Evaluation of the structural factors for the flocculation performance of a co-graft cationic starch-based flocculant

Chemosphere 240 (2020) 124866

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Evaluation of the structural factors for the flocculation performance of a co-graft cationic starch-based flocculant* Pan Hu, Zhonghua Xi, Yan Li, Aimin Li, Hu Yang* State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing, 210023, 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


Three series of starch-based flocculants with various molecular structure was prepared.
Charge density contributed evidently to flocculation.
Average graft-chain length and its distributions contributed insignificantly.
The structureeactivity relationship was established using a quantitative equation.
Flocculation mechanisms were thus discussed deeply based on the established model.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 August 2019 Received in revised form 8 September 2019 Accepted 13 September 2019 Available online 14 September 2019

Three series of co-graft cationic starch (St)-based flocculants with distinct structural characteristics, namely, charge density (CD), graft-chain length (L), and graft-chain distribution (N), were successfully synthesized through graft copolymerization of [(2-methacryloyloxyethyl) trimethyl ammonium chloride] and acrylamide. These St-based flocculants with different molecular structures were used to flocculate various kaolin suspensions with different initial turbidities and a sodium humate (NaHA) aqueous solution. The experimental results indicated that CD contributed to flocculation evidently, whereas average L and its N were insignificant in experimentally measured ranges. On the basis of phenomenological theory, a second-order polynomial equation was used to further quantitatively analyze the effects of the three structural factors on the flocculation performance of these St-based flocculants, which were fully consistent with the experimental results. Besides, the optimal dose and its corresponding removal rate could be predicted exactly, and the flocculation mechanisms were discussed in detail according to the established models. With the combination of floc properties and zeta potentials, the flocculation mechanisms of these St-based flocculants for flocculation of kaolin suspensions and NaHA aqueous solution were mainly ascribed to charge patching and simple charge neutralization, respectively. These results improve the understanding of the structureeactivity relationship of these graft St-based flocculants, which is of significant guidance for the utilization and design of novel flocculants. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Xiangru Zhang Keywords: Graft cationic starch-based flocculant Charge density Graft-chain length Graft-chain distribution Modeling of structural factor Structureeactivity relationship

* Supported by National Key R&D Program of China (No. 2016YFE0112300), the Natural Science Foundation of China (grant nos. 51978325 and 51778279) and the Major Science and Technology Program for Water Pollution Control and Treatment (grant no. 2017ZX07602-001). * Corresponding author. E-mail address: [email protected] (H. Yang).

https://doi.org/10.1016/j.chemosphere.2019.124866 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

1. Introduction Coagulation/flocculation is a widely used technique in water and wastewater treatment because of its low cost, easy operation, and

2

P. Hu et al. / Chemosphere 240 (2020) 124866

satisfying purification efficiency (Bratby, 2016; da Silva et al., 2016; Naghan et al., 2015; Pirsaheb and Ejraei, 2016; Rahni et al., 2017; Wei et al., 2018a). Generally, coagulants/flocculants play a significant role in coagulation/flocculation. Traditional inorganic metal coagulants and synthetic organic polymeric flocculants exert high efficiency and popularity in water and wastewater treatment plants. However, they have serious potentials in health risks and cause new environmental problems due to their residues of metal ions and toxic organic monomers in practical applications (Bolto and Gregory, 2007; Chen et al., 2015; Lee et al., 2014; Okuda et al., 2014; Renault et al., 2009). Therefore, the use of environmentally friendly, economical, and highly efficient flocculants is urgently needed. Natural polymeric flocculants, such as starch (St), lignin, cellulose, and chitosan, have recently attracted considerable attention due to their significant features of environmental friendliness and biodegradability (BeMiller and Whistler, 2009; Carpenter et al., 2015; Chen et al., 2004; Guibal et al., 2006; Jaafari and Yaghmaeian, 2019; Naghipour et al., 2018; Yang et al., 2016). As an important class of natural polymers, St has been increasingly popular in various fields, including water treatment, due to its low cost and wide source (BeMiller and Whistler, 2009; Lapointe and Barbeau, 2017; Lin et al., 2015; Sen et al., 2009). However, St cannot be directly used as a flocculant due to its weak water solubility, relatively low molecular weight, and insufficient surface charge. Given the abundant oxygen-containing groups on St, various chemical modification methods, such as graft copolymerization, esterification, etherification, oxidation, hydrolysis, and cross linking, can flexibly perform and improve the aforementioned weaknesses of St evidently (Guo et al., 2015; Kaith et al., 2010; Lin et al., 2012; Meshram et al., 2009; Rani et al., 2012; Tolvanen et al., 2009; Wang et al., 2013). Nevertheless, various chemical modification methods manufacture numerous St-based flocculants, which usually show different flocculation performance due to their distinct structural features (Huang et al., 2019; Jiang et al., 2010; Liu et al., 2017b), since the final application effect of a material, including flocculants, depends entirely on its internal molecular structure (Flory, 1953; Yang et al., 2016). However, previous works related to the effects of flocculants’ molecular structures on their flocculation properties have mainly focused on qualitative description, whereas quantitative analysis is relatively rare (Bratskaya et al., 2004; Guo et al., 2019; Huang et al., 2016; Liu et al., 2017b; Sen et al., 2009; Wu et al., 2016). Among the aforementioned chemical modification methods, graft copolymerization is a convenient and effective way to introduce branch chains with specific properties onto the polymeric backbone (Fanta, 1973). The flexible side chain grafted onto the polymeric backbone can increase its molecular weight and stretch freely to form a large network structure in the solution, which results in increased hydrodynamic dimensions (Flory, 1953). Thus, graft polymeric flocculants usually have improved adsorption and bridging effects, which cause an enhanced flocculation performance. Varied graft St-based flocculants have been reported in recent years by copolymerization of different monomers, including trimethyl ammonium chloride (DMC) (Wang et al., 2013), acrylamide (AM) (Gumfekar and Soares, 2018; Li et al., 2017), butyl acrylate (Wang et al., 2017), benzyl dimethylammonium chloride (Zhao et al., 2018), and acrylic acid (Yu et al., 2018). However, previous works related to the structural effects of graft polymeric flocculants have mainly focused on the contents and types of selected functional group to achieve a strong adsorption bridging and charge interactions between flocculants and contaminants (Lin et al., 2012; Rath and Singh, 1997; Vandamme et al., 2010; Wei et al., 2008). Detailed structural factors of graft polymers,

