ARTICLE pubs.acs.org/IECR
Preparation of Strong Cationic Chitosan-graft-Polyacrylamide Flocculants and Their Flocculating Properties Yaobo Lu,† Yabo Shang,‡ Xin Huang,† Aimin Chen,‡ Zhen Yang,† Yuxiang Jiang,‡ Jun Cai,† Wei Gu,‡ Xiaozhi Qian,‡ Hu Yang,*,† and Rongshi Cheng† †
Key Lab for Mesoscopic Chemistry of MOE, Department of Polymer Science & Technology, School of Chemistry & Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Zhenjiang Water Supply Company, Zhenjiang 212009, People’s Republic of China
bS Supporting Information ABSTRACT: Recently, more attentions have been paid to natural polymer-based flocculants in water treatment, since they are believed to be low-cost, nontoxic, and environmentally friendly materials. In this work, strong cationic chitosan-based graft copolymer flocculants (3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CTA) modified chitosan-graft-polyacrylamide, denoted as chitosan-CTA-g-PAM) have been prepared, and their flocculating properties were studied systematically, both at the laboratory scale and at the pilot scale. In laboratory scale, a kaolin suspension was employed as synthetic wastewater. The effects of dosage, temperature, and original turbidity of untreated wastewater were investigated, respectively. Moreover, in pilot scale, the raw water from the Zhenjiang part of the Yangtse River in China was used as wastewater. The influences of three external factors— dosage, sedimentation time, and mechanical mixing rate—on the flocculating performances have been studied by orthogonal testing, respectively. The experimental results in pilot scale were fairly consistent with those from the beaker experiment in laboratory scale. In comparison with polyferric sulfate, which is the flocculant currently used by the Zhenjiang Water Supply Company, chitosan-CTA-g-PAM showed better flocculating properties. Meanwhile, the effect of the degree of substitution of CTA on the flocculating properties has been also studied, both at the laboratory scale and at the pilot scale. It was indicated that chitosanCTA-g-PAM with higher cationic degree had better flocculating performances, which was also confirmed by the flocculation kinetics analysis. Furthermore, the flocculation mechanisms of chitosan-CTA-g-PAM have been discussed in detail. Above all, the flocculating experiments in laboratory and pilot scales both indicated that chitosan-CTA-g-PAM showed good flocculating performances in water treatment.
1. INTRODUCTION Today, water pollution has become a serious problem that is threatening the survival of human beings, plants, and animals.1 It is urgent that some strong measures should be adopted to deal with this problem. Therefore, more attention has been given to research on wastewater treatment in recent years. Various technologies have been employed, such as flocculation, adsorption, filtration, and evaporation.25 Among them, flocculation is one of the most important processes in the primary treatment for water. In comparison with traditional flocculants, such as inorganic polymeric flocculants and synthetic polymer ones, natural polymer-based flocculants have received much more attention in water treatment, since they are believed to be highly efficient, relatively inexpensive, biodegradable, and environmentally friendly materials.610 Natural polymer-based flocculants have been even acclaimed as “Green Flocculants of the 21st Century”.6 Among them, Chitosan (poly-β-(1f4)-2-amino-2deoxy-D-glucose) is one of the high-performance natural polysaccharide materials. It is derived from the deacetylation of natural chitin, which is the second-most-abundant natural polymer in the world. Chitosan contains abundant free amino groups along the chain backbone, which are cationically charged in acidic media. Because of its novel characteristics, chitosan has a prominent flocculating effect in water treatment.11,12 r 2011 American Chemical Society
However, some disadvantages of chitosan, such as poor solubility in water under neutral or alkaline conditions and low molecular weight, limit its applications in practice. To improve its performance, much effort has been made to develop suitable procedures for the preparation of water-soluble chitosan derivatives.1317 Among various methods, graft polymerization is a conventional and useful method,18 and this reaction would introduce some synthetic functional polymers as side chains to the backbone of chitosan. In addition, polyacrylamide (PAM) is a type of water-soluble polymer and has exhibited high flocculating efficiency in water treatment.19,20 Obviously, grafting PAM onto chitosan would improve the properties and increases the potential applications of chitosan.2124 In our previous work,23,24 it was found that chitosan-graft-polyacrylamide (chitosan-g-PAM) flocculants had remarkably increased solubility, because of the fact that the ordered structure of chitosan is destroyed by the grafting chain. Furthermore, the long-side PAM chain was beneficial to bridging flocculation and improved the flocculating performance. Received: January 12, 2011 Accepted: May 11, 2011 Revised: March 29, 2011 Published: May 11, 2011 7141
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Scheme 1. Synthesis Process of Chitosan-CTA-g-PAM
However, most suspensions have negatively charged surfaces, and flocculants with cationic groups are more favorable to aggregate and settle down these colloids in water for the effect of charge neutralization flocculation. As for chitosan-g-PAM, it is profitable to import the cationic groups to chitosan-g-PAM for further improvement of the flocculating performances. Wherein, quaternization is one of the simple and efficient ways to improve the positive charges of polymers.25,26 In this work, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CTA), which is a type of nontoxic quaternary ammonium reagent, was introduced to the chitosan-g-PAM chain, and a series of strong cationic chitosan-g-PAM flocculants (chitosan-CTA-g-PAM) was successfully prepared. The flocculating properties of chitosan-CTA-g-PAM have been investigated at both laboratory and pilot scales systematically. At laboratory scale, a kaolin suspension was employed as synthetic wastewater, and the effects of dosage, temperature, and original turbidity of untreated wastewater were investigated, respectively. The flocculation results were consistent with those obtained at pilot scale using the raw water from the Zhenjiang part of the Yangtse River in China as synthetic wastewater. In addition, the effects of structure factors, such as the degree of substitution of CTA, on the flocculating properties have been studied and discussed in detail, based on the flocculation mechanisms (charge neutralization and bridging flocculating mechanism).6,27 The kinetics of flocculation process has been also investigated primarily.
2. EXPERIMENTAL SECTION 2.1. Materials. Chitosan was purchased from Shangdong Aokang Biological Co., Ltd., China; its degree of deacetylation was 85.2%, and the viscosity average molecular weight was 83.4 104 g/mol.28 Acrylamide (CP grade, from Nanjing Chemical Reagent Co., China), ceric ammonium nitrate (AR grade, from Sinopharm Chemical Reagent Co., China), and 3-chloro-2hydroxypropyl trimethyl ammonium chloride (CTA) (CP grade, from Wuhan Yuancheng Technology Development, Ltd., China) were all used without further treatment. Polyferric sulfate ([Fe2(OH)n(SO4)30.5n], n < 2) was kindly supplied by the Zhenjiang Taibaifen Factory. All other chemicals were purchased from Nanjing Chemical Reagent Co., China. 2.2. Preparation of Chitosan-CTA-g-PAM. Chitosan was used as the backbone for graft copolymerization. The desired amount of solid chitosan powder was dissolved in 200 mL of a 1% acetic acid aqueous solution via agitation. After 30 min of stirring under N2, the Ce(IV) initiator and an acrylamide monomer were added to the solution. The mass ratio of chitosan and acrylamide (AM) was 1:5. After 3 h for reaction, the polymerization was stopped and chitosan-gPAM was precipitated in acetone. The white product was purified
three times by repeated dissolvingprecipitating treatment, then further by Soxhlet extraction using acetone as solvent, and finally dried at 50 °C in a vacuum oven for 48 h. The chitosan-g-PAM has been well-prepared. The more-detailed experimental process was described in our previous work.23 The grafting ratio of AM was 286%, calculated from the specific refractive index increment measurement method.23 The desired amount of chitosan-g-PAM then was added into isopropanol and a 10 wt % NaOH blending aqueous solution. The mixture was heated to 45 °C in a water bath under continuous stirring (75 rpm) and alkalized for 1 h until the solution became a thick liquid. CTA then was added into the mixture. The reaction was kept at 60 °C for 10 h under agitation. The solid product was filtered and rinsed in 90% alcohol for removal of salt and water, then by Soxhlet extraction using acetone as solvent for further treatment, and finally dried at room temperature. The amount of chitosan-g-PAM was kept constant, while the amount of CTA was changed for each synthesizing experiment, in order to prepare a series of chitosan-CTA-g-PAM samples with various degrees of substitution of CTA. The mass ratios of chitosan-g-PAM to CTA were 2:1, 2:2, 2:3, and 2:4, respectively. The degrees of substitution of CTA for various samples were detected using 1H NMR. Four final products were nominated (CCP1, CCP2, CCP3, and CCP4), based on various degrees of substitution of CTA (28.2%, 33.1%, 39.8%, and 44.7%, respectively). The detailed synthesis process of chitosan-CTA-g-PAM is shown in Scheme 1. 2.3. Instrument Analysis. Fourier Transform infrared (FTIR) spectra were recorded using a Bruker Model IFS 66/S FTIR spectrometer. The interval of tested wave numbers was 6504000 cm1. 1 H nuclear magnetic resonance spectroscopy (1H NMR) spectra were recorded on a Bruker AVANCE Model DRX-500 spectrometer, operating at 500 MHz, in a mixed solvent composed of CF3COOD and D2O with a mass ratio of 1:1. 2.4. Flocculating Experiment. 2.4.1. Beaker Experiments in Laboratory Scale. CCP4 was selected and employed to investigate the flocculating properties of chitosan-CTA-g-PAM both in laboratory and pilot scales. The beaker experiments were conducted using the same method as that described in our previous work.23 A kaolin suspension was used as synthetic wastewater (pH ∼7.0, zeta potential = 20 mV, and mean diameter = 7.4 μm). Distilled water was used in all flocculating experiments at the laboratory scale, to avoid impurities within the carrier liquid that might interfere with the final flocculation results. A flocculants solution was prepared by dissolving chitosan-CTA-g-PAM in a 1.0 wt % HCl aqueous solution. Various dosages of flocculants (ranging from 0.01 mg/L to 3.0 mg/L) were mixed with synthetic wastewater under magnetic stirring at room temperature, to investigate the effect of flocculant dosage on the flocculating properties. The 7142
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Figure 1. Flowchart of the equipment used for the flocculating experiment at the pilot scale.
