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Feb 22, 2016 - Yongzhi Liu , Huaili Zheng , Yili Wang , Xinyu Zheng , Moxi Wang , Jie Ren , Chuanliang Zhao. RSC Advances 2018 8 (50), 28329-28340 ...
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Fabricating a flocculant with controllable cationic microblock structure: Characterization and sludge conditioning behavior evaluation Wei Chen, Huaili Zheng, Qingqing Guan, Houkai Teng, Chuanliang Zhao, and Chun Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04207 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

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Fabricating a flocculant with controllable cationic microblock structure: Characterization and sludge conditioning behavior evaluation

Wei Chena,b, Huaili Zhenga,b,*, Qingqing Guana,c, Houkai Tengd, Chuanliang Zhaoa, Chun Zhaoa

a

Key laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of

Education, Chongqing University, Chongqing 400045, China; bNational

Centre for International Research of Low-carbon and Green Buildings, Chongqing

University, Chongqing 400045, China; cSchool

of Environmental Engineering, Nanjing Institute of Technology, Nanjing 211167, China;

dNational

Research Center of Industrial Water Treatment Engineering and Technology, Tianjin

Chemical Research and Design Institute, Tianjin 300131, China.

*Corresponding author. Tel.: +86 23 65120827; fax: +86 23 65120827. E-mail address: [email protected] (Huaili Zheng)

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ABSTRACT In this study, a new flocculant with cationic microblock structure was prepared by using template polymerization. AM and DAC were used as monomers, and NaPAA was used as template. The reactivity ratio, 1H NMR,

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C NMR, and TGA were employed to characterize the structural

properties of products. Results show that, with the addition of template, the reactivity ratio of DAC increased, whereas that of AM decreased. Evident microblock structure was synthesized in copolymers when the molar ratio of template to DAC is 1.0. Activated sludge dewatering experiment results reveal that the cationic microblocky flocculant exhibited superior dewatering efficiency, and a low moisture content of 71% and a specific resistance of 1.99 × 1012 m•kg−1 were acquired at pH 7.0 and dosage 40 mg L−1. The enhanced charge neutralization and bridging effect of the cationic microblocky flocculant contributed to the formation of floc with a large size and compact structure. Keywords: Flocculant, Acrylamide, Sludge dewatering, Flocculation Mechanism, Template Polymerization

1. INTRODUCTION The increasing worldwide establishment and operation of sewage treatment plants has facilitated a better aquatic environment, and has resulted in an important shift in waste streams from liquid phase to semisolid phase 1. Large amounts of waste-activated sludge, which represent 1% or 2% of treated wastewater but contain from 50% to 80% of pollution, have to be disposed of because the activated sludge process is the most important treatment technology for an extensive range of wastewaters 2, 3. Notably, sludge with >97% water content is highly compressible but difficult to dewater mainly because small negatively charged particles are distributed evenly in the form of a stable colloidal suspension 2, 4. Chemical conditioning prior to mechanical dewatering is generally required to destabilize the particulate system and reduce the sludge volume 5-7. Sludge conditioning, along with polyelectrolyte dosing, is a colloidal process of flocculation. In recent years, cationic polyacrylamide (CPAM) has been among the most commonly used polyelectrolytes for sludge conditioning because its’ low impact on the environment, and can neutralize the surface charge of solid particles and bridge particles through its long polymer chain to form larger flocs, which can reduce the sludge moisture content and diminish cake compressibility 8, 9. 2

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Acrylamide (AM) and cationic monomers were polymerized via solution polymerization, disperse polymerization, and inverse emulsion polymerization to produce CPAM 10-13. However, a deficiency in the aforementioned polymerization methods exists: The cationic units are distributed randomly in the CPAM molecular chain. The cationic units located on the loops and tails of the chain are simply wasted, as shown in Figure 1(a). Thus, insufficient cationic charges are present at the adsorption sites to neutralize counter ions. By contrast, if the cationic units exhibit a blocky distribution, as shown in Figure 1(b), then the adsorption sites between copolymer and negatively charged particle segments would be strong and the cationic charges would be more efficiently utilized, leading to a more acceptable flocculation performance. Thus, adjusting the sequence structure of the cationic units, and forming a cationic microblock structure in the polymer chain is the best course of action. Figure 1. Schematic presentation of flocculation effect of a polyelectrolyte chain: (a) randomly

distributed copolymer and (b) cationic block structure copolymer

Similar to the gene translation process, template copolymerization is generally defined as a polymerization process in which one type of monomer is organized by a preformed macromolecule (template) through hydrogen or ionic bonds, whereas the other type is not, and the monomers adsorbed on the template will form microblock structures 14. Several previous literatures have reported the synthesis of copolymers with anionic microblock structures by using template polymerization to obtain a polyacrylamide with higher thickening ability 15, 16. However, except for 3

