Enteromorpha prolifera polysaccharide based coagulant aid for humic acids removal and ultrafiltration membrane fouling control

Journal Pre-proof Enteromorpha prolifera polysaccharide based coagulant aid for humic acids removal and ultrafiltration membrane fouling control

Shuang Zhao, Qianshu Sun, Yingqiu Gu, Weihua Yang, Yun Chen, Jing Lin, Mengyao Dong, Haoyan Cheng, Hao Hu, Zhanhu Guo PII:

S0141-8130(20)30454-2

DOI:

https://doi.org/10.1016/j.ijbiomac.2020.02.273

Reference:

BIOMAC 14890

To appear in:

International Journal of Biological Macromolecules

Received date:

24 January 2020

Revised date:

18 February 2020

Accepted date:

24 February 2020

Please cite this article as: S. Zhao, Q. Sun, Y. Gu, et al., Enteromorpha prolifera polysaccharide based coagulant aid for humic acids removal and ultrafiltration membrane fouling control, International Journal of Biological Macromolecules(2020), https://doi.org/10.1016/j.ijbiomac.2020.02.273

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© 2020 Published by Elsevier.

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Enteromorpha prolifera Polysaccharide Based Coagulant aid for Humic Acids Removal and Ultrafiltration Membrane Fouling Control

Shuang Zhaoa, Qianshu Sunb, Yingqiu Gua,*, Weihua Yanga, Yun Chen,e Jing Linc,*, Mengyao Dongd,e, Haoyan Chengf, Hao Huf, Zhanhu Guoe,*

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b

School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou 221000, China

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a

School of Environmental Science and Engineering, Ocean University of China, Qingdao

School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006

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c

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266100, China

Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of

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d

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China

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Education, National Engineering Research Center for Advanced Polymer Processing Technology,

e

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Zhengzhou University, Zhengzhou 450001, China Integrated Composites Lab (ICL), Department of Chemical & Biomolecular Engineering,

University of Tennessee, Knoxville, TN 37996, USA f

College of Material Science and Engineering, Henan University of Science and Technology,

Luoyang 471023, China. Corresponding author e-mail: [email protected] (Y. G); [email protected] (J. Lin); [email protected] (Z. Guo)

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Abstract Polyacrylamide (PAM) has been used as a coagulant aid in water treatment process for past decades, but it has caused great damages to human nervous system. Developing new coagulant aid with high biological safety is urgently demanded. This study provides a natural biomacromolecule coagulant aid with good biosecurity-Enteromorpha prolifera polysaccharide

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(Ep). Its coagulant aid efficiency and mechanism were investigated in terms of organics removal,

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floc properties and membrane fouling degree. In addition, contrast experiments were conducted

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with PAM to evaluate its potential of industrial applications. Results showed that organics

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removal could be increased by 23% when 0.3 mg/L Ep was used, which exhibited comparable

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aid effects to PAM. Due to the bridging-sweep aid role of Ep, flocs sizes, growth rate and

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recovery factor reached 470 μm, 62.6 μm/min and 0.492, respectively, while only 170 μm, 14.0

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μm/min and 0.326 were obtained by PAM. Additionally, flocs exhibited more porous and

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multi-branched structures when Ep was applied, which caused less ultrafiltration membrane fouling (eventual J/J0 value=0.52). As a result, Ep could be considered as a potential substitute of PAM, since better biosecurity, higher organics removal and lower membrane fouling could be obtained simultaneously by Ep addition.

Keywords: Enteromorpha prolifera polysaccharide; Polyacrylamide; Coagulant aid

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1. Introduction Humic acid (HA), the commonest natural organic matter in surface or drinking water, has been regarded as the most important precursor to form disinfection by-products [1-3]. For HA

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eliminating, ultrafiltration (UF) technology is regarded as one of the most efficient ways, to meet

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stringent requirements of water quality regulations [4]. For example, Yang et al. [5] reported a

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UF system which demonstrated removal rates of dissolved organic carbon (DOC), UV254 (a

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water quality test parameter providing a quick measurement of the organic matter in water) and

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phosphate up to 50%, 80% and 90%, respectively. However, membrane fouling is the biggest

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obstacle for wide applications of UF technology [6-9], which may result in a flux decline and

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even a membrane permanent deterioration. To solve this problem, applying coagulation unit as

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pre-treatment measure prior to UF process was proposed recently. The coagulation-ultrafiltration hybrid system (C-UF) was expected to reduce membrane fouling and to improve HA removal simultaneously [5, 10, 11]. For a C-UF process, membrane fouling is associated with the properties of flocs generated in the coagulation unit, including floc size, strength and structure [12-14]. Generally, large and porous flocs produced by bridging mechanism can cause a slight membrane fouling, while flocs generated by charge neutralization always result in a severe pore blocking resistance due to its compact structure. Therefore, adjusting operation conditions of coagulation to get large and porous flocs becomes a necessary pathway for reducing the 3

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membrane fouling. To obtain large and porous flocs, coagulant aids are dosed in combination with coagulants, which has been confirmed to be an effective and low-cost approach. For example, polyacrylamide (PAM) is widely used at present due to its apparent coagulating aid efficiency [15, 16]. However, acrylamide (the hydrolyzed monomer of PAM) exhibits a great damage to

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human nervous system and is difficult to be biodegraded, which causes considerable risks in

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water treatment process [17]. To guarantee effluent quality and reduce PAM doses, developing

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alternative coagulant aids with a high biological safety becomes an urgent issue to be solved.

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Considering the secondary pollution of PAM, research hotspots turned to natural

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biomacromolecules coagulant aids with a better biodegradability, such as algal polysaccharides

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[18-23]. Algae is abundant in China, and some species, such as Enteromorpha prolifera (E.

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prolifera), are even outbreaking in large-scale due to water eutrophication during past decades.

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Numerous floating E. prolifera cause serious environmental problems and need to be reused urgently. For example, Yu et al. reported that E. prolifera polysaccharides (denote as Ep in this paper) accounts for more than 50% of dry weight [24]. Ep, whose molecule weight achieved 500 kDa and was mainly composed of rhamnose, xylose and glucuronic acid [25-27]. Moreover, plentiful carboxylic acid groups, hydroxyl groups and sulfonic acid groups were found in Ep [26, 28]. These groups will form a chelate network structure with coagulant metal ions [29-35], which could exert electric neutralization effect of coagulant, and meanwhile play a net trapping role to improve the HA removal. In addition, due to the adsorption bridging effect of Ep macromolecule 4

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and sweep action of gel network, large and porous flocs are possible to generate coagulation units. Therefore, Ep could be applied as a new coagulant aid in theory to eliminate HA and reduce UF membrane fouling simultaneously. Moreover, a recycle way of E. prolifera waste is also needed to solve ecological problem. Above all, as nature biological macromolecule, Ep has been widely used for regulating blood glucose and blood lipids, which exhibited preferable

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biological safety compared with PAM. For example, Sun et al. reported that when Ep was

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applied as the coagulant aid of polymeric aluminum, organics could not be fully eliminated (the

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highest HA removal was about 94%) [26]. To remove organics completely, ultrafiltration unit

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was applied on the basis of coagulation process in this study. HA removal efficiency, coagulation

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kinetics, action mechanism and membrane fouling were investigated systematically. To evaluate

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the market application potential of Ep, PAM was selected as traditional coagulant aid and a series

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of contrast experiments were conducted.

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2. Materials and methods

2.1. Water sample and coagulant Synthetic test water consists of HA and kaolin stock solution, and the detailed preparation methods could be found in a previous study [36]. The properties of water sample were as follows: HA concentration=10 mg/L, Turbidity=14.5-15.5 NTU, pH= 8.19-8.43. Polymeric aluminum (PAC) with an alum concentration of 10.0 g/L and OH/Al3+ value of 1.0 was prepared by AlCl3·6H2O and NaCO3 [36]. 2.2. Preparation and characterization of coagulant aid 5

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Cationic polyacrylamide (PAM) with a molecular weight of 3000 kDa was purchased directly. Its stock solution of 1 g/L was obtained by dissolving PAM into deionized water and then kept stirring for 4 hours. Ep was extracted from fresh E. prolifera and then applied as new coagulant aid, which was prepared as follows: (1) fresh algae was washed, dried, crushed and then mixed with deionized

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water at a mass ratio of 1:75, followed by ultrasonic treatment for 30 min with power 700 W; (2)

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the mixture was heated in water bath for 4 hours at 90 °C, and then supernatant was collected

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and enriched to 20% of its original volume; (3) excess alcohol was added followed by

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precipitation for 2 hours; (4) the precipitates were dried by a lyophilizer, which was donated as

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Ep. Before addition, Ep powders were dissolved in deionized water with continuous stirring for

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6-8 hours. The concentration, viscosity, pH and zeta potential of Ep stock solution were 1 g/L,

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1.12×10-3 Pa·s, 6.50 and -40.30 mV, respectively.