such as charge density (CD), graft-chain length (L), and graft-chain distributions on the backbone (N), play important roles in influencing their flocculation properties (Flory, 1953; Guo et al., 2019; Liu et al., 2017b). Nonetheless, only a few studies have involved such fine structural effects (Guo et al., 2019; Liu et al., 2017b; Wu et al., 2016). Recently, Guo et al. (2019) roughly assessed various lignin-based flocculants with different molecular weights, chain architecture, and CDs to flocculation of dye wastewater. In our previous work (Liu et al., 2017b; Wu et al., 2016), several graft Stbased flocculants with different values of CD and average L, various functional groups, and different positions on St backbone were designed and primarily compared in their flocculation performance. However, all the aforementioned reports (Guo et al., 2019; Liu et al., 2017b; Wu et al., 2016) were limited in qualitative description. A quantitative analysis of the effects of the structural factors on flocculation behaviors for the design and development of high-performance flocculants is urgently needed. In this work, three series of graft St-based flocculants with distinct CDs, Ls, and Ns were designed and synthesized by graft copolymerization of two monomers (i.e., AM and DMC) using ammonium persulfate (APS) as initiator. Each series of the graft flocculants was synthesized with only one structural parameter being varied by controlling the doses of two fed monomers and an initiator, which were characterized using Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (1H NMR). Kaolin particles and sodium humate (NaHA), which are typical representatives of suspended particles and water-soluble organic compounds in sewage water, respectively, were used as simulated pollutants to evaluate the flocculation performance of the well-prepared St-based flocculants. A second-order polynomial equation was used to quantitatively analyze the effects of the three aforementioned structural factors under insufficient and excessive flocculant dose conditions on the basis of phenomenological theory (Ginzburg and Landau, 1950), which theory is a generalization and refinement of experimental phenomena but has no function of indepth explanation. A proper structureeactivity relationship of this graft St-based flocculant was established. The removal rate and optimal dose were predicted and compared. The flocculation mechanisms were elucidated by combining their actual flocculation performance, floc properties, and zeta potentials. 2. Materials and methods 2.1. Materials St (weight-average molecular weight of approximately 1.5  105 g/mol) was purchased from Binzhou Jinhui Corn Development Co., Ltd. Kaolin (average particle diameter of ~4.18 mm), AM, and toluidine blue O were obtained from Sinopharm Chemical Reagent Co., Ltd. NaHA, poly(vinyl sulfate) potassium salt, and 3chloro-2-hydroxypropyltrimethylammonium chloride (CTA, 65 wt % in water) were purchased from Aladdin Industrial Corporation. DMC was obtained from Shanghai Bangcheng Bilogical Technological Co., Ltd., and APS was from Shanghai Lingfeng Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. 2.2. Preparation of graft St-based flocculants A desired amount of St was initially endued to a mild etherification using CTA as etherifying agent to increase the water solubility of St according to a previously published work (Wei et al., 2018b). The slightly etherified product (CS) with a CD of 0.228 mmol/g was used to further synthesize the graft St samples. A certain amount of CS was dissolved in water with continuous

P. Hu et al. / Chemosphere 240 (2020) 124866

stirring under N2 atmosphere at 55  C for 30 min. The initiator APS solution was rapidly added to the CS solution. After 10 min, a certain molar ratio of mixed monomers (i.e., DMC and AM) was slowly added dropwise to the CS mixture for 3 h. The final product was separated by acetone and then dried and crushed, followed by soxhlet extraction for 72 h. Three series of St-based flocculants with distinct CDs, Ls, and Ns (i.e., CS-CDs, CS-CD1-Ls, and CS-CD1-Ns) were obtained by adjusting the doses of fed initiator and two monomers. Each series of the graft flocculants was synthesized and controlled with only one structural parameter being varied. The detailed preparation conditions are listed in Table 1. 2.3. Characterization of graft St-based flocculants 2.3.1. General characterization The molecular structures of the St-based flocculants were basically characterized by FTIR (Bruker Model IFS 66/S) and 1H NMR (Bruker Ascend 400). The wavenumbers measured by FTIR ranged from 500 cm1e4000 cm1.1H NMR was operated at 400 MHz using D2O as the solvent. The CDs of the flocculants were detected by colloid titration and are listed in Table 1. The zeta potentials of different sample solutions were measured by Malvern Model NanoZ Zetasizer (UK). 2.3.2. Average L and N The average L and N are two important structural parameters of a graft copolymer, which have significant effects on its application performance (Fanta, 1973). N is expressed as the average number of graft-chain per saccharide rings of these St-based flocculants, and L is the average number of the grafted monomers per branched chain. Here, L is far different from the hydrodynamic size of polymer in a solution (Flory, 1953). However, actual N and L are difficult to experimentally determine due to the lack of appropriate detection methods (Fanta, 1973). In this current work, the two structural parameters were roughly estimated on the basis of several theoretical assumptions (Liu et al., 2017b; Yu et al., 2018). The initiation efficiency of the initiator was initially considered a constant under the same experimental conditions, and all generated free radicals were effective and propagated graft chains. The grafting reaction time was 3 h, and the half-life of APS was approximately 60 h (FMC Corporation, 2001); thus, the percentage of the actual disassociated APS (h) was approximately 3.4%. Each APS decomposition could obtain two free radicals. Therefore, N was proportional to two times

3

of the initiated APS. The efficiency of graft copolymerization was assumed as 1.0, and monomers of DMC and AM were grafted onto the branch randomly. N and L could thus be calculated by Eqs. (1) and (2) respectively, as shown in Table 1. For instance, N is equal to 0.005, i.e., 5 branched chain per 1,000 saccharide rings, while L is 1,400, namely, 1,400 grafted monomers on each branched chain.

N ¼

2,h,mðAPSÞ ,MðCSÞ MðAPSÞ ,mðCSÞ

(1)

. . mðAMÞ MðAMÞ þ mðDMCÞ MðDMCÞ . L ¼ mðCSÞ MðCSÞ  N

(2)

where m(CS), m(APS), m(DMC) and m(AM) are the mass amounts of CS, APS, DMC and AM, respectively, and M(APS),M(DMC), M(AM) and M(CS) are the molar masses of APS, DMC, AM and the saccharide ring of CS, respectively.

