Plasma-initiated polymerization of chitosan-based CS-g-P(AM-DMDAAC) flocculant for the enhanced flocculation of low-algal-turbidity water

Carbohydrate Polymers 164 (2017) 222–232

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Plasma-initiated polymerization of chitosan-based CS-g-P(AM-DMDAAC) flocculant for the enhanced flocculation of low-algal-turbidity water Yongjun Sun a,b,∗ , Chengyu Zhu a , Wenquan Sun a,b , Yanhua Xu b , Xuefeng Xiao a , Huaili Zheng c , Huifang Wu a , Cuiyun Liu a a

College of Urban Construction, Nanjing Tech University, Nanjing, 211800, China Jiangsu Key Laboratory of Industrial Water-Conservation & Emission Reduction, College of Environment, Nanjing Tech University, Nanjing, 211800, China c Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, Chongqing University, Chongqing, 400045, China b

a r t i c l e

i n f o

Article history: Received 5 November 2016 Received in revised form 5 January 2017 Accepted 2 February 2017 Keywords: Plasma initiation Chitosan Flocculants Algal removal Coagulation–flocculation

a b s t r a c t In this work, a highly efficient and environmentally friendly chitosan-based graft flocculant, namely, acrylamide- and dimethyl diallyl ammonium chloride-grafted chitosan [CS-g-P(AM-DMDAAC)], was prepared successfully through plasma initiation. FTIR results confirmed the successful polymerization of CS-g-P(AM-DMDAAC) and P(AM-DMDAAC). P(AM-DMDAAC) was the copolymer of acrylamide- and dimethyl diallyl ammonium chloride. SEM results revealed that a densely cross-linked network structure formed on the surface. XRD results verified that the ordered crystal structure of chitosan in CS-gP(AM-DMDAAC) was changed into an amorphous structure after plasma-induced polymerization. The flocculation results of low-algal-turbidity water further showed the optimal flocculation efficiency of turbidity removal rate, COD removal rate, and Chl-a removal rate were 99.02%, 96.11%, and 92.20%, respectively. The flocculation efficiency of CS-g-P(AM-DMDAAC) were significantly higher than those obtained by cationic polyacrylamide (CPAM) and Polymeric aluminum and iron (PAFC). This work provided a valuable basis for the design of eco-friendly naturally modified polymeric flocculants to enhance the flocculation of low-algal-turbidity water. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction The rapid development of industry and agriculture has caused an increase in the discharge of nitrogen, phosphorus, and other nutrients into aquatic environments; consequently, eutrophication of lakes has accelerated (Liu, Wang, Wei, Dong, & Hui, 2013). The eutrophication of lakes leads to algal blooms that pollute water sources. For instance, China’s three main lakes, namely, Taihu Lake, Chaohu Lake, and Dianchi, have experienced algal blooms (Li et al., 2016; Lu et al., 2016; Ma, Wang, Feng, & Wang, 2015). Many algae in raw waters may block filter beds in water supply treatment processes, deteriorate water quality, plug or corrode pipelines, increase the amount of chlorination by-products, and release algal toxins that threaten human health (Chen et al., 2016). Therefore, water

∗ Corresponding author at: College of Urban Construction, Nanjing Tech University, Nanjing, 211800, China. E-mail addresses: [email protected], [email protected] (Y. Sun). http://dx.doi.org/10.1016/j.carbpol.2017.02.010 0144-8617/© 2017 Elsevier Ltd. All rights reserved.

treatment methods for the removal of algae in raw waters, especially in low-algal-turbidity water, should be developed to improve and protect water quality and human health (Li, Yu, & Rittmann, 2015). Algae can be removed by using various techniques, such as air floatation (Wang, Zhao et al., 2016; Wang, Chen, Xie, Shang, & Ma, 2016), filtration (Zhao et al., 2016), adsorption (Nautiyal, Subramanian, & Dastidar, 2016), chemical peroxidation (Khalfbadam et al., 2016), and coagulation–flocculation (Sun et al., 2015). Among these methods, coagulation–flocculation is the safest, most effective, and widely used because it does not destroy algal cells and it produces disinfection by-products (Rashid, Rehman, & Han, 2013). However, algal cells and some colloidal substances in raw water are small and often negatively charged (Zeng, Wu, & Kennedy, 2008). Differences in raw water quality also result in the variation of the properties of algal cells and colloidal particles (Zohuriaan-Mehr, 2005). As such, satisfactory results are not easily obtained through algal removal via flocculation by traditional flocculants. Appropriate highly efficient flocculants should be prepared and coagulation should be optimized and strengthened on the basis

