Preparation of Amidoxime-Based Nylon-66 Fibers for Removing

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Preparation of amidoxime-based nylon 66 fibers for removing uranium from low concentration aqueous solutions and simulated nuclear industry effluents Mingxing Zhang, Qianhong Gao, Chenguang Yang, Li-Juan Pang, HongLong Wang, Hui Li, Rong Li, Lu Xu, Zhe Xing, Jiangtao Hu, and Guozhong Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02652 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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Industrial & Engineering Chemistry Research

Preparation of amidoxime-based nylon 66 fibers for removing uranium from low concentration aqueous solutions and simulated nuclear industry effluents Mingxing Zhang,†,‡ Qianhong Gao,†,‡ Chenguang Yang,†,‡,§ Lijuan Pang, †,‡ Honglong Wang,†,‡ Hui Li,† Rong Li,† Lu Xu,† Zhe Xing,† Jiangtao Hu,*,† and Guozhong Wu*,†,§ †

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No. 2019

Jialuo Rd., Jiading Dist., Shanghai, 201800, China. ‡

University of Chinese Academy of Sciences, Beijing 100049, China

§

School of Physical Science and Technology, Shanghai Tech University, Shanghai

200031, China *Corresponding author: Jiangtao Hu; Guozhong Wu Mailing address: P.O. Box 800-204, Shanghai 201800, China Tel/Fax: +86-21-39194531/+86-21-39195118. Email: [email protected]; [email protected] ABSTRACT Amidoximated nylon 66 fiber containing double amidoxime groups per repeating unit (coded as PA66-g-PGMA-IDPAO) for the purpose of removing low uranium concentrations (1-25 mg/L) from aqueous solutions and application in simulated nuclear industry effluents was prepared by a simultaneous radiation induced emulsion graft polymerization method. It was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, and single fiber tensile strength tester. Batch adsorption experiments were conducted to investigate the effect of initial pH, sorption isotherm, adsorption kinetics and

ACS Paragon Plus Environment


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thermodynamics. It was found that the optimum initial pH is 5.0. The sorption isotherm followed the Langmuir isotherm. The adsorption kinetics fitted the pseudo-second-order model. The thermodynamics parameters revealed that the adsorption process was spontaneous and endothermic. The sorption test performed in a simulated nuclear industry effluent demonstrated that it showed a high adsorption efficiency (about 91.3 %) and selectivity for uranium. 1. Introduction Uranium is the most important element in the nuclear industry because of its significant commercial use as a fuel for electricity generation1 and military application as material for nuclear weapons2. Naturally occurring uranium is generally distributed at very low concentration levels within many rocks, soils and seawater3. However, with the rapid development of nuclear science and technology, toxic levels of uranium4 are likely to contaminate the environment due to emissions from the nuclear industry5, uranium mining, and milling activities6. Concerns over uranium use has been growing because of its extensive distribution, long half-life, and carcinogenicity7. According to Gilman’s studies, the tolerable daily intake of soluble uranium is 0.6 µg/kg of body weight2. Based on that, the World Health Organization (WHO) has strictly determined that the concentration of uranium in drinking water should not exceed 15 ppb8. On the other hand, uranium is an irreplaceable raw material for nuclear energy9 and whose dwindling on land has received more attention lately10. Numerous works have been carried out to recover uranium from non-conventional resources such as seawater, industrial wastewater and other waste


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sources11. Therefore, from the perspectives of uranium conservation and environmental protection, it is desirable to develop efficient and economically viable technologies for the removal of uranium from wastewater. Multiple methods, such as membrane processing12, chemical precipitation13, solvent extraction14, exchange16,

15

, ion

17

, biosorption18, electrodeposition19, and adsorption20-22, have been

extensively employed for removing uranium from wastewater. Of these various processes, adsorption is one of the most efficient and viable options due to its cost-effectiveness, versatility, and simplicity23. In the past few decades, a wide variety of materials have been developed and modified for adsorption of high concentration of uranium, for example modified magnetic composites9, 24-26, chelating resins27, 28, activated ore materials16, 21, 29, fungi30, 31

and mesoporous materials32, 33. However, little attention has been paid to the study

of the adsorption process for low uranium concentrations from aqueous solutions. Preparation of adsorbent with high selectivity and capacity for removing low uranium concentrations remains a challenge. Additionally, adsorbents intended to remove uranium from large volumes of wastewater must meet the following requirements: inexpensive raw materials, simple preparation process, nontoxicity, and high selectivity for low uranium concentrations34. Polyamide 66, also known as nylon 66, produced by polycondensation of hexamethylene diamine and adipic acid, is of major commercial importance in the textile industry as its fiber is used for the production of yarns, garments, industrial textiles and carpets35. Compared with other shapes of the adsorbent material (like



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resin beads and TiO2 etc.), fibrous absorbent material has the following advantages in practical application36: (1) a larger contact area with the adsorbate, (2) a smaller resistance to the fluid, (3) manipulation into a variety of shapes to meet different process requirements, (4) and inexpensive raw material. In addition, compared with other most used fibers (especially PE, UHMWPE, PP/PE), nylon 66 fiber has better hydrophilic which can be in favor of the diffusion of uranyl ion. Different from the chemical grafting method, radiation induced graft polymerization (RIGP) is an attractive technique37 for the preparation of functional polymers for various reasons including the simplicity and the flexibility of reaction initiation with commercially available ionizing radiation sources38. To satisfy the above criteria and study the removal process of low uranium concentrations from aqueous solutions, a new kind of adsorbent material (PA66-g-PGMA-IDPAO) was fabricated in this work by adopting the idea of material preparation for extracting uranium from seawater39, 40. A process for fabricating a new kind of highly efficient and selective adsorbent material was investigated via the following steps: (1) introduction of epoxy groups via simultaneous radiation induced emulsion graft polymerization of glycidyl methacrylate, (2) aminolyzation via the ring-opening reaction between the epoxy group and 3, 3’-iminodipropionitrile, (3) and amidoximation of the immobilized nitrile moieties with hydroxylamine. After a series of chemical modifications, amidoxime groups can be introduced onto nylon 66 fiber for their better adsorption for removing low uranium concentrations41. And amidoximated adsorbent containing double amidoxime groups per repeating unit and


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an additional diethyl spacer unit between these double amidoxime groups has high adsorption efficiency and selectivity for low uranium concentrations42, 43, which can directionally remove or reclaim uranium. The physicochemical properties of the pristine and modified fibers were carefully characterized by Fourier transform infrared

spectroscopy

(FTIR),

scanning

electron

microscopy

(SEM),

thermogravimetric analysis (TGA), and an electronic single fiber tensile strength tester. Batch adsorption experiments of U(VI) adsorption on PA66-g-PGMA-IDPAO were carried out in uranium aqueous solutions under controlled conditions. The sorption test performed in a simulated nuclear industry effluent, containing 10 coexisting cations including uranyl ion, demonstrated that the amidoximated nylon 66 fibers showed a high adsorption efficiency (about 91.3 %) and selectivity for uranium. 2. Experimental 2.1 Materials Commercial nylon 66 fiber (PA66) with a linear density of 78 dTex/48 f(S) was used in this study. The monomer, glycidyl methacrylate (GMA), was obtained from Sigma-Aldrich Co. Ltd. 3, 3’-Iminodipropionitrile (IDPN) was obtained from TCI Co. Ltd. Hydroxylamine hydrochloride (NH2OH·HCl) was obtained from Shanghai Macklin Biochemical Co. Ltd. A uranyl nitrate standard solution was obtained from Analytical Laboratory, Beijing Research Institute of Uranium Geology. Other reagents, such as acetone, polyoxyethylene (20) sorbitan monolaurate (Tw-20), dimethyl sulfoxide (DMSO), hydrochloride (HCl) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co. Ltd and are all analytical grade. All



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reagents were used without further purification. The water used was deionized by a Water Purification system (Purelab Classic MK2). 2.2 Preparation of nylon 66 fiber adsorbent The preparation process of PA66-g-PGMA-IDPAO is illustrated in detail in Scheme 1. Firstly, glycidy methacrylate (GMA) was grafted onto nylon 66 fibers using a simultaneous radiation induced emulsion graft polymerization method. Namely, nylon 66 fiber (20 cm in length, 0.5 g) was immersed in irradiation tubes containing 5 (v) % GMA emulsions (2.5 mL GMA, 47.5 mL H2O, and 0.25 g Tw-20). Then the emulsion was bubbled with nitrogen for 15 min to remove the oxygen before irradiation. The samples were subjected to 60Co γ-radiations at a dose of 10 kGy and a dose rate of 0.59 kGy•h-1. The dose rates at different positions were measured using a Fricke dosimeter. After irradiation, the fiber was taken out of the solution and extracted with acetone in a Soxhlet for 24 h to remove homopolymer and residual monomer, and then dried at 60 oC in a vacuum oven. After drying to a constant weight, the degree of grafting (Dg) was determined according to the following equation:

D g (%) =

W1 −W 0 × 100 W0

(1)

The product was coded as PA66-g-PGMA and the Dg of the grafting material used in this study was 92.9 %, where W0 and W1 are the weight of PA66 and PA66-g-PGMA, respectively. Secondly, PA66-g-PGMA was immersed in a 20 (v) % IDPN mixture solution. The solvent used was a mixture of water and DMSO in a ratio of 1:1 by volume. The reaction was performed at 80 oC for 24 h44. The chemical modified fiber was washed


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several times with fresh ethanol and deionized water to remove the residual IDPN, and then dried in a vacuum oven at 60 oC until a constant weight was obtained. The product was coded as PA66-g-PGMA-IDPN. The conversion percent of the epoxy groups to the iminodipropilonitrile groups was 60 % calculated as follows44:

Conversion (%) =

142 .15 ( W 2 − W 1 ) × 100 123 .16 ( W 1 − W 0 )

(2)

where W2 is the weight of PA66-g-PGMA-IDPN, and the 142.15, and 123.16 are molecular weights of GMA and IDPN, respectively. Finally, a 0.5 mol•L-1 solution of hydroxylamine hydrochloride (NH2OH·HCl) was prepared using 1/1 (v/v) % water/DMSO as the solvent and solid NaOH to adjust the pH to 7. PA66-g-PGMA-IDPN reacted with 0.5 mol•L-1 NH2OH solution (pH 7.0) for 4 h at 80 oC to convert the cyano groups into amidoxime (AO) groups40. After the completion of the amidoximation reaction, the modified fiber (coded as PA66-g-PGMA-IDPAO) was washed with fresh ethanol and deionized water, and then dried in a vacuum oven at 60 oC. AO group density was evaluated as follows:

AO density ( mmol g ) =

1000 ( W 3 − W 2 ) 33 W 3

(3)

where W3 is the weight of PA66-g-PGMA-IDPAO, and the 33 is molecular weight of NH2OH. AO group density was determined to be 1.8 mmol•g-1. 2.3 Characterization of the modified fibers FT-IR spectra of the pristine PA66, PA66-g-PGMA, PA66-g-PGMA-IDPN, and PA66-g-PGMA-IDPAO were collected with an FT-IR Spectrometer (Tensor 27, Bruker Co. USA) in the range of 1000-4000 cm-1 at a resolution of 4 cm-1 and 32



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scans. A scanning electron microscope (SEM, Merlin Compact, Zeiss) was employed to observe

the

morphologies

of

the

pristine

PA66,

PA66-g-PGMA,

PA66-g-PGMA-IDPN and PA66-g-PGMA-IDPAO at an acceleration voltage of 10 kV. All samples were attached to a carbon tap and sputtered with gold to enhance the electronic conductivity under the vacuum prior to observation. Thermogravimetric

analysis

(TGA)

of

pristine

PA66,

PA66-g-PGMA,

PA66-g-PGMA-IDPN, and PA66-g-PGMA-IDPAO were performed on a NETZSCH TG 209 F3 Instruments in the range of 25 to 800 oC under a nitrogen atmosphere with a flow rate of 30 mL•min-1 and a heating rate of 10 oC•min-1. The initial decomposition temperature (Tdi) was defined as the temperature at which the weight loss was 5 (wt) %. The mechanical property tests of the pristine PA66, PA66-g-PGMA, PA66-g-PGMA-IDPN, and PA66-g-PGMA-IDPAO were performed on an electronic single fiber tensile strength tester (LLY-06E, Laizhou Electron Instrument Co. Ltd. China) according to the Chinese Standard GB/T14337-2008, with each tested at least thirty times. 2.4 Batch adsorption experiments The adsorption capacity of PA66-g-PGMA-IDPAO adsorbent for U(VI) from an aqueous solution at atmospheric pressure was studied by the batch adsorption method. Batch adsorption experiments were carried out in 100 mL of PET plastic bottles, which were placed in a constant temperature oscillator (Jintan Jinnan Instrument


