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Drinking water treatment by stepwise flocculation using polysilicate aluminum magnesium and cationic polyacrylamide
Journal of Environmental Chemical Engineering 7 (2019) 103049
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Drinking water treatment by stepwise flocculation using polysilicate aluminum magnesium and cationic polyacrylamide ⁎
Jie Ma, Runnan Wang, Xiyue Wang, Hao Zhang, Bo Zhu, Lili Lian , Dawei Lou
T
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Department of Analytical Chemistry, Jilin Institute of Chemical Technology, No. 45 Chengde Street, Jilin, 132022, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flocculation performance Composite flocculant Flocculation mechanism Charge neutralization
In the current work, polysilicate aluminum magnesium (PSAM) and cationic polyacrylamide (cPAM) were stepwise used for drinking water treatment. The flocculant was characterized by fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. PSAM-cPAM was then applied to flocculate the simulated water sample. The simulated water sample was prepared by kaolin and sodium humate. The efficiency of flocculation treatment was highly dependent on the flocculation conditions. Important parameters, such as the pH value of simulated water sample, temperature, settling time, the total dosage of flocculant, and flocculant types, were optimized to ensure an effective flocculation performance. Under optimized conditions, the removal efficiency (more than 98%) was outstanding in terms of turbidity and color removal. In addition, charge neutralization, bridging effect, and co–precipitation interaction could play crucial roles in the flocculation process. Composite flocculants consisting of PSAM and cPAM were found to be environmentally friendly, highly efficient, and fast settling in drinking water treatment.
1. Introduction Flocculation, which can remove suspended colloidal particles and various dissolved contaminants from water bodies for an efficient solid–liquid separation and drinking water purification, is an essential process in drinking water treatment [1–4]. The final flocculation performance is largely dependent on the used flocculants. Flocculants are mainly divided into three distinct classes: inorganic coagulants, synthetic organic polymeric flocculants, and composite flocculants. Traditional flocculants, such as polyaluminum chloride (PAC) or its polymer, are the most widely used inorganic coagulants in water treatment plants due to their low price, low toxicity, and abundance. However, these flocculants have shortcomings, such as the formation of small flocs, the requirement of high dosages, common removal efficiencies, and easily affected by water quality and pH conditions [5–7]. The synthetic organic polymeric flocculants, such as polyethyleneimine, polyvinylpyridinium salt, and polyacrylamide and its derivatives, have been reported in the flocculation treatment [8,9]. However, the usage of synthetic organic coagulants could lead to secondary pollution in the drinking water thereby increasing the serious health risks for human beings [10–12]. In recent years, composite flocculants have received close attentions in the field of drinking water treatment due to their high removal
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efficiency and outstanding application prospects [13]. Liu et al., prepared an inorganic–inorganic composite flocculant by adding metal magnesium salt and silicate into the traditional PAC [14]. The addition of metal magnesium can remove impurities and contaminants to improve the turbidity and color removal efficiency greatly. Moreover, the addition of silicon can promote the formation of porous structures to enhance the aggregating efficiency and to improve the effects of flocculation [15–17]. Qiu et al., incorporated ferric and silicate into the traditional PAC to obtain a new type of high–efficiency aluminum ferric silicate flocculant [18,19]. Flocculants consisting of polysilicate zinc sulfate and polysilicate titanium sulfate have also been studied [20,21]. However, high dosages of inorganic flocculants are often required for the efficient flocculation of inorganic–inorganic composite flocculants. At present, inorganic–natural organic polymeric composite flocculants are extensively reported [22]. Peng et al., presented polysilicate aluminum ferric–starch composite flocculant. The introduction of cationic starch into polysilicate aluminum flocculant through charge neutralization and net trapping interaction could improve the removal efficiency [23]. The composite flocculant consisting of chitosan and inorganic flocculants showed high removal efficiency in a kaolin and sodium humate suspension [24]. Composite flocculants, including silicon–aluminum–iron–starch and aluminum–ferrous–starch, could also achieve high removal efficiency [25,26]. However, the price of natural
Corresponding authors. E-mail addresses: [email protected] (L. Lian), [email protected] (D. Lou).
https://doi.org/10.1016/j.jece.2019.103049 Received 30 January 2019; Received in revised form 22 March 2019; Accepted 23 March 2019 Available online 29 March 2019 2213-3437/ © 2019 Published by Elsevier Ltd.
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light blue polysilicic acid (PSi) solution. Then, 0.4 mol/L of Al2(SO4)3·18H2O solution (7.4 mL) and 0.8 mol/L of MgSO4·7H2O solution (0.8 mL) were added to the prepared PSi solution. Finally, the mixture was stirred at 40 °C for 2 h and aged at room temperature for 12 h. Thus, PSAM was successfully prepared.
macromolecule limits its application. Currently, a few types of inorganic–synthetic organic polymeric composite flocculant have been developed for drinking water treatment. A new inorganic–organic composite coagulant consisting of polyferric sulfate (PFS) and polyacrylamide (PAM) was prepared via composite copolymerization [11,27,28]. The results of flocculation experiments in high–turbidity kaolin and sodium humate suspension showed that the novel composite reagent exhibits better flocculation performance in turbidity and color removal than simple PFS and PAM. However, the residual acrylamide (AM) is neurotoxic which limits its application. In view of the aforementioned aspects, the prepared cationic polyacrylamide (cPAM) and polysilicate aluminum magnesium (PSAM) were used to stepwise flocculation drinking water. More importantly, in the flocculation procedure, the positive cPAM was first added into the simulated water sample to adsorb the negative impurities through charge neutralization. Then, negative PSAM was added into the suspension to remove the impurities via bridging effect and interacting with residual cPAM further via charge neutralization. Finally, the impurities were settled by co–precipitation interaction. This stepwise flocculation process is crucial for successfully improving the flocculant performance and completely removing the residual organic flocculant as well as ultimately avoiding the possible second pollution in the drinking water sample.
