Mn oxides changed nitrogen removal process in constructed wetlands with a microbial electrolysis cell

Mn-oxidizing microalgae and woodchip-denitrifying bioreactor system for recovering manganese and removing nitrogen from electrolytic manganese metal industrial tailwater

Excess manganese (Mn) and NH4+-N emissions from electrolytic manganese metal industrial tailwater may harm the environment. However, previous studies have not combined Mn-oxidizing microalgae to reclaim Mn with woodchip substrates for nitrogen removal from tailwater. Here, a two-stage bioreactor system was constructed to recover Mn by microalgal-mediated bio-oxidation in an algae reactor (AR) and remove nitrogen by denitrification in a woodchip reactor (WR). The results showed that up to 100 % of Mn2+ in the tailwater was removed after a 3-day incubation period. The maximum amount of biogenerated Mn oxide nanoparticles reached 13.34 mg/L with Mn4+ as the main Mn valence. Mn recovery reached 65.69 % through precipitate collection, and the NH4+-N removal efficiency reached 97 % in the AR. Mn oxidation by algae might promote oxidative removal of NH4+-N. NO3--N and total nitrogen removal efficiencies in the WR reached 82-90 % and 65-87 %, respectively, which was attributed to denitrification. The predominance of the denitrification gene narG in the WR may have driven the efficient nitrate removal. Flavobacterium, Acidovora, Massilia, Arcticibacter, and Acinetobacter were the most abundant genera in the WR and represented dominant denitrifying bacteria in the woodchip microbiome, indicating their important contribution to denitrification. Overall, the combined application of Mn-oxidizing algae and woodchip-denitrifying bioreactors may represent an efficient treatment technology for electrolytic manganese wastewater remediation.

Sustained Tl(I) removal by α-MnO2: Dual role of tunnel structure incorporation and surface catalytic oxidation

Manganese oxide-based filtration technologies are considered cost-effective for thallium (Tl) removal in engineered systems. However, current gaps in understanding the heterogeneous adsorption and oxidation mechanisms of typical tunneled α-MnO2 may lead to a serious underestimation of its long-term Tl removal potential. In this study, α-MnO2 could continuously remove Tl(I) during the 584-h reaction, with its irreversible removal eventually increasing to 81 %-95 % in different anionic environments. The adsorbed low-loaded Tl(I) is preferentially oxidized, whereas the high-loaded Tl tends to be adsorbed in a nonoxidative pathway by α-MnO2. The nonoxidized Tl(I) was gradually immobilized in the stable thalliomelane-like tunnel structure. More importantly, the synergism of surface Mn(III)-oxygen vacancies (Ov) on α-MnO2 could catalyze the oxidation of Tl(I). Furthermore, the oxidized Tl(III) was bound to the tunnel surface via double edge-sharing and double corner-sharing. In addition, the phosphate anion occupied the surface active site and inhibited the oxidation of Tl(I), thereby reducing the binding strength of Tl. This study provides a new perspective on the effectiveness and stability of Tl(I) removal by MnO2 and highlights the neglected mechanism of Mn(III)-Ov mediating Tl(I) oxidation, which expands our understanding of the removal and transformation fate of Tl in MnO2-engineered systems.

Microbial manganese redox cycling drives co-removal of nitrate and ammonium

Manganese (Mn), abundant in the Earth's crust, can act as an oxidant or a reductant for diverse nitrogen biotransformation processes. However, the functional microorganisms and their metabolic pathways, as well as interactions, remain largely elusive. Here, a microbial consortium was enriched from a mixture of freshwater sediments and activated sludge by feeding ammonium, nitrate and Mn(II), which established manganese-driven co-removal of nitrate and ammonium with removal rates of 5.83 and 2.30 mg N L-1 d-1, respectively. The batch tests and metagenomic analyses revealed a nitrate-dependent anaerobic manganese oxidation (NDMO) process mediated by Anaerolineales and Phycisphaerales and a manganese reduction coupled to anaerobic ammonium oxidation (Mnammox) process mediated by Chthonomonadales. Based on identified key genes involved in the nitrogen and manganese metabolic pathways, nitrate was likely reduced to nitrite and nitrogen gas in the NDMO process while ammonium was oxidized to nitrite in the Mnammox process, which in turn fuelled the Anammox process carried out by Candidatus Brocadia. This revealed the microbial interactions of NDMO, Mnammox, and Anammox processes responsible for manganese-driven co-removal of ammonium and nitrate. These findings provide a potential solution for biological nitrogen removal and expand our understanding of the nitrogen and manganese biogeochemical cycles.

