Radical Precursor Versus Electron Acceptor

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Exploring the Role of Persulfate in the Activation Process: Radical Precursor Versus Electron Acceptor Eun-Tae Yun, Ha-Young Yoo, Hyokwan Bae, Hyung-Il Kim, and Jaesang Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02519 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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Environmental Science & Technology

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Exploring the Role of Persulfate in the Activation Process:

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Radical Precursor Versus Electron Acceptor

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Eun-Tae Yun1, Ha-Young Yoo1,Hyokwan Bae2, Hyoung-Il Kim3, and Jaesang Lee1*

4 5 6

1

School of Civil, Environmental, and Architectural Engineering, Korea University, Seoul 136-701, Korea

2

Department of Civil and Environmental Engineering, Pusan National University, Busan 46241, Korea

3

School of Civil and Environmental Engineering, Yonsei University, Seoul 120-749, Korea

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*Corresponding author: E-mail: [email protected]; phone: +82-2-3290-4864; fax: +82-2-928-7656

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Abstract. This study elucidates the mechanism behind persulfate activation by exploring the role

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of various oxyanions (e.g., peroxymonosulfate, periodate, and peracetate) in two activation

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systems utilizing iron nanoparticle (nFe0) as the reducing agent and single-wall carbon nanotubes

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(CNTs) as electron transfer mediators. Since the tested oxyanions serve as both electron

12

acceptors and radical precursors in most cases, oxidative degradation of organics was achievable

13

through one-electron reduction of oxyanions on nFe0 (leading to radical-induced oxidation) and

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electron transfer mediation from organics to oxyanions on CNTs (leading to oxidative

15

decomposition involving no radical formation). A distinction between degradative reaction

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mechanisms of the nFe0/oxyanion and CNT/oxyanion systems was made in terms of the

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oxyanion consumption efficacy, radical scavenging effect, and EPR spectral analysis. Statistical

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study of substrate-specificity and product distribution implied that the reaction route induced on

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nFe0 varies depending on the oxyanion (i.e., oxyanion-derived radical) whereas the similar

20

reaction pathway initiates organic oxidation in the CNT/oxyanion system irrespective of the

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oxyanion type. Chronoamperometric measurements further confirmed electron transfer from

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organics to oxyanions in the presence of CNTs, which was not observed when applying nFe0

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instead.

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Keywords: persulfate activation, oxidative degradation, sulfate radical, electron-transfer

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mediator

26 27

ACS Paragon Plus Environment


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INTRODUCTION

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Persulfate activation processes involving the formation of the highly reactive sulfate radical

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(SO4•−) have attracted increasing attention as alternatives to H2O2-based advanced oxidation

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processes, whereby H2O2 dissociates to form the hydroxyl radical (•OH).1,

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underpinned by potential technical advantages of the process, which include the availability of

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persulfates in a stable salt form, high-yield production of oxidizing radicals, and various strategic

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options for activation. The reduction potential of SO4•− compared to that of •OH

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(E0(SO4•−/SO42−) = 2.5 – 3.1 VNHE3; E0(•OH/OH−) = 1.8 – 2.7 VNHE4) indicates that the strong

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oxidizing power of SO4•− enables the persulfate activation system to oxidatively treat a broad

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spectrum of aquatic pollutants (e.g., herbicides, algal toxins, and heavy metals).5-7 In particular,

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the activated persulfate, known to exhibit maximized oxidizing capacity under neutral

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conditions,8 outperforms •OH in degrading select organics.9, 10 As SO4•− is more effective for

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electron abstraction than •OH, persulfate activation processes favor the one-electron oxidation of

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halide and hydroxyl ions ubiquitously present in natural waters.11,

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production of secondary radicals such as Cl• and •OH and associated acceleration of organics

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oxidation.11, 13 Similar to persulfate, select oxyanions such as peracetate (PA) and periodate (PI)

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can also undergo activation to initiate radical-induced oxidation reactions.14-16 For example,

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Since photochemical PA activation leads to •OH production, the presence of PA markedly

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improves the efficiency of UV disinfection (bacterial removal) in the tertiary effluent.17

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Additionally, various PI activation processes are accompanied by the formation of oxidizing

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radicals (i.e., •OH, iodate radical (IO3•)), which has been applied for the degradation of COD in

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wastewater,18 dye decolorization,19 and chlorophenol oxidation.15, 16


12

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Such interest is

This can lead to the

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Simple peroxides like H2O2, PA, HSO5− (peroxymonosulfate; PMS), and S2O82− (peroxydisulfate;

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PDS) are activated commonly through energy and electron transfer mechanisms. For instance,

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photon energy transfer via UV photolysis20 or one-electron reduction by ferrous ions21 results in

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homolytic fragmentation of the peroxide bond, leading to production of •OH from H2O2. UV-

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induced peroxide dissociation followed by •OH formation readily takes place with PA as a

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radical precursor.17 Similar strategies enable activation of persulfates; PMS and PDS decompose

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under UV light irradiation and in the presence of heat to form SO4•−,6, 11 and the reducing power

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of metal-based reagents (e.g., Co2+, Co3O4, and MnOx) allows homolytic cleavage of peroxide

