V
EPA-600/3-76-018 OALLA.S, T=XAS
February 1876 Ecological
RUTHENIUM: ITS BEHAVIOR !N
PLANT AND SOIL SYSTEMS
^ ^
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Las Vegas, Nevada 89114
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RESEARCH REPORTING SERiES
Reseat hv;ii": o'lht Ot'i^ •.'• R'1'.-ai '•.* ir i><. •'. i.viii'.-n1 i. 1: Enviuiiini!r',.i!:
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2 EnviKinmriital Picdectiod It'/hii'iioch
4 tiu'nonmentai Monitoring
5 Soc ioeaonomic Erivironrnenta1 Suiaiet
This repon has !i«en assigned to the tCOLCiGlC-AL RESEARCH senes Thii series
describes restarcri ori trie eftects 01 pollutio; or- fiunians plan* and anin'ial
species and materials Problems are a^sesseo 10' their long- and short-term
mfluenc'es Investigations include lormanon tiarispon aric pathway studies to
deternnne the iate o: polluiants. ano then euenj. "i inj wc>n provides the terhnira:
basis for setti'ip standards to rrnnimize jn"Jc-c,;;aoie chancjes, it. living oryamt.niE
in the dquatic tenestriai ano airnospnerK ^ri\'irfirir;i>?n'r,
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EPA-600/3-76-019
February 1976
RUTHENIUM: ITS BEHAVIOR IN PLANT AND SOIL SYSTEMS
by
K. W. Brown
Monitoring Systems Research and Development Division
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
Program Element 1FA083
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
LAS VEGAS, NEVADA 89114
-------
DISCLAIMER
This report has been reviewed by the Environmental Monitoring and
Support Laboratory-Las Vegas, U.S. Environmental Protection Agency, and
approved for publication. Mention of trade names or commercial products
does not constitute endorsement or recommendation for -use.
ii
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CONTENTS
Page
List of Tables iv
Introduction 1
Summary 2
Ruthenium in Soil Systems 3
Ruthenium in Plant Systems 11
Literature Cited 16
iii
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LIST OF TABLES
Number
1. Mechanical and chemical characteristics of acidic
soils used in laboratory investigations by Jean-Paul
Amy (1971)
2. Mechanical and chemical characteristics of the
calcareous soils used in laboratory investigations
by Jean-Paul Amy (1971)
3. Sorption percentage of cesium-1.37 and three forms of
ruthenium-106 by Piboulette soil as a function of
contact time (Amy 1971)
iv
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INTRODUCTION
Numerous investigators have studied the biological behavior of
ruthenium in both plant and soil systems. These studies- were prompted
by the release of ruthenium isotopes into the biosphere by the military
weapons and Plowshare nuclear testing programs conducted by the U.S.
Energy Research and Development Administration (ERDA), formerly the U.S.
Atomic Energy Commission.
Following the signing of the nuclear test ban treaty in 1963, the
scientific interest in radioruthenium decreased apparently because of
the discontinuance of atmospheric nuclear testing and later, due to a
reduction in the Plowshare Program's nuclear excavation experiments.
However, a number of investigators have indicated that due to the
high concentrations of radioactive materials being discarded in waste
disposal sites, and also, because of the expanding nuclear power industry,
ruthenium may ultimately become an important environmental pollutant
(Booth, 1975).
It was once thought that ruthenium, which is the rarest of the six
elements in the platinum series (Amy, 1971), contributing only 0.004
parts per million of the earth's crust, would not become incorporated
into living organisms. However, one of the first indications that
ruthenium could become biologically available, via soil, was shown by
Selders (1950), and by Cowser and Parker (1958) when they reported that
ruthenium which had been discarded as radioactive waste was quite mobile
in soils near the Oak Ridge National Laboratory. The incorporation of
ruthenium by plants was shown by Selders (1950), Neel et al. (1953),
Auerbach (1957a), (1957b), Klechkovsky (1956), Goss and Romney (1959),
and later summarized by Auerbach and Olsen (1963) when they reported
that ruthenium was found in a variety of plants grown in both soils and
hydroponic nutrient solutions containing ruthenium. These studies,
plus others similar to that conducted by Crossley (1967), which showed
that ruthenium-106 was being taken up by plants, and eventually becoming
incorporated into the food chain of herbivorous animals, accounted, to a
large extent, for the continuance of studies concerning the environmental
behavior of this pollutant.
One of the chemical characteristics, of ruthenium which, complicates
the identification of this element in its biological and environmental
behavior is that it can be found in a variety of valence states, ranging
from 0 through +8. Numerous chemical complexes of ruthenium have
already been identified in environmental samples; some of the more
common include; oxides, amines, halogens, and the nitrosyls. The two
common oxides include; dioxide, Ru02 , and tetroxide, RuO^. The halogen
elements, frequently chlorine, form compounds and -various chemical
complexes with, ruthenium, commonly in the di-, tri-and tetravalent states.
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Perhaps, the most biologically important compounds are ruthenium in
the form of amines and nitrosyl ruthenium complexes. These complexes
are formed primarily in radioactive waste systems, and during treatments
of irradiated nuclear fuels by nitric acid. There are two recognized
classes of nitrosyl ruthenium complexes: (1) the Tins-table nitrato
complexes having the general formula lRuNO(N03)xCOH:)yCH20)zI|Q where the
sum of x, y, and z is equal to 5, and the charge Q is- equal to 3—(x+y);
and (2) the more stable complex, nitro, with a general formula including
at least one N02 group, such as [RuNO(N02)x(N03)yCOH)3_x_ CH2°)Z], where
x=l. The nitro complexes can have a neutral, cationic, or anionic
character, depending on the values of x and y (Amy, 1971). Because of
their importance, a number of investigators, Fletcher et al. (1955),
Rudstam (1959), and Amy (1971), have identified and isolated many of
the ruthenium-nitrosyl compounds.
