Oxidation Of Zero-valent Iron Research Articles

Rapid Oxidative Dissolution of Zerovalent Iron Induced by Sulfite for Efficient Removal of Arsenate and Arsenite: Selective Formation of Scorodite.

In this study, we proposed a moderate oxidation strategy for accelerating the oxidative dissolution of zerovalent iron (ZVI) using sulfite (S(IV)), thereby improving the removal of As(V) and As(III). Results revealed that, in the presence of 2.0 mM S(IV), both As(V) and As(III) were selectively converted into scorodite at pH0 3.0-7.0, while As(III) oxidation and As(V) immobilization were impressed over pH0 8.0-10.0. Batch experiments, radical quenching experiments, and electron spin resonance (ESR) measurements demonstrated that ZVI initially boosted S(IV) activation to generate SO4•-, •OH, and protons, and in turn, ZVI was further oxidized more intensely by these radicals than by oxygen. Concurrently, substantial protons derived from S(IV) oxidation neutralized hydroxyls produced by ZVI oxidation, maintaining an acidic environment conducive to the generation of scorodite rather than iron (hydr)oxides. Characterizations of X-ray diffraction (XRD), Raman, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) confirmed that scorodite was formed in situ and then exfoliated from the surface of ZVI, and approximately 75% of ZVI could still be recovered, which contributed to efficient As removal in successive runs and real As-polluted wastewater. The application of S(IV) achieved a balance among ZVI reactivity improvement, As(V)/As(III) removal, and raw material consumption, making it a promising approach for addressing arsenic contamination in wastewater treatment.

Mechanisms of well iron clogging in groundwater heat pump systems: Insights from video imaging, hydrogeochemical analysis, and geochemical modeling

Groundwater heat pump (GWHP) systems are increasingly popular as low-carbon and environmentally friendly technologies, but well clogging induced by iron remains a significant issue. This study investigated the clogging characteristics and biogeochemistry of three typical wells (pumping, injection, and observation wells) in an operating GWHP system using video imaging, sampling, and analysis of hydrogeochemical and microbial data. The results revealed that iron-induced well clogging is a complex process involving physical, chemical, and microbial factors. Pumping wells experience clogging due to water mixing with varying redox conditions, resulting in hematite-based iron oxide deposits. Injection wells exhibit higher clogging severity, with transformed oxidation and accumulation of reduced iron minerals at the solid-liquid interface, resulting in darker colored clogs with magnetite. Clogging in both extraction and injection wells is closely related to iron-rich aquifer sections, where severe clogging occurs. Shallow clogging due to iron oxide is limited and attributed to the oxidation of zero-valent iron in well casing material. Iron-oxidizing bacteria and iron-reducing bacteria were detected in the consolidated deposits of clogged wells, indicating their involvement in the clogging formation process. Moreover, a strong correlation was observed between the presence of nitrate-reducing bacteria in the water phase and the severity of clogging, suggesting a possible link between iron oxidation and nitrate reduction in the system. Geochemical modeling results further supported the observed clogging severity in GWHP systems and confirmed varying clogging mechanisms in different wells and depths. These findings contribute to the understanding of clogging in GWHP operations, aiding in robust water utilization and energy-saving efforts, and supporting global carbon reduction initiatives.

Chemical and biological effects of zero-valent iron (ZVI) concentration on in-situ production of H2 from ZVI and bioconversion of CO2 into CH4 under anaerobic conditions

