Engineered nanomaterials (ENMs), which may be released to the environment due to human-related activities, can move across environmental phase boundaries and be found in most media. Given the rapid development and growing applications of nanotechnology, there is concern and thus the need to assess the potential environmental impact associated with ENMs. Accordingly, a modeling platform was developed to enable evaluation of the dynamic multimedia environmental distribution of ENMs (MendNano) and the range of potential exposure concentrations of ENMs.
The MendNano was based on a dynamic multimedia compartmental modeling approach that was guided by detailed analysis of the agglomeration of ENMs, life-cycle analysis based estimates of their potential release to the environment, and incorporation of mechanistic sub-models of various intermedia transport processes. Model simulations for various environmental scenarios indicated that ENM accumulation in the sediment increased significantly with increased ENMs attachment to suspended solids in water. Atmospheric dry and wet depositions can be important pathways for ENMs input to the terrestrial environment in the absence of direct and distributed ENM release to soil. Increased ENM concentration in water due to atmospheric deposition (wet and dry) is expected as direct ENM release to water diminishes. However, for soluble ENMs dissolution can be the dominant pathway for suspended ENM removal from water even compared to advective transport. For example, simulations for Los Angeles showed that dry deposition, rain scavenging, and wind dilution can remove 90% of ENMs from the atmospheric airshed in ~100-230 days, ~2-6 hrs, and ~0.5-2 days, respectively. For the evaluated ENMs (metal, metal oxides, carbon nanotubes (CNT), nanoclays), mass accumulation in the multimedia environment was mostly in the soil and sediment. Additionally, simulation results for TiO2 in Los Angeles demonstrates that the ENM concentrations in air and water increases rapidly to reach steady state, in 72 hrs and 8 days after the start of source release, respectively. After termination of source release, ENM concentrations would decrease by 90% in ~1 and ~4 days. In contrast, steady state for ENM concentrations in soil would not be expected to occur until after about 10 years. MendNano was further integrated with a sub-model of lifecycle environmental assessment for the release of ENMs (LearNano). Estimation of the releases of various ENMs and their environmental distributions in various regions in the U.S. and countries throughout the world revealed that the exposure concentrations for most ENMs (e.g., metal, metal oxides and carbon nanotubes) are expected to be in the range of 0.0003 - 30 ng m-3 (air), 0.006 - 150 ng L-1 (water), 0.01 - 40 μg kg-1 (soil), and 0.005 - 100 mg kg-1 (sediment).
It is important to note that the environmental transport of ENMs is governed by particulate transport processes; and thus the transport rates of ENMs are dependent on their particle size distribution. Accordingly, a computational constant-number Direct Simulation Monte Carlo (DSMC) model was also developed to assess the ENM agglomeration in aqueous systems, by solving the Smoluchowski coagulation equation coupled with particle-particle interaction energies provided by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and non-DLVO hydration repulsion interaction energy. Prediction of ENM agglomerate PSDs demonstrated excellent agreement with experimental measurements for TiO2, CeO2, α-Fe2O3, SiO2, and C60 ENMs over a wide range suspension conditions. Simulations also demonstrated, in quantitative agreement with DLS measurements, that nanoparticle agglomerate size increased both with ionic strength (IS) and as the solution pH approached the isoelectric point (IEP). Additionally, evaluation of experimental DLS measurements for TiO2, CeO2, SiO2, and α-Fe2O3 (hematite) at high IS (up to 900 mM) or low |ζ-potential| (>=1.35 mV) revealed that non-DLVO hydration repulsion energy can be above electrostatic repulsion energy such that the increased overall repulsion energy (contributed by hydration repulsion energy) can significantly lower the agglomerate diameter relative to the classical DLVO prediction. The classical DLVO theory, which is reasonably applicable for agglomeration of NPs of high |ζ-potential| (~>35 mV) in suspensions of low IS (~<1 mM), can overpredict agglomerate sizes by up to a factor of 5 at high IS (~>1 mM) or low |ζ-potential| (~< 40 mV) conditions.
In summary, the MendNano-LearNano integrated modeling platform was implemented as a web-based software application that enables rapid "what-if?" scenario analysis, in order to assess the response of environmental system to various scenarios of ENM releases, investigate the impact of geographical and meteorological parameters on ENM distribution in the environment, compare the impact of ENM production and potential releases on different regions, as well as estimate source release rates based on monitored ENM concentrations. It is envisioned that the present integrated modeling platform can serve as a decision support tool to rapidly and critically assess the potential environmental implications of ENMs and thus ensure that nanotechnology is developed in a productive and environmentally responsible manner.