UV-initiated template copolymerization of AM and MAPTAC: Microblock structure, copolymerization mechanism, and flocculation performance

Chemosphere 167 (2017) 71e81

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UV-initiated template copolymerization of AM and MAPTAC: Microblock structure, copolymerization mechanism, and flocculation performance Xiang Li a, d, Huaili Zheng a, d, *, Baoyu Gao b, Yongjun Sun c, Bingzhi Liu a, d, Chuanliang Zhao a, d a

Key Laboratory of the Three Gorges Reservoir Region's Eco-Environment, State Ministry of Education, Chongqing University, Chongqing 400045, China Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China c Jiangsu Key Laboratory of Industrial Water-Conservation & Emission Reduction, College of Urban Construction, Nanjing Tech University, Nanjing 211800, China d National Centre for International Research of Low-carbon and Green Buildings, Chongqing University, Chongqing 400045, China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A cationic polyacrylamide (CPAM) with microblock structure was synthesized through UV-initiated template copolymerization.
All of the instrumental analysis results confirmed the micro block structure of the template CPAM.
Reaction kinetics revealed I (ZIP) template polymerization mechanism of this reaction.
The template copolymer showed excellent sludge dewatering performance.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 July 2016 Received in revised form 2 August 2016 Accepted 12 September 2016

Flocculation as the core technology of sludge pretreatment can improve the dewatering performance of sludge that enables to reduce the cost of sludge transportation and the subsequent disposal costs. Therefore, synthesis of high-efficiency and economic flocculant is remarkably desired in this field. This study presents a cationic polyacrylamide (CPAM) flocculant with microblock structure synthesized through ultraviolet (UV)-initiated template copolymerization by using acrylamide (AM) and methacrylamido propyl trimethyl ammonium chloride (MAPTAC) as monomers, sodium polyacrylate (PAAS) as template, and 2,2'-azobis [2-(2-imidazolin-2-yl) propane] dihydrochloride (VA-044) as photoinitiator. The microblock structure of the CPAM was observed through nuclear magnetic resonance (1H NMR and 13 C NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) analyses. Furthermore, thermogravimetric/differential scanning calorimetry (TG/DSC) analysis was used to evaluate its thermal decomposition property. The copolymerization mechanism was investigated through the determination of the binding constant MK and study on polymerization kinetics. Results showed that the copolymerization was conducted in accordance with the I (ZIP) template polymerization mechanism, and revealed the coexistence of bimolecular termination free-radical

Handling Editor: W Mitch Keywords: Cationic polyacrylamide Sludge dewatering Polymer flocculant Microblock structure Copolymerization mechanism

* Corresponding author. Key Laboratory of the Three Gorges Reservoir Region's Eco-Environment, State Ministry of Education, Chongqing University, Chongqing 400045, China. E-mail address: [email protected] (H. Zheng). http://dx.doi.org/10.1016/j.chemosphere.2016.09.046 0045-6535/© 2016 Elsevier Ltd. All rights reserved.

