Tracking Silver Nanoparticles
Ultra-Trace Analysis of Silver Nanoparticles in the Environment in the Parts Per Trillion-Range
- Fig. 1: Scheme of the Cloud-Point-Extraction for species selective separation and enrichment of AgNPs from environmental samples.
- Fig. 2: Characterization of AgNPs in WWTP effluent: (a) AgNP concentrations in effluent of two WWTPs over the seasons; LOD = limit of detection. (b) and (c): Size distribution of AgNPs in wastewater influent (b) and effluent (c) measured by spICPMS . Environment
- Fig. 3: Occurrence of AgNPs in surface waters in South Germany. Source of map is Bayerischer Rundfunk (http://www.br.de/themen/bayern/isar-dossier-karte104.html). (a) AgNP levels of surface waters; image is obtained through ArcGIS 9. (b) Effect of WWTP effluent on the AgNP level in surface water samples of the river Isar and several lakes in southern Germany.
With his forward-looking speech “There’s Plenty of Room at the Bottom” at the California Institute of Technology in the 50ies, the physicist and Nobel Prize winner Richard Feynman laid the foundation stone for the nanotechnology that developed many years later. Today, it is one of the fastest growing technologies in our time and has already entered our everyday life. Titanium dioxide nanoparticles in sunscreen protect the skin against harmful UV radiation, nanoscale noble metals like platinum, palladium, and rhodium are used in exhaust gas catalysts, and silver nanoparticles (AgNPs) have a variety of applications due to their antimicrobial effects. Treating textiles with AgNPs prevents them from smelling. Additionally, AgNPs are used in fields of cosmetics and antibacterial coatings. The use of these products may lead to a release of AgNPs, which reach wastewater treatment plants (WWTPs), and finally natural waters.
So far, detecting AgNPs in environmental samples, like WWTP influents and effluents, river and lake water, or sewage sludge, was very challenging for scientist. The concentration of nanoparticulate silver in the samples is often that low that they are not measurable using conventional analytical methods. Furthermore, AgNPs occur in presence of ionic silver species and are embedded into very complex matrices. The research group Analytical Chemistry around Prof. Dr. Michael Schuster of the Technical University of Munich was aware of this problem and able to solve it: Using a special enrichment technique, the so called Cloud-Point-Extraction (fig. 1), AgNPs can be species selectively separated from environmental samples and enriched by a factor of 100 .
This method is based on a micellar mediated separation of AgNPs from aqueous samples. The aqueous sample is mixed with a special surfactant and heated over a certain temperature specific to this surfactant. The surfactant molecules form micelles and thereby enclose AgNPs in the micelle’s hydrophobic inside. From a macroscopic perspective, the solutions become clouded because the so-called cloud-point temperature is reached.
Ionic silver species are retained in the aqueous phase, which works almost quantitatively using special ligands. After centrifugation, the aqueous and surfactant rich phase are separated and the AgNPs can finally be found in an unmodified but strongly enriched form. Since AgNPs undergo multiple modifications in environmental samples, the influence of different NP surface modifications on the extraction efficiency during the CPE process was examined. Adverse effects of environmental relevant surface coatings, like chloride, sulfide, starch, amino acids (Cys, Lys), citrate, and additional modifications, on the extraction efficiency could be neglected. Typical matrix constituents in environmental samples, e.g. ammonium, chloride, phosphate, nitrate, inorganic colloids, and natural organic matter (NOM), pose no negative effects on the particle’s extraction efficiency .
Electrothermal atomic absorption spectroscopy (ETAAS) is used to quantify silver with a limit of detection (LOD) of 0.2 ng/L in the surfactant droplet obtained during the enrichment process . By coupling CPE with single particle mass spectrometry (sp-ICP-MS), it is possible to determine particle size distributions additionally to the quantification of nanoparticulate silver.
Application on Real Samples 
Wastewater Treatment Plants (WWTPs)
So far, existing data addressing AgNPs in environmental samples is based on model calculations or laboratory and technical experiments, respectively, using high AgNP concentrations. The presented technique enabled us to measure real environmental samples and collect reliable data on AgNPs in the environment for the first time. Within a research project financed by the Bavarian State Ministry of the Environment the actual way of AgNPs, passing WWTPs with different wastewater treatment processes, to natural water bodies was tracked. As an example for running waters, the river Isar – a 292 km long river in Southern Germany originating in the alps, passing Munich (1.5 million inhabitants), and finally reaching the river Danube near Deggendorf – was examined on the content of AgNPs. Additionally, water samples from several surrounding pre-alpine lakes were collected and their AgNP concentration was also determined. The aim of the project was to examine the influence of WWTP effluents on the occurrence of AgNPs in surface waters.
