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UVA-Induced Processes in the Aqueous Dioxide Suspensions Containing Nitrite (An EPR Spin Trapping Study)

Vlasta Brezová*, Zuzana Barbieriková, Dana Dvoranová, Andrej Staško

Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Technology, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovak Republic

Abstract: Application of TiO2 photocatalytic systems for purification and remediation is based on the generation of short-lived reactive species able to destroy a variety of contaminants, upon the ultra-bandgap irradiation of TiO2 particles in the aerated aqueous media. However the recently more profound presence of inorganic compounds can affect these processes due to the complex photochemical behavior of the nitrite and nitrate in aqueous solutions. The effect of the nitrite present in the titanium dioxide suspensions was monitored via the reactive radical intermediates detected by EPR spin trapping technique. Various spin trapping agents were applied to follow the changes in the behavior of the system caused by the nitrite upon UVA irradiation and the limits of the spin trapping technique itself were also considered. The competition reaction of the photo- generated holes and hydroxyl radicals with the nitrite was revealed as the dominant process occurring in the studied systems.

Keywords: EPR ; Spin trapping; ; Titanium dioxide; Nitrite

Introduction ) with a potential risk for public Nowadays, growing activity and interest can be health. The Environmental Protection Agency has recognized in the field of photocatalytic system adopted the 10 mg L–1 standard as the maximum application for purification and remediation of polluted contaminant level for nitrate and 1 mg L–1 for nitrite environments (1-4). Among the semiconducting for the regulated public water systems (6). High levels materials used as photocatalysts, titanium dioxide still of nitrate ions, converted in the human organism to attracts substantial attention, since a variety of toxic nitrites, may result in a decreased oxygen transport organic/inorganic pollutants can be efficiently (methaemoglobinaemia), as well as in the generation decomposed using powdered or immobilized titania of nitroso derivatives, which are potent (3-5). The unique physicochemical properties of (7). titanium dioxide polymorphs (, , ) Nitrite absorbs UVA radiation (max = 354 nm, – –1 –1 and their ability to produce electron – hole pairs (e – max = 22.7 M cm (8, 9)) and undergoes direct h+) upon UV photoexcitation, which are involved in a photolysis generating nitric and hydroxyl radical series of consecutive chemical reactions, explains the (8). The photochemical processes of nitrite play an continuous intensive research of TiO2 (3-5). The important role in the natural aquatic systems (10-15): complex processes of charge carriers (excitation, bulk NO–– + h  NO + O (1) diffusion, surface transfer) are substantially influenced 2 O–– H O OH + OH (2) by the titania structure, particle size, 2 morphology and porosity. In the aerated aqueous The recombination reaction of paramagnetic nitric  media the ultra-bandgap irradiation of TiO2 particles oxide with hydroxyl radicals, as well as OH reaction results in the generation of short-lived reactive oxygen with nitrite represent diffusion-controlled processes  –  10 –1 –1 species ( OH, O2 / O2H, singlet oxygen) which are (k3,4 = 1.010 M s (8)): able to destroy a variety of contaminants (3). NO + OH HNO (3) The worldwide expansion of agricultural and 2 –– industrial activities have caused an increased NO22 + OH  NO + OH (4) contamination of surface and ground water sources by radicals generated may dimerize the inorganic nitrogen compounds (nitrate, nitrite, to N2O4, or may enter in the reaction with

producing N2O3. The hydrolytic reactions of these *Corresponding author; E-mail address: [email protected] species produce nitrite and nitrate (8, 16):

ISSN 1203-8407 © 2016 Science & Technology Network, Inc. J. Adv. Oxid. Technol. Vol. 19, No. 2, 2016 290 V. Brezová et al.

