Degradation of the Common Aqueous Antibiotic Tetracycline using a

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Degradation of the Common Aqueous Antibiotic Tetracycline using a Carbon Nanotube Electrochemical Filter Yanbiao Liu, Han Liu, Zhi Zhou, Tianren Wang, Choon Nam Ong, and Chad David Vecitis Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00870 • Publication Date (Web): 09 Jun 2015 Downloaded from http://pubs.acs.org on June 17, 2015

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Environmental Science & Technology

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Degradation of the Common Aqueous Antibiotic Tetracycline

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using a Carbon Nanotube Electrochemical Filter

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Yanbiao Liu,†,‡ Han Liu,† Zhi Zhou, ǁ,§ Tianren Wang,ǁ

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Choon Nam Ong, ‡ and Chad D. Vecitis*,†

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Revision Submitted May 7th 2015

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†

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Massachusetts, United States 02138

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‡

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Engineering Drive 1, #02-01, Singapore 117411

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§

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Purdue University, West Lafayette, Indiana, United States 47907

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ǁ

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1 Engineering Drive 2, E1A-07-03, Singapore 117576

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*Corresponding author: Harvard School of Engineering and Applied Sciences, Pierce

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Hall 120, 29 Oxford Street, Cambridge, MA 02138, E-mail: [email protected],

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Phone: +1 617-496-1458

School of Engineering and Applied Sciences, Harvard University, Cambridge,

NUS Environmental Research Institute, National University of Singapore, 5A

Division of Environmental and Ecological Engineering and School of Civil Engineering,

Department of Civil and Environmental Engineering, National University of Singapore,

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1 ACS Paragon Plus Environment


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Abstract

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In this work, a carbon nanotube (CNT) electrochemical filter was investigated for

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treatment of aqueous antibiotics using tetracycline (TC) as a model compound.

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Electrochemical filtration of 0.2 mM TC at a total cell potential of 2.5 V and a flow rate

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of 1.5 mL min-1 (hydraulic residence time 1.5 V) required for

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the rapid degradation of more persistent aqueous contaminants. Carbon nanotubes (CNT)

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However, these carbon electrode materials are easily



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may offer a solution due to their improved mechanical & electrical properties and thermal

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& chemical resistance.15-18 CNTs can be easily formed into porous 3D networks for

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contaminant sorption (SSA ~ 30-500 m2 g-1) and electrochemical degradation (σ ~ 104-106

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S m-1).19-22 Further, the electrochemistry could reduce the filter fouling rate by in situ

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foulant destruction and biological inactivation23-25 and in turn reduce the frequency of

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physical and/or chemical cleanings required to maintain optimal permeability.26

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In this study, the removal and degradation of tetracycline (TC), a model antibiotic

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commonly detected in the aqueous environment, by a CNT electrochemical filter was

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systematically investigated. First, cyclic voltammetry (CV) and breakthrough curves were

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completed to investigate TC electron transfer and sorption processes, respectively. Then,

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the oxidation flux as a function of total cell potential, cathode material, and NOM

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concentration was examined. Finally, the removal of anti-microbial activity from reservoir

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water and wastewater treatment plant (WWTP) effluent spiked with TC by

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electrochemical filtration was investigated.

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MATERIALS AND METHODS

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Chemicals and Materials

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Sodium sulfate (≥99.0%, Na2SO4) and tetracycline (≥98.0%, C22H24N2O8·xH2O, TC)

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were purchased from Sigma-Aldrich (St. Louis, MO). Suwannee River natural organic

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matter (SR-NOM) was purchased from the International Humic Substances Society (St.

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Paul, MN). Aqueous solutions were prepared with DI water from a Barnstead Nanopure

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Infinity purification system with a resistivity of ≥18 MΩ cm−1 (DI-H2O). Porous multi-

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walled CNT networks27 ( = 15 nm and = 100 µm, as provided by manufacturer)

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were purchased from NanoTechLabs (Buckeye Composites, Yadkinville, NC) in 4 ACS Paragon Plus Environment

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preformed circles of 40 ± 10 µm thickness and 47 ± 2 mm diameter (Figure S1). The CNT

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filters were wetted and washed with 100 mL ethanol and then 250 mL DI-H2O by vacuum

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filtration. Na2SO4 was used as the background electrolyte (10 mM) to control ionic

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strength and the influent TC concentration was 0.2 mM. The reservoir water and WWTP

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effluent sample was kindly provided by Singapore Public Utilities Board (PUB), filtered

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using a 0.45-µm Nylon syringe filter (Environmental Express, Charleston, SC), and stored

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in the dark at 4 oC until use.

