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Synthesis of titanium based hetero MOF photocatalyst for reduction of Cr (VI) from wastewater
Journal of Environmental Chemical Engineering 7 (2019) 103240
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Synthesis of titanium based hetero MOF photocatalyst for reduction of Cr (VI) from wastewater Egambaram Dhivyaa, Deviga Magadevana, Yasam Palgunab, Trilochan Mishrac, Noor Amana,
T
⁎
a
Department of Chemistry, B.S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, 600048, India Department of Material Science and Metallurgical Engineering, IIT-Hyderabad, Hyderabad, 502285, India c Functional Material Group, Advance Material & Process Division, CSIR-National Metallurgical Laboratory, Jamshedpur, 831007, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cr (VI) removal Metal organic framework Photocatalyst Visible light Charge separation
Cr (VI) is one of the well-known toxic contaminant coming from leather tanning, metal finishing, textile industries etc. As per world health organization standard, limit for Cr (VI) in drinking water is 0.05 mg/l. In last few years, Metal organic framework (MOFs) based photocatalyst has find great interest for environmental remediation. In this work, titanium based MOFs NH2-MIL-125 and NTU-9 was integrated to make a hetero MOF NTU-9/NH2-MIL-125 (HMF) via refluxing method. It was characterized by XRD, TGA, FE-SEM, FT-IR, XPS, UV–vis (DRS) and PL etc. XRD indicates crystallinity and framework nature of all the synthesized MOFs. Thermal stability of HMF is improved to 520 °C in comparison to NTU-9. Diffuse reflectance spectra exhibit HMF having band gap of 2.2 eV absorbing up to 563 nm. BET surface area of NTU-9, NH2-MIL-125 and HMF were found to be 986, 1267 and 550 m²/g respectively indicating their microporous nature. In order to compare the photocatalytic activity, Cr (VI) solution was chosen a model wastewater. Because of more efficient charge separation, HMF is found to show better activity in comparison to both the contributing MOFs. Acidic condition favors the Cr (VI) reduction and HMF achieves 100% reduction within 90 min of visible light irradiation. In same time NTU-9 and NH2MIL-125 could exhibit only 35 and 55% reduction respectively. This study may give new direction to the study of hetero MOFs for environmental application.
1. Introduction Heavy metal ions namely lead, cadmium, mercury, arsenic and chromium have emerged as major anthropogenic pollutant contaminating surface and underground water [1]. Chromium is extensively used for leather tanning, electroplating, dying purpose [2,3]. In water, chromium is commonly found in (VI) and (III) oxidation states. However, wastewater discharged from the industries contain Cr (VI) ion in considerably higher level. As per the world health organization (WHO) guidelines Cr (VI) concentration in the drinking water must be less than 50 ppb. Beyond this permissible limit, it leads to severe diseases such as diarrhea, skin and kidney cancer due to its high solubility in water [4]. In comparison Cr (III) is essential nutrient and seems to be non-toxic and less soluble in water. Therefore variety of biological and chemical methods has been tried for the reductive removal of Cr (VI) into Cr (III). Bacterial reduction involving Bacillus sp., Pseudomonas sp., Acinetobactor sp., Streptomyces sp. has shown promising activity albeit at neutral and alkaline conditions [5–9]. The conventional chemical reduction involves usage of reducing
⁎
salts of iron and sulphur, however, sludge formation and its subsequent disposal is tedious and cost intensive. In retrospect, heterogeneous photocatalytic reduction involving semiconducting oxides, sulphides phosphates and nitrides is highly efficient, time saving and cost effective process [4,10]. However, large band gap, rapid electron-hole recombination and low surface area drastically reduce their activity in the visible range of the solar spectrum. Metal organic framework (MOFs) a three dimensional porous material consisting of metal-oxo clusters attached through the organic linkers has received tremendous attention and find application in gas storage, separation, drug delivery and catalysis owing to the excellent property like large surface area and porous nature [11]. By selecting appropriate metal ion and linker, MOFs property can be tuned to provide catalytic centre for carrying out the targeted reaction. It may also facilitate the transfer of photo generated charge carriers for the photocatalytic application. MOFs in the class of MIL-125 [12], MIL-68 [13], UiO-66 [14] have been studied for the photocatalytic reduction of Cr (VI). Shen et al. studied water stable UiO-66-X a Zr based MOF with different substituent X (H, NH2, NO2, and Br) on terepthalic acid. UiO-66-NH2 exhibited the maximum
Corresponding author. E-mail address: [email protected] (N. Aman).
https://doi.org/10.1016/j.jece.2019.103240 Received 21 April 2019; Received in revised form 25 June 2019; Accepted 25 June 2019 Available online 27 June 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103240
E. Dhivya, et al.
eV). C 1s peak at 285.0 eV was used as the reference peak to find the binding energy. UV–vis diffuse reflectance spectra were recorded on the Shimadzu UV-2600 spectrophotometer using BaSO4 as the reference material. Photoluminescence spectra were obtained using Agilant Carry Eclipse fluorescence spectrometer.
activity for the Cr (VI) reduction as amino substituent increase the HOMO level thereby increasing the photon absorption [15]. MIL-68(In) also exhibited similar trend with band gap decreased from 3.94 eV to 2.79 eV by substituting H with NH2 group [16]. MIL-125 (Ti) and NH2 functionalized MIL-125 (Ti) synthesized by Wang et al. also exhibited the similar trend for the Cr (VI) reduction [12]. Conduction band level of titanium-oxo cluster is considered to be more positive than that of zirconium or indium oxo clusters, therefore titanium based MOFs are expected to be better photocatalyst for the Cr (VI) reduction. In recent years heterostructure photocatalytic system involving MOF and semiconductor BiOBr/NH2-MIL-125 (Ti) [17], Bi2S3/NH2MIL-125 (Ti) [18], ZnIn2S4/ NH2-MIL-125 (Ti) [19], g-C3N4/NH2-MIL125 (Ti) [20] has displayed superior activity than the individual photocatalyst because of more efficient charge separation and extended absorption in the solar spectrum. However, heterostructure photocatalytic system involving two MOFs has not been reported till date. Integrating NTU-9 titanium based p-type semiconducting MOF with ntype NH2-MIL-125 is expected to increase the charge separation [21]. Considering the potential advantage of heterostructure MOFs, in this paper we synthesized heterostructure system involving NH2-MIL-125 (Ti) and NTU-9 (Ti) for the Cr (VI) reduction under visible light irradiation.
