Effects of the addition of nitrogen and phosphorus on anaerobic ammonium oxidation coupled with iron reduction (Feammox) in the farmland soils

High-level nitrogen removal achieved by Feammox-based autotrophic nitrogen conversion

Anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox) is an essential process in the geochemical iron and nitrogen cycling. This study explores Feammox-based nitrogen removal in a continuous laboratory up-flow bioreactor stimulated by intermittently adding 5 mM Fe(OH)3 at intervals of approximately two months. The feed was synthetic wastewater with a relatively low ammonium concentration (∼100 mg N/L), yet without organic carbon in order to test its autotrophic nitrogen removal performance. The operation of this system showed the achievement of high-level ammonium and total nitrogen removal efficiency (∼97% and ∼90% on average, respectively) within four months of operation, along with a relatively practical rate of ∼50 mg N/(L·d). The demand of Fe(Ⅲ) for ammonium removal during the whole bioreactor operation was estimated to be only 0.033, two orders of magnitude less than that calculated based on the Feammox reaction producing nitrogen gas. A series of assays on Fe(II) oxidation with different oxidants (O2, NO2 - and NO3 -) in abiotic and biotic batch tests further revealed an important role of Fe(II) oxidation processes, likely driven by microbial nitrate reduction and chemical oxygen reduction, in assisting the regeneration of Fe(III) for continuous Feammox-based nitrogen removal. This work demonstrates that Feammox-based autotrophic nitrogen conversion is a potential option for future wastewater treatment.

Enhanced ammonium oxidation and iron cycle of Feammox under micro-oxygen condition

Autotrophic anaerobic ammonium oxidation coupled to Fe(III) reduction (Feammox) is a promising technology for treating low C/N wastewater. However, Feammox still faces bottlenecks of slow ammonium oxidation rate and the continuous supply of Fe(III) source. This study adopts micro-oxygen strategy to overcome these obstacles. Micro-oxygen increased the ammonium oxidation rate up to 5.7 times higher than under anaerobic condition, and drove the iron cycle in the form of vivianite [Fe(II)] and leucophosphite [Fe(III)]. Furthermore, it was confirmed that the ammonium oxidation in Feammox relies on ammonia monooxygenase (AMO), as evidenced by 10 times increase in the relative amo expression and 1.2 times increase in AMO activity under micro-oxygen compared to anaerobic condition. Additionally, this approach enhanced the growth and co-metabolism of functional bacteria. Long-term experiments demonstrated the sustainability of the Feammox system with iron cycle under micro-oxygen condition. These findings provide valuable insights into the practical application of Feammox process.

Novel insights into Feammox coupled with the NDFO: A critical review

Ammonium oxidation coupled with Fe(III) reduction, known as Feammox, and nitrate-dependent ferrous oxidation (NDFO) are two processes that can be synergistically achieved through the Fe(III)/Fe(II) cycle. This integrated approach enables the simultaneous removal of ammonia nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) from wastewater, representing a novel method for complete nitrogen removal. This study presents a systematic and exhaustive examination of the Feammox-NDFO coupled process. An initial thorough exploration of the underlying mechanisms behind the coupling process is conducted, highlighting how the Fe(III)/Fe(II) cycle enables the concurrent occurrence of these reactions. Further, the functional microorganisms associated with and playing a crucial role in the Feammox-NDFO process are summarized. Next, the key influencing factors that govern the efficiency of the Feammox-NDFO process are explored. These include parameters such as pH, temperature, carbon source, iron source, nitrogen source, and various electron shuttles that may mediate electron transfer. Understanding the impact of these factors is essential for optimizing the process. The most recent trends and endeavors on the Feammox-NDFO coupling technology in wastewater treatment applications are also examined. This includes examining both laboratory-scale studies and field trials, highlighting their successes and challenges. Finally, an outlook is presented regarding the future advancement of the Feammox-NDFO technology. Areas of improvement and novel strategies that could further enhance the efficiency of simultaneous nitrogen removal from the iron cycle are discussed. In summary, this study aspires to offer a thorough comprehension of the Feammox-NDFO coupled process, with a focus on its mechanisms, influencing factors, applications, and prospects. It is anticipated to yield invaluable insights for the advancement of process optimization, thus sparking fresh ideas and strategies aimed at accomplishing the thorough elimination of nitrogen from wastewater via the iron cycle.

