Effect of hydrogen bonding on the humification of humic precursors with Fe-Mn concomitant minerals

Mn substitution elevates goethite's surface hydroxyl density versus pure goethite. • More surface hydroxyls on Mn-goethite accelerate humification from precursors. • Mn-goethite serves as oxidant to promote oxypolymerization of humic precursors. • DMSO hydrogen-bond shielding lowers electron transfer and Mn²⁺ release in solution. Electron transfer via hydrogen bonding is crucial for humification. s In nature settings, iron minerals usually coexist with manganese in an isomorphous-replacement manner, attributable to the equal ionic radii of ferric iron and trivalent manganese. Nevertheless, it remains unclear whether abundant surface hydroxyl groups on manganese-substituted iron minerals influence humification. Herein, the abiotic humification of humic precursors with manganese-substituted iron mineral (e.g. Mn-substituted goethite) was systematically investigated for the first time, especially in the role of hydrogen bonding interactions between the mineral and humic precursors. Mn-substituted goethite possessed a greater surface hydroxyl density compared to pure goethite, resulting in a significantly accelerated humification rate. Furthermore, the introduction of a hydrogen bonding shielding agent, such as dimethyl sulfoxide (DMSO), into the reaction system containing humic precursors and the mineral resulted in a marked decrease in both the manganese concentration within the precursor solution and the system’s overall electron transfer capability. These results demonstrated that hydrogen bonding between the mineral and humic precursors enhanced their electron transfer rate, thereby facilitating the humification process. This research offers new insights into abiotic humification mediated by iron/manganese concomitant minerals, enriching the understanding of the global carbon cycle affected by soil geochemical processes. Download: Download high-res image (243KB) Download: Download full-size image