(54)Mn Radiotracers Demonstrate Continuous Dissolution and Reprecipitation of Vernadite (δ-MnO2) during Interaction with Aqueous Mn(II)

Sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms in the aquatic environment

Microorganisms can oxidize Mn(II) to biogenic Mn oxides (BioMnOx), through enzyme-mediated processes and non-enzyme-mediated processes, which are generally considered as the source and sink of heavy metals due to highly reactive to sequestrate and oxidize heavy metals. Hence, the summary of interactions between Mn(II) oxidizing microorganisms (MnOM) and heavy metals is benefit for further work on microbial-mediated self-purification of water bodies. This review comprehensively summarizes the interactions between MnOM and heavy metals. The processes of BioMnOx production by MnOM has been firstly discussed. Moreover, the interactions between BioMnOx and various heavy metals are critically discussed. On the one hand, modes for heavy metals adsorbed on BioMnOx are summarized, such as electrostatic attraction, oxidative precipitation, ion exchange, surface complexation, and autocatalytic oxidation. On the other hand, adsorption and oxidation of representative heavy metals based on BioMnOx/Mn(II) are also discussed. Thirdly, the interactions between MnOM and heavy metals are also focused on. Finally, several perspectives which will contribute to future research are proposed. This review provides insight into the sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms. It might be helpful to understand the geochemical fate of heavy metals in the aquatic environment and the process of microbial-mediated water self-purification.

Rapid grain boundary diffusion in foraminifera tests biases paleotemperature records

The oxygen isotopic compositions of fossil foraminifera tests constitute a continuous proxy record of deep-ocean and sea-surface temperatures spanning the last 120 million years. Here, by incubating foraminifera tests in 18O-enriched artificial seawater analogues, we demonstrate that the oxygen isotopic composition of optically translucent, i.e., glassy, fossil foraminifera calcite tests can be measurably altered at low temperatures through rapid oxygen grain-boundary diffusion without any visible ultrastructural changes. Oxygen grain boundary diffusion occurs sufficiently fast in foraminifera tests that, under normal upper oceanic sediment conditions, their grain boundaries will be in oxygen isotopic equilibrium with the surrounding pore fluids on a time scale of <100 years, resulting in a notable but correctable bias of the paleotemperature record. When applied to paleotemperatures from 38,400 foraminifera tests used in paleoclimate reconstructions, grain boundary diffusion can be shown to bias prior paleotemperature estimates by as much as +0.86 to -0.46 °C. The process is general and grain boundary diffusion corrections can be applied to other polycrystalline biocarbonates composed of small nanocrystallites (<100 nm), such as those produced by corals, brachiopods, belemnites, and molluscs, the fossils of which are all highly susceptible to the effects of grain boundary diffusion.

Alteration of birnessite reactivity in dynamic anoxic/oxic environments

Although the oxidative capacity of manganese oxides has been widely investigated, potential changes of the surface reactivity in dynamic anoxic/oxic environments have been often overlooked. In this study, we showed that the reactivity of layer structured manganese oxide (birnessite) was highly sensitive to variable redox conditions within environmentally relevant ranges of pH (4.0 - 8.0), ionic strength (0-100 mM NaCl) and Mn(II)/MnO₂ molar ratio (0-0.58) using ofloxacine (OFL), a typical antibiotic, as a target contaminant. In oxic conditions, OFL removal was enhanced relative to anoxic environments under alkaline conditions. Surface-catalyzed oxidation of Mn(II) enabled the formation of more reactive Mn(III) sites for OFL oxidation. However, an increase in Mn(II)/MnO₂ molar ratio suppressed MnO₂ reactivity, probably because of competitive binding between Mn(II) and OFL and/or modification in MnO₂ surface charge. Monovalent cations (e.g., Na⁺) may compensate the charge deficiency caused by the presence of Mn(III), and affect the aggregation of MnO₂ particles, particularly under oxic conditions. An enhancement in the removal efficiency of OFL was then confirmed in the dynamic two-step anoxic/oxic process, which emulates oscillating redox conditions in environmental settings. These findings call for a thorough examination of the reactivity changes at environmental mineral surfaces (e.g., MnO₂) in natural systems that may be subjected to alternation between anaerobic and oxygenated conditions.

