Photochemical transformation and photoinduced toxicity reduction of silver nanoparticles in the presence of perfluorocarboxylic acids under UV irradiation

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.

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.

Activation of cadmium under simulated solar illumination and its impact on the mobility of Cd in flooded soils

In waterlogged paddy soils, cadmium (Cd) can be precipitated as cadmium sulfide (CdS) under reductive environment, thereby limiting the absorption of Cd by plants. Multiple environmental factors (such as water, pH, and Eh) played a role in the control of Cd mobility and bioavailability. In this study, we investigated the influence of the solar irradiation on the photodissolution of synthetic CdS-montmorillonite composites (CdS-M) in solution and the stability of Cd in natural soil. The release kinetic of Cd²⁺ showed that after the irradiation of simulated sunlight, CdS-M composites became less stable compared to the dark control. The solar irradiation seemed to enhance the release of Cd²⁺ from CdS significantly and continuously. Electron paramagnetic resonance (EPR) and quenching experiments confirmed that the photogenerated holes, •O₂^- and •OH, were possibly involved in the photo-induced release of Cd²⁺, while the holes was primarily responsible for the reaction. Irradiation under alkaline solution or the presence of DOM, PO₄^3-, CO₃^2-, and urea markedly inhibited the photodissolution process of CdS. The photo-mediated activation of Cd was further confirmed in paddy soil under natural sunlight, with a nearly threefold increase in concentration of extractable Cd during the 15 days of irradiation. This study highlights the importance of photochemical transformation of Cd in the environmental water and soil.

Polystyrene microplastics sunlight-induce oxidative dissolution, chemical transformation and toxicity enhancement of silver nanoparticles

The coexistence of microplastics (MPs) and nanomaterials has been increasingly studied, but the influence of MPs on the chemical transformation of nanomaterials remains unclear. Herein, it was demonstrated that polystyrene (PS) MPs induce the oxidative dissolution, transformation and toxicity of silver nanoparticles (Ag NPs) under simulated sunlight irradiation. The PS MPs induced the oxidation dissolution of pristine Ag NPs by ¹O₂, OH and/or acid release and simultaneously reduced the released Ag⁺ to secondary Ag NPs by O₂^-. The sizes, functional groups and ageing status of the PS MPs and pH characterized secondary Ag NPs formation. Secondary formation of Ag NPs induced by PS MPs also occurred in realistic water and was governed by dissolved organic matter (DOM) and Cl^-, rather than SO₄^2- or CO₃^2-. Moreover, PS MPs remarkably promoted Ag⁺ release, altered the Ag⁺:Ag⁰ ratio, and presented vehicle effects on Ag⁺ toxicity to Daphnia magna. The concentration addition model demonstrated that the ion-related toxicity of Ag NPs was significantly increased by PS MPs. Therefore, PS MPs induced the oxidative dissolution, transformation and toxicity enhancement of Ag NPs under sunlight irradiation, and accordingly, the coexistence of PS MPs and Ag NPs in freshwater environments should be seriously considered.

ROS-mediated photoaging pathways of nano- and micro-plastic particles under UV irradiation

Reactive oxygen species (ROS) generation is considered as an important photoaging mechanism of microplastics (MPs) and nanoplastics (NPs). To elucidate the ROS-induced MP/NP aging processes in water under UV₃₆₅ irradiation, we examined the effects of surface coatings, polymer types and grain sizes on ROS generation and photoaging intermediates. Bare polystyrene (PS) NPs generated hydroxyl radicals (•OH) and singlet oxygen (¹O₂), while coated PS NPs (carboxyl-modified PS (PS-COOH), amino-modified PS (PS-NH₂)) and PS MPs generated fewer ROS due to coating scavenging or size effects. Polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate and polycarbonate MPs only generated •OH. For aromatic polymers, •OH addition preferentially occurred at benzene rings to form monohydroxy polymers. Excess •OH resulted in H abstraction, CC scission and phenyl ring opening to generate aliphatic ketones, esters, aldehydes, and aromatic ketones. For coated PS NPs, •OH preferentially attacked the surface coatings to result in decarboxylation and deamination reactions. For aliphatic polymers, •OH attack resulted in the formation of carbonyl groups from peracid, aldehyde or ketone via H abstraction and CC scission. Moreover, ¹O₂ might participate in phenyl ring opening for PS NPs and coating degradation for coated PS NPs. This study facilitates understanding the ROS-induced weathering process of NPs/MPs in water under UV irradiation.

Inter-transformation between silver nanoparticles and Ag+ induced by humic acid under light or dark conditions

The fate and toxicity of silver nanoparticles (AgNPs) and ions in water bodies is largely determined by the natural organic matter (NOM)-mediated redox cycling. However, the process of NOM-mediated redox cycling in the day/night cycles remains elusive. In this study, the inter-transformation between AgNPs and Ag⁺ ion caused by humic acid (HA) was investigated under controlled light and dark conditions. It was shown that HA induced the reduction of Ag⁺ into AgNPs in simulated sunlight, and also oxidize AgNPs to release Ag⁺ in darkness. Kinetics data demonstrated that the rates of AgNPs formation and dissolution increased along with the increment of HA concentrations. Along with the pH increase, the reduction of Ag⁺ accelerated, but the oxidative dissolution of AgNPs was inhibited. In day-night cycles, the AgNPs and Ag⁺ concentrations exhibited similar wave-shaped change curves. The peaks of AgNPs and Ag⁺ ion appeared at 7 p.m. and 7 a.m., respectively. The toxicity of AgNPs/Ag⁺ to Escherichia coli was determined primarily by the concentration of dissolved Ag⁺, but also affected by the particle-specific toxicity. The dual role of HA implied that previous reports about the photo-reduction of Ag⁺ to AgNPs by NOM should be reconsidered, and the oxidizability of HA in darkness strongly affect the transformation and toxicity of AgNPs in water.

Mechanism of bicarbonate enhancing the photodegradation of β-blockers in natural waters

The impact of HCO₃^- on the photodegradation of β-blockers was investigated under simulated sunlight irradiation. The results show that in the presence of HCO₃^-, the photodegradation rates increase significantly for sotalol (SOT), whereas no effects on the degradation of carvedilol and arotinolol are observed. Using quenching experiments, electron paramagnetic resonance spectra and degradation product determination, we demonstrate that carbonate radical (CO₃^•-) is formed by direct oxidation of HCO₃^- by triplet-excited SOT (³SOT*) and plays a significant role in SOT photodegradation. Competition kinetics experiments show that the three β-blockers all have high second-order rate constants (10⁷-10⁸ M^-1 s^-1) for the reaction with CO₃^•-. However, only ³SOT* has a higher reduction potential that can oxidize HCO₃^- to produce CO₃^•-. Thus, enhanced SOT removal rates in the presence of HCO₃^- were observed. In addition, the results show that seawater DOM could increase HCO₃^--induced photodegradation of SOT, whereas SRNOM mainly behaves as a CO₃^•- quencher and decreases the removal rate of SOT. The results underscore the role of HCO₃^- in limiting the persistence of organic pollutants like SOT in sunlit natural waters, and especially in marine and coastal waters.

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.

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.

