Identifying the Nonradical Mechanism in the Peroxymonosulfate

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Identifying the Nonradical Mechanism in the Peroxymonosulfate Activation Process: Singlet Oxygenation Versus Mediated Electron Transfer Eun-Tae Yun, Jeong Hoon Lee, Jaesung Kim, Hee-Deung Park, and Jaesang Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00959 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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Environmental Science & Technology

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Identifying the Nonradical Mechanism in the Peroxymonosulfate Activation

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Process: Singlet Oxygenation Versus Mediated Electron Transfer

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Eun-Tae Yun1†, Jeong Hoon Lee1†, Jaesung Kim1, Hee-Deung Park1, and Jaesang Lee1*

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Abstract. Select persulfate activation processes were demonstrated to initiate oxidation not

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reliant on sulfate radicals, though the underlying mechanism has yet to be identified. This study

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explored singlet oxygenation and mediated electron transfer as plausible nonradical mechanisms

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for organic degradation by carbon nanotube (CNT)-activated peroxymonosulfate (PMS). The

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degradation of furfuryl alcohol (FFA) as a singlet oxygen (1O2) indicator and the kinetic

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retardation of FFA oxidation in the presence of L-histidine and azide as 1O2 quenchers apparently

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supported a role of 1O2 in the CNT/PMS system. However, the 1O2 scavenging effect was

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ascribed to a rapid PMS depletion by L-histidine and azide. A comparison of CNT/PMS and

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photoexcited Rose Bengal (RB) excluded the possibility of singlet oxygenation during

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heterogeneous persulfate activation. In contrast to the case of excited RB, solvent exchange (H2O

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to D2O) did not enhance FFA degradation by CNT/PMS and the pH- and substrate-dependent

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reactivity of CNT/PMS did not reflect the selective nature of 1O2. Alternatively, concomitant

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PMS reduction and trichlorophenol oxidation were achieved when PMS and trichlorophenol

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were physically separated in two chambers using a conductive vertically aligned CNT membrane.

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This result suggested that CNT-mediated electron transfer from organics to persulfate was

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primarily responsible for the nonradical degradative route.

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Keywords: persulfate activation, nonradical mechanism, singlet oxygenation, mediated electron

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transfer, carbon nanotubes

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

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

These authors contributed equally to this work.

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ACS Paragon Plus Environment


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INTRODUCTION

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The electron donating potentials of transition metals (e.g., cobalt and iron) can initiate the one-

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electron reduction of peroxymonosulfate (PMS) and the associated production of sulfate radicals

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(SO4•−), thus achieving radical-induced oxidation of organics.1-3 This process is in accordance

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with various physicochemical activation strategies (e.g., photocatalysis,4,

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radiolysis7) in which the generated (conduction band) electrons reductively cleave the peroxide

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bond of PMS to form SO4•−. In contrast, recent studies8-15 have raised the likelihood of persulfate

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activation via a degradative reaction pathway involving no radical attack based on the following

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universal experimental results: no quenching by methanol and chloride as SO4•− scavengers and

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the absence of electron paramagnetic resonance (EPR) spectra assigned to SO4•− adducts.

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Persulfate activation that does not rely on the oxidizing power of SO4•− has been demonstrated to

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proceed with metal- and carbon-based activators, and two hypotheses, i.e., singlet oxygenation

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and mediated electron transfer, have been postulated for the underlying mechanism.8-10, 12-18

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electrolysis,6 and

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Singlet oxygen (1O2) has been hypothesized to have a role in persulfate activation

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processes based on the following results: oxidation of organics by activated PMS is decelerated

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in the presence of excess azide and L-histidine as 1O2 quenchers, and PMS activation in the

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presence of 2,2,6,6-tetramethylpiperidine (TEMP) as a spin-trapping agent produces the EPR

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spectrum assigned to the corresponding 1O2 adduct.12,

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alkaline aqueous PMS solutions when phenols are selected as target substrates or benzoquinone

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is added.22, 23 As ketones (characterized by the presence of a carbonyl moiety (C=O)) have been

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found to accelerate 1O2 production associated with PMS decay under alkaline conditions (i.e.,

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HSO5− + SO52− → HSO4− + SO42− + 1O2),24 quinones formed as intermediates during phenol

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oxidation or supplied externally could provide carbonyl groups to activate PMS and produce 1O2

13, 16-22


Singlet oxygenation occurs in

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at basic pH.22, 23 The addition of benzoquinone, which serves as a O2•− scavenger, reduces the

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efficiency of PMS activation by base, suggesting an alternative 1O2 formation route that involves

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the superoxide radical anion (O2•−) as an intermediate (i.e., 2O2•− + 2H+ → H2O2 + 1O2).21

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Carbonaceous nanomaterials, such as N-doped graphene and carbon nanotubes (CNTs), could

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catalyze the self-decomposition of persulfate, even at acidic or neutral pH (note that alkaline

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conditions favor 1O2 generation through homogeneous persulfate activation21-23), mediating the

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nonphotochemical production of 1O2.13, 16, 17, 20 Surface modification with glutaraldehyde, which

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remarkably increases the surface density of carbonyl groups on CNTs, enhances 1O2 formation

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from peroxydisulfate (PDS),13 suggesting that nonradical persulfate activation occurs via

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carbonyl moieties intrinsically present on nanocarbon surfaces.13, 20 Perovskite oxide- and metal-

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derived activators also enable singlet oxygenation upon PMS addition, though the relevant

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mechanism has not been empirically verified.12, 18

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Electron transfer from organic compounds to persulfate, facilitated by select activators,

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could be responsible for the nonradical mechanism.8-10, 15, 25 PMS activation systems utilizing

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CNTs and graphitized nanodiamonds have been clearly distinguished from Co2+/PMS as a

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reference system2 based on the negligible hydroxylation characteristics of SO4•−-mediated

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oxidation and substrate-dependent degradation efficacies that contradicted the reactivity of

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SO4•−.8, 9, 25 PMS decay and current generation (in a three-electrode cell) is markedly pronounced

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when the organic substrate, PMS, and activator co-exist, revealing that the electron-transfer

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mediating action of the activator allows PMS to abstract electrons from organics.10, 25 Duan et

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al26 suggested that surface-complexed PMS on N-doped CNTs could trigger the mediated

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electron transfer from phenol to PMS involved in the complexation reaction. Our recent work25

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also supports a degradative route based on mediated electron transfer, as any oxyanion that could



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serve as an electron acceptor (e.g., periodate, peracetate, PMS, and PDS) was found to facilitate

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the oxidative degradation of organics in the presence of CNTs. The similarities in substrate-

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specificity and product distribution confirmed that the reaction pathway induced on CNTs was

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not unique to the type of oxyanion.25 In contrast, zerovalent nanosized iron (nFe0) as an activator

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causes the reductive conversion of oxyanions into the corresponding radicals; thus, the

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degradative reaction pathway is dependent on the nature of the oxyanion-derived radical.25 As

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aforementioned, two hypothetical mechanisms have been established for persulfate activation not

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involving SO4•−, but there still remains a fundamental gap in the understanding of the nonradical

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mechanism.

