Mn Cycling Research Articles

Optimized Mn cycle enhanced synchronous removal of nitrate and antibiotics driven by manganese oxides/solid carbon composites: Microbiota assembly patterns and electron transport.

The reactive substance consisting manganese oxides (MnOx) and solid carbon have been reported to be effective in polishing secondary wastewater; however, the treatment characteristics and mechanism remains limited. In this study, MnOx/carbon (Mn-C) composites were applied in biofilters to evaluate simultaneous removal of nitrate and sulfamethoxazole (SMX), with the single carbon composites as control. Results showed that the effluent concentrations of NO3--N and SMX were below 2.87 mg L-1 and 7.97 μg L-1 under hydraulic retention time (HRT) of 6 h. The intermittent aeration optimized Mn cycle with treatment performance improved under lower HRT and Mn(II) accumulation decreased. Mn-C composites could reduce the emission of N2O, CO2 and CH4. The dominant genera gradually evolved from fermentation to glycogen aggregation, and from heterotrophic/sulfur autotrophic to heterotrophic denitrifiers by intracellular substance and manganese autotrophic/heterotrophic bacteria. Microbial network analysis indicated higher antagonism, lower modularity and shorter average path among microbes in Mn-C biofilters, which highlighted microbial differentiation and faster electron transfer. Improved functions of denitrification and Mn respiration, and the increasing genes encoding electron transfer chain, including NADH dehydrogenase, Cytc and ubiquinone, further elucidated the superiority of Mn-C composites. These results improved our understanding of Mn-C composites application in low-carbon wastewater treatment.

Enhanced oxidation and in-situ coagulation Fe(Ⅱ)/peroxymonosulfate-Mn(Ⅶ) process for carbamazepine removal: Multiple promoting effects of Mn and direct/indirect regulation of Cl- on active substances transformation.

Slow transformation efficiency of Fe(III)/Fe(II) limits the generation of radicals in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs), and these radicals was easy to be interfered by the presence of water constituents. In addition, in-situ coagulation during this oxidation process was neglected. This study proposed Fe(II)/PMS-Mn(VII) in the presence of chlorides ions (FPMC) process to reveal multiple promoting effects of Mn on redox cycle of Fe(III)/Fe(II) and different reactive mechanisms of Cl- on types of radicals generation pathways, and the in-situ coagulation enhanced mechanisms was investigated. Results showed that the dual functionality of oxidation and in-situ coagulation in FPMC process was significantly enhanced that carbamazepine (CBZ) could be efficiently and quickly removed. The reduction product Mn(III) of Mn(VII) could promote the redox cycle of Mn(II)/Mn(III) and Mn(III)/Mn(IV) that facilitated Fe(III)/Fe(II), sustaining the reactivity of the system. Cl- could significantly promote the cycling of Mn(Ⅲ)/Mn(Ⅳ) that indirectly affected the cycle of Fe(III)/Fe(II).

Activation of Peroxymonosulfate by a Mn0.7Co0.5O2@C Catalyst for the Degradation of Tetracycline

AbstractPersulfate advanced oxidation technologyshows great potential in the removal of organic pollutants. Inthis study, a Mn0.7Co0.5O2@C catalyst wassynthesized from MnCO3‐ZIF‐67 precursor via calcination and employedto activate peroxymonosulfate (PMS) for the degradation of tetracycline (TC).The catalyst, characterized by a plethora of active sites and superior electrontransfer capabilities, mitigated the sluggish reaction kinetics associated withweak electron transfer. Degradation experiments demonstrated an 87.54% removalefficiency within 60 minutes using 10 mg of catalyst, outperforming PMS aloneby 2.3‐fold. The Mn0.7Co0.5O2@C/PMS system'ssynergistic effects and mechanisms were elucidated, with experiments confirmingthe generation of reactive species including singlet oxygen (1O2), sulfate radicals (SO4•−), and hydroxyl radicals (•OH).The catalyst's exceptional performance is attributed to the valence cycling ofMn and Co ions, facilitating PMS activation and the production of radicals. Additionally, the lattice oxygen within the catalyst is oxidized to form reactive oxygenspecies, which, in conjunction with oxygen vacancies, react with PMS to producenon‐radical species (1O2). The synergistic oxidationeffects of these free radicals and non‐radicals ultimately facilitate theremoval of TC, offering a novel approach for the treatment of organicwastewater with persulfate‐based advanced oxidation processes.

Microbe-forced Mn‑carbonate direct precipitation in Ediacaran micro-stromatolites of South China

The direct precipitation model of Mn‑carbonate formation, based on investigation of the modern sedimentary record and simulation experiments, is commonly used to explain the genesis of both modern and ancient Mn‑carbonate deposits. This process is considered to be primarily influenced by physicochemical conditions and lacks microbial mediation. Despite the established role of microbes in global Mn cycling and biomineralization, the specific contribution of microbial processes to Mn‑carbonate formation remains understudied. In this study, Ediacaran Mn‑carbonates from South China with well-preserved micro-stromatolites offer a novel insight into understanding the microbes involved in the formation of Mn-deposits. Petrological observations reveal that the laminated Mn-ores mainly consist of alternating layers of dark Mn‑carbonate and light-colored dolomite laminae. Mn‑carbonate minerals, dominantly rod-like rhodochrosite crystals, exhibit a close spatial relationship with micro-stromatolites, suggesting a possible link to microbial activity. Geochemical results display that these Mn‑carbonates document high δ13C values (average = −1.65 ‰) compared to typical diagenetic Mn‑carbonate indicating the seawater likely contributed to the carbon source. Combined with alabandite deposition, small-sized pyrite framboids, and positive Eu anomalies, the Ediacaran Mn‑carbonates may have formed by microbially-mediated direct precipitation in Mn-rich anoxic seawater. The Mn‑carbonate and micro-stromatolite laminae accumulated during a period of enhanced bacterial activity, driven by episodic inputs of hydrothermal Mn2+ and bioessential elements. The present study highlights the microbially-mediated significant role in the primary precipitation pathway of Mn‑carbonate. Direct precipitation of Mn‑carbonate deposits, controlled by ocean conditions as well as enhanced by microbial processes, may account for the formation of other ancient economic manganiferous sedimentary deposits.

Activation of peroxomonosulfate by manganese-containing non-homogeneous catalysts prepared via a soft template calcination strategy for efficient degradation of carbamazepine

Manganese (Mn)-containing inhomogeneous catalysts have demonstrated potential to activate peroxymonosulfate (PMS), but may be constrained by low efficiency and poor resistance to environmental disturbances. According to the soft template calcination strategy, g-C3N4 with Mn doping was synthesized to improve PMS activation through an effective Mn(II)/Mn(III)/Mn(IV) cycle. The constructed Mn25@CN/PMS system achieved complete and rapid removal of carbamazepine (CBZ) at 0.3 g/L catalyst and 2 mM PMS. Its corresponding rate constant of pseudo-first-order was 0.3424 min−1, and it was 3424 and 16.15 times higher than that of pure g-C3N4/PMS system and CN/PMS system, respectively. More strikingly, the catalysts exhibited excellent resistance to environmental impacts, with CBZ removal rates exceeding 95% under the influence of different anions, cations, aqueous pH values, and humic acid (HA) concentrations. Subsequently, it was proposed a reliable catalytic mechanism for CBZ removal and PMS activation derived from electron paramagnetic resonance (EPR) tests and quenching experiments. The O2·- and 1O2 played a major role in the CBZ removal process. This research provides a perspective soft template strategy for PMS activation using highly efficient catalysts with high resistance to environmental impacts, which provides new ideas to alleviate environmental issues.

Efficient degradation of Enrofloxacin with novel magnetic MnFe2O4/NiS2 composite as an activator of peroxymonosulfate

The development of highly active and recoverable heterogeneous catalysts for peroxymonosulfate (PMS) activation to remove the persistent presence of antibiotics in aquatic environments, such as Enrofloxacin (ENR), remains a significant challenge. Here, we introduce a pioneering magnetic nanocomposite catalyst, MnFe2O4-NiS2, synthesized via a two-step hydrothermal process, which activates PMS to efficiently degrade ENR. The optimized 0.25MnFe2O4-NiS2/PMS system reached an ENR removal efficiency of 89.9 % within 90 min, outperforming both NiS2/PMS and MnFe2O4/PMS by 5.6 and 2.5 times, respectively. Quenching experiments and electron spin response analysis confirmed SO4•- and •O2– as the primary reactive oxygen species. The enhanced mediated electron transfer capability of the 0.25MnFe2O4-NiS2 nanocomposite improved the redox cycling of Ni (Ⅱ), Mn (Ⅱ) and Fe (Ⅲ) on its surface, as verified by Density Functional Theory calculations. Furthermore, the activation of surface PMS complexes (PMS*) played a crucial role in ENR degradation. The proposed degradation pathways of ENR were confirmed, and toxicity analysis indicated a decrease in intermediate toxicity with the 0.25MnFe2O4-NiS2/PMS system. This study presents an innovative magnetically recoverable, multifunctional catalyst with easy reusability for potential applications in the degradation of organic pollutants.

