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graphene oxide composite as high performance adsorbents for U(VI) removal
Applied Surface Science 458 (2018) 226–235
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Full Length Article
Bioassembly of fungal hyphae/graphene oxide composite as high performance adsorbents for U(VI) removal
T
Yi Lia,b, Geng Zoua, Siyi Yanga, Peiheng Shia, Tao Chena, Yiren Liana, Tao Duana,b, Kui Zhenga, ⁎ ⁎ Lichun Daic, , Wenkun Zhua,b, a
State Key Laboratory of Environment-friendly Energy Materials, School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, China b Sichuan Co-Innovation Center for New Energetic Materials, Southwest University of Science and Technology, Mianyang 621010, China c Biogas Institute of Ministry of Agriculture, Chengdu 610041, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fungal hyphae Graphene oxide Uranium Adsorption Fixed-bed column
In this paper, fungus hyphae/graphene oxide (FH/GO) with interesting net structure was prepared through a simple biological culture method for U(VI) removal. SEM, Zeta potential, FT-IR and XPS were applied to characterize the physicochemical properties of the resulting FH/GO composites, and batch and column adsorption experiments were conducted to elucidate their U(VI) adsorption performances and mechanisms. Results showed that FH/GO composite was highly efficient in U(VI) adsorption with a high reusability and stability. Specifically, compared to biomass (FH), the U(VI) adsorption capacity was increased by 60% for FH/GO composite at initial pH 6.0 ( ± 0.1) and 20 °C, and the maximum U(VI) adsorption capacity for the FH/GO was 199.37 mg/g. Furthermore, the adsorption kinetics, thermodynamics and isotherms analysis demonstrated that the U(VI) adsorption on FH/GO was depended on the chemical adsorption, and it was an endothermic and spontaneous process. Therefore, these results suggested that the as-prepared FH/GO composite was promising in nuclear waste water treatment.
1. Introduction The procedures in nuclear industry, such as nuclear fuel production, nuclear power plant operation and nuclear facility decommissioning, could produce large amounts of radioactivity water. Uranium (U(VI)) is one of the most hazardous radionuclide in radioactivity water, and the free U(VI) ions in the nature environment would cause the severe damage of organisms and ecological environment [1–3]. Numerous approaches, for instance, ion exchange, precipitation, redox and adsorption, have been applied to remove and recovery of U(VI) from nuclear industry effluent [4–8]. Among these traditional methods, adsorption is concerned to be an approach with high efficiency, ease of operation and possible practical applications. Recent years, graphene oxide (GO) has received extensive attentions due to its properties of electrical conductivity, thermal conductivity, light transmission, biocompatibility and mechanical strength, and GO has been widely used in the fields of energy, biology and environment [9–12]. Such as Tang et al. significantly enhance the photoresponse from the light-controlled conductive switching based on Cu2O/rGO
[10]. GO has been applied to water treatment due to its superior absorption ability [13,14]. The characteristics of high specific surface area and abundant oxygen-containing functional groups make GO possess strong surface complexation capacity and cation exchange of metal ions [15]. However, GO is difficult to recycle for its nano-scale which restricted its practical application. Fungus hypha (FH) is a kind of unique microorganism which grows fast within several days from single cells to a macro scale of microbe, with a length of centimeter-long and a diameter less than 10 μm [16,17]. Due to the large quantities of function groups on the cell wall of FH, such as phosphonate, hydroxyl and amine groups in the cell wall, filamentous fungus is believed to be an excellent template for nanometer materials [17–20]. Therefore, the combination of FH and GO would be a superior adsorbent which extends the nano-scale GO to macro-scale level, thus enhancing the separation of adsorbents from wastewater and benefiting the practical application. In this paper, Xylaria was selected as an experimental fungus, and the fungus hypha/graphene oxide (FH/GO) composite was fabricated through a facile route for U(VI) containing wastewater treatment. The
⁎ Corresponding authors at: State Key Laboratory of Environment-friendly Energy Materials, School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, China (W. Zhu) and Biogas Institute of Ministry of Agriculture, Chengdu 610041, China (L. Dai). E-mail addresses: [email protected] (L. Dai), [email protected] (W. Zhu).
https://doi.org/10.1016/j.apsusc.2018.07.081 Received 1 May 2018; Received in revised form 29 June 2018; Accepted 11 July 2018 0169-4332/ © 2018 Published by Elsevier B.V.
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suspensions due to their spherical shape. To eliminate the effects of plastic tube walls, the adsorption experiment of U(VI) without adsorbents was also carried out under the same conditions. The concentration of U(VI) was analyzed with the Arsenazo-III spectrophotometric method on a UV2600A spectrophotometer [22]. The adsorption amount (qt, mg/g) and the adsorption percentage (Ads, %) of U(VI) were calculated by the following equations (Eq.):
specific aims of this study are to (1) prepare a novel FH/GO composite by the biological culture method, (2) to test the removal performance of FH/GO for U(VI) and its stability and reusability, (3) to explore the possible adsorption mechanisms of U(VI) on FH/GO. This paper would highlight the high adsorption capacity of FH/GO toward U(VI) and its easy separation in uranium wastewater management. 2. Experimental
qt = (C0−Ct ) V / m
(1)
2.1. Materials
Ads = (C0−Ct )/ C0 × 100%
(2)
Xylaria was purchased from Microbiology Department of East China Institute of Technology. Graphite powder (≥99.85% purity) was purchased from Shanghai Huayi Group Company. UO2(NO3)2·6H2O (≥99.9% purity) was purchased from Beijing chemical factory. All other chemicals in analytical grade were available from Chengdu Kelong Chemical Co., Ltd. All solutions were prepared using Milli-Q water and analytical grade chemicals.
where C0 (mg/mL) and Ct (mg/mL) are the concentration of U(VI) in initial solution and solution after absorption for t time. V (mL) is the volume of the solution, and m (g) is the amount of adsorbent used. Each experiment was repeated independently three times. The adsorbent loaded with U(VI) was treat by 0.1 mol/L NaOH for 2 h. Each kind of adsorbent was washed by purified water for three times before next cycle adsorption. The solutions were replaced by fresh ones mentioned above for the next desorption process.
2.2. Synthesis of GO and FH/GO
2.5. Co-existing ions and radiation
GO was prepared from nature graphite powder by a modified Hummers method.[21] Briefly, 3.75 g of NaNO3 and 5.0 g of graphite were mixed with 150 mL of H2SO4 solution under potent stirring. Then, 20 g of KMnO4 was gradually added over approximately 0.5 h. Next, 30 mL of H2O2 was added into the suspension by liquid-transferring gun after 5 days. The suspension was centrifuged at 10000 rpm for 10 min and removed supernatant. Finally, the solid were dispersed with sterile water and the solution centrifuged at 6000 rpm to collect supernatant which was used as GO solution. Repeating the above two steps until that supernatant was clear. GO concentration was control to be 0.5 mg/ mL for later use. FH/GO composites were prepared with different GO contents in this study. Briefly, the fungus (Xylaria) was dispersed evenly into culture media by stirring and ultrasonic, and the culture media for the fungus was prepared at pH 6.0 containing 2% glucose, 0.25% yeast, 0.25% peptone. 100 mL culture media was added in every Erlenmeyer flasks (250 mL). Then, these flasks were added 0, 10, 20, 30 mL GO solution (0.5 mg/L) represented as FH, FH/GO-1, FH/GO-2, and FH/GO-3, respectively. These flasks containing different GO solutions were all incubated at 293 K for 3 days on a rotary shaker (Kuhner, Switzerland ISF-1-W) at 120 rpm. After that, the fungal biomass was washed several times with deionized water to remove the rest culture solution and the production was treated by the freeze-drying method to get hybrid sphere.
Different concentrations of chlorate were added in the aqueous solution to subjoin different cations in the process of batch experiments, and the same data-handling method was used to evaluate the selectivity of FH/GO-3 on U(VI). The FH and FH/GO-3 were exposed to a 300 kGy irradiation environment for 48 h, then the maximum U(VI) adsorption capacity of them were measured. 2.6. Stability and reusability evaluation The recycle test of FH and FH/GO-3 performed 6 cycles of U(VI) adsorption, desorption and washing. The adsorbent loaded U(VI) was treated by excess HCl (1mol/L) for 3 h in a state of shock. Then, leaving the solid and liquid was drained. Next, the solid was washed by deionized water for three times before next cycle absorption experiment. In stability test, FH/GO-3 was treated with HCl solution, ultrasonic and water bath. 2.7. Column adsorption experiment A 20 cm small glass tube with 1.5 cm diameter was used as the column for all of fixed-bed column experiments at room temperature. One gram of FH and 1 g FH/GO-3 were carefully pushed into two glass tubes, respectively. 100 mg/L U(VI) solution which was adjusted to pH 6.0 ( ± 0.1) flowed into the glass tubes from the bottom of tubes and was extracted from the top of tubes by peristaltic pumps at a velocity of 1.5 mL/min. The height of FH bed was 9.4 cm and FH/GO-3 bed was 8.3 cm, respectively. The effluent samples were collected to measure the concentrations of U(VI) at 25 min interval. Breakthrough curves were obtained by plotting C/C0 against time (min) (Fig. 9a). And C0 (mg/mL) and C (mg/mL) are the influent and effluent U(VI) concentrations respectively. The breakthrough experimental data was analyzed by the following equations:
2.3. Characterization The surface morphology of samples was characterized by scanning electron microscopy (SEM, Zeiss Ultra 55). The surface functional groups were estimated with Fourier transformed infrared spectroscopy (FT-IR, PE Spectrum One) and X-ray photoelectron spectroscopy (XPS, SSX-100). Zeta potential (zetaPALS) was used to analyze the change of surface charge between FH and FH/GO.