2.3.2. Preparation of cPAM solution cPAM (0.5 g) was dissolved in 100 mL of deionized water. The mixture was stirred at 50 °C for 1.5 h and aged at room temperature for 6 h. Subsequently, the solution was vigorously stirred for approximately 1.5 h to guarantee its homogeneity. Thus, cPAM is obtained. 2.3.3. Preparation of the simulated water sample Inorganic suspended colloids and water-soluble organic contaminants mainly exist in natural water bodies, most of which are negatively charged [30]. Hence, the simulated water sample was prepared by adding 0.1 g of kaolin and 0.025 g of sodium humate into 7500 mL of tap water. Then, the mixture was vigorously mixed at the room temperature and the turbidity and color were close to the actual water sample. 2.3.4. Jar tests To evaluate the flocculation performance of the prepared flocculants, a series of standard jar tests were conducted at room temperature with a flocculation apparatus. In the flocculation process, jar tests were conducted using six 1.0 L jars that were operated synchronously, and each beaker was filled with 1.0 L of simulated water sample. First, the received cPAM was added into the water sample under rapid stirring for 2 min. Subsequently, PSAM was added into the water sample. The jar tests were designed according to the previous works with some modification [12]. The synthetic solution was stirred rapidly at 200 rpm for 3 min followed by slow stirring at 100 rpm for 15 min. Then, slow stirring at a constant speed of 60 rpm was performed for 5 min, followed by stirring at 20 rpm for 5 min. Finally, the solution was allowed to settle freely for a desired time (5, 10, 15, 20, 25 and 30 min). Subsequently, the supernatant at the depth of 1–2 cm below the liquid surface was extracted for turbidity and color measurement. After flocculation, the supernatants were removed to calculate the turbidity and color removal percentages using Eqs. (1) and (2).
2. Experimental 2.1. Chemicals and reagents Sodium silicate (Na2SiO3·9H2O), magnesium sulfate anhydrous (MgSO4·7H2O), sodium hydroxide (NaOH), concentrated sulfuric acid (20% H2SO4), and potassium bromide (KBr) were purchased from the Yongda Chemical Reagent Factory (Tianjin, China). Aluminum sulfate (Al2(SO4)3·18H2O) was obtained from the Damao Chemical Reagent Factory (Tianjin, China). cPAM was supplied by Chi Long Water Purification (Tianjin, China). Superfine kaolin (Al2Si2O9H4), humic acid sodium salt (C9H8Na2O4) were provided by Shanghai Aladdin Chemistry Co., Ltd. (Shanghai, China). All of these reagents were of analytical grade. 2.2. Instrumentation and conditions Interspec 200–X Fourier transform infrared spectrometer (FTIR) (Tianjin Tuopu Instrument Co., Ltd, China) was used for analyzing physical and chemical structures. Ultrapure water, purified with a Milli–Q Advantage A10 (Millipore, Milford, MA, USA), was utilized to prepare all solutions. Scanning electron microscopy (SEM) (JEOL, JSM–7500 F, Japan) was applied to collect the images of PSAM, cPAM, and flocs. The elemental analysis of compounds was performed via energy dispersive spectroscopy (X-Max, Oxford). The turbidity of the supernatant of the water sample after flocculation and sedimentation was measured by using optoelectronic turbidimeter (Shanghai Jinjia Scientific Instrument Co., Ltd, China). The color of the supernatant was determined by using Lovibond PFX995 colorimeter (Shanghai Minyi Electronics Co., Ltd, China). The used jar test was MY3000–6 type sixshaft agitator (Wuhan Meiyu Instrument Co., Ltd, China). The surface charges were tested using zeta–potential analyzer (Malvern, Zetasizer Nano ZS90, UK).
Turbidity Removal (%) = (To − T )/ To × 100%
(1)
Color Removal (%) = (Ho − H )/Ho × 100%
(2)
where T0 and T are the turbidity values before and after flocculation, respectively. H0 and H represent the chromaticity values in the aqueous solution before and after flocculation, respectively. 3. Results and discussion 3.1. Characterization of flocculants 3.1.1. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) In order to observe the morphologies of PSAM, cPAM, and flocs, scanning electron microscopy images of PSAM, cPAM flocculants and flocs are attained (Fig. 1). Fig. 1(a) showed that the granule surface of PSAM was smooth with an amorphous structure and high mean porosity and low density. Fig. 1(b) displayed that the cPAM surface was smooth and flat. It was noteworthy that the flocs consisting of PSAM and cPAM flocculant sediments exhibits a coarse structure with an irregular porous surface and flocculant pore tightening observed in Fig. 1(c). In that condition, the negative particles were first adsorbed on the small positive cPAM flocs, and then, the small flocs was further covered by cPAM and PSAM flocculants and formed large flocs [23]. Due to the electrostatic interaction between cPAM and PSAM, the large pores on the flocs were tightened.
2.3. Procedures 2.3.1. Preparation of PSAM PSAM was prepared in accordance with previous study with some modifications [29]. Sodium silicate (10 g) was added into 250 mL of deionized water. The pH of the solution was adjusted to 2 by using 20% H2SO4 solution. The mixture was stirred at 40 °C for 2 h. Then, the solution was allowed to stand for 12 h at room temperature to obtain a 2
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Fig. 1. SEM images of PSAM (a), cPAM (b), and flocs of PSAM and cPAM (c).
Fig. 2. EDS of polysilicate aluminum magnesium PSAM (a) and the flocs of PSAM and cPAM (b).
The distribution of all measured elements based on total weight percentages of PSAM flocculant and flocs were also evaluated through EDS analysis (Fig. 2). O, Al, Mg, Si and S were found to be the most important elements in the PSAM flocculant (Fig. 2(a)). For the S elements, the addition of Al2(SO4)3·18H2O and MgSO4·7H2O were existed in solution in the form of SO42−. Due to the Al3+ on the flocculant surface could further react with SO42− that remaining in the solution. Hence, a certain amount of S elements was detected. While N and the large amount of C (15.04%) were found in the flocs from the sediments of PSAM and cPAM flocculants (Fig. 2(b)). In addition, Al increased from 3.38 % to 13.16 %, Si increased from 6.77 % to 18.81% in the flocs. The comparison of flocs with PSAM flocculant indicated that PSAM and cPAM were co–precipitation in the flocculation process, which illustrated that the kaolin (Al2Si2O9H4) and sodium humate (C9H8Na2O4) in simulated water samples were adsorbed on the composite flocculant. Fig. 3. FTIR spectrums of cPAM (a); flocs of PSAM and cPAM (b); and PSAM (c).
3.1.2. Fourier transform infrared spectroscopy (FTIR) The FTIR spectra of PSAM, cPAM, and flocs are shown in Fig. 3. In Fig. 3a (cPAM), the broad band at 3434.86 cm−1 was due to the stretching mode of the free–NH2 and aggregated –NH2 bond; the absorption peak at 1625.08 cm-1 represented the CeO stretching vibration in cPAM [31]. The stretching of–NH2, CeO peaks were attributed to asymmetrical stretching vibrations of−CONH groups from the main
chain of cPAM [11]. As shown in Fig. 2c (PSAM), the absorption peak at 612.2 cm-1 was attributed to the stretching vibration of Al–O–Si and Mg–O–Si bonds. While the peak around 1128.42 cm−1 was induced in 3
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Fig. 4. Effect of the simulated water pH. The flocculant total dosage was 40 mg/L (cPAM: 20 mg/L; PSAM: 20 mg/L), the settling time was 25 min and the temperature of the simulated water was approximately 25℃.
Fig. 5. Effect of the simulated water sample temperature. The pH of the simulated water was approximately 7.0, the settling time was 25 min and the total dosage of flocculant was 40 mg/L (cPAM: 20 mg/L; PSAM: 20 mg/L).
the Al−OH bond and the Mg−OH bond, indicating the hydroxyl bridging of polysilicate with Al and Mg. As seen in Fig. 2b, the absorption peaks at 1625.08 cm-1 and 1090.80 cm−1 were widened with the CeN and Si–O, Al–O and Mg–O stretching vibration, respectively. The presence of these peaks verified that flocs exhibited a composite hybrid structure of PSAM and cPAM [32].
3.2. Optimization of flocculation conditions 3.2.1. Effect of pH on the flocculation performance The effect of the simulated water pH is substantial in the flocculation because it influences the formation and growth of the flocs. The removal efficiency of the developed flocculants was investigated with the pH ranging from 3.0 to 11.0. Fig. 4 showed that the removal efficiency increased dramatically with the pH, as the solution pH increased from 3.0 to 6.0. The flocculants showed excellent removal efficiency in the pH ranging from 6.0 to 8.0 with the turbidity and color removal of higher than 95%. Furthermore, the turbidity and color removal efficiency could be higher than 98% when the pH = 7.0 in solution. Removal efficiency decreased with the further increase of pH ranging from 8.0 to 11.0. The removal efficiency was low under strong acidic conditions. This was due to the fact that the H+ would compete with the cation flocculant thereby weakening the electrostatic attraction between the positively charged cPAM and the negatively charged colloids in the water [23]. Under strong alkali conditions, the macromolecular structure of PSAM was destroyed. Accordingly, the removal efficiency decreases significantly. Under the pH ranging from 6.0 to 8.0, the positively charged cPAM can fully interact with the negatively charged PSAM and impurities. These results indicated that the developed flocculants were suitable for drinking water treatment under neutral, weak acid, and weak alkaline conditions.
Fig. 6. Effect of the settling time of flocs. The pH of the simulated water was approximately 7.0, the temperature of the simulated water was approximately 25℃ and the total dosage of flocculant was 40 mg/L (cPAM: 20 mg/L; PSAM: 20 mg/L).
3.2.3. Effect of the settling time of flocs During flocculation, settling adequate time is required to form large enough flocs to allow efficient removal. The effect of settling time on the removal efficiency was illustrated in Fig. 6. With the settling time ranging from 5 min to 25 min, the removal has a steep rising tendency [4]. The removal rate reached more than 98% within 25 min. Obviously, the results were similar after 25 min, precipitation had reached saturation, and the flocs had almost settled completely. Accordingly, 25 min was chosen as the settling time. 3.2.4. Effect of the total dosage of flocculant In general, inadequate or excess dosages of flocculants can lead to poor flocculation performance. Therefore, it is crucial to evaluate the effect of the total dosage of flocculant on the removal efficiency. Fig. 7 displayed that the removal efficiency is greatly enhanced when the total dosage of flocculant increased from 20 mg/L to 40 mg/L at a mass ratio of 1:1 of PSAM and cPAM. The relatively high the total dosage of flocculant could accelerate the growth rate of flocs and increased the removal efficiency. Consequently, the removal efficiency achieved a peak of 98.8% when the total dosage of flocculant was 40 mg/L. However, the removal efficiency decreased sharply when the total dosage of flocculant increased from 40 mg/L to 70 mg/L at a mass ratio of
3.2.2. Effect of the simulated water sample temperature The effect of temperature on the removal efficiency was also study. With the simulated water sample at temperature ranging from −5 °C to 30 °C, the removal efficiency presented an overall upward trend according to the results of Fig. 5, which due to fact that the flocculation process is endothermic reaction, and elevated temperature was conducive to the flocculation process [33]. At 25 °C – 30 °C, the removal efficiency could reach 98% with a stable status. Therefore, 25 °C was chosen. However, the flocculant was also effective at a low temperature. When the simulated water sample temperature decreased to −5 °C, the removal efficiency could be higher than 90% [2]. 4
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Fig. 7. Effect of the total dosage of flocculant. (The total dosage of flocculant at the mass ratio of 1:1 of PSAM and cPAM), the pH of the simulated water was approximately 7.0, the temperature of the simulated water was approximately 25℃ and the settling time was 25 min.
Fig. 10. Effect of various flocculants. The pH of the simulated water was approximately 7.0, the temperature of the simulated water was approximately 25℃, the settling time was 25 min and the total dosage of flocculant of PSAM, cPAM and PSAM-cPAM were 40 mg/L, 40 mg/L and 40 mg/L (cPAM: 20 mg/L; PSAM: 20 mg/L), respectively.