Of the 16 ruthenium isotopes, 4 are considered to be of Biological
importance (Radiation Protection Handbook 69, NCRP, 1959). These four
nuclides include ruthenium-97, -103, -105, and -106, having radioactive
half-lives (Teff) of 2.8 days, 39.5 days, 4.4 hours, and 368 days,
respectively. The two longer-lived isotopes, lff3Ru and 106Ru, have
received more consideration in the scientific community primarily due to
their longer half-life and also because, as reported by Tikhomirov and
Petrukhin (1967), their yield, up to 20% during the fissioning of nuclear
fuels, contributes a significant amount to the growing quantities of
radioactive wastes.
This review was undertaken to assemble and summarize the available
data concerning the distribution and behavior of ruthenium in both the
soil and plant systems. The evaluation and integration of this data
will also be used to identify areas in need of investigation.
SUMMARY
Environmental levels of ruthenium in selected areas have increased
in recent years above the amounts contributed by atmospheric fallout.
These trends in ruthenium accumulation will continue to rise as the
uses of nuclear power and the reprocessing of nuclear fuels increase.
These increasing concentrations have, however, provided opportunity for
biological investigation.
Experimentation dealing with various inorganic and organic compounds
of ruthenium found in waste effluents have shown that these compounds
may become incorporated and/or taken up by plants in greater or less
quantities, or behave differently when exposed to a variety of environ-
mental media. This sometimes erratic and unpredictable behavior has
been attributed to ruthenium's various and sometimes changing chemical
forms.
As the available data concerning the biological behavior of
ruthenium are reviewed, it becomes apparent that many questions
concerning the qualitative identity and Behavior of this element still
exist. For example, previous investigations have made it clear that
ruthenium will become fixed by varying degrees on most environmental
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media. However, the identity of the fixation, incorporation, or bonding,
whether by mechanical or chemical means, is not well understood. Ionic
exchange of ruthenium in soil systems, which appears to Be nonexistent
based on available data, requires additional study. Also, the biological
availability of ruthenium, which may be influenced by exchangeable
cations, may also Be, in all probability, affected By microbial
assimilation and metabolism. Data concerning these parameters are
lacking.
Because of the chemical complexity of this element, interpretation
of its behavior in both plant and soil systems at present requires a
considerable degree of caution. This is primarily due to studies that
have not reported in sufficient detail the mechanical or chemical
makeup of the environmental media that are being treated.
Perhaps the single most important parameter that is in need of
investigation is the identity of ruthenium species. Evidence indicates
that ruthenium chemistry is very complex and identification in many
cases is extremely time consuming. However, identification of the
ruthenium species must be taken into account before valid interpretations
can be made.
It is generally agreed that ruthenium may become a greater environ-
mental liability than had been originally anticipated. As a result,
a complete documentation of its distribution, fate, and behavior in our
environment is essential, especially in view of our expanding nuclear
industry.
RUTHENIUM IN SOIL SYSTEMS
Investigations concerning the behavior of selected pollutants in
terrestrial environments have been initiated, primarily to determine
and identify a pollutant's transfer rate and pathway between various
biotic compartments within an ecosystem and, also, to determine the
magnitude of dispersion within the biota and to locate and define
abiotic and biotic sinks. Additionally, studies designed to utilize
local soils, flora, and fauna as biological sensors to measure a
pollutant's impact on the biota through cellular accumulation and
biological effects are now in progress (Crockett, 1975). Many studies
have been initiated following accidental and/or continual releases of
radioactive pollutants into our environment, while other studies have
been conducted under controlled conditions using radioisotopes as
tracers. Both types of investigations have added to our understanding
and knowledge of the complexity of ruthenium in the environment.
Other important contributions concerning the behavior of ruthenium
in soil systems have been provided by the numerous surveys conducted in
many geographical areas throughout the world. These studies, such as
conducted by Ritchie et al. (1970), Mishra and Sadasivan (1972), Plummer
and Helseth (1965), Tikhomirov and Petrukhin (1967) and Matsunami et al.
(1974), tend to show the distributions and patterns of radioruthenium in
soils and their associated ground waters following atmospheric fallout.
Ritchie et al. (1970) also point out that, due to the discontinuance of
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atmospheric testing, these sampling results may b& of historical interest
as long-range benchmark comparisons of nuclide distributions over
extensive undisturbed landscapes.
Studies involving ruthenium in soils have been conducted near nuclear
reactor sites, and on the EKDA's Nevada Tes-t Site following nuclear
testing. Extensive experimentation has- also been conducted in areas
used for the disposal of both liquid and solid radioactive was-tes.
These studies were initiated due to practices of using soils to sorb and
retain radionuclides following their disposal.
Many soils have the capacity to remove fission products from waste
solutions by filtration, adsorption, and by ionic exchange. Fission
products, especially the cations, are exchanged for various constituents
of the clay fraction of the soil. This ability of soil to remove specific
radionuclides is contingent on the type of ion, its valence state, ion
concentration, and both the salt and chemical composition of the radio-
active waste solutions. As such, the behavior, movement, and the biologi-
cal availability of ruthenium in soil systems are directly affected by
chemical transformations and reactions involving both the soil minerals
and solutions.
