Rapid magnetic removal of aqueous heavy metals and their relevant mechanisms using nanoscale zero valent iron (nZVI) particles

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.

Insights in to iron-based nanoparticles (hematite and magnetite) improving the maize growth (Zea mays L.) and iron nutrition with low environmental impacts

The possible potential application of Fe-NPs on Fe nutrition, heavy metals uptake and soil microbial community needs to be investigated. In the current research, a pot experiment was used to examine the implications of Fe-NPs (α-Fe2O3 and Fe3O4) on maize growth, Fe uptake and transportation, soil microbial community, and environmental risk. Fe3O4, α-Fe2O3, FeSO4 at a rate of 800 mg Fe kg-1 were applied in soils with four replications under a completely randomized design for a period of 60 days. Results showed that Fe uptake by maize roots were increased by 107-132% than control, with obvious variations across different treatments (Fe3O4> α-Fe2O3> FeSO4> control). Similarly, plant height, leaf surface area, and biomass were increased by 40-64%, 52-91% and 38-109% respectively, with lower values by FeSO4 application. The elevated level of chlorophyll contents and carotenoids and significant effects with control on antioxidant enzymes activities (i.e., catalase, and superoxide dismutase) suggested that application of Fe-NPs improved overall biochemical processes. The differential expression of important Fe transporters (i.e., ZmYS1 and ZmFER1) as compared to control indicated the plant strategic response for efficient uptake and distribution of Fe. Importantly, Fe-NPs reduced the heavy metals uptake (i.e., chromium, cadmium, arsenic, nickel, copper) by complex formation, and showed no toxicity to the soil microbial community. In summary, the application of Fe-NPs can be a promising approach for improving crop productivity and Fe nutrition without negatively affecting soil microbial community, and fostering sustainable agricultural production.

Synergistic promotion of microalgal growth and copper removal from synthetic wastewater by nanoscale zero-valent iron particles

The increasing concentrations of heavy metals in livestock wastewater pose a serious threat to the environmental safety and human health, limiting its resource utilisation. In the present study, microalgae and nanoscale zero-valent iron were selected to construct a coupled system for copper-containing wastewater treatment. The addition of 50 mg·L-1 nanoscale zero-valent iron (50 nm) was the optimal value for the experiment, which could significantly increase the biomass of microalgae. In addition, nanoscale zero-valent iron stimulated microalgal secretion of extracellular polymeric substances, increasing the contents of binding sites, organic ligands, and functional groups on the microalgal surfaces and ultimately promoting the settling of microalgae and binding of heavy metals. The coupled system could quickly adapt to copper-containing wastewater of 10 mg·L-1, and the copper removal rate reached 94.99%. Adsorption and uptake by organisms, together with the contribution of zero-valent iron nanoparticles, are the major copper removal pathways. Overall, this work offers a novel technical solution for enhanced treatment of copper-containing livestock wastewater, which will help improve the efficiency and quality of wastewater treatment.

Cobalt/Iron Bimetallic Biochar Composites for Lead(II) Adsorption: Mechanism and Remediation Performance

The performance of nano-zero-valent iron for heavy metal remediation can be enhanced via incorporation into bimetallic carbon composites. However, few economical and green approaches are available for preparing bimetallic composite materials. In this study, novel Co/Fe bimetallic biochar composites (BC@Co/Fe-X, where X = 5 or 10 represents the CoCl2 concentration of 0.05 or 0.1 mol L-1) were prepared for the adsorption of Pb2+. The effect of the concentration of cross-linked metal ions on Pb2+ adsorption was investigated, with the composite prepared using 0.05 mol L-1 Co2+ (BC@Co/Fe-5) exhibiting the highest adsorption performance. Various factors, including the adsorption period, Pb2+ concentration, and pH, affected the adsorption of Pb2+ by BC@Co/Fe-5. Further characterisation of BC@Co/Fe-5 before and after Pb2+ adsorption using methods such as X-ray diffraction and X-ray photoelectron spectroscopy suggested that the Pb2+ adsorption mechanism involved (i) Pb2+ reduction to Pb0 by Co/Fe, (ii) Co/Fe corrosion to generate Fe2+ and fix Pb2+ in the form of PbO, and (iii) Pb2+ adsorption by Co/Fe biochar. Notably, BC@Co/Fe-5 exhibited excellent remediation performance in simulated Pb2+-contaminated water and soil with good recyclability.

Recent progress in magnetic polydopamine composites for pollutant removal in wastewater treatment

Various water pollution issues pose a significant threat to human water safety. Magnetic polydopamine composites (MPCs), which can be separated by magnetic fields after the adsorption process, exhibit outstanding adsorption capacity and heterogeneous catalytic properties, making them promising materials for water treatment applications. In particular, by modifying the polydopamine (PDA) coating, MPCs can acquire enhanced high reactivity, antibacterial properties, and biocompatibility. This also provides an attractive platform for further fabrication of hybrid materials with specific adsorption, catalytic, antibacterial, and water-oil separation capabilities. To systematically provide the background knowledge and recent research advances in MPCs, this paper presents a critical review of MPCs for water treatment in terms of both structure and mechanisms of effect in applications. Firstly, the impact of different PDA positions within the composite structure is investigated to summarize the optimization of properties contributed by PDA when acting as the shell, core, or bridge. The roles of various secondary modifications of magnetic materials by PDA in addressing water pollution problems are explored. It is anticipated that this work will be a stimulus for further research and development of magnetic composite materials with real-world application potential.

Corrosion behaviors and kinetics of nanoscale zero-valent iron in water: A review

Knowledge on corrosion behaviors and kinetics of nanoscale zero-valent iron (nZVI) in aquatic environment is particularly significant for understanding the reactivity, longevity and stability of nZVI, as well as providing theoretical guidance for developing a cost-effective nZVI-based technology and designing large-scale applications. Herein, this review gives a holistic overview on the corrosion behaviors and kinetics of nZVI in water. Firstly, Eh-pH diagram is introduced to predict the thermodynamics trend of iron corrosion. The morphological, structural, and compositional evolution of (modified-) nZVI under different environmental conditions, assisted with microscopic and spectroscopic evidence, is then summarized. Afterwards, common analytical methods and characterization technologies are categorized to establish time-resolved corrosion kinetics of nZVI in water. Specifically, stable models for calculating the corrosion rate constant of nZVI as well as electrochemical methods for monitoring the redox reaction are discussed, emphasizing their capabilities in studying the dynamic iron corrosion processes. Finally, in the future, more efforts are encouraged to study the corrosion behaviors of nZVI in long-term practical application and further build nanoparticles with precisely tailored properties. We expect that our work can deepen the understanding of the nZVI chemistry in aquatic environment.

