Interfacial Molecular Fractionation on Ferrihydrite Reduces the Photochemical Reactivity of Dissolved Organic Matter

Mechanistic insight into the impact of interaction between goethite and humic acid on the photooxidation and photoreduction of bifenthrin

Numerous application of pyrethroid insecticides has led to their accumulation in the environment, threatening ecological environment and human health. Its fate in the presence of iron-bearing minerals and natural organic matter under light irradiation is still unknown. We found that goethite (Gt) and humic acid (HA) could improve the photodegradation of bifenthrin (BF) in proper concentration under light irradiation. The interaction between Gt and HA may further enhance BF degradation. On one hand, the adsorption of HA on Gt may decrease the photocatalytic activity of HA through decreasing HA content in solution and sequestering the functional groups related with the production of reactive species. On the other hand, HA could improve the photocatalytic activity of Gt through extending light absorption, lowing of bandgap energy, hindering the recombination of photo-generated charges, and promoting the oxidation and reduction reaction on Gt surface. The increased oxygen vacancies on Gt surface along with the reduction of trivalent iron and the nucleophilic attack of hole to surface hydroxyl group contributed to the increasing photocatalytic activity of Gt. Electron paramagnetic resonance and quenching studies demonstrated that both oxidation species, such as hydroxyl radical (•OH) and singlet oxygen (1O2), and reducing species, such as hydrogen atoms (H•) and superoxide anion radical (O2•-), contributed to BF degradation in UV-Gt-HA system. Mass spectrometry, ion chromatography, and toxicity assessment indicated that less toxic C23H22ClF3O3 (OH-BF), C9H10ClF3O (TFP), C14H14O2 (OH-MBP), C14H12O2 (MBP acid), C14H12O3 (OH-MBP acid), and chloride ions were the main degradation products. The production of OH-BF, MPB, and TFP acid through oxidation and the production of MPB and TFP via reduction were the two primary pathways of BF degradation.

High throughput identification of carbonyl compounds in natural organic matter by directional derivatization combined with ultra-high resolution mass spectrometry

Carbonyl compounds are important components of natural organic matter (NOM) with high reactivity, so that play a pivotal role in the dynamic transformation of NOM. However, due to the lack of effective analytical methods, our understanding on the molecular composition of these carbonyl compounds is still limited. Here, we developed a high-throughput screening method to detect carbonyl molecules in complex NOM samples by combining chemical derivatization with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS). In six different types of dissolved organic matter (DOM) samples tested in this study, 20-30 % of detected molecules contained at least one carbonyl group, with relative abundance accounted for 45-70 %. These carbonyl molecules displayed lower unsaturation level, lower molecular weight, and higher oxidation degree compared to non-carbonyl molecules. More importantly, the measured abundances of carbonyl molecules were consistent with the results of 13C nuclear magnetic resonance (NMR) analysis. Based on this method, we found that carbonyl molecules can be produced at DOM-ferrihydrite interface, thus playing a role in shaping the molecular diversity of DOM. This method has broad application prospects in screening carbonyl compounds from complex mixtures, and the same strategy can be used to directional identification of molecules with other functional groups as well.

Comparison of sulfide-induced transformation of biodegradable and conventional microplastics: Mechanism and environmental fate

Biodegradable plastics have been massively produced and used as potential substitutes for conventional plastics, resulting in their inevitable entry into the environment and generation of biodegradable microplastics (MPs). The sulfidation transformation of MPs is an important process for their transformation in anoxic environments (e.g., sediments, anaerobic activated sludges) that can alter their environmental effects and risks. However, how sulfides induce the transformation of biodegradable MPs and whether they are similar to conventional MPs remains unknown. In the present study, we compared the transformation and mechanism of conventional polyethylene (PE) MPs and biodegradable poly(butylene adipate-co-terephthalate) (PBAT) MPs during sulfidation. The results demonstrated that sulfidation resulted in oxidation of PE MPs, whereas PBAT MPs underwent reduction and had higher physical damage, as evidenced by fragmentation, chain scission and organic compound release. Besides, reactive oxygen species and sulfide species played important roles in the sulfidation of PE and PBAT MPs, respectively. The presence of ester groups in PBAT MPs led to their hydrolysis, causing chain scission and further reduction. Furthermore, sulfidation caused a higher degree of adsorption and toxicity alterations in PBAT MPs than in PE MPs. This work uncovers critical abiotic transformation behaviors of biodegradable microplastics and highlights the necessity of considering microplastic structural features to accurately predict microplastic occurrence.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