Degradation of rhodamine B in a novel bio-photoelectric reductive system composed of Shewanella oneidensis MR-1 and Ag3PO4

Photocatalytic catalysis is widely used for pollutant degradation. Since some pollutants with oxidative nature are readily reduced rather than oxidized and reductive reaction caused by photogenerated electrons is limited in the presence of oxygen, photocatalytic reduction process is more applicable for the degradation of pollutants with oxidative nature than oxidation. In this work, a novel bio-photoelectric reductive degradation system (BPRDS), composed of an electrochemically active bacterium Shewanella oneidensis MR-1 and a visible-light photocatalyst Ag₃PO₄, was established under anaerobic conditions and its photodegradation performance was evaluated through degrading rhodamine B (RhB), a typical organic pollutant. The as-synthesized Ag₃PO₄ nanoparticles exhibited absorption in the entire visible spectral range of 400-800 nm. RhB could be degraded in BPRDS with visible light irradiation under anaerobic conditions, but not be decomposed in the absence of Shewanella cells. Block of Mtr respiratory pathway, a transmembrane electron transport chain, resulted in a reduction in degradation rate of RHB in BPRDS. Dose of riboflavin also substantially decreased the RhB degradation. These results suggest that the electrons released by Shewanella were involved in the RhB photodegradation, which was achieved via a stepwise N-deethylation process. In BPRDS, RhB was degraded by photoreduction, rather than photooxidation. This work is useful to develop integrated physico-chemical-microbial systems for pollutant degradation, facilitate better understanding about the biophotoelectric reductive degradation mechanisms and beneficial to their applications for environmental remediation.

A Study of the Activity of Recombinant Mn-Superoxide Dismutase in the Presence of Gold and Silver Nanoparticles

Superoxide dismutase (SOD) is one of the best characterized enzyme maintaining the redox state in the cell. A bacterial expression system was used to produce human recombinant manganese SOD with a His-tag on the C-end of the protein for better purification. In addition, gold and silver nanoparticles were chemically synthesized in a variety of sizes, and then mixed with the enzyme for immobilization. Analysis by dynamic light scattering and scanning transmission electron microscopy revealed no aggregates or agglomerates of the obtained colloids. After immobilization of the protein on AuNPs and AgNPs, the conjugates were analyzed by SDS-PAGE. It was determined that SOD was adsorbed only on the gold nanoparticles. Enzyme activity was analyzed in colloids of the gold and silver nanoparticles bearing SOD. The presence of a nanoparticle did not affect enzyme activity; however, the amount of protein and size of the gold nanoparticle did influence the enzymatic activity of the conjugate. Our findings confirm that active recombinant human superoxide dismutase can be produced using a bacterial expression system, and that the enzyme can be immobilized on metal nanoparticles. The interaction between enzymes and metal nanoparticles requires further investigation.

Synthesis, characterization, and environmental behaviors of monodispersed platinum nanoparticles

The release of platinum group elements, including platinum nanoparticles (PtNPs), has been increasing over recent decades. However, few studies have investigated the fate, behavior and effects of PtNPs in environmental media. Here, we report a protocol for the synthesis of five different sizes (8.5 ± 1.2, 10.3 ± 1.3, 20.0 ± 4.8, 40.5 ± 4.1, and 70.8 ± 4.2 nm) of monodispersed citrate- and polyvinylpyrrolidone (PVP)-coated PtNPs, together with a characterization of their behaviors using a multi method approach in relevant biological and toxicological media. In general, PtNPs sizes measured using dynamic light scattering, field flow fractionation, single-particle inductively-coupled plasma-mass spectroscopy, transmission electron microscopy and atomic force microscopy, were all in good agreement when PtNP sizes were larger than the size detection limits of each analytical technique. Slight differences in sizes measured were attributable to differences in analytical techniques, measuring principles, NP shape and NP permeability. The thickness of the PVP layer increased (from 4.4 to 11.35 nm) with increases in NP size. The critical coagulation concentration of cit-PtNPs was independent of NP size, possibly due to differences in PtNPs surface charges as a function of NP size. PtNPs did not undergo significant dissolution in any media tested. PtNPs did not aggregate significantly in Dulbecco's modified Eagle's medium; but they formed aggregates in moderately hard water and in 30 ppt synthetic seawater, and aggregate size increased with increases in PtNPs concentration. Overall, this study describes a general model NP system (i.e., PtNPs) of different controlled NP sizes and coatings that is predictable, stable and useful to investigate the fate, behavior, uptake, and eco-toxicity of NPs in the environment.

