Influence of sewage sludge treatment by sulfidation on the bioavailability of silver nanomaterials when sewage sludge is applied on agricultural land subsequently

Als antibakterielle Substanz gelangen Silbernanomaterialien (AgNM) über Klärschlamm auf landwirtschaftlich genutzte Flächen. Vor ihrer Verwertung gibt es verschiedene Möglichkeiten der Klärschlammaufbereitung, die unterschiedlich auf die Schlämme und die darin enthaltenen AgNM einwirken und potentiell zu unterschiedlichen Ag-Spezies mit veränderter Bioverfügbarkeit führen können. Folgende Fragestellungen wurden im Rahmen dieser Dissertation bearbeitet: (I) Welchen Einfluss hat die Klärschlammaufbereitung auf die Bioverfügbarkeit von AgNM (Schwerpunkt Bodenmikroflora)? (II) Welche Faktoren beeinflussen die Sulfidierung von AgNM als „natürliche Detoxifizierung“ wobei die AgNM mit chemischen Verfahren speziiert wurden? (III) Können Laborversuche als Indikatoren für das Verhalten von AgNM im Freiland genutzt werden? Dazu wurden Kläranlagenversuche mit einem sphärischen AgNM und Ag-Referenzmaterialien mit verschiedenen Schwefel- und Silberkonzentrationen im Kläranlagenzulauf durchgeführt, die Klärschlämme unterschiedlich aufbereitet (Direktapplikation, Hygienisierung durch Kalkung, Hygienisierung durch Pasteurisierung und Klärschlammfaulung) und die landwirtschaftliche Verwertung in Labor- und Freilandversuchen simuliert. Untersuchungen mit dem Transmissionselektronen- mikroskop mit energiedispersiver Röntgenspektroskopie (TEM EDX) zeigten die Sulfidierung der AgNM bereits in der Kläranlage, die weitere Aufbereitung hatte keinen signifikanten Einfluss auf die Ag-Spezies und deren Bioverfügbarkeit. In allen Behandlungen der Langzeitstudien war die Nitrifikation erheblich gehemmt, nur die Hygienisierung durch Kalkung führte indirekt, durch eine Anhebung des Boden-pH-Wertes, zu weniger ausgeprägten Effekten. Dieser Einfluss muss jedoch als zeitlich begrenzte Konditionierung betrachtet werden, sodass die AgNM grundsätzlich bioverfügbar bleiben. Die aufgetretenen Effekte wurden auf instabile, in der Kläranlage sulfidierte AgNM zurückgeführt, aber auch unter Laborbedingungen hergestellte Ag2S-NM riefen ausgeprägte Effekte hervor. Das bestätigt, dass die Sulfidierung nicht zwangsläufig zu einer vollständigen Detoxifizierung der AgNM führt und die Ökotoxizität nur über längere Versuchszeiträume erfasst werden kann, indem Schwellenwerte der Ionenkonzentration überschritten werden. In einer Lysimeterstudie über drei Vegetationsperioden von April 2014 bis Juli 2016 traten Hemmungen der Nitrifikation und der Bodenatmung auf, deren Ausmaß zwar geringer als in Laborstudien ausgeprägt, aber über zwei Jahre konstant war und somit auf eine langfristige Wirkung hindeutet. Zudem wurde ein geringfügiger Austrag von Ag über das Sickerwasser und eine Aufnahme in Pflanzenwurzeln detektiert, eine Gefährdung des Grundwassers oder der Nahrungskette kann aufgrund der geringen Konzentrationen ausgeschlossen werden.Silver nanomaterials (AgNM) are used as anti-microbial substances and are immitted on agricultural areas via sewage sludge. Prior to application on the field there are several techniques to reduce pathogens in sewage sludges. These techniques affect the sewage sludges as well as the incorporated AgNM and potentially lead to different Ag-species with a different bioavailability. The following questions were addressed in this dissertation: (I) How does the sewage sludge treatment influence the bioavailability of AgNM (focus soil microflora)? (II) Which are the factors that influence the sulfidation of silver as a “natural antidote”? For this aim, the silver species was determined by chemical methods. (III) Are laboratory studies suitable indicators to assess the behavior of AgNM in the environment? For this purpose, wastewater treatment plant experiments were conducted with a spherical AgNM and various Ag-reference materials. The sewage sludges were treated by four approaches (direct application, hygienisation by liming, hygienisation by temperature and anaerobic digestion) followed by simulation of the agricultural application. Energy-dispersive X-ray spectroscopy analysis coupled to a transmission electron microscopy revealed that the particles were completely sulfidized after the wastewater treatment. The sewage sludge treatment did not significantly alter their speciation and bioavailability. In all approaches the nitrification process in long-term studies was substantial inhibited, only the setups after hygienisation by liming showed reduced inhibitions, which was related to a mid-term remediation of soil-pH, the AgNM itselves stay bioavailable. These effects were affiliated to instable AgNM after sulfidation under natural conditions. However, even Ag2S NM produced in the laboratory led to strong effects. This confirms that the sulfidation did not inevitably lead to a complete detoxification of AgNM and shows that these effects can only be observed in studies with an extended duration, after exceeding thresholds of the ion concentration. The lysimeter study over three growing seasons from April 2014 until July 2016 revealed an inhibition on the nitrification activity and on the respiratory activity in soils. The magnitudes of effects were less pronounced than in laboratory studies, but constant over the whole test period. Additionally, a slight discharge over the leachate and plant uptake into the roots were detected. Due to the low detected amounts the groundwater and food chain are not endangered

