Synthesis, characterization and evaluation of

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Nov 24, 2017 - nants from oilfield wastewater, such as inorganic-metal-based and synthetic organic ... molecular weight ≈1.6 × 105 g mol-1) and kaolinite (average par- ..... –42.3. –15.6. –31.6. –13.4. -. Colority (dilution ratio). 126. 13. 59. 5. 50 .... 10 Lou T, Wang X, Song G and Cui G, Synthesis and flocculation perfor-.
Research Article Received: 23 July 2017

Revised: 18 September 2017

Accepted article published: 26 September 2017

Published online in Wiley Online Library: 24 November 2017

(wileyonlinelibrary.com) DOI 10.1002/jctb.5448

Synthesis, characterization and evaluation of amphoteric chitosan-based grafting flocculants for removing contaminants with opposite surface charges from oilfield wastewater Shuanglei Peng, Guancheng Jiang,* Xinliang Li, Lili Yang, Fan Liu and Yinbo He Abstract BACKGROUND: Contaminants with either negative or positive surface charges in wastewater generated from oilfields are normally very difficult to remove by traditional flocculants owing to their strong pH-dependence and high health risks. Natural polymer flocculants, especially chitosan-based flocculants, have attracted much interest for their environmental friendliness, excellent flocculation efficiencies and cost-effectiveness. RESULTS: A series of amphoteric chitosan-based grafting flocculants (CM-chi)-g-PDMDAAC (denoted as CgPD) were successfully synthesized by grafting diallyl dimethyl ammonium chloride (DMDAAC) onto carboxymethyl chitosan (CM-chi) with different grafting ratios. By carboxymethyl and grafting modification, dramatically increased water solubility of chitosan was obtained. The physicochemical structure of CgPD products was characterized by 1 H NMR and elemental analysis proving that DMDAACC was grafted onto CM-chi appropriately. Flocculation effects of CgPD were studied in kaolin and hematite suspensions having opposite surface charges. These CgPD flocculants demonstrated an excellent performance in respect of flocculation window, optimal dosage and pH sensitivity. In addition, CgPD was proven to be applicable as a flocculant in water treatment of oilfield sites. CONCLUSION: The biodegradability flocculants, CgPD, can effectively remove contaminants with opposite surface charges from oilfield wastewater with low optimal dosage, wide flocculation window, low pH sensitivity and less environmental impacts than traditional flocculants. © 2017 Society of Chemical Industry Keywords: amphoteric chitosan; flocculation; water treatment; graft modification; opposite surface charges

INTRODUCTION

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Usually, the wastewater generated by an oilfield contains large amounts of solids, minerals and other additives which are difficult to convert in the natural environment. Most of them could enter water bodies and then damage the ecological balance. Therefore, it is an urgent task to remove such contaminants from oilfield wastewater effectively.1 Although a variety of technologies, such as flocculation, oxidation/reduction, adsorption, membrane filtration, and biotechnology, have been developed and employed in water treatment, flocculation is still a preferred method for wastewater treatment because of its relatively low cost, facile operation and high efficiency.2,3 Over recent decades, lots of cheap and efficient flocculants have been explored in order to remove the contaminants from oilfield wastewater, such as inorganic-metal-based and synthetic organic polymeric flocculants. However, these traditional flocculants have a strong pH-dependence and are thus applicable only to oppositely charged colloidal particles.3–5 They also carry many health risks due to residual metal ions or the release of noxious polymeric monomers into the target water.3,6 For example, J Chem Technol Biotechnol 2018; 93: 968–974

polyacrylamide (PAM) and its derivatives used in water treatment might produce or contain acrylamide (AM) and ethyleneimine monomers causing severe neurological diseases3,7 ; polyaluminum chloride (PAC) contains a large quantity of aluminum salt and may cause Alzheimer’s disease,8 so both of them have been prohibited in many countries. At present, development of more effective, safer and cheaper flocculants is ongoing. Natural biomaterials such as polysaccharides have attracted more interest because of their non-toxic, renewable, biodegradable and cost-effective aspects.9 Chitosan, an aminopolysaccharide produced by N-deacetylation of chitin, is the second most abundant natural polymer on the earth only after cellulose. It has found applications in many fields, including food processing, biomedicine, biotechnology, and



