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graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution
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Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution Yingying Zhang, Xinzhu Li, Lifeng Gong, Zhiqiang Xing, Zhenning Lou, Weijun Shan, Ying Xiong∗ College of Chemistry, Key Laboratory of Rare-scattered Elements of Liaoning Province, Liaoning University, Shenyang 110036, PR China
a r t i c l e
i n f o
Article history: Received 18 May 2019 Revised 12 August 2019 Accepted 31 August 2019 Available online xxx Keywords: Graphene oxide Germanium Selective adsorption Persimmon tannin
a b s t r a c t Herein, a new persimmon tannin/graphene oxide (GO-PT-6Glu) composite was fabricated through onestep cross-linking method. The as-prepared GO-PT-6Glu has substantial phenolic hydroxyl groups (up to 5.23 mmol g−1 ) by introducing water-soluble PT (up to 5.50 mmol g−1 ) on the GO, which provides the opportunity to use it as an adsorption material to recover germanium. The GO-PT-6Glu was then used to adsorb germanium from aqueous solution and showed the highest than among currently reported adsorption capacity (117.38 mg g− 1 ). Simultaneously, it exhibits excellent cycle performance to adsorb germanium still with a high adsorption efficiency (up to 81.42%) after 5 adsorption–desorption cycles. More importantly, the GO-PT-6Glu showed superior selectivity towards Ge(IV) as compared to As(III) (Cl− /SO4 2− /PO4 3− ), in spite of the concentration of As(III) (Cl− /SO4 2− /PO4 3− ) was 100 times higher than that of Ge(IV). Taking advantages of its low cost, selective adsorption, as well as environmental protection and high adsorption, the GO-PT-6Glu is expected to be the most promising material in recovery of Ge(IV) from water containing As(III). © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The favorable electronic properties of Ge, coupled with its quantum confinement effect that can be observed under relatively large particle sizes [1], make Ge nanoparticles attractive for energy storage [2], catalysis [3], photodetector [4], and other fields. However, the broader utilization of germanium is still limited by its source [5]. Hence, the recovery of germanium from industrial effluents has drawn particular attention of researchers. Methods, including extraction [6], flotation [7] and precipitation [8] etc. have been developed for recovery of germanium from solutions. Arrambide-Cruz et al. proposed extracting method by catechol and 8-hydroxyQuinoline from NaOH medium to recycle Ge(IV) [9]. Although these methods have focused on recycling germanium, environmental improvement are still challenging due to limitations on using large volume of acid/alkali/organic reagent with high concentration. Thus, we need to develop new and cheap, efficient as well as environmentally friendly treatment technologies for recovery germanium from waste. Adsorption method is an efficient technology for recovery metal ions from water, which possess
∗
Corresponding author. E-mail address: [email protected] (Y. Xiong).
many advantages such as low cost, simple operation, and non-toxic [10,11]. Graphene oxide (GO), a novel carbon material, possessing high surface area and abundant oxygen-containing functional groups [12]. The presence of –COOH, –C=O and –OH groups on the surface of GO could allow GO to participate in a wide range of bonding interactions. Hence, GO has fantastic potential for environmentally related application, such as wastewater treatment and water purification [13,14]. Even so, the low hydrophily of GO nanosheets, which makes GO difficult to bind metal ions freely in wastewater, is the major obstacle to adsorption applications of graphene oxidebased materials [15]. To achieve further advances in graphene oxide-based materials in adsorption field, it is important to increase the hydrophily of GO, e.g. introducing hydrophilic groups on the GO, such as amino groups, carboxyl groups and hydroxyl groups [16,17]. Persimmon tannin (PT), as an inexpensive and ubiquitous natural biomass material, could produce a strong ability of combination for metal ions due to it extremely abundant multiple adjacent phenolic hydroxyl groups [18]. Nevertheless, PT must be chemically modified or immobilized onto non-water-soluble matrixes for extraction of metal ions from water due to its high solubility in water [19]. Fan et al. prepared a novel plant tannin-modified Fe3 O4 @SiO2 microspheres, which had superior adsorption capacities for gold
https://doi.org/10.1016/j.jtice.2019.08.024 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Y. Zhang, X. Li and L. Gong et al., Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice. 2019.08.024
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and palladium [20]. In our lab, we investigated the selective recovery of molybdenum(VI) from aqueous solution containing rhenium(VII) by amine-modified PT as adsorbent [21]. However, these studies have used PT to recover metal resources, the content of the phenolic hydroxyl, which plays a key role in process of adsorption, has not been noticed in the preparation process of adsorbent. In this report, to solve the question of hydrophily of GO nanosheets and water-solubility PT, we adopted one-step crosslinking method to prepare a low-cost and environmentally “green” persimmon tannin/graphene oxide (GO-PT-6Glu) composite with high content of phenolic hydroxyl groups. The content change of phenolic hydroxyl groups in GO-PT-6Glu and its effect on adsorption of metal ions were systemically studied. Notable is, the GOPT-6Glu shows high adsorption capacity and remarkably pretty selectivity for germanium. 2. Experimental section 2.1. Materials Persimmon waste material was kindly donated by Xinshan Biological Engineering Co. Ltd., Jincheng, China. GeO2 (99.99%) was used to prepared stock solution of germanium. 10 0 0 mg L−1 of As(III) stock solution was purchased from Xingcheng Chemical reagent factory, Liaoning, China. Others reagents were purchased from Sinopharm Chemical Regent Co. Ltd. All chemicals and reagents were of analytical grade.
Scheme 1. Preparation of GO-PT-6Glu composite.
2.2. Characterization The S-3C model pH meter was applied to accurately determination the pH value of the solution. The germanium concentration in the aqueous was determined by a UV spectrophotometry (UV-2600, Shimadzu, Japan). The concentrations of coexisting metal ions in mixed solution were measured by inductively coupled plasma-optical emission spectrometry (ICP-OES) using a PE80 0 0 spectrometer (Pekin-Elmer, Wellesley, MA). SEM images were recorded using a Hitachi S-4800 scanning electron microscope at 10 kV. FTIR spectra were measured on a Spectrum One FT-IR spectrometer (Perkin-Elmer, USA). The power X-ray diffraction of persimmon tannin/graphene oxide composite was carried out on a Bruker diffractometer (Cu Ka X-ray radiation, voltages = 45 kV and current = 40 mA). XPS spectra were recorded on a thermo ESCALAB 250 X-ray photoelectron spectrometer with a standard Al Kα X-ray source and were fitted using the XPSPEAK4.1 software. The Raman spectra of adsorbents were measured with Renishaw inVia Raman system equipped with an integral microscope (Leica). 2.3. Fabrication of persimmon tannin/graphene oxide composite material In a typical procedure, GO was synthesized from graphite flakes by an improved Hummer’s method [22]. The as-prepared 0.1 g of GO was dispersed in 100 mL deionized water by continual stirring and ultrasonication for 60 min. Then, the 2.0 g of PT was added into the GO solution and the mixture was stirred at room temperature. After dissolution of PT in the water, added in 6 mL of glutaraldehyde. Successively, the mixture was stirred for 8 h at 353 K. Finally, the product was filtered, washed with water and dried, which was named as GO-PT-6Glu. Preparation procedure of GO-PT6Glu can be seen in Scheme 1.Taking into consideration of the phenolic hydroxyl groups importance to process of metal ions recovery from water [23], we must investigate the effect of crosslinking agent dosage on the phenolic hydroxyl content in the nanocomposites. Based on the volume of glutaraldehyde (3 mL, 4 mL, 5 mL,
7 mL), other four kinds of PT/GO composites with different phenolic hydroxyl contents have been fabricated, which were designated as GO-PT-3Glu (3 mL), GO-PT-4Glu (4 mL), GO-PT-5Glu (5 mL) and GO-PT-7Glu (7 mL), respectively.
