Particle coating-dependent interaction of molecular weight fractionated natural organic matter: impacts on the aggregation of silver nanoparticles

Molecular weight-dependent differences in spectral properties and metal-binding behaviors of dissolved organic matter from different lakes

Dissolved organic matter (DOM) plays an important role in governing metal speciation and migration in aquatic systems. In this study, various DOM samples were collected from Lakes Erhai, Kokonor, and Chaka, and size-fractionated into high molecular weight (HMW, 1 kDa-0.7 μm) and low molecular weight (LMW, <1 kDa) fractions for measurements of dissolved organic carbon (DOC), spectral properties, and metal binding behaviors. Our results demonstrated that samples from Lake Chaka exhibited the highest DOC concentration and fluorescence indices but the lowest percentage of carbohydrates. Regardless of sampling locations, the HMW-DOM fractions contained higher abundances of aromatic DOM, carbohydrates and protein-like substances, but lower abundance of fulvic acid-like substances compared to those in the LMW fractions. Metal titration experiments coupled with the excitation-emission matrix (EEM)-parallel factor (PARAFAC) modeling revealed that the quenching of the PARAFAC-derived fluorescent components was more pronounced in the presence of Cu(II) compared to Pb(II). Humic-like components emerged as a superior model, exhibiting higher binding affinities for Cu(II) than protein-like substances, while the opposite trend was observed for Pb(II). In samples obtained from Lakes Erhai and Kokonor, the condition stability constants (Log KM) for the binding of both Cu(II) and Pb(II) with the HMW-DOM fraction were higher than those with the LMW-DOM fraction. Conversely, a contrasting trend was observed for Lake Chaka. This study highlighted the heterogeneity in spectral properties and metal-binding behaviors of natural DOMs, contributing to an improved understanding of the molecular interactions between DOM components and metal ions and their environmental fate in aquatic ecosystems.

Environmental behavior of silver nanomaterials in aquatic environments: An updated review

The increasing applications of silver nanomaterials (nano-Ag) and their inevitable release posed great potential risks to aquatic organisms and ecosystems. Considerable attention has been attracted on their behaviors and transformations, which were critically important for their subsequent biological toxicities and ecological effects. Therefore, the summary of the recent efforts on the environmental behavior of nano-Ag would be beneficial for understanding the environmental fate and accurate risk assessment. This review summarized the studies on various physical, chemical and biological transformations of nano-Ag, meanwhile, the influencing factors (including the intrinsic properties and environmental conditions) and related mechanisms were highlighted. Surface structure and facets of nano-Ag, abiotic conditions and natural freeze-thaw cycle processes could affect the transformations of nano-Ag under different environmental scenarios (including freshwater, seawater and wastewater). The interactions with co-present components, such as chemicals and other particles, impacted the multiple processes of nano-Ag. Besides, the contradictory effects and mechanisms by several environmental factors were summarized. Lastly, the key knowledge gaps and some aspects that deserve further investigation were also addressed. Therefore, the current review aimed to provide an overall analysis of transformation processes of nano-Ag, which will provide more available information and pave the way for the future research areas.

Aggregation and stability of selenium nanoparticles: Complex roles of surface coating, electrolytes and natural organic matter

The application of selenium nanoparticles (SeNPs) as nanofertilizers may lead to the release of SeNPs into aquatic systems. However, the environmental behavior of SeNPs is rarely studied. In this study, using alginate-coated SeNPs (Alg-SeNPs) and polyvinyl alcohol-coated SeNPs (PVA-SeNPs) as models, we systematically investigated the aggregation and stability of SeNPs under various water conditions. PVA-SeNPs were highly stable in mono- and polyvalent electrolytes, probably due to the strong steric hindrance of the capping agent. Alg-SeNPs only suffered from a limited increase in size, even at 2500 mmol/L NaCl and 200 mmol/L MgCl₂, while they underwent apparent aggregation in CaCl₂ and LaCl₃ solutions. The binding of Ca²⁺ and La³⁺ with the guluronic acid part in alginate induced the formation of cross-linking aggregates. Natural organic matter enhanced the stability of Alg-SeNPs in monovalent electrolytes, while accelerated the attachment of Alg-SeNPs in polyvalent electrolytes, due to the cation bridge effects. The long-term stability of SeNPs in natural water showed that the aggregation sizes of Alg-SeNPs and PVA-SeNPs increased to several hundreds of nanometers or above 10 µm after 30 days, implying that SeNPs may be suspended in the water column or further settle down, depending on the surrounding water chemistry. The study may contribute to the deep insight into the fate and mobility of SeNPs in the aquatic environment. The varying fate of SeNPs in different natural waters also suggests that the risks of SeNPs to organisms living in diverse depths in the aquatic compartment should be concerned.

Nanoparticulate pollutants in the environment: Analytical methods, formation, and transformation

The wide application of nanomaterials and plastic products generates a substantial number of nanoparticulate pollutants in the environment. Nanoparticulate pollutants are quite different from their bulk counterparts because of their unique physicochemical properties, which may pose a threat to environmental organisms and human beings. To accurately predict the environmental risks of nanoparticulate pollutants, great efforts have been devoted to developing reliable methods to define their occurrence and track their fate and transformation in the environment. Herein, we summarized representative studies on the preconcentration, separation, formation, and transformation of nanoparticulate pollutants in environmental samples. Finally, some perspectives on future research directions are proposed.

Differences in the spectral characteristics of dissolved organic matter binding to Cu(II) in wetland soils with moisture gradients

The environmental behavior of heavy metals in soil is significantly regulated by their binding with dissolved organic matter (DOM), which is affected by soil moisture contents. However, the mechanism of this interaction in soils with varying moisture is still not well understood. Using a combination of ultrafiltration, Cu(II) titration, and multispectral (ultraviolet-visible absorption, 3D fluorescence, Fourier transform infrared) analysis techniques, we studied the differences in the spectral characteristics and Cu(II) binding properties of soil dissolved organic matter (DOM) and its different molecular weight (MW) fractions with moisture gradients. We found that the abundance and spectral characters of soil DOM changed with increasing soil moisture, i.e., the increase in abundance while the decrease in aromaticity and humification index. The components of DOM, shown by Fluorescence region-integration (FRI) analysis, also changed, with an increase in the proportion of protein-like substances and a decrease of humic-like and fulvic-like substances. The overall Cu(II) binding potential of soil DOM diminished with increasing soil moisture, as indicated by the fluorescence parallel factor (PARAFAC) analysis. This is aligns with the changes in DOM composition, as the humic-like and fulvic-like fractions exhibited higher Cu(II) binding potential compared to the protein-like fractions. The low MW fraction of the MW-fractionated samples showed a stronger binding potential for Cu(II) compared to the high MW fraction. Finally, the active binding site of Cu(II) in DOM, as revealed by UV-difference spectroscopy and 2D-FTIR-COS analysis, decreased with increasing soil moisture, with the order of preferentially functional groups shifting from OH, NH, and CO to CN and CO. This study emphasizes the impact of moisture variations on the characteristics of DOM and its interaction with Cu(II), providing insight into the environmental fate of heavy metal contaminants in soil in areas with alternating land and water conditions.

Reciprocal effects of NOM and solution electrolyte ions on aggregation of ferrihydrite nanoparticles

The effects of natural organic matter (NOM) types and electrolyte ions are crucial to the aggregation of ferrihydrite nanoparticles (Fh NPs) in the environment. Dynamic light scattering (DLS) was employed for the aggregation kinetics of Fh NPs (10 mg/L as Fe) in the present study. The critical coagulation concentration (CCC) values of Fh NPs aggregation in NaCl were obtained in the presence of 15 mg C/L NOM as SRHA (857.4 mM) > PPHA (752.3 mM) > SRFA > (420.1 mM) > ESHA (141.0 mM) > NOM-free (125.3 mM), indicating Fh NPs aggregation was inhibited as the above order. Comparatively in CaCl₂, the CCC values were measured in ESHA (0.9 mM), PPHA (2.7 mM), SRFA (3.6 mM), SRHA (5.9 mM), NOM-free (76.6 mM), implying NPs aggregation was enhanced following the order of ESHA > PPHA > SRFA > SRHA. To investigate the dominant mechanisms, the aggregation of Fh NPs was comprehensively studied under the effects of NOM types, concentrations (0-15 mg C/L) and electrolyte ions (NaCl/CaCl₂ beyond CCC). In NaCl/CaCl₂, the low concentration of NOM (<7.5 mg C/L) could accelerate NPs aggregation mainly due to patch-charge attraction. When NOM concentration was high (>7.5 mg C/L), the inhibition effect on NPs aggregation occurred in NaCl due to steric repulsion, whereas the enhancement effect in CaCl₂ of aggregation was dominated by the bridging effect. The results indicated that the effects of NOM types, concentration and electrolyte ions should be carefully considered for the environmental behavior of NPs.

