| This dissertation describes a continuing research effort that involves synthesizing new tunnel-type materials for the selective removal of cesium or strontium from various aqueous waste streams, while attempting to understand their fundamental ion exchange selectivities through the process of structural elucidation. For this study, we synthesized and characterized titanosilicate and germanium-containing analogues of the mineral pharmacosiderite. Pharmacosiderite is a naturally-occurring mineral whose framework composition is ({dollar}rm(FeOH)sb4(AsOsb4)sb3rbracksp-,{dollar} where the alternating iron octahedra and arsenic tetrahedra create a three-dimensional tunnel structure where each cube-face is composed of metal ions that form 8-membered rings with charge-balancing cations and water molecules residing in the channels.; The alkali cation derivatives, {dollar}rm HMsb3(TiO)sb4(SiOsb4)sb3{lcub}cdot{rcub}4Hsb2O (M=Ksp+, Rbsp+, Cssp+){dollar} as well as the acid form, {dollar}rm Hsb4(TiO)sb4(SiOsb4)sb3{lcub}cdot{rcub}8Hsb2O,{dollar} are isostructural with the mineral and crystallize in the cubic space group, P 4 3m, with a = 7.8212(2) A (cesium), a = 7.7644 (3) A (potassium), a = 7.8214(6) A (proton), a = 7.7718(5) A (rubidium) and Z = 1. In addition, two cesium germanium-substituted titanosilicate pharmacosiderites were hydrothermally synthesized and their respective potassium forms were prepared by ion-exchange. A mixed Si/Ti/Ge phase, {dollar}rm HCssb3(TiO)sb{lcub}3.5{rcub}(GeO)sb{lcub}0.5{rcub}(GeOsb4)sb{lcub}2.5{rcub}(SiOsb4)sb{lcub}0.5{rcub}{lcub}cdot{rcub}4Hsb2O,{dollar} crystallizes in the cubic space group, P 4 3m, with a = 7.9376(1) A, while the cesium titanogermanate, {dollar}rm HCssb3(TiO)sb4(GeOsb4)sb3{lcub}cdot{rcub}4Hsb2O,{dollar} possesses a body centered supercell belonging to space group I23, a = 15.9604(3) A. Upon ion-exchange with potassium, the resulting phases, {dollar}rm HKsb3(TiO)sb{lcub}3.5{rcub}(GeO)sb{lcub}0.5{rcub}(GeOsb4)sb{lcub}2.5{rcub}(SiO sb4)sb{lcub}0.5{rcub}{lcub}cdot{rcub}4Hsb2O{dollar} and {dollar}rm HKsb3(TiO)sb4(GeOsb4)sb3{lcub}cdot{rcub}4Hsb2O,{dollar} distorted to the tetragonal space group P 4 b2, with a = b = 11.1571(2), c = 7.9165(2) A, and a = b = 11.215(1), c = 7.9705(2) A, respectively.; The potassium germanium-substituted phases showed an increase in strontium and cesium selectivity with respect to their titanium silicate parent compounds. An increase in selectivity was ascribed to their inherent structures and bond strengths associated with the charge-neutralizing cations and framework oxygens. In comparison with leading inorganic and organic ion exchange materials, batch studies using the radiotracers {dollar}sp{lcub}137{rcub}{dollar}Cs and {dollar}sp{lcub}89{rcub}{dollar}Sr, showed that the potassium titanosilicate, {dollar}rm HKsb3(TiO)sb4(SiOsb4)sb3{lcub}cdot{rcub}4Hsb2O,{dollar} has comparable strontium (and cesium) uptake in groundwater-type solutions. However, the titanium silicate and germanium-substituted exchangers exhibited moderate strontium selectivity and no cesium selectivity in simulants that resemble tank wastes. |
