Flocculation properties and kinetic investigation of polyacrylamide with different cationic monomer content for high turbid water purification

Separation and Purification Technology 182 (2017) 134–143

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Flocculation properties and kinetic investigation of polyacrylamide with different cationic monomer content for high turbid water purification Jiangya Ma a,b,⇑, Kun Fu a,b, Xue Fu a,b, Qingqing Guan c, Lei Ding a,b, Jun Shi a,b, Guocheng Zhu d, Xinxi Zhang a,b, Shihua Zhang a,b, Liyan Jiang 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 Nanjing Institute of Technology, School of Environmental Engineering, Nanjing, Jiangsu 211167, China d College of Civil Engineering, Hunan University of Science & Technology, Xiangtan, Hunan 411201, China b c

a r t i c l e

i n f o

Article history: Received 10 December 2016 Received in revised form 21 March 2017 Accepted 25 March 2017 Available online 27 March 2017 Keywords: Polyacrylamide Characterization Flocculation Cationic monomer High turbid water

a b s t r a c t A novel cationic polyacrylamide (PAMC) with various cationic monomer contents were prepared by copolymerizing acrylamide (AM) and methacryloxyethyl trimethyl ammonium chloride aqueous solution (DMC) through low-pressure ultraviolet (UV) initiation. The chemical structure of PAMC was determined by Fourier transform infrared (FTIR), proton unclear magnetic resonance spectrometry (1H NMR), thermal gravimetric and differential analysis (TG-DTA) analysis. Flocculation performance of PAMC was tested in clarification of kaolin suspension, and flocculation kinetic and mechanism were discussed accordingly. Results showed that PAMC with 40% cationic monomer content exhibited excellent turbidity removal efficiency at pH 4.0, dosage of 0.3 mg
L1, stirring speed of 120 rpm, stirring time of 15 min, and sedimentation time of 40 min. Increased cationic monomer content and reduced kaolin pH level resulted in the increase of transmittance of the supernatant and decrease of optimal PAMC dosage. The determined kinetic constant (K) suggested that interaction of contact and collision between flocculants and kaolin particles was sufficient at the optimal PAMC dosage. ZP analytical results showed that charge neutralization and adsorption were dominant under acidic and alkaline environments, respectively. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Urbanization and industrialization have resulted in the contamination of surface water with considerable quantities of soil and sand. This process gradually forms high turbid water (especially water with turbidity more than 600 NTU), which increases difficulties of conventional drinking water treatment process worldwide [1]. Generally, flocculation is a convenient, easy to manipulate, environment-friendly, and energy-efficient technology for solid-liquid separation. This method was extensively applied in pretreating wastewater or micro-polluted raw water to remove colloidal suspended particles. For water with a specific turbidity, selecting a suitable flocculant is critical to achieve high flocculation efficiency in treatment process. Cationic polyacrylamide (CPAM) is one of the most frequently used flocculants with high intrinsic viscosity and charge density. This flocculant is a water-soluble acrylamide-based polymer having cationic quaternary ammonium groups [2]. Cationic ⇑ Corresponding author. E-mail address: [email protected] (J. Ma). http://dx.doi.org/10.1016/j.seppur.2017.03.048 1383-5866/Ó 2017 Elsevier B.V. All rights reserved.

monomer methacryloxyethyl trimethyl ammonium chloride (DMC) has higher charge density, which is favorable in destabilizing suspended particles in high turbid water. The copolymer of acrylamide (AM) and DMC is used as an effective cationic flocculant in wastewater treatment. A series of copolymers of AM and DMC (PAMC) were synthesized using different approaches. In earlier studies, c-ray irradiation with inverse emulsion polymerization was employed to copolymerize AM and DMC [3]. Additional additives, such as kerosene combined with two surfactants (Span80 and OP10), were introduced during polymerization. Shang et al. copolymerized AM, DMC, and hydrophobic monomer of methacryloxypropyltrimethoxy silane (MAPMS) using inverse emulsion polymerization with thermal initiation [4]. The results showed that the copolymer was effective in flocculating dye and kaolin wastewater, but with unfavorable water solubility. Dispersion copolymerization of AM and DMC in aqueous salt solution was accomplished by Chen et al. [5]. Ammonium sulfate and sodium chloride with poly(acryloylxyethyl trimethyl ammonium chloride) (PDAC) was added to prepare the aqueous salt solution. These traditional polymerization methods presented several disadvantages. Firstly,

