Intercalation of glycine into hydroxy double salt and its adsorption performance towards Uranium(VI)

Environmental Technology & Innovation 16 (2019) 100474

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Intercalation of glycine into hydroxy double salt and its adsorption performance towards Uranium(VI) ∗

Alemtsehay Tesfay Reda, Dongxiang Zhang , Xin Lu School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 102488, People’s Republic of China

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Article history: Received 6 December 2018 Received in revised form 14 July 2019 Accepted 31 August 2019 Available online 7 September 2019 Keywords: Uranium(VI) Adsorption Hydroxy double salts Glycine Intercalation

a b s t r a c t Human activities and natural processes lead to environmental water pollution by Uranium. In the field of environmental remediation, much attention has been paid to the development of an efficient adsorbent for removing uranyl ion (UO22+ ) from contaminated water. In this work, a new material was synthesized by intercalation of glycine (Gly) into zinc containing hydroxy double salt (Zn5 -NO3 ), and its adsorption properties towards uranyl ion were studied in an aqueous solution. The X-ray diffraction (XRD) analysis of the composite material (Zn5 -Gly) indicated that Gly was inserted into the gallery of Zn5 -NO3 with bilayer arrangement. The adsorption capacity of Zn5 -Gly towards UO22+ in an aqueous solution is 745.53 mg/g and removal rate close to 100%, these criteria make it a potential candidate for uranium removal. The adsorbent decreased 233.38 ppm of UO22+ to 23.47 ppm inside 120 min and finally to 0.79 ppm within 7 h. The adsorption kinetics of UO22+ fits with the pseudo-second-order model, suggesting a chemisorption mechanism mainly via amino and carboxyl groups interactions. The adsorption of UO22+ by the adsorbent was confirmed by XRD, Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). © 2019 Published by Elsevier B.V.

1. Introduction Human activities, including nuclear weapon developments, use of materials for nuclear energy, mining, and natural processes lead to uranium contamination of environmental waters (Wang et al., 2015b). Different health problems, such as renal damage and cancer, caused by the release of uranium decontaminated water to the environment (Şimşek et al., 2013). Main factors which have an influence on uranium removal from wastewater are the degree of speciation and reaction of UO2 2+ with OH and CO3 2− (Meinrath, 1998; Pan et al., 2017). Decontaminating of U(VI) contaminated water is essential for environmental safety and access to more energy. Ion exchange (Aly and Hamza, 2013), neutralization (Aydin and Soylak, 2007), liquid–liquid extractions (Beltrami et al., 2014), adsorption (Huynh et al., 2017; Jang et al., 2007; Webb et al., 2006) co-precipitation (Špendlíková et al., 2017) and chemical reduction (Stirling et al., 2015) are some methods of uranium uptake from wastewater. The adsorption method has the advantages of low cost, relatively simple design and strong operability, and high removal efficiency of uranium ions in aqueous solution (Yu et al., 2015; Zou et al., 2015). Adsorption properties of heavy metal ion in an aqueous solution by metal oxides (Wang et al., 2013), modified graphene oxides (Ding et al., 2017; Song et al., 2015; Sun et al., 2017; Zhang et al., 2013b), layered double hydroxide composites with different intercalants (Asiabi et al., 2018; Chen et al., 2018a,b; Ma et al., 2015; Reda and Zhang, 2019; Zou et al., 2017a), amidoxime related materials (Lu et al., 2017; Qin et al., 2017; ∗ Corresponding author. E-mail address: [email protected] (D. Zhang). https://doi.org/10.1016/j.eti.2019.100474 2352-1864/© 2019 Published by Elsevier B.V.

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A. Tesfay Reda, D. Zhang and X. Lu / Environmental Technology & Innovation 16 (2019) 100474