Evaluating the impact of microorganisms in the iron plaque and rhizosphere soils of Spartina alterniflora and Suaeda salsa on the migration of arsenic in a coastal tidal flat wetland in China

The microorganism in rhizosphere systems has the potential to regulate the migration of arsenic (As) in coastal tidal flat wetlands. This study investigates the microbial community in the iron plaque and rhizosphere soils of Spartina alterniflora (S. alterniflora) and Suaeda salsa (S. salsa), as two common coastal tidal flat wetland plants in China, and determines the impact of the As and Fe redox bacteria on As mobility using field sampling and 16S rDNA high-throughput sequencing. The results indicated that As bound to crystalline Fe in the Fe plaque of S. salsa in high tidal flat. In the Fe plaque, there was a decrease in the presence of Fe redox bacteria, while the presence of As redox bacteria increased. Thus, the formation of Fe plaque proved advantageous in promoting the growth of As redox bacteria, thereby aiding in the mobility of As from rhizosphere soils to the Fe plaque. As content in the Fe plaque and rhizosphere soils of S. alterniflora was found to be higher than that of S. salsa. In the Fe plaque, As/Fe-reducing bacteria in S. alterniflora, and As/Fe-oxidizing bacteria in S. salsa significantly affected the distribution of As in rhizosphere systems. S. alterniflora has the potential to be utilized for wetland remediation purposes.

Asynchronous characteristics of Feammox and iron reduction from paddy soils in Southern China

Recently, the newly discovered anaerobic ammonium oxidation coupled with iron reduction (i.e., Feammox) has been proven to be a widespread nitrogen (N) loss pathway in ecosystems and has an essential contribution to gaseous N loss in paddy soil. However, the mechanism of iron-nitrogen coupling transformation and the role of iron-reducing bacteria (IRB) in Feammox were poorly understood. This study investigated the Feammox and iron reduction changes and microbial community evolution in a long-term anaerobic incubation by 15N isotope labeling combined with molecular biological techniques. The average rates of Feammox and iron reduction during the whole incubation were 0.25 ± 0.04 μg N g-1 d-1 and 40.58 ± 3.28 μg Fe g-1 d-1, respectively. High iron oxide content increased the Feammox rate, but decreased the proportion of Feammox-N2 in three Feammox pathways. RBG-13-54-9, Brevundimonas, and Pelomonas played a vital role in the evolution of microbial communities. The characteristics of asynchronous changes between Feammox and iron reduction were found through long-term incubation. IRB might not be the key species directly driving Feammox, and it is necessary to reevaluate the role of IRB in Feammox process.

Hydrogel bioreactor drives Feammox and synergistically removes composite pollutants: Performance optimization, microbial communities and functional genetic differences

Mixed pollutant wastewater has been a difficult problem due to the high toxicity of water bodies and the difficulty of treatment. Rice husk biochar modified with nano-iron tetroxide (RBC-nFe3O4) by polyvinyl alcohol cross-linking internal doping was used to introduce iron-reducing bacteria Klebsiella sp. FC61 to construct a bioreactor. The results of the long-term operation of the bioreactor showed that the removal efficiency of ammonia nitrogen (NH4+-N) and chemical oxygen demand best reached 90.18 and 98.49%, respectively. In addition, in the co-presence of Ni2+, Cd2+, and ciprofloxacin, the bioreactor was still able to remove pollutants efficiently by RBC-nFe3O4 and bio-iron precipitation inside the biocarrier. During the long-term operation, Klebsiella was always the dominant species in the bioreactor. And the sequencing data for functional prediction showed that the biocarrier contained a variety of enzymes and proteins involved in Feammox-related activities to ensure the stable and efficient operation of the bioreactor.

