Combined Aops for Formaldehyde Degradation Using Heterogeneous Nanostructured Catalysts

nanomaterials

Article Combined AOPs for Formaldehyde Degradation Using Heterogeneous Nanostructured Catalysts

Renato Bonora, Carlo Boaretti *, Laura Campea, Martina Roso , Alessandro Martucci , Michele Modesti and Alessandra Lorenzetti * Department of Industrial Engineering, University of Padova, Via Marzolo 9, 35131 Padova, Italy; [email protected] (R.B.); [email protected] (L.C.); [email protected] (M.R.); [email protected] (A.M.); [email protected] (M.M.) * Correspondence: [email protected] (C.B.); [email protected] (A.L.)

Received: 4 December 2019; Accepted: 9 January 2020; Published: 14 January 2020

Abstract: In this paper we studied the combination of advanced oxidation processes (AOPs), i.e., TiO2-based photocatalysis and photo-Fenton process, on the degradation of aqueous solutions containing a low (90 ppm) concentration of formaldehyde. Heterogeneous nanostructured catalysts, supported on polymeric nanofibers, were used; for comparison, some homogeneous or partly heterogeneous systems were also analyzed. Furthermore, to make the process more sustainable (in terms of costs and safety) no hydrogen peroxide was added to the system. The results showed that the combination of AOPs gave a synergy since the presence of iron was beneficial in promoting the photocatalytic activity of TiO2 while TiO2 was beneficial in promoting the photo-Fenton reaction. Moreover, very good results were obtained using fully heterogeneous nanostructured catalysts (based on TiO2 and FeSO4), without the need to add H2O2.

Keywords: formaldehyde; titanium dioxide; photocatalysis; photo-Fenton; heterogeneous catalyst; combination; synergy; AOPs; nanoﬁbers

1. Introduction Terrorism is a threat that, in recent decades, has become increasingly fearsome and destructive. It is known that chemical weapons and toxic compounds are used by terrorists in their attacks. For this reason, toxic industrial materials (TIMs), which are commonly used in the chemical industry, are the subject of discussion in many countries, where governments want to analyze and optimize the conditions of use and disposal of these hazardous chemicals. TIMs are characterized by high toxicity and wide availability [1]. This problem has led to the introduction of restrictive regulations on trade of these materials and on safety in chemical industries which, having a large quantity of toxic materials, are a sensitive target for possible terrorist actions and, therefore, require an adequate TIMs degradation system. To limit the spread of these substances, wastewater from industries, which could contain toxic compounds in high concentrations, should also be treated. One of the most dangerous TIMs is formaldehyde, a suspected carcinogenic chemical compound characterized by high toxicity, which is widely used in various sectors of the chemical industry (synthetic resins, insulators, paints, textiles) [2]. Advanced oxidation processes (AOPs) are considered the most eﬀective treatments for the removal of these species from water [3], because they allow a complete mineralization even of the most refractory organic compounds, thus destroying wastewater pollutants and transforming them into less and even non-toxic products, thereby providing an ultimate solution for wastewater treatment. Among the most widespread radical species used in AOPs, hydroxyl radical plays a primary role, since it is the most reactive oxidizing agent in water treatment (having a high oxidizing power [3]), it is very non-selective in its behavior and rapidly reacts with numerous species. Because hydroxyl radicals

Nanomaterials 2020, 10, 148; doi:10.3390/nano10010148 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 148 2 of 16 have a very short lifetime, they are only in situ produced during application through different methods, including a combination of oxidizing agents (e.g., H2O2), irradiation (such as ultraviolet (UV) light, 2+ or ultrasound), and catalysts (such as Fenton reagent, Fe , or titanium dioxide, TiO2)[4]. Indeed, hydroxyl radicals can be initiated by photons in the presence of catalysts (the most common being titanium dioxide, TiO2) or oxidants such as H2O2 (a H2O2 molecule is cleaved by UV irradiation to generate two OH ); they are also produced in the presence of some metals (e.g., iron) which are able to • 2+ activate H2O2. For example, in the so-called Fenton process, H2O2 reacts with Fe to generate strong reactive species, like hydroxyl radicals, although other substances such as ferryl ions are proposed [5]. Several papers have already reported on the degradation of aqueous formaldehyde solution by photocatalytic process using titanium dioxide [6–9]. Some of them have used homogeneous catalysis, i.e., TiO2 particles suspended in contaminated water, which makes it necessary to recover them after treatment while the heterogeneous (i.e., supported) catalyst configurations, on the other hand, eliminate the need for catalyst filtration but generally result in a significant reduction in system efficiency [10]. Fenton and photo-Fenton processes have already been proposed for formaldehyde removal from wastewater [11–13]. In general, the Fenton and photo-Fenton processes require the use of ferrous 2+ ions (Fe ) and hydrogen peroxide (H2O2)[14]. Thus, large amounts of H2O2 need to be irreversibly consumed in real application to treat wastewater containing a high concentration of organic pollutants, and considerable iron dosage is indispensable if higher efficiency is required [15,16]. In the Fenton process, there is a need for continuous addition of ferrous ion into the reaction medium for the reaction to proceed further. This disadvantage could be overcome by the use of photo-Fenton reagent, which is a cyclic process and regenerates Fe2+ ion [15], thus limiting the deposition of ferric ion sludge. Based on the similarity between the mechanism of destruction in the case of different advanced oxidation processes (AOPs) [17], some research efforts focusing on the synergism between several AOPs have already been published. Mokhbi et al. [18] studied the effectiveness of photocatalysis with Fenton’s reagent for treating oily wastewater; Kim et al. [19] studied the combined photocatalytic- and photo-Fenton system for treating some organic substrates (i.e., phenol, benzoic acid, and methanol); 3+ Xu et al. [15] studied interactions between TiO2 and Fe /H2O2 to degrade bisphenol A. They all used 2+ unsupported catalysts, i.e., TiO2 and Fe were put in solution, in the presence of H2O2 and acidic pH, and found that synergistic effects resulted. In other works, the combination of TiO2 and Fe ions has been proposed but without exploiting Fenton or photo-Fenton reactions; the main aim of those works was, indeed, to dope titania in order to modify its photocatalytic features, such as visible light absorption and photocatalytic efficiency. For example, Khanmohammadi et al. [20] studied the use of unsupported catalyst, composed of sol-gel synthesized Fe2O3-TiO2 nano hybrid, in the degradation of liquid-phase formaldehyde in the presence of ultraviolet and visible irradiation. They found that the degradation took place according to a two-step mechanism: first, formaldehyde was converted to formic acid and then this was converted to H2O and CO2 during a photocatalytic reaction via radical mechanism in the presence of UV irradiation. In a similar way, Siddhapara et al. [21] used sol-gel TiO2 nanoparticles doped with a transition metal (Mn, Fe, Co) to degrade liquid formaldehyde solution; they found that the substitution with transition metals enhanced photocatalytic efficiency since transition metals increased defect sites and acted as a permanent space charge region, whose electric force improved the separating efficiency of electron-holes. Li et al. [22] studied the effect of the content of Fe on the photocatalytic performance of doped TiO2 on the degradation of gaseous formaldehyde and found that doped TiO2 exhibited higher photocatalytic activity under visible light irradiation. Cheng et al. [23] mixed TiO2 with iron oxide to produce modified photocatalysts (Fe/TiO2) and then coated them on a supporting medium of fiberglass and studied the degradation of gaseous formaldehyde using these supported catalysts. They concluded that the enhancement of formaldehyde decomposition by iron-doped TiO2 may be attributed to the facts that the doped iron ions could effectively enhance the absorption of UV-visible wavelength, increase the photocatalytic reactivity and retard the recombination of electron and electron-hole pairs, which thus contributed to the increase of formaldehyde decomposition efficiency. Chun et al. [24] studied the photocatalytic degradation of Nanomaterials 2020, 10, 148 3 of 16

gaseous pollutants (benzene, toluene, ethyl benzene, and o-xylene (BTEX)) by preparing the Fe-TiO2 catalyst by the sol-gel technique and electrospinning it in presence of polyvinyl pyrrolidone; they found enhanced photocatalytic activity in presence of optimal Fe:TiO2 ratio. However, authors using Fe-doped titania [20–24] did not mention the photo-Fenton reaction and did not adjust pH accordingly. Nevertheless, in literature there are already some examples of combination of photocatalysis and photo-Fenton using supported catalysts. For example, Bouras et al. [25] combined a titania-supported photocatalyst as well as Photo-Fenton oxidation, by using FeCl 4H O in solution, for the decolorization 2· 2 of aqueous solution of the azo dye Basic Blue 41. Only very few examples using a fully supported catalytic system based on TiO2 and Fe ions and exploiting photo-Fenton reaction can be found. For example, Quici et al., degraded oxalic acid [26], while Shao et al. [27] used TiO2 and Fe(III) meso-tetraphenylporphyrin on polystyrene nanofibers to degrade methyl orange under visible light. They electrospun solutions simultaneously containing polystyrene, porphyrin and TiO2 and found that this system, in presence of added H2O2, coupling photocatalysis and Fenton reaction gave the best performance. Zhang et al. [28] used TiO2/Cu2O composite coated on glass matrix to degrade methylene blue using both photocatalysis and the Fenton process, using in situ generated H2O2. In the paper of Zhu et al. [29], beta-iron oxyhydroxide (β-FeOOH) nanostructures on electrospun TiO2 nanofibers were synthesized and used to degrade methyl orange. They showed that the degradation was faster in the presence of added H2O2 but it still took place also without adding it. In this paper we study the combination of TiO2-based photocatalysis and photo-Fenton process on the degradation of aqueous solutions containing a low (90 ppm) concentration of formaldehyde to understand if synergy can be revealed. Heterogeneous nanostructured catalysts, supported on polymeric nanofibers, have been used; for comparison, some homogeneous or partly heterogeneous systems have also been analyzed. Furthermore, to make the process more sustainable (in terms of costs and safety) no hydrogen peroxide has been added to the system. To the best of our knowledge, this combination of advanced oxidation processes has never been studied in the literature for formaldehyde degradation.

2. Materials and Methods

2.1. Materials Polyacrylonitrile (PAN, Mw = 150,000 g/mol, Sigma Aldrich, St. Louis, MO, USA) was used as support for the catalytic systems in electrospun form; N,N-dimethylformamide (DMF, Sigma-Aldrich, St. Louis, MO, USA) was used as electrospinning solvent. Ferrous sulfate (FeSO 7H O) was used 4· 2 as a catalyst for the photo-Fenton process and was supplied by Sigma-Aldrich (St. Louis, MO, USA). ® A crystalline, titanium dioxide powder was used for photocatalytic process, i.e., Aeroxide TiO2 P90 (d = 14 nm, BET = 90 m2/g, 90/10 anatase/rutile, crystal size = 11 nm [30]), supplied by Evonik (Essen, Germany). We decided to use this one (instead of the most commonly used P25) because some preliminary trials (unpublished results) have shown that P90 was more active than P25 in our degradation systems. Some other authors reported similar results for different pollutants when using supported TiO2-based catalysts [30,31]. The aqueous formaldehyde solution for testing the catalytic activity was prepared by diluting formalin (37% w/w of formaldehyde and 10% w/w of methanol), supplied by Scharlau (Barcelona, Spain), with distilled water to reach a final formaldehyde concentration of 90 ppm. When the photo-Fenton process was considered, it was necessary to use 0.2 N sulfuric acid (H2SO4 at 98% w/w, Sigma Aldrich, St. Louis, MO, USA) to set the pH around 3, because this type of process is favored by acid pH [12]. Hydrogen peroxide was used as was a 30% (w/w) solution, supplied by Sigma Aldrich (St. Louis, MO, USA). Finally, pentafluorobenzylhydroxylamine hydrochlorinated (PFBHA-HCl), purchased by Alfa Aesar (Haverhill, MA, USA), and 1,2-dibromopropane 98% w/w as internal standard (Sigma-Aldrich, Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 16

Finally, pentafluorobenzylhydroxylamine hydrochlorinated (PFBHA-HCl), purchased by Alfa

AesarNanomaterials (Haverhill,2020, 10 MA,, 148 USA), and 1,2-dibromopropane 98% w/w as internal standard (Sigma-Aldrich,4 of 16 St. Louis, MO, USA) dissolved into hexane (Sigma-Aldrich, St. Louis, MO, USA) have been used for the derivatization and extraction process, needed to quantify formaldehyde content in aqueous solutionsSt. Louis, by MO, gas USA)chromatography-m dissolved intoass hexane spectrometry (Sigma-Aldrich, (GC-MS), St. accord Louis,ing MO, to the USA) U.S. haveEnvironmental been used Protectionfor the derivatization Agency (EPA) and method extraction 556. process, needed to quantify formaldehyde content in aqueous solutions by gas chromatography-mass spectrometry (GC-MS), according to the U.S. Environmental 2.2.Protection Preparation Agency of the (EPA) Nanostructured method 556. Catalytic Systems

2.2. PreparationSupported ofnanostructured the Nanostructured catalysts Catalytic have Systemsbeen used. The support was made by electrospun PAN nanofibers, while electrospraying was used for the catalysts’ deposition. In this way, the catalysts were Supportedcompletely nanostructured placed on the catalystsupper surface have been of the used. membrane The support and wasso they made were by electrospunhomogeneously PAN irradiatednanoﬁbers, by while ultraviolet electrospraying (UV) rays was [32]. used A forschematic the catalysts’ procedure deposition. is depicted In this in way, Figure the catalysts1. The use were of nanofiberscompletely membranes placed on the as uppersupport surface allowed of the to membranehave a very and large so surface they were area homogeneously where the catalysts irradiated can beby deposited. ultraviolet (UV) rays [32]. A schematic procedure is depicted in Figure1. The use of nanoﬁbers membranes as support allowed to have a very large surface area where the catalysts can be deposited.

FigureFigure 1. 1. SchematicSchematic procedure procedure for for the the preparation preparation of of nanostructured nanostructured catalytic catalytic systems. systems. The first step in preparation was the production of PAN nanofiber membranes by electrospinning, The first step in preparation was the production of PAN nanofiber membranes by using a solution of PAN in DMF at 5% w/w, which must be stirred for not less than 15 h. The electrospinning, using a solution of PAN in DMF at 5% w/w, which must be stirred for not less than homogeneous mixture was then transferred into a 5 mL syringe fitted with a 27 G gauge needle. The 15 h. The homogeneous mixture was then transferred into a 5 mL syringe fitted with a 27 G gauge syringe was put horizontally on a pump (NE-300 single syringe pump, NewEra Pump System Inc., needle. The syringe was put horizontally on a pump (NE-300 single syringe pump, NewEra Pump Farmingdale, NY, USA) and the positive electrode of the high voltage power supply (Gamma High System Inc., Farmingdale, NY, USA) and the positive electrode of the high voltage power supply Voltage Inc., Ormond Beach, FL, USA), capable of generating voltages up to 60 kV, was clamped to (Gamma High Voltage Inc., Ormond Beach, FL, USA), capable of generating voltages up to 60 kV, the metal needle tip. The nanofibers were deposited on a metallic rotating collector. The process was clamped to the metal needle tip. The nanofibers were deposited on a metallic rotating collector. parameters are reported in Table1. The process parameters are reported in Table 1.