 NO– + h NO (9) 2NO2 N 2 O 4 (5) 22

 –– NO + NO2 N 2 O 3 (6) NO2 + OH surf NO 2 + OH (10) N O H O NO– + NO – + 2 H + (7) 2 4 2 2 3 In the previous studies of UV-irradiated aqueous –+ N2 O 3 H 2 O 2 NO 2 + 2 H (8) titania suspensions, the photocatalytic oxidation of nitrite to nitrate was ascribed to the reactions of The complex photochemical behavior of nitrite photogenerated hydroxyl radicals; ammonium ions and nitrate in aqueous solutions was reviewed were not detected (23). The formation of peroxynitrite previously (8). The quantum yield of the hydroxyl (ONOO–), i.e. the product of nitric oxide reaction radical generation upon nitrite photolysis is low with radical anion was also proposed, (~ 0.01), consequently the aqueous nitrite solutions considering the nitrite reduction via the photogenerated were recommended for a long-term irradiation. electrons coupled with the production of nitric oxide Nitrate/nitrite UV actinometers were suggested based (23): on the monitoring of the reaction products of the – – 2– NO22 + e NO (11) photogenerated OH with benzoic acid, i.e. salicylic 2– – and p-hydroxybenzoic acids (9). Recent investigations NO2 + H NO + OH (12) on the photoinduced processes of nitrite in aqueous Despite a number of EPR spin trapping studies on solutions in the presence of hydroxyl radical scavengers the formation of reactive paramagnetic intermediates (phenol or 2-propanol) revealed a significant role of  upon UVA-photoactivation of nitrite in the aerated dissolved oxygen concentration and nature of OH aqueous solutions (18, 21), analogous experiments in scavenger on the nitrite phototransformation coupled the irradiated TiO2 suspensions are missing in the with the production of N-containing products in the literature. Our EPR investigations are oriented on the solution and in the gas phase (16). application of different spin trapping agents to monitor The aerated aqueous nitrite solutions and TiO2 the effect of nitrite on the spin-adduct formation, and suspensions represent systems producing reactive on the photocatalytic reduction of 2,2’-azino-bis(3- radical species upon UVA-photoactivation, consequently ethylbenzothiazoline-6-sulfonate) cation radical the EPR spin trapping technique using 5,5-dimethyl-1- (ABTS+) in the aqueous titania suspensions. pyrroline N-oxide (DMPO) spin trap was successfully  applied to monitor the photoinduced OH generation Experimental and Methods in both systems (17-20). Additionally, the formation Materials of nitrogen dioxide radical during nitrite UVA The commercial titanium dioxide Aeroxide® P25 photolysis ( = 360 nm) was monitored in alkaline (Evonik Degussa, Germany) was used and the stock media (pH > 12) using another spin trapping agent –1 suspensions containing 1 mg TiO2 mL were prepared nitromethane (NM) (18, 21). in redistilled water. Dimethylsulfoxide (DMSO; The absorption/scattering effects of TiO2 in the SeccoSolv® Merck, Germany) was used as a co- – UV-activated aqueous suspensions containing NO2 solvent. nitrite (ACS reagent), 2,2’-azino- may substantially influence the direct photolytic bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium processes of nitrite (22). Moreover, the reactive salt (ABTS), persulfate, potassium dioxide radical species generated upon TiO2 photoexcitation in obtained from Sigma-Aldrich and  –  such systems ( OH, O2 / O2H) can participate in a from Merck (Germany) were used as supplied. The variety of reactions with nitrite or its photolytic stock TiO2 suspensions were homogenized for 1 products (8). The mechanism of the photocatalytic minute using ultrasound (Ultrasonic Compact oxidation of nitrite to nitrate upon TiO2 irradiation Cleaner TESON 1; Tesla, Slovak Republic). The spin was proposed previously (23-26). The impact of TiO2 trapping agent 5,5-dimethyl-1-pyrroline N-oxide loading and pH on phenol photonitration upon UV (DMPO; Sigma-Aldrich) was distilled prior to the irradiation of nitrite was also investigated (22). The application. Nitromethane (NM; ACS reagent), 5- direct photolysis of nitrite and the generation of nitric (diisopropoxyphosphoryl)-5-methyl-1-pyrroline N- oxide is hindered in TiO2 suspensions and the oxide (DIPPMPO; Enzo Sciences, USA), α-(4- enhancement of nitrophenol generation was attributed pyridyl-1-oxide)-N-tert-butylnitrone (POBN; Janssen  to the photocatalytic production of NO2 via the Chimica, Belgium) and 3,5-dibromo-4-nitrosobenzene photogenerated holes or surface-adsorbed hydroxyl sulfonate (DBNBS, Sigma-Aldrich) were used radicals (22): without extra purification. The chemical structure

291 J. Adv. Oxid. Technol. Vol. 19, No. 2, 2016 V. Brezová et al.

Scheme 1. Chemical structures of spin trapping agents and ABTS used in the study.

H3C H C + 3 + H (i-PrO)2 P H H3C N N O O + O + - - 5,5-Dimethyl-1-pyrroline N-oxide 5-(Diisopropoxyphosphoryl)-5-methyl-1-pyrroline N-oxide DMPO DIPPMPO

- Br O - + - O N O S CH N C(CH3)3 3 NO + -(4-Pyridyl-1-oxide)-N-tert-butylnitrone Br POBN 3,5-Dibromo-4-nitrosobenzene sulfonate DBNBS