Table 1. Flocculating Experimental Results Using CCP4 as Flocculants by Orthogonal Test Turbidity (NTU) dosage of
sample a
flocculants
sedimentation
mechanical mixing
water
(mg/L) (A)
time (min) (B)
rate (rpm) (C)
temperature (°C)
before sand filtrating
after sand filtrating
raw water
40
25
150
25.1
5.78
0.57
38.9
s1
(1) 1.1
(1) 20
(1) 100
28.3
7.23
0.88
36.8
s2
(1) 1.1
(2) 10
(2) 150
28.2
9.25
1.08
42.9
s3
(1) 1.1
(3) 15
(3) 200
29.3
8.51
0.95
60.3
s4
(2) 1.3
(1) 20
(1) 150
28.5
3.56
0.34
35.6
s5 s6
(2) 1.3 (2) 1.3
(2) 10 (3) 15
(2) 200 (3) 100
27.8 27.8
6.07 4.77
0.61 0.55
45.7 56.3
s7
(3) 1.5
(1) 20
(1) 200
27.3
5.38
0.56
34.5
s8
(3) 1.5
(2) 10
(2) 100
28.7
7.98
0.87
36.9
s9
(3) 1.5
(3) 15
(3) 150
28.6
7.01
0.75
45.0
(A)
(B)
(C)
(1)
24.99
16.17
19.98
(2) (3)
14.40 20.37
23.30 20.29
19.82 19.96
(1)
8.33
5.39
6.66
(2)
4.80
7.77
6.61
(3)
6.79
6.76
6.65
R
3.53
2.38
0.05
S
18.79
8.56
0.00
s0
statistical parameter L
K
a
For sample “s0”, data were obtained for a flocculating experiment using polyferric sulfate as the flocculant.
initial turbidity of synthetic wastewater was kept at 75 NTU. [The units of turbidity from a calibrated nephelometer are called nephelometric turbidity units and are abbreviated as NTU.] After 10 min, the kaolin suspension was kept still for a certain time until the steady state of the flocculation process was reached. As for the effect of original turbidity of synthetic wastewater on the flocculating properties, the original turbidity of untreated wastewater ranged from 10 NTU to 90 NTU for various measurements. In each turbidity point, various dosages of flocculants solutions (ranging from 0.01 mg/L to 0.5 mg/L) were mixed with synthetic wastewater under magnetic stirring at pH 7.0 and 25 °C, to obtain the corresponding optimal dosage. Similarly, to investigate the effect of temperature on the flocculating properties, the temperature was ranged from 5 °C to 45 °C for various measurements, and the initial turbidity of
synthetic wastewater was kept at 75 NTU. At each temperature point, various dosages of the flocculant solutions (ranging from 0.01 mg/L to 0.5 mg/L) were mixed with synthetic wastewater under magnetic stirring at pH 7.0, to obtain the corresponding optimal dosage. All of the turbidity of the supernatant at the laboratory scale was measured by a Model 722s spectrophotometer (Shanghai Lengguang Tech., Ltd., Co.) at a wavelength of 550 nm. 2.4.2. Treatment of Water in Pilot Scale. The set of equipment used for the flocculating experiment at the pilot scale, at the Zhenjiang Water Factory of China, is depicted in Figure S1 of the Supporting Information. Figure 1 shows a flowchart for the equipment, including a raw-water tank (0.66 m in length (L), 0.66 m in width (W), and 0.80 m in height (H)), triple coagulative sedimentation tanks (L W H = 1.26 m 0.42 m 0.70 m), 7143
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Industrial & Engineering Chemistry Research an inclined-tube settling tank (L W H = 2.20 m 1.20 m 1.47 m), a sand-filtering column, a clear-water tank, and a backflushing system. The capacity of water treatment, using this equipment, was 1.0 m3/h, and the mechanical mixing rate in the raw-water tank was 150 rpm. The height of the silicious sand in the sand-filtering column was 76 cm, and the column was backflushed every 24 h. The soil discharging process in the inclined-tube settling tanks was carried out every 12 h. As for the triple coagulative sedimentation tank, the mechanical mixing rates in the No. 2 and No. 3 tanks were both kept constant, at 100 and 50 rpm, respectively, but that in the No. 1 tank varied over a range of 100200 rpm. The mechanical mixing rate was controlled by a Model Z200 agitator model (from Shanghai Weite Motor, Ltd., China). The details of the flocculating experiment at the pilot scale has been described in previous work.24 Raw water from the Zhenjiang part of the Yangtse River in China was employed as wastewater (pH ∼7.07.45, zeta potential of approximately 15 to 20 mV, mean diameter = 11 μm, and solid concentration ≈ 0.4%0.8%), and orthogonal testing has been applied to investigate the flocculating properties of chitosan-CTA-g-PAM copolymer flocculants in a continuous water treating process. Three main external factors—dosage of flocculants, sedimentation time, and mechanical mixing rate— have been investigated in detail, and three variables have been examined for each factor. The detailed experimental parameters are shown in Table 1. The dosage of flocculants ranged from 1.1 mg/L to 1.5 mg/L; the mechanical mixing rates in the triple coagulative sedimentation tank were 200100, 100, and 50 rpm, respectively; and the sedimentation time ranged from 10 min to 20 min, by controlling the flow velocity of raw water into the rawwater tank. The experimental results also have been compared with those obtained using the currently used flocculant (polyferric sulfate, in Zhenjiang Water Factory). Turbidity at the pilot scale was measured using a Model ATZ-A22 turbidity indicator (Wuxi Guangming Instrument Factory, China). 2.5. Effects of the Degree of Substitution of CTA on the Flocculating Properties. The experimental conditions for investigation of the effects of the degree of substitution of CTA on the flocculating properties at both laboratory and pilot scales were similar to those conditions as mentioned above. At the laboratory scale, the initial turbidity of each synthetic wastewater and temperature were maintained at 75 NTU and 25 °C, respectively. For each chitosan-CTA-g-PAM sample, various dosages of flocculant solutions (ranging from 0.01 mg/L to 1.5 mg/L) were mixed with synthetic wastewater under magnetic stirring, respectively, to obtain the corresponding optimal dosage. At the pilot scale, the dosage of flocculants was 1.3 mg/L, the mechanical mixing rates in the triple coagulative tanks were 150, 100, and 50 rpm, respectively, and the sedimentation time was 20 min for each measurement.
3. RESULTS AND DISCUSSION 3.1. Characterization of Chitosan-CTA-g-PAM. The detailed preparation process of chitosan-CTA-g-PAM has been described in the Experimental Section and summarized in Scheme 1. The FTIR spectra of various samples are shown in Figure 2. From spectrum a in Figure 2, the FTIR spectrum of chitosan showed a characteristic CdO peak at ∼1650 cm1, which was due to partial deacetylation, and characteristic peaks of the amino group appeared at ∼3400, 1650, and 1320 cm1.29 As for the FTIR
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Figure 2. FTIR spectra of chitosan (spectrum a), chitosan-g-PAM (spectrum b), CTA (spectrum c), and chitosan-CTA-g-PAM (spectrum d).
Figure 3. 1H NMR spectra of chitosan (spectrum a), chitosan-g-PAM (spectrum b), and chitosan-CTA-g-PAM (spectrum c).
spectrum of chitosan-g-PAM (shown as spectrum b in Figure 2), the overlapped peaks at ∼1665 and 1550 cm1 were assigned to the amide I and II bands, respectively. Furthermore, comparison of the FTIR spectra of chitosan-g-PAM and chitosan shows that a peak appeared at ∼1430 cm1, which was due to the CN stretching in the graft copolymer, which further supported that a graft reaction occurred. Comparison with the FTIR of CTA (shown as spectrum c in Figure 2) shows that a new peak at 1476 cm1 appeared, in addition to that of chitosan-CTA-g-PAM (as shown in spectrum d of in Figure 2), corresponding to the methyl of the quaternary ammonium groups.25 This confirmed that CTA was successfully introduced into chitosan-g-PAM. Furthermore, 1H NMR has been applied to investigate the structures of copolymers further. Figure 3 shows 1H NMR spectra of chitosan, chitosan-g-PAM, and chitosan-CTA-gPAM. Spectrum a in Figure 3 shows the signal of the H2 proton (at ∼2.85 ppm), the H3H6 protons (at ∼3.253.70 ppm), and the H1 proton (at ∼4.56 ppm) of chitosan. The peaks at 1.31 and 1.90 ppm that are shown in spectrum b in Figure 3, which corresponded to the H7 and H8 protons, respectively, were the characteristic signals of polyacrylamide, which supported that a graft reaction had taken place. As for the 1H NMR spectrum of 7144
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Figure 4. The dosage effect of CCP4 on the flocculating properties at 25 °C and pH ∼7.0: (a) time dependence of the transparency of the supernatant with different dosages (ranging from 0.01 mg/L to 3.0 mg/L) and (b) transparency (9) and zeta potential (2) of the suspensions in 45 min, as a function of dosage.