Magnetic iron-based nanoparticles biogeochemical behavior in soil-plant system: A critical review

Increasing attention is being given to magnetic iron-based nanoparticles (MINPs) because of their potential environmental benefits. Owing to the earth abundance and high utilization of MINPs, as well as the significant functions of Fe in sustainable agriculture and environmental remediation, an understanding of the environmental fate of MINPs is indispensable. However, there are still knowledge gaps regarding the largely unknown environmental behaviors and fate of MINPs in soil-plant system. Thus, this review summarizes recent literature on the biogeochemical behavior (uptake, transportation, and transformation) of MINPs in soil and plants. The different possible uptake (e.g., foliar and root adsorption) and translocation (e.g., xylem, phloem, symplastic/apoplastic pathway, and endocytosis) pathways are discussed. Furthermore, drivers of MINPs uptake and transportation (e.g., soil characteristics, fertilizer treatments, copresence of inorganic and organic anions, meteorological conditions, and cell wall pores) in both soil and plant environments are summarized. This review also details the physical, chemical, and biological transformations of MINPs in soil-plant system. More importantly, a metadata analysis from the existing literature was employed to investigate the distinction between MINPs and other engineering nanoparticles biogeochemical behavior. In the future, more attention should be given to understanding the behavior of MINPs in soil-plant system and improving the capabilities of predictive models. This review thus highlights the main knowledge gaps regarding MINPs behavior and fate to provide guidance for their safe application in agrochemicals, crop production, and soil health.

Sulfide-modified nanozerovalent iron for rapid decontamination of Cu(Ⅱ) complexes in high-salinity wastewater

Heavy metal complexes receive less attention, but they are more difficult to remove than the free heavy metals. Moreover, the high-salinity wastewaters from various industries hinder the removal of heavy metal complexes. Removal of the metal complexes is a top priority but a challenging task. Herein, a new strategy for removing Cu-EDTA from high-salinity wastewater with sulfide-modified nanozerovalent iron (S-NZVI) was proposed. The S-NZVI exhibited a considerable adsorption capacity for Cu-EDTA (∼83 mg Cu/g) at a high salt concentration (25 g/L NaCl). Similarly, the S-NZVI maintained excellent adsorption performance (∼83 mg Cu/g) in the presence of CaCl2, MgCl2, Na2SO4, and NaNO3 (25 g/L). The S-NZVI showed extremely high efficiency for Cu-EDTA removal; 50 mg/L of Cu-EDTA was almost completely removed in 1 min, and the kobs was approximately 1.5 g/(mg min). The S-NZVI showed an extensive pH working range, and within the pH range of 2-9, the Cu-EDTA was removed completely within 5 min. The excellent removal performance of the S-NZVI was due to the high reactivity and high affinity of NZVI for Cu, as well as the special substitution of Fe2+ and the interfacial reactions between S-NZVI and the copper complexes. Compared with other studies of Cu complex removal, removal with S-NZVI was a simpler process with higher efficiency. In brief, S-NZVI efficiently removed Cu complexes from harsh water environments and was reused many times. The process was simple and efficient and has broad application prospects.

Immobilization of cadmium in river sediments by different modified nanoscale zero-valent iron: performance, mechanisms, and Fe dissolution

Modified nanoscale zero-valent iron (NZVI) exhibited great potential for the remediation of heavy metal contaminated river sediments, but its mechanisms and environmental risks are still unclear. This study systematically discussed the performance and the mechanisms of modified NZVI materials, i.e., sodium alginate-coated NZVI (SNZVI), rhamnolipid-coated NZVI (RNZVI), and graphene oxide-loaded NZVI (GNZVI), for the stabilization of Cd in sediment, with the exploration of their stability to Cd at various pH values and Fe dissolution rate. Compared with the control, the toxicity characteristic leaching procedure (TCLP) leachable Cd decreased by 52.66-96.28%, and the physiologically based extraction test (PBET) extractable Cd decreased by 44.68-70.21% after 56 days of incubation with the immobilization efficiency varying according to GNZVI > RNZVI > SNZVI > NZVI. Besides, the adsorption behavior of Cd on materials was fitted with the Freundlich model and classified as an endothermic, spontaneous, and chemical adsorption process. SEM-EDX, XRD, and FTIR results verified that the stabilization mechanisms of Cd were principally based on the adsorption, complexation of Cd2+ with secondary Fe minerals (including Fe2O3, γ-Fe2O3, and γ-FeOOH) and precipitation (Cd(OH)2). From the risk assessment results, it was observed that the materials were favorable for Cd stabilization at a pH range from 7 to 11, meanwhile, the leaching concentration of Fe in the overlying water was detected below the limit value. These findings pave the way to developing an effective strategy to remediate Cd contaminated river sediments.

Green synthesis and characterization of iron oxide nanoparticles for the removal of heavy metals (Cd2+ and Ni2+) from aqueous solutions with Antimicrobial Investigation

Clove and green Coffee (g-Coffee) extracts were used to synthesize green iron oxide nanoparticles, which were then used to sorb Cd²⁺ and Ni²⁺ ions out of an aqueous solution. Investigations with x-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorption and desorption (BET), Zeta potential, and scanning electron microscopy were performed to know and understand more about the chemical structure and surface morphology of the produced iron oxide nanoparticles. The characterization revealed that the main component of iron nanoparticles was magnetite when the Clove extract was used as a reducing agent for Fe³⁺, but both magnetite and hematite were included when the g-Coffee extract was used. Sorption capacity for metal ions was studied as a function of sorbent dosage, metal ion concentration, and sorption period. The maximum Cd²⁺ adsorption capacity was 78 and 74 mg/g, while that of Ni²⁺ was 64.8 and 80 mg/g for iron nanoparticles prepared using Clove and g-Coffee, respectively. Different isotherm and kinetic adsorption models were used to fit experimental adsorption data. Adsorption of Cd²⁺ and Ni²⁺ on the iron oxide surface was found to be heterogeneous, and the mechanism of chemisorption is involved in the stage of determining the rate. The correlation coefficient R² and error functions like RMSE, MES and MAE were used to evaluate the best fit models to the experimental adsorption data. The adsorption mechanism was explored using FTIR analysis. Antimicrobial study showed broad spectrum antibacterial activity of the tested nanomaterials against both Gram positive (S. aureus) (25923) and Gram negative (E. coli) (25913) bacteria with increased activity against Gram positive bacteria than Gram negative one and more activity for Green iron oxide nanoparticles prepared from Clove than g-Coffee one.

Chitosan-modified magnetic carbon nanomaterials with high efficiency, controlled motility, and reusability-for removal of chromium ions from real wastewater

Hexavalent chromium Cr(VI) is one of the most hazardous oxygen-containing anions to human health and the environment. Adsorption is considered to be an effective method for the removal of Cr(VI) from aqueous solutions. Based on an environmental perspective, we used renewable biomass cellulose as carbon source and chitosan as functional material to synthesize chitosan-coated magnetic carbon (MC@CS) material. The synthesized chitosan magnetic carbons were uniform in diameter (~ 20 nm) and contain a large number of abundant hydroxyl and amino functional groups on the surface, meanwhile owning excellent magnetic separation properties. The MC@CS exhibited high adsorption capacity (83.40 mg/g) at pH 3 and excellent cycling regeneration ability when applied to Cr(VI) removal in water, removal rate of Cr(VI) (10 mg/L) was still over 70% after 10 cycles. FT-IR and XPS spectra showed that electrostatic interaction and reduction with Cr(VI) are the main mechanisms of Cr(VI) removal by MC@CS nanomaterial. This work provides an environment-friendly adsorption material that could be reused for the removal of Cr(VI) in multiple cycles.

