Computer Science
Scientific paper
Sep 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1911b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 9, pp.1911-1922
Computer Science
Scientific paper
The conditions necessary for initial clay formation have been studied in different model systems comprising different organic acids besides Si and Al. In the present paper the solid phases and the precipitation boundary characterizing the subsystem H + -Al 3+ -oxalic acid (H 2 L) are discussed. pH and tyndallometric measurements were performed in an ionic medium of 0.6 M Na(Cl) at 25 °C. The two phases Al 3 (OH) 7 (C 2 O 4 ) · 3H 2 O (phase I) and NaAl(OH) 2 (C 2 O 4 ) · 3H 2 O (phase II) determine the precipitation boundary. The following formation constants for the two phases were deduced: lg 1 = lg ([ Al 3+ ] -3 [ H 2 C 2 O 4 ] -1 [ H + ] 9 = -21.87 ± 0.08 and lg 11 = lg ([ Al 3+ ] -1 [ H 2 C 2 O 4 ] -1 [ H + ] 4 = -5.61 ± 0.06. Phase I exists in the range [ Al ] tot 10 -4.4 mol dm -3 ,[ H 2 C 2 O 4 ] tot 10 -4.9 mol dm -3 and at pH < 6.8, thus being a possible precipitate in oxalic-rich natural waters. The more soluble sodium phase is unlikely to exist in natural waters. The two phases are metastable relative to crystalline gibbsite and may be considered as the first precipitation step in the transition from aqueous Al oxalates down to stable Al hydroxide. Model calculations illustrating these competing hydrolysis-complexation reactions are discussed in terms of predominance and speciation diagrams. The solid phases have been characterized by X-ray analysis of powders, TGA and IR spectra, and tentative structures are proposed. Phase I seems to be an octahedral layer structure, in which of the octahedral sites between two close packed oxygen sheets are occupied by Al 3+ and the oxalate ion acts as a bridge ligand between two aluminium atoms. Phase II forms a more open sheet structure and has ion exchange properties. Powder data for a phase crystallized from the studied solution after a year are also presented. This phase, Na 4 Al 2 (OH) 2 (C 2 O 4 ) 4 · 10H 2 O, supports the results from the equilibrium analysis of recent solution data by and (1985), who have found the dinuclear complex Al 2 (OH) 2 (C 2 O 4 ) 4 4- to exist in a solution in which the ligand is in excess.
Bilinski Halka
Horvath Laszlo
Ingri Nils
Sjöberg Staffan
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