Quartz dissolution in organic-rich aqueous systems

Organic electrolytes are a common component of natural waters and are known to be important in many rock-water interactions. The influence of organic electrolytes on silica mobility, quartz solubility, and quartz dissolution kinetics, however, is less well understood. While there is mounting evidence supporting the presence of an aqueous organic-silica complex in natural waters, the significance of this species is difficult to characterize because of competing interactions in mixed inorganic-organic electrolyte environments. In the experiments reported here, the kinetics of quartz dissolution in dilute aqueous organic-acid solutions between 25 and 70°C were investigated to determine the influence of both organic and inorganic electrolytes. Batch-reactor dissolution experiments in inorganic and organic electrolyte solutions were designed to investigate the hypothesis that organic acids at circum-neutral pH accelerate the dissolution and increase the solubility of quartz in water. Results suggest that multi-functional organic acids such as citrate and oxalate accelerate quartz dissolution by decreasing the activation energy by approximately 20%. The increase in dissolution rate was accompanied by a 100% increase in apparent quartz solubility at 25°C. Experiments using inorganic electrolytes, in contrast, increase the rate of quartz dissolution without decreasing the activation energy, and without increasing solubility. From these data, a model for both a solution complex between dissolved organic acid and monomeric silicic acid, and an activated complex on quartz surfaces is proposed. The model suggests that dissolved organic compounds in natural waters at near-neutral pH and low temperatures are capable of accelerating the dissolution of quartz and increasing its solubility.