Gd. Turner et al., SURFACE-CHARGE PROPERTIES AND UO22+ ADSORPTION OF A SUBSURFACE SMECTITE, Geochimica et cosmochimica acta, 60(18), 1996, pp. 3399-3414
Surface charge and UO22+ adsorption were measured on a clay-sized, sub
surface mineral isolate whose mineralogy was dominated by a ferrogenou
s beidellite. Experiments were performed in batch at 25 degrees C with
N-2(g) atmosphere and sorbent suspensions (9.46 g clay/kg suspension)
that had been adjusted in pH between 4 and 9. Surface charge was defi
ned by measurements of adsorbed Na by isotopic exchange and of proton
adsorption by potentiometric titration in NaClO4 (I = 0.1, 0.01, 0.001
). Extraction of the clay with La(NO3)(3) and aqueous-phase analyses w
ere necessary to establish the contributions of Al and Si dissolution
to the proton balance and the total adsorbed cation charge (i.e., Na-a
ds(+) + 3Al(ads)(3+)). The adsorption of UO22+ (7.5 x 10(-6) mol L(-1)
) was determined in Na+ (0.1, 0.01, 0.001 mol L(-1)) and Ca2+ (0.05 an
d 0.005 mol L(-1)) electrolytes. Adsorption of UO22+ showed contributi
ons of ion exchange and edge complexation reactions in Na+ electrolyte
, but by only edge complexation reactions in Ca2+ electrolyte. A multi
ple-site surface-complexation model containing fixed-(X(-)) and variab
le-charge sites (SiOH, AlOH) was fit to adsorbed cation charge data be
tween pH 4 and 10, with the concentrations of AlOH, SiOH, and X(-) as
the adjustable parameters. Surface acidity and ion-pair formation cons
tants for gibbsite and silica were used to describe the ionization and
electrolyte binding of the AlOH and SiOH sites. The model provided an
excellent description of the surface-charge characteristics of the cl
ay as measured by sodium isotopic exchange and potentiometric titratio
n. A composite model was formulated to predict UO22+ adsorption by inc
orporating UO22+ aqueous speciation, competitive ion exchange with bac
kground electrolyte cations, and UO22+ complexation with AlOH and SiOH
sites. UO22+ complexation with AlOH and SiOH was parameterized by UO2
2+ sorption on alpha-Al(OH)(3)(s) and alpha-SiO2(s), respectively. The
composite model overpredicted UO22+ sorption across the entire pH ran
ge in both electrolytes. Acceptable predictions could be obtained if t
he UO22+ affinity for edge AlOH sites were adjusted 2.03 log units bel
ow that of gibbsite. Changes in chemical affinity arising from lattice
substitutions and edge site morphology are, therefore, concluded to c
ontribute significantly to adsorption, although the potential competit
ive effects of dissolved Al3+ and H4SiO4 could not be discounted. The
adsorption of UO22+ On the subsurface smectite was similar to that of
the reference montmorillonite, SWy-1, with the exception that Al disso
lution contributed significantly to adsorbed cation charge.