Agglomeration of Ag and TiO2 nanoparticles in surface and wastewater: Role of calcium ions and of organic carbon fractions

Colloidal stability and aggregation kinetics of nanocrystal CdSe/ZnS quantum dots in aqueous systems: Effects of ionic strength, electrolyte type, and natural organic matter

Understanding the stability and aggregation of nanoparticles in aqueous milieu is critical for assessing their behavior in the natural and engineered environmental systems and establishing their threat to human and ecosystems health. In this study, the colloidal stability and aggregation kinetics of nanocrystal quantum dots (QDs) —CdSe/ZnS QDs—were thoroughly explored under a wide range of aqueous environmental conditions. The z-average hydrodynamic diameters (z-avg. HDs) and zeta potential (ξ potential) of CdSe/ZnS QDs were measured in monovalent electrolyte (NaCl) and divalent electrolyte (CaCl2) solutions in both the absence and presence of natural organic matter (NOM)—Suwannee River natural organic matter, SRNOM to assess the dynamic growth of these nanoaggregate-QD-complexes, and the evaluation of their colloidal stability. Results show that CaCl2 was more effective to destabilize the QDs compared to NaCl at similar concentrations. An increase in NaCl concentration from 0.01 to 3.5 M increased the z-avg. HD of QD aggregates from 61.4 nm to 107.2 nm. The aggregation rates of QDs increased from 0.007 to 0.042 nm·s−1 with an increase in ionic strength from 0.5 to 3.5 M NaCl solutions, respectively. In the presence of Na+ cations, the aggregation of QDs was limited as steric forces generated by the original surface coating of QDs prevailed. In the presence of CaCl2, the aggregation of QDs was observed at a low concentration of CaCl2 (0.0001 M) with a z-avg. HD of 74.2 nm that significantly increased when the CaCl2 was higher than 0.002 M. Larger sizes of QD aggregates were observed at each level of CaCl2 concentration in suspensions of 0.002–0.1 M, as the z-avg. HDs of QDs increased from 125.1 to 560.4 nm, respectively. In the case of CaCl2, an increase in aggregation rates occurred from 0.035 to 0.865 nm·s−1 with an increase in ionic strength from 0.0001 M to 0.004 M, respectively. With Ca2+ cations, the aggregation of QDs was enhanced due to the bridging effects from the formation of complexes between Ca2+ cations in solution and the carboxyl group located on the surface coating of QDs. In the presence of SRNOM, the aggregation of QDs was enhanced in both monovalent and divalent electrolyte solutions. The degree of aggregation formation between QDs through cation-NOM bridges was superior for Ca2+ cations compared to Na+ cations. The presence of SRNOM resulted in a small increase in the size of the QD aggregates for each of NaCl concentrations tested (i.e., 0.01 to 3.5 M, except 0.1 M), and induced a monodispersed and narrower size distribution of QDs suspended in the monovalent electrolyte NaCl concentrations. In the presence of SRNOM, the aggregation rates of QDs increased from 0.01 to 0.024 nm 1 with the increase of NaCl concentrations from 0.01 to 2 M, respectively. The presence of SRNOM in QDs suspended in divalent electrolyte CaCl2 solutions enhanced the aggregation of QDs, resulting in the increase of z-avg. HDs of QDs by approximately 19.3%, 42.1%, 13.8%, 1.5%, and 24.8%, at CaCl2 concentrations of 0.002, 0.003, 0.005, 0.01, and 0.1 M, respectively. In the case of CaCl2, an increase in aggregation rates occurred from 0.035 to 0.865 nm·s−1 with an increase in ionic strength from 0.0001 to 0.004 M, respectively. Our findings demonstrated the colloidal stability of QDs and cations-NOM-QD nanoparticle complexes under a broad spectrum of conditions encountered in the natural and engineered environment, indicating and the potential risks from these nanoparticles in terms of human and ecosystem health.

Titanium nanoparticles fate in small-sized watersheds under different land-uses

Surface waters from three catchments having contrasting land-uses (forested, agricultural, and urban) were sampled monthly and analysed for nanoparticulate titanium dioxide (NPs-TiO₂) by single particle ICPMS and electron microscopy. We report one-year of data for NPs-TiO₂ having average number and mass concentrations of 9.1 × 10⁸ NPs-TiO₂ particles L^-1 and 11 µg NPs-TiO₂ L^-1 respectively. An increase in concentration during warmer months is observed in the forested and agricultural catchments. Both concentrations of NPs-TiO₂ are within the range of recently reported values using similar analytical approaches. The positive correlations for NPs-TiO₂ mass concentration or particle number with the concentration of some trace elements and DOC in the forested and agricultural catchments suggest the detected NPs-TiO₂ in these two systems are mostly from geogenic origin. Additionally, microscopy imaging confirmed the presence of NPs in the three catchments. Furthermore, the land-area normalized annual flux of NPs-TiO₂ (1.65 kg TiO₂ year^-1 km^-2) was highest for the agricultural catchment, suggesting that agricultural practices have a different impact on the NPs-TiO₂ dynamics and exports than other land-uses (urban or forestry). A similar trend is also found by the reanalysis of recent literature data.

