Template Polymerization of a Novel Cationic Polyacrylamide

Aug 18, 2016 - To make up for the defects of present researches, this paper tried to synthesize TPDA by using UV-initiated template polymerization. So...
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Template Polymerization of a Novel Cationic Polyacrylamide: Sequence Distribution, Characterization, and Flocculation Performance Zhengan Zhang,†,‡,§ Huaili Zheng,*,†,§ Fei Huang,‡,∥ Xiang Li,†,§ Shengying He,‡,∥ and Chun Zhao†,§ †

Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, and §National Centre for International Research of Low-carbon and Green Buildings, Chongqing University, Chongqing 400045, China ‡ College of Resources and Environmental Engineering, Yibin University, Yibin 644000, China ∥ Key Laboratory of the Yangtze River Water Environment, State Ministry of Education, Yibin Research Base, Yibin 400045, China S Supporting Information *

ABSTRACT: In this study, the template polymer (TPDA) of dimethyldiallylammonium chloride (DMD) and acrylamide (AM) was synthesized with UV-initiated template polymerization. The reactivity ratio, Fourier transform infrared spectroscopy, 1H NMR spectroscopy, and thermogravimetric analysis were employed to characterize the TPDA properties. The results revealed that the addition of template sodium polyacrylate improved the reactivity ratio of DMD and increased the number and length of DMD segments in polymer molecules. The results of flocculation tests demonstrated that the DMD block structure improved the flocculation performance of TPDA, especially charge neutralization. In the optimal conditions of a TPDA dose of 9.0 mg·L−1 and a wastewater pH of 5.0−7.0, the residual concentration of suspended solids in a supernatant of coal mine wastewater reached the lowest value of approximately 20.04 mg·L−1. The TPDA flocs were large, compact, rapid to settle, and difficult to restabilize. in drinking water production.6,7 Cationic modification and monomer copolymerization are the traditional methods of preparing CPAM,8 and ultraviolet (UV)-initiated polymerization, one method of monomer copolymerization, is advocated because it costs little and needs just a bit of initiator.9 However, the CPAM prepared by traditional methods like UV-initiated polymerization has a significant drawback: the randomly scattering cationic units cannot work completely. Consequently, the flocculation efficiency will be reduced.10 In template polymerization, when the special material, namely, the template, is added to the polymerization system, it interacts with polymeric monomer through electrostatic forces, van der Waals forces, or hydrogen bonds, changes the polymerization process, monomer reactivity ratio, or sequence distribution of the polymer molecule, and improves the polymer performance.11,12 Consequently, if an anionic polymer as the template is added to the reaction system of CPAM, the cationic monomer will be adsorbed and distributed directionally along the template molecular chain under the electric field force. Once the polymerization is initiated by a photoinitiator, it

1. INTRODUCTION As is well-known, deterioration of the global water quality has been extremely serious. Now the water quality of most drinking water sources in the world, especially in developing countries, cannot meet the requirements of environment quality standards because sharply increased sewage, the byproduct of population growth and industrial development, was discharged without being effectively treated. Almost all wastewater contains suspended solids (SS), which may cause serious environmental damage such as aquatic growth hindrance, river blockages, and so on.1 Common methods to remove SS from wastewater are flocculation, filtration, air flotation, sedimentation, etc.2 Among them, flocculation has been the most widely applied so far because of its good effect, low cost, and convenient operation.3 The most important factor that influences the flocculation effect is the flocculant performance;2,4 thus, fabricating a lowcost flocculant with superior performance is currently a hot research topic. Cationic polyacrylamide (CPAM), synthesized with cationic monomer and acrylamide (AM), is an organic polymer flocculant. It is extensively applied to sewage treatment because most sewage is negatively charged.5 The common cationic monomers are dimethyldiallylammonium chloride (DMD), (acryloxyethyl)trimethylammonium chloride (DAC), (methacryloyloxyethyl)trimethylammonium chloride, etc. Among them, the low toxic DMD is most popular, especially © 2016 American Chemical Society

Received: Revised: Accepted: Published: 9819

May 17, 2016 August 3, 2016 August 18, 2016 August 18, 2016 DOI: 10.1021/acs.iecr.6b01894 Ind. Eng. Chem. Res. 2016, 55, 9819−9828

