English

• 1. INTRODUCTION

  With the rapid development of economy and industry, excessive discharge of heavy metals has led to an increased concentration of various metals in aquatic systems. Heavy metal ions are not easily degraded and can be enriched with the food chain, thus endangering the ecological environment and human health^[1-3]. Therefore, the removal of heavy metal ions in wastewaters is becoming an important environmental issue. Uranium (U(Ⅵ)), an important chemical element of hexavalent actinides, is a chemically toxic and radioactive pollutant^[4]. It can be easily spilled into ecological system through a series of ways, such as unreasonable mining, nuclear weapons testing, and wastewaters from uranium mining companies^[5]. U(Ⅵ) in wastewaters can penetrate into groundwater, causing not only pollution of water resources, but also serious damage to local agriculture and fisheries^[6]. Considering the dangers of uranium to humans and the environment, it is crucial to remove U(Ⅵ) ions from polluted wastewaters effectively before they are released into the water bodies.

  Scientists have used a variety of methods such as ion exchange^[7], solvent extraction^[8], reverse osmosis^[9], membrane filtration^[10], chemical precipitation^[11], and sorption^{[12, 13]} to remove uranium from aqueous solutions. Among them, adsorption technique is one of the most available and promising methods to remove U(Ⅵ) and widely used because of its low cost, high adsorption capacity, fast adsorption speed, and recyclability of the adsorbent^[14]. In recent years, polymer hydrogel materials have been widely used for adsorption applications due to their good water permeability, biodegradability and biocompatibility^{[15, 16]}.

  It is well known that β-cyclodextrin has a special coneshaped structure of hydrophobic inner cavity and a hydrophilic external surface, which allows it to form stable inclusion complexes with guest molecules^{[17, 18]}. In addition, its coneshaped structure can enable β-CD to be encapsulated with materials of a specific size and shape, facilitating selective adsorption of different metal ions^[19]. The excellent biocompatibility of β-CD and its degradability ensure that it does not cause secondary pollution when treating sewage^[20]. Hence, β-CD has the potential to be used as an adsorbent for the removal U(Ⅵ) ions. However, when β-cyclodextrin is used as an adsorbent, there are still disadvantages of poor solubility and poor stability. To compensate for these shortages, esterification and crosslinking reaction are used to improve its solubility and thermal stability^{[21, 22]}. PAA hydrogel contains a large number of carboxyl groups, which have strong binding capacity with heavy metal cations^[23]. Permutite (PT) has attracted the attention of researches due to the outstanding properties which are low price, large specific surface area, and good thermal stability^[24]. Embedding permutite into hydrogels may be a more ideal method because it can combine the advantages of both. In this work, β-cyclodextrin/acrylic acid/permutite (CAP) hydrogel composite was synthesized via the formation of C–C single bonds between the AA monomers, ester bonds between the hydroxyl groups in β-CD and the carboxyl groups in the AA.

  The aim of this work was to study the sorption capacity of CAP composite material for removing U(Ⅵ) from aqueous solutions. A series of physical properties of the samples were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and thermogravimetric analysis (TGA). The sorption behavior of U(Ⅵ) on CAP composite was investigated by exploring the effects of pH of the solution, contact time, initial concentration, and temperature. Sorption kinetics, isotherms, and thermodynamics were also studied. Furthermore, the properties of reusability and regeneration were investigated to evaluate its application potential.

  2. EXPERIMENTAL

  2.1 Materials

  β-Cyclodextrin (β-CD) was purchased from Shanghai yuanye Bio-Technology Co., Ltd. Acrylic acid (AA), N, Nmethylenebisacrylamide (MBA), and potassium persulfate (KPS) were all obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH) was purchased from Xilong Chemical Co., Ltd. Permutite (PT) was provided by Shanghai Aladdin Bio-Chem Technology Co., LTD. All the above chemicals were of analytical grade and were used without further purification. Solutions were prepared with deionized water prepared by Fuzhou University. Stock solutions (1000 mg/L) of U (Ⅵ) ions were prepared by dissolving 1.859 g UO₂ (NO₃)₂·6H₂O in 1000 mL distilled water. The solutions of different concentrations used in various experiments were obtained by dilution of the stock solutions.

