Study of the generation and stability of thorium(IV) colloids by LIBD combined with ultrafiltration

Synthesis of “(aminomethyl)phosphonic acid-functionalized graphene oxide”, and comparison of its adsorption properties for thorium(IV) ion, with plain graphene oxide

The current study presents a simple and scalable method for the synthesis of (aminomethyl)phosphonic acid-functionalized graphene oxide (AMPA-GO) adsorbent. The chemical structure of the new material was disclosed by different instrumental analyses (e.g. FTIR, Raman, XPS, AFM, TEM, XRD, CHN, and UV), and two pertinent mechanisms namely nucleophilic substitution and condensation were suggested for its formation. Adsorption experiments revealed that both AMPA-GO and plain GO have a high affinity toward Th(IV) ions, but the AMPA-GO is superior in terms of adsorption capacity, rate of adsorption, selectivity, pH effect, etc. Indeed, the AMPA-GO can uptake Th(IV) nearly instantaneously, and coexisting Na+ ions have no effect on its adsorption. Thanks to Langmuir isotherm, the maximum adsorption capacities of the GO and AMPA-GO were obtained 151.06 and 178.67 mg g−1, respectively. Interestingly, GO and AMPA-GO both showed a higher preference for thorium over uranium so that the average “K d (Th)/K d (U)” for them was 52 and 44, respectively. This data suggests that chromatographic separation of thorium and uranium is feasible by these adsorbents.

Accountancy for intrinsic colloids on thorium solubility: The fractionation of soluble species and the characterization of solubility limiting phase

The extensive hydrolysis of tetravalent actinides leads to polynuclear formations through oxygen bridging facilitating the formation of colloids as end products. The pH, ionic strength has phenomenal effects on Thorium colloids formation. The quantitative estimation of colloids facilitates the fraction of soluble fraction into ionic, polymeric and colloidal forms of thorium. The colloids accountability and precipitate characterization explains the discrepancies in estimated solubility limits. The supernatants of long equilibrated (∼3 years) saturated thorium solution under various pH (5- 11) and ionic strengths (0-3 M NaClO₄) were analysed by Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and Ion Chromatography (IC) to determine total and ionic thorium respectively. Laser Induced Breakdown Detection (LIBD) was employed to determine the colloid size and concentrations. The precipitates were characterized by calorimetry and XRD to determine the solubility limiting phase. The results of pH, IC, ICP-MS, and LIBD measurements on the aged thorium samples are discussed with regard to the mechanism of the formation of thorium colloids. The results revealed the formation of colloids having particle size (10-40 nm) at concentrations (10⁹-10¹¹ particles/mL). The colloids accountancy resulted in estimated solubility products to 2-4 orders lower than their inclusion as soluble thorium. The soluble thorium was fractionated quantitatively into ionic, polymeric and colloidal forms of thorium. The precipitates formed are found to be semi amorphous.

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non-homogeneous samples with NPs less than 3 nm. By combining high-energy X-ray scattering (HEXS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD XANES) analysis, we have characterised for the first time both the short- and medium-range order of ThO2 NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO2 small units similar to thorium hexamer clusters mixed with 1 nm ThO2 NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO2 NPs. HERFD XANES analysis at the thorium M4 edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism.

Conditions affecting the release of thorium and uranium from the tailings of a niobium mine

Determinations of the mobility of metals from tailings is a critical part of any assessment of the environmental impacts of mining activities. The leaching of thorium and uranium from the tailings of different processing stages of a niobium mine was investigated for several pH, ionic strengths and concentrations of natural organic matter (NOM). The pH of the leaching solution did not have a noticeable impact on the extraction of Th, however, for pH values below 4, increased U mobilization was observed. Similarly, only a small fraction of Th (0.05%, ≤15 μg kg^-1) and U (1.22%, ≤6 μg kg^-1) were mobilized from the tailings in the presence of environmentally relevant concentrations of Ca, Mg or Na. However, in the presence of 10 mg L^-1 of fulvic acid, much higher concentrations of ca. 700 μg kg^-1 of Th and 35 μg kg^-1 of U could be extracted from the tailings. Generally, colloidal forms of Th and dissolved forms of U were mobilized from the tailings, however, in the presence of the fulvic acid, both dissolved and colloidal forms of the two actinides were observed. Single Particle ICP-MS was used to confirm the presence of Th (and U) containing colloids where significant numbers (up to 10⁷ mL^-1) of Th and U containing colloids were found, even in 0.2 μm filtered extracts. Although mass equivalent diameters in the range of 6-13 nm Th and 6-9 nm for U could be estimated (based upon the presence of an oxyhydroxide), most of the colloidal mass was attributed to larger (>200 nm) heterocomposite particles.

