Page 289 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

274 Principles and Methods

Figure 7.26 Illustration of nanopar-
ticles entangled in a collection of
organic macromolecules.

particular may be susceptible to the adsorption of or on other materi-
als given their high reactivity and large surface areas. If nanoparticles
are present as individual entities, adsorption of materials on these par-
ticles may not be an appropriate model since the material interacting
with the nanoparticle may be similar or even larger in size. For nanopar-
ticles with dimensions approaching several tens of nanometers or less,
it is plausible that they may become incorporated into organic macro-
molecule structures (Figure 7.26). In this scenario the organic material
acts as the adsorbing surface and thus, mobility may be dominated by
the properties of the macromolecule rather than the nanoparticle.
However, in many cases nanoparticles may be present as larger col-
loidal aggregates, presenting a clear interface for adsorption. NOM or
ions may form direct chemical bonds on the particle surface (inner
sphere adsorption). Conversely, in outer sphere adsorption, no direct
chemical bonds are formed and instead the adsorbate is held at the par-
ticle surface through electrostatic and/or hydrogen bonding forces.
Adsorption of NOM to particle surfaces is a function of the solution
chemistry (pH and ionic strength) and the characteristics of the organic
molecule, such as molecular size and charge [84]. NOM has been shown
to adsorb to negatively charged particles in aqueous media (e.g., iron
oxide colloids). Under environmentally relevant conditions, humic acids
readily adsorb to other negatively charged surfaces and in turn modify
the physicochemical characteristics (e.g., charge) of the solid-liquid
interface. The thickness of the adsorbed layer generally increases with
increasing ionic strength [84, 86] and with increasing concentration of
the organic macromolecule [86]. Adsorption of NOM on colloidal iron
particles appears to be favored by neutral pH values (pH in the range