2.4. Photocatalytic activity Activity of the as-synthesized MOFs was analyzed by studying the photocatalytic reduction of Cr (VI) to Cr (III). 200 ml of Cr (VI) solution of different concentration was prepared using ultrapure water and then transferred to the 250 ml double wall quartz reactor. 0.2 g of photocatalyst powder was suspended into the solution and then solution was stirred in the dark for half an hour to achieve adsorption equilibrium. 250 W high pressure mercury lamp was used as the light source. UV radiation was cut by supplying 2 M aqueous sodium nitrite solution through the inner jacket. 3 ml of reaction mixture was periodically withdrawn from the reaction vessel and centrifuged for the Cr (VI) analysis. Cr (VI) to Cr (III) reduction was monitored by taking UV–vis spectrum at 540 nm using 1,5-diphenylcarbazide as the colorimetric agent [10]. 3. Results and discussions
2. Materials and methods
Powder XRD pattern of NTU-9, NH2-MIL-125 and HMF is presented in Fig. 1. XRD pattern of NTU-9 and NH2-MIL-125 matches well with the previous reports confirming the crystallinity and framework structure [12,21]. HMF exhibits the most intense peak for both NH2-MIL-125 and NTU-9, thereby confirming the successful formation of heterostructure system. Peaks for rutile or anatase couldn’t be detected, confirming the absence of bulk accumulation of TiO2 in the framework. Fig. 2 shows the TGA-DTG of all three MOFs. A loss of 20% in weight is observed for NTU-9 between 40–110 °C because of the loss of solvents. NH2-MIL-125 and HMF revealed 30% loss in the same temperature range. NTU-9 exhibits another loss of 45% in the range of 250–350 °C indicating the disintegration of the framework structure [21]. In comparison, NH2-MIL-125 framework disintegration is completed at 470 °C. HMF in comparison exhibits loss of framework in two steps. Around 35% loss is observed in the temperature range of 350–460 °C and another loss of 10% is observed in the range of 460–520 °C. Higher thermal stability of HMF clearly indicates the potential of integrated MOFs for high temperature applications. FE-SEM image of NTU-9 shows agglomeration of the smaller particles (Fig. S1). NH2-MIL-125
2.1. Materials 2-aminoterephthalicacid, 2,5-dihydroxyterepthalicacid (H4DOBDC) and titanium (IV) isopropoxide were purchased from Sigma Aldrich. N,N-Dimethylformamide (DMF), methanol (CH3OH), and potassium dichromate (K2Cr2O7) were obtained from Merck chemicals. All reagents were of analytical grade and used without any further purification. Ultrapure water was used throughout the experiment (MilliQ). Cr (VI) solution was collected from the nearby electroplating unit and was subsequently diluted as per the reaction requirement. 2.2. Synthesis of MOFs Synthesis of Metal organic framework Composite is possible by integrating two different MOFs. NH2-MIL-125 and NTU-9 were synthesized separately by the method adopted from the literature [12,21]. For the synthesis of hetero-MOF, NTU-9 was suspended in a mixture of DMF (56 ml) and methanol (14 ml) and ultrasonicated for 30 min to achieve maximum miscibility. After that 2-aminoterepthalic acid (1.7645 g) was added to the mixture, and then transferred to a three-necked round bottom flask with a condenser and heated at 100 °C. Subsequently titanium (IV) isopropoxide was added and stirred for 72 h at same temperature. The resultant product was filtered, washed with DMF and methanol for about three to four times at room temperature and dried under vacuum for further use. Hetero-MOF NTU-9/NH2-MIL-125 is represented as HMF. 2.3. Characterization XRD patterns of as-synthesized MOFs were recorded on a Bruker D8 Advance powder X-ray diffractometer using Cu Kα radiation. Thermogravimetric analysis (TGA and DTG) was carried out using Q500 Hi-Res Analyzer in the range of 25 to 700 °C. NOVA 4000e (Quantachrome, USA) was used for N2 adsorption desorption studies. BET surface area was measured by considering the adsorption point in the range of P/P0 0.05 to 0.3. Pore volume and pore radius was calculated based on the BJH method using desorption points. FE-SEM of the samples were taken on a Carl Zeiss AG Supra 40 instrument. FT-IR was taken from the JASCO-6300 instrument using ATR mode. X-ray photoelectron spectra (XPS) were recorded by UHV analysis system (SPECS, Germany) with an Al-Kα twin anode X-ray source (E = 1486.6
Fig. 1. XRD Pattern of NH2-MIL-125, NTU-9 and NTU-9/NH2-MIL125 (HMF). 2
Journal of Environmental Chemical Engineering 7 (2019) 103240
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Fig. 3. N2 adsorption- desorption isotherm of the MOFs and inset their corresponding pore volume and surface area respectively.
Fig. 2. (a) TGA and (b) DTG curve of NTU-9, NH2-MIL125 and NTU-9/NH2MIL125 (HMF).
exhibits the well distinguished particles of size of 500 nm with spherical and cuboids shaped morphology. HMF also exhibits agglomerated particles indicating the integration of both the MOFs. Fig.3 shows the N2 adsorption-desorption isotherm of the MOFs. All the samples present a sharp increase in the isotherm at very low pressure, a characteristic of microporous materials. Hysteresis loop 0.4 < P/P0 < 1.0 and 0.6 < P/P0 < 1.0 is observed for NTU-9 and NH2-MIL-125 respectively, indicating the presence of mesopores with ink-bottle opening. Presence of thin hysteresis loop for HMF indicates the deposition of NH2-MIL-125 over the NTU-9, resulting in decreased mesoporosity. BET surface area of NTU-9, NH2-MIL-125 and HMF was found to be 986 m²/g, 1267 m²/g and 550 m²/g respectively. The corresponding pore volume was 0.14, 0.06 and 0.19 cc/g respectively. Porous nature of the MOFs facilitates the adsorption of more metal ions. However, for the photocatalytic electron transfer, it is also important to have the direct contact between the metal ions and the catalyst surface. The FT-IR spectra of NTU-9 presented in Fig. 4 shows a strong absorption at 3343 cm−1 indicating the presence of hydroxyl group (−OH). The vibration bands observed at 1434 and 400-800 cm-1 for the carboxylic acid group and Ti-O-Ti-O confirm the Ti-coordinated MOF Structure of NTU-9 [21]. Whereas for NH2-MIL-125, absorption band at 1530 and 1390 cm−1 can be assigned to carbonyl asymmetric stretching and symmetric stretching vibrations respectively [18]. Band at 1250 cm−1 belongs to the CeH symmetric stretching vibration of the
Fig. 4. FT-IR spectra of NTU-9, NH2-MIL-125 and NTU-9/NH2-MIL-125 (HMF).
benzene ring. NH2 shows a strong band at 3381 cm−1. FT-IR of HMF shows the presence of all the functionalities present in both the contributing MOFs. The UV–vis (DRS) of NTU-9, NH2-MIL-125 and HMF is presented in Fig. 5. NTU-9 shows two absorption edges one at 325 nm primarily due to charge transfer from O to Ti in the T-oxo cluster. Another absorption edges around 1000 nm due to electron transfer from linker to Ti. NH2-MIL-125 also exhibits two absorption edges, with first one at 325 nm due to electron transfer from O to Ti. Second absorption edges at 490 nm indicating the electron transfer from 2-aminoterepthalate linker to Ti. Similar trend is followed by HMF showing absorption up to 550 nm lying in between the contributing MOFs. The band gap energy (Eg) values calculated from the tauc plot for NTU-9, NH2-MIL-125 and HMF is found to 1.29 eV, 2.54 eV and 2.22 eV respectively. HMF seems to absorb more photon than the NH2-MIL-125 which may help in enhancing the photocatalytic efficiency. Chemical composition and local electronic structure of HMF analyzed by XPS shows characteristic peak for four elements C, Ti, O and N (Fig. 6). C1 s spectrum of HMF shows peak at 284.8 and 289.3 eV corresponding to 3
Journal of Environmental Chemical Engineering 7 (2019) 103240
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Fig. 5. (a) UV–vis (DRS) absorption spectrum of MOFs and corresponding (b) Tauc plot.
NH2-MIL-125 framework structure [12]. Photocatalytic reduction of hexavalent chromium is studied using visible light absorbing as-synthesized NTU-9, NH2-MIL-125 and HMF (Fig. 7). 10 ppm of Cr (VI) solution at pH 5 is used to compare the activity of MOFs without adding any hole scavengers and adjusting the pH. After 1 h of continuous stirring under dark, negligible amount of Cr
C]C/CeC and > C]O/−COOH group present in the organic linkers [22]. Symmetric peaks for Ti2p3/2 and Ti2p1/2 is observed at 459.1 and 464.7 eV respectively, indicating the presence of Ti4+ in the framework. Presence of O1 s peak at 529.5 and 531 eV confirms the presence of OTi-O and −OH groups. N1 s peak due to –NH2 group is observed at 399.7 eV confirming the presence of aromatic amine functionality in
Fig. 6. XPS spectra of C1 s, Ti2p, O1 s and N1 s for NTU-9/NH2-MIL-125 (HMF). 4
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Fig. 7. (a) Photocatalytic reduction of Cr (VI) at pH 3, 5 and 8 using NTU-9, NH2-MIL-125 and NTU-9/NH2-MIL-125 (HMF).
(VI) is deposited over the MOFs. After 90 min of visible light irradiation, NTU-9 could achieve only 30% reduction, whereas, NH2-MIL-125 under same reaction condition is able to reduce 42% of the Cr (VI). In comparison, HMF is able to reduce 70% of the Cr (VI). Higher activity of HMF indicates the successful integration of both the contributing MOFs. This is further confirmed by the PL spectra of MOFs, which clearly suggest the more efficient charge separation by HMF (Fig. S2). Photoluminescence intensity of NTU-9 shows maximum emission followed by NH2-MIL-125. HMF exhibits the least intensity accounting for decreased photon-induced electron-hole pair recombination. As the photocatalytic reduction of metal ions is considerably influenced by the solution pH, reduction of Cr (VI) is analyzed under acidic condition (pH 3), normal condition (pH 5) and alkaline condition (pH 8). Reduction rate of Cr (VI) is found to follow the order as pH 3 > pH 8 > pH 5. 10 ppm of Cr (VI) is completely reduced by HMF within 90 min interval under pH 3. Whereas, only 70% and 80% reduction is possible at pH 5 and 8 respectively, using HMF. Higher activity at pH 3 may be due the electrostatic interaction between negatively charged Cr2O72− ion and the positively charged Ti-oxo cluster on the HMF. NH2-MIL-125 follows the same trend achieving 55%, 42% and 50% removal at pH of 3, 5 and 8 respectively. This finding is in line with the previous report indicating acidic condition favors for the Cr (VI) removal [12]. NTU-9 is found to be least active under acidic, normal and alkaline conditions, confirming the role of charge separation for the efficient transfer of photogenerated electrons for Cr (VI) reduction. Further, NTU-9 conduction band is nearer to Cr (VI)/Cr (III) redox potential in comparasion to NH2-MIL-125. It is imperative to study the
Fig. 9. Schematic illustration of HMF for the photocatalytic reduction of Cr (VI).
effect of concentration of Cr (VI) to find the rate kinetics. Fig. 8 present the effect of Cr (VI) concentration on the rate of reaction using HMF. As the concentration is increased from 5 to 20 ppm, the rate of reaction decrease, which seems to follow Langmuir-Hinshelwood model with first order kinetics. Based on the above findings and previous reports a possible mechanism for the superior activity of HMF is illustrated in Fig. 9 [12,21]. NH2-MIL-125 with a band gap of 2.54 eV is excited by the visible light. The photogenerated electrons are used for the Cr (VI) reduction. In mean time NTU-9 absorbing till 1000 nm transfer the photogenerated electrons from its conduction band to the empty valence band of NH2MIL-125 thereby reducing the electron-hole recombinaion. This results in more efficient reduction of Cr (VI) to Cr (III). Possible reaction is as follows NH2-MIL-125 + NTU-9 → HMF HMF + hυ → h
+
+e
−
(1) (2)
Fig. 8. (a) Effect of concentration of Cr (VI) on rate of reaction using HMF at pH 3 and (b) Fitted curve between 1/r0 and 1/C0 for 5 ppm solution of Cr (VI) using HMF at pH 3. 5
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Cr2O72−+ 14H+ + 6e- → 2Cr3+ + 7H2O
(3)
[5] X. Pan, Z. Liu, Z. Chen, Y. Cheng, D. Pan, J. Shao, Z. Lin, X. Guan, Investigation of Cr (VI) reduction and Cr (III) immobilization mechanism by planktonic cells and biofilms of Bacillus subtilis ATCC-6633, Water Res. 55 (2014) 21–29. [6] C. Kang, P. Wu, L. Li, L. Yu, B. Ruan, B. Gong, N. Zhu, Cr (VI) reduction and Cr (III) immobilization by resting cells of Pseudomonas aeruginosa CCTCC AB93066: spectroscopic, microscopic, and mass balance analysis, Environ. Sci. Pollut. Res. 24 (2017) 5949–5963. [7] H.-K. Zhang, H. Lu, J. Wang, J.-T. Zhou, M. Sui, Cr(VI) reduction and Cr(III) immobilization by Acinetobacter sp. HK-1 with the assistance of a novel Quinone/ Graphene oxide composite, Environ. Sci. Technol. 48 (2014) 12876–12885. [8] A. Essahale, M. Malki, I. Marin, M. Moumni, Hexavalent chromium reduction and accumulation by Acinetobacter ab1 isolated from fez tanneries in Morocco, Indian J. Microbiol. 52 (2012) 48–53. [9] D.K. Morales, W. Ocampo, M.M. Zambrano, Efficient removal of hexavalent chromium by a tolerant Streptomyces sp. Affected by the toxic effect of metal exposure, J. Appl. Microbiol. 103 (2007) 2704–2712. [10] D. Magadevan, E. Dhivya, N.D.A. Mundari, T. Mishra, N. Aman, Development of novel TiO2-Cu2(OH)PO4 heterojunction as nanophotocatalyst for improved Cr (VI) reduction, J. Environ. Chem. Eng. 7 (2019) 102968. [11] T. Zhang, W. Lin, Metal–organic frameworks for artificial photosynthesis and photocatalysis, Chem. Soc. Rev. 43 (2014) 5982–5993. [12] H. Wang, X.Z. Yuan, Y. Wu, G. Zeng, X. Chen, L. Leng, Z. Wu, L. Jiang, H. Li, Facile synthesis of amino-functionalized titanium metal-organic frameworks and their superior visible-light photocatalytic activity for Cr (VI) reduction, J. Hazard. Mater. 286 (2015) 187–194. [13] F. Jing, R. Liang, J. Xiong, R. Chen, S. Zhang, Y. Li, L. Wu, MIL-68(Fe) as an efficient visible-light-driven photocatalyst for the treatment of a simulated waste-water contain Cr(VI) and Malachite Green, Appl. Catal. B: Environ. 206 (2017) 9–15. [14] X.-D. Du, X.-H. Yi, P. Wang, W. Zheng, J. Dengc, C.-C. Wang, Robust photocatalytic reduction of Cr(VI) on UiO-66-NH2(Zr/Hf) metal organic framework membrane under sunlight irradiation, Chem. Eng. J. 356 (2019) 393–399. [15] L. Shen, R. Liang, M. Luo, F. Jing, L. Wu, Electronic effects of ligand substitution on metal–organic framework photocatalysts: the case study of UiO-66, Phys. Chem. Chem. Phys. 17 (2015) 117–121. [16] R. Liang, R. Huang, X. Wang, S. Ying, G. Yan, L. Wu, Functionalized MIL-68(In) for the photocatalytic treatment of Cr(VI)-containing simulation wastewater: electronic effects of ligand substitution, Appl. Surf. Sci. 464 (2019) 396–403. [17] S.-R. Zhu, P.-F. Liu, M.-K. Wu, G.-C. Li, K. Tao, F.-Y. Yi, L. Han, Enhanced photocatalytic performance of BiOBr/NH2-MIL-125(Ti) composite for dye degradation under visible light, Dalton Trans. 43 (2016) 17521–17529. [18] M. Wang, L. Yang, J. Yuan, L. He, Y. Song, H. Zhang, Z. Zhang, S. Fang, Heterostructured Bi2S3@NH2-MIL-125(Ti) nanocomposite as a bifunctional photocatalyst for Cr (VI) reduction and rhodamine B degradation under visible light, RSC Adv. 8 (2018) 12459–12470. [19] H. Liu, J. Zhang, D. Ao, Construction of heterostructured ZnIn2S4@NH2-MIL125(Ti) nanocomposites for visible-light-driven H2 production, Appl. Catal. B: Environ. 221 (2018) 433–442. [20] J. Xu, J. Gao, C. Wang, Y. Yang, L. Wang, NH2-MIL-125(Ti)/graphitic carbon nitride heterostructure decorated with NiPd co-catalysts for efficient photocatalytic hydrogen production, Appl. Catal. B: Environ. 219 (2017) 101–108. [21] J. Gao, J. Miao, P.-Z. Li, W.Y. Teng, L. Yang, Y. Zhao, B. Liu, Q. Zhang, A p-type Ti (IV) based metal organic framework with visible light response, Chem. Commun. (Camb.) 50 (2014) 3786–3788. [22] Y.N. Singhbabu, K.K. Sahu, D. Dadhich, A.K. Pramanick, T. Mishra, R.K. Sahu, Capsule-embedded reduced graphene oxide: synthesis, mechanism and electrical properties, J. Mater. Chem. C Mater. Opt. Electron. Devices 1 (2013) 958–966. [23] A. Sepehri, M.-H. Sarrafzadeh, Effect of nitrifiers community on fouling mitigation and nitrification efficiency in a membrane bioreactor, Chem. Eng. Process. Process Intensif. 128 (2018) 10–18.
Efficient removal of Cr (VI) by the heterojunction MOF HMF without using sacrifical hole scavenger make it more viable option for real time application in wastewater treatment. Use of heterojunction MOFs in the memebrane bioreactors has the potential to increase its lifetime by removing the soluble microbial produts [23]. 4. Conclusions NH2-MIL-125 and NTU-9 was successfully integrated to make a hetero MOF (HMF). XRD confirms the framework structure of the assynthesized MOFs. HMF shows better thermal stability in comparison to NTU-9. All the synthesized MOFs show high surface area with micropores and mesopores. HMF shows the absorption till 563 nm corresponding to 2.2 eV. HMF is found to have better photocatlytic activity for Cr (VI) reduction in comparison to both the contributing MOFs. Acidic condition favors the Cr (VI) reduction and 100% reduction is achieved within 90 min of visible light irradiation. In same time NTU-9 and NH2-MIL-125 could achieve only 35 and 55% reduction respectively. Acknowledgements NA acknowledges DST-SERB for providing start up grant under young scientist scheme (Project No: SB/FT/CS-029/2014). Supplementary data Please see the supplementary data for the FE-SEM, PL spectra of assynthesized MOFs and image of the reactants before and after the reaction. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2019.103240. References [1] P.B. Tchounwou, C.G. Yedjou, A.K. Patlolla, D.J. Sutton, Heavy metal toxicity and the environment, Molecular, Clinical and Environmental Toxicology (2012) 133–164. [2] M. Feng, P. Zhang, H.-C. Zhou, V.K. Sharma, Water-stable metal-organic frameworks for aqueous removal of heavy metals and radionuclides: a review, Chemosphere 209 (2018) 783–800. [3] P.A. Kobielska, A.J. Howarth, O.K. Farha, S. Nayak, Metal–organic frameworks for heavy metal removal from water, Coord. Chem. Rev. 358 (2018) 92–107. [4] C.C. Wang, X.D. Du, J. Li, X.X. Guo, P. Wang, J. Zhang, Photocatalytic Cr (VI) reduction in metal–organic framework: a mini-review, Appl. Catal. B: Environ. 193 (2016) 198–216.
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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Synthesis of titanium based hetero MOF photocatalyst for reduction of Cr (VI) from wastewater Egambaram Dhivyaa, Deviga Magadevana, Yasam Palgunab, Trilochan Mishrac, Noor Amana,
T
⁎
a
Department of Chemistry, B.S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, 600048, India Department of Material Science and Metallurgical Engineering, IIT-Hyderabad, Hyderabad, 502285, India c Functional Material Group, Advance Material & Process Division, CSIR-National Metallurgical Laboratory, Jamshedpur, 831007, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cr (VI) removal Metal organic framework Photocatalyst Visible light Charge separation
Cr (VI) is one of the well-known toxic contaminant coming from leather tanning, metal finishing, textile industries etc. As per world health organization standard, limit for Cr (VI) in drinking water is 0.05 mg/l. In last few years, Metal organic framework (MOFs) based photocatalyst has find great interest for environmental remediation. In this work, titanium based MOFs NH2-MIL-125 and NTU-9 was integrated to make a hetero MOF NTU-9/NH2-MIL-125 (HMF) via refluxing method. It was characterized by XRD, TGA, FE-SEM, FT-IR, XPS, UV–vis (DRS) and PL etc. XRD indicates crystallinity and framework nature of all the synthesized MOFs. Thermal stability of HMF is improved to 520 °C in comparison to NTU-9. Diffuse reflectance spectra exhibit HMF having band gap of 2.2 eV absorbing up to 563 nm. BET surface area of NTU-9, NH2-MIL-125 and HMF were found to be 986, 1267 and 550 m²/g respectively indicating their microporous nature. In order to compare the photocatalytic activity, Cr (VI) solution was chosen a model wastewater. Because of more efficient charge separation, HMF is found to show better activity in comparison to both the contributing MOFs. Acidic condition favors the Cr (VI) reduction and HMF achieves 100% reduction within 90 min of visible light irradiation. In same time NTU-9 and NH2MIL-125 could exhibit only 35 and 55% reduction respectively. This study may give new direction to the study of hetero MOFs for environmental application.
1. Introduction Heavy metal ions namely lead, cadmium, mercury, arsenic and chromium have emerged as major anthropogenic pollutant contaminating surface and underground water [1]. Chromium is extensively used for leather tanning, electroplating, dying purpose [2,3]. In water, chromium is commonly found in (VI) and (III) oxidation states. However, wastewater discharged from the industries contain Cr (VI) ion in considerably higher level. As per the world health organization (WHO) guidelines Cr (VI) concentration in the drinking water must be less than 50 ppb. Beyond this permissible limit, it leads to severe diseases such as diarrhea, skin and kidney cancer due to its high solubility in water [4]. In comparison Cr (III) is essential nutrient and seems to be non-toxic and less soluble in water. Therefore variety of biological and chemical methods has been tried for the reductive removal of Cr (VI) into Cr (III). Bacterial reduction involving Bacillus sp., Pseudomonas sp., Acinetobactor sp., Streptomyces sp. has shown promising activity albeit at neutral and alkaline conditions [5–9]. The conventional chemical reduction involves usage of reducing
⁎
salts of iron and sulphur, however, sludge formation and its subsequent disposal is tedious and cost intensive. In retrospect, heterogeneous photocatalytic reduction involving semiconducting oxides, sulphides phosphates and nitrides is highly efficient, time saving and cost effective process [4,10]. However, large band gap, rapid electron-hole recombination and low surface area drastically reduce their activity in the visible range of the solar spectrum. Metal organic framework (MOFs) a three dimensional porous material consisting of metal-oxo clusters attached through the organic linkers has received tremendous attention and find application in gas storage, separation, drug delivery and catalysis owing to the excellent property like large surface area and porous nature [11]. By selecting appropriate metal ion and linker, MOFs property can be tuned to provide catalytic centre for carrying out the targeted reaction. It may also facilitate the transfer of photo generated charge carriers for the photocatalytic application. MOFs in the class of MIL-125 [12], MIL-68 [13], UiO-66 [14] have been studied for the photocatalytic reduction of Cr (VI). Shen et al. studied water stable UiO-66-X a Zr based MOF with different substituent X (H, NH2, NO2, and Br) on terepthalic acid. UiO-66-NH2 exhibited the maximum
Corresponding author. E-mail address: [email protected] (N. Aman).
https://doi.org/10.1016/j.jece.2019.103240 Received 21 April 2019; Received in revised form 25 June 2019; Accepted 25 June 2019 Available online 27 June 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103240
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eV). C 1s peak at 285.0 eV was used as the reference peak to find the binding energy. UV–vis diffuse reflectance spectra were recorded on the Shimadzu UV-2600 spectrophotometer using BaSO4 as the reference material. Photoluminescence spectra were obtained using Agilant Carry Eclipse fluorescence spectrometer.
activity for the Cr (VI) reduction as amino substituent increase the HOMO level thereby increasing the photon absorption [15]. MIL-68(In) also exhibited similar trend with band gap decreased from 3.94 eV to 2.79 eV by substituting H with NH2 group [16]. MIL-125 (Ti) and NH2 functionalized MIL-125 (Ti) synthesized by Wang et al. also exhibited the similar trend for the Cr (VI) reduction [12]. Conduction band level of titanium-oxo cluster is considered to be more positive than that of zirconium or indium oxo clusters, therefore titanium based MOFs are expected to be better photocatalyst for the Cr (VI) reduction. In recent years heterostructure photocatalytic system involving MOF and semiconductor BiOBr/NH2-MIL-125 (Ti) [17], Bi2S3/NH2MIL-125 (Ti) [18], ZnIn2S4/ NH2-MIL-125 (Ti) [19], g-C3N4/NH2-MIL125 (Ti) [20] has displayed superior activity than the individual photocatalyst because of more efficient charge separation and extended absorption in the solar spectrum. However, heterostructure photocatalytic system involving two MOFs has not been reported till date. Integrating NTU-9 titanium based p-type semiconducting MOF with ntype NH2-MIL-125 is expected to increase the charge separation [21]. Considering the potential advantage of heterostructure MOFs, in this paper we synthesized heterostructure system involving NH2-MIL-125 (Ti) and NTU-9 (Ti) for the Cr (VI) reduction under visible light irradiation.
2.4. Photocatalytic activity Activity of the as-synthesized MOFs was analyzed by studying the photocatalytic reduction of Cr (VI) to Cr (III). 200 ml of Cr (VI) solution of different concentration was prepared using ultrapure water and then transferred to the 250 ml double wall quartz reactor. 0.2 g of photocatalyst powder was suspended into the solution and then solution was stirred in the dark for half an hour to achieve adsorption equilibrium. 250 W high pressure mercury lamp was used as the light source. UV radiation was cut by supplying 2 M aqueous sodium nitrite solution through the inner jacket. 3 ml of reaction mixture was periodically withdrawn from the reaction vessel and centrifuged for the Cr (VI) analysis. Cr (VI) to Cr (III) reduction was monitored by taking UV–vis spectrum at 540 nm using 1,5-diphenylcarbazide as the colorimetric agent [10]. 3. Results and discussions
2. Materials and methods
Powder XRD pattern of NTU-9, NH2-MIL-125 and HMF is presented in Fig. 1. XRD pattern of NTU-9 and NH2-MIL-125 matches well with the previous reports confirming the crystallinity and framework structure [12,21]. HMF exhibits the most intense peak for both NH2-MIL-125 and NTU-9, thereby confirming the successful formation of heterostructure system. Peaks for rutile or anatase couldn’t be detected, confirming the absence of bulk accumulation of TiO2 in the framework. Fig. 2 shows the TGA-DTG of all three MOFs. A loss of 20% in weight is observed for NTU-9 between 40–110 °C because of the loss of solvents. NH2-MIL-125 and HMF revealed 30% loss in the same temperature range. NTU-9 exhibits another loss of 45% in the range of 250–350 °C indicating the disintegration of the framework structure [21]. In comparison, NH2-MIL-125 framework disintegration is completed at 470 °C. HMF in comparison exhibits loss of framework in two steps. Around 35% loss is observed in the temperature range of 350–460 °C and another loss of 10% is observed in the range of 460–520 °C. Higher thermal stability of HMF clearly indicates the potential of integrated MOFs for high temperature applications. FE-SEM image of NTU-9 shows agglomeration of the smaller particles (Fig. S1). NH2-MIL-125
