Critical Review: Biogeochemical Networking of Iron in Constructed Wetlands for Wastewater Treatment

Iron-carbon enhanced constructed wetland microbial fuel cells for tetracycline wastewater treatment: Efficacy, power generation, and the role of iron-carbon

Tetracycline (TC) antibiotics wastewater is a serious threat to human health and environment. In this study, four groups of laboratory-scale constructed wetlands (CWs) with different configurations were constructed to evaluate the removal efficiency of iron-carbon (Ic) coupled constructed wetland microbial fuel cells (CW-MFC) system for different pollutants removal and bioelectricity production. The results showed that the addition of Ic significantly promoted the removal of contaminants. The maximum removal rates of COD, TN, NH4+-N, and TP were 86.13 %, 81.60 %, 79.07 %, and 97.35 %, respectively. In particular, the removal rates of TC reached 100 %. 3D-EEM analysis further confirmed the role of Ic in promoting organic degradation. The Ic-CW-MFC system also showed superiority in power generation performance with peak power density of 7.90 mW/m2 (internal resistance is 10 Ω), 88.07 % higher than the traditional CW-MFC, while the internal resistance was 68.21 % lower. Therefore, when Ic is used as the substrate of CW-MFC system, its decontamination and electricity generation performance is the best. Analysis of RDA was used to elucidate the relationship of four CWs, dominant strains and environmental factors (pH, ORP and DO). The performance of traditional CWs decreased significantly after TC addition (5-20 mg/L), but Ic-CW-MFC could effectively alleviate the inhibition effect caused by high-concentration TC wastewater. The working mechanism of Ic-CW-MFC in TC wastewater was further analyzed through typical cycle experiment and characterization. The results showed that Ic-CW-MFC is an efficient and economical wastewater treatment technology, which has great potential application value in the treatment of wastewater containing TC.

In situ arsenic immobilization by natural iron (oxyhydr)oxide precipitates in As-contaminated groundwater irrigation canals

Arsenic-contaminated groundwater is widely used in agriculture. To meet the increasing demand for safe water in agriculture, an efficient and cost-effective method for As removal from groundwater is urgently needed. We hypothesized that Fe (oxyhydr)oxide (FeOOH) minerals precipitated in situ from indigenous Fe in groundwater may immobilize As, providing a solution for safely using As-contaminated groundwater in irrigation. To confirm this hypothesis and identify the controlling mechanisms, we comprehensively evaluated the transport, speciation changes, and immobilization of As and Fe in agricultural canals irrigated using As-contaminated groundwater. The efficiently removed As and Fe in the canals accumulated in shallow sediment rather than subsurface sediment. Linear combination fitting (LCF) analysis of X-ray absorption near edge spectroscopy (XANES) indicated that As(V) was the dominant As species, followed by As(III), and there was no FeAsO4 precipitate. Sequential extraction revealed higher contents of amorphous FeOOH and associated As in shallower sediment than in the subsurface layer. Stoichiometric molar ratio calculations, SEM‒EDS, FTIR, and fluorescence spectroscopy collectively demonstrated that the microbial reductive dissolution of amorphous FeOOH proceeded via reactive dissolved organic matter (DOM) consumption in subsurface anoxic porewater environment facilitating high labile As, whereas in surface sediment, the in situ-generated amorphous FeOOH was stable and strongly inhibited As release via adsorption. In summary, groundwater Fe2+ can efficiently precipitate in benthic surface sediment as abundant amorphous FeOOH, which immobilizes most of the dissolved As, protecting agricultural soil from contamination. This field research supports the critical roles of the phase and reactivity of in situ-generated FeOOH in As immobilization and provides new insight into the sustainable use of contaminated water.

Enhanced nitrogen removal in constructed wetlands filled with iron-carbon substrates: Reexploring unique roles of iron-cycling and electroactive microorganisms

Bacteria involved in iron cycle and electroactivity are commonly found in intensified constructed wetlands (CWs) with iron-carbon substrates. However, their roles in nitrogen (N) removal remain unclear. Here, two types of CWs with different filling modes (separate and mixed) of sponge iron and biochar substrates under micro-oxygen regulation (2 h/d, 60 mL min-1) were constructed for nitrogen removal for 240 days. The results revealed that CWs amended with iron-biochar substrates separately (CW-D) achieved a higher total nitrogen removal performance (81 %) and lower greenhouse gas emission (global warming potential reduced by 3.07 × 105 μg CO2-eq m-2h-1) compared with Fe-C micro-electrolysis CWs (CW-E). In this process, although 41 genera of nitrogen-transforming bacteria (NTB) were detected in CWs, no NTB members had a significant difference (P < 0.05) in relative abundance between CW-D and CW-E. However, 11 genera of iron-cycling bacteria (ICB, e.g. Pseudomonas) with electroactive and 5 genera of electroactive bacteria (EAB, e.g. Tetrasphaera) were significantly enriched in CW-D and CW-E, respectively, both showing significant negative correlations (P < 0.05) with NO2--N content. It indicated that ICB and EAB rather than specific NTB members were decisive in N removal in iron and carbon CWs in low C/N ratio wastewater treatment and regulated by filling modes. Our findings expand the knowledge of the application of iron and carbon substrates in CWs and provide an initial assessment of the effect of different filling modes of iron and carbon on nitrogen removal in CWs.

Tracing the electron transfer behavior driven by hydrophyte-derived carbon materials empowered autotrophic denitrification in iron-based constructed wetlands: Efficacy and enhancement mechanism

Iron-based constructed wetlands (ICWs) displayed great potential in deep nitrogen elimination for low-polluted wastewater. However, the unsatisfactory denitrification performance caused by the limited solubility and sluggish activity of iron substrates needs to be improved in an eco-effective manner. To fill this gap, the bioavailability of iron substrates (iron scraps) affected by wetland biomass-derived carbon materials with potential conductivity were explored. Results indicated that the cumulative removal of TN in biochar-added ICW (BC-ICW) and activated carbon-added ICW (AC-ICW) increased by 29.04 % and 22.96 %, respectively. The carbon matrix of AC played the geo-conductor role to facilitate the rapid release of iron ions, as indicated by the higher TN removal efficiency of AC-ICW (45.36 ± 1.45 %) at the early stage, while the reduced conductivity of AC negatively impacted the nitrogen removal. BC-ICW exhibited intensified denitrification potential, with higher TN removal capacity (52.08 ± 3.04 %) and effluent Fe2+ concentration. Electroactive bacteria (EB) (Geobacter, Desulfovibrio, Shewanella, etc.) associated with extracellular electron transfer were enriched in BC-ICW, as well as the expanded niches breadth and improved microbial community diversity. The electron-shuttling effect of BC was mainly attributed to its oxygenated functional groups (quinone/phenolic moieties), which supported the electron transfer from EB to extracellular iron oxides, as evidenced by the increased Fe(III)(hydro)oxides bioavailability. Besides, biochar concurrently up-regulated the gene expression of electron transport chains/mediators and denitrification reductases, suggesting that BC boosted the active iron cycle and iron-mediated autotrophic denitrification in ICWs by accelerating intracellular and extracellular electron transfer. This work explored the electron transfer behavior of biomass-derived carbon materials coupled with ICWs to enhance denitrification, providing insights into the sustainable application of biomass derived carbon-assisted ICWs in tertiary treatment.

The interactive application and impacts of iron/nitrogen biogeochemical cycling in distributed ponds for non-point source pollution control in a watershed

The linkages of distributed ponds are utilized in conjunction with one another to remediate non-point source (NPS) pollution in a water-scarce basin. This study provides an overview of a state-of-the-art thorough evaluation of ponds, which offers insight into the majority of topics covered by the ongoing scientific studies, including their various functions and factors affecting their functioning on the hydrological, physicochemical, and biological processes, such as environmental climate factors and basin-specific landscape configuration parameters, as well as process parameters for design, operation and management aspects. The linkages of ponds provide a variety of sustainable services (6R functions), such as resources, restoration, reduction, reuse, recycling, and recovery. The significance of regional environmental geochemical substrates in the ponds, such as red soil, as a hotspot for microbial reaction is emphasized to demonstrate the significant contribution of the migration and transformation of Fe/N cycles to the pollution removal process. In this review, 178 original research publications were thoroughly analyzed to improve our knowledge of the iron-nitrogen cycle in wetlands. From a molecular biology standpoint, the identification of functional microbe species and genes linked to microbially driven iron-nitrogen cycle activities is delved. Reliable data and homogeneous datasets from 42 studies were collected. The correlation analysis results demonstrated Feammox rates contributed to the N loss amount (r = 0.871; p < 0.01), and they had a positive correlation with Fe(III) concentration (r = 0.965; p < 0.01). The proposal for the treatment of NPS pollution by large-scale linkages of ponds in a basin involves optimizing Fe/N microbial processes to promote iron crystallization and efficient circulation of Fe(II) and Fe(III). The co-benefits of geochemistry, biotechnology, and environmental science should be considered when managing contamination in engineering applications. The linkages framework for integrated ponds, which incorporates macro (watershed management) and micro (biogeochemical cycle mechanism) investigations, provides a systematic approach to the application of integrated ponds and sustainable water management for NPS pollution control.

