Excellent organic dye adsorption capacity and recyclability of hydrothermally synthesized a-Fe₂O₃ nanoplates and nanorices

a-Fe2O3 and Fe3O4 nanoplates and nanorices were synthesized by hydrothermal reaction and used for organic pollutant adsorption and removal studies. The effect of size and morphology of a-Fe2O3 and Fe3O4 nanoplates and nanorices on the dye adsorption efficiency were investigated systematically. The adsorption studies showed a good selectivity and high adsorption capacity (~93%) of the a-Fe2O3 nanoplates towards congo red (C.R.) dye, due to the formation of hydrogen bonding between amine group (e NH2) of C.R. and surface carboxylate group (e COOH), and adsorbed e OH groups of a-Fe2O3 nanoplates. The a-Fe2O3 nanoplates also showed excellent recyclability with negligible loss of the adsorption capacity, owing to their stable morphology and crystal structure. The excellent C.R. adsorption capacity of the a-Fe2O3 nanoplates was further confirmed by conducting experiments with a solution containing a mixture of dyes. Our findings indicate that the synthesized a-Fe2O3 nanoplates have a great potential in removing industrial waste, such as C.R. from polluted water.

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ty o N P a, b, * PRATI), A c Faculty of Electrical and Electronic Engineering, Pheni d 17 Hoan my of Sc y, Vietna of Educa ures, Vie capacity of the a-Fe2O3 nanoplates was further confirmed by conducting experiments with a solution oplates have a great al University, Hanoi. ons.org/licenses/by- nc-nd/4.0/). tion, chemical oxidation, filtration, membrane separation, coagu- lation, adsorption, biological treatment, and photodegradation move organic dyes ds are expensive, ld. Among various ecent studies have technique for dye y, easy operation, nt, and the ability metal oxides with different structure, size, and morphology have been recognized to be effective solid adsorbent for wastewater treatment because of their high surface area, and presence of active sites [6,22,26e34]. Among various transition metal oxides, iron oxide (a-Fe2O3 and Fe3O4) nanoparticles are drawing substantial attentionowing to their unique properties such as excellent stability, biocompatibility, low synthesis cost, ease of synthesis, and functionalization [26e34]. * Corresponding author. Faculty of Materials Science and Engineering, Phenikaa University, Hanoi, 12116, Viet Nam. E-mail address: raja@phenikaa-uni.edu.vn (R. Das). Contents lists availab Journal of Science: Advanc journal homepage: www.el Journal of Science: Advanced Materials and Devices 6 (2021) 245e253Peer review under responsibility of Vietnam National University, Hanoi.methods such as electrochemical, chlorination, ozonation, flota- to remove different types of pollutants [18e25]. Various transitionNanorices Hydrothermal Selective adsorption Recyclability Wastewater containing a mixture of dyes. Our findings indicate that the synthesized a-Fe2O3 nan potential in removing industrial waste, such as C.R. from polluted water. © 2021 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam Nation This is an open access article under the CC BY-NC-ND license ( 1. Introduction The uncontrolled discharge of organic dyes usually azo-groups containing dyes from textile, plastic, printing, paper pulp, paint, and leather industries has contaminated water [1e6]. Removing organic dyes from wastewater has become crucial because of their adverse effect on human health and the environment. Several using photocatalyst have been used till date to re from wastewater [7e17]. However, these metho time - consuming, and difficult to apply in the fie processes available for wastewater treatment, r shown adsorption to be the most promising removal due to its convenience, high efficienc simplicity of design, minimum energy requiremeKeywords: Nanoplates 2 3 adsorption capacity, owing to their stable morphology and crystal structure. The excellent C.R. adsorptionAcademy of Military Science and Technology, e Institute of Materials Science, Vietnam Acade f Graduate University of Science and Technolog g Faculty of Physics, Hanoi National University h Center for Innovative