Correspondence to: G Jiang, College of Petroleum Engineering, China University of Petroleum-Beijing, 18 Fuxue Road, Changping, Beijing, China. Email: [email protected] College of Petroleum Engineering, China University of Petroleum-Beijing, Beijing, China

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© 2017 Society of Chemical Industry

Synthesis, characterization and evaluation of amphoteric

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OH O

OR

OR NaOH

O

HO NH2

n

O

ClCH2COOH

initiators

O

O

DADMAC

HO NH

n

R = H or -CH2COONa

R' =

n NH R' CgPD

CMC

Chitosan

O

HO

or

CH2 CH2 HC CH H2C CH2 N ClCH3 CH3 m

Figure 1. Synthesis pathway of CgPD.

wastewater treatment.3 The existence of free amino groups gives chitosan the character of typical cationic polymers and it interacts efficiently with the anionic solutes in acidic solutions.4,10,11 Owing to the large numbers of hydroxyl groups on its saccharide ring, chitosan can be easily modified chemically to satisfy various applications by the introduction of different functional groups onto its backbone.6 These excellent properties have been used for removal of contaminants in wastewater, as an alternative to the conventinal flocculants. Chitosan in acidic medium produced better flocculation performance with anionic contaminants from the perspective of charge attraction. However, oilfield wastewater is normally complex, and always contains alkaline and cationic contaminants in addition to acidic and anionic ones.12 The poor water solubility and only cationic groups of chitosan limit its utility; consequently, amphoteric modification of chitosan is important to enhance its flocculation ability.13 In this study, a series of novel amphoteric chitosan-based grafting flocculants (CgPD) were synthesized through a simple method. Molecular structure of the novel flocculants was controlled and adjusted by manipulating the feed ratio of raw materials. Kaolin, widely available in natural surface water, and hematite, widely found in mineral wastewater, are applied as model contaminants in water.14 Chitosan-based flocculants were employed to remove these two contaminants having opposite surface charges under different pH conditions and the flocculation mechanism was determined. Finally, the flocculation performance of CgPD was evaluated in oilfield wastewater.

MATERIAL AND METHODS Materials Chitosan (deacetylation degree >95 mol %, weight-average molecular weight ≈1.6 × 105 g mol-1 ) and kaolinite (average particle diameter: 3.5 𝜇m) were purchased from Aladdin Chemistry Co., Ltd (China). Hematite (average particle diameter: 0.2 𝜇m) was acquired from Alfa Aesar (China). Monochloroacetic acid (AR), redox initiators (GR) were supplied from J&K Scientific Ltd (China). Diallyl dimethyl ammonium chloride (DMDAAC, 60% in water) was obtained from Energy-Chemical (China). Other experimental chemicals were purchased from domestic reagent companies. Deionized water was used throughout the experiments.

J Chem Technol Biotechnol 2018; 93: 968–974

-NH2

a carboxymethyl

b methine of branches N+(CH3)2

methylene of branches

c 5

4

3

2

1

ppm Figure 2. 1 H NMR of chitosan (a), CM-chi (b) and CgPD5 (c).

of CgPD was prepared in accordance with the process shown in Figure 1. The detailed synthesis method is described as follows. A specified amount of dried CM-chi was dissolved in 200 mL of 1 wt% HCl solution. After 30 min of stirring under N2 atmosphere, a specified amount of redox initiators was added. The solution was kept for 3 min of pretreatment by the initiator to suppress formation of DMDAAC homopolymer. Then the DMDAAC monomer aqueous solution was dropwise added into the solution. The reaction was carried out under N2 atmosphere at 50 ∘ C for 3 h, and was then precipitated out in ethanol. The solid product obtained was filtered and extracted using ethanol as solvent in a Soxhlet apparatus for 36 h to remove impurities, and finally vacuum dried at room temperature, thereby obtaining CgPD. Five different CgPD samples with various grafting ratios of PDMDAAC groups (G(DMDAAC)) were prepared in this work and the detailed preparation recipe is listed in Table 1.