Redox-Driven Recrystallization of PbO2

Lead(IV) oxide (PbO₂) is one of the lead corrosion products that forms on the inner surface of lead pipes used for drinking water supply. It can maintain low dissolved Pb(II) concentrations when free chlorine is present. When free chlorine is depleted, PbO₂ and soluble Pb(II) will co-occur in these systems. This study used a stable lead isotope (²⁰⁷Pb) as a tracer to examine the interaction between aqueous Pb(II) and solid PbO₂ at conditions with no net change in dissolved Pb concentration. While the dissolved Pb(II) concentration remained unchanged, significant isotope exchange occurred that indicated that substantial amounts (24.3-35.0% based on the homogeneous recrystallization model) of the Pb atoms in the PbO₂ solids had been exchanged with those in solution over 264 h. Neither α-PbO₂ nor β-PbO₂ displayed a change in mineralogy, particle size, or oxidation state after reaction with aqueous Pb(II). The combined isotope exchange and solid characterization results indicate that redox-driven recrystallization of PbO₂ had occurred. Such redox-driven recrystallization is likely to occur in water that stagnates in lead pipes that contain PbO₂, and this recrystallization may alter the reactivity of PbO₂ with respect to its stability and susceptibility to reductive dissolution.

Synergistic effects of prokaryotes and oxidants in rapid sand filters treatment of groundwater versus surface water: Purification efficacy, stability and associated mechanisms

Effective elimination of manganese (Mn) and ammonium (NH₄⁺-N) from drinking water is still challenging. Utilizing oxidants to improve the simultaneous removals of Mn and NH₄⁺-N from rapid sand filter (RSF) systems has been extensively studied. However, the prokaryotes containing in the water geochemical properties greatly affected the RSF performance. In this study, groundwater and micro-polluted surface water were used to compare with/without potassium permanganate (KMnO₄) assistant on the contaminants removals and system stability. Results showed that KMnO₄ reduced the start-up period of RSF for treating groundwater and surface water to 20 and 41 days, respectively, with excellent Mn removal rates (>97%). The relative abundance of efficient ammonia-oxidizing bacteria (Nitrospira) in RSF treated groundwater without KMnO₄ was higher than that in RSFs treated micro-polluted surface water or with KMnO₄, resulting in a higher NH₄⁺-N removal rate of the former (∼57%). Notably, KMnO₄ and prokaryotes synergistically contributed to the amorphous structure, mixed phases (buserite, MnO₂ and birnessite) and mixed-valence Mn system of active manganese oxides (MnOx), whose abundant oxygen vacancies and highly reactive Mn(III) favored the autocatalytic oxidation of Mn, while NH₄⁺-N removal relied more on bacteria actions. Additionally, prokaryotes enriched the bacterial community diversity, leading to a more stable RSF system when facing hydraulic loading shock. This paper provided new insight into the synergistic effect of KMnO₄ and prokaryotes on Mn and NH₄⁺-N eliminations in RSFs and was helpful for practical applications.

Effects of Mn(II) addition on Cd(II) removal by hydrated manganese dioxide

In this study, the redox homogeneous precipitation method was applied to synthesize hydrated manganese dioxide (HMO) and to study the removal performance of Cd(II) from wastewater. Moreover, a small amount of Mn(II) could still combine with HMO without causing the desorption of Cd(II). A novel discovery was that the synergistic effect of KMnO₄ and Mn(II) could directly regenerate MnO₂ to deeply remove Cd(II), realizing the recycling of MnO₂. The influence of Mn(II) addition on the adsorption behavior of Cd(II) was discussed in terms of pH, Mn(II) addition mode, initial concentration of Mn(II) or Cd(II), and contact time. The adsorption of Cd(II) on HMO could be better in line with the Langmuir model, while that of Mn(II) accorded with the Freundlich model. The pseudo-second-order kinetic fitting results indicated that the removal of Cd(II) and Mn(II) on HMO belonged to chemisorption. SEM, XRD, FTIR, and XPS analysis demonstrated that Cd(II) was trapped by forming an inner-sphere complex on HMO, and the added Mn(II) and KMnO₄ could regenerate MnO₂ through oxidation-reduction reaction, wrapping outside of Cd(II) for a purpose of deep removal of Cd(II). HIGHLIGHTS: 1)A small amount of Mn(II) can bind to HMO without causing Cd(II) desorption 2)A large amount of Mn(II) can cause partial desorption of Cd(II) 3)Large amount of Mn(II) will partially bind to the surface of HMO or Cd(II) complexes 4)Deeply remove Cd(II) without eluting and regenerating HMO.