Table 1. Process parameters for the membranes production. Table 1. Process parameters for the membranes production. Flow Rate Voltage Electrode Relative Deposition Materials Flow Rate Voltage Electrode Relative Deposition Materials (mL/h) (kV) Distance (cm) Humidity (%) Time (min) (mL/h) (kV) Distance (cm) Humidity (%) Time (min) PAN support 2 15 20 20 25 30 40 120 ÷ ÷ ÷ CatalystsPAN support deposition 2 2 23 15 ÷ 2520 20 15÷ 25 20 30 30÷ 40 60 120 Catalysts deposition 2 23÷ ÷ 25 15 20÷ ÷ 30 60

ForFor thethe catalystscatalysts deposition deposition by by electrospraying electrospraying process, process, the catalyststhe catalysts were suspendedwere suspended into ethanol. into TiO was used at 5% w/w; this concentration was chosen in order to obtain a catalyst content of ethanol.2 TiO2 was used at 5% w/w; this concentration was chosen in order to obtain a catalyst content 2 ofabout about 0.14 0.14 mg mg/cm/cm ,2 which, which was, was, based based on on our our previous previous experienceexperience [[32],32], thethe optimaloptimal one to have have a a widespreadwidespread distribution distribution of of titanium titanium dioxide, dioxide, avoidi avoidingng great great clusters, clusters, agglomeration agglomeration and and clogging clogging of of the nanostructured substrate. Moreover, at higher concentrations, an increase in opacity and light scattering of TiO2 particles occurs, leading to a decreased passage of irradiation through the sample [10]. Nanomaterials 2020, 10, 148 5 of 16

For ferrous sulfate, based on mass balance calculation, a 4% w/w solution in ethanol should be enough to have the same catalyst content of titania and similar to that used in literature for a homogenous photo-Fenton system [11]. However, due to a partial loss of FeSO4 experienced during the electrospray 2+ process, owing probably to an interaction of the Fe with the electric field, a 10% w/w FeSO4 solution in ethanol has to be used. When the two catalysts were employed together, TiO2 was used at 5% w/w and the FeSO4 at 10% w/w dispersion in ethanol, in order to deposit a 50:50 TiO2:FeSO4 catalytic system on to the PAN support. The catalysts’ dispersions (TiO2 and/or FeSO4 in ethanol) were sonicated for 40 min at 40% of amplitude and constant temperature by using ultrasonic probe VC505® of Sonics & Materials (power of 500 W and a probe length of 254 mm, Newtown, CT, USA). Then, Dynasylan® 4144 was added at 1% w/w of ethanol, followed by a further sonication (15 min). Similarly, to the electrospinning, the catalysts’ dispersion was put in a syringe of 5 mL volume and then it was electrosprayed onto the preformed electrospun PAN membrane thanks to a voltage generated between the metallic needle and the collector. The whole production process was carried out at room temperature and the humidity was kept constant by flowing dried compressed air. In the following, membranes are named PAN_TiO2, PAN_Fe and PAN_TiO2_Fe if the catalyst deposited was TiO2, FeSO4 or TiO2+FeSO4, respectively. In this work, we referred to homogeneous 2+ process when the catalyst (Fe ) and/or oxidant (H2O2) were in the same (liquid) phase of formaldehyde (i.e., they are simply added to formaldehyde solution) while heterogeneous one referred to the use of supported TiO2 and/or Fe catalyst prepared by electro-spinning and -spraying. Concentrations of catalysts, in both homogeneous and heterogeneous experiments, have been fixed based on optimal compositions reported in literature [11] or based on our previous experience [32]: for homogeneous catalysis, FeSO4 and H2O2 were used at 24 and 400 ppm, respectively, while in the heterogeneous supported system around 0.15 mg/cm2 of catalyst has been deposited (the exact values are obtained by thermogravimetric analysis (TGA) and are reported in the following).

2.3. Testing of the Catalytic Systems in the Degradation of Aqueous Formaldehyde Solution The set-up used for the experiments is shown in Figure2. It consisted mainly of a Petri dish with a diameter of 15 cm and a height of 2 cm, in which 70 mL of aqueous formaldehyde solution (90 ppm) is placed; a magnetic stirrer provided a good mixing inside the system. The catalytic membrane, comprising titanium dioxide and/or ferrous sulfate, with a 50 cm2 area, was placed on the bottom of the Petri dish and it was kept ﬁxed using eight stainless steel weights. UV irradiation is provided by a lamp (16 W UV Stylo E16, Light Progress, Anghiari, Italy) with a spectral peak centered around 250 nm. The distance between the lamp and the membrane was constant (2 cm). For some trials, ferrous sulfate was added directly to the aqueous solution. In this case the experiment started after the complete solubilization of the salt into the solution. The catalytic process lasted 4 h. During the tests, some samples were taken from the aqueous solution. These samples were subjected to a derivatization process according to EPA method 556, before being injected into a GC-MS to quantify formaldehyde content. It is known that bubbling air within the formaldehyde solution gives some beneﬁts in terms of degradation rate [6] through the electron transfer reaction generating superoxide radical anion:

e− + O O− 2 → 2 • However, we preferred to avoid the use of air in order to avoid misleading results due to formaldehyde stripping by the bubbling air. Nonetheless, due to traces of (electron acceptor) impurities or adventitious O2 in the solution (since the solution is in direct contact with ambient air) some superoxide radical anions can still form [6]. NanomaterialsNanomaterials 20202020,, 1010,, 148x FOR PEER REVIEW 66 of 16 16

Figure 2. Experimental set-up for testing catalytic activity in formaldehyde degradation. Figure 2. Experimental set-up for testing catalytic activity in formaldehyde degradation. 2.4. Characterization It is known that bubbling air within the formaldehyde solution gives some benefits in terms of All the membranes are characterized in terms of morphology by scanning electron microscopy degradation rate [6] through the electron transfer reaction generating superoxide radical anion: (SEM, Obducat CamScan MX2500, Cambridge, UK) and transmission electron microscopy (TEM, FEI • model Tecnai G12, Hillsboro, OR, USA). Samplee +O preparation → O for TEM imaging have been carried out as follows:However, the membrane we preferred samples to were avoid embedded the use inof aair proper in order epoxy to resin avoid and misleading slices of 100 results nm thickness due to wereformaldehyde obtained bystripping ultramicrotomy by the ofbubbling the cured air. resin. Nonetheless, due to traces of (electron acceptor) Furthermore, catalytic content deposited onto the PAN support was investigated through TGA impurities or adventitious O2 in the solution (since the solution is in direct contact with ambient air) (SDTsome Q600,superoxide TA Instruments, radical anions New can Castle, still form DE, USA);[6]. the analysis was performed using alumina pan with air flow of 100 cc/min and a heating rate of 20 ◦C/min, from room temperature to 1000 ◦C. 2.4. CharacterizationThe quantification of formaldehyde was carried out by withdrawing samples from the aqueous solution during the degradation tests and derivatizing them using o-(2,3,4,5,6-Pentafluorobenzyl)hydroxylamineAll the membranes are characterized in terms hydrochloride of morphology (PFBHA-HCl). by scanning The electron oxime microscopy derivative formed(SEM, Obducat was extracted CamScan from MX2500, water withCambridge, 4 mL hexane. UK) and The transmission extract was electron processed microscopy through (TEM, an acidic FEI washmodel step Tecnai and G12, then Hillsboro, analyzed byOR, GC-MS USA). Sample (Gas Chromatograph preparation for Trace TEM 1300 imaging® coupled have been with carried the Single out Quadrupoleas follows: the Mass membrane Spectrometer samples ISQ QD were®, both embedded purchased in a from proper Thermo epoxy Scientific, resin and Waltham, slices MA,of 100 USA). nm Thethickness GC column were obtained was a non-polar by ultramicrotomy DB5 capillary of column the cured (0.25 resin. mm i.d., 30 m length, supplied by Agilent, SantaFurthermore, Clara, CA, USA). catalytic According content to deposited the EPA method onto the 556, PAN an support internal standardwas investigated (1,2-dibromopropane) through TGA has(SDT been Q600, used TA to Instruments, obtain quantitative New Castle, data. DE, A calibration USA); the curveanalysis has was also performed been built using before alumina starting pan the experimentalwith air flow tests,of 100 by cc/min analyzing and a formaldehyde heating rate of solutions 20 °C/min, of known from room concentration. temperature The to target 1000 analytes°C. The quantification of formaldehyde was carried out by withdrawing samples from the aqueous (oxime derivative, to quantify formaldehyde, and internal standard) were identified (m/z 225 and 121, solution during the degradation tests and derivatizing them using o-(2,3,4,5,6- respectively) and quantified by using a calibration curve. Each data point has been obtained as the Pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA-HCl). The oxime derivative formed was mean value of three replicates. extracted from water with 4 mL hexane. The extract was processed through an acidic wash step and 3.then Results analyzed and Discussionby GC-MS (Gas Chromatograph Trace 1300® coupled with the Single Quadrupole Mass Spectrometer ISQ QD®, both purchased from Thermo Scientific, Waltham, MA, USA). The GC 3.1.column Catalytic was a Membranes non-polar Characterization DB5 capillary column (0.25 mm i.d., 30 m length, supplied by Agilent, Santa Clara,All CA, the USA). produced According membranes to the EPA were method characterized 556, an internal by SEM standard analysis (1,2-dibromopropane) in order to gain a better has been used to obtain quantitative data. A calibration curve has also been built before starting the understanding of the morphology. The SEM micrographs of the PAN and PAN_TiO2 membranes are reportedexperimental in Figure tests,3 .by It analyzing can be observed formaldehyde that PAN soluti nanofibersons of known were smooth, concentration. with a The mean target diameter analytes of (oxime derivative, to quantify formaldehyde, and internal standard) were identified (m/z 225 and ~300 nm. The TiO2 nanoparticles had the tendency to create clusters when spread over the nanofibers 121, respectively) and quantified by using a calibration curve. Each data point has been obtained as the mean value of three replicates.

