Bovine Serum Albumin-Coated Graphene Oxide for Effective

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Bovine serum albumin-coated graphene oxide (GO) for effective adsorption of uranium(VI) from aqueous solutions Peipei Yang, Qi Liu, Jingyuan Liu, Hongsen Zhang, Zhanshuang Li, Rumin Li, Lianhe Liu, and Jun Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04532 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 11, 2017

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Industrial & Engineering Chemistry Research

Bovine serum albumin-coated graphene oxide (GO) for effective adsorption of uranium (VI) from aqueous solutions Peipei Yang a, Qi Liu a*, Jingyuan Liu a, Hongsen Zhang a, Zhanshuang Li a, Rumin Li a, Lianhe Liu a, b and Jun Wang a, b,* a

Key Laboratory of Superlight Material and Surface Technology, Ministry of Education

b

Institute of Advanced Marine Material, Harbin Engineering University, Harbin 150001,

People’s Republic of China, * Corresponding author: Email: [email protected], [email protected] ABSTRACT

Graphene oxide (GO) was modified by a carbodiimide-induced covalent cross-linking with bovine serum albumin (BSA), enriched with numerous amino and carboxyl function groups, for radioactive uranium (U(VI)) removal. The adsorbent was confirmed through SEM, TEM, FTIR, XRD, AFM and XPS. We investigated the effect of factors, such as pH, contact time and initial concentration, on the adsorption of U(VI). In this work, the adsorption process closely fitted the Langmuir isotherm model and pseudo-second-order indicate that chemical adsorption dominates the adsorption process of U(VI) onto GO-BSA composites. These results show that the optimal

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adsorption amount of U(VI) of GO-BSA composite was 389 mg g-1 at 298.15 K, pH=6, Co=200 mg L-1 and t=80 min. According to kinetics and the thermodynamic model, as well as the XPS analysis of pre- and post-adsorption of U(VI), we propose that the adsorption behavior of U(VI) onto GO-BSA adsorbent is divided into two stages: (1) chelation of the organic functional groups onto the surface of GO-BSA with U(VI); (2) movement of U(VI) into the interior of the material after adsorption onto the surface. Moreover, the composites exhibit good adsorption efficiency in simulated seawater, indicating the potential of GO-BSA composites for U(VI) removal from seawater.

Keywords: Covalent cross-linking, Uranium adsorption, BSA, GO

1. INTRODUCTION Two-dimensional (2D) materials with various specific function groups play a crucial role in the field of adsorption of heavy metals. Uranium (VI) is one of the most commonly detected radionuclides in drinking water (15 ug L-1), and represents a serious threat to ecological and human health1. Currently, numerous researchers are focusing on the availability and cost of uranium (U(VI)) to fuel nuclear reactors because conventional land-based uranium sources may become depleted by the end of the century2-4. Therefore, exploitation of alternative uranium sources, such as the extraction of U(VI) from seawater, is of great importance and will ensure the long-term availability and development of this nuclear fuel. Extraction of U(VI) from seawater embraces organic-inorganic ion exchange, electrodialysis, physical and chemical adsorption, chemical precipitation and extraction5-8. Adsorption techniques especially stand out from the aforementioned technologies because of their greater feasibility, efficiency of consumption, simple operation and facility in removing trace levels of ions9-10. In these techniques, the design


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of adsorption materials greatly influences the extraction of U(VI) from seawater. Many efficient two-dimensional (2D) materials based on graphene oxide (GO) have been utilized for the adsorption of the radiation element, such as polyaniline-graphene oxide polypyrrole

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, graphene oxide-dopamine-cysteine

graphene oxide-Ni–Al layered double hydroxide

15

13

11

, graphene oxide-

, graphene oxide-amidoxime hydrogel14,

and graphene oxide-dopamine

16

. However,

the use of these materials for the adsorption of U(VI) from large areas of aqueous solution has been hindered due to low sorption capacity. Therefore, it remains a challenge to design and prepare high-efficient, high-selective and much improved adsorption capacities of materials. GO, with abundant organic groups at the periphery of nanosheets, and with large surface-tovolume ratios, as well as other excellent physicochemical properties

17-18

, has emerged as a

promising material in the application of the adsorption of radionuclides. Li et al. proposed that the optimum capacity of GO was 299 mg g-1 for U(VI)

19

. Wang et al. revealed that GO has a

good sorption capacity for Cd (II) and Co (II) of about 0.95 and 1.16 mmol g−1, respectively 20. However, the use of GO results in irreversible coagulation because of van der Waals force operating between the layer spacing

