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International Journal of Molecular Sciences

Article Physical-Chemical Study of Selective Oxidation by a Fe(III)-Phenylporhyrin Derivative

Carlos Diaz-Uribe 1, William Vallejo 1,* and Cesar Quiñones 2

1 Grupo de Fotoquímica y Fotobiología, Universidad del Atlántico, Puerto Colombia 81007, Colombia; [email protected] 2 Facultad de Ingeniería Diseño e Innovación, Politécnico Grancolombiano, Bogotá 110231, Colombia; [email protected] * Correspondence: [email protected]; Tel.: +57-5-3599484

 Received: 30 October 2019; Accepted: 3 January 2020; Published: 5 January 2020 

Abstract: In this work, we studied the anthracene oxidation by hydroxyl radicals. Hydroxyl was generated by reaction of 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin Fe (III) (TPPFe) with under visible radiation at a atmosphere. The TPPFe was synthesized by Adler Method followed by metal complexation with Fe (III) chloride hexahydrate. was detected by fluorescence emission and we studied kinetic of anthracene selective oxidation by hydroxyl radicals through the differential method. The TPPFe was characterized by UV-Vis spectrophotometry, Dynamic Light Scattering (DLS) and Scanning Microscopy (SEM) measurements. The results indicated that TPPFE was compound by micro-particles with a size distribution of around 2500 nm. Kinetic results showed that the apparent rate constant for the oxidation of anthracene increased exponentially on as temperature increases, furthermore, the activation energy for the Anthracene oxidation by hydroxyl radicals under visible irradiation was 51.3 kJ/mol. Finally, was the main byproduct generated after oxidation of anthracene by TPP-Fe under visible irradiation.

Keywords: sensitizer; porphyrin; hydroxyl radical; polycyclic aromatic ; anthracene

1. Introduction Nowadays polycyclic aromatic hydrocarbons (PAH) originate from the incomplete of fossil fuels are an important researching subject, from the environmental point of view, PAH are very important since they are involved in metabolic processes that affect human health [1,2]. Reactive species (ROS) are an emerging class of endogenous, highly reactive, oxygen , these species can originate oxidation reactions of PAH, main ROS involve and oxygen free radicals such as hydroxyl radical (HO•), anion (O2−•), (HOO•) or other similar radicals [3,4]. Specific photodynamic oxidation reactions have become an alternative methodology for PAH oxidation; the complex primary photochemical reaction processes due to an interaction of a photosensitizer with visible light could generate several in the reaction 1 medium ( O2,O•, HO•)[5,6]. Hydroxyl radical can be achieved through different methodologies: (a) chemical trapping, (b) laser flash photolysis, (c) pulse , and (d) laser-induced fluorescence. Hydroxyl radical reaction to PHS (e.g., , Biphenyl, Anthracene) is reported for ROS detection in solution [7]. F. Goulay et al., reported the first direct measurement of the reaction rate constant of a polycyclic aromatic in the gas phase in the temperature range 58–470 K [8]. Different factors involve in ROS generation: photo-sensitizers, electromagnetic radiation and 3 the molecular oxygen O2. Under visible light irradiation, the photosensitizer (PS) excites from basal state (oPS) to singlet state (1PS*) after that, 1PS* could decay to triplet state (3PS*) through internal

Int. J. Mol. Sci. 2020, 21, 353; doi:10.3390/ijms21010353 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 2 of 11