2.4. Flocculation experiments 2.4.1. Jar tests Jar tests were conducted using 250 mL jars and a six-place programmed paddle mixer (Model TA6, Wuhan Hengling Technol. Co., Ltd.) at room temperature. Three kaolin suspensions with different initial turbidities (i.e., 55NTU, 115NTU, and 310NTU) and 50 mg/L of NaHA aqueous solution were used as synthetic wastewaters. Each synthetic wastewater and flocculant solution was freshly and accurately prepared. For three kaolin suspensions, the detailed flocculation process consists of three steps: (1) the suspensions were mixed with a desired amount of flocculant solution by 5-min stirring at a high speed of 300 rpm, (2) followed by a slow stirring at 50 rpm for 10 min, and (3) finally settled for 1.5 h. The turbidities of the kaolin suspension before (T0) and after (Tt) flocculation were measured using a WGZ-3B turbidity meter (Shanghai Xinrui Instrument Co., Ltd.), which were extracted at a jar depth of 2.0 cm. The removal rate of turbidity was calculated by

Turbidity removal rate

  T  Tt % ¼ 0  100 %: T0

(3)

For NaHA solution, the flocculation process is as follows: 2 min

Table 1 Synthesis details and structural parameters of various St-based flocculants. Samples

CS:DMC:AM molar ratio

CS:APS mass ratio

Charge density (mmol/g)

Graft-Chain number (N)

CS-CD1 CS-CD2 CS-CD3 CS-CD4 CS-CD5 CS-CD6 CS-CD7 CS-CD1-L1 CS-CD1-L2 CS-CD1-L3 CS-CD1-L4 CS-CD1-L5 CS-CD1-N1 CS-CD1-N2 CS-CD1-N3 CS-CD1-N4 CS-CD1-N5

1:0.5:6.5 1:1.0:6.0 1:1.5:5.5 1:2:5 1:3:4 1:4:3 1:5:2 1: 0.24:2.0 1: 0.39:4.5 1: 0.50:6.5 1: 0.67:9.5 1: 0.84:12.5 1: 0.20:1.20 1: 0.31:3.19 1: 0.50:6.50 1: 0.88:13.1 1: 1.63:26.4

10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 50:1 20:1 10:1 5:1 2.5:1

0.827 1.468 2.038 2.394 3.164 3.748 4.061 0.823 0.813 0.827 0.817 0.809 0.827 0.820 0.827 0.818 0.813

0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.001 0.0025 0.005 0.01 0.02

a b

The average number of graft-chain per saccharide rings estimated by Eq. (1). The average number of grafted monomers per branched chain estimated by Eq. (2).

a

Graft-Chain length (L) 1400 1400 1400 1400 1400 1400 1400 449 977 1400 2034 2668 1400 1400 1400 1400 1400

b

4

P. Hu et al. / Chemosphere 240 (2020) 124866

of rapid stirring at 250 rpm after mixing with a desired amount of flocculant solution, followed by 20 min of slow stirring at 50 rpm; then, the jars were allowed to settle for 30 min. The residual NaHA concentrations before (C0) and after (Ct) flocculation were detected using a UV-1800 spectrometer (Shimadzu Corporation, Japan) at a wavelength of 254 nm and analyzed in combination with a previously measured standard curve. The removal rate of NaHA was estimated as follows:

NaHA removal rate

  C  Ct % ¼ 0  100%: C0

(4)

Each group of flocculation experiments was repeated at least three times, and the result was the average of the three runs with the error bars representative of standard deviation. 2.4.2. Floc property measurements The floc properties, including floc size and two-dimensional fractal dimension (D2), were determined in detail according to previous works (Liu et al., 2017a; Wei et al., 2018b). The characteristic length (l) and projected area (A) were calculated with image analysis software (Imagepro® Plus 6.0) (Chakraborti et al., 2003; Huang et al., 2016). D2 is the slope of a plot of log A versus log l through linear fitting and was calculated by

A alD2

(5)

3. Results and discussion 3.1. Flocculation performance Three series of graft St-based flocculants with different structural features have been obtained, and the detailed preparation process has been described in the experimental section. The molecular structures of the St-based flocculants were basically characterized by FTIR and 1H NMR, as shown in Supporting Information Figs. S1 and S2, respectively. On the basis of the detailed analysis (Supporting Information Text S1), these cationic graft St-based flocculants were obtained successfully. Their detailed structural parameters, including CD, L, and N, were estimated (Table 1). The three series of St-based flocculants had different structural characteristics. CS-CDs had similar L and N but different CDs, CS-CD1-Ls exhibited similar CD and N but different Ls, and CS-CD1-Ns contained similar CD and L but different Ns. Scheme 1 describes the distinct structural morphologies of the various St-based flocculants schematically. 3.1.1. Dose effect The flocculation performance of the three series of graft Stbased flocculants was systematically evaluated, as shown in Fig. 1. Three kaolin suspensions with different initial turbidities and a NaHA aqueous solution were used as synthetic wastewaters. The removal rates of all St-based flocculants in flocculation of four synthetic wastewaters followed a similar dose dependence, that is, the removal rates initially increased rapidly until an optimal value was reached and then decreased. The rapid increased removal rates indicated the high efficiency of the St-based flocculants, and the decreased flocculation performance at overdose was due to the restabilization effect (Guo et al., 2015; Liu et al., 2017b; Yang et al., 2016). On the basis of Fig. 1, the optimal doses are all listed in Table 2. For three kaolin suspensions, the optimal doses of each series of Stbased flocculants roughly increased with the increase of initial turbidity because high-turbidity water needed to consume