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reaction and mono-radical termination in the polymerization process. Results of sludge dewatering tests indicated the superior flocculation performance of microblock flocculant than random distributed CPAM. The residual turbidity, filter cake moisture content, and specific resistance to filtration reached 9.37 NTU, 68.01%, and 6.24 (1012 m kg1), respectively, at 40 mg L1 of template poly(AM-MAPTAC) and pH 6.0. Furthermore, all flocculant except commercial CPAM showed a wide scope of pH application. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction With the development of industry, urbanization, and increasingly stringent environmental protection requirements, both sewage discharge and its treatment depth have increased. Consequently, large amounts of excess sludge generated in the process of wastewater treatment are now being increasingly taken seriously. Therefore, the research and development of a high-efficiency sludge dewatering agent is very crucial. Cationic acrylamide (CPAM) is the most commonly used sludge dewatering agent. This polymer usually features high positive charge density, water solubility, and high intrinsic viscosity (Chang et al., 2005; Zhao et al., 2013; Lee et al., 2014)_ENREF_1. Nevertheless, this polymer synthesized by traditional methods such as thermally induced solution polymerization usually features smooth and regular surface area and random distribution of acrylamide (AM) and cationic monomers in polymer chain (Guan et al., 2014). In general, polymers with irregular, uneven, and porous surface area would have a larger specific surface area and thus enhance the probability of contact with colloidal particles; polymers with regular microblock structure can make full use of the cationic units in the polymer chain and thus enhance their charge neutralization effect (Chen et al., 2016). Therefore, the use of a suitable synthesis method to prepare AM polymers with a large specific surface area and regular microblock structure will significantly improve the flocculation and dewatering performance. Ultraviolet (UV) radiation can induce the activation of carbon fiber surface and generation of free radicals. The free radicals react with monomer radicals to form chemical bonds, thereby increasing the surface energy and surface roughness (which is usually shown by the increase in surface porosity) of carbon fiber. (Deng et al., 2009; Akkaya, 2012; Kordoghli et al., 2012). Our previous studies have also proved the surface modification effect of UV light in the synthesis of polyacrylamide polymer (Liao et al., 2014; Zheng et al., 2014). As one of the main technologies for surface modification of polymeric materials, UV-induced polymer surface modification has been widely recognized for its simplicity, effectiveness, and versatility. Besides, UV initiation presents the advantages of low reaction temperature, short reaction time, less photoinitiator, and higher monomer conversion rates, thereby attracting increased attention. Template polymerization is a new method for the synthesis of polymers with specific sequence structure (Saito, 2008; Saito and Saito, 2011). In the process of template polymerization, the preassemble of reaction monomer on the template polymer molecular chain will improve the ordered nature of the reaction system, and chain growth progresses along the template polymer chain, which has an important effect on the polymerization kinetics and  ski, 2002). Thus far, most of the structure of the product (Połowin the research on template polymerization is limited to homopolymerization, and less research has been conducted on template copolymerization. In our previous studies, template copolymerization technology has been used for the synthesis of conventional CPAM (poly (acrylamide-acryloyloxyethyltrimethylammonium chloride) and poly (acrylamide-diallyl dimethyl ammonium

chloride)). Characterization results showed the regular microblock structure of CPAM, which proved the feasibility for the use of template copolymerization in the synthesis of CPAM with regular microblock structure. (Guan et al., 2015; Chen et al., 2016). However, the polymerization mechanism remains to be further studied to meet the needs of the development of the reaction process and the reactor design in practical engineering application. Furthermore, this template copolymerization technology should also be applied to the synthesis of CPAM or other polymer flocculant with high efficiency and wide serviceability in view of processing diverse sludge materials and stricter environmental protection policy. In this study, we report the preparation of CPAM with irregular, uneven, and porous surface area and regular microblock structure through UV-initiated template copolymerization. Nuclear magnetic resonance (1H NMR and 13C NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and thermogravimetric/differential scanning calorimetry (TG/DSC) analyses were used for comparative analysis of template copolymer and copolymer obtained by non-template copolymerization. Furthermore, the polymerization mechanism was investigated through the determination of the binding constant MK and study on polymerization kinetics. Finally, the sludge dewatering efficiency of template copolymer was evaluated in terms of residual turbidity (RT), filter cake moisture content (FCMC), specific resistance to filtration (SRF), and floc properties. 2. Material and methods 2.1. Raw materials AM, methacrylamido propyl trimethyl ammonium chloride (MAPTAC), and commercial CPAM (synthesized through copolymerization of AM and acryloyloxyethyltrimethylammonium chloride (DAC)) used in this study were of technical grade, whereas the other reagents were of analytical reagent grade. The details of reagents used in this study are as follows: AM and commercial CPAM (Chongqing Lanjie Tap Water Company, Chongqing, China); PAAS (MW ¼ 3000, Shandong Xintai Water Treatment Company, Zaozhuang, China); VA-044 (Ruihong Biological Technology, Shanghai, China); MAPTAC (50 wt% in water; Nanjing Jingruijiuan Biotechnology Co., Ltd. Nanjing, China); PAM, ethylenediaminetetraacetic acid (EDTA), urea, and other analytical reagents (Chongqing Chuandong Chemical Group Co., Ltd., Chongqing, China). PAMA (poly(AM-MAPTAC)) was synthesized by UV-initiated copolymerization without adding template PAAS. All reagents were used in the experiment without further purification. 2.2. Synthesis of template copolymers Template polymerization in combination with UV-initiated technology was adopted to synthesize template copolymers (template poly(AM-MAPTAC): TPAMA). Predetermined amounts of AM, MAPTAC, PAAS, distilled water, EDTA, and urea were added into a 10 ml Pyrex glass vessel (details of the feed ratio are listed in

