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Rapid and effective removal of uranium (VI) from aqueous solution by facile synthesized hierarchical hollow hydroxyapatite microspheres
Journal of Hazardous Materials 371 (2019) 397–405
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Rapid and effective removal of uranium (VI) from aqueous solution by facile synthesized hierarchical hollow hydroxyapatite microspheres Yanhong Wua, Diyun Chena, , Lingjun Konga, Daniel C.W. Tsangb, Minhua Sua,b, ⁎
T
⁎⁎
a
Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources, School of Environmental Science and Engineering, Guangzhou University, Guangzhou, 510006, China b Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Uranium (VI) adsorption Hollow hydroxyapatite Phosphate precipitation Sustainable remediation Radioactive wastewater treatment
Rapidly increasing development of nuclear power stimulates the exploration of low-cost and highly efficient materials to selectively remove uranium (VI) from contaminated wastewater streams. Herein, we successfully developed a novel hydroxyapatite (HAP) adsorbent by using a facile and template-free hydrothermal method. The XRD results demonstrated that the HAP was crystallized in hexagonal structure (space group P63/m(176)), and the images of SEM and TEM indicated that the HAP possessed hollow and hierarchical nanostructure. A large BET specific surface area (182.6 m2/g) and average pore size of 10.5 nm, suggested that the hierarchical hollow HAP microspheres could provide sufficient active sites for highly efficient removal of uranium from aqueous solutions, indicated the HAP might be a prompt emergency material for the remediation of nuclear leakage accident. Freundlich isotherm and pseudo-second-order kinetics model fitted well to sorption experimental data. The study was further advanced by FT-IR and XPS. The sorption mechanism was mainly attributed to surface chemisorption between U(VI) and HAP, forming a new U-containing compound, viz., autunite (Ca (UO2)2(PO4)2·3H2O).
Corresponding author. Corresponding author at: Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources, School of Environmental Science and Engineering, Guangzhou University, Guangzhou, 510006, China. E-mail addresses: [email protected] (D. Chen), [email protected] (M. Su). ⁎
⁎⁎
https://doi.org/10.1016/j.jhazmat.2019.02.110 Received 15 October 2018; Received in revised form 27 February 2019; Accepted 28 February 2019 Available online 01 March 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 371 (2019) 397–405
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1. Introduction
as reaction medium and poly aspartic acid (PASP) as both chelating and capping agents. PASP is a potential poly carboxylic sequestrant that is water soluble, nontoxic, highly biodegradable, and free of secondary pollution to the environment [36]. With the addition of PASP, the crystal structure can be tailored and the growth of crystal structure can be oriented. The presence of PASP is also known to facilitate the transition of amorphous to crystalline phases [37]. In our proposed synthetic route, via coordination bonding with Ca2+ ions, PASP acts as the nucleation centers for the newly formed amorphous calcium phosphate particles. Besides, PASP as a capping agent, through interacting with the surface of the HAP crystallite, can result in a number of nanorods and form the hydroxyapatite microspheres with a hierarchical hollow structure. The final product is supposed to be capable of achieving fast reaction and high adsorption capacity for U(VI) removal. The aim of this study was to: (i) synthesize hierarchical hollow hydroxyapatite microspheres via a green route; (ii) evaluate its adsorption performance for U(VI) removal; and (iii) investigate its adsorption mechanisms towards U(VI) removal. The processing parameters including solution pH, contact time, adsorbent dosage, initial U (VI) concentration, and temperature were well investigated. Equilibrium, kinetics, and thermodynamic studies were conducted to unravel the U(VI) adsorption behavior by hierarchical hollow hydroxyapatite microspheres.
To meet the increase of worldwide energy demand, nuclear energy has been rapidly developed in recent years in view of its environmentally friendliness and high efficiency [1]. Uranium (VI), the most important resource for nuclear power generation, is often released into the environment during the nuclear-related activities such as nuclear fuel processing, uranium mining, nuclear accidents, etc. [2]. The U(VI) is radioactive, toxic, and highly mobile. Moreover, its half-life is long, and the excessive release of U(VI) poses severe and long-term risks to the environment and human [3]. The accident at the Fukushima Daiichi Nuclear Power Plant in 2011 following the Great East Japan Earthquake resulted in the discharge of a large amount of radioactive wastewater in which uranium is the main ingredient [4]. The Chernobyl nuclear accident occurred in 1986 in Ukraine, whose environmental and radiological consequences exceeded those of the Fukushima accident [5]. Therefore, prompt and reliable elimination of U(VI) from contaminated waste streams is urgent and necessary. So far, emergency materials towards nuclear wastewater leakage accident are still under development, conventional techniques including precipitation [6], solvent extraction [7], ion exchange [8], membrane separation [9], and adsorption [10], etc., have been developed and applied to remove U(VI) from the wastewater stream. Of them, adsorption has been demonstrated to be one of the most prevalent and efficient approaches for U (VI) removal owing to its unique advantages of low-cost, simplicity, and versatility [11–13]. To achieve high efficiency and capacity of adsorption process, the nature of sorbent is a critical factor and the development of highly efficient sorbent is of great importance. Various sorbents have been explored to adsorb U(VI), such as chitosan [14], polypyrrole [15], hematite [16], zeolite [17], olive stones activated with ZnCl2 [18], alumina (Al2O3) [19], and so on. Among them, phosphate rock apatite has a great scientific and economic significance due to their naturally occurring nature, low-cost, and ease of use [20]. Apatite-like minerals can also be synthetically produced via precipitation reaction of calcium and phosphate to obtain various products with desired properties [21]. Because of its particular chemical composition and crystal structure, phosphate rock apatite has become one of the most promising materials for the treatment of wastewater and soil [22]. Our previous work has demonstrated that U(VI) can be adsorbed by phosphate rock apatite and chitosan modified phosphate rock [23,24]. However, the U(VI) adsorption efficacy and capacity is low and insufficient for use as an emergency material in the nuclear wastewater leakage accident. As a member of apatite mineral family, hydroxyapatite (HAP) is an effective material for the long-term containment of contaminants due to its high sorption capacity via forming mineral products with high stability and low solubility. Moreover, HAP is lowcost and easily attainable [25–27]. He et al. [28] prepared ultralong HAP nanowires by solvothermal method using oleic acid as a soft template to treat fluoride-contaminated water. Hou et al. [29] synthesized HAP via co-precipitation method for the removal of Congo red dye from aqueous solution. Samant et al. [30] obtained HAP nanocrystals from limacine artica shells by second precipitation method to removal fluoride. Han et al. [31] prepared Bio-HAP from freshwater grass carp bones by calcination method for uranium immobilization. Currently, hydroxyapatite mineral-like synthetic material is mainly used in bone remodeling, bone repair, and other medical fields [32,33]. The biomimetic preparation of hydroxyapatite is mostly based on the development of bone materials [34,35]. According to our best knowledge, there is no study about uranium removal using hierarchical hollow hydroxyapatite microspheres. Thus, in the current work, biomimetic synthesis technology was first introduced to synthesize a type of hydroxyapatite with hierarchical hollow microspheres that are compatible with the environment and have superior decontamination performance. Furthermore, we presented a facile and green chemistry route that is template-free to obtain hierarchical hollow hydroxyapatite microspheres, which are assembled with nanorods by use of deionized water
2. Experimental section 2.1. Materials The U(VI) stock solutions (1000 mg/L) were prepared in a glovebox by dissolving appropriate amount of uranyl nitrate (UO2(NO3)2·6H2O) (GR) into HNO3 solution. The prepared U(VI) stock solutions were then diluted into various desired concentrations but ensure that the pH is 3.0 which is adjusted by 0.1 M HNO3 for U(VI) adsorption experiments. Other chemicals were commercially purchased as analytical reagents from Chemical Reagent Co., Ltd., and used as received without further purification. All solutions were prepared with double deionized water. 2.2. HAP preparation The typical synthesis process of HAP microspheres used in this study was based on a modified method reported by Jiang et al. [38]. In brief, 0.528 g (4 mmol) of CaCl2·2H2O and 2 g PASP were totally dissolved in 40 mL of deionized water, and the mixture was stirred for 30 min to form a homogeneous solution A. Meanwhile, 0.317 g (2.4 mmol) of (NH4)2HPO4 was dissolved in 30 mL of deionized water, forming solution B under vigorous stirring. Then, solution B was added into solution A and the mixture was vigorously stirred. During this period, precipitates may form due to the slight increase of solution pH. In order to ensure homogeneous reaction, the solution pH was adjusted to 5.0 by adding a certain amount of 0.1 M HCl or diluted ammonia solution to obtain transparent solution and this solution was then transferred into a Teflon lined stainless-steel autoclave of 100 mL capacity. Then, the autoclave was maintained at 180 °C for 24 h and finally cooled down to room temperature naturally. The precipitates were collected and washed with deionized water and absolute ethanol for several times, and then dried in vacuum at 60 °C for 12 h. 2.3. Characterization and analytical methods The morphologic characterization was carried out on a field emission scanning electron microscope (FE-SEM, JEOLJSM-7001 F, Japan) and a transmission electron microscope (FEI Tecnai G2 20 Scanning Transmission Electron Microscope, USA). X-ray diffraction (XRD) data were acquired by a PANalytical PW3040/60 diffractometer using Cu Kα radiation (40 kV, 10-60°). X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo Fisher Scientific ESCALAB 398
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Scheme 1. Schematic illustration of the synthesis process of hierarchical hollow hydroxyapatite microspheres.
