Influence of Pyrophosphate on the Generation of Soluble Mn(III) from Reactions Involving Mn Oxides and Mn(VII)

The detection of soluble Mn(III) is typically accomplished using strong complexing agents to trap Mn(III), but the generation of soluble Mn(III) induced by strong complexing agents has seldom been considered. In this study, pyrophosphate (PP), a nonredox active ligand, was chosen as a typical Mn(III) chelating reagent to study the influence of ligands on soluble Mn(III) formation in reactions involving Mn oxides and Mn(VII). The presence of excess PP induced the generation of soluble Mn(III)-PP from α- and δ-MnO2 and led to the conproportionation reaction of α-, β-, δ-, or colloidal MnO2 with Mn(II) at pH 7.0. Compared to MnO2 minerals, colloidal MnO2 showed much higher reactivity toward Mn(II) in the presence of PP and the conproportionation rate of colloidal MnO2 with Mn(II) elevated with increasing PP dosage and decreasing pH. The generation of Mn(III) was not observed in MnO4–/S2O32– or MnO4–/NH3OH+ system without PP while the introduction of excess PP induced the generation of Mn(III)-PP. Thermodynamic calculation results were consistent with the experimental observations. These findings not only provide evidence for the unsuitability of using strong ligands in quantification of soluble Mn(III) in manganese-involved redox reactions, but also advance the understanding of soluble Mn(III) generation in aquatic environment.