21-22

, which will hinder sorption behavior and reduce

sorption capacity 11. To solve this problem, a large number of GO attachments was studied18, 2325

, such as grafts with nanoparticles, organic active groups, bio-based materials, and

macromolecular polymers 26-29. Protein, for example, with a large network of hydrogen bonds between amino acid residues to link up with UO22+ 30-32, is considered a promising prospect for adsorption of U(VI) 33-38. Bovine serum albumin (BSA) has 583 amino acid residues that can be used to coordinate with a variety of cationic, anionic and other small molecules. Based on the above research, we devise a facile strategy of organic synthesis to graft BSA onto the surface of GO. This method utilizes the


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covalent bonds of the GO-BSA composites to produce material of sufficient stability and to realize the effective removal of U(VI). The GO-BSA composites were characterized using SEM, FTIR, TEM XRD, AFM and XPS. The detailed effects of pH and initial concentration were examined including the adsorption kinetics and adsorption isotherms on U(VI) removal. These results show that chemical adsorption dominates the adsorption process. The survey of XPS demonstrates that the adsorption behaviour of U(VI) onto GO-BSA composites is bound up with nitrogen- and oxygen- organic groups. Meanwhile, we observe that GO-BSA composites have a good selectivity in aqueous solutions of different compositions of ions. These studies demonstrate that GO-BSA composites have a desirable application for the removal of U(VI) ion. Finally, we used this material for adsorption of U(VI) from simulated seawater to explore its potential application for real seawater. 2. EXPERIMENTAL SECTION Materials. Bovine serum albumin (BSA) was obtained from BioSharp. Hydrochloric acid, potassium permanganate, concentrated sulfuric acid and hydrogen peroxide were obtained from Sinopharm Chemical Reagent Co., Ltd. Graphite and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDCI) were obtained from Xiyan reagent. 1-Hydroxybenzotriazole (HOBt), 4Dimethylaminepyridine (DMAP) N, N- dimethyl formamid (DMF) and Pyridine (Py) were purchased from Aladdin. Synthesis of the GO-BSA composites. Graphene oxide (GO): GO was prepared by a modified Hummer’s method 39. For the reaction, 0.5 g of graphite and 115 mL of H2SO4 were stirred in an ice bath for 1 h. Then 30 g of KMnO4 was slowly added and the reaction temperature was maintained at about 0 oC for 3 h. The solution was transferred to a 50 ± 5oC water bath and stirred for 45 min. Then 400 mL of H2O was added slowly, followed with stirring for 15 min


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while the temperature was raised to 50 ± 5oC. Finally, 300 mL of H2O and 360 g of H2O2 (5%) were added to the solution with stirring for 15 min. The warm solution was then filtered and washed with H2O until the pH=6~7. The final product was dried for 3 days in a vacuum oven. Graphene oxide (GO)-Bovine serum albumin (BSA) (GO-BSA): 20 mg of GO, 60 mg of BSA and 35 uL of Py and EDCI/HOBt/DMAP (molar composition=1.5: 1: 0.5) were dissolved in 6 mL distilled water-DMF reaction mixtures (distilled water: DMF=4 mL: 2 mL) for 5 h at room temperature. Then the solution mixture was washed by mass ratio of ethanol: distilled water=3: 1. Finally, the solution mixture was freeze-dried for 3 days. Adsorption experiments. 0.01 g of GO and GO-BSA composite was added into 20 mL of UO2 (NO3)2 ·6H2O aqueous solution. In this experiment, we adjusted the pH by drops of 0.5 M HNO3 and 0.5 M NaOH. Then the solution was placed into an oscillator. Finally, the samples were isolated by centrifuge and the concentrations of U (VI) ions in solution were analyzed by ICP-AES from IRIS Intrepid II XPS instrument to obtain the concentration of adsorption U (VI) ions. The adsorption capacity of adsorbent and the removal efficiency were calculated by equations (1) and (2): = ( − ) ∙ /

(1)

= − ⁄ × 100 %

(2)

Where Qe is the adsorption capacity of adsorbent. Co (mg L-1) is original concentration of U(VI) ions and Ce (mg L-1) is the remaining concentration of U(VI) ions. V (mL) is the volume of the solution and m is the weight of adsorbent. R is the adsorption removal efficiency. Adsorption tests in simulated seawater. In the adsorption experiment, 0.01 g of GO-BSA composite was put in contact with 20 mL of simulated seawater in a conical flask and shaken for 24 h in a thermostatic shaker bath. After the adsorption process, the samples were isolated by


