Dehalogenation of trichloroethylene in microbial electrolysis cells with biogenic palladium nanoparticles.

Nanopalladium catalysts can be synthesized by the precipitation of palladium (Pd) on the surface of bacteria leading to the production of biogenic Pd nanoparticles (bio-Pd). For example, Shewanella oneidensis can reduce Pd(II) and subsequently precipitate it as Pd(0) nanocrystals on their cell wall and in their periplasmic space when a hydrogen donor is provided (De Windt et al., 2005). No expensive and harmful chemicals are required as the bacteria serve as reductant for the Pd salt and as stabilizer of the Pd nanoparticles. Bio-Pd has been reported to catalyze efficiently the dehalogenation of the groundwater contaminant trichloroethylene (TCE) (Hennebel et al., 2009). In this conversion, bio-Pd was used as catalyst. Thus an external electron donor such as hydrogen gas or formate was required. In large-scale applications, the use of hydrogen gas can give rise to large costs and technical difficulties. Microbial electrolysis cells (MECs) can be used for the production of hydrogen gas, however this innovative technology was never considered to deliver hydrogen for bio-Pd catalyzed dehalogenation reactions. In MECs, organic material is oxidized by electrochemically active microorganisms at the anode. Subsequently, the microorganisms transfer the electrons resulting from this oxidation to the anode . Through an electrical circuit, the electrons are transported to the cathode, where they are consumed for hydrogen formation (Mu et al., 2009). In contrast with microbial fuel cells (MFCs), in which electrical energy can be extracted from the electrical circuit, one needs to supply electrical energy to the electrical circuit of an MEC by means of a power source. In this study, the application of bio-Pd in the cathode of an MEC as a catalyst for pollutant reduction was compared with an MEC without bio-Pd and tested in the dehalogenation reaction of the groundwater pollutant TCE. Additionally, it was tested if different amounts of bio-Pd at the cathode influenced the dehalogenation reaction at the cathode.