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In this work, Fe3+-doped and -NH2-grafted montmorillonite-based material was prepared and the adsorption ability for uranium(VI) was verified. The microstructure and pore size distribution of the montmorillonite-based material were investigated by N2 adsorption-desorption analyzer and scanning electron microscopy. The surface groups and composition were analyzed by Fourier transform infrared spectrometer, X-ray photoelectron spectrometer and X-ray diffractometer, which proved the successful doping of Fe3+ and grafting of -NH2. In the adsorption study, the adsorption reached equilibrium within 100 min with a maximum adsorption capacity of 661.2 mg/g at pH = 6 and a high adsorption efficiency of 99.4 % at low uranium(VI) concentration (pH = 6, m/V = 0.5 g/L). The mechanism study showed that the strong synergistic complexation of -OH and -NH2 for uranium(VI) played a decisive role in the adsorption process and the transport function of interlayer bound water could also enhance the adsorption probability of uranium(VI) species. These results were far superior to other reported similar materials, which proved that the Fe3+-doped and -NH2-grafted montmorillonite-based material possessed an extremely high application potential in adsorption, providing a new route for the modification of montmorillonite.

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Extraction uranium (VI) (U(VI)) from wastewater and seawater is highly important for environmental protection and life safety, but it remains a great challenge. In this work, the growth of the zeolitic imidazolate framework-8 (ZIF-8) nanoparticles on the tannic acid (TA)-3-aminopropyltriethoxysilane (APTES) modified PVDF (TAP) membrane was designed to obtain an excellent U(VI) adsorbent. The zeolite imidazolate framework composite membrane (TAPP-ZIF-60) was prepared through polyethyleneimine (PEI) bridging strategy and temperature regulation strategy in solvothermal method. The coordination bond between PEI and ZIF-8 and the covalent bond between PEI and TAP are essential in forming stable composite membrane. TAPP-ZIF with different properties was synthesized through a temperature regulation process and the TAPP-ZIF prepared at 60 °C has the uniform morphology and good performance. The adsorption capacity of TAPP-ZIF-60 is 153.68 mg/g (C0 = 95.01 mg/L and pH = 8.0) and water permeability is 5459 L m-2 h-1 bar-1. After ten adsorption-desorption cycles, it is proved that TAPP-ZIF-60 has good repeatability and stability. In addition, the TAPP-ZIF-60 composites membrane has a good inhibitory effect on Staphylococcus aureus and Escherichia coli. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) analysis reveal that the coordination between TAPP-ZIF-60 and uranyl ions is the primary factor contributing to the high adsorption capacity.

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Uranium extraction from seawater (UES) has important strategic significance for maintaining the sustainable development of nuclear energy. This article presents the preparation of a low-cost, efficient, and highly reusable biosorbent sodium alginate/polyethyleneimine (SA/PEI) through a simple one-step crosslinking process. The chemical crosslinking between PEI and SA provides biosorbent excellent mechanical strength and thermal stability. SA/PEI was characterized by using FTIR, XRD, TGA, EDS, XPS, SEM before and after adsorption of uranium. Thermodynamic research results show that the uranium adsorption of SA/PEI is a spontaneous, entropy increasing endothermic process. The adsorption fitted the pseudo-second-order kinetic model and Langmuir model with maximum adsorption capacity reach 353.09 mg g-1, illustrating that the adsorption mechanism is monolayer chemical adsorption. The interaction between SA/PEI and uranium is synergistic chelation by amino and carboxyl, which is consistent with the results calculated by DFT. After 14 days of adsorption in 100 L natural seawater, the adsorption capacity of SA/PEI was 3.58 mg g-1, with an average adsorption efficiency of 0.256 mg g-1 day-1, which is faster than most reported alginate adsorbents. The cost of using SA/PEI to UES is $168 per kilogram of uranium. These results indicate that SA/PEI hydrogel has great potential in practical seawater application.

