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Remediation and Control Technologies
An Electroactive Modified Carbon Nanotube Filter for Simultaneous Detoxification and Sequestration of Sb(III) Yanbiao Liu, Peng Wu, Fuqiang Liu, Fang Li, Xiaoqiang An, Jianshe Liu, Zhiwei Wang, Chensi Shen, and Wolfgang Sand Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05936 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019
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An Electroactive Modified Carbon Nanotube Filter for
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Simultaneous Detoxification and Sequestration of Sb(III)
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Yanbiao Liu†‡*, Peng Wu†, Fuqiang Liu†, Fang Li†‡, Xiaoqiang An§, Jianshe Liu†‡,
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Zhiwei Wang‡ǁ, Chensi Shen†‡*, Wolfgang Sand†
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Revision Submitted December 10th 2018
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†Textile
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Protection, College of Environmental Science and Engineering, Donghua University,
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2999
13
[email protected],
[email protected]; Tel: +86 21 6779 8752.
14
‡Shanghai
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Shanghai 200092, P. R. China.
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§Center
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100084 China
18
ǁState
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Environmental Science and Engineering, Tongji University, Shanghai 200092, China.
20
Institute
21
09599, Germany.
Pollution Controlling Engineering Center of Ministry of Environmental
North
Renmin
Road,
Shanghai
201620,
P.
R.
China.
E-mail:
Institute of Pollution Control and Ecological Security, 1239 Siping Road,
for Water and Ecology, School of Environment, Tsinghua University, Beijing,
Key Laboratory of Pollution Control and Resource Reuse, School of
of Biosciences, Freiberg University of Mining and Technology, Freiberg
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Table of Content
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Abstract
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Herein, we rationally designed a dual-functional electroactive filter system for
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simultaneous detoxification and sequestration of Sb(III). Binder-free and nanoscale
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TiO2-modified carbon nanotube (CNT) filters were fabricated. Upon application of an
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external electrical field, in situ transformation of Sb(III) to less toxic Sb(V) can be
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achieved, which is further sequestered by TiO2. Sb(III) removal kinetics and capacity
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increase with applied voltage and flow rate. This can be explained by the synergistic
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effects of the filter’s flow-through design, electrochemical reactivity, small pore size,
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and increased number of exposed sorption sites. STEM characterization confirms that
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Sb were mainly sequestered by TiO2. XPS, AFS and XAFS results verify that the Sb(III)
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conversion process was accelerated by the electrical field. The proposed electroactive
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filter technology works effectively across a wide pH range. The presence of sulfate,
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chloride, and carbonate ions negligibly inhibited Sb(III) removal. Exhausted TiO2-CNT
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filters can be effectively regenerated using NaOH solution. At 2 V, 100 µg/L Sb(III)-
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spiked tap water generated ~1600 bed volumes of effluent with >90% efficiency.
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Density functional theory calculations suggest that the adsorption energy of Sb(III) onto
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TiO2 increases (from −3.81 eV to −4.18 eV) and Sb(III) becomes more positively
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charged upon application of an electrical field.
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Introduction
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The environmental pollution and ecological toxicity caused by the emerging
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contaminant antimony (Sb) has received worldwide concern recently.[1-3] As common
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raw materials for numerous products, Sb-containing compounds have been widely used
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in fields such as mining, electronics, and the textile industry. An elevated Sb
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concentration ranging from 100 to 7000 µg/L in surface and well waters near a Sb
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mining and smelting area was reported.[4] Due to its potentially high toxicity and health
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risks, the US Environmental Protection Agency has classified Sb and its compounds as
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priority pollutants and regulated their maximum contaminant level (MCL) in drinking
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water as 6 µg/L.[5]
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Compared with the extensive studies on the removal of arsenic,[6-8] a structural
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analog to Sb, research on Sb removal has received much less attention. Among the
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available Sb treatment technologies, sorption is a widely applied approach due to its
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operational simplicity and cost-effectiveness.[9, 10] To date, several metal oxide sorbents
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with large surface areas and Sb specificities have been developed, such as Fe3O4,
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FeO(OH), ZrO2, MnO2, TiO2, and FeOOH-MnO2.[11-16] Among these, nanoscale TiO2
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adsorbents are promising alternatives due to their high chemical stability and adsorption
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capacity. Extended X-ray absorption fine structure (EXAFS) and density function
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theory (DFT) suggest that antimonite (Sb(III)) and antimonate (Sb(V)) exhibit a
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bidentate binuclear surface complex on TiO2 surfaces with Ti-Sb(III) and Ti-Sb(V)
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distances of 3.47 Å and 3.70 Å, respectively.[11] However, despite certain progress in
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Sb adsorption capacity, knowledge of the adsorptive kinetics is still far from complete. 4
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The time required to reach equilibrium is usually a few hours or even a few days for
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powdered and/or granular sorbents.[12,
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cannot be directly used in practical conditions due to additional effort required for post-
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separation. Therefore, these sorbents have either been attached onto supporting
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materials, blended into a support membrane and/or encapsulated into microporous
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polymers (e.g. polystyrene anion-exchangers).[18-20] Unfortunately, these designs
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usually sacrifice performance due to inevitable blocking of sorption sites.