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of different raw water characteristics to remove algae effectively (Yang, Li, Huang, Yang, & Li, 2016). Chitosan is a naturally large molecule with many functional groups in its molecular chain, and this molecule has been extensively investigated as potential flocculants (Lu et al., 2011). Chitosan contains numerous free amino groups and hydroxyl groups in its molecular chain and yields a positive charge under weakly alkaline conditions (Dharani & Balasubramanian, 2016). Cationic active groups in chitosan chains can effectively neutralize negatively charged algal cells; as a result, charge neutralization and adsorption occur (Thakur & Thakur, 2014). However, chitosan exhibits poor solubility under neutral conditions (Jia et al., 2016). Graft copolymerization is often employed to modify chitosan and to obtain high solubility and effectiveness of chitosan-based flocculants. Chitosan can possess water-soluble groups and other functional groups through graft copolymerization to improve its flocculation performance (Wang, Chen, Ge, & Yu, 2007). Chitosan graft copolymerization is generally initiated by chemical and radiation methods (Wang, Chen, Zhang, & Yu, 2008). Plasma-initiated polymerization is highly efficient in the preparation of chitosan-based flocculants. Plasma-initiated polymerization is a new method that does not require chemical initiators (Yu et al., 2014). In this process, monomers are irradiated with plasma as single energy source for a short period and then exposed to appropriate polymerization temperatures (Baumann et al., 2013). Compared with conventional polymerization method, simple and highly efficient plasma-initiated polymerization provides various advantages; for example, external initiators are not required, reagent pollution is prevented, convenient devices are used, molecular weight is easily measured, and linear polymer products are produced (Friedrich, 2011; Zheng et al., 2013). With these characteristics, plasmainduced polymerization is suitable and beneficial for the initiation of chitosan graft copolymerization (Wang, Chen, Yuan, Sheng, & Yu, 2009). In this work, a chitosan-based flocculant, namely, acrylamideand dimethyl diallyl ammonium chloride-grafted chitosan [CSg-P(AM-DMDAAC)], was developed through plasma-induced polymerization, and their flocculation performances for low-algalturbidity water were systematically investigated. The obtained grafting copolymer was characterized through Fourier-transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), and scanning electron microscopy (SEM). Total monomer concentration, monomer ratio, discharge power, discharge time, post-polymerization temperature, post-polymerization time, and other parameters influencing grafting percentage and intrinsic viscosity were also examined. The flocculation ability of CS-gP(AM-DMDAAC) was evaluated on the basis of the removal rate of turbidity, COD, and Chl-a used for the treatment of low-algalturbidity water. 2. Materials and methods 2.1. Materials The main materials used in this study were AM, CS (≥95% degree of deacetylation, 100–200 mPa·s viscosity), and DAC (80% in water), which were obtained from Aladdin Shanghai Biochemical Technology Co., Ltd. All reagents were of analytical grade and utilized without further purification. CPAM and PAFC were obtained from Nanjing Shengjianquan Glass Instrument Co., LTD. 2.2. Synthesis of CS-g-P(AM-DMDAAC) Grafting reaction was initiated by plasma (Fig. 1). A certain mass of CS was dissolved in 10 mL of 1.0% (volume percentage) acetic acid

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solution by slowly stirring until CS was dissolved completely. Predetermined amounts of AM, DAC, and deionized water were then added to the CS solution to obtain a homogeneous reaction solution. The homogeneous reaction solution was discharged and initiated in the plasma ignition device at certain discharge power and discharge time. The resulting mixture was purged with nitrogen gas in a water bath shaker at an appropriate post-polymerization temperature. The polymerized products were purified in acetone and ethyl alcohol and dried in a vacuum oven at 40 ◦ C until a constant weight was obtained. Thus, CS-g-P(AM-DMDAAC) products were obtained. 2.3. Characterization of CS-g-P(AM-DMDAAC) A series of CS-g-P(AM-DMDAAC) flocculants was characterized through XRD, FTIR, and SEM. After the pretreatment was administered, the morphological characteristics of CS-g-P(AMDMDAAC) were examined by using a VEGA II LMU SEM (TESCAN, Czech Republic). The FTIR (ATR attachment) spectra of CS-gP(AM-DMDAAC) were obtained using potassium bromide (KBr) pellets on a 550 Series II infrared spectrometer (Bruker Company, Switzerland). An XRD spectrum was obtained with an X-ray diffractometer (SmartLabTM 3KW, Japan). The polymerization product CS-g-P(AM-DMDAAC) was characterized on the basis of intrinsic viscosity determined via a one-point method and grafting ratio and grafting efficiency identified via a gravimetric method (Wang et al., 2012). 2.4. Coagulation–flocculation tests Raw water (Jing Lake in Nanjing Tech University, Nanjing, China) was light green and slightly turbid. Raw wastewater was filtered through a graticule mesh to remove large particles and phytoplankton. Flocculation efficiency was determined by measuring turbidity, COD, and Chl-a content. Water quality analysis revealed that the turbidity, Chl-a content, COD, and pH of raw wastewater were 21–30 NTU, 0.15–0.16 mg L−1 , 126.4–130.0 mg L−1 , and pH 7.0–8.0, respectively. The coagulation–flocculation tests for the algal turbid water were conducted in 1.0 L Plexiglass beakers by using a program-controlled jar-test apparatus (ZR4-6, Zhongrun Water Industry Technology Development Co., Ltd., China) at room temperature. The pH of the algal turbid water was adjusted with 0.1 mol L−1 HCl or 0.1 mol L−1 NaOH. The dosages of different flocculants were calculated on the basis of the quantity of their effective component, that is, PAFC by Fe + Al and CS-g-P(AMDMDAAC) and CPAM by dry weight. The algal turbid water samples were stirred rapidly at 300 rpm for 2 min, slowly stirred at 70 rpm for 5 min, and precipitated for 10 min. After precipitation was completed, the supernatant samples were collected 2 cm below the surface of the tested water sample to measure COD (DR2800, Hach, USA), turbidity (2100Q turbidimeter, Hach, USA), and Chl-a (TU-901 double-beam UV–vis spectrophotometer, Beijing General Instrument Co., Ltd., China). The Chl-a content was determined according to Chinese National Standards and Water Quality Determination of Chlorophyll by Spectrophotometry (No. SL88-2012). The tests were conducted thrice, and the relative error was less than 5%. 3. Results and discussions 3.1. Characterization results of CS-g-P(AM-DMDAAC) 3.1.1. XRD spectrum results XRD was performed to obtain the XRD spectrum of CS-g-P(AMDMDAAC) and its monomers and to explore the changes in the crystal morphologies of plasma-initiated CS, AM, and DMDAAC for