The wastewater treatment of all examined WWTPs works very well because the sewage sludge retains up to 96 % of the AgNPs. This observation is consistent with experiments using a laboratory WWTP at the Bavarian Environmental Agency . Depending on the season, the influent carries 350 ng/L (winter) to 10 ng/L (summer, after heavy rain) AgNPs into the WWTPs whereby the effluent contains AgNPs up to a maximum of 11 ng/L (fig. 2a). The average particle size varies between 20 nm in the influent (fig. 2b) and 15 nm in the effluent (fig. 2c).
Based on the data of different WWTPs the estimated flux of AgNPs referred to WWTP effluent discharge is 33 kg per year in Germany. In the following, it is therefore analyzed how WWTP effluents influence the occurrence of AgNPs in running waters and lakes.
AgNPs Along the River
Isar and in Pre-Alpine Lakes
For this purpose, surface water samples of the river Isar were taken at randomly chosen sites along the river. Furthermore, water samples were collected exactly at those points where WWTPs dispose their effluent into the Isar. After each of the disposal points an additional sample was taken 1.5 km downstream to observe dissolution effects. All water samples were subjected to CPE and silver was measured using ETAAS. In the upstream areas before Munich (sites 1-9) no AgNPs are detectable but as soon as the first WWTP after Munich discharges effluent into the Isar (site 10) 1.9 ng/L AgNPs are found (fig. 3). Such AgNP load peaks are detectable at every disposal point into the Isar. Due to dissolution effects at the sampling sites downstream the discharge points, the concentration decreases again reaching a constant level until the next WWTP effluent is disposed. Nevertheless, there is a slow progressive increase in the AgNP concentration in the river from the downstream areas to its confluence with the river Danube leading to a final concentration of 2 ng/L AgNPs near the river Danube (site 33). Consequently, AgNPs in the river Isar are clearly anthropogenic but, however, AgNP concentrations remain very low.
AgNPs could also be monitored in several pre-alpine lakes. Here, the concentration of nanoparticulate silver is in a range from 0.5 to 1.3 ng/L. All examined lakes are protected against WWTP effluents in the immediate surroundings by using ring sewer systems but some lake tributaries are WWTP influenced. In summary, all examined lakes in Southern Germany contained very low concentrations of AgNPs.
It is surprising that lakes with and without WWTP influence show similar AgNPs concentrations. It can thus be concluded that AgNPs in natural water bodies are not only of anthropogenic origin. Measurements indicate that there is also a natural background. These NPs are most likely attributable to a natural formation of AgNPs from dissolved silver species.
The project was supported by the Bavarian State Ministry of the Environment, which also takes over the financing of a subsequent project. Additionally, the authors would like to thank the Bavarian Environment Agency for supporting the sp-ICP-MS measurements.
1 Technical University of Munich, Department of Chemistry, Division of Analytical Chemistry, Garching, Germany
Prof. Dr. Michael Schuster
Technical University of Munich
Department of Chemistry
Division of Analytical Chemistry
 G. Hartmann, C. Hutterer, M. Schuster, Ultra-trace determination of silver nanoparticles in water samples using cloud point extraction and ETAAS, Journal of Analytical Atomic Spectrometry, 28, 567-572 (2013), DOI: 10.1039/C3JA30365A
 G. Hartmann, T. Baumgartner, M. Schuster, Influence of Particle Coating and Matrix Constituents on the Cloud Point Extraction Efficiency of Silver Nanoparticles (Ag-NPs) and Application for Monitoring the Formation of Ag-NPs from Ag+, Analytical Chemistry, 86, 790-796 (2014), DOI: 10.1021/ac403289d
 L. Li, M. Stoiber, A. Wimmer, Z. Xu, C. Lindenblatt, B. Helmreich, M. Schuster, To What Extent Can Full-Scale Wastewater Treatment Plant Effluent Influence the Occurrence of Silver-Based Nanoparticles in Surface Waters?, Environmental Science & Technology, (2016), DOI: 10.1021/acs.est.6b00694
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