Phosphonate- and carboxylate-based chelating agents that solubilize (hydr)oxide-bound MnIII.

Recent field studies suggest that dissolved MnIII 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 MnIII via ligand-promoted dissolution at circum-neutral pHs and that their ability to release aqueous MnIII can be predicted based on the chemical structure. For two (hydr)oxides (manganite and birnessite) 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 MnIII decrease in the order PP > MDP > PAA, and at pH 5, MDP reacts equally well (with birnessite) 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 MnIII, but the aminocarboxylate-based methyliminodiacetic acid yields solely dissolved MnII via MnIII reduction.