Adsorption of Low-Concentration Uranyl Ion by Amidoxime

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Materials and Interfaces

Adsorption of Low-Concentration Uranyl Ion by Amidoxime Polyacrylonitrile Fibers Fuqiu Ma, Boran Dong, Yunyang Gui, meng cao, Lei Han, Caishan Jiao, Huitao Lv, Junjun Hou, and Yun Xue Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03509 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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Industrial & Engineering Chemistry Research

Adsorption of Low-Concentration Uranyl Ion by Amidoxime Polyacrylonitrile Fibers Fuqiu Ma, Boran Dong, Yunyang Gui, Meng Cao, Lei Han, Caishan Jiao, Huitao Lv, Junjun Hou, Yun Xue* Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, College of nuclear science and technology, Harbin Engineering University, Harbin 150001, P. R. China

ABSTRACT Amidoxime polyacrylonitrile fibers (PAO fibers) were prepared with polyacrylonitrile fibers (PAN fibers) and hydroxylamine hydrochloride through a heterogeneous reaction, which were used for low-concentration uranyl ion adsorption (under 1 mg/L). The as-prepared PAO fibers were characterized by using Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), Thermal Gravimetric Analyzer (TGA), and X-ray Photoelectron Spectroscopy (XPS) measurements. Batch adsorption experiments were conducted to explore the best adsorbent concentration, pH, contact time, and temperature. When the adsorbent concentration reaches 0.1 g/L to dispose uranium solution of 100 μg/L, the equilibrium concentration can be decreased under 10 μg/L. The optimum pH for this adsorption experiment is 5.0, and the adsorption can reach equilibrium in 8 h under 298 K. The adsorption process fit well with Intraparticle diffusion model and pseudo-second-order model. In addition, Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D-R) models were used to explain the adsorption thermodynamic process. The adsorption process is spontaneous and endothermic. Key Words: Amidoxime; Polyacrylonitrile; Adsorption; Uranyl ion;

1. Introduction As fossil energy is increasingly depleted and the world’s population is growing year by year, countries are actively developing clean energy sources. As a typical clean and efficient energy, nuclear energy has received special attention. Uranium is the main fuel used in nuclear industry, which is widely distributed in nature. With the gradual development of the nuclear power industry, the IAEA (International Atomic Energy Agency) expects that the world reactor-related uranium requirement will reach 104.74 thousand tU/yr by 20351, 2, however, the total amount of uranium mined only about 2.75 million tons before 20151. Because of the lack of uranium terrestrially mined, people turn attention to unconventional uranium resources, such as salt lake brine3, 4, seawater5, 6, etc. Furthermore, there will be large amount of uranium-containing waste water in the entire nuclear fuel cycle system. Uraniumcontaining waste water discharged without treatment will also cause great harm to the biological and ecological environment7. According to the regulations of the World Health Organization, the content of uranium in drinking-water must not exceed 14.4 μg/L, and the uranium concentration in wastewater generated during uranium mining can reach 5 mg/L 8, 9. Either extraction of uranium Corresponding author's email address: [email protected]

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from natural resources or recycle of uranium in radioactive liquid waste is the way to save and reuse the uranium resources. Furthermore, it’s related to the sustainable development of nuclear energy 10. As early as the 20th century, the selective adsorption of uranium by hundreds of functional groups of materials was examined. The amidoxime group, with an excellent selectivity for uranium adsorption, can be easily synthesized through the reaction of the nitrile group and hydroxylamine, which makes it stand out among them 11, 12. The classical method for preparing amidoxime groups by reacting nitrile groups with hydroxylamine hydrochloride was firstly proposed by Tiemann in 1884 13. The reaction process is shown in Figure. 1. In 1964, amidoxime has started to be used for the adsorption study of uranium in seawater 14.

Figure. 1 Diagrammatic drawing of modification reaction (gray ball represents carbon atom, yellow ball represents nitrogen atom, red ball represents oxygen atom, blue ball represents hydrogen atom)15.

