Adsorption of U(VI) on montmorillonite in the presence of ethylenediaminetetraacetic acid

U(VI) sorption onto rutile surface in the presence or absence of EDTA: A combined macroscopic and spectroscopic study

Knowledge of the sorption and speciation of uranium at mineral/water interface is essential to predict its long-term behavior. The sorption of uranium on minerals is well known affected by its interaction with EDTA. Here, the sorption of U(VI) on rutile in the presence or absence of EDTA was investigated by combining batch sorption, species calculation, time-resolved fluorescence spectroscopy (TRFS) and X-ray photoelectron spectroscopy (XPS). The batch sorption results suggest that the sorption of U(VI) on rutile is governed by outer-sphere surface complexation under acidic conditions, and undergoes a transformation to inner-sphere surface complexation as the pH increases. In the presence of EDTA, the sorption process is dominated by inner-sphere interactions across the entire pH range. EDTA has been observed to reduce the sorption of U(VI) on minerals, thereby enhancing the mobility of U(VI). Greater inhibition of U(VI) sorption was observed with increasing concentration of EDTA. The TRFS and XPS analysis reveal that the uranium presents as entirely distinct surface complexes on rutile in the presence or absence of EDTA. These findings are essential for understanding the sorption mechanism of U(VI) with EDTA at a molecular scale and developing a reliable assessment for the long-term storage of radioactive waste.

Effects of EDTA and Bicarbonate on U(VI) Reduction by Reduced Nontronite

Widespread Fe-bearing clay minerals are potential materials capable of reducing and immobilizing U(VI). However, the kinetics of this process and the impact of environmental factors remain unclear. Herein, we investigated U(VI) reduction by chemically reduced nontronite (rNAu-2) in the presence of EDTA and bicarbonate. U(VI) was completely reduced within 192 h by rNAu-2 alone, and higher Fe(II) in rNAu-2 resulted in a higher U(VI) reduction rate. However, the presence of EDTA and NaHCO3 initially inhibited U(VI) reduction by forming stable U(VI)-EDTA/carbonato complexes and thus preventing U(VI) from adsorbing onto the rNAu-2 surface. However, over time, EDTA facilitated the dissolution of rNAu-2, releasing Fe(II) into solution. Released Fe(II) competed with U(VI) to form Fe(II)-EDTA complexes, thus freeing U(VI) from negatively charged U(VI)-EDTA complexes to form positively charged U(VI)-OH complexes, which ultimately promoted U(VI) adsorption and triggered its reduction. In the NaHCO3 system, U(VI) complexed with carbonate to form U(VI)-carbonato complexes, which partially inhibited adsorption to the rNAu-2 surface and subsequent reduction. The reduced U(IV) largely formed uraninite nanoparticles, with a fraction present in the rNAu-2 interlayer. Our results demonstrate the important impacts of clay minerals, organic matter, and bicarbonate on U(VI) reduction, providing crucial insights into the uranium biogeochemistry in the subsurface environment and remediation strategies for uranium-contaminated environments.

Construction of montmorillonite-based materials for highly efficient uranium removal: adsorption behaviors and mechanism

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.

Adsorption of uranium(VI) by natural vermiculite: Isotherms, kinetic, thermodynamic and mechanism studies

Human activities such as mining uranium resources, hydrometallurgy, and nuclear fuel preparation inevitably produce wastewater sludge containing radionuclides, posing a severe threat to the environment around the production site. Natural clay minerals have been widely used in groundwater pollution remediation because of their high cation exchange capacity. Through static batch experiments, the optimal pH range of vermiculite for U(VI) adsorption was 6-8,the maximum adsorption capacity was 1.62 × 10-5 mol g-1. The kinetic adsorption results indicated that the adsorption mode was mainly multilayer non-homogeneous chemisorption. In addition, the adsorption of vermiculite on U(VI) was found to be a heat absorption process according to the thermodynamic model fitting, and the spontaneous reactivity of U(VI) adsorption on vermiculite surface was positively correlated with temperature and negatively correlated with the initial concentration of U(VI). Combined with SEM-EDS and FT-IR results, the adsorption process of vermiculite on U(VI) is mainly an ion exchange and complexation reaction, and U(VI) is removed in the form of ≡ SUOU22+ or ≡ SOUO2OH, etc., by XPS means. The results of this study not only investigated the adsorption behavior and mechanism of natural vermiculite in groundwater contaminated with simulated uranium but also provided theoretical support for its feasibility in remediating uranium-polluted groundwater.