chitosan-CTA-g-PAM (shown as spectrum c in Figure 3), a new peak at ∼4.01 ppm was found and corresponded to the resonance of methylene proton of CTA (H9 proton). Furthermore, an intense resonance appeared at 2.91 ppm, which was attributed to the methyl proton in a quaternary ammonium group (H10 proton). Therefore, the FTIR and 1H NMR spectra both confirmed that chitosan-CTA-g-PAM was successfully obtained. 3.2. Flocculating Properties at the Laboratory Scale. 3.2.1. Dosage Effects. It was well-known that the dosage of flocculants is a very important factor for the flocculating properties. Hence, the dosage effects of chitosan-CTA-g-PAM were investigated first. Variations of transparency of the supernatant collected from synthetic wastewater, with an original turbidity of 75 NTU at 25 °C, as a function of flocculation time, with different dosages of chitosan-CTA-g-PAM, is shown in Figure 4a. Figure 4a shows that the dosage of the flocculants, either too high or too low, could result in better flocculating performances, the results of which are similar to those of reported flocculants.2,23,30 Based on Figure 4a, most of the measuring points reached the steady state of the flocculation process within 45 min, and variations in the transparency and zeta potential of the suspensions under the same sedimentation time (∼45 min), as a function of dosage, are summarized in Figure 4b. This figure shows that chitosan-CTA-g-PAM had pretty good flocculating performances in a wider range of dosage (from 0.02 mg/L to 1.0 mg/L), in comparison to that of chitosan-g-PAM,23 and the transparency of the supernatant was above 96%. The optimal dosage was 0.1 mg/L in the measured dosage range, and the zeta potential of the suspension was near zero. On the one hand, chitosan-CTA-g-PAM presented abundant cationic quaternary ammonium groups, which could neutralize and flocculate the anionic kaolin particles efficiently at the proper amount for the effect of charge neutralization flocculation. On the other hand, the positive charge of the cationic groups made the conformation of the polymer chain more extended for intramolecular electrostatic repulsion at very low dosages of flocculant,31,32 which greatly improved the efficiency of bridging flocculation and resulted in good flocculating performances, even at very low dosage. Therefore, chitosan-CTA-g-PAM showed good flocculating performances in a wider range of dosage. However, in the overdosage region, the zeta potential of the suspension was positive (from Figure 4b), and the excessive cationic flocculants would partially restabilize the kaolin particles in the water again, thus the flocculating effects would decrease.
Figure 5. pH dependence of the transparency of the supernatant at a dosage of 0.1 mg/L and a temperature of 25 °C (sample CCP4).
Moreover, based on the optimal dosage as mentioned above, the pH effects of synthetic wastewater on the flocculating properties of chitosan-CTA-g-PAM also have been investigated, as shown in Figure 5. The initial pH varied from 1.0 to 9.0. At pH >9.0, chitosan-CTA-g-PAM was insoluble, but already had a wider range of solubility than chitosan-g-PAM. From Figure 5, it was found that, pH would have only a minor effect on the flocculating performance of chitosan-CTA-g-PAM for being strong cationic flocculants. 3.2.2. Effects of Original Turbidity of Synthetic Wastewater. Besides the dosage effect, the original turbidity of the wastewater was also a key factor for the flocculating performances. Therefore, the influences of original turbidity of synthetic wastewater to the flocculating properties have been studied. Variations in the transparency of the supernatant collected from synthetic wastewater with original turbidities (in the range of 1090 NTU at pH 7.0 and 25 °C), as a function of dosage of CCP4, is shown in Figure S2 in the Supporting Information. Based on Figure S2, the optimal dosage, and its corresponding transparency of the supernatant, at each turbidity point are summarized in Figure 6. Figure 6 shows that the optimal dosage of chitosan-CTA-gPAM increased as the original turbidity of synthetic wastewater increased, which was fully consistent with the results of traditional flocculants.2 However, it showed different increase tendencies with increasing original turbidity: Below 75 NTU, the optimal dosage increased quite slightly, but had a large jump from 7145
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Figure 6. Optimal dosage (9) and the corresponding transparency of the supernatant (2) at 25 °C and pH ∼7.0, as a function of the original turbidity of synthetic wastewater (sample CCP4), based on Figure S2 in the Supporting Information.
