The key role of Manganese (Mn) in the biogeochemical cycle of trace elements has been of great interest in recent years. A lot of effort has been put into demonstrating the oxidizing characteristics of Mn (IV)/Mn (III) oxides. Nevertheless, the redox properties of aqueous Mn (III) have been studied to a lesser extent. Mn (III) is not stable in solution by itself, but when complexed with aqueous ligands has shown its potential to oxidize and reduce trace elements. In the present study, we are exploring the redox characteristics of the complex Mn(III) with Pyrophosphate (Mn(III)-PP) and their effect on Cr mobility and speciation. The Mn(III)-PP complex is stable over a wide range of pH values but requires excess of pyrophosphate, precisely the ratio of Mn:PP needs to be greater than 1:6. Specifically, the redox reaction of Cr(OH)3 and Mn(III)-PP is investigated. Cr is present in the environment in two oxidation states: Cr(III) that is consider an essential nutrient to humans and can form insoluble solids; and Cr (VI) that is soluble and toxic to humans. We investigated as the effect of pH, ionic strength, ratio Mn(III)/Cr(III), and excess of pyrophosphate with regards to Cr(OH)3 oxidative dissolution. . Our results showed that Mn(III)-PP can effectively oxidize Cr(III) to Cr(VI)aq, taking about 15 days for the establishment of equilibrium. This reaction is highly favorable only under acidic conditions. Nevertheless, Cr(VI) is observed in the aqueous phase, which partially sorbs back on the surface of Cr(OH)3, whereas a significant amount of Cr(VI) is released in the aqueous phase. Furthermore, increase in ionic strength hinders the production of Cr(VI), implying that the mechanism behind thie phenomenon relies on sorption of the complex on the surface of chromium hydroxide. This study highlights the liability of Mn(III)-PP complexes when it comes to chromium mobility in the environment.

Cr(OH)3 oxidative dissolution to aqueous Cr(VI) by Mn(III)-pyrophosphate complex.

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