The Sorption of Polonium, Actinium and Protactinium Onto Geological Materials
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10000 MWCO filtered > 350 > 260 > 290 > 330 > 210 > 120
0.7 0.7
> 220 12 ± 1
> 160 > 140 2.0 ± 0.1 7.8 ± 0.6
> 300 > 210 > 210
9.4 8.2
> 240
10000 MWCO filtration from the experiments at 60'C were very similar to those from room temperature experiments, but results were significantly lower after 0.45[tm filtration and centrifugation. The variation with phase separation method was marked in the case of granodiorite, with average Rd values lying between -0.4 m3kg-1 after centrifugation and >200 m3kg-1 following 10000 MWCO filtration. This behaviour of protactinium was similar to that of actinium, but even more pronounced. It can again be interpreted in terms of the 'particulate' or polynuclear speciation behaviour of protactinium in aqueous systems, as described earlier for actinium. The results obtained are, where comparable, consistent with those obtained in an earlier study of the solubility and sorption of protactinium onto a range of geological and cementitious materials [12]. In this earlier study, protactinium was believed to be forming clusters or polymeric species. Predictions indicate that Pa(V) is the stable oxidation state under the experimental conditions, using the limited data available [14]. Predictions for more strongly reducing conditions indicate the possibility of Pa(IV) formation. However, Pa(IV) is reportedly unstable with respect to oxidation to Pa(V), except in the presence of strong acids or complexing agents [24] based on the reported potential for the Pa(V)/Pa(IV) couple [27]. Bermer [28] estimates thermodynamic data for the formation of a possible Pa(IV) carbonate complex Pa(C0 3)56 . Baes and Mesmer [24] summarise the data for Pa(V) hydrolysis products formed at low pH (PaO(OH) 2 + , PaO2+ and PaO 2(OH)), but report that these species undergo irreversible further hydrolysis involving polymerisation and precipitation above pH 1. They further describe the great difficulty of maintaining Pa(V) in aqueous solution as being perhaps the best known feature of the aqueous chemistry of the element. Under the experimental conditions with very low protactinium concentrations, any protactinium remaining in solution may therefore be present as higher hydrolysis products than those for which thermodynamic data are reported. The occurrence of polymeric species may be more likely with protactinium than with actinium, but there is considerable uncertainty in the aqueous speciation of protactinium under these experimental conditions.
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CONCLUSIONS Batch sorption experiments, carried out under conditions designed to simulate those around a potential HLW repository in Japan, have shown that, in general, polonium, actinium and protactinium are strongly sorbed onto bentonite, tuff and granodiorite at room temperature. Variations in Rd values with phase separation methods suggests the possibility of 'particulate' or colloid formation. Values for Rd for actinium and protactinium sorption at 60'C after ultrafiltration are similar to those at room temperature, but are significantly lower after centrifug
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