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Manufacturing Co., Ltd., mode SHA-CA) at 150 strokes per min. In detail, 15 mg of PA66-g-PGMA-IDPAO and 50 mL of U(VI) solutions with different concentrations were added to PET plastic bottles, and then the pH value of the solutions were adjusted using negligible volumes of HCl and NaOH solutions. Then the bottles were shaken for specified durations at the desired temperatures. The initial and equilibrium uranium concentrations of the supernatant were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 8000, Perkin Elmer). All the adsorption data presented here is the average value from three replicated experiments. The effects of various parameters, including pH, contact time, initial U(VI) concentration and temperature, were examined. The uranium adsorption qe (mg U•g -1 dry adsorbent) was obtained using the following equation: qe =

(C 0 − C e )V W

(4)

where qe is the adsorption capacity of adsorbent (mg•g-1), C0 is the initial concentration of U(VI) (mg•L-1), Ce is the equilibrium concentration of U(VI) (mg•L-1), V is the volume of the testing solution, and W is the weight of the adsorbent. 3. Results and Discussion 3.1 Characterizations of the pristine and modified nylon 66 fibers 3.1.1 FT-IR spectra Figure 1 shows the obvious difference between FT-IR spectra for pristine and modified nylon 66 fibers. As presented in Figure 1 (a), pure PA66 has absorption peaks at 3300.3, 1645.1, 1541.1 cm-1, which are assigned to -NH-, -C=O stretching,



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and the combined absorption of both δN-H and νC-N, respectively, while the absorption peaks at 2857.7 and 2929.2 cm-1 are due to -CH2- stretching vibration45. For PA66-g-PGMA, a new peak at 1737.9 cm-1 is due to -C=O stretching, and the peaks at 1152.9 and 908.1 cm-1 belong to stretching vibration of epoxy group in GMA. These new features indicate that GMA is grafted onto nylon 66 fibers, as expected. A sharp band at 2251.4 cm-1 due to a C≡N stretching vibration28, stemming from the 3, 3’-iminodipropionitrile structure, is clearly seen in Figure 1 (c). The broad hydroxyl band, which originates from ring opening of the epoxy group46, is observed at 3470.2 cm-1. These two bands verify the reaction between IDPN and the epoxy groups of PGMA grafting chains on nylon 66 fibers. After amidoximation, the C≡N band at 2251.4 cm-1 is diminished because of the depletion of CN groups, and a new peak presents at 922.8 cm-1, belong to the N-OH group vibration (Figure 1 (d)), indicating the formation of amidoxime groups44. 3.1.2 SEM analysis SEM images of the pristine and modified nylon 66 fibers are shown in Figure 2. It is seen that morphologies of these fibers have significant differences before and after modification. While the surface of a single fiber in the pristine PA66 fiber is very smooth (Figure 2 (a)), the surface of PA66-g-PGMA becomes rough and attaches a lot of knots (Figure 2 (b)). Compared with Figure 2 (b), Figure 2 (c) shows that the surface of PA66-g-PGMA-IDPN is still uneven and the knots attached to the fiber surface

are

interconnected.

Following

amidoximation,

the

surface

of

PA66-g-PGMA-IDPAO becomes much rougher and the knots become much smaller


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but are still significantly larger compared with those of PA66-g-PGMA (Figure 2 (d)). Additionally, the diameter of the fibers increases gradually following each modification reaction. The average values for the diameter of the nylon 66 fibers at the pristine and each subsequent modification stages are 12.3, 19.3, 21.0 and 22.0 µm, respectively. From the molecular level, the PGMA graft chains were covalently connected to the nylon 66 fiber in the grafting process, then IDPN and NH2OH were introduced on the graft chains via ring opening reaction and amidoximation, respectively. Namely, new chemical structure will introduce on the nylon 66 fiber after each modification, which results in the enhancement of the diameter of the modified fibers at the macro level. 3.1.3 TGA measurements To investigate thermal stability of the polymeric materials, thermal analysis techniques are often adopted. Figure 3 shows the TGA and DTG curves for the pristine PA66, PA66-g-PGMA, PA66-g-PGMA-IDPN, and PA66-g-PGMA-IDPAO. The thermogram of the pristine PA66 proceeds by a clean, one-step degradation47 with an initial decomposition temperature of 321

o

C, a maximum decomposition

temperature of 449 oC, and a 100 % weight degradation temperature of 500 oC. The initial decomposition temperatures of the modified fibers ― PA66-g-PGMA, PA66-g-PGMA-IDPN, and PA66-g-PGMA-IDPAO ― are 230, 180, and 130 oC, respectively, which are all lower than that of the pristine fiber. These modified fibers introduce a four-step degradation pattern as shown in the TGA and DTG curves. The set of decompositions in the range of 25 - 100 oC for all three modified fibers are due



