Particulate and colloidal silver in sewage effluent and sludge discharged from British wastewater treatment plants

Human health risk assessment and uncertainty analysis of silver nanoparticles in water

Despite frequent detection in environmental waters, literature which quantifies the health risk of silver nanoparticles (Ag NPs) through oral ingestion is scarce. This study compiled literature data to find the removal of Ag NPs from different treatment schemes (i.e., natural, engineered, or hybrid). Ag NP concentrations were found either in surface water or in groundwater based on where the effluent of treatment schemes was discharged, i.e., either in surface water or in groundwater. Monte-Carlo simulation was carried out for probabilistic assessment of health risks for children for two hypothetical exposure scenarios: (a) ingesting river water while swimming and (b) drinking groundwater. Bio-accessible fraction, dietary metal adsorption factor, and concentrations of silver ions were incorporated to simulate realistic situations. Different treatment schemes were ranked for their nanoparticles' removal efficiency with respect to (i) exceedance probability from guideline value and (ii) health risk to children. Hybrid treatment combinations, i.e., conventional primary and secondary treatment units followed by nature-based units (constructed wetlands and soil aquifer treatment), were ranked the best. The health risk value was found to be less than 1, with the 99th percentile value less than 10-3 in all cases. The maximum allowable concentration of Ag NPs was found to be as low as 1.43 mg/L for groundwater, suggesting probable potential for risk. Uncertainty analysis revealed that the uncertainty of the influent NPs concentration in raw wastewater contributes > 99% to the variance of the hazard index. The results of this work indicate that the use of natural treatment technologies with existing engineered treatments provides higher nanoparticle removal from wastewater without the requirement of any tertiary treatment unit.

Aquatic Ecosystem Risk Assessment Generated by Accidental Silver Nanoparticle Spills in Groundwater

This paper aims to create a new model for assessing the ecosystem risk in rivers and wetlands that are linked to accidental spills of silver nanoparticles (AgNPs) in soil/groundwater. Due to the uncertainty of the modeling inputs, a combination of two well-known risk assessment methodologies (Monte Carlo and fuzzy logic) were used. To test the new model, two hypothetical, accidental AgNP soil spill case studies were evaluated; both of which were located at the end of the Llobregat River basin within the metropolitan area of Barcelona (NE Spain). In both cases, the soil spill reached groundwater. In the first case, it was discharged into a river, and in the second case, it recharged a wetland. Concerning the results, in the first case study, a medium-risk assessment was achieved for most cases (83%), with just 10% of them falling below the future legal threshold concentration value. In the second case study, a high-risk assessment was obtained for most cases (84%), and none of the cases complied with the threshold value. A sensitivity analysis was conducted for the concentration and risk. The developed tool was proven capable of assessing risk in aquatic ecosystems when dealing with uncertain and variable data, which is an improvement compared to other risk assessment methodologies.

Silver contamination and its toxicity and risk management in terrestrial and aquatic ecosystems

Silver (Ag), a naturally occurring, rare and precious metal, is found in major minerals such as cerargyrite (AgCl), pyrargyrite (Ag₃SbS₃), proustite (Ag₃AsS₃), and stephanite (Ag₅SbS₄). From these minerals, Ag is released into soil and water through the weathering of rocks and mining activities. Silver also enters the environment by manufacturing and using Ag compounds in electroplating and photography, catalysts, medical devices, and batteries. With >400 t of Ag NPs produced yearly, Ag NPs have become a rapidly growing source of anthropogenic Ag input in the environment. In soils and natural waters, most Ag is sorbed to soil particles and sediments and precipitated as oxides, carbonates, sulphides, chlorides and hydroxides. Silver and its compounds are toxic, and humans and other animals are exposed to Ag through inhalation of air and the consumption of Ag-contaminated food and drinking water. Remediation of Ag-contaminated soil and water sources can be achieved through immobilization and mobilization processes. Immobilization of Ag in soil and groundwater reduces the bioavailability and mobility of Ag, while mobilization of Ag in the soil can facilitate its removal. This review provides an overview of the current understanding of the sources, geochemistry, health hazards, remediation practices and regulatory mandates of Ag contamination in complex environmental settings, including soil and aquatic ecosystems. Knowledge gaps and future research priorities in the sustainable management of Ag contamination in these settings are also discussed.

European freshwater silver monitoring data do not suggest a potential European-wide risk

European legislations frequently focus on substances that are of potential concern to human or environmental health, such as "priority substances" under the Water Framework Directive 2000/60/EC ("WFD") that are identified as substances posing a significant risk to or via the aquatic environment. The EU REACH regulation also requires the assessment of the environmental risks of chemicals put on the EU market. To properly assess the potential risk of a substance, high-quality representative monitoring data should be compared with a safe threshold concentration. The objective of this article is to evaluate different publicly available freshwater monitoring data sets for silver and investigate them for a potential European-wide risk according to the methodology used by the European Commission. Most available silver monitoring data sets contain a large proportion of undetected samples with a reported concentration below the limit of quantification (LOQ) of the analytical technique, leading to considerable uncertainty in the data set. For silver, this LOQ is often at or above the safe threshold concentration, and the method used to handle undetected samples during the data processing considerably impacts the data assessment. We demonstrate that for large data sets covering many European countries (and often a wide range of LOQs), the uncertainty in the data set does not allow us to make a general conclusion about European-wide risk. However, by examining the data sets in more detail and assessing three additional country-specific monitoring data sets, we show that silver does not pose a risk to the freshwater environment in several countries. We conclude that the available data sets need careful assessment to account for the values that are below the LOQ, and that there is currently no reliable evidence indicating a European-wide risk for silver in the aquatic environment, meaning it should not be selected as  priority substance under the WFD. Integr Environ Assess Manag 2023;00:1-10. © 2022 SETAC.

