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 –/S2O3 2– 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.