Mechanistic Insight into Electron Transfer from Fe(II)-Bearing Clay Minerals to Fe (Hydr)oxides

Electrontransfer (ET) is the essence of most biogeochemical processesrelated to element cycling and contaminant attenuation, whereas ETbetween different minerals and the controlling mechanism remain elusive.Here, we used surface-associated Fe-(II) as a proxy to explore ET betweenreduced nontronite NAu-2 (rNAu-2) and Fe (hydr)-oxides in their coexistingsystems. Results showed that ET could occur from rNAu-2 to ferrihydritebut not to goethite, and the ET amount was determined by the numberof reactive sites and the reduction potential difference between rNAu-2and ferrihydrite. ET proceeded mainly through the mineral-mineralinterface, with a negligible contribution of dissolved Fe2+/Fe3+. Control experiments by adding K+ andincreasing salinity together with characterizations by X-ray diffraction,scanning electron microscopy/energy-dispersive spectrometry, and atomicforce microscopy suggested that ferrihydrite nanoparticles insertedthe interlayer space in rNAu-2 where structural Fe-(II) in rNAu-2 transferredelectrons mainly through the basal plane to ferrihydrite. This studyimplicates the occurrence of ET between different redox-active mineralsthrough the mineral-mineral interface. As minerals at differentreduction potentials often coexist in soils/sediments, the mineral-mineralET may play an important role in subsurface biogeochemical processes. Electron transfer from Fe-(II)-bearingclay minerals to Fe(hydr)-oxides proceeds mainly through the mineral-mineral interface,with the interior structural Fe-(II) and expandable interlayer spacein clay minerals serving as the major electron reservoir and reactionsite, respectively.

Automated summary

Electron transfer is the essence of biogeochemical processes related to element cycling and contaminant attenuation, and this study explored the electron transfer between reduced nontronite and Fe-(hydr)-oxides. The results showed that electron transfer occurred mainly through the mineral-mineral interface, with a negligible contribution from dissolved Fe2+/Fe3+. The study suggests that the mineral-mineral electron transfer could play an important role in subsurface biogeochemical processes.