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Cationic starch-grafted-cationic polyacrylamide based graphene oxide ternary composite flocculant for the enhanced flocculation of oil sludge suspension
Composites Part B 177 (2019) 107416
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Cationic starch-grafted-cationic polyacrylamide based graphene oxide ternary composite flocculant for the enhanced flocculation of oil sludge suspension Ying Chen a, Gongwei Tian b, Boyin Zhai a, Huili Zhang a, Yuning Liang a, Hongbao Liang c, * a b c
Heilongjiang Provincial Key Laboratory of Chemical Engineering of Oil and Gas, Northeast Petroleum University, Daqing, China Heilongjiang Provincial Key Laboratory of Chemical Engineering of Oil and Gas, Northeast Petroleum University, Daqing, China College of Mechanical Science and Engineering, Northeast Petroleum University, Daqing, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Cationic starch Cationic polyacrylamide Graft Graphene oxide Flocculation mechanism
In this study, a novel, highly efficient and environmentally friendly ternary composites flocculant, namely, cationic starch-grafted-cationic polyacrylamide/graphene oxide (CS-g-CPAM/GO), was synthesized by ammo nium persulfate initiation polymerization and condensation reaction. First, CS-g-CPAM was polymerized with cationic starch (CS), acrylamide (AM) and diallyl dimethyl ammonium chloride (DMDAAC), and CS-g-CPAM/GO was then synthesized by condensation reaction. The influence factors of graft polymerization were investigated, including total monomer concentration, initiator dosage, monomer mass ratio of mAM: mCS: mDMDAAC, postpolymerization temperature and post-polymerization time. The chemical structures and morphologies of the samples were characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC), and scanning electron microscope (SEM). The CS-gCPAM/GO was used to flocculate the oil sludge suspension, the effects of CS-g-CPAM/GO dosage, temperature and pH value on the flocculation performance were investigated, and the flocculation mechanism of CS-g-CPAM/ GO was also analyzed. The results show that CS-g-CPAM/GO has outstanding flocculation effect, and CS-gCPAM/GO flocculates oil sludge particles by adsorption bridging and charge neutralization in acidic and alka line conditions.
1. Introduction Petroleum, as an important energy and chemical raw material, pro duces a lot of waste residue, waste water and waste gas in the process of mining, transportation, storage and processing, which is called oil sludge. Its main components are colloid, asphalt, solid granular, oil soluble organic acid [1,2], zinc, copper, chromium, lead and other heavy metals [3–6], Landfill treatment not only take up a lot of land, but also harms human health, pollutes local environment and impedes economic development [7,8]. Chemical adsorption and flocculation have good flocculation effective and relatively low cost, which is one of the com mon methods to treat oil sludge [9,10]. The cationic polyacrylamide flocculant (CPAM) has been extensively used in oil sludge treating agents due to its excellent turbidity removal, flocculation and decolorization properties [11]. Because the oil sludge particles are negatively charged, CPAM electricity neutralization and
adsorption bridging with the oil sludge particles, which lead to the destabilization, aggregation and settlement of the oil sludge particles to achieve the purpose of flocculation. At present, the research of deep and widely used CPAM mainly has polyacrylamide-dimethyl diallyl ammo nium chloride P (AM-DMDAAC), polyacrylamide-acryloyloxyethyl tri methylammonium chloride P (AM-DAC), polyacrylamide-methacry loyloxyethyl trimethylammonium chloride P (AM-DMC), other series of CPAM are still in the experimental research stage [12,13]. Although CPAM is widely utilized, it causes secondary pollution to the environ ment due to the difficulty in biodegradation [14] and has weak alkaline resistance [15]. In order to solve this problem, some natural polymer, such as chitosan, cellulose and starch, has been investigated because their derivatives are biodegradable, and their degradation intermediates are harmless for people and the environment [16–18], and starch can be further modified to improve its cationic degree to enhance charge neutralization. However, this kind of natural polymer flocculant exhibits
* Corresponding author. College of Mechanical Science and Engineering, Northeast Petroleum University, Daqing, 163318, China. E-mail address: [email protected] (H. Liang). https://doi.org/10.1016/j.compositesb.2019.107416 Received 6 July 2018; Received in revised form 9 August 2019; Accepted 4 September 2019 Available online 5 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Possible reaction mechanism of CS-g-CPAM/GO.
poor solubility under neutral conditions [19]. To make up for each other’s shortcomings and combine the advantages of each other, graft copoly-merization is often employed to modify CPAM and to obtain high solubility, environment-friendly and effectiveness of flocculants [20, 21]. In addition, the surface of graphene oxide nanosheets contains – O, many oxygen-containing functional groups, such as –COOH, –C– –OH, etc. These functional groups can improve the flocculation
performance [22–24]. Taking into account the above factors, a novel flocculant CS-gCPAM/GO was synthesized by CS, AM, DMDAAC and GO through ammonium persulfate-induced polymerization and condensation reac tion. Synthesis conditions of total monomer concentration, initiator dosage, monomer mass ratio of mAM: mCS: mDMDAAC, post-polymerization temperature and post-polymerization time were investigated for 2
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intrinsic viscosity, grafting ratio (GR) and grafting efficiency (GE). The chemical structure of the obtained CS-g-CPAM/GO was characterized by FTIR spectroscopy, XRD, TG-DSC and SEM. Finally, CS-g-CPAM/GO was used to flocculate the oil sludge suspension, the flocculation perfor mance was examined. Meanwhile, flocculation mechanism of CS-gCPAM/GO was researched via zeta potentials of supernatant and oil sludge suspension and FTIR of flocs. 2. Experimental
the ice water bath and magnetic stirring, 8 g KMnO4 was slowly added, and the reaction was continued at 5 � C for 1 h. Then the temperature of the water bath rose to 35 � C, continue stirring for 2 h, add 100 mL of deionized water, the temperature rose to 98 � C to continue the reaction for 15 min. After natural cooling, gradually add 180 mL deionized water and then add 10 mL H2O2 solution dropwise until the solution turned bright yellow. Centrifugal washing at 10000 r/min to neutrality, ultra sonic 30 min, centrifugation at 4000 r/min to take the supernatant, supernatant was placed in a vacuum oven at 60 � C for 12 h to obtain GO.
2.1. Materials
2.6. Synthesis of CS-g-CPAM/GO
Graphite powder was provided by China Oak Maanshan Chemical Industry; Acrylamide(AM, 99.0% wt), Diallyl dimethyl ammonium chloride (DMDAAC, 60% wt), H2SO4 (98% wt), H2O2 (30% wt), KMNO4, NaOH, NaCl, NaNO3, ethanol, acetone (1.0 mg/mL), Starch(ST), 1-ethyl3-(3-dimethylaminopropyl) carbodiimide (EDC), N-hydroxy sulfosucci nimide (NHS), 3-Chloro-2-hydroxypropyltrimethylammonium chloride (CTA, 65 wt% in water), Acetic acid(98% wt), poly(vinyl sulfate) po tassium salt (PVSK, average Mw ~ 162.21), toluidine blue O (TBO, Mw ~ 305.81), Ammonium persulfate and ethylene glycol (EG) were procured from China Sinopharm Chemical Reagent Co., Ltd; oil sludge from the oil sludge treatment station of China Daqing oilfield to provide; Industrial nitrogen(Purity >99%); In all experiments, all reagents were analytical grade, and we used deionized water.
100 mL EDC/NHS mixture (7 mg/mL EDC and 5 mg/mL NHS) was added to 100 mL GO solution (1 mg/mL aqueous solution) and shaken at 25 � C for 30 min. Then, 100 mg CS-g-CPAM was added into the GO so lution, the mixture was then shaken at 25 � C for 1 h and heated at 50 � C for 1 h. The product was washed three times with phosphate buffer so lution and dried in a vacuum oven at 50 � C, CS-g-CPAM/GO was obtained. 2.7. Characterization of CS-g-CPAM/GO The functional groups of CS-g-CPAM/GO was characterized by a Nicolet 380 Series infrared spectrometer (Nicolet Company, United States), KBr tablet, scanning range for 500-4000 cm 1, XRD spectrum of CS-g-CPAM/GO was obtained through D/max2500X-ray diffraction (Rigaku Company, Japan); TG-DSC were obtained on a SDT-Q600 Thermal Analyzer (DSC, TA Co., USA); to investigate the morphology of CS-g-CPAM/GO, SEM images were carried out with a S-4700 field emission scanning electron microscope. (Hitachi Limited, Japan). Grafting ratio (GR) and grafting efficiency (GE) were calculated by the following formula: � � m3 m1 GR ¼ � 100% m1
2.2. Possible reaction mechanism of CS-g-CPAM/GO The possible reaction mechanism of CS-g-CPAM/GO is shown in Fig. 1. First of all, ST was made of CS by etherification reaction. Then, CS, AM and DMDAAC monomers produced free radicals under the action of the ammonium persulfate initiator. The graft polymerization of free radicals produced CS-g-CPAM. Finally, the amino group of CS-g-CPAM [15] and the carboxyl group of GO was dehydration condensation re action to form CS-g-CPAM/GO flocculant [25].
�
� m3 m1 � 100% m2 m1
2.3. Synthesis of CS
GE ¼
The ST (10 g) was dispersed in ethanol and transferred to a threenecked flask equipped with a mechanical stirring and condensing de vice. 0.96 g NaOH and 2.5 g NaCl were prepared into 50 mL solution, which was added to the ST solution for 30 min at 50 � C, then add the CTA solution to it at 50 � C for 6 h. After the reaction was finished. First, add to 2 times the volumes of ethanol at 20 � C for 10 min 12000 rpm centrifugal 10 min, pour off the supernatant and product was washed with 70% ethanol solution until free of chloride ions. The CS was ob tained after vacuum drying at 50 � C for 24 h.
Therein m1 was the mass of CS, m2 was the mass of crude polymer ized products, m3 was the mass of fine products CS-CPAM. The intrinsic viscosity of the CS-g-CPAM was measured by the one point method accurately [26]. The positive charge density of the CS-g-CPAM was determined by titration method [26]. 2.8. Flocculation evaluation Flocculation performance of CS-g-CPAM/GO was investigated in oil sludge suspension. Preparation of oil sludge suspension was as follow: 2 g oil sludge was dissolved in a beaker containing 1000 mL deionized water under mechanical stirring 30 min at 300 r min 1, and then ul trasound 10 min, the formation of oil sludge suspension. A certain dosage of CS-g-CPAM/GO was promptly added into the oil sludge sus pension, the mixture of CS-g-CPAM/GO and oil sludge suspension was stirred at 200 r min 1 for 10 min, free-settling, and the transmittance was surveyed by V-5000h visible spectrophotometer (Shanghai Analytical Instrument Co., Ltd, China) at a λ ¼ 618 nm [27].
2.4. Synthesis of CS-g-CPAM The reaction was carried out in a 250 mL four-neck round bottom flasks containing with a stirrer, thermometer, condenser and nitrogen inlet. A certain mass of CS and 50 mL deionized water was poured into the flask to preheat at 85 � C for 30 min. After the CS gelatinization (the CS slurry became a clear solution), a certain amount of AM, DMDAAC and ammonium persulfate was added, and the mixture was reacted under N2 protection at 55 � C for 3 h. After drying and crushing, the polymerized products were washed with acetone, crude polymerized products were obtained and were then washed with a 2: 3 solution of glacial acetic acid and ethylene glycol and dried 12 h in a 50 � C vacuum oven until a constant weight was obtained, fine products CS-CPAM were obtained.
3. Results and discussions 3.1. Optimization conditions of CS-g-CPAM synthesis 3.1.1. The effects of total monomer concentration on positive charge density, intrinsic viscosity, GR and GE The total monomer concentration (m/m%) is the percentage of all monomers in the total mass of the reaction mixture, where mAM: mCS: mDMDAAC is 6: 4: 3. The effect of total monomer concentration on positive
2.5. Preparation of GO 1 g graphite powder was added to 100 mL H2SO4 to mix evenly under 3
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Fig. 2. (a) total monomer concentration, (b) initiator dosage, (c) mAM: mCS, (d) mAM: mCS: mDMDAAC, (e) post-polymerization temperature, (f) postpolymerization time.
increase of total monomer concentration, the probability of collision between CS, AM and DMDAAC increased, which promoted the chain growth reaction and made the whole reaction system in a highly active state, the chain of CS-g-CPAM was continuously extended, and the vis cosity of the reaction system was increased, when the monomer con centration reached saturation, the collision between the molecules of CS, AM and DMDAAC was saturated, and the chain growth rate cannot be further improved [28]. Furthermore, the cross linking between the su persaturated CS, AM and DMDAAC resulted in the deterioration of CS-g-CPAM solubility, and the movement and transfer rate of monomers and active free radicals was also low. Moreover, high monomer con centration may trigger a double termination reaction [29,30].
charge density, intrinsic viscosity, GR and GE of CS-g-CPAM was investigated at initiator dosage of 0.2%, post-polymerization tempera ture of 55 � C, and post-polymerization time of 24 h. The results are presented in Fig. 2(a). Positive charge density, intrinsic viscosity, GR and GE increased with the increasing monomer concentration. The optimal positive charge density, intrinsic viscosity, GR and GE obtained at 20% monomer concentration were 3.41 mmoL/g, 2981 mL/g, 61.4% and 68.9%, respectively. The collision probability between monomer was greatly lower when the concentration of CS, AM and DMDAAC was too low. The result was that the growth rate of the chain was reduced, and the entire reactivity was insufficient. Therefore, short-chain poly mers can be formed between monomers, and macromolecular graft polymer was not formed due to low monomer concentration. With the 4
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3.1.2. The effects of initiator dosage on positive charge density, intrinsic viscosity, GR and GE Fig. 2(b) shows the effect of initiator dosage on positive charge density, intrinsic viscosity, GR and GE, in which the total monomer concentration, mAM: mCS: mDMDAAC, post-polymerization temperature and post-polymerization time are 20%, 6: 4: 3, 55 � C and 24 h, respec tively. As showed in Fig. 2(b). Positive charge density, intrinsic viscos ity, GR and GE obviously increased with the increasing initiator dosage. CS, AM and DMMAC molecules were polymerized according to the free radical mechanism. The optimum positive charge density, intrinsic vis cosity, GR and GE obtained at a dosage of 0.2% initiator were 3.71 mmoL/g, 3215 mL/g, 62.7%, and 71.2%, respectively. At low initiator dosage, a small number of free radicals were surrounded by a large part of the solvent molecules, which inhibited the formation of free radicals and generating of graft polymer, called as the ‘‘cage effect” [31]. How ever, the proliferation of CS, AM and DMMAC free radicals promoted the continuous extension of CS-g-CPAM chain because of the increase of initiator dosage. Therefore, the positive charge density, intrinsic vis cosity, GR and GE were increased. Nevertheless, when the initiator was excessive, generated free radicals reached the over-saturation state, and the free radicals collided with the formed polymer chain, which caused the original property of the polymer to destroy [32]. Accordingly, the positive charge density, intrinsic viscosity, GR and GE were reduced. 0.2% were fixed at the optimum initiator dosage.
Fig. 3. FTIR spectra of various samples: (a) CS-g-CPAM/GO, (b) CS-g-CPAM, (c) CS, (d) CPAM and (e) ST.
the post-polymerization temperature of 55 � C was favorable for the synthesis of CS-g-CPAM. When the post-polymerization temperature was particularly low, the activity of the monomers was low, the diffusion rate of free radical was slow, and the free radicals were not fully con tacted, which caused that AM and DMDAAC cannot sufficiently combine the active sites of CS. The reaction activity of monomers increased with the increase of post-polymerization temperature, and there was continuous graft polymerization between CS, AM and DMDAAC. As a result, the polymer chain increased continuously, and the positive charge density, intrinsic viscosity, GR and GE also increased [33]. The active centers on cationic starch were unstable due to extremely high post-polymerization temperature, which leaded to the decrease of its own polymerization ability. The heat generated by the polymerization was not easy to reduce. Consequence, the chain transfer rate and growth rate decreased. This phenomenon was unfavorable for graft copoly merization between monomers. Secondly, excessive post-polymerization temperature could also lead to cross-linking and reduced dissolution of CS-g-CPAM. Therefore, the optimal post-polymerization temperature of CS-g-CPAM was 55 � C.
3.1.3. The effects of mAM: mCS: mDMDAAC on positive charge density, intrinsic viscosity, GR and GE Studies showed that dispersion stability of oil sludge particle can be reduced and broke via bridging effects and charge neutralization. Bridging effects can be enhanced by increasing the intrinsic viscosity [11]. The bridging capacity and charge neutralization are determined by the length of the AM main chain and the number of CS and DMDAAC cationic groups. Therefore, the effects of mAM: mCS on positive charge density, intrinsic viscosity, GR and GE were first considered, wherein the quality of the DMDAAC remains unchanged at 3 g, and then the optimal ratio of mAM: mCS: mDMDAAC was investigated, in which the total monomer concentration, initiator dosage, post-polymerization temper ature and post-polymerization time are 20%, dosage of 0.2%, 55 � C and 24 h, respectively. As shown in Fig. 2(c), the results showed that the positive charge density, intrinsic viscosity, GR and GE gradually increased with the increase of mAM: mCS mass ratio, and the intrinsic viscosity were optimal when mAM: mCS mass ratio was 6: 4. Whereas, the positive charge density, intrinsic viscosity, GR and GE reduced with the further increase of mAM: mCS mass ratio. When AM masses were low, the rate of grafting to the CS monomer was slowed down. In addition, the probability of collision of monomer molecules decreased, and the active site of AM molecules was insufficient. When the AM monomer concen tration was too high, AM monomer increased the chain transfer, resulting in a decrease in the intrinsic viscosity of the CS-g-CPAM. At the same time, the CS and DMDAAC were difficult to graft to the main chain, causing positive charge density, intrinsic viscosity, GR and GE to degrade. Furthermore, as shown in Fig. 2(d), the positive charge density, intrinsic viscosity, GR and GE exhibited the same changes under various concentrations of DMDAAC. Therefore, monomer mass ratio mAM: mCS: mDMDAAC of 6: 4: 3 was optimal for synthesis of CS-g-CPAM.
