New Synthesis of nZVI/C Composites as an Efficient Adsorbent for the Uptake of U(VI) from Aqueous Solutions

Performance and mechanism of U(vi) removal from solution by humic acid-coated Fe3O4 nanoparticle-modified biochar from filamentous green algae

The adsorbent material humic acid-coated Fe3O4 nanoparticle-modified biochar from filamentous green algae was fabricated by introducing the composites of humic acid-coated Fe3O4 nanoparticles onto biochar from filamentous green algae using the co-precipitation method. Then, the removal of U(vi) from solution by humic acid-Fe3O4/BC was carried out through batch experiments. The results of the characterization showed that the reaction conditions had an important influence on U(vi) removal by humic acid-Fe3O4/BC. The pseudo-second-order kinetic model and Langmuir model better illustrate the adsorption process of U(vi) on the surface of humic acid-Fe3O4/BC. The adsorption processes were dominated by chemisorption and monolayer adsorption. The maximum adsorption capacity of U(vi) by humic acid-Fe3O4/BC could be calculated, and it could reach 555.56 mg g-1. The probable mechanisms of U(vi) removal by humic acid-Fe3O4/BC were reduction reaction, inner-sphere surface complexation and electrostatic adsorption. The high stability and reusability of humic acid-Fe3O4/BC made it more promising in U(vi) removal applications.

Uranyl Affinity between Uranyl Cation and Different Kinds of Monovalent Anions: Density Functional Theory and Quantitative Structure-Property Relationship Model

In order to design effective extractants for uranium extraction from seawater, it is imperative to acquire a more comprehensive understanding of the bonding properties between the uranyl cation (UO22+) and various ligands. Therefore, we employed density functional theory to investigate the complexation reactions of UO22+ with 29 different monovalent anions (L-1), exploring both mono- and bidentate coordination. We proposed a novel concept called "uranyl affinity" (Eua) to facilitate the establishment of a standardized scale for assessing the ease or difficulty of coordination bond formation between UO22+ and diverse ligands. Furthermore, we conducted an in-depth investigation into the underlying mechanisms involved. During the process of uranyl complex [(UO2L)+] formation, lone pair electrons from the coordinating atom in L- are transferred to either the lowest unoccupied molecular degenerate orbitals 1ϕu or 1δu of the uranyl ion, which originate from the uranium atom's 5f unoccupied orbitals. In light of discussion concerning the mechanisms of coordination bond formation, quantitative structure-property relationship analyses were conducted to investigate the correlation between Eua and various structural descriptors associated with the 29 ligands under investigation. This analysis revealed distinct patterns in Eua values while identifying key influencing factors among the different ligands.

Statistically and visually analyzing the latest advancements and future trends of uranium removal

Uranium contamination and remediation is a very important environmental research area. Removing radioactive and toxic uranium from contaminated media requires fundamental knowledge of targets and materials. To explore the-State-of-the-Art in uranium contamination control, we employed a statistical tool called CiteSpace to visualize and statistically analyze 4203 peer-reviewed papers on uranium treatment published between 2008 and 2022. The primary content presentations of visual analysis were co-authorships, co-citations, keyword co-occurrence analysis with cluster analysis, which could offer purposeful information of research hots and trends in the field of uranium removal. The statistical analysis results indicated that studies on uranium removal have focused on adsorption of uranium from aqueous solution. From 2008 to 2022, biochar and biological treatment were firstly used to sequester uranium, then adsorption for uranium removal dominates with adsorbents of graphene oxide, primary nanofiber magnetic polymers and metal-organic frameworks (MOFs). In recent years, photocatalysts and metal-organic frameworks are expected to be two of the most popular research topics. In addition, we further highlighted the characteristics and applications of MOFs and GOs in uranium removal. Overall, a statistical review was proposed to visualize and summarize the knowledge and research trends regarding uranium treatment.

Significance of MOF adsorbents in uranium remediation from water

Uranium is considered as one of the most perilous radioactive contaminants in the aqueous environment. It has shown detrimental effects on both flora and fauna and because of its toxicities on human beings, therefore its exclusion from the aqueous environment is very essential. The utilization of metal-organic frameworks (MOFs) as an adsorbent for the removal of uranium from the aqueous environment could be a good approach. MOFs possess unique properties like high surface area, high porosity, adjustable pore size, etc. This makes them promising adsorbents for the removal of uranium from contaminated water. In this paper, sources of uranium in the water environment, human health disorders, and application of the different types of MOFs as well as the mechanisms of uranium removal have been discussed meticulously.

Intrinsically morphological effect of perovskite BaTiO3 boosting piezocatalytic uranium extraction efficiency and mechanism investigation

Developing a convenient, efficient and eco-friendly approach for the recovery of U(VI) ion is a key measure to solve the environmental problems arising from the utilization of nuclear energy. Herein, the high efficiency of uranium extraction is realized by the piezo property of perovskite BaTiO₃, revealing the intrinsically morphological engineering effect on the piezocatalytic performance. Especially, BaTiO₃ nanowires (BTO NWs) exhibit not only an excellent piezocatalytic activity with U(VI) extraction rate of 96.8% in a UO₂(NO₃)₂ aqueous solution compared to 71.3% of BaTiO₃ nanoparticles (BTO NPs), but also a promising piezocatalyst for U extraction in a real U-mining wastewater with various pH ranges. Piezo response force microscopy and finite elemental simulation show that the piezo response of BTO NWs is much higher than BTO NPs. Additionally, some factors (pH, various ions, different powers) are explored on piezocatalytic efficiency for U(VI) extraction. The results from electron spin resonance and the charge/radical capture experiments confirm that the active species (e^-, •O^2-, •OH) stemmed from the piezo induction of BTO NWs and BTO NPs in the piezocatalytic U(VI) reduction process. The present work reveals the structure-performance correlation during piezocatalysis and highlights the crucial role of piezocatalysis in dealing with environmental problems.