(oInt.PS) J. to Mol. singlet Sci. 2020 state, 21, 353(1PS*) after that, 1PS* could decay to triplet state (3PS*) through internal cross-2 of 11 system process; in this stage 3PS* could produce radical species as hydroxyl or superoxide radicals throughcross-system electron process; transfer in this reaction, stage 3 thisPS* kind could of produce reaction radical is known species as type as hydroxyl I [9]. Many or superoxide compounds radicals have beenthrough reported electron as sensitizers transfer reaction, (e.g., porphyrin, this kind of ph reactionthalocyanines, is known and as typetheir I metal [9]. Many derivatives), compounds among have these,been reportedporphyrins as sensitizers their metalcomplexes (e.g., porphyrin, are phthalocyanines, macrocyclic molecules and their with metal an derivatives), extensive amongdelocalized these, electronsporphyrins system, their metalcomplexesthey have electronic are macrocyclic and optica moleculesl properties, with anapplied extensive in delocalizedfields like artificial photodynamicsystem, they have therapy, electronic solar and cells optical and properties, photocatal appliedysis in[10–12], fields like furtherm artificialore, photodynamic metalloporphyrin therapy, derivativessolar cells andhave reported as [hydroxyl10–12], furthermore, radical generators, metalloporphyrin however, derivatives knowledge have about reported kinetic as informationhydroxyl radical is few; generators, about this however, topic, a knowledgedetailed knowledge about kinetic of oxidation information mechanisms is few; about initiated this topic, by molecular oxygen and OH• radicals is important to optimization of experimental conditions [13]. It a detailed knowledge of oxidation mechanisms initiated by molecular oxygen and OH• radicals is isimportant known that to optimization the efficient ofphoto-activity experimental of conditions both the [13simple]. It is porphyrins known that and the ethefficient metal photo-activity complexes porphyrinsof both the simpleis due to porphyrins their capability and the to metal produce complexes either superoxide porphyrins anion is due radical to their or capability singlet oxygen to produce by typeeither I and superoxide type II processes, anion radical respectively; or singlet oxygen for which bytype the type I and II type process II processes, is thought respectively; to be dominant for which in freethe base type porphyrin II process [14]. is thought Selective to oxidation be dominant of anthracene in free base to anthraquinone porphyrin [14]. before Selective by reaction oxidation with of aceticanthracene acid [15] to anthraquinoneand metachloroperbenzoic before by reaction acid by with electron-withdrawing [15] and metachloroperbenzoic metalloporphyrins [16] acid has by beenelectron-withdrawing reported. metalloporphyrins [16] has been reported. ConsideringConsidering the adverseadverse e ffeffectsects on environmentson environments and humanand human health ofhealth PAH, of we PAH, selected we anthracene selected anthraceneas the target as in the this target work in because this work of the because anthracene of the is anthracene considering is one considering of the 16 priority one of PAHsthe 16 indicatedpriority PAHsby the indicated US environmental by the US environmental protection agency protection (EPA) agency [17]. There (EPA) are [17]. reports There concerning are reports toconcerning oxidation to oxidation of anthracene with free radicals NO3•, OH• and SO4•− [18–21]. Furthermore, the formation of anthracene with free radicals NO3•, OH• and SO4•− [18–21]. Furthermore, the formation of of nitro-anthracene from the degradation of anthracene by the OH• and NO3• has been investigated nitro-anthracene from the degradation of anthracene by the OH• and NO3• has been investigated usingusing quantum quantum chemical chemical calculations calculations [22,23]. [22,23]. Karan Karan et et al. al. reported reported the the photocatalytic photocatalytic degradation degradation of of anthracene under UV irradiation on TiO2 nanoparticles, they found that the main product of anthracene under UV irradiation on TiO2 nanoparticles, they found that the main product of oxidation oxidationof Anthracene of Anthracene is 9,10-Anthraquinone, is 9,10-Anthraquinone, which is safer wh forich theis safer environment for the environment than Anthracene than [ 24Anthracene]. Kozak et [24]. Kozak et al. removed PAHs from the pretreated coking wastewater by using CaO2, Fenton al. removed PAHs from the pretreated coking wastewater by using CaO2, Fenton reagent under reagentUV irradiation under UV [25 irradiation]. The applications [25]. The applications of Fenton oxidationof Fenton ofoxidation different ofPAH different have PAH been have shown been to shownhave great to have potential great potential (e.g., photo-Fenton, (e.g., photo-Fenton, sono-Fenton sono-Fenton and electro-Fenton and electro-Fenton [26–29]). [26–29]). Fenton reactionFenton reaction mechanism relies on OH• generation by the reaction of a ferrous ion with mechanism relies on OH• generation by the reaction of a ferrous ion with hydrogen peroxide (H2O2) (Hthis2O mechanism2) this mechanism is useful is touseful break to down break a didownfferent a different . organic compound. The Scheme The1 shows Scheme the 1 generalshows thereaction general for reaction Fenton for reaction. Fenton reaction.

Fe2+ Fe3+

H2O2 OH•

SchemeScheme 1. 1. OHOH• •generationgeneration by by Fenton Fenton reaction. reaction.

TheThe photo photo Fenton Fenton reaction reaction has has been been already already used used to to degrade degrade samples samples of of phenol [30,31]. [30,31]. The The rate rate ofof this this reaction reaction could could be be increased increased when when exposed exposed to to UV UV light. light. Otherwise, Otherwise, although although the the developments developments relatedrelated to to PAH PAH photooxidation photooxidation under UVUV irradiationirradiation researchresearch have have been been significant, significant, great great attention attention in inthe the field field is directedis directed to reachto reach the PAHthe PAH photooxidation photooxidation under under solar irradiationsolar irradiation (visible (visible light) tolight) achieve to achievea practical a practical application application [32,33]. [32,33]. InIn this this work, work, we wepresented presented a kinetic a kineticstudy of study the anthracene of the anthracene oxidation trough oxidation 5,10,15,20- trough tetrakis(4-carboxyphenyl)porphyrin5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin Fe (III) under Fe visible (III) under irradiation, visible irradiation,we used the we initial used rates the initialmethod rates to determinemethod to activation determine energy activation and energykinetic andparameters kinetic parametersof the process. of the process. 2. Results and Discussion 2. Results and Discussion 2.1. UV-Vis Assay 2.1. UV-Vis Assay Figure1 shows the UV-Vis spectra of TPPFe and the TPP in solution. Results show the typicalFigure Soret 1 shows and four the QUV-Vis bands spectra for the of TPP, TPPFe see Figureand the1. TPP The in highest ethanol band solution. located Results at 419 show nm wasthe typicalassigned Soret to Soretand four band, Q andbands bands for the located TPP,between see Figure 516 1. nm The and highest 650 nm band corresponding located at 419 to nm Q bands, was

