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In ultra-low-temperature experiments at 4.7 K the luminescence of Eu(III) bound to different hydroxy- and methoxybenzoic acids and to humic substances (HS) was investigated. The benzoic acid derivatives were used as simple model compounds for common metal-binding structures in HS. The Eu(III) luminescence was directly excited by means of a pulsed dye laser, scanning through the D-5(0) -> F-7(0) transition of Eu(III) and subsequently, high-resolution total luminescence spectra (TLS) were recorded. Based on the thorough analysis of the high-resolution TLS conclusions were drawn with respect to the number of different complexes formed and to the symmetry of the complexes. The crystal-field strength parameter N-nu(B-2q) was dependent on the electrostatic forces induced by the ligands as well as on the symmetry of the complexes. The formation of thermodynamically stable complexes was found to be slow even for small model ligands such its 2-hydroxybenzoic acid. Comparison between the model compounds and HS clearly revealed that the carboxylate group is the dominant binding site in HS. Indices for the formation of chelates, e.g. similar to 2- hydroxybenzoic acid, were not found for HS.
Species-related luminescence-structure relationships in europium-exchanged mesoporous material
(2009)
Europium exchanged into a mesoporous material (Zeotile-1) was extensively characterized with respect to the Si/ Al ratio and surface silylation by using time-resolved emission spectroscopy. Qualitative as well as quantitative details of the europium species-related luminescence-structure relationships were obtained and discussed such as the decay associated spectra, local distortion and structure of the bonding environment, crystal-field strength, radiative relaxation rates, and the quantum efficiency. Thus, two europium species were found in the parent as well as in the silylated materials: one species located on the internal surface and the second inside the 2-2.5 nm pores. The species located on the internal surface is characterized by photoluminescence decay times of 105 mu s <tau < 125 mu s, an asymmetry value R of 0.6 < R < 0.8, and a quantum efficiency of 2%-2.5%. Upon silylation, the photoluminescence decay times, the asymmetry values, and the quantum efficiency were increased to 160 mu s <tau < 180 mu s, 1.7 < R < 2.1, and similar to 4%, respectively. Following silylation, the number of water molecules is reduced in the first coordination shell of the europium species located on the internal surface from eight to nine to about five. On the other hand, the europium species located inside the pores showed a much longer photoluminescence decay time (460 mu s <tau < 560 mu s) and a much higher asymmetry ratio (5 < R < 6.5). The related photoluminescence efficiency was 26%-30%. An average of one up to two water molecules in the first coordination shell of the europium species located inside the pores was estimated for both parent and silylated materials.
The adsorption of boron (boric acid) from aqueous solutions on alumina has been investigated at pH 8.0, I=0.1M NaClO4, T=22 +/- 3 degrees C, and under normal atmospheric conditions. The characterization of the adsorbed species was performed by Raman spectroscopy and the spectroscopic speciation was assisted by theoretical DFT calculations. Evaluation of the spectroscopic data points to the formation of inner-sphere surface complexes and indicates the formation of two different types of adsorbed boron species. The theoretical calculations corroborate the spectroscopic data and indicate that at low boron concentration the monodentate surface species dominates, whereas increased boron concentration favors the formation of a bidentate surface species. Assuming low coverage, the conditional formation constant for the monodentate surface species has been evaluated to be log=4.1 +/- 0.1.
In an area that contains high concentrations of natural organic matter, it is expected that it plays an important role on the behavior of rare earth elements (REE), like europium, and of trivalent actinides. Competitive interactions with H+, inorganic species, major cations, e.g. Ca(II) or Mg(II), could influence these metals transport and bioavailability. Competitive experiments between cations, which can bind differently to humic substances and Eu3+,will bring an improved understanding of the competitive mechanisms. The aim of this study is to acquire data for Eu(III)/Cu(II) and Eu(III)/Ca(II) competitive binding to a sedimentary originated humic acid (Gorleben, Germany). The NICA-Donnan parameters for Ca2+, Cu2+, and Eu3+ obtained from competitive binding experiments using Ca2+ or Cu2+ ion selective electrodes were used to model time-resolved laser fluorescence spectroscopy (TRLFS) measurements. Eu3+ and Cu2+ are in direct competition for the same type of sites,whereas Ca2+ has an indirect influence through electrostatic binding.