Early investigations concerning the behavior of ruthenium in soils
were conducted by Brown et al. (1956), Cowser and Parker (1958), Spitsyn
et al. (1958), and Brown et al. (1958). All of these studies showed
that ruthenium was quite mobile and indicated that further investigations
were needed for a more adequate containment of ruthenium. For example,
Brown et al. (1955) and (1958) found that when low-level liquid waste
containing ruthenium-106 with a fairly high salt concentration of 80
grams per liter (g/1) was added to soils in disposal cribs, the downward
movement of ruthenium-106 occurred at a much faster rate than other
disposed of radionuclides. They also reported that the results of an ion
exchange study involving ruthenium again demonstrated the rapid movement
of this isotope in soils, as its movement was delayed only slightly
through laboratory soil columns. Following the rapid movement of
ruthenium-106 in a sodium nitrate solution through a sandy clay loam
soil, Spitsyn et al. (1958) hypothesized that areas most favorable for
radioactive waste sites would be those which have low-lying subsoil
water with slow movement, and soils with both a low concentration of
calcium ions and a generally low salt content.
As the supply of radioruthenium for disposal increased, reevaluation
of existing waste sites and the development of methods for permanent
containment were required. These investigations included the testing
and evaluation of many different soils, in addition to a selected number
of humic materials, for their retentive properties. One of these
investigations, related to the disposal of low-level reactor waste into
soils, was conducted by Bryant et al. (1961). Their study was designed
to measure the fixation of ruthenium on clay soil, and also to determine
if this containment could be retained more adequately in fermentable
humic materials such as soil sawdust and soil peat mixtures. Since
ruthenium exists in a variety of valence states, as cations, complex
anions, and neutral oxides, it was anticipated by Bryant that the ruthenium
compounds may become complexed with stable organic and microbial materials
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during the soil-sawdust and peat decompositions. He further hypothesized
that nitrate ruthenium salts could Ee reduced by oxygen^onsuming
bacteria under anaerobic conditions. Ruthenium chloride, completed
with a nitric acid solution to simulate radiochemical waste, was
applied to soil columns containing clay and the soil-^sawdust and aoil-
peat mixes. At preselected times during a 120-d.ay storage period, the
columns were leached with distilled water and then sampled. Their
results showed that the retention of ruthenium increased in time for
both sawdust and peat mixes. The amount of ruthenium retained exceeded
90% in both humic mixes. The degree of fixation was determined by
leaching selected columns with three different concentrations of sodium
nitrate. The sodium which competes for the available ion-exchange
sites did not have any effect on the ruthenium fixation. The clay mix
appeared to be the most effective medium for the retention of ruthenium.
This is probably due to the abundant ion-exchange sites found in clay
soils.
Another study, conducted by Lomenick G.963) using a different form of
ruthenium determined the degree of fixation on a silty clay loam soil
collected near the Oak Ridge National Laboratory. This soil, which was
contaminated with a mixture of nitrosylruthenium hydroxide and mononitrato
nitrosylruthenium that had seeped from nearby waste pits, was leached
with six different solutions. His results showed that less than 10% of
the ruthenium was removed by leaching with water, both tap and distilled,
and also, with low normality acidic and basic solutions. Nearly 45% was
removed by KMnO^ at a pH of 8.8, while in excess of 60% was removed by the
three following solutions: NaOH at a pH of 11.3 and two acids, HC1 and
HN03 having a pH of less than 1.0. It was suggested that, of the two
forms of ruthenium, the hydroxide is retained to a larger degree than the
mononitrato nitrosylruthenium which was essentially unadsorbed by the soil.
Similar studies using a variety of inorganic and organic compounds of
ruthenium were conducted by Spitsyn et al. (1958), Nishita et al. (1956),
van der Westhuizen and van Rensburg (1973) and Amy (1971). Spitsyn et al.
(1958), using inorganic radioruthenium in solutions of NaOH, NaNO and
A1(N03)3, found in laboratory experiments that the sorption of ruthenium
in soils was extremely low. Sorption from the alkaline solutions reached
several tenths of a millicurie per 100 grams (g) of soil when cationic
ruthenium was used. However, almost no ruthenium was adsorbed from the
acidic solutions and no anionic ruthenium was absorbed by the soils. The
non-absorption of anionic ruthenium was again shown by Spitsyn et al.
(1958), when they applied ruthenium-106 to field soil which was identified
as a sandy clay loam. To determine fixation and extractability,
ruthenium-106 as the cation in the chloride form, in a HC1 solution, was
fixed in seven different soils by Nishita et al. (1956). These soils
included four loams having pH's of 4.6, 5.7, 6.5, and 6.6, one muck with
a pH of 3.9 and two clays, Bentonite pH 8.2 and Kaolinite pH 4.3, which
were leached with NH^Ac and distilled water. Their results indicated
that cationic ruthenium is also quite mobile as it, with few exceptions,
was leached from these soils in greater amounts than other tested radio-
nuclides. They also determined that the pH of leaching solutions may
affect the extractability of ruthenium from soils when they reported that,
when aliquots of acidic acid adjusted from a pE of 2.3 to a pH of 9.2 with
NH^OH were added to these soils, the amount of rutheniom-106 leached
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decreased slightly with increasing pE up to a pH of 8.5 and then the
amount of ruthenium leached increased.
The sorption of cationic ruthenium was again, shown to occur when
van der Westhuizen and 'van Rensburg (1973) added ruthenium to sandy
soils. Their investigation was conducted using the jar method COrcutt
et al., 1956) to determine the movements of radionuclides through porous
soils under the influence of ground water. The results- showed that a
downward movement of 1,000 meters by soil water through sandy soil would
result only in a 2-meter movement of ruthenium. Because of the retarda-
tion of movement caused by the sorption of ruthenium in the sand, its
relatively short half—life and the apparent dissipation of this element
in sandy soils, they- concluded that it would not present a serious
environmental hazard.
Perhaps the most ambitious and revealing investigation was conducted
by Amy 0-971). His work was conducted in an agricultural soil environ-
ment that had been contaminated by irrigation water in which radioactive
wastes from a nearby nuclear facility had been discharged. Of the
radioactivity in the effluent being discharged, ruthenium-103 and
ruthenium-106 contributed approximately 90% of this total. In addition
to the soils under cultivation, a number of acidic and calcareous soils
representative of this irrigated region were used in laboratory investiga-
tions for the determination of specific parameters which favor and/or inhibit
ruthenium migration and sorption by soil (Tables 1 and 2).
TABLE 1. MECHANICAL AND CHEMICAL CHARACTERISTICS OF ACIDIC SOILS USED
IN LABORATORY INVESTIGATIONS BY JEAN-PAUL AMY (1971)
Soils
o
i-< T3
I"S «
ill!
Levels
All
A12
(B)
(B)C
C
Al
A2
B
pH
4.1
4.1
4.1
4.2
4.4
5.4
4.25
4.8
Granulometry %
Clays
15.7
13.0
10.8
9.2
9.2
22.6
29.1
35.7
Fine
Loams
15.2
13.8
14.4
6.5
10.2
40
25
27
Coarse
Loams
2.6
6.0
5.2
4.3
3.8
22
18
16
Sands
58.9
62.1
68.3
72.0
68.3
4.5
26.5
20
Matter
C %
7.1
2.8
1.9
1.3
0.9
2.68
2.10
1.27
N 7.
0.398
0.158
0.113
0.071
0.046
1.14
0.25
0.09
C/N
17.8
17.5
17.2
18.3
19.1
10.7
16.1
13.8
Absorbent Complex
S
meq/lOOg
4.75
1.23
0.87
0.70
0.59
12.02
4.20
8.86
meq/lOOg
27.1
17.8 .
15.5
14.2
12.2
16.5
15
17,7
S/T
17.5
6.9
5.1
4.9
4.8
73.2
28.0
49.8
Free
Sesquioxide
Iron
18.8
26.0
23.2
21.4
21.6
32.1
34.5
60.1
Alumi-
num
3.9
6.2
6.9
6.5
7.0
6.5
6.8
10.8
Silica
4.6
6.0
5.0
5.1
5.5
6.8
9.7
12.9
TABLE 2. MECHANICAL AND CHEMICAL CHARACTERISTICS OF THE CALCAREOUS SOILS
USED IN LABORATORY INVESTIGATIONS BY JEAN-PAUL AMY (1971)
Soils
Miemar
Codelet
Piboulette
Pichegu
Panier
Barthelasse
pH
7.9
7.7
8.3
8.4
8.1
7.2
Granulometry (Z)
Clays
2.1
3.4
5.4
8.7
5.3
2.8
Fine
Loams
6.3
4.3
12.4
24.2
42.5
28.4
Coarse
Loams
4.4
19.9
30.2
35.4
38.2
26.1
Sands
87.2
72.4
52.0
31.7
14.0
42.7
Organic
Matter
m
3.8
2.5
4.2
2.9
5.3
12.5
Absorbent Complex
Ca
(meg/lOOg)
15.41
12.21
31.42
28.42
21.42
26.49
Mg
(meg/lOOg)
0.92
0.77
1.51
1.77
1.84
2.21
K
(meg/lOOg)
0.44
0.52
0.75
0.92
0.81
1.03
Na
(meg/lOOg)
0.99
0.12
0.19
0.22
0.27
0.34
S/T
sature
"
"
"
"
Total
Calc
(%)
25.1
15.7
20.7
35.9
28.4
34.1
Active
Calc.
C%>
6.2
2.8
5.1
9.7
7.7
3.1
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Following the radioanalysis of soils collected at various depths in
this irrigated area, it appeared that the mobility of ruthenium-106 was
associated primarily with the soil texture and the percolation rate of
the applied irrigation water. These preliminary conclusions were based
on two findings: (1) Of the nitrosyl ruthenium complexes applied to
the soil in the irrigation waters, only a small portion was found to be
sorbed on either the irrigation canal sediments or on the irrigated soil
itself, (2) Even though most of the ruthenium was associated in the top
layers of the irrigated soils, due exclusively to the continual applica-
tion of ruthenium contaminated water, it was commonly found to be evenly
distributed at the lower depths in the soil profile. Amy, similar to
van der Westhuizen and van Rensburg (1973), hypothesized that because
of the mobility and even distribution of ruthenium in these soils, a
favorable radioecological situation existed as there was not any large
accumulation and also, because of the relatively short half-life of
ruthenium, 368 days, the soil concentrations were always quite low.
Amy's laboratory experiments were conducted using three different
ruthenium-106 solutions. Two of these were the nitronitrosyl ruthenium
complexes, dinitro, RuNO(N02)2OH(H20)2, and nitrodinitrato, RuNON02~
(N03)g(H20)2. These two compounds were selected because both are very
stable and constitute 70% of the ruthenium form being released from
nuclear facilities. The other compound used was ruthenium chloride.
The two laboratory experimental methods used by Amy were the
percolation method and the agitation method. The percolation method
consisted of filling a glass soil column measuring 30 centimeters (cm)
in height and 4.5 cm in diameter with soil. The soil was then pre-
moistened with distilled water via capillary action, followed by the
addition of one of the ruthenium-106 tagged compounds. The amount of
ruthenium solution applied to each column was approximately 1-liter
with a total activity of 5 microcuries (yCi) . The duration of the
percolation varied depending on the texture of the soil used and ranged
from 1 hour to more than 70 hours. After percolation, the soil column
was dissected into nine equal sections, each then being analyzed for
its ruthenium content.
The agitation method consisted of mixing 25 g of soil into a
1-liter ruthenium contaminated solution. This mixture was continually
stirred, thereby keeping the soil in as near a homogenous suspension as
possible. At predetermined times, 15 milliliter (ml) aliquots of this
mixture were collected and then filtered. The soil and filtrate were
then analyzed for ruthenium content.