250Xi spectrometer. The FT-IR spectroscopy was performed on a Bruker Tensor 27 FT-IR spectrometer using a potassium bromide (KBr) pellet method. The specific surface areas were evaluated using a Micromeritics ASAP 2020 instrument (USA). The pore diameter was obtained from N2 desorption isotherm curves using the Barrett-JoynerHalenda (BJH) method. The residual uranium concentration in aqueous solution was determined using a WGJ-III UV-Fluorescence uranium analyzer (HD Photoelectric Instrument Co. Ltd). For each test, 5 mL uranium-bearing sample was added into the test bin firstly. A 500 μL fluorescence intensifier (BRIUG 201), which can complex with the free UO22+ in the mixed solution systems to generate a high fluorescent intensity, was mixed into the sample. After that, about 5 μL standard solution of uranium in a concentration of 1 μg/mL was injected into the above solution by micro syringe. According to the absorbance obtained, the residual concentration was calculated from the previously standard calibration curves.
Ce 1 C = + e qe bQ0 Q0
where qe is the amount of metal ion adsorbed by the adsorbent (mg g−1), Ce is ion concentration at equilibrium (mg L–1), Q0 is maximum adsorption capacity (mg g–1) and b is the Langmuir constant (L mg–1). The values of Q0 and b are calculated from the slope and intercept of the plot of Ce versus Ce, respectively. qe
The linear form of Freundlich isotherm is expressed as Eq. 3:
Logqe = LogKf +
Log (qe
Ce) × V m
(3)
qt ) = Logqe
k1 t
(4)
where qe and qt (mg/g) are the amount of metal ions adsorbed at equilibrium time and at time t, respectively and k1 is the constant value of pseudo first order model. The linear form of the pseudo-second-order model is shown in Eq. (5):
The adsorption experiments were carried out in a series of Erlenmeyer flasks. In each test, 0.05 g of adsorbent was added to uranium aqueous solution. Then, the reactive system is stirred at room temperature with a magnetic stirrer (MS-H-S10). The solution pH values of suspension were adjusted before the addition of adsorbent by injecting a negligible volume of 0.01, 0.1 and 1.0 M HNO3 or NaOH solution. Adsorption kinetics and adsorption isotherms of U(VI) on the HAP (m/v = 0.2 g/L) were determined at pH 3.0 in different reaction times (2–60 min) and different concentrations of U(VI). The U(VI) concentrations in suspensions were measured by a UV-Fluorescence uranium analyzer after the suspension was filtered through a 0.45-μm cellulose acetate membrane, respectively. Specifically, the sorption amount (Qt, mg/g) of U(VI) at t time could be calculated using Eq. (1):
(C0
1 LogCe n
where Kf is Freundlich constant and n is the adsorption intensity. The linear form of pseudo-first-order model can be expressed as follows
2.4. Adsorption experiment
Qt =
(2)
t 1 t = + qt k2 qe2 qe
(5)
where k2 is the constant value of pseudo-second-order kinetic model. 3. Results and discussion 3.1. Characterization The morphology and microstructure of the newly-prepared hydroxyapatite were first characterized by SEM and TEM (Scheme 1). A plenty of well-dispersed homogeneous microspheres were observed in Fig. 1a, and certain broken microspheres could be clearly observed (red square in Fig. 1a), indicating that the microspheres might be hollow. The microspheres were composed of many nanorods which were perpendicular on the surface of the spheres. With higher magnification of SEM image (Fig. 1b), the diameter of hydroxyapatite microsphere is approximately 2.5 μm. TEM images are shown in Fig. 1c-d, which further confirmed the hollow and hierarchical structures of the microspheres. The contrast between the light center portion and the black edge evidently indicated the hollow interiors of the sample. Overall, the SEM and TEM images confirmed that the HAP samples have unique hollow structures with nanorods, forming hierarchical structures. The XRD pattern of as-obtained HAP sample is given in Fig. 2a. The diffraction peaks located at 2θ (°) = 25.879, 28.126, 31.773, 32.196, 32.902, 39.818, 46.711, and 49.468 can be assigned to the crystal planes of (002), (102), (211), (112), (300), (310), (222), and (213) of a
(1)
where C0 (mg/L) and Ce (mg/L) are the initial concentration and equilibrated concentration after adsorption, respectively. The parameters of m (g) and V (mL) are the mass of adsorbents and the volume of the suspension, respectively. In order to unravel the adsorption behavior of U(VI) by hydroxyapatite hollow microspheres, Adsorption isotherms and Adsorption kinetics were applied to evaluate the adsorption equilibrium data. For this purpose, 50 mL of U(VI) solution with concentrations of 50, 100, 150, 200, 250 and 300 mg L–1 were prepared at room temperature. The reaction was performed at pH 3.0 with 0.01 g adsorbent and 5 min as contact time, and the U(VI) adsorption data was calculated by Eq. 1. The linear form of Langmuir isotherm is expressed as follows: 399
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Fig. 1. Field emission scanning electron microscope (FE-SEM) (a, b) and transmission electron microscope (TEM) (c, d) images of the hollow hydroxyapatite microspheres, showing hierarchical and hollow structure of as-prepared hydroxyapatite samples obtained.
typical hydroxyapatite (Ca5(PO4)3(OH)) hexagonal structure (PDF no. 09-0432), respectively. The stronger peak of (002) is probably related to HAP preferential orientation along the [001] direction. The other broadened diffraction peaks of prepared product are mainly ascribed to the nanocrystals of HAP, which is in line with the results of SEM and TEM analysis. It is noted that the slight distortions of crystal lattice may result in the broadened diffraction peaks. No impurities were detected, indicating the high purity of hydroxyapatite. The N2 adsorption-desorption isotherms and the corresponding pore size distributions of hydroxyapatite hollow microspheres are shown in Fig. 2b. The N2 adsorption-desorption isotherms belong to type IV with a distinct hysteresis loop (P/P0 > 0.4), indicating the presence of
mesopores [39]. This is further confirmed by the BJH pore size distributions shown in Fig. 2b (inset). The average pore size of the hydroxyapatite hollow spheres is 10.5 nm, which represents the nanopores in the shell, not the hollow core. The BET specific surface area yielded 182.6 m2 / g, which is much higher than those of spherical hydroxyapatite particles (34.76 m2 / g and 78.42 m2 / g, respectively) [40,41]. Such high BET surface area, mesoporous structure, and nanoscale size might be beneficial for the removal of U(VI) from aqueous solution.
Fig. 2. XRD pattern (a) and nitrogen adsorption/desorption isotherm of the as-prepared hydroxyapatite samples. The inset of Fig. 2b is the pore size distribution of hollow hydroxyapatite microspheres, showing the porous structures of the sample. 400
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Fig. 3. (a) Effect of pH on the U(VI) adsorption capacities of hollow hydroxyapatite microspheres (50 mL of 40 mg/L U(VI) solution, pH = 2.0–6.0, t = 5 min, m = 0.2 g/L, T = room temperature); (b) Zeta potential versus different pH of hollow hydroxyapatite microspheres. Table 1 Isotherm parameters for uranium ions removal by Hap. Adsorption Isotherm
Model Parameters
Langmuir
Q0 (mg g－1) b (L mg－1) R2
2659.57 0.0019 0.8220
Freundlich
n Kf (mg g－1)(L mg－1)1/n R2
1.2323 10.04 0.9909
Table 2 Maximum sorption capacity of various adsorbents towards uranium (VI).
Fig. 4. Effect of contact time on the U(VI) adsorption performance of hollow hydroxyapatite microspheres (50 mL of 34 mg/L U(VI) solution, pH = 3.0, m = 0.2 g/L, T = room temperature).
