Interactions of high-rate nitrate reduction and heavy metal mitigation in iron-carbon-based constructed wetlands for purifying contaminated groundwater

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.

Elemental sulfur facilitates co-metabolism of Cr(VI) and nitrate by autotrophic denitrifiers in constructed wetlands

The antagonistic microbial reduction of chromate and nitrate poses significant challenges for their simultaneous removal in autotrophic denitrifying systems. This study explored the treatment performance and mechanisms for the simultaneous removal of chromate (0, 2, and 10 mg·L-1) and nitrate (20 mg·L-1) in constructed wetland (CW) microcosms with elemental sulfur additions at 0-, 5-, and 25-times background levels. Results showed that sulfur-amended microcosms achieved chromate and nitrate removal up to 76.46 % and 28.12 % higher, respectively, than 0S groups. Notably, the nitrate removal rate constant was higher in the presence of chromate than its absence, exclusively in the 25S groups. In a sediment core assay, the 25S groups also exhibited substantially facilitating effect of chromate removal potential in the presence of nitrate. Metagenomic analyses revealed upregulations of denitrification and sulfur oxidation-related functional genes along with sulfur supplementation. Autotrophic denitrifiers including Dechloromonas, Thiobacillus, Sulfuricella, and Sulfuritalea made significant contributions to chromate and nitrate removal rates, as well as functional genes encoding sulfur, nitrogen, and chromium transformation, in response to sulfur addition. These findings shed first light on the co-metabolism of chromate and nitrate by sulfur-based autotrophic denitrifiers, emphasizing their pivotal role in denitrification systems, e.g., CWs, with chromate inputs.

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.

Efficient remediation of Hg in soils by iron-based materials: Environmental variable effect and regulatory mechanisms

The effectiveness of iron-based materials in soil remediation has gained significant attention. The mechanisms underlying the methylation and demethylation of mercury (Hg) by iron materials were still elusive. In this study, the effect of typical iron materials (pyrrhotite, hematite, and zero-valent iron (ZVI)) on the transformation of Hg were investigated. The supplementation of various iron-based material increased the THg removal efficiency in soil, particularly with ZVI, which was 5.6-14.2 % higher than that of the control. The iron-based materials also reduced the stress of Hg on plants and soil by decreasing the transformation and translocation of Hg and increasing oxidative enzyme activity of plants. The ZVI decreased the MeHg content in plants (0.1 mg/kg) compared to the control group (0.3 mg/kg). The relative abundances of genes that encoded Hg transportation (e.g. merA), glycolysis, TCA, and iron reduction were increased with the addition of iron materials. Iron-based materials also increased the complexity of the bacterial network, thereby enhancing the robustness of the microbial environmental systems that against Hg stress. The present study provided a comprehensive assessment of the efficacy of iron-based materials in remediating Hg-contaminated soils.

Electrochemically enhanced micro-electrolytic ceramic substrate infiltration system as an efficient approach for treatment of imidacloprid wastewater

In this study, an electrochemically coupled micro-electrolytic technology-enhanced soil infiltration system (E-ME-SIS) was proposed to address the problem of the high cost of traditional soil infiltration system (SIS) and the difficulty of removing imidacloprid (IMI) wastewater efficiently by a single treatment process. Micro-electrolytic ceramic substrates (MECS) were prepared from iron, activated carbon, aluminum, and fly ash and combined with an external power source to optimize the electrochemical and micro-electrolytic synergy and investigate their effectiveness in treating IMI wastewater. The results showed that MECS had a rough surface with a specific surface area of 2.682 m2/g, combining strong adsorption capacity (maximum adsorption of 1.149 mg/g) and wear resistance (24 h wear rate of 6.4%). The removal of total nitrogen (TN), total phosphorus (TP), and IMI by E-ME-SIS was stabilized at 99%, 98%, and 98%, respectively, at a current density (CD) of 0.625 mA/cm2 and influent C/N (COD/N) = 5. This study significantly enhanced the removal of difficult-to-degrade pollutants by SIS through an electrochemically enhanced micro-electrolysis reaction, which provides an energy-saving and stable technical reference for the efficient treatment of IMI wastewater with a potential for engineering applications.

The alleviation of Cr(Ⅵ) stress on simultaneous nitrification and denitrification process of Acinetobacter haemolyticus RH19

Bioremediation of Cr(Ⅵ) and ammonia is considered as a promising and cost-effective alternative to chemical and physical methods. However, Cr(Ⅵ) could inhibit nitrogen removal by inhibiting intra-/extracellular electron (IET/EET) transfer or nitrifying and denitrifying enzymes activity due to its higher solubility. In this study, we isolated a simultaneous nitrification and denitrification (SND) microorganism Acinetobacter haemolyticus RH19, capable of outcompeting oxygen to take nitrogen oxides/ammonia as electron acceptors, and studied a combined accelerant (cysteine, biotin and cytokinin) to relive the Cr(Ⅵ) stress. Respiratory chain inhibited experiments and intermediates showed that strain RH19 had the intact intracellular respiratory chain. Despite the inhibited complex Ⅳ favoring the electrons transfer to NOx--N, the SND process was still greatly inhibited with Cr(Ⅵ), likely attributed to lower electron flow to the electron acceptors (nitration/nitrition/denitrification enzyme). Instead, the accelerant detoxified Cr(Ⅵ) mainly at CoQ site responsible for electron transfer to AMO and NAP, as well as complex Ⅳ (related with aerobic denitrification), favoring the shortcut SND (SSND, NH4+-N→NH2ON→NO2--N→N2) process by directly converting nitrite to nitrogen gases. Additionally, accelerant could stimulate the secretion of c-Cyts and flavin mononucleotide (FMN) to improve the electron transfer. Overall, this study highlighted the accelerant-alleviated mechanism in the SND process under Cr(Ⅵ) stress, and deepened the theoretical SND basis for the treatment of co-existing pollutants.

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

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

Trade-off between electrochemical and microbial nutrient eliminations in iron anode-assisted constructed wetlands: The specificity of voltage level

Holistic understanding of electrocatalytic behaviors and microbiological mechanisms respond to voltage level (VL) benefits constructing performance-pathway-community linkages in iron anode-assisted constructed wetlands (IACWs). Herein, five solar-driven IACWs at 0, 1, 5, 10, and 15 V were established to treat secondary effluent for 109 days across moderate to low water temperatures (WTs). Results showed that total nitrogen (TN) (4.87-54.42%) and total phosphorus (TP) (20.66-97.35%) removals both ascended as VL raised, which primarily occurred in the cathodic regions and anodic upstream, respectively. More sustainable nitrogen elimination was achieved at lower VLs (≤ 5 V). Electrochemical contribution quantification revealed that electrochemical denitrogenation strengthened as VL improved (144.3-965.7 mg m-2 d-1), whereas severe anodic hardening and cathodic clogging in later operation impaired the dominant electrochemical denitrification at higher VLs (≥ 10 V). In contrast, microbial denitrogenation followed hump-shaped variational pattern with rising VL (peaked at 5 V). Microbial community and function analyses further clarified that despite VL elevation induced denitrifying microbiota evolution and up-regulated functional gene abundance, microbial denitrification function was significantly constrained at higher VLs. Particularly, the highest network complexity (at 1 V) and modularity (at 5 V) bred IACWs to better withstand low WT and high iron concentration. Overall, 5 V balanced electrochemical and microbial denitrogenation to obtain persistently effective TN removal. Additionally, intensified electro-coagulation dephosphorization was verified to remove most TP via adsorption and co-precipitation. This work provided a preferred VL regulation strategy to facilitate in situ sustainable nutrient purification of low-polluted wastewater in IACWs.

Ecological restoration orientated application and modification of constructed wetland substrates

Constructed wetlands (CWs) have gained recognition as an environmentally friendly and cost-efficient option for treating municipal, industrial, and agricultural wastewater. They treat wastewater by harnessing the combined action of physical, chemical, and biological processes within substrates, plants, and microorganisms, with substrates exerting the greatest influence on the life cycle and purification efficiency of the system. This review provides an in-depth discussion on the development and performance of various substrate types used in CWs, including natural materials, ore-based materials, biomass materials, waste materials, and modified and novel materials. Key substrate modification techniques are summarized in detail, such as acid-base treatment, metal doping, compound modification, and heat treatment, which enhance structural and functional properties to improve pollutant removal. The paper also systematically explores the mechanisms of introducing methods like inorganic electronic enhancement and describes their applications in improving pollutant removal in CW systems. This review provides a holistic evaluation of substrate classification and optimization strategies and a prospective discussion of their challenges and opportunities in practical applications. It contributes to the creation of more efficient and sustainable materials for CW systems and provides theoretical support for selecting and optimizing substrates, thereby driving progress in wastewater treatment technology.

