Stability and molecular fractionation of ferrihydrite-bound organic carbon during iron reduction by dissolved sulfide

Abstract

The interactions of organic carbon (OC) with ferric iron (Fe) oxides play an important role in OC stabilization, but reductive dissolution of Fe(III) oxides under anoxic conditions potentially compromises the stabilization of Fe-bound OC (Fe-OC). Up to date, there are very limited studies on the fate of Fe-OC in sulfidic conditions commonly encountered in OC-rich marine sediments. Herein, multi-pronged studies were implemented to quantitatively characterize the capacity of metastable iron sulfide (FeS) to sequester OC extracted from marine sediments and to unravel the fate of ferrihydrite-bound OC complexes (Fh-OCs) when exposed to dissolved sulfide. Results show that, for a given mechanism of OC association (adsorption or coprecipitation), FeS has DOC retention capacity generally comparable to Fh, implying that the metastable phase could serve as an important OC sink in anoxic marine sediments. During reduction of Fh-OCs by dissolved sulfide, Fh reduction was progressively retarded with increasing OC pre-adsorption, while Fh reduction was promoted after DOC pre-coprecipitation. Despite high extents of Fe reduction by dissolved sulfide, OC release from Fh-OC coprecipitates was low due to efficient re-sequestration of released DOC by coprecipitation with FeS, and an increase in OC pre-loading could facilitate Fe2+ sulfidization and thus re-sequestration of released DOC by FeS. When Fh-OC adsorption complexes were exposed to dissolved sulfide, substantial OC release was restricted to the complexes with lower OC pre-adsorption (C/Fe ≤ 1.5). Overall, reactive Fe reduction by dissolved sulfide may not lead to substantial release of associated OC as long as reduced Fe could be sulfidized to efficiently re-sequester concomitantly released DOC. Results of carbon-1 s near-edge X-ray absorption fine structure spectroscopy indicate that, upon reduction of Fh-OCs by dissolved sulfide and subsequent FeS formation, carboxyl-C was selectively released into the solution but aromatic-C selectively remained on the solid matrixes after Fe reduction, suggesting that OC sink switching from Fe oxides to Fe sulfides could result in molecular fractionations of porewater DOC to some extent, with potential implications for OC biogeochemical cycling.