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

Abstract

Oxidation of manganous manganese (MnII) is an important process driving manganese cycles in natural aquatic systems and leading to the formation of solid-phase MnIII,IV(hydr)oxide products. Previous research has shown that some simple ligands (e.g., phosphate, sulfate, chloride, fluoride) can bind with MnIIto 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 MnIIoxidation reactions. The objective of this study was to evaluate the rates of abiotic MnIIoxidation by O2in 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 MnIIoxidation was investigated, and initial oxidation rates were linked quantitatively to the initial MnIIspeciation in experimental solutions. Observed rates of MnIIoxidation 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 MnIIversus MnIII, and the ability of some ligands to enhance the reduction of MnIIIback to MnIIon a time scale comparable to the forward homogeneous MnIIoxidation reaction.