Novel enhancement strategy for Hg adsorption in wastewater: Nonthermal plasma-mediated advanced modification of zero-valent iron-carbon galvanic cells with thiol functionalization

Mercury (Hg) pollution poses a critical threat to human health and the environment, necessitating urgent control measures. This study introduces a novel modification method for the common zero-valent iron-carbon (ZVI-AC) galvanic cells using a two-step process, nonthermal (NTP) irradiation followed by targeted functionalization, aiming to enhance Hg adsorption potential by adjusting the physicochemical properties of the cells. The NTP irradiated functionalized adsorbent demonstrated superior Hg adsorption performance across various concentrations and pH variations. Multichannel adsorption mechanisms were confirmed by fitting a total of 22 different adsorption isotherm models, indicating the coexistence of monolayer and multilayer adsorption processes. The NTP irradiation modifies the ZVI and AC, inducing nitrogen and oxygen doping on carbon-based surfaces and oxidizing ZVI to Fe(II)-Fe(III) species. The deepened oxidation of Fe in NTP-Fe-C, coupled with Hg2+ reduction to elemental Hg by raw Fe, contributed to Hg removal. NTP irradiation facilitated electron transfer between Fe and Hg, promoting oxidation of Fe and reduction of Hg2+ cations. The emergence of diverse Hg species further supported the multichannel adsorption/removal mechanism achieved by NTP-irradiated cells. This method offers a promising solution to Hg pollution and expands the application of the traditional iron-carbon galvanic cells in treating hazardous heavy metal wastes.

Enhanced metals precipitation in acid mine drainage by thermal activation magnesite-pine shavings dispersed alkaline substrate

The acid mine drainage (AMD) is characterized by its highly acidic nature and elevated concentrations of metal ions, thereby exerting significant impacts on both human health and the environment. This study employed a dispersed alkaline substrate (DAS) consisting of thermal activation magnesite and pine shavings for the treatment of AMD. The investigation focused on determining the optimal thermal activation conditions of magnesite, evaluating the effectiveness of the DAS in regulating acidity and removing metal ions from AMD, identifying critical factors influencing treatment efficiency, and conducting toxicity assessment on the effluent. Results indicated that calcination at 700 °C for 60 min was determined to be the optimal thermal activation condition for magnesite. Under the optimal operating conditions of the DAS (flow rate: 0.4 mL/min, filling mass ratio: thermal activation magnesite to pine shavings = 1:1), the system achieved complete removal (100%) of metal ions within 396 h while maintaining a pH above 7.0. Moreover, it significantly minimized operating costs and sludge generation. Toxicity assessment experiments conducted using Chlorella proteolytica demonstrated a significant reduction in biotoxicity after treatment with this system. These findings suggest that a DAS based on thermal activation magnesite holds promise as an effective solution for treating AMD.

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.

Unraveling the synergistic promotion mechanism of Fe0 coupling phragmites australis biomass for nitrogen removal in coastal wetland: From low to moderate salinities

The simulated coastal constructed wetlands supplemented with Fe0 and phragmites australis (P.A) biomass (CW-M) were constructed to improve nitrogen removal under different salinities (0-15‰). Results showed that the denitrification performance of CW-M were improved significantly, with the higher NO3--N removal of 72-94% and lower N2O emission flux, when compared with mono-P.A biomass(CW-bio), mono-Fe0 system (CW-Fe) and control system. The nitrogen removal showed a trend of first increasing (0‰-7‰) and then decreasing (7‰-15‰) with the highest NO3--N removal of 94% and enhanced removal efficiency of 41% in CW-M. Fe0 and P.A biomass coupling could reduce the stress of salinity on denitrification. Batch experiments have demonstrated that Fe0 and P.A biomass could mutually stimulate more total organic carbon and total iron (TFe) release as electron donors for denitrification. Meanwhile, appropriate salinity could also promote the release of TFe. The typical heterotrophic denitrifying genera Bacillus and iron autotrophic denitrifying genera Thermomonas have the highest proportion in CW-M, with 21.83% and 0.10%, respectively. Fe0 and P.A biomass adding simultaneously promoted the carbon and iron metabolism, further enhancing the nitrogen metabolism process. The joint enhancement of autotrophic and heterotrophic denitrification contributes to NO3--N removal in CW-M for treating saline, low C/N wastewater in coastal wetlands.

Simulation of nitrate nitrogen concentrations and DOM characteristics in groundwater from Southwest China's Karst Wetlands using an improved GMS model

Existing models often face limitations in the understanding and prediction of nitrate nitrogen (NO3--N) concentrations in karst groundwater. In this study, to tackle this issue, a Gaussian function model was coupled with the Groundwater Modeling System (GMS) to simulate NO3--N concentration changes in the southwest karst wetland of China. Additionally, fluorescence spectroscopy was employed to measure dissolved organic matter (DOM) components in the groundwater, providing insights into their variation and influence on NO3--N dynamics. The results demonstrated that coupling the Gaussian curve fitting method with the GMS model accurately simulated NO3--N concentration changes in the study area. The simulation revealed lower NO3--N levels in the northern region, with higher concentrations in the central area, peaking at 20.73 mg/L at lower elevations. NO3--N was primarily distributed in the southwestern region and upper Mudong Lake, exhibiting a diffusion trend from west to east. DOM analysis indicated significant autochthonous contributions, particularly microbial metabolic by-products. The total fluorescence intensity and DOM components increased downstream, with the lowest values at the source and the highest values at river confluences. The humification index (HIX) was correlated with NO3--N concentrations, where lower NO3--N levels corresponded to lower HIX values, and higher NO3--N levels corresponded to higher HIX values. In conclusion, this study provides valuable insights into NO3--N prediction in groundwater and the role of DOM, offering a reference for groundwater protection in the southwest China karst basin.

Synergetic effect of pyrrhotite and zero-valent iron on Hg(Ⅱ) removal in constructed wetland: Mechanisms of electron transfer and microbial reaction

Effective removal of mercury (Hg) from wastewater is significant due to its high toxicity, especially methylmercury (MeHg). Reducing of Hg(II) to Hg(0) in constructed wetlands (CWs) using iron-based materials is an effective strategy for preventing the formation of MeHg. However, the surface passivation of zero-valent iron (ZVI) limits its application. Herein, synergetic ZVI and pyrrhotite were utilized to enhance Hg removal in CWs. Results indicated that the removal of total Hg, dissolved Hg, and particulate Hg in CWs with ZVI and pyrrhotite were improved by 21.68 ± 0.76 %, 13.02 ± 0.88 %, and 22.27 ± 0.76 % compared to that with single ZVI or pyrrhotite. Pyrrhotite increased the surface corrosion of ZVI, thereby facilitating the process of iron reduction. The redox of iron promoted the generation of EPS, which could provide electrons for Hg(II) reduction. The sulfur also participates in electron transfer by driving the methylation of Hg and provides sulfides to form FeS-Hg complexes and HgS precipitation. The abundance of key enzymes that involved in iron reduction and Hg transformation was enhanced with the addition of ZVI and pyrrhotite. The synergetic of pyrrhotite and ZVI enhances the removal of Hg in CW, offering a promising technology for high-efficiency treatment of Hg.

Enhanced in-situ sediment remediation by calcium peroxide coupled with zero-valent iron: Simultaneous nitrogen removal and phosphorus stabilization

As the potential causes of eutrophication, nitrogen (N) and phosphorus (P) in sediments have received wide attention. However, few of the in-situ sediment remediation methods can achieve simultaneous N removal and P stabilization in sediments. In this study, different impacts on N, P and organic matter (OM) properties of sediments and overlying water with different proportions of calcium peroxide (CaO2) coupling with zero-valent iron (ZVI) were explored through incubation experiments. Compared with CaO2 or ZVI alone, the total nitrogen (TN) removal ratios in the whole system at 0.6 g/kg CaO2 coupled with 40 g/kg ZVI increased by 167.91% and 152.04%, respectively. Due to the enhancement of oxidation, the removal efficiency of OM from sediments increased by 118.51%. Meanwhile, the genera related to denitrification (e.g., Anaerobacillus, Haloplasma, and Clostridium_sensu_stricto_8) were also enriched in this coupling group, which was due to the enhanced decomposition of OM and the electron donation of ZVI. In addition, CaO2 coupled with ZVI stabilized P through chemical precipitation, which converted organic phosphorus (Org-P) into more stable calcium bounded P (Ca-P) in sediments. Hence the coupling effectively increased total P (TP) content in sediments and reduced P concentration in water.