Collagen processing with mesoscale aggregates as templates and building blocks

Collagen presents a well-organized hierarchical multilevel structure. Microfibers, fibers, and fiber bundles are the aggregates of natural collagen; which achieve an ideal balance of mechanical strength and toughness at the mesoscopic scale for biological tissue. These mesostructured aggregates of collagen isolated from biological tissues retain these inherent organizational features to enable their use as building blocks for constructing new collagen materials with ideal mechanical performance, thermal and dimensional stability. This strategy is distinct from the more common bottom-up or molecular-level design and assembly approach to generating collagen materials. The present review introduces the hierarchical structure of biological collagen with a focus on mesostructural features. Isolation strategies for these collagen aggregates (CAs) are summarized. Recent progress in the use of these mesostructural components for the construction of new collagen materials with emerging applications is reviewed, including in catalysis, environmental applications, biomedicine, food packaging, electrical energy storage, and flexible sensors. Finally, challenges and prospects are assessed for controllable production of CAs as well as material designs.

Oxygen-containing groups in cellulose and lignin biochar: their roles in U(VI) adsorption

The adsorption behaviors of cellulose and lignin biochar depend on the evolution of their oxygen-containing groups to some extent. In this study, cellulose-rich pakchoi and lignin-rich corncob were selected to prepare the pyrolytic biochar at variable temperatures, named PBC and CBC, respectively. Their structure-function relationships were in-depth studied via the combination of the adsorption experiments of U(VI) and comprehensive spectral analyses. The maximal adsorption capacity of PBC 300, obtained at 300 °C, was measured as 46.62 mg g^-1 for U(VI), which was ⁓1.3 times higher than 35.60 mg g^-1 of CBC 300. U(VI) adsorption on PBC and CBC were predominantly ascribed to the coordination interaction between oxygen-containing groups and U(VI). Interestingly, the main complexation groups were distinct in both biochars due to the different inherent evolutions of cellulose and lignin. Volatile d-glucose chains in cellulose were apt to degrade rapidly, and the formed carboxyls acted as the most important sites in PBC. However, the stable aromatic network in lignin led to a slow degradation, and more hydroxyls thus remained in CBC, which controlled U(VI) adsorption. In this study, we obtained greatly cost-effective adsorbents of U(VI) and provided some essential insights into understanding the structural evolution-function relationship of cellulose and lignin biochar.

Impacts of shell structure on nitrate-reduction activity and air stability of nanoscale zero-valent iron

Nanoscale zero-valent iron (nZVI) has been intensively studied for pollution control because of its high reductive activity and environmental benignity, but the poor reaction selectivity and the aging problem have limited its practical decontamination application. Here, we shed light on the impacts of nZVI shell structure on its reactivity and air stability by systematically comparing two nZVI materials with distinct iron oxide shells. The nZVI with highly crystalline and weakly hydrophilic shell exhibited ninefold higher intrinsic activity for nitrate reduction and significantly improved air stability than that with amorphous, hydrophilic iron hydroxide oxide shell. The compact-structured crystalline shell of nZVI facilitated more efficient interfacial electronic transfer for nitrate reduction and suppressed side reaction of hydrogen evolution. The protective hematite shell endowed the nZVI with significantly improved anti-aging ability, and the reducing force remained 92.6% after exposed to air for 10 days due to decreased oxygen diffusion. This work provides a better understanding of the pollutant degradation behavior of nZVI and may guide an improved synthesis and environmental application of nZVI.

The high efficient Sb(III) removal by cauliflower like amorphous nanoscale zero-valent iron (A-nZVI)

In this study, cauliflower like amorphous nanoscale zero-valent iron (A-nZVI) was prepared and its performance on the removal of Sb(III) was investigated and compared with that of nZVI. The results indicated that the removal of Sb(III) by nZVI and A-nZVI followed the pseudo-second-order kinetic model and Langmuir isotherm model, but the removal of Sb(III) by A-nZVI was more stable and its removal capacity (558.2 mg/g) is much higher than that of nZVI (91.3 mg/g). Moreover, the effects of initial Sb(III) concentration, initial pH and anions such as Cl^-, NO₃^-, SO₄^2-, PO₄^3-, and AsO₄^3- were also investigated. A-nZVI showed extremely high selectivity towards Sb(III) in that 500 mg/L of AsO₄^3- and PO₄^3- shows little impact on its removal, while the removal of Sb(III) by nZVI was almost inhibited under the same condition. The combination of SEM-EDS, XPS, XRD and FTIR revealed the removal of Sb(III) by nZVI and A-nZVI were synergistic effects of oxidation and adsorption, but less Sb(III) (39.5%) was oxidized by A-nZVI. More γ-FeOOH and γ-Fe₂O₃ were formed at the surface of A-nZVI during the reaction. Both oxides have high affinity toward Sb(III), which might cause the higher removal capacity and selectivity for the removal of Sb(III) by A-nZVI. In conclusion, A-nZVI showed great potential for the remediation of Sb(III) in groundwater.

Mechanistic insight into the adsorption of mercury (II) on the surface of red mud supported nanoscale zero-valent iron composite

Recently, nanoscale zero-valent iron (nZVI) particles have been efficiently used in the remediation of many heavy metals, yet potential agglomeration and loss of nZVI remain a critical area of research. In this study, we used red mud as a stable supporting medium to develop red mud modified nZVI to form (RM-nZVI) composite. We assessed its sorptive/reductive removal of mercury (Hg²⁺) from aqueous solutions. The RM-nZVI was synthesized through the reduction of ferric iron by sodium borohydride (NaBH₄) in the presence of red mud. Morphological characterization of RM-nZVI confirmed its diffusion state with lesser aggregation. The RM-nZVI has the BET surface area, pore diameter, and pore volume as 111.59 m²g^-1, 3.82 nm, and 0.49 cm³g^-1, respectively. Adsorption of mercury (Hg²⁺) by RM-nZVI exhibits pH-dependent behavior with increased removal of Hg²⁺ with the increase in pH up to 5, and the removal rate decreased gradually as the pH increased from 5 to 10. Extensive characterization of RM-nZVI corroborated the evidence that the removal of Hg²⁺ was initially by rapid physical adsorption, followed by a reduction of Hg²⁺ to Hg⁰. The adsorption data were best fitted with Langmuir isotherm with R² (correlation coefficient) > 0.99 with high uptake capacity of 94.58 (mg g^-1). The novel RM-nZVI composite with enhanced sorptive and reductive capacity is an ideal alternative for removing Hg²⁺ from contaminated water.