Impact of synthetic silver nanoparticles on the biofilm microbial communities and wastewater treatment efficiency in experimental hybrid filter system treating municipal wastewater

Silver nanoparticles (AgNPs) threaten human and ecosystem health, and are among the most widely used engineered nanomaterials that reach wastewater during production, usage, and disposal phases. This study evaluated the effect of a 100-fold increase in collargol (protein-coated AgNP) and Ag⁺ ions concentrations in municipal wastewater on the microbial community composition of the filter material biofilms (FMB) and the purification efficiency of the hybrid treatment system consisting of vertical (VF) and horizontal (HF) subsurface flow filters. We found that increased amounts of collargol and AgNO₃ in wastewater had a modest effect on the prokaryotic community composition in FMB and did not significantly affect the performance of the studied system. Regardless of how Ag was introduced, 99.9% of it was removed by the system. AgNPs and AgNO₃ concentrations did not significantly affect the purification efficiency of the system. AgNO₃ induced a higher increase in the genetic potential of certain Ag resistance mechanisms in VFs than collargol; however, the increase in Ag resistance potential was similar for both substances in HF. Hence, the microbial community composition in biofilms of vertical and horizontal flow filters is largely resistant, resilient, or functionally redundant in response to AgNPs addition in the form of collargol.

Silver nanoparticles induce developmental toxicity via oxidative stress and mitochondrial dysfunction in zebrafish (Danio rerio)

Sliver nanoparticles (AgNPs) are widely used in industry, agriculture, and medicine, potentially resulting in adverse effects on human health and aquatic environments. Here, we investigated the developmental toxicity of zebrafish embryos with acute exposure to AgNPs. Our results demonstrated developmental defects in 4 hpf zebrafish embryos after exposure to different concentrations of AgNPs for 72 h. In addition, RNA-seq profiling of zebrafish embryos after AgNPs treatment. Further Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses showed that the differentially expressed genes (DEGs) were enriched in DNA replication initiation, oxidoreductase activity, DNA replication, cellular senescence, and oxidative phosphorylation signaling pathways in the AgNPs-treated group. Notably, we also found that AgNPs exposure could result in the accumulation of reactive oxygen species (ROS) and malondialdehyde (MDA), the inhibition of superoxide dismutase (SOD), catalase (CAT), and mitochondrial complex I-V activities, and the downregulated expression of SOD, CAT, and mitochondrial complex I-IV chain-related genes. Moreover, the expression of mitochondrion-mediated apoptosis signaling pathway-related genes, such as bax, bcl2, caspase-3, and caspase-9, was significantly regulated after AgNPs exposure in zebrafish. Therefore, these findings demonstrated that AgNPs exposure could cause oxidative stress, induce mitochondrial dysfunction, and ultimately lead to developmental toxicity.

Whole-lake nanosilver additions reduce northern pike (Esox lucius) growth

Nanosilver (AgNP) is an anti-microbial agent widely used in consumer products, with significant potential for these nanoparticles to be released into aquatic environments. Laboratory studies involving short-term exposures of fish to AgNP show a range of toxicological effects, but these studies do not address potential responses in long-lived organisms resulting from chronic exposures. A collaborative study involving additions of AgNP to environmentally relevant concentrations over two field seasons took place at the IISD-Experimental Lakes Area, providing an opportunity to study the impacts of chronic exposures to long-lived fish species. In the present study, we evaluated the abundance and growth of an apex predator, Northern Pike (Esox lucius), collected from Lake 222 before, during and after the AgNP dosing period and compared results to those from a nearby unmanipulated lake (Lake 239). While the abundance of Northern Pike from Lake 222 during the study period was essentially stable, per capita availability of their primary prey species, Yellow Perch (Perca flavescens) declined by over 30%. Northern Pike fork length- and weight-at-age (indices of growth rate) declined following AgNP additions, most notably in age 4 and 5 fish. No similar changes in prey availability or growth were observed in Northern Pike from the reference lake. Body condition did not change in Northern Pike collected from either Lake 222 or Lake 239. Our results indicate that declines in the growth of Northern Pike chronically exposed to AgNP likely resulted from reduced prey availability but direct sublethal effects from AgNP exposure could also have been a factor. The persistence of reduced growth in Northern Pike two years after the cessation of AgNP additions highlight the potential legacy impacts of this contaminant once released into aquatic ecosystems.

Effects of sulfidation of silver nanoparticles on the Ag uptake kinetics in Brassica rapa plants

Land application of sewage sludge containing increasing levels of silver nanoparticles (AgNPs) raises concerns about the risk for plant exposure. This study compared the uptake kinetics and distribution of Ag in Brassica rapa seedlings grown in Lufa 2.2 natural soil spiked with 20 nm Ag₂S NPs, with those from 3 to 8 nm AgNPs, 50 nm AgNPs and AgNO₃ exposures (10 mg Ag/kg dry soil). A two-compartment model was used to describe the uptake kinetics of Ag in plants, distinguishing two stages: stage I with increasing Ag uptake followed by stage II with decreasing Ag uptake. The concentration of Ag in roots from Ag₂S NPs was about 14 and 10 times lower than for the other AgNPs and AgNO₃ exposures, respectively, at the end of stage I, with root translocation rate constants being higher for Ag₂S NPs. In stage II, Ag uptake occurred only for the 50 nm AgNPs. The distribution of Ag in B. rapa exposed to pristine, ionic and sulfidized AgNPs differed at the end of exposure. This study shows that Ag uptake and distribution in plants depends on the Ag form in soil, highlighting the importance of studying the environmentally relevant chemical species in NPs risk assessment.