2.1. Materials 2-aminoterephthalicacid, 2,5-dihydroxyterepthalicacid (H4DOBDC) and titanium (IV) isopropoxide were purchased from Sigma Aldrich. N,N-Dimethylformamide (DMF), methanol (CH3OH), and potassium dichromate (K2Cr2O7) were obtained from Merck chemicals. All reagents were of analytical grade and used without any further purification. Ultrapure water was used throughout the experiment (MilliQ). Cr (VI) solution was collected from the nearby electroplating unit and was subsequently diluted as per the reaction requirement. 2.2. Synthesis of MOFs Synthesis of Metal organic framework Composite is possible by integrating two different MOFs. NH2-MIL-125 and NTU-9 were synthesized separately by the method adopted from the literature [12,21]. For the synthesis of hetero-MOF, NTU-9 was suspended in a mixture of DMF (56 ml) and methanol (14 ml) and ultrasonicated for 30 min to achieve maximum miscibility. After that 2-aminoterepthalic acid (1.7645 g) was added to the mixture, and then transferred to a three-necked round bottom flask with a condenser and heated at 100 °C. Subsequently titanium (IV) isopropoxide was added and stirred for 72 h at same temperature. The resultant product was filtered, washed with DMF and methanol for about three to four times at room temperature and dried under vacuum for further use. Hetero-MOF NTU-9/NH2-MIL-125 is represented as HMF. 2.3. Characterization XRD patterns of as-synthesized MOFs were recorded on a Bruker D8 Advance powder X-ray diffractometer using Cu Kα radiation. Thermogravimetric analysis (TGA and DTG) was carried out using Q500 Hi-Res Analyzer in the range of 25 to 700 °C. NOVA 4000e (Quantachrome, USA) was used for N2 adsorption desorption studies. BET surface area was measured by considering the adsorption point in the range of P/P0 0.05 to 0.3. Pore volume and pore radius was calculated based on the BJH method using desorption points. FE-SEM of the samples were taken on a Carl Zeiss AG Supra 40 instrument. FT-IR was taken from the JASCO-6300 instrument using ATR mode. X-ray photoelectron spectra (XPS) were recorded by UHV analysis system (SPECS, Germany) with an Al-Kα twin anode X-ray source (E = 1486.6
Fig. 1. XRD Pattern of NH2-MIL-125, NTU-9 and NTU-9/NH2-MIL125 (HMF). 2
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Fig. 3. N2 adsorption- desorption isotherm of the MOFs and inset their corresponding pore volume and surface area respectively.
Fig. 2. (a) TGA and (b) DTG curve of NTU-9, NH2-MIL125 and NTU-9/NH2MIL125 (HMF).
exhibits the well distinguished particles of size of 500 nm with spherical and cuboids shaped morphology. HMF also exhibits agglomerated particles indicating the integration of both the MOFs. Fig.3 shows the N2 adsorption-desorption isotherm of the MOFs. All the samples present a sharp increase in the isotherm at very low pressure, a characteristic of microporous materials. Hysteresis loop 0.4 < P/P0 < 1.0 and 0.6 < P/P0 < 1.0 is observed for NTU-9 and NH2-MIL-125 respectively, indicating the presence of mesopores with ink-bottle opening. Presence of thin hysteresis loop for HMF indicates the deposition of NH2-MIL-125 over the NTU-9, resulting in decreased mesoporosity. BET surface area of NTU-9, NH2-MIL-125 and HMF was found to be 986 m²/g, 1267 m²/g and 550 m²/g respectively. The corresponding pore volume was 0.14, 0.06 and 0.19 cc/g respectively. Porous nature of the MOFs facilitates the adsorption of more metal ions. However, for the photocatalytic electron transfer, it is also important to have the direct contact between the metal ions and the catalyst surface. The FT-IR spectra of NTU-9 presented in Fig. 4 shows a strong absorption at 3343 cm−1 indicating the presence of hydroxyl group (−OH). The vibration bands observed at 1434 and 400-800 cm-1 for the carboxylic acid group and Ti-O-Ti-O confirm the Ti-coordinated MOF Structure of NTU-9 [21]. Whereas for NH2-MIL-125, absorption band at 1530 and 1390 cm−1 can be assigned to carbonyl asymmetric stretching and symmetric stretching vibrations respectively [18]. Band at 1250 cm−1 belongs to the CeH symmetric stretching vibration of the
Fig. 4. FT-IR spectra of NTU-9, NH2-MIL-125 and NTU-9/NH2-MIL-125 (HMF).
benzene ring. NH2 shows a strong band at 3381 cm−1. FT-IR of HMF shows the presence of all the functionalities present in both the contributing MOFs. The UV–vis (DRS) of NTU-9, NH2-MIL-125 and HMF is presented in Fig. 5. NTU-9 shows two absorption edges one at 325 nm primarily due to charge transfer from O to Ti in the T-oxo cluster. Another absorption edges around 1000 nm due to electron transfer from linker to Ti. NH2-MIL-125 also exhibits two absorption edges, with first one at 325 nm due to electron transfer from O to Ti. Second absorption edges at 490 nm indicating the electron transfer from 2-aminoterepthalate linker to Ti. Similar trend is followed by HMF showing absorption up to 550 nm lying in between the contributing MOFs. The band gap energy (Eg) values calculated from the tauc plot for NTU-9, NH2-MIL-125 and HMF is found to 1.29 eV, 2.54 eV and 2.22 eV respectively. HMF seems to absorb more photon than the NH2-MIL-125 which may help in enhancing the photocatalytic efficiency. Chemical composition and local electronic structure of HMF analyzed by XPS shows characteristic peak for four elements C, Ti, O and N (Fig. 6). C1 s spectrum of HMF shows peak at 284.8 and 289.3 eV corresponding to 3
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Fig. 5. (a) UV–vis (DRS) absorption spectrum of MOFs and corresponding (b) Tauc plot.