Carbonaceous nanomaterial-initiated reductive transformation of silver ions in the aqueous environment under sunlight

The aquatic systems are among the major sinks for discharged substances, and these substances will likely associate with each other. The present work, therefore, aims to study the transformation of metal ions to nanoparticles by discharged carbonaceous materials of emerging concern (e.g., carbon nanotubes (CNTs)) coexisting in the aqueous environment. Here we undertook a systematic study of the reduction of silver ions by CNT suspensions under sunlight irradiation. The formation rate of silver nanoparticles (AgNPs) is suppressed by an increasing amount of dissolved oxygen or strong solution acidity, as well as the presence of other cations. The photoreduction of Ag⁺ by CNTs involves a charge transfer process between Ag⁺ and the CNTs. The way in which carbonaceous nanomaterial properties influence the formation kinetics, size, and morphology of the AgNPs was examined. An enhanced sunlight-driven formation of AgNPs with highly monodispersity was observed in CNTs with nitrogen-containing functional groups due to their active electrochemical and stabilizing nature. The compiled results reveal the importance of an understanding of not only the inherent environmental behaviors of individual substances but also their interactions with concurrent substances in the environment. We demonstrated that the transformation of silver under sunlight by carbonaceous materials with different characteristics could alter the properties and potential risks of metallic species in aquatic environments.

Effects of Chloride Ions on Dissolution, ROS Generation, and Toxicity of Silver Nanoparticles under UV Irradiation

This work investigates the effect of chloride ion (Cl^-) on dissolution, reactive oxygen species (ROS) generation, and toxicity of citrate-coated silver nanoparticles (AgNPs) under UV irradiation. The dissolution rate was decreased by 0.01 M Cl^- due to AgCl passivation on the AgNP surface. By contrast, high concentrations of Cl^- (0.1 or 0.5 M) promoted dissolution due to the formation of soluble Ag-Cl complexes (AgCl _x^{1- x}). The generation of O₂^•- in the AgNPs/Cl^-/UV system was promoted by 0.01 M Cl^-, whereas it was retarded by 0.1 or 0.5 M Cl^-, which was probably because the aggregation of AgNPs at high ionic strength reduced the nanoparticles' surface areas for radical formation. Additionally, Cl^- contributed to •OH generation in the AgNPs/Cl^-/UV system, in which the produced •OH concentrations increased with increasing Cl^- concentrations. The reduction reaction between silver ions and O₂^•- resulted in lower dissolution rates of AgNPs/Cl^- mixtures under UV irradiation than those in the dark. The phototoxicity of AgNPs toward E. coli with different concentrations of Cl^- followed the order of 0.5 M > 0 M > 0.1 M > 0.01 M. Both ROS and dissolved Ag played significant role in the phototoxicity of AgNPs. This work demonstrates the potential importance of anions in the fate and biological impact of AgNPs.

Threshold Concentrations of Silver Ions Exist for the Sunlight-Induced Formation of Silver Nanoparticles in the Presence of Natural Organic Matter

Sunlight-induced photoformation of silver nanoparticles (nAg), mediated by natural organic matter (NOM), is significantly affected by the concentration of Ag(I) and chloride. The initial photoformation rates of nAg in Suwannee River humic acid (SRHA) and Suwannee River natural organic matter (SRNOM) solutions were examined under simulated sunlight irradiation. A critical induction concentration (CIC) of Ag(I) (10 mg/L for SRHA and 5 mg/L for SRNOM, respectively) was observed, below which the nAg formation was minimal. The threshold is attributed to the interplay of reduction and oxidation reactions mediated by NOM, reflecting the need to achieve sufficiently fast growth of silver clusters to outcompete oxidative dissolution. The CIC can be reduced by scavenging oxidative radicals or be increased by promoting singlet oxygen and hydrogen peroxide generation. The presence of chloride effectively reduced the CIC by forming AgCl, which facilitates reduction reactions and provides deposition surfaces. SRNOM is more efficient in mediating photoformation of nAg than SRHA, owing to their differed phototransient generation. These results highlight prerequisites for the photoformation of nAg mediated by NOM, in which the photochemistry and solution chemistry are both important.

A rapid approach for measuring silver nanoparticle concentration and dissolution in seawater by UV-Vis

Detection and quantification of engineered nanoparticles (NPs) in environmental systems is challenging and requires sophisticated analytical equipment. Furthermore, dissolution is an important environmental transformation process for silver nanoparticles (AgNPs) which affects the size, speciation and concentration of AgNPs in natural water systems. Herein, we present a simple approach for the detection, quantification and measurement of dissolution of PVP-coated AgNPs (PVP-AgNPs) based on monitoring their optical properties (extinction spectra) using UV-vis spectroscopy. The dependence of PVP-AgNPs extinction coefficient (ɛ) and maximum absorbance wavelength (λ_max) on NP size was experimentally determined. The concentration, size, and extinction spectra of PVP-AgNPs were characterized during dissolution in 30ppt synthetic seawater. AgNPs concentration was determined as the difference between the total and dissolved Ag concentrations measured by inductively coupled plasma-mass spectroscopy (ICP-MS); extinction spectra of PVP-AgNPs were monitored by UV-vis; and size evolution was monitored by atomic force microscopy (AFM) over a period of 96h. Empirical equations for the dependence of maximum absorbance wavelength (λ_max) and extinction coefficient (ɛ) on NP size were derived. These empirical formulas were then used to calculate the size and concentration of PVP-AgNPs, and dissolved Ag concentration released from PVP-AgNPs in synthetic seawater at variable particle concentrations (i.e. 25-1500μgL^-1) and in natural seawater at particle concentration of 100μgL^-1. These results suggest that UV-vis can be used as an easy and quick approach for detection and quantification (size and concentration) of sterically stabilized PVP-AgNPs from their extinction spectra. This approach can also be used to monitor the release of Ag from PVP-AgNPs and the concurrent NP size change. Finally, in seawater, AgNPs dissolve faster and to a higher extent with the decrease in NP concentration toward environmentally relevant concentrations.

Effect of silver sulfide nanoparticles on photochemical degradation of dissolved organic matter in surface water

Silver sulfide nanoparticles (Ag₂SNPs) have shown photocatalytic activity, yet little is known about the effect of Ag₂SNPs on the photochemical degradation of dissolved organic matter (DOM) in surface water, which seriously impairs understanding of Ag₂SNPs' environmental risks. Herein, this study on the basis of electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) showed for the first time that photodegradation of natural organic matter (NOM, 2R101 N) could be accelerated by both bared and polyvinylpyrrolidone (PVP)-coated Ag₂SNPs; the NOM with Ag₂SNPs (e.g., 500 μg/L) exposed to light irradiation for 96 h showed molecular formulas with lower O/C ratios as compared to the NOM alone. Also, added number of points (ranging from 1 to 2 carboxyl groups) having the same Kendrick mass defect (KMD) (COO) values and higher intensity in smaller Kendrick mass (KM) (COO) values were observed in NOM with Ag₂SNPs compared to NOM alone. However, negligible effects of Ag₂SNPs on photodegradation of humic acid (HA, 2S101H) were observed, even when the concentration of Ag₂SNPs was as high as 5 mg/L. Besides molecular characteristics, a great reduction in organic carbon content of NOM within 96 h was only observed in the presence of Ag₂SNPs under light condition. More importantly, the enhanced photodegradation of DOM by Ag₂SNPs even at a concentration of 100 μg/L was also validated in surface water. These findings suggest that Ag₂SNPs have the potential to accelerate the photochemical degradation of DOM in surface water.

