- Email: [email protected]
Coagulation performance and floc characteristics of aluminum sulfate with cationic polyamidine as coagulant aid for kaolin-humic acid treatment
Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Coagulation performance and floc characteristics of aluminum sulfate with cationic polyamidine as coagulant aid for kaolin-humic acid treatment Bo Guo 1 , Huan Yu 1 , Baoyu Gao ∗ , Hongyan Rong, Hongyu Dong, Defang Ma, Ruihua Li, Shuang Zhao Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, No.27 Shanda South Road, Jinan 250100 Shandong, PR China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The synthesize method of polyamidine was investigated and optimized. performance of AS-polyamidine and AS was comparatively evaluated. • The characteristics of flocs generated by the two kinds of coagulants were studied. • The size and growth rate of the flocs were enhanced by the aid effect of polyamidine. • Recovery ability of flocs was improved due to the assistant role of polyamidine.
• Coagulation
a r t i c l e
i n f o
Article history: Received 12 March 2015 Received in revised form 6 June 2015 Accepted 9 June 2015 Available online 12 June 2015 Keywords: Polyamidine Aluminum sulfate Coagulation performance Floc properties
a b s t r a c t In this study, novel cationic polyamidine (Pa) was synthesized and used as coagulant aid with aluminum sulfate (AS) for kaolin-humic acid simulated water treatment. The effects of Pa on coagulation performance and floc properties were investigated. Results showed that Pa addition significantly enhanced the removal of turbidity, ultraviolet absorbance at 254 nm wavelength (UV254 ) and dissolved organic carbon (DOC). AS–Pa exhibited high coagulation efficiency at border pH range and increased floc size and growth rate significantly. Floc recovery ability was also improved, while floc fractal dimension was lower with Pa addition. This would be attributed to high cationic charge density and long chain of Pa, which could enhance the effect of charge neutralization and adsorption bridging. © 2015 Elsevier B.V. All rights reserved.
∗ Corresponding author. Fax: +86 531 88364513. E-mail addresses: baoyugao [email protected], [email protected] (B. Gao). 1 The authors contributed equally to the work. http://dx.doi.org/10.1016/j.colsurfa.2015.06.017 0927-7757/© 2015 Elsevier B.V. All rights reserved.
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
1. Introduction Humic acid (HA) as the main fraction of humic substances is the important component of natural organic matter (NOM) [1] in surface water, which can cause odor, taste, color and bacterial re-growth in potable water [2]. Moreover, HA can react with disinfectant to produce genotoxic and cytotoxic disinfection byproducts (DBPs) during drinking water treatment [3]. Thus, effective removal of HA in waters is essential. Coagulation treatment is one of the most common methods to remove NOM in water due to its high efficiency and low cost [4]. Coagulant is crucial to the efficiency of coagulation process [5]. Conventional coagulants such as aluminum and ferric salts have already been widely used in water treatment and their coagulation mechanisms have been studied for years [6,7]. However, high dosage of ferric salts with low molecular are corrosive to metal equipment and excessive aluminum salts are harmful to human and living organisms [8,9]. Therefore, it is necessary to develop effective coagulant aids to decrease the dosage of coagulants. Coagulant aids are agents for regulating the flocculation condition, promoting cohesion function or improving the structure of coagulation floc [7]. Hence, they are usually used in coagulation process to promote floc formation and increase floc density, which will subsequently improve the floc sedimentation. [10]. It has been confirmed that better coagulation efficiency, and floc characteristics could be achieved by the combination of synthetic polymers and metal coagulant [11]. Enteromorpha extract with aluminum salts [12] and lignin-acrylamide with polyferric chloride [13] were also improved to enhance coagulation efficiency, floc strength and recoverability significantly. Coagulation efficiency of aluminum and polyaluminum was enhanced with cationic polyacrylamide (PAM) addition for the treatment of pulp and paper mill wastewaters [14]. When anionic PAM was added with polyaluminum chloride or ferric sulphate, significant increase in the settling speed was achieved [15]. Polyamidine (Pa) is one kind of macromolecular polymer with amidine unit on its molecular chains. Pa could be made into chelating fiber by the wet-spinning method and the product has high binding capacity and good adsorption property for heavy metal ions [16]. Sharavanan et al. [17] have reported the hydrolytic degradation of Pa and its application in controlled release of active agents. In this study, Pa with five membered amidine rings was obtained by acid-treating the copolymer of N-vinylformamide (NVF) and acrylonitrile (AN). Due to the high density of amino groups, Pa has high cationic degree [18]. Cationic polymers have been widely used in water treatment, papermaking, textiles, rheology modifiers and oil field chemicals because of the strong hydrophilic character and ionization ability [19–22]. However, there is no previous study about the coagulation performance using Pa as coagulant aid. The objective of this study is to investigate the effect of Pa on the performance and floc properties of aluminum sulfate (AS) coagulation. The coagulation performance would be assessed in terms of turbidity, ultraviolet absorbance at 254 nm wavelength (UV254 ) and dissolved organic carbon (DOC) under different coagulant doses and initial solution pH, and the floc characteristics were evaluated by the size, growth rate, strength and recoverability of flocs. In addition, the coagulation mechanism of the aid effects of Pa was also discussed.
477
dihydrochloride (AIBA, high purity grade, purchased from BioDuly Co., Ltd., Nanjing, China). Al2 (SO4 )3 ·18H2 O and kaolin were supplied by Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Al2 (SO4 )3 ·18H2 O was analytical grade, while kaolin was chemically pure. Humic acid was purchased from Aladdin Industrial Co., Shanghai. Content of fulvic acid was more than 90%. Deionized water was used to prepare all solution. 2.2. General method for copolymerization Synthesis of cationic polyamidine was conducted in a 100 mL round bottom flask as follows: 6.0 g of a mixture comprising NVF and AN in a molar ratio 1:1, and 34.0 mL of ultrapure water were introduced into the flask. In water bath, the mixture was heated to 45 ◦ C under the flow of nitrogen gas and the flow remained for 15 min. Then, the copolymerization was initiated by adding 0.021 g AIBA and nitrogen gas stream continued for another 5 min. After the flask was held at 45 ◦ C water bath for 5 h to produce precipitated polymer, 10 mL of concentrated hydrochloric acid and 20 mL of ultrapure water was added to the suspension. Then, the reaction mixture was held at 110 ◦ C for 4 h under stirring to amidinize the polymer. The resulting polymer solution was added to acetone and the precipitated polymer was dried under vacuum. Finally, the solid polymer-cationic polyamidine was obtained. The cationic polymer had an amidine structure and the repeating amidine unit is represented by the formula (1)
*
H2 C
H2 C CH C
CH N
NH3X-
H2 C
H C
*
CN
n
Wherein, X− represents for Cl− and n represents for the degree of polymerization ranging from 100 to 1000. The molecular weight of polyamidine was 20,000–100,000 Da with 30–70% cationic charge density. Molecular weight was tested by gel chromatography (Waters, USA), and cationic charge density was measured by colloid titration. Standard solution was prepared by potassium polyvinyl sulfate (PVSK). The viscosity was measured with the rotary viscosity meter in solution of 2 g/L at 25 ◦ C, and the result was 150–250 mPa s. 2.3. Synthetic test water Synthetic test water was prepared by HA and kaolin. The stock solution of HA was prepared as follows: 1.0 g HA, and 0.4 g NaOH was dissolved in about 800 mL deionized water, and the solution was diluted to 1 L after continuous stirring for 30 min. The kaolin stock solution was prepared as follows: 5.0 g kaolin was dissolved in 1 L deionized water on magnetic stirrer for half an hour, and then the suspension was transferred to a measuring cylinder to settle for another 30 min. The superficial 500 mL was used as the stock kaolin solution. Particle size distribution of kaolin was measured by Zetasizer Nano Series (Malvern, UK) and shown in Fig. 1. Simulated water samples contained 10 mg/L HA and tap water, and suitable amount of kaolin stock solution was dosed to adjust the turbidity to 15 ± 0.5 NTU. The properties of the water samples were shown in Table 1. 2.4. Jar-test
2. Materials and methods 2.1. Materials The chemicals used in this study included N-vinylformamide (NVF), acrylonitrile (AN), 2,2 -azobis (2-methylpropionamidine)
Coagulation experiments were conducted by a jar-test apparatus (ZR-6, Zhongrun Water Industry Technology Development Co., Ltd., China) under different Al and Pa dosages. When the dualcoagulant was used, the coagulation procedure was as follows: firstly, raw water was stirred at 200 rpm for 30 s to make the solu-
478
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
Fig. 1. Particle size distribution of kaolin.