3.2. Effect of pH on U(VI) adsorption by hydroxyapatite hollow microspheres The solution pH is one of the most important parameters for the adsorption of U(VI) because uranium has different speciation at different pH values [42]. For the purpose of investigating the effect of pH on the adsorption of U(VI) by HAP, 0.01 g of adsorbent was added into 50 ml of 40 mg/L U(VI) solution at different pH values (from 2.0–6.0) at room temperature. The contact time is set at 5 min. The desired pH of the solution was adjusted by adding certain amounts of HNO3 and NaOH. The results in Fig. 3 illustrated the initial solution pH remarkably affects the U(VI) adsorption capacities of hierarchical hollow hydroxyapatite microspheres. At pH 2.0, the U(VI) adsorption capacity is much low. When the pH is elevated to 3.0, the U(VI) adsorption capacities of HAP increases markedly and reaches its maximum. Further increasing the pH, the U(VI) adsorption capacities of HAP
Adsorbent
pH
Time (min)
Dosage (g/L)
Q0 (mg/g)
Reference
Chitosan Polypyrrole Hematite Zeolite X − PAN Olive stones/ZnCl2 Alumina (Al2O3) Magnetic Fe3O4/SiO2 Fe3O4/GO Magnetite particles Anionic resin HAP microspheres
3 5 7 – 6 6.5 6 5.5 5 3 3
180 7 360 1 5 – 180 240 30 – 5
0.2 1 5 0.2 4 – 2.5 0.3 0.5 1.9 0.2
49.05 87.72 3.36 90 57.80 78 52 69.49 158 79 199
[14] [15] [16] [17] [18] [19] [50] [51] [52] [53] This work
decreased significantly. Thus, the optimum pH for the effective removal of U(VI) by hierarchical hollow hydroxyapatite microspheres should be at 3.0 in this work. It should be noted that under low pH (pH < 3.0) circumstances, uranyl ions (UO22+) dominate the existing U(VI) forms in the solution [43,44]. The greater concentration of H+ causes stronger competition with U(VI) for the active sites on the adsorbents [45]. The point of zero charge (pzc) of the hydroxyapatite hollow microspheres is 3.03 (Fig. 3b). Briefly, the zeta potential was determined by 90plus Zeta potentiometer (Brookhaven instruments, USA). First, the
Fig. 5. Langmuir isotherm (a) and Freundlich isotherm (b) for U(VI) adsorption by HAP (pH = 3, t = 5 min, m = 0.2 g/L, T = room temperature). 401
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Fig. 6. Pseudo-first-order kinetic (a) and Pseudo-second-order kinetic (b) for U(VI) adsorption by HAP (pH = 3.0, t = 5 min, m = 0.2 g/L, T = room temperature).
faster than silica materials (15 min) and carbon CMK-5 (30 min) [47,48]. The adsorption of U(VI) on hollow hydroxyapatite microspheres as a function of contact time was investigated in the treatment of actual wastewater containing uranium from No.741 uranium mine (Fig. S1). The experimental results of U(VI) adsorption are similar to those of simulated U(VI) wastewater, the removal of U(VI) was highly efficient (84.6% within 10 min) from the wastewater stream.
Table 3 Kinetic parameters for uranium ions removal by Hap. Kinetic Model
Model Parameters
Pseudo-first-order
K1 (min－1) Qe (mg g－1) R2
0.023 0.432 0.401
Pseudo-second-order
K2 (min－1) Qe (mg g－1) R2
0.050 169.49 0.999
3.4. Adsorption isotherms The amount of adsorbed uranium by hydroxyapatite hollow microspheres versus the equilibrium concentration was presented in Fig. S2. For the purpose of studying the U(VI) adsorption isotherms, the Langmuir isotherm and Freundlich isotherm were fitted with the experimental data and the results are shown in Fig. 5 and Table 1. It can be seen that the Freundlich isotherm (R2 = 0.9967) has a better agreement with the experimental data rather than Langmuir isotherm (R2 = 0.8220). The maximum adsorption capacity of hydroxyapatite hollow microspheres for U(VI) adsorption was 2659 mg g–1, which is calculated by Langmuir model. Freundlich model has been deduced by assuming an exponentially decaying sorption site energy distribution and nonuniformity surface of the adsorbents [49], which is related to the hierarchical nanostructure and surface couple with nanorods of the HAP. In order to justify the validity of hydroxyapatite hollow microspheres as a superior adsorbent for the removal of U(VI), our experimental results were compared with those reported in other works (Table 2). It is evident that the U(VI) adsorption capacities of hydroxyapatite hollow microspheres are much higher than those of other adsorbents, suggesting that hollow hydroxyapatite is a promising material for the remediation of uranium contaminated medium.
as-prepared HAP was homogenously dispersed in deionized water by ultrasound. Then the solution pH was adjusted to designated value by adding certain amount of NaOH and HNO3. Finally, the solution was transferred to the sample pool of 90plus Zeta potentiometer for measurement. The electrostatic repulsion may hinder the surface complexation between the uranyl and HAP when the solution pH was lower than 3.0. The surface charge decreased with increased pH value, leading to the decrease in electrostatic repulsion between U(VI) and HAP. At pH above 3.0, the uranyl ions start to hydrolyze, resulting in polynuclear hydroxy uranyl forms including [UO2(OH)]+, [(UO2)2(OH)2]2+, [(UO2)3(OH)4]2+, [(UO2)3(OH)5]+ and [(UO2)4(OH)7]+ [46], which are generally unfavorable for the adsorption due to the existence of −OH group by HAP. 3.3. Effect of contact time on U(VI) adsorption by hydroxyapatite hollow microspheres As can be seen in Fig. 4, the U(VI) adsorption reaches apparent equilibrium after 5 min reaction. Notably, the U(VI) adsorption by hydroxyapatite hollow microspheres was very rapid at the beginning. By extending the contact time, the U(VI) adsorption capacities of hydroxyapatite hollow microspheres remained steady, with the removal efficiency of U(VI) close to 100%. It should be noted that the adsorption process of hydroxyapatite hollow microspheres for U(VI) was much
3.5. Adsorption kinetics For the purpose of studying the U(VI) adsorption kinetics, the
Fig. 7. XRD patterns (a) and FT-IR spectra (b) of hydroxyapatite hollow microspheres before and after U(VI) adsorption. 402
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Fig. 8. (a) XPS survey spectra of hydroxyapatite hollow microspheres before and after U(VI) adsorption. (b) XPS Ca 2p spectrum of the hydroxyapatite hollow microspheres before and after U(VI) adsorption. XPS O 1 s of the hydroxyapatite hollow microspheres before (c) and after (d) U(VI) adsorption. XPS U 4f5/2 (e) and U 4f7/2 (f) of the hydroxyapatite hollow microspheres after (e) U(VI) adsorption.
