Synthesis of Amidoxime-Grafted Activated Carbon Fibers for Efficient

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Synthesis of amidoxime-grafted activated carbon fibers for efficient removal of uranium(VI) from aqueous solution Xin Lu, Dongxiang Zhang, Alemtsehay Tesfay Reda, Cong Liu, Zhi Yang, Shuaishuai Guo, Songtao Xiao, and Yinggen Ouyang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02690 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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Industrial & Engineering Chemistry Research

Synthesis of amidoxime-grafted activated carbon fibers for efficient recovery of uranium(VI) from aqueous solution Xin Lu,† Dongxiang Zhang,†,* Alemtsehay Tesfay Reda,† Cong Liu,† Zhi Yang,† Shuaishuai Guo,† Songtao Xiao,‡ Yinggen Ouyang,‡ †

School of Chemistry and Chemical Engineering, Beijing Institute of Technology,

Beijing, 102488, People’s Republic of China ‡

China Institute of Atomic Energy, P. O. Box 275 (26), Beijing 102413, People’s

Republic of China ABSTRACT A novel fibrous sorbent, amidoxime-grafted activated carbon fibers (ACFs-AO), was prepared using a chemical grafting method and tested for the efficient removal of uranium from aqueous solution. The sorbent was characterized using X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM),

elemental

analysis,

thermogravimetric

analysis

(TGA)

and

Brunauer-Emmett-Teller (BET). The effects of pH, contact time, initial concentration, and temperature on the sorption of U(VI) were investigated. The sorption of U(VI) on ACFs-AO obeyed pseudo-second-order model and Langmuir isotherm. The sorption capacity of ACFs-AO for U(VI) (about 191.6 mg/g) was much higher than that of activated carbon fibers (ACFs) (about 70.52 mg/g), which was mainly attributed to surface complexation between U(VI) and the amidoxime group on ACFs-AO. Additionally, the thermodynamic parameter results showed that the sorption process of U(VI) was spontaneous, feasible, and endothermic. Moreover, ACFs-AO adsorbed U(VI) selectively in an aqueous solution containing competitive ions, and was regenerated and reused efficiently. The findings of this work indicate that ACFs-AO 1

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could be a promising sorbent for the effective removal of U(VI) from aqueous solution. Keywords: Amidoxime; Activated carbon fibers; Uranium(VI); Sorption

1. Introduction Uranium, as the main source of nuclear fuel, has been used in nuclear power plants for electricity production in many countries. If the rapid development of the nuclear industry continues, then the depletion of uranium on land will follow in near future. While the total amount of uranium in the oceans is about 4.5 billion tons, which is approximately 1000 times the amount available on land, its concentration is approximately 3.3 ppb.1 Due to the extremely low concentration and the complex ocean environment, extracting uranium from seawater is a challenge of strategic importance to supply resources for nuclear energy. In addition, with the rapid development of the nuclear industry, toxic and weakly radioactive levels of uranium have been disposed into the environment from nuclear power plants,2 uranium mining and refining activities.3 The World Health Organization has strictly fixed the maximum uranium concentration in drinking waters at 15 ppb.4 Hence, efficient and economic methods for the selective recovery of uranium from aqueous solutions such as seawater, nuclear industry wastewater, and other waste sources have been investigated.

Methods

including

ion

exchange,5

chemical

precipitation,6

electro-deposition,7 solvent extraction,8 biosorption9 and adsorption,10, 11 have been employed for uranium removal from either seawater or nuclear wastewater. Specifically, adsorption is one of the most convenient methods due to its low cost, simplicity and versatility.12 In this method, it is critical to obtain an adsorbent with a high sorption capacity and fast sorption rate. In the past decades, a variety of adsorbent materials, including inorganic nano-materials,13

fibers,14

resins,15

biosorbents,16

metal–organic

frameworks

(MOFs),17 and layered double hydroxide composites18 have been studied and developed for the recovery of uranium by the adsorption method. To improve their selectivity and sorption capacity towards the targeted metal ions, the surface of the 2


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adsorbents is generally modified with functional groups.19 As reported in the literature, amidoxime, an excellent amphoteric functional group containing nucleophilic (-NH2) and hydroxyl (-OH) groups, has shown a strong tendency to form a chelate complex with the uranyl ion (UO22+) in aqueous solution.20-22 The lone pairs of electrons in the amino nitrogen and oxime oxygen can be donated to the UO22+ center to form a stable five-membered chelate; thus, adsorption materials that have been modified by amidoxime groups have fast sorption rates and excellent capacities for the selective sorption of uranium.23, 24 Hence, the amidoxime group is an appropriate choice to improve the adsorption performance of adsorbents towards the uranyl ion in aqueous solution. Considering their advantages in terms of economic viability, simple operation, environmental risk, and material recirculation, fibrous adsorbent materials have become popular sorbents for the recovery of uranium from seawater or nuclear wastewater. Compared with other morphologies of adsorption materials, fibrous adsorbents have a smaller resistance to the fluid, which makes them easily deployable in adsorption processes in seawater or nuclear wastewater.25 Moreover, fibers help to overcome the shortcomings of other adsorbent materials in terms of separation and recycling from aqueous solution after the adsorption, which can make them more cost effective for industrial applications.26 Additionally, because of their unique physical properties, fibrous adsorbents can be fabricated into various shapes and lengths to meet the requirements of different processes.25 To improve the sorption selectivity and capacity of fibrous adsorbents, studies of various fibrous materials prepared by chemical grafting,27 radiation-induced graft polymerization (RIGP),21,