Soil, the largest terrestrial carbon reservoir, is central to climate change and relevant feedback to environmental health. Minerals are the essential components that contribute to over 60% of soil carbon storage. However, how the interactions between minerals and organic carbon shape the carbon transformation and stability remains poorly understood. Herein, we critically review the primary interactions between organic carbon and soil minerals and the relevant mechanisms, including sorption, redox reaction, co-precipitation, dissolution, polymerization, and catalytic reaction. These interactions, highly complex with the combination of multiple processes, greatly affect the stability of organic carbon through the following processes: (1) formation or deconstruction of the mineral-organic carbon association; (2) oxidative transformation of the organic carbon with minerals; (3) catalytic polymerization of organic carbon with minerals; and (4) varying association stability of organic carbon according to the mineral transformation. Several pieces of evidence related to the carbon turnover and stability during the interaction with soil minerals in the real eco-environment are then demonstrated. We also highlight the current research gaps and outline research priorities, which may map future directions for a deeper mechanisms-based understanding of the soil carbon storage capacity considering its interactions with minerals.

Mechanism of interaction between ascorbic acid and soil iron-containing minerals for peroxydisulfate activation and organophosphorus flame retardant degradation

Soil constituents may play an important role in peroxydisulfate (PDS)-based oxidation of organic contaminants in soil. Iron-containing minerals (Fe-minerals) have been found to promote PDS activation for organics degradation. Our study found that ascorbic acid (H2A) could enhance PDS activation by soil Fe-minerals for triphenyl phosphate (TPHP) degradation. Determination and characterization analyses of Fe fractions showed that H2A could induce the reductive dissolution of solid Fe-minerals and the increasing of oxygen vacancies/hydroxyl groups content on Fe-minerals surface. The increasing of divalent Fe (Fe(II)) accelerated PDS activation to generate reactive oxygen species (ROS). Electron paramagnetic resonance (EPR) and quenching studies showed that sulfate radicals (SO4•-) and hydroxyl radicals (HO•) contributed significantly to TPHP degradation. The composition and content of Fe-minerals and soil organic matter (SOM) markedly influenced ROS transformations. Surface-bond and structural Fe played the main role in the production of Fe(II) in reaction system. The high-concentration SOM could result in ROS consumption and degradation inhibition. Density functional theory (DFT) studies revealed that H2A is preferentially adsorbed at α-Fe2O3(012) surface through Fe-O-C bridges rather than hydrogen bonds. After absorption, H atoms on H2A may further be migrated to adjacent O atoms on the α-Fe2O3(012) surface. With the transformation of H atoms to the α-Fe2O3(012) surface, the Fe-O-C bridge is broken and one electron is transferred from the O to Fe atom, inducing the reduction of trivalent Fe (Fe(III)) atom. MS/MS2 analysis, HPLC analysis, and toxicity assessment demonstrated that TPHP was transformed to less toxic 4-hydroxyphenyl diphenyl phosphate (OH-TPHP), diphenyl hydrogen phosphate (DPHP), and phenyl phosphate (PHP) through phenol-cleavage and hydroxylation processes, and even be mineralized in reaction system.

