Complexation thermodynamics and the formation of the binary and the ternary complexes of tetravalent plutonium with carboxylate and aminocarboxylate ligands in aqueous solution of high ionic strength

Exploratory and machine learning analysis of the stability constants of HgII- triazene ligands complexes

Knowing stability constants for the complexes HgII with extracting ligands is very important from environmental and therapeutic standpoints. Since the selectivity of ligands can be stated by the stability constants of cation–ligand complexes, quantitative structure–property relationship (QSPR) investigations on binding constant of HgII complexes were done. Experimental data of the stability constants in ML2 complexation of HgII and synthesized triazene ligands were used to construct and develop QSPR models. Support vector machine (SVM) and multiple linear regression (MLR) have been employed to create the QSPR models. The final model showed squared correlation coefficient of 0.917 and the standard error of calibration (SEC) value of 0.141 log K units. The proposed model presented accurate prediction with the Leave-One-Out cross validation ( Q LOO 2  = 0.756) and validated using Y-randomization and external test set. Statistical results demonstrated that the proposed models had suitable goodness of fit, predictive ability, and robustness. The results revealed the importance of charge effects and topological properties of ligand in HgII - triazene complexation.

Impact of Ca(II) on the aqueous speciation, redox behavior, and environmental mobility of Pu(IV) in the presence of EDTA

The impact of calcium on the solubility and redox behavior of the Pu(IV)-EDTA system was investigated using a combination of undersaturation solubility studies and advanced spectroscopic techniques. Batch solubility experiments were conducted in 0.1 M NaCl-NaOH-HCl-EDTA-CaCl₂ solutions at constant [EDTA] = 1∙10^-3 M, 1 ≤ pH_m ≤ 11, and 1∙10^-3 M ≤ [CaCl₂] ≤ 2∙10^-2 M. Additional samples targeted brine systems represented by 3.5 M CaCl₂ and WIPP simulated brine. Redox conditions were buffered with hydroquinone (pe + pH ≈ 9.5) with selected samples prepared in the absence of any redox buffer. All experiments were performed at T = 22 °C under Ar atmosphere. In-situ X-ray absorption spectroscopy indicated that PuO₂(ncr,hyd) was the solubility-controlling phase during the lifetime of all experiments and that aqueous plutonium was present in the +IV oxidation state across all experimental conditions except at pH_m ≈ 1, where a small fraction of Pu(III) was also identified. Current thermodynamic models overestimate Pu(IV)-EDTA solubility in the absence of calcium by approximately 1-1.5 log₁₀-units and do not describe the nearly pH-independent, increased solubility observed with increased calcium concentrations. The ternary Pu(IV)-OH-EDTA system without calcium was reevaluated using solubility data obtained in this work and reported in the literature. An updated thermodynamic model including the complexes Pu(OH)(EDTA)^-, Pu(OH)₂(EDTA)^2-, and Pu(OH)₃(EDTA)^3- was derived. Solubility data collected in the presence of calcium follows a pH-independent trend (log m(Pu)_tot vs. pH_m), which can only be explained by assuming the formation of a quaternary complex, tentatively defined as CaPu(OH)₄(EDTA)^2-, in solution. The significant enhancement of plutonium solubility observed in the investigated brine systems supports the formation of a quaternary complex that is not outcompeted by Ca(EDTA)^2-, even in concentrated CaCl₂ solutions. Although the exact stoichiometry of the complex may need to be revisited, this new quaternary complex has a pronounced impact on plutonium predominance diagrams over a broad range of pH, pe, and calcium concentrations that are relevant to nuclear waste disposal.