Materials and Architect a r t i c l e i n f o Article history: Received 9 December 2020 Received in revised form 13 February 2021 Accepted 19 February 2021 Available online 25 February 2021https://doi.org/10.1016/j.jsamd.2021.02.006 2468-2179/© 2021 The Authors. Publishing services b license ( University, Hanoi, 12116, Viet Nam g Sam, Cau Giay, Hanoi, 10000, Viet Nam ience and Technology, 18-Hoang Quoc Viet, Cau Giay, Hanoi, 10000, Viet Nam m Academy of Science and Technology, 18-Hoang Quoc Viet, Cau Giay, Hanoi, 10000, Viet Nam tion, 136 Xuan Thuy, Cau Giay, Hanoi, Viet Nam tnam National University, Ho Chi Minh City, Viet Nam a b s t r a c t a-Fe2O3 and Fe3O4 nanoplates and nanorices were synthesized by hydrothermal reaction and used for organic pollutant adsorption and removal studies. The effect of size and morphology of a-Fe2O3 and Fe3O4 nanoplates and nanorices on the dye adsorption efficiency were investigated systematically. The adsorption studies showed a good selectivity and high adsorption capacity (~93%) of the a-Fe2O3 nanoplates towards congo red (C.R.) dye, due to the formation of hydrogen bonding between amine group (e NH2) of C.R. and surface carboxylate group (e COOH), and adsorbed e OH groups of a-Fe2O3 nanoplates. The a-Fe O nanoplates also showed excellent recyclability with negligible loss of thea Faculty of Materials Science and Engineering, b Phenikaa Research and Technology Institute ( &A Green Phoenix Group, 167 Hoang Ngan, Hanoi, 13313, Viet NamRaja Das Phenikaa University, Hanoi, 12116, Viet NamOriginal Article Excellent organic dye adsorption capaci hydrothermally synthesized a-Fe2O3 nan Pham Kim Ngoc a, b, Trung Kien Mac a, b, Huu Tuan Tran Dang Thanh e, f, Pham Van Vinh g, Bach Thangy Elsevier B.V. on behalf of Vietnam d/4.0/).and recyclability of plates and nanorices guyen b, c, Do Thanh Viet d, han h, Anh Tuan Duong a, b, le at ScienceDirect ed Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY-NC-ND Fig. 2a and b reveal that the phase transformation of the as- P.K. Ngoc, T.K. Mac, H.T. Nguyen et al. Journal of Science: Advanced Materials and Devices 6 (2021) 245e253However, there are somemajor challenges associatedwith ironoxide nanostructures such as limited adsorption capacity, poor selectivity, and recyclability [31,32]. To achieve high adsorption capacity, several strategies like functionalization with organic molecules, polymer, carbon nanotubes, and nanocomposite with graphene, graphene oxide, transition metal dichalcogenide (TMD) have been used [31,35e38]. Recently, Chatterjee et al. have developed 1,2,4,5- Benzentetracarboxylic acid functionalized Fe3O4 nanoparticles exhibiting high selectivity towards C.R. and high adsorption capacity [31]. The functionalization of the nanostructures improve the adsorption capacity, nevertheless the functionalizationprocesses are complex and costly [31,35e38]. Although there have been several studies on the adsorption of dye molecules using a-Fe2O3 and Fe3O4 nanostructures, there is still no consensus on the mechanism of adsorption and particularly selectivity towards specific organic dye molecules. Moreover, in actual wastewater systems, the interaction among different dyes ubiquitously lowers the adsorption capacity of the adsorbents. No studies have been done to show the performance of dye adsorptionwhen subjected to amixture of pollutants,which is essential for practical application. From both fundamental and technological points of view, it is extremely important to develop a highly efficient, stable, environmentally friendly, and low-cost adsorbent for wastewater treatment. In our study, we have synthesized a-Fe2O3 nanoplates and nanorices by the hydrothermal method and the phase trans- formation of the a-Fe2O3 nanoplates and nanorices into Fe3O4 nanoplates and nanorices were accomplished via reduction [39e41]. To the best of our knowledge, there is no report on the adsorption properties of a-Fe2O3 and Fe3O4 nanoplates and nano- rices. The adsorption studies showed high selectivity of a-Fe2O3 towards an azo-aromatic group containing C.R. dye. The as- synthesized a-Fe2O3 nanoplates exhibited an excellent adsorption capacity of ~93% to remove C.R. dye from its aqueous solution and showed its reusability in multiple cycles with insignificant reduc- tion in the adsorption capacity. From the systematic studies, we have confirmed the significance of e NH2 groups on C.R. dye mol- ecules and e COOH group on a-Fe2O3 nanoplates in the formation of hydrogen bonding between the dye and a-Fe2O3 nanoplates. 