Int.Int.Int. J. J. J.Mol. Mol. Mol. Sci. Sci. Sci. 2020 20202020, ,21 ,2121, ,x ,x 353FOR FOR PEER PEER REVIEW REVIEW 3 3 of3 ofof 11 11 11 assignedassigned to to Soret Soret band, band, and and bands bands located located betwee betweenn 516 516 nm nm and and 650 650 nm nm corresponding corresponding to to Q Q bands, bands, * * thesethesethese signals signals signals are are are associated associated associated to to a a a2u2u((π(ππ)-e)-e)-eggg(*(ππ()π) electronic )electronic electronic excitations. excitations. excitations. The The The TPPFe TPPFe TPPFe UV-vis UV-vis UV-vis spectra spectra spectra exhibited exhibited exhibited oneoneone Soret Soret Soret band band band (413 (413 (413 nm) nm) nm) and and and only only only a a aQ Q Q band band band (535 (535 (535 nm), nm), nm), this this this result result result is is is typical typical typical of of of this this this kind kind kind of of of compound, compound, compound, afterafterafter metal metal metal complexing complexing complexing symmetry symmetry symmetry of of of porphyrin porphyrin porphyrin increases increases increases thus thus thus Q Q Q bands bands bands number number number decreases. decreases. decreases. Figure Figure Figure 1 1 showsshowsshows little little little blue blue blue shift shift shift to to to Soret Soret Soret band band band of of of TPPFe, TPPFe, TPPFe, this this this result result result indicates indicates indicates that that that energy energy energy necessary necessary necessary for for for electronic electronic electronic transitionstransitionstransitions increases increases increases after after after metal metal metal complexing, complexing, complexing, this this this could could could be be be associated associated associated to to to a a adecreasing decreasing decreasing of of of the the the average average average electronelectronelectron density density of of of the the the metalloporphyrin metalloporphyrin metalloporphyrin [34–36]. [34–36]. [34–36].

191 -3.64E-02 -0.0202492 191 11 -3.64E-02 -0.0202492 192192 9.13E-02 9.13E-02 0.050703940.05070394 TcPPH TcPPH 193193 6.29E-02 6.29E-02 0.034942890.03494289 1941940.90.9 9.27E-02 9.27E-02 0.051477220.05147722 195195 9.16E-02 9.16E-02 0.050912940.05091294 196196 -1.50E-02 -1.50E-02 -0.0083062-0.0083062 0.80.8 197197 7.14E-02 7.14E-02 0.039649560.03964956 198198 0.119726 0.119726 0.066514440.06651444 1991990.70.7 0.105768 0.105768 0.058760.05876 200200 2.58E-02 2.58E-02 0.014357280.01435728 201201 7.64E-02 7.64E-02 0.042446720.04244672 2022020.60.6 0.271817 0.271817 0.151009440.15100944 203203 5.23E-02 5.23E-02 0.029029330.02902933 204 0.115204 0.06400222 2040.50.5 0.115204 0.06400222 205205 7.02E-02 7.02E-02 0.038978280.03897828 206206 4.40E-02 4.40E-02 0.024450330.02445033 2070.4 0 0 2070.4 0 0 TPPFe 208Absorbance 7.12E-02 0.03955022 TPPFe 208Absorbance 7.12E-02 0.03955022 209 0.11547 0.06415 2090.3 0.11547 0.06415 TPPTPP 2102100.3 3.67E-02 3.67E-02 0.020370720.02037072 211211 0.117168 0.117168 0.065093330.06509333 212 4.65E-02 0.02581306 2120.20.2 4.65E-02 0.02581306 213213 -7.13E-03 -7.13E-03 -0.0039604-0.0039604 214214 0.117146 0.117146 0.065081110.06508111 2152150.10.1 7.33E-02 7.33E-02 0.04073750.0407375 216216 0.203826 0.203826 0.113236670.11323667 217 0.139399 0.07744389 2170 0.139399 0.07744389 2182180 -3.53E-02 -3.53E-02 -0.0195983-0.0195983 219219300300 0.237302 0.237302 0.13183444 3500.13183444 350 400 400 450 450 500 500 550 550 600 600 650 650 700 700 220220 0.151649 0.151649 0.084249440.08424944 221 5.62E-02 0.03123361 WavelengthWavelength (nm) (nm) 221 5.62E-02 0.03123361 FigureFigureFigure 1. 1. 1. The TheThe UV-Vis UV-Vis UV-Vis spectra spectra spectra fo fo forrr the the the TPP TPP TPP and and and TPPFe. TPPFe. TPPFe.

2.2.2.2.2.2. Dynamic Dynamic Dynamic Light Light Light Scat Scat Scatteringteringtering Characterization Characterization Characterization FigureFigureFigure 2 2 showsshowsshows the thethe DLS DLSDLS assay assayassay results resultsresults for forfor TPPFe TPPFeTPPFe af after.after.ter. Figure FigureFigure 22a 2aa shows shows the the correlationcorrelation correlation function function (fitting(fitting(fitting quality quality quality for for for the the the diameter diameter diameter of of of the the the particles particles particles in in in suspension). suspension). suspension). Figure Figure Figure 2b 22bb shows shows the the intensity intensity weighted weighted particleparticleparticle size size distribution distribution for for TPPFe,TPPFe, thethe the figurefigure figure shows sh showsows only only only one one one signal signal signal indicating indicating indicating TPPFe TPPFe TPPFe is formedis is formed formed by by oneby oneoneparticle particleparticle type typetype with with anwith average anan averageaverage size distribution sizesize distributiondistribution of around ofof aroundaround 2500 nm. 25002500 Figure nm.nm.2 FiguredidFigure not 2 show2 diddid additionalnotnot showshow additionaladditionalsignals suggesting signalssignals suggestingsuggesting that porphyrin thatthat porphyrinaggregationporphyrin aggr aggr wasegationegation not present waswas not insidenot presentpresent suspension inside inside under suspensionsuspension experimental under under experimentalexperimentalconditions [37 conditions conditions–39]. [37–39]. [37–39].