Ecotoxicity and fate of a silver nanomaterial in an outdoor lysimeter study

Sewage sludge is repeatedly applied as fertilizer on farmland due to its high nutrient content. This may lead to a significant increase of silver nanomaterials (AgNM) in soil over years. Therefore, our aim was to investigate the ecotoxicity and fate of AgNM under environmentally relevant conditions in outdoor lysimeters over 25 months. Two AgNM concentrations (1.7 and 8.0 mg/kg dry matter soil) were applied via sewage sludge into soil. In subsamples of the soil, incubated under laboratory conditions for 180 days, the comparability of outdoor and laboratory results regarding ecotoxicity was determined. The results from our long term lysimeter experiments show no detectable horizontal displacement in combination with very low remobilization to the percolate water. Thus, indicate that the sludge applied AgNM remains nearly immobile in the pathway between soils and leachate. However, Ag uptake to the roots of wheat and canola suggests that the chemical conditions in the rhizosphere induce AgNM remobilization from the incorporated sewage sludge even after two harvesting cycles. At the higher AgNM concentration a steady inhibition of the soil microflora was observed over 25 month in the lysimeter study, while there was no effect at the lower AgNM concentration. The results of the laboratory experiment reflect the findings of the lysimeter study and indicate that a risk assessment for AgNM based on data from laboratory tests is acceptable

Long-term effects of three different silver sulfide nanomaterials, silver nitrate and bulk silver sulfide on soil microorganisms and plants

Silver nanomaterials (AgNMs) are released into sewers and consequently find their way to sewage treatment plants (STPs). The AgNMs are transformed en route, mainly into silver sulfide (Ag2S), which is only sparingly soluble in water and therefore potentially less harmful than the original AgNMs. Here we investigated the toxicity and fate of different sulfidized AgNMs using an exposure scenario involving the application of five different test materials (NM-300K, AgNO3, Ag2S NM-300K, Ag2S NM and bulk Ag2S) into a simulated STP for 10 days. The sewage sludge from each treatment was either dewatered or anaerobically digested for 35 days and then mixed into soil. We then assessed the effect on soil microorganisms over the next 180 days. After 60 days, a subsample of each test soil was used to assess chronic toxicity in oat plants (Avena sativa L) and a potential uptake into the plants. The effect of each AgNM on the most sensitive test organism was also tested without the application of sewage sludge. Although Ag sulfidized species are considered poorly soluble and barely bioavailable, we observed toxic effects on soil microorganisms. Furthermore, whether or not the AgNM was sulfidized before or during the passage through the STP, comparable effects were observed on ammonium oxidizing bacteria after sewage sludge application and incubation for 180 days. We observed the uptake of Ag into oat roots following the application of all test substances, confirming their bioavailability. The oat shoots generally containing less Ag than the roots

Long-term effects of sulfidized silver nanoparticles in sewage sludge on soil microflora

The use of silver nanoparticles (AgNPs) in consumer products such as textiles leads to their discharge into wastewater and consequently to a transfer of the AgNPs to soil ecosystems via biosolids used as fertilizer. In urban wastewater systems (e.g., sewer, wastewater treatment plant [WWTP], anaerobic digesters) AgNPs are efficiently converted into sparingly soluble silver sulfides (Ag2S), mitigating the toxicity of the AgNPs. However, long-term studies on the bioavailability and effects of sulfidized AgNPs on soil microorganisms are lacking. Thus we investigated the bioavailability and long-term effects of AgNPs (spiked in a laboratory WWTP) on soil microorganisms. Before mixing the biosolids into soil, the sludges were either anaerobically digested or directly dewatered. The effects on the ammonium oxidation process were investigated over 140 d. Transmission electron microscopy (TEM) suggested an almost complete sulfidation of the AgNPs analyzed in all biosolid samples and in soil, with Ag2S predominantly detected in long-term incubation experiments. However, despite the sulfidation of the AgNPs, soil ammonium oxidation was significantly inhibited, and the degree of inhibition was independent of the sludge treatment. The results revealed that AgNPs sulfidized under environmentally relevant conditions were still bioavailable to soil microorganisms. Consequently, Ag2S may exhibit toxic effects over the long term rather than the short term