13, 17]
In addition, these nanoscale adsorbents
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Furthermore, the speciation of Sb in water bodies is significantly affected by the
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redox state of aquatic conditions. The most abundant species of Sb are inorganic Sb(V)
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and Sb(III).[21] Compared with Sb(V), Sb(III) is more toxic[22,
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efficacy by sorption is generally low due to a predominantly charge-neutral state (i.e.
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Sb(OH)3) over a broad pH range (e.g. 3–9). Therefore, pre-oxidation of Sb(III) to Sb(V)
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is a feasible option in practical conditions.[24] Some chemical oxidants, such as MnO2
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and OH• are capable of transforming Sb(III) to Sb(V).[16, 25] However, high costs of
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chemical consumption and subsequent deterioration of water quality due to high reagent
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usage seem inevitable. Therefore, recent efforts have been devoted to developing novel
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adsorptive systems that combine adsorption and oxidation for effective Sb(III) removal.
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An electroactive carbon nanotube (CNT) filter with a nanoscale TiO2 coating may
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provide a promising solution to these limitations.[26-28] Improved Sb sorption kinetics
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and enhanced sorption capacity can be expected because of the synergistic effects of
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flow-through design, electrochemical reactivity, small pore size, and more exposed
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sorption sites. The objective of this study is to develop a nano-TiO2 modified CNT filter 5
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for simultaneous oxidation and sequestration of Sb(III). We developed a facile
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electrosorption-hydrothermal route to fabricate a binder-free and nanoscale TiO2 coated
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onto a preformed CNT filter. The as-fabricated TiO2 modified CNT filter not only
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maintains excellent electrical conductivity of CNT networks but also provides
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sufficient sorption sites (due to binder-free TiO2). Various advanced characterization
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techniques were employed to obtain detailed morphological and compositional
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information of as-fabricated TiO2-CNT filter. Sb(III) sorption kinetics were
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experimentally and theoretically studied. The impact of several key operational
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parameters on Sb(III) removal were examined and optimized. Sb(III)-spiked tap water
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was used to challenge the TiO2-CNT filter to evaluate its potential use in practical
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engineering applications. We hypothesize that: 1) Sb(III) can be converted to low toxic
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Sb(V) in situ upon the application of an appropriate voltage, 2) the as-produced Sb(V)
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can then be effectively sequestrated by TiO2, 3) the electrical field can accelerate the
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electromigration kinetics of Sb(V), and 4) the high porosity of the filter and convection
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enhances the mass transport of Sb(III) and Sb(V) towards the sorption sites. This study
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provides new insights for the rational design of a continuous-flow system for efficient
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removal of Sb(III) and other similar heavy metal ions.
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Experimental Section
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Chemicals and Materials. All chemicals were used without further purification.
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N-methyl-2-pyrrolidinone (NMP, ≥99.5%), hydrochloric acid (HCl, 36–38%), sodium
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hydroxide (NaOH, ≥96%), nitric acid (HNO3, 36–38%), and ethanol (≥96%) were 6
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purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Titanium tetrachloride
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(TiCl4, 99.9% metal basis) was purchased from Aladdin (Shanghai, China). Sb(III) and
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Sb(V) stock solutions were prepared with C8H4K2O12Sb2·3H2O and KSb(OH)6, both
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purchased from Sigma-Aldrich. All chemicals used were of analytical grade. All
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aqueous solutions were prepared with ultra-pure water from a Milli-Q Direct 8
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purification system.
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TiO2-CNT Filter Preparation. Multiwalled carbon nanotubes (CNT) with =
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10–20 nm and = 10–30 µm were purchased from TimesNano Co., Ltd (Chengdu,
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China). The TiO2 modified CNT filters (TiO2-CNT) can be further prepared via a
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simple electrosorption-hydrothermal process. Firstly, 10 mL of 0.8 mol/L of TiCl4 was
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mixed with 33 mL of HCl followed by the addition of 67 mL of ultrapure water. Then,
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the solution was bath-sonicated for 15 min and used as the electrolyte for
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electrosorption. Next, the electrosorption process was performed in a conventional
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bipolar electrochemical cell with a preformed CNT filter as the cathode and a titanium
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plate (2 cm × 5 cm) as the anode. Upon the application voltage of 1 V, the CNT
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cathode becomes negatively charged and positively charged metal cations (Ti4+) can be
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adsorbed onto the filter surface by electrostatic attraction. Then, the CNT filter was
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transferred into a water bath (80 °C) and kept for 1 h to precipitate the metal oxides
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(TiO2). Finally, the TiO2-CNT filter was dried in an oven at 50 °C before use.