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Characterization analysis, including energy dispersive spectrometer (EDS), infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) were conducted. EDS detections were captured with an EDAX (PW9900) with an acceleration of 20 kV. IR spectra of Ep were measured by a Fourier Transform Infrared Spectrometer (AVATAR 370) in the frequency range of 4000-500 cm-1. Then Ep powder was dissolved in D2O and the NMR data were collected on Bruker Advance 400 spectrometer and operated at a frequency of 400.13 MHz. Chemical shifts are given in values of δ (ppm), referenced to residual solvent signals (4.79 ppm for 1H in D2O). 2.3. Coagulation procedure and efficiency 6

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Synthetic water sample of 1 L was mixed well with paddled stirrer. It was reported that a better coagulation effect was observed when alum coagulant was dosed before organic polymer [37]. Therefore, PAC was dosed firstly and then Ep or PAM was dosed 30 s later, which was defined as PAC-Ep or PAC-PAM composite coagulant. Then samples were subjected to rapid mixing (200 rpm) for 1 min, followed by slow mixing (40 rpm) for 15 min. Finally, flocs were

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allowed to precipitate for 20 min. UV254 and DOC were selected to evaluate the removal

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efficiency of organics, which could be analyzed with an UV-754 UV/VIS spectrophotometer

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(Shanghai Precision Scientific Instrument Co. Ltd., China) and TOC-VCPH analyzer (Shimadzu,

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2.4. Coagulation kinetics

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explore coagulation mechanism.

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Japan). Moreover, zeta potential was also measured by zeta sizer 3000HSa (Malvern, UK) to

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The generation, growth, breakage, regrowth and sedimentation process of flocs could be

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studied by the combination of coagulation generating device and laser diffraction instrument. Flocs generated in the coagulation unit were introduced into Mastersizer to measure sizes. As coagulation proceeding, variation of flocs sizes could be real-time monitored (Fig. 1). Coagulation procedure was listed as follows: coagulant dosed→rapid mixing for 1 min (floc generation)→slow mixing for 15 min (floc growth)→rapid mixing for 3 min (floc breakage)→ slow mixing for 5 min (floc regrowth)→sedimentation for 30 min. Breakage and regrowth procedures were repeated for three times. Strength factor (Sf), recovery factor (Rf) and fractal dimension (Df ) could be calculated by formula as stated as follows [10]: 7

Journal Pre-proof d2  100 d1 d3  d2 Rf = 100 d1  d 2

Sf =

(1) (2)

I∝Q-Df

(3)

where d1, d2 and d3 represent average sizes of flocs before breakage, after breakage and after re-growth, respectively; I and Q represent light intensity and scatter vector of Mastersizer. Larger

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2.5. Coagulation-ultrafiltration hybrid system

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generally means a more compact structure of floc.

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Sf and Rf values indicate better anti-shear and re-growth abilities, while a higher Df value

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Coagulation effluent without sedimentation was directly transferred into a dead-end batch

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ultrafiltration unit to make up a C-UF hybrid system (Fig. 1). The effluent was filtered through

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UF membrane under a constant pressure of 0.15 ± 0.02 MPa, which was supplied by high purity nitrogen gas. Filtrate weight was recorded by a counting balance every 10 s, and flux decline

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with filtration time was investigated to assess membrane fouling degree. Polyether sulfone UF

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membrane, with molecular weight cutoff of 100 kDa, should be immerged in deionized water for 48 h to achieve its optimal operating condition before being used. 3. Results and discussion 3.1. Characterization of Enteromorpha prolifera polysaccharide Ep prepared in this study was white powders with a molecular weight of 30-500 kDa, and its monosaccharide composition includes rhamnose, xylose and glucuronic acid. Ep backbone was mainly composed of (1→4)-linked rhamnose residues with partially sulfated groups at the C-3 8

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position, and small amounts of (1→3, 4)-linked rhamnose, (1→2, 4)-linked rhamnose, (1→ 4)-linked glucuronic acid and (1→4)-linked xylose [26, 27, 38]. EDS mapping and energy spectra provided in Fig. 2(a-e) clearly exhibited the signal of S element, which proved the existence of sulfated groups in Ep. Inset in Fig 2(e) presents the SEM image of Ep, which possesses a unique porous structure. In addition, N element was also found

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in the EDS image, which indicated a possible existence of monosaccharide with amino groups.

