United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA-600/S3-82-048 Sept. 1982
Project Summary
Energy Model of a Cadmium
Stream with Correlation of
Embodied Energy and Toxicity
Robert L Knight
Natural toxic substances often
occur in ecosystems as a result of
weathering of geological outcrops and
via biological defense mechanisms.
Over evolutionary time, ecosystems
may survive by incorporating compo-
nents and functional mechanisms that
maximize the rate of useful work
performed (power) in the system,
perhaps making direct use of toxicants
as control agents. Embodied energy,
that is, the energy expended to
produce a component, a controlling
biogenic toxicant, or to maintain an
interaction, may serve as a measure of
the control and amplification capacity
of the ecological entity involved.
Comparisons and analysis of relation-
ships between controlling effects and
the (energy) cost of production of
toxicants may be facilitated by reduc-
tion of both phenomena to an equiva-
lent (embodied energy) measurement
scale. The effects and consequences
of the release of anthropogenic tox-
icants and concentrated wastes may,
then, become more predictable as the
effects and strategies of ecosystems'
responses to and uses of natural
toxicants become better understood,
particularly if the controlling effects of
toxins can be estimated from prop-
erties of the compound (e.g. embodied
energy).
The heavy metal, cadmium (Cd),
was used to analyze this toxin control
hypothesis. A literatrue review indi-
cated a stimulatory (Arndt-Schulz)
effect of Cd at low concentrations in
many growth studies. Most data sets
were found to be described by a
general subsidy-stress curve. The
bioconcentration of Cd as a mechanism
in natural systems for controlling free
Cd concentration and its toxic effect
was examined.
The energy embodied in Cd storages
by three different systems was evalu-
ated. Calculations suggest that the
world geological cycle is producing
economically recoverable Cd at a very
slow pace, only 53 kg-yr '. The
energy transformation ratio of this Cd
is 2.5 x 1016 Solar Equivalent Calories
(S.E.Cal)-g Cd'1. The industrial con-
centration of Cd adds an additional
4.6 x 107 S.E.Cal-g Cd"1 in the syn-
thesis of the pure metal. A calculation
of the biological concentration in
experimental stream systems indicated
a cost of 1.3 x 109 S.E.Cal-g Cd"1 at a
concentration of only 0.8/yg-g Cd"1 on
a live weight basis.
Information collected during pre-
vious research of Cd effect in experi-
mental streams was summarized and
used to calibrate an energy and
material model of the Cd streams.
Several mechanisms of Cd toxicity
were examined and the model includes
a stimulation of system components
at low Cd levels. Simulation results
allowed a detailed correlation of the
relationship between embodied energy
in Cd and the Cd effect in equal units
(S.E.Cal-g Cd"1). This correlation was
found to be first positive, then negative,
and eventually approached zero at
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higher Cd concentrations. The results
of this study with Cd are predicted to
be general to most other toxic sub-
stances and may allow synthesis of
the burgeoning quantity of information
concerning chemicals in the environ-
ment.
This Project Summary was devel-
oped by EPA 's Environmental Research
Laboratory, Athens, GA, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The study of mechanisms controlling
environmental systems is essential for
understanding ecosystems and for their
rational management. Toxic substances
may control ecosystems and cause new
ecosystems to emerge that can directly
utilize the substances to aid their
competitive roles. Substances may
directly aid positive physiological mech-
anisms, stress a system that is not
adapted, or subsidize an adapted
system. The aim of this study was to
develop a theoretical and quantitative
mean to evaluate, compare, and utilize
controllers in environmental manage-
ment and to illustrate the approach with
one substance — the heavy metal Cd.
A theory proposed by Odum (1979)
and the author is that control action or
"amplification" ability may be a function
of the energy embodied in the controlling
agent. Embodied energy is defined as
the total energy flow of a system
necessary to form the controlling agent
through convergence of webs or con-
centrating factors. In systems selected
for maximum energy flow, controllers
may be used to manipulate productive
processes through positive amplification.