Oxidation of V(IV) by Birnessite: Kinetics and Surface Complexation

Vanadium is a redox-active metal that has been added to the EPA's Contaminant Candidate List with a notification level of 50 μg L^-1 due to mounting evidence that V^V exposure can lead to adverse health outcomes. Groundwater V concentration exceeds the notification level in many locations, yet geochemical controls on its mobility are poorly understood. Here, we examined the redox interaction between V^IV and birnessite (MnO₂), a well-characterized oxidant and a scavenger of many trace metals. In our findings, birnessite quickly oxidized sparingly soluble V^IV species such as häggite [V₂O₃(OH)₂] into highly mobile and toxic vanadate (H_nVO₄^(3-n)-) in continuously stirred batch reactors under neutral pH conditions. Synchrotron X-ray absorption spectroscopic (XAS) analysis of in situ and ex situ experiments showed that oxidation of V^IV occurs in two stages, which are both rapid relative to the measured dissolution rate of the V^IV solid. Concomitantly, the reduction of birnessite during V^IV oxidation generated soluble Mn^II, which led to the formation of the Mn^III oxyhydroxide feitknechtite (β-MnOOH) upon back-reaction with birnessite. XAS analysis confirmed a bidentate-mononuclear edge-sharing complex formed between V^V and birnessite, although retention of V^V was minimal relative to the aqueous quantities generated. In summary, we demonstrate that Mn oxides are effective oxidants of V^IV in the environment with the potential to increase dissolved V concentrations in aquifers subject to redox oscillations.

Accelerated catalytic oxidation of dissolved manganese(II) by chlorine in the presence of in situ-growing 3D manganese(III)/(IV) oxide nanosheet assembly in zeolite filter

Manganese contamination is ubiquitous in ground water. Water eutrophication also exaggerates manganese release and contamination in surface water. However, conventional manganese(II) removal process through sand filter is low-efficiency and long-term ripening. Manganese exceeding standard is still a bottleneck issue for drinking water plants. To provide a quick-setup and low-cost means, we invented an accelerated catalytic oxidation filtration process through porous zeolite filter with dynamically coating of manganese oxide nanocatalysts. In dynamic filtration process, the addition of chlorine less than redox stoichiometric consumption can efficiently remove dissolved manganese(II) from contaminated tap water, ground water and Songhua river water. Characterization results showed that a continuous manganese(III)/(IV) oxide nanosheet catalyst was dynamically in situ-growing and assembled into 3D porous superstructure in the reactive Zeolite@MnO_x(s) filter. Active Mn(III) species on the edges of MnO_x(s) nanosheets were dynamically generated and transferred into stable Mn(IV) species on the layer-structured surface. The cycling transformation of manganese(III)/(IV) species was responsible for the accelerated catalytic oxidation of dissolved manganese(II) by chlorine. Without process changes in drinking water plant, the porous Zeolite@MnO_x(s) media could be feasibly integrated onto the existing sand filtration tanks for emergence handling of manganese(II) contamination. This novel reactive Zeolite@MnO_x(s) filter with higher hydraulic conductivity provides a high-efficiency, scalable and low-cost technique for the manganese(II) removal from various of water environments.

The Energetic Potential for Undiscovered Manganese Metabolisms in Nature

Microorganisms are found in nearly every surface and near-surface environment, where they gain energy by catalyzing reactions among a wide variety of chemical compounds. The discovery of new catabolic strategies and microbial habitats can therefore be guided by determining which redox reactions can supply energy under environmentally-relevant conditions. In this study, we have explored the thermodynamic potential of redox reactions involving manganese, one of the most abundant transition metals in the Earth's crust. In particular, we have assessed the Gibbs energies of comproportionation and disproportionation reactions involving Mn²⁺ and several Mn-bearing oxide and oxyhydroxide minerals containing Mn in the +II, +III, and +IV oxidation states as a function of temperature (0-100°C) and pH (1-13). In addition, we also calculated the energetic potential of Mn²⁺ oxidation coupled to O₂, NO₂ ^-, NO₃ ^-, and FeOOH. Results show that these reactions-none of which, except O₂ + Mn²⁺, are known catabolisms-can provide energy to microorganisms, particularly at higher pH values and temperatures. Comproportionation between Mn²⁺ and pyrolusite, for example, can yield 10 s of kJ (mol Mn)^-1. Disproportionation of Mn³⁺ can yield more than 100 kJ (mol Mn)^-1 at conditions relevant to natural settings such as sediments, ferromanganese nodules and crusts, bioreactors and suboxic portions of the water column. Of the Mn²⁺ oxidation reactions, the one with nitrite as the electron acceptor is most energy yielding under most combinations of pH and temperature. We posit that several Mn redox reactions represent heretofore unknown microbial metabolisms.