CH CH CH NO 2 3 CH2CH3 3 2 Nitromethane N N NM S SO NH N-N S SO3NH4 3 4 2,2’-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt ABTS

of spin trapping agents and ABTS used is summarized Philips) with a Pyrex glass filter to eliminate radiation in Scheme 1. ABTS was oxidized by below 300 nm was used. The UVA irradiance within persulfate to the corresponding radical cation (ABTS+) the EPR cavity (5 mW cm–2) was determined using a according to the previously reported procedure (27) UVX radiometer (UVP, USA). By monitoring the and the precise concentration of ABTS+ was determined reduction of paramagnetic ABTS+ to diamagnetic 4 –1 –1 by UV-vis spectroscopy (735 nm = 1.510 M cm ) ABTS in the aqueous TiO2 suspensions containing (28, 29) using a UV-vis-NIR spectrophotometer nitrite, a monochromatic LED radiator (max = 365 nm, Shimadzu UV3600. Bluepoint LED, Hönle UV Technology, irradiance = 5 mW cm–2) was used as an irradiation EPR Experiments source for the continuous in situ photoexcitation. The The generation of paramagnetic intermediates acquisition of EPR spectra started 2 min after mixing + upon UVA irradiation of nitrite in the aerated aqueous of TiO2, ABTS and nitrite in the aerated suspensions and 5 min after mixing in systems saturated with . solutions or in the TiO2 suspensions was monitored by cw-EPR spectroscopy using the EMX X-band EPR The concentration of photogenerated paramagnetic spectrometer (Bruker, Germany). The solutions/ species was evaluated from the double-integrated EPR suspensions containing the spin trapping agent were spectra. All the EPR experiments were carried out at mixed instantly before the EPR measurements, least in triplicate; with standard deviation in the carefully aerated by a gentle air stream and relative EPR intensity of 10 %. The experimental immediately transferred to a small quartz flat cell EPR spectra were analyzed and simulated using the (WG 808-Q, Wilmad-LabGlass, USA; optical cell Bruker software (WinEPR), and Winsim2002 software length 0.04 cm) optimized for the TE102 cavity (30). The g-values were determined with an uncertainty (Bruker, Germany). The samples were irradiated at of  0.0001 by the simultaneous measurement of a 295 K directly in the EPR resonator, and the EPR reference sample containing Mn2+/MgO standard spectra were recorded in situ upon photoexcitation. As placed on the wall of the EPR cell. Typical EPR an irradiation source an HPA 400/30S lamp (400 W, spectrometer settings in a standard in situ experiment:

J. Adv. Oxid. Technol. Vol. 19, No. 2, 2016 292 V. Brezová et al. microwave frequency, ~ 9.458 GHz; microwave in situ UVA photoexcitation of nitrite (max = 365 nm) power, 10.06 mW; center field, 335.2 mT; sweep in the aqueous aerated alkaline solution (pH = 13) in width (SW), 8–16 mT; gain, 1×105–1×106; modulation the presence of NM spin trap, most probably due to amplitude, 0.05–0.1 mT; scan, 22 s; number of scans the low radiation absorption of the nitrite in the small 1–20; time constant, 10.24 ms. quartz flat EPR cell or less efficient nitrite photolysis at higher pH (32). Instead, an EPR signal (aN = 1.665 + Electronic Spectra of ABTS Monitored by UV- mT, aH(4H) = 1.182 mT; g = 2.0057) fully compatible vis Spectroscopy with an “artifact” described formerly by Pace and The UV-vis spectra were recorded at 298 K by a Carmichael (aN = 1.675 mT, aH(4H) = 1.20 mT or two UV-vis-NIR spectrophotometer (UV3600, Shimadzu) superimposed spectra (21)) assigned to the photolytic using a 1 cm quartz cell. The temperature was products of NM in alkaline solutions was observed. controlled with a thermoelectrically temperature On the other hand, the UVA exposure of aerated controlled cell holder (TCC-240A, Shimadzu). The alkaline TiO2 suspension containing nitrite and NM sets of experiments were conducted to monitor the caused an immediate generation of a ten-line EPR changes in the electronic spectra of ABTS+ under signal (Figure 1a) characterized with the hyperfine dark or upon discontinuous irradiation using a coupling constants (aN(2N) = 0.956 mT, aH = 0.409 monochromatic LED source (max = 365 nm; Bluepoint mT; g = 2.0055) typical for the –2 LED, irradiance = 5 mW cm ). The first spectrum species. The relative integral EPR intensity of was measured without irradiation and the following increased linearly with the spectra were recorded after a 1-minute exposure, until continuing irradiation, but the prolonged exposure the total exposure time reached 10 minutes. The UV- (above 8 minutes) to its gradual decline vis spectra of the irradiated solutions in the region (Figure 1b). The monitored formation of in the 200–1000 nm (sweep time 2 minutes) were taken heterogeneous TiO2-containing suspension indicates immediately after the exposure. In the case of TiO2- the involvement of the photogenerated holes and containing systems the reference cell contained TiO2 suspension in water with the corresponding titania hydroxyl radicals in the production upon an loading. effective photoactivation of the photocatalyst (Eqs. 9, 10). Results and Discussion The generation of reactive oxygen species (ROS) Nitrogen dioxide radical produced by the interaction upon UVA exposure of aerated aqueous TiO2 of photogenerated hydroxyl radicals or holes with suspensions was further followed by N-oxide and nitrite (Eqs. 4, 9, 10) represents a highly reactive nitrone spin trapping agents, i.e. DMPO, DIPPMPO  nitrogen species (31). NO2 radical was detected and POBN (Scheme 1, Figure 2). previously at room temperature by the EPR spin The experimental and simulated EPR spectra trapping in strong alkaline media using the obtained upon UVA exposure of aerated TiO2 – nitromethane aci anion ( CH22 =NO ) (18, 21). The suspensions in the presence of DMPO are fully corresponding spin-adduct, dinitromethyl radical compatible with the formation of DMPO-OH –– (Figure 2a). DMPO spin trap reacts rapidly with the dianion ( O22 N–CH=NO ) is characterized by the spin Hamiltonian parameters corresponding to the hydroxyl radicals forming the corresponding spin- interaction of an unpaired electron with two adduct characterized with the spin Hamiltonian parameters a = 1.494 mT, a β = 1.466 mT; g = 2.0057 equivalent nitrogen nuclei and one nucleus N H (aN(2N) = 0.960 mT, aH = 0.420 mT; g = 2.0055) (18, in water (Figure 2a). However, alternative reaction 21). The concentration of dinitromethyl radical pathways of DMPO-OH production not directly dianion monitored upon UV irradiation of nitrite in related with OH generation cannot be ignored, i.e. the alkaline solutions (pH in the range 12.9–13.9) oxidation of DMPO via photogenerated holes to a containing NM was proportional to the nitrite concen- radical cation DMPO+, which subsequently reacts tration, consequently the quantitative EPR spin with water forming a so-called imposter trapping method was suggested previously for the spin-adduct, or the transformation of an unstable –  determination of and NO2 from energetic DMPO-O2H spin-adduct, which is theoretically also materials (21). generated in the photocatalytic system (19). In our EPR study, no EPR signal of the The generation of surface hydroxyl radicals was dinitromethyl radical dianion was monitored upon the unambiguously confirmed by the addition of DMSO