The conversion of carbon dioxide (CO2) to methane (CH4) is a strategy for sequestering CO2. Zero-valent iron (ZVI) has been proposed as an alternative electron donor for the CO2 reduction to CH4. In this study, the effects of ZVI concentrations on the abiotic production of H2 (without the action of microorganisms) in the first part and on the biological conversion of CO2 to CH4 using ZVI as a direct electron donor in the second part were examined. In the abiotic H2 production, the increase in the ZVI concentration from 16 to 32, 64, and 96 g/L was found to have positive effects on both the amounts of H2 generated and the rates of H2 production because the extent of ZVI oxidation positively correlates with increasing surface area. Nevertheless, the increase in ZVI concentration from 96 to 224 g/L did not benefit the H2 production because the ZVI dissolution was suppressed by the increasing aqueous pH above 10. In the bioconversion of CO2 to CH4 using ZVI as an electron donor, the main methanogenesis pathway occurred via hydrogenotrophic methanogenesis at pH 8.7–9.5 driven by the genus Methanobacterium of the class Methanobacteria. At ZVI concentrations of 64 g/L and above, the production of volatile fatty acid (VFA) became clear. Acetate was the main VFA, indicating the induction of homoacetogenesis at ZVI concentrations of 64 g/L and above. In addition, the presence of propionate as the second major VFA suggests the production of propionate from CO2 and acetate under conditions with high H2 partial pressure. The results indicated that the pathway for ZVI/CO2 conversion to CH4 was competitive between hydrogenotrophic methanogenesis and homoacetogenesis.

Zero-valent iron in phosphate removal: Unraveling the role of particle size and dissolved oxygen

The effect of particle size and dissolved oxygen (DO) on zero-valent iron (ZVI) oxidation needs further study to propose strategies for improving phosphate (P) removal performance. In this study, batch and column experiments were conducted to investigate P removal by ZVI with five different particle sizes under aerobic and anaerobic conditions. The oxidation status of ZVI before and after the reaction was characterized. Under aerobic conditions, as ZVI size increased, the P removal rate initially increased and then decreased, while the difference in P removal was not obvious under anaerobic conditions. Furthermore, the P removal rate exhibited a significant positive correlation with the degree of ZVI oxidation (R2 = 0.877, p = 0.008), highlighting its pivotal role in P removal. The ZVI size and DO on ZVI oxidation primarily affect the types and quantities of iron corrosion products (FeCPs). Under aerobic conditions, the ZVI with small particle sizes (ZVIsps) (50 nm, 500 nm, and 5 μm) tended to form passivation layers composed of amorphous iron oxide (ap-FexOy) and FePO4, limiting ZVI oxidation and resulting in lower P removal rates (17.6%). Conversely, in situations with less passivation, amorphous iron hydroxides (ap-FexOHy) play a dominant role in the P removal process. The 50 μm ZVI eventually generated lepidocrocite (γ-FeOOH) under aerobic conditions, this sufficient oxidation was also the reason for its high P removal rate. Therefore, future research and applications of ZVI materials in P removal should concentrate on enhancing ZVI corrosion and regulating the formation of suitable FeCPs.

Control of odorants in swine manure and food waste co-composting via zero-valent iron /H2O2 System

Odors have posed challenges to the advancement of aerobic composting. This work aims to identify the primary components responsible for odors and assess the effectiveness and mechanisms of the zero-valent iron/H2O2 system controlling various odorants in aerobic composting. Swine manure and food waste were used as composting materials, with the addition of zero-valent iron and hydrogen peroxide to mitigate odor emissions. Results revealed that odorants included ammonia, hydrogen sulfide, and 22 types of volatile organic compounds (VOCs), with ethyl acetate, heptane, and dimethyl disulfide being predominant. Among the odorants emitted, ammonia accounted for 75.43%, hydrogen sulfide for 0.09%, and identified VOCs for 24.48%. The ZVI/H2O2 system showed a significant reduction in ammonia and VOCs emission, with the reduction of 51% (ammonia) and 41.3% (VOCs) respectively, primarily observed during the thermophilic period. The occurrence of Fenton-like reactions and changes in key microbial populations were the main mechanisms accounting for odor control. The occurrence of Fenton-like reaction was confirmed by X-ray photoelectron spectroscopy and reactive oxygen detection, showing the oxidation of zero-valent iron by H2O2 to higher valence elemental iron, and the simultaneous production of ·OH. Microbial analysis indicated that an enrichment of specific microorganisms with Bacillus contributed to feammonx and Bacillaceae contributed to organic biodegradation. Redundancy analysis highlighted the role of key microbial species (Bacillaceae, Bacillus, and Ureibacillus) in effectively reducing the level of ammonia and volatile organic compounds. These novelty findings illustrated that the potential of this system is promising for controlling the emission of odorants and aerobic composting reinforcement.

