Regulatory ecotoxicity testing of nanomaterials – proposed modifications of OECD test guidelines based on laboratory experience with silver and titanium dioxide nanoparticles

Conservation Biolog

pathway from the sewage treatment plant to the entry in soil via the agricultural use of sewage sludge

Das Ziel dieser Promotionsarbeit bestand in der Anfertigung einer Risikocharakterisierung für ein Silbernanopartikel (NM-300K) in Böden für den Klärschlammpfad. Mit Klärschlammpfad ist das Aufbringen von silbernanopartikelhaltigem Klärschlamm auf landwirtschaftliche Nutzflächen gemeint. Im Atmungshemmtest nach OECD Richtlinie 209 (1984) hat sich gezeigt, dass die Wirkung von NM-300K in der Kläranlage stark von der Zusammensetzung des Abwassers abhängig ist. Vor allem die Konzentration der verschiedenen Salze scheint eine große Rolle zu spielen. Die Kläranlagensimulationen haben bestätigt, dass bei umweltrelevanten Konzentrationen > 90% der Silbernanopartikel im Klärschlamm verbleiben und nicht in den Vorfluter gelangen. An Klärschlamm gebundene Silbernanopartikel (NM-300K) entfalten nach Abbau des Klärschlammes durch die Bodenmikroorganismen wieder die Wirkung von reinem NM-300K. Diese Aussage gilt nur für entwässerten, nicht gefaulten Klärschlamm. Es bestehen Hinweise darauf, dass durch die Faulung von Klärschlamm eine langfristige Detoxifizierung von Silbernanopartikeln bewirkt wird. Weitere Untersuchungen sind zur Untermauerung notwendig. Die Risikocharakterisierung zeigt, dass die PNEC für NM-300K in Böden bei 0,05 mg/kg TM Boden und für den Klärschlammpfad bei 30 mg/kg TS Klärschlamm liegt. Nur wenige Informationen zum Verbleib von Silbernanopartikeln in Böden sind gegenwärtig vorhanden. Es gibt Hinweise darauf, dass die Mobilität im Boden eingeschränkt ist. Bei einer Applikation von Klärschlamm aus kommunalen Kläranlagen mit Silbergehalten von 2-50 mg/kg TS Klärschlamm und einer Akkumulation an Silber in Böden, wären Gehalte die der PNEC entsprechen schnell erreicht. Der Vergleich von PEC-Werten aus Modellrechnungen für Böden, auf den mit Silbernanopartikeln belasteter Klärschlamm ausgebracht wurde, mit den in dieser Arbeit abgeleiteten PNECs für NM-300K zeigt, dass bei wiederholter Applikation von Klärschlamm ein Risiko für Organismen des terrestrischen Ökosystems nicht ausgeschlossen werden kann.The main goal of this PhD thesis was the preparation of a risk characterisation of a specific silver nanoparticle (NM-300K) in soils for the sewage-sludge pathway. The sewage-sludge pathway is, in this context, the application of sewage sludge that contains silver nanoparticles to agricultural land. In the respiration inhibition test, performed according to OECD Guideline 209 (1984), the effects of NM-300K in a wastewater treatment plants (WWTP) are highly dependent on the composition of the wastewater. The concentration of various salts seems to play a major role. The simulation of WWTPs has confirmed that at environmentally relevant concentrations, >90% of silver nanoparticles remain in sewage-sludge and may not reach the water course. The effects of NM-300K bound to sewage-sludge can be minimised to those of pure NM-300K after the sewage-sludge is processed by micro-organisms (for dewatered sewage-sludge only, not digested sludge). There is evidence that through the digestion of sewage sludge a long-term detoxification of silver nanoparticles occurs. Further studies are necessary to substantiate this hypothesis. The risk characterization also showed that the PNEC for NM- 300K in the soils is 0.05 mg/kg dry matter soil while for the sewage-sludge pathway the PNEC is 30 mg/kg dry matter sludge. Little information on the fate of silver nanoparticles in soil is currently available, however, there are indications that the mobility is limited in the soil. With accumulation of silver nanoparticles in soils from agricultural application of sewage-sludge from WWTPs, containing 2-50 mg/kg dry matter sludge, the PNEC for soils would be reached quickly. The comparison of PEC values from model calculations for silver nanoparticles in soil (applied via sewage-sludge) with PNEC values for NM-300K indicates that risk to terrestrial ecosystem organisms from repeated sewage-sludge application cannot be excluded

Influence of application techniques on the ecotoxicological effects of nanomaterials in soil

Abstract Background: In terrestrial ecotoxicological tests, the availability and ecotoxicity of solid nanomaterials may depend on the application technique. We compared five spiking procedures using solid uncoated TiO 2 and Ag nanoparticles in standardized OECD tests with earthworms, plants and soil microflora: dry spiking of soil by applying soil or silica sand as a carrier; dry spiking of food without a carrier; and wet spiking of soil and food with an aqueous nanoparticle dispersion

Influence of application techniques on the ecotoxicological effects of nanomaterials in soil