2. Experimental 2.1. Synthesis of a-Fe2O3 nanoplates In a typical reaction of a-Fe2O3 nanoplates, 0.55 g FeCl3$6H2O was dissolved in a mixture of 20 mL ethanol and 1.4 mL water [39]. To the above mixture, 2.5 g C2H3NaO2 was added and magnetically stirred for 30 min at room temperature. The solution was then transferred into a Teflon-lined stainless steel autoclave and heated to 180 C. The reaction was maintained at this temperature for two different times of 12 and 24 h. After natural cooling to room tem- perature, the precipitate was washed three times using amixture of water and ethanol (1:1). 2.2. Synthesis of a-Fe2O3 nanorices The synthesis of a-Fe2O3 nanorices was quite similar to nano- plates. In short, an aqueous solution of 0.27 g FeCl3$6H2O, 7 mg NaH2PO4$2H2O, and 19.5 mg Na2SO4 were mixed and magnetically stirred for 30 min at room temperature [40,41]. Then the reaction mixturewas transferred into a Teflon-lined stainless steel autoclave and maintained at 220 C for two different periods of 3 and 5 h. After allowing the sample to naturally cool down to room tem- perature, the precipitate was separated by centrifugation and washed three times using 1:1 mixtures of water and ethanol. 2463.2. Crystal structure The crystal phase of the as-synthesized red powder nano- structures were examined using X-ray diffraction (XRD). The XRD pattern of the as-synthesized powders in Fig. 3a matches with the rhombohedral a-Fe2O3, confirming that the product is single phase without any impurities. The XRD pattern of the reduced nanorices, as shown in Fig. 3b, reveals the formation of single phase cubic Fe3O4. The SEM (Figs. 1 and 2) and XRD (Fig. 3) results indicate that the annealing at 400 C for 5 h under hydrogen/argon (7% hydrogen) flow causes phase transformation of the nanostructures from rhombohedral a-Fe2O3 to cubic Fe3O4 without significantlysynthesized a-Fe2O3 nanoplates and nanorices into Fe3O4 nano- plates and nanorices, respectively did not alter the morphology of the nanostructures. The dimensions of the Fe3O4 nanoplates and nanorices are summarized in Tables 1 and 2. The insignificant change in the size of the nanostructures after the phase trans- formation could be due to the shrinkage during reduction.2.4. Dye adsorption experiments The adsorption experiments for the removal of C.R., Rh.B., and M.B. dyes were performed at room temperature, 27 ± 2 C. The details of the experiments are presented in the supporting information. 3. Results and discussion 3.1. Morphology Scanning electron microscope (SEM) images of the synthesized particles (Fig. 1) reveal that the as-synthesized nanoplates and nanorices have a uniform shape and size. From the SEM images (Fig. 1a and b), we can see that the as-synthesized particles consist of hexagonal plateswith width 217 ± 15, 207 ± 20 nm and thickness 21 ± 5, 23 ± 5 nm after 12 and 24 h of the hydrothermal reaction, respectively. Different sizes of the hexagonal nanoplates were ob- tained by varying the time of the hydrothermal treatment. The nanorices obtained after 3 and 5 h of hydrothermal reaction were with the length of 424 ± 35, 394 ± 35 nm, and width of 67 ± 10, 71 ± 8 nm, respectively (Fig. 1c and d). The SEM images of the nanorices suggest that the nanorices were made up of constituent self-assembled nanostructures. The width and thickness of the as- synthesized nanoplates and the length and width of the nanorices are summarized in Tables 1 and 2. Next, we investigated the morphology of the nanostructures obtained after the heat treat- ment of the as-synthesized nanoparticles. The SEM images in2.3. Synthesis of Fe3O4 nanoplates and nanorices The phase transformation of the as-synthesized a-Fe2O3 nano- plates and nanorices into Fe3O4 nanoplates and nanorices, respec- tively were completed via reduction of the a-Fe2O3 in presence of hydrogen/argon (7% hydrogen). The as-synthesized a-Fe2O3 pow- ders were annealed in a tube furnace at 400 C under hydrogen/ argon (7% hydrogen) flow for 5 h. Then, the tube furnace was allowed to cool to room temperature while still under a continuous hydrogen/argon (7% hydrogen) flow. After heat treatment in hydrogen/argon (7% hydrogen), the red powder transformed into black powder.altering the morphology of the nanostructures. P.K. Ngoc, T.K. Mac, H.T. Nguyen et al.3.3. Dye molecule removal using a-Fe2O3 nanoplates and nanorices Owing to the negatively charged surface, a-Fe2O3 nano- structures have been known to show high dye adsorption capa- bilities. The UV-vis absorption spectra of the as-synthesized a- Fe2O3 nanoplates and nanorices (Fig. 1 SI) shows remarkable ab- sorption in the visible region. It was observed that the absorption edge of the a-Fe2O3 nanoplates is red-shifted than a-Fe2O3 nano- rices. The bandgap energy of a-Fe2O3 nanoplates and nanorices is 2.2 eV, which is similar to the bulk a-Fe2O3 [42,43]. We have explored the adsorption capacity of the as-synthesized a-Fe2O3 nanoplates and nanorices for the removal of model organic water pollutants, C.R., Rh.B., and M.B. dyes [30e34]. Fig. 4aec shows the UV-vis absorption spectra of C.R., Rh.B., and M.B. aqueous solution (initial dye concentration 10 mg L1) as a function of time after incubation with 0.6 g L1 adsorbent concentration of a-Fe2O3 nanoplates after 24 h of hydrothermal reaction. From the time dependent UV-vis spectra (Fig. 4a), it was found that after 60min of shaking the dye with the 24 h a-Fe2O3 nanoplates, the absorption maximum (498 nm) of Rh.B. dye reduced significantly. The plot of absorption maxima as a function of experimental time (inset of Fig. 4a) shows that the degradation of dye was almost complete after 60 min of exposure of adsorbent to the aqueous solution of C.R. dye and remained almost constant with further increase in the experiment time. This indicates that the 24 h a-Fe2O3 nanoplates Fig. 1. SEM images of as-synthesized a,b) a-Fe2O3 nanoplates after hydrothermal reaction o 5 h, respectively. Scale bars 1 mm. Table 1 Size and surface area properties of nanoplates corresponding to the amount of time allowed in hydrothermal reaction. Sample Width (nm) Thickness (nm) Surface area (m2 g1) a-Fe2O3 nanoplates 12 h 217 ± 15 21 ± 5 a-Fe2O3 nanoplates 24 h 207 ± 20 23 ± 5 14.3 Fe3O4 nanoplates 12 h 207 ± 25 18 ± 5 Fe3O4 nanoplates 24 h 194 ± 25 20 ± 5 7.6 247Journal of Science: Advanced Materials and Devices 6 (2021) 245e253can remove 93% C.R. dye with 60 min of exposure to dye molecules. The C.R. dye removal efficiency of 24 h a-Fe2O3 nanoplates are comparable with the recently reported functionalized and com- posite nanostructures [44e47]. Next, we investigated the ability of 24 h a-Fe2O3 nanoplates to adsorb Rh.B. dye aqueous solution. Fig. 4b shows that in similar adsorption conditions, the absorption maximum of the Rh.B. dye molecule does not change significantly after 120 min of shaking the dye in the presence of the adsorbent. The plot of absorption maxima as a function of experimental time (inset of Fig. 4b) shows that the degradation of Rh.B. was almost linear as a function of time and removes only 19% of Rh.B. even after 120 min of incubation with 24 h a-Fe2O3 nanoplates. Under similar adsorption conditions, the absorption maxima of M.B. dye showed a negligible decrease as a function of experimental time (Fig. 4c). In Fig. 4d we have plotted the dye removal percentage upon shaking the 24 h a-Fe2O3 nanoplates with C.R., Rh.B., and M.B. aqueous solution of dyes after 60 min of incubation. Fig. 4d shows that 24 h a-Fe2O3 nanoplates were capable of removing 93%, 19%, 15% of C.R., Rh.B., and M.B. dye molecules, respectively. The high selectivity of the 24 h a-Fe2O3 nanoplates towards C.R. dye molecules can be used for the selective removal of organic pollutants from waste- water. The high selectivity, high adsorption capacity, low cost, low energy requirements, and easy removal process shown by a-Fe2O3 nanoplates makes it a potential contender for wastewater