Photochemical Transformation and Catalytic Activity of Dissolved Black Nitrogen Released from Environmental Black Carbon

From Bulk to Nano: Formation, Features, and Functions of Nano-Black Carbon in Biogeochemical Processes

Globally increasing wildfires and widespread applications of biochar have led to a growing amount of black carbon (BC) entering terrestrial ecosystems. The significance of BC in carbon sequestration, environmental remediation, and the agricultural industry has long been recognized. However, the formation, features, and environmental functions of nanosized BC, which is one of the most active fractions in the BC continuum during global climate change, are poorly understood. This review highlights the formation, surface reactivity (sorption, redox, and heteroaggregation), biotic, and abiotic transformations of nano-BC, and its major differences compared to other fractions of BC and engineered carbon nanomaterials. Potential applications of nano-BC including suspending agent, soil amendment, and nanofertilizer are elucidated based on its unique properties and functions. Future studies are suggested to develop more reliable detection techniques to provide multidimensional information on nano-BC in environmental samples, explore the critical role of nano-BC in promoting soil and planetary health from a one health perspective, and extend the multifield applications of nano-BC with a lower environmental footprint but higher efficiency.

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.

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.

Robust reactive oxygen species production in interfacial reaction between organic acids and biochar: The combined effect of electron acceptance and electron conduction

Interfacial electron transport and reactive oxygen species (ROS) generation in the redox action between biochar (BC) and low-molecular-weight organic acids (LMWOAs) have been overlooked during the utilization of BC in soil amelioration/remediation. Herein, BC and N-doped BC (NBC) with various physicochemical properties were prepared at pyrolysis temperatures of 350, 550 and 750 °C (namely BC/NBC350, 550 and 750) and their interactions with LMWOAs were systematically investigated. Results of ROS quenching and electrochemical cell experiments revealed that BC/NBC could strongly interact with LMWOAs, especially ascorbic acid (AA), and acted as an electron shuttle to mediate one electron transfer from AA to oxygen to generate O2•-, followed by the robust generation of H2O2 and •OH. Interestingly, the generation rates of •OH and H2O2 in the NBC-AA system were significantly higher than that in the BC-AA system, of which the NBC550-AA system exhibited the best performance, enabling its superior ability in bisphenol A degradation. Different from conventional wisdom, the electron shuttle capacity depended on a combined aromatization degree and electron-accepting capacity of BC/NBC. These findings complement the interfacial electron transfer mechanism during the natural BC-LMWOAs interaction and provide new inspiration for the development of green and efficient organic pollutant removal technologies.

Singlet Oxygen Quantum Yields of Pyrogenic Dissolved Organic Matter from Lab-Prepared and Wildfire Chars

Wildfires or prescribed fires release pyrogenic dissolved organic matter (pyDOM) into the environment, which can photochemically produce singlet oxygen (1O2) in sun-lit surface waters. 1O2 quantum yields (ΦΔ) are well-studied for non-pyrogenic DOM, but little is understood about the 1O2 generation from pyDOM, especially the ΦΔ values from real wildfire samples and their wavelength dependence. In this study, time-resolved 1O2 phosphorescence was used to determine the wavelength-dependent ΦΔ values for pyDOM generated from wildfire char and a series of lab-prepared chars produced by combusting oak and pine wood. Wildfire and most lab-prepared pyDOM generally had similar ΦΔ values (2.1-2.7%) at 365 nm compared to the reference Suwannee River Natural Organic Matter (SRNOM) isolate (2.4%). Interestingly, pyDOM from the highest combustion temperature char was found to possess extremely low ΦΔ values compared to SRNOM and other pyDOM at all excitation wavelengths. In addition, it was revealed that the predicted steady-state concentration of 1O2 from pyDOM was similar to that from SRNOM, indicating that the addition of pyDOM from wood chars may not strongly impact surface water photochemistry.

