Phosphonate- and carboxylate-based chelating agents that solubilize (hydr)oxide-bound Mn^III

Recent field studies suggest that dissolved Mn^III should be ubiquitous at oxic/anoxic interfaces in all natural waters and may play important roles in biogeochemical redox processes. Here, we uncovered environmentally relevant synthetic phosphonate-based chelators that solubilize (hydr)oxide-bound Mn^III via ligand-promoted dissolution at circum-neutral pHs and that their ability to release aqueous Mn^III can be predicted based on the chemical structure. For two (hydr)oxides (manganite and bimessite) reacting with excess concentrations of pyrophosphoric acid (PP), methylenediphosphonic acid (MDP), and phosphonoacetic acid (PAA), ligand-promoted dissolution is predominant: from pH 6-8, initial dissolution rates and plateau concentrations for dissolved Mn^III decrease in the order PP > MDP > PAA, and at pH 5, MDP reacts equally well (with bimessite) or more efficiently (with manganite) than PP, and PAA remains the least reactive chelator. For manganite reacting with an excess concentration of aminophosphonate/carboxylate-based chelators, the aminophosphonate-containing iminodimethylenephosphonic acid and glyphosate yield appreciable amounts of dissolved Mn^III, but the aminocarboxylate-based methyliminodiacetic acid yields solely dissolved Mn^II via Mn^III reduction.