3.1.5. The effects of post-polymerization time on positive charge density, intrinsic viscosity, GR and GE The effects of post-polymerization time on positive charge density, intrinsic viscosity, GR and GE are shown in Fig. 2(f) at monomer con centration of 20%, initiator dosage of 0.2%, monomer mass ratio mAM: mCS: mDMDAAC of 6: 4: 3 and post-polymerization temperature of 55 � C. The positive charge density, intrinsic viscosity, GR and GE increased relatively less when the post-polymerization time exceeded 24 h. Posi tive charge density, intrinsic viscosity, GR and GE were 4.96 mmoL/g, 3612 mL/g, 75.8% and 83.9% respectively at the 24 h postpolymerization time. These parameters tended to be balanced with the extension of post-polymerization time. Free radicals and monomers were constantly consumed with the extension of polymerization time in order to promote graft copolymerization. Thereby improving the posi tive charge density, intrinsic viscosity, GR and GE. The reaction between the monomers was completed after a certain period of time, and The effect on positive charge density, intrinsic viscosity, GR and GE was small. Therefore, the optimal post-polymerization time of 24 h was favorable for the synthesis of CS-g-CPAM.
3.1.4. The effects of post-polymerization temperature on positive charge density, intrinsic viscosity, GR and GE The effects of post-polymerization temperature on the positive charge density, intrinsic viscosity, GR and GE are shown in Fig. 2(e) at monomer concentration of 20%, initiator dosage of 0.2%, monomer mass ratio mAM: mCS: mDMDAAC of 6: 4: 3 and post-polymerization time of 24 h. The positive charge density, intrinsic viscosity, GR and GE increased gradually as the post-polymerization increased. However, the positive charge density, intrinsic viscosity, GR and GE dropped sharply when the post-polymerization temperature exceeded 55 � C. Therefore,
3.2. Characterization 3.2.1. FTIR spectra The FTIR spectrum of a series of samples are shown in Fig. 3. As can 5
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group of AM, respectively. Absorption bands of 924 cm 1 were attrib uted to –Nþ(CH3)2 group of DMDAAC, at the same time, the –Nþ(CH3)3 characteristic peaks of CS also appeared in Fig. 3(b), these data showed that CS and CAPM grafted successfully. As shown in Fig. 3(a), peak at – O of GO, and compared with the 1560 cm 1 were ascribed to the C– infrared spectrum of CS-g-CPAM in Fig. 3(b), a new absorption bands at 1218 cm 1 attributed to CO-NH- [25]. The presence of CO-NH- group indicated that the carboxyl group of GO and the amino group of CS-g-CPAM underwent a dehydration condensation reaction. Moreover, CS-g-CPAM/GO had characteristic peaks of CS, CPAM and GO, respec tively. But the position of the peak was slightly shifted [35,36], which further confirmed the successful synthesis of CS-g-CPAM/GO. 3.2.2. XRD spectrum XRD spectrum of ST, CS, CS-g-CPAM, CS-g-CPAM/GO and CPAM was obtained to investigate the crystalline structure and effects of monomer on CS-g-CPAM/GO crystal morphology. XRD spectrum is shown in Fig. 4. The characteristic diffraction peak of ST was observed in the range of 18◦-21� , this strong diffraction peak was attributed to the α-type crystal peak of starch [37]. Compared with the original ST, the XRD spectra of CS showed no obvious diffraction peak, indicating that the crystallization of CS after etherification was transformed into amorphous region. The XRD spectrum of graft copolymer was changed when the monomers of CS, AM and DMDAAC were grafted together to form graft copolymer CS-g-CPAM. The grafted polymer showed a new
Fig. 4. XRD spectra of ST, CS, CS-g-CPAM, CS-g-CPAM/GO and CPAM.
be seen from Fig. 3(c) and (d), a new peak at 1610 cm 1 attributed to the vibration absorption of –Nþ(CH3)3 group of CTA [34], which proved that CS was successfully prepared. As shown in Fig. 3(b), absorption peaks of 3190 cm 1 and 763 cm 1 were attributed to –NH2 and C-N
Fig. 5. The thermal gravimetric curves: (a) ST, (b) CS, (c) CS-g-CPAM, (d) CS-g-CPAM/GO. 6
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Table 1 Thermal gravimetric parameters of the samples. Parameter
First stage
Second stage
Third stage
Fourth stage
Flocculants
Temperature range (� C) Weight loss (%) Maximum weight loss temperature (� C) Temperature range (� C) Weight loss (%) Maximum weight loss temperature (� C) Temperature range (� C) Weight loss (%) Maximum weight loss temperature (� C) Temperature range (� C) Residual weight (%)
ST
CS
CS-gCPAM
CS-gCPAM/ GO
42–205
30–150
45–195
45–165
6.4 82.2
7.3 45
9.2 75
6.9 75
200–417
150–435
195–360
165–360
72.8 321.7
63.3 255
33.4 285
39.8 285.5
417–600
435–600
360–495
360–480
20.8 \
29.4 \
36.1 405.9
31.3 390.3
\
\
495–600
480–600
\
\
21.3
22
Fig. 6. Raman spectra of CS-g-CPAM/GO and GO.
Additionally, in order to further confirm CS-g-CPAM/GO was suc cessfully fabricated, Raman spectra of CS-g-CPAM/GO and GO was characterized. The Raman spectra is shown Fig. 6. Compared with Raman spectra of GO, Raman shift of GO appeared in Raman spectra of CS-g-CPAM/GO, characteristic peak was respectively 1377 cm 1 and 1579 cm 1. This further indicates that CS-g-CPAM/GO has been suc cessfully prepared.
diffraction peak, which was attributed to CPAM. However, compared with CPAM, the diffraction peaks of grafted polymer were reduced. This showed that CS changed the crystal structure of graft copolymer after graft polymerization [38]. CS-g-CPAM/GO formed showed character istic diffraction peaks of CPAM and CS after the introduction of GO through condensation reaction. However, the corresponding character istic peak intensity was reduced, resulting in a decrease in the crystal linity degree of CS-g-CPAM/GO. Through the above analysis, after the cationization, graft copolymerization and condensation reaction, the crystal forms of the samples had changed, which further proved that the CS-g-CPAM/GO was successfully synthesized [39].
3.2.4. SEM images analysis SEM images of (a) ST, (b) CS, (c) CS-g-CPAM, (d) GO and (e) CS-gCPAM/GO are shown in Fig. 7 to investigate the morphology. Accord ing to Fig. 7(a), ST exhibited a spherical shape with a regular structure and a smooth surface morphology. After cationization, the obtained CS morphology is as shown in Fig. 7(b). The morphology of CS was changed dramatically compared to before cationization, CS formed a rough sur face. However, the morphology and structure of CS-g-CPAM was also altered by the adding of AM and DMDAAC monomers. As shown in Fig. 7 (c). Obviously, after graft polymerization, CS and CPAM formed irreg ular morphology. After adding GO to CS-g-CPAM, the morphological structure of GO was transformed from Fig. 7(d) to Fig. 7(e). It can be seen from Fig. 7(e) that there was a significant change in the appearance of CS-g-CPAM/GO, and there were folds in some areas of the CS-gCPAM/GO surface, while the specific surface area of CS-g-CPAM/GO can be increased by the folded area. Therefore, theoretically CS-gCPAM/GO has a better flocculation effect compared with CPAM.
3.2.3. TG-DSC characterization The thermal gravimetric analysis of (a) ST, (b) CS (c) CS-g-CPAM and (d) CS-g-CPAM/GO are shown in Fig. 5, and all the analytical data are summarized in Table 1. As shown in Fig. 5 (a) and (b), ST and CS had two weight loss stages, and the weight loss of these two stages was basically similar, the first stages of weight loss was 6.4% and 7.3% respectively, and the second stage of weight loss was 72.8% and 63.3% respectively. It showed that the structure of CS had little change compared with that of the original ST. As can be seen from Fig. 5(c), CS-g-CPAM showed three distinct stages of weight loss compared with Fig. 5(b). The first stage weight loss temperature was within 45 � C–195 � C range, weight loss was 9.2%. The second stage of weight loss temperature was observed from 195 � C to 360 � C, weight loss was 33.4%. The third stage weight loss temperature was within 360 � C–495 � C range, weight loss was 36.1%. In the third stage, the main chain of graft copolymer CS-g-CPAM began to degrade rapidly. When the final temperature reached 495 � C, the grafted polymer was almost completely decomposed. After 495 � C, the curve of the weight loss was flatten, The final weight of the residue was 21.3%. And the corresponding three endothermic peaks were 75 � C, 285 � C and 405.9 � C, respectively. The CS endothermic peaks peak at 255 � C was occurred at the DSC curve of CS-g-CPAM at 285 � C. Fig. 5 (d) was compared with Fig.5 (c), Fig.5 (d) also showed three distinct weight loss stages. However, weight loss of CS-g-CPAM/GO in the second stage was significantly greater than that of CS-g-CPAM. This was attributed to the pyrolysis of GO-containing oxygen functional groups on CS-g-CPAM/GO [40]. Therefore, the successful synthesis of CS-g-CPAM/GO can be confirmed through the above analysis.