Construction of dopamine supported Mg(Ca)Al layered double hydroxides with enhanced adsorption properties for uranium

A novel dopamine-supported Mg(Ca)Al layered double hydroxide composite was synthesized by co-precipitation method. The existence of Ca²⁺ and dopamine could promote the capture of uranium on the layered double hydroxides. In batch experiments, the composite exhibited good uranium removal performance, including high adsorption capacity (687.3 mg/g), strong anti-interference and good reusability (the removal percentage was still higher than 90 % after five cycles). At low initial uranium concentration, the uranium removal percentage on the composite exceeded 99.7 % and the residual concentration of uranium in the solution was <0.03 mg/L, reaching the limited standard of the World Health Organization. The studies of adsorption kinetics and isotherm indicated that the uranium adsorption behavior on the composite conformed to the pseudo-second-order kinetic and Langmuir isotherm models, suggesting that the process was a monolayer adsorption dominated by chemical adsorption. Furthermore, the high-efficiency uranium adsorption on the Mg(Ca)Al layered double hydroxide was mainly attributed to the strong complexation between the active sites (-OH and -NH₂) and uranium, the precipitation of interlayer intercalation ions (CO₃^2- and OH^-) to uranium and the ion exchange of Ca²⁺ to uranium. Due to these advantages, the dopamine-supported Mg(Ca)Al layered double hydroxide composite is expected to be used as fine adsorbent to remove uranium from wastewater.

Building the bridge between U(VI) and Ca-bentonite - Influence of concentration, ionic strength, pH, clay composition and competing ions

In the context of a high-level nuclear waste disposal, the retention of U(VI) on non-pre-treated Ca-bentonite as potential technical barrier is studied. The objective of this study is to reveal the retention behaviour of U(VI) under extreme geochemical conditions, such as hyperalkaline pH range as well as high salinity at the same time, and taking into account other relevant parameters. This should lead to a better understanding of necessary safety precautions for avoiding a release of U(VI) in the environment. Batch experiments were conducted to determine the influence of the initial U(VI) concentration, salinity, pH value, clay composition and the presence of other elements (Ca(II), I^-, Cs(I), Eu(III)). After the sorption experiments, the remaining U(VI) concentration in solution was determined via mass spectrometry with inductively coupled plasma. U(VI) can be immobilised from 10% to 100% under all investigated conditions. Precipitation plays a role in the U(VI) retention but only at higher concentrations (≥10^-5 mol L^-1). The retention is reversible especially with decreasing pH (<10.5) as the aquo complex Ca₂UO₂(CO₃)₃(aq) is formed. Ca(II) strongly enhances the U(VI) adsorption onto Ca-bentonite in the hyperalkaline pH range, probably due to the formation of Ca(II)-bridges. The best retention could be observed on natural bentonite compared to pure montmorillonite and altered bentonite. From a waste cocktail containing important elements of the repository inventory (Cs(I), Eu(III), U(VI) and iodide), only Eu(III) as homologous element to trivalent actinoids competes with U(VI) for binding sites, especially at low metal concentrations, but also facilitates the precipitation at higher concentrations.

Leaching characteristics of heavy metals in tailings and their simultaneous immobilization with triethylenetetramine functioned montmorillonite (TETA-Mt) against simulated acid rain

For further understanding leaching characteristics of heavy metals in tailings and better immobilization on heavy metals against acid rain, batch experiments were conducted. The leaching results of Cu(II), Zn(II), Cd(II) and Mn(II) can be well fit by second-order kinetics equation, and Pb(II) can be well fit by two-constant equation. The leaching intensity of heavy metals in tailings was ranged as: Mn(II)> Cu(II)> Cd(II)> Zn(II)> Pb(II). Triethylenetetramine functioned montmorillonite (TETA-Mt) was successfully synthesized and can obtain simultaneous immobilization effect compared with Mt and TETA, and immobilization rates on Cu(II), Cd(II), Mn(II) and Zn(II) can reach above 90%, the immobilization rate on Pb(II) can reach more than 75%. The mechanisms for efficient immobilization of heavy metals on TETA-Mt included buffering and adsorption abilities. The mechanism for TETA-Mt adsorption of heavy metals included physical absorption, chelation and chemical sedimentation. The results showed that TETA-Mt can be applied to effective immobilization of heavy metals in tailings and efficient remediation of acid mine drainage (AMD) in acid rain area.

Macroscopic and spectroscopic characterization of U(VI) sorption on biotite

Knowledge of the geochemical behavior of uranium is critical for the safe disposal of radioactive wastes. Biotite, a Fe(II)-rich phyllosilicate, is a common rock-forming mineral and a major component of granite or granodiorite. This work comprehensively studied the sorption of U(VI) on biotite surface with batch experiments and analyzed the uranium speciation with various spectroscopic techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and time-resolved fluorescence spectra (TRFS). Our results indicated that uranyl ions could penetrate into the interlayer of biotite, this ion-exchange process was pH-dependent and only favorable under acidic condition. Instead of precipitation or reduction to uraninite, the TRFS results strongly suggests U(VI) forms surface complexes under the neutral and alkaline condition, though the number and structure of surface species could not be identified accurately. Besides, the oxidation of biotite with peroxide hydrogen showed that structural Fe(II) would have a very low redox reactivity. With leaching experiments, zeta potential analysis and thermodynamics calculation, we discussed the possible reasons for inhibition of U(VI) reduction at the biotite-water interface. Our results may provide insight on interaction mechanism of uranium at mineral-water interface and help us understand the migration behavior of uranium in natural environments.