Regenerated Manganese-Oxide Coated Sands: The Role of Mineral Phase in Organic Contaminant Reactivity

Manganese oxide-coated sand can oxidize electron-rich organic contaminants, but after extended exposure to contaminated water its reactivity decreases. To assess the potential for regenerating geomedia, we measured the ability of passivated manganese-oxide coated sand to oxidize bisphenol A after treatment with oxidants, acid, or methanol. Among the regenerants studied, KMnO₄, HOCl, HOBr, and pH 2 or 3 HCl solutions raised the average oxidation state of the Mn, but only HOCl and HOBr restored the reactivity of passivated geomedia to levels comparable to those of the virgin manganese-oxide coated sand. Treatment with HCl restored about one third of the reactivity of the material, likely due to dissolution of reduced Mn. Mn K-edge X-ray absorption spectroscopy data indicated that the reactive manganese oxide phases present in virgin geomedia and geomedia regenerated with HOCl or HOBr had nanocrystalline cryptomelane-like structures and diminished Mn(III) abundance relative to the passivated geomedia. KMnO₄-regenerated geomedia also had less Mn(III), but it exhibited less reactivity with bisphenol A because regeneration produced a structure with characteristics of δ-MnO₂. The results imply that manganese oxide reactivity depends on both oxidation state and crystal structure; the most effective chemical regenerants oxidize Mn(III) to Mn(IV) oxides exhibiting nanocrystalline, cryptomelane-like forms.

Inhibition of Oxyanions on Redox-driven Transformation of Layered Manganese Oxides

Layered manganese (Mn) oxides, such as birnessite, can reductively transform into other phases and thereby affect the environmental behavior of Mn oxides. Solution chemistry strongly influences the transformation, but the effects of oxyanions remain unknown. We determined the products and rates of Mn(II)-driven reductive transformation of δ-MnO₂, a nanoparticulate hexagonal birnessite, in the presence of phosphate or silicate at pH 6-8 and a wide range of Mn(II)/MnO₂ molar ratios. Without the oxyanions, δ-MnO₂ transforms into triclinic birnessite (T-bir) and 4 × 4 tunneled Mn oxide (TMO) at low Mn(II)/MnO₂ ratios (0.09 and 0.13) and into δ-MnOOH and Mn₃O₄ with minor poorly crystallized α- and γ-MnOOH at high Mn(II)/MnO₂ ratios (0.5 and 1). The presence of phosphate or silicate substantially decreases the rate and extent of the above transformation, probably due to adsorption of the oxyanions on layer edges or the formation of Mn(II,III)-oxyanion ternary complexes on vacancies of δ-MnO₂, adversely interfering with electron transfer, Mn(III) distribution, and structural rearrangements. The oxyanions also reduce the crystallinity and particle sizes of the transformation products, ascribed to adsorption of the oxyanions on the products, preventing their further particle growth. This study enriches our understanding of the solution chemistry control on redox-driven transformation of Mn oxides.

Reductive transformation of birnessite and the mobility of co-associated antimony

Manganese (Mn) oxide minerals, such as birnessite, are thought to play an important role in affecting the mobility and fate of antimony (Sb) in the environment. In this study, we investigate Sb partitioning and speciation during anoxic incubation of Sb(V)-coprecipitated birnessite in the presence and absence of Mn(II)_aq at pH 5.5 and 7.5. Antimony K-edge XANES spectroscopy revealed that Sb(V) persisted as the only solid-phase Sb species for all experimental treatments. Manganese K-edge EXAFS and XRD results showed that, in the absence of Mn(II), the Sb(V)-bearing birnessite underwent no detectable mineralogical transformation during 7 days. In contrast, the addition of 10 mM Mn(II) at pH 7.5 induced relatively rapid (within 24 h) transformation of birnessite to manganite (~93%) and hausmannite (~7%). Importantly, no detectable Sb was measured in the aqueous phase for this treatment (compared with up to ∼90 μmol L^-1 Sb in the corresponding Mn(II)-free treatment). At pH 5.5 , birnessite reacted with 10 mM Mn(II)_aq displayed no detectable mineralogical transformation, yet had substantially increased Sb retention in the solid phase, relative to the corresponding Mn(II)-free treatment. These findings suggest that the Mn(II)-induced transformation and recrystallization of birnessite can exert an important control on the mobility of co-associated Sb.