Zeng et al., 2017), novel-nanomaterials (Wu et al., 2019) and bio-sorbents (Wang et al., 2015a; Yi et al., 2016) have been studied. These materials show good adsorption capacity towards uranium, however, developing materials with better adsorption efficiency (than the existing adsorbents) is a continuous demand in the field of wastewater treatment. Hydroxy double salts (HDSs) consist of layered materials with a general formula of M2+ (OH)2−x (An )x/n · zH2 O (M = metal(II) cations; A = anionic species (e.g., NO3 − )). HDS has similar structural characteristics of positively charged layers and interlayer anions with layered double hydroxides (LDHs). In LDH, the divalent metal ions coordinated by six OH groups but in HDS some of the OH groups are replaced by other anions (Bull et al., 2011; Cardiel et al., 2017; Meyn et al., 1993; Richardson-Chong et al., 2012). The structure of HDSs held by weak forces, therefore, the anions can be easily substituted and used for different applications such as ion exchange (Meyn et al., 1993), sensing (Andreoli et al., 2011; Xia and Ning, 2011), and drugs (Taj et al., 2013). Simple amino acids, such as glycine, possess acidic and basic functional groups that can coexist in an ionized form. Glycine contains carboxylic and amino groups that bind to UO2 2+ ion (Ruby et al., 2012). Results of recent publication showed that glycine-functionalized metal hydroxides have an effective adsorption capacity for uranyl ion in aqueous solution (Tesfay Reda et al., 2018). Intercalation interactions are important to improve the chemical properties of original material. Intercalation chemistry for Layered materials such as HDS covers a large range as hosts. Glycine, as intercalant, can be inserted into the gallery of hydroxy double salt (HDS) to get chemically improved material. This composite material possesses more functional groups due to the glycine groups, and can result in higher adsorption capacity towards uranyl ion because of the combined effect of both organic and inorganic materials compared with the original layered material. Here we investigated intercalation behaviour of glycine in [Zn5 (OH)8 ](NO3 )2 .2H2 O HDS gallery, and assessed its adsorption capacity towards uranium(VI) in an aqueous solution. As far as we know, this work is the first report that HDS has been used to remove heavy metal ions from aqueous solution. It can open a door for further investigation about the HDS for such applications. 2. Experimental 2.1. Materials Reagents such as glycine, ZnO, Zn(NO3 )2 ·6H2 O, HNO3 , and NaOH purchased from corporations of Aladdin and Klamar, which were all analytically pure grades and used directly without further purification. A stock solution of uranium was supplied by Analytical Laboratory, Beijing Research Institute of Uranium Geology, China. Deionized water from a Milli-Q plus water purification system (Millipore) used in the synthesis process and UV-pure water used throughout the adsorption–desorption processes. 2.2. Synthesis Starting material and intercalation: The hydroxy double salt, [Zn5 (OH)8 ](NO3 )2 .2H2 O abbreviated as Zn5 -NO3 , synthesized according to a previous report (Richardson-Chong et al., 2012). Glycine was intercalated into the interlayer of Zn5 -NO3 as follow: Gly and Zn5 -NO3 dissolved in deionized water separately in a molar ratio of 0.81:10 and finally mixed together. An aqueous solution of NaOH added dropwise to the solution under stirring at room temperature. After ultrasonicate for about 30 min, the final solution stirred at 70 ◦ C for some time. Finally, filtered and washed with degassed deionized water, white powder (Zn5 –Gly) recovered after vacuum drying. 2.3. Adsorption experiments The adsorption properties of Zn5 -Gly on uranium (VI) in aqueous solution were studied by batch experiments. The concentrations of uranium solution were adjusted using ultra-pure water, and concentrated HNO3 or NaOH were used to adjust the pH of the solution. The solid adsorbent added to the solution, which contains uranium, followed by shaking of the mixed solution from 5 min to 48 h. After filtration by 0.22-µm pore-size filter, the concentration of uranyl ion was analysed using Microwave Plasma-Atomic Emission Spectrometer (MP-AES, 4100, Agilent). Distribution coefficient (Kd ), used to determine the selectivity of the adsorbent, removal rate (%R), and removal capacity (qm ) calculated by Eqs. (1)–(3), respectively (Ma et al., 2016): Kd = %R = qm =

(Co − Cf ) Cf (Co − Cf ) Co (Co − Cf ) m

∗

V m

∗ 100 ∗

V 1000

(1) (2) (3)

where Co and Cf are the initial and equilibrium concentrations, respectively, of the target ions (ppm) after the contact, V is the solution volume (mL), and m is the mass of the adsorbent (g).