tions more homogeneous. Secondly, rapid mixing continued for 30 s and AS was dosed at the beginning of this step. Thirdly, Pa was dosed and the solution was mixed at 200 rpm for another 30 s. Then, slow stir of 40 rpm for 15 min was introduced before sedimentation for 30 min. When AS used alone, the coagulation procedure was similar to the above process. Turbidity, DOC, UV254 and zeta potential after coagulation were measured according to the measurements presented in Table. 1. 2.5. On-line monitoring of floc formation, breakage and regrowth Dynamic floc size during coagulation process was measured by a laser diffraction instrument Mastersizer 2000 (Malvern, UK). The suspension was drawn through the optical unit of Mastersizer 2000 by a peristaltic pump (LEAD-1, Longer Precision Pump, China) and then returned to the jar tester using a 5 mm internal diameter tube at a flow rate of 2.0 L/h. The inflow and outflow tubes were positioned opposite each other at a depth just above the impeller in the holding ports. Size measurements of the flocs were taken every half minute in the process of coagulation and the results (median volumetric diameter (d50 )) were recorded automatically by computer. Coagulation procedures were conducted as follows: rapid mixing (200 rpm) for 1.5 min followed by slow mixing (40 rpm) for 15 min (floc growth stage), then the shear was increased to 200 rpm again for 5 min (floc breakage stage), and finally a slow stir at 40 rpm was reintroduced for a further 15 min (floc regrowth stage). Rapid mixing and slow mixing (floc growth stage) were same with the jar test above.
volumetric diameter (d50 ). The growth rate was calculated as follows [23]: Growthrate =
size time
(2)
where size represents for floc size variation over time. Floc strength factor (Sf ) and recovery factor (Rf ) are well established parameters for indicating strength and recoverability of floc in different flocculation systems, and they can be calculated as follows [5,24,25]: Sf =
d(2) × 100 d(1)
(3)
Rf =
d(3) − d(2) × 100 d(1) − d(2)
(4)
where d(1) is the average floc size of steady phase before breakage, d(2) is the floc size after floc breakage, and d(3) is the floc size after regrowth to the new steady phase. Higher Sf means less sensitive to breakage and larger Rf shows better recoverability of floc after being subjected to high shear force. Floc fractal dimension is measured by Laser Light Scattering in this study [26]. In this technology, the relationship among scattered light intensity I, the scattering vector Q and floc fractal dimension Df can be described as follows: I ∝ Q −Df
(5)
In which Q is determined by the refractive index of the medium (n), the laser light wavelength in vacuum () and scattering angle (), as described in Eq. (6):
2.6. Floc characteristics In this study, the size data were expressed as an equivalent volumetric diameter, and the floc size was denoted by median
Q =
4nsin(/2)
(6)
Table 1 Characteristics of synthetic test water and methods of measurement. Characteristics
Values
Methods of measurement
Turbidity (NTU) DOC (mg/L) UV254 (cm−1 ) Zeta potential (mV) pH
14.5–15.5 4.800–5.334 0.28–0.31 −16.4 to –15.6 8.18–8.45
2100P Turbidimeter (Hach Co., USA) TOC-VCPH (Shimadzu Co., Japan) TU-1810 UV–vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.) Zetasizer 3000HSa (Malvern Instruments, UK) pH Meter (Luoqite Electronics Equipment Co., Ltd., Shanghai, China)
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
100
2.2
Pa dose (mg/L)
a
0.00 0.20 0.40 0.60 0.80 1.00
Residual turbidity (NTU)
1.8 1.6
b
90
UV254 removal efficiency (%)
2.0
479
1.4 1.2 1.0 0.8
80 70
Pa dose (mg/L) 0.00 0.20 0.40 0.60 0.80 1.00
60 50 40 30 20
0.6 2
4
6
8
10
12
0
AS dose (mg/L)
2
4
6
AS dose (mg/L)
8
10
12
70
c
d
3
60
0.00 0.20 0.40 0.60 0.80 1.00
50
Zeta potential (mV)
DOC removal efficiency (%)
Pa dose (mg/L) 0
40
Pa dose (mg/L) 0.00 0.20 0.40 0.60 0.80 1.00
30
20
10
-3 -6 -9 -12 -15
0
-18 0
2
4
6
8
10
12
AS dose (mg/L)
0
2
4
6
8
10
12
AS dose (mg/L)
Fig. 2. Dosage effect of AS–Pa on coagulation performance: (a) residual turbidity; (b) UV254 removal efficiency; (c) DOC removal efficiency; (d) zeta potential.
3. Results and discussion 3.1. Effect of Pa addition on coagulation performance 3.1.1. Effect of dosage on coagulation performance Coagulation optimization tests were conducted to determine the optimum dosage for turbidity and organic matter removal under the raw water pH condition. Coagulation behaviors of AS and AS–Pa were evaluated comparatively. Zeta potential, residual turbidity and organic matter removal efficiencies as a function of coagulant dosage are shown in Fig. 2. When AS was used alone, the residual turbidity decreased rapidly at a low aluminum dosage and then increased slightly as the aluminum dosage further increased, however, which still maintained a relative low level. UV254 and DOC removal efficiency was improved with the dosage increasing and high organics removal rates achieved at coagulant dosage of 7 mg/L. As shown in Fig. 2, Pa addition significantly improved the removal efficiency of DOC and UV254 . However, adding Pa decreased the turbidity removal efficiency except when Pa dosage was 0.8 mg/L, which achieved a lower residual turbidity compared with AS used alone. According to Fig. 2(b) and (c), the best coagulation performance was also achieved at the aluminum dosage of 7 mg/L with 0.8 mg/L Pa addition, which UV254 and DOC removal rate was up to 90% and 59%, whereas the HA removal efficiency
obtained by AS alone was only 86% and 42%, respectively, under the same condition. Further increase in the dosage of Pa did not enhance the removal of organic matter. The mechanisms to explain AS coagulation performance include charge neutralization, precipitation, bridge-aggregation, adsorption and sweep-flocculation, one or their combination of which may be dominant at different conditions [8,27]. As shown in Fig. 2(d), when AS used alone, zeta potential improved as the aluminum dosage increased and the maximum value was −4.9 mV, which indicated that charge neutralization played a role in the coagulation process. Since the synthetic test water was alkaline (pH 8.18–8.45), significant precipitation of amorphous hydroxide could occur, and charge neutralization was achieved by adsorbed precipitation [28,29]. When Pa was added, zeta potential drastically increased. The maximum zeta value was 3.1 mV, which was reached at 11 mg AS with 0.8 mg Pa. It can be explained by that when Pa was used as coagulant aid with AS, high cationic degree of Pa improved the degree of charge neutralization significantly. While Pa was further dosed, excessive charge resulted in stronger inter-particle repulsion, which restrained the growth of floc and consequently decreased the coagulation efficiency. Considering coagulant cost and coagulation performance, the optimal dosage of Pa and AS was fixed at 0.8 mg/L and 5.0 mg/L for the subsequent experiments, respectively.
480
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
50
HA Kaolin AS Pa
a
40
Zeta potential (mV)
30 20 10 0
-10 -20 4
5
6
7
8
9
pH Fig. 3. Effect of pH on zeta potential for HA, kaolin, AS and Pa.
