UV-Initiated Graft Copolymerization of Cationic Chitosan-Based

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UV-initiated graft copolymerization of cationic chitosan-based flocculants for treatment of zinc phosphate-contaminated wastewater Yongjun Sun, Mengjiao Ren, Chengyu Zhu, Yanhua Xu, Huaili Zheng, Xuefeng Xiao, Huifang Wu, Ting Xia, and Zhaoyang You Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02855 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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Industrial & Engineering Chemistry Research

UV-initiated graft copolymerization of cationic chitosan-based flocculants for treatment of zinc phosphate-contaminated wastewater

Yongjun Suna,b*, Mengjiao Rena, Chengyu Zhua, Yanhua Xub, Huaili Zhengc, Xuefeng Xiaoa, Huifang Wua, Ting Xiaa, Zhaoyang Youa

a

b

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

Correspondence to: YJ. Sun (E-mail: [email protected])

Abstract: In this study, acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride were grafted onto CS by UV initiation to obtain an environmentally friendly graft copolymer, CS-g-poly(acrylamide-acryloyloxyethyl)

trimethylammonium

chloride

(CS-g-PAD).

poly(acrylamide-acryloyloxyethyl) trimethylammonium chloride was named as PAD. Nuclear magnetic resonance hydrogen spectroscopy (1H NMR), X-ray powder diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and thermal gravimetric analysis (TG-DSC) were used to characterize CS-g-PAD. Results confirmed the synthesis of CS-g-PAD. SEM results showed that CS-g-PAD exhibited a porous structure with numerous 2–6 µm micropores. The enhanced CaCl2-aided flocculation tests of wastewater contaminated by zinc phosphate coating indicated that CS-g-PAD had better flocculation performance than PAD and the commercially available cationic polyacrylamide (CPAM). The removal rates of zinc concentration, total phosphorus concentration, chemical oxygen demand were 99.3%, 98.8%, and 72.5%, respectively, at 6 mg·L–1 CS-g-PAD and pH 10. The precipitated flocs were mainly in crystalline form.

Keywords: flocculants; photo polymerization; flocculation–coagulation process; phosphorus wastewater; precipitation reaction

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1. Introduction In the manufacturing industry, zinc phosphate coating is frequently used for chemically treating the surface of metal material and workpiece to achieve the requirements of beautiful appearance, excellent corrosion resistance, and wear resistance.1,2 During zinc phosphate coating, a large amount of contaminated wastewater is produced. The pollutant concentration is often high, thereby unable to meet the emission standard, which was first level discharge standard of 《Integrated wastewater discharge Standard》(GB 8978-1996). Apart from organic pollutant, acid, alkali, and heavy metal ions, zinc phosphate coating wastewater contains a considerable amount of phosphate.3 The large amounts of phosphorus released into the water environment without treatment can cause eutrophication of rivers and lakes. 4 The treatment methods employed to remove zinc and phosphate ions from wastewater are mainly divided into two types, namely, biological and chemical methods.5 The technology used in the biological method is complicated with a removal rate of only 30%–40%.6 Currently, the chemical method is used more widely than the biological method for the removal of zinc and phosphate in zinc phosphate coating wastewater. The flocculation–coagulation–sedimentation method has higher removal efficiency and more stable treatment performance than the biological method. However, the removal rate of phosphorus by conventional flocculants is only 80%–90%.7,8 Therefore, meeting the wastewater discharge requirements is difficult when the phosphorus concentration is high. The flocculation–coagulation–sedimentation method includes two steps. The precipitation agent is initially added to react with total phosphorus (TP).9 The flocculants are subsequently used to flocculate the colloidal particles formed during the precipitation reaction and the original suspended particles in wastewater.9 The fine particles are aggregated together to form large flocs. The efficient removal of the precipitate particles and suspended colloidal particles increases, resulting in a substantially improved efficiency of the removal of phosphorus and other substances.9,10 The characteristics of flocculants are crucial to the flocculation performance.11,12 Compared with traditional flocculants and coagulants (e.g., inorganic coagulants and synthetic polyacrylamide), natural macromolecular modified flocculants were receiving more and more interests s in water environment protection

due to the advantage of low cost,

biodegradation, and green materials, especially chitosan (CS).13 CS is a natural alkaline polysaccharide; the molecular chain segment contains a number of amino, hydroxy, N-acetyl, and other reactive functional groups.14 These reactive functional groups can function with organic contaminant and heavy metal through hydrophobic association and complexation effect.15 Owing to these characteristics, CS has been promoted as a potential flocculant in water pollution control. But the disadvantages of CS have limited its flocculation performance and application fields.16