∫0
tb
∫0
te
CB =
2.4. Batch adsorption and desorption experiment
CE =
Batch adsorption experiments evaluated the effect on pH of the aqueous solution, initial U(VI) concentration, temperature and contacted time of adsorbents on the U(VI) adsorption capacity. And it was conducted in plastic tube containing 10 mg adsorbents and 20 mL U(VI) solution. The pH of the aqueous solution was adjusted from 2 to 10 by 0.01–1 mol/L HCl or NaOH in a 50 mg/L initial U(VI) concentration aqueous solution at room temperature, and then the suspensions were shaken in an orbital shaker at 100 rpm to reach adsorption equilibrium. Moreover, the adsorbents can be directly separated from the
(C0 -C)vdt/m
(3)
(C0 -C)vdt/m
(4)
η = CB / CE
(5)
where CB (mg/g) and CE (mg/g) are thebreakthrough capacity and the exhaustion capacity of the bed per unit amount of the packed sample, respectively. The breakthrough time (tb) and the exhaustion time (te) are the time when the U(VI) concentration reached 5% in the effluent and 95% in the influent, respectively. v (mL/min) is the flow rate, and η is the degree of column efficiency. 227
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Fig. 1. The synthetic process of FH/GO composites (a) and the digital photos of hydrogel sphere: (b) FH, (c) FH/GO-1, (d) FH/GO-2, (e) FH/GO-3 and the surface structure of (f) FH, (g) FH/GO-1, (h) FH/GO-2, (i) FH/GO-3 characterized by SEM.
3. Results and discussion
The SEM images of FH and FH/GO composites are shown in Fig. 1 (f–i). It can be seen that the structure of FH consisted of lots of micronsized filamentous biomasses which were crisscrossed and distributed irregularly in the space. For FH/GO-3, thin GO films appeared in the space and functioned to cover these filamentous biomasses (Fig. 1 i), and it could be found that the amount of GO covered on the surface of FH increased with the adding GO contents compared with FH/GO-1 and FH/GO-2. GO can be coated on the FH surface through hydrogen bonding and charge attraction during the growth process of FH, which could effectively reduce the risk of GO agglomeration.[24] And the FH/ GO maintains structural stability after ultrasonic and water bath, which proves the feasibility of assembly FH and GO (Figs. s2 and s3). According to SEM results, it could be inferred that the surface area and
3.1. Preparation and characterization of FH/GO The synthetic process of FH/GO composites and the macro-morphology of FH and FH/GO composite with different GO contents are shown in Fig. 1 (b–e). It can be observed that, with the increase of GO content, the color of fungal pellets was darker, and concurrently the size of the pellets was smaller. It was reported that graphene oxide sheets exhibited antimicrobial activity via extensive coverage of the cell surfaces and blocking the cell proliferation [23]. Thus higher GO doping amount resulted in smaller size of the resulting FH/GO composite.
Fig. 2. (a) Zeta potential of FH and FH/GO-3 at various solution pH values, (b) FTIR spectra of GO, FH, FH/GO composites and FH/GO-3 after adsorption of different concentrations of U(VI) solution, (c) XPS spectra of FH, FH/GO-3 and U(VI) loaded on FH/GO-3 and (d) High resolution XPS spectra of U4f5/2 and U4f7/2. 228
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higher percentage of C]O in oxygen-containing functional groups. Four peaks at 284.8, 286.4, 287.9 and 289.1 eV on C1s spectra of FH, FH/GO-3 (Fig. 3 (b, e)) were the carbon chemical bonds of CeC, CeO/ CeN, C]O and OeC]O, respectively.[27,31] The content of CeC on FH/GO-3 had an 18.0% increase as compared with FH, which indicated that GO was covered on the surface of fungus and assembled with fungus successfully. Moreover, the N1s spectra of FH, FH/GO-3 were also resolved into two peaks at 399.9 and 401.5 eV (Fig. 3 (c, f)). The peak at 399.9 eV was from the amide groups (eNH2, eNHe, O]CeN, Ne(C)3), and the peak at 401.5 eV was attributed to the protonated amine (eNH3+) [32,33]. The bonding energy of the amine groups on FH/GO-3 was lower than FH, it proved the chemical interaction between the amine groups and GO.
active sites of FH/GO-3 were improved by contrast with FH. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.07.081. The zeta potentials of FH and FH/GO-3 at different pH are shown in Fig. 2a. The values of zeta potential on FH/GO-3 were much less than FH at different pH. The zeta potential of FH/GO-3 at pH 6.0 ( ± 0.1) was about −34 eV, which indicated that a number of negative charges covered the surface of FH/GO-3. This could be beneficial for the electrostatic adsorption of positive U(VI) ion. The FTIR spectra of GO, FH and FH/GO composites are shown in Fig. 2b. Various functional groups could be found in FTIR spectra of GO, such as CeOH at 3400 cm−1, eCOOH at 1700 cm−1, C]C at 1500 cm−1, C]O at 1400 cm−1 and CeO at 1050 cm−1 [25]. Compared with GO, pure fungus showed the different functional groups, such as eCH, eCH2, and eCH3 at 2900 cm−1, C]O stretch (amide I) and eNH2 at 1648 cm−1 related to proteins, NeH and CeN stretch at 1542 cm−1 related to proteins (amide II), CeOH at 1050 cm−1 associated with phosphorylated proteins and alcohols.[26,27] For FH/GO composites, the week peak at 1542 cm−1 assigned to the amide II peaks of FH faded away with the increasing content of GO, which might be caused by the chemical bonding between amide II groups on FH with GO. As shown in Fig. 2c, the elements of C, O, N and U on the surface of FH, FH/GO-3 were characterized by XPS spectrum spectroscopy. The O1s spectra of FH, FH/GO-3 were resolved into two peaks at 531.2 and 532.8 eV (Fig. 3 (a, d)). The peak at 531.2 eV is related to the carbonyl (C]O) functional, the peak at 532.8 eV represents aryl ethers (CeOeC) and hydroxyl (CeOH) from aliphatic ether, alcohol and phenol [28–30]. Different percentages of oxygen-containing functional groups were observed for FH, FH/GO-3. In contrast to FH, FH/GO-3 showed
3.2. Batch adsorption experiments 3.2.1. Adsorption kinetics The adsorption amount of U(VI) onto FH/GO as a function of contact time is shown in Fig. 4a. From which we could easily find that the U (VI) adsorption capacity of FH and FH/GO increased rapidly at the first 30 min. And the U(VI) adsorption of FH/GO reached equilibrium faster than FH at reaction time of about 35 min. It might be attributed to the synergistic effects of the abundant oxygen-containing functional groups and CeN bonds on the surface of FH/GO, which could provide enough active sites for the immobilization and binding of U(VI) [34]. And the high electrostatic attraction between negative charged GO and positive U(VI) ion in aqueous solution might contribute to this fast adsorption. Two different kinetic models, pseudo-first-order (Fig. 4b) and pseudo-second-order model (Fig. 4c) were employed to evaluate the controlling mechanism of the adsorption process of U(VI) on FH and
Fig. 3. High resolution O 1s, C 1s and N 1s core level XPS spectrum of FH (a, b, c), FH/GO-3 (d, e, f) and U(VI) loaded on FH/GO-3 (g, h, i). 229
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Fig. 4. The effect of contact time (a), Pseudo-first-order (b) and pseudo-second-order (c) kinetic model fit for U(VI) adsorption onto FH and FH/GO at pH = 6.0 ( ± 0.1), T = 293 K, C0 = 50 mg/L, m/V = 0.375 g/L.
empirical constants related to adsorption capacity and the Freundlich isotherm nonlinearity, respectively. The constants of Langmuir and Freundlich models were summarized in Table 2. It was easy to find that Langmuir model shown a better fits for U(VI) adsorption (R2 > 0.99), which indicated that U(VI) adsorption on FH and FH/GO occurred on homogeneous monolayer surface. [37] Furthermore, the fitting results of Langmuir model showed that the maximum adsorption capacity of FH for U(VI) was 113.78 mg/g, which was sharply increased to 199.37 mg/g for FH/GO-3 (Fig. 5a, Table 2). Moreover, the maximum U(VI) adsorption on FH/GO-3 was also more than GO which was 141.43 mg/g at the same condition (Fig. s1, Table 2). Thus this result suggested that bioassembly of GO onto FH was an efficient strategy to prepare high-performance U(VI) adsorbent. The higher adsorption capacities of FH/GO composites might be attributed to their lower zeta potential compared to FH (Fig. 2a). Moreover, GO possessed large specific surface area, which would also facilitate the adsorption of pollutants on FH/GO. It is believed that the adsorption capacity of FH/GO-3 was significantly influenced by the structure of FH. Table 3 shows a comparison of the maximum capacity of U(VI), compared with other adsorbents, the FH/GO-3 had the advantages of simple production and low cost while ensuring good adsorption performance. Additionally, the adsorption capacity of FH/GO-3 could increase with the higher temperature (243.79 mg/g at 333 K), which was calculated by Langmuir model at different temperature (Fig. 6a, Table 2).
