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

Management strategies to reduce microbial mercury methylation in constructed wetlands: Potential routes and future challenges

Constructed wetlands (CWs) are widely recognized as the potential hotspots for producing highly toxic methylmercury (MeHg). This presents an obstacle to the widespread application of CWs. A comprehensive discussion on strategies to control mercury methylation in CWs is currently lacking. This review highlighted the potential impacts of differences in oxygen supply and consumption in various CWs, the characteristics of influent quality, the interactions between different substrates and mercury (including mercury adsorption, reduction), and plants on microbial mercury methylation in CWs. We also proposed the potential strategies for human intervention in regulating or controlling microbial mercury methylation in CWs, including oxygenation, nitrate inhibition, selection of substrates with high adsorption capacity, weak reducibility and low organic matter release, and plant management. Knowledge summarized in this review would help achieve a comprehensive understanding of various research gaps in previous studies and point out future research directions by focusing on CWs types, influent quality, substrates selection and plants management, to reduce the mercury methylation in CWs.

Contradictions in dissolved black carbon research: A critical review of its sources, structures, analytical methods, and environmental behaviors

Dissolved black carbon (DBC) represents the most active component within the black carbon (BC) continuum and plays a vital role in the global carbon cycle and the removal of inorganic and organic contaminants due to its prolonged residence time and unique condensed aromatic structure. Significant progress has been made in understanding DBC source, molecular structure, analytical methods, stability, and environmental behavior, particularly its photochemical and microbial transformation. However, substantial uncertainties persist, including ambiguities in its definition, limitations in isolation and quantification methods, and unidentified sources. These limitations have led to lots of inconsistencies regarding its stability, environmental transport pathways, and transformation mechanisms. This review critically examines the current landscape of DBC research, with a focus on: (1) key contradictions in DBC cycling processes, including debates over its recalcitrance, mismatched isotopic signatures, and imbalances in the marine DBC budget; (2) limitations for DBC isolation and quantification methods in natural environments; and (3) photochemical and microbial transformation processes, and its interactions with environmental pollutants. By synthesizing recent insights, this review aims to enhance the understanding of DBC's structures, turnover, and environmental behavior, as well as its implications for the global carbon cycle. To address existing challenges, future studies are suggested to prioritize resolving these contradictions, developing standardized analytical approaches, and achieving a clearer elucidation of DBC cycling processes across diverse environments.

Overlooked risks of photoaging of nitrogenous microplastics with natural organic matter in water: Augmenting the formation of nitrogenous disinfection by-products

In aqueous environments, microplastics (MPs) undergo photoaging, releasing dissolved organic matter (DOM). Disinfection byproducts (DBPs) formation from natural organic matter (NOM) phototransformation has been reported. However, the impact of NOM on the photoaging of MPs (especially nitrogen-containing MPs) and subsequent nitrogenous DBPs (N-DBPs) formation remains unknown. Herein, this study investigated polyamide (PA) with NOM (fulvic acid [FA], humic acid [HA] and biochar-derived DOM [BDOM]) on N-DBPs formation. Results showed that the levels of the main DBPs, N-nitrosamine, formed in the FA+PA, BDOM+PA, and HA+PA systems were 3.0. 2.7 and 1.6 folds higher, respectively, compared to those in the corresponding NOM treatments. NDMA was found to be the dominant N-nitrosamine species, with the highest level of 202 ng/L, exceeding the WHO guideline of 100 ng/L. The main reactive intermediates (RIs) were 1O2 and reactive nitrogen species (RNS) during the first stage (0-3d), and •OH and RNS during the second stage (3-7d), which were confirmed by quenching experiment. For the first time, we found the formation of N-DBPs during photoaging of N-containing MPs, and proposed a two-stages, two-processes, and two-pathways theory of N-DBPs formation. This work emphasizes the importance to understand the interactions between the MPs and NOM in photoaging to better assess the risk of DBPs formation in aqueous environment.

Photochemistry of microplastics-derived dissolved organic matter: Reactive species generation and organic pollutant degradation

Dissolved organic matter (DOM) originating from microplastics (MPs-DOM) is increasingly recognized as a substantial component of aquatic DOM. The photochemistry of MPs-DOM, essential for understanding its environmental fate and impacts, remains largely unexplored. This study investigates the photochemical behaviors of MPs-DOM derived from two common plastics: polystyrene (PS) and polyvinyl chloride (PVC), which represent aromatic and aliphatic plastics, respectively. Spectral and high-resolution mass spectrometry analyses demonstrated that photoreactions preferentially targeted poly-aromatic compounds within the MPs-DOM, leading to degradation products that predominantly form N-aliphatic/lipid-like substances. This transformation is characterized by decreased aromaticity and unsaturation. Additionally, irradiation of MPs-DOM generated reactive species (RS), including triplet intermediates (3DOM*) and singlet oxygen (1O2), with apparent quantum yields of 0.06-0.16 % and 0.16-0.35 %, respectively-values considerably lower than those for conventional DOM (1.19-1.56 % for 3DOM* and 1.34-1.90 % for 1O2). Despite this, the RS generated from MPs-DOM significantly enhance the degradation of coexisting organic pollutants, such as antibiotic resistance genes (ARGs). The findings shed light on the photoinduced transformation of MPs-DOM and suggest that MPs-DOM functions as a natural photocatalyst, mediating redox reactions of pollutants in sunlit aquatic settings. This highlights its previously underestimated role in natural attenuation and aquatic photochemistry.

Carbon residue from co-pyrolysis of cartons and plastics: Characteristics, environmental behaviors and applications

The continuously growing of municipal solid waste (MSW) has posed a threat to human-being. Pyrolysis is a promising technique for MSW disposal, as it can reduce its volume and produce valuable products as well. This study evaluated the potential of carbon residue (CR) derived from waste carton as soil amendment. Additionally, considering the waste plastics, such as plastic bags, plastic tape, etc., can mix with carton in MSW, the effect of polyvinyl chloride (PVC) addition on the characteristics of CR and its environmental behavior was measured. Results showed that the CR derived from carton exhibited notable efficacy in adsorbing Cd2+ (56.111 mg/g), pesticides and PO43- (0.231 mmol/g), thereby mitigating pollutants and immobilizing nutrient in soil environment. The introduction of PVC was found to enhance the adsorption of CR for Cd2+ (65.623 mg/g) and PO43- (0.524 mmol/g), albeit exhibiting diminished performance in the removal of pesticides. Different from previous studies, this study revealed that the dissolved black carbon (DBC) released from CRs did not generate reactive oxygen species (ROS) effectively under solar irradiation, and the light screening of DBC can mitigate the photodegradation of pollutants. Furthermore, both CRs increased bacterial luminescence by approximately 70%, which indicated that no toxicity produced during the pyrolysis process. And gas chromatography-mass spectrometry (GC-MS) revealed the absence of polyaromatic hydrocarbon (PAH) in CRs. In summary, this study may provide some new insights on the disposal of MSW, and the CR made by MSW could be effective in soil amendment and reduce the leaching of pollutants and nutrient.

Photochemistry of dissolved organic matter derived from compost

The extensive application of compost to enhance soil quality highlights the crucial role of dissolved organic matter (DOM) derived from compost in both terrestrial and aquatic ecosystems, influencing carbon cycling and the fate of contaminants. However, the photochemical behavior of compost-derived DOM (DOMCOM) remains poorly understood. In this study, we investigated the photochemical transformation and photoactivity of DOM derived from typical composts produced from cow manure (CDOM) and pig manure (PDOM). The results indicated that the initial CDOM exhibited higher molecular weight, aromaticity, humification, and photoactivity compared to PDOM. Under UV irradiation, both DOMCOM underwent photobleaching and photo-humification, resulting in a decrease in the average molecular weight by 23.68 % for CDOM and 3.82 % for PDOM, with CDOM being particularly affected. Meanwhile, 2D-COS analysis revealed that the fulvic-like fluorescence fraction was first to respond to photoirradiation in both DOM, followed by the protein-like and microbial humic-like fluorescence fractions, which showed contrasting response trends in CDOM and PDOM. Furthermore, CDOM with a higher concentration of humic-like substances efficiently generated 3DOM*, 1O2 and •OH (4.09 × 10-8, 1.17 × 10-8 and 7.05 × 10-12, respectively) under UV radiation, which were apparently greater than those produced by PDOM (3.30 × 10-8, 8.38 × 10-9 and 4.99 × 10-12, respectively).

Insights into the photosensitivity and photobleaching of dissolved organic matter from microplastics: Structure-activity relationship and transformation mechanism

Revealing the structure-activity relationship between physicochemical properties and photoactivities of microplastic dissolved organic matter (MPDOM) is significant for understanding the environmental fate of MPs. Here, we systematically analyzed the physicochemical properties and molecular composition of DOM derived from MPs including polystyrene (PS), polyethylene glycol terephthalate (PET), polyadipate/butylene terephthalate (PBAT), polylactic acid (PLA), polypropylene (PP), and compared their photosensitivity and photobleaching behaviors. Results indicated that PSDOM and PETDOM had more similar properties and compositions, and showed stronger photosensitivity and photobleaching effects than PBATDOM, PLADOM and PPDOM. The [3DOM∗]SS and [1O2]SS varied in the range of 0.31-13.03 × 10-14 and 1.71-5.49 × 10-13 M, respectively, which were within the reported range of DOM from other sources. The SUVA254, HIX, AImodwa, Xcwa and lignin/CRAM-like component showed positive correlation with the [3DOM∗]SS, [1O2]SS and Φ3DOM*. The negative correlation between E2/E3 and [3DOM∗]SS was due to the higher proportion of low-molecular weight components in MPDOM. The lignin/CRAM-like component was identified to be the crucial photobleaching-component. The lignin/CRAM-like in PSDOM showed a deepened oxidation degree, while its change trend in PETDOM was from unsaturated to saturated. These findings provide new insights into the relevant photochemical fate of MPDOM.

Phototransformation and photoreactivity of MPs-DOM in aqueous environment: Key role of MPs structure decoded by optical and molecular signatures

The dissolved organic matter (DOM) derived from microplastics (MPs-DOM) can be one of the photoactive components in DOM. However, information on the properties and photoreactivity of MPs-DOM during phototransformation is limited. Here, we investigated the properties and photoreactivity of MPs-DOM from polyolefins (MPs-DOM-POs), MPs-DOM derived from benzene-containing polymers (MPs-DOM-BCPs), and Suwannee River natural organic matter (SR-NOM), during a 168-hour phototransformation. After phototransformation, all examined types of DOM exhibit a decrease in concentration and molecular weight. Notably, MPs-DOM-POs display increased aromaticity and saturation, while MPs-DOM-BCPs and SR-NOM show reduced aromaticity and saturation. MPs-DOM-POs present higher steady-state concentrations of •OH but much lower steady-state concentrations of 1O2 than those of MPs-DOM-BCPs. In comparison, MPs-DOM produce more •OH but less 1O2 than SR-NOM. This study proposes that the diversification of aliphatic C─H bonds (arylation and carbonylation) by reactive intermediates (especially •OH) is the main pathway for MPs-DOM-POs phototransformation for the first time. On the other hand, the cleavage on the aromatic carboxylic acids by reactive intermediates (especially 1O2) is the main mechanism for MPs-DOM-BCPs and SR-NOM phototransformation. Our findings provide new insights into the phototransformation and photoreactivity of MPs-DOM and help to understand the potential risks of MPs in aqueous environment.

Variation in photoactivity of dissolved black carbon during the fractionation process and the role in the photodegradation of various antibiotics

The composition of dissolved black carbon (DBC) could be influenced by adsorption on minerals, subsequently affecting DBC's photoactivity and the photoconversion of contaminants. This study investigated the changes in photoactivity of DBC after absorption on ferrihydrite at Fe/C ratios of 0, 1.75, 7.50, and 11.25, compared the influences of DBC0 and DBC7.50 on the photodegradation of four typical antibiotics (AB) including sulfadiazine, tetracycline, ofloxacin, and chloramphenicol. The selective adsorption led to the compounds with high aromaticity, high oxidation states, and more oxygen-containing functional groups being more favorably adsorbed on ferrihydrite, further causing the steady-state concentrations of 3DBC*, 1O2, and •OH respectively to drop from 1.83 × 10-13 M, 7.45 × 10-13 M, and 3.32 × 10-16 M in DBC0 to 1.22 × 10-13 M, 0.93 × 10-13 M and 2.30 × 10-16 M in DBC11.25, while the light screening effect factor increased from 0.740-0.921 in DBC0 with above four antibiotics to 0.775-0.970 for that of DBC11.25. Unexpectedly, DBC after adsorption played a dual role in the photodegradation of various antibiotics. This difference might be caused by antibiotics' chemical composition, functional groups interacting with reactive intermediates, and the overlap in UV-vis spectra between antibiotics and DBC. Our data are valuable for understanding the dynamic roles of DBC in the photodegradation of antibiotics.

Photo-transformation of biochar-derived dissolved organic matter and its binding with phenanthrene/9-phenanthrol: The role of functional group and pyrolysis temperature

This study explores the physicochemical attributes of dissolved organic matter from rice straw biochar (BDOM) at varying pyrolysis temperatures and photo-irradiation conditions, focusing on the binding mechanisms of phenanthrene (PHE) and 9-phenanthrol (PTR) using multiple spectroscopic techniques and fluorescence quenching. Following 20 h of photo-irradiation, only 11.3 % of BDOM underwent mineralization, forming new CH3/CH2/CH aliphatics structures. BDOM from biochar produced by pyrolysis at 400°C exhibited a stronger binding affinity with PHE and PTR, achieving 44 % and 52 % maximum binding, respectively. Static and dynamic quenching governed PHE and PTR binding, which was influenced by temperature. Photo-irradiated BDOM showed enhanced binding with PHE, attributed to increased aliphatic content. Hydrogen bond and π-π electron-donor-acceptor (EDA) interactions dominated PTR binding, while π-π interactions and hydrophobic interactions controlled PHE. This study provides valuable insights into BDOM photochemical behaviors and their impact on the environmental fate of polycyclic aromatic hydrocarbons (PAHs) after BDOM photo-irradiation.

Photochemical behavior of dissolved organic matter in environmental surface waters: A review

As an important group of widespread organic substances in aquatic ecosystems, dissolved organic matter (DOM) plays an essential role in carbon recycling and transformation processes. The photochemical behavior of DOM is one of the main ways it participates in these processes, and it attracts extensive attention. However, due to a variety of sources and water conditions, including both freshwater and seawater environments, the photochemical properties of DOM exhibit great differences. Nowadays, a large number of studies have focused on the generation process of reactive species (RS) from sunlit DOM, while little effort has been made so far to provide a comprehensive summary of the photochemical behavior of DOM, especially in fresh and saline aquatic ecosystems. In this review, we analyzed the research hotspot on DOM photochemistry over the last 30 years, summarizing the generation of photoreactive species in natural water environments containing DOM (both freshwater and seawater) and listing the main factors affecting the rate, yield, and species of RS photoproduction. Compared with freshwater, seawater has unique characteristics such as high pH value, high ionic strength, and halide ions, which affect the photogeneration of RS, the photoconversion process, as well as the reaction pathways of various environmental substances. In general, DOM-induced surface water photochemistry has important impacts on the environmental transformation and toxic effects of aquatic pollutants and can even contribute significantly to the Earth's carbon cycle, which would have potential implications for both human and ecological health.

