- Email: [email protected]
Evaluation of structural effects on the flocculation performance of a co-graft starch-based flocculant
Water Research 118 (2017) 160e166
Contents lists available at ScienceDirect
Water Research journal homepage: www.elsevier.com/locate/watres
Evaluation of structural effects on the flocculation performance of a co-graft starch-based flocculant Zhouzhou Liu, Hua Wei, Aimin Li, Hu Yang* State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2016 Received in revised form 23 March 2017 Accepted 11 April 2017
The molecular structure of a material substantially determines its final application performance. In this work, a series of starch-based flocculants with different charge densities and average graft chain lengths were prepared by the co-graft polymerization of acrylamide and [(2-methacryloyloxyethyl) trimethyl ammonium chloride] (St-g-PAM-co-PDMC). The flocculation performance of St-g-PAM-co-PDMC was studied systematically at neutral pH using kaolin suspension and sodium humate (NaHA) aqueous solution as synthetic wastewaters. The effects of the two structural factors on the flocculation efficiency of the starch-based flocculants have been investigated. The experimental results showed that the charge density and average graft chain length contributed distinctly to flocculation performance during the removal of both kaolin particles and NaHA under insufficient and excessive flocculant dose conditions. The flocculation mechanisms of this starch-based flocculant were discussed in detail on the basis of the structure-activity relationship, which are significant to optimize the flocculation conditions and guide the development of novel high-performance flocculants. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Co-graft starch-based flocculant Charge density Average graft chain length Flocculation performance Flocculation mechanism Structure-activity relationship
1. Introduction Coagulation/flocculation removes the suspended colloidal particles and various dissolved contaminants from water bodies to achieve efficient solid-liquid separation and water purification (da Silva et al., 2016; Jiang, 2015; Yang et al., 2016). Thus, coagulation/ flocculation is considered as a highly important technique in water treatment and plays an indispensable role in many environmental strategies. Obviously, the final coagulation/flocculation performance is largely dependent on the coagulants/flocculants used. Traditional coagulants/flocculants mainly include inorganic coagulants, such as polyaluminium chloride and polyferric sulfate, as well as synthetic organic polymeric flocculants (Jiang, 2015; Lee et al., 2014; Matilainen et al., 2010), such as polyacrylamide (PAM) and its derivatives. These conventional compounds are widely applied in water treatment plants because of their high water purification efficiencies and reasonable costs (Joo et al., 2007; Li et al., 2005; Renault et al., 2009). However, the residual metal ions of inorganic coagulants and the released noxious monomers from synthetic organic polymeric flocculants in water bodies bring
* Corresponding author. E-mail address: [email protected] (H. Yang). http://dx.doi.org/10.1016/j.watres.2017.04.032 0043-1354/© 2017 Elsevier Ltd. All rights reserved.
serious health risks (Bolto and Gregory, 2007; Okuda et al., 2014; s, 2005). Renault et al., 2009; Stechemesser and Dobia Natural polymer-based flocculants, such as starch (St), chitosan, and cellulose, have recently received increased research attention because of their environmental friendliness, biodegradability, and variety of sources (Bolto and Gregory, 2007; Crini, 2005; Guibal et al., 2006; Laue and Hunkeler, 2006; Sharma et al., 2006; Vijayaraghavan and Yun, 2008; Yang et al., 2016). To improve the flocculation performance of these natural polymers, scholars have used many chemical modification methods, including graft, etherification, and esterification, and obtained various types of natural polymer-based flocculants to generate different applications (Bratskaya et al., 2004; Huang et al., 2016; Laue and Hunkeler, 2006; Liu et al., 2014; Lu et al., 2011, 2014; Rani et al., 2012; Sen et al., 2009; Song et al., 2009; Sonmez et al., 2002; Wang et al., 2011, 2013; Wu et al., 2016). The methods were realized because of the existence of numerous active groups, such as hydroxyl and amino groups, which can be easily modified (Bolto and Gregory, 2007; Crini, 2005; Guibal et al., 2006; Laue and Hunkeler, 2006; Sharma et al., 2006; Vijayaraghavan and Yun, 2008; Yang et al., 2016). As a result, different functional groups can be introduced onto the polysaccharide backbone (Bratskaya et al., 2004; Huang et al., 2016; Laue and Hunkeler, 2006; Liu et al., 2014; Lu et al., 2011, 2014; Rani et al., 2012; Sen et al., 2009; Song et al., 2009;
Z. Liu et al. / Water Research 118 (2017) 160e166
Sonmez et al., 2002; Wang et al., 2011, 2013; Wu et al., 2016). The performance of a material is highly dependent on its molecular structure (Elias, 1984; Flory, 1953; Yang et al., 2016). A material's quantitative structure-activity relationship is greatly significant to successfully facilitate the precise control of the molecular structure and thus improve the final application performance. For natural polymer-based flocculants, various structural factors are involved, such as charge density (CD), grafting ratio, degree of functional group substitution, and distributions. However, previous investigations on the structural effects of polymeric flocculants, including natural polymers, on flocculation performance were quite limited and unsystematic (Bratskaya et al., 2004; Huang et al., 2016; Liu et al., 2014; Lu et al., 2011, 2014; Rani et al., 2012; Sen et al., 2009; Song et al., 2009; Sonmez et al., 2002; Wang et al., 2011, 2013; Wu et al., 2016; Yang et al., 2016). Starch is a low-cost and highly popular natural polymer material (BeMiller and Whistler, 2009; Bolto and Gregory, 2007; Huang et al., 2016; Wu et al., 2016). In the present study, a series of different CDs and average graft chain lengths (Ls) of the co-graft Stbased flocculant, starch-graft-polyacrylamide-co-poly[(2methacryloyloxyethyl) trimethyl ammonium chloride] (St-g-PAMco-PDMC), was prepared. The procedure involved adjusting the mass feeding ratio of two co-graft monomers, namely, acrylamide (AM) and [(2-methacryloyloxyethyl) trimethyl ammonium chloride] (DMC), which are neutral and strongly cationic monomers respectively. A kaolin suspension and a sodium humate (NaHA) aqueous solution were selected as synthetic wastewaters representative of natural water mainly containing inorganic suspended colloids and water-soluble organic contaminants respectively. The flocculation efficiencies of the various St-g-PAM-co-PDMC flocculants were then studied systematically at neutral pH. The effects of both CD and L of this St-based flocculant on flocculation behavior have been investigated and discussed in detail. The two aforementioned structural factors were simultaneously considered to reveal their individual relationships with flocculation performance. The flocculation mechanisms are discussed in detail on the basis of the structure-activity relationship. 2. Materials and methods 2.1. Materials Starch (St, approximately 1.5 105 g/mol weight-average molecular weight) was obtained from Binzhou Jinhui Corn Development Co., Ltd. Kaolin (average particle diameter ~4.18 mm), AM, and toluidine blue O (TBO) were purchased from Sinopharm Chemical Reagent Co., Ltd. Poly(vinyl sulfate) potassium salt (PVSK) and NaHA were obtained from Aladdin Industrial Corporation. DMC (Shanghai Bangcheng Bilogical Technological Co., Ltd.), ammonium persulfate (APS; Shanghai Lingfeng Chemical Reagent Co., Ltd.), and other chemicals were used as received without further purification. Distilled water was used in all the experiments.