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2.4. Characterization techniques Powder X-ray diffraction (XRD) measurements for the samples before and after the adsorption experiments were collected on a Rigaku Ultima IV powder X-ray diffractometer using CuKa radiation at room temperature, with a step size of 0.0167◦ , scan time of 10 s per step and 2 theta ranging from 4.5 to 65◦ . The generator set up was 40 kV and 40 mA. Fourier transformed infrared (FT-IR) spectra for samples recorded by a Nicolet iS10 infrared spectrometer (Thermo Fisher, USA) in a frequency range of 4000–400 cm−1 using KBr pellet method. X-ray photoelectron spectroscopy (XPS) recorded on a PHI Quantera system using monochromatic Al Kα (1486.6 eV) X-rays and the C1s line at 284.6 eV used as a reference. Thermogravimetric analysis (TGA) measured using a TA SDTQ600 unit in nitrogen at a heating rate of 5 ◦ C min−1 . 2.5. Adsorption kinetics study Adsorption kinetic experiments for U(VI) ion performed under various adsorption time intervals (5–420 min). 0.08 g of solid sample added into 100 mL of the uranium solution with a constant shaking of 200 rpm. After a certain period of time, the mixtures filtered and the concentrations were analysed using MP-AES. 3. Results and discussion 3.1. Characterization Fig. 1A shows the XRD patterns of Zn5 -NO3 precursor material and Zn5 –Gly. After intercalation of glycine to the gallery of Zn5 -NO3 by ion exchange, the interlayer spacing increases from 0.97 nm to 1.31 nm. Considering the Zn5 -NO3 layer thickness of 0.5–0.73 nm (Williams et al., 2012), the gallery height can be estimated as 0.81 nm (= 1.31–0.5 nm), which is close to the double length of glycine (0.39 nm) (Ching et al., 1989), indicating double layer vertical arrangement of glycine in the Zn5 -NO3 gallery. FTIR spectra (Fig. 1B) confirmed the formation of Zn5 -NO3 structure. The strong band at 1384 cm−1 corresponds to NO3 − of the Zn5 -NO3 (Fig. 1B-b). Broadband at 3448 cm−1 due to the stretching mode of O–H (Zhang et al., 2013a; Zou et al., 2017b), stretching vibrations at 1638 cm−1 attributed to HOH mode of water molecules (Xu and Zeng, 2001), Zn–O vibrational bands below 1000 cm−1 (Williams et al., 2012) observed for Zn5 -NO3 . It was also observed that for the Zn5 -NO3 a sharp band at 3576 cm−1 recorded showing to the high basicity of the O–H groups (Xu and Zeng, 2001). After glycine intercalation (Fig. 2B), the NO3 − band at 1384 cm−1 diminished, confirming a nearly complete exchange of nitrate ion by glycine. Additionally, bands observed around 3379 cm−1 due to O–H stretching of hydroxyl groups in the Zn5 -NO3 layers and the carboxylic acid groups; 2928 and 2875 cm−1 due to C–H and N–H groups stretching vibrations, respectively (Asiabi et al., 2017; Lu et al., 2016); 1654 and 1390 cm−1 due to carbonyl groups (C=O) and carboxyl groups (–COOH), respectively (Falaise et al., 2017; Ko et al., 2013); 1618 and 1561 cm−1 due to NH2 /NH3 + deformation (Danon et al., 2011); 895 cm−1 due to C–H bending; 669 and 412 cm−1 due to the Zn–O and O-Zn-O vibrations (Asiabi et al., 2017; Lu et al., 2016) (Fig. 2A and B). The FT-IR spectra of glycine are depicted in Fig. 1B-a, consistent with a previous report (Fischer et al., 2005). Fig. 1C shows the high-resolution C1s XPS spectrum of Zn5 –Gly by deconvolution method. The C1s XPS spectrum of Zn5 –Gly contains three peaks representing C–C, C–N/C–O and COO/C=O at 284.74, 285.95, and 288.44 eV, respectively (Girard-Lauriault et al., 2015; Liao et al., 2011; Stankovich et al., 2007). The results show that the Zn5 –Gly presented different functional groups, such as COOH groups, which indicates successful intercalation of glycine with the Zn5 -NO3 . To study the thermal stabilities of Zn5 -NO3 and Zn5 –Gly, thermogravimetric analysis (TGA) was carried out. As depicted in Fig. 1D, the first weight loss (5.7 wt%) of Zn5 -NO3 observed at 58–108 ◦ C is assigned to the removal of surface and interlayer water (Khalili and Al-Banna, 2015). The further weight loss (about 28 wt%) observed in the Zn5 -NO3 curve in the range of 108–270 ◦ C is attributed to the decomposition of the functional groups of the Zn5 -NO3 . The weight loss of Zn5 –Gly observed in the temperature range of 100–423 ◦ C (about 2.56 wt%), which is due to the decomposition of functional groups of the composite material, indicating that the water content in the structure after the intercalation is minimum. This is supported by the FTIR spectrum at 3576 cm−1 due to O–H groups for Zn5 -NO3 disappeared after the intercalation. Overall, comparing Zn5 –Gly and Zn5 -NO3 , Zn5 –Gly showed less weight loss (2.56%) than Zn5 -NO3 (35%). This shows glycine intercalation in the interlayer gives high thermal stability to the Zn5 -NO3 due to the strong interaction between glycine and HDS. 3.2. Effect of pH on adsorption of U(VI) by Zn5 –Gly An important factor that affects the adsorption performance of an adsorbent is the pH of the aqueous solution. The removal of UO2 2+ by Zn5 –Gly was studied at various pH values ranging from 1 to 6 (Fig. 3). The removal rate of uranyl ion increased almost uniformly from pH 1 to 4 from <2 to >97% and reached nearly 100% at pH 5. From pH 5 to 6, the removal rate remained unchanged; thus pH 5 is taken as an equilibrium point to perform the rest adsorption experiments. In a low pH (acidic media), the COO− part of glycine picks H+ , so the net charge of the glycine would be positive as NH2 gets protonated. Thus, H+ competes with UO2 2+ during the adsorption process, resulting in a low adsorption performance

