Graduate Institute of Environmental Engineering, National Central University, Taoyuan County 320, TaiwanDepartment of Engineering and System Science, National Tsing Hua University, Hsinchu 300, TaiwanDepartment of Environmental Science and Engineering, Tunghai University, Taichung 407, Taiwan
i g h l i g h t s
Anodes of Pt-FTO and Pt/MWCNTs-FTO possessed dual functions.MWCNTs can prevent the surface ofPtNPs from sintering and provide agreater reaction sites density.Electrochemical processes for bothanodes were controlled by diffusion.Two major intermediates are deter-mined.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 28 August 2013eceived in revised form 13 February 2014ccepted 17 February 2014vailable online xxx
This study investigated applications of the electrochemical anodic oxidation process with Pt-FTO andPt/MWCNTs-FTO glasses as anodes on the treatment of one of the most important emerging contami-nants, naproxen. The anodes used in this study have been synthesized using commercial FTO, MWCNTsand Pt nanoparticles (PtNP). XRD patterns of Pt nanoparticles coated on FTO and MWCNTs revealedthat MWCNTs can prevent the surface of PtNPs from sintering and thus provide a greater reactionsites density to interact with naproxen, which have also been confirmed by higher degradation andmineralization efficiencies in the Pt/MWCNTs-FTO system. Results from the CV analysis showed thatthe Pt-FTO and Pt/MWCNTs-FTO electrodes possessed dual functions of decreasing activation energyand interactions between hydroxyl radicals to effectively degrade naproxen. The lower the solution pHvalue, the better the degradation efficiency. The existence of humic acid indeed inhibited the degrada-
ntermediates tion ability of naproxen due to the competitions in the multiple-component system. The electrochemicaldegradation processes were controlled by diffusion mechanism and two major intermediates of 2-acetyl-6-methoxynaphthalene and 2-(6-Hydroxy-2-naphthyl)propanoic acid were identified. This study hassuccessfully demonstrated new, easy, flexible and effective anodic materials which can be feasibly appliedto the electrochemical oxidation of naproxen.
xidation of naproxen by platinum nanoparticles coated FTO glass,034
Pharmaceutical compounds are regarded as micropollutants insurface waters nowadays because they are used in large quantitiesand only partially removed by the wastewater treatment plants(WWTPs), especially refractory organic compounds. Therefore,
harmaceutical compounds can be detected in the influents andffluents of the WWTPs, rivers and lakes, which have provedhat the use of pharmaceutical compounds are widespread in theurface waters due to the continuous contribution from WWTPsnd untreated wastewater [1,2]. Ubiquitous pharmaceutical com-ounds in the environment have attracted many concerns in allspects, especially the effects of potential ecological toxicity. Theesidue of the anti-inflammatory drug in the livestock has beeneported to cause the population decline >95% in the case of vulturen Pakistan [3]. Non-steroid anti-inflammatory drugs (NSAIDs) areommonly used in the treatment of fever and pain and have beeneported to be given over dose to the adults [4]. The excess amountf NSAIDs is released to the aquatic environment in the form ofither pristine compounds or metabolites. Due to the continuousontribution from WWTPs and untreated wastewater, they possessigh potential to threaten the aquatic ecology. Therefore, how toffectively treat the micropollutants of pharmaceutical compoundss an important issue in modern society and urgently needed.
Naproxen is one of NSAIDs and often detected in the effluents ofWTPs and surface waters [1,5,6]. The degradation of naproxen in
river water ecosystem has been conducted, which indicated thathe lag phase of microorganism was about 20 days [7]. Furthermore,he investigation of mineralization of naproxen in the soils was
uch dependent on the experimental conditions [8]. The white-ot fungus Trametes versicolor has been reported to be suitable forhe slow degradation of naproxen under suitable conditions [9]. Theegradation of naproxen at removal efficiency of 68% at a retentionime of 24 h was obtained in the laboratory-scale membrane biore-ctor (MBR) [10]; on contrary, naproxen was non-biodegradablend the naproxen-containing solutions was detrimental to theiofilm system the after chlorination [11]. In short, the biologi-al treatment was maybe a useful method for the degradation ofaproxen; however, the treatment efficiencies were low at a shortreatment period and much dependent on the treatment condi-ions.
