Uranium adsorption and subsequent re-oxidation under aerobic conditions by Leifsonia sp. - Coated biochar as green trapping agent

The effect of bacteria on uranium sequestration stability by different forms of phosphorus

Immobilisation of uranium (U (VI)) by direct precipitation of uranyl phosphate (U-P) exhibits a great potential application in the remediation of U (VI)-contaminated environments. However, phosphorus, vital element of bacteria's decomposition, absorption and transformationmay affect the stability of U (VI) with ageing time. The main purpose of this work is to study the effect of bacteria on uranium sequestration mechanism and stability by different forms of phosphorus in a water sedimentary system. The results showed that phosphate effectively enhanced the removal of U (VI), with 99.84%. X-Ray Diffraction (XRD), Scanning Electron Microscopy and Energy Dispersive Spectrometer (SEM-EDS), and X-ray Photoelectron Spectroscopy (XPS) analyses imply that U (VI) and U (IV) co-exist on the surface of the samples. Combined with BCR results, it demonstrated that bacteria and phosphorus have a synergistic effect on the removal of U (VI), realising the immobilisation of U (VI) from a transferable phase to a stable phase. However, from a long-term perspective, the redissolution and release of uranium immobilisation of U (VI) by pure bacteria with ageing time are worthy of attention, especially in uranium mining environments rich in sensitive substances. This observation implies that the stability of the uranium may be impacted by the prevailing environmental conditions. The novel findings could provide theoretical evidence for U (VI) bio-immobilisation in U (VI)-contaminated environments.

Utilization of nickel-graphite electrode as an electron donor for high-efficient microbial removal of solved U(VI) mediated by Leifsonia sp

Enzymatically catalyzed reduction of metals by bacteria has potential application value to uranium-mine wastewater. However, its practical implementation has long been restricted by its intrinsic drawbacks such as low efficiency and long treatment-time. This study aims to explore the effect of electrodes on U (VI) removal efficiency by a purified indigenous bacteria isolated from a uranium mining waste pile in China. The effects of current intensity, pH, initial U (Ⅵ) concentration, initial dosage of bacteria and contact time on U (Ⅵ) removal efficiency were investigated via static experiments. The results show that U(VI) removal rate was stabilized above 90% and the contact time sharply shortened within 1 h when utilized nickel-graphite electrode as an electron donor. Over the treatment ranges investigated maximum removal of U (Ⅵ) was 96.04% when the direct current was 10 mA, pH was 5, initial U (Ⅵ) concentration was 10 mg/L, and dosage of Leifsonia sp. was 0.25 g/L. In addition, it is demonstrated that U (VI) adsorption by Leifsonia sp. is mainly chemisorption and/or reduction as the quasi-secondary kinetics is more suitable for fitting the process. FTIR results indicated that amino, amide, aldehyde and phosphate -containing groups played a role in the immobilization of U (VI) more or less. SEM and EDS measurements revealed that U appeared to be more obviously aggregated on the surface of cells. A plausible explanation for this, supported by XPS, is that U (VI) was partially reduced to U (IV) by direct current then precipitated on the cells surface. These observations reveal that Nickel-graphite electrode exhibited good electro-chemical properties and synergistic capacity with Leifsonia sp. which potentially provides a new avenue for uranium enhanced removal/immobilization by indigenous bacteria.

Phosphorylated hollow carbon-based material derived from ZIF-8 and its U(VI) adsorptive performance

Inspired by its large specific surface area, and tunable chemical and physical properties, a hollow carbon-based mater8ial derived from ZIF-8 with phosphate groups (HCM-PO4) was prepared for the elimination of U(VI). The structural and surface features of HCM and HCM-PO4 were thoroughly examined using techniques such as SEM, TEM, and XRD. The resulting carbon material, HCM-PO4, exhibits a higher BET surface area of 571.2 m2·g-1, featuring a hollow structure. The removal procedure of HCM-PO4 for U(VI) aligns with the quasi-secondary kinetic model. Furthermore, the theoretical sorption capacity of HCM-PO4 was found to be 482.30 mg·g-1 at 298.15 K. The results obtained from XPS, FT-IR, and EDS analysis of HCM-PO4 after adsorption revealed the coordination of the phosphate group for U(VI), contributing significantly to the adsorption process. In brief, the HCM-PO4 demonstrates excellent adsorptive ability, positioning it as a hopeful expectant to remove U(VI) from wastewater.