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to the loss of water bound to the fibers48. The last decomposition temperatures of these modified fibers belong to the degradation of the PA66 matrix. For PA66-g-PGMA, the weight loss in the range of 230 - 350 oC is due to the decomposition of ester and the 2, 3-epoxypropyl group and the next weight loss from 350 - 430 oC is due to the grafted PGMA decomposition49. In the TGA and DTG curves of PA66-g-PGMA-IDPN, the sections in the range of 180 - 360 oC and 360 450 oC are due to the decompositions of the imino group and the grafted PGMA on the fiber, respectively. The main difference observed from the TGA and DTG curves of PA66-g-PGMA-IDPAO when compared with that of PA66-g-PGMA-IDPN is that after amidoximation, PA66-g-PGMA-IDPAO becomes thermally less stable and degradation44 starts at about 130 oC. In conclusion, thermal stability of the modified fibers gradually declines with the increase in the number of chemical reaction steps. 3.1.4 Mechanical properties The mechanical properties of the pristine and modified nylon 66 fibers are quite important in the practical application and are measured by an electronic single fiber tensile strength tester40. Excellent tensile properties are expected to be maintained to a larger extent during the lifetime of the fibers50. Figure 4 shows the tensile strengths at the break of the pristine PA66, PA66-g-PGMA, PA66-g-PGMA-IDPN, and PA66-g-PGMA-IDPAO. The tensile strengths of all modified fibers are lower than that of the pristine PA66. The tensile strength is about 4.52 cN/dTex for the pristine PA66 and decreases sharply to 3.39 cN/dTex for PA66-g-PGMA. However, the tensile strength decreases less sharply following the two subsequent steps of the chemical


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reactions. These results demonstrate that while the chemical reactions have a slight effect on the tensile properties of the materials, the main influence is the radiation damage50. 3.2 Effect of solution pH The pH value of a solution is one of the most important factors affecting the adsorption process due to its significant role in the formation of uranium species in aqueous solution26. The effect of the solution pH value on the adsorption of U(VI) by PA66-g-PGMA-IDPAO

adsorbent

was

studied

using

15

mg

of

PA66-g-PGMA-IDPAO and 50 mL 10 mg•L-1 of U(VI) solution at various pH value in the range of 3.0 - 8.0 at 25 oC for 12 h. The adsorption capacity of U(VI) versus pH is shown in Figure 5. Adsorption capacity increases with an increase in pH from 3.0 to 5.0, and achieves a maximum at pH 5.0, decreases upon further increases in pH from 5.0 to 8.0. The pH of the solution can affect both the relative distribution of U(VI) species in solution and the surface properties of the PA66-g-PGMA-IDPAO adsorbent. Free uranyl ion (UO22+) is the primary species at pH < 5.0, while UO22+, UO2OH+, (UO2)2(OH)22+, and (UO2)3(OH)5+ are the dominant species at pH 5.0. Furthermore, in the pH range of 6.0 - 8.0, schoepite ((UO2)8O2(OH)12•12H2O) is the main species51. It is normally accepted that the amidoxime group acts as a bidentate ligand system for the uranyl cation17, 24, 46. However, at pH < 5.0, a high concentration of H+ ions compete with uranyl ions for the binding sites on the sorbent surface resulting in a low adsorption capacity for U(VI)52. A lower adsorption capacity for U(VI) at pH 6.0 - 8.0 is due to the formation of the schoepite ((UO2)8O2(OH)12•12H2O). At pH 5.0, the



