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

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UVA-Induced Processes in the Aqueous Titanium Dioxide Suspensions Containing Nitrite (An EPR Spin Trapping Study) UVA-Induced Processes in the Aqueous Titanium 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 Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovak Republic Abstract: Application of TiO2 photocatalytic systems for water purification and remediation is based on the generation of short-lived reactive oxygen 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 nitrogen 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 spectroscopy; Spin trapping; Hydroxyl radical; Titanium dioxide; Nitrite Introduction ammonium ions) 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 carcinogens (3-5). The unique physicochemical properties of (7). titanium dioxide polymorphs (anatase, brookite, rutile) 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 oxide 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 crystal 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 = 1.010 M s (8)): able to destroy a variety of contaminants (3). NO + OH HNO2 (3) The worldwide expansion of agricultural and –– industrial activities have caused an increased NO22 + OH NO + OH (4) contamination of surface and ground water sources by Nitrogen dioxide radicals generated may dimerize the inorganic nitrogen compounds (nitrate, nitrite, to N2O4, or may enter in the reaction with nitric oxide 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) – – + N2 O 4 H 2 O NO 2 + NO 3 + 2 H (7) 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 superoxide 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. Sodium nitrite (ACS reagent), 2,2’-azino- ion may substantially influence the direct photolytic bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium processes of nitrite (22). Moreover, the reactive salt (ABTS), potassium persulfate, potassium dioxide radical species generated upon TiO2 photoexcitation in obtained from Sigma-Aldrich and sodium hydroxide – 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 Life 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 molecule
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