In-depth exploration of toxicity mechanism of nanoscale zero-valent iron and its aging products toward Escherichia coli under aerobic and anaerobic conditions

The bacteria toxicity of nanoscale zero-valent iron (nZVI) can be changed during its application in water treatment but the toxicity mechanism is still not well understood, particularly under anaerobic conditions. Here, the toxicity of nZVI and its aging products towards Escherichia coli (E. coli) and the mechanisms of extracellular and intracellular reactive oxygen species (ROS) damage were deeply probed in the presence and absence of oxygen in ultrapure water. Under aerobic conditions, the ROS damage primarily caused by the generation of extracellular free •OH can be a major contributor to the toxicity of nZVI to E. coli. By contrast, in anaerobic nZVI treatment system, the intracellular •OH can be quenched by benzoic acid which is a cell permeable quencher and the electron spin resonance (ESR) signals of 5,5-dimethy-1-pyrroline (DMPO)- •OH were evidently observed in system with the addition of F^- which could desorb the surface •OH into solution. It indicated that the intracellular •OH adsorbed on the particle surface can also play an indispensable role in inactivating cells under anaerobic conditions. Moreover, nZVI can steeply decline the membrane potential, causing severe membrane disruption and therefore resulting in the stronger toxicity in anaerobic conditions. Furthermore, the chemical composition transformation of nZVI and generation of benign iron corrosion products (e.g., Fe₃O₄, γ-Fe₂O₃, γ-FeOOH) are mainly responsible for the reduced toxicity with the increasing aging time. These results provide insights into the extracellular and intracellular ROS damage occurred in aerobic and anaerobic nZVI treatment systems, offering more perspective to the risk assessment of nZVI application.

Synthesis and application of starch-stablized Fe-Mn/biochar composites for the removal of lead from water and soil

Starch-stablized and Fe/Mn bimetals modified biochar derived from corn straw (SFM@CBC and SFM@CBC-350) were firstly prepared, characterized (FTIR, XRD, SEM, EDS, BET and XPS), and applied in Pb removal from water and soil. SFM@CBC and SFM@CBC-350 displayed highly effective adsorption performance of Pb²⁺ from wastewater with the maximum adsorption capacity of 170.91 mg g^-1 and 190.17 mg g^-1, respectively, which were much greater than that of FM@CBC (149.25 mg g^-1) and CBC (101.10 mg g^-1). Studies of adsorption kinetics, isotherms and thermodynamics indicated that the absorption of Pb²⁺ by SFM@CBC and SFM@CBC-350 was spontaneous and endothermic reaction, and it was controlled by monolayer chemisorption. The mechanism studies indicated that Pb²⁺ removal involved with multiple mechanism, including complexation (dominant process confirmed by XPS analysis), physical adsorption, electrostatic attraction, and cation exchange. The reusability test demonstrated that SFM@CBC and SFM@CBC-350 had very good stability and reusability. In addition, in order to further explore Pb removal performance of the modified biochar, SFM@CBC-350 was used in soil-ryegrass pot systems. Compared with the controls, the addition of SFM@CBC-350 reduced Pb content in soil and ryegrass, increased the biomass and total chlorophyll content, reduced the activity of antioxidant enzymes (CAT, SOD, MDA and POD) and ROS fluorescence intensity of ryegrass, thus alleviating Pb stress of ryegrass. Besides, the addition of SFM@CBC-350 could increase the richness and diversity of soil microorganisms, which was beneficial to the growth of ryegrass. Hence, SFM@CBC-350 has the potential of being used as a green, efficient and promising adsorbent in Pb removal from wastewater and soil.

Phase transformation of nanosized zero-valent iron modulated by As(III) determines heavy metal passivation

Nanoscale zero-valent iron (nZVI) has been extensively used for the passivation of cadmium (Cd) or arsenic (As) from wastewaters, while the underlying mechanisms of nZVI reaction with coexisting Cd and As are largely overlooked. Herein, the interactions of Cd and As during the course of nZVI transformation and the corresponding effects on respective pollutant removal have been systematically investigated. Batch experiments results show that As(III) addition significantly promotes the passivation of Cd(II) by nZVI, and the removal capacity increases by 7.8 times compared to that of Cd(II) alone. However, the adsorption and oxidative transformation of As(III) are barely affected under a relatively low Cd(II) concentration. It is conducive to the adsorption of Cd(II) and As(III) using nZVI under neutral conditions. The transformation of nZVI to lepidocrocite dominates in the Cd(II) single system, while it mainly converts to amorphous Fe oxyhydroxide with the addition of As(III). As(III) notably reduces the surface charge of Fe oxyhydroxide intermediates and to form the ternary complexes with Cd (Fe-As-Cd), which is the predominant mechanism for the promoted Cd(II) passivation. This work provides new understanding of nZVI transformation coupled to Cd(II) and As(III) passivation, which are likely contributing to the heavy metalloids regulation in waters and subsurface environments.

Multi-component removal of Pb(II), Cd(II), and As(V) over core-shell structured nanoscale zero-valent iron@mesoporous hydrated silica

The application of nanomaterials for the removal of heavy metals has received a great deal of attention because of their high efficiencies in the environment. But it is difficult to remove multiple heavy metals simultaneously with high efficiency and stability. Herein, the core-shell structured nanoscale zero-valent iron (nZVI) encapsulated with mesoporous hydrated silica (nZVI@mSiO₂) were prepared for efficient removal of heavy metals including Pb(II), Cd(II), and metalloid As(V). The material prepared uniformly with a high surface area (147.7 m² g^-1) has a nZVI core with the particle size of 20-60 nm and a modified dendritic mesoporous shell of 19 nm. 0.15 g L^-1 of the optimal material exhibited an extraordinary performance on removing Cd(II) and the maximum adsorption capacity for Pb(II), Cd(II), and As(V) reached 372.2 mg g^-1, 105.2 mg g^-1, and 115.2 mg g^-1 with a pH value at 5.0, respectively. The dissolved iron during the reaction showed that the mesoporous silica (mSiO₂) played an important role in enhancing the stability of nZVI. In addition, the competitive relationship between the coexistence of two heavy metals was discussed and it was found that the removal efficiency of the material for both was improved when Cd(II) and As(V) were removed synergistically.

A collaborative strategy for elevated reduction and immobilization of Cr(VI) using nano zero valent iron assisted by schwertmannite: Removal performance and mechanism

A novel collaborative strategy for enhanced removal of Cr(VI) using nano zero valent iron (nZVI) assisted by schwertmannite (Sch) with two synthesis methods was designed. Batch experiments demonstrated that nZVI/Sch-AP (synthesized by abiotic precipitation of Fe³⁺ species) exhibited excellent removal performance for Cr(VI) than nZVI/Sch-CO (synthesized by chemical oxidation of Fe²⁺ species). The results indicated that the removal efficiencies of Cr(VI) by nZVI/Sch-AP and nZVI/Sch-CO were highly pH-dependent and achieved to be 99.99% and 98.01% under the optimal conditions of 10 mg L^-1 Cr(VI) concentration, a pH of 6.3 and a Fe(0)/Cr(VI) molar ratio of 12. But nZVI/Sch-AP emerged greater k of 0.1097 min^-1 than that of nZVI/Sch-CO (0.0485 min^-1). Humic acid exhibited promotion effect on the Cr(VI) removal in low concentration of 1 mg L^-1. Results of XRD and XPS demonstrated that α-FeOOH was the dominant products in both incubations of nZVI/Sch-AP and nZVI/Sch-CO, accompanied with FeCr₂O₄ and CrFe mixed (oxy)hydroxides, and γ-FeOOH was found alone in the incubations of nZVI/Sch-CO. We proposed a consecutive and simultaneous process involving surface absorption-reduction and co-precipitation/immobilization for the removal. This study provides new insights into the elimination of Cr(VI) from wastewater by nZVI/Sch, especially in acid mine drainage.