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This study presents the successful synthesis of Magnesium-aluminum layered double hydroxide composite phosphate-modified hydrothermal biochar for efficient removal of U(VI) from aqueous solutions. A novel synthesis approach involving phosphate thermal polymerization-hydrothermal method was employed, deviating from conventional pyrolysis methods, to produce hydrothermal biochar. The combination of solvent thermal polymerization technique with hydrothermal process facilitated efficient loading of layered double hydroxide (LDH) components onto the biochar surface, ensuring simplicity, low energy consumption and enhanced modifiability. Bamboo waste was utilized as the precursor for biochar, highlighting its superior green and sustainable characteristics. Additionally, this study elucidated the interactions between phosphate-modified hydrothermal biochar and LDH components with U(VI). Physicochemical analysis demonstrated that the composite biochar possessed a high surface area and abundant oxygen-containing functional groups. XPS and FTIR analyses confirmed the efficient adsorption of U(VI), attributed to chelation interactions between phosphate groups, magnesium hydroxyl groups, hydroxyl groups and U(VI), as well as the co-precipitation of U(VI) with multi-hydroxyl aluminum cations captured by LDH. The composite biochar reached adsorption equilibrium with U(VI) within 80 min and exhibited excellent fitting to the pseudo-second-order kinetic model and Langmuir model. Under conditions of pH = 4 and 298 K, it displayed significantly high maximum adsorption capacity of approximately 388.81 mg g⁻1, surpassing untreated biochar by 17-fold. The adsorption process was found to be endothermic and spontaneous and even after five consecutive adsorption-desorption cycles, the removal efficiency of U(VI) remained stable at 75.46%. These findings underscore the promising application prospects of Magnesium-aluminum layered double hydroxide composite phosphate-modified hydrothermal biochar in efficiently separating U(VI) from uranium-containing wastewater, emphasizing its environmental and economic value.

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The efficient development of selective materials for uranium recovery from wastewater and seawater is crucial for the utilization of uranium resources and environmental protection. The potential of graphene oxide (GO) as an effective adsorbent for the removal of environmental contaminants has been extensively investigated. Further modification of the functional groups on the basal surface of GO can significantly enhance its adsorption performance. In this study, a novel poly(amidoxime-hydroxamic acid) functionalized graphene oxide (pAHA-GO) was synthesized via free radical polymerization followed by an oximation reaction, aiming to enhance its adsorption efficiency for U(VI). A variety of characterization techniques, including SEM, Raman spectroscopy, FT-IR, and XPS, were employed to demonstrate the successful decoration of amidoxime and hydroxamic acid functional groups onto GO. Meanwhile, the adsorption of U(VI) on pAHA-GO was studied as a function of contact time, adsorbent dosage, pH, ionic strength, initial U(VI) concentration, and interfering ions by batch-type experiments. The results indicated that the pAHA-GO exhibited excellent reuse capability, high stability, and anti-interference ability. Specially, the U(VI) adsorption reactions were consistent with pseudo-second-order and Langmuir isothermal adsorption models. The maximum U(VI) adsorption capacity was evaluated to be 178.7 mg/g at pH 3.6, displaying a higher U(VI) removal efficiency compared with other GO-based adsorbents in similar conditions. Regeneration of pAHA-GO did not significantly influence the adsorption towards U(VI) for up to four sequential cycles. In addition, pAHA-GO demonstrated good adsorption capacity stability when it was immersed in HNO3 solution at different concentrations (0.1-1.0 mol/L) for 72 h. pAHA-GO was also found to have anti-interference ability for U(VI) adsorption in seawater with high salt content at near-neutral pH condition. In simulated seawater, the adsorption efficiency was above 94% for U(VI) across various initial concentrations. The comprehensive characterization results demonstrated the involvement of oxygen- and nitrogen-containing functional groups in pAHA-GO in the adsorption process of U(VI). Overall, these findings demonstrate the feasibility of the pAHA-GO composite used for the capture of U(VI) from aqueous solutions.

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Clay minerals formations are potential geological barrier (host rocks) for the long-rerm storage of uranium tailing in deep geological repositories. However, there are still obstacles to the efficient retardation of uranium because of the competition between negatively charged regions at the clay minerals end face, surface and between layers, as well as low mineralization capacity. Herein, employing a simple method, we used sodium alginate (SA), an inexpensive natural polymer material, polyethylene (PE), and the natural clay minerals montmorillonite (Mt), nontronite (Nt), and beidellite (Bd) to prepare three hydrogel adsorbents, (denoted as Mt/PE-@SA, Nt/PE-@SA, and Bd/PE-@SA), respectively. The application of obtained hydrogel adsorbents further extends to uranium(VI) removal from aqueous. Due to the synergistic action of SA group and PE group, hydrogel adsorbents showed select adsorption and mineralization effect on uranium(VI), among which the maximum uranium(VI) adsorption capacity of Nt/PE-@SA was 133.3 mg·g-1 and Mt/PE-@SA exhibited strong selectivity for uranium(VI) in the presence of coexisting metal ions. Cyclic voltammetry studies indicated the mitigation and immobilization of uranium species onto adsorbents by both reduction and mineralization. Besides, the synergistic adsorption of SA and PE on clay minerals was hypothesized, and the idea was supported by structure optimizations results from Monte Carlo dynamics simulation (MCD). Three obtained hydrogel adsorbents structural model was constructed based on its physicochemical characterization, the low energy adsorption sites and adsorption energies are investigated using MCD simulation. The simulation results show that obtained hydrogel adsorbents have a strong interaction with uranium(VI), which ensures the high adsorption capacity of those materials. Most importantly, this work demonstrates a new strategy for preparing mineral-based hydrogel adsorbents with enough stability and provides a new perspective for uranium(VI) removal in complex environment.