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Fig. 1. (A) XRD patterns of Zn5 -NO3 and Zn5 –Gly; (B) FT-IR spectra of (a) Gly, (b) Zn5 -NO3 ; (C) XPS analysis of Zn5 –Gly and (D) TGA profile of Zn5 -NO3 and Zn5 –Gly.

of Zn5 –Gly towards uranyl ion. In an aqueous solution containing LDH or HDS, minor dissolution of layers can happen in an acidic medium so that OH groups can be washed out of the layer (Asiabi et al., 2017). We believe this could be another reason for the decrease in the removal rate of the material in low pH value of the solution. The adsorption rate increases with pH increase until the zwitterionic point of glycine because of the availability of both positive and negative sites to capture UO2 2+ . In addition to this, as pH increase, the dissolution of OH groups from the layer decreases as there are less H+ ions in the solution. 3.3. Adsorption kinetics study To study the adsorption rate and reaction properties up to the equilibrium point, the adsorption kinetics of UO2 2+ by Zn5 –Gly was investigated. As shown in Table 1, and Fig. 4A and B, 233.38 ppm of UO2 2+ ion concentration was reduced to 48.08 ppm (80% removal rate) in an hour. In the next hour, the uranium uptake speed decreased but reached 90% removal rate within 2 h. Zn5 –Gly reached equilibrium within 420 min (Fig. 4B and C) with almost 100% removal rate and 290.75 mg/g of qm value, showing that Zn5 –Gly has a high adsorption capacity for U(VI) ions in an aqueous solution. Pseudo-first-order and pseudo-second-order models were investigated to determine the nature of adsorption. The two kinetic rate equations are given as follow (Ho and McKay, 1999): Pseudo-first-order: ln (qe − qt ) = ln qe − k1 t

(4)

Pseudo-second-order: t qt

=

1 k2 qe 2

+

1 qe

(5)

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Fig. 2. FTIR spectra of Zn5 –Gly.

Fig. 3. Effect of pH on removal rate of Zn5 –Gly towards U(VI) ion.

5

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A. Tesfay Reda, D. Zhang and X. Lu / Environmental Technology & Innovation 16 (2019) 100474 Table 1 Kinetics data of UO2 2+ adsorption on Zn5 –Glya .

a

Co (ppm)

t (min)

Cf (ppm)

%R

Kd (mL/g)

qm (mg/g)

233.38

5 10 30 60 120 180 240 300 360 420

181.67 149.88 87.43 48.06 23.47 14.04 5.79 2.16 0.82 0.78

22.16 35.78 62.54 79.41 89.94 93.98 97.52 99.07 99.65 99.66

3.56×102 6.96×102 2.01×103 4.82×103 1.12×104 1.95×104 4.91×104 1.34×105 3.55×105 3.73×105