METHODS Proton nuclear magnetic resonance (1 H NMR) The 1 H NMR spectrum was recorded at room temperature on a Bruker Avance 400 spectrometer (Bruker, Switzerland), in which D2 O was used as the solvent and the spectrum was calibrated using the residual protons of the solvent. Elemental analysis The elemental percentages of carbon and nitrogen in each sample were estimated using a vario EL cube CHNOS (Elementar, Germany).

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Synthesis of amphoteric chitosan-based grafting flocculants First, O-(carboxymethyl)-chitosan (CM-chi) was prepared based upon the method reported by Liu15 and Chen.16 The degree of substitution of carboxylmethyl groups (DS(carboxylmethyl)) in CM-chi is 85%, calculated from its 1 H NMR spectrum. Then a series

-CH2OH

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CMC CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

80 ζ-potential (mV)

60 40 20 0 -20 -40

(b) Kaolin Hematite

60 ζ-potential (mV)

(a)

S Peng et al.

40 20 0 -20 -40

2

4

6

8

10

12

pH

2

4

6

pH

8

10

12

Figure 3. 𝜁 -potential curves of flocculant solutions and synthetic wastewater in the pH range from 2 to 12.

𝜁-potential analysis The 𝜁-potential of synthetic wastewater and flocculant solutions were measured by a Zetasizer Nano ZS (Malvern, England) at various pHs at room temperature. The pH of the stock suspensions and solutions was modified using 0.1 mol L-1 NaOH or 0.1 mol L-1 HCl solutions, and the pH was measured by a FE20 pH Meter (Mettler Toledo, Switzerland). Water solubility tests were performed according to Chen’s method.16 Flocculation performance 2.0 g of kaolin or hematite was added into 1 L of distilled water before the suspension was mechanically stirred at 200 rpm ultrasonically for 3 min, and this was used as the synthetic contaminated wastewater. The initial pH of wastewater was adjusted by 0.1 mol L-1 HCl or NaOH aqueous solutions. Stock solutions of the various CgPD flocculants were always freshly prepared by dissolving the solids in distilled water. Chitosan was dissolved in 0.1 mol L-1 HCl aqueous solution to prepared stock solution owing to its insolubility in neutral and alkali conditions. A jar test17–19 was applied for flocculation experiments. After adding specified amounts of flocculants into 100 mL 0.2% synthetic contaminated wastewater, the mixture was stirred at 200 rpm for 5 min. Then it was allowed to settle for 1 h. After these procedures, the turbidity and 𝜁-potential of supernatant was measured by WGZ-800 turbidimeter (Shanghai XinRui Instruments Co., Ltd, China) and Zetasizer Nano ZS, respectively.

RESULTS AND DISCUSSION

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Characterization of flocculants Water-solubility tests of CgPD The novel CgPD flocculants with different grafting ratios were prepared according to Figure 1. Water-solubility tests for CgPD samples were first carried out, as solubility is one of the fundamental characteristics of flocculants. A high water-solubility is conducive for approachability to suspended contaminants in water and bridging flocculation.14,20 As is seen in. Table 2, pure chitosan has the poorest solubility. Although CM-chi has a better solubility than chitosan due to destruction of the ordered structure of chitosan by carboxymethyl groups, it is still insoluble at some pH values. In comparison, all five CgPD samples are totally soluble at all pH tested. The excellent solubility is mainly due to the dual characteristics of carboxymethyl groups and grafted cationic groups. On the one hand, carboxymethyl

