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1. Byproduct formation in heterogeneous catalytic ozonation processes

   https://doi.org/10.1039/D2VA00216G  

   Wu, Tingting (April 2023, Environmental Science: Advances)

   Heterogeneous catalytic ozonation (HCO) is a promising advanced oxidation process (AOP) that can effectively degrade recalcitrant organic pollutants. While research efforts have been mainly devoted to the development of different catalysts to enhance the process efficiency, more studies are needed to investigate and address the other challenge faced by AOPs, i.e. generation of harmful byproducts. Bromate is the major inorganic byproduct of concern when ozone is involved. While most studies have reported less bromate formation in HCO than ozonation alone, the effects of catalysts depend on their interaction with O 3 and the dominant bromate formation pathway (direct O 3 oxidation vs. indirect ˙OH oxidation) in the system. Production of H 2 O 2 and cyclic redox reactions on the catalyst surface can also reduce different Br species leading to a lower bromate yield. Generation of organic byproducts (OBPs; e.g. aldehydes, keto-acids, carboxylic acids) in HCO depends on the reactivity of precursors ( e.g. dissolved organic matter/DOM) and OBPs with O 3 /˙OH, interactions between DOM/OBPs and catalysts, characteristics of DOM, and O 3 dose. HCO generally increased the removal of dissolved organic carbon (DOC) and the biodegradability of the bulk organics. HCO treatment may also decrease the formation potential of some disinfection byproducts (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs) but may increase the brominated species of the DBPs and also the formation potential of haloacetonitrile (HAN) under certain conditions. This review discusses the current status of studies on both organic and inorganic byproduct formation in HCO as well as transformation of bulk organics and the effects on DBP formation in the downstream disinfection process, and further provides recommendations for future research and development. A standardized experimental protocol and rigorous experimental design is important to deepen our understanding and gain insights on the byproduct formation in HCO from different studies collectively. 

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2. Photolytic reaction of dissolved organic matter and bromide ions promote the formation of manganese oxides

   Gao, Zhenwei; Liu, Jing; Skurie, Charlie; Zhu, Yaguang; Jun, Young-Shin (March 2022, American Chemical Society National Meeting)

   Manganese (Mn) oxide solids widely exist in nature, serving as both electron donors and acceptors for a variety of redox reactions. Previous studies have highlighted the adsorption of dissolved organic matter (DOM) on Mn oxides, as well as the reduction of Mn oxides by DOM. Here, we show the underappreciated roles of photolytic reactions of DOM in Mn2+(aq) oxidation and its consequential formation of Mn oxide solids. During the photolysis of DOM, reactive intermediates including excited triplet state DOM (3DOM*), hydroxyl radical (•OH), superoxide radical (O2•−), hydrogen peroxide (H2O2), and singlet oxygen (1O2) can be generated. Among them, we found that O2•− was responsible for Mn oxidation. In addition, in the presence of bromide ions (Br−), the photolytic reactions between DOM and Br− formed reactive bromide radicals and facilitated the oxidation of Mn2+(aq) to Mn oxide solids. Moreover, the composition of DOM affected its oxidative capability. When DOM contained more aromatic functional groups, we observed more oxidation of Mn2+ to Mn oxides. These new findings advance our knowledge of natural Mn2+ oxidation and Mn(III/IV) oxide formation, as well as the hitherto overlooked oxidative role of DOM in the oxidation of metal ions in surface water under sunlight illumination. 

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3. Photochemically-assisted Formation of Manganese Oxide Solids in the Presence of Dissolved Organic Matter and Bromide Ions

   Gao, Zhenwei; Liu, Jing; Skurie, Charlie; Zhu, Yaguang; Jun, Young-Shin (June 2022, the 2022 Association of Environmental Engineering and Science Professors (AEESP) Research and Education Conference)

   Dissolved natural organic matter (DOM) is a complex matrix of organic matter that is ubiquitous in natural aquatic environments. So far, substantial research has been conducted on the DOM adsorption on Mn oxides as well as the reduction processes of Mn oxides by DOM. However, little is known about the oxidative roles of DOM in oxidizing Mn2+(aq) to Mn(III/IV) oxide solids. Sunlight-driven processes can initiate the degradation of DOM accompanied by the formation of photochemically produced reactive intermediates, including excited triplet state DOM (3DOM*), hydroxyl radical (•OH), superoxide radical (O2•−), hydrogen peroxide (H2O2), and singlet oxygen (1O2). Further, in the presence of halide ions, reactive halogen species can be generated by reactions between 3DOM* and halide ions, and by reactions between •OH and halide ions. In this study, we found that the solution pH controlled the oxidation of Mn2+(aq) to Mn oxide solids during photolysis of DOM. Among the reactive oxygen species, Mn2+(aq) was found to be oxidized to Mn oxide solids mainly by O2•−. The DOM with different quantities of aromatic functional groups affected its oxidative capability. With the addition of bromide ions (Br−), Mn2+(aq) oxidation was promoted further by formation Br radicals, which can also oxidize Mn2+(aq) to Mn oxide solids. These findings can help us better understand the oxidative role of DOM in the formation of Mn oxide solids in organic-rich surface water. In addition, this study assists in comprehending the impacts of the photolytic reactions between DOM and halide ions and their resulting reactive oxygen and halogen species on the oxidation and reduction processes of other transition metal oxides in the environment. 