  2.2 Preparation of the CAP composite material

  The CAP composite was prepared by the following procedures: Firstly, β-CD (1.5 g) was added into NaOH solution (30 mL, 11%) with mechanical stirring. 5% of PT was added to a solution of β-CD and stirred for 1 h. 0.18 g MBA was dissolved in 5 mL deionized water followed by the addition of 8 mL acrylic monomer under continuous stirring. Then the solution was poured into the β-CD solution after both solutes were well distributed. Then, the reaction mixture was transferred to a water bath (50 ℃) for 1 h followed by the addition of 0.08 g KPS dissolving in 5 mL H₂O with mechanical stirring. Finally, the obtained solution above was reacted in a water bath at 70 ℃ for 2 h. The CAP hydrogel material was chopped and rinsed with deionized water for several times and dried in a vacuum freeze drier for 24 h to constant weight. The product was then milled and passed through a 100-mesh screen and stored in vacuum drying oven at 50 ℃.

  2.3 Characterization methods

  FTIR spectra of the samples were recorded with a Fourier transform infrared spectrometer (Nicolet Magna 670) in the range of 4000~400 cm⁻¹. X-ray diffraction (XRD) patterns of PT and CAP were recorded by an X-ray diffractometer (Empyrean, PANalytical, Netherlands) with Cu-Kα radiation. The XRD spectra were recorded in the range of 5~70°. The morphologies and composition of the samples were investigated by scanning electron microscope (SEM, Nova Nano 230) and energy dispersive X-ray (EDX) spectrometer. The CAP before and CAP after adsorption were dried and coated with gold. Thermogravimetric analysis (TGA, STA449C/6/G, NETZSCH) was performed under nitrogen atmosphere from 30 to 700 ℃ at a heating rate of 10 ℃/min.

  2.4 Adsorption experiments

  In order to test the adsorption performance of CAP, the adsorption experiment of U(Ⅵ) ions on CAP was carried out. All batch adsorption experiments were performed by adding 10 mg of CAP to 20 mL of UO₂²⁺ solution at 150 rpm on a thermostatic shaker. At the completion of preset time intervals, residual U(Ⅵ) cations concentration of the suspension was determined by using a UV-Vis spectrophotometer (UV-1780, Shimadzu, Japan) at 650 nm according to the calibration curve^[25]. The adsorption capacity of adsorbed U (Ⅵ) on CAP was calculated by the following equation:

  $ q_e=V\left(C_0-C_e\right) / m$

  (1)

  where q_e (mg/g) is the equilibrium adsorption capacity of adsorbent; C₀ and C_e (mg/L) are the initial and final concentration of the U (Ⅵ) ions solution, respectively; m (mg) is the weight of the dry adsorbent, and V (mL) is the volume of the U (Ⅵ) ions solution.

  The effect of pH level on U(Ⅵ) removal was studied by adding different either nitric acid or sodium hydroxide standardized solutions to U(Ⅵ) ions solution (500 mg/L) and adsorbing at 25 ℃ for 180 min. The initial concentration effect experiments were performed by adjusting the initial concentration of U(Ⅵ) ions solution. The influence of temperature was explored by adjusting the UO₂²⁺ solution (500 mg/L, pH 4.0~4.5) to different temperature and kept continuously stirred for 180 min. In order to investigate adsorption kinetics, 10 mg CAP was added to a 20 mL solution of 500 mg/L UO₂²⁺ at pH 4.0~4.5 and stirred at regular intervals. Each experiment was done in triplicate and all the data of batch adsorption experiments were the average values.

  2.5 Desorption and reusability studies

  For excellent adsorbents, good desorption and regeneration performance are important for its practical application. The regeneration experiment was carried out using 0.1 M HNO₃ solution as eluent solution. First, 10 mg CAP was added to 20 mL of UO₂²⁺ solution (500 mg/L) under optimum conditions, and then U(Ⅵ)-loaded CAP was desorbed in 10 mL of HNO₃ solution with agitating for 120 min. After that, the loaded adsorbent was sufficiently washed with distilled water to neutral for further reuse. This adsorption/desorption cycle was performed for five times under the same conditions to evaluate regeneration ability.

  3. RESULTS AND DISCUSSION

  3.1 Characterization

  Fig. 1 illustrates the XRD patterns of bare PT, CD/PAA and CAP. Compared with CD/PAA, the main characteristic peaks appearing at 7.2º, 20.4º, 26.1º and 30.8º were attributed to the crystalline structure of the PT powder, corresponding to the [220], [311], [400], [522], [511] and [400] planes of the PT powder (PDF#89-5423), respectively^{[26, 24]}. The results suggested that PT was successfully incorporated into the CAP hydrogel composite.