Insights into the long-term immobilization performances and mechanisms of CMC-Fe0/FeS with different sulfur sources for uranium under anoxic and oxic aging

Nanoscale zerovalent iron (Fe0)-based materials have been demonstrated to be a effective method for the U(VI) removal. However, limited research has been conducted on the long-term immobilization efficiency and mechanism of Fe0-based materials for U(VI), which are essential for achieving safe handling and disposal of U(VI) on a large scale. In this study, the prepared carboxymethyl cellulose (CMC) and sulfurization dual stabilized Fe0 (CMC-Fe0/FeS) exhibited excellent long-term immobilization performances for U(VI) under both anoxic and oxic conditions, with the immobilization efficiencies were respectively reached over 98.0 % and 94.8 % after 180 days of aging. Most importantly, different from the immobilization mechanisms of the fresh CMC-Fe0/FeS for U(VI) (the adsorption effect of -COOH and -OH groups, coordination effect with sulfur species, as well as reduction effect of Fe0), the re-mobilized U(VI) were finally re-immobilized by the formed FeOOH and Fe3O4 on the aged CMC-Fe0/FeS. Under anoxic conditions, more Fe3O4 was produced, which may be the main reason for the long-term immobilization U(VI). Under oxic conditions, the production of Fe3O4 and FeOOH were relatively high, which both played significant roles in re-immobilizing U(VI) through surface complexation, reduction and incorporation effects.

Microbial removal of uranyl from aqueous solution by Leifsonia sp. in the presence of different forms of iron oxides

Immobilization of uranyl by indigenous microorganisms has been proposed as an economic and clean in-situ approach for removal of uranium, but the potential mechanisms of the process and the stability of precipitated uranium in the presence of widespread Fe(III) (hydr)oxides remain elusive. The potential of iron to serve as a reductant and/or an oxidant of uranium indicates that bioemediation strategies which mainly rely on the reduction of highly soluble U(VI) to poorly soluble U(IV) minerals to retard uranium transport in groundwater may be enhanced or hindered under different environmental conditions. This study purposes to determine the effect of ubiquitous Fe(III) (hydr)oxides (two-line ferrihydrite, hematite and goethite) on the removal of U(VI) by Leifsonia sp. isolated from an acidic tailings pond in China. The removal mechanism was elucidated via SEM-EDS, XPS and Mössbauer. The results show that the removal of U(VI) was retarded by Fe(III) (hydr)oxides when the initial concentration of U(VI) was 10 mg/L, pH was 6, temperature was 25 °C. Particularly, the retardatory effect of hematite on U(VI) removal was blindingly obvious. Also, it is worth noting that the U(VI) in the precipitate slow-released in the Fe(III) (hydrodr) oxide treatment groups, accompanied by an increase in Fe(II) concentration. SEM-EDS results demonstrated that the ferrihydrite converted to goethite may be the reason for U(VI) release in the process of 15 days culture. Mössbauer spectra fitting results further imply that the metastable iron oxides were transformed into stable Fe3O4 state. XPS measurements results showed that uranium product is most likely a mixture of Iron-U(IV) and Iron-U(VI), which indicated that the hexavalent uranium was converted into tetravalent uranium. These observations imply that the stability of the uranium in groundwater may be impacted on the prevailing environmental conditions, especially the solid-phase Fe(III) (hydr)oxide in groundwater or sediment.

Techniques and mechanisms of bacteria immobilization on biochar for further environmental and agricultural applications

Bacteria immobilization on biochar is a promising approach to achieve high concentration and stability of microbial cells for several applications. The present review addressed the techniques utilized for bacteria immobilization on biochar, discussing the mechanisms involved in this process, as well as the further utilization in bioremediation and agriculture. This article presents three immobilization techniques, which vary according to their procedures and conditions, including cell growth, adsorption, and adaptation. The mechanisms for cell immobilization are primarily adsorption and biofilm formation on biochar. The favorable characteristics of biochar immobilization depend on the pyrolysis methods, raw materials, and properties of biochar, such as surface area, pore size, pH, zeta potential, hydrophobicity, functional groups, and nutrients. Scanning electron microscope (SEM) and colony forming unit (CFU) are the analyses commonly carried out to verify the efficiency of bacteria immobilization. The benefits of applying biochar-immobilized bacteria include soil decontamination and quality improvement, which can improve plant growth and crop yield. Therefore, this emerging technology represents a promising solution for environmental and agricultural purposes. However, it is important to evaluate the potential adverse impacts on native microbiota by introducing exogenous microorganisms.

Biomineralization of uranium by Desulfovibrio desulfuricans A3-21ZLL under various hydrochemical conditions

Uranium pollution in groundwater environment has become an important issue of global concern. In this study, a strain of Desulfovibrio desulfuricans was isolated from the tailings of acid heap leaching, and was shown to be able to remove uranium from water via biosorption, bio-reduction, passive biomineralization under uranium stress, and active metabolically dependent bioaccumulation. This research explored the effects of nutrients, pH, initial uranium and sulfate concentration on the functional groups, uranium valence, and crystal size and morphology of uranium immobilization products. Results showed that tetravalent and hexavalent phosphorus-containing uranium minerals was both formed. In sulfate-containing water where Desulfovibrio desulfuricans A3-21ZLL can grow, the sequestration of uranium by bio-reduction was significantly enhanced compared to that with no sulfate loading or no growth. Ungrown Desulfovibrio desulfuricans A3-21ZLL or dead ones released inorganic phosphate group in response to the stress of uranium, which associated with soluble uranyl ion to form insoluble uranium-containing precipitates. This study revealed the influence of hydrochemical conditions on the mineralogy characteristics and spatial distribution of microbial uranium immobilization products. This study is conducive to the long-term and stable bioremediation of groundwater in decommissioned uranium mining area.

