Data on the stability of Pu(V) as the dominant oxidation state of tracer concentrations of plutonium in natural waters is reviewed. Laboratory experiments for solutions of 0.1 and 1.0M (NaCl) ionic strength and pH 3–10 confirm the dominance of Pu(V) as the state in solution. Humics in the waters can cause reduction to Pu(IV).
A stopped-flow rapid mixing device interfaced to a high performanceminiature fibre optic CCD NIR detector permits the study of fast redox kineticsof actinide reactions at minimal cost. The use of fibre optics enables thestopped-flow device to be readily used in a glove-box. The system has beenevaluated at 980 nm by observing the in-growth of Np(V) from a fast reductionreaction of Np(VI), and has been shown to perform well in this spectral region.
Nuclear test explosions and nuclear reactor wastes and accidents have released large amounts of radioactivity into the environment.
Actinideions in waters often are not in a state of thermodynamic equilibrium and their solubility and migration behavior is
related to the form in which the nuclides are introduced into the aquatic system. Chemical speciation, oxidation state, redox
reactions, and sorption characteristics are necessary in predicting solubility of the different actinides, their migration
behaviors and their potential effects on marine biota. The most significant of these variables is the oxidation state of the
metal ion as the simultaneous presence of more than one oxidation state for some actinides in a solution complicates actinide
environmental behavior. Both Np(V)O2+ and Pu(V)O2+, the most significant soluble states in natural oxic waters, are relatively noncomplexing and resistant to hydrolysis and
subsequent precipitation. The solubility of NpO2+ can be as high as 10−4M while that of PuO2+ is much more limited by reduction to the insoluble tetravalent species, Pu(OH)4, (pKsp≥56) but which can be present in the pentavalent form in aqautic phases as colloidal material. The solubility of hexavalent
UO22+ in sea water is relatively high due to formation of carbonate complexes. The insoluble trivalent americium hydroxocarbonate,
Am(OH)(CO3) is the limiting species for the solubility of Am(III) in sea water. Thorium(IV) is present as Th(OH)4, in colloidal form. The chemistry of actinide ions in the environment is reviewed to show the spectrum of reactions that
can occur in natural waters which must be considered in assessing the environmental behavior of actinides. Much is understood
about sorption of actinides on surfaces, the mode of migration of actinides in such waters and the potential effects of these
radioactive species on marine biota, but much more understanding of the behavior of the actinides in the environment is needed
to allow proper and reliable modeling needed for disposition of nuclear waste over many thousands of years.
The effects of ionic strength and of ethylenediamin et etraacetic acid (EDTA) on the sorption of uranyl ion, UO22+, to SiO2·xH2O (silica gel) were investigated. It was observed that pH and the ions present in the supporting electrolytes influence the
ionic strength effects. The presence of different sodium salts in the concentration range (0.20 to 1.40M) suppressed the sorption
of UO22+ in the order: NaNO3 < NaClO4 < NaCl < NaOCOCH3 < Na2SO4 [pH 2.75(±0.05)], while the presence of perchlorate salts of Li+, Na+ and Ca2+ (0.20 to 1.40M) promoted the sorption of UO22+ on silica gel in the order: LiClO4∼NaClO4<Ca(ClO4)2 at pH 2.80(±0.05). The ionic strength effect on UO22+ sorption was studied in presence of EDTA (0–1.00·10−3M) in the pH range 2.90 to 5.57. The sorption data and speciation calculation suggest negligible complexation of UO22+ with EDTA at I≥1.00M NaClO4.
Stability constants (log b101) of Th4+, UO22+, NpO2+ and Am3+ with [NaP5W30O110]14- were determined by solvent extraction (m = 0.1M NaCl) and found to be 6.18±0.07, 3.80±0.06, 2.98±0.04, and 5.85±0.05, respectively. The order of stability constants: Th4+>Am3+>UO22+>NpO2+ is due to electrostatic repulsion between the actinyl oxygens and oxygens on the polyoxometalate surface. The order of stability constants for metal complexes with [P2W18O62]6- is Th4+>UO22+>Eu3+>NpO2+ because the steric repulsion between actinyl oxygens and oxygens on polyoxometalate are less important. Enthalpies of complexation were measured by calorimetric titration of Th4+, UO22+, Nd3+ with [NaP5W30O110]14- and [P2W18O62]6-. The results indicate that the conformation and charge distribution of the microscopic surface structures are important factors in the formation of pseudocolloids.
The diamide, N,N-dimethyl-N,N-dihexyl-3-thiopentanediamide (DMDHTPDA) was synthesized and thested for extraction of Eu(III), Am(III), Th(IV) and U(VI). DMDHTPDA shows a very weak complexation with these metal ions, which can be attributed to the soft base nature of the sulfur atom. None of the cations were extracted into the organic phase when DMDHTPDA alone was present. Synergistic extraction was measured for DMDHTPDA plus thenoyltrifluoroacetone. From the extraction dependencies on pH and extractant concentration, formation of mixed TTA+DMDHTPDA complexes were indicated. Except for the Th(IV) system, the separation and synergistic factors were smaller for the TTA+DMDHTPDA extractant than for the system of TTA+DMDHOPDA (the oxo ether analog of DMDHTPDA).