Photo-Reactivity of dissolved black carbon unveiled by combination of optical spectroscopy and FT-ICR MS analysis: Effects of pyrolysis temperature

Dissolved black carbon (DBC) has high photoactivity, which plays an important role in contaminants photodegradation. However, it is unclear how pyrolysis temperatures would affect the composition and photo-reactivity of DBC at the molecular level. Herein, we combined complementary techniques to study the characteristics of DBC pyrolyzed at 200 - 500 ℃, as well as the photoproduction of reactive species and the photodegradation of tetracycline (TC). Bulk composition characterization found that condensed aromatic carbonyl compounds (ConAC) with narrow molecular weights in DBC experienced an increase from 200 to 500 °C, which enhanced the photoproduction of 3DBC*,1O2, and ·OH. Molecular-level data suggested that 3DBC* and 1O2 were both related to the same DBC compounds. Comparatively, the patterns for ·OH were less pronounced, implying its precursor was not 3DBC* and had more complexity. Plentiful CHOx species of ConAC in DBC400 and DBC500 (DBCT, where T = pyrolysis temperature) accelerated the generation of 3DBC* and 1O2, enhancing the photodegradation of TC, and mainly triplet states of quinones reacted with TC. In contrast, DBC200 and DBC300 exhibited inhibition since massive CHOx species in lignin-like reduced 3TC* to TC. Our data revealed the diverse photochemical behavior mechanisms of DBC pyrolyzed at 200 - 500 ℃ at the molecular level and the implications for aquatic contaminants photochemistry.

Controls of Mineral Solubility on Adsorption-Induced Molecular Fractionation of Dissolved Organic Matter Revealed by 21 T FT-ICR MS

Mineral adsorption-induced molecular fractionation of dissolved organic matter (DOM) affects the composition of both DOM and OM adsorbed and thus stabilized by minerals. However, it remains unclear what mineral properties control the magnitude of DOM fractionation. Using a combined technique approach that leverages the molecular composition identified by ultrahigh resolution 21 T Fourier transform ion cyclotron resonance mass spectrometry and adsorption isotherms, we catalogue the compositional differences that occur at the molecular level that results in fractionation due to adsorption of Suwannee River fulvic acid on aluminum (Al) and iron (Fe) oxides and a phyllosilicate (allophane) species of contrasting properties. The minerals of high solubility (i.e., amorphous Al oxide, boehmite, and allophane) exhibited much stronger DOM fractionation capabilities than the minerals of low solubility (i.e., gibbsite and Fe oxides). Specifically, the former released Al3+ to solution (0.05-0.35 mM) that formed complexes with OM and likely reduced the surface hydrophobicity of the mineral-OM assemblage, thus increasing the preference for adsorbing polar DOM molecules. The impacts of mineral solubility are exacerbated by the fact that interactions with DOM also enhance metal release from minerals. For sparsely soluble minerals, the mineral surface hydrophobicity, instead of solubility, appeared to be the primary control of their DOM fractionation power. Other chemical properties seemed less directly relevant than surface hydrophobicity and solubility in fractionating DOM.

Photobleaching affects the carbon sequestration of dissolved black carbon on ferrihydrite: Perspective from molecular fractionation

Photobleaching generally changes the structure and properties of dissolved black carbon (DBC), which further affects distribution of DBC at mineral-water interface. Here, we investigated the effect mechanism by which DBC photobleaching on its sequestration on ferrihydrite (Fh) from perspective of molecular fractionation. Results indicated that continuous sunlight irradiation led to the photolysis of aromatic humic- and fulvic-like components and the carboxylation of the functional structure. DBC could be considerably sequestered on the Fh surface, and photobleached DBC (pDBC) with longer sunlight irradiation durations had lower adsorption capacity on Fh. The photo-absorption and photo-activity ability of residual DBC/pDBCs after adsorption significantly weakened, indicating that the photo-liable components with great photochemical properties were preferentially sequestered on Fh during adsorption fractionation at Fh-water interface. Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) results showed high molecular weight, high O contents and high unsaturation compounds (such as polycyclic aromatics and polyphenols) were preferentially sequestered on Fh through ligand exchange between iron-coordinated hydroxyl and substituted carboxyl/hydroxyl in DBC. Among high unsaturation compounds, aromatic ring structures (C=C) were with greater affinity with Fh surface than CO in carboxyl/ester/quinone. Photobleaching caused the decrease in aromatic ring structures and the increase in CO in carboxyl, which was the key for weakening of sequestration of pDBC on Fh. Our findings prove that the photo-liable components of DBC are more tend to be sequestered on mineral, and promote the understanding of geochemical behavior of DBC in the solid earth interfaces.