pseudo-first and second-order kinetic models were fitted with the experimental data and the results are shown in Fig. 6 and Table 3. Fig. 6 shows the linear plot of t versus t. The regression correlation coeffi-
samples before and after uranium U(VI) adsorption with EDS, XRD, FTIR and XPS. The EDS of hollow hydroxyapatite microspheres before and after uranium (VI) adsorption are shown in Fig. S2. The signals of U were detected in the sample after absorption, signifying U(VI) was accumulated on the hollow hydroxyapatite microspheres. The XRD patterns of the hydroxyapatite hollow microspheres before and after U(VI) adsorption are shown in Fig. 7a. Diffraction peaks located at 2θ (°) = 25.879, 31.773, 32.196, 32.902, 39.818, 46.711 and 49.468 matched well with the crystal planes of (002), (211), (112), (300), (130), (222) and (213) of hydroxyapatite (Ca5(PO4)3(OH), PDF# 090432), respectively. After adsorption, the diffraction peaks located at 16.432°, 20.969°, 24.571°, 25.464°, 27.567°, 34.263°, 42.717° were observed, which are due to the presence of autunite (Ca (UO2)2(PO4)2(H2O)3) (PDF no. 39-1351). These diffraction peaks of
qt
cients and constants of the pseudo-first and pseudo-second-order kinetic models are shown in Table 3. As can be seen that the correlation coefficients of the pseudo-second-order equation are high (0.999). A good fitting with the pseudo-second-order kinetic model infers that the adsorption reaction may be chemical adsorption, which may involve valency forces through sharing of electrons between and adsorbent and metal cations. 3.6. Removal mechanisms To further study the adsorption mechanism, we measured the 403
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autunite can be assigned to the crystal planes of (101), (002), (102), (200), (201), (103) and (212) of autunite (Ca(UO2)2(PO4)2·3H2O), respectively. The presence of autunite and the absence of hydroxyapatite indicated the uranyl ions were adsorbed and then eventually incorporated by hydroxyapatite hollow microspheres. Formation of Ca (UO2)2(PO4)2·3H2O by the interaction among uranyl ions and hydroxyapatite hollow microspheres is a probable mechanism contributing to the favorable sorption capacity to U(VI) with long-term stability. The FT-IR spectra of the hydroxyapatite hollow microspheres before and after adsorption of U(VI) are shown in Fig. 7b. The peaks at 3444 belong to stretching vibrations of the OeH [54]. The bands at 1039 cm−1, 1092 cm−1, and 960 cm−1 are the characteristic bands of phosphate stretching vibrations, while the bands at 605 cm−1 and 565 cm−1 are due to phosphate bending vibrations [55,56]. The peaks of 1408 cm−1 and 1591 cm−1 are related to the COO−1 in the Ca − PASP complex, suggesting that PASP molecules remain strongly bound in the HAP even after extensive washing, and the peak of 1591 cm−1 showed a shift of 54 cm−1 to higher wavenumbers [56]. Comparing the spectra of hydroxyapatite hollow microspheres before and after U(VI) adsorption, a new strong band at 913 cm−1 appeared, representing the stretching frequency of the linear structure of [O]UVI]O]2+, which further confirmed the successful chemical binding of uranyl ions with the ligands on the hydroxyapatite hollow microspheres [57]. The XPS analyses of the hydroxyapatite hollow microspheres before and after U(VI) adsorption were performed to provide more details on the chemical changes occurred on the surface of the samples before and after U(VI) adsorption (Fig. 8). From the survey spectra shown in Fig. 8a, the presence of C, O, Ca and P was detected through C 1 s (285.78 eV), O 1 s (531.30 eV), Ca 2p (346.37 eV), and P 2p (132.74) peaks for the hydroxyapatite hollow microspheres sample. Notably, a new strong double peak for the antisymmetric vibration of [O]UVI] O]2+ was observed for hydroxyapatite hollow microspheres after U(VI) adsorption. The corresponding high-resolution U 4f5/2 (393.1 eV) and U 4f7/2 (382.3 eV) core-level spectra (Fig. 8 (e)and (f)) revealed the existence of U(VI) in the hydroxyapatite hollow microspheres after U(VI) adsorption [58,59]. The binding energy value of Ca 2p on hydroxyapatite hollow microspheres increased from 347.1 eV to 347.3 eV upon U(VI) adsorption, indicating the obvious interactions between U (VI) and Ca (Fig. 8b) [56]. This was probably a consequence of the formation of covalent Ca − UO2 bonds, which resulted in a decrease of the extra nuclear electron cloud density surrounding Ca and in turn increased the binding energy of Ca 2p. As shown in Fig. 8c and d, the O 1 s spectrum was divided into three peaks, anion oxide (530.8 eV, O2−), hydroxyl bonded to calcium (531.2 eV, Ca − OH), and adsorbed H2O (532.5 eV), respectively [28]. It is obviously observed that the area ratio of Ca − OH peaks decreased from 64.4% to 60.8% after U(VI) adsorption, which confirmed that the surface hydroxyl groups (Ca − OH) played a significant role in U(VI) adsorption by sharing electrons for the formation of U − O bonds [60]. The chemical reaction between uranyl and hydroxyapatite took place upon U(VI) adsorption on the hierarchical hydroxyapatite hollow microspheres. Uranyl ion can be rapidly adsorbed and eventually incorporated into the Ca2+−PO43+−OH following the reaction as Eq. 6:
Ca10 (PO4 ) 6 (OH ) 2 + H2 O + UO22 +
Ca (UO2) 2 (PO4 ) 2 3H2 O
HAP hollow microspheres were adsorption and subsequent incorporation. Because of the incorporation of adsorbed uranyl, a new substance, viz., autunite (Ca(UO2)2(PO4)2·3H2O), was formed in the adsorbent. The adsorption kinetics followed the pseudo-second-order kinetic model, showing high performance of U(VI) removal within 5 min. The adsorption isotherm was well described by Freundlich isotherm. It is worth noting that hierarchical HAP hollow microspheres adsorb a large amount of U(VI) rapidly for its effective removal. On the basis of these evidences, the HAP hierarchical hollow microspheres are a powerful material for high-efficiency removal of uranium (VI) from the radioactive wastewater, can be supposed to a prompt emergency material for the remediation of nuclear leakage accident. Acknowledgements This work was supported by the National Natural Science Foundation of China (U1501231, 51708143, 51508116); The Project of Guangdong Provincial Key Laboratory of radioactive contamination control and resources (2017B030314182); Science and Technology Program of Guangzhou, China (201804020072, 201804010366); The Hong Kong Research Grants Council (E-PolyU503/17); Guangzhou University’s Training Program for Excellent New-recruited Doctors (YB201710); Scientific Research Foundation for the Returned Highlevel Overseas Talents (2018). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.02.110. References [1] M. Dittmar, Nuclear energy: Status and future limitations, Energy 37 (2012) 35–40. [2] D. Li, D.I. Kaplan, Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: a review, J. Hazard. Mater. 243 (2012) 1–18. [3] R. Hu, D.D. Shao, X.K. Wang, Graphene oxide/polypyrrole composites for highly selective enrichment of U(VI) from aqueous solutions, Polym. Chem. 5 (2014) 6207–6215. [4] A. Sakaguchi, P. Steier, Y. Takahashi, M. Yamamoto, Isotopic compositions of 236U and Pu isotopes in “black substances” collected from roadsides in Fukushima prefecture: fallout from the Fukushima dai-ichi nuclear power plant accident, Environ. Sci. Technol. 48 (2014) 3691–3697. [5] G. Steinhauser, A. Brandl, T.E. Johnson, Comparison of the Chernobyl and Fukushima nuclear accidents: a review of the environmental impacts, Sci. Total. Environ. 470–471 (2014) 800–817. [6] M. Kanematsu, N. Perdrial, W. Um, J. Chorover, P.A. O’Day, Influence of phosphate and silica on U(VI) precipitation from acidic and neutralized wastewaters, Environ. Sci. Technol. 48 (2014) 6097–6106. [7] T. Wannachod, T. Wongsawa, P. Ramakul, U. Pancharoen, S. Kheawhom, The synergistic extraction of uranium ions from monazite leach solution via HFSLM and its mass transfer, J. Ind. Eng. Chem. 33 (2016) 246–254. [8] Y.L. Chen, Y.Z. Wei, L.F. He, F.D. Tang, Separation of thorium and uranium in nitric acid solution using silica based anion exchange resin, J. Chromatogr. A 1466 (2016) 37–41. [9] C. Zhou, A. Ontiveros-Valencia, L. Cornette de Saint Cyr, A.S. Zevin, S.E. Carey, R. Krajmalnik-Brown, B.E. Rittmann, Uranium removal and microbial community in a H2-based membrane biofilm reactor, Water Res. 64 (2014) 255–264. [10] S.E. Bone, J.J. Dynes, J. Cliff, J.R. Bargar, Uranium(IV) adsorption by natural organic matter in anoxic sediments, P. Natl. Acad. Sci.USA. 114 (2017) 711–716. [11] S. Yu, X. Wang, H. Pang, R. Zhang, W. Song, D. Fu, T. Hayat, X. Wang, Boron nitride-based materials for the removal of pollutants from aqueous solutions: a review, Chem. Eng. J. 333 (2018) 343–360. [12] T.A. Saleh, Simultaneous adsorptive desulfurization of diesel fuel over bimetallic nanoparticles loaded on activated carbon, J. Clean. Prod. 172 (2018) 2123–2132. [13] T.A. Saleh, A. Sarı, M. Tuzen, Effective adsorption of antimony(III) from aqueous solutions by polyamide-graphene composite as a novel adsorbent, Chem. Eng. J. 307 (2017) 230–238. [14] G.H. Wang, J.S. Liu, X.G. Wang, Z.Y. Xie, N.S. Deng, Adsorption of uranium (VI) from aqueous solution onto cross-linked chitosan, J. Hazard. Mater. 168 (2009) 1053–1058. [15] S. Abdi, M. Nasiri, A. Mesbahi, M.H. Khani, Investigation of uranium (VI) adsorption by polypyrrole, J. Hazard. Mater. 332 (2017) 132–139. [16] S.B. Xie, C. Zhang, X.H. Zhou, J. Yang, X.J. Zhang, W.J. Song, Removal of uranium (VI) from aqueous solution by adsorption of hematite, J. Environ. Radioact. 100 (2009) 162–166. [17] S. Akyil, M. Eral, Preparation of composite adsorbents and their characteristics, J.
