Guoqing
Huang‡
ab,
Wenting
Li‡
a,
Qi
Liu
a,
Jingyuan
Liu
a,
Hongsen
Zhang
c,
Rumin
Li
*ad,
Zhanshuang
Li
a,
Xiaoyan
Jing
a and
Jun
Wang
*ad
aKey Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin, 150001, China. E-mail: zhqw1888@sohu.com; Fax: +86 451 8253 3026; Tel: +86 451 8253 3026
bHandan Purification Equipment Research Institute, Handan, 056027, P. R. China
cModern Analysis, Test and Research Center, Heilongjiang University of Science and Technology, Harbin 150027, P. R. China
dInstitute of Advanced Marine Materials, Harbin Engineering University, 150001, China
First published on 20th November 2017
Capturing uranium(VI) from seawater is of paramount interest for the rapid expansion of energy needs. We report on materials covalently functionalized with amine groups to improve gas adsorption and remove metal ions. In this paper, polyacrylonitrile fibers (PANFs) covalently modified with hyperbranched polyethylenimine (HPEI) of different grafting degrees were facilely synthesized, and their performance to capture U(VI) in the presence of competitive ions and simulated seawater evaluated. In addition, the adsorption properties of the as-prepared adsorbents, including thermodynamics, kinetics and the influence of critical factors affecting U(VI) adsorption from aqueous solution were examined. The fibers with the best performance were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy and Fourier transformed infrared spectroscopy. The high density of amine groups on the modified fiber surface improves the adsorption ability by electrostatic attraction and surface interaction during the reaction process. From our findings, PAN–HPEI offers wide-ranging potential in the wide field of the removal of metal ions, and, in particular, the removal of U(VI) from seawater with high throughput.
In recent years, extensive effort has been dedicated to this research field and many adsorbents have been reported for U(VI) recovery from simulated seawater or seawater. Wang et al. prepared acylamide- and carboxyl-functionalized metal–organic frameworks to extract U(VI) from seawater.7 Yamazaki et al. synthesized novel phenol-type resins embedded in highly-porous silica beads to explore the adsorption behavior in simulated seawater and seawater.8 Górka et al. investigated sonochemical functionalized mesoporous carbon to extract uranium from seawater.9 Among the potential materials, fabric and fiber adsorbents are being comprehensively researched due to their low cost and feasibility for industrialization. Dietz et al. prepared polyamide 6 fabrics modified with diallyl oxalate to remove U(VI) from seawater.10 Xie et al. designed an electrospun nanofibrous adsorbent for U(VI) extraction from seawater.11 Ladshaw et al. studied amidoxime functionalized fibers to extract U(VI) in the presence of other ions in simulated seawater.12 Herein, we use a commercial product of polyacrylonitrile fibers (PANFs) because they possesses good mechanical strength and stability, and are resistant to corrosion under concentrated salt conditions;13 the fibers have been used for the removal of heavy metal ions. To activate a tight binding with metal ions, the conversion of nitriles to carboxylic groups was necessary for the grafting of various functional moieties14 containing N and O and other atoms (amide, carboxyl, aminoxime, etc.) that have been reported to interact with U(VI) easily.
Hyperbranched polyethyleneimine (HPEI), which is believed to bind metal ions tightly owing to the substantial primary and secondary amine groups per molecule and its non-reaction with transitional metal ions, is commonly used to functionalize the surface of adsorbents to improve the adsorption ability.15,16 Recently, PEI-based adsorbents have been extensively investigated for metal binding. Tian et al. prepared PEI modified halloysite nanotubes for the adsorption of Cr(VI), in which the uptake capacity was sixty-four times higher than those of the raw materials.17 Lindén fabricated cross-linked PEI coatings for copper adsorption from seawater.18
In the present work, PANF modified with HPEI (PAN–HPEI) with different grafting degrees was facilely synthesized using a hydrothermal method. First, we explore the adsorption behavior of the obtained fibrous materials in the presence of several competing cations, and investigate the adsorption performance of the adsorbents to remove U(VI) from aqueous solution and simulated seawater; we ascertain the adsorbent with the best grafting degree. In addition, we study the parameters affecting uranium adsorption, such as pH, time and concentration, and the kinetic models and thermodynamics, to further understand the process. Finally, we propose hypothetical adsorption mechanisms.