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Fig. 1. Possible grafting polymerization scheme of CS-g-P(AM-DMDAAC).

of crystallinity degree. The change in the crystal form of chitosan before and after graft copolymerization demonstrated that CS-gP(AM-DMDAAC) was successfully synthesized (Wang, Zhao et al., 2016; Wang, Chen et al., 2016). The amorphous structure was more easily hydrated than the crystal structure. Therefore, the solubility of the graft copolymer product was prominently higher than that of CS.

Fig. 2. XRD spectra of CS-g-P(AM-DMDAAC).

CS-g-P(AM-DMDAAC) synthesis. The XRD spectra of CS-g-P(AMDMDAAC) are shown in Fig. 2. The characteristic diffraction peak of the crystalline form of chitosan was observed at 2␪ = 22◦ . This strong diffraction peak was attributed to the crystalline form II of chitosan. The characteristic diffraction peak of P(AM-DMDAAC) appeared in the range of 20◦ –23◦ . The characteristic diffraction peak of P(AM-DMDAAC) was wider with a lower intensity than the diffraction peak of chitosan. As shown in the XRD spectrum of P(AM-DMDAAC), the crystallinity degree of chitosan was significantly higher than that of P(AM-DMDAAC). The XRD spectrum of the graft copolymer varied when the olefin monomers of AM and DMDAAC were introduced to the side chain of chitosan and the graft copolymer CS-g-P(AM-DMDAAC) was polymerized. The characteristic diffraction peak of CS-g-P(AM-DMDAAC) appeared in the range of 20◦ –23◦ . The content of CS slightly affected the peak intensity of CS-g-P(AM-DMDAAC) in the XRD spectrum. The XRD spectrum of CS-g-P(AM-DMDAAC) was markedly different from that of CS, that is, the peak intensity of CS-g-P(AM-DMDAAC) was lower than that of CS. This finding indicated that the introduction of AM and DMDAAC remarkably weakened the order of the graft copolymer structure and changed the crystal structure form (Wang et al., 2015). The decrease in the peak intensity of CS-g-P(AMDMDAAC) compared with that of CS corresponded to the reduction

3.1.2. SEM results The SEM images of (a) CS-g-P(AM-DMDAAC) (40% CS content), (b) CS-g-P(AM-DMDAAC) (20% CS content), (c) P(AM-DMDAAC), and (d) CS are shown in Fig. 3. In Fig. 3(d), the chitosan surface was composed of a sheet structure with a smooth surface. P(AM-DMDAAC) contained tiny and sharp bulges on the surface. CSg-P(AM-DMDAAC) possessed a small and smooth convex structure with some fine pores and thus form a densely cross-linked network structure on the surface. The surface structure of CS-g-P(AMDMDAAC) was altered by the introduction of AM and DMDAAC. This finding suggested that modification reactions occurred on the surface of chitosan. The change in surface morphology also demonstrated that AM and DMDAAC were successfully grafted on chitosan chains, as initiated by plasma. 3.1.3. FTIR spectra The FTIR spectrum of CS-g-P(AM-DMDAAC) is shown in Fig. 4. As shown in the spectrum of CS, the stretching vibration of hydrogen bonding was caused by the hydrogen bonding in N H and O H and by the hydrogen bonding between polysaccharide molecule stretching vibrations. The absorption peaks at 2922 and 2876 cm−1 were attributed to the stretching vibrations of carbon-hydrogen bond (C H) and caused by methylene ( CH2 ) and methyl group ( CH3 ) in the residual sugar group, respectively (Ma et al., 2016). The absorption peak at 1652 cm−1 resulted from the NH bending vibration, and the absorption peak at 1601 cm−1 was ascribed to the C O stretching vibration. The absorption peaks at 1382 and 1420 cm−1 were caused by the bending vibrations of CH2 and CH3 , respectively. The absorption peak at 1322 cm−1 resulted from the stretching vibration of C N and the bending vibration of N H. The absorption peak at 1156 cm−1 was attributed to the asymmetric deformation vibration of C O C (Fast & Gude, 2015). The absorption peak at 1077 cm−1 resulted from the stretching vibration of C O (secondary alcoholic hydroxyl group), and the absorption peak at 1030 cm−1 was sourced from stretching vibration of C O (pri-