The theory suggests that controllers will
have an energy consumption from the
system that may not exceed their value
as a stimulant to productivity and that
natural selective processes will eliminate
items that use more energy than they
stimulate. In an immature system, two
values of a controller, i.e., the embodied
energy and the amplifier effect, might
be widely different, but in an adapted
system they must balance or a more
productive system will take over. Thus,
an adapted system may be able to use
toxins
Results and Discussion
A literature review of Cd toxicity and
bioconcentration was presented to
substantiate model formulations that
follow. In particular, the effect of Cd at
low concentrations was often found to
be stimulatory in a wide range of
taxonomic groups. The significance of
this "Arndt-Schulz" effect may be that
ecological systems and their components
have developed pathways to utilize toxic
substances for increased productivity at
low, naturally occurring concentrations.
Mechanisms of stimulation are variable,
but the cause of these adaptations may
be consistent — namely, the criterion of
maximum power.
Bioconcentration of Cd by many
organisms over a range of Cd concen-
trations was found to be approximately
hyperbolic. Concentration factors as
high as 80,000 X have been reported for
algae at low Cd concentrations. Through
concentrating mechanisms, biological
systems can have a large impact on
cycling of Cd in nature. When absorbed
or adsorbed by a motile organism, Cd
may be transported and relocated in a
system. Through uptake and death, an
animal or plant may store Cd for varying
time spans, effectively removing it from
biological circulation. If inputs of Cd are
low, biological uptake may greatly lower
effective concentrations. Thus, through
natural selection of species with
resistance to Cd and detoxifying mech-
anisms, ecosystems may be able to
regulate Cd concentrations to optimal
levels for maximizing productivity.
The energy embodied in Cd by three
different concentrating processes was
calculated. Thus the entire earth's
geological system, a biological system,
and industrial civilization all invest
energy in concentration of 'Cd to
different levels. For example, Figure 1
summarizes the flows of energy and
materials necessary to produce pure Cd
in the industrial system. This analysis
gave an estimate of 4.6 x 1Q7 S.E.Cal-g
Cd'1 as the energy transformation ratio
(embodied energy) of Cd. This value was
used to back-calculate the embodied
energy of Cd at other concentrations for
the correlation presented below.
An aggregated model of a stream
ecosystem was developed to predict Cd
toxicity effect over a wide range of
concentrations. The model was cali-
brated from data collected by Giesyet al.
(1979) in complex stream microcosms
receiving 5 and 10/ug Cd-L~1 fora one-
year period. Figure 2 presents the
energy and material model of Cd fate
and effects. A stimulatory action of Cd
on primary production was incorporated
as well as toxic effect on biological
storages and nutrient recycling
The predicted energy effect of Cd from
the stream model was plotted versus
the embodied energy of Cd at various
concentrations to quantify the relation-
ship between these two properties of an
ecological controller. Figure 3 presents
the correlation that was predicted for
system functioning and major biological
components. In most cases, the correla-
tion between Cd transformation ratio
and energy effect ratio was first
positive, then negative, and approached
zero at high concentrations. In addition,
in the positive part of the correlation
curve, the actual values of these
parameters were within one order of
magnitude of each other for the system
energy flow measures (gross production,
respiration, and export). Thus the
energy effect of a controlling substance
such as Cd may be directly comparable
to its energy cost of concentration
(embodied energy).
The idea of toxin effect being a
function of energy cost for naturally oc-
curring chemicals may allow a syn-
thesis of information in dealing with the
increasing load of toxic wastes. Using
the concept of embodied energy, quan-
tification of control by any toxin is
possible and comparison between
toxins in equivalent energy units can be
made. In conjunction with the recogni-
tion of natural stimulatory effects of
toxins at low concentrations, the ideas
of control presented in the final report
may strengthen our ability to serve as
ecosystem managers.