Molecular and optical signatures of photochemical transformation of dissolved organic matter: Nonnegligible role of suspended particulate matter in urban river

Natural dissolved organic matter (DOM) is one of the Earth's dynamic carbon pools and a key intermediate in the global carbon cycle. Photochemical processes potentially affect DOM composition and activity in surface water. Suspended particulate matter (SPM) is the integral component of slow-moving rivers, and holds the potential for photochemical reactivity. To further investigate the influence of SPM on DOM photochemical transformation, this study conducted experiments comparing samples with and without SPM irradiated under simulated sunlight. Surface water samples from slow-moving urban rivers were collected. DOM optical characteristics and molecular features obtained by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were investigated. Photolabile DOM was enriched in unsaturated and highly aromatic terrestrial substances. Photoproduced DOM had low aromaticity and was dominated by saturated aliphatics, protein-like substances, and carbohydrates. Study results indicated that the presence of SPM had a nonnegligible impact on the molecular traits of DOM, such as composition, molecular diversity, photolability, and bioavailability during photochemical reactions. In the environment affected by SPM, molecules containing heteroatoms exhibit higher photosensitivity. SPM promotes the photochemical transformation of a wider range of chemical types of photolabile DOM, particularly nitrogen-containing compounds. This study provides an essential insight into the more precise simulation of photochemical reactions of DOM influenced by SPM occurring in natural rivers, contributing to our understanding of the global carbon cycle from new theoretical perspectives.

Investigation with ESI FT-ICR MS on sorbent selectivity and comprehensive molecular composition of landfill leachate dissolved organic matter

Molecular characterization of landfill leachate dissolved organic matter (LDOM) is essential for developing effective processing techniques. However, the molecular selectivity of extraction method and ionization modes often leads to the bias of molecular characterization of LDOM. Here, seven representative sorbents were selected and electrospray ionization negative ion mode (ESI (-)) and positive ion mode (ESI (+)) Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were used to investigate the molecular composition of different LDOM samples. Obvious sorbent selectivity during extraction procedure was observed, resulting in the underestimation of molecular diversity of LDOM from 32.7% to 69.3%. Totally, 14,000-18,000 unique molecules were obtained in a single sample, indicating the unprecedented molecular diversity of LDOM. Lignins, proteins and lipids are three major molecular groups in LDOM, and N or S containing molecules occupied 83%. Although much of total organic carbon was removed during biochemical treatment process, the molecular diversity of LDOM was not reduced because a considerable of bio-recalcitrant molecules was produced. The results uncover the sorbents selectivity and ionization modes selectivity in LDOM analysis and provided a comprehensive change of LDOM molecular composition during biochemical treatment, which benefits the development of accurate methods to remove organic carbon in landfill leachate.

Photochemical behaviors of dissolved organic matter in aquatic environment: Generation, characterization, influencing factors and practical application

Dissolved organic matter (DOM) widely exists in aquatic environment and plays a critical role in environmental photochemical reaction. The photochemical behaviors of DOM in sunlit surface waters have received widely attention because its photochemical effects for some coexisted substances in aquatic environment, especially for organic micropollutants degradation. Therefore, to gain a comprehensive understanding of the photochemical properties and environmental effects of DOM, we reviewed the influence of sources on the structure and composition of DOM with relevant identified techniques to analysis functional groups. Additionally, identification and quantification for reactive intermediates are discussed with a focus on influencing factors to produce reactive intermediates by DOM under solar irradiation. These reactive intermediates can promote the photodegradation of organic micropollutants in the environmental system. In future, attention should be paid to the photochemical properties of DOM and environmental effects in real environmental system and development of advanced techniques to study DOM.

