membrane performance 7-8. Chemical cleaning by NaOCl, for instance, can greatly modify the surface properties and pore size of membranes potential of the membranes

8-10

, which normally increases the fouling

10-11

. On the other hand, after oxidizing the foulants on

membrane surface, residual hypochlorite passes through the membrane and ends up in the bulk sludge of MBRs 7, whose pH falls between 6 and 9 (Figure 1)

12

. The

exposure of sludge to NaOCl can trigger the excessive production of bacterial extracellular polymeric substances (EPS)

13-14

, which leads to enhanced biofilm

formation 15 or fast refouling 7. It can be expected that the continuous release of EPS will shorten contact time between residual NaOCl and each individual organic molecule. Meanwhile, NaOCl in bulk sludge would behave differently from the one in cleaning cocktail due to the pH difference, hence the released EPS may be insufficiently degraded or removed

15

. Previous attempts also documented that 3



chemical cleaning beyond an optimal duration causes refouling of the membrane due to the redeposition of the detached foulant

16-17

. Therefore, the implementation of

chemical cleaning needs to be optimized with sufficient consideration of not only membrane aging but also the foulant alteration during the cleaning process and redeposition of detached foulant during membrane filtration.

Figure 1. Schematic illustration for on-line chemical cleaning and the residual NaOCl scavenging by bacterial EPS. In a practical situation, EPS consist of polysaccharides, proteins, humic acids, nucleic acids, lipids, etc

18

, and the primary role of polysaccharides on membrane

fouling in MBRs has been well documented 3. It is reported that on-line chemical cleaning is capable of removing most protein and humic substances, but only a little part of polysaccharide in EPS

19

. Therefore, polysaccharides may be insufficiently

degraded with a given NaOCl dosage and thus play an important role in the enhanced biofilm formation and fast refouling. Alginate is an acidic polysaccharide produced by bacteria, microalgae, and macroalgae

20

, which is abundant in various

natural environments 21-22. In the past decades, alginate has been widely employed as a model substance representative of the EPS in numerous studies of membrane organic fouling

23-24

. Early studies indicated that in MBR systems for wastewater 4


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treatment, alginate can structurally support bacteria by co-depositing with cells and forming a biofilm matrix on the membrane

25-26

. During on-line chemical cleaning,

extracellular alginate produced by bacteria can help to scavenge residual hypochlorite and thus enhance the resistance to killing by hypochlorite of bacteria 27. So far, factors affecting alginate fouling, such as pH 28, cation and ion strength

29-30

,

molecular weight 31, composition and sequential structure of alginate blocks 23, have been extensively studied. However, little information is currently available regarding the effect of molecular-level degradation process of alginate induced by NaOCl on the fouling redevelopment after on-line chemical cleaning. It should be noted that changes in molecular structure of extracellular alginate during hypochlorite scavenging is responsible for the enhanced biofilm formation and fast refouling after membrane filtration restarts. Unraveling the details of such degradation process may offer a better understanding of the relation between functional group changes and refouling behavior of alginate after NaOCl exposure. Fourier transform infrared (FTIR) spectroscopy is commonly used to investigate membrane fouling by the polysaccharide fraction

32

and can offer comprehensive insights into the molecular

structure of polysaccharides

33

. By determining the FTIR spectra of alginate after

NaOCl exposure, it is possible to explore the changes of certain functional groups in alginate. However, due to peak overlap

34-36

, one-dimensional FTIR spectra have

difficulty in unveiling the susceptibility of functional groups of alginate to NaOCl and clarifying the sequential order of their changes during degradation process. Thus, an urgent need exists for new strategies to enhance the spectral resolution and 5



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explore the molecular-level degradation process of alginate upon exposure to NaOCl. Two-dimensional correlation spectroscopic (2D-COS) analysis has been documented as a useful method to probe complicated interaction processes

34-36

. The 2D-COS

analysis can resolve overlapped peaks by extending one-dimensional spectra that are collected as a function of an external perturbation (e.g., time, temperature, pH or concentration) over a second dimension. Two-dimensional spectra generated according to Noda’s rule

37

are capable of providing crucial information about the

sequential order of spectral intensity changes upon external perturbation. It can be expected that the use of 2D-FTIR-COS can provide a clearer picture of the mechanisms of alginate degradation upon exposure to NaOCl. Therefore, the objective of this study is to systematically examine the degradation process of NaOCl-exposed alginate at simulated pHs of the bulk sludge of MBRs with the 2D-FTIR-COS technique. In addition, dead-end membrane filtration was conducted to understand the effects of NaOCl exposure on the refouling behavior of alginate. This study not only helps develop understanding the efficacy of cleaning fouled membranes but also aids in finding new ways to mitigate the problem of refouling after on-line chemical cleaning.