Simultaneous removal of vanadium and nitrogen in two-stage vertical flow constructed wetlands: Performance and mechanisms

Vanadium (V(V)) and nitrate, as co-concomitant pollutants in water bodies, pose potential threats to the eco-environment and human health. This study was to reveal the feasibility of simultaneous removal of V(V) and nitrate in the series-wound vertical flow constructed wetlands (CWs) with iron ore (B-CWs)/manganese ore (C-CWs)-wood substrates. The results showed that B-CWs could achieve efficient V(V) and NO3--N removal with the influent of 2 and 10 mg/L (V(V)/NO3--N = 1:5), respectively. With the increase of V(V)/NO3--N ratio (V(V)/NO3--N = 1:1), B/C-CWs exhibited better combined pollution removal. Even when nitrate was removed (V(V)/NO3--N = 1:0), the systems could maintain a good capacity for V(V) removal. High V(V) (20 mg/L) significantly inhibited V(V) removal, with a slight recovery of the performance as the decrease of V(V) influent. High NO3--N concentration (10 mg/L) effectively enhanced V(V) removal and restored C-CWs to the better level. V(IV) precipitates/oxides were the main reducing end-products. High abundance of V(V)-reducing bacteria and iron/manganese cycling pumps ensured efficient V(V) removal.

Effects of filling substrates on remediation performance and sulfur transformation of sulfate reducing packed-bed bioreactors treating acid mine drainage

The filling substrate is one of key factors influencing effectiveness of sulfate reducing packed-bed bioreactor (SRPB) treating acid mine drainage (AMD). The effects of four substrates (i.e. quartz sand, steel residue, biochar, and peanut shell) on remediation performance and sulfur transformation of SRPB treating AMD was studied. The results showed that steel residue and biochar improved sulfate reduction efficiency (61% and 49%) compared to quartz sand (32%), whereas peanut shell inhibited sulfate reduction efficiency (19%), attributed to its decomposition process leading to a severe accumulation of acetic acid. More amounts of sulfides generated in steel residue bioreactor were converted into acid volatile sulfide and elemental sulfur, resulting in a significant decrease in dissolved sulfide in the effluent. Metals (Fe, Al, Zn, Cd and Cr) except for Mn were effectively immobilized in the bioreactors, particularly for Al and Cd. Sulfate reducing bacteria and sulfide oxidizing bacteria lived symbiotically in all bioreactors which exhibited similar heterogeneity in microbial distribution and function, i.e. bacterial sulfate reduction mainly occurring in bottom-middle layers and photoautotrophic sulfide oxidation in upper layer close to outlet. The microbial response mechanism to various substrate environments was revealed through co-occurrence networks analysis. This study suggests that attention should be paid to the inhibitory effect of acetic acid accumulation on sulfate reduction when using sole lignocellulosic waste (peanut shell), and steel residue and biochar could be utilized as filling substances to promote sulfate reduction.

Enhanced removal of tetracycline by vitamin C-modified cow manure biochar in water

Vitamin C (VC), due to its chemical properties, can provide more oxygen-containing functional groups such as hydroxyl groups for biochar (BC), which promotes the adsorption of tetracycline on biochar. Therefore, in this study, cow dung biochar (CDBC) was modified with VC and VC-modified CDBC (CDBC-VC) was synthesized. The modified biochar was characterized and related factors, adsorption kinetics, isotherms and adsorption mechanisms were investigated. Adsorption kinetics indicate a fast rate of adsorption. The adsorption isotherms showed that the maximum adsorption capacity was 31.72 mg/g (CDBC) and 50.90 mg/g (CDBC-VC), respectively, and the adsorption process was inhomogeneous with multiple molecular layers and the adsorbent has a higher affinity. Mechanistic studies showed that hydrogen bonding interactions, π-π electron donor-acceptor interactions, hydrophobic interactions, and electrostatic interactions were the key to the adsorption process. The analysis of adsorbent regeneration showed that CDBC-VC had good adsorption performance. CDBC and CDBC-VC showed the best performance in simulated industrial wastewater with removal rates of 78.81% and 93.69%. The adsorption mechanism was comprehensively analyzed using six machine learning models. The extreme gradient boosting model gave the best fit. Analysis of the weights of the input variables for predicting adsorption efficiency showed that the ratio of initial TC concentration to BC dosage (29.8%), specific surface area (23%), isoelectric point (8.8%), and ash content (7.7%) had a significant effect on the predicted results.

Enhancing total nitrogen removal in constructed wetlands: A Comparative study of iron ore and biochar amendments

Efficient nitrogen removal in constructed wetlands (CWs) remains challenging when treating agricultural runoff with a low carbon-to-nitrogen ratio (C/N). However, using biochar, iron ore, and FeCl3-modified biochar (Fe-BC) as amendments could potentially improve total nitrogen (TN) removal efficiency in CWs, but the underlying mechanisms associated with adding these substrates are unclear. In this study, five CWs: quartz sand constructed wetland (Control), biochar constructed wetland, Fe-BC constructed wetland, iron ore constructed wetland, and iron ore + biochar constructed wetland, were built to compare their treatment performance. The rhizosphere microbial community compositions and their co-occurrence networks were analyzed to reveal the underlying mechanisms driving their treatment performance. The results showed that iron ore was the most efficient amendment, although all treatments increased TN removal efficiency in the CWs. Ammonia-oxidizing, heterotrophic denitrifying, nitrate-dependent anaerobic ferrous oxidizing (NAFO), and Feammox bacteria abundance was higher in the iron ore system and led to the simultaneous removal of NH4+-N, NO3--N, and NO2--N. Visual representations of the co-occurrence networks further revealed that there was an increase in cooperative mutualism (the high proportion of positive links) and more complex interactions among genera related to the nitrogen and iron cycle (especially ammonia-oxidizing bacteria, heterotrophic denitrifying bacteria, NAFO bacteria, and Feammox bacteria) in the iron ore system, which ultimately contributed to the highest TN removal efficiency. This study provides critical insights into how different iron ore or biochar substrates could be used to treat agricultural runoff in CWs.

Distinct roles of biochar and pyrite substrates in enhancing nutrient and heavy metals removal in intermittent-aerated constructed wetlands: Performances and mechanism

Constructed wetlands have been widely employed as a cost-effective and environmentally friendly alternative for treating primary and secondary sewage effluents. In this study, biochar and pyrite were utilized as electron donor substrates in intermittent-aerated vertical flow constructed wetlands to strengthen the nutrient and heavy metals removal simultaneously, and the response of nutrient reduction and microbial community to heavy metals stress was also explored. The results indicated that biochar addition exhibited a better nitrogen removal, while pyrite addition greatly promoted the phosphorus removal. Moreover, the high removal efficiencies of Cu2+, Pb2+ and Cd2+ (above 90%) except for Zn2+ were obtained in each system. However, the exposure of heavy metals decreased phosphorus removal while had little effect on nitrogen removal. The influent load and intermittent aeration implementation led to a significant shift in microbial community structures, but microbial biodiversity and abundance decreased under the exposure of heavy metals. Particularly, Thiobacillus and Ferritrophicum, associated with sulfur autotrophic denitrification and iron autotrophic denitrification, were more abundant in pyrite-based wetland systems.

Sustainable approaches for removing toxic heavy metal from contaminated water: A comprehensive review of bioremediation and biosorption techniques

In this comprehensive study, highlights emerging environmentally friendly methods to eliminating hazardous heavy metals from contaminated water, with an emphasis on bioremediation and biosorption. Breakthroughs, such as the combination of biological remediation and nanotechnology to improve the elimination of metals effectiveness and the use of genetically modified microbes for targeted pollutant breakdown. Developing biosorption materials made from agricultural waste and biochar, this indicates interesting areas for future research and emphasizes the necessity of sustainable practices in tackling heavy metal contamination in water systems. There seems to be a surge in enthusiasm for the utilization of biological remediation and biosorption methods as sustainable and viable options for eliminating heavy metals from contaminated water in the past couple of decades. The present review intends to offer an in-depth review of the latest understanding and advances in the discipline of biological remediation methods like bioaccumulation, biofiltration, bio-slurping, and bio-venting. Biosorption is specifically explained and includes waste biomass as biosorbent with the removal mechanisms and the hindrances caused in the process are detailed. Advances in biosorption like microbes as biosorbents and the mechanism involved in it. Additionally, novel enhancement techniques like immobilization, genetic modification, and ultrasound-assisted treatment in microbial sorbent are clarified. However, the review extended with analyzing the future advances in the overall biological methods and consequences of heavy metal pollution.