Embryotoxicity of silver nanomaterials (Ag NM300k) in the soil invertebrate Enchytraeus crypticus - Functional assay detects Ca channels shutdown

Despite that silver (Ag) is among the most studied nanomaterials (NM) in environmental species and Ag's embryotoxicity is well known, there are no studies on Ag NMs embryotoxicity in soil invertebrates. Previous Full Life Cycle (FLC) studies in Enchytraeus crypticus, a standard soil invertebrate, showed that Ag materials decreased hatching success, which was confirmed to be a hatching delay effect for silver nitrate (AgNO₃) and mortality for Ag NM300K. In the present study, we aimed to investigate if the impact of Ag takes place during the embryonic development, using histology and immunohistochemistry. E. crypticus cocoons were exposed to a range of concentrations of Ag NM300K (0-10-20-60-115 mg Ag/kg) and AgNO₃ (0-20-45-60-96 mg Ag/kg) in LUFA 2.2 soil, in an embryotoxicity test, being sampled at days 1, 2, 3 and 6 (3, 4, 5 and 7 days after cocoon laying). Measured endpoints included the number of embryonic structures, expression of transferrin receptor (TfR) and L type calcium channels (LTCC) through histological and immunohistochemistry analysis, respectively. Results confirmed that Ag materials affected the embryonic development, specifically at the blastula stage (day 3). The expression and localization of TfR in E. crypticus was shown in the teloblasts cells, although this transcytosis mechanism was not activated. Ag affected calcium (Ca) metabolism during embryonic development: for AgNO_3, LTCC was initially activated, compensating the impact, for Ag NM300K, LTCC was not activated, hence no Ca balance, with irreversible consequences, i.e. terminated embryonic development. An Adverse Outcome Pathway was drafted, integrating the mechanisms here discovered with previous knowledge.

Wastewater-Aged Silver Nanoparticles in Single and Combined Exposures with Titanium Dioxide Affect the Early Development of the Marine Copepod Tisbe battagliai

In this study, the effects of aged Ag and TiO₂ nanoparticles (NPs), individually and as a mixture, in wastewater relative to their pristine counterparts on the development of the copepod nauplii (Tisbe battagliai) were investigated. NP behavior in synthetic wastewater and seawater was characterized during aging and exposure. A delayed development and subsequent mortality were observed after 6 days of exposure to aged Ag NPs, with a twofold decrease in EC₅₀ (316 μg/L) compared to pristine NPs (EC₅₀ 640 μg/L) despite the similar dissolved Ag concentrations measured for aged and pristine Ag NPs (441 and 378 μg/L, respectively). In coexposures with TiO₂ NPs, higher dissolved Ag levels were measured for aged NPs (238.3 μg/L) relative to pristine NPs (98.57 μg/L). Coexposure resulted in a slight decrease (15%) in the Ag NP EC₅₀ (270 μg/L) with a 1.9-fold increase in the Ag NP retained within the organisms after depuration (2.82% retention) compared to Ag NP single exposures as measured with sp-ICP-MS, suggesting that the particles are still bioavailable despite the heteroaggregation observed between Ag, Ti NPs, and wastewater components. This study shows that the presence of TiO₂ NPs can affect the stability and toxicity of Ag NPs in complex media that cannot be predicted solely based on ionic, total, or nanoparticulate concentrations, and the need for studying NP interactions in more complex matrices is highlighted.