J. Ma et al. / Separation and Purification Technology 182 (2017) 134–143

excessive additives increased cost, and caused sophisticated operation in industrial production, and formed additional residuals in water after flocculation. Secondly, the long reaction time (12– 24 h) was required for copolymerization, indicating low reaction efficiency. Thirdly, the heating in thermal initiation increased energy consumption and caused cross-linking of copolymer, thereby decreasing dissolubility. UV initiation is recently used to polymerize polyacrylmide because of its promotion on surface modification and improvement on adsorption capacity [6,7]. Wang et al. successfully prepared AM and DAC copolymer with high-pressure UV initiation [8]. A high-pressure mercury lamp (500 W) was used to provide UV light, and the reaction time was only 120 min. The results suggested that high-pressure UV initiation can be efficiently used to synthesize CPAM. However, further studies showed that lowpressure UV initiation (10–50 W mercury lamp) was more effective to prepare modified polyacrylamide with low energy consumption [9]. Low-pressure UV initiation displays several advantages over other similar techniques. Firstly, the amount of heat released in this process is smaller than in high-pressure UV initiation. Consequently, a complex reflux condensation device is not necessary in the low-pressure UV initiation but required in the high-pressure type [6]. The reflux condensation device is inhibited in industrial production. Secondly, the probability of cross-linking can be reduced in low-pressure UV initiation because of lower released heat, resulting in better solubility of polymers. Thirdly, the initiation efficiency of the low-pressure UV initiation is higher than thermal initiation. The former process requires 1–4 h reaction time for polymerization, whereas 5–10 h is needed in thermal initiation. Finally, remarkable amount of energy consumption is saved in lowpressure UV initiation, compared with high-pressure UV initiation and traditional thermal initiation. Therefore, low-pressure UV initiation was employed in this study to prepare the copolymer of AM and DMC. Moreover, Yang et al. prepared a biodegradable amphoteric chitosan-based flocculant and studied the flocculation properties to purify turbid kaolin suspension with original turbidity of 75 NTU [10]. The results showed that higher content of 3-chloro-2hydroxypropyl trimethyl ammonium chloride performed better flocculation to purify turbid water. The results indicated that increased cationic monomer content in polymerization would lead to increased positive charge density of polymer, which was considered as an efficient method to increase charge neutralization capacity in flocculation. Charge neutralization and adsorption capacity should be enhanced simultaneously to obtain the desired flocculation efficiency for high turbid water. Thus, cationic polyacrylamide flocculant with different cationic monomer contents was prepared through low-pressure UV initiation for high turbid water flocculation, and the flocculation property was investigated. In the research described here, PAMC with various cationic monomer contents was synthesized by copolymerization of AM and DMC using low-pressure UV initiation. FTIR, 1H NMR, and TG-DTA were employed to analyze the chemical structures. Flocculation performance of PAMC was evaluated in kaolin suspension purification, and the flocculation kinetic and mechanism were discussed for the comprehensive understanding of high turbid water flocculation.

2. Experimental section 2.1. Materials Acrylamide powder (AM, 90 wt.%), methacryloxyethyl trimethyl ammonium chloride (DMC, 75 wt.%) were obtained from Shanghai Aladdin Industrial Corporation, China. 2-hydroxy-40 -(2-hydroxye