Different Pathways for Cr(III) Oxidation: Implications for Cr(VI) Reoccurrence in Reduced Chromite Ore Processing Residue

Hexavalent chromium contamination is a global environmental issue and usually reoccurs in alkaline reduced chromite ore processing residues (rCOPR). The oxidation of Cr(III) solids in rCOPR is one possible cause but as yet little studied. Herein, we investigated the oxidation of Cr(OH)₃, a typical species of Cr(III) in rCOPR, at alkaline pH (9-11) with δ-MnO₂ under oxic/anoxic conditions. Results revealed three pathways for Cr(III) oxidation under oxic conditions: (1) oxidation by oxygen, (2) oxidation by δ-MnO₂, and (3) catalytic oxidation by Mn(II). Oxidations in the latter two were efficient, and oxidation via Pathway 3 was continuous and increased dramatically with increasing pH. XANES data indicated feitknechtite (β-MnOOH) and hausmannite (Mn₃O₄) were the reduction products and catalytic substances. Additionally, a kinetic model was established to describe the relative contributions of each pathway at a specific time. The simulation outcomes showed that Cr(VI) was mainly formed via Pathway 2 (>51%) over a short time frame (10 days), whereas in a longer-term (365 days), Pathway 3 predominated the oxidation (>78%) with an increasing proportion over time. These results suggest Cr(III) solids can be oxidized under alkaline oxic conditions even with a small amount of manganese oxides, providing new perspectives on Cr(VI) reoccurrence in rCOPR and emphasizing the environmental risks of Cr(III) solids in alkaline environments.

Catalytic oxidation removal of manganese from groundwater by iron-manganese co-oxide filter films under anaerobic conditions

Although dissolved oxygen (DO) is a key factor for the removal of manganese (Mn) from aqueous solutions, this study presents an efficient method for Mn removal without any DO consumption. We demonstrate the feasibility of using an iron (Fe)-Mn co-oxide filter film to continuously remove Mn from groundwater under anaerobic conditions. A pilot-scale filter equipped with Fe-Mn co-oxide filter media (120 cm high) was adapted to explore the Mn removal performance under three DO levels (6-7 mg/L, 1-2 mg/L, and 0-0.2 mg/L). The Fe-Mn co-oxide filter exhibited a higher Mn removal performance under anaerobic conditions (no DO consumption) than under the other two DO conditions. The morphology, structure, and Mn valence changes of the Fe-Mn co-oxide filter film were studied using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Brunauer Emmett Teller (BET) theory, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The Fe-Mn co-oxide filter film under anaerobic conditions had a large contact surface area and large pore volume, and thus possessed more adsorption sites and reaction channels for Mn removal. By considering all of the characterization and reaction data reported in this study, we conclude that H₂O ligands, hydrogen bonding (-OH), and vacant sites affect the transformation of Mn, thus play important roles in the continuous removal of Mn under anaerobic conditions. This discovery presents a new and effective approach for Mn removal during groundwater treatment.

Coupled Manganese Redox Cycling and Organic Carbon Degradation on Mineral Surfaces

Minerals, natural organic matter (NOM), and divalent manganese (Mn(II)) often coexist in suboxic/oxic environment. Multiple adsorption and oxidation processes occur in this ternary system, which are coupled to affect the fate of both OM and Mn therein and alter their chemical reactivity toward metals and other pollutants. However, the details about the coupling are poorly known although much has been gained for the binary systems. We determined the mutual influence of surface-catalyzed Mn(II) oxidation and humic acid (HA) adsorption and oxidation in a Fe(III) oxide (goethite)-HA-Mn(II) system at pH 5-8. The presence of Mn(II) substantially increased HA adsorption whereas HA greatly impaired the extent and rate of Mn(II) oxidation by O₂ on goethite surfaces. The impacts were more pronounced at higher pH. Mn(II) oxidation produced β-MnOOH, γ-MnOOH, and Mn₃O₄ which in turn oxidized HA, producing small organic acids. The presence of HA markedly altered the composition of Mn(II) oxidation products by inhibiting the formation of β-MnOOH while favoring the production of γ-MnOOH and Mn(II) adsorbed on the HA-mineral assemblage. Nonconducting γ-Al₂O₃ exhibited similar but weaker effects than semiconducting goethite in the above processes. Our results suggest that similar to Mn-oxidizing microorganisms, mineral surfaces can drive the coupling of the Mn redox cycle with NOM oxidative degradation under suboxic/oxic and circumneutral/alkaline conditions.