Revealing the reaction mechanism of the novel p-n heterojunction Mn3O4-C3N4 in efficient activation of chlorite to degrade organic pollutants under piezoelectric catalysis

This study constructed a p-n heterojunction Mn3O4-CN by loading Mn3O4 on g-C3N4 (CN), effectively activating chlorite to degrade organic pollutants under piezoelectric effect. The rate of sulfamethoxazole (SMX) degradation in the piezo/Mn3O4-CN/chlorite combination was 0.0346 min−1, surpassing piezo/CN/chlorite and Mn3O4/chlorite systems by 21.63 and 3.26 times, respectively. Experimental verification and theoretical calculation showed that Mn3O4-CN had a stronger piezoelectric response and adsorbed chlorite (-1.682 eV) better than CN (-0.615 eV) and Mn3O4 (-1.093 eV), contributing to the activation of chlorite and efficient pollutant degradation. The Mn(II)/Mn(III)/Mn(IV) cycle during piezoelectric activation of chlorite by p-n heterojunction promoted SMX degradation, with chlorine dioxide (ClO2) and Mn(IV) as dominant reactive species. Furthermore, the piezo/Mn3O4-CN/chlorite system exhibited remarkable stability, environmental safety and wide application potential. This research offered fresh insights and practical guidelines for harnessing mechanical activation of chlorite for efficient organic pollutant degradation in water, advancing water treatment technology.

One-step synthesis of magnetic catalysts containing Mn3O4-Fe3O4 from manganese slag for degradation of enrofloxacin by activation of peroxymonosulfate

Landfill disposal of manganese slag is a significant environmental concern on both local and global levels. Using manganese slag to synthesis catalysts to degrade organic pollutants in wastewater is a promising pathway for the solid waste treatment. Here, we propose a simple method to prepare magnetic catalysts containing Mn3O4-Fe3O4 (MMF) using manganese slag as raw materials. The MMF exhibits excellent degradation performance for enrofloxacin (ENR) with 97 % removal efficiency within 30 min and can be rapidly isolated from wastewater by magnet. Electron paramagnetic resonance (EPR) and quenching experiments demonstrated that 1O2 produced by the non-radical pathway is the principal active substance for ENR degradation. X-ray photoelectron spectroscopy (XPS) and density-functional theory (DFT) calculations indicate that Mn and Fe in MMF provide electrons for PMS activation and that the presence of Fe promotes the activation of the PMS and the cycling of Mn. This work paves the way for developing magnetic catalysts from manganese slag for the treatment of organic pollutants via advanced oxidation process.

Coupled effects of iron (hydr)oxides and clay minerals on the heterogeneous oxidation of aqueous Mn(II) and crystallization of manganese (hydr)oxides

The formation of nanominerals and mineral nanoparticles (NMMNs) has drawn broad attention due to their high reactivity and omnipresence in the environment. While the heterogeneous formation of NMMNs on surfaces of various minerals has been extensively studied, there is limited understanding of how mineral heteroaggregates influence this process. In this study, we investigated how heteroaggregates of iron (hydr)oxides and clay minerals affect the heterogeneous oxidation of aqueous Mn(II) and crystallization of manganese (hydr)oxides (MnOx). Our results revealed that iron (hydr)oxides (ferrihydrite) and clay minerals (kaolinite or montmorillonite) in heteroaggregates exerted coupled effects on these processes, dictating the distribution of Mn and the morphology of MnOx. Specifically, ferrihydrite catalyzed gradual oxidative removal of Mn(II) and triggered MnOx nucleation; in contrast, kaolinite/montmorillonite rapidly adsorbed Mn(II) but hardly catalyzed its oxidation. These reactions collectively resulted in fast adsorption and gradual oxidation of Mn(II) on the heteroaggregates. Further, MnOx nanoparticles formed on ferrihydrite surfaces migrated to kaolinite/montmorillonite surfaces, leading to interactions between MnOx and various component minerals within the heteroaggregates. This significantly altered the subsequent growth pathways and the eventual morphology of MnOx. Consequently, while MnOx nanoparticles in the ferrihydrite-only system aggregated freely and formed well-extended nanowires, those in the ferrihydrite-kaolinite system predominantly became short nanorods due to the immobilization by kaolinite surfaces; in the ferrihydrite-montmorillonite system, considerable MnOx nanoparticles attached to montmorillonite surfaces due to strong electrostatic attraction, and subsequently grew into blocky particles via particle attachment. These findings illustrate that surface reactivities of heteroaggregated ferrihydrite and kaolinite/montmorillonite are coupled when they interact with aqueous Mn(II) or MnOx. Our work exemplifies, for the first time, the cooperation between surfaces of various minerals during the heterogeneous formation of NMMNs. Findings from this study also enhance our understanding of MnOx formation on surfaces with diverse atomic structures, and contribute to the knowledge of Mn cycling in the environment.

A refined model for the mechanisms of Precambrian phosphorite formation

Despite the economic and scientific importance of Precambrian phosphorites, our understanding of the mechanism leading to their formation remains limited, including for the largest phosphogenic episode in the late Neoproterozoic. To improve our understanding of Precambrian phosphorite formation, we combined sedimentology, petrography, and elemental, and Fe-C isotopic analyses to study the two main phosphorite beds (the lower and upper phosphorite beds) in the Ediacaran Doushantuo Formation, Zhangcunping area, South China. The phosphorites consist mainly of granular textures characterized by densely packed grains, some of which are coated with secondary phosphate growth. However, there are notable differences in the mineralogy, microfossil assemblages, and elemental contents of the two beds. The lower phosphorites have no Ce anomaly, and relatively low Y/Ho ratios and positive δ56Fe values (0.04–0.30 ‰, average of 0.19 ‰). In contrast, the upper phosphorites have negative Ce anomalies and higher Y/Ho ratios and near-zero δ56Fe values (−0.29–0.19 ‰ (average of −0.01 ‰). These observations suggest that the lower phosphorites formed in anoxic-suboxic environments, whereas the upper phosphorites formed in relatively oxygenated environments. The δ13Ccarb values of the phosphorites range from −3.97 ‰ to 1.71 ‰ (average of −1.56 ‰), and are lighter than values in dolostones (−0.52 ‰ to 4.39 ‰, average of 2.02 ‰). This suggesting that formation of the Doushantuo phosphorites was influenced by degradation of organic matter in an ocean with high primary productivity. The lower phosphorites, which were also regulated by Fe redox pumping, have positive δ56Fe values, along with the presence of pyrite framboids and iron oxides, suggesting deposition near the Fe-redox boundary where extensive Fe cycling. The upper phosphorites show positive correlations between Mn and Fe, and Mn/Fe and P2O5, suggesting formation near the Mn boundary with extensive Mn cycling, primarily mediated by Mn redox pumping. Sedimentological observation indicate that primary phosphates were concentrated into granular phosphorites by winnowing processes following primary precipitation. Accordingly, we propose a refined model for Precambrian phosphorite formation in which degradation of organic matter, Fe and Mn pumping, and physical reworking of deposits co-evolve and interact within a dynamic Precambrian redox environment. Our model provides a reasonable explanation for the distribution of global phosphorite deposits throughout geological history.

A benthic source of isotopically heavy Ni from continental margins and implications for global ocean Ni isotope mass balance