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To improve our understanding of the PMS activation mechanism involving no radical

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attack, in this study, we examined the possibility of oxidative degradation of organics via singlet

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oxygenation and mediated electron transfer during PMS activation by CNTs (CNT-activated

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persulfate was demonstrated to cause the nonradical degradative pathway9, 13, 14). In an effort to

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identify a role of 1O2 in PMS activation, we compared CNT-activated PMS (i.e., CNT/PMS)

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with photoexcited Rose Bengal (RB), which is well-known to photosensitize singlet

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oxygenation.27 For these systems, the effects of chemical reagents able to scavenge 1O2 (L-

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histidine and azide) and extend the lifetime of 1O2 (D2O),27 the dependence of oxidizing capacity

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on substrate type and pH, and EPR spectral features were compared. Furthermore, to explore the

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electron-transfer mediating action of CNTs during PMS activation, we tested a vertically aligned

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CNT membrane (VA-CNT membrane) that physically separates the reaction system into two

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zones but allows electron transport through the CNT arrays for interzone electron delivery from

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organics to PMS across the membrane.

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MATERIALS AND METHODS


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Chemicals and Materials. Single-walled CNT (> 95%) was purchased from NanoLab Inc.

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Other chemicals were of reagent grade (see Supporting Information), and used without further

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purification or treatment. Ultrapure deionized water (>18 MΩ•cm), produced with a Millipore

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system, was used to prepare all experimental suspensions and solutions

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Preparation and Characterization of VA-CNT Membrane. VA-CNTs were synthesized by a

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water-vapor-assisted chemical vapor deposition (CVD) technique according to the previously

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reported procedure.28 Briefly, VA-CNTs were grown on a SiO2/Si wafer coated with aluminum

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and iron as a catalyst layer. The coated SiO2/Si wafer (width: 1 cm, length: 1 cm) was placed in a

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CVD reactor. The growth of millimeter-scale VA-CNT arrays (thickness: ca. 900 µm) was

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carried out at 750 °C for 2 h under a constant flow of ethylene as a carbon source and hydrogen

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and argon as carrier gases at rates of 100, 200, and 300 sccm (standard cubic centimeters per

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minute), respectively. Water vapor, a known agent for enhancing and preserving the performance

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of metal catalysts, was supplied at a feed rate of 30 sccm. Scanning electron microscopy (SEM;

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Quanta 250 FEG, Thermo Scientific) revealed a densely packed forest of aligned CNTs on the

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silicon wafer (Figure S1). The Raman spectrum of the CNTs (500–3000 cm−1; LabRam

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ARAMIS, Horiba Jobin-Yvon; argon ion laser excitation (514.5 nm)) included a G-band

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corresponding to graphite structures and a D-band sensitive to defects at 1582 and 1350 cm−1,

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respectively29 (Figure S2). Scheme S1 illustrates the procedure for VA-CNT membrane

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fabrication. An epoxy resin (Epon Resin 828, Miller-Stephenson Inc.) was mixed with a curing

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agent (Jeffamine D-230, Huntsman Corporation) at a 3:1 weight ratio.30 The resin mixture was

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infiltrated into the interstitial spaces of the as-grown CNTs. Then, the VA-CNT/epoxy composite

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was incubated under vacuum for 3 h on a mold. The resultant composite was allowed to cure at

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room temperature for 24 h. Finally, the residual resin and catalysts were removed, the CNT-



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based membrane was detached from the silicon substrate, and the VA-CNTs were uncapped by

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cutting the top surface and bottom of the composite using a microtome (HM 340 E, MICROM

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Lab.) to produce the VA-CNT membrane (with open CNT tips). X-ray photoelectron

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spectroscopy (XPS; PHI X-tool, ULVAC-PHI) confirmed that no detectable amounts of metal

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species (e.g., iron) that may activate persulfate remained on the surface of the VA-CNT

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membrane (Figure S3).

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To explore the possibility of interchamber electron transport across the VA-CNT

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membrane, we monitored concurrent oxidation of organics and reduction of PMS in a crossflow

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filtration system in which the feed water migrated tangentially across the VA-CNT membrane

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surface. The VA-CNT membrane (effective surface area: 4 cm2, thickness: 0.9 cm) was

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vertically mounted in a module to partition the reaction system into two chambers. Water flow in

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the two physically separated compartments was circulated by a gear pump (REGLO-Z,

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ISMATEC) at a feed rate of 500 mL min−1. To guarantee the complete removal of metallic

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species that may activate persulfate, the pristine VA-CNT membrane was subjected to UV-C

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treatment for 5 h (UV-C irradiation could photochemically cleave some organic linkers that

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might attach metals to the membrane, which allowed for the release of loosely bound metallic

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species) and washed thrice with deionized water prior to its application in the filtration process.

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Experimental Procedure and Analytical Methods. Oxidative degradation of organic substrates

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by activated PMS was performed in a magnetically stirred 40 mL reactor under air-equilibrated

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conditions. A typical experimental suspension contained 0.1 g L−1 CNTs, 1 mM PMS, and 0.05

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mM target pollutant. Photosensitized singlet oxygenation of organics proceeded in a 40 mL

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cylindrical quartz reactor with six fluorescent lamps (output power: 4 W; Philips Co.). The

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significant overlap between the emission spectrum of the light source and the absorption


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spectrum of RB (1O2 photosensitizer; applied at an initial concentration of 0.05 mM) indicated

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that the photoexcitation of RB readily occurred under fluorescent lamp irradiation (Figure S4).

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The incident light intensity, measured using a pyranometer (Apogee, PYR-P), was determined to

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be 1.105 mW cm−2. The suspensions (or solutions) were initially adjusted to pH 7 and buffered

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using 1 mM phosphate buffer in most cases. No significant pH change was observed over the

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course of PMS activation. To investigate pH effects, the initial pH of the aqueous suspensions

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(or solutions) was adjusted to the desired value using concentrated HClO4 and NaOH, and 1 mM

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phosphate and carbonate buffers were used for maintaining neutral and alkaline pH, respectively.