1:1 of PSAM and cPAM. This phenomenon may be due to excess dosage of flocculant agitated the sedimentation process, which caused either resuspension of the aggregated particles. Thus, color and turbidity of the treated water was greatly reduced.
3.3. Zeta potential (ZP) and flocculation mechanism ZP is an important factor in understanding the interaction of charges and the flocculation mechanism [34]. The ZP values of PSAM, cPAM, and the simulated water sample were measured at various pH values. The ZP values of the simulated water sample consisting of the kaolin and sodium humate suspension were negatively charged in a wide pH range of 3.0–11.0 based on the results of Fig. 8, with the ZP values between −14.8 mv and −46.8 mv, cPAM exhibited potent electropositivity in the pH range of 3.0–11.0, as a result of the protonation of the amino group and inhibition in the hydrolysis of quaternary ammonium groups. PSAM was negatively charged and the ZP values decreased from −1.35 mv to −29.6 mv, when the solution pH increased
Fig. 8. Zeta potential analysis of water sample, PSAM, and cPAM at pH values from 3.0 to11.0.
Fig. 9. PSAM–cPAM flocculation mechanism schematic diagram. 5
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Fig. 11. Changes in the Songhua River Water before (1a) and after flocculation using three different substances: PSAM-cPAM (2a), PSAM (3a), and cPAM (4a); Fig. 11b is a top view of Fig. 11a.
3.5. Flocculation performance in actual water sample
from 3.0 to 11.0. The possible flocculation mechanism in the water sample of kaolin and sodium humate suspension treatment by using flocculant was analyzed on the basis of the ZP investigation. Consequently, the possible flocculation mechanism schematic diagram was proposed accordingly as illustrated in Fig. 9. The simulated water sample was negative at various pH values in the ZP analysis. When adding positively charged cPAM flocculant, cPAM could interact with the negatively charged colloidal particles via charge neutralization, and small flocs were initially formed. Two minutes later, negatively charged PSAM was added into the reaction. PSAM is a macromolecular substance, which can increase the molecular weight of polymers and generate a strong bridging effect. Thus, PSAM interact with kaolin and sodium humate via strong bridging effect. The rings of PSAM can form in the water, the small flocs and kaolin and sodium humate attach to the PSAM, and the flocs become large and compact aggregates. PSAM could also react with residual cPAM through charge neutralization. Finally, during quiescent settling, co-precipitation gathered the flocs, which settled to form large flocs with a compact structure in the solution. Therefore, this stepwise drinking water treatment by flocculation can not only improve the removal efficiency but also prevent secondary pollution. Charge neutralization, bridging effect, and co-precipitation are the main flocculation mechanism. Those effects functioned in combination in the flocculation system.
The flocculation performance of the PSAM, cPAM, and PSAM–cPAM were also analyzed for the treatment of the actual water sample. The actual water sample was acquired from the Songhua River, Jilin City, China. The initial pH, temperature, turbidity, and color were 7.02, 25 °C, 17.86 NTU, and 60 Hazen, respectively. Fig. 11a showed the photographs of the flocculated water samples at the same settling time by PSAM, cPAM, and PSAM–cPAM, which indicated that the turbidity and color removal efficiencies for the river water could reach 98.4% and 98%, respectively, with the combination of PSAM and cPAM. However, the maximum turbidity and color removal efficiencies of PSAM were 80.6% and 75.4%, respectively, and those of cPAM were 82.6% and 76.7%, respectively. With regard to the combination of PSAM and cPAM, the composite flocculant had higher removal efficiency. The settling velocity of flocs can be also observed and analysed in Fig. 11b (top view of (Fig. 11a). Fig. 11(b, II) showed that after flocculation with the combination of PSAM and cPAM, large granules and thick sludge appeared in the water, and the flocs were closely packed, which led the remaining water to become clearer. However, after using single PSAM or cPAM flocculant at the same settling time (Fig. 11b) III, IV), the removal efficiency was lower, and flocs were smaller. The contrasting results directly indicated that the composite flocculant has a remarkable removal efficiency and fast settling velocity with large and dense flocs.
3.4. Comparison of the removal efficiencies of various flocculants 4. Conclusion PSAM, cPAM, and PSAM–cPAM were compared in terms of their removal efficiency in the simulated water sample (Fig. 10). Each of the three flocculants at identical dosages (PSAM : 40 mg/L; cPAM : 40 mg/ L; PSAM–cPAM : 40 mg/L) was added into 1.0 L of the simulated water sample at room temperature (25℃). Under optimal flocculation conditions, the removal efficiency of the composite flocculant PSAM–cPAM was dramatically enhanced compared with those of the single PSAM or cPAM with the turbidity and color removal efficiency of 98.8% and 98.4%, respectively.
An efficient, eco–friendly composite flocculant PSAM–cPAM was designed and successfully prepared for the treatment of drinking water via stepwise flocculation method, and it has proven to work reasonably well. The flocculation conditions were optimized by a single–factor variable test. The optimum conditions for turbidity and color removal efficiency were that the flocculant total dosage was 40 mg/L (cPAM: 20 mg/L ; PSAM: 20 mg/L); the pH of the simulated water sample was 7.0 at 25 ℃; and the settling time was 25 min. Under such conditions, the removal efficiencies were over 98% for turbidity and color respectively. Charge neutralization, bridging effect, and co–precipitation 6
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were the main flocculation mechanisms. Although cPAM have been reported as a flocculant used for drinking water treatment. Their potential risks limit its application. The residual cPAM issue have been resolved by using the combination of cPAM with PSAM in the field of drinking water treatment by stepwise flocculation. Thereby organic flocculants can be widely used in practical applications and provide a possibility. Furthermore, this composite flocculant realize the high turbidity and color removal to the kaolin and sodium humate suspension with less dosage. This greatly reduces the cost of flocculation. In summary, the current work is not only present a practically operational method for the design and application of composite flocculants but also significant in the optimization and understanding the flocculation mechanism, which has guiding significance for future application.
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Acknowledgments
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The present work is supported by the projects of National Natural Science Foundation of China (21605056), and Natural Science Foundation of Jilin Province (20180101292JC, 20170520032JH). Financial support from the Key Laboratory of Fine Chemicals of Jilin Province is also acknowledged.