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Fig. 3. SEM images: (a) CS-g-P(AM-DMDAAC) (40% CS content), (b) CS-g-P(AM-DMDAAC) (20% CS content), (c) P(AM-DMDAAC), and (d) CS.

mary alcoholic hydroxyl group). The absorption peak at 897 cm−1 was sourced from stretching vibration of 6-membered ring of chitosan. In Fig. 4, the most characteristic absorption peaks of P(AM-DMDAAC) and chitosan appeared in the FTIR spectrum

of CS-g-P(AM-DMDAAC) and thus exhibited a slight shift in wavenumber and peak intensity compared with those in the FTIR spectrum of chitosan. The absorption peak at 1597.88 cm−1 in CS showed a bathochromic effect and appeared at 1453 cm−1 in the FTIR spectrum of CS-g-P(AM-DMDAAC). The absorption peak at

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Fig. 4. FTIR spectrum of CS-g-P(AM-DMDAAC).

1025.95 cm−1 in the FTIR spectrum of CS-g-P(AM-DMDAAC) was reinforced, and this finding indicated the emergence of C N bond and the existence AM molecules. The new characteristic absorption peak at 1410 cm−1 caused by methyl connected to N+ corresponded to the presence of the DMDAAC molecule (Sun et al., 2016). Some absorption peaks with low intensity appeared in the FTIR spectrum of CS-g-P(AM-DMDAAC) compared with the FTIR spectrum of P(AM-DMDAAC). This observation confirmed that some chemical bonds in AM and DMDAAC reacted with the chemical bond in CS to generate new chemical bonds through grafting copolymerization. The emergence of the characteristic absorption peaks of chitosan and AM-DMDAAC in the FTIR spectrum of CS-P (AMDMDAAC) suggested that graft copolymerization among CS, AM, and DMDAAC occurred successfully. 3.1.4. TG-DSC characterization The thermal gravimetric curves of (a) CS, (b) CS-g-P(AMDMDAAC) with 40% CS content, (c) CS-g-P(AM-DMDAAC) with 20% CS content, and (d) P(AM-DMDAAC) are shown in Fig. 5, and the analytical data are shown in Table 1. As shown in TG curves of Fig. 5(c), the first stage of weight loss temperature was within the range of 30–190 ◦ C, and the weight loss was 5.5%. The second stage of weight loss temperature was within the range of 190–390 ◦ C, and the weight loss was 27.1%. The third stage of weight loss temperature was within the range of 390–490 ◦ C, and the weight loss was 38.6%. The graft polymer main chain began to degrade rapidly in the

Fig. 5. Thermal gravimetric curve: (a) CS, (b) CS-g-P(AM-DMDAAC) (40% CS content), (c) CS-g-P(AM-DMDAAC) (20% CS content), and (d) P(AM-DMDAAC).

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Table 1 Thermal gravimetric parameters of the graft polymer. Flocculants Parameter

First stage

Second stage

Third stage

Fourth stage

Temperature range (◦ C) Weight loss (%) Maximum weight loss temperature (◦ C) Temperature range (◦ C) Maximum weight loss temperature (◦ C) Weight loss (%) Temperature range (◦ C) Maximum weight loss temperature (◦ C) Third stage weight loss (%) Temperature range (◦ C) Residual weight (%)

third stage. The ultimate temperature of the complete decomposition was approximately 490 ◦ C. After 490 ◦ C, the TG curve began to flatten without weight loss. The final residual weight of the residue was 28.8%. From the DSC curves in Fig. 5(c), the TG curve of CS-gP(AM-DMDAAC) with 20% CS content showed obvious three stages of weight losses. Correspondingly, the three endothermic peaks occurred at 77.8, 254.1, and 403.5 ◦ C, respectively. Compared with Fig. 5(a), CS only had two weight loss stages. The exothermic peak at 288.8 ◦ C in DSC curve of CS was evident in the DSC curve of CS-g-P(AM-DMDAAC) with 20% CS content at 254.1 ◦ C. Compared with Fig. 5(d), P(AM-DMDAAC) had four weight loss stages, the endothermic peak of the third stage of weight loss in the DSC curve of P(AM-DMDAAC) was discovered in the DSC curve of CS-g-P(AM-DMDAAC) with 20% CS content at 281.8 ◦ C. The characteristics of the TG curves of CS and P(AM-DMDAAC) were found in the TG curve of CS-g-P(AM-DMDAAC) with 20% CS content. Compared with Fig. 5(b), The characteristics of the TG curves of CS-g-P(AM-DMDAAC) with 20% CS content was similar to that of CS-g-P(AM-DMDAAC) with 40% CS content. All the evidence and discussion of thermal gravimetric characterization demonstrated that the CS grafting copolymer was synthesized successfully. 3.1.5. The quality characteristics of the grafted polymer According to PRC National Standard named Water treatment chemicals – Technical specification and test method of cationic polyacrylamides (GB/T 31246-2014), Some quality indexes was determined to evaluate the graft polymer. The quality characteristics of the grafted polymer were shown in Table 2. The graft polymer CS-g-P(AM-DMDAAC) had an excellent improvement in solubility than CS, because chitosan was insoluble. CS-g-P(AM-DMDAAC) with 20% CS content had higher relative molecular weight than CS-g-P(AM-DMDAAC) with 40% CS content and P(AM-DMDAAC). 3.2. Optimization of CS-g-P(AM-DMDAAC) synthesis 3.2.1. Effects of total monomer concentration on graft copolymerization The effects of total monomer concentration on the intrinsic viscosity, grafting ratio, and grafting efficiency are shown in Fig. S1. The intrinsic viscosity exhibited a trend similar to grafting ratio and grafting efficiency. In particular, the intrinsic viscosity initially increased as the monomer concentration increased and then slightly decreased. The optimal intrinsic viscosity, grafting ratio, and grafting efficiency obtained at 15% monomer concentration were 3457 mL/g, 438%, and 70%, respectively. This phenomenon can be explained through free radical mechanism. The collision chance among chitosan, AM, and DMDAAC molecules was greatly reduced when the total monomer concentration was low. As a result, the chain growth rate was low and

