Synergistic photogeneration of reactive oxygen species by dissolved organic matter and C60 in aqueous phase

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.

The impact of dissolved organic matter on the photodegradation of tetracycline in the presence of microplastics

Microplastics (MPs), an emerging class of pollutants, significantly impact the photoconversion dynamics of tetracycline (TC). But the effect of prevalent dissolved organic matter (DOM) on TC photodegradation in the presence of MPs remains a gap in current research. In this study, the photoconversion behavior and mechanism of TC under simulated sunlight conditions were systematically investigated, both in the presence of DOM and in combination with polystyrene (PS) MPs. The results demonstrated that both DOM and MPs enhanced the photodegradation of TC when compared to its direct degradation. However, DOM, particularly humic acid (HA, 10 mg/L), exhibited a more pronounced enhancing effect on TC photodegradation within 1 h reaction, regardless of the presence or absence of MPs, reaching up to 80%. In reaction systems involving TC-HA and TC-HA-PS, the primary contributors to TC degradation were direct photolysis and HA photosensitization (free radical reactions). Conversely, photosensitization effects were not significant in the presence of fulvic acid (FA). Furthermore, even under dark reaction conditions, HA exhibited a 10% degradation effect on TC. Quenching experiments and electron spin resonance (ESR) results indicate that dark reaction processes involve free radical reactions. Additionally, toxicity test results showed a reduction in the acute toxicity of TC photodegradation products, yet the long-term cumulative risks to organisms deserved attention. In general, this investigation significantly advances our understanding of the intricate photoconversion behavior of TC in the presence of coexisting chemical components.

Promotion effect of ultraviolet light on graphene oxide aggregation in the presence of different climatic zone's humic and fulvic acid

Aggregation of graphene oxide (GO) is significantly affected by dissolved organic matter (DOM) in natural waters, while DOM's climate zone and light irradiation is seldom considered. This study investigated the effect of humic/fulvic acid (HA/FA) from various climate zones of China on aggregation of small (200 nm) and large (500 nm) GO under 120-h UV irradiation. GO aggregation was promoted by HA/FA because UV irradiation decreased hydrophilicity of GO and steric forces among particles. GO generated electron and hole pair under UV irradiation, which reduce GO with more hydrophilic oxygen-containing functional group (C-O) to rGO with high hydrophobicity and oxidize DOM into organic matter with smaller molecular weight. Most severe GO aggregation was observed with Makou HA from Subtropical Monsoon climate zone and Maqin FA from Plateau and Mountain climate zone, which was primarily because HA/FA's high molecular weight and aromaticity dispersed GO initially that facilitated UV penetration. GO aggregation ratio was positively correlated with graphitic fraction content (R2 = 0.82-0.99) and negatively correlated with C-O group content (R2 = 0.61-0.98) in the presence of DOM under UV irradiation. This work highlights different dispersity of GO during photochemical reactions in various climate zones, providing new insight into the environmental implications of nanomaterial release.

Role of biochar-derived DOM compositions in enhanced biodegradation of sulfamethoxazole and chloramphenicol

In the study, we investigated the different compositions of biochar-derived dissolved organic matter (BDOM) that play a key role in the biodegradation of sulfamethoxazole (SMX) and chloramphenicol (CAP) by P. stutzeri and S. putrefaciens, and found that aliphatic compounds in Group 4, fulvic acid like in Region III, and solid microbial byproduct like in region IV are key common factors. The growth and antibiotic degradation efficiency of P. stutzeri and S. putrefaciens are positively correlated with the content of Group 4 and Region III, and negatively correlated with Region IV. This is consistent with the optimal biodegradation results of BDOM700 with the highest content of Group 4 and Region III. Additionally, the degradation efficiency of SMX by Pseudomonas stutzeri is negatively correlated with the percentage of polycyclic aromatics in Group 1, but not with CAP. Similarly, the percentage of fatty acids in S. putrefaciens was positively correlated with Group 1, whereas P. stutzeri did not. This indicates that some components of BDOM have varying effects on different bacteria or types of antibiotics. This study provides new insights into enhancing antibiotic biodegradation by controlling the composition of BDOM.

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

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

Opposite impact of DOM on ROS generation and photoaging of aromatic and aliphatic nano- and micro-plastic particles

Dissolved organic matter (DOM) plays a significant role in the photochemical behavior of nano- and micro-plastic particles (NPs/MPs). We investigated the influence of DOM on the mechanism on the photoaging of NPs/MPs with different molecular structures under UV₃₆₅ irradiation in water. DOM components used in this study are mainly humic acid and fulvic acid. The results showed that DOM promoted the weathering of aliphatic NPs/MPs (polypropylene (PP)), but inhibited or had only a minor effect on the photoaging of aromatic NPs/MPs (polystyrene (PS) NPs/MPs, carboxyl-modified PS NPs, amino-modified PS NPs, and polycarbonate MPs). NPs with a large surface area may adsorb sufficient DOM on the particle surfaces through π-π interactions, which competes with NPs for photon absorption sites, thus, can delay the photoaging of PS NPs. Aromatic MPs may release phenolic compounds that quench •OH, thereby weakening the photoaging process. For aliphatic MPs, the detection of peracid, aldehyde, and ketone groups on the polymer surface indicated that DOM promoted weathering of PP MPs, which was primarily because the generation of •OH due to DOM photolysis may attack the polymer by C-C bond cleavage and hydrogen extraction reactions. This study provides insight into the UV irradiation weathering process of NPs/MPs of various compositions and structures, which are globally distributed in water.