the masses of the two monomers were varied for each synthesis experiment to obtain a series of co-graft St-based samples with different CDs and Ls. The detailed mass ratios of St to each of the two monomers are listed in Table 1. After a 3-h reaction under N2 atmosphere, the copolymerization was stopped by pouring the sample solution into acetone, and the obtained white precipitates were St-g-PAM-co-PDMC. The obtained crude co-graft product was then purified three times by a repeated dissolving-precipitating treatment, followed by Soxhlet extraction using ethanol as solvent for 72 h. Finally, the product was vacuum-dried at room temperature for 48 h. Six final co-graft samples were named from St-1 to St-6, with each containing increasing feeding masses of AM (Table 1). 2.3. Characterization 2.3.1. Spectral characterization The molecular structures of St-g-PAM-co-PDMC were basically characterized by Fourier transform infrared (FTIR; Bruker Model IFS 66/S) spectroscopy and 1H nuclear magnetic resonance (1H NMR; Bruker Ascend 400) spectroscopy. The wave numbers measured by FTIR ranged from 500 cm1e4000 cm1. 1H NMR was operated at 400 MHz using D2O as the solvent. 2.3.2. Charge density The CD of St-g-PAM-co-PDMC was determined by colloidal titration (Zhang et al., 2012). A St-g-PAM-co-PDMC aqueous solution with a known concentration was titrated using a PVSK standard solution, an anionic polyelectrolyte. TBO was used as indicator. The calculated CDs of the various co-graft St-based samples are all listed in Table 1. 2.3.3. Average graft chain length Here, L was defined as the average number of the grafted monomers per branched chain, which is far different from the hydrodynamic size of polymer in a solution (Elias, 1984; Flory, 1953). The actual L of a graft copolymer is highly difficult to determine precisely using current characterization methods, because the initiator's initiation efficiency and graft copolymerization efficiency are affected by various environmental factors (Fanta, 1973). Herein, L was roughly estimated in theory on the basis of some assumptions. First, all initiated free radicals would efficiently propagate graft chains for the initiator in each synthesis experiment. Besides, APS can dissociate into two free radicals after initiation. Thus, the number of graft chains was positively correlated to the twice of the mole number of the initiated initiator. Given that the reaction time was 3 h in this case and the half-life of ammonium persulfate was about 60 h (FMC Corporation, 2001), the percentage of the actual disassociated APS (h) here was about 3.4%. As aforementioned, the mass feeding ratio of St to APS was kept constant at about 1:0.1. Accordingly, the number of graft chains (N) was estimated by Eq. (1) as 0.005, i.e., 5 per 1000 saccharide rings, as follows:
2.2. Preparation of St-g-PAM-co-PDMC A desired amount of solid St was dispersed in 100 mL water with continuous stirring under N2 atmosphere at 80 C for 1.0 h. After full gelatinization, the St aqueous mixture was cooled down to 55 C. A known amount of APS aqueous solution as the initiator was then added rapidly to the mixture, and a St-to-APS mass ratio of 1:0.1 was achieved. The system was maintained for 10 min for St pretreatment by the initiator to suppress the self-polymerization of the two monomers (Sonmez et al., 2002). Next, the mixed aqueous solution of AM and DMC monomers was added to the St mixture dropwise. The amounts of St and APS were both kept constant, but
161
N ¼
2,h$mðAPSÞ $MðStÞ MðAPSÞ $mðStÞ
(1)
where m(APS) and m(St) are the mass amounts of APS and St respectively, and M(APS) and M (St) are the molar masses of APS and the saccharide ring of St respectively. Second, in the graft copolymerization, the two monomers were all grafted onto the St backbone, since the yields of products were all quite high more than 90%. The graft copolymerization efficiency was assumed as 1.0. The last one involved the two monomers AM and DMC fully randomly entering the graft chains. Therefore, the Ls
162
Z. Liu et al. / Water Research 118 (2017) 160e166
Table 1 Preparation details and structural parameters of various St-g-PAM-co-PDMC samples, and the flocs properties for flocculation of kaolin suspension and NaHA aqueous solution at the optimal doses.
in the various St-g-PAM-co-PDMC samples could be theoretically estimated based on the feeding amount of the two monomers as listed in Table 1.
L ¼
mðAMÞ =MðAMÞ þ mðDMCÞ =MðDMCÞ mðStÞ =MðStÞ N
(2)
where m(AM) and m(DMC) are the monomer mass amounts of AM and DMC respectively, and M(AM) and M (DMC) are the molar masses of AM and DMC respectively. The series of St-based flocculants were also prepared using almost identical conditions but with varying mass feeding ratios of the two graft monomers. As a result, all the theoretically estimated Ls held the same systematic error and were comparable to the corresponding real values.
2.4. Flocculation experiments Inorganic suspended colloids and water-soluble organic contaminants mainly exist in natural water bodies, most of which are negatively charged. Hence, kaolin suspension and NaHA aqueous solution were selected as typical synthetic wastewaters. A six-place programmed paddle flocculator (Model TA6, Wuhan Hengling Technology Co., Ltd.) was used in our experiments. Flocculant stock solution (0.1 wt%) was freshly prepared using distilled water as solvent. Solid content in the kaolin suspension and the NaHA concentration were 200 and 50 mg/L respectively. The flocculation process is briefly described as follows. Synthetic wastewater (1.0 L) was initially stirred at 300 rpm for 1 min to obtain a homogeneous aqueous mixture. Then, a proper volume of a well-prepared flocculant solution was rapidly added to the jar. The system was continuously stirred at 300 rpm for 10 min, then 50 rpm for 10 min, and finally left to stand for 90 min to allow floc growths to settle down. At last, the supernatant 2.0 cm below the water surface was drawn to test the flocculation performance. The turbidities of the kaolin suspension before (T0) and after (Tt) flocculation were measured using a turbidity indicator model of WGZ-200S (Shanghai Xinrui Instrument Co., Ltd.). The turbidity removal percentage was estimated by Eq. (3) as shown below:
Turbidity removal percentage ð%Þ ¼
T0 Tt 100% T0
(3)
On the basis of a previously well-prepared calibration curve, the NaHA concentrations before (C0) and after (Ct) flocculation were determined using a UV-1800 spectrometer (Shimadzu Corporation, Japan) at 254 nm wavelength. The NaHA removal percentage was then calculated by Eq. (4):
NaHA removal percentage ð%Þ ¼
C0 Ct 100% C0
(4)
Moreover, the two-dimensional fractal dimension (D2) of flocs by various St-g-PAM-co-PDMC samples at their optimal doses was measured using image analysis (Chakraborti et al., 2000, 2003), which was described in detail in our previous work (Huang et al., 2016; Wu et al., 2016). Next, D2 was calculated using Eq. (5):
AflD2
(5)
where A and l are the projected area and characteristic length of the flocs respectively, and D2 is the slope of a plot of log A versus log l by linear fitting (Chakraborti et al., 2000, 2003). 3. Results and discussion 3.1. Characterization Six St-g-PAM-co-PDMC samples, St-1 to St-6, with different CDs and Ls were synthesized by keeping the mass amounts of St and APS at constant values but changing the monomer mass ratios between AM to DMC. The detailed preparation process was described in the experimental section. The molecular structure of St-g-PAM-co-PDMC was characterized by FTIR and 1H NMR spectroscopy, as shown in Fig. 1. Three new characteristic peaks at 1718, 1483, and 956 cm1 were observed in all the FTIR spectra of the various St-g-PAM-co-PDMC samples compared with St (Fig. 1(a)). The 1718 cm1 peak corresponds to the stretching vibration of carbonyl groups on DMC. The other two peaks at 1483 and 956 cm1 could be assigned to the methyl groups on DMC (Wang
Z. Liu et al. / Water Research 118 (2017) 160e166
163
Meanwhile, the resonances at about 2.10 and 1.55 ppm are ascribed respectively to those of methylene and methine protons on the PAM chain (Fig. 1(b)) (Yuan et al., 2010). Similarly, the intensity of the characteristic peaks derived from PAM increased from St-2 to St-6 with the increasing feeding AM mass. Thus, these 1H NMR spectra further confirm the successful preparation of St-g-PAM-coPDMC. In addition, the CD and L of the St-g-PAM-co-PDMC flocculants were both determined using colloidal titration and theoretical estimation respectively, both of which were described in detail in the experimental section. All aforementioned structural parameters of the various co-graft St-based samples are listed in Table 1. Table 1 shows that the CD of the St-g-PAM-co-PDMC samples increased from 0.71 mmol/g of St-6 to 3.36 mmol/g of St-1 with the increase in feeding mass of DMC because of its characteristic strong positive charges. The feeding masses of St and APS were controlled constantly, and the number of graft chains in each St-g-PAM-coPDMC is the same in theory as approximately 5 per 1000 saccharide rings. Thus, the distribution of graft chains on the St backbone in each St-g-PAM-co-PDMC sample was regarded as the same. The effects of graft chain distribution on the flocculation performance were negligible in our study as well. Furthermore, although the total feeding masses of the two monomers were the same, the molar mass of AM was much lower than that of DMC. Thus, the Ls were estimated using Eq. (2) and were found to increase from 938 for St-1 to 2436 for St-6 (Table 1). 3.2. Flocculation performance
Fig. 1. a) FTIR and b) 1H NMR spectra of starch (St) and various St-g-PAM-co-PDMC samples.
et al., 2013; Wu et al., 2016). In the FTIR spectra of St-2 to St-6 after AM feeding, a characteristic peak at 1670 cm1 is shown, which is attributed to the C]O stretching vibration of the CONH2 groups (amide-I) on AM (Liu et al., 2014; Lu et al., 2011; Rani et al., 2012; Sen et al., 2009). The intensity of this peak clearly increased along with the increase in feeding AM mass (Fig. 1(a)). The appearance of these new characteristic peaks indicates that both AM and DMC were successfully grafted onto St. The resonance at around 3.20 ppm in the 1H NMR spectra of various St-g-PAM-co-PDMC samples likewise correspond to the methyl proton on the quaternary ammonium salt (Wu et al., 2016).
