Pyridinic- and Pyrrolic Nitrogen in Pyrogenic Carbon Improves Electron Shuttling during Microbial Fe(III) Reduction
Song Wu, Dengjun Wang, Cun Liu, Guodong Fang, Tianran Sun, Peixin Cui, Huijun Yan, Yujun Wang, Dongmei Zhou
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.