TiO2-PAM Composite Flocculant

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A molecular-based design of RGO/TiO2-PAM composite flocculant with photocatalytic self-degrading characteristics and the application of the oil sand tailings flocculant Haiwang Wang, Yukai Zhang, Guanqi Wang, Yuan Ma, Hongqin Pu, Wujiabei Xu, Dekuan Gao, Qiankang Zhang, Haichao Xu, Bingzhu Wang, Xiwei Qi, and Jun Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06041 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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ACS Sustainable Chemistry & Engineering

A molecular-based design of RGO/TiO2-PAM composite flocculant with photocatalytic self-degrading characteristics and the application of the oil sand tailings flocculant Haiwang Wang1,2,3** Yukai Zhang2 Guanqi Wang2 Yuan Ma2 Hongqin Pu2 Wujiabei Xu2 Dekuan Gao2 Qiankang Zhang2

Haichao Xu2 Bingzhu Wang4* Xiwei Qi1,2,3,* Jun Yang5*

1 School of Materials Science and Engineering, Northeastern University, No 3-11 Wenhua Road, Shenyang, Liaoning 110819, PR China 2 School of Resources and Materials, Northeastern University at Qinhuangdao, 143 Taishan Road Qinhuangdao, Hebei 066004, PR China 3 Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province, 143 Taishan Road Qinhuangdao, Hebei 066004, PR China 4 School of Materials Science and Engineering, Harbin Institute of Technology, 92 West Straight Street, Heilong Jiang, Harbin 150080, PR China. 5 Institute of Process Engineering, Chinese Academy of Sciences, No. 1 North Second Street, Zhongguancun, Beijing 100190, China **corresponding author address: Haiwang Wang:[email protected] * corresponding author address: Bingzhu Wang: [email protected] * corresponding author address: Xiwei Qi: [email protected] * corresponding author address: Jun Yang: [email protected] ACS Paragon Plus Environment

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KEYWORDS: Oil sand tailings

Flocculant

Comb structure

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Organic-inorganic composite

Photocatalytic Self-degrading ABSTRACT: Polymer flocculation technology has a very broad application in the flocculation industry of oil sand tailings at present. Nevertheless, the most commonly used commercial polyacrylamide flocculant has problems of low flocculation efficiency and secondary pollution. In this paper, we proposed an organic-inorganic composite flocculant with self-degrading properties for the flocculation treatment of oil sand tailings, which was prepared by photocatalytic surface initiation technique. Further, the functional groups of the materials before and after polymerization composites were characterized by infrared spectrum to explore the polymerization mechanism, the structure was observed by transmission electron microscope, and the molecular weight of polyacrylamide was measured by gel permeation chromatography. Then, the flocculation performance was characterized by the flocculation experiment (tested with simulated oil sand tailings). Subsequently, the flocculation mechanism was explored by testing the zeta potential of the organic-inorganic composites and analyzing images of sediment observed by transmission electron microscope and atomic force microscope. Finally, the test of selfdegradation performance was carried out under illumination. Based on the above experiments, the following conclusions were obtained. First, the structural characterization results indicate the polymerization mechanism is that under the condition of light, the surface of the inorganic photocatalyst generates free radicals to initiate the radical polymerization of the monomers, so that the monomers successfully grow on the surface of the inorganic particles into comb structure. And then, the flocculation experiment shows that reduced graphene oxide/titanium dioxidepolyacrylamide(2:40) has the best flocculation effect, of which the supernatant transmittance is

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21.4 higher and the sedimentation ratio is 8.9% higher than those of the commercial polyacrylamide. The reason for its excellent flocculation performance is that the zeta potential of the organic-inorganic composite increases, reducing repulsion of particles and flocculant molecules, simultaneously, the formed comb structure is beneficial to the expansion of the polymer chain and increases the contact area, thereby improving the flocculation effect. Ultimately, the degradation results indicates that the new organic-inorganic composite had good degradation effect, with the degradation rate up to 75.9% within 4 hours. Therefore, this work has made great contributions to solving the oil sand tailings pollution field. Introduction As oil production dwindles, opencast production—the process of extracting oil from the oil sands is widely applied, producing numerous tailings to be disposed of. These tailings not only occupy much land, and destroy the local landscape, but also pollute the groundwater, nearby rivers and air, which severely damage the ecological balance of the mining area.1-2 Therefore, the treatment of oil sand tailings has attracted extensive concern. Currently, several methods are commonly used for treating oil sand tailings, including physical/mechanical method, natural method, chemical/biological improvement method, mixing and co-processing method, and permanent storage method, among which the method using polymer flocculation technology has the best treatment effect in industrial applications.3-10 Recent research results show that polymer technology plays an effective role in dehydrating oil sands tailings.11-13 Commercial polyacrylamide (PAM) is frequently used for treating oil sand tailings by flocculation at present.