Compared with the biological treatment, physicochemical treat-ent possesses higher removal rate and efficiency at the same time
eriod. The physicochemical treatments such as sorption on acti-ated carbon [12,13], metal-organic frameworks [14], photolysis15] and combination of Fenton reagent and ultrasound irradiation16], have been explored to remove naproxen in the aqueous solu-ion. The removal of naproxen via adsorption process is only thehase transformation from liquid to the solid phase so that the useddsorbents need further treatment. Advanced oxidation processesAOPs) have been verified to effectively degrade the organic com-ounds by means of generated hydroxyl radicals. Most of the AOPseed the addition of oxidizers or photocatalysts, while the electro-hemical oxidation only require the suitable electrodes for anodicxidation, such as PbO2 [17], metal-doped SnO2 [18], boron-dopediamond (BDD) [19,20]. In this study, we report on the applicationf an easy, flexible and feasible electrochemical anodic oxidationystem on the treatment of naproxen and a new, feasible and fastethod to prepare the cheap and stable anodes. A series of anodes
Pt-FTO and Pt/MWCNTs-FTO) composed of commercial FTO con-uctive glass and Pt nanoparticles (PtNPs) were synthesized byeans of the spin coating and the polyol process. The effects of
he addition of multi-walled carbon nanotubes (MWCNTs) and theelative roughness on the degradation of naproxen were examinedo obtain the best synthesis condition. The physicochemical prop-rties of PtNPs on FTO and Pt/MWCNTs were characterized by XRDnd X-ray absorption spectroscopy (XANES and EXAFS spectra).urthermore, the operating parameters, such as the current den-
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ities, initial concentrations of naproxen, solution pH values andresence of humic acid were also investigated. The HPLC with a UVetector and TOC analyzer, were used to determine the degrada-ion and mineralization efficiencies of naproxen. The intermediates
PRESSs Materials xxx (2014) xxx–xxx
obtained from the electrochemical oxidation were determined bya UPLC-Q-TOF/MS system.
2. Materials and methods
2.1. Materials and chemicals
The commercial FTO (fluorine-doped tin oxide) glass was pur-chased from the Solaronix (Aubonne, Switzerland) and then cut intothe size of which the thickness and area were 2.2 mm and 3 cm2
(length of 3 cm and width of 1 cm), respectively. The chemicals ofH2PtCl6•(H2O)6 (98.0 wt.%, reagent grade, Showa Chemical Indus-try Co., Ltd., Tokyo, Japan), poly(N-vinyl-2-pyrrolidone) (PVP-40,99.0 wt.%, reagent grade, Sigma–Aldrich, Denmark), ethylene gly-col (EG, 99.5 wt.%, reagent grade, Showa Chemical Industry Co., Ltd.,Tokyo, Japan), acetone (99.8%, Analytic grade, Merck, Darmstadt,Germany, ethyl alcohol (99.5%, reagent grade, Shimakyu’s purechemicals, Osaka, Japan), humic acid (sodium salt, technical grade,Aldrich Chemical Co., Inc., Wisconsin, USA) and multi-walled car-bon nanotubes (99.5%, FloTubeTM9000, Industrial-grade, Cnano,Beijing, China) were used to synthesize the anodes for the elec-trochemical degradation. Naproxen (98%, reagent grade) waspurchased from Sigma–Aldrich, Denmark.
2.2. Synthesis of electrodes
2.2.1. Pre-treatment of MWCNTsThe modification of as-received CNTs was executed by means
of the acid treatment. 0.15 g CNTs was dispersed in 30 mL of mixedacid (sulfuric acid 96 wt.% and nitric acid 70 wt.% in the ratio 3:1)and then under ultrasonic treatment for 5 h at 323 K. After ultra-sonic treatment, the suspended solution was centrifuged under5000 rpm (CN-2060, Hsiangtai, Taipei, Taiwan) for 20 min. Later,the solution was poured out and then the CNTs were washed withdistilled water under vacuum. This procedure was repeated untilthe solution pH was neutral. The modified CNTs were sequentiallyfreeze-dried after a reaction time of 24 h.