Molecular-Scale Investigation on the Formation of Brown Carbon Aerosol via Iron-Phenolic Compound Reactions in the Dark

Brown carbon (BrC) is one of the most mysterious aerosol components responsible for global warming and air pollution. Iron (Fe)-induced catalytic oxidation of ubiquitous phenolic compounds has been considered as a potential pathway for BrC formation in the dark. However, the reaction mechanism and product composition are still poorly understood. Herein, 13 phenolic precursors were employed to react with Fe under environmentally relevant conditions. Using Fourier transform ion cyclotron resonance mass spectrometry, a total of 764 unique molecular formulas were identified, and over 85% of them can be found in atmospheric aerosols. In particular, products derived from precursors with catechol-, guaiacol-, and syringol-like-based structures can be distinguished by their optical and molecular characteristics, indicating the structure-dependent formation of BrC from phenolic precursors. Multiple pieces of evidence indicate that under acidic conditions, the contribution of either autoxidation or oxygen-induced free radical oxidation to BrC formation is extremely limited. Ligand-to-Fe charge transfer and subsequent phenoxy radical coupling reactions were the main mechanism for the formation of polymerization products with high molecular diversity, and the efficiency of BrC generation was linearly correlated with the ionization potential of phenolic precursors. The present study uncovered how chemically diverse BrC products were formed by the Fe-phenolic compound reactions at the molecular level and also provide a new paradigm for the study of the atmospheric aerosol formation mechanism.

Effect of adsorption on ferrihydrite on the photoreactivity of dissolved black carbon for photodegradation of sulfadiazine

The selective adsorption of dissolved black carbon (DBC) on inorganic minerals is a widespread geochemical process in the natural environment, which could change the chemical and optical properties of DBC. However, it remains unclear how selective adsorption affects the photoreactivity of DBC for photodegradation of organic pollutants. This paper was the first to investigate the effect of DBC adsorption on ferrihydrite at different Fe/C molar ratios (Fe/C molar ratios of 0, 7.50 and 11.25, and marked as DBC₀, DBC_7.50 and DBC_11.25) on the photoproduction of reactive intermediates generated from DBC and their interaction with sulfadiazine (SD). Results showed that UV absorbance, aromaticity, molecular weight and contents of phenolic antioxidants of DBC were significantly decreased after adsorption on ferrihydrite, and higher decrease was observed at higher Fe/C ratio. Photodegradation kinetics experiments showed that observed photodegradation rate constant of SD (k_obs) increased from 3.99 × 10^-5 s^-1 in DBC₀ to 5.69 × 10^-5 s^-1 in DBC_7.50 while decreased to 3.44 × 10^-5 s^-1 in DBC_11.25, in which ³DBC* played important roles and ¹O₂ played a minor role, while ·OH was not involved in the reaction. Meanwhile, the second-order reaction rate constant between ³DBC* and SD (k_{SD, 3DBC*}) increased from 0.84 × 10⁸ M^-1 s^-1 for DBC₀ to 2.53 × 10⁸ M^-1 s^-1 for DBC_7.50 while decreased to 0.90 × 10⁸ M^-1 s^-1 for DBC_11.25. The above results might be mainly attributed to the fact that the decrease of phenolic antioxidants in DBC weakened the back-reduction of ³DBC* and reactive intermediates of SD as the Fe/C ratio increased, while the decrease of quinones and ketones reduced the photoproduction of ³DBC*. The research revealed adsorption on ferrihydrite affected the photodegradation of SD by changing the reactivity of ³DBC*, which was helpful to understand the dynamic roles of DBC in the photodegradation of organic pollutants.

Photoreduction of Hg(II) by typical dissolved organic matter in paddy environments

