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 solubilities in oxalate-bearing solutions at 80-degrees-C over a wide pH range reveal that aqueous Si-oxalate complexation is of negligible importance in natural fluid-rock systems, but that Al-oxalate complexation can dramatically affect aqueous Al concentrations. The data indicate the presence of at least two Al-oxalate complexes, and the data place quantitative constraints on the stoichiometry and stability of the Al-oxalate aqueous species. However, the data do not uniquely define the 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 dissociation constants for Al(Ox)3(3-) and Al(Ox)+ to be -18.1 +/- 0.5 and -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 dominate the Al budget of formation waters. Calculations suggest that with Al(Ox)3(3-) and Al(Ox)+ dominant, the presence of a significant concentration of Ca (on the order of 200-300 ppm) does not imply a sequestering of oxalate by a Ca-oxalate precipitate. However, if Al(OH) Ox0 and Al(OH)2Ox- are the dominant Al-oxalate complexes, Ca-oxalate precipitation will occur at much lower Ca concentrations.