2.2.2. Preparation of PtNPs and Pt/MWCNTsThe preparation of PtNPs followed the polyol process. Firstly, the
metal precursor of an initial concentration of 20 mM was preparedwith ethylene glycol and PtCl62− salt, and the added stabilizer ofPVP-40 was fixed at 10 wt.%. After mixing for 1 h, subsequently, theresulting mixture was stirred at 200 rpm and refluxed at a constanttemperature of 433 K for 1 h under N2 atmosphere. Then, acetoneof certain amount was added into the suspension and the mix-ture was centrifuged under 2000 rpm for 5 min. The procedure wasrepeated until the polymer blended PtNPs formed. Later, the sus-pension of PtNPs was stripped with N2 and re-mixed with absolutealcohol in a volumetric ratio 5:2. With respect to the preparationof Pt/MWCNTs, the modified MWCNTs were added into PtNPs sus-pension until the solution turned transparent.
2.2.3. Preparation of Pt-FTO and Pt/MWCNTs-FTOThe spin coating method was use to load the PtNPs and
PtNPs/MWCNTs on the FTO glass to obtain the anodic type of Pt-FTOand Pt/MWCNTs-FTO. 0.15 mL PtNPs or PtNPs/MWCNTs suspensionwas dropped on FTO glass and then laid in the spin coater. After theprocedure of spin coating, the anodes were calcinated at a heating
xidation of naproxen by platinum nanoparticles coated FTO glass,034
rate of 10 K min−1 to the temperature of 393 K and the soaking timewas 30 min at this temperature. Later, the temperature was furtherraised to the final temperatures of 603 K and the soaking time was30 min for each anode samples.
.3. Characterization of physicochemical properties of PtNPs onlectrodes
In preventing the interferences of diffraction signals from FTOubstract to the physical properties inspection, the samples oftNPs and PtNPs coated on MWCNTs (Pt/MWCNTs) were dispersedn the silicon wafer (in the similar configuration with that of NPsn the FTO substract) for collecting the X-ray diffraction patterns.he preparation procedures were the same as those used in prepa-ation of Pt-FTO and Pt/MWCNTs-FTO electrodes. The diffractionpectra were collected by using a 2D area CCD detector (mar3450)ith the incident X-ray wavelength of 1.083 A at the beamline BL-
1C2 in National Synchrotron Radiation Research Center (NSRRC)t Taiwan. In order to investigate impact of MWCNTs support onhe surface atomic structure and the extent of surface defect sites,he Pt L3-edge X-ray absorption spectroscopy (XAS) was employedo determine the local coordination number of the two types oft NPs. The spectra of experiment samples (prepared using theame procedures with that of samples for electrochemical char-cterizations) were collected using the grazing incident fluorescesode at beamline BL-07A in NSRRC. The quantitative atomic struc-
ure parameters are determined by model simulation (the modelimulation was conducted by implementing the standard atomictructure of metallic Pt in f.c.c. phase with a space group No. of 225o the artemis program (0.8.012) basing the fitting algorithm of feff
code{Rehr, 1991 #38}).
.4. Electrolytic system
The cyclic voltammetry (CV) three-pole experimental analy-is was used to understand the electrochemical property of theesulting anodes. The degradation of naproxen was conducted in
125-mL, undivided, and thermostated cylindrical glass cell as aatch reactor with stirring by applying a constant current densityith an electrochemical apparatus (CHI-627, Model 600D Series,
lectrochemical Workstation, CH Instruments, USA). All degrada-ion experiments were carried out using a 3 cm2 stainless steel asathode and the synthesized Pt-FTO or Pt/MWCNTs-FTO glasses asnodes, having 1 cm gap between them. The solution volume andemperature were fixed at 100 mL and 298 K, respectively. The sup-orting electrolyte of sodium sulfate aqueous solution was 0.035 Mnd 0.1 M for CV analysis and naproxen degradation, respectively.
