Pyrogenic Carbon Enhances Hydroxyl Radical Generation during Microbial Transformation of Ferrihydrite at Redox Interfaces

Pyrogenic carbon (PC) is redox-active and facilitates the microbial reduction of Fe(III) (oxyhydr)oxides to Fe(II) under <i>anoxic</i> conditions. The oxygenation of Fe(II) under <i>oxic</i> conditions produces hydroxyl radicals (^•OH). However, the influence of PC on this process remains largely unknown. In this study, we investigated the effect of PC on ^•OH generation during ferrihydrite transformation by <i>Shewanella oneidensis</i> MR-1 at the redox interfaces. The results showed that PC increased ^•OH generation by 2.3-6.5-fold during the oxygenation of Fe(II) and this enhancement correlated with the redox properties of PC. Electrochemical analysis indicated that PC decreased the redox potential of Fe(II) and facilitated the interfacial electron transfer of Fe(II), thereby increasing Fe(II) oxidation by 33-86%. PC mediated electron transfer from Fe(II) to O₂ to sequentially generate O₂^•-, H₂O₂, and ^•OH through oxygen-containing functional groups and graphitic structures. During the decomposition of H₂O₂ by Fe(II) to produce ^•OH, PC inhibited Fe(IV) formation by preventing O-atom transfer from H₂O₂ to Fe(II) and promoted ^•OH generation by facilitating electron transfer from Fe(II) to H₂O₂. Consequently, PC increased ^•OH generation efficiency by 2.9-3.3-fold, making a major contribution to the enhanced ^•OH generation than to the increased Fe(II) oxidation (9-13% at most). This study improves our understanding of the effect of PC on Fe cycling and provides mechanistic insights into PC-enhanced ^•OH generation from the oxygenation of Fe(II) during the microbial transformation of Fe(III) (oxyhydr)oxides at redox interfaces.