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thoxy)-2-methylpropiophenone (IR2, 98 wt.%) was purchased from Changzhe Biotechnology Co., Ltd (Shanghai, China). Commercial flocculant of polyacrylamide (PAM, MW of 2000–14,000 kDa) was bought from Aladdin Industrial Corporation (Shanghai, China). Anhydrous ethanol and acetone were supplied by Zhenqi Chemical Reagent Co., Ltd (Shanghai, China). Kaolin (99 wt.%), sodium hydroxide (NaOH, 96 wt.%), hydrochloric acid (HCl, 1.179 g
cm3) and sodium chloride (2.165 g
cm3) without exception were sourced from Sinopharm Chemical Reagent Co., Ltd. Nitrogen (N2, purity >99.999%) was provided by Special Gas Factory Co., Ltd. The pH value of monomer solution and kaolin simulated wastewater was adjusted to a desired value using 0.1 M NaOH or 0.1 M HCl. All reagents used in this study were of analytical grade and employed directly without any further purification. Deionized water was used to prepare all the solutions through dissolution or dilution. 2.2. Preparation of polyacrylamide Flocculant PAMC was copolymerized by AM and DMC through low-pressure UV initiation in aqueous solution with IR2 as photo-initiator. The UV spectrum of IR2 was recorded and shown in supplementary data (Fig. S1). Scheme 1 shows that the freeradical copolymerization was implemented in homogeneous aqueous solution based on our previous study. In a 150 mL quartz jar, a desired amount of AM was totally dissolved in a certain volume of deionized water, and then a predetermined amount of DMC was added. The mixed liquor was stirred with a magnetic stirrer, until a uniform mixture was formed. Afterwards, the pH of the mixture was adjusted to 4.0 using HCl or NaOH aqueous solution. A known amount of initiator was added as photo-initiator under continuous mixing under pure N2 atmosphere. Then, the quartz jar was sealed immediately and transferred to a UV reaction device, and the copolymerization was performed by UV radiation with two 48 W low-pressure mercury lamps (253.7 nm, Gaojiang Scientific Co., Ltd, China) at ambient temperature for 3 h. The temperature variation during copolymerization was recorded, and is presented in supplementary dates (Fig. S2). Thereafter, colorless and transparent PAMC gel was obtained. The prepared PAMC solution was washed by mixed solution of ethanol and acetone (1:2, v/v) and purified by a soxhlet extractor, followed by drying in a vacuum oven at 60 °C for 24 h. Therefore, the final white solid product of PAMC was prepared. A possible reaction route for the synthesis of PAMC is outlined in Scheme 1. Four PAMC samples with different cationic monomer ratios (mass ratios of DMC) were synthesized by adjusting the mass ratio of AM and DMC before polymerization. The series of PAMC samples were termed from PAMC1 to PAMC4, respectively. The intrinsic viscosity and isoelectric point (PI) of the polymers were investigated, and detailed information about these samples is listed in Table 1. 2.3. Characterization of PAMC In this section, chemical structures and apparent morphologies of series PAMC samples were compared and analyzed. FTIR spectra of PAMC were recorded using KBr pellets through a TENSOR27 infrared spectrometer (Bruker Company, Germany), and the tested wave numbers was set between 900 and 4000 cm1. 1H NMR spectrograms was determined by a VANCE Model III-500 spectrometer (Bruker Company, Germany) in solvent of D2O at 500 MHz. The TGDTA of the PAMC was examined through a DTG-60H (Shimadzu, Japan) under nitrogen atmosphere, and the process was carried out in a temperature interval of 50–700 °C with heating rate of 20 °C
cm1. Moreover, the ZP of PAMC samples were measured on a Nano ZS 90 Zetasizer (Malvern Company, England).

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Scheme 1. Schematic presentation of synthesis of PAMC.

Table 1 Details of the four PAMC samples.

a

Samples

AM (g)

DMC (g)

Cationic monomer ratio (%)

Photo initiator IR2 (g)

pH

UV time (h)

Intrinsic viscositya (mg
L1)

Isoelectric point

PAMC1 PAMC2 PAMC3 PAMC4

9 8 7 6

1 2 3 4

10 20 30 40

0.1 0.1 0.1 0.1

4 4 4 4

3 3 3 3

1438 1423 1310 882

8 8.8 9.2 9.4

The measurement method of intrinsic viscosity was presented in supplementary materials.

2.4. Flocculation performance Flocculation efficiency of PAMC was evaluated to treat high turbidity water which was stimulated by kaolin suspension (1.0 g
L1). The stimulated kaolin wastewater was prepared as follows: predried 1.0 g of kaolin powder was dissolved in 1.0 L of deionized water with mechanical stirring for 5 min at 150 rpm and ultrasonically dispersed for 3 min at 40 kHz in an SB-5200 ultrasound device (Xinzhi Biotechnology Co., Ltd, Ningbo, China). The pH of the solution was adjusted to predetermined value with 0.1 M NaOH or 0.1 M HCl solution. Flocculation experiment was conducted on a ZR4-6 program-controlled jar test apparatus (Zhongrun Company, China). A certain amount of flocculant PAMC was added into 1 L of the prepared kaolin suspension filled in a beaker. A three-step mixing procedure was established as follows. Firstly, the suspension was rapidly stirred at 200 rpm for 5 min was implemented to completely mix PAMC and kaolin suspension. Secondly, the suspension was slowly stirred at 120 rpm for 15 min was conducted to generate and agglomerate flocs. Finally, the suspension was not stirred for 20 min to allow the sedimentation of precipitates and flocs. After flocculation process, the supernatant at the depth 1–2 cm below the liquid surface was extracted for measurement. The transmittance of supernatant was determined by a TU-1901 double beam Ultraviolet visible Spectrophotometer (wavelength of 550 nm, Beijing Purkinje General Instrument Co., Ltd, China), and Zeta potential (ZP) was measured through a Nano ZS 90 potential instrument (Malvern Company, UK).