Nanoscale zero-valent iron particles supported on sludge-based biochar for the removal of chromium (VI) from aqueous system

Biochar (BC) obtained by the co-pyrolysis of municipal sewage sludge (MSS) and sunflower seed shells (SSS) was utilized to support nanoscale zero-valent iron particles (nZVI) for the synthesis of a composite material (nZVI-BC) for Cr(VI) removal from aqueous systems. A series of characterization methods confirmed successful immobilization of nZVI on the surface of biochar with no aggregation. Batch experiments showed that the initial pH, initial Cr(VI) concentration, and nZVI-BC dose all significantly affected the Cr(VI) removal using nZVI-BC. The kinetics for Cr(VI) removal via nZVI-BC could be better explained by the pseudo-second-order (PSO) adsorption model. Adsorption isotherms analysis demonstrated the superior Cr(VI) removal capability of nZVI-BC in comparison to bare nZVI and BC. nZVI-BC can be reused after the regeneration process by applying 0.1 M H2SO4 and 0.1 M NaBH4 solutions. The reaction mechanism for Cr(VI) removal might involve its chemical reduction on the nZVI-BC surface. Overall, environmentally friendly nZVI-BC was highly efficient in Cr(VI) removal from aqueous systems.

Natural and artificial humic substances to manage minerals, ions, water, and soil microorganisms

The chemistry of humic substances (HSs) occurs hidden from our sight, but is of key importance to agriculture and the environment, and nowadays even to medicine and technology. HSs are nowadays not only natural, but extracted and engineered, and in the past 20 years such products have been widely used in soil improvement and environment governance. In this review, we collate and summarize the applications and working principles of such HSs in agriculture and environmental ecology, mainly to elaborate the multiple roles of this functional polymer along with physical chemical quantification. Then several of the latest synthesis technologies, including hydrothermal humification technology (HTH), hydrothermal carbonization technology (HTC) and hydrogen peroxide oxidation technology (HOT) are presented, which were introduced to prepare artificial humic substances (A-HSs). The availability of reproducible and tunable synthetic A-HSs is a new chemical tool, and effects such as solubilization of insoluble phosphorus minerals, recovery of phosphorus, improvement of soil fertility for crop growth and reduction of toxicity of typical pollutants, can now be analyzed in detail and quantified. As a result, we can provide an effective chemical technology for utilizing biomass side products ("biowaste") to generate A-HSs of different types, thus realizing improvement in agricultural production and control of environmental pollution by the macro-synthesis of A-HSs-.

Immobilization of Ni (Ⅱ) at three levels of contaminated soil by rhamnolipids modified nano zero valent iron (RL@nZVI): Effects and mechanisms

A kind of biosurfactant rhamnolipid modified zero-valent iron nanoparticles have been synthesized and applied to evaluate the immobilization efficiency of Ni (Ⅱ) contaminated soil at three concentration levels (200Ni, 600Ni and 1800Ni). The results of SEM and XRD were clearly indicative of the well-attached phenomenon of rhamnolipid on the nZVI, featuring better stability and dispersity, and FTIR analysis proposed the interactions between rhamnolipid and nZVI through monodentate chelating between carboxylate groups and nZVI or hydrogen bonding with Fe-O groups on the surface. Sequential extraction procedures (SEP) analysis illustrated that the majority of labile fractions had been transformed into less accessible fractions (Fe-Mn oxide-bound fractions and residual fractions) after 28 days of incubation. And for low-concentrations polluted soil, soil self-remediation played a dominant role, while RL@nZVI exhibited a more significant stabilizing effect for medium and high-concentrations pollution. Furthermore, XPS and XRD analyses of Ni-adsorbed RL@nZVI identified the formation of NiO, Ni(OH)₂ and revealed the possible interaction mechanisms including reduction, adsorption and precipitation/co-precipitation. These results confirmed that RL@nZVI presented a promising prospect for the immobilization of Ni polluted soil.

Metabolomic analysis reveals dose-dependent alteration of maize (Zea mays L.) metabolites and mineral nutrient profiles upon exposure to zerovalent iron nanoparticles

Nanoscale zero-valent iron (nZVI) has been widely applied in the environmental field to degrade organic pollutants. The potential risk posed from nZVI on crop species is not well understood and is critical for sustainable application in the future. In this study, maize (Zea mays L.) plants were cultivated in field soils mixed with nZVI at 0, 50, and 500 mg/kg soil for four weeks. Upon exposure to 500 mg/kg nZVI, ICP-MS results showed that Fe accumulated by roots and translocated to leaves was increased by 36% relative to untreated controls. At 50 mg/kg, root elongation was enhanced by 150-200%; at 500 mg/kg, pigments, lipid peroxidation, and polyphenolic levels in leaves were increased by 12, 87 and 23%, respectively, whereas the accumulation of Al, Ca, and P were decreased by 62.2%, 19.7%, and 13.3%, respectively. A gas chromatography-mass spectrometry (GC-MS) based metabolomics analysis of maize roots revealed that antioxidants and stress signaling-associated metabolites were downregulated at 50 mg/kg, but were upregulated at 500 mg/kg. At 50 mg/kg, the content of glutamate was increased by 11-fold, whereas glutamine was decreased by 99% with respect to controls. Interestingly, eight metabolic pathways were disturbed at 50 mg/kg, but none at 500 mg/kg. This metabolic reprogramming at the lower dose represented potential risks to the health of exposed plants, which could be particularly important although no phenotypic impacts were noted. Overall, metabolites analysis provides a deeper understanding at the molecular level of plant response to nZVI and is a powerful tool for full characterization of risk posed to crop species as part of food safety assessment.

Bibliometric analysis of zerovalent iron particles research for environmental remediation from 2000 to 2019

Zerovalent iron (ZVI) has been a major focus of research and has attracted great attention during the last 2 decades by international researchers because of its excellent pollutant removal performance and several other merits in environmental remediation. Based on Web of Science Core Collection data, we present a comprehensive bibliometric analysis of ZVI research from 2000 to 2019. We analyze 4472 publications assuming three stages of growth trend of annual publication totals. We find that "The Chemical Engineering Journal" has been the most productive journal; Noubactep C is identified as the most productive author; China has been the most active country in this field and the Chinese Academy of Science the most productive institution. The timeline of keywords shows seven distinct co-citation clusters. In addition, the top 38 keywords with strong citation bursts are also detected, suggesting that the innovation of green composite synthesis of ZVI and nanoscale ZVI and its efficient removal capacity might be the prevailing research directions in the future.