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.

Carbon content determines the aggregation of biochar colloids from various feedstocks

The aggregation kinetics of biochar colloids (BCs) play a crucial role in the fate and transport of contaminants, as well as the carbon (C) cycle in the environment. However, the colloidal stability of BCs from various feedstocks is very limited. In this study, the critical coagulation concentration (CCC) of twelve standard biochars pyrolyzed from various feedstocks (municipal source, agricultural waste, herbaceous residue, and woody feedstock) at 550 °C and 700 °C were investigated, and the relationship between the physicochemical characteristics of biochar and the colloidal stability of BCs was further analyzed. The CCC of BCs in the NaCl solution followed the trend of municipal source < agricultural waste < herbaceous residue < woody feedstock, which was similar to the order of C content in biochar. The CCC of BCs showed a strong positive correlation with the C content of various biochars, especially pyrolyzed at a higher temperature of 700 °C. The BCs derived from lignin-rich feedstock (e.g., woody feedstock) had the highest colloidal stability, followed by cellulose-rich feedstock (e.g., agricultural waste and herbaceous residue). The BCs derived from organic matter-rich feedstock (municipal source) were easy to aggregate in the aqueous environment. This study quantitatively provides new insights into the relationship between BCs stability and biochar characteristics from various feedstocks, which is critical to assess biochar environmental behavior in aqueous environments.

Characteristics and behaviors of microplastics undergoing photoaging and Advanced Oxidation Processes (AOPs) initiated aging

The fact that 94% of microplastics (MPs) ubiquitous in the environment are subject to natural weathering makes the aging study currently a research hotspot. This review summarized the physicochemical characteristics of MPs undergoing natural and artificial aging and evaluated current analytical methods used in aging studies. Besides, the differences in photoaging and aging induced by advanced oxidation processes (AOPs) were discussed, leading to a conclusion that AOPs composed of oxidant and ultraviolet (UV) irradiation can better facilitate the alteration of MPs compared to UV irradiation alone. In addition, the environmental behavior of aged MPs was outlined and their adsorption properties for organics and metals were highlighted as a result of combined effects of hydrophobic, π-π, diffusion, and hydrogen bond interaction. Furthermore, the mechanisms of photoaging and AOPs-initiated aging were analyzed, mainly the role of reactive oxygen species (ROS) and environmentally persistent free radicals (EPFRs). Finally, the applications of two-dimensional correlation spectroscopy (2D-COS) and three-dimensional fluorescence spectra using excitation emission matrix-parallel factor analysis (EEM-PARAFAC) were discussed for the aging process analysis. This overview plays an important role in explaining the aging characteristics of MPs and provides a theoretical foundation for further investigations into their toxicity and removal.

Comparisons in molecular weight distributions and size-dependent optical properties among model and reference natural dissolved organic matter

Humic acid (HA) and reference natural organic matter (NOM) have been widely used in environmental assessment, biogeochemistry, and ecotoxicity studies. Nevertheless, similarities and differences among the commonly used model/reference NOMs and bulk dissolved organic matter (DOM) have rarely been systematically evaluated. In this study, HA, SNOM (Suwannee River NOM) and MNOM (Mississippi River NOM), both from International Humic Substances Society, and freshly collected unfractionated NOM (FNOM) were concurrently characterized to evaluate their heterogeneous nature and size-dependent chemical properties. We found that molecular weight distributions, PARAFAC-derived fluorescent components, and size-dependent optical properties are NOM-specific and highly variable with pH. The < 1 kDa DOM abundance followed the order of HA < SNOM < MNOM < FNOM. In addition, FNOM was more hydrophilic and contained more protein-like and autochthonous components with a higher UV-absorbance ratio index (URI) and biological fluorescence index, whereas HA and SNOM contained more allochthonous, humic-like components with a higher aromaticity and lower URI. Significant differences in molecular composition and size spectra between FNOM and model/reference NOMs suggest that environmental role of NOMs should be evaluated at the levels of molecular weight and functionalities under the same experimental conditions and that HA and SNOM may not represent bulk NOM in the environment. This study provides new information about similarities and differences in DOM size-spectra and chemical properties between reference NOMs and in-situ NOM and highlights the need to better understand the heterogenous roles of NOMs in regulating the toxicity/bioavailability and environmental fate of pollutants in aquatic environments.

Ambivalent effects of dissolved organic matter on silver nanoparticles/silver ions transformation: A review

The numerous applications of silver nanoparticles (AgNPs) lead to their spread in aquatic systems and the release of silver ions (Ag⁺), which brings potential risks to environment and human health. Owing to the different toxicity, the mutual transformations between AgNPs and Ag⁺ has been a hot topic of research. Dissolved organic matter (DOM) is ubiquitous on the earth and almost participates in all the reactions in the nature. The previous studies have reported the roles of DOM played in the transformation between AgNPs and Ag⁺. However, different experiment conditions commonly caused contradictory results, leading to the difficulty to predict the fate of AgNPs in specific reactions. Here we summarized mechanisms of DOM-mediated AgNPs oxidation and Ag⁺ reduction, and analyzed the effects of environmental parameters. Moreover, the knowledge gaps, challenges, and new opportunities for research in this field are discussed. This review will promote the understanding of the fate and risk assessments of AgNPs in natural water systems.

Dissolved organic matter derived from biodegradable microplastic promotes photo-aging of coexisting microplastics and alters microbial metabolism

Dissolved organic matter (DOM) leaching from biodegradable microplastics (BMPs) and its characteristics and corresponding environmental implication are rarely investigated. In this study, the main component of DOM leachate from the two BMPs (polyadipate/butylene terephthalate (PBAT)/polycaprolactone (PCL)) was verified by using excitation-emission matrix-parallel factor analysis (EEM-PARAFAC). The PBAT-DOM (PBOM) was aromatized and terrestrial. Comparatively, PCL-DOM (PLOM) had low molecular weight. PBOM contained protein-like components while PLOM contained tryptophan and tyrosine components. Interestingly, both PBOM and PLOM could accelerate the decomposition and oxidation of coexisting polystyrene (PS) under light irradiation. Further, the difference in composition and the properties of BMPs-DOM significantly affected its photochemical activity. The high territoriality and protein-like component of PBOM significantly promoted the generation of ¹O₂ and O₂^•-, which caused faster disruptions to the backbone of PS. Simultaneously, the microbial community's richness, diversity, and metabolism were obviously improved under the combined pressure of aged PS and BMPs-DOM. This study threw light on the overlooked contribution of DOM derived from BMPs in the aging process of NMPs and their impact on the microbial community and provided a promising strategy for better understanding of combined MPs' fate and environmental risk.

Redispersion Behavior of 2D MoS2 Nanosheets: Unique Dependence on the Intervention Timing of Natural Organic Matter

The aggregation-redispersion behavior of nanomaterials determines their transport, transformation, and toxicity, which could be largely influenced by the ubiquitous natural organic matter (NOM). Nonetheless, the interaction mechanisms of two-dimensional (2D) MoS₂ and NOM and the subsequent influences on the redispersion behavior are not well understood. Herein, we investigated the redispersion of single-layer MoS₂ (SL-MoS₂) nanosheets as influenced by Suwannee River NOM (SRNOM). It was found that SRNOM played a decisive role on the redispersion of MoS₂ 2D nanosheets that varied distinctly from the 3D nanoparticles. Compared to the poor redispersion of MoS₂ aggregates in the absence or post-addition of SRNOM to the aggregates, co-occurrence of SRNOM in the dispersion could largely enhance the redispersion and mobility of MoS₂ by intercalating into the nanosheets. Upon adsorption to SL-MoS₂, SRNOM enhanced the hydration force and weakened the van der Waals forces between nanosheets, leading to the redispersion of the aggregates. The SRNOM fractions with higher molecular mass imparted better dispersity due to the preferable sorption of the large molecules onto SL-MoS₂ surfaces. This comprehensive study advances current understanding on the transport and fate of nanomaterials in the water system and provides fresh insights into the interaction mechanisms between NOM and 2D nanomaterials.

