Influence of aqueous Fe(III) on release of Mn(II) from low-molecular-weight organic acid-promoted dissolution of an Oxisol and γ-MnO₂

Mn(II) release from the soil mineral dissolution is a critical process to Mn availability to and uptake by plants, but little was known about how this process was affected by the coexisting Fe-containing minerals and/or aqueous Fe(III). In this study, batch dissolution experiments were conducted to obtain the kinetic of Mn(II) release from the dissolution of soil and γ-MnO₂ by oxalic acid or citric acid with the addition of aqueous Fe(III), which will shed light on the influence of Fe and root exudates on Mn geochemical behavior. The release of Mn(II) from the mineral dissolution of Oxisol by oxalic acid was inhibited in the presence of aqueous Fe(III). This result was attributed to the decrease in the concentration of uncomplexed oxalic acid, which was more efficient in the dissolution of manganese minerals than the complexed species. Such statement was mainly based on the complexation reaction between oxalic acid and aqueous Fe(III) as well as the subsequent adsorption of Fe(III)-oxalate complexes, as evidenced by the gradual decrease in the concentration of Fe(III) during the latter stage of the dissolution process. By contrast, the addition of aqueous Fe(III) or α-Fe₂O₃ could enhance the release of Mn(II) from the dissolution of soil Mn oxides by citric acid, probably resulting from the further reduction of soil Mn oxides by reductants such as Fe²⁺ and hydroxyl radicals generated from the auto-reduction of aqueous Fe(III)-citrate complexes. However, the dissolution of γ-MnO₂ by citric acid was suppressed in the presence of aqueous Fe(III), owing to the inhibition of the auto-reduction of aqueous Fe(III)-citrate complexes when they were adsorbed onto the MnO₂ surfaces. This hypothesis was confirmed by the kinetic results of Fe(III) adsorption onto γ-MnO₂ surfaces that the aqueous Fe(III) concentration decreased rapidly, nearly to zero. Overall, the present study indicated the geochemical behavior of manganese would be significantly affected by aqueous Fe(III) due to the changes in aqueous species of organic acids and/or the coupling of Mn-containing minerals dissolution with the intramolecular reaction of Fe(III)-organic acid complexes.