Natural organic matter concentration controls the coordination and redox interaction between cerium ions and manganese oxides.

Rare earth element (REE) tailings leachates can serve as long-term sources of dissolved REEs and associated metals to surrounding environments. Manganese dioxide (MnO2), as a highly reactive redox-active mineral phase, can regulate the retention and transformation of cerium (Ce) through interfacial oxidation of mobile Ce(III) to less soluble Ce(IV). However, the coupled evolution of Ce speciation and MnO2 transformation and the influence of natural organic matter (NOM), remains poorly quantified. In this study, we systematically examined how NOM concentration regulates Ce-MnO2 interactions across wide NOM/Ce ratios. Our results reveal a clear concentration-dependent pathway shift. At low NOM/Ce ratios (≤1), Ce(III) facilitates Ce-bridged coaggregation and co-immobilization of NOM and MnO2, while Ce(III)-driven MnO2 reduction dominates Mn2+ release. At the tested higher NOM/Ce ratios (2.5-5), strong Ce(III)-NOM complexation suppresses Ce(III) oxidation, and Mn2+ release occurs mainly through electron transfer from NOM to MnO2. Quantitative Ce speciation further distinguished dissolved, adsorbed, and oxidized Ce fractions, demonstrating that NOM regulates both Ce partitioning and Mn mobilization. These findings highlight that Ce immobilization and Mn oxide destabilization are coupled processes controlled by NOM/Ce ratios, providing a mechanistic basis for assessing REE mobility and contaminant-bearing Mn oxide stability in tailings-impacted environments.