Triplet-Excited\nRiboflavin Promotes Labile Fe(III)\nAccumulation and Changes Mineralization Pathways in Fe(II)-Catalyzed\nFerrihydrite Transformation

Flavins are well-known endogenous electron shuttles that\nfacilitate\nlong-distance extracellular electron transfer in dissimilatory iron\nreduction (DIR), but the effects of their photosensitivity on DIR\nand the transformation of metastable iron (oxyhydr)oxides like ferrihydrite\n(Fh) remain underexplored. This study compared the kinetics, pathways,\nand products of Fh transformation catalyzed by aqueous Fe(II) (Fe(II)<sub>aq</sub>) in the presence of oxidized riboflavin (RF<sub>ox</sub>) at pH 7 under both dark and light conditions. While RF<sub>ox</sub> has a negligible impact on Fe(II)-catalyzed Fh transformation in\nthe dark, its photoexcited triplet state (<sup>3</sup>RF*) can significantly\naccelerate interfacial electron transfer (IET) from Fe(II)<sub>aq</sub> to Fh, increasing the reductive dissolution rate of Fh and boosting\nthe accumulation rate of the key intermediate labile Fe(III) (Fe(III)<sub>labile</sub>) from 14.2 μM·h<sup>–1</sup> to 35.6\nμM·h<sup>–1</sup>. The <sup>3</sup>RF*-promoted\nFe(II)-Fh IET favors the oxolation of Fe(III)<sub>labile</sub> to\nlepidocrocite (Lp) over goethite (Gt) formation during Fh transformation\nand promotes the subsequent conversion of Lp to magnetite (Mt), altering\nthe mineral products from sole Gt to a mixture of Lp (24.1%), Gt (45.4%),\nand Mt (30.5%). These findings highlight the notable effects of riboflavin\nas a photosensitizer on Fh biotransformation, with implications for\nmicrobial respiration and elemental cycling in natural environments.