Concentrations of reactive Mn(III)-L and MnO2 in estuarine and marine waters determined using spectrophotometry and the leuco base, leucoberbelin blue

In terms of its oxidative strength, the MnO₂/Mn²⁺ couple is one of the strongest in the aquatic environment. The intermediate oxidation state, manganese(III), is stabilized by a range of organic ligands (Mn(III)-L) and some of these complexes are also strong oxidants or reductants. Here, we present improved methods for quantifying soluble reactive oxidized manganese(III) and particulate reactive oxidized manganese at ultra-low concentrations; the respective detection limits are 6.7 nM and 7 pM (100-cm spectrophotometric path length) and 260 nM and 2.6 nM (1-cm path length). The methods involve a simple, specific, spectrophotometric technique using a water-soluble leuco base (leucoberbelin blue; LBB). LBB is oxidized by manganese through a hydrogen atom transfer reaction forming a colored complex that is stoichiometrically related to the oxidation state of the manganese, either Mn(III)-L or manganese(III,IV) oxides (MnO_x). At the concentration of LBB used in this study, nitrite may be a minor interference, so we provide concentration ranges over which it interferes and suggest potential strategies to mitigate the interference. Unlike previous methods devised to quantify Mn(III)-L, which use ligand exchange reactions, the LBB oxidation requires an electron and therefore needs to physically contact manganese(III) for inner-sphere electron transfer to occur. The method for measuring soluble Mn(III)-L was evaluated in the laboratory, and LBB was found to be oxidized by an extensive suite of weak Mn(III)-L complexes, as it is by MnO_x, but could not react with or reacted very slowly with strong Mn(III)-L complexes. According to the molecular structures of the Mn(III)-L complexes tested, LBB can also be used to qualitatively assess the binding strength of Mn(III)-L complexes based on metal-chelate structural considerations. The assays for soluble Mn(III)-L (membrane filtered) and particulate manganese oxides (trapped by membrane filters) were applied to the well-oxygenated estuarine waters of the Saguenay Fjord, a major tributary of the Lower St. Lawrence Estuary, and to Western North Atlantic oceanic waters, off the continental shelf, where there is an oxygen minimum zone (< 67% O₂ saturation). The methods applied can be used in the field or onboard ships and provide important new insights into oxidized manganese speciation.

Leucoberbelin Blue (LBB); Manganese speciation; Mn(III)-L; MnO(2); Seawater; Spectrophotometry.