Uranium(VI) reduction by nanoscale zero-valent iron in anoxic batch systems: the role of Fe(II) and Fe(III)

Effects of Fe(II) and humic acid on U(VI) mobilization onto oxidized carbon nanofibers derived from the pyrolysis of bacterial cellulose

The effects of Fe(II) and humic acid on U(VI) immobilization onto oxidized carbon nanofibers (Ox-CNFs, pyrolysis of bacterial cellulose) were investigated by batch, spectroscopic and modeling techniques, with results suggesting that, Ox-CNFs exhibited fast adsorption rate (adsorption equilibrium within 3 h), high adsorption performance (maximum adsorption capacity of 208.4 mg/g), good recyclability (no notable change after five regenerations) in the presence of Fe(II) towards U(VI) from aqueous solutions (e.g., 40 % reduction and 10 % adsorption at pH 8.0), which was attributed to the various oxygen-containing functional groups, excellent chemical stability, large specific surface area and high redox effect. U(VI) adsorption increased with increasing pH from 2.0 to 5.0, then high-level plateau and remarkable decrease were observed at 5.0-6.0 and at pH > 6.0, respectively. According to FT-IR and XPS analysis, a negative correlation between U(VI) reduction and organic in the presence of Fe(II) implied that U(VI) reduction was driven by Fe(II) while inhibited by humic acid. The interaction mechanism of U(VI) on Ox-CNFs was demonstrated to be adsorption and ion exchange at low pH and reduction at high pH according to XPS and surface complexation modeling. These findings filled the knowledge gaps pertaining to the effect of Fe(II) on the transformation and fate of U(VI) in the actual environment. This carbon material with distinctive performance and unique topology offers a potential platform for actual application in environmental remediation.

Enhancement of the heterogeneous adsorption and incorporation of uraniumVI caused by the intercalation of β-cyclodextrin into the green rust

The influence of intercalated anions on the structure and composition of green rusts supplies a theoretical possibility for the investigation of the structural modification of Fe^II/Fe^III (oxyhydr)oxide materials. β-Cyclodextrin was intercalated into the mixed-valent iron-based hydroxide layers to synthesize new green rust materials (β-CD GRs), pursuing high-capacity uranium^VI (U^VI) sorption. The molar ratios of Fe^II to Fe^III and the molar ratios of β-CD GR to Fe^II + Fe^III had a significant effect on the synthesis of β-CD GRs. The synthesis process was further optimized by the quadric predictor and desirability function in a central composite design in combination. Both strong acidity and alkalinity were harmful to the adsorption of β-CD GRs towards U^VI. The pseudo-first-order kinetic model and Langmuir isotherm model were appropriate in fitting the whole adsorption process. The maximum monolayer adsorption capacity of β-CD GRs was 2548.61 mg/g. The presence of mimic groundwater constituents explicitly deteriorated the interaction between β-CD GR and U^VI species. Nanoscale nodules and particles were formed on the β-CD GR after the adsorption experiments. The peaks at 1159 and 609 cm^-1 vanished with the band at 1103 cm^-1 being left-shifted to 1117 cm^-1 in the FTIR spectra of β-CD GR during the heterogeneous process. The intercalation of β-CD brought obvious enhancement of U^VI species sorption to the GR material, which was combinedly driven by several reaction pathways and different from the unmodified GRs.

Factors influencing the reduction of U(VI) by magnetite

Under suboxic and anoxic environments, magnetite is one corrosion product of iron being used in nuclear waste canisters. Previous studies have reported a complete reduction of U(VI) on the surfaces of biogenic and natural magnetite crystals, while incomplete reductions to U(V)/U(IV)-containing species have been observed on chemosynthetic magnetite. To date, the reasons behind such disparities remain poorly studied. This study shows that uranyl nitrate or uranyl acetate is mainly reduced to UO_2+x oxides (e.g., U₄O₉, U₃O₈, etc.) by chemosynthetic magnetite under acidic conditions. When extra zero valent-iron was added, the reaction rate was significantly increased, and an improved but still incomplete U(VI) reduction was observed. Nitrate and ferric ions are ubiquitous in natural environment. Results demonstrate that the nitrate ion associated with uranyl and the ferric ion contained in magnetite or generated from U(VI) reduction have a non-negligible oxidative effect on the final products, which could mainly account for the incomplete reduction of U(VI) by chemosynthetic magnetite in the absence or presence of extra zero valent-iron observed in this study. Furthermore, the surface loading of uranium in U-Fe systems can, in part, unravel the discrepancies in various observations. An enhanced understanding of the U-Fe reaction mechanism can facilitate predictions of the extent of uranium mobility with respect to nuclear waste disposal and radioactive decontamination.