Complexation behavior of NpO2+ with ortho-silicic acid (o-SA) has been studied using solvent extraction at ionic strengths varying from 0.10 to 1.00M (NaClO4) at pcH 3.68±0.08 and 25 °C with bis-(2-ethylhexyl) phosphoric acid (HDEHP) as the extractant. The stability constant value
(log β1) for the 1:1 complex, NpO2(OSi(OH)3), was found to decrease with increase in ionic strength of the aqueous phase [6.83±0.01 at I=0.10M to 6.51±0.02 at I = 1.00M]. These values have been fitted in the SIT model expression and compared with similar values of complexation of the
metal ions Am3+, Eu3+, UO22+, PuO22+, Np4+, Ni2+ and Co2+. The speciation of NpO2+-o-silicate/carbonate system has been calculated as a function of pcH under ground water conditions.
Sorption of tracer Am3+ to silicate particles was studied as a function of pcH (4 to 9) and of ionic strength [0.20M to 1.50M (NaClO4)] at 298 K. The sorption increased with increased pcH from 4 to 6 above which saturation was observed. The insensitivity
of Am3+ sorption to increased ionic strength indicates inner-sphere complexation with the surface silicate sites. The effects of
different complexing anions such as carbonate, acetate, oxalate, phosphate, citrate, EDTA and humic acid, on Am3+ sorption were investigated. Synergistic enhancement in Am3+ sorption was observed in the presence of phosphate (4≤pcH≤7) and acetate (4≤pcH≤5) ligands at 0.20M NaClO4. The presence of the other ligands inhibited Am3+ sorption in the order: EDTA > citrate > oxalate > carbonate. Am3+ sorption in the presence of HA (25.00 mg/l) increased in the pcH range of 4.0 to 5.5, then decreased. Increased ionic strength
enhanced Am3+ sorption in the presence of 25.00 mg/l HA for 4≤pcH≤9. The sorption increased in the presence of a mixture of HA (25.00 mg/l)
and phosphate (1.00·10−3M) as compared to that of HA (25.00 mg/l) alone. The presence of Fe3+ (1.00·10−4M) enhanced Am3+ sorption at pcH∼4 but suppressed it from pcH of 5 to 9; 1.00·10−4M of Ca2+ and of UO22+ ions had no effect on the sorption profile.
Sorption of NpO2+ on silicate (10.00 g/l) particles dispersed in sodium perchlorate media was studied as a function of pcH and ionic strength
at 298 K. The sorption increased with increased pcH in the range of ∼6.5 to 9.2, above which saturation was observed. An increase
in ionic strength from 0.20M to 1.00M (NaClO4), increased the NpO2+ sorption, which then decreased at 1.50M (NaClO4) for 7<pcH<8.5. The effects of different types of ligands on the sorption of NpO2+ to suspended silicate were investigated. The types of ligands included: (i) inorganic anions (fluoride, carbonate, phosphate
(ii) N-donors (ethylenediamine, 1,10 phenanthroline (iii) carboxylic acids (oxalic acid, citric acid, iminodiacetic acid,
ethylenediaminetetraacetic acid) and (iv) humic acid. A synergistic enhancement in sorption to the suspended silicate was
observed for phosphate, oxalate, ethylenediaminetetraacetic acid, ethylenediamine, 1,10 phenanthroline (5<pcH<8) and humic
acid (6.5<pcH<8.8). This behavior was attributed to the formation of ternary NpO2+/silicate/ligand complexes. The effects of Ca(II) (1.00·10−3M) and Eu(III) (1.00·10−4 and 1.00·10−3M) ions on NpO2+ sorption to suspended silicate were also investigated.
N,N-dimethyl-N,N-dihexyl-3-oxapentanediamide, DMDHOPDA, N,N-dihexyl-3-thiopentanediamide, DHTPDA and N,N-dihexyl-3-oxapentanediamide, DHOPDA were synthesized and tested for the synergistic extraction of Eu3+, Th4+, UO
and Am3+ with thenoyltrifluoroacetone (HTTA). Although Eu3+, Th4+, UO
and Am3+ were not extracted by DHTPDA or DHOPDA alone, they were extracted synergistically when combined with HTTA. Analysis of the dependency of extraction on pH and extractant concentration indicated that the dominant extracted species were Eu(TTA)3(A), Th(TTA)3(A)(X), UO2(TTA)2(A) and Am(TTA)3(A) (where A is diamide, and X is chloroacetate or ClO