28,

29

electrospinning,30, 31 and other methods for the sorption of uranium from aqueous solution have been performed. Although RIGP is considered to be the most easily implemented, efficient, and popular technology for modifying fibrous adsorbents, chemical grafting is still an appropriate method due to its moderate reaction conditions and low cost. Carbon based sorption materials, such as activated carbon,32-34 carbon nanotubes,35 and mesoporous carbon materials36, 37 have been reported for use in the recovery of 3



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uranium and other toxic metal ions due to their chemical stability, environmental friendliness, and higher radiation and thermal resistance. An efficient adsorbent must combine high porosity, large surface area, and strong binding-site accessibility to achieve a high and selective sorption capacity.38 Among the various carbon-based sorption materials, activated carbon fibers (ACFs), as a fiber material with a large porous surface area, controllable pore structure, good thermo-stability, and low acid/base reactivity, are an ideal adsorption substrate. ACFs have been found useful for the removal of various metal ions and other pollutants due to the small amount of hydroxyl and carboxyl groups on their surface.27, 39, 40 Therefore, to enhance their selective sorption ability, it is necessary to modify the ACFs substrate by chemical grafting. In this work, nitrile groups were introduced onto the surface of the fibers via an in-situ chemical grafting reaction to obtain nitrile-modified activated carbon fibers (ACFs-DAMN), and then the nitrile was converted to amidoxime using the neutralized hydroxylamine hydrochloride. The amidoxime-modified activated carbon fibers (ACFs-AO) were characterized in detail. Additionally, batch adsorption experiments of U(VI) on ACFs-AO were carried out in aqueous uranium solutions under controlled conditions. Adsorption tests were also performed in simulated solutions containing several coexisting cations including the uranyl ion to test the sorption selectivity of ACFs-AO. Based on the experimental findings, ACFs-AO is expected to be a competitive candidate for the extraction of uranium from aqueous solutions in the nuclear industry.

2. Experimental 2.1. Materials ACFs were purchased from Jiangsu Sutong Carbon Fiber Co., Ltd. (Jiangsu, China). Diaminomaleonitrile

(DAMN),

Hydroxylamine

hydrochloride

(NH2OH·HCl),

N-Hydroxysuccinimide (NHS) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI) used in the experiment were obtained from J&K Scientific Co., Ltd. N,N-dimethylformamide (DMF), tetrahydrofuran (THF), methanol, ethanol, 4


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dichloromethane, NaOH, HNO3 and K2CO3 were purchased from Beijing chemical works, Beijing, China. Stock solution of uranium was supplied by Analytical Laboratory, Beijing Research Institute of Uranium Geology, China. Other metal nitrates were purchased from Aladdin Co. Ltd., Beijing, China. All chemicals purchased from commercial sources were of analytical grade and used without further purification. Deionized water from a milli-Q plus water purification system (Millipore) was used in all experiments.

2.2. Preparation of ACFs-AO adsorbent ACFs and concentrated nitric acid were placed in a round bottomed flask and maintained at 80 °C for 3 h. The fibers were then washed with deionized water until the washings reached neutrality. The product was dried at 60 °C in a vacuum oven, and is referred to as ACFO. Subsequently, the ACFO (0.5 g) and EDCI (1.08 g) were dispersed in THF (60 mL). Then a solution of DAMN (1.0 g) and NHS (0.66 g) in DMF (10 mL) were added to the reaction system. The resulting mixture was sonicated for 1 h and then refluxed for 24 h with stirring. The treated ACFO was removed from the solution, rinsed exhaustively with ethanol, dichloromethane, and deionized water to remove free residues until the washings became colorless, and then dried in a vacuum oven at 60 °C. This product is referred to as ACFs-DAMN. Finally, the ACFs-DAMN fibers (0.5 g) were treated with NH2OH·HCl (1.05 g) and NaOH (0.6 g) in a 40/60 (v/v)% methanol-water solution (100 mL) for 8 h at 70 °C in a closed flask. The final ACFs-AO product was washed with deionized water to remove the remaining salts and dried in a vacuum oven at 60 °C overnight. The procedure for the preparation of ACFs-AO is illustrated in detail in Scheme 1.

5



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Scheme 1. Schematic illustration of the preparation of ACFs-AO.

2.3. Characterization Methods X-ray photoelectron spectroscopy (XPS) was recorded on a PHI Quantera system using monochromatic Al Kα (1486.6 eV) X-rays and the C1s line at 284.6 eV was used as a reference. Morphological measurements were carried out using FEI Quanta-250 field-emitting scanning electron microscopy (FE-SEM) equipment. Elemental analysis for C, H, and N in fibers were performed on EuroVector EA3000 analyzer. Thermogravimetric analysis (TGA) was measured using a TA SDTQ600 unit in nitrogen at a heating rate of 10 K·min-1. The BET surface area, pore volume, and pore diameter were determined by a BELSORP-max(ANKERSMID) instrument.