3.2.1. Dose effects The series of St-g-PAM-co-PDMC samples were employed as flocculants to flocculate and purify two synthetic wastewaters, i.e., a kaolin suspension and a NaHA aqueous solution. These synthetic wastewaters were considered as representatives of common inorganic suspended colloids and water-soluble organic contaminants in natural water. Dose is a highly important environmental factor in flocculation (Guibal et al., 2006; Yang et al., 2016). The pH of natural water is near 7.0. Fig. 2 shows and compares the measured dose effects of various St-based flocculants on their flocculation performance at neutral pH. Fig. 2 generally displays the upward-climax-downward variation tendencies of the dose dependence of the two contaminant removal percentages using all the measured St-based flocculants. Initially (at the insufficient flocculant dose range), the contaminant removal percentages of both kaolin particles and NaHA increased rapidly with increasing dose. After reaching an optimal dose, the flocculation performance at the excess doses declined gradually, and restabilization effects were notable. The aforementioned phenomenon was attributed to the charge neutralization flocculation effects (Guibal et al., 2006; Wu et al., 2016; Yang et al., 2016). Moreover, charge content (CC) of the flocculant at each dose point was further calculated on the basis of Fig. 2. It was equal to the multiplication product of the CD of the employed flocculants and the fed dose. Accordingly, the CC dependence of the two contaminant removal percentages were obtained and shown in Fig. 3. The St-based flocculants with various CDs and graft chain lengths exhibited fully coincident curves of CC effects on their flocculation performance at the insufficient flocculant dose range. This result further indicates that various St-g-PAM-co-PDMC flocculants obeyed the similar flocculation mechanism. In particular, charge neutralization was the decisive flocculation mechanism for both kaolin suspension and NaHA aqueous solution. Compared with the optimal doses of various St-g-PAM-coPDMC flocculants for the flocculation of kaolin suspensions (approximately 0.2e0.6 mg/L), those for NaHA (around 15e90 mg/
100
a)
St-1 St-2 St-3 St-4 St-5 St-6
90 80 70 60
Kaolin suspension
50 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Turbidity removal percentage (%)
Z. Liu et al. / Water Research 118 (2017) 160e166
Turbidity removal percentage (%)
164
100
St-1 St-2 St-3 St-4 St-5 St-6
a) 90 80 70 60
Kaolin suspension
50
0.000
0.001
100
b)
90 80 70 St-1 St-2 St-3 St-4 St-5 St-6
60 50 40 NaHA aqueous solution
30 20 0
20
40
60
0.002
0.003
0.004
Charge contnet (mmol/L)
80 100 120 140 160 180 200
Dose (mg/L) Fig. 2. The flocculation of (a) kaolin suspension and (b) NaHA aqueous solution using various St-g-PAM-co-PDMC flocculants.
L) rose by about two orders of magnitude. This effect was due to the fact that water-soluble contaminants are more difficult to flocculate efficiently than insoluble colloidal particles in water (Guibal et al., 2006; Wu et al., 2016; Yang et al., 2016). 3.2.2. Effects of CD and graft chain length Besides flocculant dose, the effects of the two structural factors, CD and L, are compared in detail in Fig. 2. For the flocculation of both kaolin suspension and NaHA aqueous solution, the St-g-PAMco-PDMC with higher CDs exhibited better flocculation performances regardless of graft chain length (Fig. 2). Specifically, a more rapid rate in increase of contaminant removal percentages with flocculant dose in the insufficient dose range, the final lower optimal dose, and its corresponding higher maximal contaminant removal percentage were observed. Table 1 and Fig. 2 reveal that St1 attained the highest CD but lowest L, as well as the best flocculation efficiency, among the six measured St-based flocculants. These results further verify that CD contributed more substantially to the flocculation of the aforementioned two contaminants than did the graft chain length before the optimal flocculation conditions were reached. Besides, due to the one containing no AM, St-1 was the most effective graft starch-based flocculants in this current experimental system obviously. The optimal doses of St-1, St-2, and
NaHA removal percentage (%)
NaHA removal percentage (%)
Dose (mg/L) 100
St-1 St-2 St-3 St-4 St-5 St-6
b)
90 80 70 60 50 40 30
NaHA aqueous solution
20 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Charge content (mmol/L) Fig. 3. The flocculation of (a) kaolin suspension and (b) NaHA aqueous solution using various St-g-PAM-co-PDMC flocculants as a function of charge content based on Fig. 2 and Table 1. Here, charge content (CC) ¼ charge density (CD) dose.
St-3 with decreased CD but increased L were fairly similar during the flocculation of each contaminant (Table 1). This finding indicates that the two structural parameters exerted certain compensatory effects. On the other hand, given the cost of DMC and AM, the graft starch-based flocculants with suitable CD own high cost performance, especially in municipal and industrial wastewater treatment. The floc properties, namely, floc size and two-dimensional fractal dimension, produced using the various St-g-PAM-co-PDMC samples during the flocculation of kaolin suspension and NaHA aqueous solution at optimal doses were also measured and listed in Table 1. Accordingly, average D2 of the kaolin and NaHA flocs under various St-based flocculant preparations were around 1.82 and 1.78 respectively. D2 appeared structurally independent because of the steric hindrance effects of the rigid side chains of St-g-PAM-coPDMC (Wu et al., 2016). The difference in internal compactness between kaolin and NaHA flocs (different D2 values) was ascribed to the considerably divergent characteristics of the undissolved inorganic and dissolved organic contaminants. Moreover, the sizes of the kaolin and NaHA flocs both increased from St-1 to St-6 proportionally to the L but inversely to the CD of the St-g-PAMco-PDMC. This pattern implies that the graft chain length also
Z. Liu et al. / Water Research 118 (2017) 160e166
affected the flocculation performance of the co-graft copolymer. Between the kaolin and NaHA flocs, the size of kaolin flocs was much larger, and the increase in floc size from St-1 to St-6 during flocculation was also more evident in the kaolin suspension. This result was achieved because the undissolved kaolin particles with higher density and larger size were more easily flocculated than the water-soluble NaHA in water (Guibal et al., 2006; Wu et al., 2016; Yang et al., 2016). Interestingly, after reaching the optimal dose, the effective flocculation window roughly widened with the decreased CD of the St-based flocculants from St-1 to St-6 (Fig. 2). Hence, the flocculants with lower CDs were more insensitive to dose because of the dominant effects of charge neutralization flocculation (Guibal et al., 2006; Yang et al., 2016). In addition, after normalizing to the total CC of the fed flocculants (Fig. 3), the CC dependence of the two contaminant removal percentages in the excess dose range did not overlap unlike that under the insufficient dose range. Thus, the graft chain length also affected the flocculation performance of these St-based flocculants, and the effects were more evident in the excessive dose range than in the other ranges. At a given CC in the excessive dose range during the flocculation of the kaolin suspension (Fig. 3(a)), St-g-PAM-co-PDMC with longer Ls also exhibited more evident restabilization effects. On the contrary, for the NaHA aqueous solution (Fig. 3(b)), the St-based flocculants with shorter Ls roughly showed much stronger restabilization and lower flocculation efficiency. For this series of St-g-PAM-co-PDMC flocculants, St-1, which possessed the shortest L, contained the highest CD (Table 1). Thus, a lowest dose of St-1 was needed at a constant CC point compared with those of the other flocculant preparations. Conversely, a greatest dose of St-6 was required under the same CC. Moreover, the cationic groups on the graft chains were also distributed
165
unevenly in water because of the spatial connection of the macromolecular chain (Elias, 1984; Flory, 1953). It is fully different from the situation of a low-MW electrolyte, which solution can achieve an infinite dilution state. On the basis of the aggregation rate (k1 fa b), which is composed of two refined parts, i.e., the probability of collision that leads to real attachment (the collision efficiency, a) and the chances of collision at a certain time (collision frequency, b) (von Smoluchowski, 1917). Accordingly, St-6 owns the higher concentrations at a given CC and more collision chances with the contaminants (b) than those under the other preparations. Therefore, St-6 exhibited a higher aggregation rate and more primary flocs over charged. The more over charged primary flocs resulted in the more evident restabilization effects in the flocculation of the kaolin suspension at the excessive dose. For NaHA, the flocs were smaller in size, and the collision frequency with flocculants was relatively lower. However, given the abundant oxygencontaining groups, NaHA exhibited a higher probability of effective collision (a) with flocculants derived from some special interactions in addition to charge attractions (Yang et al., 2016). St-1 presented with a higher local CD in water and thus more readily collided effectively with NaHA, causing the NaHA flocs to restabilize more easily. As revealed, the graft chain length of St-g-PAM-coPDMC was inversely dependent on the restabilization effects during the flocculation of kaolin and NaHA. The detailed schematic morphologies for the flocculation of kaolin suspension by St-6 and NaHA aqueous solution using St-1 respectively at excessive doses with the same final CC are shown in Scheme 1. 4. Conclusion St-based flocculants have recently received increased attention in the field of water treatment because of their environmental
Scheme 1. Schematic morphologies of flocculation of kaolin suspension by St-6 and NaHA aqueous solution using St-1 respectively at excessive doses with the same final charge content.
