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Activation of Peroxymonosulfate by Surface-Loaded Noble Metal Nanoparticles for Oxidative Degradation of Organic Compounds Yong-Yoon Ahn, Eun-Tae Yun, Jiwon Seo, Changha Lee, Sang Hoon Kim, Jae-Hong Kim, and Jaesang Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02841 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on September 8, 2016
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Activation of peroxymonosulfate by surface-loaded noble metal
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nanoparticles for oxidative degradation of organic compounds
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Yong-Yoon Ahn1, Eun-Tae Yun1, Ji-Won Seo2, Changha Lee2, Sang Hoon Kim3, Jae-Hong Kim4,
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and Jaesang Lee1,5*
5 6 7 8 9 10
1
Civil, Environmental, and Architectural Engineering, Korea University, Seoul 136-701, Korea
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Urban and Environmental Engineering, KIST-UNIST-Ulsan Center for Convergent Materials (KUUC), Ulsan National Institute of Science and Technology, Ulsan 698-805, Korea 3
Center for Materials Architecturing, Korea Institute of Science and Technology (KIST), Seoul 136-701, Korea
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Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States
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Energy Environmental Policy and Technology, Green School, Korea University-KIST, Seoul 136-701, 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 demonstrates the capability of noble metal nanoparticles immobilized on
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Al2O3 or TiO2 support to effectively activate peroxymonosulfate (PMS) and degrade select
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organic compounds in water. The noble metals outperformed a benchmark PMS activator such as
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Co2+ (water-soluble) for PMS activation and organic compound degradation at acidic pH and
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showed the comparable activation capacity at neutral pH. The efficiency was found to depend on
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the type of noble metal (following the order of Pd > Pt ≈ Au >> Ag), the amount of noble metal
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deposited onto the support, solution pH, and the type of target organic substrate. In contrast to
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common PMS-activated oxidation processes that involve sulfate radical as a main oxidant, the
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organic compound degradation kinetics were not affected by sulfate radical scavengers and
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exhibited substrate dependency that resembled the PMS activated by carbon nanotubes. The
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results presented herein suggest that noble metals can mediate electron transfer from organic
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compounds to PMS to achieve persulfate-driven oxidation, rather than through reductive
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conversion of PMS to reactive sulfate radical.
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Keywords: noble metal, peroxymonosulfate activation, oxidative degradation, sulfate radical,
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electron transfer mediator
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INTRODUCTION
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The persulfate-based oxidation (persulfate herein represents ions or compounds
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containing SO52− or S2O82−.1) has been increasingly recognized as a viable, alternative oxidation
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process for water treatment and soil remediation.2-4 The oxidation is carried out as relatively
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stable persulfates such as peroxymonosulfate (HSO5−, PMS) and peroxydisulfate (S2O82−, PDS)
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are activated on site to generate reactive species such as a sulfate radical (SO4•−). Various
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strategies that can be readily put into practice have been developed to effectively activate
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persulfate through energy or electron transfer reactions. For instance, UV photolysis2, 5, 6 and
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thermolysis7 activate PMS and PDS by initiating intramolecular electron transfer and the
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associated homolytic cleavage of peroxide bonds within the persulfate molecule. Alternatively,
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persulfates can be electrochemically activated, for example, through cathodic reduction in an
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electrolytic cell.8 The activation through the similar reductive pathway can be also achieved
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using transition metals (e.g., Fe2+ and Co2+) and their elemental or oxide counterparts (e.g., Fe0,
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Fe(OH)3, β-MnO2, Mn2O3, and Co3O4) that reduce persulfates concomitantly with their facile
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oxidation.9-13 Conduction band electrons in semiconductors generated via photocatalysis have
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been also exploited to reduce persulfate.14
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A typical persulfate-activated oxidation process relies on sulfate radical’s strong
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oxidizing power (E0(SO4•−/SO42−) = 2.43 VNHE15) toward a broad spectrum of recalcitrant
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pollutants.2-4, 8, 16 While sulfate radical is less reactive than hydroxyl radical (•OH),17, 18 previous
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studies suggested a superior performance compared to well-established H2O2-based advanced
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oxidation for the degradation of select organic compounds.7, 19, 20 Unlike H2O2 activation that can
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be achieved with only a few transition metal ions (i.e., Fenton and Fenton-like reactions) in
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acidic condition or at circumneutral pH only when metal ions are coordinated to organic
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ligands21 or metal oxide surfaces,22 various activation strategies that are effective over a wider
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range of pH have been reported for the persulfate system. The oxidation by SO4•− favors direct
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electron abstraction and can even transform some anions (e.g., Cl− and OH−) into the
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corresponding radicals, which is not significant with •OH-induced oxidation.5 In particular,
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chlorine radical (Cl•) and active chlorine species (i.e., Cl2, HOCl) formed via one-electron
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oxidation of Cl− kinetically enhanced treatment of organic compounds and caused the occurrence
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of chlorinated intermediates.23, 24
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Recent studies16,
25, 26
have proposed that some persulfate activation schemes do not
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involve radical formation, contrasting the persulfate-based oxidation from classical advanced
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oxidation processes (AOPs). In a hypothesized non-radical mechanism postulated in these
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studies, persulfate is thought to function as an oxidant that directly accept electrons from an
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organic substrate, i.e., electron donor, with the help of an electron transfer mediator. Hence, the
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electron transfer mediator activates the organic compound degradation just like abovementioned
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persulfate activators (e.g., Co2+), but through a different mechanism. While the direct evidence
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for electron transfer through the activator is currently lacking in literature, many experimental
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observations indirectly suggest a high likelihood of this pathway occurring in a few processes.
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For example, the kinetics of 2,4-dichlorophenol oxidation by CuO/PDS (i.e., CuO as an
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activator) was found to be unaffected even when excess amounts of ethanol and chloride ions as
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a SO4•− scavenger were added.26 When carbon nanotubes (CNTs) were used to activate
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persulfate and degrade organic compounds, no radicals were detected (e.g., through electron spin
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resonance) and the degradation kinetics was also not retarded by excess radical scavenger.16 The
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mechanism involving electron transfer mediation by CNTs is also consistent with the observation
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that PMS activation was enhanced by nitrogen doping in CNT, the common approach to enhance
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conductivity of CNT.25
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We herein explore the potential use of noble metal nanoparticles as a PMS activator for
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the first time in literature, motivated by the recent emergence of persulfate-based oxidation
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schemes that involve electron transfer mediation mechanism. Noble metals are of our interest
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since they exhibit not only high electrical conductivity27 but also excellent capability to induce
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catalytic reduction28,
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mechanisms, persulfate reduction to SO4•− versus electron transfer from organics to persulfate).
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We examined whether most common noble metal catalysts such as platinum (Pt), palladium
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(Pd), gold (Au), and silver (Ag) supported on metal oxides (referred to in this study as NM-
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Al2O3 and NM-TiO2) can activate PMS and consequently degrade select organic pollutants. The
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effects of reaction parameters such as the catalyst loading and initial pH were investigated, and
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the dependence of the PMS activation efficiency on the substrate type was evaluated. In an effort
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to address the critical knowledge gap regarding the primary activation mechanism, we compared
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three PMS activating systems involving Pd-Al2O3, Co2+, and MWCNTs in terms of 1) the
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substrate specific nature of the oxidizing capacity and 2) the extent of reduction in performance
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in the presence of methanol, a radical scavenger. Finally, we assessed how NM-Al2O3 and NM-
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TiO2 would perform when used repeatedly.