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Sample aliquots were withdrawn from the reactor at fixed time intervals using a 1 mL syringe,

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filtered through a 0.45 µm PTFE filter (Millipore), and injected into a 2 mL amber glass vial. In

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PMS activation experiments, excess methanol (0.5 M) was added to quench any remaining

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radicals. The residual concentrations of organic pollutants were measured using an HPLC

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(Agilent Infinity 1260) system equipped with a C-18 column (ZORBAX Eclipse XDB-C18) and

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a UV/vis detector (G1314F 1260VWD). The mobile phase comprised 0.1% (v/v) aqueous

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phosphoric acid solution and acetonitrile at a volume ratio of 45:55. According to the method

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proposed by Liang et al., PMS was colorimetrically determined based on the amount of iodine

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(λmax = 352 nm) formed via the oxidation of iodide by PMS.31 For EPR analysis, 5,5-dimethyl-

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pyrroline N-oxide (DMPO) and TEMP were used as spin-trapping agents for SO4•−, azidyl

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radical (N3•), and 1O2, respectively. EPR spectra of the aqueous CNT/PMS suspensions (aqueous

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binary mixture of azide and PMS or fluorescent-light-irradiated RB solutions) were recorded

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using a JES-TE 300 spectrometer (JEOL) under the following conditions: microwave power = 1

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mW, microwave frequency = 9.4136 GHz, center field = 3350 G, modulation width = 0.1 mT,

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and modulation frequency = 100 kHz. Raman spectra of fresh and used CNTs were acquired on a



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LabRam ARAMIS Raman spectrometer (Horiba Jobin-Yvon) using an argon ion laser

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(excitation at 514.5 nm). Sulfur-containing chemical moieties on CNTs (after exposure to PMS

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solution) were identified by Fourier transform infrared spectroscopy (FT-IR, Thermo Scientific

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Nicolet 6700) performed in ATR (attenuated total reflectance) mode. The zeta potential of the

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aqueous-suspended CNTs was recorded as a function of pH using a Zetasizer Nano ZS

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(Malvern).

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RESULTS AND DISCUSSION

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Oxidation of furfuryl alcohol (FFA) by Activated PMS. To explore whether 1O2 was produced

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during PMS activation, oxidative degradation of FFA, as a 1O2 indicator,27 by the CNT/PMS

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system was examined (Figure 1a). Sorption on CNTs caused no noticeable FFA decay, but PMS

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alone directly oxidized FFA to a certain extent. However, CNTs in the presence of PMS

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decomposed FFA more rapidly, with k(FFA) = 0.0054 ± 0.0014 min−1 for PMS alone versus

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k(FFA) = 0.3750 ± 0.0207 min−1 for CNT/PMS. To further confirm that 1O2 had a role in CNT-

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induced PMS activation, the effects of excess azide and L-histidine as 1O2 scavengers were

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investigated (Figure 1a). FFA oxidation was completely quenched upon addition of L-histidine,

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and kinetic retardation of FFA decay was significant upon addition of azide. The results

183

appeared to corroborate the previous findings12, 16-18 that singlet oxygenation was responsible for

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the nonradical reaction pathway in PMS activation processes. However, solvent exchange (H2O

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to D2O) did not accelerate FFA degradation at all, which contradicted the usual behavior of 1O2

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in D2O, as the lifetime of 1O2 is extended up to 10 times when H2O is replaced with D2O.32 In

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particular, the solvent exchange did not kinetically affect the PMS decay in the aqueous CNT

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suspension (Figure S5). Since the highly accelerated self-decomposition of PMS was presumed

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to result in 1O2 production during PMS activation by carbocatalysts,13,


16, 17, 20

the results

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indicated that the alternative use of D2O had no influence on the kinetics of the reaction that

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might allow the CNTs to transform PMS into 1O2.

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Figure 1b shows the change in FFA degradation efficiency with quencher addition and

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solvent exchange for photoexcited RB as a benchmark 1O2 producer.27 As observed in the

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CNT/PMS system, FFA decomposition in the fluorescent-light-irradiated solution of RB was

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drastically decelerated in the presence of azide and L-histidine. However, the use of D2O as a

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solvent kinetically enhanced FFA degradation by RB under photoillumination. This behavior

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was in marked contrast to the effect of D2O on the FFA degradation efficiency of CNT/PMS but

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reflected the known properties of 1O2. Thus, this result suggested that singlet oxygenation may

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not occur during persulfate activation. We also monitored H2O2 production as an alternative

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indicator of singlet oxygenation during the oxidation of ascorbate by CNT/PMS and

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photoexcited RB. The singlet oxygenation of ascorbate is accompanied by significant H2O2

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formation.33 Though we found no difference in FFA oxidation efficiency between CNT/PMS and

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photoexcited RB (Figures 1a and 1b), a much higher H2O2 formation yield was achieved with the

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fluorescent-light-irradiated RB solution (Figure S6). The result also revealed that singlet

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oxygenation likely contributed insignificantly to the oxidizing capacity of CNT/PMS.

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The observed PMS concentrations in the PMS/quencher systems (Figure 2) revealed that and azide as 1O2 scavengers may cause misinterpretation of the

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the choice of

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experimental results on PMS activation processes, even though these compounds have been

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widely employed to evidence the role of 1O2 as an oxidant in environmental processes.27, 34 The

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inhibition of FFA degradation by CNT/PMS in the presence of excess L-histidine (Figure 1a)

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appeared to be consistent with the retarding effect of L-histidine reported in the literature.12, 19

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However, this drastic reduction in FFA oxidation efficiency resulted not from the scavenging

L-histidine



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activity of L-histidine toward 1O2 but from rapid PMS depletion by excess L-histidine (Figure 2a).

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Note that direct reaction with L-histidine at an initial concentration of 100 mM reduced the PMS

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concentration to the undetectable level within 5 min, and the reaction accelerated as L-histidine

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concentration increased (Figure 2a). The quenching effect of azide (Figure 1a) also seemed to

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support previous studies16-18 that suggest singlet oxygenation as the nonradical mechanism, but

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we found that a binary mixture of PMS (100 mM) and azide exhibited significant FFA

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degradation efficiency, which was not affected by the addition of CNTs (Figure 2b, inset). In fact,

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FFA decay by CNT/PMS in the presence of excess azide, which was initially believed to be

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kinetically hindered by the 1O2 scavenging activity of azide (Figure 1a), could instead be mainly

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ascribed to the oxidizing capacity of azide/PMS (direct PMS reduction by L-histidine was not

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accompanied by FFA oxidation (Figure S7)). In this case, the majority of PMS initially added

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was rapidly consumed through direct reaction with azide as a reducing anion35 (Figure 2b),

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rendering PMS unavailable for further reaction with residual azide (or CNTs).

