Relations entre structure et réactivité dans l’interaction entre les substances humiques, les polluants métalliques du cycle du nucléaire et les surfaces minérales

This document proposes an analysis of the structure-reactivity relationships in the interaction between humic substances, metallic pollutants from the nuclear cycle, and mineral surfaces. It composes the scientific document, which allowed the author to defend a habilitation degree. It is mainly focused on the research works into which the author have been involved in on this particular thematic. Humic substances are issued from the degradation of the living. They have an important influence onto migration of metals in the environment. They are showing particular intrinsic physic and chemical, metal complexation, and adsorption onto mineral surfaces properties, which render the global comprehension of the different mechanisms somehow difficult. These three aspects are covered in this document. The first part is dedicated to the studies on composition, structure, and organization of humic substances, which cannot be considered as a well-defined type of chemical. They are a heterogeneous degradation product with a supramolecular organization, which is showing fractal properties from fractions up to several nanometers. Second part is on the complexation reactions. The different modelling strategies come from the difficulties on apprehending composition, structure, and organization of humic substances. The different models used are showing more or less strongly empiric characteristics. They can be derived from the mass action law, or explicitly account for heterogeneity, acido-basic, or ionic strength related parameters. The third and latter part covers the adsorption studies. The main property is adsorptive fractionation, which induces modification of chemical composition of humic substances between the surface and the solution. It also induces modification of complexation properties between the adsorbed and non-adsorbed fractions. Because of adsorptive fractionation, and the particular influence of ionic strength on humic substances, and of complexed metals, adsorption, the studies on simple model molecules to represent humic substances reactivity are an illusion. The global comprehension will mainly be obtained from the understanding of dedicated studies.Ce document présente une analyse des relations entre structure et réactivité dans l’interaction entre les substances humiques, les polluants métalliques du cycle du nucléaire et les surfaces minérales. Il reprend le document scientifique qui a permis la soutenance d’une habilitation à diriger les recherches, et à ce titre se concentre principalement sur les recherches menées ou animées par l’auteur sur cette thématique. Les substances humiques sont issues de la dégradation du vivant, et possèdent une influence importante sur la migration des métaux dans l’environnement. Elles présentent des propriétés physico-chimiques intrinsèques, des capacités de complexation des métaux et d’adsorption sur les surfaces minérales qui font qu’une compréhension globale des différents mécanismes en jeu est souvent difficile. Ces trois aspects sont abordés dans ce document. La première partie est consacrée aux études sur la composition, la structure et l’organisation des substances humiques. Les substances humiques ne constituent pas un type de molécules chimiques bien définies, mais plutôt un mélange hétérogène de produits de dégradation, ayant une organisation supramoléculaire et fractale sur une grande échelle de taille, depuis la fraction de nanomètre jusqu’à plusieurs dizaines de nanomètres. La deuxième partie aborde les réactions de complexation. Les différentes stratégies de modélisation sont la conséquence des difficultés de compréhension de la composition, de la structure et de l’organisation. Les modèles utilisés possèdent des caractéristiques plus ou moins fortement empiriques. Ils peuvent être dérivés de la loi d’action des masses avec des paramètres extra-thermodynamiques, ou bien prendre en compte implicitement des paramètres liés à l’hétérogénéité, aux réactions acido-basiques, à la force ionique, et aux réactions de compétitions. La troisième et dernière partie couvre les études d’adsorption et les comportements particuliers des substances humiques. La propriété principale est le fractionnement par les surfaces qui induit des modifications de compositions des substances humiques entre la surface et la solution, et par la même des différences de propriétés de complexation des métaux entre fraction à la surface et en solution. Ce fractionnement, ainsi que l’influence particulière de la force ionique sur l’adsorption des substances humiques, et des métaux complexés, font qu’il est illusoire de vouloir représenter la réactivité des substances humiques par des composés organiques simples, et que la compréhension globale de ces systèmes ne peut être atteinte que par des études dédiées

Modelling Metal-Humic Substances/Surface Systems: Reasons for Success, Failure and Possible Routes for Peace of Mind

