Dual role of soil-derived dissolved organic matter in the sulfamethoxazole oxidation by manganese dioxide

Unravelling riverine dissolved organic matter sources using molecular fingerprints and FEAST model in a multi-tributary mountain river basin

Revealing the sources, composition and fate of riverine dissolved organic matter (DOM) is fundamental to understanding the biogeochemical cycles of aquatic ecosystems. This study aimed to reveal the impact of land uses and wastewater treatment plants (WWTPs) on riverine DOM. Spatiotemporal variations in molecular characteristics of riverine DOM in the river network containing 15 tributaries in the mainstream of upper Hanjiang River were studied. Differences in molecular characteristics of DOM in soil leachates of various land uses and the effluent of WWTPs were analyzed and their contributions to riverine DOM in both dry and wet seasons were calculated using FEAST model. DOM in soil leachates was primarily composed of lignin, protein and lipid-like compounds but was dominated by lignin and tannin-like compounds in the effluent of WWTPs. Contribution rates of the soil leachate of each land use calculated by FEAST model showed a significant positive linear correlation with the area-based proportion of each land use in the basins of tributaries. Contributions of area-based proportion of each land use to riverine DOM followed the order of grassland > forest > cropland for both seasons. DOM in the upstream of tributaries contributed more than 50 % to the molecular composition of DOM in the downstream of tributaries but the contribution of the effluent of WWTPs to riverine DOM did not exceed 3 %. These results demonstrated that FEAST model could be used for source identification of riverine DOM based on molecular fingerprint data. Accordingly, this study provides new insights into the carbon cycling and ecological health within the watershed.

Impact of Organic Carbon on Manganese Release, Colloid Formation, and Aggregation in Surface and Groundwater

The interaction of manganese (Mn) oxides with natural organic matter (NOM) can mobilize Mn, impacting groundwater quality. However, the formation of Mn colloids, critical determinants of Mn transport and aggregation, is often overlooked. To investigate Mn behavior and colloid formation upon C amendment, humic acid was reacted with Mn oxide suspensions at different C:Mn molar ratios (C:Mn = 0-15) over 200 h. The addition of organic carbon promoted the formation of highly stabilized and mixed-valence Mn colloids through reductive dissolution and complexation, with "aqueous" Mn (<450 nm, C:Mn = 15) release increasing by 56.3% from 0 to 200 h and colloidal Mn increasing by up to 31.9% (3-450 nm, C:Mn = 15, t = 200 h) compared to without carbon addition. Analysis of Mn oxidation state revealed that C-Mn colloids contained Mn in multiple oxidation states, and the percentage of Mn(II,III) relative to total Mn increased with increasing C concentrations. In surface water, the hydrodynamic diameter of both Mn and C-Mn colloids remained stable. In groundwater, C-Mn colloids (C:Mn = 3) remained stable (150 nm) over 30 days, while Mn colloids aggregated into larger particles. Analysis of natural surface and groundwaters identified a substantial fraction of Mn (up to 19.2 and 27.2%, respectively) existing in colloidal phases. These findings shed light on the intricate cycling of Mn among particulate, colloidal, and dissolved phases, which governs Mn fate and transport in the environment.

Biotic and abiotic transformations of aqueous film-forming foam (AFFF)-derived emerging polyfluoroalkyl substances in aerobic soil slurry