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Drinking water treatment by stepwise flocculation using polysilicate aluminum magnesium and cationic polyacrylamide ⁎
Jie Ma, Runnan Wang, Xiyue Wang, Hao Zhang, Bo Zhu, Lili Lian , Dawei Lou
T
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Department of Analytical Chemistry, Jilin Institute of Chemical Technology, No. 45 Chengde Street, Jilin, 132022, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flocculation performance Composite flocculant Flocculation mechanism Charge neutralization
In the current work, polysilicate aluminum magnesium (PSAM) and cationic polyacrylamide (cPAM) were stepwise used for drinking water treatment. The flocculant was characterized by fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. PSAM-cPAM was then applied to flocculate the simulated water sample. The simulated water sample was prepared by kaolin and sodium humate. The efficiency of flocculation treatment was highly dependent on the flocculation conditions. Important parameters, such as the pH value of simulated water sample, temperature, settling time, the total dosage of flocculant, and flocculant types, were optimized to ensure an effective flocculation performance. Under optimized conditions, the removal efficiency (more than 98%) was outstanding in terms of turbidity and color removal. In addition, charge neutralization, bridging effect, and co–precipitation interaction could play crucial roles in the flocculation process. Composite flocculants consisting of PSAM and cPAM were found to be environmentally friendly, highly efficient, and fast settling in drinking water treatment.
1. Introduction Flocculation, which can remove suspended colloidal particles and various dissolved contaminants from water bodies for an efficient solid–liquid separation and drinking water purification, is an essential process in drinking water treatment [1–4]. The final flocculation performance is largely dependent on the used flocculants. Flocculants are mainly divided into three distinct classes: inorganic coagulants, synthetic organic polymeric flocculants, and composite flocculants. Traditional flocculants, such as polyaluminum chloride (PAC) or its polymer, are the most widely used inorganic coagulants in water treatment plants due to their low price, low toxicity, and abundance. However, these flocculants have shortcomings, such as the formation of small flocs, the requirement of high dosages, common removal efficiencies, and easily affected by water quality and pH conditions [5–7]. The synthetic organic polymeric flocculants, such as polyethyleneimine, polyvinylpyridinium salt, and polyacrylamide and its derivatives, have been reported in the flocculation treatment [8,9]. However, the usage of synthetic organic coagulants could lead to secondary pollution in the drinking water thereby increasing the serious health risks for human beings [10–12]. In recent years, composite flocculants have received close attentions in the field of drinking water treatment due to their high removal
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efficiency and outstanding application prospects [13]. Liu et al., prepared an inorganic–inorganic composite flocculant by adding metal magnesium salt and silicate into the traditional PAC [14]. The addition of metal magnesium can remove impurities and contaminants to improve the turbidity and color removal efficiency greatly. Moreover, the addition of silicon can promote the formation of porous structures to enhance the aggregating efficiency and to improve the effects of flocculation [15–17]. Qiu et al., incorporated ferric and silicate into the traditional PAC to obtain a new type of high–efficiency aluminum ferric silicate flocculant [18,19]. Flocculants consisting of polysilicate zinc sulfate and polysilicate titanium sulfate have also been studied [20,21]. However, high dosages of inorganic flocculants are often required for the efficient flocculation of inorganic–inorganic composite flocculants. At present, inorganic–natural organic polymeric composite flocculants are extensively reported [22]. Peng et al., presented polysilicate aluminum ferric–starch composite flocculant. The introduction of cationic starch into polysilicate aluminum flocculant through charge neutralization and net trapping interaction could improve the removal efficiency [23]. The composite flocculant consisting of chitosan and inorganic flocculants showed high removal efficiency in a kaolin and sodium humate suspension [24]. Composite flocculants, including silicon–aluminum–iron–starch and aluminum–ferrous–starch, could also achieve high removal efficiency [25,26]. However, the price of natural
Corresponding authors. E-mail addresses: [email protected] (L. Lian), [email protected] (D. Lou).
https://doi.org/10.1016/j.jece.2019.103049 Received 30 January 2019; Received in revised form 22 March 2019; Accepted 23 March 2019 Available online 29 March 2019 2213-3437/ © 2019 Published by Elsevier Ltd.
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light blue polysilicic acid (PSi) solution. Then, 0.4 mol/L of Al2(SO4)3·18H2O solution (7.4 mL) and 0.8 mol/L of MgSO4·7H2O solution (0.8 mL) were added to the prepared PSi solution. Finally, the mixture was stirred at 40 °C for 2 h and aged at room temperature for 12 h. Thus, PSAM was successfully prepared.
macromolecule limits its application. Currently, a few types of inorganic–synthetic organic polymeric composite flocculant have been developed for drinking water treatment. A new inorganic–organic composite coagulant consisting of polyferric sulfate (PFS) and polyacrylamide (PAM) was prepared via composite copolymerization [11,27,28]. The results of flocculation experiments in high–turbidity kaolin and sodium humate suspension showed that the novel composite reagent exhibits better flocculation performance in turbidity and color removal than simple PFS and PAM. However, the residual acrylamide (AM) is neurotoxic which limits its application. In view of the aforementioned aspects, the prepared cationic polyacrylamide (cPAM) and polysilicate aluminum magnesium (PSAM) were used to stepwise flocculation drinking water. More importantly, in the flocculation procedure, the positive cPAM was first added into the simulated water sample to adsorb the negative impurities through charge neutralization. Then, negative PSAM was added into the suspension to remove the impurities via bridging effect and interacting with residual cPAM further via charge neutralization. Finally, the impurities were settled by co–precipitation interaction. This stepwise flocculation process is crucial for successfully improving the flocculant performance and completely removing the residual organic flocculant as well as ultimately avoiding the possible second pollution in the drinking water sample.
