Cation exchange properties of highly charged swelling micas and titanosilicates

Open Access
Author:
Noh, Young Dong
Graduate Program:
Soil Science
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 14, 2010
Committee Members:
  • Sridhar Komarneni, Dissertation Advisor
  • Sridhar Komarneni, Committee Chair
  • Maryann Victoria Bruns, Committee Member
  • Carmen Enid Martinez, Committee Member
  • Della M Roy, Committee Member
Keywords:
  • Mica
  • Titanosilicate
  • Ion exchange
  • Selectivity
Abstract:
The principal goal of this thesis work was to investigate cation exchange properties of highly charged swelling micas such as Na1Si3AlMg3O10F2 • xH2O (Na-2-mica), Na1.5Si2.5Al1.5Mg3O10F2 • xH2O (Na-3-mica), and Na2Si2Al2Mg3O10F2 • xH2O (Na-4-mica) and titanosilicates such as sodium Engelhard titanosilicate 4 (Na-ETS-4) and Na-titanosilicate. The three mica ion exchangers were synthesized using a NaCl melt method and the two Na titanosilicates were synthesized by hydrothermal treatment, and characterized by X-ray diffraction and scanning electron microscopy. In addition, 29Si and 27Al MASNMR spectroscopy was used to understand layer charge properties of Na-micas and local chemical binding environment of Si and Al. Cation exchange isotherms and Kielland plots of the above five ion exchangers were constructed in order to evaluate their selective cation exchange properties for Sr2+ and Ba2+ and heavy metals such as Hg2+ and Co2+. 2Na+→ Sr2+ exchange isotherms and Kielland plots revealed that Na-2-mica, Na-ETS-4, and Na-titanosilicate showed high selectivity for Sr2+. For the case of 2Na+→ Ba2+ exchange, Na-2-mica and Na-ETS-4 had high preference for Ba2+ over Na+ with most of the equilibrium solutions used. Na-2-mica showed higher selectivity for Sr2+ and Ba2+ than Na-3-mica and Na-4-mica because of higher expansibility of Na-2-mica resulting in easier accessibility to exchange sites in interlayer space than the other two micas. This expansibility was limited by increasing electrostatic attraction with the increase in intercalated divalent cations. In the case of 2Na+ → Hg2+ exchange reaction, Na-ETS-4 was found to have higher distribution coefficient (Kd) value than any other ion exchangers tested. The Kielland plot for Hg2+ showed that only Na-ETS-4 had selectivity for mercury (II) with most of the exchange solutions used. 2Na+ → Co2+exchange equilibria revealed that Na-2-mica and Na-ETS-4 had high selectivity for cobalt ions over sodium ions at low Co equivalent fraction in the solid phase. Two different (001) phases of ~12 Å and ~14 Å were seen in the XRD patterns of the Na-micas after 2Na+ → M2+ (M=Sr, Ba, Hg) exchange reactions with different exchange solutions. This difference in the d001-value suggested the presence of adsorbed divalent cations with different hydration states in interlayer space after ion exchange reactions, because of partial dehydration of intercalated cations by strong electrostatic attraction between adsorbed cations and negatively charged layers. Both the extent of divalent cations intercalated and layer charge density affected the attractive electrostatic force. As the uptake of adsorbed divalent cations in the solid phase and the layer charge density increased, the electrostatic attraction increased contributing to the increase in the extent of the dehydration of cations intercalated. Unlike other cases, after 2Na+ → Co2+ exchange reaction, strong ~14 Å phase of Na-2-mica was retained even at high Co2+ uptake, while ~12 Å phase decreased as the amount of Co2+ increased. This is due to stronger hydration energy of Co2+ ions than those of Sr2+, Ba2+and Hg2+ ions. The results on cation exchange properties of the above ion exchangers indicated that Na-2-mica and Na-ETS-4 could be excellent ion exchangers for separation and remediation of radioactive species such as Sr2+ and Ba2+ while only Na-ETS-4 would be useful for decontamination of Hg2+.