Fig. 5: The mechanisms influencing the persistence of OC associated with inert Fe mineral (OC–FeIM) and its transformation under the impact of redox-active substances.
OC–FeIM complexes are primarily stabilized by physical isolation, which reduces OC accessibility to microorganisms. The associated OC often comprises low-molecular-weight compounds that rely on the inert nature of Fe minerals for protection. (i) Redox-active substances act as electron shuttles, facilitating extracellular electron transfer between dissimilatory iron-reducing bacteria and Fe minerals. This process disrupts the physical protection provided by inert Fe minerals, rendering the associated OC more susceptible to microbial attack. The electron-shuttling effect of redox-active substances leads to the reduction of Fe(III) in the OC–FeIM complexes. This reduction disintegrates the complexes, releasing OC into the surrounding environment. At this stage, the OC is no longer protected and becomes vulnerable to further transformation. Once released, OC undergoes biotic and abiotic transformations. (ii) Biotic processes, driven by microbial activity, can result in the decomposition of OC. (iii) Abiotic processes involve interactions with active Fe minerals, potentially leading to the recombination of OC with active Fe minerals to form new OC–FeAM complexes. This ultimately reduces the mass of OC–FeIM.
