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.