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Characterizations of the as-fabricated filters are detailed in the SI.
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Sb Sorption Experiments. Three Sb adsorption modes: batch, recirculated
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filtration, and single-pass filtration, were comparatively studied. For the conventional 7
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batch mode, the as-fabricated TiO2-CNT filter was transferred to a flask containing 100
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mL of 750 µg/L Sb(III) at a pH 7. The flask was sealed and put into a shaker at 25 °C
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and 150 rpm for 8 h. Aliquots were sampled to examine the sorption kinetics. For
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recirculated filtration, the filter was transferred into an electrochemistry-modified
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Whatman polycarbonate filtration casing (SI Figure S1). The electrochemical filtration
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setup has been described in detail previously.[29] One hundred milliliters of Sb(III)
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solution was pumped at a flow rate of 1.5, 3, or 6 mL/min through the TiO2-CNT filter
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and then returned. The applied voltage was 0–2 V and the solution pH was adjusted to
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3–11. For single-pass filtration, the effluent was no longer returned to the influent. All
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sorption experiments were performed in triplicate. The concentration of total Sb was
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determined using a Thermo Scientific iCAP-Q inductively coupled plasma mass
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spectrometer (ICP-MS, Waltham, MA) and the concentration of Sb(III) was determined
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by an AF-610B atomic fluorescence spectrometer (AFS, Beijing, China).
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The exhausted filter was regenerated by passing 100 mL of 5 mmol/L NaOH
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through it or applying an opposite voltage (2 V) while passing through a 100 mL of 10
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mmol/L Na2SO4. Before the next cycle, the regenerated filter was then rinsed with water
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until the effluent pH became neutral. To probe the effects of coexisting anions, 1 to 10
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mmol/L of chloride, carbonate, nitrite, sulfate, or phosphate were spiked into the Sb(III)
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solution before passing through the TiO2-CNT filter in recirculation mode.
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Theoretical Sorption Analysis by DFT. The impact of an electrical field on
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Sb(III) sequestration and oxidation was performed by DFT calculations using the CP2K
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package.[30] The model building process and instructions are detailed in the SI. 8
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Results and Discussion
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Characterizations of the TiO2-CNT Filter. Figure 1 displays the FESEM and
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TEM characterizations of the CNT and TiO2-CNT filters, respectively. The CNT
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network had a smooth surface with tube diameter of 25 ± 7 nm. Meanwhile, the TiO2-
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CNT network had a much rougher surface with larger CNTs of 36 ± 9 nm in diameter.
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Coating of a thin layer of nanoscale TiO2 (0.99), indicating that chemical adsorption is the rate-controlling step.[11] The
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batch mode yielded a qe of 2.3 mg/g (or 16.8 mg/g when normalized by TiO2) over 10
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an 8 h reaction. Interestingly, the sorption kinetics increased with flow rate in
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recirculation mode. The values of qe were 3.7 mg/g (or 26.4 mg/g when normalized
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by TiO2), 4.2 mg/g (or 29.6 mg/g when normalized by TiO2), and 4.3 mg/g (or 30.5
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mg/g when normalized by TiO2), respectively, at flow rates of 1.5, 3, and 6 mL/min.
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The corresponding equilibrium times were >8, 6, and 4 h at 1.5, 3, and 6 mL/min.
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The qe at a flow rate of 6 mL/min was 1.9 times higher than that of batch mode.
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This may be due to the convection-enhanced mass transport of the flow-through
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system, compared to the diffusion-controlled mass transport of a conventional
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batch reactor. In addition, the limited pore size in the TiO2-CNT filter ( 9. The pHzpc
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for the TiO2-CNT network was determined to be 4.5, which is similar to a recently
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reported pHzpc value of 5.1 for TiO2-CNT composites.[6] TiO2 is capable to oxidize
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Sb(III) to Sb(V) and the negatively charged Sb(OH)6- could be easily sequestered by
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the positively charged TiO2-CNT filter if the solution pH is 8 h at 0 V to 4 h at 1 V, and 3 h at 2 V. It is
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noteworthy that Sb(III) sorption onto various sorbents have been recently studied. For
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example, Fe–Mn binary oxide (200 mg) with enhanced specific surface area (231.0
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m2/g) at an initial Sb(III) concentration of 60.9 mg/L produced a normalized sorption
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capacity of 1.25 mg/m2.[32] γ-Fe2O3 with a specific area of 101.5 m2/g at an initial Sb(III)
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concentration of 24.2 mg/L had a normalized sorption capacity of 0.47 mg/m2.[33] As a
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comparison, although a rather low initial Sb(III) concentration (0.75 mg/L) and very
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limited sorbents (2.4 mg) used in this work, a comparable normalized sorption capacity
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of 1.2 mg/m2 can still be obtained. Meanwhile, the Sb(III) removal efficiency (>95% at
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2 V and >83% at 0 V, SI Figure S8) was also comparable or even higher than few state-
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of-the-art batch sorption systems, as summarized in SI Table S1. A double benefit is 12
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expected when applying an external voltage to the TiO2-CNT filter. Firstly,
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conversion of Sb(III) to Sb(V) may occur, since the redox potential [Sb5+/Sb3+] is
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0.69 V vs. SHE; secondly, the negative surface charge of the filter will be
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suppressed and the migration rate of these as-transformed negatively-charged
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Sb(V) towards the surface active sites could be accelerated. It is reasonable to
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surmise that a certain amount of Sb(III) was converted to Sb(V) while passing through
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the TiO2-CNT filter upon the application of an external voltage. It has been reported
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that TiO2 is capable to oxidize As(III) to As(V)[34] and Sb(III) to Sb(V)[25] by production
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of reactive oxygen species (ROS) upon illumination. In this work, the TiO2
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nanoparticles were in amorphous phase and no UV light was provided, so the
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production of ROS via a photocatalytic route can be eliminated. The increased sorption
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kinetics and capacity can be partially ascribed to enhanced near-surface transport by
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electromigration as well as improved electrostatic interactions between the positively
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charged filter surface and the negatively charged Sb(V). Furthermore, a recent DFT
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study by Yan et al. indicates that TiO2 surface reconstruction occurs when in contact
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with adsorbed Sb due to deprotonation of Sb(OH)3 or Sb(OH)6-.[11] This also contributes
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to the stabilization of adsorbed Sb species and suppresses desorption, leading to
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increased sorption kinetics for Sb(III) and Sb(V). In addition, Sb(III) sorption isotherms
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on the TiO2-CNT filter were also performed in recirculation mode and fitted with a
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Langmuir isotherm model (SI Figure S9). At pH 7, the maximal experimental qe values
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were 82 mg/g at 0 V and 95 mg/g at 2 V. The actual maximal qe values will be even
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greater, as the obtained qe values did not achieve a plateau. 13
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STEM and XPS Analyses. To demonstrate Sb sequestration, we performed
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scanning transmission electron microscopy (STEM) to obtain high-angle annular dark-
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field imaging (HAADF) images and elemental mapping of a Sb-loaded TiO2-CNT filter.
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As shown from the elemental distribution of Ti, O and C, these nanoscale TiO2 particles
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were coated onto the CNT sidewalls (Figure 3 and SI Figure S10). This agrees well
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with the TEM results. As displayed in Figures 3b and f, the Sb elemental distribution is
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very consistent with that of Ti (or O), suggesting that Sb was mainly sequestered by the
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coated TiO2 nanoparticles.
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The chemical composition of a TiO2-CNT filter before and after Sb adsorption was
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further examined by XPS. As shown in SI Figure S3, the XPS survey pattern of a used
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TiO2-CNT filter validates the presence of C 1s, O 1s, Ti 2p, and Sb 3d, while a fresh
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TiO2-CNT filter only showed the characteristic peaks of C 1s, O 1s, and Ti 2p. The
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peak at a binding energy of 284.8 eV was assigned to C 1s, and no change to the C 1s
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pattern was identified before and after Sb sorption. The standard chemicals of Sb(III)
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(i.e. C8H4K2O12Sb2·3H2O) and Sb(V) (i.e. KSb(OH)6) were probed with XPS analysis
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and their main peaks were centered at 529.2 eV and 530.5 eV, respectively (SI Figure
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S11). Furthermore, a high-resolution scan of Sb 3d + O 1S as a function of applied
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voltage (e.g. 0, 1, and 2 V) over a small window is displayed in Figures 2g-i. At 0 V,
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the spectrum can be deconvoluted into four peaks at 539.8 eV, 532.2 eV, 531.1 eV, and
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529.7 eV, which are assigned to Sb 3d3/2 (i.e. Sb(III)), OH2O (i.e. chemisorbed oxygen
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species, like H2O), Sb 3d5/2 (i.e. Sb(V)),[14, 35] and Olatt (i.e. lattice oxygen).[14] This
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indicates that both Sb(III) and Sb(V) are present and the presence of Sb(V) may be due 14
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to oxidation by TiO2 nanoparticles. In a previous report, iron oxides were observed to
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be capable of oxidizing Sb(III) to Sb(V).[35]
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In the presence of an applied voltage, Sb(V) was always the dominant Sb species,
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and its content increased as the applied voltage increased from 0 V to 2 V. Moreover,
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the characteristic peak of Sb(III) centered at 539.8 eV disappeared and an alternative
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Sb(V) peak centered at 540.2 eV appeared. The XPS pattern of Sb(III)-2V was similar
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with that of Sb(V) sorption in the absence of applied voltage (Figure 3j), which provides
292
supportive evidence for the oxidation of Sb(III) to Sb(V) under an electrical field. This
293
indicates two things: 1) TiO2 can oxidize Sb(III) to Sb(V) to some extent, and 2) the
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applied electrical field accelerates the Sb(III) conversion process.
