Complex formation of Cm(III) with formate studied by time-resolved laser fluorescence spectroscopy

Pore waters of natural clays, which are investigated as potential host rock formations for high-level nuclear waste, are known to contain large amounts of low-molecular weight organic compounds. These small organic ligands might impact the aqueous geochemistry of the stored radionuclides and, thus, their migration behavior. In the present work, the complexation of Cm(III) with formate in aqueous NaCl solution is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) as a function of the ionic strength (0.5–3.0 mol/kg), the ligand concentration (0–0.2 mol/kg) and the temperature (20–90 °C). The Cm(III) speciation is determined by deconvolution of the emission spectra. The obtained distribution of Cm(III) species is used to calculate the conditional stability constants (log K′(T)) at a given temperature and ionic strength which are extrapolated to zero ionic strength by using the specific ion interaction theory (SIT). Thus, the thermodynamic log K⁰_n(T) values for the formation of [Cm(Form)_n]⁽³⁻ⁿ⁾⁺ (n = 1, 2) and the ion interaction coefficients (ε(i,k)) for [Cm(Form)_n]⁽³⁻ⁿ⁾⁺ (n = 1, 2) with Cl⁻ are obtained. The log K⁰₁(T) (2.11 (20 °C)–2.49 (90 °C)) and log K⁰₂(T) values (1.17 (30 °C–2.01 (90 °C)) increase continuously with increasing temperature. The log K⁰_n(T) values are used to derive the standard reaction enthalpies and entropies (Δ_rH⁰_m, Δ_rS⁰_m) of the respective complexation reactions according to the Van’t Hoff equation. In all cases, positive Δ_rH⁰_m and Δ_rS⁰_m values are obtained. Thus, both complexation steps are endothermic and entropy-driven.     

Structural incorporation of Cm(III) in trioctahedral smectite hectorite: A time-resolved laser fluorescence spectroscopy (TRLFS) study

Structural incorporation of the actinide curium in the octahedral layer of the trioctahedral Mg-rich smectite hectorite was studied using time-resolved laser fluorescence spectroscopy. Organo-hectorite nanoparticles were synthesized at 90 °C in the presence of Cm(III). Within 120 h, hectorite particles were formed. Time-resolved laser fluorescence spectroscopy was used to identify Cm(III) species during various synthesis steps and to characterize the structural incorporation mechanism. The formation of a Cm-containing Mg hydroxide precursor and the reaction with aqueous silica in a pH range of 9-10 to form TOT layers were identified to be key steps for trivalent actinide incorporation in hectorite via coprecipitation.     

Sorption of Eu(III)/Cm(III) on Ca-montmorillonite and Na-illite. Part 1: Batch sorption and time-resolved laser fluorescence spectroscopy experiments

Sorption of Cm(III) and Eu(III) at trace concentrations onto Ca-montmorillonite (SWy-1) and Na-illite (Illite du Puy) has been studied under anaerobic conditions by batch sorption experiments and time-resolved laser fluorescence spectroscopy (TRLFS). Comparison of the results from spectroscopic and batch sorption experiments with Cm and Eu indicates the existence of outer-sphere complexes at pH <4 in the experiments with Na-illite (0.25 g/L solid; 2.5 × 10⁻⁷ mol/L Cm; 0.1 mol/L NaClO₄). In the case of Ca-montmorillonite, (0.25 g/L solid, 2.5 × 10⁻⁷ mol/L Cm or 10⁻⁶ mol/L Eu, 0.066 mol/L Ca(ClO₄)₂), Cm/Eu outer-sphere complexes do not form at significant levels due to the Ca²⁺ competition for the clay mineral cation-exchange sites. TRLFS spectra indicate the formation of inner-sphere surface complexes at pH >5 for both clay minerals. Five H₂O/OH⁻ molecules remain in the first metal ion coordination sphere of the sorbed Eu/Cm. Measured fluorescence lifetimes of sorbed Eu/Cm and peak deconvolution of Cm-spectra are consistent with the formation of surface complexes of the form ≡S-O-Eu/Cm(OH)_x^(2−x)(H₂O)_5−x. At pH ≥ 12 Cm becomes incorporated into a surface precipitate at the Ca-montmorillonite surface presumably composed of Ca(OH)₂ or calcium silicate hydrate. A dramatic shift of the fluorescence emission band by more than 20 nm and a clear increase in the fluorescence lifetime suggests the almost complete displacement of coordinated H₂O and OH⁻. The pH dependent Eu sorption data obtained in batch experiments are consistent with spectroscopic data on Eu and Cm within experimental uncertainties thus demonstrating the validity of Eu as a homologue for trivalent actinides. Parameterization of a two-site protolysis nonelectrostatic surface complexation and cation exchange model using the batch sorption data and spectroscopic results is discussed in Part 2 of this work.     