CS

CS-g-P(AMDMDAAC) (40% CS content)

CS-g-P(AMDMDAAC) (20% CS content)

P(AM-DMDAAC)

30–200 5.3 61.2 200–460 306.6 54.7 460–600 \ 40.0 \ \

30–200 5.9 71.9 200–350 252.1 31.4 350–500 391.9 30.6 500–600 32.1

30–190 5.5 77.8 190–390 254.1 27.1 390–490 403.5 38.6 490–600 28.8

30–110 5.5 71.2 110–210 154.6 5.5 210–330 218.8 18.9 330–500 41.7

reactivity was insufficient. Therefore, the formed short polymer chains could not generate a graft polymer with a macromolecular backbone. As the total monomer concentration increased, chitosan, the constant collision between AM and DMDAAC monomer promoted the chain extension reaction, leading to the whole water reaction solution in a highly active state. The intrinsic viscosity constantly increased because of the continuous extension of the macromolecular chains of the polymer. However, the contact collision between the molecules of chitosan, AM, and DMDAAC reached saturation when the monomer concentration reached a certain value. Consequently, the chain growth rate could not be further improved. The viscosity of the reaction system was increased by the high concentration of the total monomer; as a result, the movement and transfer rate of monomer and active free radicals are low (Mishra, Sen, Rani, & Sinha, 2011). By contrast, over-saturated chitosan, AM, and DMDAAC molecules may result in the cross-linking of grafted copolymers and low solubility. Excessive monomer concentrations may also increase the molecular chain termination reaction rate and thus cause a double-terminated reaction. 3.2.2. Effects of monomer ratio on graft copolymerization The effects of monomer ratio, particularly AM/CS mass ratio, on the intrinsic viscosity, grafting ratio, and grafting efficiency are shown in Fig. S2. The intrinsic viscosity increased gradually as the chitosan content increased. By contrast, the intrinsic viscosity, grafting ratio, and grafting efficiency decreased drastically when the content of CS was more than 30%. The optimal intrinsic viscosity, grafting ratio, and grafting efficiency were obtained at AM:CS:DMDAAC of 5:3:2. The diffusion rates of AM and DMDAAC grafted on the CS monomer was low and the active site on the CS molecule could not be effectively utilized when the CS content was low. The intrinsic viscosity, grafting rate, and grafting efficiency were also greatly reduced. The reactive probability between AM/DMDAAC and chitosan was reduced when the AM and DMDAAC contents were high because of the olefin monomer with a high reactivity. AM and DMDAAC underwent self-polymerization when the chitosan content was low. The diffusion rate of CS molecules accelerated when the chitosan content increased and thus promoted the possibility of a collision between AM/DMDAAC and chitosan (He et al., 2007). The active site on CS was fully integrated at this time, and the intrinsic viscosity, grafting rate, and grafting efficiency increased. 3.2.3. Effects of discharge power on graft copolymerization The effects of discharge power on the intrinsic viscosity, grafting ratio, and grafting efficiency are shown in Fig. S3. The intrinsic viscosity, grafting ratio, and grafting efficiency initially increased as discharge power increased. Afterward, these parameters decreased

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Table 2 The quality characteristics of the grafted polymer. GB/T 31246-2014

Indexes

Parameters

Relative molecular weight, M Cationic degree, w/% Solid content, w1 Dissolving time (1 g/L), t/min

M ≥ 10 × 104 5.0 ≤ w ≤ 95.0 W1 ≥ 88.0 t ≤ 60

CS-g-P(AMDMDAAC) (40% CS content)

CS-g-P(AMDMDAAC) (20% CS content)

P(AM-DMDAAC)

