U(VI) sorption by silica: Effect of complexing anions

Analysis of the effect of citrate on radionuclide retention on portlandite

We analysed how citrate (CIT), a chelating agent potentially present in radioactive waste disposals, affects the mobility of four radionuclides (RN): 63Ni, 233U, 152Eu, 238Pu in portlandite, an important hydrated phase of cement, a commonly used material for waste isolation. Portlandite was synthetized in the laboratory and showed high purity and grain size of few μm. This solid, buffers the pH to 12.5 and shows high adsorption capability for the studied RNs: 152Eu and 238Pu exhibited the highest adsorption (Kd ∼1·105 mL g-1) and 233U the lowest (Kd ∼8·102 mL g-1). CIT adsorption was also experimentally evaluated by batch sorption experiments and electrophoretic (ζ-potential) measurements: a non-lineal sorption behaviour was observed, with Kd values decreasing (from ∼1·103 mL g-1) as CIT concentration increased up to 1·10-2 M, according to portlandite sorption sites saturation. In the presence of CIT, a marginal decrease for 233U adsorption in portlandite was observed, one order of magnitude reduction for 63Ni, while 238Pu and 152Eu adsorption decreased significantly. The calculated sorption reduction factors (SRF) for the four RN in the presence of CIT at a concentration of 5·10-3 M were: 2.4, 9.7, 37 and 50.9 for 233U, 63Ni, 238Pu, and 152Eu, respectively. According to the available thermodynamic databases, low complexation between CIT and RN is predicted at pH = 12.5, thus the RN adsorption decrease in the presence of CIT must be attributed to the organic adsorption on portlandite. However, current thermodynamic are still incomplete for this ligand and this pH range and this limits a precise interpretation of the experimental data.

Innovative Green Alginate-Cellulose Composite for Light Lanthanides: Experimental Design and Comprehensive Studies on Kinetics, Equilibrium, Thermodynamics, and Reusability

Nowadays, there is a great interest in efficient adsorbent development due to the recent demand for lanthanides, which are widely used in high-tech technology. Alginates, owing to their natural occurrence, gel formation capability, and safety, could be promising feasible adsorbents for lanthanide removal. This study proposes the alginate-cellulose composite as an ecological, sustainable adsorbent for light lanthanide sorption. The structure, morphology, qualitative and quantitative compositions, average diameter, and pHpzc of the composite were discussed in great detail. Using the batch approach, sorption trials were performed to evaluate the metal sorption performance. The maximum lanthanide accumulation was attained at pH 5.0 and a dosage of 0.05 g. The uptake kinetics are successfully explained by the Ho and McKay model, whereas the equilibrium data is best represented by the Langmuir equation. The presence of Cl-, NO3 -, SO4 2-, Ni(II), and Co(II) did not have any impact on the adsorption capacity. In turn, the presence of Fe(III) ions led to a 15 % reduction in the adsorption. The lanthanide ions were eluted from the adsorbent following the treatment with 0.1 M HNO3. The adsorbent retained over 95 % of its initial adsorption capacity after 6 series of sorption/desorption studies.

Uranium accumulation in environmentally relevant microplastics and agricultural soil at acidic and circumneutral pH

The co-occurrence of microplastics (MPs) with potentially toxic metals in the environment stresses the need to address their physicochemical interactions and the potential ecological and human health implications. Here, we investigated the reaction of aqueous U with agricultural soil and high-density polyethylene (HDPE) through the integration of batch experiments, microscopy, and spectroscopy. The aqueous initial concentration of U (100 μM) decreased between 98.6 and 99.2 % at pH 5 and between 86.2 and 98.9 % at pH 7.5 following the first half hour of reaction with 10 g of soil. In similar experimental conditions but with added HDPE, aqueous U decreased between 98.6 and 99.7 % at pH 5 and between 76.1 and 95.2 % at pH 7.5, suggesting that HDPE modified the accumulation of U in soil as a function of pH. Uranium-bearing precipitates on the cracked surface of HDPE were identified by SEM/EDS after two weeks of agitation in water at both pH 5 and 7.5. Accumulation of U on the near-surface region of reacted HDPE was confirmed by XPS. Our findings suggest that the precipitation of U was facilitated by the weathering of the surface of HDPE. These results provide insights about surface-mediated reactions of aqueous metals with MPs, contributing relevant information about the mobility of metals and MPs at co-contaminated agricultural sites.