2.4. Batch Adsorption Experiments The adsorption experiments were conducted in polyethylene tubes for the batch experiments to study the sorption behavior of ACFs-AO fibers towards uranium in an aqueous solution. The effects of various parameters, such as pH, initial uranyl ion concentration, adsorption time, and temperature, were investigated in detail. The concentrations of the uranium solutions were adjusted using Milli-Q water, and the pH values of the solutions were adjusted to the desired values by adding negligible amounts of concentrated HCl or NaOH. In the adsorption experiment, the flasks were shaken in an oscillator at a frequency of 200 rpm at a certain temperature to reach sorption equilibrium. The final solution was separated by filtration through a filter with a pore size of 0.22 µm. 6


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The concentrations of the metal ions were analyzed using microwave plasma-atomic emission spectroscopy (MP-AES, 4100, Agilent) and a trace uranium analyzer (WGJ-III, China) for very low ion concentrations (less than 1 ppm) of uranyl ions. The results of all the tests were reported as the average of triplicate determinations, and the relative errors were less than 5%. The adsorption results, which included the percentage removal of uranyl ions (%R), the amount of uranyl ion sorption on the solid phase at equilibrium (qe), and the distribution coefficient (Kd), were calculated on the basis of the following equations:

R(%) =

qe =

C0 − Ce ×100% C0

(1)

(C0 − Ce ) ×V m

(2)

C0 − Ce V × Ce m

(3)

Kd =

where C0 and Ce represent the initial and equilibrium concentrations of uranyl ions, respectively (mg/L). V is the volume of the solution used for adsorption (mL) and m stands for the weight of adsorbent (g).

2.5. Desorption and regeneration experiments To evaluate the practical applicability of the adsorbent, the stability of ACFs-AO was investigated by four consecutive adsorption-desorption cycles using the fresh uranium solution. A solution of 1.0 mol/L HNO3 was selected as the eluent for the regeneration of ACFs-AO. Briefly, after being saturated in the adsorption process, the uranyl-ion-loaded ACFs-AO was removed from the uranium solution. The fiber was then added to the prepared HNO3 solution under vibration at 298 K for 48 h. The resulting uranium concentration of the solution was measured to evaluate the desorption percentage. The regenerated ACFs-AO were washed thoroughly with deionized water and dried in a vacuum oven in preparation for the next adsorption-desorption cycle.

3. Results and discussion 7



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3.1. Characterization 3.1.1. XPS analysis XPS was carried out to confirm the formation of a covalent bond between DAMN and the ACFs. As shown in the XPS spectra of the different fibers (Figure 1), the two characteristic peaks of the three types of fibers had bonding energies of 284.9 eV and 532.5 eV, which were attributed to C 1s and O 1s, respectively. In comparison with the ACFs, two new bands for nitrogen (N 1s, 400.5 eV) were detected in both the ACFs-DAMN and ACFs-AO spectra, suggesting that DAMN was grafted onto the ACFs. Detailed information regarding the surface chemical composition and bonding environment of the as-prepared ACFs-DAMN and ACFs-AO were obtained from the survey and high-resolution XPS spectra. Figure 2A shows a detailed spectrum of the C 1s signal for ACFs-DAMN; the peak can be deconvoluted into five parts. The main C 1s peak was dominated by elemental carbon at 284.5 eV, corresponding to the carbon in C-C/C=C.41, 42 The two peaks at 285.3 and 286.1 eV were assigned to the nitrogen-bound species C≡N and C-N, respectively.24, 43 The appearance of these peaks suggested that the DAMN was successfully anchored onto the ACFs. Two additional peaks at 286.8 and 288.5 eV were also observed, which were attributed to the oxygen-bound species C-O and C=O, respectively.41 In the ACFs-AO spectrum shown in Figure 2B, the C 1s peaks at 284.5, 285.2, 286.1, 286.7, and 288.2 eV were ascribed to carbon in C-C/C=C, C=N, C-N, C-O, and C=O, respectively. Additionally, the N 1s spectra of ACFs-DAMN and ACFs-AO are shown in Figure 2C and 2D. For the ACFs-DAMN (Figure 2C), the N 1s peaks at 399.9 and 399.0 eV44, 45 were assigned to nitrile nitrogen atoms and the nitrogen of the amide group (-NH-CO-) involved in the binding between the carboxyl group of the ACFs and the amino group of DAMN. Similarly, in Figure 2D, for ACFs-AO, the peaks occurring at 399.2 and 400.0 eV46 were ascribed to the carbon-bound species C-N and the nitrogen of the amidoxime group that was transformed from the cyano group. The O 1s spectrum of ACFs-DAMN shown in Figure 2E can be deconvoluted into three major 8


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components centered at 533.1, 531.9, and 530.9 eV, which were attributed to oxygen atoms in hydroxyl groups, C=O, and adsorbed O2-, respectively.35, 47 Figure 2F shows the O 1s peak of ACFs-AO; the binding energies of 533.1, 531.8, and 530.9 eV corresponded to the hydroxyl groups, C=O, and adsorbed O2-. The above results clearly confirmed that ACFs-AO were prepared successfully. The elemental content of the surfaces of the ACFs, ACFs-DAMN, and ACFs-AO were calculated using the ratio of the integrated peak areas in Figure 1. The results are listed in Table S1. In comparison with the ACFs, an obvious decrease in the carbon content was observed for ACFs-DAMN and ACFs-AO. Additionally, the nitrogen contents of ACFs-DAMN and ACFs-AO increased remarkably, indicating that functional groups were grafted onto the ACFs. It can also be seen that the nitrogen content was greater in ACFs-DAMN than ACFs-AO, which might be attributed to the transformation of the cyano group to an amidoxime group. Regardless of hydrogen, the amount of the cyano group on the surface of ACFs-DAMN and of the transformed amidoxime group on ACFs-AO were calculated from the increase in nitrogen content to be about 4.36 and 2.05 mmol/g, respectively.

