Monitoring of redox state in a dechlorinating culture with immobilized redox indicators

During the dechlorination of tetrachloroethene (PCE) to ethene (ETH) in a microcosm bottle containing an enriched chlororespiring culture, redox conditions were spectrophotometrically monitored with the redox indicators thionine (THI) and cresyl violet (CV) immobilized on transparent films. Measurements were taken with a unique flow redox monitoring system based on circulating solution from the bottle through a specially constructed flow cell and returning it to the bottle. An enclosure for a peristaltic pump was constructed to minimize contamination of the solution with atmospheric O₂. The redox monitoring instrumentation was shown to be an excellent tool to evaluate the ability of an enclosure for anoxic samples to prevent diffusion of very small amounts of oxygen from the atmosphere into the sample. With careful design of the apparatus, the O₂ permeation rate into a solution of THI in a microcosm bottle during redox monitoring was reduced to less than 40 nmol h⁻¹. When monitoring a dechlorinating culture, dechlorination was not observed until immobilized THI was completely reduced and vinyl chloride was not produced until partial reduction of CV.