  Figure 1

  

  Figure 1.  XRD patterns of PT, β-CD/AA, and CAP

  DownLoad: Full-Size Img PowerPoint

  Fig. 2 shows the FTIR spectra of AA, β-CD, PT, β-CD/AA, CAP, and CAP-U. For β-CD, the characteristic absorption peaks at 3304, 2926, 1020 and 944 cm^-1 are attributed to the stretching vibration of -OH, bending vibration of -CH₂, tensile vibration of -C–O-, and α-1, 4 skeleton vibration of β-CD^{[27, 28]}. In the spectrum of AA, the peaks at 1695 and 1635 cm^-1 correspond to the stretching vibration peak of carboxyl groups C=O and C=C, respectively^[29]. The spectral bands at 994, 669 and 451 cm^-1 are assigned to the asymmetry and symmetric stretching vibration of (Si, Al)–O and the bending vibration of Si–O in the PT, as well as the peak appearing around 548 cm^-1 is attributed to the external vibration of the double quadruplex^[30]. The major peaks for β-CD/AA can be assigned as follows: 3316 cm^-1 (O–H stretching vibration), 2929 cm^-1 (–CH₂ bending vibration), 1658 cm^-1 (C=O stretching vibration of ester group) and 1254 cm^-1 (C–O–C stretching vibration of ester group). However, in the case of adding PT into β-CD/AA, it can be distinctly observed that there appears a new peak at 1030 cm^-1, which may be the asymmetric stretching vibration of (Si, Al)–O. In addition, the FTIR spectrum of CAP composite is approximately similar to that of β-CD/AA, which can be attributed to the relatively small proportion of PT in the composite. After the adsorption of UO₂²⁺, compared with CAP, the peak at about 919 cm^-1 is corresponding to the stretching vibration of O=U=O in the IR spectrum of CAP–U^[31]. The carboxyl group in acrylic acid is ionized in the presence of sodium hydroxide to form -COO-, and the structure of permutite contains exchangeable cations. There may be electrostatic interaction between them. This result indicates that UO₂²⁺ cations were adsorbed on CAP hydrogel composite.

  Figure 2

  

  Figure 2.  FT-IR spectra of AA, β-CD, PT, β-CD/AA, CAP, and CAP-U

  DownLoad: Full-Size Img PowerPoint

  The microstructures of CAP before and after adsorption were characterized from their SEM images. From Figs. 3a and 3b, the surface of CAP hydrogel composite before adsorption had rough, uneven surfaces, and some holes, which revealed that the pristine CAP composite had a large specific surface area. After adsorption of UO₂²⁺, as indicated in Figs. 3c and 3d, it can be clearly seen that the surfaces of microspheres were smooth, and in addition, the previously roughened pores were filled up by a great deal of floccules. The results proved that U(Ⅵ) was absorbed in the pores of the CAP composite and formed a layer of floccules on the composite.

  Figure 3

  

  Figure 3.  SEM images of CAP hydrogel composite

  DownLoad: Full-Size Img PowerPoint

  Energy dispersive X-ray (EDX) can analyze the elemental composition of the samples. Figs. 4a and 4b are representative EDX spectra of the CAP composite before and after adsorption of U(Ⅵ), respectively. It can be distinctly observed that the CAP composite before adsorption contains characteristic elements (Al, Si, Mg) of permutite, indicating it has been successfully prepared. By comparing Fig. 4b with Fig. 4a, a new peak of U appeared in the EDX spectrum, so UO₂²⁺ was successfully adsorbed onto CAP.

  Figure 4

  

  Figure 4.  EDX spectra of CAP

  DownLoad: Full-Size Img PowerPoint

  Fig. 5 shows the thermogravimetric curves of CD/PAA and CAP in the temperature range from 30 to 700 ℃. Clearly, the thermal stability of CAP was higher than that of CD/PAA. The thermal degradation of CAP occurred at three stages. The first weight loss ranged between 30 and 150 ℃ is due to the loss of adsorbed and bound water^[32]. The second stage of weight loss was associated with the thermal scission of carboxyl groups and ester bond formatting between β-CD and PAA^[33]. In the third stage, the loss of weight was due to the degradation of β-CD. This finding further supported the introduction of PT could enhance the thermal stability of the adsorbent.