Photocatalytic reduction of Uranium(VI) under visible light with Sn-doped In2S3 microspheres

Visible light-driven conversion of soluble U(VI) to slightly soluble U(IV) has been regarded as a efficient and environmentally friendly technology to deal with uranium containing wastewater. In this paper, we attempted to use photocatalytic technology to reduction U(VI) from aqueous solution by constructing a highly efficient photocatalysts. The novel Sn-doped In₂S₃ microspheres photocatalyst were synthesized for the first time by a simple hydrothermal method, and characterized with various analytical and spectroscopic techniques to determine their structural, morphological, compositional, optical and photocatalytic properties. In determination of photocatalytic activity, the results showed that all Sn-doped In₂S₃ samples exhibited greater photocatalytic performance in reduction of U(VI) under visible light than the pure In₂S₃. The optimum SnIn₂S₃ photocatalyst with Sn:In molar ratio of 1:4.8 (SnIn₂S₃) had the highest photocatalytic performance (95% reduction efficiency within 40 min irradiation time), which was approximately 15.60 times faster than that of pure In₂S₃. The enhanced photocatalytic activity of the optimum SnIn₂S₃ was largely ascribed to the higher specific surface area, red-shift in the absorption band, the efficient separation of photogenerated electron-hole pairs (e^-/h⁺) and the narrowed band gap with an up shifting of valence band, conduction band potentials. In addition the optimum SnIn₂S₃ photocatalyst exhibited a good recyclability and stability during the repetitive experiments. Finally, the possible active species and the possible mechanism on basis of the experimental results were discussed in detail.

Adsorption of aqueous neodymium, europium, gadolinium, terbium, and yttrium ions onto nZVI-montmorillonite: kinetics, thermodynamic mechanism, and the influence of coexisting ions

This study reports the adsorption of five rare earth elements (REEs) (belonging to light (Nd, Eu, Gd), medium (Tb), and heavy (Y) REE group) on montmorillonite-supported zero-valent iron nanoparticles (nZVI-M). Various parameters about REEs adsorption were investigated: the pH value, the adsorption kinetic, the maximum adsorption capacity, and the adsorption isotherm. The temperature (293-313 K) had a limited effect on the final adsorption equilibrium capacity and the analysis of thermodynamic studies suggests it was spontaneous (negative values of ∆G^o) and exothermic (negative values of ∆H^o). The system randomness decreased after adsorption (negative values of ∆S^o). In addition, the values of thermodynamic parameters and the activation energy were strongly dependent on the temperature range because different kinds of REEs participated in the reaction in the form of hydrated ions and followed a randomly and complexly dissociative adsorption mechanism. According to the intraparticle diffusion model analysis, the adsorption of REEs on nZVI-M was dominated by chemisorption and the nano size of nZVI-M reduced the diffusion thickness and the resistance to intraparticle diffusion. Based on the characterization of adsorbent by XPS, the adsorption mechanisms of REEs on nZVI-M were ion exchange and surface complexation.

Effect of bicarbonate on aging and reactivity of nanoscale zerovalent iron (nZVI) toward uranium removal

Bicarbonate, ubiquitous in natural and waste waters is an important factor regulating the rate and efficiency of pollutant separation and transformation. For example, it can form complexes with U(VI) in the aqueous phase and at the solid-water interface. In this work, we investigated the effect of bicarbonate on the aging of nanoscale zero-valent (nZVI) in the context of U(VI) reduction and removal from wastewater. For fresh nZVI, over 99% aqueous uranium was separated in less than 10 min, of which 83% was reduced from U(VI) to U(IV). When nZVI was aged in water, its activity for U(VI) sequestration and reduction was significantly reduced. Batch experiments showed that for nZVI aged in the presence of 10 mM bicarbonate, only 20.3% uranium was reduced to U(IV) after 6 h reactions. Characterizations of the iron nanoparticles with spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) suggest that in fresh nZVI, uranium was concentrated at the nanoparticle center; whereas in nZVI aged in bicarbonate, uranium was largely deposited on the outer surface of the nanoparticles. Furthermore, aged nZVI without bicarbonate contained more lepidocrocite (γ-FeOOH) while aged nZVI in the presence of bicarbonate had more magnetite/maghemite (Fe₃O₄/γ-Fe₂O₃). This could be attributed to the formation of carbonate green rust and pH buffer effect of . Primary mechanisms for U(VI) removal with nZVI include reduction, sorption and/or precipitation. Results demonstrate that bicarbonate alter the aging products of nZVI, and reduces the separation efficiency and reduction capability for uranium removal.