35 0.90.9 0.9350.9

0.80.8 0.8 30300.8 0.70.7 0.70.7 2525 0.60.6 0.60.6 20 0.50.5 0.5200.5 0.4 0.4 150.40.4 PSD (%) (%) PSD 15 PSD (%) (%) PSD 0.30.3 0.30.3 1010 10 10 correlation function correlation 10

correlation function 10 correlation function correlation 0.20.2 2020 correlation 0.2function 0.2 2020 3030 3030 5 0.10.1 4040 0.10.15 4040 5050 5050 0 0 0 7070 0 0 7070 1 10 100 1000 1000010 100000 0 0 1 10 100 1000 1000010 100000 0 1 10 100 1000 1000010 100000 0 111 10 10 10 100 100 100 1000 1000 1000 1000010 100000 10000 10000 0 2020 0 0 2020 0 0 (a) TimeTime (μs) (μs) 60 0 (b) TimesizeTimesize (nm) (nm)(μs) (μs) 60 0 (a) 60 0 (b) 60 0 Figure 2. Results for DLS assay: (a) correlation function and (b) Intensity weight particle size FigureFigure 2.2. ResultsResults forfor DLSDLS assay:assay: (a(a) ) correlationcorrelation functionfunction andand (b(b) ) IntensityIntensity weightweight particleparticle sizesize distribution TPPFe obtained from correlation function. distributiondistribution TPPFe TPPFe obtained obtained from from correlation correlation function. function. 2.3. SEM Characterization 2.3.2.3. SEM SEM Characterization Characterization The Figure3 shows the SEM image for TPPFe powders. According to DLS results, the main size of the particlesTheThe Figure Figure is around3 3 shows shows 2.5 the theµ m,SEM SEM however, image image for thefor TPPFe FigureTPPFe 3powder powder showss. thats. According According samples areto to DLS DLS formed results, results, by micro-particles the the main main size size ofof thethe particlesparticles isis aroundaround 2.52.5 μμm,m, however,however, thethe FigureFigure 33 showsshows thatthat samplessamples areare formedformed byby micro-micro-

Int.Int. J. J. Mol. Mol. Sci. Sci. 20202020, ,2121, ,x 353 FOR PEER REVIEW 4 4 of of 11 11 Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 4 of 11 particles and (Figure 3), the micro-particles show little aggregates on the surface around 300–400 nm, and (Figure3), the micro-particles show little aggregates on the surface around 300–400 nm, besides, besides,particles Figure and 3(Figure shows 3), that the TPPFe micro-particles surface was show regular little aggregates and uniform. on the surface around 300–400 nm, Figurebesides,Int.3 J.shows Mol. Figure Sci. that 2020 3 ,shows TPPFe21, x FOR that surface PEER TPPFe REVIEW was surface regular was and regular uniform. and uniform. 4 of 11

particles and (Figure 3), the micro-particles show little aggregates on the surface around 300–400 nm, besides, Figure 3 shows that TPPFe surface was regular and uniform.

FigureFigureFigure 3. 3. 3. SEMSEM SEM micrograph micrographmicrograph for for TPPFe,TPPFe, magnification magnification 80 80 80 K. K. K.

2.4.2.4.2.4. Identification Identification Identification of of ofHydroxyl Hydroxyl HydroxylFigure Radical Radical Radical 3. SEM micrograph for TPPFe, magnification 80 K. HydroxylHydroxyl2.4.Hydroxyl Identification radical radical radical of detectionHydroxyl detection detection Radical was was was performed performed performed by by selectiveselective reaction reaction ofof of disodium disodium disodium terephthalate terephthalate terephthalate with with with hydroxylhydroxyl radical radical to to form form 2-hydroxyterephthalate, 2-hydroxyterephthalate, the Figure Figure 4 shows the the chemical chemical reaction reaction and and the the hydroxyl Hydroxylradical to radical form 2-hydroxyterephthalate,detection was performed by the selective Figure reaction 4 shows of disodiumthe chemical terephthalate reaction with and the Figure 5 shows the emission spectra of by product obtained for the reaction of hydroxyl FigureFigurehydroxyl 5 shows radical the fluorescencefluorescence to form 2-hydroxyterephthalate, emissionemission spectra of the by Figure product 4 shows obtained the chemical for the reaction reaction ofand hydroxyl the radical and disodium terephthalate recorded at different reaction times. Results shows an increase of radicalradicalFigure and and disodium5 disodium shows the terephthalate terephthalatefluorescence emis recorded recordedsion spectra at at differ di offferent byent product reaction reaction obtained times. for Results Results the reaction shows shows of an anhydroxyl increase increase of of the maximum emission band of the 2-hydroxyterephthalic acid during the reaction time. thethe maximum maximumradical and emission emission disodium band band terephthalate of of the the 2-hydroxyterephthalic 2-hydroxyterephthalic recorded at different reaction acid acid during duringtimes. Results the reaction shows time.an increase of the maximum emission band of the 2-hydroxyterephthalic acid during the reaction time.

Figure 4. Specific oxidation chemical reaction of terephthalic acid by hydroxyl radical generated after FigureFigurevisibleFigure 4. 4. SpecificirradiationSpecific 4. Specific oxidation oxidation of oxidation TPPFe. chemical chemical chemical reaction reaction reaction of ofof terephthalic terephthalic acidacid acid by by by hydroxyl hydroxyl hydroxyl radical radical radical generated generated generated after after after visiblevisiblevisible irradiation irradiation irradiation of of TPPFe. TPPFe. of TPPFe. 180 180 180160 0 min 160 0 min 160140 0 min 140 10 min 140120 10 min 120 10 min 120100 30 min 100 30 min 10080 30 min 80 60 min Intensity 60 min 80Intensity 60 60 60 min Intensity 40 60 40 402020 20 00 350350 400 400 450 450 500 500 550 550 600 600 0 Wavelenght (nm) 350 400Wavelenght 450 500(nm) 550 600

Wavelenght (nm) FigureFigure 5. 5.The The fluorescence fluorescence emission emission spectraspectra of the productuct obtainedobtained for for the the reaction reaction of ofhydroxyl hydroxyl Figure 5. The fluorescence emission spectra of the product obtained for the reaction of hydroxyl radical andradical disodiumradical and and disodium terephthalate disodium terephthalate terephthalate recorded reco reco at dirdedrdedfferent atat different irradiation irradiationirradiation times. times. times. Figure 5. The fluorescence emission spectra of the product obtained for the reaction of hydroxyl radical and disodium terephthalate recorded at different irradiation times.