a turbidity of 75 NTU to a turbidity of 90 NTU. It may be expected that the dosage would continue to increase at a faster rate for a more-turbid feed. In addition, the transparency of the supernatant at each optimal condition was similar and kept at a high level, but slightly decreased as the original turbidity increased. This indicated that the flocculants requirement increased as the original turbidity of synthetic wastewater increased. 3.2.3. Temperature Effects. In addition, the temperature of the raw water was also important to the flocculating effects in the actual water treatment. Variations of transparency of the supernatant collected from synthetic wastewater with a original turbidity of 75 NTU at pH 7.0, as a function of dosage of sample CCP4, at various temperatures (ranging from 5 °C to 45 °C) is shown in Figure S3 in the Supporting Information. Based on Figure S3, the optimal dosage and the corresponding transparency of the supernatant at each temperature point are summarized in Figure 7. It was found that the optimal dosage of chitosan-CTA-g-PAM decreased, but its corresponding transparency of the supernatant increased, as temperature of raw water increased. On the one hand, the viscosity of carrier liquid decreased with temperature increase, which may result in the increase of colloids movements, and collision to form larger particles thus became easier. On the other hand, the more extended conformation of polymeric flocculants due to the improved solubility for the increase of temperature was beneficial to bridging flocculation effects. Both of which resulted in the decrease of dosage requirements with temperature increase. It might be the reason that higher dosage of flocculants in winter than that in summer was needed.24 3.3. Flocculating Properties at the Pilot Scale. In addition to the beaker experiment, the flocculating properties of chitosanCTA-g-PAM were studied at the pilot scale for real applications. Raw water from the Zhenjiang part of the Yangtze River in China was used as wastewater. The details regarding this experimental process at the pilot scale has been described in the Experimental Section and in previous work.24 Figure 1 indicates that the raw water and flocculants are mixed in the raw-water tank, then flocculated in triple coagulative sedimentation tanks, and settled in an inclined-tube settling tank; after filtration in a sand-filtering column, the treated water was stored in clear-water tank. The effects of three main external factors for water treatment— including flocculant dosage, sedimentation time, and mechanical
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Figure 7. Optimal dosage (9) and the corresponding transparency of the supernatant (2) at pH ∼7.0, as a function of temperature (sample CCP4), based on Figure S3 in the Supporting Information.
mixing rate—on the flocculating properties were investigated, and the three variables have been examined for each factor by orthogonal testing. The detailed experimental parameters are shown in Table 1. Orthogonal testing is a scientific method that is used to study the effects of many different factors on certain properties of materials, which could reduce the number of the repeated experiments, largely based on the statistic principle.33 The statistical parameters in Table 1 include the following: • Lij is the sum of the turbidity of the treated water before sand-filtrating at a constant variable under certain conditions of j. Lij ¼
∑TjðiÞ
ði ¼ 1; 2; 3; j ¼ A; B; CÞ
ð1Þ
• Kij is the average of Lij. Kij ¼
Lij 3
ði ¼ 1; 2; 3; j ¼ A; B; CÞ
ð2Þ
• Rj is the largest difference of Kij under certain conditions of j. Rj ¼ maxðKij Þ minðKij Þ
ði ¼ 1; 2; 3; j ¼ A; B; CÞ
ð3Þ • Sj is the square sum of each difference of Kij under certain conditions of j. Sj ¼
∑
1 ðKij Klj Þ2 2 i6¼ l
ði ¼ 1; 2; 3; j ¼ A; B; C; l ¼ 1; 2; 3Þ
ð4Þ
To evaluate the flocculating performances of chitosan-CTA-gPAM itself, the turbidity of treated water before sand-filtering was adopted in the data analysis during the orthogonal testing. All of the experimental data at the pilot scale are listed in Table 1. Based in Table 1, the effects of the dosage, sedimentation time, and mechanical mixing rate on the flocculating properties are shown in Figure 8. The following flocculation conditions were determined to be optimal: dosage of flocculants, 1.3 mg/L; sedimentation time, 20 min; and mechanical mixing rates of the No. 1, No. 2, and No. 3 7146
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Figure 8. Effects of (a) the dosage, (b) the sedimentation time, and (c) mechanical mixing rate on the flocculating properties at the pilot scale, according to K (see Table 1).
tanks in the triple coagulative sedimentation tanks, 150, 100, and 50 rpm, respectively. According to Figure 8a, it was indicated that the dosage of chitosan-CTA-g-PAM at the pilot scale as well as at the laboratory scale should be maintained at a proper range for better flocculating performance. In terms of the charge neutralization flocculation mechanism, at a proper amount of the flocculants, the surface anionic colloids could be efficiently neutralized and aggregated into sediments. However, if the amount of the flocculants was too high, the excessive cationic flocculants would enwrap and restabilize the suspension partially in water, which made the final flocculating effects decrease. In addition, the optimal dosage of chitosan-CTA-g-PAM at the pilot scale was much higher than that at the laboratory scale, which might be because the water quality of the raw water from the Yangtse River was more complicated than that of kaolin suspensions. However, in comparison with the optimal dosage of polyferric sulfate currently used in the Zhenjiang Water Factory (∼40 mg/L), which also was obtained by orthogonal testing, the dosage of chitosan-CTA-g-PAM was much lower (1.3 mg/L). Although the chitosan-based flocculant was more expensive than the inorganic polymeric flocculant, it still showed higher costeffectiveness in water treatment. Furthermore, the sedimentation time was also very important to the flocculating effects, and the sedimentation time could be adjusted by controlling the flow velocity of raw water into the rawwater tank in the continuous water treating process. Based in Figure 8b, it was well understood that the flocculating properties could be improved by prolonging the sedimentation time, since the suspension was unstable. Nevertheless, in order to save energy, the sedimentation time should be controlled in a suitable range. Aside from dosage and sedimentation time, the effect of the mechanical mixing rate in the No. 1 tank in the triple coagulative
