Pyridinic- and Pyrrolic Nitrogen in Pyrogenic Carbon Improves Electron Shuttling during Microbial Fe(III) Reduction

Abstract

Black nitrogen (N) is ubiquitous in natural pyrogenic carbon, and doping-induced heterocyclic N structures are often redox-active. However, our current knowledge about how the N functionalities affect the redox property of pyrogenic carbon remains limited. Here, we explored the redox impact of N functionalities on pyrogenic carbon with a particular focus on how heterocyclic N structures affect microbial extracellular electron transfer during Fe(III) reduction. The results showed that N functionalities were present in 10 different types of biomass-derived pyrogenic carbons (0.36–7.72 wt %) and were dominated by heterocyclic pyridinic-N and pyrrolic-N species (0.27–7.59 wt %). The statistical analyses suggested that the ferrihydrite reduction rate (kmax) was positively related to pyridinic-N and pyrrolic-N contents (R2 = 0.970, p < 0.001) of biomass-derived pyrogenic carbons, which indicated that pyridinic-N and pyrrolic-N structures significantly contributed to the enhanced Fe(III) reduction. The observed improvement in electron shuttling due to heterocyclic N structures was confirmed after analyzing specifically synthesized N-doped pyrogenic carbons, which also accelerated the successive precipitation of vivianite and siderite. Furthermore, experimental observations paired with density functional theory calculations revealed that pyridinic-N and pyrrolic-N structures showed a higher electron shuttling efficiency than the quinone group. These results demonstrate the improved electron transfer rate due to heterocyclic N structures and advance our understanding of the geochemical and environmental significance of N functionalities in pyrogenic carbon from a biogeochemical redox perspective.