Int. J. Mol. Sci. 2020, 21, 353 5 of 11

Due to the fluorescent product (2- hydroxyterephthalic acid) formed during irradiation are generated due to the specific reaction between hydroxyl radicals and terephthalic acid, these results corroborate that hydroxyl radicals were generated by the TCPPFe under visible light irradiation according to reaction in Figure4[40].

2.5. Kinetic Study We determined the initial rates of reaction through the differential method for each of the experiments for determining anthracene concentration. The determination of the slopes was solved numerically using finite differences. Taking into account that the derivative of the concentration with respect to time is defined by: dC C(t + h) C(t) = lim − (1) dt h h →∞ where C represents de concentration, t the time and h is the spacing constant. Because of h is a fixed value near to zero then, Equation (1) can be rewritten as:

dC ∆[C](t) = (2) dt h The difference can be considered a differential operator, which matches the function C with ∆C and, by using the Taylor theorem:

1 1 ∆ = hD + h2D2 + h3D3 + ... = ehD 1 (3) 2 3 − where D is the operator derivative, which matches C with its derivative (C’) finally, by solving the exponential: 1 1 hD = log(1 + ∆) = ∆ ∆2 + ∆3 + ... (4) − 2 3 The first two terms of the series lead to:

dC C(t + 2h) 4C(t + h) + 3C(t) = − (5) dt − 2h for using the first three points (t , C ), (t , c ), (t , C ) whose abscissae equidistant h (h = t t = t t 0 0 1 1 2 2 2 − 1 1 − 0 = 5 min) then, we can obtain: C 4C + 3C initial rate(v) = 2 − 1 0 (6) 2h Using the experimental data and applying Equation (7), we can obtain the initial rates at different temperatures for the reactions with hydroxyl radicals [41]. The initial rates for the oxidation of anthracene by hydroxyl radical as a function of the concentration and temperature are shown in Figure6. The results show that the initial rates increase as the concentration of anthracene increases due to a higher probability of collision between anthracene molecules and the oxygen species present in the solution. Furthermore, Figure6 shows that the initial rate increases with the temperature; however, no significant change is observed in the initial rate to very low concentrations of reagent. Under the experimental conditions of this study, the following rate law equation could be considered:

α α r = k[Anthracene] [ OH∗] (7) where r is the initial rate as function of the reagent concentration, α and β are the reaction orders from each reagent and k is the rate constant. We can determinate the values of α, β in order to analyze how the initial rate depends on the initial concentrations of reagent through of initial rates method. If the Int. J. Mol. Sci. 2020, 21, 353 6 of 11 initial rate depends only on the reagent concentration which is being varied [Anthracene] and if the concentration of the oxidant is in excess, we can express Equation (8) as follows:

= [ ]α Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW r kapp Anthracene 6 of(8) 11

β where kapp is the apparent constant representing the product k*[HO ] β. If the anthracene concentration where kapp is the apparent constant representing the product k*[HO•]• . If the anthracene concentration α isis changed, the the reaction reaction order order α andand the the k kvaluevalue can can be be obtained obtained from from a linear a linear fit fitof ofthe the ln lnv vs.v vs.ln ln[Anthracene]. [Anthracene]. Kinetic Kinetic results results indicated indicated that that the the reaction reaction order order αα forfor the the oxidation oxidation reaction reaction between the anthraceneanthracene andand thethe hydroxylhydroxyl radicalradical isis approximatelyapproximately 1.0.1.0.

0.4

0.35 0.020 M 0.050 M 0.3 0.100 M 0.150 M 0.200 M 0.25

0.2

0.15 rate (mol/min)

0.1

0.05

0 280 290 300 310 320 330 Temperature (K)

Figure 6.6. InitialInitial rates rates for for the the oxidation of anthracene at didifferentfferent temperaturestemperatures andand anthraceneanthracene concentration.

Figure7 a7a shows shows the thekapp kappvalues values for for oxidation oxidation of anthraceneof anthracene at di atfferent different temperatures. temperatures. Results Results show thatshow the that apparent the apparent rate constants rate constants increase withincrease temperature, with temperature, this result indicatesthis result that indicates as temperatures that as increasetemperatures more increase particles more acquired partic enoughles acquired energy enough to overpass energy activation to overpass energy activation of the process energy and of ratethe constantprocess and increases rate constant also. The increases activation also. energy The acti canvation be determined energy can by be the determined Arrhenius equationby the Arrhenius [41]: equation [41]: Ea RT kapp(T) = Ae (9) 𝑘() =𝐴𝑒 (9) k A wherewhere kappapp(T)(T) isis the the apparent apparent rate rate of of the the reaction reaction (temperature (temperature dependent), dependent), A isis a a constant constant related with E R the initial reagentreagent concentration,concentration, Ea isis the activation energy, andand R isis thethe molarmolar gasgas constant.constant. We cancan rewrite thethe EquationEquation (9)(9) asas follows:follows:   𝐸Ea 11 Ln𝐿𝑛(𝑘(k ()))=𝐿𝑛𝐴− = LnA (10)(10) app(T) − 𝑅R 𝑇T The activation energy can be calculated from the slope of the linear fit of the natural logarithm The activation energy can be calculated from the slope of the linear fit of the natural logarithm of the apparent constant as function of 1/T. Figure 7a shows the graphic kapp vs. Temperature for of the apparent constant as function of 1/T. Figure7a shows the graphic kapp vs. Temperature for anthracene oxidation, and Figure 7b shows Ln v vs. 1/T fitting. The value of Ea obtained for oxidation anthracene oxidation, and Figure7b shows Ln v vs. 1/T fitting. The value of Ea obtained for oxidation of anthracene by hydroxyl radical is 51.3 kJ/mol. of anthracene by hydroxyl radical is 51.3 kJ/mol.