The photochemical behavior of dissolved organic matter (DOM) in surface water and its effect on Hg(II) photoreduction has been extensively studied, but the contribution of DOM in paddy water to Hg(II) photoreduction is largely unknown. Herein, the effect of DOM from biochar (BC_DOM), rice straw (RS_DOM), and chicken manure (CM_DOM) on Hg(II) photoreduction were examined. The comparable reduction efficiency of Hg(II) suggested that DOM-like fraction (62.3-63.7%) contributes more than suspended particulate matter-like fraction (17.7-23.4%) and bacteria-like fraction (13.0-20.0%) in paddy water. Under irradiation, the typical DOM significantly promoted Hg(II) photoreduction, and the reduction efficiency of BC_DOM (65.5 ± 2.1%) was higher than that of CM_DOM (48.3 ± 2.6%) and RS_DOM (32.8 ± 2.4%) in 6 h. The quenching and kinetics experiments showed that superoxide anion (O₂^•-) was the main reactive species for Hg(II) photoreduction. Fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry revealed that DOM with a higher degree of lignin/carboxy-rich acyclic molecules, condensed aromatics structures, and phenolic compounds could promote the formation of O₂^•-. These findings highlight the importance of DOM in Hg(II) photoreduction and provide new ideas for regulating Hg cycling and bioavailability in paddy environments.

New insights into the mechanism of phosphate release during particulate organic matter photodegradation based on optical and molecular signatures

Phosphate release from particulate organic matter (POM) dominates phosphorus (P) cycling in aquatic ecosystems. However, the mechanisms underlying P release from POM remain poorly understood because of complex fractionation and analytical challenges. In this study, the release of dissolved inorganic phosphate (DIP) during POM photodegradation was assessed using excitation-emission matrix (EEM) fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). POM in suspension was significantly photodegraded under light irradiation, concomitantly with the production and release of DIP in the aqueous solution. Chemical sequential extraction revealed that organic phosphorus (OP) in POM participated in photochemical reactions. Moreover, FT-ICR MS analysis revealed that the average molecular weight of P-containing formulas decreased from 374.2 to 340.1 Da. Formulas containing P with a lower oxidation degree and unsaturation were preferentially photodegraded, generating oxygen-enriched and saturated formula compounds, such as protein- and carbohydrate-like P-containing formulas, benefiting further utilization of P by organisms. Reactive oxygen species played an important role in the photodegradation of POM, and excited triplet state chromophoric dissolved organic matter (³CDOM*) was mainly responsible for POM photodegradation. These results provide new insights into the P biogeochemical cycle and POM photodegradation in aquatic ecosystems.

Novel insights into the temporal molecular fractionation of dissolved black carbon at the iron oxyhydroxide - water interface

As the most reactive and mobile fraction of black carbon, dissolved black carbon (DBC) inexorably interacts with minerals in the biosphere. Nevertheless, the research on the mechanisms and compositions of DBC assembly at the mineral-water interface remains limited. In this study, we revealed the "kinetic architecture" of DBC on iron oxyhydroxide at novel insights based on quantitative and qualitative approaches. The results indicated that high molecular weight, highly unsaturated, oxygen-rich (such as carboxyl-rich fraction, phenolics), aliphatics, and long C chains compounds were preferentially adsorbed on the iron oxyhydroxide. 2D-COS analyses directly disclosed the sequential fractionation: aromatic and phenolic groups > aliphatic groups, and few aromatics were continuously adsorbed after the rapid adsorption. Quantitative determinations identified that aromatic and phenolic components were adsorbed rapidly over the first 60 min, while aromatics achieved the dynamic equilibrium until ∼300 min, which was consistent with the 2D-COS observations. Our findings supported the hypothesis that "mineral-OM" and "OM-OM" interactions worked simultaneously, and the adsorption might be co-driven by ligand exchange, hydrophobic interactions, and other mechanisms. This work provided the theoretical basis for organic carbon storage and turnover, and it was valuable for predicting the behaviors and fates of contaminants at the soil-water interface and surface water.