3.1.2. Effect of pH on coagulation performance The initial pH of synthetic water was adjusted to 4.0–9.0 by adding HCl and NaOH to investigate the influence of pH on coagulation performance. Residual turbidity, UV254 , DOC removal efficiency and zeta potential as a function of pH are shown in Fig. 4. For both AS and AS–Pa, the suitable initial pH range was 6.0–7.0, which corresponded well with the reported optimums for Al coagulant [30]. The coagulation performance of AS–Pa was better than that of AS within pH investigated. In Fig. 4(a), for both AS and AS–Pa, residual turbidity reduced significantly when pH was increased from 4.0 to 7.0, and reached steady with pH exceeding 7.0. As indicated in Fig. 4(b) and (c), UV254 and DOC removal efficiency increased obviously with pH ranging from 4.0 to 6.0, and the optimal organics removal efficiency was achieved at pH 6.0. under this condition, the UV254 and DOC removal efficiency reached 93% and 62% for As–Pa, which was better than the removal efficiency of 90% and 56% for AS. When initial pH increased from 7.0 to 9.0, the coagulant efficiency with and without Pa addition decreased accordingly. In previous reports, surface tension of HA increased above pH 6, suggesting that HA became to be difficult to aggregate above pH 6 [31]. In addition, ion-pair formation between fulvic acid and alkylammonium surfactants is optimal at pH 7, and the higher pH resulted in the decrease of the formation [32]. These issues suggest that HA becomes to be difficult to coagulate in neutral to weak alkaline condition. Fig. 3 shows pH-dependence of zeta potential for HA, kaolin, AS and Pa alone. Zeta potential of AS and Pa was much higher than that of HA and kaolin in whole pH range. As pH increasing, zeta potential for AS and Pa decreased significantly while HA and kaolin decreased slightly. Fig. 4(d) shows that, under the condition of pH lower than 6, Pa addition significantly increased zeta potential compared with AS used alone owing to high zeta potential of Pa. Because of the lack of OH− at acidic condition, hydrolysis of Al was restrained and Al existed in the form of monomeric and dimeric ones with positive charge [30]. Therefore, charge neutralization was the dominated mechanism under acidic condition. When pH was higher than 6, zeta potential continuously decreased for both coagulants. However, Pa addition made the zeta potential of AS–Pa higher than that of AS alone in the whole pH scale. It indicated that the effect of charge neutralization decreased as pH increased. It was reported that aluminum hydrolysis to voluminous Al(OH)3 would occur under high pH condition and then HA carboxyl groups-hydroxide irons complexes formed, in which
hydrolysates played a role in sweeping and finally resulted in precipitation [33,34]. Therefore, high HA removal efficiency could also be realized when pH was higher than 6 due to sweeping and entrapment. Zeta potential of AS–Pa kept higher than that of AS and the HA removal efficiency was also better. It means that high cationic degree of Pa could also contribute to charge neutralization. Zeta potential of Pa decreased with pH increasing under the alkaline condition. Hence, Pa performed better under neutral or mild acid condition. 3.2. Effect of Pa addition on floc characteristic 3.2.1. Floc size The results of floc growth, breakage and regrowth at different aluminum dosages are shown in Fig. 5, where the floc size was represented by d50 . As shown in Fig. 1, particle size of kaolin was 1.2 m in average, which was too small compared to flocs. Floc growth rate and average floc size at steady state are shown in Fig. 6. The dosages of Pa were constant at 0.8 mg/L based on Section 3.1.1, and the solution pH was adjusted to 6.0 according to Section 3.1.2. It was obviously that floc growth and breakage reached appropriate balance in steady phase [13]. When the shear force was introduced by increasing the mixing speed up to 200 rpm, floc size showed an obvious drop. The flocs began to regrow when the slow stir was reintroduced after following 5 min of quick stir. However, the floc could not recover to the initial floc sizes for both coagulants. As shown in Fig. 5, flocs grew rapidly once the coagulant was dosed until a state of steady floc size distribution was reached. As shown in Fig. 6(a), the floc sizes for AS–Pa with aluminum dosage variation were much bigger than that for AS. When aluminum dosed 5–9 mg/L, floc sizes of AS were about 270–330 m, while floc sizes of AS–Pa could reach 700–750 m. The variation of floc growth rates as a function of aluminum dosage was shown in Fig. 6(b). Floc formed more quickly at the aluminum dosage of 5 mg/L for AS while the floc gave faster aggregation at the aluminum dosage of 3–9 mg/L when Pa was used as coagulant aid with AS. Growth rate of AS–Pa floc (100–110 m/min) was much larger than that of AS floc (65–70 m/min). When AS was dosed firstly, it was adsorbed on the surface of the micro floc quickly and neutralized the negative charge on it subsequently, which resulted in weaker repulsion force between the
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
481
100
7
a
90
UV254 removal efficiency (%)
6
Residual turbidity (NTU)
b
AS AS-Pa
5 4 3 2
80
70
AS AS-Pa
60
50
40
1
30
0 4
5
6
7
8
4
9
5
6
pH
7
8
9
pH
70
c 60
d
AS AS-Pa
8
50
Zeta potential (mV)
DOC removal efficiency (%)
12
AS AS-Pa
40
30
4 0 -4 -8
20
-12 10 4
5
6
7
8
9
4
5
6
7
8
9
pH
pH
Fig. 4. pH Effect on coagulation performance: (a) residual turbidity; (b) UV254 removal efficiency; (c) DOC removal efficiency; (d) zeta potential.
colloids. Therefore, colloids began to aggregate and the floc size increased gradually. As mentioned in Section 2.2, synthetic macromolecular compound Pa has long chain with five-membered ring like structure called amidine unit and there was a positive charge for each amidine. When Pa was used as coagulant aid with AS in
a
coagulation process, long chain of the polymer could adsorb more suspension colloid particles and promote bridging effectively. Thus, large floc with preferable settling velocity was generated. Because of the positive charge of amidine unit, it was easily for Pa to adsorb micro floc with negative charge. As a result, the speed of aggrega-
b
Fig. 5. Growth, breakage and regrowth of flocs for different aluminum dosages: (a) AS; (b) AS + 0.8 mg/L Pa.
482
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
a
b
Fig. 6. Flocs size and flocs growth rate for different aluminum dosages: (a) Floc size d50 ; (b) Floc growth rate. Table 2 Strength and recovery factors of flocs formed by different coagulant with different doses. Parameter
Coagulant
Strength factor
AS AS–Pa AS AS–Pa
Recovery factor
Alum dose (mg/L) 3
5
7
9
11
31.85 ± 1.01 28.91 ± 1.13 20.37 ± 1.32 56.45 ± 3.15
36.21 ± 0.67 34.52 ± 1.51 18.98 ± 1.05 53.12 ± 2.49
34.21 ± 1.24 30.85 ± 2.25 20.34 ± 1.57 47.34 ± 1.47
39.69 ± 1.07 32.64 ± 1.29 20.84 ± 0.95 48.11 ± 1.07
42.12 ± 0.76 38.28 ± 2.04 15.74 ± 1.24 29.11 ± 0.96
tion enhanced a lot. As stated above, the adsorption bridging ability played a positive role in flocculation process. 3.2.2. Floc strength factor and recovery factor The strength and recovery factors of floc formed by different coagulant with different doses were calculated according to Eqs. (3)–(4), and the results are shown in Table 2. The floc particle size distribution of different coagulant was also analyzed. The result is shown in Fig. 7. It could be seen from Fig. 7 that the major peak shifted apparently after breakage for both AS and AS–Pa. And the strength factors shown in Table 2 indicated that the floc formed by AS–Pa was not stronger than that of AS. However, for recovery factor, a gradual decline as the coagulant dosage increased is shown in Table 2, and
a
the values of AS–Pa were much larger than that of AS. The result could also be illustrated with Fig. 7, which shows that, when AS–Pa was used the curve of floc particle size distribution after regrowth was closer to the initial one than that of AS, and this stated that the floc after regrowth formed by AS–Pa was not much smaller than that of initial steady period, and stronger recoverability was also indicated. It is reported that floc formed by charge neutralization have total recoverability [35], while sweep floc have poor regrowth after breakage [34], so the floc reversibility became lower at high AS dose for the hydroxide precipitation of aluminum. And as mentioned above, Pa enhanced the effect of charge neutralization in coagulation process, so floc formed by AS–Pa have better recoverability than that of AS.
b
Fig. 7. Partial size distributions of the flocs after growth, breakage and regrowth: (a) AS; (b) AS + 0.8 mg/L Pa.
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
483
Table 3 Fractal dimension of flocs formed by different coagulant with different doses. Coagulant dosage (mg/L)
AS Before breakage
3 5 7 9 11
2.44 ± 0.04 2.43 ± 0.03 2.67 ± 0.06 2.66 ± 0.02 2.69 ± 0.03
As + 0.8 mg/L Pa After breakage 2.73 ± 0.03 2.69 ± 0.01 2.81 ± 0.03 2.83 ± 0.02 2.86 ± 0.02
3.2.3. Floc fractal dimension Fractal dimensions of flocs formed in different condition was calculated and the result was shown in Table 3. It is obviously that fractal dimensions in the three stages were in the order: before breakage < after regrowth < after breakage. A larger Df means more compact structure of flocs [13]. Hence, floc structure became more compact after flocs broken and reformed. When flocs exposed to a high shear force, floc size decreased rapidly. Smaller floc had a more compact structure as well as a larger Df value [5]. After the break of weak point, fragments arranged into aggregates with more stable and compact structure. When AS used alone, Df meanly increased as AS dosage rose. Increasing AS dosage resulted in an enhancement of charge neutralization, and the zeta potential was closer to zero. As a result, repulsive force decreased and the particle combined more tightly. Hence, more compact flocs with higher Df value were produced. For AS–Pa, Df value decreased as AS dosage rose. When Pa was introduced to the coagulation procedure, effect of charge neutralization was enhanced because of high cationic degree of Pa. At low AS dosage, zeta potential would be closer to zero for both effect of charge neutralization of AS and Pa. Electrostatic repulsion between complexes decreased and more compact flocs were formed than that of AS. While as AS dosage further increased, zeta potential would exceed zero and repulsive force strengthened again. Long chain of Pa also led to the formation of large, high branched flocs with looser structure, which had lower Df value. 4. Conclusion Pa addition improved coagulation performance of AS under investigated dosage and pH condition. The best coagulation performance was reached at the dosage of 5 mg/L Al and 0.8 mg/L Pa. And optimal pH was 6. Under the optimal condition, floc generated by AS–Pa had larger size and higher growth rate than that of AS used alone. Although strength of these floc was similar, the recovery ability of floc formed by AS–Pa was much better than that of AS. In the system of AS–Pa, more compact floc formed at low dosage of AS while looser floc formed at high dosage of AS than that of AS system. Conclusively, the introduction of Pa enhanced charge neutralization effect because of high cationic degree of Pa, and long chain of Pa also provided a function of adsorption bridging in coagulation process. References [1] M. Kabsch-Korbutowicz, Application of ultrafiltration integrated with coagulation for improved NOM removal, Desalination 174 (2005) 13–22. [2] Y.X. Zhao, B.Y. Gao, H.K. Shon, Q.B. Qi, S. Phuntsho, Y. Wang, Q.Y. Yue, Q. Li, J.H. Kim, Characterization of coagulation behavior of titanium tetrachloride coagulant for high and low molecule weight natural organic matter removal: the effect of second dosing, Chem. Eng. J. 228 (2013) 516–525. [3] W. Wang, W. Wang, Q. Fan, Y. Wang, Z. Qiao, X. Wang, Effects of UV radiation on humic acid coagulation characteristics in drinking water treatment processes, Chem. Eng. J. 256 (2014) 137–143. [4] C.Z. Hu, H.J. Liu, J.H. Qu, D.S. Wang, J. Ru, Coagulation behavior of aluminum salts in eutrophic water: significance of Al-13 species and pH control, Environ. Sci. Technol. 40 (2006) 325–331.
