Application of coumarin and coumarin-3-carboxylic acid for the determination of hydroxyl radicals during different advanced oxidation processes

• Published in: Radiation physics and chemistry (Oxford, England : 1993) England : 1993), 2020-05, Vol.170, p.108610, Article 108610
• Main Authors: Náfrádi, Máté, Farkas, Luca, Alapi, Tünde, Hernádi, Klára, Kovács, Krisztina, Wojnárovits, László, Takács, Erzsébet
• Format: Article
• Language: English
• Subjects: 172 nm VUV photolysis; Adsorption; Alcohols; Aqueous solutions; Carboxylic acids; Coumarin; Fluorescence; Heterogeneous photocatalysis; Hydroxyl radical; Hydroxyl radicals; Oxidation; Photocatalysis; Photolysis; Radiolysis; Rate constants; Titanium dioxide
• ISSN: 0969-806X; 1879-0895
• DOI: 10.1016/j.radphyschem.2019.108610

Transformation of coumarin (COU) and coumarin-3-carboxylic acid (3-CCA), the formation of their fluorescent hydroxylated products, (7-hydroxy-coumarin (7-HO-COU) and 7-hydroxy-3-carboxycoumarinic acid (7-HO-3-CCA)) were investigated and compared using three different advanced oxidation processes: gamma radiolysis, VUV (172 nm) photolysis and heterogeneous photocatalysis (TiO2/UV (300–400 nm)). Beside •OH (hydroxyl radical), other reactive species: H• (H-atom, in VUV irradiated aqueous solution and in gamma radiolysis with low yield) and eaq− (hydrated electron, in gamma radiolysis) also contributed to the degradation. The reaction rate constants of COU and 3-CCA with each reactive species were determined via pulse radiolysis. The values obtained were: 6.88 × 109 and 4.9 × 109 with •OH; 2.5 × 109 and 1.3 × 109 with H•; 11.4 × 109 and 14.3 × 1010 mol−1 dm3 s−1 with eaq−, in the case of COU and 3-CCA, respectively. Based on the results of radiolysis it was suggested, that the formation of fluorescent products are initiated only by •OH. The effects of dissolved O2 and •OH scavengers (MeOH and t-BuOH) were also investigated. In radiolysis dissolved O2 increased the formation rate of fluorescent products. In VUV photolysis of COU and 3-CCA solutions the inhibition effect of alcohols was more pronounced in the presence of O2 than in its absence. In heterogeneous photocatalysis both MeOH and t-BuOH decreased the transformation rate of COU, while they had no observable effect on that of 3-CCA, which is well adsorbed on TiO2 surface. [Display omitted] •Rate constants of reactions with •OH, H• and eaq− were determined•Formation of hydroxylated fluorescent products requires •OH•Reaction with H• and eaq− does not results in fluorescent products•Hydroxylated products are more sensitive to •OH in bulk than coumarin•Adsorption on TiO2 has to be considered when evaluating •OH scavenging effect