Loss and Increase of the Electron Exchange Capacity of Natural Organic Matter during Its Reduction and Reoxidation: The Role of Quinone and Nonquinone Moieties

Redox-active quinone and nonquinone moieties represent the electron exchange capacity (EEC) of natural organic matter (NOM), playing an important role in the electron transfer link of microbes and transformation of contaminants/metal minerals. However, the corresponding transformation of quinone/phenol and their respective influence on the EECs during reduction and reoxidation remain poorly characterized. Besides, it is still controversial whether nonquinones donate or accept electrons. Herein, we demonstrated that reoxidation of NOM after reduction can form new phenolic/quinone moieties, thus increasing the EEC. The assessment for the EEC, including the electron-donating capacity (EDC) and electron-accepting capacity (EAC), of nonquinones reflects the contribution of sulfur-containing moieties with considerable EDCs and EACs. In contrast, nitrogen-containing moieties donate negligible electrons even at E_h = +0.73 V. The contributions of both thiol and amine moieties to the EEC are greatly affected by adjacent functional groups. Meanwhile, aldehydes/ketones did not display an EAC during the electron transfer process of NOM. Furthermore, substantially increased EDC at E_h from +0.61 to +0.73 V could not be fully explained using thiol and phenolic moieties, suggesting the contribution of unknown moieties with high oxidation potential. The overall findings suggest that the roles of new quinones/phenol (derived from the addition of oxygen to condensed aromatic/lignin-like components) during redox dynamic cycling and thiol species should be considered in assessing the electron transfer processes of NOM.

The transformation of Benzophenone-3 in natural waters and AOPs: The roles of reactive oxygen species and potential environmental risks of products

Benzophenone-3 (BP-3) is a widespread emerging organic pollutant. However, little is known about the synergistic effect of various reactive oxygen species (ROS) in natural waters and wastewater treatment plants on its transformation. In this study, the indirect photochemical behavior of BP-3 in the natural aquatic environments and the degradation process in the AOPs system were investigated by theoretical chemistry calculations. Besides the potential eco-toxicity effects, health effects, and bioaccumulation of the transformation products were assessed by computational toxicology. Results of transformation mechanism and kinetics showed that OH· and ¹O₂ are the keys to the transformation of BP-3, whereas the role of HO₂· and O₃ can be ignored. AOPs based on OH· and ¹O₂ could lead to the rapid transformation of BP-3, while the transformation of BP-3 in natural waters is slow, and even environmental persistence can be observed. However, dissolved organic matter (DOM) promotes the indirect phototransformation of BP-3 in natural waters. A variety of transformation products are generated under the synergistic effects of ROS, H₂O, and ³O₂. Assessments of environmental risks indicated that the potential eco-toxicity and health effects of the main products are significantly lower than that of the parent BP-3. More importantly, low bioaccumulation of transformation products would not enlarge their eco-toxicity and health effects. This study not only gives valuable insights into the indirect phototransformation of BP-3 in natural waters but also provides theoretical support for the feasibility of BP-3 degradation in industrial wastewater by AOPs based on OH· and ¹O₂.

Comparing Photoactivities of Dissolved Organic Matter Released from Rice Straw-Pyrolyzed Biochar and Composted Rice Straw

Here, we systematically compared the photoactivity and photobleaching behavior between dissolved black carbon (DBC) from rice straw biochar and leached dissolved organic carbon (LDOC) from rice straw compost using complementary techniques. The Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis showed that DBC was dominated by polycyclic aromatic (55.1%) and tannin-like molecules (24.1%), while LDOC was dominated by lignin-like (58.9%) and tannin-like molecules (19.7%). Under simulated sunlight conditions, DBC had much higher apparent quantum yields for ³DOM* and ¹O₂ but much lower apparent quantum yields for ^•OH than LDOC. After a 168 h irradiation, the total number of LDOC formulas identified by FT-ICR MS decreased by 40.1% with concurrent increases in O/C and H/C ratios and also decreases in double bond equivalence minus oxygen (DBE - O) and average molecular weight identified by gel permeation chromatography. However, despite the large decreases in UVA₂₅₄ and DOC, the total number of DBC formulas decreased only by 12.0% with nearly unchanged O/C ratio, DBE - O values, molecular weight distribution, and benzenepolycarboxylic aromatic condensation (BACon) index regardless of the decreased percentage of condensed aromatic carbon (ConAC %). Compared with LDOC, the photolysis of DBC was much less oxidative and destructive mainly via breakup of a small portion of the highly condensed aromatic rings, probably accompanied by photodecarboxylation.