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However, commercial polyacrylamide (PAM) were originally developed for other applications, not for flocculation of oil sands tailings, resulting in limitation in the improvement of flocculation efficiency.

14-17

In this context, how to improve the flocculation efficiency of PAM has aroused

widespread concerns. Some reports indicate that the flocculation effect of organic-inorganic composite flocculant, which is usually a carrier typically coated by organic compounds with high molecular weights (e.g. PAM), is generally superior to that of traditional PAM hybrid materials.1821 Organic-inorganic

composite flocculant are synthesized to obtain hybrid flocculants with strong

ability of solid/liquid separation, CaCl2-PAM, Al(OH)3-PAM, etc. which is proven to have great flocculation effect.22-24 Additionally, further study shows that the close integration of polymer and inorganic particles capable of significantly improving the effects of flocculation and water reduction have attracted extensive attention.22 On the other hand, the sewage treatment by flocculation has been ending up with abundant, PAM polymer chains are left in water, causing serious secondary pollution.25 Several methods have been usually used to solve this problem, among which photocatalytic degradation is relatively efficient and the open-air environment in which oil sand tailings are located in industrial production provides sufficient light conditions for photocatalysis.26-28 Plus, TiO2 is the most widely applied in the field of photocatalysis.29-34 Furthermore, extensive researches prove that with the TiO2 modified by graphene, etc. photocatalyst can significantly improve electronic conductivity, avoid integration of electrons and holes, and enhance its photocatalytic activity.35-42 However, RGO/TiO2 photocatalytic materials can’t be dispersed in polyacrylamide solution uniformly, resulting in undesirable degradation effect, thereby limiting further promotion and application.


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In this paper, a new RGO/TiO2-PAM composite flocculant with self-degradable property is proposed. This composite flocculant is synthesized using RGO/TiO2 inorganic composite particles as a carrier and acrylamide monomer is polymerized by free radical on the surface of RGO/TiO2 composite under light condition, promoting the growth of polyacrylamide polymer chain on the surface of carrier. After flocculation, RGO/TiO2 composites can decompose the PAM into small molecules, such as CO2, and H2O. The main innovations of this research are as follows. First, the mechanism for initiating monomer polymerization is novel. To be more specific, the RGO/TiO2PAM composite flocculants are obtained through photocatalytic surface initiation polymerization of acrylamide monomer, which is characterized by the close integration of polymer and inorganic particles. Moreover, compared with the present polymerization technology, the introduction of the initiator was omitted to simplify the polymerization process and make it more environmentally friendly. Secondly, flocculant is of the function of photocatalytic degradation: the polymer is coated on the surface of RGO/TiO2, to make the photocatalyst (RGO/TiO2) is uniformly dispersed in the solution uniformly to obtain higher photocatalytic degradation efficiency. Experiment Material Acrylamide (>98.0%) is made by Sinopharm Chemical Reagent Co.,Ltd. ,Acetone is made by Tianjin jingdongtianzheng Precision Chemical Reagent Factory ,Graphene oxide (>99%) is made by Suzhou Carbon Technology Co., Ltd. ,Butyl titanate ( ≥ 98%) was made by Tianjin Fuchen Chemical Reagent Factory ,Glacial acetic acid (≥99.5%) is from TIANJIN GUANGFU TECHNILIGY DEVELOP MENT CO.,LTD. ,sodium hypochlorite (≥5%) is manufacture at TIANJIN NORTH TIANYI CHEMICAL Reagent Plant



