Novel thymine-functionalized MIL-101 prepared by post-synthesis and enhanced removal of Hg2+ from water
Graphical abstract
Introduction
For many decades, heavy metal contamination has been a challenge throughout the world, as heavy metal ions dissolved in water are highly toxic to organisms, even at very low concentration [1]. Among various types of heavy metal ions, the water-soluble mercuric ion is generally considered to be one of the most toxic types because of its accumulative properties in the ecosystem [2]. Mercuric ions can cause kidney/liver diseases, bone softening, neurological disorder (such as Minamata disease, pulmonary edema, cyanosis and nephritic syndrome [3]), immune damage, and other ill effects that are extremely hazardous to human health [2]. Actually, as early as 2013, a global effort to address mercury was began at the Minamata Convention on Mercury [4]. Therefore, various methods have been used to remove Hg2+ from the aqueous environment, such as reverse osmosis, chemical precipitation, ion exchange, coagulation and adsorption [5]. Among them, the adsorption method is considered to be the most facile and effective [6], [7]. Because the adsorption efficiency is mainly dependent on the adsorbent properties, developing an effective adsorbent is of vital importance. Furthermore, coexisting ions in real samples greatly influence the removal efficiency of Hg2+, so developing a specific adsorption material for Hg adsorption is essential.
In recent years, many chelating adsorbents containing functional groups such as aminos, amidoximes, and sulphydryls have been synthesized for the selective removal of Hg2+ [8], [9], [10]; however, those adsorbents are not good enough because the interaction of their functional groups with Hg2+ is not specific. Thymine in DNA sequences has recently been reported to selectively bind with Hg2+ by forming T-Hg2+–T interactions [11], [12], [13]. A number of Hg2+ probes or sensors have been developed based on the specific interaction of Hg2+ with thymine [14]. All of those studies focused on single nucleotide polymorphism analysis and Hg2+ detection; however, no attempts have been made to adsorb Hg2+ using thymine modified MOFs through this specific interaction [15]. In this study, we extend the application of this specific interaction to the removal of Hg2+ from water.
Metal-organic frameworks (MOFs), a subset of two or three-dimensional coordination polymers that are composed of metal ions or metal ion clusters and bridge organic ligands, have recently emerged as an important family of porous materials due to their high specific surface area and their potential application in gas storage, separation, and heterogeneous catalysis [16]. However, systematic study and further research in selective adsorption of metal ions from wastewater by MOFs are still relatively deficient.
Among the many MOFs synthesized, chromium-based MOFs have been developed largely by the group of Férey under the name Material Institut Lavoisier (MIL) and exhibit particularly attractive features, including high surface area, thermal stability, and chemical stability [17]. Herein, we chose a well-known MOF—MIL-101-NH2. Functionalization in MOFs are routinely conducted using diverse functional groups for various applications. Functionalized MOFs often exhibit highly selective separation, chemisorption, and catalytic performance. The modification method can be broadly classified as direct and post-synthesis modification (PSM) [18]. Compared to direct-synthesis modification, PSM allows MOFs modified by new pendant groups to expand their use while retaining their crystallinity.
Inspired by the T-Hg2+–T interaction and MOFs, we successfully synthesized a new material, thymine-functionalized MIL-101, by the post-synthesis method, and its application for mercury adsorption was investigated. The thymine-functionalized MIL-101 shows a high Hg adsorption capacity because the material retains the physical and chemical properties of MOFs. Moreover, it exhibits excellent selectivity towards Hg2+ due to its modification by thymine groups. The Hg2+ adsorption mechanism was investigated by XPS. The novel adsorbent was also used to remove trace Hg2+ from real water samples.
Section snippets
Materials
Chromium nitrate hydrate, sodium hydroxide, N,Ń-dimethylformamide (DMF), ethanol, mercury chloride, nickel nitrate hexahydrate, cupric nitrate trihydrate, cobaltous nitrate hexahydrate, and cadmium chloride hydrate were purchased from Xilong Chemical Reagent Co., Ltd. Thymine-1-acetic acid, N,N-disopropylethylamine (DIEA), O-(Benzotriazol-1-yl)-N,N,N',N'-tetramethyluronium tetrafluoroborate (TBTU) and 2-aminoterephthalic acid were supplied by Aladdin (Shanghai, China). Deionized water was
Characterization of the adsorbent
PXRD was used to characterize the MIL-101-NH2 and MIL-101-Thymine synthesized in this work. Fig. 2 shows the PXRD patterns of MIL-101-Thymine, MIL-101-NH2 and simulated MIL-101. The PXRD of MIL-101-Thymine and MIL-101-NH2 are similar to the simulated pattern of MIL-101 reported previously [20], which indicates that the resulting nanoscale MOFs have a typical MIL-101 structure. Interestingly, the PXRD of MIL-101-Thymine and MIL-101-NH2 exhibit very broad Bragg reflections, indicating the
Conclusions
In summary, thymine-functionalized MIL-101 was synthesized for the first time by the post-synthetic modification method. Batch equilibrium adsorption of Hg2+ onto MIL-101-Thymine was carried out in this work. The Hg2+ adsorption agreed well with the Langmuir model, and the maximum adsorption capacity was 51.27 mg/g. The adsorption rate fit with the pseudo-second-order kinetic model, and intraparticle diffusion was rate limiting. More importantly, MIL-101-Thymine exhibited excellent selectivity
Acknowledgements
This work was financially supported by Natural Science Foundation of China (51178213, 51238002 and 51272099), the National Science Fund for Excellent Young Scholars (51422807), and the Program for New Century Excellent Talents in University (NCET-11-1004).
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