4. Flocculation performance and mechanism 4.1. Effect of dosage on flocculation performance Flocculant dosages play the vital role in the transmittance of oil sludge supernatant under the conditions of T ¼ 40 � C, pH ¼ 8. As can be seen from Fig. 8(a), the light transmittance of the supernatant gradually increased with increasing dosage, and then the light transmittance gradually decreased with increasing dosage. The maximum trans mittance of 87.7%, 84.7% and 75.3% was obtained by CS-g-CPAM/GO, CS-g-CPAM and CS at a dosage of 6 mg L 1, 6 mg L 1 and 8 mg L 1, respectively. The above phenomenons are explained by the following reasons. At low dosage, it can not be fully contacted with the oil sludge particles, and the positive charge was not enough, as well as it can not effectively play the adsorption bridging and charge neutralization to destroy the stabilization of oil sludge suspension. The sludge particles were wrapped by the flocculant when the flocculant overdosage, and the 7
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Fig. 7. SEM images of (a) ST, (b) CS, (c) CS-g-CPAM, (d)GO and CS-g-CPAM/GO.
electrostatic repulsion between the flocs that were not enough to settle was gradually increased. Due to electrostatic repulsion, these flocs were not sufficient to form larger flocs [41]. Therefore, the optimal dosage of CS-g-CPAM/GO, CS-g-CPAM and CS was 6 mg L 1, 6 mg L 1 and 8 mg L 1, respectively. In addition, the flocculation performance of CS-g-CPAM/GO was consistently better than CS-g-CPAM and CS during the whole flocculation process, and CS-g-CPAM/GO also exhibited the advantages of low dosage. The main reasons for the above phenomenon were because GO based CS-g-CPAM/GO can enhance adsorption bridging, sweeping and enmeshment due to oxygen-containing func tional groups and sheet structure of GO. Therefore, the flocculation performance of CS-g-CPAM/GO was consistently better than CS-g-CPAM and CS. This result indicated that flocculation performance can be enhanced through the introduction of CS and GO.
neutralization function. When the temperature was too high, the CS-based in CS-g-CPAM/GO was easy to be gelatinized, resulting in the decrease of CS-g-CPAM/GO viscosity, which leaded to the decrease of the adsorption bridge performance, but the flocculation performance of CS-g-CPAM/GO was always higher than CS-g-CPAM and CS. Therefore, the best flocculation temperature is 40 � C. In addition, in order to determine the above conclusions, the influ ence of temperature on the viscosity of flocculant was studied (the vis cosity of flocculant was determined by RDV-2 digital rotational viscometer), and the results were shown in the inset in Fig. 8(b). As can be seen from the illustration in Fig. 8(b), the viscosity of CS-g-CPAM/GO reduced with the increase of temperature. At lower temperatures, vis cosity of CS-g-CPAM/GO decreased less. The higher the temperature, The viscosity of CS-g-CPAM/GO was reduced more. This further proves that the viscosity of flocculant decreased with the increase of temperature.
4.2. Effect of temperature on flocculation performance
4.3. Effect of pH on flocculation performance
The effect of temperature on the transmittance of oil sludge super natant was investigated under the condition of the optimal dosage and pH. The results are shown in Fig. 8(b). It was found that the maximum transmittance was reached 90.1%, 87.1% and 79.3% on T ¼ 40 � C, respectively. At low temperature, the activity of flocculant was low, the brownian motion was slow and the collision probability between each other was reduced, which was unfavorable to flocculation [42]; floc culant activity was increased with the temperature rising, and the probability of flocculant colliding with the oil sludge particles was augmented, which promoted flocculant adsorption bridging and charge
The effect of pH value was evaluated under the conditions of optimal dosage and temperature. The results are described in Fig. 8(c). The ul timate transmittance of CS-g-CPAM/GO were stabilized around 90.2% at pH range of 3–12. Therefore, CS-g-CPAM/GO is suitable for wide range of pH. Flocculation performance of CS-g-CPAM/GO was always better than that of CS-g-CPAM and CS under acidic and alkaline condi tions. In addition, the amino group of cationic polyacrylamide on CS-gCPAM was deprotonated under alkaline conditions [43], resulting in a 8
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Fig. 9. Zeta potential of CS-g-CPAM/GO, supernatant and oil sludge suspension at various pH.
Fig. 10. FTIR spectra of CS-g-CPAM/GO, floc and oil sludge.
decrease in charge neutralization, the flocculation performance of CS-g-CPAM decreased with the increase of alkalinity. But the GO-based on CS-g-CPAM/GO can make up the flocculation performance under alkaline conditions via adsorption and bridging. Therefore, CS-g-CPAM/GO showed better flocculation effect than CS-g-CPAM and CS under alkaline conditions. From what has been discussed above, optimized pH of oil sludge suspension for CS-g-CPAM/GO is 3–12. Moreover, zeta potential of CS-g-CPAM/GO, supernatant and oil sludge suspension are shown in Fig. 9. The potential of oil sludge sus pension was always electronegativity in the pH range of 3–12, the su pernatant zeta potential was significantly improved after adding CS-gCPAM/GO. This result indicated that the negative charge of the oil sludge particles was neutralized by CS-g-CPAM/GO in the flocculation process. Fig. 8. Effect of (a)dosage, (b) temperature and (c) pH.
4.4. FTIR analysis of flocs FTIR spectra of CS-g-CPAM/GO, flocs and oil sludge are shown in Fig. 10 in order to analyze the interaction between CS-g-CPAM/GO and oil sludge particles. As shown in Fig. 10 (1) and (3), spectral bands at 9
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Fig. 11. Possible flocculation mechanism of CS-g-CPAM/GO.
1022 cm 1 and 596 cm 1 attributed to Si-O-Si and Si-O groups on oil sludge particles. The results showed that no chemical reaction was found between oil sludge particles and CS-g-CPAM/GO [44]. As shown in Fig. 10 (2), peaks at roughly 1600 cm 1 and 924 cm 1 were attributed to –Nþ(CH3)3 and –Nþ(CH3)2 of CS-g-CPAM/GO. However, these two vi bration peaks observed in the spectrum of the flocs were very weak. The possible reason for this phenomenon was that the two positive charge groups were wrapped by oil sludge particles, which made it difficult to be detected by infrared spectrometers. These oil sludge particles were adsorbed to the surface of positive charge groups by charge neutrali zation. From Fig. 9 (1) and (3), we can see that the (a), (b) and (c) strong peaks of oil sludge particles were weakened when the oil sludge particles were flocculated into flocs, the one possible cause was due to the branched chain adsorption and bridging effect on the surface of CS-g-CPAM/GO, another reason was charge neutralization, resulting in the oil sludge particles being agglomerated, the transmittance decreased and the strong peak decreased.
5. Conclusion CS-g-CPAM/GO was obtained by Ammonium persulfate-initiated polymerization and EDC–NHS–induced condensation reaction. The in fluence factors of the positive charge density, intrinsic viscosity, GR and GE were investigated in this study, including total monomer concen tration, initiator dosage, monomer ratio, post-polymerization tempera ture and post-polymerization time. Optimal synthetic conditions were achieved at under conditions of monomer concentration of 20%, initi ator of 0.2%, monomer ratio of 6: 4: 3, post-polymerization tempera tures of 55 � C and post-polymerization time of 24 h. The chemical structure and morphological characteristics of CS-g-CPAM/GO was confirmed by FTIR spectra, XRD, TG-DSC and SEM. The FTIR spectrum showed that CS-g-CPAM/GO was successfully synthesized. Meanwhile, XRD confirmed that crystallinity degree of CS-g-CPAM/GO decreased, and further revealed the successful synthesis of CS-g-CPAM/GO. In addition, thermal stability of CS-g-CPAM/GO was confirmed by TG-DSC. SEM indicated that surface of CS-g-CPAM/GO formed folded area and had better flocculation effect. Then influence factors of CS-g-CPAM/GO flocculation performance were examined in flocculation oil sludge sus pension, including CS-g-CPAM/GO dosage, temperature and pH. Opti mum flocculation conditions were reached at CS-g-CPAM/GO dosage of 6 mg L 1, T ¼ 40 � C and pH of 3–12, wherein the transmittance of 90.2–97.5% can be finally attained. Finally, the possible flocculation mechanism of CS-g-CPAM/GO was summarized through the above analysis. Charge neutralization and adsorption bridging played an important role in the whole flocculation process.
4.5. Flocculation mechanism According to the effects of CS-g-CPAM/GO dosage, temperature and pH on flocculation performance and the changes of zeta potential and infrared spectrum analysis of flocs and oil sludge, the flocculation mechanism of CS-g-CPAM/GO is summarized in Fig. 11. As shown in Fig. 11, since the CS-based of CS-g-CPAM/GO was degraded under acidic conditions, charge neutralization and adsorption bridging prop erties were weakened. However, There was not enough positive charge and were not sufficient to neutralize the negative charge of the oil sludge particles, a small amount of positive charge acted only to enhance the initial adsorption properties of oxygen-containing functional groups of CS-g-CPAM/GO by reducing the negative charge of the surface on the oil sludge particles. First, negative charges on oil sludge particles were reduced by a small amount of positive charge, the oil sludge particles that were reduced the charge were adsorbed by via supply-reception electronic mechanisms [45], the long chain on CS-g-CPAM/GO bridged oil sludge particles and larger particles were generated, large particles formed flocs by sweeping and enmeshment. Oil sludge particles were combined with CS-g-CPAM/GO by charge neutralization and ab sorption bridging&patching. Then they form flocs and settlement under pH > 7 conditions. The flocs particles will continue to sweep and enmesh the remaining oil sludge particles in the settlement process.