Chemical transformation of silver nanoparticles in aquatic environments: Mechanism, morphology and toxicity

Silver nanoparticles (Ag NPs) have been inevitably introduced into ecological environment during their extensive applications in daily human life. Thermodynamically, Ag NPs are unstable and transform into other species under various aqueous conditions. Ag NPs and their transformation products pose potential threats to environment and humans. However, the complex environmental conditions and transformations of Ag NPs complicate their human health and environmental risk assessment. To bridge the knowledge gap, four essential environmental transformations, oxidative dissolution, sulfidation, chlorination and photoreduction, of Ag NPs are reviewed herein. The mechanism, morphology and size change, as well as the toxicity of Ag NPs during these transformations under certain aqueous conditions are detailed. In particular, these environmental transformations have shown strong correlations that are discussed. The transformation, fate, bioavailability, morphology and toxicity of Ag NPs are critical factors and should be considered in a complete human health and environmental risk assessment of Ag NPs. The fluctuation of these factors in the realistic environment is also vital and should be considered.

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.

Aqueous Aggregation Behavior of Engineered Superparamagnetic Iron Oxide Nanoparticles: Effects of Oxidative Surface Aging

For successful aqueous-based applications, it is necessary to fundamentally understand and control nanoparticle dispersivity and stability over a range of dynamic conditions, including variable ionic strengths/types, redox chemistries, and surface ligand reactivity/degradation states (i.e., surface aging). Here, we quantitatively describe the behavior of artificially aged, oleic acid (OA) bilayer coated iron oxide nanoparticles (IONPs) under different scenarios. Hydrogen peroxide (H₂O₂), used here as a model oxidant under both dark and light ultraviolet (UVA) conditions, was employed to "age" materials, to varying degrees, without increasing ionic strength. Short-term stability experiments indicate that OA-IONPs, while stable in the dark, are effectively destabilized when exposed to UVA/H₂O₂/•OH based oxidation processes. Compared to bicarbonate, phosphate (1.0 mM) has a net stabilizing effect on OA-IONPs under oxidative conditions, which can be attributed to (surface-based) functional adsorption. Corresponding aggregation kinetics in the presence of monovalent (Na⁺) and divalent cations (Ca²⁺) show that attachment efficiencies (α) are strongly dependent on the cation concentrations/types and degree of surface aging. Taken together, our findings directly highlight the need to understand the critical role of particle surface transformation(s), via oxidative aging, among other routes, with regard to the ultimate stability and environmental fate of surface functionalized engineered nanoparticles.

Inactivation of Escherichia coli planktonic cells by multi-walled carbon nanotubes in suspensions: Effect of surface functionalization coupled with medium nutrition level

While earlier studies have identified the antibacterial activity of carbon nanotubes (CNTs) and proposed that cell membrane damage by direct contact with CNTs is likely the main toxicity mechanism, the relative importance of chemical versus physical properties of CNTs in controlling their bacterial cytotoxicity is understudied. Given that CNT is commonly modified via acid treatment to enhance its dispersivity and surface chemistry, in this study commercially available multi-walled carbon nanotubes (MWCNTs) with high purity were processed carefully by acid reflux, resulting in differences in surface charge of MWCNTs without altering their physical properties. The surface condition of MWCNTs was also modified by adsorption of organic matter to compare bacterial toxicity of functionalized and non-functionalized MWCNTs in suspensions. Results show that although overall electrostatic repulsion and steric obstruction resulted from surface modifications led to elevated dispersivity of MWCNTs and mitigated toxicity on planktonic Escherichia coli cultures, no correlation between the dispersivity and bacterial toxicity of MWCNTs was observed, suggesting that dispersity alone may not be a proper index to estimate the CNT antibacterial effect on planktonic cells in the aqueous phase. In addition, viability recovery of MWCNT-treated cells was observed to be nutrition level-dependent, implying that availability of proper nutrients may be another important factor to be considered when assessing the ecotoxicity of CNTs in the aquatic system.