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Characterization Raman signal (HORIBA LabRAM HR Evolution) was measured in the range of 200-2000 using a laser source of 514nm. Infrared absorption spectrum was measured through Fourier transform infrared spectrometer spectrum 100 made by Perkinelmer Co.,LTD. In addition, Powder X-ray diffraction (XRD) characterization was carried out on a D8 advance X-ray diffractometer operated at 40kV and 40mA through Cu Kα monochromatized radiation. Besides, XPS analysis was conducted on a multifunctional surface analysis system ESCALAB250 with Mg/Al dual-anode target X-ray source. Moreover, transmission electron microscopy (TEM) with high-resolution transmission electron microscopy (HRTEM) were collected on a transmission electron microscope electron microscope (Tecnal G2 F20, FEI) with accelerating voltage of 200kV. Thermogravimetric and Differential Thermal characterization experiments were conducted using Thermogravimetric analyzer TGA4000 and Simultaneous Thermal Analyzer HCT-2. Flocculation precipitate was observed by Prima atomic force microscope produced by NT-MDT. The zeta potential was measured by the Malvern Zetasizer Nano ZS90 Zeta potential analyzer. The molecular weight of the composite RGO/TiO2-PAM (2:40) was determined by dissolving RGO/TiO2 particles with hydrofluoric acid and then purifying the PAM polymer with ethanol to prepare the solution, which was finally characterized by agilent 1260 infinity liquid chromatograph.

Synthesis of RGO/TiO2 composites RGO/TiO2 composites were prepared by sol-gel method. 9g butyl titanate, 25ml ethanol and 20mg GO were mixed in a 100ml beaker. The mixture was stirred in 35℃ of water bath


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and evenly dispersed by ultrasonic. The obtained mixture was called A. Subsequently, 150ml absolute ethanol, 36ml deionized water and 5.4ml hydrochloric acid were added to a beaker, and the solution was evenly mixed by magnetic stirring. The above mixed solution was called B. Afterwards, 10ml B was dropped into A until gel-formation under the magnetic stirring so that the titanium precursor could load on GO. After the gel had aged for 6h, it was dried and ground uniformly for the next step. Finally, RGO/TiO2(1%) composites were obtained after calcining in nitrogen atmosphere at a tubular furnace for 3h at 400℃. Synthesis of RGO/TiO2-PAM organic-inorganic composites material First, the above RGO/TiO2 (1%) samples were ground for the next photocatalytic reaction. Subsequently, configuring 40ml solution by mixing 90% a certain amount of acetone solution with Xg (X = 0.1, 0.2, 0.3) RGO/TiO2 and 4g acrylamide uniformly in a quartz tube. Next, photocatalytic reaction was conducted: 300W ultraviolet light was used for irradiating it for 2h under the nitrogen atmosphere to make the RGO/TiO2 generate free radical, which prompted the acrylamide monomer to be polymerized on the surface of RGO/TiO2.Further, the generated polymer is insoluble in acetone and precipitates. Therefore, the precipitate sample was obtained through centrifugation and several times of ethanol washing. At last, by drying the above precipitates, RGO/TiO2-PAM (1:40,2:40,3:40) organic-inorganic composites material was obtained. Detection of flocculation performance The preparation procedure of the simulated oil sand tailings was described as follows: according to mass ratio of 14%:4%:37%:22%:18%:6%, asphalt, water, bentonite, quartz,



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kaolin and montmorillonite were evenly mixed at high temperature to prepare the simulated oil sand ore. Afterwards,0.3% concentration of sodium hydroxide at water: sand mass ratio =1:1 was added to the obtained mixture as eluent, and oil sand bitumen was separated out from the sand surface completely by stirring at 90 ℃ for 90 minutes. Subsequently, all the oil was poured into a large beaker to simulate flotation cell. After adding a certain amount of water, the mixture was stirred while nitrogen was passed. Finally, by removing the crude oil sand at the bottom, the stable suspended oil sand tailings (MTF with Solid content 2.4%) was obtained.43 Flocculation effect determination: firstly, removing 25ml of oil sand tailing to the measuring cylinder. Then, Yppm(Y = 45,75,150,225) of four components [RGO/TiO2-PAM (1:40/2:40/3:40) and commercial PAM] were added into oil sand tailings. After stirring at a certain power for 90s and resting for 12min, uniform supernatant was taken and the measured the transmittance to obtain evaluation of flocculation efficiency of different components and different dosage. And then, the sediment was taken out and weighed. Afterwards, the sedimentation ratio was obtained by calculating the mass ratio of sediment mass to solid mass of the oil sand tailings. Furthermore, 150 ppm of RGO/TiO2-PAM (2:40) and commercial PAM are added to the oil sand tailings, and the supernatant was took at different times to test the transmission and the transmission - Time curve was obtained. Ultimately, the flocculation mechanism was discussed through analyzing the characterization of AFM and TEM tests of the flocculation sediment. Detection of the self-degradation effect of PAM


