Pyrogenic carbon has redox properties that allow it to participate in a series of biogeochemical redox reactions. However, studies on the redox properties of pyrogenic carbon have, until recently, been concerned with the development of electrochemical quantification methods.

In this study, we investigated the redox properties of pyrogenic carbon and its enhanced performance in a microbial extracellular electron transfer system when it is functionalized with oxygen (HNO₃ oxidation) or nitrogen (pyrolysis in NH₃ atmosphere)-containing functional groups. Both oxygen-functionalized and N-doped pyrogenic carbons promoted the rate and extent of ferrihydrite reduction by Shewanella oneidensis MR-1. For the oxygen-functionalization, it increased the electron exchange capacity (EEC) of pyrogenic carbon, which was responsible for the enhancement of the microbial reduction of ferrihydrite. This conclusion was supported by the presence of quinone/hydroquinone groups and strongly positively correlation between the EEC and the content of C=O groups. For the N-doping, it increased the EEC and the electrochemical capacitance of pyrogenic carbon, which were responsible for the enhanced ferrihydrite reduction. X-ray photoelectron spectroscopy, electrochemical, and electron paramagnetic resonance analyses suggested the increase in the pyrrolic-N and pyridinic-N group contents in pyrogenic carbon played a dominant role in elevating the EEC and the electrochemical capacitance of pyrogenic carbon, and finally the rate for ferrihydrite reduction. The improved electron transfer rate due to surface functionalization favors the environmental and agronomic applications of pyrogenic carbon from a biogeochemical redox perspective.

Microbial reduction of arsenic (As)-bearing Fe(III)-(oxyhydr)oxides is one of the major processes for the release of As in various environmental settings such as acid mine drainage, groundwater, and flooded paddy soil. Then, the fate and transformation of As during pyrogenic carbon facilitated microbial reduction of As-bearing ferrihydrite were investigated. Our results show that the pyrogenic carbon facilitated reduction caused the release of As(III) into the solution, whereas it caused the preferential immobilization of As(V) on the solid phase. Furthermore, pyrogenic carbon accelerated the precipitation of vivianite and siderite in sequence during microbial reduction processes. Both of the formed vivianite and siderite had an insignificant capacity for capturing As(III); however, As(V) was selectively immobilized by vivianite compared to that of siderite. Taken together, our findings provide crucial insights into understanding the role of pyrogenic carbon on the redox and immobilization of Fe and As in suboxic and anoxic environments and thus their environmental fate when it is employed for agronomic and environmental applications.