substantial flocculants; for NaHA aqueous solutions, considerably higher doses were required than those for three kaolin suspensions due to the fact that the water-soluble organic matters owned considerably more complicated surface structure; however, the inorganic kaolin particles with higher compactness and density were easier to settle down after efficient compression of the electric double layer during flocculation (Huang et al., 2016; Yang et al., 2016). 3.1.2. Effect of CD The curves for dose dependences of removal rates of varied CSCDs with different CDs showed different patterns (Fig. 1aed), and those for CS-CD1-Ls and CS-CD1-Ns nearly overlapped each other and exhibited similar trends (Fig. 1eel). These phenomena indicated qualitatively that the CD of the St-based flocculants (i.e., charge characteristics) influenced their flocculation performance evidently; however, L and N were insignificant in our experimentally measured ranges. Charge content (CC), which is defined as multiplication of the CD of flocculant and its actual fed dose, was introduced. After normalizing to the total CC of the fed flocculants, the curves for CC dependences of removal rates of varied CS-CDs in the insufficient dose range fully overlapped (Fig. 1aed). Thus, the St-based flocculants may obey a similar flocculation mechanism, and charge neutralization played a dominant role in the flocculation. Specifically, on the basis of the synchronously measured zeta potentials of the supernatants in the flocculation processes (Fig. 1), the zeta potentials at the optimal doses for three kaolin suspensions all deviated from zero and maintained negative values, whereas those for NaHA solutions were all close to zero. These experimental facts indicated that the flocculation of kaolin suspensions was mainly attributed to the charge-patching mechanism; however, simple charge neutralization was dominant in the flocculation of NaHA aqueous solution (Guibal et al., 2006; Yang et al., 2016). Meanwhile, the curves for CC dependences of removal rates of varied CS-CDs in the excessive dose range were different and did not overlap (Fig. 1aed). The different degrees of restabilization at overdose may be associated with other mechanisms involved, such as bridging effect. Although CS-CDs had similar L and N, the different CDs resulted in different rigidities and actual hydrodynamic sizes of polymeric flocculants in solution, which lead to different bridging effects (Cho et al., 2006; Flory, 1953; Liu et al., 2017b; Yang et al., 2016). On the basis of Fig. 1aed and Table 2, CS-CDs with high CD exhibited high flocculation efficiency and a low optimal dose but were sensitive to flocculant dose and showed a strong restabilization effect because of the high positive CD and efficient charge neutralization flocculation effects (Liu et al., 2017b; Yang et al., 2016). Because of the one containing the lowest AM, which is a highly toxic monomer, CS-CD7 was the most effective graft St-based flocculants evidently in this current experimental system. The floc properties at optimal doses corresponding to the highest removal efficiency were determined and are listed in Table 2. Floc properties are important indicators and parameters in the actual water treatment (Moghaddam et al., 2010). CS-CDs with small CD produced large flocs in each synthetic wastewater, which may be due to the fact that they had low aggregation efficiency and slow precipitation, which were beneficial to the full growth of floc. 3.1.3. Effects of L and N As mentioned previously, the L and N of the graft St-based flocculants were insignificant for contaminant removal rates (Fig. 1eel) in our measured range. However, their floc properties were evidently influenced by L and N on the basis of Table 2. The produced floc size by CS-CD1-Ls increased with L in each synthetic

P. Hu et al. / Chemosphere 240 (2020) 124866

5

Scheme 1. The structural morphologies of various St-based flocculants:(a) and (b) with the same L and N but different CDs, (a) low, and (b) high CDs, respectively; (c) and (d) with the same CD and N but different Ls, (c) shorter, and (d) longer Ls, respectively; (e) and (f) with the same CD and L but different Ns, (e) fewer, and (f) more Ns, respectively.

wastewater when CD and N were kept constant. CS-CD1-Ls with long branch chains exhibited an extended configuration in solutions, were helpful to approach the contaminants and showed strong bridging flocculation effects (Flory, 1953; Yang et al., 2016). Similarly, CS-CD1-Ns with dense N on the St backbone produced large flocs in flocculation of not only three kaolin suspensions but also the NaHA aqueous solution. Numerous branched chains on the polymeric flocculants were advantageous to the aggregation of contaminants and promotion of floc growth. It was worth noting

that CS-CD1-Ls with short L had similar floc properties to CS-CD1Ns with sparse N while CS-CD1-Ls with long L were also similar to CS-CD1-Ns with dense N due to their similar total graft amounts (N  L) and bridging effects. This experimental fact indicated that the above two structural parameters had certain compensatory effects. The average D2 of the kaolin and NaHA flocs under their optimal doses of flocculants were approximately 1.86 and 1.80, respectively, regardless of CD, L, and N (Table 2). The internal compactness of

40

20

20 1.5 0

1.0

(e)

-5

CS-CD1-L1 CS-CD1-L2 CS-CD1-L3 CS-CD1-L4 CS-CD1-L5

-10 -15 -20 100 80 60

Kaolin suspension 55NTU

40 20 0.0

0.2

0.0

3

0.4

0.6

0.8

1.0

1.2

0.5

1.0

(i)

-5

CS-CD1-N1 CS-CD1-N2 CS-CD1-N3 CS-CD1-N4 CS-CD1-N5

-10 -15 -20 100 80 60

Kaolin suspension 55NTU

40 20 0.0

0.2

0.4

0.6

1

2

3

CS-CD1-L1 CS-CD1-L2 CS-CD1-L3 CS-CD1-L4 CS-CD1-L5

100 80 Kaolin suspension 115NTU

40 20 0.0

0.5

60 40 20 0.0 0.5 1.0 1.5 2.0 2.5 0

4

1.0

1.5

2.0

0.8

Dose (mg/L)

1.0

1.2

5 0 -5

CS-CD1-N1 CS-CD1-N2 CS-CD1-N3 CS-CD1-N4 CS-CD1-N5

-10 -15 -20 100 80 Kaolin suspension 115NTU

40 20 0.0

0.5

1.0

Dose (mg/L)

3

(g) CS-CD1-L1 CS-CD1-L2 CS-CD1-L3 CS-CD1-L4 CS-CD1-L5

-10 -15 -20 100 80 60 Kaolin suspension 310NTU

20 0

1

2

3

4

5

1.5

2.0

10

40

60 40

20

20 0

30

60

90

(k) CS-CD1-N1 CS-CD1-N2 CS-CD1-N3 CS-CD1-N4 CS-CD1-N5

-10 -20 100 80 60

Kaolin suspension 310NTU

40 20 0

1

2

3

4

Dose(mg/L)

5

6

120

20

0

100

200

300

Charge content (μmol/L)

Dose (mg/L)

(h)