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components can enhance solubility under alkaline conditions due to deprotonation at high pH value2,14 ; on the other hand, cationic PDMDAAC groups in the branches are also responsible for better water-solubility, owing to their hydrophilic properties in all pH regions. The desired solubility illustrates the potential applicability of chitosan-based flocculants in a wide range of conditions. Physicochemical structure of CgPD The 1 H NMR spectrum is one of the most common characterization tools for the investigation of chemical modification. Figure 2 depicts the 1 H NMR spectra of chitosan, CM-chi and CgPD5. The basic assignment of chitosan resonance is shown in Figure 2(a): 𝜎 = 3.52 and 3.73 ppm for protons in -CH2 OH, 𝜎 = 2.98 ppm for –CH– in the backbone, 𝜎 = 1.86 ppm for acetyl-protons -NH2 16,20 and the peak at 4.67 ppm is for the solvent D2 O. In Figure 2(b), the region between 3.75 and 4.35 ppm is the resonance of substituted carboxymethyl.16,21 The peak at 1.86 ppm indicates that carboxymethyl groups are mostly at the -OH position. In Figure 2(c), the intense resonances at 3.03 and 3.65 ppm are attributed to the methylene and methane in DMDAAC groups, respectively. The new peak at 4.00 ppm is the resonance of -CH3 in the quaternary ammonium group. Mass ratios between carbon and nitrogen elements in each CgPD sample are given by elemental analysis as shown in Table 1. Therefore, G of DMDAAC components in each CgPD sample can be calculated according to the elements mass ratios based on Equation (1), and the results are also listed in Table 1. m (C) = m (N)

12×n(C)CM−chi Mw (CM−chi) 14×n(N)CM−chi Mw (CM−chi)

+

12×n(C)DMDAAC

+

14×n(N)DMDAAC

Mw (DMDAAC) Mw (DMDAAC)

· G (DMDAAC) (1) · G (DMDAAC)

where G(DMDAAC) is defined as the weight ratio of DMDAAC to CM-chi backbone. n(C)CM-chi and n(C)DMDAAC are the average number of carbon atoms in one CM-chi carbohydrate unit and DMDAAC molecule, respectively. n(N)CM-chi and n(N)DMDAAC are the average number of nitrogen atoms in one CM-chi carbohydrate unit and DMDAAC molecule, respectively. Mw (CM-chi) and Mw (DMDAAC) are the average molecular weight of CM-chi carbohydrate unit and DMDAAC, respectively. Based on the G of PDMDAAC, weight-average molecular weights of different CgPD samples are also calculated and listed in Table 1. It is found from Table 1 that G(DMDAAC) and average molecular weight increase with increasing feed amount of DMDAAC from CgPD1 to CgPD5.

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Synthesis, characterization and evaluation of amphoteric

CMC CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

300 Kaolin pH=4

250 200 150 100

(d)

Turbidity (NTU)

Turbidity (NTU)

(a)

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50 0 0.5

1.0 1.5 2.0 Dosage (mg/L)

400 350 300 250 200 150 100 50 0

Kaolin pH=7

Turbidity (NTU)

400 350

Hematite pH=7

300 250

CMC CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

200 150 100 50 0

0

(c)

(e)

CMC CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

CMC CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Dosage (mg/L)

1

2 3 4 Dosage (mg/L)

CMC CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

600 Kaolin pH=10

500

5

400 300 200 100 0

0 1 2 3 4 5 6 7 8 9 10 Dosage (mg/L)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Dosage (mg/L)

(f) Turbidity (NTU)

Turbidity (NTU)

(b) 450

Hematite pH=4

2.5

Turbidity (NTU)

0.0

450 400 350 300 250 200 150 100 50 0

400 350 300 250 200 150 100 50 0

Hematite pH=10

0.0

0.4

0.8 1.2 1.6 Dosage (mg/L)

