Environmental Engineering Reference
In-Depth Information
1.0
NaCl only
CaCl 2 -Approach
0.6
CaCl 2 -Separation
0.5
NaCl-Approach
NaCl-Separation
0.4
0.0
Approach
Separation
0.2
0
-0.5
Bridging attraction
-1.0
10
20
30
40
5 0 5 0 5 0
Separation (nm)
Separation (nm)
(a)
(b)
Figure 4.12 Normalized force as a function of surface separation distance for both approach
and separation for alumina surface; (a) in 0.001 M NaCl solution at pH 6.2 and (b) with
adsorbed SRHA layer at pH 6 in 0.01 M CaCl 2 and 0.01 M NaCl solution. (Reprinted with
permission from S. Sander, L.M. Mosley, K.A. Hunter, Investigation of interparticle forces in
natural waters: effects of adsorbed humic acids on iron oxide and alumina surface properties,
Environmental Science & Technology , 38 , 4791-6. Copyright 2004, American Chemical
Society.)
4.6
Fate and Behaviour of Colloids in Aquatic Systems
4.6.1
Aggregation
4.6.1.1
General
Electrostatic stabilization and steric stabilization are the two main mechanisms for
colloid stabilization in aquatic systems, as described in Section 4.5. Unstable col-
loidal systems form fl ocs or aggregates due to interparticle attractions. Aggregation
can be accomplished by a number of following methods:
(i) Removal of the electrostatic barrier (Section 4.5.3) that prevents aggregation
of particles. This can occur due to the increase in salt concentration (Baalousha
et al. , 2006b) or alteration of the pH (Baalousha et al. , 2008) of a suspension
to effectively neutralize or screen the surface charge of the particles. This
reduces the repulsive forces that keep colloidal particles separate and allows
for coagulation due to van der Waals and possibly other forces. Estuarine
systems, which exhibit a sharp increase in salinity, often exhibit rapid and nearly
complete aggregation, a process to be borne in mind when considering manu-
factured nanoparticle behaviour. Nevertheless, see the discussion of non-DLVO
forces which may maintain dispersion even under these conditions.
(ii) In water treatment facilities, addition of a charged polymer fl occulant. Polymer
fl occulants can bridge individual colloidal particles by attractive electrostatic
 
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