Differences in properties and Cm(III) complexation behavior of isolated humic and fulvic acid derived from Opalinus clay and Callovo-Oxfordian argillite

Fulvic acids from deep clay formations have been isolated by the International Humic Substances Society (IHSS) standard protocol and analyzed. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy reveals, that the basic structural features relating to the origin of the clay organic matter (i.e., terrestrial or marine) are preserved even after the around 150 Ma since deposition (Jurassic sediment). Analysis by asymmetrical flow field flow fractionation (AFFFF) shows the size distribution peak found for typical fulvic acids. In addition, a second larger size peak is found for the fulvic acids influenced by marine deposition. These fulvic acids also have a considerable content of organic material which does not absorb in the visible range. The Cm(III) complexation behavior has been studied by time-resolved laser fluorescence spectroscopy (TRLFS). Despite considerable fluorescence quenching, the complexation constant is shown to be in the same range as published values found for different typical fulvic acids.     

Sorption of Eu(III)/Cm(III) on Ca-montmorillonite and Na-illite. Part 2: Surface complexation modelling

Sorption edges and isotherms for Eu(III) uptake on Ca-montmorillonite and Na-illite in 0.066 mol/L Ca(ClO₄)₂ and 0.1 mol/L NaClO₄ background electrolytes, respectively, were modelled using a quasi-mechanistic sorption model (the two site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model). For both clay minerals the Eu sorption edges could be quantitatively modelled in the pH range ∼3 to ∼10 using cation exchange reactions for Eu³⁺/Na⁺ and Eu³⁺/Ca²⁺ and three surface complexation reactions on the strong sorption sites forming ≡S^SOEu²⁺, ≡S^SOEuOH⁺ and ≡S^SOEu(OH)₂° inner sphere complexes which appear successively with increasing pH. Time resolved laser fluorescence spectroscopy (TRLFS) measurements of Cm(III) loaded Ca-montmorillonite and Na-illite were available from Part 1 of this work. De-convolution of the normalised fluorescence spectra measured at different pH values indicated three distinct Cm surface complexes, Cm complexes 1, 2 and 3 for both clay minerals, in agreement with model predictions, but with different distribution functions for the individual species. Under the assumption that Eu and Cm exhibit essentially the same hydrolysis and sorption behaviour, the Eu surface complexation constants were used to predict surface species distribution functions for Cm under the same experimental conditions used in the TRLFS measurements. Comparison of modelled and experimentally deduced species distributions indicated that for both clay minerals peak heights and widths of the three peaks did not correspond particularly well. It is shown that the calculated species distribution functions are sensitive to the values of the hydrolysis constants used in the calculations, whereas modelling the sorption edge measurements by applying the 2SPNE SC/CE approach is much less sensitive. By modifying the values of the hydrolysis constants within their uncertainty range and re-modelling the sorption edges, considerably better correspondence between the modelled and TRLFS species distribution functions was found. In particular, peak positions, heights and widths for the model predicted peaks for the ≡S^SOCm²⁺ and ≡S^SOCmOH⁺ species distribution, and those for Cm complexes 1 and 2 derived from TRLFS, were found to be very close for both clay minerals. However, discrepancies were still apparent between the profile for the calculated ≡S^SOEu(OH)₂° surface species and the Cm complex 3 species, especially in the case of Na-illite.     

