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Polyaniline Nanofiber Electrodes for Reversible Capture and Release of Mercury(II) from Water

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Polyaniline Nanofiber Electrodes for Reversible Capture and Release of Mercury(II) from Water Polyaniline Nanofiber Electrodes for Reversible Capture and Release of Mercury(II) from Water The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Kim, Yoonseob et al. "Polyaniline Nanofiber Electrodes for Reversible Capture and Release of Mercury(II) from Water." Journal of the American Chemical Society 140, 43 (October 2018): 14413– 14420 © 2018 American Chemical Society As Published http://dx.doi.org/10.1021/jacs.8b09119 Publisher American Chemical Society (ACS) Version Author's final manuscript Citable link https://hdl.handle.net/1721.1/128126 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Page 1 of 8 Journal of the American Chemical Society 1 2 3 4 5 6 7 Polyaniline Nanofiber Electrodes for Reversible Capture and Release 8 of Mercury(II) from Water 9 10 Yoonseob Kim1, Zhou Lin1, Intak Jeon2, Troy Van Voorhis1, and Timothy M. Swager1* 11 12 1 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139 USA 13 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 USA 14 KEYWORDS. Redox-active system, polyaniline nanofiber, heavy metal ion removal, intermolecular binding energy 15 16 17 ABSTRACT: π-Conjugated polyaniline nanofiber networks are an attractive material platform for reversible and selective capture and release of toxic heavy metal ions from water. Their nanofiber geometry facilitates high sorption kinetics, sulfur functionaliza 18 tion of the backbone for improved adsorption, and electrochemical control of the oxidation (charge) state for reversible triggered 19 sorption/desorption of metal ions. These active materials also function as sensors in that the sorption of mercury ions can be de 20 tected by analysis of cyclic voltammograms. Calculation of binding energies between polyaniline and metal ions using molecular 21 dynamics and density functional theory support the electrochemically controlled reversible sorption/desorption mechanism. These 22 redox active materials for removing Hg2+ from water create an attractive system that combines efficiency, capacity, selectivity, and 23 reusability. 24 25 26 economic perspective, the reuse of adsorbents is critical for 27 INTRODUCTION real-world applications. Most often adsorbents are recycled 28 Mercury (Hg) pollution and poisoning cause serious human through demetalation using strong acid12,22 or base,18 some 29 diseases such as the Minamata disease, and have long been a times at elevated temperatures (70 ˚C).23 Limitations associat- 30 major threat to human health. In spite of this recognition, Hg ed with present-day methods include potential safety issues 31 ion pollution is growing world-wide from expanded manufac associated with manipulations, extra treatment steps, and lim- 32 turing and processing of chemicals, electronic materials, and ited regeneration yields. Numerous adsorbents have been 33 petroleum.1 Additionally, Hg2+, which has long been dormant developed with simple and effective chemistry, however, sys- 34 in arctic ice, is being released into the environment as a result tems simultaneously achieving low cost, high uptake, fast ki- 2 2+ 35 of global warming. Hg pollution in drinking water is be netics, selectivity, long-term usability, and facile recyclability, 36 coming a critical issue. Therefore, there is an urgent need to remain elusive. As a result, there remains a need for new ma develop efficient technologies that can remove Hg2+ from pol- 2+ 37 terials and methods for Hg removal. luted water to concentrations less than 2 ppb, and hence meet Electrochemically mediated capture and release of contami- 38 drinkable water standards.3 39 nants is attractive for the creation of systems that can be actu- Among existing Hg2+ mitigation methods, adsorption as a ated without reagents.24,25 Hence we have targeted the crea 40 separation method is the most facile, convenient, and practical tion of high-performance redox-active selective sorption mate 41 technology.4 It has proven cost-effective and can have high rials that display facile regeneration by simple application of 42 uptake capacity, fast kinetics, and good selectivity. If the ab- an electrical potential. These systems were also designed to 43 sorption materials are stable in water over a wide pH range, compare favorably relative to existing systems in terms of 44 the Hg2+ can be removed, allowing for reuse. Traditional ad- water economy and downstream waste sustainability. The 45 sorption methods include use of clay, activated carbons, zeo- electrical transport properties of conducting polymers enable 46 lites, and ion exchange resins, which are often thiol- both the efficient charging and discharging of bulk membrane 5–7 47 functionalized to enhance performance. However, the