2.3.2. Preparation of cPAM solution cPAM (0.5 g) was dissolved in 100 mL of deionized water. The mixture was stirred at 50 °C for 1.5 h and aged at room temperature for 6 h. Subsequently, the solution was vigorously stirred for approximately 1.5 h to guarantee its homogeneity. Thus, cPAM is obtained. 2.3.3. Preparation of the simulated water sample Inorganic suspended colloids and water-soluble organic contaminants mainly exist in natural water bodies, most of which are negatively charged [30]. Hence, the simulated water sample was prepared by adding 0.1 g of kaolin and 0.025 g of sodium humate into 7500 mL of tap water. Then, the mixture was vigorously mixed at the room temperature and the turbidity and color were close to the actual water sample. 2.3.4. Jar tests To evaluate the flocculation performance of the prepared flocculants, a series of standard jar tests were conducted at room temperature with a flocculation apparatus. In the flocculation process, jar tests were conducted using six 1.0 L jars that were operated synchronously, and each beaker was filled with 1.0 L of simulated water sample. First, the received cPAM was added into the water sample under rapid stirring for 2 min. Subsequently, PSAM was added into the water sample. The jar tests were designed according to the previous works with some modification [12]. The synthetic solution was stirred rapidly at 200 rpm for 3 min followed by slow stirring at 100 rpm for 15 min. Then, slow stirring at a constant speed of 60 rpm was performed for 5 min, followed by stirring at 20 rpm for 5 min. Finally, the solution was allowed to settle freely for a desired time (5, 10, 15, 20, 25 and 30 min). Subsequently, the supernatant at the depth of 1–2 cm below the liquid surface was extracted for turbidity and color measurement. After flocculation, the supernatants were removed to calculate the turbidity and color removal percentages using Eqs. (1) and (2).
2. Experimental 2.1. Chemicals and reagents Sodium silicate (Na2SiO3·9H2O), magnesium sulfate anhydrous (MgSO4·7H2O), sodium hydroxide (NaOH), concentrated sulfuric acid (20% H2SO4), and potassium bromide (KBr) were purchased from the Yongda Chemical Reagent Factory (Tianjin, China). Aluminum sulfate (Al2(SO4)3·18H2O) was obtained from the Damao Chemical Reagent Factory (Tianjin, China). cPAM was supplied by Chi Long Water Purification (Tianjin, China). Superfine kaolin (Al2Si2O9H4), humic acid sodium salt (C9H8Na2O4) were provided by Shanghai Aladdin Chemistry Co., Ltd. (Shanghai, China). All of these reagents were of analytical grade. 2.2. Instrumentation and conditions Interspec 200–X Fourier transform infrared spectrometer (FTIR) (Tianjin Tuopu Instrument Co., Ltd, China) was used for analyzing physical and chemical structures. Ultrapure water, purified with a Milli–Q Advantage A10 (Millipore, Milford, MA, USA), was utilized to prepare all solutions. Scanning electron microscopy (SEM) (JEOL, JSM–7500 F, Japan) was applied to collect the images of PSAM, cPAM, and flocs. The elemental analysis of compounds was performed via energy dispersive spectroscopy (X-Max, Oxford). The turbidity of the supernatant of the water sample after flocculation and sedimentation was measured by using optoelectronic turbidimeter (Shanghai Jinjia Scientific Instrument Co., Ltd, China). The color of the supernatant was determined by using Lovibond PFX995 colorimeter (Shanghai Minyi Electronics Co., Ltd, China). The used jar test was MY3000–6 type sixshaft agitator (Wuhan Meiyu Instrument Co., Ltd, China). The surface charges were tested using zeta–potential analyzer (Malvern, Zetasizer Nano ZS90, UK).
Turbidity Removal (%) = (To − T )/ To × 100%
(1)
Color Removal (%) = (Ho − H )/Ho × 100%
(2)
where T0 and T are the turbidity values before and after flocculation, respectively. H0 and H represent the chromaticity values in the aqueous solution before and after flocculation, respectively. 3. Results and discussion 3.1. Characterization of flocculants 3.1.1. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) In order to observe the morphologies of PSAM, cPAM, and flocs, scanning electron microscopy images of PSAM, cPAM flocculants and flocs are attained (Fig. 1). Fig. 1(a) showed that the granule surface of PSAM was smooth with an amorphous structure and high mean porosity and low density. Fig. 1(b) displayed that the cPAM surface was smooth and flat. It was noteworthy that the flocs consisting of PSAM and cPAM flocculant sediments exhibits a coarse structure with an irregular porous surface and flocculant pore tightening observed in Fig. 1(c). In that condition, the negative particles were first adsorbed on the small positive cPAM flocs, and then, the small flocs was further covered by cPAM and PSAM flocculants and formed large flocs [23]. Due to the electrostatic interaction between cPAM and PSAM, the large pores on the flocs were tightened.
2.3. Procedures 2.3.1. Preparation of PSAM PSAM was prepared in accordance with previous study with some modifications [29]. Sodium silicate (10 g) was added into 250 mL of deionized water. The pH of the solution was adjusted to 2 by using 20% H2SO4 solution. The mixture was stirred at 40 °C for 2 h. Then, the solution was allowed to stand for 12 h at room temperature to obtain a 2
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Fig. 1. SEM images of PSAM (a), cPAM (b), and flocs of PSAM and cPAM (c).
Fig. 2. EDS of polysilicate aluminum magnesium PSAM (a) and the flocs of PSAM and cPAM (b).
The distribution of all measured elements based on total weight percentages of PSAM flocculant and flocs were also evaluated through EDS analysis (Fig. 2). O, Al, Mg, Si and S were found to be the most important elements in the PSAM flocculant (Fig. 2(a)). For the S elements, the addition of Al2(SO4)3·18H2O and MgSO4·7H2O were existed in solution in the form of SO42−. Due to the Al3+ on the flocculant surface could further react with SO42− that remaining in the solution. Hence, a certain amount of S elements was detected. While N and the large amount of C (15.04%) were found in the flocs from the sediments of PSAM and cPAM flocculants (Fig. 2(b)). In addition, Al increased from 3.38 % to 13.16 %, Si increased from 6.77 % to 18.81% in the flocs. The comparison of flocs with PSAM flocculant indicated that PSAM and cPAM were co–precipitation in the flocculation process, which illustrated that the kaolin (Al2Si2O9H4) and sodium humate (C9H8Na2O4) in simulated water samples were adsorbed on the composite flocculant. Fig. 3. FTIR spectrums of cPAM (a); flocs of PSAM and cPAM (b); and PSAM (c).
