Modelling engineered nanoparticle fate in aquatic systems: Application of attachment rate
Engineered nanoparticles (ENPs) are currently being used in a variety of commercial products and processes, however the material characteristics employed in standard regulatory fate models (e.g. FOCUS surface and ground water) do not fully capture nano-specific behaviours. One such instance is the use of partitioning coefficients (e.g. koc) to describe fate. ENPs do not reach equilibrium in the way “traditional” chemicals do, therefore attachment efficiency (α) is a more appropriate measure of adsorption to different surfaces. In this study we demonstrate the use of attachment efficiency and collision frequency in predicting the fate of PVP-coated silver nanoparticles in three model aquatic systems: a wastewater activated sludge unit, river and lake. Heteroaggregation with background environmental particles was considered removal of ENPs, with the collision rate (β) defined in terms of system (mixing rate; residence time), background particle (concentration; size; density) and ENP characteristics (size; density). The attachment efficiency (α) was defined in terms of system (ionic strength; dissolved organic carbon) and material (surface chemistry) characteristics. Complete removal (> 10 order; t50 5.25 min) was predicted for the activated sludge unit (assuming a residence time of 6 hr), while 50 % (t50 24.2 hr) and 73 % (t50 88 hr) removal were predicted for the river (residence time 24 hr) and lake (residence time 7 days) systems, respectively. A sensitivity analysis found background particle size and concentration, attachment efficiency and residence time had the greatest influence on predicted removal. This study demonstrates the utility of ENP attachment efficiency in predicting fate in the environment, employing both material and system characteristics. For this method to be applied as part of a regulatory scheme, a database of attachment efficiencies for different surfaces under varied conditions (e.g. ionic strength, pH, OM content, etc.) must be developed, while typical aquatic and porous systems must be further defined and standardized in terms of mixing rate and background material concentration and type.