Acknowledgements The work was financially supported by the National Natural Science Foundation of China(Project Nos. 51146008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107416. References [1] Yang X, Tan W, Bu Y. Demulsification of asphaltenes and resins stabilized emulsions via the freeze/thaw method. Energy Fuel 2009;23(1):481–6.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Cationic starch-grafted-cationic polyacrylamide based graphene oxide ternary composite flocculant for the enhanced flocculation of oil sludge suspension Ying Chen a, Gongwei Tian b, Boyin Zhai a, Huili Zhang a, Yuning Liang a, Hongbao Liang c, * a b c
Heilongjiang Provincial Key Laboratory of Chemical Engineering of Oil and Gas, Northeast Petroleum University, Daqing, China Heilongjiang Provincial Key Laboratory of Chemical Engineering of Oil and Gas, Northeast Petroleum University, Daqing, China College of Mechanical Science and Engineering, Northeast Petroleum University, Daqing, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Cationic starch Cationic polyacrylamide Graft Graphene oxide Flocculation mechanism
In this study, a novel, highly efficient and environmentally friendly ternary composites flocculant, namely, cationic starch-grafted-cationic polyacrylamide/graphene oxide (CS-g-CPAM/GO), was synthesized by ammo nium persulfate initiation polymerization and condensation reaction. First, CS-g-CPAM was polymerized with cationic starch (CS), acrylamide (AM) and diallyl dimethyl ammonium chloride (DMDAAC), and CS-g-CPAM/GO was then synthesized by condensation reaction. The influence factors of graft polymerization were investigated, including total monomer concentration, initiator dosage, monomer mass ratio of mAM: mCS: mDMDAAC, postpolymerization temperature and post-polymerization time. The chemical structures and morphologies of the samples were characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC), and scanning electron microscope (SEM). The CS-gCPAM/GO was used to flocculate the oil sludge suspension, the effects of CS-g-CPAM/GO dosage, temperature and pH value on the flocculation performance were investigated, and the flocculation mechanism of CS-g-CPAM/ GO was also analyzed. The results show that CS-g-CPAM/GO has outstanding flocculation effect, and CS-gCPAM/GO flocculates oil sludge particles by adsorption bridging and charge neutralization in acidic and alka line conditions.
1. Introduction Petroleum, as an important energy and chemical raw material, pro duces a lot of waste residue, waste water and waste gas in the process of mining, transportation, storage and processing, which is called oil sludge. Its main components are colloid, asphalt, solid granular, oil soluble organic acid [1,2], zinc, copper, chromium, lead and other heavy metals [3–6], Landfill treatment not only take up a lot of land, but also harms human health, pollutes local environment and impedes economic development [7,8]. Chemical adsorption and flocculation have good flocculation effective and relatively low cost, which is one of the com mon methods to treat oil sludge [9,10]. The cationic polyacrylamide flocculant (CPAM) has been extensively used in oil sludge treating agents due to its excellent turbidity removal, flocculation and decolorization properties [11]. Because the oil sludge particles are negatively charged, CPAM electricity neutralization and
adsorption bridging with the oil sludge particles, which lead to the destabilization, aggregation and settlement of the oil sludge particles to achieve the purpose of flocculation. At present, the research of deep and widely used CPAM mainly has polyacrylamide-dimethyl diallyl ammo nium chloride P (AM-DMDAAC), polyacrylamide-acryloyloxyethyl tri methylammonium chloride P (AM-DAC), polyacrylamide-methacry loyloxyethyl trimethylammonium chloride P (AM-DMC), other series of CPAM are still in the experimental research stage [12,13]. Although CPAM is widely utilized, it causes secondary pollution to the environ ment due to the difficulty in biodegradation [14] and has weak alkaline resistance [15]. In order to solve this problem, some natural polymer, such as chitosan, cellulose and starch, has been investigated because their derivatives are biodegradable, and their degradation intermediates are harmless for people and the environment [16–18], and starch can be further modified to improve its cationic degree to enhance charge neutralization. However, this kind of natural polymer flocculant exhibits
* Corresponding author. College of Mechanical Science and Engineering, Northeast Petroleum University, Daqing, 163318, China. E-mail address: [email protected] (H. Liang). https://doi.org/10.1016/j.compositesb.2019.107416 Received 6 July 2018; Received in revised form 9 August 2019; Accepted 4 September 2019 Available online 5 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Possible reaction mechanism of CS-g-CPAM/GO.
poor solubility under neutral conditions [19]. To make up for each other’s shortcomings and combine the advantages of each other, graft copoly-merization is often employed to modify CPAM and to obtain high solubility, environment-friendly and effectiveness of flocculants [20, 21]. In addition, the surface of graphene oxide nanosheets contains – O, many oxygen-containing functional groups, such as –COOH, –C– –OH, etc. These functional groups can improve the flocculation
performance [22–24]. Taking into account the above factors, a novel flocculant CS-gCPAM/GO was synthesized by CS, AM, DMDAAC and GO through ammonium persulfate-induced polymerization and condensation reac tion. Synthesis conditions of total monomer concentration, initiator dosage, monomer mass ratio of mAM: mCS: mDMDAAC, post-polymerization temperature and post-polymerization time were investigated for 2
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intrinsic viscosity, grafting ratio (GR) and grafting efficiency (GE). The chemical structure of the obtained CS-g-CPAM/GO was characterized by FTIR spectroscopy, XRD, TG-DSC and SEM. Finally, CS-g-CPAM/GO was used to flocculate the oil sludge suspension, the flocculation perfor mance was examined. Meanwhile, flocculation mechanism of CS-gCPAM/GO was researched via zeta potentials of supernatant and oil sludge suspension and FTIR of flocs. 2. Experimental
the ice water bath and magnetic stirring, 8 g KMnO4 was slowly added, and the reaction was continued at 5 � C for 1 h. Then the temperature of the water bath rose to 35 � C, continue stirring for 2 h, add 100 mL of deionized water, the temperature rose to 98 � C to continue the reaction for 15 min. After natural cooling, gradually add 180 mL deionized water and then add 10 mL H2O2 solution dropwise until the solution turned bright yellow. Centrifugal washing at 10000 r/min to neutrality, ultra sonic 30 min, centrifugation at 4000 r/min to take the supernatant, supernatant was placed in a vacuum oven at 60 � C for 12 h to obtain GO.
2.1. Materials
2.6. Synthesis of CS-g-CPAM/GO
Graphite powder was provided by China Oak Maanshan Chemical Industry; Acrylamide(AM, 99.0% wt), Diallyl dimethyl ammonium chloride (DMDAAC, 60% wt), H2SO4 (98% wt), H2O2 (30% wt), KMNO4, NaOH, NaCl, NaNO3, ethanol, acetone (1.0 mg/mL), Starch(ST), 1-ethyl3-(3-dimethylaminopropyl) carbodiimide (EDC), N-hydroxy sulfosucci nimide (NHS), 3-Chloro-2-hydroxypropyltrimethylammonium chloride (CTA, 65 wt% in water), Acetic acid(98% wt), poly(vinyl sulfate) po tassium salt (PVSK, average Mw ~ 162.21), toluidine blue O (TBO, Mw ~ 305.81), Ammonium persulfate and ethylene glycol (EG) were procured from China Sinopharm Chemical Reagent Co., Ltd; oil sludge from the oil sludge treatment station of China Daqing oilfield to provide; Industrial nitrogen(Purity >99%); In all experiments, all reagents were analytical grade, and we used deionized water.
100 mL EDC/NHS mixture (7 mg/mL EDC and 5 mg/mL NHS) was added to 100 mL GO solution (1 mg/mL aqueous solution) and shaken at 25 � C for 30 min. Then, 100 mg CS-g-CPAM was added into the GO so lution, the mixture was then shaken at 25 � C for 1 h and heated at 50 � C for 1 h. The product was washed three times with phosphate buffer so lution and dried in a vacuum oven at 50 � C, CS-g-CPAM/GO was obtained. 2.7. Characterization of CS-g-CPAM/GO The functional groups of CS-g-CPAM/GO was characterized by a Nicolet 380 Series infrared spectrometer (Nicolet Company, United States), KBr tablet, scanning range for 500-4000 cm 1, XRD spectrum of CS-g-CPAM/GO was obtained through D/max2500X-ray diffraction (Rigaku Company, Japan); TG-DSC were obtained on a SDT-Q600 Thermal Analyzer (DSC, TA Co., USA); to investigate the morphology of CS-g-CPAM/GO, SEM images were carried out with a S-4700 field emission scanning electron microscope. (Hitachi Limited, Japan). Grafting ratio (GR) and grafting efficiency (GE) were calculated by the following formula: � � m3 m1 GR ¼ � 100% m1
2.2. Possible reaction mechanism of CS-g-CPAM/GO The possible reaction mechanism of CS-g-CPAM/GO is shown in Fig. 1. First of all, ST was made of CS by etherification reaction. Then, CS, AM and DMDAAC monomers produced free radicals under the action of the ammonium persulfate initiator. The graft polymerization of free radicals produced CS-g-CPAM. Finally, the amino group of CS-g-CPAM [15] and the carboxyl group of GO was dehydration condensation re action to form CS-g-CPAM/GO flocculant [25].