293 J. Adv. Oxid. Technol. Vol. 19, No. 2, 2016 V. Brezová et al.

(a) (a)

a N a H a N aH

(b)

(b)

aP aN a.u. , a H

Relativeintegral EPR intensity

(c) 0 120 240 360 480 600 Irradiation time, s Figure 1. (a) Experimental (solid line) and simulated (dotted line) EPR spectra (magnetic field sweep SW = 6 mT) of dinitromethyl radical dianion –O N–CH=NO •– (along with the reconstruction 22 a diagram indicating the corresponding hyperfine coupling constants): N aH aN(2N) = 0.956 mT, aH = 0.409 mT; g = 2.0055) obtained upon continuous photoexcitation of aerated alkaline TiO2 suspensions containing nitrite ions and nitromethane spin trapping agent. (b) Changes in the relative integral EPR signal intensity monitored during irradiation. (pH = 13; c0,nitrite = 0.3 M; c0,NM = 0.02 M; –1 –2 cTiO2 = 0.17 mg mL ; UVA irradiance 5 mW cm ).

to the system, since the diffusion-limited reaction of Figure 2. Experimental (solid line) and simulated (dotted line) EPR hydroxyl radicals with DMSO produces methyl spectra of spin-adducts (along with the reconstruction diagrams radicals (19), easily detectable using water-soluble indicating the corresponding hyperfine coupling constants) obtained nitroso spin trapping agent DBNBS as the upon continuous photoexcitation of aerated aqueous TiO2  suspensions containing various spin trapping agents: (a) DMPO, corresponding DBNBS-CH3 spin-adduct (aN = 1.430 SW = 7 mT; (b) DIPPMPO, SW = 14 mT; (c) POBN, SW = 5 mT β m –1 mT, aH (3H) = 1.326 mT, aH(2H ) = 0.070 mT; (cTiO2 = 0.17 mg mL ; c0,spin trap = 0.035 M; UVA irradiance g = 2.0063); the experimental and simulated spectra 5 mW cm–2). are presented in Figure 3a. The EPR spin trapping experiments using The chiral centre in DIPPMPO molecule may DIPPMPO and POBN spin trapping agents suitable result in the production of trans and cis spin-adduct for the detection of oxygen-centered radicals in the diastereoisomers (33). The detailed simulation analysis irradiated aqueous TiO2 suspensions brought further of the experimental EPR spectra measured in the evidence on the dominant generation of hydroxyl TiO2/DIPPMPO/water/air photocatalytic systems radical spin-adducts under air (Figures 2b,c). (Figure 2b) reveals the superposition of three individual