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the anionic polymer, studies relating to the synthesis and application of copolymers with cationic microblock structure in the sludge dewatering process are limited. Acryloyloxyethyl trimethyl ammonium chloride (DAC) is an extensively used cationic unit, and several previous papers have reported the copolymerization of AM and DAC by solution polymerization 8, 17. Therefore, studies adjusting the sequence structure of DAC groups in the copolymer chain, resulting in the cationic microblock structure in polymers, and enhancing dewatering efficiency effectively will be more significant. Based on the aforementioned findings, the primary focus of the present study is to (1) investigate the possibility of synthesizing the flocculant with cationic microblock structures by template polymerization using AM and DAC as monomers; (2) determine the influence of the template on the reactivity ratios of AM and DAC of the template copolymer; (3) examine the structural properties of the template copolymer by using 1H nuclear magnetic resonance (1H NMR) spectroscopy, 13C nuclear magnetic resonance (13C NMR) spectroscopy, and thermal gravimetric analysis (TGA) methods; (4) evaluate the activated sludge dewatering efficiency of new flocculants in terms of filter cake moisture content, specific resistance to filtration, and floc properties; and (5) acquire the optimal parameters, such as preparation parameter, flocculant dosage, intrinsic viscosity, and pH, resulting in high dewatering efficiency.

2. MATERIALS AND METHODS 2.1. Materials AM, DAC, and template sodium polyacrylate (NaPAA) used in this experiment were of technical grade, whereas the remaining reagents were of analytical grade. The monomer AM was purchased from Chongqing Lanjie Tap Water Company (Chongqing, China), DAC was obtained from Guangchuangjing Company (Shanghai, China), NaPAA was a gift from Shandong Xintai Water Treatment (Zaozhuang, China), and the molecular weight of which is 3000. photoinitiator 2,2′azobis (2-methylpropionamide) dihydrochloride (V-50) was purchased from Ruihong Biological Technology (Shanghai, China). All aqueous and standard solutions used in the experiment were prepared with deionized water.

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2.2. Synthesis of copolymers The template polymerization technique was used to synthesize the copolymers. The reaction was initiated by ultraviolet (UV) irradiation 8, 18. Figure S1 in the Supporting Information illustrates the overview of template polymerization and the scheme of the reaction route for the preparation of copolymers. Predetermined amounts of monomers (AM, 126.61 mmol; DAC, 42.21 mmol) and urea (0.80 mmol) were added to a quartz reaction vessel, and NaPAA was added according to the molar ratio of template NaPAA to cationic monomer DAC (T/D) of 0.0, 0.5, 1.0, and 1.5. Deionized water was subsequently poured into the reaction vessel to reach a monomer mass ratio of 30%. Then, the pH of the reaction solution was adjusted to 4.5 by 0.5 M HCl and NaOH. Prior to the addition of the photoinitiator, the aqueous solution was completely deoxygenated by bubbling with nitrogen gas for 30 min. The reaction vessel was exposed to radiation from a 500 W high-pressure mercury lamp 8, 19. After 60 min of radiation, 2 h of aging at room temperature was conducted to increase the polymerization degree. The copolymer was dissolved in water, and the pH was adjusted to TPAD-0.5 > TPAD-1.0. A lower sediment height indicates a lower sludge 20

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volume. Thus, the small sludge volume of TPAD-1.0 indicates that the flocs formed by cationic microblocky flocculants possess a compact and dense structure. In addition to floc density, the floc diameter of conditioned sludge of the four flocculants was also investigated. Figure 9 illustrates the floc size distribution of the conditioned sludge of the four flocculants at the end of the flocculation process. The results show that the diameter of TPAD-1.0 is the largest and that of TPAD-0.0 is the smallest. As a result, the combined effect of the compact structure and large size of floc formed by TPAD-1.0 effectively enhanced settleability.

