Pure Appl. Chem., 2001, Vol. 73, No. 12, pp. 2017-2026
http://dx.doi.org/10.1351/pac200173122017
Hydrolyzing metal salts as coagulants
Abstract:
Aluminium and ferric salts are widely used as coagulants
in water and wastewater treatment. They are effective in removing a
broad range of impurities from water, including colloidal particles
and dissolved organic substances. Their mode of action is broadly understood
in terms of essentially two mechanisms: charge neutralization of negatively
charged colloids by cationic hydrolysis products and incorporation of
impurities in an amorphous precipitate of metal hydroxide. The relative
importance of these two mechanisms depends on many factors, especially
pH and coagulant dosage.
Alternative coagulants based on prehydrolyzed forms of aluminium or
iron can be more effective than the traditional materials in many cases,
but their mode of action is not completely understood, especially with
regard to the role of charge neutralization and hydroxide precipitation.
Basic principles of colloid stability and metal ion hydrolysis are
briefly reviewed, and the action of hydrolyzing metal coagulants is
then discussed, with some examples from recent experimental studies.
Although it is possible to interpret results reasonably well in terms
of established ideas, there are still some uncertainties that need to
be resolved