FH/GO-3 composites. And the linear forms of the two models can be described by (Eqs. (6) and (7)).
ln(q e-qt ) =lnq e-k1t
(6)
t/qt =1/k2q 2e+t/q e
(7)
where qt and qe are respectively correspond to the sorption amount at time t and equilibrium. The k1 (min−1) and k2 (g/mg·min) represent respectively the adsorption rate constants of the pseudo-first-order and the pseudo-second-order models. The kinetics constants of U(VI) adsorption were summarized in Table 1. The higher correlation coefficient value (R2 > 0.99) suggested that the pseudo-second-order models could be used for a better description of the adsorption process than pseudo-first-order, indicating that the chemical adsorption took place between the composite and U(VI) through ion exchange and electron sharing between the adsorbents and anionic U(VI)-hydroxo complexes. [35,36] 3.2.2. Adsorption isotherm In order to ascertain the relationship between the maximum adsorption capacity of composite material and the equilibrium concentration, it was very effective to analyze the data at adsorption equilibrium state under different initial U(VI) concentrations. It can be found from Fig. 5a that the maximum adsorption capacity increased gradually with increasing initial concentration. This was presumably due to the higher driving force between U(VI) and adsorbent brought by the high U(VI) concentration. The adsorption isotherms of U(VI) were described through Langmuir (Fig. 5b) and Freundlich (Fig. 5c) models, and these models were displayed as Eqs. (8) and (9), respectively.
q e=(b·qmax ·Ce )/(1 + b·Ce ) qe = kf
3.2.3. Thermodynamic The effect of temperature on adsorption was shown in Fig. 6a. It could be found that the adsorption capacity increased significantly with the increasing temperatures from 293 K to 333 K. This was because of the elevated temperature might produce a swelling effect within the internal structure of FH and FH/GO, penetrating the U(VI) and its complexes further [13]. The free energy change (ΔG0), enthalpy change (ΔH) and entropy change (ΔS) provided information about the interaction between the surface of FH, FH/GO and U(VI). And these thermodynamic parameters can be calculated by Eqs. (10)–(12) and Fig. 6b and c.
(8)
1 Cn
(9)
e
where qe (mg/g) and Ce (mg/L) are the adsorption of U(VI) per weight until reaching the adsorption equilibrium and the equilibrium concentration of U(VI), respectively. qmax (mg/g) is the maximum amount of adsorption monolayer. b (L/mg) is a constant related to the energy and affinity of the sorbent in Langmuir formula. kf and n are the Table 1 Kinetic of pseudo first-order and second-order models. Sample
FH FH/GO-1 FH/GO-2 FH/GO-3
Pseudo-first-order
Pseudo-second-order
qe (mg/L)
k1 (min−1)
R2
qe (mg/L)
k2 (g/mg·h)
R2
75.33 91.83 103.04 116.17
0.015 0.017 0.016 0.019
0.981 0.976 0.975 0.975
76.53 95.72 106.41 121.07
0.114 0.128 0.133 0.149
0.999 0.998 0.999 0.998
K d=q e/Ce
(10)
ΔG 0 = - RTlnK 0 = ΔH 0 - T ·ΔS 0
(11)
lnK 0 = - ΔH 0/RT + ΔS 0/ R
(12)
where T and R are the temperature in Kelvin and the ideal gas constant (8.314 J/(mol·K)), respectively. The sorption equilibrium constants at different temperature, K0, can be calculated from intercept of the plot lnKd versus Ce (Fig. 6b). Then, the values of enthalpy (ΔH0) and entropy (ΔS0) can be calculated from the slope and intercept by the plot of lnK0 versus 1/T (Fig. 6c). At last, the values of free energy change (ΔG0) at different temperature were calculated by Eq. (11). 230
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Fig. 5. The adsorption isotherms (a), Langmuir (b) and Freundlich (c) model fit for U(VI) adsorption onto FH and FH/GO at pH = 6.0 ( ± 0.1), T = 293 K, m/ V = 0.375 g/L. Table 2 The Langmuir and Freundlich parameters of adsorption isotherms. Sample
FH FH/GO-1 FH/GO-2 FH/GO-3 FH/GO-3 FH/GO-3 GO
T(K)
293 293 293 293 313 333 293
Langmuir
Table 4 Thermodynamic parameters for U(VI) sorption on FH/GO-3.
Freundlich
qmax (mg/g)
b (L/mg)
R2
KF (mg1−nLng−1)
n
R2
113.78 153.58 163.21 199.37 211.43 243.79 141.43
0.139 0.098 0.133 0.076 0.088 0.105 0.081
0.988 0.997 0.997 0.999 0.995 0.997 0.994
24.79 27.18 29.39 31.44 32.39 33.35 30.18
2.74 2.56 2.41 2.25 2.14 2.09 2.36
0.981 0.968 0.967 0.956 0.974 0.975 0.987
T (K)
ΔG0 (kJ/mol)
ΔS0 (J/mol/K)
ΔH0 (kJ/mol)
293 313 333
−5.64 −6.77 −7.90
56.57
10.94
spontaneous process and benefitted from the higher temperature [6,38]. Moreover, the positive value of ΔH0 (10.94 kJ/mol) and ΔS0 (56.57 J/mol/K) shown that U(VI) adsorption on FH and FH/GO was an endothermic process, and the affinity of the adsorbent material for U (VI) [39]. In general, the adsorption of U(VI) on the adsorbents was an endothermic and spontaneous process.
Table 3 The maximum U(VI) sorption capacity on different adsorbents. Adsorbents
Experimental conditions
Qmax (mg/g)
Reference
MIL-101 PANI/GO FH/ATP FH/GO-3 (Xylaria) Magnetite particles Mesoporous silica POMN Polypyrrole MX-80 bentonite Ferrihydrite Fe3O4/GO
pH = 5.5, T = 298 K pH = 3, T = 298 K pH = 4, T = 303 K pH = 7, T = 293 K
350 242 125 199.37 113.78 158 153 141.4 87.72 32.37 50.62 69.49
[52] [2] [6] This work
pH = 5, T = 323 K pH = 4, T = 293 K pH = 8, T = 298 K pH = 5, T = 298 K pH = 5.5, T = 303 K pH = 5.5, T = 298 K pH = 5.5, T = 293 K
3.2.4. Effects of initial pH and co-existing ions The effects of pH on U(VI) adsorption on FH and FH/GO are shown in Fig. 7a. The adsorption of U(VI) onto FH/GO was increased with increasing pH value from 2 to 6, peaked at pH 7 (116 mg/g), then maintained a high level until pH > 9. This might be result from the competitive binding of H+ or H3O+ and UO2(OH)n(2−n)+ [40,41]. This law of U(VI) adsorbed by FH and FH/GO fitted the Zeta potential closely, the region with negative potential promoted the U(VI) adsorption. While the formation of anionic U(VI)-hydroxyl complexes (i.e. (UO2)3(OH)7− and UO2(OH)42−) would enhance the electrostatic repulsion process and hold back the adsorption capacity of U(VI) as a barrier when pH > 8. Furthermore the deprotonation of functional groups of FH and FH/GO increased with pH, which would contributed
[57] [58] [51] [59] [60] [61] [62]
Fig. 6. (a) Adsorption isotherms of FH/GO-3 at different temperatures, pH = 5.0 ( ± 0.1), T = 293 K, m/V = 0.375 g/L, (b) Linear plots of LnKd versus Ce at three different temperatures and (c) Linear plots of LnK0 versus 1/T.