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The oxidative transformation of Sb(III) to Sb(V) was further verified by
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quantitative determination of changes in the effluent Sb species with time. The
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oxidation of Sb(III) by a CNT-alone filter at an applied voltage of 2 V provides
298
supportive evidence on the feasibility of Sb(III) oxidation to Sb(V) (SI Figure S12). As
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shown in Figure 4a, at an applied voltage of 2 V, both Sbtotal and Sb(III) decreased
300
exponentially with time. In contrast, the Sb(V) content increased linearly in the first 2
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h, and then showed a linear declining trend. The initial increase in Sb(V) concentration
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suggest that a continuous oxidation process occurred during the filtration. Sb(V) can
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also be sequestered by the nanoscale TiO2 simultaneously. Initially, the Sb(III)
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oxidation rate was higher than the Sb(V) sequestration rate, so the Sb(V) concentration
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kept rising. After 3 h, Sb(V) became the dominant Sb species, which was further
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sequestered by the TiO2-CNT filter. This changing trend in Sb species confirms the 15
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capability of the electroactive filter system to simultaneously sequester and detoxify
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Sb(III). Another set of solid evidence for Sb(III) oxidation is resulting from the XAFS
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characterization, as displayed in the inset of Figure 4a and SI Figure S13. Comparing
310
the Sb K-edge XANES spectra of an exhausted TiO2-CNT filter at 0 V with pure Sb
311
reference compounds (KSb(OH)6 and C8H4K2O12Sb2 were used as the reference
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compounds for Sb(V) and Sb(III), respectively), suggesting that Sb was in a mixture
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oxidative states of Sb(V) and Sb(III). However, the XANES spectra of an exhausted
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TiO2-CNT filter at 2 V coincided with Sb(V) only, suggesting a complete Sb(III)
315
conversion at this case. This is in well accordance with the XPS and AFS results. Based
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on this discussion, we believe two pathways are involved in Sb(III) removal. The
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dominant pathway is Sb(III) oxidation to Sb(V) by the electrical field followed by Sb(V)
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sequestration by TiO2. However, another pathway is direct Sb(III) sequestration by
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TiO2 and then oxidation to Sb(V), which cannot be excluded.
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Effect of Solution Chemistry. Figure 4b presents the impact of certain ubiquitous
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anions on Sb(III) removal by the electroactive TiO2-CNT filter. The results suggest that
322
regardless of an applied voltage of 2 V, sulfate, chloride, and carbonate ions negligibly
323
inhibit Sb(III) removal in the range 1–10 mmol/L. This can be explained by the different
324
sorption mechanisms of Sb(III) and these competing anions by the TiO2-CNT filter.
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Sb(III) was chemisorbed onto the nanoscale TiO2, whereas sulfate, chloride, and
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carbonate were mainly removed by electrostatic interaction. In contrast, the presence
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of 1–10 mmol/L phosphate suppressed 10–13% of the Sb(III) removal efficiency in the
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absence of an applied voltage. A similar phosphate inhibition effect on Sb sequestration 16
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has been previously reported by Yan and co-workers.[11] Phosphorus (P) and Sb are
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from the same group 15 (VA) of the periodic table, so they share similar
331
physicochemical properties. Due to the limited active sites available on the TiO2-CNT
332
filter surface, competitive adsorption of phosphate greatly suppressed the sorption of
333
Sb(III) and Sb(V).[36, 37] Such inhibition effect became very pronounced in the presence
334
of an applied voltage, and the Sb(III) removal efficiency decreased considerably, by
335
74%, at 2 V. This indicates that electrostatic attraction further contributes to phosphate
336
uptake (or suppresses Sb uptake).