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Developing highly efficient photocatalysts based on conjugated microporous polymers (CMPs) are often impeded by the intrinsically large exciton binding energy and sluggish charge transfer kinetics that result from their vulnerable driving force. Herein, a family of pyrene-based nitrogen-implanted CMPs were constructed, where the nitrogen gradient was regulated. Accordingly, the built-in electric field endowed by the nitrogen gradient dramatically accelerates the dissociation of exciton into free carriers, thereby enhancing charge separation efficiency. As a result, PyCMP-3N generated by polymerization of 1,3,6,8-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyrene and 2,4,6-tris(4-bromophenyl)-1,3,5-triazine featured an optimized built-in electric field and exhibited the highest photocatalytic removal efficiency of uranium (VI) (99.5 %). Our proposed strategy not only provides inspiration for constructing the built-in electric field by controlling nitrogen concentration gradients, but also offers an in-depth understanding the crucial role of built-in electric field in exciton dissociation and charge transfer, efficiently promoting CMPs photocatalysis.

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This study synthesized (3-aminopropyl)triethoxysilane-functionalized porous silica (AP@MPS) to adsorb aqueous uranium (U(VI)). To comprehensively analyze the surface properties of the AP@MPS materials, a combination of SEM, BET, XPS, NMR, and zeta potential tests were conducted. The adsorption experiments for U(VI) revealed the rapid and efficient adsorption capacity of AP@MPS, with the solution condition of a constant solution pH = 6.5, an initial U(VI) concentration of 600 mg × L-1, a maximum U(VI) capacity of AP@MPS reaching 381.44 mg-U per gram of adsorbent, and a removal rate = 63.6%. Among the four types of AP@MPS with different average pore sizes tested, the one with an average pore size of 2.7 nm exhibited the highest U(VI) capacity, particularly at a pH of 6.5. The adsorption data exhibited a strong fit with the Langmuir model, and the calculated adsorption energy aligned closely with the findings from the Potential of Mean Force (PMF) analysis. The outcomes obtained using the Surface Complex Formation Model (SCFM) highlight the dominance of the coulombic force ΔG0coul as the principal component of the adsorption energy (ΔG0ads). This work garnered insights into the adsorption mechanism by meticulously examining the ΔG0ads across a pH ranging from 4 to 8. In essence, this study's findings furnish crucial insights for the future design of analogous adsorbents, thereby advancing the realm of uranium(VI) removal methodologies.

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The complexation of uranyl hydroxides with orthosilicic acid was investigated by experimental and theoretical methods. Spectroluminescence titration was performed in a glovebox under argon atmosphere at pH 9.2, 10.5 and 11.5, with [U(VI)] = 10-6 and 5 × 10-6 mol kgw-1. The polymerization effects of silicic acid were minimized by ruling out samples with less than 90 % monomeric silicic acid present, identified via UV-Vis spectrometry using the molybdate blue method. Linear regression analysis based on time-resolved laser-induced fluorescence spectroscopy (TRLFS) results yielded the conditional stepwise formation constants of U(VI)-OH-Si(OH)4 complexes at 0.05 mol kgw-1 NaNO3. The main spectroscopic features - characteristic peak positions and decay-time - are reported for the first time for the UO2(OH)2SiO(OH)3- species observed at pH 9.2 and 10.5 and UO2(OH)2SiO2(OH)22- predominant at pH 11.5. Quantum chemical calculations successfully computed the theoretical luminescence spectrum of the complex UO2(OH)2SiO(OH)3- species, thus underpinning the proposed chemical model for weakly alkaline systems. The conditional stability constants were extrapolated to infinite dilution using the Davies equation, resulting in log10ß°(UO2(OH)2SiO(OH)3-) and log10ß°(UO2(OH)2SiO2(OH)22-). Implications for U(VI) speciation in the presence and absence of competing carbonate are discussed for silicate-rich environments expected in certain repository concepts for nuclear waste disposal.