In the past few decades, many kinds of materials have been reported to be used as efficient adsorbents to adsorb uranium in aqueous solution, such as molecular sieve 16, 17, mesoporous silica 18, graphene oxide 19, activated carbon 20, resins 21, 22, zeolite 23, novel composite material 24, etc. Some frequently used materials are costly, such as graphene oxides 25 and some composite materials 26. Other less costly materials usually have low adsorbability for low-concentration uranium solution, such as activated carbon 20 and zeolite 23. On the other hand, fibrous material is a kind of outstanding material for adsorbing uranium27, which exhibits satisfying adsorption capacity. Moreover, the mechanical strength of the fibers is tolerable enough for the scour in water environment for a long time. The adsorbent also needs to have strong complexing ability to uranium and reach a certain adsorption capacity for uranium. Above all, we chose the fibrous polymers which contain many nitrile groups as the raw material to prepare adsorbent for uranium. As the purpose is to dispose the waste water of China National Nuclear Corporation Beifang Nuclear Fuel Co., Ltd, we must consider about the feasibility, and save cost. Several methods were chosen to prepared amidoxime polyacrylonitrile fibers (PAO fibers) before, such as suspension polymerization, radiation-induced grafting polymerization, and sonochemical functionalization28, 29. Through comparison, the method we used are straightforward and cheap, and it is realizable in industry. In this work, we focused on the low-concentration uranium adsorption based on the PAO fibers. When the concentration of uranium in industrial wastewater can be decreased under 10 μg/L or lower, it is an essential improvement for uranium-containing waste water. We prepared PAO fibers with PAN fibers and hydroxylamine hydrochloride through a heterogeneous reaction. The prepared PAO fibers showed an excellent adsorption ability for the uranium in waste water. 2. Materials and methods 2


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2.1 Reagents PAN fibers were afforded by Changzhou Tianyi engineering fiber Co.,Ltd, China, which is a kind of industrial fibers. Uranyl nitrate hexahydrate [UO2(NO3)2·6H2O] was bought from Hubei Chushengwei Chemical Co.,Ltd. All other chemical reagents [NH2OH·HCl, C2H5OH, Na2CO3, NH3·H2O, HNO3] were obtained from Sinopharm Chemical Reagent Co.,Ltd. All chemicals except PAN fibers were reagent-grade or higher. All the chemicals were used as received and all the experimental water was deionized water, which was prepared by DBRO-SYS (1840A). 2.2 Synthesis of materials We referred to references 29 and 30 to designed our synthetic method, and the synthetic method was reconsidered. Extensive experiments were finished by us firstly to explore the applicability between methanol and ethanol, sodium hydroxide and sodium carbonate, and contact time. Because of the solubility of sodium carbonate and the hydrophilicity of PAN fibers, dosages of reagents were determined. PAO fibers were prepared in hydroxylamine hydrochloride mixed solution of ethanol and water. In a typical procedure, 13.9 g of hydroxylamine hydrochloride and 10.6 g of sodium carbonate were dissolved in 200 mL of deionized water, and then 200 mL ethyl alcohol was slowly added in the mixed aqueous solution under stirring. 10.6 g of PAN fibers were put into the mixed solution for reacting30, 31. The reaction was conducted at 323 K. After 14 days, PAO fibers was filtered, and then washed with deionized water and ethyl alcohol. Finally, PAO fibers were dried at 323 K for 2 h. 2.3 Batch adsorption experiments Batch adsorption experiments were performed by a thermostatically controlled air bath shaker (TS-111C), and the speed of shaking was 200 rpm. Multiple factors were studied in order as follows: adsorbent concentration, pH, contact time, and temperature. Adsorbents of different dosages were added in 500 mL aqueous solution of 100 μg/L uranium for different durations. After adsorption, adsorbent was separated from solution through filter membrane. The aperture of filter membrane is 0.45 micrometer. Solution concentrations were then measured with ICP-MS (ThermoFisher, X-II), and the adsorption capacity of UO22+ ions adsorbed by PAO fibers was calculated using the following equation: 𝑉 (1) 𝑞 = (𝐶0 ― 𝐶𝑒) × 𝑚 where q is adsorption capacity [the quality of uranium element adsorbed onto per unit mass of the adsorbent (mg/g)], C0 and Ce are respectively the initial and the equilibrium uranium element concentration (mg/L), V is solution volume (L), and m is the mass of used adsorbent (g). Percentage adsorption was calculated as follows: 𝐶0 ― 𝐶𝑒 (2) 𝜂= × 100% 𝐶0 where η is percentage adsorption (%), C0 and Ce are the same as above. The explored experimental conditions are shown in Table 1. Table 1 Explored Experimental Conditions Conditions

Adsorbent Concentration

Solution pH

Contact Time 3


Uranium Concentration

Temperature


3. Results and discussion 3.1 Characterization of PAN fiber, PAO fiber, and PAO fiber after adsorption The chemical structure, morphologies, and composition of the prepared materials were studied by means of test, e.g., FT-IR, SEM, EDS, TGA, and XPS measurements. The FT-IR spectra (Perkin Elmer, Spectrum 100) of the samples were recorded in pressed KBr pellets at room temperature. The spectrogram of PAN fibers and PAO fibers are compared in Figure. 2. The typical absorption peaks of PAN fibers were located at 2242 cm-1 (C≡N), 1730 cm-1 (>C=O), and 1451 cm-1 [(CH2)n]19, 32. The peak at 1730 cm-1 corresponds to carbonyl stretch 33, which comes from other blends in industrial polyacrylonitrile products. After the modification reaction, the absorption peak of C≡N at 2242 cm-1 disappeared, while three new peaks appeared at 1637 cm-1, 1386 cm-1 and 923 cm-1, corresponding to the vibration absorption of the C=N bond, the stretch absorption of C-N bond, and the vibration absorption of the N–O bond in the amidoxime group, respectively 10, 32. The appearance of peak 1258 cm-1 (tertiary amide group) was due to the residual DMF in industrial fibers. The peak at 1451 cm-1 [(CH2)n] underwent a shift to 1445 cm-1 [(CH2)n] in PAO fibers spectra, demonstrating the influence of the formation of C=N and N-O bonds.

Figure. 2 FT-IR spectra of PAN fibers and PAO fibers.

The SEM measurements were conducted on a QUANTA 200 scanning electron microscope operated at a beam energy of 12.5 kV or 15.0 kV. SEM micrographs presented the differences among PAN fibers, PAO fibers, and PAO fibers after adsorption. Figure. 3 shows morphologies of materials. The fibers before modification reaction looked uniform and perfectly straight, and there were superficial wrinkle all over the surface. PAN fibers possess better mechanical strength seemingly. In fact, the mechanical strength of PAN fibers is better than PAO fibers 34, 35. The surface 4


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of PAO fibers is soft and irregular. PAO fibers superficial wrinkle became salient, and it was not straight any more. Compared with PAN fibers, the prepared PAO fibers turned coarser and crisper. In addition, more irregular surface can be seen on PAO fibers after adsorption, which may be related to the adsorbed uranyl.

Figure. 3 SEM images: (a)&(b) PAN fibers; (c)&(d) PAO fibers; (e)&(f) PAO fibers after adsorption. 5



EDS (EDAX, QUANTA 200) was also tested to prove that uranyl had been adsorbed on PAO fibers, and the results are shown in Figure. 4. In order to make fibers electric, samples were disposed by metal spraying with gold dust. Thereby, there were peaks of gold in results. Obviously, there are two peaks for uranium in the spectrogram of PAO fibers after adsorption. Through the comparation between Figure. 4 (b) and Figure. 4 (d), the ratio of atom% for U in PAO fibers and PAO fibers after adsorption are 0.00 and 1.13, respectively. It’s assertive evidence that PAO fibers have ability to adsorb uranium. And there are several Elemental Mapping Images (EMI) shown in Figure. 4 (e, f, g, h, i, j). The profile of PAO fibers after adsorption can be recognized in images of C, N, and O. EMI indicates a homogeneous distribution of uranium on PAO fibers after adsorption as the profile of PAO fiber couldn’t be recognized in the image of U. There is other fiber couldn’t be seen clearly in the black area in SEM image, therefore, many points can be seen in Figure. 4 (f, g, h, i, j).

Figure. 4 EDS Spectroscopy: (a&b) PAO fibers; (c&d) PAO fibers after adsorption; EMI of (e) PAO fiber after 6


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adsorption; (f) C; (g) N; (h) O; (i) U; (j) Composition of C, N, O, and U.

TGA (TA, Q50) was used to analyze the heat stability of PAN fibers and PAO fibers, respectively. The thermogravimetric curves of PAN fibers and PAO fibers are shown in Figure. 5. By comparison, the fibers after modification became labile. Maximum decomposition rate of PAN fibers appeared at 297°C and 444°C, respectively, as for PAO fibers, it appeared at 266°C and 405°C, respectively. PAN fibers would disintegrate at 250°C, and PAO fibers would disintegrate at 150°C. There is a peak under 100°C on both dotted lines, which results from the loss of water within samples.

Figure. 5 Thermogravimetric curves of PAN fibers and PAO fibers.

The prepared materials were analyzed with XPS (ESCLAB, 250Xi) measurement, and the results are shown in Figure. 6. The surveys of PAN fiber, PAO fiber and PAO fiber after adsorption are shown in Figure. 6 (a). The peaks of C, N, and O respectively appear at 284.08 eV, 398.08 eV, and 531.08 eV. By comparison, there are two new peaks at 381.08 eV and 392.08 eV after the adsorption of uranyl on the surface of PAO fibers, and both peaks are related to uranium 36, 37. This result implies that uranyl has been adsorbed by PAO fibers. The peaks of carbon atoms for nitrile group (C≡N), carbon chain [(CH2)n], and carbonyl (>C=O) in PAN fibers appear at 284.31 eV, 285.36 eV, and 286.60 eV, respectively 38 [Figure. 6 (b)]. As shown in Figure. 6 (c), there are two peaks at 398.63 eV and 400.60 eV, which are related to the nitrogen atoms in PAN fibers. The peak at 398.63 eV belongs to nitrile group (C≡N), and the other peak is shake-up line 37. There are masses of nitrile groups that are unsaturated groups on the side chain of PAN molecule. That’s why there is a shake-up peak. The peaks of oxygen atoms for carbonyl (>C=O) and hydroxy (C-O-H) in PAN fibers appear at 531.25 eV and 533.26 eV, respectively 39, which is shown in Figure. 6 (d). Both carbonyl (>C=O) and hydroxy (C-O-H) come from other blends in industrial PAN products. For PAO fibers, the peaks of carbon atoms for carbon chain [(CH2)n] and carbon and nitrogen double 7



bond (C=N) appear at 283.60 eV and 285.34 eV, respectively, which is shown in Figure. 6 (b). As shown in Figure. 6 (c), the peak of nitrogen atoms for C=N-O appears at 398.37 eV, and the peak for C-NH2 appears at 398.82 eV. In Figure. 6 (d), the appearance of peak at 530.75 eV is related to oxygen atoms contained in N-O-H, and the peak for carbonyl (>C=O) appears at 531.50 eV.

Figure. 6 XPS spectra: (a) Surveys of PAN fibers, PAO fibers and PAO fibers after adsorption; (b) C 1s of PAN fibers and PAO fibers; (c) N 1s of PAN fibers and PAO fibers; (d) O 1s of PAN fibers and PAO fibers.

3.2 Effect of adsorbent concentration 0.02 g/L, 0.04 g/L, 0.1 g/L, 0.15 g/L, and 0.2 g/L of PAO fibers concentrations were used to adsorb uranyl for 24 h, respectively. As seen from Figure. 7, the adsorbent concentration ≥ 0.1 g/L, the concentration of uranium is below 10 μg/L. Furthermore, with the increasing of adsorbent mass, the equilibrium concentration has no obvious change. Long Chen et al. tested amidoxime appended metal-organic framework in real seawater sample from Bohai sea with the adsorbent concentration of 1g/L, and it’s much higher than 0.1g/L40. Certainly, species, pH, and ionic strength of real seawater sample are different from pure uranyl nitrate solution, for example there must be carbonate in seawater complex with uranyl ion result in other speciation of uranium. 3.3 Effect of solution pH Different pH were researched from 2.0 to 9.0, and the pH was adjusted with nitric acid or ammonium hydroxide. The percentage adsorption under different pH are shown in Figure. 8. When pH is higher than 5.0, the percentage adsorption are all around 90%. The best percentage adsorption is higher than 90%, which appears at pH=5.0. And with the decrement of pH, the percentage adsorption declines. However, the percentage adsorption under pH=2.0 is obviously better than 8


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pH=3.0 and 4.0. Amidogen will be protonated when hydrogen ion concentration is high enough. If amino combines with hydrogen ions, it will have positive charges. Thus, amidoxime groups will reject uranyl ions in solution, so the percentage adsorption is minimum in this case. When the concentration of hydrogen ions becomes higher, amidogen will fall off from fibers, and amidoxime groups are broken. If so, hydroxy and nitrogen atom in amidoxime will combine with uranyl ions together, therefore, the adsorbability for uranyl ions of PAO fibers increases 41, 42. This part of work was repeated three times, and the results of the three times were similar. M. Josick Comarmond et al. adsorbed uranium solution of 1000 mg/L with titanium dioxide, and the influence of pH on adsorption results are same as ours except pH 243. There were no amidoxime group on the titanium dioxide, that’s why there is no aforementioned abnormal situation.

Figure. 7 Adsorbent concentration effect on adsorption of uranyl from aqueous solution by PAO fibers (pH: 5, T: 298K, C0: 99.8μg/L, V: 500mL, t: 24h).

Figure. 8 pH effect on adsorption of uranyl from aqueous solution by PAO fibers (T: 298K, m: 0.05 g, C0: 95.37 9



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μg/L, V: 500 mL, t: 24 h).