Figure 1. XPS spectra of ACFs, ACFs-DAMN and ACFs-AO.

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Figure 2. High resolution XPS spectra of C 1s for (A) ACFs-DAMN and (B) ACFs-AO, N 1s for (C) ACFs-DAMN and (D) ACFs-AO and O 1s for (E) ACFs-DAMN and (F) ACFs-AO.

3.1.2. Elemental analysis To further confirm the carbon, nitrogen, and hydrogen contents in the pristine and modified ACFs, elemental analysis was carried out. According to the results shown in Table 1, negligible nitrogen content was detected for the ACFs; this could be attributed to N-containing impurities in the pristine ACFs. In comparison with the ACFs, an obvious increase of nitrogen content was observed for ACFs-DAMN. This indicated that cyano groups were grafted onto ACFs, in agreement with the previous XPS analysis. The nitrogen content of ACFs-AO was found to be somewhat decreased in comparison with ACFs-DAMN, which might be attributed to the 10


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transformation of the cyano group to the amidoxime group. This same trend was observed in the XPS analysis above. Similar to in the XPS analysis, the amount of C≡N on ACFs-DAMN and the amount of transformed C(NH2)=N-OH on ACFs-AO were calculated from the increase in the nitrogen content to be about 2.33 mmol/g and 1.31 mmol/g, respectively. The difference between the results of the XPS analysis and those of the elemental analysis indicated that most of the nitrogen-containing groups were directly anchored on the surface of the ACFs, suggesting that the surface chelating sites were easily accessible. Table 1. Ratio of elements for ACFs, ACFs-DAMN and ACFs-AO by elemental analysis. Sample

C Wt%

N Wt%

H Wt%

ACFs ACFs-DAMN ACFs-AO

84.89 65.60 61.35

0.68 7.21 6.19

1.561 2.609 2.643

3.1.3. SEM analysis The surface morphologies of the pristine and modified ACFs were characterized using FE-SEM. As shown in Figure 3, the ACFs were not damaged during the grafting process. Obvious differences among the surface morphologies of the ACFs, ACFs-DAMN, and ACFs-AO were observed. The surface of a single fiber in the ACFs was smooth (Figure 3A). However, as shown in Figure 3B, the surface of ACFs-DAMN was rougher and uneven, with many small knots and a coating covering the surface of the fiber, which was related to the modification by DAMN. Compared to the ACFs-DAMN surface, the surface of ACFs-AO (Figure 3C) became much rougher and much more coating on the fiber was observed due to the amidoximation. The chemical components of ACFs-AO were characterized using the EDX spectra. As shown in Figure 3D, the amount of the nitrogen in ACFs-AO determined using EDX was similar to the results from XPS and elemental analysis, suggesting the successful grafting of amidoxime groups onto the ACFs.

11



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Figure 3. FE-SEM images: (A) ACFs, (B) ACFs-DAMN, (C) ACFs-AO and EDX spectra (D): ACFs-AO.

3.1.4. Thermal analysis Thermal gravimetric analysis (TGA) was carried out to investigate the thermal stabilities of the as-prepared fibers. The TGA curves for the ACFs, ACFs-DAMN, and ACFs-AO are shown in Figure 4. Compared to those of the ACFs, the TGA curves of the ACFs-DAMN and ACFs-AO exhibited an obvious weight loss process from room temperature to 120 °C; this may be attributed to physically sorbed water, and possibly other residual solvents, on the surfaces of the fibers.48 At 150-400 °C, the further weight loss can be observed in the ACFs-DAMN and ACFs-AO curves (about 26.11 wt% for ACFs-DAMN and 23.97 wt% for ACFs-AO), which could be assigned to the decomposition of their functional groups. When the temperature was above 400 °C, the weight loss behavior of ACFs-DAMN and ACFs-AO were similar to that of the ACFs due to the decomposition of carbon in the pristine fiber. The TGA results confirmed that a large amount of functional groups were successfully grafted onto the ACFs.

12


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Figure 4. TGA profiles of ACFs, ACFs-DAMN and ACFs-AO.

3.1.5. BET study The BET surface area, pore volume, and pore diameter of the ACFs and ACFs-AO are listed in Table 2. Compared to the ACFs, the pore diameter of ACFs-AO was slightly increased. This increase could be attributed to the pores generated between the groups grafted on the surface. However, it was noteworthy that when the amidoxime groups were modified onto the ACFs, both the surface area and pore volume decreased drastically. This could be due to the coating of the grafted functional groups covering some of the micropores on the fibers. Table 2. Pore structure parameters of ACFs and ACFs-AO. Adsorbent

BET surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

ACFs ACFs-AO

1231 140

0.6090 0.0997

1.98 2.78

3.2. Adsorption behavior studies 3.2.1. Effect of pH The initial pH value of the solution had a significant influence on the sorption properties of the amidoxime-functional-group-modified adsorbent for the recovery of U(VI). Figure 5 shows the effect of different pH values on the uranium sorption by the ACFs and ACFs-AO. The efficiency of U(VI) removal from the solution by both adsorbents increased gradually as the pH value increased, and the removal efficiency of U(VI) on ACFs-AO was higher than that on the ACF. The lower sorption efficiency at low pH can be attributed to the strongly acidic environment. The greater concentration of H+ created stronger competition with the uranyl ions for the binding 13



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active sites on the adsorbents. The amidoxime functional groups were highly protonated, resulting in more positive charges on the adsorbent surface, which was unfavorable for the adsorption of the uranyl ions. When the initial pH values were increased, the amount of H+ decreased gradually, resulting in fewer cations that could compete with the uranyl ions in the adsorption process. Meanwhile, the degree of protonation of the amidoxime group was reduced. The hydroxyl proton on the oxime groups could be easily stripped off, allowing the lone-pair electrons of the negatively charged oxygen and the amino nitrogen to easily occupy the empty orbitals of the uranium atom, consequently promoting the formation of the complex.35, 49 Hence, the uranyl ions could be attached to the surface easily due to the formation of metal complexes by chelation. Furthermore, it is known that uranyl ions in aqueous solution can precipitate at higher pH values, according to the species distribution for U(VI) hydrolysis.50 It has been found that the free uranyl ion (UO22+) is the primary species at pH < 5.0. However, other uranium species, such as UO2OH+, (UO2)2(OH)22+, and (UO2)3(OH)5+, also exist, and become dominant species as the pH value increases. Furthermore, in the pH range of 6.0-8.0, schoepite ((UO2)8O2(OH)12·12H2O) is the main species.25, 51 Therefore, to eliminate the effect of hydrolysis at high pH values, the subsequent adsorption experiments were carried out at pH = 5.0. The changes in pH values after adsorption on ACFs-AO are shown in Table 3. The blank tests were performed in deionized water. The pHe value clearly increased when the initial pH was within the range of 4.0-5.0 due to the protonation of the amine groups to form -NH3+ groups. However, the pH values showed almost no change in uranyl nitrate solution or deionized water at low pH, because the protonation of the limited functional groups had little influence on the acidity of the solution. Compared to the pHe in deionized water, it was noteworthy that the pHe in uranyl nitrate solution showed a small decrease, which was attributed to the formation of metal complexes between the uranyl ions and amidoxime groups. This chelation could compete more strongly than the protonation reaction for access to the amidoxime groups on the adsorbents. Additionally, the pH change results after the absorption of 14


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U(VI) on the ACFs are shown in Table S2. The pH values all showed little decrease in the initial pH range of 1.0-5.0. This could be due to the exchange between the uranyl ions and H+, which was ionized from the carboxyl groups on the ACFs.

Figure 5. Effect of pH on U(VI) adsorption by ACFs and ACFs-AO. (C0 = 15.0 mg/L, t = 48 hrs, T = 298 ± 0.5 K, m/V = 10 mg/30 mL) Table 3. The changes in pH after the U(VI) adsorption on ACFs-AOa.

a

pH0

pHe (in uranyl nitrate solution)

pH0

pHe (in deionized water)

1.21 2.07 3.04 4.15 5.11

1.25 2.06 3.18 5.99 6.74

1.33 2.27 3.08 4.08 5.09

1.37 2.28 3.16 6.37 7.32

pH0: the initial pH value, pHe: the pH value after adsorption, m: 0.01 g, V: 30 mL, T = 298±0.5 K, t = 48 hrs.

3.2.2. Effect of contact time To study the sorption kinetics, the effect of contact time on the U(VI) sorption of the ACFs and ACFs-AO was investigated for contact times ranging from 10 min to 72 h. As shown in Figure 6, for ACFs-AO, the sorption amount increased rapidly from 10 min to 12 h, resulting from the strong chelation of U(VI) from the higher concentration uranyl ion solution during the primary adsorption process. However, after 12 h, the sorption rate decreased obviously due to the increase in the diffusive resistance and the decrease of sorption sites on the internal and external surfaces of the material during the process of the chelation of uranyl ions by the amidoxime groups. After 48 h, there was no obvious change to the sorption amounts with further increases in time, meaning that the adsorption process had reached the equilibrium state. Compared with ACFs-AO, the ACFs had a lower U(VI) adsorption capacity due 15



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to the limited amount of carboxyl groups as sorption sites, and the adsorption of uranium on the ACFs reached equilibrium after 24 h. It was worth noting that the longer equilibrium time and slower diffusion were attributed to the small specific surface area and internal microporous structures of ACFs-AO.49 Based on the sorption kinetics data, 48 h was selected as the contact time for reaching U(VI) sorption equilibrium for the following adsorption experiments.

Figure 6. Effect of contact time on U(VI) adsorption by ACFs and ACFs-AO. (C0 = 12.0 mg/L, pH = 5.0, T = 298 ± 0.5 K, m/V = 10 mg/30 mL)

To investigate the underlying sorption mechanism, the sorption kinetics of U(VI) on

the

ACFs

and

ACFs-AO

were

simulated

using

pseudo-first-order,

pseudo-second-order, and intra-particle diffusion models. The linear forms of these models can be respectively represented as follows: Pseudo-first-order model

ln(qe − qt ) = ln qe − k1t

(5)

Pseudo-second-order model

t 1 t = + 2 qt k2 qe qe

(6)

qt = kint t1/ 2 + θ

(7)

Intra-particle diffusion model

where qe (mg/g) and qt (mg/g) are the amounts of U(VI) adsorbed on per gram of ACFs-AO at equilibrium time and at any contact time t (h), respectively; k1 (1/h), k2 [g/(mg·h)] and kint [mg/(g·h1/2)] represent the pseudo-first-order rate constant, the pseudo-second-order rate constant and intra-particle diffusion rate constant, 16


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respectively; θ (mg/g) is a constant of the intra-particle diffusion model. The linear plots of the pseudo-first-order, pseudo-second-order, and intra-particle diffusion models, and the values of these kinetic parameters for the ACFs are shown in Figure S1, S2, and Table S3. For ACFs-AO, the kinetic parameters of the pseudo-first-order (Figure S3A) and pseudo-second-order (Figure S3B) models can be calculated easily from the slopes and intercepts of linear plots of ln(qe-qt) versus t, and t/qt versus t in Figure S3. The relevant kinetic parameters of these models for simulating the uranyl ion sorption data are listed in Table 4. The correlation coefficient (R2) of the pseudo-second-order model was higher than that of the pseudo-first-order model, indicating that the pseudo-second-order model was better able to describe the sorption behavior over the whole adsorption process. The pseudo-second-order model is based on the assumption that the rate-limiting step of the adsorption is a chemisorption process involving valence forces through the sharing or exchanging of electrons between the uranyl ions and the adsorbent.52 Moreover, the calculated qe value of the pseudo-second-order model was about 35.74 mg/g, which was similar to the experimental value of qe. Therefore, it can be inferred the sorption between the uranyl ions and ACFs-AO was mainly controlled by the chemical interaction in the adsorption process. An intra-particle diffusion model was tested to analyze the experimental kinetic data. In this model, if the plot of qt versus t1/2 is a straight line that passes through the origin, it indicates that the adsorption is controlled by intra-particle diffusion alone. If the data exhibit a multilinear plot, it implies that this process is influenced by several different steps.53 As shown in Figure 7, the adsorption plot consisted of three different portions with different slopes. The first, steeply sloped line from 0 to 12 h denoted the external surface mass transfer and instantaneous adsorption stage, including the removal of uranyl ions from the aqueous solution to the solid phase. The second, more gently sloping line from 12 to 48 h was attributed to the intra-particle diffusion, including the kinetic process of the uranyl ions reacting with the amidoxime groups on ACFs-AO. The third line represented the final equilibrium sorption stage. It can be concluded that the plot does not pass through the origin, suggesting that the 17



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intra-particle diffusion was not the only rate-limiting step.

Figure 7. Plots of U(VI) adsorption on ACFs-AO according to the intra-particle diffusion model. Table 4. Adsorption Kinetics Fitting Results for removal of U(VI) on ACFs-AO by Pseudo-First-Order, Pseudo-Second-Order, and Intraparticle Diffusion Models. Pseudo-First-Order Model qe(mg/g) 25.80

k1(1/h) 0.0743

R2 0.9385

Pseudo-second-Order Model qe(mg/g) 35.47

k2[g/(mg·h)] 0.0071

R2 0.9992

Intra-particle Diffusion Model θ(mg/g) 18.73

kint[mg/(g·h1/2)] 2.131

R2 0.9484

3.2.3. Effect of equilibrium uranium (VI) concentration To evaluate the sorption capacities of the ACFs and ACFs-AO, equilibrium adsorption experiments were performed in aqueous solutions with different initial uranium concentrations. As shown in Figure 8, the amount of uranium adsorbed on both the ACFs and ACFs-AO initially increased with increasing initial uranium concentration, but eventually reached a plateau, which meant that the sorption amount had reached the maximum sorption capacity of the sorbent. This was attributed to the greater chance for the adsorbent to be in contact with the uranyl ions at higher uranium concentrations, until the maximum sorption capacity was reached. In addition, it was obvious that the uranium sorption capacity of ACFs-AO was much higher than that of the ACFs, which was attributed to the grafted amidoxime groups. The distribution coefficient (Kd) is one of the main parameters for evaluating the adsorption performance of an adsorbent. Generally, a material with a large Kd value (greater than 104 mL/g) is considered to be an excellent adsorbent material.18, 54 As 18


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shown in Table S4, compared with the ACFs, ACFs-AO had consistently high values of 3.77×104-1.11×104 in uranium solutions with different initial concentrations. The uranium removal values and the distribution coefficient values clearly indicated that the ACFs-AO adsorbent had an excellent adsorption performance in uranium solution.

Figure 8. Effect of initial uranium concentration for the U(VI) adsorption on ACFs and ACFs-AO. (pH = 5.0, T = 298 ± 0.5 K, t = 48 hrs, m/V = 10 mg/30 mL)

To quantify the sorption capacity of the ACFs-AO adsorbent for uranyl ions, the Langmuir and Freundlich isotherm models were used to treat the adsorption data at different initial uranium concentrations. The Langmuir isotherm model assumes that sorption occurs on a homogenous surface by monolayer sorption with a finite number of homogeneous sites, and that there is no interaction between the adsorbates on adjacent sites. In contrast, the Freundlich isotherm model is based on the assumption that the uptake of metal ions occurs on heterogeneous surfaces and that there are several types of sorption active sites.38 The linear equations of the Langmuir and Freundlich isotherm models can be respectively expressed as

Ce C 1 = + e qe bqm qm

(8)

1 ln qe = ln K F + ln Ce n

(9)

where qm and b are the maximun sorption capacity (mg/g) and the Langmuir constant (L/mg), respectively. KF is the Freundlich constant (mg/g)(L/mg)1/n which is related to sorption capacity of the sorbent. And n is an empirical parameter related to the intensity of sorption. The fitting results for the ACFs and ACFs-AO are shown in Figure S4 and S5, 19