Nickel (Ni) is a bio-essential element for phytoplankton in the modern oceans, and yet, the global ocean mass balance of Ni has puzzled scientists for decades. Many estimates of total Ni output flux are larger than the total Ni input flux to the ocean. The measurement of Ni stable isotopes (δ60Ni) in earth materials may inform our understanding of ocean biogeochemistry and redox conditions in both the modern ocean and the geological past. An ocean Ni stable isotope imbalance has also concerned scientists, with a global ocean δ60Ni of +1.4 ‰ which is notably heavier than the major source of Ni to the oceans from rivers. Recent efforts to resolve Ni and δ60Ni imbalances have focused on the role which manganese (Mn) oxides play in marine Ni cycling, both in the water column and in marine sediments. Manganese oxide rich marine sediments can supply isotopically heavy dissolved Ni to porewater fluids and seawater, but the source of the isotopically heavy porewater Ni remains unclear. Here we present porewater trace metal concentrations and δ60Ni from two sites from the continental margin off southern California. Porewater Ni concentrations are up to roughly 100-fold higher than deep seawater concentrations (∼1 μM compared to 10 nM, respectively). Porewater δ60Ni is near 0 ‰ in the subsurface region where Ni concentrations are highest, suggesting dissolution of lithogenic material from sediments. Porewater δ60Ni increases dramatically towards the sediment-water interface with values of up to +2.66 ‰, which to our knowledge are the heaviest Ni isotope compositions ever reported for natural materials. Measurements of porewater and sediment Ni and Mn concentrations suggest that Ni is released to porewaters in the Mn-reducing zone, and then removed by newly precipitated Mn oxides as Mn and Ni move towards the oxygenated zone of sediments. A simple Rayleigh model suggests a Ni isotope fractionation of factor of -0.61 ‰ for Ni sorption onto Mn oxides, while a diffusion-reaction model suggests a Ni isotope fractionation of -1.80 to -0.96 ‰, always with preferential adsorption of lighter Ni isotopes. This flux of Ni toward the sediment-water interface, coupled with preferential removal of lighter Ni isotopes in marine sediments, provides evidence supporting an isotopically heavy benthic source of new Ni to the oceans, which we estimate has a magnitude of 0.65 × 108 to 1.66 × 108 moles/year. We find that a benthic flux such as measured here over just 0.27-2.70 % of the ocean seafloor could mix with the river δ60Ni of +0.8 ‰ to achieve the observed deep ocean δ60Ni of +1.4 ‰. This study reveals the similarities in how rivers and continental margin sediments supply heavy Ni isotopes to the ocean. In both continental rivers and margin sediments, terrigenous Ni is released with an δ60Ni near 0.1 ‰, but lighter isotopes are captured by precipitation onto oxides so that the dissolved flux to the ocean is isotopically heavier than the terrigenous material from which Ni was released, and in the case of margin sediments may be heavier than even bulk seawater δ60Ni. We thus recognize continental margin sediments with active Mn cycling as a ‘new’ source of isotopically heavy dissolved Ni to the ocean.

Ca2+-controlled Mn(II) removal process in Aurantimonas sp. HBX-1: Microbially-induced carbonate precipitation (MICP) versus Mn(II) oxidation

The application of manganese-oxidizing bacteria (MnOB) to produce manganese oxides (MnOx) has been widely studied, but often overlooking the concurrent formation of MnCO3. In this study, we found Ca2+ plays a crucial role in controlling Mn(II) removal in the bacterium Aurantimonas sp. HBX-1. Under conditions with 6.8 mM Ca2+ and without adding Ca2+, 100 μM Mn(II) was removed by 96.96 % and 38.28 % within 8 days, respectively. X-ray photoelectron spectroscopy (XPS) showed that adding Ca2+ increased the average oxidation state (AOS) of the solid products from 2.05 to 2.37. X-ray absorption fine structure (XAFS) analysis revealed the product proportions as follows: under Ca2+-supplemented condition, the ratio of MnOx (1 < x ≤ 2) to MnCO₃ was 52 % to 28.1 %, while under Ca2+-free condition, the ratio shifted to 4.6 % for MnOx (1 < x ≤ 2) and 55.2 % for MnCO₃. Urease activity assay and proteomic analysis confirmed the expression of urease and carbonic anhydrase, leading to the formation of MnCO3. Additionally, animal heme peroxidase (AHP) in strain HBX-1 was found to be responsible for Mn(II) oxidation through superoxide production, with Ca2+ addition promoting its expression level. Given the widespread presence of Ca2+ in wastewater, its potential impact on the biogeochemical Mn(II) cycle driven by bacteria should be reconsidered.