Environmental Risk Assessment of Silver Nanoparticles in Aquatic Ecosystems Using Fuzzy Logic

The rapid development of nanotechnology has stimulated the use of silver nanoparticles (AgNPs) in various fields that leads to their presence in different ecosystem compartments, in particular aquatic ecosystems. Several studies have shown that a variety of living organisms are affected by AgNPs. Therefore, a methodology to assess the risk of AgNPs for aquatic ecosystems was developed. The methodology is based on fuzzy logic, a proven method for dealing with variables with an associated uncertainty, as is the case with many variables related to AgNPs. After a careful literature search, a selection of relevant variables was carried out and the fuzzy model was designed. From inputs such as AgNPs’ size, shape, and coating, it is possible to determine their level of toxicity which, together with their level of concentration, are sufficient to create a risk assessment. Two case studies to assess this methodology are presented, one involving continuous effluent from a wastewater treatment plant and the second involving an accidental spill. The results showed that the accidental spills have a higher risk than WWTP release, with the combination of Plates–BPEI being the most toxic one. This approach can be adapted to different situations and types of nanoparticles, making it highly useful for both stakeholders and decision makers.

Removal of Extended-Spectrum Beta-Lactamase-Producing Escherichia coli, ST98, in Water for Human Consumption by Black Ceramic Water Filters in Low-Income Ecuadorian Highlands

Fecal contamination in natural water sources is a common problem in low-income countries. Several health risks are associated with unprotected water sources, such as gastrointestinal infections caused by parasites, viruses, and bacteria. Moreover, antibiotic-resistant bacteria in water sources have become an increasing problem worldwide. This study aimed to evaluate the bacterial pathogens present in water within a rural context in Ecuador, along with the efficiency of black ceramic water filters (BCWFs) as a sustainable household water treatment. We monitored five natural water sources that were used for human consumption in the highlands of Ecuador and analyzed the total coliforms and E. coli before and after BCWF installation. The results indicated a variable bacterial contamination (29–300 colony-forming units/100mL) in all unfiltered samples, and they were considered as high risk for human consumption, but after filtration, no bacteria were present. Moreover, extended-spectrum beta-lactamase-producing E. coli with bla_TEM, bla_CTX-M9, and bla_CTX-M1 genes, and two E. coli classified in the clonal complex ST10 (ST98) were detected in two of the locations sampled; these strains can severely impact public health. The clonal complex ST10, found in the E. coli isolates, possesses the potential to spread bacteria-resistant genes to humans and animals. The results of the use of BCWFs, however, argue for the filters’ potential impact within those contexts, as the BCWFs completely removed even antibiotic-resistant contaminants from the water.

Biotic Process Dominated the Uptake and Transformation of Ag+ by Shewanella oneidensis MR-1

Silver ions (Ag⁺) directly emitted from industrial sources or released from manufactured Ag nanoparticles (AgNPs) in biosolid-amended soils have raised concern about the risk to ecosystems. However, our knowledge of Ag⁺ toxicity, internalization, and transformation mechanisms to bacteria is still insufficient. Here, we combine the advanced technologies of hyperspectral imaging (HSI) and single-particle inductively coupled plasma mass spectrometry to visualize the potential formed AgNPs inside the bacteria and evaluate the contributions of biological and non-biological processes in the uptake and transformation of Ag⁺ by Shewanella oneidensis MR-1. The results showed a dose-dependent toxicity of Ag⁺ to S. oneidensis MR-1 in the ferrihydrite bioreduction process, which was primarily induced by the actively internalized Ag. Moreover, both HSI and cross-section high-resolution transmission electron microscopy results confirmed that Ag inside the bacteria existed in the form of particulate. The Ag mass distribution in and around live and inactivated cells demonstrated that the uptake and transformation of Ag⁺ by S. oneidensis MR-1 were mainly via biological process. The bioaccumulation of Ag⁺ may be lethal to bacteria. A better understanding of the uptake and transformation of Ag⁺ in bacteria is central to predict and monitor the key factors that control Ag partitioning dynamics at the biointerface, which is critical to develop practical risk assessment and mitigation strategies.

Occurrence and size distribution of silver nanoparticles in wastewater effluents from various treatment processes in Canada

The occurrence of silver (Ag) in urban effluents is partly associated with the increasing use of silver nanoparticles (Ag NPs) as an antiseptic agent in various consumer products. Distinction among Ag forms must be taken into account in the assessment of exposure and toxicological effects to aquatic organisms. Wastewater treatment processes effectively remove Ag particles and colloids (mostly > 95%), but this still leaves notable concentrations (in order of ng/L) escaping to effluent-receiving waters. Total suspended Ag concentrations in various studied effluents ranged from 0.1 to 6 ng/L. The purpose of this study was then to measure and characterize Ag NPs in urban effluents for their concentrations and size distribution using the single particle ICP-MS technique (SP-ICP-MS). Wastewater influents and effluents from various treatment plants-from aerated lagoons to advanced treatment technology-were collected for three sampling days. Our results showed the presence of Ag NP in all samples with concentrations reaching 0.5 ng/L on a mass basis. However, on a particle number basis, Ag NP concentrations (expressed in particle/mL) in the 20-34-nm fraction (up to 3400 particles/mL) were much more abundant (> 700%) than in the > 35-nm larger fraction. The proportion of Ag at the nanoscale (1-100 nm) represents less than 8% of the total suspended Ag for all effluent samples, regardless of their origins. A significant correlation (linear regression: r² > 0.7) was observed between Ag NP and total suspended Ag concentrations in investigated effluents. Because Ag nanotoxicity is size dependent, the determination of size distribution and exposure concentration on a particle number basis is urgently needed for risk assessment of this class of nanoparticles.