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bonds in PMS and PDS, thus resulting in SO4•− production and associated degradation of organic

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substrates.13,

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behind various persulfate activation approaches through a number of previous observations,

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including: through the quenching (knock-out) effects of alcohols (e.g., ethanol),24 formation of

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chlorinated intermediates in the presence of excess Cl−,13 and electron paramagnetic resonance

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(EPR) detection of radical adduct(s).1 On the other hand, evidence of SO4•− formation was not

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identified

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nanodiamond,27 and noble metals (i.e., Pd and Au)28 were used as persulfate activators, despite

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observed reactivity. This suggests the possibility of an alternative reaction pathway that does not

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involve SO4•−. While such a non-radical reaction mechanism remains poorly characterized, it

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seems to preferentially take place on conductive materials (e.g., carbon nanotubes and noble

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metals) that likely enhance the electron delivery from organics to persulfates.

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Persulfates have a peroxide bonding environment that is vulnerable to one-electron reduction,

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allowing for SO4•− formation through reductive transformation. Alternatively, PMS and PDS

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show a strong tendency to withdraw electrons owing to their sufficiently negative reduction

22, 23

when

The SO4•−-induced oxidation has been confirmed as the main mechanism

CuO,25

multi-walled

carbon

nanotubes


(MWCNTs),26

graphitized


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potentials (E0(HSO5−/HSO4−) = 1.82 VNHE29; E0(S2O82−/HSO4−) = 2.08 VNHE30). The dual

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functionality of persulfate as a precursor of oxidizing radicals and as an electron acceptor has

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been utilized in environmental remediation systems based on redox chemistry. For example,

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persulfate markedly improves the photocatalytic activity of TiO2 and WO3 in two ways: by

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producing oxidizing radicals (i.e., SO4•−) through reductive conversion using conduction band

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(CB) electrons (radical precursor) as well as impeding charge recombination by quenching CB

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electrons (electron acceptor).31, 32 This leads us to hypothesize a dual role of persulfate in the

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activation mechanism that varies depending on the choice of activator: Transition metals initiate

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one-electron reduction of persulfate to SO4•− (radical mechanism), whereas carbonaceous

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nanomaterials mediate the electron transfer from organic pollutants to persulfate (non-radical

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mechanism). In particular, given that the role of persulfate as an electron acceptor is fundamental

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to the non-radical mechanism, the degradative reaction that does not rely on oxidizing radical

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species would take place on the carbon-based materials when appropriate electron acceptors are

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present (as an alternative to persulfate).

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To identify the role of persulfate in the activation process, herein we explore the mechanisms

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behind 4-chlorophenol (4-CP) oxidation by various oxyanions (i.e., PMS, PDS, and their proxy

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compounds like PA, PI, pyrophosphate (PP), H2O2, and bromate (BR)) along with two model

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activators: zerovalent iron nanoparticles (nFe0) (reducing agent) and single-walled carbon

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nanotubes (CNTs) (electron transfer mediator). Similar to the well understood radical

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mechanism of persulfate activation, the one-electron reduction of select oxyanions as radical

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precursors (e.g., PMS, PA, and PI) on nFe0 is hypothesized to initiate radical-induced

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degradation. On the other hand, when applying CNTs as an activator, oxidative degradation

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relying on electron transfer mediation is hypothesized to occur with any tested oxyanion that


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serves as an appropriate electron acceptor. For these systems, we describe and discern a

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distinction between the activation mechanisms of the nFe0/oxyanion and CNT/oxyanion systems

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based on oxyanion stability, methanol quenching effect, EPR spectra, substrate-specificity, and

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intermediate/product distribution. Further, by monitoring current change upon addition of 4-CP

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we explore and confirm electron shuttling from the organic substrate to oxyanions via CNTs.

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MATERIALS AND METHODS

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Chemicals and Materials. The following reagents were used as received: Potassium

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monopersulfate (OXONE®, Sigma-Aldrich), potassium peroxydisulfate (Sigma-Aldrich), sodium

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periodate (Sigma-Aldrich), sodium pyrophosphate tetrabasic decahydrate (Sigma-Aldrich),

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peracetic acid solution (32 wt. % in diluted acetic acid, Sigma-Aldrich), sodium bromate (Sigma-

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Aldrich), hydrogen peroxide solution (30 wt. % in water, Sigma-Aldrich), benzoic acid (Sigma-

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Aldrich), bisphenol A (Aldrich), carbamazepine (Sigma-Aldrich), 4-chlorophenol (Aldrich), 4-

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nitrophenol (Aldrich), phenol (Sigma-Aldrich), nitrobenzene (Sigma-Aldrich), methanol (Sigma-

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Aldrich), perchloric acid (Sigma-Aldrich), sodium hydroxide (Sigma-Aldrich), sodium carbonate

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(Sigma-Aldrich), sodium bicarbonate (Sigma-Aldrich), acetone (Samchun Chemical), SWCNT

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(Nanolab. Inc.), 5,5- dimethyl-1-pyrrolidine-N-oxide (DMPO) (Sigma-Aldrich), iron(III)

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perchlorate hydrate (Sigma-Aldrich), sodium borohydride(Sigma-Aldrich), phosphoric acid

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(Sigma-Aldrich), deuterium oxide (99.9 at. % D, Aldrich), and acetonitrile (J.T. Baker).