Vitamin C mediates the activation of green tea extract to modify nanozero-valent iron composites: Enhanced transport in heterogeneous porous media and the removal of hexavalent chromium

Both green tea (GT) extract and vitamin C (VC) were used for the reduction of Fe³⁺ to Fe⁰ using a green synthesis method. Modified nanozero-valent iron (GT-nZVI@VC nanocomposites) was successfully obtained and characterized as α-Fe⁰-iron oxide/VC by multiple analytical methods. The GT-nZVI@VC nanocomposites showed better transportability than nZVI, in that transport behavior was slightly dependent on various ratios of sand/soil in water-saturated heterogeneous porous media. Breakthrough curves of GT-nZVI@VC nanocomposites in paddy soil exhibited "blocking effects" and were well described using a first-order straining coefficient (k₂) on site 2 obtained from a two-site kinetic attachment model. In particular, GT-nZVI@VC (VC/Fe = 0.6) showed higher Cr(VI) removal (especially reducibility) in both paddy soil and water compared to that of nZVI and VC. It is likely that the synergistic effects of VC (ascorbic acid) and tea polyphenols can increase the released free electrons into solution, favoring the high reduction of Cr(VI) into Cr(III) (i.e., FeOCr₂O₃, Cr(OH)₃ and Cr₂O₃), where Cr(III) is prone to be immobilized by the nanocomposites in soil. This research highlights that VC can mediate the activation of GT extract to successfully modify nZVI, which could be beneficial for efficient transport in subsurface and remediation of Cr(VI)-contaminated soil and underground water.

Integrated remediation of sulfate reducing bacteria and nano zero valent iron on cadmium contaminated sediments

Integrated-remediation technologies on heavy metal polluted sediments have received much attention. In this study, Cd contaminated sediments were treated with various conditions: sulfate reducing bacteria (SRB) only and SRB combined with different dosages of nano zero valent iron (nZVI (0.5-10 mg/g)). The immobilization of Cd was found in all remediation treatments according to the decreases of mobile Cd and the increases of more stable Cd compared with control. Five typical SRBs (Desulfobulbaceae, Desulfobacteraceae, Syntrophobacteraceae, Desulfovibrionaceae and Desulfomicrobiaceae) were identified having significant influences on Cd speciation transformation and they could stabilize Cd into sulfide precipitation through dissimilatory sulfate reduction (DSR). The ANOVA results of mobilization index and Cd concentration in overlying water both demonstrated that integrated-remediation systems with 5 mg/g and 10 mg/g of nZVI (Fe5 and Fe10 systems, respectively) presented better immobilization performance than conventional SRB only system (P < 0.05). It is confirmed that nZVI could stimulate the SRB bio-immobilization possibily through providing electrons and enhancing enzyme activities during DSR. The XPS analyses and Pourbaix diagrams revealed that mackinawite may be produced in the Fe10, resulting in the possible formation of Cd-S-Fe. This study indicates that integrated-remediation of SRB and nZVI have great potential in Cd immobilization of sediments, especially with higher addition of nZVI.

Does soluble starch improve the removal of Cr(VI) by nZVI loaded on biochar?

A novel material that nano zero valent iron (nZVI) loaded on biochar with stable starch stabilization (nZVI/SS/BC) was synthesized and used for the removal of hexavalent chromium [Cr(VI)] in simulated wastewater. It was indicated that as the pyrolysis temperature of rice straw increased, the removal rate of Cr(VI) by nZVI/SS/BC first increased and then decreased. nZVI/SS/BC made from biochar pyrolyzed at 600 °C (nZVI/SS/BC600) had the highest removal efficiency and was suitable for a wide pH range (pH 2.1-10.0). The results showed that 99.67% of Cr(VI) was removed by nZVI/SS/BC600, an increase of 45.93% compared to the control group, which did not add soluble starch during synthesis. The pseudo-second-order model and the Langmuir model were more in line with reaction. The maximum adsorption capacity for Cr(VI) by nZVI/SS/BC600 was 122.86 mg·g^-1. The properties of the material were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) mapping, Brunauer-Emmett-Teller (BET), Fourier-transform infrared (FTIR), and X-ray diffraction (XRD). The results showed that the nZVI particles were uniformly supported on the biochar, and the BET surface areas of nZVI/SS/BC was 40.4837 m²·g^-1, an increase of 8.79 times compared with the control group. Mechanism studies showed that soluble starch reduced the formation of metal oxides, thereby improving the reducibility of the material, and co-precipitates were formed during the reaction. All results indicated that nZVI/SS/BC was a potential repair material that can effectively overcome the limitations of nZVI and achieve efficient and rapid repair of Cr(VI).

Enhanced molybdenum(VI) removal using sulfide-modified nanoscale zerovalent iron: kinetics and influencing factors

The overall goal of this study is to investigate the effect of sulfidated nanoscale zerovalent iron (S-nZVI) on the removal of hexavalent molybdate (MoO₄^2-) under different aquatic chemistry conditions. Surface analysis suggests that Mo(VI) is removed mainly by adsorption and co-precipitation onto the surface of S-nZVI and a small amount of Mo(VI) can be reduced to Mo(V) species. The results of batch tests show that Mo(VI) removal by S-nZVI are well described with the pseudo-second-order adsorption model. The removal rate increases with a decrease in solution pH (4.0-9.0) and is significantly affected by the S/Fe ratio of S-nZVI, with the optimal S/Fe ratio being 0.5. The presence of anions WO₄^2- or CrO₄^2- can reduce the Mo(VI) removal, which is likely because they compete for adsorption sites on the solid surfaces. The divalent cations Ni²⁺, Cu²⁺ and Co²⁺ also inhibit the removal of Mo(VI) whereas Zn²⁺, Ca²⁺ and Mg²⁺ enhance it. After being aged for 35 d in water, S-nZVI still exhibits high reactivity towards Mo(VI) removal (57.39%). The study demonstrates that S-nZVI can be used as an environmentally friendly material for effectively removing Mo(VI) from contaminated water.

One-pot synthesis of spherical nanoscale zero-valent iron/biochar composites for efficient removal of Pb(ii)

In this study, a spherical Fe/C composite (AIBC) was successfully prepared via carbonization of Fe³⁺-crosslinked sodium alginate. The removal capacity and mechanism of AIBC were evaluated for the adsorption of Pb(ii) from aqueous solution and compared with that of commercial nanoscale zero-valent iron (nZVI). The effects of the initial concentration, pH of Pb(ii) solution, the contact time, coexisting anions, and aging under air were investigated. The results showed that the pH had a strong impact on the adsorption of Pb(ii) by AIBC. The adsorption data for AIBC followed the Langmuir model, while the maximum adsorption capacity at pH 5 was 1881.73 mg g⁻¹. The AIBC had a higher adsorption capability than nZVI, especially under the condition of relatively high Pb(ii) concentrations. The oxidation–reduction reaction between Fe and Pb(ii) was the main mechanism for the adsorption of Pb(ii) onto nZVI. AIBC converted the largest amount of Pb(ii) into PbO·XH₂O/Pb(OH)₂ mainly by generating Fe²⁺.

In this study, a spherical Fe/C composite (AIBC) was successfully prepared via carbonization of Fe³⁺-crosslinked sodium alginate.

A cost effective method for immobilization of Cu and Ni polluted river sediment with nZVI synthesized from leaf extract

This study investigates the performance of oak (OL) and mulberry (ML) leaves for synthesized of nanoscale zero-valent iron (nZVI), in immobilizing Cu and Ni in contaminated sediment. Characterization of synthesized Fe nanoparticles from oak and mulberry leaf extracts demonstrated that they are nontoxic and stabile nanomaterials for application in the sediment remediation. Effectiveness of stabilization process was performed by microwave-assisted sequential extraction procedure (MWSE) and single-step leaching tests which have been applied to evaluate the metal extraction potential. This research showed that OL-nZVI and ML-nZVI were effective in transforming available Cu and Ni to stable fraction. The maximum residual percentage of Cu increased by 76% and 73%, and for Ni 81% and 80%, respectively, with addition of 5% OL-nZVI and 5% ML-nZVI. Used single-step leaching tests (Toxicity Characteristic Leaching Procedure-TCLP and German standard test- DIN) indicated that all stabilized samples can be considered as non-hazardous waste, as all leached metal concentrations met the appropriate set criteria. Cost analysis showed that the operating cost for contaminated sediment treatment with green synthesized nZVI are 50.37 €/m3/per year. This work provides a new insight into the immobilization mechanism and environmental impact of Cu and Ni in contaminated sediment and potential way of treatment with OL-nZVI and ML-nZVI. Generally, nZVI can be an effective and versatile tool for stabilization of sediment polluted with toxic metals.