3. Results and discussion 3.1. Characterization of PAMC 3.1.1. FTIR spectra analysis The FTIR spectra of PAMC1, PAMC2, PAMC3, PAMC4, and PAM are illustrated in Fig. 1(a)–(e), respectively. As shown in Fig. 1, the bending vibration absorption band of 2927 cm1 is derived from methyl -CH3 and methylene -CH2- in various flocculant samples [11]. Moreover, few offsets of this vibration absorption to 2926 cm1, 2928 cm1, 2946 cm1, and 2947 cm1 were observed in PAMC [12]. The result is ascribed to the introduction of cationic monomer DMC with branched chain. Absorption bands at 1738 cm1 was assigned to the stretching vibration of C@O in the ester groups of DMC [4,8], and the absorption peak at

Fig. 1. FTIR spectra of (a) PAMC1, (b) PAMC2, (c) PAMC3, (d) PAMC4, and (e) PAM.

1665 cm1 was assigned to stretching vibration of CAN in amide groups [8]. The vibrating absorption peak at 1445 is assigned to the methyl groups of quaternary ammonium in the cationic monomer [13–14]. Except for Fig. 1(e), Fig. 1(a)–(d) all exhibit an obvious and similar adsorption peak at 952 cm1 or 953 cm1, which is sourced from characteristic group of quaternary ammonium AN+(CH3)3 in DMC [15]. The result suggests that DMC was successfully copolymerized with AM in the polymerization of PAMC. However, it is also found that the strength of these peaks at 952 cm1 is different with each other. From PAMC1 to PAMC4, these adsorption peaks become stronger as increase of DMC content, which indicates more quaternary ammonium groups were grafted onto PAMC. Therefore, the FTIR spectra provide a sufficient evidence for copolymerization of AM and DMC.

3.1.2. 1H-NMR spectra analysis 1 H NMR spectra were measured in order to further analyze the chemical structures of PAMC, and the results are presented in Fig. 2. As demonstrated in Fig. 2, an intensive resonance peak is observed at d = 4.79 ppm, which is assigned to proton of solvent D2O. The resonance peaks at da = 1.65 ppm and db = 2.22 ppm

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Fig. 2. 1H NMR spectra of (a) PAMC1, (b) PAMC2, (c) PAMC3, (d) PAMC4, and (e) PAM.

are derived from protons of methylene ACH2A and methine ACHA in copolymer backbone [16]. However, compared with Fig. 2(e), several new resonance peaks appear in Fig. 2(a) and (b). The most obvious one is the resonance peak at dg = 3.19 ppm, which is attributed to the proton of quaternary ammonium AN+(CH3)3 [5,17]. The peaks at dc = 4.04 ppm and dd = 3.51 ppm correspond to ACH2A in backbone of DMC and ACH3 in branched chain of DMC, respectively. And the peaks at de = 4.04 ppm and df = 3.51 ppm are sourced from different positional proton of AOCH2CH2A [17]. In addition, it can be found that, the resonance peaks at da and db weaken with the increase of DMC content, whereas the peaks at dc-dg strengthen with the increased cationic monomer. These variation tendencies between different PAMC samples are in accordance with our desired results. Therefore, the analysis of 1H NMR spectra indicates that PAMC was successfully synthesized in our preparation.

3.1.3. TG/DTA analysis Method of differential scanning calorimetry (DSC) was used for thermal analysis of PAMC. The thermal gravimetric analysis (TG) and differential thermal analysis (DTA) of PAM and PAMC were accomplished with a TGA instrument from 50 °C to 700 °C. The results are presented in Fig. 3, and the detailed data was summarized in supplementary material Table S1. The results show that there are four stages of weight loss (WL) for PAM and PAMC1, whereas three stages for PAMC2, PAMC3 and PAMC4. In the first stage, the WL of PAM and PAMC1-4 was observed at the range of 31.85–192.59 °C, 23.99–188.24 °C, 31.29–177.72 °C, 28.41– 185.18 °C, and 34.26–174.66 °C, respectively. The WL in this period was considered as the moisture evaporation. It was found that the WL gradually increased from PAM to PAMC4 within 18.02–22.2%, which was attributed to the hydrophilic groups in DMC [18]. As increase of DMC content, the polymer PAMC became easier to absorb water from the air. The second stage of WL for flocculants

