Sorption of trivalent actinides onto gibbsite, γ-alumina, and kaolinite : A spectroscopic study of An(III) interactions at the mineral-water interfaces

This Ph.D. thesis concerns investigations on the interactions of trivalent actinides with the mineral phases γ-alumina, gibbsite, and kaolinite. Spectroscopic methods, namely time-resolved laser fluorescence spectroscopy (TRLFS) and nuclear magnetic resonance (NMR) have been employed together with macroscopic sorption investigations to obtain molecular level process understanding of the solid-water interface reactions occurring in the mineral suspensions. \n\nResults obtained in the macroscopic sorption investigations with gadolinium(III) and the spectroscopic (TRLFS) investigations with curium(III) onto the aluminum hydroxide phase gibbsite, α-Al(OH)3 showed that two interaction mechanisms, inner sphere surface complexation and incorporation, are required to explain the metal ion speciation at the gibbsite surface over the investigated pH range 5-11.5. Sorption of Gd3+/Cm3+ commences at pH values close to 5 through formation of an inner sphere surface complex. When the pH is raised towards the alkaline pH range another inner sphere complex can be detected from the acquired TRLFS data. The continuous increase of suspension pH, however, also influences the solubility of aluminum with a direct impact on the trivalent metal ion speciation. Upon moving from higher aluminum solubility regions in the acidic pH range towards the solubility minimum at pH 6.3, aluminum hydroxide precipitates from oversaturated solutions and consequently incorporates surface bound curium on the gibbsite surface. When further increasing the pH towards the alkaline pH range, aluminum solubility increases again which results in dissolution of the incorporating precipitate uncovering the curium surface complex. This sorption behavior has not been encountered previously in investigations with other aluminum oxide/hydroxide minerals as sorbent phases for trivalent metal ion attachment. Identification of an incorporated curium species as a consequence of pH variations is a clear proof that mineral surfaces cannot be considered as inert with regard to chemical variations as done in many studies. \n\nIn macroscopic (Eu3+) and TRLFS (Cm3+) sorption investigations with kaolinite as sorbent phase, both outer sphere and inner sphere surface complexes could be identified. Outer sphere complexation was pronounced in background electrolyte concentrations of 1 mM NaClO4, while outer sphere attachment was found only for low (less than 10-7 M) europium concentrations on a natural kaolinite batch and not at all in synthetic kaolinite suspensions when 100 mM NaClO4 was used in the experiments. Three inner sphere curium complexes differing only in the degree of hydrolysis were found to dominate the speciation over a large pH range (5.5-11.5). These species were assigned to &gt;Cm(H2O)52+, &gt;Cm(OH)(H2O)4+ and &gt;Cm(OH)2(H2O)3 based on fluorescence lifetime data and they correlate very well in terms of emission peak positions and fluorescence lifetimes values to curium species found on other clay mineral surfaces. Above pH 11.5 a further curium complex that could not be assigned to a pure hydrolyzed curium surface complex became prevailing. The TRLFS experiments showed that the dissolution of kaolinite in the alkaline pH-range (&gt;10) resulted in the formation of a ternary complex between surface-sorbed curium and dissolved silicates in solution. Thus, the dissolution of the sorbent phase could again be shown to influence the metal ion speciation verifying the need for spectroscopic investigations for a correct assignment of the formed complexes at the solid/water interface. \n\nThe sorption of diamagnetic yttrium(III) and paramagnetic europium(III) on the various surface hydroxyl groups of γ-alumina (γ-Al2O3) and kaolinite was investigated in NMR studies. Upon Eu3+/Y3+ attachment onto the mineral surfaces the 1H NMR signal could be seen to decrease. In difference spectra produced by subtracting the acquired proton spectra of samples with high metal ion concentrations from spectra of clean mineral samples or samples with lower metal ion loadings, a manifold of surface hydroxyl groups influenced by the metal ion attachment to the surface could be identified. These surface groups could be assigned to doubly coordinated Al2-OH and singly coordinated Al-OH groups on the respective mineral surfaces. In the γ-alumina study the wide distribution of chemically very similar OH protons was attributed to variations in O-H bond lengths and to variations in the Al coordination (AlIV/AlVI). In the kaolinite study removed protons in the metal ion sorption reaction were assigned to singly coordinated Al-OH groups at the kaolinite edge surfaces. In addition, indications of trivalent metal ion attachment to bridging Al-OH-Al groups at the gibbsite-like basal plane of the mineral were obtained.

Topic: Radioactive element chemistry and processing
Subfield: Inorganic Chemistry
Field: Chemistry
Domain: Physical Sciences