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(as aforementioned, persulfate activation is initiated via two distinct
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MATERIALS AND METHODS
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Chemicals and Materials. The chemicals that were used as-received in this study include:
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aluminum oxide (Al2O3, PURALOX TH 100/150, Sasol; BET surface area = 150 m2/g; average
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particle size = 35 µm), titanium dioxide (TiO2, DT-51, Cristal Global; BET surface area = 90
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m2/g; average particle size = 1.7 µm), cobalt nitrate hexahydrate (Sigma-Aldrich), multiwall
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carbon nanotubes (MWCNTs, Hanwha), potassium monopersulfate (OXONE®, Sigma-Aldrich),
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potassium peroxydisulfate (Sigma-Aldrich), benzoic acid (Sigma-Aldrich), bisphenol A
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(Aldrich), carbamazepine (Sigma-Aldrich), 4-chlorophenol (Aldrich), 4-nitrophenol (Aldrich),
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phenol (Sigma-Aldrich), 2,4,6-trichlorophenol (Aldrich), pentachlorophenol (Aldrich), methanol
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(J.T. Baker), potassium iodide (Sigma-Aldrich), perchloric acid (Sigma-Aldrich), sodium
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bicarbonate (Sigma-Aldrich), sodium hydroxide (Fluka), phosphoric acid (Aldrich), and
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acetonitrile (J.T. Baker). All chemicals used in this study were of the highest purity available,
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and used without further treatment. Ultrapure water (>18 MΩ•cm) produced by a Milli-Q Water
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Purification System (Millipore) was used to prepare all the solutions.
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Catalyst Preparation and Characterization. Noble metals were deposited onto the surface of
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Al2O3 or TiO2 support using a coaxial pulsed arc-plasma deposition (APD) system (ULVAC,
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ARL-300) equipped with a rod-shaped noble metal cathode, a trigger electrode at the center of a
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reaction chamber, and a cylindrical anode coaxially aligned to surround the cathode.30 Metal
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oxide powders (Al2O3 or TiO2) were placed at the bottom of the chamber, directed toward the
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plasma source, and constantly stirred for effective dispersion. An electric charge was
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accumulated in a discharge condenser (capacity = 1080 µF; connected to the cathode) at the
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discharge voltage of 200 V. An ionized metal plasma was then instantly produced at room
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temperature under vacuum (10-5 Torr) by a trigger pulse on the cathode which subsequently
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formed nanoparticles on the surface of Al2O3 or TiO2.
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Energy dispersive spectroscopy (EDS) analysis (Figures S1-4) confirmed successful
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surface loading of noble metals on Al2O3. The transmission electron microscopy (TEM;
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TECNAI G2 F30ST, JEOL Ltd.) images in Figure 1 (Al2O3) and Figure S5 (TiO2) show that
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noble metal nanoparticles with an average size of approximately 2–5 nm were uniformly
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deposited on the surface of metal oxide supports, though a minor fraction of Au and Ag particles
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were observed to undergo aggregation (Pd (on Al2O3) = 3.10 ± 0.965 nm; Pt (on Al2O3) = 3.02 ±
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0.735 nm; Au (on Al2O3) = 3.35 ± 0.933 nm; Ag (on Al2O3) = 2.09 ± 1.124 nm; Pd (on TiO2) =
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2.76 ± 0.576 nm; Pt (on TiO2) = 2.02 ± 0.480 nm). The loading of Pd, Pt, Au, and Ag on both
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Al2O3 and TiO2 surface was estimated to be approximately 1 wt% using inductively coupled
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plasma mass spectrometry (ICP-MS, ELAN DRC-II, Perkin Elmer). The oxidation states of the
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noble metals deposited on Al2O3 were analyzed by X-ray photoelectron spectroscopy (XPS, PHI
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X-tool, ULVAC-PHI, Inc.) using the Al Kα line (1486.7 eV) as an excitation source. The
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spectrum of freshly-prepared Pd-Al2O3, which was essentially the same when TiO2 was an
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alternative support, is characterized by a double peak centered at 335.1 eV for Pd 3d5/2 and
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340.35 eV for Pd 3d3/2, suggesting the presence of Pd in a metallic form (Figure S6a).31 The Pt 4f
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band in the XPS spectrum of Pt-Al2O3 deconvolutes into three peaks at 74.3, 72.5, and 71.2 eV
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that correspond to Pt(IV), Pt(II), and Pt(0), respectively, indicating the co-existence of metallic
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and oxidized Pt (Figure S6b).32 The XPS spectrum of Au deposited on Al2O3 (Figure S6c) shows
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the Au 4f peaks at a binding energy of 83.8 for Au 4f7/2 and 87.5 eV for Au 4f5/2; these peaks are
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distinctive for Au(0).33 The XPS signal of Ag consisted of a doublet with two peaks at 368 and
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374 eV, respectively assigned to Ag 3d5/2 and Ag 3d3/2 (Figure S6d); this signal is attributed to
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surface deposition of metallic Ag.34 The immobilized noble metal particles were stored in the
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sealed borosilicate glass vials in the dark. PMS activation capacity of NM-Al2O3 (or NM-TiO2)
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samples was confirmed to remain unchanged after 2 years of storage. The zeta potentials of the
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aqueous-suspended NM-Al2O3 were measured as a function of pH using an electrophoretic light
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scattering spectrophotometer (ELSZ-1000, Otsuka Electronics).
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To explore the possible leaching of noble metals from NM-Al2O3, we prepared aqueous
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suspensions containing 0.25 g/L NM-Al2O3, 0.25 mM PMS and 0.1 mM 4-CP (4-chlorophenol;
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initially adjusted to pH 3, pH 7, and pH 10), performed PMS activation, and quantified the
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dissolved metal ions in the filtrates using ICP-MS. Chemical stability of noble metal-based
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activators was also examined based on determination of the leached metal ions during multiple
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catalytic cycles.
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Experimental Procedure and Analytical Methods. Degradation of various target organic
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compounds (0.1 mM) was monitored in a magnetically-stirred 40-mL reactor containing air-
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equilibrated aqueous suspensions of 0.25 g/L NM-Al2O3 (or NM-TiO2) and 0.25 mM PMS. The
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reaction suspensions were typically buffered at ca. 7.0 using 1 mM bicarbonate buffer and found
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to change only marginally over the course of reaction. We confirmed that bicarbonate could
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neither kinetically affect PMS activation processes (Figure S7) nor activate PMS by itself
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(Figure S8) (note that some anions (e.g., HPO42−, Cl−, HCO3−) were demonstrated to active
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PMS7 and carbonate/bicarbonate can react with SO4•− to produce some radical species (e.g.,
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CO3•−).35 For select experiments performed to evaluate pH effect, the suspensions were
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unbuffered and the initial pH was adjusted using 0.1 M HClO4 or NaOH solution. Sample
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aliquots were periodically withdrawn from the reactor using a 1 mL syringe, filtered through a
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0.45 µm PTFE filter (Millipore), and injected into a 2 mL amber glass vial containing excess
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methanol (0.5 M) to quench any residual radicals. The concentration of organic compounds were
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determined using a HPLC (Agilent Infinity 1260) system equipped with a C-18 column
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(ZORBAX Eclipse XDB-C18) and a UV/Vis detector (G1314F 1260VWD). The typical eluent
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consisted of a binary mixture of 0.1% (v/v) aqueous phosphoric acid solution and acetonitrile
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(45/55 by volume). Intermediates formed during 4-CP oxidation by the activated PMS were
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qualitatively identified using the Rapid Separation Liquid Chromatography (RSLC)/orbitrap
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MS/MS system. The RSLC separation was carried out on an acclaimTM C18 column (150 mm ×
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2.1 mm, 2.2 µm; Thermo Fisher Scientific Inc.) with a mobile phase consisting of 0.1 % aqueous
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formic acid solution and acetonitrile. The mass analysis was performed in a negative electrospray
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ionization (ESI) mode. The relative abundances of oxidation intermediates were estimated based
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on their mass spectroscopy peak areas. Accurate mass measurements were guaranteed with the
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low ppm range (< 10 ppm of the theoretical mass). Evolution of chloride ions as a result of
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dechlorination of 4-chlorophenol was monitored using an ion chromatography (IC, Dionex DX-
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120) equipped with a Dionex IonPac AS-14 and a conductivity detector. PMS was
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spectrophotometrically quantified using the method proposed by Liang et al., based on an iodine
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(λmax = 352 nm) forming reaction between PMS and iodide.36 A calibration plot of absorbance at
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352 nm versus PMS concentration showed a linear relationship over the concentration range of
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30 µM to 500 µM, suggesting 30 µM as the detection limit for PMS. Formaldehyde that forms as
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a result of methanol oxidation was quantitatively measured using the HPLC after derivatization
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with 2,4,-dinitrophenylhydrazine (DNPH).37 Gaseous CO2 production as an evidence for
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mineralization of organic compounds was monitored using a gas chromatography (GC, Agilent
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HP6890A) that was equipped with a flame ionization detector (FID), a Carboxen 1010 PLOT
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capillary column, and a CO2 methanizer (Agilent G2474A). Dissolved organic carbon was
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measured using a total organic carbon analyzer (TOC-VCPH, Shimadzu). For electron
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paramagnetic resonance (EPR) analysis, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used as
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a spin-trapping agent for SO4•−. The EPR spectra were monitored in the Pd-Al2O3/PMS system
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using a JES-TE 300 spectrometer (JEOL, Japan) under the following conditions: microwave
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power = 1 mW, microwave frequency = 9.421 GHz, center field = 3375 G, modulation width =
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0.2 mT, and modulation frequency = 100 kHz.