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First, 96mg/L flocculant solution was prepared. Then 2/4/6/8/10ml of the above solution was taken out and diluted to 10ml with deionized water to prepare standard solution. Next, according to PAM solution: 5mol/L glacial acetic acid: 1.31% sodium hypochlorite = 1:1:1 volume ratio, the PAM concentration of standard solution was measured through turbidity method.44-45 The absorbance of each concentration of the PAM solution was measured with an ultraviolet spectrophotometer and then a standard curve was drawn to verify the linear correlation between PAM concentration and the absorbance. In addition, 50ml of 800mg/L flocculant (RGO/TiO2-PAM) solution was prepared in the quartz tube for self-degradation detection. Thereafter, the sample was irradiated with ultraviolet light of 500W at 30℃ to self-degrade. And every 30mins, 3ml of the sample was taken and diluted to the 25ml as the sample solution to measure the absorbance value during 0-240mins according to the turbidity method. By calibrating the absorbance, the relative concentration of PAM(Ct) and degradation rate at different degradation times was obtained: Ct=At*C0/A0

(1)

degradation rate=(C0-Ct)/C0 *100%=(A0-At/A0)*100%

(2)

And then the dynamics mechanism of degradation reaction was inferred by analyzing the change of degradation rate with time.



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Results and discussion Sample synthesis and characterization

Fig.1 Schematic representation of the preparation process.

Fig.1 is Schematic representation of the preparation process. Firstly, the RGO/TiO2 inorganic composite materials were prepared by sol gel method, and then photocatalytic surface polymerization was initiated on the surface of RGO/TiO2 to make polyacrylamide was closely combined with inorganic composites and get comb structure RGO/TiO2-PAM composite flocculant, in which the polymer chains elongate each other, thereby increasing the contact area between the polymer chain and the particles. Therefore, it is conducive to adsorption, bridging, entanglement and improvement of flocculation efficiency. What’s more, the polyacrylamide on the surface of the inorganic composite could be selfdegraded under light after the flocculation.


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Fig.2 Raman spectrum of (a) uncalcined RGO/TiO2 (1%), (b) calcined RGO/TiO2 (1%).

Fig.2 shows the Raman spectrum of samples before and after calcination (200cm-12000cm-1). There were 3 peaks corresponding to B1g, A1g+B1g and Eg of TiO2 at about 393cm-1, 513cm-1 and 640cm-1. Besides, the peaks at around 1363cm-1 and 1595cm-1 respectively correspond to the D and G band of GO. D band was assigned to the edge or the sp3 hybridization or surface defects and disorder carbon, while G band was produced by a first-order E2g vibration caused by the scattering of sp2 bonded carbon atoms.46-47 Therefore, the degree of reduction of GO could be inferred based on the intensity ratio of D band and G band.48-49 According to calculation, ID/IG sharply reduced to 0.697 from 0.948 after being calcined, testifying the sufficient reduction of GO during the calcining process.



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A

B A

C


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Fig.3.A FT-IR spectra of (a) calcined GO/TiO2 (1%), (b) calcined RGO/TiO2 (1%); Fig.3.B FT-IR spectra of (c) RGO/TiO2 and AM (2:40)mixture do not occur photocatalytic initiation polymerization, (d) RGO/TiO2-PAM(3:40), (e) RGO/TiO2-PAM(2:40), (f) RGO/TiO2-PAM(1:40); Fig.3.C the enlargement of B at the 2830cm-1-2940cm-1 wave band.

Fig. 3 are the infrared absorption spectrums of each sample during the synthesis process. As shown in Fig.3.A, the intensity of characteristic absorption peaks of hydroxyl and ketones at 3320 cm-1, 1671 cm-1 and 1076 cm-1 in (a) was significantly decreased in comparison with (b). Moreover, apart from C=C double bond, there were basically no other functional groups in (b). Thus, in the process of high temperature calcination, the oxygen containing functional groups on the surface of GO were pyrogenic decomposed, which is similar to the result obtained by Raman spectrum. In Fig3.B(c-f) the absorption peaks at around 1660cm-1, 1610cm-1, 1184cm-1 and 1122cm1

were produced by amide vibration. And the strength of C-H out-of-plane bending

vibration peak of olefin at 986cm-1 and 960cm-1 was considerably reduced while the strength of three new methylene vibration peaks at around 2922cm-1, 2852cm-1 and 1454cm-1 was increased, meaning the decrease of the content of C=C double bonds and the content increase of C-C single bond in the photocatalytic reaction. These changes proved that the photocatalyst produced surface free radicals to combine with acrylamide under the condition of light, so that double bond of acrylamide monomer was broken to form free radicals, and the single bond content was greatly increased after polymerization. Fig.3.C is the enlargement of B at the 2830cm-1-2940cm-1 wave band. In Fig.3.C, the methylene peak at around 2928 cm-1 and 2852 cm-1 gradually red shifts and the absorption peak became sharper, which indicated that the degree of polymerization of the formed polymer



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increased from (d) to (f) sequentially; The reason of the above phenomenon is that as the addition of the photocatalyst increases, surface free radicals generated are increased. Thus, a certain concentration of acrylamide monomer grows along a plurality of polymer chains, resulting in shortening of the polymer chain and a decrease in polymerization degree.