Article

Industrial & Engineering Chemistry Research will generate a special polymer with a cationic microblock structure, which contributes to the cationic monomer working to improve the flocculation performance of the polymer.13 So far there have been a number of researches about the template or UV-initiated polymerization of organic flocculants. For example, Zhang et al. successfully synthesized anionic polyacrylamide through template polymerization and studied the reaction kinetics of template polymerization.14 Sun et al. prepared the terpolymer of AM, DAC, and butyl acrylate by UV-initiated polymerization and researched its molecular structure and sludge dewatering performance.9 However, studies on the UV-initiated template polymerization for the preparation of CPAM have rarely been reported in the literature so far. To make up for the defects of present researches, this paper tried to synthesize TPDA by using UVinitiated template polymerization. Sodium polyacrylate (PAAS) was selected as the template because of its advantage as a linear configuration in solution. The reactivity ratios of AM and DMD were calculated, and the sequence distribution of the TPDA molecule was statistically analyzed. The properties of TPDA were characterized by Fourier transform infrared (FTIR), 1H NMR spectroscopy, and thermogravimetric analysis (TGA). Finally, the flocculation tests for treating coal mine wastewater were carried out to investigate the flocculation performance and mechanism of TPDA. Meanwhile, the common polymer (CPDA) of DMD and AM was prepared with UV-initiated polymerization, the necessary controlled trials were also done, and the results were compared with those of TPDA to research the template impact on the polymer.

wavelength 365 nm; Shanghai Jiguang Special Lighting Electric Factory, Shanghai, China) to initiate the polymerization. Then, the prepared polymer was completely dissolved with deionized water, and the polymer solution was adjusted to a pH of less than 2 and then purified with acetone and ethanol to obtain a pure TPDA product.10,11 The intrinsic viscosity of TPDA could be adjusted by adding different doses of the initiator.15 The preparation method of CPDA was nearly identical with that of TPDA except that no PAAS was added to the reaction vessel. Poly(dimethyldiallylammonium chloride) (PDMD) and polyacrylamide (PAM) were also synthesized with UV-initiated homopolymerization according to the study demand. The polymers from this preparation would be used for characterization and the flocculation test. The possible synthesis scheme of TPDA is shown in Figure 1.

2. EXPERIMENTAL SECTION 2.1. Materials. The monomer AM (>99%) was supplied by Chongqing Lanjie Tap Water Company (Chongqing, China). The cationic monomer DMD (60% in water) was obtained from Jinan Yifan Chemical Co., Ltd. (Jinan, China). Commercial cationic polyacrylamide (CCPAM; >90%; the molecular weight is approximately 3000000) was purchased from Gongyi Yiqing Water-Purifying Material Co., Ltd. (Gongyi, China). The template PAAS (30% in water; the molecular weight is approximately 3000) was offered as a gift from Shandong Xintai Water Treatment Co., Ltd. (Zaozhuang, China). The photoinitiator 2,2′-azobis(2-methylpropionamide) dihydrochloride (V-50) was purchased from Ruihong Biological Technology Co., Ltd. (Shanghai, China). Urea [CO(NH2)2] was obtained from Tianjing Kaitong Chemical Reagent Co., Ltd. (Tianjing, China). DMD, AM, PAAS, and CCPAM were of technical grade, and other reagents, including ethanol, V-50, urea, hydrochloric acid (HCl), and sodium hydroxide (NaOH), were of analytical grade. All aqueous and standard solutions were prepared with homemade deionized water. The purity of nitrogen gas was higher than 99.99%. 2.2. Methods. 2.2.1. Synthesis of Polymers. TPDA was prepared as follows. When the predetermined dosages of the monomers (AM, 57.0 mmol; DMD, 24.4 mmol) and PAAS (24.4 mmol) were put in a glass reaction vessel, a certain amount of distilled water was immediately added into the vessel to make a total monomer mass ratio of 30%. Then urea (4.0 wt % of total mass) was added to increase the solubility. After that, the solution was purged with nitrogen bubbling for 20 min to remove oxygen. In the meantime, the prearranged dosage of the V-50 initiator was added into the solution. After another 10 min purge with nitrogen, the reaction vessel was sealed, and the solution was irradiated for 1 h with a UV lamp (main

Figure 1. Possible synthesis reaction of TPDA.

2.2.2. Determination of the Monomer Reactivity Ratio. The reactivity ratio (r) reflects the competitiveness of the monomer homopolymerization ability and copolymerization. The theoretical value of the reactivity ratio of a monomer equals the ratio of its homopolymerization rate constant to the copolymerization rate constant.16 When r is more than 1, the monomer tends to homopolymerize with the same monomers; otherwise, the monomer tends to copolymerize with other monomers. It is reported that the relative activity of AM is much stronger than that of DMD.17,18 Measures not taken, the copolymer of DMD and AM will have an undesirable molecular sequence distribution and cannot fully exert the function of DMD. In this experiment, two groups of special polymer samples, one containing five TPDA samples and the other five CPDA samples, were prepared. The reaction material ratios, i.e., the mole ratios of PAAS, DMD, and AM, were 1:1:9, 3:3:7, 5:5:5, 7:7:3, and 9:9:1 for TPDA and 0:1:9, 0:3:7, 0:5:5, 0:7:9, and 0:9:1 for CPDA. Other synthesis conditions were identical with those in section 2.2.1 except that the monomer conversions of all samples were limited to less than 15% by shortening the UV radiation time in this experiment.19 The reactivity ratios of AM and DMD were calculated with the Yezrielev−Brokhina− Roskin (Y−B−R) method, which is formula (S1) in the Supporting Information,20, and the related calculating data were presented in Table S1 of the Supporting Information. The polymers from this preparation would also be used for analysis of the composition equation and sequence distribution besides calculation of the reactivity ratio. 2.2.3. Composition and Sequence Distributions of Polymers. The copolymer composition equation is usually 9820