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amidoxime groups can undergo deprotonation24, then the R-NO- anions and the lone pairs of electron on the amino nitrogen combine with uranyl cations via electrostatic interaction and complexation to form stable chelates, resulting in the maximum adsorption capacity for U(VI). Therefore, the optimum initial pH is 5.0 and all further adsorption experiments were carried out at an initial pH of 5.0. 3.3 Sorption isotherms Equilibrium isotherm studies were carried out to evaluate the maximum adsorption capacity of PA66-g-PGMA-IDPAO adsorbent for uranium, in which, the initial concentrations of U(VI) were varied from 1 to 25 mg•L-1 and the other parameters (15 mg of PA66-g-PGMA-IDPAO, retention time 36 h, pH = 5.0, T = 25 55 oC and V = 50 mL) were kept constant20. The adsorption isotherm is illustrated in Figure 6. The equilibrium data are analyzed using the Langmuir and Freundlich equilibrium models to determine the adsorption mechanism of the adsorbent53 (Figure 7). The Langmuir equation is given as: 1 Ce C = + e qe b q m q m

(5)

where Ce is the equilibrium concentration in the solution (mg•L-1), qe (mg•g-1) is the adsorbed value of U(VI) at equilibrium concentration, qm is the maximum adsorption capacity (mg•g-1), and b is the Langmuir constant. The Freundlich equation is defined as: 1 ln q e = ln K F + ln C e n

(6)

where KF ((mg/g)•(L/mg)1/n) and n are the Freundlich constants, which represent sorption capacity and sorption intensity, respectively.


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The values of qm and b, and KF and n, can be determined from the intercept and slope of the plots corresponding to eq. (5) and (6), respectively, and are given in Table 1. Comparing the values of correlation coefficients R2 obtained by the two isotherm models, we conclude that the adsorption experimental results are better conforming to the Langmuir model, which assumes that the uptake of metal ions occurs on a homogeneous surface by monolayer adsorption. According to the Langmuir isotherm23, 52, the monolayer maximum capacity of PA66-g-PGMA-IDPAO adsorbent for U(VI) increases from 21.09 to 41.98 mg/g-adsorbent with an increase in temperature from 25 to 55 oC. 3.4 Adsorption kinetics To assess the adsorption rate of PA66-g-PGMA-IDPAO adsorbent towards uranium, the time-dependent curves of uranium uptake at 25, 35, 45 and 55 oC are illustrated in Figure 8. The uranium adsorption was rapid with 70 % uptake in the first 120 min and then it became slower until equilibrium was reached34. To offer useful information for understanding the underlying sorption mechanisms, the experimental kinetic data of uranium adsorption on PA66-g-PGMA-IDPAO adsorbent were simulated by pseudo-first-order and pseudo-second-order models52 (Figure 9). The pseudo-first-order is expressed as:

ln (q e − q t ) = ln q e − k 1t

(7)

where qe and qt are sorption amounts of U(VI) (mg•L-1) at equilibrium time (min) and time t (min), respectively; k1 (min-1) represent the kinetic rate constant of the pseudo-first-order. The pseudo-second-order model is written as:



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t 1 t = + 2 qt k 2 q e q e

Page 16 of 36

(8)

where k2 (g•mg-1•min-1) represent the kinetic rate constant of pseudo-second-order model. The values of qe, k1 and k2 obtained by linear regression according to the two models are summarized in Table 2. The values of correlation coefficients R2 of the pseudo-second-order model are all higher than that of the pseudo-first-order model especially at higher temperatures. The calculated adsorption capacity values are almost equal to those from experimental data. Therefore, adsorption kinetics of U(VI) on the PA66-g-PGMA-IDPAO adsorbent are in better agreement with the pseudo-second-order model, verifying that the process is chemical adsorption54. Based on above results, the possible adsorption mechanism of PA66-g-PGMA-IDPAO adsorbent is that at initial pH 5, the amidoxime groups can undergo deprotonation, then the R-NO- anions and the lone pairs of electron on the amino nitrogen combine with uranyl cations via electrostatic interaction and complexation to form stable chelates, respectively. 3.5 Thermodynamic studies The thermodynamic parameters such as standard free energy, standard enthalpy and standard entropy, were calculated based on Van’t Hoff plot (Figure 10) and the Gibbs-Helmholtz equation55, respectively. Van’t Hoff equation: ln K C =

∆S o

∆ − H R RT

o

Gibbs-Helmholtz equation: ∆Go = ∆ H o − T∆S o


(9) (10)