166
Z. Liu et al. / Water Research 118 (2017) 160e166
friendliness and low cost. In this work, a series of co-graft St-based flocculants with different CDs and Ls were designed and prepared. Besides the dose effects, the flocculation performances of the various St-g-PAM-co-PDMC flocculants in kaolin suspension and NaHA aqueous solution were evaluated in detail at neutral pH on the basis of two structural factors. The experimental results indicated that the charge density and average graft chain length contributed distinctly to flocculation performance at different flocculant dose conditions. Under insufficient flocculant dose, CD played an important role in the flocculation of the two contaminants, and the flocculation performance of St-g-PAM-co-PDMC enhanced with increased CD. Hence, charge neutralization was the dominant flocculation mechanism. Under excessive dose, further increasing the CC induced a restabilization phenomenon in both the kaolin suspension and NaHA solution. The graft chain length exerted a notable but opposite contribution in this situation as well. The St-g-PAM-co-PDMC with longer Ls resulted in more significant restabilization in the flocculation of the kaolin suspension but to a lower degree in the NaHA aqueous solution. This discrepancy was due to the wide difference in characteristics of the dispersed inorganic kaolin particles and the water-soluble organic NaHA in the flocculation process. This study provided a detailed description of the effects of the two important structural factors on the flocculation performance of co-graft St-based flocculants. Given this information, one can feasibly obtain a predicted flocculant by precisely controlling the flocculant's molecular structure to achieve the desired flocculation performance aiming at target contaminants in water. Acknowledgement Supported by the Natural Science Foundation of China (grant nos. 51378250 and 51438008), the Natural Science Foundation of Jiangsu Province (grant no. BK20161405), and Six Talent Peaks Project in Jiangsu Province of China (grant no. 2015-JNHB-003). References BeMiller, J.N., Whistler, R.L., 2009. Starch: Chemistry and Technology, 3nd Ed. Academic Press, New York. Bolto, B., Gregory, J., 2007. Organic polyelectrolytes in water treatment. Water Res. 41 (11), 2301e2324. Bratskaya, S., Schwarz, S., Chervonetsky, D., 2004. Comparative study of humic acids flocculation with chitosan hydrochloride and chitosan glutamate. Water Res. 38 (12), 2955e2961. Chakraborti, R.K., Atkinson, J.F., Van Benschoten, J.E., 2000. Characterization of alum floc by image analysis. Environ. Sci. Technol. 34 (18), 3969e3976. Chakraborti, R.K., Gardner, K.H., Atkinson, J.F., Van Benschoten, J.E., 2003. Changes in fractal dimension during aggregation. Water Res. 37 (4), 873e883. Crini, G., 2005. Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment. Prog. Polym. Sci. 30 (1), 38e70. da Silva, L.F., Barbosa, A.D., de Paula, H.M., Romualdo, L.L., Andrade, L.S., 2016. Treatment of paint manufacturing wastewater by coagulation/electrochemical methods: proposals for disposal and/or reuse of treated water. Water Res. 101, 467e475. Elias, H.G., 1984. Macromolecules. 1 Structure and Properties. Plenum Press, New York. Fanta, G.F., 1973. In block and graft copolymerization. Wiley Inter. Science (London). Flory, P.J., 1953. Principles of Polymer Chemistry. Cornell University Press, New York. FMC Corporation, 2001. Persulfates Technical Information. http://www. peroxychem.com/media/90826/aod_brochure_persulfate.pdf. Guibal, E., Van Vooren, M., Dempsey, B.A., Roussy, J., 2006. A review of the use of
chitosan for the removal of particulate and dissolved contaminants. Sep. Sci. Technol. 41 (11), 2487e2514. Huang, M., Wang, Y.W., Cai, J., Bai, J.F., Yang, H., Li, A.M., 2016. Preparation of dualfunction starch-based flocculants for the simultaneous removal of turbidity and inhibition of Escherichia coli in water. Water Res. 98, 128e137. Jiang, J.Q., 2015. The role of coagulation in water treatment. Curr. Opin. Chem. Eng. 8, 36e44. Joo, D.J., Shin, W.S., Choi, J.H., Choi, S.J., Kim, M.C., Han, M.H., Ha, T.W., Kim, Y.H., 2007. Decolorization of reactive dyes using inorganic coagulants and synthetic polymer. Dyes Pigments 73 (1), 59e64. Laue, C., Hunkeler, D., 2006. Chitosan-graft-acrylamide polyelectrolytes: synthesis, flocculation, and modeling. J. Appl. Polym. Sci. 102 (1), 885e896. Lee, C.S., Robinson, J., Chong, M.F., 2014. A review on application of flocculants in wastewater treatment. Process Saf. Environ. Prot. 92 (6), 489e508. Li, F.T., Zhao, S.F., Zhao, Y., 2005. Coagulants and Flocculants. Chemical Industry Press, Beijing. Liu, H., Yang, X., Zhang, Y., Zhu, H., Yao, J., 2014. Flocculation characteristics of polyacrylamide grafted cellulose from Phyllostachys heterocycla: an efficient and eco-friendly flocculant. Water Res. 59, 165e171. Lu, L., Pan, Z., Hao, N., Peng, W., 2014. A novel acrylamide-free flocculant and its application for sludge dewatering. Water Res. 57, 304e312. Lu, Y.B., Shang, Y.B., Huang, X., Chen, A.M., Yang, Z., Jiang, Y.X., Cai, J., Gu, W., Qian, X.Z., Yang, H., Cheng, R.S., 2011. Preparation of strong cationic chitosangraft-polyacrylamide flocculants and their flocculating properties. Ind. Eng. Chem. Res. 50 (12), 7141e7149. Matilainen, A., Vepsalainen, M., Sillanpaa, M., 2010. Natural organic matter removal by coagulation during drinking water treatment: a review. Adv. Colloid Interface 159 (2), 189e197. Okuda, T., Nishijima, W., Sugimoto, M., Saka, N., Nakai, S., Tanabe, K., Ito, J., Takenaka, K., Okada, M., 2014. Removal of coagulant aluminum from water treatment residuals by acid. Water Res. 60, 75e81. Rani, G.U., Mishra, S., Sen, G., Jha, U., 2012. Polyacrylamide grafted Agar: synthesis and applications of conventional and microwave assisted technique. Carbohydr. Polym. 90 (2), 784e791. Renault, F., Sancey, B., Badot, P.M., Crini, G., 2009. Chitosan for coagulation/flocculation processes - an eco-friendly approach. Eur. Polym. J. 45 (5), 1337e1348. Sen, G., Kumar, R., Ghosh, S., Pal, S., 2009. A novel polymeric flocculant based on polyacrylamide grafted carboxymethylstarch. Carbohydr. Polym. 77 (4), 822e831. Sharma, B.R., Dhuldhoya, N.C., Merchant, U.C., 2006. Flocculants - an ecofriendly approach. J. Polym. Environ. 14 (2), 195e202. Song, H., Wu, D., Zhang, R.Q., Qiao, L.Y., Zhang, S.H., Lin, S., Ye, J., 2009. Synthesis and application of amphoteric starch graft polymer. Carbohydr. Polym. 78 (2), 253e257. Sonmez, H.B., Senkal, B.F., Bicak, N., 2002. Poly(acrylamide) grafts on spherical bead polymers for extremely selective removal of mercuric ions from aqueous solutions. J. Polym. Sci. Part A-Polym. Chem. 40 (17), 3068e3078. s, B., 2005. Coagulation and Flocculation, second ed. CRC Stechemesser, B., Dobia Press. Vijayaraghavan, K., Yun, Y.S., 2008. Bacterial biosorbents and biosorption. Biotechnol. Adv. 26 (3), 266e291. von Smoluchowski, M., 1917. Versuch einer mathematischen theorie der koagulations kinetic kolloider losungen. Z. Phys. Chem. 92, 129e168. Wang, J.P., Chen, Y.Z., Wang, Y., Yuan, S.J., Yu, H.Q., 2011. Optimization of the coagulation-flocculation process for pulp mill wastewater treatment using a combination of uniform design and response surface methodology. Water Res. 45, 5633e5640. Wang, J.P., Yuan, S.J., Wang, Y., Yu, H.Q., 2013. Synthesis, characterization and application of a novel starch-based flocculant with high flocculation and dewatering properties. Water Res. 47 (8), 2643e2648. Wu, H., Liu, Z.Z., Yang, H., Li, A.M., 2016. Evaluation of chain architectures and charge properties of various starch-based flocculants for flocculation of humic acid from water. Water Res. 96, 126e135. Yang, R., Li, H.J., Huang, M., Yang, H., Li, A.M., 2016. A review on chitosan-based flocculants and their applications in water treatment. Water Res. 95, 59e89. Yuan, B., Shang, Y.B., Lu, Y.B., Qin, Z.Q., Jiang, Y.X., Chen, A.M., Qian, X.Z., Wang, G.Z., Yang, H., Cheng, R.S., 2010. The flocculating properties of chitosan-graftpolyacrylamide flocculants (I) e effect of the grafting ratio. J. Appl. Polym. Sci. 117 (4), 1876e1882. Zhang, W.X., Yang, H., Dong, L., Yan, H., Li, H.J., Jiang, Z.W., Kan, X.W., Li, A.M., Cheng, R.S., 2012. Efficient removal of both cationic and anionic dyes from aqueous solutions using a novel amphoteric straw-based adsorbent. Carbohydr. Polym. 90 (2), 887e893.