Simultaneous removal of nitrate, manganese, zinc, and bisphenol a by manganese redox cycling system: Performance and mechanism

The manganese(Mn) redox cycling system in this work was created by combining Mn(IV)-reducing bacteria MFG10 with Mn(II)-oxidizing bacteria HY129. The biomanganese oxides (BMO) generated by strain HY129 were transformed by strain MFG10 to Mn(II), finishing the Mn redox cycling, in which nitrate (NO3–-N) was converted to nitrite, which was further reduced to nitrogen gas. The system could achieve 85.7 % and 98.8 % elimination efficiencies of Mn(ⅠⅠ) and NO3–-N, respectively, at Mn(ⅠⅠ) = 20.0 mg/L, C/N = 2.0, pH = 6.5, and NO3–-N = 16.0 mg/L. The removal of bisphenol A (BPA) and zinc (Zn(II)) at 36 h reached 91.7 % and 89.7 % under the optimal condition, respectively. Furthermore, the Mn redox cycling system can reinforce the metabolic activity and electron transfer activity of microorganisms. The findings showed that the adsorption by bioprecipitation throughout the Mn cycling was responsible for the elimination of Zn(II) and BPA.

Sediment colour as a marker of syn-depositional and early diagenetic processes in glaciofluvial sediments

The continental red beds, encompassing a broad spectrum of genetic types, can serve as important palaeoclimatological and palaeoenvironmental archives. The origin of sediment colouration is a complex process involving abiotic processes (e.g., breakdown of original and precipitation of newly-formed minerals), which, together with biogenic factors, lead to mobilisation of redox-sensitive elements and precipitation of Fe- and Mn-(oxy)hydroxides. There is still discussion about the interpretation of the continental red beds as palaeoclimatological archives or the colour patterns reflecting ancient redox gradients. The layers coloured in red, yellow or black can be found in the Quaternary glaciofluvial sediments in the Czech Republic. We are using a combination of field study with multi-spectral petrophysical, petrological and geochemical analyses to investigate the mechanism and timing of the origin of coloured coatings in glaciofluvial sediments, and causes of cycling of Fe, Mn, and other redox sensitive elements and isotopes. The results show that both syn-depositional and early diagenetic processes are responsible for the origin of colour patterns in the Quaternary glaciofluvial sediments. The stable molybdenum and iron isotope fractionation is primarily driven by the breakdown of the primary Fe and Mn-bearing silicates and the precipitation of the secondary Fe- and Mn-(oxy)hydroxides, such as goethite and birnessite. These precipitates are the main components of colouring coatings on the detrital grains and are able to bind other redox-sensitive elements, such as Cu, As, Mo, U, and REEs. The textural patterns and geochemistry suggest that the colour features were developed in the time range of decades to several thousand years after the deposition along ancient subsurface redox gradients due to changes in groundwater flow associated with primary lithology, glaciotectonics, and seasonal changes in the active layer of permafrost. The coatings show morphological features (rods, botryoids) and geochemical signatures (e.g., increased P contents) suggesting involvement of microorganisms to their precipitation.

The underestimated role of manganese in modulating the nutrient structure in a eutrophic seasonally-stratified reservoir

Accumulation and subsequent release of nutrients have great potential to trigger algal blooms in lakes and reservoirs. We conducted high vertical resolution (2 m interval) monitoring at ∼monthly intervals over a year for hydrological parameters, Chl-a, ammonium (NH4+), nitrate (NO3−) and different species of phosphorus (P) and manganese (Mn) in a 40-meter-deep subtropical reservoir (Shanmei Reservoir) in Fujian, southern China. In this seasonally stratified reservoir featured with high nutrient loading, the consistent trend in the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) between the euphotic zone and the hypolimnion, coupled with its mirrored correlation with Chl-a concentration indicates that upward flux from the hypolimnion affects phytoplankton growth in the euphotic zone. The monthly variation of the depth-integrated multiple species of N and P indicates that during the stratification period in the hypoxic hypolimnion, approximately 80% of the DIP is removed, leading to a remarkable decoupling phenomenon between NH4+ and DIP. This process effectively increases the ratio of DIN to DIP in the hypolimnion, thereby significantly reducing the potential of algal blooms caused by the upward flux. A robust positive linear correlation between iron-manganese bound phosphorus (CBD-P) and particulate Mn was observed during stratification period implying that DIP was scavenged by sediment-released Mn throughout the water column. Vertical profiles during stratification showed that upward diffusion of Mn2+ facilitated the formation of Mn oxide zones near the oxycline. The most significant decrease in DIP inventory occurs when Mn oxide zones migrate either upwards from the bottom or downwards from the oxycline, indicating that the migration of Mn oxides on the vertical profile is a key factor in the decoupling of NH4+and DIP.Our findings underscore the importance of Mn cycling as an underappreciated DIP self-immobilization process in the water column of reservoirs characterized by high nutrient loading. Furthermore, we propose that denitrification and Mn cycling establish a consecutive feedback mechanism, preventing excessive nutrient accumulation in low oxygen bottom water. In the context of global changes, we anticipate a heightened prominence of this feedback mechanism.

