Synthesis of cationic polyacrylamide by low-pressure UV initiation for turbidity water flocculation

Chemical Engineering Journal xxx (2016) xxx–xxx

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Synthesis of cationic polyacrylamide by low-pressure UV initiation for turbidity water flocculation Jiangya Ma a,b,⇑, Jun Shi a,b, Houcheng Ding a,b,⇑, Guocheng Zhu c, Kun Fu a,b, Xue Fu a,b a

School of Civil Engineering and Architecture, Anhui University of Technology, Maanshan, Anhui 243002, China Engineering Research Center of Biomembrane Water Purification and Utilization Technology, Ministry of Education, Maanshan, Anhui 243002, China c College of Civil Engineering, Hunan University of Science & Technology, Xiangtan, Hunan 411201, 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 novel flocculants of PADC was

synthesized through low-pressure initiation.
Hydrophobic monomer of CDEA was introduced to synthesis of polyacrylamide.
Intrinsic viscosity and positive charge density of PADC was investigated.
Chemical structures of PADC were characterized and analyzed.
Flocculation performance and mechanism were evaluated and summarized.

a r t i c l e

i n f o

Article history: Received 29 September 2016 Received in revised form 16 November 2016 Accepted 17 November 2016 Available online xxxx Keywords: Low-pressure ultraviolet Cationic polyacrylamide Characterization Flocculation Kaolin

a b s t r a c t This work presents a preparation process and flocculation performance evaluation of a novel cationic polyacrylamide PADC synthesized by low-pressure ultraviolet (UV) initiation. Flocculant of PADC was polymerized with acrylamide (AM), diallyl dimethyl ammonium chloride (DMDAAC) and coconut diethanolamide (CDEA) in homogeneous aqueous solution under the irradiation of low-pressure UV, and 2-h ydroxy-40 -(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure2959) was used as photo sensitizer. The factors that affect polymerization were investigated; these include monomer concentration, initiator dosage, mass ratio of mAM:mDMDAAC and irradiation time. Structures and morphologies were characterized by Fourier transform-infrared (FTIR) spectroscopy, nuclear magnetic resonance hydrogen (1H NMR) spectra, scanning electron microscope (SEM), and thermo gravimetric and differential thermal analysis (TGDTA). The flocculation performance was evaluated by removing high turbidity kaolin suspension; flocculation mechanism was also correspondingly analyzed. Experimental results show that the PADC efficiently removes the suspended particles efficiently by charge neutralization and bridging effect in an environment with wide range of pH. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction

⇑ Corresponding authors at: School of Civil Engineering and Architecture, Anhui University of Technology, Maanshan, Anhui 243002, China. E-mail addresses: [email protected] (J. Ma), [email protected] (H. Ding).

One of the severe problems in water treatment technology facing is the purification of high turbidity wastewater caused by the breeding industry [1,2], stormwater pollution attributed to development of large residential housing, commercial buildings, institutions, agriculture, as well as livestock and industrial areas [3]. To

http://dx.doi.org/10.1016/j.cej.2016.11.114 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: J. Ma et al., Synthesis of cationic polyacrylamide by low-pressure UV initiation for turbidity water flocculation, Chem. Eng. J. (2016), http://dx.doi.org/10.1016/j.cej.2016.11.114

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solve this problem, certain technologies were researched and applied, such as coagulation with iron or aluminum, electrochemical oxidation, adsorption, membrane filtration and biotechnology for water treatment [4,5]. Flocculation used in wastewater treatment process is an important purification technique because of its low cost, high efficiency, and simple execution compared with other technologies [6]. The most critical issue is selecting a suitable flocculant for a specific wastewater. At present, various inorganic flocculant are widely used in purifying highly turbid suspension. However, problems of massive dosage, exorbitant cost, and residual chemicals limit the use of inorganic flocculant in turbid water treatment. Meanwhile, effective organic polymer flocculant, especially cationic polyacrylamide (CPAM), was developed and extensively used for turbidity separation because of their satisfactory solid-water separation performance [7]. Generally, cationic polyacrylamide is copolymerized by acrylamide (AM) and cationic monomers in an aqueous solution. Among various cationic monomers, (3acrylamidopropyl) trimethyl ammonium chloride (ATMAC) is the first used cationic monomer because of the rich amine groups. Moreover, research on [2-(methacryloyloxy) ethyl] trimethyl ammonium chloride (METAC), methacryloxy ethyl trimethyl ammonium chloride (DMC), and [2-(acryloyloxy) ethyl]-trimethyl ammonium chloride (AOTAC) grafted with AM were reported afterwards [8,9]. Compared with these monomers, diallyl dimethyl ammonium chloride (DMDAAC) received consistent approval as an excellent cationic monomer for polymerization of CPAM because of the two double bonds contained in these molecules [10]. Yang et al. successfully synthesized a cationic polyacrylamide flocculant by grafted polymerization of DMDAAC, AM, and hydrophobic monomer butylacrylate (BA); the flocculation performance of this flocculant was evidently improved because of hydrophobic association. However, the synthesis became complex because of the hydrophobic monomer that is difficult to dissolve in the reaction solution [11]. In addition, surfactants are currently used as cosolvent currently for reformation of the interface state between different materials [12]. Thus, this problem can be solved by using coconut diethanolamide (CDEA) with excellent solubility, and contains both hydrophobic and hydrophilic group. At present, various methods for grafting copolymer synthesis were reported, such as aqueous solution, inverse emulsion, and radiation polymerizations [13,14]. Furthermore, aqueous solution polymerization is the most popular technique in industry production because of its low requirement for equipment [15]. In recent years, ultraviolet (UV) initiation based on aqueous solution polymerization has attracted scholar’s attention because this method is environment-friendly, energy-efficient, and has rapid reaction rate in production [16]. Zheng et al. reported that a novel anion polyacrylamide was obtained under high-pressure ultraviolet initiation after 60 min; the copolymer is effective in dioctyl phthalate removal from water [17]. The utilized high-pressure ultraviolet lamp is characterized by power of 500–1000 W and predominant wavelength of 365 nm. Another cationic polyacrylamide with intrinsic viscosity of 4.2 dL g1 was synthesized in 120 min from aqueous photo-polymerization; a 200-W high-pressure mercury lamp was used in polymerization [18]. Moreover, these studies also reported the improvement in surface modification under UV initiation. However, several new problems emerged with the development of high-pressure UV initiation technique. Extreme temperature is the biggest problem in high-pressure UV initiation, which was recorded up to 60–70 °C [17]. The addition of complicated condensing equipment is necessary to reduce the temperature and ensure smooth reaction in the polymerization. However, the equipment would increase production cost and operational difficulties in industrial production. On the other hand, extreme temperature results in cross linking of polymers, and further leads to

decreases in flocculant’s solubility. Furthermore, excellent penetration of high-pressure UV with higher wavelength is considered a significant security threat to laborers in the workshop, and higher energy consumption is not suggested in industrial production. Therefore, low-pressure UV initiation was used in this study to avoid these problems. Considering all aforementioned factors, a novel flocculant PADC was synthesized by AM, DMDAAC, and CDEA under low-pressure UV initiation. The predominant wavelength and power of the low-pressure UV were 253.7 nm and 24 W, respectively. Synthesis conditions of monomer concentration, mass ratio, photo-initiator dosage, and irradiation time were investigated for intrinsic viscosity and positive charge density. The chemical structure of the obtained copolymer was characterized by FTIR spectroscopy, 1H NMR spectra, SEM and TG-DTA. Finally, flocculation performance of PADC for kaolin suspension treatment was examined. FTIR of generated flocs and zeta potentials of supernatant and flocculant were also analyzed for the summary of flocculation mechanism. 2. Materials and methods 2.1. Experimental materials Acrylamide (AM, 99.0% wt.), Coconut diethanolamide (CDEA, 99.5% wt.) and sodium chloride (NaCl, 99.8% wt.) without exception were obtained from Sinopharm Chemical Reagent Co., Ltd. Diallyl dimethyl ammonium chloride solution (DMDAAC, 60% wt. aqueous solution), poly(vinyl sulfate) potassium salt (PVSK, average Mw  162.21) and toluidine blue O (TBO, Mw  305.81) all were purchased from aladdin Reagent Co., Ltd. Acetic acid (AA, 36% wt.) was produced by Lingfeng Chnt emical Reage Co., Ltd (Shanghai, China). Moreover, ethanol and acetone were sourced from Zhenqin Chemical Reagent Co., Ltd (Shanghai, China). Photo-initiator 2-hydroxy-40 -(2-hydroxyethoxy)-2-methylpropio phenone (Irgacure2959, 98% wt.) was supplied by Changzhe Biological Technology Co., Ltd. (Shanghai, China). Except for AM, all reagents used in this study were of analytical grade and employed directly without any further purification. 2.2. Synthesis of PADC PADC was synthesized by aqueous solution polymerization via low-pressure UV initiation. Fig. 1(a) shows the experimental setup used in this research. Predetermined amounts of main chain monomer acrylamide, cationic monomer DMDAAC, and hydrophobic monomer CDEA were dissolved in a quartz wide-mouth jar with 20 mL deionized water. The pH of the solution was adjusted with 0.1 molL1 sodium hydroxide and 0.1 molL1 hydrochloric acid. The miscible liquids were thoroughly mixed under vigorous magnetic stirring for 30 min at room temperature. Subsequently, 0.017 g photo-initiator Irgacure2959 was completely dissolved in the mixed aqueous solution under continuous stirring and inert nitrogen (N2) atmosphere for 15 min. The aqueous solution was exposed to low-pressure ultraviolet light (predominant wavelength of 253.7 nm, and power of 24 W) for 90 min. Finally, solid product was obtained after desiccation in an oven at 105 °C for 24 h. Temperature and humidity variation of the polymerization were also recorded in Supplementary Fig. 1(b). 2.3. Measurement of intrinsic viscosity and positive charge density The intrinsic viscosity of the PADC was measured by one point method accurately [19]. The flow time of the copolymer solutions in capillary tube was acquired using Ubbelohde viscometer (capillary diameter 0.5–0.6 mm) at (30 ± 0.1) °C. The positive charge

Please cite this article in press as: J. Ma et al., Synthesis of cationic polyacrylamide by low-pressure UV initiation for turbidity water flocculation, Chem. Eng. J. (2016), http://dx.doi.org/10.1016/j.cej.2016.11.114

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Fig. 1. Schematic of (a) experimental setup, and (b) variations of temperature and humidity in polymerization.

density of the PDAC was determined by titration method [20]. Poly (vinyl sulfate) potassium salt (PVSK) and toluidine blue O (TBO) was used as titrantas and indicator, respectively. 2.4. Characterization In order to explore the structure of PADC, several samples with different monomer mass ratio were characterized, including mAM: mDMDAAC = 4:1 (CPAM), mAM:mDMDAAC = 1:1 (CPAM1), mAM: mDMDAAC:mCDEA = 1:1:0.05 (PADC1), mAM:mDMDAAC: mCDEA = 4:1:0.15 (PADC2), mAM:mDMDAAC:mCDEA = 4:1:1 (PADC3), and PAM from homopolymerization of AM. The functional groups of PADC were indicated by FTIR spectra using a 550 Series II infrared spectrometer (Bruker Company, Switzerland), and wave numbers was between 500 and 4000 cm1. The 1H NMR spectrograms of polymer were recorded in D2O by an AVANCE 500 NMR spectrometer (Bruker Company, Germany). The morphologies of copolymer particles were obtained by a scanning electron microscopy (SEM, JSM-6510 Japan). The thermogravimetric and differential thermal analysis (TG-DTA) of the copolymer particles were performed on a DTG-60 H (Shimadzu, Japan) under a heating rate of 10 °Cmin1, a nitrogen flow of 40 mLmin1, and a temperature interval of 20–600 °C. 2.5. Flocculation evaluation Flocculation efficiency of PADC was evaluated in simulated kaolin wastewater treatment. The preparation of simulated kaolin suspension in the laboratory is as follows: 0.5 g kaolin was dissolved in a ‘jar tester’ apparatus (ZR4-6 Zhongrun Co., Ltd, China) with 1000 mL deionized water under stirring for 5 min at 200 r min1, followed by ultrasonic cavitation in an ultrasonic oscillation (SB5200 DTDN Xinzhi Co., Ltd, China) for 1 min. In the flocculation process, a certain dosage of PADC was immediately added into the kaolin suspensions. The mixture solution was first stirred at a high speed of 250 r min1 for 5 min and then mixed at low speed of 50 r min1 for 15 min. Subsequently, supernatant was obtained after free-settling for 30 min. The light transmittance of the supernatant was measured by a UV–visible spectrophotometer (T6 Pgeneral Co., Ltd, China) at a wavelength of 600 nm [21]. 3. Result and discussion 3.1. Synthesis conditions optimization 3.1.1. Effect of monomer concentration on polymerization Monomer concentration (v/v%) is the percentage of monomer in total volume of the reaction mixture, which is a critical factor in free-radical polymerization [17]. The effect of monomer concentra-