International audienceIron oxides and oxy-hydroxides are commonly of considerable importance in the sorption of ions onto rocks, soils and sediments. They can be the controlling sorption phase even if they are present in relatively small quantities. In common with other oxides and clay minerals, the sorption pH-edge of metals is directly linked to their hydrolysis: the higher the residual charge on the metal ion, the lower the pH-edge. Modelling of this process has been successfully carried out using different microscopic or macroscopic definitions of the interface (e.g. surface complexation or ion exchange models that may or may not include mineralogical descriptions). The influence of organics on the sorption of many metals is significant. This organic material includes simple organic molecules and more complex exopolymeric substances (e.g., humic substances) produced by the decay of natural organic matter. Sorption of these organics materials to mineral surfaces has also been the subject of a large body of work. The various types of organics do not share the same affinities for minerals in general, and for iron oxides and oxy-hydroxides in particular. In those cases in which successful models of the component binary systems (i.e., metal/surface, metal/organic, organic/surface) have been developed, the formation of mixed surface complexes, the evolution of the surface itself, the addition order in laboratory systems, and the evolution of natural organic matter fractions during sorption, have often precluded a satisfying description of the metal/surface/organic ternary system over a sufficiently wide ranges of parameter values (i.e. pH, ionic strength, concentration of humic substances). This manuscript describes the reasons for some successes and failures in the modelling of the ternary systems. Promising recent advances and possible methods of providing more complete description of these intricate systems are also discussed

Contribution of capillary electrophoresis to an integrated vision of humic substances size and charge characterizations

International audienceThe physicochemical properties of three different humic substances (HS) are probed using capillary zone electrophoresis in alkaline carbonate buffers, pH 10. Special attention is drawn to the impact of the electrolyte ionic strength and counter-ion nature, chosen within the alkali-metal series, on HS electrophoretic mobility. Taylor-Aris dispersion analysis provides insights into the hydrodynamic radius (R$_H$) distributions of HS. The smallest characterized entities are of nanometric dimensions, showing neither ionic strength- nor alkali-metal-induced aggregation. These results are compared with the entities evidenced in dynamic light scattering measurements, the size of which is two order of magnitude higher, $ca$. 100 nm. The extended Onsager model provides a reasonable description of measured electrophoretic mobilities in the ionic strength range 1-50 mM, thus allowing the estimation of limiting mobilities and ionic charge numbers for the different HS samples. An unexpected HS electrophoretic mobility increase (in absolute value) is observed in the order Li$^+$ < Na$^+$ < K$^+$ < Cs$^+$ and discussed either in terms of retarding forces or in terms of ion-ion interactions

Bi-exponential decay of Eu(III) complexed by Suwannee River humic substances: Spectroscopic evidence of two different excited species

International audienceThe bi-exponential luminescence decay of europium (III) complexed by Suwannee River fulvic acid (SRFA) and humic acid (SRHA), is studied in time-resolved luminescence spectroscopy using two different gratings at varying delay after the laser pulse, increasing accumulation time in order to obtain comparable signals. The two hypotheses found in the literature to interpret this bi-exponential decay are (i) a back transfer from the metal to the triplet state of the organic ligand and (ii) the radiative decay of two different excited species. It is shown that evolutions of the $^5$D0$\rightarrow$$^7$F$_0$ and $^5$D0$\rightarrow$$^7$F$_2$ luminescent transitions are occurring between 10 and 300 $\mu$s delay. First, the $^5$D0$\rightarrow$$^7$F$_0$ transition is decreasing relative to the $^5$D0$\rightarrow$$^7$F$_1$ showing a slightly greater symmetry of the ‘slow' component, and is also slightly red shifted. Second, a slight modification of the $^5$D0$\rightarrow$$^7$F$_2$ transition is also evidencing a slightly different ligand field splitting. No significant modification of the $^5$D0$\rightarrow$$^7$F$_1$ magnetic dipole, which is less susceptible to symmetry changes, is noted in line with expectations. The $^5$D0$\rightarrow$$^7$F$_0$ transitions are adjusted with either one or two components. The use of a simple component fit seems to be well adapted for representing an average comportment of these heterogeneous compounds, and a two-component fit constrained by the bi-exponential decay parameters and accumulation times yields in the proposition of the spectra for the fast and slow components

Influence of addition order and contact time on thorium(IV) retention by hematite in the presence of humic acids