2. Materials and methods 2.1. Chemicals and NaOCl exposure assays All chemicals used in this study were analytical reagents. NaOCl stock solutions (available chlorine, > 7.50%) were purchased from the Guangzhou Chemical 6


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Reagent Factory (Guangzhou, China). The concentration of free chlorine was determined by the DPD (N,N-diethyl-p-phenylenediamine) method

38

before use.

Powdered sodium alginate (~398.31 kDa) from the Aladdin Industrial Corporation (Shanghai, China) was directly used without any purification. Stock alginate solutions (222 mg/L) were prepared with phosphate buffer (57.88 mM ionic strength, Table S1) to control the solution pH (i.e., pH = 6, 7, and 8) and avoid having different compressions of the electrical double layer in alginate solution

39

.

Afterwards, NaOCl stock solution (total free chlorine concentration, 2000 mg/L) was diluted into the alginate solution at a volume ratio of 1:9 to obtain final concentrations of both alginate and total free chlorine at 200 mg/L. Use of such low concentrations in NaOCl exposure assays allow the chemical reaction to proceed more slowly in order to probe the detailed degradation process of alginate molecules. NaOCl exposure assays of alginate were carried out at 30 °C in dark under gentle stirring. Solution samples were collected at 10, 30, 60, 120, 240, and 480 min. Note that the chemical reaction in the above samples was immediately terminated by the addition of 150 mM Na2SO3 solution into the mixed solution at a volume ratio of 1:30. Additionally, in order to collect the alginate sample at 0 min and maintain constant ionic strength with other samples, NaOCl stock solution was mixed with Na2SO3 solution prior to its addition to alginate solution. In this study, exposure time points of 0, 30, and 480 min were selected in order to obtain samples with different degrees of oxidation, which help us to investigate the effects of alginate degradation on fouling redevelopment. Therein, exposure time of 30 min was selected as 7



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previously described by Cai et al. to obtain partially oxidized alginate 7. Details on the chemicals used in this study can be found in Supporting Information (Text S1). 2.2. FTIR measurement and 2D-COS analysis The hypochlorite-treated samples were freeze-dried at −80 °C for 2 days (Alpha 1-4 LSCplus, Christ Co., Germany). Afterwards, a mixture of 1 mg of freeze-dried powder and 100 mg of KBr was ground, homogenized, and then pressed twice between two clean, polished iron anvils to form a KBr window under vacuum conditions. The FTIR spectra were obtained using a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific Co., USA). Each spectrum was recorded with a total of 16 scans over a range from 4000-700 cm−1 at a resolution of 2 cm−1. In this study, exposure time was applied as the external perturbation for the structural changes of alginate molecules. A set of time-dependent FTIR spectra were therefore collected. Prior to 2D-COS analysis, the FTIR spectra were baseline-corrected and denoised by using OMNIC 8.0 (Thermo Fisher Scientific Co., USA).

The

2D-COS computation

was

performed

by

2Dshige

software

(Kwansei-Gakuin University, Japan) to generate synchronous (Zmax = 1500, contour = 8) and asynchronous spectra (Zmax = 150, contour = 8) based on the work of Noda and Ozaki

37

. Detailed information about the 2D-COS computation could be

found in elsewhere

35, 40

. The synchronous and asynchronous spectra can be

interpreted by a set of well-established principles (a demo can be found in Supporting Information). In brief, autopeaks (denoted as Φ (v1, v1)), which are located on the diagonal of synchronous spectra, represent the susceptibility of 8