2.4. Boehm titration The surface functional groups of persimmon tannin/graphene oxide composites were detected by Boehm titration [24–27]. The process of Boehm titrations is as follows: Firstly, the GO-PT-6Glu was filtered, washed to neutrality and dried. Then, three GO-PT6Glu samples, each of which is 100 mg, were taken along with 25 mL of 0.05 mol L−1 solutions of sodium hydroxide, sodium carbonate and sodium bicarbonate in three different conical flasks. Subsequently, the solutions were shaken for 24 h. Next, the solutions were filtered and then the excess of base titrated with standard HCl. Herein, the number of various types of acidic sites was calculated under the assumption that NaOH neutralizes carboxylic, phenolic, and lactonic groups, Na2 CO3 neutralizes carboxylic and lactonic groups, and NaHCO3 neutralizes only carboxylic groups
2.5. Adsorption studies The adsorption activity of these adsorbents was examined by the recovery of germanium, and the adsorption capacity of adsorbents for germanium was calculated according to Eqs. (1) and (2). The effect of initial pH of solution on adsorption efficiency was investigated by adding 10 mg adsorbent to various acidity of 10 mL 20 mg L−1 germanium solution (pH = 3.0, 5.0, 6.0, 8.0 and 10.0) for shaking 24 h at 180 rpm and 303 K. Germanium usually as the form of compounds exists in the photodetectors and semiconductor, such as gallium arsenide (GeAs) [28,29]. Arsenic is also exist in some germanium-containing minerals, such as brown coal [30], Wulantuga high-germanium coal [31]. Thus, the competitive
Please cite this article as: Y. Zhang, X. Li and L. Gong et al., Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice. 2019.08.024
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Fig. 1. SEM image of (A) GO nanosheets, (B) PT, (C) GO-PT-6Glu composite. (D) PXRD patterns. (E) Raman spectra and (F) FTIR spectra of the pristine GO sample and PT/GO composites.
adsorption experiments were also researched by preparing Ge(IV)– As(III) solution. The selectivity coefficient was calculated according to Eq. (3).
qe =
(Ci − Ce )
A (% ) =
W
×V
(Ci − Ce ) Ci
SelGe/X = log
× 100%
(qe /Ce )Ge (qe /Ce )X
(1) (2)
Table 1 Property changes of GO and GO/PT composites.
GO GO-PT-3Glu GO-PT-4Glu GO-PT-5Glu GO-PT-6Glu GO-PT-7Glu
˚ d (A)
ID /IG
8.19 4.46 4.45 4.45 4.43 4.44
0.96 1.04 1.24 1.03 0.92 1.00
(3)
Where V stands for volume of solution, Ce stands for the equilibrium concentration measured after adsorption, Ci is the initial concentration of adsorbate, and W is the weight of adsorbent, X represents As(III), and Sel represents germanium adsorption selectivity when there are other metal ions in aqueous solution. 3. Results and discussion 3.1. Materials characterization 3.1.1. SEM Compared with the GO (Fig. 1(A)) and PT (Fig. 1(B)), we can obviously observe that the GO-PT-6Glu (Fig. 1(C)) has the lamella morphology of GO and the features of PT. It indicates that the composite of GO with PT has been successfully prepared. Moreover, we can also see that the PT mainly located in the border of laminar GO (Fig. 1(C)), which may result from the linking of –OH on the border of GO flake and –OH of PT by glutaraldehyde. Most importantly, the SEM image of the GO-PT-6Glu (Fig. 1(C)) showed that GO nanosheets in the GO-PT-6Glu composite still presents distinguishable layer with slight wrinkles, which illustrated that the laminar structure of GO nanosheet was almost not destroyed in our work. 3.1.2. XRD and Raman XRD profiles of pristine GO and PT/GO composites were compared in Fig. 1(D). By comparing spectrum of PT/GO composites to
that of GO, a new wide peak showed up at ∼19.94°, which maybe because GO undergoes a certain degree of reduction by persimmon tannin during modification. Additionally, the broad XRD peaks of PT/GO nanocomposites (2θ values in the range of 10–30°) indicate that the crystallinity of PT/GO nanocomposites is not as good as that of pure GO. Furthermore, XRD spectrum also demonstrate that the d-spacing distance decreased from 8.19 A˚ for the pure GO to ∼4.45 A˚ for the PT/GO composites (as listed in Table 1). It is clear that the glutaraldehyde cross-linking reagent reduce the d-spacing distance of GO by linking the hydroxyl groups on different layers of a GO platelet, rather than linking the hydroxyl groups on interfacing GO platelets [32]. Fig. 1(E) presents the Raman spectrum of the GO and the as-prepared PT/GO nanocomposites. The GO-PT-6Glu composite shows similar spectrum and D/G ratio to GO (based on the Raman measurement results in Fig. 1(E) and Table 1), indicating that the addition of PT introduces negligible disorder. These phenomena indicate that the composite of PT and GO has been successfully prepared. 3.1.3. FTIR Fig. 1(F) shows the FTIR spectrum of PT/GO composites with different contents of phenolic hydroxyl groups, and the spectrum of GO and PT are also shown in the figure for comparison. The absorption peaks at around 1110 cm−1 , 1630 cm−1 , 1730 cm−1 and 3403 cm−1 in the FTIR spectrum of the GO belongs to the stretching vibrations of C–O [33], C=C [34], C=O and –OH [35], respectively. After modification by PT, the –OH stretching vibrations
Please cite this article as: Y. Zhang, X. Li and L. Gong et al., Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice. 2019.08.024
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Y. Zhang, X. Li and L. Gong et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx Table 2 Effect of the content of hydroxyl functional group of adsorbents on the adsorption of Ge(IV). Adsorbent
Carboxylic group (mmol g−1 )
Phenolic hydroxyl group (mmol g− 1 )
Lactonic group (mmol g− 1 )
Total functional group (mmol g− 1 )
A%
PT GO GO-PT-3Glu GO-PT-4Glu GO-PT-5Glu GO-PT-6Glu GO-PT-7Glu
5.75 3.98 4.24 4.35 4.25 5.36 5.13
5.50 2.01 3.88 4.53 4.70 5.23 5.10
0.38 0.17 0.25 0.13 0.51 0.25 0.25
11.63 6.16 8.37 9.01 9.46 10.84 10.48
– 46.03 72.18 75.43 85.24 100 91.82