Assessment of surface reactivity of thorium oxide in conditions close to chemical equilibrium by isotope exchange 229Th/232Th method

This work aims to assess the solubility and the surface reactivity of crystallized thorium at pH 3.0 in presence of three types of solids: synthesized powder at 1300°C, crushed kernel, and intact kernel. In this study, the kernel is composed by the core solid from high temperature reactors (HTR) sphere particles. The originality of this work consisted in following in a sequential order the kinetic of dissolution, the surface reactivity in presence of isotope tracer 229Th, and its desorption process. Long time experiments (634 days) allowed to get deeper understanding on the behavior of the surface reactivity in contact with the solution. Solubility values are ranging from 0.3×10−7 mol·L−1 to 3×10−7 mol·L−1 with a dissolution rate of 10−6–10−4 g·m−2 day−1. PHREEQC modeling showed that crystallized ThO2(cr, 20 nm) phase controls the equilibrium in solution. Isotope exchange between 229Th and 232Th indicated that well-crystallized phase exist as an inert surface regarding to the absence of exchange between surface solid and solution.

Oxyhydroxy Silicate Colloids: A New Type of Waterborne Actinide(IV) Colloids

At the near-neutral and reducing aquatic conditions expected in undisturbed ore deposits or in closed nuclear waste repositories, the actinides Th, U, Np, and Pu are primarily tetravalent. These tetravalent actinides (AnIV) are sparingly soluble in aquatic systems and, hence, are often assumed to be immobile. However, AnIV could become mobile if they occur as colloids. This review focuses on a new type of AnIV colloids, oxyhydroxy silicate colloids. We herein discuss the chemical characteristics of these colloids and the potential implication for their environmental behavior. The binary oxyhydroxy silicate colloids of AnIV could be potentially more mobile as a waterborne species than the well-known mono-component oxyhydroxide colloids.

Colloid-borne forms of tetravalent actinides: a brief review

Tetravalent actinides, An(IV), are usually assumed to be little mobile in near-neutral environmental waters because of their low solubility. However, there are certain geochemical scenarios during which mobilization of An(IV) in a colloid-borne (waterborne) form cannot be ruled out. A compilation of colloid-borne forms of tetravalent actinides described so far for laboratory experiments together with several examples of An(IV) colloids observed in field experiments and real-world scenarios are given. They are intended to be a knowledge base and a tool for those who have to interpret actinide behavior under environmental conditions. Synthetic colloids containing structural An(IV) and synthetic colloids carrying adsorbed An(IV) are considered. Their behavior is compared with the behavior of An(IV) colloids observed after the intentional or unintentional release of actinides into the environment. A list of knowledge gaps as to the behavior of An(IV) colloids is provided and items which need further research are highlighted.

Adsorption of tetravalent thorium by geomedia

We measured the pH-dependent adsorption of Th(IV), an analogue for Pu(IV) and other tetravalent actinides, to two geomedia: goethite (α-FeOOH(s)) and a heterogeneous Fe-containing sand from the southeastern USA. The goal was to examine whether or not the Th(IV)-goethite adsorption data could be used to predict the adsorption of Th(IV) by the heterogeneous sand. In the absence of either geomedia, after forty-eight hours the measured pH-dependent “adsorption” was consistent with the solubility of solid amorphous ThO2(am, aged), despite the fact that ThO2(am, aged) is generally not formed until approximately seventy days. We concluded that ThO2(am, aged) was stabilized by precipitating on the walls of the reaction vessels. Ignoring this phenomenon could lead to experimental artifacts in Th(IV) adsorption studies. Thorium adsorption by both goethite and the sand was strongly pH dependent, with adsorption increasing sharply from pH ∼ 2 to pH ∼ 4. Two methods were utilized to predict the pH-dependent adsorption of Th(IV) by the sand using the Th(IV)-goethite adsorption data. Using the Fe content of the sand and the Th(IV)-goethite adsorption data, we were able to predict the maximum amount of Th(IV) adsorption by the sand within 78% of the actual value (i.e., an error of 22%). In contrast, on a surface-area-normalized basis, we were only able to predict the maximum adsorption of Th(IV) by the sand within a factor of two. These results have important implications to scaling and extrapolating the results of batch-scale tetravalent actinide adsorption studies with pure minerals to predict their field-scale adsorption by and transport in heterogeneous subsurface media.

