TY - JOUR
T1 - Sulfidization of lepidocrocite and its effect on uranium phase distribution and reduction
AU - Alexandratos, Vasso G.
AU - Behrends, Thilo
AU - Van Cappellen, Philippe
PY - 2014/10
Y1 - 2014/10
N2 - Sulfidization of iron oxyhydroxides can be accompanied by a release of
adsorbed uranium, thus enhancing the mobility of uranium in systems
undergoing a shift in redox conditions. We investigated the phase
distribution and redox state of uranium in batch experiments, in which
lepidocrocite with adsorbed U(VI) was reacted with sulfide. The amount
of added sulfide was varied in the experiments performed, at pH 8 and
ionic strength of 0.1 M. Sulfide, when not added in excess, was removed
from solution within less than 1 h of reaction time. Consumption of
dissolved sulfide was accompanied by reduction of Fe(III) and formation
of iron sulfide. Each addition of sulfide led to an instantaneous
release of uranium into solution. This release is most likely caused by
the exchange of hydroxide groups at the lepidocrocite surface by thiol
groups which have a lower tendency to bind uranium. Along with the
consumption of dissolved sulfide, part of the released uranium became
reassociated with the solid phase. This can be explained by a reversal
of the ligand exchange process at the solid surfaces. However, steady
state concentrations of dissolved uranium remained higher than before
sulfide addition, indicating that the product of lepidocrocite
sulfidization has a lower affinity for uranium than the starting
material. Reduction of U(VI) also contributed to the transfer of
dissolved uranium back to the solid phase. X-ray absorption spectroscopy
revealed that reduction of U(VI) occurred in all experiments. The extent
of U(VI) reduction depended on sulfide addition, however, formation of
UO2 occurred within a period of 48 h only when sulfide was
added in excess. This suggests that the presence of dissolved sulfide is
a prerequisite for fast reduction of U(VI) and formation of
UO2. This would imply that the fast reaction of lepidocrocite
with sulfide outcompetes reduction of U(VI) and, by this, kinetically
inhibits the thermodynamically more favorable reduction of U(VI) to
uraninite. Our results demonstrate that the transition from oxic to
sulfidic conditions can lead to intermittent mobilization of uranium
which is not expected based on equilibrium thermodynamics.
AB - Sulfidization of iron oxyhydroxides can be accompanied by a release of
adsorbed uranium, thus enhancing the mobility of uranium in systems
undergoing a shift in redox conditions. We investigated the phase
distribution and redox state of uranium in batch experiments, in which
lepidocrocite with adsorbed U(VI) was reacted with sulfide. The amount
of added sulfide was varied in the experiments performed, at pH 8 and
ionic strength of 0.1 M. Sulfide, when not added in excess, was removed
from solution within less than 1 h of reaction time. Consumption of
dissolved sulfide was accompanied by reduction of Fe(III) and formation
of iron sulfide. Each addition of sulfide led to an instantaneous
release of uranium into solution. This release is most likely caused by
the exchange of hydroxide groups at the lepidocrocite surface by thiol
groups which have a lower tendency to bind uranium. Along with the
consumption of dissolved sulfide, part of the released uranium became
reassociated with the solid phase. This can be explained by a reversal
of the ligand exchange process at the solid surfaces. However, steady
state concentrations of dissolved uranium remained higher than before
sulfide addition, indicating that the product of lepidocrocite
sulfidization has a lower affinity for uranium than the starting
material. Reduction of U(VI) also contributed to the transfer of
dissolved uranium back to the solid phase. X-ray absorption spectroscopy
revealed that reduction of U(VI) occurred in all experiments. The extent
of U(VI) reduction depended on sulfide addition, however, formation of
UO2 occurred within a period of 48 h only when sulfide was
added in excess. This suggests that the presence of dissolved sulfide is
a prerequisite for fast reduction of U(VI) and formation of
UO2. This would imply that the fast reaction of lepidocrocite
with sulfide outcompetes reduction of U(VI) and, by this, kinetically
inhibits the thermodynamically more favorable reduction of U(VI) to
uraninite. Our results demonstrate that the transition from oxic to
sulfidic conditions can lead to intermittent mobilization of uranium
which is not expected based on equilibrium thermodynamics.
U2 - 10.1016/j.gca.2014.08.009
DO - 10.1016/j.gca.2014.08.009
M3 - Article
SN - 0016-7037
VL - 142
SP - 570
EP - 586
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -