Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 156

Wastewater contains both soluble and insoluble substances that need to be removed.
These substances that (solutes) are in either molecular (such as benzene, etc.) or
ionic (such as Na + , Cl − , Mg ++ , K + , Fe ++ , etc.) form. The concentrations are given in
various units:

Weight/volume mg L −1 ; kg m −3

Weight/w mg kg −1 ; parts per million (ppm); parts per billion (ppb)

Molarity mol L −1

Normality equivalents L −1

The specific unit used depends on the amounts present. The unit used for a trace amount
of a solute, such as benzene, is given in ppm or ppb. The hardness of drinking water
(mostly Na, Ca-Mg) concentration is given as mg L −1 . The typical values as found are
in the range of less than 10 mg L −1 (soft water) or over 20 mg L −1 (hard water).
The presence of a net charge at the particle surface produces an asymmetric dis-
tribution of ions in the surrounding region. This means that the concentration of
counterions close to the surface are higher than the ions with the same charge as the
particle. Thus, an electrical double layer is measured around such a particle placed
in water.
The solids can be removed by filtration and precipitation methods. The precipita-
tion (of charged particles) is controlled by making the particles flocculate by control-
ling the pH and ionic strength. The latter gives rise to a decrease in charge-charge
repulsion, and can lead to precipitation and removal of finely divided suspended sol-
ids. The most important factor that effects zeta potential is found to be pH. Therefore,
all zeta-potential data must include a note on its pH. Imagine a particle in suspen-
sion with a negative zeta-potential. If more alkali is added to this suspension, then
the particle will exhibit an increase in negative charge. On the other hand, if acid is
added to the colloidal suspension, then the particle will acquire increasing positive
charge. During this process, the particle will undergo a change from negative charge
to zero charge (where the number of positive charge is equal to the negative charge
( point-of-zero-charge [PZC]). In other words, one can control the magnitude and
sign of the surface charge by a potential-determining ion.
The stability is dependent on the magnitude of electrostatic potential, ψ, at the
surface of the colloid, ψ o . The magnitude of ψ o is estimated by using the microelec-
trophoresis method. When an electric field is applied across an electrolyte, charged
particles suspended in the electrolyte are attracted toward the electrode of opposite
charge. Viscous forces acting on the particles tend to oppose this movement. When
equilibrium is reached between these two opposing forces, the particles move with
constant velocity. In this technique, the movement (or rather the speed) of a particle is
observed under a microscope subjected to an electric field. The field is related to the
applied voltage, V , divided by the distance between the electrodes (in centimeters).
The velocity is dependent on the strength of the electric field or voltage gradient, the
dielectric constant of the medium, viscosity, and zeta-potential. Commercially avail-
able electrophoresis instruments are used where the quartz cells designed for any
specific system are available.
The magnitude of the zeta potential, ζ, is obtained from the following relation: