Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 137

+

Figure 4.14 Schematic representation of the diffusion limited aggregation (DLA) and a
reaction limited aggregation (RLA) mechanisms, leading to the formation of either loose
aggregates of low fractal dimension or denser aggregates of higher fractal dimension.
(Reprinted with permission from J. Buffl e and G.G. Leppard, Characterization of aquatic
colloids and macromolecules. 1. Structure and behavior of colloidal material, Environmental
Science & Technology , 29 , 2169-75. Copyright 1995, American Chemical Society.)

Table 4.1 Comparison between diffusion and reaction limited aggregation regimes.

DLA RLA

Energy barrier Absence or primary minimum Secondary minimum

Type Reversible Irreversible

Collision effi ciency 1

< 1

Aggregation rate Rapid (Kd) Slow (KR)

Fractal dimension ∼ 1.8 ∼ 2.3

In addition, salts (sodium chloride, magnesium chloride and calcium chloride)
induce the aggregation of alginate coated haematite particles through electrostatic
destabilization (Figure 4.15a). However, in the presence of calcium chloride, aggre-
gation rate was much higher than that which conventional diffusive aggregation
predicts (Figure 4.15b). The observation of higher aggregation rate than that which
would be observed in the presence of a simple electrolyte (Chen et al. , 2006 ) sug-
gests that charge neutralization mechanism is not suffi cient to explain the aggrega-
tion mechanism. ' Excess ' aggregation was explained by the formation of an alginate
coated hematite gel network and the cross-linking between unadsorbed alginate,
via Ca 2+ bridging, that might form bridges between hematite-alginate gel structures
(Chen et al. , 2006 ).