Integration of manganese ores with activated carbon into constructed wetland for greenhouse gas emissions reduction

Manganese oxide and activated carbon (AC) are widely employed in constructed wetlands (CWs) to remove nutrients and reduce greenhouse gas (GHG) emissions, however, the effect and mechanism of AC combined with manganese ores (MO) on GHG emissions remain unclear. In this study, the mechanisms of nutrient removal and GHG emissions reduction were investigated by three vertical subsurface-flow CWs: gravel (CW-B), manganese ores (MO) uniformly mixing with gravel (CW-M), or activated carbon (CW-MC). The average removal efficiencies of chemical oxygen demand, total nitrogen and total phosphorus in CW-MC were markedly improved compared to CW-B and CW-M, reaching 82.72%, 95.72% and 93.43%, respectively. Moreover, the global warming potential (CO2 equivalent) of CW-MC was reduced by 52.80% and 36.88% relative to CW-B and CW-M, respectively. Mixing of MO with AC reduced the loss of manganese and further enhanced the manganese cycling process by X-ray photoelectron spectroscope and concentration of Mn(Ⅱ) in CWs analysis. The introduction of MO and AC enhanced the PN/PS ratio of extracellular polymeric substances and facilitated extracellular electron transfer (EET). Furthermore, metagenomic analysis showed that the abundances of denitrifying, manganese oxidizing and electroactive bacteria genera were enhanced in the CW-MC, which promoted the transformation of nitrogen and manganese. Meanwhile, high abundances of denitrification and EET related genes were observed in CW-MC, improving denitrification efficiency and reducing N2O emission. This study elucidated the impacts and mechanisms of MO and AC on GHG emissions, providing a new insight to improve manganese-based CW performance.

Fulvic acid impact on constructed wetland-microbial electrolysis cell system performance: Metagenomic insights

This study explores the roles of fulvic acid (FA) in both a conventionally constructed wetland (CCW) and a newly constructed wetland-microbial electrolysis cell (ECW). The results showed that FA increased the average removal efficiency of chemical oxygen demand, total phosphorus, total nitrogen, and ammonia nitrogen in ECW by 8.6, 46.2, 33.0, and 27.9 %, respectively, compared to CCW, and reduced the global warming potential by > 60 %. FA promoted the proliferation of electroactive bacteria (e.g., Chlorobaculum and Candidatus Tenderia) and FA-degrading bacteria (e.g., Anaerolineaceae and Gammaproteobacteria) and reduced methanogens (e.g., Methanothrix) via type-changing. The study's findings suggest that FA influences pollutant removal and microbiome dynamics by altering dissolved oxygen levels and redox potential. In summary, FA and ECW enhanced the efficiency of constructed wetlands by facilitating electron transfer and consumption, and supporting microbial growth and metabolism.

Structure and molecular-level transformation for oxidation of effluent organic matters by manganese oxides

As important organic components in water environments, effluent organic matters (EfOMs) from wastewater treatment plants are widely present in Mn-rich environments or engineered treatment systems. The redox interaction between manganese oxides (MnOx) and EfOMs can lead to their structural changes, which are crucial for ensuring the safety of water environments. Herein, the reactivities of MnOx with EfOMs were evaluated, and it was found that MnOx with high specific surface area, active high-valent manganese content and lattice oxygen content (i.e., amorphous MnO2) possessed stronger oxidizing ability towards EfOMs. Accompanying by EfOMs oxidation, Mn(IV) and Mn(III) were reduced into Mn(II), with Mn(III) as the significant active species. Through molecular-level transformation analysis by ultrahigh mass spectrometry (FT-ICR MS), the highly reactive compounds in EfOMs were clearly determined to be that with more aromatic and unsaturated structures, especially lignin-like compounds (the highest content in EfOMs (over 60 %)). EfOMs were oxidized by amorphous MnO2 into products with lower humification index (0.60 vs. 0.46), smaller apparent molecular weight (386.94 Da vs. 368.68 Da), and higher biodegradability (BOD5/COD: 0.12 vs. 0.78). This finding suggested that redox reactions between MnOx and EfOMs might alter their abiotic and biotic behaviors in receiving water environments.