CMC CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

2.0

Figure 4. Turbidity of the supernatants as a function of dosage of different CgPD samples at pH 4.0, 7.0 and 10.0 for kaolin (a, b, c) and hematite (d, e, f ) suspension. Table 1. Details of the different CgPD samples

Sample name CM-chi CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

m(CM-chi): m(DMDAAC) 1:0 1:0.5 1:1 1:1.5 1:2 1:3

G(DMDAAC)/%

IEP

m(C)/m(N)

Weight-average molecular weight/(g mol-1 )

0 25 51 88 118 174

4.9 6.3 7.1 8.3 10.0 >12

6.60 6.65 6.71 6.74 6.76 6.78

2.1 × 105 2.6 × 105 3.2 × 105 3.9 × 105 4.6 × 105 5.8 × 105

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is found from Figure 3(a) that the 𝜁 -potential of all flocculants decreases as pH rises owing to the deprotonation of carboxyl groups as well as increased number of hydroxyl anions surrounding the flocculant molecular chains. The isoelectric point (IEP) of different samples, one of the most important parameters of

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𝜁-potential of synthetic wastewater and flocculant solutions 𝜁-potential of different samples was measured since 𝜁 -potential plays a vital role in predicting and understanding the flocculation mechanism in flocculation process.22–25 The 𝜁-potential curves of various samples at different pHs are illustrated in Figure 3. It

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0.4

0.8

1.2

1.6

2.0

Kaolin pH=10 Hematite

15 10 5 0 -5 -10 -15 -20 -25 -30

(b)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

-16

Kaolin pH=7 Hematite

19 18 ζ-potential (mV)

0.0

ζ-potential (mV)

15 10 5 0 -5 -10 -15 -20 -25 -30

ζ-potential (mV)

ζ-potential (mV)

(a)

S Peng et al.

17

-18

16

-20

15

-22

14

(b)

13

-24

12 0

0 1 2 3 4 5 6 7 8 9 10 CgPD5 Dosage (mg/L)

1 2 3 4 CgPD2 Dosage (mg/L)

5

Figure 5. 𝜁 -potential curves of the supernatants with different dosage of CgPD flocculants at pH 7 and 10. Table 2. Water-solubility of different chitosan-based samples at various pHs pH

1

2

3

4

5

6

7

8

9

10

11

12

13

Chitosan CM-chi CgPD1 CgPD2 CgPD3 CgPD4 CgPD5

+ + + + + + +

+ + + + + + +

+ + + + + + +

– ± + + + + +

– – + + + + +

– ± + + + + +

– + + + + + +

– + + + + + +

– + + + + + +

– + + + + + +

– + + + + + +

– + + + + + +

– + + + + + +

+: soluble; ±: partially soluble; – insoluble.

flocculants and strongly dependent on the composition of ionic groups, are summarized in Table 1 according to Figure 3(a). Taking into account the G(DMDAAC), more quaternary ammonium groups cause IEP to increase. 𝜁 -potential curves of two types of synthetic wastewater at different pHs were also tested and are plotted in Figure 3(b). The surface charge of kaolin particles is always negative in water at pH from 2.0 to 12.0. In contrast, hematite particles show an IEP of approximate 9.0, suggesting that it bears different surface charges at different pHs. Flocculation performance of CgPD Flocculation performance of the novel chitosan-based flocculants was studied systematically. Flocculant dosage and pH are the two most important external factors affecting the flocculation efficiencies according to previous studies,5,7,17–19,26–29 thus, jar tests were carried out at various flocculant dosages at different pHs in both kaolin and hematite suspensions and the results are depicted in Figure 4.

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Flocculation performance of CgPD in kaolin suspensions Figure 4(a)–(c) illustrates the flocculation performances of various CgPD samples in kaolin suspensions. As expected, all of the flocculants with a positive 𝜁-potential at pH 4 exhibit desirable flocculation efficiencies since kaolin particles carry a negative surface charge. In the same way, CgPD3–5 at pH 7 and CgPD5 at pH 9 also provide excellent flocculation performance. However, CgPD2 at pH 7 and CgPD4 at pH 10 with 𝜁 -potential close to zero also wield good contaminant removal efficiencies, which may be due to the bridging mechanism. For these effective flocculants, three influencing factors: dosage, G(DMDAAC) and pH were studied in detail.