(6)
Similar results were reported by Han et al. [31] in which uranium immobilized by wasted bio-hydroxyapatite. 4. Conclusions In this study, novel hierarchical HAP hollow microspheres were successfully synthesized via a facile template-free hydrothermal process and this versatile material was used as an effective adsorbent to eliminate U(VI) from aqueous solution. The experimental results indicated that the main interaction mechanisms of U(VI) removal by hierarchical 404
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Rapid and effective removal of uranium (VI) from aqueous solution by facile synthesized hierarchical hollow hydroxyapatite microspheres Yanhong Wua, Diyun Chena, , Lingjun Konga, Daniel C.W. Tsangb, Minhua Sua,b, ⁎
T
⁎⁎
a
Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources, School of Environmental Science and Engineering, Guangzhou University, Guangzhou, 510006, China b Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Uranium (VI) adsorption Hollow hydroxyapatite Phosphate precipitation Sustainable remediation Radioactive wastewater treatment
Rapidly increasing development of nuclear power stimulates the exploration of low-cost and highly efficient materials to selectively remove uranium (VI) from contaminated wastewater streams. Herein, we successfully developed a novel hydroxyapatite (HAP) adsorbent by using a facile and template-free hydrothermal method. The XRD results demonstrated that the HAP was crystallized in hexagonal structure (space group P63/m(176)), and the images of SEM and TEM indicated that the HAP possessed hollow and hierarchical nanostructure. A large BET specific surface area (182.6 m2/g) and average pore size of 10.5 nm, suggested that the hierarchical hollow HAP microspheres could provide sufficient active sites for highly efficient removal of uranium from aqueous solutions, indicated the HAP might be a prompt emergency material for the remediation of nuclear leakage accident. Freundlich isotherm and pseudo-second-order kinetics model fitted well to sorption experimental data. The study was further advanced by FT-IR and XPS. The sorption mechanism was mainly attributed to surface chemisorption between U(VI) and HAP, forming a new U-containing compound, viz., autunite (Ca (UO2)2(PO4)2·3H2O).
Corresponding author. Corresponding author at: Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources, School of Environmental Science and Engineering, Guangzhou University, Guangzhou, 510006, China. E-mail addresses: [email protected] (D. Chen), [email protected] (M. Su). ⁎
⁎⁎
https://doi.org/10.1016/j.jhazmat.2019.02.110 Received 15 October 2018; Received in revised form 27 February 2019; Accepted 28 February 2019 Available online 01 March 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
as reaction medium and poly aspartic acid (PASP) as both chelating and capping agents. PASP is a potential poly carboxylic sequestrant that is water soluble, nontoxic, highly biodegradable, and free of secondary pollution to the environment [36]. With the addition of PASP, the crystal structure can be tailored and the growth of crystal structure can be oriented. The presence of PASP is also known to facilitate the transition of amorphous to crystalline phases [37]. In our proposed synthetic route, via coordination bonding with Ca2+ ions, PASP acts as the nucleation centers for the newly formed amorphous calcium phosphate particles. Besides, PASP as a capping agent, through interacting with the surface of the HAP crystallite, can result in a number of nanorods and form the hydroxyapatite microspheres with a hierarchical hollow structure. The final product is supposed to be capable of achieving fast reaction and high adsorption capacity for U(VI) removal. The aim of this study was to: (i) synthesize hierarchical hollow hydroxyapatite microspheres via a green route; (ii) evaluate its adsorption performance for U(VI) removal; and (iii) investigate its adsorption mechanisms towards U(VI) removal. The processing parameters including solution pH, contact time, adsorbent dosage, initial U (VI) concentration, and temperature were well investigated. Equilibrium, kinetics, and thermodynamic studies were conducted to unravel the U(VI) adsorption behavior by hierarchical hollow hydroxyapatite microspheres.
To meet the increase of worldwide energy demand, nuclear energy has been rapidly developed in recent years in view of its environmentally friendliness and high efficiency [1]. Uranium (VI), the most important resource for nuclear power generation, is often released into the environment during the nuclear-related activities such as nuclear fuel processing, uranium mining, nuclear accidents, etc. [2]. The U(VI) is radioactive, toxic, and highly mobile. Moreover, its half-life is long, and the excessive release of U(VI) poses severe and long-term risks to the environment and human [3]. The accident at the Fukushima Daiichi Nuclear Power Plant in 2011 following the Great East Japan Earthquake resulted in the discharge of a large amount of radioactive wastewater in which uranium is the main ingredient [4]. The Chernobyl nuclear accident occurred in 1986 in Ukraine, whose environmental and radiological consequences exceeded those of the Fukushima accident [5]. Therefore, prompt and reliable elimination of U(VI) from contaminated waste streams is urgent and necessary. So far, emergency materials towards nuclear wastewater leakage accident are still under development, conventional techniques including precipitation [6], solvent extraction [7], ion exchange [8], membrane separation [9], and adsorption [10], etc., have been developed and applied to remove U(VI) from the wastewater stream. Of them, adsorption has been demonstrated to be one of the most prevalent and efficient approaches for U (VI) removal owing to its unique advantages of low-cost, simplicity, and versatility [11–13]. To achieve high efficiency and capacity of adsorption process, the nature of sorbent is a critical factor and the development of highly efficient sorbent is of great importance. Various sorbents have been explored to adsorb U(VI), such as chitosan [14], polypyrrole [15], hematite [16], zeolite [17], olive stones activated with ZnCl2 [18], alumina (Al2O3) [19], and so on. Among them, phosphate rock apatite has a great scientific and economic significance due to their naturally occurring nature, low-cost, and ease of use [20]. Apatite-like minerals can also be synthetically produced via precipitation reaction of calcium and phosphate to obtain various products with desired properties [21]. Because of its particular chemical composition and crystal structure, phosphate rock apatite has become one of the most promising materials for the treatment of wastewater and soil [22]. Our previous work has demonstrated that U(VI) can be adsorbed by phosphate rock apatite and chitosan modified phosphate rock [23,24]. However, the U(VI) adsorption efficacy and capacity is low and insufficient for use as an emergency material in the nuclear wastewater leakage accident. As a member of apatite mineral family, hydroxyapatite (HAP) is an effective material for the long-term containment of contaminants due to its high sorption capacity via forming mineral products with high stability and low solubility. Moreover, HAP is lowcost and easily attainable [25–27]. He et al. [28] prepared ultralong HAP nanowires by solvothermal method using oleic acid as a soft template to treat fluoride-contaminated water. Hou et al. [29] synthesized HAP via co-precipitation method for the removal of Congo red dye from aqueous solution. Samant et al. [30] obtained HAP nanocrystals from limacine artica shells by second precipitation method to removal fluoride. Han et al. [31] prepared Bio-HAP from freshwater grass carp bones by calcination method for uranium immobilization. Currently, hydroxyapatite mineral-like synthetic material is mainly used in bone remodeling, bone repair, and other medical fields [32,33]. The biomimetic preparation of hydroxyapatite is mostly based on the development of bone materials [34,35]. According to our best knowledge, there is no study about uranium removal using hierarchical hollow hydroxyapatite microspheres. Thus, in the current work, biomimetic synthesis technology was first introduced to synthesize a type of hydroxyapatite with hierarchical hollow microspheres that are compatible with the environment and have superior decontamination performance. Furthermore, we presented a facile and green chemistry route that is template-free to obtain hierarchical hollow hydroxyapatite microspheres, which are assembled with nanorods by use of deionized water
2. Experimental section 2.1. Materials The U(VI) stock solutions (1000 mg/L) were prepared in a glovebox by dissolving appropriate amount of uranyl nitrate (UO2(NO3)2·6H2O) (GR) into HNO3 solution. The prepared U(VI) stock solutions were then diluted into various desired concentrations but ensure that the pH is 3.0 which is adjusted by 0.1 M HNO3 for U(VI) adsorption experiments. Other chemicals were commercially purchased as analytical reagents from Chemical Reagent Co., Ltd., and used as received without further purification. All solutions were prepared with double deionized water. 2.2. HAP preparation The typical synthesis process of HAP microspheres used in this study was based on a modified method reported by Jiang et al. [38]. In brief, 0.528 g (4 mmol) of CaCl2·2H2O and 2 g PASP were totally dissolved in 40 mL of deionized water, and the mixture was stirred for 30 min to form a homogeneous solution A. Meanwhile, 0.317 g (2.4 mmol) of (NH4)2HPO4 was dissolved in 30 mL of deionized water, forming solution B under vigorous stirring. Then, solution B was added into solution A and the mixture was vigorously stirred. During this period, precipitates may form due to the slight increase of solution pH. In order to ensure homogeneous reaction, the solution pH was adjusted to 5.0 by adding a certain amount of 0.1 M HCl or diluted ammonia solution to obtain transparent solution and this solution was then transferred into a Teflon lined stainless-steel autoclave of 100 mL capacity. Then, the autoclave was maintained at 180 °C for 24 h and finally cooled down to room temperature naturally. The precipitates were collected and washed with deionized water and absolute ethanol for several times, and then dried in vacuum at 60 °C for 12 h. 2.3. Characterization and analytical methods The morphologic characterization was carried out on a field emission scanning electron microscope (FE-SEM, JEOLJSM-7001 F, Japan) and a transmission electron microscope (FEI Tecnai G2 20 Scanning Transmission Electron Microscope, USA). X-ray diffraction (XRD) data were acquired by a PANalytical PW3040/60 diffractometer using Cu Kα radiation (40 kV, 10-60°). X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo Fisher Scientific ESCALAB 398
Journal of Hazardous Materials 371 (2019) 397–405
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Scheme 1. Schematic illustration of the synthesis process of hierarchical hollow hydroxyapatite microspheres.