Enhanced efficiencies on purifying acid mine drainage in constructed wetlands based on synergistic adsorption of attapulgite-soda residue composites and microbial sulfate reduction

Constructed wetlands (CWs) are a promising approach for treating acid mine drainage (AMD). However, the extreme acidity and high loads of heavy metals in AMD can easily lead to the collapse of CWs without proper pre-treatment. Therefore, it is considered essential to maintain efficient and stable performance for AMD treatment in CWs. In this study, pre-prepared attapulgite-soda residue (ASR) composites were used to improve the substrate of CWs. Compared with CWs filled with gravel (CWs-G), the removal efficiencies of sulfate and Fe, Mn, Cu, Zn Cd and Pb in CWs filled with ASR composites (CWs-ASR) were increased by 30% and 10-70%, respectively. These metals were mainly retained in the substrate in stable forms, such as carbonate-, Fe/Mn (oxide)hydroxide-, and sulfide-bound forms. Additionally, higher levels of photosynthetic pigments and antioxidant enzyme activities in plants, along with a richer microbial community, were observed in CWs-ASR than in CWs-G. The application of ASR composites alleviated the adverse effects of AMD stresses on wetland plants and microorganisms. In return, the increased bacteria abundance, particularly SRB genera (e.g., Thermodesulfovibrionia and Desulfobacca), promoted the formation of metal sulfides, enabling the saturated ASR adsorbed with metals to regenerate and continuously capture heavy metals. The synergistic adsorption of ASR composites and microbial sulfate reduction maintained the stable and efficient operation of CWs. This study contributes to the resource utilization of industrial alkaline by-products and promotes the breakthrough of new techniques for low-cost and passive treatment systems such as CWs.

Microbial community structure in a constructed wetland based on a recirculating aquaculture system: Exploring spatio-temporal variations and assembly mechanisms

The diversity, composition and performance of microbial communities within constructed wetlands (CW) were markedly influenced by spatio-temporal variations. A pilot-scale integrated vertical-flow constructed wetland (IVCW) as the biological purification unit within a recirculating aquaculture system (RAS) was established and monitored in this study. The investigation aimed to elucidate the responses of community structure, co-occurrence networks, and assembly mechanisms of the microbial community to spatial and temporal changes. Spatially, all a-diversity indices and microbial networks complexity were significantly higher in the upstream pool of the IVCW than in the downstream pool. Temporally, the richness increased over time, while the evenness showed a decreasing trend. The number of nodes and edges of microbial networks increased over time. Notably, the stable pollutant removal efficiencies were observed during IVCW operations, despite a-diversity and bacterial community networks exhibited significant variations across time. Functional redundancy emerged as a likely mechanism contributing to the stability of microbial ecosystem functions. Null model and neutral model analyses revealed the dominance of deterministic processes shaping microbial communities over time, with deterministic influences being more pronounced at lower a-diversity levels. DO and inorganic nitrogen emerged as the principal environmental factor influencing microbial community dynamics. This study provides a theoretical foundation for the regulation of microbial communities and environmental factors within the context of IVCW.

Study on the transformation of nitrate nitrogen by manganese-catalyzed iron-carbon micro-electrolysis and microbial coupling

Nitrate-nitrogen pertains to the nitrogen component of the overall nitrate present in a given sample in order to reduce nitrate nitrogen pollution in water, nitrate nitrogen removal methods based on iron-carbon micro-electrolysis have become a key research focus. The process and mechanism of nitrate nitrogen removal by microbial coupling was comprehensively explored in a novel iron-carbon micro-electrolysis (ICME) system. In order to establish the transformation pathway of nitrate nitrogen in water, the transformation paths of nitrate nitrogen in water before and after coupling microorganisms in three groups of continuous flow reaction devices, namely sponge iron (s-Fe0), sponge iron + biochar (s-Fe0/BC) and sponge iron + biochar + manganese sand (s-Fe0/BC/MS), were studied. The morphology and composition changes of sponge iron were analyzed by means of characterization, and the microbial population changes in the three groups were analyzed by high-throughput sequencing. Results showed that the nitrate conversion rate in the s-Fe0, s-Fe0/BC and s-Fe0/BC/MS systems reached 99.48%, 99.57% and 99.36%, respectively, with corresponding ammonia nitrogen generation, rates of 3.77%, 9.34% and 11.24% and nitrogen generation rates of 95.71%, 90.23% and 88.12%. Scanning electron microscopy imaging showed that in the s-Fe0/BC and s-Fe0/BC/MS systems the surface of sponge iron was highly corroded, with granular substances in the corrosion product clusters. X-ray photoelectron spectroscopy analysis found that the relative contents of Fe2O3 in the surface oxides of sponge iron after microbial coupling were 38.02% and 71.27% in the s-Fe0/BC and s-Fe0/BC/MS systems, while the relative Fe3O4 contents were 61.98% and 28.72%, respectively. Microbial high-throughput sequencing analysis revealed that the Chao and Ace index values in the s-Fe0 system were 871.89 and 880.78, while in the s-Fe0/BC system they were 1012.05 and 1017.29, and in the s-Fe0/BC/MS system were 1241.09 and 1198.29, respectively. The relative proportion of Thauera in the s-Fe0, s-Fe0/BC, and s-Fe0/BC/MS systems was 16.76%,14.25% and 10.01%, while the proportion of Acetoanaerobium was 15.36%, 13.27% and 11.11%, and the proportion of Chloroflexi was 0%, 1.11% and 2.18%, respectively. Furthermore, FAPROTAX function annotation found that the expression levels of chemoheterotrophs in the s-Fe0, s-Fe0/BC and s-Fe0/BC/MS systems were 43 316 OTU, 37 289 OTU and 34 205 OTU, while nitrate respiration expression levels were 16 230 OTU, 15 483 OTU and 9149 OTU, with nitrogen respiration expression levels of 16 328 OTU, 15 493 OTU and 9154 OTU, respectively. These findings suggest that nitrate is converted into nitrogen gas and ammonia nitrogen through the actions of the coupled system of sponge iron/biochar/manganese sand and microorganisms. The catalytic effect of MnO2 promotes the conversion of Fe2+ to Fe3+, generating more electrons, allowing denitrifying bacteria to reduce more nitrate nitrogen, effectively coupling the manganese-catalyzed ICME reaction and microbial denitrification. The micro-electrolysis system and the addition of manganese sand enhanced biodiversity within the s-Fe0/BC/MS system. The heterotrophic bacteria Thauera and Acetoanaerobium were the dominant microorganisms in all three systems, although the micro-electrolysis system with added manganese sand significantly reduced the proportion of facultative bacteria Thauera and Acetoanaerobium and promoted the growth of autotrophic Chloroflexi bacteria. The ecological functions of the three systems were mainly nitrate respiration and nitrogen respiration. By comparing the expression levels of nitrate respiration and nitrogen respiration in s-Fe0/BC and s-Fe0/BC/MS systems, it can be seen that the addition of manganese sand reduced microbial activity.

Boosting the denitrification efficiency of iron-based constructed wetlands in-situ via plant biomass-derived biochar: Intensified iron redox cycle and microbial responses

Considering the unsatisfied denitrification performance of carbon-limited wastewater in iron-based constructed wetlands (ICWs) caused by low electron transfer efficiency of iron substrates, utilization of plant-based conductive materials in-situ for improving the long-term reactivity of iron substrates was proposed to boost the Fe (III)/Fe (II) redox cycle thus enhance the nitrogen elimination. Here, we investigated the effects of withered Iris Pseudacorus biomass and its derived biochar on nitrogen removal for 165 days in ICWs. Results revealed that accumulate TN removal capacity in biochar-added ICW (BC-ICW) increased by 14.7 % compared to biomass-added ICW (BM-ICW), which was mainly attributed to the synergistic strengthening of iron scraps and biochar. The denitrification efficiency of BM-ICW improved by 11.6 % compared to ICWs, while its removal capacity declined with biomass consumption. Autotrophic and heterotrophic denitrifiers were enriched in BM-ICW and BC-ICW, especially biochar increased the abundance of electroactive species (Geobacter and Shewanella, etc.). An active iron cycle exhibited in BC-ICW, which can be confirmed by the presence of more liable iron minerals on iron scraps surface, the lowest Fe (III)/Fe (II) ratio (0.51), and the improved proportions of iron cycling genes (feoABC, korB, fhuF, TC.FEV.OM, etc.). The nitrate removal efficiency was positively correlated with the nitrogen, iron metabolism functional genes and the electron transfer capacity (ETC) of carbon materials (P < 0.05), indicating that redox-active carbon materials addition improved the iron scraps bioavailability by promoting electron transfer, thus enhancing the autotrophic nitrogen removal. Our findings provided a green perspective to better understand the redox properties of plant-based carbon materials in ICWs for deep bioremediation in-situ.