Thallium Sorption onto Manganese Oxides

The sorption of thallium (Tl) onto manganese (Mn) oxides critically influences its environmental fate and geochemical cycling and is also of interest in water treatment. Combined quantitative and mechanistic understanding of Tl sorption onto Mn oxides, however, is limited. We investigated the uptake of dissolved Tl(I) by environmentally relevant phyllo- and tectomanganates and used X-ray absorption spectroscopy to determine the oxidation state and local coordination of sorbed Tl. We show that extremely strong sorption of Tl onto vacancy-containing layered δ-MnO₂ at low dissolved Tl(I) concentrations (log K_d ≥ 7.4 for ≤10^-8 M Tl(I); K_d in (L/kg)) is due to oxidative uptake of Tl and that less specific nonoxidative Tl uptake only becomes dominant at very high Tl(I) concentrations (>10^-6 M). Partial reduction of δ-MnO₂ induces phase changes that result in inhibited oxidative Tl uptake and lower Tl sorption affinity (log K_d 6.2-6.4 at 10^-8 M Tl(I)) and capacity. Triclinic birnessite, which features no vacancy sites, and todorokite, a 3 × 3 tectomanganate, bind Tl with lower sorption affinity than δ-MnO₂, mainly as hydrated Tl⁺ in interlayers (triclinic birnessite; log K_d 5.5 at 10^-8 M Tl(I)) or tunnels (todorokite). In cryptomelane, a 2 × 2 tectomanganate, dehydrated Tl⁺ replaces structural K⁺. The new quantitative and mechanistic insights from this study contribute to an improved understanding of the uptake of Tl by key Mn oxides and its relevance in natural and engineered systems.

Mandelic acid and phenyllactic acid "Reaction Sets" for exploring the kinetics and mechanism of oxidations by hydrous manganese oxide (HMO)

At pH 4.0, hydrous manganese oxide (HMO) oxidizes mandelic acid by two mole-equivalents of electrons, yielding phenylglyoxylic acid and benzaldehyde. These intermediates, in turn, are oxidized by two mole-equivalents of electrons to the same ultimate oxidation product, benzoic acid. The four compounds of the "reaction set" just described are conveniently monitored using capillary electrophoresis (CE) and HPLC. Extents of adsorption are negligible and their sum exhibits mass balance. Concentrations of phenylglyoxylic acid, benzaldehyde, and benzoic acid can therefore be used to calculate mole-equivalents delivered to HMO for comparison with experimentally-determined dissolved MnII concentrations. Semi-log plots (ln[substrate] versus time) and numerical analysis can also be used to explore rates of oxidation of the functional groups represented, i.e. an α-hydroxycarboxylic acid, an α-ketocarboxylic acid, and an aldehyde. Inserting a -CH2- group between the benzene ring and the functional groups just described yields a new reaction set comprised of phenyllactic acid, phenylpyruvic acid, and phenylacetaldehyde, plus the C-1 ultimate oxidation product, phenylacetic acid. At pH 4, mass balance for phenyllactic acid oxidation fell short by ∼9%. Phenyllactic acid was oxidized 2.7-times more slowly than mandelic acid, while phenylpyruvic acid was oxidized 12.7-times faster than phenylglyoxylic acid. Unlike benzaldehyde, oxidation rates for phenylacetaldehyde were too fast to measure. Under pH 4.0 conditions, this reaction set approach was used to explore the acceleratory effects of increases in HMO loading and inhibitory effects of 500 μM phosphate and pyrophosphate additions. Mandelic acid and phenyllactic acid were oxidized by HMO far more slowly at pH 7.0 than at pH 4.0. At pH 7.0, 2 mM MOPS and phosphate buffers did not yield appreciable dissolved MnII, despite oxidation of organic substrate. 2 mM pyrophosphate, in contrast, solubilized HMO-bound MnII and MnIII.