Discrepancies in thorium oxide solubility values: study of attachment/detachment processes at the solid/solution interface

The solubility of thorium under oxide and/or hydroxide forms has been extensively studied for many years. Nevertheless, a large discrepancy in the solubility values is noticed in the literature. We study Th atom exchange between thorium oxide surfaces and various aqueous solutions (0.01 mol·L(-1) NaCl for 0.0 < pH < 5.2) to address this issue. By solid-state characterization [X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and atomic force microscopy], we determined that 80% of the XPS accessible near the surface region of sintered thorium oxide is represented by the less reactive ThO(2)(cr) grains. The remaining 20% corresponds to ThO(x)(OH)(y)(H(2)O)(z), which is largely associated with grain boundaries. Only the latter fraction is involved in solid/solution exchange mechanisms. Local conditions (thorium concentrations, pH values, etc.) in grain boundaries lead to an adjustment of the "local solubility constraints" and explain the thorium concentration measured in our experiments. For pH <5.2, the thorium concentration and pH gradient between the bulk solution and grain-boundary regions imply that the solubility values mainly depend on the availability and accessibility of ThO(x)(OH)(y)(H(2)O)(z). We have performed two solubility experiments with a (232)ThO(2)(cr) solid in a 0.01 mol·L(-1) NaCl solution for 300 days. In a first experiment, we measured (232)Th concentrations in dissolution experiments in order to determine the detachment rates of Th atoms from the solid surface. In a subsequent step, we added (229)Th to the solution in order to measure the surface attachment rate for dissolved Th atoms. This allowed an assessment of the net balance of Th atom exchange at the solid/solution interface. The empirical solubility data do not correspond to the thermodynamic bulk phase/solution equilibrium because measured solution concentrations are controlled by site-specific exchange mechanisms at the solid/solution interface. Therefore, for sparingly soluble solids, one needs to quantify site-specific surface attachment and detachment rates if one wants to assess solubility constraints.

Discrepancies in thorium oxide solubility values: a new experimental approach to improve understanding of oxide surface at solid/solution interface

The solubility of ThO2(cr) was studied since many years but a large discrepancy in solubility values is noticed in the literature. The present work suggests that this discrepancy is related to differences in the surface properties of thorium oxide.

To understand the role of surface properties on solubility values, we conducted experiments with ThO2(cr) spheres with reproducable surface properties. Batch dissolution experiments were conducted in 0.01 M NaCl solution at pH=3.0 and 4.0 for periods of time up to 270 days. The solutions were spiked with229Th to determine precipitation (sorption) rates of thorium, while dissolution rates were determined by measuring232Th released from ThO2(cr) spheres. We assume that229Th atoms are exchanged only with active sites involved in Th-dissolution. Using229Th as local sensor of attachement and detachment processes at the ThO2(cr) surface under close-to-equilibrium conditions, allows to assess surface reactivity of the solid during solubility experiments.

Formation and hydrolysis of polynuclear Th(IV) complexes – a nano-electrospray mass-spectrometry study

Polynuclear hydroxide complexes play an important role for the hydrolysis of tetravalent thorium ions in aqueous solution, in particular for Th(IV) concentrations exceeding some [Th(IV)]=10−4 M. Consequently, these polymers must be considered when describing hydrolysis of Th(IV) or dissolution processes of Th(IV) solids. In the past, considerable efforts were made to obtain equilibrium formation constants of these polymers and different stoichiometries for dimers, tetramers and hexamers have been suggested. However, most information was obtained from indirect methods, in particular, from potentiometric titrations. In the present work, we present an approach of directly quantifying polymeric metal hydroxide complexes in solution. By nano-electrospray mass-spectrometry the degrees of polymerization, i.e. the numbers of Th4+ ions and the numbers of hydroxide ligands, and as a consequence, also the charges of the complexes are measured. All mono- and polynuclear species which are present in solution are quantified simultaneously down to species contributing less than 0.1% of the total [Th(IV)] concentration. Solutions of [Th(IV)]=6×10−6–10−1 M are investigated in HCl at [H+]=10−4–0.1 M. More than 30 different polymeric complexes are observed with the general trend of increasing number of hydroxide ligands with decreasing acidity. A surprising finding is the presence of the pentamer Th5(OH)y z +, which was not described in the literature before. With decreasing Th(IV) concentration the stability field of polymers narrows continuously until polymers can no longer be detected below [Th(IV)]=10−5 M.