.5. Analytical methods
The concentrations of pristine naproxen were measured by aPLC analyzer with a UV detector (UDV 170U, Dionex Corpora-
ion, Sunnyvale, USA) under wavelength of 228 nm. Total organicarbon (TOC) of solutions was determined by using a TOC ana-yzer (liquiTOCII, Elementar, Hanau, Germany). The intermediatesf naproxen degradation were determined using UPLC-Q-TOF/MS,hich were Thermo Scientific UHPLC+: Dionex UltiMate 3000 andruker micrOTOF Q-III with electrospray ion (ESI) source in pos-
tive polarity. In order to avoid the interference of electrolytes inhe analysis of intermediates, the Oasis HLB cartridge (1 mL/30 mg,
aters Corp., Pennsylvania, USA) was used to extract the inter-ediates from the solution. The processes for the solid phase
xtraction of the samples were: the cartridge was conditioned with mL of methanol, followed by washing the cartridge with 3 mLeionized water; the sample of 0.5 mL was added dropwise into theartridge for 60 min, followed by washing the cartridge with 1 mLeionized water; the cartridge was eluted with 1 mL methanol and
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he eluent was collected in a brown vials for analysis of UPLC-Q-OF/MS. The rapid separation column used in UPLC was C18 columnith 2.2 �m particles (Acclaim® RSLC 120, Dionex, Thermo Scien-
ific) and kept at 303 K. The mobile phase was composed of Solution
A (0.2% HCOOH in H2O) and Solution B (0.2% HCOOH in acetoni-trile), as shown in Table 1. The sample of 2 �L was injected andmixed with the mobile phase at 0.3 mL min−1. Operating conditionsof the source were scanning range of 50–1000 m/z, capillary volt-age of 4500 V, nebulizer pressure of 2 bars with nitrogen and dryinggas flow of 8 L min−1 at a temperature of 473 K. All experimentalconditions were repeated three times and the average values arereported.
3. Results and discussion
3.1. Characterization of PtNPs coated on FTO and MWCNTs andelectrodes
The diffraction spectra of PtNPs and Pt/MWCNTs are demon-strated in Fig. 1a and b, respectively. As shown in Fig. 1, the twodiffraction lines A and B centered at 27.49◦ and 31.88◦ correspondsto the metallic characters of (1 1 1) and (2 0 0) facets of PtNPs. Theaverage particle size (Davg) was estimated by using the Scherrerequation and results are summarized in Table 2. It is important tonote that the Davg of (1 1 1) facet is determined to be 7.19 nm whichis 31.4% larger than that of (2 0 0) facet (5.76 nm) at PtNPs. Suchasymmetric crystal growth is the direct evidence which indicatesthat the Pt atoms will restructure to the low surface energy facet(i.e. (1 1 1)) due to the inter-particle sintering [21]. On the otherhand, the interparticle sintering between PtNPs on the MWCNTsis insignificant since their Davg difference between the (1 1 1) and(2 0 0) facets is insignificant (∼5.2%). The XRD results elucidate thesupporting materials of MWCNTs can prevent the surface of PtNPsfrom sintering and thus provide a high density reaction sites tointeract with the narpoxen in the solution.
Fig. 2 demonstrates the X-ray absorption near-edge structure(XANES) of PtNPs (Pt-FTO) and MWCNTs (Pt/MWCNTs) comparedwith that of Pt foil. In general, the intensity of L3-edge white line(WL) refers to the extent of core-level electron transition probabil-ity from 2p5/2 to 5d states. Therefore, the higher the WL intensitythe more the empty states at the outmost core level state (5d) willbe found. In our system, the PtNPs in Pt/MWCNTs possesses thehighest WL intensity and thus the density of empty states in the out-most core level among all samples (Pt foil, Pt-FTO, and Pt/MWCNTs).This is the indication for the high density of defect sites (high sur-face energy sites for activating the electrochemical reaction) for thePtNPs on MWCNTs surface. The presence of high density defect siteson the Pt/MWCNTs surface can be further revealed by the results
xidation of naproxen by platinum nanoparticles coated FTO glass,034
of atomic structure analysis (extended X-ray absorption fine struc-ture, EXAFS). Fig. 3 demonstrates the Fourier transformed EXAFSspectra (denoted as radial structure function, RSF) of Pt-FTO andPt/MWCNTs with the corresponding fitting curves (solid curves in
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Fig. 1. The XRD diffraction patterns of (a) PtNPs on silicon wafer (Pt NPs) and (b) PtNPs on MWCNTs (Pt/MWCNTs). The solid lines in red, blue, and green denote thefitting curves of diffraction peaks from (1 1 1) and (2 0 0) facets of PtNPs, and the amorphous carbon signal (from the modified CNTs) by using Lorentzian peak function. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
FPs
rdttipT
Table 3EXAFS model analysis determined atomic structure parameters of PtNPs on Pt-FTOand Pt/MWCNTs.