Kaolin-Supported Silver Nanoparticles as an Effective Catalyst for the Removal of Methylene Blue Dye from Aqueous Solutions

Water contamination by organic dyes has become a reason for severe environmental pollution and has been threatening the aquatic ecosystem. In this study, kaolin-supported silver nanoparticle (Ag-NP) composites were synthesized by a facile two-step adsorption-reduction method through the reduction of silver ions adsorbed onto locally available, inexpensive, and easily pretreated kaolin surfaces by using sodium borohydride (NaBH₄) for the catalytic degradation of methylene blue (MB) dye in aqueous solution. The morphology, structure, surface area, and interaction of the synthesized materials were investigated by scanning electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller, and Fourier transform infrared spectroscopy, respectively. Characterization results showed the successful growth of Ag-NPs on the kaolin surface. To understand the catalytic degradation performance of the catalyst, batch experiments were carried out using MB dye as a model dye. The catalytic reduction tests confirmed the importance of Ag-NPs and the high catalytic activities of the synthesized Ag-NPs/kaolin composite toward MB dye reduction. The degradation results indicated that the increased Ag-NP content on the kaolin surface through repeating cycles could effectively enhance the removal of MB dye from an aqueous solution. The kinetic analysis of the MB dye degradation of the catalyst has fitted the pseudo-first-order kinetic model. More than 97% removal efficiency was still present after five reuse cycles, demonstrating exceptional stability and reusability of the composite. In conclusion, the Ag-NPs supported kaolin (Ag-NPs/kaolin) composite was found to be a promising catalyst for the excellent catalytic activity to reduce a model dye MB from the aqueous solution in the presence of NaBH₄ with catalytic efficiency higher than 97% and a reduction rate constant, k _red, higher than 0.86 min^-1.

Eco-Corona Dictates Mobility of Nanoplastics in Saturated Porous Media: The Critical Role of Preferential Binding of Macromolecules

Nanoplastics are an increasing environmental concern. In aquatic environments, nanoplastics will acquire an eco-corona by interacting with macromolecules (e.g., humic substances and extracellular polymeric substances (EPS)). Here, we show that the properties of the eco-corona and, consequently, its ability to enhance the transport of nanoplastics vary significantly with the surface functionality of nanoplastics and sources of macromolecules. The eco-corona derived from the EPS of Gram-negative Escherichia coli MG1655 enhances the transport of polystyrene (PS) nanospheres in saturated porous media to a much greater extent than the eco-corona derived from soil humic acid and fulvic acid. In comparison, the eco-corona from all three sources significantly enhance the transport of carboxylated PS (HOOC-PS). We show that the eco-corona inhibits the deposition of the two types of nanoplastics to the porous media mainly via steric repulsion. Accordingly, an eco-corona consisting of a higher mass of larger-sized macromolecules is generally more effective in enhancing transport. Notably, HOOC-PS tends to acquire macromolecules of lower hydrophobicity than PS. The more disordered and flexible structures of such macromolecules may result in greater elastic repulsion between the nanoplastics and sand grains and, consequently, greater transport enhancement. The findings of this study highlight the critical role of eco-corona formation in regulating the mobility of nanoplastics, as well as the complexity of this process.

Insight into the formation and biological effects of natural organic matter corona on silver nanoparticles in water environment using biased cyclical electrical field-flow fractionation

Natural organic matter (NOM) readily interacts with nanoparticles, leading to the formation of NOM corona structures on their surface. NOM corona formation is closely related to the surface coatings and bioavailability of nanoparticles. However, the mechanism underlying NOM corona formation on silver nanoparticles (AgNPs) remains largely unknown due to the lack of effective analytical methods for identifying the changes in the AgNP surface. Herein, the separation ability of biased cyclical electrical field-flow fractionation (BCyElFFF) for same-sized polyvinyl pyrrolidone-coated and poly(ethylene glycol)-coated silver nanoparticles (AgNPs) with different electrophoretic mobilities was evaluated under various electrical conditions. Then, the mechanism behind the NOM corona formation on these AgNP surfaces was elucidated based on the changes in the elution time and off-line characterization of the collected fractions during their elution time in a BCyElFFF run. Finally, the survival rates of E. coli exposed to polyvinyl pyrrolidone-coated and poly(ethylene glycol)-coated AgNPs with or without NOM collected during repeated BCyElFFF runs were observed to increase with increasing NOM concentration, clearly demonstrating the negative effect of NOM corona structures on the bioavailability of AgNPs. These findings highlight the powerful separation and isolation ability of BCyElFFF in studying the transformation and fate of nanoparticles in aqueous environments.

Metabolomics analysis of salt tolerance of Zygosaccharomyces rouxii and guided exogenous fatty acid addition for improved salt tolerance

BACKGROUND

Zygosaccharomyces rouxii plays an irreplaceable role in the manufacture of traditional fermented foods, which are produced in a high-salt environment. However, there is little research on strategies for improving salt tolerance of Z. rouxii.

RESULTS

In this study, metabolomics was used to reveal the changes in intracellular metabolites under salt stress, and the results show that most of the carbohydrate contents decreased, the contents of xanthohumol and glycerol increased (fold change 4.07 and 5.35, respectively), while the contents of galactinol, xylitol and d-threitol decreased (fold change -9.43, -5.83 and -3.59, respectively). In addition, the content of four amino acids and six organic acids decreased, while that of the ten nucleotides increased. Notably, except for stearic acid (C18:0), all fatty acid contents increased. Guided by the metabolomics results, the effect of addition of seven exogenous fatty acids (C12:0, C14:0, C16:0, C18:0, C16:1, C18:1, and C18:2) on the salt tolerance of Z. rouxii was analyzed, and the results suggested that four exogenous fatty acids (C12:0, C16:0, C16:1, and C18:1) can increase the biomass yield and maximum growth rate. Physiological analyses demonstrated that exogenous fatty acids could regulate the distribution of fatty acids in the cell membrane, increase the degree of unsaturation, improve membrane fluidity, and maintain cell integrity, morphology and surface roughness.

CONCLUSION

These results are applicable to revealing the metabolic mechanisms of Z. rouxii under salt stress and screening potential protective agents to improve stress resistance by adding exogenous fatty acids. © 2022 Society of Chemical Industry.

Leveraging the potential of silver nanoparticles-based materials towards sustainable water treatment

Increasing demand of pure and accessible water and improper disposal of waste into the existing water resources are the major challenges for sustainable development. Nanoscale technology is an effective approach that is increasingly being applied to water remediation. Compared to conventional water treatment processes, silver nanotechnology has been demonstrated to have advantages due to its anti-microbial and oligodynamic (biocidal) properties. This review is focused on environmentally friendly green syntheses of silver nanoparticles (AgNPs) and their applications for the disinfection and microbial control of wastewater. A bibliometric keyword analysis is conducted to unveil important keywords and topics in the utilisation of AgNPs for water treatment applications. The effectiveness of AgNPs, as both free nanoparticles (NPs) or as supported NPs (nanocomposites), to deal with noxious pollutants like complex dyes, heavy metals as well as emerging pollutants of concern is also discussed. This knowledge dataset will be helpful for researchers to identify and utilise the distinctive features of AgNPs and will hopefully stimulate the development of novel solutions to improve wastewater treatment. This review will also help researchers to prepare effective water management strategies using nano silver-based systems manufactured using green chemistry.