Autotrophic ammonium nitrogen removal process mediated by manganese oxides: Bioreactors performance optimization and potential mechanisms

Manganese(IV) (Mn(IV)) reduction coupled with ammonium (NH4+-N) oxidation (Mnammox) has been found to play a significant role in the nitrogen (N) cycle within natural ecosystems. However, research and application of the autotrophic NH4+-N removal process mediated by manganese oxides (MnOx) in wastewater treatment are currently limited. This study established autotrophic NH4+-N removal sludge reactors mediated by various MnOx types, including δ-MnO2 (δ-MSR), β-MnO2 (β-MSR), α-MnO2 (α-MSR), and natural Mn ore (MOSR), investigating their NH4+-N removal performances and mechanisms under different initial N loading and pH conditions. During the 330 d operation, the reactors exhibited NH4+-N removal efficiencies in the order of δ-MSR > α-MSR > β-MSR > MOSR. Notably, metal-reducing bacteria (Candidatus Brocadia, Dechloromonas, and Rhodocyclaceae) and Mn(II) oxidizing bacteria (Pseudomonas and Zoogloea) were enriched in the reactors, especially in the δ-MSR. The presence of these microorganisms facilitated the reduction of Mn(IV) and utilized the generated Mn(II) to drive autotrophic denitrification (MnOAD), thereby completing the Mn(IV)/Mn(II) cycle and enhancing N removal in the system. An active Mn cycle displayed in δ-MSR, which could be demonstrated by the formation of petal-shaped biogenic MnOx and the increased abundance of Mn cycling genes (MtrCDE, MtrA, MtrB, and CotA, etc.). Meanwhile, genes involved in N metabolism were enriched, particularly functional genes associated with nitrification and denitrification. In this study, the coupling of Mnammox and MnOAD was realized via the Mn cycle, providing a new perspective on the application of autotrophic N removal technologies in wastewater treatment.

Flooding increases plant-derived carbon accumulation in soils of aquatic-terrestrial ecotone

Soils in the aquatic-terrestrial zone undergo periodic flooding and act as significant carbon sinks. However, the mechanisms governing soil organic carbon (SOC) formation in these zones are not well understood. This study elucidates the effects of periodic flooding on SOC accumulation at the water level drawdown zone of the Three Gorges Reservoir, using lignin phenols and amino sugars as indicators of plant- and microbial-derived carbon. Results showed that SOC content averaged at 7.52, 8.31, and 8.76 g kg-1 in 0-20 cm soils at low, intermediate, and high flooding levels, respectively, compared to 5.87 g kg-1 in control soils. Total lignin phenols and amino sugars averaged at 0.351, 0.377, 0.337 g kg-1 and 0.697, 0.718, 0.756 g kg-1 in 0-20 cm soils at high, intermediate, and low flooding levels, respectively, compared to 0.161 and 0.624 g kg-1 in control soils. Similar patterns were observed in 20-40 cm soils. Periodic flooding significantly enhanced the accumulation of plant-derived carbon and its contribution to SOC accumulation by decreasing lignin phenol oxidation, while microbial-derived carbon contribution remained unaffected. Ratios of cinnamyl to vanillyl (1.13 in flooded soils vs. 1.08 in control) and syringyl to vanillyl (0.20 in flooded soils vs. 0.17 in control) indicated that lignin phenols originated primarily from woody angiosperms and remained stable. Flooding also modified edaphic variables, such as clay mineral and particle feature, enhancing organic compound accumulation. Clay minerals, particularly chlorite and kaolinite, played more pivotal roles than illite in regulating SOC accumulation. These findings underscore the potential for managing flooding regimes as a strategy to enhance carbon sequestration and improve ecosystem resilience in aquatic-terrestrial zones.

A comprehensive review on iron‒carbon microelectrolysis constructed wetlands: Efficiency, mechanism and prospects

The traditional constructed wetlands (CWs) face challenges such as significant seasonal fluctuations in decontamination performance and susceptibility to clogging, with the bottlenecks in advanced wastewater treatment becoming increasingly prominent. The iron‒carbon microelectrolysis coupled with constructed wetlands (ICME‒CWs) represents a promising new type of CWs, capable of removing typical and emerging pollutants in water through various mechanisms including adsorption, precipitation, oxidation‒reduction, microelectrolysis, and plant‒microbial synergy. Therefore, this review summarizes the sources, preparation, and basic properties of the ICME substrate commonly used in ICME‒CWs in recent years. It systematically outlines the decontamination mechanisms of ICME‒CWs and their removal performance for pollutants. Additionally, the potential ecological effects of ICME on wetland organisms (microorganisms and plants) are discussed. Finally, the prospects and challenges of ICME‒CWs in applications such as greenhouse gas reduction, groundwater remediation, and the removal of emerging pollutants are proposed. This review aims to advance the development of ICME‒CWs technology for efficient wastewater treatment and provide prospects and guidance for the sustainable and environmentally friendly development of CWs.

Enhanced remediation of real agricultural runoff in surface-flow constructed wetlands by coupling composite substrate-packed bio-balls, submerged plants and functional bacteria: Performance and mechanisms

Import of agricultural runoff containing nutrients considerably contributes to eutrophication of receiving water bodies. Surface-flow constructed wetlands (SFCWs) are commonly applied for agricultural runoff purification, but the performance is usually unsatisfactory. In this study, suspended bio-balls filled with zeolite and iron-carbon (Fe-C) composite substrates, submerged macrophyte (Ceratophyllum demersum) and functional denitrifying bacteria were collectively added into SFCW microcosms to enhance the remediation efficiency for real agricultural runoff with high nutrient concentrations and low content of bioavailable organic matter. The bio-ball added SFCWs achieved notably higher pollutant removal efficiencies (21.1%, 80.2% and 47.5% for chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP), respectively) than the control (COD: 6.9%, TN: 64.4%, TP: 27.9%), because of the versatile functions of filling materials for pollutant removal. C. demersum plantation (COD: 44.2%, TN: 82.8% and TP: 53.7%) and functional bacteria inoculation (COD: 51.8%, TN: 85.8% and TP: 55.1%) further enhanced the efficiency of the SFCWs for agricultural runoff remediation. Bio-ball addition and C. demersum plantation significantly increased the humification degree and reduced the molecular weight of dissolved organic matter (DOM) in the agricultural runoff. Moreover, the two intensification measures also notably reduced organic and nitrogen contents in the wetland sediment. Remarkable distinction in bacterial community distribution patterns was observed in the SFCW sediment and filling substrates in bio-balls. Keystone genera including Clostridium_sensu_stricto_1 and Bacillus in the zeolite, Sphingomonas and Exiguobacterium in the Fe-C substrates and Sediminibacterium in the sediment might be critical for agricultural runoff remediation in the SFCW microcosms. The study highlights a high potential of the intensified SFCWs by these coupling measures for agricultural runoff remediation.

A critical review of Mnammox coupled with the NDMO for innovative nitrogen removal

In the context of increasing global nitrogen pollution, traditional biological nitrogen removal technologies like nitrification and denitrification are hindered by high energy consumption. Additionally, the deployment of anaerobic ammonium oxidation (Anammox) technology is constrained due to the slow growth rate of Anammox bacteria and there is a bottleneck in nitrogen removal efficiency. To overcome these technical bottlenecks, researchers have discovered a revolutionary nitrogen removal technology that cleverly combines the redox cycling of manganese with nitrification and denitrification reactions. In this new process, manganese dependent anaerobic ammonium oxidation (Mnammox) bacteria can convert NH4+ to N2 under anaerobic conditions, while nitrate/nitrite dependent manganese oxidation (NDMO) bacteria use NO3-/NO2- as electron acceptors to oxidize Mn2+ to Mn4+. Mn4+ acts as an electron acceptor in Mnammox reaction, thereby realizing the autotrophic nitrogen removal process. This innovative method not only simplifies the steps of biological denitrification, but also significantly reduces the consumption of oxygen and organic carbon, providing a more efficient and environmentally friendly solution to the problem of nitrogen pollution. The article initially provides a concise overview of prevalent nitrogen removal technologies and the application of manganese in these processes, and discusses the role of manganese in biogeochemical cycles, including its discovery, mechanism of action, microbial communities involved, and its impact on these key factors in the process. Subsequently, metabolic principles, benefits, advantages, and environmental considerations of Mnammox coupled with the NDMO process are analyzed in detail. Finally, this article summarizes the shortcomings of current research and looks forward to future research directions. The goal of this article is to provide a valuable reference for researchers to fully understand the application of manganese in nitrogen removal processes.

Constructed wetlands and hyperaccumulators for the removal of heavy metal and metalloids: A review

Water pollutions of heavy metal and metalloids (HMMs), typically including As, Cd, Cu, Cr, Mn, Ni, Pb, and Zn, are becoming a severe environmental problem to be controlled. Constructed wetlands (CWs) have been intensively investigated and applied for the removal of HMMs. By analyzing a mass of data from the existing literatures, this review found that the HMM removals in CWs varied from 12.35 % to 91.01 %, depending upon the HMM species and CW conditions. Nonetheless, 88.50 % of the influent HMMs were eventually immobilized in the CW sediments, while the common wetland plants are inefficient (i.e., accounting for 4.64 %) to uptake and accumulate the HMMs. It was also found that the concentrations of certain HMMs in the CW sediments have already exceeded up to 100 % of various environmental standards, indicating the urgency of introducing HMM hyperaccumulators in the systems. Through comparison, both the aboveground and belowground HMM accumulating capacities of reported hyperaccumulators were higher by magnitudes than common wetland plants. Following this, the efficacies and mechanisms of candidate hyperaccumulators were provided for the various scenarios of HMM control in CWs. Further, the selection principals, culture methods, and harvest strategies of hyperaccumulator in CWs were discussed. Finally, several perspectives were suggested for the future research. Overall, this review provided guiding information for the utilization of hyperaccumulators in CWs, which can improve the efficiency and sustainability of HMM removal in the CW systems.