Lead(II) complexation by fulvic acid: how it differs from fulvic acid complexation of copper(II) and cadmium(II)

Pb²⁺, like Cu²⁺, forms strong complexes with fulvic acids (Cd²⁺-fulvate complexes are much weaker), but Pb-fulvate precipitates at a much lower mole ratio of metal ion to fulvic acid than either Cu-fulvate or Cd-fulvate does. Physical association of Pb²⁺ with Pb-fulvate solids as well as complexation by sites still available in the precipitates probably causes the increased removal of free Pb²⁺ from solution after precipitation begins.     

光化学降解黄腐酸及其对黄腐酸-苯并(α)芘结合性质的影响

在不同照射时间、pH和溶解氧浓度下,研究了从加拿大魁北克省Rimouski河提取的黄腐酸(RRFA)在模拟太阳光照射过程中对苯并(α)芘结合性质的影响.结果表明,RRFA经模拟太阳光照射后,其与苯并(α)芘的结合系数KOC下降.光化过程中RRFA的相对分子质量或SUVA270与KOC之间有良好的指数回归关系.在pH 4.1～8.0范围内,以光照后的pH条件下测定照射样品的KOC,KOC平均下降47.4%±17.4%;以光照前的pH条件下测定照射样品的KOC,KOC平均下降48.7%±16.9%.溶解氧浓度的升高能加速RRFA的光氧化降解过程,与相应的未照射样品相比,空气饱和与氧气饱和条件下照射后的KOC分别下降30%和74%,氮气饱和条件下照射后的KOC升高122%.     

Effect of phosphate, silicate, and Ca on Fe(III)-precipitates formed in aerated Fe(II)- and As(III)-containing water studied by X-ray absorption spectroscopy

We studied the local coordination and structure of Fe(III)-precipitates formed in aerated Fe(II)- and As(III)-containing water (buffered to pH 7 by 8 mM bicarbonate) using synchrotron-based X-ray absorption spectroscopy (XAS) at the K-edges of Fe, P, Ca, and As. Dissolved phosphate, silicate, and Ca at different ratios relative to each other and to Fe affect the forming Fe(III)-phases in a complex manner. The high affinity of phosphate for Fe(III) results in the predominant precipitation of Fe(III)-phosphate as long as dissolved phosphate is present, with Fe(III) polymerization limited to small oligomers. In Ca-containing solution, Ca uptake by Fe(III)-Ca-phosphate involves the linkage and coagulation of negatively charged Fe(III)-phosphate oligomers via Ca-O-P bonds. In the absence of phosphate, dissolved silicate at Si/Fe ratios above ∼0.5 results in the formation of hydrous ferric oxide (HFO) with mainly edge-sharing Fe-Fe linkage. At lower Si/Fe ratios of ∼0.5-0.1, mainly 2-line ferrihydrite (2L-Fh) with both edge- and corner-sharing Fe-Fe linkage forms. Only in the absence of phosphate at low Si/Fe ratio, lepidocrocite (Lp) forms. In solutions containing sufficient Fe(II), aeration results in the sequential precipitation of Fe(III)-(Ca-)phosphate, HFO or 2L-Fh (depending on solution Si/Fe), and finally Lp. The amount and oxidation state of As co-precipitated with Fe(III) are controlled by the co-oxidation of As(III) with Fe(II), which increases with initial Fe/As ratio, and the competitive uptake of phosphate, As(V) and less strongly sorbing silicate and As(III). This study demonstrates that the diversity and sequence of short-range-ordered Fe(III)-precipitates forming by Fe(II) oxidation in near-neutral natural waters depend on water chemistry. Because differences in the colloidal stability and biogeochemical reactivity of these phases will affect the fate of associated major and trace elements, the different Fe(III)-precipitates and their specific biogeochemical properties must be taken into account when addressing nutrient and contaminant dynamics at redox boundaries in natural and engineered systems.     