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centrifuge at 8000 rpm for 10 min at room temperature. Then the supernatant solutions were analysed with ICP-MS from X series II. 3. RESULTS AND DISCUSSION 3.1. The synthesis lines and mechanism of the GO-BSA composites. Scheme 1 shows the steps involved in the cross-linked assemblies of the amide bond and carboxyl group of BSA with GO: 1) GO was obtained by a modified Hummer’s method; 2) GO-BSA composites were synthesized by covalent bonding of GO and BSA

40

. This process includes EDCI/ HOBt/ Py/

DMAP as reaction reagent (see the Experimental Section for details) and DMF as the solution. The lines of reaction mechanism are shown in Supporting Information Scheme S1 41: 1) GO and EDCI form reactive intermediates through electron transfer under alkaline conditions; 2) As a nucleophilic reagent DMOP reacts with the active intermediates to obtain the ester; 3) The successful synthesis of GO-BSA composites through ammonia ester exchange of ester and BSA.

Scheme 1. The reaction lines of the GO-BSA composites 3.2. Characterization of GO-BSA composites. To determine the measure of success of the graft of GO, SEM, TEM and AFM measurements were performed. The patterns of the assynthesized GO and GO-BSA are shown in Fig. 1, respectively. For the SEM of GO, the wrinkle nanosheets are found on the morphology. Meanwhile, the morphology also illustrated that the GO are no-monolayer. For the SEM of GO-BSA, the change in the shape is not obvious, mainly


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because the introduction of organic functional groups did not affect the GO shape. For a clearer indication of the successful grafting of BSA, the TEM is used to describe the microstructure of GO. From the TEM of GO and GO-BSA, we can observed that the ledge of GO-BSA was become thicker than GO, which was confirmed by AFM. In addition, the presence of the wrinkle is consistent with the SEM results42. AFM analysis in Fig. 1e and Fig. 1f reveal the height increase of GO-BSA composites (~4.0 nm) as compared to pristine GO (~ 1.0 nm).

Fig. 1 SEM images of GO (a) and GO-BSA composites (b), TEM images of GO (c) and GOBSA composites (d), and AFM images of GO (e) and GO-BSA composites (f) According to the above description, FTIR spectra was employed to examine the materials GO, BSA and GO-BSA, as shown in Fig. 2a. For GO, the characteristic peaks at 1050 cm-1 and 1220 cm-1 correspond to C-O-C stretching vibration 43, the feature peaks at 1500 cm-1 and 1600 cm-1 belong to the stretching of the benzene frame vibration, the peak at 3300 cm-1 is assigned to the O-H stretching vibrations with hydrogen bonding 44,and the peak at 1750 cm-1 belongs to the C=O stretching vibrations45. However, compared with GO, the chemical bond of GO-BSA composites relates to the O=C-NH in-plane bending vibration at 630 cm-1. The new band located at 1650 cm-1 explains the disappearance of the carboxyl peak at 1750 cm-1 in the blue curve; this is due to cross-linking of O=C-OH with amide groups, which indicates that the amine groups of


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BSA are successfully grafted onto the surface of GO

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. The new peak at 2850 cm-1 is

characteristic of the C-H stretching vibrations of methylene group of BSA. To sum up, for GO and GO-BSA composites, the significant appearance of most of the peaks for the GO-BSA composites (2850 cm-1 and 630 cm-1), in contrast to GO powder, suggest the formation of GOBSA composites via covalent bonding. XRD was introduced to further confirm that the GO-BSA composites were successfully synthesized. From Fig. 2b, a sharp diffraction peak of GO curve at 11.4o (interlayer spacing: 0.7732 nm) is ascribed to GO (001) spacing

46

. At the same time, there is a wide and weak

diffraction peak at 22.5º from which an interlayer spacing of 0.3911 nm is obtained corresponding to the (002) plane of GO 47-48. In conclusion, the interlayer spacing increases from 0.3911 to 0.7732 nm showing total exfoliation of GO

49

. As to the GO-BSA composites, the

broad and moderately strong peak located at the scope of 20-27º is observed because of the amide bond of BSA. Meanwhile, there is no typical GO peak in the XRD patterns of the GOBSA composites, which means long range disorder or full exfoliation of GO in the GO-BSA due to the uniform dispersion of GO within BSA. These results are similar to those found from previous studies.