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Ultrapure deionized water (> 18 MΩ•cm), produced with a Millipore system, was used for the

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preparation of all experimental solutions. All chemicals were of reagent grade and were used

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without further purification or treatment.



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Preparation of nFe0. Zerovalent iron nanoparticles were prepared using a mild chemical

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reduction method published by Lee et al.33 Briefly, a 4 g/L aqueous sodium borohydride solution

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was slowly added into a magnetically-stirred 200 mL beaker containing 5 g/L ferrous sulfate. To

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minimize surface oxidation of the resultant iron nanoparticles, reduction was performed under an

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anoxic condition. The product – a black powder – was washed thrice with N2-saturated deionized

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water and acetone and dried for 2 h at room temperature. The nanoscale iron particles were

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stored in a completely-sealed amber bottle prior to use. TEM images presented in Figure S1

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show that the nFe0 particles with sizes ranging from 10 to 100 nm were mostly spherical and

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formed chain-like clusters.

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Characterization of CNTs. To determine metal contents in pristine CNTs, we dissolved 0.3 g

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pristine CNTs in 0.1 L 10 % (v/v) aqueous HCl solution and quantified the dissolved metal ions

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using inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 700-ES)

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after 24 h. Our previous study demonstrated no difference in persulfate activating capacity

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between fresh CNTs and HCl-treated CNTs.26 The metal impurities comprised ca. 5 % of the

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total mass of CNTs (Figure S2). Dynamic light scattering (DLS) analysis for particle size

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characterization of suspended CNTs was performed with ELSZ-1000 (Otsuka Electronics). The

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average particle size of CNTs was determined to be ca. 1.86 µm. Transmission electron

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microscopic analysis (TEM, JEOL JEM-2200FS) showed that the CNTs were less than ca. 2 nm

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in diameter and were up to several micrometers in length (Figure S3). Raman spectra of fresh

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and used CNTs were acquired on a LabRam ARAMIS Raman spectrometer (Horiba Jobin-Yvon)

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using an argon ion laser (excitation at 514.5 nm). In the spectrum of fresh CNTs (Figure S4), the

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graphite structure-derived G-band exhibited strong intensity at 1582 cm-1 whereas very weak D-

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band sensitive to structural defects appeared at 1350 cm-1.34 The intensity ratio of D-band to G-


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band (ID/IG) as a measure of the structural disorder of carbon-based materials was determined to

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be 0.033, which reflected that the CNTs had very low defect density and predominant graphitic

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nature. Raman spectrum in the low wavenumber range of 100 to 350 cm-1 (Figure S5a)

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demonstrated that Raman peaks around 160 – 200 cm-1 were more significant compared to those

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around 200 – 280 cm-1, which revealed the enrichment of semiconducting CNTs.35 In addition,

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the broad G-band typical of metallic CNTs was not observed in Figure S5b.35 To explore the

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change in surface chemical composition of CNTs after their use in oxyanion (i.e., PMS)

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activation, surface oxygen content of CNTs was analyzed by X-ray photoelectron spectroscopy

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(XPS, Thermo Scientific K alpha) using Al Kα lines as an excitation source, and surface

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functional groups on CNTs were identified by Fourier transform infrared spectroscopy (FT-IR,

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Thermo Scientific Nicolet 6700) performed in ATR (attenuated total reflectance) mode.

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Experimental Procedure and Analytical Methods. Oxyanion activation was performed under

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air-equilibrated conditions in a magnetically-stirred flask with a working volume of 100 mL.

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Typical experimental suspensions consisted of aliquots of 0.05 – 0.1 g/L activator (i.e., nFe0,

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CNT), 1 mM oxyanion (e.g., PMS, PI), and 0.1 mM organic substrate. A 1 mM phosphate buffer

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was used to avoid a drastic pH change over the course of oxyanion activation. Addition of nFe0

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or CNT as an activator initiated the reaction. Neither surfactant nor sonication was applied to

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enhance the dispersion of CNTs in water. Addition of anionic and non-ionic surfactants (sodium

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dodecyl sulfate and Brij 35) led to drastic kinetic retardation in phenol oxidation by the PMS

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activated with CNTs (Figure S6). Ultrasound irradiation significantly decreased the average

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particle size of the CNT aggregates (from ca. 1.86 µm to ca. 0.54 µm) (Figure S7), but it caused

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no enhancement in PMS activating capacity (Figure S8). 1 mL aliquots of the reaction

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suspension were withdrawn at predetermined time intervals as the reaction progressed, filtered



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using a 0.45- µm PTFE syringe filter, and transferred to a 1 mL amber vial containing 200 mM

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methanol as a quencher that prevented further radical-induced reaction. Degradation of target

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organic contaminants and PI was monitored by a high performance liquid chromatograph (HPLC,

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Shimadzu LC-20AD) equipped with a UV/vis detector (SPD-20AV) and a C-18 column

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(ZORBAX Eclipse XDB-C18). HPLC measurements were carried out using a binary eluent

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comprising 0.1 % (v/v) aqueous phosphoric acid and acetonitrile (typically 60:40 by volume).

