Rates of Abiotic MnII Oxidation by O₂: Influence of Various Multidentate Ligands at High pH

Oxidation of manganous manganese (Mn^(II)) is an important process driving manganese cycles in natural aquatic systems and leading to the formation of solid-phase Mn^(III,IV) (hydr)oxide products. Previous research has shown that some simple ligands (e.g., phosphate, sulfate, chloride, fluoride) can bind with Mn^(II) to make it unreactive to oxidation by dissolved oxygen. However, there is little to no understanding of the role played by stronger, complex-forming ligands in Mn^(II) oxidation reactions. The objective of this study was to evaluate the rates of abiotic Mn^(II) oxidation by O₂ in the presence of low concentrations of several complex-forming model ligands (pyrophosphate, tripolyphosphate, ethylenediaminetetraacetic acid, oxalate) in bicarbonate-carbonate buffered laboratory solutions of pH 9.42, 9.65, and 10.19. The influence of increasing ligand concentrations on observed autocatalytic profiles of Mn^(II) oxidation was investigated, and initial oxidation rates were linked quantitatively to the initial Mn^(II) speciation in experimental solutions. Observed rates of Mn^(II) oxidation decreased with increasing ligand concentration for all four ligands tested. However, the profiles observed with time and the magnitudes of decrease in initial oxidation rates were different for the different ligands. Likely explanations for these observations include the denticity of the tested ligands, the relative strength of the ligands to complex Mn^(II) versus Mn^(III), and the ability of some ligands to enhance the reduction of Mn^(III) back to Mn^(II) on a time scale comparable to the forward homogeneous Mn^(II) oxidation reaction.