Freeze-Thaw Cycle Events Enable the Deep Disintegration of Biochar: Release of Dissolved Black Carbon and Its Structural-Dependent Carbon Sequestration Capacity

Biochar is widely regarded as a recalcitrant carbon pool. However, the impact of freeze-thaw cycle events on its storage capacity, particularly on the release of dissolved black carbon (DBC), has remained poorly investigated. This study investigated the release behavior of DBC from biochar pyrolyzed at 300-700 °C during freeze-thaw cycles and their retention capacity in soil. Freeze-thaw cycles dramatically promoted DBC release (33.08-230.74 mg C L-1), exhibiting an order of magnitude higher than those without freeze-thaw process. The release kinetics of freeze-thaw-induced DBC varied depending on the pyrolysis temperature of biochar due to the different disintegration mechanisms. Interestingly, the retention capacity of freeze-thaw-induced DBC in soil showed a reduction ranging from 7.7 to 29.5% compared to DBC without the freeze-thaw process. This reduction can be attributed to numerous hydrophilic low-molecular-weight compounds (16.97-75.31%) in freeze-thaw-induced DBC, as evidenced by the results of size exclusion chromatography, fluorescence excitation/emission matrix, Fourier transform infrared spectroscopy, and nuclear magnetic resonance. These compounds tend to concentrate in the aqueous phase rather than being retained in the soil, potentially exacerbating the outflow of dissolved organic carbon. These findings clarify the release behavior of DBC during freeze-thaw cycles and reveal their contribution to the attenuation of carbon pools in cold regions.

Effects of dissolved organic matter from different sources on ritonavir photolysis

With the misuse of antiviral drugs, the residual levels of ritonavir (RTV) in aquatic environments continue to increase, potentially posing threats to ecosystems and human health. However, the current understanding of the photochemical behavior of RTV in water, especially the mechanism by which dissolved organic matter (DOM) from different sources affects the indirect photolysis of RTV, remains limited. This study systematically investigated the effects of DOM from different sources (including sludge, algae, dustfall, and soil, namely SL-DOM, AL-DOM, DF-DOM, and SO-DOM, respectively) on the photodegradation of RTV for the first time. DOM exhibited a dual role in RTV degradation, with SL-DOM and AL-DOM accelerating the degradation process, while DF-DOM and SO-DOM inhibited it. Direct photolysis accounted for 40-53% of the overall photodegradation, underscoring its significant contribution to the degradation process. Quenching and competitive kinetics experiments revealed that 3DOM⁎ is the dominant contributor to the indirect photolysis of RTV. Exogenous DOM (DF-DOM, SO-DOM) exhibited higher generation rate and steady-state concentraiton of 3DOM⁎, while endogenous DOM (SL-DOM, AL-DOM) exhibited higher quantum yields of 3DOM⁎ and reactivity, leading to distinct mechanisms for the indirect photodegradation of RTV. This study explored the effects of DOM from different sources on the photodegradation of RTV, providing important insights into how DOM affects the photochemical behavior and ecological risk of RTV. It also provides a reference for exploring the photochemical behavior of other drugs.

Sunlight-Driven Direct/Mediated Electron Transfer for Cr(VI) Reductive Sequestration on Dissolved Black Carbon-Ferrihydrite Coprecipitates

Surface runoff horizontally distributed chromium (Cr) pollution into various surface environments. Sunlight is a vital factor for the Cr cycle in the surface environment, which may be affected by photoactive substances such as ferrihydrite (Fh) and dissolved black carbon (DBC). Herein, sunlight-driven transformation dynamics of Cr species on DBC-Fh coprecipitates were studied. Under sunlight, the removal of aqueous Cr(VI) by DBC-Fh coprecipitates occurred through sunlight-driven reductive sequestration including adsorption, followed by surface reduction (pathway 1) and aqueous reduction, followed by precipitation (pathway 2). Additionally, coprecipitates with a higher DBC content exhibited a more effective reduction of both adsorbed (kapp,S_red) and aqueous Cr(VI) (kapp,A_red). Photoelectrons facilitated Cr(VI) reduction through direct electron transfer; notably, electron donating DBC promoted the production of photoelectrons by consuming photogenerated holes. Photogenerated Fe(II) species (mineral-phase and aqueous Fe(II)) mediated electron transfer for Cr(VI) reduction, which was reinforced via a ligand-to-metal charge transfer (LMCT) process between DBC-organic ligands and mineral Fe(III). Furthermore, ·O2- also mediated Cr(VI) reduction, although this impact was limited. Overall, this study demonstrates that photoelectrons and photogenerated electron mediators play a crucial role in Cr(VI) reductive sequestration on DBC-Fh coprecipitates, providing new insights into the geochemical cycle of Cr pollution in sunlight-influenced surface environments.

Photoinduced Evolutions of Permafrost-Derived Carbon in Subarctic Thermokarst Pond Surface Waters

In subarctic regions, rising temperature and permafrost thaw lead to the formation of thermokarst ponds, where organics from eroding permafrost accumulate. Despite its environmental significance, limited knowledge exists regarding the photosensitivity of permafrost-derived carbon in these ponds. In this study, laboratory experiments were conducted to explore the photochemical transformations of organic matter in surface water samples from thermokarst ponds from different environments in northern Quebec, Canada. One pond near Kuujjuarapik is characterized by the presence of a collapsing palsa and is therefore organically rich, while the other pond near Umiujaq is adjacent to a collapsing lithalsa and thus contains fewer organic matters. Photobleaching occurred in the Umiujaq sample upon irradiation, whereas the Kuujjuarapik sample exhibited an increase in light absorbance at wavelength related to aromatic functionalities, indicating different photochemical aging processes. Ultrahigh-resolution mass spectrometry analysis reveals that the Kuujjuarapik sample preferentially photoproduced highly unsaturated CHO compounds with great aromaticity, while the irradiated Umiujaq sample produced a higher proportion of CHON aromatics with reduced nitrogen functionalities. Overall, this study illustrates that the photochemical reactivity of thermokarst pond water varies with the source of organic matter. The observed differences in reactivity contribute to an improved understanding of the photochemical emission of volatile organic compounds discovered earlier. Further insights into the photoinduced evolutions in thermokarst ponds may require the classification of permafrost-derived carbon therein.

Novel insights into the relationship between the functional groups and photoactivity of biochar-derived dissolved organic matter

Due to the production of a large amount of biochar, highly photoactive biochar-derived dissolved organic matter (BDOM) from different sources is released into surface water. This study investigated the molecular composition of BDOM (sludge, bamboo and stalk BDOM) using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and used tetracycline (TC) as model pollutant to investigate the relationship between molecular composition and BDOM photoactivity, specifically focusing on reactive oxygen species (ROS) production. The results indicate that the fluorescence signal intensity of humic acid-like and aromatic proteins in the plant-derived BDOM are significantly higher than that derived from sewage sludge. FT-ICR MS results also showed that plant-derived BDOM contained more CHO molecular formula. Photodegradation experiments of TC mediated by various BDOM analogues demonstrated the photoactivity is highly correlated with the components and functional groups. The electrochemical experiments and density functional theory (DFT) calculations further verified that the aromatic moiety, sulfydryl group and amino group of BDOM affected the electronic supply and energy transfer. Higher electron and energy transfer favor the reaction of BDOM with the ground state oxygen to generate ROS, thus promoting photodegradation of TC. This study provides a new basis for better assessing the ecological risks of BDOM.

Dissolved Black Carbon Facilitates the Photodegradation of Microplastics via Molecular Weight-Dependent Generation of Reactive Intermediates

Photodegradation of microplastics (MPs) induced by sunlight plays a crucial role in determining their transport, fate, and impacts in aquatic environments. Dissolved black carbon (DBC), originating from pyrolyzed carbon, can potentially mediate the photodegradation of MPs owing to its potent photosensitization capacity. This study examined the impact of pyrolyzed wood derived DBC (5 mg C/L) on the photodegradation of polystyrene (PS) MPs in aquatic solutions under UV radiation. It revealed that the photodegradation of PS MPs primarily occurred at the benzene ring rather than the aliphatic segments due to the fast attack of hydroxyl radical (•OH) and singlet oxygen (1O2) on the benzene ring. The photosensitivity of DBC accelerated the degradation of PS MPs, primarily attributed to the increased production of •OH, 1O2, and triplet-excited state DBC (3DBC*). Notably, DBC-mediated photodegradation was related to its molecular weight (MW) and chemical properties. Low MW DBC (<3 kDa) containing more carbonyl groups generated more •OH and 1O2, accelerating the photodegradation of MPs. Nevertheless, higher aromatic phenols in high MW DBC (>30 kDa) scavenged •OH and generated more O2•-, inhibiting the photodegradation of MPs. Overall, this study offered valuable insights into UV-induced photodegradation of MPs and highlighted potential impacts of DBC on the transformation of MPs.