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The band at 3362 and 1635 cm-1 in Fig. 3(a) refers to the stretching vibration of -OH and

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carbonyl group (C=O), while the characteristic peaks at 1138 and 859 cm-1 can be assigned to the

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vibration of S=O and C-O-S stretching. In addition, 1H NMR spectrum of Ep also showed

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signals for anomeric protons corresponding to different residues (Fig. 3(b)). The anomeric proton

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signals at 4.43 ppm and 4.15 ppm are assigned to (1→4)-linked β-xylopyranose residues and (1

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→4)-linked 3-sulfated-α-rhamnopyranose; chemical shift of 3.00-4.00 ppm can be attributed to

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the protons of C-2-C-5 in sugar rings; the signal at 1.21 ppm corresponds to the proton of CH3 group of rhamnose residues [26, 38]. These results indicated that Ep was a kind of water-soluble sulfate polysaccharide, and contained abundant hydroxyl groups, carboxylic acid groups and sulfonic acid groups. 3.2. Coagulation mechanism of PAC-Ep and PAC-PAM Pre-experiment optimized dosage range of PAC (2-12 mg/L), and the influence of Ep dosage on variations of organics removal and zeta potential are presented in Fig. 4. As seen, the maximum UV254 and DOC removal (87% and 47%) were found when 12 mg/L of PAC was used. 9

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In addition, zeta potential increased gradually as PAC dosage increased and finally reached about 0 mV. This result demonstrated that negative charges of HA could be neutralized by positively charged hydrolysate of PAC, and electrostatic repulsion between HA collides decreased dramatically. Therefore, charge neutralization was considered as dominate mechanism of PAC. Fig. 4(a-c) shows that Ep application was conducive to the removal of HA throughout PAC

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dosage ranges (2-12 mg/L). When 0.3 mg/L Ep was dosed, the highest UV254 and DOC removal

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achieved 90% and 51%, while the highest UV254 and DOC removal obtained by PAC alone was

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only 88% and 47%. Moreover, Ep could also play an aid role in lower PAC dosage conditions:

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when PAC dosage was 4.0 mg/L, UV254 and DOC removal were increased by 7% and 23% due

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to 0.3 mg/L Ep addition (Fig. 4(a-c)). Relevant reaction mechanism between PAC-Ep and HA

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was inferred as follows: hydrolysis reaction occurred quickly when PAC was dosed and then

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polynuclear hydrolysates neutralized the negative charges of HA, which weakened the repulsive

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forces of coagulation system to generate micro-flocs. Then Ep was dosed, hydroxyl, carboxyl and sulfonic groups in Ep could react with PAC hydrolysate to form egg-box structure and then generated a gel network by the effective bridging role of Ep. This gel network could not only enhance the precipitates aggregation but also capture the unabsorbed HA. As a result, flocs with larger sizes and higher coagulation efficiency were obtained. However, when excess Ep was dosed, neutralized particle surface was recharged with negative charges again, which increased the repulsion energy between flocs and further resulted in a "re-stabilization" phenomenon. When PAM was used as the coagulant aid, coagulation efficiency could also be enhanced. 10

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Furthermore, as PAM dose increased, zeta potential exhibited an increasing tendency, which can be ascribed to positive charges introduced by PAM. In coagulation process, these charges enhanced the electric neutralization role, accelerated the aggregation and destabilization of colloidal particles, and finally improved the coagulation effect [15, 18]. Therefore, the aid mechanism of PAM was considered as charge neutralization, which was different from that of Ep.

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Considering coagulation efficiency and coagulant cost simultaneously, the optimal dosage of

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PAC, Ep and PAM were constant at 10 mg/L, 0.3 mg/L and 0.5 mg/L, respectively.

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Fig. 4 (g-h) shows the comparison results of UV254 and DOC removal when Ep or PAM was

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applied in optimal dosage conditions. As seen, when Ep was used, the coagulation efficiencies

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were equivalent to that of traditional coagulant aid-PAM, even exhibited a higher UV254 removal

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rate: UV254 removal reached 77% when Ep was dosed with 2.0 mg/L PAC, while only 69% was

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obtained when PAM was used. Moreover, the cost of Ep preparation was much lower than that of

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PAM, since raw material-E. prolifera could be obtained for free. In addition, preparation process of Ep was simple, both high temperature and pressure conditions were not involved. Therefore, as a substitute of PAM, Ep could be applied for the purpose of improving coagulation effect and reducing the cost of water treatment process. This conclusion provides the theoretical basis for Ep applications, and meanwhile this points out a new recycle way of E. prolifera disaster in view of “treating waste by waste”. 3.3. Coagulation kinetics process 3.3.1 Floc growth characteristics 11

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Coagulation kinetics was studied in different Ep or PAM dosages conditions, and PAC dose was constant at 10 mg/L. Floc size variation is shown in Fig. 5. As seen, the flocs size increased rapidly once PAC was dosed, and then entered to stable phase ten minutes later, which indicated that flocs growth and breakage achieved an equilibrium condition. As shown in Table 1, when 0.3 mg/L of Ep was applied as coagulant aid, the flocs size in the steady-state region increased

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dramatically: corresponding size and growth rate of flocs in a steady stage were about 470 μm

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and 62.6 μm/min, which were much larger than that of PAC used alone (150 μm and 13.6

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μm/min). In addition, particle size distribution curves indicated that the volume of bigger flocs

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(larger than 100 μm) increased to a large extent when Ep was dosed. It is a great advantage in

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actual water treatment process since larger particles were settled down more quickly and resulted

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in a higher coagulation efficiency [39, 40].

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Sizes of flocs generated by PAC-PAM were slightly larger than those of PAC, and the

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maximum size (178 μm) was obtained when 0.5 mg/L PAM was dosed (Table 1). This should be attributed to different coagulation mechanisms when Ep or PAM was applied. Various long carbon chains of Ep macromolecules could absorb more suspension colloid particles and promote the bridging role effectively, which usually generates flocs with larger sizes. Moreover, the polar and unsaturated functional groups in Ep could also enhance the charged particle’s adsorption effect. However, positive charges introduced by PAM enhanced the charge neutralization role and generated more compact flocs with smaller sizes. The comparison result was consistent with previous study [41], which indicated that the flocs produced by bridging 12

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flocculation were usually much larger than those formed by charge neutralization. Table 1. Growth properties of flocs formed by PAC, PAC-Ep and PAC-PAM. PAC

PAC-Ep

PAC-PAM

Dosage (mg/L)

10

0.1

0.3

0.5

0.1

0.3

0.5

Lag time (min)

120

120

120

120

120

120

120

Growth time (min)

11.0

10.0

7.5

7.5

11.5

13.0

13.5

Floc size (μm)

150

307

470

417

161

170

178

Growth rate (μm/s)

13.6

30.7

62.6

55.6

14.0

13.1

13.1

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Coagulant

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3.3.2 Floc breakage and regrowth characteristics

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Floc breakage was inevitable in water treatment process due to the variation of hydraulic

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conditions. In this section, simulated shear force was introduced and lasted for 5 min. As a result,

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sharp reducing of flocs sizes was observed. Then the flocs began to regrow as shear force disappeared, and finally reached a stable stage again. Fig. 5(a, b) indicated that when Ep or PAM

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was applied as coagulant aid, the floc sizes of re-stabilization stage were much larger than those

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of PAC used alone. The largest flocs (340 μm) were obtained when 0.3 mg/L Ep was applied, but

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it was still lower than floc sizes of initial stabilized stage. Table 2 shows that Sf of flocs decreased gradually as the shear time increased, but the values were basically maintained at 0.35-0.40 in both PAC-Ep and PAC-PAM coagulation systems, indicating their good capabilities to resist the shear forces. However, the Rf of flocs generated by PAC-Ep was much higher than that resulted by PAC, and meanwhile enlarged gradually with increasing the Ep dosage. This indicated that Ep could improve the recovery and regeneration ability of broken flocs, since the structure attraction on newly exposed flocs surface could bond floc fragments together. On the 13

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other hand, the unsaturated functional group in Ep could enhance the adsorption effect of the charged particles [42, 43]. Particle size distributions of flocs before breakage, after breakage and regrowth were also analyzed when PAC, Ep and PAM dosage were 10.0 mg/L, 0.3 mg/L and 0.5 mg/L, respectively. Fig. 5(c-e) shows that the shift distance from initial peak to the peak after first breakage (“a1”) was almost equality. Moreover, the data derived from Fig. 5(c＆d), Fig. 5(c

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＆e) and Fig. 5(d＆e) were analyzed with statistic methods. The P values of T-test were 0.32,

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0.21 and 0.06, respectively, which indicated that the difference of flocs anti-shear ability was not

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significant (P>0.05). However, the distance between the peak after regrowth and initial peak

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(“a2”) showed the longest distance when PAC was used alone, following by PAC-PAM and

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PAC-Ep. The result indicated that a better recovery ability of flocs could be obtained due to the

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Ep addition, which was in accordance with the calculation results listed in Table 2.

