Transformation of Divalent Manganese at Humic Acid–Calcite–Bacteria Interfaces: Kinetics, Thermodynamics, and Mechanisms

The immobilization and transformation of manganese in soil environments primarily depend on its interactions with soil mineral components, organic matter, and microorganisms. To investigate the migration and transformation of manganese in the water–soil system of loess regions, we used quartz sand (SiO2) and calcite (CaCO3)—the main components of loess—as soil matrices, along with humic acid (HA) and a typical bacterium (Bacillus subtilis) as influencing factors. Laboratory experiments combined with instrumental characterization were employed to examine Mn transformation. The results indicate that the presence of humic acid and bacteria significantly inhibits the cation exchange reaction between Mn and Ca in calcite while enhancing the binding of Mn to organic functional groups (–OH and –COOH). In particular, biofilms formed by bacteria and their metabolites exhibited a more pronounced inhibitory effect on cation exchange and promoted Mn oxidation. The effects of pH and temperature were more evident in the composite systems (quartz sand–calcite–humic acid (QS-CL-HA) and quartz sand–calcite–humic acid–Bacillus subtilis (QS-CL-HA-B.S.)). Our thermodynamic results show that the transformation of Mn2+ in the composite systems best fits the pseudo-second-order kinetic model (chemical adsorption) and the Freundlich model (monolayer adsorption). The values of ΔH (15.22, 5.29 kJ·mol−1) and ΔG (0.82–2.76 kJ·mol−1) confirm that the transformation of Mn2+ in these composite systems is non-spontaneous and endothermic. This study demonstrates that, in addition to the effects of minerals, trace organic matter and microorganisms in soil significantly influence the transformation of metallic Mn. The findings also provide a theoretical basis for designing bio-enhanced soil remediation strategies.