up- materials and the modulation of the binding of ions. The con- take capacities and binding affinities of most materials strug- trol of polymer morphologies is known to be critical for the 48 8– 49 gle to meet the necessary high standards for drinkable water. formation of conducting polymer adsorbents for the removal 11 Recent advances in Hg2+ adsorbent materials addressing of metal cations.23,26,27 Polyaniline (PANI) is an especially 50 this challenge include the use of highly porous organic poly- notable material for this application, as a result of its facile 51 mers,12–14 metal−organic frameworks (MOFs),15–17 and cova synthesis, environmental stability, and simple dop- 52 lent−organic frameworks (COFs).18–21 The enhanced adsorp- ing/dedoping chemistry, the latter of which can be sensitive to 53 tion using these adsorbents over traditional materials is at- its chemical environment.28–31 As a result of the nitrogen 54 tributed to their high surface area. MOFs have further dis- groups in PANI, it naturally absorbs metal cations in its neu- 55 played luminescence Hg2+ sensing capabilities, however this tral state, but when oxidized its highly positively charged na 56 system has limited instability in aqueous solutions at low pH ture is expected to repel the cations to give a release mecha 16 57 needed for regeneration. COFs showed increased stability, nism. 58 fast kinetics, and high uptake capacity, however cumbersome 59 chemical treatments are required to enable reuse. From the RESULTS AND DISCUSSION 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Figure 1. a; Transmission electron microscopy image of PANI nanofibers. b-d; Photographic images of polypropylene support, PANI- 32 coated absorbent, polyaniline nanofiber electrode (PNE), respectively. A 1 cm scale bar for (b-d) is placed between (c) and (d). e; Repre- 33 sentation of deprotonated PANI and its cyclic voltammogram. f; Drawing showing the addition of mercaptopropionic acid into the PANI 34 backbone and its cyclic voltammogram. The numbers in the cyclic voltammogram represent scan cycles after the addition of the reagent. 35 Cyclic voltammograms were recorded in 0.5 M H2SO4 electrolyte solution at a scan rate of 10 mV/s with pyrolytic graphite sheet as a 36 counter electrode. 37 Preparation of polyaniline nanofiber electrodes. We polyaniline per unit area (cm2) of the substrate and samples 38 demonstrate herein a working electrode of PANI nanofibers from single dipping were used for further experiments. Final- 39 that can selectively adsorb Hg2+ from water, then subsequently ly, a graphite sheet was used to establish electrical contact to 40 release these ions upon application of an oxidizing electro- create a polyaniline nanofiber working electrode (denoted as 41 chemical potential. Nanoscale PANI fibers provide high inter- PNE) with an active area of 1.5 cm2 (Fig. 1b-d, and S1d, f). 32,33 42 facial area to enhance transport kinetics of ions. These Cyclic voltammogram profiles of PNE (Fig. 1e, S3) display 43 nanofibers are produced by an interfacial polymerization two characteristic peaks (0.36 and 0.94 V) in the oxidation 44 method using ammonium persulfate as an oxidant and cam- scans, and two reduction peaks (0.88 and 0.22 V) with other 45 phorsulfonic acid to give polymer filaments of ca. 80 nm di- processes providing small shoulders in between. The first ameter (Fig. 1a, S1).34 Fibrous polypropylene substrates 46 oxidation peak corresponds to the oxidization of leu- (thickness: 0.285 ± 0.05 mm, n=5) were used as the supports, coemeraldine to emeraldine, and the second maximum at the 47 and were immersed directly in the PANI synthesis solution.35 48 higher potential is attributed to the oxidization of emeraldine Upon contact, strong van der Waals forces result in tight bind- to pernigraniline. PNEs showed very stable cyclic voltamme 49 ing of the PANI nanofibers to the polypropylene. Coating of try profiles even after the 15th repetitive scan cycles, and are 50 nanomaterials on an economical porous substrate is a proven indicative of a tight binding between the nanofibers and the 51 approach for the construction of lightweight, flexible and substrates (Fig. S3). This aspect is important, because often 36 52 functional materials. A scanning electron microscope (SEM) electrochemically active electrodes prepared by drop casting 53 image shows conformal coating of a percolated network of of active materials on carbon (or general graphite) electrode, 54 PANI nanofibers on the substrate (Fig. S1e), and mass of the suffer from unstable long-term performance, which is general- 55 active materials increased linearly with each dipping cycles ly attributed to delamination of the active materials. (Fig. S2). Single dipping coated 0.83 ± 0.084 mg (n=5) of 56 57 58 59 60 ACS Paragon Plus Environment Page 3 of 8 Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2+ Figure 2.
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