Molten salt synthesis of gadolinium boride nanocrystals suitable for methyl blue removal

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In addition to reduction, MB decolorization mainly results from adsorption on the surface of GdB6 based on the adsorption-desorption tests. The adsorption behavior follows the pseudo-second-order kinetics and Freundlich isotherm model.

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Nội dung Text: Molten salt synthesis of gadolinium boride nanocrystals suitable for methyl blue removal

Environmental Advances 4 (2021) 100055 Contents lists available at ScienceDirect Environmental Advances journal homepage: www.elsevier.com/locate/envadv Molten salt synthesis of gadolinium boride nanocrystals suitable for methyl blue removal Fengquan Zhang a,b, Xiuting Chen a, Chenyang Wang a,c, Xiyan Liu a,c,∗ a Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China b Department of Microelectronic Science and Engineering, School of Physical Science and Technology, Ningbo University, Ningbo, Zhejiang 315211, China c School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China a r t i c l e i n f o a b s t r a c t Keywords: Eﬃcient removal of cationic dyes such as methyl blue (MB) is of great importance from the environmental per- Gadolinium boride spective. Although adsorption methods have been employed for this purpose, it is not easy to ﬁnd inorganic Molten salt synthesis materials featuring both high adsorption capacity and ratio. Herein, a comprehensive study on the MB removal Methylene blue performance by GdB6 nanoparticles was provided. GdB6 was conveniently synthesized via the reaction of GdF3 Adsorption and NaBH4 at 900 °C in molten LiCl-KCl, and the resulting nanoparticles exhibit cubic-shaped structures with an Wastewater treatment average size of 100 nm and BET surface area of 51.5 m2 /g. During the evaluation of MB removal performance, reduction of MB to colorless leucomethylene (LMB) on the surface of GdB6 was observed for the ﬁrst time, which makes it more reliable to determine the capability of GdB6 in the treatment of MB. In addition to reduction, MB decolorization mainly results from adsorption on the surface of GdB6 based on the adsorption-desorption tests. The adsorption behavior follows the pseudo-second-order kinetics and Freundlich isotherm model. An adsorp- tion capacity of 3108.8 mg/g with an adsorption ratio of 77.7% was obtained, and the performance remained unchanged after six adsorption-desorption cycles, which demonstrate the excellent MB removal performance of GdB6 in comparison to other inorganic materials. 1. Introduction novative inorganic particles with nanoscale structures for better re- moval performance of dye contaminants has attracted growing interest. Methylene blue (MB) is known as a cationic dye that has a wide in- There has been remarkable attention to novel transition metal and lan- dustrial application in the development of pharmaceuticals, plastics and thanide metal-based materials, such as oxides (Fe3 O4 (Gautam et al., cosmetics (Rafatullah et al., 2010). However, the extensive use of MB 2020), CeO2 (Kurian, 2020), Gd2 O3 (Ledwaba et al., 2015; Das and has brought about serious environmental issues such as deterioration Sharma, 2020)), carbides (Ti3 C2 Tx (Wei et al., 2018)) and sulﬁdes (Ni-S of water quality due to its high aqueous solubility. Its chemically sta- (Kumari et al., 2020), Ni-Co-S (Chowdhury et al., 2021)). ble character also makes it diﬃcult to decompose, which could eventu- Apart from these inorganic nanomaterials, the application of metal ally threaten the health of humans upon bioaccumulation (Zhang et al., borides in the removal of organic dyes has attracted attention owing to a 2019). Therefore, it is of great importance to ﬁnd an eﬃcient method variety of unique properties of these inorganic materials including high for the treatment of MB. There have been numerous methods includ- mechanical strength, thermal and chemical stability, and those with ing photocatalysis, ion-exchange, and biological treatment that were nanostructures are considered ideal adsorbents for their high porosity employed to treat MB over the past decades (Khaksar et al., 2015; and large surface areas (Hang et al., 2018; Liu et al., 2019; Wang et al., Das et al., 2017; Bilal et al., 2020; Fazal et al., 2020; Thakur et al., 2019). Regarding the treatment of MB, it was ﬁrst reported that porous 2020). Compared with other methods, adsorption is more commonly CeB6 prepared at 700 °C via the reaction of CeCl3 and NaBH4 in molten utilized for this purpose as a result of its simple, fast and econom- NaCl-KCl is capable of treating MB although the capacity for MB adsorp- ical features, and various adsorbents like zeolites, activated carbon, tion was only 6.75 mg⋅g−1 (Hang et al., 2018). A similar Zn-assisted re- graphene, biosorbents, and metal-organic frameworks have been con- action produced LaB6 powders that showed rapid MB adsorption with a sidered (Rafatullah et al., 2010; Xie et al., 2014; Fronczak et al., 2017; high capacity of 606.2 mg⋅g−1 , but the adsorption ratio was only 55.1% Liew et al., 2018; Shang et al., 2019; Azari et al., 2020; Khaleque et al., (Wang et al., 2019). Apparently, there is no boride material that features 2020). In addition to the above materials, the development of in- ∗ Corresponding author at: Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail address: liuxiyan@sinap.ac.cn (X. Liu). https://doi.org/10.1016/j.envadv.2021.100055 Received 25 January 2021; Received in revised form 16 March 2021; Accepted 12 April 2021 2666-7657/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)
F. Zhang, X. Chen, C. Wang et al. Environmental Advances 4 (2021) 100055 both high adsorption capacity and ratio that are important regarding the X-ray photoelectron spectroscopic (XPS) studies were performed us- application in MB removal (Chang et al., 2016; Huang et al., 2019). In ing a spectrometer (Escalab 250Xi) with Al K𝛼 X-ray source to deter- addition, the very few studies that have been reported limit the under- mine the chemical nature of the surface species in the prepared sam- standing on the roles of lanthanide boride in this process, which prevents ple. The Brunauer-Emmett-Teller (BET) surface area were measured us- the potential application of boride materials. A particular aspect is that ing a V-Sorb 2800TP instrument. Electron spin resonance (ESR) experi- it is not clear whether other processes such as reduction occur when ments were performed on a JES-FA200 instrument using 5, 5 dimethyl- MB is in contact with lanthanide boride. Such eﬀect has been observed 1-pyrroline-N-oxide (DMPO) as a spin-trapping reagent to characterize in the cases of mixed-valent iron oxide (Khan et al., 2017), Fe0 /H2 O the radical species. Electrospray ionization (ESI) mass spectra were ob- (Noubactep, 2009), N2 H4 with Au nanoparticles (He and Zheng, 2017), tained using a Bruker Daltonics SolariX XR 7.0T Fourier transform ion NaBH4 with CLPUR (Sultan et al., 2018), NaBH4 with cotton@Ag NPs cyclotron resonance mass spectrometer (FTICR-MS) equipped with a (Qi et al., 2020), and NaBH4 with LPR@Ag (Chen et al., 2020b), and it heated ESI source for the identiﬁcation of the structure of dye molecules could have inﬂuence on the evaluation of MB removal performance. The before and after reacting with GdB6 . previously reported lanthanide borides for dye contaminants removal are usually compounds of cerium group elements. In fact, heavy lan- 2.4. MB removal performance thanide nanomaterials like Gd2 O3 , also possess promising performance in the treatment of organic pollutants in water. Here, gadolinium was The MB removal experiments were carried out by mixing GdB6 with selected as the representative of heavy lanthanide element to investigate MB aqueous solutions at 25 °C. For the kinetic study, 5 mg GdB6 was the dye removal performance of heavy lanthanide boride. mixed with 10 mL aqueous solution containing 100, 200, 300, 400 and In this paper, the synthesis of GdB6 nanoparticles by a simple one- 500 mg/L MB. The mixture was sampled every 5 min within 30 min to pot molten salt approach which favors the preparation of metal borides determine the concentration of MB. To obtain the adsorption isotherm, with nanostructures was reported (Portehault et al., 2011; Gouget et al., 5 mg GdB6 was added into 10 mL MB solutions with diﬀerent initial 2016). Pure GdB6 phase was successfully obtained at 900 °C at a GdF3 concentrations from 100 to 2000 mg/L. 0.5 mL mixture was collected to NaBH4 molar ratio of 1: 12. The structure and morphology of the after 30 min, and the MB concentration in the supernatant can be de- GdB6 sample were characterized, and the MB removal performance was termination after the mixture was centrifuged. Adsorption-desorption investigated. The kinetic and isotherm adsorption behaviors show the experiments were also performed to evaluate the reusability of GdB6 . excellent performance of GdB6 nanoparticles for MB removal in com- Typically, 10 mg solid GdB6 powder was added into a 10 ml 500 mg/L parison to the known inorganic materials. In addition, reduction from MB aqueous solution in a plastic centrifuge tube. After 30 min reaction, blue MB to transparent leucomethylene (LMB) was observed in the pres- the mixture was centrifuged and the sediment was separated. The ad- ence of boride for the ﬁrst time. sorbed MB was desorbed two times with fresh ethanol for each sample. And then solid GdB6 powder was processed for the next adsorption- 2. Experimental details desorption cycle after being washed with water to remove residual ethanol. 