  Figure 5

  

  Figure 5.  TG curves of CD/PAA and CAP

  DownLoad: Full-Size Img PowerPoint

  3.2 Effect of pH on U(Ⅵ) adsorption

  The solution pH is an important factor in affecting the adsorption of U(Ⅵ) by the adsorbent^[34]. The experiments were carried out in the pH range of 2~6 and the results of U(Ⅵ) adsorption by the CAP at various pH levels are illustrated in Fig. 6a. It can be seen that the adsorption capacity of U(Ⅵ) increased with increasing the solution pH from 2.0 to 3.5, and the adsorption capacity of the hydrogel for U(Ⅵ) was gradually increased. And then the adsorption capacity increases to 730.26 mg/g as pH reached to 4.5. Afterwards, the sorption capacity gradually decreased as the pH was increased from 4.5 to 6. These results may be ascribed to the following reasons. At low pH, the hydroxy groups on β-CD are positively charged and the carboxyl groups on the surface of the CAP composite are protonated, leading to the electrostatic repulsion between adsorbed H⁺ and UO₂^{2+[28, 35]}. When the pH increases, the hydroxyl and carboxyl groups tend to deprotonate and more activated deprotonation carboxyl groups can form electrostatic interaction with UO₂²⁺, leading to an increase in the adsorption capacity. In the pH range of 3.5~4.5, U(Ⅵ) can form different complexes with the carboxyl and hydroxy groups and -COO^- is the main existing form of carboxyl groups. However, the ion exchange effect of UO₂²⁺ ions is reduced, so the adsorption capacity reaches a plateau in general. As shown in Fig. 7, at pH > 4.5, the predominant U(Ⅵ) species are UO₂OH⁺, (UO₂)₂(OH)₂²⁺ and (UO₂)₃(OH)⁵⁺. As the pH of the U(Ⅵ) ions solution increases, the UO₂²⁺ ions may be hydrolyzed to form negative groups and they are difficult to be immobilized on the negatively charged surfaces of CAP composite due to electrostatic repulsion, leading to a serious decrease of adsorption capacity. Therefore, the following studies were conducted at the range value of pH 4.0~4.5.

  Figure 6

  

  Figure 6.  Effect of initial solution pH, initial concentrations and adsorption time; Adsorption kinetics plots of U(Ⅵ) ions onto CAP

  DownLoad: Full-Size Img PowerPoint

  Figure 7

  

  Figure 7.  Species distribution of U (Ⅵ) as a function of pH values 3.3Effect of initial concentration on the U(Ⅵ) adsorption

  DownLoad: Full-Size Img PowerPoint

  3.3 Effect of initial concentration on the U(VI) adsorption

  Fig. 6b illustrates the effect of initial concentration of U(Ⅵ) ions on the adsorption by the CAP composite. Clearly, with the increase of initial concentration, the adsorption capacities of U(Ⅵ) ions also increased, and then the adsorption capacity did not change a lot after the concentration reached 600 mg/L. This may be because at lower initial concentration, there were enough available adsorption sites on the adsorbents for the U(Ⅵ) ions. At enough high initial concentration of U(Ⅵ) ions solution, the adsorption almost reached equilibrium because the adsorption sites were saturated.

  3.4 Adsorption kinetics

  The adsorption equilibrium time has been shown to be an important indicator that evaluates the adsorption process. Fig. 6c shows the adsorption capacity of CAP hydrogel for U(Ⅵ) ions changing over time. It can be observed that the adsorption of U(Ⅵ) ions increased rapidly during the first 30 min of contact time, and then increased slowly until the sorption process finally approached equilibrium. The fast removal rate during the initial stage may be due to a large number of empty adsorption sites on the surface of the adsorbents. As these adsorption sites were gradually occupied, the adsorption rate of U(Ⅵ) ions was gradually slowed down. Then the adsorption sites were saturated, so the adsorption capacity became nearly constant. Overall, the adsorption process is quite fast and 3 h is enough to reach equilibrium. This phenomenon indicates that CAP hydrogels can be used in continuous wastewater treatment.

  In order to further evaluate the adsorption mechanism of CAP, the adsorption kinetics was studied by pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models. These three models can be expressed using the following equations, respectively^[36]:

  $ \ln \left(q_e-q_t\right)=\ln q_e-k_1 t $

  (2)

  $ t / q_t=1 / k_2 q_e^2+t / q_e $

  (3)

  $ q_t=k_i t^{1 / 2}+C $

  (4)

  Where t (min) is the adsorption time; q_e and q_t are the adsorption capacity (mg/g) at adsorption equilibrium and time t, respectively; k₁ and k₂ are pseudo-first-order and pseudosecond-order rate constants, respectively; k_i is the intraparticle diffusion rate constants; and C is the thickness of the boundary layer.