Contents lists available at ScienceDirect
Water Research journal homepage: www.elsevier.com/locate/watres
Evaluation of structural effects on the flocculation performance of a co-graft starch-based flocculant Zhouzhou Liu, Hua Wei, Aimin Li, Hu Yang* State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2016 Received in revised form 23 March 2017 Accepted 11 April 2017
The molecular structure of a material substantially determines its final application performance. In this work, a series of starch-based flocculants with different charge densities and average graft chain lengths were prepared by the co-graft polymerization of acrylamide and [(2-methacryloyloxyethyl) trimethyl ammonium chloride] (St-g-PAM-co-PDMC). The flocculation performance of St-g-PAM-co-PDMC was studied systematically at neutral pH using kaolin suspension and sodium humate (NaHA) aqueous solution as synthetic wastewaters. The effects of the two structural factors on the flocculation efficiency of the starch-based flocculants have been investigated. The experimental results showed that the charge density and average graft chain length contributed distinctly to flocculation performance during the removal of both kaolin particles and NaHA under insufficient and excessive flocculant dose conditions. The flocculation mechanisms of this starch-based flocculant were discussed in detail on the basis of the structure-activity relationship, which are significant to optimize the flocculation conditions and guide the development of novel high-performance flocculants. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Co-graft starch-based flocculant Charge density Average graft chain length Flocculation performance Flocculation mechanism Structure-activity relationship
1. Introduction Coagulation/flocculation removes the suspended colloidal particles and various dissolved contaminants from water bodies to achieve efficient solid-liquid separation and water purification (da Silva et al., 2016; Jiang, 2015; Yang et al., 2016). Thus, coagulation/ flocculation is considered as a highly important technique in water treatment and plays an indispensable role in many environmental strategies. Obviously, the final coagulation/flocculation performance is largely dependent on the coagulants/flocculants used. Traditional coagulants/flocculants mainly include inorganic coagulants, such as polyaluminium chloride and polyferric sulfate, as well as synthetic organic polymeric flocculants (Jiang, 2015; Lee et al., 2014; Matilainen et al., 2010), such as polyacrylamide (PAM) and its derivatives. These conventional compounds are widely applied in water treatment plants because of their high water purification efficiencies and reasonable costs (Joo et al., 2007; Li et al., 2005; Renault et al., 2009). However, the residual metal ions of inorganic coagulants and the released noxious monomers from synthetic organic polymeric flocculants in water bodies bring
* Corresponding author. E-mail address: [email protected] (H. Yang). http://dx.doi.org/10.1016/j.watres.2017.04.032 0043-1354/© 2017 Elsevier Ltd. All rights reserved.
serious health risks (Bolto and Gregory, 2007; Okuda et al., 2014; s, 2005). Renault et al., 2009; Stechemesser and Dobia Natural polymer-based flocculants, such as starch (St), chitosan, and cellulose, have recently received increased research attention because of their environmental friendliness, biodegradability, and variety of sources (Bolto and Gregory, 2007; Crini, 2005; Guibal et al., 2006; Laue and Hunkeler, 2006; Sharma et al., 2006; Vijayaraghavan and Yun, 2008; Yang et al., 2016). To improve the flocculation performance of these natural polymers, scholars have used many chemical modification methods, including graft, etherification, and esterification, and obtained various types of natural polymer-based flocculants to generate different applications (Bratskaya et al., 2004; Huang et al., 2016; Laue and Hunkeler, 2006; Liu et al., 2014; Lu et al., 2011, 2014; Rani et al., 2012; Sen et al., 2009; Song et al., 2009; Sonmez et al., 2002; Wang et al., 2011, 2013; Wu et al., 2016). The methods were realized because of the existence of numerous active groups, such as hydroxyl and amino groups, which can be easily modified (Bolto and Gregory, 2007; Crini, 2005; Guibal et al., 2006; Laue and Hunkeler, 2006; Sharma et al., 2006; Vijayaraghavan and Yun, 2008; Yang et al., 2016). As a result, different functional groups can be introduced onto the polysaccharide backbone (Bratskaya et al., 2004; Huang et al., 2016; Laue and Hunkeler, 2006; Liu et al., 2014; Lu et al., 2011, 2014; Rani et al., 2012; Sen et al., 2009; Song et al., 2009;
Z. Liu et al. / Water Research 118 (2017) 160e166
Sonmez et al., 2002; Wang et al., 2011, 2013; Wu et al., 2016). The performance of a material is highly dependent on its molecular structure (Elias, 1984; Flory, 1953; Yang et al., 2016). A material's quantitative structure-activity relationship is greatly significant to successfully facilitate the precise control of the molecular structure and thus improve the final application performance. For natural polymer-based flocculants, various structural factors are involved, such as charge density (CD), grafting ratio, degree of functional group substitution, and distributions. However, previous investigations on the structural effects of polymeric flocculants, including natural polymers, on flocculation performance were quite limited and unsystematic (Bratskaya et al., 2004; Huang et al., 2016; Liu et al., 2014; Lu et al., 2011, 2014; Rani et al., 2012; Sen et al., 2009; Song et al., 2009; Sonmez et al., 2002; Wang et al., 2011, 2013; Wu et al., 2016; Yang et al., 2016). Starch is a low-cost and highly popular natural polymer material (BeMiller and Whistler, 2009; Bolto and Gregory, 2007; Huang et al., 2016; Wu et al., 2016). In the present study, a series of different CDs and average graft chain lengths (Ls) of the co-graft Stbased flocculant, starch-graft-polyacrylamide-co-poly[(2methacryloyloxyethyl) trimethyl ammonium chloride] (St-g-PAMco-PDMC), was prepared. The procedure involved adjusting the mass feeding ratio of two co-graft monomers, namely, acrylamide (AM) and [(2-methacryloyloxyethyl) trimethyl ammonium chloride] (DMC), which are neutral and strongly cationic monomers respectively. A kaolin suspension and a sodium humate (NaHA) aqueous solution were selected as synthetic wastewaters representative of natural water mainly containing inorganic suspended colloids and water-soluble organic contaminants respectively. The flocculation efficiencies of the various St-g-PAM-co-PDMC flocculants were then studied systematically at neutral pH. The effects of both CD and L of this St-based flocculant on flocculation behavior have been investigated and discussed in detail. The two aforementioned structural factors were simultaneously considered to reveal their individual relationships with flocculation performance. The flocculation mechanisms are discussed in detail on the basis of the structure-activity relationship. 2. Materials and methods 2.1. Materials Starch (St, approximately 1.5 105 g/mol weight-average molecular weight) was obtained from Binzhou Jinhui Corn Development Co., Ltd. Kaolin (average particle diameter ~4.18 mm), AM, and toluidine blue O (TBO) were purchased from Sinopharm Chemical Reagent Co., Ltd. Poly(vinyl sulfate) potassium salt (PVSK) and NaHA were obtained from Aladdin Industrial Corporation. DMC (Shanghai Bangcheng Bilogical Technological Co., Ltd.), ammonium persulfate (APS; Shanghai Lingfeng Chemical Reagent Co., Ltd.), and other chemicals were used as received without further purification. Distilled water was used in all the experiments.
