14 research outputs found
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)
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
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
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
Studies on some divalent metal α-isosaccharinic acid complexes
The presence of organic complexants, such as α-isosaccharinic acid (ISA) in an intermediate-level
radioactive-waste (ILW) repository may have a detrimental effect on the sorption of radionuclides by
forming organic complexes in solution. To assess this, stability constants are required for the
complexes formed. Constants have been determined for the divalent metal ions, Cd2+, Co2+ and UO2
2+
with α-isosaccharinic acid (ISA) in near-neutral and alkaline conditions. The effect of these complexes
on radionuclide solubility in the near-field and alkaline-disturbed zone is shown. The constants were
measured by the ion-exchange (Schubert) method [1] with the exception of U(VI) at pH 13.3, for
which the solubility product method [2] was used. At pH 7 each complex was of the form M1ISA1,
with log β values suggestive of salt formation. At high pH, log β values were between 13 and 20. The
number of hydroxide ions involved in the complexation reactions was determined using Bjerrum plots,
conductometric titrations and spectrophotometric methods [1]. The constants have enabled speciation
calculations to be performed showing the effect of ISA on the metal’s solubility. Solubility is predicted
to increase in the presence of ISA from pH 9 to 13.5, suggesting that it may have an impact on
radionuclide behaviour. The largest solubility increases are for Cd and Co, the smallest for U(VI)
The solubility of technetium(IV) at high pH
Technetium-99 is a β-emitting fission product of 238U. It is important when considering a safety case for nuclear
waste disposal because of its high yield in radioactive waste and long half-life. The aqueous chemistry of Tc in a
cementitious repository is likely to be dominated by Tc(VII), as TcO4
-, in aerobic areas and by Tc(IV), as TcO2 (am),
in anaerobic.
Some previous studies have shown an increase in Tc(IV) solubiltiy with increasing pH above pH 9.5 whilst others
have not observed this increase. This study found that the solubility of Tc(IV) remains independent of pH until
around 13.5, when a small increase can be seen which continues to increase linearly with pH. Modelling suggests
that this increase occurs as the species TcO(OH)3
- is formed. The formation constant was estimated with data from
this study and was found to be log K2 = -21.6 ± 0.3
Stability constants of uranium(IV)-α-isosaccharinic acid and gluconic acid complexes
Conditional and pH independent stability constants
have been determined for U(IV) α-isosaccharinic acid
(ISA) and gluconic acid (Gl) complexes, under anaerobic
and carbonate-free conditions, from pH 3 to 14. The constants
are needed for nuclear waste repository performance
assessment purposes. The constants were obtained by developing
an approach based on the solubility product of
amorphous UO2 ·2H2O. The derived pH independent log β
values for U(OH)4ISA and U(OH)4Gl were 49±2 and 50±1
respectively
Prediction and measurement of complexation of radionuclide mixtures by α-isosaccharinic, gluconic and picolinic acids
The purpose of this study was to investigate the effects of competition between cobalt, europium and strontium for isosaccharinate, gluconate and picolinate. Systems where results indicated that competitive effects were significant have been identified. Thermodynamic calculations were performed for each system for comparison with the experimental results. Some exceptions may be due to precipitation of some species, or presence of species not in databases, or formation of mixed-metal complexes, or sorption to the solid phase(s). In some of the experiments, the complexity of the systems studied caused difficulty in identifying consistent trends. By concentrating on the results for simpler systems (i.e. for solubilities in the presence and absence of organic complexants and with just one competing metal ion), the evidence for competition effects has been investigated. Evidence for solubility enhancement due to organic ligands was apparent in the data for the systems Co with gluconate and Eu with isosaccharinate and gluconate. Of these above cases, the systems in which the effects of the competing ion are consistent with competition were limited to the cases of Eu with isosaccharinate and Sr as the competing ion, and Eu with gluconate and either Co or Sr as the competing ion