COMPLEXATION OF HEAVY METALS BY LIGANDS IN
INDUSTRIAL WASTEWATER-MEASUREMENT AND EFFECT
ON METALS REMOVAL
Michael L. Crosser, Research Chemist
Amoco Chemicals Corporation
Naperville, Illinois 60540
Herbert E. Allen, Associate Professor
Department of Environmental Engineering
Illinois Institute of Technology
Chicago, Illinois 60616
INTRODUCTION
Complexation of heavy metals by inorganic and organic ligands is important for two
reasons.    The first is that the biological availability and toxicity of heavy metals are dependent on the chemical form of the metal.   Siegel [ 1 ] showed that after addition of a
strong complexing agent to an algal medium, growth was inhibited due to iron deficiency,
and other studies [2-5] have shown that solutions containing toxic metals are detoxified
by the addition of natural and synthetic ligands.   These studies suggest that the free metal
ion is the available and toxic chemical species.
The second reason is that soluble complex formation competes with precipitation.
Metals normally removed by hydroxide or carbonate precipitation will remain soluble if
strong complexing ligands are present.  This point is especially important to environmental
engineers because they often depend on precipitation to remove metals from solution.
This paper describes methods for measuring complexation and predicting its effects on
metal removal from solution.
If, for the system in Figure 1, the pH, the EDTA concentration and the constants were
known, the amount of nickel remaining in solution could be calculated.   More important,
for removing soluble nickel, one could (a) evaluate process alternatives; for example, separation or mixing waste streams, concentration or dilution of waste streams and ligand
destruction and (b) predict the effect of a new waste stream on present treatment efficiency.   The second point would be important to both municipal and industrial treatment
engineers.
NIEDTA2-JL EDTA4- + Ni2+
soluble
+
2(OH)
K Figure 1.  Competing reactions of nickel in an
aqueous solution of EDTA.  (S is the complex
.,.._„,.       K$3    _                         Ks,                „ stabUity constant.   K9, Ksj and Ks3 are the
N„OH, 3=i=^ OH" ♦ Ni(OH)2 ^ Ni(OH)°, ^^ constMtI,&r *e indicated OH" -
soluble Ni reactions.
Although equilibrium values for the hydroxide reactions are generally available, the
ligand concentration and complex stability constant are not, for several reasons.   First,
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