sedimentation tank on the flocculating properties was also measured. According to Figure 8c, it was also demonstrated that both higher and lower mechanical mixing rates were detrimental. At lower rate, the small particles were not able to have effective collisions, whereas, at higher rate, the sediments with large size mass were easily broken. Moreover, from the statistical parameters R and S, the weight of the three investigated factors could be sorted as dosage of flocculants > sedimentation time > mechanical mixing rate
which indicated that the dosage of flocculants was the most important factor to the flocculating properties. In addition, the turbidity of the treated water after sand-filtering, under most conditions, has been already lower than 1.0 NTU, which fully met the Chinese standard for drinking water quality (Chinese standard GB 5749-2006).34 3.4. Effect of the Degree of Substitution of CTA. Recall that the aforementioned factors are all external factors. In fact, the structure factors are much more important, because the structure will determine the final performance of the materials. Based on this viewpoint, the effects of the degree of substitution of CTA on the flocculating properties of chitosan-CTA-g-PAM flocculants have been investigated further at both the laboratory and pilot scales. Variations of transparency of the supernatant collected from synthetic wastewater with a original turbidity of 75 NTU at pH 7.0 and 25 °C, as a function of dosage by various flocculants with different CTA contents, are shown in Figure S4 in the Supporting Information. Based on Figure S4 and the calculated degree of substitution of CTA, the CTA content dependence of the optimal dosage and its corresponding transparency of the supernatant are summarized in Figure 9. 7147
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Table 2. Effect of the Degree of Substitution of CTA on the Flocculating Properties of Chitosan-CTA-g-PAM at the Pilot Scale Turbidity (NTU) degree of substitution
Figure 9. Optimal dosage (9) and its corresponding transparency of the supernatant (2) at 25 °C and pH ∼7.0 by various chitosan-CTA-gPAM samples with different degrees of substitution of CTA, based on Figure S4 in the Supporting Information.
Figure 9 shows that, as the CTA content of flocculants increased, higher transparency of the supernatant was obtained, and the corresponding optimal dosage of flocculants shifted to lower value. CCP4, which had the highest degree of substitution of CTA, showed the best flocculating properties of the four samples. For further comparison, the flocculating properties of chitosan-CTA-g-PAM with different degrees of substitution of CTA under the same experimental conditions at the pilot scale were also measured, and the final results are summarized in Table 2. It was found that the flocculating properties were also proportional to the degree of substitution of CTA. The flocculating effect increased as the degree of substitution of CTA increased, which was fully consistent with the results from the beaker experiment at the laboratory scale, using a kaolin suspension as synthetic wastewater. Based on the charge neutralization mechanisms of polymer flocculants, a higher degree of substitution of CTA was beneficial to improve the actual flocculating capacity of flocculants. Furthermore, more positive charges on polymers made the chain conformation more extended, which was also helpful to the bridging flocculation effect. Furthermore, in order to investigate the CTA content effect further, a flocculation kinetics model of the particle collisions was employed. It was well-known that the order of flocculation process was mostly bimolecular:3537 1=2 N0 1 ¼ 1þ ð5Þ kN0 t 2 Nt where N0 is the initial number concentration of kaolin particles, Nt the number concentration of kaolin particles at time t, and k the rate constant for collisions between the singlets. Moreover, the relationship between transparency and the number concentration of kaolin particles was N0 100 T0 ¼ Nt 100 Tt
ð6Þ
where T0 is the transparency of the initial water and Tt is the transparency of the supernatant at time t. Therefore, a plot of [(100 T0)/(100 Tt)]1/2 against t would give a straight line with an intercept of 1 for a bimolecular process. kN0 were also calculated via computer fitting and are shown in Table 3. In this work, N0 was a fixed value for the same synthetic wastewater.
water
before
after
of CTA
dosage
temperature
sand
sand
sample
(%)
(mg/L)
(°C)
CCP1
28.2
1.3
28.5
5.33
0.68
55.4
CCP2 CCP3
33.1 39.8
1.3 1.3
28.8 28.3
4.77 4.28
0.51 0.45
44.7 62.3
CCP4
44.7
1.3
27.8
3.56
0.34
35.6
raw
filtrating filtrating water
Table 3. Results of Flocculation Kinetics for Flocculation Processes with Their Optimal Dosages of Different ChitosanCTA-g-PAM Samples optimal dosage sample
(mg/L)
kN0 ( 104 s1)
R2
CCP1
0.15
5.96
0.991
CCP2
0.12
7.27
0.996
CCP3
0.12
8.16
0.994
CCP4
0.10
9.34
0.995
From Table 3, it was found obviously that the rate constant k of various chitosan-CTA-g-PAM, under their optimal conditions, increased as the CTA content increased. In the case of low cationic content, the interaction between the flocculants and kaolin particles was weak, which resulted in low k. The enhancement in k with the increase of CTA content could be due to the greater availability of flocculating sites on the flocculants and stronger interaction between chitosan-CTA-g-PAM and kaolin particles. It was beneficial to both bridging and charge neutralization flocculation effects. Hence, the chances for molecular collisions between the flocculants and kaolin increased.
