Gibbs free energies of formation at 298 K for imogolite and gibbsite from solubility measurements

Details Gibbs free energies of formation at 298 K for imogolite and gibbsite from solubility measurements Gibbs free energies of formation at 298 K for imogolite and gibbsite from solubility measurements

Mar 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1667s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 6, pp.1667-1677

Physics

2

Scientific paper

The aqueous solubility of synthetic imogolite at 298 K and 1 bar pressure and at 373, 393, 408 and 423 K and equilibrium vapor pressure was determined in 0.01 M NaCl at two initial pH levels. Samples were run with and without pretreatment with HCl and in the presence and absence of gibbsite or boehmite. At 298 K and 1 bar pressure, dissolution of non-HCl-washed synthetic imogolite at the initial pH values of 1.8 and 2.2, with or without addition of AlCl 3 and H 4 SiO 4 , approached equilibrium within 335 and 33 days, respectively. Dissolution of HCl-washed imogolite and gibbsite at initial pH 2.5 and 3.0 reached equilibrium within 332 and 487 days, respectively. There was no difference between the log IAP values at equilibrium from non-HCl-washed and HCl-washed imogolite samples. Dissolution of non-HCl-washed synthetic imogolite and gibbsite at an initial pH 2.2 attained equilibrium within 485 days of equilibration, but equilibrium was not reached after 861 days for samples at an initial pH 1.8. Dissolution of HCl-washed imogolite at initial pH values of 2.5 and 3.0 did not reach equilibrium within 766 days. At 373 and 393 K and an initial pH 3.0, dissolution of imogolite and synthetic boehmite reached equilibrium. The calculated Gibbs free energies of formation at 298 K were 2923.79 ± 3.38 (synthetic imogolite), -2920.83 ± 3.92 (natural imogolite), -1155.06 ± 1.43 (gibbsite), -915.10 ± 1.83 (boehmite, extrapolated from elevated temperature) and -920.64 ± 1.41 kJ mol -1 (boehmite, from 298 K solubility). The results indicate that synthetic imogolite is more soluble than earlier reports suggest and natural imogolite is less stable than its synthetic counterpart.

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