Control the greenhouse gas emission via mediating the dissimilatory iron reduction: Fulvic acid inhibit secondary mineralization of ferrihydrite

Extracellular electron shuttles induced transformation and mobilization of Fe/As with the occurrence of biogenic vivianite

Microorganisms that utilize organic matter to reduce Fe oxides/hydroxides constitute the primary geochemical processes controlling the formation of high-arsenic (As) groundwater. Biogenic secondary iron minerals play a significant role in As migration. However, the influence of quinone electron shuttles and competitive anionic phosphate on this process has not been thoroughly studied. In this study, 10 mM phosphate effectively increased the growth and reproduction of the indigenous metal-reducing bacterium Bacillus D2201, ensuring high biomass participation in goethite reduction. Three forms of goethite (pure goethite [Gt], goethite with coprecipitated As [Gt-As], and goethite with adsorbed As [Gt*As]) were synthesized and reduced by strain D2201 to investigate the fate of As/Fe. The results showed that the amount of Fe(II) released and precipitated in the Gt-As group with the addition of 9,10-anthraquinone-2,6-disulfonic acid (AQDS) and phosphate was the highest. Various solid-phase analytical techniques revealed that a significant amount of dissolved Fe(II) precipitated and formed the secondary mineral vivianite owing to phosphate input. Vivianite formation was pH-dependent, with high pH levels inhibiting vivianite development. As migration in the Gt-As system exhibited desorption and re-adsorption phenomena. The total As content decreased by 59.0 %, 53.7 %, and 49.4 %, at pH 6.0, 7.0, and 8.0, respectively, compared to the maximum As content values. The As re-adsorption percentage in the Gt*As group was lower than that in the Gt-As group, with decreases of 30.2 %, 16 %, and 10.3 % at pH, 6.0, 7.0, and 8.0, respectively. The results indicated that phosphate and AQDS enhanced goethite bioreduction and facilitated the migration of As and Fe. However, the subsequent formation of secondary vivianite resulted in the re-fixation of As and Fe. Our research suggested that metal-reducing bacteria may not universally facilitate As migration from sediments to groundwater, as previously assumed. This study highlights the effects of phosphate, As doping methods, and pH levels on As migration and transformation and refines theories on microbiologically induced high-As groundwater formation.

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

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

Microbial community assemblage altered by coprecipitation of artificial humic substances and ferrihydrite: Implications for carbon fixation pathway transformation

The suppression of soil carbon mineralization has been demonstrated to be effectively facilitated by carbon‑iron interactions, yet the specific mechanisms by which artificial humic substances (A-HS) coupled with ferrihydrite influence this process remain insufficiently explored. This study is to investigate how the A-HS, specifically artificial fulvic acid (A-FA) and artificial humic acid (A-HA), coupled with ferrihydrite, affect carbon mineralization under anaerobic system that simulates paddy flooding conditions. The object is to investigate trends in carbon emissions and to delineate microbial community structure and functional pathways. The findings indicate that A-HA and A-FA substantially reduce CO2 and CH4 emissions, with A-FA having a particularly pronounced effect on carbon fixation, halving CO2 concentrations. The low concentration of Fe(II) observed suggest that A-FA and A-HA impede the dissimilatory iron reduction (DIR) process. Detailed 16S rDNA sequencing and gene prediction analyses reveal changes in microbial community structures and functions, highlighting Methanobacterium as the dominant hydrogenotrophic methanogens. The reductive citric acid cycle, predominantly utilized by Clostridium carboxidivorans, was identified as the principal carbon fixation pathway. This work provides a novel insight into the microbial mechanisms of carbon sequestration and highlights the potential of A-HS in improving soil fertility and contributing to climate change mitigation through enhancing soil carbon storage.