A pilot study of Mn redox driven nitrogen removal in municipal tailwater: Nitrogen metabolic pathway and electron transport mechanism

Microbial-mediated manganese (Mn) cycle coupled with nitrogen removal in municipal tailwater has received increasing attention. Herein, this study successfully extended the application of this technique to an 800 L biofilter, achieving a daily capacity of 2.4 m3 for treating municipal tailwater with a low C/N ratio. The operation results over 180 days indicated that the MnOx biofilter performed more efficiently than gravel control biofilter, achieving ∼ 40.2 % for nitrate and ∼ 35.9 % for total nitrogen (TN) removal efficiencies, respectively, under the conditions of hydraulic retention time (HRT) of 4 ∼ 5 h and low temperature (6.3–10.5 °C). To better elucidate the potential mechanisms, various functional bacteria involved in nitrogen and Mn transformation were identified, including heterotrophic denitrifying bacteria, such as Denitratisoma, Thauera, and Anaerolineae (cumulative relative abundance: 16.94–23.14 % in MnOx system vs. 9.86–11.90 % in control system) and Mn(II) oxidizing denitrifying bacteria, such as Pseudomonas and Acinetobacter (12.90–18.13 % vs. 3.59–4.64 %). Furthermore, metagenomic analysis indicated that the Mn cycle enhanced the expression of genes responsible for electron generation in organics/Mn metabolic pathways (e.g. TCA cycle and Mn oxidation), promoted electron transfer by enriching the Mn(II)-oxidizing denitrifying bacteria, and improved the abundance of genes associated with electron utilization (e.g. nitrate and nitrite reduction). The enhancement of the electron generation-transport-utilization due to Mn circulation contributed to efficient nitrate removal. This study provides further guidance of large-scale application and expands the understanding of the nitrogen removal mechanism in the Mn cycle-mediated process in municipal tailwater.

Biogeochemical mechanisms of iron (Fe) and manganese (Mn) in groundwater and soil profiles in the Zhongning section of the Weining Plain (northwest China)

High levels of Iron (Fe) and manganese (Mn) in soils may contribute to secondary contamination of groundwater. However, there is limited understanding of the cycling mechanisms of Fe and Mn in groundwater and soil. This study aimed to investigate the biogeochemical processes constituting the Fe and Mn cycle by combining hydrochemistry, sequential extraction and microbiological techniques. The results indicated a similar vertical distribution pattern of Fe and Mn, with lower levels of the effective form (EFC-Fe/Mn) observed at the oxygenated surface, increasing near the groundwater table and decreasing below it. Generally, there was a tendency for accumulation above the water table, with Mn exhibiting a higher release potential compared to Fe. Iron‑manganese oxides (Ox-Fe/Mn) dominated the effective forms, with Fe and Mn in the soil entering groundwater through the reduction dissolution of Ox-Fe/Mn and the oxidative degradation of organic matter or sulfide (OM-Fe/Mn). Correlation analysis revealed that Fe and Mn tend to accumulate in media with fine particles and high organic carbon (TOC) contents. 16S rRNA sequencing analysis disclosed significant variation in the abundance of microorganisms associated with Fe and Mn transformations among unsaturated zone soils, saturated zone media and groundwater, with Fe/Mn content exerting an influence on microbial communities. Furthermore, functional bacterial identification results from the FAPROTAX database show a higher abundance of iron-oxidizing bacteria (9.3 %) in groundwater, while iron and manganese-reducing bacteria are scarce in both groundwater and soil environments. Finally, a conceptual model of Fe and Mn cycling was constructed, elucidating the biogeochemical processes in groundwater and soil environments. This study provides a new perspective for a deeper understanding of the environmental fate of Fe and Mn, which is crucial for mitigating Fe and Mn pollution in groundwater.

