The Geochemistry of Uranium

THE GEOCHEMISTRY
OF URANIUM U
RADIO CHE MISTRY
ELALAOUICHAIMAE
BOUMANSOUKAINA
92
uranium
238.029

01
02
03
04
Introduction
Uranium
Geochemical Properties
of Uranium
Conclusion

• A large number of radioactive and stable isotopes that occur in the environment have provided a
tremendous wealth of information towards unraveling many secrets of our Earth and its environmental
health.
• These isotopes, because of their suitable geochemical and nuclear properties, serve as tracers and
chronometers to investigate a variety of topics that includes :
chronology of
rocks and
minerals
reconstruction
of sea-level
changes
erosion and
weathering
rates of rocks
and minerals
rock-water
interactions
magmatic
processes
material transport
within and
between various
reservoirs of earth

• Arguably the most important milestone on the application of isotopes to earth science is
the determination of the age of the Earth and our solar system. Isotope-based dating methods serve
as the gold-standard and are routinely used to validate other non-isotope-based dating methods.
• The field of isotope geochemistry started taking its roots shortly after the
discovery of “radioactivity”. Stable isotope geochemistry is largely concerned with isotopic
variations arising from mass-dependent isotope fractionation, whereas radiogenic
isotope geochemistry is concerned with the products of natural radioactivity.
In every chemical or
physical reaction, e.g.
oxidation or evaporation, the
light isotopes react at a
slightly higher rate than
heavy ones, because the
bonds in a molecule or
crystal between lighter
atoms vibrate with a higher
frequency and therefore they
split easier than those
between heavier atoms
Why does fractionationoccur?
Isotopic fractionation is
defined as the relative
partitioning of the
heavier and lighter
isotopes between two
coexisting phases in a
natural system.
What is isotopic fractionation?
Fractionation ratios
and isotopic ratios
are useful in
determining
palaeotemperatures,
geologic processes,
and the modes of
formation of rocks
and minerals
Whatis isotopicfractionation
used for?

Discovery : 1789 by Martin Heinrich Klaproth.
Uranium is a silver-gray radioactive metal in the actinide
series of the periodic table.
Key isotopes : U238
, U235
and U234
. Their relative
abundances are 99.2739, 0.7204 and 0.0057, respectively.
They are predominantly α-emitters and decay through two
different series headed by U235
and U238
.
Oxidation States : 0, 3, 4, 5,6
Geochemical Properties :
• Lithophile : Lithophile elements have a strong
affinity for oxygen, and thus mostly form silicate
minerals, and thus are the elements that make up
Earth's crust and mantle.
• Incompatible : An incompatible element is one that
is unsuitable in size and/or charge to the cation sites of
the minerals of which it is included.
• Soluble (+6)
• Insoluble (+4)

• In aqueous solutions, the chemistry of Uranium is highly dependent on its ability to form complexes
with other ions in solution. In non-complexing acid media, they generally exists as 𝑀𝑛+
. At higher
oxidation states (e.g., 𝑈6+
), the cations will react with water, a reaction known as hydrolysis :
𝐌𝐧+
+ 𝐱𝐇𝟐𝐎 → 𝐌𝐎𝐱
𝐧−𝟐𝐱 +
+ 𝟐𝐱𝐇+
• 𝐔𝟔+
in the aqueous solutions is thus present as the Uranyl ion 𝐔𝐎𝟐
𝟐+
. As pH increases, the form of these
ions evolves to 𝐌(𝐎𝐇) 𝐧−𝟏 +
or M𝐎𝐱𝐎𝐇 𝐧−𝟐𝐱−𝟏 +

03. Geochemical Properties of
Uranium

• The flow of matter from continents to
oceans and to the ocean floor is obviously
visible: it is frequently termed 'exogenous
cycle'. It describes the transfer of matter
from the lithosphere to the hydrosphere
and then back to the lithosphere.
• Chemically, the molecular state of matter
in sediments differs from that in
crystalline rocks. Matter does not revert to
its initial conditions by sedimentation.
The geochemical cycle is not closed in the
exogenous process: metamorphism,
magmatism and anatexis must take place
to bring matter back to its initial state.
This forms the endogenous geologic
cycle.
Thus, one could say the union of the
exogenous and the endogenous geologic
cycles corresponds with the geochemical
cycle.
Geochemical Cycle :

Exogenous Process
• Weathering (the breaking down or dissolving of rocks
and minerals on Earths surface.)
• Erosion (process in which earthen materials are worn
away and transported by naturalforces such as wind or
water)
• Transportation (the movement of material across the
Earth's surface by water, wind, ice or gravity.)
• Sedimentation (process of allowing particles in
suspension in water to settle out of the suspension under
the effect of gravity. )
Endogenous Process
• Tectonic movements of the crust
• Magmatism (The emplacement of magma formed by
partial melting of silicate rocks)
• Metamorphism (the change of minerals or geologic
texture which occurs primarily due to heat, pressure,
and the introduction of chemically active fluids.)
• Seismic activity
Exogenous refers to all the
processes that are produced at
the surface of the Earth (and
other planets).
Endogenic is a processes driven
by the internal heat of the Earth
causing radioactive decay of
elements deep beneath the
surface.