Simultaneous removal of nitrate, oxytetracycline and copper by ferrous-manganese co-driven immobilized bioreactor

The treatment of polluted water contaminated by nitrate, antibiotics, and heavy metals is a difficult problem in the current water treatment process. In this study, MnFe2O4 modified illite was mixed with sodium alginate (SA) to prepare a biological carrier illite@MnFe2O4@SA (IMFSA), which was used to immobilize strain Zoogloea sp. MFQ7 and construct a bioreactor. The bioreactor can use sodium acetate as a carbon source as well as ferrous and manganese ions as additional electron donors to achieve efficient nitrate removal. The denitrification capability of bioreactor was considerably enhanced by the addition of illite@MnFe2O4 (IMF) in comparison to SA biological carrier. The bioreactor was able to achieve a nitrate removal efficiency of 97.2% when hydraulic retention time is 5.0 h, C/N ratio is 2.0, and the concentration of Fe2+ and Mn2+ were 5.0 mg L-1. Furthermore, the bioreactor can achieve efficient removal of oxytetracycline (91.8%) and copper (85.6%) through the adsorption by IMF and biological iron-manganese precipitates. High-throughput sequencing results indicated that Zoogloea was successfully immobilized into the biocarrier. According to the KEGG database, it is suggested that the addition of modified IMF enhances denitrification and stimulates the expression of genes associated with the iron-manganese redox cycle.

Recovery mechanism of bio-promoters on Cr(VI) suppressed denitrification: Toxicity remediation and enhanced electron transmission

Although the biotoxicity of heavy metals has been widely studied, there are few reports on the recovery strategy of the inhibited bio-system. This study proposed a combined promoter-I (Primary promoter: l-cysteine, biotin, and cytokinin + Electron-shuttle: PMo12) to recover the denitrification suppressed by Cr(VI). Compared with self-recovery, combined promoter-I shortened the recovery time of 28 cycles, and the recovered reactor possessed more stable long-term operation performance with >95 % nitrogen removal. The biomass increased by 7.07 mg VSS/(cm3 carrier) than self-recovery due to the promoted bacterial reproduction, thereby reducing the toxicity load of chromium per unit biomass. The combined promoter-I strengthened the toxicity remediation by promoting 92.84 % of the intracellular chromium release and rapidly activating anti-oxidative stress response. During toxicity remediation, ROS content quickly decreased, and the PN/PS value was 2.27 times that of self-recovery. PMo12 relieved Cr(VI) inhibition on NO3--N reduction by increasing NAR activity. The enhanced intracellular and intercellular electron transmission benefited from the stimulated NADH, FMN, and Cyt.c secretion by the primary promoter and the improved transmembrane electron transmission by Mo. PMo12 and the primary promoter synergized in regulating community structure and improving microbial richness. This study provided practical approaches for microbial toxicity remediation and maintaining high-efficiency denitrification.

Enhanced adaptability of pyrite-based constructed wetlands for low carbon to nitrogen ratio wastewater treatments: Modulation of nitrogen removal mechanisms and reduction of carbon emissions

Pyrite-based constructed wetlands (CWs) stimulated nitrate removal performance at low carbon to nitrogen (C/N) ratio has been gaining widely attention. However, the combined effects of pyrite and C/N on the nitrate removal mechanisms and greenhouse gases (GHGs) reduction were ignored. This study found that pyrite-based CWs significantly enhanced nitrate removal in C/N of 0, 1.5 and 3 by effectively driving autotrophic denitrification with high abundance of autotrophs denitrifiers (Rhodanobacter) and nitrate reductase (EC 1.7.7.2), while the enhancement was weakened in C/N of 6 by combined effect of mixotrophic denitrification and dissimilatory nitrate reduction to ammonium (DNRA) with high abundance of organic carbon-degrading bacteria (Stenotrophobacter) and DNRA-related nitrite reductase genes (nrf). Moreover, pyrite addition significantly reduced GHGs emissions from CWs in all stages with the occurrence of iron-coupled autotrophic denitrification. The study shed light on the potential mechanism for pyrite-based CWs for treating low C/N ratio wastewater.

Revealing the generation of reactive oxygen species in hydrochar and pyrochar: Insight into rational regulation of free radicals and catalytic mechanism

The removal of organic pollutants by biochar has been extensively studied. However, the differences in the removal mechanisms of contaminants by biochar obtained from different preparation techniques have not been thoroughly elucidated. In this study, the catalytic performances of hydrochar (HC) and pyrochar (PC) were compared in the dark and light. Owing to more persistent free radicals (PFRs), greater defects and stronger charge transfer ability on the surface, PC could produce a certain concentration of superoxide radicals (•O2-) even in the dark, making its degradation efficiency for benzoic acid (BA) 11% higher than that of HC. On the contrary, when the light was turned on, HC rather than PC can generate a higher amount of hydroxyl radical (•OH), resulting in an 11% higher degradation efficiency of BA compared to PC. The improvement of catalytic performance in HC originated from its oxygen-containing functional groups (OFGs), which was beneficial for its effective production of singlet oxygen (1O2) and ·OH under light exposure. For PC, its photocatalytic activity depended mainly on the formation of 1O2 induced by the triplet of DOM (dissolved organic matter), but the lack of oxidative ·OH in its system leads to a lower degradation efficiency than that of HC. To prove the universal applicability of this rule for biochar materials, HC and PC materials obtained from soybean residue were also prepared for degrading BA. This work is devoted to an in-depth exploration of the catalytic activation mechanism of biochar obtained by different technological methods, and can create conditions for the generation of more dominant reactive oxygen species (ROS) on biochar, thus providing the guidance for environmental remediation.

Simultaneously enhanced autotrophic-heterotrophic denitrification in iron-based ecological floating bed by plant biomass: Metagenomics insights into microbial communities, functional genes and nitrogen metabolic pathways

In this study, the ecological floating bed supporting with zero-valent iron (ZVI) and plant biomass (EFB-IB) was constructed to improve nitrogen removal from low-polluted water. The effects of ZVI coupling with plant biomass on microbial community structure, metabolic pathways and functional genes were analyzed by metagenomic sequencing, and the mechanism for nitrogen removal was revealed. Results showed that compared with mono-ZVI system (EFB-C), the denitrification efficiencies of EFB-IB were effectively enhanced, with the higher average NO3--N removal efficiencies of 22.60-59.19%. Simultaneously, the average NH4+-N removal efficiencies were 73.08-91.10%. Metagenomic analyses showed that EFB-IB enriched microbes that involved in iron cycle, lignocellulosic degradation and nitrogen metabolism. Plant biomass addition simultaneously increased the relative abundances of autotrophic and heterotrophic denitrifying bacteria. Network analysis showed the cooperation between autotrophic and heterotrophic denitrifying bacteria in EFB-IB. Moreover, compared with EFB-C, plant biomass addition increased the relative abundances of genes related to iron cycle, lignocellulose degradation and glycolysis processes, ensuring the production of autotrophic and heterotrophic electron donors. Therefore, the relative abundances of key enzymes and functional genes related to denitrification were higher in EFB-IB, being beneficial to the NO3--N removal. Additionally, the correlation analysis of nitrogen removal and functional genes verified the synergistic mechanism of iron-based autotrophic denitrification and plant biomass-mediated heterotrophic denitrification in EFB-IB. In summary, plant biomass has excellent potential to improve the nitrogen removal of iron-based EFB from low-polluted water.

Highly efficient nitrate removal in sulfur-based constructed wetlands: Microbial mechanisms and environmental risks

Nitrate is widely distributed in groundwater, posing an increasing threat to both water resources and human health. In this study, the treatment performance, removal mechanisms and environmental risks of sulfur-based constructed wetlands (CWs) for purifying nitrate-contaminated groundwater were investigated. Results showed that sulfur-based CWs could achieve the highest nitrate removal (95%). However, sulfate was largely produced as a by-product in sulfur-based CWs, which declined the nitrogen and phosphorus assimilation by plants. Metagenomic analysis indicated that autotrophs denitrifiers (e.g., Thiobacillus) were enriched, and the abundance of nitrate removal genes was enhanced in sulfur-based CWs. Additionally, sulfur cycle was formed in sulfur-based CWs, which explained the highest nitrate removal reasonably. This study provides comprehensive insights into the nitrate removal mechanisms in sulfur-based CWs and the associated environmental risks in purifying the polluted groundwater.

