Biological indicators to assess the impacts of silver nanoparticles in different forms on agroecosystems

Abstract

Silver nanoparticles (AgNPs) have been used increasingly in consumer products from medical supplies, clothing, cleaning equipment, plant protection products, to household appliances. This increased use leads to an increased presence in the environment, especially in agroecosystems by direct utilisation of plant protection products in addition to the application of biosolids to soils as a fertiliser. Silver in biosolids has increased in parallel with more consumer usage, as an increasingly larger amount of AgNPs is washed into the waste-water system. However, during wastewater treatment, AgNPs react with sulphur and are transformed into silver sulphide (Ag2S), and research concerning this substance is less prevalent than it could be. In this thesis, a variety of aspects concerning potential effects of silver in the agricultural environment have been investigated, mainly through the use of Aporrectodea caliginosa (an endogeic earthworm species) as a representative of the soil faunal community.

Laboratory-based experiments investigated the toxicity of AgNPs in soil on A. caliginosa and AgNPs were found to be fatal at high doses (20% at 500; 60% at 750; and 80% at 1000 mg kg-1), but only if the particles were pristine, as aged AgNPs had little to no effect. Earthworms also lost mass with increased levels of AgNPs. Pristine AgNPs caused avoidance behaviour of the earthworms, however in a reproductive study it was found that over the course of several weeks, A. caliginosa acclimatised to the adverse conditions, and initial negative effects diminished.

These experiments were repeated with Ag2S, the form of silver more likely to occur after biosolid application, so that findings between the effects of AgNPs and Ag2S, could be directly compared. Ag2S caused no mortality and had limited negative effects on mass and reproduction of A. caliginosa. However, they still avoided Ag2S-containing soil in an avoidance experiment, suggesting a low level of toxicity.

A range of plant-based experiments were undertaken to assess the impact of AgNP-containing plant protection products in addition to soil containing Ag2S. In a germination experiment, silver products caused a significant decrease in germination and plumule emergence of lettuce seeds. A spray application of silver-containing products onto lettuce caused a decrease in mass at high silver concentration, but Ag2S-containing soil caused an increased mass in radish plants in line with the increased concentration. Results from these experiments overall posed more questions worthy of investigation.

From the given experimental work, the potential hazards of increased spread of AgNPs onto agricultural fields via the application of biosolids have been shown to be low due to their transformation to Ag2S. However, more research should be undertaken which acknowledges the prevalence of Ag2S in agroecosystems.