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Intermediates involved in 4-CP oxidation by the activated oxyanions were qualitatively

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identified by a Rapid Separation Liquid Chromatography (RSLC) (UltiMate 3000, Dionex Co.)

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coupled with a Q Exactive™ quadrupole-Orbitrap mass spectrometer (Thermo Fisher Scientific

173

Inc.). The separation was performed on an acclaimTM C18 column (150 mm × 2.1 mm, 2.2 µm;

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Thermo Fisher Scientific Inc.) using a mixture of 0.1 % aqueous formic acid solution and

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acetonitrile as the mobile phase. Mass analysis was carried out in the negative electrospray

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ionization (ESI) mode. Accurate mass measurements were guaranteed with the low ppm range (
PDS > PI >> PP ≈ PA, resulting in the following order of

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dechlorination efficiency: PMS > PDS ≈ PI >> PP ≈ PA. In contrast, the 4-CP degradation rate

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was not necessarily proportional to the dechlorination efficiency in the nFe0-based systems

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(Figure 1a and Figure S10a). For instance, the nFe0/PI system caused a higher efficiency for 4-

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CP removal than the systems using PDS or PA as a radical precursor, but formation of chloride

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ions was basically absent. This is in line with the previous finding regarding the inability of IO3•

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to oxidatively dechlorinate chlorophenols.16 More significant chlorine release took place in the

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nFe0/PP system that in the nFe0/PDS system, with the extent of dechlorination being 41.71 ±

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3.28 % for PP versus 15.23 ± 3.28% for PDS, while nFe0/PDS degraded 4-CP around two-fold

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faster than nFe0/PP (it is noted that the combination of nFe0 with PP leads to effective

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dechlorination via •OH). Overall, such poor correlation suggests that radical-induced

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mechanism(s) likely dominates in the nFe0/oxyanion systems since the oxidative reaction

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pathway depends on the nature of the oxidizing radical itself. In contrast, the positive correlation

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observed in CNT/oxyanion implies the occurrence of a non-radical mechanism; an identical

for CNT/BR) (Figure S9).


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degradative route (i.e., electron transfer from 4-CP to oxyanion via CNT) occurs irrespective of

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the oxyanion type, assuming that all tested oxyanions play similar role(s) of electron acceptors.

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Reduction of oxyanions by carbon nanotubes and zerovalent iron. Figures S11a and S11b

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compare the rates of consumption of select oxyanions such as PMS, PDS, PI, and PA in the

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nFe0-based systems versus in the CNT-based systems. Oxyanion decomposition proceeded

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rapidly on nFe0 (PA completely disappeared within 20 min), whereas only 10 to 20% of

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oxyanions initially added were consumed in the aqueous CNT suspensions. Accordingly, high

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efficiency for degradation of 4-CP was maintained over five cycles without external oxyanion

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addition when CNTs were used as an activator (Figure 2b). In comparison, as oxyanions

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underwent rapid depletion by nFe0 in the first cycle, no residual oxyanions were available for

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further activation by nFe0, which resulted in no significant 4-CP degradation in the following

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cycles (Figure 2a). 4-CP was injected at an initial concentration of 100 µM and the used

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activators were exchanged with fresh ones in each cycle. Results presented here suggest that the

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amount of oxyanion required to achieve a certain level of treatment efficiency should be

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substantially reduced when CNTs are applied as the activator, highlighting the difference in the

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activation mechanism between nFe0/oxyanion versus CNT/oxyanion. Reductive conversion of

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oxyanions to reactive radicals by nFe0 probably takes place regardless of the presence of

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organics (e.g., 4-CP), eventually decreasing oxyanion concentrations to undetectable levels. In

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contrast, since consumption of oxyanions as electron acceptors in the CNT-based systems

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requires addition of organics as an electron donor, oxyanion reduction would not proceed further

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when 4-CP decomposes completely.



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To explore the chemical transformation of CNTs (indirect indication of oxidizing radical

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production) during oxyanion activation, characterization of exposed CNTs (collected after being

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subjected to 4-CP oxidation by the activated PMS for 1 h and 2 h) was performed with TEM and

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multiple spectroscopic techniques (i.e., XPS, FT-IR, and Raman). Comparison of TEM images

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(Figures S3a-S3c versus Figures S3d-S3f) shows no significant change in morphological features

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before and after the use of CNTs in PMS activation. Fresh and used CNTs showed almost similar

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ID/IG ratio (Figure S4), which confirmed that the oxidizing capacity of the activated oxyanion

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was unlikely to cause the structural defects in CNTs. The surface atomic composition determined

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by XPS analysis shows the minor change of surface oxygen content (i.e., ca. 4.52 atomic %

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(fresh CNTs) versus ca. 4.36 atomic % (used CNTs)) (Figure S12). The FT-IR spectra of used

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CNTs recorded in the range of 400 to 4000 cm-1 (Figure S13) also indicate low surface

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functionalization (comparatively); a broad –OH stretch band as an indication of surface

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hydroxylation41 did not appear between 3100 and 3600 cm-1 and new band at 1710 cm-1 that is

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attributed to carbonyl (C=O) stretching in the carboxylic group41 was not observed. These results

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collectively indicated that the exposure to oxidation reactions by the activated oxyanions did not

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lead to significant changes in morphological features, structural defect level, or surface

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functionalities, which suggest low, if any, contribution of oxidizing radicals to the CNT-

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mediated oxyanion activation.

