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indicate that Sm 3+ is present as an outer-sphere complex with nine water molecules
surrounding the cation. Comparative QENS measurements of the mobility of the hydration
water associated with Ni and Sm sorbed on fluorated hectorite show similar behaviour, with
mobility in Sm-hectorite being somewhat greater than in the Ni-hectorite.
The results of EXAFS-based studies of Y, Lu and U(VI) sorption on clay minerals
(montmorillonite, hectorite) by CEA are coherent with formation of inner-sphere complexes,
but the location of these complexes at clay layer edges could be proposed only by analogy
with other sorbate cations. Evidence of differences in U(VI) sorption mechanisms was also
observed for the two minerals. TRLIF data for Eu sorption on both clays show increasing
formation of inner sphere complexes as pH increases.
The redox reactivity under anoxic conditions of Se(IV) with Fe(II) adsorbed on synthetic,
structural Fe-free montmorillonite was studied by UJF, in collaboration with BRGM, LPEC,
LMPC/LMM. The results show slow reduction of Se and formation of a nano-particulate
Se(0) solid phase when selenite is added to a montmorillonite previously equilibrated with
Fe 2+ solution; this was not observed in Fe-free systems. These, and other, results clearly
suggest that the Se and Fe redox reactions are not directly coupled, leading to the hypothesis
that electrons produced in the absence of Se by oxidation of sorbed Fe(II) are stored, for
example by formation of surface H2 species, and are then available for the later Se(IV)
reduction.
Regarding thermodynamic modelling of sorption:
The results of batch sorption studies (CIEMAT) of Sr, Pu, selenite and europium sorption
onto Na-smectite, Na-illite and mixed systems showed for selenite that (i) sorption was
higher in smectite than in illite and, in both clays, was independent of ionic strength and
decreased with pH, (ii) linear sorption isotherms over a broad concentration range (1•10 -10
to 1•10 -4 M), and (iii) that data could be satisfactorily modelled (from pH 3 to 8) considering
the formation of surface complexes at the edge sites of the clay and using a one site, non
electrostatic model. Regarding Eu (III), results show that (i) ionic exchange is important at
pH < 4, (ii) surface complexation becomes increasingly important as pH increases to 10, and
(iii) that data could be represented satisfactorily using a model incorporating nonelectrostatic,
two site surface complexation and cation exchange.
Measurements and modelling of Ni(II), Co(II) and U(VI) sorption on Opalinus clay, and of
Co(II) on illite were carried out by PSI, with the results for U(VI) being particularly
illustrative. Here, predictive modelling of sorption was carried out using the 2 site
protolysis, non electrostatic surface complexation and cation exchange sorption model used
in previous studies to represent U(VI) sorption on purified Na-illite assuming that (i) illite is
the main sorbing phase in the Opalinus clay and (ii) only the UO2 2+ and the hydrolyzed
species sorb. In the case of the clayrock, it was found that the U(VI) sorption isotherm could
only be modelled if the neutral Ca2UO2(CO3)3(aq) complex was included in the calculations
and assumed to be non-sorbing.
Sorption data for Cs, Sr, Am and Th on dispersed Boom Clay were gathered and interpreted
(SCK•CEN) in terms of surface complexation models, and sorption experiments on
compacted samples (clay disks) were performed for Cs and Sr to check the impact of
compaction (decreased accessibility to sorption sites) on Kd, with no significant differences
being observed. Cs sorption could be modelled using the same 3 site, cation-exchange
model used by PSI for modelling sorption on Opa. SCK•CEN also studied the influence of
the natural organic matter (NOM) present in the porewater on sorption of Am(III) and
Th(IV) onto Boom Clay, including the potential role of colloids. Data on Am(III) and
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