As the behavior of ruthenium in soils is quite complex, because of
its many changing chemical forms and its ability to form both cationic
and anionic complexes, the ionic nature of these three solutions was
determined. After passing these compounds through both anionic and
cationic resins, the nitrodinitrato complex, RuNON02(N03)2(H20)2, was
identified to have a cationic character, whereas the dinitro complex,
RuNO(N02)2OH(H20)2, had an anionic character. The ruthenium chloride
caused a predominance of anionic forms to appear which had previously
been documented by Bhagat and Gloyna (1965). It was also reported by Amy
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that these compounds can change in their ionic character as a function of
time. As a result, the behavior of the ruthenium was studied in both the
acidic and calcareous soils simultaneously.
Amy's percolation experimental results showed that all of the
ruthenium compounds used were quite mobile. As expected, the movement
of the anionic form through all of the soils tested exceeded both the
ruthenium chloride and the cationic nitrodinitrato form. In fact, two of
the soils, Miemar and Codelet, retained only 11% and 36% of the total
anionic ruthenium, respectively. In contrast to the anionic form 38%
to 80% of the cationic ruthenium was sorbed in the surface layers
(0-4.5 cm in depth). Even though the retention in the surface layer
was generally quite high, at no time did the cationic ruthenium fail to
migrate throughout the soil column. The minimum amount retained in the
bottom soil layer varied between 1% and 2%, with a minimum of 2% and a
maximum of 33% being passed into the percolate.
The data also showed that generally ruthenium chloride was more
mobile than the nitrodinitrato ruthenium complex, with 10% to 50% of the
applied ruthenium being retained in the 0- to 4.5-cm surface layers.
The amount of ruthenium chloride that passed into the percolate following
its migration throughout the soil columns varied from a low of 3% to a
high of 68% of the amount applied. The mobility of ruthenium chloride
was found to be similar to the nitrodinitrato complex in three of the
six soils tested, Piboulette, Pichegu, and Panier, with greater mobility,
similar to the dinitro form, observed in the Miemar and Codelet soils.
As is seen in Table 1, the former three soils are similar in both their
mechanical and chemical makeup. The latter two soils exhibit a high
permeability having an excess of 72% sand with less than 3.4% clay and
3.9% organic matter.
As is evident, the ionic nature of ruthenium is a very important
parameter in its rate of migration. To augment the data obtained from
these soil column migration studies, the rate of ruthenium sorption by
the Piboulette soil as a function of time was determined by the
agitation method. As is seen in Table 3, the nitrodinitrato complex with
its cationic character corresponds to some extent to the high sorption
rate of the cation cesium-137, whereas the sorption rate of the dinitro
complex with its anionic character is relatively slow. However,
irrespective of the three forms of ruthenium used, the rate of sorption
is progressive, reaching 90% sorb after a 3-day period. Similar
sorption percentages were observed by Bryant et al. (1961) during 120-day
exposure of humic materials.
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TABLE 3. SORPTION PERCENTAGE OT CESI.UM-137 AND THREE FORMS OF RTTTHENHJM-106
BY PIBOULETTE SOIL AS A FUNCTION OF CONTACT TIME (AMY 1971)
Radionuclides
Cesiun-137
Ruthenium- 106
Complexes
nitronidinltrato
RuNO N02(N03)2(H20)2
Ruthenium Chloride
105RuCl3
Dinitro
RuNO(N02)2CH(H20)2
Contact Tine (min)
1
min
95.2
49.2
7.9
5.2
5
min
98.1
60.0
9.4
6.0
10
min
99.2
64.9
12.4
7.2
40
min
99.3
73.2
21.9
10.5
100
min
99.6
80.3
36.1
18.9
500
min
99.5
88.4
85.7
42.0
1
dav
99.7
94.4
94.0
76.2
3
davs
99.7
98.1
97.4
89.6
7
davs
99.6
97.4
97.5
92.1
Other studies conducted by Amy showed that the effect of organic
matter and soil particle size on the sorption rate during ruthenium
exposures was quite variable. For example, the Barthelasse soil, which
is fairly rich in organic matter, 12.5%, was exposed to the three
ruthenium compounds both before and after the destruction of its organic
matter. The data showed that the presence of the organic matter
increased the amount of ruthenium sorbed in all three compounds with a
two-fold increase in the retention of the dinitro complex. The effect
of the sorption rate, relative to the soil particle size, was determined
by separating the treated soil into various sizes ranging from clay to
sands. He reported that the ruthenium content decreased as the particle
size increased. The clays, which make up only 5.9% of the Piboulette
soil, sorbed in an excess of 84% of the ruthenium, whereas the sands, 52%,
sorbed less than 1%, the remainder being associated with the silt
fractions.
Following these investigations, Amy conducted a study to determine
if the sorption or fixation of the three forms of ruthenium resulted
from an ionic exchange phenomenon, or if a physical bonding occurred,
depending only on the surface characteristics of the soil particles. Amy
concluded, following the data collection and analysis that the fixation
of the cationic form on the soil particle is a result of physical binding,
depending on both the particle's surface area and its physical character-
istics, whereas the anionic ruthenium compounds appear to be fixed
chemically.
Acidic soils appeared to enhance the rate of ruthenium migration
when compared to the migratior rates in calcareous soils. Amy reported
that, irrespective of a soil's texture and organic matter, the amount of
ruthenium sorbed by the acidic soils as a function of time was: always
less than that sorbed by calcareous soils. For example, depending on
soil type, the sorbtion of 50% of the radioactive ruthenium-106 applied to
the calcareous soils reqnired from 1 to 4 minutes for the cationic form,
1.5 to 5.0 hours for the ruthenium chloride, and from 2.5 hours to l.Q day
for the anionic forms, whereas for the acidic soils, the cationic form
-------
took from 30 minutes to 2.5 hours, ruthenium chloride 1 minute to 3 days,
and for the fixation of the anionic form, 3 minutes to 7 days. He also
found that the maximum amount of ruthentam sorted occurred between a
solution pR of 7-8. This finding agreed in large part with the data
reported by Nishita et al. 0-956) when they found that an increase in the
soil sorption rate occurred Tip to a pH of 8.5.