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intensity changes to the perturbation. Cross-peaks (denoted as Φ (v1, v2)), located off the diagonal, suggest the coordinated changes of spectral intensities observed at two different spectral variables (v1, v2). Cross-peaks with positive signs indicate that the intensity changes at the corresponding spectral coordinates are in the same direction, while negative signs indicate the opposite direction. In asynchronous spectra, cross-peaks (denoted as Ψ (v1, v2)) represent the sequence of the events (or variable changes) caused by given perturbations. In brief, if Φ (v1, v2) and Ψ (v1, v2) are of same sign, changes at peak v1 occur prior to those at peak v2; if they are of opposite signs, changes at v2 occur prior to those at v1; if Φ (v1, v2) or Ψ (v1, v2) is zero, the changes at v1 and v2 occur simultaneously. 2.3. Dead-end filtration tests A dead-end stirred cell (MSC300, Mosu Co., China) with a volume of 300 mL and an effective filtration area of 37.4 cm2 was used for the ultrafiltration experiments. Flat-sheet ultrafiltration membranes (PES, Microdyn-Nadir, Germany) with a molecular weight cut-off (MWCO) of 50 kDa and a diameter of 80 mm were used for the dead-end filtration test. Prior to ultrafiltration, the pristine membranes were first soaked in deionized water for 12 h and then filtered 300 mL of deionized water to remove impurities. The pure water permeability of the pristine membrane was determined with 200 mL of deionized water. Then, 200 mL of a fresh or NaOCl-treated alginate sample was introduced to the cell for filtration. The filtration pressure was fixed at 69 kPa for the whole experiment, and the rotating speed was maintained at 300 rpm. An electronic scale was connected to a computer to record 9



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the permeate production. In the present study, the extent of alginate fouling was evaluated by the unified membrane fouling index (UMFI) (Text S2). Recent studies have shown that UMFI is a robust parameter to describe fouling potential during low-pressure membrane filtration 41. 2.4. Analysis of molecular weight distribution In order to determine detailed MW distribution of alginate upon exposure to NaOCl, liquid chromatography with two size exclusion chromatography (SEC) columns, Toyopearl HW-50S and Toyopearl HW-65S (TOSOH Co., Japan), and an organic carbon detector (LC-OCD) (D-O-C Labor, German) were used. Details about LC-OCD can be found elsewhere 42. In brief, 1000 µL of sample was injected into the mobile phase of pH 6.72 (2.5 g/L KH2PO4 and 1.88 g/L Na2HPO4) after in-line filtration using a 0.45 µm filter. The LC columns then separated the injected organics according to their molecular size at a flow rate of 1.5 mL/min. The chromatography

subdivides

the

organics into

six

sub-fractions including

biopolymers (>20 kDa), humics (~1000 Da), building blocks (300~500 Da), low molecular weight acids ( vibration of C-O-C bonds (1122 cm−1) > C-O stretching of uronic acids (970 cm−1) > vibration of C-O-C bonds (1250 cm−1) > vibration of C-O-C bonds (1170 cm−1). Of note, changes of some functional groups occurred simultaneously due to the cross-peaks in asynchronous map exhibited zero correlation (i.e., Ψ = 0) at 1725/1605, 1725/1425, 1725/1320, 1725/1070, 1725/1020, 1725/880, 1605/1425, 1605/1320, 1605/1070, 1605/1020, 1605/880, 1425/1320, 1425/1070, 1425/1020, 1425/880, 1320/1070, 1320/1020, 1320/880, 1070/880, and 1020/880 cm−1. This could be related to two reasons: i) high oxidation-reduction potentials of NaOCl at lower pH induced the fast degradation of alginate

55

; ii) limited number of FTIR

spectra for alginate degradation at short-term (0-30 min) was not able to solve the overlapped peaks problem. However, 2D-COS analysis with all FTIR spectra (0-480 min) could, to a certain extent, provide some sequential information regarding the NaOCl-induced degradation of alginate at pH 6. The sequential order showed that NaOCl induced the hydrolyzation of polysaccharides at pH 6 alginate network caused by carboxyl groups

56

39

, which weakens

and hydrogen bonds

16


31

. As a result,

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the carboxylate groups (1725, 1605, and 1425 cm−1) were greatly impacted. The open structure of dissociated alginate provided more sites for NaOCl attack, resulting in an enhancement in C–C–H and O–C–H deformation (1320 cm−1), C-O-C rupture (1070 and 1020 cm−1), and C-H deformation (880 cm−1). With increasing exposure time, uronic acids (970 and 930 cm−1) were degraded by NaOCl, leading to the breakdown of hemiacetals. A previous study reported that the hemiacetals formed by urinates can protect the alginate backbone from oxidation