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Page 20 of 30

respectively, and the values of the correlation coefficients (R2) and corresponding parameters for uranium adsorption on the ACFs calculated from these models are listed in Table S5. In Table 5, for ACFs-AO, the correlation coefficient of the Langmuir model (R2 = 0.9982) was higher than that of the Freundlich model (R2 = 0.9593), indicating that the U(VI) adsorbed on the surfaces of ACFs-AO formed a homogenous and monolayer coverage. Moreover, the qm calculated using the Langmuir model was about 191.57 mg/g, which was compared to those of other similar uranium-selective sorbents that have been previously reported in Table S6. As shown in this table, the large sorption capacity of ACFs-AO indicated that it had great potential for applications in uranium removal and recovery from aqueous solutions. Table 5. Langmuir and Freundlich model fitting parameters for uranium adsorption on ACFs-AO. Langmuir parameters Isotherms parameters

Values

Freundlich parameters

qm (mg/g)

b (L/mg)

R2

KF (mg L1/n g-1)

n

R2

191.6

0.1996

0.9982

22.56

1.465

0.9593

1-1/n

3.2.4. Effect of temperature To investigate the effect of temperature on U(VI) adsorption by ACFs-AO, adsorption experiments were performed at 288, 298, 308, and 318 K. As shown in Figure 9, the amount of U(VI) sorption on ACFs-AO increased with increasing temperature at the equilibrium time. The results suggested that the amount of adsorbed U(VI) increased as the temperature was increased within the specified interval.

Figure 9. Effect of temperature on the U(VI) adsorption on ACFs-AO (C0 = 13.4 mg/L, pH = 5.0, t = 48 hrs, m/V = 10 mg/30 mL). 20


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The thermodynamic parameters of standard enthalpy change (∆H°), standard entropy change (∆S°) and standard Gibbs free energy change (∆G°), can be calculated from the temperature dependence of the sorption amounts. Standard enthalpy change (∆H°) and standard entropy change (∆S°) were calculated from the slope and intercept, of a plot of ln Kd versus 1/T (Figure S6), using the Van’t Hoff equation (Eq. 10). The Gibbs free energy change (∆G°) was obtained using the Gibbs-Helmholtz equation (Eq. 11). ln K d = −

∆H ο ∆S ο + RT R

(10)

∆G ο = ∆H ο − T ∆S ο

(11)

where Kd is the distribution coefficient and R is the ideal gas constant [8.314 J/(mol·K)]. The calculated values of ∆H°, ∆S°, and ∆G° are listed in Table 6. The positive value of the standard enthalpy change (∆H° = 59.27 kJ/mol) indicated that the sorption of U(VI) on ACFs-AO was an endothermic process in nature, which agreed with the observation that the adsorbed amount increased with increasing temperature. The positive value of the standard entropy change (∆S° = 285.08 J/K·mol) demonstrated the increased randomness at the adsorbent-solution interface during the U(VI) adsorption process for ACFs-AO. In addition, the Gibbs free energy changes were all negative, suggesting that the sorption of U(VI) on ACFs-AO was a spontaneous process under all the conditions studied. The values of ∆G° became even more negative at increased temperature, implying that the adsorption process was more favorable at higher temperatures. Table 6. Thermodynamic Parameters for the U (VI) Adsorption on ACFs-AO. ∆H° (kJ/mol)

∆S° (J/K·mol)

59.27

285.08

∆G° (kJ/mol) 288 K -22.83

298 K -25.68

308 K -28.53

318 K -31.39

3.2.5. Selective sorption of ACFs-AO towards uranium To investigate the sorption selectivity of both the ACFs and ACFs-AO towards uranium, an adsorption experiment was carried out in a uranium solution containing some representative alkaline earth metal and transition metal ions. The distribution 21



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Page 22 of 30

coefficient (Kd) and the selective adsorption coefficient (KU/M) were calculated from Eq. 3 and Eq. 12, respectively. K U/M =

K d(U) K d(M)

(12)

where Kd(U) and Kd(M) represent the distribution coefficient of uranium and the distribution coefficient of other metal ions, respectively. The results are shown in Figure 10. The chemical modification led to a distinct increase in the U(VI) sorption capacity. The sorption amount of U(VI) on ACFs-AO was obviously higher than that of the ACFs. The sorption of U(VI) was also much higher than those of other competitive cations on ACFs-AO, which indicated the excellent selective sorption property of the ACFs-AO. The outstanding selectivity of ACFs-AO compared with the selectivity of the ACFs towards uranium was mainly attributed to the coordination ability of the amidoxime group for uranyl ions. The average U-N distances were shorter than the corresponding distances for the other competitive cations, which led to the strong selectivity of amidoxime groups towards uranyl ions.26 As shown in Table S7, the value of the distribution coefficient (Kd) of ACFs-AO towards U(VI) reached up to 19980 mL/g in the solution containing competitive ions, which was greater than for any of the other cations. Thus, it was clearly demonstrated that the KU/M values were greater than 1. Therefore, ACFs-AO could be a promising adsorbent for the selective sorption of uranyl ions from an aqueous solution containing several competitive ions.

Figure 10. Competitive sorption capacities of coexistent ions on ACFs and ACFs-AO (C0 = 8.0 mg/L, pH = 5.0, t = 48 hrs, m/V = 10 mg/30 mL).