The disinfection performance and mechanisms of Ag/lysozyme nanoparticles supported with montmorillonite clay

The fabrication of montmorillonite (Mt) decorated with lysozyme-modified silver nanoparticles (Ag/lyz-Mt) was reported. The lysozyme (lyz) was served as both reducing and capping reagent. Coupling the bactericidal activity of the lyz with AgNPs, along with the high porous structure and large specific surface area of the Mt, prevented aggregation of AgNPs and promoted nanomaterial-bacteria interactions, resulting in a greatly enhanced bactericidal capability against both Gram positive and Gram negative bacteria. This paper systematically elucidated the bactericidal mechanisms of Ag/lyz-Mt. Direct contact between the Ag/lyz-Mt surface and the bacterial cell was essential to the disinfection. Physical disruption of bacterial membrane was considered to be one of the bactericidal mechanisms of Ag/lyz-Mt. Results revealed that Ag(+) was involved in the bactericidal activity of Ag/lyz-Mt via tests conducted using Ag(+) scavengers. A positive ROS (reactive oxygen species) scavenging test indirectly confirmed the involvement of ROS (O2(-), H2O2, and OH) in the bactericidal mechanism. Furthermore, the concentrations of individual ROS were quantified. Results showed that Ag/lyz-Mt nanomaterial could be a promising bactericide for water disinfection.

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.

Photo-enhanced oxidizability of tetrazolium salts and its impact on superoxide assaying

Herein, we report for the first time the enhanced oxidation properties of tetrazolium salts induced by UV-irradiation, and demonstrate that there is real deviation in the photo-induced superoxide anion radical assay based on tetrazolium salts.

Influence of Perfluorooctanoic Acid on the Transport and Deposition Behaviors of Bacteria in Quartz Sand

The significance of perfluorooctanoic acid (PFOA) on the transport and deposition behaviors of bacteria (Gram-negative Escherichia coli and Gram-positive Bacillus subtilis) in quartz sand is examined in both NaCl and CaCl2 solutions at pH 5.6 by comparing both breakthrough curves and retained profiles with PFOA in solutions versus those without PFOA. All test conditions are found to be highly unfavorable for cell deposition regardless of the presence of PFOA; however, 7%-46% cell deposition is observed depending on the conditions. The cell deposition may be attributed to micro- or nanoscale roughness and/or to chemical heterogeneity of the sand surface. The results show that, under all examined conditions, PFOA in suspensions increases cell transport and decreases cell deposition in porous media regardless of cell type, presence or absence of extracellular polymeric substances, ionic strength, and ion valence. We find that the additional repulsion between bacteria and quartz sand caused by both acid-base interaction and steric repulsion as well as the competition for deposition sites on quartz sand surfaces by PFOA are responsible for the enhanced transport and decreased deposition of bacteria with PFOA in solutions.

Silver nanoparticles in aquatic environments: Physiochemical behavior and antimicrobial mechanisms

Nanosilver (silver nanoparticles or AgNPs) has unique physiochemical properties and strong antimicrobial activities. This paper provides a comprehensive review of the physicochemical behavior (e.g., dissolution and aggregation) and antimicrobial mechanisms of nanosilver in aquatic environments. The inconsistency in calculating the Gibbs free energy of formation of nanosilver [ΔGf(AgNPs)] in aquatic environments highlights the research needed to carefully determine the thermodynamic stability of nanosilver. The dissolutive release of silver ion (Ag(+)) in the literature is often described using a pseudo-first-order kinetics, but the fit is generally poor. This paper proposes a two-stage model that could better predict silver ion release kinetics. The theoretical analysis suggests that nanosilver dissolution could occur under anoxic conditions and that nanosilver may be sulfidized to form silver sulfide (Ag2S) under strict anaerobic conditions, but more investigation with carefully-designed experiments is required to confirm the analysis. Although silver ion release is likely the main antimicrobial mechanism of nanosilver, the contributions of (ion-free) AgNPs and reactive oxygen species (ROS) generation to the overall toxicity of nanosilver must not be neglected. Several research directions are proposed to better understand the dissolution kinetics of nanosilver and its antimicrobial mechanisms under various aquatic environmental conditions.