tion on polymerization was investigated at CDEA mass ratio of 3%, initiator dosage of 0.3%, illumination time of 120 min, and mAM: mDMDAAC of 4:1. The results are presented in Fig. 2(a). As shown in Fig. 2(a), the intrinsic viscosity of PADC was evidently improved with the increase of monomer concentration, and it remained stable with small fluctuations of monomer concentration between 20% and 30%. However, the intrinsic viscosity of PADC was significantly reduced with further increase on monomer concentration. The results also reveal that the variation of positive charge density under different monomer concentration is similar to the intrinsic viscosity of PADC, and the maximum positive charge density of PADC was finally obtained at monomer concentration of 30%. Generally, chain growth is determined by the account of free radicals and chance of monomer collision in initial stage of reaction [20]. Fewer free radicals and fewer opportunities for monomer collision can be generated at a low concentration of monomer. The situation is rapidly improved with the increase of monomer concentration. Thus, a striking increase in intrinsic viscosity was observed in the experiment as the monomer concentration increased from 10% to 30%. The maximum intrinsic viscosity of 975 mL g1 and positive charge density of 3.13 mmol g1 were achieved at monomer concentration of 30%. However, chain transfer and chain termination always emerged at excessive monomer concentration, thereby leading to the decrease of intrinsic viscosity and positive charge density [22]. Consequently, the optimal monomer concentration was deemed to be 30% in this study. 3.1.2. Effect of initiator dosage on polymerization Irgacure 2959 was chosen as the photo-initiator to induce the polymerization. Fig. 2(b) shows the effect of initiator dosage on intrinsic viscosity and positive charge density, in which the mAM: mDMDAAC, illumination time, CDEA concentration, and monomer concentration are 4:1, 120 min, 3%, and 30%, respectively. The results were obtained as the dosage was changed from 0.1% (wt., 0.01 g) to 0.8% (wt., 0.08 g). Results reveal that intrinsic viscosity significantly increased with the increase of initiator dosage in the low concentration stage, in accordance with the free radicals mechanism of polymerization. When the initiator dosage is lower than 0.1%, few primary free radicals generated from UV irradiation were surrounded by a large number of solvent molecules, called as the ‘‘cage effect”, which inhibits the formation of monomeric free radicals and further chain growth [19]. Moreover, the increased initiator dosage induces the surge of primary and monomeric free radicals, and more cationic groups from DMDAAC are continually grafted onto the AM, leading to the increase in intrinsic viscosity and positive charge density. The maximum intrinsic viscosity of 1255 mL g1 was observed at photo-initiator dosage of 0.2% and the positive charge density of 4.43 mmol g1 was obtained at 0.25%. Both parameters were clearly reduced with the increased

Please cite this article in press as: J. Ma et al., Synthesis of cationic polyacrylamide by low-pressure UV initiation for turbidity water flocculation, Chem. Eng. J. (2016), http://dx.doi.org/10.1016/j.cej.2016.11.114

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Fig. 2. Effect of (a) monomer concentration, (b) initiator dosage, (c) UV irradiation time, (d) mAM:mDMDAAC, (e) monomer mass ratio on intrinsic viscosity and positive charge density of PADC.

irgacure 2959 dosage from 0.25% to 0.8%. By contrast, supernumerary photo-initiator concentration greatly increased the heat in a short time. Thus, opportunity of cross-linking and implosion was considerably increased [17], and PADC with desirable intrinsic viscosity and positive charge density is difficult to obtain. Accordingly, Irgacure 2959 dosage of 0.2% was favorable in this research. 3.1.3. Effect of UV irradiation time on polymerization UV initiation is more efficient and energy saving compared with thermal initiator system [20]. UV irradiation time attracted much attention in industrial production of flocculant. Therefore, the effect of UV irradiation time on polymerization was evaluated whereas other factors remained constant. As presented in Fig. 2 (c), the intrinsic viscosity of PADC evidently improved with the increase in UV irradiation time in less than 75 min; the maximum intrinsic viscosity of 1380 mL g1 was obtained after irradiation for 75 min. However, the intrinsic viscosity was slightly reduced at first and then reached a constant value with continuous UV irradiation time. The positive charge density always increases with the increase of UV irradiation time and eventually remains stable. Photon energy in low-pressure UV can reach up to 471 kJ mol1. Thus, activation energy of the polymerization can be reduced under lowpressure UV irradiation, and molecular bond breakage is accelerated. As a result, a large number of graft sites are exposed for combination with free radicals, thereby resulting in rapid increase of intrinsic viscosity and positive charge density under constant illumination. According to Fig. 1(b), temperature in the reactor was elevated when UV irradiation time was further increased, leading to chain transfer and termination [17]. A slight decrease, followed by stability of intrinsic viscosity and positive charge density for PADC was observed after 90 min of UV irradiation time. Consequently, 90 min was deemed the optimum illumination time in this study. In addition, Fig. 1(b) indicates that temperature sourced from polymerization increased from 24 °C (initial ambient temperature) to 34 °C with the increase of illumination time within 90 min, and humidity was reduced accordingly from 62.5% to 42.1%. Furthermore, variation of temperature and humidity was not observed after 90 min. Thus, low-pressure UV initiation released minimal heat than high-pressure UV initiation, and complicated condensing

