Electroactive Modified Carbon Nanotube Filter for Simultaneous

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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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Environmental Science & Technology

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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†

6 7


8

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

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[email protected], [email protected]; Tel: +86 21 6779 8752.

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‡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.

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Institute

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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

1

ACS Paragon Plus Environment


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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

50

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]

53

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-

68

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


23]

and its removal


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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.

104 105

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

113

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

127

(TiO2). Finally, the TiO2-CNT filter was dried in an oven at 50 °C before use.

128

Characterizations of the as-fabricated filters are detailed in the SI.

129

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

132

mL of 750 µg/L Sb(III) at a pH 7. The flask was sealed and put into a shaker at 25 °C

133

and 150 rpm for 8 h. Aliquots were sampled to examine the sorption kinetics. For

134

recirculated filtration, the filter was transferred into an electrochemistry-modified

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Whatman polycarbonate filtration casing (SI Figure S1). The electrochemical filtration

136

setup has been described in detail previously.[29] One hundred milliliters of Sb(III)

137

solution was pumped at a flow rate of 1.5, 3, or 6 mL/min through the TiO2-CNT filter

138

and then returned. The applied voltage was 0–2 V and the solution pH was adjusted to

139

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

142

spectrometer (ICP-MS, Waltham, MA) and the concentration of Sb(III) was determined

143

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

196

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

199

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

237

limited sorbents (2.4 mg) used in this work, a comparable normalized sorption capacity

238

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

251

production of ROS via a photocatalytic route can be eliminated. The increased sorption

252

kinetics and capacity can be partially ascribed to enhanced near-surface transport by

253

electromigration as well as improved electrostatic interactions between the positively

254

charged filter surface and the negatively charged Sb(V). Furthermore, a recent DFT

255

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

257

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

262

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

268

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.

271

The chemical composition of a TiO2-CNT filter before and after Sb adsorption was

272

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

275

peak at a binding energy of 284.8 eV was assigned to C 1s, and no change to the C 1s

276

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

280

voltage (e.g. 0, 1, and 2 V) over a small window is displayed in Figures 2g-i. At 0 V,

281

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

283

species, like H2O), Sb 3d5/2 (i.e. Sb(V)),[14, 35] and Olatt (i.e. lattice oxygen).[14] This

284

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,

288

and its content increased as the applied voltage increased from 0 V to 2 V. Moreover,

289

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

291

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

299

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

301

h, and then showed a linear declining trend. The initial increase in Sb(V) concentration

302

suggest that a continuous oxidation process occurred during the filtration. Sb(V) can

303

also be sequestered by the nanoscale TiO2 simultaneously. Initially, the Sb(III)

304

oxidation rate was higher than the Sb(V) sequestration rate, so the Sb(V) concentration

305

kept rising. After 3 h, Sb(V) became the dominant Sb species, which was further

306

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

309

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

312

compounds for Sb(V) and Sb(III), respectively), suggesting that Sb was in a mixture

313

oxidative states of Sb(V) and Sb(III). However, the XANES spectra of an exhausted

314

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

316

on this discussion, we believe two pathways are involved in Sb(III) removal. The

317

dominant pathway is Sb(III) oxidation to Sb(V) by the electrical field followed by Sb(V)

318

sequestration by TiO2. However, another pathway is direct Sb(III) sequestration by

319

TiO2 and then oxidation to Sb(V), which cannot be excluded.

320

Effect of Solution Chemistry. Figure 4b presents the impact of certain ubiquitous

321

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.

325

Sb(III) was chemisorbed onto the nanoscale TiO2, whereas sulfate, chloride, and

326

carbonate were mainly removed by electrostatic interaction. In contrast, the presence

327

of 1–10 mmol/L phosphate suppressed 10–13% of the Sb(III) removal efficiency in the

328

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

330

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

338

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

353

examine the practical application potential of the proposed technology, the TiO2-CNT

354

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%.

357

A removal efficiency of 50% could still be obtained after 4000 bed volumes. Such

358

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


Page 18 of 31

Page 19 of 31


373

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



395

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: …...

416 20


Page 20 of 31

Page 21 of 31


417

Notes

418

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-

429

005710).

430 431

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432

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550

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


Page 26 of 31

Page 27 of 31

579


Figures

580 581

Figure 1. FESEM and TEM characterizations of CNT (a, b) and TiO2-CNT (c, d) filter.

27



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


Page 28 of 31

Page 29 of 31


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).

29



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)


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)


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.

30


Page 31 of 31


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.

31


Electroactive Modified Carbon Nanotube Filter for Simultaneous

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