Photochemical Transformation of Free Chlorine Induced by Triplet State Dissolved Organic Matter

Making waves: Sustainable control of micropollutants via NOM-mediated photosensitized activation of oxidants

While photo-based advanced oxidation processes (AOPs) are promising for the abatement of micropollutants in water and wastewater, they are inevitably influenced by the components of the background water matrix. As one of the major water matrix components, natural organic matter (NOM) generates photochemically produced reactive intermediates (PPRIs, e.g., 3NOM* and NOM•-) upon photolysis. PPRIs have recently been found to activate oxidants (e.g., H2O2) and generate reactive species (e.g., HO•), offering a novel and sustainable approach to degrade micropollutants in water. To facilitate the application of this NOM-mediated process, we summarize the fundamentals from the relevant literature, including PPRI generation, the mechanism of photosensitized activation of oxidants, performance of the processes for micropollutant degradation, and the factors influencing photosensitized activation. NOM•- is the PPRI activating H2O2 whereas the rest of the oxidants are primarily activated by 3NOM*. Resulting from the photosensitized activation, NOM and oxidant can exhibit synergism for micropollutant degradation under solar irradiation. Various factors, such as NOM properties, irradiation wavelength, pH, and other water matrix components (e.g., inorganic carbon and metal ions), affect the efficiency of photosensitized activation. Accordingly, we identify several future research directions: (1) investigating the wavelength dependency of photosensitized activation, (2) manipulating NOM structures in pre-treated processes, and (3) evaluating the formation of undesired byproducts.

Overlooked Pathway of UV Filter Degradation in the UV/H2O2: The Important Role of Triplet State UV Filter

The large production and usage of UV filters resulted in pervasive contamination in the aquatic environment. UV/H2O2 is a widely used advanced oxidation process (AOP) to eliminate contaminants including UV filters relying on the generated HO•. In this study, the degradation of UV filters in the UV/H2O2 AOP was investigated by using benzophenone (BP) as a representative. A previously overlooked pathway for BP degradation and HO• formation was newly identified. The triplet state species generated from BP photosensitization (i.e., 3BP*) was found to react with H2O2 to produce HO•, and this pathway contributed to 31% of BP degradation. The second-order rate constant of 3BP* with H2O2 was determined to be 8.7(±1.0) × 107 M-1 s-1 at pH 7.0 by using a laser flash photolysis system. 3BP* acted as a reductant, and it was transformed into a radical cation (BP•+). The further hydrolysis of BP•+ produced hydroxylated BPs as products. This pathway can be barely influenced by pH and inorganic ions in the real water matrix. This work not only reported an unrecognized pathway for pollutant degradation in the UV/H2O2 AOP but also inspired ideas to develop novel technologies to abate pollutants by taking advantage of their triplet states.

Solar-Induced Phototransformation of Peracetic Acid in Chromophoric Dissolved Organic Matter Solutions

Peracetic acid (PAA) has been extensively investigated as an alternative disinfectant for water and wastewater treatments. However, the photochemical transformation of PAA in sunlit surface waters has not been previously investigated. For the first time, the photosensitized transformation of PAA was here observed in chromophoric dissolved organic matter (CDOM)-enriched solutions under simulated solar irradiation. Triplet CDOM (3CDOM*) is here proposed to play a key role in the phototransformation processes. Using triplet model compounds, the reaction rate constant of 3CDOM* with PAA can be estimated to around 1.1 × 108 M-1 s-1. The reaction mechanism of triplet with PAA involves both energy- and electron-transfer. CH3COO• was generated from the reaction between PAA and excited triplets. Furthermore, HO• and organic reactive species (i.e., CH3COOO• and CH3COO•) are involved in the photochemical transformation of PAA in sunlit CDOM-enriched solutions. These reactive species (including HO•, CH3COOO•, and CH3COO•) also play a role in the removal of contaminants of emerging concerns (CECs). Overall, the current study provided new insights into the photochemical transformation of PAA in CDOM-enriched solutions. The solar irradiation of wastewater with PAA enhancement could be a useful and economically beneficial advanced oxidation process for CEC abatement.

