Cj. Chisholm-brause et al., Uranium(VI) sorption complexes on montmorillonite as a function of solution chemistry, J COLL I SC, 233(1), 2001, pp. 38-49
We have investigated the effect of changes in solution chemistry on the nat
ure of uranyl sorption complexes on montmorillonite (SAz-1) at different su
rface coverages (1.43-53.6 mu mol/g). Uranyl uptake onto SAz-1 between pH 3
and 7 was determined in both titration and batch-mode experiments. These p
H values result in solutions that contain a range of monomeric and oligomer
ic aqueous uranyl species. Continuous-wave and time-resolved emission spect
roscopies were used to investigate the nature of U(VI) sorbed to SAz-1. A d
iscrete set of uranyl surface complexes has been identified over a wide ran
ge of pH values at these low to moderate coverages. For ail samples, two su
rface complexes are detected with spectral characteristics commensurate wit
h an inner-sphere complex and an exchange-site complex; the relative abunda
nce of these two species is similar over these pH values at low coverage (1
.43-2.00 mu mol/g). In addition, surface species having spectra consistent
with polymeric hydroxide-like sorption complexes form at the moderate cover
ages (similar to 34-54 mu mol/g), increasing in abundance as the capacity o
f the amphoteric surface sites is exceeded. Furthermore, a species with spe
ctral characteristics anticipated for an outer-sphere surface complex is ob
served for wet paste samples at low pH (3.7-4.4) and both low (similar to2
mu mol/g) and moderate (similar to 40 mu mol/g) coverage. There are only su
btle differences in the nature of sorption complexes formed at different pH
values but similar coverages, despite markedly different uranyl speciation
in solution. These results indicate that the speciation in the solution ha
s minimal influence on the nature of the sorption complex under these exper
imental conditions. The primary control on the nature and abundance of the
different uranyl sorption complexes appears to be the relative abundance an
d reactivity of the different sorption sites. (C) 2001 Academic Press.