原子荧光光谱法直接测定载金炭样品中的砷

建立了原子荧光光谱法直接测定载金炭样品中砷的分析方法。采用硝酸溶样,原子荧光光谱法直接测定载金炭样品中的砷。该方法具有准确度高、流程短、精密度好、速度快等优点。比对试验结果表明,该方法测定结果与全谱直读ICP法结果一致。     

Coordination chemistry and hydrolysis of Fe(III) in a peat humic acid studied by X-ray absorption spectroscopy

The speciation of iron (Fe) in soils, sediments and surface waters is highly dependent on chemical interactions with natural organic matter (NOM). However, the molecular structure and hydrolysis of the Fe species formed in association with NOM is still poorly described. In this study extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the coordination chemistry and hydrolysis of Fe(III) in solution of a peat humic acid (5010-49,200 μg Fe g⁻¹ dry weight, pH 3.0-7.2). Data were analyzed by both conventional EXAFS data fitting and by wavelet transforms in order to facilitate the identification of the nature of backscattering atoms. Our results show that Fe occurs predominantly in the oxidized form as ferric ions and that the speciation varies with pH and Fe concentration. At low Fe concentrations (5010-9920 μg g⁻¹; pH 3.0-7.2) mononuclear Fe(III)-NOM complexes completely dominates the speciation. The determined bond distances for the Fe(III)-NOM complexes are similar to distances obtained for Fe(III) complexed by desferrioxamine B and oxalate indicating the formation of a five-membered chelate ring structure. At higher Fe concentrations (49,200 μg g⁻¹; pH 4.2-6.9) we detect a mixture of mononuclear Fe(III)-NOM complexes and polymeric Fe(III) (hydr)oxides with an increasing amount of Fe(III) (hydr)oxides at higher pH. However, even at pH 6.9 and a Fe concentration of 49,200 μg g⁻¹ our data indicates that a substantial amount of the total Fe (>50%) is in the form of organic complexes. Thus, in environments with significant amounts of organic matter organic Fe complexes will be of great importance for the geochemistry of Fe. Furthermore, the formation of five-membered chelate ring structures is in line with the strong complexation and limited hydrolytic polymerization of Fe(III) in our samples and also agrees with EXAFS derived structures of Fe(III) in organic soils.     

Biosorption of Cr(III) and Cr(VI) species from aqueous solution by Cabomba caroliniana: kinetic and equilibrium study

This study reports the potential ability of non-living biomass of Cabomba caroliniana for biosorption of Cr(III) and Cr(VI) from aqueous solutions. Effects of contact time, biosorbent dosage, pH of the medium, initial concentration of metal ion and protonation of the biosorbent on heavy metal–biosorbent interactions were studied through batch sorption experiments. Cr(III) was sorbed more rapidly than Cr(VI) and the pH of the medium significantly affected the extent of biosorption of the two metal species differently. Surface titrations showed that the surface of the biosorbent is positively charged at low pH while it is negatively charged at pH higher than 4.0. Protonation of the biosorbent increased its capacity for removal of Cr(III), while decreasing that of Cr(VI). FT-IR spectra of the biosorbent confirmed the involvement of –OH groups on the biosorbent surface in the chromium removal process. Kinetic and equilibrium data showed that the sorption process of each chromium species followed pseudo second-order kinetic model and both Langmuir and Freundlich isothermal models. A possible mechanism for the biosorption of chromium species by non-living C. caroliniana is suggested.     

Influence of natural fulvic acids on the solubility of sulfide ores (experimental study)

Model experiments were performed on the solubility of sulfide ores from the Kyzyl-Tashtyg deposit in distilled water and fulvic-acid (FA) solutions of different concentrations. It has been established that the oxidation of sulfide minerals in ores under atmospheric conditions might produce acid drainage waters with toxic heavy metals (Cu, Zn, Cd). The influence of natural organic acids depends on the capability of ore and host rock to neutralize the acidity of solutions. If carbonate content is enough at the first stage, a considerable increase in the pH value of FA-free solutions ensures Fe and Cu removal into the solid phase, whereas the formation of metal fulvate complexes hinders this process. However, when the rocks exhaust their neutralizing potential, all the solutions remain acidic for > 100 days of leaching. In this case, FAs, on the contrary, inhibit the oxidation of the surface of sulfide minerals and reduce the removal of heavy metals into the solution.     

