Polyphosphonate chelating agents such as aminotrismethylene phosphonate (ATMP) are substances of rising concern. They are increasingly used to replace polycarboxylates like ethylenediaminetetraacetate (EDTA) and polyphosphates in various domestic and industrial applications. One degradation process likely relevant for the environmental fate of polyphosphonates is manganese(II)-catalyzed oxidation by molecular oxygen. However, the underlying degradation mechanisms are yet not fully understood as the role of manganese(III) intermediates is unclear. To tackle these open questions, compound specific stable carbon isotope analysis (carbon CSIA) is a promising approach, to gain deeper insights into the underlying mechanisms of transformation reactions due to the footprint of the transition state in the observable isotope fractionation. In the presented study we investigated the influence of the manganese-ATMP speciation (i.e. manganese-complexed vs. free, uncomplexed ATMP) on the observed kinetic carbon isotope fractionation in laboratory batch experiments conducted at varying (i) pH as well as (ii) initial manganese-ATMP ratios. Obtained carbon isotope enrichment factors (εc) varied from minor values (-0.8 ‰ ± 0.1 ‰) to pronounced values (-9.4 ‰ ± 0.6 ‰) and were inversely correlated with the calculated fraction of ATMP complexed with manganese(II). Based on these findings, we propose two mechanisms responsible for ATMP degradation associated with distinctly different magnitudes of carbon isotope fractionation, namely (i) inner-sphere electron transfer (εc ≈ -10 ‰), leading to Mn(III)ATMP complex breakdown and (ii) oxidation of free ATMP by Mn(III)ATMP intermediates via outer-sphere electron transfer (εc ≈ -1 ‰). This so far overlooked reactivity of Mn(III)ATMP suggests a diminished potential for ATMP breakdown in technical and environmental systems, as Mn-ATMP complexes may act as catalytically active oxidants for naturally occurring reductants. Our study contributes to an deeper understanding of manganese(II)-catalyzed oxidation of polyphosphonates and therefore help to develop a more profound assessment of the environmental fate of these compounds.

Using compound specific carbon isotope analysis to gain deeper insights into the underlying mechanisms of manganese-catalyzed aminotrismethylene phosphonate (ATMP) oxidation

ACS Spring Meeting 2021

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