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Fig. 2 FTIR images of GO, BSA and GO-BSA composites (a), XRD images of GO and GOBSA composites (b). 3.3. Adsorption studies on GO-BSA composites. As well-known from the literature, pH is a crucial factor in controlling U(VI) ion adsorption. A series of adsorption experiments were carried out to determine the optimal pH. According to the pH value (Fig. 3a) and Zeta potential (Supporting Information, Fig. S1) measurement, in the low pH value, the active groups of GOBSA were protonated and the H+ competed active sites with UO22+, which were situated on the surface of the adsorbent. The GO-BSA composites adsorption capacity on U(VI) increases sharply in the ranging of 3 to 6. As the concentration of H+ ions decreases with increasing pH, more active sites on the GO-BSA material are exposed, leading to an increase in adsorption capacity. The zeta potential of GO-BSA also demonstrated this characteristic (Supporting Information Fig S1). In the Fig S1, the ζ-potential value becoming more and more negative. It provided more binding sites and was in favor of the combination of U(VI), thereby the adsorption capacity of GO-BSA increased gradually. However, the decreased adsorption of U(VI) on GO-BSA composites with increasing the pH from 6 to 10. In the pH of 6 to 10, uranium combine with hydroxide and formed a different complex such as UO2(OH)3- and (UO2)3(OH)7- . Meanwhile, we can observed that the ζ-potential value was negative from 7 to 10, which leading to electrostatic repulsion between these ions and the adsorbent surface. In summary, the pH=6 was an optimal pH for the adsorption of U(VI) on GO-BSA composites 22. Concentration of U(VI), is also considered as a significant factor in the adsorption of U(VI), which we employed to determine the saturated adsorption of GO and GO-BSA composites for U(VI). To examine the influence of concentration of U(VI), we set up various amounts of constant parameters, such as 0.01 g adsorbent (Supporting Information, Fig. S2), 20 mL of U(VI)


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solution, pH=6, respectively. From the Fig.3b, the increased adsorption of U(VI) on GO and GOBSA composites with increasing the U(VI) concentration from 50 mg g-1 to 500 mg g-1. It is proposed that the excess active sites of GO-BSA composites were supplied for sorption of U(VI), the adsorption amount increased along with U(VI) concentration promoted. In contrast, for the removal rate in the Fig.3b, these curves of remove rate about GO and GO-BSA were decreased with increasing the U(VI) concentration. The remove rate of GO was decreased from 95% to 50% and GO-BSA was decreased from 98% to 75%. It is means that the removal efficiency of U(VI) onto the surface of GO-BSA is better than the GO, indicated that GO-BSA is suitable for the extraction of uranium from aqueous solution. However, the decrease of removal rate of adsorbent with the increase of U(VI) concentration may result in a decrease in the adsorption efficiency of the adsorbent. Therefore, taking into account the above reasons, the 200 mg L-1 of initial concentration was chosen as an optimal concentration for the following studies.

Fig. 3 Effect of initial pH (a) and concentration (b) of U(VI) adsorption onto GO and GO-BSA composites. The contact time of U(VI) adsorption onto GO and GO-BSA composites was examined to determine the optimal equilibrium time of the adsorption process. Contact times of 3 to 200 min were investigated under the conditions of 20 mL of U(VI) solution at 25 oC, 0.01 g adsorbents,


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pH=6 and Co=200 mg L-1. These results (Supporting Information, Fig. S3) show clearly that the adsorption amount tends to increase with the increase of adsorption time until 80 min, at which time the adsorption capacity of the GO and GO-BSA composites reaches 300 mg g-1 and 389 mg g-1, respectively, indicating that 80 min is the optimum time to select for adsorption of U(VI) in our research. 3.4. Adsorption kinetics and adsorption isotherms on GO-BSA composites. To evaluate the adsorption process of U(VI) onto GO-BSA composites, adsorption kinetics and isotherms were explored. Pseudo-first-order, pseudo-second-order kinetics and the Weber-Morris (W-M) model 50-51 were applied to analyze the kinetics process (Supporting Information, SI. 4). From Fig. 4 and Table S1, the results show that the adsorption kinetics of U(VI) onto GO-BSA follows best a pseudo-second-order (R2=0.9998), indicating that chemical adsorption dominates. According to the Weber-Morris formula, the adsorption behavior of U(VI) at the edges of GOBSA is examined (Fig. 4c). In the formula (Supporting Information, SI. 4), the value of C is associated with the contribution of the edge of the adsorbent for U(VI). When the value of C increases, the boundary layer of sorbent for U(VI) provides a greater contribution. From Fig. 4c, the values of two stages in C of GO and GO-BSA are 233.18, 391.15 and 270.9, 391.16, respectively, indicating that the edges of GO have some contribution for U(VI) adsorption. Meanwhile, for the first-step of intra-particle diffusion, the values for the constant Kip for GO and GO-BSA are 2.01 and 2.35, respectively, which indicate that GO-BSA has a higher diffusion rate than GO. The curve does not pass through the origin, which implies that external diffusion of ions and internal diffusion dominate the whole adsorption process. As for the second-step, the values of Kip are 0.14 and 0.14, respectively, for GO and GO-BSA, indicating lower rates of this procedure compared to the first-step. Moreover, the fitted curves of intercept are close to 0,


