The pH-dependent photochemistry of anthraquinone-2-sulfonate

Abstract

The photochemistry of anthraquinone-2-sulfonate (AQ2S) was studied as a function of pH, combining laser flash photolysis and steady-state irradiation experiments, with the additional help of a computational study of energy levels. Two out of the three transient species produced upon irradiation of AQ2S can be involved into the degradation of dissolved molecules, and also AQ2S in its ground state is degraded. The reactive transients are less stable but often more reactive under acidic conditions, which modulates the pH trend of the photodegradation of the adopted organic substrates (furfuryl alcohol, benzene, nitrobenzene). The ability of the excited states of irradiated AQ2S to simulate the reactivity of singlet oxygen upon degradation of furfuryl alcohol, and that of the hydroxyl radical by producing phenol from benzene, can have important consequences. Furfuryl alcohol and benzene are widely adopted probe molecules for the respective quantification of singlet oxygen and the hydroxyl radical in many systems, among which are natural waters under irradiation. This study shows that the interference of AQ2S on singlet oxygen determination would be higher in acidic or basic than in approximately neutral conditions, while in the case of the hydroxyl radical the interference would increase with pH. Processes analogous to those studied could account for the interference of coloured dissolved organic matter on the quantification of singlet oxygen, observed in previous studies.

Modelling the photochemical fate of ibuprofen in surface waters

“…The initial transformation rate of FFA under the adopted conditions was Rate FFA = (1.26±0.06)⋅10 (Loeff et al, 1983;Maurino et al, 2008, Maddigapu et al, 2010c. at pH 2 and (1.59±0.04)⋅10 2 [IBP] at pH 8.…”

“…The availability of the direct photolysis quantum yield and of the reaction rate constants of the (Albinet et al, 2010a;Maddigapu et al, 2010a;Hatipoglu et al, 2010;Maddigapu et al, 2011). Figure S2-SM reports the sunlight spectrum adopted for the calculations, which corresponds to a 22 W m −2 irradiance in the UV (Frank and Klöpffer, 1988).…”

Could triplet-sensitised transformation of phenolic compounds represent a source of fulvic-like substances in natural waters?

“…They are most likely due to AQ2S phototransformation intermediates, possibly the hydroxyderivatives. Indeed, the evolution of AQ2S triplet state upon reaction with water produces transient adducts with water (where water is attached to either the aromatic ring or an oxygen atom), which either yield back ground-state AQ2S, or evolve into AQ2S-OH isomers (Maurino et al, 2008;Maddigapu et al, 2010;Bedini et al, in press). To support this, we observed the formation of a broad absorption band in the 350-550 nm range (that is, right in the range of EEM excitation wavelengths), which is fully compatible with the occurrence of α and β-hydroxy-AQ2S (Loeff et al, 1983; see also Figure SM1).…”

“…Maddigapu et al, 2010). The transient absorption experiments were performed by using the third harmonic (λ = 355 nm) of a Quanta Ray Nd:YAG laser system, operating with an output of 60 mJ per pulse.…”

Phototransformation of the sunlight filter benzophenone-3 (2-hydroxy-4-methoxybenzophenone) under conditions relevant to surface waters

“…AQ2S was used as CDOM proxy, to assess reactivity between BP3 and 3 CDOM*. Reasons for this choice are the widespread occurrence of quinones in CDOM and the fact that irradiation of AQ2S, unlike other triplet sensitisers, does not yield interfering transients such as • OH and 1 O 2 (Cory and McKnight, 2005;Maddigapu et al, 2010). AQ2S initial concentration was 0.1 mM, to limit the additional complication represented by reaction between 3 AQ2S* and ground-state AQ2S (Bedini et al, 2012a).…”