ROUND TABLES: 2. ENVIRONMENTAL BIOINORGANIC CHEMISTRY
will depend on: i) the relative abundance of the
solid in natural waters, ii) the wavelength dependent quantum yield of surface complexes and/or
of the solid for the given chemical reaction, and
iii) the wavelength dependent light intensity in the
water of interest.
The band model has proven to be a useful model
for understanding the electronic properties of
some solids. In order to assess the potential of a
wide variety of naturally occurring solids for participating in surface photochemical reactions, an
examination of the band gap energies of some
well characterized solids may be useful, provided
one realizes that the band theory may not be the
most appropriate model for the electronic properties of all solids. Within the framework of band
theory, for a solid to participate in surface photochemical reactions of interest in natural waters, its
band gap energy must be constrained to values
less than 4.20 eV, which corresponds to light of
wavelength 295 nm. The minimum energy required to activate the solid may be altered by surface
coordination, stoichiometric deficiencies, impurities, and crystal defects, all of which may introduce energy levels within the band gap.
RT2.4 — MO
FRANÇOIS M.M. MOREL
the effective free ferric ion concentration near the
cell surface as controlled by coordination and
redox reactions, and by transport processes. In
media containing high concentrations of chelating
agents, uptake rates in the dark are simply proportional to the equilibrium free ferric ion concentration in the bulk solution. Many Fe(III) chelates are photoreactive, so that in the light, a rapid reduction of the chelated Fe(III) to Fe(II) and
reoxidation of Fe(II) by oxygen can take place,
leading to an increase in the effective free ferric
ion activity and in uptake rate. In the absence of
chelating agents, iron transport can become limited by the dissolution rate of iron oxide or by the
diffusion rate near the cell surface depending on
concentrations and mixing conditions. The kinetics of uptake are also directly affected by other
trace metals (e.g. Cd) which compete with iron
for cellular transport sites.
Under all conditions, a sizeable fraction of the
cellular iron (20-80%) is bound to the surface of
the cells, and can be readily removed by acidification, reduction or chelation. Under steady conditions, that iron is not measurably transported into
the cell and thus the surface bound iron does not
then serve as an iron storage mechanism. However, upon decreases in pH, a fraction of the surface bound iron is released, a large portion of
which is transported into the cell. There is thus a
two step mechanism providing for surface accumulation of iron at high pH (e.g. during the day
when the algae photosynthesize and deplete the
CO 2 locally) and subsequent uptake at low pH
(e.g. during the night when the algae respire and
increase the local CO 2 concentration).
Ralph M. Parsons Laboratory
Department of Civil Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
U.S.A.
IRON UPTAKE AND PHYTOPLANKTON
GROWTH
The kinetics of iron uptake have been studied in
cultures of the coastal diatom Thalassiosira weissflogii, both in chelated (buffered) and unchelated
media. The availability of the iron is governed by
140
The maximum iron uptake rate is dependent on
the previous iron nutrition of the algae. Under
iron limitation, one can observe increases in maximum (short term) uptake rates, decreases in cellular iron, and/or decreases in growth rates depending on the free ferric ion concentration. The
general relationship among these physiological
parameters has been elucidated quantitatively. At
a "median" free ferric ion concentration (ca. a
10 -20 NO both the short term uptake rate and the
cellular iron concentration are near their maximum values and result in maximum growth rate.
At higher free ferric ion concentrations, the maxiRev. Port. Quím., 27 (1985)
2nd INTERNATIONAL CONFERENCE ON BIOINORGANIC CHEMISTRY
mum short term uptake rate is decreased in such a
way that the actual steady iron uptake rate, the
iron cellular quota and the growth rate all remain
constant (and maximum). At lower free ferric ion
concentration, the cellular quota decreases in such
a way as to maintain maximum growth rate.
When the cellular iron quota reaches some minimum value, the growth rate itself decreases proportionally to the free ferric iron concentration.
The mathematical model corresponding to these
interactive processes appears generally applicable
to all limiting phytoplankton nutrients. The model
can be further applied to formulate the uptake
and growth kinetic effects of toxicants and to predict the stable ambient nutrient and toxicant concentrations in systems under geobiological control.
RT2.5 — MO
J.E. CROSS
D. READ
G.L. SMITH
D.R. WILLIAMS
Department of Applied Chemistry
UWIST PO Box 13
Cardiff CFI 3XF
U.K.
PLUTONIUM SPECIATION
FROM DISPOSAL VAULT INTO MAN
Plutonium is one of the most important components of radioactive waste arising from the
nuclear power programme. It is the major transuranic by-product of the nuclear fuel cycle and,
although a considerable proportion of the radioelement is removed during reprocessing, nuclear
wastes may contain significant quantities of this
element. The chemistry of plutonium — its long
half-life (2.41 x 10 4 years for Pu-239 and
6.55 x 10 3 years for Pu-240), its high specific activity of a-emission and its chemical toxicity —
make this element of paramount importance in
radiological assessments. Hence, the predicted
Rev. Port. Quím., 27 (1985)
chemical behaviour of plutonium in any proposed
radioactive waste disposal site must be carefully considered.
The disposal of any radioactive material involves
the encapsulation of the waste within a suitable
matrix and its subsequent placement in either a
natural or engineered repository, known as the
vault. It is intended that this barrier will contain
the waste until such a time that it no longer
poses an environmental threat [1]. However, the
situation in which the vault fails, and the radioactive material enters the surrounding geosphere
and is ultimately incorporated into biosphere food
chains, must be considered when assessing the
safety of a proposed site.
In order to evaluate the behaviour of plutonium
along the pathway from the vault through the
geosphere to the biosphere, a knowledge of its
physical and chemical forms at each stage is
required. Computer simulation modelling may be
used to predict such information concerning the
speciation of plutonium. Chemical speciation models use thermodynamic formation constant data
for all possible complexes which may be present
and compute the equilibrium species distribution
in a given scenario, enabling the most important
plutonium species to be identified. To model a
particular system, it must be fully characterised in
terms of component concentrations, Eh and pH,
together with the physical properties of the disposal environment pertaining to ion exchange and
sorption phenomena.
Cement is being considered as a matrix for waste
containment for low and intermediate level waste
disposal facilities [2]. Hence, as a first approximation, the flooded vault may be considered as a
cement solution. Calculation of the speciation of
plutonium in this media has shown that the behaviour of this radioelement in the vault is dominated by the high pH encountered therein. Subsequent release of this solution into the geosphere is
accompanied by a decrease in pH to the near neutral conditions typically encountered in groundwaters. In this situation, competitive complexing by
inorganic ligands, such as carbonate and fluoride,
and by organic material, becomes important.
The bioavailability of a metal is dependent on its
chemical speciation. Thus, the extent to which
plutonium is taken up by man will be determined
141