3.4 Effect of contact time Figure. 9 (a) shows the changing situation of percentage adsorption with contact time for PAN fibers and PAO fibers. The adsorption will reach equilibrium in 8 h when PAO fibers were used as adsorbent, and the adsorption rate is much higher than some similar materials35. The adsorption rate in first 5 min is much higher than the later. About 43% of uranyl can be adsorbed in first 5 min, and percentage adsorption was increasing step by step until 8 h. After 8 h, the adsorption reaches a stable equilibrium. By comparison, pristine PAN fibers almost have no adsorption ability for uranyl ion. The percentage adsorption start fluctuates after 30 min, and the value is below 20%. This result indicates that PAO fibers are much better than PAN fibers for uranyl ion adsorption. 3.5 Kinetic studies Three kinetic models including pseudo-second-order, Elovich 44, and intraparticle diffusion models were used in order to evaluate the kinetics and mechanism of adsorption 45, 46. The adsorption process can reach equilibrium within 8 h, so we concentrated on the step of the first 8 h to calculate. Pseudo-second-order kinetic model can be expressed by following equations: 𝑡 𝑡 1 = + (3) 𝑞𝑡 𝑞𝑒 𝛼 𝛼 = 𝑘𝑎𝑑𝑞𝑒2

(4) where t is time (min), and qt and qe are the adsorption capacity at time t and at equilibrium (mg/g), respectively. α is the initial adsorption rate (mg/g·min), and kad is the rate constant of pseudosecond-order kinetic model (g/mg·min). The fitting result are depicted in Figure. 9 (b). The expression for the linear form of the Elovich kinetic equation is as follow: 𝑙𝑛 (𝑡) 𝑙𝑛 (𝛼𝛽) (5) 𝑞𝑡 = + 𝛽 𝛽 where β is the adsorption constant (mg/g·min). The Elovich model assumes the adsorbent surface is energetically heterogeneous. The fitting result can be seen in Figure. 9 (c). And intraparticle diffusion model is expressed as follow: 1 2

(6)

𝑞𝑡 = 𝑘𝑖𝑛𝑡𝑟𝑎𝑡 + 𝐶

where C is a rate constant, and kintra is the intraparticle diffusion rate factor (mg/g·min). The fitting result is shown in the Figure. 9 (d). The kinetic parameters of pseudo-second-order, Elovich, and intraparticle diffusion models were calculated from the slopes and intercepts of the linear plots between t and qt, between qt and ln(t), and between qt and t1/2, respectively. All the fitting results are shown in Table 2. Consequently, intraparticle diffusion model is adopted to describe the adsorption process. Meanwhile, pseudosecond-order model is appropriate enough to explain the process, and it can prove that the adsorption for uranium is chemisorption. Uranium uptake by adsorbent can be described in four steps: (i) interparticle diffusion of uranium species through fibers, (ii) external-film (interphase) mass transfer, (iii) intraparticle diffusion of uranium species through adsorbent fibers, and (iv) binding reaction of uranyl ions with amidoxime ligands47. Because the fibers were fluidized in conical flask, interparticle diffusion and external-film (interphase) mass transfer were assumed couldn’t limited the adsorption rate. Therefore, intraparticle diffusion was considered as rate-limiting step. Long Chen et al. analyzed shows that modified polymers have relatively slow adsorption kinetics, which 10


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likely originates from their irregular pore size and low surface area, further hindering effective transportation of uranium throughout the material40. PAO fibers were far below the adsorption saturation state due to the low standard of uranium concentration, so pseudo-second-order model described the adsorption process apropos as well.

Figure. 9 (a) Contact time effect on adsorption of uranyl from aqueous solution by PAN fibers and PAO fibers (pH: 5, T: 298 K, m: 0.05 g, C0 for PAN fibers: 99.82 μg/L, C0 for PAO fibers: 99.14 μg/L, V: 500 mL); Plots for the uranyl adsorption on PAO fibers: (b) t/qt versus Time; (c) qt versus ln(t); (d) qt versus t1/2 (pH: 5, T: 298 K, m: 0.05 g, C0: 99.14 μg/L, V: 500 mL, t: 8 h).