After regrowth 2.62 ± 0.03 2.62 ± 0.02 2.74 ± 0.02 2.73 ± 0.03 2.74 ± 0.04
Before breakage 2.64 ± 0.05 2.61 ± 0.04 2.58 ± 0.05 2.50 ± 0.04 2.54 ± 0.04
After breakage 2.78 ± 0.02 2.78 ± 0.01 2.77 ± 0.03 2.74 ± 0.02 2.73 ± 0.01
After regrowth 2.70 ± 0.03 2.67 ± 0.03 2.59 ± 0.02 2.58 ± 0.02 2.60 ± 0.04
[5] B.J.P. Jarvis, S. Parsons, Breakage, regrowth, and fractal nature of natural organic matter flocs, Environ. Sci. Technol. 39 (2005) 2307–2314. [6] J. Wei, B. Gao, Q. Yue, Y. Wang, W. Li, X. Zhu, Comparison of coagulation behavior and floc structure characteristic of different polyferric-cationic polymer dual-coagulants in humic acid solution, Water Res. 43 (2009) 724–732. [7] S. Zhao, B. Gao, Y. Wang, Z. Yang, Influence of a new coagulant aid-enteromorpha extract on coagulation performance and floc characteristics of aluminum sulfate coagulant in kaolin-humic acid solution treatment, Colloids Surf. A: Physicochem. Eng. Aspects 417 (2013) 161–169. [8] W.P. Cheng, F.H. Chi, A study of coagulation mechanisms of polyferric sulfate reacting with humic acid using a fluorescence-quenching method, Water Res. 36 (2002) 4583–4591. [9] F. Ge, H. Shu, Y. Dai, Removal of bromide by aluminum chloride coagulant in the presence of humic acid, J. Hazard. Mater. 147 (2007) 457–462. [10] M.A. Aboulhassan, S. Souabi, A. Yaacoubi, M. Baudu, Improvement of paint effluents coagulation using natural and synthetic coagulant aids, J. Hazard. Mater. 138 (2006) 40–45. [11] J.-F. Lee, P.-M. Liao, D.-H. seng, P.-T. Wen, Behavior of organic polymers in drinking water purification, Chemosphere 37 (1998) 1045–1061. [12] S. Zhao, B. Gao, X. Li, M. Dong, Influence of using enteromorpha extract as a coagulant aid on coagulation behavior and floc characteristics of traditional coagulant in Yellow River water treatment, Chem. Eng. J. 200–202 (2012) 569–576. [13] H. Rong, B. Gao, J. Li, B. Zhang, S. Sun, Y. Wang, Q. Yue, Q. Li, Floc characterization and membrane fouling of polyferric–polymer dual/composite coagulants in coagulation/ultrafiltration hybrid process, J. Colloid Interface Sci. 412 (2013) 39–45. [14] A. Ahmad, S. Wong, T. Teng, A. Zuhairi, Improvement of alum and PACl coagulation by polyacrylamides (PAMs) for the treatment of pulp and paper mill wastewater, Chem. Eng. J. 137 (2008) 510–517. ˜ Microscopic observation [15] M.I. Aguilar, J. Sáez, M. Lloréns, A. Soler, J.F. Ortuno, of particle reduction in slaughterhouse wastewater by coagulation–flocculation using ferric sulphate as coagulant and different coagulant aids, Water Res. 37 (2003) 2233–2241. [16] H. Lin, M. Kimura, K. Hanabusa, H. Shirai, N. Ueno, Y. Mori, Preparation and adsorption properties of poly(N-vinylformamide/acrylonitrile) chelating fiber for heavy metal ions, J. Appl. Polym. Sci. 85 (2002) 1378–1386. [17] K. Sharavanan, H. Komber, D. Fischer, F. Böhme, Hydrolytic degradation of polyamidines and its potential application in controlled release of active agents, Polymer 45 (2004) 2127–2132. [18] R.J. Anthony, R.C. Sims, Optimization of cationic amino starch synthesis using biogenic amines, Carbohydr. Polym. 98 (2013) 1409–1415. [19] Y.X. Zhao, B.Y. Gao, H.Y. Rong, H.K. Shon, J.H. Kim, Q.Y. Yue, Y. Wang, The impacts of coagulant aid-polydimethyldiallylammonium chloride on coagulation performances and floc characteristics in humic acid-kaolin synthetic water treatment with titanium tetrachloride, Chem. Eng. J. 173 (2011) 376–384. [20] R. Wu, B. He, G. Zhao, X. Li, Immobilization of pectinase on polyethyleneimine-coated pulp fiber for treatment of whitewater from papermaking, J. Mol. Catal. B Enzym. 99 (2014) 163–168. [21] R.K. Yap, M. Whittaker, M. Diao, R.M. Stuetz, B. Jefferson, V. Bulmus, W.L. Peirson, A.V. Nguyen, R.K. Henderson, Hydrophobically-associating cationic polymers as micro-bubble surface modifiers in dissolved air flotation for cyanobacteria cell separation, Water Res. 61 (2014) 253–262. [22] G.P. ‘t Lam, M.H. Vermuë, G. Olivieri, L.A.M. van den Broek, M.J. Barbosa, M.H.M. Eppink, R.H. Wijffels, D.M.M. Kleinegris, Cationic polymers for successful flocculation of marine microalgae, Bioresour. Technol. (2014). [23] F. Xiao, P. Yi, X.-R. Pan, B.-J. Zhang, C. Lee, Comparative study of the effects of experimental variables on growth rates of aluminum and iron hydroxide flocs during coagulation and their structural characteristics, Desalination 250 (2010) 902–907. [24] M.A. Yukselen, J. Gregory, The reversibility of floc breakage, Int. J. Miner. Process. 73 (2004) 251–259. [25] R.J. Franc¸ois, Strength of aluminum hydroxide flocs, Water Res. 21 (1987) 1023–1030. [26] T. Harif, A. Adin, Size and structure evolution of kaolin-Al(OH) 3 flocs in the electroflocculation process: a study using static light scattering, Water Res. 45 (2011) 6195–6206. [27] Z. Li, S. Zhong, H. -y. Lei, R. -w. Chen, Q. Yu, H.-L. Li, Production of a novel bioflocculant by Bacillus licheniformis X14 and its application to low
484
[28]
[29]
[30]
[31]
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484 temperature drinking water treatment, Bioresour. Technol. 100 (2009) 3650–3656. P. Xiao, F. Xiao, W. Zhang, B. Zhao, D. Wang, Insight into the combined colloidal-humic acid fouling on the hybrid coagulation microfiltration membrane process: the importance of aluminum, Colloids Surf. A 461 (2014) 98–104. R. Liu, L. Zhu, W. Gong, H. Lan, H. Liu, J. Qu, Effects of fluoride on coagulation performance of aluminum chloride towards Kaolin suspension, Colloids Surf. A 421 (2013) 84–90. H. Zhao, H. Liu, J. Qu, Effect of pH on the aluminum salts hydrolysis during coagulation process: formation and decomposition of polymeric aluminum species, J. Colloid Interface Sci. 330 (2009) 105–112. M. Terashima, M. Fukushima, S. Tanaka, Influence of pH on the surface activity of humic acid: micelle-like aggregate formation and interfacial adsorption, Colloids Surf. A 247 (2004) 77–83.