2.1. Chemicals The concentrations of MB in all the aqueous and ethanol solu- tions were determined according to the intensity of the 664 (H2 O) and GdF3 (99.99%), LiCl (99%) and KCl (≥99%) were purchased from 655 (ethanol) nm bands using a UV–Vis-NIR spectrophotometer (Cary Sigma-Aldrich. Methylene blue (MB), NaBH4 and ethanol were obtained 6000i, Agilent). The removal capacity Qt (mg/g), desorption capacity from Sinopharm. All chemicals were used as received without further Qd (mg/g) and removal ratio R (%) were calculated by the following puriﬁcation. The MB solution was prepared by dissolving MB in deion- equations (Gupta et al., 2017; Sadegh et al., 2017; Seyed Arabi et al., ized water. 2019). 𝐶0 − 𝐶𝑡 𝐶 𝑉 𝐶 − 𝐶𝑡 𝑄𝑡 = 𝑉 (1) 𝑄𝑑 = 𝑑 𝑑 (2) 𝑅 = 0 × 100%(3) (1) 2.2. Synthesis 𝑚 𝑚 𝐶𝑜 where Co and Ct (mg/L) represent the initial MB concentration and the GdB6 was synthesized in eutectic LiCl-KCl (45/55 wt., Tf = 353 °C) concentration of MB at time t in aqueous solutions; Cd (mg/L) is the MB molten salt under argon atmosphere (Scheme 1). The GdF3 and NaBH4 concentration in desorbed ethanol solution; m (g) is the mass of GdB6 , precursor powders as well as eutectic LiCl-KCl were mixed and grounded V and Vd (L) are the volumes of aqueous solution and desorbed ethanol in an agate mortar. The ﬁne powders were then transferred into a corun- solution. dum crucible that were heated to a given temperature in an electric fur- nace at a heating rate of 8 °C/min. After being kept at this temperature 3. Results and discussions for 4 h, the mixture was cooled, and the chloride salt was removed by deionized water. The as-prepared boride powders separated by centrifu- 3.1. Structure and morphology of GdB6 gation were left in an argon glovebox after being dried under vacuum at 35 °C. Gadolinium borides were synthesized between 600 and 1000 °C with the precursor molar ratio (GdF3 /NaBH4 ) ﬁxed at 1: 12. The XRD pat- 2.3. Characterizations terns of the as-prepared samples at diﬀerent temperatures were dis- played in Figure S1. Many weak diﬀraction peaks were observed for Powder X-ray diﬀraction (XRD) patterns were obtained on a diﬀrac- the samples prepared at 600 and 700 °C, which would be indexed as a tometer (X’Pert Pro MPD) using Cu K𝛼 1 radiation to identify the sam- known phase GdB4 . But some parasite peaks were also presented that ple’s crystalline phases. The surface morphologies, sizes and microstruc- could not be indexed. As temperature increases, the GdB6 peaks domi- ture of the sample were investigated by means of electron microscope nate while the peaks arising from diﬀerent crystalline phases completely techniques. The scanning electron microscopy (SEM) images were ac- disappear. The transformation phenomenon of lanthanide tetraboride quired by a ﬁled-emission scanning electron microscope (Merlin com- to hexaboride has been observed in the controllable molten salt synthe- pact, Zeiss). The transmission electron microscopy (TEM) images, se- sis of samarium borides (Liu and Gong, 2021). The XRD pattern of the lective area electron diﬀraction (SAED) pattern, and energy dispersive sample prepared at 900 °C is shown in Fig. 1, and all the diﬀraction spectrum (EDS) were obtained on a transmission electron microscope peaks are well indexed to cubic GdB6 with a space group of Pm-3m. The (Tecnai G2F20S-Twin, FEI) at an accelerating voltage of 200 kV. The absence of impurity peaks indicates the capability for the synthesis of 2