1862.8 × 104 30 95 163

2125.9 × 104 30 95 56

650.2 × 104 30 95 21

slowly. The optimum intrinsic viscosity, grafting ratio, and grafting efficiency were obtained at a discharge power of 30–60 W. Plasma-generated high energy activated and excavated free radicals to promote the reaction, and the energy intensity and quantity of free radicals were determined on the basis of plasma discharge power. At low discharge power, a low amount of free radicals was generated with a slow chain growth rate. Thus, low grafting ratio and grafting efficiency were obtained. As discharge power increased, more active free radicals were produced by plasma to increase the chain growth rate and promote graft copolymerization. Consequently, high grafting ratio and grafting efficiency were observed. The intrinsic viscosity also increased. At a discharge power of more than 40 W, the graft active sites on the CS chain were grafted fully by AM and DMDAAC monomer molecules to reach saturation. The excessive amount of free radicals generated at this time induced the homopolymerization and copolymerization of AM and DMDAAC (Carton, Salem, Pulpytel, & Arefi-Khonsari, 2015). The cross-linking reaction and decomposition of the formed grafted polymer structure also occurred because of excessive free radicals. Thus, the intrinsic viscosity, grafting ratio, and grafting efficiency decreased. 3.2.4. Effects of discharge time on graft copolymerization The effects of discharge time on the intrinsic viscosity, grafting ratio, and grafting efficiency are shown in Fig. S4. The intrinsic viscosity increased as the discharge time increased. The intrinsic viscosity decreased quickly when the discharge time was more than 90 s. The optimum intrinsic viscosity, graft ratio, and graft efficiency obtained at 90 s were 3324 mL/g, 430%, and 68%, respectively. Therefore, 90 s was set as the optimal discharge time in the subsequent synthesis experiments. Few active free radicals were generated by plasma energy when the discharge time was short, but few free radicals could not fully initiate graft copolymerization. As a consequence, the intrinsic viscosity, graft ratio, and graft efficiency were low. By contrast, numerous active free radicals were produced when the discharge time was extended. Thus, AM and DMDAAC molecules were effectively grafted to the active site of CS monomers and the molecular chain was constantly lengthened (Joshi, Friedrich, & Krishna-Subramanian, 2013). After the discharge time was further extended, the number of active free radicals reached a supersaturated status. The collision between the free radicals and the formed grated polymer chains destroyed the original properties of the polymer. Therefore, the intrinsic viscosity and grafting rate were reduced. 3.2.5. Effects of post-polymerization temperature on graft copolymerization The effects of post-polymerization temperature on the intrinsic viscosity, grafting ratio, and grafting efficiency are shown in Fig. S5. The intrinsic viscosity increased as the post-polymerization temperature slightly increased but decreased sharply as the post-polymerization temperature further increased. The optimum post-polymerization temperature was 50 ◦ C.

At low post-polymerization temperature, the activity of monomers and the rate of radical movement and diffusion were low and thus yielded a low reaction rate. AM and DMDAAC molecules did not sufficiently bind to the active site of CS. As the postpolymerization temperature increased, the reactivity of AM and DMDAAC monomers was enhanced and the rate of chain propagation was continuously accelerated; as a result, the graft ratio and intrinsic viscosity increased (Zheng et al., 2013). The active centers on chitosan became unstable when the post-polymerization temperature exceeded 40 ◦ C. Consequently, the polymerization ability of amino and hydroxyl groups on the chitosan chains was reduced. The heat of polymerization reaction was not easily decreased. As such, the chain termination was observed and the chain transfer rate were decreased. This result was not conducive to graft copolymerization. The high post-polymerization temperature also caused cross-linking and reduced the solubility of CS-g-P(AM-DMDAAC). Therefore, 50 ◦ C was set as the optimum post-polymerization temperature. 3.2.6. Effects of post-polymerization time on graft copolymerization The effects of post-polymerization time on the intrinsic viscosity, grafting ratio, and grafting efficiency are shown in Fig. S6. The intrinsic viscosity, grafting ratio, and grafting efficiency were decreased when the post-polymerization time was more than 24 h. The intrinsic viscosity, grafting ratio, and grafting efficiency of the polymer were 3457 mL/g, 443%, and 71.2%, respectively, and these parameters reached equilibrium as the post-polymerization time was prolonged. As time was extended, the free radicals were consumed continuously to promote graft copolymerization and thus increased the intrinsic viscosity, grafting ratio, and grafting efficiency (Zheng et al., 2014). After a certain time was reached, the polymerization reaction between monomers was completed and caused a low effect on the intrinsic viscosity and grafting rate. Therefore, 24 h was set as the optimum post-polymerization time. 3.3. Flocculation tests 3.3.1. Effects of initial water pH on flocculation performance The effects of initial water pH on the flocculation performance are shown in Fig. 6. The turbidity removal rates of CPAM and PAFC increased rapidly as pH increased [Fig. 6(a)]. At pH > 8, the turbidity removal rate decreased sharply. However, the turbidity removal rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content and CS-g-P(AM-DMDAAC) with 20% CS content increased almost uniformly within the pH range of 1–12. The turbidity removal rate obtained by CS-g-P(AM-DMDAAC) was higher than that by PAM and PAFC within the investigated pH range. The optimal turbidity removal rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content and CS-g-P(AM-DMDAAC) with 20% CS content at pH 11 were 99.02% and 97.5%, respectively. These results indicated that the turbidity removal performance of CS-g-P(AM-DMDAAC) was higher and its pH range was wider than those of PAM and PAFC. The COD removal rates obtained by CS-g-P(AM-DMDAAC)