(∼3403 cm−1 ) of neat GO was stronger and broader peak, which is attributed to much more hydroxyls group (–OH) of the PT than that of the GO. The peaks at around 1441 cm−1 and 1019 cm−1 in the FTIR spectrum of all composites correspond to the O–H inplane bending vibrations and the C–O vibrations of the phenolic hydroxyl in the PT [36,37], respectively. These data demonstrated that the PT and GO were successful composited. In addition, in the spectrum of all composites, it was obviously observed that the intensity of the adsorption peaks at 1441 cm−1 and 1019 cm−1 was weaker than that of the PT. Moreover, the –OH peak (∼3403 cm−1 ) of these composites is up-shifted compared with that of the PT (3314 cm−1 ). These data demonstrated the aldol reaction of – OH on the PT with –CHO on the glutaraldehyde, as shown in Scheme 1. 3.1.4. Boehm titrations In order to investigate the actual contents of phenolic hydroxyl groups in the PT/GO composites, the Boehm titrations were carried out. The results of adsorbents were shown as followed in Table 2. The content of phenolic hydroxyl functional groups in the GO was 2.01 mmol g−1 . After PT modification, the contents of phenolic hydroxyl functional groups in the GO-PT-3Glu, GO-PT4Glu, GO-PT-5Glu, GO-PT-6Glu and GO-PT-7Glu were decreased to 3.88, 4.53, 4.70, 5.23 and 5.10 mmol g−1 , respectively, demonstrating the reaction of the –OH groups of the PT with –CHO of glutaraldehyde. In addition, we found that the content of phenolic hydroxyl functional groups of GO-PT-6Glu (5.23 mmol g−1 ) was higher than that of the GO-PT-3Glu, GO-PT-4Glu and GO-PT-5Glu. This might be because the content of PT on the PT/GO composites increased gradually with increasing the volume of glutaraldehyde. However, the content of phenolic hydroxyl functional groups decreased from 5.23 mmol g−1 for GO-PT-6Glu to 5.10 mmol g−1 for GO-PT-7Glu, which may be because of the excessive addition of glutaraldehyde making the phenolic hydroxyl functional groups of the PT consuming again. Thus, the content of phenolic hydroxyl functional group of GO/PT materials have strong regularity, which proves that the Bohr method can accurately determine the content of surface acidic functional groups of graphene-based materials. Comparing with the GO (10.48 mmol g−1 ), the contents of total functional group in the GO-PT-3Glu, GO-PT-4Glu, GO-PT-5Glu, GOPT-6Glu and GO-PT-7Glu were significantly increased to 8.37, 9.01, 9.46, 10.84 and 10.48 mmol g−1 , respectively, which increases hydrophilicity of GO by introducing tannins. 3.2. Adsorption performance of germanium on the permission tannin/graphene oxide composites 3.2.1. Effect of pH on the GO-PT-Glu with different contents of phenolic hydroxyl groups pH in aqueous solution is an important parameter in the chemical adsorption process, as it influence the existence form of metal ions in solution [38]. To specifically analyze the existence form of germanium under different pH conditions, the distribution speciation of germanium under different pH values was shown in
Table 3 Langmuir, Freundlich and Temkin isotherm constants. Isotherms
Formulas
Langmuir
qe =
Freundlich
qe = KLCe1/n
Temkin
qe =
qmax KL Ce 1+KL Ce
RT b
ln(ACe )
Constants
GO-PT-6Glu
qmax (mg g− 1 ) KL (mg L− 1 ) R2 KF (L mg−1 ) n R2 A (L g− 1 ) b R2
123.12 0.01 0.99 8.21 2.38 0.98 0.08 90.67 0.98
Fig. 2(A) [5]. It can be seen that Ge(OH)4 was the predominant germanium speciation during pH 1–7, GeO(OH)3 − during pH 7–11, and GeO2 (OH)2 2− during pH 11–14. Fig. 2(C) shows the effect of initial pH of solution on the adsorption efficiency of Ge(IV) with the initial Ge(IV) concentration of 20 mg g−1 on GO, GO-PT-3Glu, GO-PT-4Glu, GO-PT-5Glu, GO-PT-6Glu and GO-PT-7Glu. It can be obviously seen that the adsorption efficiency of Ge(IV) on GO-PT6Glu is sharply increased with the increase of the value of initial pH of solution, and reached highest at pH = 10 (100%). Thus, we suspect that the adsorption mechanism of Ge(IV) on the PT/GO composites was the ion-exchange reaction between germanium ions and the phenolic hydroxyl containing in the PT/GO composites. Moreover, it was observed that the order of adsorption capacity of Ge(IV) on the adsorbents was GO-PT-6Glu (100%) > GO-PT-7Glu (91.82%) > GO-PT-5Glu (85.24%) > GO-PT-4Glu (75.43%) > GO-PT-3Glu (72.18%), which was consistent with the order of the contents of phenolic hydroxyl groups of adsorbents. Thus, we can determine that the content of hydroxyl functional groups in tannin-based adsorbents has a great impact on the adsorption capacity for germanium ions. In addition, we could also find that the GO-PT-6Glu (100%) has higher adsorption efficiency than that of the GO (40.31%), the existence of PT could significantly improve the adsorption capacity of initial GO for germanium. 3.2.2. Adsorption isotherms Adsorption isotherm could show the equilibrium relationship between the adsorbate and adsorbent and give an idea of the adsorption capacity of the adsorbent [39]. The adsorption isotherm was determined by varying the initial concentration of germanium from 20 to 450 mg g−1 at pH 10 aqueous solutions, and the detail data were illustrated in Fig. 2(D). The Langmuir, Freundlich and Temkin equations were applied to fit the experimental data of Ge(IV) adsorption on the GO-PT-6Glu [40]. From Table 3, we can see the correlation coefficient (R2 = 0.99) of Langmuir model was larger than the other models, which indicates that the GO-PT6Glu has a uniform adsorption site for germanium. The maximum adsorption capacity of the GO-PT-6Glu for Ge(IV) (117.38 mg g−1 ) is relatively modest compared to that of previous reported adsorbents (such as Kelex-100 resin [5], T-10157 activated carbons and so on) for Ge(IV) (Fig. 3(A)) [41–45]. Thus, the GO-PT-6Glu can be
Please cite this article as: Y. Zhang, X. Li and L. Gong et al., Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice. 2019.08.024
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Fig. 2. (A) Distribution of germanium. (B) Distribution of arsenic. (C) The effect of pH on adsorption of Ge(IV) by the GO-PT-3Glu, GO-PT-4Glu, GO-PT-5Glu, GO-PT-6Glu and GO-PT-7Glu. (D) Adsorption isotherms of Ge(IV) by GO-PT-6Glu.
Fig. 3. (A) Comparison of adsorption capacity of the GO-PT-6Glu for Ge(IV) with those reported previously. (B) Adsorption of Ge(IV) on the GO-PT-6Glu in Ge-As (Cl− /SO4 2− /PO4 3− ) binary system. (C) The elution of Ge(IV) by HCl at different concentrations. (D) The elution of Ge(IV) by NaOH solution with different concentrations.