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To explore the reactivity of azide/PMS, we examined the binary mixture of azide and

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PMS for the oxidative degradation of diverse organic substrates including 4-chlorophenol (4-CP),

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2,4,6-trichlorophenol (TCP), 4-nitrophenol, and carbamazepine (Figure S8). Unlike FFA, which

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was rapidly decomposed by azide/PMS, the other organic compounds were barely degraded in

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aqueous azide/PMS solutions, which implied that direct azide oxidation by PMS led to the

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production of a highly selective oxidant. The previous finding that N3• exhibited a very low

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reactivity toward aromatic compounds substituted with electron-withdrawing groups36 likely

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reveals the possibility that PMS could oxidatively convert azide into N3•. Further, the EPR

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spectrum obtained for the aqueous azide/PMS mixture showed features that are assignable to the

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formation of N3• (Figure S9).37 Although the kinetic rate of PMS degradation increased


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proportionally with the initial azide concentration (Figure 2b), the FFA removal efficiency

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decreased with increasing azide concentration (inset of Figure 2b). This concentration-dependent

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efficiency may be because excess azide likely favored the reaction routes that can deactivate N3•

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(e.g., 2N3• → 3N2 (k = 4.5 × 109 M−1 s−1)38; N3• + N3− → N6•− (k = 1.0 × 106 M−1 s−1)38), but a

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further study is required for an in-depth investigation into the mechanism underlying the PMS-

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mediated production of N3• from PMS. Overall, the apparent inhibitory effects of 1O2 quenchers

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that have been considered as evidence for 1O2 formation in persulfate activation systems are

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attributable to the reactivity of L-histidine and azide toward PMS, which was consistent with the

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previous report39 on the effective PMS consumption by L-histidine and azide.

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FFA degradation was insignificant in aqueous suspensions of CNTs when PDS was

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applied instead of PMS (Figure S10), which is consistent with the previous finding14 that

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CNT/PMS was more effective for decomposing FFA than CNT/PDS. In contrast, a comparison

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of the 4-CP degradation efficiencies for PMS and PDS activated with CNTs indicated that the

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oxidizing powers of these two systems were similar (Figure S11). To examine the possibility of

250

1

251

4-CP oxidation efficiency was investigated (Figure S11). These reagents, as a 1O2 scavenger and

252

a singlet oxygenation enhancer, respectively, had no effect on the kinetic rate of 4-CP oxidation;

253

L-histidine

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alternative solvent did not accelerate the 4-CP decay (Figure S11). The results eliminated the

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possible contribution of 1O2 to the decomposition of organics by activated PDS. In contrast to

256

CNT/PDS, 4-CP was barely oxidized when excess

257

suspension of CNT/PMS (Figure S11). Although PMS was reductively decomposed by L-

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histidine (Figure 2a), no loss of PDS was observed in the presence of 100 mM L-histidine

O2 formation during CNT-induced PDS activation, the effect of L-histidine and D2O addition on

negligibly quenched the degradation of 4-CP by CNT/PDS, and the use of D2O as an

L-histidine


was added to an aqueous


259

(Figure S12). This result confirmed that the significantly inhibited oxidation of organics by

260

CNT/PMS (Figures 1a and S11) was due to rapid PMS degradation by excess L-histidine rather

261

than 1O2 scavenging. No acceleration in 4-CP degradation occurred in aqueous CNT/PMS

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suspension when H2O was replaced with D2O (applied as an enhancer for singlet oxygenation)

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(Figure S11).

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While CNT/PMS caused much more rapid FFA oxidation than CNT/PDS (Figures 1a and

265

S10), similar 4-CP degradation efficiencies were observed, irrespective of whether PMS or PDS

266

was used (Figure S11). This reveals that the production of the reactive oxygen species (e.g., 1O2

267

and SO4•−) through persulfate activation may not be primarily responsible for the oxidizing

268

capacities of CNT/PMS and CNT/PDS. If the CNT-induced activation of persulfate involved 1O2

269

formation, the experiments using FFA as a 1O2 probe (Figures 1a and S10) would suggest that

270

CNT/PMS was much superior to CNT/PDS with respect to singlet oxygenation, which

271

contradicted with the lack of difference observed in the 4-CP treatment efficiency between

272

CNT/PMS and CNT/PDS (Figure S11). On the other hand, the nonradical mechanism in which

273

the CNTs effectively facilitated the transfer of electrons from organics to persulfates may

274

provide a plausible explanation for the substrate-specific reactivity of CNT/persulfate. The

275

mediated electron transfer should occur depending on how the electron flow from the organic

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substrate to persulfate is favored. The observation that PMS achieved more rapid direct FFA

277

oxidation than PDS (Figure S13) clearly indicated that electrons were more preferentially

278

transferred from FFA to PMS than to PDS. This likely led to a much higher efficiency of

279

CNT/PMS for FFA degradation. In contrast, since 4-CP is susceptible to oxidative degradation

280

via direct electron transfer40, the electron delivery from 4-CP to either PMS or PDS appears to be


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thermodynamically plausible, which could render CNT/PMS and CNT/PDS with comparable 4-

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CP degradation efficiencies.

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Dependence of Reactivity on pH and Substrate Type. The substrate specificity of CNT/PMS

284

(in the dark) versus RB (under visible light irradiation) was compared (Figure 3). The systems

285

that produce identical reactive species are expected to exhibit similar substrate specificity during

286

oxidative degradation. The reactivity of the photoexcited RB varied considerably depending on

287

the type of organic compound (Figure 3b), which is consistent with the selective nature of 1O2.34,

288

41

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degradation, whereas the other compounds, namely, benzoic acid, bisphenol A, and

290

carbamazepine, were highly persistent (Figure 3b). The oxidizing capacity of CNT/PMS was

291

also dependent on the type of substrate, but CNT/PMS caused significant degradation of all the

292

organics tested in this study, except benzoic acid (Figure 3a). In particular, organic compounds

293

that exhibited negligible or slow decomposition by photoexcited RB (e.g., bisphenol A,

294

carbamazepine, and propranolol) were effectively removed by aqueous CNT/PMS suspensions.

295

The substrate-specific reactivities of the CNT/PMS and photoexcited RB systems were clearly

296

distinguishable, suggesting that singlet oxygenation was minor (if present at all) in the

297

CNT/PMS system.

For example, cimetidine rapidly decomposed and propranolol underwent relatively slow

298

Three model substrates (4-CP, TCP, and pentachlorophenol (PCP)) were selected to

299

explore the effect of pH on the kinetic rates of chlorophenol oxidation by the CNT/PMS and

300

photoexcited RB systems (Figure 4). In general, alkaline conditions favor oxidative degradation

301

of phenolic compounds by 1O2 because phenolates, which are more electron-rich forms of

302

phenols that become predominant as the pH increases, are more susceptible to singlet

303

oxygenation than neutral phenols.42 The rate of phenol oxidation by 1O2 increases by two orders



304

of magnitude at pH values above the pKa, with k(phenolate + 1O2) = 1.8 × 108 M−1 s−1 and

305

k(neutral phenol + 1O2) = 3.0 × 106 M−1 s−1.43 Consistent with the intrinsic reactivity of 1O2,34, 42,

306

43

307

solutions accelerated significantly with increasing pH up to or above the pKa (pKa(4-CP) =

