4 research outputs found
New Insights into Formation of Trivalent Actinides Complexes with DTPA
Complexation of trivalent actinides with DTPA (diethylenetriamine
pentaacetic acid) was studied as a function of pcH and temperature
in (Na,H)Cl medium of 0.1 M ionic strength. Formation constants of
both complexes AnHDTPA<sup>ā</sup> and AnDTPA<sup>2ā</sup> (where An stands for Am, Cm, and Cf) were determined by TRLFS, CE-ICP-MS,
spectrophotometry, and solvent extraction. The values of formation
constants obtained from the different techniques are coherent and
consistent with reinterpreted literature data, showing a higher stability
of Cf complexes than Am and Cm complexes. The effect of temperature
indicates that formation constants of protonated and nonprotonated
complexes are exothermic with a high positive entropic contribution.
DFT calculations were also performed on the An/DTPA system. Geometry
optimizations were conducted on AnDTPA<sup>2ā</sup> and AnHDTPA<sup>ā</sup> considering all possible protonation sites. For both
complexes, one and two water molecules in the first coordination sphere
of curium were also considered. DFT calculations indicate that the
lowest energy structures correspond to protonation on oxygen that
is not involved in AnāDTPA bonds and that the structures with
two water molecules are not stable
Role of Silver Nanoparticles in Enhanced Xenon Adsorption Using Silver-Loaded Zeolites
Molecular simulation is used to unravel
the adsorption mechanisms
of xenon on Ag-doped ZSM-5 zeolite. We show that silver nanoparticles,
which form at the external surface of zeolite crystallites, are responsible
for enhanced xenon physisorption at very low pressure. We also propose
a simple model of adsorption on such composite materials made up of
silver-exchanged zeolites and silver nanoparticles adsorbed at the
zeolite surface. This model, which allows predicting the adsorption
of other gases without any additional parameters, provides a tool
to characterize the amount of reduced silver as well as the silver
particle size distribution (in good agreement with transmission electron
microscopy images). The presence of a majority of silver nanoparticles
is further characterized by means of X-ray diffraction and X-ray Absorption
Spectroscopy at the silver K edge
Thermodynamic Study of the Complexation of Protactinium(V) with Diethylenetriaminepentaacetic Acid
The
complex formation of protactiniumĀ(V) with DTPA was studied at different
temperatures (25ā50 Ā°C) and ionic strengths (0.1ā1
M) with the element at tracer scale. Irrespective of the temperature
and ionic strength studied, only one neutral complex with (1:1) stoichiometry
was identified from solvent extraction and capillary electrophoresis
coupled to ICP-MS (CE-ICP-MS) experiments. Density Functional Theory
(DFT) calculations revealed that two complexes can be considered:
PaĀ(DTPA) and PaOĀ(H<sub>2</sub>DTPA). The associated formation constants
were determined from solvent extraction data at different ionic strengths
and temperatures and then extrapolated to zero ionic strength by SIT
methodology. These constants are valid, regardless of complex form,
PaĀ(DTPA) or PaOĀ(H<sub>2</sub>DTPA). The standard thermodynamic data
determined with these extrapolated constants revealed a very stable
complex formed energetically by an endothermic contribution which
is counter balanced by a strong entropic contribution. Both, the positive
enthalpy and entropy energy terms suggest the formation of an inner
sphere complex
Xenon Capture on Silver-Loaded Zeolites: Characterization of Very Strong Adsorption Sites
The
number and strength of adsorption sites for Xe in silver-modified
zeolites are estimated from isotherm measurements at various temperatures
over a broad range of pressure (from 1 ppm to atmospheric pressure).
Fully and partially exchanged silver zeolites were synthesized starting
from Na-ZSM-5(25), Na-ZSM-5(40), Na-Beta, NaX, and NaY. We have discovered
that silver-modified zeolites may present one or two distinct adsorption
sites depending on the nature of the material and silver loadings.
The strongest adsorption sites are characterized by isosteric heat
of adsorption in the order of ā40 to ā50 kJĀ·mol<sup>ā1</sup>. For Pentasil-type zeolites, we observe a linear
2:1 correlation between the total amount of silver and the number
of strong sites. The highest concentration of strong sites is found
for fully silver exchanged ZSM-5 (5.7 Ć 10<sup>ā4</sup> mol/g), which presents the largest silver content for Pentasil-type
zeolite. The equilibrium constant of Ag-ZSM-5 at low pressure is about
50 times larger than that of AgX. Qualitative correlations were established
between Xe adsorption isotherms and Xe NMR signals. We show that Xe
NMR could be used as a quantitative method for the characterization
of the strength and of the number of strong Xe adsorption sites on
silver-exchanged zeolites. The numbers of strong adsorption sites
responsible for the Xe adsorption at 10ā1000 ppm can be determined
by the length of the plateau observed at low Xe uptake. In practice,
our findings give guidelines for the discovery and optimization of
silver-loaded zeolites for the capture of Xe at ppm levels. It appears
that the amount of silver is a key parameter. Silver-modified ZSM-5
shows adsorption capacities 2ā3 orders of magnitude larger
than currently applied adsorbents for atmospheric Xe capture