Uranium carbonates removal by Layered Double Hydroxides

Uranium as a radionuclide and heavy metal has strong negative health effects on all living beings.
Since uranium salts display a high solubility in water, its mobility in aquifers is immense
and leads to the risk of ingestion by water consuming organisms. Sources of uranium contamination
in the environment are military and mining activities as well as leaking repositories.
The objective of this work is to analyze the uranium removal properties of two double layered
hydroxides (LDH) with different redox properties in absence and presence of carbonate. The
LDH phases selected are Ca(II)-Al(III)-Cl and Fe(II)-Al(III)-Cl, hereafter named Ca-LDH and
Fe-LDH. These LDHs play a crucial role in the geosphere, as they consist of the most abundant
elements in the earth crust. Furthermore, Ca-LDH is a product of bentonite weathering, which
is an essential process considered in repository safety management.
The first aim of this work is to synthesize and characterize Ca-LDH and Fe-LDH with respect
to their stoichiometry (inductively coupled plasma mass spectrometry, Mössbauer spectroscopy,
thermogravimetric analysis, energy-dispersive X-ray spectroscopy), their structure
(X-ray diffraction, Brunauer-Emmett-Teller theory, dynamic light scattering, scanning electron
microscopy, Raman microscopy) and their electronic state (X-ray photoelectron spectroscopy,
electrophoretic mobility).
The ultimate goal is to determine the best conditions under which uranium is removed from
solution by these LDHs. This will be achieved by analyzing the parameters that influence the
process (carbonate presence, redox processes, pH, ionic strength and uranium concentration).
In comparison to Ca-LDH, Fe(II)-LDH contains a redox active moiety, so that a different mechanism
for the interaction of these LDH phases with aqueous uranium is expected. For a comprehensive
understanding of these molecular uranium reactions occuring at the LDH phase,
various spectroscopic techniques (attenuated total reflexion Fourier-transform infrared, cryo
time-resolved laser-induced fluorescence, energy-dispersive X-ray, X-ray photelectron spectroscopy)
and microscopies (Raman microscopy, scanning electron microscopy) are applied
and combined. The synthesis of Ca-LDH and Fe-LDH was successful and structurally characterized by different
techniques (XRD, DLS, electrophoretic mobility, BET, SEM, EDXS, Raman microscopy,
XPS and Mössbauer). The stoichiometry under consideration of the corresponding oxidation
states was determined as
Ca(II)₀.₅₉Al(III)₀.₄₁Cl₀.₈₂(OH)₂ · 3.0 H2O and
Fe(II)₀.₆₀Al(III)₀.₄₀Cl₀.₈₀(OH)₂ · 1.8 H2O.
Uranium removal by Ca-LDH and Fe-LDH was evaluated as a function of pH (5.5 to 11.0),
ionic strength (H2O, 0.01 M NaCl, and 0.1 M NaCl), carbonate concentration (0, 0.013 mM,
0.2 mM, 0.24 mM and 2 mM) and U(VI) concentration (from nM to mM). As a general statement,
uranium removal was higher than 95% for pH > 6.0 for both Ca-LDH and Fe-LDH under
all studied conditions. Uranium removal decreased for pH < 6 in both LDH, as these mineral
phases are only stable under alkaline conditions. Uranium removal by Ca-LDH decreased at high ionic strength (0.1 M NaCl) and carbonate
concentration (2 mM and 20 mM). For the sorption mechanism of uranium to Ca-LDH, redox
potential measurements indicate a pH, carbonate and ionic strength dependency of the sorbed
minerals. In all studied cases, uranium associated to Ca-LDH is found as U(VI). Two different
U(VI) species are detected by ATR-IR measurements at pH 9.5 in presence of carbonate. Most
tentatively, one species is corresponding to U(VI) precipitation, which is also suggested by Raman
microscopy. The other species could be correlated to U(VI) outer-sphere complexation,
which is also supported by the lack of changes in the isoelectric point of Ca-LDH in presence
of U(VI) and the decrease of chloride content on the Ca-LDH after being in contact with U(VI).
The presence of outer-sphere complexation might be the reason of the decreased U(VI) removal
at higher ionic strengths and carbonate concentrations.
Three different species of U(VI) associated to Ca-LDH are detected by TRLFS from pH 8.0
to pH 11.0 in presence and absence of carbonate. Species 1 could be related to [UO₂(OH)₃]⁻
complexation according to uranium speciation diagrams. Species 2 identity is challenging to
hypothesize. It is assumed that U(VI) incorporation occurs due to an increased Ca concentrationin solution. Species 3 is assigned to [UO₂(CO₃)₂]²⁻ complexation. A reliable identification of
this species would need the use of additional techniques, like XAS.
In contrast, uranium removal by Fe-LDH occurs via Fe(II) promoted reduction of U(VI) to
U(IV). This is confirmed by redox potential values, the detection of Fe(III) by XPS and the
observation of Fe(III) minerals (ferrihydrite, hematite and iron aluminate) by Raman. Changes
on the Fe-LDH structure after contact with U(VI) are also observed in SEM images. The confirmation
of possible stepwise uranium removal by Fe-LDH (anion exchange followed by U(VI)
reduction) would need further verification by ATR FT-IR.
To sum up, the synthesized Ca-LDH and Fe-LDH phases are found to exhibit excellent and effective
uranium removal properties under alkaline conditions, being able to remove negatively
charged uranium species from solutions. Sorption mechanisms could be suggested in a multispectroscopic
approach as outer-sphere surface complexation and incorporaton for Ca-LDH and
as uranium reductive immobilization for Fe-LDH.
This study shows, that the examined Ca-LDH and Fe-LDH can act as a naturally occuring retention
barrier in geosphere against uranium release from repositories. Therefore, these LDH
phases can possibly be part of a technical multi-barrier system preventing uranium leaking into
the biosphere. Further experiments need to be carried out by TRLFS, ATR-IR and XAS in
order to have a comprehensive identification of the uranium sorption mechanisms on Ca-LDH
and Fe-LDH.