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EPR signals belonging to trans and cis diastereo- of aerated aqueous TiO2 suspensions (Eq. 12) was isomers of hydroxyl radical spin-adduct (trans- trapped by complex of D-N-methylglucamine  β dithiocarbamate and the characteristic three-line signal DIPPMPO-OH: aP = 4.672 mT, aN = 1.403 mT, aH = 1.316 mT; g = 2.0057; cis-DIPPMPO-OH: (g = 2.041) was detected by EPR (23). The previous EPR experiments with DBNBS spin aP = 3.639 mT, aN = 1.584 mT, = 1.209 mT; g = 2.0057), along with a low-intensity EPR signal trapping agent with nitric oxide produced from the nitrite demonstrated the formation of a three-line assigned to the trans-DIPPMPO-O H (a = 4.948 mT, 2 P signal (a = 0.96 mT) characterized with large a = 1.337 mT, = 1.086 mT, a  = 0.083 mT; N N H Gaussian and Lorentzian linewidths, attributed to the g = 2.0057) (33, 34). DBNBS-NO product (40-42). The additional triplet DIPPMPO represents a spin trapping agent m signal (aN = 1.27 mT; aH(2H ) = 0.064 mT) was specially designed to detect a superoxide radical anion tentatively assigned to a DBNBS degradation product in aqueous media and biological systems (33).  – + DBNBS-SO3 (40, 42) or a radical cation DBNBS Consequently, despite the transformation of superoxide produced by the spin trap oxidation (41). radical anions to hydrogen by a dis- The EPR spectra obtained in our study upon UVA proportionation with protons (35), and significantly –  irradiation of TiO2/DBNBS/nitrite/water/air systems lower rate constants for the addition of O2 / O2H to are fully compatible with those monitored during the nitrone spin trap (36) (causing a limited production  nitric oxide spin trapping (Figure 3b), and the spin of the spin-adducts), the trans- DIPPMPO-O2H was –  Hamiltonian parameters evaluated by the simulation observed confirming the formation of O2 / O2H in the  analysis ( DBNBS-NO: aN = 0.977 mT; g = 2.0063; irradiated TiO2 suspensions via the reactions of  – m DBNBS-SO3 : aN = 1.264 mT, aH(2H ) = 0.064 mT; photogenerated electrons with molecular oxygen. g = 2.0063) correlate well with the published data The sufficient water solubility of POBN, its rapid (40-42). Yet, exactly the same EPR spectra were reaction with hydroxyl radical (second-order rate measured upon the irradiation of nitrite-free TiO2 9 –1 –1 constant of 3.510 M s (37)), as well as a rather suspensions under analogous experimental conditions. high stability of POBN-OH spin-adduct allow the Consequently, we can conclude that the paramagnetic detection of photogenerated hydroxyl radicals in the species monitored in TiO2/DBNBS/nitrite/water/air TiO2 suspensions using this spin trap. The EPR does not reflect the nitric oxide generation and its spectrum measured in the irradiated TiO2/POBN/ trapping, but the species originate from the processes water/air systems (Figure 2c) represents a dominant coupled with DBNBS spin trap oxidation and  β signal of POBN-OH (aN = 1.497 mT, aH = 0.161 mT, subsequent decomposition most probably caused by 13 a13C(4 C) = 0.440 mT; g = 2.0057) superimposed the hydroxyl radicals/holes generated in the system as with a low-intensity signal of a -centered spin- confirmed previously (40). adduct indicating the POBN degradation (38), most The addition of nitrite anions in the TiO2  probably POBN-C(CH3)3 (aN = 1.506 mT, = 0.215 suspensions containing spin trapping agents DMPO, mT; g = 2.0056). DIPPMPO or POBN caused no changes in the Under given experimental conditions using in situ character of EPR spectra, evidencing that radicals photoexcitation in the EPR resonator, the dispersed produced by reaction with holes or hydroxyl radicals titania photocatalyst is efficiently activated and the (Eqs. 9, 10), were not “trapped”, or the spin-adducts ROS produced were successfully detected by EPR are not stable enough for the detection using cw-EPR spin-trapping technique (Figure 2). In which ways spectroscopy. does the presence of the nitrite affect the processes in Figure 4 shows the effect of the growing nitrite the photocatalytic system and are these changes concentration (initial nitrite concentration range 0.83– observable by the EPR spin trapping technique? What 4.17 mM) on the relative integral intensity of DMPO- is the effect of nitrite and its paramagnetic products on OH spin-adduct monitored upon continuous irradiation the generation and stability of the spin-adducts? of aerated TiO2 suspensions in the presence of DMPO   Previous investigations of the NO and NO2 effects spin trap in comparison to the nitrite-free reference on the stability of DMPO-OH spin-adduct indicated system. A significant decrease of a relative integral that the direct interaction of the nitroxide spin-adduct intensity of DMPO-OH was found already for the with nitric oxide, rather than with nitrogen dioxide lowest nitrite concentration, and this effect was even radical, is responsible for the destruction of the spin- more pronounced when the nitrite concentration adduct (39). Nitric oxide produced upon UV irradiation increased (Figure 4). This phenomenon may be

