No abstract

Research efforts on advanced oxidation processes (AOPs) have long been focused on the fundamental chemistry of activation processes and free radical reactions. Little attention has been paid to the chemistry of the precursor oxidants. Herein, we found that the precursor oxidants could lead to quite different outcomes. A counterintuitive result was observed in the photoreduction of bromate/iodate: the combination of H 2 O 2 and UV enhanced the reduction of bromate/iodate, whereas the addition of persulfate to the UV system led to an inhibitory effect. Thermodynamic and kinetic evidence suggests that the reduction of bromate in UV/H 2 O 2 was attributable to the direct reaction between HOBr and H 2 O 2 . Both experimental determination and kinetic simulation demonstrate that the reaction between HOBr and H 2 O 2 dominated over the • OH-mediated reactions. These results suggest that H 2 O 2 possesses some particular redox properties that distinguish it from other peroxides. The prototypical UV/H 2 O 2 process is not always an AOP: it can also be an enhanced reduction process for chemicals with intermediates that are reducible by H 2 O 2 . Considering the increasing interest in persulfate-based AOPs, the results of this study identify some novel advantages of the classical H 2 O 2 -based AOPs.

Selective inhibition of photosynthesis is a fundamental strategy to solve the global challenge caused by harmful cyanobacterial blooms. However, there is a lack of specificity of the currently used cyanocides, because most of them act on cyanobacteria by generating nontargeted oxidative stress. Here, for the first time, we find that the simplest β-diketone, acetylacetone, is a promising specific cyanocide, which acts on Microcystis aeruginosa through targeted binding on bound iron species in the photosynthetic electron transport chain, rather than by oxidizing the components of the photosynthetic apparatus. The targeted binding approach outperforms the general oxidation mechanism in terms of specificity and eco-safety. Given the essential role of photosynthesis in both natural and artificial systems, this finding not only provides a unique solution for the selective control of cyanobacteria but also sheds new light on the ways to modulate photosynthesis.

Whether superoxide radical anion (O) was a key reactive species in the oxidation of arsenite (As(III)) in photochemical processes has long been a controversial issue. With hydroquinone (BQH) and 1,4-benzoquinone (BQ) as redox mediators, the photochemical oxidation of As(III) and reduction of nitrate (NO) was carefully investigated. O, singlet oxygen (O), HO, and semiquinone radical (BQH) were all possible reactive species in the irradiated system. However, since the formation of As(IV) is a necessary step in the oxidation of As(III), taking the standard reduction potentials into account, the reactions between the above species and As(III) were thermodynamically unfavorable. On the basis of radical scavenging experiments, hydroxyl radical (OH) was proved as the key species that led to the oxidation of As(III) in the UV/BQH system. It should be noted that the OH radicals were generated from the photolysis of HO, which came from the disproportionation of O and the reaction of O with BQH. Both the photoejected e from (BQH)* and the direct electron transfer with (BQH)* contributed to the reduction of NO in the UV/BQH process. No synergistic effect was observed in the redox conversion of As(III) and NO, further demonstrating that the role of BQH was negligible in the studied systems. The results here are helpful for a better understanding of the photochemical behaviors of quinones in the aquatic environment.

Quinones are important electron shuttles as well as micropollutants in the nature. Acetylacetone (AA) is a newly recognized electron shuttle in aqueous media exposed to UV irradiation. Herein, we studied the interactions between AA and hydroquinone (QH 2 ) under steady-state and transient photochemical conditions to clarify the possible reactions and consequences if QH 2 and AA coexist in a solution. Steady-state experimental results demonstrate that the interactions between AA and QH 2 were strongly affected by dissolved oxygen. In O 2 -rich solutions, the phototransformation of QH 2 was AA-independent. Both QH 2 and AA utilize O 2 as the electron acceptor, but in O 2 -insufficient solutions, AA became an important electron acceptor for the oxidation of QH 2 . In all cases, the coexistence of AA increased the phototransformation of QH 2 , whereas the decomposition of AA in O 2saturated and oversaturated solutions was inhibited by the presence of QH 2 . The underlying mechanisms were investigated by a combination of laser flash photolysis (LFP) and reduction potential analysis. The LFP results show that the excited AA serves as a better electron shuttle than QH 2 . As a consequence, AA might regulate the redox cycling of quinones, leading to significant effects on many processes, ranging from photosynthesis and respiration to photodegradation.

No abstract

Reduction of bromate to bromide is of great significance for the remediation of bromate-containing waters. In natural water, some low-molecular-weight organics (LMWOs) ubiquitously coexist with bromate. However, LMWOs-mediated photoreduction of bromate has been rarely studied. Herein, we found that acids (formic acid, acetic acid, pyruvic acid, lactic acid, and oxalic acid), aldehydes (formaldehyde and acetaldehyde), alcohols (tert-butyl and isopropyl), ketones (acetone, 2,3butanedione, acetylacetone (AA), and 2,5-hexanedione), and quinones (hydroquinone and benzoquinone) could all enhance the photoreduction of bromate under UV irradiation. The mechanisms of LMWOs-mediated photoreduction of bromate were systematically elucidated. Special attention was paid to AA, because the photolysis of AA generates most of the other tested aliphatic LMWOs. The presence of LMWOs exerted complicated effects on the formation of disinfection byproducts (DBPs): On the one hand, the presence of AA inhibited the formation of hypobromous acid (HBrO), which is believed to be helpful to control the potential formation of DBPs, because HBrO is a key precursor of halogenated DBPs. On the other hand, the LMWOs possibly functioned as precursors of organic halogens. The results here may help us to better understand the fate of bromine species in the natural aquatic environment.

No abstract