REMOVAL AND FIXATION OF HARMFUL OXOMETALATES IONS BY FORMATION OF LAYERED HYDROTALCITE-LIKE COMPOUNDS AND THEIR HEAT-TREATMENT

The removal of chromium(VI) or manganese(VII) oxometalate ion from aqu eous solution by the magnesium-aluminium oxide solid solution, Mg0.58A l0.28 square 0.14O, prepared by the thermal decomposition of a synthet ic hydrotalcite was investigated at 25-degrees-C. The oxide adsorbed h ighly CrO42- and MnO4- to reconstruct the hydrotalcite-type structure with the basal spacing of 8.7 angstrom. As seen in Fig. 2, the Freundl ich's adsorption isotherm can be well applied in both cases. The Freun dlich parameters, k and n values, were calculated to be 55.2 mg.g-1 an d 5.0 for CrO42-, and 7.7 mg.g-1 and 2.5 for MnO4-. Thus the amount of divalent anion adsorbed was larger than that of monovalent anion in t he wide region of equilibrium concentration. In addition, the crystall inity of the solid product was found to be poor with the increase in t he amount of the guest anions adsorbed as shown in Figs. 3 and 4. Then , the hydrotalcite-like compounds incorporating these oxometalates ion s were heated at 300-900-degrees-C for 1 h and the dissolution behavio rs of Cr or Mn ion from the heated products in aqueous solutions were tested. These metal ions were fixed in the matrix of the resulting MgO (Periclase) phase at 500-750-degrees-C and in the chemically stable M gAl2O4 (Spinel) phase as the solid solutions at 800-900-degrees-C. By these heat-treatment, neither metal ions were leached into the solutio ns of pH 4-10 and pH 2-12 f rom the products heated above 500-degrees- C and 800-degrees-C, respectively.