Background: In terrestrial ecotoxicological tests, the availability and ecotoxicity of solid nanomaterials may depend on the application technique. We compared five spiking procedures using solid uncoated TiO2 and Ag nanoparticles in standardized OECD tests with earthworms, plants and soil microflora: dry spiking of soil by applying soil or silica sand as a carrier; dry spiking of food without a carrier; and wet spiking of soil and food with an aqueous nanoparticle dispersion. Results: The effects of the nanomaterials were influenced by the application technique. The differences were independent of the test organism (which represented different habitats and exposure pathways) and the specificity of the effect (stimulation or inhibition). Wet spiking resulted in stronger effects than dry spiking, but the bioavailability of the particles appeared to be limited when highly-concentrated nanoparticle suspensions were used for wet spiking. The availability of the nanoparticles was slightly lower when silica sand rather than soil was used as the carrier for dry spiking, but the matrix itself (soil or food) had no effect. Conclusion: There are indications that the concentrations of the stock suspensions influence the test results, so dry spiking is preferred for solid TiO2 and Ag nanoparticles. We achieved satisfactory spiking homogeneity with Ag nanoparticles using soil as a solid carrier. Further experiments with other carriers and soil types are required to confirm that the observed differences are universal in character. There was no difference in effect when TiO2 nanoparticles were applied via food or soil. The spiking of soil instead of food is preferred for TiO2 nanoparticles, as is the case for conventional chemicals

Single versus repeated applications of CuO and Ag nanomaterials and their effect on soil microflora

Nanomaterials enter the terrestrial environment via the repeated application of sludge to soils over many years. The goal of this investigation was to compare the effects of CuO and Ag nanomaterials on soil microorganisms after a single application and after repeated applications ultimately resulting in the same test concentrations. The effect on soil microorganisms was determined using the ammonium oxidation (ISO 15685), enzymatic activity patterns (ISO 22939) and MicroResp™ tests on days 28, 56 and 84. The comparability of single and repeated applications of ion-releasing nanomaterials depended on the test endpoint and duration. No significant differences between single and repeated applications were observed when testing nitrifying microorganisms and exoenzymes, but differences were observed in the substrate-induced respiration test. The three test systems used together provide more comprehensive information about the impact of different nanomaterials on the soil microflora and its diversity.peerReviewe

Hazard assessment of a silver nanoparticle in soil applied via sewage sludge

Background: Silver nanoparticles (AgNPs) are widely used in many fields of application and consumer products due to their antibacterial properties. The aim of this study was to prepare a hazard assessment for one specific AgNP in soil, incorporated via sewage sludge (the sewage sludge pathway). The effects of pristine AgNPs on microorganisms, plants and earthworms were first determined in screening tests. Long-term tests over 140 days were then conducted with AgNPs added to soil via sewage sludge. AgNPs were incorporated into the sludge through a simulated sewage treatment plant (STP) over 10 days to allow transformation to occur and also by manual spiking over 2 h. The results of the most sensitive organism from the long-term tests, the soil microorganisms, are presented. Results: The STP simulations confirmed that at environmentally relevant concentrations >90% of AgNPs remain bound to sewage sludge. Effects of AgNPs bound to sewage sludge and added to soil were similar to that of pristine NM-300K after degradation of the sludge. The predicted no-effect concentration for NM-300K in soil of 0.05 mg/kg dry soil determined a maximum threshold of 30 mg/kg dry sludge per application, considering the maximum addition of sewage sludge in Germany (5 tons per hectare every 3 years). Conclusion: At environmentally relevant concentrations, AgNP absorption to sludge and aging in soil (even after transformation) cause toxic effects on soil microorganisms of the terrestrial ecosystem

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

Silver nanoparticles in sewage sludge: Bioavailability of sulfidized silver to the terrestrial isopod Porcellio scaber

Silver nanoparticles (AgNPs) are efficiently converted during the wastewater‐treatment process into sparingly soluble Ag sulfides (Ag2S). In several countries, sewage sludge is used as a fertilizer in agriculture. The bioavailability of sulfidized Ag to the terrestrial isopod Porcellio scaber was investigated. Sewage sludge containing transformed AgNPs was obtained from a laboratory‐scale sewage‐treatment plant operated according to Organisation for Economic Co‐operation and Development (OECD) guideline 303a. The results of transmission electron microscopy with energy dispersive X‐ray of sludge samples suggest that AgNPs were completely transformed to Ag2S. Adult isopods were exposed to OECD 207 soil substrate amended with the AgNP spiked sludge for 14 d (uptake phase) followed by an elimination phase in unspiked soil of equal duration. Most of the Ag measured in P. scaber at the end of the uptake phase was found in the hindgut (71%), indicating that only a minor part of the estimated Ag content was actually assimilated by the isopods with 16.3 and 12.7% found in the carcass and hepatopancreas, respectively. As a result of this, the Ag content of the animals dropped following transition to unspiked sludge within 2 d to one‐third of the previously measured Ag concentration and remained stable at this level until the end of the elimination period. The present study shows that Ag2S in sewage sludge is bioavailable to the terrestrial isopod P. scaber