Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002gecoa..66.4203p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 66, Issue 24, pp.4203-4222
Other
Scientific paper
The influence of aqueous silica on gallium(III) hydrolysis in dilute (2 × 10 -4 m Ga 5 × 10 -3 ) and moderately concentrated (0.02 m Ga 0.3) aqueous solutions was studied at ambient temperature, using high resolution X-ray absorption fine structure (XAFS) and nuclear magnetic resonance (NMR) spectroscopies, respectively. Results show that, in Si-free acidic solutions (pH < 3), Ga is hexa-coordinated with oxygens of H 2 O molecules and/or OH groups in the first coordination sphere of the metal. With increasing pH, these hydroxyl groups are progressively replaced by bridging oxygens (-O-), and polymerized Ga-hydroxide complexes form via Ga-O-Ga chemical bonds. In the 2.5-3.5 pH range, both XAFS and NMR spectra are consistent with the dominant presence of the Ga 13 Keggin polycation, which has the same local structure as A1 13 . Under basic pH (pH > 8), Ga exhibits a tetrahedral coordination, corresponding to Ga(OH) 4 - species, in agreement with previous NMR and potentiometric studies. Major changes in Ga hydrolysis have been detected in the presence of aqueous silica. Ga is tetra-coordinated, both in basic and acid (i.e., at pH > 2.7) Si-bearing solutions (0.01 m Si 0.2), and forms stable gallium-silicate complexes. In these species, Ga binds via bridging oxygen to 2 ± 1 silicons, with an average Ga-Si distance of 3.16 ± 0.05 Å, and to 2 ± 1 silicons, with an average Ga-Si distance of 3.39 ± 0.03 Å. These two sets of Ga-Si distances imply the formation of two types of Ga-silicate aqueous complex, cyclic Ga-Si 2-3 species (formed by the substitution of Si in its tri-, tetra- or hexa-cyclic polymers by Ga atoms), and chainlike GaSi 2-4 species (similar to those found for A1), respectively. The increase in the number of Si neighbors (a measure of the complex concentration and stability), in alkaline media, with increasing SiO 2 (aq) content and decreasing pH is similar to that for A1-Si complexes found in neutral to basic solutions. At very acid pH and moderate silica concentrations, the presence of another type of Ga-Si complex, in which Ga remains hexa-coordinated and binds to the silicon tetrahedra via the GaO 6 octahedron corners, has also been detected. These species are similar to those found for Al 3+ in acid solutions. Thus, as for aluminum, silicic acid greatly hampers Ga hydrolysis and enhances Ga mobility in natural waters via the formation of gallium-silicate complexes.
Farges François
Hazemann Jean-Louis
Pokrovski Gleb S.
Pokrovsky Oleg S.
Schott Jacques
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