  The sorption curves of three kinetics models are shown in Figs. 6d, 6e and 6f. Table 1 summarizes the kinetic parameters for the pseudo-first order model, pseudo-second order model and intraparticle diffusion model. It can be observed that correlation coefficients (R²) of the pseudo-second-order model are much higher than the R² value of the pseudo-first-order model, and the q_e values calculated from the former model agreed with the experimental data (q_{e, exp}). The results suggested that the adsorption behavior of UO₂²⁺ onto the CAP hydrogels could be explained by a pseudo-second-order model, and the adsorption mechanism of U(Ⅵ) on CAP is chemisorption^[37]. Fig. 6f illustrates the intraparticle diffusion plots of the U(Ⅵ) ions adsorption by CAP composite. It can be seen that the whole adsorption process can be divided into three parts. The first portion was the surface diffusion stage, UO₂²⁺ in solution was absorbed onto the exterior surface of the adsorbents and the adsorption rate was the fastest. The second stage represented the U(Ⅵ) ions began to diffuse into the pores of the sorbents. In this phase, the adsorption rate slowed down. The third portion was the final equilibrium stage, and the adsorption capacity almost did not change with time, demonstrating that adsorption has reached equilibrium. Overall, the adsorption process of U (Ⅵ) may be affected by both chemical chelating reaction and intra-particle diffusion.

  Table 1

  

  Table 1.  Kinetic Parameters of U (Ⅵ) Adsorption onto CAP

  DownLoad: CSV

  Kinetic	Parameter	UO22+
  Pseudo-first-order	qe, exp (mg/g)	730.77
  	qe, cal (mg/g)	602.76
  	k1 (min-1)	0.0351
  	R2	0.9682
  Pseudo-second-order	qe, exp (mg/g)	730.77
  	qe, cal (mg/g)	781.25
  	k2 (g/mg/min)	9.70×10-5
  	R2	0.9993
  Intra-particle diffusion	ki1 (mg/g/min)	113.6033
  	ki2 (mg/g/min)	29.3928
  	ki2 (mg/g/min)	3.1906

  3.5 Adsorption isotherms

  In order to better study the U(Ⅵ) adsorption mechanism, the Langmuir and Freundlich isotherm models were used to investigate the adsorption isotherms. The Langmuir isotherm model is a theoretical model of monolayer adsorption and can be expressed as^[38]:

  $ C_e / q_e=1 / q_m K_L+C_e / q_m $

  (5)

  The Freundlich isotherm model is a semi-empirical equation describing multilayer adsorption and it is usually written as^[39]:

  $ \lg q_e=\lg K_F+1 / n \lg C_e $

  (6)

  Where C_e (mg/L) is the equilibrium concentration of U(Ⅵ) ions; q_e and q_m (mg/g) are the equilibrium and maximum adsorption capacity, respectively; K_L (L/mg) is the Langmuir binding equilibrium constant; K_F and n represent the Freundlich adsorption equilibrium constant, respectively.

  R_L is an important dimensionless parameter of the Langmuir isothermal model, which can be used to evaluate the suitability of adsorbents for U(Ⅵ) ions^[40]. The equation is expressed by Eq. (7):

  $ R_L=1 /\left(1+K_L C_0\right) $

  (7)

  Where C₀ (mg/L) is the initial concentration of U(Ⅵ) ions. R_L can be used to determine the adsorption process to be unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1), and irreversible (R_L = 0)^[41].

  The experimental data of U(Ⅵ) adsorption by CAP hydrogel were simulated by two isotherm models, and the results are shown in Figs. 8a and 8b. The relative values calculated from the two models are summarized in Table 2. It could be clearly observed that correlation coefficients of the Langmuir model were much higher than that of the Freundlich model, which suggested that the adsorption of U(Ⅵ) ions was monolayer adsorption^[35]. Based on the Langmuir model, the theoretical maximum adsorption capacity q_m of CAP hydrogel was the highest at 308.15 K and the lowest at 288.15 K, indicating that the adsorption capacity increased with increasing the temperature. In addition, the values of separation factor (R_L) at different temperature were in the range of 0 to 1, thereby confirming that the adsorption of uranium ions by CAP hydrogel was favorable^{[42, 28]}.