Geochemical Stability of Dissolved Mn(III) in the Presence of Pyrophosphate as a Model Ligand: Complexation and Disproportionation

Dissolved Mn(III) species have recently been recognized as a significant form of Mn in redox transition zones, but their speciation, stability, and reactivity are poorly understood. Besides acting as the intermediate for Mn redox chemistry, Mn(III) can undergo disproportionation producing insoluble Mn oxides and aqueous Mn(II). Using pyrophosphate(PP) as a model ligand, we evaluated the thermodynamic and kinetic stability of Mn(III) complexes. They were stable at circumneutral pH and were prone to (partial) disproportionation at acidic or basic pH. With an initial lag phase, the kinetics of Mn(III)-PP disproportionation was autocatalytic with the produced Mn oxides promoting the disproportionation. X-ray diffraction and the average Mn oxidation state indicated that the solid products were not pure Mn(IV) oxides but a mixture of triclinic birnessite and δ-MnO₂. Addition of synthetic analogs of the precipitates eliminated the lag phase, confirming their catalytic roles. Thermodynamic calculations adequately predicted the stability regime of Mn(III)-PP. The present results refined the constant for Mn(PP)₂^5- formation, which allows a consistent and quantitative prediction of equilibrium speciation of Mn(III)-Mn(II)-birnessite with PP. A simple and robust model, which incorporated the thermodynamic constraints, autocatalytic rate law, and verified reaction stoichiometry, successfully simulated all kinetic data.

Chemical Regeneration of Manganese Oxide-Coated Sand for Oxidation of Organic Stormwater Contaminants

Urban stormwater, municipal wastewater effluent, and agricultural runoff contain trace amounts of organic contaminants that can compromise water quality. To provide a passive, low-cost means of oxidizing substituted phenols, aromatic amines, and other electron-rich organic compounds during infiltration of contaminated waters, we coated sand with manganese oxide using a new approach involving the room-temperature oxidation of Mn²⁺ with permanganate. Manganese oxide-coated sand effectively oxidized bisphenol A under typical infiltration conditions and sustained reactivity longer than previously described geomedia. Because geomedia reactivity decreased after extended operation, chlorine was evaluated for use as an in situ geomedia regenerant. Geomedia regenerated by HOCl demonstrated similar reactivity and longevity to that of virgin geomedia. Chemical analyses indicated that the average manganese oxidation state of the coatings decreased as the geomedia passivated. X-ray absorption spectroscopy and X-ray diffraction showed that the reactive virgin and regenerated geomedia coatings had nanocrystalline manganese oxide structures, whereas the failed geomedia coating exhibited greater crystallinity and resembled cryptomelane. These results suggest that it is possible to regenerate the oxidative capacity of manganese oxide-coated sands without excavating stormwater infiltration systems. These results also suggest that manganese oxide geomedia may be a cost-effective means of treating urban stormwater and other contaminated waters.

Effect of carbonate on U(VI) sorption by nano-crystalline α-MnO2

U(VI) sorption on nano-crystalline α-MnO2was studied in NaClO4medium as a function of pH by batch sorption method in presence and absence of carbonate and subsequently employing surface complexation modeling (SCM) to predict species responsible for U(VI) sorption. The kinetic study of U(VI) sorption on nano-crystalline α-MnO2was carried out to fix the time of equilibration. In presence of carbonate, U(VI) sorption on nano-crystalline α-MnO2increases with pH of the suspension, leveling off in the pH range 5–8.5 thereafter decreasing at higher pH. However, in absence of carbonate, U(VI) sorption on nano-crystalline α-MnO2remains close to 100% at pH>5. The difference in sorption behavior of uranium in the presence and absence of carbonate can be explained in terms of uranium speciation in the two systems. The dissolution of nano-crystalline α-MnO2was studied in presence and absence of carbonate to ascertain its role in sorption. Surface complexation modeling was satisfactorily able to explain the sorption phenomena in all the systems. In addition, U(VI) sorption on nano-crystalline α-MnO2was compared with literature data on U(VI) sorption by δ-MnO2.

Structural Transformation of Birnessite by Fulvic Acid under Anoxic Conditions

The structure and Mn(III) concentration of birnessite dictate its reactivity and can be changed by birnessite partial reduction, but effects of pH and reductant/birnessite ratios on the changes by reduction remain unclear. We found that the two factors strongly affect the structure of birnessite (δ-MnO₂) and its Mn(III) content during its reduction by fulvic acid (FA) at pH 4-8 and FA/solid mass ratios of 0.01-10 under anoxic conditions over 600 h. During the reduction, the structure of δ-MnO₂ is increasingly accumulated with both Mn(III) and Mn(II) but much more with Mn(III) at pH 8, whereas the accumulated Mn is mainly Mn(II) with little Mn(III) at pH 4 and 6. Mn(III) accumulation, either in layers or over vacancies, is stronger at higher FA/solid ratios. At FA/solid ratios ≥1 and pH 6 and 8, additional hausmannite and MnOOH phases form. The altered birnessite favorably adsorbs FA because of the structural accumulation of Mn(II, III). Like during microbially mediated oxidative precipitation of birnessite, the dynamic changes during its reduction are ascribed to the birnessite-Mn(II) redox reactions. Our work suggests low reactivity of birnessite coexisting with organic matter and severe decline of its reactivity by partial reduction in alkaline environment.