Constructed wetlands combined with microbial fuel cells (CW-MFCs) as a sustainable technology for leachate treatment and power generation

The physical and chemical treatment processes of leachate are not only costly but can also possibly produce harmful by products. Constructed wetlands (CW) has been considered a promising alternative technology for leachate treatment due to less demand for energy, economic, ecological benefits, and simplicity of operations. Various trends and approaches for the application of CW for leachate treatment have been discussed in this review along with offering an informatics peek of the recent innovative developments in CW technology and its perspectives. In addition, coupling CW with microbial fuel cells (MFCs) has proven to produce renewable energy (electricity) while treating contaminants in leachate wastewaters (CW-MFC). The combination of CW-MFC is a promising bio electrochemical that plays symbiotic among plant microorganisms in the rhizosphere of an aquatic plant that convert sun electricity is transformed into bioelectricity with the aid of using the formation of radical secretions, as endogenous substrates, and microbial activity. Several researchers study and try to find out the application of CW-MFC for leachate treatment, along with this system and performance. Several key elements for the advancement of CW-MFC technology such as bioelectricity, reactor configurations, plant species, and electrode materials, has been comprehensively discussed and future research directions were suggested for further improving the performance. Overall, CW-MFC may offer an eco-friendly approach to protecting the aquatic environment and come with built-in advantages for visual appeal and animal habitats using natural materials such as gravel, soil, electroactive bacteria, and plants under controlled condition.

Biotic/abiotic transformation mechanisms of phenanthrene in iron-rich constructed wetland under redox fluctuation

Iron-rich constructed wetlands (CWs) could promote phenanthrene bioremediation efficiently through biotic and abiotic pathways, which have gained increasing attention. However, the biotic/abiotic transformation mechanisms of trace organic contaminants in iron-rich CW are still ambiguous. Herein, three CWs (i.e., CW-A: Control; CW-B: Iron-rich CW, CW-C: Iron-rich CW + tidal flow) were constructed to investigate the transformation mechanisms of phenanthrene through Mössbauer spectroscopy and metagenomics. Results demonstrated CW-C achieved the highest phenanthrene removal (94.0 %) and bacterial toxicity reduction (92.1 %) due to the optimized degradation pathway, and subsequently achieved the safe transformation of phenanthrene. Surface-bound/low-crystalline iron regulated hydroxyl radical (·OH) production predominantly, and its utilization was promoted in CW-C, which also improved electron transfer capacity. The enhanced electron transfer capacity led to the enrichment of PAH-degrading microorganisms (e.g., Thauera) and keystone species (Sphingobacteriales bacterium 46-32) in CW-C. Additionally, the abundances of phenanthrene transformation (e.g., EC:1.14.12.-) and tricarboxylic-acid-cycle (e.g., EC:2.3.3.1) enzyme were up-regulated in CW-C. Further analysis indicated that the safe transformation of phenanthrene was mainly attributed to the combined effect of abiotic (·OH and surface-bound/low-crystalline iron) and biotic (microbial community and diversity) mechanisms in CW-C, which contributed similarly. Our study revealed the essential role of active iron in the safe transformation of phenanthrene, and was beneficial for enhanced performance of iron-rich CW.

Recovery capacity of constructed wetlands in response to multiple disturbances: Microbial interaction perspective

Previous studies have predominantly explored the response mechanisms of constructed wetlands (CWs) to singular disturbances. In practical applications, CWs are frequently subject to multiple disturbances, resulting in complex interference mechanisms. Therefore, this study aims to select harmful algal blooms and microalga ZM-5 as disturbances to investigate the response mechanisms of CWs. Results revealed a dynamic pattern in COD removal efficiency of CWs, with fluctuations at 39.0 ± 6.2 % and 80.1 ± 4.7 % during the disturbances, followed by a recovery to approximately 65.7 ± 3.2 %. Additionally, the CWs exhibited a capacity for self-recovery and enhanced stability by selectively promoting specific microbial communities through the regulation of the genes responsible for indole-3-acetic acid (IAA) and vitamin production. Importantly, this study underscored the establishment of a resilient microbial community structure within CWs following multiple disturbances, characterized by a more interconnected microbial network. These findings shed light on the adaptive mechanisms of CWs in the face of complex environmental challenges.

Free water surface constructed wetlands: review of pollutant removal performance and modeling approaches

Free water surface constructed wetlands (FWSCWs) for the treatment of various wastewater types have evolved significantly over the last few decades. With an increasing need and interest in FWSCWs applications worldwide due to their cost-effectiveness and other benefits, this paper reviews recent literature on FWSCWs' ability to remove different types of pollutants such as nutrients (i.e., TN, TP, NH4-N), heavy metals (i.e., Fe, Zn, and Ni), antibiotics (i.e., oxytetracycline, ciprofloxacin, doxycycline, sulfamethazine, and ofloxacin), and pesticides (i.e., Atrazine, S-Metolachlor, imidacloprid, lambda-cyhalothrin, diuron 3,4-dichloroanilin, Simazine, and Atrazine) that may co-exist in wetland inflow, and discusses approaches for simulating hydraulic and pollutant removal processes. A bibliometric analysis of recent literature reveals that China has the highest number of publications, followed by the USA. The collected data show that FWSCWs can remove an average of 61.6%, 67.8%, 54.7%, and 72.85% of inflowing nutrients, heavy metals, antibiotics, and pesticides, respectively. Optimizing each pollutant removal process requires specific design parameters. Removing heavy metal requires the lowest hydraulic retention time (HRT) (average of 4.78 days), removing pesticides requires the lowest water depth (average of 0.34 m), and nutrient removal requires the largest system size. Vegetation, especially Typha spp. and Phragmites spp., play an important role in FWSCWs' system performance, making significant contributions to the removal process. Various modeling approaches (i.e., black-box and process-based) were comprehensively reviewed, revealing the need for including the internal process mechanisms related to the biological processes along with plants spp., that supported by a further research with field study validations. This work presents a state-of-the-art, systematic, and comparative discussion on the efficiency of FWSCWs in removing different pollutants, main design factors, the vegetation, and well-described models for performance prediction.

Biochemical mechanisms underlying iron plaque-mediated phosphorus accumulation and uptake in rice roots

The iron oxyhydroxides of iron plaque on the surface of rice root are crucial for the uptake of nutrition elements, especially phosphorus (P), but the effects of iron oxyhydroxides of iron plaque on the accumulation and uptake of P remain largely unknown. In this study, we investigated the regulatory mechanism of iron plaque on P uptake in rice via hydroponics of whole plant and simulation of iron oxyhydroxides-coated suspension cells in rice. The hydroponic experiment results showed that the presence of iron plaque increased the P content in rice shoots. The simulation experiment results further confirmed that after iron plaque coating, the P contents in the whole cell and on the cell wall were significantly increased from 5.16 mg/g and 2.73 mg/g to 8.85 mg/g and 5.27 mg/g, respectively. In addition, our data also showed that iron plaque coating led to an increase in cell surface potentials from -380 ± 40 mV to -200 ± 30 mV, thus promoting the adsorption of more P. Taken together, this study demonstrated that the iron plaque coating increased the surface potential of the cells, thus enhancing cellular P enrichment, eventually promoting P efficient adsorption in rice. Deciphering these regulatory mechanisms provide an insight into P biogeochemical cycling at the soil-plant interface and offer theoretical basis and practical references for the improvement of P bioavailability in rice production.

Iron Plaque: A Shield against Soil Contamination and Key to Sustainable Agriculture

Soils play a dominant role in supporting the survival and growth of crops and they are also extremely important for human health and food safety. At present, the contamination of soil by heavy metals remains a globally concerning environmental issue that needs to be resolved. In the environment, iron plaque, naturally occurring on the root surface of wetland plants, is found to be equipped with an excellent ability at blocking the migration of heavy metals from soils to plants, which can be further developed as an environmentally friendly strategy for soil remediation to ensure food security. Because of its large surface-to-volume porous structure, iron plaque exhibits high binding affinity to heavy metals. Moreover, iron plaque can be seen as a reservoir to store nutrients to support the growth of plants. In this review, the formation process of iron plaque, the ecological role that iron plaque plays in the environment and the interaction between iron plaque, plants and microbes, are summarized.

Root-mediated acidification, phosphatase activity and the phosphorus-cycling microbial community enhance phosphorus mobilization in the rhizosphere of wetland plants

Rhizoremediation of wetland plants is an environmentally friendly strategy for sediment phosphorous (P) removal, the basic underlying principle of which is the complex interactions between roots and microorganisms. This study investigated the immobilization and mobilization mechanisms of P in the rhizosphere of wetland plants using high-resolution spatial visualization techniques and metagenomic sequencing. Two-dimensional visualization of the spatial distribution of P, iron (Fe) and manganese (Mn) indicated that the sequestration of Fe-oxides rather than Mn-oxides caused the depletion of labile P, resulting in an increase in the Fe-adsorbed P fraction. Plants altered the rhizospheric environments and P-cycling microbial community to mobilize low-availability P from sediments. Mineral P solubilization and organic P mineralization were enhanced by local acidification and increased phosphatase activity, respectively. Microbial P mobilization also increased with increasing relative abundances of P solubilization and mineralization genes (gcd and phnW) and decreasing P transportation genes (ugpA, ugpB, and pit) genes in the rhizosphere. These processes led to the remobilization of 10.04 % of inorganic P, and 15.23 % of organic P, in the rhizosphere during the incubation period. However, the resupply of P via the above processes did not compensate for the depletion of rhizospheric P via root uptake and mineral sequestration. Our results provide novel insights into the mechanisms of rhizospheric P cycling, which will help to inform future phytoremediation strategies.