occurred at 192.59–354.97 °C, 188.24–336.73 °C, 177.72– 319.7 °C, 185.18–321.58 °C, and 174.66–308.9 °C for PAM and PAMC1-4, respectively. This was ascribed to the decomposition of acryl amide group ACOANHA in AM and quaternary ammonium AN+(CH3)3 in DMC [19]. The increase of DMC content caused cross-linking by branched chain, resulting in the increase of WL from PAMC1 to PAMC4. In the third stage, the WL of 33% for PAM, 20.16% for PAMC1, 57.38% for PAMC2, 53.14% for PAMC3, and 51.93% for PAMC4 was observed above 308.9 °C. The WL of PAMC2-4 was obvious greater than that of PAM and PAMC1, which was resulted from the length of molecular chain [17]. According to intrinsic viscosity of flocculants, PAM and PAMC1 have a longer and more complex molecular chain than PAMC2-4, which was difficult to be decomposed. A certain amount of low molecular polymers were generated from decomposition of high molecular polymers [20–21]. For this reason, there was the fourth WL stage for PAM and PAMC1 between 444.77 °C and 700 °C. These low molecular polymers were further decomposed in this period. Finally, WL of PAM and PAMC1 was found to be 34.78% and 40.58%, respectively. Moreover, Fig. 3(b) shows that the endothermic peaks of PAM and PAMC1-4 are observed at 85 °C, 290 °C, 402 °C for PAM (or 431 °C for PAMC1), and 496 °C, which are correspondent to the above three or four WL stages. Overall, the synthesized PAMC exhibited an excellent thermal stability at ordinary temperature.

3.1.4. ZP analysis of flocculants and kaolin suspension In practical engineering, the presence of the numerous electric charges distributed on the surface of pollutants is the primary reason for the suspension stability of colloidal particles in water. Charge properties of cationic flocculants greatly affect flocculation performance. Therefore, the ZP of PAMC and kaolin suspension at various pH levels was investigated and the results are illustrated in Fig. 4. As shown in Fig. 4, the ZP of all PAMC samples first

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Fig. 3. Comparison of (a) TG and (b) DTA of PAM and PAMC.

3.2. Flocculation properties

Fig. 4. Zeta potential of flocculants, and kaolin suspension at various pH

increased and then decreased with increased pH from 2.0 to 11. All flocculants were obviously electropositive as pH < 9.0, whereas the ZP near zero was observed at approximately pH 9.0. As displayed in Table 1, the PIs for PAMC1-4 emerged at pH levels of 8.0, 8.8, 9.2, and 9.4, respectively. The maximum ZP appeared at pH level of 4.0–5.0 because of the characteristics of cationic polyelectrolyte. Under acidic conditions, the ionization of quaternary ammonium salts in DMC was heavily promoted by numerous hydrogen ions, and amide groups ANH2 in AM was converted to ANH+3 through protonation [22]. Thus, the electropositivity of these polymers aqueous solution was increased, showing higher ZP. By contrast, deprotonation of ANH+3 and hydrolysis of quaternary ammonium salts would be promoted under alkaline conditions, resulting in reduced positive charges and ZP [23]. Moreover, as shown in Fig. 4, the ZP of PAMC at the same pH increases from PAMC1 to PAMC4 because of the increased DMC content. The results also provide an indirectly evidence to successfully copolymerize AM and DMC. In addition, Fig. 4 illustrates that the ZP of the kaolin suspension in this study decreased with increased pH from 2.0 to 12, and exhibited electronegativity within the entire pH measuring range. The negative ZP was smaller in magnitude for kaolin suspension, which was attributed to the surface charges formed by the isomorphic substitution of low-valency metal ions were neutralized by the pH-dependent positive edge charges [24].

3.2.1. Effect of dosage and pH The effect of PAMC dosage and pH of kaolin suspension on flocculation was investigated to evaluate the flocculation performance of PAMC. The results are presented in Fig. 5 and listed in the supplementary material Table S2. In addition, the images of water samples before and after flocculation were taken and are shown in Fig. S3. As shown in Fig. 5(a), under acidic environment (pH = 4.0), the transmittance of the supernatant with PAMC1-4 exhibited similar variation trend with increased dosage, but with much higher dosage than that using PAM. The transmittances with PAMC1-4 rapidly increased with dosage, and slowly decreased with further increase in dosage. Collision between kaolin particles was inadequate under lower dosage, and the stability between kaolin colloid particles was difficult to break, resulting in low removal efficiency [25]. Once stability was disrupted as dosage increased, kaolin particles would agglomerate immediately. However, excess dosage of PAMC would cause ‘‘cage effect”, preventing the growth of flocs and reducing settling efficiency [26]. Finally, the optimal dosage was 2.3 mg
L1 of PAMC1, 0.5 mg
L1 of PAMC2, 0.6 mg
L1 of PAMC3, and 0.3 mg
L1 of PAMC4 with maximum transmittance of 56.34%, 68.43%, 69.08%, and 87.96%, respectively. The optimal transmittance of 11.21% with PAM was achieved at dosage of 1.0 mg
L1. The results also suggested that the removal efficiency was improved significantly, and the optimal dosage decreased evidently as cationic monomer content in PAMC increased. A few differences emerged under neutral environment (pH = 7.0) as illustrated in Fig. 5(b). Compared with acidic conditions, despite the evident changes in the variation tendency in transmittance with increased dosage, the optimal dosage of PAMC increased obviously, and the corresponding transmittance decreased slightly. The corresponding optimal dosages of PAMC14 were 2.7 mg
L1, 2.3 mg
L1, 2 mg
L1, and 1 mg
L1, respectively. The result was attributed to the decrease in ZP of PAMC with increase in pH from 4.0 to 7.0 according to Fig. 4(a), indicating the decrease in positive charge density for PAMC. Thus, charge neutralization between flocculants and surface charge of kaolin colloid particles was reduced accordingly. However, the molecular chain of PAMC would be incompletely stretched in a neutral aqueous solution because of the electrostatic repulsion between positive electrocharges. This characteristic was not favorable for the adsorption of kaolin particles, resulting in decreased transmittance of supernatant [27]. Moreover, flocculant of PAMC4 still showed relatively excellent flocculation efficiency because of higher