Characterization and mechanism of p-nitrophenol removal based on modified nanoscale zero-valent iron electrocoagulation

p-Nitrophenol (PNP) is a toxic chemical and is difficult to degrade naturally when it enters the environment. It is of great importance to develop methods to remove PNP efficiently and safely. Electrocoagulation has been widely used for the removal of PNP, but challenges remain in the improvement of anode materials and solid-liquid separation of PNP-containing precipitates. In this work, the characteristics and mechanism of activated biochar (ABC) and sulfide dual-modified nanoscale zero-valent iron (nZVI) electrocoagulation anode for the removal of PNP were systematically investigated. The synergistic effect of magnetic iron oxide generated by zero-valent iron oxidation and electrochemical oxidation technique was used to rapidly reduce the PNP concentration for water purification. The results showed that the modified nZVI anode had higher electrocoagulation activity than the iron sheet anode, and the S-nZNI/ABC anode had the highest PNP removal efficiency. The main component of the precipitate after the reaction was Fe3O4, which facilitated the magnetic solid-liquid separation of the precipitate and scum. The seed germination test confirmed that the electrocoagulation material synthesized in this work had almost no adverse effects on the ecological environment. The electrocoagulation process had good degradation performance for PNP, which could significantly attenuate the ecotoxicity of PNP in the aquatic environment and thus improve the water quality. It is expected that this work may provide design references and ideas for the efficient degradation of PNP and the application of electrocoagulation.

Treatment of the insensitive munitions compound, 3-nitro-1,2,4-triazol-5-one (NTO), in flow-through columns packed with zero-valent iron.

The need for effective technologies to remediate the insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) is emerging due to the increasing use by the US Army and environmental concerns about the toxicity and aqueous mobility of NTO. Reductive treatment is essential for the complete degradation of NTO to environmentally safe products. The objective of this study is to investigate the feasibility of applying zero-valent iron (ZVI) in a continuous-flow packed bed reactor as an effective NTO remediation technology. The ZVI-packed columns treated an acidic influent (pH 3.0) or a circumneutral influent (pH 6.0) for 6months (ca. 11,000 pore volumes, PVs). Both columns effectively reduced NTO to the amine product, 3-amino-1,2,4-triazol-5-one (ATO). The column treating the pH-3.0 influent exhibited prolonged longevity in reducing NTO, treating 11-fold more PVs than the column treating pH-6.0 influent until the breakthrough point (defined as when 85% of NTO was removed). The exhausted columns (defined as when only 10% of NTO was removed) regained the NTO reducing capacity by reactivation using 1M HCl, fully removing NTO. After the experiment, solid-phase analysis of the packed-bed material showed that ZVI was oxidized to iron (oxyhydr)oxide minerals such as magnetite, lepidocrocite, and goethite during NTO treatment. This is the first report on the reduction of NTO and the concomitant oxidation of ZVI in continuous-flow column experiments. The evidence indicates that treatment in a ZVI-packed bed reactor is an effective approach for the removal of NTO.

Enhancing zerovalent iron-based Fenton-like chemistry by copper sulfide: Insight into the active sites for sustainable Fe(II) supply

Zerovalent iron (ZVI)-based Fenton-like processes have been widely applied in degrading organic contaminants. However, the surface oxyhydroxide passivation layer produced during the preparation and oxidation of ZVI hinders its dissolution and Fe(III)/Fe(II) cycling, and restricts the generation of reactive oxygen species (ROS). In this study, copper sulfide (CuS) was found to effectively enhance the degradation of diverse organic pollutants in the ZVI/H2O2 system. Moreover, the degradation performance for the actual industrial wastewater (i.e., dinitrodiazophenol wastewater) in the ZVI/H2O2 system was impressively improved by 41% with CuS addition, and the COD removal efficiency could reach 97% after 2 h of treatment. Mechanism investigation revealed that the introduction of CuS accelerated the sustainable supply of Fe(II) in the ZVI/H2O2 system. Specifically, Cu(I) and reductive sulfur species (i.e., S2–, S22–, Sn2– and H2S (aq)) from CuS directly induced efficient Fe(III)/Fe(II) cycling. The iron-copper synergistic effect between Cu(II) from CuS and ZVI expedited Fe(II) generation from ZVI dissolution and Fe(III) reduction by formed Cu(I). This study not only elucidates the promotion effects of CuS on ZVI dissolution and Fe(III)/Fe(II) cycling in ZVI-based Fenton-like processes, but also provides a sustainable and high-efficiency iron-based oxidation system for removal of organic contaminants.

Bioconversion of Glycerol to 1,3-Propanediol Using Klebsiella pneumoniae L17 with the Microbially Influenced Corrosion of Zero-Valent Iron

The bacterial redox state is essential for controlling the titer and yield of the final metabolites in most bioconversion processes. Glycerol conversion to 1,3-propanediol (PDO) requires a large amount of reducing equivalent and the expression of reductive pathways. Zero-valent iron (ZVI) was used in the glycerol bioconversion of Klebsiella pneumoniae L17. The level of 1,3-PDO production increased with the oxidation of ZVI (31.8 ± 1.2 vs. 25.7 ± 0.5, ZVI vs. no ZVI) while the cellular NADH/NAD+ level increased (0.6 vs. 0.3, ZVI vs. no ZVI). X-ray diffraction showed that the iron oxide (Fe2O3) was formed during glycerol fermentation. L17 obtained electrons from ZVI and dissolved the iron continuously to form cracks on the surface, suggesting microbially influenced corrosion (MIC) was involved on the surface of ZVI. The ZVI-implemented fermentation shifted bioconversion to a more glycerol-reductive pathway. The qPCR-presented glycerol dehydratase (DhaB) with ZVI implementation was strongly expressed compared to the control. These results suggest that ZVI can contribute to the biotransformation of PDO by inducing intracellular metabolic shifts. This study could also suggest a novel microbial fermentation strategy with the application of MIC.

Effective adsorption of Pb(II) from wastewater using facile enclosed pyrolysis strategy for defect-rich lignite-based carbon-coated zero-valent iron

In recent years, nanoscale zero-valent iron has been widely applied to treat the Pb(II)-laden wastewater. And the catalytic pyrolysis is an important way to convert coal into carbon based functional materials. However, it still remains challenging for designing a novel carbon-based composite structure to effectively control nanoscale zero-valent iron oxidation and agglomeration problems. For this purpose, a defect-rich lignite-based carbon-coated nano-zero-valent iron structure (AC4) was synthesized by the one-step pyrolysis of lignite catalyzed by K2FeO4 in a closed environment. During the pyrolysis process, K2FeO4 not only promoted the formation of defective carbon and zero-valent iron, but also formed a special structure of carbon-coated iron. The larger the ratio of K2FeO4, the easier it is to generate adsorption sites. Adsorption experiments showed that AC4 exhibits a large adsorption capacity. The higher adsorption amount was due to the synergistic effect between zero-valent iron and defective carbon. The Langmuir isotherm model fitted the true experimental data successfully and the best adsorption amount of Pb(II) was 1118.54 mg/g at 298.15 K and pH 5. The true experimental situation was more accurately fitted by the pseudo-second-order model and chemisorption was the main reason for the adsorption. From a thermodynamic point of view, adsorption was an endothermic reaction and a spontaneous process. This work demonstrates that lignite resources can be converted into high value-added adsorbents.