equipment is not necessary in the polymerization. Overall, this synthesis method is cost-effective, easy to execute, and can reduce the chance of cross-linking in polymerization. 3.1.4. Effect of mAM:mDMDAAC on polymerization Previous research showed that dispersion stability of kaolin suspended particles can be decreased and destroyed by adding a polymer with high positive charge via charge neutralization, whereas the ‘‘bridge effect” for agglomeration of flocs is enhanced with increased intrinsic viscosity [23]. Neutralization and bridging capacity always depend on the length of AM main chain and numbers of cationic groups, respectively. Thus, the effect of mAM: mDMDAAC on polymerization was studied while other factors remained constant with optimal values; the obtained results are illustrated in Fig. 2(d). The result reveals that intrinsic viscosity perpendicularly increased to 1577 mL g1 as the mAM:mDMDAAC mass ratio was increased from 1:1 to 4:1, but intrinsic viscosity decreased with further increase of mAM:mDMDAAC. However, positive charge density was significantly reduced from 3.75 mmol g1 with increased mass ratio. Based on the free radicals mechanism of polymerization, intrinsic viscosity of the polymer is determined by the number of monomeric free radicals in the early stage of reaction [22]. AM has a higher reactivity ratio compared with other monomers. With the increase of AM concentration, chance of collision increases between AM and primary free radicals, and chain propagation is more likely to occur. On the contrary, excessive AM monomer concentration also increases chain transfer, resulting in reduced intrinsic viscosity of PADC. Similarly, with decline of DMDAAC concentration, cationic monomer is difficult to graft onto the main chain and weakens the charge density of polymer. Moreover, as shown in Fig. 2(e), the intrinsic viscosity and positive charge density exhibited similar change under varying concentrations of CDEA. Therefore, the monomer mass ratio mAM:mDMDAAC of (1–4):1 was favorable for synthesis of PADC. 3.2. FTIR spectra analysis A series of polymers with different mass ratio were prepared in laboratory, and the detailed synthesis methods were previously described. The FTIR spectrum of each sample is shown in Fig. 3.

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Fig. 3. FTIR spectra of various samples: (a) CPAM, (b) PADC1, (c) PADC2, (d) PADC3, and (e) PAM.

As shown in Fig. 3(a), the bending vibration absorption of CPAM was derived from AM and DMDAAC. Absorption bands of 3396 cm1 and 1665 cm1 were attributed to –N–H and –C@O group of AM, respectively. The structure of DMDAAC was characteristic by –N+–(CH3)3 group appearing at 953 cm1 [11,24]. Fig. 3(b)–(d) illustrate that the copolymers of PADC1, PADC2, and PADC3 show similar structures. The absorption bands, 2942 cm1, 1665 cm1, and 953 cm1, sourced from the characteristics groups of AM and DMDAAC already appeared in these copolymers. Absorption band of 3341 cm1 in PADC were slightly shifted from 3396 cm1 in CPAM assigned to the primary amino –NH2 [25]. The peaks at 3193 cm1 and 633 cm1 were ascribed to the –OH and tertiary amine –C–N of CDEA, respectively. Fig. 3 (e) shows that the polymer was polymerized only by AM, and peaks at 3461 cm1, 2942 cm1, and 1665 cm1 are characteristic vibrations of the amide groups of PAM. In conclusion, the analyses of the FTIR spectra provide sufficient evidence for copolymerization of AM, DMDAAC, and CDEA. 3.3. Thermo gravimetric and differential thermal analysis The thermal gravimetric analysis (TG) and differential thermal analysis (DTA) of PAM, CPAM, and PADC were implemented with a TGA instrument from 20 °C to 600 °C in an inert atmosphere (N2). During the measurement, the heating rate was controlled as 10 °C min1; results are presented in Fig. 4. All detailed informa-