Enhancing assimilatory sulfate reduction with ferrihydrite-humic acid coprecipitate in anaerobic sulfate-containing wastewater treatment

Sulfide produced from dissimilatory sulfate reduction can combine with hydrogen to form hydrogen sulfide, causing odor issues and environmental pollution. To address this problem, ferrihydrite-humic acid coprecipitate was added to improve assimilatory sulfate reduction (ASR), resulting in a decrease in sulfide production (190.2 ± 14.6 mg/L in the Fh-HA group vs. 246.3 ± 8.1 mg/L in the Fh group) with high sulfate removal. Humic acid, adsorbed on the surface of ferrihydrite, delayed secondary mineralization of ferrihydrite under sulfate reduction condition. Therefore, more iron-reducing species (e.g. Trichococcus, Geobacter) were enriched with ferrihydrite-humic acid coprecipitate to transfer more electrons to other species, which led to more COD reduction, an increase in electron transfer capacity, and a decrease in the NADH/NAD+ ratio. Metagenomic analysis also indicated that functional genes related to ASR was enhanced with ferrihydrite-humic acid coprecipitate. Thus, the addition of ferrihydrite-humic acid coprecipitate can be considered as a promising candidate for anaerobic sulfate wastewater treatment.

Iron driven organic carbon capture, pretreatment, recovery and upgrade in wastewater: Process technologies, mechanisms, and implications

Wastewater treatment plants face significant challenges in transitioning from energy-intensive systems to carbon-neutral, energy-saving systems, and a large amount of chemical energy in wastewater remains untapped. Iron is widely used in modern wastewater treatment. Research shows that leveraging the coupled redox relationship of iron and carbon can redirect this energy (in the form of carbon) towards resource utilization. Therefore, re-examining the application of iron in existing wastewater carbon processes is particularly important. In this review, we investigate the latest research progress on iron for wastewater carbon flow restructuring. During the iron-based chemically enhanced primary treatment (CEPT) process, organic carbon is captured into sludge and its bioavailability is enhanced through iron-based advanced oxidation processes (AOP) pretreatment, further being recovered or upgraded to value-added products in anaerobic biological processes. We discuss the roles and mechanisms of iron in CEPT, AOP, anaerobic biological processes, and biorefining in driving organic carbon conversion. The dosage of iron, as a critical parameter, significantly affects the recovery and utilization of sludge carbon resources, particularly by promoting effective electron transfer. We propose a pathway for beneficial conversion of wastewater organic carbon driven by iron and analyze the benefits of the main products in detail. Through this review, we hope to provide new insights into the application of iron chemicals and current wastewater treatment models.

Interactions between Iron Minerals and Dissolved Organic Matter Derived from Microplastics Inhibited the Ferrihydrite Transformation as Revealed at the Molecular Scale

Microplastic-derived dissolved organic matter (MP-DOM) is an emerging carbon source in the environment. Interactions between MP-DOM and iron minerals alter the transformation of ferrihydrite (Fh) as well as the distribution and fate of MP-DOM. However, these interactions and their effects on both two components are not fully elucidated. In this study, we selected three types of MP-DOM as model substances and utilized Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and extended X-ray absorption fine structure (EXAFS) spectroscopy to characterize the structural features of DOMs and DOM-mineral complexes at the molecular and atomic levels. Our results suggest that carboxyl and hydroxyl groups in MP-DOM increased the Fe-O bond length by 0.02-0.03 Å through interacting with Fe atoms in the first shell, thereby inhibiting the transformation of Fh to hematite (Hm). The most significant inhibition of Fh transformation was found in PS-DOM, followed by PBAT-DOM and PE-DOM. MP-DOM components, such as phenolic compounds and condensed polycyclic aromatics (MW > 360 Da) with high oxygen content and high unsaturation, exhibited stronger mineral adsorption affinity. These findings provide a profound theoretical basis for accurately predicting the behavior and fate of iron minerals as well as MP-DOM in complex natural environments.

Effective remediation of agricultural drainage at three influent strengths by bioaugmented constructed wetlands filled with mixture of iron‑carbon and organic solid substrates: Performance and mechanisms

