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

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 modelling of batch reactor dissolutions, which relies upon simple and sum of exponentials models. This is applied to archived data, and to some new data for Cyclotella cryptica, to stiffen an earlier four-point classification of dissolution behaviour which relies upon plots of ln{(C∞ − Ct)/C∞} versus time. The models are also used to demonstrate the potential for serious errors in the earlier rate constants derived from the batch approach. By extending recent modelling of non-linearity in the back reaction of the O’Connor and Greenberg equation, it is shown how simple exponential behaviour during dissolution might still be approximated, even though a net reaction is involved. The earlier approach is further extended to introduce non-linearity in the forward term of the O’Connor and Greenberg equation, caused by adsorption of silicic acid on the silica surface. The study of marine biogenic silica dissolution is reviewed in the wider context of general mineral dissolution, with an explanation of the relative merits of Transition State Theory used generally in mineral dissolution, and the more pragmatic approach to kinetics used here. The relative strength of chemo-stat (flow-through) and batch reactors is discussed.