54

. Consequently, the

degradation of hemiacetals could be highly responsible for the subsequent degradation of the alginate backbone, e.g., C-O-C bonds (1250 and 1170 cm−1). Likewise, the sequence of characteristic bands at pH 7 followed the order 1632 → 1095 → 960 → 1295 → 1235 → 885 cm−1 according to the 2D-COS result (Table 2). This result demonstrated that at pH 7, the asymmetric C-O stretching of free COOH groups (1632 cm−1) decreased, which indicated the rupture of hydrogen bonds and dissociation of alginate blocks at the initial exposure to NaOCl. C-O-C (1095 cm−1) and C-O bonds of uronic acids (960 cm−1) were subsequently attacked, leading to the degradation of more C-O-C bonds (1295 and 1235 cm−1). At pH 8 (Table 3), the sequence followed the order 855 → 940 → 1050 → 1100 → 1190 cm−1 (i.e., C-H deformation > C-O stretching of uronic acids > C-O-C stretching > vibration of C-O-C bonds > vibration of C-O-C bonds). Similarly, NaOCl triggered the degradation of C-O bonds in uronic acids (940 cm−1) at pH 8, followed by the degradation of C-O-C bonds (1190, 1100, and 1050 cm−1). However, the degradation of C-O in free COOH groups was not detected at pH 8. 17



Figure 4. Synchronous (a, c, and e) and asynchronous (b, d, and f) maps in the 1750-800 cm−1 region generated from the FTIR spectra of alginate upon hypochlorite exposure at three pHs (i.e., 6, 7, and 8) with exposure time as the perturbation. Red represents positive correlation, while blue represents negative correlation. Darker color indicates a stronger correlation. The signs of corresponding autopeaks (on the diagonal) and cross-peaks (off the diagonal) in both synchronous and asynchronous maps are presented in Table 1, 2, and 3 (pH = 6, 18


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7, and 8, respectively) for further interpretation.

19


ACS Applied Bio Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 20 of 40

Table 1. Signs of cross-peaks in synchronous and asynchronous (in the bracket) maps of alginate upon exposure to NaOCl at pH 6

Peak (cm−1) 1725

Assignment C=O stretch of COOH

Sign* at pH 6 1725

1605

1425

1320

1250

1170

1122

1070

1020

970

930

880

+

+(0)

+(0)

+(0)

-(-)

-(-)

+(+)

+(0)

+(0)

-(-)

+(+)

+(0)

+

+(0)

+(0)

-(-)

-(-)

+(+)

+(0)

+(0)

-(-)

+(+)

+(0)

+

+(0)

+(-)

-(-)

+(+)

+(0)

+(0)

-(-)

+(+)

+(0)

+

+(0)

-(-)

+(+)

+(0)

+(0)

-(-)

+(+)

+(0)

+

+(+)

-(+)

-(+)

-(0)

+(-)

-(+)

-(+)

+

-(+)

-(+)

-(+)

+(-)

-(+)

-(+)

+

+(-)

+(-)

-(-)

+(-)

+(-)

+

+(+)

-(-)

+(+)

+(0)

asymmetric

1605

vibration of COOH symmetric

1425

vibration of COOH C–C–H and

1320

O–C–H deformation

1250 1170 1122

vibration of C-O-C bonds vibration of C-O-C bonds vibration of C-O-C bonds antisymmetric

1070

C-O-C stretching 20


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C-O-C

1020

+

stretching

-(-)

+(+)

+(0)

+

-(+)

-(+)

+

+(-)

C-O

970

stretching of uronic acids C-O

930

stretching of uronic acids C-H

880

+

deformation Note: (i)

“+” means a positive sign; “-” means a negative sign; “0” denotes zero correlation.

(ii)

“*” denotes that signs were obtained in the upper-left corner of the maps.

(iii) If Φ (v1, v2) and Ψ (v1, v2) are of same sign, changes at peak v1 occur prior to those at peak v2; if they are of opposite signs, changes at v2 occur prior to those at v1; if Φ (v1, v2) or Ψ (v1, v2) is zero, the changes at v1 and v2 occur simultaneously. (iv) Ψ (v1, v2) are zero at 1725/1605, 1725/1425, 1725/1320, 1725/1070, 1725/1020, 1725/880, 1605/1425, 1605/1320, 1605/1070, 1605/1020, 1605/880, 1425/1320, 1425/1070, 1425/1020, 1425/880, 1320/1070, 1320/1020, 1320/880, 1070/880, and 1020/880 cm−1. Therefore, the changes of corresponding functional groups may occur simultaneously. This is possibly due to the fact that limited number of FTIR spectra for fast alginate degradation at short-term (0-30 min) was not able to solve the overlapped peaks problem.