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flocculants and negative kaolin particles and resulted in reduced of charge neutralization [27]. Thus, the formation of heavy and denser flocs was constrained during flocculation process. However, the molecular chains of flocculants could be sufficiently stretched because of this electrostatic repulsion. Moreover, the adsorption effect was improved accordingly and played a dominant role in alkaline environment. 3.2.2. Effect of initial concentration of kaolin suspension The effect of initial concentration (0.25–1.25 g
L1) of kaolin suspension was investigated. The more efficient PAMC4 was used in this process, and the initial pH of kaolin suspension was set to 4.0. The results are shown in Fig. 6. The transmittance of the supernatant rapidly increased with flocculants dosage before 0.5 mg
L1, and then slowly decreased with further increase in flocculant dosage. The optimal dosage for PAMC4 increased with increased initial concentration of kaolin suspension, and the corresponding transmittance of supernatant increased slightly. Higher initial concentration of kaolin suspensions indicates more electronegative colloid particles dispersed in water, and the dosage should be increased to neutralize these negative charges and break the stability of kaolin suspension [28]. Moreover, Fig. 6 also shows that the kaolin suspension with higher concentration was easier to purify at higher flocculant dosage. The kaolin particles with lower concentration could be easily wrapped by excess dosage of flocculants and caused cage effect. This phenomenon would prevent the further agglomeration of kaolin particles with flocculants, resulting in deceased transmittance [26]. By contrast, the cage effect could be weakened at higher concentration because of more kaolin particles, and floc can continue to grow during flocculation.

Fig. 5. Transmittance of supernatant after flocculation at (a) pH = 4.0, (b) pH = 7.0, and (c) pH = 10.

cationic monomer DMC content, and almost no obvious removal efficiency for kaolin suspension was observed. As presented in Fig. 5(c), worse flocculation efficiency was observed under alkaline environment (pH = 10) at various dosages of flocculants. Under this condition, 3.0 mg
L1of PAMC1, 3.1 mg
L1 of PAMC2, 3.0 mg
L1of PAMC3, and 0.6 mg
L1of PAMC4 was necessary to achieve optimal transmittances of 39.42%, 46.25%, 53.41%, and 69.71%, respectively. According to the Fig. 4(a), the decrease in removal efficiency was due to the increased negative charges in the flocculants, while pH was higher than the IPs of flocculants (negative ACOO generated from the hydrolysis of acylamino group ACONH2). This phenomenon caused the steric hindrance and electrostatic repulsion between

3.2.3. Effect of stirring speed and time During flocculation, rapid stirring was used first to uniformly distribute flocculants in water, whereas slow stirring was employed to promote the contact and collision of flocculants and kaolin particles. This phenomenon accelerates the generation of agglomerates and the growth of flocs. Therefore, slow stirring was an important stage in flocculation, and the effect of slow stirring speed and stirring time was investigated in this study. The dosage of used PAMC4 was 0.3 mg
L1 and pH 4.0, and the obtained results are presented in Fig. 7(a) and (b). As shown in Fig. 7(a), the transmittance of supernatant was at a low level as the stirring speed was set to within 10–40 rpm. A significant improvement in transmittance appeared at stirring speed of 50 rpm and further slowly increased with increase in stirring speed at the range of 50–160 rpm. In this case, the probability of contact and collision between PAMC and kaolin particles was promoted, resulting in complete destabilization of kaolin suspension [29]. However, the transmittance of supernatant was evidently reduced as stirring speed became higher than 160 rpm. The hydrodynamic shear stress was increased at high-speed stirring. The formed stable flocs may be disintegrated and destabilized through this hydrodynamic shear, resulting in reduced of settling property [28]. Thus, the stirring speed was determined as 120–150 rpm in experiments. As presented in Fig. 7(b), the transmittance of the supernatant sustained increased with the increase of stirring time within 5– 20 min. No evident increase in transmittance was observed with further increase in stirring time. The maximum transmittance of supernatant of 87.4% was achieved at stirring time of 15 min. Generally, insufficient stirring time may cause insufficient contact and collision between PAMC and kaolin particles, and the bridging effect sourced from long-chain flocculants was difficult to use in the agglomeration and growth of flocs [30]. By contrast, the generated flocs were easy to break into smaller particles at excess stirring time. A previous report indicated that the regenerated flocs

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Fig. 6. Effect of initial concentration of kaolin suspension on transmittance (flocculant of PAMC4 was used).