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 phosphorus amendments reduces metal uptake and increases yield of Oryza saliva L. (rice) in Cd/Cu-contaminated paddy field

To alleviate worldwide food safety issues caused by metal contamination, an easily available material is urgently needed for extensive application. In this study, calcium magnesium phosphate fertiliser (Pcm) was applied to a Cd/Cu co-contaminated paddy field in comparison with limestone and organic fertiliser. The results showed that only Pcm is effective in simultaneously reducing Cd uptake by 56.7% and Cu uptake by 36.2% in Oryza saliva L. (rice). The rice yield, reduced mainly by Cu, also increased by 30.1% with respect to the enhancement of soil pH, cation exchange capacity and availability of phosphorus, as well as the reduction in availabilities of Cd and Cu. Additionally, Pcm dramatically shaped the bacterial community structure, with Proteobacteria and Firmicutes predominant in the soils. The beneficial genera Exiguobacterium, Citrobacter, and Acinetobacter, which are vital for phosphate dissolution and Cd/Cu immobilisation, were also enriched. The results demonstrated that the application of Pcm at 0.4% (w:w) was able to enhance both crop quantity and quality in Cd/Cu co-contaminated paddy fields by reducing Cu/Cd availability, promoting rice yield, and reshaping bacterial community structures.

Integration of Microbial Transformation Mechanism of Polyphosphate Accumulation and Sulfur Cycle in Subtropical Marine Mangrove Ecosystems with Spartina alterniflora Invasion

Excessive phosphorus can lead to eutrophication in marine and coastal ecosystems. Sulfur metabolism-associated microorganisms stimulate biological phosphorous removal. However, the integrating co-biotransformation mechanism of phosphorus and sulfur in subtropical marine mangrove ecosystems with Spartina alterniflora invasion is poorly understood. In this study, an ecological model of the coupling biotransformation of sulfur and phosphorus is constructed using metagenomic analysis and quantitative polymerase chain reaction strategies. Phylogenetic analysis profiling, a distinctive microbiome with high frequencies of Gammaproteobacteria and Deltaproteobacteria, appears to be an adaptive characteristic of microbial structures in subtropical mangrove ecosystems. Functional analysis reveals that the levels of sulfate reduction, sulfur oxidation, and poly-phosphate (Poly-P) aggregation decrease with increasing depth. However, at depths of 25-50 cm in the mangrove ecosystems with S. alterniflora invasion, the abundance of sulfate reduction genes, sulfur oxidation genes, and polyphosphate kinase (ppk) significantly increased. A strong positive correlation was found among ppk, sulfate reduction, sulfur oxidation, and sulfur metabolizing microorganisms, and the content of sulfide was significantly and positively correlated with the abundance of ppk. Further microbial identification suggested that Desulfobacterales, Anaerolineales, and Chromatiales potentially drove the coupling biotransformation of phosphorus and sulfur cycling. In particular, Desulfobacterales exhibited dominance in the microbial community structure. Our findings provided insights into the simultaneous co-biotransformation of phosphorus and sulfur bioconversions in subtropical marine mangrove ecosystems with S. alterniflora invasion.

Tracing Fe cycle isotopically in soils based on different land uses: Insight from a typical karst catchment, Southwest China

Iron (Fe) isotopes can effectively unveil the Fe cycle mechanisms under redox and biological conditions during the weathering and pedogenic processes. Fe contents and Fe isotope compositions (defined as δ⁵⁶Fe) in the soil profiles under secondary forest land, abandoned cropland and shrubland were investigated in a typical karst area in Southwest China. The results showed that the Fe content ranged from 23.92 to 38.56 g/kg, 21.92 to 33.02 g/kg and 12.98 to 27.93 g/kg, and the δ⁵⁶Fe levels varied from -0.48 ‰ to 0.21 ‰, -0.24 ‰ to 0.11 ‰ and - 0.11 ‰ to 0.16 ‰ from the secondary forest land, abandoned cropland and shrubland, respectively. The correlation analysis results showed that Fe transportation and isotopic fractionation were regulated by the redox processes through soil pH and soil organic matter (SOM) in the abandoned cropland and shrubland. Heavier Fe isotope may be accumulated in the deeper soil of secondary forest land due to Fe-oxide precipitation. The Fe isotope fractionations were greatly altered by soil organic carbon (SOC) in surface soils due to negative surface charges. Soil pH also plays a key role in enriching lighter Fe in a medium-acidic environment (shrubland) by ligand-controlled dissolution and reductive dissolution. Long-term cultivation in abandoned cropland and grazing in shrubland reshaped the Fe cycle in soil profiles by changing soil pH and SOC contents. However, the similar values of δ⁵⁶Fe in different land use soils indicated that the agricultural activities have no significant impact on the Fe transformation in karst soil ecosystems. The land utilization is reasonable in the Yinjiang County. This study provided effective data and insightful analysis to understand the Fe cycle processes in the karst soils under varied land uses.

