Optical properties and oxidative potential of aqueous-phase products from OH and ³C^*-initiated photolysis of eugenol

In ambient air, aqueous-phase oxidation may turn precursors into more light-absorbing and toxic products, leading to air quality deterioration and adverse health effects. In this study, we investigated eugenol degradation in aqueous phase under direct photolysis, and triplet excited organic (3C*) and hydroxyl radical (OH) as oxidants. Results showed degradation rates of eugenol followed the order of 3C* > OH > direct photolysis. Relative contributions of reactive oxygen species (ROS) and 3C* were evaluated via quenching and O2-free experiments. 3C* played a dominant role in eugenol degradation for 3C*-initiated oxidation, while both O2 and O2•-generated were important for eugenol degradation for OH-initiated oxidation. Rate constants under O2, air and N2 followed the order of ko2＞kAir＞kN2 under both direct photolysis and OH oxidation, and it changed to kAir＞kN2＞ko2 in 3C*-initiated oxidation. Light absorption spectra showed absorbance at 300–400 nm increased as photolysis progressed, and there were new broad fluorescent spectra at excitation/emission (Ex/Em) = 250/(400–500) nm, suggesting the formation of new chromophores and fluorophores, such as humic-like substances (HULIS). Additionally, distinct fluorescence peaks appeared at Ex/Em=(300–350)/300 nm at different stages. Concentration of generated HULIS increased gradually over time, then leveled off. Dithiothreitol (DTT) assay was applied to assess the oxidation potential of products, which was greater than pure eugenol, suggesting more harmful species were produced during oxidation. Detailed reaction pathways were elucidated via analyses of chemical characteristics of the products.

UV-LED Combined with Small Bioreactor Platform (SBP) for Degradation of 17α-Ethynylestradiol (EE2) at Very Short Hydraulic Retention Time

Degradation of 17α-ethynylestradiol (EE2) and estrogenicity were examined in a novel oxidative bioreactor (OBR) that combines small bioreactor platform (SBP) capsules and UV-LED (ultraviolet light emission diode) simultaneously, using enriched water and secondary effluent. Preliminary experiments examined three UV-LED wavelengths—267, 279, and 286 nm, with (indirect photolysis) and without (direct photolysis) H2O2. The major degradation wavelength for both direct and indirect photolysis was 279 nm, while the major removal gap for direct vs. indirect degradation was at 267 nm. Reduction of EE2 was observed together with reduction of estrogenicity and mineralization, indicating that the EE2 degradation products are not estrogens. Furthermore, slight mineralization occurred with direct photolysis and more significant mineralization with the indirect process. The physical–biological OBR process showed major improvement over other processes studied here, at a very short hydraulic retention time. The OBR can feasibly replace the advanced oxidation process of UV-LED radiation with catalyst in secondary sedimentation tanks with respect to reduction ratio, and with no residual H2O2. Further research into this OBR system is warranted, not only for EE2 degradation, but also to determine its capabilities for degrading mixtures of pharmaceuticals and pesticides, both of which have a significant impact on the environment and public health.

Enhanced UV Direct Photolysis And UV/H2O2 For Oxidation of Triclosan And Ibuprofen In Synthetic Effluent: An Experimental Study

This study aimed to evaluate the implementation of an advanced oxidation system based on UV radiation and UV/H2O2 for degradation of TCS and IBU in synthetic effluent. The assays occurred in a 2L reactor, protected from external light and equipped with a UV lamp (λ = 254nm). The effect of contaminant concentration, fractions of chemical species present, and mineralization were evaluated. In the UV/ H2O2 system, different concentrations of H2O2 were studied for oxidation of the contaminants. The kinetic experiments took place between 75 - 270 min of UV irradiation. The results showed > 99% oxidation of TCS in the direct photolysis system at pH 9.4 after 12 min. The degradation of IBU in the UV/H2O2 system, when 10mg L-1 of H2O2 was used, obtained 97.39% oxidation. We obtained k' values of 0.189 min-1 for TCS when its highest oxidation occurred, and k' values of 0.0219 min-1 for IBU. The system was not able to completely mineralize the contaminants, presenting high values of TOC and COD after treatment, thus suggesting the occurrence of phototransformation.

Kinetics and Mechanistic Studies of Photochemical and Oxidative Stability of Galaxolide

Studies on kinetics of galaxolide (HHCB) degradation under influence of UV, simulated sunlight and some advanced oxidation processes (H2O2, UV/H2O2, and Vis/H2O2) were conducted. Galaxolide appeared to be a photolabile compound. The first-order kinetics model was assumed for all studied processes. It was observed that basic pH favored HHCB degradation. The influence of natural matrices (river water and artificial sweat) on direct photolysis of HHCB was examined. It was stated that the process of the photodegradation proceeded slower at the presence of each matrix. HHCB lactone was identified using the GC-MS technique. The recorded chromatograms showed that apart from the lactone, other degradation products were formed that we could not identify. In order to deeper understand the HHCB degradation process, DFT calculations were performed. The results pointed out that OH radicals play a key role in HHCB decomposition, which mainly proceeds via H abstractions as well as OH additions. It follows from the calculations that the visible light is sufficient to initiate the advanced oxidation processes (AOPs) under the oxidative conditions, whereas UV irradiation is needed to start decay with no oxidative agents.

UV/Vis-Based Persulphate Activation for p-Nitrophenol Degradation

In the present work, the degradation of p-nitrophenol (PNP) and its mineralization by a UV/Vis-based persulphate activation process was investigated. Firstly, a screening of processes as direct photolysis, persulphate alone and persulphate activated by radiation was performed. The incidence of radiation demonstrated to have an important role in the oxidant activation, allowing to achieve the highest PNP and total organic carbon (TOC) removals. The maximum PNP oxidation (100%) and mineralization (61.6%)—both after 2 h of reaction time—were reached when using T = 70 °C, (S2O82−) = 6.4 g/L and I = 500 W/m2. The influence of radiation type (ultraviolet/visible, visible or simulated solar light) was also evaluated, being found that the source with the highest emission of ultraviolet radiation (UV/visible) allowed to achieve the best oxidation efficiency; however, solar radiation also reached very-good performance. According to quenching experiments, the sulphate radical is key in the activated persulphate oxidation process, but the hydroxyl radical also plays an important role.