Fiber optical oxygen measurement
Henrik Hecht (now a1-envirotech) worked as a PhD in our group in a DFG granted project (Ko1656/2) on pyrite weathering processes.The specific topic was the detection of the diffusive oxygen recharge in open pit mine overburden material. The idea was to measure the oxgen profile in soils affected by pyrite weathering to determine the pyrite weathering kinetics.
We adapted a fiber optical microtechnique developed at the Max Planck Institute for Marine Microbiology (MPI) for the more rough use in silt and sand on land.
While the MPI sensors were developed to replace Clark-type microelectrodes working at depth resolutions of 50 µm we needed sensors that work for depth resolutions of cm to m. And they needed to be cheap, stable and easy to manufacture, so we could leave them in the sediments of interest for long-term monitoring.
In close cooperation with Gerhard Holst (MPI, now PCO, Kelheim, Germany) we developed an oxygen meter and sensors based on 1mm plastic fibers. The fully digital version of this fiber optical oxygen meter is now available from Presens, Regensburg, Germany.
The same type of sensors are used on the MPI landers.
Byproducts of these investigations were fiber optical sensor switches with extremely low attenuation that were needed for the automated use of one oxygen meter with multiple sensors. Henrik developed both a switch for 1 mm plastic optical fibers (POF) and another switch for standard 140µm quartz fibers. For the prototypes of these switches we successfully abused technolgy antiques such as 5.25" disk drives and dot matrix printers.
Results from field oxygen measurements with POF optode array.
Age of overburden aprrox 45yr, pyrite content 0.4 wt% (after Hecht & Kölling, 2001a)
Henrik sticking the 8m long oxygen sensor array into a borehole in a overburden dump site from an Eastern German lignite mine
Design of 1mm plastic fiber based oxygen sensors (Hecht & Kölling, 2001).
Acid residual lake in lignite mine overburden material. The acid mine water contains extremely high iron and sulfate at pH values down to less than pH2. (Photo: N. Geissler)
Pyrite oxidation front burning down into an aerated sediment as expressed in the oxygen concentrations over time and depth. The sedimet column is initially fully oxic. Over the first 20 days pyrite oxydation evolves in the upper 20 cm. After the pyrite is "burnt" in the surface layer, a pyrite oxidation front burns down with decreasing speed due to decreasing diffusive oxygen recharge. This kind of behaviour with the resulting pH, Fe and SO4 concentrations was modelled with a coupled Reaction-Diffusion-Model (H. Hecht, M. Kölling and N. Geissler , 2002).
- Hecht, H., Kölling, M. & Schulz, H.D. (2003): Pyrite weathering in the unsaturated zone of lignite mine tailings: Release of As, Cd, Ni and Pb in a soil column experiment.- In: Hadeler, A. & H.D. Schulz (Eds.): Geochemical Processes in Soil and Groundwater: Measurement - Modelling - Upscaling. Wiley.
- Hecht, H. & Kölling, M. (2002): Investigation of pyrite weathering processes in the vadose zone using optical oxygen sensors.- Environmental Geology, 42/7, pp 800-809.
- H. Hecht, M. Kölling and N. Geissler (2002): DiffMod7 - modeling oxygen diffusion and pyrite decomposition in the unsaturated zone based on ground air oxygen distribution.- pp 55-78, in: Schulz, H.D. & Teutsch, G. (Eds): "Geochemical Processes - Concepts for Modeling Reactive Transport in Soils and Groundwater", Wiley-VCH, Weinheim.
- Koelling, M. Hecht, H and Holst, G.A. (2002): Simple plastic fiber based optode array for the in-situ measurement of ground air oxygen concentrations.- in: Advanced Environmental Sensing Technology II.- Tuan Vo-Dinh, Stephanus Büttgenbach (Eds.), Proceedings of SPIE Vol 4576, pp 75-86; ISBN 0-8194-4304-2
- Hecht, H. & Kölling, M. (2001a): A low-cost optode-array measuring system based on 1 mm plastic optical fibers - new technique for in situ detection and quantification of pyrite weathering processes.- Sensors and Actuators B 81, pp.76-82, Elsevier
- Hecht, H. & Kölling, M. (2001b): A very low attenuation fiber-optical sensor switch (LAFOSS).- Sensors and Actuators B 81, pp.128-131, Elsevier
We also experimented with planar optodes. The picture shows a transparent column with a planar optode inside filled with sediment. Both the intensity and lifetime of the red fluorescence excited by the blue LEDs are decreasing with increasing oxygen concentration. For the measurement a very fast camera synchronized with the pulsing excitation LEDs is needed.
Gerhard Holst even had an application where he was measurning 3D oxygen distributions over time in the water sorrounding a living coral using oxygen sensitive fluorophors immobilized in floating microbeads. He suspended the microbeads with the fluorescent dye in the water and moved a camera/excitation-laser assembly over the coral to get 3D images. Gerhard now works for the company (PCO) who builds the extremely fast cameras needed for this kind of application (Photo: H.Hecht)