First Isolation and Characterization of Bacteria from the Core's Cooling Pool of an Operating Nuclear Reactor

Microbial life can thrive in the most inhospitable places, such as nuclear facilities with high levels of ionizing radiation. Using direct meta-analyses, we have previously highlighted the presence of bacteria belonging to twenty-five different genera in the highly radioactive water of the cooling pool of an operating nuclear reactor core. In the present study, we further characterize this specific environment by isolating and identifying some of these microorganisms and assessing their radiotolerance and their ability to decontaminate uranium. This metal is one of the major radioactive contaminants of anthropogenic origin in the environment due to the nuclear and mining industries and agricultural practices. The microorganisms isolated when sampling was performed during the reactor operation consisted mainly of Actinobacteria and Firmicutes, whereas Proteobacteria were dominant when sampling was performed during the reactor shutdown. We investigated their tolerance to gamma radiation under different conditions. Most of the bacterial strains studied were able to survive 200 Gy irradiation. Some were even able to withstand 1 kGy, with four of them showing more than 10% survival at this dose. We also assessed their uranium uptake capacity. Seven strains were able to remove almost all the uranium from a 5 µM solution. Four strains displayed high efficiency in decontaminating a 50 µM uranium solution, demonstrating promising potential for use in bioremediation processes in environments contaminated by radionuclides.

Preparation of biochar@chitosan-polyethyleneimine for the efficient removal of uranium from water environment

A novel chitosan-based composite with rich active sites was synthesized by uniformly dispersing biochar into the cross-linked network structure formed by chitosan and polyethyleneimine. Due to the synergistic effect of biochar (minerals) and chitosan-polyethyleneimine interpenetrating network (amino and hydroxyl), the chitosan-based composite possessed an excellent adsorption performance for uranium(VI). It could rapidly (<60 min) achieve a high adsorption efficiency (96.7 %) for uranium(VI) from water and a high static saturated adsorption capacity (633.4 mg/g), which was far superior to other chitosan-based adsorbents. Moreover, the separation for uranium(VI) on the chitosan-based composite was suitable for a variety of actual water environments and the adsorption efficiencies all exceeded 70 % in different water bodies. The soluble uranium(VI) could be completely removed by the chitosan-based composite in the continuous adsorption process, which could meet the permissible limits of the World Health Organization. In sum, the novel chitosan-based composite could overcome the bottleneck of current chitosan-based adsorption materials and become a potential adsorbent for the remediation of actual uranium(VI) contaminated wastewater.

Enhanced mechanism of calcium towards uranium incorporation and stability in magnetite during electromineralization

Doping uranium into a room-temperature stable Fe₃O₄ lattice structure effectively reduces its migration. However, the synergistic or competitive effects of coexisting ions in an aqueous solution directly affect the uranium mineralization efficiency and the structural stability of uranium-bearing Fe₃O₄. The effects of calcium, carbonate, and phosphate on uranium electromineralization were investigated via batch experiments and theoretical calculations. Calcium incorporated into the Fe₃O₄ lattice increased the level and stability of doped uranium in Fe₃O₄. Uranium and calcium occupied the octahedral and tetrahedral sites of Fe₃O₄, respectively; the formation energy was only -10.23 eV due to strong hybridization effects between Fe1s, U4f, O2p, and Ca3d orbitals. Compared to the uranium-doped Fe₃O₄, uranium leaching ratios decreased by 19.2 % and 48.9 % under strongly acidic and alkaline conditions after 120 days. However, high concentrations of phosphate inhibited Fe₃O₄ crystallization. These results should provide new avenues for the development of multi-metal co-doping technologies and mineralization optimization to treat uranium-containing complex wastewater.