3.6 Effect of uranium concentration and adsorption isotherm The equilibrium data were evaluated according to the Freundlich, Langmuir 20, 48, Temkin and Dubinin–Radushkevich 49 isotherm models, which are all commonly used to describe adsorption equilibrium in aqueous solution, to study the adsorption properties of PAO fibers. Freundlich isotherm model describes adsorption equilibrium better than other three isotherms for adsorption in low-concentration uranyl solution. The Freundlich equation is given by: 1 𝑛

1 (7) 𝑙𝑜𝑔 (𝑞𝑒) = 𝑙𝑜𝑔 (𝐶𝑒) + 𝑙𝑜𝑔 (𝐾𝐹) 𝑛 where qe is adsorption capacity of adsorbent (mg/g), Ce is the equilibrium concentration (mg/L), KF and n are the Freundlich constants characteristic of a particular adsorption isotherm. Langmuir isotherm model is used to describe monolayer adsorption equilibrium on uniform surface at constant temperature. Langmuir isotherm equation is given by: 𝐶𝑒 𝐶𝑒 1 (8) = + 𝑞𝑒 𝑞𝑚𝑎𝑥 𝑞𝑚𝑎𝑥𝐾𝐿

𝑞𝑒 = 𝐾𝐹𝐶𝑒

where qmax and KL are Langmuir constants, corresponding to maximum adsorption capacity at complete monolayer coverage (mg/g) and equilibrium constant (L/mg), respectively. Through 11



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calculation, qmax is equal to 17.34 mg/g, and the replication experiment under optimum condition was did for comparing. The adsorption capacity of PAO fiber is 15.92 mg/g. It is higher than some similar materials28, 29, 50, 51. Temkin Isotherm equation is given by the following equation: 𝑞𝑒 = 𝐵𝑙𝑛(𝐶𝑒) + 𝐵𝑙𝑛(𝐴𝑇) (9) where B is constant related to heat of sorption (J/mol), and AT is Temkin isotherm equilibrium binding constant (L/g). Temkin shows the relationship between qe and Ce, meanwhile, the constant related to heat of sorption can be calculated. B demonstrates the complexity of adsorption process. The Dubinin–Radushkevich (D-R) isotherm model can be used to describe sorption on both homogeneous and heterogeneous surfaces 16. The equation is expressed by the following equation: 𝑙𝑛 (𝑞𝑒) = ―𝐾𝜀2 + 𝑙𝑛 (𝑞𝑚) (10) 1 𝜀 = 𝑅𝑇𝑙𝑛(1 + ) (11) 𝐶𝑒 where qm is the theoretical adsorption capacity (mg/g), K is the constant related to adsorption energy, ε is Dubinin–Radushkevich isotherm constant, R is the universal gas constant that is equal to 8.314 J/mol·K, and T is Kelvin temperature. All the fitting results are depicted in the Figure. 10. The resulting parameters from the four isotherm models are presented in Table 3. Table 2 Results of Kinetic Fitting Kinetic Models Pseudo-second-order kad (g/mg·min) qe (mg/g) α (mg/g·min) R2

Elovich

3.29×10-2

β (mg/g·min)

9.60

0.90 2.69×10-2 0.9802

R2

0.8600

Intraparticle diffusion kintra (mg/g·min) C R2

2.50×10-2 0.34 0.9876

Figure. 10 Plots for the uranium adsorption on PAO fibers: qe versus Ce; (pH: 5, T: 298K, m: 0.05g, V: 500mL, t: 12


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24h).

Table 3 Results of Isotherm Fitting Isotherm Models Freundlich n KF (L/g) R2

1.82 17.23 0.9886

Langmuir qmax (mg/g) KL (L/mg) R2

17.34 3.79 0.9406

Temkin B (J/mol) AT (L/g) R2

D-R

1.92 267.51 0.8278

1.23×10-8 9.06 0.9400

K qm (mg/g) R2

3.7 Effect of temperature and thermodynamic studies Firstly, the equilibrium concentration trend with the change of temperature was studied. The result is shown in Figure. 11. Obviously, the percentage adsorption will rise with the temperature going up, and the adsorption is endothermic process. Austin P. Ladshaw et al. got the same conclusion that amidoxime-based adsorbents tend to produce better adorability as temperature increases52. Li-Jung Kuo et al. reported that uranyl complexation by amidoxime is endothermic, and amidoxime-based polymeric adsorbents display a strong positive temperature response to U adsorption capacity53. According to the general rules, this process is chemisorption 54. Secondly, the values of free energy change, enthalpy change and entropy change were calculated for the analyzation. Figure. 12 (a) shows the adsorption isotherms under different temperatures, and they are fitted and drawn by Origin. Figure. 12 (b) shows the fitting result of this part. 318 K and 338 K were chosen to do the same work as we did under 298 K for fitting Langmuir model in chapter 3.6 16, 21, 45, 55. The results are listed in Table 4. The free energy change was calculated from following equation: ∆𝐺𝑜 = ―𝑅𝑇𝑙𝑛(𝐾𝐿) (12) where ΔGo is Gibbs free energy change (J/mol), and the unit of KL was transformed into L/g. Then the values of entropy change and enthalpy change can be fitted from the slope and intercept of the plot of ΔGo against T using the following equation: (13) 𝛥𝐺𝑜 = 𝛥𝐻𝑜 ― 𝑇𝛥𝑆𝑜 o o where ΔS is entropy change (kJ/mol·K), and ΔH is enthalpy change (kJ/mol). The unit of ΔGo was transformed into kJ/mol here. The results of thermodynamic parameters are shown in Table 4. Table 4 Results of Thermodynamic Fitting Langmuir Isotherm Parameters Temp (K) 298 318 338