3.1.2. Fourier transform infrared spectroscopy (FTIR) The FTIR spectra of PSAM, cPAM, and flocs are shown in Fig. 3. In Fig. 3a (cPAM), the broad band at 3434.86 cm−1 was due to the stretching mode of the free–NH2 and aggregated –NH2 bond; the absorption peak at 1625.08 cm-1 represented the CeO stretching vibration in cPAM [31]. The stretching of–NH2, CeO peaks were attributed to asymmetrical stretching vibrations of−CONH groups from the main
chain of cPAM [11]. As shown in Fig. 2c (PSAM), the absorption peak at 612.2 cm-1 was attributed to the stretching vibration of Al–O–Si and Mg–O–Si bonds. While the peak around 1128.42 cm−1 was induced in 3
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Fig. 4. Effect of the simulated water pH. The flocculant total dosage was 40 mg/L (cPAM: 20 mg/L; PSAM: 20 mg/L), the settling time was 25 min and the temperature of the simulated water was approximately 25℃.
Fig. 5. Effect of the simulated water sample temperature. The pH of the simulated water was approximately 7.0, the settling time was 25 min and the total dosage of flocculant was 40 mg/L (cPAM: 20 mg/L; PSAM: 20 mg/L).
the Al−OH bond and the Mg−OH bond, indicating the hydroxyl bridging of polysilicate with Al and Mg. As seen in Fig. 2b, the absorption peaks at 1625.08 cm-1 and 1090.80 cm−1 were widened with the CeN and Si–O, Al–O and Mg–O stretching vibration, respectively. The presence of these peaks verified that flocs exhibited a composite hybrid structure of PSAM and cPAM [32].
3.2. Optimization of flocculation conditions 3.2.1. Effect of pH on the flocculation performance The effect of the simulated water pH is substantial in the flocculation because it influences the formation and growth of the flocs. The removal efficiency of the developed flocculants was investigated with the pH ranging from 3.0 to 11.0. Fig. 4 showed that the removal efficiency increased dramatically with the pH, as the solution pH increased from 3.0 to 6.0. The flocculants showed excellent removal efficiency in the pH ranging from 6.0 to 8.0 with the turbidity and color removal of higher than 95%. Furthermore, the turbidity and color removal efficiency could be higher than 98% when the pH = 7.0 in solution. Removal efficiency decreased with the further increase of pH ranging from 8.0 to 11.0. The removal efficiency was low under strong acidic conditions. This was due to the fact that the H+ would compete with the cation flocculant thereby weakening the electrostatic attraction between the positively charged cPAM and the negatively charged colloids in the water [23]. Under strong alkali conditions, the macromolecular structure of PSAM was destroyed. Accordingly, the removal efficiency decreases significantly. Under the pH ranging from 6.0 to 8.0, the positively charged cPAM can fully interact with the negatively charged PSAM and impurities. These results indicated that the developed flocculants were suitable for drinking water treatment under neutral, weak acid, and weak alkaline conditions.
Fig. 6. Effect of the settling time of flocs. The pH of the simulated water was approximately 7.0, the temperature of the simulated water was approximately 25℃ and the total dosage of flocculant was 40 mg/L (cPAM: 20 mg/L; PSAM: 20 mg/L).
3.2.3. Effect of the settling time of flocs During flocculation, settling adequate time is required to form large enough flocs to allow efficient removal. The effect of settling time on the removal efficiency was illustrated in Fig. 6. With the settling time ranging from 5 min to 25 min, the removal has a steep rising tendency [4]. The removal rate reached more than 98% within 25 min. Obviously, the results were similar after 25 min, precipitation had reached saturation, and the flocs had almost settled completely. Accordingly, 25 min was chosen as the settling time. 3.2.4. Effect of the total dosage of flocculant In general, inadequate or excess dosages of flocculants can lead to poor flocculation performance. Therefore, it is crucial to evaluate the effect of the total dosage of flocculant on the removal efficiency. Fig. 7 displayed that the removal efficiency is greatly enhanced when the total dosage of flocculant increased from 20 mg/L to 40 mg/L at a mass ratio of 1:1 of PSAM and cPAM. The relatively high the total dosage of flocculant could accelerate the growth rate of flocs and increased the removal efficiency. Consequently, the removal efficiency achieved a peak of 98.8% when the total dosage of flocculant was 40 mg/L. However, the removal efficiency decreased sharply when the total dosage of flocculant increased from 40 mg/L to 70 mg/L at a mass ratio of
3.2.2. Effect of the simulated water sample temperature The effect of temperature on the removal efficiency was also study. With the simulated water sample at temperature ranging from −5 °C to 30 °C, the removal efficiency presented an overall upward trend according to the results of Fig. 5, which due to fact that the flocculation process is endothermic reaction, and elevated temperature was conducive to the flocculation process [33]. At 25 °C – 30 °C, the removal efficiency could reach 98% with a stable status. Therefore, 25 °C was chosen. However, the flocculant was also effective at a low temperature. When the simulated water sample temperature decreased to −5 °C, the removal efficiency could be higher than 90% [2]. 4
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Fig. 7. Effect of the total dosage of flocculant. (The total dosage of flocculant at the mass ratio of 1:1 of PSAM and cPAM), the pH of the simulated water was approximately 7.0, the temperature of the simulated water was approximately 25℃ and the settling time was 25 min.
Fig. 10. Effect of various flocculants. The pH of the simulated water was approximately 7.0, the temperature of the simulated water was approximately 25℃, the settling time was 25 min and the total dosage of flocculant of PSAM, cPAM and PSAM-cPAM were 40 mg/L, 40 mg/L and 40 mg/L (cPAM: 20 mg/L; PSAM: 20 mg/L), respectively.
1:1 of PSAM and cPAM. This phenomenon may be due to excess dosage of flocculant agitated the sedimentation process, which caused either resuspension of the aggregated particles. Thus, color and turbidity of the treated water was greatly reduced.