Influence of ethylenediaminetetraacetic acid on the long-term oxidation state distribution of plutonium

Spectrophotometry was used to study the effect of EDTA on plutonium oxidation state distribution as a function of time, pH, and ligand-to-metal ratio (L/M) under anoxic conditions. Novel Pu(V)-EDTA absorption bands were identified at 571, 993, 1105, and 1150 nm with molar absorption coefficients of 15 ± 1, 6 ± 1, 10 ± 1, and 10 ± 1 cm^-1M^-1, respectively. Pu(V)-EDTA spectral changes occurred at L/M < 1, indicating only Pu^VO₂(EDTA)^3- formed with logK = 3.6 ± 0.3. Time-resolved experiments showed EDTA drastically increased the Pu(V/VI) reduction rate, which we propose is driven by amine lone-pair electron donation and the oxidative decarboxylation of EDTA. Oxidation of Pu(III)-EDTA to Pu(IV)-EDTA occurred on a slower time scale (110-237 days) than previously reported (<15 min) and is hypothesized to be radiolysis driven. Pu(V/VI)-EDTA and Pu(III)-EDTA both approached Pu(IV)-EDTA stabilization over time, yet Pu(V/VI)-EDTA solubility data was ≥ 1.0 log₁₀ units higher than predicted by Pu(IV)-EDTA solubility models, indicating that current thermodynamic models are incomplete. Ultimately, the data show EDTA preferentially stabilizes Pu(IV) over time regardless of initial oxidation state, but Pu(V)-EDTA can persist under environmentally-relevant conditions, emphasizing the need to continue investigating redox reactions, speciation, and behavior of these complexes to support the transuranic waste disposal and surface remediation/containment efforts.

How Does Iron Storage Protein Ferritin Interact with Plutonium (and Thorium)?

The impact of the contamination of living organisms by actinide elements has been a constant subject of attention since the 1950s. But to date still little is understood. Ferritin is the major storage and regulation protein of iron in many organisms, it consists of a protein ring and a ferrihydric core at the center. This work sheds light on the interactions of early actinides (Th, Pu) at oxidation state +IV with ferritin and its ability to store those elements at physiological pH compared to Fe. The ferritin-thorium load curve suggests that Th^IV saturates the protein (2840 Th atoms per ferritin) in a similar way that Fe does on the protein ring. Complementary spectroscopic techniques (spectrophotometry, infrared spectroscopy, and X-ray absorption spectroscopy) were combined with molecular dynamics to provide a structural model of the interaction of Th^IV and Pu^IV with ferritin. Comparison of spectroscopic data together with MD calculations suggests that Th^IV and Pu^IV are complexed mainly on the protein ring and not on the ferrihydric core. Indeed from XAS data, there is no evidence of Fe neighbors in the Th and Pu environments. On the other hand, carboxylates from amino acids of the protein ring and a possible additional carbonate anion are shaping the cation coordination spheres. This thorough description from a molecular view point of Th^IV and Pu^IV interaction with ferritin, an essential iron storage protein, is a cornerstone in comprehensive nuclear toxicology.

Enhanced performance for Eu(iii) ion remediation using magnetic multiwalled carbon nanotubes functionalized with carboxymethyl cellulose nanoparticles synthesized by plasma technology

Synthesis of sodium carboxymethyl cellulose/iron oxides/MWCNTs composites by a plasma technique and their application to the decontamination of europium ions from aqueous solutions under controlled laboratory conditions.

Complexation of protactinium(v) with nitrilotriacetic acid: a study at the tracer scale

Complexation of Pa(v) with nitrilotriacetic acid (NTA) in aqueous solution (1 M (Na,H)ClO₄) was studied by solvent extraction at different acidities (pcH = 0.6; 1.0; 2.0 and 2.5) with the element at the tracer scale (C_Pa< 10⁻¹⁰M).

