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Microfluidic Flow through Lamellar-Structured GrapheneSupported Polyaniline for Mass Transfer Enhanced Electrocatalytic Reduction of Hexavalent Chromium Qinghua Ji, Dawei Yu, Gong Zhang, Huachun Lan, Huijuan Liu, and Jiuhui Qu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03314 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on October 28, 2015
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Environmental Science & Technology
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Microfluidic Flow through Lamellar-structured Graphene-supported Polyaniline
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for Mass Transfer Enhanced Electrocatalytic Reduction of Hexavalent Chromium
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Qinghua Jia,b, Dawei Yua,b, Gong Zhanga,b, Huachun Lana, Huijuan Liu a,* and Jiuhui
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Qua.
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a
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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China
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b
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KEYWORDS: Electrocatalytic • Polyaniline • Graphene • Hexavalent chromium •
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Flow-through
Key Laboratory of Drinking Water Science and Technology, Research Center for
University of Chinese Academy of Sciences, Beijing, China
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ABSTRACT
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Owing to its high efficiency and environmental compatibility, electroreduction
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holds great promise for the detoxification of aqueous Cr(VI). However, the typical
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electroreduction system often shows poor mass transfer, which results in slow
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reduction kinetics and hence higher energy consumption. Here, we demonstrate a
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lamellar-structured
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electrode for electrocatalytic reduction of Cr(VI). The reaction kinetics of the
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LGS-PANI flow-through electrodes are 6.4 times (at acidic condition) and 17.3 times
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(at neutral condition) faster than traditional immersed parallel-plate electrodes.
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Computational Fluid Dynamics simulation suggests that the flow-through mode greatly
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enhances the mass transfer and the nanoscale convection induced by the PANI
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nanodots increases the nanoscale mass transport in the interfacial region of the
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electrode/solution. In situ Raman spectroscopy shows that the PANI-Cr(VI) redox
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reactions are dominated by the leucoemeraldine/emeraldine transition at 1.5 V cell
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voltage, which also remarkably contributes to the fast reaction kinetics. Using
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single-pass flow-through mode, the LGS-PANI electrode reaches an average reduction
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efficiency of 99.8% with residual Cr(VI) of 22.3 ppb [initial Cr(VI)=10 ppm, flux=20
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L h-1 m-2]. Long-term stability test shows that the LGS-PANI maintains stable
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performance over 40 days’ operation and achieves >98% reduction efficiency with
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average current efficiency of as high as 99.1 % [initial Cr(VI)=10 ppm, flux=50 L h-1
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m-2].
graphene-supported
polyaniline
(LGS-PANI)
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Table of Contents (TOC) Art
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INTRODUCTION
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The presence of Chromium (Cr) in the aquatic environment has been a serious
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threat to human health and ecosystems because of its high toxicity and
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non-degradability.1 Owing to its harmful effects on human health, hexavalent
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chromium [Cr(VI)] has been classified as a Group 1 carcinogen by the International
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Agency for Research on Cancer.2 The release of Cr(VI) species into aquatic
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environments is mainly through industrial discharge from leather tanning, electronic
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device manufacturing, chrome plating, and other industrial activities.3 Cr(III) is less-
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toxic and can be easily precipitated or adsorbed on various adsorbents.4 Therefore, the
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reduction of Cr(VI) to Cr(III) is considered to be a promising strategy for
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detoxification of Cr(VI).5-7
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Current approaches for the reduction of Cr(VI) include chemical reduction,
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photocatalytic reduction, bio-reduction and electroreduction.8-11 Electroreduction
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demonstrates high efficiency, easy operation and environmental compatibility, which
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could be especially useful for practical application.12-13 However, since the reduction
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reaction occurs at the cathode/solution interface, electrostatic repulsion will hinder
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Cr(VI) species from migrating to the active sites on the cathode. On the other hand, the
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product [mostly Cr(III) species] will probably inhibit the further reduction reaction
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unless it is transferred to the bulk solution.14-15 Therefore, mass transfer is considered
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to play a critical role in electroreduction of Cr(VI).16-17 Unfortunately, the classical
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parallel-plate electrodes often show poor mass transfer.13 In such cases, the
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detoxification of aqueous Cr(VI), particularly when Cr(VI) is present at low
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concentration, often shows low current efficiency and results in high energy demand.17
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Flow-through porous electrodes demonstrate high current efficiency, enhanced
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mass transfer and high volumetric rates of reaction and have been used in fuel cells,18
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electrooxidation,19 desalination20 and adsorption.21 This flow-through architecture
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enables optimal utilization of the active sites inside the porous electrodes and provides
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enhanced rates of mass transport, which could be especially useful for treatment of
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dilute solutions.22 In theory, flow-through electrodes with continuous and
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nanostructured active materials supported on a porous architecture can facilitate mass
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transport and active materials utilization. As one of the most-promising porous
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framework materials, the three-dimensional (3D) graphene-based framework with
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large surface area and interconnected macro-pores enables it to be an ideal support for
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nano-materials.23-24
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Due to its environmental stability and multiple intrinsic redox states associated
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with its good conductivity, polyaniline (PANI) has attracted a great deal of attention in
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electrocatalytic reduction of Cr(VI).25-30 Nanostructured PANI (e. g., nanowires,
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nanodots and films) shows improved performance because it increases the material
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usage and surface area.31-32 A previous study showed that graphene-supported PANI
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exhibits superior electrochemical activities toward Cr(VI) reduction due to the
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abundant active sites and effective electron transfer.33 Here, we present a novel
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lamellar-structured graphene nanosheet hydrogel (LGS) supported PANI flow-through
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electrode (LGS-PANI) (Scheme 1). This flow-through electrode demonstrates fast
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reaction kinetics for electrocatalytic reduction of Cr(VI), which enables it to achieve
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high reduction efficiency even in single-pass mode. Computational Fluid Dynamics
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(CFD) simulation and in situ Raman spectroscopy were performed to gain insights into
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the mechanism of Cr(VI) reduction by the electrode.
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Scheme 1. Schematic representation of the LGS-PANI flow-through electrode for
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electrocatalytic reduction of Cr(VI).
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EXPERIMENTAL SECTION
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Materials Synthesis. Graphene oxide (GO) was synthesized from synthetic graphite
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powder (