0

CS-CD1-L1 CS-CD1-L2 CS-CD1-L3 CS-CD1-L4 CS-CD1-L5

-20 -40 100 80 60

NaHA aqueous solution

40 20 0

6

0

60

80

4

0 -5

40

NaHA aqueous solution

-40 100

50

100

150

200

Dose (mg/L)

Dose (mg/L)

(j)

60

2

20

CS-CD1 CS-CD2 CS-CD3 CS-CD4 80 CS-CD5 CS-CD6 CS-CD7

0 -20

Charge content (μmol/L)

Dose (mg/L) 5

1

40

100

(d)

20

Removal rate (%)

CS-CD1 CS-CD2 CS-CD3 CS-CD4 CS-CD5 CS-CD6 CS-CD7

80

Charge content (μmol/L)

(f)

60

60

Dose (mg/L)

Removal rate (%) Zeta potential (mV)

Removal rate (%) Zeta potential (mV)

0

2.0 0

Dose (mg/L)

5 0 -5 -10 -15 -20

Dose (mg/L) 5

1.5

20

-20 100

40

20

(l)

0

CS-CD1-N1 CS-CD1-N2 CS-CD1-N3 CS-CD1-N4 CS-CD1-N5

-20 -40 100 80 60

NaHA aqueous solution

40 20 0

50

100

150

200

Dose (mg/L)

Fig. 1. Zeta potentials and removal rate of various kaolin suspensions with different initial turbidities: (a, e, i) 55 NTU, (b, f, j) 115 NTU, and (c, g, k) 310 NTU as well as those of (d, h, l) 50 mg/L NaHA aqueous solution flocculated by using (aed) CS-CDs, (eeh) CS-CD1-Ls, and (iel) CS-CD1-Ns respectively. Here, charge content (CC) ¼ charge density (CD)  dose.

P. Hu et al. / Chemosphere 240 (2020) 124866

0

2

20

Charge content (μmol/L)

Dose (mg/L)

5

1

40

40

Removal rate (%) Zeta potential (mV)

0.5

60

Kaolin suspension 310NTU

Removal rate (%)

40

60

CS-CD1 CS-CD2 CS-CD3 CS-CD4 CS-CD5 CS-CD6 CS-CD7

80

80 -10

Removal rate (%) Zeta potential (mV)

80

100

( c)

0

Removal rate (%) Zeta potential (mV)

100

60

100

10

Removal rate (%) Zeta potential (mV)

60

Kaolin suspension 115NTU

Removal rate (%) Zeta potential (mV)

Kaolin suspension 55NTU

-15

80

0 -10

Removal rate (%)

-10

100

(b)

Removal rate (%) Zeta potential (mV)

0

20 10

Removal rate (%)

CS-CD1 CS-CD2 CS-CD3 CS-CD4 80 CS-CD5 CS-CD6 CS-CD7

-5

0.0

Removal rate (%) Zeta potential (mV)

Zeta potential (mV)

100

(a )

Removal rate (%)

5

Removal rate (%) Zeta potential (mV)

Removal rate (%) Zeta potential (mV)

6

10

P. Hu et al. / Chemosphere 240 (2020) 124866

7

Table 2 Floc properties for flocculation of various kaolin suspensions and NaHA aqueous solution by using different St-based flocculants at their optimal doses. Samples Kaolin suspension

NaHA aqueous solution

55 NTU

115 NTU

Optimal D2 dose (mg/L) CS-CD1 CS-CD2 CS-CD3 CS-CD4 CS-CD5 CS-CD6 CS-CD7 CS-CD1L1 CS-CD1L2 CS-CD1L3 CS-CD1L4 CS-CD1L5 CS-CD1N1 CS-CD1N2 CS-CD1N3 CS-CD1N4 CS-CD1N5

310 NTU

l (mm)

Optimal D2 dose (mg/L)

102.0 ± 3.5 105.2 ± 1.3 101.2 ± 3.1 90.8 ± 2.7 89.6 ± 1.9 78.0 ± 1.2 71.7 ± 2.3 77.8 ± 2.1

0.35 0.25 0.20 0.15 0.10 0.10 0.075 0.30

1.847 ± 0.009 1.847 ± 0.021 1.814 ± 0.011 1.885 ± 0.024 1.880 ± 0.021 1.836 ± 0.017 1.874 ± 0.021 1.908 ± 0.008

l (mm)

Optimal D2 dose (mg/L)

149.4 ± 1.6 150.1 ± 1.7 151.2 ± 3.3 142.3 ± 1.5 120.1 ± 1.7 108.7 ± 2.1 100.0 ± 2.7 121.1 ± 2.1

0.45 0.30 0.20 0.20 0.15 0.10 0.10 0.40

Optimal D2 dose (mg/L)

279.0 ± 3.6 282.9 ± 2.5 268.8 ± 2.1 263.7 ± 2.4 254.1 ± 1.6 248.9 ± 1.3 250.2 ± 2.0 250.0 ± 1.2

50.0 30.0 20.0 20.0 15.0 15.0 10.0 45.0

0.25

1.894 ± 0.021 91.0 ± 1.8

0.30

1.827 ± 0.024 122.4 ± 1.7 0.40

1.854 ± 0.029 252.5 ± 2.8 45.0

1.839 ± 0.021 64.1 ± 1.6

0.25

1.862 ± 0.029 97.9 ± 0.7

0.30

1.847 ± 0.031 144.6 ± 2.3 0.40

1.866 ± 0.023 272.7 ± 3.6 45.0

1.815 ± 0.018 73.4 ± 2.6

0.25

1.867 ± 0.012 105.8 ± 0.7 0.30

1.833 ± 0.025 167.2 ± 2.1 0.40

1.874 ± 0.007 284.6 ± 1.1 45.0

1.771 ± 0.031 73.8 ± 3.5

0.25

1.789 ± 0.023 116.5 ± 0.2 0.30

1.837 ± 0.028 176.2 ± 0.5 0.40

1.852 ± 0.025 292.9 ± 0.7 45.0

1.785 ± 0.013 79.4 ± 2.6

0.20

1.827 ± 0.026 74.5 ± 3.3

0.30

1.877 ± 0.020 114.2 ± 3.7 0.40

1.869 ± 0.021 249.1 ± 2.7 45.0

1.805 ± 0.009 54.3 ± 4.2

0.20

1.894 ± 0.013 78.5 ± 2.3

0.30

1.841 ± 0.021 136.9 ± 0.8 0.40

1.911 ± 0.033 267.4 ± 0.7 45.0

1.835 ± 0.018 61.1 ± 3.1

0.20

1.862 ± 0.029 97.9 ± 0.7

0.30

1.927 ± 0.009 155.3 ± 1.7 0.40

1.860 ± 0.023 272.7 ± 3.6 45.0

1.815 ± 0.029 70.8 ± 2.1

0.20

1.879 ± 0.027 107.1 ± 1.1 0.30

1.824 ± 0.021 171.1 ± 2.8 0.40

1.864 ± 0.019 284.6 ± 2.5 45.0

1.765 ± 0.021 79.2 ± 1.7

0.20

1.904 ± 0.026 116.1 ± 0.9 0.30

1.886 ± 0.015 178.8 ± 1.7 0.40

1.874 ± 0.030 298.6 ± 4.1 45.0

1.823 ± 0.014 80.9 ± 3.2

3.2.1. Modeling of structural factors The preceding qualitative discussion on the structural effects of St-based flocculants on flocculation performance at different dose ranges (i.e., insufficient and excessive doses) presented various structural dependences of their flocculation behaviors. The influences of the charge characteristics on the flocculation effect were greater than those of L and N. A second-order polynomial model, as shown in Eq. (6), was used on the basis of phenomenological theory to investigate the relationship between the structural factors of flocculants and their flocculation behavior (Ginzburg and Landau, 1950). Each coefficient was defined by combining L, CD, and flocculant dose, as shown in Eq. (7). The multiplication product of the CD and fed dose is the CC; hence, Eq. (7) was further converted into Eq. (8). n X i¼1

bi x i þ

n X i¼1

bii x2i þ

n1 X

n X

bij xi xj

(6)

i¼1 j¼iþ1

Y ¼ b0 þ bCC $CD$dose þ bL $L þ bCCL $CD$dose$L þ bCC2 $CD2 $dose2 þ bL2 $L2

(7)

1.815 ± 0.006 1.802 ± 0.028 1.847 ± 0.021 1.767 ± 0.019 1.797 ± 0.018 1.849 ± 0.007 1.819 ± 0.017 1.826 ± 0.070

l (mm)

1.862 ± 0.029 1.823 ± 0.022 1.850 ± 0.018 1.874 ± 0.028 1.909 ± 0.021 1.874 ± 0.019 1.901 ± 0.009 1.885 ± 0.027

3.2. Modeling

1.860 ± 0.023 1.882 ± 0.024 1.839 ± 0.025 1.855 ± 0.011 1.869 ± 0.024 1.904 ± 0.029 1.836 ± 0.021 1.887 ± 0.031

l (mm)

0.30 0.20 0.15 0.10 0.10 0.075 0.075 0.25

kaolin and NaHA flocs mainly depended on their considerably discrepant intrinsic characteristics of the undissolved inorganic and dissolved organic contaminants. Kaolin suspensions formed higher density and larger sized flocs in comparison with NaHA aqueous solution due to the complicated structural characteristics of watersoluble organic matters (Guibal et al., 2006; Wu et al., 2016).

Y ¼ b0 þ

50 mg/L

73.1 ± 2.6 70.6 ± 2.1 63.3 ± 0.6 65.4 ± 1.5 50.8 ± 2.4 53.1 ± 1.3 45.7 ± 3.1 50.5 ± 3.1

Y ¼ b0 þ bCC $CC þ bL $L þ bCCL $CC$L þ bCC2 $CC2 þ bL2 $L2 (8) ð CC ¼ CD$dose Þ where Y refers to the turbidity or NaHA removal rate; b0 stands for the constant coefficient; bCC and bL are the linear coefficients corresponding to CC and L respectively; and bCCL, b2CC, and b2L are the quadratic coefficients. Equation (8) was used as a basis to separately fit and evaluate the flocculation curves of the three series of graft St-based flocculants at insufficient and excessive dose ranges, since different doses fed are usually resultant in different flocculation effects due to various physicochemical processes involved at different dose ranges. The original and simulation data for the flocculation of various kaolin suspensions and NaHA aqueous solution are presented and compared in Fig. 2. The fitting was accomplished using the Matlab cftool function. The obtained simulation coefficients are listed in Table 3. The fitting and actual experimental data agree well (Fig. 2), and most of the correlation coefficients (R2) were greater than 0.9 (Table 2). Hence, the second-order polynomial model described the experimental results thoroughly. As previously mentioned, CC dominantly contributed to the contaminant removal efficiency. At insufficient flocculant dose ranges, all values of bCC for the flocculation of various kaolin suspensions and NaHA aqueous solution were positive, which confirmed the favorable conditions for CC. This finding was attributed to the notions that a high CC reflects an effective charge neutralization (Guibal et al., 2006; Yang et al., 2016). However, when the flocculant doses were higher than the optimal levels at the excessive dose range, bCC became negative. Hence, CC holds an inverse relationship with the contaminant removal rate, which

40 20 0

D1

120 100 80

CS-C

D2

CS-C

Insufficient

Original data

Simulative data

60 40 20 120 0

Excessive

Kaolin suspension 55NTU

80 60 40 20 0

CS-C

D

120 100 80

(i)

1-L1

CS-C

D

1-L2

CS-C

D

1-L3

CS-C

D

Original data

Insufficient

1-L4

CS-C

D

1-L5

Simulative data

60 40 20 120 0 100

Excessive

Kaolin suspension 55NTU

80 60 40 20 0

CS

-N1 -CD1

CS

-N2 -CD1

1200

CS

-N3 -CD1

CS

-N4 -CD1

CS

-N5 -CD1

Excessive

100

Kaolin suspension 115NTU

80 60 40 20 0

D3 S-CD4 S-CD5 S-CD6 S-CD7 C C C C

(e)

100

20

CS-C

D1

120 100 80

CS-C

D2

CS-C

Original data

Insufficient

Simulative data

60 40 20 0 120

Excessive

100

Kaolin suspension 115NTU

80 60 40 20 0

CS-C 120 100

D

(j)