Flow and fate of silver nanoparticles in small French catchments under different land-uses: The first one-year study

This study focused on surface waters from three small creeks, within the Seine River watershed, which are characterized by different land-uses, namely forested, agricultural and urban. Silver nanoparticles (Ag-NPs) in these waters were detected and quantified by single-particle ICPMS during one-year of monthly sampling. Their temporal and spatial variations were investigated. Ag-NPs, in the three types of surface water, were found to range from 1.5 × 10⁷ to 2.3 × 10⁹ particles L^-1 and from 0.4 to 28.3 ng L^-1 at number and mass concentrations, respectively. These values are in consistent with the very few previous studies. In addition, the role of factors driving process and potential sources are discussed with correlations between Ag-NPs concentrations and biogeochemical parameters, like dissolved organic carbon concentration and divalent cations concentrations. For the forested watershed NOM controls the stability (number and mass) of the Ag-NPs as recently observed in the field in lake water in Germany. In the case of the agricultural and urban watersheds major cations such as Ca would control the number and mass of Ag-NPs. Dilution processes are rejected as conductivity and Cl^- ions do not show significant correlations with Ag-NPs or other major geochemical parameters. The specific exportation rates of Ag-NPs for artificial, agricultural and forested areas were calculated based on the monthly data for the full year and are equal to 5.5 ± 3.0, 0.5 ± 0.3 and 0.2 ± 0.2 gy^-1km^-2, respectively. These data suggest a constant release of Ag-NPs from consumer products into freshwaters in artificial areas, for instance, from textiles, washing machines, domestic tap-water filters, outdoor paints. These first data of Ag-NPs fluxes in surface waters of France enlarge the very limited database of field measurements. Moreover, for the first time, the influence of time, land-use and aquatic geochemistry parameters on Ag-NPs in real natural water samples is reported. It is also helpful to further understand the fate and the process of Ag-NPs in natural waters, as well as to the ecotoxicity studies in real-world environment.

Transport and retention of engineered silver nanoparticles in carbonate-rich sediments in the presence and absence of soil organic matter

The transport and retention behavior of polymer- (PVP-AgNP) and surfactant-stabilized (AgPURE) silver nanoparticles in carbonate-dominated saturated and unconsolidated porous media was studied at the laboratory scale. Initial column experiments were conducted to investigate the influence of chemical heterogeneity (CH) and nano-scale surface roughness (NR) arising from mixtures of clean, positively charged calcium carbonate sand (CCS), and negatively charged quartz sands. Additional column experiments were performed to elucidate the impact of CH and NR arising from the presence and absence of soil organic matter (SOM) on a natural carbonate-dominated aquifer material. The role of the nanoparticle capping agent was examined under all conditions tested in the column experiments. Nanoparticle transport was well described using a numerical model that facilitated blocking on one or two retention sites. Results demonstrate that an increase in CCS content in the artificially mixed porous medium leads to delayed breakthrough of the AgNPs, although AgPURE was much less affected by the CCS content than PVP-AgNPs. Interestingly, only a small portion of the solid surface area contributed to AgNP retention, even on positively charged CCS, due to the presence of NR which weakened the adhesive interaction. The presence of SOM enhanced the retention of AgPURE on the natural carbonate-dominated aquifer material, which can be a result of hydrophobic or hydrophilic interactions or due to cation bridging. Surprisingly, SOM had no significant impact on PVP-AgNP retention, which suggests that a reduction in electrostatic repulsion due to the presence of SOM outweighs the relative importance of other binding mechanisms. Our findings are important for future studies related to AgNP transport in shallow unconsolidated calcareous and siliceous sands.

Toxicity of silver nanoparticles to Lumbriculus variegatus is a function of dissolved silver and promoted by low sediment pH

Toxicity of silver nanoparticles (AgNPs) to benthic organisms is a major concern. The use of AgNPs in industry and consumer products leads to increasing release of AgNPs into the aquatic environment-sediments being the major sink. Effects of sediment pH on the toxicity of AgNPs to benthic oligochaeta Lumbriculus variegatus were studied in a 23-d toxicity test. Artificially prepared sediments (pH 5 and 7) were spiked with varying concentrations of uncoated AgNP, polyvinylpyrrolidone (PVP)-coated AgNP, and silver nitrate (AgNO₃ ) as dissolved Ag reference. Number of individuals and biomass change were used as endpoints for the toxicity. The toxic effects were related to the bioaccessible concentration of dissolved Ag in the sediments, assessed with a 2-step extraction procedure. The toxicity of 2 AgNPs was similar and greatly enhanced in the acidic sediment. Because the toxic effects were well related to the bioaccessible concentration of dissolved Ag in the sediments, the toxicity of sediment-associated AgNPs to L. variegatus is suggested to be a function of dissolved Ag rather than a result from NP-specific modes of toxicity. Environ Toxicol Chem 2018;37:1889-1897. © 2018 SETAC.