Characteristic and mechanism of biological nitrogen and phosphorus removal facilitated by biogenic manganese oxides (BioMnOx) at various concentrations of Mn(II)

Biogenic manganese oxides (BioMnOx) have attracted considerable attention as active oxidants, adsorbents, and catalysts. However, characteristics and mechanisms of nitrification-denitrification in biological redox reactions mediated by different concentrations of BioMnOx are still unclear. Fate of nutrients (e.g., NH4+-N, TP, NO3--N) and COD were investigated through different concentrations of BioMnOx produced by Mn(II) in the moving bed biofilm reactor (MBBR). 34% and 89.2%, 37.8% and 89.8%, 57.3% and 88.9%, and 62.1% and 90.4% of TN and COD by MBBR were synchronously removed in four phases, respectively. The result suggested that Mn(II) significantly improved the performance of simultaneous nitrification and denitrification (SND) and TP removal based on manganese (Mn) redox cycling. Characteristics of glutathione peroxidase (GSH-Px), reactive oxygen species (ROS), and electron transfer system activity (ETSA) were discussed, demonstrating that ROS accumulation reduced the ETSA and GSH-Px activities when Mn(II) concentration increased. Extracellular polymeric substance (EPS) function and metabolic pathway of Mn(II) were explored. Furthermore, effect of cellular components on denitrification was evaluated including BioMnOx performances, indicating that Mn(II) promoted the non-enzymatic action of cell fragments. Finally, mechanism of nitrification and denitrification, denitrifying phosphorus and Mn removal was further elucidated through X-ray photoelectron spectroscopy (XPS), high throughput sequencing, and fourier transform infrared reflection (FTIR). This results can bringing new vision for controlling nutrient pollution in redox process of Mn(II).

Current progress on manganese in constructed wetlands: Bibliometrics, effects on wastewater treatment, and plant uptake

Constructed wetlands (CWs) are a pollutant treatment design inspired by natural wetlands and are widely utilized for the removal of common pollutants. The research focus lies in the circulation of manganese (Mn) in the environment to enhance pollutant removal within CWs. This paper provides a comprehensive review of recent advancements in understanding the role and effects of Mn in chemical weapons, based on literature retrieval from 2002 to 2021. Ecological risk assessment and heavy metals within CWs emerge as current areas of research interest. Mn sources within CWs primarily include natural deposition, heavy metal wastewater, and intentional addition. The cycling between Mn(II) and Mn(IV) facilitates enhanced wastewater treatment within CWs. Moreover, employing a Mn matrix proves effective in reducing ammonia nitrogen wastewater, organic pollutants, as well as heavy metals such as Cd and Pb, thereby addressing complex pollution challenges practically. To comprehensively analyze influencing factors on the system's performance, both internal factors (biological species, design parameters, pH levels, etc.) and external factors (seasonal climate variations, precipitation patterns, ultraviolet radiation exposure, etc.) were discussed. Among these factors, microorganisms, pollutants, and temperature are the most important influencing factors, which emphasizes the importance of these factors for wetland operation. Lastly, this paper delves into plant absorption of Mn along with coping strategies employed by plants when faced with Mn poisoning or deficiency scenarios. When utilizing Mn for the regulation of constructed wetlands, it is crucial to consider the tolerance levels of associated plant species. Furthermore, the study predicts future research hotspots encompass high-efficiency catalysis techniques, matrix-filling approaches, and preparation of resource utilization methods involving Mn nanomaterials.