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Quenching effect of methanol on oxidation of 4-chlorophenol. To explore the involvement of

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radical species in 4-CP oxidation, we monitored kinetic rates of 4-CP degradation by the

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nFe0/oxyanion and CNT/oxyanion systems in the absence and presence of excess methanol as a

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radical scavenger. The addition of methanol caused drastic retardation of 4-CP degradation in the


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nFe0/oxyanion systems in most cases (Figure 3a). The quenching effect of methanol was

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pronounced when combining nFe0 with PP or PA as •OH is primarily responsible for the

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oxidizing power of nFe0/PP or nFe0/PA. Even though SO4•− exhibits much lower reactivity

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toward methanol than •OH (k(MeOH + SO4•−) = 3.2 × 106 M-1s-1 42; k(MeOH + •OH) = 9.7 × 108

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M-1s-1 4), 4-CP decomposition was still significantly decelerated in the nFe0/persulfate systems

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when excess methanol was added. In contrast, the performance of nFe0/PI, where IO3• acts as the

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main oxidant, was negligibly reduced in the presence of methanol. This is similar to our previous

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findings, whereby methanol did not interfere with IO3•-induced oxidation of organic

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compounds.16

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In comparison to the drastically-decreased degradation efficiency of nFe0/oxyanion in the

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presence of methanol, methanol did not affect systems for which CNTs were applied to activate

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oxyanions (Figure 3b). Specifically, methanol addition led to no change in 4-CP degradation by

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CNT/PDS and CNT/PI. The performance reduction in 4-CP oxidation in the presence of

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methanol was much less significant with CNTs, with Ym/Y (Ym: 4-CP degradation efficiency in

337

the presence of methanol; Y: 4-CP degradation efficiency in the absence of methanol) = 0.717 ±

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0.057 for CNT/PMS versus Ym/Y = 0.332 ± 0.002 for nFe0/PMS. The inability of CNT/oxyanion

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to react with methanol was confirmed by monitoring the formation of formaldehyde as a result of

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methanol oxidation (Figure S14); nFe0 effectively converted methanol to formaldehyde in the

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presence of oxyanions, whereas production of formaldehyde was absent with CNT/oxyanion in

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most cases. These results collectively imply that the use of nFe0 allows oxyanions to act as

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radical precursors and initiate radical-induced oxidation while radical species marginally

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contribute to oxidative degradation in the CNT/oxyanion systems.

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EPR study. An EPR spin trapping technique was used to explore and identify the main oxidizing

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species in the select nFe0- and CNT-based activation systems. Figure 4a demonstrates that SO4•−

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forms when nFe0 activates PMS based on the EPR spectrum characteristic of the SO4•− adduct of

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DMPO. DMPOX (5,5-dimethylpyrrolidone-2-(oxy)-(1)) known to be generated as a result of

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direct DMPO oxidation involving no radical species43 was detected in the aqueous suspension of

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CNT/PMS or aqueous PMS solution. nFe0 coupled with PA caused the EPR spectral features

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assigned to •OH whereas no signal was found in the EPR spectrum of CNT/PA (Figure 4b).

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These results further confirm that oxyanions are transformed to oxidizing radicals through one-

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electron reduction on nFe0 whereas radicals are not involved in the oxidative degradation of

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organics by oxyanions activated with CNT.

356 357

Statistical analysis of substrate-specificity and product distribution. The oxidizing capacity

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of nFe0/oxyanion and CNT/oxyanion toward various organics was evaluated by monitoring the

359

initial rate of degradation of the target substrates including benzoic acid, bisphenol A, 4-CP, 4-

360

nitrobenzene, 4-nitrophenol, phenol, and 2,4,6-trichlorophenol (Figures S15 and S16). Substrate

361

dependence of the activated oxyanion reactivity was visualized by combined two-way clustering

362

analysis with a heat map to represent the relative treatment efficiency using variations in the

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color intensity (Figure S17). The inter-sample distance as a similarity measure index in the

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NMDS plot reveals that nFe0/BR and nFe0/H2O2 differ significantly from other systems utilizing

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nFe0 as an oxyanion activator in terms of substrate-specificity (Figure 5a). This is in marked

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contrast to the possible similarity among a group of the CNT/oxyanion systems (Figure 5a),

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which likely supports that CNT/oxyanion performs decomposition of organics via an identical

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degradative mechanism (i.e., electron transfer from organics to oxyanions). However, it is


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noteworthy that degradation of organics was almost absent in the nFe0/BR and nFe0/H2O2

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systems. Thus, the long distance between nFe0/BR and nFe0/H2O2 samples versus other

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nFe0/oxyanion samples may be attributed to significant dissimilarity in terms of oxyanion

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activating capacity rather than substrate-specificity.

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Tables S1 and S2 compare the distribution of intermediates produced when applying the

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nFe0/oxyanion and CNT/oxyanion systems for 4-CP degradation. The dendogram combined with

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the heat map in Figure S18 shows the relative abundances of major reaction intermediates. All

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CNT/oxyanion samples form a cluster of points that are close together in the NMDS plot (Figure

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5b), which suggests the similarity among the CNT/oxyanion systems in terms of the relative

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quantity and variety of intermediates. On the other hand, the distance between the nFe0/PI

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sample versus other nFe0/oxyanion samples presents a clear difference in the intermediate

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distribution (Figure 5b). It is of note that the reactivity of IO3• (main oxidant in nFe0/PI) is

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considerably different from that of SO4•− and •OH (main oxidants in nFe0/PMS, nFe0/PDS, and

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nFe0/PA), based on the dechlorination efficiency and quenching effect (Figures S10a and 3a).

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Collectively, CNT-mediated electron transfer as the common degradative mechanism causes a

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similar intermediate distribution whereas nFe0/oxyanion initiates the radical-induced reaction

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pathway that reflects the nature of the oxyanion-derived radical.

386 387

Effect of activators on current generation. Figures 6a and 6b show chronoamperometric

388

results for the nFe0/CP electrode versus the CNT/CP electrode recorded at an applied potential of

389

+ 0.8 V (vs Ag/AgCl) during the sequential addition of oxyanions and 4-CP. The current

390

response monitored at the nFe0/CP electrode shows that oxyanion injection results in a strong

391

current spike followed by gradual increase in the current intensity regardless of the oxyanion



392

type. Such continuous current increase is likely attributed to the oxidative conversion of nFe0 on

393

the electrodes; it is probable that zerovalent iron deposits were gradually oxidized (i.e., nFe0 →

394

FexOy) over the course of the electrochemical measurement and the oxidative transformation was

395

accelerated in the presence of oxyanions as oxidants. The electrons released from nFe0 would be

396

transferred to oxyanions (leading to oxyanion activation) and be concomitantly captured by the

397

electrode substrate (leading to the increase in the background current). No significant current

398

change occurred in response to subsequent 4-CP addition (Figure 6a). In contrast, the current

399

generation at the CNT/CP electrode was almost absent upon oxyanion injection but a significant

400

current jump occurred immediately after 4-CP addition (Figure 6b). In particular, there was a

401

rough correlation between the extent of increase in current density on 4-CP injection (Figure 6b)

402

and the efficiency of CNT/oxyanion for 4-CP degradation (Figure 1b). These results imply that

403

CNT effectively mediates the delivery of electrons from 4-CP to oxyanions, which is crucial in

404

the oxidative treatment of organics by CNT/oxyanion through the non-radical mechanism. LSV

405

analysis further confirmed that current generation was synergistically improved at the CNT/CP

406

electrode in the co-presence of PMS and 4-CP whereas no enhancing effect was observed at the

407

nFe0/CP electrode (Figure S19).

408 409

Environmental applications. This study was designed to test the hypothesis that the electron

410

accepting action of persulfate plays a key role in the non-radical mechanism underlying

411

persulfate activation. To substantiate this, we explored the possibility that oxidative degradation

412

of organics on CNTs (as electron transfer mediators) was achievable with any (as tested)

413

oxyanion able to function as an electron scavenger. Comparison of nFe0/oxyanion versus

414

CNT/oxyanion collectively suggests that persulfate activation takes place through different


Page 18 of 33

Page 19 of 33


415

processes according to the activator type present. One-electron reduction of select oxyanions by

416

nFe0 led to the production of the corresponding oxidizing radicals and the associated degradation

417

of organics. For this, the role of oxyanions as radical precursors is implicated in persulfate

418

activation processes (e.g., Co2+/PMS and Fe2+/PDS) involving the reductive conversion of

419

persulfate to SO4•−. When CNTs were applied instead of nFe0, organic compounds were also

420

oxidized in the presence of all tested oxyanions. However, when considering 1) marginal

421

oxyanion consumption in the absence of 4-CP, 2) insignificant quenching effect(s), 3) no radical

422

detection, 4) substrate-specificity and product distribution that are not unique to the oxyanions,

423

and 5) current generation upon 4-CP injection, the CNT/oxyanion system fundamentally differs

424

from the nFe0/oxyanion system in terms of the primary degradative mechanism. The results

425

suggest an alternative degradative pathway in which CNTs act as electron shuttles and facilitate

426

the transfer of electrons from organics to oxyanions, which is fundamental to the non-radical

427

mechanism of persulfate activation. In other studies,44, 45 persulfate activation via surface-bound

428

ketone and quinone moieties on carbonaceous nanomaterials was observed to initiate singlet

429

oxygenation of organics as a non-radical mechanism. However, such a mechanism can be ruled

430

out for the described systems based on the following reasons. First, since singlet oxygen (1O2) is

431

unlikely to oxidize neutral phenols that predominantly exist at a pH below the pKa,46 effective

432

degradation of 4-CP, bisphenol A, and phenol at neutral pH (Figure S16) would not be achieved

433

with 1O2-generating systems (the pKa values of 4-CP and bisphenol A are 9.41 and 9.6,

434

respectively46). Second, the alternative use of D2O as a solvent kinetically enhances singlet

435

oxygenation because the lifetime of 1O2 is extended by up to ca. 10 times in D2O solution.47 Here,

436

the efficiency of CNT/PMS for 4-CP degradation was not enhanced when D2O was used instead

437

of H2O (Figure S20). Finally, the hypothetical role of 1O2 in the persulfate activation cannot offer



438

reasonable explanations for the results presented in this study, which include 1) oxidative

439

degradation by CNT in the presence of not only persulfate but also other oxyanions, 2)

440

significant persulfate decay only when 4-CP was present, and 3) current change upon 4-CP

441

addition.