Fig. 7. Effect of (a) stirring speed, (b) stirring time, and (c) sedimentation time on transmittance (flocculant of PAMC4 was used).

with smaller particle size and looser structure were observed after fragmentation [31]. Thus, the growth rate of transmittance decreased with further increase in stirring time after 20 min. Finally, the stirring time of 15 min was selected for the optimal stirring time during flocculation.

3.2.4. Effect of sedimentation time Sedimentation time was studied to estimate the settling property of flocs. This parameter was critical in evaluation of flocculation performance. The flocculant dosage of PAMC4 was controlled to 0.3 mg
L1 and pH of 4.0. As shown in Fig. 7(c), the transmittance of supernatant achieved 84.52% in the initial 5 min of sedimentation process. The results suggest that the sedimentation of most flocs was completed in a short time, and the particle size and compaction degrees of the generated flocs exceeded the necessary conditions for gravity settling. The transmittance did not evidently vary as sedimentation time further increased from 5 min to 60 min. The transmittance slightly fluctuated in the range of 84–85%, and this fluctuation was ascribed to the slow growth and settlement of small particle flocs [30]. Finally, 40 min was selected as the optimal sedimentation time.

3.2.5. ZP analysis The ZP of supernatant after flocculation was investigated at various pH environments to better understand the flocculation process and mechanism of PAMC. The results are displayed in Fig. 8. As illustrated from Fig. 8(a)–(c), ZP increased with flocculant dosage under acidic and neutral environments, whereas ZP first increased and then decreased with increase in flocculant dosage under alkaline environment. The ZP of kaolin supernatant increased from -8 mV to 15 mV with the increase in PAMC dosage from 0 mg
L1 to 3.0 mg
L1 at pH 4.0, whereas ZP increased from 23 mV to 7 mV with increased PAMC dosage at pH 7.0. The maximum ZP of kaolin supernatant was observed at pH of 10 and PAMC dosage of 0.5 mg
L1. The increase in ZP was due to the fact that the electronegative charges on the surface of kaolin particles were neutralized by the added electropositive PAMC. The decrease in ZP under alkaline dosage of more than 1 mg
L1 was attributed to the deprotonation of amide group and hydrolysis of quaternary ammonium salts [32], leading to increased negative electrocharges. Moreover, Fig. 8(a) shows that ZP was close to zero at pH 4.0 with optimal PAMC dosage, suggesting that charge neutralization played a dominant role in flocculation under acidic environment [10]. By contrast, Fig. 8(c) shows that the maximum ZP of 18.3 mV was achieved at pH 10, which was much less than

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141

Fig. 8. ZP of supernatant after flocculation at (a) pH = 4.0, (b) pH = 7.0, (c) pH = 10.

zero and showed intensive electronegativity. The result indicated that compared with charge neutralization, adsorption and bridging effect exerted a primary role in flocculation process under alkaline environment.

(s1) is the kinetic constant for particle collisions and aggregation. In addition, the relationship between concentration of kaolin suspension and transmittance of supernatant is exhibited as Eq. (2).

3.3. Kinetic investigation

where T0 (%) is the transmittance of initial kaolin suspension, and Tt (%) is the transmittance of supernatant at time t (s). therefore, the functional relationship between N0/Nt and t can be linear fitted based on original date in Fig. 9(a). The fitting results are presented in Fig. 9(b). The theoretically simulated curves were fitted based on Eqs. (1) and (2), and the results are presented in Fig. 9. The error bars in Fig. 9 were not obvious because of small errors in the transmittance measurement. As illustrated in Fig. 9(a), the transmittances of supernatant at different PAMC dosages gradually increased and then remained generally stable with increased flocculation time. This result indicated that an appropriate flocculation time was required for the generation and continued growth of flocs [35]. The transmittances at lower dosage of 0.2 mg
L1 and higher dosage of 1 mg
L1 were less than those at the optimal dosage (0.3–0.6 mg
L1). This phenomenon was probably because the carried positive charges on PAMC were not sufficient to completely neutralize the negative charges on the surface of kaolin particles at the low dosage, and excessive dosage resulted in cage effect

Furthermore, in order to better understand the flocculation process, flocculation kinetic of PAMC was investigated at various flocculant dosages. Flocculant sample of PAMC4 was selected as representative cationic polyacrylamide for kinetic research in this work. Predetermined amount of flocculant was added into kaolin suspension, followed by rapid stirring at 200 rpm for different times and no stirring for 20 min for settling. The supernatant of these different samples was collected for transmittance measurement and kinetic analysis. The results are illustrated in Fig. 9(a). A flocculation kinetics model of the particles collisions was employed in present work. According to the literature [33–34], the flocculation process was mostly considered as a bimolecular reaction, which was expressed as Eq. (1).