  Figure 8

  

  Figure 8.  Sorption isotherms plots and the effect of adsorption temperature

  DownLoad: Full-Size Img PowerPoint

  Table 2

  

  Table 2.  Isotherm Parameters for the Adsorption of U (Ⅵ) onto CAP

  DownLoad: CSV

  Isotherm	Parameter	288.15 K	298.15 K	308.15 K
  Langmuir	qm	800.06	833.33	862.07
  	KL	0.0403	0.0480	0.0638
  	R2	0.99935	0.99943	0.99969
  	RL	0.0242~0.1104	0.0204~0.1041	0.0154~0.0727
  Freundlich	KF	207.20	226.93	271.17
  	1/n	0.2200	0.2137	0.1930
  	R2	0.7325	0.8204	0.8106

  3.6 Effect of temperature and thermodynamics studies

  The effect of temperature on the adsorption of CAP hydrogel is shown in Fig. 7c. The adsorption capacity of CAP increased with increasing temperature, which represented that the adsorption may be an endothermic process. To further evaluate the effect of temperature on adsorption process, three thermodynamic parameters, enthalpy change (ΔH), entropy change (ΔS), and Gibbs free energy (ΔG), were calculated using the following equations^[43]:

  $ \Delta G=-R T \ln K_C $

  (8)

  $\ln K_C=\Delta S / R-\Delta H / R T $

  (9)

  $ K_C=q_e / C_e $

  (10)

  Where R is the gas constant (8.314 J/mol/K), T (K) is the absolute temperature, and K_C is the adsorption equilibrium constant. The values of ΔH and ΔS were calculated from the slopes and intercepts of the linear variation of ln K_C versus 1/T, as shown in Fig. 7d. The results are summarized in Table 3.

  Table 3

  

  Table 3.  Values of Thermodynamic Parameters for U(Ⅵ) Sorption on CAP

  DownLoad: CSV

  ΔG (kJ/mol)				ΔH (kJ/mol)	ΔS (J/mol/K)	R2
  288.15K	298.15 K	308.15 K	318.15 K
  –3.82	–4.21	–4.59	–4.98	7.33	38.63	0.965

  The ΔG values were negative at all the studied temperature, indicating the adsorption process of U(Ⅵ) ions was spontaneous^[44]. The positive value of ΔH confirmed that the adsorption of U(Ⅵ) ions by CAP hydrogels was an endothermic process and the adsorption would be enhanced with the rise of temperature. It also indicated that the adsorption of U(Ⅵ) on CAP was mainly chemisorption mechanism, which may be due to the chelation of groups and U(Ⅵ) ions^[45]. The positive value of ΔS indicated that the adsorption of U(Ⅵ) ions was a process from disorder to order^[46].

  3.7 Elution and reusability studies

  From the perspective of cost effective and environmental sustainability, the regeneration and reuses of adsorbents are the key to reduce the cost of sewage treatment. In this work, the reusability of CAP composite was evaluated by five consecutive adsorption-desorption cycles. The results clarified that the sorption capacity was slightly decreased after five cycles as judged from Fig. 9, confirming that CAP composite can be recycled and reused to adsorb UO₂²⁺ in solution.

  Figure 9

  

  Figure 9.  Adsorption capacity of U(Ⅵ) on the CAP composite in five successive cycles of desorption-adsorption

  DownLoad: Full-Size Img PowerPoint

  3.8 Comparison with other adsorbents

  Table 4 shows the U(Ⅵ) sorption capacity by CAP hydrogel composite and other adsorbents. It can be seen in Table 4, the U(Ⅵ) absorption capacity of CAP hydrogel composite is higher than that of activated carbon^[47], TiO₂/β-zeolite^[48], polyacrylic acid hydrogels^[29], CD/MMT/iron oxide^[34], PAAAM hydrogels^[49], and magnetic chitosan/graphene oxide nanocomposites^[50]. Although direct comparison of CAP with other adsorbents is very difficult owing to different experimental conditions, it is concluded that the sorption capacity of CAP is higher than that of other adsorbents. The results confirm CAP hydrogel composite can be a prospective adsorbent due to its superior adsorption capacity and excellent regeneration capacity.