High removal efficiencies of antibiotics and low accumulation of antibiotic resistant genes obtained in microbial fuel cell-constructed wetlands intensified by sponge iron

Using microbial fuel cells with constructed wetlands (MFC-CWs) for eliminating antibiotics has recently attracted extensive attention. However, antibiotic removal efficiencies in MFC-CWs must be enhanced, and the accumulation of antibiotic resistant genes (ARGs) remains an unmanageable issue. This study tries to enhance the antibiotic removal in synthetic wastewater and reduce ARGs by adding sponge iron (s-Fe⁰) and calcium peroxide to the anode and cathode of MFC-CWs, respectively, and/or simultaneously. The results demonstrated that adding s-Fe⁰ and calcium peroxide to MFC-CWs could improve the removal efficiencies of sulfamethoxazole (SMX) and tetracycline (TC) by 0.8-1.3% and 6.0-8.7%. Therein, s-Fe⁰ also significantly reduced 84.10-94.11% and 49.61-60.63% of total sul and tet genes, respectively. Furthermore, s-Fe⁰ improved the voltage output, power density, columbic efficiency, and reduced the internal resistance of reactors. The intensification to the electrode layers posed a significant effect on the microbial community composition and functions, which motivated the shift of antibiotic removal, accumulation of ARGs and bioelectricity generation in MFC-CWs. Given the overall performance of MFC-CWs, adding s-Fe⁰ to the anode region of MFC-CWs was found to be an effective strategy for removing antibiotics and reducing the accumulation of ARGs.

Citrate iron complex induced dramatically enhanced oxidation of atrazine with bimetallic Bi/Fe0: Reactivity, oxidation and mechanism

The oxidative degradation of atrazine (ATR) using bimetallic Bi/Fe⁰ nanoparticles cooperated with citric acid (CA) and sodium citrate (NaCA) without extra addition of H₂O₂ or another oxidant was conducted. Almost 73% of ATR was removed in Bi/Fe⁰+NaCA + CA buffer system in 3 h, and the bimetallic Bi/Fe⁰ performs high stability and long service life in the buffer system according to the results of cyclic degradation experiments. The citrate iron complex of Fe(II)[Cit]^- played the key role for the degradation process since it could quickly react with the generated H₂O₂ to produce free radicals in the Bi/Fe⁰+NaCA + CA system, which broadened the applicable pH range of the traditional Fenton reaction and promoted the oxidative degradation process of ATR. The possible degradation pathways of ATR were also investigated. In the Bi/Fe⁰+NaCA + CA buffer system, twelve kinds of ATR intermediate products were detected, of which the main products were dechlorination products and alkyl oxidative products. Due to the pH controllable of the Bi/Fe⁰+NaCA + CA system, it could reduce the acidity impact on the environment and makes the additional impact on the environment lower. Therefore, this work provides a new strategy for the degradation of ATR.

Constructing new Fe3O4@MnOx with 3D hollow structure for efficient recovery of uranium from simulated seawater

Enrichment of uranium from seawater is a promising method for addressing the energy crisis. Current technologies are generally not effective for enriching uranium from seawater because its concentration in seawater is low. In this study, new Fe₃O₄@MnO_x with 3D hollow structure, which is capable of enriching low concentration uranium, was prepared via a novel redox etching method. The physicochemical characteristics of Fe₃O₄@MnO_x were studied with TEM, HRTEM, SEAD, FTIR, XRD, and N₂ adsorption-desorption analysis. Dynamic kinetic studies of different initial U(VI) concentrations revealed that the pseudo-second-order model fit the sorption process better, and the sorption rates of Fe₃O₄@MnO_x in 1, 10, and 25 mg/L U(VI) solution were 0.0124, 0.00298, and 0.000867 g/mg·min, respectively. Isothermal studies showed that the maximum sorption amounts were 50.09, 56.27, and 64.62 mg/g for 1, 10, and 25 mg/L U(VI), respectively, at pH 5.0 and 313 K, suggesting that Fe₃O₄@MnO_x could effectively enrich low concentration U(VI) from water. The sorption amount of U(VI) did not significantly decrease in the presence of Na⁺, Mg²⁺, and Ca²⁺. HRTEM, FTIR, and XPS results demonstrated that Fe(II) and Mn/Fe-O-H active sites in Fe₃O₄@MnO_x were accounted for the high and specific enrichment efficiency. A column experiment was conducted to evaluate the U(VI) sorption efficiency of Fe₃O₄@MnO_x in simulated seawater. The U(VI) sorption efficiency remained above 80% in 28 days run. Our findings demonstrate that Fe₃O₄@MnO_x has extraordinary potential for the enrichment of uranium from simulated seawater.