Hafnium(IV) complexation with oxalate at variable temperatures

Appropriate management of fission products in the reprocessing of spent nuclear fuel (SNF) is crucial in developing advanced reprocessing schemes. The addition of aqueous phase complexing agents can prevent the co-extraction of these fission products. A solvent extraction technique was used to study the complexation of Hf(IV) – an analog to fission product Zr(IV) – with oxalate at 15, 25, and 35°C in 1 M HClO4 utilizing a 175+181Hf radiotracer. The mechanism of the solvent extraction system of 10−5 M Hf(IV) in 1 M HClO4 to thenoyltrifluoroacetone (TTA) in toluene demonstrated a 4th-power dependence in both TTA and H+, with Hf(TTA)4 the only extractable species. The equilibrium constant for the extraction of Hf(TTA)4 was determined to be log K ex=7.67±0.07 (25±1°C, 1 M HClO4). The addition of oxalate to the aqueous phase decreased the distribution ratio, indicating aqueous Hf(IV)-oxalate complex formation. Polynomial fits to the distribution data identified the formation of Hf(ox)2+ and Hf(ox)2 (aq) and their stability constants were measured at 15, 25, and 35°C in 1 M HClO4. van’t Hoff analysis was used to calculate Δr G, Δr H, and Δr S for these species. Stability constants were observed to increase at higher temperature, an indication that Hf(IV)-oxalate complexation is endothermic and driven by entropy.

A proteomic approach to identification of plutonium-binding proteins in mammalian cells

Plutonium can enter the body through different routes and remains there for decades; however its specific biochemical interactions are poorly defined. We, for the first time, have studied plutonium-binding proteins using a metalloproteomic approach with rat PC12 cells. A combination of immobilized metal ion chromatography, 2D gel electrophoresis, and mass spectrometry was employed to analyze potential plutonium-binding proteins. Our results show that several proteins from PC12 cells show affinity towards Pu(4+)-NTA (plutonium bound to nitrilotriacetic acid). Proteins from seven different spots in the 2D gel were identified. In contrast to the previously known plutonium-binding proteins transferrin and ferritin, which bind ferric ions, most identified proteins in our experiment are known to bind calcium, magnesium, or divalent transition metal ions. The identified plutonium interacting proteins also have functional roles in downregulation of apoptosis and other pro-proliferative processes. MetaCore™ analysis based on this group of proteins produced a pathway with a statistically significant association with development of neoplastic diseases.

Redox behavior of Ce(IV)/Ce(III) in the presence of nitrilotriacetic acid: A surrogate study for An(IV)/An(III) redox behavior

Using cyclic voltammetry, we investigated the redox behavior of Ce(IV)/Ce(III), which is a surrogate for An(IV)/An(III) (An=actinides), in a solution of nitrilotriacetic acid (NTA) at 25 °C. The cyclic voltammogram of Ce in a 0.1 M NTA solution at pH 6 showed a reversible one-electron redox reaction for Ce(IV)/Ce(III) at 0.51 V vs. Ag/AgCl. This redox potential was much lower than that obtained in 1 M nitric acid, indicating that Ce(IV) was preferentially stabilized by complexation with NTA. The redox potential in the NTA solution was independent of the Ce concentration from 2 to 20 mM, NTA concentration from 5 to 200 mM and pH between 3 and 7. These results indicated that no polymerization and no additional coordination of NTA and OH− to the Ce(III)-NTA complex took place during the redox reaction. As the speciation calculation of Ce(III) in the NTA solution showed that the predominant species was CeIII(nta)2 3− (H3nta=NTA), the redox reaction of the Ce-NTA complex was expressed by the following: CeIV(nta)2 2−+e−⇋CeIII(nta)2 3−. The logarithm of the stability constant of CeIV(nta)2 2− was calculated to be 38.6±0.8 for I=0 from the redox potential shift of Ce(IV)/Ce(III) in the NTA solution. The value was in good accordance with the stability constant of the NpIV(nta)2 2− complex, demonstrating that the aqueous coordination chemistry of Ce(IV) with NTA is quite similar to that of An(IV). These results strongly suggest that a negative shift of the Pu(IV)/Pu(III) redox potential in the NTA solution should make Pu(IV) more stable than Pu(III) even in a reducing environment.