Adsorption of U(VI) on the natural soil around a very low-level waste repository

The behaviors of U(VI) in environmental media around radioactive waste disposal site are important for safety assessment of geological repositories. However, the estimation of environmental behaviors of U(VI) in natural media was insufficient. This work aimed to determine the adsorption of U(VI) on natural soil surrounding a candidate very low-level radioactive waste (VLLW) disposal site in southwest China. Results showed that the adsorption process of U(VI) on soils could be well supported by pseudo-second-order kinetic and Freundlich model. The adsorption of U(VI) was pH-dependent but temperature-independent. High ionic strength (NaCl) strongly affected the adsorption process at low pH (2.0-5.5). CO₃^2- remarkably inhibited the U(VI) adsorption, while the adsorption of U(VI) was promoted by PO₄^3- and SO₄^2-. Naturally occurred soil organic matters (SOMs) showed high affinity for U(VI), while the presence of additional humic acid (HA) strongly inhibited U(VI) adsorption. The occurrence of ferrous iron could result in the reduction of U(VI) at low pH values (pH < 4), leading to the promotion of immobilization of U(VI). These findings would provide some guidance for the safety assessments of the VLLW disposal as well as the remediation of contaminated soil.

Sorption-desorption coefficients of uranium in contaminated soils collected around Fukushima Daiichi Nuclear Power Station

Various radionuclides including fission products and heavy nuclides were released into the environment during the Fukushima Daiichi Nuclear Power Station (FDNPS) accident. The dissolution followed by migration of deposited radionuclides of Cs, Sr and U on soils could take place to the local environment. Therefore, it is necessary to determine sorption-desorption coefficients of U in soil-water system around the FDNPS from a migration viewpoint. The determination of sorption coefficient K_d(S) as well as desorption coefficient K_d(D) for U has been carried out in the present study using a laboratory batch method. Stable U was used for sorption from simulated ground water onto contaminated soil samples collected from Okuma Town, Fukushima. Different soil parameters were measured to understand their effects on sorption and desorption processes. The obtained K_d(S) and K_d(D) values of U were compared with values of K_d(S) and K_d(D) of Cs and Sr and K_d(S)-U in known Fukushima accident contaminated soils reported in the literature for better understanding. It was observed that K_d(S)-U varied from 160 to 5100 L/kg, whereas K_d(D)-U ranged from 200 to 11000 L/kg. K_d(D) was higher than K_d(S) for U in these soils implying irreversibility of the sorption process. Pearson's correlation of K_d(S) values suggested that U sorption is affected by various soil parameters. However, desorption is decided by the nature of U species formed in sorption process and soil parameters like pH, presence of carbonates, Ca ions, clay minerals etc. to some extent. The comparison between K_d(S) and K_d(D) values for Cs, Sr and U revealed that unsorbed Sr could migrate farther than unsorbed Cs or U under the present experimental conditions. Both sorption and desorption studies are of great importance to understand migration of metal ions from contaminated sites to local uncontaminated areas.

Simulating uranium sorption onto inorganic particles: The effect of redox potential

An analytical expression is proposed to simulate the effects of pH and redox potential (E) on the sorption of uranium onto model inorganic particles in aquatic environments instead of following an experimental approach providing a list of empirical sorption data. The expression provides a distribution coefficient (Kd) as function of pH, E and ligand concentration (complex formation) applying a surface complexation model on one type of surface sites (>SuOH). The formulation makes use of the complexation and hydrolysis constants for all species in solution and those sorbed at the surface, using correlations between hydrolysis constants and surface complexation constants, for the specific sorption sites. The model was applied for the sorption of uranium onto aluminol, iron hydroxide and silanol sites, mimicking respectively 'clean' clay or 'dirty' clay and 'clean' sand or 'dirty' sand ('dirty' referring to iron hydroxide contaminated), in absence or presence of carbonates in solution. The calculated distribution coefficients are very sensitive with the presence or absence of carbonates. The Kd values obtained by applying the model are compared with values reported in the literature for the sorption of uranium onto specific adsorbents. It is known that in surface water, U(VI) and its hydroxides are the primary stable species usually observed. However, reduction to U(IV) is possible and may be simulated during sorption or when the redox potential (E) decreases. Similar simulations are also applicable to study the sorption of other redox sensitive elements.

Measurement of uranium distribution coefficient and 235U/238U ratio in soils affected by Fukushima dai-ichi nuclear power plant accident

Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted radioactive contamination in soil due to deposition of mainly radiocesium as well as many long-lived radionuclides surrounding a large area around FDNPP. Depending upon environmental conditions, radionuclides in soil can be mobilized in aquatic systems. Therefore, the fate and transfer of these radionuclides in the soil water system is very important for radiation protection and dose assessment. In the present study, soil and water samples were collected from contaminated areas around FDNPP. Inductively coupled plasma mass spectrometry (ICP-MS) is used for total uranium concentration. Emphasis has been given on isotope ratio measurement of ²³⁵U/²³⁸U ratio using thermal ionization mass spectrometry (TIMS) that gives us the idea about its contamination during accident. For the migration behavior, its distribution coefficient (K_d) has been determined using laboratory batch method. Chemical characterization of soil with respect to different parameters has been carried out. The effect of these soil parameters on distribution coefficient of uranium has been studied in order to explain the radionuclide mobility in this particular area. The distribution coefficient values for uranium are found to vary from 30 to 36000 L/kg. A large variation in the distribution coefficient values shows the retention or mobility of uranium is highly dependent on soil characteristics in the particular area. This variation is explained with respect to soil pH, Fe, Mn, CaCO₃ and organic content. There is a very good correlation of uranium K_d obtained with Fe content. There is no enrichment of ²³⁵U has been noticed in the studied area.