1-L1

CS-C

D

1-L2

CS-C

D

1-L3

CS-C

D

Original data

Insufficient

1-L4

CS-C

D

1-L5

Simulative data

80 60 40 20 0 120 100

Kaolin suspension 115NTU

Excessive

80 60 40 20 0

CS

-N1 -CD1

CS

80 60 40 20 1200

-N2 -CD1

CS

-N3 -CD1

CS

-N4 -CD1

CS

-N5 -CD1

Kaolin suspension 310NTU

Excessive

100 80 60 40 20 0

D3 S-CD4 S-CD5 S-CD6 S-CD7 C C C C

(f)

Simulative data

CS-C

D1

120 100

CS-C

D2

CS-C

D3

CS-C

D4 S-CD5 S-CD6 S-CD7 C C C

Original data

Insufficient

(g)

Simulative data

80 60 40 20 0 120

Excessive

100

Kaolin suspension 310NTU

80 60 40 20 0

1-L1

D

CS-C 120 100

(k)

1-L2

D

CS-C

CS-C

Insufficient

D

1-L3

CS-C

D

Original data

1-L4

D

CS-C

Simulative data

60 40 20 0 120 100

Kaolin suspension 310NTU

80 60 40 20 0

CS

-N1 -CD1

CS

-N2 -CD1

CS

-N3 -CD1

CS

-N4 -CD1

CS

-N5 -CD1

120 100

(d)

Original data

Insufficient

Simulative data

80 60 40 20 1200

NaHA aqueous solution

Excessive

100 80 60 40 20 0

CS-C

1-L5

80

Excessive

Removal rate (%) Removal rate (%)

60

40

Original data

Insufficient

(c)

100

Removal rate (%) Removal rate (%)

Kaolin suspension 55NTU

80

(b)

60

120

D1

120 100

(h)

CS-C

D2

CS-C

D3 S-CD4 S-CD5 S-CD6 S-CD7 C C C C Original data

Insufficient

Simulative data

80 60 40 20 0 120

Excessive

100

NaHA aqueous solution

80 60 40 20 0

CS-C

Removal rate (%) Removal rate (%)

Excessive

100

Simulative data

Removal rate (%) Removal rate (%)

1200

80

Original data

Insufficient

Removal rate (%) Removal rate (%)

20

100

Removal rate (%) Removal rate (%)

40

120

D1-L

120 100

(l)

1 CS-C

D1-L

2

CS-C

D1-L

3

CS-C

5 4 D1-L CS-CD1-L

Original data

Insufficient

Simulative data

80 60 40 20 120 0 100

Excessive

NaHA aqueous solution

80 60 40 20 0

CS-C

D1-N

1 CS-C

D1-N

2

CS-C

D1-N

3

CS-C

5 4 D1-N CS-CD1-N

Fig. 2. The original and simulative data for the flocculation of various Kaolin suspensions with different initial turbidities: (a, e, i) 55 NTU, (b, f, j) 115 NTU, and (c, g, k) 310 NTU as well as those of (d, h, l) 50 mg/L NaHA aqueous solution by using (aed) CS-CDs, (eeh) CS-CD1-Ls, and (iel) CS-CD1-Ns at insufficient and excessive dose conditions respectively.

P. Hu et al. / Chemosphere 240 (2020) 124866

Removal rate (%) Removal rate (%)

Simulative data

60

CS-C

Removal rate (%) Removal rate (%)

Original data

Removal rate (%) Removal rate (%)

80

Insufficient

(a)

Removal rate (%) Removal rate (%)

100

Removal rate (%) Removal rate (%)

Removal rate (%) Removal rate (%)

8

120

P. Hu et al. / Chemosphere 240 (2020) 124866

9

Table 3 The simulative coefficients of St-based flocculants for flocculation of various kaolin suspensions and NaHA aqueous solution by using Eq. (6). Samples

Contaminants

Dose

b0

bCC (L/mmol)

bL

bCCL (L/mmol)

b2CC (L2/mmol2)

b2L

R2

CS-CDs

55 NTU Kaolin suspension

insufficient excessive insufficient excessive insufficient excessive insufficient excessive insufficient excessive insufficient excessive insufficient excessive insufficient excessive insufficient excessive insufficient excessive insufficient excessive insufficient excessive

8.662 30.24 12.41 32.34 16.37 32.94 3.659 4939 21.05 77.15 37.29 95.04 49.15 103.4 17.20 108.5 9.126 25.93 12.48 29.16 14.99 34.28 8.407 54.19

178.4 16.23 175.2 6.141 133.6 1.019 1.895 50.24 567.3 9.348 507.8 7.842 308.0 7.731 5.984 0.2247 355.6 1.534 224.1 8.360 167.3 4.368 3.032 8.964E-2

8.662E-2 0.3024 0.1241 0.3234 0.1637 0.3294 3.659E-2 1.830E-3 1.630E-2 7.360E-3 1.524E-3 3.320E-2 1.680 E2 2.161E-2 4.710E-3 5.295E-4 9.130E-2 0.2593 0.1248 0.2916 0.1499 0.3428 6.568E-3 2.509E-2

1.784 0.1623 1.752 6.141E-2 1.336 1.020E-2 1.895E-2 0.4975 0.1424 2.291E-2 5.926E-2 2.350E-2 3.544E-2 2.640E-3 1.320E-3 7.091E-5 3.556 0.2126 2.312 0.1981 1.673 4.368E-2 3.032E-2 9.000E-4

615.6 3.215 538.5 5.019E-2 340.8 1.191 4.239E-2 4.440E-4 1338 21.31 1023 8.316 463.2 0.2206 8.209E-2 3.020E-4 2168 44.85 950.4 14.33 510.0 7.670E-2 8.217E-2 4.100E-4

8.66E-4 3.02E-3 1.24E-3 3.23E-3 1.64E-3 3.29E-3 3.659E-4 0.506 3.44E-5 7.80E-5 2.60E-5 4.72E-5 1.52E-5 5.11E-5 2.25E-4 1.72E-5 9.13E-4 2.59E-3 1.25E-3 2.92E-3 1.50E 3 3.43E-3 8.41E-4 5.42E-3

0.983 0.874 0.949 0.845 0.935 0.888 0.976 0.913 0.994 0.982 0.920 0.976 0.862 0.987 0.982 0.987 0.980 0.989 0.935 0.987 0.880 0.985 0.976 0.975