the masses of the two monomers were varied for each synthesis experiment to obtain a series of co-graft St-based samples with different CDs and Ls. The detailed mass ratios of St to each of the two monomers are listed in Table 1. After a 3-h reaction under N2 atmosphere, the copolymerization was stopped by pouring the sample solution into acetone, and the obtained white precipitates were St-g-PAM-co-PDMC. The obtained crude co-graft product was then purified three times by a repeated dissolving-precipitating treatment, followed by Soxhlet extraction using ethanol as solvent for 72 h. Finally, the product was vacuum-dried at room temperature for 48 h. Six final co-graft samples were named from St-1 to St-6, with each containing increasing feeding masses of AM (Table 1). 2.3. Characterization 2.3.1. Spectral characterization The molecular structures of St-g-PAM-co-PDMC were basically characterized by Fourier transform infrared (FTIR; Bruker Model IFS 66/S) spectroscopy and 1H nuclear magnetic resonance (1H NMR; Bruker Ascend 400) spectroscopy. The wave numbers measured by FTIR ranged from 500 cm1e4000 cm1. 1H NMR was operated at 400 MHz using D2O as the solvent. 2.3.2. Charge density The CD of St-g-PAM-co-PDMC was determined by colloidal titration (Zhang et al., 2012). A St-g-PAM-co-PDMC aqueous solution with a known concentration was titrated using a PVSK standard solution, an anionic polyelectrolyte. TBO was used as indicator. The calculated CDs of the various co-graft St-based samples are all listed in Table 1. 2.3.3. Average graft chain length Here, L was defined as the average number of the grafted monomers per branched chain, which is far different from the hydrodynamic size of polymer in a solution (Elias, 1984; Flory, 1953). The actual L of a graft copolymer is highly difficult to determine precisely using current characterization methods, because the initiator's initiation efficiency and graft copolymerization efficiency are affected by various environmental factors (Fanta, 1973). Herein, L was roughly estimated in theory on the basis of some assumptions. First, all initiated free radicals would efficiently propagate graft chains for the initiator in each synthesis experiment. Besides, APS can dissociate into two free radicals after initiation. Thus, the number of graft chains was positively correlated to the twice of the mole number of the initiated initiator. Given that the reaction time was 3 h in this case and the half-life of ammonium persulfate was about 60 h (FMC Corporation, 2001), the percentage of the actual disassociated APS (h) here was about 3.4%. As aforementioned, the mass feeding ratio of St to APS was kept constant at about 1:0.1. Accordingly, the number of graft chains (N) was estimated by Eq. (1) as 0.005, i.e., 5 per 1000 saccharide rings, as follows:
2.2. Preparation of St-g-PAM-co-PDMC A desired amount of solid St was dispersed in 100 mL water with continuous stirring under N2 atmosphere at 80 C for 1.0 h. After full gelatinization, the St aqueous mixture was cooled down to 55 C. A known amount of APS aqueous solution as the initiator was then added rapidly to the mixture, and a St-to-APS mass ratio of 1:0.1 was achieved. The system was maintained for 10 min for St pretreatment by the initiator to suppress the self-polymerization of the two monomers (Sonmez et al., 2002). Next, the mixed aqueous solution of AM and DMC monomers was added to the St mixture dropwise. The amounts of St and APS were both kept constant, but
161
N ¼
2,h$mðAPSÞ $MðStÞ MðAPSÞ $mðStÞ
(1)
where m(APS) and m(St) are the mass amounts of APS and St respectively, and M(APS) and M (St) are the molar masses of APS and the saccharide ring of St respectively. Second, in the graft copolymerization, the two monomers were all grafted onto the St backbone, since the yields of products were all quite high more than 90%. The graft copolymerization efficiency was assumed as 1.0. The last one involved the two monomers AM and DMC fully randomly entering the graft chains. Therefore, the Ls
162
Z. Liu et al. / Water Research 118 (2017) 160e166
Table 1 Preparation details and structural parameters of various St-g-PAM-co-PDMC samples, and the flocs properties for flocculation of kaolin suspension and NaHA aqueous solution at the optimal doses.
in the various St-g-PAM-co-PDMC samples could be theoretically estimated based on the feeding amount of the two monomers as listed in Table 1.
L ¼
mðAMÞ =MðAMÞ þ mðDMCÞ =MðDMCÞ mðStÞ =MðStÞ N
(2)
where m(AM) and m(DMC) are the monomer mass amounts of AM and DMC respectively, and M(AM) and M (DMC) are the molar masses of AM and DMC respectively. The series of St-based flocculants were also prepared using almost identical conditions but with varying mass feeding ratios of the two graft monomers. As a result, all the theoretically estimated Ls held the same systematic error and were comparable to the corresponding real values.
2.4. Flocculation experiments Inorganic suspended colloids and water-soluble organic contaminants mainly exist in natural water bodies, most of which are negatively charged. Hence, kaolin suspension and NaHA aqueous solution were selected as typical synthetic wastewaters. A six-place programmed paddle flocculator (Model TA6, Wuhan Hengling Technology Co., Ltd.) was used in our experiments. Flocculant stock solution (0.1 wt%) was freshly prepared using distilled water as solvent. Solid content in the kaolin suspension and the NaHA concentration were 200 and 50 mg/L respectively. The flocculation process is briefly described as follows. Synthetic wastewater (1.0 L) was initially stirred at 300 rpm for 1 min to obtain a homogeneous aqueous mixture. Then, a proper volume of a well-prepared flocculant solution was rapidly added to the jar. The system was continuously stirred at 300 rpm for 10 min, then 50 rpm for 10 min, and finally left to stand for 90 min to allow floc growths to settle down. At last, the supernatant 2.0 cm below the water surface was drawn to test the flocculation performance. The turbidities of the kaolin suspension before (T0) and after (Tt) flocculation were measured using a turbidity indicator model of WGZ-200S (Shanghai Xinrui Instrument Co., Ltd.). The turbidity removal percentage was estimated by Eq. (3) as shown below:
Turbidity removal percentage ð%Þ ¼
T0 Tt 100% T0
(3)
On the basis of a previously well-prepared calibration curve, the NaHA concentrations before (C0) and after (Ct) flocculation were determined using a UV-1800 spectrometer (Shimadzu Corporation, Japan) at 254 nm wavelength. The NaHA removal percentage was then calculated by Eq. (4):
NaHA removal percentage ð%Þ ¼
C0 Ct 100% C0
(4)
Moreover, the two-dimensional fractal dimension (D2) of flocs by various St-g-PAM-co-PDMC samples at their optimal doses was measured using image analysis (Chakraborti et al., 2000, 2003), which was described in detail in our previous work (Huang et al., 2016; Wu et al., 2016). Next, D2 was calculated using Eq. (5):
AflD2
(5)
where A and l are the projected area and characteristic length of the flocs respectively, and D2 is the slope of a plot of log A versus log l by linear fitting (Chakraborti et al., 2000, 2003). 3. Results and discussion 3.1. Characterization Six St-g-PAM-co-PDMC samples, St-1 to St-6, with different CDs and Ls were synthesized by keeping the mass amounts of St and APS at constant values but changing the monomer mass ratios between AM to DMC. The detailed preparation process was described in the experimental section. The molecular structure of St-g-PAM-co-PDMC was characterized by FTIR and 1H NMR spectroscopy, as shown in Fig. 1. Three new characteristic peaks at 1718, 1483, and 956 cm1 were observed in all the FTIR spectra of the various St-g-PAM-co-PDMC samples compared with St (Fig. 1(a)). The 1718 cm1 peak corresponds to the stretching vibration of carbonyl groups on DMC. The other two peaks at 1483 and 956 cm1 could be assigned to the methyl groups on DMC (Wang
Z. Liu et al. / Water Research 118 (2017) 160e166
163
Meanwhile, the resonances at about 2.10 and 1.55 ppm are ascribed respectively to those of methylene and methine protons on the PAM chain (Fig. 1(b)) (Yuan et al., 2010). Similarly, the intensity of the characteristic peaks derived from PAM increased from St-2 to St-6 with the increasing feeding AM mass. Thus, these 1H NMR spectra further confirm the successful preparation of St-g-PAM-coPDMC. In addition, the CD and L of the St-g-PAM-co-PDMC flocculants were both determined using colloidal titration and theoretical estimation respectively, both of which were described in detail in the experimental section. All aforementioned structural parameters of the various co-graft St-based samples are listed in Table 1. Table 1 shows that the CD of the St-g-PAM-co-PDMC samples increased from 0.71 mmol/g of St-6 to 3.36 mmol/g of St-1 with the increase in feeding mass of DMC because of its characteristic strong positive charges. The feeding masses of St and APS were controlled constantly, and the number of graft chains in each St-g-PAM-coPDMC is the same in theory as approximately 5 per 1000 saccharide rings. Thus, the distribution of graft chains on the St backbone in each St-g-PAM-co-PDMC sample was regarded as the same. The effects of graft chain distribution on the flocculation performance were negligible in our study as well. Furthermore, although the total feeding masses of the two monomers were the same, the molar mass of AM was much lower than that of DMC. Thus, the Ls were estimated using Eq. (2) and were found to increase from 938 for St-1 to 2436 for St-6 (Table 1). 3.2. Flocculation performance
Fig. 1. a) FTIR and b) 1H NMR spectra of starch (St) and various St-g-PAM-co-PDMC samples.
et al., 2013; Wu et al., 2016). In the FTIR spectra of St-2 to St-6 after AM feeding, a characteristic peak at 1670 cm1 is shown, which is attributed to the C]O stretching vibration of the CONH2 groups (amide-I) on AM (Liu et al., 2014; Lu et al., 2011; Rani et al., 2012; Sen et al., 2009). The intensity of this peak clearly increased along with the increase in feeding AM mass (Fig. 1(a)). The appearance of these new characteristic peaks indicates that both AM and DMC were successfully grafted onto St. The resonance at around 3.20 ppm in the 1H NMR spectra of various St-g-PAM-co-PDMC samples likewise correspond to the methyl proton on the quaternary ammonium salt (Wu et al., 2016).