Tide-Triggered Production of Reactive Oxygen Species in Coastal Soils

We report an unrecognized, tidal source of reactive oxygen species (ROS). Using a newly developed ROS-trapping gel film, we observed hot spots for ROS generation within ∼2.5 mm of coastal surface soil. Kinetic analyses showed rapid production of hydroxyl radicals (^•OH), superoxide (O₂^•-), and hydrogen peroxide (H₂O₂) upon a shift from high tide to low tide. The ROS production exhibited a distinct rhythmic fluctuation. The oscillations of the redox potential and dissolved oxygen concentration followed the same pattern as the ^•OH production, suggesting the alternating oxic-anoxic conditions as the main geochemical drive for ROS production. Nationwide coastal field investigations confirmed the widespread and sustainable production of ROS via tidal processes (22.1-117.4 μmol/m²/day), which was 5- to 36-fold more efficient than those via classical photochemical routes (1.5-7.6 μmol/m²/day). Analyses of soil physicochemical properties demonstrated that soil redox-metastable components such as redox-active iron minerals and organic matter played a key role in storing electrons at high tide and shuttling electrons to infiltrated oxygen at low tide for ROS production. Our work sheds light on a ubiquitous but previously overlooked tidal source of ROS, which may accelerate carbon and metal cycles as well as pollutant degradation in coastal soils.

Influence of Fe(III) source, light quality, photon flux and presence of oxygen on photoreduction of Fe(III)-organic complexes - Implications for light-influenced coastal freshwater and marine sediments

Iron(III) photoreduction is an important source of Fe(II) in illuminated aquatic and sedimentary environments. Under oxic conditions, the Fe(II) can be re-oxidized by oxygen (O₂) forming reactive O-species such as hydrogen peroxide (H₂O₂) which further react with Fe(II) thus enhancing Fe(II) oxidation rates. However, it is unknown by aquatic sediments how the parameters wavelength of radiation, photon flux, origin of Fe(III) source and presence or absence of O₂ influence the extent of Fe(II) and H₂O₂ turnover. We studied this using batch experiments with different Fe(III)-organic complexes mimicking sedimentary conditions. We found that wavelengths <500 nm are necessary to initiate Fe(III) photoreduction and that the photon flux, wavelength and identity of Fe(III)-complexing organic acids control the kinetics of Fe(III) photoreduction. The formation of photo-susceptible Fe(III)-organic complexes did not depend on whether the Fe(III) source was biogenically produced, poorly-crystalline Fe(III) oxyhydroxides or chemically synthesized ferrihydrite. Oxic conditions caused chemical re-oxidation of Fe(II) and accumulation of H₂O₂. The photon flux, wavelength and availability of Fe(III)-complexing organic molecules are critical for the balance between concurrent Fe(III) photoreduction and abiotic Fe(II) oxidation and may even lead to a steady-state concentration of Fe(II) in the micromolar range. These results help understand and predict Fe(III) photoreduction dynamics and in-situ formation of Fe(II) in oxic or anoxic, illuminated and organic-rich environments.

Improving nanoplastic removal by coagulation: Impact mechanism of particle size and water chemical conditions

Plastic particles may bring potential threats to the ecosystem. Coagulation, as a widely used method to remove particles, has been rarely studied for plastic particles in the nanometer range. In this work, the coagulation removal of polystyrene nanoplastic particles (PSNPs, 50-1000 nm) was conducted in a model system containing coagulants aluminum chlorohydrate (PAC) and polyacrylamide (PAM). The optimal removal efficiency (98.5%) was observed in the coagulation process at pH= 8.0, 0.4 g·L^-1 PAC and 20 mg·L^-1 PAM. The inhibition impact of humic acid was also noticed, due to its competitive adsorption with PSNPs onto flocs. The interaction energies between PSNPs and PAC were calculated by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, which showed that electrical neutralization resulted in the difference of the remove efficiency in different sizes and coagulant concentrations. The formation of Al-O bond between PSNPs and PAC/PAM flocs promoted the removal of PSNPs. Excessive PAM (> 20 mg·L^-1) increased clusters size and solution viscosity, which resulted in the settling of clusters being controlled by buoyancy and the reduced remove efficiency. The findings suggest that the chemical coagulation dominants the removal of NPs, and the coagulation efficiency can be optimized by choosing suitable coagulant and water chemical conditions.