Conclusions
Cadmium consistently stimulates
growth parameters of biological systems
at concentrations slightly above ambient
levels. This stimulatory role of Cd may
be useful in maximizing the productivity
of human-perturbed ecosystems. Cad-
mium-adapted systems may be useful
in the recovery of Cd wastes by
bioconcentration.
At higher concentrations, Cd is
extremely toxic to biological systems. A
continuous input of only 5 yug Cd-L~1
lowered average gross production and
respiration by 40% in soft-water stream
systems over a 1 -yr period. Thus,
intermediate levels of Cd may be useful
as a toxic control of biological systems.
Cadmium is an easily depleted re-
source because of its extremely low
natural production rates. The embodied
energy of Cd storage is high, makingthe
conservation and recovery of Cd impor-
tant for long-term survival of human
systems.
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Cadmium W-
Production j
\
Other
By-Products
Results of all toxicity studies should
be organized under one general system
such as the embodied energy-energy
effect curves presented in this report.
References
Giesy, J.P., H J. Kania, J.W. Bowling, R.L
Knight, S Mashburn, and S. Clarkm.
1979. Fate and biological effects of
cadmium introduced into channel
microcosms. U.S. Environmental Pro-
tection Agency, Athens, GA. EPA-
600/3-79-039. 156 p.
Odum, H T. 1979. Energy quality control
of ecosystem design. Pages 221-235
in R F Dame (ed.) Marsh-estuarine
system simulation. University of
South Carolina Press, Columbia, SC.
Figure 1. Evaluated model of Zn and Cd production by the electrolytic process
used to calculate the embodied energy of purified Cd. Note that Cd is
produced as a by-product ofZn production and therefore, the embodied
energy of Cd includes the energy necessary to generate pure Zn.
An energy analysis of the data from a
calibrated Cd-stream model indicated a
positive-negative correlation between
the energy cost of Cd and its energy
effect. The model data indicated a
possible equivalence between the
stimulatory and toxic actions of Cd and
its energy cost of concentration at
naturally-occurring concentrations.
Recommendations
Tables of embodied energy should be
calculated for all chemical substances
of potential environmental impact as a
way to organize the understanding of
which wastes are important.
Toxicity studies of chemicals should
include careful examination of effects at
low, stimulatory levels as well as at
higher toxic levels. More studies
concerned with the stimulatory and
toxic effects of chemicals on ecosystem-
level parameters are needed Large-
scale microcosms may provide the best
means to study hierarchical effects of
chemicals.
Additional data concerning energy
inputs to toxicity research should be
routinely reported These inputs include:
energy (illumination, stirring, heating,
etc.); materials (nutrients, gases, mocula,
etc.), structure (cost); and human
services In addition, ambient concen-
trations of toxins should be monitored
and reported in batch studies.
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Figure 2. Energy and material model of stream microcosms receiving Cd inputs. Sunlight and nutrients in stream inflow
generate a productive system of algae, macrophytes, consumers, and detritus-microbes. Cadmium in inflow
water has both stimulatory and toxic action, and water concentration is regulated by biological storage and cycling.
Model was stimulated to predict Cd toxicity effect at concentrations not actually tested.
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701-
Gross Production
4 6 8 10 12
Cadmium Transformation Ratio
(S.E. Cal-Cd-' x
14
16
Figure 3. Predicted correlation between Cd transformation ratio (embodied
energy) and Cd energy effect ratio for system-level parameters and for
storages. Values are calculated from 1 -yr averages of simulation data
from Cd-streams model.
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Robert L Knight is with the Center for Wetlands. University of Florida, Gaines-
ville, FL 32601.
L. A. Burns is the EPA Project Officer (see below).
The complete report, entitled "Energy Model of a Cadmium Stream with Correla-
tion of Embodied Energy and Toxicity." (Order No. PB 82-256 876; Cost:
$13.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
College Station Road
Athens, GA 30613
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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