Reductive and adsorptive elimination of U(VI) ions in aqueous solution by SFeS@Biochar composites

The novel biochar supported by starch and nanoscale iron sulfide (SFeS@Biochar) composites were successfully prepared through coupling of biochar derived from peanut shell with nanoscale ferrous sulfide and starch under nitrogen atmosphere. It had the advantages of biochar, starch, and nanoscale ferrous sulfide. Therefore, it could overcome some shortcomings. The nanoscale ferrous sulfide particles and starch were thought to be loaded successfully on the surface of the biochar by SEM, EDS, BET, XRD, FT-IR, and XPS techniques. High uptake capacity of U(VI) by SFeS@Biochar could be attributed to reactive reaction of FeS nanoparticles and adsorptive of a lot of functional groups. The proposed reaction mechanisms of the U(VI) uptake by SFeS@Biochar were electrostatic attraction, surface complexation, precipitation, and reductive reaction. It also might be an improved environmentally friendly material for U(VI) removal.

Metal-Organic-Framework Based Functional Materials for Uranium Recovery: Performance Optimization and Structure/Functionality-Activity Relationships

Uranium recovery has profound significance in both uranium resource acquisition and pollution treatment. In recent years, metal-organic frameworks (MOFs) have attracted much attention as potential uranium adsorbents owing to their tunable structural topology and designable functionalities. This review explores the research progress in representative classic MOFs (MIL-101, UiO-66, ZIF-8/ZIF-67) and other advanced MOF-based materials for efficient uranium extraction in aqueous or seawater environments. The uranium uptake mechanism of the MOF-based materials is refined, and the structure/functionality-property relationship is further systematically elucidated. By summarizing the typical functionalization and structure design methods, the performance improvement strategies for MOF-based adsorbents are emphasized. Finally, the present challenges and potential opportunities are proposed for the breakthrough of high-performance MOF-based materials in uranium extraction.

Orderly Porous Covalent Organic Frameworks-based Materials: Superior Adsorbents for Pollutants Removal from Aqueous Solutions

Covalent organic frameworks (COFs) are a new type of crystalline porous polymers known for chemical stability, excellent structural regularity, robust framework, and inherent porosity, making them promising materials for capturing various types of pollutants from aqueous solutions. This review thoroughly presents the recent progress and advances of COFs and COF-based materials as superior adsorbents for the efficient removal of toxic heavy metal ions, radionuclides, and organic pollutants. Information about the interaction mechanisms between various pollutants and COF-based materials are summarized from the macroscopic and microscopic standpoints, including batch experiments, theoretical calculations, and advanced spectroscopy analysis. The adsorption properties of various COF-based materials are assessed and compared with other widely used adsorbents. Several commonly used strategies to enhance COF-based materials' adsorption performance and the relationship between structural property and sorption ability are also discussed. Finally, a summary and perspective on the opportunities and challenges of COFs and COF-based materials are proposed to provide some inspiring information on designing and fabricating COFs and COF-based materials for environmental pollution management.

Mechanisms of enhanced hexavalent chromium removal from groundwater by sodium carboxymethyl cellulose stabilized zerovalent iron nanoparticles

Chromium (Cr) contamination poses serious threats to the environment and human health. Thus, batch and column experiments were performed to investigate hexavalent chromium [Cr (VI)] removal from solution and porous media using nanoscale zerovalent iron nanoparticles (NZVI) stabilized by sodium carboxymethyl cellulose (CMC). Batch experiments indicated that the mass ratio of Fe/CMC = 1, the presence of 150-200 mg L^-1 CMC and lower ionic strength led to optimum Cr (VI) removal in aqueous solution. Column experiments demonstrated that Cr (VI) removal was enhanced with decreasing solution pH and increasing CMC-NZVI concentration. The presence of CMC can increase Cr (VI) removal by NZVI in both aqueous solution and porous media by complexation precipitation of Cr (VI) compounds and better dispersion of NZVI. X-ray photoelectron spectroscopy (XPS) analysis revealed that an appropriate amount of CMC supported the redox reaction of Cr (VI) and NZVI. The removal of Cr (VI) through columns was 20.8% and 88.5% under no additional CMC and optimized CMC content, respectively. However, Cr (VI) removal decreased to 64.6% under excessive CMC content. The CMC modified NZVI nanoparticles were characterized by XRD, XPS and TEM techniques. These findings imply that CMC can be used as an effective stabilizer on NZVI which can in turn be applied for the efficient removal of Cr (VI) from industrial wastewater and groundwater.

Waste-to-resources: Green preparation of magnetic biogas residues-based biochar for effective heavy metal removals

The agricultural wastes disposal and polluted water purification are always the key issues of environmental restoration. In this work, a magnetic biogas residue-based biochar (mBR-C) by direct pyrolysis and sonochemical method was prepared from biogas residue (BR). Response design methodology based on Box-Behnken design was used for the preparation parameters optimization. The characterization results identified that mBR-C had well-developed pore structure and surface area, which was beneficial to diffuse and capture heavy metal ions. Traces of toxic heavy metal in mBR-C was leached (˂0.04 mg/L) through TCLP method, indicating the environmental safety of the magnetic biochar. Meanwhile, the mBR-C exhibited excellent solid-liquid separation efficiency because of its strong magnetism. The series of adsorption experiments indicated that mBR-C could capture Cu²⁺ and Pb²⁺ rapidly, and the maximum adsorption capacity for Cu²⁺ and Pb²⁺ was 75.76 and 181.82 mg/g, respectively, which was higher than some other biochars previously reported. mBR-C was further applied in the synthetic wastewater treatment, which could effectively purify at least 600 mL (150 BV) to meet emission standards. After several column adsorption-desorption cycles, the adsorption capacity could still reach 85%, implying that mBR-C has good reusability and stability. Overall, the mBR-C can be used as an eco-friendly, desirable, economic and recyclable biosorbent in heavy metal polluted water treatment, providing a new idea for a combination of biogas residue recycle and wastewater treatment.