Synergistic denitrification and Mn cycle driven by sulfur autotrophy significantly enhanced nitrogen removal

Due to the low C/N and high NO3−-N content in tailwater of wastewater treatment plants, the need for further denitrification treatment attracted wide attention. To achieve deep denitrification of tailwater with low C/N and high NO3−-N, a novel sulfur-manganese carbonate ore denitrification (SMCD) biological reactor was constructed. The denitrification performance of SMCD reactor was significantly higher than that of sulfur denitrification and manganese carbonate ore denitrification reactors. Across the study, the removal efficiencies of TN and NO3−-N were above 90%, and without NO2−-N accumulation in the SMCD reactor. Quantitative results of functional genes showed that SMCD reactor were conducive to realize complete denitrification. Besides, manganese autotrophic denitrification (MAD) process occurred in the SMCD reactor. By analyzing the nitrogen as well as SO42- and Mn(Ⅱ) concentrations along the height, it was found that the conversion between Mn(II) and Mn(IV) was stable during Stage Ⅲ (C/N=1). The microbiota in the SMCD reactor was distinctly different from that in the controls. Thiobacillus, Thermomonas, and an unclassified member in family cvE6 were the dominant members of SMCD reactor. Besides, Thermomonas, Dokdonella, and Simplicispira were identified as potential Mn oxidizers. The SMCD reactor exhibited a significant synergistic denitrification effect by coupling sulfur autotrophic denitrification and MAD process, which provided an effective solution for the deep denitrification of low C/N tailwater.

Effects of wind-driven current and thermal dynamics in a temperate monomictic reservoir: Implications for manganese transport and treatment in water supply systems

Increasing manganese (Mn) concentrations in source water contribute to aesthetic and health-related concerns in drinking water. The challenges with Mn in drinking water primarily arise from elevated Mn concentrations in the water supply reservoir, with the inefficacy of Mn treatment largely attributed to fluctuating Mn levels in the water source. A three-dimensional Mn cycle model in a temperate monomictic reservoir, Tarago Reservoir, and a decision support system reflecting Mn variations in the local water treatment plant have been established in previous research. This study aimed to examine Mn variations from the reservoir to raw water and treated water under the influence of wind conditions during different stages of thermal structure, and discover valuable recommendations for Mn treatment in the local water supply system. We crafted 12 scenarios to scrutinize the impact of varying intensities of offshore and onshore winds on hydrodynamic processes and Mn transport during strong thermal stratification, weak thermal stratification, and turnover. The scenario analysis revealed that, during the gradual weakening of thermal stratification, offshore wind induced a substantial amount of Mn to the upper layers near the water intake point. Conversely, onshore wind hindered the upward transport of Mn. The simulated Mn in the raw water under the 12 scenarios indicated that the timing of turnover in the Tarago Reservoir is the primary concern for Mn treatment in the water treatment plant. Additionally, close attention should be given to the frequency and intensity of offshore winds during the weakening of thermal stratification.

Superoxide radical-induced rapid valence cycling of Mn(II/III/IV) for efficient degradation of organic pollutants

Transition metal oxides are one of the commonly used catalysts for ozone oxidation, and the slow redox cycling of different valence metal components inside the catalysts currently becomes a key bottleneck limiting their catalytic activity. In this work, the superoxide radicals generated at the N/C sites were used to reduce the neighboring high-valence metal components to low-valence states to construct a new mechanism for the rapid cyclic transition of Mn (II/III/IV) valence states. What's more, N/C sites enhanced the ozone adsorption and conversion ability, synergized with the adjacent MnO2 sites to generate interfacial hydroxyl radicals and superoxide radicals, and effectively inhibited the quenching effect of inorganic salt ions (e.g., Cl-, SO42-) on the free radicals. For the actual printing and dyeing wastewater, the COD (chemical oxygen demand) removal rate was as high as 70 % in 60 min and remained stable in 10 cycles with almost no manganese ions leaching. In addition, characterization such as EPR combined with DFT calculations showed that the dual-site structure significantly increased the oxygen vacancy content and Lewis acidity, and the synergistic interaction between graphite N and MnO2 was the most favorable for ozone conversion with the lowest reaction energy barriers.