The adsorption mechanism of U(VI) and Eu(III) on carbonaceous nanofibers (CNFs) was investigated using batch, IR, XPS, XANES, and EXAFS techniques. The pH-dependent adsorption indicated that the adsorption of U(VI) on the CNFs was significantly higher than the adsorption of Eu(III) at pH < 7.0. The maximum adsorption capacity of the CNFs calculated from the Langmuir model at pH 4.5 and 298 K for U(VI) and Eu(III) were 125 and 91 mg/g, respectively. The CNFs displayed good recyclability and recoverability by regeneration experiments. Based on XPS and XANES analyses, the enrichment of U(VI) and Eu(III) was attributed to the abundant adsorption sites (e.g., −OH and −COOH groups) of the CNFs. IR analysis further demonstrated that −COOH groups were more responsible for U(VI) adsorption. In addition, the remarkable reducing agents of the R-CH2OH groups were responsible for the highly efficient adsorption of U(VI) on the CNFs. The adsorption mechanism of U(VI) on the CNFs at pH 4.5 was shifted from inner- to out...

Macroscopic and Microscopic Investigation of U(VI) and Eu(III) Adsorption on Carbonaceous Nanofibers.

The sorption of Eu(III) on anatase and rutile was studied as a function of ionic strength, humic acid (HA, 7.5 mg/L), and electrolyte anions over a large range of pH (2−12). The presence of HA significantly affected Eu(III) sorption to anatase and rutile. The sorption of Eu(III) on anatase and rutile was independent of ionic strength. Results of an X-ray photoelectron spectroscopy (XPS) analysis showed that Eu(III) was chemically present within the near-surface of TiO2 due to the formation of ≡SOEu and ≡SOHAEu complexes. An extended X-ray absorption fine structure (EXAFS) technique was applied to characterize the local structural environment of the adsorbed Eu(III), and the results indicated that Eu(III) was bound to about seven or eight O atoms at a distance of about 2.40 A. The functional groups of surface-bound HA were expected to be involved in the sorption process. The measured Eu−Ti distance confirmed the formation of inner-sphere sorption complexes on a TiO2 surface.

Eu(III) sorption to TiO2 (anatase and rutile): batch, XPS, and EXAFS studies.

Biogeochemical controls on metal behaviour in freshwater environments

To identify the effect of humic acid (HA) and fulvic acid (FA) on the sorption mechanism of Eu(III) on organic--inorganic colloids in the environment at a molecular level, surface adsorbed/ complexed Eu(III) on hydrous alumina, HA-, and FA-hydrous alumina hybrids were characterized by using X-ray photoelectron spectroscopy (XPS) and time-resolved laser fluorescence spectroscopy (TRLFS). The experiments were performed in 0.1 mol/L KNO3 or 0.1 mol/L NaClO4 under ambient conditions. The pH values were varied between 2 and 11 at a fixed Eu(III) concentration of 6.0 x 10(-7) mol/L and 4.3 x 10(-5) mol/L. The different Eu(III)/FA(HA)/hydrous alumina complexes were characterized by their fluorescence emission spectra ((5D0-F1)/ (5D0 --> 7F2)) and binding energy of Eu(III). Inner-sphere surface complexation may contribute mainly to Eu(III) sorption on hydrous alumina, and a ternary surface complex is formed at the HA/ FA-hydrous alumina hybrid surfaces. The sorption and species of Eu(III) in ternary Eu-HA/FA-hydrous alumina systems are not dominated by either HA/FA or hydrous alumina, but are dominated by both HA/FA and hydrous alumina. The results are important for understanding the sorption mechanisms and the nature of surface adsorbed Eu(III) species and trivalent chemical homologues of Eu(III) in the natural environment.

Sorption of Eu(III) on humic acid or fulvic acid bound to hydrous alumina studied by SEM-EDS, XPS, TRLFS, and batch techniques.