64.64 104.38 182.42 231.65 262.39 274.18 284.49 289.03 290.70 290.75

V = 100 ml, m = 0.08 g, V/m ratio = 100/0.08 = 1250. Table 2 Kinetic parameters (pseudo-second-order) of U(VI) adsorption by Zn5 –Gly. qe,exp (mg/g)

qe,cal (mg/g)

K2

290.75

292.34

0.0002148

Table 3 Adsorption performance of Zn5 –Gly towards UO2 2+ b .

b

Co (ppm)

Cf (ppm)

%R

Kd (mL/g)

20.65 67.88 114.51 233.38 333.25 433.88 484.85 638.29

0.56 0.61 0.68 0.72 0.78 0.85 0.94 41.87

97.29 99.10 99.41 99.69 99.77 99.80 99.81 93.44

4.48 1.38 2.10 4.04 5.33 6.37 6.43 1.80

× × × × × × × ×

104 105 105 105 105 105 105 104

qm (mg/g) 25.11 84.09 142.29 290.83 415.59 541.29 604.89 745.53

V = 25 ml, m = 0.02 g, V /m ratio = 25/0.02 = 1250, contact time = 12 h, pH = 5.

where qe (mg/g) is the amount of UO2 2+ adsorbed per unit mass of adsorbent at equilibrium, and qt (mg/g) is the UO2 2+ adsorbed at time t, while K1 (min−1 ) and K2 (g/mg min−1 ) are equilibrium rate constants of pseudo-first-order and pseudosecond-order adsorption interactions, respectively. K1 and K2 values were obtained by plotting ln(qe -qt ) against t and t/qt versus t, respectively. The calculated adsorption capacities (qe,cal ) obtained from pseudo-second-order model and the experimental value (qe,exp ) are close to each other (Table 2 and Fig. 4C). As shown in Fig. 4D, the correlation coefficient (R2 ), from the pseudosecond-order model is very close to one, reveals that the adsorption system is well-fitted with the pseudo-second-order model, suggesting that the adsorption mechanism is chemisorption (Liu et al., 2012). 3.4. Adsorption performance of Zn5 –Gly towards uranyl ion Maximum uptake capacity of Zn5 –Gly was calculated from the adsorption equilibrium study. As shown in Table 3, the uptake capacity of UO2 2+ by Zn5 –Gly increased with increasing concentration (20–638 ppm). In a wide initial concentration range (68–485 ppm), the removal rate of uranyl ion reached > 99% with a Kd value of > 105 beyond that, the removal rate decreased. The material has shown maximum adsorption capacity (qm ) for UO2 2+ ∼746 mg/g, which is particularly high compared to existing materials (Table 4). It has also been observed that Zn5 –Gly can adsorb uranium in relatively higher concentrations with excellent efficiency. To assess if the high adsorption capacity of the material is a synergistic effect of Zn5 -NO3 and glycine, a control adsorption test was conducted using Zn5 -NO3 as an adsorbent for UO2 2+ ions. As indicated in Table A.1 and Table 3, the intercalation of glycine into the gallery of Zn5 -NO3 contributed a considerable value to enhance the removal capacity of Zn5 -Gly for UO2 2+ ion. The Zn5 -NO3 alone showed a significant adsorption capacity (213.41 mg/g at pH 5), suggesting surface complexation via the hydroxide groups of the surface layer was one of the adsorption mechanisms in the composite material. In general, the adsorption capacity of Zn5 -Gly is higher than that of the linear combination of Zn5 -NO3 and glycine in an aqueous solution assuming glycine alone is not appropriate adsorbent towards uranyl ion in water because of its solubility. 3.5. Selective adsorption of Zn5 –Gly towards uranyl ion To study the selectivity of the material for uranium ions, adsorption experiments were carried out under mixed ion state condition containing uranium and other representative metal ions. Separation factor for U and M (M represents the

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Fig. 4. Adsorption kinetics curves: (A) UO2 2+ concentration change versus contact time, (B) Removal % against contact time, (C) Adsorption capacity (qt ) (experimental and calculated) with contact time, (D) Pseudo-second-order kinetic plot.

Table 4 Comparison of adsorption capacity of selected adsorbents towards U(VI) ions. Adsorbent

qm (mg/g)

Reference

Sx -LDHa MS-LDHb TMP-g-AOc Ca-Mg-Al-LDOd KMS-1e U-passf HSDCg HTC-btgh PA/PANI/FeOOHi Zn5 -NO3 -Gly

330.00 657.90 35.37 486.80 380.00 148.00 373.00 307.00 555.80 745.53

Ma et al. (2015) Asiabi et al. (2018) Zeng et al. (2017) Zhang et al. (2013a) Manos and Kanatzidis (2012) Monier and Elsayed (2014) Liu et al. (2013) Li et al. (2014) Wei et al. (2018) This work

a

Polysulfide/Layered Double Hydroxide. Layered double hydroxide intercalated with 2-mercaptoethanesulfonate. c Titanium-molybdopyrophosphate-g-amidoxime. d Ca-Mg-Al Layered Double Oxides. e K2 MnSn2 S6 . f P-aminostyrene and salicyaldehyde, g ghydroxyl styrene-divinylbenzenecopolymer micro-particles. h Catechol-like phenolic ligand functionalized hydrothermal carbon. i Phytic acid/polyaniline/FeOOH composites. b

selected metal ions) (SFU/M ), which can be estimated by Kd(U) /Kd(M) , is served to check if Zn5 –Gly can be used to separate U(VI) and M from each other (Mertz et al., 2013). When the SF is >100, then the material is considered as good by

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Fig. 5. Competitive adsorption capacities in mixed ion state on Zn5 –Gly (C0 = 20 ppm, pH = 5, t = 48 h, V = 25 ml, m = 0.01 g, V/m = 2500.

considering the factors and uses for which the SF is measured (Ma et al., 2016). As shown in Table A.2, Zn5 –Gly could be a good candidate for the selective adsorption of uranyl ion in an aqueous solution containing Ag1+ , La3+ , Ba2+ , Eu3+ , Nd3+ and Sr2+ . The adsorption capacity of the material for the target ions is also given in Fig. 5. The selective adsorption of U(VI) is significantly higher than that of the other ion, except zirconium, which indicates how fine selective the adsorbent is for uranium ion in an aqueous solution. 3.6. XPS analysis Fig. 6 shows XPS spectra of Zn5 –Gly before and after UO2 2+ ion adsorption at pH 5.0. In the total scan XPS spectra, O1s and C1s were observed at 530.5 and 284.5 eV, respectively, before adsorption. Before adsorption, the Zn2p spectrum showed low energy band at 1021.2 eV (Zn2p3/2) and high-energy band at 1044.5 eV (Zn2p1/2)(Mai et al., 2013) (Fig. 6A-b). Two strong U4f7 peaks after adsorption at 381.2 and 392.2 eV (Fig. 6A-b) were attributed to the binding energies of U4f7/2 and U4f5/2, respectively (Sun et al., 2016), confirming the successful removal of U(VI) from aqueous solution. Before the adsorption, the O1s peak appeared at 529.8 eV (O2− ), Schindler et al. (2009) but after adsorption, the peaks appeared at 530.5 (O associated with uranium, O=U=O) (Schindler et al., 2009) and 530.3 eV (with Zn or U-bonded intrinsic O atoms) (Xu et al., 2013), as shown by the high-resolution spectrum of O1s (Fig. 6B). Fig. 6C shows the high-resolution XPS spectra of C1s in the sample, with high (at 284.6 eV) and low (at 288.7 eV) peaks, which are assigned for C=C and –COO− , respectively (Shao et al., 2014) before adsorption. After adsorption, the peaks are shifted to 284.8 and 288.5 eV, respectively. The N1s high-resolution XPS spectra of the sample contained two main peaks at 400.5 eV (due to amine, –NH2 /NH3 + ) (Feng et al., 2016) and at 398.4 eV (due to imine, –N=) (Wen et al., 2016) before adsorption (Fig. 6D). After adsorption of uranyl ion, the N1s XPS spectrum showed three main peaks with binding energies of 399.4, 399.8 and 401.7 eV corresponding to NH2 (free amine) (Graf et al., 2009), amides (Gengenbach et al., 1996; Truica-Marasescu and Wertheimer, 2008) and –NH2 /NH3 + , respectively (Graf et al., 2009). The fact that the binding energies of the amine and imine groups shifted to higher binding energies after uranium adsorption indicated a decrease in the electron density of the nitrogen atom. The high-resolution spectra of U4f shows double peaks at 391.8 eV and 380.9 eV, corresponding to the binding energies of U4f5/2 and U4f7/2 , respectively (Manos and Kanatzidis, 2012) (Fig. 6E), shows the bonding between U(VI) and the adsorbent is chemically stable (Pang et al., 2019). In general, a shift in the peak value of the binding energy of O1s, C1s and N1s to a higher value means that the electron density of each atom in the structure is lowered by the interaction of the uranyl ion with the adsorbent. 3.7. Characterization of the structure after adsorption After the adsorption of uranyl ion by Zn5 –Gly, the residue was collected by centrifugation, dried, and subjected to XPS, FT-IR, XRD, and TG observations. The XRD patterns of Zn5 –Gly after adsorption have a decreasing peak compared to that of before adsorption (Fig. A.1A). These weak reflections, especially at the lower angles, maybe due to the adsorbed ion (Ma et al., 2016). After uranium adsorption, the basal spacing of the structure was increased from 1.31 to 1.43 nm due to the interaction of uranyl ions in the interlayer of the structure. The difference between the two basal spacing, 0.12 nm (1.43 nm–1.31 nm), is close to the diameter of U6+ (0.14 to 0.18 nm) (Manos and Kanatzidis, 2012), suggesting linear parallel alignment of uranyl ion in the gallery. The small difference could be due to a strong interaction between