Intrinsic strain of defect sites steering chlorination reaction for water purification

Carbon nanotube (CNT)-based heterogeneous advanced oxidation processes (AOPs) used for water purification have been exploited for several decades. Many strategies for modifying CNTs have been utilized to improve their catalytic performance in remediation processes. However, the strain fields of the intrinsic defect sites on CNT steering AOPs (such as chlorination) have not yet been reported. Here, we explored the strained defect sites for steering the chlorination process for water purification. The strained defect sites with the elongated sp2 hybridized C-C bonds boost electronic reactivity with the chlorine molecules via the initial Yeager-type adsorption. As a result, the reactive species in chlorination can be regulated on demand, such as the ratio of high-selectivity ClO• ranging from 38.8% in conventional defect-based systems to 87.5% in our strain-dominated process, which results in the generation of harmless intermediates and even deep mineralization during 2,4-DCP abatement. This work highlights the role that strain fields have on controlling the extent of chlorination reactions.

Probing the Photochemical Formation of Hydroxyl Radical from Dissolved Organic Matter: Insights into the H2O2-Dependent Pathway

This study quantifies the contribution of the H2O2-dependent pathway to hydroxyl radical (•OH) production from the photolysis of dissolved organic matter (DOM). •OH formation rates were cross-validated using benzoate and terephthalate as probe compounds for diverse DOM sources (reference isolates and whole waters). Catalase addition revealed that the H2O2-dependent pathway accounts for 10-20% of the total •OH production in DOM isolate materials, but no significant correlation was observed between ambient iron (Fe) concentrations and H2O2-dependent •OH formation. This lack of correlation was likely due to lower total Fe levels in isolated materials, thus limiting the concentration of photochemically produced Fe(II) available for reaction with H2O2. Notably, the H2O2-dependent pathway contributed 11 ± 3% to •OH formation from Pony Lake fulvic acid, which had the lowest Fe content, implicating additional H2O2-driven formation mechanisms independent of Fe. Experiments with the DOM model compounds acetophenone and p-benzoquinone indicated no •OH production from triplet DOM reactions with H2O2. However, •OH formation rate increased 6-fold when H2O2 was reduced by ketyl radicals formed from the reaction between excited triplet acetophenone and 2,4,6-trimethylphenol. This study advances the knowledge of •OH production mechanisms from DOM photolysis, providing insight into the role of H2O2 in aquatic photochemical processes.

Molecular composition and formation mechanism of chlorinated organic compounds in biological waste leachate treated by electrochemical oxidation with a boron-doped diamond anode

The use of electrochemical oxidation with boron-doped diamond (BDD) as an anode has been demonstrated to be an effective means of removing dissolved organic matter (DOM) from biologically treated waste leachate. However, in the presence of chloride ions, undesired chlorine evolution occurs on the anode; this forms chlorinated DOM, mostly of unknown molecular composition. We investigate the molecular composition and formation mechanism of chlorinated DOM during electrochemical oxidation process of biologically treated leachate DOM. At a current density of 8 mA/cm2, after 120 min of electrolysis, 479 unknown chlorinated DOMs were detected in the treated effluent, comprising 21.55% of the total. The unknown species are dominated by oxygen-rich, highly unsaturated structures, and exhibit higher oxidation degrees, lower unsaturation, and lower aromaticity compared to the removed nonchlorinated DOM. An additional 43.63 mg/L of known chlorinated DOM species, predominantly dichloroacetic and trichloroacetic acids, also accumulate in the treated effluent. Introducing hydroxyl radicals (HO•) to the anode surface forms reactive chlorine species including chlorine radical (Cl•), dichlorine radical (Cl2•-), and hypochlorous acid/hypochlorite (HOCl/OCl-); the concentration of HOCl/OCl- reaches 529.2 mg/L. These species react with reduced and aromatic dissolved organic matter via reaction pathways such as chlorine substitution for hydrogen (Cl+H-) and the HOCl addition reaction (HO+Cl+) to generate unknown chlorinated DOM species; the known chlorinated DOM are formed afterward via ring opening and dealkylation pathways. Our results provide a theory for the prevention and control of chlorinated DOM during treatment of chlorine-laden organic wastewater by an electrochemical oxidation system with a boron-doped diamond anode.