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where KC (CAe/Ce) is the equilibrium constant, CAe and Ce (mg•L-1) are the equilibrium concentration of U(VI) on the adsorbent and solution, respectively, ∆So is the standard entropy (J•mol-1•K-1), ∆Ho is the standard enthalpy (kJ•mol-1), ∆Go is the standard Gibbs free energy (kJ•mol-1), T is the absolute temperature (K), and R is the gas constant (8.314 J•mol-1•K-1). From Table 3, the positive value of ∆So suggests increased randomness at the solid/liquid interface during the adsorption of U(VI) on PA66-g-PGMA-IDPAO adsorbent52. The positive value of ∆Ho indicates that the adsorption process is endothermic; consequently, U(VI) adsorption is more favorable at higher temperature23. The negative values of ∆Go suggest that the adsorption process is feasible and spontaneous. The decreasing value of ∆Go with an increase in temperature reiterates that the adsorption process is more favorable at higher temperatures56. 3.6 Application in simulated nuclear industry effluents The

simulated

PA66-g-PGMA-IDPAO

nuclear

industry

effluent

sample

was

treated

with

adsorbent to evaluate its properties and industrial

applicability. The nuclear industry effluent sample was prepared based on previous works57. Namely, a controlled quality of

PA66-g-PGMA-IDPAO adsorbent and 50

mL simulated nuclear industry effluents containing UO22+ (77.8 mg/L), Na+ (56625 mg/L), Sr2+ (97.1 mg/L), Cr3+ (148.6 mg/L), Mn2+ (105.9 mg/L), Ni2+ (106.1 mg/L), Co2+ (119.3 mg/L), Zn2+ (153.2 mg/L), Ce3+ (107.0 mg/L) and La3+ (92.4 mg/L) were added into a PET plastic bottle, and then the pH value of the solution was adjusted to



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4.0 using negligible volumes of HCl and NaOH solutions. Subsequently, adsorption experiments were performed at 30 oC on a constant temperature oscillator at 150 strokes per min for 36 h. As shown in Figure 11(a), the sorption (%) of U(VI) increases with the increase of solid-to-liquid ratio, and it can reach up to 91.3 % at the solid-to-liquid ratio of 5 g/L, indicating that PA66-g-PGMA-IDPAO adsorbent has a high adsorption efficiency for uranium. Figure 11(b) shows that the adsorption selectivity for uranium is much higher than that of the competing metal ions. The ion selectivity order for adsorption is UO22+ >> Ni2+ > Ce3+ > La3+ > Co2+ > Zn2+ > Cr3+ > Mn2+ > Na+ >> Sr2+, suggesting that PA66-g-PGMA-IDPAO adsorbent has a remarkable adsorption selectivity for uranium over a range of the coexisting cations. PA66-g-PGMA-IDPAO adsorbent contains double amidoxime groups per repeating unit of PGMA graft chains and an additional diethyl spacer unit between these double amidoxime groups58, which provides four active sites and adequate space to combine with UO22+ ion. Previous literature papers43, 59 state that proper distance between the neighboring amidoxime groups should have been required to chelate UO22+ and these neighboring amidoxime groups having diethyl spacer unit are more accessible for the adsorption of UO22+ than those without diethyl spacers. Besides that, UO22+ ion possesses enough unoccupied orbitals and suitable ionic radii in the aqueous solution42 compared with other metal ions. All these contribute to form stable chelates between double amidoxime groups and UO22+ ions and improve the selectivity to UO22+ ion. Above results demonstrate that amidoximated nylon 66 fiber has a high efficiency and selectivity for removing uranium from simulated nuclear industry


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effluents40, implying that it has a potential industrial applicability. 4. Conclusions In this study, amidoximated nylon 66 fiber (PA66-g-PGMA-IDPAO) containing double amidoxime groups per repeating unit and an additional diethyl spacer unit between these double amidoxime groups for the purpose of removing low uranium concentrations from aqueous solutions and simulated nuclear industry effluents was prepared by a simultaneous radiation induced emulsion graft polymerization method. High adsorption affinity and selectivity for aqueous U(VI) is achieved through the complexation of metal ions by amidoxime groups on PA66-g-PGMA-IDPAO adsorbent. Batch adsorption experiments show that the optimum initial pH is 5.0, the adsorption process is spontaneous and endothermic, the sorption isotherm agrees with the Langmuir model, the monolayer maximum capacity of PA66-g-PGMA-IDPAO adsorbent for U(VI) increases from 21.09 to 41.98 mg•g-1 with an increase in temperature from 25 to 55 oC, and the adsorption kinetics are in congruence with the pseudo-second-order model. High adsorption efficiency and selectivity for uranium over a range of competing metal ions was determined by the application in simulated nuclear industry effluents. All these properties indicate that the amidoximated nylon 66 fiber is a potential candidate for an adsorbent with high efficiency and selectivity to remove uranium from nuclear industry effluents and has a potential industrial applicability. Acknowledgments Funding for this research was provided by the National Natural Science



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Foundation of China (grant 11305243, 11275252, 11405249, 11605275 and 11675247). References 1.