295 J. Adv. Oxid. Technol. Vol. 19, No. 2, 2016 V. Brezová et al.

(a)

a.u. ,

aN a (3H) H a H(2H)

Relativeintegral EPR intensity

0 120 240 360 480 600 Irradiation time, s Figure 4. Effect of nitrite concentration on the relative integral  intensity of DMPO-OH spin-adduct monitored upon continuous irradiation of aerated TiO2 suspensions in the presence of DMPO spin trap. Initial nitrite concentration in mM:  0;  0.83;  1.67;  2.5;  3.33;  4.17. The inset represents the EPR spectrum (SW = 6 mT) obtained in the nitrite-free TiO2 suspensions upon –1 (b) 180-s exposure. (cTiO2 = 0.17 mg mL ; c0,DMPO =0.035 M; UVA irradiance 5 mW cm–2).

influenced by TiO2 loading, UVA radiation dose, as well as by the initial oxygen and spin trap concen- trations. Under given experimental conditions, we a N observed an instantaneous formation of the adduct when the radiation started, and a gradual decrease of the EPR signal after reaching the maximum at about 120–240 seconds of exposure (Figure 4). This trend may be explained by the interaction of DMPO-OH a (2H) H with other radical species generated in the complex photocatalytic system leading to the formation of a N EPR-silent species. Figure 5 and 6 show the influence of an increased Figure 3. Experimental (solid line) and simulated (dotted line) EPR spectra of spin-adducts (along with the reconstruction diagram initial nitrite concentration on the relative integral indicating the corresponding hyperfine coupling constants) obtained intensity of the spin-adducts monitored upon upon continuous photoexcitation of aerated TiO2 suspensions continuous irradiation of aerated TiO2 suspensions in containing DBNBS spin trap: (a) in nitrite-free mixed solvent the presence of DIPPMPO or POBN spin traps in water/DMSO (16.7% vol. DMSO, SW = 8 mT); (b) in water comparison to the nitrite-free reference systems. Also containing nitrite ions (c0,nitrite = 3.33 mM; SW = 6 mT). –1 here a substantial decrease of the relative integral (cTiO2 = 0.17 mg mL ; c0,DBNBS = 0.017 M; UVA irradiance 5 mW cm–2). intensity of the spin-adducts was found, the extent of which correlates with the growing nitrite content in explained by a competitive reaction of hydroxyl the suspensions and demonstrates again the competitive radicals towards DMPO spin trapping agent generating reactions of hydroxyl radicals with spin traps and a paramagnetic spin-adduct and towards nitrite (the nitrite. The spin-adducts DIPPMPO-OH and POBN- second-order rate constants of hydroxyl radicals with OH are generated immediately after the start of the 9 –1 –1 DMPO and nitrite are 2.8–4.310 M s and radiation and their relative integral intensity time- 1.01010 M–1 s–1, respectively (8, 43)). An alternative profile during exposure were described by the sigmoidal interpretation may involve also an inhibition of functions (Figures 5 and 6). Due to the improved hydroxyl radical generation due to the competitive stability of DIPPMPO and POBN spin-adducts in reactions of photogenerated holes with nitrite (Eq. 9). comparison with DMPO spin trapping agent, as well The concentration profile of DMPO-OH during the as due to the rather high reactivity towards hydroxyl in situ EPR spin trapping experiments is strongly radicals (the second-order rate constants of hydroxyl