The severe contamination of per- and polyfluoroalkyl substances (PFAS) in aqueous film-forming foam (AFFF)-affected soil and groundwater has raised global concerns. Although extensive studies on the transformation of electrochemical fluorination (ECF)-based PFAS in soil exist, limited research on AFFF-derived emerging fluorotelomer (FT) compounds has been conducted. Herein, a total of 38 PFAS were identified in a composite AFFF formulation through suspect and nontarget screening using high-resolution mass spectrometry (HRMS), and emerging 6:2 FT compounds were particularly prominent. Subsequently, the composite AFFF formulation was introduced to aerobic soil slurry to investigate the transformation behaviors of nine high-abundance polyfluoroalkyl substances. After a 150-day incubation, polyfluorinated sulfonamide betaine and quaternary ammonium compounds showed significant recalcitrance. The tertiary amine- and thioether-based PFAS underwent biotic and abiotic transformations, with half-lives ranging from 2 to 56 days and from 38 to 248 days, respectively. On the basis of the products identified using HRMS, the transformation pathways of FT- and ECF-based PFAS were proposed. Notably, the hydroxylation of tertiary amines and the oxidation of thioethers were two major abiotic reactions. Toxicity prediction revealed that certain transformation products exhibited higher toxicity toward aquatic organisms compared with the parent compounds. This study provides valuable insights into the stability and transformation of emerging PFAS in aerobic soil.

From Quencher to Promoter: Revisiting the Role of 2,4,6-Trimethylphenol (TMP) in Triplet-State Photochemistry of Dissolved Organic Matter

Triplet-state dissolved organic matter (3DOM*) plays a crucial role in environmental aquatic photochemistry, with 2,4,6-trimethylphenol (TMP) frequently used as a chemical probe or quencher due to its high reactivity with 3DOM*. However, the influence of TMP-derived oxidation intermediates on the target photochemical reactions has not been comprehensively examined. This study investigated TMP's effect on the photolysis of sulfamethoxazole (SMX), a common antibiotic found in natural waters, in the presence of different DOM sources or model photosensitizer. Contrary to expectation, TMP significantly accelerated SMX photolysis, with the extent of enhancement depending on TMP and DOM concentrations. Laser flash photolysis and kinetic modeling suggested the long-lived TMP-derived reactive species (TMP-RS), including phenoxyl radicals, semiquinone radicals, and quinones, as the key factors in this process. Unlike 3DOM*, TMP-RS may react with SMX with the formation of non-SMX•+ intermediates. This process prevents the reduction of SMX•+ and the subsequent regeneration of SMX. The kinetic model successfully predicts the dynamic contributions of various factors to SMX oxidation during the reaction, highlighting the critical role of TMP-RS. This study advances our understanding of TMP's involvement in triplet-state photochemistry and suggests a reconsideration of the role long-lived organic RSs play in the transformation of environmental micropollutants.

Unraveling the dual roles of dissolved organic matter on the photodegradation of aquatic contaminants: Molecular weight- and type-dependent heterogeneities

Dissolved organic matter (DOM) in natural waters can regulate the behaviors and fates of aquatic contaminants, while the specific effects on contaminant attenuation are highly dependent on its inherent properties [e.g., molecular weights (MW) and types]. In this study, the algae-derived organic matter (AOM) and humic acid (HA) were selected as the representative autochthonous and allochthonous DOMs, which were further fractionated into low MW (LMW, <1 kDa) and high MW (HMW, <1 kDa∼0.45 μm) fractions to evaluate the MW- and type-dependent heterogeneities in the photodegradation of sulfadiazine (SDZ). Results showed that presence of bulk AOM promoted SDZ photodegradation by 2.45 folds while those of the bulk HA inhibited SDZ photodegradation by 1.70 folds due to the higher light screening effects and phenolic antioxidant concentrations. Further analysis revealed obvious MW-dependent heterogeneities that, regardless of DOM types, the HMW-fraction always inhibited SDZ photodegradation while the LMW-fraction promoted photodegradation efficiencies owing to higher carbonyl contents and electron transfer capabilities. In addition, the MW-dependent heterogeneities within DOM samples resulted in different photodegradation pathways and Ecological Structure-Activity Relationship (ECOSAR) calculation showed that most of photodegradation products in the LMW-fraction were more ecotoxicity than the parent SDZ while those in the HMW-fraction exhibited alleviated ecotoxicity. This study indicated that the dual roles of aquatic DOMs on contaminant photodegradation were MW- and type-dependent, and detailed structural composition analysis on DOM matrix was needed for a better assessment of the behaviors and fates of contaminants in aquatic ecosystems.