21



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Peak (cm−1)

1632

1295 1235 1095

960

885

Assignment

Sign* at pH 7 1632

1295

1235

1095

960

885

+

+(+)

+(+)

+(+)

+(+)

+(+)

+

+(+)

+(-)

+(-)

+(+)

+

+(-)

+(-)

+(+)

+

+(+)

+(+)

+

+(+)

asymmetric C-O stretching of COOH vibration of C-O-C bonds vibration of C-O-C bonds C-O-C stretching C-O stretching of uronic acids C-H deformation

+

Note: (i) “+” means a positive sign; “-” means a negative sign; “0” denotes zero correlation. (ii) “*” denotes that signs were obtained in the upper-left corner of the maps.


Peak (cm−1) 1190 1100 1050

940

855

Assignment vibration of C-O-C bonds vibration of C-O-C bonds C-O-C stretching C-O stretching of uronic acids C-H deformation

Sign* at pH 8 1190

1100

1050

940

855

+

+(-)

+(-)

+(-)

+(-)

+

+(-)

+(-)

+(-)

+

+(-)

+(-)

+

+(-)

+

Note: (i) “+” means a positive sign; “-” means a negative sign; “0” denotes zero correlation. (ii) “*” denotes that signs were obtained in the upper-left corner of the maps.

22


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3.3. Fouling propensity of the fresh and NaOCl-treated alginate

As presented in Figure 5, the UMFI of fresh alginate solution declined as the pH increased, indicating that alkaline conditions could help mitigate the fouling of polysaccharides. This result can be explained by previous attempts in which alginate network in the cake layer trends to shrink at lower pH due to restricted electrostatic repulsion among the protonated carboxylates

51, 57

. It is interesting to note that the

initial treatment with NaOCl (30 min) increased the UMFI of alginate by 20.70, 85.89, and 116.01% at pH 6, 7, and 8, respectively; after long-term exposure (480 min) at pH 6 and 7, the UMFI were 86.87 and 8.80% lower than those of untreated alginate, respectively; however, the UMFI remained constant after long-term exposure at pH 8. It implies that the use of sufficient exposure time is recommended during chemical cleaning to achieve a satisfying cleaning performance. In addition, after 480 min of exposure, the alginate solution at pH 7 was of a high fouling propensity, followed by that at pH 8 and pH 6. This phenomenon was quite different from that of untreated alginate (pH 6 > pH 7 > pH 8), attributable to the NaOCl-induced changes in alginates.

23



Figure 5. Normalized flux decline (J/J0) (a, b, and c) and UMFI (d) of alginate as a function of time with increasing time of NaOCl exposure at pH 6, 7, and 8. Error bars of UMFI represent the standard deviation based on triplicate filtration tests.

Molecular size can be considered as one of the items affecting the fouling propensity of biopolymers and organic matter in water. In this study, LC-OCD chromatograms of fresh and NaOCl-treated alginate were analyzed. As seen in Figure 6, short-term exposure to NaOCl (30 min) at the three pHs did not lead to significant

changes in the molecular size distribution of the alginate from that of untreated alginate at the corresponding pH. These results indicated that the LC-OCD compositions of treated alginate did not differ significantly with those of fresh alginate (Figure 7). It thus suggests that the increased fouling propensity of alginate after short-term exposure to NaOCl should be related to the changes in functional groups rather than changes in molecular size. Under long-term exposure to NaOCl

24


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(480 min), however, obvious increase in the elution time (from 1.5-2 min, Figure 6) and percentages of building blocks and biopolymers (20~100 kDa) (increased by 5~10%, Figure 7), indicates a significant decrease in the molecular size of alginate at pH 6 and 7. In comparison, the exposure to NaOCl at pH 8 led to slight shift of the molecules to be larger in size as the elution time of the chromatogram peak decreased by one minute and percentages of biopolymers increased by 8%.

