title: Structural insight into iodide uptake by AFm phases.
abstract: The ability of cement phases carrying positively charged surfaces to retard the mobility of (129)I, present as iodide (I(-)) in groundwater, was investigated in the context of safe disposal of radioactive waste. (125)I sorption experiments on ettringite, hydrotalcite, chloride-, carbonate- and sulfate-containing AFm phases indicated that calcium-monosulfate (AFm-SO(4)) is the only phase that takes up trace levels of iodide. The structures of AFm phases prepared by coprecipitating iodide with other anions were investigated in order to understand this preferential uptake mechanism. X-ray diffraction (XRD) investigations showed a segregation of monoiodide (AFm-I(2)) and Friedel's salt (AFm-Cl(2)) for I-Cl mixtures, whereas interstratifications of AFm-I(2) and hemicarboaluminate (AFm-OH-(CO(3))(0.5)) were observed for the I-CO(3) systems. In contrast, XRD measurements indicated the formation of a solid solution between AFm-I(2) and AFm-SO(4) for the I-SO(4) mixtures. Extended X-ray absorption fine structure spectroscopy showed a modification of the coordination environment of iodine in I-CO(3) and in I-SO(4) samples compared to pure AFm-I(2). This is assumed to be due to the introduction of stacking faults in I-CO(3) samples on one hand and due to the presence of sulfate and associated space-filling water molecules as close neighbors in I-SO(4) samples on the other hand. The formation of a solid solution between AFm-I(2) and AFm-SO(4), with a short-range mixing of iodide and sulfate, implies that AFm-SO(4) bears the potential to retard (129)I.
authors.affiliation: Laboratory for Waste Management, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. laure.aimoz@psi.ch
authors.forename: Laure
authors.initials: L
authors.lastname: Aimoz
authors.forename: Erich
authors.initials: E
authors.lastname: Wieland
authors.forename: Christine
authors.initials: C
authors.lastname: Taviot-Guého
authors.forename: Rainer
authors.initials: R
authors.lastname: Dähn
authors.forename: Marika
authors.initials: M
authors.lastname: Vespa
authors.forename: Sergey V
authors.initials: SV
authors.lastname: Churakov
mesh_terms: D011232:Chemical Precipitation; D003252:Construction Materials; D007454:Iodides / Q000737:chemistry*; D008956:Models, Chemical; D011850:Radioactive Waste / Q000032:analysis; D012037:Refuse Disposal; D012996:Solutions; D013431:Sulfates / Q000737:chemistry; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D007454:Iodides; D011850:Radioactive Waste; D012996:Solutions; D013431:Sulfates