Transformation of Iminodi(methylene phosphonate) on Manganese Dioxides – Passivation of the Mineral Surface by (Formed) Mn²⁺

Aminopolyphosphonates (APPs) are strong chelating agents with growing use in industrial and household applications. In this study, we investigated the oxidation of the bisphosphonate iminodi(methylene phosphonate) (IDMP) – a major transformation product (TP) of numerous commercially used APPs and a potential precursor for aminomethylphosphonate (AMPA) – on manganese dioxide (MnO₂). Transformation batch experiments at pH 6 revealed AMPA and phosphate as main TPs, with a phosphorus mass balance of 80 to 92% throughout all experiments. Our results suggest initial cleavage of the C–P bond and formation of the stable intermediate N-formyl-AMPA. Next, C–N bond cleavage leads to the formation of AMPA, which exhibits lower reactivity than IDMP. Reaction rates together with IDMP and Mn²⁺ sorption data indicate formation of IDMP-Mn²⁺ surface bridging complexes with progressing MnO₂ reduction, leading to the passivation of the mineral surface regarding IDMP oxidation. Compound-specific stable carbon isotope analysis of IDMP in both sorbed and aqueous fractions further supported this hypothesis. Depending on the extent of Mn²⁺ surface concentration, the isotope data indicated either sorption of IDMP to the mineral surface or electron transfer from IDMP to Mn^IV to be the rate-limiting step of the overall reaction. Our study sheds further light on the complex surface processes during MnO₂ redox reactions and reveals abiotic oxidative transformation of APPs by MnO₂ as a potential process contributing to widespread elevated AMPA concentrations in the environment.