337
Regeneration of the Exhausted TiO2-CNT Filter. Regeneration of the exhausted
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TiO2-CNT filter was evaluated comparatively by passing through 100 mL of 5 mM
339
NaOH solution (i.e. chemical washing) or applying a −2 V voltage while passing
340
through 100 mL of 10 mmol/L Na2SO4 solution (i.e. electrostatic repulsion). As
341
illustrated in Figure 4c, the exhausted TiO2-CNT membrane could be effectively
342
regenerated by passing through NaOH solution. After two single-pass filtration cycles
343
(i.e. 8000 bed volumes), the Sb(III) removal efficiency was still as high as 90% under
344
given conditions, and the efficiency dropped slightly to 80% in the third cycle. In
345
comparison, the efficacy of regeneration by electrostatic repulsion of the adsorbed Sb(V)
346
ions was limited; only 61% Sb(III) removal was achieved over 3 consecutive cycles.
347
This indicates that desorption of the exhausted TiO2-CNT filter by chemical washing
348
is an effective choice. The optimization of operational parameters (e.g. flow rate and
349
NaOH concentration), and regeneration of exhausted filters by a combination of
350
electrostatic repulsion and chemical washing warrant further investigation. 17
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Removal Performance of Sb(III)-Spiked Tap Water. Tap water contains some
352
salt and organic matter and is believed to be more complex than deionized water.[38] To
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examine the practical application potential of the proposed technology, the TiO2-CNT
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filter was challenged with 100 µg/L Sb(III)-spiked tap water (Figure 4d). At an applied
355
voltage of 2 V and in single-pass filtration mode, the TiO2-CNT filter generated 1600
356
bed volumes of effluent before the Sb(III) removal efficiency became lower than 90%.
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A removal efficiency of 50% could still be obtained after 4000 bed volumes. Such
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performance was much less than in DI water (1600 vs. 8000 bed volumes at 90%
359
removal efficiency), as displayed in Figure 4c. Although the tap water TOC was only
360
2.0 mg/L, the much lower conductivity (524 vs. 2180 μS/cm) and relatively complex
361
organic matrix (compared with DI water) may account for the more significant decrease
362
in removal efficiency. It is noteworthy that the current electroactive filter technology
363
only requires a hydraulic retention time (HRT) of 2 sec to achieve efficacy similar to
364
that of a conventional fixed-bed filter with a typical HRT of a few minutes or hours.[12]
365
Moreover, the energy consumption for this process, at an applied voltage of 2 V and
366
assuming 2 electrons transferred per Sb(III) molecule, is calculated to be 0.93
367
kwh/kgCOD. Alternatively, the energy per volume treated is only 0.01 kwh/m3, much
368
lower than state-of-the-art electrochemical processes.[39] The improved removal
369
efficiency, regenerable materials, and affordable cost suggest that the proposed
370
electroactive filtration system can be served as a promising unit for Sb decontamination.
371
DFT Calculations. To understand the impact of an applied voltage on the
372
sequestration and detoxification of Sb(III), DFT calculations were performed. We 18
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applied a 0.2 V/Å electric field along the Z-direction in our simulation, which generated
374
an approximately 1 V potential that was similar to our experimental conditions. In the
375
absence of an electrical field, the calculated Sb(III) adsorption energy on the {001} and
376
{100} facets were −3.81 eV and −1.90 eV, respectively. The different adsorption
377
energies can be explained by the different Sb(III) adsorption configurations on the facet,
378
as displayed in Figure 5. Sb(III) combines with surface oxygen via Sb-O bonds on the
379
001 facet. Similar to the [OH] group of Sb(OH)3, oxygen from -OH groups forms
380
chemical bonds with exposed Ti via Ti-O(H) bonding. The relevant Sb-O bond length
381
is 2.69 Å and those of Ti-O(H) are 2.09 Å and 2.12 Å. However, on the {100} facet,
382
the surface conditions do not provide a unique structure that forms stable bonds as does
383
the {001} facet. This result is consistent with Yan et al that TiO2 exposed to a high
384
energy {001} facet exhibits favorable Sb sorption performance.[11]
385
In the presence of an electrical field of ~1 V, Sb(III) sorption on both the {001}
386
and {100} facets was enhanced, with increased adsorption energies of −4.18 eV and
387
−2.04 eV, respectively. Mulliken charge analysis suggests that the Sb(OH)3 became
388
more positively charged on both the {001} and {100} facets. This indicates more
389
electrons (0.07 electron) from the TiO2 transfers to the Sb(OH)3 species, as well as
390
stronger adsorption. Similarly, charge analysis reveals that the Sb(III) became more
391
positively charged by about 0.03 e when adsorbed on the {001} facet and 0.01 e on the
392
{100} facet, suggesting a higher oxidation state. From the density of states (DOS)
393
shown in Figures 5c and d, the DOS of Sb shifted to relatively lower energies under an
394
electric field. This result suggests that the valence band of Sb is bent and such band 19
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alignment under an electrical field is favorable for Sb(III) oxidation. These DFT results
396
are consistent with the macroscopic experimental results discussed above, as well as
397
the STEM and XPS analyses.