Efficient removal of uranium(VI) from aqueous solution by a novel phosphate-modified biochar supporting zero-valent iron composite

Uranium (U) is an important strategic resource as well as a heavy metal element with both chemical and radiotoxicity. At present, the rapid and efficient removal of uranium from wastewater remains a huge challenge for environmental protection and ecological security. In this paper, phosphate-modified biochar supporting nano zero-valent iron (PBC/nZVI) was triumphantly prepared and fully characterized. The introduction of polyphosphate can greatly increase the specific surface area of biochar pores, and then the zero-valent iron can be evenly distributed on the surface of material, thus leading to excellent removal performance of the PBC/nZVI for U(VI). The theoretical maximum U(VI) removal capacity of PBC/nZVI was up to 967.53 mg/g at pH 5. The results of adsorption kinetics, isotherm, and thermodynamics showed that the adsorption of uranium by PBC/nZVI was a monolayer physical adsorption and endothermic reaction. And the PBC/nZVI has favorable selectivity toward uranium against the interference of coexisting metal ions. Further mechanism studies show that the excellent uranium removal performance of PBC/nZVI is mainly attributed to the synergistic effect of physical adsorption and chemical reduction. This work proves that the PBC/nZVI has a wide application prospect in the field of uranium wastewater treatment.

Construction of an egg-like DTAB/SiO2 composite for the enhanced removal of uranium

For the past few years, the environmental safety problems of radioactive nuclides caused wide public concern. In this work, the dodecyl trimethyl ammonium bromide-modified silicon dioxide composite (DTAB/SiO₂) was synthesized for the elimination of uranium. The dodecyl trimethyl ammonium bromide can decorate the surface of the silicon dioxide and change its surface topography, which can offer more active sites and functional groups for the combination of U(VI). The removal capacity of U(VI) on DTAB/SiO₂ reached 78.1 mg/g, which was greater than that of the silicon dioxide nanopowder. In the adsorption process, the surface oxygen-containing functional groups formed surface complexation with uranium. The results may provide helpful content to eliminate U(VI) and expand the application of surfactant in radioactive nuclide cleanup.

Enhancement of Uranium Recycling from Tailings Caused by the Microwave Irradiation-Induced Composite Oxidation of the Fe-Mn Binary System

The extraction of uranium (U)-related minerals from raw ore sands via a leaching procedure would produce enormous amounts of tailings, not only causing radioactivity contamination to surroundings but also wasting the potential U utilization. Effective recycling of U from U tailings is propitious to the current issues in U mining industries. In this study, the influence of the composite oxidation of Fe(III) and Mn(VII) intensified by microwave (MW) irradiation on the acid leaching of U from tailings was comprehensively explored in sequential and coupling systems. The U leaching activities from the tailing specimens were explicitly enhanced by MW irradiation. The composite oxidation caused by Fe(III) and Mn(VII) further facilitated the leaching of U ions from the tailing under MW irradiation in two systems. Maximum leaching efficiencies of 84.61, 80.56, and 92.95% for U ions were achieved in the Fe(III)-, Mn(VII)-, and Fe(III)-Mn(VII)-participated coupling systems, respectively. The inappropriateness of the shrinking core model (SCM) demonstrated by the linear fittings and analysis of variance (ANOVA) for the two systems explained a reverse increase of solid cores in the later stage of leaching experiments. The internal migration of oxidant ions into the particle cores enhanced by MW accelerated the dissolution of Al, Fe, and Mn constituents under acidic conditions, which further strengthened U extraction from tailing specimens.

Bismuth impregnated biochar for efficient uranium removal from solution: Adsorption behavior and interfacial mechanism

In this work, Bi₂O₃ doped horse manure-derived biochar was obtained by carbonizing the H₂O₂-modified horse manure loaded with bismuth nitrate under nitrogen atmosphere at 500 °C. The results showed that there was a sharp response between the as-prepared bismuth impregnated biochar and uranium(VI) species in solution, which resulted in a short equilibrium time (<80 min), a fast adsorption rate (about 5.0 mg/(g·min)), a high removal efficiency (93.9%) and a large adsorption capacity (516.5 mg/g) (T = 298 K, pH = 4, C_i = 10 mg/L and m/V = 0.1 g/L). Besides, the removal behavior of the bismuth impregnated biochar for uranium(VI) did not depend on the interfering ions and ion strength, except Al³⁺, Ca²⁺, CO₃^2- and PO₄^3-. These results indicated that the modified biochar might possess the potential of remediating the actual uranium(VI)-containing wastewater. Moreover, the interaction mechanism between Bi₂O₃ doped biochar and uranium(VI) species was further explored. The results demonstrated that the enrichment of uranium(VI) on the surface of the as-prepared biochar was controlled by various factors, such as surface complexation, ion exchange, electrostatic attraction, precipitation and reduction, which facilitated the adsorption of uranium(VI) on the bismuth impregnated biochar.

