Most of the existing
exposure models for engineered nanomaterials
(ENMs) do not consider particle size, crystalline forms, and coating
materials that all may influence the material’s fate, transport, and toxicity. Our work aimed
to incorporate particle size distributions into a material flow analysis
(MFA) to develop a size-specific, dynamic, probabilistic MFA model (ss-DPMFA). Using titanium dioxide (TiO2) as a
first case study, we aimed to determine the contribution of conventional
TiO2 pigments to the total amount of nanoscale TiO2 released into the environment. Besides providing information
on mass flows, the new model used particle size distributions and
crystalline forms to describe the stocks and flows of TiO2. The most striking modeling result to emerge was that before TiO2 ENMs came onto the market as such in 2000, 22,400 tons of
nanosized (<100 nm) TiO2 particles had already been
released into the environment, originating from conventional TiO2 pigments. Even in 2016, 50% of the nanosized TiO2 particles released into wastewater came from the nanosized fraction
of TiO2 particles in pigments. Quantitative data on the
particle size distribution of TiO2 particles released into
the environment can be used as input for environmental fate models.
Our new ss-DPMFA model’s additional insights about crystalline
forms and coatings could pave the way for advanced size- and form-specific
hazard and risk assessments for other nanomaterials in ecological
systems.