25



4

0 min

OCD-Signal

3.5

30 min

pH = 6

480 min

3 Bypass

2.5

Polysacchrides

2 1.5 1 0

10

20

4

OCD-Signal

30 40 50 60 70 Retention time / min

80

90 100

0 min 30 min

3.5

pH = 7

480 min

3 Bypass

2.5

Polysacchrides

2 1.5 1 0

10

20

4


80

90 100

0 min

3.5 OCD-Signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

30 min

pH = 8

480 min

3 Bypass

2.5

Polysacchrides

2 1.5 1 0

10

20


80

90 100

Figure 6. LC-OCD chromatograms of alginate upon exposure to NaOCl at pH 6, 7, and 8. In general, the organics with a shorter elution time have a larger molecular size. Given that two size exclusion chromatography columns were used to separate the dissolved organic matters in this study, the retention time of each fraction is longer than the reported retention time. Meanwhile, due to the less complex compositions of fresh and treated alginate compared to natural water, no distinct peaks corresponding to humic substances, building blocks, low molecular organics and neutrals can be observed in the LC-OCD chromatograms. 26


100%

80%

80%

60%

60%

pH = 7

100%

40% 20%

40% 20%

0%

0% 0 min

30 min

480 min

0 min

30 min

480 min

100% 80% pH = 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pH = 6

Page 27 of 40

LMW Neutrals

60%

Building Blocks

40%

Biopolymers 20~100 kDa

20%

Biopolymers >100 kDa

0% 0 min

30 min

480 min

Figure 7. Quantification of organic fractions from the treated alginate at different pHs as a function of time determined by LC-OCD and dead-end filtration. Values expressed as percent of dissolved organic matters. The organic compositions of treated alginate, including biopolymers, building blocks, and LMW neutrals were characterized by LC-OCD. Notably, no LMW acids was identified in this study. Furthermore, molecular size distributions of biopolymers were characterized by dead-end filtration test using PVDF membrane with a MWCO of 100 kDa.

4. Discussion 4.1. NaOCl-induced degradation of alginate molecules at different pH

Overall, 2D-FTIR-COS analysis of the degradation of alginate molecules upon exposure to NaOCl differed somewhat at different pH values. At pH 6 and 7, NaOCl preferentially reacted with carboxylate groups (-COOH) over other groups. Nevertheless, this phenomenon was not found at pH 8. Similar degradation sequences of alginate were roughly identified at all three pHs (i.e., C-O-C bonds, C-O bonds of uronic acids > C-O-C bonds). However, the susceptibility of these functional groups 27



Page 28 of 40

in alginate to NaOCl increased with decreasing pH according to synchronous maps, suggesting a more severe degradation of alginate molecule at pH 6. Several reasons may explain the different results obtained at the three pHs. On one hand, alginate tends to be more cross-linked at lower pHs 58 due to the bridging of carboxyl groups 59, but tends to have a more open structure under caustic conditions

39

. Meanwhile,

deprotonation of carboxylate groups at pH 8 can lead to effective anion-anion repulsion between OCl- and alginate molecules. Therefore, the dissociation of cross-linked alginate during the initial degradation at lower pH resulted in greater changes in carboxylate groups. On the other hand, the oxidation-reduction potentials of NaOCl largely depend on pH

55

. According to ionization equilibrium

52

, the

available chlorine in bulk solution existing in the form of HOCl is approximately ~97.2% at pH 6, ~77.5% at pH 7, but only ~25.6% at pH 8. Given that HOCl is a stronger electrophilic chlorinating agent than OCl-, oxidation-reduction potential of NaOCl is much higher at a lower pH. Therefore, NaOCl can induce a more drastic degradation of alginate chains at low pH. 4.2. Consequence of NaOCl treatment to the redevelopment of alginate fouling

Changes in the functional groups of alginate upon exposure to NaOCl are responsible for the detachment of polysaccharides from fouled membranes as well as refouling after chemical cleaning. It has been documented that fouling propensity of polysaccharides linearly increases with increasing content of uronic acids

60

. As

expected, variations of C-O bonds in uronic acids as a result of chemical cleaning should play an important role in the removal of alginate foulants. A number of studies 28


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reported that the reaction between aldehyde groups in uronates and alcohol groups in the vicinity yielded hemiacetals, which can protect alginate chain from the oxidation by NaOCl or HOCl

54, 61

. As such, the degree of alginate chain scission was low

during short-term (30 min) exposure to NaOCl, which was responsible for the constant MW distribution (Figure 6 and Figure 7). Additionally, the rupture of C-O-C bonds in pyranosyl rings during the initial treatment by NaOCl (30 min) decreased the stiffness of alginate molecules