Des substances humiques de reference (acides fulviques et humiques) ont ete utilisees pour mettre en evidence l'adsorption de la matiere organique dissoute naturelle (MOD) sur des surfaces biologiques. Les experiences ont ete realisees a des valeurs de pH (4-7) et a des concentrations en MOD (0-20 mg.L -1 ) representatives de celles rencontrees dans le milieu naturel, et a des forces ioniques entre 0,02 a 0,1 M. Comme modeles biologiques nous avons employe des cellules phytoplanctoniques (Chlorella pyrenoidosa, Synechococcus leopoliensis) ainsi que des cellules de branchies de poisson (Salmo salar). La charge superficielle des cellules etait initialement negative; en presence de la MOD elle devenait plus negative, tel que demontre par des mesures de mobilite electrophoretique. Cet effet de la MOD etait plus important a pH 4 qu'a pH 7, ce qui suggere que l'interaction de la MOD a la surface de la cellule implique soit la formation de liens hydrogene (entre des groupements fonctionnels electronegatifs presents au sein de la MOD et a la surface biologique), soit la formation de liaisons hydrophobes entre la surface cellulaire et des regions hydrophobes de la MOD. L'adsorption de la MOD a la surface des cellules a ete confirmee par des mesures directes, ou des pertes de MOD de la phase dissoute ont ete suivies apres addition de cellules phytoplanctoniques, de meme que par l'observation de l'association de substances humiques aux surfaces algales a l'aide de la microscopie electronique a transmission. Les resultats de ces experiences demontrent que la MOD interagit avec des surfaces vivantes; cette interaction pourrait influer sur les processus physiques et chimiques importants qui se deroulent a l'interface cellule-solution.

Accumulation of natural organic matter on the surfaces of living cells: implications for the interaction of toxic solutes with aquatic biota

The influence of humic acid on the adsorption of europium onto inorganic colloids as a function of pH

The adsorption of humic acid (HA) and Eu onto bentonite, kaolin, montmorillonite and quartz in the presence of HA has been studied as a function of pH (2 to 10). HA adsorption onto the mineral surfaces decreased with increasing pH, probably due to both electrostatic and specific site interactions. The zeta potentials of the clay systems were also investigated and the results showed that the zeta potentials decreased with increasing pH due to deprotonation and that the mineral surfaces were negatively charged across the pH range. HA only affected the zeta potential of the montmorillonite system, conferring a more negative charge on the surface. Eu (1 X10\" mol Γ) adsorption to HA free mineral surfaces generally increased with increasing pH. The mechanism appeared not to be purely electrostatic, but due to metal speciation and the nature of the surface, i.e. specific site binding. In the presence of HA, Eu adsorption generally increased or stayed similar at pHs up to 5. This was due to adsorption of EuHA complexes on the mineral surface. At higher pH, adsorption was decreased, reaching a minimum at around pH 6 to 7 and then increasing again up to a pH of around 10. This was probably due to both the desorption of EuHA complexes from the mineral surface and possibly because inorganic Eu species were being blocked from adsorbing to the colloid surface by adsorbed HA. It may be concluded that if mineral surfaces, either colloidal or stationary, are to be included in metal speciation and transport codes, a thorough understanding of the ternary interactions of natural organic matter (NOM) with toxic metals and minerals must be gained. Ternary metal interactions may also occur. For example, the possibility of mixed complexes of humic acid, europium and low molecular weight organics have been examined [7], Previous work has suggested that both colloidal and non-colloidal mineral surfaces in the environment may be coated to some extent by natural organic matter (NOM) [8—23], conferring a more negative surface charge to the mineral [24] and influencing colloidal stability [25, 26] and mobility [27]. If such HA coatings do exist on mineral surfaces then ternary NOM-metal mineral interactions must be considered and understood if they are to be included in metal speciation and migration codes. However, at present, this information is still limited to a few ternary systems which have been investigated under a limited range of conditions [28—38]. Inorganic surfaces in the environment often occur as colloidal and non-colloidal oxy/hydroxides of Mn, Fe, Al and Si, as well as carbonates and clays [39— 41], In this paper we report investigations on the adsorption of humic acid (HA) to a range of minerals (quartz, bentonite, kaolin and montmorillonite), over a range of pH and report the effect this has on the zeta potential of the clays. The influence of humic acid on the adsorption of europium to these minerals is also reported, europium being chosen as an analogue to the trivalent actinides (i.e. Am and Pu).

Steve Richardson

Papers

The influence of humic acid on the adsorption of europium onto inorganic colloids as a function of pH

The Effect of pH on Europium-Mineral Interactions in the Presence of Humic Acid