New insights for simultaneous nutrient removal enhancement and greenhouse gas emissions reduction of constructed wetland by optimizing its redox environment through manganese oxide addition

Manganese oxide (MnOx) is receiving increased interest in the nutrient removal of constructed wetlands (CWs); however, its service effectiveness for simultaneous greenhouse gas (GHG) emissions reduction is still vague. In this study, three vertical flow CWs, i.e., volcanics (CCW), manganese sand uniformly mixing with volcanics (Mn-CW) and MnOx doped volcanics (MnV-CW), were constructed to investigate the underlying mechanisms of MnOx on nutrient removal enhancement and greenhouse gas (GHG) emissions reduction. The results showed that the MnOx doped volcanics optimized the oxidation-reduction potential surrounding the substrate (-164.0 ∼ +141.1 mv), and resulted in the lowest GHG emissions (CO2-equivalent) from MnV-CW, 16.8-36.5 % lower than that of Mn-CW and CCW. This was mainly ascribed to mitigation of N2O produced during the NO3--N reduction process, according to results of 15N stable isotope labeling. Analysis of the microbial community structure revealed that due to the optimized redox conditions through chemical doping of MnOx on volcanics, the abundance of microbe involved in denitrification and Mn-oxidizing process in the MnV-CW was significantly increased at genus level, which led to a higher Mn cycling efficiency between biogenic MnOx and Mn2+, and enhanced denitrification efficiency and N2O emission reduction. This study would help to understand and provide a preferable reference for future applications for manganese-based CW.

Anodic ammonium oxidation in microbial electrolysis cell: Towards nitrogen removal in low C/N environment

Biological nitrogen removal in low C/N environment is challenging in wastewater treatment for a long time. Autotrophic ammonium oxidation is promising due to the no need of carbon source addition, but alternative electron acceptors other than oxygen has to be widely investigated. Recently, microbial electrolysis cell (MEC), which applies a polarized inert electrode as the electron harvester, has been proved effective to oxidize ammonium with electroactive biofilm. That is, anodic microbes stimulated by exogenous low power can extract electron from ammonium and transfer electron to electrodes. This review aims to consolidate the recent advances in anodic ammonium oxidation in MEC. Various technologies based on different functional microbes and mechanisms of these processes are reviewed. Thereafter, the crucial factors influencing the ammonium oxidation technology are discussed. Challenges and prospects of anodic ammonium oxidation in ammonium-containing wastewater treatment are also proposed to provide valuable insights on the technologic reference and potential value of MEC in ammonium-containing wastewater treatment.

Application of manganese oxides in wastewater treatment: Biogeochemical Mn cycling driven by bacteria

Manganese oxides (MnOx) are recognized as a strongest oxidant and adsorbent, of which composites have been proved to be effective in the removal of contaminants from wastewater. This review provides a comprehensive analysis of Mn biochemistry in water environment including Mn oxidation and Mn reduction. The recent research on the application of MnOx in the wastewater treatment was summarized, including the involvement of organic micropollutant degradation, the transformation of nitrogen and phosphorus, the fate of sulfur and the methane mitigation. In addition to the adsorption capacity, the Mn cycling mediated by Mn(II) oxidizing bacteria and Mn(IV) reducing bacteria is the driving force for the MnOx utilization. The common category, characteristics and functions of Mn microorganisms in recent studies were also reviewed. Finally, the discussion on the influence factors, microbial response, reaction mechanism and potential risk of MnOx application in pollutants' transformation were proposed, which might be the promising opportunities for the future investigation of MnOx application in wastewater treatment.