250Xi spectrometer. The FT-IR spectroscopy was performed on a Bruker Tensor 27 FT-IR spectrometer using a potassium bromide (KBr) pellet method. The specific surface areas were evaluated using a Micromeritics ASAP 2020 instrument (USA). The pore diameter was obtained from N2 desorption isotherm curves using the Barrett-JoynerHalenda (BJH) method. The residual uranium concentration in aqueous solution was determined using a WGJ-III UV-Fluorescence uranium analyzer (HD Photoelectric Instrument Co. Ltd). For each test, 5 mL uranium-bearing sample was added into the test bin firstly. A 500 μL fluorescence intensifier (BRIUG 201), which can complex with the free UO22+ in the mixed solution systems to generate a high fluorescent intensity, was mixed into the sample. After that, about 5 μL standard solution of uranium in a concentration of 1 μg/mL was injected into the above solution by micro syringe. According to the absorbance obtained, the residual concentration was calculated from the previously standard calibration curves.
Ce 1 C = + e qe bQ0 Q0
where qe is the amount of metal ion adsorbed by the adsorbent (mg g−1), Ce is ion concentration at equilibrium (mg L–1), Q0 is maximum adsorption capacity (mg g–1) and b is the Langmuir constant (L mg–1). The values of Q0 and b are calculated from the slope and intercept of the plot of Ce versus Ce, respectively. qe
The linear form of Freundlich isotherm is expressed as Eq. 3:
Logqe = LogKf +
Log (qe
Ce) × V m
(3)
qt ) = Logqe
k1 t
(4)
where qe and qt (mg/g) are the amount of metal ions adsorbed at equilibrium time and at time t, respectively and k1 is the constant value of pseudo first order model. The linear form of the pseudo-second-order model is shown in Eq. (5):
The adsorption experiments were carried out in a series of Erlenmeyer flasks. In each test, 0.05 g of adsorbent was added to uranium aqueous solution. Then, the reactive system is stirred at room temperature with a magnetic stirrer (MS-H-S10). The solution pH values of suspension were adjusted before the addition of adsorbent by injecting a negligible volume of 0.01, 0.1 and 1.0 M HNO3 or NaOH solution. Adsorption kinetics and adsorption isotherms of U(VI) on the HAP (m/v = 0.2 g/L) were determined at pH 3.0 in different reaction times (2–60 min) and different concentrations of U(VI). The U(VI) concentrations in suspensions were measured by a UV-Fluorescence uranium analyzer after the suspension was filtered through a 0.45-μm cellulose acetate membrane, respectively. Specifically, the sorption amount (Qt, mg/g) of U(VI) at t time could be calculated using Eq. (1):
(C0
1 LogCe n
where Kf is Freundlich constant and n is the adsorption intensity. The linear form of pseudo-first-order model can be expressed as follows
2.4. Adsorption experiment
Qt =
(2)
t 1 t = + qt k2 qe2 qe
(5)
where k2 is the constant value of pseudo-second-order kinetic model. 3. Results and discussion 3.1. Characterization The morphology and microstructure of the newly-prepared hydroxyapatite were first characterized by SEM and TEM (Scheme 1). A plenty of well-dispersed homogeneous microspheres were observed in Fig. 1a, and certain broken microspheres could be clearly observed (red square in Fig. 1a), indicating that the microspheres might be hollow. The microspheres were composed of many nanorods which were perpendicular on the surface of the spheres. With higher magnification of SEM image (Fig. 1b), the diameter of hydroxyapatite microsphere is approximately 2.5 μm. TEM images are shown in Fig. 1c-d, which further confirmed the hollow and hierarchical structures of the microspheres. The contrast between the light center portion and the black edge evidently indicated the hollow interiors of the sample. Overall, the SEM and TEM images confirmed that the HAP samples have unique hollow structures with nanorods, forming hierarchical structures. The XRD pattern of as-obtained HAP sample is given in Fig. 2a. The diffraction peaks located at 2θ (°) = 25.879, 28.126, 31.773, 32.196, 32.902, 39.818, 46.711, and 49.468 can be assigned to the crystal planes of (002), (102), (211), (112), (300), (310), (222), and (213) of a
(1)
where C0 (mg/L) and Ce (mg/L) are the initial concentration and equilibrated concentration after adsorption, respectively. The parameters of m (g) and V (mL) are the mass of adsorbents and the volume of the suspension, respectively. In order to unravel the adsorption behavior of U(VI) by hydroxyapatite hollow microspheres, Adsorption isotherms and Adsorption kinetics were applied to evaluate the adsorption equilibrium data. For this purpose, 50 mL of U(VI) solution with concentrations of 50, 100, 150, 200, 250 and 300 mg L–1 were prepared at room temperature. The reaction was performed at pH 3.0 with 0.01 g adsorbent and 5 min as contact time, and the U(VI) adsorption data was calculated by Eq. 1. The linear form of Langmuir isotherm is expressed as follows: 399
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Fig. 1. Field emission scanning electron microscope (FE-SEM) (a, b) and transmission electron microscope (TEM) (c, d) images of the hollow hydroxyapatite microspheres, showing hierarchical and hollow structure of as-prepared hydroxyapatite samples obtained.
typical hydroxyapatite (Ca5(PO4)3(OH)) hexagonal structure (PDF no. 09-0432), respectively. The stronger peak of (002) is probably related to HAP preferential orientation along the [001] direction. The other broadened diffraction peaks of prepared product are mainly ascribed to the nanocrystals of HAP, which is in line with the results of SEM and TEM analysis. It is noted that the slight distortions of crystal lattice may result in the broadened diffraction peaks. No impurities were detected, indicating the high purity of hydroxyapatite. The N2 adsorption-desorption isotherms and the corresponding pore size distributions of hydroxyapatite hollow microspheres are shown in Fig. 2b. The N2 adsorption-desorption isotherms belong to type IV with a distinct hysteresis loop (P/P0 > 0.4), indicating the presence of
mesopores [39]. This is further confirmed by the BJH pore size distributions shown in Fig. 2b (inset). The average pore size of the hydroxyapatite hollow spheres is 10.5 nm, which represents the nanopores in the shell, not the hollow core. The BET specific surface area yielded 182.6 m2 / g, which is much higher than those of spherical hydroxyapatite particles (34.76 m2 / g and 78.42 m2 / g, respectively) [40,41]. Such high BET surface area, mesoporous structure, and nanoscale size might be beneficial for the removal of U(VI) from aqueous solution.
Fig. 2. XRD pattern (a) and nitrogen adsorption/desorption isotherm of the as-prepared hydroxyapatite samples. The inset of Fig. 2b is the pore size distribution of hollow hydroxyapatite microspheres, showing the porous structures of the sample. 400
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Fig. 3. (a) Effect of pH on the U(VI) adsorption capacities of hollow hydroxyapatite microspheres (50 mL of 40 mg/L U(VI) solution, pH = 2.0–6.0, t = 5 min, m = 0.2 g/L, T = room temperature); (b) Zeta potential versus different pH of hollow hydroxyapatite microspheres. Table 1 Isotherm parameters for uranium ions removal by Hap. Adsorption Isotherm
Model Parameters
Langmuir
Q0 (mg g－1) b (L mg－1) R2
2659.57 0.0019 0.8220
Freundlich
n Kf (mg g－1)(L mg－1)1/n R2
1.2323 10.04 0.9909
Table 2 Maximum sorption capacity of various adsorbents towards uranium (VI).
Fig. 4. Effect of contact time on the U(VI) adsorption performance of hollow hydroxyapatite microspheres (50 mL of 34 mg/L U(VI) solution, pH = 3.0, m = 0.2 g/L, T = room temperature).
