Factors affecting the dissolution kinetics of volcanic ash soils: dependencies on pH, CO 2, and oxalate

Abstract

Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO 2 (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m −2 h −1 at pH 2.78 in the batch experiments to 0.009 μmol Si m −2 h −1 at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H + consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO 2 demonstrated that dissolution rates were independent of CO 2 within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO 2 have been elevated over the last decade. This release of magmatic CO 2 has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO 2 may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO 2 can be excluded.