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Fig. 6. XPS analyses of Zn5 - Gly before and after adsorption of UO2 2+ , pH = 5.0, T = 293 K. (A) total scans; (B) O1s; (C) C1s; (D) N1s and (E) U4f7.

UO2 2+ ions and the adsorbent. A new diffraction peak observed around 2θ = 11◦ with a spacing of 0.76 nm (Fig. A.1A). This peaks could be due to uranyl ion which is associated with other functional groups (Sun et al., 2018), which provides additional evidence for the successful removal of uranium ion. TGA curve of Zn5 -Gly showed that the weight loss rate after adsorption (∼6%) was higher than the weight loss rate before adsorption (∼2.56%) (Fig. A.1B-b). The curve reveals

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Fig. 7. Regeneration capacity of Zn5 -Gly for UO2 2+ ion adsorption at a concentration of 433.88 ppm.

approximately four main steps of weight loss after adsorption. The first step (2%) assigned to the loss of water molecules of the interlayer and occurred between room temperature and 112 ◦ C. The second weight loss occurred up to 300 ◦ C, corresponds to the removal of adsorbed water. The third weight loss (about 5.5% of the total mass) occurred at about 400 ◦ C assigned for the decomposition of functional groups of the structure. A further weight loss observed at around 453 ◦ C which maybe due to further decomposition of remaining species associated with uranium. In general, Zn5 -Gly-U showed a weight loss of about 6% after heating from room temperature up to 600 ◦ C, and this shows that the material is stable after the adsorption though less stable than that of before adsorption. At 913 cm−1 , a strong peak appeared due to (O=U=O)2+ vibration (Amayri et al., 2004; Manos and Kanatzidis, 2012), as shown by the FT-IR spectra of Zn5 –Gly after uranium(VI) ion adsorption (Fig. A.2). The peaks at 414 and 668 cm−1 , due to Zn–O and O–Zn–O vibration modes, respectively, are almost similar with the peaks before adsorption. These results indicate that the material is stable after the adsorption. Additionally, peaks for vibrations of carbonyl and carboxyl are shifted from 1654 and 1390 cm−1 to 1650 and 1384, respectively, after the adsorption. Similarly, peaks representing the amine/imine groups also shifted after the adsorption of uranyl ion. An important point is that new peak appeared at 466 cm−1 after adsorption (Fig. A.2B) assigned for the U–N stretching band (O’Brien, 1982). All these results lead to the conclusion that the amine and carboxyl groups played a significant role for UO2 2+ uptake. 3.8. Desorption and reusability It is important to study desorption efficiency and reusability of the adsorbent from an economic point of view. After the adsorption of the UO2 2+ ion at a concentration of 433.88 ppm, the sample was collected and dried, and then the U(VI) 0.01M HNO3 solution was desorbed. Four-cycles of adsorption–desorption processes carried out to investigate regeneration and reusability of Zn5 -Gly. The efficiency of the first and second cycles were 89.14 and 81.00%, respectively, beyond that the desorption rate dropped to < 75% (Fig. 7). Considering the high initial concentration of uranyl ion, it is possible to conclude that the material has good potential to remove uranyl ions repeatedly (at least three times) from the aqueous solution. 3.9. Proposed adsorption mechanism XPS is an effective method to study the interaction mechanism between the target ions and adsorbent (Duan et al., 2017). Based on the XPS and FTIR studies, UO2 2+ is probably adsorbed by electrostatic interaction with –COO− and covalent interaction by –NH2 groups simultaneously. It is indicated that Zn5 -NO3 has the potential to adsorb UO2 2+ ion (Table A.1) so surface adsorption through hydroxyl groups of the Zn5 -Gly is additional adsorption mechanism. As discussed in Section 3.6, C, N and O elements exhibited a shift in XPS binding energy after the adsorption. These results indicate that carboxyl and amine functional groups are the primary driving forces for the uranyl ion removal from the aqueous solution. We proposed trans-isomer complex of glycine with uranium because of the FT-IR between 800 and 1200 cm−1 is centrally symmetric (Fig. A.2B) (O’Brien, 1982).

