Studies on oxygen concentrations in uranium contaminated biofilms: Comparing electrochemical and fiber-optic sensors

Within the framework of ReCosy first comparative investigations of electrochemical and fiber-optic sensors were performed in collaboration with the University Potsdam (UPPC). Fiber-optic sensor measurements in biofilms have some advantages compared to those made by electro-chemical microelectrodes. As described in Beyenal et al. (2000) fiber-optic sensors are immune to environmental changes in pH, salinity, and ionic strength and immune to interference from moisture, carbon dioxide, methane, and other substances. A range of fiber-optic sensor studies were carried out to measure in situ geochemical gradients in biofilms. To our knowledge optical sensors applied to biofilm samples exposed to heavy metals are so far not reported in the literature. In our first collaboration with the UPPC we compared the oxygen concentration profiles obtained by electrochemical and optical microsensors, respectively. Oxygen concentration profiles were measured in the biofilms by electrochemical microsensors of the Clark design (Unisense, Denmark). These Clark-type oxygen microsensors are generally used for microbial ecology studies and contain a guard electrode. They have a tip diameter of 10 µm, a stirring sensitivity of <1–2%, and a response time of <1 s. The used multispecies biofilms (microbial diversity described in Krawczyk-Bärsch et al., 2008) were cultivated in the laboratory under non-sterile conditions in different biofilm reactors. One biofilm reactor was adjusted to a uranium concentration of (1x10-6 M), which is typical for uranium contaminated sites. The data achieved from both sensor methods are directly comparable. Fiber-optic sensor measurements showed high concentrations of oxygen over the total thickness of the biofilms, which were not in contact with uranium. In contrast, biofilms exposed to uranium revealed a much lower oxygen concentration in the bottom parts of the biofilm as described before in Krawczyk-Bärsch et al. (2008). Already at a depth of approximately 400 µm no oxygen was detectable. The comparative studies revealed that depending on the existing environmental conditions during the measurements a decision for the appropriate method is possible. However, further developments, e.g. miniaturization of the sensor equipments, are needed and aimed for within ReCosy.

References:

Beyenal, H. et al. (2000) Appl Opt 39, 3408.
Krawczyk-Bärsch, E. et al. (2008) Geochimica et Cosmochimica Acta 72, 5251–5265