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RESULTS AND DISCUSSION
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PMS Activation by Noble Metals. Results in Figure 2 demonstrate that PMS effectively
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degraded the model organic compound, 4-CP, but only when Pd-Al2O3 or Pd-TiO2 was present.
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Neither Pd-Al2O3, Pd-TiO2, nor PMS alone caused any decrease in 4-CP concentration (Figure
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S8). PMS also did not affect 4-CP concentration at all when bare Al2O3 or TiO2 without noble
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metals was used (Figure S8). The oxidation of 4-CP by the Pd-Al2O3 or Pd-TiO2 was
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accompanied by the loss of PMS that is related to the stoichiometric reduction into sulfate ions
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(SO42−) (Figure 2). Pd-Al2O3 (or Pd-TiO2) decomposed 4-CP in the presence of PMS at a rate
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comparable to Co2+, a benchmark PMS activator,3 while PMS consumption was 2.5 times faster
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with Co2+ than with Pd-Al2O3 (Figure S9). The rapid PMS depletion in the Co2+/PMS system is
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ascribed to oxidative conversion of PMS by Co3+ and concurrent regeneration of Co2+ (HSO5− +
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Co3+ → HSO5• + Co2+).38 Approximately 33.7 % of chlorine initially present in 4-CP was found
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to be released as chloride ion (Figure 2), which is consistent with 38.2 % dechlorination of 4-CP
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by Co2+/PMS system (Figure S9). The comparable 4-CP degradation between Pd and Co2+ is
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noteworthy, considering that Pd is in an immobilized, solid form in contrast to soluble Co2+. The
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gaseous CO2 produced after 8 h of 4-CP oxidation by either Pd-Al2O3/PMS or Co2+/PMS
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corresponded to only ca. 1.2-1.3 % mineralization of the initial 4-CP concentration (Figure S10).
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Other noble metals also induced PMS activation and 4-CP degradation when deposited
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onto Al2O3 (a more inert, non-reducible support used in this set of experiments to isolate the
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noble metal effects39), but to a widely varying degrees depending on both the type and amount of
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noble metal (Figure 3 and Figure S11). Over the range of metal loadings investigated, Pd was
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found most effective in 4-CP degradation. While a greater difference was observed at lower
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metal loadings, on average, the initial 4-CP degradation rate was 31% faster with Pd compared to
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the second most effective Au. No significant difference in the 4-CP degradation kinetics was
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found between Au and Pt. Unlike other metals, no reduction in the 4-CP concentration was
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observed when Ag was used, similar to the report that silver ion cannot activate PMS and
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therefore is ineffective in chlorophenol degradation.9
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The reason behind the effect of metal type is unknown, while the efficient PMS
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activation by Pd-, Au-, and Pt-Al2O3 might be related to the intrinsic surface catalytic property of
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these metals, for example, in reduction of nitrophenol29, 40 and nitrate.28 When Pd was used as an
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activator, the reaction rate reaches near plateau values at the 0.375 g/L, above which further
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loading induced only a marginal improvement in 4-CP removal kinetics. On the other hand, 4-CP
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degradation efficiency of Au- and Pt-Al2O3 continued to increase in proportion to the loading
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amount, becoming comparable to that of Pd-Al2O3 at 0.5 g/L loading. The PMS decomposition
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showed a similar trend as 4-CP degradation (Figure 3 and Figure S12), with the decay rate
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following the same order of Pd > Au ≈ Pt >> Ag and increasing with greater metal loading.
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These results collectively suggest that select noble metals, when deposited onto metal oxide
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surface, can activate PMS to result in PMS reduction and subsequent degradation of organic
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compound.
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The Effect of Initial pH on PMS Activation Efficiency. The kinetics of 4-CP degradation and
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PMS reduction were not significantly affected by the change in solution pH between 3 to 9
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(Figure 4 and Figure S13), when PMS was activated by Pd-Al2O3 or Pt-Al2O3. When the
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experimental suspensions were initially adjusted to an acidic or a basic pH and were buffered at
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pH 7 using 1 mM bicarbonate, the change in solution pH was marginal over the course of PMS
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activation. This relative pH independence, as confirmed in other persulfate-based oxidation
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processes, is one notable advantage compared to other peroxide-based oxidation processes such
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as H2O2/Fe2+ the efficiency of which is highly pH sensitive. When pH was raised to 11, however,
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4-CP degradation kinetics was significantly inhibited in both cases. These results are in contrast
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to the pH dependence of 4-CP oxidation with Au-Al2O3, in which alkaline and neutral pH
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favored but acidic pH retarded 4-CP degradation. Almost no 4-CP decomposition was observed
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with Ag-Al2O3, regardless of the initial pH. These 4-CP degradation kinetics correlated generally
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well with PMS decomposition rates for all the composite catalysts over all pH range investigated
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(Figure 4 and Figure S13). For example, when Pd-Al2O3 or Pt-Al2O3 was used as a PMS
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activator, the efficient 4-CP degradation was accompanied by the fast PMS decomposition
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between pH 3 and 9 and the slow 4-CP degradation by the slow PMS decomposition at pH 11.
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Likewise, an increase in pH accelerated both 4-CP degradation and PMS decomposition in Au-
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Al2O3 case. The inability of Ag-Al2O3 to degrade 4-CP in the presence of PMS accords with no
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detectable decomposition of PMS in the pH range investigated.
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The quantitative monitoring of noble metal leaching from NM-Al2O3 under varying pH
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conditions (Table S1) confirms negligible loss of noble metal mass (less than 0.03 % of the
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initial mass) during PMS activation, regardless of initial pH. This rules out the possibility that
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the pH-dependent PMS activation efficiency of NM-Al2O3 (Figure 4) is attributable to the
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chemical stability of activators that may be sensitive to pH conditions. The decreased efficiency
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of 4-CP degradation by Pd (or Pt)-Al2O3 at pH 11 could be partly related to the change of
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activator particle surface charge from positive to negative; the PZC (point of zero charge) of
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activator particle was found to be at around 10 (Figure S14), while PMS exists as anion over the
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entire pH range (pKa1 = 0.4 and pKa2 = 9.3).41, 42 We exclude the onset of electrostatic repulsion
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between negatively charged activator surface and deprotonated 4-CP as the major cause of
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decreased kinetics at pH 11. We found that when Pd-Al2O3 was employed as an activator, other
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chlorophenols, 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP) also underwent
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rapid degradation in the pH range of 3 to 9 and showed drastically retarded decomposition
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kinetics only at pH 11, despite widely varying pKa values (i.e., pKa(4-CP) = 9.4143; pKa(2,4,6-
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TCP) = 6.2343; pKa(PCP) = 4.7044) (Figure S15). Alternatively, hydrolysis of PMS under
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alkaline conditions (SO52− + H2O → HO2− + SO42− + H+)45 might be responsible for futile loss of
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PMS. However, we found that PMS does not decay over the entire pH range in the absence of
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noble metal (but in the presence of bare Al2O3). When Pd-Al2O3 was added alternatively, PMS
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decayed over the entire pH range except only pH 11 (Figure S16), further excluding this
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hypothesis. These collectively suggest that the ineffective PMS activation by Pd (Pt)-Al2O3 is a
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major cause of the decelerated 4-CP degradation at high pH, possibly due to electrostatic
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repulsion between NM and PMS or through a mechanism currently unknown (e.g., the formation
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of poisoning oxide layer on Pd (Pt) surface at strongly alkaline pH). It is noteworthy that Au that
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underwent negligible reduction in PMS activation capacity at pH 11 (Figure 4) is much less
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prone to surface oxidation than Pd and Pt and is superior over Pt and Pd in (electro)catalytic O2
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reduction and alcohol oxidation in alkaline media.46, 47 Considering its relative irrelevance to
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realistic conditions, we did not further delve into the phenomenon occurring at such an extreme
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high pH condition.
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The above pH dependence of 4-CP degradation kinetics presents a unique feature of PMS
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activation by NM-Al2O3 compared to benchmark Co2+ activator (Figure S15). The Co2+ showed
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the optimal 4-CP degradation in the near-neutral pH range, due to predominance of CoOH+
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species in the neutral pH region which is known to be an efficient PMS activator48 and
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precipitation of cobalt ions under even slightly alkaline conditions.49, 50 At low pH, the loss of
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oxidation capacity has been attributed to the decrease of oxidizing power of SO4•−,49 a main
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radical oxidant in typical persulfate-based oxidation processes such as the one employing Co2+ as
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an activator. It is therefore interesting to note that chlorophenols were effectively degraded when
297
PMS was activated by Pd-Al2O3 and Pt-Al2O3 at pHs as low as 3.0 (Figure 4 and Figure S15).
298
This not only highlights the advantage of using NM-Al2O3 as an activator for PMS but indicates
299
likely involvement of a different persulfate reduction mechanism that does not result in SO4•−.
300
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Degradation of Various Organics. The efficacy of PMS activated by NMs, deposited onto
302
either Al2O3 or TiO2, to degrade various organic compounds was further evaluated (Figure 5).
303
The degradation rate was consistently in the order of Pd > Pt ≈ Au >> Ag, regardless of the type
304
of target organic compounds and metal oxide support, while some variations were observed. In
305
general, TiO2 support caused faster organic compound degradation for the same NM loading
306
under acidic and neutral conditions (Figure S17 and Figure 5). This effect was more pronounced
307
for phenolic compounds such as phenol (PH) and bisphenol A (BPA), presumably due to binding
308
of phenolic hydroxyl group with surface titanol moiety (>TiOH).51 Most notable in Figure 5 is
309
the dependence of degradation rate on the type of organic compounds. PH and chlorophenols (4-
310
CP and 2,4,6-TCP) were readily oxidized, while other compounds were degraded only
311
moderately (BPA and carbamazepine (CBZ)) or almost negligibly (benzoic acid (BZA) and 4-
312
nitrophenol (4-NP)). This feature once again appears unique to NM-Al2O3 (or NM-TiO2)/PMS
313
system, since PMS activated through other routes has been claimed to be effective for a wide
314
range of organic compound degradation.3, 8, 9, 52 For example, our previous study also suggested
315
that, through activation with zero-valent iron, PMS forms SO4•− that effectively oxidized various
316
organic compounds following rather constant kinetics rates.52 Selective nature toward organic
317
degradation observed in Figure 5 once again suggests that activation of PMS by NM-Al2O3 (or
318
NM-TiO2)/PMS and resulting organic compound oxidation might not involve SO4•− as a primary
319
oxidant.
320 321
PMS Activation Mechanism. To confirm the above speculation, excess methanol was added as
322
a SO4•− quencher and the kinetics of 4-CP oxidation was evaluated (Figure 6a). If SO4•− is a
323
major oxidant that results from PMS activation, the addition of methanol would inhibit the
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organic compound degradation.52 Consistently, we observed that excess methanol completely
325
quenched 4-CP oxidation in the Co2+/PMS system, where PMS is known to be reductively
326
converted into SO4•−.3, 9 In contrast, the same concentration (i.e., 0.25 M) of methanol as used in
327
Co2+/PMS control experiment did not cause any noticeable retardation in the rate of 4-CP
328
degradation when Pd-Al2O3 and Au-Al2O3 were used for PMS activation. The experiments to
329
evaluate the conversion of methanol to formaldehyde (inset of Figure 6a) demonstrate that
330
Co2+/PMS initially caused a steep increase in formaldehyde concentration whereas methanol
331
oxidation very slowly proceeded or was absent within 15 min in the NM-Al2O3/PMS systems.
332
These results suggest that the role of SO4•− in the oxidation of organics by NM-Al2O3/PMS is
333
most likely negligible.
334 335
We further compared the oxidizing capacity of Pd-Al2O3/PMS to that of Co2+/PMS and
336
MWCNT/PMS in terms of substrate specificity (Figure 6b). Two activating systems, Pd-
337
Al2O3/PMS and MWCNT/PMS, showed the similar substrate specificity; i.e., organics that were
338
readily oxidized in Pd-Al2O3/PMS was also in general well oxidized in MWCNT/PMS. In
339
contrast, when Co2+ was employed to activate PMS, a significant difference in the substrate
340
specificity was observed. For example, CBZ that was recalcitrant to the degradation by both Pd-
341
Al2O3/PMS and MWCNT/PMS underwent the fastest decay with Co2+. The higher chlorinated
342
phenol underwent more rapid decomposition in the systems of Pd-Al2O3/PMS and
343
MWCNT/PMS, which is in contrast to the substrate-specificity of Co2+/PMS. It has been
344
reported that MWCNT activates PMS through a different mechanism in which electron is
345
transferred from the organics to PMS through CNTs that functions as an electron shuttle.16, 25 As
346
a result, organics are oxidized and PMS are reduced to sulfate. We believe that similar non-
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radical pathway is highly plausible for noble metals considering the materials’ superior electrical
348
conductivity.26 Accordingly, we did not observe any DMPO-SO4•− adduct in the EPR spectrum
349
of Pd-Al2O3/PMS (Figure S18). Instead, we observed the occurrence of peaks assigned to 5,5-
350
dimethylpyrrolidone-2-(oxy)-(1) (DMPOX) as a product of direct DMPO oxidation,53 which was
351
also observed when CNTs were alternative used for PMS activation.16 In contrast, the spectral
352
characteristics of DMPO-SO4•− adduct appeared in the Co2+/PMS system (Figure S18). There is
353
a likelihood that quinone intermediates (likely produced from 4-CP oxidation) may reductively
354
convert PMS to SO4•− or may mediate the transfer of electrons from 4-CP to PMS.54 However,
355
Figure S19 demonstrates that 4-CP degradation was not achieved when benzoquinone or
356
catechol was used as a PMS activator, which rules out a role of quinone intermediates in the
357
PMS activation mechanism.
358 359
The comparative analysis of reaction intermediates also confirmed the difference in PMS
360
activation mechanism between noble metals versus Co2+. For instance, hydroxybenzoquinone,
361
(2E,4Z)-3-hydroxyhexa-2,4-dienedioic acid, and 4-hydroxyphenylbenzoquinone appeared only
362
during oxidative degradation of 4-CP by Co2+/PMS (Table S3). In contrast, PMS activation by
363
Pd-Al2O3 led to formation of (Z)-4,5-dioxopent-2-enoic acid and some unidentified intermediates
364
that were not observed when Co2+/PMS decomposed 4-CP (Table 3). In particular, the time-
365
dependent profiles of the relative abundances of all detected compounds (Figure S20) showed
366
that the distribution of major intermediates varied depending on the type of activator: (Z)-4,5-
367
dioxopent-2-enoic acid and an unknown compound with m/z 186.11 formed as primary products
368
of 4-CP oxidation by Pd-Al2O3/PMS whereas they were not detectable in the Co2+/PMS system.
369
The quantification of quinone intermediates using HPLC (Figure S21) suggested that formation
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of hydroquinone (HQ) seemed more pronounced when Co2+ was used as an activator; almost
371
threefold higher concentration of HQ was detected in the Co2+/PMS system after 30 min of 4-CP
372
oxidation than in the Pd-Al2O3/PMS system.
