UPTAKE OF ARSENITE AND ARSENATE BY LAYERED DOUBLE HYDROXIDES

Open Access
Author:
Grover, Kanchan B
Graduate Program:
Soil Science
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 09, 2008
Committee Members:
  • Sridhar Komarneni, Thesis Advisor
Keywords:
  • remediation
  • Arsenate
  • Arsenite
  • Layered double hydroxides
  • soil and water
Abstract:
The objectives of this thesis research were to (i) synthesize cost-effective layered double hydroxides (LDH) or anionic clays to remove arsenite and arsenate oxyanions from water and (ii) quantify oxyanion uptake by LDH and understand the mechanisms of uptake. Two types of LDH were synthesized: hydrotalcite and hydrocalumite by coprecipitation at room temperature or higher with different compositions of layers and interlayers. Removal of 100% arsenate and 87.5% removal of arsenite with nitrate form of hydrotalcite and 99.9% of arsenate and 83.6% of arsenite with nitrate form of hydrocalumite were achieved. Chloride and carbonate forms of hydrotalcite and chloride form of hydrocalumite removed 50-90% of these oxyanions. The uptake capacity of LDH synthesized by different methods was also compared. Layered double hydroxides synthesized by co-precipitation method had greater uptake capacity than those synthesized by hydrothermal method because of smaller crystal size in the former. Powder X-ray diffraction (XRD) patterns showed greater crystallinity of LDH synthesized by hydrothermal method than those synthesized by coprecipitation method. These results suggest that oxyanion uptake is less in the case of well crystallized hydrothermally synthesized LDH compared to that of the poorly crystallized materials because it is more difficult to break electrostatic bonds in the former than in the latter. Higher oxyanion uptake was observed with calcined LDH than with commercially available un-calcined carbonate form of LDH. Calcination of LDH produced intermediate non-stoichiometric oxides, which undergo rehydration and regeneration of the structure with the incorporation of these oxyanions. iv The mechanism of arsenate and arsenite uptake is by topotactic anion exchange for all forms of hydrotalcites while the mechanism of uptake of these ions in hydrocalumites appears to be initial anion exchange followed by dissolution-precipitation. The results of uptake were confirmed by XRD, Raman Spectroscopy, and Scanning Electron Microscopy. This study indicated that LDH are efficient and probably cost-effective materials for removal of arsenite and arsenate contaminants from water.