Catherine Galindo

Surface reactivity of α-Al2O3 and mechanisms of phosphate sorption: In situ ATR-FTIR spectroscopy and ζ potential studies

We have investigated the effect of solution parameters on the adsorption of phosphate ions and on charges and structures, i.e., on the nature of species, at the alpha-Al(2)O(3) colloid/solution interface by using the batch method, zeta potential measurements, and in situ ATR-FTIR spectroscopy. The uptake of phosphate decreases with the extent of surface deprotonation (i.e., pH), imparts negative charges to the colloid surface, and induces IEP shifts showing chemical sorption. Use of complementary techniques provides evidence that phosphate is sorbed at low pH (3.3) by a combination of surface reactions of complexation and precipitation, whose relative contributions depend on phosphate loading. Surface complexation includes fast reactions of ligand exchange with single coordinated hydroxyls, and electrostatic attraction of H(2)PO(4)(-) ions at positively charged surface sites. This is supported by experiments at low coverage showing sharp and linear decrease of zeta potential (i.e., surface charge) with amount of phosphate sorbed. At high coverage, zeta potential values are low and independent of phosphate loading. Formation of surface precipitates of Al-phosphate is inferred from the assignment of the ATR-FTIR absorption band at 1137cm(-1), whose intensity increases with phosphate solution content and reaction time, to the P-O-stretching vibration mode for phosphate sorbed at high concentrations on alpha-Al(2)O(3). In situ ATR-FTIR spectroscopy reveals also structural reorganizations of surface hydroxyls with time, due to surface hydration and to surface precipitation continuing over extended periods along alumina dissolution.

TRLFS evidence for precipitation of uranyl phosphate on the surface of alumina: environmental implications.

We studied the ligand-enhanced sorption of uranyl ions (1-12 μM) on α-alumina colloids suspended in (and pre-equilibrated with) solutions at various concentrations of phosphate ions (P(T) = 0-900 μM). A highly sensitive technique, time resolved laser-induced fluorescence spectroscopy (TRLFS), was used to examine the chemical speciation of uranyl sorbed at trace concentrations (0.4-4 μmol U·g⁻¹). The suspensions with P(T) ≥ 100 μM exhibited high uranyl adsorption, and a very high intensity of fluorescence that increased with the sorbed amounts of phosphate and uranyl. These samples exhibited similar spectral and temporal characteristics of fluorescence emission, evidencing a uniform speciation pattern and a single coordination environment for sorbed U, despite large variation in parameters such as aqueous uranyl speciation, U loading, and extent of coverage of alumina by secondary Al phosphates precipitating on the surface. The results pointed formation of surface precipitates of uranyl phosphates, which are characterized by high quantum yield, peak maxima at positions similar to those of U(VI) phosphate minerals and four lifetimes indicating distortions, in-homogeneities or varying number of water molecules in the lattice. The findings have major implications for our understanding of the mechanisms of immobilization of U at trace levels on surfaces of oxides submitted to phosphated solutions in soils with low pH.

Mechanisms of uranyl and phosphate (co)sorption: Complexation and precipitation at α-Al2O3 surfaces

This study presents new in situ electrophoretic and ATR-FTIR data on the surface species controlling the cosorption of uranyl and phosphate ions in alpha-Al(2)O(3) suspensions at acidic pH (3.3). It was shown that the uranyl sorption (i) was promoted in the presence of phosphate, (ii) induced significant changes in zeta potential of P-loaded alumina, and (iii) was governed by two mechanisms, surface complexation and surface precipitation, with the predominant species being mainly dependent on phosphate surface coverage. Formation of surface precipitates of uranyl phosphate at high phosphate surface coverage was inferred from the high negative charges imparted to the surface by uranyl and phosphate (co)sorption, and from assignments of IR bands at 1107, 1024, and 971 cm(-1) to P-O-stretching vibrations for phosphate coordinated to uranyl, at the alumina surface. The ATR-FTIR study showed that the precipitates of uranyl phosphate formed at the surface of alpha-Al(2)O(3) for aqueous concentrations of uranyl at trace levels. It also evidenced that formation of surface precipitates of U(VI)-phosphate was occurring along with the transformation of alumina into secondary surface precipitates of Al-phosphate, at very high phosphate concentrations. These findings are relevant to the mechanisms of adsorption of trace uranyl on naturally occurring oxide surfaces, in soils with low pH where cosorption of phosphate and uranyl ions is known to play a crucial role in the long-term retention of U.

Molecular level description of the sorptive fractionation of a fulvic acid on aluminum oxide using electrospray ionization Fourier transform mass spectrometry.

We addressed here, by means of electrospray ionization mass spectrometry (ESI-MS) with ultrahigh resolution, the molecular level fractionation of a reference fulvic acid (SRFA) during its sorption at an alumina surface, taken as a model for surfaces of natural aluminum oxide hydrates. Examination of ESI-MS spectra of a native SRFA solution and of supernatants collected in sorption experiments at acidic pH showed that the ∼5700 compounds identified in the native solution were partitioned between the solution and alumina surface to quite varying degrees. Compounds showing the highest affinity for the surface were aromatic compounds with multiple oxygenated functionalities, polycyclic aromatic compounds depleted of hydrogen and carrying few oxygenated groups, and aliphatic compounds with very high O/C values, highlighting the fact that SRFA constituents were sorbed mainly via chemical sorption involving their oxygenated functionalities. We observed an inverse correlation between the degree of sorption of a molecule within a CH2 series and its number of CH2 groups and a positive correlation between the degree of sorption and the number of CO2 groups in a COO series, which was remarkable. These correlations provide evidence at the molecular scale that molecule acidity is the key parameter governing fulvic acid (FA) sorptive fractionation, and they are useful for predicting sorption of FA at a natural oxide surface.

