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Ecotoxicity of metal nanoparticles in soils

Final Report Summary - NANO-ECOTOXICITY (Ecotoxicity of metal nanoparticles in soils)

Nanotechnology is based on the fact that, by engineering the size and shape of materials at the scale of atoms (nanometres), distinct optical, electronic, or magnetic properties can be tuned to produce novel properties of commercial value, but also of potential toxicological concern. Following use and release in the environment, soils will become the biggest environmental sink for nanoparticles (NPs), as a result of sludge application to land. Little data however, exist on the effects of NPs in soil. This project deals with the toxicokinetics of metal NPs to soil-dwelling organisms with the aim of linking their fate and effects in terrestrial ecosystems. The work was based on case studies with ZnO and Ag NPs, representing different fate kinetics.

The experiments I have conducted have allowed me to investigate the influence of soil pH on the toxicity of ZnO NPs to earthworms, to determine uptake rate and routes and to assess internal distribution and speciation including the analysis of sectioned tissues at the DIAMOND Light Source STFC facility (in collaboration with Prof John Morgan and Dr Peter Kille, Cardiff University). Results so far show that for ZnO NP bioavailability and toxicity are governed to a considerable extent by soil pH and are related to the effects on the surface properties which I measured in conjunction with Oxford University. In this study, I also found that Zn toxicity was higher in earthworms exposed to ionic Zn compared to nano ZnO, despite greater tissue concentrations in the nano-exposed worms. In my studies on Ag NP and ZnO NP exposure routes, contrary to published data for ionic forms of trace elements, where dermal uptake generally dominates, I found that for NPs soil ingestion is probably the main exposure route. Sealing the mouth of earthworms with surgical glue I observed uptake decreasing by almost 75% compared to unsealed worms. These results also showed that earthworms exposed to uncoated and coated AgNP can accumulate Ag to concentrations above the “critical Ag body burden” estimated for earthworms exposed to Ag ions, without causing toxicity. Hence, using standard testing protocols for NPs could possibly provide a false sense of safety as there is no toxicity "above ionic toxicity". Furthermore, the increased uptake does raise issues of concern for both the food chain safety aspects.

In an effort to predict whether soils testing in time (after aging) may provide different answers and input data for risk assessment, we did run toxicity experiments in soils dosed with AgNP at different incubation times over a year. Our results revealed that AgNP toxicity increased over time confirming that standard toxicity tests usually focus on measuring toxicity over short-term (day to week) durations might not be valid to assess AgNP risk assessment.