Sample RPtPt CNPtPt Sigma-square ×103 R2 × 103
Fc
ig. 2. The Pt L3-edge X-ray absorption near-edge structure (XANES) spectra oftNPs coated on FTO (Pt-FTO) and MWCNTs (Pt/MWCNTs) compared with that oftandard samples of Pt foil.
ed). In the RSF spectra, the position and intensity of the radial peaksenote the interatomic bond distance and coordination numbers ofhe center atoms (Pt in our study), respectively. As shown in Fig. 3,
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he RSF peak across 2.6–3.1 A is originated from the interatomicnterference of Pt Pt bond pairs. The quantitative atomic structurearameters are determined by model simulation, as summarized inable 3. Accordingly, the coordination number of Pt Pt bond pairs
ig. 3. The Fourier transformed EXAFS spectra (RSF) of (a) Pt-FTO and (b) Pt/MWCNTs
orrecting the phase off set. (For interpretation of the references to color in this figure leg
for PtNPs in Pt/FTO and Pt/MWCNTs-FTO is determined to be 10.21and 9.25, respectively. Before discussing the experiment samples,the general information of the atomic coordination structure insidea bulk and at the surface will provide an easy guidance for inter-preting the obtained EXAFS spectra. The atomic structure of an idealf.c.c. crystal is shown in Fig. 4. As indicated the coordination num-ber (CN) of atoms in the bulk is 12 at the nearest atomic shell, onthe other hand that of surface atoms is reduced to 9 due to the for-mation of break bonds. Basing on this understanding, we can notethat the majority of atoms in Pt/MWCNTs are located at the particlesurface therefore resulting in the high density of defects at the NPs.On the other hand, the PtNPs in Pt/FTO possesses the relative lowerextent of surface atoms (defects).
Cyclic voltammetric analyses for various electrodes are shownin Fig. 5, in which Fig. 5a was used for estimation of their relative
xidation of naproxen by platinum nanoparticles coated FTO glass,034
roughness factor (Rf) and double layer capacitance (C), and Fig. 5brevealed the reduction or oxidation potentials of the electrodes.The capacitance of electrode/solution interface was determinedby the linear correlation between charging currents (I, A) and
with the corresponding atomic model fitting curves (solid curves in red) withoutend, the reader is referred to the web version of this article.)
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F e coor
sttooaaitpPtaohtrtho
3o
d
TC
ig. 4. Schematic representation of atomic structure of f.c.c. crystal. As indicated, thespectively.
canning rates (�, mV s−1), given that the slope was closely relatedo the double layer capacitance (as shown in Table 4). And, the rela-ive roughness factor was estimated by comparing the capacitancef studied electrode with that of an oxide with a smooth surfacef 60 �F cm−2 [22]. The results indicated that the addition of PtNPsnd Pt/MWCNTs onto FTO glass increased the capacitance as wells the relative roughness of the electrode materials. The increasen roughness of the anode would increase its resistance and lowerhe oxidation potential of the anode. The original FTO glass did notresent reduction and oxidation potentials, whereas Pt-FTO andt/MWCNTs-FTO anodes showed oxidation peaks of naproxen inhe electrolyte solution. Therefore, Pt-FTO and Pt/MWCNTs-FTOnodes demonstrated their ability to degrade naproxen. Depositionf PtNPs onto FTO contributed to high surface to volume ratio andigh surface energy, which were directly related to the overpoten-ial for oxygen evolution and to the adsorption enthalpy of hydroxyladicals on the anode surface. In addition, PtNPs posed high oxida-ion potential which resulted in lower activation energy betweenydroxyl radicals and organics and to a high chemical reactivity forrganics oxidation.
.2. Effects of solution pH values and humic acid on degradation
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f naproxen
In order to investigate the effect of solution pH values on theegradation of naproxen, the solution pH values of 4.6 and 3.0
able 4apacitance (C) and relative roughness factor (Rf) for studied electrodes.