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which indicates that intra-particle diffusion may be the main rate-controlling step in the adsorption process, with the larger edges having a negligible contribution. In summary, we infer that the adsorption behavior of U(VI) onto GO and GO-BSA adsorbent is divided into two stages. The first stage is the chelation of the organic functional groups on the surface of GO and GOBSA with U(VI), when U(VI) reaches the surfaces of GO and GO-BSA. In the second stage, U(VI) enters into the interior of the material because of intra-particle diffusion, so that the adsorption capacity of U(VI) is further improved.

Fig. 4 Pseudo-second-order (a), pseudo-first-order (b) and Weber–Morris order (c) plot for the removal of U (VI) by GO and GO-BSA composites, pH= 6.00; T= 25 oC; amount of adsorbent 0.01 g and Co= 200 mg L-1 Adsorption isotherms of GO and GO-BSA for U(VI) were also investigated under the conditions of 298.15, 308.15 and 318.15 K, as shown in Fig. 5a. The Langmuir, Freundlich and Temkin models were used to illustrate the process of adsorption. Fig. 5b, c, Fig. S4 and Table S2 show that the Langmuir isotherm model (R2=0.9979, 0.9986, 0.9997 at 298.15, 308.15, 318.15 K for GO-BSA composites, respectively) fits better than the Freundlich model (R2=0.9099, 0.8428, 0.8644 at 298.15, 308.15, 318.15 K for GO-BSA composites, respectively and Temkin model (R2=0.7282, 0.8226, 0.9668 at 298.15, 308.15, 318.15 K for GO-BSA composites, respectively). It indicates that U(VI) is adsorbed by a monolayer chemical adsorption process onto GO-BSA


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composites. The result is confirmed by the kinetics of U(VI) adsorption onto GO-BSA composites. In addition, in accordance with the formula and fitted curve of the Langmuir model, the optimal adsorption amount of GO-BSA composites are evaluated at pH= 6 as 389, 398 and 419 mg g-1 under the conditions of 298.15 K, 308.15 K, 318.15 K (Supporting Information, Table S2), respectively. These results demonstrate that the adsorption capacity of GO-BSA composites is greater than that of GO nanosheets, the phenomenon of which is attributed to certain organic groups of GO-BSA composites, which exert the main influence on the extraction of U(VI) from aqueous solutions 52. We also used the Dubinin-Radushkevich model to better explain the U(VI) adsorption behaviour (physical adsorption or chemical adsorption) onto the surfaces of GO and GO-BSA. In this process, the following equations are used53: = − (3) Where is activity coefficient and ε is the Polanyi potential. ε was calculated by the following equation: = (1 + 1⁄ )

(4)

In Fig. 5d, the ln qe vs plot was used to determine the values of to ascertain the value of E (adsorption energy). E is used to estimate the type of adsorption mechanism. A value greater than 8 KJ mol-1 is considered to be due to chemical adsorption while a value less than 8 KJ mol-1 is due to physical adsorption. For GO and GO-BSA adsorbents, E was calculated by the following equation: = 1⁄2

(5)

The values of E for GO are 3.5, 5 and 7.06 KJ mol-1 under the conditions of 298.15 K, 308.15 K, 318.15 K, respectively, which indicates that the adsorption process of U(VI) onto GO is largely due to a physical adsorption mechanism. The introduction of BSA to the lamella


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interface of GO of E are calculated as 8.06, 9.32 and 11.12 KJ mol-1 under the conditions of 298.15 K, 308.15 K, 318.15 K, respectively, indicating that the process of adsorption of U(VI) onto GO-BSA is mainly through organic groups via a chemical adsorption mechanism. In addition, we also do the nonlinear fitting of isotherm, which was shown in the Fig. S5 and Table S3. These results show that the Langmuir isotherm model fits better than the Freundlich model and Temkin model.

