Promoting interspecies hydrogen/electron transfer in Bio-PdNPs-mediated denitrification with the selectivity towards N2

The palladium nanoparticles (Bio-PdNPs) were in-situ biosynthesized in denitrifying sludge at three Pd loading levels, and the denitrification performance was evaluated in terms of interspecies hydrogen/electron transfer (IHT/IET) and their pathway for the first time. The results showed that high selectivity towards N-2 (76.96%, 97.10% and 99.30%) were observed when conventional denitrifying sludge was embedded with appropriate amount of Bio-PdNPs at three C/N ratios (3, 7 and 10), respectively. In-situ fermented hydrogen could be consecutively provided for IHT at low H/N ratio, avoiding the production of NH4+. IHT related with Pd[H] mainly catalyzed the enzymic/nonenzymic transformation of NO2-, NO and N2O, in which NO could be directly/ abiotically cracked to N-2 and O-2/H2O. Moreover, IET was greatly accelerated via the bounded flavin (FBcyt) and cytochrome, riboflavin, as well as electron hopping. Nevertheless, enzymatic denitrification was depressed with excess Bio-PdNPs (Bio-PdNPs-10). This study shed light on IET/IHT and denitrification pathway in Bio-PdNPs-mediated denitrification system, and provided basis for engineering practice.