The introduction of influent sulfamethoxazole loads induces changes in the removal pathways of sulfamethoxazole in vertical flow constructed wetlands featuring hematite substrate

High frequent detection of sulfamethoxazole (SMX) in wastewater cannot be effectively removed by constructed wetlands (CWs) with a traditional river sand substrate. The role of emerging substrate of hematite in promoting SMX removal and the effect of influent SMX loads remain unclear. The removal efficiency of SMX in hematite CWs was significantly higher than that in river sand CWs by 12.7-13.8% by improving substrate adsorption capacity, plant uptake and microbial degradation. With increasing influent SMX load, the removal efficiency of SMX in hematite CWs slightly increased, and the removal pathways varied significantly. The contribution of plant uptake was relatively small (< 0.1%) under different influent SMX loads. Substrate adsorption (37.8%) primarily contributed to SMX removal in hematite CWs treated with low-influent SMX. Higher influent SMX loads decreased the contribution of substrate adsorption, and microbial degradation (67.0%) became the main removal pathway. Metagenomic analyses revealed that the rising influent load increased the abundance of SMX-degrading relative bacteria and the activity of key enzymes. Moreover, the abundance of high-risk ARGs and sulfonamide resistance genes in hematite CWs did not increase with the increasing influent load. This study elucidates the potential improvements in CWs with hematite introduction under different influent SMX loads.

Biochar imparted constructed wetlands (CWs) for enhanced biodegradation of organic and inorganic pollutants along with its limitation

The remediation of polluted soil and water stands as a paramount task in safeguarding environmental sustainability and ensuring a dependable water source. Biochar, celebrated for its capacity to enhance soil quality, stimulate plant growth, and adsorb a wide spectrum of contaminants, including organic and inorganic pollutants, within constructed wetlands, emerges as a promising solution. This review article is dedicated to examining the effects of biochar amendments on the efficiency of wastewater purification within constructed wetlands. This comprehensive review entails an extensive investigation of biochar's feedstock selection, production processes, characterization methods, and its application within constructed wetlands. It also encompasses an exploration of the design criteria necessary for the integration of biochar into constructed wetland systems. Moreover, a comprehensive analysis of recent research findings pertains to the role of biochar-based wetlands in the removal of both organic and inorganic pollutants. The principal objectives of this review are to provide novel and thorough perspectives on the conceptualization and implementation of biochar-based constructed wetlands for the treatment of organic and inorganic pollutants. Additionally, it seeks to identify potential directions for future research and application while addressing prevailing gaps in knowledge and limitations. Furthermore, the study delves into the potential limitations and risks associated with employing biochar in environmental remediation. Nevertheless, it is crucial to highlight that there is a significant paucity of data regarding the influence of biochar on the efficiency of wastewater treatment in constructed wetlands, with particular regard to its impact on the removal of both organic and inorganic pollutants.

Photosynthetic pretreatment increases membrane-based rejection of boron and arsenic

The metalloids boron and arsenic are ubiquitous and difficult to remove during water treatment. As chemical pretreatment using strong base and oxidants can increase their rejection during membrane-based nanofiltration (NF), we examined a nature-based pretreatment approach using benthic photosynthetic processes inherent in a unique type of constructed wetland to assess whether analogous gains can be achieved without the need for exogenous chemical dosing. During peak photosynthesis, the pH of the overlying clear water column above a photosynthetic microbial mat (biomat) that naturally colonizes shallow, open water constructed wetlands climbs from circumneutral to approximately 10. This biological increase in pH was reproduced in a laboratory bioreactor and resulted in analogous increases in NF rejection of boron and arsenic that is comparable to chemical dosing. Rejection across the studied pH range was captured using a monoprotic speciation model. In addition to this mechanism, the biomat accelerated the oxidation of introduced arsenite through a combination of abiotic and biotic reactions. This resulted in increases in introduced arsenite rejection that eclipsed those achieved solely by pH. Capital, operation, and maintenance costs were used to benchmark the integration of this constructed wetland against chemical dosing for water pretreatment, manifesting long-term (sub-decadal) economic benefits for the wetland-based strategy in addition to social and environmental benefits. These results suggest that the integration of nature-based pretreatment approaches can increase the sustainability of membrane-based and potentially other engineered treatment approaches for challenging water contaminants.

Uptake patterns for nitrogen and sulfur source by aquatic plants and various nitrogen acquisition strategies: Affected by mining activities

Understanding the nitrogen and sulfur uptake strategies of mine plants, including sources and preferences for nitrogen forms (ammonium nitrogen (NH4+) vs nitrate nitrogen (NO3-)), is critical to improving understanding of the role of plants in participating in the biogeochemical cycles of nitrogen and sulfur in mining areas. In this study, the stable N and S isotopic compositions of two species of aquatic plants (calamus and reed) in Linhuan mining area were analyzed to determine their absorption strategies for different nitrogen and sulfur sources. The results showed that river water was the largest source of nitrogen and sulfur, contributing 54.6% and 53.9% respectively. NO3- is the main form of nitrogen uptake by reed and calamus, followed by NH4+. In order to adapt to the change of nitrogen form in the environment, reed and calamus tend to absorb and utilize NO3- to maintain their absorption of nitrogen. Mine effluents from mining activities provide at least 12.9% and 16.8% sulfate to reed and calamus respectively, and the effect of mine effluents on reed and calamus sulfur has been underestimated. This study reveals the key factors controlling plant isotope composition, and the use of nitrogen and sulfur isotope composition of aquatic plants can help quantify the level of influence of mining activities, and understand the biogeochemical cycle of nitrogen and sulfur in mining areas.

Dual Regulatory Role of Penicillium oxalicum SL2 in Soil: Phosphorus Solubilization and Pb Stabilization

The mechanisms of the P. oxalicum SL2-mediated microbial community on phosphorus solubilization and Pb stabilization were investigated through a 90-day soil experiment. In the treatments inoculated with P. oxalicum SL2, the amount of P. oxalicum SL2-GFP remained at 77.8%-138.6% of the initial inoculation amount after 90 days, and the available phosphorus (AP) content increased 21.7%-40.8% while EDTA-Pb decreased 29.9%-43.2% compared with CK treatment. SEM-EDS results showed that P. oxalicum SL2 changed the agglomeration degree of microaggregates and promoted the combination of Pb with C and O elements. These phenomena were enhanced when applied with Ca3(PO4)2. Microbial community analysis showed that P. oxalicum SL2 improved soil microbial activity, in which the fungi absolute abundance increased about 15 times within 90 days. Correlation analyses and a partial least-squares path model showed that the activation of Penicillium, Ascobolus, Humicola, and Spizellomyces in a fungal community increased the content of oxalate and AP, which directly decreased EDTA-Pb content, while the change of Bacillus, Ramlibacter, Gemmatimonas, and Candidatus Solibacter in the bacterial community regulated Fe/Mn/S/N cycle-related functions, thus promoting the conversion of Pb to oxidizable state. Our findings highlight that P. oxalicum SL2 enhanced the microbial-induced phosphate precipitation process by activating soil microbial communities and regulating their ecological functions.

Highly efficient phosphorous removal in constructed wetland with iron scrap: Insights into the microbial removal mechanism

Excessive phosphorus (P) in surface water can lead to serious eutrophication and economic losses. Iron-based constructed wetland (CW) is considered as a promising solution to eliminate P effectively due to the advantage of low-cost. However, there is limited available information on the microbial removal mechanism of P in iron-based CW up to now. Therefore, CW with iron scrap was constructed to investigate the treatment performance and microbial removal mechanism in this study. Results showed that efficient and stable P removal (97.09 ± 1.90%) was achieved in iron scrap-based CW during the experiment period, which was attributed to the precipitation of iron and P and improved microbially mediated P removal. Metagenomic analysis showed that microbial diversity was enhanced and phosphate accumulating organisms (e.g., Dechloromonas and Tetrasphaera) were enriched in CW with iron scrap, which explained higher P removal reasonably. In addition, the abundance of genes involved in the P starvation (e.g., phoB), uptake and transport (e.g., pstB) were enhanced in iron scrap-based CW. Enrichment analysis demonstrated that phosphotransferase pathway was also significantly up-regulated in CW with iron scraps, indicating that the energy supply of microbial P removal was enhanced. These findings provide a better understanding of the microbial removal mechanism of P in iron-based CW.

Synergistic improvement of nitrogen and phosphorus removal in constructed wetlands by the addition of solid iron substrates and ferrous irons

Sanjiang Plain is intensively used for rice production, and ditch drainage diffuse pollution prevention is crucial. Groundwater, rich in Fe ions, is the main source of irrigation water in this region. In this study, pyrite and zero-valent iron (ZVI) (sponge iron and iron scraps) were used as substrates to identify the synergistic influence of exogenous Fe2+ addition and solid iron substrates on pollutant removal in constructed wetlands. Based on the results, iron substrates hardly improved the ammonia removal, mainly because of the physical structure and oxidation activity. At a hydraulic retention time longer than 8 h, the pollution removal efficiency in the zero-valent iron (ZVI) substrate treatment increased significantly, and the removal of nitrate (NO3 --N) and total phosphorus (TP) in the iron scrap substrate treatment reached about 60% and 70%, respectively. The high-throughput sequencing results showed a significant increase in the abundance of microorganisms involved in denitrification and phosphate accumulation in biofilms on ZVI substrates. The highest diversities of such microorganisms in biofilms on iron scraps were found for denitrifying bacteria (Pseudomonas), nitrate-reducing Fe (II)-oxidizing bacteria (Acidovorax), and Dechloromonas with autotrophic denitrification and phosphate accumulation, with a 43% cumulative abundance. Dechloromonas dominated in the iron sponge substrate treatment. The highest relative abundance of Acidovorax was found in the mixed iron substrate (pyrite, sponge iron, and iron scraps) treatment. The addition of ZVI substrate significantly improved the removal of NO3 --N and TP and reduced the hydraulic retention time through the continuous release of Fe2+ and the promotion of microbial growth. When designing constructed wetlands for treating paddy field drainage, the appropriate addition of iron scrap substrates is recommended to enhance the pollutant removal efficiency and shock load resistance of CWs.

