EFFECTS OF g RADIATION ON CHROMATE IMMOBILIZATION BY CALCINED HYDROTALCITES

Abstract—The ability of hydrotalcites to retain anionic wastes was studied. In particular, Cr(VI)-adsorbed hydrotalcites were heated to immobilize Cr(VI) in the solid sample. When the heating temperature increased up to 500ºC, the lamellar structure of hydrotalcite was lost. At high temperatures (1200ºC), the solids were recrystallized in the form of a spinel. Lixiviation with 1 N and 5 N NaCl solutions were utilized to simulate the effect of sea water and of concentrated NaCl solutions in salt mines on the immobilization of Cr. Radiation damage on the solid containing the immobilized Cr was studied by g-irradiating with a 60Co source at 1000 and 6000 kGy. The Cr-containing samples heated at 1200ºC, whether irradiated or not, safely immobilized Cr in the hydrotalcite mainly in the form of MgCr2O4 spinel. Irradiation of hydrotalcites revealed two different effects: (1) samples heated up to 1200ºC evolved as a solid in which chromium was retained more firmly than in the non-irradiated material, irradiation enhancing the spinel formation; (2) the structure of samples heated at 1200ºC developed a preferential crystallite orientation rather than a purely random one or new location of chromium ions, this effect did not affect Cr immobilization in the solid. Chromium lixiviation with 5 N NaCl solution was always less than the corresponding value with 1 N NaCl solution, probably due to the poor mobility of Cl ions in the highly concentrated NaCl solution