  Table 4

  

  Table 4.  Comparison of U(Ⅵ) Adsorption Capacity by Various Sorbents

  DownLoad: CSV

  Adsorbent	qm (mg/g)	References
  Activated carbon	45.24	[47]
  TiO2/β-zeolite	49.25	[48]
  Polyacrylic acid hydrogels	445.11	[29]
  CD/MMT/iron oxide	109.94	[34]
  PAAAM hydrogels	713.24	[49]
  Magnetic chitosan/graphene oxide Nanocomposites	204.10	[50]
  CAP hydrogels	833.33	This work

  4. CONCLUSION

  In this research, the CAP composite hydrogel was successfully prepared and used as an excellent adsorbent for the removal of UO₂²⁺ ions from aqueous solution. The physicochemical properties of the composites were characterized by FTIR, XRD, SEM, EDX, and TGA. The sorption of U(Ⅵ) relied on contact time, pH, and initial UO₂²⁺ ions concentration. The adsorption process fitted well with the pseudo-second-order model, suggesting that the adsorption mechanism of U(Ⅵ) on CAP is chemisorption. The Langmuir isotherm model fitted the experimental data and the thermodynamic parameters demonstrated the sorption process was endothermic and spontaneous. From the results of reusability, this adsorbent material showed high recyclable removal efficiency. Considering the obtained results, CAP composite hydrogel may be widely applied as a promising adsorbent for the cost-effective treatment of U(Ⅵ) in wastewater.

  
  
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• 

  Figure 1  XRD patterns of PT, β-CD/AA, and CAP

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  Figure 2  FT-IR spectra of AA, β-CD, PT, β-CD/AA, CAP, and CAP-U

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  Figure 3  SEM images of CAP hydrogel composite

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  Figure 4  EDX spectra of CAP

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  Figure 5  TG curves of CD/PAA and CAP

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  Figure 6  Effect of initial solution pH, initial concentrations and adsorption time; Adsorption kinetics plots of U(Ⅵ) ions onto CAP

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  Figure 7  Species distribution of U (Ⅵ) as a function of pH values 3.3Effect of initial concentration on the U(Ⅵ) adsorption

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  Figure 8  Sorption isotherms plots and the effect of adsorption temperature

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  Figure 9  Adsorption capacity of U(Ⅵ) on the CAP composite in five successive cycles of desorption-adsorption

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  Table 1.  Kinetic Parameters of U (Ⅵ) Adsorption onto CAP

  Kinetic	Parameter	UO22+
  Pseudo-first-order	qe, exp (mg/g)	730.77
  	qe, cal (mg/g)	602.76
  	k1 (min-1)	0.0351
  	R2	0.9682
  Pseudo-second-order	qe, exp (mg/g)	730.77
  	qe, cal (mg/g)	781.25
  	k2 (g/mg/min)	9.70×10-5
  	R2	0.9993
  Intra-particle diffusion	ki1 (mg/g/min)	113.6033
  	ki2 (mg/g/min)	29.3928
  	ki2 (mg/g/min)	3.1906

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  Table 2.  Isotherm Parameters for the Adsorption of U (Ⅵ) onto CAP

  Isotherm	Parameter	288.15 K	298.15 K	308.15 K
  Langmuir	qm	800.06	833.33	862.07
  	KL	0.0403	0.0480	0.0638
  	R2	0.99935	0.99943	0.99969
  	RL	0.0242~0.1104	0.0204~0.1041	0.0154~0.0727
  Freundlich	KF	207.20	226.93	271.17
  	1/n	0.2200	0.2137	0.1930
  	R2	0.7325	0.8204	0.8106

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  Table 3.  Values of Thermodynamic Parameters for U(Ⅵ) Sorption on CAP

  ΔG (kJ/mol)				ΔH (kJ/mol)	ΔS (J/mol/K)	R2
  288.15K	298.15 K	308.15 K	318.15 K
  –3.82	–4.21	–4.59	–4.98	7.33	38.63	0.965

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  Table 4.  Comparison of U(Ⅵ) Adsorption Capacity by Various Sorbents

  Adsorbent	qm (mg/g)	References
  Activated carbon	45.24	[47]
  TiO2/β-zeolite	49.25	[48]
  Polyacrylic acid hydrogels	445.11	[29]
  CD/MMT/iron oxide	109.94	[34]
  PAAAM hydrogels	713.24	[49]
  Magnetic chitosan/graphene oxide Nanocomposites	204.10	[50]
  CAP hydrogels	833.33	This work

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