[32] M. Fukushima, Separation of fulvic acid from soil extracts based on ion-pair formation with a cationic surfactant, Anal. Sci. 22 (2006) 229– 233. [33] J.E. Gregor, C.J. Nokes, E. Fenton, Optimising natural organic matter removal from low turbidity waters by controlled pH adjustment of aluminium coagulation, Water Res. 31 (1997) 2949–2958. [34] W. Xu, B. Gao, Q. Yue, Y. Wang, Effect of shear force and solution pH on flocs breakage and re-growth formed by nano-Al13 polymer, Water Res. 44 (2010) 1893–1899. [35] V. Chaignon, B.S. Lartiges, A. El Samrani, C. Mustin, Evolution of size distribution and transfer of mineral particles between flocs in activated sludges: an insight into floc exchange dynamics, Water Res. 36 (2002) 676–684.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Coagulation performance and floc characteristics of aluminum sulfate with cationic polyamidine as coagulant aid for kaolin-humic acid treatment Bo Guo 1 , Huan Yu 1 , Baoyu Gao ∗ , Hongyan Rong, Hongyu Dong, Defang Ma, Ruihua Li, Shuang Zhao Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, No.27 Shanda South Road, Jinan 250100 Shandong, PR China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The synthesize method of polyamidine was investigated and optimized. performance of AS-polyamidine and AS was comparatively evaluated. • The characteristics of flocs generated by the two kinds of coagulants were studied. • The size and growth rate of the flocs were enhanced by the aid effect of polyamidine. • Recovery ability of flocs was improved due to the assistant role of polyamidine.
• Coagulation
a r t i c l e
i n f o
Article history: Received 12 March 2015 Received in revised form 6 June 2015 Accepted 9 June 2015 Available online 12 June 2015 Keywords: Polyamidine Aluminum sulfate Coagulation performance Floc properties
a b s t r a c t In this study, novel cationic polyamidine (Pa) was synthesized and used as coagulant aid with aluminum sulfate (AS) for kaolin-humic acid simulated water treatment. The effects of Pa on coagulation performance and floc properties were investigated. Results showed that Pa addition significantly enhanced the removal of turbidity, ultraviolet absorbance at 254 nm wavelength (UV254 ) and dissolved organic carbon (DOC). AS–Pa exhibited high coagulation efficiency at border pH range and increased floc size and growth rate significantly. Floc recovery ability was also improved, while floc fractal dimension was lower with Pa addition. This would be attributed to high cationic charge density and long chain of Pa, which could enhance the effect of charge neutralization and adsorption bridging. © 2015 Elsevier B.V. All rights reserved.
∗ Corresponding author. Fax: +86 531 88364513. E-mail addresses: baoyugao [email protected], [email protected] (B. Gao). 1 The authors contributed equally to the work. http://dx.doi.org/10.1016/j.colsurfa.2015.06.017 0927-7757/© 2015 Elsevier B.V. All rights reserved.
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
1. Introduction Humic acid (HA) as the main fraction of humic substances is the important component of natural organic matter (NOM) [1] in surface water, which can cause odor, taste, color and bacterial re-growth in potable water [2]. Moreover, HA can react with disinfectant to produce genotoxic and cytotoxic disinfection byproducts (DBPs) during drinking water treatment [3]. Thus, effective removal of HA in waters is essential. Coagulation treatment is one of the most common methods to remove NOM in water due to its high efficiency and low cost [4]. Coagulant is crucial to the efficiency of coagulation process [5]. Conventional coagulants such as aluminum and ferric salts have already been widely used in water treatment and their coagulation mechanisms have been studied for years [6,7]. However, high dosage of ferric salts with low molecular are corrosive to metal equipment and excessive aluminum salts are harmful to human and living organisms [8,9]. Therefore, it is necessary to develop effective coagulant aids to decrease the dosage of coagulants. Coagulant aids are agents for regulating the flocculation condition, promoting cohesion function or improving the structure of coagulation floc [7]. Hence, they are usually used in coagulation process to promote floc formation and increase floc density, which will subsequently improve the floc sedimentation. [10]. It has been confirmed that better coagulation efficiency, and floc characteristics could be achieved by the combination of synthetic polymers and metal coagulant [11]. Enteromorpha extract with aluminum salts [12] and lignin-acrylamide with polyferric chloride [13] were also improved to enhance coagulation efficiency, floc strength and recoverability significantly. Coagulation efficiency of aluminum and polyaluminum was enhanced with cationic polyacrylamide (PAM) addition for the treatment of pulp and paper mill wastewaters [14]. When anionic PAM was added with polyaluminum chloride or ferric sulphate, significant increase in the settling speed was achieved [15]. Polyamidine (Pa) is one kind of macromolecular polymer with amidine unit on its molecular chains. Pa could be made into chelating fiber by the wet-spinning method and the product has high binding capacity and good adsorption property for heavy metal ions [16]. Sharavanan et al. [17] have reported the hydrolytic degradation of Pa and its application in controlled release of active agents. In this study, Pa with five membered amidine rings was obtained by acid-treating the copolymer of N-vinylformamide (NVF) and acrylonitrile (AN). Due to the high density of amino groups, Pa has high cationic degree [18]. Cationic polymers have been widely used in water treatment, papermaking, textiles, rheology modifiers and oil field chemicals because of the strong hydrophilic character and ionization ability [19–22]. However, there is no previous study about the coagulation performance using Pa as coagulant aid. The objective of this study is to investigate the effect of Pa on the performance and floc properties of aluminum sulfate (AS) coagulation. The coagulation performance would be assessed in terms of turbidity, ultraviolet absorbance at 254 nm wavelength (UV254 ) and dissolved organic carbon (DOC) under different coagulant doses and initial solution pH, and the floc characteristics were evaluated by the size, growth rate, strength and recoverability of flocs. In addition, the coagulation mechanism of the aid effects of Pa was also discussed.
477
dihydrochloride (AIBA, high purity grade, purchased from BioDuly Co., Ltd., Nanjing, China). Al2 (SO4 )3 ·18H2 O and kaolin were supplied by Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Al2 (SO4 )3 ·18H2 O was analytical grade, while kaolin was chemically pure. Humic acid was purchased from Aladdin Industrial Co., Shanghai. Content of fulvic acid was more than 90%. Deionized water was used to prepare all solution. 2.2. General method for copolymerization Synthesis of cationic polyamidine was conducted in a 100 mL round bottom flask as follows: 6.0 g of a mixture comprising NVF and AN in a molar ratio 1:1, and 34.0 mL of ultrapure water were introduced into the flask. In water bath, the mixture was heated to 45 ◦ C under the flow of nitrogen gas and the flow remained for 15 min. Then, the copolymerization was initiated by adding 0.021 g AIBA and nitrogen gas stream continued for another 5 min. After the flask was held at 45 ◦ C water bath for 5 h to produce precipitated polymer, 10 mL of concentrated hydrochloric acid and 20 mL of ultrapure water was added to the suspension. Then, the reaction mixture was held at 110 ◦ C for 4 h under stirring to amidinize the polymer. The resulting polymer solution was added to acetone and the precipitated polymer was dried under vacuum. Finally, the solid polymer-cationic polyamidine was obtained. The cationic polymer had an amidine structure and the repeating amidine unit is represented by the formula (1)
*
H2 C
H2 C CH C
CH N
NH3X-
H2 C
H C
*
CN
n
Wherein, X− represents for Cl− and n represents for the degree of polymerization ranging from 100 to 1000. The molecular weight of polyamidine was 20,000–100,000 Da with 30–70% cationic charge density. Molecular weight was tested by gel chromatography (Waters, USA), and cationic charge density was measured by colloid titration. Standard solution was prepared by potassium polyvinyl sulfate (PVSK). The viscosity was measured with the rotary viscosity meter in solution of 2 g/L at 25 ◦ C, and the result was 150–250 mPa s. 2.3. Synthetic test water Synthetic test water was prepared by HA and kaolin. The stock solution of HA was prepared as follows: 1.0 g HA, and 0.4 g NaOH was dissolved in about 800 mL deionized water, and the solution was diluted to 1 L after continuous stirring for 30 min. The kaolin stock solution was prepared as follows: 5.0 g kaolin was dissolved in 1 L deionized water on magnetic stirrer for half an hour, and then the suspension was transferred to a measuring cylinder to settle for another 30 min. The superficial 500 mL was used as the stock kaolin solution. Particle size distribution of kaolin was measured by Zetasizer Nano Series (Malvern, UK) and shown in Fig. 1. Simulated water samples contained 10 mg/L HA and tap water, and suitable amount of kaolin stock solution was dosed to adjust the turbidity to 15 ± 0.5 NTU. The properties of the water samples were shown in Table 1. 2.4. Jar-test
2. Materials and methods 2.1. Materials The chemicals used in this study included N-vinylformamide (NVF), acrylonitrile (AN), 2,2 -azobis (2-methylpropionamidine)
Coagulation experiments were conducted by a jar-test apparatus (ZR-6, Zhongrun Water Industry Technology Development Co., Ltd., China) under different Al and Pa dosages. When the dualcoagulant was used, the coagulation procedure was as follows: firstly, raw water was stirred at 200 rpm for 30 s to make the solu-
478
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
Fig. 1. Particle size distribution of kaolin.