• The geochemical cycle provides a useful conceptual framework for considering the course followed by an individual
element in proceeding through the different stages. For a specific element, a complete understanding of its behavior
throughout the cycle is one of the major objectives of geochemical research. An element may tend to concentrate in
a specific type of deposit at a particular stage in the cycle, or it may remain dispersed throughout the entire cycle.
• The exogenous cycle of uranium is shown in the figure below :
The Exogenous Cycle of Uranium

• Igneous or, more generally, crystalline rocks are assumed to be the initial state of matter, the starting point of
the exogenous cycle.
• The upper arrow, (1), connecting the igneous rocks box with the sedimentary rocks box, represents flow of
unaltered solids. The mechanical weathering is the physical break down of a material without alerting its
chemical composition.
The final product is a resistate sediment. (any of the class of sediments, as sand or sandstone,
consisting chiefly of minerals resistant to weathering.)
Mechanical Weathering :

• The lower arrow, (2), depicts chemical alteration-that is, the change of matter in igneous mineral lattices to the
dissolved species in water. There are different types of chemical weathering, the most important are:
a) Solution - removal of rock in solution by acidic rainwater. In particular, limestone is weathered by rainwater
containing dissolved CO2, (this process is sometimes called carbonation).
b) Hydrolysis - the breakdown of rock by acidic water to produce clay and soluble salts.
c) Oxidation - the breakdown of rock by oxygen and water, often giving iron-rich rocks a rusty-coloured
weathered surface.
Chemical Weathering :

• Transport across phase boundaries requires molecular diffusion. (the process by which, under the influence of a
chemical potential gradient, atoms, molecules, ions or lattice vacancies move from one position to another in a
solvent phase.)
• The surface waters box is a sub-field of the hydrosphere. Surface water flow to the oceans provides large-scale
transport on the continental scale. No great change of state of matter is involved: it is just physical flow, similar to
the movement of unaltered solid rock particles.

• Chemical precipitation is depicted by the arrow from the hydrosphere sea-water sub-box to marine sediments. The
physical migration of suspended solids to sediments and to sedimentary rocks is left implicit.
• One arrow is drawn to connect the surface water to the solid phase box: it represents chemical deposition, which
also involves diffusion across phase boundaries. From the viewpoint of physical transport, however, it may
correspond with either uranium going back to stream sediments or uranium remaining trapped in aquifer rocks.

• The relative ratio between flows in processes mechanical weathering and chemical weathering varies according to
the distribution of uranium atoms in the different molecular sites in the source rock. Such distribution determines
the way in which uranium mobilization occurs, Uranium contained in insoluble accessories is leached to a very
limited extent, so it is presumed to be transported to and concentrated in resistate sediments as clastic material.
• A large part of the uranium in igneous rocks is contained in heavy chemically resistant minerals the weathering of
which is mainly mechanical. Thus, uranium in such form is transported by rivers and streams as clastic particles
that are ultimately found in residual soils, in stream sediments and in common sedimentary rocks, either
continental or marine.
Chemical or mechanical weathering :

• Ocean water contains uranium at a broadly uniform concentration (0.001-0.004 ppm). The average uranium
concentration in stream water is less than 1 ppb U. Groundwater shows remarkable variability of concentration as
a result of, for example, the presence of enriched mineralization, the time of contact of the water with the source
rocks and the concentration of ligands that either form soluble uranium complexes or insoluble uranium
compounds.
• Uranium exhibits radically different geochemical properties in its two oxidation states. Hexavalent uranium is
quite soluble, especially when complexed by carbonate ions in alkaline solutions. Tetravalent uranium, by
contrast, is very insoluble, and it would be expected to be rapidly removed from natural waters either by
precipitation of a mineral phase such as UO2 or by adsorption to solid surfaces. It is important to know the
oxidation state in which uranium will occur under specified conditions of Eh and pH as this relates both to
processes forming economically valuable uranium deposits and to the mobility of uranium released to the
environment during various stages of the nuclear fuel cycle.
Uranium in natural waters :

• pH is a measure of the acidity or alkalinity of a solution, where pH = - log a H+. pH is thus less in more acidic
(less basic) solutions and greater in less acidic (more basic) solutions. A change of 1 (i.e., from pH=3 t o pH=4)
represents a ten-fold change in the activity of H+.
• Eh is a measure of the redox (oxidation-reduction) state of a solution or, more exactly, its solutes. Eh is a
measurement of electrical potential and thus commonly expressed in volts. Values of Eh in nature range from -0.6
to +0.9V, with 0.0 characterizing a solution with no drive to either oxidize or reduce.
!! A Small Reminder :

• Uranium occurs in natural waters as 𝐔𝟒+
, 𝐔𝟓+
and 𝐔𝟔+
. Species relationships in aqueous equilibria of the 𝑈 − 𝑂2 −
𝐻2𝑂 − 𝐶𝑂2 sub-system, as a function of Eh and pH, are shown in the figure below for a temperature of 25°C and 1 atm
pressure (the shaded area shows the stability field of Uraninite (UO2)).
• Dissolved uranium in water is mainly in the form of stable Uranyl dicarbonate and tricarbonate complexes.