3.2.1. Dose effects The series of St-g-PAM-co-PDMC samples were employed as flocculants to flocculate and purify two synthetic wastewaters, i.e., a kaolin suspension and a NaHA aqueous solution. These synthetic wastewaters were considered as representatives of common inorganic suspended colloids and water-soluble organic contaminants in natural water. Dose is a highly important environmental factor in flocculation (Guibal et al., 2006; Yang et al., 2016). The pH of natural water is near 7.0. Fig. 2 shows and compares the measured dose effects of various St-based flocculants on their flocculation performance at neutral pH. Fig. 2 generally displays the upward-climax-downward variation tendencies of the dose dependence of the two contaminant removal percentages using all the measured St-based flocculants. Initially (at the insufficient flocculant dose range), the contaminant removal percentages of both kaolin particles and NaHA increased rapidly with increasing dose. After reaching an optimal dose, the flocculation performance at the excess doses declined gradually, and restabilization effects were notable. The aforementioned phenomenon was attributed to the charge neutralization flocculation effects (Guibal et al., 2006; Wu et al., 2016; Yang et al., 2016). Moreover, charge content (CC) of the flocculant at each dose point was further calculated on the basis of Fig. 2. It was equal to the multiplication product of the CD of the employed flocculants and the fed dose. Accordingly, the CC dependence of the two contaminant removal percentages were obtained and shown in Fig. 3. The St-based flocculants with various CDs and graft chain lengths exhibited fully coincident curves of CC effects on their flocculation performance at the insufficient flocculant dose range. This result further indicates that various St-g-PAM-co-PDMC flocculants obeyed the similar flocculation mechanism. In particular, charge neutralization was the decisive flocculation mechanism for both kaolin suspension and NaHA aqueous solution. Compared with the optimal doses of various St-g-PAM-coPDMC flocculants for the flocculation of kaolin suspensions (approximately 0.2e0.6 mg/L), those for NaHA (around 15e90 mg/
100
a)
St-1 St-2 St-3 St-4 St-5 St-6
90 80 70 60
Kaolin suspension
50 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Turbidity removal percentage (%)
Z. Liu et al. / Water Research 118 (2017) 160e166
Turbidity removal percentage (%)
164
100
St-1 St-2 St-3 St-4 St-5 St-6
a) 90 80 70 60
Kaolin suspension
50
0.000
0.001
100
b)
90 80 70 St-1 St-2 St-3 St-4 St-5 St-6
60 50 40 NaHA aqueous solution
30 20 0
20
40
60
0.002
0.003
0.004
Charge contnet (mmol/L)
80 100 120 140 160 180 200
Dose (mg/L) Fig. 2. The flocculation of (a) kaolin suspension and (b) NaHA aqueous solution using various St-g-PAM-co-PDMC flocculants.
L) rose by about two orders of magnitude. This effect was due to the fact that water-soluble contaminants are more difficult to flocculate efficiently than insoluble colloidal particles in water (Guibal et al., 2006; Wu et al., 2016; Yang et al., 2016). 3.2.2. Effects of CD and graft chain length Besides flocculant dose, the effects of the two structural factors, CD and L, are compared in detail in Fig. 2. For the flocculation of both kaolin suspension and NaHA aqueous solution, the St-g-PAMco-PDMC with higher CDs exhibited better flocculation performances regardless of graft chain length (Fig. 2). Specifically, a more rapid rate in increase of contaminant removal percentages with flocculant dose in the insufficient dose range, the final lower optimal dose, and its corresponding higher maximal contaminant removal percentage were observed. Table 1 and Fig. 2 reveal that St1 attained the highest CD but lowest L, as well as the best flocculation efficiency, among the six measured St-based flocculants. These results further verify that CD contributed more substantially to the flocculation of the aforementioned two contaminants than did the graft chain length before the optimal flocculation conditions were reached. Besides, due to the one containing no AM, St-1 was the most effective graft starch-based flocculants in this current experimental system obviously. The optimal doses of St-1, St-2, and
NaHA removal percentage (%)
NaHA removal percentage (%)
Dose (mg/L) 100
St-1 St-2 St-3 St-4 St-5 St-6
b)
90 80 70 60 50 40 30
NaHA aqueous solution
20 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Charge content (mmol/L) Fig. 3. The flocculation of (a) kaolin suspension and (b) NaHA aqueous solution using various St-g-PAM-co-PDMC flocculants as a function of charge content based on Fig. 2 and Table 1. Here, charge content (CC) ¼ charge density (CD) dose.
St-3 with decreased CD but increased L were fairly similar during the flocculation of each contaminant (Table 1). This finding indicates that the two structural parameters exerted certain compensatory effects. On the other hand, given the cost of DMC and AM, the graft starch-based flocculants with suitable CD own high cost performance, especially in municipal and industrial wastewater treatment. The floc properties, namely, floc size and two-dimensional fractal dimension, produced using the various St-g-PAM-co-PDMC samples during the flocculation of kaolin suspension and NaHA aqueous solution at optimal doses were also measured and listed in Table 1. Accordingly, average D2 of the kaolin and NaHA flocs under various St-based flocculant preparations were around 1.82 and 1.78 respectively. D2 appeared structurally independent because of the steric hindrance effects of the rigid side chains of St-g-PAM-coPDMC (Wu et al., 2016). The difference in internal compactness between kaolin and NaHA flocs (different D2 values) was ascribed to the considerably divergent characteristics of the undissolved inorganic and dissolved organic contaminants. Moreover, the sizes of the kaolin and NaHA flocs both increased from St-1 to St-6 proportionally to the L but inversely to the CD of the St-g-PAMco-PDMC. This pattern implies that the graft chain length also
Z. Liu et al. / Water Research 118 (2017) 160e166
affected the flocculation performance of the co-graft copolymer. Between the kaolin and NaHA flocs, the size of kaolin flocs was much larger, and the increase in floc size from St-1 to St-6 during flocculation was also more evident in the kaolin suspension. This result was achieved because the undissolved kaolin particles with higher density and larger size were more easily flocculated than the water-soluble NaHA in water (Guibal et al., 2006; Wu et al., 2016; Yang et al., 2016). Interestingly, after reaching the optimal dose, the effective flocculation window roughly widened with the decreased CD of the St-based flocculants from St-1 to St-6 (Fig. 2). Hence, the flocculants with lower CDs were more insensitive to dose because of the dominant effects of charge neutralization flocculation (Guibal et al., 2006; Yang et al., 2016). In addition, after normalizing to the total CC of the fed flocculants (Fig. 3), the CC dependence of the two contaminant removal percentages in the excess dose range did not overlap unlike that under the insufficient dose range. Thus, the graft chain length also affected the flocculation performance of these St-based flocculants, and the effects were more evident in the excessive dose range than in the other ranges. At a given CC in the excessive dose range during the flocculation of the kaolin suspension (Fig. 3(a)), St-g-PAM-co-PDMC with longer Ls also exhibited more evident restabilization effects. On the contrary, for the NaHA aqueous solution (Fig. 3(b)), the St-based flocculants with shorter Ls roughly showed much stronger restabilization and lower flocculation efficiency. For this series of St-g-PAM-co-PDMC flocculants, St-1, which possessed the shortest L, contained the highest CD (Table 1). Thus, a lowest dose of St-1 was needed at a constant CC point compared with those of the other flocculant preparations. Conversely, a greatest dose of St-6 was required under the same CC. Moreover, the cationic groups on the graft chains were also distributed
165
unevenly in water because of the spatial connection of the macromolecular chain (Elias, 1984; Flory, 1953). It is fully different from the situation of a low-MW electrolyte, which solution can achieve an infinite dilution state. On the basis of the aggregation rate (k1 fa b), which is composed of two refined parts, i.e., the probability of collision that leads to real attachment (the collision efficiency, a) and the chances of collision at a certain time (collision frequency, b) (von Smoluchowski, 1917). Accordingly, St-6 owns the higher concentrations at a given CC and more collision chances with the contaminants (b) than those under the other preparations. Therefore, St-6 exhibited a higher aggregation rate and more primary flocs over charged. The more over charged primary flocs resulted in the more evident restabilization effects in the flocculation of the kaolin suspension at the excessive dose. For NaHA, the flocs were smaller in size, and the collision frequency with flocculants was relatively lower. However, given the abundant oxygencontaining groups, NaHA exhibited a higher probability of effective collision (a) with flocculants derived from some special interactions in addition to charge attractions (Yang et al., 2016). St-1 presented with a higher local CD in water and thus more readily collided effectively with NaHA, causing the NaHA flocs to restabilize more easily. As revealed, the graft chain length of St-g-PAM-coPDMC was inversely dependent on the restabilization effects during the flocculation of kaolin and NaHA. The detailed schematic morphologies for the flocculation of kaolin suspension by St-6 and NaHA aqueous solution using St-1 respectively at excessive doses with the same final CC are shown in Scheme 1. 4. Conclusion St-based flocculants have recently received increased attention in the field of water treatment because of their environmental
Scheme 1. Schematic morphologies of flocculation of kaolin suspension by St-6 and NaHA aqueous solution using St-1 respectively at excessive doses with the same final charge content.
166
Z. Liu et al. / Water Research 118 (2017) 160e166
friendliness and low cost. In this work, a series of co-graft St-based flocculants with different CDs and Ls were designed and prepared. Besides the dose effects, the flocculation performances of the various St-g-PAM-co-PDMC flocculants in kaolin suspension and NaHA aqueous solution were evaluated in detail at neutral pH on the basis of two structural factors. The experimental results indicated that the charge density and average graft chain length contributed distinctly to flocculation performance at different flocculant dose conditions. Under insufficient flocculant dose, CD played an important role in the flocculation of the two contaminants, and the flocculation performance of St-g-PAM-co-PDMC enhanced with increased CD. Hence, charge neutralization was the dominant flocculation mechanism. Under excessive dose, further increasing the CC induced a restabilization phenomenon in both the kaolin suspension and NaHA solution. The graft chain length exerted a notable but opposite contribution in this situation as well. The St-g-PAM-co-PDMC with longer Ls resulted in more significant restabilization in the flocculation of the kaolin suspension but to a lower degree in the NaHA aqueous solution. This discrepancy was due to the wide difference in characteristics of the dispersed inorganic kaolin particles and the water-soluble organic NaHA in the flocculation process. This study provided a detailed description of the effects of the two important structural factors on the flocculation performance of co-graft St-based flocculants. Given this information, one can feasibly obtain a predicted flocculant by precisely controlling the flocculant's molecular structure to achieve the desired flocculation performance aiming at target contaminants in water. Acknowledgement Supported by the Natural Science Foundation of China (grant nos. 51378250 and 51438008), the Natural Science Foundation of Jiangsu Province (grant no. BK20161405), and Six Talent Peaks Project in Jiangsu Province of China (grant no. 2015-JNHB-003). References BeMiller, J.N., Whistler, R.L., 2009. Starch: Chemistry and Technology, 3nd Ed. Academic Press, New York. Bolto, B., Gregory, J., 2007. Organic polyelectrolytes in water treatment. Water Res. 41 (11), 2301e2324. Bratskaya, S., Schwarz, S., Chervonetsky, D., 2004. Comparative study of humic acids flocculation with chitosan hydrochloride and chitosan glutamate. Water Res. 38 (12), 2955e2961. Chakraborti, R.K., Atkinson, J.F., Van Benschoten, J.E., 2000. Characterization of alum floc by image analysis. Environ. Sci. Technol. 34 (18), 3969e3976. Chakraborti, R.K., Gardner, K.H., Atkinson, J.F., Van Benschoten, J.E., 2003. Changes in fractal dimension during aggregation. Water Res. 37 (4), 873e883. Crini, G., 2005. Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment. Prog. Polym. Sci. 30 (1), 38e70. da Silva, L.F., Barbosa, A.D., de Paula, H.M., Romualdo, L.L., Andrade, L.S., 2016. Treatment of paint manufacturing wastewater by coagulation/electrochemical methods: proposals for disposal and/or reuse of treated water. Water Res. 101, 467e475. Elias, H.G., 1984. Macromolecules. 1 Structure and Properties. Plenum Press, New York. Fanta, G.F., 1973. In block and graft copolymerization. Wiley Inter. Science (London). Flory, P.J., 1953. Principles of Polymer Chemistry. Cornell University Press, New York. FMC Corporation, 2001. Persulfates Technical Information. http://www. peroxychem.com/media/90826/aod_brochure_persulfate.pdf. Guibal, E., Van Vooren, M., Dempsey, B.A., Roussy, J., 2006. A review of the use of
chitosan for the removal of particulate and dissolved contaminants. Sep. Sci. Technol. 41 (11), 2487e2514. Huang, M., Wang, Y.W., Cai, J., Bai, J.F., Yang, H., Li, A.M., 2016. Preparation of dualfunction starch-based flocculants for the simultaneous removal of turbidity and inhibition of Escherichia coli in water. Water Res. 98, 128e137. Jiang, J.Q., 2015. The role of coagulation in water treatment. Curr. Opin. Chem. Eng. 8, 36e44. Joo, D.J., Shin, W.S., Choi, J.H., Choi, S.J., Kim, M.C., Han, M.H., Ha, T.W., Kim, Y.H., 2007. Decolorization of reactive dyes using inorganic coagulants and synthetic polymer. Dyes Pigments 73 (1), 59e64. Laue, C., Hunkeler, D., 2006. Chitosan-graft-acrylamide polyelectrolytes: synthesis, flocculation, and modeling. J. Appl. Polym. Sci. 102 (1), 885e896. Lee, C.S., Robinson, J., Chong, M.F., 2014. A review on application of flocculants in wastewater treatment. Process Saf. Environ. Prot. 92 (6), 489e508. Li, F.T., Zhao, S.F., Zhao, Y., 2005. Coagulants and Flocculants. Chemical Industry Press, Beijing. Liu, H., Yang, X., Zhang, Y., Zhu, H., Yao, J., 2014. Flocculation characteristics of polyacrylamide grafted cellulose from Phyllostachys heterocycla: an efficient and eco-friendly flocculant. Water Res. 59, 165e171. Lu, L., Pan, Z., Hao, N., Peng, W., 2014. A novel acrylamide-free flocculant and its application for sludge dewatering. Water Res. 57, 304e312. Lu, Y.B., Shang, Y.B., Huang, X., Chen, A.M., Yang, Z., Jiang, Y.X., Cai, J., Gu, W., Qian, X.Z., Yang, H., Cheng, R.S., 2011. Preparation of strong cationic chitosangraft-polyacrylamide flocculants and their flocculating properties. Ind. Eng. Chem. Res. 50 (12), 7141e7149. Matilainen, A., Vepsalainen, M., Sillanpaa, M., 2010. Natural organic matter removal by coagulation during drinking water treatment: a review. Adv. Colloid Interface 159 (2), 189e197. Okuda, T., Nishijima, W., Sugimoto, M., Saka, N., Nakai, S., Tanabe, K., Ito, J., Takenaka, K., Okada, M., 2014. Removal of coagulant aluminum from water treatment residuals by acid. Water Res. 60, 75e81. Rani, G.U., Mishra, S., Sen, G., Jha, U., 2012. Polyacrylamide grafted Agar: synthesis and applications of conventional and microwave assisted technique. Carbohydr. Polym. 90 (2), 784e791. Renault, F., Sancey, B., Badot, P.M., Crini, G., 2009. Chitosan for coagulation/flocculation processes - an eco-friendly approach. Eur. Polym. J. 45 (5), 1337e1348. Sen, G., Kumar, R., Ghosh, S., Pal, S., 2009. A novel polymeric flocculant based on polyacrylamide grafted carboxymethylstarch. Carbohydr. Polym. 77 (4), 822e831. Sharma, B.R., Dhuldhoya, N.C., Merchant, U.C., 2006. Flocculants - an ecofriendly approach. J. Polym. Environ. 14 (2), 195e202. Song, H., Wu, D., Zhang, R.Q., Qiao, L.Y., Zhang, S.H., Lin, S., Ye, J., 2009. Synthesis and application of amphoteric starch graft polymer. Carbohydr. Polym. 78 (2), 253e257. Sonmez, H.B., Senkal, B.F., Bicak, N., 2002. Poly(acrylamide) grafts on spherical bead polymers for extremely selective removal of mercuric ions from aqueous solutions. J. Polym. Sci. Part A-Polym. Chem. 40 (17), 3068e3078. s, B., 2005. Coagulation and Flocculation, second ed. CRC Stechemesser, B., Dobia Press. Vijayaraghavan, K., Yun, Y.S., 2008. Bacterial biosorbents and biosorption. Biotechnol. Adv. 26 (3), 266e291. von Smoluchowski, M., 1917. Versuch einer mathematischen theorie der koagulations kinetic kolloider losungen. Z. Phys. Chem. 92, 129e168. Wang, J.P., Chen, Y.Z., Wang, Y., Yuan, S.J., Yu, H.Q., 2011. Optimization of the coagulation-flocculation process for pulp mill wastewater treatment using a combination of uniform design and response surface methodology. Water Res. 45, 5633e5640. Wang, J.P., Yuan, S.J., Wang, Y., Yu, H.Q., 2013. Synthesis, characterization and application of a novel starch-based flocculant with high flocculation and dewatering properties. Water Res. 47 (8), 2643e2648. Wu, H., Liu, Z.Z., Yang, H., Li, A.M., 2016. Evaluation of chain architectures and charge properties of various starch-based flocculants for flocculation of humic acid from water. Water Res. 96, 126e135. Yang, R., Li, H.J., Huang, M., Yang, H., Li, A.M., 2016. A review on chitosan-based flocculants and their applications in water treatment. Water Res. 95, 59e89. Yuan, B., Shang, Y.B., Lu, Y.B., Qin, Z.Q., Jiang, Y.X., Chen, A.M., Qian, X.Z., Wang, G.Z., Yang, H., Cheng, R.S., 2010. The flocculating properties of chitosan-graftpolyacrylamide flocculants (I) e effect of the grafting ratio. J. Appl. Polym. Sci. 117 (4), 1876e1882. Zhang, W.X., Yang, H., Dong, L., Yan, H., Li, H.J., Jiang, Z.W., Kan, X.W., Li, A.M., Cheng, R.S., 2012. Efficient removal of both cationic and anionic dyes from aqueous solutions using a novel amphoteric straw-based adsorbent. Carbohydr. Polym. 90 (2), 887e893.