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CNT Electrochemical Filtration Apparatus

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TC electrochemistry and/or filtration experiments were conducted as previously

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described.27,

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(Omnipore JWMP; Millipore; dpore = 5 µm; filtration area = 706 mm2) and placed into an

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electrochemistry-modified polycarbonate filtration casing (Whatman). The fluid to be

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treated flowed first through either a perforated Ti (Figure S2A) or a CNT network (Figure

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S2B) cathode, followed by an insulating Si-rubber O-ring or a PTFE membrane that

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separated the electrodes, respectively, and finally an anodic CNT network. The CNT

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networks were connected to a power supply via mechanical contact with a Ti current

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collector. A difference between the two setups is the inter-electrode distance; with the Ti

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cathode the distance was the rubber O-ring thickness (0.46 mm) and with a CNT cathode

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the distance was twice the thickness of the PTFE membrane (0.11 mm).

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Briefly, a porous CNT network was supported by a PTFE membrane

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After sealing the filtration casing and priming with water, a peristaltic pump

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(Masterflex) was used to flow DI-H2O through the filter at 1.5 mL min-1 to rinse and

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calibrate flow rate. The electrochemistry was driven by a DC power supply (Agilent

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E3646A). Effluent aliquots were collected and analyzed by an Agilent 1290 UPLC



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(Waldbronn, Germany) coupled to a 6540 quadrupole-time of flight (Q-TOF) mass

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detector equipped with a dual jet stream electrospray ionization source and managed by a

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MassHunter workstation (details were available in SI).

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Electrochemical Characterization

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The cyclic voltammetry, open-circuit potential and electrochemical impedance

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spectroscopy were completed with a CHI604D electrochemical analyzer (Austin, TX)

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using a three-electrode system: a CNT working electrode, an Ag/AgCl reference electrode,

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and a Ti or CNT counter electrode. Characterization details can be found in SI.

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Breakthrough Curve Measurement

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The breakthrough curve experiments were carried out using a CNT filter with an influent

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flow rate of 1.5 mL min-1 in the absence of applied potential. The TC influent

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concentration was 0.2 mM and effluent aliquots were collected every min until the effluent

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concentration was equal to the influent concentration.

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Material Characterization and Effluent Analysis

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Details of the CNT network characterization, liquid chromatography-mass spectrometry

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(LC-MS) effluent analysis, antimicrobial susceptibility tests, minimum inhibitory

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concentration (MIC) tests, and total organic carbon analysis were available in SI.

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RESULTS AND DISCUSSION

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Tetracycline Electrochemistry and Filtration

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Cyclic Voltammetry and Breakthrough Curve

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The fundamental TC electrochemical and sorption processes on a CNT network were

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examined by cyclic voltammetry and breakthrough curve analysis, respectively, as

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displayed in Figure 1. As shown in Figure 1A, the anodic peak observed at +0.52 V (vs. 6 ACS Paragon Plus Environment

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Ag/AgCl) may be ascribed to TC oxidation (e.g., dimethylamino group)29-31 and the rise in

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oxidative current when V > +0.8 V (vs. Ag/AgCl) is likely due to water oxidation (2H2O

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→ O2 + 4H+ + 4e-).32 Meanwhile, the reduction peaks at +0.45 V (vs. Ag/AgCl) and +0.66

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V (vs. Ag/AgCl) are likely due to oxygen reduction (O2 + 2H+ + 2e- → H2O2, E0 = 0.465 V

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vs. Ag/AgCl) and TC oxidation product or hydroxyl group reduction, respectively.33, 34

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The adsorption breakthrough curves for [TC]in = 0.2 mM onto the CNT filter in the

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absence of electrochemistry is shown in Figure 1B. For the first few (7-12) minutes, TC

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was not detected in the effluent indicating quantitative sorption within the hydraulic

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residence time of τ = 1.2 s. TC is an amphoteric molecule with multiple functional

