Photosensitized Transformation of Hydrogen Peroxide in Dissolved Organic Matter Solutions under Simulated Solar Irradiation.

Hydrogen peroxide plays an important role in photochemical processes in aquatic environments. However, whether it can be transformed by photoexcited chromophoric dissolved organic matter (CDOM) remains unclear. Therefore, this study examined the photosensitized degradation of H2O2 in CDOM-enriched solutions under simulated solar irradiation. Our results suggest that the presence of CDOM enhances the photodegradation rate of H2O2 via the photosensitization process and ·OH is generated stoichiometrically with H2O2 attenuation. Experimental results with model photosensitizers indicate that one-electron reducing species of CDOM (CDOM·-), not triplet CDOM, is the primary reactive species that reduces H2O2 to yield ·OH. By monitoring the variation of CDOM·-, the reaction rate constant of CDOM·- with H2O2 was estimated to be 1.5-fold greater than that with O2. Furthermore, a wastewater effluent was exposed to simulated solar irradiation with the addition of H2O2, and the results demonstrated that the photodegradation of trace organic contaminants (TrOCs) was significantly enhanced by the increased ·OH level. Overall, the current study provided new insights into the photochemical formation of ·OH via the one-electron reduction of H2O2 by CDOM·-. The solar irradiation of wastewater with H2O2 enhancement could be a useful and economically beneficial advanced oxidation process for TrOC abatement.

Microheterogeneous Distribution of Hydroxyl Radicals in Illuminated Dissolved Organic Matter Solutions.

Hydroxyl radicals (•OH) are important reactive species that are photochemically generated through solar irradiation of chromophoric dissolved organic matter (CDOM) in surface waters. However, the spatial distribution within the complex three-dimensional structure of CDOM has not been examined. In this study, we used a series of hydrophobic chlorinated paraffins as chemical probes to elucidate the microheterogeneous distribution of •OH in illuminated CDOM solutions. The steady-state concentration of •OH inside the CDOM microphase is 210 ± 31-fold higher than the concentration in the aqueous phase. Our results suggest that the most photochemically generated •OH are confined into the CDOM microphase. Thus, illuminated CDOM behaves as a natural microreactor for •OH-based oxidations. By including intra-CDOM •OH, the quantum yield of •OH for CDOM solutions was estimated to be 2.2 ± 0.5 × 10-3, which is 2 orders of magnitude greater than previously thought. The elevated concentrations of photogenerated •OH within the CDOM microphase may improve the understanding of hydrophobic pollutant degradation in aqueous environments. Moreover, our results also suggest that •OH oxidation may play more important roles in the phototransformation of CDOM than previously expected.

Phototransformation of an emerging cyanotoxin (Aerucyclamide A) in simulated natural waters.

Aerucyclamide A (ACA) is an emerging cyanopeptide toxin produced by cyanobacteria, and its transformation pathway has rarely been reported. In the present study, ACA was purified from cyanobacterial extracts, and photodegradation processes were investigated in dissolved organic matter (DOM) solutions. Under simulated solar irradiation, the photodegradation of ACA was dominated by •OH oxidation, accounting for ~72% of the indirect photodegradation. The bimolecular reaction rate constant of ACA with •OH was (6.4 ± 0.2) × 109M - 1s - 1. Our results indicated that the major reactive sites of ACA toward •OH are thiazoline and thiazole moieties. Product analysis via high-resolution mass spectrometry suggested that hydrogen abstraction and gradual hydroxylation are the main photodegradation pathways. The acute toxicity assessment indicate that the products generated in photolysis process did not show any measurable toxicity to Thamnocephalus platyurus. Photodegradation experiments with various DOM-phycocyanin mixtures demonstrated that the half-life of ACA is much longer than that of microcystin-LR.

Characterization of EDTA-cross-linked ß-cyclodextrin grafted onto Fe-Al hydroxides as an efficient adsorbent for methylene blue.

The synthesis process of Fe-Al hydroxides coated with ethylenediaminetetraacetic acid (EDTA)-cross-linked ß-cyclodextrin polymer (FA-ECD) and its utilization in the adsorption of methylene blue (MB) were investigated. The FA-ECD before and after adsorption was characterized by scanning electron microscopy (SEM), Brunauer-Emmet-Teller (BET) analysis, Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric (TG) analysis, and X-ray diffraction (XRD). In addition, the EDTA and ß-cyclodextrin contents were determined quantitatively. The optimization of several variables such as contact time, pH, initial concentration, and adsorbent dosage achieved the maximum removal percentages in mild conditions. The results revealed that the adsorption process mainly depended on the pH value and the optimal adsorption capacity of the MB was 60.71 mg/g at pH 8. Subsequently, the experimental equilibrium data at different temperatures were fitted with the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models; the Freundlich model provided the best results. In addition, the pseudo-second-order kinetic model best described the adsorption of the MB. The thermodynamic analysis proved that the adsorption process was endothermic and spontaneous.