Fig. 5 Langmuir, Freundlich and D-R models for the removal of U (VI) on GO (inset) and GOBSA composites, pH= 6.00; T= 25 oC; amount of adsorbent 0.01 g; Co= 200 mg L-1 and t= 80 min


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The uptake of U(VI) shows that the adsorption of U(VI) is enhanced with the increase of temperature, indicating an endothermic nature of the adsorption process. To evaluate the feasibility of adsorption progress, the thermodynamic parameters of standard free energy (∆G°), standard enthalpy (∆H°) and standard entropy (∆S°) are calculated using the Van’t Hoff equation (Supporting Information, SI.5). The values of the thermodynamic parameters in the adsorption of U (VI) under the three temperatures imply that the adsorption process of U (VI) is endothermic. The value of ∆H° is used to estimate the type of adsorption mechanism. The range of 20.9-418.4 KJ mol-1 reflects the chemical nature of adsorption while a value less than 20.9 KJ mol-1 corresponds with physical adsorption. From Fig. 6 and Table S4, the values of ∆H° are 5.24 and 21.84, respectively, for GO and GO-BSA, indicating that BSA is successfully introduced into the lamella interface of GO mainly via chemical adsorption.

Fig. 6 The influence of temperature on U (VI) sorption onto GO (inset) and GO-BSA composites relationship curves between ln Kd and 1000/T (K), pH= 6.0; T= 25 oC; amount of adsorbent 0.01 g; Co= 200 mg L-1 and t= 80 min


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3.5. Ion competition experiment. To further illustrate the adsorption process, which is dominated by chemical adsorption and the selective adsorption of the U(VI), we investigated the effect of different ions co-existing in aqueous solutions

54-55

(i.e., Ca2+, Mn2+, Pb2+, Fe3+, K+,

Cu2+, Al3+, Ba2+, Na+, Zn2+, Ni2+, Mg2+, Sr2+) on removal efficiency of U(VI) by GO-BSA composites. From Fig 7a, GO-BSA composites exhibit good selectivity and still retain high efficiency of more than 98% even with certain quantities of background metal ions. From the Fig. 7a, the selective change is assumed to be responsibility for adsorption of metal ions onto the surface of GO-BSA composites. Pb2+, Al3+ and Fe3+ are easy to form hydroxides and hence to produce precipitates, which can be only physically adsorbed onto the surface of adsorbent. However, the uranium ions and adsorbents are mainly chemically adsorbed, making these ions have lower effect than other ions on the selectivity of the adsorption. In this experiment, there are exists the interaction between Cu2+, Ni2+, Zn2+ and amino acid residues of BSA. They form the complex was stable than the Ba2+, Ca2+ and Mg2+. Base the adsorption of other metal ions, we observed that the adsorption amount of U(VI) mainly affected by Zn2+, Ni2+ ions in the aqueous solutions. In addition, the U-N bond length is significantly shorter than the length of other metalnitrogen cations (i.e. K+, Na+, Ca2+, Ba2+, Cu2+, Ni2+, Pb2+, Mn2+ and Sr2+ ions), which indicate that a strong affinity of nitrogen organic groups toward U(VI) ions. U-N bond length is longer than other metal-nitrogen cations (i.e. Zn2+, Mg2+, Al3+, Fe3+ ions), leading to a depth effect for the N- combined with U(VI) ions. To sum up, we proposed that the adsorption amount of U(VI) mainly affected by Na+, Zn2+, Mg2+ and Ni2+ions in the co-exits ion aqueous solutions56. Meanwhile, we also investigated the effect of individual ions on removal efficiency of U(VI) by GO-BSA composites in aqueous solutions. From our results, GO-BSA composites retain good


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selectivity. Good selectivity suggests that GO-BSA composite has a strong affinity toward U(VI) in aqueous solutions.

removal rate

140

b 100 80

120 60

100 80

40

60 40

20

20 0

0 Ca Mn Pb Fe K Cu Al Ba Na Sr Zn Ni Mg U

450

absorption capacity

removal rate

400 350

100 80

300 250

60

200 40

150 100

20

Removal Rate (%)

160

before after

Q e (mg g -1 )

180

Removal Rate (%)

a Concentration (mg L -1 )

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


50 0

0 Na(U)Ca(U) K(U) Ni(U) Sr(U) Al(U) Mg(U)Ba(U)

Element

Element

Fig. 7 Effect of co-existing ions (a) and individual ions (b) on the extraction of U (VI) by GOBSA composites. 3.6. Adsorption mechanism of GO-BSA composites. Based on the above research, The adsorption mechanisms of the U(VI) onto the surface of GO-BSA composites was proposed by the Fig. 8. The result is supported by XPS spectra and FTIR of the GO-BSA composites preand post-treatment of U(VI) adsorption that explain the interaction mechanism between U(VI) and GO-BSA composites, as shown in Supporting Information Fig. S7 and Fig. 9. In Fig. S7 of the FTIR, a new peak at 912 cm-1 is assigned to the asymmetric stretching vibration of the O=U=O, and the peaks at 1650 cm-1 have a red shift, which give a clear indication of the chelation between the U(VI) and -COOH on the surface of GO-BSA composites57. From Fig. 9a, we reveal that the introduction of U(VI) results in higher binding energy peaks of N 1s and O 1s (Table 1). By using an eluent for U(VI), the binding energy of the desorption-GO-BSA sample shifts to a lower energy compared to the GO-BSA-U sample. Therefore, the XPS spectra clearly demonstrate that the adsorption behaviour of U(VI) onto GO-BSA composites cannot be