Coupled mixotrophic denitrification and utilization of refractory organics driven by Mn redox circulation for significantly enhanced nitrogen removal

Coupled mixotrophic denitrification and degradation of organics driven by redox transition of Mn for nitrogen removal has attracted much attention. Herein, this study explored the removal performance and mechanisms for nitrogen and refractory organics from secondary effluent in up-flow MnOx biofilter. Results showed that the removal of organics and nitrate was significantly enhanced by the synergistic process of heterotrophic denitrification and Mn(II)-driven autotrophic denitrification (MnAD), which were originated from the facilitation of Mn circulation. But nitrate removal was closely related to the types of carbon source and Mn(II) concentration. Single small molecular carbon source (glucose) performed better than mixed carbon source (humic acid and glucose) in nitrate removal process (74.8% in stage 1-2 vs. 54.1% in stage 3-5). And raising external Mn(II) concentration increased the contribution of MnAD (60.2% in stage 5 vs. 46.5% in stage 3) to nitrate removal. Furthermore, the relationship between Mn/N transformation and microbial community structure shifts revealed that the redox transition between Mn(II) and Mn(IV) promoted the enrichment of denitrogenation bacteria and functional genes, thus contributing to pollutants removal. Our studies expand the knowledge of MnOx-mediated pollutants removal processes and support the potential application of MnOx for removal of residual refractory organics and nitrogen.

A review: Manganese-driven bioprocess for simultaneous removal of nitrogen and organic contaminants from polluted waters

Water pollutants, such as nitrate and organics have received much attention for their harms to ecological environment and human health. The redox transformation between Mn(Ⅱ) and Mn(Ⅳ) for nitrogen and organics removal have been recognized for a long time. Mn(Ⅱ) can act as inorganic electron donor to drive autotrophic denitrification so as to realize simultaneous removal of Mn(Ⅱ), nitrate and organic pollutants. Mn oxides (MnOx) also play an important role in the adsorption and degradation of some organic contaminants and they can change or create new oxidation pathways in the nitrogen cycle. Herein, this paper provides a comprehensive review of nitrogen and organic contaminants removal pathways through applying Mn(Ⅱ) or MnOx as forerunners. The main current knowledge, developments and applications, pollutants removal efficiency, as well as microbiology and biochemistry mechanisms are summarized. Also reviewed the effects of factors such as the carbon source, the environmental factors and operation conditions have on the process. Research gaps and application potential are further proposed and discussed. Overall, Mn-based biotechnology towards advanced wastewater treatment has a promising prospect, which can achieve simultaneous removal of nitrogen and organic contaminants, and minimize sludge production.

Mn oxides enhanced pyrene removal with both rhizosphere and non-rhizosphere microorganisms in subsurface flow constructed wetlands

The polycyclic aromatic hydrocarbons (PAHs) are substantial wastewater pollutants emitted mostly by petroleum refineries and petrochemical industries, and their environmental fate has been of increasing concern among the public. Consequently, subsurface flow constructed wetlands (SFCWs) filled with Mn oxides (W-CW) or without Mn oxides (K-CW) were established to investigate the performance and mechanisms of pyrene (PYR) removal. The average removal rates of PYR in W-CW and K-CW were 96.00% and 92.33%, respectively. The PYR removal via other pathways (microbial degradation, photolysis, volatilisation, etc.) occupied a sizeable proportion, while the total PYR content in K-CW plant roots was significantly higher than that of W-CW. The microorganisms on the root surface and rhizosphere played an important role in PYR degradation in W-CW and K-CW and were higher in W-CW than that in K-CW in all matrix zones. The microorganisms between the 10-16 cm zone from the bottom of W-CW filled with Mn oxides (W-16) were positively correlated with PYR-degrading microorganisms, aerobic bacteria and facultative anaerobes, whereas K-16 without birnessite-coated sand was negatively correlated with these microorganisms.

Effects of Mn2+ and humic acid on microbial community structures, functional genes for nitrogen and phosphorus removal, and heavy metal resistance genes in wastewater treatment