A BiOBr/Bi4MoO9 edge-on heterostructure with fast electron transport for efficient photocatalytic activity

This study demonstrates the rational design and construction of a BiOBr/Bi₄MoO₉ edge-on heterostructure by growing fish scale-like BiOBr nanosheets on the surface of Bi₄MoO₉. Such structural and compositional merits expedite electron transport and offer a large interfacial contact area and abundant reactive sites. Optimized BiOBr/Bi₄MoO₉ exhibited outstanding TC degradation activity.

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

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

Mechanistic Investigation of Enhanced Photoreactivity of Dissolved Organic Matter after Chlorination

Chlorine is commonly used in disinfection processes in wastewater treatment plants prior to discharge of the effluents into receiving waters. Effluent organic matter and humic substances constitute up to 90% of dissolved organic matter (DOM) in receiving water, which induces photogeneration of reactive species (RS) such as excited triplet state of DOM (³DOM*), singlet oxygen (¹O₂), and hydroxyl radical (^•OH). The RS plays an important role in the attenuation of trace pollutants. However, the effect of chlorine disinfection on the photoreactivity of the DOM has remained unclear. Here, we investigated the physicochemical properties and subsequent RS variation after chlorination of DOM. Solid-state ¹³C cross-polarization/magic angle-spinning NMR and Fourier transform ion cyclotron resonance mass spectrometry verified that the aromaticity, electron-donating capacity (EDC), and average molecular weight of DOM decreased markedly after chlorination. It was found for the first time that the photoproduction of ³DOM*, ¹O₂, and ^•OH increased markedly after chlorination of DOM upon irradiation of simulated sunlight. The quantum yields of ³DOM*, ¹O₂, and ^•OH were positively correlated with E2/E3 (ratio of the absorbance at 254 to 365 nm) while negatively correlated with EDC before and after chlorination. These findings highlight the synergetic effect of chlorine disinfection on the photosensitization of DOM under irradiation of sunlight, which will promote the removal of trace pollutants in surface waters.

Weathering of microplastics and interaction with other coexisting constituents in terrestrial and aquatic environments

Weathering of microplastics (MPs, < 5 mm) in terrestrial and aquatic environments affects MP transport and distribution. This paper first summarizes the sources of MPs, including refuse in landfills, biowastes, plastic films, and wastewater discharge. Once MPs enter water and soil, they undergo different weathering processes. MPs can be converted into small molecules (e.g., oligomers and monomers), and may be completely mineralized under the action of free radicals or microorganisms. The rate and extent of weathering of MPs depend on their physicochemical properties and environmental conditions of the media to which they are exposed. In general, water dissipates heat better, and has a lower temperature, than land; thus, the weathering rate of MPs in the aquatic environment is slower than in the terrestrial environment. These weathering processes increase oxygen-containing functional groups and the specific surface area of MPs, which influence the sorption and aggregation that occur between weathered MPs and their co-existing constituents. More studies are needed to investigate the various weathering processes of diverse MPs under natural field conditions in soils, sediments, and aquatic environments, to understand the impact of weathered MPs in the environment.

Enhanced and selective phototransformation of chlorophene on aluminum hydroxide-humic complexes

Mineral-humic complexes, known as mineral-associated organic matter (MAOM), are ubiquitous in natural waters. However, the interaction between organic pollutants and MAOM remains elusive, which may affect their degradation process. In this study, photochemical transformation of chlorophene (CP) in the presence of MAOM, prepared by coating aluminum hydroxide with humic acid (HA-HAO), was investigated. Our results showed that the degradation of CP was significantly enhanced in the presence of HA-HAO, and the degradation rate constant was ~5 times as that with HA only. It was because the adsorption of CP to HA-HAO particles was greatly enhanced, and concentration of reactive oxygen species (ROS) was increased on HA-HAO surfaces, which further promoted the reactions between CP and ROS. The quenching experiments combined with EPR technology confirmed that superoxide anion (O₂^·-) was the primary reactive radical on CP photodegradation. More importantly, the degradation of CP with HA-HAO followed a hydroxylation process, rather than the oligomerization reaction with HA only. Spectroscopic analysis provided direct evidence for the formation of hydrogen bonding between CP phenolic hydroxyl group and surface oxygen of HAO, which would suppress the reactivity of phenolic hydroxyl group, consequently the ortho- and meta-positions of CP became more facile for the hydroxylation reaction. This study shows the importance of MAOM in altering the photochemical behavior and transformation pathway of organic contaminants.