Solubility and colloid formation of Th(IV) in concentrated NaCl and MgCl2solution

The solubility of crystalline ThO2(cr) and amorphous hydrated Th(IV) oxyhydroxide ThOn(OH)4-2n·xH2O(am) has been measured in dilute to concentrated NaCl and MgCl2solutions equilibrated with magnesium hydroxide or hydroxychloride at 22±2 °C. The contributions of colloids to the total thorium concentrations observed in both over- and undersaturation experiments with amorphous Th(IV) precipitates have been analysed by ultracentrifugation. The solubility increasing effect of long-time stable Th(IV) eigencolloids, previously investigated in 0.5 M NaCl solutions, is also observed in concentrated 5 M NaCl. Ionic strength and chloride concentration have no effect on the stability of these hydrophilic Th(IV) oxyhydroxide eigencolloids, which are the predominant species in solution. They cause relatively high total thorium concentration in neutral to alkaline steady state solutions, independent of ionic strength: log[Th]tot≈log[Th]coll=-6.3±0.5. In concentrated MgCl2solutions saturated with magnesium hydroxychloride colloids, the formation of pseudocolloids,i.e., Th(IV) sorbed onto Mg2(OH)3Cl·4H2O(coll), leads to a further increase of the total thorium concentration up to 10-5M. The present results are discussed with regard to maximum Th(IV) and Pu(IV) concentrations in performance assessment calculations.

Hydrolysis of plutonium(IV) in acidic solutions: no effect of hydrolysis on absorption-spectra of mononuclear hydroxide complexes

Tetravalent plutonium readily undergoes hydrolysis even in highly acidic aqueous solutions. In the past, many attempts were made to quantify hydrolysis species by means of optical absorption spectroscopy. In the present work solutions ranging from 10−5M to 10-2M (total Pu) concentration in 0.5 M HCl/NaCl (0.3 < pHc< 2.1) are carefully investigated by combining absorption-spectroscopy (UV-Vis, liquid core waveguide capillary) and laser-induced breakdown detection, with special emphasis on the limited solubility of Pu(IV). The results clearly indicate that all changes in the absorption spectra originate from the formation of Pu-polyspecies and colloids. The molar absorptivity of mononuclear Pu(IV) hydroxide complexes does not vary with increasing pHcand ongoing hydrolysis. The normalized absorption spectra of at least the first and the second hydroxide complex (Pu(OH)n4-nn= 1, 2) do not differ from those of the hydrated Pu4+ion.

Probing particle size distributions in natural surface waters from 15 nm to 2 μm by a combination of LIBD and single-particle counting

We present a technique for measuring colloid size distributions between 15 nm and 2 microm at concentrations relevant to natural surface waters. Two particle-measuring methods are combined: laser-induced breakdown detection (LIBD), which allows the quantification of colloid size distributions below 400 nm, and a commercial single-particle counter that extends the accessible size range up to two mum. Centrifugation was used in order to separate micrometer sized particles for the LIBD measurement. The feasibility is demonstrated on water of Lake Brienz (Switzerland) and the River Pfinz (Germany) and the particle size distributions follow Pareto's law even down to 15 nm in both cases.