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groups/moieties (e.g. phenol, amino, alcohol, diketone) and will be zwitterionic at neutral

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pH due to the tricarbonyl hydroxyl (pKa1=3.32; negative), phenolic diketone (pKa2=7.78;

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neutral), and dimethylamine (pKa3=9.58; positive) functional groups.35, 36 The rapid TC

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adsorption can be attributed to its relatively strong van der Waals, π-π, and cation-π

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interactions with the sp2-conjugated CNT sidewalls.37 The specific surface area (m2 g-1),

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TC sorption capacity (mg g-1), and area per TC sorbed (Å2 per molecule) were 88.5, 142.2,

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and 48.5, respectively (Table S1). It is of note that the TC sorption capacity of 142.2 mg g-

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1

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capacity of 168.8 mg g-1 obtained from batch isotherms using the same material. Although

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TC appears planar in 2D representations, it has significant 3D molecular curvature

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(Scheme S1).38 The molecular area of TC has been estimated to be 17-52 Å2,39 with the

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low values corresponding to the smallest 2D cross-section of the 3D structure and the high

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values corresponding to surface area of the curved 3D structure. The calculated surface

to the CNT network in flow through mode was comparable to the equilibrium sorption



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area (48.5 Å2) per TC molecule sorbed falls into the higher end of this range indicating

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that TC adsorption occurs until monolayer formation.

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TC sorption onto CNTs has been previously studied.40-42 For example, CNTs (320

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mg L-1) with high specific surface area (>500 m2 g-1) at an initial TC concentration of 0.11

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mM had a batch sorption capacity of 3.4 × 10-4 mg m-2 after 15 min of contact time.41 For

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comparison, in this work although the CNTs had a 5-fold lower specific surface area (88

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m2 g-1) at a similar initial TC concentration (0.20 mM), a greater sorption capacity of 0.67

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mg m-2 was observed after 15 min of flow through the filter. The improved sorption

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capacity in the current work can be ascribed to the enhanced mass transfer in the filtration

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as compared to the batch system that will rapidly transport TC to the CNT surface.32

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Additionally, in the batch system the CNTs will aggregate due to their high surface

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energy, which will block sorption sites reducing sorption capacity. In contrast, the filter

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CNTs are well-dispersed since they were vacuum filtered from a recently sonicated

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DMSO solution.43 However, the thinness of the CNT filter (~40 µm) results in an absolute

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sorption capacity that is relatively low and in turn TC breakthrough occurs within 0.5 h.

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Thus, further experiments were conducted in an attempt to electrochemically-degrade the

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TC and regenerate adsorption sites.

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Tetracycline Electrooxidative Filtration Kinetics

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The electrooxidative filtration of TC was examined as a function of total cell potential and

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cathode material as displayed in Figure 2A. For the Ti cathode and CNT anode, minimal

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oxidation was observed at a total cell potential of 1.5 V and at >1.5 V the TC oxidation

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flux increased with increasing cell potential to a maximum of 0.025±0.001 mol hr-1 m-2 at

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2.5 V. The LC-MS results indicate that the characteristic TC peak (m/z = 445.16175) was


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observed in the influent sample and was significantly decreased by 60.9% and 96.3% at

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cell potentials of 2.0 and 2.5 V (Figure S3), respectively, indicating the parent TC

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molecule had been degraded. Further increase of the total cell potential over 2.5 V reduced

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the filter performance due to the increased gas bubble formation (O2 at anode and H2 at

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cathode), which may clog CNT pores, block the electroactive sites, or even degrade filter

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integrity at higher potentials.44

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In regards to cathode material, a recent study indicated that by utilizing a CNT

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network cathode rather than a perforated Ti cathode, the overall cell potential could be

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reduced by 1 V.28 Open circuit potential measurements over a range of cell potentials for

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both Ti and CNT cathodes were conducted with [TC]in = 0.2 mM as displayed in Figure

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2B. At similar total cell potentials, greater average anode potentials were obtained with the

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CNT cathode as compared to the Ti cathode. For example, to achieve an anode potential

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high enough (>+0.5 V) for TC oxidation, a total cell potential of 1.0 V was necessary with

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the CNT cathode, whereas a total cell potential of 1.5 V was required with the Ti cathode.