The value of these parameters were summarized in Table 4, the negative ΔG0 (−5.64 kJ/mol at 293 K, −6.77 kJ/mol at 313 K and −7.90 kJ/mol at 333 K) indicated that the adsorption was a
the adsorption on metal ion U(VI). The effect of ionic strength on U(VI) adsorption on FH/GO from 0 to 0.1 mol/L is shown in Fig. 7b. It could be easily found that there was no
231
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Fig. 7. The effect of initial pH on FH and FH/GO (d), the effect of ionic (e) and cation (f) strength on FH/GO-3 at pH = 6.0 ( ± 0.1), T = 293 K, C0 = 50 mg/L, m/ V = 0.375 g/L.
recognized [49,50], indicating that significant amount of uranium (VI) was bound to FH/GO. The main elemental components of FH/GO were C, N and O (Fig. 2c). In the case of O1s spectra, FH/GO-3-U(VI) (Fig. 3g) underwent a significant shift compared to FH/GO (Fig. 3d). According to the adsorption isotherm results (Table 2), all the hydroxyl, oxoacetate and oxime groups contributed to binding uranium (VI) [51]. The bonding energy of CeO/CeN, C]O and OeC]O peaks of C1s shifted to the lower location (Fig. 3 e, h), which was attributed to the chemical interaction between the functional group on the surface of FH/GO-3 and the U(VI)-hydroxy compounds [2] Referring to the binding energies of nitrogen-containing functional groups, there was no clear change before and after adsorption. The percentage of NH3+ decreased after U(VI) adsorption in Fig. 3i. This could be illustrated that NH3+ flew away and replaced with H+ during the pretreatment before XPS test. In summary, the adsorption of U(VI) on FH/GO-3 was depended on the oxygen- and nitrogen-containing groups, while the nitrogen-containing functional groups in FH/GO-3 had little effect on the adsorption of U(VI) in sympathy with the previous research [8].
effect on U(VI) adsorption, which indicated that the inner-sphere surface complexation dominated U(VI) adsorption rather than ion exchange on FH/GO [42–44]. In addition, the U(VI) adsorption was also independent of different negative ions. However, when some cations, Cu2+, Co2+, Cs+, Mn2+ and Sr2+, were added in aqueous solution, the adsorption capacity was reduced to 106 mg/g comparing with 121 mg/ g at the same conditions without extra ions. It could be found that the competing adsorption test showed the affinity sequence of U(VI) > Sr > Mn > Co > Cu > Cs, indicating that FH/GO had preferentially affinity for highly selective binding U(VI) (Fig. 7c). The capture of U(VI) mainly via chelation with the active site on the FH/GO surface correspond to XPS [45,46]. The results suggested the FH/GO with abundant active sites possessed excellent adsorption selectivity towards U(VI). 3.3. Adsorption mechanism FTIR and XPS spectra were further applied to investigate the mechanism of U(VI) adsorption on FH/GO-3. As shown in Fig. 2b, FH/GO3a, FH/GO-3b and FH/GO-3c represented FH/GO-3 after adsorption of U(VI) at the initial concentration of 20, 50 and 80 mg/L, respectively. By comparison of FTIR spectra of FH/GO-3 before and after adsorption of U(VI), it could be observed that the vibration signals CeO at 1050 cm−1 gradually declined with the increase of U(VI). This could be explained that the chemical interaction of U(VI) with CeO groups in epoxy or alkoxy from GO and CeOH from phosphorylated proteins and alcohols [47]. The peak at 907 cm−1 could be seen from the FTIR spectra of FH/GO-3a, FH/GO-3b and FH/GO-3c, which was not appear on FH/GO composites spectra, which might be caused by the red-shift of aqueous UO22+ complexes (at 963 cm−1) [48,49]. As shown in Fig. 2d, the peaks of U4f5/2 at 393.0 eV and U4f7/2 at 382.1 eV were decomposed into three components, and these binding energies can be
3.4. Stability and reusability evaluation In practical application, the stability of adsorbent is also an important factor. As shown in Fig. s2, the hybrid sphere of FH/GO-3 was very stable in aqueous solution after different treatments (HCl, ultrasound and water bath). This was further confirmed by the results in Fig. s3, which showed that the ultraviolet absorption peak of GO didn’t appear in these filter liquors from different treatments of FH/GO-3, indicating the strong interactions between fungus and GO. Moreover, Fig. s4 shows the change of FH/GO-3 after irradiation treatment. It could be found that the network structure and functional groups on the surface of the FH/GO-3 showed almost no change, and the maximum U (VI) adsorption amounts of FH/GO and FH were not affected by the
Fig. 8. The maximum adsorption capacity of FH and FH/GO before and after irradiation (a) and effect of cycle on the U(VI) adsorption (b) at pH = 6.0 ( ± 0.1), T = 293 K, C0 = 80 mg/L, m/V = 0.375 g/L. 232
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Fig. 9. The process of fixed-bed column experiments (a) and breakthrough curves of U(VI) adsorption on FH and FH/GO-3 (b) at pH = 6.0 ( ± 0.1), T = 293 K, C0 = 100 mg/L, v = 1.5 mL/min.
Thomas equations were expressed as Eqs. (13) and (14), respectively.
irradiation (Fig. 8a). Thus FH/GO composites were stable under various conditions, which was beneficial to the practical application. The regeneration and reusability of adsorbents facilitate the reduction in the economic cost in practical application. Here, the repetitive usage of the FH and FH/GO-3 were evaluated by cycle experiments. From Fig. 8b, it could be easily seen that the adsorption of U (VI) at 80 mg/L on FH and FH/GO-3 were slightly reduced after 6 cycles (about 6 mg/g). This suggested that FH/GO-3 possessed the characteristic of high reusability and was superior to many other adsorbents from the treatment cost and other environmental concerns such as disposal of spent adsorbents [51,52].
C/C0 =1-1/(1+ ((C0 /qY m) × vt )a
(13)
C/C0 =1/(1+exp((KTh qTh m/v)-KTh C0 t))
(14)
where qY and qTh are the maximum adsorption capacity (mg/g) for Yan and Thomas models, respectively. m is the adsorbent mass (g) and KTh are the kinetic constants of adsorption rate (mL/mg/min) in Thomas model. a is a constant which can be obtained after the fitting of Yan model. From Table 6, the Yan and Thomas model constants both displayed excellent agreement between these models and the experimental results, with correlation coefficients (R2) of > 0.998. Furthermore, the maximum adsorption capacities, calculated by the Yan and Thomas models were also closed to the experimental one.
3.5. Fixed-bed column experiment The function of effluent concentration and breakthrough time was used to characterize the adsorption equilibrium relationship between mobile phase (U(VI) aqueous solution) and the stationary phase (absorbents). Fig. 9b displayed the breakthrough curves of U(VI) removal by FH and FH/GO-3 and the major parameters were summarized in Table 5. It can be seen that the value of C/C0 on FH/GO-3 remained below 5% at t < 875 min, while the value of C/C0 on FH was observed reaching 5% at t = 500 min. This can be explained by the fact that FH/ GO-3 requires more U(VI) aqueous solution to pass through the adsorption column than FH in order to achieve adsorption equilibrium, which meant that the FH/GO-3 presented the more excellent adsorption performance than FH since that the breakthrough point (defined as C/ C0 ≤ 5%) of FH/GO-3 was significantly higher than that of FH. And the efficiency of the U(VI) removal (η, 81%) on FH/GO-3 by fixed-bed column was obviously higher than by batch adsorption on the same reaction conditions. The high column efficiency further demonstrated that FH/GO can be a potential adsorbent for the removal and recovery of U(VI). In practical application, it is vital to predict accurately the breakthrough curves of target metal ions from the fixed-bed system for the facility design and operation [53,54]. The Yan and Thomas models are always applied to analyze the adsorption process in a dynamic system [55,56]. As shown in Fig. 9b, the fixed-bed column data was fitted with Yan and Thomas models by the plot of C/C0 versus t. The Yan and
4. Conclusion In this work, the material of FH/GO was synthesized successfully by the biological assemble method. Adsorption of U(VI) onto FH and FH/ GO under various conditions were investigated to elucidate their adsorption performance and mechanisms. The results showed that the adsorption of U(VI) on the adsorbents was a quickly, endothermic and spontaneous chemical process, and the FH/GO showed a better adsorption capacity of U(VI) (199.37 mg/g) than pure fungus in U(VI) aqueous solution. It also showed satisfactory performance in the stability under the condition of irradiation, water bath and ultrasound in acid solution. FH/GO as U(VI) adsorbent was explained further by the high reusability and excellent performance in fixed-bed column system. Due to its simple and low cost synthetic method, eco-friendly and outstanding stability, the material of FH/GO would be a potential adsorbent for the removal and recovery of U(VI) in nuclear waste water. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21601147, 21771002 and 21707074), Table 6 The parameters of Yan and Thomas model on FH and FH/GO-3. Sample
Table 5 The parameters of breakthrough curves on FH and FH/GO-3. Sample
tb (min)
te (min)
CB (mg/g)
CE (mg/g)
η
FH FH/GO-3
500 875
1100 1375
79.94 130.89
115.41 161.12
0.69 0.81
FH FH/GO-3
233
Yan model
Thomas model
qe (mg/g)
a
R2
qe (mg/g)
KTh × 10−4 (mL/min/mg)
R2
117.16 157.82
7.48 12.03
0.998 0.999
118.31 158.49
1.106 1.188
0.998 0.999
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Sichuan Province Science and Technology Program (No. 2017GZ0342,2016NZ0051, 2016GZ0259 and 2016GZ0277), Sichuan Province Education Department Program (No. 17zd1131, 14ZA0104 and 18za0494), Plan Projects of Mianyang Science and Technology (No. N-01-7), China Postdoctoral Fund (No. 2018M630715) and Southwest University of Science and Technology Longshan Academic Talent Research Support Plan (No. 17LZX526).