Facile construction of Fe, N and P co-doped carbon spheres by carbothermal strategy for the adsorption and reduction of U(vi)

In this work, nitrogen and phosphorus co-doped magnetic carbon spheres encapsulating well-dispersed active Fe nanocrystals (Fe/P-CN) were fabricated via a simple copolymer pyrolysis strategy. Benefiting from heteroatoms doping, Fe/P-CN could primarily adsorb soluble U(vi) ions through abundant functional groups, and subsequently, the adsorbed U(vi) could be reduced to insoluble U(iv) by Fe nanocrystals. Fe/P-CN pyrolyzed at 800 °C (Fe/P-CN-800) exhibited excellent U(vi) removal capacity of 306.76 mg g⁻¹, surpassing nitrogen and phosphorus co-doped carbon spheres and nano zero-valent iron. In addition, the magnetic separation and thermal reactivation properties endow Fe/P-CN-800 with excellent reusability. This research, especially, provides a promising synergistic adsorption and reduction strategy to effectively remove U(vi) using heteroatom-doped composites.

The constructed novel magnetic carbon sphere co-doped by N, P, Fe (Fe/P-CN) exhibits high U(vi) removal efficiency, excellent magnetic separation and reusability, evidencing the potential practical applications in environmental remediation.

The removal of uranium using novel temperature sensitive urea-formaldehyde resin: adsorption and fast regeneration

A novel adsorbent of temperature sensitive urea-formaldehyde (TS-UF) resin was synthesized by base/acid two-step synthetic strategy with low formaldehyde/urea mole ratio of 0.8. The sorption kinetics of TS-UF resin obeys the pseudo-second-order model, and the adsorption is an endothermic process. The Langmuir model can well describe the sorption isotherms, through which the Q_max is calculated to be 99.2 mg/g for uranium (VI) at pH 6.0 and T = 298 K. The characterized results show that the functional groups -NH- and -CH₂OH in TS-UF resin have been involved in uranium sorption via chemical interaction. The temperature sensitive property of TS-UF resin significantly accelerates the regeneration of TS-UF resin, which can be fast regenerated within 15 min at its low critical solution temperature 333 K and exhibits high removal efficiency of uranium (VI) (>90%) over 5 cycles. Therefore, TS-UF resin can be as a promising sorbent for the uranium (VI) removal from wastewater due to its low-cost, easy-fabrication, high-efficiency and fast regeneration. This work can not only boost the exploration of novel adsorbent materials, but also promote the investigations of the regeneration and reusability of adsorbents.

Untangling the nitrate removal pathways for a constructed wetland- sponge iron coupled system and the impacts of sponge iron on a wetland ecosystem

Sponge iron (s-Fe⁰) is a potential alternative electron donor for nitrate reduction. To gain insight into the mechanism of denitrification in a constructed wetland- sponge iron coupled system (CW-Fe⁰ system), the removal performance and reduction characteristics of nitrate in constructed wetlands (CWs) with and without s-Fe⁰ application were compared. Results indicated that s-Fe⁰ intensified the removal of nitrate with a 6h-HRT. The nitrate removal efficiency was improved by 16-76 % with various influent NO₃^--N concentrations (10-30 mg L^-1) and at a chemical oxygen demand(COD)/N ratio of 5. The rates of chemical denitrification were positively correlated with the dosage of s-Fe⁰ and negatively correlated with the influent COD concentration. 16S rDNA sequencing revealed that hydrogen-utilizing autotrophic denitrifier of Hydrogenophaga was highly enriched (accounting for 10 % of the total OTUs) only in CW-Fe⁰ system. The micro-environment created by s-Fe⁰ was suitable for heterotrophic denitrifiers of Thauera, Tessaracoccus and Simplicispira. The determination of physiological indicators for plants showed that the application of s-Fe⁰ causes abiotic stress to wetland plants (Canna indica L.). Nevertheless, s-Fe⁰ can be used as a substrate for CWs, since it allows a high-efficiency removal of nitrate by mediating chemical denitrification and hydrogen-driven autotrophic denitrification.

Ultrafast and highly capture of U(VI) by hierarchical mesoporous carbon

In this study, the hierarchical mesoporous carbon (HMC) was synthesized by the hydrothermal method. The batch adsorption experiments showed that HMC exhibited the ultrafast equilibrium fate (80 % U(VI) capture efficiency within 5 min), high UO2 2+ capture capacity (210 mg/g, pH = 4.5) and well recyclability. The investigations of XPS techniques indicated the oxygen-containing functional groups were responsible for high efficient UO2 2+ adsorption. The pH-dependent adsorption was simulated by three surface complexation modellings, revealing that UO2 2+ adsorption on HMC was excellently fitted by triple layer model using two inner-sphere complexes (i. e. SOUO2 + and SOUO2(CO3)3 5− species) compared to constant capacitance model and diffuse layer model. These findings are crucial for expanding actual applications of HMC towards the removal of radionuclides under environmental cleanup.

Optimizing nanocarbon shell in zero-valent iron nanoparticles for improved electron utilization in Cr(VI) reduction

A core-shell structured zero-valent iron@carbon (ZVI@C) nanocompoiste was designed to improve the electron utilization of ZVI in the Cr(VI) reduction. The porosity of carbon layer in ZVI@C was optimized for improving the efficiency of electron utilization of ZVI in the Cr(VI) reduction process. The porous structure of carbon layer was controllably synthesized by adjusting the carbon source and the ratio of C/Fe in the precursor. The glucose was suggested as the optimal carbon source, and a high specific surface area (37.067 m²/g) was reached for the prepared ZVI@C when the ratio of C/Fe was controlled at 20. These ZVI@C performed well on Cr(VI) reduction, e.g. a complete reduction of Cr(VI) (2 mg/L) to Cr(III) within 10 min. The removal capacity (800 mg/g) exceeded previously recorded ZVI based adsorbents. The pH and initial Cr(VI) concentration were demonstrated as the key factors for the efficient electron utilization of ZVI. Furthermore, the efficiency of electron utilization of the ZVI increased up to 80% when the concentration of Cr(VI) was 2000 mg/L and the pH was controlled at 3, which was much higher than 8% of the naked ZVI.