Applying sheet iron to enhance the treatment efficiency of digested effluent with continuous flow and the corresponding mechanism

Because of the unstable wastewater quantity and quality, the biological treatment efficiency of digested effluent was not as expected. A convenient and effective way was eagerly required to improve the efficiency of biological treatment. By sheet iron addition (R1), the COD and TN removal efficiencies under continuous flow condition increased by 59% and 37% respectively. The bulk pH maintained at around 7.5 which benefited most bacteria, while in the control (R0, without sheet iron addition) the pH decreased to 5.0. Both chemical and bio-removal of COD existed in R1, but the chemical removal dominated (63.71%). The enhanced COD removal efficiency came from the chemical oxidation by Fe3+ (47.43%) and Fe0 (10.86%). For the TN removal, the enhancement mainly came from the improvement of anammox activity by Fe3+ (14.87%), the bio-oxidation of ammonium with Fe3+ as electron acceptor (8.78%), and the bio-reduction of nitrate/nitrite with Fe2+ and H2 as electron donor (35.76%). By the first-order kinetic fitting analysis, the COD and TN removal rate in R1 was higher than that in R0. Thus, for a quick and high COD and TN removal from digested effluent, the addition of Fe0/Fe2+/Fe3+ was suggested, and the best form should be Fe0 (e.g., sheet iron). The addition of sheet iron reduces the cost of nitrogen removal and improves the efficiency of COD and TN removal. Comparing with the combined processes, this novel approach has potential advantages with simple operation and high efficiency. It endows the biological process much broader application in digested effluent treatment.

Exploring the resilience of constructed wetlands to harmful algal blooms disturbances: A study on microbial response mechanisms

Constructed wetlands (CWs) have emerged as a promising environmentally sustainable technique for wastewater treatment. However, the susceptibility of CWs to disturbances caused by harmful algal blooms (HABs) raises concerns. This study aimed to investigate the impact of HABs on the pollutants' removal performance of CWs and the response of rhizosphere microbial community. Results revealed that CWs possessed an adaptive capacity that enabled them to recover caused by HABs. The rhizosphere was found to stimulate the occurrence of Acinetobacter, which played a critical role to help resist HABs disturbance. This study also observed an increased dissimilatory nitrate reduction metabolic pathway which promoted denitrification and enhanced the nitrogen removal efficiency of CWs. Additionally, the structural equation model further suggested that dissolved oxygen exerted a significant influence on the microbial activities and then affected the pollutants removal performance. Overall, our findings shed light on the mechanism for CW stability maintenance during HABs disturbance.

Adsorption effects and mechanisms of phosphorus by nanosized laponite

Phosphorus (P), an important macroelement for crops, may be lost into water systems by human activities and subsequently cause serious environmental problems such as eutrophication. Thus, the recovery of P from wastewater is essential. P can be adsorbed and recovered from wastewater using many natural, environmentally friendly clay minerals, however the adsorption ability is limited. Here we applied a synthesis nanosized clay mineral, laponite, to evaluate the P adsorption ability and molecular mechanisms of the adsorption process. We apply X-ray Photoelectron Spectroscopy (XPS) to observe the adsorption of inorganic phosphate onto laponite, and then measure the adsorption content of phosphate by laponite via batch experiments in different solution conditions, including pH, ionic species and concentrations. Then the molecular mechanisms of adsorption are analyzed by Transmission Electron Microscopy (TEM) and molecular modeling using Density Functional Theory (DFT). The results show that phosphate adsorbs to the surface and interlayer of laponite via hydrogen bonding, and the adsorption energies of the interlayer are greater than those of the surface. These bulk solution and molecular-scale results in a model system may provide new insights into the recovery of phosphorus by nanosized clay, with possible environmental engineering applications for P-pollution control and sustainable utilization of P sources.

Iron plaque formation and its effect on key elements cycling in constructed wetlands: Functions and outlooks

Ecological restoration of wetland plants has emerged as an environmentally-friendly and less carbon footprint method for treating secondary effluent wastewater. Root iron plaque (IP) is located at the important ecological niches in constructed wetlands (CWs) ecosystem and is the critical micro-zone for pollutants migration and transformation. Root IP can affect the chemical behaviors and bioavailability of key elements (C, N, P) since its formation/dissolution is a dynamic equilibrium process jointly influenced by rhizosphere habitats. However, as an efficient approach to further explore the mechanism of pollutant removal in CWs, the dynamic formation of root IP and its function have not been fully studied, especially in substrate-enhanced CWs. This article concentrates on the biogeochemical processes between Fe cycling involved in root IP with carbon turnover, nitrogen transformation, and phosphorus availability in CWs rhizosphere. As IP has the potential to enhance pollutant removal by being regulated and managed, we summarized the critical factors affecting the IP formation from the perspective of wetland design and operation, as well as emphasizing the heterogeneity of rhizosphere redox and the role of key microbes in nutrient cycling. Subsequently, interactions between redox-controlled root IP and biogeochemical elements (C, N, P) are emphatically discussed. Additionally, the effects of IP on emerging contaminants and heavy metals in CWs rhizosphere are assessed. Finally, major challenges and outlooks for future research in regards to root IP are proposed. It is expected that this review can provide a new perspective for the efficient removal of target pollutants in CWs.

Performance of constructed wetlands with different water level for treating graphene oxide wastewater: Characteristics of plants and microorganisms

Constructed wetlands (CWs) have been expected advantages in emerging pollutant removal, but with less known on their characteristic when treating wastewater containing graphene oxide (GO). In present study, we investigated characteristics of Iris pseudacorus, microorganisms, and pollutant removal in CWs with 60 cm and 37 cm water level (termed HCW and LCW). Plants in LCW had higher chlorophyll content and lower activities of antioxidant enzyme (superoxide dismutase, catalase, peroxidase) as well as malondialdehyde content. Substrate enzyme activities were affected by time and CW type. LCW increased only dehydrogenase activities, while HCW increased catalase, urease, neutral phosphatase, and arylsulfatase activities. Sequencing analysis revealed that microbial community showed higher richness and diversity in LCW, but this dissimilarity could be eased by time-effect. Proteobacteria (25.62-60.36%) and Actinobacteria (13.86-56.20%) were stable dominant phyla in CWs. Ratio of Proteobacteria/Acidobacteria indicated that trophic status of plant rhizosphere zone was lower in LCW. Nitrospirae were enriched to 0.16-0.68% and 0.75-1.42% in HCW and LCW. The enrichment of phyla Proteobacteria and Firmicutes in HCW was attributed to class Gammaproteobacteria and genus Enterococcus. GO transformation showed some reductions in CWs, which could be affected by water depth and substrate depth. Overall, HCW achieved nitrogen and phosphorus removal for 48.78-62.99% and 95.01%, which decreased by 8.41% and 7.31% in LCW. COD removal was less affected reaching 93%. This study could provide some new evidence for CWs to treat wastewater containing GO.

Pyrogenic Carbon Improves Cd Retention during Microbial Transformation of Ferrihydrite under Varying Redox Conditions

Fe(III) (oxyhydr)oxides are ubiquitous in paddy soils and play a key role in Cd retention. Recent studies report that pyrogenic carbon (PC) may largely affect the microbial transformation processes of Fe(III) (oxyhydr)oxides, yet the impact of PC on the fate of Fe(III) (oxyhydr)oxide-associated Cd during redox fluctuations remains unclear. Here, we investigated the effects of PC on Cd retention during microbial (Shewanella oneidensis MR-1) transformation of Cd(II)-bearing ferrihydrite under varying redox conditions. The results showed that in the absence of PC, microbial reduction of ferrihydrite resulted in Cd release under anoxic conditions and Fe(II) oxidation by oxygen resulted in Cd retention under subsequent oxic conditions. The presence of PC facilitated microbial ferrihydrite reductive dissolution under anoxic conditions, promoted Fe(II) oxidative precipitation under oxic conditions, and inhibited Cd release under both anoxic and oxic conditions. The presence of PC and frequent shifts in redox conditions (i.e., redox cycling) inhibited the transformation of ferrihydrite to highly crystalline goethite and magnetite that exhibited less Cd adsorption. As a result, PC enhanced Cd retention by 41-59% and 55-77% after the redox shift and redox cycling, respectively, while in the absence of PC, Cd retention decreased by 5% after the redox shift and increased by 11% after redox cycling. Sequential extraction analysis revealed that 63-78% of Cd was associated with Fe minerals, while 3-12% of Cd was bound to PC, indicating that PC promoted Cd retention mainly through inhibiting ferrihydrite transformation. Our results demonstrate the great impacts of PC on improving Cd retention under dynamic redox conditions, which is essential for applying PC in remediating Cd-contaminated paddy soils.