3.3. Zeta potential (ZP) and flocculation mechanism ZP is an important factor in understanding the interaction of charges and the flocculation mechanism [34]. The ZP values of PSAM, cPAM, and the simulated water sample were measured at various pH values. The ZP values of the simulated water sample consisting of the kaolin and sodium humate suspension were negatively charged in a wide pH range of 3.0–11.0 based on the results of Fig. 8, with the ZP values between −14.8 mv and −46.8 mv, cPAM exhibited potent electropositivity in the pH range of 3.0–11.0, as a result of the protonation of the amino group and inhibition in the hydrolysis of quaternary ammonium groups. PSAM was negatively charged and the ZP values decreased from −1.35 mv to −29.6 mv, when the solution pH increased
Fig. 8. Zeta potential analysis of water sample, PSAM, and cPAM at pH values from 3.0 to11.0.
Fig. 9. PSAM–cPAM flocculation mechanism schematic diagram. 5
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Fig. 11. Changes in the Songhua River Water before (1a) and after flocculation using three different substances: PSAM-cPAM (2a), PSAM (3a), and cPAM (4a); Fig. 11b is a top view of Fig. 11a.
3.5. Flocculation performance in actual water sample
from 3.0 to 11.0. The possible flocculation mechanism in the water sample of kaolin and sodium humate suspension treatment by using flocculant was analyzed on the basis of the ZP investigation. Consequently, the possible flocculation mechanism schematic diagram was proposed accordingly as illustrated in Fig. 9. The simulated water sample was negative at various pH values in the ZP analysis. When adding positively charged cPAM flocculant, cPAM could interact with the negatively charged colloidal particles via charge neutralization, and small flocs were initially formed. Two minutes later, negatively charged PSAM was added into the reaction. PSAM is a macromolecular substance, which can increase the molecular weight of polymers and generate a strong bridging effect. Thus, PSAM interact with kaolin and sodium humate via strong bridging effect. The rings of PSAM can form in the water, the small flocs and kaolin and sodium humate attach to the PSAM, and the flocs become large and compact aggregates. PSAM could also react with residual cPAM through charge neutralization. Finally, during quiescent settling, co-precipitation gathered the flocs, which settled to form large flocs with a compact structure in the solution. Therefore, this stepwise drinking water treatment by flocculation can not only improve the removal efficiency but also prevent secondary pollution. Charge neutralization, bridging effect, and co-precipitation are the main flocculation mechanism. Those effects functioned in combination in the flocculation system.
The flocculation performance of the PSAM, cPAM, and PSAM–cPAM were also analyzed for the treatment of the actual water sample. The actual water sample was acquired from the Songhua River, Jilin City, China. The initial pH, temperature, turbidity, and color were 7.02, 25 °C, 17.86 NTU, and 60 Hazen, respectively. Fig. 11a showed the photographs of the flocculated water samples at the same settling time by PSAM, cPAM, and PSAM–cPAM, which indicated that the turbidity and color removal efficiencies for the river water could reach 98.4% and 98%, respectively, with the combination of PSAM and cPAM. However, the maximum turbidity and color removal efficiencies of PSAM were 80.6% and 75.4%, respectively, and those of cPAM were 82.6% and 76.7%, respectively. With regard to the combination of PSAM and cPAM, the composite flocculant had higher removal efficiency. The settling velocity of flocs can be also observed and analysed in Fig. 11b (top view of (Fig. 11a). Fig. 11(b, II) showed that after flocculation with the combination of PSAM and cPAM, large granules and thick sludge appeared in the water, and the flocs were closely packed, which led the remaining water to become clearer. However, after using single PSAM or cPAM flocculant at the same settling time (Fig. 11b) III, IV), the removal efficiency was lower, and flocs were smaller. The contrasting results directly indicated that the composite flocculant has a remarkable removal efficiency and fast settling velocity with large and dense flocs.
3.4. Comparison of the removal efficiencies of various flocculants 4. Conclusion PSAM, cPAM, and PSAM–cPAM were compared in terms of their removal efficiency in the simulated water sample (Fig. 10). Each of the three flocculants at identical dosages (PSAM : 40 mg/L; cPAM : 40 mg/ L; PSAM–cPAM : 40 mg/L) was added into 1.0 L of the simulated water sample at room temperature (25℃). Under optimal flocculation conditions, the removal efficiency of the composite flocculant PSAM–cPAM was dramatically enhanced compared with those of the single PSAM or cPAM with the turbidity and color removal efficiency of 98.8% and 98.4%, respectively.
An efficient, eco–friendly composite flocculant PSAM–cPAM was designed and successfully prepared for the treatment of drinking water via stepwise flocculation method, and it has proven to work reasonably well. The flocculation conditions were optimized by a single–factor variable test. The optimum conditions for turbidity and color removal efficiency were that the flocculant total dosage was 40 mg/L (cPAM: 20 mg/L ; PSAM: 20 mg/L); the pH of the simulated water sample was 7.0 at 25 ℃; and the settling time was 25 min. Under such conditions, the removal efficiencies were over 98% for turbidity and color respectively. Charge neutralization, bridging effect, and co–precipitation 6
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were the main flocculation mechanisms. Although cPAM have been reported as a flocculant used for drinking water treatment. Their potential risks limit its application. The residual cPAM issue have been resolved by using the combination of cPAM with PSAM in the field of drinking water treatment by stepwise flocculation. Thereby organic flocculants can be widely used in practical applications and provide a possibility. Furthermore, this composite flocculant realize the high turbidity and color removal to the kaolin and sodium humate suspension with less dosage. This greatly reduces the cost of flocculation. In summary, the current work is not only present a practically operational method for the design and application of composite flocculants but also significant in the optimization and understanding the flocculation mechanism, which has guiding significance for future application.
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Acknowledgments
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The present work is supported by the projects of National Natural Science Foundation of China (21605056), and Natural Science Foundation of Jilin Province (20180101292JC, 20170520032JH). Financial support from the Key Laboratory of Fine Chemicals of Jilin Province is also acknowledged.
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