Rates of abiotic Mn^(II) oxidation by O₂: Influence of various multidentate ligands at high pH

Abstract

Oxidn. 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 oxidn. by dissolved oxygen. However, there is little to no understanding of the role played by strong, complex-forming ligands in Mn^(II) oxidn. reactions. The objective of this study was to evaluate the rates of abiotic Mn^(II) oxidn. by O₂ in the presence of several strong complex-forming ligands (pyrophosphate, tripolyphosphate, EDTA, oxalate) in bicarbonate-carbonate buffered lab. solns. of pH 9.42, 9.65 and 10.19. The influence of increasing ligand concns. on obsd. autocatalytic patterns of Mn^(II) oxidn. was investigated, and initial oxidn. rates have been tentatively linked to the initial Mn^(II) speciation in exptl. solns. Obsd. rates of Mn^(II) oxidn. decreased with increasing ligand concn. for all four ligands tested. However, the profiles obsd. and the magnitudes of decrease in initial oxidn. 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) vs. Mn^(III), and the ability of some ligands to act as an electron donor and thereby enhance the back reaction.