308

9.41;44 pKa(TCP) = 6.23;44 pKa(PCP) = 4.7045). Fast 4-CP degradation proceeded in the visible-

309

light-irradiated RB solution at basic pH (pH = 11), whereas no removal of 4-CP was observed at

310

pH 7 or 4.5 (Figures 4b and S14b). However, photoexcited RB still mediated the rapid oxidation

311

of TCP at neutral pH, and the PCP decomposition efficiency was relatively constant, regardless

312

of the pH, i.e., k(PCP) = 0.0469 ± 0.0015 min−1 at pH 4.5, k(PCP) = 0.0671 ± 0.0056 min−1 at pH

313

7, and k(PCP) = 0.0711 ± 0.0025 min−1 at pH 11 (Figures 4b and S14b). In contrast, the pH

314

dependence of chlorophenol oxidation by activated PMS (Figures 4a and S14a) was different

315

from that of photosensitized chlorophenol oxidation by RB. All the tested chlorophenols were

316

significantly degraded in aqueous CNT/PMS suspensions under acidic and neutral pH conditions

317

(Figures 4a and S14a) (note that the scale of the y-axis in Figure 4a is 10 times greater than that

318

of Figure 4b). The rate of chlorophenol degradation by CNT/PMS at acidic pH was comparable

319

to or higher than the maximal rate observed in photoirradiated RB solution (k(4-CP at pH 4.5) =

320

0.159 ± 0.009 min−1 for CNT/PMS versus k(4-CP at pH 11) = 0.165 ± 0.006 min−1 for

321

photoexcited RB; k(TCP at pH 4.5) = 0.380 ± 0.017 min−1 for CNT/PMS versus k(TCP at pH 7)

322

= 0.111 ± 0.009 min−1 for photoexcited RB). Further, alkaline conditions in which phenolate

323

anions preferentially exist inhibited oxidative degradation of TCP and PCP by activated PMS.

324

These results also contradicted the possibility that heterogeneous PMS activation was

325

accompanied by singlet oxygenation.

Figure 4b demonstrates that the oxidation of chlorophenols in visible-light-irradiated RB


Page 14 of 36

Page 15 of 36


326

Considering the possibility that the phenolate anion may have a stronger tendency to lose

327

electrons than neutral phenol, the oxidative degradation of chlorophenols could also be

328

accelerated in alkaline suspensions of CNT/PMS. However, the pH of the heterogeneous

329

persulfate activation system affects not only the interconversion between the phenolate anion and

330

the un-ionized phenol but also the surface charge of the carbocatalysts. The zeta potential

331

measured as a function of pH demonstrated that the CNT surface was negatively charged when

332

pH increased above ca. 6.2 (Figure S15), which implied that the alkaline conditions in which the

333

phenolate anion dominates the speciation caused electrostatic repulsions between the CNTs and

334

chlorophenol or PMS, kinetically hindering the electron transfer process in the ternary system.

335

EPR Study. PMS activation by CNTs in the presence of DMPO as a spin trapping agent led to

336

the formation of 5,5-dimethylpyrrolidone-2-(oxy)-(1) (DMPOX), a known product of direct

337

DMPO oxidation,46 which confirmed that SO4•− and hydroxyl radicals (•OH) were not involved

338

in the degradative mechanism (Figure S16). No EPR signals corresponding to DMPO adducts of

339

free radicals were observed for the photoilluminated RB solution (Figure S16). Signals

340

corresponding to 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), which is assignable to the

341

formation of a TEMP-1O2 adduct, have been demonstrated in the EPR spectra of persulfates in

342

the presence of activators (e.g., N-doped graphene,16,

343

provided a basis for suggesting singlet oxygenation as an alternative degradative route. We also

344

observed EPR spectral features corresponding to TEMPO generation in aqueous CNT/PMS

345

suspensions and photoirradiated RB solutions (Figure 5). However, D2O as a singlet oxygenation

346

enhancer affected the EPR spectral patterns in a different way. The peaks assigned to TEMPO

347

for CNT/PMS slightly decreased in the presence of D2O (Figure 5a), but the solvent exchange

348

caused a ca. 50% increase in the intensity of the EPR signal for photoexcited RB (Figure 5b).

17

CNTs,13 and Pd/g-C3N412), which



349

This result implies that the TEMPO signal observed during PMS activation in the presence of

350

TEMP as a spin trap may not indicate the formation of 1O2. Nardi et al.47 suggested that EPR

351

detection of TEMPO may not be associated with 1O2 production; abstraction of one electron

352

from TEMP by the excited sensitizer results in the formation of the TEMP radical cation

353

(TEMP•+), which undergoes deprotonation and combination with dissolved oxygen to form

354

TEMPO. Therefore, TEMPO signals considered as evidence for singlet oxygenation during

355

persulfate activation could instead correspond to an electron-transfer mechanism, in which the

356

CNTs mediate electron transfer from TEMP to persulfate, leading to TEMP•+ generation.

357

Intermediate Distribution. In order to further clarify the difference in the degradative

358

mechanism between CNT/PMS (mediated electron transfer) and photoexcited RB (singlet

359

oxygenation), we compared the intermediate distribution from TCP oxidation by CNT/PMS and

360

fluorescent-light-irradiated RB. LC/MS analysis demonstrated a clear distinction in the

361

intermediate distribution between the two systems (Table S1). For instance, the main products

362

formed during the photosensitized singlet oxygenation of TCP included 1,2,3-trihydroxybenzene

363

2,6-dichloro-3-hydroxy-1,4-benzoquinone, which were not detectable over the course of TCP

364

decomposition by CNT/PMS. On the other hand, TCP oxidation in the aqueous CNT/PMS

365

suspension led to the formation of dihydroxydiphenyl ether and trichlorobenzene, which barely

366

formed when applying the photoirradiated RB for photochemical TCP degradation. The result

367

further confirmed that the degradative mechanism induced by CNT/PMS could be distinguished

368

from singlet oxygenation.