tions more homogeneous. Secondly, rapid mixing continued for 30 s and AS was dosed at the beginning of this step. Thirdly, Pa was dosed and the solution was mixed at 200 rpm for another 30 s. Then, slow stir of 40 rpm for 15 min was introduced before sedimentation for 30 min. When AS used alone, the coagulation procedure was similar to the above process. Turbidity, DOC, UV254 and zeta potential after coagulation were measured according to the measurements presented in Table. 1. 2.5. On-line monitoring of floc formation, breakage and regrowth Dynamic floc size during coagulation process was measured by a laser diffraction instrument Mastersizer 2000 (Malvern, UK). The suspension was drawn through the optical unit of Mastersizer 2000 by a peristaltic pump (LEAD-1, Longer Precision Pump, China) and then returned to the jar tester using a 5 mm internal diameter tube at a flow rate of 2.0 L/h. The inflow and outflow tubes were positioned opposite each other at a depth just above the impeller in the holding ports. Size measurements of the flocs were taken every half minute in the process of coagulation and the results (median volumetric diameter (d50 )) were recorded automatically by computer. Coagulation procedures were conducted as follows: rapid mixing (200 rpm) for 1.5 min followed by slow mixing (40 rpm) for 15 min (floc growth stage), then the shear was increased to 200 rpm again for 5 min (floc breakage stage), and finally a slow stir at 40 rpm was reintroduced for a further 15 min (floc regrowth stage). Rapid mixing and slow mixing (floc growth stage) were same with the jar test above.
volumetric diameter (d50 ). The growth rate was calculated as follows [23]: Growthrate =
size time
(2)
where size represents for floc size variation over time. Floc strength factor (Sf ) and recovery factor (Rf ) are well established parameters for indicating strength and recoverability of floc in different flocculation systems, and they can be calculated as follows [5,24,25]: Sf =
d(2) × 100 d(1)
(3)
Rf =
d(3) − d(2) × 100 d(1) − d(2)
(4)
where d(1) is the average floc size of steady phase before breakage, d(2) is the floc size after floc breakage, and d(3) is the floc size after regrowth to the new steady phase. Higher Sf means less sensitive to breakage and larger Rf shows better recoverability of floc after being subjected to high shear force. Floc fractal dimension is measured by Laser Light Scattering in this study [26]. In this technology, the relationship among scattered light intensity I, the scattering vector Q and floc fractal dimension Df can be described as follows: I ∝ Q −Df
(5)
In which Q is determined by the refractive index of the medium (n), the laser light wavelength in vacuum () and scattering angle (), as described in Eq. (6):
2.6. Floc characteristics In this study, the size data were expressed as an equivalent volumetric diameter, and the floc size was denoted by median
Q =
4nsin(/2)
(6)
Table 1 Characteristics of synthetic test water and methods of measurement. Characteristics
Values
Methods of measurement
Turbidity (NTU) DOC (mg/L) UV254 (cm−1 ) Zeta potential (mV) pH
14.5–15.5 4.800–5.334 0.28–0.31 −16.4 to –15.6 8.18–8.45
2100P Turbidimeter (Hach Co., USA) TOC-VCPH (Shimadzu Co., Japan) TU-1810 UV–vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.) Zetasizer 3000HSa (Malvern Instruments, UK) pH Meter (Luoqite Electronics Equipment Co., Ltd., Shanghai, China)
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
100
2.2
Pa dose (mg/L)
a
0.00 0.20 0.40 0.60 0.80 1.00
Residual turbidity (NTU)
1.8 1.6
b
90
UV254 removal efficiency (%)
2.0
479
1.4 1.2 1.0 0.8
80 70
Pa dose (mg/L) 0.00 0.20 0.40 0.60 0.80 1.00
60 50 40 30 20
0.6 2
4
6
8
10
12
0
AS dose (mg/L)
2
4
6
AS dose (mg/L)
8
10
12
70
c
d
3
60
0.00 0.20 0.40 0.60 0.80 1.00
50
Zeta potential (mV)
DOC removal efficiency (%)
Pa dose (mg/L) 0
40
Pa dose (mg/L) 0.00 0.20 0.40 0.60 0.80 1.00
30
20
10
-3 -6 -9 -12 -15
0
-18 0
2
4
6
8
10
12
AS dose (mg/L)
0
2
4
6
8
10
12
AS dose (mg/L)
Fig. 2. Dosage effect of AS–Pa on coagulation performance: (a) residual turbidity; (b) UV254 removal efficiency; (c) DOC removal efficiency; (d) zeta potential.
3. Results and discussion 3.1. Effect of Pa addition on coagulation performance 3.1.1. Effect of dosage on coagulation performance Coagulation optimization tests were conducted to determine the optimum dosage for turbidity and organic matter removal under the raw water pH condition. Coagulation behaviors of AS and AS–Pa were evaluated comparatively. Zeta potential, residual turbidity and organic matter removal efficiencies as a function of coagulant dosage are shown in Fig. 2. When AS was used alone, the residual turbidity decreased rapidly at a low aluminum dosage and then increased slightly as the aluminum dosage further increased, however, which still maintained a relative low level. UV254 and DOC removal efficiency was improved with the dosage increasing and high organics removal rates achieved at coagulant dosage of 7 mg/L. As shown in Fig. 2, Pa addition significantly improved the removal efficiency of DOC and UV254 . However, adding Pa decreased the turbidity removal efficiency except when Pa dosage was 0.8 mg/L, which achieved a lower residual turbidity compared with AS used alone. According to Fig. 2(b) and (c), the best coagulation performance was also achieved at the aluminum dosage of 7 mg/L with 0.8 mg/L Pa addition, which UV254 and DOC removal rate was up to 90% and 59%, whereas the HA removal efficiency
obtained by AS alone was only 86% and 42%, respectively, under the same condition. Further increase in the dosage of Pa did not enhance the removal of organic matter. The mechanisms to explain AS coagulation performance include charge neutralization, precipitation, bridge-aggregation, adsorption and sweep-flocculation, one or their combination of which may be dominant at different conditions [8,27]. As shown in Fig. 2(d), when AS used alone, zeta potential improved as the aluminum dosage increased and the maximum value was −4.9 mV, which indicated that charge neutralization played a role in the coagulation process. Since the synthetic test water was alkaline (pH 8.18–8.45), significant precipitation of amorphous hydroxide could occur, and charge neutralization was achieved by adsorbed precipitation [28,29]. When Pa was added, zeta potential drastically increased. The maximum zeta value was 3.1 mV, which was reached at 11 mg AS with 0.8 mg Pa. It can be explained by that when Pa was used as coagulant aid with AS, high cationic degree of Pa improved the degree of charge neutralization significantly. While Pa was further dosed, excessive charge resulted in stronger inter-particle repulsion, which restrained the growth of floc and consequently decreased the coagulation efficiency. Considering coagulant cost and coagulation performance, the optimal dosage of Pa and AS was fixed at 0.8 mg/L and 5.0 mg/L for the subsequent experiments, respectively.
480
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
50
HA Kaolin AS Pa
a
40
Zeta potential (mV)
30 20 10 0
-10 -20 4
5
6
7
8
9
pH Fig. 3. Effect of pH on zeta potential for HA, kaolin, AS and Pa.
3.1.2. Effect of pH on coagulation performance The initial pH of synthetic water was adjusted to 4.0–9.0 by adding HCl and NaOH to investigate the influence of pH on coagulation performance. Residual turbidity, UV254 , DOC removal efficiency and zeta potential as a function of pH are shown in Fig. 4. For both AS and AS–Pa, the suitable initial pH range was 6.0–7.0, which corresponded well with the reported optimums for Al coagulant [30]. The coagulation performance of AS–Pa was better than that of AS within pH investigated. In Fig. 4(a), for both AS and AS–Pa, residual turbidity reduced significantly when pH was increased from 4.0 to 7.0, and reached steady with pH exceeding 7.0. As indicated in Fig. 4(b) and (c), UV254 and DOC removal efficiency increased obviously with pH ranging from 4.0 to 6.0, and the optimal organics removal efficiency was achieved at pH 6.0. under this condition, the UV254 and DOC removal efficiency reached 93% and 62% for As–Pa, which was better than the removal efficiency of 90% and 56% for AS. When initial pH increased from 7.0 to 9.0, the coagulant efficiency with and without Pa addition decreased accordingly. In previous reports, surface tension of HA increased above pH 6, suggesting that HA became to be difficult to aggregate above pH 6 [31]. In addition, ion-pair formation between fulvic acid and alkylammonium surfactants is optimal at pH 7, and the higher pH resulted in the decrease of the formation [32]. These issues suggest that HA becomes to be difficult to coagulate in neutral to weak alkaline condition. Fig. 3 shows pH-dependence of zeta potential for HA, kaolin, AS and Pa alone. Zeta potential of AS and Pa was much higher than that of HA and kaolin in whole pH range. As pH increasing, zeta potential for AS and Pa decreased significantly while HA and kaolin decreased slightly. Fig. 4(d) shows that, under the condition of pH lower than 6, Pa addition significantly increased zeta potential compared with AS used alone owing to high zeta potential of Pa. Because of the lack of OH− at acidic condition, hydrolysis of Al was restrained and Al existed in the form of monomeric and dimeric ones with positive charge [30]. Therefore, charge neutralization was the dominated mechanism under acidic condition. When pH was higher than 6, zeta potential continuously decreased for both coagulants. However, Pa addition made the zeta potential of AS–Pa higher than that of AS alone in the whole pH scale. It indicated that the effect of charge neutralization decreased as pH increased. It was reported that aluminum hydrolysis to voluminous Al(OH)3 would occur under high pH condition and then HA carboxyl groups-hydroxide irons complexes formed, in which
hydrolysates played a role in sweeping and finally resulted in precipitation [33,34]. Therefore, high HA removal efficiency could also be realized when pH was higher than 6 due to sweeping and entrapment. Zeta potential of AS–Pa kept higher than that of AS and the HA removal efficiency was also better. It means that high cationic degree of Pa could also contribute to charge neutralization. Zeta potential of Pa decreased with pH increasing under the alkaline condition. Hence, Pa performed better under neutral or mild acid condition. 3.2. Effect of Pa addition on floc characteristic 3.2.1. Floc size The results of floc growth, breakage and regrowth at different aluminum dosages are shown in Fig. 5, where the floc size was represented by d50 . As shown in Fig. 1, particle size of kaolin was 1.2 m in average, which was too small compared to flocs. Floc growth rate and average floc size at steady state are shown in Fig. 6. The dosages of Pa were constant at 0.8 mg/L based on Section 3.1.1, and the solution pH was adjusted to 6.0 according to Section 3.1.2. It was obviously that floc growth and breakage reached appropriate balance in steady phase [13]. When the shear force was introduced by increasing the mixing speed up to 200 rpm, floc size showed an obvious drop. The flocs began to regrow when the slow stir was reintroduced after following 5 min of quick stir. However, the floc could not recover to the initial floc sizes for both coagulants. As shown in Fig. 5, flocs grew rapidly once the coagulant was dosed until a state of steady floc size distribution was reached. As shown in Fig. 6(a), the floc sizes for AS–Pa with aluminum dosage variation were much bigger than that for AS. When aluminum dosed 5–9 mg/L, floc sizes of AS were about 270–330 m, while floc sizes of AS–Pa could reach 700–750 m. The variation of floc growth rates as a function of aluminum dosage was shown in Fig. 6(b). Floc formed more quickly at the aluminum dosage of 5 mg/L for AS while the floc gave faster aggregation at the aluminum dosage of 3–9 mg/L when Pa was used as coagulant aid with AS. Growth rate of AS–Pa floc (100–110 m/min) was much larger than that of AS floc (65–70 m/min). When AS was dosed firstly, it was adsorbed on the surface of the micro floc quickly and neutralized the negative charge on it subsequently, which resulted in weaker repulsion force between the
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
481
100
7
a
90
UV254 removal efficiency (%)
6
Residual turbidity (NTU)
b
AS AS-Pa
5 4 3 2
80
70
AS AS-Pa
60
50
40
1
30
0 4
5
6
7
8
4
9
5
6
pH
7
8
9
pH
70
c 60
d
AS AS-Pa
8
50
Zeta potential (mV)
DOC removal efficiency (%)
12
AS AS-Pa
40
30
4 0 -4 -8
20
-12 10 4
5
6
7
8
9
4
5
6
7
8
9
pH
pH
Fig. 4. pH Effect on coagulation performance: (a) residual turbidity; (b) UV254 removal efficiency; (c) DOC removal efficiency; (d) zeta potential.
colloids. Therefore, colloids began to aggregate and the floc size increased gradually. As mentioned in Section 2.2, synthetic macromolecular compound Pa has long chain with five-membered ring like structure called amidine unit and there was a positive charge for each amidine. When Pa was used as coagulant aid with AS in
a
coagulation process, long chain of the polymer could adsorb more suspension colloid particles and promote bridging effectively. Thus, large floc with preferable settling velocity was generated. Because of the positive charge of amidine unit, it was easily for Pa to adsorb micro floc with negative charge. As a result, the speed of aggrega-
b
Fig. 5. Growth, breakage and regrowth of flocs for different aluminum dosages: (a) AS; (b) AS + 0.8 mg/L Pa.
482
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
a
b
Fig. 6. Flocs size and flocs growth rate for different aluminum dosages: (a) Floc size d50 ; (b) Floc growth rate. Table 2 Strength and recovery factors of flocs formed by different coagulant with different doses. Parameter
Coagulant
Strength factor
AS AS–Pa AS AS–Pa
Recovery factor
Alum dose (mg/L) 3
5
7
9
11
31.85 ± 1.01 28.91 ± 1.13 20.37 ± 1.32 56.45 ± 3.15
36.21 ± 0.67 34.52 ± 1.51 18.98 ± 1.05 53.12 ± 2.49
34.21 ± 1.24 30.85 ± 2.25 20.34 ± 1.57 47.34 ± 1.47
39.69 ± 1.07 32.64 ± 1.29 20.84 ± 0.95 48.11 ± 1.07
42.12 ± 0.76 38.28 ± 2.04 15.74 ± 1.24 29.11 ± 0.96
tion enhanced a lot. As stated above, the adsorption bridging ability played a positive role in flocculation process. 3.2.2. Floc strength factor and recovery factor The strength and recovery factors of floc formed by different coagulant with different doses were calculated according to Eqs. (3)–(4), and the results are shown in Table 2. The floc particle size distribution of different coagulant was also analyzed. The result is shown in Fig. 7. It could be seen from Fig. 7 that the major peak shifted apparently after breakage for both AS and AS–Pa. And the strength factors shown in Table 2 indicated that the floc formed by AS–Pa was not stronger than that of AS. However, for recovery factor, a gradual decline as the coagulant dosage increased is shown in Table 2, and
a
the values of AS–Pa were much larger than that of AS. The result could also be illustrated with Fig. 7, which shows that, when AS–Pa was used the curve of floc particle size distribution after regrowth was closer to the initial one than that of AS, and this stated that the floc after regrowth formed by AS–Pa was not much smaller than that of initial steady period, and stronger recoverability was also indicated. It is reported that floc formed by charge neutralization have total recoverability [35], while sweep floc have poor regrowth after breakage [34], so the floc reversibility became lower at high AS dose for the hydroxide precipitation of aluminum. And as mentioned above, Pa enhanced the effect of charge neutralization in coagulation process, so floc formed by AS–Pa have better recoverability than that of AS.
b
Fig. 7. Partial size distributions of the flocs after growth, breakage and regrowth: (a) AS; (b) AS + 0.8 mg/L Pa.
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484
483
Table 3 Fractal dimension of flocs formed by different coagulant with different doses. Coagulant dosage (mg/L)
AS Before breakage
3 5 7 9 11
2.44 ± 0.04 2.43 ± 0.03 2.67 ± 0.06 2.66 ± 0.02 2.69 ± 0.03
As + 0.8 mg/L Pa After breakage 2.73 ± 0.03 2.69 ± 0.01 2.81 ± 0.03 2.83 ± 0.02 2.86 ± 0.02
3.2.3. Floc fractal dimension Fractal dimensions of flocs formed in different condition was calculated and the result was shown in Table 3. It is obviously that fractal dimensions in the three stages were in the order: before breakage < after regrowth < after breakage. A larger Df means more compact structure of flocs [13]. Hence, floc structure became more compact after flocs broken and reformed. When flocs exposed to a high shear force, floc size decreased rapidly. Smaller floc had a more compact structure as well as a larger Df value [5]. After the break of weak point, fragments arranged into aggregates with more stable and compact structure. When AS used alone, Df meanly increased as AS dosage rose. Increasing AS dosage resulted in an enhancement of charge neutralization, and the zeta potential was closer to zero. As a result, repulsive force decreased and the particle combined more tightly. Hence, more compact flocs with higher Df value were produced. For AS–Pa, Df value decreased as AS dosage rose. When Pa was introduced to the coagulation procedure, effect of charge neutralization was enhanced because of high cationic degree of Pa. At low AS dosage, zeta potential would be closer to zero for both effect of charge neutralization of AS and Pa. Electrostatic repulsion between complexes decreased and more compact flocs were formed than that of AS. While as AS dosage further increased, zeta potential would exceed zero and repulsive force strengthened again. Long chain of Pa also led to the formation of large, high branched flocs with looser structure, which had lower Df value. 4. Conclusion Pa addition improved coagulation performance of AS under investigated dosage and pH condition. The best coagulation performance was reached at the dosage of 5 mg/L Al and 0.8 mg/L Pa. And optimal pH was 6. Under the optimal condition, floc generated by AS–Pa had larger size and higher growth rate than that of AS used alone. Although strength of these floc was similar, the recovery ability of floc formed by AS–Pa was much better than that of AS. In the system of AS–Pa, more compact floc formed at low dosage of AS while looser floc formed at high dosage of AS than that of AS system. Conclusively, the introduction of Pa enhanced charge neutralization effect because of high cationic degree of Pa, and long chain of Pa also provided a function of adsorption bridging in coagulation process. References [1] M. Kabsch-Korbutowicz, Application of ultrafiltration integrated with coagulation for improved NOM removal, Desalination 174 (2005) 13–22. [2] Y.X. Zhao, B.Y. Gao, H.K. Shon, Q.B. Qi, S. Phuntsho, Y. Wang, Q.Y. Yue, Q. Li, J.H. Kim, Characterization of coagulation behavior of titanium tetrachloride coagulant for high and low molecule weight natural organic matter removal: the effect of second dosing, Chem. Eng. J. 228 (2013) 516–525. [3] W. Wang, W. Wang, Q. Fan, Y. Wang, Z. Qiao, X. Wang, Effects of UV radiation on humic acid coagulation characteristics in drinking water treatment processes, Chem. Eng. J. 256 (2014) 137–143. [4] C.Z. Hu, H.J. Liu, J.H. Qu, D.S. Wang, J. Ru, Coagulation behavior of aluminum salts in eutrophic water: significance of Al-13 species and pH control, Environ. Sci. Technol. 40 (2006) 325–331.
After regrowth 2.62 ± 0.03 2.62 ± 0.02 2.74 ± 0.02 2.73 ± 0.03 2.74 ± 0.04
Before breakage 2.64 ± 0.05 2.61 ± 0.04 2.58 ± 0.05 2.50 ± 0.04 2.54 ± 0.04
After breakage 2.78 ± 0.02 2.78 ± 0.01 2.77 ± 0.03 2.74 ± 0.02 2.73 ± 0.01
After regrowth 2.70 ± 0.03 2.67 ± 0.03 2.59 ± 0.02 2.58 ± 0.02 2.60 ± 0.04
[5] B.J.P. Jarvis, S. Parsons, Breakage, regrowth, and fractal nature of natural organic matter flocs, Environ. Sci. Technol. 39 (2005) 2307–2314. [6] J. Wei, B. Gao, Q. Yue, Y. Wang, W. Li, X. Zhu, Comparison of coagulation behavior and floc structure characteristic of different polyferric-cationic polymer dual-coagulants in humic acid solution, Water Res. 43 (2009) 724–732. [7] S. Zhao, B. Gao, Y. Wang, Z. Yang, Influence of a new coagulant aid-enteromorpha extract on coagulation performance and floc characteristics of aluminum sulfate coagulant in kaolin-humic acid solution treatment, Colloids Surf. A: Physicochem. Eng. Aspects 417 (2013) 161–169. [8] W.P. Cheng, F.H. Chi, A study of coagulation mechanisms of polyferric sulfate reacting with humic acid using a fluorescence-quenching method, Water Res. 36 (2002) 4583–4591. [9] F. Ge, H. Shu, Y. Dai, Removal of bromide by aluminum chloride coagulant in the presence of humic acid, J. Hazard. Mater. 147 (2007) 457–462. [10] M.A. Aboulhassan, S. Souabi, A. Yaacoubi, M. Baudu, Improvement of paint effluents coagulation using natural and synthetic coagulant aids, J. Hazard. Mater. 138 (2006) 40–45. [11] J.-F. Lee, P.-M. Liao, D.-H. seng, P.-T. Wen, Behavior of organic polymers in drinking water purification, Chemosphere 37 (1998) 1045–1061. [12] S. Zhao, B. Gao, X. Li, M. Dong, Influence of using enteromorpha extract as a coagulant aid on coagulation behavior and floc characteristics of traditional coagulant in Yellow River water treatment, Chem. Eng. J. 200–202 (2012) 569–576. [13] H. Rong, B. Gao, J. Li, B. Zhang, S. Sun, Y. Wang, Q. Yue, Q. Li, Floc characterization and membrane fouling of polyferric–polymer dual/composite coagulants in coagulation/ultrafiltration hybrid process, J. Colloid Interface Sci. 412 (2013) 39–45. [14] A. Ahmad, S. Wong, T. Teng, A. Zuhairi, Improvement of alum and PACl coagulation by polyacrylamides (PAMs) for the treatment of pulp and paper mill wastewater, Chem. Eng. J. 137 (2008) 510–517. ˜ Microscopic observation [15] M.I. Aguilar, J. Sáez, M. Lloréns, A. Soler, J.F. Ortuno, of particle reduction in slaughterhouse wastewater by coagulation–flocculation using ferric sulphate as coagulant and different coagulant aids, Water Res. 37 (2003) 2233–2241. [16] H. Lin, M. Kimura, K. Hanabusa, H. Shirai, N. Ueno, Y. Mori, Preparation and adsorption properties of poly(N-vinylformamide/acrylonitrile) chelating fiber for heavy metal ions, J. Appl. Polym. Sci. 85 (2002) 1378–1386. [17] K. Sharavanan, H. Komber, D. Fischer, F. Böhme, Hydrolytic degradation of polyamidines and its potential application in controlled release of active agents, Polymer 45 (2004) 2127–2132. [18] R.J. Anthony, R.C. Sims, Optimization of cationic amino starch synthesis using biogenic amines, Carbohydr. Polym. 98 (2013) 1409–1415. [19] Y.X. Zhao, B.Y. Gao, H.Y. Rong, H.K. Shon, J.H. Kim, Q.Y. Yue, Y. Wang, The impacts of coagulant aid-polydimethyldiallylammonium chloride on coagulation performances and floc characteristics in humic acid-kaolin synthetic water treatment with titanium tetrachloride, Chem. Eng. J. 173 (2011) 376–384. [20] R. Wu, B. He, G. Zhao, X. Li, Immobilization of pectinase on polyethyleneimine-coated pulp fiber for treatment of whitewater from papermaking, J. Mol. Catal. B Enzym. 99 (2014) 163–168. [21] R.K. Yap, M. Whittaker, M. Diao, R.M. Stuetz, B. Jefferson, V. Bulmus, W.L. Peirson, A.V. Nguyen, R.K. Henderson, Hydrophobically-associating cationic polymers as micro-bubble surface modifiers in dissolved air flotation for cyanobacteria cell separation, Water Res. 61 (2014) 253–262. [22] G.P. ‘t Lam, M.H. Vermuë, G. Olivieri, L.A.M. van den Broek, M.J. Barbosa, M.H.M. Eppink, R.H. Wijffels, D.M.M. Kleinegris, Cationic polymers for successful flocculation of marine microalgae, Bioresour. Technol. (2014). [23] F. Xiao, P. Yi, X.-R. Pan, B.-J. Zhang, C. Lee, Comparative study of the effects of experimental variables on growth rates of aluminum and iron hydroxide flocs during coagulation and their structural characteristics, Desalination 250 (2010) 902–907. [24] M.A. Yukselen, J. Gregory, The reversibility of floc breakage, Int. J. Miner. Process. 73 (2004) 251–259. [25] R.J. Franc¸ois, Strength of aluminum hydroxide flocs, Water Res. 21 (1987) 1023–1030. [26] T. Harif, A. Adin, Size and structure evolution of kaolin-Al(OH) 3 flocs in the electroflocculation process: a study using static light scattering, Water Res. 45 (2011) 6195–6206. [27] Z. Li, S. Zhong, H. -y. Lei, R. -w. Chen, Q. Yu, H.-L. Li, Production of a novel bioflocculant by Bacillus licheniformis X14 and its application to low
484
[28]
[29]
[30]
[31]
B. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 476–484 temperature drinking water treatment, Bioresour. Technol. 100 (2009) 3650–3656. P. Xiao, F. Xiao, W. Zhang, B. Zhao, D. Wang, Insight into the combined colloidal-humic acid fouling on the hybrid coagulation microfiltration membrane process: the importance of aluminum, Colloids Surf. A 461 (2014) 98–104. R. Liu, L. Zhu, W. Gong, H. Lan, H. Liu, J. Qu, Effects of fluoride on coagulation performance of aluminum chloride towards Kaolin suspension, Colloids Surf. A 421 (2013) 84–90. H. Zhao, H. Liu, J. Qu, Effect of pH on the aluminum salts hydrolysis during coagulation process: formation and decomposition of polymeric aluminum species, J. Colloid Interface Sci. 330 (2009) 105–112. M. Terashima, M. Fukushima, S. Tanaka, Influence of pH on the surface activity of humic acid: micelle-like aggregate formation and interfacial adsorption, Colloids Surf. A 247 (2004) 77–83.
[32] M. Fukushima, Separation of fulvic acid from soil extracts based on ion-pair formation with a cationic surfactant, Anal. Sci. 22 (2006) 229– 233. [33] J.E. Gregor, C.J. Nokes, E. Fenton, Optimising natural organic matter removal from low turbidity waters by controlled pH adjustment of aluminium coagulation, Water Res. 31 (1997) 2949–2958. [34] W. Xu, B. Gao, Q. Yue, Y. Wang, Effect of shear force and solution pH on flocs breakage and re-growth formed by nano-Al13 polymer, Water Res. 44 (2010) 1893–1899. [35] V. Chaignon, B.S. Lartiges, A. El Samrani, C. Mustin, Evolution of size distribution and transfer of mineral particles between flocs in activated sludges: an insight into floc exchange dynamics, Water Res. 36 (2002) 676–684.