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.

Biochar-derived organic carbon promoting the dehydrochlorination of 1,1,2,2-tetrachloroethane and its molecular size effects: Synergies of dipole-dipole and conjugate bases

The environmental effects of biochar-derived organic carbon (BDOC) have attracted increasing attention. Nevertheless, it is unknown how BDOC might affect the natural attenuation of widely distributed chloroalkanes (e.g., 1,1,2,2-tetrachloroethane (TeCA)) in aqueous environments. We firstly observed that the kinetic constants (ke) of TeCA dehydrochlorination in the presence of BDOC samples or their different molecular size fractions (<1 kDa, 1∼10 kDa, and >10 kDa) ranged from 9.16×103 to 26.63×103 M-1h-1, which was significantly greater than the ke (3.53×103 M-1h-1) of TeCA dehydrochlorination in the aqueous solution at pH 8.0, indicating that BDOC samples and their different molecular size fractions all could promote TeCA dehydrochlorination. For a given BDOC sample, the kinetic constants (ke) of TeCA dehydrochlorination in the initial pH 9.0 solution was 2∼3 times greater than that in the initial pH 8.0 solution due to more formation of conjugate bases. Interestingly, their DOC concentration normalized kinetic constants (ke/[DOC]) were negatively correlated with SUVA254, and positively correlated with A220/A254 and the abundance of aromatic protein-like/polyphenol-like matters. A novel mechanism was proposed that the CH dipole of BDOC aliphatic structure first bound with the CCl dipole of TeCA to capture the TeCA molecule, then the conjugate bases (-NH-/-NH2 and deprotonated phenol-OH of BDOC) could attack the H atom attached to the β-C atom of bound TeCA, causing a CCl bond breaking and the trichloroethylene formation. Furthermore, a fraction of >1 kDa had significantly greater ke/[DOC] values of TeCA dehydrochlorination than the fraction of <1 kDa because >1 kDa fraction had higher aliphiticity (more dipole-dipole sites) as well as more N-containing species and aromatic protein-like/polyphenol-like matters (more conjugate bases). The results are helpful for profoundly understanding the BDOC-mediated natural attenuation and fate change of chloroalkanes in the environment.

Revealing Molecular Structures of Nitrogen-Containing Compounds in Dissolved Black Carbon Using Ultrahigh-Resolution Mass Spectrometry Combined with Thermodynamic Calculations

Landscape wildfires generate a substantial amount of dissolved black carbon (DBC) annually, yet the molecular nitrogen (N) structures in DBC are poorly understood. Here, we systematically compared the chemodiversity of N-containing molecules among three different DBC samples from rice straw biochar pyrolyzed at 300, 400, and 500 °C, one leached dissolved organic carbon (LDOC) sample from composted rice straw, and one fire-affected soil dissolved organic matter (SDOMFire) sample using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). N-Containing molecules contributed 20.0%, 36.1%, and 43.7% of total compounds in Combined DBC (pooling together the three DBC), LDOC, and SDOMFire, respectively, and molecules with fewer N atoms had higher proportions (i.e., N1 > N2 > N3). The N-containing molecules in Combined DBC were dominated by polycyclic aromatic (62.2%) and aromatic (14.4%) components, while those in LDOC were dominated by lignin-like (50.4%) and aromatic (30.1%) components. The composition and structures of N-containing molecules in SDOMFire were more similar to those in DBC than in LDOC. As the temperature rose, the proportion of the nitrogenous polycyclic aromatic component in DBC significantly increased with concurrent enhanced oxidation and unsaturation of N. As indicated by density functional theory (DFT)-based thermodynamic calculations, the proportion of aliphatic amide N decreased from 23.2% to 7.9%, whereas that of nitroaromatic N increased from 10.0% to 39.5% as the temperature increased from 300 to 500 °C; alternatively, the proportion of aromatic N in the 5/6 membered ring remained relatively stable (∼31%) and that of aromatic amide N peaked at 400 °C (32.7%). Our work first provides a comprehensive and thorough description of molecular N structures of DBC, which helps to better understand and predict their fate and biogeochemical behavior.

The molecular binding sequence transformation of soil organic matter and biochar dissolved black carbon antagonizes the transport of 2,4,6-trichlorophenol

Dissolved organic matter (DOM) and dissolved black carbon (DBC) are significant environmental factors that influence the transport of organic pollutants. However, the mechanisms by which their molecular diversity affects pollutant transport remain unclear. This study elucidates the molecular binding sequence and adsorption sites through which DOM/DBC compounds antagonize the transport of 2,4,6-trichlorophenol (TCP) using column experiments and modelling. DBC exhibits a high TCP adsorption rate (kn = 5.32 × 10-22 mol1-n∙Ln-1∙min-1) and conditional stability constant (logK = 5.19-5.74), indicating a strong binding affinity and antagonistic effect on TCP. This is attributed to the high relative content of lipid/protein compounds in DBC (25.65 % and 30.28 %, respectively). Moreover, the small molecule lipid compounds showed stronger TCP adsorption energy (Ead = -0.0071 eV/-0.0093 eV) in DOM/DBC, combined with two-dimensional correlation spectroscopy model found that DOM/DBC antagonized TCP transport in the environment through binding sequences that transformed from lipid/protein small molecule compounds to lignin/tannin compounds. This study used a multifaceted approach to comprehensively assess the impact of DOM/DBC on TCP transport. It reveals that the molecular diversity of DOM/DBC is a critical factor affecting pollutant transport, providing important insights into the environmental trend and ecological effects of pollutants.

Insights into the Crystallinity-Dependent Photochemical Productions of Reactive Oxygen Species from Iron Minerals

Iron minerals are widespread in earth's surface water and soil. Recent studies have revealed that under sunlight irradiation, iron minerals are photoactive on producing reactive oxygen species (ROS), a group of key species in regulating elemental cycling, microbe inactivation, and pollutant degradation. In nature, iron minerals exhibit varying crystallinity under different hydrogeological conditions. While crystallinity is a known key parameter determining the overall activity of iron minerals, the impact of iron mineral crystallinity on photochemical ROS production remains unknown. Here, we assessed the photochemical ROS production from ferrihydrites with different degrees of crystallinity. All examined ferrihydrites demonstrated photoactivity under irradiation, resulting in the generation of hydrogen peroxide (H2O2) and hydroxyl radical (•OH). The photochemical ROS production from ferrihydrites increased with decreasing ferrihydrite crystallinity. The crystallinity-dependent photochemical •OH production was primarily attributed to conduction band reduction reactions, with the reduction of O2 by conduction band electrons being the rate-limiting key process. Conversely, the crystallinity of iron minerals had a negligible influence on photon-to-electron conversion efficiency or surface Fenton-like activity. The difference in ROS productions led to a discrepant degradation efficiency of organic pollutants on iron mineral surfaces. Our study provides valuable insights into the crystallinity-dependent ROS productions from iron minerals in natural systems, emphasizing the significance of iron mineral photochemistry in natural sites with abundant lower-crystallinity iron minerals such as wetland water and surface soils.