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(1) For the dosage effect, turbidity of kaolin suspensions reduces rapidly at the beginning, then slows down and finally rises slightly with increasing flocculant dosages. In industrial applications larger dosage means more cost, therefore it is important to find the optimal dosage range. The down-valley-up trend indicates that there must be an optimal dosage. When flocculant dosage is lower than the optimal range, it is incapable of destabilizing or bridging colloidal particles, while overdose would bring about redispersion of the flocs because of electrostatic repulsion among colloidal particles covered by excessive flocculants. Nonetheless, the redispersion effect in CgPD is less remarkable than in traditional flocculants reported previous.5,17,30 The wider flocculation window is attributed to the amphoteric groups having a charge screening effect. Also, those optimal dosages of effective CgPD samples for kaolin suspension are much lower than that of PAM and PAC under the same conditions as previously reported.17,31 Both the wider flocculation window and the smaller optimal dosage prove the potential applicability of CgPD as an efficient alternative. (2) For the G(DMDAAC) effect, it is observed from Figure 4 that flocculation performance improves and requires lower optimal dosage as G(DMDAAC) increases. This is due to the fact that CgPD with larger G(DMDAAC) represents more cation groups, thus triggering stronger adsorption between flocculants and kaolin particles. (3) For the pH effect, these optimal dosages of CgPD flocculants become larger as pH increases. On the one hand, higher pH values cause diminishing 𝜁-potential of flocculants and enhancing stability of colloidal particles, resulting in weakening charge attraction between flocculant and contaminant particles as well as stronger repulsion among colloidal

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Synthesis, characterization and evaluation of amphoteric

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Table 3. Characteristics of water from Shengli Oilfield W1 Parameter pH 𝜁 -potential Colority (dilution ratio) Solid content (mg L-1 ) CODCr (mg L-1 )

W2

R* 9.4 –42.3 126 4608 3340

T** 8.2 –15.6 13 2.5 56.4

R

T

GB 8978–1996***

7.4 –31.6 59 154 3162

7.2 –13.4 5 1.1 35.2

6.0–9.0 50 70 60

*Raw water. **Treated water after filtration under optimal dosages of CgPD5. ***Class A of integrated wastewater discharge standard in China.

particles. On the other hand, higher pH produces more ionogenic carboxylic groups and enhances the intra-chain electrostatic interaction, which will further give rise to more collapsed conformation of polymer chains and reduction of active flocculation sites. Both aspects mentioned above are responsible for an increase in optimal dosages.32 Nevertheless, the optimal dosages of CgPD samples are still much lower than those of traditional flocculants even at pH 10, suggesting that the novel chitosan-based flocculants enjoy a lower pH sensitivity. Flocculation effect of CgPD in hematite suspensions For hematite suspensions, the situation is a bit more intricate as shown in Figure 4(d)–(f ). On combining the 𝜁-potential curves of various CgPD samples with that of hematite in Figure 3, similar findings to that in kaolin suspension can be concluded. Flocculants with opposite charges to hematite (CgpD1 at pH 7 in Figure 4(e); CgPD5 at pH 10 in Figure 4(f )) or with 𝜁 -potential close to zero (CgPD2 at pH 7 in Figure 4(e); CgPD4 at pH 10 in Figure 4(f )) would enjoy good contaminant removal. Similarly to the results in kaolin suspension, the CgPD flocculants have advantages of wider flocculation window, lower optimal dosage and lower pH sensitively over the commercial flocculants, PAM and PAC. All the results in Figure 4 illustrate that these novel amphoteric CgPD flocculants have an extensive potential to remove contaminants, even those with opposite surface charges in wastewater.