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This was further supported by the TC electrooxidation kinetics and current efficiency as a

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function of cathode material as displayed in Figure 2A and Figure S4. At low cell

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potentials, the system with the CNT cathode had significantly higher TC oxidation flux as

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compared to the Ti cathode (Figure S5). For instance, at 1.0 V cell potential, the TC

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oxidation flux for the CNT cathode was 0.020±0.001 mol hr-1 m-2, 2.3-fold higher than

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that of Ti cathode under similar conditions. At 1.5 V cell potential, the oxidation flux

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using a CNT cathode was 0.024±0.001 mol hr-1 m-2, which was still 1.8-fold higher than

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that of Ti cathode under similar conditions. A similar phenomenon was also observed in

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the change of total organic carbon (TOC) during TC oxidation (Figure S6).



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There are two possible reasons for the enhanced performance when using a CNT

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cathode. First, the CNTs catalyze water reduction (0.4 V underpotential) to hydrogen, as

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previously reported for a range of elemental carbon materials,45 increasing the fraction of

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the total cell potential going towards the anode for TC electrooxidation. Second, the

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increased specific surface area of the CNT cathode will result in increased anode

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potential.46 For example, the Ti cathode has a total surface area of 0.024 mol hr-1 m-2 oxidation flux of influent TC (>95%) in a single pass through the

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CNT filter at a relatively low cell potential (98% degradation over the 3 h experiment.

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Antimicrobial Activity of Tetracycline Electrooxidation Products

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Major concerns with TC release e.g., in WWTP effluent, is its potential antimicrobial

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effects on natural ecosystems and the evolution of pathogen resistance to antimicrobial

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agents.49 To evaluate the antimicrobial activity of the influent and effluent samples, the

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disk agar-diffusion biocidal susceptibility method was performed by drop addition of the

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sample to an Escherichia coli (E. coli) Luria-Bertani (LB) plate inoculum. The E. coli

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susceptibility results are presented in Figure S16. The influent TC samples have an

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inhibition zone with a diameter of ~13.0 mm, while an inhibition zone is not visually

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present in the three effluent samples, indicating the antibacterial activity of the TC

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electrooxidation products was significantly reduced as compared to influent TC, or

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possibly because the effluent TC was below the minimum inhibitory concentration (MIC)


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of E. coli (Figure S17 and Table S4). Hence, at all cell potentials, the TC electrooxidative

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degradation is sufficient to significantly reduce its antimicrobial activity.

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Electrochemical Filtration of Tetracycline Spiked Reservoir Water and WWTP

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Effluent

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Effect of NOM

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SR-NOM was selected as a model dissolved natural organic matter due to the prevalence

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of humic-like substances in surface waters and wastewater effluent.50 Electrochemical

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filtration was completed with three different SR-NOM concentrations (1, 5, 10 mg L-1)

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using CNT networks as both the anode and cathode (Figure 4). Addition of SR-NOM

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negatively effects the TC oxidation flux; the oxidation flux at 1 mg L-1 SR-NOM

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(NOM:TC = 1.1%) and 5 mg L-1 SR-NOM (NOM:TC = 5.7%) was decreased by 18-22%

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in comparison with flux obtained in the absence of SR-NOM, in accordance with previous

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studies.51 A further increase in NOM concentration to 10 mg L-1 (NOM:TC = 11.4%)

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resulted in a slight further decrease in TC oxidation flux to 0.017 and 0.019 mol hr-1 m-2,

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at cell potentials of 1.5 V (30% decrease) and 2.0 V (21% decrease). This decrease is

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caused by the competitive adsorption and electrooxidation of NOM by the CNT

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electrochemical filter.52,

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NOM:TC ratio suggesting that the larger NOM fraction likely adsorbs strongly to the CNT

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and may block a large number of surface sites.

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Effect of Reservoir Water and WWTP Effluent Matrices

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To explore electrochemical filtration of TC in real water samples, 0.2 mM TC was spiked

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into both water collected from a Singapore reservoir and WWTP effluent (characteristics

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in Table S2) before the filtration experiment. The electrooxidation kinetics in both water

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As the decrease in TC oxidative flux is greater than the



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samples displays a similar trend to that in DI water. At 2.0 V, the TC spiked reservoir

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water and WWTP effluent oxidation flux was 0.015±0.001 and 0.022±0.001 mol hr-1 m-2,

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respectively, 40% and 10% lesser than in DI water (Figure 5A). Although the reservoir

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water TOC was only 4.1 mg L-1 (4.6% of TC) lesser than that of the WWTP effluent at 7.5

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mg L-1 (8.4% of TC), the lower conductivity (269 vs. 508 µS cm-1) and relatively complex

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natural reservoir organic matrix may account for the more significant decrease in reaction

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rate. The average TC oxidation flux over 3 h in reservoir water and WWTP effluent was,

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respectively, 0.019 and 0.021 mol hr-1 m-2 (Figure S18), 24% and 12% less than that in DI

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water matrix as displayed in Figure 3. The biocidal susceptibility results presented in

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Figure 5B and Figure S19 demonstrates that the electrochemical filtration process also

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significantly decreases the anti-microbial activity of TC in both real water samples. Thus

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the CNT electrochemical filter is a promising technology for removal of antibiotics

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biocidal activity from various water matrixes.

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Energy Consumption

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The energy consumption for electrochemical TC filtration is calculated at a total cell

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potential of 2.5 V, by assuming 31 electrons transferred per TC molecule, to be 1.34 kWh

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kgCOD-1 for the CNT cathode, and by assuming 15 electrons transferred per TC molecule,

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to be 1.75 kWh kgCOD-1 for the Ti cathode. The CNT cathode filter can achieve similar

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performance at a lower cell voltage (e.g. 1.0 and 1.5 V) and in turn the energy consumed is

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decreased to ~0.7 kWh kgCOD-1 at 1.5 V. These values are comparative to or lower than

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state-of-the-art electrochemical oxidation processes with energy consumptions in the range

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of 5-100 kWh kgCOD-1.54, 55 Alternatively, the energy per volume treated is calculated to

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be 0.084 kWh m-3, lower than the state-of-the-art electrochemical processes with energy


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consumption in the range of 0.1-40 kWh m-3,56-58 as well as that for AOPs used for

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wastewater effluent polishing with typical energy consumptions of 1-10 kWh m-3.7,

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Compared to the electrical energy consumption per cubic meter of conventional municipal

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wastewater treatment (0.80 kWh m-3),59 addition of electrochemical filtration as an

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effluent polishing step would not add a significant operational cost (~10%) to the overall

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treatment process.

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Environmental Applications of an Electrochemical CNT Filter

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Trace levels of antibiotics in wastewater effluent and drinking water may present both an

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ecosystem and human health risk due to the rise of antibacterial activity, pathogen

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antibiotic resistance, and toxicity to the downstream environments. Therefore, there is an

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urgent need to develop cost-effective and efficient antibiotic treatment technologies. In

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this study, effective, efficient, and rapid tetracycline oxidation and antibiotic activity

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removal was achieved during a single pass through an electrochemical CNT filter and

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there was minimal performance loss after 3 h of continuous operation regardless of the

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water matrixes. Additionally, the electrochemical filter could utilize organic contaminants

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such as antibiotics as sacrificial electron donors for cathodic hydrogen production further

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reducing energy requirements. A solar panel could provide required electricity, thus CNT

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electrochemical filters could be used as a point-of-use device for efficient removal of

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antibiotics in water and wastewater in rural areas or developing countries. Further

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experiments are currently underway to better understand and further optimize the

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electrochemical filtration process.

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Supporting Information


8


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Detailed descriptions of the electrochemical process, material characterization, LC-MS

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method and results, antimicrobial susceptibility tests, minimum inhibitory concentration

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(MIC) tests, schematic and electrochemical characterizations of the CNT electrochemical

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filter can be found in the supporting information. This material is available free of charge

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via the Internet at http://pubs.acs.org.