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The modification of CS is required to improve its flocculation performance. Among most of the modification methods, grafting technology has been confirmed a useful and convenient method which is in favor of improving the water solubility and molecular weight of CS.17 Graft copolymerization can introduce a functional hydrophilic group to improve the solubility of CS as well as increase its potential applications. Simultaneously, the graft copolymer can combine the main properties of introduced monomers to strengthen the absorption bridging effect and charge neutralization ability of CS, resulting in the increase of its flocculation efficiency.18 Furthermore, most of the suspended colloidal particles are negatively charged. The CS with cationic groups becomes effective in destabilizing and aggregating the suspended particles or precipitating them into large flocs and adsorbing dissolved contaminants. Therefore, the grafted CS flocculants have better flocculation performance than unmodified CS and traditional flocculants. Meanwhile, the existing literature demonstrated that grafted CS flocculants have also beneficial to destabilize the colloidal particles for bridge-neutralizing flocculation.19,20 Thus far, most of graft copolymerization has been initiated by heat, ultrasonic wave and γ rays in the literature.21–23 UV-initiated copolymerization has shorter reaction time, lower polymerization temperature, faster polymerization rate, and

better

solubility than other

24

initiation systems. As an environmentally friendly graft copolymerization method, UV-initiated copolymerization is also easy to operate and beneficial to improve the flocculation performance. In our previous work, the flocculation ability of cationic flocculants synthesized by UV initiation was more efficient than that of commercially available flocculants in wastewater treatment and sludge dewatering.25 Therefore, the UV initiation method is also appropriate for grafting a functional cationic monomer onto CS to synthesize a high efficient flocculant. Meanwhile, information about grafting a cationic monomer onto CS by UV-irradiation remains sparse. In this work, acrylamide (AM) and [2-(acryloyloxy)ethyl]trimethylammonium chloride (DAC) were used as functional monomers, which were grafted onto CS by UV initiation to synthesize the flocculant CS-g-poly(acrylamide-acryloyloxyethyl) trimethylammonium chloride (CS-g-PAD) for treatment of zinc phosphate-contaminated wastewater. The grafting of AM and DAC was expected to improve water solubility, the absorption bridging effect, and charge neutralization ability efficiently. The grafting reaction induced by UV radiation had higher reaction efficiency and shorter polymerization time than those by other initiation systems.26,27 The graft copolymer induced by UV radiation was characterized by FTIR, 1H NMR, XRD, SEM, and TG-DSC. The flocculation ability of the graft copolymer was used to treat zinc phosphate coating wastewater. The effects of CS-g-PAD dosage and initial wastewater pH value on the flocculation efficiency were systematically investigated. The optimum flocculation of the zinc phosphate

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coating wastewater by CS-g-PAD was obtained.

2. Materials and methods 2.1 Materials 2′-azobis (2-methylpropionamide) dihydrochloride (V-50), DAC (80% in water) and, AM were industrial grade, which were obtained from Chongqing LanJie tap water material co ., LTD. CS (≥95% degree of deacetylation, 100–200 mPa·s viscosity) and all the other reagents were analytical grade used without further purification, which were sourced from Aladdin Shanghai Biochemical Technology Co., Ltd..