Electrodes Linear regression equation C, F cm−2 Rf
rdination number of atoms in crystal bulk (C) and at crystal surface (S) is 12 and 9,
were fixed to compare the degradation efficiencies. In addition, anassay, which the solution pH value was not controlled, was alsoconducted. The results are shown in Fig. 6. When the solution pHvalue was not controlled, the solution pH spontaneously decreaseswith reaction time and the degradation efficiency is 84.9% at reac-tion time of 120 min. At reaction time of 120 min, the degradationefficiencies at solution pH values of 4.6 and 3 are 79.3% and 89.1%,respectively, indicating the lower the solution pH value, the betterthe degradation efficiency. The results obtained in this study aresimilar to those presented in reports [23,24]. The naproxen presentsin the molecular form at a solution pH value of 3 since the pKa ofnaproxen is 4.12. Nevertheless, with the data available we cannotspeculate that the promotion of degradation efficiencies at solutionpH of 3 was due to the reduction of the electrostatic interactionsbetween the surface of catalysts and naproxen because the struc-ture of the target compound and the nature of the electrode alsosignificantly influence the degradation efficiency.
In order to obtain the degradation results to evaluate the prac-tical use, the solution pH value was not fixed in the followingexperiments. In order to clarify the effects of natural compounds(i.e., humic acid) on the degradation efficiency of naproxen, theaddition of humic acid in the naproxen-containing solution wasexecuted in the degradation experiments, as illustrated in Fig. 6.The degradation efficiencies of naproxen in the existence of humicacid of 5 and 50 mg L−1 are 68.3% and 63.5%, respectively There-fore, the existence of humic acid indeed reduces the degradationefficiency of naproxen in the multiple-compound system but theadverse effect is limited to a certain extent.
3.3. Degradation and mineralization of naproxen
xidation of naproxen by platinum nanoparticles coated FTO glass,034
Degradation efficiencies of naproxen by the Pt-FTO andPt/MWCNTs-FTO anodes at different applied current densitiesof 10, 30, 50 and 70 mA cm−2 are illustrated in Fig. 7. The
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E, V v s. Ag/ AgCl
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
I, 1
0-5
mA
-20
-15
-10
-5
0
5
10
15
FTOPt-FTOPt/MCNT-FTO
E, V v s. Ag/ AgCl
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
I, 1
0-5
mA
-60
-40
-20
0
20
40
60
80
100FTOPt-FTOPt/MWC NTs -FTO, MWC NTs =0.002 g
(a)
(b)
Fig. 5. Cyclic voltamogramms (CV) of FTO, Pt-FTO and Pt/MWCNTs-FTO electrodesobtained with a counter electrode of Pt microelectrode and a reference electrodeoNs
f Ag/AgCl, 3.0 M under a scanning rate of 20 mV s−1. (a) CV obtained at in a 0.5 Ma2SO4 solution and in the absence of naproxen; (b) CV obtained in a 0.35 M Na2SO4
olution with an initial concentration of naproxen at 50 mg L−1.
seudo-first-order equation was used to model the degradationinetics, as shown in Eq. (1) and Table 5.
0 = −dC
dt= −kobs × C (1)
here kobs is the observed degradation rate constant, and C is theaproxen concentration at time = t.
Approximately 96% of the naproxen was degraded by Pt/FTOnode within two hours at the highest applied current density of0 mA cm−2. The results showed that naproxen degradation rate
ncreased with applied current density. The increase in appliedurrent density facilitated the formation of hydroxyl radicals andesulted in electrodegradation as favoring conversion reactions.oreover, Pt/MWCNTs-FTO electrode demonstrated greater degra-
ation ability than Pt-FTO electrode because it has more highensity reaction sites. The highest degradation and mineraliza-
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ion efficiencies of naproxen using Pt/MWCNTs-FTO anode for 2 hf reaction period were 96% and 51%, respectively. This revealedhat the Pt/MWCNTs-FTO anode demonstrated good ability to
able 5he kinetic parameters for electrochemical degradation of naproxen under variousurrent densities.
of 5 mg L−1 (without pH control), solution pH fixed at 4.6, solution pH fixed at 3.0and initial solution pH at 4.6 without pH control, respectively.
