Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 120

(a) Ionisation of surface groups (c) Dissolution of ionic solids

Agl

Al—OH 2 pH < 7

I -

Ag +

Al—O - Ag + pH > 7

+

I - I -

Ag +

I -

(b) Ion adsorption (d) Isomorphous substitution

Cla y

-

Al 3+ Si 4+

e.g. SDS,
CH 3 (CH 2 ) 10 CH 2 OSO 3 Na +

-

Figure 4.4 The methods of charging a solid surface immersed in electrolyte. (T. Cosgrove,
Charge in colloidal systems, in Colloid science: principles, methods and applications, 2005 .
Reproduced with permission from Blackwell Publishing.)

size and crystal structure, and so can be positively charged in many environmental
compartments. In the presence of natural organic matter (i.e. humic substances),
colloids generally become negatively charged and the point of zero charge shifts
to lower values (Amal et al. , 1992 ; Baalousha et al. , 2008 ; Ramos - Tejada et al. , 2003 ).
In natural systems, for example freshwater, estuarine, marine and groundwaters,
colloids have been observed to have a narrow range of electrophoretic mobilities
consistent with the formation of NOM surface coating on all other types of colloids
(Beckett and Le, 1990; Hunter and Liss, 1982). Thus, adsorbed NOM molecules
dominate colloids surface charge and will have important consequences on their
environmental functions and their fate and behaviour. In a few cases, colloids rich
in iron oxides (Kaplan et al. , 1995; Loder and Liss, 1985; Newton and Liss, 1987)
were reported to have a positive surface charge.
In aqueous media, the colloidal system as a whole is electrically neutral; oppo-
sitely charged ions surround charged particles which balance their surface charge.
The distribution of ions in the vicinity of charged particle surfaces may be described
by the electric double layer theory (e.g. Stern- Grahame - Gouy - Chapman), which
describes the development of the potential with increasing distance from the
surface (Figure 4.5). In this model ions are distributed across two layers, a compact
inner layer (Stern layer), where the counterions are immobile and a diffuse outer
layer, which extends over a certain distance from the particle surface and decays
exponentially with increasing distance into the bulk liquid phase. The distribution
of ions in the diffuse layer depends on the concentration of the electrolyte, the
charge of the ions and the potential at the boundary between the compact inner
layer and the diffuse outer layer. The potential at this interface is called the Stern
potential. The potential at the shear plane, that is the transition plane from fi xed
ions and water molecules to those which can be sheared of by fl uid motion, is called
the zeta potential ( ζ ), which can be measured by electrokinetic methods (e.g. elec-