The enhancement roles of layered double hydroxide on the reductive immobilization of selenate by nanoscale zero valent iron: Macroscopic and microscopic approaches

Herein, we utilized nanoscale zero-valent iron loaded on layered double hydroxide (NZVI/LDH) to immobilize Se(VI) and evaluated the enhancement role of LDH in the NZVI reaction system. The structural characterization indicated that LDH could stabilize and disperse NZVI as well as prevent NZVI from oxidation, thereby increasing iron reactivity. Batch experiments displayed that, compared with those by NZVI, both extent and rate of Se(VI) immobilized by NZVI/LDH significantly increased, owing to the prominent synergistic effect ascribing from adsorption and reduction. Kinetics studies under a series of conditions showed that Se(VI) reaction could be well described by pseudo first-order model. The performance of Se(VI) immobilization was inhibited to a considerable extent by most of co-existing ions, Nevertheless, the presence of Cu²⁺ improved performance of NZVI/LDH due to its role as a catalyst or medium of charge transfer during reduction. XANES revealed that LDH acted as a promoter for complete reduction of Se(VI) into Se(0)/Se(-II) over a wide pH range, whereas EXAFS suggested that LDH acted as a scavenger for insoluble products, making more reactive sites exposure to Se(VI) for reduction. These results suggested that NZVI/LDH as a promising candidate exhibited potential application in remediation of wastewaters containing Se(VI).

The application of illite supported nanoscale zero valent iron for the treatment of uranium contaminated groundwater

In this study, nanoscale zero valent iron I-NZVI was investigated as a remediation strategy for uranium contaminated groundwater from the former Cimarron Fuel Fabrication Site in Oklahoma, USA. The 1 L batch-treatment system was applied in the study. The result shows that 99.9% of uranium in groundwater was removed by I-NZVI within 2 h. Uranium concentration in the groundwater stayed around 27 μg/L, and there was no sign of uranium release into groundwater after seven days of reaction time. Meanwhile the release of iron was significantly decreased compared to NZVI which can reduce the treatment impact on the water environment. To study the influence of background pH of the treatment system on removal efficiency of uranium, the groundwater was adjusted from pH 2-10 before the addition of I-NZVI. The pH of the groundwater was from 2.1 to 10.7 after treatment. The removal efficiency of uranium achieved a maximum in neutral pH of groundwater. The desorption of uranium on the residual solid phase after treatment was investigated in order to discuss the stability of uranium on residual solids. After 2 h of leaching, 0.07% of the total uranium on residual solid phase was leached out in a HNO₃ leaching solution with a pH of 4.03. The concentration of uranium in the acid leachate was under 3.2 μg/L which is below the EPA's maximum contaminant level of 30 μg/L. Otherwise, the concentration of uranium was negligible in distilled water leaching solution (pH = 6.44) and NaOH leaching solution (pH = 8.52). A desorption study shows that an acceptable amount of uranium on the residuals can be released into water system under strong acid conditions in short terms. For long term disposal management of the residual solids, the leachate needs to be monitored and treated before discharge into a hazardous landfill or the water system. For the first time, I-NZVI was applied for the treatment of uranium contaminated groundwater. These results provide proof that I-NZVI has improved performance compared to NZVI and is a promising technology for the restoration of complex uranium contaminated water resources.

The application of iron-based technologies in uranium remediation: A review

Remediating uranium contamination is of worldwide interest because of the increasing release of uranium from mining and processing, nuclear power leaks, depleted uranium components in weapons production and disposal, and phosphate fertilizer in agriculture activities. Iron-based technologies are attractive because they are highly efficient, inexpensive, and readily available. This paper provides an overview of the current literature that addresses the application of iron-based technologies in the remediation of sites with elevated uranium levels. The application of iron-based materials, the current remediation technologies and mechanisms, and the effectiveness and environmental safety considerations of these approaches were discussed. Because uranium can be reduced and reoxidized in the environment, the review also proposes strategies for long-term in situ remediation of uranium. Unfortunately, iron-based materials (nanoscale zerovalent iron and iron oxides) can be toxic to microorganisms. As such, further studies exploring the links among the fates, ecological impacts, and other environmentally relevant factors are needed to better understand the constraints on using iron-based technologies for remediation.