Developing understanding of the fate and behaviour of silver in fresh waters and waste waters

The Windermere Humic Aqueous Model (WHAM) is often used for risk assessment of metals; WHAM can be used to estimate the potential bioavailability of dissolved metals, where metals complexed to dissolved organic matter (DOM) are expected to be less toxic than ionic forms. Silver is a potential metal of concern but WHAM has not been rigorously tested against experimental measurements. This study compares WHAM predictions to measured ionic silver during fixed pH (4, 8 or 10) argentometric titrations of DOM from diverse origins. There were almost two orders of magnitude variation in free silver between sources but, within model uncertainty, WHAM captured this variability. This agreement, between measurements and models, suggests that WHAM is an appropriate tool for silver risk assessment in surface receiving waters when DOM is predominantly in the form of humic/fulvic acids. In sewage samples WHAM dramatically underestimated silver binding by approximately 3 orders of magnitude. Simulations with additional specific strong silver binding sulphide-like binding sites could explain Ag binding at low loadings, but not at higher loadings. This suggests the presence of additional intermediate strength binding sites. These additional ligands would represent components of the raw sewage largely absent in natural waters unimpacted by sewage effluents. A revised empirical model was proposed to account for these sewage-specific binding sites. Further, it is suspected that as sewage organic matter is degraded, either by natural attenuation or by engineered treatment, that sewage organic matter will degrade to a form more readily modelled by WHAM; i.e., humic-like substances. These ageing experiments were performed starting from raw sewage, and the material did in fact become more humic-like, but even after 30 days of aerobic incubation still showed greater Ag⁺ binding than WHAM predictions. In these incubation experiments it was found that silver (up to 1000 μg/L) had minimal impact on ammonia oxidation kinetics.

Removal of silver nanoparticles in aqueous solution by activated sludge: Mechanism and characteristics

The increasing production and use of silver nanoparticles (AgNPs) have attracted more and more attention due to their environmental and health risks. Municipal sewage biological treatment unit has been playing an important role in the removal of AgNPs. This study investigated the mechanism and characteristics of AgNPs and their removal from aqueous solution by activated sludge. Results from Scanning Electron Microscope and Energy Dispersive Spectrometer (SEM/EDS) showed that mixed AgNPs were immobilized by activated sludge. It was shown by X-ray photoelectron spectroscopy (XPS) that the fixed AgNPs had an oxidation state of +1. It was inferred by fourier transform infra-red (FTIR) spectra that AgNPs were adsorbed by activated sludge via binding with its primary amino (R-NH₂) radical groups on the surface. These results revealed that the major mechanism for the removal of AgNPs by activated sludge was adsorption. The experiment data were in agreement with the Langmuir and Redlich-Peterson isotherms. The maximum adsorption capacity ranged from 12-32 mg g^-1 at temperatures of 10-30 °C. Thermodynamic experiment showed that the adsorption of AgNPs by activated sludge was a spontaneous and endothermic reaction. The adsorption kinetics data were in good agreement with the pseudo-second-order model. The factor results indicated that the adsorption of AgNPs onto activated sludge was influenced by electrostatic repulsion, agglomeration, and the process of oxidation and sulfurization.

Occurrence and leaching of silver in municipal sewage sludge in China

Sewage treatment plants effectively remove silver (Ag) from sewage. Sewage sludge can therefore be important Ag sinks, polluting the environment with this element. In this work, we report a nation-wide survey on the Ag content of sewage sludge in China (0.23-19.02 mg kg^-1, average 2.72 mg kg^-1). Furthermore, we identify that sludge disposal represents an important Ag pollution source (84.48 tons in 2016) for the environment by estimating the national and provincial inventories of sludge-borne Ag in China. Also the positive correlations between the per capita gross domestic product (GDP)/provincial GDP and the content/mass loadings of Ag highlighted the impact of human activities on Ag pollution. In different samples, strong complexation of thiosulfate contributed to the highest leaching concentration (95.00-438.15 μg kg^-1) and ratio (1.9-8.8%) of Ag, emphasizing the necessity of a long-term risk assessment for landfill and land application of sludge.

Mercury/silver resistance genes and their association with antibiotic resistance genes and microbial community in a municipal wastewater treatment plant

Municipal wastewater treatment plants (WWTPs) are an important reservoir for heavy metal (e.g., Hg and Ag) resistance genes and antibiotic resistance genes (ARGs). However, current knowledge on Hg/Ag resistance genes and their association with ARGs in WWTPs remains largely unknown. In this study, the fates of five Hg/Ag resistance genes (merB, merD, merR, silE, and silR), five ARGs (sulI, sulII, tetO, tetQ, tetW), and class 1 integrase (intI1) in a WWTP were investigated. Results show that the absolute abundances of all target genes were greatly reduced through the treatment systems. The dynamics of merB, merD and silE were significantly correlated with tetW and sulII. Based on network analysis, Hg/Ag resistance genes might share the same microbial hosts with tetQ and tetW, implying the potential importance of Hg/Ag in ARGs evolution and spread. These findings advanced our understanding of the occurrence of Hg/Ag resistance genes and ARGs in WWTPs.