NanomaterialsNanomaterials 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 7 7 of of 16 16

3.3. ResultsResults andand DiscussionDiscussion

3.1.3.1. CatalyticCatalytic MembranesMembranes CharacterizationCharacterization AllAll thethe producedproduced membranesmembranes werewere characterizedcharacterized byby SEMSEM analysisanalysis inin orderorder toto gaingain aa betterbetter understanding of the morphology. The SEM micrographs of the PAN and PAN_TiO2 membranes are Nanomaterialsunderstanding2020, 10of, 148the morphology. The SEM micrographs of the PAN and PAN_TiO2 membranes7 of are 16 reportedreported inin FigureFigure 3.3. ItIt cancan bebe observedobserved thatthat PAPANN nanofibersnanofibers werewere smooth,smooth, withwith aa meanmean diameterdiameter ofof ~300~300 nm.nm. TheThe TiOTiO22 nanoparticlesnanoparticles hadhad thethe tendencytendency toto createcreate clustersclusters whenwhen spreadspread overover thethe nanofibersnanofibers surfacesurfacesurface andandand somesomesome largelargelarge aggregatesaggregatesaggregates werewerewere formed.formedformed.. However, However, thethe catalyticcatalytic particlesparticles hadhad ananan almostalmostalmost homogeneoushomogeneoushomogeneous distributiondistributiondistribution upon uponupon PAN PANPAN support supportsupport without withoutwithout clogging cloggingclogging it. itit..

((aa)) ((bb)) ((cc))

FigureFigure 3.3. ScanningScanning electron electron microscope microscope (SEM)(SEM) micrographsmicrographs ofof ((aa)) PAN,PAN, ( ((bbb,,c,cc))) PAN_TiO PAN_TiOPAN_TiO222 membranes. membranes.membranes.

TEMTEMTEM images imagesimages of ofof PAN-TiO PAN-TiOPAN-TiO2 are22 are reportedare reportedreported in Figure inin FigureFigure4. In Figure 4.4. InIn 4 Figurea,Figure the cross 4a,4a, sectionsthethe crosscross of sections thesections nanofibers ofof thethe cannanofibersnanofibers be observed; cancan bebe in observed;observed; Figure4b, inin TiO FigureFigure2 nanoparticles 4b,4b, TiOTiO22 nanoparticlesnanoparticles can be seen around cancan bebe seen theseen nanofibers aroundaround thethe (the nanofibersnanofibers cross section (the(the ofcrosscross the section fibersection is ofnotof thethe circular fiberfiber isis due notnot to circularcircular cutting duedue direction); toto cuttingcutting TiO direction);direction);2 clusters were, TiOTiO22 clusters mainly,clusters less were,were, than mainly,mainly, 500 nm. lessless From thanthan SEM500500 nm.nm. and FromFrom TEM SEMSEM analysis andand of TEMTEM PAN_Fe analysisanalysis membranes ofof PAN_FePAN_Fe (Figure membranesmembranes5), it was ((FigureFigure evident 55),), that itit waswas this evidentevident kind of thatthat catalyst thisthis alsokindkind coveredofof catalystcatalyst almost alsoalso completelycoveredcovered almostalmost the nanofibers completelycompletely of thethe PAN; nanofibersnanofibers in contrast ofof PAN; toPAN; what inin happens contrastcontrast fortoto whatwhat TiO2 , happenshappens the clusters forfor hadTiOTiO2 larger2,, thethe clusters dimensionclusters hadhad and largerlarger no spherical dimensiondimension shape. andand The nono dispspffhericalhericalerence betweenshape.shape. TheThe the difference twodifference catalysts betweenbetween is more thethe clear twotwo if thecatalystscatalysts TEM imageisis moremore is analysed:clearclear ifif thethe ferrous TEMTEM imageimage sulfate isis had analysed:analysed: bigger dimensionsferrousferrous sulfatesulfate and hadhad irregular biggerbigger shape, dimensionsdimensions while TiO andand2 appearedirregularirregular shape, inshape, spherical whilewhileclusters TiOTiO22 appearedappeared with submicron inin sphericalspherical mean clustersclusters diameter. withwith submicronsubmicron meanmean diameter.diameter.

((aa)) ((bb))

FigureFigureFigure 4.4.4. TransmissionTransmission electronelectron microscope microscope (TEM) (TEM) images images at at different didifferentﬀerent magniﬁcationsmagnificationsmagnifications ((aa,,,bb)) ofofof NanomaterialsPAN_TiO 2020, 102, membrane.x FOR PEER REVIEW 8 of 16 PAN_TiOPAN_TiO2 2membrane. membrane.

(a) (b) (c)

FigureFigure 5. SEM 5. SEM images images of (a of) PAN_Fe (a) PAN_Fe and and (b) (PAN_TiOb) PAN_TiO2_Fe2 _Femembranes; membranes; (c) (TEMc) TEM image image of of PAN_TiOPAN_TiO2_Fe2_Fe membrane. membrane.

The catalyst content of each prepared membrane was measured by TGA analysis, analyzing the residue at high temperature (1000 °C), taking into account that TiO2 did not lose any weight at that temperature while FeSO4 lost 70% of its initial weight. Based on TGA results, the specific catalyst content for each membrane has been measured; the results are reported in Table 2.

Table 2. Catalyst content of the membranes.

Membrane Catalyst Content (mg/cm2) PAN_TiO2 0.14 PAN_Fe 0.12 PAN_TiO2_Fe 0.19 (53% TiO2 + 47% FeSO4)

3.2. Photocatalytic Degradation of the Formaldehyde

The photocatalytic degradation of formaldehyde, in the presence of TiO2, takes place according to the following reactions (Scheme 1, [33]):

TiO +hυ →e + h (1)

h +OH → OH (2)

e + O → O  (3)

OH +CHO → HO+CHO (4)

CHO + OH → HCOOH (5)

(6) CHO + O  →HCO HCOOOH ⎯⎯ 2 HCOOH

 (7) HCOOH ⎯ HCOO ⎯ HO+CO 

(8) HCOO ⎯ H +CO 

, , (9) CO  ⎯⎯⎯⎯⎯⎯⎯⎯⎯ CO

Scheme 1. Mechanism of photocatalytic degradation of formaldehyde in aqueous solution.

Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 16

(a) (b) (c)

Figure 5. SEM images of (a) PAN_Fe and (b) PAN_TiO2_Fe membranes; (c) TEM image of

PAN_TiONanomaterials2_Fe membrane.2020, 10, 148 8 of 16

The catalyst content of each prepared membrane was measured by TGA analysis, analyzing the residue at highThe temperature catalyst content (1000 of°C), each taking prepared into account membrane that was TiO measured2 did not by lose TGA any analysis, weightanalyzing at that the temperatureresidue while at FeSO high4temperature lost 70% of (1000its initial◦C), takingweight. into Based account on TGA thatTiO results,2 did notthe losespecific any weightcatalyst at that content fortemperature each membrane while has FeSO been4 lost measur 70% ofed; its the initial results weight. are reported Based on in TGATable results, 2. the speciﬁc catalyst content for each membrane has been measured; the results are reported in Table2. Table 2. Catalyst content of the membranes. Table 2. Catalyst content of the membranes. Membrane Catalyst Content (mg/cm2) Membrane Catalyst Content (mg/cm2) PAN_TiO2 0.14 PAN_FePAN_TiO 2 0.12 0.14 PAN_Fe 0.12 PAN_TiO2_Fe 0.19 PAN_TiO2_Fe 0.19 (53% TiO2(53% + 47% TiO FeSO2 + 47%4) FeSO4)

3.2. Photocatalytic3.2. Photocatalytic Degradation Degradation of the Formaldehyde of the Formaldehyde

The photocatalyticThe photocatalytic degradation degradation of formaldehyde, of formaldehyde, in the presence in the presence of TiO of2, TiOtakes2, takes place place according according to to the followingthe following reactions reactions (Scheme (Scheme 1, [33]):1,[ 33]):

TiO +hυ →e + h (1)

h +OH → OH (2)

e + O → O  (3)

OH +CHO → HO+CHO (4)

CHO + OH → HCOOH (5)

(6) CHO + O  →HCO HCOOOH ⎯⎯ 2 HCOOH

 (7) HCOOH ⎯ HCOO ⎯ HO+CO 

(8) HCOO ⎯ H +CO 

, , (9) CO  ⎯⎯⎯⎯⎯⎯⎯⎯⎯ CO

Scheme 1. Mechanism of photocatalytic degradation of formaldehyde in aqueous solution. Scheme 1. Mechanism of photocatalytic degradation of formaldehyde in aqueous solution.

