GEOC
results model Sr sorption predominantly as innersphere complexes and not as outersphere
complexes. Sr sorption is best modeled as >SiOSr+ to amorphous silica, as >(Si,AI)OSr+ to
kaolinite, and as >AIOSr+ to gibbsite. In goethite suspensions saturated with ambient atmosphere
carbonate sorption is very important, however Sr sorption maybe modeled equally well as
>FeOSr+ or >FeOCOOSr+. In goethite suspensions saturated with 100% nitrogen, Sr sorption is
best modeled as a combination of innersphere >FeOSr+ and outersphere >FeO-Sr++ complexes.
Innersphere Sr sorption to kaolinite, amorphous silica, and goethite in 100% nitrogen contrasts
our spectroscopy results, which show no evidence of Sr-Si, Sr-AI, or Sr-Fe bonds.
025.
N l S O R P T I O N O N P Y R O P H Y L L I T E : E V I D E N C E F O R T H E T R A N S F O R M AT I O N O F L AY E R E D
NI-AL DOUBLEHYDROXIDE INTO A PHYLLOSILICATE PRECURSOR. A.C. Scheinost, R.G.
Ford, K.S. Scheckel, and D.L. Sparks), Department of Plant and Soil Sciences, University of
Delaware, Newark, DE, 19711
Former EXAFS studies have shown that precipitates formed when Ni or Co were reacted with Alcontaining clay minerals. The exact structure and composition of these precipitates is, however,
still disputed. We used a combination of four techniques, EXAFS, DRS, FTIR, and high-resolution
thermogravimetry to monitor the reaction of Ni with pyrophyllite over 12 months. Within 5 minutes,
an Al-containing double hydroxide with nitrate in the interlayer formed. Release of Si from the
pyrophyllite caused the exchange of interlayer nitrate by silicate, and the subsequent gradual
polymerization of silicate led to the formation of a Ni pyllosilicate precursor. This transformation
substantially enhanced the stability of the Ni-containing phase, and may create a relatively stable
sink for Ni in soils and sediments.
026.
E X P E R I M E N TA L A N D M O D E L I N G S T U D I E S O F U R A N Y L A D S O R P T I O N O N T O B I N A R Y
MIXTURES OF SILICATE MINERALS. Alka Jain, James D. Prikryl, Roberto T. Pabalan, and
David R. Turner, CNWRA, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX
78238-5166
Batch experiments on uranyl sorption onto quartz, clinoptilolite, and mixtures of quartz and
clinoptilolite were conducted as a function of pH and in equilibrium with atmospheric C02{g). The
results show that uranyl sorption is strongly dependent on pH, reaching a maximum at nearneutral pH and decreasing sharply towards more acidic or more alkaline conditions. The pHdependence of uranyl sorption are the same for the different substrates. Parameters for the
diffuse-layer surface complexation model were derived from data on single minerals, and were
used to predict uranyl sorption onto binary mineral mixtures.
027.
S T R U C T U R E O F A T E R N A R Y G O E T H I T E - C I T R AT E - U R A N Y L S U R F A C E C O M P L E X . G . D .
Redden, J.R. Bargar, P. Persson, Idaho National Engineering and Environmental Laboratory,
Idaho Falls, ID 83415
Organic ligands can alter the surface chemical properties of mineral surfaces and influence the
solid-solution partitioning of cationic heavy metals and radionuclides. Extended X-ray Fine
Structure and FT-IR spectroscopies, and electrokinetic mobility have been used to analyze the
effect of citric acid on uranyl partitioning on a goethite surface. At low pH citrate promotes uranyl
sorption and the effect depends on the ligand:surface rather than the ligand:metal ratio.
Spectroscopic results reveal that U022+ partitions as an outer-sphere species on the citrate-
coated surface. At near-neutral pH, and as surface-bound citrate increases, this species appears
to replace the existing inner-sphere uranyl-goethite complex. We conclude that citrate influences
the solid-solution partitioning of cationic actinides by altering the surface chemical properties of the
aoethite surface. Models should caoture the macroscooic exoression of these orocesses.