Review of soluble uranium removal by nanoscale zero valent iron

Uranium (U) has been released to surface soil and groundwater through military and industrial activities. Soluble forms of U transferred to drinking water sources and food supplements can potentially threaten humans and the biosphere due to its chemical toxicity and radioactivity. The immobilization of aqueous U onto iron-based minerals is one of the most vital geochemical processes controlling the transport of U. As a consequence, much research has been focused on the use of iron-based materials for the treatment of U contaminated waters. One material currently being tested is nanoscale zero-valent iron (nZVI). However, understanding the removal mechanism of U onto nZVI is crucial to develop new technologies for contaminated water resources. This review article aims to provide information on the removal mechanism of U onto nZVI under different conditions (pH, U concentration, solution ion strength, humic acid, presence of O₂ and CO₂, microorganism effect) pertinent to environmental and engineered systems, and to provide risk or performance assessment results with the stability of nZVI products after removal of U in environmental restoration.

Structural Evolution of Nanoscale Zero-Valent Iron (nZVI) in Anoxic Co(2+) Solution: Interactional Performance and Mechanism

The structures of nanoscale zero-valent iron (nZVI) particles evolving during reactions, and the reactions are influenced by the evolved structures. To understand the removal process in detail, it is important to investigate the relationships between the reactions and structural evolution. Using high resolution-transmission electron microscopy (HR-TEM), typical evolved structures (sheet coprecipitation and cavity corrosion) of nZVI in anoxic Co(2+) solutions were revealed. The system pH (pH measured in mixture), which controls the stability of coprecipitation and the nZVI corrosion rate, were found to be the determining factors of structural evolutions. X-ray photoelectron spectroscopy (XPS) results indicated that the formation and dissolution of sheet structure impacts on the ratio of Fe(0) on the nZVI surface and the surface Co(2+) reduction. The cavity structure provides the possibility of Co migration from the surface to the bulk of nZVI, leading to continuous removal. Subacidity conditions could accelerate the evolution and improve the removal; the results of structurally controlled reactions further indicated that the removal was suspended by the sheet structure and enhanced by cavity structure. The results and discussion in this paper revealed the "structural influence" crucial for the full and dynamical understanding of nZVI reactions.

The removal of uranium onto carbon-supported nanoscale zero-valent iron particles

In the current work carbon-supported nanoscale zero-valent iron particles (CS nZVI), synthesised by the vacuum heat treatment of ferric citrate trihydrate absorbed onto carbon black, have been tested for the removal of uranium (U) from natural and synthetic waters. Two types of CS nZVI were tested, one vacuum annealed at 600 °C for 4 h and the other vacuum annealed at 700 °C for 4 h, with their U removal behaviour compared to nZVI synthesised via the reduction of ferrous iron using sodium borohydride. The batch systems were analysed over a 28-day reaction period during which the liquid and nanoparticulate solids were periodically analysed to determine chemical evolution of the solutions and particulates. Results demonstrate a well-defined difference between the two types of CS nZVI, with greater U removal exhibited by the nanomaterial synthesised at 700 °C. The mechanism has been attributed to the CS nZVI synthesised at 700 °C exhibiting (i) a greater proportion of surface oxide Fe²⁺ to Fe³⁺ (0.34 compared to 0.28); (ii) a greater conversion of ferric citrate trihydrate [2Fe(C₆H₅O₇)·H₂O] to Fe⁰; and (iii) a larger surface area (108.67 compared to 88.61 m² g^-1). Lower maximum U uptake was recorded for both types of CS nZVI in comparison with the borohydride-reduced nZVI. A lower decrease in solution Eh and DO was also recorded, indicating that less chemical reduction of U was achieved by the CS nZVI. Despite this, lower U desorption in the latter stages of the experiment (>7 days) was recorded for the CS nZVI synthesised at 700 °C, indicating that carbon black in the CS nZVI is likely to have contributed towards U sorption and retention. Overall, it can be stated that the borohydride-reduced nZVI were significantly more effective than CS nZVI for U removal over relatively short timescales (e.g. <48 h), however, they were more susceptible to U desorption over extended time periods.