54

, giving rise to intermolecular binding. These

phenomena were further confirmed by the increased viscosity of treated alginate (Figure S2). Taken together, because of the unchanged molecular size and increased viscosity, the fouling potential of alginate increased after short-term (30 min) exposure to NaOCl (Figure 5). With increasing exposure time (480 min), hemiacetals were damaged, thus leading to a higher degree of chain scission (i.e., C-O-C bond rupture) in the alginate backbone. The chain scission of alginate molecules provided larger rotational freedom and smaller molecular size of the alginate (Figure 6 and 7) 54, as well as a decrease in the viscosity of alginate solution (Figure S2); these changes in alginate properties resulted in a lower fouling propensity of the alginate solution (Figure 5). As expected, NaOCl with higher oxidation-reduction potentials at pH 6 induced a greater degree of degradation in the alginate chain and thus achieved a higher cleaning efficiency than that at pH 7. Intriguingly, the molecular size and fouling potential of alginate increased even after long-term exposure to NaOCl (480 min) at pH 8. Possible mechanisms underlying these increases of molecular weight and fouling potential at 29



pH 8 may be due to the low degree of oxidation as well as stronger intermolecular binding of alginate after 480 min exposure. At pH 8, the low HOCl concentration led to only minor structural change in alginate molecules (Figure 4e), which maintain a constant molecular size of alginate after long-term exposure. Meanwhile, the low degree of oxidation of alginate can result in relatively stronger intermolecular binding, evidenced by increased viscosity of alginate solution (Figure S2). Therefore, the alginate will preferentially form larger-size aggregate and compact network in cake layer (i.e., high fouling potential) after long-term exposure at pH 8, which is consistent with the previous reports of hydrogels formation by the partially oxidized alginate 62-63. 4.3. Significance for the chemical cleaning of fouled membranes

Alginate is regarded as a model extracellular polymeric substances due to its abundance in various natural environments

21-22

, and one of the key fouling-causing

substance 64-65. Detailed information about interaction between NaOCl and alginate at a molecular level is needed for better understanding on enhanced fouling propensity as well as improvement of on-line cleaning. In present study, 2D-FTIR-COS analysis was used to explore the degradation process of alginate exposed to NaOCl (i.e., sequential orders of functional group variation). Our results revealed that during the initial degradation of alginate, partial oxidation by hypochlorite does not induce significant changes in the molecular size of alginate but does induce stronger intermolecular binding. It can be expected that network of cake layer formed by partially degraded alginate are more compact, which provide a highly hydrated 30


Page 30 of 40

Page 31 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


biofilm matrix for microorganisms to embed

66

. As such, partial degradation of

extracellular alginate during hypochlorite scavenging would lead to a fast refouling after on-line chemical cleaning. Unfortunately, due to the complex nature of real sludge and complex process of fouling occurrence, the simulation of actual MBR systems in this study may not be convincing enough. For instances, the interaction between cations (e.g., calcium) and partially degraded alginate, the NaOCl-induced degradation process of other kinds of foulants (e.g., different kinds of polysaccharides, protein, humic substance, etc) are not studied in present study, hence their effects on fouling redevelopment remain an open question and motivate further investigation in the future. However, the results presented in this study do, to some extents, shed a light on the adverse impacts on membrane performance imposed by on-line chemical cleaning Such adverse sides of on-line chemical cleaning are inevitable in the practical operation of large-scale MBRs. Generally, in order to mitigate the problem of enhanced fouling propensity and maintain a satisfying cleaning performance, chemical cleaning is performed with concentrated hypochlorite solutions. Whereas, the harsh environment during chemical cleaning can result in deteriorated mechanical strength

67

as well as the profound fouling of post-cleaned membranes 68. Therefore,

the currently used cleaning protocols should be optimized, or novel cleaning strategies should be developed 7. In the future, more attention needs to be focused on the combination of different cleaning technologies and chemical cleaning. For instance, quorum quenching can reduce the production of polysaccharide (e.g. alginate) 31


69-70

,


Page 32 of 40

which guarantees the complete oxidation of orgainc molecules with a given NaOCl dosage. Additionally, enzymatic cleaning can promote the degradation of EPS

17

,

which may help remove the residual foulants after the partial oxidation of alginate by NaOCl. If these technologies can be integrated into chemical cleaning, it should greatly contribute to the improvement of on-line cleaning.