ðN0 =Nt Þ1=2 ¼ 1 þ 0:5  kN 0 t

ð1Þ

where N0 is the initial concentration of kaolin particles, Nt is the concentration of kaolin particles with respect to time t (s), and k

N0 =Nt ¼ ð100  T 0 Þ=ð100  T t Þ

ð2Þ

Fig. 9. The result of (a) Transmittance variation with flocculation time under different PAMC dosage and (b) (N0/Nt)1/2 as a function of setting time at various flocculation time.

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J. Ma et al. / Separation and Purification Technology 182 (2017) 134–143

Fig. 10. Flocculation mechanism of kaolin suspension with PAMC.

Table 2 Simulated results of flocculation kinetics for PAMC with various dosages according to Fig. 9(b). Flocculant dosage

0.2 mg
L1 4

KN0 (10 PAMC

9.08 ± 0.02

0.6 mg
L1 1

s

)

R

2

0.750

and electrostatic repulsion. Thus, the transmittance of supernatant decreased in both cases. According to Eq. (1), kinetic constants of particle aggregation (K) can be calculated using the slope of the fitting curve between (N0/ Nt)1/2 and flocculation time t, as shown in Fig. 9(b). The results of slope KNi are presented in Table 2, and N0 is a fixed value for all the same kaolin suspensions in this section. Table 2 shows that the maximum KN0 of 13.15  104 s1 was found at optimal dosage of 0.6 mg
L1. The result suggested that collision between particles was effective at the optimal dosage, and lower and higher dosages could cause the decrease of K [33]. In the case of low dosage, the interaction between PAMC and kaolin particles was very weak because of the low positive charge density in the solution system, resulting in lower K (9.08  104 s1). The enhancement in K with increased PAMC would be ascribed to more positive charges and adsorption sites on the flocculants. This characteristic is beneficial to accelerate contact and collision. However, overdose of PAMC caused steric and static repulsion and reduced the number of effective junction points, leading to decreased K (12.42  104 s1) [36]. The competition between adsorption of PAMC and particle bridging effect also increased in flocculation, which could affect the properties of flocs. 3.4. Flocculation mechanism According to the analysis above, a possible flocculation mechanism with PAMC was assumed and illustrated in Fig. 10. Overall, three different principals were found may be applicable to acidic, neutral, and alkaline environments, respeictively. As shown in Fig. 10, in acidic environment, the stabilization of kaolin suspen-

KN0 (10

4

1 mg
L1 s

1

)

13.15 ± 0.04

R

KN0 (104 s1)

R2

0.921

12.42 ± 0.03

0.993

2

sion was destroyed by PAMC with higher positive charges through charge neutralization and numerous legible small flocculating particles generated in the water. These particles were further agglomerated with regional static electricity and long molecular chain of PAMC through patching and bridging effect, forming heavier and denser flocs. In neutral environment, charge neutralization in flocculation was weakened due to the partially decrease of positive charges of PAMC, and adsorption also played a certain role in this process. In alkaline environment, charge neutralization was eliminated by the hydrolysis of cationic quaternary ammonium groups. Instead, adhesion and adsorption through van der Waals force played a critical role in contact and collision process, generating smaller and finer precipitates with decrease of floc strength. Finally, looser and lighter flocs formed through bridging effect in agglomeration period. 4. Conclusions In this study, the flocculants of PAMC with different cationic monomer contents were synthesized through low-pressure initiation and were characterized through FTIR, 1H NMR, TG/DTA and ZP analysis. The performance of PAMC in the flocculation of high turbid kaolin suspension, and the flocculation kinetic and mechanism were systematically investigated. Evaluation of flocculation performance showed that PAMC had much higher turbidity removal efficiency under acidic conditions, and their optimal dosage decreased with increased cationic monomer content. Stirring speed of 120 rpm, stirring time of 15 min, and sedimentation time of 40 were selected as the optimum conditions for flocculation, and the maximum transmittance of supernatant at

J. Ma et al. / Separation and Purification Technology 182 (2017) 134–143

85%-90% was achieved at pH 4.0 and PAMC4 dosage of 0.3 mg
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