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A novel and efficient mesoporous nano-absorbent for U(VI) removal was developed through an environment-friendly route by inducing the biomimetic mineralization of hydroxyapatite (HAP) on the bioinspired surface of polydopamine-graphene oxide (PDA-GO). PDA-GO/HAP exhibited the greatly rapid and efficient U(VI) removal within 2 min, and much higher U(VI) adsorption capacity of 433.07 mg·g-1 than that of GO and PDA-GO. The enhanced adsorption capacity was mainly attributed to the synergistic effect of O-H, -C=N-, and PO43- functional groups and the incorporation of uranyl ions by the formation of a new phase (chernikovite, H2(UO2)2(PO4)2·8H2O). The adsorption process of U(VI) fitted well with pseudo-second-order kinetic and Langmuir isotherm model. Moreover, PDA-GO/HAP showed a high U(VI) adsorption capacity in a broad range of pH values and owned good thermal stability. PDA-GO/HAP with various excellent properties made it a greatly promising adsorbent for extracting uranium. Our work developed a good strategy for constructing fast and efficient uranium-adsorptive biomimetic materials.

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The effect of oxygen on the reduction of uranyl and photocorrosion of CdS remains a pressing issue when CdS is used as a photocatalyst for the removal of uranyl in uranium-containing wastewater. In this study, composites (CdS/PCN) were prepared by designing N-deficient g-C3N4 composite with CdS for efficient photocatalytic reduction of uranyl under aerobic condition. Meanwhile, a series of characterizations of the CdS/PCN composites were carried out by XRD, FT-IR, XPS, EDS and UV-vis. Surprisingly, the CdS/PCN not only showed very high photocatalytic reduction activity for uranyl under aerobic condition, but also the photocorrosion of CdS by oxygen and h+ was inhibited. With a starting uranium (VI) concentration of 20 ppm, the uranium (VI) removal efficiency could reach 97.33% (dark: 30 min, light: 10 min). Interestingly, the removal efficiency was better in air condition than in pure nitrogen or 30% oxygen atmosphere, i.e. a proper amount of oxygen has accelerated the reduction reaction, while excess oxygen weakened the reduction. Finally, a new mechanism of reduction of uranyl by CdS/PCN photocatalyst was given under aerobic condit ions. This work presents a novel strategy for reduction of U(VI) by photocatalysis and the inhibition of photocorrosion of photocatalysts under aerobic conditions.

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As a novel adsorbent for the separation of uranium(VI) from wastewater, Magnetite/ß-cyclodextrin/fly ash composite (Fe3O4/ß-CD/FA) was first prepared via a chemical coprecipitation technology. The characterization results indicated that Fe3O4 and ß-CD had been successfully loaded on FA, which had brought abundant oxygen-containing functional groups, providing numerous adsorptive sites for the removal of uranium(VI). At pH = 5.0 and T = 25 °C, the maximum uranium(VI) removal efficiency and capacity of Fe3O4/ß-CD/FA were higher to 97.8% and 444.4 mg g-1, respectively. Pseudo-second-order and Langmuir models fitted better with the experimental data, illustrating that chemical adsorption dominated the uranium(VI) removal process. In addition, Fe3O4/ß-CD/FA showed good anti-interference ability and recoverability. After five cycles, the removal rate of uranium(VI) on Fe3O4/ß-CD/FA was still higher to 90.4%. The immobilization of uranium(VI) on Fe3O4/ß-CD/FA was mainly ascribed to the synergism of redox reaction, complex reaction, chemical reaction and electrostatic interaction. Given the above, Fe3O4/ß-CD/FA would be regarded as an efficacious, green and promising adsorbent for uranium(VI) separation from wastewater.

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Significant selectivity, reasonable surface modification and increased structural porosity were three key factors to improve the competitiveness of biochar in the adsorption field. In this study, a hydrothermal bamboo-derived biochar modified with phosphate groups (HPBC) was synthesized using "one-can" strategy. BET showed that this method could effectively increase the specific surface area (137.32 m2 g-1) and simulation of wastewater experiments indicated HPBC had an excellent selectivity for U(VI) (70.35%), which was conducive to removal of U(VI) in real and complex environments. The accurate matchings of pseudo-second-order kinetic model, thermodynamic model and Langmuir isotherm showed that at 298 K, pH = 4.0, the adsorption process dominated by chemical complexation and monolayer adsorption was spontaneous, endothermic and disordered. Saturated adsorption capacity of HPBC could reach 781.02 mg g-1 within 2 h. The introduction of phosphoric acid and citric acid by "one-can" method not only provided abundant -PO4 to assist adsorption, but also activated oxygen-containing groups on the surface of the bamboo matrix. Results showed that adsorption mechanism of U(VI) by HPBC included electrostatic action and chemical complexation involving P-O, PO and ample oxygen-containing functional groups. Therefore, HPBC with high phosphorus content, outstanding adsorption performance, excellent regeneration, remarkable selectivity and green value provided a novel solution for the field of radioactive wastewater treatment.

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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.

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The co-presence of chromium (VI) [Cr(VI)] and uranium (VI) [U(VI)] is widely found in groundwater, imposing severe risks on human health. Although zerovalent iron [Fe(0)] supports superb performance for bioreduction of Cr(VI) and U(VI) individually, the biogeochemical process involving their concurrent removal with Fe(0) as electron donor remains unexplored. In the 6-d batch study, 86.1% ± 0.7% of Cr(VI) was preferentially eliminated, while 78.4% ± 0.5% of U(VI) removal was achieved simultaneously. Efficient removal of Cr(VI) (100%) and U(VI) (51.2% ∼ 100%) was also obtained in a continuous 160-d column experiment. As a result, Cr(VI) and U(VI) were reduced to less mobile Cr(III) and insoluble U(IV), respectively. 16 S rRNA sequencing was performed to investigate the dynamics of microbial community. Delftia, Acinetobacter, Pseudomonas and Desulfomicrobium were the major contributors mediating the bioreduction process. The initial Cr(VI) and hydraulic retention time (HRT) incurred pronounced effects on community diversity, which in turn altered the reactor's performance. The enrichment of Cr(VI) resistance (chrA), U(VI) reduction (dsrA) and Fe(II) oxidation (mtrA) genes were observed by reverse transcription qPCR. Cytochrome c, glutathione and NADH as well as VFAs and gas metabolites also involved in the bioprocess. This study demonstrated a promising approach for removing the combined contaminants of Cr(VI) and U(VI) in groundwater.

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Zerovalent iron [Fe(0)] can donate electron for bioprocess, but microbial uranium (VI) [U(VI)] reduction driven by Fe(0) is still poorly understood. In this study, Fe(0) supported U(VI) bio-reduction was steadily achieved in the 160-d continuous-flow biological column. The maximum removal efficiency and capacity of U(VI) were 100% and 46.4 ± 0.52 g/(m3·d) respectively, and the longevity of Fe(0) increased by 3.09 times. U(VI) was reduced to solid UO2, while Fe(0) was finally oxidized to Fe(III). Autotrophic Thiobacillus achieved U(VI) reduction coupled to Fe(0) oxidation, verified by pure culture. H2 produced from Fe(0) corrosion was consumed by autotrophic Clostridium for U(VI) reduction. The detected residual organic intermediates were biosynthesized with energy released from Fe(0) oxidation and utilized by heterotrophic Desulfomicrobium, Bacillus and Pseudomonas to reduce U(VI). Metagenomic analysis found the upregulated genes for U(VI) reduction (e.g., dsrA and dsrB) and Fe(II) oxidation (e.g., CYC1 and mtrA). These functional genes were also transcriptionally expressed. Cytochrome c and glutathione responsible for electron transfer also contributed to U(VI) reduction. This study reveals the independent and synergistic pathways for Fe(0)-dependent U(VI) bio-reduction, providing promising remediation strategy for U(VI)-polluted aquifers.