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trend similar to the COD removal rate. The optimum Chl-a removal rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content and CS-g-P(AM-DMDAAC) with 20% CS content at pH 9 were 92.20% and 88.90%, respectively. The COD and Chl-a removal rates of CPAM and PAFC were lower than those of CS-g-P(AM-DMDAAC) with 40% CS content and CS-g-P(AM-DMDAAC) with 20% CS content at the same pH. The COD and Chl-a removal rates obtained by CPAM increased as pH increased and then decreased as pH further increased. The optimum COD and Chl-a removal rates obtained by CPAM at pH 7 were 85.5% and 84.17%, respectively. By comparison, the optimum COD and Chl-a removal rates obtained by PAFC at pH 8 were 90.21% and 86.7%, respectively. These experimental results revealed that the flocculation performance of CS-g-P(AM-DMDAAC) for COD and Chl-a removal was more efficient than those of CPAM and PAFC. This result indicated that the introduction of CS could improve the COD and Chl-a removal performance in low-algal-turbidity water treatment. Organic flocculants likely undergo hydrolysis under acidic and alkaline conditions; as a consequence, their flocculation performance is reduced (Liu et al., 2014). Polymeric aluminum-ferric in PAFC is easily hydrolyzed into aluminum and iron ions under acidic conditions. Therefore, flocculation performance is decreased. Polymeric aluminum-ferric in PAFC also participates in aluminum and iron precipitation under alkaline conditions and thus reduces the effective constituent in PAFC (Wu et al., 2015). The pH range of CS-g-P(AM-DMDAAC) was broader than those of CPAM and PAFC coagulation–flocculation, and it exhibited a good flocculation performance under strong alkaline conditions. This result is attributed to the presence of numerous CS molecules on CSg-P(AM-DMDAAC) and the amino and hydroxyl groups on CS molecules. pH elicited a cushioning effect on changes in pH and consequently withstand the variations in raw water pH. Colloidal particles in water were positively charged under acidic conditions, and cationic flocculants/coagulants cannot efficiently cause charge neutralization; the adsorption-bridging effect is the main mechanism in the destabilization and cohesion of colloidal particles (Das, Ghorai, & Pal, 2013). Negatively charged colloidal particles under alkaline conditions are easily destabilized by charge neutralization and adsorption bridging effect because CS-g-P(AM-DMDAAC) with a certain amount of positively charged amino groups can efficiently undergo electrostatic neutralization (Dong, Chen, & Liu, 2014).

Fig. 6. Effects of initial water pH on flocculation performance.

with 40% CS content and CS-g-P(AM-DMDAAC) with 20% CS content increased continuously as pH increased (Fig. 6(b)). At pH > 10, the COD removal rate reduced drastically. The COD removal rates obtained by CPAM and PAFC decreased sharply at pH 7 and 9, respectively. The optimal COD removal rates obtained by CS-g-P(AMDMDAAC) with 40% CS content and CS-g-P(AM-DMDAAC) with 20% CS content at pH 9 were 96.11% and 93.08%, respectively. The Chl-a removal rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content and CS-g-P(AM-DMDAAC) with 20% CS content exhibited a

3.3.2. Effects of dosage on flocculation performance Fig. 7 shows the effect of dosage on flocculation performance. In Fig. 7(a), the turbidity removal rate initially increased as the flocculant dosage increased. Afterward, the turbidity removal rate decreased remarkably. The optimal turbidity removal rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content, CS-gP(AM-DMDAAC) with 20% CS content, and 6 mg/L CPAM were 99.02%, 97.5%, and 88.3%, respectively. At less than 40 mg/L PAFC, the turbidity removal rate increased as the dosage increased [Fig. 7(b)]. At higher than 40 mg/L PAFC, the turbidity removal rate increased as the dosage increased [Fig. 7(b)]. The optimum turbidity removal rate by PAFC was 90.09% at 40 mg/L. The turbidity removal performance obtained by CS-g-P(AM-DMDAAC) was higher than those by CPAM and PAFC. Nevertheless, the turbidity removal performance could be significantly improved by the introduction of CS. In Fig. 7(c), the COD removal rate initially increased as the flocculant dosage increased. However, the COD removal rate decreased remarkably when the dosage was more than 10 mg/L. The optimum COD removal rate obtained by CS-g-P(AM-DMDAAC) with 40% CS content, CS-g-P(AM-DMDAAC) with 20% CS content, and 10 mg/L CPAM were 96.11%, 93.08%, and 85.5%, respectively. In Fig. 7(e), the Chl-a removal rate increased as the flocculant dosage increase continuously until 10 mg/L was reached. The Chl-a removal rate

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Fig. 7. Effects of dosage on flocculation performance.