DOI: 10.1021/acs.iecr.6b01894 Ind. Eng. Chem. Res. 2016, 55, 9819−9828

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Industrial & Engineering Chemistry Research Table 1. Characteristics of the Coal Mine Wastewater Sample SS (mg·L−1)

COD (mg·L−1)

pH

ζ potential (mV)

turbidity (NTU)

conductivity (ms·cm−1)

color

465.8 ± 1.4

47.8 ± 0.6

7.59 ± 0.32

−43.6 ± 0.4

212.7 ± 0.6

3.2 ± 0.5

black

refrigerator. The initial characteristics of wastewater were measured and are shown in Table 1, and the main pollutant was SS, which is usually removed by flocculation. According to the established testing scheme, a solution of three flocculants of given concentration was separately prepared. The same amount of coal mine wastewater was poured into several beakers; a prearranged dose of a flocculant solution was orderly added; the wastewater pH was adjusted by adding NaOH (1.0 mol·L−1) or HCl (1.0 mol·L−1) as needed; the mixture was stirred at a high speed of 450 rpm for 3 min and then at a low rate of 200 rpm for 3 min with a ZR4-6 coagulation experiment blender (Zhongrun Water Industry Technology and Development, Shenzhen, China). When all of these were done, flocs began to form and kept settling for 30 min. In order to evaluate the flocculation efficiencies and mechanisms of TPDA, CPDA, and CCPAM, the residual concentrations of SS and the ζ potentials in the supernatant were measured by a gravimetric method and a Zetasizer Nano ZS90 (Malvern Instruments Ltd., Malvern, U.K.), respectively. In addition, another group of tests were carried out to assess the floc properties of TPDA, CPDA, and CCPAM. A total of 1 L of mine wastewater was poured into each graduated cylinder with a total scale height of 29.8 cm. TPDA, CPDA, and CCPAM were separately added and completely mixed with wastewater under the action of an agitator. After flocs formed and started settling, the heights of the floc−liquid interface were regularly recorded within 60 min for calculation of the settling rates. The size distributions of the flocs were measured with a laser diffraction instrument (Mastersizer 2000, Malvern, U.K.). The flocculation behaviors of TPDA, CPDA, and CCPAM were judged in terms of the settling rates and the size distributions of the flocs.

used to describe the quantitative relationship between the copolymer composition and material monomer composition.21,22 The composition equation for the bipolymer can be expressed by formula (1) as follows: F1 =

r1f12 + f1 f2 r1f12 + 2f1 f2 + r2f2 2

(1)

In formula (1), F is the molar ratio of units of one monomer to the total copolymer units, f is the molar ratio of one monomer to all material monomers before polymerization, and r is the monomer reactivity ratio. In this study, the composition equations of TPDA and CPDA were obtained by the monomer reactivity ratio and formula (1). The distribution of the segment and monomeric units are important information to characterize the microstructure of the polymer.23−25 In order to investigate the influence of template addition on the polymer microstructure, two copolymer samples with mole ratios of PAAS, DMD, and AM of 5:5:5 and 0:5:5 were statistically analyzed. Their average lengths and sequence distributions of monomer segments were obtained and are exhibited in Figure 3, and the calculating equations are formulas (S6)−(S11) in the Supporting Information.23−25 2.2.4. Characterization of the Polymers. The intrinsic viscosities of the polymers were measured with an Ubbelohde viscosity meter (Shanghai Shenyi Glass Instrument Co., Ltd., China). The molecular weights of the polymers were calculated according to their intrinsic viscosities, and the calculation equation was formula (S12) in the Supporting Information. The cationic degrees and monomer conversions of the copolymers were determined by colloid titration and gravimetry, respectively.10 The calculation methods of the cationic degree and monomer conversion are formulas (S13) and (S14) in the Supporting Information. The polymers were dried and ground into a powder for characterization. The FTIR spectra of the copolymers were recorded using KBr pellets on a 550 Series II IR spectrometer (Mettler Toledo Instruments Co., Ltd., Küsnacht, Switzerland); 1 H NMR spectra of the polymers with deuterium oxide (D2O) as the solvent were obtained with an AVANCE500 NMR spectrometer (Bruker Company, Karlsruhe, Germany); the thermostabilities of the copolymers were investigated through differential scanning calorimetry and thermogravimetric analysis (DSC−TGA), which were conducted with a DTG-60H synchronal thermal analyzer (Shimadzu, Kyoto, Japan) at a heating rate of 10 °C·min−1, a nitrogen flow of 20 mL·min−1, and a temperature range of 20−600 °C. 2.2.5. Flocculation Test. In this part, TPDA, CPDA, and CCPAM were used for the treatment of coal mine wastewater through a flocculation test at room temperature to investigate their flocculation performance. Among them, CPDA and CCPAM were used to facilitate comparison with TPDA. In order to make the test results comparable, TPDA, CPDA, and CCPAM samples with similar molecular weights of approximately 3000000 were prepared or purchased.2 The wastewater sample used for the test was collected from Yibin Gongquan Coal Mine (Yibin, China), immediately transported to the laboratory within 60 min, and subsequently stored at 4 °C in a

