Hydrolyzing metal salts as coagulants*
John Gregory1, and Jinming Duan2
1Department of Civil and Environmental Engineering,
University College London, Gower Street, London WC1E 6BT, UK; 2Ian Wark
Research Institute, University of South Australia, Adelaide, SA 5095,
Australia
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
* An issue of reviews and research papers based on
presentations made at the IUPAC/ICSU Workshop on
Electrochemistry and Interfacial Chemistry in Environmental Clean-up
and Green Chemical Processes, Coimbra, Portugal, 6-7 April, 2001.
** Corresponding author.
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