Adatom Fe(III) on the hematite surface: Observation of a key reactive surface species

The reactivity of a mineral surface is determined by the variety and population of different types of surface sites (e.g., step, kink, adatom, and defect sites). The concept of "adsorbed nutrient" has been built into crystal growth theories, and many other studies of mineral surface reactivity appeal to ill-defined "active sites." Despite their theoretical importance, there has been little direct experimental or analytical investigation of the structure and properties of such species. Here, we use ex-situ and in-situ scanning tunneling microcopy (STM) combined with calculated images based on a resonant tunneling model to show that observed nonperiodic protrusions and depressions on the hematite (001) surface can be explained as Fe in an adsorbed or adatom state occupying sites different from those that result from simple termination of the bulk mineral. The number of such sites varies with sample preparation history, consistent with their removal from the surface in low pH solutions.     

腐殖酸三维荧光光谱特性研究

腐殖质的荧光特性被广泛用来解析其在各种天然环境中的来源及分布.由于荧光光谱分析具有灵敏度高,选择性好,且不破坏样品结构的优点,非常适合用来研究腐殖质的结构和官能团等特征.利用三维荧光激发-发射光谱研究了 Fluka腐殖酸的荧光光谱特性,结果显示,离子强度(0～0.05 mol/L KClO4)对 Fluka腐殖酸的三维荧光光谱特性影响非常小,而腐殖酸的浓度(5～100 mg/L)和溶液 pH(2～12)对其三维荧光光谱特性影响显著.当腐殖酸浓度增大时,荧光峰出现明显红移现象.荧光强度一般随着 pH的升高而增大,当 pH大于 10后呈下降趋势,我们从垃圾渗滤液溶解有机质的实验结果中也得到相同结论;在浓度为 50 mg/L和 100 mg/L的 Fluka腐殖酸中,荧光峰 B(fulvic-like)的荧光强度却在 pH=5.0左右时达到最大值,与前人报道的土壤富里酸的行为一致,质子化常数 lgK′ HL分别为 3.57和 3.13,与二羧基化合物接近,说明荧光峰 B可能与 Fluka腐殖酸结构中的羧基有关;荧光峰 A与荧光峰 B的荧光强度比值 r(A/B)在 0.61～2.59之间,并且在 pH=2～11范围内, r(A/B)与 pH具有较好的线性相关关系,表明荧光峰 A和荧光峰 B随着 pH值的改变有着相同的变化趋势.     

Incorporation of trivalent actinides into calcite: A time resolved laser fluorescence spectroscopy (TRLFS) study

In order to characterize and quantify the substitution of Ca(II) by Cm(III) (coordination, charge compensation), homogeneous Cm(III) coprecipitated calcite was synthesized in a mixed-flow-through experiment. Two sets of experiments were conducted at pH 8.1 and at pH 12.5.At pH 8.1 two calcites, a calcite with a low Cm³⁺ concentration (LCMpH8.1) and a calcite with a high M³⁺ (Gd³⁺ and Cm³⁺) concentration (HCMpH8.1) were grown and investigated by time resolved laser fluorescence spectroscopy. The Cm(III) emission spectra of LCMpH8.1 and HCMpH8.1 show the same Cm(III) fluorescence signals for two Cm(III) species; Cm(III) species (1) with a peak maximum at 606.2 nm and Cm(III) species (2) with a peak maximum at 620.3 nm. Cm(III) species (1) has a mean lifetime of τ = 386 ± 40 μs and Cm(III) species (2) has a mean lifetime of τ = 1874 ± 200 μs. A lifetime of 386 μs correlates with 1.3 water molecule in the first coordination sphere of the Cm ion whereas a lifetime of 1874 μs indicates the total loss of the Cm(III) hydration sphere. According to the fluorescence emission peak position and the fluorescence emission lifetime, Cm(III) species (1) is identified as a surface sorbed species whereas Cm(III) species (2) is identified as a Cm(III) incorporated into the calcite lattice.Cm(III) fluorescence emission spectra of Cm(III) doped calcite grown at pH 12.5 (LCMpH12.5) show the same peak maxima which are found for LCMpH8.1 and HCMpH8.1 grown at pH 8.1 but an additional emission band at 608.2 nm (3) is found, which can be assigned to a further Cm(III) species. Fluorescence emission lifetime measurements show that this Cm(III) species (3) has a lifetime of τ = 477 ± 25 μs, which correlates with 0.9 water molecules in the first coordination sphere. Cm(III) species (3) is suggested to be a CmOH²⁺ incorporated species.     

Effect of hydrolysed aluminum(III) and chromium(III) cations on the lipophilicity of talc

Liquid-liquid extraction and electrokinetic studies show that both the talc particles and the isooctane droplets are affected by the presence of hydrolysable metal species of Al(III) and Cr(III). The inherent lipophilicity of talc is unchanged by the hydroxo complexes of these metal species, while positive shifts in zeta potentials of talc and isooctane take place due partly to the strong adsorption of these hydroxo complexes. In the pH range where the hydroxide precipitates, the talc particles are rendered hydrophillic between the precipitation pH and the point-of-zero charge of the metal hydroxide. This hydrophile-lipophile transition of talc in the isooctane-water system relates to heterocoagulation among talc particles, isooctane droplets and hydroxide precipitates.     

Advances in lithium analysis in solids by means of laser-induced breakdown spectroscopy: an exploratory study

Lithium is an important geochemical tracer for fluids or solids. However, because the electron microprobe cannot detect Li, variations of Li abundance at the micrometric scale are most often estimated from bulk analyses. In this study, the Li intense emission line at 670.706 nm in optical emission spectroscopy was used to perfect the analysis of Li at the micrometric scale by means of laser-induced breakdown spectroscopy (LIBS). To estimate lithium content for different geological materials, LIBS calibration of the emission line at 670.706 nm was achieved by use of synthetic glasses and natural minerals. The detection limit for this method is ∼5 ppm Li. Three applications to geological materials show the potential of LIBS for lithium determination, namely for Li-bearing minerals, melt inclusions, quartz, and associated fluid inclusions.For spodumene and petalite from granite pegmatite dikes (Portugal), the Li₂O concentrations are 7.6 ± 1.6 wt% and 6.3 ± 1.3 wt%, respectively, by use of LIBS. These values agree with ion microprobe analyses, bulk analyses, or both. For eucryptite crystals, the Li concentrations are scattered because grain size is smaller than the LIBS spatial resolution (6 to 8 μm). Lithium concentrations of melt inclusions from the Streltsovka U deposit (Siberia) are in the range of 2 to 6.2 wt% (Li₂O) for Li-rich daughter minerals. Lithium estimations on silicate glasses display values between 90 and 400 ppm.Lithium was also analyzed as a trace element in quartz. Transverse profiles were performed in hydrothermal barren quartz veins from the Spanish Central System (Sierra de Guadarrama). The highest Li concentrations (250 to 370 ppm) were found in specific growth bands in conjunction with the observed variation in optical cathodoluminescence intensity. Considering the fluid inclusion analysis, the source of fluid responsible to the Li enrichment in quartz is probably high-salinity fluids derived from sedimentary basins.     

Computer simulation of the low-molecular-weight inorganic species distribution of antimony(III) and antimony(V) in natural waters

Computer speciation models of antimony in aqueous, multicomponent solutions of environmental and biological interest have been developed. Antimony is an element of increasing environmental significance but one whose chemical speciation has not previously been modelled in any comprehensive way. The available thermodynamic data have been critically evaluated and entered into a thermodynamic database. The JESS suite of computer programs has been used to develop the thermodynamically-consistent mass balance equations used for modelling purposes. The calculated speciation distributions successfully account for some but not all key known properties of antimony in natural waters. It is believed that the main difference is most likely due to kinetic factors.     