Polyethylene glycol-stabilized nano zero-valent iron supported by biochar for highly efficient removal of Cr(VI)

In this study, polyethylene glycol (PEG)-stabilized nano zero-valent iron (nZVI) supported by biochar (BC) (PEG-nZVI@BC) was prepared to remedy Cr(VI) with high efficiency. The morphology, functional groups, and crystalline structure of PEG-nZVI@BC composites were characterized, revealing that when PEG was added, a large number of -OH functional groups were introduced, and nZVI was effectively dispersed on the BC surface with a smaller particle size. The results of Cr(VI) remediation experiments showed Cr(VI) removal rate by PEG-nZVI@BC (97.38%) was much greater than that by BC-loaded nZVI (nZVI@BC) (51.73%). The pseudo second-order and Sips isotherm models provide the best simulation for Cr(VI) removal experimental data, respectively. The main remediation mechanism of Cr(VI) was reduction and co-precipitation of Cr-containing metal deposits onto PEG-nZVI@BC. Ecotoxicity assessment revealed PEG-nZVI@BC (1.00 g/L) has little influence on rice germination and growth, but resisted the toxicity of Cr(VI) to rice. The modified Community Bureau of Reference (BCR) sequential extraction showed pyrolysis could increase the percentage of oxidizable and residual Cr and diminish the environmental risk of Cr release from post-removal composites.

Mechanisms of U(VI) removal by biochar derived from Ficus microcarpa aerial root: A comparison between raw and modified biochar

Uranium (U) is a toxic and radioactive element. Excessive amounts of aqueous U(VI) generated from U mining, processing and nuclear industry may result in severe and irreversible damage to the environment. Herein, Ficus microcarpa aerial root (FMAR), a biowaste material, was used to adsorb U(VI) from aqueous solutions for the first time. Potassium permanganate (KMnO₄)-modified FMAR biochar was synthesised, characterised and compared with raw (unmodified) biochar with respect to U(VI) adsorption. The results showed that the adsorption capability of the modified FMAR biochar was evidently higher than that of the raw biochar. Multiple characterisation techniques confirmed that the discrepancy was mainly due to the increased content of O-H and formation of irregular sheet-like nanostructure with the ultrafine MnO₂ nanoparticles on the biochar surfaces after KMnO₄ modification. The abundance of O-H and nanoscale MnO₂ notably enhanced the adsorption of U(VI) by means of coordination and Lewis acid-base interaction. The results indicate that KMnO₄-modified FMAR biochar has a good potential to serve as an environment-friendly adsorbent for the removal of U(VI) from solution.

Construction of novel graphene-based materials GO@SiO2@C@Ni for Cr(VI) removal from aqueous solution

A series of novel sandwich-like GO@SiO₂@C@Ni composites were developed. The morphologies and adsorption capacities of the materials sintered at different carbonization temperatures were investigated. The formed GO@SiO₂@C@Ni-400 possessed of wonderful dispersion, large surface area (229.88 m²/g) and high saturation magnetization. Batch experimental results revealed that maximum adsorption capacities of these materials towards Cr(VI) were as follows: GO@SiO₂@C@Ni-400 (299.20 mg/g) > GO@SiO₂@C@Ni-500 (244.05 mg/g) > GO (202.39 mg/g) > Graphene@C@Ni (188.80 mg/g) > GO@SiO₂@C@Ni-600 (165.51 mg/g) > GO@SiO₂@C@Ni-700 (93.36 mg/g). Moreover, the influence of hydrochemistry, such as contact time, pH, co-existing ions and solution temperature, on Cr(VI) adsorption was researched as well. It was demonstrated that GO@SiO₂@C@Ni-400 had remarkable adsorption capacity for Cr(VI) removal under the acidic condition, hardly disturbed by other anions, and showed better adsorption performance at 328 K. Besides, On the base of X-ray photoelectron spectroscopy analysis, mechanisms of adsorption could be explained that Cr(VI) was reduced to Cr(III) by nitrogen dopant, and the complexation was existed between Cr(VI) and oxygen-containing functional groups. Additionally, GO@SiO₂@C@Ni-400 could be easily separated under the external magnetic field and displayed outstanding reusability. Herein, GO@SiO₂@C@Ni-400 opens up the possibility of future practical applications.

Self-assembly of graphene oxide/PEDOT:PSS nanocomposite as a novel adsorbent for uranium immobilization from wastewater

In recent years, water pollution caused by radionuclides has become a rising concern, among which uranium is a representative class of actinide element. Since hexavalent uranium, i.e. U(VI), is biologically hazardous with high migration, it's essential to develop efficient adsorbents to minimize the impact on the environment. Towards this end, we have synthesized a novel material (GO/PEDOT:PSS) by direct assembling graphene oxide (GO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) through a facile ball milling method, which shows impressing performance for the immobilization of U(VI). On the basis of the batch experiments, GO/PEDOT:PSS exhibits ionic strength-independent sorption edges and temperature-promoted sorption isotherms, revealing an inner-sphere complexation with endothermic nature. The sorption kinetics can be illustrated by the pseudo-second-order model, yielding a rate constant of 1.09. × 10^-2 g mg^-1∙min^-1, while the sorption isotherms are in coincidence with the Langmuir model, according to which the maximum sorption capacity is measured to be 384.51 mg g^-1 at pH 4.5 under 298 K, indicating a monolayer sorption mechanism. In the light of the FT-IR and XPS investigations, the surface carboxyl/sulfonate group is responsible to the chelation of U(VI), indicating that the enhanced sorption capacity may be ascribed to the PSS moiety. These findings can greatly contribute to the design strategy for developing highly efficient adsorbents in the field of radioactive wastewater treatment.