decreased when the dosage was more than 10 mg/L. The optimal Chl-a removal rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content, CS-g-P(AM-DMDAAC) with 20% CS content, and 10 mg/L CPAM were 92.21%, 88.90%, and 84.17%, respectively. In Fig. 7(d) and (f), the optimum removal rates of COD and Chl-a obtained by PAFC were 90.21% and 86.7%, respectively. In Fig. 7, CS-g-P(AM-DMDAAC) exhibited a higher flocculation performance than CPAM and PAFC did. This result confirmed that the flocculation performance could be improved by the introduction of chitosan. The flocculation performance of CS-gP(AM-DMDAAC) with 40% CS content was higher than that of CS-g-P(AM-DMDAAC) with 20% CS content. Colloidal particles

and algae cannot be sufficiently destabilized when the CS-gP(AM-DMDAAC) flocculant dosage is high (Rashid et al., 2013). Destabilized colloidal particles aggregate into large flocs for settlement as the CS-g-P(AM-DMDAAC) dosage increases (Granados, Acién, Gómez, Fernández-Sevilla, & Grima, 2012). However, flocculation deteriorates because of excessive dosage. The Chl-a removal performance of CS-g-P(AM-DMDAAC) was significantly higher than those of CPAM and PAFC because of the distribution of numerous active groups on chitosan molecules in CS-g-P(AM-DMDAAC). Active groups promote the occurrence of special adsorption and complexation between algae and CS-g-

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CS-g-P(AM-DMDAAC) with 20% CS content, CPAM, and PAFC at G of 250 s−1 were 99.02%, 97.5%, 88.3%, and 90.09%, respectively. In Fig. 8(b) and (c), the flocculation efficiency increased as G increased. At G of more than 350 s−1 , the flocculation efficiency decreased. The optimal COD removal rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content, CS-g-P(AM-DMDAAC) with 20% CS content, CPAM, and PAFC at G of 350 s−1 were 96.11%, 93.08%, 85.5%, and 90.21%, respectively. The optimum Chl-a removal rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content, CS-g-P(AM-DMDAAC) with 20% CS content, CPAM, and PAFC at G of 250 s−1 were 92.20%, 88.90%, 84.17%, and 86.7%, respectively. Appropriate G could improve the flocculation efficiency (Fig. 8). A low G can lead to insufficient contact and mix between colloidal particles (algae) and flocculants (Yang et al., 2014). At very high G, the formed flocs break, disperse, stay in water, and increase the COD and Chl-a content; consequently, flocculation efficiency is reduced (Fast, Kokabian, & Gude, 2014). The high COD removal rate and Chl-a removal rate were obtained at G of 350 s−1 rather than at G of 250 s−1 . Therefore, higher G was more suitable to flocculation and COD and Chl-a removal.

4. Conclusions In this study, the chitosan-grafted polymer CS-g-P(AMDMDAAC) was synthesized through plasma-initiated polymerization. Key influencing factors, such as monomer concentration, monomer ratio, discharge power, discharge time, postpolymerization temperature, and post-polymerization time, were systemically investigated on the basis of intrinsic viscosity, grafting ratio, and grafting efficiency.

Fig. 8. Effects of hydraulic condition on flocculation performance.

P(AM-DMDAAC) to strengthen adsorption-bridging flocculation (Delrue, Imbert, Fleury, Peltier, & Sassi, 2015). 3.3.3. Effects of hydraulic condition on flocculation performance The effects of hydraulic condition on flocculation performance are shown in Fig. 8. In Fig. 8(a), the turbidity removal rate increased as G increased. By contrast, the turbidity removal rate decreased when G was more than 250 s−1 . The optimal turbidity rates obtained by CS-g-P(AM-DMDAAC) with 40% CS content,

(1) The following optimum synthesis conditions were obtained: monomer concentration of 15%, CS:AM:DMDAAC monomer ratio of 3:5:2, discharge power of 40 W, discharge time of 90 s, post-polymerization temperature of 50 ◦ C, postpolymerization time of 24 h, intrinsic viscosity of 3457 mL/g, graft rate of 438%, and graft efficiency of 70%. (2) The structure and morphological characteristics of chitosan-gPDMDAAC were examined through XRD, SEM, and FTIR. XRD results confirmed that the crystallinity degree of CS-g-P(AMDMDAAC) decreased, and the crystal structure of chitosan in CS-g-P(AM-DMDAAC) was changed into an amorphous structure after plasma-induced polymerization occurred. The FTIR spectrum also revealed the successful polymerization of CSg-P(AM-DMDAAC). SEM results showed that the surface of CS-g-P(AM-DMDAAC) contained a small and smooth convex structure with some fine pores. Thus, a densely cross-linked network structure was formed on the surface. (3) The flocculation ability of low-algal-turbidity water was investigated and compared with those of CPAM and PAFC. Experimental results demonstrated that the optimum flocculation efficiency was obtained at a dosage of 8–12 mg·L−1 , pH 4–10, and G of 250–550 s−1 with 99.02% turbidity removal rate, 96.11% COD removal rate, and 92.20% Chl-a removal rate. CSg-P(AM-DMDAAC) with a higher CS content exhibited a higher flocculation performance, and this finding was also confirmed by flocculation tests. The flocculation performance of CS-gP(AM-DMDAAC) was also superior to those of CPAM and PAFC. Therefore, CS-g-P(AM-DMDAAC) could be applied as a highly efficient and environmentally friendly flocculant in low-algalturbidity water treatment.

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