Heating-Induced Changes in Content and Molecular Characteristics of Pyrogenic Dissolved Organic Matter across Soil Types

Wildfires remarkably alter the quantity and quality of dissolved organic matter (DOM) that regulates postfire biogeochemical processes and environmental quality. However, it remains unclear how the heating-induced percent changes (%HIC) in DOM quantity and quality differ among soil types on a wide geographic scale. Here, we used dissolved organic carbon (DOC) quantification, absorption, and fluorescence spectroscopies, and Fourier transform ion cyclotron resonance mass spectrometry to investigate the variations in %HIC in DOM quantity and quality of Chinese soil reference materials after heating at 250 and 400 °C. Our results reveal that as soil pH increased, %HIC in DOC content increased, while %HIC in aromaticity-related indices of DOM decreased for both heating temperatures. Moreover, the %HIC in DOM biolability and contents of aliphatics increased with soil pH for 250 °C heating but remained relatively stable for 400 °C heating. Results suggest that compared to those in acidic soil-dominated forests, wildfires in alkaline soil-dominated forests may cause greater DOM content and biolability in soils, which may facilitate postfire microbial recovery. These findings deepen our understanding of the site-specific impacts of wildfires on DOM and the subsequent implications for biogeochemical cycling and environmental quality across different geographic regions.

Temporal Variations in Fire Impacts on Characteristics and Composition of Soil-Derived Dissolved Organic Matter at Qipan Mountain, China

Dissolved organic matter (DOM), the most reactive fraction of forest soil organic matter, is increasingly impacted by wildfires worldwide. However, few studies have quantified the temporal changes in soil DOM quantity and quality after fire. Here, soil samples were collected after the Qipan Mountain Fire (3-36 months) from pairs of burned and unburned sites. DOM contents and characteristics were analyzed using carbon quantification and various spectroscopic and spectrometric techniques. Compared with the unburned sites, burned sites showed higher contents of bulk DOM and most DOM components 3 months after the fire but lower contents of them 6-36 months after the fire. During the sharp drop of DOM from 3 to 6 months after the fire, carboxyl-rich alicyclic molecule-like and highly unsaturated compounds had greater losses than condensed aromatics. Notably, the burned sites had consistently higher abundances of oxygen-poor dissolved black nitrogen and fluorescent DOM 3-36 months after the fire, particularly the abundance of pyrogenic C2 (excitation/emission maxima of <250/∼400 nm) that increased by 150% before gradually declining. This study advances the understanding of temporal variations in the effects of fire on different soil DOM components, which is crucial for future postfire environmental management.

Reassessing the Quantum Yield and Reactivity of Triplet-State Dissolved Organic Matter via Global Kinetic Modeling

Measuring the quantum yield and reactivity of triplet-state dissolved organic matter (3DOM*) is essential for assessing the impact of DOM on aquatic photochemical processes. However, current 3DOM* quantification methods require multiple fitting steps and rely on steady-state approximations under stringent application criteria, which may introduce certain inaccuracies in the estimation of DOM photoreactivity parameters. Here, we developed a global kinetic model to simulate the reaction kinetics of the hv/DOM system using four DOM types and 2,4,6-trimethylphenol as the probe for 3DOM*. Analyses of residuals and the root-mean-square error validated the exceptional precision of the new model compared to conventional methods. 3DOM* in the global kinetic model consistently displayed a lower quantum yield and higher reactivity than those in local regression models, indicating that the generation and reactivity of 3DOM* have often been overestimated and underestimated, respectively. The global kinetic model derives parameters by simultaneously fitting probe degradation kinetics under different conditions and considers the temporally increasing concentrations of the involved reactive species. It minimizes error propagation and offers insights into the interactions of different species, thereby providing advantages in accuracy, robustness, and interpretability. This study significantly advances the understanding of 3DOM* behavior and provides a valuable kinetic model for aquatic photochemistry research.