373 374
If a role as a mediator in transferring electron from organics to PMS were critical to the
375
NM-induced PMS activation, PMS reduction would not occur without organic substances. Note
376
that PMS was reduced in the aqueous suspensions of CNTs only when organics were added.16
377
Figure S22 showed that PMS was not decomposed at all by Au-Al2O3, Pt-Al2O3, or CNTs in the
378
absence of 4-CP whereas 4-CP addition drastically decreased PMS concentration. This contrasts
379
with PMS degradation by Co2+ that was kinetically retarded but still significant in the absence of
380
4-CP (Figure S22). The acceleration of PMS reduction in the ternary mixture, Co2+/PMS/4-CP, is
381
likely attributable to PMS activation or Co2+ regeneration from Co3+ by quinone intermediates.55
382
Accordingly, the results confirm that NM-Al2O3 effectively facilitate electron transfer between
383
pollutants (electron donor) and PMS (electron acceptor), contributing to oxidative degradation
384
via non-radical mechanism. Unlike other tested activators, Pd-Al2O3 exhibited almost the same
385
but significant PMS degradation efficiency regardless of the presence of 4-CP. This may result
386
from the catalytic activity of Pd in chemical transformation of peroxide compounds.56 It is
387
noteworthy that Pd-Al2O3 repeatedly achieved PMS decomposition without significant loss of
388
catalytic activity when 4-CP was absent (Figure S23).
389 390
Repetitive Use. Figure 7 explores the possibility of using immobilized Pd and Pt more than one
391
time for PMS activation. Repetitive uses caused a gradual decline in the capability of the noble
392
metals to decompose 4-CP in the presence of PMS, but the extent of reduction of the 4-CP
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393
degradation rate (Figure 7) or the PMS reduction rate (Figure S24) considerably varied
394
depending on the noble metal and support material employed. Drastic deactivation of Pd-Al2O3
395
for oxidative degradation of 4-CP was observed after single use, with the removal rate being
396
decreased by 68 % only after the first cycle. Similarly, when Pt was used as the activator, the 4-
397
CP degradation efficiency was decreased by up to 80 % in the second cycle. When TiO2 was
398
used as an alternate host for the noble metals, the reusability of Pd and Pt in PMS activation was
399
significantly improved. For instance, the Pt-TiO2 samples recovered after the first and second
400
cycles were respectively still capable of oxidizing 47 % and 25 % of 4-CP within 60 min. But
401
Pd-TiO2/PMS was able to maintain most of its catalytic property for the first three cycles, while
402
subsequent reuse decreased the efficiency down to 75 % of its original oxidation capacity.
403 404
In the recycling experiments, each cycle was carried out in different batches using NM-
405
Al2O3 (or NM-TiO2) that was recovered after previous cycle by using a 0.45 µm PTFE
406
membrane filter, washed with 2 L distilled water, and resuspended in a freshly-prepared aqueous
407
solution of PMS. Therefore, it is not likely that the steady reduction in the 4-CP degradation
408
efficiency is caused by accumulation of oxidation products in the experimental suspensions. We
409
found that the morphological change of noble metals did not occur before and after PMS
410
activation (Figures S25, S26, and S27) and therefore would not be responsible for the gradual
411
reduction in PMS activation performance. We also did not detect any meaningful accumulation
412
of noble metals in the filtrates in trace amounts (less than 0.012 % of the initial mass) (Table S2).
413
The EDS analysis of five randomly-selected regions from fresh Pd-Al2O3 versus Pd-Al2O3
414
reused for five cycles showed marginal loss of Pd during the repeated uses in PMS activation
415
(i.e., 2.40 ± 1.76 atomic % for fresh Pd-Al2O3; 2.08 ± 1.03 atomic % for reused Pd-Al2O3). The
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comparison of their TEM images (Figure S28) further confirmed that Pd particles still remained
417
uniformly distributed on Al2O3 after five cycles. The results collectively exclude metal
418
dissolution as a primary cause. We suspect that reduced efficiency with repeated applications
419
may involve a change in the oxidation state of the noble metal, for example, through non-
420
conducting metal oxide formation. Note that PMS was reduced by Pd-Al2O3 (or Pd-TiO2) even
421
when 4-CP, an electron donor, was absent (Figures S8, S16, and S22), indicating potential
422
oxidation of Pd due to PMS. Also surface deposition of organic intermediates may hinder noble
423
metals from being available for PMS activation, resulting in a gradual performance reduction
424
during the catalytic use of NM-Al2O3 (or NM-TiO2). As opposed to steady retardation in PMS
425
degradation in the presence of 4-CP (Figure S24), the efficiency of Pd-Al2O3 for PMS
426
decomposition was constantly maintained even after the fifth cycle when 4-CP was absent
427
(Figure S23). Furthermore, significant TOC reduction (ca. 60 % mineralization) after 4-CP
428
oxidation by Pd-Al2O3/PMS (Figure S29) is contradictory to the negligible conversion of 4-CP to
429
CO2 (ca. 1.2 % mineralization) (Figure S10). Note that neither TOC reduction (Figure S29) nor
430
CO2 generation (Figure S10) could be achieved with the Co2+/PMS system. The results
431
collectively corroborate the accumulation of oxidation intermediates on the surface of noble
432
metal-based activators.
433 434
Environmental Applications. This study presents the first instance of successfully employing
435
noble metals for PMS activation. It is noteworthy that noble metal ions such as Pd2+, Pt4+, and
436
Ag+ were found ineffective for PMS decomposition (not PMS activation).57 Comparative
437
analysis of the activation process using various noble metal nanoparticles immobilized on Al2O3
438
and TiO2 suggested a few interesting features that contrast this strategy from existing ones. First,
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439
the Pd-Al2O3 exhibited comparable or even better efficiency compared to the conventional
440
activation system involving Co2+ in degrading phenolic compounds when the same amount of
441
activating reagent (i.e., 25 µM) was employed (Figure S30), while consuming much less PMS
442
(Figure S9). Second, Pd- and Pt-Al2O3 exhibited significant PMS activating capacity over a wide
443
pH range (pH 3 to 9), especially at low pH in which SO4•−-mediated oxidation does not
444
efficiently function.49 Third, contrary to the previously observed non-selective reactivity of
445
SO4•− toward phenolic compounds,52 the rate of oxidative decomposition by PMS activated with
446
NM-Al2O3 was found to be highly dependent on the target substrate. Particularly, the substrate-
447
specific reactivity likely allows the NM-Al2O3/PMS systems to maintain significant treatment
448
efficiency in the presence of background organic matter; we found that Pd-Al2O3/PMS was still
449
capable of effective 4-CP degradation in the presence of excess methanol or humic acid whereas
450
Co2+/PMS caused no 4-CP decomposition (Figures 6a and S31). Finally, the mechanism appears
451
to involve facile transfer of electrons from organics (electron donor) to PMS (electron acceptor)
452
through electron transfer mediation by NM. We expect that the same mechanism will function
453
when different persulfate species are used; for example, we found that significant degradation of
454
4-CP was also observed with Pd-Al2O3 (Figure S32) when PDS, instead of PMS, was used.
455
While this study introduces a new materials approach for persulfate-based oxidation process, it
456
also raises many questions that have not been addressed in literature and thus require further
457
studies, particularly on ways to effectively regenerate spent noble metal activators (given that
458
reduction in activation performance is attributed to the changes in their oxidation states). These
459
might include: 1) alternative use of other metal oxide supports (e.g., SiO2, CeO2, and Nb2O5) 2)
460
reduction using chemical reagents (e.g., NaBH4, H2 gas), and 3) photocatalytic treatment.58
461
Acknowledgements
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This research was supported by the Basic Science Research Program through the National
463
Research
464
(2014R1A1A2056935). This work was also partially supported by a grant from the future R&D
465
Program (2E26120) funded by Korea Institute of Science and Technology, by Korea Ministry of
466
Environment as “The GAIA Project” (2016000550007), and by the Ministry of Trade, Industry,
467
and Energy (MOTIE), Korea, as "Encouragement Program for the Industries of Economic
468
Cooperation Region (R0004881)".
