Development of microfluidic technology for in-situ determination of iron and manganese in natural aquatic systems

Abstract

In-situ sensors are crucially important for understanding the physico-chemical processes that occur in natural water environments. Manual sampling with laboratory analysis cannot provide the temporal and spatial resolution required to characterize marine and fresh water ecosystems, and this approach is both expensive and time consuming, and may also be affected by artefacts during handling and storage. In-situ sensors minimize these drawbacks and provide a tool to obtain long-term data banks which will allow a more synoptic interpretation of the biogeochemical cycles of key elements in water systems. The trace metals iron and manganese are examples of key elements that shape the
biogeochemistry of aquatic systems. Processes influenced by them include phytoplankton growth, deep-sea vent chemistry and redox equilibria in environments with strong oxygen concentration gradients.

This thesis describes the development, optimisation and application in environment of two sequential prototypes of a Lab-On-A-Chip microfluidic autonomous analyser for the in-situ determination iron and manganese in aquatic environments. A first prototype (Prototype 1,P1) of the device existed at the beginning of this project. It was labtested and deployed at depth in the Lucky Strike Vents Field (Mid Atlantic Ridge) for the determination of Fe(II).
An operative fault during the deployment triggered a trouble shooting process which highlighted some weak points in the device. Those weaknesses were addressed and solved in a second version of the device (Prototype 2, P2) whose novel feature was the in-line mixing by diffusion of reagents and samples. Total Fe, Fe(II) and Mn could be measured with a frequency of up to 12 and 6 samples per hour respectively, with limits of detection of 35 nM and 27 nM for Total Fe and Fe(II) and 28 nM for Mn.

The robustness and reliability of P2 was tested in the laboratory and in the environment in both marine (Baltic Sea) and fresh (Beaulieu River) waters. The results of these deployments are presented and directions for further developments of the technology are proposed.

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