EXPERIMENTAL STUDIES OF OXALATE COMPLEXATION AT 80-DEGREES-C - GIBBSITE, AMORPHOUS SILICA, AND QUARTZ SOLUBILITIES IN OXALATE-BEARING FLUIDS

Experimental measurements of amorphous silica, quartz, and gibbsite so lubilities in oxalate-bearing solutions at 80-degrees-C over a wide pH range reveal that aqueous Si-oxalate complexation is of negligible im portance in natural fluid-rock systems, but that Al-oxalate complexati on can dramatically affect aqueous Al concentrations. The data indicat e the presence of at least two Al-oxalate complexes, and the data plac e quantitative constraints on the stoichiometry and stability of the A l-oxalate aqueous species. However, the data do not uniquely define th e stoichiometries of the important Al-oxalate complexes. The two most likely possibilities are (1) Al(Ox)3(3-) and Al(Ox)+ as the important complexes or (2) Al(OH)2Ox-1 and Al(OH)Ox0. For the first speciation, the observed solubilities constraint the values for the log of the dis sociation constants for Al(Ox)3(3-) and Al(Ox)+ to be -18.1 +/- 0.5 an d -8.3 +/- 0.7, respectively. If Al(OH)2Ox- and Al(OH)Ox0 are dominant , the data define the dissociation constants for these complexes to be -24.5 +/- 0.2 and -15.8 +/- 0.5, respectively. Thermodynamic modeling , using these results, indicates that Al-oxalate complexation can domi nate the Al budget of formation waters. Calculations suggest that with Al(Ox)3(3-) and Al(Ox)+ dominant, the presence of a significant conce ntration of Ca (on the order of 200-300 ppm) does not imply a sequeste ring of oxalate by a Ca-oxalate precipitate. However, if Al(OH) Ox0 an d Al(OH)2Ox- are the dominant Al-oxalate complexes, Ca-oxalate precipi tation will occur at much lower Ca concentrations.