2.3 Agglomeration
Nanoparticles properties in natural environments are expected to be altered by
complex factors, including pH, ionic strength (IS) or NOM and one of the critical
changes is agglomeration (Petosa et al. 2010). The formation of aggregates in the
solution is greatly favored for small NPs because of a high number of primary
particles at the same mass concentration, while larger ones maintained
unagglomerated or settled to the bottom (Ludwig et al. 2005). Detailed reviews
about agglomeration mechanisms can be found elsewhere (Batley et al. 2013; Collin
et al. 2017; Petosa et al. 2010). The deep understandings on agglomeration enable a
better assessment of NPs transport and ecological consequences through environmental compartments.
There is growing evidence that notable agglomeration yields higher rates of
sedimentation and less mobility and surface area. It is also clear that NPs surface
area is predictably related to the surface reactivity and this relationship transfers well
to the most toxicity studies (Pelletier et al. 2010; Rogers et al. 2010; van Hoecke
et al. 2009). Therefore, in many past studies, CNPs with a greater agglomeration
state during the exposure experiment have a less inhibitory effect on the tested
organism (Limbach et al. 2005; Petosa et al. 2010; Xu et al. 2018; Zeyons et al.
2009). As a function of agglomeration state, the direct transport through the cell wall
of aquatic organisms and the phagocytosis process by membrane of human lung
fibroblasts may be prevented and the individual CNPs diffusion across the cell
membrane might also be reduced (Auffan et al. 2010; Ludwig et al. 2005). Additionally, heteroaggregation between CNPs and NOM or inorganic colloids in the
environment could hinder the direct contact between CNPs and bacteria (Thill et al.
2006; Collin et al. 2014; Zeyons et al. 2009), which mitigates the toxicity of CNPs.
In addition to the natural compositions, the biomacromolecule like polymeric or
humic substances secreted by both microorganisms and plants as a defense response
plays key roles in CNPs agglomeration, which is an exciting outcome to reduce the
ecological risk (Ma et al. 2015; Wang et al. 2018a; You et al. 2017). On the other
hand, heteroaggregation between CNPs and soft biogenic particles may increase
their bioavailability for filter feeders, which need further attention to provide clear
mechanisms.
Currently, agglomeration is demonstrated to reduce CNPs toxicity when the toxic
response is resulted from a surface area-mediated reaction. However, there still exist
challenges and limitations in delineating the influence of agglomeration on uptake
and subsequent toxicity, because it is a dynamic process influenced by chemical,
hydrodynamic, and biological conditions. Also, agglomeration may serve to promote the persistence of CNPs as it decreases the rate of dissolution and transformation, which may shift the exposure pathway to a different location compared to the
dispersed CNPs.
Surface Properties and Environmental Transformations Controlling the. . .
171
Nanoparticles properties in natural environments are expected to be altered by
complex factors, including pH, ionic strength (IS) or NOM and one of the critical
changes is agglomeration (Petosa et al. 2010). The formation of aggregates in the
solution is greatly favored for small NPs because of a high number of primary
particles at the same mass concentration, while larger ones maintained
unagglomerated or settled to the bottom (Ludwig et al. 2005). Detailed reviews
about agglomeration mechanisms can be found elsewhere (Batley et al. 2013; Collin
et al. 2017; Petosa et al. 2010). The deep understandings on agglomeration enable a
better assessment of NPs transport and ecological consequences through environmental compartments.
There is growing evidence that notable agglomeration yields higher rates of
sedimentation and less mobility and surface area. It is also clear that NPs surface
area is predictably related to the surface reactivity and this relationship transfers well
to the most toxicity studies (Pelletier et al. 2010; Rogers et al. 2010; van Hoecke
et al. 2009). Therefore, in many past studies, CNPs with a greater agglomeration
state during the exposure experiment have a less inhibitory effect on the tested
organism (Limbach et al. 2005; Petosa et al. 2010; Xu et al. 2018; Zeyons et al.
2009). As a function of agglomeration state, the direct transport through the cell wall
of aquatic organisms and the phagocytosis process by membrane of human lung
fibroblasts may be prevented and the individual CNPs diffusion across the cell
membrane might also be reduced (Auffan et al. 2010; Ludwig et al. 2005). Additionally, heteroaggregation between CNPs and NOM or inorganic colloids in the
environment could hinder the direct contact between CNPs and bacteria (Thill et al.
2006; Collin et al. 2014; Zeyons et al. 2009), which mitigates the toxicity of CNPs.
In addition to the natural compositions, the biomacromolecule like polymeric or
humic substances secreted by both microorganisms and plants as a defense response
plays key roles in CNPs agglomeration, which is an exciting outcome to reduce the
ecological risk (Ma et al. 2015; Wang et al. 2018a; You et al. 2017). On the other
hand, heteroaggregation between CNPs and soft biogenic particles may increase
their bioavailability for filter feeders, which need further attention to provide clear
mechanisms.
Currently, agglomeration is demonstrated to reduce CNPs toxicity when the toxic
response is resulted from a surface area-mediated reaction. However, there still exist
challenges and limitations in delineating the influence of agglomeration on uptake
and subsequent toxicity, because it is a dynamic process influenced by chemical,
hydrodynamic, and biological conditions. Also, agglomeration may serve to promote the persistence of CNPs as it decreases the rate of dissolution and transformation, which may shift the exposure pathway to a different location compared to the
dispersed CNPs.
Surface Properties and Environmental Transformations Controlling the. . .
171