398
In summary, a dual-functional electroactive filter system was designed rationally
399
for simultaneous oxidation and sequestration of toxic Sb(III) compounds. To do so, we
400
developed a facile route to fabricate nanoscale TiO2 modified CNT filter anodes. The
401
as-fabricated filters are conductive, regenerable, possess a small pore size, and have an
402
increased number of exposed sorption sites. Upon application of an external potential,
403
an in situ conversion of Sb(III) to less toxic Sb(V) can be achieved. The latter are
404
sequestered further by TiO2. Various advanced characterization techniques were
405
employed to provide convincing evidences for Sb(III) transformation and sequestration.
406
Moreover, this technology works effectively within a wide pH range. The impact of the
407
electrical field was verified further by DFT calculations. Overall, this proof-of-concept
408
study provides new insights on the decontamination of Sb(III) compounds and similar
409
toxic heavy metals.
410 411
Supporting Information
412
Detailed descriptions of the filter fabrication, material characterization results, sorption
413
experiments, DFT calculations, XAFS measurements, optimization of the sorption
414
conditions, and sorption isotherms. The Supporting Information is available free of
415
charge on the ACS Publications website at DOI: …...
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Notes
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The authors declare no competing financial interest.
419 420
Acknowledgements
421
We thank Dr. Zenglu Qi of Research Center for Eco-Environmental Sciences, Chinese
422
Academy of Sciences for his kind help in AFS analysis. This work was supported by
423
the Natural Science Foundation of Shanghai, China (No. 18ZR1401000), the Shanghai
424
Pujiang Program (No. 18PJ1400400), the National Natural Science Foundation of
425
China (No. 21777023), the National Key Research and Development Program of China
426
(No. 2018YFF0215703 and No. 2016YFC0400501), and the State Key Laboratory of
427
Separation Membranes and Membrane Processes (Tianjin Polytechnic University, No.
428
M2-201709). Y.L. thanks Donghua University for the start-up grant (No. 113-07-
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005710).
430 431
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[24]Shan, C.; Dong, H.; Huang, P.; Hua, M.; Liu, Y.; Gao, G.; Zhang, W.; Lv, L.; Pan,
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Figure Captions
551
Figure 1. FESEM and TEM characterizations of CNT (a, b) and TiO2-CNT (c, d) filter.
552
Figure 2. Sb(III) sorption kinetics on the TiO2-CNT filter. (a) Comparison of batch
553
mode (black) and recirculated filtration (flow rate 1.5–6 mL/min); Effect of (b) pH (3–
554
11) and (c) applied voltage (0–2 V) on Sb (III) sorption. Unless noted, the experiments
555
were completed in the recirculation mode using a 100 mL reservoir of 750 μg/L Sb(III)
556
and [Na2SO4] of 10 mmol/L at pH 7. The batch sorption kinetics were completed in a
557
shaker at 150 rpm with a fresh TiO2-CNT filter.
558
Figure 3. HAADF image (a) and corresponding EDS mapping of Sb-loaded TiO2-CNT
559
filter: (b) Ti +Sb, (c) C, (d) O, (e) Ti, and (f) Sb; and XPS Sb 3d + O 1s spectra of the
560
Sb-loaded TiO2-CNT filter at 0 V (g), 1 (h), and 2V (i), and XPS Sb 3d + O 1s spectra
561
of Sb(V)-loaded TiO2-CNT filter at 0 V (j).
562
Figure 4. (a) Changes in Sbtotal and Sb species as a function of time. (b) Effect of
563
competing anions on Sb(III) sorption. Experimental conditions: [Sb(III)]in of 200 μg/L,
564
flow rate of 1.5 mL/min, pH of 7, applied voltage of 2 V, [Na2SO4] of 10 mmol/L, and
565
recirculation mode. Sb K-edge EXAFS spectra and linear combination fits for
566
Sb@CNT-TiO2 at 0 V and 2 V (inset). (c) Comparison of two regeneration methods for
567
exhausted TiO2-CNT filters. Experimental conditions: [Sb(III)]in of 200 μg/L, flow rate
568
of 1.5 mL/min, pH of 7, and recirculation mode. (d) Performance of the TiO2-CNT
569
filter when loaded with Sb(III)-spiked tap water. Experimental conditions: [Sb(III)]in of
570
100 μg/L, applied voltage of 2 V, flow rate of 1.5 mL/min, pH of 7, and single-pass
571
filtration mode.
