Dissolved trivalent manganese in forest soils – interactions of natural organic ligands with manganese oxides

The presence of dissolved trivalent manganese (Mn³⁺) in soils has been neglected largely due to its rapid disproportionation. However, natural organic ligands (NOLs) complex and stabilize Mn³⁺ by the formation of stable Mn³⁺–NOL complexes. Our objectives were (i) to investigate the influence of NOLs on the dissolution of synthetic Mn oxides, (ii) to perform the speciation analysis of the resulting dissolved total Mn (Mn_T) pool, and (iii) to elucidate the principles governing abiotic formation of Mn³⁺–NOL complexes. NOLs were obtained by extraction (0.001 M CaCl₂, 24 h) from a terrestrial forest floor Oe horizon (moder-like raw humus). In batch operations, NOLs reacted with either birnessite (containing Mn^IV and minor Mn^III) or manganite (containing solely Mn^III). The interaction between NOLs and Mn (hydr)oxides was investigated as a function of time (1–168 h, 7 steps), and pH (3–7, 5 steps). Mn speciation analysis was performed using a spectrophotometric protocol based on kinetic modeling. Results show that the dissolution of the Mn oxides increased with decreasing pH and increasing time. Mean proportions of Mn³⁺–NOL complexes relative to the Mn_T pool ranged from 0 to 87 ± 18% (birnessite), and from 0 to 69 ± 14% (manganite). A pH-dependent formation of Mn³⁺–NOL complexes was observed, highlighting pH as the critical parameter. Complex stability decreased with decreasing pH, while an influence of time was only assumed for strongly acidic conditions. Overall, Mn³⁺–NOL complexes appeared to be metastable at pH 3–5 (birnessite) and below pH 7 (manganite). In addition, the formation of Mn³⁺–NOL complexes was influenced by the individual properties of the Mn oxides as they were differing in their average oxidation state, point of zero charge, specific surface area and morphology and structure. These properties influence the formation mechanisms of Mn³⁺–NOL complexes and, consequently, the Mn speciation. For example, they affect NOL adsorption rates and capacities, as well as the transformation and degradation of NOLs. We suggest (i) ligand-promoted non-reductive dissolution, (ii) ligand-promoted reductive dissolution, (iii) H⁺-promoted dissolution, and (iv) ligand exchange as the four possible abiotic dissolution mechanisms for Mn release and Mn³⁺–NOL complex formation. Following dissolution, either Mn³⁺–NOL complexes were released, or released Mn²⁺ and Mn³⁺ may be complexed by additional NOLs with and without oxidation. We demonstrate that Mn³⁺–NOL complexes are important, previously underestimated, constituents of the Mn_T pool in forest floor solutions and propose that they are a non-negligible component in terrestrial environments.