The interplay between microalgae and toxic metal(loid)s: mechanisms and implications in AMD phycoremediation coupled with Fe/Mn mineralization

Acid mine drainage (AMD) is low-pH with high concentration of sulfates and toxic metal(loid)s (e.g. As, Cd, Pb, Cu, Zn), thereby posing a global environmental problem. For decades, microalgae have been used to remediate metal(loid)s in AMD, as they have various adaptive mechanisms for tolerating extreme environmental stress. Their main phycoremediation mechanisms are biosorption, bioaccumulation, coupling with sulfate-reducing bacteria, alkalization, biotransformation, and Fe/Mn mineral formation. This review summarizes how microalgae cope with metal(loid) stress and their specific mechanisms of phycoremediation in AMD. Based on the universal physiological characteristics of microalgae and the properties of their secretions, several Fe/Mn mineralization mechanisms induced by photosynthesis, free radicals, microalgal-bacterial reciprocity, and algal organic matter are proposed. Notably, microalgae can also reduce Fe(III) and inhibit mineralization, which is environmentally unfavorable. Therefore, the comprehensive environmental effects of microalgal co-occurring and cyclical opposing processes must be carefully considered. Using chemical and biological perspectives, this review innovatively proposes several specific processes and mechanisms of Fe/Mn mineralization that are mediated by microalgae, providing a theoretical basis for the geochemistry of metal(loid)s and natural attenuation of pollutants in AMD.

Iron plaque formation, characteristics, and its role as a barrier and/or facilitator to heavy metal uptake in hydrophyte rice (Oryza sativa L.)

The persistent bioavailability of toxic metal(oids) (TM) is undeniably the leading source of serious environmental problems. Through the transfer of these contaminants into food networks, sediments and the aquatic environmental pollution by TM serve as key routes for potential risks to soil and human health. The formation of iron oxyhydroxide plaque (IP) on the root surface of hydrophytes, particularly rice, has been linked to the impact of various abiotic and biotic factors. Radial oxygen loss has been identified as a key driver for the oxidation of rhizosphere ferrous iron (Fe²⁺) and its subsequent precipitation as low-to-high crystalline and/or amorphous Fe minerals on root surfaces as IP. Considering that each plant species has its unique capability of creating an oxidised rhizosphere under anaerobic conditions, the abundance of rhizosphere Fe²⁺, functional groups from organic matter decomposition and variations in binding capacities of Fe oxides, thus, impacting the mobility and interaction of several contaminants as well as toxic/non-toxic metals on the specific surface areas of the IP. More insight from wet extraction and advanced synchrotron-based analytical techniques has provided further evidence on how IP formation could significantly affect the fate of plant physiology and biomass production, particularly in contaminated settings. Collectively, this information sets the stage for the possible implementation of IP and related analytical protocols as a strategic framework for the management of rice and other hydrophytes, particularly in contaminated sceneries. Other confounding variables involved in IP formation, as well as operational issues related to some advanced analytical processes, should be considered.

Strategy of nitrate removal in anaerobic ammonia oxidation-dependent processes

The anaerobic ammonium oxidation (anammox), a microbial process that is considered as a low-cost and high efficient wastewater treatment, has received extensive attention with an attractive application prospect. The anammox process reduces nitrite (NO₂^-) to nitrogen gas (N₂) with ammonium (NH₄⁺) as the electron donor. However, some nitrate (NO₃^-) equivalent to 11% of total nitrogen (TN) is generated in this process, which limits the development of anammox. To overcome this problem, many efforts have been made in this regard, mainly combining with other biological treatment methods (denitrification, denitrifying anaerobic methane oxidation, etc.), introducing the substance into anammox process, etc. Herein, we summarized a detailed review of previous researches on the removal of NO₃^- in the anammox-dependent processes. It is hoped that this review could serve as valuable guidance in future research and practical applications of anammox.

Impact of clogging on accumulation and stability of phosphorus in the subsurface flow constructed wetland

Substrate clogging is one of the major operation challenges of subsurface flow constructed wetlands (SSF-CWs). And the phosphorus (P) removal performance and stability of P accumulation of SSF-CWs would be varied with the development of substrate clogging. In this study, three horizontal SSF-CWs microcosms with different clogging degrees were conducted to explore the mechanism of P accumulation behavior influenced by substrate clogging. Increase in clogging degree resulted in hydraulic retention time (HRT) diminution and adsorption sites increase, which jointly led to reduced P removal efficiency at low clogging degree (L-CW), however, higher P removal efficiency was obtained as adsorption sites increase offset HRT diminution at high clogging degree (H-CW). Substrate adsorption was the primary removal pathway in all SSF-CW systems. It accounted for 77.86 ± 2.63% of the P input in the H-CW, significantly higher than the control (60.08 ± 4.79%). This was attributed to a higher proportion of Fe/Al-P accumulated on the substrate of H-CW, since clogging aggravated the anaerobic condition and promoted the generation of Fe ions. The increase in clogging degree also elevated the release risk of the accrued P in SSF-CWs, since Fe/Al-P was considered bioavailable and readily released under environmental disturbance. The obtained results provide new insights into the P transport and transformation in SSF-CWs and would be helpful to optimize substrate clogging management.

Integrated treatment of suburb diffuse pollution using large-scale multistage constructed wetlands based on novel solid carbon: Nutrients removal and microbial interactions

In this study, an integrated treatment system was proposed and applied in situ, including detention tank, multistage constructed wetlands (CWs) and wastewater treatment plants (WWTPs), preventing nutrients flowing into Dianchi Lake, in which the treatment performance of multistage CWs were evaluated principally. Results skillfully realized the bypass purification of upstream river at dry reasons, as well as the effective management and treatment of the collected diffuse pollution at rainy reasons. The purified water flowing into water bodies could satisfy the Grade III of environmental quality standards for surface water in China with the average effluent concentrations of COD, NH₄⁺-N, TN and TP decreased to 10 (51.2-72.7%), 0.5 (67.2-83.0%), 1.0 (71.2-79.6%) and 0.15 (72.3-89.4%) mg L^-1, respectively. High-throughput sequencing results indicated that the application of poly-3-hydroxybutyrate-cohyroxyvelate-sawdust (PS) blends could enrich norank_f_Anaerolineaceae (7.95%) and Bradyrhizobium (10.2%), which were distinct from the dominant genera of Pleurocapsa (13.0%) in gravel-based CWs. Functional genes and metabolism analysis uncovered that the heterotrophic denitrification was the main pathway of nitrogen removal with the abundance of genes encoding TCA cycle, glycolysis and denitrification process up-regulated. In addition, molecular ecological network (MEN) analysis suggested the denitrification genes were positively correlated with the predominant microbes in PS-based CWs, favorable for denitrifiers to transfer and utilize electron donors during denitrification process. This study proved that the developed PS blends as carbon supplies in CWs and the proposed integrated treatment system are effective methods for watershed management, providing valuable reference to low-pollution wastewater treatment in practical engineering projects.

Removal of pharmaceutical active compounds in wastewater by constructed wetlands: Performance and mechanisms

The occurrence of pharmaceutical active compounds (PhACs) in aquatic environments is a cause for concern due to potential adverse effects on human and ecosystem health. Constructed wetlands (CWs) are cost-efficient and sustainable wastewater treatment systems for the removal of these PhACs. The removal processes and mechanisms comprise a complex interplay of photodegradation, biodegradation, phytoremediation, and sorption. This review synthesized the current knowledge on CWs for the removal of 20 widely detected PhACs in wastewater. In addition, the major removal mechanisms and influencing factors are discussed, enabling comprehensive and critical understanding for optimizing the removal of PhACs in CWs. Consequently, potential strategies for intensifying CWs system performance for PhACs removal are discussed. Overall, the results of this review showed that CWs performance in the elimination of some pharmaceuticals was on a par with conventional wastewater treatment plants (WWTPs) and, for others, it was above par. Furthermore, the findings indicated that system design, operational, and environmental factors played important but highly variable roles in the removal of pharmaceuticals. Nonetheless, although CWs were proven to be a more cost-efficient and sustainable technology for pharmaceuticals removal than other engineered treatment systems, there were still several research gaps to be addressed, mainly including the fate of a broad range of emerging contaminants in CWs, identification of specific functional microorganisms, transformation pathways of specific pharmaceuticals, assessment of transformation products and the ecotoxicity evaluation of CWs effluents.

Wastewater irrigation: An opportunity for improving soil phosphorus availability; PHREEQC modeling and adsorption studies

Wastewater, an alternative supply of water and nutrients, is being allocated as a priority for human population sustainability in arid and semi-arid regions. This work proposes phosphorus (P), a vital growth-limiting nutrient, adsorption behavior in wastewater irrigated agricultural soils in comparison to non-irrigated soils using laboratory batch experiments. The adsorption mechanism was assessed using different adsorption isotherm models. Saturation indices were modeled, using the hydro-geochemical transport code PHREEQC and MINTEQ geochemical software. Phosphorus buffering parameters were also calculated based on the standard equations. The equilibrium data were well fitted with the Freundlich isotherm model. The physical adsorption mechanism was found based on the calculated isotherm parameters. The maximum adsorption capacity was two times more in non-wastewater irrigated soils than irrigated. Results highlighted the effectiveness of wastewater irrigation in P availability in soil. Based on the PHREEQC modeling data, precipitation of Pb and Zn mineral phases was probable in soils by wastewater influence. Meanwhile, the precipitation of stable calcium phases, that affect the P sorption and/or co-precipitation, in non-wastewater irrigated soils was highlighted in the PHREEQC calculations. The standard buffer capacity (SBC) was 43 and 64 L kg^-1 in wastewater irrigated soils and non-irrigated soils, respectively. Findings of the present study demonstrate the importance of wastewater reuse opportunities for agricultural application, especially soil P availability, and are helpful to minimize the environmental impacts of wastewater and solid waste.

