LIGAND PROMOTED DISSOLUTION OF OLIVINE BY OXALATE IONS AT LOW PH

Because minerals often show enhanced solubility in the presence of organic complexing agents, there is a widely held idea that these organic ligands cause those minerals to dissolve faster. Although ligand promoted dissolution is generally accepted within the geologic community, few studies have documented this phenomenon using silicate minerals. Additionally, no study has produced a rate law that can be used to quantify the effect of ligand concentration on dissolution rate. In order to further understand both the reaction rate and reaction mechanism governing ligand-promoted dissolution, we have undertaken a quantitative study of olivine dissolution rates in the presence of organic ligands.

Oxalate was chosen as the ligand for these experiments. All experiments were done using oxalic acid at concentrations between 0.03 M and 0.08 M and at ionic strengths around 0.1. Olivine was chosen because it is one of the few silicates that dissolves congruently, making it a useful model system from which generalizations about all silicate minerals can be made. Dissolution experiments were conducted in batch reactors within a shaker bath. Five grams of olivine with a grain size of 40-60 mesh and a BET surface area of 0.0356 m²/g were placed in a 250 mL Erlenmeyer flask with 100 mL of solution and placed in a shaker bath for two hours. Samples were taken every 20 minutes. Samples were analyzed for magnesium by atomic absorption and for silica by the silicomolybdate blue colorimetric method. The dissolution rate was calculated using the initial rate method. Experiments were conducted over a pH range of 0 to 7.

Our data shows that oxalate enhances dissolution rates by up to ten times. This effect is greatest above the second pK of 4.19 for oxalic acid where oxalate ions dominate. The rate enhancement declines at pH values below 4.19 and eventually converges with the rate law for olivine dissolution in acidic pH’s. This indicates that the ligand promoted dissolution proceeds via a reaction path that is independent of proton promoted dissolution so the dissolution rate law involves the sum of two terms.