Transformation of Iminodi(methylene phosphonate) on Manganese Dioxides – Passivation of the Mineral Surface by (Formed) Mn<sup>2+</sup>
Anna M. Röhnelt, Philipp R. Martin, Daniel Büchner, Stefan B. Haderlein
Abstract
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 (MnO 2 ). 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 Mn 2+ sorption data indicate formation of IDMP-Mn 2+ surface bridging complexes with progressing MnO 2 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 Mn 2+ 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 MnO 2 redox reactions and reveals abiotic oxidative transformation of APPs by MnO 2 as a potential process contributing to widespread elevated AMPA concentrations in the environment.