Light-driven differences in bacterial networks and organic matter decomposition: Insights from an analysis of the harmful cyanobacterium Microcystis aeruginosa PCC 7806

Freshwater systems are critical yet often underestimated components of global carbon cycling, functioning both as carbon sinks and sources. Cyanobacteria play a key role in this cycle by capturing atmospheric carbon dioxide through photosynthesis. The captured carbon is either released back into the atmosphere or sequestered in sediments following organismal decay. This study examines the pivotal role of cyanobacteria, specifically Microcystis aeruginosa PCC 7806, in the biogeochemical cycling of carbon in freshwater ecosystems, with a focus on how light influences the degradation of cyanobacteria-derived organic matter. Using a combination of 16S rDNA sequencing and excitation-emission matrix coupled with parallel factor (EEM-PARAFAC) analysis, we conducted a 50-day experiment to investigate the dynamics of dissolved organic matter (DOM) and lysate organic matter (LOM) derived from M. aeruginosa PCC 7806 under light and dark conditions. Our results demonstrate that light significantly impacts bacterial community composition, gene functionality, and the decomposition of organic matter. The findings emphasize the crucial role of light in facilitating microbial adaptation, stabilizing microbial networks and driving organic substrate transformation. These insights underscore the influence of light on microbial community dynamics and organic matter degradation, revealing shifts in microbial populations under varying light conditions. This suggests a strong link between photochemical processes and microbial activity, with significant ecological implications.

Molecular Weights of Dissolved Organic Matter Significantly Affect Photoaging of Microplastics

The fate of ubiquitous microplastics (MPs) is largely influenced by dissolved organic matter (DOM) in aquatic environments, which has garnered significant attention. The reactivity of DOM is reported to be greatly regulated by molecular weights (MWs), yet little is known about the effects of different MW DOM on MP aging. Here, the aging behavior of polystyrene MPs (PSMPs) in the presence of different MW fulvic acids (FAs) and humic acids (HAs) was systematically investigated. Under ultraviolet (UV) illumination, O/C of PSMPs aged for 96 h surged from 0.008 to 0.146 in the lower MW FA (FA<1kDa) treatment, suggesting significant PSMP aging. However, FA exhibited a stronger effect on facilitating PSMP photoaging than HA, which can be attributed to the fact that FA<1kDa contains more quinone and phenolic moieties, demonstrating a higher redox capacity. Meanwhile, compared to other fractions, FA<1kDa was more actively involved in the increase of different reactive species yields by 50-290%, including •OH, which plays a key role in PSMP photoaging, and contributed to a 25% increase in electron-donating capacity (EDC). This study lays a theoretical foundation for a better understanding of the environmental fate of MPs.

Photochemical Transformation of Monochloramine Induced by Triplet State Dissolved Organic Matter

The photoexcited dissolved organic matter (DOM) could produce reactive intermediates, affecting chemical oxidant transformation in UV based advanced oxidation processes (AOPs). This study confirmed the critical role of triplet state DOM (3DOM*), generated from DOM photoexcitation, in the transformation of monochloramine (NH2Cl), a commonly used chemical oxidant and disinfectant in water treatment. NH2Cl (42.25 μM, as Cl2) was decayed by 17.4-73.4 % within 60 min, primarily due to 3DOM* , in DOM (2-30 mgC L-1) solutions irradiated by 365 nm, where NH2Cl has no absorption. The second-order quenching rate constants of triplet state model photosensitizers by NH2Cl were determined to be 0.95(± 0.04)-4.49(± 0.04)× 108 M-1 s-1 by using laser flash photolysis. As a reductant, 3DOM* reacted with NH2Cl through one-transfer mechanism, leading to amino radical (NH2•) generation, which then transferred to ammonia (NH4+, pKa 9.25) through H-abstraction by the phenolic moieties in DOM. Additionally, the intermediate product of 3DOM* oxidized by NH2Cl or those triplet state quinones can hydrolyze to form phenolic moieties, elevating NH4+ yield to higher than 99% upon 365 nm irradiation. These findings suggest that the widespread DOM can be applied to convert NH2Cl via 3DOM* with minimal toxic risks.

