J. I. Kim

Complexation of the Uranyl Ion with Aquatic Humic Acid

The complexation of the uranyl(VI) ion with aquatic humic acid from Gorleben groundwater (Gohy-573) is studied in 0.1 Μ NaC104 at pH 4 under argon atmosphere. The concentration range investigated is from 1 Χ 10 to 1 X 10~ mol/1 for the uranyl ion and from 1 Χ10 to 1.12 Χ 10 mol/1 for the humic acid. Three different experimental methods are applied to separate the uncomplexed uranyl ion from the uranyl humate complex, using differences of the two species in size by ultrafiltration at the pore size of about 1 nm, in charge by anion exchange separation and in spectroscopic properties by time resolved laser fluorescence spectroscopy (TRLFS). No reduction of U(VI) to U(IV) is observed in the course of the humate complexation process. The experimental results confirm that the prevailing complexation reaction is charge neutralization of the UC>2 ion with two proton exchanging sites of humic acid. At pH 4 in 0.1 Μ NaC104, where the hydrolysis of the uranyl ion appears negligible, the fraction of proton exchanging sites of the humic acid that can be occupied by the uranyl ion (called the loading capacity (LC) of the uranyl ion) is determined to be 0.185 ±0.003. The complexation constant evaluated by taking the loading capacity into account is found to be log/? = 6.16±0.13, which is a grand average of all values determined by the three experimental procedures under consideration. This value is valid for the humate complexation of the non-hydrolyzed uranyl ion, being independent of pH and ionic strength.

Complexation of Trivalent Actinide Ions (Am3+, Cm3+) with Humic Acid: A Comparison of Different Experimental Methods

The complexation of trivalent metal ions with humic acid has been studied at pH 4 and 5 in 0.1 Μ NaC104 by three different experimental methods, i.e. UV spectroscopy, time resolved laser fluorescence spectroscopy (TRLFS) and ultrafiltration. The direct speciation of the metal ion and its humate complex in the reaction process has been made by UV spectroscopy for Am(III) in the micromolar concentration range and by TRLFS for Cm(III) in the nanomolar concentration range. The ultrafiltration is used with the lowest pore size of filter (ca. 1 nm) to separate the uncomplexed metal ion from its complexed species. The concentrations of both metal ion and humic acid are varied in such a manner that the effective functional groups of the humic acid becomes loaded with metal ions from 1 % to nearly 100%. The loading capacity of the humic acid for the trivalent metal ion, determined separately at each pH, is introduced into the evaluation of complexation constants. The variation of the metal ion concentration from 6 χ 10 8 mol/1 to 4 χ 10 mol/1 does not show any effect on the complexation reaction. The three different methods give rise to constants being comparable with one another. The average value of the constants thus determined is log β = 6.24 + 0.28 for the trivalent actinide ions.

Complexatíon of Trivalent Actinide Ions (Am3+, Cm3+) with Humic Acid: The Effect of Ionic Strength

The effect of ionic strength on the complexatíon of Am and Cm by humic acid is investigated at pH 6 with three different experimental methods: ultrafiltration, UV-Visible spectroscopy, and time resolved laser fluorescence spectroscopy (TRLFS). The ionic strength is adjusted by NaCl04 and varied from 0.001 M to 5.0 M. The metal ion concentration is changed from 9.95 X10 mol/L Cm to 1.04X 10 mol/L Am, while the humic acid concentration is varied from 5.5 X10 mol/L to 1.87 XI 0~ mol/L. This assures a wide range of metal ion to humic acid concentration ratios to be examined. At pH 6, the loading capacity (LC) of Gohy-573 humic acid for trivalent actinide ions is found to change with ionic strength : LC = -(0.126+0.012) J~1 + (0.683±0.014). A relatively small effect of ionic strength on the complexatíon constant can be ascertained. The constant at 7 = 0 evaluated by the SIT model is : log ß(0) = 6.52 ± 0.05 and an average value of the constants determined over the whole range of ionic strength under investigation is: log ßUJ = 6.24±0.14.

A Study of Hydrolysis Reaction of Curium(III) by Time Resolved Laser Fluorescence Spectroscopy

T h e hydrolys is reac t ion of C m 3 + has been invest igated by t ime resolved laser f luorescence spec t roscopy ( T R L F S ) in 0.1 M N a C 1 0 4 a t 25 C u n d e r a r g o n a t m o s p h e r e . T h e exper iment is carr ied ou t in the p H range f r o m 6 to 10 fo r Cm ( III ) concen t ra t ions f r o m 1.21 χ 1 0 7 mol/1 d o w n to 3 χ I O 9 mol/1, which are subs tan t ia l ly lower than solubi l i ty l imits of the cu r ium hydrox ide at each given p H . T h e p r i m a r y a n d seconda ry hydrolys is cons t an t s d e t e r m i n e d a re : log/?i ι = 6.67 ± 0 . 1 8 fo r C m O H 2 + a n d l o g 1 2 = 12.06 ± 0 . 2 8 fo r C m ( O H ) J . These values are f o u n d to be in g o o d ag reemen t with the corres p o n d i n g hydro lys i s c o n s t a n t s o f A m 3 + d e t e r m i n e d previous ly in o u r l a b o r a t o r y by the solubi l i ty expe r imen t . However , the present va lues a re o n e or m a n y o r d e r s of m a g n i t u d e smal ler t h a n the da t a k n o w n fo r Cm (III) in the l i tera ture .