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Table S1) and stirred until they completely dissolved. The pH of the reaction solution was adjusted to 4e5 by HCl (1 þ 1) and NaOH (0.1 mol L1). The predetermined amount of VA-044 was added after the reaction solution was completely deoxygenated by bubbling with pure N2 (99.99%) for 30 min. Finally, the reaction vessel was sealed immediately and exposed to radiation with a 500-W high-pressure mercury lamp (Tianyuanhuiteng, China) at room temperature until the set time. After UV irradiation, the gels were kept for 2 h. The TPAMA was purified by first dissolving in distilled water and adjusting the pH to 2.0, and then washing with acetone and ethanol several times. Finally, the white product was dried in a vacuum oven at 60  C until constant weight. 2.3. Characterization of copolymers In this section, PAMA and TPAMA with the same cationic degree (~21%) and intrinsic viscosity were chosen for comparative analysis. After copolymers were milled into fine powder, FTIR, 1H NMR, and 13 C NMR spectra of copolymer were recorded by a 550Series II infrared spectrometer (Bruker, Switzerland) using KBr pellets and Avance-500 NMR spectrometer (Bruker, Switzerland) in deuterium oxide (D2O) with tetramethylsilane as an internal standard. The thermal decomposition property of the copolymer was determined by an STA449C instrument (Netzsch Group, Germany) under argon atmosphere at a heating rate of 10  C min1. SEM analysis was performed on MIRA 3 LMU SEM system (TES-CAN Company, Czech Republic). All the instrumental analyses (including FTIR, NMR, TG/ DSC, and SEM) were repeated (at least three samples per instrumental analysis). Moreover, the peak area of each characteristic peak is the average value of three parallel samples. 2.4. Determination of KM and reaction kinetics 2.4.1. Determination of KM Dialysis and conductance titration method were used to determine the association constant of cationic monomer MAPTAC and template PAAS in aqueous solution. Certain amount of PAAS (MW ¼ 3000) was added into a dialysis bag (intercepted MW ¼ 3000, MD44-3000-01, USA) and then soaked in distilled water for 48 h. Subsequently, ethanol was added, and the intercepted PAAS was precipitated and dried in a vacuum oven at 60  C until constant weight. Certain amount of dried PAAS aqueous solution (1 g L1) was added into 100 mL known concentration of MAPTAC aqueous solution. The pH was adjusted to 4e5 by HCl (1 þ 1) and NaOH (0.1 mol L1). The mixed solution was then sealed and kept for 20 h at room temperature. The concentration of free MAPTAC was then determined through the conductance titration method, and the KM was calculated by Equation (1):

KM ¼

½PAA/MAPTAC ½PAASf  ½MAPTACf

(1)

where [PAAS/MAPTAC] is the association concentration of PAAS and MAPTAC and [PAAS]f and [MAPTAC]f are the concentrations of free PAAS and MAPTAC, respectively. 2.4.2. Reaction kinetics Predetermined amounts of AM, MAPTAC, PAAS, distilled water, EDTA, and urea were added into a dilatometer packed with a capillary. Next, according to the synthesis steps described in Section 2.2 and the design formula of reaction kinetics, liquid level in capillary at 0, 10, 20, 30, 40, 50, 60, and 70 min of UV irradiation were read and recorded for the calculation of conversion rate (a). All the synthetic experiments were conducted in triplicate, and the average value was calculated as the result for the subsequent

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analysis. Calculation methods of Rp (polymerization rate) are illustrated in Supporting Text S1. 2.5. Flocculation performance To evaluate the flocculation efficiency of template copolymer, municipal sludge was chosen as the study material. The municipal sludge (97.26% of moisture content and pH ¼ 6.5) was sampled from sludge thickener of Dadukou Drainage Co., Ltd., and the details of the used flocculants are listed in Table 1. The flocculation tests were carried out on a program-controlled jar test apparatus (ZR4-6, Zhongrun, China) at room temperature. Set dose of flocculants (1 g L1) was added into 100 ml of original municipal sludge, and the pH was adjusted by adding HCl (0.1 mol L1) and NaOH (0.1 mol L1). Stirring at 120 rpm for 20 s was conducted to achieve complete mixing of the flocculants and sludge, then at 40 rpm for 60 s for the flocs to grow, and no stirring for 10 min for them to settle. Supernatant at the depth 1 cm below the water surface was collected for measurement. Analytical methods for RT, FCMC, and SRF are described in Supporting Text S2. 3. Result and discussion 3.1. Characterization of copolymers 3.1.1. FTIR spectra of copolymers The FTIR spectra of PAMA, TPAMA, and PAM are shown in Fig. 1. As shown in Fig. 1(b), the absorption peaks at 3445 and 1669 cm1 were assigned to stretching vibration of amino and carbonyl groups of amide in AM and MAPTAC, respectively (Guan et al., 2014; Zheng et al., 2014). The peaks at 2941, 2843, and 1543 cm1 were attributed to asymmetry stretching vibration of methyl, methylene, and NeH bending of mono-substituted amide. Peaks at 1492, 1452, and 965 cm1 and a slight peak at 3746 cm1, which originated from methyl groups of quaternary ammonium, deformation vibration of the methylene group in MAPTAC unit, Nþ (CH3)3 stretching vibration, and stretching band of amide NeH in MAPTAC, respectively (Pourjavadi et al., 2013), and were invisible in Fig. 1(c), indicated the successful introduction of cation monomer MAPTAC into the polymer chain. Comparing Fig. 1 (a) and (b), we find that the absorption peaks in TPAMA and PAMA are almost the same, which indicates that the two polymers had the same structure. 3.1.2. 1H NMR and 13C NMR spectra of copolymers The characteristic peaks in the 1H NMR spectra of PAMA, TPAMA, and PAM are indicated in Fig. 2. As shown in Fig. 2 (b) and (c), compared with PAM, each corresponding absorption peak had a certain shift. Furthermore, five new peaks were observed at 1.90, 3.12, 1.34, 3.28, and 3.06 ppm in the spectra of TPAMA. These five peaks were assigned to protons (Hd1-Hh1) in MAPTAC (Pourjavadi et al., 2013). Obviously, these new peaks and peak shift indicated the successful copolymerization of AM with MAPTAC. Comparing Fig. 2 (a) and (b), the absorption peaks of PAMA and TPAMA were

Table 1 Details of used flocculants in sludge dewatering test. Flocculanta

Intrinsic viscosity dL g1

Cationic degree (%)

TPAMA PAMA FTPAMA CPAM

11.20 11.24 12.03 11.37

25 25 25 25

a TPAMA: template poly(AM-MAPTAC); PAMA: non-template poly(AM-MAPTAC); FTPAMA: purified template poly(AM-MAPTAC); CPAM: commercial poly(AMDAC).

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which indicates that TPAMA had higher proportion of cationic microblock than PAMA. The 13C NMR spectra of PAMA, TPAMA, and PAM are shown in Fig. 3. Compared with Fig. 3(c), the characteristic peaks of 13Cd-13Ci in MAPTAC are observed at Fig. 3(a) and (b), which provided another evidence for the successful synthesis of PAMA and TPAMA. In general, the sequence structure of the polymer molecular chain will affect the chemical shift of C atoms, which is manifested as severe split in the corresponding carbon characteristic peaks (Truong et al., 1986b, 1986a; Zhang et al., 2007). As shown in Fig. S2, peaks b1 and d originated from eCHe in AM unit and eCH3 in MAPTAC unit, respectively, split into three peaks in the 13C NMR spectra of TPAMA.

Fig. 1. FTIR spectra of PAMA (a), TPAMA (b), and PAM (c).

almost identical, which indicates that the two polymers had the same structure. However, some minor difference observed from the integral quantity of characteristic peaks cannot be ignored. The peak area ratios of H(a1þa10 ), Hb1, and Hd1 in PAMA and TPAMA were 1:0.400:0.300 and 1:0.404:0.287, respectively. This change was caused by the trace template PAAS, which had been grafted onto the polymer chain through chain transfer (Al-Alawi and Saeed, 1990), and it's percent content was about 4.2%. The cationic degree calculated (20%) from the peak area ratio was very close to the experimental value of 20.8%. Furthermore, in the polymer chain, two types of link mode of MAPTAC shown in Fig. S1 affect the chemical group environment of eCH2 of MAPTAC, thereby producing peak a10, which is different from a1 of AM. The change in the relative proportion of the a1 and a10 peaks also contributes to the proof of the microblock structure of the template polymer. The larger the relative peak area of a10, the larger is the proportion of cationic microblock in the polymer chain. The peak area ratios of a1 to a10 in Fig. 2 (a) and (b) were 1:0.08 and 1:0.22, respectively,

3.1.3. SEM of copolymers Fig. 4 illustrates the SEM images of (a) PAMA and (b) TPAMA. Thus, two different surface morphologies were observed. TPAMA had a regular, chainlike, and porous structure with a prodigious surface area (Fig. 4(b)), whereas PAMA showed a comparatively smooth and less porous surface area (Fig. 4(a)). UV, which has a certain surface modification effect, had been used to modify carbon fiber and proved to increase the surface porosity of the modified polymer (Hong et al., 2009; Kordoghli et al., 2012). Therefore, the porous structure of PAMA and TPAMA may partially attribute to UV light modification. Furthermore, in TPAMA without template separation, the template occupies a certain spatial location. Then, with the removal of template, a certain pore is formed at the original position of the template; thus, the porosity of TPAMA was significantly increased. Therefore, the prodigious surface area of TPAMA can be attributed to the UV modification and template polymerization mode. In addition, Fig. 4 also shows the linear correlation of the logarithm of projected area (A) and the characteristic length (L). From the two-dimensional fractal dimension calculation using Image-Pro Plus 6.0 software (Supporting Text S3), the fractal dimensions of PAMA and TPAMA were found to be 1.4517 and 1.5928, respectively, and this result also showed that TPAMA had a larger pore structure than PAMA.

Fig. 2. 1H NMR spectra of PAMA (a), TPAMA (b), and PAM (c).

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Fig. 3.

13

C NMR spectra of PAMA (a), TPAMA (b), and PAM (c).

Fig. 4. SEM micrographs of PAMA (a) and TPAMA (b).

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3.1.4. TG/DSC of copolymers Thermal analysis of copolymer (DSC) can also be an indirect analysis method of polymer microstructure (Guan et al., 2014). Fig. 5 shows the TG/DSC analysis of PAMA (a) and TPAMA (b). There were three obvious weight loss stages on the thermal weight curve of PAMA and TPAMA. The first stage (36.5e194  C, 12.4% for PAMA; 33.5e222.7  C, 17.5% for TPAMA) was considered for the loss of moisture. The second stage (194e334.3  C, 28.9% for PAMA; 222.7e334.0  C, 21.6% for TPAMA) was considered for the thermal decomposition and imidization of amide groups (eCOeNHe), detachment of methyl from quaternary ammonium (eC (CH3)3NþCl), and the removal of hydrogen chloride (Ghimici et al., 2001). The last stage (334.3e494.0  C, 46.26% for PAMA; 334.0e478.9  C, 35.45% for TPAMA) was decomposition of polymer backbone. Obviously, there were two slight endothermic peaks (at 376.1 and 460.6  C) in the DSC curve of TPAMA but only one (at 356.1  C) in that of PAMA, which indicates that TPAMA had a twophase structure and microblock sequence of MAPTAC existed in the main chain of TPAMA; yet, the main chain of PAMA was mostly PAM unit, and monomer MAPTAC in the main chain was randomly distributed with seldom microblock sequence of MAPTAC (Guan et al., 2014; Chen et al., 2016). 3.2. Binding constant (KM) and reaction mechanism The strength of the interaction between the template and the cationic monomer can be measured by the binding constant (KM). Moreover, this constant can also reflect the mechanism of template  ski, 2002). In this study, the KM was 12.01 polymerization (Połowin and about 3/4 of MAPTAC was pre-adsorbed on PAAS at the PAAS to MAPTAC molar ratio of 1:1, which indicate that the polymerization mechanism was I (zip) template polymerization mechanism (Fig. 6). Cationic monomer MAPTAC was pre-absorbed on the template PAAS to form multimonomer by electrostatic attraction. The photoinitiator then decomposed to form primary radicals under UV radiation and initiated monomers to polymerization. The chain growth process of monomer polymerization was occurred along the template molecular chain. 3.3. Reaction kinetics 3.3.1. Effect of monomer concentration on polymerization rate (Rp) The experiment was conducted with mole ratios of AM to MAPTAC (n(AM):n(MAPTAC)) at 3:1, 2:1, and 1:1; concentration of initiator (CI) at 8.66  104 mol/L; mole ratio of PAAS to MAPTAC (n(PAAS):n(MAPTAC)) at 1:1; pH ¼ 4.5; and total monomer