• The field of existence of soluble uranium complexes becomes wider as pH increases, owing to the formation of Uranyl
carbonate complexes. This means that carbonate ions control uranium circulation. One should remember that dissolved
CO2 is always present one way or another in natural waters-even in rain water.
• Upper and lower boundaries within diagram (dolled lines) are limits within which water itself is chemically stable.
Above upper limit water is oxidized to give oxygen and below lower limit reduced to yield hydrogen.

• Stability field for crystalline Uraninite shaded; predominant U species in solution indicated in various unshaded areas;
unbroken lines represent equilibrium conditions.
• Such 𝐔𝟒+
compounds as UO2 and U(OH)4 are very insoluble, so the concentration of 𝐔𝟒+
in water is extremely small.
The field of stability of Uraninite corresponds broadly with the fields in the Eh-pH graph where 𝐔𝟒+
species are
dominant.

Marine sediments :
In the supergene cycle uranium is removed from sea water by several processes, in which the ability of uranium to form
stable complexes with various species may play an important role.
Generally speaking, a direct relationship between uranium concentration and increasing organic carbon content exists in
marine black shales. Uranium content is also directly related to the colloidal size ranges of such sediments. Many therefore
believe that the enrichment of uranium in marine black shales is strictly related to the presence of organic matter in the
sediments. This last, along with the H2S, is deemed to be ultimately responsible for the reduction of Uranyl ion to the
insoluble form, Uraninite.
Obtained experimental data results explain the difference in uranium accumulation in bottom sediments in oxic
(environments contain free molecular oxygen) and anoxic (an aquatic environment with no dissolved oxygen)
environments in the examples of the White Sea and the Black Sea.

Marine sediments :
In oxidizing conditions, the seawater contains uranium (6+) in soluble forms; typical concentrations vary in the range of 0.002–
0.003 ppm. In such conditions, part of the uranium is accumulated in marine organisms and absorbed in the organic matter of
sediments; however, the total content of uranium in oxidizing layers of sediments does not exceed 1–1.5 ppm, including
uranium contained in the inorganic matter of continental run-off and uranium accumulated in organic matter. The content of
uranium in deeper layers of sediments may be slightly (up to 2.5–3 ppm) higher than in upper oxidizing layers due to the
change of redox conditions from oxidizing to reducing, which results in the fixation of uranium contained in sludge water in
organic and inorganic particles of the bottom sediments. This uranium behavior has been observed in the White Sea and is
typical for water reservoirs characterizing oxygen in the bottom layer.

Marine sediments :
In the case of reducing conditions in the bottom layer, if thermodynamic equilibria are reached, most of the uranium in the
water–bottom sediment system is accumulated in the forms of insoluble compounds in the solid phase. A considerable part of
uranium could also be absorbed by organic matter, uranium sorption is increased in reducing conditions.
In this case, the concentrations of uranium in bottom sediments could be at least one order of magnitude higher, depending on
sedimentation conditions, including the concentration of uranium in water, redox conditions, sedimentation rate, the content of
organic matter, and other factors. This behavior has been found in the Black Sea and is typical for reducing conditions in a
water reservoir’s bottom layer.
The considerable variations of uranium content in marine source rocks could be explained by the variations in redox
conditions at the sedimentation stage; however, other factors affecting uranium accumulation could also be taken into
account.

Non-marine sediments :
• Minerogenetic studies and thermodynamic data suggest that uranium can be removed from weathering solutions
by many processes, one of which is reduction-notably by organic matter among a number of reducing agents.
Reduction commonly results in the formation of 𝐔𝐎𝟐 or one of its hydrates. Uranium may also undergo
precipitation directly in its hexavalent state by a variety of anions.
• Among the geochemical factors possibly responsible for the reduction and precipitation of uranium from
groundwater, Eh is most effective. 𝐅𝐞𝟐+ and sulphides, in addition to organic matter, deserve mention. For
example insoluble Uraninite may be precipitated according to the redox reaction :
𝐂𝐇𝟒 symbolizes organic matter in general.

Non-marine sediments :
• Anaerobic bacteria prevalent in reducing environment are believed to play a significant role. Humic constituents of
alluvia and soils are very effective trapping material and remove uranium from natural waters. Their molecule
consist of a polyaromatic skeleton that carries phenolic hydroxyl and carboxyl groups. Acidic hydrogen of the
carboxyl group is exchanged with the Uranyl ion: this process is so effective that very high enrichment factors may
be obtained.

In the Environmental Sciences Isotope geochemistry has become an essential tool for the
environmental sciences, providing clearly defined tracers of sources, quantitative information
on mixing, identification of physical and chemical processes, and information on the rates of
environmental processes. Clearly, this tool will continue to be important in all aspects of the
field, including studies of contamination, resource management, climate change, bio-
geochemistry, exploration geochemistry, archaeology, and ecology. In addition to further
utilization of established methods, new applications will continue to be developed.

The Geochemistry of Uranium

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The Geochemistry of Uranium