Agricultural drainage containing a large quantity of nutrients can cause quality deterioration and algal blooming of receiving water bodies, thus needs to be effectively remediated. In this study, iron‑carbon (FeC) composite-filled constructed wetlands (Fe-C-CWs) were employed to treat farmland drainage at three pollution levels, and organic solid substrates (walnut shells) and phosphate-accumulating denitrifying bacteria (Pseudomonas sp. DWP1) were supplemented to enhance the treatment performance. The results showed that the Fe-C-CWs exhibited notably superior removal efficiency for total nitrogen (TN, 52.0-58.2 %), total phosphorus (TP, 67.8-70.2 %) and chemical oxygen demand (COD, 56.7-70.4 %) than the control systems filled solely with gravel (28.5-32.5 % for TN, 33.2-40.5 % for TP and 30.2-55.0 % for COD) at all influent strengths, through driving autotrophic denitrification, Fe-based dephosphorization, and organic degradation processes. The addition of organic substrates and functional bacteria markedly enhanced pollutant removal in the Fe-C-CWs. Furthermore, use of FeC and organic substrates and denitrifier inoculation decreased CO2 and CH4 emissions from the CWs, and reduced global warming potential of the CWs at low influent strength. Pollutant removal efficiencies in the CWs were only marginally impacted by the increasing influent loads except for NO3--N, and pollutant removal mass was largely increased with the increase of influent strengths. The microbial community in the FeC composite-filled CWs exhibited distinct distribution patterns compared to the gravel-filled CWs regardless of the influent strengths, with obviously higher proportions of dominant genera Trichococcus, Geobacter and Ferritrophicum. Keystone taxa associated with pollutant removal in the Fe-C-filled CWs were identified to be Pseudomonas, Geobacter, Ferritrophicum, Denitratisoma and Sediminibacterium. The developed augmented Fe-C-filled CWs show great promises for remediating agricultural drainage with varied pollutant loads.

Insights into the Crystallinity-Dependent Photochemical Productions of Reactive Oxygen Species from Iron Minerals

Iron minerals are widespread in earth's surface water and soil. Recent studies have revealed that under sunlight irradiation, iron minerals are photoactive on producing reactive oxygen species (ROS), a group of key species in regulating elemental cycling, microbe inactivation, and pollutant degradation. In nature, iron minerals exhibit varying crystallinity under different hydrogeological conditions. While crystallinity is a known key parameter determining the overall activity of iron minerals, the impact of iron mineral crystallinity on photochemical ROS production remains unknown. Here, we assessed the photochemical ROS production from ferrihydrites with different degrees of crystallinity. All examined ferrihydrites demonstrated photoactivity under irradiation, resulting in the generation of hydrogen peroxide (H2O2) and hydroxyl radical (•OH). The photochemical ROS production from ferrihydrites increased with decreasing ferrihydrite crystallinity. The crystallinity-dependent photochemical •OH production was primarily attributed to conduction band reduction reactions, with the reduction of O2 by conduction band electrons being the rate-limiting key process. Conversely, the crystallinity of iron minerals had a negligible influence on photon-to-electron conversion efficiency or surface Fenton-like activity. The difference in ROS productions led to a discrepant degradation efficiency of organic pollutants on iron mineral surfaces. Our study provides valuable insights into the crystallinity-dependent ROS productions from iron minerals in natural systems, emphasizing the significance of iron mineral photochemistry in natural sites with abundant lower-crystallinity iron minerals such as wetland water and surface soils.

New insights into inhibition of high Fe(III) content on anaerobic digestion of waste-activated sludge

The impacts of the increased iron in the waste-activated sludge (WAS) on its anaerobic digestion were investigated. It was found that low Fe(III) content (< 750 mg/L) promoted WAS anaerobic digestion, while the continual increase of Fe(III) inhibited CH4 production and total chemical oxygen demand (TCOD) removal. As the Fe(III) content increased to 1470 mg/L, methane production has been slightly inhibited about 5 % compared with the group containing 35 mg/L Fe(III). Particularly, as Fe(III) concentration was up to 2900 mg/L, CH4 production, and TCOD removal decreased by 43.6 % and 37.5 %, respectively, compared with the group with 35 mg/L Fe(III). Furthermore, the percentage of CO2 of the group with 2900 mg/L Fe(III) decreased by 52.8 % compared with the group containing 35 mg/L Fe(III). It indicated that Fe(II) generated by the dissimilatory iron reduction might cause CO2 consumption, which was confirmed by X-ray diffraction that siderite (FeCO3) was generated in the group with 2900 mg/L Fe(III). Further study revealed that Fe(III) promoted the WAS solubilization and hydrolysis, but inhibited acidification and methane production. The methanogenesis test with H2/CO2 as a substrate showed that CO2 consumption weakened hydrogenotrophic methanogenesis and then increased H2 partial pressure, further causing VFA accumulation. Microbial community analysis indicated that the abundance of hydrogen-utilizing methanogens decreased with the high Fe(III) content. Our study suggested that the increase of Fe(III) in sludge might inhibit methanogenesis by consuming or precipitating CO2. To achieve maximum bioenergy conversion, the iron content should be controlled to lower than 750 mg/L. The study may provide new insights into the mechanistic understanding of the inhibition of high Fe(III) content on the anaerobic digestion of WAS.