Diversity distribution and characteristics of comammox in different ecosystems

The discovery of complete ammonia oxidizers (comammox), which can oxidize ammonia into nitrate, has recently changed the concept of traditional nitrification. However, comparative studies on the analysis of comammox microbial community in different ecosystems are still scarce. In this study, the distribution and diversity of the comammox microbial community in farmlands, riparian zones, and river sediments in summer and winter were investigated by high-throughput sequencing. And the relative abundance of ammonia-oxidizing microorganisms was measured via their amoA genes of real-time quantitative polymerase chain reaction (qPCR). The relationships between ammonia oxidation microorganisms and the environmental factors were further analyzed. The abundance of comammox clade A was one order of magnitude lower than that of ammonia-oxidizing archaea (AOA) but higher than that of ammonia-oxidizing bacteria (AOB). The abundance of comammox was higher in summer than in winter and higher in farmland soils (1.81 ± 0.95 × 10⁷ copies g^-1) than in riparian zones and river sediments. Meanwhile, Candidatus Nitrospira nitrosa were the most widespread comammox in most samples (up to 86.31%), followed by Candidatus Nitrospira nitrificans, with a low abundance of Candidatus Nitrospira inopinata (lower than 0.61%). Furthermore, the abundance of comammox clade A had a significantly negative correlation with pH and NH₄⁺ concentration (P < 0.05). The study revealed the potential advantages of comammox in farmlands and may be conducive to further research on comammox in microbial nitrogen cycling.

Recent progress in applications of Feammox technology for nitrogen removal from wastewaters: A review

Feammox process is crucial for the global nitrogen cycle and has great potentials for the treatment of low COD/NH₄⁺-N wastewaters. This work provides a systematic and comprehensive overview of the Feammox process. Specifically, underlying mechanisms and functional microbes mediating the Feammox process are summarized in detail. And key influencing factors including pH, temperature, dissolved oxygen, organic carbon, source of Fe(III) as well as various electron shuttles are discussed. Additionally, recent development trends and attempts of the Feammox technology in wastewater treatment applications are reviewed, and perspectives for future development are presented. A thorough review of the recent progress in Feammox process is expected to provide valuable information for further process optimization, which is helpful to achieve a more economical operation and better nitrogen removal performance in future field applications.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

Anaerobic ammonium oxidation coupled to Fe(III) reduction: Discovery, mechanism and application prospects in wastewater treatment

Fe(III) reduction coupled with anaerobic ammonium oxidation is known as Feammox. Feammox, which was first discovered in wetland ecosystems, has the potential to be used in wastewater treatment systems due to its ability to remove ammonium. Feammox can produce N₂, NO₂^- or NO₃^- through the reduction of Fe(III) and oxidation of ammonium, which is a potential process to nitrogen loss from aquatic ecosystems and terrestrial ecosystems. The Acidimicrobiaceae sp. A6 was the first Feammox functional bacteria that was successfully isolated from wetlands. The nitrogen removal effect of Feammox can be influenced by many environmental factors, such as pH, organic matter, and different sources of Fe(III). Feammox has broad application prospects, but more exploration is needed to apply this principle to wastewater treatment. This review introduces the development, mechanism, functional microbes and factors affecting the Feammox process, and discusses its potential applications.