Intrinsic peroxidase-like activity of dissolved black carbon released from biochar

Dissolved black carbon (DBC) is an important constituent of the natural organic carbon pool, influencing the global carbon cycling and the fate processes of many pollutants. In this work, we discovered that DBC released from biochar has intrinsic peroxidase-like activity. DBC samples were derived from four biomass stocks, including corn, peanut, rice, and sorghum straws. All DBC samples catalyze H2O2 decomposition into hydroxyl radicals, as determined by the electron paramagnetic resonance and the molecular probe. Similar to enzymes that exhibit saturation kinetics, the steady-state reaction rates follow the Michaelis-Menten equation. The peroxidase-like activity of DBC is controlled by the ping-pong mechanism, as suggested by parallel Lineweaver-Burk plots. Its activity increases with temperature from 10 to 80 °C and has an optimum at pH 5. The peroxidase-like activity of DBC is positively correlated with its aromaticity as aromatics can stabilize the reactive intermediates. The active sites in DBC also involve oxygen-containing groups, as inferred by increased activity after the chemical reduction of carbonyls. The peroxidase-like activity of DBC has significant implications for biogeochemical processing of carbon and potential health and ecological impacts of black carbon. It also highlights the need to advance the understanding of the occurrence and role of organic catalysts in natural systems.

Dissolved black carbon incorporating with ferric minerals promoted photo-Fenton-like degradation of triclosan in acidic conditions

Dissolved black carbon (DBC) has been recognized as an important organic matter that influences the photochemical processes of organic pollutants. The excited triplet state (3DBC*) of DBC usually exhibits activity in neutral and basic aqueous conditions, rather than in acidic conditions. In this study, we found the crop (wheat, rice, maize) straw sourced DBC can substantially enhance the photodegradation of triclosan in relatively acidic conditions, and in the presence of ferric minerals (ferrihydrite and lepidocrocite), when exposed to simulated sunlight irradiation. This should be ascribed to the rapid non-reductive dissolution of ferric minerals by DBC, which leads to the generation of abundant hydrogen peroxides (H2O2) and hydroxyl radicals (•OH) through photo Fenton-like reactions. •OH is the dominant reactive species that leads to triclosan degradation in acidic conditions. Otherwise, triclosan itself is resistant to direct photolysis at pH < 5.0. The triplet state (3DBC*) plays a critical role in accelerating the Fe3+/Fe2+ cycling, which further promotes •OH generation. This study provides a new perspective on the role of DBC in surface water or mineral-water interfaces with acidic conditions and adds a more comprehensive understanding about the environmental implications of the DBC-ferric mineral system in sunlit surface water.

Vacancy pairs regulate BiOBr microstructure for efficient dimethyl phthalate removal under visible light irradiation

Developing new photocatalysts to achieve efficient removal of phthalate esters (PAEs) in water is an important research task in environmental science. However, existing modification strategies for photocatalysts often focus on enhancing the efficiency of material photogenerated charge separation, neglecting the degradation characteristics of PAEs. In this work, we proposed an effective strategy for the photodegradation process of PAEs: introducing vacancy pair defects. We developed a BiOBr photocatalyst containing "Bi-Br" vacancy pairs, and confirmed that it has an excellent photocatalytic activity in removing phthalate esters (PAEs). Through a combination of experimental and theoretical calculations, it is proved that "Bi-Br" vacancy pairs can not only improve the charge separation efficiency, but also alter the adsorption configuration of O₂, thus accelerating the formation and transformation of reactive oxygen species. Moreover, "Bi-Br" vacancy pairs can effectively improve the adsorption and activation of PAEs on the surface of samples, surpassing the effect of O vacancies. This work enriches the design concept of constructing highly active photocatalysts based on defect engineering, and provides a new idea for the treatment of PAEs in water.

Emerging Chemical Methods for Petroleum and Petroleum-Derived Dissolved Organic Matter Following the Deepwater Horizon Oil Spill

Despite the fact that oil chemistry and oils spills have been studied for many years, there are still emerging techniques and unknown processes to be explored. The 2010 Deepwater Horizon oil spill in the Gulf of Mexico resulted in a revival of oil spill research across a wide range of fields. These studies provided many new insights, but unanswered questions remain. Over 1,000 journal articles related to the Deepwater Horizon spill are indexed by the Chemical Abstract Service. Numerous ecological, human health, and organismal studies were published. Analytical tools applied to the spill include mass spectrometry, chromatography, and optical spectroscopy. Owing to the large scale of studies, this review focuses on three emerging areas that have been explored but remain underutilized in oil spill characterization: excitation-emission matrix spectroscopy, black carbon analysis, and trace metal analysis using inductively coupled plasma mass spectrometry.