Please cite this article as: Y. Zhang, X. Li and L. Gong et al., Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice. 2019.08.024
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Fig. 4. (A) UV spectra of GO-PT-6Glu were soaked in pH =10 NaOH and 0.5 mol L−1 HCl solution for 24 h. (B) Performance of GO-PT-6Glu in adsorption-elution cycles. (C) XPS spectra of the GO-PT-6Glu, (D) O1s spectra of the GO-PT-6Glu before (a) and after (b) adsorption of Ge(IV).
used as an alternative material to recovery Ge(IV) from solution due to its high adsorption capacity for germanium. 3.2.3. Selectivity tests It should be pointed out that not only high absorptivity but also good selectivity is essential for an excellent adsorbent. As shown in Fig. 3(B), it could be obviously seen that the GO-PT-6Glu still has high adsorption efficiency for Ge(IV) (> 90%), in spite of the concentration of As(III) (Cl− /SO4 2− /PO4 3− ) was 100 times higher than that of Ge(IV). These results showed that the GO-PT-6Glu possesses perfect separation for Ge(IV) from the actual solutions of semiconductor waste in alkaline conditions 3.2.4. Desorption and regeneration The adsorption reproducibility of GO-PT-6Glu was also studied. The GO-PT-6Glu loaded germanium sample was washed with different concentrations of hydrochloric acid (0.5, 1.0, 2.0 and 3.0 mol L−1 ) and sodium hydroxide (0.5, 1.0, 2.0 and 3.0 mol L−1 ) (Fig. 3(C) and (D)). It can be seen that the elution efficiency of 0.5 mol L−1 HCl for germanium was 87.15%. This elution efficiency of germanium is relatively modest compared to that of other several elution agents in this work. Thus, The Ge(IV)-adsorbed sample was washed with 0.5 mol L−1 HCl aqueous solution to get the regenerated adsorbent. After five recycles (Fig. 4(B)), the GO-PT6Glu still maintain good adsorption performance for germanium (> 75%). This shows that the GO-PT-6Glu can be almost fully regenerated and the PT was well immobilized on the GO. 3.2.5. Stability of GO-PT-6Glu In order to study the stability of adsorption material, the UV spectrum of GO-PT-6Glu was conducted by immersing it at NaOH (pH =10) or HCl (0.5 mol L−1 ) solution for 24 h, which were shown in Fig. 4(A). The UV spectra of GO-PT-6Glu at NaOH (pH =10) and
HCl (0.5 mol L−1 ) solution were not observed the peak of glutaraldehyde, which showed that GO-PT-6Glu had better stability within the study.
3.2.6. Adsorption mechanism To investigate the mechanism of germanium onto the GO-PT6Glu. The FTIR spectrum of the GO-PT-6Glu before and after loaded Ge(IV) was carried out and shown in Fig. 2(A). After adsorption of germanium, the peak at 3403 cm−1 (–OH vibration) for the GOPT-6Glu is obviously decreasing, and the peak at 1019 cm−1 , corresponding to the C–O vibration of phenolic hydroxyl groups, was almost disappear. The results of FTIR indicated that the phenolic hydroxyl groups on the GO-PT-6Glu plays an important role in the adsorption process of germanium. The XPS was used to further study the adsorption mechanism of Ge(IV) on the GO-PT-6Glu. By comparison of the spectrum changes before and after adsorbed Ge(IV) (Fig. 4(C)), we can see that a new peak corresponding to Ge3d of GeO3 2− appears at 31.60 eV, which indicated that Ge(IV) has been adsorbed successfully onto the GO-PT-6Glu and the adsorbing forms of Ge is Ge(IV) ion. Besides, the O1s peak on the GO-PT-6Glu could be fitted by two peaks at 532.16 eV and 533.02 eV, corresponding to O–H and O–C, respectively (Fig. 4(D) (b)). After adsorption of germanium (Fig. 4(D) (a)), a new peak corresponding to the O–Ge vibration, appears at 532.31 eV, demonstrating that the O elements on the GO-PT-6Glu reacted with germanium ions. Moreover, after adsorption, the binding energy of O–H of GO-PT-6Glu is up-shifted from 532.16 eV to 531.67 eV with the area ratio decreased from 57.66% to 32.20%, and the binding energy of O–C almost no change but the area ratio of O–C decreased from 42.34% to 27.48%. Thus, we suggest that the adsorption mechanism of Ge(IV) on the GO-PT-6Glu was ion exchange reaction between the germanium ions and the phenolic hydroxyl on the GO-PT-6Glu, which is similar to previous
Please cite this article as: Y. Zhang, X. Li and L. Gong et al., Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice. 2019.08.024
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reports [46–48]. The mechanism was expressed by the following equation.
(4) 4. Conclusions Based on aldol reaction, bio-waste persimmon as the hydrophilic precursor and graphene oxide as curing agent were used for preparing persimmon tannin/graphene oxide composite with high phenolic hydroxyl groups by the one pot method. This study not only improves the water stability of bio-waste persimmon but also increase the hydrophilicity of GO. More importantly, GO-PT6Glu exhibited excellent adsorptivity and good selective property toward germanium at alkaline condition, and it also showed good reproducibility for germanium. All of these make GO-PT-6Glu become an ideal candidate for the recovery of germanium in water system. Acknowledgments This project is supported by National Natural Science Foundation of China (51674131 and 21373005), Natural Science Foundation of Liaoning Province of China (2019157), Project supported National Science Technology Ministry (2015BAB02B03) and Natural Science Foundation of Liaoning University (LDGY2019009 and LDGY2019014) Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.08.024. References [1] Vaughn DD, Schaak RE. Synthesis, properties and applications of colloidal germanium and germanium-based nanomaterials. Chem Soc Rev 2013;42:2861–79. [2] Zhang CJ, Lin Z, Yang ZZ, Xiao DD, Hu P, Xu HX, et al. Hierarchically designed germanium microcubes with high initial coulombic efficiency toward highly reversible lithium storage. Chem Mater 2015;27:2189–94. [3] Ji YJ, Dong HL, Hou TJ, Li YY. Monolayer graphitic germanium carbide (g-GeC): the promising cathode catalyst for fuel cell and lithium-oxygen battery applications. J Mater Chem A 2018;6:2212–18. [4] Ren FF, Ang KW, Ye JD, Yu MB, Lo G-Q, Kwong DL. Split bull’s eye shaped aluminum antenna for plasmon-enhanced nanometer scale germanium photodetector. Nano Lett 2011;11:1289–93. [5] Park HJ, Tavlarides-Lawrence L. Germanium(IV) adsorption from aqueous solution using a kelex-100 functional adsorbent. Ind Eng Chem Res 2009;48:4014–21. [6] Liu F, Yang YZ, Lu YM, Shang K, Lu WJ, Zhao XD. Extraction of germanium by the AOT microemulsion with N235 system. Ind Eng Chem Res 2010;49:10 0 05–8. [7] Hernandez-Exposito A, Chimenos JM, Fernandez AI, Font O, Querol X, Coca P, et al. Ion flotation of germanium from fly ash aqueous leachates. Chem Eng J 2006;118:69–75. [8] Kul M, Topkaya Y. Recovery of germanium and other valuable metals from zinc plant residues. Hydrometallurgy 2008;92:87–94. [9] Arrambide-Cruz C, Marie S, Arrachart G, Pellet-Rostaing S. Selective extraction and separation of germanium by catechol based resins. Sep Purif Technol 2018;193:214–19. [10] Wang FC, Zhao JM, Liu HZ, Luo Y. Preparation of double carboxylic corn stalk gels and their adsorption properties towards rare earths (III). Waste Biomass Valori 2018;9:1945–54. [11] Wang FC, Zhao JM, Wang WK, Liu HZ. Superior AU-adsorption performance of aminothiourea-modified waste cellulosic biomass. J Cent South Univ 2018;25:2992–3003.
