Investigating the stability of aqueous Mn(III) in the presence of humic substances and measuring manganese speciation with alpha, beta, gamma, delta-tetrakis (4-carboxyphenyl)porphine

Abstract

Manganese is a significant component of natural waters, and is found as Mn(II), (III), and (IV). Mn(III) had long been considered too unstable to be a dissolved species in natural waters, and ligand stabilization is required to prevent it from disproportionating. However, Mn(III) is present under certain conditions in ocean waters. Our work addressed the ability of humic substances to stabilize Mn(III). Mn(II) and (III) concentrations were measured by the modeling of complexation kinetics of both manganese species with the porphyrin ligand α,β,γ,δ-tetrakis(4-carboxyphenyl)porphine (TCPP) to form Mn(III)-TCPP. We found that dissolved Mn(III) was not stabilized in solution by humic substances and that, instead, a colloidal manganese species formed that was reactive with TCPP. In addition, we found that high concentrations of pyrophosphate and citrate interfere with the TCPP method by binding to cadmium, which is required to catalyze the reaction between TCPP and Mn. This was found to slow the rates of complexation of Mn(II) and (III).