Oxidation of Mn(III) Species by Pb(IV) Oxide as a Surrogate Oxidant in Aquatic Systems

Abstract

Dissolved Mn­(III) species have been recognized as a significant form of Mn in redox transition environments, but a holistic understanding of their geochemical properties still lacks the characterization of their reactivity as reductants. Through using PbO2 as a surrogate oxidant and pyrophosphate (PP) as a model ligand, we evaluated the thermodynamic and kinetic constrains of dissolved Mn­(III) oxidation under environmentally relevant pH. Without disproportionation, Mn­(III) complexes could be directly oxidized by PbO2 to produce Mn oxides. The reaction rates decreased with increasing PP:Mn­(III) ratio and became negligible when the ratio was above a threshold value. Particulate manganite could also be oxidized by PbO2 with detectable production of Pb­(II). The favorability of Mn­(III) oxidation by PbO2 as a function of the PP:Mn ratio could be predicted by the stability constant of the Mn­(III)–PP complex. We developed kinetic models that couple multiple pathways of Mn oxidation by PbO2 to simulate the dynamics of Pb release, loss of dissolved Mn, as well as Mn­(III) production and consumption. Beyond the context of Mn geochemistry, the interactions between Pb and various Mn species, including its trivalent forms, may also have important implications to the water quality in lead service lines within distribution systems.

Methylmercury Degradation by Trivalent Manganese

“…The Mn­(III)-PP solution, with 9.10 ± 0.03 mM Mn, was prepared by dissolving Mn­(III)­(CH 3 COO) 3 ·2H 2 O in Na 4 P 2 O 7 ·10H 2 O solution at a molar ratio of 1:5. ,,, Various concentrations of Mn­(III)-PP complexes were prepared immediately before use by diluting the above bulk Mn­(III)-PP solution in PBS buffer.…”

“…The relatively high thermodynamic stability of the Mn(III)-PP complexes at neutral pH (7.1) than those at pH 4.0 and 7.8 observed here was also previously confirmed. 21,39 All these results suggest that pH plays a critical role in MeHg degradation by Mn(III)-PP complexes by mediating the Mn(III) species and behaviors. At similar dissolved Mn(III) concentrations, the pH 4.0 system degraded more MeHg than the pH 7.1 system (Figure 3A,B).…”

“…Mn­(III), mainly generated through comproportionation reaction between Mn­(IV) and Mn­(II), can locate on surfaces or in the bulk of MnO 2– x and occur as Mn­(III) oxyhydroxides and Mn 3 O 4 or a substituent in other minerals . Mn­(III) can also be complexed by various ligands to form soluble Mn­(III)-ligand complexes that have been found in various natural (seawater, estuarine, soil, and sediments) and engineered (water treatment works) systems, with concentrations up to several hundred μM. − Being capable of accepting and/or donating electrons, MnO 2– x and dissolved Mn­(III) complexes can significantly influence the surrounding redox chemistry and have high reactivity toward various redox-sensitive metals and organic pollutants. ,− MnO 2– x minerals can greatly affect Hg cycling in Hg­(II) or MeHg-containing environments by mediating the microbial activity and/or adsorbing these Hg species on their surfaces. Inorganic colloids (e.g., phyllosilicates and Fe and Mn (hydr)­oxides) might affect the dynamic balance of methylation and demethylation by stimulating or inhibiting microbial growth and altering the microbial community compositions .…”

Effects of Cu(II) and Zn(II) on PbO2 Reductive Dissolution under Drinking Water Conditions: Short-term Inhibition and Long-term Enhancement

“…15,18,19,25−27 First, certain solutes can reduce PbO 2 to Pb(II); these solutes include Br − , I − , Fe(II), Mn(II), and Mn(III). 14,18,25,27,28 Natural organic matter (NOM) is a collection of molecules, and some of these molecules can also reduce PbO 2 . 26,29 PbO 2 reduction by water is even thermodynamically favorable.…”

“…Coexisting solutes in the water can affect the reductive dissolution process in two ways. ,,,− First, certain solutes can reduce PbO 2 to Pb­(II); these solutes include Br – , I – , Fe­(II), Mn­(II), and Mn­(III). ,,,, Natural organic matter (NOM) is a collection of molecules, and some of these molecules can also reduce PbO 2 . , PbO 2 reduction by water is even thermodynamically favorable . Second, some ions can inhibit reductive dissolution via adsorption and precipitation. ,, For example, Pb­(II) and PO 4 3– can adsorb onto the PbO 2 surface, which decreases the proportion of Pb­(IV) surface species that are accessible to reductants and inhibits the reductive dissolution of PbO 2 . , The presence of Ca­(II) and phosphate can form a precipitate on the PbO 2 surface, which blocks the reductant from active sites …”