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Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution Yingying Zhang, Xinzhu Li, Lifeng Gong, Zhiqiang Xing, Zhenning Lou, Weijun Shan, Ying Xiong∗ College of Chemistry, Key Laboratory of Rare-scattered Elements of Liaoning Province, Liaoning University, Shenyang 110036, PR China
a r t i c l e
i n f o
Article history: Received 18 May 2019 Revised 12 August 2019 Accepted 31 August 2019 Available online xxx Keywords: Graphene oxide Germanium Selective adsorption Persimmon tannin
a b s t r a c t Herein, a new persimmon tannin/graphene oxide (GO-PT-6Glu) composite was fabricated through onestep cross-linking method. The as-prepared GO-PT-6Glu has substantial phenolic hydroxyl groups (up to 5.23 mmol g−1 ) by introducing water-soluble PT (up to 5.50 mmol g−1 ) on the GO, which provides the opportunity to use it as an adsorption material to recover germanium. The GO-PT-6Glu was then used to adsorb germanium from aqueous solution and showed the highest than among currently reported adsorption capacity (117.38 mg g− 1 ). Simultaneously, it exhibits excellent cycle performance to adsorb germanium still with a high adsorption efficiency (up to 81.42%) after 5 adsorption–desorption cycles. More importantly, the GO-PT-6Glu showed superior selectivity towards Ge(IV) as compared to As(III) (Cl− /SO4 2− /PO4 3− ), in spite of the concentration of As(III) (Cl− /SO4 2− /PO4 3− ) was 100 times higher than that of Ge(IV). Taking advantages of its low cost, selective adsorption, as well as environmental protection and high adsorption, the GO-PT-6Glu is expected to be the most promising material in recovery of Ge(IV) from water containing As(III). © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The favorable electronic properties of Ge, coupled with its quantum confinement effect that can be observed under relatively large particle sizes [1], make Ge nanoparticles attractive for energy storage [2], catalysis [3], photodetector [4], and other fields. However, the broader utilization of germanium is still limited by its source [5]. Hence, the recovery of germanium from industrial effluents has drawn particular attention of researchers. Methods, including extraction [6], flotation [7] and precipitation [8] etc. have been developed for recovery of germanium from solutions. Arrambide-Cruz et al. proposed extracting method by catechol and 8-hydroxyQuinoline from NaOH medium to recycle Ge(IV) [9]. Although these methods have focused on recycling germanium, environmental improvement are still challenging due to limitations on using large volume of acid/alkali/organic reagent with high concentration. Thus, we need to develop new and cheap, efficient as well as environmentally friendly treatment technologies for recovery germanium from waste. Adsorption method is an efficient technology for recovery metal ions from water, which possess
∗
Corresponding author. E-mail address: [email protected] (Y. Xiong).
many advantages such as low cost, simple operation, and non-toxic [10,11]. Graphene oxide (GO), a novel carbon material, possessing high surface area and abundant oxygen-containing functional groups [12]. The presence of –COOH, –C=O and –OH groups on the surface of GO could allow GO to participate in a wide range of bonding interactions. Hence, GO has fantastic potential for environmentally related application, such as wastewater treatment and water purification [13,14]. Even so, the low hydrophily of GO nanosheets, which makes GO difficult to bind metal ions freely in wastewater, is the major obstacle to adsorption applications of graphene oxidebased materials [15]. To achieve further advances in graphene oxide-based materials in adsorption field, it is important to increase the hydrophily of GO, e.g. introducing hydrophilic groups on the GO, such as amino groups, carboxyl groups and hydroxyl groups [16,17]. Persimmon tannin (PT), as an inexpensive and ubiquitous natural biomass material, could produce a strong ability of combination for metal ions due to it extremely abundant multiple adjacent phenolic hydroxyl groups [18]. Nevertheless, PT must be chemically modified or immobilized onto non-water-soluble matrixes for extraction of metal ions from water due to its high solubility in water [19]. Fan et al. prepared a novel plant tannin-modified Fe3 O4 @SiO2 microspheres, which had superior adsorption capacities for gold
https://doi.org/10.1016/j.jtice.2019.08.024 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Y. Zhang, X. Li and L. Gong et al., Persimmon tannin/graphene oxide composites: Fabrication and superior adsorption of germanium ions in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice. 2019.08.024
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and palladium [20]. In our lab, we investigated the selective recovery of molybdenum(VI) from aqueous solution containing rhenium(VII) by amine-modified PT as adsorbent [21]. However, these studies have used PT to recover metal resources, the content of the phenolic hydroxyl, which plays a key role in process of adsorption, has not been noticed in the preparation process of adsorbent. In this report, to solve the question of hydrophily of GO nanosheets and water-solubility PT, we adopted one-step crosslinking method to prepare a low-cost and environmentally “green” persimmon tannin/graphene oxide (GO-PT-6Glu) composite with high content of phenolic hydroxyl groups. The content change of phenolic hydroxyl groups in GO-PT-6Glu and its effect on adsorption of metal ions were systemically studied. Notable is, the GOPT-6Glu shows high adsorption capacity and remarkably pretty selectivity for germanium. 2. Experimental section 2.1. Materials Persimmon waste material was kindly donated by Xinshan Biological Engineering Co. Ltd., Jincheng, China. GeO2 (99.99%) was used to prepared stock solution of germanium. 10 0 0 mg L−1 of As(III) stock solution was purchased from Xingcheng Chemical reagent factory, Liaoning, China. Others reagents were purchased from Sinopharm Chemical Regent Co. Ltd. All chemicals and reagents were of analytical grade.
Scheme 1. Preparation of GO-PT-6Glu composite.
2.2. Characterization The S-3C model pH meter was applied to accurately determination the pH value of the solution. The germanium concentration in the aqueous was determined by a UV spectrophotometry (UV-2600, Shimadzu, Japan). The concentrations of coexisting metal ions in mixed solution were measured by inductively coupled plasma-optical emission spectrometry (ICP-OES) using a PE80 0 0 spectrometer (Pekin-Elmer, Wellesley, MA). SEM images were recorded using a Hitachi S-4800 scanning electron microscope at 10 kV. FTIR spectra were measured on a Spectrum One FT-IR spectrometer (Perkin-Elmer, USA). The power X-ray diffraction of persimmon tannin/graphene oxide composite was carried out on a Bruker diffractometer (Cu Ka X-ray radiation, voltages = 45 kV and current = 40 mA). XPS spectra were recorded on a thermo ESCALAB 250 X-ray photoelectron spectrometer with a standard Al Kα X-ray source and were fitted using the XPSPEAK4.1 software. The Raman spectra of adsorbents were measured with Renishaw inVia Raman system equipped with an integral microscope (Leica). 2.3. Fabrication of persimmon tannin/graphene oxide composite material In a typical procedure, GO was synthesized from graphite flakes by an improved Hummer’s method [22]. The as-prepared 0.1 g of GO was dispersed in 100 mL deionized water by continual stirring and ultrasonication for 60 min. Then, the 2.0 g of PT was added into the GO solution and the mixture was stirred at room temperature. After dissolution of PT in the water, added in 6 mL of glutaraldehyde. Successively, the mixture was stirred for 8 h at 353 K. Finally, the product was filtered, washed with water and dried, which was named as GO-PT-6Glu. Preparation procedure of GO-PT6Glu can be seen in Scheme 1.Taking into consideration of the phenolic hydroxyl groups importance to process of metal ions recovery from water [23], we must investigate the effect of crosslinking agent dosage on the phenolic hydroxyl content in the nanocomposites. Based on the volume of glutaraldehyde (3 mL, 4 mL, 5 mL,
7 mL), other four kinds of PT/GO composites with different phenolic hydroxyl contents have been fabricated, which were designated as GO-PT-3Glu (3 mL), GO-PT-4Glu (4 mL), GO-PT-5Glu (5 mL) and GO-PT-7Glu (7 mL), respectively.
