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 conditions. The oxygenation of Fe(II) under 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 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 O to sequentially generate O, HO, and OH through oxygen-containing functional groups and graphitic structures. During the decomposition of HO by Fe(II) to produce OH, PC inhibited Fe(IV) formation by preventing O-atom transfer from HO to Fe(II) and promoted OH generation by facilitating electron transfer from Fe(II) to HO. 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.