Impacts of hydrous manganese oxide on the retention and lability of dissolved organic matter

Minerals constitute a primary ecosystem control on organic C decomposition in soils, and therefore on greenhouse gas fluxes to the atmosphere. Secondary minerals, in particular, Fe and Al (oxyhydr)oxides-collectively referred to as "oxides" hereafter-are prominent protectors of organic C against microbial decomposition through sorption and complexation reactions. However, the impacts of Mn oxides on organic C retention and lability in soils are poorly understood. Here we show that hydrous Mn oxide (HMO), a poorly crystalline δ-MnO₂, has a greater maximum sorption capacity for dissolved organic matter (DOM) derived from a deciduous forest composite O_i, O_e, and O_a horizon leachate ("O horizon leachate" hereafter) than does goethite under acidic (pH 5) conditions. Nonetheless, goethite has a stronger sorption capacity for DOM at low initial C:(Mn or Fe) molar ratios compared to HMO, probably due to ligand exchange with carboxylate groups as revealed by attenuated total reflectance-Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy and scanning transmission X-ray microscopy-near-edge X-ray absorption fine structure spectroscopy coupled with Mn mass balance calculations reveal that DOM sorption onto HMO induces partial Mn reductive dissolution and Mn reduction of the residual HMO. X-ray photoelectron spectroscopy further shows increasing Mn(II) concentrations are correlated with increasing oxidized C (C=O) content (r = 0.78, P < 0.0006) on the DOM-HMO complexes. We posit that DOM is the more probable reductant of HMO, as Mn(II)-induced HMO dissolution does not alter the Mn speciation of the residual HMO at pH 5. At a lower C loading (2 × 10² μg C m^-2), DOM desorption-assessed by 0.1 M NaH₂PO₄ extraction-is lower for HMO than for goethite, whereas the extent of desorption is the same at a higher C loading (4 × 10² μg C m^-2). No significant differences are observed in the impacts of HMO and goethite on the biodegradability of the DOM remaining in solution after DOM sorption reaches steady state. Overall, HMO shows a relatively strong capacity to sorb DOM and resist phosphate-induced desorption, but DOM-HMO complexes may be more vulnerable to reductive dissolution than DOM-goethite complexes.

Recrystallization of Manganite (γ-MnOOH) and Implications for Trace Element Cycling

The recrystallization of Mn(III,IV) oxides is catalyzed by aqueous Mn(II) (Mn(II)_aq) during (bio)geochemical Mn redox cycling. It is poorly understood how trace metals associated with Mn oxides (e.g., Ni) are cycled during such recrystallization. Here, we use X-ray absorption spectroscopy (XAS) to examine the speciation of Ni associated with Manganite (γ-Mn(III)OOH) suspensions in the presence or absence of Mn(II)_aq under variable pH conditions (pH 5.5 and 7.5). In a second set of experiments, we used a ⁶²Ni isotope tracer to quantify the amount of dissolved Ni that exchanges with Ni incorporated in the Manganite crystal structure during reactions in 1 mM Mn(II)_aq and in Mn(II)-free solutions. XAS spectra show that Ni is initially sorbed on the Manganite mineral surface and is progressively incorporated into the mineral structure over time (13% after 51 days) even in the absence of dissolved Mn(II). The amount of Ni incorporation significantly increases to about 40% over a period of 51 days when Mn(II)_aq is present in solution. Similarly, Mn(II)_aq promotes Ni exchange between Ni-substituted Manganite and dissolved Ni(II), with around 30% of Ni exchanged at pH 7.5 over the duration of the experiment. No new mineral phases are detected following recrystallization as determined by X-ray diffraction and XAS. Our results reveal that Mn(II)-catalyzed mineral recrystallization partitions Ni between Mn oxides and aqueous fluids and can therefore affect Ni speciation and mobility in the environment.

Rates of Cr(VI) Generation from CrxFe1-x(OH)3 Solids upon Reaction with Manganese Oxide

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 MnO₂ 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-MnO₂. The Fe/Cr ratio (x = 0.055-0.23 in Cr_xFe_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_xFe_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_xFe_1-x(OH)₃-MnO₂ 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_xFe_1-x(OH)₃ solids to aqueous solution can migrate to MnO₂ surfaces where it is oxidized.