Hornblende dissolution kinetics at 25°C

Abstract

The dissolution kinetics of hornblende from Kragerö, Norway, were studied in plug-flow columns at 25°C in order to determine the rate of dissolution. Grains with a size of 125–250 μm and a BET surface area of 0.28 m 2 g −1 were dissolved in solutions of deionized water/HCl at pH 1–5. Run duration's were about 6000 h. Experiments were also conducted with 0.01 M KCl/HCl at pH 1, 2, to investigate the effects of an added electrolyte on the reaction. The duration of these runs was about 3000 h. The added KCl had an insignificant effect on the release rates of Si, but slowed down the release of Al, Mg and Fe initially. The rates and the non-stoichiometry of dissolution were found to decrease with time in experiments with pH 1 and 2. Based on an apparent coupling of dissolution stoichiometry and rate in these runs, we propose that reconstruction of an Si-enriched surface layer that follows after a preferential loss of Al, and to a minor extent also Mg and Fe, will control `steady-state' dissolution rates in acidic solutions. It took a minimum of 3000 h until a quasi steady-state was reached at pH 1. At pH 2 at least 6000 h are required to develop the leached surface layer to its maximum thickness. For the experiments at pH>2 the runtimes were insufficient to reach a steady-state. The validity of the pH-dependence of the dissolution rate, described as log Rate proportional to −0.47 pH between pH 3 and 4, is therefore highly uncertain. Referring to the O 22(OH) 2 as 1 mol of hornblende, a dissolution rate of 4.0×10 −13 mol m −2 s −1 was measured at pH 4 after 3000 h dissolution, which is lower than earlier reported.