3.2.6. Regeneration and stability studies 22


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Regeneration and stability are important factors for an effective and economical adsorbent in practical application. The regeneration and reusability of ACFs-AO were investigated to evaluate their suitability for use in practical U(VI) sorption applications. In this work, 1.0 mol/L HNO3 was chosen as the desorbing agent to regenerate ACFs-AO because that the desorption percentage could reach up to more than 95.0% when 1.0 mol/L HNO3 was used. The adsorption–desorption cycle was repeated four times to investigate the regeneration properties, and the results are shown in Figure 11. After four cycles, only a slight decrease in the adsorption efficiency in the U(VI) sorption process was found. The results indicated that ACFs-AO could be efficiently regenerated using 1.0 mol/L HNO3, and they support the long-term use of ACFs-AO for the removal of U(VI) from large volumes of aqueous solutions.

Figure 11. Recycling of ACFs-AO in the adsorption of U(VI) (C0 = 13.4 mg/L, pH = 5.0, t = 48 hrs, m/V = 10 mg/30 mL).

3.2.7. Proposed sorption mechanism XPS analysis can provide information about the oxidation states, chemical compositions, relative concentrations, and bonding relationships of all surface and near-surface elements. Thus, it is an efficient method for exploring the mechanism of interaction between the adsorbate and adsorbent.55 To further investigate the mechanism of interaction in the sorption between ACFs-AO and uranyl ions, the XPS survey spectra and high-resolution scans of ACFs-AO after U(VI) sorption (ACFs-AO-U(VI)) for N 1s and O 1s were recorded. In Figure 12A, unlike for bare ACFs-AO, the peak of U 4f can be clearly observed, which indicates that U(VI) was adsorbed onto the surface of ACFs-AO successfully. The detailed spectrum of the N 23



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1s signal for ACFs-AO-U(VI) is shown in Figure 12B, and the peak was deconvoluted into two peaks. They were very similar to the peaks in the spectrum of ACFs-AO, which were analyzed in section 3.1.1. However, compared to the corresponding peaks of ACFs-AO, it was found that the two peaks of ACFs-AO-U(VI) corresponding to the -C-NH2 and -C=N bonds were shifted to a higher binding energy, suggesting that the electron density of the nitrogen atoms decreased. The result demonstrated that U(VI) can interact with the -NH2 and -C=NOH groups simultaneously. Analogously, as shown in Figure 12C, the terminal OH peak of ACFs-AO-U(VI) was also shifted to a higher binding energy, while the other peaks remained almost unchanged, indicating that U(VI) can also interact with the hydroxyl group. Based on the comparison of the XPS spectra of ACFs-AO before and after the adsorption of U(VI), it can be concluded that the uranyl ions were adsorbed onto ACFs-AO through the complexation of U(VI) with both the amine and hydroxyl groups of the amidoxime groups.

Figure 12. (A) The typical XPS survey spectra of ACFs-AO and ACFs-AO-U(VI). High resolution XPS spectra of (B) N 1s and (C) O 1s for ACFs-AO and ACFs-AO-U(VI), respectively.

4. Conclusion 24


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In this work, a novel and efficient U(VI) sorbent, amidoxime grafted activated carbon fibers (ACFs-AO) was prepared using a chemical grafting method. The effects of pH, contact time, initial uranium (VI) concentration, temperature, and the presence of competing ions on the sorption behavior of the adsorbent were investigated. It was found that, compared to raw activated carbon fibers (ACFs), the capacity of the prepared ACFs-AO towards U(VI) was enhanced significantly due to the strong chelation of the amidoxime group to U(VI), and that ACFs-AO had desirable distribution coefficient values (Kd) for uranium. The sorption results revealed that the U(VI)

adsorption

process

on

ACFs-AO

was

modelled

well

using

the

pseudo-second-order model and Langmuir model. The thermodynamic study suggested that the sorption process was spontaneous, feasible, and endothermic. In addition, ACFs-AO exhibited excellent selectivity in an aqueous solution containing competitive ions, and was regenerated and reused with high sorption efficiency for U(VI) sorption. The results indicated that ACFs-AO could be a promising candidate for the selective removal and recovery of U(VI) from aqueous solution.

ASSOCIATED CONTENT Supporting Information This Supporting Information is available free of charge on the ACS Publication website. Ratio of elements for ACFs, ACFs-DAMN and ACFs-AO by XPS, The pH changes after the U(VI) adsorption on ACFs, three kinds of kinetic models and parameters for removal of U(VI) on ACFs, pseudo-first-order and pseudo-second-order kinetic models for removal of U(VI) on ACFs-AO, effect of Initial Uranium concentration on U(VI) Adsorption by ACFs and ACFs-AO, Langmuir and Freundlich model fitting plots and parameters for uranium adsorption on ACFs, comparison of sorption capacity of U(VI) on various uranium sorbents, plot of lnKd versus 1/T for the U(VI) adsorption on ACFs-AO, distribution ratios and selectivity coefficients of ACFs-AO.

AUTHOR INFORMATION 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Corresponding Author * Corresponding Author at: School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 102488, People’s Republic of China. Tel.: +86-010-68941331; fax: +86-010-68914503; e-mail address: [email protected].

ORCID Dongxiang Zhang: 0000-0002-5159-1184

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the International Science & Technology Cooperation Program of China (2014DFR61080).

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