17


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separated from nitrogen- and oxygen-containing organic groups. The peak positions of N 1s and O 1s of GO-BSA, GO-BSA-U, and desorption-GO-BSA are shown in Supporting Information Fig. S6.

Fig. 8 Proposed adsorption mechanisms of the GO-BSA composites


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


Fig. 9 XPS spectra of GO, GO-BSA, GO-BSA-U, and desorption-GO-BSA-U (a) whole figure; (b) single oxygen peak’s figure; (c) single nitrogen peak’s figure; (d) single U 4f peak’s figure From Fig. 9b, O 1s of GO-BSA-U (at 530.81 eV) has a higher binding energy than O 1s of GO-BSA (at 530.30 eV). From Supporting Information Fig. S6c, d, f, the binding energy of -O(at 530.60 eV) and -OH (at 532.20 eV) in the desorption-GO-BSA samples is located at a lower site than GO-BSA-U (-O-: 530.60 eV and -OH: 532.20 eV), which indicates that good adsorption is greatly attributed to the large number of -OH organic groups of -COOH and the surface of GO-BSA

58

. From Fig. 9c and Supporting Information Fig. S6a, b, e, the -NH (at

399.50 eV) and -NH2 (at 400.65 eV) of GO-BSA-U are also of higher binding energy than the -


19


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NH (at 398.48 eV) and -NH2 (at 399.88 eV) of GO-BSA and the -NH (at 398.56 eV) and -NH2 (at 400.08 eV) of desorption-GO-BSA (at 398.68 eV), which gives the same result as O 1s. This shows that the large number of nitrogen-containing organic groups of -CONH and BSA of GOBSA composites are chelated with U(VI) ions. However, the changed binding energy of N 1s (∼1.06 eV) is higher than that of O 1s (~0.5 eV) between GO-BSA-U and GO-BSA, indicating that the generation of coordination bonds is mainly due to nitrogen- organic groups and U(VI) ions. Meanwhile, from Fig. 9b and Fig. 9c, we also observe that the binding energies of O 1s (at 530.33 eV) and N 1s (at 398.68 eV) of desorption-GO-BSA are similar to those of GO-BSA. The changed binding energies of N 1s and O 1s indicate that the binding of nitrogen-containing functional groups combined with U(VI) is more stable than that of the oxygen-containing groups. Therefore, the nitrogen-containing organic groups are considered the main group of adsorptiondesorption of GO-BSA composites. In brief, the adsorption behaviour of GO-BSA composites is due to the presence of oxygen- and nitrogen-containing groups on the surface of GO-BSA composites59. Table. 1 The change of binding energies of GO, GO-BSA, and GO-BSA-U composites adsorbents

O 1s

N 1s

U 4f 5/2 U 4f 7/2

(eV)

(eV)

(eV)

(eV)

392.29

381.42

GO

530.74

GO-BSA

530.30 398.15

GO-BSA-U

530.81

399.21

Desorption-GO-BSA 530.33

398.68

3.7. Desorption of U(VI) from GO-BSA composites. For practical application, the regeneration of GO-BSA composites is of great importance. The elution rate of the GOBSA composites adsorbent after U(VI) uptake was carried out by H2O, 0.1 M HCl, 0.1 M


20

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NaHCO3, 0.1 M NaOH aqueous solutions, respectively (Supporting Information, Table S5). From Table S5, 0.1 M HCl is the ideal eluent for U(VI) in aqueous solutions. Therefore, HCl was chosen as the desorption solution in the following test. Different concentrations of HCl (0.01 to 1.0 M) were tested to determine the optimum eluent concentration of HCl for the cycle of adsorption-desorption (Fig 10a), which we find is 0.5 M HCl (Supporting Information, SI.7.2). Meanwhile, from Fig. 10b, when the GOBSA composites were recycled five times, eluent efficiency is maintained at 79 %. Moreover, with FTIR (Supporting Information, Fig. S7) the appearance of a small band at 912 cm−1 clearly represents the organic groups of UO2. We also show that the structure of GO-BSA composites after elution with high eluent efficiency are the same as the pristine GO-BSA composites. To sum up, good regeneration efficiency of the GO-BSA composites shows the potential application for removal of U(VI) from seawater.