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, a.u. ,

a.u. ,

intensity EPR integral Relative

intensity EPR integral Relative 0 120 240 360 480 600 Irradiation time, s 0 120 240 360 480 600 Figure 5. Effect of nitrite concentration on the relative integral Irradiation time, s intensity of DIPPMPO-adducts monitored upon continuous irradiation of aerated TiO suspensions in the presence of DIPPMPO Figure 6. Effect of nitrite concentration on the relative integral 2  spin trap. Initial nitrite concentration in mM:  0;  0.83;  1.67; intensity of POBN-adducts monitored upon continuous irradiation  2.5;  3.33;  4.17. The inset represents the EPR spectrum of aerated TiO2 suspensions in the presence of POBN spin trap. Initial nitrite concentration in mM:  0;  0.83;  1.67;  3.33; (SW = 14 mT) obtained in the nitrite-free TiO2 suspensions upon –1  8.33. The inset represents the EPR spectrum (SW = 5 mT) 660-s exposure. (cTiO2 = 0.17 mg mL ; c0,DIPPMPO = 0.04 M; UVA irradiance 5 mW cm–2). obtained in the nitrite-free TiO2 suspensions upon 660-s exposure. (c = 0.17 mg mL–1; c = 0.035 M; UVA irradiance TiO2 0,POBN 5 mW cm–2). radicals with DIPPMPO and POBN are 4.6109 M–1 s–1 9 –1 –1 (43) and 3.810 M s (37), respectively), we can + recommend DIPPMPO and POBN spin trapping of the ABTS signal (Figure 7). The experiments conducted under inert atmosphere revealed a slower agents for the detection of reactive free radicals + generated in the irradiated TiO suspensions. reduction of ABTS (formal first-order rate constant 2 + –1 + To complete investigations of the processes kf(ABTS ) = 0.029 s under argon vs. kf(ABTS ) = 0.042 s–1 under air) indicating the role of oxygen in in the irradiated TiO2 suspensions containing nitrite, we monitored the effect of an increasing nitrite the reduction process. While in the de-aerated TiO2 concentration on the reduction of a water-soluble suspensions we consider the reduction via photo- semi-stable radical cation ABTS+, which is generated electrons only, under air the photogenerated characterized by a highly-resolved EPR spectrum superoxide radical anion involvement is proposed as (44). Additionally, the electronic spectrum of ABTS+ well, due to the suitable position of standard redox reveals selective absorption bands at 735 nm and 415 potentials of both redox couples (Table 1). Additionally, nm, well distinguishable from the absorption maxima in order to test this hypothesis, we added potassium 2+ dioxide as a source of superoxide into the aqueous of its reduced ABTS (340 nm) or oxidized ABTS + + TiO2 suspensions containing ABTS and the immediate (518 nm) form (27). Despite the ABTS reduction + approach being broadly applicable mainly in the loss of color reflects the reduction of ABTS to antioxidant studies (29), we previously successfully ABTS via superoxide. evidenced the photocatalytic reduction of ABTS+ to The addition of nitrite (initial nitrite concentration + ABTS in the aerated aqueous titania suspensions (27, range 0.8–16.7 mM) to the TiO2/ABTS /water system 45). caused decline of ABTS+ EPR signal already prior to The decrease in the ABTS+ concentration in the the irradiation, and the extent of this dark reaction was aerated or argon saturated TiO2 suspensions in water enhanced by the increased initial nitrite concentration (Figure 7) was followed in situ upon continuous and by the reaction time. (The acquisition of EPR –2 + photoexcitation (max = 365 nm; 5 mW cm ) measuring spectra of ABTS started 2 min after mixing in the the over-modulated EPR spectra of ABTS+ as a aerated suspensions and 5 min after mixing in systems single-line (g = 2.0036, inset of Figure 7a). Blank saturated with argon). Thermodynamically, the standard – – experiments in the TiO2-free systems confirmed a redox potential of redox couple NO3 / NO2 is lower sufficient photostability of ABTS+ under both air and than ABTS+/ABTS (Table 1), consequently we inert atmospheres. However, the irradiation of the assigned this dark reaction to the electron transfer + TiO2 suspensions caused almost immediate decrease between ABTS and nitrite.

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Table 1. Standard reduction potentials of selected redox couples (a) vs. NHE 25

Standard 20

Redox couple reduction Reference , mM ,

potential, V 15 NO / NO– 1.00 (49) 2 2

ABTS+ / ABTS 0.68 (50,51) 10 concentration

– •+ NO3 / 0.43 (52) 5 –

O / O –0.33 (49) ABTS 2 2 0 The presence of nitrite in the UVA-photoactivated 0 44 88 132 176 220 + system TiO2/ABTS /nitrite/water (initial nitrite Irradiation time, s concentrations 0.8–16.7 mM) appears to hinder the (b) + + photocatalytic ABTS reduction (kf(ABTS ) = –1 25 0.022–0.016 s under air or argon) and thus indirectly demonstrates the participation of nitrite on the

M 20  photoinduced redox processes. The limited reduction of ABTS+ in the presence of nitrites may be 15 interpreted as a consequence of ABTS re-oxidation to ABTS•+ via photogenerated radicals (46-48), in 10 good correlation with the higher standard reduction concentration, •+ potential of the redox couple / (Table 1). 5

ABTS In order to evaluate the involvement of nitrite in + the ABTS reduction, additional experiments were 0