F. Zhang, X. Chen, C. Wang et al. Environmental Advances 4 (2021) 100055 whole process. The spectrum of MB aqueous solution shows three major absorptions at 246, 292, and 664 nm the last of which contributes to the blue color of MB (Khan et al., 2017). As the contact time increases (Fig. 4b), the MB absorptions decrease with concurrent appearance of a new band at 258 nm in the UV region which is due to the reduced form of MB (LMB) (Basu et al., 2012; Khan et al., 2017). This is consistent with the change in color from blue to colorless since there is no absorption in the visible region for LMB. The oxidation of the colorless supernatant in air after it was separated from GdB6 is shown in Fig. 4c, which demon- strates the disappearance of LMB and appearance of MB according to the change in absorption intensity with the increase in time (Basu et al., 2012; Khan et al., 2017). Note that the intensities of the MB absorptions in Fig. 4c are lower than those in Fig. 4b, in line with the change in color of MB from dark to light blue as shown in Fig.4a. The discovery of the reduction and reoxidation phenomenon of MB is of great signiﬁcance for accurately evaluating the performance of metal boride in the treat- ment of MB wastewater. In addition, the discovery also reminds people that there may be misjudgment caused by reduction when evaluating Fig. 1. Powder XRD pattern of the GdB6 sample synthesized at 900 °C. the treatment performance of methylene blue during the development of new adsorptive or catalytic materials. To explore the mechanism of MB reduction in the presence of GdB6 , pure crystalline boride phase by molten salt approach at a temperature the boride samples after being in contact with MB were immersed in of 900 °C. ethanol such that the adsorbed dye molecules on the surface of GdB6 The full XPS spectrum (Fig. 2a) shows the dominance of Gd, B, C and could be desorbed and dissolved in ethanol. The UV–Vis spectrum of the O with the latter two being common impurities in the spectra of borides ethanol solution taken immediately after GdB6 was removed is shown in (Selvan et al., 2008). The spectrum in the Gd 3d region is shown in Fig. 5a which reveals the presence of both MB and LMB. This is further Fig. 2b with the 3d3/2 and 3d5/2 peaks observed at 1219.8 and 1187.1 eV conﬁrmed by the appearance of two peaks due to MB+ and LMB+ in (Patel et al., 2016). The single peak in the B 1s region can be ﬁtted the ESI mass spectra (Fig. 5b). However, the MB absorptions increase by two distinct components at 187.1 and 188.1 eV ascribed to boron at the cost of the LMB absorption, and the latter completely disap- in GdB6 (Bao et al., 2016; Han et al., 2017). For the weak peak lo- pears after the ethanol solution was left in air for 30 min. Such spectral cated at 192.6 eV, it is probably due to very small amount of borate change indicates that MB reduction occurs on the surface of GdB6 , and that was also observed in the spectra of other hexaborides (Han et al., both MB and LMB were desorbed upon contacting ethanol. The ESR 2017; Yu et al., 2018). Nitrogen adsorption-desorption experiment was measurement of the aqueous supernatant separated from GdB6 using performed to determine the BET surface area of GdB6 . As shown in DMPO (5, 5-dimethyl-1-pyrroline-N-oxide) as the trapping reagent was Fig. 2d, the adsorption-desorption isotherm follows type IV isotherm also carried out, but no signal was observed, indicating that MB reduc- (Hang et al., 2018). The BET surface area of GdB6 was determined to be tion did not arise from the reaction between MB and hydrated elec- 51.5 m2 /g. trons generated upon visible-light irradiation of GdB6 in aqueous solu- The morphology and microstructure of GdB6 was investigated by tion, but occurred on the surface of GdB6 nanoparticles as illustrated in SEM and TEM. As depicted in Fig. 3a and b, the GdB6 powders are Scheme 2 (Khan et al., 2017; Liu et al., 2019). homogeneously dispersed with the cubic particles possess diameters In addition to adsorption and reduction, it is possible for MB to around 100 nm. The observed fringes with spacing of 0.410 and 0.291 undergo decomposition upon contacting lanthanide materials such as nm correspond to the d values of the (100) and (110) planes (Fig. 3c), cerium oxide (Ma et al., 2019; Zeleke and Kuo, 2019; Wei et al., 2020). which is further supported by the characteristic rings revealed from the On the basis of the fact that the adsorption capacity (177.5 mg/g) is SEAD pattern (Fig. 3d). The HAADF-STEM image and elemental map- about the same as the desorption capacity (181.3 mg/g) (Fig. S2). It can ping demonstrate the homogeneous distribution of Gd and B over the be concluded that adsorption and reduction of MB dominate on the sur- GdB6 sample (Fig. 3e–g) with negligible contaminations as revealed by face of GdB6 while MB degradation is negligible. All of the adsorption the EDS spectrum (Fig. 3h). and desorption results reported hereafter exclude the contribution from MB reduction. 