treat- ment. The exceptional C.R. dye adsorption capabilities of the 24 h a- f 12 and 24 h, respectively c,d) a-Fe2O3 nanorices after hydrothermal reaction of 3 and Table 2 Size and surface area properties of nanorices corresponding to the amount of time allowed in hydrothermal reaction. Sample Length (nm) Width (nm) Surface area (m2 g1) a-Fe2O3 nanorices 3 h 424 ± 35 67 ± 10 a-Fe2O3 nanorices 5 h 394 ± 35 71 ± 8 38.9 Fe3O4 nanorices 3 h 402 ± 35 64 ± 15 Fe3O4 nanorices 5 h 392 ± 30 61 ± 15 41.8 P.K. Ngoc, T.K. Mac, H.T. Nguyen et al.Fe2O3 nanoplates motivated us to investigate in detail its kinetics and recyclability. To understand the kinetics of the adsorption capacity of the 24 h a-Fe2O3 nanoplates (0.6 g L1) towards C.R. dye (initial dye con- centration 10 mg L1), adsorption studies were undertaken after shaking the adsorbent with dye molecules. From Fig. 5a it can be seen that the kinetics of the dye adsorption is rapid and 88% of C.R. is removed after 1 min of exposure to 24 h a-Fe2O3 nanoplates and it remained almost constant after a further increase in time. 91% C.R. dye removal efficiency is achieved after 10 min of incubation. Next, we varied the quantity of 24 h a-Fe2O3 nanoplates (0.4, 0.6, 0.4 g L1) with initial C.R. concentration fixed at 10 mg L1 (Fig. 5a). We found that with an increase in the amount of 24 h a-Fe2O3 nanoplates, the C.R. removal efficiency increased but the increase Fig. 3. X-ray diffraction (XRD) patterns of a) as-synthesized a-Fe2O3 nanoplates, nanorices, b) as-synthesized a-Fe2O3 and Fe3O4 nanoplates (24 h), nanorices (5 h). The lower patterns in blue and red are for bulk a-Fe2O3 and Fe3O4, respectively, from JCPDS data. Fig. 2. SEM images of a) Fe3O4 nanoplates after hydrothermal reaction of 24 h 248was not significant as the adsorbent concentration of 0.4 g L1 was sufficient for 91% removal of C.R. (10 mg L1) after 10 min. To become cost-effective, the adsorbent should have the ability to be recycled without undergoing a significant reduction in the adsorption capacity [31]. To check the recyclability of the as- synthesized 24 h a-Fe2O3 nanoplates, we have performed three cycles of adsorption/desorption studies. The recyclability experi- ments were carried out by recording the UV-vis spectra of C.R. aqueous solution with adsorbent recovered from the initial cycles. For recyclability experiments, the concentration of 24 h a-Fe2O3 nanoplates was 0.6 g L1 and initial dye concentration was 10 mg L1, and incubation time 10 min. The corresponding dye removal efficiency of the adsorbent is plotted as bar graphs in Fig. 5b. From Fig. 5b it can be seen that after three cycles, there is no significant reduction in dye removal ability of the as-synthesized 24 h a-Fe2O3 nanoplates. The slight change in the dye removal ef- ficiency has resulted from the loss of the particles during the extraction of adsorbent from the earlier cycles. In the process of dye removal using adsorption, the charges on the adsorbent (a-Fe2O3 nanoplates) and adsorbate (C.R. dye) play an important role [30e34]. The pH value affects the adsorption capacity of the adsorbent because it affects the surface charge of the adsorbent, the degree of ionization of the dye molecule, the dissociation of functional groups on the adsorbent, and the struc- ture of the dye molecules [31,32]. C.R. is a dipolar dye which exists in an anionic form (red color) at alkaline and neutral pH and cationic form (blue color) at acidic pH [32]. The blue color of the C.R. dye is due to the protonation of the azo group (eN¼Ne) which shifts the absorption maxima to higher wavelength (570 nm) [32]. The pH dependence of the C.R. adsorption on 24 h a-Fe O nano- , b) Fe3O4 nanorices after hydrothermal reaction of 5 h. Scale bars 1 mm. Journal of Science: Advanced Materials and Devices 6 (2021) 245e2532 3 plates is shown in Fig. 6. It is evident from the Fig. that the adsorption efficiency is highest at acidic pH (3) which remained almost constant at neutral pH (~7) and decreased in the alkaline pH (10) (Fig. 6aec). Fig. 6d show the bar diagram for the percentage removal of C.R. dyemol
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