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And Cr-Isosaccharinic Acid Under Alkaline Conditions View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library Radiochim. Acta 84, 221-224 (1999) © by Oldenbourg Wissenschaftsverlag, München Rapid Communication Evidence for the Existence of Complexes between Th(IV) and cr-Isosaccharinic Acid under Alkaline Conditions By K. Vercammen, M. A. Glaus and L. R. Van Loon Laboratory for Waste Management, Paul Scherrer Institute, CH-5232 Villigen, Switzerland Received February 8, 1999; accepted March 9, 1999 Alkaline pH / Complexation / Isosaccharinic acid / nuclide complexes with isosaccharinic acid in a reposi- Organic ligand / Radionuclide / Thorium tory might therefore enhance the release of radio- nuclides to the geo- and biosphere. Only indirect evidence for the formation of com- Summary. plexes with radionuclides and ISA at alkaline pH ex- The complexation of Th(IV) with a-isosaccharinic acid (ISA) ists [2, 6—9]. It is therefore the aim of the present was studied under alkaline conditions (pH 12.8 and 13.3) by two work to study the complexation of Th(IV) with ISA complementary techniques. Using high performance ion ex- under alkaline conditions and to demonstrate the exist- change chromatography, evidence was found for the formation ence of Th-ISA complexes. Th(IV) is an ideal rep- of complexes between Th(IV), Ca and ISA. In the presence of Ca, Th is assumed to coordinate with two ISA ligands and two resentative for safety-relevant tetravalent actinides Ca ions. The ratio between Th(IV) and ISA in this complex in such as U(IV) or Pu(IV). The experimental handling the presence of Ca was confirmed by batch sorption experi- of Th(IV) is easier than of the latter two radionuclides, ments, in which the sorption of Th was studied as a function because Th(IV) is redox-insensitive. Moreover, the ef- of the concentration of ISA. Moreover, it was shown in these fect of Ca2+ on the complexation was studied, because experiments that, in the absence of Ca, a 1:1 Th:ISA complex 2+ is formed. Conditional stability constants for the formation of the concentration of Ca in a cementitious repository both complexes are calculated. is high, owing to the presence of Ca(OH)2 in cement. Two different techniques were applied: (i) high per- formance ion exchange chromatography (HPIEC) and Introduction (ii) a batch technique, in which the sorption equilib- rium of Th(IV) on an ion exchange resin was investi- In the planned repository for low and intermediate lev- gated as a function of the concentration of ISA. el waste in Switzerland substantial amounts of organic waste are cellulosic [1]. Under the alkaline conditions of the cementitious repository, cellulose degrades rap- HPIEC method idly via a peeling off reaction [2], The main degra- dation products are the erythro- and threo-form Using HPIEC, the complexed Th species were separ- of 2,4,5-trihydroxy-2-hydroxymethyl-pentanoic acid, ated from free ISA and hydrolysed Th. The total con- called «-isosaccharinic acid (further denoted as ISA) centrations of Th, ISA and Ca were determined in the and /?-isosaccharinic acid respectively (Fig. 1). Isosac- fraction containing the complexed Th species. The ra- charinic acid belongs to the class of polyhydroxy li- tio of these three parameters can elucidate the stoichi- gands. These are known to form stable complexes with ometry of the complex formed. Briefly, the experi- various metal cations, especially under alkaline con- ments were carried out as follows: a mixture of 4 ditions [3—5], The formation of water soluble radio- defined amounts of Th(N03)4 (IO" M) and ISA (10~2 M) was allowed to equilibrate at pH 12.8 (0.08 M NaOH). In order to study the influence of Ca, COOH COOH ISA was supplied either as the sodium salt (prepared by alkaline hydrolysis of ISA lactone in NaOH) or as HOH2C •C •OH HO Ο- CH20H the Ca salt. The solution was spiked with Th-234 and I Ι 1 1 Ca-45. After six days of equilibration, the clear solu- H •C •H Η Ο- Η tion was injected into the HPIEC device (Dionex DX- I Ι 1 I 500), equipped with a Carbopac PA-100 analytical col- H •c -OH Η C- OH umn (Dionex) filled with an anion exchange resin. The eluent (0.08 M NaOH) was forced through the column CH2OH CH2OH at a flow rate of 1 ml/min. After passing through the α-isosaccharinic acid ß-isosaccharinic acid column, the eluate was collected in 2 ml fractions and each fraction was analysed for Th, ISA and Ca. The Fig. 1. Molecular structure of a- and ^-isosaccharinic acid. 222 Κ. Vercammen, M. A. Glaus and L. R. Van Loon Table 1. The ratios of the total measured Th, ISA and Ca con- with Ca. The reasons for including the formation of centration in the fractions of the eluate from 0—4 min. protons are the hydrolysis of Th(IV) and the obser- average std. dev. number of exp. vation that the coordination of polyhydroxy ligands with higher charged cations at alkaline pH is ac- ISA/Th 2.3 0.4 18 companied by deprotonation of the coordinating hy- ISA/Ca 1.0 0.1 11 droxo groups [4,11]. Because the dissociation con- stants of these groups are unknown, it is of no use to Ca/Th 2.4 0.5 11 further subdivide eqn's. (1) and (2) into deprotonation of the hydroxo groups of ISA and interaction of Th(IV) with the deprotonated ISA. total concentrations of Th and Ca were radiochem- ically determined. The activity of Th-234 was deter- Batch method mined by measuring the Cerenkov radiation of its daughter isotope Pa-234m. ISA was determined using In the batch sorption experiments, the distribution of essentially the same HPIEC system as described, but Th(IV) between a solid and a liquid phase was investi- with a pulsed amperometric detector connected (gold gated as a function of the concentration of ISA. This working electrode). technique was used to further clarify the existence of As had been shown in independent experiments, Th-ISA complexes in the absence of Ca. In addition, hydrolysed Th(IV) is completely retained by the this technique allows the calculation of thermodyn- HPIEC column in the absence of the ISA ligand. amic stability data for the complexes formed, which is Further, Ca and free ISA are eluted at retention times not possible for HPIEC owing to the possible changes longer than 4 min (data not shown). However, if the of chemical equilibria involved therein. mixture of Th, Ca, and ISA is injected, Th and small For the present purpose, the sorption of Th(IV) on amounts of ISA and Ca are eluted between 0 and BioRad 50W-X2 cation exchange resin was measured 4 min. This is a direct indication that a complex be- as a function of the concentration of total ISA at pH tween these ions is formed, which elutes during the 13.3 in the presence and absence of Ca. 15 ml aliquots first four minutes after injection. The stoichiometry of of ISA in 0.3 M NaOH, spiked with Th-234 were ad- the complexes present in the fractions eluted from 0— ded to 100 mg of resin. In some experiments, ISA was 4 min. in a series of replicates was calculated from the equilibrated with a 0.3 M NaOH solution saturated ratios of the total measured Th, ISA and Ca concen- w.r.t. Ca(CO)2 and spiked with Th-234. After equili- trations (Table 1). The ratio between ISA and Ca was brating the systems for 3 days on a end-over-end found to be 1:1. Owing to the larger standard devi- shaker, the solid phase was separated from the solution ation, the ratio between. Th and ISA is less clear, but by centrifuging at 48360 g for 30 min. The activity of is probably near 1:2. These observations can be ex- Th-234 in the supernatant was radiochemically deter- plained by the formation of a 1:2:2 Th:ISA:Ca com- mined as described in the HPIEC experiment. The am- plex. ount of sorbed Th was then calculated from the differ- If a mixture of Th and the sodium salt of ISA is ence between this value and the total activity of Th injected, no Th is eluted from the column. This obser- added to the systems, taking into account the loss of vation can be interpreted in two ways : (i) no complex Th by wall sorption. between Th(IV) and ISA is formed or (ii) a complex The system obeys the required conditions for ap- is formed, however it dissociates completely when plying this batch technique: it is reasonable to assume passing through the column. Independent of these in- that the reference sorption isotherm is linear in view terpretation possibilities, it can be stated that the pres- of the low Th concentration (^ IO"15 M) and that the ence of Ca2+ in a mixture of Th(IV) and ISA has a sorption of Th on the resin is reversible. Further, under stabilising effect on possible complexes between these the experimental conditions the resin is known to be ions. Similar complexes between Ca2+, Al(III) and the chemically stable (manufacturer's specifications and structurally related aldarate ligands were earlier de- reference [12]). Test experiments have shown the ab- scribed by Venema et al. [10]. sence of sorption of ISA on the resin. Based on these results, two tentative general com- Based on the results obtained by HPIEC (eqn's.
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