�
� m3 m1 � 100% m2 m1
2.3. Synthesis of CS
GE ¼
The ST (10 g) was dispersed in ethanol and transferred to a threenecked flask equipped with a mechanical stirring and condensing de vice. 0.96 g NaOH and 2.5 g NaCl were prepared into 50 mL solution, which was added to the ST solution for 30 min at 50 � C, then add the CTA solution to it at 50 � C for 6 h. After the reaction was finished. First, add to 2 times the volumes of ethanol at 20 � C for 10 min 12000 rpm centrifugal 10 min, pour off the supernatant and product was washed with 70% ethanol solution until free of chloride ions. The CS was ob tained after vacuum drying at 50 � C for 24 h.
Therein m1 was the mass of CS, m2 was the mass of crude polymer ized products, m3 was the mass of fine products CS-CPAM. The intrinsic viscosity of the CS-g-CPAM was measured by the one point method accurately [26]. The positive charge density of the CS-g-CPAM was determined by titration method [26]. 2.8. Flocculation evaluation Flocculation performance of CS-g-CPAM/GO was investigated in oil sludge suspension. Preparation of oil sludge suspension was as follow: 2 g oil sludge was dissolved in a beaker containing 1000 mL deionized water under mechanical stirring 30 min at 300 r min 1, and then ul trasound 10 min, the formation of oil sludge suspension. A certain dosage of CS-g-CPAM/GO was promptly added into the oil sludge sus pension, the mixture of CS-g-CPAM/GO and oil sludge suspension was stirred at 200 r min 1 for 10 min, free-settling, and the transmittance was surveyed by V-5000h visible spectrophotometer (Shanghai Analytical Instrument Co., Ltd, China) at a λ ¼ 618 nm [27].
2.4. Synthesis of CS-g-CPAM The reaction was carried out in a 250 mL four-neck round bottom flasks containing with a stirrer, thermometer, condenser and nitrogen inlet. A certain mass of CS and 50 mL deionized water was poured into the flask to preheat at 85 � C for 30 min. After the CS gelatinization (the CS slurry became a clear solution), a certain amount of AM, DMDAAC and ammonium persulfate was added, and the mixture was reacted under N2 protection at 55 � C for 3 h. After drying and crushing, the polymerized products were washed with acetone, crude polymerized products were obtained and were then washed with a 2: 3 solution of glacial acetic acid and ethylene glycol and dried 12 h in a 50 � C vacuum oven until a constant weight was obtained, fine products CS-CPAM were obtained.
3. Results and discussions 3.1. Optimization conditions of CS-g-CPAM synthesis 3.1.1. The effects of total monomer concentration on positive charge density, intrinsic viscosity, GR and GE The total monomer concentration (m/m%) is the percentage of all monomers in the total mass of the reaction mixture, where mAM: mCS: mDMDAAC is 6: 4: 3. The effect of total monomer concentration on positive
2.5. Preparation of GO 1 g graphite powder was added to 100 mL H2SO4 to mix evenly under 3
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Fig. 2. (a) total monomer concentration, (b) initiator dosage, (c) mAM: mCS, (d) mAM: mCS: mDMDAAC, (e) post-polymerization temperature, (f) postpolymerization time.
increase of total monomer concentration, the probability of collision between CS, AM and DMDAAC increased, which promoted the chain growth reaction and made the whole reaction system in a highly active state, the chain of CS-g-CPAM was continuously extended, and the vis cosity of the reaction system was increased, when the monomer con centration reached saturation, the collision between the molecules of CS, AM and DMDAAC was saturated, and the chain growth rate cannot be further improved [28]. Furthermore, the cross linking between the su persaturated CS, AM and DMDAAC resulted in the deterioration of CS-g-CPAM solubility, and the movement and transfer rate of monomers and active free radicals was also low. Moreover, high monomer con centration may trigger a double termination reaction [29,30].
charge density, intrinsic viscosity, GR and GE of CS-g-CPAM was investigated at initiator dosage of 0.2%, post-polymerization tempera ture of 55 � C, and post-polymerization time of 24 h. The results are presented in Fig. 2(a). Positive charge density, intrinsic viscosity, GR and GE increased with the increasing monomer concentration. The optimal positive charge density, intrinsic viscosity, GR and GE obtained at 20% monomer concentration were 3.41 mmoL/g, 2981 mL/g, 61.4% and 68.9%, respectively. The collision probability between monomer was greatly lower when the concentration of CS, AM and DMDAAC was too low. The result was that the growth rate of the chain was reduced, and the entire reactivity was insufficient. Therefore, short-chain poly mers can be formed between monomers, and macromolecular graft polymer was not formed due to low monomer concentration. With the 4
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3.1.2. The effects of initiator dosage on positive charge density, intrinsic viscosity, GR and GE Fig. 2(b) shows the effect of initiator dosage on positive charge density, intrinsic viscosity, GR and GE, in which the total monomer concentration, mAM: mCS: mDMDAAC, post-polymerization temperature and post-polymerization time are 20%, 6: 4: 3, 55 � C and 24 h, respec tively. As showed in Fig. 2(b). Positive charge density, intrinsic viscos ity, GR and GE obviously increased with the increasing initiator dosage. CS, AM and DMMAC molecules were polymerized according to the free radical mechanism. The optimum positive charge density, intrinsic vis cosity, GR and GE obtained at a dosage of 0.2% initiator were 3.71 mmoL/g, 3215 mL/g, 62.7%, and 71.2%, respectively. At low initiator dosage, a small number of free radicals were surrounded by a large part of the solvent molecules, which inhibited the formation of free radicals and generating of graft polymer, called as the ‘‘cage effect” [31]. How ever, the proliferation of CS, AM and DMMAC free radicals promoted the continuous extension of CS-g-CPAM chain because of the increase of initiator dosage. Therefore, the positive charge density, intrinsic vis cosity, GR and GE were increased. Nevertheless, when the initiator was excessive, generated free radicals reached the over-saturation state, and the free radicals collided with the formed polymer chain, which caused the original property of the polymer to destroy [32]. Accordingly, the positive charge density, intrinsic viscosity, GR and GE were reduced. 0.2% were fixed at the optimum initiator dosage.
Fig. 3. FTIR spectra of various samples: (a) CS-g-CPAM/GO, (b) CS-g-CPAM, (c) CS, (d) CPAM and (e) ST.
the post-polymerization temperature of 55 � C was favorable for the synthesis of CS-g-CPAM. When the post-polymerization temperature was particularly low, the activity of the monomers was low, the diffusion rate of free radical was slow, and the free radicals were not fully con tacted, which caused that AM and DMDAAC cannot sufficiently combine the active sites of CS. The reaction activity of monomers increased with the increase of post-polymerization temperature, and there was continuous graft polymerization between CS, AM and DMDAAC. As a result, the polymer chain increased continuously, and the positive charge density, intrinsic viscosity, GR and GE also increased [33]. The active centers on cationic starch were unstable due to extremely high post-polymerization temperature, which leaded to the decrease of its own polymerization ability. The heat generated by the polymerization was not easy to reduce. Consequence, the chain transfer rate and growth rate decreased. This phenomenon was unfavorable for graft copoly merization between monomers. Secondly, excessive post-polymerization temperature could also lead to cross-linking and reduced dissolution of CS-g-CPAM. Therefore, the optimal post-polymerization temperature of CS-g-CPAM was 55 � C.
3.1.3. The effects of mAM: mCS: mDMDAAC on positive charge density, intrinsic viscosity, GR and GE Studies showed that dispersion stability of oil sludge particle can be reduced and broke via bridging effects and charge neutralization. Bridging effects can be enhanced by increasing the intrinsic viscosity [11]. The bridging capacity and charge neutralization are determined by the length of the AM main chain and the number of CS and DMDAAC cationic groups. Therefore, the effects of mAM: mCS on positive charge density, intrinsic viscosity, GR and GE were first considered, wherein the quality of the DMDAAC remains unchanged at 3 g, and then the optimal ratio of mAM: mCS: mDMDAAC was investigated, in which the total monomer concentration, initiator dosage, post-polymerization temper ature and post-polymerization time are 20%, dosage of 0.2%, 55 � C and 24 h, respectively. As shown in Fig. 2(c), the results showed that the positive charge density, intrinsic viscosity, GR and GE gradually increased with the increase of mAM: mCS mass ratio, and the intrinsic viscosity were optimal when mAM: mCS mass ratio was 6: 4. Whereas, the positive charge density, intrinsic viscosity, GR and GE reduced with the further increase of mAM: mCS mass ratio. When AM masses were low, the rate of grafting to the CS monomer was slowed down. In addition, the probability of collision of monomer molecules decreased, and the active site of AM molecules was insufficient. When the AM monomer concen tration was too high, AM monomer increased the chain transfer, resulting in a decrease in the intrinsic viscosity of the CS-g-CPAM. At the same time, the CS and DMDAAC were difficult to graft to the main chain, causing positive charge density, intrinsic viscosity, GR and GE to degrade. Furthermore, as shown in Fig. 2(d), the positive charge density, intrinsic viscosity, GR and GE exhibited the same changes under various concentrations of DMDAAC. Therefore, monomer mass ratio mAM: mCS: mDMDAAC of 6: 4: 3 was optimal for synthesis of CS-g-CPAM.