2.2. Synthesis of CS-g-PAD The grafting polymerziation of synthesizing CS-PAD was initiated by ultraviolet light. As an active nucleophile, the NH2 groups of CS can react with electrophilic reagents readily.20 The possible graft reaction is illustrated in Figure 1. Firstly, 2.000 g CS was dissolved in 10 mL acetic acid solution with slowly stirring, and the volume concentration of acetic acid was 1.0%. The CS solution was made in this step. Secondly, 3.75 mL DAC (80% in water), 5.000 g AM, and 30.0 mL deionized water were added into the CS solution with slowly stirring to obtain the reaction solution. While the content of AM and DAC relative to the total monomer was 50% and 30% at this time, respectively. Nitrogen was used to remove the oxygen at ambient temperatures for 30 min. Thirdly, 1‰ photoinitiator (V-50) relative to the total monomer (AM, DAC, and CS) was added during the nitrogen bubbling. V-50 was solid powder. 0.1000 g V-50 was dissolved in 10.0 mL water, and 0.1 mL V-50 solution (1% mass percent) was taken into the reaction solution by the pipette. At last, the quartz reactor containing the reaction solution was irradiated in an ultraviolet light reaction device controlled at room temperature under nitrogen atmosphere for 120 min. Afterward, the polymerization products were purified in ethyl alcohol and dried until a constant weight was obtained. The dried products were made into powder. The powders of the dried products were taken for FTIR analysis, XRD analysis, SEM analysis, and TG-DSC analysis. The powder of the dried product was dissolved into D2O for the 1H-NMR spectrum analysis, and the mass percent of the dried product powder was 0.1‰. 2.3. Characterization of the CS-based flocculant The FTIR (ATR attachment) results, 1H-NMR results, TG-DSC results, and SEM results were obtained by a 550 Series II infrared spectrometer (BRUKER Company, Switzerland), an AVANCE 500 nuclear magnetic resonance spectrometer (BRUKER Company, Germany), a DTG-60H synchronal thermal analyzer (SHIMADZU, Japan), Scanning electron microscope 4


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(TES-CAN Company, Chech), respectively. The XRD results were conducted on D max/RB device (Rigaku Industrial Corporation).

2.4. Zinc phosphate coating wastewater treatment experiments In this study, phosphorus wastewater was selected as the flocculation treatment object, whereas the COD, TP concentration, and Zn concentration were used as indexes to investigate flocculation efficiency. The phosphorus wastewater samples were were collected from the storage pool in Pushi Co., Ltd. (Yibin, China). The characteristics of the zinc phosphate coating wastewater are presented in Table 1. The flocculation process of raw water was conducted with 1.0L wastewater samples, and the mixture stirring process was in the following steps: slow agitation at 70 rpm for 5 min after rapid agitation at 300 rpm for 1 min. The treated wastewater was precipitated for 15 min at last. 0.1000 g CS-g-PAD polymer products powder was dissolved in 100.0 mL deionized water to obtain the flocculant solution. Meanwhile 0.1000 g PAD (CPAM) was also dissolved in 100.0 mL deionized water to obtain the PAD (CPAM) solution. In the flocculation process, 1.0~8.0 mL flocculant solution was added into 1.0 L phosphorus wastewater as a single injection before the rapid agitation process according to the dosage of the experimental designs. The flocculants solution

was

added

as

a

single

injection

before

the

rapid

agitation.

After

the

coagulation-flocculation process, the Zn concentration, and TP concentration, and COD were determined. The tests were conducted in triplicate, and the relative error was less than 5%. PAD and commercially available CPAM were employed as the flocculants in the two independent jar tests in order to compare their flocculation performance. The characteristics of flocculants used in the flocculation experiments are listed in Table 2. 3. Results and discussions 3.1. Characterization of CS-g-PAD 3.1.1. FTIR spectrum The FTIR spectrum of CS-g-PAD is illustrated in Figure 2. The absorption peak at 3175 cm–1 and 3299 cm–1 originated from the hydroxyl group (–OH) (including NH2) in CS and the amide group (–NH2) in AM, respectively. The absorption peak between 3000 –and 3500 cm–1 was wide because of the hydrogen bonding effect. The peak at 1646 cm–1 originated from the –COOCH2 groups in AM monomer and C=O goups in CS monomer. The peak at 1404 cm–1 originated from – CH2–N+(CH3)3 group in DAC. The peak at 1160 cm–1 represented C–O–C in the –COOCH2– of AM and the glycosidic bond in CS. The absorption peak at 947 cm–1 originated from the quaternary ammonium group in DAC.24 The peak of N–H group in CS appeared at 1602 cm–1, while the peak at 1072 cm–1 represented the primary alcohol (C–OH) group in CS.28 Compared