qmax (mg/g) 17.34 13.82 14.42

R2 0.9406 0.9621 0.9705

KL (L/g) 3789.70 8412.50 10339.05

Thermodynamic Parameters Temp (K) 298 318 338

ΔGo (kJ/mol)

ΔHo (kJ/mol)

ΔSo (kJ/mol·K)

R2

-20.42 -23.89 -25.98

20.78

0.14

0.9588

The adsorption process is spontaneous, because Gibbs free energy change is negative. Both of 13



enthalpy change and entropy change of this process is positive, therefore, the spontaneous process is controlled by entropy increasing. 3.8 Contrast experiment with some common adsorbent Silica gel, activated carbon, and zeolite were chosen to compare with PAO fibers under the following conditions: pH=5, T=298 K, m: 0.05 g, C0=95.32 μg/L, V=500 mL, t=24 h. Equilibrium concentrations of these three adsorbents were all higher than 60 μg/L. It proves that PAO fibers are more effective adsorbent for uranyl ion compared with silica gel, activated carbon, and zeolite.

Figure. 11 Temperature effect on adsorption of uranyl from aqueous solution by PAO fibers (pH: 5, m: 0.02g, C0: 99.89 μg/L, V: 500mL, t: 24h).

Figure. 12 (a) Adsorption isotherms of uranium on PAO fibers surface (pH: 5, m: 0.05g, V: 500mL, t: 24h); (b) Plots of ΔGo versus T for the uranyl adsorption on PAO fibers (pH: 5, T: 298K, 318K, 338K, m: 0.05g, V: 500mL, 14


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t: 24h).

4. Conclusion The PAN fibers used in this work can be successfully modified with amidoxime groups for uranium adsorption. When the adsorbent concentration reaches 0.1 g/L to dispose uranium solution of 100 μg/L, the equilibrium concentration can be decreased under 10 μg/L. Through kinetics and thermodynamics fitting, intraparticle diffusion model and pseudo-second-order model are adopted to describe the adsorption process, and Freundlich model describes the adsorption better than other thermodynamic models. The adsorption process is spontaneous and endothermic. All the experimental results demonstrate that PAO fibers are a kind of excellent adsorbent to dispose lowconcentration uranium solution. Using the PAO fibers to decrease the uranium concentration under 10 μg/L in industry is realizable. Acknowledgement This work was financially supported by the Foundation of Heilongjiang Postdoctoral Science Foundation (LBH-Z17050), the National Natural Science Foundation of China (21771045), the Fundamental Research funds for the Central Universities (HEUCFG201842), and Decommissioning of nuclear facilities and special funds for radioactive waste management ([2017]955). Authors are grateful to all support. References (1) Agency, N. E. Uranium 2016. 2016. (2) Asif, M.; Muneer, T. Energy supply, its demand and security issues for developed and emerging economies. Renew. Sust. Energ. Rev. 2007, 11, 1388-1413. (3) Bai, J.; Yin, X.; Zhu, Y.; Fan, F.; Wu, X.; Tian, W.; Tan, C.; Zhang, X.; Wang, Y.; Cao, S.; Fan, F.; Qin, Z.; Guo, J. Selective uranium sorption from salt lake brines by amidoximated Saccharomyces cerevisiae. Chem. Eng. J. 2016, 283, 889-895. (4) Milja, T. E.; Prathish, K. P.; Prasada Rao. T. Synthesis of surface imprinted nanospheres for selective removal of uranium from simulants of Sambhar salt lake and ground water. J. Hazard. Mater. 2011, 188, 384-390. (5) Guo, X.; Xiong, X.-G.; Li, C.; Gong, H.; Huai, P.; Hu, J.; Jin, C.; Huang, L.; Wu, G. DFT investigations of uranium complexation with amidoxime-, carboxyl- and mixed amidoxime/carboxyl-based host architectures for sequestering uranium from seawater. Inorg. Chim. Acta 2016, 441, 117-125. (6) Ladshaw, A. P.; Ivanov, A. S.; Das, S.; Bryantsev, V. S.; Tsouris, C.; Yiacoumi, S. First-Principles Integrated Adsorption Modeling for Selective Capture of Uranium from Seawater by Polyamidoxime Sorbent Materials. ACS Appl. Mater. Interfaces 2018, 10, 12580-12593. (7) Olivelli, M. S.; Curutchet, G. A.; Torres Sánchez, R. M. Uranium Uptake by Montmorillonite-Biomass Complexes. Ind. Eng. Chem. Res. 2013, 52, 2273-2279. (8) Rao, T. P.; Metilda, P.; Gladis, J. M. Preconcentration techniques for uranium(VI) and thorium(IV) prior to analytical determination-an overview. Talanta 2006, 68, 1047-1064. (9) W. H. O. Guidelines for Drinking Water Quality. Eng.sanit.ambient 2011, 16, 04-05. (10) Zhao, Y.; Wang, X.; Li, J.; Wang, X. Amidoxime functionalization of mesoporous silica and its high removal of U(VI). Polym. Chem. 2015, 6, 5376-5384.

