| Summary: | Dissolution rates and stoichiometry of the dissolution of basaltic andesites to trachydacitic ash from five volcanic eruptions (1980 Mount St. Helens, USA; 1991 Mt. Pinatubo, Philippines; 2010 Eyjafjallajökull, Iceland; 2010 Pacaya, Guatemala; 2010 Tungurahua, Ecuador) were investigated as part of a study to determine the impact of ash weathering on the potential drawdown of atmospheric CO2. All ash samples except for the Pinatubo ash were collected within days of deposition. Pinatubo ash was collected in 2008 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, 5 and 7) for approximately six months. Dissolution rates and concentrations of major elements and ions (H4SiO4, PO43-, Ca2+, Mg2+) were determined from the evolution of solution composition over time. Here after, major elements and ions are referred as Si, PO4, Ca, and Mg. Ash samples were characterized before and after the experiments by BET surface area analysis, scanning electron microscopy and X-ray fluorescence to determine changes in physical, chemical and mineralogical properties of the ash. Dissolution kinetics are dependent on the composition, mineralogy, texture, particle size of the ash, and solution pH. Reaction rates increased with increasing acidity, although 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 are more variable in solution in comparison to dissolved silica. 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. The dissolution of trace minerals, such as apatite which is commonly found in igneous materials, may be important because they release nutrients such as phosphate into solution, suggesting an influence in the short-term carbon consumption as biomass. Equilibrium solubility calculations show that solutions are greatly undersaturated with respect to silicate minerals and apatite. The presence of dissolved iron suggests that the solubility of secondary phosphate minerals, such as iron-phosphate present in solubility calculations, may be limiting phosphate release. The solubility and reactivity are key components in determining the volcanic ashes’ potential for short-term and long-term CO2 sequestration. Shell Exploration and Production Company Arts and Sciences Undergraduate Research Scholarship A one-year embargo was granted for this item.
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