Toward an Understanding of Biogenic-silica Dissolution in Seawater – An Initial Rate Approach Applied between 40 and 90 °C

Abstract

The kinetics of phytoplankton frustule dissolution has generally been studied as the appearance of silicic acid in a batch reactor. Unfortunately, this approach, though often illuminating, has not so far been successful because of the difficulty of parameterising the full reaction curve. This current study shows how the initial rate approach to chemical kinetics offers a way around this bottleneck, thereby allowing much chemical kinetics information about frustule dissolution to be collected. The technique is shown to be flexible and suited to short reaction times which facilitate detailed quantitative kinetics investigation, indeed, as would be expected in a solution phase, kinetics study. The technique is exemplified by a dissolution study of uncleaned frustules of Cyclotella crypticaat 40 °C and above. The frustules were found to yield the same dissolution rate after 5 weeks dark storage, at 4 °C. Meanwhile, log dissolution rate was found to vary linearly with pH, with gradient 0.38 ± 0.01 (r 2=0.990). Linearity was upheld even at pHs as high as 14. Finally, a robust Arrhenius plot was established between 40 and 90 °C yielding an activation energy for dissolution of 84 ± 3 kJ mol −1. Follow through with the Eyring equation yielded an activation enthalpy, ΔH ‡, and an activation entropy, ΔS ‡, of 81 and 85 J mol −1 K −1, respectively. The discussion brings salient aspects of existing knowledge about diatom frustule dissolution kinetics into the wider context of silicate mineral dissolution.