The sea salt was purchased from Haiyang Guangzhou aquarium Technology Co., Ltd. The HNO3 (UPS, 68%) was obtained from Suzhou Crystal clear chemical Co., Ltd. Na2EDTA (AR), NaOH (AR), HCl (AR, 36%), NaHCO3 (AR), and Na2SO4 (AR) were acquired from Tianjin Dongli chemical Co., Ltd.
The water used was deionized.
Scheme 1 Synthesis routine of PAN–HPEI based on PAN: (a) in the structural formula of the molecule of which the red circle is HPEI; (b) in the stick type; (c) in 3D representation. |
First, dried PAN (1.0 g), NaOH (10 g), and deionized water (100 mL) were added to a conical flask and stirred in a bath oscillator at 80 °C for 15 min. Then the hydrolyzed adsorbent was immersed in HCl until the supernatant was neutral, followed by drying at 60 °C under vacuum overnight to give PAN–COOH.
Quintessentially, the as-prepared fibers and HPEI (mass ratio of the fiber to HPEI was 1:3) were put into a 100 mL Teflon-lined autoclave. Ample de-ionized water was poured into the mixture and stirred for 2 h to make sure that all HPEI was dissolved. Then the autoclave was put into an oven at 140 °C for a specified time to obtain PAN–HPEI with different grafting degrees. After cooling, the fibers were separated and washed with warm water until the wash water was neutral, followed by drying at 60 °C under vacuum overnight.
The grafting degrees (GDs) of HPEI onto the PANFs were assessed by the mass increase of the raw fibres from the following eqn (1):19
(1) |
Then adsorption tests in simulated seawater were carried out for 48 h to determine the adsorption performance. Simulated seawater was prepared according to a previous specification in the literature20 with a minor change. By dissolving 2.11 g uranyl nitrate and 33 g sea salt in 1 L DI water, artificial seawater with a concentration of 1000 mg L−1 was synthesized. Herein, the as-prepared seawater was diluted to the μg L−1 level ranging from 3 to 30 μg L−1 to mimic the seawater environment.
To further study the adsorption process, batch experiments were implemented. Classically, U(VI) removal was executed in a Erlenmeyer flask (100 mL) in which a specified dosage of the fibres was vibrated with U(VI)-containing solution (50 mL) of an assigned pH and concentration in a thermostatic oscillator at a speed of 150 rpm. The solution pH was monitored with trace volumes of 0.5 M HNO3 or NaOH solution at first. After the fibrous material separated, the concentration of U(VI) in the supernatant was measured for further calculation.
The loading capacity Qe (mg g−1), solid phase distribution coefficient Kd (mL g−1), and removal rate R (%) were determined on the basis of eqn (2)–(4):21,22
(2) |
(3) |
(4) |
The concentration of U(VI) at mg L−1 or μg L−1 level was determined using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) with an IRIS Intrepid II XSP instrument, or using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) with a X SeriesII instrument. All the experiments mentioned above were carried out in triplicate, and the reported results were the mean values of three data sets.
