TY - JOUR
T1 - 23Na and 133Cs NMR study of cation adsorption on mineral surfaces
T2 - Local environments, dynamics, and effects of mixed cations
AU - Kim, Yeongkyoo
AU - Kirkpatrick, R. James
PY - 1997/12
Y1 - 1997/12
N2 - 23Na and 133Cs MAS NMR methods were used to study the adsorption sites and atomic-scale dynamics of Cs, Na, and their mixtures on the surfaces of illite, kaolinite, boehmite, and silica gel reacted with 0.1 M NaCl and mixed NaCl and CsCl solutions. The 23Na spectra were collected at relative humidities (R.H.) from ≈0 to 100% and temperatures from room temperature to -80°C and compared to previously published 133Cs spectra collected under the same conditions. Surface Cs+ and Na+ behave differently due mainly to their different hydration energies, smaller for Cs+ than for Na+ . Cs+ is adsorbed in two distinguishable environments, (1) inner-sphere complexes and (2) outer-sphere complexes and in the diffuse layer. Na+ is adsorbed only as outer-sphere complexes and in the diffuse layer. Na+ undergoes dynamical averaging at frequencies ≫10 kHz at room temperature and high R.H., as indicated by low temperature data. For Na+ the increasing peak width with decreasing temperature indicates a decreasing frequency of motional averaging among Na+ sites with similar nearest-neighbor environment but a range of next nearest neighbor (NNN) environment. In contrast, for Cs+ there is exchange between two distinguishable sites. Experiments with mixed solutions support our peak assignments and interpretations. Surface Na+ successfully competes with Cs+ for outer-sphere sites and the diffuse layer, whereas Cs+ prefers inner-sphere complexes. This differences results in a surface Cs/(Cs + Na) ratio greater than the bulk solution. Chemical analysis shows that Cs+ is preferentially adsorbed by illite relative to silica gel, and this is due to the large permanent surface charge of illite caused by Al substitution for Si in the tetrahedral sites.
AB - 23Na and 133Cs MAS NMR methods were used to study the adsorption sites and atomic-scale dynamics of Cs, Na, and their mixtures on the surfaces of illite, kaolinite, boehmite, and silica gel reacted with 0.1 M NaCl and mixed NaCl and CsCl solutions. The 23Na spectra were collected at relative humidities (R.H.) from ≈0 to 100% and temperatures from room temperature to -80°C and compared to previously published 133Cs spectra collected under the same conditions. Surface Cs+ and Na+ behave differently due mainly to their different hydration energies, smaller for Cs+ than for Na+ . Cs+ is adsorbed in two distinguishable environments, (1) inner-sphere complexes and (2) outer-sphere complexes and in the diffuse layer. Na+ is adsorbed only as outer-sphere complexes and in the diffuse layer. Na+ undergoes dynamical averaging at frequencies ≫10 kHz at room temperature and high R.H., as indicated by low temperature data. For Na+ the increasing peak width with decreasing temperature indicates a decreasing frequency of motional averaging among Na+ sites with similar nearest-neighbor environment but a range of next nearest neighbor (NNN) environment. In contrast, for Cs+ there is exchange between two distinguishable sites. Experiments with mixed solutions support our peak assignments and interpretations. Surface Na+ successfully competes with Cs+ for outer-sphere sites and the diffuse layer, whereas Cs+ prefers inner-sphere complexes. This differences results in a surface Cs/(Cs + Na) ratio greater than the bulk solution. Chemical analysis shows that Cs+ is preferentially adsorbed by illite relative to silica gel, and this is due to the large permanent surface charge of illite caused by Al substitution for Si in the tetrahedral sites.
UR - http://www.scopus.com/inward/record.url?scp=0031412480&partnerID=8YFLogxK
U2 - 10.1016/S0016-7037(97)00347-5
DO - 10.1016/S0016-7037(97)00347-5
M3 - Article
AN - SCOPUS:0031412480
SN - 0016-7037
VL - 61
SP - 5199
EP - 5208
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 24
ER -