Silver engineered nanoparticles in freshwater systems - Likely fate and behaviour through natural attenuation processes

Growth in the nanotechnology sector is likely introducing unnatural formations of materials on the nanoscale (10^-9m) to the environment. Disposal and degradation of products incorporating engineered nanomaterials (ENMs) are likely being released into natural aquatic systems un-intentionally primarily via waste water effluents. The fate and behaviour of metallic based nanoparticles (NPs) such as silver (Ag) in aquatic waters is complex with high levels of variability and uncertainty. In-situ physical, biological and chemical (natural attenuation) processes are likely to influence ENM fate and behaviour in freshwater systems. Surfaced functionalized particles may inhibit or limit environmental transformations which influence particle aggregation, mobility, dissolution and eco-toxic potential. This paper focuses on ENM characteristics and the influence of physical, chemical and biological processes occurring in aquatic systems that are likely to impact metallic ENMs fate. A focus on silver NPs (while for comparison, reporting about other metallic ENMs as appropriate) released to aquatic systems is discussed relating to their likely fate and behaviour in this dynamic and complex environment. This paper further highlights the need for specific risk assessment approaches for metallic ENMs and puts this into context with regard to informing environmental policy and potential NP influence on environmental/human health.

Particle-impact analysis of the degree of cluster formation of rutile nanoparticles in aqueous solution

Cluster formation can profoundly influence the bioavailability and (bio)geochemical activity of nanoparticles in natural aquatic systems. While colloidal properties of nanoparticles are commonly investigated using light-scattering techniques, the requirement to dilute samples can affect the fundamental nature and extent of the cluster size. Hence, an alternative in situ approach that can cover a much higher and wider concentration range of particles is desirable. In this study, particle impact chronoamperometry is employed to probe the degree of cluster formation of Alizarin Red S modified rutile nanoparticles of diameter ca. 167 nm in conditions approximating those existing in the environment. Random collisions of individual clusters of the modified rutile particles with a stationary electrode result in transient current signals during a chronoamperometric measurement, indicative of the reduction of the adsorbed Alizarin Red S dye molecules. The results from the particle-impact analysis reveal that the nanoparticles are heavily clustered with an average 91 monomeric particles per cluster. As the spherical equivalent size of the clusters (ca. 754 nm in diameter) is considerably larger than that from nanoparticle tracking analysis (ca. 117 nm), the present work highlights the impact of the dilution on the fundamental nature of the colloidal suspension and introduces the electrochemical determination of the size distribution of inert mineral nanoparticles in highly concentrated media.

The effect of soil properties on the toxicity and bioaccumulation of Ag nanoparticles and Ag ions in Enchytraeus crypticus

Standard natural Lufa soils (2.2, 2.3 and 5M) with different organic carbon contents (0.67-1.61%) and pH_CaCl2 (5.5-7.3) were spiked with ionic Ag (AgNO₃) and polyvinyl pyrrolidone (AgNP-PVP) and citrate (AgNP-Cit) coated Ag nanoparticles (NPs). Enchytraeus crypticus were exposed for 21 days to assess effects on survival and reproduction. Soil, pore water and animals were analyzed for Ag. AgNP-Cit had a strong increasing effect on soil pH, leading to high enchytraeid mortality at concentrations higher than 60-100mg Ag/kg dry soil which made it impossible to determine the influence of soil properties on its toxicity. LC50s were lower for AgNO₃ than for AgNP-PVP (92-112 and 335-425mg Ag/kg dry soil, respectively) and were not affected by soil properties. AgNO₃ and AgNP-PVP had comparable reproductive toxicity with EC50s of 26.9-75.2 and 28.2-92.3mg Ag/kg dry soil, respectively; toxicity linearly increased with decreasing organic carbon content of the soils but did not show a clear effect of soil pH. Ag uptake in the enchytraeids was higher at higher organic carbon content, but could not explain differences in toxicity between soils. This study indicates that the bioavailability of both ionic and nanoparticulate Ag is mainly affected by soil organic carbon, with little effect of soil pH.

Detection of TiO2 Nanoparticles in Municipal Sewage Treatment Plant and Their Characterization Using Single Particle ICP-MS

Establishment of analytical methods for detection and characterization of nanoparticles in the environment are gaining prominence across the globe. The present study was designed to quantify titanium (Ti) and to characterize titanium dioxide nanoparticles (TNP) from a municipal sewage treatment plant, by inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of Ti & TNP were 1085 & 13.6 mg/kg in the influent sewage and 298 & 3.3 mg/kg in the aeration tank contents, respectively. The size of TNP ranged between 71-145 nm in the sludge fraction. Determining environmentally realistic concentrations of TNP could serve as a tracer material for characterization of those nanomaterials with similar size and aggregation properties. Furthermore, inference of Ti and TNP in municipal sewage in the study will also help in environmental risk assessment of nanomaterials.