The sorption of ruthenium—106 by three different soils was investi-
gated by Klechkovsky (1958). His method, similar to Amy*s agitation
technique, consisted of mixing 0.5 g of each of the following soils:
red earth, black earth, and turf podzol, with a 10-ml solution containing
ruthenium—106. After stirring and allowing the soil to settle, aliquots
of the solution were taken and analyzed. A direct comparison of his
results with those of Amy's unfortunately cannot be made, as neither the
form of ruthenium used nor the physical or chemical identity of the three
soils was presented. His data did show, however, that between 30% and
50%, depending on soil type, was sorbed on the soils. A limited effort to
determine if the ruthenium could be replaced by calcium was accomplished
by adding CaCl2 to the ruthenium-contaminated soils. These results
showed that the calcium was ineffective in replacing the ruthenium. Two
conclusions can possibly be drawn from Klechkovsky's data: ruthenium
under proper conditions can reach a sorption equilibrium with selected
soils in a relatively short time, perhaps less than 2 hours, and once
fixed, the attraction, if ionic, is quite strong as ionic Ca++ can
replace most other ions. In 1958, Klechkovsky repeated the CaCl2
desorption treatment using two additional soils. His results were
identical to those obtained in 1957.
A limited number of investigations has been initiated concerning
the effect of various complexing and chelating agents on the mobility
of ruthenium in soils and its uptake by plants. Some of the original
work was conducted by Essington et al. (1963) when they applied and
mixed the chelating agents, cyclohexane-1, 2-diaminetetraacetic acid,
(CDTA), diethylenetriaminepentaacetic acid (DTPA), and ethylenediamine
di(0-hydroxyphenylacetic acid) (EDDHA), into a Sorrento loam soil that
had been previously contaminated by ruthenium-106. After watering, the
soil was stored for a 2—week period and then planted. The degree of
fixation and solubility of the ruthenium in the soil as a function of the
addition of each of the chelating agents was indicated by the amount of
plant uptake. A comparison of ruthenium concentration in the plants
grown in uncontaminated soil versus those grown in the soils treated with
the chelating agents showed virtually no difference in tissue concentra-
tions. They concluded that as a cation, the ruthenium may have been
strongly sorbed on the soil colloids and thus be relatively unavailable
for chelate formation. This hypothesis is probably correct in view of
the rate of ruthenium fixation with time as reported by Amy 0-971) and
Bryant et al. (1961) on a variety of soils and humic materials. They
further suggested that if the ruthenium was chelated, it may have
become fixed on the soil colloids in such. a. manner as to remain unavaila-
ble for plant assimilation.
Another study to determine the effect of soil amendments on the
movement of ruthenium in soils- was conducted by Essington and Nishita
(1966). They prepared and applied leaching solutions consisting of DTPA,
10
-------
EDDHA, and CDTA to aoll columns containing 260 g of uncontaminated soil
covered with a ^inch layer of rutheniwn^ontaminated soil. After leach-
ing, the columns were fractioned into %^inch increments and analyzed for
their ruthenium content. The results snowed that the effects- of the
chelating agents- in leaching the ruthenium were in the following order:
DTP A. > EDDHA > CDTA. A control, leached with 30 inches of distilled
water, removed only 61 of the. ruthenium from the zone of contamination
with 0.030% of the applied ruthenium occurring in the leachate where only
9.5% was removed -using the same amount of DTPA, with 0.042% occurring in
the leachate. They concluded that as only cationic metal ions are
chelataBle, this small chelate effect may have Been related to a low
concentration of ruthenium cations in the soil solution.
The transfer, moMlity, and Biological availaBility of ruthenium in
soils may be enhanced By soil microorganisms. Tor example, it is common
knowledge that soil microorganisms aid in plant assimilation of various
minerals By chemically altering their form. A study By Au G-974) showed
that plutonium was assimilated and taken up By the common fungus,
Aspergillus sp., from a plutonium—contaminated malt agar. His work, along
with AuerBach and Olson CL963), indicates that other pollutants, including
ruthenium, may Become more soluBle and moBile in soil systems following
microBial action. "Unfortunately, there does not appear to Be any
conclusive data in the literature concerning microBial effects on the soil
moBility of ruthenium.
RUTHENIUM IN PLANT SYSTEMS
In 1955, Rediske et al. indicated that the majority of the radioactive
pollutants that Became incorporated into Biological systems have entered
via plants. One means of incorporation is By the interception and retention
of airBorne contaminants By aerial portions of plants with suBsequent
transport Between the different trophic levels. Another method is the
deposition of contaminants on soils with eventual root aBsorption.
With its tendency to Be moBile in most soils, ruthenium is readily
taken up and translocated in a variety of plant species. However,
ruthenium Belongs in a group of metals identified By Menzel (1965) which
include iron, cesium, and Beryllium, and which apparently become less
concentrated in plant tissues than that in the surrounding media.
Several studies have shown that ruthenium can Be incorporated into
the aerial organs of plants following exposures to airborne ruthenium
in Both the particulate and gaseous forms. In addition to the numerous
surveys which have reported a variety of ruthenium concentrations in Both
plants and soils, related directly to airBorne radioactive fallout, a
number of local studies have Been conducted. For example, Culver (1960)
reported that growing grass, which had Become exposed to an accidental
release of particulate ruthenium, contained over twice the amount of
ruthenium per gram than did other environmental samples (litter, root mat,
and suBsurface and surface soil] collected in the same area.
