Issues in assessing environmental exposures to manufactured nanomaterials

Abstract

Manufactured nanomaterials (MNs) are commonly considered to be commercial products possessing at least one dimension in the size range of 10(-9) m to 10(-7) m. As particles in this size range represent the smaller fraction of colloidal particles characterized by dimensions of 10(-9) m to 10(-6) m, they differ from both molecular species and bulk particulate matter in the sense that they are unlikely to exhibit significant settling under normal gravitational conditions and they are also likely to exhibit significantly diminished diffusivities (when compared to truly dissolved species) in environmental media. As air/water, air/soil, and water/soil intermedium transport is governed by diffusive processes in the absence of significant gravitational and inertial impaction processes in environmental systems, models of MN environmental intermedium transport behavior will likely require an emphasis on kinetic approaches. This review focuses on the likely environmental fate and transport of MNs in atmospheric and aquatic systems. Should significant atmospheric MNs emission occur, previous observations suggest that MNs may likely exhibit an atmospheric residence time of ten to twenty days. Moreover, while atmospheric MN aggregates in a size range of 10(-7) m to 10(-6) m will likely be most mobile, they are least likely to deposit in the human respiratory system. An examination of various procedures including the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal particle suspension stability in water indicates that more sophisticated approaches may be necessary in order to develop aquatic exposure models of acceptable uncertainty. In addition, concepts such as Critical Coagulation Concentrations and Critical Zeta Potentials may prove to be quite useful in environmental aquatic exposure assessments.

Keywords: Critical Coagulation Concentration; DLVO theory; aquatic emissions; atmospheric emissions; manufactured nanomaterials; ultrafine particles; zeta potential.

Figure 1

Average, representative estimated atmospheric particle deposition velocities as a function of particle diameter (particle density = 10 g/cm³; windspeed friction velocities range from 2.3 to 145 cm/s; data used in calculating averages obtained from Sehmel [33]).

Figure 2

Enhanced MIT Diffuse Layer Model [44,15] predicted site distributions and diffuse layer potentials for reactive ionizable sites on the surface of amorphous iron oxide particles in world average river water. Site concentrations less than one percent are not included in this figure. Temperature = 20 °C, pCO₂ = 3.8×10⁻⁴ atm. Simulated conditions given in Loux [15].

Figure 3

Evaluation of consistency between the Ross and Morrison [49] CCC expression [Equation (6)] and the critical zeta potential expression given by Fowkes [[51]; Equation (8)]. The log¹⁰(CCC/Ionic Strength) term should equal zero with perfect agreement between the two approaches. As materials with lower Hamaker constants are most likely to be mobile at ionic strengths below 0.01 M, these two approaches do approach agreement to within 18% in systems where colloidal mobility is more likely.

Figure 4

Comparison of the world average river water pH-dependent amorphous iron oxide DLM diffuse layer potentials depicted in Figure 2 with zeta potential estimates obtained using “correction” procedures given by Lyklema and Overbeek [46]. An estimated maximum zeta potential for this system using a procedure from Lyklema and Overbeek [46] is ±103 mV and an estimated critical zeta potential for this system using a procedure from Fowkes [51] is ±33 mV.