Figure 8. Effect of types of flocculants on sludge floc settling behaviors

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Figure 9. Sludge floc size distribution for (a) TPAD-0.0, (b) TPAD-0.5, (c) TPAD-1.0, and (d) TPAD-1.5 23

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Other than settleability and size distribution, the morphology of sludge flocs conditioned by the four flocculants was captured by a camera, and the results were illustrated in Figure 10. The morphology of sludge flocs conditioned by TPAD-1.0 in a beaker presented a beehive-shaped model that is large and densely granulated and with many large voids that were distributed on the floc surface. The sludge aggregates were tightly linked together and closely overlapped. By contrast, the morphology of sludge flocs conditioned by TPAD-0.0 presented a fine sand shape and uniformly filled the beaker, which was loose and small and with a loosely reticular structure.

Figure 10. Morphology and photomicrographs of sludge flocs for (a) TPAD-0.0, (b) TPAD-0.5, (c) TPAD-1.0, and (d) TPAD-1.5

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The density, diameter, and morphology of sludge flocs are a combined effect of charge neutrality, bridging effect, and patch mechanism 17. The zeta potential of the four flocculants shown in Figure 5(c) revealed that the main mechanism of TPAD-1.0 in the flocculation process is charge neutralization. The flocculants with blocky distributed cationic groups presented a stronger adsorption/charge neutralization capacity, whereas the flocculants with randomly distributed cationic units did not. Therefore, more particles were tightly absorbed on the polymer chain, and a larger and denser structure of flocs was formed by the cationic microblocky flocculant. In addition to the charge neutralization mechanism, the bridging effect also played an important role in floc growth. The repulsion between charged segments induced chain expansion and embedding into the sludge solution because of the number of cationic microblock segments distributed along the polymer chain 42. The elongated linear molecular chains induced a significant bridging effect. Therefore, the number of particles adsorbed by the cationic microblock structure was more than the number adsorbed by the random cationic segments. As a result, a large and compact aggregation was formed with the enhanced charge neutralization ability and bridging effect of the cationic microblocky flocculant.

4. CONCLUSION In this study, high dewatering efficiency flocculants with a cationic microblock structure was synthesized by a UV-initiated template polymerization technique. The characterization test such that reactivity ratio, 1H NMR, 13C NMR, and TGA showed that a copolymer with microblock structure was successfully synthesized. When T/D is 1.0, evident block structures were observed in the copolymer chain. The activated sludge dewatering experiment result demonstrated that the cationic microblocky flocculants improved the dewatering behavior effectively. High intrinsic viscosity of flocculants and pH 3–8 are the optimal conditions for sludge dewatering. When all cationic units exhibited a blocky distribution, a lower moisture content of 71% and a specific resistance of 1.99 × 1012 m kg−1 were acquired at the dosage of 40 mg L−1 in the sludge dewatering process. In addition, sludge floc formed by cationic microblocky flocculant presented a higher settling rate, larger size, and denser floc structure, which can be attributed to the enhanced charge neutralization ability and absorption/bridging effect of the cationic microblock structure in the flocculant polymer chain.

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■ SUPPORTING INFORMATION Scheme of Template polymerization and Reaction route for the preparation of copolymer The calculation process of Kelen-Tüdös methods in reactivity ratio measurement

■ AUTHOR INFORMATION *Corresponding author Tel.: +86 23 65120827; fax: +86 23 65120827. E-mail address: [email protected](Huaili Zheng) Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Project Nos. 21177164 and 21477010) and Graduate Student Research Innovation Project in Chongqing (CYB14045).

■ ABBREVIATIONS AM

Acrylamide

DAC

Acryloyloxyethyltrimethylammonium chloride

NaPAA

Sodium polyacrylate

1H

Proton nuclear magnetic resonance

NMR

13C

NMR

Carbon nuclear magnetic resonance spectroscopy

TGA

Thermal gravimetric analysis

CPAM

Cationic polyacrylamide

UV

Ultraviolet

T/D or T:D

Molar ratio of template NaPAA to cationic monomer DAC

TPAD

Template Polymer P(AM-DAC)

VSS/TSS

Proportion of microorganism in the activated sludge

FCMC

Filter cake moisture content

SRF

Specific resistance to filtration 26

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𝑟𝐴𝑀

The reactivity ratio of AM monomer in AM and DAC monomer pair

𝑟𝐷𝐴𝐶

The reactivity ratio of DAC monomer in AM and DAC monomer pair

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(46) Chen, Y.; Chen, Y.-S.; Gu, G. Influence of pretreating activated sludge with acid and surfactant prior to conventional conditioning on filtration dewatering. Chem. Eng. J. 2004, 99, 137.

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