Interactions between cerium dioxide nanoparticles and humic acid: Influence of light intensities and molecular weight fractions

Cerium dioxide nanoparticles (CeO₂ NPs) are ubiquitous in the water environment due to the extensive commercial applications. The complexity of heterogeneous humic acid (HA) plays a significant role in affecting the physicochemical properties of CeO₂ NPs in aqueous environments. However, the effects of light intensities and HA fractions on the interaction mechanism between CeO₂ NPs and HA are poorly understood. Here, we provided the evidence that both light intensities (>3 E L^-1 s^-1) and molecular weights (>10 kDa) can effectively affect the interactions between CeO₂ NPs and HA. The absolute content of reactive oxygen species (ROS) and quantum yield (Φ) of ³HA* were inhibited when HA (10 mg of C L^-1) interacts with CeO₂ NPs. However, they were positively correlated with the increasing irradiation time and simulated sunlight intensities. High molecular weights of HA fraction (>100 kDa) restrained the ROS generation and Φ of ³HA* due to surface adsorption between HA and CeO₂ NPs blocking reactive sites, competitive absorption for simulated sunlight. Fourier transform infrared and three-dimensional excitation-emission matrix fluorescence spectroscopy confirmed that the carboxylic groups of HA have high complexation capacity with CeO₂ NPs. These findings are essential for us to improve the understanding of the impacts of HA on CeO₂ NPs under different conditions in natural waters.

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

The selective sorption of dissolved organic matter (DOM) on minerals is a widespread geochemical process in the natural environment. Recent studies have explored the influence of this process on the molecular fractionation of DOM at water-mineral interfaces. However, it remains unclear how molecular fractionation affects the photochemistry of DOM. Here, we demonstrate that the adsorptive fractionation of DOM on ferrihydrite greatly reduces its photoproduction of reactive oxygen species (ROS) including ¹O₂, O₂^•-, and •OH normalized to organic carbon (ROS_OC). The ROS_OC for ¹O₂, O₂^•-, and •OH were positively correlated with the abundances of polyphenols and oxygenated polycyclic aromatics, which were also observed using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis to be preferentially sequestered by ferrihydrite. The molecules that preferentially remained in the solution after adsorption displayed low levels of ROS_OC. The molecular fractionation of DOM induced by adsorption on ferrihydrite therefore influenced the molecular components and also significantly reduced the photoreactive fractions of DOM in waters. These results are very important in promoting our understanding of the effects of molecular fractionation on the biogeochemical features, behaviors, and implications of DOM in the environment.

Supporting ultrathin “fish scale-like” BiOBr nanosheets on Bi6Mo2O15 sub-microwires for boosting photocatalytic performance

A BiOBr/Bi6Mo2O15 edge-on heterostructure with fast electron transport could improve interface conductivity and accelerate charge-separation efficiency.

Climate-zone-dependent effect mechanism of humic acid and fulvic acid extracted from river sediments on aggregation behavior of graphene oxide

Climate factors could affect the physicochemical properties of dissolved organic matter (DOM) in river sediments, which potentially influence the stability of nanoparticles in natural waters. In this study, we extracted humic acid (HA) and fulvic acid (FA) from river sediments in different climate zones of China. Their effect with different concentrations (0.2 and 1 mg·C·L^-1) on the aggregation kinetics of large (589 nm) and small graphene oxide (GO, 200 nm) in NaCl solutions was investigated. Both concentrations of HA/FA significantly inhibited the aggregation of small GO because of the steric forces rendered by DOM. For large GO, the inhibition effect of HA on aggregation was higher than FA because of the higher molecular weight, longer carbon chain length, and more structure complexity of HA. Interestingly, with 0.2 mg·C·L^-1 HA and large GO, Makou in Subtropical monsoon climate zone decreased the aggregation rate more significantly due to its larger molecular weight, while, Maqin in the Plateau and mountain (PM) climate zone with smaller molecular weight and greater hydrophobicity showed lower inhibition effect on the aggregation. One mg·C·L^-1 FA with high polarity from Tangke in PM climate zone and Panjin from Temperate monsoon climate zone showed more stability ability towards large GO. Derjaguin-landau-verwey-overbeek (DLVO) theory indicated that the interaction energy barrier between GO particles dependent on physicochemical characteristics of DOM and GO size. Understanding the climate-zone-dependent effect of sediment DOM on stability of GO is essential for anticipating its fate in natural systems.

Impact of Environmental Conditions on Secondary Organic Aerosol Production from Photosensitized Humic Acid

Recent studies have shown the potential of the photosensitizer chemistry of humic acid, as a proxy for humic-like substances in atmospheric aerosols, to contribute to secondary organic aerosol mass. The mechanism requires particle-phase humic acid to absorb solar radiation and become photoexcited, then directly or indirectly oxidize a volatile organic compound (VOC), resulting in a lower volatility product in the particle phase. We performed experiments in a photochemical chamber, with aerosol-phase humic acid as the photosensitizer and limonene as the VOC. In the presence of 26 ppb limonene and under atmospherically relevant UV-visible irradiation levels, there is no significant change in particle diameter. Calculations show that SOA production via this pathway is highly sensitive to VOC precursor concentrations. Under the assumption that HULIS is equally or less reactive than the humic acid used in these experiments, the results suggest that the photosensitizer chemistry of HULIS in ambient atmospheric aerosols is unlikely to be a significant source of secondary organic aerosol mass.

Generation and properties of aqu/nC60: the combined effects of humic acid, sunlight, and agitation intensity

Once released into natural water, the environmental behavior and fate of C₆₀ could inevitably been affected by humic acid (HA), sunlight, and hydrodynamic conditions. However, the combined effects of these factors are not so clear. Therefore, in the present study, effects of HA, sunlight, and agitation intensity on generation and properties of aqu/nC₆₀ were investigated. The results indicated that HA could increase the concentration of aqu/nC₆₀ mainly through the steric hindrance effect. The higher agitation intensity led to higher concentrations of aqu/nC₆₀ and more efficient steric stabilization was formed by HA. Sunlight irradiation promoted the surface oxidization and consequently enhanced the dispersion of C₆₀. The relative order of the influence on the UV/vis concentration was sunlight > agitation intensity > HA. In addition, HA might not always enhance the dispersion of aqu/nC₆₀ due to light screening/ROS scavenging, over-coating, or chain-like bridging mechanism. Therefore, evaluating the environmental behavior and fate of C₆₀ should take these factors into account together.

An atomic insight into BiOBr/La2Ti2O7 p–n heterojunctions: interfacial charge transfer pathway and photocatalysis mechanism

Mechanisms of the enhancement photocatalytic activity of p–n heterojunction BiOBr/La₂Ti₂O₇ and photocatalytic NO oxidation are proposed.

The dispersion, stability, and resuspension of C60 in environmental water matrices

Environmental waters cover a range of water quality characteristics which could greatly affect the behavior and fate of C₆₀ in the aquatic environment. In this study, the dispersion and stability of C₆₀ in several environmental water matrices during a 70-day extended mixing period were investigated to better understand its environmental behavior and fate in environmental waters. Relatively stable nanoscale aggregates in water (aqu/nC₆₀) could be formed in wastewater influent, while unstable suspensions were obtained in river water, wastewater effluent, seawater, and estuarine water. During the extended mixing under sunlight, oxygen-containing moieties were produced on the surface of the C₆₀ aggregates, independent of the kind of environmental water matrices. Once the mixed system went under quiescent condition, aggregation and sedimentation of aqu/nC₆₀ occurred. However, an extremely short-time disturbance could easily resuspend the C₆₀ aggregates deposited and increase the concentration of aqu/nC₆₀ in the overlying water column. Therefore, the effects of resuspension should be considered when investigating the environmental behavior and fate of C₆₀.

The missing link between carbon nanotubes, dissolved organic matter and organic pollutants

Ternary interactions between carbon nanotubes (CNTs), dissolved organic matter (DOM) and small organic molecules (namely low molecular mass organic pollutants) are of great importance since they can affect the reactivity and fate of all involved compartments in the environment. This review thoroughly assesses existing knowledge on the adsorption of DOM and small organic molecules by CNTs, while giving special attention to (i) the complex nature of DOM, (ii) the ternary rather than binary interactions between CNTs, DOM and the small organic molecules and (iii) the DOM-organic molecule interactions. We discuss in detail the main factors influencing DOM adsorption by CNTs and attempt to differentiate between the role of DOM composition and conformation. We then outline how the presence of DOM influences the adsorption of small organic molecules by CNTs, considering the introduction stage of DOM and the impact of the organic molecule's properties. DOM adsorption by CNTs is highly dependent on its composition and is governed by the size, hydrophobicity and aromaticity of DOM. DOM adsorption was found to alter the assembly of the CNTs, resulting in changes in the distribution of adsorption sites. Small organic molecules may adsorb to residual surface area on the CNTs, to DOM-coating the CNTs or remain in solution, possibly complexed with DOM. This results in their suppressed or enhanced adsorption in comparison to DOM-free media. The physicochemical properties of the organic molecules (hydrophobicity, size, structure and charge) also play a major role in this process. We present knowledge gaps that need clarification such as the extent of DOM desorption from CNTs, the amount of co-adsorbed DOM during competition with small organic molecules for adsorption sites on the CNTs and the behavior of CNTs under realistic conditions. More data generated from experiments using natural DOM rather than dissolved humic substances are required to improve our understanding of the interactions between CNTs and small organic molecules in realistic environmental scenarios. This review provides conclusions and research directions needed to evaluate the nature of interactions between CNTs, DOM and organic pollutants in aquatic systems affected by anthropogenic activities.

Significant contribution of metastable particulate organic matter to natural formation of silver nanoparticles in soils

Particulate organic matter (POM) is distributed worldwide in high abundance. Although insoluble, it could serve as a redox mediator for microbial reductive dehalogenation and mineral transformation. Quantitative information on the role of POM in the natural occurrence of silver nanoparticles (AgNPs) is lacking, but is needed to re-evaluate the sources of AgNPs in soils, which are commonly considered to derive from anthropogenic inputs. Here we demonstrate that POM reduces silver ions to AgNPs under solar irradiation, by producing superoxide radicals from phenol-like groups. The contribution of POM to the naturally occurring AgNPs is estimated to be 11-31%. By providing fresh insight into the sources of AgNPs in soils, our study facilitates unbiased assessments of the fate and impacts of anthropogenic AgNPs. Moreover, the reducing role of POM is likely widespread within surface environments and is expected to significantly influence the biogeochemical cycling of Ag and other contaminants that are reactive towards phenol-like groups.

Cotransformation of Carbon Dots and Contaminant under Light in Aqueous Solutions: A Mechanistic Study

In this study, the photochemistry of carbon dots (CDs) and their effects on pollutant transformation were systematically examined. Diethyl phthalate (DEP) degradation was strongly enhanced by CDs under UV light, with the observed reaction rate constant ( k_obs) increased by 2.4-15.1-fold by CDs at a concentration of 0.5-10 mg/L. Electron paramagnetic resonance (EPR) spectrometry combined with free radical quenching experiments with various chemical probes indicated the production of reactive oxygen species (ROS), including hydroxyl radicals (^•OH), singlet oxygen (¹O₂), and superoxide radical anions (O₂^•-), and these contributed to the enhanced DEP degradation. Meanwhile, CDs were also degraded to low-molecular-weight species and partially mineralized to CO₂ by ROS, as evidenced by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and total organic carbon (TOC) analysis, and transformation of CDs was accelerated by DEP. Furthermore, CDs were degraded rapidly under natural sunlight, accompanied by the formation of ^•OH and ¹O₂. Anions such as CO₃^2-, NO₃^-, and Cl^- had limited effects on transformation of CDs, while humic substances greatly inhibited this process. Our results indicate that photoreactions of CDs play an important role in influencing the transformation of pollutants and CDs themselves in the natural aquatic environment. The findings provide invaluable information for evaluating risks associated with the release of CDs into the natural environment.

Mechanistic Insight into the Effect of Metal Ions on Photogeneration of Reactive Species from Dissolved Organic Matter

The photogeneration of reactive species (RS) from dissolved organic matter (DOM) exhibits a great impact on the attenuation of pollutants in natural waters. However, the effect of metal ions on the photogeneration of excited triplet-state DOM (³DOM*), singlet oxygen (¹O₂), and hydroxyl radical (^•OH) by effluent organic matter (EfOM), fulvic acid (FA), and humic acid (HA) is poorly understood. Here, we provided the first evidence that the quenching of ³DOM* was positively correlated with the complexation capacity of metal ions with DOM. Generally, the paramagnetic metal ions (Cr³⁺, Mn²⁺, Fe³⁺, and Cu²⁺) exhibited higher conditional stability constants (log K_ML) with DOM and stronger inhibition for RS than the others (Mg²⁺, Ca²⁺, Al³⁺, and Zn²⁺). For DOM of different sources, the metal binding capacity increased in the order of EfOM < HA < FA and the humic substances were more susceptible to metal ions. The inhibition was attributed to both static and dynamic quenching of ³DOM* by metal ions. The dynamic quenching rate constants of metal ions for ³DOM* were estimated as ∼10⁹ M^-1 s^-1, which was positively related to the corresponding log K_ML. These findings highlight crucial links between metal-DOM complexation and ³DOM* quenching and, consequently, the inhibition of RS.

Phototransformation of 2,3-Dibromopropyl-2,4,6-tribromophenyl ether (DPTE) in Natural Waters: Important Roles of Dissolved Organic Matter and Chloride Ion

Novel brominated flame retardants (NBFRs) have become ubiquitous emerging organic pollutants. However, little is known about their transformation in natural waters. In this study, aquatic photochemical behavior of a representative NBFR, 2,3-dibromopropyl-2,4,6-tribromophenyl ether (DPTE), was investigated by simulated sunlight irradiation experiment. Results show that DPTE can undergo direct photolysis (apparent quantum yield 0.008 ± 0.001) and hydroxyl radical (·OH) initiated oxidation (second order reaction rate constant 2.4 × 10⁹ M^-1·s^-1). Dissolved organic matter (DOM) promotes the photodegradation due to generation of excited triplet DOM and ·OH. Two chlorinated intermediates were identified in the photodegradation of DPTE in seawaters. Density functional theory calculation showed that ·Cl or ·Cl₂^- addition reactions on C-Br sites of the phenyl group and H-abstraction reactions from the propyl group are main reaction pathways of DPTE with the chlorine radicals. The ·Cl or ·Cl₂^- addition proceeds via a replacement mechanism to form chlorinated intermediates. Environmental half-lives of DPTE relevant with photodegradation are estimated to be 6.5-1153.9 days in waters of the Yellow River estuarine region. This study provides valuable insights into the phototransformation behavior of DPTE in natural waters, which is helpful for persistence assessment of the NBFRs.