Porous biochar-nanoscale zero-valent iron composites: Synthesis, characterization and application for lead ion removal

Nano-zero-valent iron has been used in combination with a variety of support carriers to remove heavy metals in solution. However, pre-treatment of the carrier can reflect a better synergistic effect and thus achieve high heavy metal removal capabilities. In this study, the hydrophilic biochar obtained by an acid ammonium persulfate oxidation has an adsorption capacity of up to 135.4 mg g^-1 for Pb²⁺ (25 °C, pH = 6 with adsorbent amount of 10 mg and Pb²⁺ concentration of 50 mg L^-1). Due to the strong Fe-C-O covalent bond, nZVI increases the binding force with the carbon matrix. Benefitting from the high specific surface area, porous structure and rich oxygen-containing functional groups, the resultant nZVI-HPB samples are favourable for Pb²⁺ diffusion and adsorption, exhibiting maximum adsorption capacity of 480.9 mg g^-1 (pH = 6, 25 °C with adsorbent amount of 10 mg and Pb²⁺ concentration of 200 mg L^-1). The multiple interaction mechanisms in the Pb²⁺ removal process such as the reduction reaction, complexation and co-precipitation proceed simultaneously are concluded by the analyses of Fourier-Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) spectra.

Insight into the Mechanism of Arsenic(III/V) Uptake on Mesoporous Zerovalent Iron-Magnetite Nanocomposites: Adsorption and Microscopic Studies

Mesoporous zerovalent iron-magnetite nanocomposites (ZVI-MNCs) were developed to circumvent the limitations of magnetite, such as its susceptibility to phase transition in air-water interfaces. High-resolution transmission electron microscopy images revealed the presence of Fe⁰ and Fe₃O₄ in the as-prepared adsorbent. High-resolution X-ray photoelectron spectroscopy (HR-XPS) Fe 2p deconvoluted spectra showed that electron transfer between Fe⁰ and Fe₃O₄ controlled the magnetite transformation. The isotherm equilibrium data for As(III) and As(V) are described by the Sips model, which suggests single- and multilayer formation onto a heterogeneous surface with different binding sites, whereas adsorption is controlled by a pseudo-second-order kinetic model, which indicates chemisorption. The maximum sorption capacities (q_m) for As(III) and As(V) are 632.6 and 1000 μmol g^-1, respectively, which are larger than the q_m of similar adsorbents. The greater q_m for As(V) is attributed to a higher multilayer formation and a stronger bonding force compared with As(III). The arsenic uptake capacity showed that the as-prepared adsorbent was effective over a wide pH range, and an optimal uptake capacity was recorded between pH 5.0 and 9.0 for As(III) and 3.0 and 7.0 for As(V). The adsorbent exhibited a remarkable regeneration performance for As(III) and As(V) uptake. Several microscopic analytical tools, including Fourier transform infrared spectroscopy, HR-XPS, and X-ray absorption near-edge structure together with zeta potential, confirmed that the binding mode of As(III) and As(V) on ZVI-MNCs was predominantly inner-sphere coordination. Partial redox transformation occurred for As(III) and As(V) on nearly 10 nm of the adsorbent, which indicates that a surface redox mechanism contributed partially to arsenic uptake on the near surface of the ZVI-MNCs. Extended X-ray absorption fine structure spectral analysis proposed that a corner-sharing monodentate mononuclear (¹V) complex occurred for As(III) with a small portion of a corner-sharing bidentate binuclear (²C) complex, whereas As(V) formed a corner-sharing bidentate binuclear (²C) complex with octahedral Fe bonding.

Simultaneous removal of chromium(VI) and tetracycline hydrochloride from simulated wastewater by nanoscale zero-valent iron/copper-activated persulfate

In this paper, metallic copper (Cu) was supported on nanoscale zero-valent iron (nZVI) to form a nanoscale bimetallic composite (nZVI-Cu), which was used to activate persulfate (PS) to simultaneously remove the compound contaminants Cr(VI) and tetracycline hydrochloride (TCH) in simulated wastewater. nZVI, nZVI-Cu, and nZVI-Cu-activated PS (nZVI-Cu/PS) were characterized by SEM, TEM, XRD, and XPS. The effects of the bimetallic composite on Cr(VI) and TCH removal were compared in the nZVI, nZVI-activated PS (nZVI/PS), nZVI-Cu, and nZVI-Cu/PS systems. The results showed that nZVI and Cu can form a nanobimetallic system, which can create galvanic cells; thus, the galvanic corrosion of nZVI and the transfer of electrons are accelerated. For a single contaminant, the removal efficiency of Cr(VI) and TCH is the highest when nZVI is loaded with 3 wt% and 1 wt% Cu, respectively. The ratio of nZVI-Cu with 3 wt% Cu to PS is 7:1, and the removal efficiency of Cr(VI) and TCH compound contaminants is ~ 100% after 60 min under acidic conditions, which indicates that the Cr(VI) reduction and TCH oxidation were complete in the nZVI-Cu/PS system. The mechanisms of simultaneous removal of Cr(VI) and TCH in the nZVI-Cu/PS system are proposed. The removal of Cr is because of the adsorption-reduction effects of the nZVI-Cu bimetallic material. The degradation of TCH is mainly due to the action of oxidative free radicals generated by Fe²⁺-activated PS. The free radical capture experiments showed that SO- 4· plays a major role in the process of TCH degradation.

Removal of heavy metals and radionuclides from water using nanomaterials: current scenario and future prospects

There is an increase in concern about the hazardous effects of radioactivity due to the presence of undesirable radioactive substances in our vicinity. Nuclear accidents such as Chernobyl (1986) and Fukushima (2011) have further raised concerns towards such incidents which have led to contamination of water bodies. Conventional methods of water purification are less efficient in decontamination of radioisotopes. They are usually neither cost-effective nor environmentally friendly. However, nanotechnology can play a vital role in providing practical solutions to this problem. Nano-engineered materials like metal oxides, metallic organic frameworks, and nanoparticle-impregnated membranes have proven to be highly efficient in treating contaminated water. Their unique characteristics such as high adsorption capacity, large specific surface area, high tensile strength, and excellent biocompatibility properties make them useful in the field of water purification. This review explores the present status and future prospects of nanomaterials as the next-generation water purification systems that can play an important role in the removal of heavy metals and radioactive contaminants from aqueous solutions.

Remediation of iron oxide bound Pb and Pb-contaminated soils using a combination of acid washing agents and l-ascorbic acid

Soil washing is an efficient, rapid, and cost-effective remediation technique to dissolve target pollutants from contaminated soil. Here we studied the effects of leaching agents: hydrochloric acid (HCl), ethylenediamine tetraacetic acid disodium salt (Na₂EDTA) and citric acid (CA), and reductants: hydroxylamine hydrochloride (NH₂OH·HCl) and l-ascorbic acid (VC) on the leaching of Pb from synthetic iron oxide; the changes in mineralogy, morphology, and occurrence of Pb were shown by XRD, SEM, and sequential extraction analyses. Although the washing efficiency of Pb follows the trend HCl (44.24%) > Na₂EDTA (39.04%) > CA (28.85%), the cooperation of the leaching agent with reductant further improves the efficiency. VC is more suitable as a reductant considering the higher washing efficiency by HCl-VC (98.6%) than HCl-NH₂OH·HCl (88.8%). Moreover, increasing the temperature can promote the decomposition and dehydrogenation reaction of VC with more H⁺. Among the mixture agents, Na₂EDTA + VC is the most effective agent to remediate the two kinds of contaminated soils owing to the formation of Fe(ii)-EDTA, a powerful reducing agent so that the efficiencies can reach up to 98.03% and 92.81%, respectively. As a result, these mixture agents have a great prospect to remediate Pb-contaminated soils.

Effects of environmental factors on the removal of heavy metals by sulfide-modified nanoscale zerovalent iron

Sulfide-modified nanoscale zerovalent iron (S-nZVI) has excellent reducing performance for heavy metals in water. The influence of environmental factors on the reactivity can be used to explore the practical feasibility of S-nZVI and analyze the reaction mechanism in depth. This study compared the removal effect and mechanism of Cu²⁺ and Ni²⁺ by nanoscale zerovalent iron (nZVI), S-nZVI, and carboxymethyl cellulose-modified nanoscale zerovalent iron (CMC-nZVI). The results show that the pseudo-first-order kinetic constant of Cu²⁺ removal by nZVI, S-nZVI, and CMC-nZVI was 1.384, 1.919, and 2.890 min^-1, respectively, and the rate of Ni²⁺ removal was 0.304, 0.931, and 0.360 min^-1, respectively. The removal mechanism of S-nZVI was similar to that of nZVI and CMC-nZVI. Specifically, Cu²⁺ was predominantly removed by reduction, while Ni²⁺ removal included adsorption and reduction. Environmental factors had a specific inhibitory effect on the removal of Cu²⁺ but had a negligible impact on Ni²⁺. The condition of low pH, the presence of Cl^- and humic acid (HA) promoted the corrosion consumption of Fe⁰, in which H⁺ directly corroded Fe⁰ at low pH. At the same time, Cl^- and HA inhibited the adsorption or binding of heavy metal ions on the particle surface, thereby reducing the electron transfer and utilization efficiency. The passivation of NO₃^- reduced the anaerobic corrosion of the material in water but suppressed the release of electrons, thereby reducing the reduction efficiency of the three types of materials. The anaerobic corrosion of S-nZVI was less affected by environmental factors, and it can still maintain more than 80% of the electronic utilization efficiency under different environmental factors, which illustrates that S-nZVI has broad prospects for practical applications in heavy metal polluted water.

Efficient reduction of Cr(VI) and immobilization of Cr driven by an iron-air fuel cell: Reaction mechanisms and electricity generation

The iron-air fuel cell (IAFC) has been successfully employed for the oxidative removal of many pollutants, but its feasibility for reductive immobilization of Cr(VI) is still unknown. Herein, we developed an IAFC system consisting of an iron anode and an activated carbon-PTFE based air-cathode, and evaluated its performance for Cr(VI) removal and power generation. In this reaction system, cathodic reduction and Fe(II) reduction both contributed to the reductive removal of Cr(VI). It was found that the decrease of solution pH from 6.0 to 3.0 promoted the removal of Cr(VI) due to the enhanced yield of Fe(II) ions and cathodic reduction, accompanying the increased power generation from 1040 mW m^-2 to 2880 mW m^-2. Besides, the Cr(VI) removal and power generation could be also promoted by elevating Na₂SO₄ concentration from 0.01 M to 0.1 M. In the IAFC process, Cr(VI) was initially reduced to less soluble ionic Cr(III) homogeneously and heterogeneously and then Cr(III) was immobilized by adsorption and/or co-precipitation with the fresh Fe(III) (oxy)hydroxides. Generally, this study is of great interest for the engineering community to design the environmentally benign and cost-effective strategy for the treatment of wastewater in remote areas, where the electricity is not easily available.

Applying modified biochar with nZVI/nFe3O4 to immobilize Pb in contaminated soil

Lead (Pb) pollution in soil has become one of the most serious environmental problems, and it is more urgent in areas where acid rain is prevalent. Curing agents to solidify heavy metals in soil are efficiently applied to remediate Pb-contaminated soil. In this study, we prepared biochar, biochar loaded with nano-zero-valent iron (BC-nZVI), and biochar loaded with nano-ferroferric oxide (BC-nFe₃O₄), and investigated the Pb-immobilizing efficiency in contaminated soil in the condition of acid rain by them. The results showed that 8 g/kg is the best added dosage of curing agents for immobilizing Pb, which of the immobilizing efficiency of Pb were 19% (biochar), 42% (BC-nZVI), and 23% (BC-nFe₃O₄), respectively. Besides, the curing agents had positive effects on immobilizing Pb under acid rain condition, which could significantly reduce the content of acid extractable Pb, especially BC-nZVI (1.5%). And the immobilization efficiency of modified biochar was better than biochar, especially BC-nZVI (66%). BC-nZVI showed a more ideal effect on decreasing the leaching amount of Pb in the condition of acid rain. The results highlighted that biochar-loaded nano-iron-based materials, especially BC-nZVI, was promising and environmentally friendly materials for remediating Pb-contaminated soils, which provided scientific reference and theoretical basis for the treatment of Pb-contaminated soils around industrial sites particularly in acid rain area.

Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron

To reduce the toxicity of vanadium(V) [V(V)] and inhibit the desorption of adsorbed vanadium in groundwater, we synthesized nanoscale zerovalent iron (nZVI) dispersed on layered double hydroxide (LDH) composites (nZVI@LDH) to remove V(V) from simulated groundwater. We found that nZVI@LDH could reduce high-valence vanadium to low-valence vanadium, then forming vanadium-containing precipitation to reduce the toxicity and inhibiting vanadium from returning to groundwater. SEM and XRD characterizations exhibited the uniform dispersal of nZVI on the surface of LDH. nZVI@LDH with nZVI/LDH at a mass ratio of 1:2 provided the maximum adsorption capacity of 93.7 mg g^-1 at pH 3.0. Coexisting anions and dissolved oxygen in groundwater have little effect on V(V) removal. nZVI@LDH performed well across a wide pH range (3.0-8.0). The surface characterizations and XPS analysis revealed that LDH as supporting materials inhibited the aggregation and passivation of nZVI. The adsorbed V(V) was reduced to V(IV) and V(III) by nZVI and spontaneously transformed into insoluble VO₂ and V₂O₃. The DFT calculations indicated the strong complexation and better stability of the V(IV) and V(III) species with nZVI@LDH than V(V). This work suggests that nZVI@LDH has the potential to serve as an efficient material for the immobilization of V(V) in groundwater.

