JRP CALL information
Supported By

European Commission

Short description of the work
This JRP is dedicated to study the clay colloidal size effect on the sorption capacities and sorption reversibility of radionuclides (RNs) onto montmorillonite colloids under glacial melt water conditions (synthetic carbonated water at pH 8-9 and low ionic strength).

During summer 2012, montmorillonite colloids, originating from raw MX80, were separated into different colloidal size fractions, in presence or not of organic matter, by sequential or direct (ultra-)centrifugation and thereafter characterized (ICP-OES, XRD, IC, PCS, AsFlFFF-ICPMS). Even though the colloidal size fractions remain polydisperse after separation, a decrease in the mean and the mode of the colloids size distributions were found throughout the separation steps. The mean diameter decreases from approx. 1030 nm down to approx. 220 nm for the different colloidal fractions. According to literature data and thanks to the characterization by several different techniques, the mean edge site density for each colloidal fraction has been estimated. This reveals an increase in the mean edge site density by a factor of 10 maximum between the largest and the smallest colloidal fractions. These results are summarized in a first publication (to be submitted).

The colloidal fractions were used to prepare batch sorption experiments using the following RNs: 232-Th(IV), 242-Pu(IV), 99-Tc(VII), 237-Np(V) and 233-U(VI). The sorption samples were monitored after 3 days, 2 weeks, 1 month and 6 months contact time. During this stay at KIT-INE, the last analysis of the batch sorption and sorption reversibility samples has been performed after 1 year contact time and sorption reversibility time.

All samples were prepared in a glove box. The initial concentrations of colloids and RNs were first determined. After varying contact time, the amount of stable montmorillonite colloids in the suspension was analyzed by ICP-MS. Thereafter, the samples were transferred into ultra-centrifugation vials in which they were ultra-centrifuged during 1h at 90.000 rpm (centrifugal force of approx. 7∙10^5 xG). The resulting supernatants were analyzed by ICP-MS. Furthermore, Eh-measurements of all sorption samples were performed in the glove box after 1 year contact time. In parallel, the sorption reversibility was tested by i) decreasing the pH, ii) increasing the ionic strength by addition of CaCl2, iii) introducing fracture filling material or organic matter as competing ligands. The reversibility tests were prepared from the batch sorption samples after varying sorption times, and were let to equilibrate before sampling during 1 week and 1 year reversibility time. The sampling procedure to test the reversibility was similar to the protocol applied to follow the sorption.

The sorption study shows that both 232-Th(IV) and 242-Pu(IV) are sorbed onto the montmorillonite colloids independently of the clay colloid fractions used. The amount of 242-Pu(IV) sorbed increases from approx. 85% up to 95% during the time investigated, i.e. from 3 days up to 1 year, whereas the amount of 232-Th(IV) sorbed is constant at 98% over the same time period. The largest montmorillonite colloids are instable over time. Both 232-Th(IV) and 242-Pu(IV) remain associated to these colloids. In presence of organic matter, a small, but significant decrease in the amount of sorbed 232-Th(IV) and 242-Pu(IV) is observed. 99-Tc(VII), 237-Np(V) and 233-U(VI) do not sorb to the montmorillonite colloids in the carbonated synthetic ground water. After 1 year, particulates of 237-Np and 233-U are formed in the samples. This is attributed to slow reduction kinetics.

Interestingly, different behaviors of the studied RNs are observed in the sorption reversibility experiments. 233-U(VI) is sorbed, as expected, onto the clay colloids while pH is decreased from pH 9 to 7, independently of the clay colloidal fractions used. Clay colloids are destabilized at high ionic strength, as expected, which does not affect the 232-Th(IV) and 242-Pu(IV) sorption. After a delayed addition of organic matter, a slightly decrease in the sorption of 232-Th(IV) and 242-Pu(IV) is observed. Addition of fracture filling material induces formation of particulates for 99-Tc and 237-Np due to reduction and precipitation and/or sorption to fracture filling materials. Here, the amount of 99-Tc and 237-Np particulates increases with the sorption reversibility time. Nevertheless, no clear colloidal size effect is noticeable in the sorption reversibility tests.

Data treatment of all the results obtained so far is being finalized. At a first glance, no noticeable clay colloidal size effect is observed. A closer look is necessary to determine if it is due to the use of colloidal clay suspensions which still contain various colloidal sizes after the separation, due to the low trace concentration of RNs or analytical errors.

Manuscripts are now under preparation and planned to be submitted within the next weeks. A detailed summary will be presented in JRP report related to this work.

 

Main visitor contact data
Name: Karin Norrfors
Organisation: KTH Royal Institute of Technology

JRP Identification
JRP nr: TALI-C02-10
JRP title: Impact of bentonite colloid size on actinide sorption/desorption properties
JRP scope: Scope 2: Actinide in the geological environment

Visited Associated Pooled Facility
Visited APF during the stay: KIT-INE - Laboratories
Name of the APF Contact Person: Thorsten Schäfer

Other APF and organisation involved in the JRP
Other organisations involved:
Other APF involved in the project:

Description of the work done at the associated pooled facility
Start date of the stay: 3/7/2014
End date of the stay: 3/24/2014
Quantity of access: 11
Access Unit: Days
Misc.: