Molten salts for efficient removal of radioactive contaminants from stainless steel surface: Mechanisms and applications.

Here, we have demonstrated an innovative decontamination strategy using molten salts as a solvent to clean stubborn uranium contaminants on stainless steel surfaces. The aim of this work was to investigate the evolutionary path of contaminants in molten salts to reveal the decontamination mechanism, thus providing a basis for the practical application of the method. Thermodynamic analysis revealed that alkali metal hydroxides, carbonates, chlorides and nitrates can react with uranium oxides (UO3 and U3O8) to form various uranates. Notably, the decontamination mechanism was elucidated by analyzing the chemical composition of the contaminants in the molten salts and the surface morphology of the specimens considering NaOH-Na2CO3-NaCl melt as the decontaminant. The decontamination process involved two stages: a rapid decontamination stage dominated by the thermal effect of molten salt, and a stable decontamination stage governed by the chemical reactions and diffusion of molten salt. Subsequently, a multiple decontamination strategy was implemented to achieve high decontamination rates and low residual radioactivity. Within the actual cleaning time of 30 min, the decontamination efficiency (DE) of UO3-contaminated specimens reached 97.8% and 93.0% for U3O8-contaminated specimens. Simultaneously, the radioactivity levels of all specimens were reduced to below the control level for reuse in the nuclear domain. Particularly, the actual radioactive waste from the nuclear industry reached a reusable level of radioactivity after decontamination. The NaOH-Na2CO3-NaCl melt outperforms conventional chemical solvents and may be one of the most rapid and efficient decontaminants for stubborn uranium contamination of metal surfaces, which provides insights in regard to handling nuclear waste.

Efficient capture of uranium by a hydroxyapatite-modified polyethyleneimine@carbon nanotube composite from radioactive nuclear waste.

The toxicity and radioactivity of uranium (U)-containing wastewater pose a serious threat to the environment of humans, animals, and plants. It is necessary to remove U from contaminated wastewater. With high adsorption capacity and fast adsorption rate, a composite CNT-P/HAP, which comprises carbon nanotubes (CNT) modified with polyethyleneimine (PEI), was functionalized further by hydroxyapatite (HAP) using the hydrothermal method. Adsorption experiments indicated that the optimal performance for CNT-P/HAP was 1330.64 mg g-1 of adsorption capacity and 40 min of adsorption equilibrium at a pH of 3. In addition, the adsorption capacity of CNT-P/HAP was over 2 times that of HAP at a pH of 7. The synergistic effect in both synthesis and adsorption gave CNT-P/HAP an excellent adsorption capacity for U. The XRD and FT-IR analysis indicated that the adsorption mechanism of CNT-P/HAP for U is decided by the pH of the solution. CNT-P/HAP could be used in multiple conditions to remediate U-containing wastewater.

The effect of air on oxidation decomposition of uranium-containing cationic exchange resins in Li2CO3-Na2CO3-K2CO3 molten-salt system.

With the development of nuclear energy, spent cationic exchange resins after purification of radioactive wastewater must be treated. Molten-salt oxidation (MSO) can minimize the disposal content of resins and capture SO2. In this work, the decomposition of uranium-containing resins in carbonate molten salt in N2 and air atmospheres was investigated. Compared to N2 atmosphere, the content of SO2 released from the decomposition of resins was relatively low at 386-454 °C in an air atmosphere. The SEM morphology indicated that the presence of air facilitated the decomposition of the resin cross-linked structure. The decomposition efficiency of resins in an air atmosphere was 82.6% at 800 °C. The XRD analysis revealed that uranium compounds had the reaction paths of UO3 → UO2.92 → U3O8 and UO3 → K2U2O7 → K2UO4 in the carbonate melt, and sulfur elements in resins were fixed in the form of K3Na(SO4)2. The XPS result illustrated that peroxide and superoxide ions accelerated the conversion of sulfone sulfur to thiophene sulfur and further oxidized to CO2 and SO2. Besides, the ion bond formed by uranyl ions on the sulfonic acid group was decomposed at high temperature. Finally, the decomposition of uranium-containing resins in the carbonate melt in an air atmosphere was explained. This study provided more theoretical guidance and technical support for the industrial treatment of uranium-containing resins.

Electrocatalytic and green system coupling strategy for simultaneous recovery and purification of uranium from uranium-containing wastewater.