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Full Length Article
Bioassembly of fungal hyphae/graphene oxide composite as high performance adsorbents for U(VI) removal
T
Yi Lia,b, Geng Zoua, Siyi Yanga, Peiheng Shia, Tao Chena, Yiren Liana, Tao Duana,b, Kui Zhenga, ⁎ ⁎ Lichun Daic, , Wenkun Zhua,b, a
State Key Laboratory of Environment-friendly Energy Materials, School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, China b Sichuan Co-Innovation Center for New Energetic Materials, Southwest University of Science and Technology, Mianyang 621010, China c Biogas Institute of Ministry of Agriculture, Chengdu 610041, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fungal hyphae Graphene oxide Uranium Adsorption Fixed-bed column
In this paper, fungus hyphae/graphene oxide (FH/GO) with interesting net structure was prepared through a simple biological culture method for U(VI) removal. SEM, Zeta potential, FT-IR and XPS were applied to characterize the physicochemical properties of the resulting FH/GO composites, and batch and column adsorption experiments were conducted to elucidate their U(VI) adsorption performances and mechanisms. Results showed that FH/GO composite was highly efficient in U(VI) adsorption with a high reusability and stability. Specifically, compared to biomass (FH), the U(VI) adsorption capacity was increased by 60% for FH/GO composite at initial pH 6.0 ( ± 0.1) and 20 °C, and the maximum U(VI) adsorption capacity for the FH/GO was 199.37 mg/g. Furthermore, the adsorption kinetics, thermodynamics and isotherms analysis demonstrated that the U(VI) adsorption on FH/GO was depended on the chemical adsorption, and it was an endothermic and spontaneous process. Therefore, these results suggested that the as-prepared FH/GO composite was promising in nuclear waste water treatment.
1. Introduction The procedures in nuclear industry, such as nuclear fuel production, nuclear power plant operation and nuclear facility decommissioning, could produce large amounts of radioactivity water. Uranium (U(VI)) is one of the most hazardous radionuclide in radioactivity water, and the free U(VI) ions in the nature environment would cause the severe damage of organisms and ecological environment [1–3]. Numerous approaches, for instance, ion exchange, precipitation, redox and adsorption, have been applied to remove and recovery of U(VI) from nuclear industry effluent [4–8]. Among these traditional methods, adsorption is concerned to be an approach with high efficiency, ease of operation and possible practical applications. Recent years, graphene oxide (GO) has received extensive attentions due to its properties of electrical conductivity, thermal conductivity, light transmission, biocompatibility and mechanical strength, and GO has been widely used in the fields of energy, biology and environment [9–12]. Such as Tang et al. significantly enhance the photoresponse from the light-controlled conductive switching based on Cu2O/rGO
[10]. GO has been applied to water treatment due to its superior absorption ability [13,14]. The characteristics of high specific surface area and abundant oxygen-containing functional groups make GO possess strong surface complexation capacity and cation exchange of metal ions [15]. However, GO is difficult to recycle for its nano-scale which restricted its practical application. Fungus hypha (FH) is a kind of unique microorganism which grows fast within several days from single cells to a macro scale of microbe, with a length of centimeter-long and a diameter less than 10 μm [16,17]. Due to the large quantities of function groups on the cell wall of FH, such as phosphonate, hydroxyl and amine groups in the cell wall, filamentous fungus is believed to be an excellent template for nanometer materials [17–20]. Therefore, the combination of FH and GO would be a superior adsorbent which extends the nano-scale GO to macro-scale level, thus enhancing the separation of adsorbents from wastewater and benefiting the practical application. In this paper, Xylaria was selected as an experimental fungus, and the fungus hypha/graphene oxide (FH/GO) composite was fabricated through a facile route for U(VI) containing wastewater treatment. The
⁎ Corresponding authors at: State Key Laboratory of Environment-friendly Energy Materials, School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, China (W. Zhu) and Biogas Institute of Ministry of Agriculture, Chengdu 610041, China (L. Dai). E-mail addresses: [email protected] (L. Dai), [email protected] (W. Zhu).
https://doi.org/10.1016/j.apsusc.2018.07.081 Received 1 May 2018; Received in revised form 29 June 2018; Accepted 11 July 2018 0169-4332/ © 2018 Published by Elsevier B.V.
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suspensions due to their spherical shape. To eliminate the effects of plastic tube walls, the adsorption experiment of U(VI) without adsorbents was also carried out under the same conditions. The concentration of U(VI) was analyzed with the Arsenazo-III spectrophotometric method on a UV2600A spectrophotometer [22]. The adsorption amount (qt, mg/g) and the adsorption percentage (Ads, %) of U(VI) were calculated by the following equations (Eq.):
specific aims of this study are to (1) prepare a novel FH/GO composite by the biological culture method, (2) to test the removal performance of FH/GO for U(VI) and its stability and reusability, (3) to explore the possible adsorption mechanisms of U(VI) on FH/GO. This paper would highlight the high adsorption capacity of FH/GO toward U(VI) and its easy separation in uranium wastewater management. 2. Experimental
qt = (C0−Ct ) V / m
(1)
2.1. Materials
Ads = (C0−Ct )/ C0 × 100%
(2)
Xylaria was purchased from Microbiology Department of East China Institute of Technology. Graphite powder (≥99.85% purity) was purchased from Shanghai Huayi Group Company. UO2(NO3)2·6H2O (≥99.9% purity) was purchased from Beijing chemical factory. All other chemicals in analytical grade were available from Chengdu Kelong Chemical Co., Ltd. All solutions were prepared using Milli-Q water and analytical grade chemicals.
where C0 (mg/mL) and Ct (mg/mL) are the concentration of U(VI) in initial solution and solution after absorption for t time. V (mL) is the volume of the solution, and m (g) is the amount of adsorbent used. Each experiment was repeated independently three times. The adsorbent loaded with U(VI) was treat by 0.1 mol/L NaOH for 2 h. Each kind of adsorbent was washed by purified water for three times before next cycle adsorption. The solutions were replaced by fresh ones mentioned above for the next desorption process.
2.2. Synthesis of GO and FH/GO
2.5. Co-existing ions and radiation
GO was prepared from nature graphite powder by a modified Hummers method.[21] Briefly, 3.75 g of NaNO3 and 5.0 g of graphite were mixed with 150 mL of H2SO4 solution under potent stirring. Then, 20 g of KMnO4 was gradually added over approximately 0.5 h. Next, 30 mL of H2O2 was added into the suspension by liquid-transferring gun after 5 days. The suspension was centrifuged at 10000 rpm for 10 min and removed supernatant. Finally, the solid were dispersed with sterile water and the solution centrifuged at 6000 rpm to collect supernatant which was used as GO solution. Repeating the above two steps until that supernatant was clear. GO concentration was control to be 0.5 mg/ mL for later use. FH/GO composites were prepared with different GO contents in this study. Briefly, the fungus (Xylaria) was dispersed evenly into culture media by stirring and ultrasonic, and the culture media for the fungus was prepared at pH 6.0 containing 2% glucose, 0.25% yeast, 0.25% peptone. 100 mL culture media was added in every Erlenmeyer flasks (250 mL). Then, these flasks were added 0, 10, 20, 30 mL GO solution (0.5 mg/L) represented as FH, FH/GO-1, FH/GO-2, and FH/GO-3, respectively. These flasks containing different GO solutions were all incubated at 293 K for 3 days on a rotary shaker (Kuhner, Switzerland ISF-1-W) at 120 rpm. After that, the fungal biomass was washed several times with deionized water to remove the rest culture solution and the production was treated by the freeze-drying method to get hybrid sphere.
Different concentrations of chlorate were added in the aqueous solution to subjoin different cations in the process of batch experiments, and the same data-handling method was used to evaluate the selectivity of FH/GO-3 on U(VI). The FH and FH/GO-3 were exposed to a 300 kGy irradiation environment for 48 h, then the maximum U(VI) adsorption capacity of them were measured. 2.6. Stability and reusability evaluation The recycle test of FH and FH/GO-3 performed 6 cycles of U(VI) adsorption, desorption and washing. The adsorbent loaded U(VI) was treated by excess HCl (1mol/L) for 3 h in a state of shock. Then, leaving the solid and liquid was drained. Next, the solid was washed by deionized water for three times before next cycle absorption experiment. In stability test, FH/GO-3 was treated with HCl solution, ultrasonic and water bath. 2.7. Column adsorption experiment A 20 cm small glass tube with 1.5 cm diameter was used as the column for all of fixed-bed column experiments at room temperature. One gram of FH and 1 g FH/GO-3 were carefully pushed into two glass tubes, respectively. 100 mg/L U(VI) solution which was adjusted to pH 6.0 ( ± 0.1) flowed into the glass tubes from the bottom of tubes and was extracted from the top of tubes by peristaltic pumps at a velocity of 1.5 mL/min. The height of FH bed was 9.4 cm and FH/GO-3 bed was 8.3 cm, respectively. The effluent samples were collected to measure the concentrations of U(VI) at 25 min interval. Breakthrough curves were obtained by plotting C/C0 against time (min) (Fig. 9a). And C0 (mg/mL) and C (mg/mL) are the influent and effluent U(VI) concentrations respectively. The breakthrough experimental data was analyzed by the following equations:
2.3. Characterization The surface morphology of samples was characterized by scanning electron microscopy (SEM, Zeiss Ultra 55). The surface functional groups were estimated with Fourier transformed infrared spectroscopy (FT-IR, PE Spectrum One) and X-ray photoelectron spectroscopy (XPS, SSX-100). Zeta potential (zetaPALS) was used to analyze the change of surface charge between FH and FH/GO.