3.2. MB reduction phenomenon and mechanism 3.3. MB removal performance MB is known to be reduced to colorless leucomethylene (LMB) in the presence of reducing materials such as Cu2 O (Basu et al., 2012) The time proﬁle of MB adsorption at diﬀerent initial concentrations and NaBH4 (Qi et al., 2020), but there is no report regarding such phe- is shown in Fig. S3. The adsorption capacity increases quickly within the nomenon in the presence of metal borides that could aﬀect the accurate ﬁrst 5 min for all groups and remains unchanged after about 15 min, determination of the capability for MB removal. In order to minimize indicating a great adsorption rate of MB on GdB6 (Tian et al., 2013). this unusual eﬀect, the inﬂuence of GdB6 on the form of MB when they A good linearity was obtained by plotting t/Qt versus time, suggest- were in contact was studied for the ﬁrst time. Decolorization of the MB ing that MB adsorption should follow the pseudo-second-order kinetic solution (10 mL, 100 mg/L) was observed upon GdB6 addition, and the model (Fig. 6a). Besides the adsorption rate, it is also of importance solution became almost colorless after a contact time of 30 min (Fig. 4a). to know the adsorption capacity and ratio that are key to the applica- After the supernatant was separated and left in the air, a slow change in tion of GdB6 in the treatment of MB. The adsorption capacity of GdB6 color back to blue was observed. This contrasts the general phenomenon in 100 mg/L MB solution was determined to be 168.3 mg/g with an of irreversible color change of dye solutions caused by adsorption or adsorption ratio of 84.1%. A much higher capacity of 3108.8 mg/g degradation even if they were exposed to air. The decolorization of MB was obtained in 2000 mg/L MB solution with an adsorption ratio of by GdB6 is not only a result of adsorption or degradation, but also in- 77.7% (Figs. S4 and S5). Such high adsorption capacity is related to volves reduction induced by boride. To get more evidence on the reduc- the large BET area of GdB6 nanocrystals with more active adsorption tion of MB by GdB6 , UV–vis spectroscopy was employed to monitor the sites (Huang et al., 2019; Chen et al., 2020a). The rough surface struc- 3
F. Zhang, X. Chen, C. Wang et al. Environmental Advances 4 (2021) 100055 Fig. 2. (a) XPS survey spectrum of GdB6 , high resolution XPS spectra of (b) Gd 3d and (c) B 1s, (d) the nitrogen adsorption-desorption isotherm of GdB6 . Fig. 3. (a) SEM, (b) TEM, (c) HRTEM images, (d) SEAD pattern, (e) HAADF-STEM, (f-g) map- ping, and (h) EDS spectrum of GdB6 . ture (Fig. 3a) and good dispersion of nanoparticles may also helpful drops once the equilibrium is reached (Ghosal et al., 2013; Chang et al., for the adsorption of MB molecules through promoting the mass trans- 2016). For example, the titanate nanosheets prepared by chemical exfo- fer process (Agarwal et al., 2016; Maazinejad et al., 2020). As listed liation of layered protonated titanate exhibit high adsorption capacity in Tables 1 and S1, the adsorption capacity of MB by GdB6 is much (2236 mg/g) but low adsorption ratio (28.0%) (Yuan et al., 2019), while higher than those of other lanthanide borides and inorganic materials the situation is reversed for the hierarchical ﬂower-like sodium titanate while a reasonably high adsorption ratio is maintained under compara- where an adsorption ratio of 98.6% and a capacity of 49.3 mg/g were ble conditions, which demonstrates that GdB6 is a promising material obtained (Feng et al., 2013). for MB removal. In general, adsorption capacity increases with the in- The equilibrium adsorption data were ﬁtted using the Langmuir and crease in initial concentration until saturation, while adsorption ratio Freundlich isotherm models to understand the nature of MB adsorp- 4
F. Zhang, X. Chen, C. Wang et al. Environmental Advances 4 (2021) 100055 Fig. 4. (a) Reversible color change of 100 mg/L MB aqueous solution, (b) UV–vis spectra of the MB aqueous solutions upon contacting GdB6 for diﬀerent durations. (c) UV–Vis spectra of the aqueous solutions left in air for diﬀerent periods. The solution was separated from GdB6 after they were in contact for 30 min. (For interpretation of the references to color in this ﬁgure, the reader is referred to the web version of this article.). Fig. 5. (a) UV–Vis spectra of desorbed MB and LMB in ethanol (red) and the same solution after being left in air for 30 min (blue), (b) ESI mass spectra of the MB aqueous solution (top) and aqueous solution separated from the GdB6 /MB mixture (bottom). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) Table 1 Comparison of the MB removal performance by diﬀerent lanthanide borides. Material Condition Q (mg/g) R (%) Ref. GdB6 [dye] = 10 mL, 100 mg/L [material] = 5 mg 168.3 84.1 this [dye] = 10 mL, 2000 mg/L [material] = 5 mg 3108.8 77.7 work CeB6 [dye] = 100 mL, 10 mg/L [material] = 120 mg 6.75 80.0 (Hang et al., 2018) LaB6 [dye] = 100 mL, 220 mg/L [material] = 20 mg 606.2 55.1 (Wang et al., 2019) 5