Spatiotemporal dynamics in soil iron affected by wetland conversion on the Sanjiang Plain

Since the 1950s, nearly 80% of wetlands in the Sanjiang Plain have been converted into paddy fields. The conversion might affect the solubility and mobility of soil iron, influencing the export of iron into the Amur River and the primary production of the Okhotsk Sea. However, information regarding long‐term studies of the spatiotemporal dynamics of soil iron after cultivation is limited. In this study, six regions, including 18 plots in the Sanjiang Plain, were selected as sampling sites covering natural wetlands and paddy fields with planting ages of 2, 5, 11, 18, and 25 years after conversion from the wetland. Samples were collected at six different depths (0–10, 10–20, 20–40, 40–60, 60–90, and 90–120 cm) analyzed for water‐soluble ferrous iron (Fe[II]), water‐soluble iron (Few), complex iron (Fep), amorphous iron oxides (Feo), free iron oxides (Fed), and total iron (Fet) and six soil physicochemical characteristics. Two years after the conversion of wetlands to rice fields led to an immediate decrease in Fe(II), Few, Fep/Fed, and Feo/Fed, while the Fep and Feo contents decreased at 5 years. Both the concentrations and stocks of soil Fet increased gradually during the first 18 years. The findings in the Sanjiang Plain suggest that the function of wetlands after conversion as a source of iron might decrease with increasing time, with potential ecological effects on the neighboring marine environment. Recently initiated wetland restoration would protect the land ecosystems in the Sanjiang Plain and promote the future sustainability of the Amur Basin.

Anaerobic-petroleum degrading bacteria: Diversity and biotechnological applications for improving coastal soil

Due to the industrial emissions and accidental spills, the critical material for modern industrial society petroleum pollution causes severe ecological damage. The prosperous oil exploitation and transportation causes the recalcitrant, hazardous, and carcinogenic sludge widespread in the coastal wetlands. The costly physicochemical-based remediation remains the secondary and inadequate treatment for the derivatives along with the tailings. Anaerobic microbial petroleum degrading biotechnology has received extensive attention for its cost acceptable, eco-friendly, and fewer health hazards. As a result of the advances in biotechnology and microbiology, the anaerobic oil-degrading bacteria have been well developing to achieve the same remediation effects with lower operating costs. This review summarizes the advantages and potential scenarios of the anaerobic degrading bacteria, such as sulfate-reducing bacteria, denitrifying bacteria, and metal-reducing bacteria in the coastal area decomposing the alkanes, alkenes, aromatic hydrocarbons, polycyclic aromatic, and related derivatives. In the future, a complete theoretical basis of microbiological biotechnology, molecular biology, and electrochemistry is necessary to make efficient and environmental-friendly use of anaerobic degradation bacteria to mineralize oil sludge organic wastes.

Nitrogen loss through denitrification, anammox and Feammox in a paddy soil

Since the process of anaerobic ammonium oxidation (anammox) coupled with ferric iron reduction (termed Feammox) was discovered, it has been observed in various natural environments. However, besides the vertical distribution of Feammox in paddy soils, its differences and relationships with traditional nitrogen loss processes, including denitrification and anammox, remain unclear. Here, we studied the distribution of nitrogen loss pathways in different layers (0-50 cm) of paddy soil in southeastern China using ¹⁵N isotope tracer technology and molecular analysis. Our study showed that denitrification had a rate of 2.19 ± 0.39 mg N·kg^-1·d^-1, which was the highest activity in the surface layer (0-10 cm). The activities of anammox and Feammox reached peak values in the 10-20 cm (1.13 ± 0.16 mg N·kg^-1·d^-1) and 20-30 cm (0.23 ± 0.02 mg N·kg^-1·d^-1) soil layer, respectively. The nitrogen loss in the surface layer was more serious than that in the deep layer under paddy cultivation. In this study, denitrification was the main nitrogen loss pathway in the surface soil, but Feammox became an important nitrogen loss pathway (up to 26.1%) in the 20-40 cm depth. Overall, our research could improve and perfect the nitrogen cycle pathways in paddy soil.