degrade and mineralize naproxen. Compared with the previousstudy [16], which adopted the Fenton process, the higher degra-dation efficiency of naproxen at the similar initial solution pH wasobtained in this study. The previous study [25] has also conductedthe electrochemical degradation of naproxen with the commer-cial boron-doped diamond electrode, in which the results showedthe optimum conditions for continuously complete removal werepH of 10.70, flowrate of 4.10 cm3 min−1, and the current densityof 194 mA cm−2 using a supporting electrolyte concentration of0.392 mol L−1. Obviously, the applied current density used in thisstudy is much smaller than that used in the study mentioned above;therefore, the synthesized anodes in this study are proved to be veryeffective.
The intermediates were identified during the electrochem-ical oxidation of naproxen by Pt-FTO and Pt/MWCNTs-FTOanodes using UPLC-Q-TOF/MS analysis and were illustrated inTable 6 and Fig. 8. The 2-acetyl-6-methoxynaphthalene and2-(6-hydroxy-2-naphthyl)propanoic acid are the two major inter-mediates determined during the degradation of naproxen byPt-FTO and Pt/MWCNTs-FTO anodes. The existence of 2-acetyl-6-methoxynaphthalene at a reaction time of 0 may be due tothe rapid photolysis via photodecarboxylation process in theenvironment. The amount of intermediates increases with increas-ing reaction time to the maximum, and then decreases as the
xidation of naproxen by platinum nanoparticles coated FTO glass,034
reaction goes further. Two possible pathways for degradation ofnaproxen judged by the intermediates determined in this studyare shown in Fig. 9. In Route A, the attack of OH· radical in
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Table 6The possible intermediates of naproxen by electrochemical oxidation.
Compound Structure R.T., min Molecular weight [M+H]+, m/z
Naproxen 8.1 230 231
2-Acetyl-6-methoxynaphthalene 9.6 200 201
mpm[ar
FPopr
2-(6-Hydroxy-2-naphthyl)propanoic acid
ethyl position initially occurred which sequentially yields theeroxyl radical so as to further produce the compounds of 1-(6-ethoxy-2-napthyl)ethanol and 2-acetyl-6-methoxynaphthalene
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15]. However, 1-(6-methoxy-2-napthyl)ethanol is not observed inll experiments in this study. On the other hand, the attack of OH·adical in O-methyl position causes the demethylation so as to from
the 2-(6-hydroxy-2-naphthyl)propanoic acid in the reaction RouteB [26].
3.4. Mineralization current efficiency of naproxen
Two possible mechanisms are controlling the electrochemicaldegradation of organic compounds, namely diffusion control (masstransfer from bulk solution to surface of the electrode) and currentcontrol (electron transfer to anode’s surface). Favoring diffusion orcurrent controlled reaction can be determined by comparing thelimiting current density (jlim) and applied current density (jappl). Ifjappl > jlim, the electrochemical process is controlled by diffusion; onthe contrary, when jappl < jlim, the process is current controlled. Thelimiting current density is defined as follows:
jlim = nFkmC0 (2)
where n is number of electrodes, F is the Faraday constant, km is
xidation of naproxen by platinum nanoparticles coated FTO glass,034
the medium mass transfer coefficient (m s−1) calculated from dataof TOC analysis, and TOC0 is the initial concentration of naproxen(mol m−3). km was obtained from the correlation between con-centrations (TOC/TOC0), electrode area (A, 3 × 10−4 m2), volume of
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oxen
tf
nfwcIbclt
tt
M
It
TTo
goal of complete mineralization over a period of reaction time. Ingeneral, the MCE decreased with applied specific charge increased,and, similar MCE decay was found for anode at 50 and 70 mA cm−2.
0, %
80
100
120
Fig. 9. Two possible pathways for degradation of napr
he treated solution (V, 10−4 m3) and reaction time (t) using theollowing formula, as present in Eq. (3).
TOCTOC0
= exp(
− A
Vkmt
)(3)
Electrochemical kinetic parameters for mineralization ofaproxen are shown in Table 7 and Fig. 10. The medium mass trans-
er coefficient km increased as increase in applied current density,hich confirmed the results that electrodes with higher applied
urrent density posed the most effective degradation performance.n addition, for all of the studied conditions, jappl were found toe greater than jlim, which implied that the electrochemical pro-esses were controlled by diffusion. And, according to Fick’s firstaw, higher level of diffusion rate generally lead to higher degrada-ion efficiency [24].