The results for catalytic degradation with PAN_TiO2 membrane are reported in Figure6; PAN_TiO 2 2 membrane contained 0.14 mg/cm of TiO2 (i.e., optimal composition deﬁned elsewhere [32]) which corresponded to a total amount of TiO2 of 7 mg in the system. Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 16

Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 16 The results for catalytic degradation with PAN_TiO2 membrane are reported in Figure 6; PAN_TiO2 membrane contained 0.14 mg/cm2 of TiO2 (i.e., optimal composition defined elsewhere The results for catalytic degradation with PAN_TiO2 membrane are reported in Figure 6; [32]) which corresponded to a total amount of TiO2 of 7 mg in the system. PAN_TiO2 membrane contained 0.14 mg/cm2 of TiO2 (i.e., optimal composition defined elsewhere [32]) which corresponded to a total amount of TiO2 of 7 mg in the system. Nanomaterials 2020, 10, 148 9 of 16

PAN_TiO2 H2O2 PAN_TiO2+H2O2 100 PAN_TiO2 H2O2 PAN_TiO2+H2O2 100 80

80 60

60 40

formaldehyde conc [ppm] 20

formaldehyde conc [ppm] 20 0 0 50 100 150 200 250 0 time [min] 0 50 100 150 200 250 time [min]

Figure 6. FormaldehydeFigure 6. Formaldehyde concentration concentrationduring photocatalytic during photocatalyticexperiment with experiment and without with added and without added H2O2 (400 ppm). The maximum standard deviation of the experimental points was 1.9 ppm. HFigure2O2 (400 6. ppm).Formaldehyde The maximum concentration standard du deviationring photocatalytic of the experimental experime pointsnt with was and 1.9 without ppm. added

H2O2 (400 ppm). The maximum standard deviation of the experimental points was 1.9 ppm. For heterogeneousFor heterogeneous catalytic system catalytic PAN_TiO system2, an PAN_TiO increase2, in an the increase formaldehyde in the formaldehyde content was content was observed duringobserved the first during stage the of ﬁrst degradation. stage of degradation. This can be Thisrationalized can be rationalizedconsidering consideringthat the that the For heterogeneous catalytic system PAN_TiO2, an increase in the formaldehyde content was degradation degradationofobserved methanol during of(which methanol the was first contained (which stage was ofat degradation.10 contained wt% in the at This10 formalin wt% can in besolution the rationalized formalin as stabilizer solution considering to as stabilizer that the limit formaldehydetodegradation limit polymerization) formaldehyde of methanol led polymerization) to(which the form wasation contained led of toformaldehyde, the at formation10 wt% accordingin ofthe formaldehyde, formalin to the solutionfollowing according as stabilizer to the to reactions (Schemefollowinglimit 2, formaldehyde [34]): reactions (Scheme polymerization)2,[34]): led to the formation of formaldehyde, according to the following reactions (Scheme 2, [34]): CHOH + OH → CHOH + HO (10)

CHOH + OCH → OHCH +O+HO OH → CHOH + HO (11) (10)

CHO+OH →HCOOH+HCHOH + O → CHO+HO (12) (11)

Scheme 2. MethanolScheme CHphotocatalytic 2.MethanolO+OH photocatalytic→HCOOH+Hdegradation path. degradation path. (12) Scheme 2. Methanol photocatalytic degradation path. The degradationThe of degradation the methanol of thecan methanolstart as soon can as start a hydroxyl as soon radical as a hydroxyl is produced radical and isit may produced and it may compete with formaldehyde to consume hydroxyl radicals resulting in the retardation of the compete with formaldehydeThe degradation to consume of the methanol hydroxyl can radicals start as resultingsoon as a hydroxylin the retardation radical is producedof the and it may oxidation reaction of formaldehyde. Both methanol and formaldehyde formed formic acid during oxidation reactioncompete of formaldehyde. with formaldehyde Both methanol to consume and formaldehydehydroxyl radicals formed resulting formic inacid the during retardation of the their degradation. Indeed, the initial pH of the solution was around 6.5–7, and it was not adjusted, their degradation.oxidation Indeed, reaction the initial of formaldehyde. pH of the solution Both wasmethanol around and 6.5–7, formaldehyde and it was formednot adjusted, formic acid during owing to the pH-independent photocatalytic activity of TiO [19], but during the test, it decreased owing to the pH-independenttheir degradation. ph Indeed,otocatalytic the initialactivity pH of ofTiO the2 [19], solution but during was2 around the test, 6.5–7, it decreased and it was to not adjusted, 4.2 and thento rose 4.2 to and 5, thenas already rose to reported 5, as already in similar reported studies in similar [6,7], studiesthus implying [6,7], thus the implyingformation the of formation of owing to the pH-independent photocatalytic activity of TiO2 [19], but during the test, it decreased to formic acid. Therefore, we can assess that, since both methanol and formaldehyde were oxidized to formic acid. Therefore,4.2 and then we canrose assess to 5, asthat, already since bothreported methanol in similar and formaldehydestudies [6,7], thus were implying oxidized the to formation of formic acid (and then to CO and water), these compounds were responsible for the drop in the pH of formic acid (andformic then acid. to CO Therefore,2 and water), we2 canthese assess compounds that, since were both responsible methanol forand the formaldehyde drop in the pHwere oxidized to of the solution.the As solution. CO2 was As liberated CO2 was from liberated the solution, from the pH solution, increased pH [7]. increased [7]. formic acid (and then to CO2 and water), these compounds were responsible for the drop in the pH However, it wasHowever, evident it wasthat evidentafter 4 h that the afterabatem 4 hent the of abatement the formaldehyde of the formaldehyde content was contentstill low. was still low. of the solution. As CO2 was liberated from the solution, pH increased [7]. The addition of H O (400 ppm, according to optimal content reported in [11]) in aqueous solution The addition However,of H2O2 (400 it was ppm,2 evident2 according that after to optimal4 h the abatem contentent reported of the formaldehyde in [11]) in aqueous content was still low. solution to theto thePAN-TiO PAN-TiO2 system2 system (PAN_TiO (PAN_TiO2 + 2H+2OH2 2inO 2Figurein Figure 6) resulted6) resulted in ingreater greater oxidation oxidation with with respect to The addition of H2O2 (400 ppm, according to optimal content reported in [11]) in aqueous respect to neatneat PAN-TiO PAN-TiO2 and2 and this this was was likely likely attributed attributed to the to the enhanced enhanced charge charge separation separation induced induced by H2O2 solution to the PAN-TiO2 system (PAN_TiO2 + H2O2 in Figure 6) resulted in greater oxidation with by H2O2 as anas anelectron electron acceptor acceptor and and the the reductive reductive conversion conversion of ofH2 HO2 Oto2 OH•to OH [7].[7 Moreover,]. Moreover, the the methanol respect to neat PAN-TiO2 and this was likely attributed to the enhanced• charge separation induced degradation was very fast and the increase in the formaldehyde content was not revealed (since the by H2O2 as an electron acceptor and the reductive conversion of H2O2 to OH• [7]. Moreover, the ﬁrst sampling point was after 1 h). Comparing homogeneous system using H2O2 only (400 ppm, without catalytic membrane, curve H 2O2 in Figure6) with the previous heterogeneous one (PAN_TiO 2 + H2O2), it can be noted that the higher removal rate was mainly due, especially in the ﬁrst 2 h, to the direct oxidation of H2O2 onto the organic substrate. The trend of the degradation of formaldehyde with only H2O2 was very similar to that reported by Guimarães et al. [7]. Since also in H2O2-system the pH decreased from 6.5 to 3.8, Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 16 methanol degradation was very fast and the increase in the formaldehyde content was not revealed (since the first sampling point was after 1 h). Comparing homogeneous system using H2O2 only (400 ppm, without catalytic membrane, curve H2O2 in Figure 6) with the previous heterogeneous one (PAN_TiO2 + H2O2), it can be noted that the higher removal rate was mainly due, especially in the first 2 h, to the direct oxidation of H2O2 onto the organic substrate. The trend of the degradation of formaldehyde with only H2O2 was very similar Nanomaterials 2020, 10, 148 10 of 16 to that reported by Guimarães et al. [7]. Since also in H2O2-system the pH decreased from 6.5 to 3.8, the formation of formic acid can still be speculated. After the H2O2 was completely consumed, the degradation thereaction formation stopped of formicand the acid CH2 canO concentration still be speculated. reached After a plateau the H while2O2 was in the completely presence consumed,of the TiO2 it seemeddegradation to further reactiondecrease. stopped and the CH2O concentration reached a plateau while in the presence of TiO2 it seemed to further decrease. 3.3. Photo-Fenton Degradation of Formaldehyde 3.3. Photo-Fenton Degradation of Formaldehyde Photo-Fenton degradation of formaldehyde was also studied. Several catalytic, homogeneous and heterogeneousPhoto-Fenton systems have degradation been compared, of formaldehyde to gain deep was understanding also studied. of Severalthe process. catalytic, In this homogeneous work, we referredand heterogeneous to homogeneous systems process have when been the compared, catalyst to(Fe gain2+) and/or deep understanding oxidant (H2O2) of were the process.in In this 2+ the same (liquid)work, wephase referred of formaldehyde to homogeneous while process the whenheterogeneous the catalyst one (Fe referred) and/or to oxidant the use (H2 Oof2 ) were in the supported Fesame catalyst. (liquid) phase of formaldehyde while the heterogeneous one referred to the use of supported GenerallyFe speaking, catalyst. photo-Fenton systems are based on the contemporary use of Fe ions and hydrogen peroxide:Generally ferrous speaking,ions react photo-Fenton with hydrogen systems peroxide are under based onthe theirradiation contemporary of UV uselight, of Fe ions and producing hydroxylhydrogen radicals peroxide: with ferrouspowerful ions oxidizing react with abilities. hydrogen While peroxide in the Fenton under reaction the irradiation Fe3+ ions of UV light, accumulate inproducing the system hydroxyl and theradicals reaction with stops powerful once all oxidizingFe2+ ions are abilities. consumed, While in in the the photo-Fenton Fenton reaction Fe3+ ions reaction the regenerationaccumulate in of the ferrous system ions and by thephoto-re reactionduction stops of once ferric all ions Fe2+ occursions are according consumed, to Scheme in the photo-Fenton 3: reaction the regeneration of ferrous ions by photo-reduction of ferric ions occurs according to Scheme3:

