Nitrogen-Rich Pyrogenic Carbon-Induced Hydroxyl Radical Generation During Sulfide Oxidation: An Unrecognized Polysulfide Radical Chain Propagation Mechanism

Abstract

Sulfide oxidation mediated by pyrogenic carbon (PyC) is critical for initiating reactive oxygen species (ROS) generation in subsurface systems. Nitrogen-rich pyrogenic carbon (N-PyC), containing abundant heterocyclic N, serves as a highly reactive component of the PyC pool; yet, its role in regulating sulfide oxidation remains unclear. The study systematically investigated the overlooked role of N-PyC in dark •OH generation during sulfide oxidation. Compared to counterpart PyC, N-PyC significantly increased •OH production during sulfide oxidation by ∼2–3 times, elevating pollutant degradation efficiencies in soils by >50%. Spectroscopic and electron paramagnetic resonance analyses revealed a novel electron transfer pathway mediated by N-PyC, which effectively facilitated the formation and subsequent propagation of polysulfide radicals. Unlike the previously reported two-electron transfer mechanisms on conductive graphitic domains, these polysulfide radicals induced by N-PyC enhanced •OH generation by >80% via a single-electron transfer mechanism with oxygen, as evidenced by electrochemical results. Theoretical calculations demonstrated that pyridinic N thermodynamically promoted the binding and homolytic cleavage of reactive sulfur structures by 0.7 and 0.13 eV, respectively, thereby activating polysulfide radicals. Soil incubations confirmed a significant positive correlation between N-PyC-mediated sulfide oxidation and •OH production in natural sulfide-rich zones (R2 = 0.87, p < 0.01). This study reveals a polysulfide radical-mediated electron transfer pathway for •OH formation, offering insights into the biogeochemical processes of sulfide oxidation and the involved ROS dynamics in pyrogenic-impacted environments.