Foundation
of
Korea
(NRF)
funded
by
the
Ministry
of
Education
469 470
Supporting Information Available.
471
This information is available free of charge via the Internet at http://pubs.acs.org/.
472 473
Literature Cited
474
1. Harald Jakob; Stefan Leininger; Thomas Lehmann; Sylvia Jacobi; Gutewort, S., Peroxo
475
Compounds, Inorganic. In Ullmann's Encyclopedia of Industrial Chemistry, John Wiley and
476
Sons: 2012; pp 293-324.
477
2. Liu, X. W.; Zhang, T. Q.; Zhou, Y. C.; Fang, L.; Shao, Y., Degradation of atenolol by
478
UV/peroxymonosulfate: Kinetics, effect of operational, parameters and mechanism.
479
Chemosphere 2013, 93, (11), 2717-2724.
480
3. Anipsitakis, G. P.; Dionysiou, D. D., Degradation of organic contaminants in water with
481
sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci.
482
Technol. 2003, 37, (20), 4790-4797.
483
4. Rastogi, A.; Ai-Abed, S. R.; Dionysiou, D. D., Sulfate radical-based ferrous-
484
peroxymonosulfate oxidative system for PCBs degradation in aqueous and sediment systems.
485
Appl. Catal. B. Environ. 2009, 85, (3-4), 171-179.
21 Environment ACS Paragon Plus
Environmental Science & Technology
486
5. Yang, Y.; Pignatello, J. J.; Ma, J.; Mitch, W. A., Comparison of halide impacts on the
487
efficiency of contaminant degradation by sulfate and hydroxyl radical-based advanced oxidation
488
processes (AOPs). Environ. Sci. Technol. 2014, 48, (4), 2344-2351.
489
6. Liu, X. W.; Fang, L.; Zhou, Y. C.; Zhang, T. Q.; Shao, Y., Comparison of UV/PDS and
490
UV/H2O2 processes for the degradation of atenolol in water. J. Environ. Sci. 2013, 25, (8), 1519-
491
1528.
492
7. Yang, S. Y.; Wang, P.; Yang, X.; Shan, L.; Zhang, W. Y.; Shao, X. T.; Niu, R., Degradation
493
efficiencies of azo dye Acid Orange 7 by the interaction of heat, UV and anions with common
494
oxidants: Persulfate, peroxymonosulfate and hydrogen peroxide. J. Hazard. Mater. 2010, 179,
495
(1-3), 552-558.
496
8. Chen, W. S.; Huang, C. P., Mineralization of aniline in aqueous solution by electrochemical
497
activation of persulfate. Chemosphere 2015, 125, 175-181.
498
9. Anipsitakis, G. P.; Dionysiou, D. D., Radical generation by the interaction of transition metals
499
with common oxidants. Environ. Sci. Technol. 2004, 38, (13), 3705-3712.
500
10. Liu, H. Z.; Bruton, T. A.; Doyle, F. M.; Sedlak, D. L., In situ chemical oxidation of
501
contaminated groundwater by persulfate: Decomposition by Fe(III)- and Mn(IV)-containing
502
oxides and aquifer materials. Environ. Sci. Technol. 2014, 48, (17), 10330-10336.
503
11. Oh, S. Y.; Kim, H. W.; Park, J. M.; Park, H. S.; Yoon, C., Oxidation of polyvinyl alcohol by
504
persulfate activated with heat, Fe2+, and zero-valent iron. J. Hazard. Mater. 2009, 168, (1), 346-
505
351.
506
12. Saputra, E.; Muhammad, S.; Sun, H. Q.; Ang, H. M.; Tade, M. O.; Wang, S. B., Manganese
507
oxides at different oxidation states for heterogeneous activation of peroxymonosulfate for phenol
508
degradation in aqueous solutions. Appl. Catal. B. Environ. 2013, 142, 729-735.
509
13. Yang, Q. J.; Choi, H.; Dionysiou, D. D., Nanocrystalline cobalt oxide immobilized on
510
titanium dioxide nanoparticles for the heterogeneous activation of peroxymonosulfate. Appl.
511
Catal. B. Environ. 2007, 74, (1-2), 170-178.
512
14. Kim, H.; Yoo, H. Y.; Hong, S.; Lee, S.; Lee, S.; Park, B. S.; Park, H.; Lee, C.; Lee, J.,
513
Effects of inorganic oxidants on kinetics and mechanisms of WO3 mediated photocatalytic
514
degradation. Appl. Catal. B. Environ. 2015, 162, 515-523.
515
15. Huie, R. E.; Clifton, C. L.; Neta, P., Electron transfer reaction rates and equilibria of the
516
carbonate and sulfate radical anions. Radiat. Phys. Chem. 1991, 38, (5), 477-481.
22 Environment ACS Paragon Plus
Page 22 of 34
Page 23 of 34
Environmental Science & Technology
517
16. Lee, H.; Lee, H. J.; Jeong, J.; Lee, J.; Park, N. B.; Lee, C., Activation of persulfates by
518
carbon nanotubes: Oxidation of organic compounds by nonradical mechanism. Chem. Eng. J.
519
2015, 266, 28-33.
520
17. Zhou, D. N.; Zhang, H.; Chen, L., Sulfur-replaced Fenton systems: can sulfate radical
521
substitute hydroxyl radical for advanced oxidation technologies? J. Chem. Technol. Biotechnol.
522
2015, 90, (5), 775-779.
523
18. Toth, J. E.; Rickman, K. A.; Venter, A. R.; Kiddle, J. J.; Mezyk, S. P., Reaction kinetics and
524
efficiencies for the hydroxyl and sulfate radical based oxidation of artificial sweeteners in water.
525
J. Phys. Chem. A 2012, 116, (40), 9819-9824.
526
19. Yoon, S. H.; Jeong, S.; Lee, S., Oxidation of bisphenol A by UV/S2O82-: Comparison with
527
UV/H2O2. Environ. Technol. 2012, 33, (1), 123-128.
528
20. Shah, N. S.; He, X. X.; Khan, H. M.; Khan, J. A.; O'Shea, K. E.; Boccelli, D. L.; Dionysiou,
529
D. D., Efficient removal of endosulfan from aqueous solution by UV-C/peroxides: A
530
comparative study. J. Hazard. Mater. 2013, 263, 584-592.
531
21. Keenan, C. R.; Sedlak, D. L., Ligand-enhanced reactive oxidant generation by
532
nanoparticulate zero-valent iron and oxygen. Environ. Sci. Technol. 2008, 42, (18), 6936-6941.
533
22. Pham, A. L. T.; Doyle, F. M.; Sedlak, D. L., Kinetics and efficiency of H2O2 activation by
534
iron-containing minerals and aquifer materials. Wat. Res. 2012, 46, (19), 6454-6462.
535
23. Anipsitakis, G. P.; Dionysiou, D. D.; Gonzalez, M. A., Cobalt-mediated activation of
536
peroxymonosulfate and sulfate radical attack on phenolic compounds. Implications of chloride
537
ions. Environ. Sci. Technol. 2006, 40, (3), 1000-1007.
538
24. Anipsitakis, G. P.; Tufano, T. P.; Dionysiou, D. D., Chemical and microbial
539
decontamination of pool water using activated potassium peroxymonosulfate. Wat. Res. 2008,
540
42, (12), 2899-2910.
541
25. Duan, X. G.; Sun, H. Q.; Wang, Y. X.; Kang, J.; Wang, S. B., N-doping-induced nonradical
542
reaction on single-walled carbon nanotubes for catalytic phenol oxidation. ACS Catal. 2015, 5,
543
(2), 553-559.
544
26. Zhang, T.; Chen, Y.; Wang, Y. R.; Le Roux, J.; Yang, Y.; Croue, J. P., Efficient
545
peroxydisulfate activation process not relying on sulfate radical generation for water pollutant
546
degradation. Environ. Sci. Technol. 2014, 48, (10), 5868-5875.