High-Resolution Mass Spectrometry Combined with Reactive Oxygen Species Reveals Differences in Photoreactivity of Dissolved Organic Matter from Microplastic Sources in Aqueous Environments

Microplastics (MPs)-derived dissolved organic matter (MPs-DOM) is becoming a non-negligible source of DOM pools in aquatic systems, but there is limited understanding about the photoreactivity of different MPs-DOM. Herein, MPs-DOM from polystyrene (PS), polyethylene terephthalate (PET), poly(butylene adipate-co-terephthalate) (PBAT), PE, and polypropylene (PP), representing aromatic, biodegradable, and aliphatic plastics, were prepared to examine their photoreactivity. Spectral and high-resolution mass spectrometry analyses revealed that PS/PET/PBAT-DOM contained more unsaturated aromatic components, whereas PE/PP-DOM was richer in saturated aliphatic components. Photodegradation experiments observed that unsaturated aromatic molecules were prone to be degraded compared to saturated aliphatic molecules, leading to a higher degradation of PS/PET/PBAT-DOM than PE/PP-DOM. PS/PET/PBAT-DOM was mainly degraded by hydroxyl (•OH) via attacking unsaturated aromatic structures, whereas PE/PP-DOM by singlet oxygen (1O2) through oxidizing aliphatic side chains. The [•OH]ss was 1.21-1.60 × 10-4 M in PS/PET/PBAT-DOM and 0.97-1.14 × 10-4 M in PE/PP-DOM, while the [1O2]ss was 0.90-1.35 × 10-12 and 0.33-0.44 × 10-12 M, respectively. This contributes to the stronger photoreactivity of PS/PET/PBAT-DOM with a higher unsaturated aromatic degree than PE/PP-DOM. The photodegradation of MPs-DOM reflected a decreasing tendency from aromatic-unsaturated molecules to aliphatic-saturated molecules. Special attention should be paid to the photoreactivity and environmental impacts associated with MPs-DOM containing highly unsaturated aromatic compounds.

Microplastic and adhesive free, multifunctional, circular economy approach-based biomass-derived drinking straws

Generation of voluminous single-use plastic waste and byproducts from agricultural harvests such as rice straws (RSs) are major global challenges due to their disposal issues, contributing to greenhouse gas emissions, and affecting the ecological system with threats to human health. A scalable, low-cost, and eco-friendly strategy for fabricating cellulose-silica-based drinking straws, free from microplastics and adhesive, through strategic valorization of RS is reported. Functionalization by delignification-cum-crosslinking of RS leads to development of straws with high water stability (∼5 days), solvothermal stability (0°C-95°C), tensile strength (128 MPa), low migration values (<60 mg/kg), improved biodegradability (∼126 days) with reduced wettability and hydrophobicity. RS drinking straws show antibacterial, self-cleaning, self-healing, anti-fizzing, reusable, and generate significantly lower carbon footprint (<99.8% and <53.34% global warming potential than metal and polylactic acid straws). Repurposing of agro-wastes from farms to commercially viable drinking straws which biodegrades after its consumption achieves the goal of circular economy and sustainable development.

Influence of microplastics and environmentally persistent free radicals on the ability of biochar components to promote degradation of antibiotics by activated peroxymonosulfate

Microplastics (MPs) in sludge can affect the ability of biochar-activated peroxymonosulfate (PMS) to degrade antibiotics. In this work, biochar was prepared by mixing sludge and polystyrene (PS) through hydrothermal carbonization (HTC) and high-temperature pyrolysis processes. The resulting biochar was used to activate PMS to degrade ofloxacin (OFX), levofloxacin (LEV), and pefloxacin (PFX). The addition of PS significantly enhanced the ability of biochar/PMS to degrade antibiotics and the levels of environmentally persistent free radicals (EPFRs, 4.59 × 1020 spin/g) due to the decomposition of PS. The addition of PS resulted in a slight decrease in the specific surface area of biochar (2-3 m2/g on average), but a significant increase in the concentration of EPFRs increased the removal efficiency. The activation of PMS by biochar is dominated by free radicals, accounting for about 70%, in which SO4•- and •OH contribute the most and O2•- the least. However, 1O2 contributes 15-20% to the degradation of antibiotics in non-free radical processes. Overall, the process of biochar/PMS degradation of antibiotics is mainly dominated by free radicals, and the effect of non-free radicals is not obvious. Both hydrochar and pyrocarbon samples showed good hydrophilicity, and this property should improve the ability of active sites on biochar to degrade antibiotics. In the HTC process, PS can decompose during hydrochar preparation, with a maximum reduction value of 40.09%. The three-dimension excitation emission matrix fluorescence spectroscopy (3D-EEM) and total organic carbon (TOC) results show that the protein content in sludge plays a major role in reducing PS, with little effect of polysaccharide and SiO2. There are six to seven degradation intermediates of quinolone antibiotics, which are eventually degraded into CO2, H2O, and inorganic substances. The regeneration experiment showed good reusability of hydrochar and pyrocarbon, further demonstrating the suitability of biochar for the degradation of antibiotics.

New Insights into Aggregation Behaviors of the UV-Irradiated Dissolved Biochars (DBioCs) in Aqueous Environments: Effects of Water Chemistries and Variation in the Hamaker Constant

The aggregation behavior of ubiquitous dissolved black carbon (DBC) largely affects the fate and transport of its own contaminants and the attached contaminants. However, the photoaging processes and resulting effects on its colloidal stability remain yet unknown. Herein, dissolved biochars (DBioCs) were extracted from common wheat straw biochar as a proxy for an anthropogenic DBC. The influences of UV radiation on their aggregation kinetics were systematically investigated under various water chemistries (pH, electrolytes, and protein). The environmental stability of the DBioCs before and after radiation was further verified in two natural water samples. Hamaker constants of pristine and photoaged DBioCs were derived according to Derjaguin-Landau-Verwey-Overbeek (DLVO) prediction, and its attenuation (3.19 ± 0.15 × 10-21 J to 1.55 ± 0.07 × 10-21 J after 7 days of radiation) was described with decay kinetic models. Pearson correlation analysis revealed that the surface properties and aggregation behaviors of DBioCs were significantly correlated with radiation time (p < 0.05), indicating its profound effects. Based on characterization and experimental results, we proposed a three-stage mechanism (contended by photodecarboxylation, photo-oxidation, and mineral exposure) that DBioCs might experience under UV radiation. These findings would provide an important reference for potential phototransformation processes and relevant behavioral changes that DBC may encounter.

Peroxydisulfate-Driven Reductive Dechlorination as Affected by Soil Constituents: Free Radical Formation and Conversion

We report a previously unrecognized but efficient reductive degradation pathway in peroxydisulfate (PDS)-driven soil remediation. With supplements of naturally occurring low-molecular-weight organic acids (LMWOAs) in anaerobic biochar-activated PDS systems, degradation rates of 12 γ-hexachlorocyclohexanes (HCH)-spiked soils boosted from 40% without LMWOAs to a maximum of 99% with 1 mM malic acid. Structural analysis revealed that an increase in α-hydroxyl groups and a diminution in pKa1 values of LMWOAs facilitated the formation of reductive carboxyl anion radicals (COO•-) via electrophilic attack by SO4•-/•OH. Furthermore, degradation kinetics were strongly correlated with soil organic matter (SOM) contents than iron minerals. Combining a newly developed in situ fluorescence detector of reductive radicals with quenching experiments, we showed that for soils with high, medium, and low SOM contents, dominant reactive species switched from singlet oxygen/semiquinone radicals to SO4•-/•OH and then to COO•- (contribution increased from 30.8 to 66.7%), yielding superior HCH degradation. Validation experiments using SOM model compounds highlighted critical roles of redox-active moieties, such as phenolic - OH and quinones, in radical formation and conversion. Our study provides insights into environmental behaviors related to radical activation of persulfate in a broader soil horizon and inspiration for more advanced reduction technologies.