Table 2 Breakage and regrowth properties of flocs formed by PAC, PAC-Ep and PAC-PAM Coagulant

PAC

PAC-Ep

PAC-PAM

Dosage (mg/L)

10

0.1

0.3

0.5

0.1

0.3

0.5

First 𝐒𝐟

0.400

0.386

0.372

0.374

0.412

0.418

0.422

Second 𝐒𝐟

0.363

0.348

0.341

0.330

0.407

0.388

0.388

Third 𝐒𝐟

0.360

0.357

0.322

0.335

0.403

0.377

0.381

First 𝐑 𝐟

0.118

0.182

0.349

0.492

0.289

0.325

0.326

Second 𝐑 𝐟

0.083

0.118

0.373

0.256

0.219

0.260

0.277

Third 𝐑 𝐟

0.019

0.061

0.280

0.191

0.196

0.221

0.227

3.3.3 Floc sedimentation characteristics 14

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Flocs sedimentation experiments were performed by a light scattering particle analyzer. Flocs began to settle down naturally, as the stirring stopped, and sedimentation curves were recorded automatically, which means that the coagulant aid application could reduce the turbidity of supernatant after precipitation. Fig. 6(a, b) also shows that the addition of Ep or PAM improved the sedimentation performance of flocs, and a faster sedimentation velocity was observed. When

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PAC was used alone, it took flocs about 42 minutes to precipitate completely, while it only took

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37 and 32 minutes to reach stable stages when PAM or Ep was applied as coagulant aid. It was a

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great advantage in actual water treatment process since sedimentation time could be shortened

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and then the entire water treatment process could be accelerated. This conclusion was consistent

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with previous research by Biggs and Lant [41], who found that larger flocs generally had a better

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sedimentation performance.

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3.4. Effect of Ep or PAM on ultrafiltration membrane fouling

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The influence of Ep or PAM application on ultrafiltration process was discussed in this section. To avoid the breaking of flocs, coagulation effluent was transferred to ultrafiltration beaker lightly, and the UF process lasted for more than 2 hours. Residual UV254 and DOC of ultrafiltration effluent were measured and removal rates were shown in Fig. 7(a, b). As seen, DOC removal achieved 0.68 when PAC (10 mg/L) and Ep (0.3 mg/L) were dosed, which was much higher than that of coagulant effluent (0.49). For UV254 removal, the residual concentration was lower than the detection limit, meaning that the removal rate exceeded 99%. Therefore, UV254 and DOC removal increased significantly when ultrafiltration unit was applied on the 15

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basis of coagulation.

The data in Fig. 7(a) and Fig. 7(b) were also analyzed with statistic

methods. P values of T-test were 4.33×10-3 and 4.86×10-5 separately, which indicated that UV254 and DOC removal existed significant differences before and after UF operation (P<0.05). These achievements demonstrated that applying ultrafiltration unit after coagulant process was efficient for organic matter removal.

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Membrane fouling was analyzed emphatically since it is the biggest barrier for UF technology

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application. Permeate flux (J) variations of UF effluents were monitored, and normalized

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permeate flux (J/J0) was selected to represent the fouling degree of UF membrane. Fig. 7(c)

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showed that fluxes curves exhibited a gradual decreasing tendency as the filtration time was

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increased. The lowest J/J0 occurred when raw water was filtrated, since HA entered into

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membrane pores under the pressure of N2, which resulted in a serious pore size reduction and the

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pore blocking [44]. While the coagulation process could remove HA particles effectively, hence