Fe + HO → Fe +OH +OH (13)

Fe +hυ+HO → OH + Fe +H (14)

Scheme 3. Photo-FentonScheme 3. reactions.Photo-Fenton reactions.

The newly generated ferrous ions react again with H2O2 generating hydroxyl radical and ferric The newly generated ferrous ions react again with H2O2 generating hydroxyl radical and ferric ion and, in this way, the cycle continues. Therefore, the rate of degradation of organic pollutants in ion and, in this way, the cycle continues. Therefore, the rate of degradation of organic pollutants in photo-Fenton reaction increases with respect to the Fenton one [35]. The photo-Fenton process offers photo-Fenton reaction increases with respect to the Fenton one [35]. The photo-Fenton process offers better performance at pH 3.0, when the hydroxyl-Fe3+ complexes are more soluble and Fe(OH)2+ is better performance at pH 3.0, when the hydroxyl-Fe3+ complexes are more soluble and Fe(OH)2+ is more photoactive [36]. However, although ferrous ions are regenerated, generally, a large amount of more photoactive [36]. However, although ferrous ions are regenerated, generally, a large amount of H2O2 needs to be irreversibly consumed. H2O2 needs to be irreversibly consumed. Results for formaldehyde degradation in a homogeneous photo-Fenton process, with and without Results for formaldehyde degradation in a homogeneous photo-Fenton process, with and added H2O2 (Fe/H2O2 and Fe curves, respectively) as well as heterogeneous one (PAN_Fe curve) are without added H2O2 (Fe/H2O2 and Fe curves, respectively) as well as heterogeneous one (PAN_Fe reported in Figure7; homogeneous photolytic oxidation (H 2O2 curve in Figure7) is also reported for curve) are reported in Figure 7; homogeneous photolytic oxidation (H2O2 curve in Figure 7) is also comparison. In photo-Fenton processes (i.e., whenever Fe was present) the pH has been adjusted to 3 reported for comparison. In photo-Fenton processes (i.e., whenever Fe was present) the pH has been using 0.2 N sulfuric acid. When ferrous sulfate and H2O2 were used in solution, their concentrations adjusted to 3 using 0.2 N sulfuric acid. When ferrous sulfate and H2O2 were used in solution, their were 24 and 400 ppm, respectively, i.e., set at the optimal conditions identified elsewhere [11]. The concentrations were 24 and 400 ppm, respectively, i.e., set at 2the optimal conditions identified heterogeneous PAN-Fe catalyst comprised 0.12 mg/cm of FeSO4 (i.e., similar content to the TiO2 content elsewhere [11]. The heterogeneous PAN-Fe catalyst comprised 0.12 mg/cm2 of FeSO4 (i.e., similar of the previous experiment, i.e., optimal composition defined elsewhere [32]), which corresponded to a content to the TiO2 content of the previous experiment, i.e., optimal composition defined elsewhere total of 6 mg of ferrous sulfate in the system. All the trials have been carried out under UV irradiation; [32]), which corresponded to a total of 6 mg of ferrous sulfate in the system. All the trials have been it shall be kept in mind that in presence of iron sulfate the UV absorbance of the system around 290 nm carried out under UV irradiation; it shall be kept in mind that in presence of iron sulfate the UV was quite high, owing to the absorption properties of this compound [23]. absorbance of the system around 290 nm was quite high, owing to the absorption properties of this Within the homogeneous processes (curves Fe, H O and Fe/H O in Figure7), the photo-Fenton compound [23]. 2 2 2 2 Fe/H2O2 demonstrated the best performance; however, also the use of iron compound alone (Fe curve), without adding H2O2, gave good results since the final formaldehyde concentration was the same as for Fe/H2O2, although the initial rate was lower. Similar results have already been reported by other authors [29,37] for different compounds, i.e., the addition of H2O2 to Fe catalyst was beneficial, but the degradation took place also without adding H2O2.

Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 16

100

Fe/H2O2 Fe 80 PAN_Fe Nanomaterials 2020, 10, x FOR PEER REVIEW H2O2 11 of 16 Nanomaterials 2020, 10, 148 11 of 16 60

100 40 Fe/H2O2 Fe 80 20 PAN_Fe H2O2 Formaldehyde concentration [ppm] 60 0 0 50 100 150 200 250 time [min] 40

Figure 7. Formaldehyde concentration during photolysis and photo-Fenton experiment. The 20 maximum standard deviation of the experimental points was 2.5 ppm. Formaldehyde concentration [ppm]

Within the homogeneous processes 0(curves Fe, H2O2 and Fe/H2O2 in Figure 7), the photo-Fenton 0 50 100 150 200 250 Fe/H2O2 demonstrated the best performance; however, also the use of iron compound alone (Fe time [min] curve), without adding H2O2, gave good results since the final formaldehyde concentration was the same as for Fe/H2O2,Figure although 7. Formaldehyde the initial rate concentration was lower. during Similar photolysis results and have photo-Fenton already been experiment. reported The maximum by other authors [29,37]standardFigure for deviation7. different Formaldehyde of thecompounds, experimental concentration i.e., points the during wasaddition 2.5 photol ppm. ofysis H2 Oand2 to photo-Fenton Fe catalyst wasexperiment. The beneficial, but the degradationmaximum tookstandard place deviation also without of the experimentaladding H2O points2. was 2.5 ppm. There can be severalThere reasons can be severalfor this. reasons First, it forshould this. also First, be it kept should in mind also be that kept small in mind amount that of small amount of

H2O2 can also beH 2developedO2 canWithin also theduring be developedhomogeneous UV irradiation during processes UV ofirradiation formaldehyde (curves Fe, of formaldehyde H [9].2O2 andSecondly, Fe/H [92 ].Oferrous2 Secondly, in Figure ion ferrous 7),can the photo-Fenton ion can react react with dissolvedwithFe/H dissolved 2oxygenO2 demonstrated in oxygen water in (Scheme waterthe best (Scheme 4)perfor forming4)mance; forming reactive however, reactive oxidants oxidantsalso thecapable capableuse ofof iron ofoxidative oxidative compound degradation. alone (Fe 2+2+ degradation. Indeed,Indeed,curve), superoxide superoxidewithout adding radical radical Hanions2 anionsO2, gave can can goodform, form, results by by reaction reaction since ofthe of Fe Fe final withwith formaldehyde oxygen oxygen of of the theconcentration air air dissolved was into the 22++ dissolved into thethesame solution solution as for [6], [ 6Fe/H], leading 2O2, although to the formationformation the initial ofof rate HH22O Owas22 [[38];38 lower.]; moreover,moreover, Similar FeFe resultsinin the the have UV already light conditions, been reported can 3+ 3+ 3+ 3+ 2+ conditions, canbe bybe partially otherpartia llyauthors transformed transformed [29,37] into forinto Fe different ,Fe the, Fethe compounds, inFe solution in solution i.e., can bethe can hydrolyzed addition be hydrolyzed of to H hydroxylable2O2 toto Fe catalyst Fe(OH) was, 2+ 2+ 2+ 2+ 2+ hydroxylable Fe(OH)Febeneficial, (OH), Fealso but(OH) canthe bedegradationalso transformed can be transformedtook into place Fe also underinto without Fe ultraviolet under adding ultraviolet light, H2O while2. light, producing while OH radicals to 3+ 2+ producing OH radicalsform aThere Fe to form/ Fecan abecycle Fe several3+/Fe of2+ reactions cycle reasons of reactions [for11, 39this.,40 ]: First,[11,39,40]: it should also be kept in mind that small amount of H2O2 can also be developed during UV irradiation of formaldehyde [9]. Secondly, ferrous ion can react with dissolved Feoxygen+ O in →water Fe (Scheme+ O  4) forming reactive oxidants(15) capable of oxidative degradation. Indeed, superoxide radical anions can form, by reaction of Fe2+ with oxygen of the air Fe + O  + 2H → Fe + HO (16) dissolved into the solution [6], leading to the formation of H2O2 [38]; moreover, Fe2+ in the UV light conditions, can beFe partia+ HllyO transformed → Fe(OH) +into H Fe 3+, the Fe3+ in solution can(17) be hydrolyzed to hydroxylable Fe(OH)2+Fe(OH), Fe (OH) →2+ alsoFe can+OH be transformed into Fe2+ under ultraviolet(18) light, while producing OH radicals to form a Fe3+/Fe2+ cycle of reactions [11,39,40]: Scheme 4. Reactions between ferrous ion and dissolved oxygen. Scheme 4. Reactions between ferrous ion and dissolved oxygen. Fe + O → Fe + O (15) Therefore, in the presence of Fe2+ and UV light several reactions can take place to generate 2+ Therefore, in the presence of Fe and UV light several reactions can take place to generate oxidizing species- (OH ,O2− and HFe2O2+) without O  + the2H need → Fe to add+ externalHO hydrogen peroxide. (16) oxidizing species (OH•,O  and H2O2•) without• the need to add external hydrogen peroxide. Lastly,2 it could also be possible that the formic acid, deriving from degradation of formaldehyde, Lastly, it could also be possible that the formic acid,Fe deriving+ H O from → Fe(OH) degradation + H of formaldehyde, (17) created a complex compound with iron [41] which could be photolyzed and even photocatalyzed in created a complex compound with iron [41] which 2could+ be photolyzed and even photocatalyzed in the reaction medium, generating Fe ,HFe(OH)2O2 andother → Fe active+OH radical species [38], in a similar way(18) to the reaction medium, generating Fe2+, H2O2 and other active radical species [38], in a similar way to that reported for oxalic acid by Quici et al. [26]. that reported for oxalic acid by Quici etScheme al. [26]. 4. Reactions between ferrous ion and dissolved oxygen. The eﬀectiveness of Fe catalyst also without H2O2 is very important since avoiding the use of The effectiveness of Fe catalyst also without H2O2 is very important since avoiding the use of H2O2 makes the process more economic and safer. H2O2 makes the processTherefore, more economic in the presenceand safer. of Fe2+ and UV light several reactions can take place to generate Also, a heterogeneous photo-Fenton- process, i.e., using a supported Fe catalyst (PAN_Fe), has been oxidizing species (OH•,O  and H2O2) without the need to add external hydrogen peroxide. shown to be active in formaldehyde2 degradation; however, as expected, the reaction rate was slower Lastly, it could also be possible that the formic acid, deriving from degradation of formaldehyde, with respect to homogenous systems. This PAN_Fe system reached the same ﬁnal formaldehyde content created a complex compound with iron [41] which could be photolyzed and even photocatalyzed in as photolytic oxidation (H2O2 curve) process (but without the use of costly and hazardous reagents the reaction medium, generating Fe2+, H2O2 and other active radical species [38], in a similar way to like water peroxide) and only slightly higher concentration than homogenous photo-Fenton processes. that reported for oxalic acid by Quici et al. [26]. The effectiveness of Fe catalyst also without H2O2 is very important since avoiding the use of H2O2 makes the process more economic and safer.

Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 16

Also, a heterogeneous photo-Fenton process, i.e., using a supported Fe catalyst (PAN_Fe), has been shown to be active in formaldehyde degradation; however, as expected, the reaction rate was slower with respect to homogenous systems. This PAN_Fe system reached the same final formaldehyde content as photolytic oxidation (H2O2 curve) process (but without the use of costly and hazardous reagents like water peroxide) and only slightly higher concentration than homogenous photo-Fenton processes. Nanomaterials 2020, 10, 148 12 of 16 3.4. Photo-Catalytic-Fenton Degradation of Formaldehyde 3.4. Photo-Catalytic-FentonAfter the separate analysis Degradation of photocatalytic of Formaldehyde and photo-Fenton processes, the combination of suchAfter AOPs the has separate been studied. analysis of photocatalytic and photo-Fenton processes, the combination of such AOPsFigure has been 8 studied.reports the results when using a heterogeneous system based on photo-Fenton (PAN_Fe),Figure8 reportsphotocatalysis the results (PAN_TiO when using2) aand heterogeneous their combination system based (PAN_TiO on photo-Fenton2_Fe), under (PAN_Fe), UV illumination, pH = 3, without the use of any added H2O2. It can be noted that the combination of TiO2 photocatalysis (PAN_TiO2) and their combination (PAN_TiO2_Fe), under UV illumination, pH = 3, and Fe2+ showed the best result, i.e., there was a synergy deriving from the combination2+ of without the use of any added H2O2. It can be noted that the combination of TiO2 and Fe showed the bestphotocatalysis result, i.e., there and photo-Fenton was a synergy since deriving the combined from the combination process gave of better photocatalysis results than and the photo-Fenton simple sum of the two distinct processes. since the combined process gave better results than the simple sum of the two distinct processes.

100

PAN_TiO formaldehyde conc [ppm] formaldehyde 2 20 PAN_Fe PAN_TiO2 _Fe 0 0 50 100 150 200 250 time [min]

Figure 8. Formaldehyde concentration profile using heterogeneous photo-Fenton (PAN_Fe), photocatalyticFigure 8. Formaldehyde (PAN_TiO2) andconcentration their combination profile (PAN_TiO using heterogeneous2_Fe) catalysts. photo-Fenton The maximum (PAN_Fe), standard deviation of the experimental points was 1.9 ppm. photocatalytic (PAN_TiO2) and their combination (PAN_TiO2_Fe) catalysts. The maximum standard deviation of the experimental points was 1.9 ppm. This synergy can be due to the fact that in presence of iron sulfate the UV absorbance of the system around 290 nm increased, owing to the absorption properties of this compound, therefore This synergy can be due to the fact that in presence of iron sulfate the UV absorbance of the favoring UV activation [23]. Moreover, this synergy can be related to the roles of iron as an electron system around 290 nm increased, owing to the absorption properties of this compound, therefore acceptor, to facilitate charge separation in TiO photocatalyst, thereby reducing the non-desired favoring UV activation [23]. Moreover, this synergy2 can be related to the roles of iron as an electron electron/hole recombination and thus increasing the rate of OH formation through the first and second acceptor, to facilitate charge separation in TiO2 photocatalyst,• thereby reducing the non-desired reaction in Scheme1. Similar results have been reported for homogeneous TiO -Fe system in phenol electron/hole recombination and thus increasing the rate of OH• formation2 through the first and degradation [19], for Azo dye [25] and for bisphenol A [15] and for Fe doped TiO2 [20,21,23]. second reaction in Scheme 1. Similar results have been reported for homogeneous TiO2-Fe system in 3+ 2+ It was also reported that in this system TiO2 is beneficial to the fast conversion of Fe Fe [15], phenol degradation [19], for Azo dye [25] and for bisphenol A [15] and for Fe doped TiO→2 [20,21,23]. thus favoring the closing the Fe3+/Fe2+ cycle of reactions mentioned before when dealing with It was also reported that in this system TiO2 is beneficial to the fast conversion of Fe3+→Fe2+ [15], photo-Fenton. Also, the accumulated electrons in the conduction band of TiO are transferred to thus favoring the closing the Fe3+/Fe2+ cycle of reactions mentioned before when 2dealing with photo- oxygen (coming from the air in contact with formaldehyde solution) on the TiO2 surface for the Fenton. Also, the accumulated electrons in the conduction band of TiO2 are transferred to oxygen 2 2 + formation of O − or O2 −, which combines with H to form H2O2 [28]. The evolved H2O2 with FeSO4 (coming from the air in contact with formaldehyde solution) on the TiO2 surface for the formation of formed Fenton reagent which can further degrade formaldehyde [28]. O2− or O22−, which combines with H+ to form H2O2 [28]. The evolved H2O2 with FeSO4 formed Fenton reagentSummarizing, which can thefurther presence degrade of iron formaldehyde was beneficial [28]. in promoting the photocatalytic activity of TiO2 while TiO2 was beneficial in promoting the photo-Fenton reaction. If we compare the results obtained with a fully heterogeneous system (PAN_TiO2_Fe, where Fe is deposited onto the support) and partly homogeneous one (PAN_TiO2+Fe, where 24 ppm of ferrous sulfate was in the aqueous solution, as before in photo-Fenton trials) (Figure9), it can be noted that the degradation rate was slightly faster for the latter, probably owing to a higher photo-Fenton reaction rate; however, both systems reached a final CH2O concentration of around 10 ppm. Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 16

Summarizing, the presence of iron was beneficial in promoting the photocatalytic activity of TiO2 while TiO2 was beneficial in promoting the photo-Fenton reaction. If we compare the results obtained with a fully heterogeneous system (PAN_TiO2_Fe, where Fe is deposited onto the support) and partly homogeneous one (PAN_TiO2+Fe, where 24 ppm of ferrous sulfate was in the aqueous solution, as before in photo-Fenton trials) (Figure 9), it can be noted that the degradation rate was slightly faster for the latter, probably owing to a higher photo-Fenton reaction rate; however, both systems reached a final CH2O concentration of around 10 ppm. Nanomaterials 2020, 10, 148 13 of 16

100

PAN_TiO2 + Fe

PAN_TiO2 _ Fe 80

formaldehyde conc [ppm] conc formaldehyde 20

0 0 50 100 150 200 250 time [min]

Figure 9. Formaldehyde concentration profile using heterogeneous photo-catalytic-Fenton with Fe supportedFigure 9. (PAN_TiOFormaldehyde2_Fe) orconcentration dispersed in profile solution using (PAN_TiO heterogeneous2+Fe). The photo-catalytic-Fenton maximum standard deviation with Fe of the experimental points was 1.8 ppm. supported (PAN_TiO2_Fe) or dispersed in solution (PAN_TiO2+Fe). The maximum standard deviation of the experimental points was 1.8 ppm. However, if we compare the results on a catalyst weight basis, i.e., considering the moles of formaldehydeHowever, degraded if we compare per mg the of results catalyst, on it a can catalyst be shown weight (Figure basis, 10 i.e.,) that considering up to 3 h the there moles were of Nanomaterials 2020, 10, x FOR PEER REVIEW 14 of 16 noformaldehyde significant di degradedfferences per for PAN_Femg of catalyst, and PAN_TiO it can be2 shown_Fe; however, (Figure 10) over that a longerup to 3 period h there the were best no performing system was PAN_Fe. Obviously, this was an initial work and further studies on the optimal significant differences for PAN_Fe and PAN_TiO2_Fe; however, over a longer period the best ironperforming content needsystem to bewas carried PAN_Fe. out. Obviously, this was an initial work and further studies on the

optimal iron content need12 to be carried out. PAN_TiO2 10 PAN_Fe PAN_TiO2 _Fe 8

0 moles degraded/mg catalyst [mol/mg] catalyst degraded/mg moles

0 50 100 150 200 250 time [min]

Figure 10. Formaldehyde degraded per mg of catalyst for heterogeneous photo-Fenton (PAN_Fe), photocatalytic (PAN_TiO ) and their combination (PAN_TiO _Fe) catalysts. Figure 10. Formaldehyde2 degraded per mg of catalyst for2 heterogeneous photo-Fenton (PAN_Fe), 4. Conclusionsphotocatalytic (PAN_TiO2) and their combination (PAN_TiO2_Fe) catalysts.

4. ConclusionsPhotocatalytic and photo-Fenton processes as well as their combination were studied to degrade formaldehyde aqueous solution; both homogeneous and heterogeneous catalysts were used. ThePhotocatalytic results showed and that photo-Fenton the combination processes of AOPs as we gavell as their a synergy combination since the were presence studied of ironto degrade was formaldehyde aqueous solution; both homogeneous and heterogeneous catalysts were used. beneﬁcial in promoting the photocatalytic activity of TiO2 while TiO2 was beneﬁcial in promoting the photo-FentonThe results reaction. showed that the combination of AOPs gave a synergy since the presence of iron was beneficial in promoting the photocatalytic activity of TiO2 while TiO2 was beneficial in promoting the photo-Fenton reaction. Moreover, very good results have been obtained using fully heterogeneous nanostructured catalysts (based on TiO2 and FeSO4) without the need to add H2O2. In this way, a more economic formaldehyde treatment process has been developed.

Funding: The research leading to these results has received funding from University of Padova, Department of Industrial Engineering, Twinning Project 2017.

Conflicts of Interest: The authors declare no conflict of interest.

References

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Nanomaterials 2020, 10, 148 14 of 16

Moreover, very good results have been obtained using fully heterogeneous nanostructured catalysts (based on TiO2 and FeSO4) without the need to add H2O2. In this way, a more economic formaldehyde treatment process has been developed.

Author Contributions: Conceptualization, R.B., M.R., A.M. and A.L.; Data curation, R.B. and C.B.; Funding acquisition, A.L.; Investigation, C.B. and L.C.; Methodology, R.B., M.R., A.M. and A.L.; Project administration, A.L.; Supervision, R.B.; Visualization, R.B.; Writing—original draft, R.B., C.B. and L.C.; Writing—review and editing, M.R., A.M., M.M. and A.L. All authors have read and agreed to the published version of the manuscript. Funding: The research leading to these results has received funding from University of Padova, Department of Industrial Engineering, Twinning Project 2017. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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