Asynchronous reductive release of iron and organic carbon from hematite–humic acid complexes

•Fe–organic carbon association governs the release and stability of carbon.•Upon reduction of hematite, Fe-bound organic carbon releases asynchronously with Fe.•Release of carbon was much faster than Fe at the beginning stage.•Aromatic carbon was released much faster than other components of organic carbon.•Conformation and structure of organic carbon regulates its stability during reduction.

Solid-phase iron (Fe) plays an important role in the accumulation and stabilization of soil organic matter (SOM). However, ferric minerals are subject to redox reactions, which can compromise the stability of Fe-bound SOM. To date, there is limited information available concerning the fate of Fe-bound SOM during redox reactions at Fe mineral surfaces. In this study, we investigated the release kinetics of hematite-bound organic carbon (OC) during the abiotic reduction of hematite–humic acid (HA) complexes by dithionite, to elucidate important processes governing the stability and fate of organic matter during the redox processes. Our results indicate that the reductive release of Fe obeyed first-order kinetics with release rate constants of 6.67–13.0 × 10− 3 min− 1. The Fe-bound OC was released rapidly during the initial stage with release rate constants of 0.011–1.49 min− 1, and then became stable with residual fractions of 4.6–58.2% between 120 and 240 min. The release rate of aromatic OC was much faster than for the non-aromatic fraction of HA, and 90% of aromatic OC was released within the first hour for most samples. Our findings show that in the reductive reaction the mobilization of Fe-bound OC was asynchronous with the reduction of Fe, and aromatic OC was released more readily than other components of SOM. This study highlights the importance of evaluating the release of SOM bound with Fe during the redox reactions, especially the influence of the physicochemical properties of SOM.

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