442

Powerful oxidizing capacity of SO4•− enables degradation of a broad spectrum of organic

443

pollutants when reducing agents are used as persulfate activators. This implies that persulfate

444

activation processes based on SO4•− as a main oxidant outperform those initiated through

445

electron transfer mediation in terms of treatment efficiency. However, the non-selective nature of

446

SO4•− likely raises the possibility that oxidation by the activated persulfate would undergo

447

drastic kinetic retardation in the presence of naturally-occurring organic and inorganic substrates

448

(e.g., humic substance, chloride ion) as radical scavengers. Since non-radical mechanism

449

proceeds only with the ternary system consisting of electron donor, electron acceptor, and

450

electron transfer mediator, a kinetic rate of persulfate reduction depends on residual

451

concentration of organic substrate; no further consumption of persulfate would occur once

452

organic pollutant as an electron donor is not available anymore. On the other hand, one-electron

453

reducing agents as activators continue to fruitlessly decompose persulfate even after achieving

454

complete removal of organics.

455

The mechanisms behind persulfate activation concern electron transfer reaction initiated or

456

mediated by activators: one-electron reduction of persulfate by an activator (i.e., radical

457

mechanism) versus electron shuttling from organics to persulfate via an activator (i.e., non-

458

radical mechanism). Accordingly, in order to screen and design persulfate activating catalysts,

459

surface properties including surface affinity, complexing activity, and electrostatic charge should

460

be explored considering that direct contact or close proximity with a redox active surface favors


Page 20 of 33

Page 21 of 33


461

heterogeneous electron transfer. Surface modification of the supported iron oxide catalyst caused

462

a switch between the reaction pathways for H2O2 decomposition: one-electron transfer from

463

surficial Fe(II) to H2O2 (leading to •OH formation) and two-electron transfer from surficial Fe(II)

464

to H2O2 (leading to Fe(IV) formation).48 Similarly, the strategies to control the surface

465

characteristics of activators (e.g., surface functionalization of carbocatalysts, surface deposition

466

of nanoscale metal particles on metal oxide supports) may alter the mechanism of activating

467

persulfate.

468 469

Acknowledgements

470

This study was supported by a National Research Foundation of Korea grant funded by the

471

Korea Government (NRF-2017R1A2B4002235); a grant from the National Research Foundation

472

of Korea, funded by the Ministry of Science, ICT, and Future Planning (No.

473

2016M3A7B4909318), and by Korea Ministry of Environment as “The GAIA Project”

474

(2016000550007).

475

Supporting Information Available.

476

Intermediate and product distributions obtained in the course of 4-CP oxidation by the

477

nFe0/oxyanion and CNT/oxyanion systems (Tables S1 and S2), TEM images of iron

478

nanoparticles (Figure S1), Metal contents in pristine CNTs (Figure S2), TEM images of fresh

479

and used CNTs (Figure S3), Raman spectra of fresh and used CNTs (Figure S4), Raman spectra

480

of CNTs in the short and long wavenumber ranges (Figure S5), Effects of surfactants on CNTs-

481

mediating persulfate activation (Figure S6), DLS size distributions of pristine CNTs and CNTs

482

exposed to ultrasound (Figure S7), Effect of sonication on persulfate activating capacity of CNTs

483

(Figure S8), sorption test with CNTs (Figure S9), Release of chloride ions during 4-CP



Page 22 of 33

484

degradation by the activated oxyanions (Figure S10), Rate of oxyanion consumption by nFe0 and

485

CNTs (Figure S11), XPS spectra of fresh and used CNTs (Figure S12), FT-IR spectra of fresh

486

and used CNTs (Figure S13), Formation of formaldehyde during methanol oxidation by the

487

activated oxyanions (Figure S14), Initial rates of degradation of various organics by the activated

488

oxyanions (Figures S15-S16), Combined two-way clustering analysis of kinetic data and

489

degradation intermediate data, with a heat map, for two oxyanion-activating groups (Figures

490

S17-S18), linear sweep voltammograms of nFe0- and CNT-coated carbon paper electrodes

491

(Figure S19), Rates of 4-CP degradation by CNT/PMS in water and in deuterium oxide (Figure

492

S20).

493

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494

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495

heterogeneous catalysis of persulfate activation on dimensional-structured nanocarbons. ACS

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catalysis for organic contaminants removal: Current development, challenges and prospects.