∫0
tb
∫0
te
CB =
2.4. Batch adsorption and desorption experiment
CE =
Batch adsorption experiments evaluated the effect on pH of the aqueous solution, initial U(VI) concentration, temperature and contacted time of adsorbents on the U(VI) adsorption capacity. And it was conducted in plastic tube containing 10 mg adsorbents and 20 mL U(VI) solution. The pH of the aqueous solution was adjusted from 2 to 10 by 0.01–1 mol/L HCl or NaOH in a 50 mg/L initial U(VI) concentration aqueous solution at room temperature, and then the suspensions were shaken in an orbital shaker at 100 rpm to reach adsorption equilibrium. Moreover, the adsorbents can be directly separated from the
(C0 -C)vdt/m
(3)
(C0 -C)vdt/m
(4)
η = CB / CE
(5)
where CB (mg/g) and CE (mg/g) are thebreakthrough capacity and the exhaustion capacity of the bed per unit amount of the packed sample, respectively. The breakthrough time (tb) and the exhaustion time (te) are the time when the U(VI) concentration reached 5% in the effluent and 95% in the influent, respectively. v (mL/min) is the flow rate, and η is the degree of column efficiency. 227
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Fig. 1. The synthetic process of FH/GO composites (a) and the digital photos of hydrogel sphere: (b) FH, (c) FH/GO-1, (d) FH/GO-2, (e) FH/GO-3 and the surface structure of (f) FH, (g) FH/GO-1, (h) FH/GO-2, (i) FH/GO-3 characterized by SEM.
3. Results and discussion
The SEM images of FH and FH/GO composites are shown in Fig. 1 (f–i). It can be seen that the structure of FH consisted of lots of micronsized filamentous biomasses which were crisscrossed and distributed irregularly in the space. For FH/GO-3, thin GO films appeared in the space and functioned to cover these filamentous biomasses (Fig. 1 i), and it could be found that the amount of GO covered on the surface of FH increased with the adding GO contents compared with FH/GO-1 and FH/GO-2. GO can be coated on the FH surface through hydrogen bonding and charge attraction during the growth process of FH, which could effectively reduce the risk of GO agglomeration.[24] And the FH/ GO maintains structural stability after ultrasonic and water bath, which proves the feasibility of assembly FH and GO (Figs. s2 and s3). According to SEM results, it could be inferred that the surface area and
3.1. Preparation and characterization of FH/GO The synthetic process of FH/GO composites and the macro-morphology of FH and FH/GO composite with different GO contents are shown in Fig. 1 (b–e). It can be observed that, with the increase of GO content, the color of fungal pellets was darker, and concurrently the size of the pellets was smaller. It was reported that graphene oxide sheets exhibited antimicrobial activity via extensive coverage of the cell surfaces and blocking the cell proliferation [23]. Thus higher GO doping amount resulted in smaller size of the resulting FH/GO composite.
Fig. 2. (a) Zeta potential of FH and FH/GO-3 at various solution pH values, (b) FTIR spectra of GO, FH, FH/GO composites and FH/GO-3 after adsorption of different concentrations of U(VI) solution, (c) XPS spectra of FH, FH/GO-3 and U(VI) loaded on FH/GO-3 and (d) High resolution XPS spectra of U4f5/2 and U4f7/2. 228
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higher percentage of C]O in oxygen-containing functional groups. Four peaks at 284.8, 286.4, 287.9 and 289.1 eV on C1s spectra of FH, FH/GO-3 (Fig. 3 (b, e)) were the carbon chemical bonds of CeC, CeO/ CeN, C]O and OeC]O, respectively.[27,31] The content of CeC on FH/GO-3 had an 18.0% increase as compared with FH, which indicated that GO was covered on the surface of fungus and assembled with fungus successfully. Moreover, the N1s spectra of FH, FH/GO-3 were also resolved into two peaks at 399.9 and 401.5 eV (Fig. 3 (c, f)). The peak at 399.9 eV was from the amide groups (eNH2, eNHe, O]CeN, Ne(C)3), and the peak at 401.5 eV was attributed to the protonated amine (eNH3+) [32,33]. The bonding energy of the amine groups on FH/GO-3 was lower than FH, it proved the chemical interaction between the amine groups and GO.
active sites of FH/GO-3 were improved by contrast with FH. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.07.081. The zeta potentials of FH and FH/GO-3 at different pH are shown in Fig. 2a. The values of zeta potential on FH/GO-3 were much less than FH at different pH. The zeta potential of FH/GO-3 at pH 6.0 ( ± 0.1) was about −34 eV, which indicated that a number of negative charges covered the surface of FH/GO-3. This could be beneficial for the electrostatic adsorption of positive U(VI) ion. The FTIR spectra of GO, FH and FH/GO composites are shown in Fig. 2b. Various functional groups could be found in FTIR spectra of GO, such as CeOH at 3400 cm−1, eCOOH at 1700 cm−1, C]C at 1500 cm−1, C]O at 1400 cm−1 and CeO at 1050 cm−1 [25]. Compared with GO, pure fungus showed the different functional groups, such as eCH, eCH2, and eCH3 at 2900 cm−1, C]O stretch (amide I) and eNH2 at 1648 cm−1 related to proteins, NeH and CeN stretch at 1542 cm−1 related to proteins (amide II), CeOH at 1050 cm−1 associated with phosphorylated proteins and alcohols.[26,27] For FH/GO composites, the week peak at 1542 cm−1 assigned to the amide II peaks of FH faded away with the increasing content of GO, which might be caused by the chemical bonding between amide II groups on FH with GO. As shown in Fig. 2c, the elements of C, O, N and U on the surface of FH, FH/GO-3 were characterized by XPS spectrum spectroscopy. The O1s spectra of FH, FH/GO-3 were resolved into two peaks at 531.2 and 532.8 eV (Fig. 3 (a, d)). The peak at 531.2 eV is related to the carbonyl (C]O) functional, the peak at 532.8 eV represents aryl ethers (CeOeC) and hydroxyl (CeOH) from aliphatic ether, alcohol and phenol [28–30]. Different percentages of oxygen-containing functional groups were observed for FH, FH/GO-3. In contrast to FH, FH/GO-3 showed
3.2. Batch adsorption experiments 3.2.1. Adsorption kinetics The adsorption amount of U(VI) onto FH/GO as a function of contact time is shown in Fig. 4a. From which we could easily find that the U (VI) adsorption capacity of FH and FH/GO increased rapidly at the first 30 min. And the U(VI) adsorption of FH/GO reached equilibrium faster than FH at reaction time of about 35 min. It might be attributed to the synergistic effects of the abundant oxygen-containing functional groups and CeN bonds on the surface of FH/GO, which could provide enough active sites for the immobilization and binding of U(VI) [34]. And the high electrostatic attraction between negative charged GO and positive U(VI) ion in aqueous solution might contribute to this fast adsorption. Two different kinetic models, pseudo-first-order (Fig. 4b) and pseudo-second-order model (Fig. 4c) were employed to evaluate the controlling mechanism of the adsorption process of U(VI) on FH and
Fig. 3. High resolution O 1s, C 1s and N 1s core level XPS spectrum of FH (a, b, c), FH/GO-3 (d, e, f) and U(VI) loaded on FH/GO-3 (g, h, i). 229
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Fig. 4. The effect of contact time (a), Pseudo-first-order (b) and pseudo-second-order (c) kinetic model fit for U(VI) adsorption onto FH and FH/GO at pH = 6.0 ( ± 0.1), T = 293 K, C0 = 50 mg/L, m/V = 0.375 g/L.
empirical constants related to adsorption capacity and the Freundlich isotherm nonlinearity, respectively. The constants of Langmuir and Freundlich models were summarized in Table 2. It was easy to find that Langmuir model shown a better fits for U(VI) adsorption (R2 > 0.99), which indicated that U(VI) adsorption on FH and FH/GO occurred on homogeneous monolayer surface. [37] Furthermore, the fitting results of Langmuir model showed that the maximum adsorption capacity of FH for U(VI) was 113.78 mg/g, which was sharply increased to 199.37 mg/g for FH/GO-3 (Fig. 5a, Table 2). Moreover, the maximum U(VI) adsorption on FH/GO-3 was also more than GO which was 141.43 mg/g at the same condition (Fig. s1, Table 2). Thus this result suggested that bioassembly of GO onto FH was an efficient strategy to prepare high-performance U(VI) adsorbent. The higher adsorption capacities of FH/GO composites might be attributed to their lower zeta potential compared to FH (Fig. 2a). Moreover, GO possessed large specific surface area, which would also facilitate the adsorption of pollutants on FH/GO. It is believed that the adsorption capacity of FH/GO-3 was significantly influenced by the structure of FH. Table 3 shows a comparison of the maximum capacity of U(VI), compared with other adsorbents, the FH/GO-3 had the advantages of simple production and low cost while ensuring good adsorption performance. Additionally, the adsorption capacity of FH/GO-3 could increase with the higher temperature (243.79 mg/g at 333 K), which was calculated by Langmuir model at different temperature (Fig. 6a, Table 2).