The recycling of uranium in wastewater is not only beneficial to the protection of ecological safety but also has great significance for the sustainable development of nuclear energy. However, there is no satisfactory method to recover and reuse uranium efficiently up to now. Here, we have developed an efficient and economical strategy that can achieve uranium recovery and direct reuse in wastewater. The feasibility analysis verified that the strategy still had good separation and recovery ability in acidic, alkaline, and high-salinity environments. The purity of uranium recovered from the separated liquid phase after electrochemical purification was up to about 99.95%. Ultrasonication could greatly increase the efficiency of this strategy, and 99.00% of high-purity uranium could be recovered within 2 h. We further improved the overall recovery rate by recovering the residual solid-phase uranium, and the overall recovery of uranium was increased to 99.40%. Moreover, the concentration of impurity ions in the recovered solution met the World Health Organization guidelines. In summary, the development of this strategy is of great importance for the sustainable use of uranium resources and environmental protection.

Effect of copper ions on transformation of organic sulfur in cationic exchange resins in Li2CO3-Na2CO3-K2CO3 molten-salt system.

Cationic exchange resins (CERs) were applied for purification and clarifying process of radioactive wastewater in nuclear industry, which was a kind of sulfur-containing organic material. Molten-salt oxidation (MSO) method can be applied for the treatment of spent CERs and the absorption of acid gas (such as SO2). The experiments about the molten salt destruction of the original resin and Cu ions doped resin were conducted. The transformation of organic sulfur in Cu ions doped resin was investigated. Compared with the original resin, the content of tail gas (such as CH4, C2H4, H2S and SO2) released from the decomposition of Cu ions doped resin was relatively high at 323-657 °C. Sulfur elements in the form of sulfates and copper sulfides were fixed in spent salt through XRD analysis. The XPS result revealed that the portion of functional sulfonic acid groups (-SO3H) in Cu ions doped resin was converted into sulfonyl bridges (-SO2-) at 325 °C. With the enhancement of temperature, sulfonyl bridges (-SO2-) were further decomposed to sulfoxides sulfur (-SO-) and organic sulfide sulfur. The destruction of thiophenic sulfur to H2S and CH4 was prompted by copper ions in copper sulfide. Sulfoxide were oxidized to the sulfone sulfur in molten salt. Sulfones sulfur consumed by reduction of Cu ions at 720 °C was more than it produced by oxidation of sulfoxide through XPS analysis, and the relative proportion of sulfone sulfur was 16.51%.

Decontamination of uranium-containing radioactive waste from stainless steel surfaces using NaOH-based molten salts.

Herein, we report a new strategy for the rapid removal of uranium-containing contaminants from metal surfaces, and it relies on decontaminants made of NaOH-based molten salts. The addition of Na2CO3 and NaCl to NaOH exhibited superior decontamination performance, with a decontamination rate of 93.8% within 12 min, outdoing the performance of the single NaOH molten salt. The experimental results demonstrated that the synergistic effects between CO32- and Cl- promoted the corrosion efficiency of the molten salt on the substrate, which accelerated the decontamination rate. Additionally, benefiting from the optimization of the experimental conditions by the response surface method (RSM), the decontamination efficiency was improved to 94.9%. Notably, it also showed remarkable results in the decontamination of specimens containing different uranium oxides at low and high levels of radioactivity. This technology is promising for broadening the path in rapid decontamination of radioactive contaminants on metal surfaces.

Effects of oxygen content on gaseous and solid products during molten salt oxidation of cation exchange resins.

Molten salt oxidation (MSO) is an advanced method for waste resins treatment; nevertheless, the research about gas product variations of resins under different stoichiometric air feed coefficient (α) is rare. The optimal working condition of hazardous waste disposal is obtained through thermodynamic equilibrium calculation, and the method to improve the treatment efficiency is found to guide the optimization of the actual experiment. In this paper, Fact Sage was used to calculate the oxidation products of cation exchange resins (CERs) at different temperatures and α, focusing on the similarities and differences through the contents of CO, CH4, CO2, and SO2 during the oxidation of CERs, the MSO of CERs, and the theoretical calculation. The results indicated that the gas products of the calculation and reality of the oxidation process of CERs are quite different, while the CO contents of CERs during MSO are close to the calculated values. The main reason for this consequence is that in the oxidation process of CERs, the S in the sulfonic acid group will form thermally stable C-S with the styrene-divinylbenzene skeleton. Moreover, the introduction of carbonate can promote the destruction of C-S and absorb SO2 as sulfate, weakening the influence of C-S on the oxidation products of CERs. The gas chromatograph results indicated that the SO2 content is reduced from 0.66% in the process of CERs oxidation to 0.28% in MSO of CERs. When 1.25 times stoichiometric air feed coefficient is fed, the sulfate content in the carbonate is the highest at 900 °C, which is 23.4%.