Exploring the mechanisms of humic acid mediated degradation of polystyrene microplastics under ultraviolet light conditions

Microplastics (MPs) are emerging pollutants that interact extensively with dissolved organic matter (DOM) and this influences the environmental behavior of MPs in aqueous ecosystems. However, the effect of DOM on the photodegradation of MPs in aqueous systems is still unclear. The photodegradation characteristics of polystyrene microplastics (PS-MPs) in an aqueous system in the presence of humic acid (HA, a signature compound of DOM) under ultraviolet light conditions were investigated in this study through Fourier transform infrared spectroscopy coupled with two-dimensional correlation analysis, electron paramagnetic resonance, and gas chromatography-mass spectrometry (GC/MS). HA was found to promote higher levels of reactive oxygen species (0.631 mM of ▪OH), which accelerated the photodegradation of PS-MPs, with a higher degree of weight loss (4.3%), higher level of oxygen-containing functional groups, and lower average particle size (89.5 μm). Likewise, GC/MS analysis showed that HA contributed to a higher content of oxygen-containing compounds (42.62%) in the photodegradation of PS-MPs. Moreover, the intermediates and final degradation products of PS-MPs with HA were significantly different in the absence of HA during 40 days of irradiation. These results provide an insight into the co-existing compounds on the degradation and migration processes of MP and also support further research toward the remediation of MPs pollution in aqueous ecosystems.

Exposure Order to Photoaging and Humic Acids Significantly Modifies the Aggregation and Transformation of Nanoplastics in Aqueous Solutions

The colloidal stability of nanoplastics in aqueous solutions is greatly regulated by photoaging and dissolved organic matter (DOM). However, how the exposure order to sunlight and DOM modifies the environmental behavior of nanoplastics is seldomly determined. Here, with two different exposure orders, we investigated the impact of molecular-weight (MW)-fractionated humic acids (HAs) derived from biochar and the Suwannee River, respectively, on the aggregation of poly(ethylene terephthalate) nanoplastics (PET-NPs) in mono- and divalent electrolyte solutions. For exposure pattern (i) (photoaging followed by HA coating), photoaged PET-NPs had more oxidized surfaces and exhibited 22-320% higher binding affinity to HAs (especially the higher MW fractions) than the pristine counterparts, which greatly improved the dispersion of PET-NPs. For exposure pattern (ii) (HA coating followed by photoaging), HA-PET assemblies were formed, the dispersion of which increased with increasing irradiation time and was significantly higher than that of the samples in the exposure pattern (i) at the end of the experiment. This high dispersion of photoaged HA-PET assemblies was ascribed to the extra oxidation of PET by reactive oxygen species generated in the PET-HA interfaces during photoaging. These findings highlight the "active nature" of HA-PET assemblies, which provide new insight into the reaction of HA with nanoplastics beyond adsorption in the natural environment.

Assessing the quantum yield spectrum of photochemically produced reactive intermediates from black carbon of various sources and properties

Black carbon (BC) is ubiquitous in sunlit waters and atomosphere. Recent studies revealed that under sunlight irradiation BC is photoactive on producing photochemically produced reactive intermediates (PPRIs), a group of key species in accelerating earth's surface biogeochemical processes and pollutant dynamics. Nevertheless, reported PPRIs productions from BC exhibit large inconsistency and the intrinsic capacities of BC in producing PPRIs remain poorly characterized. This work provided a wavelength-dependent quantum yields (QYs) assessment of four environmentally-relevant PPRIs (excited triplet state BC (³BC*), singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), and hydroxyl radical (^·OH)) from various BC. The QYs of all investigated PPRIs exhibit high dependence on incident light wavelength. For instance, the QYs of ¹O₂ dramatically decreased from 4.4% to 0.4% with light wavelength increasing from 375 to 490 nm and decreased to 0 above 490 nm. Suprisingly, PPRIs QYs only varied by 2.0-2.5-fold among BC prepared from different biomasses (i.e., pine needle, shell, straw, and wood), while the pyrolysis temperature and size of BC demonstrate higher impacts on the PPRIs QYs by up to 30.3- and 7.1-fold variations, respectively. Analyses on the physicochemical properties of BC demonstrate that QYs of ³BC* and ¹O₂ were linked to the optical properties of BC, while the QYs of H₂O₂ and ^·OH were determined by multiple factors including the surface redox characteristics. Further, PPRIs productions from BC follow similar paths and efficiencies compared to those from natural organic matter. The revealed QYs of BC-derived PPRIs establish a key basis for evaluating PPRIs-mediated element cycles and pollutant transformation in natural waters, which are becoming increasingly important in the context of higher BC input from more frequent wildfires and artificial sources.