23 Environment ACS Paragon Plus
Environmental Science & Technology
547
27. Lee, J.; Kim, J.; Choi, W., TiO2 photocatalysis for the redox conversion of aquatic
548
pollutants. In Aquatic Redox Chemistry (ACS Symposium Series), Tratnyek, P. G.; Grundl, T. J.;
549
Haderlein, S. B., Eds. 2011; Vol. 1071, pp 199-222.
550
28. Gauthard, F.; Epron, F.; Barbier, J., Palladium and platinum-based catalysts in the catalytic
551
reduction of nitrate in water: effect of copper, silver, or gold addition. J. Catal. 2003, 220, (1),
552
182-191.
553
29. Li, H. Q.; Han, L. N.; Cooper-White, J.; Kim, I., Palladium nanoparticles decorated carbon
554
nanotubes: facile synthesis and their applications as highly efficient catalysts for the reduction of
555
4-nitrophenol. Green Chem. 2012, 14, (3), 586-591.
556
30. Kim, S. H.; Jeong, Y. E.; Ha, H.; Byun, J. Y.; Kim, Y. D., Ultra-small platinum and gold
557
nanoparticles by arc plasma deposition. Appl. Surf. Sci. 2014, 297, 52-58.
558
31. Lee, K. Y.; Byeon, H. S.; Yang, J. K.; Cheong, G. W.; Han, S. W., Photosynthesis of
559
palladium nanoparticles at the water/oil interface. Bull. Korean Chem. Soc. 2007, 28, (5), 880-
560
882.
561
32. Qin, H. M.; Qian, X. S.; Meng, T.; Lin, Y.; Ma, Z., Pt/MOx/SiO2, Pt/MOx/TiO2, and
562
Pt/MOx/Al2O3 catalysts for CO oxidation. Catalysts 2015, 5, (2), 606-633.
563
33. Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R., Synthesis of thiol-
564
derivatized gold nanoparticles in a 2-phase liquid-liquid system. J. Chem. Soc. Chem. Commun.
565
1994, (7), 801-802.
566
34. Pol, V. G.; Srivastava, D. N.; Palchik, O.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.;
567
Gedanken, A., Sonochemical deposition of silver nanoparticles on silica spheres. Langmuir
568
2002, 18, (8), 3352-3357.
569
35. Sanchez-Polo, M.; Daiem, M. M. A.; Ocampo-Perez, R.; Rivera-Utrilla, J.; Mota, A. J.,
570
Comparative study of the photodegradation of bisphenol A by HO•, SO4•- and CO3•-/HCO3•
571
radicals in aqueous phase. Sci. Total Environ. 2013, 463, 423-431.
572
36. Liang, C. J.; Huang, C. F.; Mohanty, N.; Kurakalva, R. M., A rapid spectrophotometric
573
determination of persulfate anion in ISCO. Chemosphere 2008, 73, (9), 1540-1543.
574
37. Sun, L. Z.; Bolton, J. R., Determination of the quantum yield for the photochemical
575
generation of hydroxyl radicals in TiO2 suspensions. J. Phys. Chem. 1996, 100, (10), 4127-4134.
576
38. Thompson, R. C., Catalytic decomposition of peroxymonosulfate in aqueous perchloric acid
577
by the dual catalysts Ag+ and S2O82- and Co2+. Inorg. Chem. 1981, 20, (4), 1005-1010.
24 Environment ACS Paragon Plus
Page 24 of 34
Page 25 of 34
Environmental Science & Technology
578
39. Widmann, D.; Liu, Y.; Schuth, F.; Behm, R. J., Support effects in the Au-catalyzed CO
579
oxidation - Correlation between activity, oxygen storage capacity, and support reducibility. J.
580
Catal. 2010, 276, (2), 292-305.
581
40. Zhang, Z. Y.; Shao, C. L.; Zou, P.; Zhang, P.; Zhang, M. Y.; Mu, J. B.; Guo, Z. C.; Li, X.
582
H.; Wang, C. H.; Liu, Y. C., In situ assembly of well-dispersed gold nanoparticles on electrospun
583
silica nanotubes for catalytic reduction of 4-nitrophenol. Chem. Commun. 2011, 47, (13), 3906-
584
3908.
585
41. Elias, H.; Gotz, U.; Wannowius, K. J., Kinetics and mechanism of the oxidation of
586
sulfur(IV) by peroxomonosulfuric acid anion. Atmos. Environ. 1994, 28, (3), 439-448.
587
42. Evans, D. F.; Upton, M. W., Studies on singlet oxygen in aqueous-solution. 3. The
588
decomposition of peroxy-acids. J. Chem. Soc. Dalton Trans. 1985, (6), 1151-1153.
589
43. Serjeant, E. P.; Dempsey, B., Ionisation Constants of Organic Acids in 435 Aqueous
590
Solution. Pergamon: Oxford, U.K., 1979.
591
44. Cessna, A. J.; Grover, R., Spectrophotometric determination of dissociation constants of
592
selected acidic herbicides. J. Agric. Food Chem. 1978, 26, (1), 289-292.
593
45. Furman, O. S.; Teel, A. L.; Watts, R. J., Mechanism of base activation of persulfate.
594
Environ. Sci. Technol. 2010, 44, (16), 6423-6428.
595
46. Kwon, Y.; Lai, S. C. S.; Rodriguez, P.; Koper, M. T. M., Electrocatalytic oxidation of
596
alcohols on gold in alkaline media: Base or gold catalysis? J. Amer. Chem. Soc. 2011, 133, (18),
597
6914-6917.
598
47. Quaino, P.; Luque, N. B.; Nazmutdinov, R.; Santos, E.; Schmickler, W., Why is gold such a
599
good catalyst for oxygen reduction in alkaline media? Angew. Chem. Int. Ed. 2012, 51, (52),
600
12997-13000.
601
48. Xu, L.; Yuan, R. X.; Guo, Y. G.; Xiao, D. X.; Cao, Y.; Wang, Z. H.; Liu, J. S., Sulfate
602
radical-induced degradation of 2,4,6-trichlorophenol: A de novo formation of chlorinated
603
compounds. Chem. Eng. J. 2013, 217, 169-173.
604
49. Chan, K. H.; Chu, W., Degradation of atrazine by cobalt-mediated activation of
605
peroxymonosulfate: Different cobalt counteranions in homogenous process and cobalt oxide
606
catalysts in photolytic heterogeneous process. Wat. Res. 2009, 43, (9), 2513-2521.
25 Environment ACS Paragon Plus
Environmental Science & Technology
607
50. Ji, Y. F.; Dong, C. X.; Kong, D. A.; Lu, J. H., New insights into atrazine degradation by
608
cobalt catalyzed peroxymonosulfate oxidation: Kinetics, reaction products and transformation
609
mechanisms. J. Hazard. Mater. 2015, 285, 491-500.
610
51. Kim, S.; Choi, W., Visible-light-induced photocatalytic degradation of 4-chlorophenol and
611
phenolic compounds in aqueous suspension of pure titania: demonstrating the existence of a
612
surface-complex-mediated path. J. Phys. Chem. B. 2005, 109, (11), 5143-9.
613
52. Lee, H.; Yoo, H. Y.; Choi, J.; Nam, I. H.; Lee, S.; Lee, S.; Kim, J. H.; Lee, C.; Lee, J.,
614
Oxidizing capacity of periodate activated with iron-based bimetallic nanoparticles. Environ. Sci.
615
Technol. 2014, 48, (14), 8086-8093.
616
53. Wang, Y. X.; Sun, H. Q.; Ang, H. M.; Tade, M. O.; Wang, S. B., 3D-hierarchically
617
structured MnO2 for catalytic oxidation of phenol solutions by activation of peroxymonosulfate:
618
Structure dependence and mechanism. Appl. Catal. B Environ. 2015, 164, 159-167.
619
54. Fang, G. D.; Gao, J.; Dionysiou, D. D.; Liu, C.; Zhou, D. M., Activation of persulfate by
620
quinones: Free radical reactions and implication for the degradation of PCBs. Environ. Sci.
621
Technol. 2013, 47, (9), 4605-4611.
622
55. Zhou, Y.; Jiang, J.; Gao, Y.; Ma, J.; Pang, S. Y.; Li, J.; Lu, X. T.; Yuan, L. P., Activation of
623
peroxymonosulfate by benzoquinone: A novel nonradical oxidation process. Environ. Sci.