F. Zhang, X. Chen, C. Wang et al. Environmental Advances 4 (2021) 100055 Fig. 6. (a) Pseudo-second-order kinetics of MB adsorption on GdB6 , (b) ﬁtting curve of Freundlich isotherm, (c) reusability of GdB6 for MB adsorption. tion on GdB6 . According to the results presented in Figs. 6b and S4, terms of the reaction mechanism, it has been veriﬁed that the reduction a much better ﬁt was found for the Freundlich isotherm than Langmuir of MB was occurred on the surface of GdB6 nanoparticle, but the essence isotherm, implying that MB adsorption is better described by a multi- of transformation process of MB to LMB brought by boride need to be layer adsorption mechanism (Yang et al., 2018). The Freundlich param- studied in-depth. eters KF and n were determined to be 16.8 mg1-n ⋅Ln /g and 1.16, showing favorable adsorption of MB on GdB6 (Zhang et al., 2019). Conclusions Given the importance of stability and reusability for the application of an adsorbent, a six-run adsorption-desorption experiment was car- In summary, single-phase gadolinium hexaboride nanoparticles were ried out. The adsorption capacity of MB slightly decreases from 486.1 successfully synthesized by a molten salt approach. GdF3 and NaBH4 to 455.5 mg/g after six adsorption-desorption cycles (Fig. 6c) with the were employed as precursors at a molar ratio of 1: 12, and cubic- decrease in capacity less than 7%, indicating a good stability for GdB6 shaped GdB6 with an average size of 100 nm and a BET surface area of in the treatment of MB (Huang et al., 2019). 51.5 m2 /g was obtained at 900 °C. On the basis of the results from UV- The morphology and structure of GdB6 after the 6th MB adsorption- vis spectroscopy and ESI-mass spectrometry, reduction of MB to LMB desorption cycle (denoted as GdB6 -MB) was characterized using TEM in aqueous solution upon contacting GdB6 was observed for the ﬁrst and XPS (Figure S6). As displayed in Fig. 7a–c, the nanoparticles still time, which allows for the accurate evaluation of the MB removal per- possess cubic shape with crystalline structure. The Gd 3d and B 1s XPS formance by boride materials. The adsorption behaviors of MB on lan- spectra of GdB6 -MB (Fig. 7d and e) exhibit no change in peak position thanide boride nanomaterial were studied in detail. The adsorption pro- and intensity in comparison to those of freshly synthesized GdB6 sample. cess follows the pseudo-second-order kinetics and Freundlich isotherm The above results demonstrate negligible change in morphology and model, and an adsorption capacity of 3108.8 mg/g with a reasonably structure of GdB6 even after MB treatment, which is consistent with the high adsorption ratio was obtained in 2000 mg/L MB solution. These re- good stability of this boride material as revealed by the reusability test. sults are superior in comparison to other inorganic materials used for MB GdB6 nanocrystals exhibit promising adsorption properties of MB, removal, and the performance remains unchanged after six adsorption- a cationic dye, including a high adsorption capacity with a reasonably desorption cycles. The present study provides a facile method for the high adsorption ratio and good stability based on the laboratory discov- synthesis of GdB6 nanocrystals that may contribute to the development eries. The basic adsorption kinetic and thermodynamic behaviors were of the functional diversity of such boride in the ﬁeld of nanomaterials. revealed, but the present study was mainly limited to laboratories batch This work also demonstrates the capability of metal borides for the eﬃ- adsorption experiments in simple aqueous solutions, so larger scale work cient removal of dye wastes, which could serve as a promising strategy such as ﬂow adsorption should be conducted to evaluate the potential for water remediation. applications in real dye wastewater treatment of lanthanide borides. In 6
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