International audienceThe influence of addition order and contact time in the system hematite (alpha-Fe2O3)-humic acid (HA)-thorium(IV) (Th(IV)) was studied in batch experiments. Th(IV) is considered here as a chemical analogue of other actinides (IV). The sorption isotherms were acquired varying pH in the range 2-10 and HA concentration in the range 1-100 mg/L. As already observed by numerous authors, Th(IV) retention was hindered when HA and hematite were equilibrated beforehand during 24 h. As it has been observed in a previous study, this effect was drastic when the ratio between humic and surface (iron oxide) sites exceeds a critical value. However, when HA was added after a 24-h equilibration of the hematite-Th(IV) system, Th(IV) was barely desorbed from the iron oxide surface. Furthermore, no drastic effect of the ratio between humic and surface sites could be evidenced, as the increase of HA concentration only results in a slight monotonic decrease in Th(IV) retention. Increasing contact time between components of the systems only indicated slight Th(IV) retention variation. This was interpreted as a consequence of slow kinetic controls of both the Th(IV)-HA complexation and HA-hematite sorption

Sorption of Aldrich Humic Acids onto Hematite: Insights into Fractionation Phenomena by Electrospray Ionization with Quadrupole Time-of-Flight Mass Spectrometry

International audienceSorption induced fractionation of purified Aldrich humic acid (PAHA) on hematite is studied through the modification of electrospray ionization (ESI)quadrupole time-of-flight (QToF) mass spectra of supernatants from retention experiments. The ESI mass spectra show an increase of the “mean molecular masses” of the molecules that constitutes humic aggregates. The low molecular weight fraction (LMWF; m/z ≤ 600 Da) is preferentially sorbed compared to two other fractions. The resolution provided by ESI-QToF mass spectrometer in the low-mass range provided evidence of further fractionation induced by sorption within the LMWF. Among the two latter fractions, the high molecular weight fraction (HMWF; m/z ≈ 1700 Da) seems to be more prone to sorption compared to the intermediate molecular weight fraction (IMWF; m/z ≈ 900 Da). The IMWF seems to be more hydrophilic as it should be richer in O, N and alkyl C from the proportion of even mass, and poorer in aromatic structures from mass defect analysis in ESI mass spectra

Uptake of anionic radionuclides onto degraded cement pastes and competing effect of organic ligands

http://www.radiochimacta.deInternational audienceHardened cement pastes (HCP) present a high affinity with a lot of radionuclides (RN) and can be used as waste confining materials in radioactive waste repository. Indeed, in cementitious media, RN can be removed from solution via (co)precipitation reactions or via sorption/diffusion mechanisms. In this study, the affinity of anionic RN (Cl−, I−, SeO32− and CO32− chemical forms) with a CEM-I HCP has been studied vs. the degradation of the HCP particles. These RN are considered as mobile in repository media and it is important to have a set of distribution ratio (Rd) in cement environment. The Rd values have been measured in batch experiments as a function of the pH, used as the degraded state parameter of the HCP suspensions. The Rd values increase in all cases, from the unaltered state (pH 13.3) to the altered state of HCP, i.e. until all portlandite is dissolved, corresponding to pH 12.6. Then, Rd values decrease until degraded states (pH 12.0), corresponding to the decalcification of the calcium silicate hydrate (C-S-H) phases. The behaviour of anionic RN seems to be correlated to the evolution of calcium concentration and is opposed to the evolution of sulphate concentration in solution which could have a competing effect. Comparison is done with the behaviour of caesium and uranium(VI), which is a cationic RN but has a major negative hydrolysed species at high pH. As awaited, the uranium(VI) behaviour is very different from purely anionic RN one in accord with spectroscopic analyses from literature works. The Rd values have also been measured for the organic ligands isosaccharinate (ISA) and EDTA. The uptake of ISA can be important and competing effect with the sorption of SeO32− has been evidenced in HCP suspensions as a function of the ISA concentration

Conditional stability parameters for the actinides (IV)-humic acid system: a search for consistency

http://www.radiochimacta.deInternational audienceThe coherence of actinide(IV) complexation by humic substances (HS) is reviewed and new data are proposed. In a first attempt, the values of independent data from literature on Th(IV), U(IV), and Pu(IV) are collected, selected, and compiled. The data obtained follow the “classical” trend of increasing conditional formation “constants” with pH, led both by the increasing ionisation of HS and by the extensive hydrolysis of the tetravalent actinides. Even though a fair agreement is evident, the experimental uncertainties do not permit a full analogy between the actinides(IV) to be ascertained. In a second attempt, the experiments from which the original data are available were reinterpreted using only one hydrolysis constant set for U(IV) as an example, considering that all actinides(IV) have analogous humic complexation behaviour. Hence, the obtained evolution of conditional formation “constants” is much more coherent and the uncertainties do not permit to distinguish an actinide(IV) from one another. The obtained data are then applied to independent laboratory and in situ experiments in order to delimit the domain of possible applicability. This exercise demonstrates the treatment of data through analogy in the case of actinides(IV) and would permit to limit and orientate the number of necessary, but difficult, experiment with redox sensitive elements like U, Np, or Pu. It also demonstrates that complexation-only mechanisms may not be sufficient to understand field observations

