98When aluminum sulfate is added to water, the aluminum ions enter into a series of complicated
reactions. The aluminum ions become hydrated, meaning that water molecules attach themselves
to the aluminum ions. In addition, anions present in the water, such as hydroxide and sulfate ions
can attach to the aluminum ions.
These reactions result in large, positively charged molecules having aluminum ions at their center.
These particles may have charges as high as +4. Following these reactions, a second type of
reaction occurs, called Olation. This reaction involves the bridging of two or more of these large
molecules to form even larger, positively charged ions. A typical molecule can contain eight
aluminum ions, twenty hydroxide ions, and will have a +4 charge. Iron salts behave in a similar
manner when added to water.
Once these large polymeric aluminum or iron compounds are formed, the magnitude of their high
positive charge allows these species to rapidly move toward the colloid, where they are adsorbed
onto the negatively charged surface of the turbidity particle. The coagulant compounds can
penetrate the bound water layer because of their high positive charge.
This rapid adsorption results in the compression of the electrical double layer, and results in the
colloid becoming coated with the coagulant compounds. The net result of this process is that the
electrical charges on the particle are reduced.
The suspension is now considered to be destabilized, and the particles can be brought together
through, among other forces, Brownian Movement, and will be held together by the Van der Waals
forces.
An additional process occurs which assists this process. As the coagulant continues to undergo
the hydrolyzation and olation reactions, progressively larger masses of flocculent material are
formed. These compounds can become large enough to settle on their own, and tend to trap
turbidity particles as they settle. This is commonly referred to as sweep floc.
As the coagulation reactions and destabilization are occurring, the Zeta Potential at the surface of
the colloid is also found to be reducing. Typically, the Zeta Potential for a naturally occurring water
may be in the range of -10 to -25 millivolts. As the reactions occur, this Zeta Potential will be
reduced to approximately -5 millivolts.
These figures are only examples of what might be considered typical waters. Since all waters
exhibit a specific set of characteristics, these numbers will vary. It is interesting to note that the Zeta
Potential does not have to be reduced to zero in order for coagulation to occur, because the forces
of attraction can become predominant before complete destabilization occurs.
Hydrophilic colloids participate in the coagulation process in a slightly different way. These colloids
tend to attract water molecules and attach these water molecules to their surfaces. This is also a
hydration process, and the water molecules act as a barrier to contact between particles.
Also attached to the surfaces are hydroxyl, carboxyl, and phosphate groups, all to which are
negatively charged. Coagulant products react chemically with the negatively charged groups
attached to the hydrophilic colloids, forming an insoluble product which is electrically neutral and
destabilized.