REMOVAL OF TOXIC METALS FROM POWER GENERATION
WASTE STREAMS BY ADSORPTION AND CO-PRECIPITATION
Mark M. Benjamin, Assistant Professor
Dept. of Civil Engineering
University of Washington
Seattle, Washington 98195
Kim F. Hayes, Graduate Student
James O. Leckie, Associate Professor
Environmental Science and Engineering
Stanford University
Stanford, California 94305
INTRODUCTION
Release of toxic substances and their dispersal in the environment can have tragic effects
on exposed populations. It is axiomatic that these substances are more easdy controUed
when they are generated than after they are dispersed. Hence, our success in minimizing
then- adverse impacts depends largely on our abiUty to design treatment processes which
isolate and remove the contaminants at their source. To describe and predict the fate of
these substances in treatment systems and the natural environment requires a fundamental
understanding of the mechanisms controUing the chemical reactions in which they participate.
Liquid waste streams in coal-fired power plants contain a wide spectrum of trace elements, most of which originate in the coal and remain in the fly ash or bottom ash when
the coal is burned. In addition, trace contaminants from sources other than the coal may
be concentrated in cooling tower blowdown and other waste streams. We have conducted
studies to gain insight into the physical-chemical behavior in heterogeneous aqueous environments of such trace elements as Cd, Cu, Pb, Zn, As, Se and Cr. AU of these elements are
known to be trace contaminants in waste fluids from various operations in coal-fired power
plants. In particular, we have assessed the potential to treat these streams by adsorption
of the various contaminants on hydrous iron oxide surfaces.
There are many factors which affect the adsorbability of a dissolved element. Among
these are its chemical form (e.g., oxidation state, degree of complexation), solution pH,
ionic strength, the presence of competing adsorbates, identity and crystaUinity of the
solid substrate, time of contact, etc. In this paper we present results of adsorption studies
using complex waste solutions from full-scale, operating power plants and compare these
with the results in simple, weU-characterized solutions. Our emphasis is on the appUcation
of adsorption as a waste treatment process in coal-fired plants. DetaUs of a mathematical
model to describe the adsorption process, and the effects of competing metals and com-
Plexing ligands, are presented elsewhere [ 1-3 ].
ADSORPTION ON OXIDES: IMPORTANT VARIABLES AND TRENDS
There have been numerous adsorption studies describing the binding of environmentaUy
significant chemical species on oxide surfaces [4-11]. In most of these, binding of a single
adsorbate has been determined as a function of solution pH, soUds concentration, or ad-
sorbate concentration. WhUe there are some differences among systems, enough data are
avadable to generaUze certain conclusions. In this section a brief description is given of the
primary factors affecting adsorption of potentiaUy toxic trace cations and anions.
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