Jb. Fein et al., THE EFFECT OF AQUEOUS COMPLEXATION AND GIBBSITE SURFACE SITES ON THE DECARBOXYLATION RATE OF MALONATE, Geochimica et cosmochimica acta, 59(24), 1995, pp. 5071-5080
The effect of metal-malonate complexation on the decarboxylation rate
of malonate was studied by comparing the decarboxylation rates observe
d for malonate-bearing solutions with and without aqueous metal at 55,
80, and 90 degrees C. The presence of high concentrations of an aqueo
us metal significantly slows malonate decarboxylation and the effect a
ppears to increase with increasing concentration of metal in solution.
This suggests that aqueous complexation significantly slows malonate
decarboxylation. The change in decarboxylation rate due to complexatio
n correlates reasonably well with the percent of malonate present in s
olution as a metal-malonate complex, and a single correlation appears
to hold for all of the metals investigated. Malonate decarboxylation r
ates were also measured in the presence of powdered gibbsite. Due to a
queous Al-malonate complexation, the fluid in the presence of gibbsite
contained elevated Al concentrations. However, comparison of malonate
decarboxylation in solutions with gibbsite to the decarboxylation rat
es measured in gibbsite-free, aqueous Al-bearing solutions allows for
isolation of the effect of the mineral surface on the decarboxylation
rate. The results indicate that the effect of aluminum hydroxide miner
al surface sites on malonate decarboxylation is negligible and that th
e primary controls on decarboxylation, in the absence of other mineral
surface catalysts, are pH, temperature, and the extent of aqueous met
al-malonate complexation.