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tion of weight loss and corresponding temperature are summarized in Table S2. As displayed in Fig. 4(a) and Table S2, all samples exhibit three weight loss stages in their TG curves except PAM. The initial weight loss of 6.9%, 14.7%, 11.89%, 12.54%, and 8.2% for PAM, CPAM, PADC1, PADC2, and PADC3 was observed from 21.5 °C to 150.8 °C, respectively. DTA curves also show an endothermic peak at 73.4 °C in Fig. 4(b). This finding is attributed to moisture evaporation existed intra- and inter-molecules [26]. Moreover, the weight loss of PADC3 in this period is obviously less than that of PADC1 and PADC2, which means there is less moisture in PADC3. The result is ascribed to hydrophobic effect in synthesis process caused by a higher mass ratio of hydrophobic monomer CDEA in PADC3. As the temperature was further increased, the second weight loss stage was observed from 150.8 °C to 351.1 °C, thereby corresponding to the decomposition of amide groups and [19]. Weight loss of CPAM and PADC is evidently higher than that of PAM, which is attributed to the decomposition of methyl from quaternary ammonium groups in cationic monomer DMDAAC. Accordingly, as illustrated in Fig. 4(b), the endothermic peak at 279.84 °C is assigned to PADC and CPAM, whereas 301.1 °C is ascribed to PAM. The third weight loss stage of PADC and CPAM appeared at 350.2 °C to 600 °C, which is caused by the rupture of the main chain and accompanied by carbon dioxide production [19]. However, PAM exhibited a continuous weight loss in the third and fourth stage. The weight loss of 32.72% and 13.3% were observed at 349.8–469.1 °C and 469.1–600 °C, respectively. This difference was caused by cross-linking in molecular chain of PAM [26], and the weight loss in these two stages is attributed to decomposition in branched and main chain, respectively. 3.4. 1H NMR spectrum analysis The 1H NMR spectrum was analyzed to further confirm the molecular structure of copolymer; results are displayed in Fig. 5. As shown in Fig. 5, several resonance peaks of hydrogen nucleus derived from different monomer were observed. A strong and broad resonance peak at d  4.8 ppm is attributed to the solvent proton of D2O [17,27]. The sharp signal at 1.65 ppm and 2.21 ppm are ascribed to the protons –CH2 and –CH in acrylamide, respectively [28]. The peak at 3 ppm and 3.87 ppm was attributed to protons of –N+–CH3 and –N–CH2 in cationic monomer DMDAAC and hydrophobic monomer CDEA, respectively [20]. The proton signal of double bond –CH@CH2 in DMDAAC was observed at 5.69 ppm. Furthermore, impurities in the sample were separated during washing and purification; the detected groups were all sourced from polymers. Overall, these groups identified in 1HNMR spectra further confirm the molecular structure of copolymer.

Fig. 4. Comparison of (a) TG and (b) DTA for CPAM, PAM and PADC.

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Fig. 5. 1H NMR spectra of CPAM, PAM and PADC.

According to the structure analysis of FTIR, TG-DTA, and 1H NMR, a possible reaction mechanism for the synthesis of PADC is outlined in Scheme 1. Two routes (I and II) for polymerization of DMDAAC were identified; one or two –CH@CH2 in DMDAAC engaged in polymerization is the difference of these two routes. However, based on the analysis of 1H NMR, double bond –CH@CH2 was observed in both PADC and CPAM, revealing that a certain portion PADC was synthesized by route (I), and there is no sufficient evidence for PADC synthesized by route (II) which is needed further research. Therefore, the possible product of the polymerization is (a) or (c) as illustrated in Scheme 1.

ples. Flocculation performance was measured by light transmittance, and the initial light transmittance of simulated kaolin suspension was 4.1–5.0%. The synthesized polymers samples except PAM were tested in this study. Moreover, as shown in Fig. 2(e), the results of basic parameters of PADC used in flocculation are as follows: intrinsic viscosity and positive charge density of PADC1 sample was 722 mL g1 and 3.795 mmol g1, PADC2 sample was 1577 mL g1 and 1.875 mmol g1, and PADC3 sample was 1082 mL g1 and 2.29 mmol g1. In addition, the flocculation experiment was processed at room temperature. 4.1. Effect of dosage on flocculation

3.5. SEM images analysis SEM images were obtained and shown in Fig. 6 to investigate the amorphous morphology and intuitively observe the surface visualized information of copolymer. The dimension of particles was selected by using SEM at magnifications of 800. As presented in Fig. 6, each samples exhibited a different surface morphologies because of a distinct monomer. According to Fig. 6(a), PAM shows a relative regular structure with smooth surface morphology. After adding DMDAAC in polymerization of CPAM, branch chains containing quaternary ammonium were grafted onto the PAM chain. Obviously, folded areas appeared at surface of CPAM as shown in Fig. 6(b). More spherical bulges in Fig. 6(c)–(e) reveal that monomer CDEA was added in the synthesis process. A similar result was reported in the research of S. Iglauera et al. [29]. Among these three PADC samples, structure with more folded areas and less porous can be found in PADC2 with highest DMDAAC content and initiation of low-pressure UV. Short branched chain in copolymer causes the folded area, whereas long branched chain results in pore-forming as illustrated in Fig. 6(e) [11]. However, specific surface area of PADC can be increased by both folded and porous areas, which are expected to be more effective in flocculation process.