Catalytic effect of cesium on the oxidation behavior of cation exchange resins in Li2CO3-Na2CO3-K2CO3 melt.

After the treatment of liquid radioactive waste, there is a certain amount of Cs in the waste resin, and these Cs-doped resins are prone to volatilize during the thermal treatment process and cause radionuclide leakage. The molten salt oxidation (MSO) can effectively prevent the volatilization of toxic metal, especially the volatilization of Cs. Under nitrogen and air conditions, it is found that the oxidation behavior between Cs-doped and clean cation exchange resins (CERs) is quite different. In the presence of oxygen and molten carbonate salt, Cs2CO3 is generated by the destruction of functional groups in Cs-doped CERs. The Cs2CO3 in Na2CO3-K2CO3-Li2CO3 reacts with oxygen to form Li2O2, which reduces the content of S in residue from 26.33 to 13.38% in air conditions at 400 °C and promotes the generation of sulfate in the molten carbonate salt. The elements Cs and S in the Cs doped CERs spontaneously form thermally stable Cs2SO4 in the molten carbonate salt.

Valence regulation investigation of key factors on the electrochemical immobilization uranyl from wastewater.

Electrochemical techniques are considered promising applications to immobilize uranium in alkaline wastewater in order to prevent its migration into groundwater and soil. In this work, the results of electrochemical and Atomic Force Microscope (AFM) demonstrate a successful immobilization of uranyl in the carbonate system by U(VI)-U(V), U(V)-U(IV) reduction, and U(V) disproportionation reactions. The results indicated that the electrochemical fixation rate in alkaline system could reach more than 99%. The valence state of uranium is the key factor affecting its migration in the working system. Where, the analysis of the immobilized samples by X-ray photoelectron spectroscopy (XPS) revealed that pHs, current density, and the presence of foreign cations significantly affect the valence state of uranium in the immobilized samples. Under same conditions, the reduction reactions of U(VI)-U(V) and U(V)-U(IV) occurred easily. Where, at pH higher than 3.4 or the current density in the range of 0.5-20 mA/cm2, high content of U(V) and U(IV) in the immobilized products was obtained. Other conditions favored the occurrence of the electrolytic water reaction, and the immobilized samples were dominated by U(VI). It was found that the temperature showed the greatest effect on the electrochemical immobilization rate. Where, the electrochemical immobilization rate increased by about 1.8 times when the ambient temperature increased from 293.15 to 328.15 K. This study provides a new idea for the immobilization of uranium in alkaline wastewater and demonstrates the feasibility of electrochemical immobilization of uranium in alkaline systems.

Molten salt/liquid metal extraction: Electrochemical behaviors and thermodynamics properties of La, Pr, U and separation factors of La/U and Pr/U couples in liquid gallium cathode.

The electrochemical behavior of lanthanides (La, Pr) and actinide (U) on inert W and liquid Ga electrodes in LiCl-KCl molten salt as well as their related thermodynamic properties were experimentally determined for further Lns/Ans separation. The results indicate that the reductions of La3+ and Pr3+ in LiCl-KCl melts are both one-step process with three electrons exchanged, and the reactions are quasi-reversible processes at low scan rate. Temperature dependencies of apparent standard redox potentials of La(Ga), Pr(Ga) and U(Ga) alloys were determined by open-circuit chronopotentiometry versus Cl-/Cl2 reference electrode. The activity and activity coefficients of lanthanum, praseodymium and uranium on the liquid Ga electrode in the temperature interval 723-813 K were calculated. The separation factors for La/U and Pr/U on the liquid Ga electrode in the molten salt were determined by logθU/La=-10.39+11440.69T±0.0125 and logθU/Pr=-5.84+7763.27T±0.07. The separation factors of La/U and Pr/U on the liquid Ga electrode indicate that lower temperature should be more effective for separating uranium.

Construction of reduced graphene oxide coupled with CoSe2-MoSe2 heterostructure for enhanced electrocatalytic hydrogen production.