Fig.4 X-ray diffraction (XRD) patterns of (a) RGO/TiO2 (1%); (b) RGO/TiO2-PAM(3:40); (c) RGO/TiO2-PAM(2:40); (d) RGO/TiO2-PAM(1:40).

Fig.4 shows X-ray diffraction (XRD) patterns of RGO/TiO2 and RGO/TiO2-PAM. In Fig.4.(a), several spikes appeared in the original TiO2, which were concentrated in 25.3, 37.9, 48.1, 54.0, 55.1, 62.7, 69.0, 70.3, 75.2 and 82.7, respectively representing (101),(004), (200), (105), (221), (204), (116), (220), (215) and (224) crystal faces. These results proved the formation of anatase TiO2 in RGO/TiO2. The peak of RGO at 25.6 degrees might be covered by the peak of TiO2 (101) crystal face at 25.3 degrees. In addition, the grain size was approximately 12.1nm based on the calculation according to the diffraction Debye-Scherrer formula (D = Kλ /(B cos θ)).


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Fig.4 (b-d) respectively represent the X-ray diffraction patterns of RGO/TiO2-PAM (1:40/2:40/3:40), main peaks of which were consistent with those in (a) reflected by anatase TiO2. Moreover, RGO/TiO2-PAM materials exhibited a board peak in the range of 6-32 degrees. These phenomena proved that the content of PAM in the prepared RGO/TiO2PAM flocculant increased gradually with the addition decrease of RGO/TiO2 when the addition of acrylamide monomer is constant.

A

B

C

D C

D

Fig.5 X-ray photoelectron spectroscopy of A RGO/TiO2-PAM(2:40), B XPS spectra of C1s, C XPS spectra of N 1s, D XPS spectra of O1s.



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Fig.5 is X-ray photoelectron spectroscopy of RGO/TiO2-PAM (2:40).As seen from Fig.5.A, there were three obvious peaks at 284.6, 399.4 and 531.2, which corresponded to the constituent elements of polyacrylamide C, N and O. As Fig.5.B shows, there were peak splits of C1, C2 and C3, and the area ratio of which was 1:0.995:0.988 corresponding to the three valence of carbon polyacrylamide as shown in Fig.5.B. Simultaneously, the amide group and titanium dioxide can be observed from the Fig.5.(C-D). Thus, under the condition of light, acrylamide monomer polymerized to form the polyacrylamide. A

B

C

D

E

F

Fig.6.A the transmission electron microscopy images of RGO/TiO2 (1%); Fig.6.B High-resolution transmission electron microscopy of RGO/TiO2(1%); Fig.6.C the transmission electron microscopy images of RGO/TiO2-PAM(2:40); Fig.6.D


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High-resolution transmission electron microscopy of RGO/TiO2-PAM(2:40); Fig.6.E EDX energy spectrum; Fig.6.F Scanning energy spectrum of C, N, O, Ti elements.



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Fig.6 describes the transmission electron microscopy images of each sample during the synthesis process. Fig.6.A shows the transmission electron microscopy image of RGO/TiO2. In the figure, TiO2 was evenly distributed on the RGO layer, which increased the specific surface area of the reaction, and enhanced the electron conductivity, which was conducive to the improvement of photocatalytic efficiency.38 The interplanar spacing in Fig6.B was about 0.350nm after repeated measurement which corresponded to the (101) crystal face of anatase TiO2. The grain size was close to the grain size calculated by XRD approximate between 12.80nm and 9.60nm. And Fig.6.C show that the high molecular polymer was uniformly grown on the surface of the inorganic composite material, and as can be seen from their junction, the polymer and the inorganic material were closely combined, forming comb structure, which improved its flocculation efficiency. In Fig.6.D, several crystal faces were evenly distributed in the polyacrylamide, which also belongs to the (101) crystal face of anatase TiO2. And it proved that the photocatalyst was uniformly dispersed in the polyacrylamide, which increased the contact area of the photocatalytic reaction and was beneficial to photocatalytic self-degradation performance. As seen from the Fig.6.E, RGO/TiO2-PAM mainly consisted of C, O, N and Ti, with the result being the same as that obtained by XPS. Subsequently, according to the four surface sweep spectra in Fig.6.F, the main three constituent elements (C,N and O) of the polyacrylamides were uniformly distributed in the structure, while Ti the constituent