concentrations (CM) of 1.5, 2.0, 2.5, 3.0, and 3.5 mol/L. Fig. 7 shows the effect of monomer concentration on conversion and Rp (102 mol L1 S, internal illustration). LnRp showed a good linear relationship with LnCM (internal illustration of Fig. 7 (aec)). The slopes of the linear regression line were 1.804, 1.801, and 1.698, respectively, which indicate the relationship between Rp and CM under the three types of monomer molar ratio: RpfC1.804 , M 1.698 RpfC1.801 . The reaction orders of monomer conM , and RpfCM centration all exceeded 1.0, and this phenomenon can be attributed to complex interactions between monomers such as ion pair and polarity effect (Liu et al., 2011) and production of precipitation in the polymerization process, which is the common phenomenon in precipitation polymerization (Al-Alawi and Saeed, 1990). 3.3.2. Effect of initiator concentration on Rp The experiment was conducted with n(AM):n(MAPTAC) ¼ 3:1; CM ¼ 3 mol/L; n(PAAS):n(MAPTAC) ¼ 1:1; pH ¼ 4.5; CI ¼ 2.17e10.83 (  104) mol/L. Fig. 8 shows the effect of initiator concentration on conversion (a) and Rp (b). LnRp also showed a good linear relationship with LnCM, and the slope of the linear regression line was 1.05. Therefore, the relationship between Rp and CI can be expressed as RpfC1.05 , and this concentration index is higher than I that of conventional bimolecular termination free-radical reaction (two free radicals interact with each other to form a stable molecule, and RpfC0.5 I ), which indicate the existence of mono-radical termination (chain-free radicals terminate reaction when an atom is captured from monomer, solvent, initiator, and other low molecules or macromolecules) in the polymerization process. Relatively tight complex formed by MAPTAC and template PAAS could reduce the diffusion rate of growth chain free radical and hinder the encounter of radicals, thereby significantly reducing the probability of bimolecular termination reaction (Bdumstein and Kakivaya, 1977). 3.3.3. Effect of mole ratio of PAAS and MAPTAC on Rp The experiment was conducted with n(AM):n(MAPTAC) ¼ 3:1; CM ¼ 3 mol/L; CI ¼ 8.66  104 mol/L; pH ¼ 4.5; n(PAAS):n(MAPTAC) ¼ 0.4e1.4. Fig. 9 shows the effect of mole ratio of PAAS and MAPTAC on conversion (a) and Rp (b). With the increase in n(PAAS)/n(MAPTAC), Rp showed a trend of first increasing and then decreasing, which was typical I (zip) template polymerization mechanism, and the Rp reached the maximum 5.298 at n(PAAS)/ n(MAPTAC) ¼ 1. This result was consistent with that presented in Section 3.2 and previous report (Bamford and Shiiki, 1968; He et al., 2007). The value of n(PAAS)/n(MAPTAC) reflects the filling degree of PAAS by MAPTAC. At low n(PAAS)/n(MAPTAC) (<1), the proportion of

Fig. 5. TG/DSC analysis of PAMA (a) and TPAMA (b) as a function of temperature.

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Fig. 6. Mechanism of UV-initiated template copolymerization.

Fig. 7. Effect of monomer concentration on conversion and Rp. (CI ¼ 8.66  104 mol/L, n(PAAS):n(MAPTAC) ¼ 1:1, pH ¼ 4.5; n(AM):n(MAPTAC) ¼ 3:1 (a), 2:1 (b), 1:1 (c)).

Fig. 8. Effect of initiator concentration on conversion (a) and Rp (b) (n(AM):n(MAPTAC) ¼ 3:1; CM ¼ 3 mol/L; n(PAAS):n(MAPTAC) ¼ 1:1; pH ¼ 4.5).

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Fig. 9. Effect of mole ratio of PAAS and MAPTAC on conversion (a) and Rp (b). (n(AM):n(MAPTAC) ¼ 3:1; CM ¼ 3 mol/L; CI ¼ 8.66  104 mol/L; pH ¼ 4.5).

MAPTAC adsorbed on PAAS increased with the increase of proportion, thus Rp showed an increasing trend. However, at high n(PAAS)/n(MAPTAC) (>1), further increase in the proportion decreased the filling degree of template molecule chain, thereby increasing the distance of MAPTAC and eventually led to the decline in Rp. 4. Flocculation performance 4.1. Effect of flocculant category on dewatering performance To evaluate the flocculation performance of copolymer with microblock structure (TPAMA) and the influence of template, randomly distributed PAMA, commercial CPAM (synthesized through copolymerization of AM and DAC), and purified template copolymer (FTPAMA) were chosen for comparison in the sludge dewatering test. Fig. 10 shows the effect of flocculant category on dewatering performance. RT, FCMC, and SRF of the four flocculants in dewatering tests first decreased and then increased with the increase in the dose, and the optimal doses were 40 mg/L. The dewatering performance of FTPAMA (3.37 NTU, 59.01% of FCMC and 4.76  1012 m kg1 of SRF at 40 mg/L dose) was superior to that of PAMA, TPAMA, and CPAM. At low dose, the surface of flocculant, which can effectively adsorb sludge particles, was insufficient, resulting in high turbidity, FCMC, and SRF. However, excess