Unraveling the photochemical behavior of dissolved organic matter derived from hydrothermal carbonization process water: Insights from molecular transformation and photoactive species

Hydrothermal carbonization process water (HTPW) has been utilized as a substitute for chemical fertilizers in agricultural applications. However, the input of HTPW into paddy water, particularly the significant proportion of dissolved organic matter (DOM) in HTPW (DOM-HTPW), directly engages in photochemical transformations, a phenomenon often overlooked. This study observed a consistent decrease in humification (SUVA280, 7.7-53.9%) and aromaticity (SUVA254, 6.1-40.0%) of DOM-HTPW after irradiation. The primary active photobleaching components of DOM-HTPW varied depending on the feedstock, such as protein for chicken manure DOM-HTPW and lignin for rice straw DOM-HTPW. The photochemical activity of DOM-HTPW was augmented by its lower molecular weight and higher hydrophilic composition, particularly evident in chicken manure DOM-HTPW, which exhibited higher generation rates for 1O2 (35.1-37.1%), 3DOM* (32.8-43.9%), and O2•- (28.6-48.8%) as measured by molecular probes. DOM-HTPW effectively facilitated the phototransformation of tetracycline, with the contribution of O2•- being more significant than 3DOM* and 1O2. These findings shed new light on the understanding the photochemical processes of DOM-HTPW as exogenous DOM and the interconnected fate of contaminants in aquatic environments.

Comparison of water-soluble organic matter (WSOM)-containing and WSOM-free biochars for simultaneous sorption of lead and cadmium

The effects of individual biochar constituents and natural environmental media on the immobilization behaviors and chemical activities of toxic heavy metals are still poorly understood. In this work, the physicochemical properties of raw corn straw (CS) and CS-derived biochar materials as well as their sorption abilities and retention mechanisms for lead (Pb) and cadmium (Cd) were evaluated by combining batch experiments and spectral approaches. According to the spectral analysis results and single variable principle, the setting of biochars after soaking in solution as the control group was suggested when evaluating their retention mechanisms for Pb and Cd. The rising of ionic strength did not apparently affect the immobilization of Pb by biochar prepared at 500 °C (i.e., CB500) and Pb/Cd by water-soluble organic matter (WSOM)-free CB500 (i.e., DCB500), while slightly inhibited the sorption of Cd by CB500. Pb and Cd exhibited a mutual inhibition effect on their sorption trends with a higher sorption preference of Pb. The dominant fixation mechanism of Pb by CB500 and DCB500 was identified to be mineral precipitation. In contrast, the main sorption mechanism of Cd changed from mineral precipitation in the single-metal system to surface complexation in the binary-metal system. The sorption ratios of Pb and Cd on CB500 were comparable to those on DCB500 with the coexistence of mixed natural organic matters (NOM) and ferrihydrite. The current experimental findings suggested that DCB500 was a suitable remediation agent for regulating the migration behaviors of toxic Pb and Cd in acidic and NOM-rich soil and water systems.

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.

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.

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.

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.

Molecular-level transformations of biomass burning-derived water-soluble organic carbon during dark aqueous OH oxidation: Insights from absorption, fluorescence, high-performance size exclusion chromatography and high-resolution mass spectrometry analysis

Biomass burning (BB) releases large amounts of water-soluble organic carbon (WSOC), which would undergo heterogenous oxidation processes that induce transformations in both molecular structures and compositions within BB WSOC. This study designed an aqueous oxidation initiated by OH radicals in the absence of light for WSOC extracted from smoke particles generated by burning of corn straw and fir wood. The BB WSOC was comprehensively characterized using a combination of UV-visible spectra, excitation-emission matrix fluorescence in conjunction with parallel factor analysis (EEM-PARAFAC), high-performance size exclusion chromatography (HPSEC), and high-resolution mass spectrometry (HRMS) analyses. Over the course of oxidation, both chromophores and fluorophores exhibited gradual decreases. Moreover, EEM-PARAFAC revealed a preferential degradation of larger-sized protein-like/phenol-like organic matters, accompanied by the accumulation and/or formation of humic-like substances in aged BB WSOC. HPSEC analysis showed notable changes in molecular weight (MW) distributions for both types of BB WSOC during oxidation. Specifically, high MW species (>1 kDa) displayed a tendency to form along with oxidation, possibly attributed to the formation of assemblies via intermolecular weak forces. After oxidation, evidence of CHO compound degradation and enrichment/formation of CHON compounds was observed for both types of BB WSOC. Remarkably, the resistant, degraded and produced molecules for BB WSOC were dominated by CHO (38-73 %) and lignin-like molecules (41-47 %), suggesting diverse responses to oxidation within these two groups. Furthermore, polyphenols experienced selective degradation, while CHON, aliphatic and poly-aromatic molecules tended to form during the oxidation process for both types of BB WSOC. In summary, this study provides a comprehensive understanding of the molecular-level transformations undergone by BB WSOC during dark aqueous OH oxidation. The findings significantly contribute to our insights into atmospheric evolution of BB WSOC, thereby playing a crucial role in accurately assessing their effects within climate models and informing policy decisions.

Differential influences of forest floor-pyrolyzed biochar-derived and leached dissolved organic matter interaction with natural iron-bearing minerals in forest subsoil on the formation of mineral-associated soil organic matter

The vertical sequestration of dissolved organic matter (DOM) by iron minerals along the soil profile is assumed to be central to the long-term storage of the soil organic matter (SOM) pool. However, there is limited information available about how the interaction between DOM and natural iron-bearing minerals shape mineral SOM associations quantitatively and qualitatively in forest subsoils. Here, we systematically investigated the influences of forest organic layer-pyrolyzed biochar-derived DOM (BDOM) and leached DOM (LDOM) on quantity, molecular composition, and diversity of deposition layer-derived iron minerals-associated OM by using Fourier transform ion cyclotron resonance mass spectrometry and other complementary spectroscopy. Results indicated natural iron minerals (FeOx1 and FeOx2) had a greater capacity for sorbing LDOM with higher aromaticity and molecular weight than those of BDOM, and the higher proportion of goethite and short-order-range phase in natural iron minerals was closely related to the increased OM adsorption capacity. We also observed the preferential sorption of oxygen/nitrogen-rich polycyclic aromatic compounds and carboxylic-containing compounds in LDOM and concurrent the potential release of lignin-like/aromatics compounds and carboxyl/nitrogen-less aliphatic compounds from native OM coprecipitates into the solution. However, unsaturated and oxidized phenolic compounds in BDOM had a stronger affinity for FeOx through hydrophobic partitioning and specific polar interactions, and concomitantly the partial release of nitrogen-free aliphatic and other carboxyl-rich compounds. More nitrogen structures in aromatic-containing compounds can improve the saturation level and polarity of BDOM. Compared with BDOM, LDOM exerted a stronger control over the exchange of native OM from subsoil natural iron-bearing minerals and substantially enhanced the molecular diversity of the reconstituted mineral-associated OM during the adsorptive fractionation. Overall, these findings suggest the compositional evolution of DOM profoundly shapes SOM formation and persistence in forest subsoils, which is the key to understanding DOM cycling and contaminant fate during its passage through the soil.

Benzenepoly(carboxylic acid)s as Exclusive Intrinsic Markers to Assess Riverine Export of Dissolved Black Carbon

Landscape fires annually generate large quantities of black carbon. The water-soluble fraction of black carbon (i.e., dissolved black carbon/DBC) is an important constituent of the dissolved organic carbon (DOC) pool, playing a crucial role in the global budget of refractory carbon and climate change. A key challenge in constraining the flux and fate of riverine DBC is to develop targeted and accurate quantification methods. Herein, we report that benzenepentacarboxylic acid (B5CA) intrinsically present in DBC can be used as an exclusive and holistic marker (representing both condensed aromatics and less-/nonaromatic fractions) for DBC quantification. B5CA was universally detected in water extractions of biochar and fire-affected soils with relatively large abundance but not produced by nonthermogenic processes. It has good mobility in the environment as it is not readily precipitated by cations or adsorbed by common geosorbents. B5CA also represents the recalcitrant components of DBC with excellent stability against photodegradation and biodegradation. Applying B5CA as the DBC marker in surface waters of the Changjiang River (i.e., the third largest river in the world), we calculate the DBC concentration in the downstream Changjiang River to be 4.8 ± 5.5% of the DOC flux. Our work provides a simple and reliable approach for the accurate quantification and source tracking of DBC in the soil and aquatic carbon pools.