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membrane flux increased apparently when the PAC was dosed as coagulant. Therefore, applying coagulation unit as pre-treatment method was necessary and helpful. When coagulant aid was applied with PAC, membrane fouling degree could be further reduced, especially when 0.3 mg/L Ep was dosed: eventual J/J0 value of effluent water achieved 0.52, which was much higher than those results when PAC-PAM or PAC was used alone. This result could be attributed to three reasons. Firstly, HA removal by coagulation was efficient when Ep was applied, so HA concentration in ultrafiltration raw water was relatively low. Secondly, the results of section 3.2.1 indicated that Ep addition enlarged the flocs sizes and reduced the amount of small aggregation, 16

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which was in favor of decreasing membrane fouling. Since smaller flocs usually contribute to a higher specific cake resistance and a more severe membrane fouling [44, 45]. Last but not the least, the Df values in Fig. 8(a) indicated that flocs exhibited more multi-branched structures when Ep was dosed. SEM image in Fig. 8(b, c) also proved that the flocs produced by PAC-Ep exhibited more loose and porous structure than those by PAC-PAM. It is a great advantage for

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UF process since the compact flocs always result in a serious membrane fouling, while loosely

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structured aggregates are more beneficial for membrane permeability [46]. As a result, the cake

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layer resistance was much lower than that when PAM was dosed. In summary, when 0.3 mg/L of

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Ep was added, coagulation performance could be enhanced apparently, and meanwhile the UF

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membrane fouling could be reduced. Therefore, Ep could be applied as a new coagulant aid in

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4. Conclusion

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the C-UF process.

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HA could be removed effectively via C-UF process when Ep was used as coagulant aid, which exhibited comparable to or superior aid effect of commercial PAM. The flocs sizes reached 470 μm due to the Ep addition, while only 178 μm was obtained when PAM was dosed. In addition, the flocs generated by PAC-Ep showed faster growth rates, a better recovery ability and more multi-branched structures than those of PAC-PAM, which brought less ultrafiltration membrane fouling. As a result, eliminating organic matter and reducing membrane fouling could be simultaneously achieved by Ep addition. Moreover, as a natural polymer coagulant aid, Ep exhibited better biosafety and biodegradability than PAM. Therefore, Ep could be considered as 17

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a promising substitute of commercial PAM in water treatment process. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51908256), and Natural Science Foundation Youth Program of Jiangsu (BK20170238, BK20170232). The kind suggestions from the anonymous reviewers are highly appreciated.

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Declaration of interests

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The authors declare no conflict of interest.

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References

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Figures and Figure Captions

Fig. 1 Schematic of C-UF hybrid system.

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Fig. 2 Characterization of Enteromorpha prolifera polysaccharide: (a) C, (b) S, (c) N and (d) O

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elements images obtained from EDAX (PW9900); (e) SEM images and energy spectrum

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obtained from HITACHI (SU8010) and EDAX (PW9900)

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Fig. 3 (a) FTIR spectrum and (b) 1H NMR spectra of Enteromorpha prolifera polysaccharide.

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Fig. 4 Coagulation aid performance of Ep and PAM: (a-c) coagulation efficiency of PAC-Ep; (d-f) coagulation efficiency of PAC-PAM; (g-h) comparation of coagulation aid effect of 0.3 mg/L Ep and 0.5 mg/L PAM; (i) EDX of flocs generated by PAC-Ep.

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Fig. 5 Floc formation, breakage and re-growth curves of PAC-Ep and PAC-PAM: (a, b)

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variations of flocs sizes; (c-e) sizes distribution of flocs generated by PAC, PAC-PAM and

Fig. 6 Sedimentation curves of flocs generated by PAC-Ep and PAC-PAM: (a) PAC-Ep; (b) PAC-PAM 26

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Fig. 7 Ultrafiltration efficiency and membrane fouling when Ep or PAM was applied: (a) UV254 removal; (b) DOC removal; (c) membrane fouling curves

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Fig. 8 Floc structure analysis of PAC-Ep and PAC-PAM: (a) Fractal dimension calculation; (b)

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SEM images of PAC-Ep flocs; (c) SEM images of PAC-PAM flocs

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Graphical Abstract

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Highlights  Enteromorpha prolifera polysaccharide (Ep) was prepared from hazardous waste  Ep was used as a new coagulant aid to remove humic acid (HA).  Ep exhibited the same coagulation aid effects as traditional coagulant aid-PAM.  Floc with a larger size and better recovery ability occurred when Ep was applied.  Chelated reticular structure was formed by the addition of Ep.

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 Membrane fouling was effectively reduced due to application of Ep.

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