3.2. Effect of pH on U(VI) adsorption by hydroxyapatite hollow microspheres The solution pH is one of the most important parameters for the adsorption of U(VI) because uranium has different speciation at different pH values [42]. For the purpose of investigating the effect of pH on the adsorption of U(VI) by HAP, 0.01 g of adsorbent was added into 50 ml of 40 mg/L U(VI) solution at different pH values (from 2.0–6.0) at room temperature. The contact time is set at 5 min. The desired pH of the solution was adjusted by adding certain amounts of HNO3 and NaOH. The results in Fig. 3 illustrated the initial solution pH remarkably affects the U(VI) adsorption capacities of hierarchical hollow hydroxyapatite microspheres. At pH 2.0, the U(VI) adsorption capacity is much low. When the pH is elevated to 3.0, the U(VI) adsorption capacities of HAP increases markedly and reaches its maximum. Further increasing the pH, the U(VI) adsorption capacities of HAP
Adsorbent
pH
Time (min)
Dosage (g/L)
Q0 (mg/g)
Reference
Chitosan Polypyrrole Hematite Zeolite X − PAN Olive stones/ZnCl2 Alumina (Al2O3) Magnetic Fe3O4/SiO2 Fe3O4/GO Magnetite particles Anionic resin HAP microspheres
3 5 7 – 6 6.5 6 5.5 5 3 3
180 7 360 1 5 – 180 240 30 – 5
0.2 1 5 0.2 4 – 2.5 0.3 0.5 1.9 0.2
49.05 87.72 3.36 90 57.80 78 52 69.49 158 79 199
[14] [15] [16] [17] [18] [19] [50] [51] [52] [53] This work
decreased significantly. Thus, the optimum pH for the effective removal of U(VI) by hierarchical hollow hydroxyapatite microspheres should be at 3.0 in this work. It should be noted that under low pH (pH < 3.0) circumstances, uranyl ions (UO22+) dominate the existing U(VI) forms in the solution [43,44]. The greater concentration of H+ causes stronger competition with U(VI) for the active sites on the adsorbents [45]. The point of zero charge (pzc) of the hydroxyapatite hollow microspheres is 3.03 (Fig. 3b). Briefly, the zeta potential was determined by 90plus Zeta potentiometer (Brookhaven instruments, USA). First, the
Fig. 5. Langmuir isotherm (a) and Freundlich isotherm (b) for U(VI) adsorption by HAP (pH = 3, t = 5 min, m = 0.2 g/L, T = room temperature). 401
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Fig. 6. Pseudo-first-order kinetic (a) and Pseudo-second-order kinetic (b) for U(VI) adsorption by HAP (pH = 3.0, t = 5 min, m = 0.2 g/L, T = room temperature).
faster than silica materials (15 min) and carbon CMK-5 (30 min) [47,48]. The adsorption of U(VI) on hollow hydroxyapatite microspheres as a function of contact time was investigated in the treatment of actual wastewater containing uranium from No.741 uranium mine (Fig. S1). The experimental results of U(VI) adsorption are similar to those of simulated U(VI) wastewater, the removal of U(VI) was highly efficient (84.6% within 10 min) from the wastewater stream.
Table 3 Kinetic parameters for uranium ions removal by Hap. Kinetic Model
Model Parameters
Pseudo-first-order
K1 (min－1) Qe (mg g－1) R2
0.023 0.432 0.401
Pseudo-second-order
K2 (min－1) Qe (mg g－1) R2
0.050 169.49 0.999
3.4. Adsorption isotherms The amount of adsorbed uranium by hydroxyapatite hollow microspheres versus the equilibrium concentration was presented in Fig. S2. For the purpose of studying the U(VI) adsorption isotherms, the Langmuir isotherm and Freundlich isotherm were fitted with the experimental data and the results are shown in Fig. 5 and Table 1. It can be seen that the Freundlich isotherm (R2 = 0.9967) has a better agreement with the experimental data rather than Langmuir isotherm (R2 = 0.8220). The maximum adsorption capacity of hydroxyapatite hollow microspheres for U(VI) adsorption was 2659 mg g–1, which is calculated by Langmuir model. Freundlich model has been deduced by assuming an exponentially decaying sorption site energy distribution and nonuniformity surface of the adsorbents [49], which is related to the hierarchical nanostructure and surface couple with nanorods of the HAP. In order to justify the validity of hydroxyapatite hollow microspheres as a superior adsorbent for the removal of U(VI), our experimental results were compared with those reported in other works (Table 2). It is evident that the U(VI) adsorption capacities of hydroxyapatite hollow microspheres are much higher than those of other adsorbents, suggesting that hollow hydroxyapatite is a promising material for the remediation of uranium contaminated medium.
as-prepared HAP was homogenously dispersed in deionized water by ultrasound. Then the solution pH was adjusted to designated value by adding certain amount of NaOH and HNO3. Finally, the solution was transferred to the sample pool of 90plus Zeta potentiometer for measurement. The electrostatic repulsion may hinder the surface complexation between the uranyl and HAP when the solution pH was lower than 3.0. The surface charge decreased with increased pH value, leading to the decrease in electrostatic repulsion between U(VI) and HAP. At pH above 3.0, the uranyl ions start to hydrolyze, resulting in polynuclear hydroxy uranyl forms including [UO2(OH)]+, [(UO2)2(OH)2]2+, [(UO2)3(OH)4]2+, [(UO2)3(OH)5]+ and [(UO2)4(OH)7]+ [46], which are generally unfavorable for the adsorption due to the existence of −OH group by HAP. 3.3. Effect of contact time on U(VI) adsorption by hydroxyapatite hollow microspheres As can be seen in Fig. 4, the U(VI) adsorption reaches apparent equilibrium after 5 min reaction. Notably, the U(VI) adsorption by hydroxyapatite hollow microspheres was very rapid at the beginning. By extending the contact time, the U(VI) adsorption capacities of hydroxyapatite hollow microspheres remained steady, with the removal efficiency of U(VI) close to 100%. It should be noted that the adsorption process of hydroxyapatite hollow microspheres for U(VI) was much
3.5. Adsorption kinetics For the purpose of studying the U(VI) adsorption kinetics, the
Fig. 7. XRD patterns (a) and FT-IR spectra (b) of hydroxyapatite hollow microspheres before and after U(VI) adsorption. 402
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Fig. 8. (a) XPS survey spectra of hydroxyapatite hollow microspheres before and after U(VI) adsorption. (b) XPS Ca 2p spectrum of the hydroxyapatite hollow microspheres before and after U(VI) adsorption. XPS O 1 s of the hydroxyapatite hollow microspheres before (c) and after (d) U(VI) adsorption. XPS U 4f5/2 (e) and U 4f7/2 (f) of the hydroxyapatite hollow microspheres after (e) U(VI) adsorption.
