Assessing the environmental occurrence and risk of nano-silver in Hunan, China using probabilistic material flow modeling.

Using a probabilistic material flow analysis model, we investigate the mass balance and occurrence of nano-silver in the environment of Hunan, renowned as "the home to non-ferrous metals" in China. The model builds on China-specific production and environmental information and incorporates sulfidation of nano-silver in urban wastewater treatment systems. We predict that the bulk of nano-silver (>90%), primarily originating from production and consumption of nano-silver products, ends up in sewage treatment plants (STPs, 5.3t/a), followed by the landfill (4.7t/a). More than 99% of nano-silver in STPs reacts with sulfide and thus does not appear in effluents. Direct release of nano-silver from production and consumption is identified as the dominant source of nano-silver in the environment, most of which enters surface water (0.71t/a). As such, regulation of direct emissions from production and consumption of nano-silver products can be of priority for local environmental management. The modeled regional concentrations (modes) of nano-silver are 0.9ng/L in surface water, 20ng/kg in soil, 6.9μg/kg in sediment, and negligible in the air, which agree well with measurements in the modeled region. Based on the modeled concentrations, we calculate that the risk characterization ratio is <1 in the air, surface water and soil, which means that nano-silver currently poses no risk to organisms living in these environmental compartments.