Ferrocene-catalyzed heterogeneous Fenton-like degradation mechanisms and pathways of antibiotics under simulated sunlight: A case study of sulfamethoxazole

Readily-available and efficient catalyst is essential for activating oxidants to produce reactive species for deeply remediating water bodies contaminated by antibiotics. In this study, Ferrocene (Fc) was introduced to establish a heterogeneous photo-Fenton system for the degradation of sulfonamide antibiotics, taking sulfamethoxazole as a representative. Results showed that the removal of sulfamethoxazole was effective in Fc-catalyzed photo-Fenton system. Electron spin resonance and radical scavenging experiments verified that there was a photoindued electron transfer process from Fc to H₂O₂ and dissolved oxygen resulting in the formation of OH that was primarily responsible for the degradation of sulfamethoxazole. The reactions of OH with substructure model compounds of sulfamethoxazole unveiled that aniline moiety was the preferable reaction site of sulfamethoxazole, which was verified by the formation of hydroxylated product and the dimer of sulfamethoxazole in Fc-catalyzed photo-Fenton system. This heterogeneous photo-Fenton system displayed an effective degradation efficiency even in a complex water matrices, and Fc represented a long-term stability by using the catalyst for multiple cycles. These results demonstrate that Fc-catalyzed photo-Fenton oxidation may be an efficient approach for remediation of wastewater containing antibiotics.

Relative importance of humic and fulvic acid on ROS generation, dissolution, and toxicity of sulfide nanoparticles

In this study, the effect of natural organic matter (NOM) composition (humic acid (HA) or fulvic acid (FA)) on dissolution, reactive oxygen species (ROS) generation, and toxicity of sulfide nanoparticles (NPs) was investigated under UV irradiation. NOM acted as a UV filter or antioxidant, decreasing ROS (O₂^-, OH, and ¹O₂) generation by WS₂ and MoS₂ NPs. The higher light-absorbing fractions of HA in NP/HA mixtures and the faster reaction rate of HA with ROS resulted in higher inhibition effect of HA than FA on O₂^- and OH generation by WS₂ and MoS₂ NPs. Both HA and FA completely inhibited ¹O₂ generation by WS₂ and MoS₂ NPs. NOM could transfer electrons to CdS and promote its O₂^- generation. No measurable amount of OH was generated by CdS with or without NOM. FA decreased ¹O₂ generation by CdS more significantly than HA due to the higher reaction rate between FA and ¹O₂. HA showed a higher inhibition effect on the dissolution rate of CdS and WS₂ NPs than FA. Both HA and FA played minor roles in the toxicity of CdS toward Escherichia coli but decreased the toxicity of MoS₂ and WS₂ due to the reduced ROS generation and/or dissolution concentrations.

Development of Novel Chemical Probes for Examining Triplet Natural Organic Matter under Solar Illumination

Excited triplet states of chromophoric dissolved organic matter (³CDOM*) are critical transient species in environmental photochemistry. In the present study, sorbic amine (2,4-hexadien-1-amine) and sorbic alcohol were employed as new probe molecules for triplet measurements and compared to the results measured from sorbic acid under identical conditions. Unlike sorbic acid, sorbic amine and sorbic alcohol were not directly photolyzed under solar irradiation. Photosensitized isomerization of the probes with the conjugated diene structure could yield four geometrical isomers. The separation and quantitative determination of the geometrical isomers were accomplished using HPLC and high-resolution NMR analyses. When photoirradiated Suwannee River natural organic matter (SRNOM) was employed as a source of ³CDOM*, significantly different photosensitized isomerization rates were observed for the diverse charged probes. The bimolecular reaction rate constants between ³SRNOM* and the probes were calculated as (0.42 ± 0.1) × 10⁹ M^-1 s^-1 for sorbic acid, (1.1 ± 0.1) × 10⁹ M^-1 s^-1 for sorbic alcohol, and (5.2 ± 0.4) × 10⁹ M^-1 s^-1 for sorbic amine, respectively. The average apparent Φ_triplet was (0.96 ± 0.03)% based on an irradiation range of 290 to 400 nm. We developed highly selective and efficient probes for triplet determination and elucidated the different reaction behaviors of these conjugated dienes containing different charged substituents within the photochemical energy transfer process.

Comparative toxicity of Cd, Mo, and W sulphide nanomaterials toward E. coli under UV irradiation

In this study, the phototoxicity of cadmium sulfide (CdS), molybdenum disulfide (MoS₂), and tungsten disulfide (WS₂) nanoparticles (NPs) toward Escherichia coli (E. coli) under UV irradiation (365 nm) was investigated. At the same mass concentration of NPs, the toxicity of three NPs decreased in the order of CdS > MoS₂ > WS₂. For example, the death rates of E. coli exposed to 50 mg/L CdS, MoS₂, and WS₂ were 96.7%, 38.5%, and 31.2%, respectively. Transmission electron microscope and laser scanning confocal microscope images of E. coli exposed to three NPs showed the damage of cell walls and release of intracellular components. The CdS-treated cell wall was more extensively damaged than those of MoS₂-treated and WS₂-treated bacteria. WS₂ and MoS₂ generated superoxide radical (O₂^-), singlet oxygen (¹O₂), and hydroxyl radical under UV irradiation, CdS produced only O₂^- and ¹O₂. CdS and WS₂ released ions under UV irradiation, while MoS₂ did not. Reactive oxygen species (ROS) generation and toxic ion release jointly resulted in the antibacterial activities of CdS and WS₂. ROS generation was the dominant toxic mechanism of MoS₂ toward the bacteria. This study highlighted the importance of considering the hazardous effect of sulfide NPs after their release into natural waters under light irradiation condition.

