Tricky cyclic voltammetry peaks in glycerol anaerobic fermentation: Microbial excreted organics or others?

• Ferrous was responsible for redox peaks in cyclic voltammograms.
• Clear explanation about the shift of redox peaks and its disappearance
• Verification of the hypothesis with simulation of ferrous sulfate in PBS

Cyclic voltammetry (CV) is a widely used and powerful technique for detection and investigation of redox active substances. However, the interpretation of cyclic voltammograms is challenging as redox peaks could be attributed to metabolites and inorganic compounds. In this study, a CV redox peak pair was observed in the anaerobic fermentation broth of glycerol and the amplitude of the peak current increased as fermentation proceeded. Moreover, the central potential of the redox pair shifted towards a more positive potential as pH decreased. Iron was found responsible for the observed redox peaks and its precipitation led to the disappearance of redox peaks. The proposed electrode and precipitation reactions were supported by the simulation of ferrous sulfate in a phosphate buffer solution. This study provided an example that an inorganic compound, instead of microbial excreted organics, resulted in redox peaks in the cyclic voltammograms of glycerol fermentation. Moreover, the results also highlighted that acidic pH enhanced the bio-availability of trace minerals in anaerobic fermentation.

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