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with the FTIR results of CS and PAD, the characteristic absorption peaks of CS appeared in the infrared spectrum of polymerization product. Based on the analysis of the absorption peaks of CS and CS-g-PAD, the CS backbone structure did not change after polymerization, indicating that the grafting polymerization reaction did not destroy the original structure and the degree of deacetylation of CS. The FTIR spectrum of CS-g-PAD demonstrated that AM and DAC were successfully grafted onto CS, and the polymerization product was the copolymer of CS, AM, and DAC. 3.1.2. 1H NMR spectroscopy The 1H NMR spectra of CTS-g-PAD and PAD are illustrated in Figures 3 and 4, respectively. The characteristic peaks of CS skeleton appeared at 4.6 (H1), 3.8–3.3 (H3, H4, H5, and H6), and 2.9 ppm (H2).29,30 The characteristic peaks at 3.13, 1.55, 3.42, 3.66, 3.97, and 4.47 ppm were supported by the protons on 2, 3–6 positions of the CS backbone. Compared with Figure 4, the signals at δ = 1.68 ppm (Ha) and δ = 2.12 ppm (Hb) originated from –CH2–CH– groups –CH2– CH– in AM and DAC, respectively. The protons in the three equivalent methyl groups of ammonium –N(CH3)3 in DAC were observed at δ = 3.13 (Hc) ppm (overlapped by H2).25 Therefore, the 1H NMR spectrum of CTS-g-PAD demonstrated that the graft polymerization of CS, DAC, and AM occurred.31 3.1.3. XRD patterns The XRD spectrum results of CS-g-PAD are presented in Figure 5. The intense diffraction peak at 2θ=20° was identified as representative of the crystallinity of form II. The orderly crystalline form was destroyed because of the introduction of AM and DAC to the CS backbone; thus, the diffraction peak at 2θ=20° in the XRD spectrum of CS-g-PAD significantly decreased, which attributed to the decrease in crystallinity.32 Compared with the XRD spectrum of CS, the characteristic peak became weaker and wider. The overall structural order was significantly weakened by graft copolymerization, resulting in the right shift of the diffraction peak. However, compared with the XRD spectrum of PAD, the characteristic peak becomes stronger and narrower because of the absence of CS in PAD. Therefore, the XRD spectrum results also demonstrated that AM and DAC were successfully grafted onto CS. In addition, the amorphous structure is easier to hydrate compared with the crystal structure, so the water solubility of the grafted copolymer product may also be increased. 3.1.4 SEM images The SEM results are indicated in Figure 6. The surface morphology of the three types of flocculants was clearly different. The surface of CS was generally flat and ellipsoidal with 6


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rounded edges. The surface of PAD was relatively flat with a large number of fine projections in the lamellar structure (Figure 6(b)). The surface morphology of CS-g-PAD was significantly different from that of the two other types of flocculants. CS-g-PAD had many pores with large porous structures. The range of the pore size was 2–6 µm. The introduction of the cationic groups to the CS destroyed the originally ordered structure of CS crystal. Compared with Figure 6(b), the surface of PAD was more compact and smoother. The change of surface morphology also indicated that AM and DAC were successfully grafted onto the CS backbone. 3.1.5 TG-DSC The thermal gravimetric curves of (a) CS, (b) CS-g-PAD, and (c) PAD are illustrated in Figure 7, and the analytical data are presented in Table 3. As shown in TG curves of Figure 7(b), the first stage of weight loss temperature was within the range of 30–210 °C, and the weight loss was 8.3%. The weight loss in the first range was due to the evaporation of adsorbed water. The second stage was within the range of 210–340 °C, and the weight loss was 28.3%. The weight loss in the second stage may be due to the deprivation of methyl and dehydrochlorination from a quaternary ammonium group (–C(CH3)3N+Cl–). The third stage was within the range of 340– 480 °C, and the weight loss was 41.3%. The polymer main chain began to degrade rapidly with heat in the third stage. The ultimate temperature of the complete decomposition was approximately 480 °C. After 480 °C, the TG curve began to flatten without weight loss. The final weight of the residue was 21.8%. From the DSC curves in Figure 7(b), the TG curve of CS-g-PAD exhibited three stages of weight losses. Correspondingly, the three endothermic peaks occurred at 95.2, 276.2, and 414.2 °C, respectively.25,26 Compared with Figure 7(a), CS only had two weight loss stages. The exothermic peak at 302.2 °C in DSC curve of CS was evident in the DSC curve of CS-g-PAD at 382.2 °C. In addition, the endothermic peak of the third stage of weight loss in the DSC curve of PAD was discovered in the DSC curve of CS-g-PAD at 414.2 °C. The characteristics of the TG curves of CS and PAD were found in the TG curve of CS-g-PAD. All the evidence and discussion of thermal gravimetric curves confirmed the copolymer of CS, DAC, and AM was synthesized successfully. Meanwhile, the TG of CS-g-PAD was 210.2 °C, which was higher than that of CS and PAD (Table 3), indicating that the grafting of AM and DAC onto the CS backbone significantly changed the thermal stability of CS.33 3.2. Treatment of phosphate-contaminated wastewater 3.2.1 Effect of pH value on the removal of pollutants

The effect of pH value on the removal of Zn and TP is demonstrated in Figure 8 (a) and (b).

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Raw water with low pH value contains many ions that can be precipitated. Therefore, the wastewater was pretreated by adjusting the pH and dosing CaCl2. The dosage of CaCl2 was 25 g·L–1 in the pretreatment. With the increase of pH value, the removal rate of Zn increased gradually (Figure 7). When the pH value was greater than 8, the removal rate stabilized. The removal rate of Zn after adding CaCl2 was greater than 85.2%, and the residual Zn concentration was 6.9 mg/L. Without adding CaCl2, the removal rate was only 81.5% and the residual concentration was 8.6 mg/L. The removal rate of TP increased remarkably with the increase of pH value as shown in Figure 8. When the pH value was greater than 5, the removal rate of TP tended to be stable by adding CaCl2. The removal efficiency was greater than 92%, and the residual concentration of TP was approximately 5.3 mg/L. The removal rate of TP was stabilized at pH greater than 10 without adding CaCl2. The removal rate was only 84%, and the residual concentration was 10.42 mg/L. The addition of CaCl2 clearly contributed to the increase of the removal rate of Zn and TP. In addition, high removal rates were obtained at the alkaline conditions. Zn can react with OH– and PO43–, forming the precipitate of Zn(OH)2 and Zn3(PO4)2 under the alkaline conditions.34 Then, Zn and TP were removed by precipitation reaction. Meanwhile, phosphate reacted with Ca2+ to produce several types of calcium phosphate precipitation, and the optimum reaction pH was 10.35 Further, the phosphate reacted with Ca2+ to form Ca10(OH)2(PO4)6 to remove TP. Through comprehensive comparison and discussion, pH 10 was selected as the optimum pH value. 3.2.3 Effect of flocculant dosage on flocculation efficiency To further remove Zn, TP, COD, and suspended colloid particles produced in the pretreatment, flocculation was tested after pretreatment by CaCl2. As shown in Figure 9(a), the removal rate of Zn increased with the increase of flocculant dosage in the investigated dosage range. When the dosage of CS-g-PAD was greater than 4.0 mg/L, the removal rate of Zn tended to stabilize. The removal rate of Zn was 99.2%, and the residual concentration was 0.36 mg/L. The removal efficiency of Zn flocculated by PAD and CPAM was distinctly lower than that by CS-g-PAD. This outcome was due to the ability of the CS units in CS-g-PAD to engage in a complex with Zn ions to improve the removal rate of Zn.36 The removal efficiency of TP increased with the increase of the flocculant dosage as shown in Figure 9(b). The removal rate of TP flocculated by CS-g-PAD increased gradually, but it was prominently higher than that by PAD and CPAM. The removal rate of TP tended to stabilize when the dosage of CS-g-PAD was more than 4 mg/L. The removal rate of TP was 98.6% with 4 mg/L CS-g-PAD, and the residual concentration was 0.86mg/L. The removal rate of TP by PAD and CPAM at 7 mg/L was 95.1% and 93.6%, respectively. The residual concentration of TP by PAD 8