Dual Effect of Acetic Acid Efficiently Enhances Sludge-Based Biochar to Recover Uranium From Aqueous Solution

Excess sludge (ES) treatment and that related to the uranium recovery from uranium-containing wastewater (UCW) are two hot topics in the field of environmental engineering. Sludge-based biochar (SBB) prepared from ES was used to recover uranium from UCW. Excellent effects were achieved when SBB was modified by acetic acid. Compared with SBB, acetic acid-modified SBB (ASBB) has shown three characteristics deserving interest: 1) high sorption efficiency, in which the sorption ratio of U(VI) was increased by as high as 35.0%; 2) fast sorption rate, as the equilibrium could be achieved within 5.0 min; 3) satisfied sorption/desorption behavior; as a matter of fact, the sorption rate of U(VI) could still be maintained at 93.0% during the test cycles. In addition, based on the test conditions and various characterization results, it emerged as a dual effect of acetic acid on the surface of SBB, i.e., to increase the porosity and add (−COOH) groups. It was revealed that U(VI) and −COO− combined in the surface aperture of ASBB via single-dentate coordination. Altogether, a new utilization mode for SBB is here proposed, as a means of efficient uranium sorption from UCW.

Application Progress of Deinococcus radiodurans in Biological Treatment of Radioactive Uranium-Containing Wastewater

Radioactive uranium wastewater contains a large amount of radionuclide uranium and other heavy metal ions. The radioactive uranium wastewater discharged into the environment will not only pollute the natural environment, but also threat human health. Therefore, the treatment of radioactive uranium wastewater is a current research focus for many researchers. The treatment in radioactive uranium wastewater mainly includes physical, chemical and biological methods. At present, the using of biological treatment to treat uranium in radioactive uranium wastewater has been gradually shown its superiority and advantages. Deinococcus radiodurans is a famous microorganism with the most radiation resistant to ionizing radiation in the world, and can also resist various other extreme pressures. D. radiodurans can be directly used for the adsorption of uranium in radioactive waste water, and it can also transform other functional genes into D. radiodurans to construct genetically engineered bacteria, and then applied to the treatment of radioactive uranium containing wastewater. Radionuclides uranium in radioactive uranium-containing wastewater treated by D. radiodurans involves a lot of mechanisms. This article reviews currently the application of D. radiodurans that directly or construct genetically engineered bacteria in the treatment of radioactive uranium wastewater and discusses the mechanism of D. radiodurans in bioremediation of uranium. The application of constructing an engineered bacteria of D. radiodurans with powerful functions in uranium-containing wastewater is prospected.

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.

X-Ray Photoelectron Spectroscopy on Microbial Cell Surfaces: A Forgotten Method for the Characterization of Microorganisms Encapsulated With Surface-Engineered Shells

Encapsulation of single microbial cells by surface-engineered shells has great potential for the protection of yeasts and bacteria against harsh environmental conditions, such as elevated temperatures, UV light, extreme pH values, and antimicrobials. Encapsulation with functionalized shells can also alter the surface characteristics of cells in a way that can make them more suitable to perform their function in complex environments, including bio-reactors, bio-fuel production, biosensors, and the human body. Surface-engineered shells bear as an advantage above genetically-engineered microorganisms that the protection and functionalization added are temporary and disappear upon microbial growth, ultimately breaking a shell. Therewith, the danger of creating a "super-bug," resistant to all known antimicrobial measures does not exist for surface-engineered shells. Encapsulating shells around single microorganisms are predominantly characterized by electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, particulate micro-electrophoresis, nitrogen adsorption-desorption isotherms, and X-ray diffraction. It is amazing that X-ray Photoelectron Spectroscopy (XPS) is forgotten as a method to characterize encapsulated yeasts and bacteria. XPS was introduced several decades ago to characterize the elemental composition of microbial cell surfaces. Microbial sample preparation requires freeze-drying which leaves microorganisms intact. Freeze-dried microorganisms form a powder that can be easily pressed in small cups, suitable for insertion in the high vacuum of an XPS machine and obtaining high resolution spectra. Typically, XPS measures carbon, nitrogen, oxygen and phosphorus as the most common elements in microbial cell surfaces. Models exist to transform these compositions into well-known, biochemical cell surface components, including proteins, polysaccharides, chitin, glucan, teichoic acid, peptidoglycan, and hydrocarbon like components. Moreover, elemental surface compositions of many different microbial strains and species in freeze-dried conditions, related with zeta potentials of microbial cells, measured in a hydrated state. Relationships between elemental surface compositions measured using XPS in vacuum with characteristics measured in a hydrated state have been taken as a validation of microbial cell surface XPS. Despite the merits of microbial cell surface XPS, XPS has seldom been applied to characterize the many different types of surface-engineered shells around yeasts and bacteria currently described in the literature. In this review, we aim to advocate the use of XPS as a forgotten method for microbial cell surface characterization, for use on surface-engineered shells encapsulating microorganisms.