Fig. 1 Grafting degree of HPEI to PAN influenced by reaction time at 140 °C with a mass ratio of 3:1. |
As an adsorbent material, hydrophilicity and porosity are paramount factors that influence the adsorption performance of the absorbents and directly relate to the quantity of ions in the water that contacts the functional groups. Taking the practical application into account, the mechanical properties are also very important. Herein, the hydrophilicity, porosity, and mechanical properties have been explored11 and displayed in the ESI.†
Fig. 2 (A) Adsorption capacity and (B) removal rate and Kd for uranium removal with different grafting degrees of PAN–HPEI in the presence of several competing ions. |
Fig. 3 Removal of U(VI) by PAN–HPEI with different grafting degrees under simulated seawater. (V = 50 mL, T = 298 K, m = 0.02 g). |
Fig. 4 Effect of pH on the adsorption of U(VI). (C0 = 200 mg L−1, V = 50 mL, T = 298 K, m = 0.02 g). |
When the pH is under 5.0, UO22+ is the main form of U(VI) existing in the solution. The adsorption process between U(VI) ions and PAN–HPEI is restricted by electrostatic repulsion and the effective competition of H+ on the binding sites.27 With a pH higher than 5.0 and electrostatic repulsion decreasing the coordinating ability of the functional ligands (such as amine, imine, and carbonyl) is promoted,28 chemical complexation takes a major part in the adsorption process. In the pH range of 6.0–8.0, uranium combined with hydroxide and formed different complexes such as UO2(OH)3− and (UO2)3(OH)7−. Meanwhile, it is observed that the ζ-potential value was negative from 6 to 8, leading to electrostatic repulsion between these ions and the adsorbent surface. Accordingly, the optimum pH during the adsorption process is 6.0, which we select for further experiments.
Fig. 5 Effect of contact time on the adsorption of U(VI). (C0 = 200 mg L−1, pH = 6, V = 50 mL, T = 298 K, m = 0.02 g). |
To deeply understand the physical chemistry process and to establish the kinetics, we investigated the kinetic mechanism of the process. The kinetics data were tested and modeled with pseudo-first order and pseudo-second order rate models, and the intra-particle diffusion model.29,30 The details of these equations are displayed in the ESI.†
The kinetic parameters computed from the above models are listed in Table 1, from which we see that the calculated Qe value in pseudo-second order kinetics is close to the experimental data with a regression coefficient of 0.9772, which verifies that the extraction of uranyl ions onto PAN–HPEI is more accurately depicted by a pseudo-second order equation. These results are suggestive that a pseudo-second order model is the uppermost mechanism and the calculated rate constant seems to be governed by a chemisorption process, which implies that chemical adsorption occurs involving electronic sharing and transfer between PAN–HPEI and U(VI) ions.
Kinetic model | T (°C) | C 0 (mg L−1) | Q expe (mg g−1) | Q cale (mg g−1) | k 1 (min−1)/k2 (g (mg min)−1) | R 2 |
---|---|---|---|---|---|---|
Pseudo-first-order | 25 | 200 | 475 | 441.6 | 1.1518 | 0.9314 |
Pseudo-second-order | 25 | 200 | 475 | 463.3 | 0.0038 | 0.9772 |
Intra-particle diffusion | 25 | 200 | 475 | 471.0 | 33.30/0.52 | 0.9622/0.9953 |
As depicted in Fig. 5B, the plot of Qtversus t1/2 presents two straight lines, showing that the adsorption process of uranyl on PAN–HPEI includes two stages. The removal rate is fastest in the initial twenty minutes, in which the uranium ions move toward the surface of PAN–HPEI with a k3 value of 33.30 mg g−1 min−0.5. For the second stage, the fitted curve exhibits a comparatively low slope by virtue of a decrease in available active sites, implying an approaching adsorption equilibrium state, which further indicates that the adsorption on PAN–HPEI is via chemical bonding between the active sites on the surface of PAN–HPEI and U(VI).
The interaction between uranyl and active sites on the surface of PAN–HPEI may be interpreted as a chelation mechanism. For the design and manipulation of extraction systems, coefficient of equilibrium data is essential. Langmuir, Freundlich, Sips and D–R isotherm models were used for the acquired adsorption values to verify the adsorption parameters that drive the adsorption process. The details of these equations are explored in the ESI,† while the results are displayed in Fig. 6A and D. Fig. 6B depicts the relationship of θ, RL and initial concentration of U(VI) ions. Evidently, the adsorption process proceeds fast in the initial stage (low coverage of the fiber surface and plenty of free active sites are available for binding with U(VI) ions) and then tends toward a plateau at a higher surface coverage where most of the active sites are occupied. This reveals the applicability of the Langmuir model to describe the adsorption of uranyl onto PAN–HPEI.