3. RESULTS AND DISCUSSION 3.1. Monomer Reactivity Ratios of the Polymers. The monomer reactivity ratio supplies important information for researching the composition and sequence distribution of the copolymers.16 According to the Y−B−R method, the monomer reactivity ratios of TPDA and CPDA were calculated and are shown in Table 2. The template addition caused significant Table 2. Monomer Reactivity Ratios of the Polymers reactivity ratio copolymer

rAM

rDMD

TPDA CPDA

3.84 5.62

0.93 0.42

changes of the monomer reactivity ratios. The reactivity ratio of DMD (rDMD) increased from 0.42 for CPDA to 0.93 for TPDA, whereas the reactivity ratio of AM (rAM) decreased from 5.62 for CPDA to 3.84 for TPDA. These changes implied that the homopolymerization ability of DMD increased and that of AM declined.26 When two monomer reactivity ratios are both greater than 1 in the binary polymerization, the reaction can be considered to be block copolymerization, which generates a block structure in the polymer molecular chain.27 In this study, rDMD and rAM were respectively 0.93 and 3.84 after template 9821

DOI: 10.1021/acs.iecr.6b01894 Ind. Eng. Chem. Res. 2016, 55, 9819−9828

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was greater than that for CPDA given the same f DMD, and FAM was just the opposite, indicating that template addition strengthened the activity of DMD and improved its conversion under the experimental control conditions; however, the activity of the AM monomer was weakened, so its change was just the reverse. 3.3. Sequence Distributions of the Polymers. The sequence distributions of the monomer segments of TPDA and CPDA were statistically investigated, and more statistical data are listed in Tables S2 and S3 in the Supporting Information. Figure 3 reveals the following conclusions: (1) The long DMD segments with length greater than 1 accounted for 48.2% of all DMD segments in TPDA and 29.6% in CPDA, indicating that TPDA had more long DMD segments than CPDA, whereas the situation of the AM segments was just the opposite. (2) The average length of all DMD segments in TPDA (N̅ DMD-TPDA) was 1.93, which was greater than that in CPDA, yet the average length of all AM segments (N̅ AM) was the reverse. These results prove that template addition prompted more DMD monomers to form long segments, thus increasing the number and average length of DMD blocks. N̅ AM was shortened because the AM activity decreased. This phenomenon could be explained by the mechanism of template polymerization demonstrated in Figure 4.28,29 When the anion template PAAS was added to the

addition, which nearly met the requirement of block polymerization. Consequently, it was inferred that there were more DMD blocks in the molecular chain of TPDA than in that of CPDA. 3.2. Composition Equations of the Polymers. The composition equations of TPDA and CPDA, namely, formulas (S2)−(S5) in the Supporting Information, were obtained according to the reactivity ratios of the monomers and formula (1). The composition curves of TPDA and CPDA were drawn in terms of their composition equations to more intuitively display the relationship between the copolymer and material. As shown in Figure 2, FDMD values were less than f DMD values

Figure 2. Composition curves and test results of the polymers. Note that the squares stand for the test results, the four solid curves are composition curves of the polymers, and the dashed line represents an ideal copolymerization. F is the molar ratio of AM or DMD units to the total copolymer units; f is the molar ratio of AM or DMD to total material monomers before polymerization.

Figure 4. Overview of the template polymerization of TPDA.

reaction system, large amounts of cationic monomers, namely DMD, were arranged along the molecular chain of the template under the action of electrostatic force, whereas the AM monomers were almost unaffected and still randomly distributed. When UV irradiation was started, a DMD molecule on the template was attacked by a free radical and formed a

for both TPDA and CPDA, whereas the status of AM was just the opposite. A comparison found that the composition curves of TPDA were evidently closer to the ideal copolymerization curve than those of CPDA. In other words, FDMD for TPDA

Figure 3. Sequence distributions of (a) DMD and (b) AM in the polymers. 9822

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reason for the difference probably still lies in the template addition. 3.4.3. 1H NMR. 1H NMR is an effective method to identify the molecular structure of matter. The chemical shift and area of the characteristic peak in a spectrum reflect the type and number of corresponding protons. For the polymer with an uncertain chemical formula, the peak area ratio of different protons is equal to their number ratio in the chemical formula.32 The 1H NMR spectra of CPDA, TPDA, PDMD, and PAM are all exhibited in Figure 6 for convenient comparison and analysis. As the TPDA 1H NMR spectrum shows, multiple overlapping peaks appear at the chemical shifts (δ) = 1.65−1.85 and 2.20−2.35 ppm derived from the protons at backbone methane −CH2− (a and c) and methylene −CH− (b) groups, respectively; the proton peaks of −CH− (d), −(CH2)2−N+ (e), and N+−(CH3)2 (f) in DMD appear at δ = 2.60−2.70, 3.70−3.80, and 3.15−3.30 ppm, respectively. The comparison exposed that the chemical shifts of protons in the TPDA spectra were somewhat different from those in the PDMD and PAM spectra. For instance, the chemical shift of −CH2− (a) was 1.65−1.85 ppm in the TPDA spectrum but 1.55−1.70 ppm in the PAM spectrum. The reason for this phenomenon was as follows. TPDA was the copolymer of DMD and AM rather than their homopolymer, namely, PDMD or PAM. The chemical environments of the corresponding protons in TPDA were different from those in PDMD or PAM; consequently, the chemical shifts of the corresponding protons were not identical.6 The result of 1H NMR confirmed that TPDA was successfully copolymerized with DMD and AM. This view was also confirmed by FTIR spectra of TPDA. Figure 6 demonstrates not only many similarities between the 1H NMR spectra of CPDA and TPDA, such as the same proton type and similar absorption peak shapes, but also many differences, which was vital to the study. First, the proton chemical shifts of −CH2− (e), −CH− (d), and N+−(CH3)2 (f) in TPDA were all much closer to those in PDMD than those in CPDA. Next, CPDA had more split peaks than TPDA and PDMD because of the interference of adjacent protons. For example, the split peak number of the −CH2− (c) proton was 3 for CPDA but 2 for both TPDA and PDMD. These phenomena can be explained by the fact that TPDA had large amounts of continuous adjacent DMD units, i.e., DMD blocks, just as PDMD, and many protons such as −CH2− (c, e), −CH− (d), and N+−(CH3)2 (f), had chemical environments similar to or even identical with those in PDMD, whereas the DMD and AM units in CPDA were randomly distributed, resulting in more diverse chemical environments; therefore, CPDA possessed more split peaks than TPDA and PDMD. Finally, the peak areas of some protons in TPDA were different from those in CPDA, indicating that their numbers of corresponding protons were not equal. For further quantitative analysis, the protons of −CH3 (f) from DMD and −CH− (b) from AM were selected as study subjects, and their peak areas in CPDA and TPDA were calculated with MestReNova software and are presented in Table 4. The results show that the peak area ratios of N+−(CH3)2 (f) to −CH− (b) in TPDA and CPDA were 0.63 and 0.58, respectively, implying that TPDA had more DMD units in its molecule than CPDA. This phenomenon proved once again that template addition contributed to the improvement of the reactivity and conversion of DMD. The differences between TPDA and CPDA 1H NMR spectra suggested that template addition contributed to the generation of the polymer with block

new free-radical chain, which easily reacted with adjacent DMD monomers. Thus, the reaction continuously reproduced and generated polymers with a long DMD segment, namely, a microblock structure. However, when CPDA was synthesized, AM and DMD were both randomly distributed in the reaction system, with the result of few DMD segments in its molecular chain. The survey results of sequence distributions also confirmed the inference in section 3.1 that TPDA had more DMD blocks in its molecular chain than CPDA. 3.4. Characterizations of the Polymers. 3.4.1. Intrinsic Viscosity and Cationic Degree. Table 3 reveals that the Table 3. Intrinsic Viscosities and Cationic Degrees of the Polymers polymer

intrinsic viscosity (mL·g−1)

molecular weight

cationic degree (%)