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element of TiO2 was mainly distributed in the center. Moreover, it could be inferred that RGO/TiO2 was mainly located in the middle of the composite in RGO/TiO2-PAM, and the outer layer was uniformly coated with a layer of polyacrylamide, forming comb structure.

Fig.7 TG and DTA of RGO/TiO2-PAM(2:40).

Fig.7 shows Thermal Gravity Analysis and Differential Thermal Analysis of RGO/TiO2PAM (2:40). In Fig.7, at 30-97 ℃, the TG curve appeared to be weightless, and there was an endothermic peak on the DTA curve, which was caused by the vaporization of small molecules such as acetone, water adsorbed on the surface of polymer endotherms. Then, the sample presented a bit of weight loss in the stage of 253-300℃, simultaneously, the endothermic peak 2 showed that the process was an endothermal reaction. Because the polyacrylamide C-N bond energy was weaker, the branch cracks with decalescence and released ammonia gas. As the temperature further increased, the polyacrylamide backbone began break (350-410℃), accelerating the rate of weight loss of the sample.


19


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According to the endothermic peak 3, the carbon skeleton fracture endotherm could be inferred. Subsequently, the remaining skeleton underwent an oxidation reaction (480-650 ℃) with a large amount of weight loss and released a lot of heat, as seen in the obvious exothermic peak in the DTA curve. Ultimately, the mass did not change at 650-950℃, and the surface coated PAM was decomposed and removed, remaining the inorganic particles.

Fig.8 GPC spectrum of TiO2-PAM(2:40).

Fig.8 is the GPC spectrum of PAM in RGO/TiO2-PAM(2:40). As can be seen from the Figure that the molecular weight of the polymer was widely distributed. And the molecular weight was mainly concentrated at about 4.3*105, distribution width was 2.02 on the basis of Fig.8. Subsequently, combined with the data analysis of the Thermal Gravity, it is obtained that the high molecular weight polymer accounts for about 96% of the composite material. Therefore, it can be inferred that the graft density of the polymer on the inorganic material is very high.


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Fig.9 Zeta potential of (a) particles in oil sand tailings, (b) Commercial PAM, (c) RGO/TiO2-PAM(2:40).

Fig. 9 shows the zeta potential of the particles in oil sand tailings, commercial PAM and RGO/TiO2-PAM(2:40). It could be seen from the figure that the suspended particles in the oil sand tailings had a large negative zeta potential (-32.4 mV) and good stability in solution. And in the system, most of the particles had a negative zeta potential larger than -30 mV, which have strong mutual repulsion and can be stably dispersed in the solution.5051

Simultaneously, the zeta potential of commercial polyacrylamide is -10.4 mV as the

particles is negatively charged. Hence, there existed a strong mutual ion mutual repulsion, which made it difficult for commercial polyacrylamide to adsorb particles. However, the RGO/TiO2-PAM material zeta potential was significantly lower than the commercial PAM potential, only -0.889mV due to the introduction of the inorganic particles RGO/TiO2. Thereby, the polymer chain is more likely to be in contact with the particles, and promotes adsorption and bridging function, which is beneficial to the improvement of flocculation effect.


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Flocculation efficiency and mechanism analysis

A

B

C

D

Fig.10.A Different composition of flocculant dosages-transmission ratio map of oil sands tailings (a) RGO/TiO2PAM(1:40), (b) RGO/TiO2-PAM(2:40), (c) RGO/TiO2-PAM(3:40), (d) Commercial PAM; Fig.10.B Sedimentation Ratio of (a) RGO/TiO2-PAM(2:40), (b) Commercial PAM with different flocculation dosage; Fig.10.C The curve of the transmittance of the supernatant over time when the oil sand tailings are treated with 150ppm flocculant dosage of (a) RGO/TiO2-PAM(2:40), (b) Commercial PAM; Fig.10.D Comparison diagram of oil sand tailings before and after flocculation as well as optical microscope images of the settled flocs.