Transformation of AgCl Particles under Conditions Typical of Natural Waters: Implications for Oxidant Generation

The engineered silver nanoparticles (AgNPs) used in consumer products are ultimately released to the environment either as Ag(0), silver sulfide (Ag₂S(s)), silver chloride (AgCl(s)), and/or dissolved Ag(I) complexes. Of these, AgCl(s) and Ag₂S(s) exhibit semiconducting properties and hence may have significant implications to oxidant generation and subsequent redox transformations in natural waters. In this work, we investigate the transformation and photoreactivity of AgCl(s) under simulated natural water conditions with the photoreactivity probed by measuring the oxidation of formate (HCOO^-), a simple compound with a well-defined oxidation pathway. Our results show that AgCl(s) undergoes rapid dissolution in the presence of chloride concentrations representative of seawater (ca. 0.5 M NaCl) forming dissolved Ag(I) complexes but is stable in fresh waters and slightly brackish waters (≤200 mM NaCl). We further show that under these lower salinity conditions in which AgCl(s) is stable, pH has a significant impact on the reactivity of semiconducting AgCl(s). The photoreactivity (measured as initial HCOO^- oxidation rate) of AgCl(s) is relatively constant at pH 4.0 for periods of 24 h or more; however, it decreases rapidly under alkaline conditions. The rapid transformation (or "aging") of AgCl(s) under alkaline conditions suggests that AgCl(s), potentially transported through wastewater effluent to fresh or brackish water environments, may not have a significant impact in such environments. In comparison, in situ formed AgCl(s), potentially formed as a result of the oxidation of high concentrations (≥60 μg Ag·L^-1) of Ag(0) and/or Ag₂S(s), may have significant implications to oxidant generation in natural waters. Our results further show that rapid cycling of Ag between the 0 and +I redox states in sunlit surface waters as a result of the presence of AgNP oxidants (such as H₂O₂ and organic radicals) will further enhance the rate and extent of oxidant generation by AgCl(s).

Occurrence, characterisation and fate of (nano)particulate Ti and Ag in two Norwegian wastewater treatment plants

Due to their widespread application in consumer products, elemental titanium (e.g., titanium dioxide, TiO₂) and silver (Ag), also in nanoparticulate form, are increasingly released from households and industrial facilities to urban wastewater treatment plants (WWTPs). A seven-day sampling campaign was conducted in two full-scale WWTPs in Trondheim (Norway) employing only primary treatment. We assessed the occurrence and elimination of Ti and Ag, and conducted size-based fractionation using sequential filtration of influent samples to separate particulate, colloidal and dissolved fractions. Eight-hour composite influent samples were collected to assess diurnal variations in total Ti and Ag influx. Measured influent Ti concentrations (up to 290 μg L^-1) were significantly higher than Ag (<0.15-2.1 μg L^-1), being mostly associated with suspended solids (>0.7 μm). Removal efficiencies ≥70% were observed for both elements, requiring for one WWTP to account for the high Ti content (∼2 g L^-1) in the flocculant. Nano- and micron-sized Ti particles were observed with scanning transmission electron microscopy (STEM) in influent, effluent and biosolids, while Ag nanoparticles were detected in biosolids only. Diurnal profiles of influent Ti were correlated to flow and pollutant concentration patterns (especially total suspended solids), with peaks during the morning and/or evening and minima at night, indicating household discharges as predominant source. Irregular profiles were exhibited by influent Ag, with periodic concentration spikes suggesting short-term discharges from one or few point sources (e.g., industry). Influent Ti and Ag dynamics were reproduced using a disturbance scenario generator model, and we estimated per capita loads of Ti (42-45 mg cap^-1 d^-1) and Ag (0.11 mg cap^-1 d^-1) from households as well as additional Ag load (14-22 g d^-1) from point discharge. This is the first study to experimentally and mathematically describe short-term release dynamics and dry-weather sources of emissions of Ti and Ag in municipal WWTPs and receiving environments.

Ecotoxicological Effects of Transformed Silver and Titanium Dioxide Nanoparticles in the Effluent from a Lab-Scale Wastewater Treatment System

In this study, a lab-scale wastewater treatment plant (WWTP), simulating biological treatment, received 10 μg/L Ag and 100 μg/L TiO₂ nanoparticles (NPs) for 5 weeks. NP partitioning was evaluated by size fractionation (>0.7 μm, 0.1-0.7 μm, 3 kDa-0.1 μm, < 3 kDa) using inductively coupled plasma mass spectrometry (ICP-MS), single particle ICP-MS and transmission electron microscopy. The ecotoxicological effects of the transformed NPs in the effluent were assessed using a battery of marine and freshwater bioassays (algae and crustaceans) and an in vitro gill cell line model (RTgill-W1). TiO₂ aggregates were detected in the effluent, whereas Ag NPs (0.1-0.22 μg/L) were associated with S, Cu, Zn. Fractionation showed that >80% of Ag and Ti were associated with the effluent solids. Increased toxicity was observed during weeks 2-3 and the effects were species-dependent; with marine epibenthic copepods and algae being the most sensitive. Increased reactive oxygen species formation was observed in vitro followed by an increase in epithelial permeability. The effluent affected the gill epithelium integrity in vitro and impacted defense pathways (upregulation of multixenobiotic resistance genes). To our knowledge, this is the first study to combine a lab-scale activated sludge WWTP with extensive characterization techniques and ecotoxicological assays to study the effects of transformed NPs in the effluent.