Sorption of Eu(III) on Fe–montmorillonite relevant to geological disposal of HLW

Montmorillonite (Mt) is the major clay mineral of bentonite, which is the candidate buffer material in the engineered barrier system for geological disposal of high level waste (HLW). The alteration of Mt due to its interaction with carbon steel (overpack) can produce Fe–Mt. In order to understand the basic properties of Fe–Mt, the sorption studies using Eu(III) are reported here. For this, Fe(III)–Mt was prepared by conventional cation exchange method using FeCl3 with Na–Mt. The obtained Fe(III)–Mt was then reduced to Fe(II)–Mt using ascorbic acid. Both the samples were characterized based on their X-ray diffraction, Fourier transform infrared spectra, cation exchange capacity and specific surface area. The batch sorption studies of Eu(III) were conducted for both Fe(III)–Mt and Fe(II)–Mt as a function of pH (3–10), ionic strength (0.001 M–1 M) and Eu(III) concentration (10−8–10−3 M). The distribution coefficient (Kd) was found to be higher for Fe(III)–Mt compared to Fe(II)–Mt and Na–Mt. The sudden increase in sorption in the pH range 4.5–6 and remaining constant beyond it indicates ion exchange mechanism at pH<4.5, with surface complexation mechanism dominating the sorption at pH>4.5. This is further corroborated by ionic strength dependent sorption data which shows decrease in sorption capacity of Fe–Mt with increasing ionic strength at low pH, but remaining more or less unchanged at higher pH. Eu(III) adsorption isotherm on Fe–Mt increased linearly with [Eu(III)] reaching saturation at 10−5 M and 10−4 M for Fe(III)–Mt and Fe(II)–Mt, respectively. The amount of iron released from Fe–Mt and Fe(II)/Fetotal during sorption were estimated to understand the effect on Eu(III) sorption behaviour by release of interlayer iron in Fe–Mt.

Effect of carbonate on U(VI) sorption by nano-crystalline α-MnO2

U(VI) sorption on nano-crystalline α-MnO2was studied in NaClO4medium as a function of pH by batch sorption method in presence and absence of carbonate and subsequently employing surface complexation modeling (SCM) to predict species responsible for U(VI) sorption. The kinetic study of U(VI) sorption on nano-crystalline α-MnO2was carried out to fix the time of equilibration. In presence of carbonate, U(VI) sorption on nano-crystalline α-MnO2increases with pH of the suspension, leveling off in the pH range 5–8.5 thereafter decreasing at higher pH. However, in absence of carbonate, U(VI) sorption on nano-crystalline α-MnO2remains close to 100% at pH>5. The difference in sorption behavior of uranium in the presence and absence of carbonate can be explained in terms of uranium speciation in the two systems. The dissolution of nano-crystalline α-MnO2was studied in presence and absence of carbonate to ascertain its role in sorption. Surface complexation modeling was satisfactorily able to explain the sorption phenomena in all the systems. In addition, U(VI) sorption on nano-crystalline α-MnO2was compared with literature data on U(VI) sorption by δ-MnO2.

Uranium(VI) sorption complexes on silica in the presence of calcium and carbonate

Uranium sorption on minerals and related solids depends to a large degree on its aqueous speciation. The present work attempts to understand the U(VI) sorption behavior on silica under environmentally relevant conditions, i.e. at neutral to weakly alkaline pH and in the presence of dissolved calcium and carbonate. Under these conditions, Ca(UO₂)(CO₃)₃^2- and Ca₂(UO₂)(CO₃)₃(aq) complexes emerge as the dominant aqueous U(VI) species. The U(VI) sorption affinity was measured as a function of contact time, solution pH, and humic acid. The U(VI) sorption decreased with increase of pH and was not affected by the addition of 50 mg/L humic acid. On the other hand, nitric acid was more effective than EDTA and carbonate at desorbing U(VI). Generally, the U(VI) sorbed on silica at neutral pH was less readily desorbed than that sorbed at higher pH values. Therefore, the U(VI) complex favorably sorbed on silica at the neutral pH is more strongly bound to the silica surface than that sorbed at higher pH values. Time-resolved laser fluorescence spectroscopy confirmed the results of the batch sorption experiments and revealed the presence of two surface U(VI) complexes with fluorescence lifetimes 251 ± 8 μs and 807 ± 24 μs.

Adsorption of uranyl on hydroxylated α-SiO2(001): a first-principle study

The effects of pH, CO₂, aqua solution and anionic ligands on the adsorption of uranyl on α-SiO₂(001) were investigated.