Another study conducted by Hungate et al. (1960), following a
simulated reactor Burnup, reported that vegetation had Become contaminated
11
-------
by both ruthenium-Ida and rutheniMm^lQ6 following an aerial exposure.
Their data showed that initial retention of ruthenium on the vegetation
only 0.3% to Q.7% of the total radieaetiyity applied. However, the hlnding
of these contaminants on the leaf tisane appeared to be ejntte weak, as
after washing the plants- witfL water only half the deposited ruthenium
remained. This apparently- slow-permanent Binding characteristic of
ruthenium on leaf tissue was- also Indicated By Russell (1965). He
reported that growing caSBage treated witFL a soluBle form of ruthenium-106
lost nearly 90% of the original ruthenium deposited after 28 days.
Another 27% of this; was- removed By washing in water for a relatively
short period of 90 seconds-. The fairly long period In which, this soluBle
contaminant was Being lost from the leaves- is evidence that much of this
material remained superficially on the leaf surface. This conclusion is
supported By the fact that, even after 28 days/, an appreciable fraction
of the contamination remaining on the leaves could still Be removed by
washing with water. Based on these data, Russell hypothesized that from
the viewpoint of a ruthenium radiation dose to the Biota, the extent of
absorption by leaves is of small importance as compared with the extent
of its retention.
The absorption and retention of ruthenium into Both seeds and fleshy
fruits were reported By Wittwer et al. O-956) following an aerial
exposure to radioactive fallout. This accumulation of ruthenium in the
reproductive organs is quite small, however, as most of the ruthenium
entering the above-ground portions concentrate mainly in the vegetative
organs (leaves and stems) (Klechkovsky, 1956).
As with the early investigations concerning the behavior of ruthenium
in soils conducted in the vicinity of the Oak Ridge National Laboratory,
studies were in progress to determine if this contaminant was being
taken up by the local flora, Auerbach and Olson (1963) reported from
their previous investigations that concentrations of ruthenium-106 from
10 to 10 3 microcuries per gram (yCi/g) of dry weight were found in
trees growing in known seepage areas near the National Laboratory. A
similar study reported by Olson (1960) indicated that ruthenium is
readily translocated into various plant parts. His data showed that
ruthenium-106 was the main component of the radioactivity in both the
hulls and nuts collected from the local walnut, Juglans nigra, having
concentrations of 117 x 10~6 and 31 x 10~6 pCi/g dry tissue weight,
respectively.
Obviously, plant uptake of ruthenium by root absorption via a soil
exposure can be affected by soil treatments. This should not be surprising
in respect to the complex soil chemical Behavior exhibited by this
element. The soil makeup, changes in pH, additions of various organic
humic materials, and/or complexing agents, can become associated with
ruthenium forming various catlonic or anionic compounds which may serve
to limit or facilitate its availability.
Many of the early investigators attempted to identify the form of
ruthenium used in "controlled" soil-plant kinetic studies, but neglected
to Identify and investigate its chemical behavior once mixed in soil.
This fact is probably due to the extreme difficulties encountered in
12
-------
qualitative analysis of its many complex forms; howeyer, as a result, most
of the data reported is quantitative in nature.
A study conducted by Auerhach (1957b) showed that the concentration
of ruthenium-l(I6 was greater in the leaves and roots than in the stems of
bean plants, Phaselas vnlgaris, grown in pots filled with shale soil and
watered with ruthenium-containing wastes. The form of the ruthenium
contamination was thought to be nitrosyl ruthenium complexes- from wastes
which had been derived from nitric acid treatments of fuel elements.
Similar data were obtained following a series of root -uptake studies by
Klechkovsky (1956) using wheat, Triticum persicum, exposed to ruthenium-106.
He also found that the majority of ruthenium was accumulated in the
roots. His data showed that Between 88% to 95% of the ruthenium, depending
on the development stage of the plant at the time of exposure, accumulated
in this plant organ. Mature plants accumulated far less ruthenium in
both srtem and leaf tissue (1.9% and 1.5%, respectively), whereas both
stem and leaf tissues of younger plants each contained approximately 4.9%.
Other studies, which have contributed to an understanding of the root
uptake and retention of ruthenium by plants, have been conducted by
Klechkovsky and Gulyakin (1958), Goss and Romney (1959), Mills and Shields
(1961), Neel et al. (1953), Romney et al. (1957) (1966), and Ng and
Thompson (1966). Most of these investigators used crop plants in their
studies as indicators of the possible hazards associated with human
consumption of cultivated species following their exposure to soils
contaminated with radioruthenium. Generally, the results of these
studies, similar to those reported previously, showed that the greatest
amount of ruthenium incorporated by the plants occurred in the roots.
The largest amount of ruthenium found in the roots occurred in carrots,
whereas red bromegrass, Bromus rubens, which had been germinated and
grown on soils contaminated by a nuclear detonation, had the greatest
amount of ruthenium in the leaf and stem organs. It also appears from
the data of these investigators that ruthenium is taken up in much smaller
quantities than either strontium or cesium. The reason for this is
perhaps related to the fact that whereas strontium and cesium are closely
related to the plant micronutrients, calcium and potassium, ruthenium
apparently lacks any relationship with nutrient elements.
All of these studies have shown that ruthenium is taken up and
translocated into a variety of plant species. However, it is abundantly
evident that, of the quantity of ruthenium incorporated in the plant,
the majority is retained in the root system. The amount of ruthenium
that becomes fixed, either by chemical or mechanical means, on the root
surface versus that which becomes biologically incorporated into the
root tissues, however, was not fully investigated.