Microbially mediated Fe-N coupled cycling at different hydrological regimes in riparian wetland

Although the significance of the coupled Fe- and N- cycling processes on biogeochemical transformation in riparian wetlands is well-known, the regulation associated with the changes on the microbiotas during different hydrological regimes remains unclear. This study performed field investigations on the bacterial community compositions (BCC) and specific genera associated to Fe- and N- cycling in the rhizosphere soil and sediments in a riparian wetland in Poyang lake, China. The predominant phyla Proteobacteria, Acidobacteria, and Nitrospirae from all the samples remarkably decreased after long-term continuous flooding, while Actinobacteria, Firmicutes and Bacteroidetes were enriched. For the family level, the relative abundances of iron-oxidizing bacteria (FeOB) Gallionellaceae, and N fixing bacteria Nitrospiraceae and Bradyrhizobiaceae significantly declined upon the long-term flooding and then increased with dewatering, which were consistent with the functional genes sequencing analysis. In which, the Bradyrhizobiaceae (RA 2.0 %-34.6 %) was the dominant nirS denitrifier and potential iron-reducing bacteria (FeRB), Sideroxydans lithotrophicus was one of the dominant FeOB (RA 1.7 %-23 %), which was also identified to be the nirS dentrifier (RA 0.2 %-4.3 %). The absolute quantification of the functional genes levels including nirS, nirK, FeRB (Geobacter spp.) showed their significant increases by 3-7 times upon desiccation compared to that under post-CF. The PCA and RDA results indicated the linkage between redox changes of N and Fe during inundation mediated by FeRB, NOB, and FeOB, which were closely related to hydrochemical indices NO₃^-, Fe²⁺ and SO₄^2-. These evidences all implied the likely occurrence of nitrate reduction coupled to Fe(II) oxidation (NRFeOx) under oligotrophic conditions, which was potentially facilitated by metabolizers consisting of highly correlated Bradyrhizobiaceae and Sideroxydans (rho = 0.86, p < 0.01). These findings provide an interpretation of the biological reactions in the microbially mediated NRFeOx processes driven by hydrological change, which could assist the mechanistic understanding of the global biogeochemical cycles of iron and nitrogen in riparian wetlands.

Immobile Iron-Rich Particles Promote Arsenic Retention and Regulate Arsenic Biotransformation in Treatment Wetlands

Remediation of arsenic (As)-contaminated wastewater by treatment wetlands (TWs) remains a technological challenge due to the low As adsorption capacity of wetland substrates and the release of adsorbed As to pore water. This study investigated the feasibility of using immobile iron-rich particles (IIRP) to promote As retention and to regulate As biotransformation in TWs. Iron-rich particles prepared were immobilized in the interspace of a gravel substrate. TWs with IIRP amendment (IIRP-TWs) achieved a stable As removal efficiency of 63 ± 4% over 300 days, while no As removal or release was observed in TWs without IIRP after 180 days of continuous operation. IIRP amendment provided additional adsorption sites and increased the stability of adsorbed As due to the strong binding affinity between As and Fe oxides. Microbially mediated As(III) oxidation was intensified by iron-rich particles in the anaerobic bottom layer of IIRP-TWs. Myxococcus and Fimbriimonadaceae were identified as As(III) oxidizers. Further, metagenomic binning suggested that these two bacterial taxa may have the capability for anaerobic As(III) oxidation. Overall, this study demonstrated that abiotic and biotic effects of IIRP contribute to As retention in TWs and provided insights into the role of IIRP for the remediation of As contamination.

Interactive effects of aquatic nitrogen and plant biomass on nitrous oxide emission from constructed wetlands

Understanding of mechanisms in nitrous oxide (N₂O) emission from constructed wetland (CW) is particularly important for the establishment of related strategies to reduce greenhouse gas (GHG) production during its wastewater treatment. However, plant biomass accumulation, microbial communities and nitrogen transformation genes distribution and their effects on N₂O emission from CW as affected by different nitrogen forms in aquatic environment have not been reported. This study investigated the interactive effects of aquatic nitrogen and plant biomass on N₂O emission from subsurface CW with NH₄⁺-N (CW-A) or NO₃^--N (CW-B) wastewater. The experimental results show that NH₄⁺-N and NO₃^--N removal efficiencies from CW mesocosms were 49.4% and 87.6%, which indirectly lead to N₂O emission fluxes of CW-A and CW-B maintained at 213 ± 67 and 462 ± 71 μg-N/(m²·h), respectively. Correlation analysis of nitrogen conversion dynamic indicated that NO₂^--N accumulation closely related to N₂O emission from CW. Aquatic NH₄⁺-N could up-regulate plant biomass accumulation by intensifying citric acid cycle, glycine-serine-threonine metabolism etc., resulting in more nitrogen uptake and lower N₂O emission/total nitrogen (TN) removal ratio of CW-A compared to CW-B. Although the abundance of denitrifying bacteria and N₂O reductase nosZ in CW-B were significantly higher than that of CW-A, after fed with mixed NH₄⁺-N and NO₃^--N influent, N₂O fluxes and N₂O emission/TN removal ratio in CW-A were extremely close to that of CW-B, suggesting that nitrogen form rather than nitrogen transformation microbial communities and N₂O reductase nosZ determines N₂O emission from CW. Hence, the selection of nitrate-loving plants will play an important role in inhibiting N₂O emission from CW.

Impact analysis of hydraulic loading rate on constructed wetland: Insight into the response of bulk substrate and root-associated microbiota

Constructed wetland (CW) is an environment-friendly and low-cost technology for nutrients removal from domestic wastewater. For a well-tuned CW, hydraulic loading rate (HLR) is one of the critical factors, particularly under the challenging circumstance of more frequent heavy rainfall events brought by global warming. In this study, a comprehensive investigation was conducted to explore the influence of different HLRs on the CW's bulk substrate and root-associated microbiota aiming to yield new insight for CW management from a hybrid perspective of environmental microbiology and engineering science. The response of the microbial community and associated nutrients removal performance under different HLR settings were analyzed after a one-year operation. Results showed that the bulk substrate and rhizosphere genera involved in desulfurization and denitrification, such as Ferritrophicum, Sulfurimonas, and Sulfurisoma, were enriched in the higher HLR condition and associated with the higher total nitrogen (TN) and nitrate nitrogen (NO₃^--N) removal compared to the lower HLR condition. Co-occurrence network analysis demonstrated a more complex network under the higher HLR condition. Besides, it was observed that more stochastic in microbial assembly under the higher HLR condition. Surprisingly, zoonotic pathogens were observed and showed a greater prevalence under the higher HLR condition, indicating the potential correlation between HLR and pathogen intrusion. Collectively, this study revealed that the microbiota could be significantly altered under different HLR conditions, thereby resulting in differences in nutrients removal performance.

New insight into ammonium oxidation processes and mechanisms mediated by manganese oxide in constructed wetlands

Manganese oxide (MnOx) mediated ammonium (NH₄⁺) oxidation in wetlands is receiving increased interest; however, the biochemical mechanisms of this process are vague due to only few studies have focused on terrestrial ecosystems. In this study, three subsurface flow constructed wetlands (CWs), high/low content of Mn-sand CW (HMn-CW/LMn-CW) and quartz sand CW (C-CWs), were set up to explore the extent of ammonium nitrogen (NH₄⁺-N) removal and underlying mechanisms. According to the surface characteristics of Mn-sand, MnOx nanospheres were loaded as birnessite on the sand, while changes of the Mn/N contents indicated involvement of Mn-sand in NH₄⁺-N removal. During the 120-day operation, higher extent of NH₄⁺-N removal with decreased nitrous oxide (N₂O) emission was achieved in the HMn-CW (76%) than in the LMn-CW (73%) and C-CW (67%). According to the distribution of nitrogen compounds and Mn²⁺, Mn-sand in the HMn-CW delayed oxidation of NH₄⁺ and production of nitrate and nitrite. High abundance of Zooloea and Psychrobacter was observed in the Mn-sand layer of HMn-CW, corresponding to a higher observed NH₄⁺-N removal. NH₄⁺ oxidation to hydroxylamine and then to nitrite was enhanced in HMn-CW due to ammonia monooxygenase genes being promoted. The decrease of N₂O emission was closely related to the genus TM7a, verified by Pearson correlation analysis. Our findings expand the knowledge of MnOx-mediated NH₄⁺ oxidation in wetlands and support the potential application of manganese oxide for effective nitrogen removal in CWs.

Simultaneously enhanced removal of PAHs and nitrogen driven by Fe2+/Fe3+ cycle in constructed wetland through automatic tidal operation

The lack of dissolved oxygen and weak substrate removal capacity in constructed wetlands (CW) leads to terrible removal of nitrogen and polycyclic aromatic hydrocarbons (PAHs). In this study, automatic tidal flow CW microcosms were constructed by improving the oxygen environment (siphon and air-duct) and substrate (magnetite) to enhance purification performance and the mechanism was explored. The results showed that the addition of air-duct could improve the oxygen collection and thus improved the NH₄⁺ removal efficiency. Additionally, nitrogen removal was improved greatly due to the simultaneous nitrification and denitrification in aerobic layer with the addition of magnetite. Mass balance indicated the microbial degradation dominated (32-62%) the removal of PAHs. Metagenomic analysis proved the existence of magnetite enhanced the number of PAHs-degrading bacteria, functional groups and metabolic pathways and thus greatly improved the microbial degradation of PAHs. Furthermore, Fe²⁺/Fe³⁺ cycle played an important role in promoting the anaerobic degradation of PAHs, which might be served as an electron conduit to establish the direct interspecies electron transfer between iron-reducing bacteria (e.g. Deltaproteobacteria bacterium) and Anaerolineae bacterium to degrade PAHs efficiently. This study provided better understanding of the simultaneous removal of PAHs and nitrogen in tidal flow CWs.

Experimental Research on the Remediation Ability of Four Wetland Plants on Acid Mine Drainage

In order to study the economical, efficient, and environmentally friendly techniques for the treatment of acid mine drainage (AMD), this paper investigated the effects of watering with AMD on the growth condition, the resilience of four wetland plants, as well as the uptake and transport of pollutants by plants. The results showed that Typha orientalis was more resistant to AMD (irrigation with AMD increased its catalase activity and glutathione content and promoted its growth), so it was suitable for treating high concentrations of AMD (SO42− ≈ 9400 mg/L); Cyperus glomeratus was suitable for treating medium concentrations of AMD (SO42− ≈ 4600 mg/L); and Scirpus validus and Phragmites australis could be used to treat low concentrations of AMD (SO42− ≈ 2300 mg/L). All four plants could be used for phytoextraction for Mn-contaminated water (TF > 1). Phragmites australis could be used for phytoextraction for Zn-contaminated water, and the other three plants could be used for phytostabilisation for Zn-contaminated water (TF < 1); the microbial biomass in the soil was affected not only by the concentration of AMD but also by plant species. This study provides a scientific basis for the phytoremediation technology of AMD.

Fate of Dissolved Nitrogen in a Horizontal Levee: Seasonal Fluctuations in Nitrate Removal Processes

Horizontal levees are a nature-based approach for removing nitrogen from municipal wastewater effluent while simultaneously providing additional benefits, such as flood control. To assess nitrogen removal mechanisms and the efficacy of a horizontal levee, we monitored an experimental system receiving nitrified municipal wastewater effluent for 2 years. Based on mass balances and microbial gene abundance data, we determined that much of the applied nitrogen was most likely removed by heterotrophic denitrifiers that consumed labile organic carbon from decaying plants and added wood chips. Fe(III) and sulfate reduction driven by decay of labile organic carbon also produced Fe(II) sulfide minerals. During winter months, when heterotrophic activity was lower, strong correlations between sulfate release and nitrogen removal suggested that autotrophic denitrifiers oxidized Fe(II) sulfides using nitrate as an electron acceptor. These trends were seasonal, with Fe(II) sulfide minerals formed during summer fueling denitrification during the subsequent winter. Overall, around 30% of gaseous nitrogen losses in the winter were attributable to autotrophic denitrifiers. To predict long-term nitrogen removal, we developed an electron-transfer model that accounted for the production and consumption of electron donors. The model indicated that the labile organic carbon released from wood chips may be capable of supporting nitrogen removal from wastewater effluent for several decades with sulfide minerals, decaying vegetation, and root exudates likely sustaining nitrogen removal over a longer timescale.

Can we use mine waste as substrate in constructed wetlands to intensify nutrient removal? A critical assessment of key removal mechanisms and long-term environmental risks

The utilization of natural ores and/or mine waste as substrate in constructed wetlands (CWs) to enhance nutrient removal performance has been gaining high popularity recently. However, the knowledge regarding the long-term feasibility and key removal mechanisms, particularly the potential negative environmental effects of contaminants leached from mine waste is far insufficient. This study, for the first time, performed a critical assessment by using different CWs with three mine waste (coal gangue, iron ore and manganese ore) as substrates in a 385-day experiment treating wastewater with varying nutrient loadings. The results showed that the addition of mine waste in CWs increased removal of total phosphorus (TP) by 17-34%, and total nitrogen (TN) by 11-51%. The higher removal of TP is mainly attributed to the strong binding mechanism of phosphate with the oxides and hydroxides of Mn, Fe and/or Al, which are leached out of mine waste. Moreover, integration of mine waste in CWs also significantly stimulated biofilm establishment and enriched the relative abundance of key functional genes related to the nitrogen cycle, supporting the observed high-rate nitrogen removal. However, leaching of heavy metals (Fe, Mn, Cu and Cr) from the beded mine waste in the experimented CWs was monitored, which further influenced cytoplasmic enzymes and created oxidative stress damage to plants, resulting in a decline of nutrient uptake by plants.

Unveiling the role of sediments in phosphorus removal in pilot-scale constructed wetlands for swine wastewater treatment

The accumulation rate, fractions, and sorption capacity of phosphorus in sediments determine the removal efficiency and service life of constructed wetlands (CWs). Nine pilot-scale three-stage surface flow CWs were constructed to treat three loading rates of lagoon-pretreated swine wastewater, and surface sediment samples at initial and one-year treatment were collected to analyze the phosphorus fractions and sorption capacity. After one-year treatment, concentration of total phosphorus (TP) in sediments increased for high loading rates of wastewater, but remained stable for low loading rates. The annual accumulation rate of TP in sediments (M_a) was -43-445 mg kg^-1 yr^-1 at surface loading rate (SLR) of 36-355 g P m^-2 yr^-1. Their association could be described well using a sigmoid model, i.e., M_a = -23 + 538/(1 + exp.(-(SLR-262)/48)) (R²_adj = 0.897, RMSE = 40.8, p < 0.01), indicating that the phosphorus accumulation rates in sediments were loading rate-dependent. The sum of inorganic phosphorus fractions contributed to 80-100% of the TP concentration, and accumulation of aluminum-bound phosphorus (AlP) and iron-bound phosphorus (FeP) was responsible for variability of TP concentration in sediments. Phosphorus sorption capacity of CW1 sediments increased by 1.3-1.8 times, attributed to increased pH, and concentrations of ammonium oxalate-extractable aluminum and iron in sediments due to the wastewater input. Selecting iron and aluminum-rich materials preferentially as substrates and regulating the ratio of metal ions to phosphorus in wastewater should be alternative enhancement strategies of CWs for phosphorus removal.

Enhanced sequestration of molybdenum(VI) using composite constructed wetlands and responses of microbial communities

The molybdenum (Mo) non-point source pollution in the mining area has an irreversible impact on the surrounding water and soil ecosystems. Herein, three integrated vertical subsurface flow constructed wetlands (CWs) were constructed to assess the effects of combination substrates and plant on the removal of Mo(VI). Results showed that CW1 with combination substrates and cattail exhibited a favorable removal performance for Mo(VI) at 80.90%. Moreover, most Mo(VI) retained in the CWs was retained in the substrate (58.13-88.04%), and the largest fraction of Mo(VI) retained was the water-soluble fraction on the surface of the combination substrates. Mo(VI) removal was also influenced by the microbial community composition in substrate, especially their co-occurrence networks. The species that showed significant positive correlation with Mo(VI) removal were Planctomycetes, Latescibacteria, Armatimonadetes, and Gemmatimonadetes. Moreover, CWs added plants showed that more co-occurrences interaction between taxa occurs, which means that the wetlands efficiently select recruitment of potential microbial consortia and change the co-occurrences to remove pollution in the substrate. These results could be useful in providing an ecology-based solution for the treatment of Mo(VI) in wastewater, especially in adjusting the microbial communities for Mo(VI) removal at the genetic level.

Effectiveness of Exogenous Fe2+ on Nutrient Removal in Gravel-Based Constructed Wetlands

A group of microcosm-scale unplanted constructed wetlands (CWs) were established to evaluate the effectiveness of exogenous Fe²⁺ addition on ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), and total phosphorus (TP) removal. The addition of Fe²⁺ concentrations were 5 mg/L (CW-Fe5), 10 mg/L (CW-Fe10), 20 mg/L (CW-Fe20), 30 mg/L (CW-Fe30), and 0 mg/L (CW-CK). The microbial community in CWs was also analyzed to reveal the enhancement mechanism of pollutant removal. The results showed that the addition of Fe²⁺ could significantly (p < 0.05) reduce the NO₃^--N concentration in the CWs. When 10 mg/L Fe²⁺ was added and the hydraulic retention time (HRT) was 8 h, the highest removal rate of NO₃^--N was 88.66%. For NH₄⁺-N, when the HRT was 8-24 h, the removal rate of CW-Fe5 was the highest (35.23% at 8 h and 59.24% at 24 h). When the HRT was 48-72 h, the removal rate of NH₄⁺-N in CWs with 10 mg/L Fe²⁺ addition was the highest (85.19% at 48 h and 88.66% and 72 h). The removal rate of TP in all CWs was higher than 57.06%, compared with CW-CK, it increased 0.63-31.62% in CWs with Fe²⁺ addition; the final effluent TP concentration in CW-Fe5 (0.13 mg/L) and CW-Fe10 (0.16 mg/L) met the class III water standards in Surface Water Environmental Quality Standards of China (GB3838-2002). Microbical diversity indexes, including Shannon and Chao1, were significantly lower (p < 0.05) in Fe²⁺ amended treatment than that in CW-CK treatment. Furthermore, phylum Firmicutes, family Carnobacteriaceae, and genus Trichococcus in Fe²⁺ amended treatments was significantly (p < 0.05) higher than that in CW-CK treatment. Fe³⁺ reducing bacteria, such as Trichococcus genus, belonging to the Carnobacteriaceae in family-level, and Lactobacillales order affiliated to Firmicutes in the phylum-level, can reduce the oxidized Fe³⁺ to Fe²⁺ and continue to provide electrons for nitrate. It is recommended to consider adding an appropriate amount of iron into the water to strengthen its purifying capacity effect for constructed artificial wetlands in the future.