2.4. Boehm titration The surface functional groups of persimmon tannin/graphene oxide composites were detected by Boehm titration [24–27]. The process of Boehm titrations is as follows: Firstly, the GO-PT-6Glu was filtered, washed to neutrality and dried. Then, three GO-PT6Glu samples, each of which is 100 mg, were taken along with 25 mL of 0.05 mol L−1 solutions of sodium hydroxide, sodium carbonate and sodium bicarbonate in three different conical flasks. Subsequently, the solutions were shaken for 24 h. Next, the solutions were filtered and then the excess of base titrated with standard HCl. Herein, the number of various types of acidic sites was calculated under the assumption that NaOH neutralizes carboxylic, phenolic, and lactonic groups, Na2 CO3 neutralizes carboxylic and lactonic groups, and NaHCO3 neutralizes only carboxylic groups
2.5. Adsorption studies The adsorption activity of these adsorbents was examined by the recovery of germanium, and the adsorption capacity of adsorbents for germanium was calculated according to Eqs. (1) and (2). The effect of initial pH of solution on adsorption efficiency was investigated by adding 10 mg adsorbent to various acidity of 10 mL 20 mg L−1 germanium solution (pH = 3.0, 5.0, 6.0, 8.0 and 10.0) for shaking 24 h at 180 rpm and 303 K. Germanium usually as the form of compounds exists in the photodetectors and semiconductor, such as gallium arsenide (GeAs) [28,29]. Arsenic is also exist in some germanium-containing minerals, such as brown coal [30], Wulantuga high-germanium coal [31]. Thus, the competitive
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3
Fig. 1. SEM image of (A) GO nanosheets, (B) PT, (C) GO-PT-6Glu composite. (D) PXRD patterns. (E) Raman spectra and (F) FTIR spectra of the pristine GO sample and PT/GO composites.
adsorption experiments were also researched by preparing Ge(IV)– As(III) solution. The selectivity coefficient was calculated according to Eq. (3).
qe =
(Ci − Ce )
A (% ) =
W
×V
(Ci − Ce ) Ci
SelGe/X = log
× 100%
(qe /Ce )Ge (qe /Ce )X
(1) (2)
Table 1 Property changes of GO and GO/PT composites.
GO GO-PT-3Glu GO-PT-4Glu GO-PT-5Glu GO-PT-6Glu GO-PT-7Glu
˚ d (A)
ID /IG
8.19 4.46 4.45 4.45 4.43 4.44
0.96 1.04 1.24 1.03 0.92 1.00
(3)
Where V stands for volume of solution, Ce stands for the equilibrium concentration measured after adsorption, Ci is the initial concentration of adsorbate, and W is the weight of adsorbent, X represents As(III), and Sel represents germanium adsorption selectivity when there are other metal ions in aqueous solution. 3. Results and discussion 3.1. Materials characterization 3.1.1. SEM Compared with the GO (Fig. 1(A)) and PT (Fig. 1(B)), we can obviously observe that the GO-PT-6Glu (Fig. 1(C)) has the lamella morphology of GO and the features of PT. It indicates that the composite of GO with PT has been successfully prepared. Moreover, we can also see that the PT mainly located in the border of laminar GO (Fig. 1(C)), which may result from the linking of –OH on the border of GO flake and –OH of PT by glutaraldehyde. Most importantly, the SEM image of the GO-PT-6Glu (Fig. 1(C)) showed that GO nanosheets in the GO-PT-6Glu composite still presents distinguishable layer with slight wrinkles, which illustrated that the laminar structure of GO nanosheet was almost not destroyed in our work. 3.1.2. XRD and Raman XRD profiles of pristine GO and PT/GO composites were compared in Fig. 1(D). By comparing spectrum of PT/GO composites to
that of GO, a new wide peak showed up at ∼19.94°, which maybe because GO undergoes a certain degree of reduction by persimmon tannin during modification. Additionally, the broad XRD peaks of PT/GO nanocomposites (2θ values in the range of 10–30°) indicate that the crystallinity of PT/GO nanocomposites is not as good as that of pure GO. Furthermore, XRD spectrum also demonstrate that the d-spacing distance decreased from 8.19 A˚ for the pure GO to ∼4.45 A˚ for the PT/GO composites (as listed in Table 1). It is clear that the glutaraldehyde cross-linking reagent reduce the d-spacing distance of GO by linking the hydroxyl groups on different layers of a GO platelet, rather than linking the hydroxyl groups on interfacing GO platelets [32]. Fig. 1(E) presents the Raman spectrum of the GO and the as-prepared PT/GO nanocomposites. The GO-PT-6Glu composite shows similar spectrum and D/G ratio to GO (based on the Raman measurement results in Fig. 1(E) and Table 1), indicating that the addition of PT introduces negligible disorder. These phenomena indicate that the composite of PT and GO has been successfully prepared. 3.1.3. FTIR Fig. 1(F) shows the FTIR spectrum of PT/GO composites with different contents of phenolic hydroxyl groups, and the spectrum of GO and PT are also shown in the figure for comparison. The absorption peaks at around 1110 cm−1 , 1630 cm−1 , 1730 cm−1 and 3403 cm−1 in the FTIR spectrum of the GO belongs to the stretching vibrations of C–O [33], C=C [34], C=O and –OH [35], respectively. After modification by PT, the –OH stretching vibrations
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Y. Zhang, X. Li and L. Gong et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx Table 2 Effect of the content of hydroxyl functional group of adsorbents on the adsorption of Ge(IV). Adsorbent
Carboxylic group (mmol g−1 )
Phenolic hydroxyl group (mmol g− 1 )
Lactonic group (mmol g− 1 )
Total functional group (mmol g− 1 )
A%
PT GO GO-PT-3Glu GO-PT-4Glu GO-PT-5Glu GO-PT-6Glu GO-PT-7Glu
5.75 3.98 4.24 4.35 4.25 5.36 5.13
5.50 2.01 3.88 4.53 4.70 5.23 5.10
0.38 0.17 0.25 0.13 0.51 0.25 0.25
11.63 6.16 8.37 9.01 9.46 10.84 10.48
– 46.03 72.18 75.43 85.24 100 91.82
