Silver Nanoparticles In Natural Waters: Is it Only Humans Who Are To Blame?

Nanotechnology is a rapidly-growing technology in our time. Nanoparticles are very small objects with a diameter ranging from 1 to 100 nm, where 1 nm is equal to one-billionth of a meter (10^-9 m). In comparison, nanoparticle size can range between that of simple molecules and complex viruses.

Silver nanoparticles (AgNPs) provide antimicrobial benefits and are thus present in a vast range of consumer products — for instance, fabrics for functional clothing, medicinal applications like dressings and surgical instruments, cosmetics, and food storage systems. Usage and eventual disposal of these products inevitably lead to a release of AgNPs in the environment. Wastewater treatment plants (WWTPs) serve as a kind of basin, collecting AgNPs and distributing them via WWTP effluents within natural waters like rivers and lakes. Although WWTPs retain more than 95% of all AgNPs present in the influents, AgNP traces are present in environmental waters.

Detection, characterization, and quantification of these nanomaterials in river and lake water have been major challenges for analytical chemists so far, as AgNPs are embedded into a complex environmental matrix of real water bodies and coexist with dissolved Ag species at extremely low concentrations of few nanograms (10^-9 gram) per liter. The working group around Prof. Dr. Michael Schuster at the Technical University of Munich (Germany) developed a method to extract, quantify, and characterize AgNPs at environmentally-relevant concentrations (few ng L^-1) in wastewater, rivers, and lake water for the first time.^1-4 Using the so-called Cloud Point Extraction (CPE), AgNPs are species selectively extracted from aqueous samples into a surfactant droplet, simultaneously separated from dissolved silver species and the majority of matrix constituents and enriched with a factor of about 100. Only after this kind of extraction are AgNPs measurable in real water samples via mass spectrometry or atomic absorption spectrometry.

In the first subsequent study, the authors were in search of AgNPs in the river Isar⁵: This river originates in the Austrian Alps, then passes more rural areas in Germany before it reaches the metropolitan area of Munich and finally meets several other smaller cities until its estuary into the river Danube. The river Isar was sampled all over its whole reach of almost 300 km, whereas the study investigated to what extend WWTPs influence the occurrence of AgNPs in natural water bodies. In the upstream parts of the river, no AgNPs could be detected beyond the method’s limit of detection of 0.2 ng L^-1 – in contrast to the downstream parts behind Munich. Here, AgNP concentration reached up to 10 ng L^-1 at sampling sites directly next to a WWTP discharge spot. Even these load peaks are far from concentrations being harmful to health or the environment.

Nevertheless, AgNP concentrations were quickly diluted to a constant level of 1-2 ng L^-1 within the reach of the river. As a consequence, the authors drew the conclusion that AgNPs present in the river Isar are clearly of anthropogenic origin and can be attributed to WWTP discharge. Investigating AgNPs in lakes with and without any anthropogenic influence, the authors came across a curious observation: even lakes without anthropogenic influences contained detectable traces (0.5-1 ng L^-1) of AgNPs. Is there a kind of geogenic background for AgNPs in the environment?

Therefore, the authors conducted another study, focusing on two lakes in Southern Germany⁶: lake Waginger See, a eutrophic lake, and lake Königssee, one of the most oligotrophic lakes available in Germany. Both lakes are protected from anthropogenic influences by ring sewer systems. For the first time, the natural formation of AgNPs could be studied in this field investigation. Both lakes contained comparable amounts of total silver (one-digit ng L^-1 range), presumably attributable to geogenic traces, whereas AgNPs could be only found in the eutrophic lake containing significant amounts of natural organic matter (NOM). At the surface of this lake, approximately 40% of total silver (5.7 ng L^-1) was present in nanoparticulate form.

To substantiate the conclusion that NOM is able to reduce Ag(I) to nanoparticles, additional lab experiments with nature-related Ag(I) concentrations mixed with 5 mg L^-1 NOM – the NOM content of the eutrophic lake – were conducted. The measurements revealed that AgNPs are formed from dissolved species under the influence of NOM. After an incubation time of 24 hours for 50 ng L^-1 Ag(I) and 5 mg L^-1 NOM, 46% of the originally used dissolved silver was transformed into nanoparticles. Particle size (about 20 nm) increased with increasing reaction time, showing that Ostwald ripening even occurs at such low concentrations. In the case of the coexisting sulfide, particle size distributions for the formed AgNPs were smaller and narrower.

The presented studies were financed by the Bavarian Ministry of the Environment and Consumer Protection. They show,\ that AgNPs are measurable in river water at very low concentrations (ng L^-1), attributed mainly to anthropogenic sources like WWTP effluents. Nevertheless, detailed investigations of AgNPs in lakes combined with lab experiments mimicking environmental circumstances revealed that AgNPs are also of geogenic origin and may have been present for a long time, even before mankind purposely used and facilitated nanotechnology.

Based on current measurements, AgNPs in the environment don’t pose any risk to human or environmental health due to their extremely low concentrations. Nevertheless, AgNP concentrations should be steadily monitored in the future, especially with regard to the still-growing importance and increasing production quantities of nanoparticles.

These findings are described in the article entitled New insights into the formation of silver-based nanoparticles under natural and semi-natural conditions, recently published in the journal Water Research. This work was conducted by Andreas Wimmer, Anna Kalinnik, and Michael Schuster from the Technical University of Munich.

References:

1. Hartmann, G.; Baumgartner, T.; Schuster, M., 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 2014, 86 (1), 790-796.
2. Hartmann, G.; Hutterer, C.; Schuster, M., Ultra-trace determination of silver nanoparticles in water samples using cloud point extraction and ETAAS. Journal of Analytical Atomic Spectrometry 2013, 28 (4), 567-572.
3. Hartmann, G.; Schuster, M., Species selective preconcentration and quantification of gold nanoparticles using cloud point extraction and electrothermal atomic absorption spectrometry. Analytica Chimica Acta 2013, 761, 27-33.
4. Li, L.; Hartmann, G.; Döblinger, M.; Schuster, M., Quantification of Nanoscale Silver Particles Removal and Release from Municipal Wastewater Treatment Plants in Germany. Environmental Science & Technology 2013, 47 (13), 7317-7323.
5. Li, L.; Stoiber, M.; Wimmer, A.; Xu, Z.; Lindenblatt, C.; Helmreich, B.; Schuster, M., To What Extent Can Full-Scale Wastewater Treatment Plant Effluent Influence the Occurrence of Silver-Based Nanoparticles in Surface Waters? Environmental Science & Technology 2016, 50 (12), 6327-6333.
6. Wimmer, A.; Kalinnik, A.; Schuster, M., New insights into the formation of silver-based nanoparticles under natural and semi-natural conditions. Water Research 2018, 141, 227-234.