Reduction of Manganese Oxides: Thermodynamic, Kinetic and Mechanistic Considerations for One- Versus Two-Electron Transfer Steps

Adsorptive-Oxidative Removal of Sulfides from Water by MnO2-Loaded Carboxylic Cation Exchangers

“…The contact between hydrosulfite ions present in the aqueous phase and the structure of HIX beads was hindered because HS − ions were repelled by electrostatic interactions of fixed carboxylic groups of the polymeric matrix (–COO − ) according to the Donnan exclusion effect [ 68 , 69 ]. Moreover, the inorganic deposit in the mentioned pH range and in the case of hybrid materials without modified functional groups also had negative, Mn-O − , speciation (MnO 2 exhibits pH zpc of 4.6), which affected the transport of HS − anions toward MnO 2 surface active sites [ 25 , 26 , 70 , 71 ]. Nevertheless, the sulfides were removed by MnO 2 by oxidation to S 2 O 3 2− , and in a form permanently bound to the HIX beads.…”

“…There are several possible mechanisms of interactions between MnO 2 and sulfide species in an aquatic environment [ 25 , 26 , 72 , 73 , 74 , 75 ]. Based on obtained experimental data, we suggest that in the case of HIX without modified functional groups, despite electrostatic –COO − /MnO − :HS − repulsion, a direct Mn-S surface complex was formed.…”

“…Based on obtained experimental data, we suggest that in the case of HIX without modified functional groups, despite electrostatic –COO − /MnO − :HS − repulsion, a direct Mn-S surface complex was formed. The formation of such a complex is considered possible even in a strongly basic pH and consists of formation of a precursor (5) and a main surface complex (6) [ 25 , 26 ]: ≡Mn (IV)− O − + HS − → ≡Mn (IV) -O − —HS − ≡Mn (IV) -O − —HS − → ≡Mn (IV) S − + OH − within the formed complex, donation of two electrons occurs with a S∙ − free radical being the possible intermediate product [ 75 ], elemental sulfur the final product, and a decrease in the Mn atom’s oxidation state in the MnO 2 structure: ≡Mn (IV) S − → ≡Mn (III) S ∙ ≡Mn (III) S ∙ → ≡Mn (II) S 0 …”

“…In the present study, we deposited manganese(IV) oxide (MnO 2 ) in the structure of tested commercial gel-type and macroporous carboxylic cation exchangers. The MnO 2 was selected since it is the strongest and most abundant known oxidizer in natural systems after oxygen [ 25 , 26 ], and has been widely used, as fine or ultrafine particles, for oxidation/removal of toxic substances from aquatic environments [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ]. Moreover, MnO 2 has been successfully introduced into the structure of different carriers—for instance cellulose, biochar, chitin, chitosan, alginate beads, activated carbon, carbon fibers and nanotubes, graphene, polyester fiber, ion exchange resins, as well as poly- (vinyl alcohol) and (vinyl pyrrolidone).…”

Novel insights into the taxonomic diversity and molecular mechanisms of bacterial Mn(III) reduction

“…Ligand-bound Mn(III) is often the most abundant dissolved Mn species in sediment pore waters (Madison et al, 2013;Oldham et al, 2019) and soils (Heintze and Mann, 1947), with the potential to facilitate one-electron redox reactions in a variety of biogeochemical cycles (Luther III et al, 2015). Microbes accelerate the oxidation and reduction of Mn by orders of magnitude compared with abiotic mechanisms (Hem, 1963;Diem and Stumm, 1984;Morgan, 2005;Tebo et al, 2005;Learman et al, 2011;Luther et al, 2018;Jung et al, 2020;Yu and Leadbetter 2020). Yet, despite clear evidence for the environmental importance of Mn(III), knowledge about microbial Mn(III) cycling pathways remains fragmented.…”