4. CONCLUSION Above all, the flocculating test at the laboratory and pilot scales both indicated that chitosan-CTA-g-PAM was a type of efficient flocculant in water treatment. Compared to the traditional flocculant of polyferric sulfate, chitosan-CTA-g-PAM showed higher efficient in flocculating effects, and the dosage was much lower. Furthermore, chitosan-CTA-g-PAM had much wider flocculation windows than chitosan-g-PAM for more-extended conformation of polymer chain. In addition, the dosage of flocculants was proved to be the most important factor to the flocculating properties of chitosan-CTA-g-PAM flocculants. Moreover, the beaker experiment at the laboratory scale demonstrated that the temperature and original turbidity of synthetic wastewater had remarkable influences on the flocculating performances. Furthermore, it was also found that the degree of substitution of CTA was one of the key structure factors to the flocculating properties. The flocculating performance increased as the CTA content increased, which could efficiently improve both charge neutralization and the bridging flocculation effect. The results were also confirmed by the flocculation kinetics analysis. 7148
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’ ASSOCIATED CONTENT
bS
Supporting Information. Photographs of the equipment used for flocculating experiments at the pilot scale (Figure S1), and plots showing the effect of turbidity (Figure S2), temperature (Figure S3), and the degree of substitution of CTA (Figure S4). (PDF) This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: 86-25-83686350. Fax: 86-25-83317761. E-mail: yanghu@ nju.edu.cn.
’ ACKNOWLEDGMENT Supported by the Key Natural Science Foundation of China (Grant No. 51073077 and 50633030), and Research Project of Ministry of Housing and Urban-Rural Development of China (Grant No. 2009-K7-11). ’ REFERENCES (1) Schwarzenbach, R.; Escher, B.; Fenner, K.; Hofstetter, T.; Johnson, C.; Von Gunten, U.; Wehrli, B. The challenge of micropollutants in aquatic systems. Science 2006, 313 (5790), 1072. (2) Li, F. T.; Zhang, S. F.; Zhao, Y. Coagulants and Flocculants; Chemical Industry Press: Beijing, 2005. (3) Yuan, B. L.; Wang, H. J. Principle and Application of New Technology in Water Treatment; Chemical Industry Press: Beijing, 2006. (4) Kraume, M.; Drews, A. Membrane Bioreactors in Waste Water Treatment—Status and Trends. Chem. Eng. Technol. 2010, 33 (8), 1251. (5) Torove, L. The effect of coupling coagulation and flocculation with membrane filtration in water treatment: A review. J. Environ. Sci. 2009, 21 (1), 8. (6) Xiao, J.; Zhou, Q. Natural Polymer Flocculants; Chemical Industry Press: Beijing, 2005. (7) Zhang, L. N. Modified Natural Polymer Materials and Their Application; Chemical Industry Press: Beijing, 2006. (8) Jiang, T. D. Chitosan; Chemical Industry Press: Beijing, 2007. (9) Song, Y. B.; Zhang, J.; Gan, W. P.; Zhou, J. P.; Zhang, L. N. Flocculation properties and antimicrobial activities of quaternized celluloses synthesized in NaOH/urea aqueous solution. Ind. Eng. Chem. Res. 2010, 49 (3), 1242. (10) Bhatnagar, A.; Sillanpaa, M. Applications of chitin- and chitosan-derivatives for the detoxification of water and wastewater—A short review. Adv. Colloid Interface Sci. 2009, 151 (12), 26. (11) Guibal, E.; Van Vooren, M.; Dempsey, B. A.; Roussy, J. A review of the use of chitosan for the removal of particulate and dissolved contaminants. Sep. Sci. Technol. 2006, 41 (11), 2487. (12) Brown, T. J.; Emelko, M. B. Chitosan and metal salt coagulant impacts on Cryptosporidium and microsphere removal by filtration. Water Res. 2009, 43 (2), 331. (13) Rojas-Reyna, R.; Schwarz, S.; Heinrich, G.; Petzold, G.; Schutze, S.; Bohrisch, J. Flocculation efficiency of modified water soluble chitosan versus commonly used commercial polyelectrolytes. Carbohydr. Polym. 2010, 81 (2), 317. (14) Wang, J. P.; Chen, Y. Z.; Yuan, S. J.; Sheng, G. P.; Yu, H. Q. Synthesis and characterization of a novel cationic chitosan-based flocculant with a high water-solubility for pulp mill wastewater treatment. Water Res. 2009, 43 (20), 5267. (15) Francois, R.; Bertrand, S.; Jeremie, C.; Nadia, M. C.; Pierre, B.; Peter, W.; Gregorio, C. Chitosan flocculation of cardboard-mill secondary biological wastewater. Chem. Eng. J. 2009, 155, 775.
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