TPDA CPDA

6936.43 7219.67

2856760 3001552

23.86 22.47

intrinsic viscosity and molecular weight of TPDA were less than those of CPDA under the same polymerization conditions. The possible reason was that the polymerization retardation of PAAS to AM or DMD inhibited free-radical reaction, preventing longer molecule chains from forming,14 whereas a flocculant with a long molecular chain possesses strong bridging ability.30 The cationic degree of TPDA was slightly higher than that of CPDA, implying that TPDA had stronger charge neutralization than CPDA.31 3.4.2. FTIR. As shown in Figure 5, most characteristic peaks of chemical groups in DMD and AM molecules, except CC,

Figure 5. FTIR spectra of CPDA and TPDA.

were observed in the FTIR spectrum of TPDA. For instance, the strong absorption peaks at 3432 and 1670 cm−1 were attributed to the stretching vibrations of −NH2 and−CO in AM, respectively; the absorption peaks at 1453 and 2845 cm−1 were ascribed to the bending vibrations −CH2 and −CH3 of −(CH2)2N+(CH3)2 in DMD, respectively; the characteristic absorption peak at 2492 cm−1 was due to the stretching vibration of −CH2. However, the adsorption peak of CC did not appear in the FTIR spectrum of TPDA, indicating that all CC in AM and DMD took part in the polymerization. The FTIR spectrum of CPDA was almost the same as that of TPDA except for a slight shift in the peak area. For example, the absorption peak area of −(CH2)2N+(CH3)2 at 1453 cm−1 in TPDA was larger than that in CPDA, demonstrating that their molecular structures were similar but not exactly identical. The 9823

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Figure 6. 1H NMR spectra of the polymers.

mass loss occurred in the ranges of 330−550 °C for TPDA and 340−550 °C for CPDA with mass losses of about 51.0% and 53.5%, respectively, which resulted from thermal decomposition of the copolymer backbone. In the last stage, two peaks of heat adsorption appeared at 395.0 and 418.3 °C in the TPDA DSC curve, whereas only a single peak at 416.7 °C could be observed in the CPDA DSC curve. It was inferred that the two absorption peaks of TPDA probably resulted from two different structural blocks in the TPDA molecular chain, namely, the DMD and AM blocks, and their decomposition gave rise to two different heat adsorption peaks. Similar findings were reported.35,36 The DSC−TGA result manifested that both TPDA and CPDA have satisfactory thermal stabilities. 3.5. Flocculation Test. The flocculant performance is influenced by many factors, including the intrinsic viscosity, dose, pH, stirring, and so on, but only the dose and pH have noticeable effects and are easily controlled artificially.37,38 In this study, TPDA, CPDA, and CCPAM were used to remove SS from coal mine wastewater by flocculation under the conditions of various doses and wastewater pH levels. 3.5.1. Effect of the Dose on the Polymer Performances. As shown in Figure 8, the flocculation effects of TPDA, CPDA, and CCPAM presented similar change rules. The residual SS concentrations in the supernatant all sharply decreased first and then increased slightly with increasing polymer dose, but TPDA

Table 4. Peak Areas of −CH3 (f) and −CH− (b) in 1H NMR Spectra of the Polymers peak area of protons polymer

N+−(CH3)2 (f)

−CH− (b)

peak area ratio

TPDA CPDA

1.00 1.00

1.59 1.72

1.00:1.59 = 0.63 1.00:1.72 = 0.58

structure. This view was identical with the analysis result of sequence distributions of the polymers. 3.4.4. DSC−TGA. Figure 7 shows that the processes of mass losses of both TPDA and CPDA experienced three stages. In the first stage, the mass losses were both about 10% (w/w) in the ranges of 40−205 °C for TPDA and 40−165 °C for CPDA. These losses could be ascribed to the evaporation of water in polymer samples.33 Because of heat absorption of water evaporation, two apparent heat adsorption peaks appeared at 82.7 °C for TPDA and 71.0 °C for CPDA in their DSC curves. The second stage occurred in the ranges of 205−330 °C for TPDA and 245−340 °C for CPDA with the mass losses of 19.3% and 18.0%, respectively. These losses may be due to dehydration of the amide groups and thermal decomposition of the methyl groups in the quaternary ammonium groups, and the corresponding peaks of heat adsorption appeared at 279.7 °C for TPDA and 294.0 °C for CPDA.34 The third stage of 9824

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Figure 7. TGA curves of the polymers.

molecular chain, so bridging also played an important role in the TPDA flocculation behavior besides charge neutralization,40 which was also the main reason why restabilization of TPDA was less significant than that of CPDA and CCPAM in Figure 8. 3.5.2. Effect of the pH on the Polymer Performances. In this study, the impacts of the pH on the flocculant performances of the polymers were investigated. The doses of TPDA, CPDA, and CCPAM were their optimum values, i.e., 9.0, 10.0, and 11.0 mg·L−1, respectively, and the results are shown in Figure 9.

