20 research outputs found
Binding mechanisms of radionuclides to cement
Radionuclide solubility is one of the factors that can determine the 'source term' for potential migration to the far field. Hence, the mechanisms of immobilisation of radionuclides in cements need close study in order to quantify the nature of the binding. The nature of the binding forces in a cement matrix is very species sensitive, so it is difficult to produce a general description. This review summarises the main sorption mechanisms and then details immobilisation mechanisms for many elements
Modelling the sorption of 63Ni to granitic materials: application of the component additive model
The component additive modelling approach is based on summing the results from models already calibrated with pure
mineral phases. The summation can occur as the sum of results for thermodynamic surface speciation models or as the sum of
pseudo-thermodynamic models for adsorption on individual mineral phases. Static batch sorption experiments of 63Ni are with
different granitic rocks and component minerals. XRD analyses have been used to calculate the percentage mineralogical
composition of the granitic rocks. Sorption data has been modelled using non electrostatic correction models to obtain Rd for the
granitic rocks and mineral. Rd values for the granitic rocks predicted from the component additive model have been compared to
experimental values. Results showed that predicted Rd values for granite adamellite, biotite granite and rapakivi granite were
identical to the experimentally determined values, whereas, for graphic granite and grey Granite, the predicted and experimentally
determined Rd values were much different. The results also showed a greater contribution to the bulk Rd by feldspar while quartz
showed the least contribution to the Rd
Sorption of Ni And Eu in a multi-element system
Laboratory studies on the retardation of radionuclides in the far-field of a radioactive waste repository are often performed
using a single metal in the presence of the solid surfaces in solution. However, in the event of failure in the engineered barrier system
(EBS), groundwater flowing pass the EBS will likely contain more than one long live radionuclide. The aim of this paper is to
understand sorption in systems containing more than one radionuclide in solution. The work presented is carried out to understand how
the presence of Ni or Eu in solution will affect the sorption of each other. Static batch sorption experiments of Ni and Eu sorption to
different granitic materials have been performed. Radiometric analysis was done using Cobra (II) Auto gamma for 152Eu and TriCarb
liquid scintillation for Ni63. Experimental results were described initially using the empirical models from which Rd values were
obtained for sorption in single systems of Ni and Eu, and of multi-element systems of Ni and Eu. The experimental data was fitted to the
surface complexation using JChess Geochemical Code to obtained complexation constants for the interactions between Ni and Eu and
the solid surfaces. The effect on sorption in a multi-element system was expressed by the ratio of the sorption capacity of one metal ions
in the presence of the other metal ions Qmix
to the sorption capacity of same metal ions when it is present alone in solution Qsing.
s in
1 m ix
g
Q
Q
> , the sorption is promoted by the presence of other metal ions, whereas if:
sin
1 mix
g
Q
Q
= , there appears to be no observable
effect, and if:
s in
1 m ix
g
Q
Q
< , it would indicate that sorption is suppressed by the presence of other metal ions in solution. Results obtained
showed that sorption was suppressed in systems containing Eu and Ni. Ratios of Qmix and Qsing were calculated as approximately 1and
Eu sorption to Muscovite Mica and for Ni sorption to Plagioclase Feldspar, showing that there was no effect on the sorption capacity.
The ratio was calculated as < 1 for Eu sorption to Granite Adamellite, Rose Quartz, Plagioclase Feldspar and Muscovite Mica, showing
that sorption of Eu in the presence of both Ni was suppressed. Results also showed suppression of Ni sorption in the presence of Eu with
the most suppression occurring with for Ni sorption to Rose Quartz in the presence of Eu with a mixed/single ratio << 1. A correlation
between the Rd and determined surface complexation constant is determined for the granitic minerals
Studies on metal gluconic acid complexes
The presence of organic complexants, such as gluconic acid, in an intermediate-level
radioactive-waste (ILW) repository may have a detrimental effect on the sorption of
radionuclides, by forming organic complexes in solution. In order to assess this, stability
constants are required for the complexes formed with radionuclides at high pH. This study
reports the stability constants for the reactions of metals with gluconic acid (Gl). The metals
studied were Cd, Ce, Co, Eu, Fe(II), Fe(III), Ho and U(VI) at pH 13.3; and Ce, Co and U(VI) at
pH 7. The constants were measured by the Schubert (ion-exchange) or solubility product
methods. Stoichiometries of the complexes were also determined. At pH 7 each complex was of
the form M1Gl1, with log β values suggestive of salt formation. The M2+ log β values were
between 13 and 20. For M3+, there was less consistency. The M2Gl1 complexes (Ho & Ce) had
values of 49.8 and 43.9, whereas the M1Gl1 type (Fe(III) & Eu) range from 24 to 38. The
constants have enabled speciation calculations to be performed showing the effect of gluconic
acid on the metal’s solubility. Solubility is predicted to increase in the presence of gluconic acid
from pH 9 to 13.5, suggesting that gluconic acid may have an impact on radionuclide behaviour.
The largest increases in solubility are for Cd, Co, Eu, Ho and Ni, the smallest is with U(VI)
Degradation of tetraphenylphosphonium bromide at high pH and its effect on radionuclide solubility
Recently, tetraphenylphosphonium bromide (TPPB) has been used to remove
technetium from some radioactive waste streams. However, before TPPB could
15 be approved for use it was necessary to show that TPPB and its degradation
products would not have a significant detrimental effect on post-closure
performance of a radioactive waste repository. TPPB is known to be stable at
neutral pH, however, under alkaline conditions it degrades by an alkaline
hydrolysis mechanism to triphenylphosphonium oxide (TPPO). Degradation can
20 also occur by radiolysis to produce triphenylphosphine (TPP). The kinetics of
the alkaline hydrolysis degradation of TPPB is described and the solubility of
europium, iodine, nickel, technetium(VII) and uranium(VI) in aqueous solutions
of TPPB and its degradation products is reported. These results were used to
support the use of TPPB in removing technetium from some waste streams
Competitive effect of iron(III) on metal complexation by humic substances : characterisation of ageing processes
Aiming at an assessment of counteractive effects on colloid-borne migration of actinides in
the event of release from an underground repository, competition by Fe(III) in respect of
metal complexation by dissolved organic matter was investigated for the example of Eu(III)
as an analogue of trivalent actinides. Complexation with different humic materials was
examined in cation exchange experiments, using 59Fe and 152Eu as radioactive tracers for
measurements in dilute systems as encountered in nature. Competitive effects proved to be
significant when Fe is present at micromolar concentrations. Flocculation as a limiting
process was attributed to charge compensation of humic colloids. Fe fractions bound to humic
acids (HA) were higher than 90%, exceeding the capacity of binding sites at high Fe
concentrations. It is thus concluded that the polynuclear structure of hydrolysed Fe(III) is
maintained when bound to HA, which is also inferred from UV-Vis spectrometry. The
competitive effect was found to be enhanced if Fe and HA were contacted before Eu was
added. Depending on the time of Fe/HA pre-equilibration, Eu complexation decreased
asymptotically over a time period of several weeks, the amount of bound Fe being unchanged.
Time-dependent observations of UV-Vis spectra and pH values revealed that the ageing effect
was due to a decline in Fe hydrolysis rather than structural changes within HA molecules. Fe
polycations are slowly degraded in contact with humic colloids, and more binding sites are
occupied as a consequence of dispersion. The extent of degradation as derived from pH shifts
depended on the Fe/HA ratio
Sorption of selected radionuclides to clay in the presence of humic acid
Within the framework of the FUNMIG programme, Loughborough University is performing work to increase understanding of the sorption behaviour of selected radionuclides with various minerals in the absence and presence of competing complexing ligands, such as humic acid (HA).
The determination of the distribution ratios (Rd) of binary (metal- and humic-solid), and ternary (metal-solid-humic) systems using a batch adsorption technique is reported. Four radionuclides have been used; 137Cs, 63Ni, 152Eu and 109Cd, to facilitate modelling. Montmorillonite, kaolinite and α-goethite have been used as solids.
Humic acid concentrations (2 - 300 ppm (w/v)) were determined using UV spectrophotometry. Radiometric analysis was used for radionuclide measurement.
Construction of sorption isotherms using the Langmuir and Freundlich Equations has allowed characterisation of sorption types, and has provided maximum sorption capacities of the solid surfaces for each of the metals in the binary systems. Distribution relationships between metal and humic acid for each ternary system have been established and correlated
A procedure to assess the importance of chemical kinetics in the humic-mediated transport of radionuclides in radiological performance assessment calculations
Previous work has shown that humic substances can bind metal ions in two
fractions: the exchangeable, where it is available instantaneously for reaction with
other sinks (such as mineral surfaces); and the non-exchangeable, from which it may
only dissociate slowly. In the absence of metal ion/humic/mineral surface ternary
complexes, if the dissociation rate is slow compared to the solution residence time in
the groundwater column, then metal in the non-exchangeable will have a
significantly higher mobility than that in the exchangeable. The critical factor is the
ratio of the non-exchangeable first order dissociation rate constant and the residence
time in the groundwater column, metal ion mobility increasing with decreasing rate
constant.
Sorption of humic/metal complexes at mineral surfaces may reduce mobility.
In addition to direct retardation, sorption also increases the residence time of the
non-exchangeable fraction, giving more time for dissociation and immobilisation.
The magnitude of the effect depends upon the concentrations of the mineral surface
humic binding sites and the humic in solution, along with the magnitudes of the
equilibrium constant and the forward and backward rate constants.
The non-exchangeable dissociation reaction and the sorption reaction may be
classified in terms of two Damkohler numbers, which can be used to determine the
importance of chemical kinetics during transport calculations. These numbers could
be used to determine when full chemical kinetic calculations are required for a
reliable prediction, and when equilibrium may be assumed, or when the reactions
are sufficiently slow that they may be ignored completely
The role of humic non-exchangeable binding in the promotion of metal ion
Metal ions form strong complexes with humic substances. When the metal ion is first complexed
by humic material, it is bound in an ‘exchangeable’ mode. The metal ion in this fraction is
strongly bound, however, if the metal–humic complex encounters a stronger binding site on a
surface, then the metal ion may dissociate from the humic substance and be immobilised.
However, over time, exchangeably-bound metal may transfer to a ‘non-exchangeable’ mode.
Transfer into this mode and dissociation from it are slow, regardless of the strength of the
competing sink, and so immobilisation may be hindered. A series of coupled chemical transport
calculations has been performed to investigate the likely effects of non-exchangeable binding upon
the transport of metal ions in the environment. The calculations show that metal in the nonexchangeable
mode will have a significantly higher mobility than that in the exchangeable mode.
The critical factor is the ratio of the non-exchangeable first-order dissociation rate constant and
the residence time in the groundwater column, metal ion mobility increasing with decreasing rate
constant. A second series of calculations has investigated the effect of the sorption to surfaces of
humic/metal complexes on the transport of the non-exchangeably bound metal. It was found that
such sorption may reduce mobility, depending upon the humic fraction to which the metal ion is
bound. For the more weakly sorbing humic fractions, under ambient conditions (humic
concentration etc.) the non-exchangeable fraction may still transport significantly. However, for
the more strongly sorbed fractions, the non-exchangeable fraction has little effect upon mobility.
In addition to direct retardation, sorption also increases the residence time of the nonexchangeable
fraction, giving more time for dissociation and immobilisation. The nonexchangeable
dissociation reaction, and the sorption reaction have been classified in terms of two
Damkohler numbers, which can be used to determine the importance of chemical kinetics during
transport calculations. These numbers have been used to develop a set of rules that determine
when full chemical kinetic calculations are required for a reliable prediction, and when
equilibrium may be assumed, or when the reactions are sufficiently slow that they may be ignored
completely
Understanding the solution behavior of minor actinides in the presence of EDTA(4-), carbonate, and hydroxide ligands
Understanding the solution behavior of minor actinides in the presence of EDTA(4-), carbonate, and hydroxide ligand