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.

Facet-Dependent Productions of Reactive Oxygen Species from Pyrite Oxidation

Reactive oxygen species (ROS) are widespread in nature and play central roles in numerous biogeochemical processes and pollutant dynamics. Recent studies have revealed ROS productions triggered by electron transfer from naturally abundant reduced iron minerals to oxygen. Here, we report that ROS productions from pyrite oxidation exhibit a high facet dependence. Pyrites with various facet compositions displayed distinct efficiencies in producing superoxide (O2• -), hydrogen peroxide (H2O2), and hydroxyl radical (•OH). The 48 h •OH production rates varied by 3.1-fold from 11.7 ± 0.4 to 36.2 ± 0.6 nM h-1, showing a strong correlation with the ratio of the {210} facet. Such facet dependence in ROS productions primarily stems from the different surface electron-donating capacities (2.2-8.6 mmol e- g-1) and kinetics (from 1.2 × 10-4 to 5.8 × 10-4 s-1) of various faceted pyrites. Further, the Fenton-like activity also displayed 10.1-fold variations among faceted pyrites, contributing to the facet depedence of •OH productions. The facet dependence of ROS production can greatly affect ROS-driven pollutant transformations. As a paradigm, the degradation rates of carbamazepine, phenol, and bisphenol A varied by 3.5-5.3-fold from oxidation of pyrites with different facet compositions, where the kinetics were in good agreement with the pyrite {210} facet ratio. These findings highlight the crucial role of facet composition in determining ROS production and subsequent ROS-driven reactions during iron mineral oxidation.

Microbial metabolism influences microplastic perturbation of dissolved organic matter in agricultural soils

An estimated 258 million tons of plastic enter the soil annually. Joining persistent types of microplastic (MP), there will be an increasing demand for biodegradable plastics. There are still many unknowns about plastic pollution by either type, and one large gap is the fate and composition of dissolved organic matter (DOM) released from MPs as well as how they interact with soil microbiomes in agricultural systems. In this study, polyethylene MPs, photoaged to different degrees, and virgin polylactic acid MPs were added to agricultural soil at different levels and incubated for 100 days to address this knowledge gap. We find that, upon MP addition, labile components of low aromaticity were degraded and transformed, resulting in increased aromaticity and oxidation degree, reduced molecular diversity, and changed nitrogen and sulfur contents of soil DOM. Terephthalate, acetate, oxalate, and L-lactate in DOM released by polylactic acid MPs and 4-nitrophenol, propanoate, and nitrate in DOM released by polyethylene MPs were the major molecules available to the soil microbiomes. The bacteria involved in the metabolism of DOM released by MPs are mainly concentrated in Proteobacteria, Actinobacteriota, and Bacteroidota, and fungi are mainly in Ascomycota and Basidiomycota. Our study provides an in-depth understanding of the microbial transformation of DOM released by MPs and its effects of DOM evolution in agricultural soils.

Photochemical behavior of dissolved organic matter derived from Alternanthera philoxeroides hydrochar: Insights from molecular transformation and photochemically reactive intermediates

Hydrochar-derived dissolved organic matter (HDOM) enters aquatic ecosystems through soil leaching and surface runoff following the application of hydrochar. However, the photochemical behavior of HDOM remains unclear. The photo-transformation of HDOM was analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), multiple spectroscopy methods, high-performance liquid chromatography, and combining synchronous fluorescence and Fourier-transform infrared spectroscopy with two-dimensional correlation spectroscopy. The results showed that with the increase of carbonization temperature, amide II in protein-like substances were observed to be preferentially photolyzed, and the protein-like substances were more sensitive to low irradiation time, while the duration time of the photochemical behavior of amide II and aliphatic C-H were more persistent. FT-ICR MS results showed that N and S-containing molecules, including lignins and lipids were more sensitive to ultraviolet irradiation. Furthermore, the photo-transformation of HDOMs was accompanied by the generation of triple excited state dissolved organic matter and singlet oxygen. Our findings will be beneficial for understanding the mechanisms of photo-transformation of HDOM and for predicting the possible behaviors of hydrochar produced at different temperatures before large-scale application.

Characterizing photochemical production carboxyl content of dissolved organic matters using absorbance spectroscopy combined with FT-ICR MS

Irradiation can significantly impact the structure, reactivity and environmental behavior of dissolved organic matter (DOM). The extent of these processes remains to be ascertained in more detail but the heterogeneity and site-specificity of DOM, and the lack of methods to characterize DOM at its environmentally-relevant concentrations make it a challenge. In this study, the differences of DOM response to photodegradation in four typical origins (i.e., surface water, sediment and intracellular and extracellular algal DOM) were tracked on the molecular-level using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). Changes of the carboxyl and phenolic DOM moieties induced by irradiation were quantified by spectroscopic titrations, and the mechanism of functional groups affecting the changes of specific molecular composition was qualitatively proposed. The results demonstrated that intracellular algal organic matter (I-DOM) was most susceptible to photodegradation (ca. 63% DOM loss), then came extracellular algal organic matter (E-DOM) and surface water DOM (W-DOM) (ca. 15% DOM loss). Sediment DOM (S-DOM) was most resistant to irradiation, with a very small level of its mineralization. Lipids, lignin-like compounds and tannin-like compounds in I-DOM and E-DOM were relatively photo-labile. The photodegradation of lipids was related to the decarboxylation of carboxyl functional groups, while the photodegradation of tannin-like compounds was related to the rupture of phenolic functional groups. In comparison, the molecular composition of W-DOM and S-DOM was less affected by irradiation, which was also reflected in the fact that the carboxyl and phenolic functional groups were highly photo-resistant. This study showed that the photoactivity of DOM in surface water was closely related to the abundance of algae, so controlling the excessive reproduction of algae may have a positive effect on stability of quality and quantity of organic matter in surface water.

Evaluation of the fate of wastewater effluent organic matter in receiving water: Effect of sequential photochemical and biological processes

Effluent organic matter (EfOM) discharged from wastewater treatment plants (WWTPs) carry substantial risks to river ecosystems. The fate and role of EfOM in the receiving water is affected by its exposure to sunlight and microbial processes, but the extent of these processes remains unclear. In this study, three-phase sequence of irradiation and microbial incubation with EfOM were conducted to compare the behavior of EfOM with that of natural organic matter in receiving rivers (RNOM). The dissolved organic carbon (DOC) in EfOM was degraded by 23% after three sequential phases, while that in RNOM was degraded by 19%. In the first phase, the irradiation of EfOM stimulated microbial respiration and growth by producing easily metabolizable less aromatic lignin-type molecules, leading to a 21% increase in biodegradation. Conversely, the irradiation of RNOM removed biodegradable lignin-type molecules, causing a 50% decreased in biodegradation. The second and third irradiation phases of EfOM and RNOM produced biodegradable lignin-type molecules, making their molecular compositions increasingly similar. The acute toxicity of EfOM decreased by 55%, and differences in microbial species composition between EfOM and RNOM waters decreased by 82% after the three-phase sequence. These findings can improve understanding of the fate of EfOM discharged into receiving rivers.