Dissolved Organic Matter-Mediated Photosensitized Activation of Monochloramine for Micropollutant Abatement in Wastewater Effluent

Utilizing solar light and water matrix components in situ to reduce the chemical and energy demands would make treatment technologies more sustainable for micropollutant abatement in wastewater effluents. We herein propose a new strategy for micropollutant abatement through dissolved organic matter (DOM)-mediated photosensitized activation of monochloramine (NH2Cl). Exposing the chlorinated wastewater effluent with residual NH2Cl to solar irradiation (solar/DOM/NH2Cl process) degrades six structurally diverse micropollutants at rate constants 1.26-34.2 times of those by the solar photolysis of the dechlorinated effluent (solar/DOM process). Notably, among the six micropollutants, the degradation rate constants of estradiol, acetaminophen, bisphenol A, and atenolol by the solar/DOM/NH2Cl process are 1.13-4.32 times the summation of those by the solar/DOM and solar/NH2Cl processes. The synergism in micropollutant degradation is attributed to the generation of reactive nitrogen species (RNS) and hydroxyl radicals (HO·) from the photosensitized activation of NH2Cl. Triplet state-excited DOM (3DOM*) dominates the activation of NH2Cl, leading to the generation of RNS, while HO· is produced from the interactions between RNS and other photochemically produced reactive intermediates (e.g., O2·- and DOM·+/·-). The findings advance the knowledge of DOM-mediated photosensitization and offer a sustainable method for micropollutant abatement in wastewater effluents containing residual NH2Cl.

Effects of pH on the triplet state dissolved organic matter induced free available chlorine decay and radical formation

Triplet state dissolved organic matter (3DOM*) plays a significant role in inducing oxidant decay and radical generation in light-based advanced oxidation processes. However, the effects of pH still need investigation. This work quantitatively analyzed the pH-dependent free available chlorine (FAC) decay and radical formation (i.e., HO• and Cl•) induced by 3DOM* or triplet state photosensitizer (3PS*). Upon UV irradiation at 254 nm, the decay rate of FAC by 3DOM* or 3PS* was the highest at neutral pH, while those by dark reaction of DOM and the direct photolysis of FAC were the highest at acidic conditions. This is attributed to the variation of FAC species, 3DOM* or 3PS* formation, and the reaction rate constants of FAC with 3DOM* or 3PS* at pH 5.0-10.0. 3DOM* and 3PS* formed increasingly with pH varying from 5.0 to 10.0, while their reactivity with FAC decreased due to the speciation from HOCl to OCl-. Radical formation (i.e., HO• and Cl•) from FAC reaction with 3DOM* or 3PS* occurred at all the testing pH range (5.0-10.0). This work highlighted the pH-dependent role of 3DOM* in oxidant decay and radical formation in treating DOM containing waters through oxidant photolysis. ENVIRONMENTAL IMPLICATIONS: Triplet state dissolved organic matter (3DOM*) plays a significant role in inducing oxidant decay and radical generation in light-based AOPs. This study revealed the effects of pH in 3DOM* induced free available chlorine (FAC) decay and radical formation (i.e., HO• and Cl•). With DOM at 3 mgC L-1, FAC decayed fastest under neutral conditions and radical formation (i.e., HO• and Cl•) was enhanced at 5.0-10.0 due to 3DOM* reaction with FAC. These results highlighted the pH-dependent role of 3DOM* in oxidant transformation and radical formation in treating DOM containing waters by AOPs based on oxidant photolysis.

Boosting the photocatalytic activity of g-C3N4 via loading bio-synthesized Ag0 nanoparticles and imidazole modification for the degradation and mineralization of fluconazole