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ACKNOWLEDGEMENTS

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This work is supported by the Singapore National Research Foundation under its

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Environment and Water Technologies Strategic Research Programme and administered by

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the Environment and Water Industry Programme Office (EWI) of the Public Utilities

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Board on project 1102-IRIS-14-03. We thank Professor Guandao Gao at Nankai

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University, Ms. Qiaoying Zhang and Ms. Mary Schnoor at Harvard University for their

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valuable technical discussions, thank Dr. Liang Gao, Dr. Hui Zhang and Ms. Per Poh

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Geok at NUS Environmental Research Institute for their kind guidance on the HPLC-

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MS/QTOF, and thank Dr. Xu Li and Dr. Chao Chen at Institute of Materials Research and

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Engineering (IMRE) for their kind help on the XPS analysis. Y. Liu also thanks the NERI-

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GE research support (GE-NERI 2014, Grant number of R-706-005-004-592).


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Figures:

Current (A)

0.03

A

0.02

0.01

0.00 0.0

0.2

0.4

0.6

0.8

1.0

Potential (V vs. Ag/AgCl) 1.0

B

CEff / CIn

0.8 0.6 0.4 0.2 0.0 0

5

10

15

20

25

30

Time (min)

Figure 1. Tetracycline CV and Breakthrough Curve on a CNT Filter. (A) TC and Na2SO4 (in dash) cyclic voltammetry at a scan rate of 20 mV s-1 and (B) TC adsorption breakthrough curve with [TC]in = 0.2 mM, [Na2SO4]in = 10 mM, and J = 1.5 mL min−1 for all experiments.


Oxidation Flux (mol hr-1 m-2)


0.03

0.02

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Ti Cathode CNT Cathode

A

0.01

0.00

0.5

1.0

1.5

2.0

2.5

3.0

Potential (V vs. Ag/AgCl)

Total Cell Potential (V) 1

B Anode Cathode (CNT) Anode Cathode (Ti)

0

-1

-2 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Total Cell Potential (V) Figure 2. Tetracycline Electrooxidative Flux and Anode/Cathode Potential as a Function of Total Cell Potential and Cathode Material. (A) TC electroxidative flux (mol h-1 m-2) and (B) anode/cathode potential (V) at total cell potentials of 0-3 V with [TC]in = 0.2 mM, [Na2SO4] = 10 mM, and J = 1.5 mL min−1 for all experiments. Red denotes use of CNT cathode and black denotes use of Ti cathode.



Oxidation Flux (mol hr-1 m-2)

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0.03

0.02

0.01

0.00

0

30

60

90

120

150

180

Time (min) Figure 3. Tetracycline electrooxidation flux as a function of time. CNT networks were used for both the cathode and anode, the cell potential was 1.5 V, [TC]in = 0.2 mM, [Na2SO4] = 10 mM, and J = 1.5 mL min−1. The inset is an SEM image of the anodic CNT filter after 3 h of continuous operation. The white circles and squares showed, respectively, some extent of inorganic or organic build-upon the CNT surface. Scale bar was 2 µm.


Oxidation Flux (mol hr-1m-2)


0.03 1.5 V 2.0 V

0.02

0.01

0.00 0

1

5

NOM Concentration

10

-1 (mg L )

Figure 4. Tetracycline Electrooxidation Flux as a Function of NOM Concentration and Cell Potential. [TC]in = 0.2 mM, [Na2SO4] = 10 mM, and J = 1.5 mL min−1 for all experiments. The black bars are for a cell potential of 1.5 V and the red bars are for a cell potential of 2.0 V. The influent SR-NOM concentration (0, 1, 5, and 10 mg L-1) was varied over the typical range found in surface waters.


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Oxidation Flux (mol hr-1m-2)

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0.03

Reservoir Water WWTP Effluent

A

0.02

0.01

0.00

1.0

1.5

2.0

2.5

Total Cell Potential (V) B

Figure 5. Electrooxidative Flux (completed in at least duplicate) and Anti-Microbial Activity of TC Spiked Reservoir Water and WWTP Effluent. (A) Electrooxidative flux as a function of cell potential for TC spiked reservoir water (black bars) and TC spiked WWTP effluent (red bars) and (B) anti-microbial activity by disk agar-diffusion susceptibility method of the influent and effluent of the TC spiked WWTP where ‘BF’ represents the sample before filtration and ‘AF’ represents the sample after filtration. [TC]in = 0.2 mM, [Na2SO4] = 10 mM, cell potential of 2.0 V, and J = 1.5 mL min−1 for all experiments.



TOC Art


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Degradation of the Common Aqueous Antibiotic Tetracycline using a

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