Fate and Transport of Nanomaterials in Aquatic Environments - Nanotechnologies for Water Environment Applications - page 503

If the two-film model is used, Henry's law can be used to link the concentration
of a chemical in the gas phase with that in the liquid phase. Within the soil matrix,
Henry's law can be used to estimate partitioning between the soil-water and the soil-gas
interphases (Eweis et al., 1998). However, it is unknown whether NMs still follow
Henry's law, under what conditions and how many of them follow Henry's law.
Currently, sufficient information is not available on the relationship between the
aqueous phase NM concentration and that in the other phases (e.g., oil, grease, tar, soils,
and pure NMs themselves); a reasonable assumption is that NMs obey the partitioning
model (Table 15.6).
15.4 Transformation Processes
Transformation can occur as a result of biological, chemical, and physical
processes. In this section, the most important transformation processes in aquatic
environments are discussed, including biotic (e.g., biodegradation, biologically-mediated
reaction, bioaccumulation) and abiotic transformation (e.g., photochemical, oxidation-
reduction, and hydrolysis reaction, ion exchange).
15.4.1 Biotic Transformation
Organisms (e.g., microbes, plants) can play a significant role in fate and
transformation of NMs in the environment. Their metabolism depends on the
availability of electron donors/acceptors (substrate), the essential nutrients (nutrients),
and necessary conditions (pH, temperature, moisture, redox potential) for growth.
Organisms have evolved a wide range of mechanisms for the immobilization,
mobilization, uptake and transformation of NMs. In general, biotic transformation of
NMs or other contaminants in the environment can be classified into biosorption,
bioaccumulation/uptake, and bio-transformation (Table 15.7). Reactions involved in
such transformation processes typically involve intracellular or extracellular enzymes.
Biosorption. NMs in the environment can be taken by microbes or plants
through a passive physical absorption process (referred to as biosorption here). Under
the term biosorption, a number of mechanisms allow interactions between living
organisms and NMs, including physical adsorption, ion exchange, complexation and
precipitation. The bioadsorption rate of NMs varies with different types of organisms as
well as types and concentration of NMs.
Physical adsorption takes place due to van der Waals' forces and can also occur
via electrostatic interactions between the NM in solution and the cell walls of the
microorganisms (Kuyucak and Volesky, 1988). Most of NMs themselves are good
adsorbents due to their larger surface areas and surface charges. For example, all AP-