624
Technol. 2015, 49, (21), 12941-12950.
625
56. Ingram, A. J.; Walker, K. L.; Zare, R. N.; Waymouth, R. M., Catalytic role of multinuclear
626
palladium-oxygen intermediates in aerobic oxidation followed by hydrogen peroxide
627
disproportionation. J. Amer. Chem. Soc. 2015, 137, (42), 13632-13646.
628
57. Ball, D. L.; Edwards, J. O., The catalysis of the decomposition of Caro's acid. J. Amer.
629
Chem. Soc. 1958, 62, 343-345.
630
58. Lee, J. S.; Choi, W. Y., Photocatalytic reactivity of surface platinized TiO2: Substrate
631
specificity and the effect of Pt oxidation state. J. Phys. Chem. B. 2005, 109, (15), 7399-7406.
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FIGURE 1. Representative TEM images of (a) palladium, (b) platinum, (c) gold, and (d) silver particles loaded on alumina supports.
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0.30
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100 4-CP Conc. Pd-Al2 O3 /PMS
1.0 80
0.20
0.15
0.10
0.05
60 0.6 40
0.4
20
0.2
Pd-TiO2 /PMS Chloride Ion Conc. ( µM)
0.8 4-CP Conc. (C/C0 )
PMS or Sulf ate Ion Conc. (mM)
0.25
Chloride ion Conc. Pd-Al2 O3 /PMS Pd-TiO2 /PMS PMS Conc. Pd-Al2 O3 /PMS Pd-TiO2 /PMS Sulf ate ion Conc. Pd-Al2 O3 /PMS Pd-TiO2 /PMS
0.00
0.0
0 0
10
20
30
40
50
60
Time (min)
FIGURE 2. Degradation of 4-CP, reduction of PMS, formation of sulfate ions, and evolution of chloride ions (as a result of 4-CP dechlorination) by Pd-Al2O3 and Pd-TiO2 combined with PMS ([Pd-Al2O3]0 = [Pd-TiO2]0 = 0.25 g/L; [PMS]0 = 0.25 mM; [4-CP]0 = 0.1 mM; [bicarbonate]0 = 1 mM; pHi = 7.0). Concentration of sulfate ions was calculated by subtracting 0.25 mM from the measured sulfate concentration (sulfate ions were initially present due to addition of 0.25 mM PMS (i.e., KHSO5•0.5KHSO4•0.5K2SO4)).
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20 4-CP Conc. Pd-Al2 O3
8
15
6 10 4 5 2
0
PMS Reduction Rate Rate ( µM/min)
Initial Rate of 4-CP Degradation ( µM/min)
10
Pt-Al2 O3 Au-Al2 O3 PMS Conc. Pd-Al2 O3 Pt-Al2 O3 Au-Al2 O3
0 0.125 g/L
0.25 g/L
0.375 g/L
0.5 g/L
FIGURE 3. Initial rate of 4-CP degradation in the presence of PMS by Pd-, Pt-, and Au-Al2O3 systems as a function of the activator loading ([NM-Al2O3]0 = 0.125, 0.25, 0.375, and 0.5 g/L; [PMS]0 = 0.25 mM; [4-CP]0 = 0.1 mM; [bicarbonate]0 = 1 mM; pHi = 7.0).
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14 4-CP Conc. Pd-Al2 O3
12 8 10 6
8 6
4 4 2
Pt-Al2 O3 PMS Reduction Rate ( µM/min)
Initial Rate of 4-CP Degradation ( µM/min)
10
Au-Al2 O3 Ag-Al2 O3 PMS Conc. Pd-Al2 O3 Pt-Al2 O3 Au-Al2 O3 Ag-Al2 O3
2 0
0 pH 3
pH 6
pH 7 (b uffered)
pH 9
pH 11
FIGURE 4. Effect of initial pH on the kinetic rate of 4-CP degradation and PMS reduction in the aqueous suspensions of Pd-, Pt-, Au-, and Ag-Al2O3 ([NM-Al2O3]0 = 0.25 g/L; [PMS]0 = 0.25 mM; [4-CP]0 = 0.1 mM; pHi = 3, 6, 7 (buffered using 1 mM bicarbonate), 9, and 11).
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Pd-Al2O3
100
Pt-Al2O3 Au-Al2 O3 Ag-Al2 O3
Degradation Eff iciency ( %)
80
Pd-TiO2 Pt-TiO2
60
40
20
0 BPA
PH
BZA
4-NP
CBZ
4-CP
2,4,6-TCP
FIGURE 5. Oxidative degradation of various organic compounds by PMS activated with noble metals loaded on alumina and titania ([NM-Al2O3]0 = [NM-TiO2)]0 = 0.25 g/L; [PMS]0 = 0.25 mM; [bisphenol A (BPA)]0 = [phenol (PH)]0 =[benzoic acid (BZA)]0 = [4-nitrophenol (4-NP)]0 = [carbamazepine (CBZ)]0 = [4-chlorophenol (4-CP)]0 = [2,4,6-trichlorophenol (2,4,6-TCP)]0 = 0.1 mM; [bicarbonate]0 = 1 mM; pHi = 7.0).
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(a)
Pd-Al2O3 /PMS
1.0
w /o MeOH w / MeOH Au-Al2O3 /PMS
150
0.8
Co 2+/PMS HCHO Conc . (µ M)
4-CP Conc. (C/C0)
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0.6
w /o MeOH w / MeOH 2+ Co /PMS w /o MeOH w / MeOH
100
50 Pd-Al 2O3/PMS
0.4
Au-Al 2O3/PMS
0 0
5
10
15
T ime (min)
0.2
0.0 0
10
20
30
40
50
60
2+
(b) 6
3
5 4
2
3 2
1
1 0
Rate of Substrate Degradation by MWCNT/PMS and Pd-Al2O3/PMS ( µM/min)
Rate of Substrate Degradation by Co2+/PMS ( µM/min)
Time (min) 7 Co /PMS MWCNT/PMS Pd-Al2O3 /PMS
0 BZA CBZ
BPA 4-NP
PH
4-CP 2,4,6-TCP
FIGURE 6. (a) Effect of methanol on the rate of 4-CP degradation by the Pd-Al2O3/PMS, AuAl2O3/PMS, and Co2+/PMS systems ([Pd-Al2O3]0 = [Au-Al2O3]0 = 0.25 g/L; [Co2+]0 = 25 µM; [PMS]0 = 0.25 mM; [4-CP]0 = 0.1 mM; [methanol]0 = 0.25 M; [bicarbonate]0 = 1 mM; pHi = 7.0). Inset: oxidative conversion of methanol to formaldehyde ([Pd-Al2O3]0 = [Au-Al2O3]0 = 0.25 g/L; [Co2+]0 = 25 µM; [PMS]0 = 1 mM; [methanol]0 = 1 mM; [bicarbonate]0 = 1 mM; pHi = 7.0) (b) Comparison of PMS activators (i.e., Co2+, MWCNT, and Pd-Al2O3) in terms of substrate specificity ([Pd-Al2O3]0 = [MWCNT]0 = 0.25 g/L; [Co2+]0 = 0.25 mM; [PMS]0 = 0.25 mM; [target substrate]0 = 0.1 mM; pHi = 3.85). 32 Environment ACS Paragon Plus
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1st
2nd
3rd
4th
5th
Pd-Al2 O3
1.0
Pt-Al2 O3 Pd-TiO2
4-CP Conc. (C/C0 )
0.8
Pt-TiO2
0.6
0.4
0.2
0.0 0
50
100
150
200
250
300
Time (min)
FIGURE 7. Repeated degradation of 4-CP by Pd and Pt deposited on alumina and titania supports ([Pd-Al2O3]0 = [Pt-Al2O3]0 = [Pd-TiO2]0 = [Pt-TiO2]0 = 0.25 g/L; [PMS]0 = 0.25 mM; [4-CP]0 = 0.1 mM; [bicarbonate]0 = 1 mM; pHi = 7.0).
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Table of Contents Figure:
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