Reductive removal of pertechnetate and chromate by zero valent iron under variable ionic strength conditions

Radioactive technetium-99 (Tc) present in waste streams and subsurface plumes at legacy nuclear reprocessing sites worldwide poses potential risks to human health and environment. This research comparatively evaluated efficiency of zero-valent iron (ZVI) toward reductive removal of Tc(VII) in presence of Cr(VI) from NaCl and Na2SO4 electrolyte solutions under ambient atmospheric conditions. In both electrolytes, anticorrosive Cr(VI) suppressed oxidation of ZVI at elevated concentrations resulting in the delay of initiation of Tc(VII) reduction to Tc(IV). In the absence of Cr(VI), no delay was observed in the analogous systems. At low ionic strength (IS), retarded ZVI oxidation inhibited Tc(VII) reduction. Higher IS favored reduction of both Tc(VII) and Cr(VI), which followed second-order reaction rates in both electrolytes attributed to the more efficient iron oxidation as evident from solids characterization studies. Magnetite was the primary iron oxide phase, and its higher fraction in the SO42- solutions facilitated reductive removal of Tc(VII) and Cr(VI). In the Cl- matrix, Cr(VI) promoted further oxidation of magnetite as well as formation of chromite diminishing overall reductive capacity of this system and resulting in less effective removal of Tc(VII) compared to the SO42- solutions.

A novel way for hydroxyl radicals generation: Biochar-supported zero-valent iron composite activates oxygen to generate hydroxyl radicals

As the cheapest oxidant, it is significant to investigate how to use oxygen to generate hydroxyl radicals (·OH) effectively. Though the oxidation of zero-valent iron (ZVI) by O2 can generate ·OH, the yield is too low for application. Here, we successfully prepared the biochar-supported zero-valent iron composites (BC-ZVI) and applied them to generate ·OH. The ·OH quantitative experiments presented that the BC-ZVI (1:1) has the strongest ability to activate oxygen to generate ·OH among the different mass ratios of BC-ZVI, which can reach 20.19 μM at the concentration of 1.0 g/L. Materials characterization results demonstrated that the uniform dispersion of ZVI particles and the catalytic capacity of biochar led to the superior performance of BC-ZVI (1:1) in ·OH generation. The O2·- and H2O2 are proved to be the intermediate reactive oxygen species in the generation of ·OH through reactive oxygen species (ROS) capture experiments. Our findings presented that the BC-ZVI (1:1) exhibited high performance in TC removal (186.26 mg/g) and the role of ·OH was demonstrated by quenching experiments. Overall, those results indicated that BC-ZVI (1:1) composite could be a promising material to generate ·OH and remove pollutants.

In-situ reduction-passivation synthesis of magnetic octahedron accumulated by Fe@Fe3O4-C core@complex-shell for the activation of persulfate

The iron-based heterogeneous catalysts, especially the zero valent iron (ZVI) is an eco-friendly and cost-effective activator for persulfate (PS) excitation. However, the oxidation of surface ZVI reduces its service lifetime. Herein, a series of magnetic octahedrons accumulated by Fe@Fe3O4-C core@complex-shell were constructed by in-situ reduction-passivation of the MIL-101(Fe). The catalysts exhibited good activity, high stability and magnetic recyclability. The optimized Fe@Fe3O4-C-60 with largest proportion of active iron achieved a 100 % conversion of Rhodamine B (RhB) within 4 min and high mineralization efficiency of 667.34 % in 30 min. Moreover, Fe@Fe3O4-C-60 maintained its activity even after 3 times of cycling test and could be easily magnetic separation. The rate constant was 1.23 min−1 for Fe@Fe3O4-C-60+PS in first 3 min, which was 1.6, 5 and 323 times of Fe0+PS, FeSO4+PS and Fe3O4+PS. Fe0 and Fe2+ were identified as active sites for SO4•- generating, while the carbon shell was responsible for the high stability. The synergistic effect between Fe0 core and Fe3O4-C complex shell realized the sustained regeneration of active surface Fe2+ and inhibition leaching of Fe.

Study on preparation methodology of zero-valent iron decorated on graphene oxide for highly efficient sonocatalytic dye degradation

Various preparation methodologies (stirring, hydrothermal, and high energy ball milling methods) were used to prepare zero-valent iron (ZVI) catalysts decorated on graphene oxide (GO) for highly efficient sonochemical dye degradation, and their dye degradation efficiencies were compared. For the effective activation of ZVI, N,S co-doped GO (N,S-GO) prepared by high-energy ball milling synthesis was also tested. According to the preparation methodologies, the ZVI catalysts showed different degrees of surface passivation, morphologies, and zeta-potentials because of the different degrees of aerial oxidation of ZVI, which are the key factors determining their activities in sonocatalytic dye degradation. The ZVI catalysts on GO prepared using the stirring and hydrothermal methods showed a high oxidation state with low sonocatalytic activity. In contrast, the ZVI catalysts on GO prepared using the high-energy ball milling method exhibited moderate sonocatalytic activity due to moderate passivation. The ZVI on N,S-GO prepared by stepwise high-energy ball milling synthesis showed the maximum activity (100% removal efficiency within 1 min) due to the minimum aerial oxidation. The rapid generation of •OH radicals by high-energy sonication, the existence of mixed-valence states of iron via activation, and N,S-doping facilitated electron transfer for the dye degradation process. The complete mineralization and degradation pathway of crystal violet dye was confirmed by electrospray ionization-mass spectrometry.

Anammox-synchronous zero-valent iron oxidation promoting synergistic nitrogen and phosphorus removal from wastewater

In this study, a novel process, anammox–synchronous zero-valent iron (ZVI) oxidationin whichnitrate byproductsare usedfor in situoxidization of ZVIwas first developed to simultaneously remove nitrogen and phosphorus from wastewater. The optimal ZVI dosage of 4 g/L significantly improved the nitrogen removal efficiency to 86.02 ± 1.98%, with the highest phosphorus removal efficiency enhanced from 39.62% to 98.97%. Several analytical techniques revealed that iron phosphate minerals formed by biologically induced mineralization promoted the phosphorus removal of the system and enhanced the settleability of granules. Candidatus Brocadia was the main anammox functional bacteria, which could promote the formation of iron phosphorus minerals. The increase of key functional genes involved in denitrification, especially narG, played a pivotal role in reducing nitrate to improve nitrogen removal performance. In addition, abundance regulation of gene fur enabled anammox bacteria always maintain high activity under different pH and ZVI dosages.

Improved performance and applicability of copper-iron bimetal by sulfidation for Cr(VI) removal

The reactivity of zero-valent iron (ZVI) for the Cr(VI) removal in groundwater is mainly limited by the formation of a passivation layer during its application in permeable reactive barrier (PRB). A kind of sulfidated copper-iron bimetal (S-ZVICu) with high reactivity for Cr(VI) removal was prepared by depositing FeSx onto copper modified ZVI via a one-pot method. The surface characteristic, reactivity and Cr(VI) removal performance of S-ZVICu were investigated. It was found that S-ZVICu had a Cr(VI) removal capacity as high as 67.5 mg/g and little risk of secondary contaminant of Cu(II). The optimal Cu/Fe mass ratio and S/Fe molar ratio were 0.0125 and 0.084, respectively. The S-ZVICu exhibited great superiority of Cr(VI) removal compared with ZVI, sulfidated ZVI (SZVI) and coper-iron bimetal (ZVICu). Mineralogy and morphology analysis showed that S-ZVICu had a hierarchical structure of Fe0/Cu0/FeSx, which could effectively reduce the risk of secondary contaminant of copper ions. The mechanism analysis suggested that the copper and FeSx successively plated on the surface of ZVI played a dual role in promoting the corrosion of zero-valent iron, and was facilitated to electron transfer between Fe0, Cu0, FeSx and Cr(VI). In addition, the loose FeSx layer had a positive effect on alleviating the oxidation of ZVI in air, which was helpful in maintaining the reactivity of S-ZVICu in the air. S-ZVICu is an environmentally friendly material for sustainable and effective removal of Cr(VI) in groundwater.