Organic binding iron formation and its mitigation in cation exchange resin assisted anaerobic digestion of chemically enhanced primary sedimentation sludge

Fe based chemically enhanced primary sedimentation (CEPS) is an effective method of capturing the colloidal particles and inorganic phosphorous (P) from wastewater but also produces Fe-CEPS sludge. Anaerobic digestion is recommended to treat the sludge for energy and phosphorus recovery. However, the aggregated sludge flocs caused by the coagulation limited sludge hydrolysis and P release during anaerobic digestion process. In this study, cation exchange resin (CER) was employed during anaerobic digestion of Fe-CEPS sludge with aims of prompting P release and carbon recovery. CER addition effectively dispersed the sludge flocs. However, the greater dispersion of sludge flocs could not translate to higher sludge hydrolysis. The maximum hydrolysis and acidification achieved at lower CER dosage of 0.5 g CER/g TS. It was observed that the extents of sludge hydrolysis and acidification had a strongly negative correlation with the organic binding iron (OBI) concentration. The presence of CER during anaerobic digestion favored Fe(III) reduction to Fe(II), and then further induced iron phase transformation, leading to the OBI formation from the released organic matters. Meanwhile, higher CER dosage resulted in higher P release efficiency and the maximum efficiency at 4 g CER/g TS was four times than that of the control. The reduction of BD-P, NaOH-P and HCl-P in solid phase contributed most P release into the supernatant. A new two-stage treatment process was further developed to immigrate the OBI formation and improve the carbon recovery efficiency. Through this process, approximately 45% of P was released, and 63% of carbon was recovered as methane from Fe-CEPS sludge via CER pretreatment.

Biochar promoted microbial iron reduction in competition with methanogenesis in anaerobic digestion

Microbial Fe (III) reduction generally could outcompete methanogenesis due to its thermodynamic advantage, while the low bioavailability of Fe (III) compounds limits this process in the anaerobic digestion system, which could result in the low recovery of vivianite. Therefore, this study investigated the competition between Fe (III) reduction and methanogenesis in the presence of different biochar (pyrochar and hydrochar). The results showed that pyrochar obtained at 500 °C (P5) resulted in the highest Fe (III) reduction (80.3%) compared to the control experiment (29.1%). P5 also decreased methane production by 9.4%. Both conductivity and surface oxygen-containing functional groups contributed to the promotion of direct electron transfer for Fe (III) reduction. Genomic-centric metatranscriptomics analysis showed that P5 led to the highest enrichment of Geobacter soli A19 and induced the significant expression of out membrane cytochrome c and pilA in Geobacter soli A19, which was related to higher Fe (III) reduction.

Sustainable disposal of Fenton sludge and enhanced organics degradation based on dissimilatory iron reduction in the hydrolytic acidification process

Fenton sludge generated in the flocculation stage of the Fenton oxidation process contains significant amounts of ferric iron and organic pollutants, which require proper treatment. Previous studies have demonstrated that adding Fenton sludge to an anaerobic digester can decompose some of the organic pollutants in the Fenton sludge to lower its environmental risk, but iron gradually accumulates in the reactor, which weakens the sustainability of the method. In this study, Fenton sludge was introduced into a hydrolytic acidification reactor with a weak acid environment to relieve the iron accumulation as well as improve the degradation of organic matter. The results showed that the added Fenton sludge acted as an extracellular electron acceptor to induce dissimilatory iron reduction, which increased chemical oxygen demand (COD) removal and acidification efficiency by 16.1% and 19.8%, respectively, compared to the group without Fenton sludge. Along with the operation, more than 90% of the Fe(III) in Fenton sludge was reduced to Fe(II), and part of them was released to the effluent. Moreover, the Fe(II) in the effluent could be used as flocculants and Fenton reagents to further decrease the effluent COD by 29.8% and 44.5%, respectively. It provided a sustainable strategy to reuse Fenton sludge to enhance organic degradation based on the iron cycle.

