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Antimony trioxide (Sb 2 O 3 ) is one of the primary forms of Sb in the environment, and its dissolution significantly impacts the migration and bioavailability of Sb. However, the dissolution of Sb 2 O 3 coupled with abiotic redox of Mn processes is unclear. Here, we investigated the kinetics of Sb 2 O 3 dissolution in the presence of the ubiquitous Mn(II) by kinetic experiments, spectroscopies, density functional theory calculations and the chemical kinetic modeling. The oxidative dissolution of Sb 2 O 3 was catalyzed by Mn(II) through the in-situ generated amorphous Mn oxides (MnO x ) under oxic conditions, during which the generation of Mn(III) is a critical step in Sb(V) release. The released Sb(V) was partially retained on MnO x through bidentate-binuclear (corner-sharing) complexes as revealed by extended X-ray absorption fine structure analysis. The coexistent morphological forms of Sb 2 O 3 , i.e., senarmontite and valentinite exhibited distinct dissolution patterns. Valentinite showed higher activity in catalyzing Mn(II) oxidation and faster oxidative dissolution than senarmontite, due to its higher surface energy and lower conduction band minimum of its exposed facets. These abiotic processes can extrapolate to other metal(loid)s (hydr)oxides, further supplying for the comprehensive understanding of the redox transformation of Mn., (Copyright © 2022. Published by Elsevier Ltd.)

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 PbO 2 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 PbO 2 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 PbO 2 with detectable production of Pb(II). The favorability of Mn(III) oxidation by PbO 2 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 PbO 2 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.

The reaction of manganese oxides with Cr(III)-bearing solids in soils and sediments can lead to the natural production of Cr(VI) in groundwater. Building on previous knowledge of MnO 2 as an oxidant for Cr(III)-containing solids, this study systematically evaluated the rates and mechanisms of the oxidation of Cr(III) in iron oxides by δ-MnO 2 . The Fe/Cr ratio (x = 0.055-0.23 in Cr x Fe 1-x (OH) 3 ) and pH (5-9) greatly influenced the Cr(VI) production rates by controlling the solubility of Cr(III) in iron oxides. We established a quantitative relationship between Cr(VI) production rates and Cr(III) solubility of Cr x Fe 1-x (OH) 3 , which can help predict Cr(VI) production rates at different conditions. The adsorption of Cr(VI) and Mn(II) on solids shows a typical pH dependence for anions and cations. A multichamber reactor was used to assess the role of solid-solid contact in Cr x Fe 1-x (OH) 3 -MnO 2 interactions, which eliminates the contact of the two solids while still allowing aqueous species transport across a permeable membrane. Cr(VI) production rates were much lower in multichamber than in completely mixed batch experiments, indicating that the redox interaction is accelerated by mixing of the solids. Our results suggest that soluble Cr(III) released from Cr x Fe 1-x (OH) 3 solids to aqueous solution can migrate to MnO 2 surfaces where it is oxidized.

The mobility of hexavalent uranium in soil and groundwater is strongly governed by adsorption to mineral surfaces. As strong naturally occurring adsorbents, manganese oxides may significantly influence the fate and transport of uranium. Models for U(VI) adsorption over a broad range of chemical conditions can improve predictive capabilities for uranium transport in the subsurface. This study integrated batch experiments of U(VI) adsorption to synthetic and biogenic MnO(2), surface complexation modeling, ζ-potential analysis, and molecular-scale characterization of adsorbed U(VI) with extended X-ray absorption fine structure (EXAFS) spectroscopy. The surface complexation model included inner-sphere monodentate and bidentate surface complexes and a ternary uranyl-carbonato surface complex, which was consistent with the EXAFS analysis. The model could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. U(VI) adsorption to synthetic δ-MnO(2) appears to be stronger than to biogenic MnO(2), and the differences in adsorption affinity and capacity are not associated with any substantial difference in U(VI) coordination.

Lead(IV) oxide (PbO(2)) is a corrosion product on lead pipes used for drinking water distribution, and its dissolution can control lead release to drinking water. This study evaluated the adsorption of Pb(II) to PbO(2) and its impact on the dissolution rate of PbO(2). The dissolution rate of PbO(2) was determined as a function of pH in the absence and presence of free chlorine using continuously-stirred tank reactors. Pb(II) adsorption was examined as a function of pH and initial Pb(II) concentrations. The dissolution rate of PbO(2) increased with decreasing pH. The presence of free chlorine inhibited PbO(2) dissolution. The dissolution of PbO(2) involves a coupled reduction-detachment process, and a model was developed that accounts for the adsorption of Pb(II) from the reduction. The extent of Pb(II) adsorption to PbO(2) increased with increasing pH and Pb(II) concentrations until reaching a plateau. Adsorption was interpreted with a surface complexation model using the diffuse double-layer model and a single surface complex. The dissolution rate of PbO(2) was directly related to the distribution of the PbO(2) surface species predicted by the surface complexation model. The dissolution rate was predominantly controlled by >Pb(IV)OH(2)(+) for acidic conditions and by>Pb(IV)OH and>Pb(IV)O(-) at neutral to basic conditions., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Lead(IV) oxide (PbO(2)) is a corrosion product that can develop on lead pipes used for drinking water supply, and its stability can control lead concentrations in tap water. A set of batch experiments were performed to determine the extent of PbO(2) formation as a function of time, pH, the presence of dissolved inorganic carbon (DIC), and free chlorine concentration. Experiments were conducted with four lead(II) compounds that are precursors of PbO(2) formation: dissolved lead(II) chloride, massicot (β-PbO), cerussite (PbCO(3)), and hydrocerussite (Pb(3)(OH)(2)(CO(3))(2)). While PbO(2) formed in the presence and absence of DIC, the presence of DIC accelerated PbO(2) formation and affected the identity of the PbO(2) (scrutinyite vs plattnerite) product. For some conditions, intermediate solids formed that affected the identity of the PbO(2) produced. When no intermediate solids formed, hydrocerussite led to the formation of pure scrutinyite, and lead(II) chloride and massicot led to mixtures of scrutinyite and plattnerite. Based on the experimental results, a conceptual model of lead(IV) oxide formation pathways was proposed.