(∼3403 cm−1 ) of neat GO was stronger and broader peak, which is attributed to much more hydroxyls group (–OH) of the PT than that of the GO. The peaks at around 1441 cm−1 and 1019 cm−1 in the FTIR spectrum of all composites correspond to the O–H inplane bending vibrations and the C–O vibrations of the phenolic hydroxyl in the PT [36,37], respectively. These data demonstrated that the PT and GO were successful composited. In addition, in the spectrum of all composites, it was obviously observed that the intensity of the adsorption peaks at 1441 cm−1 and 1019 cm−1 was weaker than that of the PT. Moreover, the –OH peak (∼3403 cm−1 ) of these composites is up-shifted compared with that of the PT (3314 cm−1 ). These data demonstrated the aldol reaction of – OH on the PT with –CHO on the glutaraldehyde, as shown in Scheme 1. 3.1.4. Boehm titrations In order to investigate the actual contents of phenolic hydroxyl groups in the PT/GO composites, the Boehm titrations were carried out. The results of adsorbents were shown as followed in Table 2. The content of phenolic hydroxyl functional groups in the GO was 2.01 mmol g−1 . After PT modification, the contents of phenolic hydroxyl functional groups in the GO-PT-3Glu, GO-PT4Glu, GO-PT-5Glu, GO-PT-6Glu and GO-PT-7Glu were decreased to 3.88, 4.53, 4.70, 5.23 and 5.10 mmol g−1 , respectively, demonstrating the reaction of the –OH groups of the PT with –CHO of glutaraldehyde. In addition, we found that the content of phenolic hydroxyl functional groups of GO-PT-6Glu (5.23 mmol g−1 ) was higher than that of the GO-PT-3Glu, GO-PT-4Glu and GO-PT-5Glu. This might be because the content of PT on the PT/GO composites increased gradually with increasing the volume of glutaraldehyde. However, the content of phenolic hydroxyl functional groups decreased from 5.23 mmol g−1 for GO-PT-6Glu to 5.10 mmol g−1 for GO-PT-7Glu, which may be because of the excessive addition of glutaraldehyde making the phenolic hydroxyl functional groups of the PT consuming again. Thus, the content of phenolic hydroxyl functional group of GO/PT materials have strong regularity, which proves that the Bohr method can accurately determine the content of surface acidic functional groups of graphene-based materials. Comparing with the GO (10.48 mmol g−1 ), the contents of total functional group in the GO-PT-3Glu, GO-PT-4Glu, GO-PT-5Glu, GOPT-6Glu and GO-PT-7Glu were significantly increased to 8.37, 9.01, 9.46, 10.84 and 10.48 mmol g−1 , respectively, which increases hydrophilicity of GO by introducing tannins. 3.2. Adsorption performance of germanium on the permission tannin/graphene oxide composites 3.2.1. Effect of pH on the GO-PT-Glu with different contents of phenolic hydroxyl groups pH in aqueous solution is an important parameter in the chemical adsorption process, as it influence the existence form of metal ions in solution [38]. To specifically analyze the existence form of germanium under different pH conditions, the distribution speciation of germanium under different pH values was shown in
Table 3 Langmuir, Freundlich and Temkin isotherm constants. Isotherms
Formulas
Langmuir
qe =
Freundlich
qe = KLCe1/n
Temkin
qe =
qmax KL Ce 1+KL Ce
RT b
ln(ACe )
Constants
GO-PT-6Glu
qmax (mg g− 1 ) KL (mg L− 1 ) R2 KF (L mg−1 ) n R2 A (L g− 1 ) b R2
123.12 0.01 0.99 8.21 2.38 0.98 0.08 90.67 0.98
Fig. 2(A) [5]. It can be seen that Ge(OH)4 was the predominant germanium speciation during pH 1–7, GeO(OH)3 − during pH 7–11, and GeO2 (OH)2 2− during pH 11–14. Fig. 2(C) shows the effect of initial pH of solution on the adsorption efficiency of Ge(IV) with the initial Ge(IV) concentration of 20 mg g−1 on GO, GO-PT-3Glu, GO-PT-4Glu, GO-PT-5Glu, GO-PT-6Glu and GO-PT-7Glu. It can be obviously seen that the adsorption efficiency of Ge(IV) on GO-PT6Glu is sharply increased with the increase of the value of initial pH of solution, and reached highest at pH = 10 (100%). Thus, we suspect that the adsorption mechanism of Ge(IV) on the PT/GO composites was the ion-exchange reaction between germanium ions and the phenolic hydroxyl containing in the PT/GO composites. Moreover, it was observed that the order of adsorption capacity of Ge(IV) on the adsorbents was GO-PT-6Glu (100%) > GO-PT-7Glu (91.82%) > GO-PT-5Glu (85.24%) > GO-PT-4Glu (75.43%) > GO-PT-3Glu (72.18%), which was consistent with the order of the contents of phenolic hydroxyl groups of adsorbents. Thus, we can determine that the content of hydroxyl functional groups in tannin-based adsorbents has a great impact on the adsorption capacity for germanium ions. In addition, we could also find that the GO-PT-6Glu (100%) has higher adsorption efficiency than that of the GO (40.31%), the existence of PT could significantly improve the adsorption capacity of initial GO for germanium. 3.2.2. Adsorption isotherms Adsorption isotherm could show the equilibrium relationship between the adsorbate and adsorbent and give an idea of the adsorption capacity of the adsorbent [39]. The adsorption isotherm was determined by varying the initial concentration of germanium from 20 to 450 mg g−1 at pH 10 aqueous solutions, and the detail data were illustrated in Fig. 2(D). The Langmuir, Freundlich and Temkin equations were applied to fit the experimental data of Ge(IV) adsorption on the GO-PT-6Glu [40]. From Table 3, we can see the correlation coefficient (R2 = 0.99) of Langmuir model was larger than the other models, which indicates that the GO-PT6Glu has a uniform adsorption site for germanium. The maximum adsorption capacity of the GO-PT-6Glu for Ge(IV) (117.38 mg g−1 ) is relatively modest compared to that of previous reported adsorbents (such as Kelex-100 resin [5], T-10157 activated carbons and so on) for Ge(IV) (Fig. 3(A)) [41–45]. Thus, the GO-PT-6Glu can be
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Fig. 2. (A) Distribution of germanium. (B) Distribution of arsenic. (C) The effect of pH on adsorption of Ge(IV) by the GO-PT-3Glu, GO-PT-4Glu, GO-PT-5Glu, GO-PT-6Glu and GO-PT-7Glu. (D) Adsorption isotherms of Ge(IV) by GO-PT-6Glu.
Fig. 3. (A) Comparison of adsorption capacity of the GO-PT-6Glu for Ge(IV) with those reported previously. (B) Adsorption of Ge(IV) on the GO-PT-6Glu in Ge-As (Cl− /SO4 2− /PO4 3− ) binary system. (C) The elution of Ge(IV) by HCl at different concentrations. (D) The elution of Ge(IV) by NaOH solution with different concentrations.