5. Conclusions The NaOCl-induced degradation of alginate was investigated at a molecular level using 2D-FTIR spectroscopy. The main conclusions of this study are summarized as follows: 2D-FTIR-COS results demonstrated that alginate is degraded by NaOCl at three pHs (i.e., 6, 7, and 8) in a similar sequence: carboxylate groups (-COOH found at pH 6 and 7) > C-O-C bond, C-O bonds of uronic acids > C-O-C bond. During the initial treatment with NaOCl (30 min), presence of hemiacetals, which are formed by uronic acids, protected the alginate chains from oxidation, maintaining the molecular size of alginate at a constant value. Meanwhile, C-O-C bond rupture during the initial degradation of alginate decreased the stiffness of the polymer chain and induced stronger intermolecular binding. With increasing NaOCl exposure time (480 min), uronic acids and C-O-C bonds of alginate molecules were largely degraded, thus decreasing molecular weight and viscosity of alginate. Due to the higher oxidation-reduction potentials of NaOCl at low pH, susceptibility of functional groups of alginate to NaOCl was higher at pH 6 and 7 than that at pH 8. Initial degradation of alginate foulants by NaOCl enhanced the fouling propensity 32


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of alginate solution, hence contributing to fast refouling after on-line chemical cleaning. With the aid of information gained from the 2D-FTIR-COS analysis, we can look for new ways to mitigate the problem of refouling after on-line chemical cleaning.

Acknowledgements This study was supported by the National Natural Science Foundation of China (No. 51622813, 51608546 and 51738012), the Natural Science Foundation of Guangdong Province (No. 2014A030306002), and the Science and Technology Planning Project of Guangdong Province (No. 2015A020215014).

Supporting Information Text S1. Chemicals purchasing. Text S2: Interpretation method for 2D-COS maps. Text S3. Assessment of the UMFI. Text S4. Viscosity measurement. Table S1.

Formulas for buffer solutions with different pH. Figure S1. A demo for interpretation of 2D-COS maps. Figure S2. Changes in viscosity of alginate solution at different pH during chlorine exposure. Detailed information on viscosity measurements can be found in Text S3

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Figure Captions: 37



Figure 1. Schematic illustration for on-line chemical cleaning and the residual NaOCl scavenging by bacterial EPS. Figure 2. FTIR spectra of fresh alginate at pH 6, 7, and 8 in the region of 1800-800 cm−1. Figure 3. FTIR spectra of alginate as a function of hypochlorite exposure time (0-480 min) at pH 6, 7, and 8 in the region of 1800-800 cm−1. Figure 4. Synchronous (a, c, and e) and asynchronous (b, d, and f) maps in the 1750-800 cm−1 region generated from the FTIR spectra of alginate upon hypochlorite exposure at three pHs (i.e., 6, 7, and 8) with exposure time as the perturbation. Red represents positive correlation, while blue represents negative correlation. Darker color indicates a stronger correlation. The signs of corresponding autopeaks and cross-peaks in both synchronous and asynchronous maps are presented in Table 1, 2, and 3 (pH = 6, 7, and 8, respectively) for further interpretation. Figure 5. Normalized flux decline (J/J0) (a, b, and c) and UMFI (d) of alginate as a function of time with increasing time of NaOCl exposure at pH 6, 7, and 8. Error bars of UMFI represent the standard deviation based on triplicate filtration tests. Figure 6. LC-OCD chromatograms of alginate upon exposure to NaOCl at pH 6, 7, and 8. In general, the organics with a shorter elution time have a larger molecular size. Given that two size exclusion chromatography columns were used to separate the dissolved organic matters in this study, the retention time of each fraction is longer than the reported retention time. Meanwhile, due to the less complex compositions of fresh and treated alginate compared to natural water, no distinct peaks corresponding to humic substances, building blocks, low molecular organics and neutrals can be observed in the LC-OCD chromatograms. Figure 7. Quantification of organic fractions from the treated alginate at different pHs as a 38


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function of time determined by LC-OCD and dead-end filtration. Values expressed as percent of dissolved organic matters. The organic compositions of treated alginate, including biopolymers, building blocks, and LMW neutrals were characterized by LC-OCD. Notably, no LMW acids was identified in this study. Furthermore, molecular size distributions of biopolymers were characterized by dead-end filtration test using PVDF membrane with a MWCO of 100 kDa.

Table Captions: Table 1. Signs of cross-peaks in synchronous and asynchronous (in the bracket) maps of alginate upon exposure to NaOCl at pH 6. Table 2. Signs of cross-peaks in synchronous and asynchronous (in the bracket) maps of alginate upon exposure to NaOCl at pH 7. Table 3. Signs of cross-peaks in synchronous and asynchronous (in the bracket) maps of alginate upon exposure to NaOCl at pH 8.

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Two-dimensional FTIR spectroscopic characterization of functional

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