26    THE USE OF THE SOLUBILITY DOMAIN APPROACH
FOR THE MODELING OF THE HYDROXIDE
PRECIPITATION OF LEAD FROM WASTEWATER
K. A. Baltpurvins, Graduate Research Student
R. C. Burns, Senior Lecturer of Chemistry
G. A. Lawrance, Professor of Chemistry
University of Newcastle, Callaghan,
NSW, Australia
A. D. Stuart, Principal Scientist
BHP Research (Newcastle Laboratories)
Wallsend, Australia
INTRODUCTION
One of the principal sources of heavy metal pollution is discharge from industrial operations.
In response to this, industry has developed a number of specialized treatment processes for the
removal of the heavy metals prior to their discharge into the environment.1 Of all such processes,
heavy metal hydroxide precipitation is the most commonly employed due to its low cost and
simplicity. This involves the addition of a suitable base to the influent stream such that the heavy
metals are immobilized as their respective "hydroxides."
In order for hydroxide precipitation to be considered as an effective treatment option, it must
satisfy three fundamental requirements:
• The residual heavy metal ion concentrations after treatment must be below those
deemed permissible for discharge by the authorities.
• The precipitated sludge produced by the precipitation process must demonstrate acceptable filtration and dehydration properties.
• The sludges produced must have a minimal potential for secondary leaching back
into the environment.
Although all three criterion are of significant importance, the first of these criteria must be
considered to be the principal requirement, as it constitutes the primary objective of the treatment process. Thus, treatment efficiency may be considered to be largely defined by this criteria.
In principle, hydroxide precipitation appears to be a simple process; however, in reality the efficiency of the treatment performance is affected by a multitude of parameters. These include
factors such as the chemical nature of the solubility limiting solid phase, the crystal structure of
the solid phase, aqueous phase complex formation, ionic strength, electrical potential, temperature and the time of reaction. In order to provide the optimum treatment performance, the composite effect of all such parameters must be identified, and accounted for.
Various studies have attempted to predict the effect of these controlling parameters on efficiency of hydroxide precipitation on a system specific basis;2^1 however, little emphasis has
been placed on the development of a generalized model for the prediction of the treatment performance. One method which appears to be potentially useful for the prediction of effluent composition effects employs the use of the solubility domain concept.5 This involves the calculation
of the treatment efficiency in terms of residual metal ion concentration versus pH for the chemical extremes of the potential effluent compositions. Providing that these boundaries represent the
treatment efficiency extremes, the experimentally observed precipitation profiles will be encompassed within the defined solubility domain. Thus, the solubility domain provides a convenient
50th Purdue Industrial Waste Conference Proceedings, 1995. Ann Arbor Press, inc., Chelsea, Michigan 48118.
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