Micron zero-valent iron chitosan hydrogel balls boosts nitrate removal in constructed wetlands for secondary effluent treatment

Reducing nitrate in the secondary effluent from municipal wastewater treatment plants can prevent eutrophication, which can be achieved by constructed wetlands. Zero-valent iron has been used as electron donors for nitrate removal in constructed wetlands to deal with the low carbon-to-nitrogen ratio (C/N) problem, but the effects are often limited by passivation. In this study, micron zero-valent iron chitosan hydrogel balls were prepared as part of the substrate. The total nitrogen removal efficiency maintained at 85 %-96 % in 70 days. The chelating ability of chitosan could reduce the formation of iron oxides on the surface of iron particles and microbial cells, thus eliminating the passivation. Denitrification microorganisms were enriched and the expressions of denitrification genes were increased. The study provides new understandings of further improving the nitrate removal efficiency of constructed wetlands under low C/N and efficient use of iron materials.

Promoting aerobic denitrification in reservoir water with iron-activated carbon: Enhanced nitrogen and organics removal efficiency, and biological mechanisms

Carbon scarcity limits denitrification in micropolluted water, especially in drinking water reservoirs. Therefore, a Fe-activated carbon (AC) carrier was used in this study to enhance the nitrogen removal capacity of aboriginal denitrification in drinking water reservoirs under aerobic conditions. Following carrier addition, total nitrogen (TN) and permanganate index (CODMn) removal efficiencies reached 81.89% and 72.66%, respectively, and were enhanced by 40.45% and 39.65%. Nitrogen balance analysis indicated that 77.86% of the initial TN was converted into gaseous nitrogen. Biolog analysis suggested that the metabolic activity of denitrifying bacteria was substantially enhanced. 16S rRNA gene sequencing indicated that organic degradation bacteria, hydrogen-consuming, Fe-oxidizing, and Fe-reducing denitrifying bacteria (e.g., Arenimonas, Hydrogenophaga, Zoogloea, Methylibium, and Piscinibacter) evolved into the dominant species. Additionally, napA, nirS, nirK, and nosZ genes were enriched by 3.17, 6.68, 0.40, and 6.70 folds, respectively, which is conducive to complete denitrification. These results provide a novel pathway for the use of Fe-AC to promote aerobic denitrification in micropolluted drinking water reservoirs.

Effects of chromate on nitrogen removal and microbial community in two-stage vertical-flow constructed wetlands

Nitrogen and chromium (Cr(VI)) pollution in waterbodies pose great threats to human health, and a cost-effective alternative with Cr(VI) and nitrogen simultaneous removal is still needed. This study investigated the influence of Cr(VI) on nitrogen removal in the two-stage vertical-flow constructed wetlands (TS-VFCWs) along with iron ore and woodchip, and explored relationship between Cr(VI) and nitrogen removal. The results showed that efficient Cr(VI) and nitrogen removal were simultaneously achieved in TS-VFCWs together with iron-ore and woodchip under 2 mg/L-Cr(VI), whereas 10 mg/L-Cr(VI) gave significant and recoverable inhibition of nitrogen removal. Cr(VI) supplementation promoted the beneficiation of Cr(VI)-reducing/resistant bacteria IMCC26207 and Bryobacter on iron-ore. Woodchip enriched Cr(VI)-reducing bacteria Streptomyces and Thiobacillus. XRD and XPS showed that abundant bound-Cr existed in the surface of iron ore and woodchip, and Cr(III) precipitation/oxide was the major product. High abundances of nitrifying and autotrophic/heterotrophic denitrifying bacteria ensured good nitrogen removal at Cr(VI) stress.

Magnetic loofah sponge biochar facilitates microbial interspecies cooperation in surface and subsurface sediments for enhanced PAH biodegradation

Magnetic biochar had been used for the bioremediation of polycyclic aromatic hydrocarbon (PAH)-contaminated sediments. However, the long-term remediation pattern of vertical stratification driven by the application of magnetic biochar and the assembly of microbes had received little attention. In this study, magnetic loofah sponge biochar (MagLsBC), magnetic iron oxide (MagOx) and magnetic coconut shell activated carbon (MagCoAC) were applied for the 900-day remediation of contaminated sediments. Significant (p < 0.05) PAH biodegradation was observed in both the surface and subsurface sediments with MagLsBC addition. However, enhanced PAH biodegradation was observed only in the surface sediments with MagOx and MagCoAC treatments. Magnetotactic bacteria (Magnetococcus) was dominant genera in surface sediments and indigenous PAH degradation bacteria were more abundant in subsurface sediments of MagLsBC relative to other bacterial communities. The network interaction between microbes in surface and subsurface sediments with MagLsBC treatments was a less complex and tighter than those with MagCoAC, MagOx or Control treatments. Long-distance electron transfer rates could be enhanced through cooperation between magnetotactic bacteria and indigenous degradation bacteria, thus accelerating PAH degradation in sediment with MagLsBC treatment, especially in the underlying sediment.

Efficient purification of a nitrate and chlorate mixture in water via photoredox activated intermediate coupling-decoupling pathway

Nitrate (NO3-) is a widespread contaminant that threatens human health and ecological safety. Meanwhile, the disinfection byproducts chlorate (ClO3-) is generated inevitably in conventional wastewater treatment. Therefore, the contaminants mixture of NO3- and ClO3- are universal in common emission units. Photocatalysis technology is a feasible approach for the synergistic abatement of contaminant mixture, where matching suitable oxidation reactions is a potential strategy to improve the photocatalytic reduction reactions. Herein, formate (HCOOH) oxidation is introduced to facilitate the photocatalytic reduction of the NO3- and ClO3- mixture. As a result, high purification efficiency of NO3- and ClO3- mixture are achieved, evidenced by 84.6% e--dependent removal of the mixture at a reaction time of 30 min, with 94.5% N2 selectivity and 100% Cl- selectivity, respectively. Specifically, by the close combination of in-situ characterizations and theoretical calculations, the detailed reaction mechanism is revealed, in which the intermediate coupling-decoupling route from NO3- reduction and HCOOH oxidation is established by the chlorate-induced photoredox activation, leading to the significantly enhanced efficiency for the wastewater mixture purification. The practical application of this pathway is established for simulated wastewater to show its wide applicability. This work provides new insights into photoredox catalysis technology for its environmental application.

Enhanced nutrient removal in agro-industrial wastes-amended hybrid floating treatment wetlands treating real sewage: Laboratory microcosms to field-scale studies

The use of natural agro-industrial materials as suspended fillers (SFs) in floating treatment wetlands (FTWs) to enhance nutrient removal performance has recently been gaining significant attention. However, the knowledge concerning the nutrient removal performance enhancement by different SFs (alone and in mixtures) and the major removal pathways is so far inadequate. The current research, for the first time, carried out a critical analysis using five different natural agro-industrial materials (biochar, zeolite, alum sludge, woodchip, flexible solid packing) as SFs in various FTWs of 20 L microcosm tanks, 450 L outdoor mesocosms, and a field-scale urban pond treating real wastewater over 180 d. The findings demonstrated that the incorporation of SFs in FTWs enhanced the removal performance of total nitrogen (TN) by 20-57% and total phosphorus (TP) by 23-63%. SFs further enhanced macrophyte growth and biomass production, leading to considerable increases in nutrient standing stocks. Although all the hybrid FTWs showed acceptable treatment performances, FTWs set up with mixtures of all five SFs significantly enhanced biofilm formation and enriched the abundances of the microbial community related to nitrification and denitrification processes, supporting the detected excellent N retention. N mass balance assessment demonstrated that nitrification-denitrification was the major N removal pathway in reinforced FTWs, and the high removal efficiency of TP was attributable to the incorporation of SFs into the FTWs. Nutrient removal efficiencies ranked in the following order among the various trials: microcosm scale (TN: 99.3% and TP: 98.4%) > mesocosm scale (TN: 84.0% and TP: 95.0%) > field scale (TN: -15.0-73.7% and TP: -31.5-77.1%). These findings demonstrate that hybrid FTWs could be easily scaled up for the removal of pollutants from eutrophic freshwater systems over the medium term in an environmentally-friendly way in regions with similar environmental conditions. Moreover, it demonstrates hybrid FTW as a novel way of disposing of significant quantities of wastes, showing a win-win means with a huge potential for large-scale application.