Figure 8. Effect of the dose on the polymer performances.

had a better efficiency than the other two in the full dose range. Their lowest SS concentrations were 26.7 mg·L−1 with a dose of 9.0 mg·L−1 for TPDA, 31.8 mg·L−1 with a dose of 10.0 mg· L−1 for CPDA, and 33.2 mg·L−1 with a dose of 11.0 mg·L−1 for CCPAM. After TPDA, CPDA, and CCPAM reached their optimal doses, their flocs, especially CPDA and CCPAM, all gradually recovered stabilization with increasing doses, leading to increases in the SS concentrations. This phenomenon is also called restabilization, which means that the excessive addition of a charged flocculant or polymer into the colloidal solution enhances the electrostatic repulsion between colloidal particles, resulting in another floc dissolution.10 As Figure 8 shows, the ζ potentials of the supernatant all continuously increased with growing doses of TPDA, CPDA, and CCPAM, but that of TPDA was the highest of the three. This phenomenon should be attributed to the TPDA microblock structure, which had concentrated positive charges and benefited the charge neutralization of cationic monomers, and it was also the main reason why TPDA had a better removal efficiency of SS than CPDA and CCPAM. In addition, when polymer doses were at optimal values, the ζ potentials for both CPDA and CCPAM were nearly equal to zero, but not that for TPDA, indicating that charge neutralization played the main role for CPDA and CCPAM during their flocculation processes.39 The strong electrostatic repulsion of the cationic microblock contributed to the linear extension of the TPDA

Figure 9. Effect of the pH on the polymer performances.

Figure 9 illustrates the flocculation effect of TPDA. The residual SS concentration in the supernatant sharply decreased at pH values of 2.0−5.0, then maintained its stability at pH values of 5.0−7.0, and increased markedly at pH values of 7.0− 11.0; when the pH was in the range from 5.0 to 7.0, the residual SS concentration always remained close to the minimum of 20.4 mg·L−1, indicating that pH values of 5.0−7.0 were beneficial to the TPDA flocculation performance. Because of the electrostatic repulsion, too high or too low pH would lead to colloid restabilization and a SS concentration increase.41 A comparison found that TPDA, CPDA, and CCPAM presented similar change rules of the flocculation behavior, but TPDA had the lowest SS concentration at pH values of 4−11 and the highest ζ potential in the full pH range. The cationic 9825

DOI: 10.1021/acs.iecr.6b01894 Ind. Eng. Chem. Res. 2016, 55, 9819−9828

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Industrial & Engineering Chemistry Research microblock structure and strong charge neutralization ability of TPDA accounted for these phenomena again. The residual SS concentration for TPDA was greater than that for CPDA and CCPAM at pH values of 2.0−3.0 because of the stronger electrostatic repulsion of the cationic microblock. After the optimum pH 5.0 for TPDA was reached, the residual SS concentration stood stable at pH values of 5.0−7.0 rather than rising immediately, whereas the ζ potential consistently declined during this period. Hence, merely bridging and charge neutralization could not completely explain such flocculation behavior of TPDA, but maybe the electrostatic patching could do it.42 The positive charges of the cationic microblock structure of TPDA could not be neutralized completely because of its high density and nonuniformity of charge.43 When locally positively charged colloidal particles contacted the locally negatively charged ones under the action of electrostatic attraction, flocculation occurred and lasted at pH values of 5.0− 7.0. The cationic groups of CPDA and CCPAM, by contrast, were both randomly distributed, so electrostatic patching did not occur during their flocculation. However, when the pH surpassed the optimum value of 6.0, both CPDA and CCPAM restabilization arose, leading to a significant increase of residual SS concentrations, as shown in Figure 9. Thus, TPDA will have a good application prospect because of its excellent steady flocculation performance in a wide pH range. Because positive charges of the polymer colloid were neutralized and offset by the hydroxyl,44 the ζ potentials for TPDA, CPDA, and CCPAM all continued to decline with increasing pH, as shown in Figure 9. 3.5.3. Settlement of Flocs. The size distribution and settlement of flocs can provide information to judge the flocculation behavior of the flocculant. In general, the average size of flocs produced by bridging is larger than that produced by charge neutralization,5 and the fast settling rate implies a good flocculation effect.45,46 Another set of flocculation experiments were conducted under the optimum conditions with a dose of 9.0 mg·L−1 and a wastewater pH at 6.0 for TPDA, a dose of 10.0 mg·L−1 and a wastewater pH at 6.0 for CPDA, and a dose of 11.0 mg·L−1 and a wastewater pH at 6.0 for CCPAM. The size distributions and settling rates of flocs were determined to evaluate their flocculation behaviors and performances. As shown in Figure 10, the flocs produced by TPDA, CPDA, and CCPAM all first sank fast, then slowed down gradually, and eventually stopped sinking. The difference was that the settling rates of TPDA flocs were faster than the other two, indicating that TPDA flocs needed a shorter settling time. As Figure 11 shows, when the flocs stopped settling and finally remained stable, the heights of floc sedimentations of TPDA, CPDA, and CCPAM were 4.69, 6.98, and 7.90 cm, respectively. On the basis of the floc sedimentation volume and the test results of section 3.2, the densities of floc sedimentation of TPDA, CPDA, and CCPAM were roughly calculated, and the values were approximately 2.85, 1.90, and 1.67 kg·m−3, respectively. The related computing details are listed in Table S4 in the Supporting Information. The calculation results implied that floc sedimentation of TPDA was the most compact of the three. 3.5.4. Size Distributions of Flocs. As shown in Figure 12, the percentage of large flocs with the sizes greater than 69.2 μm is 58.0% for TPDA, 43.1% for CPDA, and 40.1 for CCPAM, and their average sizes (d50) are 84.05, 72.55, and 70.95 μm, respectively. This result indicated that TPDA produced more