Direct observation of microcrack development in marble caused by thermal weathering

One of the properties that makes marble such an excellent construction and ornamental material is its low porosity. It is very difficult for water or decay agents to penetrate the internal structure of materials with no or few pores, so enhancing the durability of these materials. However, environmental temperature fluctuations bring about significant physical changes in marbles that result in an increase in porosity, due to the appearance of new microcracks and the expansion of existing ones. These cracks offer new paths into the marble which make it easier for solutions containing pollutants to penetrate the material. Thermal expansion tests were performed on three different types of marble known as White, Tranco, and Yellow Macael (Almeria, Spain), after which an increase in porosity (from 17 to 73% depending on marble type) was observed, mainly due to crack formation. The structural changes occurring during thermal expansion tests were more significant in the case of White Macael samples, a fact that is not only related to its mineralogical composition but also to the morphology of the grains, grain boundaries and crystal size. Our research suggests that thermally weathered White Macael marble could be more susceptible to decay by other contaminant agents than Tranco or Yellow Macael. The use of hot-stage environmental scanning electron microscopy is proposed as a valid tool for observing, both in situ and at high magnification, changes in the fracture system of building stones induced by thermal stress.     

The effect of isosaccharinic acid and gluconic acid on the retention of Eu(III), Am(III) and Th(IV) by calcite

Calcite is an important component of many potential host rocks currently under consideration for the disposal of radioactive wastes. Even in the chemically disturbed zone formed around a cementitious repository, this mineral remains largely unaffected by the hyper-alkaline waters migrating out of the near field. Thus, due to its abundance and geochemical stability, calcite could play an important role in the retardation of radionuclides released from a repository for nuclear wastes. Actinides are an important class of elements present in almost all radioactive waste streams, and for this reason, investigations of their retention behaviour under representative chemical conditions are particularly relevant to assessing safe disposal in the long term. Organic ligands originating from the degradation of cellulosic materials in the repository or present as cement additives could possibly reduce the retardation of tri- and tetravalent actinides due to the formation of stable metal–ligand complexes in solution. In this study, isosaccharinic acid (ISA) and gluconic acid (GLU) have been taken as representatives of cellulose degradation products and concrete admixtures, respectively. Batch-type sorption experiments have been conducted to investigate the effect of ISA and GLU on the retardation of ¹⁵²Eu, ²⁴¹Am and ²²⁸Th by calcite. ¹⁵²Eu and ²⁴¹Am are representatives of the trivalent lanthanides and actinides, respectively, and ²²⁸Th is a representative of the tetravalent actinides.High ISA and GLU concentrations in solution were found to significantly affect the sorption of the radionuclides. R_d values for Eu(III) and Am(III) decreased significantly at ISA concentrations above 10⁻⁵ M and at GLU concentrations above 10⁻⁷ M. The critical concentration limits were similar for Th(IV), that is 2 × 10⁻⁵ M in the case of ISA and 10⁻⁶ M in the case of GLU. The effects of ISA and GLU on the immobilisation of Eu(III), Am(III) and Th(IV) were interpreted in terms of complex formation in solution. In the case of Eu(III) and Am(III) in ACW, the metal–ligand complexes were found to have a 1:1 stoichiometry. Complexation constants of these aqueous ISA and GLU complexes with Eu(III) and Am(III) were determined and discussed in connection with the presently unclear situation concerning the stability constant of the $\mathrm{Eu}(\mathrm{OH}{)}_3^0$ species. In the case of Th(IV) in ACW, it was assumed that a Th(IV)–ISA–Ca complex and a Th(IV)–GLU–Ca complex are formed, both having a 1:2:1 stoichiometry. The complexation constants of these complexes were determined and compared with the literature data.Assuming maximum concentrations for ISA and GLU in the pore water of the disturbed zone of a cementitious repository based on representative near-field conditions ([ISA]_aq = 3 × 10⁻⁶ M, [GLU]_aq = 10⁻⁷ M), it is predicted that the formation of aqueous ISA and GLU complexes would not significantly affect Eu(III), Am(III) and Th(IV) sorption on calcite.