Toward an understanding of biogenic-silica dissolution in seawater – An initial rate approach applied between 40 and 90°C

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 5weeks dark storage, at 4°C. Meanwhile, log dissolution rate was found to vary linearly with pH, with gradient 0.38±0.01 (r2=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±3kJmol–1. Follow through with the Eyring equation yielded an activation enthalpy, H, and an activation entropy, S, of 81 and 85Jmol–1K–1, respectively. The discussion brings salient aspects of existing knowledge about diatom frustule dissolution kinetics into the wider context of silicate mineral dissolution. [KEYWORDS: biogenic silica ; dissolution kinetics ; initial rate]

In vitro, batch-dissolution of biogenic silica in seawater – the application of recent modelling to real data

Abstract Both for straight-forward oceanographic needs as well as application to climate-change amelioration, there is a need for quantitative description of the spatial and temporal variability in the recycled flux of silicic acid in the ocean water column in terms of governing processes. As part of that, the need for a fuller chemical kinetics treatment of biogenic silica dissolution, involving laboratory rate measurements, rate equations and mechanisms, is stressed. Advantage is taken of recent modell [KEYWORDS: Silica dissolution ; Biogenic silica ; In vitro dissolution ; Chemical kinetics ; Batch dissolutions ; Mineral dissolution]

The Rate-equation for Biogenic Silica Dissolution in Seawater – New Hypotheses

This paper investigates the kinetics of biogenic silica dissolution in seawater, through batch dissolution, where the reaction is observed as the increase in dissolved silicic acid concentration with time. It utilises new data from dissolution of the marine diatom Cyclotella cryptica, and the freshwater diatom C. meneghiniana, as well as literature results. The sum of exponentials form:, is hypothesised as the most general rate equation, with the single exponential form occurring in a minority of cases. The consistency of this behaviour with a near-exponential decay of surface area with time, an appropriate mathematical integration, and surface heterogeneity, is discussed. (Serious errors in some existing integrations are identified.) The rate of dissolution at constant surface area is shown to decrease non-linearly as the ambient concentration of silicic acid increases. A fractional order with respect to silicic acid in the back reaction, close to 0.5, leads to a mechanism in which an intermediate is formed from the surface and an, as yet, unidentified molecule, probably water. Good preliminary fits are found between the model and literature results found using entirely different methods. A parallel treatment of hydrogen ion dependency is suggested. The likely distortion of full reaction curves from exponential behaviour imposed by the back reaction, is considered in detail. [KEYWORDS: biogenic silica ; dissolution kinetics ; diatom frustules ; silica cycling]