Enhancing effects of dissolved and media surface-bound organic matter on titanium dioxide nanoparticles transport in iron oxide-coated porous media under acidic conditions

Natural organic matter (NOM) and iron oxides have been proved to be crucial factors controlling the behaviors of nanoparticles in heterogenous environment. Here, we conducted experimental and modeling study on the transport of titanium dioxide nanoparticles (TiO₂ NPs) in iron oxide-coated quartz in the presence of NOM under acidic conditions. Results showed the antagonistic effects of iron oxides and NOM on TiO₂ NPs mobility. The inhibition of iron oxides coated on quartz was crystal form-dependent other than quantity-dependent. Amorphous ferric oxyhydroxide with higher specific surface area brought more positive charge and favorable deposition sites onto quartz, and induced more retention of nanoparticles than two crystalline iron oxides, goethite and hematite. Dissolved organic matter (DOM) facilitated TiO₂ NPs transport in iron oxide-coated quartz. In comparation with the limited enhancing effects of DOM, the NOM coatings on media surface partially or largely offset the inhibition of goethite on nanoparticles mobility through direct occupation of attachment sites and sites screening due to the steric repulsion of the macromolecules. Owing to the higher steric hindrance, humic acid, both in dissolved and media surface-bound states, exerted stronger facilitating effects on TiO₂ NPs mobility relative to fulvic acid.

Effects of environmental factor fulvic acid on AgNPs food chain delivery and bioavailability

Due to its antimicrobial activity, silver nanoparticles (AgNPs) have become the most commonly applied nanomaterials. However, the potential ecotoxicological toxicity of AgNPs in the environment is still unclear. Here we assessed the trophic transfer and toxicity of commercially manufactured polyvinyl pyrrolidone (PVP)-coated AgNPs using a model food chain from Escherichia coli (E. coli) to Caenorhabditis elegans (C. elegans). Our results demonstrated that AgNPs could be accumulated in E. coli and transferred to C. elegans that preyed on the bacteria. Although low concentration of AgNPs had no significant inhibition on E. coli, they could affect germ cell apoptosis, reproduction ability and population size of C. elegans through food chain. Importantly, natural organic matter (NOM), which is omnipresent in environmental system, could increase the accumulation of AgNPs in E. coli and C. elegans, and significantly enhance the ecotoxicity of AgNPs. Our findings indicated that potential risks of nanomaterial through food chain should be considered for higher trophic organisms. And environmental factors could play an important role in transport of nanomaterials and altering their accumulation and toxicity in ecosystem.

Spectroscopic Characteristics and Speciation Distribution of Fe(III) Binding to Molecular Weight-Dependent Standard Pahokee Peat Fulvic Acid

Peat-derived organic matter, as powerful chelators, is of great significance for the transport of Fe to the ocean and the enhancement of dissolved Fe. However, the iron binding capacity of molecular weight (MW)-fractionated dissolved organic matter is variable, due to its structure and composition heterogeneity. In this work, we used the standard Pahokee Peat fulvic acid (PPFA) as an example, and investigated the spectroscopy properties and Fe(III) binding ability of PPFA and different molecular weight fractions by UV−Vis absorbance and fluorescence spectroscopy and the Donnan Membrane Technique (DMT). The results showed binding sites for Fe(III) at the 263 nm and >320 nm regions in differential absorbance spectra. Upon increasing the iron concentration to 18.00 μmol·L−1, the critical binding capacity was exceeded, which resulted in a decrease in absorbance. Fe(III) was found to prefer to bind to humic-like components, and ultraviolet humic-like fluorophores displayed stronger binding strength. High molecular weight PPFA fractions (>10 kDa) possessed more aromatic and hydrophobic components, displayed a higher degree of humification, and exhibited higher metal binding potential. Furthermore, the speciation analysis and stability constant (cK) were calculated using Donnan membrane equilibrium. The correlation between cK values and PPFA spectral properties demonstrated that aromaticity, hydrophobicity, molecular weight and humification degree were crucial indices of PPFA−Fe(III) affinity. Significantly, the humification degree, represented by HIX, showed the strongest correlation (r = 0.929, p = 0.003), which could be used to estimate the binding strength. This study provides further understanding of the complexation mechanism of iron and DOM in the peat environment and identifies the considerable effect of molecular weight.

Insight into the hetero-interactions of 4-nonylphenol with dissolved organic matter: multiple spectroscopic methods, 1H NMR study and principal component analysis

Understanding the interactions between heterogeneous dissolved organic matter (DOM) and nonylphenols (NPs) is essential for predicting their behavior and fate in the environment. Herein, we firstly obtained different MW-fractionated humic acids (HAs) using the ultrafiltration method. Afterward, the molecular weight (MW)-dependent interactions of HAs with 4-nonylphenol (4-NP) were analysed by excitation emission matrix (EEM) fluorescence spectroscopy, fluorescence quenching, UV-vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and principal component analysis (PCA). EEM spectra indicated that the quenching mechanism was static. In the binding process, the higher MW fractions showed stronger interaction with 4-NP than the lower MW counterparts, exhibiting a clear MW-dependent interaction heterogeneity. The interaction constants for the 4-NP–HAs system were suppressed as the ionic strength decreased and pH increased, which was especially obvious in the binding of 4-NP to the lower MW-fractionated HAs. The FTIR spectra revealed that hydroxyl and aromatics were involved in the interaction process of HA fractions with 4-NP. It was also found from ¹H NMR that π–π interactions between aromatic rings of 4-NP and MW-fractionated HAs were responsible for the complexation. The correlation analysis and PCA results indicated that aromaticity and MW play important roles in the interaction process and confirmed an obvious interaction heterogeneity among MW-fractionated HAs samples. This work highlighted MW-dependent interaction heterogeneities of HA, which suggested that heterogeneity in MW distribution should be taken into consideration when exploring the fate and biogeochemistry cycling of 4-NP from contaminated environments.

Multiple spectroscopic methods, ¹H NMR study and PCA were used to investigate the heterointeractions of 4-nonylphenol with humic acids.

Formation and mechanisms of hydroxyl radicals during the oxygenation of sediments in Lake Poyang, China

Seasonal flooding-drought transformation process of lake sediments lead to changes of dissolved oxygen and redox conditions and the resultant generation of hydroxyl radical (HO^•). To date, information on HO^• formation and its regulators in seasonal lake sediments is largely unexplored. In this study, a total of nineteen sediments were collected from Lake Poyang, China, with the formation and mechanisms of HO^• during the oxygenation process exploring via the incubation experiments, Fe K-edge X-ray adsorption spectroscopy, ultrafiltration, and fluorescent spectroscopy. Results showed that the concentrations of HO^• generated ranged from 3.75 ± 1.13 to 271.8 ± 22.81 μmol kg^-1, demonstrating high formation potential and obvious spatial heterogeneity. The yield of HO^• formed was positively correlated with the contents of Fe(II), sedimentary organic carbon, and dissolved organic carbon, showing a general contribution of these reduced substances to HO^• formation. Furthermore, application of Fe K-edge X-ray adsorption spectroscopy revealed the key species of sedimentary Fe-smectite for HO^• formation due to its high peroxidase-like activity. Besides inorganic Fe(II), the sedimentary dissolved organic matters (DOMs) represented an important regulator for HO^• formation, which contributed about 2-11% of the total HO^• generation. Moreover, the DOM-induced formation potential was found to be highly related to the molecular weight distribution that the low molecular weight- (LMW, <1 kDa) fraction exhibited higher HO^• formation potential than the bulk and high molecular weight- (HMW, 1 kDa-0.45 μm) counterparts. In addition, the omnipresent mineral Fe(II)-DOM interaction in sediment matrix exhibited another 2-6% of contribution to the total HO^• production. This study highlighted the importance of contents and species of Fe(II) and DOM in manipulating the HO^• yield, providing new insight into understanding the formation mechanisms of HO^• in the seasonal lake sediment.

The release inhibition of organic substances from microplastics in the presence of algal derived organic matters: Influence of the molecular weight-dependent inhibition heterogeneities

As the emerging contaminants, the behavior and fate of microplastics (MPs) were highly related to the interactions with surrounding organic matters. However, information on the effects of molecular sizes of organic matters on the interaction is still lacking. In this study, the bulk algal-derived organic matter (AOM) samples were obtained and further fractionated into high molecular weight (HMW-, 1kDa-0.45 μm) and low molecular weight (LMW-, < 1 kDa) fractions. The interaction between MPs [polyethylene (PE) and polystyrene (PS)] and these MW-fractionated AOMs were characterized by dissolved organic carbon, fluorescence and absorbance spectroscopy, and fourier transform infrared (FTIR) analysis. Results showed that presence of AOM could effectively inhibit the release of additives from MPs. Further analysis found that the inhibition extents decreased in the order of HMW- > bulk > LMW-AOM. The absorbance and fluorescence spectroscopy showed that aromatic protein-like substances in HMW fraction exhibited higher adsorption affinity to MPs than the bulk and LMW counterparts. The strong sorption of aromatic substances may offer more binding sites for additives to inhibit the release of organic substances. Moreover, two dimensional FTIR correlation spectroscopy revealed that the HMW non-aromatic substances were preferentially adsorbed onto PS, which led to an enhanced adsorption capacity to additives by forming H-bonding. Therefore, the MW- and component-dependent heterogeneities of AOM samples must be fully considered in evaluating the environmental behavior of MPs.

