Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 136

Small Aggregates
(Stable Suspension)

Gels

Large Aggregates
(Untable Suspension)

Figure 4.13 Major types of aggregates formed in the three-colloidal component system:
fulvic acid = small points; inorganic colloids = circles; rigid biopolymers = lines. Both fulvic
acids and polysaccharides can also form gels, which are represented here as gray areas into
which inorganic colloids can be embedded. (Reprinted with permission from J. Buffl e, K.J.
Wilkinson, S. Stoll, M. Filella, J. Zhang, A generalized description of aquatic colloidal interac-
tions: the three-colloidal component approach, Environmental Science & Technology , 32 ,
2887-99. Copyright 1998, American Chemical Society.)

similarly to inorganic colloids. Such an aggregation process can be modelled numer-
ically (Stoll and Buffl e, 1996 ).

4.6.1.3 Role of Ionic Strength and Cations

According to DLVO theory, an increase in ionic strength results in the screening
of surface charge, shrinkage of double layer and, subsequently, colloid aggregation.
In low ionic strength freshwaters, compact reaction limited aggregation (RLA) type
aggregate (Figure 4.14, Table 4.1) structures should be observed more frequently
due to the low collision effi ciency. However, at high ionic strengths such as marine
systems, loose aggregates with a diffusion limited aggregation (DLA) type structure
are more likely to be observed due to the higher collision effi ciency (Leppard
et al. , 1986, 1997 ; Wilkinson et al. , 1999). In the presence of high ionic strength, humic
substances aggregate to small spheroids of about 10 nm or large, fractal aggregates
of several micrometers (Baalousha et al. , 2005, 2006b ).