Section Three
WASTE TREATMENT PROCESS
A. PHYSICAL/CHEMICAL PROCESS
47    MIXED SOLID PHASE EQUILIBRIUM
James W. Van Nortwick, Jr., Research Assistant
Pritzker Department of Environmental Engineering
Illinois Institute of Technology, Chicago, Illinois 60614
Charles N. Haas, Betz Chair Professor
Drexel University, Philadelphia, Pennsylvania 19104
INTRODUCTION
In many industrial wastewater treatment systems soluble metal contaminants are often removed
from the waste stream by alkaline precipitation. These processes result in the formation of mixed solid
phases containing a number of metal hydroxides. In the past, it has been shown that (1) mixed solid
phases generally have different solubilities than pure solid phases'"3 and (2) precipitation processes do
not generally obey simple equilibrium relationships.3"5 Attempts to describe this deviation from
ideality have included studies on kinetics and surface adsorption.5"7 However, no studies have been
reported on solid phase non-idealities of environmentally important systems, which are known to
exist.80 The recent emphasis on kinetic and surface adsorption studies has somewhat obscured the
fact that a knowledge of thermodynamic properties is still of fundamental importance. This arises
from the dependence of theoretical dissolution models on solubilities, which in turn depend on their
thermodynamic properties.
The practical significance of non-idealities in metal precipitation processes is that there is potential
for a change in the point of minimum solubility of all the components in a solid mixture. The existence
of such phenomena may make possible new separation processes based upon exploitation of differential non-ideal behavior among the components, however, no data on mixed solid phase non-idealities
are available for the metal hydroxide solid phases encountered in wastewater treatment.
The specific objectives of this research project were to investigate the existence of non-ideal behavior (with respect to ideal solution theory) in the hydroxide system composed of mixtures of cupric and
cadmium hydroxide and to determine the dependency of solid phase activity corrections (non-
idealities) on facets of the solution such as ionic strength, and total metal concentration. Cadmium
and copper were chosen in light of their prevalence in industrial wastewaters, current environmental
concerns regarding their disposal and their physical properties.
BACKGROUND
Traditionally, classical solubility products have been used extensively to describe the solid-solution
phase equilibrium in single and mixed solid phases. This classical theory assumes that the activity of
the solid phase is equal to unity, i.e., the solid mixture behaves as an ideal solid solution. Although
this approach has become a versatile tool in rationalizing the dissolution and precipitation behavior of
many systems, direct application of the solubility product principle and the assumption of solid phase
ideality may not adequately describe mixed solid equilibrium relationships. The solid solution is in
equilibrium with its ions but the equilibrium relationships are not always immediately obvious and
quite often the assumption of unit activities is inappropriate.
According to the Gibbs equilibrium condition,21 if a mixed solid and a solution are in equilibrium,
the chemical potential of each component is the same in both phases. The solubility of the solid is
therefore dependent on the activity of the solid phase. A number of researchers have shown that the
mixed solid-solution equilibrium varies as a function of solid phase composition."16,19,20 These
deviations from the ideal solid solution theory have often been attributed to solid phase non-
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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