Ultra-thin iron phosphate nanosheets for high efficient U(VI) adsorption

In this study, the ultra-thin iron phosphate Fe7(PO4)6 nanosheets (FP1) with fine-controlled morphology, has been designed as a new two-dimensional (2D) material for uranium adsorption. Due to its unique high accessible 2D structure, atom-dispersed phosphate/iron anchor groups and high specific surface area (27.77 m2⋅g-1), FP1 shows an extreme-high U(VI) adsorption capacity (704.23 mg·g-1 at 298 K, pH = 5.0 ± 0.1), which is about 27 times of conventional 3D Fe7(PO4)6 (24.51 mg·g-1 -sample FP2) and higher than most 2D absorbent materials, showing a great value in the treatment of radioactive wastewater. According to the adsorption results, the sorption between U(VI) and FP1 is spontaneous and endothermic, and can be conformed to single molecular layer adsorption. Based on the analyses of FESEM, EDS, Mapping, FT-IR and XRD after adsorption, the possibile adsorption mechanism can be described as a Monolayer Surface Complexation and Stacking mode (MSCS-Mode). Additionally, the research not only provide a novel preparing method for 2D phosphate materials but also pave a new pathway to study other two-dimensional adsorption materials.

Enhanced Photoreduction of U(VI) on C3N4 by Cr(VI) and Bisphenol A: ESR, XPS, and EXAFS Investigation

The effect of Cr(VI) and bisphenol A (BPA) on U(VI) photoreduction by C₃N₄ photocatalyst was demonstrated by the batch experiments, electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) techniques. The batch experiments manifested that Cr(VI) and BPA enhanced the photocatalytic activity of C₃N₄ for U(VI) photoreduction, whereas U(VI) photoreduction was significantly diminished with increased pH from 4.0 to 8.0. According to radical scavengers and ESR analysis, U(VI) was photoreduced to U(IV) by photogenerated electrons of conduction band edge, whereas Cr(VI) was reduced to Cr(III) by H₂O₂. BPA and its products such as organic acid and alcohols can capture photoinduced holes, which resulted in the enhancement of U(VI) photoreduction to U(IV). XPS and XANES analyses demonstrated that U(VI) was gradually photoreduced to U(IV) by C₃N₄ within irradiation 60 min, whereas U(IV) was reoxidized to U(VI) with increasing irradiation time. EXAFS analysis determined that the dominant interaction mechanisms of U(VI) on C₃N₄ after irradiation for 240 min were reductive precipitation and inner-sphere surface complexation. This work highlights the synergistic removal of radionuclides, heavy metals, and persistent organic pollutants by C₃N₄, which is crucial for the design and application of a high-performance photocatalyst in actual environmental cleanup.

Decontamination of U(VI) on graphene oxide/Al2O3 composites investigated by XRD, FT-IR and XPS techniques

The decontamination of U(VI) on graphene oxide/nano-alumina (GO/Al₂O₃) composites were investigated by batch, XRD, FT-IR and XPS techniques. The characterization results showed that GO/Al₂O₃ composites presented a variety of oxygen-containing functional groups, which provided the more surface reactive sites. The batch experiments indicated that sorption equilibrium of U(VI) on GO/Al₂O₃ composites was achieved within 30 min, and the maximum sorption capacity derived from Langmuir model was 142.8 mg/g at pH 6.5. In addition, the slight decrease of sorption capacity was observed even after fifth recycling times. These results indicated that GO/Al₂O₃ composites displayed the fast sorption rate, high sorption capacity and good regeneration performance. No effect of ionic strength revealed the inner-sphere surface complexation of U(VI) on GO/Al₂O₃ composites. FT-IR and XPS analysis demonstrated that the high adsorption of U(VI) on GO/Al₂O₃ was attributed to the various oxygen-bearing functional groups. In addition, the nano Al₂O₃ was transferred to amorphous AlO(OH) mineral phase by XRD pattern, which provided the additional reactive sorption sites. These observations indicated that GO-based composites can be regarded as a promising adsorbent for immobilization and pre-concentration of U(VI) from aqueous solutions in the environmental remediation.

Effect of Staphylococcus epidermidis on U(VI) sequestration by Al-goethite

Effect of Staphylococcus epidermidis (S. epidermidis) on U(VI) sequestration by Al-goethite were conducted under different geologic conditions. The batch experiments showed that S. epidermidis significantly enhanced the adsorption rates of U(VI) at pH < 9.0 due to the additional metal binding sites. The maximum adsorption capacities of U(VI) on Al-goethite and Al-goethite +S. epidermidis at pH 4.0 and 310 K were calculated from Langmuir equation to be 13.16 and 47.86 mg/g, respectively. The decreased adsorption of U(VI) on Al-goethite+ S. epidermidis at high carbonate and pH conditions were primarily driven by the electrostatic repulsion between negatively charged U(VI)-carbonate complexes and the negatively charged adsorbents. According to XPS analysis, the adsorbed U(VI) can be reduced to U(IV) by S. epidermidis, whereas inhibited reduction of U(VI) on Al-goethite + S. epidermidis at high pH could be attributed to the complexation of structural Fe(III) with the oxygen-containing functional groups of S. epidermidis. FT-IR analysis further demonstrated that the bonding of structural Fe(III) with functional groups (e.g., carboxyl and phosphate groups) of S. epidermidis. These results herein are important to understand the fate and transport of U(VI) on the mineral-bacteria ternary systems in the near-surface environment.

Hierarchical Structure Kaolinite Nanospheres with Remarkably Enhanced Adsorption Properties for Methylene Blue

Kaolinite nanospheres with hierarchical structures were synthesized via dehydration-rehydration technique through calcined-hydrothermal route. The microstructure of samples were characterized and analyzed by diverse techniques. The results show that after hydrothermal treatment, the layered pseudo-hexagonal kaolinite particles transformed to hierarchical structure nanospheres. The hierarchical structures exhibit large specific surface area of 157.1 m² g^-1 and narrow mesoporous size distribution. The adsorption properties of kaolinite nanospheres were systematically investigated by the removal of methylene blue (MB) from water. It was found that the nanospheres can rapidly adsorb MB with a higher adsorption capacity (184.9 mg/g), and adsorption data followed Langmuir isotherm model and pseudo-second-order kinetic model. Furthermore, the adsorbent can be regenerated by washing with methanol-HCl solution and shown removal efficiency of more than 95% up to 4 cycles.