Heteroaggregation of different surface-modified polystyrene nanoparticles with model natural colloids

This study investigated heteroaggregation of three surface-functionalized polystyrene nanoparticles (PSNPs), i.e. negatively charged unfunctionalized nanoparticles (Bare-PS) and carboxylated nanoparticles (COOH-PS), and positively charged amino-functionalized nanoparticles (NH₂-PS), with two model natural colloids, positively charged hematite and negatively charged kaolin, respectively. Heteroaggregation was conducted at a constant natural colloid concentration and variable NP/colloid concentration ratios. Electrostatic interaction was the main mechanism driving the formation of heteroaggregates. In binary systems containing hematite and Bare-PS/COOH-PS, a charge neutralization - charge inverse mechanism was observed with the increase of PSNP concentration. At NP/hemetite concentration ratios much smaller or larger than the full charge neutralization point, the primary heteroaggregates were stable, while full charge neutralization induced the formation of large secondary heteroaggregates. Large aggregates were not observed in suspensions containing kaolin and NH₂-PS, as highly positively charged NH₂-PS reversed surface charges of kaolin at extremely low concentrations. Heteroaggregation between PSNPs and natural colloids with the same charge is unfavorable due to strong electrostatic repulsion. In the presence of electrolytes, homoaggregation and heteroaggregation both occurred, and homoaggregation of hematite played a key role when the concentration of PSNPs was low. The presence of Suwannee River natural organic matter (SRNOM) could modify surface charges of nanoparticles, and thus affect heteroaggregation behaviors of the binary suspension. When SRNOM and electrolytes were both present, whether SRNOM inducing or hindering the stability of the binary system was a combined effect of NP/colloid concentration ratios, SRNOM concentrations, electrolyte types and ionic strength. Mechanisms extensively reported in homoaggregation such as steric hindrance and cation bridging effects between SRNOM and Ca²⁺ also stand for heteroaggregation. These results highlight the critical role of surface modification on the environmental behaviors of NPs, and will underpin our understanding of the fate and transport of NPs in the aquatic environment.

Mitigation of silver nanoparticle toxicity by humic acids in gills of Piaractus mesopotamicus fish

Silver nanoparticles (AgNPs) are one of the most produced nanoproducts due to their unique biocide properties. The natural organic matter has an important impact on nanoparticle's dispersion as it may alter their fate and transport, as well as their bioavailability and toxicity. Therefore, this study aimed to evaluate the mitigatory effect of humic acids (HAs) on AgNP toxicity. For this purpose, we carried out an ex vivo exposure of gill of Piaractus mesopotamicus fish to 100 μg L^-1 of AgNPs or AgNO₃, alone and in combination with 10 mg L^-1 of HAs. In parallel, a complete AgNP characterization in the media, including the presence of HAs, was provided, and the Ag⁺ release was measured. We analyzed Ag bioaccumulation, antioxidant enzymes activities, lipid peroxidation, antioxidant capacity against peroxyl radicals, and reduced glutathione levels in fish tissue. Our results indicated the Ag⁺ release from AgNPs decreased 28% when the HAs were present in the media. The Ag accumulation in gill tissue exposed to AgNPs alone was higher than the AgNO₃ exposure, and sixfold higher than the treatment with the HA addition. Moreover, after both Ag forms, the catalase enzyme augmented its activity. However, those responses were mitigated when the HAs were present in the media. Then, our results suggested the mitigation by HAs under the exposure to both Ag forms, providing valuable information about the fate and behavior of this emergent pollutant.

Reduction of Ionic Silver by Sulfur Dioxide as a Source of Silver Nanoparticles in the Environment

The natural formation of silver nanoparticles (AgNPs) via biotic and abiotic pathways in water and soil media contributes to the biogeochemical cycle of silver metal in the environment. However, the formation of AgNPs in the atmosphere has not been reported. Here, we describe a previously unreported source of AgNPs via the reduction of Ag(I) by SO₂ in the atmosphere, especially in moist environments, using multipronged advanced analytical and surface techniques. The rapid reduction of Ag(I) in the atmospheric aqueous phase was mainly caused by the sulfite ions formed from the dissolution of SO₂ in water, which contributed to the formation of AgNPs and was consistent with the Finke-Watzky model with a major contribution of the reduction-nucleation process. Sunlight irradiation excited SO₂ to form triplet SO₂, which reacted with water to form H₂SO₃ and greatly enhanced Ag(I) reduction and AgNP formation. Different pH values affected the speciation of Ag(I) and S(IV), which were jointly involved in the reduction of Ag(I). The formation of AgNPs was also observed in the atmospheric gas phase via direct reduction of Ag(I) by SO₂(gas), which occurred even in 50 ppbv SO₂(gas). The natural occurrence of AgNPs in the atmosphere may also be involved in silver corrosion, AgNP transformation and regeneration, detoxification of gaseous pollutants, and the sulfur cycle in the environment.

Research progress of nanoplastics in freshwater

With the mass production and use of plastic products, which leads to their continuous entry into the water environment, the problem of environmental pollution has been paid more and more attention by scholars from different countries. In recent years, a large number of studies have focused on microplastics, but few on nanoplastics (NPs). However, NPs are smaller in size, have a higher affinity for cells, and surface and volume ratios are higher than those of microplastics. NPs may also enter biological tissues, blood and cells, which may cause greater potential harm to organisms. In this paper, firstly, the environmental fate of NPs accumulation and deposition is summarized, and further research is needed in the future; secondly, the current techniques for NPs extraction and characterization of NPs extraction and characterization are summarized. At present, the analytical methods of NPs are in the primary stage, and lack of standardized and accurate methods; finally, the toxic effects of NPs on biological morphology, behavior and reproduction are discussed. It has been found that the small size and high surface area of NPs make them more toxic to organisms than microplastics. However, most of the current toxicological studies of NPs on freshwater organisms could not be simulated in real environment.

Multimedia distribution and trophic transfer of PPCPs in the middle and lower reaches of the Yarlung Zangbo River

The increasing human presence is having an impact on plateau ecosystems, but the special environment and lack of data make it difficult to assess the real ecological risks of pharmaceutical and personal care products (PPCPs) in the river of plateau. The occurrence, distribution and trophic transfer of nineteen PPCPs were investigated in the middle and lower reaches of the Yarlung Zangbo River on the Tibetan Plateau. All the targeted PPCPs were detected in filtrated water, and seventeen PPCPs were detected in the colloid, sediment and suspended particulate matter (SPM). The distribution coefficients of colloid-infiltration water (IFW) were 1-2 orders of magnitude larger than those in the SPM-IFW, which were 1-2 orders of magnitude greater than those in the sediment-IFW. Colloids are sinks for PPCPs with up to 78.55% of the water being in the colloidal phase, in which important factors such as protein and protein-like substances are found. PPCPs in the rivers of the plateau showed high bioaccumulation ability. The fugacity-based bioaccumulation model was established and revealed that the fish in the Tibetan Plateau ingested PPCPs mainly through water instead of food and excreted them mainly through metabolism. In addition, the trophic dilution effect in the food web was observed with trophic magnification factors ranging from 0.06 to 0.22. The positive correlation between the K_d in the colloid-IFW and the bioaccumulation factors implied that natural colloids can not only regulate the behaviour of PPCPs in the environment, but also play an important role in bioaccumulation, which may affect the scientific nature of biological risk assessment.

Analytical methods and environmental processes of nanoplastics

The degradation of plastic debris may result in the generation of nanoplastics (NPs). Their high specific surface area for the sorption of organic pollutions and toxic heavy metals and possible transfer between organisms at different nutrient levels make the study of NPs an urgent priority. However, there is very limited understanding on the occurrence, distribution, abundant, and fate of NPs in the environment, partially due to the lack of suitable techniques for the separation and identification of NPs from complex environmental matrices. In this review, we first overviewed the state-of-the-art methods for the extraction, separation, identification and quantification of NPs in the environment. Some of them have been successfully applied for the field determination of NPs, while some are borrowed from the detection of microplastics or engineered nanomaterials. Then the possible fate and transport of NPs in the environment are thoroughly described. Although great efforts have been made during the recent years, large knowledge gaps still exist, such as the relatively high detection limit of existing method failing to detect ultralow masses of NPs in the environment, and spherical polystyrene NP models failing to represent the various compositions of NPs with different irregular shapes, which needs further investigation.

Molecular weight-dependent heterogeneities in photochemical formation of hydroxyl radical from dissolved organic matters with different sources

Dissolved organic matter (DOM) is ubiquitous in aquatic ecosystem and characterized by a wide range of molecular weight (MW) distribution. In this study, a total of nine bulk DOM samples, including five International Humic Substances Society (IHSS) standards and four naturally collected samples, were fractionated into low MW (LMW-, <1 kDa) and high MW (HMW-, 1 kDa~0.45 μm) fractions, with MW-dependent heterogeneities in photochemical formation of hydroxyl radical (HO) was investigated. The formation rate of HO (R^HO) for the bulk samples were 4.60-7.27 × 10^-12 M/s/mg-C/L for IHSS standards and 4.63-7.66 × 10^-12 M/s/mg-C/L for naturally collected samples. Regardless of sample types, the LMW fraction was found to exhibit generally higher R^HO values than the HMW counterparts. For IHSS standards, the R^HO decreased from 4.68-8.46 × 10^-12 M/s/mg-C/L for LMW fraction to 3.67-6.66 × 10^-12 M/s/mg-C/L for HMW fraction, and for naturally collected samples, the value of R^HO decreased from 5.21-12.04 × 10^-12 M/s/mg-C/L for LMW fraction to 3.25-6.49 × 10^-12 M/s/mg-C/L for HMW counterpart. A positive correlation between the net R^HO and the normalized intensities of fluorescent peak [Em/Ex: (400-500)/(230-250) nm] was found, showing that the HO formation was strongly coupled to the abundance of humic-like substances. The results indicate that aquatic DOM is an important pool for HO formation, and characterization of MW distribution rather than average MW is thus required to explain the DOM-induced formation potential of HO.

Effects of Extracellular Polymeric Substances on the Formation and Methylation of Mercury Sulfide Nanoparticles

Growing evidence has suggested that microbial biofilms are potential environmental "hotspots" for the production and accumulation of a bioaccumulative neurotoxin, methylmercury. Here, we demonstrate that extracellular polymeric substances (EPS), the main components of biofilm matrices, significantly interfere with mercury sulfide precipitation and lead to the formation of nanoparticulate metacinnabar available for microbial methylation, a natural process predominantly responsible for the environmental occurrence of methylmercury. EPS derived from mercury methylating bacteria, particularly Desulfovibrio desulfuricans ND132, substantially increase the methylation potential of nanoparticulate mercury. This is likely due to the abundant aromatic biomolecules in EPS that strongly interact with mercury sulfide via inner-sphere complexation and consequently enhance the short-range structural disorder while mitigating the aggregation of nanoparticulate mercury. The EPS-elevated bioavailability of nanoparticulate mercury to D. desulfuricans ND132 is not induced by dissolution of these nanoparticles in aqueous phase, and may be dictated by cell-nanoparticle interfacial reactions. Our discovery is the first step of mechanistically understanding methylmercury production in biofilms. These new mechanistic insights will help incorporate microbial EPS and particulate-phase mercury into mercury methylation models, and may facilitate the assessment of biogeochemical cycling of other nutrient or toxic elements driven by EPS-producing microorganisms that are prevalent in nature.

Exploring the potential of laser desorption ionisation time-of-flight mass spectrometry to analyse organic capping agents on inorganic nanoparticle surfaces

Analytical techniques are in high demand for the determination of organic capping agents on surfaces of metallic nanoparticles (NPs) such as gold (Au) and silver (Ag). In this study, the potential of laser desorption ionisation time-of-flight mass spectrometry (LDI-ToF-MS) as a technique fit for this purpose is demonstrated. First, a collection of reference spectra of most commonly used organic capping agents, including small molecules and polymers was established. Second, the robustness of the method was tested towards parameters like NP core material and NP size. In a third step, the quantitative capabilities of LDI-ToF-MS were determined. Finally, the potential to detect chemical alterations of the organic capping agent was evaluated. LDI-ToF-MS is able to detect capping agents ranging from small molecules (citric acid, tannic acid, lipoic acid) to large polymers (polyvinylpyrrolidone, branched polyethylenimine and methoxy polyethylene glycol sulfhydryl) on Au and Ag NPs based on characteristic signals for each capping agent. Small molecules showed characteristic fragment ions with low intensities, whereas polymers showed intense signals of the monomeric subunit. The NP concentration range comprises about two orders of magnitude with lowest detection limits of 5 mg/L or a capping agent concentration in the lower nM range. Changes in capping agent composition are detectable at NP concentrations in the g/L range. Thus, LDI-ToF-MS is particularly suitable for characterisation of polymer-capped NPs with high NP concentrations. This may be the case for quality control as part of the material synthesis and testing. Graphical abstract.

Natural Organic Matter (NOM) Imparts Molecular-Weight-Dependent Steric Stabilization or Electrostatic Destabilization to Ferrihydrite Nanoparticles

Ferrihydrite nanoparticles (Fh NPs) are ubiquitous in natural environments. However, their colloidal stability, and fate and transport behavior are difficult to predict in the presence of heterogeneous natural organic matter (NOM) mixtures. Here, we investigated the adsorption and aggregation behavior of Fh NPs exposed to NOM fractions with different molecular weights (MW). The NOM fraction with MW < 3 kDa destabilized the NPs, resulting in accelerated aggregation even at high C/Fe mass ratios, whereas higher MW NOM fractions imparted better colloidal stability with increasing MW and C/Fe ratio. Despite differences in the functional group composition of the bulk (dissolved) NOM fractions, all NOM fractions produced similar adsorbed layer compositions on the NPs, suggesting minimal contribution of chemical properties to the distinctive aggregation behavior. Rather, the higher adsorbed mass and larger size of the higher MW fractions were key factors in stabilizing the NPs through steric repulsion, whereas the lowest MW fraction had low adsorbed mass and was unable to counter electrostatic patch-charge attraction when the NPs are positively charged. This mechanistic understanding helps us predict the transport and fate of Fh NPs and the associated contaminants in natural environments with varying NOM compositions.

Impacts of Proteins on Dissolution and Sulfidation of Silver Nanowires in an Aquatic Environment: Importance of Surface Charges

With increasing utilization of silver nanomaterials, growing concerns are raised on their deleterious effects to the environment. Once discharged in an aquatic environment, the interactions between silver nanowires (AgNWs) and proteins may significantly affect the environmental behaviors, fate, and toxicities of AgNWs. In the present study, three representative model proteins, including ovalbumin (OVA), bovine serum albumin (BSA), and lysozyme (LYZ), were applied to investigate the impacts of the interactions between proteins and AgNWs on the transformations (oxidative dissolution and sulfidation) of AgNWs in an aquatic environment. Fluorescence spectroscopy and isothermal titration calorimetry analyses indicated that there was very weak interaction between OVA or BSA and AgNWs, but there was a strong interaction between the positively charged LYZ and the negatively charged AgNWs. The presence of LYZ not only reversed the surface charge of AgNWs but also resulted in the breakup of the nanowire structure and increased the reactive surface area. The positively charged surface of AgNWs in the presence of LYZ favored the access of sulfide ions. As a consequence, the kinetics of oxidative dissolution and sulfidation of AgNWs were not affected by OVA and BSA but were significantly facilitated by LYZ. The results shed light on the important roles of electrostatic interactions between AgNWs and proteins, which may have important implications for evaluating the fate and effects of silver nanomaterials in complicated environments.