Fig. 10 The elution rate with different concentration of HCl (a) and the cycle number of GOBSA composites (b) 3.8. Comparison of sorbent performance of GO-composites with literature data. On comparison of U(VI) adsorption capacity with other adsorbents, GO-BSA composites exhibit good adsorption performance, which is shown in Table 2 in which the adsorbent materials are


21


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Page 22 of 35

modified GO materials with organic compounds, conventional GO materials, modified GO materials with organic impurity atoms, and inorganic functional GO materials. The adsorption amount of GO-BSA equals 389 mg g-1, which is higher than those of other adsorbents materials, suggesting that GO-BSA as an adsorbent is suitable for the adsorption of U(VI). Table. 2 Comparison of sorbent performance of GO-composites with literature data Adsorbents

Qm

R

pH

(mg g-1) (%)

t

Isotherm

Ref.

(min)

magnetic 141.2

90

6

100

endothermic

60

Phosphate-functionalized graphene 251.7 oxide

78

4

240

endothermic

61

Sulfonated graphene oxide

309.09

84

6

180

endothermic

62

Halloysite@graphene oxide

161

50

5.6

240

--

63

Polydopamine/graphene oxide

145.39

72

4

120

--

64

Layered double hydroxide/

277.80

92

4

100

endothermic

65

Graphene oxide/polypyrrole

147.06

88

5

600

--

66

Manganese dioxide/graphene oxide

185.2

55

3.8

20

--

67

Polyacrylamide/graphene oxide

166

70

5

480

endothermic

68

Magnetic cucurbit [6]uril/graphene 122.5 oxide

64

5

150

endothermic

69

Manganese dioxide/iron oxide

73

6

360

endothermic

70

88

6

80

endothermic

present study

Amino functionalized graphene oxide

graphene

108.7

/reduced graphite oxide Graphene albumin

oxide-bovine

serum 389

3.9. U(VI) adsorption in simulated seawater. To evaluate the practicality of this material for removal of U(VI) from seawater, simulated seawater was prepared according to a method


22

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previously reported in the literature 71. 17 mg of uranyl nitrate ([UO2(NO3)2]), 25.6 g of sodium chloride (NaCl) and 193 mg of sodium bicarbonate (NaHCO3) were dissolved in 1 L of deionized water, then the solution was diluted to 5 ppb, 10 ppb, 15 ppb, 50 ppb, 100 ppb and 500 ppb. The GO-BSA composites were immersed in simulated seawater for 24 h at 25 oC. The results in Fig. 11 show that GO-BSA has a promising ability for removal of U(VI) from real seawater.

Fig. 11 The adsorption amount and removal rate of U(VI) by GO-BSA composites in simulated seawater. 4. CONCLUSIONS In our research, we have demonstrated an organic synthesis method to prepare composites of BSA and GO by covalent crosslinking with amide bonds using EDCI coupling. Importantly, the GO-BSA composites are different from previously reported GO-based materials; they depend on chemical groups grafted onto the surface of GO, resulting in a maximum adsorption of U(VI) as high as 389 mg g-1 at 298.15 K, pH=6, Co=200 mg L-1 and t=80 min. Meanwhile, the adsorption process closely fitted the Langmuir isotherm model and pseudo-second-order show


23


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that chemisorption dominates the process in aqueous solution. From XPS analysis of U(VI) adsorption-desorption of the GO-BSA composites, the distinctive adsorption capacity is predominately attributed to the coordination of U(VI) ions with organic groups. In addition, the adsorption data of U(VI) in simulated seawater demonstrate that GO-BSA composites have good adsorption efficiency and have promising potential as economical adsorbents for removing U(VI) from seawater. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Authors *Email: [email protected], *Email: [email protected] ADDITIONAL INFORMATION The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China [NSFC 51402065], Fundamental Research Funds of the Central University [HEUCFZ], Natural Science Foundation of Heilongjiang Province [B201404], International Science & Technology Cooperation Program of China [2015DFR50050] and the Major Project of Science and Technology of Heilongjiang Province [GA14A101]. REFERENCES


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A facile method was developed to prepare the GO-BSA composite as a highly efficient, recyclable U (VI) adsorbent from aqueous solutions with the removal rate up to 85%. 159x159mm (300 x 300 DPI)


Bovine Serum Albumin-Coated Graphene Oxide for Effective

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