The emergence of fluorinated organic compounds in the pharmaceutical, agrochemical, and textile industries has led to a potential increase in the environmental issues and health problems. Herein, a modified heterojunction of bio-synthesized Ag nanoparticles (Ag0 NPs) immobilized on imidazole-modified graphite carbon nitride (Im/g-C3N4) as a suitable support (Ag0/Im/g-C3N4) was hydrothermally synthesized and studied for the photocatalytic removal of the most widely used antifungal organo-fluorine compound-fluconazole (FCZ). The optical properties were thoroughly investigated in the present study, and it was observed that the proposed modification to g-C3N4 has led to the shifting of conduction and valance band edge position (for g-C3N4, -0.73 and 1.54 eV and for ICA, -1.14 and 1.28 eV), narrowing of band gap energies, i.e., 2.01 eV, and reduced charge recombination rate. The external and internal surface morphologies were scrutinized through FE-SEM and HR-TEM analyses. Functionalities and potential crystallinity were investigated using FTIR and XRD techniques. The elemental state and composition of the composite were analyzed via XPS. The obtained results substantiate the intended modifications in the ICA composite. The photocatalyst Ag0/Im/g-C3N4 (ICA) was able to degrade 95.74% of FCZ with a high degradation rate (k1) of 0.0289 min-1 within 2-h of the solar illumination experiment. The overall degradation process was observed to be governed by a pseudo-first-order kinetic model. Detailed parameters such as effects of ions, pH (optimized pH 4, highest degradation rate k1 =0.039 min-1), dissolved organic matter (DOM), and optimization of catalysts dosage were studied. The major reactive oxygen species (ROS) was identified as super-oxide radicals (O2●-). The HR-MS and COD-TOC analysis were used to evaluate the degradation and mineralization of FCZ forced by ICA catalysts. The ICA catalyst was found to be stable and reusable for up to five cycles suggesting towards its potential towards the mitigation of environmental pollutants.

Insights into the Enhanced Photogeneration of Hydroxyl Radicals from Chlorinated Dissolved Organic Matter

Free available chlorine has been and is being applied in global water treatment and readily reacts with dissolved organic matter (DOM) in aquatic environments, leading to the formation of chlorinated products. Chlorination enhances the photoreactivity of DOM, but the influence of chlorinated compounds on the photogeneration of hydroxyl radicals (•OH) has remained unexplored. In this study, a range of chlorinated carboxylate-substituted phenolic model compounds were employed to assess their •OH photogeneration capabilities. These compounds demonstrated a substantial capacity for •OH production, exhibiting quantum yields of 0.1-5.9 × 10-3 through direct photolysis under 305 nm and 0.2-9.5 × 10-3 through a triplet sensitizer (4-benzoylbenzoic acid)-inducing reaction under 365 nm LED irradiation. Moreover, the chlorinated compounds exhibited higher light absorption and •OH quantum yields compared to those of their unchlorinated counterparts. The •OH photogeneration capacity of these compounds exhibited a positive correlation with their triplet state one-electron oxidation potentials. Molecular-level compositional analysis revealed that aromatic structures rich in hydroxyl and carboxyl groups (e.g., O/C > 0.5 with H/C < 1.5) within DOM serve as crucial sources of •OH, and chlorination of these compounds significantly enhances their capacity to generate •OH upon irradiation. This study provides novel insights into the enhanced photogeneration of •OH from chlorinated DOM, which is helpful for understanding the fate of trace pollutants in chlorinated waters.

Freezing-Enhanced Photoreduction of Iodate by Fulvic Acid

Iodate is a stable form of iodine species in the natural environment. This work found that the abiotic photosensitized reduction of iodate by fulvic acid (FA) is highly enhanced in frozen solution compared to that in aqueous solution. The freezing-induced removal of iodate by FA at an initial pH of 3.0 in 24 h was lower than 10% in the dark but enhanced under UV (77.7%) or visible light (31.6%) irradiation. This process was accompanied by the production of iodide, reactive iodine (RI), and organoiodine compounds (OICs). The photoreduction of iodate in ice increased with lowering pH (pH 3-7 range) or increasing FA concentration (1-10 mg/L range). It was also observed that coexisting iodide or chloride ions enhanced the photoreduction of iodate in ice. Fourier transform ion cyclotron resonance mass spectrometric analysis showed that 129 and 403 species of OICs (mainly highly unsaturated and phenolic compounds) were newly produced in frozen UV/iodate/FA and UV/iodate/FA/Cl- solution, respectively. In the frozen UV/iodate/FA/Cl- solution, approximately 97% of generated organochlorine compounds (98 species) were identified as typical chlorinated disinfection byproducts. These results call for further studies of the fate of iodate, especially in the presence of chloride, which may be overlooked in frozen environments.