3.1.5. The effects of post-polymerization time on positive charge density, intrinsic viscosity, GR and GE The effects of post-polymerization time on positive charge density, intrinsic viscosity, GR and GE are shown in Fig. 2(f) at monomer con centration of 20%, initiator dosage of 0.2%, monomer mass ratio mAM: mCS: mDMDAAC of 6: 4: 3 and post-polymerization temperature of 55 � C. The positive charge density, intrinsic viscosity, GR and GE increased relatively less when the post-polymerization time exceeded 24 h. Posi tive charge density, intrinsic viscosity, GR and GE were 4.96 mmoL/g, 3612 mL/g, 75.8% and 83.9% respectively at the 24 h postpolymerization time. These parameters tended to be balanced with the extension of post-polymerization time. Free radicals and monomers were constantly consumed with the extension of polymerization time in order to promote graft copolymerization. Thereby improving the posi tive charge density, intrinsic viscosity, GR and GE. The reaction between the monomers was completed after a certain period of time, and The effect on positive charge density, intrinsic viscosity, GR and GE was small. Therefore, the optimal post-polymerization time of 24 h was favorable for the synthesis of CS-g-CPAM.
3.1.4. The effects of post-polymerization temperature on positive charge density, intrinsic viscosity, GR and GE The effects of post-polymerization temperature on the positive charge density, intrinsic viscosity, GR and GE are shown in Fig. 2(e) at monomer concentration of 20%, initiator dosage of 0.2%, monomer mass ratio mAM: mCS: mDMDAAC of 6: 4: 3 and post-polymerization time of 24 h. The positive charge density, intrinsic viscosity, GR and GE increased gradually as the post-polymerization increased. However, the positive charge density, intrinsic viscosity, GR and GE dropped sharply when the post-polymerization temperature exceeded 55 � C. Therefore,
3.2. Characterization 3.2.1. FTIR spectra The FTIR spectrum of a series of samples are shown in Fig. 3. As can 5
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group of AM, respectively. Absorption bands of 924 cm 1 were attrib uted to –Nþ(CH3)2 group of DMDAAC, at the same time, the –Nþ(CH3)3 characteristic peaks of CS also appeared in Fig. 3(b), these data showed that CS and CAPM grafted successfully. As shown in Fig. 3(a), peak at – O of GO, and compared with the 1560 cm 1 were ascribed to the C– infrared spectrum of CS-g-CPAM in Fig. 3(b), a new absorption bands at 1218 cm 1 attributed to CO-NH- [25]. The presence of CO-NH- group indicated that the carboxyl group of GO and the amino group of CS-g-CPAM underwent a dehydration condensation reaction. Moreover, CS-g-CPAM/GO had characteristic peaks of CS, CPAM and GO, respec tively. But the position of the peak was slightly shifted [35,36], which further confirmed the successful synthesis of CS-g-CPAM/GO. 3.2.2. XRD spectrum XRD spectrum of ST, CS, CS-g-CPAM, CS-g-CPAM/GO and CPAM was obtained to investigate the crystalline structure and effects of monomer on CS-g-CPAM/GO crystal morphology. XRD spectrum is shown in Fig. 4. The characteristic diffraction peak of ST was observed in the range of 18◦-21� , this strong diffraction peak was attributed to the α-type crystal peak of starch [37]. Compared with the original ST, the XRD spectra of CS showed no obvious diffraction peak, indicating that the crystallization of CS after etherification was transformed into amorphous region. The XRD spectrum of graft copolymer was changed when the monomers of CS, AM and DMDAAC were grafted together to form graft copolymer CS-g-CPAM. The grafted polymer showed a new
Fig. 4. XRD spectra of ST, CS, CS-g-CPAM, CS-g-CPAM/GO and CPAM.
be seen from Fig. 3(c) and (d), a new peak at 1610 cm 1 attributed to the vibration absorption of –Nþ(CH3)3 group of CTA [34], which proved that CS was successfully prepared. As shown in Fig. 3(b), absorption peaks of 3190 cm 1 and 763 cm 1 were attributed to –NH2 and C-N
Fig. 5. The thermal gravimetric curves: (a) ST, (b) CS, (c) CS-g-CPAM, (d) CS-g-CPAM/GO. 6
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Table 1 Thermal gravimetric parameters of the samples. Parameter
First stage
Second stage
Third stage
Fourth stage
Flocculants
Temperature range (� C) Weight loss (%) Maximum weight loss temperature (� C) Temperature range (� C) Weight loss (%) Maximum weight loss temperature (� C) Temperature range (� C) Weight loss (%) Maximum weight loss temperature (� C) Temperature range (� C) Residual weight (%)
ST
CS
CS-gCPAM
CS-gCPAM/ GO
42–205
30–150
45–195
45–165
6.4 82.2
7.3 45
9.2 75
6.9 75
200–417
150–435
195–360
165–360
72.8 321.7
63.3 255
33.4 285
39.8 285.5
417–600
435–600
360–495
360–480
20.8 \
29.4 \
36.1 405.9
31.3 390.3
\
\
495–600
480–600
\
\
21.3
22
Fig. 6. Raman spectra of CS-g-CPAM/GO and GO.
Additionally, in order to further confirm CS-g-CPAM/GO was suc cessfully fabricated, Raman spectra of CS-g-CPAM/GO and GO was characterized. The Raman spectra is shown Fig. 6. Compared with Raman spectra of GO, Raman shift of GO appeared in Raman spectra of CS-g-CPAM/GO, characteristic peak was respectively 1377 cm 1 and 1579 cm 1. This further indicates that CS-g-CPAM/GO has been suc cessfully prepared.
diffraction peak, which was attributed to CPAM. However, compared with CPAM, the diffraction peaks of grafted polymer were reduced. This showed that CS changed the crystal structure of graft copolymer after graft polymerization [38]. CS-g-CPAM/GO formed showed character istic diffraction peaks of CPAM and CS after the introduction of GO through condensation reaction. However, the corresponding character istic peak intensity was reduced, resulting in a decrease in the crystal linity degree of CS-g-CPAM/GO. Through the above analysis, after the cationization, graft copolymerization and condensation reaction, the crystal forms of the samples had changed, which further proved that the CS-g-CPAM/GO was successfully synthesized [39].
3.2.4. SEM images analysis SEM images of (a) ST, (b) CS, (c) CS-g-CPAM, (d) GO and (e) CS-gCPAM/GO are shown in Fig. 7 to investigate the morphology. Accord ing to Fig. 7(a), ST exhibited a spherical shape with a regular structure and a smooth surface morphology. After cationization, the obtained CS morphology is as shown in Fig. 7(b). The morphology of CS was changed dramatically compared to before cationization, CS formed a rough sur face. However, the morphology and structure of CS-g-CPAM was also altered by the adding of AM and DMDAAC monomers. As shown in Fig. 7 (c). Obviously, after graft polymerization, CS and CPAM formed irreg ular morphology. After adding GO to CS-g-CPAM, the morphological structure of GO was transformed from Fig. 7(d) to Fig. 7(e). It can be seen from Fig. 7(e) that there was a significant change in the appearance of CS-g-CPAM/GO, and there were folds in some areas of the CS-gCPAM/GO surface, while the specific surface area of CS-g-CPAM/GO can be increased by the folded area. Therefore, theoretically CS-gCPAM/GO has a better flocculation effect compared with CPAM.
3.2.3. TG-DSC characterization The thermal gravimetric analysis of (a) ST, (b) CS (c) CS-g-CPAM and (d) CS-g-CPAM/GO are shown in Fig. 5, and all the analytical data are summarized in Table 1. As shown in Fig. 5 (a) and (b), ST and CS had two weight loss stages, and the weight loss of these two stages was basically similar, the first stages of weight loss was 6.4% and 7.3% respectively, and the second stage of weight loss was 72.8% and 63.3% respectively. It showed that the structure of CS had little change compared with that of the original ST. As can be seen from Fig. 5(c), CS-g-CPAM showed three distinct stages of weight loss compared with Fig. 5(b). The first stage weight loss temperature was within 45 � C–195 � C range, weight loss was 9.2%. The second stage of weight loss temperature was observed from 195 � C to 360 � C, weight loss was 33.4%. The third stage weight loss temperature was within 360 � C–495 � C range, weight loss was 36.1%. In the third stage, the main chain of graft copolymer CS-g-CPAM began to degrade rapidly. When the final temperature reached 495 � C, the grafted polymer was almost completely decomposed. After 495 � C, the curve of the weight loss was flatten, The final weight of the residue was 21.3%. And the corresponding three endothermic peaks were 75 � C, 285 � C and 405.9 � C, respectively. The CS endothermic peaks peak at 255 � C was occurred at the DSC curve of CS-g-CPAM at 285 � C. Fig. 5 (d) was compared with Fig.5 (c), Fig.5 (d) also showed three distinct weight loss stages. However, weight loss of CS-g-CPAM/GO in the second stage was significantly greater than that of CS-g-CPAM. This was attributed to the pyrolysis of GO-containing oxygen functional groups on CS-g-CPAM/GO [40]. Therefore, the successful synthesis of CS-g-CPAM/GO can be confirmed through the above analysis.