J Chem Technol Biotechnol 2018; 93: 968–974

Flocculation effect of CgPD in wastewater from oilfield Aside from the tests in synthetic wastewater, the flocculation properties of CgPD were also studied in wastewater from an oilfield. Raw water samples from Shengli Oilfield in China were used for this test in the lab and its characteristics are shown in Table 3. W1 sample was obtained from the drilling site and W2 sample was oil recovery wastewater from polymer flooding. The experimental process was similar to the jar test at the beginning, but after settlement for 1 h, these wastewaters were filtered through a 200-mesh sieve which is always used in drilling site to remove drilling cuttings from drilling fluids. From the analysis of flocculation mechanisms above, we know that opposite surface charge and larger G of DMDAAC benefit the flocculation effect. Therefore, W1 and W2 were both treated with CgPD5 in this test at the optimal dosages of 4.2 mg L-1 and 2.8 mg L-1 , respectively. The detailed characteristics of the treated water after filtration under the optimal dosages are shown in Table 3. It can be seen that all of the parameters fully meet the Chinese integrated wastewater discharge standard-Class A (GB 8978-1996).

CONCLUSION A series of novel amphoteric chitosan-based grafting flocculants, CgPD, have been successfully prepared in the present work. The novel flocculants exhibit high flocculation effect with low optimal

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Flocculation mechanism The flocculation mechanism was studied to better understand and control the flocculation process. The 𝜁 -potential curves of supernatants after sedimentation with various flocculant dosages were measured, and Figure 5 suggests that three flocculation processes obeyed different mechanisms. As illustrated in Figure 5(a), Figure 4(c)–CgPD5 and (f )–CgPD5, 𝜁-potential of the supernatants after sedimentation goes across zero and becomes the opposite when flocculant is overdosed. This is because of opposite surface charges on the flocculant and contaminant particles validating the dominant effect of charge attraction. It is known that two kinds of flocculation mechanism can be caused by charge attraction, charge neutralization and patching.22 In the charge neutralization process, first, colloid particles are neutralized and coated by oppositely-charged flocculants and when totally wrapped by the flocculants, the destabilized colloid particles with surface charge near to zero aggregate and form larger flocs. Differently, in patching flocculation, the surfaces of

colloidal particles are not fully covered by flocculant molecules and many small regions with opposite charges still exist on the surfaces, resulting in further particle aggregation due to electrostatic interactions.2 Consequently, if charge neutralization is dominant, the 𝜁 -potential of supernatants is near zero at near optimal dosage and will pass rapidly through this point with further slight dosage increase. For the patching process, 𝜁 -potential will still be remote from the zero point at the optimal dosage. Thus, patching and charge neutralization mechanisms, resprctively, are found in Figure 5(a) in the flocculation process of kaolin and hematite. However, the result shown in Figure 5(b) is quite different. 𝜁 -potential of the flocculant CgPD2 at pH 7 as shown in Figure 3(a) is quite near to zero, and regarding the two synthetic wastewaters, one carries a very positive surface charge and the other carries a very negative charge, the electrostatic repulsion between flocculants and contaminant particles is negligible and therefore, bridging mechanisms prevail here. The surface charges of colloidal particles are partially screened by the flocculant molecules, resulting in absolute values of 𝜁 -potential decreasing but not crossing zero.

www.soci.org dosage, a wide flocculation window and low pH sensitivity in the removal of various colloidal contaminants, even those with opposite surface charges, from wastewater. On the basis of 𝜁-potential analysis, various flocculation mechanisms were studied in detail. Based on the experimental results, CgPD is proven to be applicable as a flocculant in water treatment at oilfield sites.

ACKNOWLEDGEMENTS We would like to acknowledge the financial support from the National Science and Technology Major Project during the 13th Five-Year Plan Period of China (Project No. 2016ZX05040-005 and 2016D-4503) for this work.

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