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and CPAM was 3.23 and 4.23 mg/L, respectively. The COD removal rate increased slowly with the increase of dosage as shown in Figure 9(c). The removal rate of COD by CS-g-PAD at 5 mg/L was 71.1%, and the remaining COD was 53.7 mg/L. The optimal removal efficiency of COD by PAD and CPAM at 6 and 7 mg/L were 61.8% and 57.7%, respectively. The removal efficiency of COD decreased slightly at 7 mg/L CPAM because of the overdosing effect resulting in the restabilization of flocs. As shown in Figures 9(b) and 9(c), the removal rate of COD and TP by CS-g-PAD was significantly higher than that by PAD and CPAM (p value ＜0.01). According to the above discussions, CS-g-PAD had better flocculation performance for the removal of TP and COD. The suspended colloidal particles of phosphate precipitation could be effectively destabilized and aggregated to form large flocs for sedimentation.37 The porous flocs could further adsorb the residual suspended sediments of Zn and TP during the precipitation, thereby raising the removal efficiency of COD, Zn, and TP.38 In summary, the optimal dosage of CS-g-PAD was 4–6

mg·L–1.

3.2.3 Analysis of the flocs The images of floc (Figure 10) were observed. The flocs formed by adding alkali were illustrated in

Figure 10(a). The crystalline flocs were mainly Zn(OH)2 precipitates because of the zinc salt precipitation reaction under alkaline condition. The flocs formed by alkali and CaCl2 was calcium phosphate caused by a high concentration of TP in the wastewater. The flocs after flocculation were denser and larger in Figure 10(c). The tiny suspended particles were destabilized by CS-g-PAD to be aggregated into large flocs. Zinc in the wastewater was partially removed through the complexation and chelation effect of CS units in the CS-g-PAD chain.39,40 The most of the colloidal state of TP and COD were removed by CS-g-PAD through adsorption bridging action and charge neutralization effect in the flocculation process. Owing to the porous structure of flocs, some other colloidal state of TP and COD were further removed by adsorbing on the flocs during the precipitation process.41,42 As shown in

Figure 6, the surface of CS-g-PAD contained a lot of porous structure, which were conducive to improve the solubility and adsorption-bridging ability. It can be seen from Figure 10, the flocs were formed by apparent crystalline structure and less dense flocculate, and the crystalline were wrapped by the less dense flocculate. The less organized flocculates were the aggregates of suspended colloidal particles formed in the flocculation process. Figure 11 shows the FTIR results of flocs. The FTIR spectrum of flocs after treatment by alkali+CaCl2+CS-g-PAD illustrates that the absorption peaks at 3432 and 1636 cm–1 were the stretching and bending vibration absorption peaks of the hydroxyl group, respectively. The absorption peak observed at 564 cm–1 was attributed to Zn-O. The bending vibration absorption peak of 605 cm–1 was attributed to PO43–. Similar to Figure 2,

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the characteristic absorption peak of CS-g-PAD was clearly observed in Figure 11. The composition of crystalline flocs was mainly zinc hydroxide and calcium phosphate crystals from the analyses of the microscopic photograph and infrared spectrum. The discussion in this section indicated that CS-g-PAD has a good flocculation performance with an efficient removal rate of the characteristic contaminants in

zinc phosphate coating wastewater (Figure 12).