Effect of interaction between dissolved organic matter and iron/manganese (hydrogen) oxides on the degradation of organic pollutants by in-situ advanced oxidation techniques

Iron and manganese (hydrogen) oxides (IMHOs) exhibit excellent redox capabilities for environmental pollutants and are commonly used in situ chemical oxidation (ISCO) technologies for the degradation of organic pollutants. However, the coexisting dissolved organic matter (DOMs) in surface environments would influence the degradation behavior and fate of organic pollutants in IMHOs-based ISCO. This review has summarized the interactions and mechanisms between DOMs and IMHOs, as well as the properties of DOM-IMHOs complexes. Importantly, the promotion or inhibition impact of DOM was discussed from three perspectives. First, the presence of DOMs may hinder the accessibility of active sites on IMHOs, thus reducing their efficiency in degrading organic pollutants. The formation of compounds between DOMs and IMHOs alters their stability and activity in the degradation process. Second, the presence of DOMs may also affect the generation and transport of active species, thereby influencing the oxidative degradation process of organic pollutants. Third, specific components within DOMs also participate and affect the degradation pathways and rates. A comprehensive understanding of the interaction between DOMs and IMHOs helps to better understand and predict the degradation process of organic pollutants mediated by IMHOs in real environmental conditions and contributes to the further development and application of IMHO-mediated ISCO technology.

Self-motivated photoaging of microplastics by biochar-dissolved organic matter under different pyrolysis temperatures

Dissolved organic matter (DOM) released from biochar (BDOM) can interact with microplastics (MPs) in the environment, inevitably affecting their environmental behaviour. Information regarding the influence of BDOM on MPs during photoaging and associated variations in the MP aging mechanism remains unclear. This study evaluated the effect of BDOM on the aging of polystyrene (PS) MPs. The results showed that among three pyrolysis temperatures, low-temperature BDOM significantly enhanced the photoaging process of PS MPs, with the smallest average particle size and highest carbonyl index value after 15 days of aging under light conditions. The DOM level decreased after 5 days, increased after 5-10 days, and stabilised after 15 d. BDOM accelerates PS MPs aging, leading to more DOM released from PS, which can be transformed into 1O2 via triplet-excited state (3DOM⁎ and 3PS⁎) to further enhance PS MPs aging, resulting in the realisation of the self-accelerated aging process of PS MPs. 1O2 plays a crucial role in the self-motivated accelerated aging process of PS MPs. These findings provide new insights into the effects of the DOM structure and composition on reactive oxygen species generation during MPs aging.

Advanced treatment of antibiotic-polluted wastewater by a consortium composed of bacteria and mixed cyanobacteria

This study constructed a cyanobacteria-bacteria consortium using a mixture of non-toxic cyanobacteria (Synechococcus sp. and Chroococcus sp.) immobilized in calcium alginate and native bacteria in wastewater. The consortium was used for the advanced treatment of sulfamethoxazole-polluted wastewater and the production of cyanobacterial lipid. Mixed cyanobacteria increased the abundances of denitrifying bacteria and phosphorus-accumulating bacteria as well as stimulated various functional enzymes in the wastewater bacterial community, which efficiently removed 70.01-71.86% of TN, 91.45-97.04% of TP and 70.72-76.85% of COD from the wastewater. The removal efficiency of 55.29-69.90% for sulfamethoxazole was mainly attributed to the upregulation of genes encoding oxidases, reductases, oxidoreductases and transferases in two cyanobacterial species as well as the increased abundances of Stenotrophomonas, Sediminibacterium, Arenimonas, Novosphingobium, Flavobacterium and Hydrogenophaga in wastewater bacterial community. Transcriptomic responses proved that mixed cyanobacteria presented an elevated lipid productivity of 33.90 mg/L/day as an adaptive stress response to sulfamethoxazole. Sediminibacterium, Flavobacterium and Exiguobacterium in the wastewater bacterial community may also promote cyanobacterial lipid synthesis through symbiosis. Results of this study proved that the mixed cyanobacteria-bacteria consortium was a promising approach for advanced wastewater treatment coupled to cyanobacterial lipid production.