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Microbial U(VI) reduction influences uranium mobility in contaminated subsurface environments and can affect the disposal of high-level radioactive waste by transforming the water-soluble U(VI) to less mobile U(IV). The reduction of U(VI) by the sulfate-reducing bacterium Desulfosporosinus hippei DSM 8344T, a close phylogenetic relative to naturally occurring microorganism present in clay rock and bentonite, was investigated. D. hippei DSM 8344T showed a relatively fast removal of uranium from the supernatants in artificial Opalinus Clay pore water, but no removal in 30 mM bicarbonate solution. Combined speciation calculations and luminescence spectroscopic investigations showed the dependence of U(VI) reduction on the initial U(VI) species. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy showed uranium-containing aggregates on the cell surface and some membrane vesicles. By combining different spectroscopic techniques, including UV/Vis spectroscopy, as well as uranium M4-edge X-ray absorption near-edge structure recorded in high-energy-resolution fluorescence-detection mode and extended X-ray absorption fine structure analysis, the partial reduction of U(VI) could be verified, whereby the formed U(IV) product has an unknown structure. Furthermore, the U M4 HERFD-XANES showed the presence of U(V) during the process. These findings offer new insights into U(VI) reduction by sulfate-reducing bacteria and contribute to a comprehensive safety concept for a repository for high-level radioactive waste.

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In this study, we analyzed the removal efficiency of uranium(U(VI)) in wastewater at relatively low concentrations using strong alkaline ion exchange fiber (SAIEF). Static tests showed that the strong alkali fibers can purify U(VI) containing wastewater in a concentration range of 20-100 mg L-1 with an optimal pH of 10.5 and contact time of 15-30 min. Adsorption and desorption cycling tests indicated that, adsorbed uranium is easily desorbed by 0.1 mol L-1 HCl, and the fiber still maintained the original adsorption efficiency after eight cycles. According to dynamic penetration test results, the SAIEF saturation adsorption capacity was 423.9 mg g-1, and the effluent concentration of uranium through two series columns was less than 0.05 mg L-1, reaching the national standard for non-receiving water (GB23727-2009) SEM-EDS and FTIR analysis revealed that the functional group of SAIEF is CH2N+(CH3)3Cl-. Addotionally, the major forms of fiber exchange adsorption are (UO2)2CO3(OH)3-, UO2(CO)34- and UO2(OH)3-. The results indicate that the SAIEF is an excellent material for uranium removal.

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The removal and recovery of uranium (VI) from water solutions are critical for energy and environmental security. In this study, hydrochar at 100, 150, and 190 °C (HC100, HC150, and HC190) and pyrochar at 250 °C (BC250) were prepared from residual sludge (RS). The uranium (VI) adsorption behavior, recovery, and heavy metal risk of RS and its biochars were assessed. The sorption distribution coefficient of RS was higher than those of its biochars within the tested concentration range. The maximum adsorption capacity of uranium (VI) by HC190 was 121.26 mg/g at acidic pH (pH 4.5), which was higher than those of other tested biochars, previously reported unmodified biochars, and activated carbon. The zeta potential, FTIR, and XPS results implied that the adsorption of uranium (VI) by RS and its biochars was regulated by electrostatic attraction and the complexation with oxygen- and phosphorus-containing functional groups. Besides, partial reduction of uranium (VI) into uranium (IV) happened during the process of adsorption. More than 86% of the adsorbed uranium (VI) was recovered by 0.01 M hydrochloric acid and 100% by 0.01 M sodium carbonate. The leaching amount of heavy metals was greatly reduced after the sludge was converted to biochar, indicating that hydrothermal carbonization and pyrolysis can promote the stabilization of heavy metals. This work demonstrates that RS and its biochars can be implemented as low-cost, environment-friendly, and high-efficient materials for the purification of uranium (VI)-containing solutions by means of adsorption and desorption.

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The separation and enrichment of uranium from various resources such as seawater is of great significance to the sustainable development of nuclear energy. Therefore, there is an urgent need to develop an adsorbent with low cost and easy availability, simple preparation process, and environment friendly. In this work, we used inexpensive natural polymer material (sodium alginate) and natural adsorbents (ß-cyclodextrin and diatomite) to prepare a novel biosorbent ß-cyclodextrin/sodium alginate/diatomite (CSD) hydrogel beads for uranium adsorption by a simple method. The results of FTIR, XRD, SEM, EDX and XPS characterization proved that the CSD were prepared. Static adsorption experiments showed that pH, contact time, initial concentration of U(VI) and temperature has significant impact on the adsorption capacity of CSD. Adsorption fitted the pseudo-second-order kinetic model and Langmuir model and the adsorption mechanism is chemical adsorption. In the presence of other metal ions (Cu2+, Mg2+, Ca2+, Na+, K+), CSD has obvious selectivity for uranium(VI) adsorption. In addition, the elution and reusability studies show that CSD has excellent reusability. Overall, CSD is an inexpensive, green preparation, efficient, and environment friendly biosorbent with obvious selectivity for U(VI). It has potential application in U(VI) enrichment from seawater or salt lake water.

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