The effect of dosage on flocculation was examined at an initial pH of 7.0 and the results are displayed in Fig. 7(a). The transmittance of liquid supernatant after flocculation was initially increased and subsequently reduced with the increased addition of PADC from 1.0 mg L1 to 3.5 mg L1. The maximum transmittance of 83.88%, 82.12% and 79.87% was achieved by PADC1, PADC2, and PADC3 at a dosage of 1.5 mg L1, 2.5 mg L1 and 3.0 mg L1, respectively. These results are explained by the following reasons. At low dosage, bare polymers surface available for adhesion of kaolin suspended particles, as well as positive charges are both insufficient to destroy the stabilization of kaolin suspension via charge neutralization [19]. With excessive dosage of PADC, kaolin suspended particles are surrounded by cationic polymers, and electrostatic repulsion between fine aggregates was gradually increases. Flocculation of these fine aggregates and formation of larger flocs with sufficient weight for settling down are difficult because of the electrostatic repulsion [17]. The optimal dosage of PADC1, PADC2 and PADC3 was 1.5 mg L1, 2.5 mg L1 and 3.0 mg L1, respectively, and PADC1 is the most effective in flocculation of kaolin suspension at a low dosage. In addition, CPAM only exhibited minimal advantage at higher dosage, which is not cost-effective in practical applications.

4. Flocculation performance and mechanism

4.2. Effect of pH on flocculation

Effect of dosage and pH on flocculation experiments in kaolin suspension treatment was investigated and described in Fig. 7 to compare and evaluate the flocculation performance of these sam-

The effect of pH was evaluated in a dosage of 1.5 mg L1, and results are described in Fig. 7(b). The initial transmittance of kaolin liquid suspension was steadily maintained at approximately 10%

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Scheme 1. Possible reaction mechanism for synthesis of PADC.

Fig. 6. SEM images of (a) PAM, (b) CPAM, (c) PADC1, (d) PADC2, and (e) PADC3.

Please cite this article in press as: J. Ma et al., Synthesis of cationic polyacrylamide by low-pressure UV initiation for turbidity water flocculation, Chem. Eng. J. (2016), http://dx.doi.org/10.1016/j.cej.2016.11.114

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Fig. 7. Effect of (a) dosage and (b) pH on flocculation of kaolin suspension.

Fig. 8. Zeta potential of (a) flocculant, (b) kaolin suspension and supernatant at various pH.

without any exception at pH > 4; the ultimate transmittance was stabilized around 83% at pH from 2 to 10. Thus, PADC is appropriate in a wide range of pH. In addition, flocculant of PADC2 and PADC3 showed better flocculation performance than PADC1 in an acidic environment. The result suggests that protonation of –NH2 groups in PADC was generated at pH < 7, thereby forming positive groups of –NH+3 and promoting charge neutralization property. The intrinsic viscosity of PADC2 and PADC3, as well as the ratio of CDEA are higher than those of PADC1. Thus, PADC2 and PADC3 exert better bridging effect and hydrophobic association after generation of fine aggregates [30]. On the contrary, PADC1 showed an obvious advantage in an alkaline environment because of the partial electrostatic region existing in aggregates from adsorption. The remaining kaolin particles were combined with aggregates by patching and bridging effect; large flocs were generated and precipitated to the bottom. Especially, compared with CPAM1 (mAM: mDMDAAC = 1:1), PADC1 (mAM:mDMDAAC:mCDEA = 1:1:0.05) exhibited an effective flocculation performance in an alkaline environment because of hydrophobic association from CDEA. Overall, PADC1 was favorable at a wide pH range (2–10), and the optimized pH of kaolin suspension for both PADC2 and PADC3 was 2–3. Moreover, the relationship between pH and zeta potential also confirm the conclusion. As seen from Fig. 8(a), the zeta potential of all samples were initially maintained and promptly reduced

with pH > 10; flocculant solution was changed from electropositivity to electro-negativity because of the deprotonation in – NH2 induced by OH in alkaline environment [24]. PADC1 has a

Fig. 9. FTIR comparison of PAM, kaolin and flocs.

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Fig. 10. Possible flocculation mechanism for PADC.

higher zeta potential compared with PADC2 and PADC3; the higher potential was ascribed to higher positive charge density of PADC1 according to Fig. 2(e). Furthermore, the zeta potential of PADC, kaolin solution, and flocculation suspension were illustrated in Fig. 7 (b). Kaolin suspension shows electro-negativity at various pH. However, the zeta potential of supernatant of kaolin wastewater was significantly improved after flocculation and was higher than zero as pH < 5. The results reveal that the negative charge was neutralized in flocculation by positive PADC, and charge neutralization has an important role in flocculation.

results suggest that no chemical reaction was observed between PDAC and kaolin particles, and electrostatic interaction was the primary reason for formation of flocs [15]. As shown in Fig. 9(a), the peaks observed at roughly 3341 cm1 and 1665 cm1 were attributed to –NH2 and –C@O of PADC, respectively. However, these two vibration peaks are very weak as observed in the FTIR spectrum of flocs. A potential explanation is that the dosage of PADC (1.5 mg L1) was considerably lower than initial concentration of kaolin suspension (500 mg L1). Thus, PADC is wrapped and sheltered in flocs, and PADC is difficult to be detected by infrared spectrometer.

