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 no...

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Published in:Environmental science & technology 2022-04, Vol.56 (7), p.4091-4100
Main Authors: Martin, Philipp R, Buchner, Daniel, Jochmann, Maik A, Elsner, Martin, Haderlein, Stefan B
Format: Article
Language:English
Subjects:
Aqueous environments
Carbon
Carbon Isotopes
Catalysis
Chelating agents
Chelation
Chemical Fractionation
Contaminants in Aquatic and Terrestrial Environments
Cyclic AMP - analogs & derivatives
Fractionation
Isotope fractionation
Isotopes
Isotopic enrichment
Kinetics
Organophosphonates
Oxidants
Oxidation
Oxidation-Reduction
Oxidizing agents
Oxygen
Phosphonates
Rate constants
Reaction kinetics
Solutes
Speciation
Species
Transformations
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Summary: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 (εc-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 (εc ≈ −10 ‰) and (ii) oxidation of free ATMP by such Mn­(III)­ATMP species (εc ≈ −1 ‰) 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.
ISSN:0013-936X
1520-5851
DOI:10.1021/acs.est.1c06407