The thermodynamics parameters as well as the correlation coefficients are listed in Table 2, which indicates that the Sips model better fits the experimental results over the experimental range, with high correlation coefficients (>0.97). The Sips model shows that the extraction process is a union of Langmuir and Freundlich models. The maximum loading capacity of PAN–HPEI is determined as 481.26 mg g−1 at 298 K.
Isotherm parameters | 298 K | 308 K | 318 K |
---|---|---|---|
Langmuir isotherm | |||
q m (mg U g−1) | 470.67 | 476.49 | 486.87 |
K L (L mg−1) | 0.1669 | 0.2639 | 0.4237 |
R 2 | 0.9407 | 0.9160 | 0.8724 |
Freundlich isotherm | |||
K F (L g−1) | 131.85 | 147.36 | 164.74 |
N | 0.2561 | 0.2451 | 0.2344 |
R 2 | 0.9085 | 0.9218 | 0.9171 |
Dubinin–Radushkevich (D–R) | |||
B | 6.97 × 10−7 | 4.24 × 10−7 | 2.61 × 10−7 |
Q DR | 388.23 | 424.02 | 449.78 |
E | 846.94 | 1086.16 | 1383.27 |
R 2 | 0.8677 | 0.9199 | 0.8976 |
Langmuir–Freundlich (Sips) | |||
Q s | 481.26 | 510.18 | 574.63 |
K s | 0.2009 | 0.3009 | 0.3730 |
M | 0.0617 | 0.1599 | 0.3718 |
R 2 | 0.9733 | 0.9663 | 0.9366 |
The thermodynamic parameters, enthalpy (ΔH°) and entropy (ΔS°) changes and Gibbs free energy (ΔG°) were also computed and the formulae were listed in the ESI.† The results of ΔS°and ΔH° are appraised from the slope and intercept of the fitting line of ln KDvs. 1/T (Fig. 7). Positive standard entropy (ΔS°) implies that randomness increases at the fiber/solution interface in the course of extraction, which is in virtue of the liberation of free water molecules in the chelation sphere as a consequence of chelation of uranyl at the carboxyl and amine-active sites. A positive value of ΔH° suggests that the essence of the extraction process of UO22+ is endothermic and the binding of U(VI) ions increases as the temperature rises.
ΔG° values at diverse temperatures are also attained. The values of ΔH°, ΔS°, and ΔG° are listed in Table 3. Minus values of ΔG° manifest that the process accords with a spontaneous and feasible tendency. ΔG°, in the range of −25.063 and −23.486, reduces with increase in temperature, which implies that higher temperatures facilitate adsorption of U(VI) ions onto PAN–HPEI due to a greater driving force.
ΔH° (kJ mol−1) | ΔS° (J mol−1 K−1) | ΔG° (kJ mol−1) | ||
---|---|---|---|---|
4.357 | 78.83 | 298 K | 308 K | 318 K |
−23.486 | −24.275 | −25.063 |
Fig. 8 (A) Desorption efficiency for the removal of U(VI) by different desorption agents. (B) Adsorption–desorption cycles of U(VI) from aqueous solutions. |
To estimate the cyclic utilization of the fiber, an adsorption–desorption cycle test with 0.5 M HCl was carried out five times, which was shown in Fig. 8B. After five circulations, the elution efficiency of the PAN–HPEI is still more than 82%, which suggests that the fiber can be utilized availably in a real process, for instance, extraction of uranyl from large volumes of aqueous solutions.