A greenhouse study to determine if the amount of ruthenium-lQ6
extracted from soils could be used in assessing the biological availability
of this isotope was conducted by Schulz and Babcock (1974). Basically,
their method involved planting five different soils that had been
previously mixed with rutheniura-106 in the chloride form, with ladino
clover. At 2-month intervals throughout a 10-month period, the clover
was harvested and regrowth allowed. The soils were then extracted with
the following solutions: water, IN NH^OAc, lfl[ NaOAc, and IN CaCl2. Their
13
-------
study, unfortunately, did not show any conclusive pattern as to the
availability of ruthenium. However, it did indicate that exposure of plants
to ruthenium for a considerable length of time does not necessarily result in
an increase in tissue concentration as a function of time. This is contrary
to other reported data, such as by Romney et al. (1970) who found that a
consistent increase of other radionuclides in plant leaf tissue occurred
during an alfalfa cropping study. This lack of an increase in ruthenium
uptake as a function of time is probably related to its ability to become
strongly fixed with soil colloids with time, thereby becoming biologically
unavailable.
The use of ruthenium-106 as a tracer in translocation investigations
was recently reported by Handley and Babcock (1970, 1972). Ruthenium-106,
in the form of ^6RuClo, was applied in some cases to the foliage, and in
other cases to hydroponic solutions containing a variety of crop plants
and woody shrub species. Following these treatments, the ruthenium content
of roots, and both old and newly formed plant tissue, was determined.
Their results showed that in excess of 99% of the ruthenium applied to the
foliage remained fixed on the treated area. Of the amount translocated,
no evidence of preferential movement into new growth was observed. The
results of the hydroponic treatment were similar to those obtained in the
foliage treatment with an excess of 99% of the ruthenium being retained
on the roots. Over 80% was found in newly formed tissue of corn. From
these results, even though translocation is evident, ruthenium in the
chloride form at least becomes bound very rapidly to surface tissues,
thereby reducing its availability.
The effects of ruthenium on plants have apparently received very
limited study. One study in plant nutrition was conducted by Menzel and
Brown (1959). They found that ruthenium at 0.0018 and 0.18 micrograms
per milliliter in hydroponic solutions had no effect on the metabolism of
iron by red clover, Trifolium protense.
The need for an adequate supply of water, and the relatively easy
means of disposing of radioactive wastes into aquatic environments, have
led to the construction of many nuclear power facilities along both fresh
water and marine shorelines. As a result of reactor effluent being dis-
charged into these environments, studies to determine the fate and
behavior of ruthenium in aquatic biota are being initiated.
A comparison between the biological behavior of nitrosylnitrato
ruthenium-106, and complexes of nitrosyl ruthenium-106 occurring in waste
effluents commonly discharged into the sea, was conducted by Jones (1960).
His data showed that the accumulation of the synthesized complex, nitrato
ruthenium, was always greater on marine algae than were the other two
ruthenium complexes. He also found that the majority of the accumulated
ruthenium on the algae was due to a surface binding characteristic
rather than to actual incorporation. Desorption of 60% of the accumulated
ruthenium on the algae occurred after a 1-hour emergence in clean sea
water. Contrary to the relatively weak binding on the algae, he found
that ruthenium-106 which became fixed on particles of marine sediments was
not easily removed. For example, less than 7% was removed by washing in
sea water or in water adjusted to pH's of 2, 4, or 6. However, a solution
of 50% nitric acid, or 5% solution of potassium hydroxide and potassium
periodate, removed over 80%. He concluded that the adsorption on the
14
-------
particles was predominantly the result of a chemical binding rather than
mechanical. His data further indicated that marine diatoms were as
efficient as fine particles of sand in removing ruthenium-106 from
solution in sea water. The mode of binding appeared to be about the same
for both collecting media.
A similar study by van der Borght and van Puymbroeck (1970), using
the same synthesized ruthenium complex as Jones (1960), also found that
ruthenium could be removed from marine algae fairly easily. Their data
showed that the marine algae Fucus. spp. lost 15% to 20% of the bound
ruthenium-106 after six changes of water within a 10-day period.
A number of other studies, such as by Burkholder (1963), Beninson
et al. (1965), and Simek et al. (1967), along with a study by Hampson
(1967), who used the common seaweed Porphyia umbilicalis as an indicator
species, have shown that marine flora will accumulate ruthenium. However,
these studies, being more quantitative in nature, serve to illustrate
the need for additional investigations concerning the adsorption and
fixation of defined complexes of ruthenium on biological surfaces.
15
-------
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17
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20
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-76-019
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
RUTHENIUM: ITS BEHAVIOR IN PLANT AND SOIL SYSTEMS
5. B£P.ORT DATE
February 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
K. W. Brown
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORG'WIZATION NAME AND ADDRESS
Environmental Monitoring & Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Las Vegas, NV 89114
10. PROGRAM ELEMENT NO.
1FA083
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Same as Above
13, TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD, Office of Health and
Ecological Effects
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The information published concerning the behavior of ruthenium in plant
and soil systems is reviewed and areas needing further investigation are identified.
Studies in the literature indicate that ruthenium is one of the most chemically
complex elements, thereby challenging the initiative and investigative abilities
of both physical and biological scientists.
Ruthenium can become extremely mobile in soils at one time, and then become
tightly bound the next. The retention and binding of ruthenium on soil colloids
and other environmental media have been demonstrated to be both a physical and
chemical phenomenon; however, these binding mechanisms have largely remained
unidentified and uninvestigated.
Evidence indicates that ruthenium can become incorporated into plants
through either a root or foliar exposure. Mechanisms of vegetative incorporation
and retention of ruthenium are still not fully understood, thereby requiring
continued study.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Ruthenium
Absorption (biology)
Plant metabolism
Radioactive tracers
Leaching
Agronomy
Soil chemistry
Soil physics
Radioisotopes
Ruthenium uptake
Plant systems
Soil systems
06F
07E
08M
18B
13. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
28
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
*GP O 690- 358-1 976
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