Recently, the occurrence of aqueous Mn(III) complexed by organic ligands has been documented in various soil and sediment environments. These observations emphasize that aqueous Mn(III), which is a transient intermediate between Mn(II) and Mn(IV), may play a significant role not only in the biogeochemical cycling of Mn, but also in environmental redox processes. However, knowledge of the formation mechanisms, stability and reactivity of Mn(III)-Ligand (Mn-L) complexes remains largely unknown. In this study, we investigated the formation of Mn-L complexes and their stability in mixed manganese and iron (hydr) oxide systems.<br/>A series of kinetic batch dissolution experiments were conducted in which desferrioxamine B (DFOB), a terrestrial bacterial siderophore, was added to suspensions of Mn(III,IV) (hydr)oxides (manganite and birnessite) or mixtures of Mn(III,IV) (hydr)oxides and Fe(III) (hydr)oxides (lepdicrocite and 2-line ferrihydrite) under oxic conditions at circumneutral pH. Aqueous Mn(III)-DFOB (Mn-L), Mn(II) and Fe(III)-DFOB (Fe-L) concentrations were measured over time. The corresponding rates of mobilization and decomposition of metal-L complexes were determined. In addition, Mn-K edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of Mn-L interacted Fe (hydr)oxides were collected in order to examine the redox state and mineralogy of the precipiated Mn species.<br/>The mobilization rate and stability of Mn-L were strongly dependent on the type of Mn (hydr)oxide. The Mn-L mobilization rates from manganite were significantly faster (24 - 37 times for pH 7.0 - 8.0) than the mobilization rates from birnessite. After the mobilized Mn-L reached a maximum concentration, it remained constant in the presence of manganite. While, in the presence of birnessite Mn-L concentrations decreased slowly but completely between 2 hours and 72 hours and the rates increased with decreasing the pH. The mobilization and decomposition rates of the Mn-L complexes were also affected by the presence of Fe (hydr)oxides. 2-line ferrihydrite suppressed the mobilization of Mn-L from both manganite and birnessite, while the effects of lepidocrocite were marginal. Additionally, both Fe(III) (hydr)oxide minerals accelerated the decomposition of Mn-L complexes. Our results show that the redox properties of the Mn (hydr)oxide, the presence of other soil minerals and pH can regulate Mn-L concentrations in terrestrial ecosystems.

Formation and reactivity of Mn(III)ligand complexes in mixed manganese and iron (hydr)oxide systems

ACS Spring 2022

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