Triplet-Excited Riboflavin Promotes Labile Fe(III) Accumulation and Changes Mineralization Pathways in Fe(II)-Catalyzed Ferrihydrite Transformation

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