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Fig.10.A shows the relation curve between the addition of flocculant in oil sands tailings and the transmittance of supernatant. As seen from the figure, the transmission increased at the beginning with the addition of flocculant, and then weakened. When the flocculation dosage of the RGO/TiO2-PAM(1:40/2:40/3:40) was 75ppm,150ppm and 45ppm , the transmission ratio of the processed oil sands tailings reached the maximum of 83.6, 88.7 and 74.0,which were higher than the highest value of commercial PAM(67.3).From Fig.10.B, it can be seen that the sedimentation ratio is positively correlated with the supernatant transmittance, and both values of the RGO/TiO2-PAM (2:40) are significantly improved, which proves RGO/ TiO2-PAM (2:40) has better solidliquid separation capacity compared to commercial PAM. Fig.10.C is a plot of transmission versus time for commercial PAM and RGO/TiO2-PAM (2:40) flocculation. As time goes by, the rate of increase in transmittance gradually slows down and eventually approaches the same. And by comparing the final transmission, it is obtained that RGO/ TiO2-PAM (2:40) has better flocculation effect.


23


A

B

C

D

Page 24 of 37

Fig.11 TEM images of flocculation sediment with different resolutions.

Fig.11 depicts TEM images of flocculation sediment. As shown in Fig.11.(A-B), the large particles(as shown in Fig.11.A) were formed by the agglomeration of small particles due to the entanglement and bridging of polyacrylamide chains. Simultaneously, based on the high-resolution transmission electron microscopy of Fig.11.(C-D), a large number of small particles(as shown in Fig.11.D) were uniformly adsorbed on the surface of polyacrylamide, which proved that the flocculant was fully utilized.


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A

B

C

D

Fig.12 Atomic force microscope of flocculation precipitation of A two-dimensional atomic force microscope photograph of flocculation precipitation, B height distribution curve along the line in A, C and D atomic force microscope 3D photographs of flocculation precipitation.

Fig.12.A is a two-dimensional AFM photograph of flocculation precipitation and Fig.10.B is a height distribution curve along the line in Fig.12.A. The uneven shape on the curve was formed by the suspension of small particles attached to the polymer chain. And the particle size obtained by calculating the radius of curvature was about 34-47nm. The two 3D images in Fig.12.(C-D) clearly and intuitively revealed that there were numerous small particles attached to the polymer convex surface. Thus, it could be speculated that the small particles suspended in the solution were first absorbed by the polymer chain, and then aggregated into large


25


Page 26 of 37

particles after the macromolecular bridging and gathering action. Finally, the particles precipitated together with the polymer, thereby achieving higher flocculation efficiency.


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Fig.13 Schematic representation of the flocculation mechanism.

Therefore, according to the flocculation test curve, TEM and AFM it can be speculated that the flocculation mechanism of flocculant might be similar to the image shown in Fig.13.When the addition of flocculant did not reach the optimal value, the flocculation process was shown in (a) and flocculant acted as bridging and gathering. As the amount of flocculant increased, the amount of particles absorbed by flocculant increased. But when the addition of flocculant was larger than the optimal value, the flocculation process developed as shown in (b). The polymer chain was coated on the surface of particles to form the spatial three-dimensional resistance of particle agglomeration, thus acting as an anti-flocculant. In addition, with the increase addition of high polymer flocculant, the water viscosity also increased. According to the relationship between average velocities gradient in the same direction flocculation and the agitation power applied in the flocculation reactor: G=

du 𝑑𝑧

=

𝑃1

( )

1 2

𝑉×𝜇

(3)


27


Page 28 of 37

In the formula, P1 is stirring power; V refers to water volume, and μ denotes water viscosity coefficient. Under the same conditions of mixing time, power and size of oil sands tailings, the addition of polymer flocculation could reduce the collision frequency of particles in water and make the particles not easy to settle. Self-degradation efficiency and the dynamics of process analysis

Fig.14 Photocatalytic reaction time-relative concentration curve of RGO/TiO2-PAM(2:40). And the picture in Fig.14 is relative concentration-absorbance curve.