Unexpected molecular diversity of brown carbon formed by Maillard-like reactions in aqueous aerosols

Atmospheric brown carbon (BrC) exerts a key impact on the global radiative balance due to its light-absorbing properties. Maillard-like reactions between carbonyl and amino compounds have been identified as an important pathway for forming secondary BrC. Although optical properties have been widely studied, the molecular composition of secondary BrC generated in Maillard chemistry remains unclear, resulting in a knowledge gap to understand its formation and light-absorbing mechanism. In this study, a combination of optical spectroscopy, ¹H nuclear magnetic resonance (NMR), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was employed to comprehensively characterize the chemical and light-absorbing characteristics of secondary BrC. The results indicate that both the light-absorbing and molecular characteristics of secondary BrC were highly related to the structures of their precursors. Organic amine precursors consistently result in enhanced light-absorbing capacities of BrC compared to ammonium, but have inconsistent effects on the molecular diversity of BrC. Compared to amino precursors (i.e., glycine, ethylamine, propylamine, and ammonium), carbonyl precursors play a more important role in determining the molecular diversity of BrC. Different from black carbon, the light-absorbing products from Maillard-like reactions are mainly nitrogen-containing heterocycles. Unexpectedly, 35–64% of molecular formulae detected in real atmospheric samples were found in simulated Maillard reaction products, implying a potentially important contribution of Maillard chemistry to the atmospheric organic molecular pool. These results will improve our understanding of the formation and molecular diversity of BrC, and further help to manage emissions of secondary aerosol precursors.

We found unexpected molecular diversity of brown carbon formed by Maillard-like reactions in aqueous aerosols, and carbonyl precursors play a more important role in determining the molecular diversity of brown carbon.

Photosensitized formation of sulfate and volatile sulfur gases from dissolved organic sulfur: Roles of pH, dissolved oxygen, and salinity

The photodegradation of dissolved organic sulfur (DOS) is a potential source of aqueous sulfate and its chemical precursors in surface water. However, the photochemical fate of DOS and factors that control its fate still remain unclear. Herein, we employed a DOS model featuring a photosensitizer (humic acids, HA) to investigate the photochemical degradation pathways of DOS in various natural water sources, from which we observed the substantial photosensitized formation of sulfate, methanesulfonic acid (MSA), carbonyl sulfide (COS), and carbon disulfide (CS₂). However, the photochemical production of sulfate and MSA tends to be more efficient than COS and CS₂. The formation of sulfur-containing photodegradation products was also strongly affected by the identity of the organic sulfur precursor, the oxygen concentration, and the pH, while the salinity did not significantly influence the production ratios. Our results revealed that the photosensitization of DOS contributed significantly to the overall production of sulfate and MSA production, especially in acidic and oxygen-enriched environments, which was attributed to the photochemical production of reactive intermediates, such as excited CDOM (³CDOM*) and reactive oxygen species (ROS). Considering the coexistence of DOS and photosensitizers in aquatic environments, photochemistry may play an essential role in the fate of aquatic DOS.

Singlet Oxygen Quantum Yields in Environmental Waters

Singlet oxygen (¹O₂) is a reactive oxygen species produced in sunlit waters via energy transfer from the triplet states of natural sensitizers. There has been an increasing interest in measuring apparent ¹O₂ quantum yields (Φ_Δ) of aquatic and atmospheric organic matter samples, driven in part by the fact that this parameter can be used for environmental fate modeling of organic contaminants and to advance our understanding of dissolved organic matter photophysics. However, the lack of reproducibility across research groups and publications remains a challenge that significantly limits the usability of literature data. In the first part of this review, we critically evaluate the experimental techniques that have been used to determine Φ_Δ values of natural organic matter, we identify and quantify sources of errors that potentially explain the large variability in the literature, and we provide general experimental recommendations for future studies. In the second part, we provide a qualitative overview of known Φ_Δ trends as a function of organic matter type, isolation and extraction procedures, bulk water chemistry parameters, molecular and spectroscopic organic matter features, chemical treatments, wavelength, season, and location. This review is supplemented with a comprehensive database of Φ_Δ values of environmental samples.