369

PMS Reduction and TCP Oxidation in Two Chambers Separated by a CNT Membrane. As

370

presented above, the empirical results supporting a role of 1O2 in persulfate activation, including

371

kinetic retardation in the presence of 1O2 quenchers and EPR spectral features characteristic of


Page 16 of 36

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372

1

373

pathway. Further, the lack of a D2O enhancing effect and the incompatibility of the reactivity of

374

CNT-activated PMS with the pH-dependent and substrate-specific oxidizing capacity of 1O2

375

collectively implied that oxidative degradation during the heterogeneous activation of persulfate

376

was unlikely to involve 1O2. Thus, as an alternative nonradical degradative route, we examined

377

CNT-mediated electron transfer from organic substrates to PMS using a VA-CNT membrane

378

that physically partitioned the reaction system into two chambers containing PMS and TCP

379

(Figure 6a). Pilgrim et al.48 demonstrated electron exchange between photoexcited CdSe

380

quantum dots and methyl viologen located on opposite sides of a VA-CNT membrane. In this

381

system, photogenerated electrons were transported over hundreds of micrometers across the

382

CNT-based membrane. In contrast, as an extreme pressure (>120 bar) is required to allow water

383

entry into the inner pores of superhydrophobic virgin CNT membranes (with unmodified

384

entrances and exits),49 we found that VA-CNT membranes blocked the passage of water

385

molecules under ambient pressure (even with an external pressure of 5 bar, water did not pass

386

through the nanotube channels). As a VA-CNT membrane with high electric conductivity48, 50

387

and water impermeability is likely to reject organic/inorganic impurities and reactive oxygen

388

species (e.g., SO4•− and 1O2), concomitant TCP oxidation and PMS reduction in the physically

389

separated chambers (i.e., chambers A and B in Figure 6a) would be strong proof for an electron-

390

transfer mechanism, in which CNTs mediate electron transfer from organics to persulfates.

O2 formation, were not convincing evidence for singlet oxygenation as the nonradical reaction

391

When an aqueous chloride solution (1 M) and pure water were placed in the separated

392

compartments, no movement of Cl− from one side of the membrane (chamber A) to the other

393

side (chamber B) occurred as the conductivity of the pure water side did not change at all (Figure

394

S17). This result indicated that hydrophilic ions are unable to pass through the nonwettable pores



395

of the VA-CNT membrane. Further, the PMS concentration decreased negligibly over 8 h when

396

Co2+ and PMS were placed on opposite sides of membrane, which confirmed that there was no

397

simultaneous transport of the ionic species across the CNT membrane (Figure S18). PMS

398

transport across the VA-CNT membrane would lead to a substantial increase in the PMS

399

concentration in chamber A, and the delivery of Co2+ to the other side would also cause

400

significant PMS reduction in chamber B. In contrast, when PMS and TCP were placed in the

401

separated chambers, noticeable decomposition was observed (Figure 6b). This result suggested

402

that electrons released from TCP on one side of the membrane (leading to TCP oxidation in

403

chamber A) crossed the conductive VA-CNT membrane (serving as an electron-transfer

404

mediator), eventually being accepted by PMS on the other side of the membrane (leading to PMS

405

reduction in chamber B). PMS was overconsumed for the observed TCP degradation efficiency

406

(~60%). When we repeated the experiment without the phosphate buffer, the performance with

407

respect to TCP treatment was almost unchanged, though PMS consumption was drastically

408

reduced (Figure S19). Considering ca. 10% of PMS was removable via sorption (Figure S18),

409

the observed PMS decay was insignificant. Therefore, the exposure to the phosphate buffer over

410

a relatively long reaction time (8 h) likely resulted in PMS being consumed in excessive

411

quantities (anions such as bicarbonate and phosphate added in high concentrations are capable of

412

direct PMS reduction51). On the other hand, PDS decay was minor when the TCP oxidation

413

associated with PDS activation (presented later) was performed even in the case of the buffered

414

solution (Figure S20), since PDS was unreactive toward anions present in excess

415

concentrations.51 Neither PMS nor TCP was detected on the opposite side of the membrane

416

(Figure 6b), which implied that interchamber transport did not contribute to the significant loss

417

of PMS or TCP in each chamber. TCP removal by sorption on the VA-CNT membrane was


Page 18 of 36

Page 19 of 36


418

marginal (Figure S21). TCP degradation would not occur with PMS-derived oxidants (e.g.,

419

SO4•−, 1O2) (if any formed) owing to the impermeability of the VA-CNT membrane to water-

420

soluble species.

421

The distribution of the intermediates resulting from TCP oxidation in one chamber

422

physically separated from the other chamber in which PMS reduction concurrently occurred was

423

fairly similar to that observed when TCP was subjected to oxidation in the aqueous CNT/PMS

424

suspension (Table S1). The result confirmed that the organic oxidation associated with PMS

425

activation was achieved in the same manner irrespective of the type of CNTs used (i.e., an

426

aqueous CNT suspension versus VA-CNT membrane). Minor differences in the intermediate

427

distribution may rule out the possible role of the radical-induced reaction pathway in the CNT-

428

induced PMS activation process. If the surface-bound or free SO4•− derived from the PMS

429

molecules (if any) contributed to oxidative TCP decomposition, there would be a significant

430

distinction in the intermediate distribution between the two PMS activation systems: aqueous

431

CNT/PMS/TCP mixture and CNT and PMS physically separated via the VA-CNT membrane.

432

Note that no other (transient) chemical species (except electrons and protons) is transferable to

433

the other chamber across the VA-CNT membrane. TCP decomposition was noticeable, though it

434

was kinetically retarded when we used PDS alternatively (Figure S20), which supports our

435

hypothetic nonradical reaction pathway based on mediated electron transfer; CNT-induced

436

activation involving no radical formation is not unique to PMS and is achievable using proper

437

chemicals that can serve as electron acceptors.

438

Spectroscopic analysis suggested the possibility that PMS could form a complex on the

439

surface of graphitized nanodiamond10, and the resultant complex could effectively facilitate the

440

transfer of electrons from organic substrates to the PMS molecules involved in the surface



441

complexation. More electronegative nitrogen atoms doped to CNTs could induce positive

442

charges on the adjacent carbons, which allowed the anionic PMS to form a reactive complex

443

with the N-doped CNTs.52 The surface complex was also suggested to initiate organic oxidation

444

by abstracting electrons from the organics through the conductive doped CNTs.52 The nonradical

445

mechanism is not far from our hypothetic mechanism, in which the CNTs likely mediate the

446

delivery of electrons from the organic compound to PMS, apart from the assumption that surface

447

complexation involving PMS would be a prerequisite for the electron exchange between the

448

organics and PMS. However, the formation of the reactive complex of PMS with CNTs is

449

unlikely to be indispensable to the electron transfer mechanism on account of the following

450

reasons. First, our previous work25 demonstrated that not only persulfate (PMS and PDS) but

451

also any oxyanions that serve as effective electron acceptors (e.g., periodate and percarboxylate)

452

could initiate oxidative organic degradation not reliant on the reactive radicals in the aqueous

453

CNT suspensions. Second, surface characterization of CNTs (after exposure to the PMS solution

454

for 1 h) using ATR-FTIR showed no occurrence of the spectral features assignable to the sulfur-

455

containing chemical moieties (Figure S22a); the infrared absorption peak at 1160 cm−1 is

456

attributed to the asymmetric stretching vibrations of C-S-C and S=O.53 Moreover, the

457

comparison of Raman spectra (Figure S22b) showed that the intensity of G-band, indicative of

458

graphitic carbon (1582 cm−1), did not change significantly after the use of CNTs in PMS

459

activation, implying no variation in defect density. Overall, the surface complexation of PMS can

460

promote the nonradical reaction pathway on carbonaceous activators, but will not contribute as

461

an essential step to PMS activation not relying on SO4•−.