Adsorption of cyanobacterial extracellular polymeric substance on colloidal particle: Influence of molecular weight

Cyanobacterial extracellular polymeric substances (EPSs) in aquatic environments are easily adsorbed onto colloidal particles, whereas the adsorption behavior as affected by molecular weight (MW) properties remained unknown till now. Herein, the bulk cyanobacterial EPS matrix (<0.45 μm) was fractionated into high MW (HMW-, 1 kDa~0.45 μm) and low MW (LMW-, <1 kDa) fractions, with MW-dependent adsorption heterogeneities onto TiO₂ colloids exploring through batch experiment, UV-Vis and fluorescence spectroscopy, and two dimensional Fourier transform infrared correlation spectroscopy (2D-FTIR-COS). About two-thirds of total organic matters within bulk EPS matrix were distributed in the HMW fraction, leaving one-third in the LMW fraction. Compared to LMW-EPS, the HMW counterpart exhibited higher aromaticity and richness of autochthonous protein-like substances, showing evident MW-dependent differences in abundance and composition. The adsorption capacity based on the measurement of total abundance, UV-Vis and fluorescent spectra all decreased in sequence of HMW- > Bulk > LMW-EPS, demonstrating obvious MW-dependent adsorption heterogeneities. During adsorption, the values of SUVA₂₅₄ in residual supernatants exhibited an initial decrease followed by gradual increase for all samples, suggesting that the preferentially adsorbed aromatic substances can be subsequently replaced by the non-aromatic moieties. 2D-FTIR-COS further revealed that the carboxylic groups of proteins were preferentially adsorbed onto colloidal surface, followed by the CC functional groups and then the CH groups of polysaccharides, which accounted for the variations of SUVA₂₅₄ values in the supernatants. This study demonstrated that the adsorption behavior of EPS matrix was highly MW-dependent, and detailed characterization on size fractionation is thus needed in future studies.

Quantitative insight into dispersity and antibactericidal capability of silver nanoparticles noncovalently conjugated by polysaccharide-protein complexes

Precise prediction and measurement of dispersibility of silver nanoparticles (AgNPs) under atmospheric conditions are extremely vital for their potential commercial application. In the present work, the dispersibility of AgNPs capped by polysaccharide-protein from viscera of abalone (PSP-AgNPs) was studied using the combination of ultraviolet-visible spectroscopy (UV-vis), dynamic light scattering (DLS) and multiple-light-scattering (MLS) techniques. The results showed that the combination of UV/vis, DLS and MLS not only accurately determined the dispersibility of PSP-AgNPs, but also provided detailed information about the aggregation behavior of PSP-AgNPs. Furthermore, the results revealed a high dispersibility of PSP-AgNPs in the studied environment. The system temperature, pH value and thermal treatment (pasteurization and sterilization) had no effect on the dispersion of PSP-AgNPs in the effective concentration range against the pathogenic bacteria. Besides, an excellent stable dispersion and antibacterial activity against common pathogenic vibrio was also found in the dispersed PSP-AgNPs in seawater. Overall, the study provides a suitable method for the precise measurement of the dispersibility of AgNPs in environment. The AgNPs act as a potential bactericide with good dispersion and antibacterial activity in mariculture and other fields.

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.

Biodynamics of Silver Nanoparticles in an Estuarine Oyster Revealed by 110mAgNP Tracing

The prevalence of silver nanoparticles (AgNPs) requires a comprehensive understanding of their biological impacts especially in marine and estuarine environments. Nevertheless, the background Ag concentration in organisms may impede the accuracy of Ag detection if the net accumulated Ag is low over a short exposure period. Here, a radio-synthesizing method was employed to trace the behavior of AgNPs with two sizes (15 and 60 nm) and two coatings (humic acid and citrate) in an estuarine oyster Crassostrea hongkongensis. This method was sensitive to detect the bioaccumulation and depuration of AgNPs in the oysters over a short period of exposure, which was necessary given the significant changes of particle aggregation in saline water environments. Through radioactive AgNP tracing and biokinetic modeling, we for the first time demonstrated the differential uptake mechanisms of different-sized AgNPs in oysters. Specifically, the ingestion of particles dominated the uptake of 60 nm AgNPs, whereas dermal uptake and ingestion contributed equally to 15 nm AgNPs. Surface coating (humic acid vs citrate) did not significantly affect the uptake of AgNPs by the oysters. The depuration of AgNPs from the oysters was relatively faster than that for the Ag ion. The digestive gland was the key detoxification organ of AgNPs with the greatest loss of Ag by the end of depuration. The findings of this study provide fundamental knowledge for nano-specific risk assessment in marine and estuarine environments.

Freezing Facilitates Formation of Silver Nanoparticles under Natural and Simulated Sunlight Conditions

Freezing is essential in the light-mediated transformation of organic pollutants. However, the effects of the freezing process on the reduction of Ag⁺ by natural organic matter (NOM) remains unclear, causing significant uncertainties in the natural formation of silver nanoparticles (AgNPs). This study investigated the sunlight-induced reduction of Ag⁺ by NOM under natural or controlled freezing processes. Natural (outdoor) freezing experiments demonstrated intense aggregation and precipitation of AgNPs in three aqueous media, including a NOM solution and two river water samples, under natural sunlight irradiation. Indoor experiments under simulated sunlight irradiation and controlled freezing processes showed that freezing at -20 °C and repeated freeze-thaw cycles (-20 to 4 °C) drastically accelerated the formation and growth of AgNPs compared to maintenance at 4 °C. Finally, under the natural freezing process, commercial AgNPs were found to influence the redox reduction of Ag⁺ probably through a reduction in dissolution rates and homoaggregation with AgNPs newly formed in the river water samples. Additionally, the enhancement effect of freezing on AgNP formation was confirmed in the presence of Ag⁺ and AgNPs both at environmentally relevant concentration levels, especially upon light irradiation. This work emphasizes the importance of freezing processes on the natural formation of AgNPs.

Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent electrolytes

The intentional production and degradation of plastic debris may result in the formation of nanoplastics. Currently, the scarce information on the environmental behaviors of nanoplastics hinders accurate assessment of their potential risks. Herein, the aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent electrolytes was investigated to shed some light on the fate of nanoplastics in the aquatic environment. Three monodisperse nanoparticles including unmodified nanoparticles (PS-Bare), carboxylated nanoparticles (PS-COOH) and amino modified nanoparticles (PS-NH₂), as well as one polydisperse nanoparticles that formed by laser ablation of polystyrene films (PS-Laser) were used as models to understand the effects of surface groups and morphology. Results showed that aggregation kinetics of negatively charged PS-Bare and PS-COOH obeyed the DLVO theory in NaCl and CaCl₂ solutions. The presence of Suwannee river natural organic matters (SRNOM) suppressed the aggregation of PS-Bare and PS-COOH in monovalent electrolytes by steric hindrance. However, in divalent electrolytes, their stability was enhanced at low concentrations of SRNOM (below 5 mg C L^-1), while became worse at high concentrations of SRNOM (above 5 mg C L^-1) due to the interparticle bridging effect caused by Ca²⁺ and carboxyl groups of SRNOM. The cation bridging effect was also observed for PS-laser in the presence of high concentrations of divalent electrolytes and SRNOM. The adsorption of SRNOM could neutralize or even reverse surface charges of positively charged PS-NH₂ at high concentrations, thus enhanced or inhibited the aggregation of PS-NH₂. No synergistic effect of Ca²⁺ and SRNOM was observed on the aggregation of PS-NH₂, probably due to the steric repulsion imparted by the surface modification. Our results highlight that surface charge and surface modification significantly influence aggregation behaviors of nanoplastics in aquatic systems.