Fabrication of novel p–n heterojunction BiOI/La2Ti2O7 composite photocatalysts for enhanced photocatalytic performance under visible light irradiation

A novel BiOI/La₂Ti₂O₇ composite photocatalyst with p–n heterojunctions was prepared for the first time. The as-obtained photocatalysts exhibit efficient activity under visible light irradiation for degradation of organic pollutants.

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.

Simulation and comparative study on the oxidation kinetics of atrazine by UV/H₂O₂, UV/HSO₅⁻ and UV/S₂O₈²⁻

This study comparatively investigated atrazine (ATZ) degradation by irradiation at the wavelength of 254 nm in the presence of peroxides including hydrogen peroxide (H2O2), peroxymonosulfate (HSO5(-)), and persulfate (S2O8(2-)) at various initial ATZ concentrations and oxidant dosages. The effects of water matrix, such as carbonate/bicarbonate (HCO3(-)/CO3(2-)), chloride ions (Cl(-)), and natural organic matter (NOM), were evaluated on these three advanced oxidation processes. A simple steady-state kinetic model was developed based on the initial rates of ATZ destruction, which could well describe the apparent pseudo-first-order rate constants (k(app), s(-1)) of ATZ degradation in these three processes. The specific roles of reactive species (i.e., HO·, SO4(-·), CO3(-·), and Cl2(-·)) under various experimental conditions were quantitatively evaluated based on their steady-state concentrations obtained from this model. Modeling results showed that the steady-state concentrations of HO· and SO4(-·) decreased with the increase of CO3(2-)/HCO3(-) concentration, and the relative contribution of HO· to ATZ degradation significantly decreased in UV/H2O2 and UV/HSO5(-) systems. On the other hand, the scavenging effect of HCO3(-)/CO3(2-) on the relative contribution of SO4(-·) to ATZ degradation was lower than that on HO·. The presence of Cl(-) (0.5-10 mM) significantly scavenged SO4(-·) but had slightly scavenging effect on HO· at the present experimental pH, resulting in greater decrease of k(app) in the UV/S2O8(2-) than UV/H2O2 and UV/HSO5(-) systems. Higher levels of Cl2(-·) were generated in the UV/S2O8(2-) than those in the UV/H2O2 and UV/HSO5(-) systems at the same Cl(-) concentrations. NOM significantly decreased k(app) due to its effects of competitive UV absorption and radical scavenging with the latter one being dominant. These results improve the understanding of the effects of water constituents for ATZ degradation in the UV-based oxidation processes.

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

We investigated the photogeneration of reactive oxygen species (ROS) by C60 under UV irradiation, when humic acid (HA) or fulvic acid (FA) is present. When C60 and dissolved organic matter (DOM) were present as a mixture, singlet oxygen ((1)O2) generation concentrations were 1.2–1.5 times higher than the sum of (1)O2 concentrations that were produced when C60 and DOM were present in water by themselves. When C60 and HA were present as a mixture, superoxide radicals (O2(•–)) were 2.2–2.6 times more than when C60 and HA were present in water by themselves. A synergistic ROS photogeneration mechanism involved in energy and electron transfer between DOM and C60 was proposed. Enhanced (1)O2 generation in the mixtures was partly due to (3)DOM* energy transfer to O2. However, it was mostly due to (3)DOM* energy transfer to C60 producing (3)C60*. (3)C60* has a prolonged lifetime (>4 μs) in the mixture and provides sufficient time for energy transfer to O2, which produces (1)O2. The enhanced O2(•–) generation for HA/C60 mixture was because (3)C60* mediated electron transfer from photoionized HA to O2. This study demonstrates the importance of considering DOM when investigating ROS production by C60.