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Figure. 1

Diagrammatic drawing of modification reaction (gray ball represents carbon atom, yellow ball represents nitrogen atom, red ball represents oxygen atom, blue ball represents hydrogen atom).

Figure. 2

FT-IR spectra of PAN fibers and PAO fibers.

Figure. 3

SEM images: (a)&(b) PAN fibers; (c)&(d) PAO fibers; (e)&(f) PAO fibers after adsorption.

Figure. 4 Figure. 5 Figure. 6 Figure. 7 Figure. 8

EDS Spectroscopy: (a&b) PAO fibers; (c&d) PAO fibers after adsorption; EMI of (e) PAO fiber after adsorption; (f) C; (g) N; (h) O; (i) U; (j) Composition of C, N, O, and U. Thermogravimetric curves of PAN fibers and PAO fibers. XPS spectra: (a) Surveys of PAN fibers, PAO fibers and PAO fibers after adsorption; (b) C 1s of PAN fibers and PAO fibers; (c) N 1s of PAN fibers and PAO fibers; (d) O 1s of PAN fibers and PAO fibers. Adsorbent concentration effect on adsorption of uranyl from aqueous solution by PAO fibers (pH: 5, T: 298K, C0: 99.8μg/L, V: 500mL, t: 24h). pH effect on adsorption of uranyl from aqueous solution by PAO fibers (T: 298K, m: 0.05 g, C0: 95.37 μg/L, V: 500 mL, t: 24 h). (a) Contact time effect on adsorption of uranyl from aqueous solution by PAN fibers and PAO fibers (pH: 5, T: 298

Figure. 9

K, m: 0.05 g, C0 for PAN fibers: 99.82 μg/L, C0 for PAO fibers: 99.14 μg/L, V: 500 mL); Plots for the uranyl adsorption on PAO fibers: (b) t/qt versus Time; (c) qt versus ln(t); (d) qt versus t1/2 (pH: 5, T: 298 K, m: 0.05 g, C0: 99.14 μg/L, V: 500 mL, t: 8 h).

Figure. 10 Figure. 11 Figure. 12

Plots for the uranium adsorption on PAO fibers: qe versus Ce; (pH: 5, T: 298K, m: 0.05g, V: 500mL, t: 24h). Temperature effect on adsorption of uranyl from aqueous solution by PAO fibers (pH: 5, m: 0.02g, C0: 99.89μg/L, V: 500mL, t: 24h). (a) Adsorption isotherms of uranium on PAO fibers surface (pH: 5, m: 0.05g, V: 500mL, t: 24h); (b) Plots of ΔGo versus T for the uranyl adsorption on PAO fibers (pH: 5, T: 298K, 318K, 338K, m: 0.05g, V: 500mL, t: 24h).

Table 1

Explored Experimental Conditions

Table 2

Results of Kinetic Fitting

Table 3

Results of Isotherm Fitting

Table 4

Results of Thermodynamic Fitting

19



Fig. 1 177x41mm (300 x 300 DPI)


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Figure. 3



Figure. 4


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Fig. 5 59x41mm (600 x 600 DPI)



Figure. 6


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Fig. 7 59x41mm (600 x 600 DPI)



Fig. 8 59x41mm (600 x 600 DPI)


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Fig. 9 177x126mm (300 x 300 DPI)



Fig. 10 59x41mm (600 x 600 DPI)


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Fig. 11 59x41mm (600 x 600 DPI)



Fig. 12 177x67mm (300 x 300 DPI)


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Adsorption of Low-Concentration Uranyl Ion by Amidoxime

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