The limited facilitating effect of dissolved organic matter extracted from organic wastes on the transport of titanium dioxide nanoparticles in acidic saturated porous media

The complexity of natural dissolved organic matter (DOM) motivates the determination of how DOM from diverse sources affects the environmental behaviors of engineered nanoparticles. Here, three types of DOM, DOM extracted from swine manure (SWDOM), sludge (SLDOM) and sediment (SEDOM), were characterized, and their effects on the transport of titanium dioxide nanoparticles (TiO₂ NPs, 30 nm in diameter) were evaluated and compared with those of humic acid (HA). Characterization tests showed differences in the aromaticity and weight-average molecular weight (M_w) properties among the three extracted DOM solutions, and greater distinctions were found between the extracted DOM and HA. All the extracted DOM facilitated TiO₂ NPs transport in acidic porous media. Nevertheless, the enhancing effects varied among the different extracted DOM types. SWDOM had a promoting effect on TiO₂ NPs mobility that was equivalent to that of SEDOM and much higher than that of SLDOM. However, the facilitating effects of all three extracted DOM types were limited compared to that of HA. Based on the combined analysis of DOM properties and TiO₂ NPs transport behaviors, it could be concluded that aromaticity and M_w were the key properties determining the limited promoting effects of DOM on TiO₂ NPs mobility, and the specific UV absorbance at 280 nm (normalized by concentration, SUVA₂₈₀) was a facile and useful indicator of the DOM-promoted transport of TiO₂ NPs. These findings revealed that transport potential in the presence of DOM would be overestimated if either HA or fulvic acid were chosen as the DOM model in studies.

Effects of molecular weight fractions and chemical properties of time-series cyanobacterial extracellular polymeric substances on the aggregation of lake colloidal particles

Colloidal particles in lake waters interact inevitably with cyanobacterial extracellular polymeric substance (EPS), which will change their behavior and fate. Quantitative prediction of the effects of cyanobacterial EPS on colloidal behavior is difficult due to its variability and heterogeneity. To explore the effects of molecular weight (MW) fractions and chemical properties of cyanobacterial EPS on aggregation kinetics of colloidal particles, time-series cyanobacterial samples were collected in Lake Taihu, China, from April to November (during blooming and maintenance period), with the bulk EPS matrix fractionating into low MW (LMW-, <1 nm) and high MW (HMW-, 1 nm-0.45 μm) fractions. HMW-EPS was generally characterized with higher absorbance and predominant distribution of protein-like substances, while LMW-EPS contained mainly the humic- and fulvic-like substances. The absorbance, molecular size, and humification degree for each MW fraction consistently increased from April to November, showing obvious temporal variations from blooming period to maintenance period. As for the MW-dependent aggregation behaviors, the HMW-EPS provided better stability against aggregation than the LMW-EPS, and the bulk EPS matrix that consisted of HMW- and LMW-fractions exhibited the effects intermediate between that of each fraction alone. Regardless of MW fractions, the effects of EPS-induced stability enhancement were more evident in maintenance period than in blooming period. Further analysis showed that the colloidal stability was correlated positively with SUVA₂₅₄ (R² = 0.82-0.93) but negatively with Slope_275-295 (R² = 0.53-0.91) of UV-Vis absorption spectra, indicating that aromaticity and MWs were two critical parameters controlling colloidal aggregation. Therefore, cyanobacterial EPS can exhibit variable effects on colloidal stability, and characterization of MW distribution is strongly required in predicating the behavior and fate of colloidal particles in water environments.

Maize (Zea mays L.) root exudates modify the surface chemistry of CuO nanoparticles: Altered aggregation, dissolution and toxicity

Copper oxide nanoparticles (CuO NPs), as an antimicrobial nanomaterial, have found many applications in agriculture. Ubiquitous and complex root exudates (RE) in the plant root zone motivates the determination of how specific components of RE interact with CuO NPs. This work aims to reveal the role of maize (Zea mays L.)-derived RE and their components on the aggregation and dissolution of CuO NPs in the rhizosphere. We observed that RE significantly inhibited the aggregation of CuO NPs regardless of ionic strength and electrolyte type. In the presence of RE, the CCC of CuO NPs in NaCl shifted from 30 to 125 mM and the value in CaCl₂ shifted from 4 to 20 mM. Furthermore, this inhibition was correlated with molecular weight (MW) of RE fractions. Higher MW fraction (>10 kDa) reduced the aggregation most. We also discovered that RE significantly promoted the dissolution of CuO NPs and lower MW fraction (<3 kDa) RE mainly contributed to this process. Additionally, phytotoxicity of CuO NPs in the presence of RE and different fractions of RE was evaluated. The addition of 20 mg/L RE reduced the seedlings growth rate to 1.89% after 7 days exposure to 25 mg/L CuO NPs, which were significantly lower than the control group (4.82%). Notably, Cu accumulation in plant root tissues was significantly enhanced by 20 mg/L RE. This study provides useful insights into the interactions between RE and CuO NPs, which is of significance for the safe use of CuO NPs-based antimicrobial products in agricultural production.

One-step synthesized fluorescent nitrogen doped carbon dots from thymidine for Cr (VI) detection in water

A novel, simple and low-cost nitrogen doped carbon dots (N-CDs) fluorescent sensor for sensitive detection of Cr (VI) was developed via one-step hydrothermal method using thymidine as carbon source. As-prepared N-CDs exhibited the ability of sensitive and selective detection of Cr (VI) through the inner filter effect (IFE). The performances of N-CDs were investigated with the characterization methods of transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Under the optimized conditions, a good logarithm correlation between the fluorescence intensity of N-CD_S and the concentration of Cr (VI) was obtained ranging from 0.1 μM to 430 μM (R² = 0.992), with a low detection limit (LOD; S/N = 3) of 1.26 nM. The fluorescent sensor showed good repeatability, reproducibility and stability. Furthermore, N-CDs fluorescent sensor had a good applicability for Cr (VI) detection in real water samples with acceptable recoveries, and the detection results were consistent with the inductively coupled plasma mass spectrometry (ICP-MS) results, indicating this fluorescent sensor has a great potential for the environmental monitoring.

Study of the Catalytic Strengthening of a Vacuum Carburized Layer on Alloy Steel by Rare Earth Pre-Implantation

Conventional carburizing has disadvantages, such as high energy consumption, large deformation of parts, and an imperfect structure of the carburizing layer. Hence, a rare earth ion pre-implantation method was used to catalyze and strengthen the carburized layer of 20Cr2Ni4A alloy steel. In this study, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive microanalysis (EDS), transmission electron microscopy (TEM), and Rockwell/Vickers hardness testing were used to analyze the microstructure, phase composition, retained austenite content, hardness, carburized layer thickness, and carbon diffusion. The results showed that lanthanum and yttrium ions implanted into the 20Cr2Ni4A steel formed solid solutions of rare earth ions and a large number of dislocations, which improved the diffusion coefficient of carbon elements on the carburized surface and the uniformity of the carbon distribution. Simultaneously, rare earth ion implantation improved the structure and hardness of the vacuum carburized layer. Compared to the lanthanum ion implantation, yttrium ion implantation caused the structure of the carburized layer to be finer, and the carbon diffusion coefficient increased by 1.17 times; in addition, the surface hardness of the carburized layer was 61.8 HRC.

Impact of light and Suwanee River Fulvic Acid on O2 and H2O2 Mediated Oxidation of Silver Nanoparticles in Simulated Natural Waters

In this work, we investigate the impact of natural organic matter (NOM) and light on silver nanoparticle (AgNP) dissolution kinetics with particular emphasis on determining the (i) mechanism via which NOM affects the oxidative dissolution of AgNPs, (ii) the role of photogenerated organic radicals and reactive oxygen species (ROS) in oxidative dissolution of AgNPs, and (iii) the mechanism of formation of AgNPs in NOM solution under dark and irradiated conditions. We measured the oxidation of citrate stabilized AgNPs by O₂ and hydrogen peroxide (H₂O₂) in the dark and in irradiated Suwannee River fulvic acid (SRFA) solutions at pH 8.0. Results show that the reactivity of AgNPs toward O₂ and H₂O₂ in the dark decreased in the presence of SRFA as a result of blocking of AgNP surface sites through either steric or electrostatic effects. Irradiation promoted dissolution of AgNPs by O₂ and H₂O₂ in the presence of low concentrations (≤20 mg·L^-1) of SRFA as a result of contribution from photogenerated H₂O₂ formed on irradiation of SRFA as well as photofragmentation of AgNPs. Furthermore, our results show that photogenerated superoxide can induce formation of AgNPs by reducing Ag(I) ions. Based on our experimental results, we have developed a kinetic model to explain AgNP transformation by O₂ and H₂O₂ in the dark and in irradiated SRFA solutions with this model of use in predicting the transformation and fate of AgNPs in natural waters.