Fig. 9 FT-IR spectra of (A) raw PAN, (B) PAN–HPEI0.33, (C) PAN–HPEI0.33Ad-U, and (D) PAN–HPEI0.33De-U. |
The surface morphologies of (A) PAN–HPEI0.33, (B) PAN–HPEI0.33Ad-U, and (C) PAN–HPEI0.33De-U were observed by SEM and are recorded in Fig. 10. Fig. S2 (ESI†) presents the SEM images of raw PAN and PAN–HPEI with different grafting degrees. We observe that the surface of raw PAN is somewhat smooth. After U(VI) adsorption, however, the surface roughens with a covering of a dense layer of small particles, whereas, after desorption, the fiber body restores its smoothness. In the interest of evaluating the change of surface elemental composition, the EDS spectra of (D) PAN–HPEI0.33, (E) PAN–HPEI0.33Ad-U, and (F) PAN–HPEI0.33De-U were also recorded (Fig. 10). The presence of U(VI) with a wt(%) of 9.45 on the surface of the fiber dramatically changes the morphology of PAN–HPEI0.33, confirming the adsorption of U(VI) onto the fiber surface. After the desorption, the wt(%) of U(VI) decreases to 0.48, signifying an incomplete removal of U(VI), which is agreement with the results of desorption experiments and FT-IR.
Fig. 10 SEM images and EDS spectra of (A) and (D) PAN–HPEI0.33, (B) and (E) PAN–HPEI0.33Ad-U, and (C) and (F) PAN–HPEI0.33 De-U. |
To ascertain the surface chemical composition and bonding environment after U(VI) adsorption and desorption, the XPS spectra of (a) PAN–HPEI0.33, (b) PAN–HPEI0.33Ad-U, and (c) PAN–HPEI0.33De-U were recorded (Fig. 11). The results assist in verifying the binding mechanisms of PAN–HPEI and U(VI) ions. Compared with the survey spectra of PAN–HPEI0.33 (Fig. 11A), the spectra of PAN–HPEI0.33-U obviously show a new and strong double U 4f peak, and a corresponding high resolution of U 4f (Fig. 11B), characterized with U 4f5/2 (∼392 eV) and U 4f7/2 (∼382 eV) core-level spectra, disclosing the existence of U(VI) in PAN–HPEI0.33Ad-U,34 respectively. Meanwhile, we observe pronounced shifts of N 1s and O 1s (Fig. 11C and D), which indicate chemical complexing reactions between nitrogen and oxygen atoms and U(VI) ions, respectively. Before the adsorption of U(VI) the N 1s spectra can be fitted to two peaks as seen on Curve a in Fig. 11C, of which peaks 1 and 2 at binding energies of 399.6 and 398.4 eV are ascribed to NH2 or NH groups and CN,35 respectively. After the extraction of uranyl, a new peak appears at 402.1 eV (Curve b in Fig. 11C), while peak 1 experiences a positive shift of 0.3 eV and does not change, which further demonstrates that uranyl ions and amine groups chemically interact.36 After desorption, peak 1 and peak 2 return to their original positions. Before the adsorption of U(VI), the O 1s spectra can be fitted to two peaks as seen on Curve a in Fig. 11D, of which peaks 1 and 2 at 399.60 and 398.40 eV belong to –OH and CO,20,37 respectively. After the adsorption of U(VI) (Curve b in Fig. 11D), peak 1 encounters a negative shift of 0.2 eV and peak 2 experiences a negative shift as much as 0.8 eV, both of which indicate chemical combinations between uranyl ions and oxygen-containing functional groups on PAN–HPEI.38 After the desorption of U(VI), peak 1 and peak 2 undergo a positive shift to almost their original positions.
In addition, the FT-IR and XPS spectra of PAN–HPEI and PAN–HPEI0.33Ad-U were recorded to better understand the mechanism for the adsorption of U(VI) onto PAN–HPEI. Comparing the FT-IR spectra of PAN–HPEI and PAN–HPEI0.33Ad-U it is evident that PAN–HPEI extracts uranyl ions primarily through chemical bonding. Of more importance, the XPS spectra of PAN–HPEI and PAN–HPEI0.33Ad-U strongly confirm the existence of complexation between U(VI) ions and oxygen and nitrogen ligands onto PAN–HPEI.4 Combined with the result of ζ-potential, an electrostatic attraction exists with pH < 5.8, and chemical bonding most likely plays a leading role with partial electrostatic attraction, as displayed in Scheme 2.
Scheme 2 Schematic diagram of the proposed mechanism for the adsorption of uranyl ions onto PAN–HPEI fibers. |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj03243a |
‡ These authors contributed to the work equally and should be regarded as co-first authors. |
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