Probabilistic modelling of engineered nanomaterial emissions to the environment: a spatio-temporal approach

For the environmental risk assessment of engineered nanomaterials (ENM) knowledge about environmental concentrations is crucial. Soils and sediments are considered sinks for ENM and thus a better understanding of the spatial and temporal variability of concentrations is needed. In this work we use South Australia as a case study for a region with significant biosolids and treated wastewater application on soils, representing a system with almost "closed loops". The probabilistic material flow modelling approach was extended to include a temporal modelling of ENM production and biosolids handling and transfer onto soils, focusing on nano-TiO2, nano-ZnO, nano-Ag, Carbon Nanotubes(CNT) and fullerenes. The results thus not only incorporate the uncertainty on ENM flows but also the spatial and temporal variability of ENM concentrations between 2005 and 2012. The ENM concentrations in different waste amended soils vary by more than 2 orders of magnitude due to different biosolids and wastewater application rates. Because of the almost complete transformation of nano-ZnO and nano-Ag during wastewater treatment, we also modelled the total flows of Zn and Ag derived from the nanoparticles and compared their modelled concentrations to measured total Ag and Zn concentration in biosolids and soils in South Australia. The modelled Ag concentration derived from nano-Ag is 50-times smaller than measured Ag in soils and 10-times in biosolids. For Zn the respective values are 250 and 7. If in the future the accumulation continues with the same rate as in 2012 it would take about 170 years until a regulatory threshold value of 500 ug Ag per kg of soil would be reached. For Zn, it will take 930 years. The results from this modelling highlight that regional and site-specific conditions need to be considered when assessing the environmental risks of nanomaterials.