Fig.14 shows the relationship between relative concentration of flocculant and photocatalytic reaction time. According to the relative concentration-time curve, from 0 to 60 minutes, the concentration did not decrease with time, the surface free radical(-OH,O2-, etc.) produced by photocatalysis reacts with other groups in the solution; from 60 to 150 minutes, the concentration of reactants was basically linear with time; and from 180 to 240 minutes, the rate of degradation in


28

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concentration reduced as the concentration decreases. And the above relationship can be explained by langmuir-hinshwood kinetic equation: (4)

r = κθ K AC A

(5)

θ = 1 + 𝐾𝐴𝐶𝐴

In the formula, r denotes the apparent reaction rate,  represents a constant, θ means surface coverage rate, and KA refers to the adsorption coefficient of polyacrylamide on the surface of inorganic materials, and CA signifies the concentration of polyacrylamide. When the degradation of polyacrylamide began (60min), owing to the tightly combination and high coverage rate of comb structure polyacrylamide was equivalent to saturate adsorption in the surface of the photocatalyst: CA = C0 ― k0t

(6)

After calculation, it was obtained that the level 0 reaction rate constant was obtained as below: k0==0.5197mg/(L*min). When polyacrylamide was partially degraded (at 180min), the concentration of polyacrylamide was relatively low. The reaction rate was controlled by diffusion, the rate of which was directly proportional to the concentration, so the reaction turned to the level 1 reaction, by integrating 𝐫 = ln

𝐝𝐂𝐀 𝐝𝐭

we obtained:

( ) = kt C0 Ct

(7)


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Page 30 of 37

In the formula, Ct denotes the polyacrylamide concentration at t moment, k=KA is the reaction rate constant. Based on the image method, it was obtained that the level 1 reaction rate constant k=3.03*10-3min-1 through calculation. Conclusion In this paper, a novel organic-inorganic composite flocculant was prepared by photocatalytic surface initiation polymerization. Through the characterization of infrared spectra and TEM, the polymerization mechanism is inferred that under the light condition, the surface of the inorganic photocatalyst generates free radicals to induce the monomer grow on its surface, forming polymer chains to become comb structure. And the flocculation experiment shows that RGO/TiO2-PAM(2:40) has the best flocculation effect, of which the supernatant transmittance is 21.4 higher and the sedimentation ratio is 8.9% higher than those of the commercial PAM. The study found there are two reasons for its good flocculation effect. On the one side, the zeta potential of the organic-inorganic composite increases, reducing repulsion of particles and flocculant molecules. On the other side, the formed comb structure is beneficial to the expansion of the polymer chain and further increases the contact area, thereby improving the flocculation effect. Ultimately, the degradation experiments show that the degradation process of RGO/TiO2PAM (1:20) firstly conforms to the 0 level reaction, then to the 1 level reaction, and the degradation rate can reach 75.9% within 4 hours. In summary, this work is of great significance in the field of oilfield flocculation treatment of oil sand tailings pollution.


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ACKNOWLEDGMENT The main work was done with the joint efforts of the photochemical organization. This work was supported by Natural Science Foundation of China No. 21604007; No. 51474061, No 51374083; the Special Fund for Basic Scientific Research of Central Colleges, Northeastern University under Grant Nos. N162304010. Science and technology research projects of Colleges and universities in Hebei province (ZD2016207). The core authors of the article belong to the photochemical organization of Northeastern University at Qinhuangdao. REFERENCES (1) Dibike, Y. B.; Shakibaeinia, A.; Droppo, I. G.; Caron, E. Modelling the potential effects of Oil-Sands tailings pond breach on the water and sediment quality of the Lower Athabasca River. Sci. Total Environ. 642 (2018) 1263–1281, DOI 10.1016/j.scitotenv.2018.06.163. (2) Rooney, R. C.; Bayley, S. E.; Schindler, D. W. Oil sands mining and reclamation cause massive loss of peatland and stored carbon. Proc. Natl. Acad.Sci.U.S.A. March 27, 2012 109 (13) 4933-4937, DOI 10.1073/pnas.1117693108. (3) Ghorai, S.; Sarkar, A.; Raoufi, M. Enhanced Removal of Methylene Blue and Methyl Violet Dyes from Aqueous Solution Using a Nanocomposite of Hydrolyzed Polyacrylamide Grafted Xanthan Gum and Incorporated Nanosilica. ACS Appl. Mater. Interfaces. 2014, 6, 4766−4777, DOI 10.1021/am4055657. (4) Lapointe, M.; Barbeau, B. Dual starch-polyacrylamide polymer system for improved flocculation. Water Research. 124 (2017) 202-209, DOI 10.1016/j.watres.2017.07.044.


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（ 51 ） Hanaor, D.; Michelazzi, M.; Leonelli, C. The effects of carboxylic acids on the aqueous dispersion and electrophoretic

deposition

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Graphical Abstract

Flocculant with self-degrading properties was designed to treat oil sand tailings, and ecological sustainable development was realized by green chemistry.


37

TiO2-PAM Composite Flocculant

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