Risk of Silver Transfer from Soil to the Food Chain Is Low after Long-Term (20 Years) Field Applications of Sewage Sludge

The increasingly widespread usage of silver (Ag) nanoparticles has raised concerns regarding their environmental risk. The behavior of Ag and its transfer risk to the food chain were investigated using a long-term field experiment that commenced in 1942 in which Ag-containing sewage sludge was repeatedly applied to the soil (25 applications during 20 years). The speciation of the Ag in both the sludge and the soils retrieved from the long-term experimental archive was examined using synchrotron-based X-ray absorption spectroscopy, and extractable Ag concentrations from soils were determined using 0.01 M Ca(NO₃)₂ and 0.005 M DTPA. The total Ag in the sludge during the time period of 1942-1961 ranged from 155 to 463 mg kg^-1. These values are 1-2 orders of magnitude higher than those in currently produced sludge (ca. 0.5-20 mg kg^-1). Long-term repeated applications of these sludges resulted in an increase of Ag in soils from 1.9 mg kg^-1 in the control to up to 51 mg kg^-1. The majority (>80%) of the Ag in both the sludge and the sludge-treated soils was present as insoluble Ag₂S, thereby markedly reducing the bioavailability of this Ag. Concentrations of Ag in the archived crop samples were generally <0.70 mg kg^-1 in edible tissues, much less than those in diets that may cause an adverse effects in animals and humans (>100 mg kg^-1). These data indicate that the transfer of Ag (derived from both traditional Ag industry and current nano Ag industry) to the terrestrial food chain is limited.

Comparison of hyperspectral classification methods for the analysis of cerium oxide nanoparticles in histological and aqueous samples

Hyperspectral imaging (HSI) and classification are established methods that are being applied in new ways to the analysis of nanoscale materials in a variety of matrices. Typically, enhanced darkfield microscopy (EDFM)-based HSI data (also known as image datacubes) are collected in the wavelength range of 400-1000 nm for each pixel in a datacube. Utilising different spectral library (SL) creation methods, spectra from pixels in the datacube corresponding to known materials can be collected into reference spectral libraries (RSLs), which can be used to classify materials in datacubes of experimental samples using existing classification algorithms. In this study, EDFM-HSI was used to visualise and analyse industrial cerium oxide (CeO₂ ; ceria) nanoparticles (NPs) in rat lung tissues and in aqueous suspension. Rats were exposed to ceria NPs via inhalation, mimicking potential real-world occupational exposures. The lung tissues were histologically prepared: some tissues were stained with hematoxylin and eosin (H&E) and some were left unstained. The goal of this study was to determine how HSI and classification results for ceria NPs were influenced by (1) the use of different RSL creation and classification methods and (2) the application of those methods to samples in different matrices (stained tissue, unstained tissue, or aqueous solution). Three different RSL creation methods - particle filtering (PF), manual selection, and spectral hourglass wizard (SHW) - were utilised to create the RSLs of known materials in unstained and stained tissue, and aqueous suspensions, which were then used to classify the NPs in the different matrices. Two classification algorithms - spectral angle mapper (SAM) and spectral feature fitting (SFF) - were utilised to determine the presence or absence of ceria NPs in each sample. The results from the classification algorithms were compared to determine how each influenced the classification results for samples in different matrices. The results showed that sample matrix and sample preparation significantly influenced the NP classification thresholds in the complex matrices. Moreover, considerable differences were observed in the classification results when utilising each RSL creation and classification method for each type of sample. Results from this study illustrate the importance of appropriately selecting HSI algorithms based on specific material and matrix characteristics in order to obtain optimal classification results. As HSI is increasingly utilised for NP characterisation for clinical, environmental and health and safety applications, this investigation is important for further refining HSI protocols while ensuring appropriate data collection and analysis.

Delivering environmental benefit from the use of Environmental Quality Standards: why we need to focus on implementation

The UK has adopted a broader approach to the introduction of Environmental Quality Standard (EQS) for the aquatic environment than many other jurisdictions around the world, with a greater focus on the implementation of scientifically derived standards. This follows the publication of a report by the Royal Commission on Environmental Pollution in 1998 which drew attention to the need to recognise that whilst an EQS is often just viewed as a numerical value, it also has other important characteristics that need to be recognised if it is to be a practical and effective regulatory tool. One of the aspects that has not always been recognised was that of implementation assessment, i.e. the steps needed to ensure that a standard actually delivers environmental benefit or improvements. In many jurisdictions, there is considerable technical and sometimes political emphasis on the numerical value of the EQS (e.g. the critical concentration in an environmental matrix like water), including the method of derivation, the scrutiny of the reliability and relevance of the ecotoxicity test data and extensive deliberations of unquantified uncertainties in relation to the choice of assessment factor. The regulatory value of an EQS only comes through a comparison against a measured environmental concentration, yet only relatively limited regulatory effort has historically been expended on this component of the classic environmental risk assessment paradigm. For example, there needs to be an acceptable (i.e. small) uncertainty in the EQS, an appropriate analytical method and detection limit in the correct matrix, a method to deliver a comparison with the EQS and a robust statistical method to draw unbiased conclusions about environmental risk. In addition, we argue that there is a case for checking the consequences of introducing a standard against field data, wherever possible. This validation of the EQS rarely happens currently. We explain what implementation assessment is and why it is needed. We give examples of how implementation assessment can be integrated with EQS derivation and also present examples of what happens when the focus is only upon the derivation of a numerical value. It is clear from this evidence that advances in derivation methods need to be coupled with practical solutions of implementation if we are to realise environmental benefit from an EQS in a cost-effective manner.