flocculant would make the colloidal system highly positive charged, which re-stabilizes sludge particles through electrostatic repulsion, thereby reducing the dewatering effect (Yang et al., 2013). TPAMA showed superior dewatering performance to PAMA and CPAM, attributing the success to its microblock structure, which can make full use of cationic units in polymer chain and enhance its charge neutralization effect (the zeta potential of the supernatant after treatment with TPAMA was much higher than those of PAMA and CPAM at the same dose) (Chen et al., 2016), and regular, chainlike, and porous structure with a prodigious surface area, which is conducive to the bridging effect (Zheng et al., 2014). Furthermore, FTPAMA, the purified product of TPAMA, showed better dewatering performance than TPAMA. The supernatant zeta potential of FTPAMA was close to that of TPAMA at the same dose, which could be considered as the two flocculants having the same charge neutralization effect. However, FTPAMA showed better dewatering performance at lower RT, FCMC, and SRF. The slight better performance might originate from the bridging effect because of the higher intrinsic viscosity of FTPAMA (intrinsic viscosities of FTPAMA and TPAMA were 12.03 and 11.20 dL g1, respectively, listed in Table 1). However, the tedious template separation process and additional cost limit the use of TPAMA in practical applications.

Fig. 10. Effect of flocculant category on dewatering performance. (pH ¼ 6.5, 120 rpm for 20 s, 40 rpm for 60 s, no stirring for 10 min).

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Fig. 11. Effect of pH on dewatering performance. (40 mg/L of dose, 120 rpm for 20 s, 40 rpm for 60 s, no stirring for 10 min).

Fig. 12. Sludge floc size distribution for a-FTPAMA, b-TPAMA, c-PAMA, and d-CPAM. (40 mg/L of dose, pH ¼ 6.0, 120 rpm for 20 s, 40 rpm for 60 s, no stirring for 10 min).

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4.2. Effect of sludge pH on dewatering performance In the coagulationeflocculation sludge dewatering process, pH plays a crucial role in the dewatering efficiency. Therefore, it is of utmost importance to study the relationship between the dewatering performance and pH of the sludge. In this section, the dose of flocculant was 40 mg/L and pH of sludge was adjusted to 2e12. Fig. 11 shows the effect of sludge pH on the RT and zeta potential of supernatant, FCMC, and SRF. Zeta potential of the original sludge was significantly influenced by pH. With the increase in pH, zeta potential decreased sharply. However, the supernatant zeta potential after the coagulationeflocculation process increased significantly compared with the corresponding origin sludge, and the supernatant zeta potential after treatment with TPAMA was much higher than that after treatment with PAMA. These results indicated that charge neutralization played an important role in the flocculation process of this type of flocculants, and confirmed the enhancement of electric neutralization effect by the microblock structure in TPAMA. The four evaluation indices of CPAM sludge dewatering performance significantly fluctuated at pH 2.0e12.0 of the original sludge. However, those of other three flocculants changed slightly. This phenomenon indicated that the PAMA series flocculant has excellent acid and alkali endurance, thus broadening the scope of pH application. 4.3. Floc properties Floc size is one of the main evaluating indicators of sludge dewatering performance. Larger floc is more conducive to dewatering, while small floc can easily plug the filter holes and increase specific filtration resistance, thus reduce the dewatering efficiency. Fig. 12 illustrates the floc size distribution of the conditioned sludge of the four flocculants at the end of the flocculation process. The sludge conditioning tests were conducted with 40 mg/L of FTPAMA, TPAMA, PAMA, and CPAM, and the pH were fixed at 6.0. After flocculation and settling process, the sludge floc was collected, and the floc size was measured using a laser diffraction instrument (Mastersizer, 2000; Malvern, UK). The results show that the floc size of the sludge of the four flocculants decreased in the following order: FTPAMA > TPAMA > PAMA > CPAM. This indicated that FTPAMA shows the optimal dewatering performance, which was in line with the previous conclusion. 5. Conclusions In this study, a cationic polyacrylamide (TPAMA) with microblock structure was synthesized through UV-initiated template copolymerization. The success of template copolymerization was confirmed by characterization of the copolymer through 1H NMR, 13 C NMR, FTIR, and TG/DSC analyses, whose results verified the microblock structure of TPAMA. SEM analysis indicated that TPAMA has a regular, chainlike, and porous structure with a prodigious surface area. A binding constant of 12.01 between PAAS and MAPTAC reflected I (zip) template polymerization mechanism of this UV-initiated template copolymerization. Copolymerization reaction kinetics demonstrated: (1) ion pair, polarity, and precipitation effect in the polymerization process made the reaction order of monomer concentration on Rp higher than 1.0; (2) reaction order of initiator concentration on Rp of 1.05 proved the coexistence of bimolecular termination free-radical reaction and mono-radical termination in the polymerization process; and (3) the changing trend of Rp with n(PAAS)/n(MAPTAC) again proved I (zip) template polymerization mechanism of this UV-initiated template copolymerization. Sludge dewatering tests showed that the UV-initiated template copolymer with porous, prodigious surface area