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Fig. A.1. (A) XRD patterns (B) TG of Zn5 –Gly after adsorption of UO22+ in an aqueous solution. Table A.1 Adsorption performance of Zn5 -HDS towards UO22+ .a

a

Co (ppm)

Cf (ppm)

%R

Kd (mg/L)

qm (mg/g)

20.65 67.88 114.51 233.38 333.25 433.88 484.85

6.58 22.89 45.54 96.18 170.46 263.15 344.05

68.12 66.27 60.23 58.79 48.85 39.35 29.04

2.67×103 2.46×103 1.89×103 1.78×103 1.19×103 8.11×102 5.11×102

17.58 56.23 86.212 171.51 203.49 213.41 176.00

V = 25 ml, m = 0.02 g, V /m = 1250, contact time = 48 h

Table A.2 Selective adsorption of Zn5 –Gly towards different metal ions (mixed ions case).a

a

Ion

Co (ppm)

Cf (ppm)

%R

Kd (mL/g)

SFU/M (-)

qm (mg/g)

Ag+ Nd3+ Sr2+ Ba2+ Eu3+ La3+ Pb2+ Ca2+ Mg2+ Cu2+ Zr4+ UO2 2+

20 20 20 20 20 20 20 20 20 20 20 20

19.21 19.15 19.13 18.51 18.03 17.96 17.35 16.88 15.93 15.19 2.19 1.41

3.95 4.25 4.35 7.45 9.85 10.2 13.25 15.60 20.35 24.05 89.05 92.95

1.03×102 1.11×102 1.14×102 2.01×102 2.73×102 2.84×102 3.82×102 4.62×102 6.39×102 7.92×102 2.03×103 3.30×104

320.60 297.04 289.91 163.79 120.67 116.07 86.32 71.33 51.60 41.64 1.62 -

1.98 2.13 2.18 3.73 4.93 5.10 6.63 7.80 10.18 12.03 44.53 46.48

V = 25 ml, contact time = 48 h, m = 0.01g, V/m = 25/0.01 = 2500

4. Conclusions Glycine was intercalated into the gallery of Zn-HDS by ion exchange successfully, and the adsorption capacity of the composite material towards uranyl ion in an aqueous solution was studied. In aqueous solution, Zn5 -Gly showed significantly high adsorption capacity (∼746 mg/g) towards UO2 2+ ion, this criterion make it a potential candidate (among the top materials) for uranium removal. the Zn5 -Gly also exhibits high selectivity against other coexisting ions (mixed ion state) such Ag+ , La3+ , Ba2+ , Eu3+ , Nd3+ and Sr2+ in an aqueous solution. The adsorption kinetics for UO2 2+ conforms to the pseudo-second-order model, hinting chemisorption adsorption mechanism mainly through interaction of amine and carboxyl groups. As investigated by FTIR and TGA, the adsorbent showed good stability after adsorption. The main limitation of this material is that it needs relatively longer contact time (7 h) to reach equilibrium. Analytical methods such as XRD, FTIR and XPS were used to confirm the attachment of UO2 2+ to the adsorbent. Acknowledgement This work was supported by the International Science & Technology Cooperation Program of China (2014DFR61080).

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Fig. A.2. FTIR spectra of sample obtained after Zn5 - Gly adsorbed UO22+ in an aqueous solution.

Appendix See Tables A.1 and A.2 and Figs. A.1 and A.2.

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