Influence of Divalent Cation Inhibition and Dissolved Organic Matter Enhancement on Phenol Oxidation Kinetics by Manganese Oxides

Manganese oxides can oxidize organic compounds, such as phenols, and may potentially be used in passive water treatment applications. However, the impact of common water constituents, including cations and dissolved organic matter (DOM), on this reaction is poorly understood. For example, the presence of DOM can increase or decrease phenol oxidation rates with manganese oxides. Furthermore, the interactions of DOM and cations and their impact on the phenol oxidation rates have not been examined. Therefore, we investigated the oxidation kinetics of six phenolic contaminants with acid birnessite in ten whole water samples. The oxidation rate constants of 4-chlorophenol, 4-tert-octylphenol, 4-bromophenol, and phenol consistently decreased in all waters relative to buffered ultrapure water, whereas the oxidation rate of bisphenol A and triclosan increased by up to 260% in some waters. Linear regression analyses and targeted experiments demonstrated that the inhibition of phenol oxidation is largely determined by cations. Furthermore, quencher experiments indicated that radical-mediated interactions from oxidized DOM contributed to enhanced oxidation of bisphenol A. The variable changes between compounds and water samples demonstrate the challenge of accurately predicting contaminant transformation rates in environmentally relevant systems based on experiments conducted in the absence of natural water constituents.

Oxidation of Sb(III) by Shewanella species with the assistance of extracellular organic matter

Antimony (Sb) is a toxic substance that poses a serious ecological threat when released into the environment. The species and redox state of Sb determine its environmental toxicity and fate. Understanding the redox transformations and biogeochemical cycling of Sb is crucial for analyzing and predicting its environmental behavior. Dissolved organic matter (DOM) in the environment greatly affects the fate of Sb. Microbially produced DOM is a vital component of environmental DOM; however, its specific role in Sb(III) oxidation has not been experimentally confirmed. In this work, the oxidation capacity of several Shewanella strains and their derived DOM to Sb(III) was confirmed. The oxidation rate of Sb(III) shows a positive correlation with DOM concentration, with higher rates observed under neutral and weak alkaline conditions, regardless of the presence of light. Incubation experiments indicated that extracellular enzymes and common reactive oxygen species were not involved in the oxidation of Sb(III). Characteristics of DOM suggests that microbial humic acid-like and fulvic acid-like substances are the potential contributors to Sb(III) oxidation. These findings not only experimentally validate the role of bacterial-derived DOM in Sb(III) oxidation but also reveal the significance of Shewanella and biogenic DOM in the biogeochemical cycling of Sb.

Understanding molecular-level reactions between permanganate/ferrate and dissolved effluent organic matter from municipal secondary effluent

Permanganate (Mn(VII)) and ferrate ((Fe(VI)) oxidations are well-recognized as advanced treatment processes to degrade organic components detected in secondary effluents from municipal wastewater treatment plant (WWTP). However, Mn(VII) or Fe(VI) reactions are sensitive to the water matrix components, especially the dissolved effluent organic matter (EfOM) contained in secondary effluent. Here, we found that Mn(VII) or Fe(VI) hardly mineralize EfOM, but do change its molecular-level composition. Tests with representative trace organic contaminants (TrOCs) spiked to EfOM-rich wastewater effluent showed that Mn(VII) and Fe(VI) preferentially oxidize phenolic TrOCs. UV-vis and fluorescence spectroscopy analyses suggested that molecular properties associated with optical characteristics of reaction solutions are altered by treatment, including decreases in aromaticity, molecular weight, and electron-donating capacity of EfOM. At the molecular-level, the observed phenomenon is ascribed to preferential oxidation of aromatic structures and electron-rich functional groups (e.g., phenolic structures and hydroxyl groups) within EfOM, as well as the transformation and decomposition of macromolecular sulfur- and nitrogen-containing compounds (most likely proteins or microbial byproduct-like material). These findings advance the application of Mn(VII) and Fe(VI) for optimization and proper control of EfOM of emerging concern within treatment trains being developed for advanced treatment facilities.