In-situ methane enrichment of biogas from anaerobic digestion of palm oil mill effluent by addition of zero valent iron (ZVI)

Palm oil mill effluent (POME) is a wastewater effluent that is generated from palm oil milling. Treatment of POME, especially using biological treatment methods, is a challenge as it contains high amounts of organic and sulfur compounds, and it is highly acidic. In this research, the effects of zero-valent iron (ZVI) on the enhancement of methane production from POME via anaerobic digestion were investigated. Furthermore, to identify the reactor operation modes that were suitable for the addition of ZVI, anaerobic digestion of POME was tested in three reaction configurations: batch reactor, fed-batch reactor, and continuous stirred-tank reactor (CSTR). In the batch mode, where acidic POME was fed with 16 g/L of ZVI dose just once, methane production increased by 74%. However, as the oxidation of ZVI under anaerobic conditions led to the production of hydroxyl ions, the pH of the medium continuously increased from approximately 7 to 9, which is not suitable for methanogenesis. In the fed-batch mode that involved intermittent feeding of acidic POME, the pH of the culture media was maintained at 6.8. This is because the extra hydroxyl ions generated from the oxidation reaction of ZVI tended to neutralize the acids in the feeding substrate. In addition, ZVI promoted the production of methane from POME and increased the average methane content in biogas from 62% to 76%. In the CSTR mode, which involved continuous feeding of acidic POME, ZVI increased methane production by 86% (from 1.79 to 3.32 L/day), methane content in biogas from 60 to 75%, and total chemical oxygen demand (tCOD) removal efficiency from 78 to 89 to 88–95%. Thus, the addition of ZVI can be a potential strategy for in-situ methane enrichment of biogas by anaerobic digestion of POME. This is because ZVI acts as a buffer for acid generation and provides extra electrons, ferrous ions, and ferric ions, which promote key microbial activities in the anaerobic digestion process.

Increased structural defects of graphene oxide compromised reductive capacity of ZVI towards hexavalent chromium

Graphene oxide (GO) was treated with irradiation beams to understand the defective degree of carbon structure of GO in relation to electron transfer property of impregnated zerovalent iron (ZVI). The GO-supported ZVI (ZVI/GO) was synthesized and then characterized by an X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The results showed that the oxygen-bearing functional groups, oxygen content and structural disorder were increased as a function of irradiation beam intensity. ZVI was dominant in the composites, but proportion of iron oxide increased with greater oxygen content. Batch sorption revealed that Cr(VI) removal decreased from 20.11 g kg−1 to 2.30 g kg−1 as solution pH rose from 3 to 9. Cr(VI) removal capacity was 26.39 g kg−1, 23.12 g kg−1 and 12.35 g kg−1 for ZVI/GO0, ZVI/GO12.3 and ZVI/GO36.9, respectively. The reduction capacity of sorbents followed similar trends as Cr(VI) sorption as per desorption experiment, which accounted for a major Cr(VI) detoxification mechanism by ZVI/GO composites. The electrochemical tests demonstrated that unfavorable electron transfer rate of ZVI/GO composites was aggravated by greater structural disorder of GO. Thus, higher dose of irradiations could create more disorder in graphitic carbon and promote oxidation of ZVI, which hindered Cr(VI) reduction.