462

Environmental Applications. In this study, we tested singlet oxygenation and mediated electron

463

transfer as hypothetical nonradical mechanisms underlying heterogeneous PMS activation. A


Page 20 of 36

Page 21 of 36


464

comparison of CNT/PMS and photoirradiated RB showed that the reactivity of CNT/PMS

465

contradicted the intrinsic properties of 1O2 in terms of the effects of chemical reagents that

466

scavenge or enhance singlet oxygenation, substrate specificity, the pH dependence of

467

chlorophenol oxidation efficiency, and the effects of solvent exchange on EPR signal intensity.

468

However, simultaneous PMS reduction and TCP oxidation were demonstrated to occur when

469

these compounds were placed in two chambers physically separated by a VA-CNT membrane

470

that prevented interchamber transport of chemical species (e.g., water, hydrophilic anions, and

471

reactive oxygen species) but allowed electron conduction. Collectively, these results implied that

472

electron transfer from organics to persulfate, effectively facilitated by nanocarbon activators, was

473

primarily responsible for persulfate activation not involving reactive radicals. Here, we

474

demonstrated that PMS was superior to PDS in terms of the organic oxidation associated with

475

persulfate activation (i.e., FFA oxidation in aqueous CNT suspensions; TCP decomposition in

476

the two-chamber system). The observation was consistent with the previous finding that CNTs

477

achieved a more rapid destruction of a variety of organics in the presence of PMS rather than

478

PDS.25 Together with the general recognition that metal-induced activation results in more

479

effective SO4•− production from PMS than PDS,8, 54 the results justified the use of PMS in the

480

persulfate activation processes, even though the choice of PDS is more economically feasible.

481

The persulfate activation process utilizing SO4•− as the main oxidant (e.g., Co2+/PMS)

482

enables effective the oxidative degradation and mineralization of a wide range of organic

483

pollutants, whereas nonradical persulfate activation leads to substrate-specific oxidation. On the

484

other hand, the selective nature of the nonradical degradation pathway (typically induced by

485

carbocatalysts) has a competitive advantage over the radical-induced pathway on account of the

486

following reasons. First, a substrate-dependent oxidizing capacity allows the persulfate activation



487

system to better target the priority pollutants present at trace levels in the complicated water

488

matrix; the majority of non-selective radicals (e.g., SO4•−, •OH) would be fruitlessly consumed

489

through the reactions with background organic and inorganic substrates. Second, since halide

490

ions are highly susceptible to one-electron oxidation by SO4•−, it is probable that the oxidative

491

treatment by SO4•− in the presence of bromide ions is inevitably accompanied by the high-yield

492

production of bromate.55 In contrast, persulfate activation based on the mediated electron transfer

493

mechanism would not result in bromate production from bromide.56 Finally, the nonradical

494

persulfate activation takes place in the co-presence of the organic pollutant (electron donor),

495

persulfate (electron acceptor), and activator (electron-transfer mediator), which likely minimizes

496

persulfate consumption; persulfate barely decomposes once the pollutant concentration is

497

significantly reduced. On the other hand, since the SO4•− in the heterogeneous persulfate

498

activation process is generated through the one-electron reduction of persulfate by the activators,

499

persulfate continues to be degraded until the residual PMS concentration reduces to virtually

500

zero.

501

The oxidative degradation of organics by CNT/PMS not reliant on SO4•− was suggested

502

to result from the CNT-induced electron exchange between the organic pollutant and PMS.

503

Carbon-based activators can kinetically enhance the transfer of electrons from organic substrates

504

to persulfates only when the electron flow is thermodynamically favored, which implies that the

505

thermodynamics of the electron transfer process is likely responsible for the selective nature of

506

the nonradical degradation pathway induced by carbocatalysts. As a result, a comparison of the

507

redox potentials of the target contaminant and persulfate will allow us to explore the treatability

508

of organic pollutants in the persulfate activation processes based on a mechanism that is not

509

radical-induced. Considering the key role of activators in the nonradical mechanism, the


Page 22 of 36

Page 23 of 36


510

properties of the carbonaceous activators (e.g., electric conductivity, surface charge, and surface

511

affinity) that affect their capability of electron transfer mediation should be considered while

512

evaluating the performance of nanocarbon materials in heterogeneous persulfate activation. The

513

persulfate activation capacity of carbonaceous materials that can initiate oxidative degradation

514

without oxidizing radicals could possibly be improved in two ways: i) improving the electric

515

conductivity and ii) increasing the surface affinity toward organic contaminants (i.e., electron

516

donor) and persulfate (i.e., electron acceptor). Accordingly, possible strategies for developing

517

high-performance nanocarbon activators include doping with heteroatoms, combining with

518

metals/metal oxides, and surface modification with chemical moieties that interact

519

electrostatically with persulfate or organics.

520

Acknowledgements

521

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

522

Korean Government (No. 2017R1A2B4002235) and a grant from the National Research

523

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

524

2016M3A7B4909318).

525

Supporting Information Available

526

Description of chemicals used in this study (Text S1), VA-CNT membrane fabrication (Scheme

527

S1), SEM images and Raman spectrum of aligned CNTs (Figures S1 and S2), XPS survey

528

spectrum of VA-CNT membrane (Figure S3), emission spectrum of fluorescent lamp and

529

absorption spectrum of RB (Figure S4), PMS decay by CNTs in H2O and D2O (Figure S5), H2O2

530

production during ascorbate oxidation by CNT/PMS and photoexcited RB (Figure S6), FFA

531

oxidation by PMS with excess L-histidine (Figure S7), reactivity of azide/PMS towards organics

532

(Figure S8), EPR spectra of azide only, PMS only, and azide/PMS with DMPO as a spin trap



533

(Figure S9), FFA degradation by CNT/PMS and CNT/PDS (Figure S10), effects of L-histidine

534

and D2O on 4-CP oxidation by CNT/PMS and CNT/PDS (Figure S11), direct reduction of PMS

535

and PDS by L-histidine (Figure S12), direct FFA oxidation by PMS and PDS (Figure S13), effect

536

of pH on chlorophenol degradation by CNT/PMS and photoexcited RB (Figure S14), zeta

537

potential of CNTs (Figure S15), EPR spectra of CNT/PMS and photoirradiated RB with DMPO

538

as a spin trap (Figure S16), chloride permeability of VA-CNT membrane (Figure S17), time-

539

dependent changes in PMS concentration with Co2+ and PMS in chambers A and B (Figure S18),

540

concurrent TCP oxidation (chamber A) and PMS reduction (chamber B) without a phosphate

541

buffer (Figure S19), concurrent TCP oxidation (chamber A) and PDS reduction (chamber B)

542

(Figure S20), time-dependent changes in TCP concentration with aqueous TCP and pure water in

543

chambers A and B (Figure S21), and ATR-FTIR and Raman spectra of fresh and used CNTs

544

(Figure S22). This information is available free of charge via the Internet at http://pubs.acs.org/.

