Chemical speciation in tropical waters — A cautionary tale

Abstract

Speciation studies in natural waters have revealed a number of shortcomings in the application of the traditional equilibrium approach. Conventionally most work has been concentrated at 25°C and 1 atmosphere pressure — conditions typical of tropical surface waters. However, even where complete equilibrium can be assumed and only dissolved inorganic species are involved, considerable uncertainties remain in many of the stability constants for complexation, hydrolysis and protonation equilibria. Despite such difficulties the tendency has been for a proliferation of modelling excercises, with species distributions often quoted to a few tenths of a percent, instead of a systematic comparison of model predictions with experimental measurements. Realistic equilibrium calculations must also take account of the interactions of dissolved species with colloidal material and with (frequently uncharacterised) organic ligands. Problems concerned with distributions of complexing sites with varying properties must be recognised in assessing the importance of such equilibria. Finally, many chemical systems are pushed far from equilibrium by biological and photochemical processes so that account must be taken of the presence of metastable components and of the rate with which such species attain their equilibrium state. Examples are provided by the presence of the reduced forms of many elements in air-saturated waters. To overcome some of these problems, numerous ‘operational’ procedures have been developed which divide the total element concentrations into various fractions by techniques such as filtration, ion-exchange and UV irradiation. Since the total concentrations of many elements may not exceed 10 −9 M, such procedures are prone to experimental error. In addition, their relationship to the molecular concept of speciation is not always clear. A re-assessment is required of our approach to chemical speciation studies that will provide a much closer link between the processes we intend to study and the procedures we use for elucidating the chemistry of the elements in natural waters.