Synthesis of magnetic biochar composites for enhanced uranium(VI) adsorption

Magnetic biochar composites were successfully fabricated by pyrolysis of siderite and rice husk under N₂ condition. The results of a variety of characterization implied magnetic biochar displayed porous structures with larger specific surface area. The batch adsorption experiments showed high adsorption properties of magnetic biochar composites toward U(VI) (52.63 mg/g at pH 4.0), whereas U(VI) adsorption was significantly influenced by Na₂CO₃ and HA. U(VI) adsorbed onto magnetic biochar was reduced to U(IV) by Fe₃O₄ according to XPS and XANES analyses. In addition, no significant effect of ionic strength of NaCl and EXAFS results, illustrated the inner-sphere surface complexation of U(VI) on magnetic biochar. Owing to the simple synthesis procedure, low cost, high adsorption efficiency, easy separation and environmental friendly, magnetic biochar can be considered as a potential adsorbent for the purification of U(VI)-bearing wastewater in environmental remediation.

Nanoscale zero-valent iron/magnetite carbon composites for highly efficient immobilization of U(VI)

Nanoscale zerovalent iron/magnetic carbon (NZVI/MC) composites were successfully synthesized by simply calcining yellow pine and iron precursors. NZVI/MC pyrolyzed at 800°C (NZVI/MC800) had a higher percentage of NZVI and displayed better resistance to aggregation and oxidation of NZVI than samples prepared at other temperatures. The NZVI/MC800 material was applied for the elimination of U(VI) from aqueous solutions. The results suggested that the NZVI/MC800 displayed excellent adsorption capacity (203.94 mg/g) toward U(VI). The significant adsorption capacity and fast adsorption kinetics were attributed to the presence of well-dispersed NZVI, which could quickly reduce U(VI) into U(IV), trapping the guest U(IV) in the porous biocarbon matrix. The removal of U(VI) on the NZVI/MC samples was strongly affected by solution pH. The NZVI/MC samples also displayed outstanding reusability for U(VI) removal after multiple cycles. These findings indicate that NZVI/MC has great potential for remediation of wastewater containing U(VI).

High dispersions of nano zero valent iron supported on biochar by one-step carbothermal synthesis and its application in chromate removal

A one-step carbothermal synthesis and characterization of biochar-supported nanoscale zero-valent iron (nZVI/BC) was performed for the removal of hexavalent chromium (Cr(vi)) from aqueous solution. High dispersions of nanoscale zero-valent iron supported on biochar were successfully synthesized by the pyrolysis of an iron-impregnated biomass (corn stover) as the carbon and iron source under nitrogen atmosphere. The effects of the pyrolytic temperature on the Fe mineralogies formed on the biochar are discussed. In general, the effects of the treatment time, initial solution pH, and nZVI/BC dosage on the Cr(vi) removal are presented. The results showed high crystallinity and purity, and nZVI/BC was obtained at a pyrolytic temperature of 800 °C. The batch experimental results determined that the adsorption capacity of Cr(vi) decreases with the increase in the initial pH value from 4.0 to 10.0. The Cr(vi) adsorption kinetics data effectively followed a pseudo-second-order kinetics with a calculated rate constant of 0.0.3396 g mg⁻¹ min⁻¹. The calculated thermodynamic parameters, such as ΔG°, ΔH°, and ΔS°, were evaluated, and the results indicated that the Cr(vi) reduction on nZVI/BC was a spontaneous and endothermic process. The adsorption mechanism of Cr(vi) was investigated by XRD and XPS analyses and the results demonstrated that Cr(vi) was reduced to Cr(iii) and the oxidation of nZVI occurred during the reaction process. These results prove that nZVI/BC synthesized by a one-step carbothermal method can be considered as a potential candidate for the removal of Cr(vi) from aqueous solutions.

A one-step carbothermal synthesis and characterization of biochar-supported nanoscale zero-valent iron (nZVI/BC) was performed for the removal of hexavalent chromium (Cr(vi)) from aqueous solution.

Highly efficient carbonaceous nanofiber/layered double hydroxide nanocomposites for removal of U(VI) from aqueous solutions

The three-dimensional (3D) carbonaceous nanofiber and Ni-Al layered double hydroxide (CNF/LDH) nanocomposite was successfully prepared by a facile one-step hydrothermal methodology. Characterization of scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), XRD, and Fourier transformed infrared spectroscopy (FTIR) provided a demonstration that the modified CNF/LDH nanocomposite possessed abundant functional groups, for instance, metal-oxygen surface bonding sites (Ni–O as well as Al–O) and free-metal surface bonding sites (C–O, C–O–C, as well as O–C=O). The elimination of representative radionuclide (i.e. U(VI)) on the CNF/LDH nanocomposite from aqueous solutions was explored as a key function of pH, ionic strength, contact time, reaction temperature as well as radionuclide preliminary concentrations with the use of the batch methodology. As revealed by the findings, the sorption of radionuclides on CNF/LDH nanocomposite adhered to the pseudo-second-order kinetic model as well as Langmuir model. The maximum elimination capacity of U(VI) amounted to be 0.7 mmol/g. The independent of ionic strength shed light on the fact that inner-sphere surface complexation mainly overpowered radionuclide uptake by the CNF/LDH nanocomposite, which was further verified through the combination of FTIR and XPS spectral analyses. The abovementioned analyses shed light on the fact that the CNF/LDH nanocomposite can be regarded as a latent material to preconcentration radionuclides for environmental remediation.