It is important to develop novel energy to solve energy shortage and environmental problems. Hydrogen evolution reaction (HER) is envisaged as a viable technology that can be used to develop sustainable clean energy. Herein, we report a catalyst with CoSe2-MoSe2 heterostructure grown on reduced graphene oxide with an optimum Co/Mo proportion of 1:1 (CoSe2-MoSe2(1-1)/rGO). It exhibits good HER activities in both acidic and alkaline conditions. The CoSe2-MoSe2(1-1)/rGO shows an overpotential of 107 mV at 10 mA cm-2 with a Tafel slope of 56 mV dec-1 under acidic condition. Meanwhile, CoSe2-MoSe2(1-1)/rGO also presents an overpotential of 182 mV at 10 mA cm-2 and with a Tafel slope of 89 mV dec-1 under alkaline condition. These impressive performances of the catalyst are mainly due to the excellent electronic transmission capability of rGO and the abundant active sites of CoSe2-MoSe2 heterostructure as well as the optimized hydrogen adsorption energy of CoSe2-MoSe2 interface. The design of CoSe2-MoSe2(1-1)/rGO provides a meaningful guide for manufacturing electrode in energy storage and conversion.

Preparation and Corrosion Performance of PPy/Silane Film on AZ31 Magnesium Alloy via One-Step Cyclic Voltammetry.

PPy/silane composite film on a magnesium alloy surface was prepared by one-step cycle voltammetry. The mixed solution of methanol and water was used as the hydrolysis solvent of a γ-(2,3-glycidoxypropyl) trimethoxysilane coupling agent (KH-560). The surface morphology of the PPy/silane film, the electro-polymerization progress of KH-560 and PPy, the influence of the silane coupling agent and the corrosion behavior of the coated AZ31 Mg alloy were all investigated. The results indicated that the PPy/silane film on AZ31 Mg alloy via one-step cyclic voltammetry could provide better corrosion protection for an Mg alloy when the volume fraction of KH-560 in the hydrolysis solution was 15% and the time span of hydrolysis was 24 h with the 5.935 × 10-10 A cm-2 corrosion current density.

Iron molybdenum selenide supported on reduced graphene oxide as an efficient hydrogen electrocatalyst in acidic and alkaline media.

It is of great significance to develop inexpensive and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER). In this work, we synthesized iron molybdenum selenide (FeSe2-MoSe2) loaded on reduced graphene oxide (FeSe2-MoSe2/rGO) by a one-step hydrothermal method. We further optimized the Fe/Mo ratio and determined the best ratio to be 1-1. In acidic (or alkaline) solution, the optimized FeSe2-MoSe2(1-1)/rGO has a small Tafel slope of 55 (or 80) mV dec-1 and needs an overpotential of 101 (or 178) mV to achieve 10 mA cm-2. These good properties are mainly due to the structure of bimetallic selenides combining rGO. Moreover, rGO enhances the electrical conductivity. Furthermore, the synergistic effect between FeSe2-MoSe2(1-1) and rGO results in better HER performance. Density functional theory (DFT) calculation proves that FeSe2-MoSe2(1-1)/rGO has a small work function. Based on our reasonable design and analysis, FeSe2-MoSe2(1-1)/rGO is expected to be an efficient and robust catalyst for large-scale applications.

Water-locking molecule-assisted fabrication of nature-inspired Mg(OH)2 for highly efficient and economical uranium capture.

With the depletion of uranium terrestrial deposits, researchers have focused on the development of adsorbents to extract radioactive uranium from seawater/wastewater. However, the artificial manipulation of adsorbents for the cost-effective extraction of radioactive uranium from large numbers of water samples is still significantly challenging. Herein, a facile yet versatile stepwise strategy has been reported for the fabrication of adsorbents. Magnesium hydroxide (Mg(OH)2) was fabricated via the in situ conversion of a natural ore powder (magnesite), whose unique internal pore structure is highly suitable for the development of highly efficient sorbents. The coordination interaction of the synthesized adsorbent with uranium was enhanced by further introducing inexpensive molecules with water-locking properties, which resulted in superior extraction capacity and low production cost. After careful calculation, the cost per kilogram of the adsorbent was found to be about $0.21. The adsorption behaviors of the synthesized adsorbent CMC-PAM/Mg(OH)2 were investigated by batch adsorption, flow-through column adsorption (in laboratory), and field adsorption experiments in natural seawater and river. Representatively, CMC-PAM/Mg(OH)2 was exceptional in extracting uranium not only at high concentrations with sufficient capacities in a wide pH range (1584.67 mg g-1 and 454.55 mg g-1 at pH = 5 and pH = 8, respectively), but also in trace quantities including uranium in a flow-through column (55.68 mg g-1), natural seawater (8.6 mg g-1), and river (6.7 mg g-1). Inspired by this excellent performance, the effects of competitive ions on the selective adsorption of uranium by CMC-PAM/Mg(OH)2 in simulated wastewater and seawater environments were further studied. Using a combination of FTIR spectroscopic and XPS studies, it was revealed that the amine and hydroxyl groups enhanced the overall uranyl affinity of the CMC-PAM/Mg(OH)2 composite.