DISSOLUTION KINETICS OF VOLCANIC ASH- IMPLICATIONS FOR CO2 DRAWDOWN FROM WEATHERING OF VOLCANIC ASH

The dissolution rates and stoichiometry of andesitic-dacitic ash from five volcanic eruptions (1980 Mount St. Helens, USA, 1991 Mt. Pinatubo, Philippines, and the 2010 Eyjafjallajökull, Iceland, Mt. Pacaya, Guatemala, and Tungurahua, Ecuador) were investigated to determine the impact of ash weathering on the drawdown of atmospheric CO₂. All the ash samples except for the Pinatubo ash were collected within days of deposition, Pinatubo ash was collected in 2009 from the side of a valley that had experienced rapid physical erosion. Ash dissolution experiments were conducted in batch reactors with water or dilute hydrochloric acid over a range of pH (pH ~ 3, 4, and 5). Dissolution rates were determined from the evolution of solution composition over time. Ash samples were characterized before and after the experiments by scanning electron microscopy and X-ray fluorescence to determine changes in physical, chemical and mineralogical properties of the ash.

Batch dissolution experiments reveal that dissolution kinetics are dependent on the composition, mineralogy, texture, and particle size of the ash. Overall, reaction rates increased with increasing acidity, though the pH-dependence of the ash dissolution is complex. Silica concentrations increase approximately linearly over time, and total Si in the experiments increases ~ 2 to 5-fold with increasing acidity. Phosphate concentrations in solution were comparable to silica, however, net phosphate release to solution was more variable. All experiments showed an initial rapid release of phosphate, and then concentrations either increased more slowly, remained constant, or decreased slightly over time depending on the experiment. Thus far, equilibrium solubility calculations show that solutions are greatly undersaturated with respect to silicate minerals and apatite. However, solubility calculations further reveal that the solubility of secondary phosphate minerals, such as iron- phosphate, may be limiting phosphate release. The solubility and reactivity are key components in determining the volcanic ashes’ potential for long-term CO₂ sequestration.