+ 0 44 88 132 176 220 carried out where the transformation of ABTS to ABTS was monitored by the UV-vis spectroscopy Irradiation time, s under different conditions, i.e., in aqueous solutions or Figure 7. Effect of nitrite concentration on the photocatalytic + TiO2 suspensions with/without nitrite under dark or reduction of radical cation ABTS monitored upon continuous upon photoexcitation ( = 365 nm; 5 mW cm–2). UVA irradiation in the (a) aerated or (b) argon saturated TiO2 max suspensions. Initial nitrite concentration in mM:  0;  0.83;  The sets of UV-vis spectra obtained are summarized in 4.17;  12.5;  16.7 ( represents TiO2- and nitrite-free blank Figure 8 (the experimental conditions were optimized solution). The inset shows the EPR spectrum of ABTS+ (SW = 10 + for UV-vis spectroscopy, e.g., TiO2 loading; ABTS mT; modulation amplitude 0.2 mT) monitored before exposure in –1 and nitrite initial concentration, discontinuous exposure). the nitrite-free TiO2 suspensions. (cTiO2 = 0.17 mg mL ; –2 In the aerated aqueous solutions paramagnetic c0,ABTS+ = 25 M; max = 365 nm; 5 mW cm ). + ABTS is stable; no changes in the electronic spectra + ABTS revealed sufficient stability under given were found during a 20-minute measurement at 298 K. experimental conditions (total exposure time 10 min). However, the addition of nitrite (initial concentration –1 The presence of titania (0.017 mg mL ) caused the 5 mM) caused a decline in ABTS+ concentration progressive reduction of ABTS+ to ABTS upon evidenced as the absorbance decrease at 735 and 415 discontinuous exposure, and this process is even more nm, coupled with the simultaneous ABTS absorbance pronounced in the suspensions containing nitrite. The increase at 340 nm. An isosbestic point at 368 nm can trends observed in the UV-vis experiments (Figure 8) be clearly recognized in the spectral set (Figure 8). –1 however cannot be quantitatively compared with the The addition of TiO2 (0.017 mg mL ) into solution + + results obtained by EPR experiment (Figure 7), due to ABTS /nitrite has no effect on the ABTS reduction the differences in the experimental conditions, via nitrite. The sets of UV-vis spectra monitored under + concerning mainly the photoexcitation, i.e. continuous dark for the TiO2/ABTS /nitrite suspensions correlate or discontinuous irradiation, and the varying initial well with the dark reaction of ABTS+ and nitrite concentrations of the nitrites and TiO2 chosen to meet monitored by EPR spectroscopy (Figure 7). to the requirements of each experimental technique. Also upon discontinuous UVA-exposure (max = The UVA-irradiated aqueous titania suspensions 365 nm; 5 mW cm–2) the aerated aqueous solutions of containing nitrite represent complex systems producing

J. Adv. Oxid. Technol. Vol. 19, No. 2, 2016 298 V. Brezová et al.

2.0 2.0 •+ •+ ABTS ABTS + UVA 1.5 1.5

1.0 1.0 Absorbance Absorbance

0.5 0.5

0.0 0.0 400 600 800 1000 400 600 800 1000 Wavelength, nm Wavelength, nm

2.0 2.0 •+ •+ ABTS + TiO + UVA ABTS + nitrite 2

1.5 1.5

1.0 1.0 Absorbance Absorbance

0.5 0.5

0.0 0.0 400 600 800 1000 400 600 800 1000 Wavelength, nm Wavelength, nm

2.0 2.0 ABTS•+ + nitrite + TiO ABTS•+ + nitrite + TiO + UVA 2 2 1.5 1.5

1.0 1.0 Absorbance Absorbance

0.5 0.5

0.0 0.0 400 600 800 1000 400 600 800 1000 Wavelength, nm Wavelength, nm Figure 8. Time evolution of the electronic absorption spectra monitored under dark (time interval between individual spectra was 2 min) + in the aqueous solution of ABTS alone or with the addition of the nitrite or nitrite/TiO2, along with the changes in the electronic the spectra obtained upon discontinuous irradiation (time interval between individual spectra was 3 min, including 1 min exposure) of ABTS+ in water solution, TiO2 water suspension or TiO2 water suspension containing nitrite. In the case of TiO2-containing systems the reference cell –1 –2 + contained TiO2 dispersed in water. (c0,ABTS = 50 M; c0,NaNO2 = 5 mM ; cTiO2 = 0.017 mg mL ; max = 365 nm; irradiance 5 mW cm ). nitrogen and oxygen reactive species. The results of 1/0041/15) and Slovak University of Technology in our EPR spin trapping study confirmed the significance Bratislava (Young Researcher Grant 1371, of a competitive reaction of nitrite with photogenerated Z. Barbieriková). The authors gratefully acknowledge hydroxyl radicals or holes, resulting in the substantial Michael Kenneth Lawson for helpful discussion. inhibition of hydroxyl radical spin-adduct formation. References The nitrogen dioxide radicals so produced have (1) Lu, M.; Pichat, P. and Water potential to enter a variety of reactions with inorganic Purification: From Fundamentals to Recent and organic substances in aqueous systems. Applications; Wiley, 2013. (2) Pelaez, M.; Nolan, N.; Pillai, S.; Seery, M.; Falaras, Acknowledgements P.; Kontos, A.; Dunlop, P.; Hamilton, J.; Byrne, J.; This study was financially supported by Scientific O'Shea, K.; Entezari, M.; Dionysiou, D. Appl. Catal., Grant Agency of the Slovak Republic (VEGA Project B 2012, 125, 331-349.

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