Kinetic modelling of the uranium biosorption by Deinococcus radiodurans biofilm

Increasing number of reports on uranium contamination in groundwater bodies is a growing concern. Deinococcus radiodurans biofilm-based U(VI) bioremediation has great potential to provide solution. This study focuses on the kinetic modelling of uranium biosorption by D. radiodurans biofilm biomass and identification of the functional groups involved in the sequestration process. The effect of temperature, pH and amount of biofilm dry mass were studied using two uranyl ion concentrations (100 and 1000 mg/L). D. radiodurans dry biomass showed good affinity for uranyl ion adsorption. The kinetic experiments revealed that the biosorption process was spontaneous and exothermic in nature. The modelling of kinetic adsorption data revealed that U(VI) sorption by D. radiodurans biofilm biomass follows a pseudo-second-order reaction. Mechanism of U(VI) sorption was suggested to follow an intra-particle diffusion model, which includes covalent bonding between U(VI) and functional groups present on the surface of biofilm biomass, and diffusional barrier acts as a rate limiting step. External mass transfer was the rate-limiting step as evident from Boyd and Elovich plot. Chemical modifications in surface functional groups of biofilm biomass, confirmed the involvement of carboxyl, phosphate, and hydroxyl groups in uranium binding as a significant loss in U(VI) sorption capacity was recorded in these chemically modified biomasses. XRD data indicated the formation of metal deposits, predominantly as uranyl phosphates.

Uranium removal from aqueous solution using macauba endocarp-derived biochar: Effect of physical activation

The main aim of this study was to evaluate options for addressing two pressing challenges related to environmental quality and circular economy stemming from wastage or underutilization of abundant biomass residue resources and contamination of water by industrial effluents. In this study we focused on residues (endocarp) from Macaúba palm (Acrocomia aculeata) used for oil production, its conversion to activated biochar, and its potential use in uranium (U) removal from aqueous solutions. Batch adsorption experiments showed a much higher uranyl ions (U(VI)) removal efficiency of activated biochar compared to untreated biochar. As a result of activation, an increase in removal efficiency from 80.5% (untreated biochar) to 99.2% (after activation) was observed for a 5 mg L^-1 initial U(VI) concentration solution adjusted to pH 3 using a 10 g L^-1 adsorbent dosage. The BET surface area increased from 0.83 to 643 m² g^-1 with activation. Surface topography of the activated biochar showed a very characteristic morphology with high porosity. Activation significantly affected chemical surface of the biochar. FTIR analysis indicated that U(VI) was removed by physisorption from the aqueous solution. The adsorbed U(VI) was detected by micro X-ray fluorescence technique. Adsorption isotherms were employed to represent the results of the U adsorption onto the activated biochar. An estimation of the best fit was performed by calculating different deviation equations, also called error functions. The Redlich-Peterson isotherm model was the most appropriate for fitting the experimental data, suggesting heterogeneity of adsorption sites with different affinities for uranium setting up as a hybrid adsorption. These results demonstrated that physical activation significantly increases the adsorption capacity of macauba endocarp-derived biochar for uranium in aqueous solutions, and therefore open up a potential new application for this type of waste-derived biochar.

Natural and anthropogenic radionuclides in water and wastewater: Sources, treatments and recoveries

Water-energy nexus in the context of changing climate amplifies the importance of comprehending the transport, fate and recovery of radioisotope. While countries have been more interested for zero/low greenhouse gas emission technologies, energy production from nuclear power plant (NPP) can be a prominent solution. Moreover, radioisotopes are also used for other benefits such as in medical science, industrial activities and many more. These radionuclides are blended accidently or intentionally with water or wastewater because of inefficacious management of the nuclear waste; and therefore, it is an imperative task to manage nuclear waste so that the harmful consequences of the waste on environment, ecology and human health can be dispelled. Due to generation of significant amount of waste throughout its utilization, a noticeable number of physical, chemical and biological processes has been introduced as remediation processes although mechanisms of optimum removal process are still under investigation. Removal mechanisms and influencing factors for radionuclide removal are elucidated in this review so that, further, operation and process development can be promoted. Again, resource recovery, opportunities and challenges are also discussed for elevating the removal rates and minimizing the knowledge gaps existing in development and applications of novel decontamination processes.

Size-dependent biochar breaking under compaction: Implications on clogging and pathogen removal in biofilters

Breaking of biochar during compaction of amended soil in roadside biofilters or landfill cover can affect infiltration and pollutant removal capacity. It is unknown how the initial biochar size affects the biochar breaking, clogging potential, and contaminant removal capacity of the biochar-amended soil. We compacted a mixture of coarse sand and biochar with sizes smaller than, similar to, or larger than the sand in columns and applied stormwater contaminated with E. coli. Packing columns with biochar pre-coated with a dye and analyzing the dye concentration in the broken biochar particles eluted from the columns, we proved that biochar predominantly breaks under compaction by disintegration or splitting, not by abrasion. Increases in biochar size decrease the likelihood of biochar breaking. We attribute this result to the effective dissipation of compaction energy through a greater number of contact points between a large biochar particle and the adjacent particles. Most of the broken biochar particles are deposited in the pore spaces of the background geomedia, resulting in an exponential decrease in hydraulic conductivity of amended sand with an increase in suspended sediment loading. The clogging rate was higher in the columns with small biochar. The columns with small biochar also exhibited high E. coli removal capacity, partly because of an increase in bacterial straining at reduced pore size after compaction. These results are useful in selecting appropriate biochar size for its application in soils and roadside biofilters for stormwater treatment.