Spatio-temporal variation of groundwater pollution in urban wetlands and management strategies for zoning

Groundwater is an important resource to maintain the sustainable development of urban wetlands. The Jixi National Wetland Park (JNWP) was studied to realize the refined prevention and control of groundwater. The self-organizing map-K-means algorithm (SOM-KM), improved water quality index (IWQI), health risk assessment model and forward model were used comprehensively to evaluate the groundwater status and solute sources in different periods. The results showed that the groundwater chemical type in most areas was the HCO₃-Ca type. Groundwater chemistry data from different periods were clustered into five groups. Groups 1 and 5 are affected by agricultural and industrial activities, respectively. The IWQI value in the normal period was higher in most areas due to the influence of spring ploughing. The east side of the JNWP was disturbed by human activities, and the quality of drinking water continued to deteriorate from the wet period to the dry period. 64.29% of the monitoring points showed good irrigation suitability. The health risk assessment model showed that the health risk was the largest in the dry period and the smallest in the wet period. The main factors causing health risks in the wet period and other periods were NO₃^- and F^-, respectively. The overall cancer risk was within acceptable limits. The forward model and ion ratio analysis showed that the weathering of carbonate rocks was the main factor affecting the evolution of groundwater chemistry, accounting for 67.16%. The high-risk areas of pollution were mainly concentrated in the east of the JNWP. K⁺ and Cl^- were the key monitoring ions in the risk-free zone and potential risk zone, respectively. The research can be used to help decision-makers carry out fine zoning control of groundwater.

Microelectrolysis-integrated constructed wetland with sponge iron filler to simultaneously enhance nitrogen and phosphorus removal

Integrating sponge iron (SI) and microelectrolysis individually into constructed wetlands (CWs) to enhance nitrogen and phosphorus removal are challenged by ammonia (NH₄⁺-N) accumulation and limited total phosphorus (TP) removal efficiency, respectively. In this study, a microelectrolysis-assisted CW using SI as filler surrounding the cathode (e-SICW) was successfully established. Results indicated that e-SICW reduced NH₄⁺-N accumulation and intensified nitrate (NO₃^--N), the total nitrogen (TN) and TP removal. The concentration of NH₄⁺-N in the effluent from e-SICW was lower than that from SICW in the whole process with 39.2-53.2 % decrease, and as the influent NO₃^--N concentration of 15 mg/L and COD/N ratio of 3, the removal efficiencies of NO₃^--N, TN and TP in e-SICW achieved 95.7 ± 1.9 %, 79.8 ± 2.5 % and 98.0 ± 1.3 %, respectively. Microbial community analysis revealed that hydrogen autotrophic denitrifying bacteria of Hydrogenophaga was highly enriched in e-SICW.

Treatment of micro-polluted water with low C/N ratio by immobilized bioreactor using PVA/sintered ores@sponge cube: Performance effects and potential removal pathways

The widespread use of industrial products containing lead (Pb²⁺) and tetracycline (TC) medications led to the combined pollution of nitrate, Pb²⁺, and TC in water. A novel biomaterial containing polyvinyl alcohol (PVA) and sponge cube with sintered ores (PVA/sintered ores@sponge cube) was prepared to ensure the maximum NO₃^--N removal efficiency (96.21 %) of the bioreactor under the hydraulic retention time (HRT) of 7.0 h, pH of 6.0, and the carbon to nitrogen (C/N) of 1.5 that had the ability to remove TC and Pb²⁺ synergistically. Composite pollutants slightly decreased denitrification performance in the combined pollution system on account of the addition of sintered ores. Results of scanning electron microscopy (SEM) showed that the sintered ores in the biocarrier induced denitrification and the adsorption of bio‑iron oxides were involved in the removal of TC and Pb²⁺. The simultaneous removal of composite pollutants during denitrification was facilitated by extracellular polymeric substances (EPS) as revealed by Fourier transform infrared spectroscopy (FTIR) and fluorescence excitation-emission matrix (EEM). In addition, high-throughput sequencing results showed that Zoogloea had the highest proportion in the bioreactor.

Identification of microbial consortia for sustainable disposal of constructed wetland reed litter wastes

Reed is a typical emerged plant in constructed wetlands (CWs). Its litters were used as raw materials for preparing Fe-C ceramic-filler (Fe-C-CF). The physical and chemical properties of Fe-C-CF were studied under different conditions, including the mass ration of Fe to carbon (Fe/C ratio), sintering temperature, and time, to determine the optimum preparing conditions. Meanwhile, the denitrification performance and CO₂ emission flux of the surface flow constructed wetland (SFCW) systems were investigated when using Fe-C-CF as the matrix. The optimum preparing conditions for Fe-C-CF were Fe/C ratio of 1:1, sintering temperature and time of 500 °C and 20 min, respectively. The SFCW system with Fe-C-CF obtained a higher total nitrogen (TN), nitrate nitrogen (NO₃^--N), and ammonia nitrogen (NH₃-N) removal efficiencies than the control SFCW system without Fe-C-CF. Compared with the heterotrophic denitrification process, the SFCW system with Fe-C-CF decreased CO₂ emission by 67.9 g m^-2 per year. The results of microbial community analysis indicated that addition of Fe-C-CF increased the diversity and abundance of microbial communities in the SFCW systems. The dominant genus of the SFCW system with Fe-C-CF was Bacillus, while Uliginosibacterium was the dominant genus in the system without the filler.

Reducing the inhibitive effect of fluorine and heavy metals on nitrate reduction by hydroxyapatite substrate in constructed wetlands

Bio-toxic inorganic pollutants, e.g., fluorine (F) and heavy metals (HMs), in wastewaters are the potential threats to nitrate (NO₃^--N) reduction by microorganisms in constructed wetlands (CWs). Selection of suitable substrate with high F and HMs adsorption efficiency and capacity is a potential alternative for simultaneous removal of these pollutants in CWs. Herein, this study investigated the feasibility of applying hydroxyapatite (HA)-gravel media for F and HMs adsorption and its effect on NO₃^--N reduction in CWs (HA CWs) by comparing the CWs filled with gravel substrate (CK CWs). The results indicated that the removal efficiency of F, Cr, As, and NO₃^--N in HA CWs increased by 113.6-, 3.3-, 2.7-, and 0.6-folds, respectively, compared to CK CWs. The NO₃^--N reduction rate decreased by 11-46% in CK CWs after the presence of F and HMs in influent, while for HA CWs, it was only 13-22%. Excellent F and HMs adsorption capacity of HA substrate availed for wetland plants resisting F/HMs toxicity and making catalase activity lower. The HA substrate in CWs resulted in the certain succession of nitrogen-transforming bacteria, e.g., nitrifiers (Nitrospira) and denitrifiers (Thiobacillus and Desulfobacterium). More importantly, key functional genes, including nirK/nirS, korA/korB, ChrA/ChrD, arsA/arsB, catalyzing the processes of nitrogen biotransformation, energy metabolism, NO₃^--N and metal ions reduction were also enriched in HA CWs. This study highlights HA substrate reduce the inhibitive effect of F and HMs on NO₃^--N reduction, and provides new insights into how microbiota structurally and functionally respond to different substrates in CWs.

Transition of source/sink processes and fate of ammonium in groundwater along with redox gradients

Ammonium (NH₄⁺) retention/removal processes in groundwater are of great interest because of the continuous increase in nitrogenous compound loading due to anthropogenic activities. However, the transition of multiple co-occurring transformation processes that determine the fate of NH₄⁺ in groundwater along a redox gradient remains underexplored. We selected a high nitrogen (N) groundwater system in the western Hetao Basin, China, to identify and quantify NH₄⁺ source and sink processes, including mineralization, dissimilatory nitrate reduction to ammonium (DNRA), nitrification, and anammox, to better understand the dynamics of NH₄⁺. Based on redox-sensitive parameters, that is, the oxidation-reduction potential (ORP) and NH₄⁺ and nitrate (NO₃^-) contents, etc., the groundwater system was classified into three zones from upstream to downstream: zone I (oxidizing), zone II (moderately reducing), and zone III (strongly reducing). Using the ¹⁵N tracing technique, we found that NH₄⁺ was mainly produced by mineralization while < 2% was produced by DNRA throughout the study area. Mineralization increased downstream because the supply of biodegradable N-containing compounds was augmented, which created a strong redox gradient to host a serial reaction chain. In zone I, NH₄⁺ was mainly transferred to NO₃^- via nitrification, whereas in zones II and III, NH₄⁺ was mainly transferred to N₂ via anammox. The average NH₄⁺ production/consumption ratios (P/C) in zones I, II, and III were 0.7, 6.9, and 51.1, respectively. Obviously, the NH₄⁺ purification ability can only exceed the supply under aerobic conditions, thus suggesting that NH₄⁺ will accumulate without limitation and be retained in strongly reducing groundwater. The situation of NH₄⁺ accumulation would deteriorate over space and time in groundwater as human activities increase without an additional artificial supply of oxidants. The results provide mechanistic insights for quantitatively comprehending the dynamics and fate of NH₄⁺ in groundwater, shedding light on groundwater NH₄⁺ mitigation techniques.