Int. J. Mol. Sci. 21 Int. J. Mol. Sci.2020 2020, , 21,, 353 x FOR PEER REVIEW 7 7 of of 11 11 Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 7 of 11

70 4.5 70 4.5 60 4 60 4 50 50 3.5 3.5 ) 40 ) -1 ) 40 ) 3 app -1 3 app

(min 30

(min 2.5

30 Ln (K 2.5 Ln (K app k app

k 20 20 2 2 y =y -6116.1x = -6116.1x + 22.95 + 22.95 R² = 0.9978 1010 R² = 0.9978 1.51.5

00 1 1 280280 300 300 320 320 340 340 0.0030.003 0.0032 0.0032 0.0034 0.0034 0.0036 0.0036 (a)(a) TemperatureTemperature (K)(K) (b)(b) 1/T 1/T (K -1(K) -1)

Figure 7. a k b Ln T FigureFigure 7. 7.( (()aa Apparent)) ApparentApparent rate raterate constants constantsconstants k appkappapp asas as function function function of of of temperature. temperature. temperature. (b( )( bLn)) Lnv vs.1/v vvs.1 vs.1/T /fitting.Tfitting. fitting. 2.6. Analysis of the Products of Oxidation 2.6.2.6. Analysis Analysis of of the the ProductsProducts of Oxidation After photooxidation process, the anthraquinone was identified by CG-ME as the only one AfterAfter photooxidationphotooxidation process, thethe anthraquinoneanthraquinone was was identified identified by by CG-ME CG-ME as asthe theonly only one one product for anthracene reaction. In the presence of hydroxyl radical, the anthraquinone formation productproduct for for anthraceneanthracene reaction. InIn thethe presencepresence of of hydroxyl hydroxyl radical, radical, the the anthraquinone anthraquinone formation formation could be explained via the formation of 9(10H)anthracenone. The latter could be obtained by couldcould be be explainedexplained viavia the formation ofof 9(10H)anthracenone.9(10H)anthracenone. The The latter latter could could be beobtained obtained by aby a a hydrogenhydrogen atomatom abstraction reaction reaction following following by by su subsequentbsequent addition addition of ofhydroxyl hydroxyl radical radical to toyields yields hydrogen atom abstraction reaction following by subsequent addition of hydroxyl radical to yields 9,10-dihydroxyanthracene9,10-dihydroxyanthracene which which can can be be readily readily oxidized oxidized toanthraquinone. to anthraquinone. These These primary primary step step events 9,10-dihydroxyanthracene which can be readily oxidized to anthraquinone. These primary step couldevents be consideredcould be considered as the rate-determining as the rate-determining the stage the of stage the process, of the process, this mechanism this mechanism is an alternate is an events could be considered as the rate-determining the stage of the process, this mechanism is an routealternate to the route mechanism to the mechanism proposed proposed to anthracene to anthracene oxidation oxidation in the advanced in the advanced oxidation oxidation process process [42–44 ]. alternate route to the mechanism proposed to anthracene oxidation in the advanced oxidation process Figure[42–44].8 proposes Figure 8 a proposes mechanism a mechanism leading to leading the formation to the formation of 9,10-anthraquinone. of 9,10-anthraquinone. [42–44]. Figure 8 proposes a mechanism leading to the formation of 9,10-anthraquinone.

hv(visible) * Fe(III)TPPFehv(visible) [Fe(III)TPPFe] [Fe(II)TPPFe] Fe(III)TPPFeI [Fe(III)TPPFe]* II [Fe(II)TPPFe] I II III III • OH H2O2 • OH H2O2