Nitrate promoted defluorination of perfluorooctanoic acid in UV/sulfite system: Coupling hydrated electron/reactive nitrogen species-mediated reduction and oxidation

A significantly accelerated defluorination of recalcitrant perfluorooctanoic acid (PFOA) was explored with the co-present nitrate (20 mg L^-1) by UV/sulfite treatment (UV/sulfite-nitrate). The deep defluorination of PFOA and complete denitrification of nitrate were simultaneously achieved in UV/sulfite-nitrate system. At the initial 30 min, PFOA defluorination exhibited an induction period, exactly corresponding to the removal of the co-existed nitrate. Upon the induction period passed, an accelerated removal of PFOA (5 mg L^-1) occurred, nearly 100% defluorination ratio reached within 2 h. Compared with those in UV/sulfite, the kinetics of PFOA decay, defluorination, and transformation product formations were greatly enhanced in UV/sulfite-nitrate system. Reactive nitrogen species (RNS) generated from e_aq^--induced reduction of nitrate were found to play significant roles on the promoted defluorination apart from e_aq^--mediated reductive defluorination. The investigations on solution pH (7.0-11.0) confirmed that the reductive defluorination of PFOA was more efficient under alkaline conditions, however, the presence of nitrate can promote the defluorination even under neutral pH. Theoretical calculations of Fukui function demonstrated that RNS could easily launch electrophilic attack toward H-rich moieties of fluorotelomer carboxylates (FTCAs, C_nF_2n+1-(CH₂)_m-COO^-), more persistent intermediates (formed via H/F exchange), and convert FTCAs into shorter-chain perfluorinated carboxylic acids, thus facilitating the deep defluorination. Along with the analysis on the denitrification products, the liberation of fluoride ions and generated intermediates, possible decomposition pathways were proposed. This work highlights the indispensable synergy from e_aq^-/RNS with integrated reduction and oxidation on PFOA defluorination and will advance remediation technologies of perfluorinated compound contaminated water.

Dissolved Organic Matter Promotes the Aging Process of Polystyrene Microplastics under Dark and Ultraviolet Light Conditions: The Crucial Role of Reactive Oxygen Species

Microplastics (MPs) interact frequently with dissolved organic matter (DOM) commonly found in the environment, but information on the aging behavior of MPs under the participation of DOM is still lacking. Thus, the polystyrene microplastic (PSMP) aging process with DOM participation was systematically studied by electron paramagnetic resonance spectroscopy, high-performance liquid chromatography, Fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy analyses under dark and ultraviolet (UV) light conditions. DOM was found to promote electron transfer to generate reactive oxygen species (ROS) under dark conditions and the aging of PSMPs, while the process of DOM generating ROS under UV light was more susceptible to photoelectrons and accelerated the aging process of PSMPs. However, among the four DOM types, fulvic acid (FA) has a more significant promoting effect on the aging process of PSMPs than humic acid, which can be attributed to the stronger conversion ability of FA to semiquinone radicals. Density functional theory calculations are used to describe the difference in the aging process of different structures of plastics with the participation of DOM. This study provides a necessary theoretical basis for the study of the migration of MPs in groundwater and deep surface water.

Comparative study on heterogeneous activation of peroxydisulfate and peroxymonosulfate with black carbon derived from coal tar residues: Contribution of free radical, 1O2 and surface-bound radicals

Carbon materials draw increasing attention as metal-free catalysts for persulfates activation. Herein, the potential of black carbon (BC) derived from coal tar residues on heterogeneous activation of peroxydisulfate (PDS) and peroxymonosulfate (PMS) to eliminate organic pollutants was investigated. Compared with UV/persulfates systems, persulfates/BC systems degraded 3 selected phenolic compounds (i.e. phenol, 4-chlorophenol (4-CP) and bisphenol A (BPA)) with an order of magnitude higher oxidation rates, and removed dissolved organics (DOC) with over 27% higher efficiency. In the PDS/BC system, ¹O₂ and surface-bound radicals were proved to be the dominant active species, while free radicals, ¹O₂, and surface-bound radicals were responsible for organics oxidation in the PMS/BC system. Relative contribution of different reactive species in persulfates/BC systems was pH-dependent. Surface oxygen functionalities of BC were involved in ¹O₂ generation, and its structural defects played a critical role in forming free radicals and surface-bound radicals. This study provided an in-depth insight into carbon-driven persulfates activation processes.