Chemical fractionation of a terrestrial humic acid upon sorption on alumina by high resolution mass spectrometry

Understanding the fractionation of humic acids (HA) during their sorption at mineral-solution interfaces is one of the major issues of soil and environmental sciences. Molecular-scale investigations have been conducted on the fractionation of a terrestrial HA (more precisely, of the water-soluble fraction of Aldrich HA denoted WSAHA) – rich in highly condensed aromatic compounds – during its sorption at acidic pH on alumina particles, which were taken as surrogates of Al oxide hydrates existing in soils. High-resolution mass spectrometry combined with electrospray ionization and atmospheric pressure chemical ionization was used for analysing WSAHA solutions before and after their contact with alumina. The sorption process was found to lead to enrichment of the highly reactive, acidic, oxygen-functionalized aromatic and aliphatic molecules, and of highly condensed aromatic compounds depleted in hydrogen carrying only a few oxygenated groups on the alumina surface. In contrast, the poorly oxygenated aliphatic constituents and aromatic compounds of O/C values in the range 0.2 to 0.5 remained preferentially in the solution. By comparing results obtained for homologous compounds whose elemental composition differed only by the number of CO2 groups, evidence is found that both molecular acidity and degree of molecular hydrophobicity influence the degree of sorption (via ligand exchange on the surface and hydrophobic interactions, respectively) of WSAHA compounds of highly condensed aromatic type. Evidence at the molecular scale is provided that molecular acidity and hydrophobicity are the determining factors that control the size-fractionation of WSAHA during sorption on alumina.

Trace Level Uranyl Complexation with Phenylphosphonic Acid in Aqueous Solution: Direct Speciation by High Resolution Mass Spectrometry

The complexation of U(VI) by organic P-containing ligands in humic substances (HS) is an important issue of uranyl mobility in soil. We have investigated the complexation of uranyl by a model ligand for aromatic phosphorus functionalities in HS, phenylphosphonic acid, by using ultrahigh resolution electrospray ionization-mass spectrometry (ESI-MS). The high sensitivity permitted to investigate the complexation of trace level uranyl and to explore directly in the native aqueous solutions the nature of the uranyl-phenylphosphonate complexes. Positive identification of the complexes coexisting in solutions with low pH and varying ligand-to-metal ratio was achieved thanks to the high resolving power, high mass accuracy, and reliability of ion abundance of the technique. The positively charged and neutral uranyl species were detected simultaneously on negative ion mass spectra, evidencing formation of three types of U(VI)-phenylphosphonate complexes. Two complexes with a metal-to-ligand stoichiometry of 1:1 (in the monoprotonated and nonprotonated forms) existed in solutions at pH 3-5, and a 1:2 complex was additionally formed at relatively high ligand-to-metal ratio. A strategy based on the use of uranyl-phosphate solution complexes as internal standards was developed to determine from the ESI(-)MS results the stability constants of the complexes, which were calculated to be log K111 = 3.4 ± 0.2 for UO2(HPhPO3)(+), log K101 = 7.1 ± 0.1 for UO2PhPO3, and log K112 = 7.2 ± 0.2 for UO2(HPhPO3)2. The speciation model presented here suggests that organic P existing at low concentration in HS is involved significantly in binding by humic and fulvic acids of trace level uranyl in soil.

Sequential Extraction of U and Th Isotopes: Study of Their Intrinsic Distribution in Phosphate and Limestone Sedimentary Rock in Comparison with Black Shale

In this work, a study of the intrinsic behavior of U and Th isotopes in sedimentary rocks was performed by sequential extraction. The results for the phosphate layers and limestone interlayers of Oulad Abdoun Basin are compared to those obtained previously for Timahdit black Shale rocks. Characterization car-ried out by ICP-AES and SEM-EDX showed that apatite, calcite, dolomite, quartz and clays are the essential components of the inorganic matrix. The selective leached phases are treated radiochemically before measuring U and Th isotopes by alpha spectrometry. Among the results, it should be noted that the U isotopes were concentrated in an anaerobic environment. Humic acids that are the richest organic phase in U are responsible for the concentration of 234U, 238U in the carbonates and apatite. In the mineral phase, 232Th, of terrigenous origin is partitioned between the silicate and pyrite minerals.

Radiochemical methods analysis of U and Th: metrological and geochemical applications

A present conference describes the methods for determination of U and thorium isotopes in natural samples and the study of their adsorption by the new activated adsorbents. The first part was carried out by using alpha spectrometry. The thin alpha sources were prepared by chemical electroplating of U and Th. Both elements were separated by using the ion-exchanges chromatography before their deposition. The dissolution of major solid sample was made by using alkaline fusion by LiB4O7 and LiBO2. The proposed methods have been applied for U, Th isotopes determination in the sedimentary rock: phosphate and its derivatives products and in the extracted solution from the black shale. The second part of the work was focused on the study of U, Th isotopes adsorption by new oil shale activated adsorbents.