Tautomerization

Tautomerization IV V 9(10H)Anthracenone IV V 9(10H)Anthracenone

VI VII 9,10-dihydroxyanthracene Anthraquinone VI VII FigureFigure 8. 8.Mechanism Mechanism for for anthracene anthracene oxidationoxidation9,10-dihydroxyanthracene by hydroxyl radical: radical: TCPPFe TCPPFe (AnthraquinoneIII (III) absorbs) absorbs radiation radiation in in 3+ visiblevisible region region and and then then an an electron electron is is transferred transferred fromfrom the porphyrin to to Fe Fe3+ (I).(I ).The The metal metal is reduced is reduced Figure3 8.+ Mechanism2+ for anthracene2+ oxidation by hydroxyl radical: TCPPFe (III) absorbs radiation in fromfrom Fe Fe3+to to Fe Fe2+ ((IIII),), thethe FeFe2+ interactsinteracts with with H H2O2O2 which2 which produces produces hydroxyl hydroxyl radicals radicals (III ().III Afterward,). Afterward, 3+ thevisiblethe hydroxyl hydroxyl region radicals radicalsand then react react an with electronwith anthracene anthra is transferredcene generatinggenerating from 9-hydroxyanthracene 9-hydroxyanthracenethe porphyrin to Fe (IV (IVI).), ),theThe the tautumeration metal tautumeration is reduced 3+ 2+ 2+ equilibriumfromequilibrium Fe tois Fe is reached reached (II), the generating generating Fe interacts 9(10H)anthracenone9(10H)anthracenone with H2O2 which (produces (VV).). The The subsequent subsequenthydroxyl radicalsaddition addition ( IIIof of).hydroxyl Afterward, hydroxyl radicaltheradical hydroxyl to to yields yields radicals 9,10-dihydroxyanthracene 9,10-dihydroxyanthracene react with anthracene ((VIVI generating)) which can can 9-hydroxyanthracene be be readily readily oxidized oxidized to to anthraquinone(IV anthraquinone), the tautumeration (VII (VII). ). equilibrium is reached generating 9(10H)anthracenone (V). The subsequent addition of hydroxyl radical to yields 9,10-dihydroxyanthracene (VI) which can be readily oxidized to anthraquinone (VII).

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3. Materials and Methods 3. Materials and Methods

3.1.3.1. SynthesisSynthesis and Characterization AllAll reagents reagents used used in inthis this work work were wereanalytical analytical grade, the grade, UV-Vis the spectra UV-Vis were spectra taken were in a Hewlett- taken in aPackard Hewlett-Packard 8453 spectrophotometer. 8453 spectrophotometer. FT-IR spectra FT-IR spectra(KBr) were (KBr) wererecorded recorded in a inBruker a Bruker Tensor Tensor 27 27 spectrometer, in this assay, we dissolved 2.0 × 10−5 g5 of each compound in ethyl acetate. The NMR 1H spectrometer, in this assay, we dissolved 2.0 10− g of each compound in ethyl acetate. The NMR 1spectra were performed in a Bruker AC-400 spectrometer× using DMSO-d6 as solvent. The 1H MNR1 H spectra were performed in a Bruker AC-400 spectrometer using DMSO-d6 as solvent. The H chemical shifts were reported as ppm (δ), relative to CDCl3 (signal located at 7.28 ppm). Mass MNR chemical shifts were reported as ppm (δ), relative to CDCl3 (signal located at 7.28 ppm). spectrometry assay was measured in ESI (LC)-MS/MS ion Trap Amazon, Bruker spectrometer. The Mass spectrometry assay was measured in ESI (LC)-MS/MS ion Trap Amazon, Bruker spectrometer. Scheme 2 shows the synthesis procedure. The Scheme2 shows the synthesis procedure.

Scheme 2. The synthesis procedure: 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (1) and Scheme 2. The synthesis procedure: 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (1) and 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin Fe (III) (2). 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin Fe (III) (2). 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (1) was synthesized by mixing pyrrole and 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (1) was synthesized by mixing pyrrole and 4- 4-carboxybenzaldehyde (9 mmol) in propionic acid (100 mL) and nitrobenzene (45 mL). The mixture carboxybenzaldehyde (9 mmol) in propionic acid (100 mL) and nitrobenzene (45 mL). The mixture was heated for 1h at 120 C. After cooling and solvent removal under vacuum, the porphyrin was was heated for 1h at 120 ◦°C. After cooling and solvent removal under vacuum, the porphyrin was dissolved in 250 mL of sodium (0.10 M) and then it was precipitated by adding hydrochloric dissolved in 250 mL of (0.10 M) and then it was precipitated by adding acid (1.0 M) [45]. The obtained powder was purified using column chromatography; we used silica gel hydrochloric acid (1.0 M) [45]. The obtained powder was purified using column chromatography; we (2.5 24 cm) as stationary phase and ether-ethyl acetate 20:2 as mobile phase (rf. 079); yield used× silica gel (2.5 × 24 cm) as stationary phase and petroleum ether-ethyl acetate 20:2 as mobile phase 7501 g (33%); > 300 C; UV-Vis (ethyl acetate) 419, 516, 552, 594, 650; FT-IR (cm 1): O-H (rf. 079); yield 7501 g (33%); melting◦ point >300 °C; UV-Vis (ethyl acetate) 419, 516, 552, 594, 650;− FT- (3200–3500), N-H (3500 band overlapped by O-H), C=O (1687), C=C (1603), C=N (1211); 1H RMN IR (cm−1): O-H (3200–3500), N-H (3500 band overlapped by O-H), C=O (1687), C=C (1603), C=N (1211); (400 MHz, DMSO-d6) δ (ppm): = 7.76 (8 H, d; J = 7.66 Hz, 3-Ar), 7.85 (8 H, d; J = 7.66, 2-Ar), 8.22 (8 H, 1 H RMN (400 MHz, DMSO-d6) δ (ppm): =7.76 (8 H, d; J = 7.66 Hz, 3-Ar), 7.85 (8 H, d; J = 7.66, 2-Ar), d; J = 7.6 Hz, Py), 8.86 (4 H, s, O-H), 12.85 (2 H, s; N-H); MS (ESI-IT), m/z: 791.2 [M + H]+. 8.22 (8 H, d; J = 7.6 Hz, Py), 8.86 (4 H, s, O-H), 12.85 (2 H, s; N-H); MS (ESI-IT), m/z: 791.2 [M + H]+. 5,10,15,20-tetrakis(4-carboxyphenyl)5,10,15,20-tetrakis(4-carboxyphenyl) porphyrinporphyrin Fe Fe (III) (III)(2) (2) was was synthesized synthesized by mixingby mixing (1) (0.330 (1) (0.330 mmol) andmmol) iron and (III) iron chloride (III) chloride hexahydrate hexahydrate in 70 mL in 70 of mL N,N’-dimethylformamide of N,N’-dimethylformamide (DMF) (DMF) for 2 for h in 2 refluxh in system.reflux system. The DMF The was DMF removed was removed by distillation by distillati and theon (2) and was the precipitated (2) was precipitated in . The in TCPPFewater. The was dissolvedTCPPFe was in dissolution dissolved in of dissolution solid sodium of hydroxidesolid sodium (0.10 hydroxide M) and after (0.10 that, M) and the TCPPFeafter that, precipitated the TCPPFe by addingprecipitated hydrochloric by adding acid hydrochloric (1.0 M). Finally, acid (1.0 the compoundM). Finally, was the purifiedcompound by was column purified chromatography; by column we used silica gel (2.5 24 cm) as the stationary phase and petroleum ether-ethyl acetate 5:1 as the chromatography; we used× silica gel (2.5 × 24 cm) as the stationary phase and petroleum ether-ethyl mobile,acetate 5:1 melting as the point mobile,> 300 melting◦C; UV-Vis: point > 413,300 °C; 535. UV-Vis: 413, 535.