Simultaneous removal of nitrate, lead, and tetracycline by a fixed-biofilm reactor assembled with kapok fiber and sponge iron: Comparative analysis of operating conditions and biotic community

In recent years, under the condition of lack of carbon source, the presence of composite micro-pollutants make the removal of nitrate seriously damaged, and to find a suitable way to solve this problem is imminent. A fixed-biofilm carrier modified by mixing sponge iron (SI) and kapok fiber (KF) combined with strain Zoogloea sp. FY6 was constructed in this study to get a fixed-biofilm reactor with merit denitrification performance. By adjusting the operation parameters, it can be concluded that when the carbon to nitrogen (C/N) ratio was 1.5, the hydraulic retention time (HRT) was 6.0 h, and the pH was 6.0, the nitrate removal efficiency (NRE) of the fixed-biofilm reactor was up to 95.4% (2.95 mg L^-1 h^-1). In addition, the fixed-biofilm reactor constructed in this study can remove lead (Pb²⁺) and tetracycline (TC) excellently in the presence of SI and Zoogloea sp. FY6, and the denitrification performance can still maintain a high level under the influence of different concentrations of Pb²⁺ and TC. Furthermore, the addition of SI not only removes the compound pollutants, but also protects the toxicity of the pollutant inflow in the bioreactor, and the metabolic process of microorganisms in the bioreactor also removes some of the compound pollutants. The high-throughput data showed the abundance of strain Zoogloea sp. FY6 was still the highest value under the influence of various pollutants, and the metagenomic prediction showed that the fixed-biofilm reactor had perfect denitrification process and iron redox cycle benefits. This study provides a valuable reference for sustainable utilization of natural biological resources and reduction of material costs in wastewater treatment plants (WWTPs).

Comparison of nitrogen removal performance and mechanism from low-polluted wastewater by constructed wetlands with two oxygen supply strategies: Tidal flow and intermittent aeration

Due to dissolved oxygen (DO) limited nitrogen removal efficiency in constructed wetlands (CWs), two representative oxygen-suppling CWs, i.e., tidal flow constructed wetlands (TFCWs) and intermittently aerated constructed wetlands (IACWs) were proposed to compare the effect of oxygen supply strategies on the nitrogen removal performance and mechanism. Results showed that the removal efficiencies of NH₄⁺-N and COD in IACWs were as high as 90.35-97.14% and 91.14-92.44%, respectively. In terms of TN, TFCWs (83.82%) showed a significantly higher removal efficiency than IACWs, and this result was derived with the flooded/drained phase (FP/DP) ratio of 21 h:3 h in TFCWs, because rhythmic FP and DP formed a high oxygen gradient at different depths of the system, which intensified the nitrification and denitrification simultaneously. The potential nitrifying and denitrifying bacteria (e.g., Nitrospira, Azospira, Haliangium, Bradyrhizobium and Arenimonas) were enriched more significantly in TFCWs compared with IACWs, as well as Bacillus for simultaneous nitrification and denitrification, which promoted nitrogen transformation together. Also, the results of molecular ecological network analysis showed that bacterial community structure in IACWs was more complex and robust than in TFCWs, because there were obviously more nodes and links as well as a higher proportion of negative interference. However, the relationship between genera in TFCWs was closer depending on shorter path distances, and the keystone genus (Nitrosomonas) in related to nitrification was considered to play an important role in nitrogen transformation performance.

A novel constructed wetland based on iron carbon substrates: performance optimization and mechanisms of simultaneous removal of nitrogen and phosphorus

In recent years, the combination of iron carbon micro-electrolysis (ICME) with constructed wetlands (CWs) for removal of nitrogen and phosphorus has attracted more and more attention. However, the removal mechanisms by CWs with iron carbon (Fe-C) substrates are still unclear. In this study, the Fe-C based CW (CW-A) was established to improve the removal efficiencies of nitrogen and phosphorus by optimizing the operating conditions. And the removal mechanisms of nitrogen and phosphorus were explored. The results shown that the removal rates of COD, NH₄⁺-N, NO₃^--N, TN, and TP in CW-A could reach up to 84.4%, 94.0%, 81.1%, 86.6%, and 84.3%, respectively. Wetland plants and intermittent aeration have dominant effects on the removal of NH₄⁺-N, while the removal efficiencies of NO₃^--N, TN, and TP were mainly affected by Fe-C substrates, wetland plants, and HRT. XPS analysis revealed that Fe(0)/Fe²⁺ and their valence transformation played important roles on the pollutants removal. High-throughput sequencing results showed that Fe-C substrates and wetland plants had considerable impacts on the microbial community structures, such as richness and diversity of microorganism. The relative abundance of autotrophic denitrification bacteria (e.g., Denitatsoma, Thauera, and Sulfuritalea) increased in CW-A than CW-C. The electrons and H₂/[H] produced from Fe-C substrates were utilized by autotrophic denitrification bacteria for NO₃^--N reduction. Microbial degradation was the main removal mechanism of nitrogen in CW-A. Removal efficiency of phosphorus was enhanced resulted from the reaction of phosphate with iron ion. The application of CWs with Fe-C substrates and plants presented great potential for simultaneous removal of nitrogen and phosphorus.

Quantifying biochar-induced greenhouse gases emission reduction effects in constructed wetlands and its heterogeneity: A multi-level meta-analysis

Zero-waste biochar is an emerging tool for carbon neutralization, but the role of biochar in reducing greenhouse gases (GHGs) emissions from CWs were controversy and uncertainty. Yet, no previous study has integrated multiple research systems to quantitatively examine biochar-mediated GHGs emission reduction potential in CWs. Here we synthesized 114 studies to quantify biochar-induced declines ability of GHGs in the CWs by using the multi-level meta-analysis, reveal the variation of GHGs emission effect in different biochar-CWs and its response relationship with biochar, and identify the moderating variables that had a strong explanatory effect on the emission reduction effect of biochar. We showed that biochar remarkably affect CO₂ mitigation (p < 0.05), but has insignificant and heterogeneous effects on CH₄ and N₂O. Pyrolysis time, influent dissolved oxygen (DO), influent NO₃^--N concentration, hydraulic retention time (HRT) and wetland type can significantly affect the effect of biochar on CH₄ emission reduction. Particularly, the importance of HRT and wetland type was 0.89 and 0.85, respectively. Specially, the surface batch CWs modified by biochar could significantly promote the emission of CH₄ (p < 0.001), and the effect size was up to 89.59. For N₂O, biochar diameter, biochar addition ratio, influent COD/TN ratio, plant name, and removal efficiency of NO₃^--N/TN/COD were significant moderators. Among them, influent COD/TN ratio and plant name showed a stronger explanation. Planting Cyperus alternifolius L. significantly enhanced the N₂O emission reduction capacity by biochar (p < 0.001), and effect size was as low as -24.32. 700-900 °C biochar can promote CH₄ flux but inhibit N₂O flux. This study provides an important theoretical basis and valuable strategic guidance for more accurate estimation and improvement of synergistic emission reduction benefits between CH₄ and N₂O of biochar in CWs.

Iron mediated autotrophic denitrification for low C/N ratio wastewater: A review

In recent years, iron mediated autotrophic denitrification has been a concern because it overcomes the absence of organic carbon and has been successfully used in denitrification for low C/N ratio wastewater. However, there is currently a lack of a more systematic summary of iron-based materials that can be used for denitrification, and no detailed overview about the mechanism of iron mediated autotrophic denitrification has been reported. In this study, the iron materials with different valence states that can be used for denitrification were summarized, and emphasized, as well as the mechanism in different interaction systems were emphasize. In addition, the contribution of various microorganisms in nitrate reduction were analyzed and the effects of operating conditions and water quality were evaluated. Finally, the challenges and shortcomings of the denitrification process were discussed aiming to find better practical engineering applications of iron-based denitrification.