FH/GO-3 composites. And the linear forms of the two models can be described by (Eqs. (6) and (7)).
ln(q e-qt ) =lnq e-k1t
(6)
t/qt =1/k2q 2e+t/q e
(7)
where qt and qe are respectively correspond to the sorption amount at time t and equilibrium. The k1 (min−1) and k2 (g/mg·min) represent respectively the adsorption rate constants of the pseudo-first-order and the pseudo-second-order models. The kinetics constants of U(VI) adsorption were summarized in Table 1. The higher correlation coefficient value (R2 > 0.99) suggested that the pseudo-second-order models could be used for a better description of the adsorption process than pseudo-first-order, indicating that the chemical adsorption took place between the composite and U(VI) through ion exchange and electron sharing between the adsorbents and anionic U(VI)-hydroxo complexes. [35,36] 3.2.2. Adsorption isotherm In order to ascertain the relationship between the maximum adsorption capacity of composite material and the equilibrium concentration, it was very effective to analyze the data at adsorption equilibrium state under different initial U(VI) concentrations. It can be found from Fig. 5a that the maximum adsorption capacity increased gradually with increasing initial concentration. This was presumably due to the higher driving force between U(VI) and adsorbent brought by the high U(VI) concentration. The adsorption isotherms of U(VI) were described through Langmuir (Fig. 5b) and Freundlich (Fig. 5c) models, and these models were displayed as Eqs. (8) and (9), respectively.
q e=(b·qmax ·Ce )/(1 + b·Ce ) qe = kf
3.2.3. Thermodynamic The effect of temperature on adsorption was shown in Fig. 6a. It could be found that the adsorption capacity increased significantly with the increasing temperatures from 293 K to 333 K. This was because of the elevated temperature might produce a swelling effect within the internal structure of FH and FH/GO, penetrating the U(VI) and its complexes further [13]. The free energy change (ΔG0), enthalpy change (ΔH) and entropy change (ΔS) provided information about the interaction between the surface of FH, FH/GO and U(VI). And these thermodynamic parameters can be calculated by Eqs. (10)–(12) and Fig. 6b and c.
(8)
1 Cn
(9)
e
where qe (mg/g) and Ce (mg/L) are the adsorption of U(VI) per weight until reaching the adsorption equilibrium and the equilibrium concentration of U(VI), respectively. qmax (mg/g) is the maximum amount of adsorption monolayer. b (L/mg) is a constant related to the energy and affinity of the sorbent in Langmuir formula. kf and n are the Table 1 Kinetic of pseudo first-order and second-order models. Sample
FH FH/GO-1 FH/GO-2 FH/GO-3
Pseudo-first-order
Pseudo-second-order
qe (mg/L)
k1 (min−1)
R2
qe (mg/L)
k2 (g/mg·h)
R2
75.33 91.83 103.04 116.17
0.015 0.017 0.016 0.019
0.981 0.976 0.975 0.975
76.53 95.72 106.41 121.07
0.114 0.128 0.133 0.149
0.999 0.998 0.999 0.998
K d=q e/Ce
(10)
ΔG 0 = - RTlnK 0 = ΔH 0 - T ·ΔS 0
(11)
lnK 0 = - ΔH 0/RT + ΔS 0/ R
(12)
where T and R are the temperature in Kelvin and the ideal gas constant (8.314 J/(mol·K)), respectively. The sorption equilibrium constants at different temperature, K0, can be calculated from intercept of the plot lnKd versus Ce (Fig. 6b). Then, the values of enthalpy (ΔH0) and entropy (ΔS0) can be calculated from the slope and intercept by the plot of lnK0 versus 1/T (Fig. 6c). At last, the values of free energy change (ΔG0) at different temperature were calculated by Eq. (11). 230
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Fig. 5. The adsorption isotherms (a), Langmuir (b) and Freundlich (c) model fit for U(VI) adsorption onto FH and FH/GO at pH = 6.0 ( ± 0.1), T = 293 K, m/ V = 0.375 g/L. Table 2 The Langmuir and Freundlich parameters of adsorption isotherms. Sample
FH FH/GO-1 FH/GO-2 FH/GO-3 FH/GO-3 FH/GO-3 GO
T(K)
293 293 293 293 313 333 293
Langmuir
Table 4 Thermodynamic parameters for U(VI) sorption on FH/GO-3.
Freundlich
qmax (mg/g)
b (L/mg)
R2
KF (mg1−nLng−1)
n
R2
113.78 153.58 163.21 199.37 211.43 243.79 141.43
0.139 0.098 0.133 0.076 0.088 0.105 0.081
0.988 0.997 0.997 0.999 0.995 0.997 0.994
24.79 27.18 29.39 31.44 32.39 33.35 30.18
2.74 2.56 2.41 2.25 2.14 2.09 2.36
0.981 0.968 0.967 0.956 0.974 0.975 0.987
T (K)
ΔG0 (kJ/mol)
ΔS0 (J/mol/K)
ΔH0 (kJ/mol)
293 313 333
−5.64 −6.77 −7.90
56.57
10.94
spontaneous process and benefitted from the higher temperature [6,38]. Moreover, the positive value of ΔH0 (10.94 kJ/mol) and ΔS0 (56.57 J/mol/K) shown that U(VI) adsorption on FH and FH/GO was an endothermic process, and the affinity of the adsorbent material for U (VI) [39]. In general, the adsorption of U(VI) on the adsorbents was an endothermic and spontaneous process.
Table 3 The maximum U(VI) sorption capacity on different adsorbents. Adsorbents
Experimental conditions
Qmax (mg/g)
Reference
MIL-101 PANI/GO FH/ATP FH/GO-3 (Xylaria) Magnetite particles Mesoporous silica POMN Polypyrrole MX-80 bentonite Ferrihydrite Fe3O4/GO
pH = 5.5, T = 298 K pH = 3, T = 298 K pH = 4, T = 303 K pH = 7, T = 293 K
350 242 125 199.37 113.78 158 153 141.4 87.72 32.37 50.62 69.49
[52] [2] [6] This work
pH = 5, T = 323 K pH = 4, T = 293 K pH = 8, T = 298 K pH = 5, T = 298 K pH = 5.5, T = 303 K pH = 5.5, T = 298 K pH = 5.5, T = 293 K
3.2.4. Effects of initial pH and co-existing ions The effects of pH on U(VI) adsorption on FH and FH/GO are shown in Fig. 7a. The adsorption of U(VI) onto FH/GO was increased with increasing pH value from 2 to 6, peaked at pH 7 (116 mg/g), then maintained a high level until pH > 9. This might be result from the competitive binding of H+ or H3O+ and UO2(OH)n(2−n)+ [40,41]. This law of U(VI) adsorbed by FH and FH/GO fitted the Zeta potential closely, the region with negative potential promoted the U(VI) adsorption. While the formation of anionic U(VI)-hydroxyl complexes (i.e. (UO2)3(OH)7− and UO2(OH)42−) would enhance the electrostatic repulsion process and hold back the adsorption capacity of U(VI) as a barrier when pH > 8. Furthermore the deprotonation of functional groups of FH and FH/GO increased with pH, which would contributed
[57] [58] [51] [59] [60] [61] [62]
Fig. 6. (a) Adsorption isotherms of FH/GO-3 at different temperatures, pH = 5.0 ( ± 0.1), T = 293 K, m/V = 0.375 g/L, (b) Linear plots of LnKd versus Ce at three different temperatures and (c) Linear plots of LnK0 versus 1/T.