Photo-transformation of wastewater effluent organic matter reduces the formation potential and toxicity of chlorinated disinfection byproducts

Sunlight exposure can degrade and transform discharged wastewater effluent organic matter (EfOM) in aquatic systems, potentially enhancing the feasibility of reusing wastewater for drinking purposes. However, there remains a lack of comprehensive understanding regarding the sunlight-induced changes in the molecular-level composition, characteristics, and chlorine reactivity of EfOM. Herein, we investigated the impact of sunlight on the optical properties, chemical composition, and formation of disinfection byproducts of EfOM using multiple spectroscopic analyses, high-resolution mass spectrometry, chlorination experiments, and in vitro bioassays. Upon natural sunlight exposure, we observed significant decreases in ultraviolet-visible absorbance and fluorescence intensity of EfOM, indicating the destruction of chromophores and fluorophores. Photolysis generally yields products with lower molecular weight and aromaticity, and with higher saturation and oxidation levels. Moreover, a shift within the EfOM from condensed aromatic-like compounds to tannin-like components was observed. Furthermore, sunlight exposure reduced the reactivity of EfOM toward the formation of trihalomethanes and haloacetonitriles during chlorination, while there was a slight increase in the specific formation potential of haloketones. Importantly, the disinfection byproducts resulting from chlorination of the irradiated EfOM exhibited reduced microtoxicity. Overall, this study provides new insights into alterations in EfOM under sunlight exposure and aids in predicting the health risks of effluent discharge in water environments.

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.

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.

Temperature-Dependent Molecular Evolution of Biochar-Derived Dissolved Black Carbon and Its Interaction Mechanism with Polyvinyl Chloride Microplastics

Biochar-derived dissolved black carbon (DBC) molecules are dependent on the BC formation temperature and affect the fate of emerging contaminants in waters, such as polyvinyl chloride microplastic (MP_PVC). However, the temperature-dependent evolution and MP_PVC-interaction of DBC molecules remain unclear. Herein, we propose a novel DBC-MP_PVC interaction mechanism by systematically interpreting heterogeneous correlations, sequential responses, and synergistic relationships of thousands of molecules and their linking functional groups. Two-dimensional correlation spectroscopy was proposed to combine Fourier transform-ion cyclotron resonance mass spectrometry and spectroscopic datasets. Increased temperature caused diverse DBC molecules and fluorophores, accompanied by molecular transformation from saturation/reduction to unsaturation/oxidation with high carbon oxidation states, especially for molecules with acidic functional groups. The temperature response of DBC molecules detected via negative-/positive-ion electrospray ionization sequentially occurred in unsaturated hydrocarbons → lignin-like → condensed aromatic → lipid-/aliphatic-/peptide-like → tannin-like → carbohydrate-like molecules. DBC molecular changes induced by temperature and MP_PVC interaction were closely coordinated, with lignin-like molecules contributing the most to the interaction. Functional groups in DBC molecules with m/z < 500 showed a sequential MP_PVC-interaction response of phenol/aromatic ether C-O, alkene C═C/amide C═O → polysaccharides C-O → alcohol/ether/carbohydrate C-O groups. These findings help to elucidate the critical role of DBCs in MP environmental behaviors.

Sunlight-Driven Production of Reactive Oxygen Species from Natural Iron Minerals: Quantum Yield and Wavelength Dependence

Photochemically generated reactive oxygen species (ROS) play numerous key roles in earth's surface biogeochemical processes and pollutant dynamics. ROS production has historically been linked to the photosensitization of natural organic matter. Here, we report the photochemical ROS production from three naturally abundant iron minerals. All investigated iron minerals are photoactive toward sunlight irradiation, with photogenerated currents linearly correlated with incident light intensity. Hydroxyl radicals (^•OH) and hydrogen peroxide (H₂O₂) are identified as the major ROS species, with apparent quantum yields ranging from 1.4 × 10^-8 to 3.9 × 10^-8 and 5.8 × 10^-8 to 2.5 × 10^-6, respectively. Photochemical ROS production exhibits high wavelength dependence, for instance, the ^•OH quantum yield decreases with the increase of light wavelength from 375 to 425 nm, and above 425 nm it sharply decreases to zero. The temperature shows a positive impact on ^•OH production, with apparent activation energies ranging from 8.0 to 17.8 kJ/mol. Interestingly, natural iron minerals with impurities exhibit higher ROS production than their pure crystal counterparts. Compared with organic photosensitizers, iron minerals exhibit higher wavelength dependence, higher selectivity, lower efficiency, and long-term stability in photochemical ROS production. Our study highlights natural inorganic iron mineral photochemistry as a ubiquitous yet previously overlooked source of ROS.

Deciphering the structural characteristics and molecular transformation of dissolved organic matter during the electrolytic oxygen aerobic composting process

Electrolytic oxygen aerobic composting (EOAC) effectively treats organic solid waste by using in-situ electrolytic oxygen for aeration. However, the fundamental mechanism of compost maturity is still unclear. Therefore, we comprehensively characterized dissolved organic matter (DOM) transformation closely related to compost maturity during EOAC. Excitation-emission matrix-parallel factor (EEM-PARAFAC) and Fourier transform infrared (FTIR) analysis confirmed that EOAC quickly decreased organic matter and increased humus substances, accelerating the compost humification process compared with conventional aerobic composting. Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis reveals that the double bound equivalent and aromaticity index during EOAC are higher than in conventional aerobic composting (CAC), suggesting more aromatic compounds in EOAC. DOM's detailed transformation investigation suggested that low O/C and high H/C compounds were preferentially decomposed during EOAC. Our investigation firstly extends the in-depth molecular mechanisms of humification during EOAC, and reveals its practical engineering applications.

Photochemical behavior of constructed wetlands-derived dissolved organic matter and its effects on Bisphenol A photodegradation in secondary treated wastewater

Free water surface flow (FWS) constructed wetlands (CWs) have been broadly applied for polishing secondary treated effluents. Dissolved organic matter derived from FWS CWs (WDOM) plays key roles in contaminants transformations. Conversely, photodegradation could shape the quantity and quality of WDOM, thereby affecting its roles in the photolysis of organic micropollutants (OMPs). Nevertheless, whether and how solar irradiation-induced photodegradation modify the properties of WDOM, and the effects of WDOM on the photodegradation of OMPs remain unclear. This study elucidates the photochemical behavior of two WDOM isolated from field-scale FWS CWs for effluent polishing under simulated sunlight irradiation using spectroscopic tools and high-resolution mass spectra. Furthermore, the roles of WDOM in the photodegradation of Bisphenol A (BPA), as a representative endocrine-disrupting compound (EDC), were comprehensively investigated. Solar irradiation was demonstrated to lower the molecular weight and aromaticity of WDOM, as well as weaken its light absorption. Ultrahigh-resolution mass spectra further confirmed that aromatic and unsaturated structures were susceptible to solar irradiation-induced photodegradation reactions. Subsequently, less aromatic and more saturated structures eventually formed under sunlight irradiation, consistent with the result from spectroscopic characterization. The reactive species produced from WDOM significantly enhanced the photodegradation of BPA with the k_obs noticeably increasing 4-fold compared with the k_obs for direct photolysis. Additionally, ³WDOM* was identified as the dominant reactive species leading to the photolysis of BPA in the presence of WDOM. These findings improve understanding of the phototransformation behavior of WDOM under sunlight irradiation and the roles that WDOM plays in the photochemical fate of coexisting OMPs in CWs treatment systems.

Molecular transformation of dissolved organic matter and formation pathway of humic substances in dredged sludge under aerobic composting

Using Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) and molecular reaction network analysis, this study investigated molecular transformation of dissolved organic matter (DOM) and formation pathway of humic substances (HS) in dredged sludge during aerobic composting. The results showed that macromolecular N-containing compounds in dredged sludge are abundantly transformed into unsaturated and aromatic oxygenated compounds, exhibiting physicochemical properties similar to those of humus. Especially, N-containing compounds with one nitrogen atom are susceptible to oxidative deamination. Furthermore, assemblages of reactive fragments (e.g., -C7H8O2, -C10H12O2, -C2H2O2, and -C4H6O2) were identified as potential precursors to HS formed by the following reactions: starting with protein deamination and desulfurization, lignin delignification cascaded, finally decarbonylation occurred. This work provides novel insight for optimizing the process of stabilization and humification of dredged sludge.