Considering the wide occurrence of Mn²⁺ and humic acid (HA) in environmental media, the effects of Mn²⁺ (5-16 mg/L) and HA (10 mg/L) on microbial community structures, functional genes for nitrogen and phosphorus removal, and heavy metal resistance genes (HMRGs) were investigated in wastewater treatment using sequencing batch bioreactors (SBRs). The treatment efficiencies of influent chemical oxygen demands (COD), NH₄⁺-N, and PO₄^3--P were unaffected during the entire operational processes irrespective of whether Mn²⁺ and HA were supplied. Although the functional prediction of genetic information via sequencing analysis showed that the microbial activity was not influenced by Mn²⁺ and HA from different SBRs, the abundance of dominant phyla (Proteobacteria, Actinobacteriota, Firmicutes, and Bacteroidota), classes (Saccharimonadia, Gammaproteobacteria, and Bacilli), and genera (unidentified_Chloroplast, TM7a, Micropruina, Candidatus_Competibacter, Lactobacillus, OLB12, and Pediococcus) was different. Compared to the SBR without Mn²⁺ and HA supplementation, the abundance of functional genes for nitrogen and phosphorus removal (narG, nirS, nosZ, ppk, and phoD) and HMRGs (corA and mntA) significantly increased under Mn²⁺ stress, but significantly decreased with the addition of HA except for genes nirS and ppk. The abundance of genes corA and mntA was related to the partially dominant microbes and functional genes, and might be reduced by supplying HA. This study provides insight into the effects of Mn²⁺ and HA on functional genes for nitrogen and phosphorus removal and HMRGs in wastewater treatment.

Manganese oxides in Phragmites rhizosphere accelerates ammonia oxidation in constructed wetlands

Phragmites reeds are widely used in constructed wetlands (CWs) for treating wastewater. The enrichment of microorganisms and Fe/Mn plaque in Phragmites rhizospheres largely contributes to pollutant removal. However, their interactions and potential synergistic roles in water purification are poorly understood. To address the issue, we first compared the microbial community traits in the Phragmites rhizosphere and adjacent bulk soil in six long-term operated CWs. Results showed that enriched microbes and functional genes in the Phragmites rhizosphere were largely involved in Mn oxidation, resulting in a two to three times enrichment of Mn oxides in the rhizosphere. In turn, the enriched Mn oxides played significant roles in driving microbial community composition and function. To further understand the biological manganese oxidation in the rhizosphere, we identified Mn-oxidizing bacteria using genome-centric analysis and found that 92% of identified Mn-oxidizing bacteria potentially participated in nitrogen cycling. We then conducted relationships between Mn-oxidizing genes and different nitrogen cycling genes and found Mn-oxidizing gene abundance was significantly correlated with ammonia oxidation gene amoA (R = 0.65). Remarkably, complete ammonia oxidation (comammox) Nitrospira, accounting for 39.11% of ammonia oxidizers, also positively correlated with Mn-oxidizing microbes. Based on the above observations, we inferred that the use of Mn oxides as a substrate in CWs may enhance ammonia oxidation. To apply this to actual engineering, we explored treatment performance in a pilot-scale Mn-amending CW. As expected, ammonia removal capacity improved by 23.34%, on average, in the Mn-amending CW. In addition, the abundance of amoA genes increased significantly in the Mn-amending CW, indicating improved biological processes rather than chemical reactions.

An integrated migration and transformation model to evaluate the occurrence characteristics and environmental risks of Nitrogen and phosphorus in constructed wetland

In this study, an integrated migration and transformation (IMT) model based on microbial action, plant absorption, sediment release and substrate adsorption was firstly established to evaluate the temporal-spatial distribution of N and P in Lingang hybrid constructed wetland (CW), Tianjin. Compared to the conventional transformation model that only considers the microbial action, the IMT model could accurately predict the occurrence characteristics of N and P. In Lingang CW, NO₃^--N (0.56-3.63 mg/L) was the most important form of N, and the TP was at a relatively low concentration level (0.04-0.07 mg/L). The spatial distribution results showed that a certain amount of N and P could be removed by CW. Form the temporal perspective, the N and P concentrations were greatly affected by the dissolved oxygen (DO). The simulated values obtained by IMT model indicated that the distribution of N and P was more affected by the temporality compared with the spatiality, which was consistent with measured values. Besides, the PCA indicated that TN, NO₃^--N and DO were important factors, which affected the water quality of CW. The Nemerow pollution index method based on the simulated values indicated that Lingang CW was overall moderately polluted, and the subsurface area was the main functional unit of pollutants removal in CW. This work provides a new model for accurately predicting the occurrence characteristics of N and P pollutants in CW, which is of great significance for identifying its environmental risks and optimizing the construction of wetlands.