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.

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.

Governing factors affecting the impacts of silver nanoparticles on wastewater treatment

Silver nanoparticles (nanosilver or AgNPs) enter municipal wastewater from various sources, raising concerns about their potential adverse effects on wastewater treatment processes. We argue that the biological effects of silver nanoparticles at environmentally realistic concentrations (μgL^-1 or lower) on the performance of a full-scale municipal water resource recovery facility (WRRF) are minimal. Reactor configuration is a critical factor that reduces or even mutes the toxicity of silver nanoparticles towards wastewater microbes in a full-scale WRRF. Municipal sewage collection networks transform silver nanoparticles into silver(I)-complexes/precipitates with low ecotoxicity, and preliminary/primary treatment processes in front of biological treatment utilities partially remove silver nanoparticles to sludge. Microbial functional redundancy and microbial adaptability to silver nanoparticles also greatly alleviate the adverse effects of silver nanoparticles on the performance of a full-scale WRRF. Silver nanoparticles in a lab-scale bioreactor without a sewage collection system and/or a preliminary/primary treatment process, in contrast to being in a full scale system, may deteriorate the reactor performance at relatively high concentrations (e.g., mgL^-1 levels or higher). However, in many cases, silver nanoparticles have minimal impacts on lab-scale bioreactors, such as sequencing batch bioreactors (SBRs), especially when at relatively low concentrations (e.g., less than 1mgL^-1). The susceptibility of wastewater microbes to silver nanoparticles is species-specific. In general, silver nanoparticles have higher toxicity towards nitrifying bacteria than heterotrophic bacteria.

Elemental profiles of freshwater mussels treated with silver nanoparticles: A metallomic approach

Nanoparticles released into the environment could pose a risk to resident organisms that feed on suspended particles in aquatic ecosystems. The purpose of this study was to examine the effects of silver nanoparticles (nanoAg) of different sizes in freshwater mussels using a multi-elemental (metallomic) approach in order to determine signature effects of nanoparticulate and ionic Ag. Mussels were exposed to three concentrations (0.8, 4 and 20μg/L) of 20-nm and 80-nm nanoAg and AgNO3 for 48h at 15°C. After the exposure period, mussels were placed in clean, aerated water for a depuration step and analyzed for the following total elements in gill, digestive gland and gonad tissues: Al, Ag, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Pb, Na, Ni, Se, Sr, Th, U, V and Zn. Metallothioneins (MT; digestive gland only) and lipid peroxidation (LPO) were also determined in gills, digestive glands and gonads. The 20-nm-diameter nanoAg was detected in all three tissues at 20μg/L, while the 80-nm nanoAg was detected more strongly in the digestive gland. Ionic Ag was found at higher levels in gills than in other tissues. Correlation analysis revealed that gonad Ag levels were significantly correlated with Al (r=0.28), V (r=0.28), Cr (r=0.31), Co (r=0.32), Se (r=0.34) and MT levels (r=0.28). Indeed, the MT levels in the digestive gland were significantly increased by 20-nm nanoAg (20μg/L) and 80-nm nanoAg (4μg/L) and AgNO3 (<0.8μg/L). LPO was observed in gills, digestive glands and even gonads for all Ag forms. Discriminant function analysis revealed that all forms of Ag differed from each other and from unexposed mussels, where ionic Ag was more closely related to the 80-nm-diameter nanoAg. Factorial analysis revealed that Ba, Ca, Co, Mn, Sr, U and Zn had consistently high factorial weights in all tissues; that explained 80% of the total variance. Moreover, the following elements showed strong correlations (r>0.7) with each other: Sr, Ba, Zn, Ca, Mg Cr, Mn and U. Comparisons of these elements with other elements showing low or no correlations (e.g., transition elements) revealed that these elements had significantly lower standard reduction potential and electronegativity, suggesting that stronger reducing elements were most influenced by the oxidizing effects of nanoAg and ionic Ag in tissues. Indeed, tissues with oxidative stress (LPO) had decreased levels for most of these reducing elements. We conclude that exposure to Ag nanoparticles produces a characteristic change in the elemental composition of gills, digestive gland and gonad tissues in freshwater mussels. Elements most responsive to oxidative stress were more influenced by both nanoAg and ionic Ag. Sr and Ba were readily decreased by Ag and appeared to respond more sensitively to nanoAg than to ionic Ag. The metallomic approach could contribute in the understanding of fundamental mode of action of nanoparticles in mussels.

Sediment chemistry and toxicity in Barnegat Bay, New Jersey: Pre- and post-Hurricane Sandy, 2012-13

Hurricane Sandy made landfall in Barnegat Bay, October, 29, 2012, damaging shorelines and infrastructure. Estuarine sediment chemistry and toxicity were investigated before and after to evaluate potential environmental health impacts and to establish post-event baseline sediment-quality conditions. Trace element concentrations increased throughout Barnegat Bay up to two orders of magnitude, especially north of Barnegat Inlet, consistent with northward redistribution of silt. Loss of organic compounds, clay, and organic carbon is consistent with sediment winnowing and transport through the inlets and sediment transport modeling results. The number of sites exceeding sediment quality guidance levels for trace elements tripled post-Sandy. Sediment toxicity post-Sandy was mostly unaffected relative to pre-Sandy conditions, but at the site with the greatest relative increase for trace elements, survival rate of the test amphipod decreased (indicating degradation). This study would not have been possible without comprehensive baseline data enabling the evaluation of storm-derived changes in sediment quality.