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Scheme 1. Preparation of amidoximated nylon 66 fiber

Figure 1. FT-IR spectra for (a) PA66, (b) PA66-g-PGMA, (c) PA66-g-PGMA-IDPN, (d) PA66-g-PGMA-IDPAO.



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Figure 2. SEM images for (a) PA66, (b) PA66-g-PGMA, (c) PA66-g-PGMA-IDPN, (d) PA66-g-PGMA-IDPAO.

Figure 3. TGA and DTG curves for pristine and modified nylon 66 fibers in N2 atmosphere


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Figure

4.

Tensile

strength

plot for (a) PA66,

(b) PA66-g-PGMA,

(c)

PA66-g-PGMA-IDPN, (d) PA66-g-PGMA-IDPAO.

Figure 5. Effect of initial pH on adsorption of U(VI) by PA66-g-PGMA-IDPAO adsorbent (initial uranium concentration 10 mg•L-1, 15 mg of PA66-g-PGMA-IDPAO, retention time 12 h, T = 25 oC , V = 50 mL, and pH = 3 - 8).



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Figure 6. Adsorption isotherm of PA66-g-PGMA-IDPAO adsorbent for U(VI) (15 mg of PA66-g-PGMA-IDPAO, retention time 36 h, pH = 5.0, T = 25 - 55 oC, and V = 50 mL)

Figure 7. Langmuir and Freundlich isotherms for removal of U(VI) by PA66-g-PGMA-IDPAO adsorbent.


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Figure 8. Effect of adsorption time on the adsorption of U(VI) by PA66-g-PGMA-IDPAO adsorbent (initial concentration 10 mg•L-1, 15 mg of PA66-g-PGMA-IDPAO, pH = 5.0, T = 25 - 55 oC, and V = 50 mL).

Figure 9. Pseudo- first-order and pseudo-second-order kinetics for removal of U(VI) by PA66-g-PGMA-IDPAO adsorbent.



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Figure 10. Van’t Hoff plot for removal of U(VI) by PA66-g-PGMA-IDPAO adsorbent.

Figure 11. Adsorption of U(VI) in simulated nuclear industry effluents by PA66-g-PGMA-IDPAO adsorbent.


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Table 1 Isotherm parameters of U(VI) adsorbed on PA66-g-PGMA-IDPAO adsorbent. Isotherm model o

Langmuir -1

Freundlich -1

2

T ( C)

qm (mg•g )

b (L•mg )

R

KF (mg/g)•(L/mg)1/n

1/n

R2

25 35 45 55

21.09 29.43 40.57 41.98

1.90 1.28 1.84 3.32

0.9975 0.9951 0.9988 0.9990

10.88 13.36 18.31 21.84

3.84 3.29 2.58 2.74

0.7988 0.8653 0.8834 0.7968

Table 2 Kinetic parameters of U(VI) adsorbed on PA66-g-PGMA-IDPAO adsorbent. Pseudo-first-order

Pseudo-second-order

T (oC)

qe,exp (mg•g-1)

k1 × 10-3 (min-1)

qe,cal (mg•g-1)

R2

k2× 10-4 (g•mg-1•min-1)

qe,cal (mg•g-1)

R2

25 35 45 55

13.98 17.62 19.17 21.45

3.74 4.08 3.56 7.55

9.31 11.45 9.33 10.82

0.9495 0.9645 0.9666 0.7551

8.36 7.62 11.24 14.75

14.83 18.66 19.57 22.29

0.9834 0.9889 0.9904 0.9975

Table

3

Thermodynamic

parameters

for

uranium

adsorption

on

PA66-g-PGMA-IDPAO adsorbent. ∆Ho (kJ•mol-1)

∆So (J•mol-1•K-1)

∆Go (kJ•mol-1)

75.17

252.99

25 oC

35 oC

45 oC

55 oC

-0.26

-2.79

-5.32

-7.85



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Abstract Graphic


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Preparation of Amidoxime-Based Nylon-66 Fibers for Removing

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