Spectral and temporal luminescent properties of Eu(III) in humic substances solutions from different origins

International audienceAlthough a high heterogeneity of composition is awaited for humic substances, their complexation properties do not seem to greatly depend on their origins. The information on the difference in the structure of these complexes is scarce. To participate in the filling of this lack, a study of the spectral and temporal evolution of the Eu(III) luminescence implied in humic substance (HS) complexes is presented. Seven different extracts, namely Suwannee River fulvic acid (SRFA) and humic acid (SRHA), and Leonardite HA (LHA) from the International Humic Substances Society (USA), humic acid from Gorleben (GohyHA), and from the Kleiner Kranichsee bog (KFA, KHA) from Germany, and purified commercial Aldrich HA (PAHA), were made to contact with Eu(III). Eu(III)-HS time-resolved luminescence properties were compared with aqueous Eu$^{3+}$ at pH 5. Using an excitation wavelength of 394 nm, the typical bi-exponential luminescence decay for Eu(III)-HS complexes is common to all the samples. The components $\tau$1 and $\tau$τ2 are in the same order of magnitude for all the samples, i.e., 40 $\leq$ $\tau$1 ($\mu$s) $\leq$ 60, and 145 $\leq$ $\tau$2 ($\mu$s) $\leq$ 190, but significantly different. It is shown that different spectra are obtained from the different groups of samples. Terrestrial extract on the one hand, i.e. LHA/GohyHA, plus PAHA, and purely aquatic extracts on the other hand, i.e., SRFA/SRHA/KFA/KHA, induce inner coherent luminescent properties of Eu(III) within each group. The $^5$D0 $\rightarrow$ $^7$F$_2$ transition exhibits the most striking differences. A slight blue shift is observed compared to aqueous Eu$^{3+}$ ($\lambda_{max}$ = 615.4 nm), and the humic samples share almost the same $\lambda_{max}$ $\approx$ 614.5 nm. The main differences between the samples reside in a shoulder around $\lambda$$\approx$ 612.5 nm, modelled by a mixed Gaussian–Lorentzian band around $\lambda$$\approx$ 612 nm. SRFA shows the most intense shoulder with an intensity ratio of $I_{612.5}$/$I_{614.7}$ = 1.1, KFA/KHA/SRHA share almost the same ratio $I_{612.5}$/$I_{614.7}$ = 1.2–1.3, whilst the LHA/GohyHA/PAHA group has a $I_{612.5}$/$I_{614.5}$ = 1.5–1.6. This shows that for the two groups of complexes, despite comparable complexing properties, slightly different symmetries are awaited

Reproducibility of the uptake of U(VI) onto degraded cement pastes and calcium silicate hydrate phases

http://www.radiochimacta.deInternational audienceThe U(VI) uptake in degraded cement pastes was undertaken in the laboratories of CEA/L3MR and SUBATECH in order to check the reproducibility of the study. Two well hydrated cement pastes, CEM I (Ordinary Portland Cement, OPC) and CEM V (blast furnace slag (BFS) and fly ash added to OPC) were degraded using similar protocols. Equilibrium solutions and solid materials were characterised for three degradation states for each paste. All samples are free of portlandite and the pH of the equilibrated cement solutions vary in the range 9.8–12.2. Three calcium silicate hydrate phases (C-S-H) were synthesised in order to compare the sorption properties of degraded cement pastes and of hydrate phases in similar pH conditions. In order to avoid precipitation processes, the operational solubility limit was evaluated before batch experiments. These solubility values vary significantly in the pH range [9–13] with a 2.4×10−7 mol/L minimum at pH close to 10.5. In batch sorption experiments, the distribution ratio Rd values are high: 30000–150000 mL/g. The uptake of U(VI) increases when comparing the least and the most degraded cement pastes whereas the initial composition of cement has relatively insensitive effect. Sorption isotherms, expressed as a log [U(VI)solid]/ log [U(VI)solution] plots are linear. A slope of 1 is calculated indicating the predominance of sorption processes. As sorption and desorption values are close, the uptake mechanism seems reversible. The Rd values measured in C-S-H suspensions are in good agreement with Rd values of degraded cement pastes, and C-S-H materials could be one of the cementitious phases which control U(VI) uptake in cement pastes