Enhanced Anaerobic Wastewater Treatment by a Binary Electroactive Material: Pseudocapacitance/Conductance-Mediated Microbial Interspecies Electron Transfer

Anaerobic digestion (AD) is a promising method to treat organic matter. However, AD performance was limited by the inefficient electron transfer and metabolism imbalance between acid-producing bacteria and methanogens. In this study, a novel binary electroactive material (Fe3O4@biochar) with pseudocapacitance (1.4 F/g) and conductance (10.2 μS/cm) was exploited to store-release electrons as well as enhance the direct electron transfer between acid-producing bacteria and methanogens during the AD process. The mechanism of pseudocapacitance/conductance on mediating interspecies electron transfer was deeply studied at each stage of AD. In the hydrolysis acidification stage, the pseudocapacitance of Fe3O4@biochar acting as electron acceptors proceeded NADH/NAD+ transformation of bacteria to promote ATP synthesis by 21% which supported energy for organics decomposition. In the methanogenesis stage, the conductance of Fe3O4@biochar helped the microbes establish direct interspecies electron transfer (DIET) to increase the coenzyme F420 content by 66% and then improve methane production by 13%. In the complete AD experiment, electrons generated from acid-producing bacteria were rapidly transported to methanogens via conductors. Excess electrons were buffered by the pseudocapacitor and then gradually released to methanogens which alleviated the drastic drop in pH. These findings provided a strategy to enhance the electron transfer in anaerobic treatment as well as guided the design of electroactive materials.

Effects of Fe oxides and their redox cycling on Cd activity in paddy soils: A review

Cadmium (Cd) contamination of soils is a global problem, particularly in paddy soils. Fe oxides, as a key fraction of paddy soils, can significantly affect the environmental behavior of Cd, which is controlled by complicated environmental factors. Therefore, it is necessary to systematically collect and generalize relevant knowledge, which can provide more insight into the migration mechanism of Cd and a theoretical basis for future remediation of Cd contaminated paddy soils. This paper summarized that (1) Fe oxides influence Cd activity through adsorption, complexation, and coprecipitation during transformation; (2) compared with the flooded period, the activity of Cd during the drainage period is stronger in paddy soils, and the affinity of different Fe components for Cd was distinct; (3) Fe plaque reduced Cd activity but was associated with plant Fe²⁺ nutritional status; (4) the physicochemical properties of paddy soils have the greatest impact on the interaction between Fe oxides and Cd, especially with pH and water fluctuations.

Continuously feeding fenton sludge into anaerobic digesters: Iron species change and operating stability

Fenton sludge generated from the Fenton process contains a large number of ferric species and organic pollutants, which need to be properly treated before discharge. In this study, Fenton sludge as an Fe(III) source for dissimilatory iron reduction (DIR) was continuously added with increasing dosage into an anaerobic digester to enhance the treatment. Results showed continuously feeding Fenton sludge to the anaerobic digester did not deteriorate the performance and increased methane production and COD removal rate by 2.2 folds and 14.0%, respectively. The Fe content of sludge in the digester increased from 40.25 mg/g (dry weight) to 131.53 mg/g after continuously feeding for 77days, and then declined to 109.17 mg/g when the feeding was stopped. Mass balance analysis showed that 20.5 to 48.4% of Fe in the Fenton sludge was released to the effluent. After experiment, the ratio of reducible Fe species to the total Fe was 75.1%, which maintained the high activity in DIR. Microbial community analysis showed that iron-reducing bacteria were enriched with the addition of Fenton sludge and the sludge in the digester had a higher conductivity and capacitance to strengthen the electron transfer of DIR. All results suggested that feeding Fenton sludge into anaerobic digesters was a feasible method to dispose of Fenton sludge as well as to enhance the performance of anaerobic digestion.