Figure 10. Floc settling rates of the polymers.

Figure 11. Floc interface heights of the polymers.

Figure 12. Floc size distributions of the polymers.

large sized flocs than the other two, which also should be attributed to the cationic microblock structure of TPDA. Because of the strong electrostatic repulsion between DMD segments of TPDA, its molecular chain expanded well,47 and the linear-expanding molecular chains improved the TPDA 9826

DOI: 10.1021/acs.iecr.6b01894 Ind. Eng. Chem. Res. 2016, 55, 9819−9828

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Industrial & Engineering Chemistry Research

also received and appreciates funding from the Fermentation Resources and Application of Sichuan College (Grant 2009KFJ002) and Education Department of Sichuan Province (Grant 09ZA152).

bridging ability. Besides, the high charge density of the DMD block also contributed to attracting negatively charged particles,48 so TPDA adsorbed amounts of particles and formed large and compact flocs, as shown in Figure 13. However, CPDA and CCPAM do not have the abovementioned performances because of the random distribution of the cationic monomers in their molecules.

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Figure 13. Schematic diagrams of flocs of TPDA (a) and CPDA or CCPAM (b).

4. CONCLUSIONS In this study, a novel flocculant TPDA were successfully synthesized with UV-initiated template polymerization. The reactivity ratio, FTIR, 1H NMR, and DSC−TGA were employed to characterize its properties, and the flocculation tests were performed to evaluate its flocculation performance. The main conclusions were as follows: (1) The addition of the template PAAS improved the DMD reactivity ratio, increased the number and length of DMD segments in the polymer molecules, but reduced the intrinsic viscosity of the polymer. (2) The DMD block structure improved the flocculation performance of TPDA, especially charge neutralization. In the optimal conditions of a TPDA dose of 9.0 mg·L−1 and a wastewater at pH value of 5.0−7.0, the SS residual concentration of coal mine wastewater reached the lowest value of approximately 20.09 mg·L−1. (3) TPDA had a wider pH range for application than CPDA and CCAM; flocs produced by TPDA were not only large, compact, and rapid to settle but also difficult to restabilize.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01894. Y−B−R method and calculation of the monomer reactivity ratios, composition equations of TPDA and CPDA, sequence distributions of monomer segments of TPDA and CPDA, calculation formulas for the molecular weight (Mr), conversion, cationic degree and calculation data relating to the densities of flocs sedimentation (PDF)

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ABBREVIATIONS AM =acrylamide CCPAM =commercial cationic polyacrylamide CPAM =cationic polyacrylamide CPDA =UV-initiated polymer P(DMD−AM) δ =chemical shift D2O =deuterium oxide DMD =dimethyldiallylammonium chloride DSC−TGA =differential scanning calorimetry and thermogravimetric analysis FTIR =Fourier transform infrared spectroscopy 1 H NMR =hydrogen proton nuclear magnetic resonance N̅ DMD =average length of the DMD segments N̅ AM =average length of the AM segments PAAS =sodium polyacrylate rAM =reactivity ratio of an AM monomer in a AM and DMD monomer pair rDMD =reactivity ratio of a DMD monomer in a AM and DMD monomer pair SS =suspended solids TPDA =UV-initiated template polymer P(DMD−AM) UV =ultraviolet Y−B−R =Yezrielev−Brokhina−Roskin method ζ potential =zeta potential

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 23 65120827. Fax: +86 23 65120827. E-mail: [email protected] (H.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All authors received and appreciate funding from the National Natural Science Foundation of China (Grant 21477010). Z.Z. 9827

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DOI: 10.1021/acs.iecr.6b01894 Ind. Eng. Chem. Res. 2016, 55, 9819−9828