4. Conclusions In this study, an enhanced coagulation with CS-g-PAD aided by CaCl2 was employed for treatment of zinc phosphate-contaminated wastewater. The graft copolymerization of AM and DAC onto CS was initiated by UV. The FTIR spectrum and 1H NMR spectrum analyses demonstrated that the graft copolymer CS-g-PAD was synthesized successfully. SEM images indicated that CS-g-PAD had a porous structure with many 2–6µm micropores. The TG-DSC curve analysis revealed that the UV-initiated graft copolymerization of CS, AM, and DAC had better thermostability than that of PAD. The wastewater treatment results showed that after alkali and CaCl2 pretreatments, CS-g-PAD can significantly reduce the concentration of TP, COD, and Zn. When the dosage of CS-g-PAD was 6 mg·L–1 with pH 10, the removal rate of TP, COD , and Zn was 98.8%, 72.5%, and 99.3%, respectively. The analysis of floc microscopic images indicated that the flocs were dense with precipitated crystals. The flocculation tests demonstrated that CS-g-PAD had better flocculation performance than PAD and CPAM. CS-g-PAD was confirmed as an alternative flocculant that can be used in the treatment of wastewater contaminated with zinc phosphate coating.

Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51508268), the Natural Science Foundation of the Jiangsu Province in China (No. BK20150951), China Postdoctoral Science Foundation (No. 2016M591835), Natural Science Foundation of the Jiangsu Higher Education Institution of China (12KJA610001), and Science and Technology Program of the Ministry of Housing and Urban-Rural Development of China (2014-K7-010).

References (1) Zhang, M; Peng, Y.; Wang, C.; Wang, C.; Zhao, W.; Zeng, W. Optimization denitrifying phosphorus removal at different hydraulic retention times in a novel anaerobic anoxic oxic-biological contact oxidation process. Biochem. Eng. J. 2016, 106, 26-36.

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Their Flocculating Properties. Ind. Eng. Chem. Res. 2011, 50, 7141-7149. (17) Yang, X.; Zhanga, C.; Qiao, C.; Mu, X.; Li, T.; Xu, J.; Shi, L.; Zhang, D. A simple and convenient method to synthesize N-[(2-hydroxyl)-propyl-3-trimethylammonium] chitosan chloride inan ionic liquid. Carbohyd. Polym. 2015, 130, 325-332. (18) Nair, V.; Panigrahy, A.; Vinu, R. Development of novel chitosan–lignin composites for adsorption of dyes and metal ions from wastewater. Chem. Eng. J. 2014, 254, 491-502. (19) Dao, V. H.; Cameron, N. R.; Saito, K. Synthesis, properties and performance of organic polymers employed in flocculation applications. Polym. Chem. 2016, 7, 11-25. (20) Wang, J.; Chen, Y.; Wang, Y.; Yuan, S.; Sheng, G.; Yu, H. A novel efficient cationic flocculant prepared through grafting two monomers onto chitosan induced by Gamma radiation. RSC Adv. 2012, 2, 494-500. (21) Nasef, M. M.; Güven, O. Radiation-grafted copolymers for separation and purification purposes: Status, challenges and future directions. Prog. Polym. Sci. 2012, 37, 1597-1656. (22) Wang, J.; Chen, Y.; Zhang, S.; Yu, H. A chitosan-based flocculant prepared with gamma-irradiation-induced grafting. Bioresource Technol. 2008, 99, 3397-3402. (23) Santos, D. M.; Bukzem, A. L.; Campana-Filho, S. P. Response surface methodology applied to the study of the microwave-assisted synthesis of quaternized chitosan. Carbohyd. Polym. 2016, 138, 317-326. (24) Guan, Q.; Zheng, H.; Zhai, J.; Zhao, C.; Zheng, X.; Tang, X.; Chen, W.; Sun, Y. Effect of Template on Structure and Properties of Cationic Polyacrylamide: Characterization and Mechanism. Ind. Eng. Chem. Res. 2014, 53, 5624-5635. (25) Zheng, H.; Sun, Y.; Guo, J.; Li, F.; Fan, W.; Liao, Y.; Guan, Q. Characterization and Evaluation of dewatering properties of PADB, a highly efficient cationic flocculant. Ind. Eng. Chem. Res. 2014, 53: 2572-2582. (26) Zheng, H.; Sun, Y.; Zhu, C.; Guo, J.; Zhao, C.; Liao, Y.; Guan, Q. UV-initiated polymerization of hydrophobically associating cationic flocculants: Synthesis, characterization, and dewatering properties. Chem. Eng. J. 2013, 234: 318-326. (27) Ma, J.; Fu, K.; Shi, J.; Sun, Y.; Zhang, X.; Ding, L. Ultraviolet-assisted synthesis of polyacrylamide-grafted chitosan nanoparticles and flocculation performance. Carbohyd. Polym., doi:10.1016/j.carbpol.2016.06.002 (28) Yang, Z.; Shang, Y.; Lu, Y.; Chen, Y.; Huang, X.; Chen, A.; Jiang, Y.; Gu, W.; Qian, X.; Yang, H.; Cheng, R. Flocculation properties of biodegradable amphoteric chitosan-based flocculants. Chem. Eng. J. 2011, 172, 287- 295. (29) Mourya, V. K.; Inamdar, N. Trimethyl chitosan and its applications in drug delivery. J.

