Page 226 - Water and wastewater engineering

COAGULATION AND FLOCCULATION 6-3

condition the small particles for subsequent processing by flocculation. * Flocculation is the
process of aggregation of the destabilized particles and precipitation products.

6-2 CHARACTERISTICS OF PARTICLES

Electrical Properties
The most important electrical property of the colloidal and suspended particles is their surface
charge. This charge causes the particles to remain in suspension without aggregating for long
periods of time. Surface water particle suspensions are thermodynamically unstable and, given
enough time, they will flocculate and settle. However, the aggregation process is very slow, and
the particles cannot be removed by sedimentation in a reasonable amount of time, that is, a short
enough time that would allow production of a sufficient amount of water for a community of
more than a few people.
For most particles in water the sign of the charge is negative (Niehof and Loeb, 1972; Hunter
and Liss, 1979). This charge arises in four principal ways (Stumm and Morgan, 1970):
• Ionization. For example, silica has hydroxyl groups on its exterior surface. Depending on
the pH, these can accept or donate protons:

OH 2 OH O
Si
Si
Si
pH 2 pH 2 pH 2
• Adsorption. In this case, a solute becomes bound to the solid surface, for example, a humic
acid or natural color on a silica surface. These large macromolecules have carboxylic acid
groups that dissociate at pH values greater then 5 to form negative ions.
• Isomorphous replacement. Under geologic conditions, the metal in a metal oxide is re-
placed by a metal atom with a lower valence. For example, if, in an array of solid SiO 2
3
4
tetrahedra, an Si atom is replaced by an Al atom (Al has one less electron than Si ), the
lattice becomes negatively charged.
• Structural imperfections. In the formation of the mineral crystal, bonds are broken on the
edge of the crystal. These lead to development of surface charge.

Electrical Double Layer. A colloidal dispersion in solution does not have a net charge. This is
because the negatively charged particles accumulate positive counterions on and near the particle
surface. Thus, as shown in Figure 6-2 , a double layer forms. The adsorbed layer of cations (known
as the Helmholtz layer or the Stern layer) is bound to the particle surface by electrostatic and adsorp-
tion forces. It is about 0.5 nanometers (nm) thick. A loose diffuse layer forms beyond the Helmholtz
layer. The double layer (Helmholtz plus diffuse) has a net negative charge over the bulk solution.
Depending on the solution characteristics, it can extend up to 30 nm into the solution (Kruyt, 1952).

Zeta Potential. When a charged particle is placed in an electric field, it will migrate to the pole
of opposite charge. This movement is called electrophoresis. As the particle moves, a portion of
the water near the surface moves with it. This movement displaces the ion cloud and gives it the
*Although the conditioning of colloidal and suspended matter is the primary function of the coagulation process, the precipita-
tion of dissolved NOM is a concurrent objective.