Removal of trace Cr(VI) from aqueous solution by porous activated carbon balls supported by nanoscale zero-valent iron composites

In this study, porous activated carbon balls supported by nanoscale zero-valent iron composites (Fe@PACB-700) were used for the first time for the removal of trace Cr(VI) from aqueous solutions. The Fe@PACB-700 composites were prepared by a facile carbothermal reduction method and then characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS). The results show that nZVI particles have been successfully loaded onto PACBs. Fe@PACB-700 shows an excellent Cr(VI) removal efficiency of 91.2%. The maximum adsorption capacity of Fe@PACB-700 for Cr(VI) is 22.24 mg/g, which is 4.36 times that of PACB. The residual Cr(VI) concentration is below 20 ppb with the use of 0.15 g of Fe@PACB-700, which is much lower than the allowable concentration for Cr(VI) in drinking water (0.05 mg/L). The adsorption of Cr(VI) can be well described by the Langmuir isotherm model and pseudo-second-order kinetic model. Fe@PACB-700 still has a high removal efficiency of 80% after five cycles. Thus, Fe@PACB-700 has a great potential for Cr(VI) removal from aqueous solution. Graphical abstract.

High-dispersion zero-valent iron particles stabilized by artificial humic acid for lead ion removal

Nano zero-valent iron (nZVI), as a high-efficiency adsorbent for heavy metals, often suffers being oxidized and assembling together due to small size and super reactivity, further decreasing its adsorption performance and limiting application ranges. Herein, we have designed a novel adsorbent with high-dispersion nZVI stabilized by as-prepared artificial humic acid (AHA-nZVI) derived from hydrothermal humification (HTH) technology. Introduction of artificial humic acid (A-HA) can effectively reduce the oxidation and agglomeration of nZVI, leading to superior kinetic removal efficiency of Pb²⁺ (> 99.2%) and huge Langmuir removal capacity of 649.0 mg/g. The combination of nZVI and A-HA (contained abundant functional groups, i.e. -OH and -COOH) via C-O-Fe bonding makes nZVI have good dispersion and oxidation resistance. Multiple interaction mechanisms including reduction reaction, complexation and co-precipitation between heavy metals and AHA-nZVI samples are realized. Overall, AHA-nZVI is a promising material for high-performance heavy metal contaminated water treatment.

Experimental and theoretical aspects of biochar-supported nanoscale zero-valent iron activating H2O2 for ciprofloxacin removal from aqueous solution

Ciprofloxacin has been frequently detected in water environment, and its removal has become a significant public concern. Biochar-supported nanoscale zero-valent iron (BC/nZVI) to activate hydrogen peroxide (H₂O₂) has many advantages on promoting the removal of organic contaminants. In this paper, the BC/nZVI activating H₂O₂ degradation of ciprofloxacin was systematically investigated by experimental and theoretical approaches. The morphologies and property analysis showed that nZVI particles distributed uniformly on the biochar surface, which mainly include ^-OH, >CO and COC and CO groups. Different reaction conditions were compared to define the optimal conditions for ciprofloxacin removal in BC/nZVI/H₂O₂ system. More than 70% of ciprofloxacin was removed in the optimal conditions: acidic condition (pH 3∼4), low doses of H₂O₂ (20 mM), and temperature of 298 K. The hydroxyl radical (^•OH) oxidation was the primary pathway in BC/nZVI/H₂O₂ degradation of ciprofloxacin process. The theoretical calculation indicated that hydrogen atom abstraction (HAA) pathways were the dominant oxidation pathways contributing 92.3% in overall second‒order rate constants (k) of ^•OH and ciprofloxacin. The current results are valuable to evaluate the application of BC/nZVI activating H₂O₂ degradation of ciprofloxacin and other fluoroquinolone antibiotics in water treatment plants.

Utilization of nanomaterials for in-situ remediation of heavy metal(loid) contaminated sediments: A review

Heavy metal(loid)s are toxic and non-biodegradable environmental pollutants. The contamination of sediments with heavy metal(loid)s has attracted increasing attention due to the negative environmental effects of heavy metal(loid)s and the development of new remediation techniques for metal(loid) contaminated sediments. As a result of rapid nanotechnology development, nanomaterials are also being increasingly utilized for the remediation of contaminated sediments due to their excellent capacity of immobilizing/adsorbing metal(loid) ions. This review summarizes recent studies that have used various nanomaterials such as nanoscale zero-valent iron (nZVI), stabilizer-modified nZVI, nano apatite based-materials including nano-hydroxyapatite particles (nHAp) and stabilized nano-chlorapatite (nCLAP), carbon nanotubes (CNTs), and titanium dioxide nanoparticles (TiO₂ NPs) for the remediation of heavy metal(loid) contaminated sediments. We also review the analysis of potential mechanisms involved in the interaction of nanomaterials with metal(loid) ions. Subsequently, we discuss the factors affecting the nanoparticle-heavy metal(loid)s interaction, the environmental impacts resulting from the application of nanomaterials, the knowledge gaps, and potential future research.

Catalytic oxidation of diclofenac by hydroxylamine-enhanced Cu nanoparticles and the efficient neutral heterogeneous-homogeneous reactive copper cycle

This study has demonstrated that hydroxylamine (HA) could greatly enhance Cu nanoparticles (nCu) in activating molecular oxygen and significantly elevate the diclofenac (DCF) degradation rate about two orders of magnitude in neutral circumstances. Effects of several important parameters on the DCF degradation such as nCu loading, HA dosage, pH and reaction temperature were investigated in the nCu/HA/O₂ system. Multiple examinations revealed that the reactive Cu(III) species instead of OH• would be predominant in the nCu/HA/O₂ system, despite their similar DCF degradation pathways. Based on a HA-enhanced copper cycle depending on the pristine Cu⁰@Cu(I) (hydro)oxides core-shell structure, the heterogeneous-homogeneous reaction mechanism was proposed. It included solid-liquid interfacial and bulk reactions, e.g. heterogeneous activation of O₂ by Cu(I) to produce H₂O₂ and homogeneous Cu(I)-catalytic generation of Cu(III) from H₂O₂. Further quantitative investigation of the main reactive species in the cycle revealed that the Cu(I) regeneration instead of the O₂ activation would be rate-limited. Besides, nCu could be recycled to effectively degrade DCF in four consecutive cycles in the raw neural nCu/HA/O₂ system. It suggested that the nCu/HA/O₂ system with a more efficient copper cycle would be a good alternative Fenton-like system in treating neutral recalcitrant organic wastewaters.

Addition of iron materials for improving the removal efficiencies of multiple contaminants from wastewater with a low C/N ratio in constructed wetlands at low temperatures

Constructed wetlands (CWs) are widely used in wastewater treatment. Wastewater generally contains multiple contaminants. In this study, CWs were applied to treat wastewater with a low COD/TN ratio and containing heavy metals. Iron-based material was added in CWs to enhance the treatment efficiency. The contaminant removal efficiency was positively correlated with the dosage of iron-based material. Considering the operation cost, we added 1 g of iron-based material in CW and realized the multi-contaminant removal efficiency after 4-day treatment at low temperature: 99.51% of Cu(II), 87.22% of Cr(VI), 65.62% of TN, and 60.23% of COD. Microbial community analysis and kinetic analysis predicted that the removal mechanism involved ion exchange and microbial denitrification. Specific bacteria were found in CWs with iron-based material, such as Thiobacillus spp. and Thauera spp., indicating that the nitrate removal in the denitrification process was triggered by carbon sources and that Fe²⁺ worked as both the electron donor and the adjuster of the abundances of specific bacteria. The addition of iron-based material into CWs was a green option to improve the pollutant removal performance.