Effects of C60 on the Photochemical Formation of Reactive Oxygen Species from Natural Organic Matter

Buckminsterfullerenes (C₆₀) are widely used nanomaterials that are present in surface water. The combination of C₆₀ and humic acid (HA) generates reactive oxygen species (ROS) under solar irradiation, but this process is not well understood. Thus, the present study focused on the photochemical formation of singlet oxygen (¹O₂), hydroxyl radical (HO^•)-like species, superoxide radicals (O₂^•-), hydrogen peroxide (H₂O₂), and triplet excited states (³C₆₀*/³HA*) in solutions containing both C₆₀ and HA. The quantum yield coefficients of excited triplet states (f_TMP) and apparent quantum yields of ROS were measured and compared to the calculated values, which were based on the conservative mixing model. Although C₆₀ proved to have only a slight impact on the ¹O₂ formation from HA, C₆₀ played a key role in the inhibition of O₂^•-. The photochemical formation of H₂O₂ followed the conservative mixing model due to the reaction of C₆₀^•- with HO₂^•/O₂^•-, and the biomolecular reaction rate constant has been measured as (7.4 ± 0.6) × 10⁶ M^-1 s^-1. The apparent f_TMP was significantly lower than the calculated value, indicating that the steric effect of HA was significant in the reaction of ³C₆₀* with the TMP probe. In contrast, C₆₀ did not have an effect on the photochemical formation of HO^• from HA, suggesting that HO^• is elevated from the hydrophilic surface of HA. The aforementioned results may be useful for predicting the photochemical influence of C₆₀ on aqueous environments.

Influence of dissolved organic matter on photogenerated reactive oxygen species and metal-oxide nanoparticle toxicity

The effect of humic acid (HA) or fulvic acid (FA) on reactive oxygen species (ROS) generation by six metal-oxide nanoparticles (NPs) and their toxicities toward Escherichia coli was investigated under UV irradiation. Dissolved organic matter (DOM) decreased OH generation by TiO2, ZnO, and Fe2O3, with FA inhibiting OH generation more than HA. The generated OH in NPs/DOM mixtures was higher than the measured concentrations because DOM consumes OH faster than its molecular probe. None of NPs/FA mixtures produced O2(-). The generated O2(-) concentrations in NPs/HA mixtures (except Fe2O3/HA) were higher than the sum of O2(-) concentrations that produced as NPs and HA were presented by themselves. Synergistic O2(-) generation in NPs/HA mixtures resulted from O2 reduction by electron transferred from photoionized HA to NPs. DOM increased (1)O2 generation by TiO2, CuO, CeO2, and SiO2, and FA promoted (1)O2 generation more than HA. Enhanced (1)O2 generation resulted from DOM sensitization of NPs. HA did not increase (1)O2 generation by ZnO and Fe2O3 primarily because released ions quenched (1)O2 precursor ((3)HA*). Linear correlation was developed between total ROS concentrations generated by NPs/DOM mixtures and bacterial survival rates (R(2) ≥ 0.80). The results implied the necessity of considering DOM when investigating the photoreactivity of NPs.

Effect of aqueous media on the copper-ion-mediated phototoxicity of CuO nanoparticles toward green fluorescent protein-expressing Escherichia coli

Quantitative comparison of different aqueous media on the phototoxicity of copper oxide nanoparticles (CuO NPs) is crucial for understanding their ecological effects. In this study, the phototoxicity of CuO NPs toward the green fluorescent protein-expressing Escherichia coli (GFP-E. coli) under UV irradiation (365 nm) was investigated in Luria-Bertani medium (LB), NaCl solution, deionized water (DI) and phosphate-buffered saline (PBS). The phototoxicity of CuO NPs toward GFP-E. coli decreased in the order of DI>NaCl>PBS>LB because of different released concentrations of Cu(2+). The 3h released Cu(2+) concentrations by 10mg/L CuO NPs in DI water, NaCl solution, LB medium, and PBS were 1946.3 ± 75.6, 1242.5 ± 47.6, 1023.4 ± 41.2, and 1162.1 ± 41.9 μg/L, respectively. Transmission electron microscope and laser scanning confocal microscope images of E. coli exposed to CuO NPs demonstrated that the released Cu(2+) resulted in fragmentation of bacterial cell walls, leakage of intracellular components, and finally death of bacteria in four media after UV light irradiation. In each medium, the bacterial mortality rate logarithmically increased with the releasing concentrations of Cu(2+) by CuO NPs (R(2)>0.90) exposed to 3h UV light. This study highlights the importance of taking into consideration of water chemistry when the phototoxicity of CuO NPs is assessed in nanotoxicity research.