Probabilistic modelling of prospective environmental concentrations of gold nanoparticles from medical applications as a basis for risk assessment

BACKGROUND

The use of gold nanoparticles (Au-NP) based medical applications is rising due to their unique physical and chemical properties. Diagnostic devices based on Au-NP are already available in the market or are in clinical trials and Au-NP based therapeutics and theranostics (combined diagnostic and treatment modality) are in the research and development phase. Currently, no information on Au-NP consumption, material flows to and concentrations in the environment are available. Therefore, we estimated prospective maximal consumption of Au-NP from medical applications in the UK and US. We then modelled the Au-NP flows post-use and predicted their environmental concentrations. Furthermore, we assessed the environment risks of Au-NP by comparing the predicted environmental concentrations (PECs) with ecological threshold (PNEC) values.

RESULTS

The mean annual estimated consumption of Au-NP from medical applications is 540 kg for the UK and 2700 kg for the US. Among the modelled concentrations of Au-NP in environmental compartments, the mean annual PEC of Au-NP in sludge for both the UK and US was estimated at 124 and 145 μg kg(-1), respectively. The mean PEC in surface water was estimated at 468 and 4.7 pg L(-1), respectively for the UK and US. The NOEC value for the water compartment ranged from 0.12 up to 26,800 μg L(-1), with most values in the range of 1000 μg L(-1).

CONCLUSION

The results using the current set of data indicate that the environmental risk from Au-NP used in nanomedicine in surface waters and from agricultural use of biosolids is minimal in the near future, especially because we have used a worst-case use assessment. More Au-NP toxicity studies are needed for the soil compartment.

Silver nanoparticle exposure induced mitochondrial stress, caspase-3 activation and cell death: amelioration by sodium selenite

Silver nanoparticles (AgNP), one of the most commonly used engineered nanomaterial for biomedical and industrial applications, has shown a toxic potential to our ecosystems and humans. In this study, murine hippocampal neuronal HT22 cells were used to delineate subcellular responses and mechanisms to AgNP by assessing the response levels of caspase-3, mitochondrial oxygen consumption, reactive oxygen species (ROS), and mitochondrial membrane potential in addition to cell viability testing. Selenium, an essential trace element that has been known to carry protecting property from heavy metals, was tested for its ameliorating potential in the cells exposed to AgNP. Results showed that AgNP reduced cell viability. The toxicity was associated with mitochondrial membrane depolarization, increased accumulation of ROS, elevated mitochondrial oxygen consumption, and caspase-3 activation. Treatment with sodium selenite reduced cell death, stabilized mitochondrial membrane potential and oxygen consumption rate, and prevented accumulation of ROS and activation of caspase-3. It is concluded that AgNP induces mitochondrial stress and treatment with selenite is capable of preventing the adverse effects of AgNP on the mitochondria.

Removal of colloidal biogenic selenium from wastewater

Biogenic selenium, Se(0), has colloidal properties and thus poses solid-liquid separation problems, such as poor settling and membrane fouling. The separation of Se(0) from the bulk liquid was assessed by centrifugation, filtration, and coagulation-flocculation. Se(0) particles produced by an anaerobic granular sludge are normally distributed, ranging from 50 nm to 250 nm, with an average size of 166±29 nm and a polydispersity index of 0.18. Due to its nanosize range and protein coating-associated negative zeta potential (-15 mV to -23 mV) between pH 2 and 12, biogenic Se(0) exhibits colloidal properties, hampering its removal from suspension. Centrifugation at different centrifugal speeds achieved 22±3% (1500 rpm), 73±2% (3000 rpm) and 91±2% (4500 rpm) removal. Separation by filtration through 0.45 μm filters resulted in 87±1% Se(0) removal. Ferric chloride and aluminum sulfate were used as coagulants in coagulation-flocculation experiments. Aluminum sulfate achieved the highest turbidity removal (92±2%) at a dose of 10(-3) M, whereas ferric chloride achieved a maximum turbidity removal efficiency of only 43±4% at 2.7×10(-4) M. Charge repression plays a minor role in particle neutralization. The sediment volume resulting from Al2(SO3)4 treatment is three times larger than that produced by FeCl3.

Nano silver and nano zinc-oxide in surface waters - exposure estimation for Europe at high spatial and temporal resolution

Nano silver and nano zinc-oxide monthly concentrations in surface waters across Europe were modeled at ~6 x 9 km spatial resolution. Nano-particle loadings from households to rivers were simulated considering household connectivity to sewerage, sewage treatment efficiency, the spatial distribution of sewage treatment plants, and their associated populations. These loadings were used to model temporally varying nano-particle concentrations in rivers, lakes and wetlands by considering dilution, downstream transport, water evaporation, water abstraction, and nano-particle sedimentation. Temporal variability in concentrations caused by weather variation was simulated using monthly weather data for a representative 31-year period. Modeled concentrations represent current levels of nano-particle production.Two scenarios were modeled. In the most likely scenario, half the river stretches had long-term average concentrations exceeding 0.002 ng L(-1) nano silver and 1.5 ng L(-1) nano zinc oxide. In 10% of the river stretches, these concentrations exceeded 0.18 ng L(-1) and 150 ng L(-1), respectively. Predicted concentrations were usually highest in July.