The value of these parameters were summarized in Table 4, the negative ΔG0 (−5.64 kJ/mol at 293 K, −6.77 kJ/mol at 313 K and −7.90 kJ/mol at 333 K) indicated that the adsorption was a
the adsorption on metal ion U(VI). The effect of ionic strength on U(VI) adsorption on FH/GO from 0 to 0.1 mol/L is shown in Fig. 7b. It could be easily found that there was no
231
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Fig. 7. The effect of initial pH on FH and FH/GO (d), the effect of ionic (e) and cation (f) strength on FH/GO-3 at pH = 6.0 ( ± 0.1), T = 293 K, C0 = 50 mg/L, m/ V = 0.375 g/L.
recognized [49,50], indicating that significant amount of uranium (VI) was bound to FH/GO. The main elemental components of FH/GO were C, N and O (Fig. 2c). In the case of O1s spectra, FH/GO-3-U(VI) (Fig. 3g) underwent a significant shift compared to FH/GO (Fig. 3d). According to the adsorption isotherm results (Table 2), all the hydroxyl, oxoacetate and oxime groups contributed to binding uranium (VI) [51]. The bonding energy of CeO/CeN, C]O and OeC]O peaks of C1s shifted to the lower location (Fig. 3 e, h), which was attributed to the chemical interaction between the functional group on the surface of FH/GO-3 and the U(VI)-hydroxy compounds [2] Referring to the binding energies of nitrogen-containing functional groups, there was no clear change before and after adsorption. The percentage of NH3+ decreased after U(VI) adsorption in Fig. 3i. This could be illustrated that NH3+ flew away and replaced with H+ during the pretreatment before XPS test. In summary, the adsorption of U(VI) on FH/GO-3 was depended on the oxygen- and nitrogen-containing groups, while the nitrogen-containing functional groups in FH/GO-3 had little effect on the adsorption of U(VI) in sympathy with the previous research [8].
effect on U(VI) adsorption, which indicated that the inner-sphere surface complexation dominated U(VI) adsorption rather than ion exchange on FH/GO [42–44]. In addition, the U(VI) adsorption was also independent of different negative ions. However, when some cations, Cu2+, Co2+, Cs+, Mn2+ and Sr2+, were added in aqueous solution, the adsorption capacity was reduced to 106 mg/g comparing with 121 mg/ g at the same conditions without extra ions. It could be found that the competing adsorption test showed the affinity sequence of U(VI) > Sr > Mn > Co > Cu > Cs, indicating that FH/GO had preferentially affinity for highly selective binding U(VI) (Fig. 7c). The capture of U(VI) mainly via chelation with the active site on the FH/GO surface correspond to XPS [45,46]. The results suggested the FH/GO with abundant active sites possessed excellent adsorption selectivity towards U(VI). 3.3. Adsorption mechanism FTIR and XPS spectra were further applied to investigate the mechanism of U(VI) adsorption on FH/GO-3. As shown in Fig. 2b, FH/GO3a, FH/GO-3b and FH/GO-3c represented FH/GO-3 after adsorption of U(VI) at the initial concentration of 20, 50 and 80 mg/L, respectively. By comparison of FTIR spectra of FH/GO-3 before and after adsorption of U(VI), it could be observed that the vibration signals CeO at 1050 cm−1 gradually declined with the increase of U(VI). This could be explained that the chemical interaction of U(VI) with CeO groups in epoxy or alkoxy from GO and CeOH from phosphorylated proteins and alcohols [47]. The peak at 907 cm−1 could be seen from the FTIR spectra of FH/GO-3a, FH/GO-3b and FH/GO-3c, which was not appear on FH/GO composites spectra, which might be caused by the red-shift of aqueous UO22+ complexes (at 963 cm−1) [48,49]. As shown in Fig. 2d, the peaks of U4f5/2 at 393.0 eV and U4f7/2 at 382.1 eV were decomposed into three components, and these binding energies can be
3.4. Stability and reusability evaluation In practical application, the stability of adsorbent is also an important factor. As shown in Fig. s2, the hybrid sphere of FH/GO-3 was very stable in aqueous solution after different treatments (HCl, ultrasound and water bath). This was further confirmed by the results in Fig. s3, which showed that the ultraviolet absorption peak of GO didn’t appear in these filter liquors from different treatments of FH/GO-3, indicating the strong interactions between fungus and GO. Moreover, Fig. s4 shows the change of FH/GO-3 after irradiation treatment. It could be found that the network structure and functional groups on the surface of the FH/GO-3 showed almost no change, and the maximum U (VI) adsorption amounts of FH/GO and FH were not affected by the
Fig. 8. The maximum adsorption capacity of FH and FH/GO before and after irradiation (a) and effect of cycle on the U(VI) adsorption (b) at pH = 6.0 ( ± 0.1), T = 293 K, C0 = 80 mg/L, m/V = 0.375 g/L. 232
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Fig. 9. The process of fixed-bed column experiments (a) and breakthrough curves of U(VI) adsorption on FH and FH/GO-3 (b) at pH = 6.0 ( ± 0.1), T = 293 K, C0 = 100 mg/L, v = 1.5 mL/min.
Thomas equations were expressed as Eqs. (13) and (14), respectively.
irradiation (Fig. 8a). Thus FH/GO composites were stable under various conditions, which was beneficial to the practical application. The regeneration and reusability of adsorbents facilitate the reduction in the economic cost in practical application. Here, the repetitive usage of the FH and FH/GO-3 were evaluated by cycle experiments. From Fig. 8b, it could be easily seen that the adsorption of U (VI) at 80 mg/L on FH and FH/GO-3 were slightly reduced after 6 cycles (about 6 mg/g). This suggested that FH/GO-3 possessed the characteristic of high reusability and was superior to many other adsorbents from the treatment cost and other environmental concerns such as disposal of spent adsorbents [51,52].
C/C0 =1-1/(1+ ((C0 /qY m) × vt )a
(13)
C/C0 =1/(1+exp((KTh qTh m/v)-KTh C0 t))
(14)
where qY and qTh are the maximum adsorption capacity (mg/g) for Yan and Thomas models, respectively. m is the adsorbent mass (g) and KTh are the kinetic constants of adsorption rate (mL/mg/min) in Thomas model. a is a constant which can be obtained after the fitting of Yan model. From Table 6, the Yan and Thomas model constants both displayed excellent agreement between these models and the experimental results, with correlation coefficients (R2) of > 0.998. Furthermore, the maximum adsorption capacities, calculated by the Yan and Thomas models were also closed to the experimental one.
3.5. Fixed-bed column experiment The function of effluent concentration and breakthrough time was used to characterize the adsorption equilibrium relationship between mobile phase (U(VI) aqueous solution) and the stationary phase (absorbents). Fig. 9b displayed the breakthrough curves of U(VI) removal by FH and FH/GO-3 and the major parameters were summarized in Table 5. It can be seen that the value of C/C0 on FH/GO-3 remained below 5% at t < 875 min, while the value of C/C0 on FH was observed reaching 5% at t = 500 min. This can be explained by the fact that FH/ GO-3 requires more U(VI) aqueous solution to pass through the adsorption column than FH in order to achieve adsorption equilibrium, which meant that the FH/GO-3 presented the more excellent adsorption performance than FH since that the breakthrough point (defined as C/ C0 ≤ 5%) of FH/GO-3 was significantly higher than that of FH. And the efficiency of the U(VI) removal (η, 81%) on FH/GO-3 by fixed-bed column was obviously higher than by batch adsorption on the same reaction conditions. The high column efficiency further demonstrated that FH/GO can be a potential adsorbent for the removal and recovery of U(VI). In practical application, it is vital to predict accurately the breakthrough curves of target metal ions from the fixed-bed system for the facility design and operation [53,54]. The Yan and Thomas models are always applied to analyze the adsorption process in a dynamic system [55,56]. As shown in Fig. 9b, the fixed-bed column data was fitted with Yan and Thomas models by the plot of C/C0 versus t. The Yan and
4. Conclusion In this work, the material of FH/GO was synthesized successfully by the biological assemble method. Adsorption of U(VI) onto FH and FH/ GO under various conditions were investigated to elucidate their adsorption performance and mechanisms. The results showed that the adsorption of U(VI) on the adsorbents was a quickly, endothermic and spontaneous chemical process, and the FH/GO showed a better adsorption capacity of U(VI) (199.37 mg/g) than pure fungus in U(VI) aqueous solution. It also showed satisfactory performance in the stability under the condition of irradiation, water bath and ultrasound in acid solution. FH/GO as U(VI) adsorbent was explained further by the high reusability and excellent performance in fixed-bed column system. Due to its simple and low cost synthetic method, eco-friendly and outstanding stability, the material of FH/GO would be a potential adsorbent for the removal and recovery of U(VI) in nuclear waste water. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21601147, 21771002 and 21707074), Table 6 The parameters of Yan and Thomas model on FH and FH/GO-3. Sample
Table 5 The parameters of breakthrough curves on FH and FH/GO-3. Sample
tb (min)
te (min)
CB (mg/g)
CE (mg/g)
η
FH FH/GO-3
500 875
1100 1375
79.94 130.89
115.41 161.12
0.69 0.81
FH FH/GO-3
233
Yan model
Thomas model
qe (mg/g)
a
R2
qe (mg/g)
KTh × 10−4 (mL/min/mg)
R2
117.16 157.82
7.48 12.03
0.998 0.999
118.31 158.49
1.106 1.188
0.998 0.999
Applied Surface Science 458 (2018) 226–235
Y. Li et al.
Sichuan Province Science and Technology Program (No. 2017GZ0342,2016NZ0051, 2016GZ0259 and 2016GZ0277), Sichuan Province Education Department Program (No. 17zd1131, 14ZA0104 and 18za0494), Plan Projects of Mianyang Science and Technology (No. N-01-7), China Postdoctoral Fund (No. 2018M630715) and Southwest University of Science and Technology Longshan Academic Talent Research Support Plan (No. 17LZX526).
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235