The effect of chronic silver nanoparticles on aquatic system in microcosms

Silver nanoparticles (AgNPs) inevitably discharge into aquatic environments due to their abundant use in antibacterial products. It was reported that in laboratory conditions, AgNPs display dose-dependent toxicity to aquatic organisms, such as bacteria, algae, macrophytes, snails and fishes. However, AgNPs could behave differently in natural complex environments. In the present study, a series of microcosms were established to investigate the distribution and toxicity of AgNPs at approximately 500 μg L^-1 in aquatic systems. As a comparison, the distribution and toxicity of the same concentration of AgNO₃ were also determined. The results showed that the surface layer of sediment was the main sink of Ag element for both AgNPs and AgNO₃. Both aquatic plant (Hydrilla verticillata) and animals (Gambusia affinis and Radix spp) significantly accumulated Ag. With short-term treatment, phytoplankton biomass was affected by AgNO₃ but not by AgNPs. Chlorophyll content of H. verticillata increased with both AgNPs and AgNO₃ short-term exposure. However, the biomass of phytoplankton, aquatic plant and animals was not significantly different between control and samples treated with AgNPs or AgNO₃ for 90 d. The communities, diversity and richness of microbes were not significantly affected by AgNPs and AgNO₃; in contrast, the nitrification rate and its related microbe (Nitrospira) abundance significantly decreased. AgNPs and AgNO₃ may affect the nitrogen cycle and affect the environment and, since they might be also transferred to food web, they represent a risk for health.

Natural water as the test medium for Ag and CuO nanoparticle hazard evaluation: An interlaboratory case study

Engineered nanoparticles (NPs) have realistic potential of reaching natural waterbodies and of exerting toxicity to freshwater organisms. The toxicity may be influenced by the composition of natural waters as crucial NP properties are influenced by water constituents. To tackle this issue, a case study was set up in the framework of EU FP7 NanoValid project, performing an interlaboratory hazard evaluation of NPs in natural freshwater. Ag and CuO NPs were selected as model NPs because of their potentially high toxicity in the freshwater. Daphnia magna (OECD202) and Danio rerio embryo (OECD236) assays were used to evaluate NP toxicity in natural water, sampled from Lake Greifen and Lake Lucerne (Switzerland). Dissolution of the NPs was evaluated by ultrafiltration, ultracentrifugation and metal specific sensor bacteria. Ag NP size was stable in natural water while CuO NPs agglomerated and settled rapidly. Ag NP suspensions contained a large fraction of Ag(+) ions and CuO NP suspensions had low concentration of Cu(2+) ions. Ag NPs were very toxic (48 h EC50 1-5.5 μg Ag/L) to D. magna as well as to D. rerio embryos (96 h EC50 8.8-61 μg Ag/L) in both standard media and natural waters with results in good agreement between laboratories. CuO NP toxicity to D. magna differed significantly between the laboratories with 48 h EC50 0.9-11 mg Cu/L in standard media, 5.7-75 mg Cu/L in Lake Greifen and 5.5-26 mg Cu/L in Lake Lucerne. No toxicity of CuO NP to zebrafish embryos was detected up to 100 mg/L independent of the medium used. The results show that Ag and CuO NP toxicity may be higher in natural water than in the standard media due to differences in composition. NP environmental hazard evaluation can and should be carried out in natural water to obtain more realistic estimates on the toxicity.

An approach for environmental risk assessment of engineered nanomaterials using Analytical Hierarchy Process (AHP) and fuzzy inference rules

The usage of Engineered Nanoparticles (ENPs) in consumer products is relatively new and there is a need to conduct environmental risk assessment (ERA) to evaluate their impacts on the environment. However, alternative approaches are required for ERA of ENPs because of the huge gap in data and knowledge compared to conventional pollutants and their unique properties that make it difficult to apply existing approaches. This study aims to propose an ERA approach for ENPs by integrating Analytical Hierarchy Process (AHP) and fuzzy inference models which provide a systematic evaluation of risk factors and reducing uncertainty about the data and information, respectively. Risk is assumed to be the combination of occurrence likelihood, exposure potential and toxic effects in the environment. A hierarchy was established to evaluate the sub factors of these components. Evaluation was made with fuzzy numbers to reduce uncertainty and incorporate the expert judgements. Overall score of each component was combined with fuzzy inference rules by using expert judgements. Proposed approach reports the risk class and its membership degree such as Minor (0.7). Therefore, results are precise and helpful to determine the risk management strategies. Moreover, priority weights calculated by comparing the risk factors based on their importance for the risk enable users to understand which factor is effective on the risk. Proposed approach was applied for Ag (two nanoparticles with different coating) and TiO2 nanoparticles for different case studies. Results verified the proposed benefits of the approach.