The state of iron in coal gangue minerals is an important factor in determining the potential for value-added utilization of this solid waste; this is especially true for the coal gangue coming from the Pingshuo open-pit mine in China. The objective of the present study was to characterize the petrological, mineralogical, and chemical states of Fe in the coal gangue from the Carboniferous Taiyuan Formation. Methods used included polarizing microscopy, X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS), X-ray fluorescence, micro-Fourier-transform infrared (micro-FTIR) spectroscopy, and Mössbauer spectroscopy. The coal gangues are mudstones, silty mudstones, and pelitic siltstones, which are composed primarily of kaolinite, quartz, feldspar, pyrite, illite, and magnesite. In coal gangue, the Fe was found to occur in ferruginous minerals, in crystal-lattice substitutions, or in a colloidal state. The ferruginous minerals in the coal gangue are pyrite and marcasite, and the pyrite morphologies are framboidal, euhedral octahedral crystals, subhedral granular crystals, and irregular crystals. The results of SEM–EDS and micro-FTIR confirmed that the lattice substitution of Fe in the coal gangue minerals occurred mainly in kaolinite, resulting in two types of kaolinite: iron-containing and iron-free kaolinite. The former may be transformed from volcanic biotite and the latter from volcanic feldspar. The Mössbauer spectra of kaolinite showed intense doublets with isomer shift and quadrupole splitting values consistent with tetrahedrally coordinated Fe3+ and ocahedrally coordinated Fe2+, suggesting the presence of two types of substitution sites: (1) Fe2+ replacing Al3+ in the octahedral sheet; and (2) Fe3+ replacing Si4+ in the tetrahedral sheet. This study has important theoretical significance for the high-value utilization of coal gangue.

Various secondary phases formed during alteration/dissolution of HLW (high-level nuclear waste) borosilicate glass represent a significant retention potential for radionuclides including divalent actinides. The trioctahedral smectite, hectorite, Na0.7[Li0.7Mg5.3Si8O20(OH)4], is one of the secondary phases identified within the alteration layer of corroded HLW glass. Numerous studies have clearly shown that many radionuclides are associated with clay minerals and the migration of radionuclides is strongly reduced by complexation. Due to the structural complexity and chemical variability of smectites, sorption of radionuclides involves several sorption mechanisms: (1) adsorption via inner-sphere and outer-sphere complexation; (2) cation exchange in the interlayer; and (3) incorporation into the smectite structure. Up to now, it was not known whether trivalent actinides such as Cm(III) and Am(III) become incorporated into the crystal structure of clay minerals like hectorite. We have used a new method, developed by Carrado et al. (1997b), to synthesize a Eu- and a Cm-containing hectorite, utilizing Cm(III) and chemically homologous Eu(III) coprecipitated with Mg(OH)2 as a precursor. X-ray diffraction, Fourier transform infrared spectroscopy and atomic force microscopy identified the reaction products unambiguously as hectorite. The sorption mechanisms of Eu associated with the synthesized hectorite were investigated by time-resolved laser fluorescence spectroscopy (TRLFS). An unhydrated Eu species (fluorescence lifetime 930 µs) and a partly hydrated Eu species (fluorescence lifetime 381 µs) could be identified. The unhydrated Eu species can be interpreted as incorporating Eu(III) into the hectorite structure or a remaining X-ray amorphous silica phase. The spectra of Eu hectorite and the Eu silica complexation are too similar to permit differentiation between these species, but dialysis experiments demonstrated the close association of the unhydrated Eu species with the crystalline hectorite phase. Time-resolved laser fluorescence spectroscopy (TRLFS) measurements identified the same incorporated Eu species as long as the Eu hectorite was stable under acidic conditions. The stability of the Eu hectorite could be shown by the dialysis experiment over a time period of 160 h. Between 160 und 500 h, hectorite became unstable and a new silica phase was detected. In addition, TRLFS measurements of the Cm-containing hectorite confirmed the incorporation of actinides in the smectite structure. The Cm-hectorite and Cm-silica species can be differentiated unambiguously by TRLFS. In order to differentiate between coprecipitated and surface-sorbed Eu species, batch sorption studies were performed with synthetic Eu-free hectorite. For the surface-sorbed Eu species, a fluorescence lifetime of 284 µs (3.1 H2O molecules) was found, which clearly differs from the coprecipitated species with a fluorescence lifetime of 930 µs. The different lifetimes indicate a different chemical environment. Based on all observations it seems to be very likely that trace amounts of Cm/Eu occupy a distorted octahedral site in the hectorite.