3.2.3.2. PhysicochemicalPhysicochemical Assay For hydroxyl radical generation, photosensitizer was added to H O solution at pH = 3.0; For hydroxyl radical generation, photosensitizer was added to H2O2 2solution2 at pH = 3.0; the themixture mixture was was introduced introduced in ina batch a batch photo-reactor photo-reactor in innitrogen nitrogen atmosphere atmosphere under under visible visible irradiation irradiation (100(100 WW OSRAMOSRAM halogenhalogen immersion lamp). Hydroxyl Hydroxyl radical radical generation generation was was monitored monitored through through the the fluorescencefluorescence emissionemission signal of 2-hydroxyterephthali 2-hydroxyterephthalicc disodium disodium (excitation (excitation at at 425 425 nm) nm) of of the the solution solution inin aa Shimadzu Shimadzu RF-5301PC RF-5301PC spectrofluorometer. spectrofluorometer. The The reaction reaction products products were were identified identified in a 5890 in a Hewlett 5890 PackardHewlett gasPackard chromatographer, gas chromatographer, with a 5972 with mass a 5972 selective mass detectorselective anddetector a HP5-MS and a columnHP5-MS (30 column m long and(30m 0.25 long mm and internal 0.25 mm diameter) internal and diameter) He was and the He carrier was gasthe carrier (1 mL/min). gas (1 Sample mL/min). aliquots Sample of aliquots 1,0 µL were of injected1,0 μL were with injected split (1:30). with The split temperature (1:30). The temperature program: heated program: at 200 heated◦C for at 5 200 min, °C then for 105 min,◦C/min then up 10 to 300°C/min◦C and up keptto 300 at °C 300 and◦C forkept 15 at min. 300 Detector°C for 15 conditionsmin. Detector were conditions 70 eV,electronic were 70 impact, eV, electronic 35–400 mimpact,/z mass

Int. J. Mol. Sci. 2020, 21, 353 9 of 11

range, 200 V EM voltage (A-tune), 20 Hz Sweep Frequency at 230 ◦C. The particle size in suspension was determined by the Dynamic Light Scattering (DLS). The equipment used was a Zetasizer version 6.2 of Malvern Instruments. For the analysis, 1g/L of TCPPFe was scattered in water at pH = 3. The intensity distribution seen in the DLS program was obtained by the analysis of the correlation function by the analysis of the smaller non-negative squares. Besides, the scanning electron microscopy (SEM) characteristics for TPPFe were measured in SEM Tescan VEGA3 SB equipment under excitation energy of 5 and 1 kV. Finally, reaction rates of the anthracene oxidation were obtained by using the initial slope method. In this study, we changed the concentration of anthracene from 0.02 M to 2.0 M and the activation energies for each process was calculated at different temperatures (283–323 K).

4. Conclusions In this work, we studied the kinetics of the oxidation of anthracene on ferric tetracarboxyphenylporphyrin under visible irradiation. Spectrophotometric results verified metal complexation porphyrin and DSL and SEM results indicated that TPPFe was compound by micro-particles with a narrow size 2500 nm. The corresponding rate constants and activation energy were estimated for selective anthracene oxidation. Results showed an activation energy value of 51.3 kJ/mol for anthracene oxidation by hydroxyl radical. Furthermore, the fluorescent product proved that hydroxyl radical was generated after visible irradiation of TPPFe sensitizer. Finally, anthraquinone was identified as the only product during anthracene oxidation by hydroxyl radical and we proposed a possible mechanism for the oxidation process. The anthracene removal by using TPPFe under visible irradiation is an alternative procedure to removal dangerous PAH.

Author Contributions: Conceptualization, C.D.-U. and W.V.; methodology, C.D.-U. and W.V.; validation, C.D.-U. and W.V.; formal analysis, C.D.-U., W.V. and C.Q.; data curation, C.D.-U. and W.V.; writing-original draft preparation, C.D.-U., W.V. and C.Q. All authors have read and agreed to the published version of the manuscript. Funding: This research did not receive specific funding. Acknowledgments: C.D.-U. and W.V. thank to Universidad del Atlántico for supporting the publishing process. Conflicts of Interest: The authors declare no conflict of interest.

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