A Comparative Spectroscopic Study of the Fulvate Complexation of Trivalent Transuranium Ions

A spectroscopic study has been carried out at pH 6 in 0.1 M NaClO« for the complexation of trivalent transuranium ions with a fulvic acid from the Gorleben aquifer system. For comparison of methods, two different spectroscopic procedures are applied, i.e. conventional UV/Vis absorption spectroscopy for Am(III) and time-resolved laser fluorescence spectroscopy (TRLFS) for Cm(III). Both absorption and emission spectroscopy result in comparable constants of the fulvate complexation; log)3 = 5.78±0.07 for Am(III) and log;S=5.90±0.11 for Cm(III). These values, being independent of pH and ionic strength, are evaluated by taking into account the loading capacity of the fulvic acid, which amounts 64.9% of the total proton exchange capacity at the given pH and ionic strength. A large concentration variety of the metal ions, i.e. from 4.6x 10 mol r to 5.6x10® m o i r does not show any concentration effect on the complexation reaction and hence leads to nearly the same complexation constants.

Colloid Generation and the Actinide Migration in Gorleben Groundwaters

The generation and behaviour of realcolloids and pseudocolloids of the actinides, 238 pu, 241 Am and 244 Cm, are investigated in four different Gorleben Groundwaters. Colloids are characterized by ultrafiltration at different pore sizes from 1 nm to 450 nm and various analytical instrumentations. Behaviour of pseudocolloids has been analysed by determining their effects on the solubility and the distribution coefficient of actinides between groundwater and geomatrix. Sorption of actinide ions on groundwater-colloids (generation of pseudocolloid) are examined and the migration of actinides as pseudocolloids is discussed.

Hydrolysis Reactions of Neptunium (V)

Hydrolysis of Np(V) in Ι.ΟΛί NaCIO, was investigated by radiometric pH-titration of 2 3 , N p O j under C0 2 f ree Ar atmosphere. From the resulting solubility curve the first and second hydrolysis constant (3, and ß2 as well as the solubility product Ksp were evaluated: log ß1 = 2.3, log ß2 = 4.89 and log Ksp = 8.81.

AMERICIUM(III)-HUMATE INTERACTION IN NATURAL GROUNDWATER : INFLUENCE OF PURIFICATION ON COMPLEXATION PROPERTIES

The complexation of Am(III) is investigated with humic acid from one of the Gorleben groundwaters to describe whether or not the isolation and purification of humic acid influences its complexation behaviour with metal ions. Two different experimental systems are taken for comparison, which comprise (i) natural humic acid separated from groundwater by ultracentrifugation at 6 Χ 10 g and (ii) humic acid purified by the standard method of acid-base treatment. 241 Am is used for the complexation study in solutions (0.1 Μ NaC104) buffered at pH 6.0. Speciation is made by UV spectroscopy with the help of peak deconvolution for the two absorption bands at 503.2 nm and 505.5 nm for the A m ion and Am humate complex, respectively. The complexation is found to undergo a charge neutralization reaction of the Am ion with three proton exchanging sites of humic acid. With respect to the complexation reaction, loading capacity and complexation constant, no significant difference is found for the natural and purified humic acids. An average complexation constant is evaluated to be log β = 6.26±0.15. The effect of humic acid purification is also characterized by analysis of inorganic impurities and IR-spectroscopy. The results lead to the conclusion that humate complexation studies made on purified humic acid are directly applicable to natural systems.

Complexation of Americium(III) with Humic Acid

The complexation of the A m 3 + ion with humic acid has been investigated at different pH (5.0, 5.5, 6.0) in 0.1 M and 1.0 M NaC104 by laser-induced photoacoustic spectroscopy (LPAS), UV spectroscopy and ultrafiltration. Two humic acids are included in the study: one is extracted from lake Bradford in Florida and the other is the commercially available Aldrich humic acid, which are purified and protonated. The loading capacity of the Bradford humic acid for the Am 3 + ion is found to be a function of the pH and ionic strength. In solution with the equivalent concentration ratio of humic acid to Am being larger than one, a tridentate complexation is prevalent. The húmate complexation constants determined by taking account of the loading capacity are shown to be independent of the pH and ionic strength. The different methods have resulted in comparable constants: log β = 6.27 ± 0.04 for Aldrich humic acid and log β = 6.36 + 0.14 for Bradford humic acid.

A Direct Speciation of Cm(III) in Natural Aquatic Systems by Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS)

A direct speciation of Cm(III) traced in three different Gorleben groundwaters is performed by time resolved laser fluorescence spectroscopy (TRLFS) in very dilute concentrations from 7.8 χ 10~ mol 1 to 6.3 χ IO mol Γ 1 , which are below the solubility of Cm(III) at the given aquatic neutral pH. As spectroscopic references, fluorescence spectra are produced for individual hydroxides, carbonates, húmate and fulvate of Cm(III). Parallel to the spectroscopic speciation, a thermodynamic speciation is also made based on complexation constants of the predominant aquatic chemical reactions, i.e. hydrolysis, carbonate complexation and húmate complexation. The speciation results from thermodynamic calculation and spectroscopy are compared with one another.