Two Pathways Compete in the Mn(II)-Catalyzed Oxidation of Aminotrismethylene Phosphonate (ATMP)

Mn(II)-catalyzed oxidation by molecular oxygen is considered a relevant process for the environmental fate of aminopolyphosphonate chelating agents such as aminotrismethylene phosphonate (ATMP). However, the potential roles of Mn(III)ATMP-species in the underlying transformation mechanisms are not fully understood. We combined kinetic studies, compound-specific stable carbon isotope analysis, and equilibrium speciation modeling to shed light on the significance of such Mn-ATMP species for the overall ATMP oxidation by molecular oxygen. The fraction of ATMP complexed with Mn(II) inversely correlated with both (i) the Mn(II)-normalized transformation rate constants of ATMP and (ii) the observed carbon isotope enrichment factors (epsilon(c)-values). These findings provide evidence for two parallel ATMP transformation pathways exhibiting distinctly different reaction kinetics and carbon isotope fractionation: (i) oxidation of ATMP present in Mn(III)ATMP complexes (epsilon(c) approximate to -10 parts per thousand) and (ii) oxidation of free ATMP by such Mn(III)ATMP species (epsilon(c) approximate to -1 parts per thousand) in a catalytic cycle. The higher reaction rate of the latter pathway implies that aminopolyphosphonates can be trapped in catalytic Mn-complexes before being transformed and suggests that Mn(III)ATMP might be a potent oxidant also for other reducible solutes in aqueous environments.

Automated summary

The oxidation of aminopolyphosphonate chelating agents such as aminotrismethylene phosphonate (ATMP) by molecular oxygen catalyzed by Mn(II) is an important process in environmental fate. The potential roles of Mn(III)ATMP species in the transformation mechanisms are not fully understood. This study revealed two parallel ATMP transformation pathways, one involving the oxidation of ATMP in Mn(III)ATMP complexes and the other involving the oxidation of free ATMP by Mn(III)ATMP species. These findings highlight the significance of Mn-ATMP species in the overall ATMP oxidation and suggest that Mn(III)ATMP could also act as a potent oxidant for other reducible solutes in aqueous environments.