macromolecular structure, and microblock structure of cation monomer showed excellent dewatering performance. This excellent flocculant should attribute to the success of the combination of UV-initiated mode and template copolymerization technology. Acknowledgments This study was supported by the National Natural Science Foundation of China (Project No. 21477010) and 111 Project (Project No. B13041). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2016.09.046. Abbreviations AM Acrylamide MAPTAC Methacrylamido propyl trimethyl ammonium chloride DAC Acryloyloxyethyltrimethylammonium chloride DMDAAC Diallyl dimethyl ammonium chloride CPAM Cationic polyacrylamide UV Ultraviolet PAAS Sodium polyacrylate VA-044 2, 20 -Azobis [2-(2-imidazolin-2-yl) propane] dihydrochloride 1 H (13C) NMR 1H (13C) nuclear magnetic resonance FTIR Fourier transform infrared spectroscopy SEM Scanning electron microscopy TG/DSC Thermogravimetric/differential scanning calorimetry RT Residual turbidity FCMC Filter cake moisture content SRF Specific resistance to filtration TPAMA Template poly(AM-MAPTAC) PAMA Poly(AM-MAPTAC) FTPAMA Purified template poly(AM-MAPTAC) CI Concentration of initiator CM Concentration of monomer References Akkaya, R., 2012. Synthesis and characterization of a new low-cost composite for the adsorption of rare earth ions from aqueous solutions. Chem. Eng. J. 200e202, 186e191. http://dx.doi.org/10.1016/j.cej.2012.06.042. Al-Alawi, S., Saeed, N.A., 1990. Preparation and separation of complexes prepared by template polymerization. Macromolecules 23, 4474e4476. http://dx.doi.org/ 10.1021/ma00222a023. Bamford, C.H., Shiiki, Z., 1968. Free-radical template polymerization. Polymer 9, 595e598. http://dx.doi.org/10.1016/0032-3861(68)90079-7. Bdumstein, A., Kakivaya, S., 1977. Polymerization of Organized System. Gordon&Breach, Paris, pp. 189e211. Chang, E.E., Chiang, P.-C., Tang, W.-Y., Chao, S.-H., Hsing, H.-J., 2005. Effects of polyelectrolytes on reduction of model compounds via coagulation. Chemosphere 58, 1141e1150. http://dx.doi.org/10.1016/j.chemosphere.2004.08.008. Chen, W., Zheng, H., Guan, Q., Teng, H., Zhao, C., Zhao, C., 2016. Fabricating a flocculant with controllable cationic microblock structure: characterization and sludge conditioning behavior evaluation. Ind. Eng. Chem. Res. 55, 2892e2902. http://dx.doi.org/10.1021/acs.iecr.5b04207. Deng, J., Wang, L., Liu, L., Yang, W., 2009. Developments and new applications of UVinduced surface graft polymerizations. Prog. Polym. Sci. 34, 156e193. http:// dx.doi.org/10.1016/j.progpolymsci.2008.06.002. Ghimici, L., Dranca, I., Dragan, S., Lupascu, T., Maftuleac, A., 2001. Hydrophobically modified cationic polyelectrolytes. Eur. Polym. J. 37, 227e231. http://dx.doi.org/ 10.1016/S0014-3057(00)00101-4. Guan, Q., Zheng, H., Zhai, J., Liu, B., Sun, Y., Wang, Y., Xu, Z., Zhao, C., 2015. Preparation, characterization, and flocculation performance of P(acrylamide-codiallyldimethylammonium chloride) by UV-initiated template polymerization. J. Appl. Polym. Sci. 132 http://dx.doi.org/10.1002/app.41747 n/a-n/a. Guan, Q., Zheng, H., Zhai, J., Zhao, C., Zheng, X., Tang, X., Chen, W., Sun, Y., 2014. Effect of template on structure and properties of cationic polyacrylamide: characterization and mechanism. Ind. Eng. Chem. Res. 53, 5624e5635. http://

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