4.3. FTIR analysis of flocs 4.4. Flocculation mechanism FTIR spectrum of PAM, kaolin particles, and flocs were obtained and illustrated in Fig. 9 to analyze the interaction between flocculant of PADC and kaolin particles. As shown in Fig. 9(b), (c), bands assigned to Si-O-Si and Si-O groups at 1098 cm1 and 471 cm1 of kaolin particles were observed in the FTIR spectrum of flocs. The

According to the effects of dosage and pH on the flocculation performance and characterization of flocs and supernatant by FTIR and zeta potential, respectively, the flocculation mechanism in this study was summarized and reported in Fig. 10.

Please cite this article in press as: J. Ma et al., Synthesis of cationic polyacrylamide by low-pressure UV initiation for turbidity water flocculation, Chem. Eng. J. (2016), http://dx.doi.org/10.1016/j.cej.2016.11.114

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J. Ma et al. / Chemical Engineering Journal xxx (2016) xxx–xxx

As shown in Fig. 10, kaolin suspended solids combined with PADC by charge neutralization, thereby forming large amounts of fine aggregates under acidic and neutral conditions. These fine aggregates collided and cross-linked together by bridging and hydrophobic association with continuous stirring; lager net-like particles were observed in the process. These larger net-like particles can further seize the kaolin suspended solids by enmeshment and sweeping effect, and insoluble complexes flocs emerged and settle down to the bottom. By contrast, adsorption played a predominant role in earlier period of flocculation because of the decrease in positive charge density under alkaline conditions. However, many partial electrostatic regions remain; thus, patching and bridging effect gradually promoted the formation of larger particles. Finally, flocs were also generated by enmeshment and sweeping effect in the process settlement. 5. Conclusion A practical flocculant of PADC was synthesized by copolymerization of AM, DMDAAC and CDEA via a low-pressure UV initiation technique. The synthesis conditions were examined in this study, including monomer concentration, initiator dosage, UV irradiation time and mAM:mDMDAAC. The results show that the increase in AM engaged in the polymerization results in higher intrinsic viscosity and the lower positive charge density. The intrinsic viscosity and positive charge density were achieved at 1577 mL g1 and 4.43 mmol g1 under different synthesis conditions, respectively. The chemical structure of polymer PDAC was confirmed via FTIR spectra, and 1H NMR spectrum displayed the characteristic proton of different groups. In addition, amorphous morphology of polymer was surveyed by SEM, and thermal stability of PDAC was determined by TG-DTA. A possible synthesis mechanism of PDAC was proposed based on the above analysis. Finally, the flocculation performance was tested in treatment of high turbid kaolin suspension. Optimum flocculation conditions were confirmed as pH of 2–10 and PDAC1 dosage of 1.5 mg L1, wherein the transmittance of 82.12–86.64% can be eventually achieved. Furthermore, charge neutralization and adsorption are predominant in acidic and alkaline environments, respectively. Acknowledgements The work was financially supported by the National Nature Science Foundation of China (Project Nos. 51408004 and 51408215) and the University Natural Science Research Project of Anhui Province (Project Nos. KJ2016A086). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2016.11.114. References [1] R. Mores, H. Treichel, C.A. Zakrzevski, A. Kunz, J. Steffens, R.M. Dallago, Remove of phosphorous and turbidity of swine wastewater using electrocoagulation under continuous flow, Sep. Purif. Technol. 171 (2016) 112–117. [2] A.L. Mather, R.L. Johnson, Event-based prediction of stream turbidity using a combined cluster analysis and classification tree approach, J. Hydrol. 530 (2015) 751–761. [3] S. Yahyapour, A. Golshan, A. Halim, B. Ghazali, Removal of total suspended solids and turbidity within experimental vegetated channel: optimization through response surface methodology, J. Hydro-Environ. Res. 8 (2014) 260– 269. [4] F. Arena, R.D. Chio, B. Gumina, L. Spadaro, G. Trunfio, Recent advances on wet air oxidation catalysts for treatment of industrial wastewaters, Inorg. Chim. Acta 431 (2015) 101–109.

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Please cite this article in press as: J. Ma et al., Synthesis of cationic polyacrylamide by low-pressure UV initiation for turbidity water flocculation, Chem. Eng. J. (2016), http://dx.doi.org/10.1016/j.cej.2016.11.114