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4. Photooxidants from brown carbon and other chromophores in illuminated particle extracts

   https://doi.org/10.5194/acp-19-6579-2019  

   Kaur, Richie; Labins, Jacqueline R.; Helbock, Scarlett S.; Jiang, Wenqing; Bein, Keith J.; Zhang, Qi; Anastasio, Cort (January 2019, Atmospheric Chemistry and Physics)

   Abstract. While photooxidants are important in atmospheric condensed phases, there arevery few measurements in particulate matter (PM). Here we measure lightabsorption and the concentrations of three photooxidants – hydroxyl radical(⚫OH), singlet molecular oxygen (1O2*),and oxidizing triplet excited states of organic matter (3C*) –in illuminated aqueous extracts of wintertime particles from Davis,California. 1O2* and 3C*, which are formedfrom photoexcitation of brown carbon (BrC), have not been previously measuredin PM. In the extracts, mass absorption coefficients for dissolved organiccompounds (MACDOC) at 300 nm range between 13 000 and30 000 cm2 (g C)−1 are approximately twice ashigh as previous values in Davis fogs. The average (±1σ)⚫OH steady-state concentration in particle extracts is4.4(±2.3)×10-16 M, which is very similar to previous valuesin fog, cloud, and rain: although our particle extracts are moreconcentrated, the resulting enhancement in the rate of ⚫OHphotoproduction is essentially canceled out by a corresponding enhancement inconcentrations of natural sinks for ⚫OH. In contrast,concentrations of the two oxidants formed primarily from brown carbon (i.e.,1O2* and 3C*) are both enhanced in theparticle extracts compared to Davis fogs, a result of higher concentrationsof dissolved organic carbon and faster rates of light absorption in theextracts. The average 1O2* concentration in the PM extractsis 1.6(±0.5)×10-12 M, 7 times higher than past fogmeasurements, while the average concentration of oxidizing triplets is 1.0(±0.4)×10-13 M, nearly double the average Davis fog value.Additionally, the rates of 1O2* and 3C*photoproduction are both well correlated with the rate of sunlightabsorption. Since we cannot experimentally measure photooxidants under ambient particlewater conditions, we measured the effect of PM dilution on oxidantconcentrations and then extrapolated to ambient particle conditions. As theparticle mass concentration in the extracts increases, measuredconcentrations of ⚫OH remain relatively unchanged,1O2* increases linearly, and 3C* concentrations increase lessthan linearly, likely due to quenching by dissolved organics. Based on ourmeasurements, and accounting for additional sources and sinks that should beimportant under PM conditions, we estimate that [⚫OH] inparticles is somewhat lower than in dilute cloud/fog drops, while [3C*]is 30 to 2000 times higher in PM than in drops, and [1O2*] isenhanced by a factor of roughly 2400 in PM compared to drops. Because ofthese enhancements in 1O2* and 3C* concentrations,the lifetimes of some highly soluble organics appear to be much shorter inparticle liquid water than under foggy/cloudy conditions. Based onextrapolating our measured rates of formation in PM extracts, BrC-derivedsinglet molecular oxygen and triplet excited states are overall the dominantsinks for organic compounds in particle liquid water, with an aggregate rateof reaction for each oxidant that is approximately 200–300 times higherthan the aggregate rate of reactions for organics with ⚫OH. Forindividual, highly soluble reactive organic compounds it appears that1O2* is often the major sink in particle water, which is a newfinding. Triplet excited states are likely also important in the fate ofindividual particulate organics, but assessing this requires additionalmeasurements of triplet interactions with dissolved organic carbon innatural samples. 

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5. DOM Molecular Weight Fractionation and Fluorescence Quantum Yield Assessment Using a Coupled In-Line SEC Optical Property System

   https://doi.org/10.1021/acsestwater.2c00318  

   Hanson, Blair; Wünsch, Urban; Buckley, Shelby; Fischer, Sarah; Leresche, Frank; Murphy, Kathleen; D’Andrilli, Juliana; Rosario-Ortiz, Fernando L. (December 2022, ACS ES&T Water)

   Size exclusion chromatography (SEC) in combination with optical measurements has become a popular form of analysis to characterize dissolved organic matter (DOM) as a function of molecular size. Here, SEC coupled with in-line absorbance scans and fluorescence emission scans was utilized to derive apparent fluorescence quantum yield (Φf) as a function of molecular weight (MW) for DOM. Individual instrument-specific SEC-fluorescence detector correction factors were developed by comparison of an SEC-based excitation emission matrix (EEM) to an EEM generated by a calibrated benchtop fluorometer. The method was then applied to several sample sets to demonstrate how to measure the Φf of unknown DOM samples and to observe changes to Φf following a processing mechanism (ozonation). The Φf of riverine water samples and DOM fulvic acid isolates from Suwannee River and Pony Lake increased from < 0.5% to a maximum of ∼2.5–3% across the medium- to low-MW range. Following ozonation of PLFA, Φf increased most notably in the large-MW fractions (elution volumes < 40 mL). Overall, this method provides a means by which highly fluorescent size fractions of DOM can be identified for more detailed analyses of chemical composition and its changes through different processing mechanisms. 

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