Formation of Aquatic Th(IV) Colloids and Stabilization by Interaction with Cm(III)/Eu(III)

The present investigation is to ascertain under what conditions actinide ions undergo aggregation via oxo-bridging to form stable colloidal species. Eu and Th are taken for this purpose as trivalent and tetravalent actinide homologue ions, respectively. For verification of the effects of impurities in chemicals on the actinide colloid generation, pH is adjusted either by a conventional acid-base titration or by coulometry without addition of NaOH. The colloid generation is monitored by highly sensitive laser-induced breakdown detection in varying pH from 3 to 7, first in dilute Eu and Th solutions separately and then in a mixture of both, all in 0.5 M HCl/NaCl. The formation of stable colloids is observed particularly in a mixed solution of Eu and Th, suggesting that aggregation via mutual oxo-bridging of trivalent and tetravalent metal ions results in surface polarization, leading to stable hydrophilic particles of 20-30 nm in diameter. When Eu is replaced by Cm in the mixed solution in favor of the high fluorescence intensity of the latter, the chemical speciation is determined on colloid-borne Cm by time-resolved laser fluorescence spectroscopy. Two different colloid-borne Cm species, oxo-bridged with Th, are identified: a minor amount at 598.0 nm (denoted as Cm-Th(1)) and a major amount at 604.8 nm (Cm-Th(2)). The former is found as a transitional state, which converts to the latter with increasing pH and prevails at pH > 5.5. Both colloid-borne species (Cm-Th) are distinctively different from hydrolyzed Cm or its carbonate complexes with respect to their fluorescence peak positions and lifetimes. In conclusion, a mixed oxo-bridging of trivalent and tetravalent actinides elicits the generation of stable colloids, whereas individual ions in their pure state form colloids under oversaturation at near neutral pH only as a transitional state for precipitation.

Combined LIBD and XAFS investigation of the formation and structure of Zr(IV) colloids

The solubility of Zr(OH)4(am)--in other words hydrated Zr(IV) oxyhydroxide--is determined by means of coulometric titration (CT), and colloids are detected by laser-induced breakdown when the solubility limit is exceeded. Our results at pH 3-8 demonstrate that the solubility of Zr(OH)4(am) is several orders of magnitude higher than reported classical solubility data for acidic solutions, determined from undersaturation with a less soluble microcrystalline Zr(IV) oxide precipitate. Analysis of extended X-ray absorption fine structure (EXAFS) data shows that the microcrystalline colloids in a 0.1 mol l(-1) Zr aqueous solution at pH 0.2 contain tetrameric units, similar to those present in the structure of ZrOCl2.8H2O. Characterization of the CT solutions by means of EXAFS shows that oligomeric species form as the solubility limit is approached. The current lack of data on equilibrium constants for polynuclear hydroxide complexes prohibits the use of a realistic speciation model to describe the solubility of pH-dependent Zr(OH)4(am). However, the solubility curve is obtained using the mononuclear hydrolysis constants estimated in the present paper, along with the solubility constant (log K'sp=-49.9+/-0.5 in 0.5 mol l(-1) NaCl; log K degrees(sp)=-53.1+/-0.5 at I=0).

XAFS and LIBD Investigation of the Formation and Structure of Colloidal Pu(IV) Hydrolysis Products

Pu(IV) oxyhydroxide colloid growth is investigated with XAFS and LIBD. From combined results a model of colloid formation is proposed, which leads to a face-centered cubic Pu sublattice having cation defects, as observed with EXAFS, and a linear dependency of log [Pu(IV)] on -log [H+] with slope -2, in accord with LIBD. The solubility for Pu(IV) measured with LIBD is close to the lower limit of the solubility curve from previously reported data.

Mechanisms of uranyl sorption

Detailed knowledge of the reactions at the water/colloid/mineral interface is crucial to model accurately actinide behaviour in nature. In this paper, we review current knowledge of the sorption of actinides and of the mechanisms of sorption, with a particular focus on uranyl. Of major interest is the influence of the aqueous uranyl species (e.g., carbonate complexes, polynuclear species, colloids) on the uranyl sorption species. We present extended X-ray absorption fine structure (EXAFS) and X-ray photoelectron spectroscopy (XPS) studies on the coordination of uranyl onto an amorphous Al phase and onto quartz, respectively. Our XPS investigations show that two components having uranyl ions in very distinct coordination environments co-exist on quartz at high uranyl surface coverage, independently of the presence of uranyl carbonate complexes or uranyl colloids in solution. One component corresponds to polynuclear surface species and/or schoepite-like surface precipitates. In the case of similar uranyl concentrations and of high carbonate solution concentrations, polymeric uranyl species are formed on quartz, whereas no such surface species occurs on the Al phase. Uranyl is found on the Al phase as mononuclear uranyl carbonato surface complexes only. These results are of importance because they suggest that mineral surface characteristics strongly control the uranyl surface species in aquifers.