High-efficient microbial immobilization of solved U(VI) by the Stenotrophomonas strain Br8

The environmental impact of uranium released during nuclear power production and related mining activity is an issue of great concern. Innovative environmental-friendly water remediation strategies, like those based on U biomineralization through phosphatase activity, are desirable. Here, we report the great U biomineralization potential of Stenotrophomonas sp. Br8 CECT 9810 over a wide range of physicochemical and biological conditions. Br8 cells exhibited high phosphatase activity which mediated the release of orthophosphate in the presence of glycerol-2-phosphate around pH 6.3. Mobile uranyl ions were bioprecipitated as needle-like fibrils at the cell surface and in the extracellular space, as observed by Scanning Transmission Electron Microscopy (STEM). Extended X-Ray Absorption Fine Structure (EXAFS) and X-Ray Diffraction (XRD) analyses showed the local structure of biogenic U precipitates to be similar to that of meta-autunite. In addition to the active U phosphate biomineralization process, the cells interact with this radionuclide through passive biosorption, removing up to 373 mg of U per g of bacterial dry biomass. The high U biomineralization capacity of the studied strain was also observed under different conditions of pH, temperature, etc. Results presented in this work will help to design efficient U bioremediation strategies for real polluted waters.

Mn-substituted goethite for uranium immobilization: A study of adsorption behavior and mechanisms

Goethite is a common iron hydroxide, which can be substituted by manganese (Mn) in the goethite structure. It is important to investigate the immobilization of uranium(VI) on Mn-substituted goethite (Mn-Goe) to understand the fate and migration of uranium in soils and sediments. In this study, the sorption of uranium(VI) by Mn-Goe was investigated as a function of pH, adsorbent dosage, contact time, and initial uranium concentration in batch experiments. Several material analysis techniques were used to characterize manganese substituted materials. Results indicated that Mn was successfully introduced into the goethite structure, the length of particles increased gradually, the surface clearly exhibited higher roughness with increasing Mn content, and that uranium(VI) sorption of synthetic Mn-Goe appeared to be higher than that of goethite. The sorption kinetics supported the results presented by the pseudo-second-order model. The sorption capacity of uranium on Mn-Goe was circa 77 mg g^-1 at pH = 4.0 and 25 °C. Fourier transform-infrared spectroscopy (FT-IR) analyses revealed that uranium ions were adsorbed through functional groups containing oxygen on the Mn-Goe structure. The enhancement of Mn-substitution for the uranium(VI) sorption capacity of goethite was revealed. This study suggests that goethite and Mn-Goe can both play a significant role in controlling the mobility and transport of uranium(VI) in the subsurface environment, which is helpful for material development in environmental remediation.

Effect of Leifsonia sp. on retardation of uranium in natural soil and its potential mechanisms

Uranium mining and milling activities for many years resulted in release of uranium into the adjoining soil in varying degrees. Bioremediation approaches (i.e., immobilization via the action of bacteria) resulting in uranium bearing solid is supposed as an economic and clean in-situ approach for the treatment of uranium contaminated sites. This study purposes to determine the immobilization efficiency of uranium in soil by Leifsonia sp. The results demonstrated that cells have a good proliferation ability under the stress of uranium and play a role in retaining uranium in soil. Residual uranium in active Leifsonia-medium group (66%) was higher than that in the controls, which was 31% in the deionised water control, 46% in the Leifsonia group, and 47% in the medium group, respectively. This indicated that Leifsonia sp. facilitates the immobilization efficiency of uranium in soil by converting part of the reducible and oxidizable fraction of uranium into the residual fraction. X-ray photoelectron fitting results showed that tetravalent states uranium existed in the soil samples, which indicated that the hexavalent uranium was converted into tetravalent by cells. This is the first report of effect of Leifsonia sp. on uranium immobilization in soil. The findings implied that Leifsonia sp. could, to some extent, prevent the migration and diffusion of uranium in soil by changing the chemical states into less toxicity and less risky forms.