115 NTU Kaolin suspension 310 NTU Kaolin suspension NaHA aqueous solution CS-CD1-Ls

55 NTU Kaolin suspension 115 NTU Kaolin suspension 310 NTU Kaolin suspension NaHA aqueous solution

CS-CD1-Ns

55 NTU Kaolin suspension 115 NTU Kaolin suspension 310 NTU Kaolin suspension NaHA aqueous solution

accounts for the restabilization effect at overdoses. Moreover, all values of bCC for the flocculation of various kaolin suspensions were considerably higher than those for NaHA aqueous solution, which further confirmed that inorganic kaolin particles in water were more sensitive to the charge characteristics of flocculants and that more flocculants were required to achieve desired efficiency in the removal of water-soluble organic matters. All values of bL for the flocculation of various kaolin suspensions and NaHA aqueous solution, not only at the insufficient but also excessive flocculant dose ranges, always kept a small value around zero regardless of the distributions of the branched chains on the St backbone in our experimental measured range. This result indicated that L and N played minor effects on the contaminant removal rate of the graft St-based flocculants, which is fully consistent with the preceding experimental results.

3.2.2. Theoretical estimation The CC data always significantly affected the flocculation performance of the St-based flocculants in the entire measured dose range. Therefore, a new equation was obtained by differentiating Eq. (8) on the basis of CC, as shown as follows:

doseopt

! bcc þ bccL $L CC ¼ CD$dose : ¼ 2bCC2 $CD

(9)

The optimal dose (doseopt) for the flocculation of the target contaminants was estimated from the structural parameters CD and L using Eq. (9). As a result, the structureeactivity relationship was established. On the basis of Tables 2 and 3, the calculated optimal doses using Eq. (9) for the flocculation of various kaolin suspensions and NaHA aqueous solution were primarily obtained and compared with the experimental ones, as shown in Fig. 3. The theoretical optimal doses were fairly consistent with the experimental values (Fig. 3), especially for the St-based flocculants with relatively high CDs. The increased deviation in the St-based flocculants with low CDs may be due to the reduced charge neutralization effects; thus, other contributions to the flocculation performance, such as bridging flocculation, evidently influenced by L may not be ignored.

In short, the flocculation performance of the graft St-based flocculants could be predicted on the basis of the established structureeactivity relationship (Eqs. (8) and (9)).

4. Conclusion In this work, a second-order polynomial equation based on phenomenological theory was used to quantitatively analyze the structural effects of graft St-based flocculants, including CD, L, and N, on their performance for flocculation of three kaolin suspensions with different initial turbidities and a NaHA aqueous solution. The flocculation performance was well interpreted by this theoretical model. The simulation data and actual experimental results were in good agreement. The structureeactivity relationship was thus established. In short. (1) The CD of the St-based flocculants played a dominant role in the removal of the aforementioned various contaminants from water. The flocculation efficiency increased with the increase of CD. The L and N showed insignificant effects on the contaminant removal rates in experimentally measured ranges. Meanwhile, long L and dense N on the St backbone were beneficial to the promotion of floc growth and formation of large flocs due to the enhanced adsorption bridging effects. (2) Charge neutralization was the main flocculation mechanism throughout the flocculation process in this case. Specifically, the flocculation of kaolin suspensions was dominantly caused by charge patching, whereas that of NaHA aqueous solution was mainly ascribed to a simple charge neutralization mechanism by the combination of zeta potentials' analysis. (3) The flocculation performance of the graft St-based flocculants, including the contaminant removal rate and optimal dose, could be predicted correctly on the basis of the established structureeactivity relationship, which is of significant guidance for the usage and design of novel and highperformance flocculants.

0.1 0.0 60 50 NaHA aqueous solution 40 30 20 10 0

0.4 0.3 0.2

kaolin suspension 310NTU

0.1 0.0

Experimental Calculated

(c)

0.5 0.4

kaolin suspension 55NTU

0.3

0.2

0.1

0.1

0.0

0.0

kaolin suspension 310NTU

0.5

NaHA aqueous solution 60

50 40 30 20 10 0

0.4 0.3 0.2 0.1 0.0

kaolin suspension 115NTU

0.5 0.4 0.3

0.2

0.2

0.1

0.1

0.0

kaolin suspension 310NTU

0.5

0.3

0.2

0.0 NaHA aqueous solution 60

50 40 30 20 10 0

0.4 0.3 0.2 0.1 1 1N -C 2 D CS 1-N -C 3 D C S 1 -N -C 4 D 1N 5 CS -C D CS 1-N -C 1 D CS 1-N -C 2 D CS 1-N -C 3 D CS 1-N -C 4 D 1N 5

0.0 D

CS

-C

CS

CS

-C D

1N

Optimal dose (mg/L) Optimal dose (mg/L)

CS C S CD 1 -C CS D - 2 C S CD 3 -C CS D4 CS CD -C 5 CS D6 -C D CS 7 -C CS D1 -C CS D 2 C S CD 3 CS CD -C 4 CS D5 CS CD -C 6 D7

Optimal dose (mg/L)

0.0

0.4

Optimal dose (mg/L) Optimal dose (mg/L)

0.1

0.3

0.5 kaolin suspension 115NTU

kaolin suspension 55NTU

Optimal dose (mg/L) Optimal dose (mg/L)

kaolin suspension 115NTU 0.2

kaolin suspension 55NTU

0.4

Experimental Calculated

D1 -C L 1 CS D 1 -C L 2 D C S 1- C L3 D C S 1-C L 4 D1 -L CS 5 -C D C S 1-C L 1 D C S 1-L -C 2 D C S 1 -L -C 3 D C S 1L -C 4 D1 -L 5

0.2

0.3

(b)

0.5

CS

0.3

Optimal dose (mg/L) Optimal dose (mg/L)

0.4

-C

Experimental Calculated

CS

(a)

Optimal dose (mg/L)

0.4

Optimal dose (mg/L)

P. Hu et al. / Chemosphere 240 (2020) 124866

Optimal dose (mg/L)

10

Fig. 3. Comparison of the experimental and calculated optimal doses based on Eq. (7) using various St-based flocculants, (a) CS-CDs, (b) CS-CD1-Ls and (c) CS-CD1-Ns, for the flocculation of different kaolin suspensions and NaHA aqueous solution respectively.

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