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3. Neta, P.; Huie, R. E.; Ross, A. B., Rate constants for reactions of inorganic radicals in

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8. Chan, K. H.; Chu, W., Degradation of atrazine by cobalt-mediated activation of

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9. Shah, N. S.; He, X. X.; Khan, H. M.; Khan, J. A.; O'Shea, K. E.; Boccelli, D. L.; Dionysiou,

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Activation of persulfates by graphitized nanodiamonds for removal of organic compounds.

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Page 26 of 33

Page 27 of 33


(a)

0

nFe /PMS 0 nFe /PDS 0 nFe /PI 0 nFe /PA 0 nFe /PP 0 nFe /H2 O2

1.0

4-CP Conc. (C/C0 )

0.8

0.6

nFe0 /BR

0.4

0.2

0.0 (b)

1.0

CNT/PMS CNT/PDS CNT/PI CNT/PA CNT/PP CNT/H2 O2

4-CP Conc. (C/C0 )

0.8

0.6

CNT/BR

0.4

0.2

0.0 0

10

20

30

40

50

60

Reaction Time (min)

632 633

FIGURE 1. Degradation of 4-CP by oxyanions activated with (a) nFe0 and (b) CNT ([nFe0]0 =

634

0.05 g/L; [CNT]0 = 0.1 g/L; [oxyanion]0 = 1 mM; [4-CP]0 = 0.1 mM; [phosphate buffer]0 = 1

635

mM; pHi = 7.0).

636 637 638



(a) 1st

2nd

3rd

4th

5th

(b) 1st

2nd

3rd

4th

5th

4-CP Conc. (C/C0 )

1.0

0.8

Page 28 of 33

0

nFe /PMS 0 nFe /PDS 0 nFe /PI 0 nFe /PA

0.6

0.4

0.2

0.0

4-CP Conc. (C/C0 )

1.0

0.8

CNT/PMS CNT/PDS CNT/PI CNT/PA

0.6

0.4

0.2

0.0 0

639

50

100

150

200

250

300

Reaction Time (min)

640

FIGURE 2. Repeated degradation of 4-CP by oxyanions activated with (a) nFe0 and (b) CNT

641

(used activators were exchanged with fresh ones in each cycle, but 4-CP degradation was

642

repeatedly performed without further oxyanion addition) ([nFe0]0 = 0.05 g/L; [CNT]0 = 0.1 g/L;

643

[oxyanion]0 = 1 mM; [4-CP]0 = 0.1 mM; [phosphate buffer]0 = 1 mM; pHi = 7.0).

644 645


Page 29 of 33


(a)

w /o MeOH w / MeOH

4-CP Degradation Ef f iciency (%)

100

80

60

40

20

0 (b) 4-CP Degradation Ef f iciency (%)

100

80

60

40

20

0

646

PMS

PDS

PI

PA

PP

H2O2

647

FIGURE 3. Effect of excess methanol on 4-CP degradation efficiency of (a) nFe0/oxyanion and

648

(b) CNT/oxyanion ([nFe0]0 = 0.05 g/L; [CNT]0 = 0.1 g/L; [oxyanion]0 = 1 mM; [4-CP]0 = 0.1

649

mM; [methanol]0 = 200 mM; [phosphate buffer]0 = 1 mM; pHi = 7.0).

650 651 652



Page 30 of 33

(a)

nFe /PMS CNT/PMS PMS only

(b)

nFe /PA CNT/PA PA only

Intensity (Arb. Unit)

0

Intensity (Arb. Unit)

0

332

653

334

336

338

340

342

344

Magnetic Field (G)

654

FIGURE 4. EPR spectra recorded in aqueous (a) PMS and (b) PA suspensions containing

655

activators (e.g., nFe0, CNT) and DMPO as a spin-trapping agent (1 min after PMS or PA

656

activation) ([nFe0]0 = 0.05 g/L; [CNT]0 = 0.1 g/L; [oxyanion]0 = 1 mM; [4-CP]0 = 0.1 mM;

657

[DMPO]0 = 10 mM; [phosphate buffer]0 = 1 mM; pHi = 7.0).

658 659


Page 31 of 33


660 661 662 663 664 665 666 667 668 669

FIGURE 5. Nonmetric multidimensional scaling (NMDS) analysis (based on Bray-Curtis

670

dissimilarity) of two groups (nFe0/oxyanion and CNT/oxyanion) for (a) initial degradation rate

671

data and (b) oxidation intermediate data.

672 673 674



Page 32 of 33

0.025 (a)

PMS PDS PI PA

Current (mA)

0.020

PP BR w /o oxyanion

0.015

4-CP

0.010

0.005

Oxyanion

0.000 50

100

150

200

250

300

0.025 (b)

PMS PDS PI PA PP BR w /o oxyanion

Current (mA)

0.020

0.015

0.010

0.005

Oxyanion

4-CP

0.000 350

675

400

450

500

550

Time (s)

676

FIGURE 6. Current response upon addition of oxyanion and 4-CP at the carbon paper surface-

677

coated with (a) nFe0 and (b) CNT as the working electrode ([oxyanion]0 = 1 mM; [4-CP]0 = 0.1

678

mM; [phosphate buffer]0 = 10 mM; pHi = 7.0).

679 680 681 682


Page 33 of 33

683


Table of Contents Figure:

684 685 686 687 688 689 690 691


Radical Precursor Versus Electron Acceptor

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