Low concentration of Fe(II) to enhance the precipitation of U(VI) under neutral oxygen-rich conditions

Immobilization of U(VI) by naturally ubiquitous ferrous ions (Fe(II)) has been considered as an efficient and ecofriendly method to retard the migration of aqueous U(VI) at many nuclear sites and surface environments. In this study, we conducted Fe-U coprecipitation experiments to investigate the mechanism and stability of uranium (U) precipitation induced by a small quantity of Fe(II) under oxygen-rich conditions. The experimental results suggest that the sedimentation rates of U(VI) by Fe(II) under neutral oxygen-rich conditions are more than 96%, which are about 36% higher than those without Fe(II) and 16% higher than those under oxygen-free conditions. The Fe-U coprecipitates were observed to remain stable under slightly acidic to neutral and oxygen-rich conditions. Fe(II) primarily settles down as low-crystalline iron oxide hydroxide. U(VI) mainly precipitates as three forms: 16-20% of U forms uranyl hydroxide and metaschoepite, which is absorbed on the surface of the solids; 52-56% of U is absorbed as discrete uranyl phases at the internal pores of iron oxide hydroxide; and 27-29% of U is probably incorporated into the FeO(OH) structure as U(V) and U(VI). The U(V) generated via one-electron reduction is somewhat resistant to the oxidation of O₂ and the acid dissolution. In addition, nearly 70% of U and only about 15% of Fe could be extracted in 24 h by a hydrochloric acid solution with the H⁺ concentration ([H⁺]) of 0.01 M, revealing that U(VI) immobilization by low concentration of Fe(II) combined with O₂ has potential applications in the separation and recycling of aqueous uranium.

Influence of Leifsonia sp. on U(VI) removal efficiency and the Fe-U precipitates by zero-valent iron

Zero-valent iron (ZVI) has been widely applied to the remediation of uranium (U)-contaminated water. Notably, indigenous bacteria may possess potential positive or unfavorable influence on the mechanism and stability of Fe-U precipitates. However, the focus of the researches in this field has mainly been on physical and/or chemical aspects. In this study, batch experiments were conducted to explore the effects of an indigenous bacterium (Leifsonia sp.) on Fe-U precipitates and the corresponding removal efficiency by ZVI under different environmental factors. The results showed that the removal rate and capacity of U(VI) was significantly inhibited and decreased by ZVI when the pH increased to near-neutral level (pH = 6~8). However, in the ZVI + Leifsonia sp. coexistence system, the U(VI) removal efficiency were maintained at high levels (over 90%) within the experimental scope (pH = 3~8). This revealed that Leifsonia sp. had a synergistic effect on U(VI) remove by ZVI. According to scanning electron microscope and energy dispersive X-ray detector (SEM-EDX) analysis, dense scaly uranium-phosphate precipitation was observed on ZVI + Leifsonia sp. surface. The X-photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis indicated that Leifsonia sp. facilitated the generation of U(VI)-phosphates precipitates. The X-ray diffraction (XRD) analyses further revealed that new substances, such as (Fe(II)Fe(III)₂(PO₄)₂(OH)₂), Fe(II)(UO₂)₂(PO₄)₂·8H₂O, Fe(II)Fe(III)₅(PO₄)₄(OH)₂·4H₂O, etc., were produced in the coexisting system of ZVI and Leifsonia sp. This study provides new insights on the feasibility and validity of site application of ZVI to U(VI)-contaminated subsurface water in situ. Graphical abstract.

Influence on Uranium(VI) migration in soil by iron and manganese salts of humic acid: Mechanism and behavior

Soil contains large amounts of humic acid (HA), iron ions and manganese ions, all of which affect U(VI) migration in the soil. HA interacts with iron and manganese ions to form HA salts (called HA-Fe and HA-Mn in this paper); however, the effects of HA-Fe and HA-Mn on the migration of U(VI) is not fully understood. In this study, HA-Fe and HA-Mn were compounded by HA interactions with ferric chloride hexahydrate and manganese chloride tetrahydrate, respectively. The influence of HA, HA-Fe and HA-Mn on U(VI) immobilization and migration was investigated by bath adsorption experiments and adsorption-desorption experiments using soil columns. The results showed that the presence of HA, HA-Fe and HA-Mn retarded the migration of U(VI) in soil. Supported by X-ray photoelectron spectroscopy (XPS) and BCR sequential extraction analyses, a plausible explanation for the retardation was that HA-Fe and HA-Mn could reduce hexavalent uranium to stable tetravalent uranium and increase the specific gravity of Fe/Mn oxide-bound uranium and organic/sulfide-bound uranium, which made it difficult for them to longitudinally migrate in soil. Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and surface area and pore size analyses indicated that the complex formed between the hydroxyl, amino and carboxyl groups of HA-Fe and U(VI) increased the crystallinity of HA-Fe. The reaction between U(VI) and the hydroxyl, amino, aldehyde, keto and chlorine-containing groups of HA-Mn had no effect on the crystallinity of HA-Mn. Notably, the column desorption experiment found that the U(VI) immobilized in the soil remigrated under the effect of rain leaching, and acid rain promoted uranium remigration better than neutral rain. The findings provide some guidance for the decommissioning disposal of uranium contaminated site and it's risk assessments.

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.