545 546

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547

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27. Haag, W. R.; Hoigne, J., Singlet oxygen in surface waters .3. Photochemical formation and

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36. Alfassi, Z. B.; Schuler, R. H., Reaction of azide radicals with aromatic compounds. Azide as

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azide/catalase/H2O2 and various azide/peroxidase/H2O2 peroxidizing systems. J. Biol. Chem.

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Structure dependence and mechanism. Appl. Catal. B: Environ. 2015, 164, 159-167.

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Electron conductive and proton permeable vertically aligned carbon nanotube membranes. Nano

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press.

704



(a)

CNT/PMS CNT/PMS w / L-histidine CNT/PMS w / azide

1.0

CNT/PMS w / D2O

0.8 FFA Conc. (C/C0 )

Page 30 of 36

CNT only PMS only 0.6

0.4

0.2

0.0 0

10

20

30

40

50

60

Reaction Time (min) (b)

RB RB w / L-histidine RB w / azide RB w / D2 O

1.0

FFA Conc. (C/C0)

0.8

0.6

0.4

0.2

0.0 0

705

10

20

30

40

50

60

Fluorescent Light Irradiation Time (min)

706

FIGURE 1. Degradation of furfuryl alcohol (FFA) by (a) CNT/PMS (in the dark) and (b) RB

707

(under photoirradiation) in the absence and presence of L-histidine, azide, and D2O ([CNT]0 =

708

0.1 g L−1; [RB]0 = 0.05 mM; [PMS]0 = 1 mM; [FFA]0 = 0.05 mM; [L-histidine]0 = [azide]0 = 100

709

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


Page 31 of 36


(a)

1.0

5 mM L-histidine 10 mM L-histidine 25 mM L-histidine 100 mM L-histidine

PMS Conc. (C/C0 )

0.8

0.6

0.4

0.2

0.0 0

10

20

30

40

50

60

(b) 1.0 FFA Conc. (C/C0)

1.0

PMS Conc. (C/C0 )

0.8

0.6

Azide only PMS/5 mM azide PMS/25 mM azide PMS/100 mM azide CNT /PMS/100 mM azide

0.8

0.6

5 mM azide 10 mM azide 25 mM azide 100 mM azide

0.4

0.2

0.0

0.4

0

10

20

30

40

50

60

Reaction Time (min)

0.2

0.0 0

10

20

30

40

50

60

Reaction Time (min)

710 711

FIGURE 2. Effects of initial concentrations of (a)

712

decomposition ([PMS]0 = 1 mM; [furfuryl alcohol (FFA)]0 = 0.05 mM; [phosphate buffer]0 = 1

713

mM; pHi = 7.0). Inset: FFA degradation during PMS activation with increasing concentrations of

714

azide.

L-histidine

715


and (b) azide on PMS


(a)

Benzoic acid

1.0 Organic Compound Conc. (C/C0 )

Page 32 of 36

Bisphenol A Carbamazepine Cimetidine

0.8

Propranolol 0.6

0.4

0.2

0.0 0

10

20

30

40

50

60

Reaction Time (min) (b)

Benzoic acid Bisphenol A

Organic Compound Conc. (C/C0)

1.0

Carbamazepine Cimetidine

0.8

Propranolol

0.6

0.4

0.2

0.0 0

716

10

20

30

40

50

60

Fluorescent Light Irradiation Time (min)

717

FIGURE 3. Degradation of various organic compounds by (a) CNT/PMS (in the dark) and (b)

718

RB (under photoirradiation) ([CNT]0 = 0.1 g L−1; [RB]0 = 0.05 mM; [PMS]0 = 1 mM; [benzoic

719

acid]0 = [bisphenol A]0 = [cimetidine]0 = [propranolol]0 = [carbamazepine]0 = 0.05 mM;

720

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


Page 33 of 36


Pseudo-First-Order Rate Constant (min-1 )

2.0

(a)

pH 4.5 pH 7 pH 11

1.5

1.0

0.5

0.0 4-CP

TCP

PCP

0.20 Pseudo-First-Order Rate Constant (min-1 )

(b)

pH 4.5 pH 7 pH 11

0.15

0.10

0.05

0.00 4-CP

TCP

PCP

721 722

FIGURE 4. Pseudo-first-order rate constants for chlorophenol degradation by (a) CNT/PMS (in

723

the dark) and (b) RB (under photoirradiation) under various pH conditions ([CNT]0 = 0.1 g L−1;

724

[RB]0 = 0.05 mM; [PMS]0 = 1 mM; [4-chlorophenol (4-CP)]0 = [trichlorophenol (TCP)]0 = 0.05

725

mM; [pentachlorophenol (PCP)]0 = 0.04 mM; [phosphate buffer]0 = [carbonate buffer]0 = 1 mM).



(a)

Page 34 of 36

CNT only PMS only CNT/PMS (in H2O)

Intensity (Arb. Unit)

CNT/PMS (in D2O)

330.4

330.6

330.8

331.0

331.2

(b)

RB (in H2O)

Intensity (Arb. Unit)

RB (in D2O)

330.6

330.7

330.8

330.9

331.0

331.1

Magnetic Field (mT)

726 727

FIGURE 5. EPR spectra recorded in H2O- and D2O-based (a) CNT/PMS suspensions (in the

728

dark) and (b) RB solutions (under photoirradiation) ([CNT]0 = 0.1 g L−1; [RB]0 = 0.05 mM;

729

[PMS]0 = 1 mM; [TEMP]0 = 1 mM; [phosphate buffer]0 = 1 mM; pHi = 7.0). TEMP was used as

730

a spin trap.

731 732


Page 35 of 36


733 (a)

734 735 736 737 738 739 740 741 1.5

(b)

1.0

TCP (chamber A) TCP (chamber B) PMS (chamber A) PMS (chamber B)

1.2

0.9

0.6

0.6

0.4

0.3

0.2

0.0 0

742

PMS Conc. (mM)

TCP Conc. (C/C0 )

0.8

100

200

300

400

0.0 500

Reaction Time (min)

743

FIGURE 6. (a) Experimental set-up for PMS activation in the reaction system partitioned into

744

two chambers by a vertically aligned CNT membrane and (b) simultaneous trichlorophenol (TCP)

745

oxidation (chamber A) and PMS reduction (chamber B) ([PMS]0 = 1.5 mM; [TCP]0 = 0.01 mM;

746

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



747

Table of Contents Figure:

748 749 750 751 752 753


Page 36 of 36

Identifying the Nonradical Mechanism in the Peroxymonosulfate

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