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Figure 1 Possible scheme of graft copolymerization

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Figure 2 FTIR spectrum: (a) CS, (b) CS-g-PAD and PAD

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Figure 3 1H NMR spectrum of CTS-g-PAD.

Figure 4 1H NMR spectrum of PAD.

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Figure 5 XRD patterns of the polymers.

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Figure 6 SEM images: (a) CS, (b) CS-g-PAD, (c) PAD

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Figure 7 Thermal gravimetric curve: (a) CS, (b) CS-g-PAD, (c) PAD.

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Figure 8 Effect of pH value on the removal of (a) Zn and (b) TP

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Figure 9 Effect of flocculant dosage on removal of (a) Zn, (b) TP, and (c) COD

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Figure 10 Photo (×200) of flocs formed by: (a) alkali treatment, (b) (alkali + CaCl2) treatment, (c) CS-g-PAD after flocculation.

Figure 11 FTIR spectrum of flocs

Figure 12 Photo of the zinc phosphate coating wastewater before and after flocculation and precipitation 22


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Table 1. Characteristics of the zinc phosphate coating wastewater Turbidity

COD

Total phosphorus (TP)

Zn concetration

Total suspended solids

pH

(NTU)

(mg·L-1)

(mg·L-1)

(mg·L-1)

(TSS) (mg·L )

value

46.5+2

185.8±8.0

66.2±2.9

46.5±1.5

72.7±3.2

3.9±0.2

-1

Table 2. Characteristics of flocculants utilized in the phosphating wastewater treatment experiments Full name of Flocculants

Flocculants

Theoretical

content

Cationic

(%)

Degree (%)

CS-g-PAD

20

PAD

CPAM

Abbreviation

Chitosan-graft-

CS

Intrinsic Viscosity -1

Dissolving

(dL·g )

Time (h)

30.0

1.88

1.8

0

30.0

1.90

2.9

0

30

2.00

3.0

Poly(acrylamide-2-(acryloyl oxy)ethyl)trimethylammoniu m chloride) Poly(acrylamide-2-(acryloyl oxy)ethyl)trimethylammoniu m chloride) commercially available cationic polyacrylamide

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Table 3. Thermal gravimetric parameters by the four initiation methods Flocculants CS

CS-g-PAD

PAD

Temperature range (°C)

30–200

30–210

30–200

Weight loss (%)

6.4

8.3

9.4

78.1

95.2

95.2

200–410

210–340

200–340

274.8

276.2

278.2

Weight loss (%)

47.0

28.3

29.1


410–600

340–480

340–490

\

414.2

396.2

Third stage weight loss (%)

46.4

41.3

40.7


\

480–600

490–600

Residual weight (%)

\

21.8

20.8

Temperature of endothermic peak (°C)

302.2

382.2

\

Tg (°C)

127.9

210.2

176

Parameter

First stage Maximum weight loss temperature (°C) Temperature range (°C) Second

Maximum weight loss

stage

temperature (°C)

Maximum weight loss Third stage temperature (°C)

Fourth stage

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UV-Initiated Graft Copolymerization of Cationic Chitosan-Based

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