Mineralization current efficiency (MCE) was used to determinehe degree of mineralization and presence of intermediates duringhe electrochemical degradation reaction [27], as shown in Eq. (4).
�(TOC)exp
Please cite this article in press as: C.-J.M. Chin, et al., Effective anodic oJ. Hazard. Mater. (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.02.
CE =�(TOC)theor
× 100 (4)
n which �(TOC)exp is the experimental decrease of TOC at a reac-ion time of t and �(TOC)theor is the theoretical TOC removal under
able 7he kinetic parameters for electrochemical mineralization of naproxen under vari-us current densities.
jappl, mA cm−2 Pt-FTO electrode Pt/MWCNTs-FTO electrode
judged by the intermediates determined in this study.
the assumption that the applied electrical charge is only consumedby naproxen in the mineralization process.
MCE as a function of the specific charge is shown in Fig. 11, whichindicated higher MCE was obtained by means of Pt/MWCNTs-FTOanode. The results suggested that the proposed Pt-FTO anode mayintroduce high oxygen evolution near the anode’s surface that pre-vent organics from approaching the anode in the initial stage;however, high concentration hydroxyl radicals still led to the final
xidation of naproxen by platinum nanoparticles coated FTO glass,034
he finding validated a fact found in literatures that occurrence ofaste in current was positive correlated to current density [23].
. Conclusion
The proposed anodes (Pt-FTO and Pt/MWCNTs-FTO) have beenuccessfully synthesized using FTO glass, Pt nanoparticles andWCNTs, and possessed two features of decreasing activation
nergy in degradation of naproxen and increasing adsorption capa-ility of hydroxyl radicals, which were two major advantages forffective mineralization the naproxen in aqueous environment. Thet atoms coated on Pt-FTO and Pt/MWCNTs-FTO restructured tohe low surface energy facet due to the inter-particle sintering.urthermore, the results elucidated that the supporting materi-ls of MWCNTs can prevent the surface of PtNPs from sinteringnd thus provide a high density reaction sites to interact withhe naproxen in the solution, which was also confirmed by theesults obtained from EXAFS analysis. The largest intensity of L3-dge white line from the XANES analysis obtained by the PtNPsn the Pt/MWCNTs-FTO proved that the greatest ability to acti-ate the electrochemical reaction for naproxen degradation. TheV analysis revealed that the addition of PtNPs and Pt/MWCNTsn the FTO glass increased the capacitance as well as the relativeoughness of the electrode material so as to lower oxidation poten-ial and to create the oxidation peaks of naproxen in the electrolyteolution. The lower the solution pH value, the better the degra-ation efficiency of naproxen. The existence of humic acid indeed
nhibited the degradation ability of naproxen due to the compe-ition in the multiple-component system. The pseudo-first-orderquation can well describe the degradation and mineralization ofaproxen, and the greatest efficiencies and mineralization currentfficiency existed in the anode of Pt/MWCNTs-FTO. Furthermore,he electrochemical processes were controlled by diffusion mech-nism. Two possible pathways for degradation of naproxen wereroposed by the identified intermediates, which were 2-acetyl--methoxynaphthalene and 2-(6-Hydroxy-2-naphthyl)propanoiccid in this study. To conclude, this study has successfully devel-ped new and effective anodic materials which can be feasiblypplied on the electrochemical degradation of naproxen.
Please cite this article in press as: C.-J.M. Chin, et al., Effective anodic oJ. Hazard. Mater. (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.02.
cknowledgements
The authors thank the National Science Council Taiwan forhe financial support under Grant NSC 101-2221-E-029-015-MY3,
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PRESSs Materials xxx (2014) xxx–xxx 9
the Global Research and Education on Environment and Soci-ety (GREEnS) of Tunghai University for the financial supportunder Grant 101GREEnS006-2. The authors thank the help fromProf. Tsang-Lang Lin and Chih-Hao Lee for sharing the beam-time for data collection and interpretation. We also appreciatethe staffs of NRRC for providing the facilities and helping the datacollection.
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