pseudo-first and second-order kinetic models were fitted with the experimental data and the results are shown in Fig. 6 and Table 3. Fig. 6 shows the linear plot of t versus t. The regression correlation coeffi-
samples before and after uranium U(VI) adsorption with EDS, XRD, FTIR and XPS. The EDS of hollow hydroxyapatite microspheres before and after uranium (VI) adsorption are shown in Fig. S2. The signals of U were detected in the sample after absorption, signifying U(VI) was accumulated on the hollow hydroxyapatite microspheres. The XRD patterns of the hydroxyapatite hollow microspheres before and after U(VI) adsorption are shown in Fig. 7a. Diffraction peaks located at 2θ (°) = 25.879, 31.773, 32.196, 32.902, 39.818, 46.711 and 49.468 matched well with the crystal planes of (002), (211), (112), (300), (130), (222) and (213) of hydroxyapatite (Ca5(PO4)3(OH), PDF# 090432), respectively. After adsorption, the diffraction peaks located at 16.432°, 20.969°, 24.571°, 25.464°, 27.567°, 34.263°, 42.717° were observed, which are due to the presence of autunite (Ca (UO2)2(PO4)2(H2O)3) (PDF no. 39-1351). These diffraction peaks of
qt
cients and constants of the pseudo-first and pseudo-second-order kinetic models are shown in Table 3. As can be seen that the correlation coefficients of the pseudo-second-order equation are high (0.999). A good fitting with the pseudo-second-order kinetic model infers that the adsorption reaction may be chemical adsorption, which may involve valency forces through sharing of electrons between and adsorbent and metal cations. 3.6. Removal mechanisms To further study the adsorption mechanism, we measured the 403
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autunite can be assigned to the crystal planes of (101), (002), (102), (200), (201), (103) and (212) of autunite (Ca(UO2)2(PO4)2·3H2O), respectively. The presence of autunite and the absence of hydroxyapatite indicated the uranyl ions were adsorbed and then eventually incorporated by hydroxyapatite hollow microspheres. Formation of Ca (UO2)2(PO4)2·3H2O by the interaction among uranyl ions and hydroxyapatite hollow microspheres is a probable mechanism contributing to the favorable sorption capacity to U(VI) with long-term stability. The FT-IR spectra of the hydroxyapatite hollow microspheres before and after adsorption of U(VI) are shown in Fig. 7b. The peaks at 3444 belong to stretching vibrations of the OeH [54]. The bands at 1039 cm−1, 1092 cm−1, and 960 cm−1 are the characteristic bands of phosphate stretching vibrations, while the bands at 605 cm−1 and 565 cm−1 are due to phosphate bending vibrations [55,56]. The peaks of 1408 cm−1 and 1591 cm−1 are related to the COO−1 in the Ca − PASP complex, suggesting that PASP molecules remain strongly bound in the HAP even after extensive washing, and the peak of 1591 cm−1 showed a shift of 54 cm−1 to higher wavenumbers [56]. Comparing the spectra of hydroxyapatite hollow microspheres before and after U(VI) adsorption, a new strong band at 913 cm−1 appeared, representing the stretching frequency of the linear structure of [O]UVI]O]2+, which further confirmed the successful chemical binding of uranyl ions with the ligands on the hydroxyapatite hollow microspheres [57]. The XPS analyses of the hydroxyapatite hollow microspheres before and after U(VI) adsorption were performed to provide more details on the chemical changes occurred on the surface of the samples before and after U(VI) adsorption (Fig. 8). From the survey spectra shown in Fig. 8a, the presence of C, O, Ca and P was detected through C 1 s (285.78 eV), O 1 s (531.30 eV), Ca 2p (346.37 eV), and P 2p (132.74) peaks for the hydroxyapatite hollow microspheres sample. Notably, a new strong double peak for the antisymmetric vibration of [O]UVI] O]2+ was observed for hydroxyapatite hollow microspheres after U(VI) adsorption. The corresponding high-resolution U 4f5/2 (393.1 eV) and U 4f7/2 (382.3 eV) core-level spectra (Fig. 8 (e)and (f)) revealed the existence of U(VI) in the hydroxyapatite hollow microspheres after U(VI) adsorption [58,59]. The binding energy value of Ca 2p on hydroxyapatite hollow microspheres increased from 347.1 eV to 347.3 eV upon U(VI) adsorption, indicating the obvious interactions between U (VI) and Ca (Fig. 8b) [56]. This was probably a consequence of the formation of covalent Ca − UO2 bonds, which resulted in a decrease of the extra nuclear electron cloud density surrounding Ca and in turn increased the binding energy of Ca 2p. As shown in Fig. 8c and d, the O 1 s spectrum was divided into three peaks, anion oxide (530.8 eV, O2−), hydroxyl bonded to calcium (531.2 eV, Ca − OH), and adsorbed H2O (532.5 eV), respectively [28]. It is obviously observed that the area ratio of Ca − OH peaks decreased from 64.4% to 60.8% after U(VI) adsorption, which confirmed that the surface hydroxyl groups (Ca − OH) played a significant role in U(VI) adsorption by sharing electrons for the formation of U − O bonds [60]. The chemical reaction between uranyl and hydroxyapatite took place upon U(VI) adsorption on the hierarchical hydroxyapatite hollow microspheres. Uranyl ion can be rapidly adsorbed and eventually incorporated into the Ca2+−PO43+−OH following the reaction as Eq. 6:
Ca10 (PO4 ) 6 (OH ) 2 + H2 O + UO22 +
Ca (UO2) 2 (PO4 ) 2 3H2 O
HAP hollow microspheres were adsorption and subsequent incorporation. Because of the incorporation of adsorbed uranyl, a new substance, viz., autunite (Ca(UO2)2(PO4)2·3H2O), was formed in the adsorbent. The adsorption kinetics followed the pseudo-second-order kinetic model, showing high performance of U(VI) removal within 5 min. The adsorption isotherm was well described by Freundlich isotherm. It is worth noting that hierarchical HAP hollow microspheres adsorb a large amount of U(VI) rapidly for its effective removal. On the basis of these evidences, the HAP hierarchical hollow microspheres are a powerful material for high-efficiency removal of uranium (VI) from the radioactive wastewater, can be supposed to a prompt emergency material for the remediation of nuclear leakage accident. Acknowledgements This work was supported by the National Natural Science Foundation of China (U1501231, 51708143, 51508116); The Project of Guangdong Provincial Key Laboratory of radioactive contamination control and resources (2017B030314182); Science and Technology Program of Guangzhou, China (201804020072, 201804010366); The Hong Kong Research Grants Council (E-PolyU503/17); Guangzhou University’s Training Program for Excellent New-recruited Doctors (YB201710); Scientific Research Foundation for the Returned Highlevel Overseas Talents (2018). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.02.110. References [1] M. Dittmar, Nuclear energy: Status and future limitations, Energy 37 (2012) 35–40. [2] D. Li, D.I. Kaplan, Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: a review, J. Hazard. Mater. 243 (2012) 1–18. [3] R. Hu, D.D. Shao, X.K. Wang, Graphene oxide/polypyrrole composites for highly selective enrichment of U(VI) from aqueous solutions, Polym. Chem. 5 (2014) 6207–6215. [4] A. Sakaguchi, P. Steier, Y. Takahashi, M. Yamamoto, Isotopic compositions of 236U and Pu isotopes in “black substances” collected from roadsides in Fukushima prefecture: fallout from the Fukushima dai-ichi nuclear power plant accident, Environ. Sci. Technol. 48 (2014) 3691–3697. [5] G. Steinhauser, A. Brandl, T.E. Johnson, Comparison of the Chernobyl and Fukushima nuclear accidents: a review of the environmental impacts, Sci. Total. Environ. 470–471 (2014) 800–817. [6] M. Kanematsu, N. Perdrial, W. Um, J. Chorover, P.A. O’Day, Influence of phosphate and silica on U(VI) precipitation from acidic and neutralized wastewaters, Environ. Sci. Technol. 48 (2014) 6097–6106. [7] T. Wannachod, T. Wongsawa, P. Ramakul, U. Pancharoen, S. Kheawhom, The synergistic extraction of uranium ions from monazite leach solution via HFSLM and its mass transfer, J. Ind. Eng. Chem. 33 (2016) 246–254. [8] Y.L. Chen, Y.Z. Wei, L.F. He, F.D. Tang, Separation of thorium and uranium in nitric acid solution using silica based anion exchange resin, J. Chromatogr. A 1466 (2016) 37–41. [9] C. Zhou, A. Ontiveros-Valencia, L. Cornette de Saint Cyr, A.S. Zevin, S.E. Carey, R. Krajmalnik-Brown, B.E. Rittmann, Uranium removal and microbial community in a H2-based membrane biofilm reactor, Water Res. 64 (2014) 255–264. [10] S.E. Bone, J.J. Dynes, J. Cliff, J.R. Bargar, Uranium(IV) adsorption by natural organic matter in anoxic sediments, P. Natl. Acad. Sci.USA. 114 (2017) 711–716. [11] S. Yu, X. Wang, H. Pang, R. Zhang, W. Song, D. Fu, T. Hayat, X. Wang, Boron nitride-based materials for the removal of pollutants from aqueous solutions: a review, Chem. Eng. J. 333 (2018) 343–360. [12] T.A. Saleh, Simultaneous adsorptive desulfurization of diesel fuel over bimetallic nanoparticles loaded on activated carbon, J. Clean. Prod. 172 (2018) 2123–2132. [13] T.A. Saleh, A. Sarı, M. Tuzen, Effective adsorption of antimony(III) from aqueous solutions by polyamide-graphene composite as a novel adsorbent, Chem. Eng. J. 307 (2017) 230–238. [14] G.H. Wang, J.S. Liu, X.G. Wang, Z.Y. Xie, N.S. Deng, Adsorption of uranium (VI) from aqueous solution onto cross-linked chitosan, J. Hazard. Mater. 168 (2009) 1053–1058. [15] S. Abdi, M. Nasiri, A. Mesbahi, M.H. Khani, Investigation of uranium (VI) adsorption by polypyrrole, J. Hazard. Mater. 332 (2017) 132–139. [16] S.B. Xie, C. Zhang, X.H. Zhou, J. Yang, X.J. Zhang, W.J. Song, Removal of uranium (VI) from aqueous solution by adsorption of hematite, J. Environ. Radioact. 100 (2009) 162–166. [17] S. Akyil, M. Eral, Preparation of composite adsorbents and their characteristics, J.
(6)
Similar results were reported by Han et al. [31] in which uranium immobilized by wasted bio-hydroxyapatite. 4. Conclusions In this study, novel hierarchical HAP hollow microspheres were successfully synthesized via a facile template-free hydrothermal process and this versatile material was used as an effective adsorbent to eliminate U(VI) from aqueous solution. The experimental results indicated that the main interaction mechanisms of U(VI) removal by hierarchical 404
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[45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59]
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