NH2-MIL-125 framework structure [12]. Photocatalytic reduction of hexavalent chromium is studied using visible light absorbing as-synthesized NTU-9, NH2-MIL-125 and HMF (Fig. 7). 10 ppm of Cr (VI) solution at pH 5 is used to compare the activity of MOFs without adding any hole scavengers and adjusting the pH. After 1 h of continuous stirring under dark, negligible amount of Cr
C]C/CeC and > C]O/−COOH group present in the organic linkers [22]. Symmetric peaks for Ti2p3/2 and Ti2p1/2 is observed at 459.1 and 464.7 eV respectively, indicating the presence of Ti4+ in the framework. Presence of O1 s peak at 529.5 and 531 eV confirms the presence of OTi-O and −OH groups. N1 s peak due to –NH2 group is observed at 399.7 eV confirming the presence of aromatic amine functionality in
Fig. 6. XPS spectra of C1 s, Ti2p, O1 s and N1 s for NTU-9/NH2-MIL-125 (HMF). 4
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Fig. 7. (a) Photocatalytic reduction of Cr (VI) at pH 3, 5 and 8 using NTU-9, NH2-MIL-125 and NTU-9/NH2-MIL-125 (HMF).
(VI) is deposited over the MOFs. After 90 min of visible light irradiation, NTU-9 could achieve only 30% reduction, whereas, NH2-MIL-125 under same reaction condition is able to reduce 42% of the Cr (VI). In comparison, HMF is able to reduce 70% of the Cr (VI). Higher activity of HMF indicates the successful integration of both the contributing MOFs. This is further confirmed by the PL spectra of MOFs, which clearly suggest the more efficient charge separation by HMF (Fig. S2). Photoluminescence intensity of NTU-9 shows maximum emission followed by NH2-MIL-125. HMF exhibits the least intensity accounting for decreased photon-induced electron-hole pair recombination. As the photocatalytic reduction of metal ions is considerably influenced by the solution pH, reduction of Cr (VI) is analyzed under acidic condition (pH 3), normal condition (pH 5) and alkaline condition (pH 8). Reduction rate of Cr (VI) is found to follow the order as pH 3 > pH 8 > pH 5. 10 ppm of Cr (VI) is completely reduced by HMF within 90 min interval under pH 3. Whereas, only 70% and 80% reduction is possible at pH 5 and 8 respectively, using HMF. Higher activity at pH 3 may be due the electrostatic interaction between negatively charged Cr2O72− ion and the positively charged Ti-oxo cluster on the HMF. NH2-MIL-125 follows the same trend achieving 55%, 42% and 50% removal at pH of 3, 5 and 8 respectively. This finding is in line with the previous report indicating acidic condition favors for the Cr (VI) removal [12]. NTU-9 is found to be least active under acidic, normal and alkaline conditions, confirming the role of charge separation for the efficient transfer of photogenerated electrons for Cr (VI) reduction. Further, NTU-9 conduction band is nearer to Cr (VI)/Cr (III) redox potential in comparasion to NH2-MIL-125. It is imperative to study the
Fig. 9. Schematic illustration of HMF for the photocatalytic reduction of Cr (VI).
effect of concentration of Cr (VI) to find the rate kinetics. Fig. 8 present the effect of Cr (VI) concentration on the rate of reaction using HMF. As the concentration is increased from 5 to 20 ppm, the rate of reaction decrease, which seems to follow Langmuir-Hinshelwood model with first order kinetics. Based on the above findings and previous reports a possible mechanism for the superior activity of HMF is illustrated in Fig. 9 [12,21]. NH2-MIL-125 with a band gap of 2.54 eV is excited by the visible light. The photogenerated electrons are used for the Cr (VI) reduction. In mean time NTU-9 absorbing till 1000 nm transfer the photogenerated electrons from its conduction band to the empty valence band of NH2MIL-125 thereby reducing the electron-hole recombinaion. This results in more efficient reduction of Cr (VI) to Cr (III). Possible reaction is as follows NH2-MIL-125 + NTU-9 → HMF HMF + hυ → h
+
+e
−
(1) (2)
Fig. 8. (a) Effect of concentration of Cr (VI) on rate of reaction using HMF at pH 3 and (b) Fitted curve between 1/r0 and 1/C0 for 5 ppm solution of Cr (VI) using HMF at pH 3. 5
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Cr2O72−+ 14H+ + 6e- → 2Cr3+ + 7H2O
(3)
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Efficient removal of Cr (VI) by the heterojunction MOF HMF without using sacrifical hole scavenger make it more viable option for real time application in wastewater treatment. Use of heterojunction MOFs in the memebrane bioreactors has the potential to increase its lifetime by removing the soluble microbial produts [23]. 4. Conclusions NH2-MIL-125 and NTU-9 was successfully integrated to make a hetero MOF (HMF). XRD confirms the framework structure of the assynthesized MOFs. HMF shows better thermal stability in comparison to NTU-9. All the synthesized MOFs show high surface area with micropores and mesopores. HMF shows the absorption till 563 nm corresponding to 2.2 eV. HMF is found to have better photocatlytic activity for Cr (VI) reduction in comparison to both the contributing MOFs. Acidic condition favors the Cr (VI) reduction and 100% reduction is achieved within 90 min of visible light irradiation. In same time NTU-9 and NH2-MIL-125 could achieve only 35 and 55% reduction respectively. Acknowledgements NA acknowledges DST-SERB for providing start up grant under young scientist scheme (Project No: SB/FT/CS-029/2014). Supplementary data Please see the supplementary data for the FE-SEM, PL spectra of assynthesized MOFs and image of the reactants before and after the reaction. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2019.103240. References [1] P.B. Tchounwou, C.G. Yedjou, A.K. Patlolla, D.J. Sutton, Heavy metal toxicity and the environment, Molecular, Clinical and Environmental Toxicology (2012) 133–164. [2] M. Feng, P. Zhang, H.-C. Zhou, V.K. Sharma, Water-stable metal-organic frameworks for aqueous removal of heavy metals and radionuclides: a review, Chemosphere 209 (2018) 783–800. [3] P.A. Kobielska, A.J. Howarth, O.K. Farha, S. Nayak, Metal–organic frameworks for heavy metal removal from water, Coord. Chem. Rev. 358 (2018) 92–107. [4] C.C. Wang, X.D. Du, J. Li, X.X. Guo, P. Wang, J. Zhang, Photocatalytic Cr (VI) reduction in metal–organic framework: a mini-review, Appl. Catal. B: Environ. 193 (2016) 198–216.
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