4. Flocculation performance and mechanism 4.1. Effect of dosage on flocculation performance Flocculant dosages play the vital role in the transmittance of oil sludge supernatant under the conditions of T ¼ 40 � C, pH ¼ 8. As can be seen from Fig. 8(a), the light transmittance of the supernatant gradually increased with increasing dosage, and then the light transmittance gradually decreased with increasing dosage. The maximum trans mittance of 87.7%, 84.7% and 75.3% was obtained by CS-g-CPAM/GO, CS-g-CPAM and CS at a dosage of 6 mg L 1, 6 mg L 1 and 8 mg L 1, respectively. The above phenomenons are explained by the following reasons. At low dosage, it can not be fully contacted with the oil sludge particles, and the positive charge was not enough, as well as it can not effectively play the adsorption bridging and charge neutralization to destroy the stabilization of oil sludge suspension. The sludge particles were wrapped by the flocculant when the flocculant overdosage, and the 7
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Fig. 7. SEM images of (a) ST, (b) CS, (c) CS-g-CPAM, (d)GO and CS-g-CPAM/GO.
electrostatic repulsion between the flocs that were not enough to settle was gradually increased. Due to electrostatic repulsion, these flocs were not sufficient to form larger flocs [41]. Therefore, the optimal dosage of CS-g-CPAM/GO, CS-g-CPAM and CS was 6 mg L 1, 6 mg L 1 and 8 mg L 1, respectively. In addition, the flocculation performance of CS-g-CPAM/GO was consistently better than CS-g-CPAM and CS during the whole flocculation process, and CS-g-CPAM/GO also exhibited the advantages of low dosage. The main reasons for the above phenomenon were because GO based CS-g-CPAM/GO can enhance adsorption bridging, sweeping and enmeshment due to oxygen-containing func tional groups and sheet structure of GO. Therefore, the flocculation performance of CS-g-CPAM/GO was consistently better than CS-g-CPAM and CS. This result indicated that flocculation performance can be enhanced through the introduction of CS and GO.
neutralization function. When the temperature was too high, the CS-based in CS-g-CPAM/GO was easy to be gelatinized, resulting in the decrease of CS-g-CPAM/GO viscosity, which leaded to the decrease of the adsorption bridge performance, but the flocculation performance of CS-g-CPAM/GO was always higher than CS-g-CPAM and CS. Therefore, the best flocculation temperature is 40 � C. In addition, in order to determine the above conclusions, the influ ence of temperature on the viscosity of flocculant was studied (the vis cosity of flocculant was determined by RDV-2 digital rotational viscometer), and the results were shown in the inset in Fig. 8(b). As can be seen from the illustration in Fig. 8(b), the viscosity of CS-g-CPAM/GO reduced with the increase of temperature. At lower temperatures, vis cosity of CS-g-CPAM/GO decreased less. The higher the temperature, The viscosity of CS-g-CPAM/GO was reduced more. This further proves that the viscosity of flocculant decreased with the increase of temperature.
4.2. Effect of temperature on flocculation performance
4.3. Effect of pH on flocculation performance
The effect of temperature on the transmittance of oil sludge super natant was investigated under the condition of the optimal dosage and pH. The results are shown in Fig. 8(b). It was found that the maximum transmittance was reached 90.1%, 87.1% and 79.3% on T ¼ 40 � C, respectively. At low temperature, the activity of flocculant was low, the brownian motion was slow and the collision probability between each other was reduced, which was unfavorable to flocculation [42]; floc culant activity was increased with the temperature rising, and the probability of flocculant colliding with the oil sludge particles was augmented, which promoted flocculant adsorption bridging and charge
The effect of pH value was evaluated under the conditions of optimal dosage and temperature. The results are described in Fig. 8(c). The ul timate transmittance of CS-g-CPAM/GO were stabilized around 90.2% at pH range of 3–12. Therefore, CS-g-CPAM/GO is suitable for wide range of pH. Flocculation performance of CS-g-CPAM/GO was always better than that of CS-g-CPAM and CS under acidic and alkaline condi tions. In addition, the amino group of cationic polyacrylamide on CS-gCPAM was deprotonated under alkaline conditions [43], resulting in a 8
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Fig. 9. Zeta potential of CS-g-CPAM/GO, supernatant and oil sludge suspension at various pH.
Fig. 10. FTIR spectra of CS-g-CPAM/GO, floc and oil sludge.
decrease in charge neutralization, the flocculation performance of CS-g-CPAM decreased with the increase of alkalinity. But the GO-based on CS-g-CPAM/GO can make up the flocculation performance under alkaline conditions via adsorption and bridging. Therefore, CS-g-CPAM/GO showed better flocculation effect than CS-g-CPAM and CS under alkaline conditions. From what has been discussed above, optimized pH of oil sludge suspension for CS-g-CPAM/GO is 3–12. Moreover, zeta potential of CS-g-CPAM/GO, supernatant and oil sludge suspension are shown in Fig. 9. The potential of oil sludge sus pension was always electronegativity in the pH range of 3–12, the su pernatant zeta potential was significantly improved after adding CS-gCPAM/GO. This result indicated that the negative charge of the oil sludge particles was neutralized by CS-g-CPAM/GO in the flocculation process. Fig. 8. Effect of (a)dosage, (b) temperature and (c) pH.
4.4. FTIR analysis of flocs FTIR spectra of CS-g-CPAM/GO, flocs and oil sludge are shown in Fig. 10 in order to analyze the interaction between CS-g-CPAM/GO and oil sludge particles. As shown in Fig. 10 (1) and (3), spectral bands at 9
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Fig. 11. Possible flocculation mechanism of CS-g-CPAM/GO.
1022 cm 1 and 596 cm 1 attributed to Si-O-Si and Si-O groups on oil sludge particles. The results showed that no chemical reaction was found between oil sludge particles and CS-g-CPAM/GO [44]. As shown in Fig. 10 (2), peaks at roughly 1600 cm 1 and 924 cm 1 were attributed to –Nþ(CH3)3 and –Nþ(CH3)2 of CS-g-CPAM/GO. However, these two vi bration peaks observed in the spectrum of the flocs were very weak. The possible reason for this phenomenon was that the two positive charge groups were wrapped by oil sludge particles, which made it difficult to be detected by infrared spectrometers. These oil sludge particles were adsorbed to the surface of positive charge groups by charge neutrali zation. From Fig. 9 (1) and (3), we can see that the (a), (b) and (c) strong peaks of oil sludge particles were weakened when the oil sludge particles were flocculated into flocs, the one possible cause was due to the branched chain adsorption and bridging effect on the surface of CS-g-CPAM/GO, another reason was charge neutralization, resulting in the oil sludge particles being agglomerated, the transmittance decreased and the strong peak decreased.
5. Conclusion CS-g-CPAM/GO was obtained by Ammonium persulfate-initiated polymerization and EDC–NHS–induced condensation reaction. The in fluence factors of the positive charge density, intrinsic viscosity, GR and GE were investigated in this study, including total monomer concen tration, initiator dosage, monomer ratio, post-polymerization tempera ture and post-polymerization time. Optimal synthetic conditions were achieved at under conditions of monomer concentration of 20%, initi ator of 0.2%, monomer ratio of 6: 4: 3, post-polymerization tempera tures of 55 � C and post-polymerization time of 24 h. The chemical structure and morphological characteristics of CS-g-CPAM/GO was confirmed by FTIR spectra, XRD, TG-DSC and SEM. The FTIR spectrum showed that CS-g-CPAM/GO was successfully synthesized. Meanwhile, XRD confirmed that crystallinity degree of CS-g-CPAM/GO decreased, and further revealed the successful synthesis of CS-g-CPAM/GO. In addition, thermal stability of CS-g-CPAM/GO was confirmed by TG-DSC. SEM indicated that surface of CS-g-CPAM/GO formed folded area and had better flocculation effect. Then influence factors of CS-g-CPAM/GO flocculation performance were examined in flocculation oil sludge sus pension, including CS-g-CPAM/GO dosage, temperature and pH. Opti mum flocculation conditions were reached at CS-g-CPAM/GO dosage of 6 mg L 1, T ¼ 40 � C and pH of 3–12, wherein the transmittance of 90.2–97.5% can be finally attained. Finally, the possible flocculation mechanism of CS-g-CPAM/GO was summarized through the above analysis. Charge neutralization and adsorption bridging played an important role in the whole flocculation process.
4.5. Flocculation mechanism According to the effects of CS-g-CPAM/GO dosage, temperature and pH on flocculation performance and the changes of zeta potential and infrared spectrum analysis of flocs and oil sludge, the flocculation mechanism of CS-g-CPAM/GO is summarized in Fig. 11. As shown in Fig. 11, since the CS-based of CS-g-CPAM/GO was degraded under acidic conditions, charge neutralization and adsorption bridging prop erties were weakened. However, There was not enough positive charge and were not sufficient to neutralize the negative charge of the oil sludge particles, a small amount of positive charge acted only to enhance the initial adsorption properties of oxygen-containing functional groups of CS-g-CPAM/GO by reducing the negative charge of the surface on the oil sludge particles. First, negative charges on oil sludge particles were reduced by a small amount of positive charge, the oil sludge particles that were reduced the charge were adsorbed by via supply-reception electronic mechanisms [45], the long chain on CS-g-CPAM/GO bridged oil sludge particles and larger particles were generated, large particles formed flocs by sweeping and enmeshment. Oil sludge particles were combined with CS-g-CPAM/GO by charge neutralization and ab sorption bridging&patching. Then they form flocs and settlement under pH > 7 conditions. The flocs particles will continue to sweep and enmesh the remaining oil sludge particles in the settlement process.
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