Enhanced simultaneous removal of nitrate and phosphate using novel solid carbon source/zero-valent iron composite

Abstract Excessive discharge of nitrate and phosphate to aquatic environment can induce bad eutrophication phenomenon. Simultaneous removal of nitrate and phosphate is challenging for that the low C/N ratios and trace phosphate in wastewater concentration limit advanced nitrate and phosphate removal, respectively. In this study, a novel iron based solid carbon source composite namely solid carbon source/zero-valent iron was prepared by solid carbon sources and zero-valent iron for advanced simultaneous removal of nitrate and phosphate. The novel composite with 30% zero-valent iron weight ratio presented best simultaneous removal performance with 1.1 ± 0.1 mg NO3–N/(L·h) and 0.21 ± 0.07 mg PO43--P/(L·h). The initial pH effects on the removal performance of the novel composite showed that initial pH = 7 remarkably enhanced the nitrate removal (1.1 ± 0.1 mg NO3–N/(L·h)) and phosphate concentration declined fastest (0.14 ± 0.07 mg PO43--P/(L·h)) at initial pH = 5.5. Physical and chemical characterization of the composite confirmed the zero-valent iron oxidation and hydroxidation process after used and phosphate adsorption and precipitation were involved in this process. Microbial communities at genus level on the surface of the composite were identified to be capable of complex carbon hydrolysis and decomposition and denitrification, demonstrating the dominant role of microbial denitrification in nitrate removal. Interestingly, the observation of nitrate reducing Fe(II)-oxidizing bacteria suggested the synergistic effect of autotrophic and heterotrophic denitrification. The novel composite exhibited simultaneous removal of nitrate and phosphate effectively and can be applied in nutrients control in wastewater such as secondary effluent.

Enhancement of microbial redox cycling of iron in zero-valent iron oxidation coupling with deca-brominated diphenyl ether removal

Iron-redox cycling microorganisms are important for understanding the biogeochemical iron and play key roles in zero-valent iron (ZVI) mediated environmental bioremediation. Their influence on ZVI oxidation coupling with organic contaminant removal is of particular interest but is still poorly understood. The objective of this research was to study microbial redox cycles of iron in ZVI oxidation and deca-brominated diphenyl ether (deca-BDE) removal. It was found that iron-oxidizing bacteria (IOB) enhanced ZVI oxidation by using iron as the sole electron donor. Iron-reducing bacteria (IRB) with high activity of Fe (III) reduction, also significantly accelerated rather than inhibited ZVI oxidation. ZVI oxidation activity was increased from 3.42% to 24.28% by IOB and 19.49% by IRB. When deca-BDE was present in the medium, ZVI oxidation activity by IOB and IRB was increased from 2.67% to 48.33% and 64.33%, respectively. However, no co-accelerating effect of IOB and IRB occurred but rather a neutralizing influence on ZVI oxidation was detected with iron-redox cycling bacteria (IORB). ZVI oxidation activity by IORB only increased to 13.14% and 37.0% in the absence and presence of deca-BDE, respectively. Meanwhile, IRB also exhibited the highest removal activity of deca-BDE. Approximately 71.67% of deca-BDE was removed by IRB, compared to 18.91% by IOB and 43.24% by IORB. Deca-BDE significantly influenced the effects of iron-metabolizing microorganisms on ZVI oxidation by altering the composition of microbial communities. Pseudomonas, Paenibacillus, and Sporolactobacillus were the key genera influencing ZVI oxidation and deca-BDE removal. Sporolactobacillus was firstly reported to be able to stimulate both ZVI oxidation and deca-BDE removal. Pseudomonas accelerated ZVI oxidation but had no significant contribution to deca-BDE removal. However, Paenibacillus inhibited both Fe(III) reduction and deca-BDE removal. It is expected that continuous integration of ZVI oxidation and organic contaminant removal can be achieved by regulating the key genera in iron-metabolizing microbial communities.