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Fig. 4. (A) UV spectra of GO-PT-6Glu were soaked in pH =10 NaOH and 0.5 mol L−1 HCl solution for 24 h. (B) Performance of GO-PT-6Glu in adsorption-elution cycles. (C) XPS spectra of the GO-PT-6Glu, (D) O1s spectra of the GO-PT-6Glu before (a) and after (b) adsorption of Ge(IV).
used as an alternative material to recovery Ge(IV) from solution due to its high adsorption capacity for germanium. 3.2.3. Selectivity tests It should be pointed out that not only high absorptivity but also good selectivity is essential for an excellent adsorbent. As shown in Fig. 3(B), it could be obviously seen that the GO-PT-6Glu still has high adsorption efficiency for Ge(IV) (> 90%), in spite of the concentration of As(III) (Cl− /SO4 2− /PO4 3− ) was 100 times higher than that of Ge(IV). These results showed that the GO-PT-6Glu possesses perfect separation for Ge(IV) from the actual solutions of semiconductor waste in alkaline conditions 3.2.4. Desorption and regeneration The adsorption reproducibility of GO-PT-6Glu was also studied. The GO-PT-6Glu loaded germanium sample was washed with different concentrations of hydrochloric acid (0.5, 1.0, 2.0 and 3.0 mol L−1 ) and sodium hydroxide (0.5, 1.0, 2.0 and 3.0 mol L−1 ) (Fig. 3(C) and (D)). It can be seen that the elution efficiency of 0.5 mol L−1 HCl for germanium was 87.15%. This elution efficiency of germanium is relatively modest compared to that of other several elution agents in this work. Thus, The Ge(IV)-adsorbed sample was washed with 0.5 mol L−1 HCl aqueous solution to get the regenerated adsorbent. After five recycles (Fig. 4(B)), the GO-PT6Glu still maintain good adsorption performance for germanium (> 75%). This shows that the GO-PT-6Glu can be almost fully regenerated and the PT was well immobilized on the GO. 3.2.5. Stability of GO-PT-6Glu In order to study the stability of adsorption material, the UV spectrum of GO-PT-6Glu was conducted by immersing it at NaOH (pH =10) or HCl (0.5 mol L−1 ) solution for 24 h, which were shown in Fig. 4(A). The UV spectra of GO-PT-6Glu at NaOH (pH =10) and
HCl (0.5 mol L−1 ) solution were not observed the peak of glutaraldehyde, which showed that GO-PT-6Glu had better stability within the study.
3.2.6. Adsorption mechanism To investigate the mechanism of germanium onto the GO-PT6Glu. The FTIR spectrum of the GO-PT-6Glu before and after loaded Ge(IV) was carried out and shown in Fig. 2(A). After adsorption of germanium, the peak at 3403 cm−1 (–OH vibration) for the GOPT-6Glu is obviously decreasing, and the peak at 1019 cm−1 , corresponding to the C–O vibration of phenolic hydroxyl groups, was almost disappear. The results of FTIR indicated that the phenolic hydroxyl groups on the GO-PT-6Glu plays an important role in the adsorption process of germanium. The XPS was used to further study the adsorption mechanism of Ge(IV) on the GO-PT-6Glu. By comparison of the spectrum changes before and after adsorbed Ge(IV) (Fig. 4(C)), we can see that a new peak corresponding to Ge3d of GeO3 2− appears at 31.60 eV, which indicated that Ge(IV) has been adsorbed successfully onto the GO-PT-6Glu and the adsorbing forms of Ge is Ge(IV) ion. Besides, the O1s peak on the GO-PT-6Glu could be fitted by two peaks at 532.16 eV and 533.02 eV, corresponding to O–H and O–C, respectively (Fig. 4(D) (b)). After adsorption of germanium (Fig. 4(D) (a)), a new peak corresponding to the O–Ge vibration, appears at 532.31 eV, demonstrating that the O elements on the GO-PT-6Glu reacted with germanium ions. Moreover, after adsorption, the binding energy of O–H of GO-PT-6Glu is up-shifted from 532.16 eV to 531.67 eV with the area ratio decreased from 57.66% to 32.20%, and the binding energy of O–C almost no change but the area ratio of O–C decreased from 42.34% to 27.48%. Thus, we suggest that the adsorption mechanism of Ge(IV) on the GO-PT-6Glu was ion exchange reaction between the germanium ions and the phenolic hydroxyl on the GO-PT-6Glu, which is similar to previous
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reports [46–48]. The mechanism was expressed by the following equation.
(4) 4. Conclusions Based on aldol reaction, bio-waste persimmon as the hydrophilic precursor and graphene oxide as curing agent were used for preparing persimmon tannin/graphene oxide composite with high phenolic hydroxyl groups by the one pot method. This study not only improves the water stability of bio-waste persimmon but also increase the hydrophilicity of GO. More importantly, GO-PT6Glu exhibited excellent adsorptivity and good selective property toward germanium at alkaline condition, and it also showed good reproducibility for germanium. All of these make GO-PT-6Glu become an ideal candidate for the recovery of germanium in water system. Acknowledgments This project is supported by National Natural Science Foundation of China (51674131 and 21373005), Natural Science Foundation of Liaoning Province of China (2019157), Project supported National Science Technology Ministry (2015BAB02B03) and Natural Science Foundation of Liaoning University (LDGY2019009 and LDGY2019014) Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.08.024. References [1] Vaughn DD, Schaak RE. Synthesis, properties and applications of colloidal germanium and germanium-based nanomaterials. Chem Soc Rev 2013;42:2861–79. [2] Zhang CJ, Lin Z, Yang ZZ, Xiao DD, Hu P, Xu HX, et al. Hierarchically designed germanium microcubes with high initial coulombic efficiency toward highly reversible lithium storage. Chem Mater 2015;27:2189–94. [3] Ji YJ, Dong HL, Hou TJ, Li YY. Monolayer graphitic germanium carbide (g-GeC): the promising cathode catalyst for fuel cell and lithium-oxygen battery applications. J Mater Chem A 2018;6:2212–18. [4] Ren FF, Ang KW, Ye JD, Yu MB, Lo G-Q, Kwong DL. Split bull’s eye shaped aluminum antenna for plasmon-enhanced nanometer scale germanium photodetector. Nano Lett 2011;11:1289–93. [5] Park HJ, Tavlarides-Lawrence L. Germanium(IV) adsorption from aqueous solution using a kelex-100 functional adsorbent. Ind Eng Chem Res 2009;48:4014–21. [6] Liu F, Yang YZ, Lu YM, Shang K, Lu WJ, Zhao XD. Extraction of germanium by the AOT microemulsion with N235 system. Ind Eng Chem Res 2010;49:10 0 05–8. [7] Hernandez-Exposito A, Chimenos JM, Fernandez AI, Font O, Querol X, Coca P, et al. Ion flotation of germanium from fly ash aqueous leachates. Chem Eng J 2006;118:69–75. [8] Kul M, Topkaya Y. Recovery of germanium and other valuable metals from zinc plant residues. Hydrometallurgy 2008;92:87–94. [9] Arrambide-Cruz C, Marie S, Arrachart G, Pellet-Rostaing S. Selective extraction and separation of germanium by catechol based resins. Sep Purif Technol 2018;193:214–19. [10] Wang FC, Zhao JM, Liu HZ, Luo Y. Preparation of double carboxylic corn stalk gels and their adsorption properties towards rare earths (III). Waste Biomass Valori 2018;9:1945–54. [11] Wang FC, Zhao JM, Wang WK, Liu HZ. Superior AU-adsorption performance of aminothiourea-modified waste cellulosic biomass. J Cent South Univ 2018;25:2992–3003.
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