572
Figure 5. (a) Sb(III) species adsorbed onto the TiO2 {001}facet (the bond length of Sb-
573
O is about 2.06 Å and those of Ti-O(H) are about 2.00 Å and 2.06 Å), and (b) Sb(III)
574
species adsorbed onto the TiO2 {100} facet (Sb-O bond length is about 2.69 Å and
575
those of Ti-O(H) are 2.09 Å and 2.12 Å). Color code: white sphere = H atom, red = O,
576
pink = Ti and yellow = Sb. Relatively large atoms are from the surface, smaller ones
577
are adsorbates. Density of state (DOS) for Sb(III) on the TiO2 {001} facet (c) and {100}
578
facet (d). The Fermi level energy is set to 0 for comparison. 26
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Figures
580 581
Figure 1. FESEM and TEM characterizations of CNT (a, b) and TiO2-CNT (c, d) filter.
27
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582 583
Figure 2. Sb(III) sorption kinetics on the TiO2-CNT filter. (a) Comparison of batch
584
mode (black) and recirculated filtration (flow rate 1.5–6 mL/min); Effect of (b) pH (3–
585
11) and (c) applied voltage (0–2 V) on Sb (III) sorption. Unless noted, the experiments
586
were completed in the recirculation mode using a 100 mL reservoir of 750 μg/L Sb(III)
587
and [Na2SO4] of 10 mmol/L at pH 7. The batch sorption kinetics were completed in a
588
shaker at 150 rpm with a fresh TiO2-CNT filter. 28
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589 590
Figure 3. HAADF image (a) and corresponding EDS mapping of Sb-loaded TiO2-CNT
591
filter: (b) Ti +Sb, (c) C, (d) O, (e) Ti, and (f) Sb; and XPS Sb 3d + O 1s spectra of the
592
Sb-loaded TiO2-CNT filter at 0 V (g), 1 (h), and 2V (i), and XPS Sb 3d + O 1s spectra
593
of Sb(V)-loaded TiO2-CNT filter at 0 V (j).
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0 mM
400
Sb@CNT-TiO2 2V Sb@CNT-TiO2 0V C8H4K2O12Sb2 KSb(OH)6
0.5
30470
30490
30510
30530
30550
Energy (eV)
200
0
Effluent Concentration (g/L)
2
4
6
(b)
60
40
0
8
120
Cycle 2
Cycle 3
80
40
90% 0
4000
8000
12000
HPO42-
(d)
80
60
40
20
0
90% 0
Bed Volumes
594
HCO3ClCompeting Anions
2V
160
Cycle 1
SO42-
100
(c)
NaOH Na2SO4+ (-2V)
Effluent Concentration (g/L)
0
0
10 mM +2 V
20
Sb(V)
200
10 mM
80 1.0
0.0
Sb(III)
5 mM
1.5
Sb (III) Removal (%)
Sbtotal
600
1 mM
100
(a)
2.0
Nomalized (E)
Effluent Concentration (g/L)
800
Page 30 of 31
1000
2000 3000 Bed Volumes
4000
595
Figure 4. (a) Changes in Sbtotal and Sb species as a function of time. (b) Effect of
596
competing anions on Sb(III) sorption. Experimental conditions: [Sb(III)]in of 200 μg/L,
597
flow rate of 1.5 mL/min, pH of 7, applied voltage of 2 V, [Na2SO4] of 10 mmol/L, and
598
recirculation mode. Sb K-edge EXAFS spectra and linear combination fits for
599
Sb@CNT-TiO2 at 0 V and 2 V (inset). (c) Comparison of two regeneration methods for
600
exhausted TiO2-CNT filters. Experimental conditions: [Sb(III)]in of 200 μg/L, flow rate
601
of 1.5 mL/min, pH of 7, and recirculation mode. (d) Performance of the TiO2-CNT
602
filter when loaded with Sb(III)-spiked tap water. Experimental conditions: [Sb(III)]in of
603
100 μg/L, applied voltage of 2 V, flow rate of 1.5 mL/min, pH of 7, and single-pass
604
filtration mode.
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605 606
Figure 5. (a) Sb(III) species adsorbed onto the TiO2 {001}facet (the bond length of Sb-
607
O is about 2.06 Å and those of Ti-O(H) are about 2.00 Å and 2.06 Å), and (b) Sb(III)
608
species adsorbed onto the TiO2 {100} facet (Sb-O bond length is about 2.69 Å and
609
those of Ti-O(H) are 2.09 Å and 2.12 Å). Color code: white sphere = H atom, red = O,
610
pink = Ti and yellow = Sb. Relatively large atoms are from the surface, smaller ones
611
are adsorbates. Density of state (DOS) for Sb(III) on the TiO2 {001} facet (c) and {100}
612
facet (d). The Fermi level energy is set to 0 for comparison.
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